06 - 1 Neural Sciences

01 - 1.1 Introduction
1.1 Introduction
Neural Sciences
1.1 Introduction
The human brain is responsible for our cognitive processes, emotions, and behaviors—
that is, everything we think, feel, and do. Although the early development and adult
function of the brain are shaped by multiple factors (e.g., epigenetic, environmental,
and psychosocial experiences), the brain is the final integrator of these influences.
Despite the many advances in neural sciences over the last several decades, including
the “decade of the brain” in the 1990s, and the wide acceptance of the brain as the
biological substrate for normal and abnormal mental functions, there has not been a
true transformational advance in the treatment of mental disorders for more than half a
century. The most obvious reason for the absence of more progress is the profound
complexity of the human brain. A perhaps less obvious reason is the current practice of
psychiatric diagnosis, which, for most clinicians, is based on syndrome-based
classification systems.
The purpose of this chapter is to introduce the neural sciences sections, which describe
the anatomy and function of the human brain, and then to discuss how an evolution of
thinking toward a brain-based or biologically based diagnostic system for mental illness
might facilitate our efforts to advance brain research, to develop better treatments, and
to improve patient care.
In other fields of medicine, diagnosis is based on physical signs and symptoms, a
medical history, and results of laboratory and radiological tests. In psychiatry, a
diagnosis is based primarily on the clinician’s impression of the patient’s interpretation
of his or her thoughts and feelings. The patient’s symptoms are then cross-referenced to
a diagnostic or classification manual (e.g., Diagnostic and Statistical Manual of Mental
Disorders [DSM-5], International Statistical Classification of Diseases and Related Health
Problems [ICD]) containing hundreds of potential syndromes, and one or more diagnoses
are applied to the particular patient. These standard classification systems represent
significant improvements in reliability over previous diagnostic systems, but there is
little reason to believe that these diagnostic categories are valid, in the sense that they
represent discrete, biologically distinct entities. Although a patient with no symptoms or
complaints can be diagnosed as having diabetes, cancer, or hypertension on the basis of
blood tests, X-rays, or vital signs, a patient with no symptoms cannot be diagnosed with
schizophrenia, for example, because there are no currently recognized objective,
independent assessments.
The goals of clinicians and researchers are to reduce human suffering by increasing
our understanding of diseases, developing new treatments to prevent or cure diseases,
and caring for patients in an optimal manner. If the brain is the organ of focus for
mental illnesses, then it may be time to be more ambitious in building the classification
of patients with mental illnesses directly from our understanding of biology, rather than
only from the assessment of a patient’s symptoms.
THE HUMAN BRAIN
The following neural sciences sections each address a field of brain biology. Each of
these fields could be relevant to the pathophysiology and treatment of mental illnesses.
Although the complexity of the human brain is daunting compared with other organs of
the body, progress can only be made if one approaches this complexity consistently,
methodically, and bravely.
The neuronal and glial cells of the human brain are organized in a characteristic
manner, which has been increasingly clarified through modern neuroanatomical
techniques. In addition, our knowledge of normal human brain development has become
more robust in the last decade. The human brain clearly evolved from the brain of lower
animal species, allowing inferences to be made about the human brain from animal
studies. Neurons communicate with one another through chemical and electrical
neurotransmission. The major neurotransmitters are the monoamines, amino acids, and
neuropeptides. Other chemical messengers include neurotrophic factors and an array of
other molecules, such as nitric oxide. Electrical neurotransmission occurs through a wide
range of ion channels. Chemical and electrical signals received by a neuron
subsequently initiate various molecular pathways within other neurons that regulate the
biology and function of individual neurons, including the expression of individual genes
and the production of proteins.
In addition to the central nervous system (CNS), the human body contains two other
systems that have complex, internal communicative networks: the endocrine system and
the immune system. The recognition that these three systems communicate with each
other
has
given
birth
to
the
fields
of
psychoneuroendocrinology
and
psychoneuroimmunology. Another property shared by the CNS, the endocrine system,
and the immune system is the regular changes they undergo with the passage of time
(e.g., daily, monthly), which is the basis of the field of chronobiology.
PSYCHIATRY AND THE HUMAN BRAIN
In the first half of the 20th century, the advances in psychodynamic psychiatry, as well
as in social and epidemiological psychiatry, led to a separation of psychiatric research
from the study of the human brain. Since the 1950s, the appreciation of the effectiveness
of medications in treating mental disorders and the mental effects of illicit drugs, have
reestablished a biological view of mental illness, which had already been seeded by the
introduction of electroconvulsive therapy (ECT) and James Papez’s description of the
limbic circuit in the 1930s. This biological view has been reinforced further by the
development of brain imaging techniques that have helped reveal how the brain
performs in normal and abnormal conditions. During this period, countless discoveries
have been made in basic neural science research using experimental techniques to assess
the development, structure, biology, and function of the CNS of humans and animals.
Psychopharmacology
The effectiveness of drugs in the treatment of mental illness has been a major feature of
the last half-century of psychiatric practice. The first five editions of this textbook
divided psychopharmacological treatment into four chapters on antipsychotic,
antidepressant, antianxiety, and mood-stabilizing drugs. The prior division of
psychiatric drugs into four classes is less valid now than it was in the past for the
following reasons: (1) Many drugs of one class are used to treat disorders previously
assigned to another class; (2) drugs from all four categories are used to treat disorders
not previously treatable by drugs (for example, eating disorders, panic disorders, and
impulse control disorders); and (3) drugs such as clonidine (Catapres), propranolol
(Inderal), and verapamil (Isoptin) can effectively treat a variety of psychiatric disorders
and do not fit easily into the aforementioned classification of drugs.
The primary motivation for this change was that the variety and application of the
drug treatments no longer clearly fit a division of disorders into psychosis, depression,
anxiety, and mania. In other words, the clinical applications of biologically based
treatments did not neatly align with our syndrome-based diagnostic system. An
implication of this observation could be that drug response might be a better indicator
of underlying biological brain dysfunction than any particular group of symptoms. For
example, although the DSM-5 distinguishes major depressive disorder from generalized
anxiety disorder, most clinicians are aware that these are often overlapping symptoms
and conditions in clinical practice. Moreover, the same drugs are used to treat both
conditions.
The animal models that are used to identify new drug treatments may also have
affected our ability to advance research and treatment. Many major classes of
psychiatric drugs were discovered serendipitously. Specifically, the drugs were
developed originally for nonpsychiatric indications, but observant clinicians and
researchers noted that psychiatric symptoms improved in some patients, which led to
focused study of these drugs in psychiatric patients. The availability of these effective
drugs, including monoaminergic antidepressants and antipsychotics, led to the
development of animal models that could detect the effects of these drugs (e.g., tricyclic
antidepressants increase the time mice spend trying to find a submerged platform in a
“forced swim” test). These animal models were then used to screen new compounds in
an attempt to identify drugs that were active in the same animal models. The potential
risk of this overall strategy is that these animal models are merely a method for
detecting a particular molecular mechanism of action (e.g., increasing serotonin
concentrations), rather than a model for a true behavioral analog of a human mental
illness (e.g., behavioral despair in a depressed patient).
Endophenotypes
A possible diagnosis-related parallel to how this textbook separated the four classes of
psychotropic drugs into approximately 30 different categories is the topic of
endophenotypes in psychiatric patients. An endophenotype is an internal phenotype,
which is a set of objective characteristics of an individual that are not visible to the
unaided eye. Because there are so many steps and variables that separate a particular
set of genes from the final functioning of a whole human brain, it may be more
tractable to consider intermediate assessments such as endophenotypes. This hypothesis
is based on the assumption that the number of genes that are involved in an
endophenotype might be fewer than the number of genes involved in causing what we
would conceptualize as a disease. The nature of an endophenotype, as considered in
psychiatry, is biologically defined on the basis of neuropsychological, cognitive,
neurophysiological, neuroanatomical, biochemical, and brain imaging data. Such an
endophenotype, for example, might include specific cognitive impairments as just one of
its objectively measured features. This endophenotype would not be limited to patients
with a diagnosis of schizophrenia because it might also be found in some patients with
depression or bipolar disorder.
The potential role of an endophenotype can be further clarified by stating what it is
not. An endophenotype is not a symptom, and it is not a diagnostic marker. A
classification based on the presence or absence of one or more endophenotypes would
be based on objective biological and neuropsychological measures with specific
relationships to genes and brain function. A classification based on endophenotypes
might also be a productive approach toward the development of more relevant animal
models of mental illnesses, and thus the development of novel treatments.
PSYCHIATRY AND THE HUMAN GENOME
Perhaps 70 to 80 percent of the 25,000 human genes are expressed in the brain, and
because most genes code for more than one protein, there may be 100,000 different
proteins in the brain. Perhaps 10,000 of these are known proteins with somewhat
identified functions, and no more than 100 of these are the targets for existing
psychotherapeutic drugs.
The study of families with the use of population genetic methods over the last 50
years has consistently supported a genetic, heritable component to mental disorders.
More recent techniques in molecular biology have revealed that specific chromosomal
regions and genes are associated with particular diagnoses. A potentially very powerful
application of these techniques has been to study transgenic models of behavior in
animals. These transgenic models can help us understand the effects of individual genes
as well as discover completely novel molecular targets for drug development.
It may be a natural response to resist “simple” genetic explanations for human
features. Nonetheless, research on humans generally has found that approximately 40 to
70 percent of aspects of cognition, temperament, and personality are attributable to
genetic factors. Because these are the very domains that are affected in mentally ill
patients, it would not be surprising to discover a similar level of genetic influence on
mental illness, especially if we were able to assess this impact at a more discrete level,
such as with endophenotypes.
Individual Genes and Mental Disorders
Several types of data and observations suggest that any single gene is likely to have
only a modest effect on the development of a mental disorder, and that when a mental
disorder is present in an individual, it represents the effects of multiple genes,
speculatively on the order of five to ten genes. This hypothesis is also supported by our
failure to find single genes with major effects in mental illnesses. Some researchers,
however, still consider it a possibility that genes with major effects will be identified.
“Nature” and “Nurture” within the CNS
In 1977, George Engel, at the University of Rochester, published a paper that articulated
the biopsychosocial model of disease, which stressed an integrated approach to human
behavior and disease. The biological system refers to the anatomical, structural, and
molecular substrates of disease; the psychological system refers to the effects of
psychodynamic factors; and the social system examines cultural, environmental, and
familial influences. Engel postulated that each system affects and is affected by the
others.
The observation that a significant percentage of identical twins are discordant for
schizophrenia is one example of the type of data that support the understanding that
there are many significant interactions between the genome and the environment (i.e.,
the biological basis of the biopsychosocial concept). Studies in animals have also
demonstrated that many factors—including activity, stress, drug exposure, and
environmental toxins—can regulate the expression of genes and the development and
functioning of the brain.
Mental Disorders Reflect Abnormalities in Neuroanatomical Circuits and
Synaptic Regulation
Although genes lead to the production of proteins, the actual functioning of the brain
needs to be understood at the level of regulation of complex pathways of
neurotransmission and intraneuronal signaling, and of networks of neurons within and
between brain regions. In other words, the downstream effects of abnormal genes are
modifications in discrete attributes such as axonal projections, synaptic integrity, and
specific steps in intraneuronal molecular signaling.
Why Not a Genetic-Based Diagnostic System?
Some researchers have proposed moving psychiatry toward a completely genetic-based
diagnostic system. This proposal, however, seems premature based on the complexity of
the genetic factors presumably involved in psychiatric disorders, the current absence of
sufficient data to make these genetic connections, and the importance of epigenetic and
environmental influences on the final behavioral outcomes resulting from an
individual’s genetic information.
LESSONS FROM NEUROLOGY
Clinical and research neurologists seem to have been able to think more clearly than
psychiatrists about their diseases of interest and their causes, perhaps because the
symptoms are generally nonbehavioral. Neurologists have biologically grounded
differential diagnoses and treatment choices. This clarity of approach has helped lead to
significant advances in neurology in the last two decades, for example, clarification of
the amyloid precursor protein abnormalities in some patients with Alzheimer’s disease,
the presence of trinucleotide repeat mutations in Huntington’s disease and
spinocerebellar ataxia, and the appreciation of alpha-synucleinopathies, such as
Parkinson’s disease and Lewy body dementia.
The continued separation of psychiatry from neurology is in itself a potential
impediment to good patient care and research. Many neurological disorders have
psychiatric symptoms (e.g., depression in patients following a stroke or with multiple
sclerosis or Parkinson’s disease), and several of the most severe psychiatric disorders
have been associated with neurological symptoms (e.g., movement disorders in
schizophrenia). This is not surprising given that the brain is the organ shared by
psychiatric and neurological diseases, and the division between these two disease areas
is arbitrary. For example, patients with Huntington’s disease are at much greater risk
for a wide range of psychiatric symptoms and syndromes, and thus many different DSM5 diagnoses. Because we know that Huntington’s disease is an autosomal dominant
genetic disorder, the observation that it can manifest with so many different DSM-5
diagnoses does not speak to a very strong biological distinction among the existing
DSM-5 categories.
EXAMPLES OF COMPLEX HUMAN BEHAVIORS
The goal to understand the human brain and its normal and abnormal functioning is
truly one of the last frontiers for humans to explore. Trying to explain why a particular
individual is the way he or she is, or what causes schizophrenia, for example, will
remain too large a challenge for some decades. It is more approachable to consider
more discrete aspects of human behavior.
It is not the role of textbooks to set policies or to write diagnostic manuals, but rather
to share knowledge, generate ideas, and encourage innovation. The authors believe,
however, that it is time to reap the insights of decades of neural science and clinical
brain research and to build the classification of mental illnesses on fundamental
principles of biology and medicine. Regardless of official diagnostic systems, however,
clinicians and researchers should fully understand the biological component of the

02 - 1.2 Functional Neuroanatomy
1.2 Functional Neuroanatomy
biopsychosocial model, and not let research or patient care suffer because of a
diagnostic system that is not founded on biological principles.
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1.2 Functional Neuroanatomy
The sensory, behavioral, affective, and cognitive phenomena and attributes experienced
by humans are mediated through the brain. It is the organ that perceives and affects the
environment and integrates past and present. The brain is the organ of the mind that
enables persons to sense, do, feel, and think.
Sensory systems create an internal representation of the external world by processing
external stimuli into neuronal impulses. A separate map is formed for each sensory
modality. Motor systems enable persons to manipulate their environment and to
influence the behavior of others through communication. In the brain, sensory input,
representing the external world, is integrated with internal drivers, memories, and
emotional stimuli in association units, which in turn drive the actions of motor units.
Although psychiatry is concerned primarily with the brain’s association function, an
appreciation of information processing of the sensory and motor systems is essential for
sorting logical thought from the distortions introduced by psychopathology.
BRAIN ORGANIZATION
The human brain contains approximately 1011 neurons (nerve cells) and approximately
1012 glial cells. Neurons most classically consist of a soma, or cell body, which contains
the nucleus; usually multiple dendrites, which are processes that extend from the cell
body and receive signals from other neurons; and a single axon, which extends from the
cell body and transmits signals to other neurons. Connections between neurons are
made at axon terminals; there the axons of one neuron generally contact the dendrite or
cell body of another neuron. Neurotransmitter release occurs within axon terminals and
is one of the major mechanisms for intraneuronal communications, and also for the
effects of psychotropic drugs.
There are three types of glial cells, and although they have often been thought of as
having only a supportive role for neuronal functioning, glia have been increasingly
appreciated as potentially involved in brain functions that may contribute more directly
to both normal and disease mental conditions. The most common type of glial cell are
the astrocytes, which have a number of functions, including nutrition of neurons,
deactivation of some neurotransmitters, and integration with the blood–brain barrier.
The oligodendrocytes in the central nervous system and the Schwann cells in the
peripheral nervous system wrap their processes around neuronal axons, resulting in
myelin sheaths that facilitate the conduction of electrical signals. The third type of glial
cells, the microglia, which are derived from macrophages, are involved in removing
cellular debris following neuronal death.
The neurons and glial cells are arranged in regionally distinct patterns within the brain. Neurons and their processes
form groupings in many different ways, and these patterns of organization, or architecture, can be evaluated by several
approaches. The pattern of distribution of nerve cell bodies, called cytoarchitecture, is revealed by aniline dyes called Nissl
stains that stain ribonucleotides in the nuclei and the cytoplasm of neuronal cell bodies. The Nissl stains show the relative
size and packing density of the neurons and, consequently, reveal the organization of the neurons into the different layers
of the cerebral cortex.
SENSORY SYSTEMS
The external world offers an infinite amount of potentially relevant information. In this
overwhelming volume of sensory information in the environment, the sensory systems
must both detect and discriminate stimuli; they winnow relevant information from the
mass of confounding input by applying filtration at all levels. Sensory systems first
transform external stimuli into neural impulses and then filter out irrelevant
information to create an internal image of the environment, which serves as a basis for
reasoned thought. Feature extraction is the quintessential role of sensory systems, which
achieve this goal with their hierarchical organizations, first by transforming physical
stimuli into neural activity in the primary sense organs and then by refining and
narrowing the neural activity in a series of higher cortical processing areas. This neural
processing eliminates irrelevant data from higher representations and reinforces crucial
features. At the highest levels of sensory processing, neural images are transmitted to
the association areas to be acted on in the light of emotions, memories, and drives.
Somatosensory System
The somatosensory system, an intricate array of parallel point-to-point connections from
the body surface to the brain, was the first sensory system to be understood in
anatomical detail. The six somatosensory modalities are light touch, pressure, pain,
temperature, vibration, and proprioception (position sense). The organization of nerve
bundles and synaptic connections in the somatosensory system encodes spatial
relationships at all levels, so that the organization is strictly somatotopic (Fig. 1.2-1).
FIGURE 1.2-1
Pathway of somatosensory information processing. (Adapted from Patestas MA, Gartner
LP. A Textbook of Neuroanatomy. Malden, MA: Blackwell; 2006:149.)
Within a given patch of skin, various receptor nerve terminals act in concert to mediate distinct modalities. The
mechanical properties of the skin’s mechanoreceptors and thermoreceptors generate neural impulses in response to
dynamic variations in the environment while they suppress static input. Nerve endings are either fast or slow responders;
their depth in the skin also determines their sensitivity to sharp or blunt stimuli. Thus the representation of the external
world is significantly refined at the level of the primary sensory organs.
The receptor organs generate coded neural impulses that travel proximally along the sensory nerve axons to the spinal
cord. These far-flung routes are susceptible to varying systemic medical conditions and to pressure palsies. Pain, tingling,
and numbness are the typical presenting symptoms of peripheral neuropathies.
All somatosensory fibers project to, and synapse in, the thalamus. The thalamic neurons preserve the somatotopic
representation by projecting fibers to the somatosensory cortex, located immediately posterior to the sylvian fissure in the
parietal lobe. Despite considerable overlap, several bands of cortex roughly parallel to the sylvian fissure are segregated by
a somatosensory modality. Within each band is the sensory “homunculus,” the culmination of the careful somatotopic
segregation of the sensory fibers at the lower levels. The clinical syndrome of tactile agnosia (astereognosis) is defined by
the inability to recognize objects based on touch, although the primary somatosensory modalities—light touch, pressure,
pain, temperature, vibration, and proprioception—are intact. This syndrome, localized at the border of the somatosensory
and association areas in the posterior parietal lobe, appears to represent an isolated failure of only the highest order of
feature extraction, with preservation of the more basic levels of the somatosensory pathway.
Reciprocal connections are a key anatomical feature of crucial importance to
conscious perception—as many fibers project down from the cortex to the thalamus as
project up from the thalamus to the cortex. These reciprocal fibers play a critical role in
filtering sensory input. In normal states, they facilitate the sharpening of internal
representations, but in pathological states, they can generate false signals or
inappropriately suppress sensation. Such cortical interference with sensory perception is
thought to underlie many psychosomatic syndromes, such as the hemisensory loss that
characterizes conversion disorder.
The prenatal development of the strict point-to-point pattern that characterizes the
somatosensory system remains an area of active study. Patterns of sensory innervation
result from a combination of axonal guidance by particular molecular cues and pruning
of exuberant synaptogenesis on the basis of an organism’s experience. Leading
hypotheses weigh contributions from a genetically determined molecular map—in which
the arrangement of fiber projections is organized by fixed and diffusible chemical cues—
against contributions from the modeling and remodeling of projections on the basis of
coordinated neural activity. Thumbnail calculations suggest that the 30,000 to 40,000
genes in human deoxyribonucleic acid (DNA) are far too few to encode completely the
position of all the trillions of synapses in the brain. In fact, genetically determined
positional cues probably steer growing fibers toward the general target, and the pattern
of projections is fine-tuned by activity-dependent mechanisms. Recent data suggest that
well-established adult thalamocortical sensory projections can be gradually remodeled
as a result of a reorientation of coordinated sensory input or in response to loss of part
of the somatosensory cortex, for instance, in stroke.
Development of the Somatosensory System
A strict somatotopic representation exists at each level of the somatosensory system.
During development, neurons extend axons to connect to distant brain regions; after
arriving at the destination, a set of axons must therefore sort itself to preserve the
somatotopic organization. A classic experimental paradigm for this developmental
process is the representation of a mouse’s whiskers in the somatosensory cortex. The
murine somatosensory cortex contains a barrel field of cortical columns, each of which
corresponds to one whisker. When mice are inbred to produce fewer whiskers, fewer
somatosensory cortex barrels appear. Each barrel is expanded in area, and the entire
barrel field covers the same area of the somatosensory cortex as it does in normal
animals. This experiment demonstrates that certain higher cortical structures can form
in response to peripheral input and that different input complexities determine different
patterns of synaptic connectivity. Although the mechanisms by which peripheral input
molds cortical architecture are largely unknown, animal model paradigms are beginning
to yield clues. For example, in a mutant mouse that lacks monoamine oxidase A and,
thus, has extremely high cortical levels of serotonin, barrels fail to form in the
somatosensory cortex. This result indirectly implicates serotonin in the mechanism of
barrel field development.
In adults, the classic mapping studies of Wilder Penfield suggested the existence of a
homunculus, an immutable cortical representation of the body surface. More recent
experimental evidence from primate studies and from stroke patients, however, has
promoted a more plastic conception than that of Penfield. Minor variations exist in the
cortical pattern of normal individuals, yet dramatic shifts in the map can occur in
response to loss of cortex from stroke or injury. When a stroke ablates a significant
fraction of the somatosensory homunculus, the homuncular representation begins to
contract and shift proportionately to fill the remaining intact cortex.
Moreover, the cortical map can be rearranged solely in response to a change in the
pattern of tactile stimulation of the fingers. The somatotopic representation of the
proximal and distal segments of each finger normally forms a contiguous map,
presumably because both segments contact surfaces simultaneously. However, under
experimental conditions in which the distal segments of all fingers are simultaneously
stimulated while contact of the distal and proximal parts of each finger is separated, the
cortical map gradually shifts 90 degrees to reflect the new sensory experience. In the
revised map, the cortical representation of the proximal segment of each finger is no
longer contiguous with that of the distal segment.
These data support the notion that the internal representation of the external world, although static in gross structure,
can be continuously modified at the level of synaptic connectivity to reflect relevant sensory experiences. The cortical
representation also tends to shift to fit entirely into the available amount of cortex.
These results also support the notion that cortical representations of sensory input, or of memories, may be holographic
rather than spatially fixed: The pattern of activity, rather than the physical structure, may encode information. In sensory
systems, this plasticity of cortical representation allows recovery from brain lesions; the phenomenon may also underlie
learning.
Visual System
Visual images are transduced into neural activity within the retina and are processed
through a series of brain cells, which respond to increasingly complex features, from the
eye to the higher visual cortex. The neurobiological basis of feature extraction is best
understood in finest detail in the visual system. Beginning with classic work in the
1960s, research in the visual pathway has produced two main paradigms for all sensory
systems. The first paradigm, mentioned earlier with respect to the somatosensory
system, evaluates the contributions of genetics and experience—or nature and nurture—
in the formation of the final synaptic arrangement. Transplantation experiments,
resulting in an accurate point-to-point pattern of connectivity, even when the eye was
surgically inverted, have suggested an innate, genetically determined mechanism of
synaptic pattern formation. The crucial role of early visual experience in establishing
the adult pattern of visual connections, on the other hand, crystallized the hypothesis of
activity-dependent formation of synaptic connectivity. The final adult pattern is the
result of both factors.
The second main paradigm, most clearly revealed in the visual system, is that of
highly specialized brain cells that respond exclusively to extremely specific stimuli.
Recent work, for example, has identified cells in the inferior temporal cortex that
respond only to faces viewed at a specific angle. An individual’s response to a particular
face requires the activity of large neural networks and may not be limited to a single
neuron. Nevertheless, the cellular localization of specific feature extraction is of critical
importance in defining the boundary between sensory and association systems, but only
in the visual system has this significant question been posed experimentally.
In the primary visual cortex, columns of cells respond specifically to lines of a specific
orientation. The cells of the primary visual cortex project to the secondary visual cortex,
where cells respond specifically to particular movements of lines and to angles. In turn,
these cells project to two association areas, where additional features are extracted and
conscious awareness of images forms.
The inferior temporal lobe detects the shape, form, and color of the object—the what
questions; the posterior parietal lobe tracks the location, motion, and distance—the
where questions. The posterior parietal lobe contains distinct sets of neurons that signal
the intention either to look into a certain part of visual space or to reach for a particular
object. In the inferior temporal cortices (ITCs), adjacent cortical columns respond to
complex forms. Responses to facial features tend to occur in the left ITC, and responses
to complex shapes tend to occur in the right ITC. The brain devotes specific cells to the
recognition of facial expressions and to the aspect and position of faces of others with
respect to the individual.
The crucial connections between the feature-specific cells and the association areas
involved in memory and conscious thought remain to be delineated. Much elucidation of
feature recognition is based on invasive animal studies. In humans, the clinical
syndrome of prosopagnosia describes the inability to recognize faces, in the presence of
preserved recognition of other environmental objects. On the basis of pathological and
radiological examination of individual patients, prosopagnosia is thought to result from
disconnection of the left ITC from the visual association area in the left parietal lobe.
Such lesional studies are useful in identifying necessary components of a mental
pathway, but they may be inadequate to define the entire pathway. One noninvasive
technique that is still being perfected and is beginning to reveal the full anatomical
relation of the human visual system to conscious thought and memory is functional
neuroimaging.
As is true for language, there appears to be a hemispheric asymmetry for certain
components of visuospatial orientation. Although both hemispheres cooperate in
perceiving and drawing complex images, the right hemisphere, especially the parietal
lobe, contributes the overall contour, perspective, and right-left orientation, and the left
hemisphere adds internal detail, embellishment, and complexity. The brain can be
fooled in optical illusions.
Neurological conditions such as strokes and other focal lesions have permitted the definition of several disorders of
visual perception. Apperceptive visual agnosia is the inability to identify and draw items using visual cues, with
preservation of other sensory modalities. It represents a failure of transmission of information from the higher visual
sensory pathway to the association areas and is caused by bilateral lesions in the visual association areas. Associative visual
agnosia is the inability to name or use objects despite the ability to draw them. It is caused by bilateral medial
occipitotemporal lesions and can occur along with other visual impairments. Color perception may be ablated in lesions of
the dominant occipital lobe that include the splenium of the corpus callosum. Color agnosia is the inability to recognize a
color despite being able to match it. Color anomia is the inability to name a color despite being able to point to it. Central
achromatopsia is a complete inability to perceive color. Anton’s syndrome is a failure to acknowledge blindness, possibly
owing to interruption of fibers involved in self-assessment. It is seen with bilateral occipital lobe lesions. The most
common causes are hypoxic injury, stroke, metabolic encephalopathy, migraine, herniation resulting from mass lesions,
trauma, and leukodystrophy. Balint’s syndrome consists of a triad of optic ataxia (the inability to direct optically guided
movements), oculomotor apraxia (inability to direct gaze rapidly), and simultanagnosia (inability to integrate a visual scene
to perceive it as a whole). Balint’s syndrome is seen in bilateral parietooccipital lesions. Gerstmann’s syndrome includes
agraphia, calculation difficulties (acalculia), right–left disorientation, and finger agnosia. It has been attributed to lesions of
the dominant parietal lobe.
Development of the Visual System
In humans, the initial projections from both eyes intermingle in the cortex. During the
development of visual connections in the early postnatal period, there is a window of
time during which binocular visual input is required for development of ocular
dominance columns in the primary visual cortex. Ocular dominance columns are stripes
of cortex that receive input from only one eye, separated by stripes innervated only by
fibers from the other eye. Occlusion of one eye during this critical period completely
eliminates the persistence of its fibers in the cortex and allows the fibers of the active
eye to innervate the entire visual cortex. In contrast, when normal binocular vision is
allowed during the critical development window, the usual dominance columns form;
occluding one eye after the completion of innervation of the cortex produces no
subsequent alteration of the ocular dominance columns. This paradigm crystallizes the
importance of early childhood experience on the formation of adult brain circuitry.
Auditory System
Sounds are instantaneous, incremental changes in ambient air pressure. The pressure
changes cause the ear’s tympanic membrane to vibrate; the vibration is then transmitted
to the ossicles (malleus, incus, and stapes) and thereby to the endolymph or fluid of the
cochlear spiral. Vibrations of the endolymph move cilia on hair cells, which generate
neural impulses. The hair cells respond to sounds of different frequency in a tonotopic
manner within the cochlea, like a long, spiral piano keyboard. Neural impulses from the
hair cells travel in a tonotopic arrangement to the brain in the fibers of the cochlear
nerve. They enter the brainstem cochlear nuclei, are relayed through the lateral
lemniscus to the inferior colliculi, and then to the medial geniculate nucleus (MGN) of
the thalamus. MGN neurons project to the primary auditory cortex in the posterior
temporal lobe. Dichotic listening tests, in which different stimuli are presented to each
ear simultaneously, demonstrate that most of the input from one ear activates the
contralateral auditory cortex and that the left hemisphere tends to be dominant for
auditory processing.
Sonic features are extracted through a combination of mechanical and neural filters. The representation of sound is
roughly tonotopic in the primary auditory cortex, whereas lexical processing (i.e., the extraction of vowels, consonants,
and words from the auditory input) occurs in higher language association areas, especially in the left temporal lobe. The
syndrome of word deafness, characterized by intact hearing for voices but an inability to recognize speech, may reflect
damage to the left parietal cortex. This syndrome is thought to result from disconnection of the auditory cortex from
Wernicke’s area. A rare, complementary syndrome, auditory sound agnosia, is defined as the inability to recognize
nonverbal sounds, such as a horn or a cat’s meow, in the presence of intact hearing and speech recognition. Researchers
consider this syndrome the right hemisphere correlate of pure word deafness.
Development of the Auditory System
Certain children are unable to process auditory input clearly and therefore have
impaired speech and comprehension of spoken language. Studies on some of these
children have determined that, in fact, they can discriminate speech if the consonants
and vowels—the phonemes—are slowed twofold to fivefold by a computer. Based on
this observation, a tutorial computer program was designed that initially asked
questions in a slowed voice and, as subjects answered questions correctly, gradually
increased the rate of phoneme presentation to approximate normal rates of speech.
Subjects gained some ability to discriminate routine speech over a period of 2 to 6 weeks
and appeared to retain these skills after the tutoring period was completed. This finding
probably has therapeutic applicability to 5 to 8 percent of children with speech delay,
but ongoing studies may expand the eligible group of students. This finding, moreover,
suggests that neuronal circuits required for auditory processing can be recruited and be
made more efficient long after language is normally learned, provided that the circuits
are allowed to finish their task properly, even if this requires slowing the rate of input.
Circuits thus functioning with high fidelity can then be trained to speed their processing.
A recent report has extended the age at which language acquisition may be acquired
for the first time.
A boy who had intractable epilepsy of one hemisphere was mute because the
uncontrolled seizure activity precluded the development of organized language
functions. At the age of 9 years he had the abnormal hemisphere removed to cure the
epilepsy. Although up to that point in his life he had not spoken, he initiated an
accelerated acquisition of language milestones beginning at that age and ultimately
gained language abilities only a few years delayed relative to his chronological age.
Researchers cannot place an absolute upper limit on the age at which language
abilities can be learned, although acquisition at ages beyond the usual childhood period
is usually incomplete. Anecdotal reports document acquisition of reading skills after the
age of 80 years.
Olfaction
Odorants, or volatile chemical cues, enter the nose, are solubilized in the nasal mucus,
and bind to odorant receptors displayed on the surface of the sensory neurons of the
olfactory epithelium. Each neuron in the epithelium displays a unique odorant receptor,
and cells displaying a given receptor are arranged randomly within the olfactory
epithelium. Humans possess several hundred distinct receptor molecules that bind the
huge variety of environmental odorants; researchers estimate that humans can
discriminate 10,000 different odors. Odorant binding generates neural impulses, which
travel along the axons of the sensory nerves through the cribriform plate to the olfactory
bulb. Within the bulb, all axons corresponding to a given receptor converge onto only 1
or 2 of 3,000 processing units called glomeruli. Because each odorant activates several
receptors that activate a characteristic pattern of glomeruli, the identity of external
chemical molecules is represented internally by a spatial pattern of neural activity in
the olfactory bulb.
Each glomerulus projects to a unique set of 20 to 50 separate columns in the olfactory cortex. In turn, each olfactory
cortical column receives projections from a unique combination of glomeruli. The connectivity of the olfactory system is
genetically determined. Because each odorant activates a unique set of several receptors and thus a unique set of olfactory
bulb glomeruli, each olfactory cortical column is tuned to detect a different odorant of some evolutionary significance to
the species. Unlike the signals of the somatosensory, visual, and auditory systems, olfactory signals do not pass through the
thalamus but project directly to the frontal lobe and the limbic system, especially the pyriform cortex. The connections to
the limbic system (amygdala, hippocampus, and pyriform cortex) are significant. Olfactory cues stimulate strong
emotional responses and can evoke powerful memories.
Olfaction, the most ancient sense in evolutionary terms, is tightly associated with sexual and reproductive responses. A
related chemosensory structure, the vomeronasal organ, is thought to detect pheromones, chemical cues that trigger
unconscious, stereotyped responses. In some animals, ablation of the vomeronasal organ in early life may prevent the onset
of puberty. Recent studies have suggested that humans also respond to pheromones in a manner that varies according to
the menstrual cycle. The structures of higher olfactory processing in phylogenetically more primitive animals have
evolved in humans into the limbic system, the center of the emotional brain and the gate through which experience is
admitted into memory according to emotional significance. The elusive basic animal drives with which clinical psychiatry
constantly grapples may therefore, in fact, originate from the ancient centers of higher olfactory processing.
Development of the Olfactory System
During normal development, axons from the nasal olfactory epithelium project to the
olfactory bulb and segregate into about 3,000 equivalent glomeruli. If an animal is
exposed to a single dominant scent in the early postnatal period, then one glomerulus
expands massively within the bulb at the expense of the surrounding glomeruli. Thus, as
discussed earlier with reference to the barrel fields of the somatosensory cortex, the size
of brain structures may reflect the environmental input.
Taste
Soluble chemical cues in the mouth bind to receptors in the tongue and stimulate the
gustatory nerves, which project to the nucleus solitarius in the brainstem. The sense of
taste is believed to discriminate only broad classes of stimuli: sweet, sour, bitter, and
salty. Each modality is mediated through a unique set of cellular receptors and channels,
of which several may be expressed in each taste neuron. The detection and the
discrimination of foods, for example, involve a combination of the senses of taste,
olfaction, touch, vision, and hearing. Taste fibers activate the medial temporal lobe, but
the higher cortical localization of taste is only poorly understood.
Autonomic Sensory System
The autonomic nervous system (ANS) monitors the basic functions necessary for life. The
activity of visceral organs, blood pressure, cardiac output, blood glucose levels, and
body temperature are all transmitted to the brain by autonomic fibers. Most autonomic
sensory information remains unconscious; if such information rises to conscious levels, it
is only as a vague sensation, in contrast to the capacity of the primary senses to
transmit sensations rapidly and exactly.
Alteration of Conscious Sensory Perception through Hypnosis
Hypnosis is a state of heightened suggestibility attainable by a certain proportion of the
population. Under a state of hypnosis, gross distortions of perception in any sensory
modality and changes in the ANS can be achieved instantaneously. The anatomy of the
sensory system does not change, yet the same specific stimuli may be perceived with
diametrically opposed emotional value before and after induction of the hypnotic state.
For example, under hypnosis a person may savor an onion as if it were a luscious
chocolate truffle, only to reject the onion as abhorrently pungent seconds later, when
the hypnotic suggestion is reversed. The localization of the hypnotic switch has not been
determined, but it presumably involves both sensory and association areas of the brain.
Experiments tracing neural pathways in human volunteers via functional neuroimaging
have demonstrated that shifts in attention in an environmental setting determine
changes in the regions of the brain that are activated, on an instantaneous time scale.
Thus the organizing centers of the brain may route conscious and unconscious thoughts
through different sequences of neural processing centers, depending on a person’s
ultimate goals and emotional state. These attention-mediated variations in synaptic
utilization can occur instantaneously, much like the alteration in the routing of
associational processing that may occur in hypnotic states.
MOTOR SYSTEMS
Body muscle movements are controlled by the lower motor neurons, which extend axons
—some as long as 1 meter—to the muscle fibers. Lower motor neuron firing is regulated
by the summation of upper motor neuron activity. In the brainstem, primitive systems
produce gross coordinated movements of the entire body. Activation of the rubrospinal
tract stimulates flexion of all limbs, whereas activation of the vestibulospinal tract
causes all limbs to extend. Newborn infants, for example, have all limbs tightly flexed,
presumably through the dominance of the rubrospinal system. In fact, the movements of
an anencephalic infant, who completely lacks a cerebral cortex, may be
indistinguishable from the movements of a normal newborn. In the first few months of
life, the flexor spasticity is gradually mitigated by the opposite actions of the
vestibulospinal fibers, and more limb mobility occurs.
At the top of the motor hierarchy is the corticospinal tract, which controls fine movements and which eventually
dominates the brainstem system during the first years of life. The upper motor neurons of the corticospinal tract reside in
the posterior frontal lobe, in a section of cortex known as the motor strip. Planned movements are conceived in the
association areas of the brain, and in consultation with the basal ganglia and cerebellum, the motor cortex directs their
smooth execution. The importance of the corticospinal system becomes immediately evident in strokes, in which
spasticity returns as the cortical influence is ablated and the actions of the brainstem motor systems are released from
cortical modulation.
Basal Ganglia
The basal ganglia, a subcortical group of gray matter nuclei, appear to mediate postural
tone. The four functionally distinct ganglia are the striatum, the pallidum, the substantia
nigra, and the subthalamic nucleus. Collectively known as the corpus striatum, the
caudate and putamen harbor components of both motor and association systems. The
caudate nucleus plays an important role in the modulation of motor acts. Anatomical
and functional neuroimaging studies have correlated decreased activation of the caudate
with obsessive-compulsive behavior. When functioning properly, the caudate nucleus
acts as a gatekeeper to allow the motor system to perform only those acts that are goal
directed. When it fails to perform its gatekeeper function, extraneous acts are
performed, as in obsessive-compulsive disorder or in the tic disorders, such as Tourette’s
disorder. Overactivity of the striatum owing to lack of dopaminergic inhibition (e.g., in
parkinsonian conditions) results in bradykinesia, an inability to initiate movements. The
caudate, in particular, shrinks dramatically in Huntington’s disease. This disorder is
characterized by rigidity, on which is gradually superimposed choreiform, or “dancing,”
movements. Psychosis may be a prominent feature of Huntington’s disease, and suicide
is not uncommon. The caudate is also thought to influence associative, or cognitive,
processes.
The globus pallidus contains two parts linked in series. In a cross section of the brain,
the internal and external parts of the globus pallidus are nested within the concavity of
the putamen. The globus pallidus receives input from the corpus striatum and projects
fibers to the thalamus. This structure may be severely damaged in Wilson’s disease and
in carbon monoxide poisoning, which are characterized by dystonic posturing and
flapping movements of the arms and legs.
The substantia nigra is named the black substance because the presence of melanin
pigment causes it to appear black to the naked eye. It has two parts, one of which is
functionally equivalent to the globus pallidus interna. The other part degenerates in
Parkinson’s disease. Parkinsonism is characterized by rigidity and tremor and is
associated with depression in more than 30 percent of cases.
Finally, lesions in the subthalamic nucleus yield ballistic movements, sudden limb
jerks of such velocity that they are compared to projectile movement.
Together, the nuclei of the basal ganglia appear capable of initiating and maintaining
the full range of useful movements. Investigators have speculated that the nuclei serve
to configure the activity of the overlying motor cortex to fit the purpose of the
association areas. In addition, they appear to integrate proprioceptive feedback to
maintain an intended movement.
Cerebellum
The cerebellum consists of a simple six-cell pattern of circuitry that is replicated roughly
10 million times. Simultaneous recordings of the cerebral cortex and the cerebellum have
shown that the cerebellum is activated several milliseconds before a planned movement.
Moreover, ablation of the cerebellum renders intentional movements coarse and
tremulous. These data suggest that the cerebellum carefully modulates the tone of
agonistic and antagonistic muscles by predicting the relative contraction needed for
smooth motion. This prepared motor plan is used to ensure that exactly the right
amount of flexor and extensor stimuli is sent to the muscles. Recent functional imaging
data have shown that the cerebellum is active, even during the mere imagination of
motor acts when no movements ultimately result from its calculations. The cerebellum
harbors two, and possibly more, distinct “homunculi” or cortical representations of the
body plan.
Motor Cortex
Penfield’s groundbreaking work defined a motor homunculus in the precentral gyrus,
Brodmann’s area 4 (Fig. 1.2-2), where a somatotopic map of the motor neurons is
found. Individual cells within the motor strip cause contraction of single muscles. The
brain region immediately anterior to the motor strip is called the supplementary motor
area, Brodmann’s area 6. This region contains cells that when individually stimulated
can trigger more complex movements by influencing a firing sequence of motor strip
cells. Recent studies have demonstrated wide representation of motor movements in the
brain.
FIGURE 1.2-2
Drawing of the lateral view (A) and medial view (B) of the cytoarchitectonic
subdivisions of the human brain as determined by Brodmann. (From Sadock BJ, Sadock
VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009.)
The skillful use of the hands is called praxis, and deficits in skilled movements are termed apraxias. The three levels of
apraxia are limb-kinetic, ideomotor, and ideational. Limb-kinetic apraxia is the inability to use the contralateral hand in
the presence of preserved strength; it results from isolated lesions in the supplementary motor area, which contains
neurons that stimulate functional sequences of neurons in the motor strip.
Ideomotor apraxia is the inability to perform an isolated motor act on command, despite preserved comprehension,
strength, and spontaneous performance of the same act. Ideomotor apraxia simultaneously affects both limbs and involves
functions so specialized that they are localized to only one hemisphere. Conditions in two separate areas can produce this
apraxia. Disconnection of the language comprehension area, Wernicke’s area, from the motor regions causes an inability to
follow spoken commands, and lesions to the left premotor area may impair the actual motor program as it is generated by
the higher-order motor neurons. This program is transmitted across the corpus callosum to the right premotor area,
which directs the movements of the left hand. A lesion in this callosal projection can also cause an isolated ideomotor
apraxia in the left hand. This syndrome implies the representation of specific motor acts within discrete sections of the
left premotor cortex. Thus just as some cells respond selectively to specific environmental features in the higher sensory
cortices, some cells in the premotor cortex direct specific complex motor tasks.
Ideational apraxia occurs when the individual components of a sequence of skilled acts can be performed in isolation,
but the entire series cannot be organized and executed as a whole. For example, the sequence of opening an envelope,
removing the letter, unfolding it, and placing it on the table cannot be performed in order, even though the individual acts
can be performed in isolation. The representation of the concept of a motor sequence may involve several areas,
specifically the left parietal cortex, but it likely also relies on the sequencing and executive functions of the prefrontal
cortex. This apraxia is a typical finding of diffuse cortical degeneration, such as Alzheimer’s disease.
Autonomic Motor System
The autonomic system is divided into a sensory component (described earlier) and a
motor component. The autonomic motor system is divided into two branches: the
sympathetic and the parasympathetic. As a rule, organs are innervated by both types of
fibers, which often serve antagonistic roles. The parasympathetic system slows the heart
rate and begins the process of digestion. In contrast, the sympathetic system mediates the
fight or flight response, with increased heart rate, shunting of blood away from the
viscera, and increased respiration. The sympathetic system is highly activated by
sympathomimetic drugs, such as amphetamine and cocaine, and may also be activated
by withdrawal from sedating drugs such as alcohol, benzodiazepines, and opioids.
Investigators who have found an increased risk of heart attacks in persons with high
levels of hostility have suggested that chronic activation of the sympathetic fight or
flight response, with elevated secretion of adrenaline, may underlie this association.
The brain center that drives the autonomic motor system is the hypothalamus, which houses a set of paired nuclei that
appear to control appetite, rage, temperature, blood pressure, perspiration, and sexual drive. For example, lesions to the
ventromedial nucleus, the satiety center, produce a voracious appetite and rage. In contrast, lesions to the upper region of
the lateral nucleus, the hunger center, produce a profound loss of appetite. Numerous research groups are making intense
efforts to define the biochemical regulation of appetite and obesity and frequently target the role of the hypothalamus.
In the regulation of sexual attraction, the role of the hypothalamus has also become an area of active research. In the
1990s, three groups independently reported neuroanatomical differences between certain of the hypothalamic nuclei of
heterosexual and homosexual men. Researchers interpreted this finding to suggest that human sexual orientation has a
neuroanatomical basis, and this result has stimulated several follow-up studies of the biological basis of sexual orientation.
At present, however, these controversial findings are not accepted without question, and no clear consensus has emerged
about whether the structure of the hypothalamus consistently correlates with sexual orientation. In animal studies, early
nurturing and sexual experiences consistently alter the size of specific hypothalamic nuclei.
Primitive Reflex Circuit
Sensory pathways function as extractors of specific features from the overwhelming
multitude of environmental stimuli, whereas motor pathways carry out the wishes of the
organism. These pathways may be linked directly, for example, in the spinal cord,
where a primitive reflex arc may mediate the brisk withdrawal of a limb from a painful
stimulus, without immediate conscious awareness. In this loop, the peripheral stimulus
activates the sensory nerve, the sensory neuron synapses on and directly activates the
motor neuron, and the motor neuron drives the muscle to contract. This response is
strictly local and all-or-none. Such primitive reflex arcs, however, rarely generate an
organism’s behaviors. In most behaviors, sensory systems project to association areas,
where sensory information is interpreted in terms of internally determined memories,
motivations, and drives. The exhibited behavior results from a plan of action determined
by the association components and carried out by the motor systems.
Localization of Brain Functions
Many theorists have subdivided the brain into functional systems. Brodmann defined 47
areas on the basis of cytoarchitectonic distinctions, a cataloging that has been
remarkably durable as the functional anatomy of the brain has been elucidated. A
separate function, based on data from lesion studies and from functional neuroimaging,
has been assigned to nearly all Brodmann’s areas. At the other extreme, certain experts
have distinguished only three processing blocks: The brainstem and the thalamic
reticular activating system provide arousal and set up attention; the posterior cortex
integrates perceptions and generates language; and, at the highest level, the frontal
cortex generates programs and executes plans like an orchestra conductor.
Hemispheric lateralization of function is a key feature of higher cortical processing.
The primary sensory cortices for touch, vision, hearing, smell, and taste are represented
bilaterally, and the first level of abstraction for these modalities is also usually
represented bilaterally. The highest levels of feature extraction, however, are generally
unified in one brain hemisphere only. For example, recognition of familiar and
unfamiliar faces seems localized to the left inferior temporal cortex, and cortical
processing of olfaction occurs in the right frontal lobe.
Hypotheses about the flow of thought in the brain are based on few experimental data, although this scarcity of findings
has not impeded numerous theoreticians from speculating about functional neuroanatomy. Several roles have been
tentatively assigned to specific lobes of the brain, on the basis of the functional deficits resulting from localized injury.
These data indicate that certain regions of cortex may be necessary for a specific function, but they do not define the
complete set of structures that suffices for a complex task. Anecdotal evidence from surface electrocorticography for the
study of epilepsy, for example, suggests that a right parietal seizure impulse may shoot immediately to the left frontal lobe
and then to the right temporal lobe before spreading locally to the remainder of the parietal lobe. This evidence illustrates
the limitations of naively assigning a mental function to a single brain region. Functional neuroimaging studies frequently
reveal simultaneous activation of disparate brain regions during the performance of even a simple cognitive task.
Nevertheless, particularly in the processing of vision and language, fairly well-defined lobar syndromes have been
confirmed.
Language
The clearest known example of hemispheric lateralization is the localization of language
functions to the left hemisphere. Starting with the work of Pierre Broca and Karl
Wernicke in the 19th century, researchers have drawn a detailed map of language
comprehension and expression.
At least eight types of aphasias in which one or more components of the language
pathway are inured have been defined. Prosody, the emotional and affective components
of language, or “body language,” appears to be localized in a mirror set of brain units in
the right hemisphere.
Because of the major role of verbal and written language in human communication,
the neuroanatomical basis of language is the most completely understood association
function. Language disorders, also called aphasias, are readily diagnosed in routine
conversation, whereas perceptual disorders may escape notice, except during detailed
neuropsychological testing, although these disorders may be caused by injury of an
equal volume of cortex. Among the earliest models of cortical localization of function
were Broca’s 1865 description of a loss of fluent speech caused by a lesion in the left
inferior frontal lobe and Wernicke’s 1874 localization of language comprehension to the
left superior temporal lobe. Subsequent analyses of patients rendered aphasic by
strokes, trauma, or tumors have led to the definition of the entire language association
pathway from sensory input through the motor output.
Language most clearly demonstrates hemispheric localization of function. In most
persons, the hemisphere dominant for language also directs the dominant hand. Ninety
percent of the population is right-handed, and 99 percent of right-handers have left
hemispheric dominance for language. Of the 10 percent who are left-handers, 67
percent also have left hemispheric language dominance; the other 33 percent have
either mixed or right hemispheric language dominance. This innate tendency to
lateralization of language in the left hemisphere is highly associated with an asymmetry
of the planum temporale, a triangular cortical patch on the superior surface of the
temporal lobe that appears to harbor Wernicke’s area. Patients with mixed hemispheric
dominance for language lack the expected asymmetry of the planum temporale. That
asymmetry has been observed in prenatal brains suggests a genetic determinant.
Indeed, the absence of asymmetry runs in families, although both genetic and
intrauterine influences probably contribute to the final pattern.
Language comprehension is processed at three levels. First, in phonological processing,
individual sounds, such as vowels or consonants, are recognized in the inferior gyrus of
the frontal lobes. Phonological processing improves if lip reading is allowed, if speech is
slowed, or if contextual clues are provided. Second, lexical processing matches the
phonological input with recognized words or sounds in the individual’s memory. Lexical
processing determines whether a sound is a word. Recent evidence has localized lexical
processing to the left temporal lobe, where the representations of lexical data are
organized according to semantic category. Third, semantic processing connects the words
to their meaning. Persons with an isolated defect in semantic processing may retain the
ability to repeat words in the absence of an ability to understand or spontaneously
generate speech. Semantic processing activates the middle and superior gyri of the left
temporal lobe, whereas the representation of the conceptual content of words is widely
distributed in the cortex. Language production proceeds in the opposite direction, from
the cortical semantic representations through the left temporal lexical nodes to either
the oromotor phonological processing area (for speech) or the graphomotor system (for
writing). Each of these areas can be independently or simultaneously damaged by
stroke, trauma, infection, or tumor, resulting in a specific type of aphasia.
The garbled word salad or illogical utterances of an aphasic patient leave little
uncertainty about the diagnosis of left-sided cortical injury, but the right hemisphere
contributes a somewhat more subtle, but equally important, affective quality to
language. For example, the phrase “I feel good” may be spoken with an infinite variety
of shadings, each of which is understood differently. The perception of prosody and the
appreciation of the associated gestures, or “body language,” appear to require an intact
right hemisphere. Behavioral neurologists have mapped an entire pathway for prosody
association in the right hemisphere that mirrors the language pathway of the left
hemisphere. Patients with right hemisphere lesions, who have impaired comprehension
or expression of prosody, may find it difficult to function in society despite their intact
language skills.
Developmental dyslexia is defined as an unexpected difficulty with learning in the
context of adequate intelligence, motivation, and education. Whereas speech consists of
the logical combination of 44 basic phonemes of sounds, reading requires a broader set
of brain functions and, thus, is more susceptible to disruption. The awareness of specific
phonemes develops at about the age of 4 to 6 years and appears to be prerequisite to
acquisition of reading skills. Inability to recognize distinct phonemes is the best
predictor of a reading disability. Functional neuroimaging studies have localized the
identification of letters to the occipital lobe adjacent to the primary visual cortex.
Phonological processing occurs in the inferior frontal lobe, and semantic processing
requires the superior and middle gyri of the left temporal lobe. A recent finding of
uncertain significance is that phonological processing in men activates only the left
inferior frontal gyrus, whereas phonological processing in women activates the inferior
frontal gyrus bilaterally. Careful analysis of an individual’s particular reading deficits
can guide remedial tutoring efforts that can focus on weaknesses and thus attempt to
bring the reading skills up to the general level of intelligence and verbal skills.
In children, developmental nonverbal learning disorder is postulated to result from
right hemisphere dysfunction. Nonverbal learning disorder is characterized by poor finemotor control in the left hand, deficits in visuoperceptual organization, problems with
mathematics, and incomplete or disturbed socialization.
Patients with nonfluent aphasia, who cannot complete a simple sentence, may be able
to sing an entire song, apparently because many aspects of music production are
localized to the right hemisphere. Music is represented predominantly in the right
hemisphere, but the full complexity of musical ability seems to involve both
hemispheres. Trained musicians appear to transfer many musical skills from the right
hemisphere to the left as they gain proficiency in musical analysis and performance.
Arousal and Attention
Arousal, or the establishment and maintenance of an awake state, appears to require at
least three brain regions. Within the brainstem, the ascending reticular activating
system (ARAS), a diffuse set of neurons, appears to set the level of consciousness. The
ARAS projects to the intralaminar nuclei of the thalamus, and these nuclei in turn
project widely throughout the cortex. Electrophysiological studies show that both the
thalamus and the cortex fire rhythmical bursts of neuronal activity at rates of 20 to 40
cycles per second. During sleep, these bursts are not synchronized. During wakefulness,
the ARAS stimulates the thalamic intralaminar nuclei, which in turn coordinate the
oscillations of different cortical regions. The greater the synchronization, the higher the
level of wakefulness. The absence of arousal produces stupor and coma. In general,
small discrete lesions of the ARAS can produce a stuporous state, whereas at the
hemispheric level, large bilateral lesions are required to cause the same depression in
alertness. One particularly unfortunate but instructive condition involving extensive,
permanent, bilateral cortical dysfunction is the persistent vegetative state. Sleep–wake
cycles may be preserved, and the eyes may appear to gaze; but the external world does
not register and no evidence of conscious thought exists. This condition represents the
expression of the isolated actions of the ARAS and the thalamus.
The maintenance of attention appears to require an intact right frontal lobe. For example, a widely used test of
persistence requires scanning and identifying only the letter A from a long list of random letters. Healthy persons can
usually maintain performance of such a task for several minutes, but in patients with right frontal lobe dysfunction, this
capacity is severely curtailed. Lesions of similar size in other regions of the cortex usually do not affect persistence tasks.
In contrast, the more generally adaptive skill of maintaining a coherent line of thought is diffusely distributed throughout
the cortex. Many medical conditions can affect this skill and may produce acute confusion or delirium.
One widely diagnosed disorder of attention is attention-deficit/hyperactivity disorder (ADHD). No pathological findings
have been consistently associated with this disorder. Functional neuroimaging studies, however, have variously
documented either frontal lobe or right hemisphere hypometabolism in patients with ADHD, compared with normal
controls. These findings strengthen the notion that the frontal lobes—especially the right frontal lobe—are essential to the
maintenance of attention.
Memory
The clinical assessment of memory should test three periods, which have distinct
anatomical correlates. Immediate memory functions over a period of seconds; recent
memory applies on a scale of minutes to days; and remote memory encompasses months
to years. Immediate memory is implicit in the concept of attention and the ability to
follow a train of thought. This ability has been divided into phonological and
visuospatial components, and functional imaging has localized them to the left and right
hemispheres, respectively. A related concept, incorporating immediate and recent
memory, is working memory, which is the ability to store information for several seconds,
whereas other, related cognitive operations take place on this information. Recent
studies have shown that single neurons in the dorsolateral prefrontal cortex not only
record features necessary for working memory, but also record the certainty with which
the information is known and the degree of expectation assigned to the permanence of
a particular environmental feature. Some neurons fire rapidly for an item that is eagerly
awaited, but may cease firing if hopes are dashed unexpectedly. The encoding of the
emotional value of an item contained in the working memory may be of great
usefulness in determining goal-directed behavior. Some researchers localize working
memory predominantly to the left frontal cortex. Clinically, however, bilateral
prefrontal cortex lesions are required for severe impairment of working memory. Other
types of memory have been described: episodic, semantic, and procedural.
Three brain structures are critical to the formation of memories: the medial temporal
lobe, certain diencephalic nuclei, and the basal forebrain. The medial temporal lobe
houses the hippocampus, an elongated, highly repetitive network. The amygdala is
adjacent to the anterior end of the hippocampus. The amygdala has been suggested to
rate the emotional importance of an experience and to activate the level of
hippocampal activity accordingly. Thus an emotionally intense experience is indelibly
etched in memory, but indifferent stimuli are quickly disregarded.
Animal studies have defined a hippocampal place code, a pattern of cellular
activation in the hippocampus that corresponds to the animal’s location in space. When
the animal is introduced to a novel environment, the hippocampus is broadly activated.
As the animal explores and roams, the firing of certain hippocampal regions begins to
correspond to specific locations in the environment. In about 1 hour, a highly detailed
internal representation of the external space (a “cognitive map”) appears in the form of
specific firing patterns of the hippocampal cells. These patterns of neuronal firing may
bear little spatial resemblance to the environment they represent; rather, they may seem
randomly arranged in the hippocampus. If the animal is manually placed in a certain
part of a familiar space, only the corresponding hippocampal regions show intense
neural activity. When recording continues into sleep periods, firing sequences of
hippocampal cells outlining a coherent path of navigation through the environment are
registered, even though the animal is motionless. If the animal is removed from the
environment for several days and then returned, the previously registered hippocampal
place code is immediately reactivated. A series of animal experiments have dissociated
the formation of the hippocampal place code from either visual, auditory, or olfactory
cues, although each of these modalities may contribute to place code generation. Other
factors may include internal calculations of distances based on counting footsteps or
other proprioceptive information. Data from targeted genetic mutations in mice have
implicated both the N-methyl-D-aspartate (NMDA) glutamate receptors and the calciumcalmodulin kinase II (CaMKII) in the proper formation of hippocampal place fields.
These data suggest that the hippocampus is a significant site for formation and storage
of immediate and recent memories. Although no data yet support the notion, it is
conceivable that the hippocampal cognitive map is inappropriately reactivated during a
déjà vu experience.
The most famous human subject in the study of memory is H. M., a man with
intractable epilepsy, who had both his hippocampi and amygdalae surgically removed
to alleviate his condition. The epilepsy was controlled, but he was left with a
complete inability to form and recall memories of facts. H. M.’s learning and memory
skills were relatively preserved, which led to the suggestion that declarative or factual
memory may be separate within the brain from procedural or skill-related memory. A
complementary deficit in procedural memory with preservation of declarative
memory may be seen in persons with Parkinson’s disease, in whom dopaminergic
neurons of the nigrostriatal tract degenerate. Because this deficit in procedural
memory can be ameliorated with treatment with levodopa (Larodopa), which is
thought to potentiate dopaminergic neurotransmission in the nigrostriatal pathway, a
role has been postulated for dopamine in procedural memory. Additional case reports
have further implicated the amygdala and the afferent and efferent fiber tracts of the
hippocampus as essential to the formation of memories. In addition, lesional studies
have suggested a mild lateralization of hippocampal function in which the left
hippocampus is more efficient at forming verbal memories and the right hippocampus
tends to form nonverbal memories. After unilateral lesions in humans, however, the
remaining hippocampus may compensate to a large extent. Medical causes of amnesia
include alcoholism, seizures, migraine, drugs, vitamin deficiencies, trauma, strokes,
tumors, infections, and degenerative diseases.
The motor system within the cortex receives directives from the association areas. The
performance of a novel act requires constant feedback from the sensory and association
areas for completion, and functional neuroimaging studies have demonstrated
widespread activation of the cortex during unskilled acts. Memorized motor acts initially
require activation of the medial temporal lobe. With practice, however, the performance
of ever-larger segments of an act necessary to achieve a goal become encoded within
discrete areas of the premotor and parietal cortices, particularly the left parietal cortex,
with the result that a much more limited activation of the cortex is seen during highly
skilled acts, and the medial temporal lobe is bypassed. This process is called the
corticalization of motor commands. In lay terms, the process suggests a neuroanatomical
basis for the adage “practice makes perfect.”
Within the diencephalon, the dorsal medial nucleus of the thalamus and the
mammillary bodies appear necessary for memory formation. These two structures are
damaged in thiamine deficiency states usually seen in chronic alcoholics, and their
inactivation is associated with Korsakoff’s syndrome. This syndrome is characterized by
severe inability to form new memories and a variable inability to recall remote
memories.
The most common clinical disorder of memory is Alzheimer’s disease. Alzheimer’s disease is characterized
pathologically by the degeneration of neurons and their replacement by senile plaques and neurofibrillary tangles.
Clinicopathological studies have suggested that the cognitive decline is best correlated with the loss of synapses. Initially,
the parietal and temporal lobes are affected, with relative sparing of the frontal lobes. This pattern of degeneration
correlates with the early loss of memory, which is largely a temporal lobe function. Also, syntactical language
comprehension and visuospatial organization, functions that rely heavily on the parietal lobe, are impaired early in the
course of Alzheimer’s disease. In contrast, personality changes, which reflect frontal lobe function, are relatively late
consequences of Alzheimer’s disease. A rarer, complementary cortical degeneration syndrome, Pick’s disease, first affects
the frontal lobes while sparing the temporal and parietal lobes. In Pick’s disease, disinhibition and impaired language
expression, which are signs of frontal dysfunction, appear early, with relatively preserved language comprehension and
memory.
Memory loss can also result from disorders of the subcortical gray matter structures, specifically the basal ganglia and
the brainstem nuclei, from disease of the white matter, or from disorders that affect both gray and white matter.
Emotion
Individual emotional experiences occupy the attention of all mental health
professionals. Emotion derives from basic drives, such as feeding, sex, reproduction,
pleasure, pain, fear, and aggression, which all animals share. The neuroanatomical
basis for these drives appears to be centered in the limbic system. Distinctly human
emotions, such as affection, pride, guilt, pity, envy, and resentment, are largely learned
and most likely are represented in the cortex (see Color Plate 1.2-3). The regulation of
drives appears to require an intact frontal cortex. The complex interplay of the
emotions, however, is far beyond the understanding of functional neuroanatomists.
Where, for example, are the representations of the id, the ego, and the superego?
Through what pathway are ethical and moral judgments shepherded? What processes
allow beauty to be in the eye of the beholder? These philosophical questions represent a
true frontier of human discovery.
Several studies have suggested that within the cortex exists a hemispheric dichotomy
of emotional representation. The left hemisphere houses the analytical mind but may
have a limited emotional repertoire. For example, lesions to the right hemisphere, which
cause profound functional deficits, may be noted with indifference by the intact left
hemisphere. The denial of illness and of the inability to move the left hand in cases of
right hemisphere injury is called anosognosia. In contrast, left hemisphere lesions, which
cause profound aphasia, can trigger a catastrophic depression, as the intact right
hemisphere struggles with the realization of the loss. The right hemisphere also appears
dominant for affect, socialization, and body image.
Damage to the left hemisphere produces intellectual disorder and loss of the narrative
aspect of dreams. Damage to the right hemisphere produces affective disorders, loss of
the visual aspects of dreams, and a failure to respond to humor, shadings of metaphor,
and connotations. In dichotic vision experiments, two scenes of varied emotional
content were displayed simultaneously to each half of the visual field and were
perceived separately by each hemisphere. A more intense emotional response attended
the scenes displayed to the left visual field that were processed by the right hemisphere.
Moreover, hemisensory changes representing conversion disorders have been repeatedly
noted to involve the left half of the body more often than the right, an observation that
suggests an origin in the right hemisphere.
Within the hemispheres, the temporal and frontal lobes play a prominent role in emotion. The temporal lobe exhibits a
high frequency of epileptic foci, and temporal lobe epilepsy (TLE) presents an interesting model for the role of the
temporal lobe in behavior. In studies of epilepsy, abnormal brain activation is analyzed, rather than the deficits in activity
analyzed in classic lesional studies. TLE is of particular interest in psychiatry because patients with temporal lobe seizures
often manifest bizarre behavior without the classic grand mal shaking movements caused by seizures in the motor cortex.
A proposed TLE personality is characterized by hyposexuality, emotional intensity, and a perseverative approach to
interactions, termed viscosity. Patients with left TLE may generate references to personal destiny and philosophical themes
and display a humorless approach to life. In contrast, patients with right TLE may display excessive emotionality, ranging
from elation to sadness. Although patients with TLE may display excessive aggression between seizures, the seizure itself
may evoke fear.
The inverse of a TLE personality appears in persons with bilateral injury to the temporal lobes after head trauma,
cardiac arrest, herpes simplex encephalitis, or Pick’s disease. This lesion resembles the one described in the Klüver-Bucy
syndrome, an experimental model of temporal lobe ablation in monkeys. Behavior in this syndrome is characterized by
hypersexuality, placidity, a tendency to explore the environment with the mouth, inability to recognize the emotional
significance of visual stimuli, and constantly shifting attention, called hypermetamorphosis. In contrast to the aggression–
fear spectrum sometimes seen in patients with TLE, complete experimental ablation of the temporal lobes appears to
produce a uniform, bland reaction to the environment, possibly because of an inability to access memories.
The prefrontal cortices influence mood in a complementary way. Whereas activation of the left prefrontal cortex appears
to lift the mood, activation of the right prefrontal cortex causes depression. A lesion to the left prefrontal area, at either the
cortical or the subcortical level, abolishes the normal mood-elevating influences and produces depression and
uncontrollable crying. In contrast, a comparable lesion to the right prefrontal area may produce laughter, euphoria, and
witzelsucht, a tendency to joke and make puns. Effects opposite to those caused by lesions appear during seizures, in which
occurs abnormal, excessive activation of either prefrontal cortex. A seizure focus within the left prefrontal cortex can
cause gelastic seizures, for example, in which the ictal event is laughter. Functional neuroimaging has documented left
prefrontal hypoperfusion during depressive states, which normalized after the depression was treated successfully.
Limbic System Function
The limbic system was delineated by James Papez in 1937. The Papez circuit consists of
the hippocampus, the fornix, the mammillary bodies, the anterior nucleus of the
thalamus, and the cingulate gyrus (Fig. 1.2-4). The boundaries of the limbic system were
subsequently expanded to include the amygdala, septum, basal forebrain, nucleus
accumbens, and orbitofrontal cortex.
FIGURE 1.2-4
Schematic drawing of the major anatomic structures of the limbic system. The cingulate
and parahippocampal gyri form the “limbic lobe,” a rim of tissue located along the
junction of the diencephalon and the cerebral hemispheres. (Adapted from Hendelman
WJ. Student’s Atlas of Neuroanatomy. Philadelphia: WB Saunders; 1994:179.)
Although this schema creates an anatomical loop for emotional processing, the
specific contributions of the individual components other than the hippocampus or even
whether a given train of neural impulses actually travels along the entire pathway is
unknown.
The amygdala appears to be a critically important gate through which internal and
external stimuli are integrated. Information from the primary senses is interwoven with
internal drives, such as hunger and thirst, to assign emotional significance to sensory
experiences. The amygdala may mediate learned fear responses, such as anxiety and
panic, and may direct the expression of certain emotions by producing a particular
affect. Neuroanatomical data suggest that the amygdala exerts a more powerful
influence on the cortex, to stimulate or suppress cortical activity, than the cortex exerts
on the amygdala. Pathways from the sensory thalamic relay stations separately send
sensory data to the amygdala and the cortex, but the subsequent effect of the amygdala
on the cortex is the more potent of the two reciprocal connections. In contrast, damage
to the amygdala has been reported to ablate the ability to distinguish fear and anger in
other persons’ voices and facial expressions. Persons with such injuries may have a
preserved ability to recognize happiness, sadness, or disgust. The limbic system appears
to house the emotional association areas, which direct the hypothalamus to express the
motor and endocrine components of the emotional state.
Fear and Aggression
Electrical stimulation of animals throughout the subcortical area involving the limbic
system produces rage reactions (e.g., growling, spitting, and arching of the back).
Whether the animal flees or attacks depends on the intensity of the stimulation.
Limbic System and Schizophrenia
The limbic system has been particularly implicated in neuropathological studies of
schizophrenia. Eugen Bleuler’s well-known four A’s of schizophrenia—affect,
associations, ambivalence, and autism—refer to brain functions served in part by limbic
structures. Several clinicopathological studies have found a reduction in the brain weight
of the gray matter but not of the white matter in persons with schizophrenia. In
pathological as well as in magnetic resonance imaging (MRI) reports, persons with
schizophrenia may have reduced volume of the hippocampus, amygdala, and
parahippocampal gyrus. Schizophrenia may be a late sequela of a temporal epileptic
focus, with some studies reporting an association in 7 percent of patients with TLE.
Functional neuroimaging studies have demonstrated decreased activation of the
frontal lobes in many patients with schizophrenia, particularly during tasks requiring
willed action. A reciprocal increase in activation of the temporal lobe can occur during
willed actions, such as finger movements or speaking, in persons with schizophrenia.
Neuropathological studies have shown a decreased density of neuropil, the intertwined
axons and dendrites of the neurons, in the frontal lobes of these patients. During
development, the density of neuropil is highest around age 1 year and then is reduced
somewhat through synaptic pruning; the density plateaus throughout childhood and is
further reduced to adult levels in adolescence. One hypothesis of the appearance of
schizophrenia in the late teenage years is that excessive adolescent synaptic pruning
occurs and results in too little frontolimbic activity. Some experts have suggested that
hypometabolism and paucity of interneuronal connections in the prefrontal cortex may
reflect inefficiencies in working memory, which permits the disjointed discourse and
loosening of associations that characterize schizophrenia. At present, the molecular basis
for the regulation of the density of synapses within the neuropil is unknown. Other lines
of investigation aimed at understanding the biological basis of schizophrenia have
documented inefficiencies in the formation of cortical synaptic connections in the
middle of the second trimester of gestation, which may result from a viral infection or
malnutrition. Neurodevelopmental surveys administered during childhood have found
an increased incidence of subtle neurological abnormalities before the appearance of the
thought disorder in persons who subsequently exhibited signs of schizophrenia.
In one intriguing study, positron emission tomography (PET) scanning was used to
identify the brain regions that are activated when a person hears spoken language. A
consistent set of cortical and subcortical structures demonstrated increased metabolism
when speech was processed. The researchers then studied a group of patients with
schizophrenia who were experiencing active auditory hallucinations. During the
hallucinations, the same cortical and subcortical structures were activated as were
activated by the actual sounds, including the primary auditory cortex. At the same time,
decreased activation was seen of areas thought to monitor speech, including the left
middle temporal gyrus and the supplementary motor area. This study raises the
questions of what brain structure is activating the hallucinations and by what
mechanism do neuroleptic drugs suppress the hallucinations. Clearly, functional imaging
has much to tell about the neuroanatomical basis of schizophrenia.
Frontal Lobe Function
The frontal lobes, the region that determines how the brain acts on its knowledge,
constitute a category unto themselves. In comparative neuroanatomical studies, the
massive size of the frontal lobes is the main feature that distinguishes the human brain
from that of other primates and that lends it uniquely human qualities. There are four
subdivisions of the frontal lobes. The first three—the motor strip, the supplemental
motor area, and Broca’s area—are mentioned in the preceding discussion of the motor
system and language. The fourth, most anterior, division is the prefrontal cortex. The
prefrontal cortex contains three regions in which lesions produce distinct syndromes: the
orbitofrontal, the dorsolateral, and the medial. Dye-tracing studies have defined dense
reciprocal connections between the prefrontal cortex and all other brain regions.
Therefore, to the extent that anatomy can predict function, the prefrontal cortex is
ideally connected to allow sequential use of the entire palette of brain functions in
executing goal-directed activity. Indeed, frontal lobe injury usually impairs the executive
functions: motivation, attention, and sequencing of actions.
Bilateral lesions of the frontal lobes are characterized by changes in personality—how
persons interact with the world. The frontal lobe syndrome, which is most commonly
produced by trauma, infarcts, tumors, lobotomy, multiple sclerosis, or Pick’s disease,
consists of slowed thinking, poor judgment, decreased curiosity, social withdrawal, and
irritability. Patients typically display apathetic indifference to experience that can
suddenly explode into impulsive disinhibition. Unilateral frontal lobe lesions may be
largely unnoticed because the intact lobe can compensate with high efficiency.
Frontal lobe dysfunction may be difficult to detect by means of highly structured,
formal neuropsychological tests. Intelligence, as reflected in the intelligence quotient
(IQ), may be normal, and functional neuroimaging studies have shown that the IQ
seems to require mostly parietal lobe activation. For example, during administration of
the Wechsler Adult Intelligence Scale-Revised (WAIS-R), the highest levels of increased
metabolic activity during verbal tasks occurred in the left parietal lobe, whereas the
highest levels of increased metabolic activity during performance skills occurred in the
right parietal lobe. In contrast, frontal lobe pathology may become apparent only under
unstructured, stressful, real-life situations.
A famous case illustrating the result of frontal lobe damage involves Phineas Gage,
a 25-year-old railroad worker. While he was working with explosives, an accident
drove an iron rod through Gage’s head. He survived, but both frontal lobes were
severely damaged. After the accident, his behavior changed dramatically. The case
was written up by J. M. Harlow, M.D., in 1868, as follows: [George] is fitfull,
irreverent, indulging at times in the grossest profanity (which was not previously his
custom), manifesting but little deference for his fellows, impatient of restraint or
advice when it conflicts with his desires...His mind was radically changed, so
decidedly that his friends and acquaintances said he was “no longer Gage.” (see Fig.
1.2-5)
FIGURE 1.2-5
The life mask and skull of Phineas Gage. Note damage to the frontal region. “A famous
case illustrating the result of frontal lobe damage involves Phineas Gage, a 25-year-old
railroad worker. While he was working with explosives, an accident drove an iron rod
through Gage’s head. He survived, but both frontal lobes were severely damaged. After
the accident, his behavior changed dramatically. The case was written up by J.M.
Harlow, M.D., in 1868, as follows: [Gage] is fitful, irreverent, indulging at times in the
grossest profanity (which was not previously his custom), manifesting but little
deference for his fellows, impatient of restraint or advice when it conflicts his desires…
His mind was radically changed, so decidedly that his friends and acquaintances said he
was ‘no longer Gage.’” (Courtesy of Anthony A. Walsh, Ph.D.)
In one study of right-handed males, lesions of the right prefrontal cortex eliminated the tendency to use internal,
associative memory cues and led to an extreme tendency to interpret the task at hand in terms of its immediate context. In
contrast, right-handed males who had lesions of the left prefrontal cortex produced no context-dependent interpretations
and interpreted the tasks entirely in terms of their own internal drives. A mirror image of the functional lateralization
appeared in left-handed subjects. This test thus revealed the clearest known association of higher cortical functional
lateralization with the subjects’ dominant hand. Future experiments in this vein will attempt to reproduce these findings
with functional neuroimaging. If corroborated, these studies suggest a remarkable complexity of functional localization
within the prefrontal cortex and may also have implications for the understanding of psychiatric diseases in which
prefrontal pathology has been postulated, such as schizophrenia and mood disorders.
The heavy innervation of the frontal lobes by dopamine-containing nerve fibers is of interest because of the action of
antipsychotic medications. At the clinical level, antipsychotic medications may help to organize the rambling associations
of a patient with schizophrenia. At the neurochemical level, most typical antipsychotic medications block the actions of
dopamine at the D2 receptors. The frontal lobes, therefore, may be a major therapeutic site of action for antipsychotic
medications.
DEVELOPMENT
The nervous system is divided into the central and peripheral nervous systems (CNS and
PNS). The CNS consists of the brain and spinal cord; the PNS refers to all the sensory,
motor, and autonomic fibers and ganglia outside the CNS. During development, both
divisions arise from a common precursor, the neural tube, which in turn is formed
through folding of the neural plate, a specialization of the ectoderm, the outermost of
the three layers of the primitive embryo. During embryonic development, the neural
tube itself becomes the CNS; the ectoderm immediately superficial to the neural tube
becomes the neural crest, which gives rise to the PNS. The formation of these structures
requires chemical communication between the neighboring tissues in the form of cell
surface molecules and diffusible chemical signals. In many cases, an earlier-formed
structure, such as the notochord, is said to induce the surrounding ectoderm to form a
later structure, in this case the neural plate (see Color Plate 1.2-6). Identification of the
chemical mediators of tissue induction is an active area of research. Investigators have
begun to examine whether failures of the interactions of these mediators and their
receptors could underlie errors in brain development that cause psychopathology.
Neuronal Migration and Connections
The life cycle of a neuron consists of cell birth, migration to the adult position, extension
of an axon, elaboration of dendrites, synaptogenesis, and, finally, the onset of chemical
neurotransmission. Individual neurons are born in proliferative zones generally located
along the inner surface of the neural tube. At the peak of neuronal proliferation in the
middle of the second trimester, 250,000 neurons are born each minute. Postmitotic
neurons migrate outward to their adult locations in the cortex, guided by radially
oriented astrocytic glial fibers. Glia-guided neuronal migration in the cerebral cortex
occupies much of the first 6 months of gestation. For some neurons in the prefrontal
cortex, migration occurs over a distance 5,000 times the diameter of the neuronal cell
body. Neuronal migration requires a complex set of cell–cell interactions and is
susceptible to errors in which neurons fail to reach the cortex and instead reside in
ectopic positions. A group of such incorrectly placed neurons is called a heterotopia.
Neuronal heterotopias have been shown to cause epilepsy and are highly associated
with mental retardation. In a neuropathological study of the planum temporale of four
consecutive patients with dyslexia, heterotopias were a common finding. Recently,
heterotopic neurons within the frontal lobe have been postulated to play a causal role in
some cases of schizophrenia.
Many neurons lay down an axon as they migrate, whereas others do not initiate axon
outgrowth until they have reached their cortical targets. Thalamic axons that project to
the cortex initially synapse on a transient layer of neurons called the subplate neurons.
In normal development, the axons subsequently detach from the subplate neurons and
proceed superficially to synapse on the true cortical cells. The subplate neurons then
degenerate. Some brains from persons with schizophrenia reveal an abnormal
persistence of subplate neurons, suggesting a failure to complete axonal pathfinding in
the brains of these persons. This finding does not correlate with the presence of
schizophrenia in every case, however. A characteristic branched dendritic tree elaborates
once the neuron has completed migration. Synaptogenesis occurs at a furious rate from
the second trimester through the first 10 years or so of life. The peak of synaptogenesis
occurs within the first 2 postnatal years, when as many as 30 million synapses form
each second. Ensheathment of axons by myelin begins prenatally; it is largely complete
in early childhood, but does not reach its full extent until late in the third decade of life.
Myelination of the brain is also sequential.
Neuroscientists are tremendously interested in the effect of experience on the formation of brain circuitry in the first
years of life. As noted earlier, many examples are seen of the impact of early sensory experience on the wiring of cortical
sensory processing areas. Similarly, early movement patterns are known to reinforce neural connections in the
supplemental motor area that drive specific motor acts. Neurons rapidly form a fivefold excess of synaptic connections;
then, through a Darwinian process of elimination, only those synapses that serve a relevant function persist. This synaptic
pruning appears to preserve input in which the presynaptic cell fires in synchrony with the postsynaptic cell, a process
that reinforces repeatedly activated neural circuits. One molecular component that is thought to mediate synaptic
reinforcement is the postsynaptic NMDA glutamate receptor. This receptor allows the influx of calcium ions only when
activated by glutamate at the same time as the membrane in which it sits is depolarized. Thus, glutamate binding without
membrane depolarization or membrane depolarization without glutamate binding fails to trigger calcium influx. NMDA
receptors open in dendrites that are exposed to repeated activation, and their activation stimulates stabilization of the
synapse. Calcium is a crucial intracellular messenger that initiates a cascade of events, including gene regulation and the
release of trophic factors that strengthen particular synaptic connections. Although less experimental evidence exists for
the role of experience in modulating synaptic connectivity of association areas than has been demonstrated in sensory and
motor areas, neuroscientists assume that similar activity-dependent mechanisms may apply in all areas of the brain.
Adult Neurogenesis
A remarkable recent discovery has been that new neurons can be generated in certain
brain regions (particularly the dentate gyrus of the hippocampus) in adult animals,
including humans. This is in marked contrast to the previous belief that no neurons were
produced after birth in most species. This discovery has a potentially profound impact
on our understanding of normal development, incorporation of experiences, as well as
the ability of the brain to repair itself after various types of injuries (see Color Plates
1.2-7 and 1.2-8).
Neurological Basis of Development Theories
In the realm of emotion, early childhood experiences have been suspected to be at the
root of psychopathology since the earliest theories of Sigmund Freud. Freud’s
psychoanalytic method aimed at tracing the threads of a patient’s earliest childhood
memories. Franz Alexander added the goal of allowing the patient to relive these
memories in a less pathological environment, a process known as a corrective emotional
experience. Although neuroscientists have no data demonstrating that this method
operates at the level of neurons and circuits, emerging results reveal a profound effect
of early caregivers on an adult individual’s emotional repertoire. For example, the
concept of attunement is defined as the process by which caregivers “play back a child’s
inner feelings.” If a baby’s emotional expressions are reciprocated in a consistent and
sensitive manner, certain emotional circuits are reinforced. These circuits likely include
the limbic system, in particular, the amygdala, which serves as a gate to the
hippocampal memory circuits for emotional stimuli. In one anecdote, for example, a
baby whose mother repeatedly failed to mirror her level of excitement emerged from
childhood an extremely passive girl, who was unable to experience a thrill or a feeling
of joy.
The relative contributions of nature and nurture are perhaps nowhere more indistinct
than in the maturation of emotional responses, partly because the localization of
emotion within the adult brain is only poorly understood. It is reasonable to assume,
however, that the reactions of caregivers during a child’s first 2 years of life are
eventually internalized as distinct neural circuits, which may be only incompletely
subject to modification through subsequent experience. For example, axonal connections
between the prefrontal cortex and the limbic system, which probably play a role in
modulating basic drives, are established between the ages of 10 and 18 months. Recent
work suggests that a pattern of terrifying experiences in infancy may flood the
amygdala and drive memory circuits to be specifically alert to threatening stimuli, at the
expense of circuits for language and other academic skills. Thus infants raised in a
chaotic and frightening home may be neurologically disadvantaged for the acquisition
of complex cognitive skills in school.
An adult correlate to this cascade of detrimental overactivity of the fear response is
found in posttraumatic stress disorder (PTSD), in which persons exposed to an intense
trauma involving death or injury may have feelings of fear and helplessness for years
after the event. A PET scanning study of patients with PTSD revealed abnormally high
activity in the right amygdala while the patients were reliving their traumatic
memories. The researchers hypothesized that the stressful hormonal milieu present
during the registration of the memories may have served to burn the memories into the
brain and to prevent their erasure by the usual memory modulation circuits. As a result,
the traumatic memories exerted a pervasive influence and led to a state of constant
vigilance, even in safe, familiar settings.
Workers in the related realms of mathematics have produced results documenting the organizing effects of early
experiences on internal representations of the external world. Since the time of Pythagoras, music has been considered a
branch of mathematics. A series of recent studies has shown that groups of children who were given 8 months of intensive
classical music lessons during preschool years later had significantly better spatial and mathematical reasoning in school
than a control group. Nonmusical tasks, such as navigating mazes, drawing geometric figures, and copying patterns of twocolor blocks, were performed significantly more skillfully by the musical children. Early exposure to music, thus, may be
ideal preparation for later acquisition of complex mathematical and engineering skills.
These tantalizing observations suggest a neurological basis for the developmental theories of Jean Piaget, Erik Erikson,
Margaret Mahler, John Bowlby, Sigmund Freud, and others. Erikson’s epigenetic theory states that normal adult behavior
results from the successful, sequential completion of each of several infantile and childhood stages. According to the
epigenetic model, failure to complete an early stage is reflected in subsequent physical, cognitive, social, or emotional
maladjustment. By analogy, the experimental data just discussed suggest that early experience, particularly during the
critical window of opportunity for establishing neural connections, primes the basic circuitry for language, emotions, and
other advanced behaviors. Clearly, miswiring of an infant’s brain may lead to severe handicaps later when the person
attempts to relate to the world as an adult. These findings support the vital need for adequate public financing of Early
Intervention and Head Start programs, programs that may be the most cost-effective means of improving persons’ mental
health.
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03 - 1.3 Neural Development and Neurogenesis
1.3 Neural Development and Neurogenesis
Zilles K, Amunts K, Smaers JB. Three brain collections for comparative neuroanatomy and neuroimaging. Ann N Y Acad
Sci. 2011;1225:E94.
1.3 Neural Development and Neurogenesis
The human brain is a structurally and functionally complex system that exhibits ongoing
modification in response to both experience and disease. The anatomical and
neurochemical systems that underlie the cognitive, social, emotional, and sensorimotor
functions of the mature nervous system emerge from neuronal and glial cell populations
that arise during the earliest periods of development.
An understanding of molecular and cellular mechanisms mediating nervous system
development is critical in psychiatry because abnormalities of developmental processes
contribute to many brain disorders. Although a developmental basis may not be
surprising in early childhood disorders, such as autism, fragile X mental retardation, and
Rett syndrome, even mature diseases including schizophrenia and depression reflect
ontogenetic factors. For example, evidence from brain pathology and neuroimaging
indicates that there are reductions in forebrain region volumes, neuron and glial cell
numbers, and some classes of interneurons in schizophrenia that are apparent at the
time of diagnosis. Similarly, in autism, early brain growth is abnormally increased, and
abnormalities of cellular organization are observed that reflect disturbances in the basic
processes of cell proliferation and migration. When there is abnormal regulation of
early brain development, a foundation of altered neuron populations that may differ in
cell types, numbers, and positions is laid down, or abnormal connections, with
consequences for interacting glial populations, may be elaborated. With progressive
postnatal development, the maturing brain systems call upon component neurons to
achieve increasing levels of complex information processing, which may be deficient
should initial conditions be disturbed. New neural properties emerge during maturation
as neuron populations elaborate additional functional networks based on and modified
by ongoing experience. Given the brain’s dynamic character, we may expect that
developmental abnormalities in neural populations and systems, caused by genetic as
well as environmental factors, will manifest at diverse times in a person’s life.
OVERVIEW OF NERVOUS SYSTEM MORPHOLOGICAL DEVELOPMENT
In considering brain development, several overarching principles need to be considered.
First, different brain regions and neuron populations are generated at distinct times of
development and exhibit specific temporal schedules. This has implications for the
consequences of specific developmental insults, such as the production of autism
following fetal exposure to the drug thalidomide only during days 20 to 24 of gestation.
Second, the sequence of cellular processes comprising ontogeny predicts that
abnormalities in early events necessarily lead to differences in subsequent stages,
although not all abnormalities may be accessible to our clinical tools. For example, a
deficit in the number of neurons will likely lead to reductions in axonal processes and
ensheathing white matter in the mature brain. However, at the clinical level, since glial
cells outnumber neurons 8 to 1, the glial cell population, the oligodendrocytes, and their
myelin appear as altered white matter on neuroimaging with little evidence of a
neuronal disturbance. Third, it is clear that specific molecular signals, such as
extracellular growth factors and cognate receptors or transcription factors, play roles at
multiple developmental stages of the cell. For example, both insulin-like growth factor I
(IGF-I) and brain-derived neurotrophic factor (BDNF) regulate multiple cellular
processes during the developmental generation and mature function of neurons,
including cell proliferation, survival promotion, neuron migration, process outgrowth,
and the momentary synaptic modifications (plasticity) underlying learning and
memory. Thus changes in expression or regulation of a ligand or its receptor, by
experience, environmental insults, or genetic mechanisms, will have effects on multiple
developmental and mature processes.
The Neural Plate and Neurulation
The nervous system of the human embryo first appears between 2½ and 4 weeks of
gestation. During development, emergence of new cell types, including neurons, results
from interactions between neighboring layers of cells. On gestational day 13, the
embryo consists of a sheet of cells. Following gastrulation (days 14 to 15), which forms
a two-cell-layered embryo consisting of ectoderm and endoderm, the neural plate region
of the ectoderm is delineated by the underlying mesoderm, which appears on day 16.
The mesoderm forms by cells entering a midline cleft in the ectoderm called the
primitive streak. After migration, the mesodermal layer lies between ectoderm and
endoderm and induces overlying ectoderm to become neural plate. Induction usually
involves release of soluble growth factors from one group of cells, which in turn bind
receptors on neighboring cells, eliciting changes in nuclear transcription factors that
control downstream gene expression. In some cases, cell–cell contact-mediated
mechanisms are involved. In the gene-patterning section below, the important roles of
soluble growth factors and transcription factor expression are described.
The neural plate, the induction of which is complete by 18 days, is a sheet of
columnar epithelium and is surrounded by ectodermal epithelium. After formation, the
edges of the neural plate elevate, forming the neural ridges. Subsequently, changes in
intracellular cytoskeleton and cell–extracellular matrix attachment cause the ridges to
merge in the midline and fuse, a process termed neurulation, forming the neural tube,
with a central cavity presaging the ventricular system (Fig. 1.3-1). Fusion begins in the
cervical region at the hindbrain level (medulla and pons) and continues rostrally and
caudally. Neurulation occurs at 3 to 4 weeks of gestation in humans, and its failure
results in anencephaly rostrally and spina bifida caudally. Neurulation defects are well
known following exposure to retinoic acid in dermatological preparations and
anticonvulsants, especially valproic acid, as well as diets deficient in folic acid.
FIGURE 1.3-1
Mechanisms of neurulation. Neurulation begins with the formation of a neural plate in
response to soluble growth factors released by the underlying notochord. The neural
plate originates as a thickening of the ectoderm that results from cuboidal epithelial
cells becoming columnar in shape. With further changes in cell shape and adhesion, the
edges of the plate fold and rise, meeting in the midline to form a tube. Cells at the tips
of the neural folds come to lie between the neural tube and overlying epidermis,
forming the neural crest that gives rise to the peripheral nervous system and other
structures. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive
Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:44.)
Another product of neurulation is the neural crest, the cells of which derive from the edges of the neural plate and dorsal
neural tube. From this position, neural crest cells migrate dorsolaterally under the skin to form melanocytes and
ventromedially to form dorsal root sensory ganglia and sympathetic chains of the peripheral nervous system and ganglia of
the enteric nervous system. However, neural crest gives rise to diverse tissues including cells of neuroendocrine, cardiac,
mesenchymal, and skeletal systems, forming the basis of many congenital syndromes involving brain and other organs. The
neural crest origin at the border of neural and epidermal ectoderm and its generation of melanocytes forms the basis of the
neurocutaneous disorders, including tuberous sclerosis and neurofibromatosis. Finally, another nonneuronal structure of
mesodermal origin formed during neurulation is the notochord found on the ventral side of the neural tube. As seen in
subsequent text of this section, the notochord plays a critical role during neural tube differentiation, since it is a signaling
source of soluble growth factors, such as sonic hedgehog (Shh), which affect gene patterning and cell determination.
Regional Differentiation of the Embryonic Nervous System
After closure, the neural tube expands differentially to form major morphological
subdivisions that precede the major functional divisions of the brain. These subdivisions
are important developmentally, because different regions are generated according to
specific schedules of proliferation and subsequent migration and differentiation. The
neural
tube
can
be
described
in
three
dimensions,
including
longitudinal,
circumferential, and radial. The longitudinal dimension reflects the rostrocaudal
(anterior–posterior) organization, which most simply consists of brain and spinal cord.
Organization in the circumferential dimension, tangential to the surface, represents two
major axes: In the dorsoventral axis, cell groups are uniquely positioned from top to
bottom. On the other hand, in the medial to lateral axis, there is mirror image
symmetry, consistent with right–left symmetry of the body. Finally, the radial dimension
represents organization from the innermost cell layer adjacent to the ventricles to the
outermost surface and exhibits region-specific cell layering. At 4 weeks, the human brain
is divided longitudinally into the prosencephalon (forebrain), mesencephalon
(midbrain), and rhombencephalon (hindbrain). These three subdivisions or “vesicles”
divide further into five divisions by 5 weeks, consisting of the prosencephalon, which
forms the telencephalon (including cortex, hippocampus, and basal ganglia) and
diencephalon (thalamus and hypothalamus), the mesencephalon, (midbrain), and the
rhombencephalon, yielding metencephalon (pons and cerebellum) and myelencephalon
(medulla). Morphological transformation into five vesicles depends on region-specific
proliferation of precursor cells adjacent to the ventricles, the so-called ventricular zones
(VZs). As discussed later, proliferation intimately depends on soluble growth factors
made by proliferating cells themselves or released from regional signaling centers. In
turn, growth factor production and cognate receptor expression also depend on regionspecific patterning genes. We now know that VZ precursors, which appear
morphologically homogeneous, express a checkerboard array of molecular genetic
determinants that control the generation of specific types of neurons in each domain
(Fig. 1.3-2).
FIGURE 1.3-2
Progression of brain regional differentiation. Early after neurulation, the neural tube
differentiates into four regions (forebrain, midbrain, hindbrain, and spinal cord) that
give rise following later divisions and maturation to the different brain structures.
(From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of
Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:45.)
In the circumferential dimension, organization begins very early and extends over many rostrocaudal subdivisions. In the
spinal cord, the majority of tissue comprises the lateral plates, which later divide into dorsal or alar plates, composed of
sensory interneurons, and motor or basal plates, consisting of ventral motor neurons. Two other diminutive plates, termed
the roof plate and floor plate, are virtually absent in maturity; however, they play critical regulatory roles as growth factor
signaling centers in the embryo. Indeed, the floor plate, in response to Shh from the ventrally located notochord, produces
its own Shh, which in turn induces neighboring cells in ventral spinal cord and brainstem to express region-specific
transcription factors that specify cell phenotype and function. For example, in combination with other factors, floor plate
Shh induces midbrain precursors to differentiate into dopamine-secreting neurons of the substantia nigra. Similarly, the
roof plate secretes growth factors, such as bone morphogenetic proteins (BMPs), which induce dorsal neuron cell fate in
spinal cord. In the absence of roof plate, dorsal structures fail to form, such as cerebellum, and midline hippocampal
structures are missing. Finally, in the radial dimension, the organization of layers is subdivision specific, produced by
differential proliferation of VZ precursors and cell migration, as described later.
The Ventricular and Subventricular Proliferative Zones
The distinct patterns of precursor proliferation and migration in different regions
generate the radial organization of the nervous system. In each longitudinal subdivision,
the final population size of a brain region depends on the interplay of regulated
neurogenesis with programmed cell death. Traditional concepts had suggested that there
was excess cell production everywhere and that final cell number regulation was
achieved primarily after neurogenesis through selective cell death mediated by targetderived survival (trophic) factors. We now know that the patterning genes discussed
later play major roles in directing regional precursor proliferation that is coordinated
with final structural requirements, and that programmed cell death occurs at multiple
stages. Consequently, in diseases characterized by brain regions smaller than normal,
such as schizophrenia, there may be a failure to generate neurons initially, as opposed
to normal generation with subsequent cell loss.
Radial and Tangential Patterns of Neurogenesis and Migration
Of interest to psychiatry, the cerebral cortex is the paradigmatic model of inside-tooutside neurogenesis. A large number of studies now relate specific genetic mutations to
distinct cortical malformations that alter neurogenesis, migration, and cellular
organization, thereby increasing our knowledge of both normal and pathophysiologic
cortical development. Derived from the embryonic forebrain telencephalic vesicles, the
characteristic six-cell layers represent a common cytoarchitectural and physiological
basis for neocortical function. Within each layer, neurons exhibit related axodendritic
morphologies, use common neurotransmitters, and establish similar afferent and
efferent connections. In general, pyramidal neurons in layer 3 establish synapses within
and between cortical hemispheres, whereas deeper layer 5/6 neurons project primarily
to subcortical nuclei, including thalamus, brainstem, and spinal cord. The majority of
cortical neurons originate from the forebrain VZ. At the earliest stages, the first
postmitotic cells migrate outward from the VZ to establish a superficial layer termed the
preplate. Two important cell types comprise the preplate—Cajal–Retzius cells, which
form outermost layer 1 or marginal zone, and subplate neurons, which lay beneath
future layer 6. These distinct regions form when later-born cortical plate neurons
migrate within and divide the preplate in two (Fig. 1.3-3).
FIGURE 1.3-3
Schematic drawing of radial and tangential migration during cerebral cortex
development. A. A coronal section of one half of the developing rat forebrain. The
dorsal forebrain gives rise to the cerebral cortex. Medial ganglionic eminences (MGEs)
and lateral ganglionic eminences (LGEs) of the ventral forebrain generate neurons of
the basal ganglia and the cortical interneurons. The arrows indicate the tangential
migration route for γ-aminobutyric acid (GABA) interneurons to the cortex. The boxed
area (enlarged in B and C) shows the developing cortex at early and late stages. B. In
the dorsal forebrain, the first cohort of postmitotic neurons migrate out from the
ventricular zone (VZ) and create a preplate (PP) below the pial surface. C. Subsequent
postmitotic neurons will migrate along radial glia through the intermediate zone (IZ)
and take position in the middle of the preplate, creating a cortical plate (CP) between
the outer marginal zone (MZ) and inner subplate (SP). Ultimately, the CP will be
composed of six layers that are born sequentially, migrating in an inside-to-outside
pattern. Horizontal processes in the IZ represent axon terminals of thalamic afferents.
(From Nadarajah B, Parnavelas JG. Modes of neuronal migration in the developing
cerebral cortex. Nat Neurosci. 2002;3:423, with permission.)
A recent discovery, postulated for years, has changed the view of the origins of
cortical neuron populations involved in human brain disease. Neuron tracing
experiments in culture and in vivo demonstrate that the neocortex, a dorsal forebrain
derivative, is also populated by neurons generated in the ventral forebrain (see Fig. 1.33). Molecular studies of patterning genes, especially Dlx, strongly support this model
(see below). In contrast to excitatory pyramidal neurons, the overwhelming majority of
inhibitory γ-aminobutyric acid (GABA)–secreting interneurons originate from mitotic
precursors of the ganglionic eminences that generate the neurons of the basal ganglia.
Subsets of interneurons also secrete neuropeptides, such as neuropeptide Y (NPY) and
somatostatin, and express nitrous oxide (NOS)-generating enzyme. Not associated with
cortical VZ radial glia, these GABA interneurons reach the cortical plate by migrating
tangentially, in either the superficial marginal zone or a deep position above the VZ, the
subplate region where thalamic afferents are also growing. Significantly, in brains from
patients with schizophrenia, the prefrontal cortex exhibits a reduced density of
interneurons in layer 2. In addition, there is upregulation of GABAA-receptor binding, a
potential functional compensation, as well as a relative deficiency of NOS-expressing
neurons. These observations have led to the hypothesis that schizophrenia is due to
reduced GABAergic activity. The origin of GABA interneurons from the ganglionic
eminences and their association with specific patterning genes raises new genetic
models of disease causation and possible strategies for disease intervention. Thus, more
broadly, normal cortical development depends on a balance of two principal patterns of
neurogenesis and migration, consisting of radial migration of excitatory neurons from
the dorsal forebrain VZ and tangential migration of inhibitory neurons from the ventral
forebrain.
In contrast to inside-to-outside neurogenesis observed in cortex, phylogenetically older
regions, such as hypothalamus, spinal cord, and hippocampal dentate gyrus, exhibit the
reverse order of cell generation. First-formed postmitotic neurons lie superficially, and
last-generated cells localize toward the center. Although this outside-to-inside pattern
might reflect passive cell displacement, radial glia and specific migration signaling
molecules clearly are involved. Furthermore, cells do not always lie in direct extension
from their locus of VZ generation. Rather, some groups of cells migrate to specific
locations, as observed for neurons of the inferior olivary nuclei.
Of prime importance in psychiatry, the hippocampus demonstrates both radial and nonradial patterns of neurogenesis
and migration. The pyramidal cell layer, Ammon’s horn Cornu Ammonis (CA) 1 to 3 neurons, is generated in a typical
outside-to-inside fashion in the dorsomedial forebrain for a discrete period, from 7 to 15 weeks of gestation, and exhibits
complex migration patterns. In contrast, the other major population, dentate gyrus granule neurons, starts appearing at 18
weeks and exhibits prolonged postnatal neurogenesis, originating from several migrating secondary proliferative zones. In
rats, for instance, granule neurogenesis starts at embryonic day 16 (E16) with proliferation in the forebrain VZ. At E18, an
aggregate of precursors migrates along a subpial route into the dentate gyrus itself where they generate granule neurons in
situ. After birth, there is another migration, localizing proliferative precursors to the dentate hilus, which persists until 1
month of life. Thereafter, granule precursors move to a layer just under the dentate gyrus, termed the subgranular zone
(SGZ), which produces neurons throughout life in adult rats, primates, and humans. In rodents, SGZ precursors proliferate
in response to cerebral ischemia, tissue injury, and seizures, as well as growth factors. Finally, the diminished
hippocampal volume reported in schizophrenia raises the possibility that disordered neurogenesis plays a role in
pathogenesis, as either a basis for dysfunction or a consequence of brain injuries, consistent with associations of gestational
infections with disease manifestation.
Finally, a different combination of radial and nonradial migration is observed in cerebellum, a brain region recently
recognized to play important functions in nonmotor tasks, with particular significance for autism spectrum disorders.
Except for granule cells, the other major neurons, including Purkinje and deep nuclei, originate from the primary VZ of the
fourth ventricle, coincident with other brainstem neurons. In rats, this occurs at E13 to E15, and in humans, at 5 to 7
weeks of gestation. The granule neurons, as well as basket and stellate interneurons, originate in the secondary proliferative
zone, the external germinal cell layer (EGL), which covers newborn cerebellum at birth. EGL precursors originate in the
fourth ventricle VZ and migrate dorsally through the brainstem to reach this superficial position. The rat EGL proliferates
for 3 weeks, generating more neurons than in any other structure, whereas in humans, EGL precursors exist for at least 7
weeks and up to 2 years. When an EGL precursor stops proliferating, the cell body sinks below the surface and grows
bilateral processes that extend transversely in the molecular layer, and then the soma migrates further down into the
internal granule layer (IGL). Cells reach the IGL along specialized Bergmann glia, which serve guidance functions similar to
those of the radial glia. However, in this case, cells originate from a secondary proliferative zone that generates neurons
exclusively of the granule cell lineage, indicating a restricted neural fate. Clinically, this postnatal population in infants
makes cerebellar granule neurogenesis vulnerable to infectious insults of early childhood and an undesirable target of
several therapeutic drugs, such as steroids, well known to inhibit cell proliferation. In addition, proliferative control of this
stem cell population is lost in the common childhood brain tumor, medulloblastoma (see Fig. 1.3-4).
FIGURE 1.3-4
Neurogenesis, migration, and differentiation of granule cells during cerebellar
development. Granule cell precursors proliferate in the external germinal layer. After
exiting the cell cycle, they migrate through the molecular layer and past the Purkinje
neurons to reach the internal granule layer where they differentiate and make synapses.
Neurons that do not migrate properly or that do not establish proper synaptic
connections undergo apoptosis. EGL, external germinal cell layer; Mol, molecular layer;
P, Purkinje cell layer; IGL, internal granule cell layer; Wm, white matter. (From Sadock
BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed.
Philadelphia: Lippincott Williams & Wilkins; 2009:48.)
Developmental Cell Death
During nervous system development, cell elimination is apparently required to
coordinate the proportions of interacting neural cells. Developmental cell death is a
reproducible, spatially and temporally restricted death of cells that occurs during the
organism’s development. Three types of developmental cell death have been described:
(1) phylogenetic cell death that removes structures in one species that served
evolutionarily earlier ones, such as the tail or the vomeronasal nerves; (2)
morphogenetic cell death, which sculpts the fingers from the embryonic paddle and is
required to form the optic vesicles, as well as the caudal neural tube; and (3)
histogenetic cell death, a widespread process that allows the removal of select cells
during development of specific brain regions. Numerous studies have focused on
histogenetic cell death, the impact of which varies among brain regions but can affect
20 to 80 percent of neurons in some populations. A major role for developmental cell
death was proposed in the 1980s based on the paradigm of nerve growth factor,
suggesting that following neurogenesis, neurons compete for trophic factors. In this
model, survival of differentiating neurons depended absolutely on establishing axonal
connections to the correct targets in order to obtain survival-promoting (trophic) growth
factors, such as the neurotrophins. Otherwise, they would be eliminated by programmed
cell death. This competitive process was thought to ensure proper matching of new
neuronal populations with the size of its target field. Although such interactions are
involved in controlling cell degeneration, this model is overly simplistic: Developmental
cell death also occurs in neural precursors and immature neurons, before any synaptic
contacts are established.
Apoptosis.
Apoptotic cell death, or apoptosis, is the major type of developmental
cell degeneration. Apoptosis or “programmed cell death” involves specific molecules
that possess enzymatic activities such as cysteine-containing aspartate-specific
proteases, also called “caspases,” which participate in complex intracellular
mechanisms. A large number of signals (both proapoptotic and antiapoptotic) converge
to regulate common signaling pathways. Of importance for psychiatry, both
developmental as well as pathological cell death involve many of the same signaling
cascades. A failure to inhibit apoptosis is involved in cancers and autoimmune diseases
(multiple sclerosis), whereas excess stimulation of apoptosis is observed in
neurodegenerative diseases during both development (Huntington’s disease, lysosomal
diseases, and leukodystrophy) and aging (Alzheimer’s and Parkinson’s diseases).
Massive apoptotic cell death is also observed during acquired developmental brain
injuries such as hypoxia-ischemia, fetal alcohol syndrome, and exposure to ionizing
radiations and neurotoxicants. Thus dysregulation of apoptotic cell death during
development can lead to severe brain abnormalities, which may only manifest later as
mature functional impairments.
Programmed cell death is a necessary process during neurodevelopment, as genetic
deletion of caspases in embryonic mice produces enlarged and disorganized brains with
marked regional specificity. Programmed cell death occurs at multiple stages of nervous
system development, interacting with neurogenesis and differentiation with precise and
complex mechanisms. As many neuropathologies also involve dysregulation of
apoptosis, future studies hold promise for elucidation and treatment of neurological
diseases.
THE CONCEPT OF NEURAL PATTERNING
Principles of Function
The morphological conversion of the nervous system through the embryonic stages, from
neural plate through neural tube to brain vesicles, is controlled by interactions between
extracellular factors and intrinsic genetic programs. In many cases, extracellular signals
are soluble growth factors secreted from regional signaling centers, such as the
notochord, floor, or roof plates, or surrounding mesenchymal tissues. The precursor’s
ability to respond (competence) depends on cognate receptor expression, which is
determined by patterning genes whose proteins regulate gene transcription. The
remarkable new observation is that the subdivisions of the embryonic telencephalon
that were initially based on mature differences in morphology, connectivity, and
neurochemical profiles are also distinguished embryonically by distinct patterns of gene
expression. Classical models had suggested that the cerebral cortex was generated as a
fairly homogeneous structure, unlike most epithelia, with individual functional areas
specified relatively late, after cortical layer formation, by the ingrowth of afferent axons
from thalamus. In marked contrast, recent studies indicate that proliferative VZ
precursors themselves display regional molecular determinants, a “protomap,” which
the postmitotic neurons carry with them as they migrate along radial glia to the cortical
plate. Consequently, innervating thalamic afferents may serve to modulate only
intrinsic molecular determinants of the protomap. Indeed, in two different genetic
mutants, Gbx2 and Mash1, in which thalamocortical innervation is disrupted, expression
of cortical patterning genes proceeds unaltered. On the other hand, thalamic afferent
growth may be directed by patterning genes and subsequently play roles in modulating
regional expression patterns. Thus experience-dependent processes may contribute less
to cortical specialization than originally postulated.
The term patterning genes connotes families of proteins that serve primarily to
control transcription of other genes, the products of which include other transcription
factors or proteins involved in cellular processes, such as proliferation, migration, or
differentiation. Characteristically, transcription factor proteins contain two principal
domains, one that binds DNA promoter regions of genes and the other that interacts
with other proteins, either transcription factors or components of intracellular second
messengers. It is notable that transcription factors form multimeric protein complexes to
control gene activation. Therefore, a single transcription factor will play diverse roles in
multiple cell types and processes, according to what other factors are present, the socalled cellular environment. The combinatorial nature of gene promoter regulation
leads to a diversity of functional outcomes when a single patterning gene is altered.
Furthermore, because protein interactions depend on protein–protein affinities, there
may be complex changes as a single factor’s expression level is altered. This may be one
important
mechanism
of
human
variation
and
disease
susceptibility,
since
polymorphisms in gene promoters, known to be associated with human disease, can
alter levels of gene protein products. A transcription factor may associate primarily with
one partner at a low concentration but with another at a higher titer. The multimeric
nature of regulatory complexes allows a single factor to stimulate one process while
simultaneously inhibiting another. During development, a patterning gene may thus
promote one event, say generation of neurons, by stimulating one gene promoter, while
simultaneously sequestering another factor from a different promoter whose activity is
required for an alternative phenotype, such as glial cell fate. Finally, the factors
frequently exhibit cross-regulatory functions, where one factor negatively regulates
expression of another. This activity leads to the establishment of tissue boundaries,
allowing the formation of regional subdivisions, such as basal ganglia and cerebral
cortex in the forebrain.
In addition to combinatorial interactions, patterning genes exhibit distinct temporal
sequences of expression and function, acting in hierarchical fashion. Functional
hierarchies were established experimentally by using genetic approaches, either deleting
a gene (loss of function) or over-/ectopically expressing it (gain of function), and
defining developmental consequences. At the most general level, genetic analyses
indicate that regionally restricted patterning genes participate in specifying the
identity, and therefore function, of cells in which they are expressed. Subdivisions of the
brain, and of cerebral cortex specifically, are identified by regionalized gene expression
in the proliferative VZ of the neural tube, leading to subsequent differentiation of
distinct types of neurons in each mature (postmitotic) region. Thus the protomap of the
embryonic VZ apparently predicts the cortical regions it will generate and may instruct
the hierarchical temporal sequence of patterning gene expression. It appears that the
different genes underlie multiple stages of brain development including the following:
(1) determining that ectoderm will give rise to nervous system (as opposed to skin); (2)
defining the dimensional character of a region, such as positional identity in
dorsoventral or rostrocaudal axes; (3) specifying cell class, such as neuron or glia; (4)
defining when proliferation ceases and differentiation begins, (5) determining specific
cell subtype, such as GABA interneuron, as well as projection pattern; and (6) defining
laminar position in the region, such as cerebral cortex. Although investigations are
ongoing, studies indicate that these many steps depend on interactions of transcription
factors from multiple families. Furthermore, a single transcription factor plays
regulatory roles at multiple stages in the developmental life of a cell, yielding complex
outcomes, for instance, in genetic loss of function studies and human disease.
Recent advances in molecular biology have led to identification of another principle of nervous system organization,
which if sustained by further studies, may provide a molecular basis for brain system diseases, such as Parkinson’s disease
and autism. Using molecular techniques to permanently identify cells that had expressed during development of a specific
gene, in this case the soluble growth factor, Wnt3a, investigators were able to determine where cells originated
embryonically and could trace their path of migration along the neuraxis during development. These genetic-fate mapping
studies indicate that cells that expressed Wnt3a migrated widely from the dorsal midline into the dorsal regions of the
brain and spinal cord, thereby contributing to diverse adult structures in the diencephalon, midbrain, and brainstem and
rostral spinal cord. Of interest, most of these structures were linked into a functional neural network, specifically the
auditory system. The observation that a single functional system emerges from a specific group of fated cells would allow
for restricted neurological-system–based disorders, such as deficits in dopamine or catecholamine neurons, or for the
dysfunction of inter-related brain regions that subserve social cognition and interaction, a core symptom of the autism
spectrum disorders. Other adult system degenerations may also be considered. This new observation may change the way
that we consider temporal changes in patterning gene expression of specific brain regions during development.
Finally, patterning gene expression in nervous system subdivisions is not insensitive
to environmental factors. To the contrary, expression is intimately regulated by growth
factors released from regional signaling centers. Indeed, although a century of classical
experimental embryology described morphologically the induction of new tissues
between neighboring cell layers, we have only recently defined molecular identities of
soluble protein morphogens and cell response genes underlying development. Signaling
molecules from discrete centers establish tissue gradients that provide positional
information (dorsal or ventral), impart cell specification, and/or control regional
growth. Signals include the BMPs, the Wingless-Int proteins (Wnts), Shh, fibroblast
growth factors (FGFs), and epidermal growth factors (EGFs), to name a few. These
signals set up developmental domains characterized by expression of specific
transcription factors, which in turn control further regional gene transcription and
developmental processes. The importance of these mechanisms for cerebral cortical
development is only now emerging, altering our concepts of the roles of subsequent
thalamic innervation and experience-dependent processes. In light of the temporal and
combinatorial principles discussed earlier, brain development can be viewed as a
complex and evolving interaction of extrinsic and intrinsic information.
SPECIFIC INDUCTIVE SIGNALS AND PATTERNING GENES IN
DEVELOPMENT
Induction of the central nervous system (CNS) begins at the neural plate stage when the
notochord, underlying mesenchyme, and surrounding epidermal ectoderm produce
signaling molecules that affect the identity of neighboring cells. Specifically, the
ectoderm produces BMPs that prevent neural fate determination by promoting and
maintaining epidermal differentiation. In other words, neural differentiation is a default
state that manifests unless it is inhibited. In turn, neural induction proceeds when BMP’s
epidermis-inducing activity is blocked by inhibitory proteins, such as noggin, follistatin,
and chordin, which are secreted by Hensen’s node (homologous to the amphibian
Spemann organizer), a signaling center at the rostral end of the primitive streak. Once
the neural tube closes, the roof plate and floor plate become new signaling centers,
organizing dorsal and ventral neural tube, respectively. The same ligand/receptor
system is used sequentially for multiple functions during development. BMPs are a case
in point, since they prevent neural development at the neural plate stage, whereas after
neurulation the factors are produced by the dorsal neural tube itself to induce sensory
neuron fates.
The Spinal Cord
The spinal cord is a prime example of the interaction of soluble signaling factors with
intrinsic patterning gene expression and function. The synthesis, release, and diffusion
of inductive signals from signaling sources produce concentration gradients that impose
distinct neural fates in the spinal cord (Fig. 1.3-5). The notochord and floor plate secrete
Shh, which induces motoneurons and interneurons ventrally, whereas the epidermal
ectoderm and roof plate release several BMPs that impart neural crest and sensory relay
interneuron fates dorsally. Growth factor inductive signals act to initiate discrete
regions of transcription factor gene expression. For instance, high concentrations of Shh
induce winged helix transcription factor Hnf3β gene in floor plate cells and Nkx6.1 and
Nkx2.2 in ventral neural tube, whereas the expression of more dorsal genes, Pax6,
Dbx1/2, Irx3, and Pax7, is repressed. In response to Shh, ventral motoneurons express
transcription factor gene Isl1, whose protein product is essential for neuron
differentiation. Subsequently, ventral interneurons differentiate, expressing En1 or
Lim1/2 independent of Shh signaling. In contrast, the release of BMPs by dorsal cord
and roof plate induces a distinct cascade of patterning genes to elicit sensory
interneuron differentiation. In aggregate, the coordinated actions of Shh and BMPs
induce the dorsoventral dimension of the spinal cord. Similarly, other inductive signals
determine rostrocaudal organization of the CNS, such as retinoic acid, an upstream
regulator of hox patterning genes, anteriorly, and the FGFs posteriorly. The overlapping
and unique expression of the many hox gene family members are important for
establishing the segmental pattern in the anterior–posterior axis of the hindbrain and
spinal cord, now classic models well described in previous reviews.
FIGURE 1.3-5
Patterning genes in the spinal cord. A. Diagram illustrating the localization of gene
expression in the developing “trunk.” Rhombomere boundaries are specified by specific
combinations of transcription factors. (Modified from Darnell, 2005.) B. Morphogen
induction of spinal cord cell fate. Dorsoventral gradients of sonic hedgehog (Shh) and
bone morphogenetic protein (BMP) induce expression of several position identity genes.
Combinatorial effects of these factors establish progenitor domains and result in the
expression of specific downstream molecular markers. D, dorsal neurons; V, ventral
neurons. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook
of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:51.)
Recent advances in spinal cord transcription factor expression and function support
the principle that these factors play roles at multiple stages of a cell’s development,
likely due to their participation in diverse protein regulatory complexes: The
transcription factors Pax6, Olig2, and Nkx2.2, which define the positional identity of
multipotent progenitors early in development, also play crucial roles in controlling the
timing of neurogenesis and gliogenesis in the developing ventral spinal cord.
The Cerebral Cortex
Recent evidence suggests that forebrain development also depends on inductive signals
and patterning genes as observed in more caudal neural structures. In the embryo, the
dorsal forebrain structures include the hippocampus medially, the cerebral cortex
dorsolaterally, and the entorhinal cortex ventrolaterally, whereas in basal forebrain, the
globus pallidus lies medially and the striatum laterally. On the basis of gene expression
and morphological criteria, it has been hypothesized that the forebrain is divided into a
checkerboard-like grid pattern of domains generated by the intersection of longitudinal
columns and transverse segments, perpendicular to the longitudinal axis. The columns
and segments (prosomeres) exhibit restricted expression of patterning genes, allowing
for unique combinations of factors within each embryonic subdivision. Many of these
genes, including Hnf3β, Emx2, Pax6, and Dlx2, are first expressed even before
neurulation in the neural plate and are then maintained, providing the “protomap”
determinants of the VZ described earlier. As in spinal cord, initial forebrain gene
expression is influenced by a similar array of signaling center soluble factors—Shh,
BMP, and retinoic acid. As the telencephalic vesicles form, signaling centers localize to
the edges of the cortex. In the dorsal midline there is the anterior neural ridge, an
anterior cranial mesenchyme secreting FGF8, the roof plate, and, at the junction of the
roof plate with the telencephalic vesicle, the cortical hem (Fig. 1.3-6). Other factors
originate laterally from the dorsal–ventral forebrain junction, as well as from basal
forebrain structures themselves.
FIGURE 1.3-6
Patterning genes and signaling centers in the developing cerebral cortex. This schematic
diagram shows a lateral–superior view of the two cerebral hemispheres of the embryonic
mouse, sitting above the midbrain and hindbrain (broken lines). The anterior–lateral
extent of Pax6 gene expression is indicated by circles. The posterior–medial domain of
Emx2 expression is indicated by stripes. The genes exhibit continuous gradients of
expression that decrease as they extend to opposite poles. The signaling factor fibroblast
growth factor 8 (FGF8) is produced by and released from mesenchymal tissue in the
anterior neural ridge, which regulates Pax6 and Emx2 expression. In the midline, bone
morphogenetic proteins (BMPs) and Wingless-Int proteins (Wnts) are secreted from
other signaling centers, including the roof plate and the cortical hems. (Courtesy of E.
DiCicco-Bloom and K. Forgash.)
Do molecular studies identify how different cortical regions interact with thalamic neurons to establish specific
functional modalities, such as vision and sensation? And once regional identity is established, can it be modified by later
developmental events? It has been proposed that initially there are no functional distinctions in the cortex but that they are
induced by the ingrowth of extrinsic thalamic axons, which convey positional and functional specifications, the so-called
“protocortex model.” However, in contrast, the abundant molecular evidence provided earlier suggests that intrinsic
differences are established early in the neuroepithelium by molecular determinants that regulate areal specification,
including the targeting of thalamic axons, termed the “protomap” model. The foregoing mutants now provide
experimental tests of these two alternative models and indicate that neither model is completely correct. Although there is
early molecular regionalization of the cortex, the initial targeting of thalamic axons to the cortex is independent of these
molecular differences. In the rodent, thalamic afferents first target to their usual cortical regions prenatally in the late
embryo. However, once thalamic afferents reach the cortex, which occurs several days after birth, interactions of thalamic
axon branches with local regional cues leads to modifications of initial outgrowth and the establishment of connections
that conform to areal molecular identities. Furthermore, the developing cortex exhibits a remarkable and unexpected level
of flexibility in mediating modality-specific functions: In the ferret, surgical elimination of visual pathway (lateral
geniculate nucleus) in postnatal pups results in the transfer of visual signaling to the auditory cortex, which successfully
mediates vision! Thus the animal’s visual information is effectively processed by their auditory cortex.
The Hippocampus
The hippocampus is a region of major importance in schizophrenia, depression, autism,
and other disorders, and defining mechanisms regulating hippocampal formation may
provide clues to the developmental bases of these disorders. In mice, the hippocampus is
located in the medial wall of the telencephalic vesicle. Where it joins the roof plate, the
future roof of the third ventricle, there is a newly defined signaling center, the cortical
hem, which secretes BMPs, Wnts, and FGFs (see Fig. 1.3-6). Genetic experiments have
defined patterning genes localized to the cortical hem and hippocampal primordia,
whose deletions result in a variety of morphogenetic defects. In mice lacking Wnt3a,
which is expressed in the cortical hem, the hippocampus is either completely missing or
greatly reduced, whereas neighboring cerebral cortex is mainly preserved. A similar
phenotype is produced by deleting an intracellular factor downstream to Wnt receptor
activation, the Lef1 gene, suggesting that the Wnt3a–Lef1 pathway is required for
hippocampal cell specification and/or proliferation, issues remaining to be defined.
When another cortical hem gene, Lhx5, is deleted, mice lack both the hem and
neighboring choroid plexus, both sources of growth factors. However, in this case, the
cortical hem cells may in fact proliferate in excess, and the hippocampal primordia may
be present but disorganized, exhibiting abnormalities in cell proliferation, migration,
and differentiation. A related abnormality is observed with Lhx2 mutation. Finally, a
sequence of bHLH transcription factors plays roles in hippocampal neurogenesis:
Dentate gyrus differentiation is defective in NeuroD and Mash1 mutants. Significantly,
expression of all these hippocampal patterning genes is regulated by factors secreted by
anterior neural ridge, roof plate, and the cortical hem, including FGF8, Shh, BMPs, and
Wnts. Moreover, the basal forebrain region secretes an EGF-related protein,
transforming growth factor α (TGF-α), which can stimulate expression of the classical
limbic marker protein, lysosomal-associated membrane protein (LAMP). These various
signals and genes now serve as candidates for disruption in human diseases of the
hippocampus.
The Basal Ganglia
In addition to motor and cognitive functions, the basal ganglia take on new importance
in neocortical function, since they appear to be the embryonic origin of virtually all
adult GABA interneurons, reaching the neocortex through tangential migration. Gene
expression studies have identified several transcription factors that appear in precursors
originating in the ventral forebrain ganglionic eminences, allowing interneurons to be
followed as they migrate dorsally into the cortical layers. Conversely, genetic deletion
mutants exhibit diminished or absent interneurons, yielding results consistent with other
tracing techniques. These transcription factors, including Pax6, Gsh2, and Nkx2.1,
establish boundaries between different precursor zones in the ventral forebrain VZ,
through mechanisms involving mutual repression. As a simplified model, the medial
ganglionic eminence (MGE) expresses primarily Nkx2.1 and gives rise to most GABA
interneurons of the cortex and hippocampus, whereas the lateral ganglionic eminence
(LGE) expresses Gsh2 and generates GABA interneurons of the SVZ and olfactory bulb.
The boundary between ventral and dorsal forebrain then depends on LGE interaction
with the dorsal neocortex, which expresses Pax6. When Nkx2.1 is deleted, LGE
transcription factor expression spreads ventrally into the MGE territory, and there is a
50 percent reduction in neocortical and striatal GABA interneurons. In contrast, deletion
of Gsh2 leads to ventral expansion of the dorsal cortical molecular markers and
concomitant decreases in olfactory interneurons. Finally, Pax6 mutation causes both
MGE and LGE to spread laterally and into dorsal cortical areas, yielding increased
interneuron migration. The final phenotypic changes are complex, as these factors
exhibit unique and overlapping expression and interact to control cell fate.
Neuronal Specification
As indicated for basal ganglia, throughout the nervous system transcription factors
participate in decisions at multiple levels, including determining the generic neural cell,
such as neuron or glial cell, as well as neuron subtypes. Mash1 can promote a neuronal
fate over a glial fate as well as induce the GABA interneuron phenotype. However,
another bHLH factor, Olig1/2, can promote oligodendrocyte development, whereas it
promotes motor neuron differentiation elsewhere, indicating that the variety of factors
expressed in a specific cell leads to combinatorial effects and thus diverse outcomes for
cell differentiation. The bHLH inhibitory factor, Id, is expressed at the transition from
somatosensory to motor cortex, implying roles of family members in areal
characteristics. In the hippocampus, granule neuron fate is dependent on NeuroD and
Math1, with deficient cell numbers when either one is deleted. The role of specific factors
in cortical cell layer determination remains an area of active investigation but likely
includes Tbr1, Otx1, and Pax6.
A NEW MECHANISM FOR REGULATING GENE EXPRESSION: MIRNAS
Over the last decade a new mechanism for regulating messenger ribonucleic acid
(mRNA) has been explored in simple to complex organisms that involves microRNAs
(miRNAs). We now know that miRNAs contribute not only to normal development and
brain function but also to brain disorders, such as Parkinson’s and Alzheimer’s disease,
tauopathies, and brain cancer. miRNAs can affect the regulation of RNA transcription,
alternative splicing, molecular modifications, or RNA translation. miRNAs are 21- to 23nucleotide-long single-strand RNA molecules. Unlike mRNAs that encode the instructions
for ribosome complex translation into proteins, miRNAs are noncoding RNAs that are
not translated but are instead processed to form loop structures. miRNAs exhibit a
sequence that is partially complementary to one or several other cellular mRNAs. By
binding to target mRNA transcripts, the miRNAs serve to interfere with their function,
thereby downregulating expression of these gene products. This gene silencing involves
a complex mechanism: The larger miRNA primary transcript is first processed by the
Microprocessor, an enzymatic complex consisting of the nuclease Drosha and the doublestranded RNA binding protein Pasha. The mature miRNA binds to its complementary
RNA and then interacts with the endonuclease Dicer that is part of the RNA-induced
silencing complex (RISC), resulting in the cleavage of the target mRNA and gene
silencing (Fig. 1.3-7).
FIGURE 1.3-7
Processing and function of micro RNA (miRNA). After transcription, the primary miRNA
forms a hairpin conformation. This structure allows the enzyme Drosha to cleave the
transcript, producing a pre-miRNA that then exits the nucleus through nuclear pores. In
the cytoplasm, Dicer cleaves the pre-miRNA stem loop, resulting in the formation of two
complementary short RNA molecules. Only one of these molecules is integrated in the
RNA-induced silencing complex (RISC) and serves as a guide strand that allows
recognition and specificity for target RNA due to its sequence complementarity. After
integration into the RISC complex, the miRNA matches with the complementary mRNA
strand and induces mRNA duplex degradation by the argonaute protein, the catalytic
enzyme of the RISC complex. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s
Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams &
Wilkins; 2009:55.)
Currently, 475 miRNAs have been identified in humans, and their total number is estimated to be between 600 and
3,441. Potentially, up to 30 percent of all genes might be regulated by miRNAs, a whole new layer of molecular
complexity. A connection between miRNAs and several brain diseases has already been made. For example, miR-133b,
which is specifically expressed in midbrain dopaminergic neurons, is deficient in midbrain tissue from patients with
Parkinson’s disease. Furthermore, the miRNAs encoding miR-9, miR-124a, miR-125b, miR-128, miR-132, and miR-219 are
abundantly represented in fetal hippocampus, are differentially regulated in the aged brain, and are altered in Alzheimer’s
disease hippocampus. Similar RNA species termed short-interfering RNAs (siRNAs) have been discovered in plants where
they prevent the transcription of viral RNA. The mechanisms involved in these effects are closely related to those of
miRNA. Thus siRNAs are now being used in both basic and clinical research to downregulate specific cellular gene
products, thereby advancing the study of pathways involved in neurodevelopment and providing new selective tools to
regulate disease-causing genes or therapeutic molecular targets.
REGULATION OF NEURODEVELOPMENT BY EXTRACELLULAR FACTORS
The interaction of extracellular factors with intrinsic genetic determinants controlling
region-specific neurogenesis includes signals that regulate cell proliferation, migration,
differentiation, and survival (Table 1.3-1). Patterning genes control the expression of
growth factor receptors and the molecular machinery of the cell division cycle.
Extracellular factors are known to stimulate or inhibit proliferation of VZ precursors and
originate from the cells themselves, termed autocrine, neighboring cells/tissues, or
paracrine, or from the general circulation, as in endocrine, all sources known to affect
proliferation in prenatal and postnatal developing brain. Although defined initially in
cell culture, a number of mitogenic growth factors are now well-characterized in vivo,
including those stimulating proliferation, such as basic FGF (bFGF), EGF, IGF-I, Shh, and
signals inhibiting cell division, such as pituitary adenylate-cyclase-activating
polypeptide (PACAP), GABA and glutamate, and members of the TGF-β superfamily.
However, in addition to stimulating re-entry of cells into the cell cycle, termed a
mitogenic effect, extracellular signals also enhance proliferation by promoting survival
of the mitotic population, a trophic action. Stimulation of both pathways is necessary to
produce maximal cell numbers. These mitogenic and trophic mechanisms during
development parallel those identified in carcinogenesis, reflecting roles of c-myc and
bcl-2, respectively. Several of the neurotrophins, especially BDNF and neurotrophin-3
(NT3), promote survival of mitotic precursors as well as the newly generated progeny.
Table 1.3-1
Regulation of Neurodevelopment by Extracellular Factors
The developmental significance of extracellular mitogens is demonstrated by the expression of the factors and their
receptors in regions of neurogenesis, and by the profound and permanent consequences of altering their activities during
development. For example, by administering growth factors to developing embryos or pups, one can induce changes in
proliferation in prenatal cortical VZ, postnatal cerebellar EGL, and hippocampal dentate gyrus that produce lifelong
modifications in brain region population size and cell composition. Such changes may be relevant to structural differences
observed in neuropsychiatric disorders, such as depression, schizophrenia, and autism. Specifically, in the cerebral cortex
VZ of the embryonic rat, proliferation is controlled by promitogenic bFGF and antimitogenic PACAP, which are expressed
as autocrine/paracrine signals. Positive and negative effects were shown in living embryos in utero by performing
intracerebroventricular (ICV) injections of the factors or antagonists. ICV injection of bFGF produced a larger adult cortex
composed of 87 percent more neurons, which employed glutamate, thus increasing the ratio of excitatory pyramidal
neurons to GABA inhibitory neurons, which were unchanged. Conversely, embryonic PACAP injection inhibited
proliferation of cortical precursors by 26 percent, reducing the number of labeled layer 5/6 neurons in the cortical plate 5
days later. A similar reduction was accomplished by genetically deleting promitogenic bFGF or leukocyte inhibitory factor
(LIF)/ciliary neurotrophic factor (CNTF)/gp130 signaling, diminishing cortical size. Furthermore, effects of mitogenic
signals depended critically on the stage-specific program of regional development, since bFGF injection at later ages when
gliogenesis predominates affected glial numbers selectively. Thus developmental dysregulation of mitogenic pathways due
to genetic or environmental factors (hypoxia, maternal/fetal infection, or drug or toxicant exposure) will likely produce
subtle changes in the size and composition of the developing cortex. Other signals likely to play proliferative roles include
Wnt’s, TGF-α, IGF-I, and BMPs. Although interactions between intrinsic cortical programs and extrinsic factors remain to
be defined, a remarkable new study of mouse embryonic stem cells suggests that embryonic mammalian forebrain
specification may be a developmentally ancestral intrinsic program that emerges in the absence of extrinsic signals. In
specific culture conditions that block endogenous Shh signaling, mouse embryonic stem cells can sequentially generate the
various types of neurons that display most salient features of genuine cortical pyramidal neurons. When grafted into the
cerebral cortex, these cells differentiate into neurons that project to select cortical (visual and limbic regions) and
subcortical targets, corresponding to a wide range of pyramidal layer neurons. Insight into precision control of neuronal
differentiation will open new avenues to perform neuronal grafts in humans for cellular replacement in various acquired
and neurodegenerative diseases.
Similar to cerebral cortex, later generated populations of granule neurons, such as in
cerebellum and hippocampal dentate gyrus, are also sensitive to growth factor
manipulation, which is especially relevant to therapies administered intravenously to
premature and newborn infants in the neonatal nursery. Like in humans, cerebellar
granule neurons are produced postnatally in rats, but for only 3 weeks, whereas in both
species dentate gyrus neurons are produced throughout life. Remarkably, a single
peripheral injection of bFGF into newborn rat pups rapidly crossed into the
cerebrospinal fluid (CSF) and stimulated proliferation in the cerebellar EGL by 30
percent as well as hippocampal dentate gyrus by twofold by 8 hours, consistent with an
endocrine mechanism of action. The consequence of mitogenic stimulation in cerebellum
was a 33 percent increase in the number of internal granule layer neurons and a 22
percent larger cerebellum. In hippocampus, mitotic stimulation elicited by a single bFGF
injection increased the absolute number of dentate gyrus granule neurons by 33 percent
at 3 weeks, defined stereologically, producing a 25 percent larger hippocampus
containing more neurons and astrocytes, a change that persisted lifelong. Conversely,
genetic deletion of bFGF resulted in smaller cerebellum and hippocampus at birth and
throughout life, indicating that levels of the growth factor were critical for normal brain
region formation. Other proliferative signals regulating cerebellar granule neurogenesis
include Shh and PACAP, the disruption of which contributes to human medulloblastoma,
whereas in hippocampus the Wnt family may be involved.
Clinical Implications
There are several clinical implications of these surprising growth factor effects observed
in newborns. First, we may need to investigate possible neurogenetic effects of
therapeutic agents we administer in the newborn nursery for long-term consequences.
Second, because bFGF is as effective in stimulating adult neurogenesis (see subsequent
text) as in newborns because of specific transport across the mature blood–brain barrier
(BBB), there is the possibility that other protein growth factors are also preferentially
transported into the brain and alter ongoing neurogenesis. Indeed, in rats, IGF-I also
stimulates mature hippocampal dentate gyrus neurogenesis. Third, other therapeutics
cross the BBB efficiently due to their lipid solubility, such as steroids, which inhibit
neurogenesis across the age spectrum. Steroids are frequently used perinatally to
promote lung maturation and treat infections and trauma, but effects on human brain
formation have not been examined. Fourth, it is well known that neurological
development may be delayed in children who experience serious systemic illness that is
associated with numerous inflammatory cytokines, and one may wonder to what degree
this reflects interference with neurogenesis and concomitant processes, potentially
producing long-term differences in cognitive and motor functional development.
Finally, maternal infection during pregnancy is a known risk factor for schizophrenia,
and cytokines that cross the placental barrier may directly affect fetal brain cell
proliferation and differentiation as well as cell migration, target selection, and synapse
maturation, as shown in animal models, eventually leading to multiple brain and
behavioral abnormalities in the adult offspring.
CELL MIGRATION
Throughout the nervous system, newly generated neurons normally migrate away from
proliferative zones to achieve final destinations. If this process is disrupted, abnormal
cell localization and function result. In humans, more than 25 syndromes with disturbed
neuronal migration have been described. As noted earlier, neurons migrate in both
radial and tangential fashions during development and may establish cell layers that are
inside-to-outside, or the reverse, according to region. In developing cerebral cortex, the
most well-characterized mechanism is radial migration from underlying VZ to
appropriate cortical layers in an inside-to-outside fashion. In addition, however, the
inhibitory GABA interneurons that are generated in ventrally located medial ganglionic
eminences reach the cortex through tangential migration in the intermediate zone along
axonal processes or other neurons. The neurons in developing cerebellum also exhibit
both radial and tangential migration. Purkinje cells leave the fourth ventricle VZ and
exhibit radial migration, whereas other precursors from the rhombic lip migrate
tangentially to cover the cerebellar surface, establishing the EGL, a secondary
proliferative zone. From EGL, newly generated granule cells migrate radially inward to
create the internal granule cell layer. Finally, granule interneurons of the olfactory bulb
exhibit a different kind of migration, originating in the SVZ of the lateral ventricles
overlying the striatum. These neuroblasts divide and migrate simultaneously in the
rostral migratory stream in transit to the bulb, on a path comprising chains of cells that
support forward movements. The most commonly recognized disorders of human
neuronal migration are the extensive lissencephalies (see subsequent text), although
incomplete migration of more restricted neuron aggregates (heterotopias) frequently
underlies focal seizure disorders.
Animal models have defined molecular pathways involved in neuronal migration. Cell movement requires signals to start
and stop migration, adhesion molecules to guide migration, and functional cytoskeleton to mediate cell translocation. The
best-characterized mouse model of aberrant neuronal migration is reeler, a spontaneous mutant in which cortical neuron
laminar position is inverted, being generated in outside-to-inside fashion. Reelin is a large, secreted extracellular
glycoprotein produced embryonically by the earliest neurons in the cortical preplate, Cajal–Retzius cells, and
hippocampus and cerebellum. Molecular and genetic analysis has established a signaling sequence in reelin activity that
includes at least two receptors, the very low-density lipoprotein receptor (VLDLR) and the apoprotein E receptor 2
(ApoER2), and the intracellular adapter protein, disabled 1 (Dab1), initially identified in the scrambler mutant mouse, a
reelin phenocopy. Current thoughts consider the reelin system as one mediator of radial glia-guided neuron migration,
although its specific functions in starting or stopping migration remain controversial. The roles of the VLDL and ApoE2
receptors are intriguing for their possible contributions to Alzheimer’s disease risk. Recent studies have found human
reelin gene (RELN) mutations associated with autosomal recessive lissencephaly with cerebellar hypoplasia, exhibiting a
markedly thickened cortex with pachygyria, abnormal hippocampal formations, and severe cerebellar hypoplasia with
absent folia. Additional studies suggest that reelin polymorphisms may contribute to autism spectrum disorder (ASD) risk
as well.
With regard to cytoskeletal proteins, studies of the filamentous fungus Aspergillus nidulans surprisingly provided insight
into the molecular machinery underlying the human migration disorder, Miller-Dieker syndrome, a lissencephaly
associated with abnormal chromosome 17q13.3. Lissencephaly is a diverse disorder characterized by a smooth cortical
surface lacking in gyri and sulci, with markedly reduced brain surface area. The absence of convolutions results from a
migration defect: the majority of neurons fail to reach their final destinations. In classical lissencephaly (type I), cerebral
cortex is thick and usually four-layered, whereas in cobblestone lissencephaly (type II), the cortex is chaotically organized
with a partly smooth and partly pebbled surface and deficient lamination. The most severely affected parts of the brain are
the cerebral cortex and hippocampus, with the cerebellum less affected. In fungus, the gene NudF was found to be essential
for intracellular nuclear distribution, a translocation process also involved in mammalian cell migration. The human
homologue of NudF is LIS-1 or PAFAH1B1, a mutation of which accounts for up to 60 percent of lissencephaly cases of
type I pathology. The LIS-1 gene product interacts with microtubules and related motor components dynein and dynactin
as well as doublecortin (DCX), which may regulate microtubule stability. Mutations in DCX gene result in X-linked
lissencephaly in males and bands of heterotopic neurons in white matter in females, appearing as a “double cortex” on
imaging studies, producing severe mental retardation and epilepsy. Other migratory defects occur when proteins
associated with the actin cytoskeleton are affected, such as mutation in filamin 1 gene responsible for periventricular
nodular heterotopias in humans and mutations of a regulatory phosphokinase enzyme, the CDK5/p35 complex.
Cell migration also depends on molecules mediating cellular interactions, which
provide cell adhesion to establish neuron–neuron and neuron–glia relationships or
induce attraction or repulsion. Astrotactin is a major glial protein involved in neuronal
migration on radial glial processes, whereas neuregulins and their receptors, ErbB2-4,
play roles in neuronal–glial migratory interactions. Recent genetic studies associate
neuregulin polymorphisms with schizophrenia, suggesting that this developmental
disease may depend on altered oligodendrocyte numbers and activities and synaptic
functions. Furthermore, some work suggests that early appearing neurotransmitters
themselves, GABA and glutamate, and platelet-derived growth factor (PDGF) regulate
migration speed. In contrast to radial migration from cortical VZ, GABA interneurons
generated in ganglionic eminences employ different mechanisms to leave the ventral
forebrain and migrate dorsally into the cerebral cortex. Several signaling systems have
been identified, including the Slit protein and Robo receptor, the semaphorins and their
neuropilin receptors, and hepatocyte growth factor and its c-Met receptor, all of which
appear to repel GABA interneurons from basal forebrain, promoting tangential
migration into cortex. Significantly, the c-Met receptor has recently been associated with
autism spectrum disorders, suggesting that altered GABA interneuron migration into
cortex and deficits in inhibitory signaling may contribute to the phenotype, including
seizures and abnormal cognitive processing. Finally, several human forms of congenital
muscular dystrophy with severe brain and eye migration defects result from gene
mutations in enzymes that transfer mannose sugars to serine/threonine –OH groups in
glycoproteins, thereby interrupting interactions with several extracellular matrix
molecules and producing type II cobblestone lissencephalies.
DIFFERENTIATION AND NEURONAL PROCESS OUTGROWTH
After newly produced neurons and glial cells reach their final destinations, they
differentiate into mature cells. For neurons, this involves outgrowth of dendrites and
extension
of
axonal
processes,
formation
of
synapses,
and
production
of
neurotransmitter systems, including receptors and selective reuptake sites. Most axons
will become insulated by myelin sheaths produced by oligodendroglial cells. Many of
these events occur with a peak period from 5 months of gestation onward. During the
first several years of life, many neuronal systems exhibit exuberant process growth and
branching, which is later decreased by selective “pruning” of axons and synapses,
dependent on experience, whereas myelination continues for several years after birth
and into adulthood.
Although there is tremendous synapse plasticity in adult brain, a fundamental feature
of the nervous system is the point-to-point or topographic mapping of one neuron
population to another. During development, neurons extend axons to innervate diverse
distant targets, such as cortex and spinal cord. The structure that recognizes and
responds to cues in the environment is the growth cone, located at the axon tip. The
axonal process is structurally supported by microtubules that are regulated by numerous
microtubule-associated proteins (MAPs), whereas the terminal growth cone exhibits a
transition to actin-containing microfilaments. The growth cone has rod-like extensions
called filopodia that bear receptors for specific guidance cues present on cell surfaces
and
in
extracellular
matrix.
Interactions
between
filopodial
receptors
and
environmental cues cause growth cones to move forward, turn, or retract. Recent studies
have identified the actin-modulating proteins and kinases involved in rapid growth cone
movements, such as LIMK kinase that causes the language phenotype associated with
Williams’ syndrome. Perhaps surprising is that activation of growth cone receptors leads
to local mRNA translation to produce synaptic proteins, whereas traditional concepts
assumed that all proteins were transported to axon terminals from distant neuronal cell
somas. The region-specific expression of extracellular guidance molecules, such as
cadherins, regulated by patterning genes Pax6 and Emx2, results in highly directed
outgrowth of axons, termed axonal pathfinding. These molecules affect the direction,
speed, and fasciculation of axons, acting through either positive or negative regulation.
Guidance molecules may be soluble extracellular factors or, alternatively, may be bound
to extracellular matrix or cell membranes. In the latter class of signal is the newly
discovered family of transmembrane proteins, the ephrins. Playing major roles in
topographic mapping between neuron populations and their targets, ephrins act via the
largest known family of tyrosine kinase receptors in brain, Eph receptors. Ephrins
frequently serve as chemorepellent cues, negatively regulating growth by preventing
developing axons from entering incorrect target fields. For example, the optic tectum
expresses ephrins A2 and A5 in a gradient that decreases along the posterior to anterior
axis, whereas innervating retinal ganglion cells express a gradient of Eph receptors.
Ganglion cell axons from posterior retina, which possess high Eph A3 receptor levels,
will preferentially innervate the anterior tectum because the low level ephrin expression
does not activate the Eph kinase that causes growth cone retraction. In the category of
soluble molecules, netrins serve primarily as chemoattractant proteins secreted, for
instance, by the spinal cord floor plate to stimulate spinothalamic sensory interneurons
to grow into the anterior commissure, whereas Slit is a secreted chemorepulsive factor
that through its roundabout (Robo) receptor regulates midline crossing and axonal
fasciculation and pathfinding.
THE NEURODEVELOPMENTAL BASIS OF PSYCHIATRIC DISEASE
An increasing number of neuropsychiatric conditions are considered to originate during
brain development, including schizophrenia, depression, autism, and attentiondeficit/hyperactivity disorder. Defining when a condition begins helps direct attention
to underlying pathogenic mechanisms. The term neurodevelopmental suggests that the
brain is abnormally formed from the very beginning due to disruption of fundamental
processes, in contrast to a normally formed brain that is injured secondarily or that
undergoes degenerative changes. However, the value of the term neurodevelopmental
needs to be reconsidered, because of different use by clinicians and pathologists. In
addition, given that the same molecular signals function in both development and
maturity, altering an early ontogenetic process by changes in growth factor signaling,
for instance, probably means that other adult functions exhibit ongoing dysregulation as
well. For example, clinical researchers of schizophrenia consider the disorder
neurodevelopmental because at the time of onset and diagnosis, the prefrontal cortex
and hippocampus are smaller and ventricles enlarged already at adolescent
presentation. In contrast, the neuropathologist uses the term neurodevelopmental for
certain morphological changes in neurons. If a brain region exhibits a normal
cytoarchitecture but with neurons of smaller than normal diameter, reminiscent of
“immature” stages, then this may be considered an arrest of development. However, if
the same cellular changes are accompanied by inflammatory signs, such as gliosis and
white blood cell infiltrate, then this is termed neurodegeneration. These morphological
and cellular changes may no longer be adequate to distinguish disorders that originate
from development versus adulthood, especially given the roles of glial cells, including
astrocytes, oligodendrocytes, and microglia, as sources of neurotrophic support during
both periods of life. Thus abnormalities in glial cells may occur in both epochs to
promote disease or act as mechanisms of repair. Many neurodegenerative processes such
as in Alzheimer’s and Parkinson’s diseases are associated with microglial cells. On the
other hand, neuronal dysfunction in adulthood such as cell shrinkage may occur without
inflammatory changes. In animal models, interruption of BDNF neurotrophic signaling
in adult brain results in neuron and dendrite atrophy in cerebral cortex without eliciting
glial cell proliferation. Thus finding small neurons without gliosis in the brains of
patients with schizophrenia or autism does not necessarily mean that the condition is
only or primarily developmental in origin. In turn, several etiological assumptions
about clinical brain conditions may require reexamination.
Because the same processes that mediate development, including neurogenesis, gliogenesis, axonal growth and
retraction, synaptogenesis, and cell death, also function during adulthood, a new synthesis has been proposed. All of these
processes, although perhaps in more subtle forms, contribute to adaptive and pathological processes. Successful aging of
the nervous system may require precise regulation of these processes, allowing the brain to adapt properly and counteract
the numerous intrinsic and extrinsic events that could potentially lead to neuropathology. For example, adult neurogenesis
and synaptic plasticity are necessary to maintain neuronal circuitry and ensure proper cognitive functions. Programmed
cell death is crucial to prevent tumorigenesis that can occur as cells accumulate mutations throughout life. Thus
dysregulation of these ontogenetic processes in adulthood will lead to disruption of brain homeostasis, expressing itself as
various neuropsychiatric diseases.
Schizophrenia
The neurodevelopmental hypothesis of schizophrenia postulates that etiologic and
pathogenetic factors occurring before the formal onset of the illness, that is, during
gestation, disrupt the course of normal development. These subtle early alterations in
specific neurons, glia, and circuits confer vulnerability to other later developmental
factors, ultimately leading to malfunctions. Schizophrenia is clearly a multifactorial
disorder, including both genetic and environmental factors. Clinical studies using risk
assessment have identified some relevant factors, including prenatal and birth
complications (hypoxia, infection, or substance and toxicant exposure), family history,
body dysmorphia, especially structures of neural crest origin, and presence of mild
premorbid deficits in social, motor, and cognitive functions. These risk factors may
affect ongoing developmental processes such as experience-dependent axonal and
dendritic production, programmed cell death, myelination, and synaptic pruning. An
intriguing animal model using human influenza–induced pneumonia of pregnant mice
shows that the inflammatory cytokine response produced by the mother may directly
affect the offspring’s brain development, with no evidence of the virus in the fetus or
placenta.
Neuroimaging and pathology studies identify structural abnormalities at disease
presentation, including smaller prefrontal cortex and hippocampus and enlarged
ventricles, suggesting abnormal development. More severely affected patients exhibit a
greater number of affected regions with larger changes. In some cases, ventricular
enlargement and cortical gray matter atrophy increase with time. These ongoing
progressive changes should lead us to reconsider the potential role of active
degeneration in schizophrenia, whether due to the disease or its consequences, such as
stress or drug treatment. However, classic signs of neurodegeneration with
inflammatory cells are not present.
Structural neuroimaging strongly supports the conclusion that the hippocampus in schizophrenia is significantly
smaller, perhaps by 5 percent. In turn, brain morphology has been used to assess etiological contributions of genetic and
environmental factors. Comparisons of concordance for schizophrenia in monozygotic and dizygotic twins support roles
for both factors. Among monozygotic twins, only 40 to 50 percent of both twins have the illness, indicating that genetic
constitution alone does not ensure disease and suggesting that the embryonic environment also contributes. Neuroimaging,
pharmacological, and pathological studies suggest that some genetic factors allow for susceptibility and that secondary
insults, such as birth trauma or perinatal viral infection, provide the other. This model is consistent with imaging studies
showing small hippocampus in both affected and unaffected monozygotic twins. Moreover, healthy, genetically at risk
individuals show hippocampal volumes (smaller) more similar to affected probands than normal controls. Thus
hippocampal volume reduction is not pathognomonic of schizophrenia but rather may represent a biological marker of
genetic susceptibility. It is not difficult to envision roles for altered developmental regulators in producing a smaller
hippocampus, which in turn limits functional capacity. A smaller hippocampus may result from subtle differences in the
levels of transcription factors, such as NeuroD, Math1, or Lhx, signaling by Wnt3a and downstream mediator Lef1, or
proliferative control mediated by bFGF, the family members of which exhibit altered expression levels in schizophrenia
brain samples. Such genetic limitations may only become manifest following another developmental challenge, such as
gestational infection, stressors, or toxicant exposure.
A regional locus of schizophrenia pathology remains uncertain but may include hippocampus, entorhinal cortex,
multimodal association cortex, limbic system, amygdala, cingulate cortex, thalamus, and medial temporal lobe. Despite
size reductions in specific regions, attempts to define changes in cell numbers have been unrewarding, since most studies
do not quantify the entire cell population but assess only regional cell density. Without assessing a region’s total volume,
cell density measures alone are limited in revealing population size. Most studies have found no changes in cell density in
diverse regions. A single study successfully examining total cell number in hippocampus found normal neuron density and
a 5 percent volume reduction on the left and 2 percent on the right, yielding no significant change in total cell number.
In contrast to total neuron numbers, using neuronal cell-type–specific markers, many studies have found a decreased
density of nonpyramidal GABA interneurons in cortex and hippocampus. In particular, parvalbumin-expressing
interneurons are reduced, whereas calretinin-containing cells are normal, suggesting a deficiency of an interneuron
subtype. These morphometric data are supported by molecular evidence for decreased GABA neurons, including reduced
mRNA and protein levels of the GABA-synthesizing enzyme, GAD67, in cortex and hippocampus. Another product of the
adult GABA-secreting neurons, reelin, which initially appears in Cajal–Retzius cells in embryonic brain, is reduced 30 to 50
percent in schizophrenia and bipolar disorder with psychotic symptoms. Such a deficiency, leading to diminished GABA
signaling, may underlie a potential compensatory increase in GABAA receptor binding detected in hippocampal CA 2 to 4
fields by both pyramidal and nonpyramidal neurons, apparently selective since benzodiazepine binding is unchanged. More
generally, deficiency in a subpopulation of GABA interneurons raises intriguing new possibilities for schizophrenia
etiology. As indicated in the preceding gene patterning section, different subpopulations of forebrain GABA interneurons
originate from distinct precursors located in the embryonic basal forebrain. Thus cortical and hippocampal GABA
interneurons may derive primarily from the MGE under control of the patterning gene Nkx2.1, whereas SVZ and olfactory
neurons derive from Gsh2-expressing LGE precursors. Furthermore, the timing and sequence of GABA interneuron
generation may depend on a regulatory network including Mash1, Dlx1/2, and Dlx5/6, all gene candidates for schizophrenia
risk. Indeed, DLX1 gene expression is reduced in the thalamus of patients with psychosis. Thus abnormal regulation of
these factors may diminish selectively GABA interneuron formation, which in turn may represent a genetically determined
vulnerability, and may contribute to diminished regional brain size and/or function.
The most compelling neuropathological evidence for a developmental basis is the
finding of aberrantly localized or clustered neurons especially in lamina II of the
entorhinal cortex and in the white matter underlying prefrontal cortex and temporal
and
parahippocampal
regions.
These
abnormalities
represent
alterations
of
developmental neuronal migration, survival, and connectivity. In addition, in
hippocampus and neocortex, pyramidal neurons appear smaller in many studies,
exhibiting fewer dendritic arborizations and spines with reduced neuropil, findings that
are associated with reductions in neuronal molecules, including MAP2, spinophilin,
synaptophysin, and SNAP25. Although the genes associated with schizophrenia are
reviewed extensively in other chapters, worth mentioning here is a particularly
intriguing candidate gene DISC1, whose protein has roles during development including
regulating cell migration, neurite outgrowth, and neuronal maturation as well as in
adult brain, where it modulates cytoskeletal function, neurotransmission, and synaptic
plasticity. DISC1 protein interacts with many other proteins intimately involved in
neuronal cell migration and forms a protein complex with Lis1 and NudEL that is
downstream of reelin signaling.
Autism Spectrum Disorders
Another condition that is clearly neurodevelopmental in origin is autism spectrum
disorders (ASDs), a complex and heterogeneous group of disorders characterized by
abnormalities in social interaction and communication and the presence of restricted or
repetitive interests and activities. In the last edition of DSM (DSM-IV) the ASDs included
classic autistic disorder, Asperger’s syndrome, and pervasive developmental disorder not
otherwise specified. These three disorders were grouped together due to their common
occurrence in families, indicating related genetic factors and shared signs and
symptoms. Recent conceptualizations of ASDs propose that there are multiple “autisms”
differing in underlying pathogenetic mechanisms and manifestations. It is likely that the
different core symptom domains (or other endophenotypes) will be more heritable than
the syndromic diagnosis, which was constructed to be inclusive. The large diversity of
ASD signs and symptoms reflects the multiplicity of abnormalities observed in
pathological and functional studies and include both forebrain and hindbrain regions.
Forebrain neurons in the cerebral cortex and limbic system play critical roles in social
interaction, communication, and learning and memory. For example, the amygdala,
which connects to prefrontal and temporal cortices and fusiform gyrus, plays a
prominent role in social and emotional cognition. In ASDs, the amygdala and fusiform
gyrus demonstrate abnormal activation during facial recognition and emotional
attribution tasks. Some investigators hypothesize that ASDs reflect dysfunctions in
specific neural networks, such as the social network. On the other hand,
neurophysiological tests of evoked cortical potentials and oculomotor responses indicate
normal perception of primary sensory information but disturbed higher cognitive
processing. The functional impairments in higher-order cognitive processing and
neocortical circuitry suggest a developmental disorder involving synaptic organization,
a mechanism that may be uniformly present throughout the brain, a model in distinct
contrast to abnormalities of specific neural networks. Earlier reference to the expression
of Wnt3a in cells that migrated widely during development and appear in auditory
systems is one example of how developmental changes may affect single functional
networks, whereas changes in common and widely expressed synaptic molecules, such
as the neuroligins, would represent the other mechanism.
The most important recent discovery in ASD pathogenesis has been the widely
reported and replicated brain growth phenotype: Starting with probably normal size at
birth, the brain exhibits an accelerated increase in volume by the end of the first year
compared to the typically developing child, and this process continues from ages 2 to 4
years. These data derive from both neuroimaging studies as well as measures of head
circumference performed by multiple labs. It is not known whether this reflects an
acceleration of normal developmental processes or, alternatively, a disease-specific
aberration in postnatal development, including changes in cell numbers, neuronal
processes, synapse formation and modifications, or glial cell dysfunction, to name a
few. The most prominent differences are observed in frontal and parietal cortex,
cerebellar hemispheres, as well as the amygdala. These findings are also consistent with
recent reports of macrocephaly in up to ∼20 percent of ASD cases in brain and DNA
banks. These findings raise many questions to be addressed by developmental
neuroscientists.
Functional neuroimaging studies indicate broad forebrain but also cerebellar dysfunctions in ASD, and classical
pathological studies have suggested abnormalities restricted to limbic and cerebellar structures. However, classical studies
were hampered by small sample sizes, poor control for comorbidities such as epilepsy and mental retardation that affect
neuroanatomy, and the use of tissue cell density measures as opposed to unbiased stereological methods to estimate
regional neuron numbers. Although previous studies described increased densities of small neurons in interconnecting
limbic nuclei, including CA fields, septum, mammillary bodies, and amygdala, these results have not been replicated by
other laboratories. In contrast, the most consistent neuropathology has been observed in the cerebellum (21 of 29 brains),
showing reductions in the number of Purkinje neurons without signs of acquired postnatal lesions, such as gliosis, empty
baskets, and retrograde loss of afferent inferior olive neurons, suggesting prenatal origins.
A more recent study identifies widespread and nonuniform abnormalities, suggesting dysregulation of many processes,
including neuron proliferation, migration, survival, organization, and programmed cell death. Four of six brains were
macrocephalic, consistent with increased size defined by numerous pathology and neuroimaging studies. In cerebral
cortex, there was thickened or diminished gray matter, disorganized laminar patterns, misoriented pyramidal neurons,
ectopic neurons in both superficial and deep white matter, and increased or decreased neuron densities. This evidence of
abnormal cortical neurogenesis and migration accords well with the deficits in cognitive functions. In brainstem, neuronal
disorganization appeared as discontinuous and malpositioned neurons in olivary and dentate nuclei, ectopic neurons in
medulla and cerebellar peduncles, and aberrant fiber tracts. There were widespread patchy or diffuse decreases of Purkinje
neurons, sometimes associated with increased Bergmann glia, or ectopic Purkinje neurons in the molecular layer.
Hippocampal neuronal atrophy was not observed, and quantitative stereology found no consistent change in neuron
density or number. Moreover, a single recent neuropathological study using multiple immunological indices has reported
increased levels of immune cytokines in the cerebrospinal fluid of patients and in brain tissues as well as astrocytes
expressing high levels of glial fibrillary acidic protein in frontal and cingulated cortex, white matter, and cerebellum, all
suggesting potential immune activation without evidence of an inflammatory process. We await confirmation of these
important findings.
Although seemingly incompatible, these various data support a model of
developmental abnormalities occurring at different times, altering regions according to
specific schedules of neurogenesis and differentiation. It is notable that a similar range
of abnormalities was found in classical studies but was excluded, since these
abnormalities did not occur in every brain examined. Moreover, in 15 children exposed
to the teratogen thalidomide during days 20 to 24 of gestation, when cranial and
Purkinje neurogenesis occurs in brainstem, four cases exhibited autism. On the basis of
these data, autism is associated with insults at 3 weeks for thalidomide, 12 weeks when
inferior olivary neurons are migrating, and ∼30 weeks when olivary axons make
synapses with Purkinje cells. These diverse abnormalities in cell production, survival,
migration, organization, and differentiation in both hindbrain and forebrain indicate
disturbed brain development over a range of stages. Recent genetic studies have defined
two genetic polymorphisms associated reproducibly with ASD in several datasets, both
of which influence brain developmental processes. The first is ENGRAILED-2, the
cerebellar patterning gene whose dysregulation causes deficits in Purkinje and granule
neurons in animal models and acts to control proliferation and differentiation. The
second is the hepatocyte growth factor receptor cMET, whose function affects tangential
migration of GABA interneurons from the ventral forebrain ganglionic eminences,
potentially leading to imbalances of excitatory and inhibitory neurotransmission.
Furthermore, although the cellular derangements may be directly responsible for the
core symptoms of autism, there is an alternative hypothesis: Disturbed regulation of
developmental processes produces an as-yet unidentified biochemical cellular lesion that
may be associated with autism. This proposal is supported by the currently known
genetic causes of autism that account for 10 percent of cases, including tuberous
sclerosis, neurofibromatosis, Smith-Lemli-Opitz syndrome, Rett syndrome, and fragile X
mental retardation. These genetic etiologies interfere with cell proliferation control,
cholesterol biosynthesis and Shh function, and synaptic and dendrite protein translation
and function, fundamental processes in the sequence of development. An intriguing
potential link in these monogenetic causes of autism symptoms is their participation in
protein synthesis in the synapse, especially as regulated via the PI3K/Akt signaling
pathway and the mammalian target of rapamycin (mTOR) complex, an area of active
research.
THE REMARKABLE DISCOVERY OF ADULT NEUROGENESIS
In the last decade, there has been a fundamental shift in paradigm regarding the limits
of neurogenesis in the brain, with important implications for neural plasticity,
mechanisms of disease etiology and therapy, and possibilities of repair. Until recently, it
has generally been maintained that we do not produce new neurons in the brain after
birth (or soon thereafter, considering cerebellar EGL); thus brain plasticity and repair
depend on modifications of a numerically static neural network. We now have strong
evidence to the contrary: new neurons are generated throughout life in certain regions,
well documented across the phylogenetic tree, including birds, rodents, primates, and
humans. As an area of intense interest and investigation, we may expect rapid progress
over the next two decades, likely altering models described herein.
The term neurogenesis has been used inconsistently in different contexts, indicating
sequential production of neural elements during development, first neurons then glial
cells, but frequently connoting only neuron generation in adult brain, in contrast to
gliogenesis. For this discussion, we use the first, more general meaning, and distinguish
cell types as needed. The first evidence of mammalian neurogenesis, or birth of new
neurons, in adult hippocampus was reported in the 1960s in which 3H-thymidine-labeled
neurons were documented. As a common marker for cell production, these studies used
nuclear incorporation of 3H-thymidine into newly synthesized DNA during chromosome
replication, which occurs before cells undergo division. After a delay, cells divide,
producing two 3H-thymidine-labeled progeny. Cell proliferation is defined as an
absolute increase in cell number, which occurs only if cell production is not balanced by
cell death. Because there is currently little evidence for a progressive increase in brain
size with age, except perhaps for rodent hippocampus, most neurogenesis in adult brain
is apparently compensated for by cell loss. More recent studies of neurogenesis employ
the more convenient thymidine analog BrdU, which can be injected into living animals
and then detected by immunohistochemistry.
During embryonic development, neurons are produced from almost all regions of the
ventricular neuroepithelium. Neurogenesis in the adult, however, is largely restricted to
two regions: the SVZ lining the lateral ventricles and a narrow proliferative zone
underlying the dentate gyrus granule layer (subgranular zone) in hippocampus. In mice,
rodents, and monkeys, newly produced neurons migrate from the SVZ in an anterior
direction into the olfactory bulb to become GABA interneurons. The process has been
elegantly characterized at both ultrastructural and molecular levels. In the SVZ, the
neuroblasts (A cells) on their way to olfactory bulb create chains of cells and migrate
through a scaffold of glial cells supplied by slowly dividing astrocytes (B cells). Within
this network of cell chains, there are groups of rapidly dividing neural precursors (C
cells). Evidence suggests that the B cells give rise to the C cells, which in turn develop
into the A cells, the future olfactory bulb interneurons. The existence of a sequence of
precursors with progressively restricted abilities to generate diverse neural cell types
makes defining mechanisms that regulate adult neurogenesis in vivo a great challenge.
As in developing brain, adult neurogenesis is also subject to regulation by extracellular signals that control precursor
proliferation and survival and in many cases the very same factors. After initial discovery of adult neural stem cells
generated under EGF stimulation, other regulatory factors were defined including bFGF, IGF-I, BDNF, and LIF/CNTF.
Although the hallmark of neural stem cells includes the capacity to generate neurons, astrocytes, and oligodendroglia,
termed multipotentiality, specific signals appear to produce relatively different profiles of cells that may migrate to
distinct sites. Intraventricular infusion of EGF promotes primarily gliogenesis in the SVZ, with cells migrating to olfactory
bulb, striatum, and corpus callosum, whereas bFGF favors the generation of neurons destined for the olfactory bulb. Both
factors appear to stimulate mitosis directly, with differential effects on the cell lineage produced. In contrast, BDNF may
increase neuron formation in SVZ as well as striatum and hypothalamus, though effects may be primarily through
promoting survival of newly generated neurons that otherwise undergo cell death. Finally, CNTF and related LIF may
promote gliogenesis or, alternatively, support self-renewal of adult stem cells rather than enhancing a specific cell category.
Remarkably, in addition to direct intraventricular infusions, adult neurogenesis is also affected by peripheral levels of
growth factors, hormones, and neuropeptides. Peripheral administration of both bFGF and IGF-I stimulate neurogenesis,
increasing selectively mitotic labeling in the SVZ and hippocampal subgranular zone, respectively, suggesting that there
are specific mechanisms for factor transport across the BBB. Of interest, elevated prolactin levels, induced by peripheral
injection or natural pregnancy, stimulate proliferation of progenitors in the mouse SVZ, leading to increased olfactory bulb
interneurons, potentially playing roles in learning new infant scents. This may be relevant to changes in prolactin seen in
psychiatric disease. Conversely, in behavioral paradigms of social stress, such as territorial challenge by male intruders,
activation of the hypothalamic-pituitary-adrenal axis with increased glucocorticoids leads to reduced neurogenesis in the
hippocampus, apparently through local glutamate signaling. Inhibition is also observed after peripheral opiate
administration, a model for substance abuse. Thus neurogenesis may be one target process affected by changes of hormones
and neuropeptides associated with several psychiatric conditions.
The discovery of adult neurogenesis naturally leads to questions about whether new
neurons can integrate into the complex cytoarchitecture of the mature brain and to
speculation about its functional significance, if any. In rodents, primates, and humans,
new neurons are generated in the dentate gyrus of the hippocampus, an area important
for learning and memory. Some adult-generated neurons in humans have been shown to
survive for at least 2 years. Furthermore, newly generated cells in adult mouse
hippocampus indeed elaborate extensive dendritic and axonal arborizations appropriate
to the neural circuit and display functional synaptic inputs and action potentials. From
a functional perspective, the generation and/or survival of new neurons correlates
strongly with multiple instances of behavioral learning and experience. For example,
survival of newly generated neurons is markedly enhanced by hippocampal-dependent
learning tasks and by an enriched, behaviorally complex environment. Of perhaps
greater importance, a reduction in dentate gyrus neurogenesis impairs the formation of
trace memories, that is, when an animal must associate stimuli that are separated in
time, a hippocampal-dependent task. Finally, in songbirds, neurogenesis is activity
dependent and is increased by foraging for food and learning new song, whether it
occurs seasonally or is induced by steroid hormone administration.
From clinical and therapeutic perspectives, fundamental questions are whether
changes in neurogenesis contribute to disease and whether newly formed neurons
undergo migration to and integration into regions of injury, replacing dead cells and
leading to functional recovery. A neurogenetic response has now been shown for
multiple conditions in the adult, including brain trauma, stroke, and epilepsy. For
instance, ischemic stroke in the striatum stimulates adjacent SVZ neurogenesis with
neurons migrating to the injury site. Furthermore, in a highly selective paradigm not
involving local tissue damage, degeneration of layer 3 cortical neurons elicited SVZ
neurogenesis and cell replacement. These studies raise the possibility that newly
produced neurons normally participate in recovery and may be stimulated as a novel
therapeutic strategy. However, in contrast to potential reconstructive functions,
neurogenesis may also play roles in pathogenesis: In a kindling model of epilepsy,
newly generated neurons were found to migrate to incorrect positions and participate in
aberrant neuronal circuits, thereby reinforcing the epileptic state. Conversely,
reductions in neurogenesis may contribute to several conditions that implicate
dysfunction or degeneration of the hippocampal formation. Dentate gyrus neurogenesis
is inhibited by increased glucocorticoid levels observed in aged rats and can be reversed
by steroid antagonists and adrenalectomy, observations potentially relevant to the
correlation of elevated human cortisol levels with reduced hippocampal volumes and the
presence of memory deficits. Similarly, stress-induced increases in human glucocorticoids
may contribute to decreased hippocampal volumes seen in schizophrenia, depression,
and posttraumatic stress disorder.
A potential role for altered neurogenesis in disease has gained the most support in recent studies of depression. A
number of studies in animals and humans suggest a correlation of decreased hippocampal size with depressive symptoms,
whereas clinically effective antidepressant therapy elicits increased hippocampal volume and enhanced neurogenesis, with
causal relationships still being defined. For example, postmortem and brain imaging studies indicate cell loss in
corticolimbic regions in bipolar disorder and major depression. Significantly, mood stabilizers, such as lithium ion and
valproic acid, as well as antidepressants and electroconvulsive therapy activate intracellular pathways that promote
neurogenesis and synaptic plasticity. Furthermore, in a useful primate model, the adult tree shrew, the chronic
psychosocial stress model of depression elicited ∼15 percent reductions in brain metabolites and a 33 percent decrease in
neurogenesis (BrdU mitotic labeling), effects that were prevented by coadministration of antidepressant, tianeptine. More
importantly, although stress exposure elicited small reductions in hippocampal volumes, stressed animals treated with
antidepressant exhibited increased hippocampal volumes. Similar effects have been found in rodent models of depression.
In addition to the foregoing structural relationships, recent evidence has begun defining the roles of relevant
neurotransmitter systems to antidepressant effects on behavior and neurogenesis. In a most exciting finding, a causal link
between antidepressant-induced neurogenesis and a positive behavioral response has been demonstrated. In the serotonin
1A receptor null mouse, fluoxetine, a selective serotonin reuptake inhibitor [SSRI], produced neither enhanced
neurogenesis nor behavioral improvement. Furthermore, when hippocampal neuronal precursors were selectively reduced
(85 percent) by X-irradiation, neither fluoxetine nor imipramine induced neurogenesis or behavioral recovery. Finally, one
study using hippocampal cultures from normal and mutant rodents strongly supports a neurogenetic role for endogenous
NPY, which is contained in dentate gyrus hilar interneurons. NPY stimulates precursor proliferation selectively via the Y1
(not Y2 or Y5) receptor, a finding consistent with the receptor-mediating antidepressive effects of NPY in animal models
and the impact of NPY levels on both hippocampal-dependent learning and responses to stress. In aggregate, these
observations suggest that volume changes observed with human depression and therapy may directly relate to alterations
in ongoing neurogenesis. More generally, the discovery of adult neurogenesis has led to major changes in our perspectives

04 - 1.4 Neurophysiology and Neurochemistry
1.4 Neurophysiology and Neurochemistry
on the regenerative capacities of the human brain.
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1.4 Neurophysiology and Neurochemistry
The study of chemical interneuronal communication is called neurochemistry, and in
recent years there has been an explosion of knowledge in understanding chemical
transmission between neurons and the receptors affected by those chemicals. Similarly,
advances in the science of physiology as applied to the brain and how the brain
functions have been equally influenced. This chapter focuses on the complex
heterogeneity of both these areas to help explain the complexity of thoughts, feelings,
and behaviors that make up the human experience.
MONOAMINE NEUROTRANSMITTERS
The monoamine neurotransmitters and acetylcholine have been historically implicated
in the pathophysiology and treatment of a wide variety of neuropsychiatric disorders.
Each monoamine neurotransmitter system modulates many different neural pathways,
which themselves subserve multiple behavioral and physiological processes. Conversely,
each central nervous system (CNS) neurobehavioral process is likely modulated by
multiple interacting neurotransmitter systems, including monoamines.
This complexity poses a major challenge to understanding the precise molecular, cellular, and systems level pathways
through which various monoamine neurotransmitters affect neuropsychiatric disorders. However, recent advances in
human genetics and genomics, as well as in experimental neuroscience, have shed light on this question. Molecular cloning
has identified a large number of genes that regulate monoaminergic neurotransmission, such as the enzymes, receptors,
and transporters that mediate the synthesis, cellular actions, and cellular reuptake of these neurotransmitters, respectively.
Human genetics studies have provided evidence of tantalizing links between allelic variants in specific monoamine-related
genes and psychiatric disorders and trait abnormalities, whereas the ability to modify gene function and cellular activity in
experimental animals has clarified the roles of specific genes and neural pathways in mediating behavioral processes.
Monoamines act on target cells by binding to specific cell-surface receptors. There are multiple receptor subtypes for
each monoamine, which are expressed in diverse regions and subcellular locales and which engage a variety of intracellular
signaling pathways. This panoply of receptors thus allows each monoamine neurotransmitter to modulate target cells in
many ways; the same molecule may activate some cells while inhibiting others, depending on which receptor subtype is
expressed by each cell. The various monoamines are discussed below.
Serotonin
Although only one in a million CNS neurons produces serotonin, these cells influence
virtually all aspects of CNS function. The cell bodies of these serotonergic neurons are
clustered in the midline raphe nuclei of the brainstem; the rostral raphe nuclei send
ascending axonal projections throughout the brain, whereas the descending caudal
raphe nuclei send projections into the medulla, cerebellum, and spinal cord (Fig. 1.4-1).
The descending serotonergic fibers that innervate the dorsal horn of the spinal cord have
been implicated in the suppression of nociceptive pathways, a finding that may relate to
the pain-relieving effects of some antidepressants. The tonic firing of CNS serotonin
neurons varies across the sleep–wake cycle, with an absence of activity during rapid eye
movement (REM) sleep. Increased serotonergic firing is observed during rhythmic motor
behaviors and suggests that serotonin modulates some forms of motor output.
FIGURE 1.4-1
Brain serotonergic pathways (in rats). Serotonergic neurons are located in brainstem
midline raphe nuclei and project throughout the neuraxis. (There is an approximate
similarity between monoamine pathways in rats and in humans.) AMG, amygdala;
CBM, cerebellum; cc, corpus callosum; CP, caudate putamen; CRN, caudal raphe nuclei;
CTX, neocortex; DR, dorsal raphe nucleus; HI, hippocampus; HY, hypothalamus; LC,
locus ceruleus; MR, median raphe nucleus; NAc, nucleus accumbens; OB, olfactory bulb;
SN, substantia nigra; TE, tectum; TH, thalamus; TM, tuberomammillary nucleus of
hypothalamus. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive
Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:65.)
Most serotonergic innervation of the cortex and limbic system arises from the dorsal and median raphe nuclei in the
midbrain; the serotonergic neurons in these areas send projections through the medial forebrain bundle into target
forebrain regions. The median raphe provides most of the serotonergic fibers that innervate the limbic system, whereas
the dorsal raphe nucleus provides most of the serotonergic fibers that innervate the striatum and thalamus.
In addition to the different target fields of these serotonergic nuclei, there are also cellular differences between their
constituent neurons. Dorsal raphe serotonergic fibers are fine, with small vesicle-coated swellings called varicosities,
whereas median raphe fibers have large spherical or beaded varicosities. It is unclear to what extent serotonin acts as a
true synaptic or “private” neurotransmitter versus action as a local endocrine hormone or “social transmitter,” or whether
its roles differ depending on the fiber type from which it is released. These fibers show differential sensitivity to the
neurotoxic effects of the amphetamine analog 3,4-methylenedioxy-methamphetamine (MDMA, “ecstasy”), which lesions
the fine axons of the dorsal raphe while sparing the thick beaded axons of the median raphe. The significance of these
morphological differences is unclear, although recent work has identified functional differences between the serotonergic
neurons of the dorsal and median raphe nuclei.
Dopamine
Dopamine neurons are more widely distributed than those of other monoamines,
residing in the midbrain substantia nigra and ventral tegmental area and in the
periaqueductal gray, hypothalamus, olfactory bulb, and retina. In the periphery,
dopamine is found in the kidney where it functions to produce renal vasodilation,
diuresis, and natriuresis. Three dopamine systems are highly relevant to psychiatry: The
nigrostriatal,
mesocorticolimbic,
and
tuberohypophyseal
system
(Fig.
1.4-2).
Degeneration of the nigrostriatal system causes Parkinson’s disease and has led to an
intense research focus on the development and function of dopamine neurons in the
midbrain substantia nigra nuclei. Dopamine cell bodies in the pars compacta division of
this region send ascending projections to the dorsal striatum (especially to the caudate
and putamen) and thereby modulate motor control. The extrapyramidal effects of
antipsychotic drugs are thought to result from the blockade of these striatal dopamine
receptors.
FIGURE 1.4-2
Brain dopaminergic pathways (in rats).The three principal dopaminergic pathways: (1)
nigrostriatal pathway, (2) mesocorticolimbic pathway, and (3) tuberohypophyseal
pathway. AMG, amygdala; CBM, cerebellum; cc, corpus callosum; CP, caudate putamen;
CTX, neocortex; HI, hippocampus; HY, hypothalamus; LC, locus ceruleus; NAc, nucleus
accumbens; OB, olfactory bulb; PFC, prefrontal cortex; PI, pituitary; SNC, substantia
nigra pars compacta; TE, tectum; TH, thalamus; VTA, ventral tegmental area. (From
Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry.
9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:66.)
The midbrain ventral tegmental area (VTA) lies medial to the substantia nigra and
contains dopaminergic neurons that give rise to the mesocorticolimbic dopamine system.
These neurons send ascending projections that innervate limbic structures, such as the
nucleus accumbens and amygdala; the mesoaccumbens pathway is a central element in
the neural representation of reward, and intense research has been devoted to this area
in recent years. All known drugs of abuse activate the mesoaccumbens dopamine
pathway, and plastic changes in this pathway are thought to underlie drug addiction.
The mesolimbic projection is believed to be a major target for the antipsychotic
properties of dopamine receptor antagonist drugs in controlling the positive symptoms
of schizophrenia, such as hallucinations and delusions.
VTA dopamine neurons also project to cortical structures, such as the prefrontal cortex, and modulate working memory
and attention; decreased activity in this pathway is proposed to underlie negative symptoms of schizophrenia. Thus
antipsychotic drugs that decrease positive symptoms by blocking dopamine receptors in the mesolimbic pathway may
simultaneously worsen these negative symptoms by blocking similar dopamine receptors in the mesocortical pathway.
The decreased risk of extrapyramidal side effects seen with clozapine (Clozaril; versus other typical antipsychotic
medications) is thought to be due to its relatively selective effects on this mesocortical projection. The tuberohypophyseal
system consists of dopamine neurons in the hypothalamic arcuate and paraventricular nuclei that project to the pituitary
gland and thereby inhibit prolactin release. Antipsychotic drugs that block dopamine receptors in the pituitary may thus
disinhibit prolactin release and cause galactorrhea.
Norepinephrine and Epinephrine
The postganglionic sympathetic neurons of the autonomic nervous system release
norepinephrine, resulting in widespread peripheral effects including tachycardia and
elevated blood pressure. The adrenal medulla releases epinephrine, which produces
similar effects; epinephrine-secreting pheochromocytoma tumors produce bursts of
sympathetic activation, central arousal, and anxiety.
Norepinephrine-producing neurons are found within the brain in the pons and medulla in two major clusterings: The
locus ceruleus (LC) and the lateral tegmental noradrenergic nuclei (Fig. 1.4-3). Noradrenergic projections from both of
these regions ramify extensively as they project throughout the neuraxis. In humans, the LC is found in the dorsal portion
of the caudal pons and contains approximately 12,000 tightly packed neurons on each side of the brain. These cells provide
the major noradrenergic projections to the neocortex, hippocampus, thalamus, and midbrain tectum. The activity of LC
neurons varies with the animal’s level of wakefulness. Firing rates are responsive to novel and/or stressful stimuli, with
largest responses to stimuli that disrupt ongoing behavior and reorient attention. Altogether, physiological studies indicate
a role for this structure in the regulation of arousal state, vigilance, and stress response. The projections from lateral
tegmental nucleus neurons, which are loosely scattered throughout the ventral pons and medulla, partially overlap those of
the LC. Fibers from both cell groups innervate the amygdala, septum, and spinal cord. Other regions, such as the
hypothalamus and lower brainstem, receive adrenergic inputs predominantly from the lateral tegmental nucleus. The
relatively few neurons that utilize epinephrine as a neurotransmitter are located in the caudal pons and medulla,
intermingled with noradrenergic neurons. Projections from these groups ascend to innervate the hypothalamus, LC, and
visceral efferent and afferent nuclei of the midbrain.
FIGURE 1.4-3
Brain noradrenergic pathways (in rats). Projections of noradrenergic neurons located in
the locus ceruleus (LC) and lateral tegmental noradrenergic nuclei (LTN). AMG,
amygdala; CBM, cerebellum; cc, corpus callosum; CP, caudate putamen; CTX, neocortex;
HI, hippocampus; HY, hypothalamus; OB, olfactory bulb; TE, tectum; TH, thalamus.
(From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of
Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:66.)
Histamine
Histamine is perhaps best known for its role in allergies. It is an inflammatory mediator
stored in mast cells and released upon cellular interaction with allergens. Once released,
histamine causes vascular leakage and edema and other facial and topical allergy
symptoms. In contrast, central histaminergic neural pathways have only more recently
been characterized by immunocytochemistry using antibodies to the synthetic enzyme
histidine decarboxylase and to histamine. Histaminergic cell bodies are located within a
region of the posterior hypothalamus termed the tuberomammillary nucleus. The
activity of tuberomammillary neurons is characterized by firing that varies across the
sleep–wake cycle, with the highest activity during the waking state, slowed firing during
slow-wave sleep, and absence of firing during REM sleep. Histaminergic fibers project
diffusely throughout the brain and spinal cord (Fig. 1.4-4). Ventral ascending
projections course through the medial forebrain bundle and then innervate the
hypothalamus, diagonal band, septum, and olfactory bulb. Dorsal ascending projections
innervate the thalamus, hippocampus, amygdala, and rostral forebrain. Descending
projections travel through the midbrain central gray to the dorsal hindbrain and spinal
cord. The fibers have varicosities that are seldom associated with classical synapses, and
histamine has been proposed to act at a distance from its sites of release, like a local
hormone. The hypothalamus receives the densest histaminergic innervation, consistent
with a role for this transmitter in the regulation of autonomic and neuroendocrine
processes. In addition, strong histaminergic innervation is seen in monoaminergic and
cholinergic nuclei.
FIGURE 1.4-4
Brain histaminergic pathways (in rats). Histaminergic neurons are located in the
tuberomammillary nucleus of the caudal hypothalamus (TM) and project to the
hypothalamus (HY) and more distant brain regions. CBM, cerebellum; cc, corpus
callosum; CP, caudate putamen; CTX, neocortex; HI, hippocampus; NAc, nucleus
accumbens; OB, olfactory bulb; TE, tectum; TH, thalamus. (From Sadock BJ, Sadock VA,
Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009:67.)
Acetylcholine
Within the brain, the axonal processes of cholinergic neurons may either project to
distant brain regions (projection neurons) or contact local cells within the same
structure (interneurons). Two large clusters of cholinergic projection neurons are found
within the brain: The basal forebrain complex and the mesopontine complex (Fig. 1.45). The basal forebrain complex provides most of the cholinergic innervation to the
nonstriatal telencephalon. It consists of cholinergic neurons within the nucleus basalis of
Meynert, the horizontal and vertical diagonal bands of Broca, and the medial septal
nucleus. These neurons project to widespread areas of the cortex and amygdala, to the
anterior cingulate gyrus and olfactory bulb, and to the hippocampus, respectively. In
Alzheimer’s disease there is significant degeneration of neurons in the nucleus basalis,
leading to substantial reduction in cortical cholinergic innervation. The extent of
neuronal loss correlates with the degree of dementia, and the cholinergic deficit may
contribute to the cognitive decline in this disease, consistent with the beneficial effects
of drugs that promote acetylcholine signaling in this disorder.
FIGURE 1.4-5
Brain cholinergic projection pathways (in rats). The majority of cholinergic projection
neurons are located in the basal forebrain complex (BFC) and the mesopontine complex
(MPC). AMG, amygdala; CBM, cerebellum; cc, corpus callosum; CP, caudate putamen;
CTX, neocortex; HI, hippocampus; HY, hypothalamus; LC, locus ceruleus; NAc, nucleus
accumbens; OB, olfactory bulb; SN, substantia nigra; TE, tectum; TH, thalamus. (From
Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry.
9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:67.)
The mesopontine complex consists of cholinergic neurons within the pedunculopontine and laterodorsal tegmental
nuclei of the midbrain and pons and provides cholinergic innervation to the thalamus and midbrain areas (including the
dopaminergic neurons of the ventral tegmental area and substantia nigra) and descending innervation to other brainstem
regions such as the LC, dorsal raphe, and cranial nerve nuclei. In contrast to central serotonergic, noradrenergic, and
histaminergic neurons, cholinergic neurons may continue to fire during REM sleep and have been proposed to play a role
in REM sleep induction. Acetylcholine is also found within interneurons of several brain regions, including the striatum.
The modulation of striatal cholinergic transmission has been implicated in the antiparkinsonian actions of anticholinergic
agents. Within the periphery, acetylcholine is a prominent neurotransmitter, located in motoneurons innervating skeletal
muscle, preganglionic autonomic neurons, and postganglionic parasympathetic neurons. Peripheral acetylcholine mediates
the characteristic postsynaptic effects of the parasympathetic system, including bradycardia and reduced blood pressure,
and enhanced digestive function.
MONOAMINE SYNTHESIS, STORAGE, AND DEGRADATION
In addition to neuroanatomic similarities, monoamines are also synthesized, stored, and
degraded in similar ways (Fig. 1.4-6). Monoamines are synthesized within neurons from
common amino acid precursors (Fig. 1.4-6, step 1) and taken up into synaptic vesicles
by way of a vesicular monoamine transporter (Fig. 1.4-6, step 2). On stimulation,
vesicles within nerve terminals fuse with the presynaptic terminal and release the
neurotransmitter into the synaptic cleft (Fig. 1.4-6, step 3). Once released, the
monoamines interact with postsynaptic receptors to alter the function of postsynaptic
cells (Fig. 1.4-6, step 4), and they may also act on presynaptic autoreceptors on the
nerve terminal to suppress further release (Fig. 1.4-6, step 5). In addition, released
monoamines may be taken back up from the synaptic cleft into the nerve terminal by
plasma membrane transporter proteins (Fig. 1.4-6, step 6), a process known as
reuptake. Reuptake plays an important role in limiting the total magnitude and
temporal duration of monoamine signaling. Once monoamines are taken up, they may
be subject to enzymatic degradation (Fig. 1.4-6, step 7), or they may be protected from
degradation by uptake into vesicles. The processing of acetylcholine differs from this
scheme and is described later in this section.
FIGURE 1.4-6
Schematic diagram of a monoaminergic synapse. Steps involved in synaptic
transmission are described in the text. MAO, monoamine oxidase. (From Sadock BJ,
Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed.
Philadelphia: Lippincott Williams & Wilkins; 2009:68.)
SEROTONIN
The CNS contains less than 2 percent of the serotonin in the body; peripheral serotonin
is located in platelets, mast cells, and enterochromaffin cells. More than 80 percent of
all the serotonin in the body is found in the gastrointestinal system, where it modulates
motility and digestive functions. Platelet serotonin promotes aggregation and clotting
through a most unusual mechanism: The covalent linkage of serotonin molecules to
small GTP-binding proteins, which can then activate these proteins, is a process termed
“serotonylation.” Peripheral serotonin cannot cross the blood–brain barrier, so serotonin
is synthesized within the brain as well. Serotonin is synthesized from the amino acid
tryptophan, which is derived from the diet. The rate-limiting step in serotonin synthesis
is the hydroxylation of tryptophan by the enzyme tryptophan hydroxylase to form 5hydroxytryptophan (4-HT) (Fig. 1.4-7). Two isoforms of tryptophan hydroxylase exist—
one isoform is found mainly in the periphery, whereas the second isoform is restricted to
the CNS.
FIGURE 1.4-7
Synthesis and catabolism of serotonin. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan &
Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams
& Wilkins; 2009:68.)
Under normal circumstances, tryptophan concentration is rate limiting in serotonin synthesis. Therefore, much attention
has focused on the factors that determine tryptophan availability. Unlike serotonin, tryptophan is taken up into the brain
by way of a saturable active carrier mechanism. Because tryptophan competes with other large neutral amino acids for
transport, brain uptake of this amino acid is determined both by the amount of circulating tryptophan and by the ratio of
tryptophan to other large neutral amino acids. This ratio may be elevated by carbohydrate intake, which induces insulin
release and the uptake of many large neutral amino acids into peripheral tissues. Conversely, high-protein foods tend to be
relatively low in tryptophan, thus lowering this ratio. Moreover, the administration of specialized low tryptophan diets
produces significant declines in brain serotonin levels. After tryptophan hydroxylation, 5-hydroxytryptophan is rapidly
decarboxylated by aromatic amino acid decarboxylase (an enzyme also involved in dopamine synthesis) to form serotonin.
The first step in the degradation of serotonin is mediated by monoamine oxidase type A (MAOA), which oxidizes the
amino group to form an aldehyde. MAOA is located in mitochondrial membranes and is nonspecific in its substrate
specificity; in addition to serotonin, it oxidizes norepinephrine. The elevation of serotonin levels by MAO inhibitors
(MAOIs) is believed to underlie the antidepressant efficacy of these drugs. After oxidation by MAOA, the resulting
aldehyde is further oxidized to 5-hydroxyindoleacetic acid (5-HIAA). Levels of 5-HIAA are often measured as a correlate of
serotonergic system activity, although the relationship of these levels to serotonergic neuronal activity remains unclear.
Catecholamines
The catecholamines are synthesized from the amino acid tyrosine, which is taken up into
the brain via an active transport mechanism (Fig. 1.4-8). Within catecholaminergic
neurons, tyrosine hydroxylase catalyzes the addition of a hydroxyl group to the meta
position of tyrosine, yielding L-dopa. This rate-limiting step in catecholamine synthesis
is subject to inhibition by high levels of catecholamines (end-product inhibition).
Because tyrosine hydroxylase is normally saturated with substrate, manipulation of
tyrosine levels does not readily affect the rate of catecholamine synthesis. Once formed,
L-dopa is rapidly converted to dopamine by dopa decarboxylase, which is located in the
cytoplasm. It is now recognized that this enzyme acts not only on L-dopa but also on all
naturally occurring aromatic L-amino acids, including tryptophan, and thus it is more
properly termed aromatic amino acid decarboxylase. In noradrenergic and adrenergic
neurons, dopamine is actively transported into storage vesicles, where it is oxidized by
dopamine β-hydroxylase to form norepinephrine. In adrenergic neurons and the adrenal
medulla, norepinephrine is converted to epinephrine by phenylethanolamine Nmethyltransferase (PNMT), which is located within the cytoplasmic compartment.
FIGURE 1.4-8
Synthesis of catecholamines. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s
Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams &
Wilkins; 2009:69.)
Two enzymes that play major roles in the degradation of catecholamines are monoamine oxidase and catechol-Omethyltransferase (COMT). MAO is located on the outer membrane of mitochondria, including those within the terminals
of adrenergic fibers, and oxidatively deaminates catecholamines to their corresponding aldehydes. Two MAO isozymes
with differing substrate specificities have been identified: MAOA, which preferentially deaminates serotonin and
norepinephrine, and MAO type B (MAOB), which deaminates dopamine, histamine, and a broad spectrum of
phenylethylamines. Neurons contain both MAO isoforms. The blockade of monoamine catabolism by MAO inhibitors
produces elevations in brain monoamine levels. MAO is also found in peripheral tissues such as the gastrointestinal tract
and liver, where it prevents the accumulation of toxic amines. For example, peripheral MAO degrades dietary tyramine, an
amine that can displace norepinephrine from sympathetic postganglionic nerve endings, producing hypertension if
tyramine is present in sufficient quantities. Thus patients treated with MAO inhibitors are cautioned to avoid pickled and
fermented foods that typically have high levels of tyramine. Catechol-O-methyltransferase (COMT) is located in the
cytoplasm and is widely distributed throughout the brain and peripheral tissues, although little to none is found in
adrenergic neurons. It has a wide substrate specificity, catalyzing the transfer of methyl groups from S-adenosyl
methionine to the m-hydroxyl group of most catechol compounds. The catecholamine metabolites produced by these and
other enzymes are frequently measured as indicators of the activity of catecholaminergic systems. In humans, the
predominant metabolites of dopamine and norepinephrine are homovanillic acid (HVA) and 3-methoxy-4hydroxyphenylglycol (MHPG), respectively.
Histamine
As is the case for serotonin, the brain contains only a small portion of the histamine
found in the body. Histamine is distributed throughout most tissues of the body,
predominantly in mast cells. Because it does not readily cross the blood–brain barrier, it
is believed that histamine is synthesized within the brain. In the brain, histamine is
formed by the decarboxylation of the amino acid histidine by a specific L-histidine
decarboxylase. This enzyme is not normally saturated with substrate, so synthesis is
sensitive to histidine levels. This is consistent with the observation that the peripheral
administration of histidine elevates brain histamine levels. Histamine is metabolized in
the brain by histamine N-methyltransferase, producing methylhistamine. In turn,
methylhistamine undergoes oxidative deamination by MAOB.
Acetylcholine
Acetylcholine is synthesized by the transfer of an acetyl group from acetyl coenzyme A
(ACoA) to choline in a reaction mediated by the enzyme choline acetyltransferase
(ChAT). The majority of choline within the brain is transported from the blood rather
than being synthesized de novo. Choline is taken up into cholinergic neurons by a highaffinity active transport mechanism, and this uptake is the rate-limiting step in
acetylcholine synthesis. The rate of choline transport is regulated such that increased
cholinergic neural activity is associated with enhanced choline uptake. After synthesis,
acetylcholine is stored in synaptic vesicles through the action of a vesicular
acetylcholine transporter. After vesicular release, acetylcholine is rapidly broken down
by hydrolysis by acetylcholinesterase, located in the synaptic cleft. Much of the choline
produced by this hydrolysis is then taken back into the presynaptic terminal via the
choline transporter. Of note, although acetylcholinesterase is localized primarily to
cholinergic neurons and synapses, a second class of cholinesterase termed
butyrylcholinesterase is found primarily in the liver and plasma as well as in glia. In the
treatment of Alzheimer’s disease, strategies aimed at enhancing cholinergic function,
primarily through the use of cholinesterase inhibitors to prevent normal degradation of
acetylcholine, have shown moderate efficacy in ameliorating cognitive dysfunction as
well as behavioral disturbances. Cholinesterase inhibitors are also used in the treatment
of myasthenia gravis, a disease characterized by weakness due to blockade of
neuromuscular transmission by autoantibodies to acetylcholine receptors.
Transporters
A great deal of progress has been made in the molecular characterization of the
monoamine plasma membrane transporter proteins. These membrane proteins mediate
the reuptake of synaptically released monoamines into the presynaptic terminal. This
process also involves cotransport of Na+ and Cl− ions and is driven by the ion
concentration gradient generated by the plasma membrane Na+/K+ ATPase.
Monoamine reuptake is an important mechanism for limiting the extent and duration of
activation of monoaminergic receptors. Reuptake is also a primary mechanism for
replenishing terminal monoamine neurotransmitter stores. Moreover, transporters serve
as molecular targets for a number of antidepressant drugs, psychostimulants, and
monoaminergic neurotoxins. Whereas transporter molecules for serotonin (SERT),
dopamine (DAT), and norepinephrine (NET) have been well characterized, transporters
selective for histamine and epinephrine have not been demonstrated.
Among drugs of abuse, cocaine binds with high affinity to all three known monoamine transporters, although the
stimulant properties of the drug have been attributed primarily to its blockade of DAT. This view has been recently
supported by the absence of cocaine-induced locomotor stimulation in a strain of mutant mice engineered to lack this
molecule. In fact, psychostimulants produce a paradoxical locomotor suppression in these animals that has been attributed
to their blockade of the serotonin transporter. The rewarding properties of cocaine have also been attributed primarily to
dopamine transporter inhibition, although other targets mediate these effects as well, since cocaine still has rewarding
effects in mice lacking the dopamine transporter. It appears that serotonergic as well as dopaminergic mechanisms may be
involved. Transporters may also provide routes that allow neurotoxins to enter and damage monoaminergic neurons;
examples include the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and the
serotonergic neurotoxin MDMA.
Vesicular Monoamine Transporter
In addition to the reuptake of monoamines into the presynaptic nerve terminal, a
second transport process serves to concentrate and store monoamines within synaptic
vesicles. The transport and storage of monoamines in vesicles may serve several
purposes: (1) to enable the regulated release of transmitter under appropriate
physiological stimulation, (2) to protect monoamines from degradation by MAO, and
(3) to protect neurons from the toxic effects of free radicals produced by the oxidation of
cytoplasmic monoamines. In contrast with the plasma membrane transporters, a single
type of vesicular monoamine transporter is believed to mediate the uptake of
monoamines into synaptic vesicles within the brain. Consistent with this, blockade of
this vesicular monoamine transporter by the antihypertensive drug reserpine (Serpasil)
has been found to deplete brain levels of serotonin, norepinephrine, and dopamine and
to increase the risk of suicide and affective dysfunction.
RECEPTORS
Ultimately, the effects of monoamines on CNS function and behavior depend on their
interactions with receptor molecules. The binding of monoamines to these plasma
membrane proteins initiates a series of intracellular events that modulate neuronal
excitability. Unlike the transporters, multiple receptor subtypes exist for each
monoamine neurotransmitter (Table 1.4-1).
Table 1.4-1
Monoamine Receptors: Overview
Serotonin Receptors
The 5-hydroxytryptophan type 1 (5-HT1) receptors comprise the largest serotonin
receptor subfamily, with human subtypes designated 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E,
and 5-HT1F. All five 5-HT1 receptor subtypes display intronless gene structures, high
affinities for serotonin, and adenylate cyclase inhibition. The most intensively studied of
these has been the 5-HT1A receptor. This subtype is found on postsynaptic membranes of
forebrain neurons, primarily in the hippocampus, cortex, and septum and on
serotonergic neurons, where it functions as an inhibitory somatodendritic autoreceptor.
There is significant interest in the 5-HT1A receptor as a modulator of both anxiety and
depression. The downregulation of 5-HT1A autoreceptors by the chronic administration
of serotonin reuptake inhibitors has been implicated in their antidepressant effects, and
SSRIs may produce some behavioral effects via increases in hippocampal neurogenesis
mediated by postsynaptic 5-HT1A receptor activation. In addition, partial 5-HT1A
receptor agonists such as buspirone (BuSpar) display both anxiolytic and antidepressant
properties.
Much recent attention has focused on the contributions of 5-HT2A/C receptors to the
actions of atypical antipsychotic drugs such as clozapine (Clozaril), risperidone
(Risperdal), and olanzapine (Zyprexa). Analysis of the receptor-binding properties of
these drugs has led to the hypothesis that 5-HT2A receptor blockade correlates with the
therapeutic effectiveness of atypical antipsychotics. Of interest, the 5-HT2A receptor has
also been implicated in the cognitive process of working memory, a function believed to
be impaired in schizophrenia.
The 5-HT2C receptor is expressed at high levels in many CNS regions, including the
hippocampal formation, prefrontal cortex, amygdala, striatum, hypothalamus, and
choroid plexus. Stimulation of 5-HT2C receptors has been proposed to produce
anxiogenic effects as well as anorectic effects, which may result from interactions with
the hypothalamic melanocortin and leptin pathways. 5-HT2C receptors may also play a
role in the weight gain and development of type 2 diabetes mellitus associated with
atypical antipsychotic treatment. Indeed, a line of mice lacking this receptor subtype
exhibits an obesity syndrome associated with overeating and enhanced seizure
susceptibility, suggesting that this receptor regulates neuronal network excitability. A
variety of antidepressant and antipsychotic drugs antagonize 5-HT2C receptors with high
affinity. Conversely, hallucinogens such as lysergic acid diethylamide (LSD) display
agonist activity at 5-HT2 (and other) serotonin receptor subtypes. 5-HT2C receptor
transcripts also undergo RNA editing, producing isoforms of the receptor with
significantly altered basal versus serotonin-induced activity. Alterations in 5-HT2C
receptor messenger ribonucleic acid (mRNA) editing have been found in the brains of
suicide victims with a history of major depression, and SSRIs have been shown to alter
these editing patterns.
Dopamine Receptors
In 1979, it was clearly recognized that the actions of dopamine are mediated by more
than one receptor subtype. Two dopamine receptors, termed D1 and D2, were
distinguished on the basis of differential binding affinities of a series of agonists and
antagonists, distinct effector mechanisms, and distinct distribution patterns within the
CNS. It was subsequently found that the therapeutic efficacy of antipsychotic drugs
correlated strongly with their affinities for the D2 receptor, implicating this subtype as
an important site of antipsychotic drug action. Recent molecular cloning studies have
identified three additional dopamine receptor genes encoding the D3, D4, and D5
dopamine receptors. On the basis of their structure, pharmacology, and primary effector
mechanisms, the D3 and D4 receptors are considered to be “D2-like,” and the D5 receptor
“D1-like.” The functional roles of the recently discovered subtypes remain to be
definitively elucidated.
The D1 receptor was initially distinguished from the D2 subtype by its high affinity for
the antagonist SCH 23390 and relatively low affinity for butyrophenones such as
haloperidol (Haldol). Whereas D1 receptor activation stimulates cyclic adenosine
monophosphate (cAMP) formation, D2 receptor stimulation produces the opposite effect.
Adrenergic Receptors
As for the α1 receptors, the functions of α2 receptor subtypes (designated α2A, α2B, and
α2C) have been difficult to determine due to a lack of selective agonists and antagonists;
α2 receptors display both presynaptic autoreceptor and postsynaptic actions, and all
appear to inhibit cAMP formation and to activate potassium channels with resultant
membrane hyperpolarization. These receptors regulate neurotransmitter release from
peripheral sympathetic nerve endings. Within the brain the stimulation of α2
autoreceptors (likely the α2A subtype) inhibits firing of the noradrenergic neurons of the
LC, which have been implicated in arousal states. This mechanism has been proposed to
underlie the sedative effects of the α2 receptor agonist clonidine (Catapres). In addition,
the stimulation of brainstem α2 receptors has been proposed to reduce sympathetic and
to augment parasympathetic nervous system activity. This action may relate to the
utility of clonidine in lowering blood pressure and in suppressing the sympathetic
hyperactivity associated with opiate withdrawal. Activation of α2 receptors inhibits the
activity of serotonin neurons of the dorsal raphe nucleus, whereas activation of local α1
receptors stimulates the activity of these neurons, and this is thought to be a major
activating input to the serotonergic system.
Histamine Receptors
Histaminergic systems have been proposed to modulate arousal, wakefulness, feeding
behavior, and neuroendocrine responsiveness. Four histaminergic receptor subtypes
have been identified and termed H1, H2, H3, and H4. The H4 receptor was identified
recently and is detected predominantly in the periphery, in regions such as the spleen,
bone marrow, and leukocytes. The other three histamine receptors have prominent
expression in the CNS. H1 receptors are expressed throughout the body, particularly in
smooth muscle of the gastrointestinal tract and bronchial walls as well as on vascular
endothelial cells. H1 receptors are widely distributed within the CNS, with particularly
high levels in the thalamus, cortex, and cerebellum. H1 receptor activation is associated
with Gq activation and stimulation of phosphoinositide turnover and tends to increase
excitatory neuronal responses. These receptors are the targets of classical
antihistaminergic agents used in the treatment of allergic rhinitis and conjunctivitis. The
well-known sedative effects of these compounds have been attributed to their actions in
the CNS and have implicated histamine in the regulation of arousal and the sleep–wake
cycle. Accordingly, a line of mutant mice lacking histamine displays deficits in waking
and attention. In addition, the sedation and weight gain produced by a number of
antipsychotic and antidepressant drugs have been attributed to H1 receptor antagonism.
Conversely, H1 receptor agonists stimulate arousal and suppress food intake in animal
models.
Cholinergic Receptors
M1 receptors are the most abundantly expressed muscarinic receptors in the forebrain,
including the cortex, hippocampus, and striatum. Pharmacological evidence has
suggested their involvement in memory and synaptic plasticity, and recent evaluation of
mice lacking the M1 receptor gene revealed deficits in memory tasks believed to require
interactions between the cortex and the hippocampus.
Nicotinic receptors have been implicated in cognitive function, especially working
memory, attention, and processing speed. Cortical and hippocampal nicotinic
acetylcholine receptors appear to be significantly decreased in Alzheimer’s disease, and
nicotine
administration
improves
attention
deficits
in
some
patients.
The
acetylcholinesterase inhibitor galantamine used in the treatment of Alzheimer’s disease
also acts to positively modulate nicotinic receptor function. The α7 nicotinic
acetylcholine receptor subtype has been implicated as one of many possible
susceptibility genes for schizophrenia, with lower levels of this receptor being associated
with impaired sensory gating. Some rare forms of the familial epilepsy syndrome
autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) are associated with
mutations in the α4 or β2 subunits of the nicotinic acetylcholine receptor. Finally, the
reinforcing properties of tobacco use are proposed to involve the stimulation of nicotinic
acetylcholine receptors located in mesolimbic dopaminergic reward pathways.
AMINO ACID NEUROTRANSMITTERS
For more than 50 years, biogenic amines have dominated thinking about the role of
neurotransmitters in the pathophysiology of psychiatric disorders. However, over the
last decade, evidence has accumulated from postmortem, brain imaging, and genetic
studies that the amino acid neurotransmitters, in particular glutamic acid and γaminobutyric acid (GABA), play an important, if not central, role in the
pathophysiology of a broad range of psychiatric disorders including schizophrenia,
bipolar disorder, major depression, Alzheimer’s disease, and anxiety disorders.
Glutamic Acid
Glutamate mediates fast excitatory neurotransmission in the brain and is the transmitter
for approximately 80 percent of brain synapses, particularly those associated with
dendritic spines. The repolarization of neuronal membranes that have been depolarized
by glutamatergic neurotransmission may account for as much as 80 percent of the
energy expenditure in the brain. The concentration of glutamate in brain is 10 mM, the
highest of all amino acids, of which approximately 20 percent represents the
neurotransmitter pool of glutamate.
The postsynaptic effects of glutamate are mediated by two families of receptors. The
first are the glutamate-gated cation channels that are responsible for fast
neurotransmission. The second type of glutamate receptor are the metabotropic
glutamate receptors (mGluR), which are G-protein-coupled receptors like α-adrenergic
receptors and dopamine receptors. The mGluRs primarily modulate glutamatergic
neurotransmission.
Major Glutamatergic Pathways in the Brain.
All primary sensory afferent systems appear to use glutamate as their neurotransmitter including retinal ganglion cells,
cochlear cells, trigeminal nerve, and spinal afferents. The thalamocortical projections that distribute afferent information
broadly to the cortex are glutamatergic. The pyramidal neurons of the corticolimbic regions, the major source of intrinsic,
associational, and efferent excitatory projections from the cortex, are glutamatergic. A temporal lobe circuit that figures
importantly in the development of new memories is a series of four glutamatergic synapses: The perforant path innervates
the hippocampal granule cells that innervate CA3 pyramidal cells that innervate CA1 pyramidal cells. The climbing fibers
innervating the cerebellar cortex are glutamatergic as well as the corticospinal tracks.
Ionotropic Glutamate Receptors.
Three families of ionotropic glutamate receptors have been identified on the basis of selective activation by
conformationally restricted or synthetic analogs of glutamate. These include α-amino-3-hydroxy-5-methyl-4-isoxazole
propionic acid (AMPA), kainic acid (KA), and N-methyl-D-aspartic acid (NMDA) receptors. Subsequent cloning revealed
16 mammalian genes that encode structurally related proteins, which represent subunits that assemble into functional
receptors. Glutamate-gated ion channel receptors appear to be tetramers, and subunit composition affects both the
pharmacologic and the biophysical features of the receptor.
Metabotropic Glutamate Receptors.
These receptors are so designated because their effects are mediated by G proteins. All mGluRs are activated by
glutamate, although their sensitivities vary remarkably. To date, eight mGluRs have been cloned. These genes encode for
seven membrane-spanning proteins that are members of the superfamily of G-protein-coupled receptors.
The Role of Astrocytes.
Specialized end-feet of the astrocyte surround glutamatergic synapses. The astrocyte expresses the two Na+-dependent
glutamate transporters that play the primary role in removing glutamate from the synapse, thereby terminating its action:
EAAT1 and EAAT2 (excitatory amino acid transporter). The neuronal glutamate transporter, EAAT3, is expressed in upper
motor neurons, whereas EAAT4 is expressed primarily in cerebellar Purkinje cells and EAAT5 in retina. Mice homozygous
for null mutations of either EAAT1 or EAAT2 exhibit elevated extracellular glutamate and excitotoxic neurodegeneration.
Notably, several studies have described the loss of EAAT2 protein and transport activity in the ventral horn in
amyotrophic lateral sclerosis.
The astrocytes express AMPA receptors so that they can monitor synaptic glutamate release. GlyT1, which maintains
subsaturating concentrations of glycine in the synapse, is expressed on the astrocyte plasma membrane. GlyT1 transports
three Na+ out for each molecule of glycine transported into the astrocyte. This stoichiometry results in a robust reversal
of the direction of transport when glutamate released in the synapse activates the AMPA receptors on the astrocyte, thus
depolarizing the astrocyte. Thus glycine release in the synapse by GlyT1 is coordinated with glutamatergic
neurotransmission. Similarly, activation of the astrocyte AMPA receptors causes GRIP to dissociate from the AMPA
receptor and bind to serine racemase, activating it to synthesize D-serine. D-Serine levels are also determined by D-amino
acid oxidase (DAAO) with low D-serine levels in the cerebellum and brainstem where DAAO expression is high, and high
D-serine levels are found in corticolimbic brain regions where DAAO expression is quite low. In contrast, the expression of
GlyT1 is highest in the cerebellum and brainstem. This distribution suggests that D-serine is the primary modulator of the
NMDA receptor in the forebrain, whereas glycine is more prominent in the brainstem and cerebellum.
Plasticity in Glutamatergic Neurotransmission.
The extinction of conditioned fear has been shown to be an active process mediated by the activation of NMDA receptors
in the amygdala. Treatment of rats with NMDA receptor antagonists prevents the extinction of conditioned fear, whereas
treatment with the glycine modulatory site partial agonist D-cycloserine facilitates the extinction of conditioned fear. (DCycloserine is an antibiotic used to treat tuberculosis that has 50 percent of the efficacy of glycine at the NMDA receptor.)
To determine whether the phenomenon generalizes to humans, patients with acrophobia were administered either placebo
or a single dose of D-cycloserine along with cognitive behavioral therapy (CBT). D-Cycloserine plus CBT resulted in a
highly significant reduction in acrophobic symptoms that persisted for at least 3 months as compared to placebo plus CBT.
Other placebo-controlled clinical trials support the notion that D-cycloserine is a robust enhancer of CBT, suggesting that
pharmacologically augmenting neural plasticity may be used to bolster psychological interventions.
Fragile X mental retardation protein (FMRP), which is deficient in individuals with fragile X syndrome, appears to be
synthesized locally within the spine during times of NMDA receptor activation and also plays a role in transporting
specific mRNAs to the spine for translation. Notably, mice in which the FMRP gene has been inactivated through a null
mutation as well as patients with fragile X syndrome have fewer dendritic spines, the preponderance of which have an
immature morphology. Loss of FMRP exaggerates responses of mGluR5, which stimulates dendritic protein synthesis, and
treatment with an mGluR5 antagonist reverses the fragile-X-like phenotype in mice with the FMRP gene inactivated.
Excitotoxicity.
In the early 1970s, it was shown that the systemic administration
of large amounts of monosodium glutamate to immature animals resulted in the
degeneration of neurons in brain regions where the blood–brain barrier was deficient.
Excitotoxicity has also been implicated in the proximate cause of neuronal
degeneration in Alzheimer’s disease. Most evidence points to the toxic consequences of
aggregates of β-amyloid, especially β-amyloid1–42. The β-amyloid fibrils depolarize
neurons, resulting in loss of the Mg2+ block and enhanced NMDA receptor sensitivity to
glutamate. The fibrils also impair glutamate transport into astrocytes, thereby
increasing the extracellular concentration of glutamate. β-Amyloid directly promotes
oxidative stress through inflammation that further contributes to neuronal vulnerability
to glutamate. Thus, several mechanisms contribute to neuronal vulnerability to NMDAreceptor-mediated excitotoxicity in Alzheimer’s disease. Memantine, a recently approved
treatment for mild to moderate Alzheimer’s disease, is a weak noncompetitive inhibitor
of NMDA receptors. It reduces tonic sensitivity of NMDA receptors to excitotoxicity but
does not interfere with “phasic” neurotransmission, thereby attenuating neuronal
degeneration in Alzheimer’s disease.
Inhibitory Amino Acids: GABA
GABA is the major inhibitory neurotransmitter in the brain, where it is broadly
distributed and occurs in millimolar concentrations. In view of its physiological effects
and
distributions,
it
is
not
surprising
that
the
dysfunction
of
GABAergic
neurotransmission has been implicated in a broad range of neuropsychiatric disorders
including anxiety disorders, schizophrenia, alcohol dependence, and seizure disorders.
Chemically, GABA differs from glutamic acid, the major excitatory neurotransmitter,
simply by the removal of a single carboxyl group from the latter.
GABA is synthesized from glutamic acid by glutamic acid decarboxylase (GAD), which
catalyzes the removal of the α-carboxyl group. In the CNS, the expression of GAD
appears to be restricted to GABAergic neurons, although in the periphery it is expressed
in pancreatic islet cells. Two distinct but related genes encode GAD. GAD65 is localized
to nerve terminals, where it is responsible for synthesizing GABA that is concentrated in
the synaptic vesicles. Consistent with its role in fast inhibitory neurotransmission, mice
homozygous for a null mutation of GAD65 have an elevated risk for seizures. GAD67
appears to be the primary source for neuronal GABA because mice homozygous for a
null mutation of GAD67 die at birth, have a cleft pallet, and exhibit major reductions in
brain GABA.
GABA is catabolized by GABA transaminase (GABA-T) to yield succinic semialdehyde.
Transamination generally occurs when the parent compound, α-ketoglutarate, is present
to receive the amino group, thereby regenerating glutamic acid. Succinic semialdehyde
is oxidized by succinic semialdehyde dehydrogenase (SSADH) into succinic acid, which
re-enters the Krebs cycle. GABA-T is a cell surface, membrane-bound enzyme expressed
by neurons and glia, which is oriented toward the extracellular compartment. As would
be anticipated, drugs that inhibit the catabolism of GABA have anticonvulsant
properties. One of the mechanisms of action of valproic acid is the competitive
inhibition of GABA-T. γ-Vinyl-GABA is a suicide substrate inhibitor of GABA-T that is
used as an anticonvulsant in Europe (vigabatrin [Sabril]).
The synaptic action of GABA is also terminated by high-affinity transport back into
the presynaptic terminal as well as into astrocytes. Four genetically distinct GABA high-
affinity transporters have been identified with differing kinetic and pharmacological
characteristics. They all share homology with other neurotransmitter transporters with
the characteristic of 12 membrane-spanning domains. The active transport is driven by
the sodium gradient so that upon depolarization, transportation of GABA out of the
neuron is favored. GABA transported into astrocytes is catabolyzed by GABA-T and
ultimately converted to glutamic acid and then to glutamine, which is transported back
into the presynaptic terminal for GABA synthesis. Tiagabine (Gabitril) is a potent GABA
transport inhibitor that is used to treat epilepsy. Preliminary results suggest that it also
may be effective in panic disorder.
GABAA Receptors.
GABAA receptors are distributed throughout the brain. The
GABAA complex, when activated, mediates an increase in membrane conductance with
an equilibrium potential near the resting membrane potential of –70 mV (Fig. 1.4-9). In
the mature neuron, this typically results with an influx of Cl−, causing membrane
hyperpolarization. Hyperpolarization is inhibitory because it increases the threshold for
generating an action potential. In immature neurons, which have unusually high levels
of intracellular Cl−, activating the GABAA receptor can counterintuitively cause
depolarization. For this reason, anticonvulsants that act by enhancing GABAA receptor
activity may actually exacerbate seizures in the neonatal period.
FIGURE 1.4-9
Schematic representation of the GABAA receptor. The receptor-channel complex is a
heteropentamer. The GABA binding site is at the interface of the α and β subunits. The
benzodiazepine binding site is at the interface between the γ and α subunits. (From
Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry.
9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:81.)
Barbiturates such as phenobarbital and pentobarbital are noted for their sedative and anticonvulsant activities.
Barbiturates allosterically increase the affinities of the binding sites for GABA and benzodiazepines at concentrations that
are pharmacologically relevant. Barbiturates also affect channel dynamics by markedly increasing the long open state and
reducing the short open state, thereby increasing Cl− inhibition. Chemically modified analogs of progesterone and
corticosterone have been shown in behavioral studies to have sedative and anxiolytic effects through their interaction with
the GABAA receptor complex. They share features with barbiturates, although they act at a distinctly different site. Thus,
they allosterically enhance agonist ligand binding to the receptor and increase the duration of chloride channel opening. A
variety of behavioral effects associated with steroid administration or fluctuation of endogenous steroids and sex-specific
effects of GABAergic drugs have been linked to the action of endogenous neurosteroids.
With regard to GABAA receptor antagonists, picrotoxin, like the barbiturates, alters
channel dynamics but in the opposite direction by reducing long open states and
favoring the briefest open state. The proconvulsant pentylenetetrazol also acts by
reducing chloride channel permeability. Penicillin, which at high concentrations is
proconvulsant, binds to the positively charged residues in the channel, thereby occluding
it. As a general class, anesthetics including barbiturates, steroids, and volatile
anesthetics increase chloride conductance, thereby inhibiting neurotransmissions. Amino
acids in the membrane-spanning domain of the GABA receptor subunits confer
sensitivity to anesthetics. The precise mechanism whereby ethanol enhances GABAA
receptor function remains unclear due to inconsistent results, suggesting that subunit
composition may be important. However, recent studies suggest that ethanol increases
the response of the tonic GABA-activated currents, which contain the δ subunit and
exhibit remarkably high affinity to GABA.
Recently, recombinant DNA strategies exploiting site-directed mutagenesis have
permitted the identification of sites on the specific subunits that mediate the
pharmacological action of drugs such as the benzodiazepines. Removal of the binding
ability for benzodiazepines has established that the α1 subunit plays a major role in the
sedative and amnestic effects of benzodiazepines, whereas inactivating the
benzodiazepine site on the α2 subunit eliminates the anxiolytic effect of benzodiazepines
GABAB Receptors.
The GABAB receptors are distinguished pharmacologically from
GABAA receptors by the fact that they are insensitive to the canonical GABAA receptor
antagonist bicuculline and that they are potently activated by baclofen [β-(4chlorophenyl)-γ-aminobutyric acid], which is inactive at GABAA receptors. They are
members of the G-protein-coupled superfamily of receptors but are highly unusual, as
they are made of a dimer of two seven-transmembrane-spanning subunits. GABAB
receptors are widely distributed throughout the nervous system and are localized both
presynaptically and postsynaptically. The postsynaptic GABAB receptors cause a longlasting hyperpolarization by activating potassium channels. Presynaptically, they act as
autoreceptors and heteroreceptors to inhibit neurotransmitter release.
Glycine as a Neurotransmitter.
Glycine is an inhibitory neurotransmitter
primarily in the brainstem and spinal cord, although the expression of glycine receptor
subunits in the thalamus, cortex, and hippocampus suggest a broader role. Glycine is a
nonessential amino acid that is synthesized in the brain from L-serine by serine
hydroxymethyltransferase. Glycine is concentrated within synaptic vesicles by H+-
dependent vesicular inhibitory amino acid transporter (VIAAT or VGAT), which also
transports GABA. Termination of the synaptic action of glycine is through reuptake into
the presynaptic terminal by the glycine transporter II (GlyT2), which is quite distinct
from GlyT1 that is expressed in astrocytes and modulates NMDA receptor function.
The inhibitory effects of glycine are mediated by a ligand-gated chloride channel,
which can also respond to β-alanine, taurine, L-alanine, L-serine, and proline, but not to
GABA. The canonical antagonist for the glycine receptor is the plant alkaloid strychnine.
The receptor was first identified through the specific binding of [3H]strychnine.
[3H]Glycine binds to two sites: One that is displaceable by strychnine and represents the
glycine A receptor and a second that is insensitive to strychnine and is designated the
glycine B receptor, representing the glycine modulatory site on the NMDA receptor.
Neuropsychiatric Implications of Amino Acid Transmitters
Schizophrenia.
Evidence accumulating from postmortem, pharmacological, and
genetic studies is shifting the focus of the pathophysiology of schizophrenia from
dopamine to glutamate and GABA. Indeed, after the use of D2 receptor antagonists as
the sole treatment of schizophrenia for the last 50 years, more than two thirds of the
treated patients remain substantially disabled. Early postmortem studies indicated a
reduction in the activity of GAD in the cortex in patients with schizophrenia as
compared to suitable controls. With the advent of immunocytochemistry and gene
expression techniques, it has been possible to more precisely define the GABAergic
deficit in schizophrenia. It appears that the parvalbumin-positive GABAergic
interneurons in the intermediate layers of the cortex bear the brunt of the pathology,
which includes reduced expression of GAD67, parvalbumin, and the GABA transporter
(GAT). The finding that GABAA receptors are upregulated, as measured by
autoradiography or with antibodies, supports the theory that these changes reflect
hypofunction of the presynaptic GABAergic neurons. These particular GABAergic
interneurons, which include the chandelier cells, play an important role in negative
feedback inhibition to the pyramidal cells in the cortex. Despite this highly reproducible
neuropathology, genes related to GABAergic function have not figured prominently in
genomewide searches, suggesting that GABAergic deficits may be a downstream
consequence of some more proximal genetic defects.
The theory that hypofunction of NMDA receptors is an etiologic factor in schizophrenia initially arose from the
observation that phencyclidine (PCP) and related dissociative anesthetics that block NMDA receptors produce a syndrome
that can be indistinguishable from schizophrenia (Fig. 1.4-10). Dissociative anesthetics are so named because they prevent
the acquisition of new memories while the patient is apparently conscious. In fact under laboratory conditions, low-dose
infusion of ketamine can produce the positive symptoms, negative symptoms, and specific cognitive deficits associated
with schizophrenia in clear consciousness. Subsequent studies indicated that low-dose ketamine can also cause enhanced
amphetamine-induced subcortical dopamine release as is observed in schizophrenia as well as abnormal cortical eventrelated potentials (ERPs) and disruption of prepulse inhibition in experimental animals.
FIGURE 1.4-10
Pathological circuit in schizophrenia. The NMDA receptors on the rapidly firing
parvalbumin (PV) expressing GABAergic interneurons in the intermediate levels of the
cortex are disproportionately sensitive to antagonists or loss of the coagonist, D-serine.
NMDA receptor hypofunction causes reduced expression of PV, GAD67, and the GABA
transporter and upregulation of GABAA receptors on pyramidal neurons. Disinhibition of
the pyramidal neurons causes cognitive dysfunction and negative symptoms and drives
excessive subcortical dopamine release resulting in psychosis. (From Sadock BJ, Sadock
VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009:83.)
A number of putative risk genes for schizophrenia are closely associated with NMDA receptor function. DAAO, which
encodes a protein that activates D-amino acid oxidase, has been repeatedly linked to the risk of schizophrenia. D-Amino
acid oxidase itself has been associated with increased risk. Recently an allelic variant of serine racemase in the promoter
region has also been associated with the risk for schizophrenia. Each of these gene variants could reduce the availability of
D-serine in the cortex, thereby impairing NMDA receptor function. Notably, cerebrospinal fluid (CSF) and blood levels of
D-serine are significantly reduced in patients with schizophrenia. Neuregulin 1 appears to be a convincing risk gene and
interacts directly with NMDA receptors. Dysbindin, another risk gene, is expressed in glutamatergic terminals. mGluR3,
which downregulates glutamate release, has also been associated with schizophrenia.
Recent findings have provided a link between the GABAergic neuropathology and NMDA receptor hypofunction.
Chronic treatment of rats with NMDA receptor antagonists causes a downregulation of GAD67, parvalbumin, and GAT. The
sensitive subpopulation of GABAergic neurons is the rapidly firing interneurons that provide the perisomatic innervation
of the pyramidal cells. Their NMDA receptors appear to be much more sensitive to antagonists than those less active
GABAergic neurons and pyramidal cells. The subtly reduced GABAergic inhibition results in a disinhibition of the
glutamatergic pyramidal output. This degradation of the inhibitory feedback could account for the cognitive deficits and
negative symptoms in schizophrenia, and the disinhibited output also results in elevated subcortical dopamine release and
psychosis. Thus psychosis would be considered a downstream event resulting from a disruption in critical glutamatergic–
GABAergic synaptic function in the cerebral cortex.
Anxiety and Depression.
GABAergic dysfunction has been associated with
anxiety disorders, especially panic disorder, as well as with major depressive disorder.
Clinically, there is considerable comorbidity between anxiety and affective disorders.
Decreased levels of the GABAA receptor modulators, the three α-reduced neuroactive
steroids, have been found both in plasma and in CSF in major depressive disorder.
Effective treatment with selective serotonin reuptake inhibitors (SSRIs) increases the
neurosteroid levels. In contrast, in patients with panic disorder, the plasma neurosteroid
levels were significantly elevated, perhaps as a compensatory mechanism. Magnetic
resonance spectroscopy (MRS) has disclosed significant reductions in GABA levels in the
anterior cingulate and in the basal ganglia of medicated patients with panic disorder.
Positron emission tomography (PET) scanning reveals a highly selective reduction in
benzodiazepine receptor sites bilaterally in the insular cortex in panic disorder. A
genomewide screen has shown significant linkage at 15q in a region containing GABAA
receptor subunit genes and panic disorder. MRS reveals significant reductions in both
GABA and glutamate/glutamine (Glx) in the prefrontal cortex in major depressive
disorder. Postmortem studies indicate upregulation of the GABAA receptor α1 and β3
subunits in the cerebral cortices of depressed patients who committed suicide, consistent
with a reduction in GABAergic neurotransmission. The reduced levels of GABA in the
occipital cortex in episodes of major depressive disorder normalized with effective
treatment with SSRI or with electroconvulsive therapy.
Glutamatergic dysfunction has also been implicated in depression. NMDA receptor
antagonists have antidepressant effects in several animal models of depression
including forced swim, tail suspension, and learned helplessness. A single injection of
ketamine provides protection from the induction of behavioral despair in rats for up to
10 days. Chronic treatment with antidepressants alters the expression of NMDA receptor
subunits and decreases glycine receptor B binding. Two placebo-controlled clinical trials
have shown that a single dose of ketamine can produce a rapid, substantial, and
persistent reduction in symptoms in patients with major depressive disorder.
Alcoholism.
Ethanol at concentrations associated with intoxication has a dual
action of enhancing GABAergic receptor function and attenuating NMDA receptor
function. The GABA receptor effects may be associated with the anxiolytic effects of
ethanol. Persistent abuse and dependency on ethanol result in a downregulation of
GABAA receptors and an upregulation of NMDA receptors such that acute
discontinuation of ethanol results in a hyperexcitable state characterized by delirium
tremens. Furthermore, supersensitive NMDA receptors in the context of thiamine
deficiency may contribute to the excitotoxic neuron degeneration of Wernicke–
Korsakoff’s syndrome.
Acamprosate is a derivative of homotaurine that was developed as an agent to reduce
alcohol consumption, craving, and relapse in alcoholic patients, for which it exhibits
moderate efficacy in clinical trials. Because of taurine’s resemblance to GABA, it was
thought that acamprosate acted via GABAA receptors, but electrophysiological studies
found little evidence to support this hypothesis. Subsequent studies demonstrated that it
inhibited NMDA receptor responses in cortical slices and recombinant NMDA receptors.
The precise mechanism whereby acamprosate alters NMDA receptor function, however,
remains unclear.
Fetal alcohol syndrome is the most common preventable cause of mental retardation.
Convincing evidence has been developed that the microencephaly associated with fetal
alcohol exposure results from inhibition of NMDA receptor function, resulting in
widespread neuronal apoptosis in the immature cortex. NMDA receptor activation is
essential for immature neuronal survival and differentiation.
NEUROPEPTIDES
Neuropeptides represent the most diverse class of signaling molecules in the CNS.
Initially discovered for their role in the hypothalamic regulation of pituitary hormone
secretion, the complex role of peptides in brain function has emerged over the last 30
years. Many neuropeptides and their receptors are widely distributed within the CNS
where they have an extraordinary array of direct or neuromodulatory effects, ranging
from modulating neurotransmitter release and neuronal firing patterns to the regulation
of emotionality and complex behaviors. More than 100 unique biologically active
neuropeptides have been identified in the brain, a subset of which is presented in Table
1.4-2. Adding to the complexity of neuropeptide systems in the CNS, the actions of many
peptides are mediated via multiple receptor subtypes localized in different brain
regions. In fact, the discovery of new peptides and receptor subtypes has outpaced our
understanding of the roles of these peptides in normal or aberrant CNS function.
Pharmacological, molecular, and genetic approaches are now leading the way in our
understanding of the contribution of neuropeptide systems in psychiatric disorders.
Table 1.4-2
Selected Neuropeptide Transmitters
Neuropeptides have been implicated in the regulation of a variety of behavioral and
physiological processes, including thermoregulation, food and water consumption, sex,
sleep, locomotion, learning and memory, responses to stress and pain, emotion, and
social
cognition.
Involvement
in
such
behavioral
processes
suggests
that
neuropeptidergic systems may contribute to the symptoms and behaviors exhibited in
major psychiatric illnesses such as psychoses, mood disorders, dementias, and autism
spectrum disorders.
Investigating Neuropeptide Function
The roles of neuropeptides in CNS function and behavior have been examined using a
multitude of experimental techniques. The levels of analysis include the following:
Molecular structure and biosynthesis of the peptide and its receptor(s), the
neuroanatomical localization of the peptide and its receptor(s), the regulation of the
expression and release of the peptide, and the behavioral effects of the peptide. Most
information on neuropeptide biology is derived from laboratory animal studies;
however, there is a growing database on the localization, activity, and potential
psychiatric relevance of several neuropeptide systems in humans.
Most neuropeptide structures have been identified based on the chemical analysis of
purified biologically active peptides, leading ultimately to the cloning and
characterization of the genes encoding them. Characterization of the gene structure of
peptides and their receptors has provided insight into the molecular regulation of these
systems, and their chromosomal localization is useful in genetic studies examining the
potential roles of these genes in psychiatric disorders. Structural characterization
permits the production of immunological and molecular probes that are useful in
determining
peptide
distribution
and
regulation
in
the
brain.
Quantitative
radioimmunoassays on microdissected brain regions or immunocytochemistry on brain
sections are typically used to localize the distribution of peptide within the brain. Both
techniques use specific antibodies generated against the neuropeptide to detect the
presence of the peptide. Immunocytochemistry allows researchers to visualize the
precise cellular localization of peptide-synthesizing cells as well as their projections
throughout the brain, although the technique is generally not quantitative. With
molecular probes homologous to the mRNA encoding the peptides or receptor, in situ
hybridization can be used to localize and quantify gene expression in brain sections.
This is a powerful technique for examining the molecular regulation of neuropeptide
synthesis with precise neuroanatomical resolution, which is impossible for other classes
of nonpeptide neurotransmitters that are not derived directly from the translation of
mRNAs, such as dopamine, serotonin, and norepinephrine.
Generally, the behavioral effects of neuropeptides are initially investigated by infusions of the peptide directly into the
brain. Unlike many nonpeptide neurotransmitters, most neuropeptides do not penetrate the blood–brain barrier in
amounts sufficient enough to produce CNS effects. Furthermore, serum and tissue enzymes tend to degrade the peptides
before they reach their target sites. The degradation is usually the result of the cleavage of specific amino acid sequences
targeted by a specific peptidase designed for that purpose. Thus, intracerebroventricular (ICV) or site-specific infusions of
peptide in animal models are generally required to probe for behavioral effects of peptides. However, there are some
examples of delivery of neuropeptides via intranasal infusions in human subjects, which in some cases has been shown to
permit access of the peptide to the brain.
One of the greatest impediments for exploring the roles and potential therapeutic values of neuropeptides is the
inability of the peptides or their agonists/antagonists to penetrate the blood–brain barrier. Thus the behavioral effects of
most peptides in humans are largely uninvestigated, with the exception of a few studies utilizing intranasal delivery.
However, in some instances small-molecule, nonpeptide agonists/antagonists have been developed that can be administered
peripherally and permeate the blood–brain barrier in sufficient quantities to affect receptor activation.
The use of pretreatment and posttreatment CSF samples or of samples obtained
during the active disease state versus when the patient is in remission addresses some of
the serious limitations in study design. For such progressive diseases as schizophrenia or
Alzheimer’s disease, serial CSF samples may be a valuable indicator of disease
progression or response to treatment. Even with these constraints, significant progress
has been made in describing the effects of various psychiatric disease states on
neuropeptide systems in the CNS.
Biosynthesis
Unlike other neurotransmitters, the biosynthesis of a neuropeptide involves the
transcription of an mRNA from a specific gene, translation of a polypeptide
preprohormone encoded by that mRNA, and then posttranslational processing involving
proteolytic cleavage of the preprohormone to yield the active neuropeptide. Over the
last 25 years the gene structures and biosynthetic pathways of many neuropeptides have
been elucidated. The gene structure of selected neuropeptides is illustrated in Figure 1.411. Neuropeptide genes are generally composed of multiple exons that encode a protein
preprohormone. The N-terminus of the preprohormone contains a signal peptide
sequence, which guides the growing polypeptide to the rough endoplasmic reticulum
(RER) membrane. The single preprohormone molecule often contains the sequences of
multiple peptides that are subsequently separated by proteolytic cleavage by specific
enzymes. For example, translation of the gene encoding NT yields a preprohormone,
which upon enzymatic cleavage produces both NT and neuromedin N.
FIGURE 1.4-11
Schematics illustrating the gene structure, preprohormone messenger RNA (mRNA), and
processed neuropeptides of thyrotropin-releasing hormone (TRH), corticotropinreleasing factor (CRF), oxytocin (OT), arginine vasopressin (AVP), and neurotensin
(NT). Boxed regions indicate the locations of the exons in the respective genes. Shaded or
hatched regions indicate coding regions. Each preprohormone begins with a signal
peptide (SP) sequence. Black boxes indicate the locations of the sequences encoding the
neuropeptide. (From Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive
Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:87.)
Distribution and Regulation
Although many neuropeptides were originally isolated from pituitary and peripheral
tissues, the majority of neuropeptides were subsequently found to be widely distributed
throughout the brain. Those peptides involved in regulating pituitary secretion are
concentrated in the hypothalamus. Hypothalamic releasing and inhibiting factors are
produced in neurosecretory neurons adjacent to the third ventricle that send projections
to the median eminence where they contact and release peptide into the
hypothalamohypophysial portal circulatory system. Peptides produced in these neurons
are often subject to regulation by the peripheral hormones that they regulate. For
example, thyrotropin-releasing hormone (TRH) regulates the secretion of thyroid
hormones, and thyroid hormones negatively feedback on TRH gene expression.
However, neuropeptide-expressing neurons and their projections are found in many
other brain regions, including limbic structures, midbrain, hindbrain, and spinal cord.
Neuropeptide Signaling
Neuropeptides may act as neurotransmitters, neuromodulators, or neurohormones.
Neurotransmitters are typically released from axonal terminals into a synapse where
they
change
the
postsynaptic
membrane
potential,
either
depolarizing
or
hyperpolarizing the cell. For classical neurotransmitters, this often involves direct
modulation of voltage-gated ion channels. In contrast, neuromodulators and
neurohormones do not directly affect the firing of the target cell itself but may alter the
response of the cell to other neurotransmitters through the modulation of second
messenger pathways. Neuropeptide release is not restricted to synapses or axon
terminals but may occur throughout the axon or even from dendrites.
The cellular signaling of neuropeptides is mediated by specific neuropeptide
receptors. Thus understanding neuropeptide receptor function is essential for
understanding neuropeptide biology. Neuropeptide receptors have undergone the same
process of discovery and characterization that receptors for other neurotransmitters
have
enjoyed.
Most
neuropeptide
receptors
are
G-protein-coupled,
seventransmembrane domain receptors belonging to the same family of proteins as the
monoamine receptors.
Molecular technology has made it possible to clone and characterize neuropeptide receptor genes and complementary
DNAs (cDNAs). This is most often accomplished in one of three ways. First, the neuropeptide receptor protein is
biochemically purified and partially sequenced, which allows the development of oligonucleotide probes that can be used
to isolate the cDNA encoding the protein from a cDNA library. A second approach involves producing expression libraries
in which cells containing the receptor cDNA can be isolated based on their ability to bind to a radiolabeled peptide ligand.
Finally, many neuropeptide receptors are now isolated based on their sequence homology with other known peptide
receptors. Once the cDNA of the receptor has been isolated, it can be used to produce purified receptor protein for
structural and functional studies. By mutation of specific amino acids in the receptor structure and determination of
relative binding affinities of peptides with various amino acid substitutions, it is possible to elucidate the nature of the
ligand–receptor interaction. This information facilitates the development of drugs that specifically modulate receptor
function, including nonpeptide drugs, leading to the ability to manipulate peptide systems in ways that are currently
enjoyed by the more classic neurotransmitters. The availability of cDNAs encoding the receptor also permits the
neuroanatomical mapping of the receptor-producing cells in the brain, which is critical for understanding the neural
circuits modulated by the peptide. Finally, with the cloned receptor in hand, it is possible to use transgenic techniques,
such as targeted gene overexpression or gene knockouts, to further elucidate the functions of these receptors. siRNA
techniques now allow the targeted synthesis disruption of specific receptor populations, allowing researchers to examine
the roles of these receptor populations on physiology and behavior.
The following three factors determine the biological roles of a neuropeptide hormone:
(1) the temporal–anatomical release of the peptide, (2) functional coupling of the
neuropeptide receptor to intracellular signaling pathways, and (3) the cell type and
circuits in which the receptor is expressed. Genetic studies have demonstrated that
regulatory sequences flanking the receptor coding region determine the expression
pattern of the receptor and thus the physiological and behavioral response to the
neuropeptide.
Peptidases
Unlike monoamine neurotransmitters, peptides are not actively taken up by presynaptic
nerve terminals. Rather, released peptides are degraded into smaller fragments, and
eventually into single amino acids by specific enzymes termed peptidases. The enzymes
may be found bound to presynaptic or postsynaptic neural membranes or in solution in
the cytoplasm and extracellular fluid, and they are distributed widely in peripheral
organs and serum as well as in the CNS. As a result, neuropeptides generally have halflives on the order of minutes once released.
Specific Neuropeptides as Prototypes of Neuropeptide Biology
Thyrotropin-Releasing Hormone.
In 1969, TRH, a
pyroglutamylhistidylprolinamide tripeptide (Table 1.4-3), became the first of the
hypothalamic releasing hormones to be isolated and characterized. The discovery of the
structure of this hormone led to the conclusive demonstration that peptide hormones
secreted from the hypothalamus regulate the secretion of hormones from the anterior
pituitary. The gene for TRH in humans resides on chromosome 3q13.3-q21. In the rat it
consists of three exons (coding regions) separated by two introns (noncoding sequences)
(see Fig. 1.4-11). The first exon contains the 5′ untranslated region of the mRNA
encoding the TRH preprohormone, the second exon contains the signal peptide (SP)
sequence and much of the remaining N-terminal end of the precursor peptide, and the
third contains the remainder of the sequence, including five copies of the TRH precursor
sequence, the C-terminal region, and the 3′ untranslated region. The 5′ flanking of the
gene, or promoter, contains sequences homologous to the glucocorticoid receptor and
the thyroid hormone receptor DNA binding sites, providing a mechanism for the
regulation of this gene by cortisol and negative feedback by thyroid hormone.
Enzymatic processing of TRH begins with excision of the progenitor peptides by
carboxypeptidases, amidation of the C-terminal proline, and cyclization of the Nterminal glutamine to yield five TRH molecules per prohormone molecule. TRH is
widely distributed in the CNS with TRH immunoreactive neurons being located in the
olfactory bulbs, entorhinal cortices, hippocampus, extended amygdala, hypothalamus,
and midbrain structures. As is the case for most neuropeptides, the TRH receptor is also
a member of the seven-transmembrane domain, G-protein-coupled receptor family.
Table 1.4-3
Selected Neuropeptide Structures
Hypothalamic TRH neurons project nerve terminals to the median eminence; there
they release TRH into the hypothalamohypophyseal portal system where it is
transported to the adenohypophysis, causing the release of thyroid-stimulating hormone
(TSH) into systemic circulation. TSH subsequently stimulates the release of the thyroid
hormones triiodothyronine (T3) and thyroxine (T4) from the thyroid gland. TRH neurons
in the paraventricular nucleus (PVN) contain thyroid hormone receptors and respond to
increases in thyroid hormone secretion with a decrease in TRH gene expression and
synthesis. This negative feedback of thyroid hormones on the TRH-synthesizing neurons
was first demonstrated by a decrease in TRH content in the median eminence, but not in
the PVN of the hypothalamus, after thyroidectomy. This effect can be reversed with
exogenous thyroid hormone treatment. The treatment of normal rats with exogenous
thyroid hormone decreases TRH concentration in the PVN and the posterior nucleus of
the hypothalamus. With a probe against the TRH preprohormone mRNA, in situ
hybridization studies have demonstrated that TRH mRNA is increased in the PVN 14
days after thyroidectomy. The ability of thyroid hormones to regulate TRH mRNA can be
superseded by other stimuli that activate the hypothalamic–pituitary–thyroid (HPT) axis.
In that regard, repeated exposure to cold (which releases TRH from the median
eminence) induces increases in the levels of TRH mRNA in the PVN despite
concomitantly elevated concentrations of thyroid hormones. Further evidence of the
different levels of communication of the HPT axis are seen in the ability of TRH to
regulate the production of mRNA for the pituitary TRH receptor and for TRH
concentrations to regulate the mRNA coding for both the α and β subunits of the
thyrotropin (TSH) molecule. In addition, TRH-containing synaptic boutons have been
observed in contact with TRH-containing cell bodies in the medial and periventricular
subdivisions of the paraventricular nucleus, thus providing anatomical evidence for
ultrashort feedback regulation of TRH release. Negative feedback by thyroid hormones
may be limited to the hypothalamic TRH neurons because negative feedback on TRH
synthesis by thyroid hormones has not been found in extrahypothalamic TRH neurons.
The early availability of adequate tools to assess HPT axis function (i.e., radioimmunoassays and synthetic peptides),
coupled with observations that primary hypothyroidism is associated with depressive symptomatology, ensured extensive
investigation of the involvement of this axis in affective disorders. Early studies established the hypothalamic and
extrahypothalamic distribution of TRH. This extrahypothalamic presence of TRH quickly led to speculation that TRH
might function as a neurotransmitter or neuromodulator. Indeed, a large body of evidence supports such a role for TRH.
Within the CNS, TRH is known to modulate several different neurotransmitters, including dopamine, serotonin,
acetylcholine, and the opioids. TRH has been shown to arouse hibernating animals and counteracts the behavioral response
and hypothermia produced by a variety of CNS depressants including barbiturates and ethanol.
The use of TRH as a provocative agent for the assessment of HPT axis function evolved rapidly after its isolation and
synthesis. Clinical use of a standardized TRH stimulation test, which measures negative feedback responses, revealed
blunting of the TSH response in approximately 25 percent of euthyroid patients with major depression. These data have
been widely confirmed. The observed TSH blunting in depressed patients does not appear to be the result of excessive
negative feedback due to hyperthyroidism because thyroid measures such as basal plasma concentrations of TSH and
thyroid hormones are generally in the normal range in these patients. It is possible that TSH blunting is a reflection of
pituitary TRH receptor downregulation as a result of median eminence hypersecretion of endogenous TRH. Indeed, the
observation that CSF TRH concentrations are elevated in depressed patients as compared to those of controls supports the
hypothesis of TRH hypersecretion but does not elucidate the regional CNS origin of this tripeptide. In fact, TRH mRNA
expression in the PVN of the hypothalamus is decreased in patients with major depression. However, it is not clear
whether the altered HPT axis represents a causal mechanism underlying the symptoms of depression or simply a
secondary effect of depression-associated alterations in other neural systems.
Corticotropin-Releasing Factor (CRF) and Urocortins.
There is convincing
evidence to support the hypothesis that CRF and the urocortins play a complex role in
integrating the endocrine, autonomic, immunological, and behavioral responses of an
organism to stress.
Although it was originally isolated because of its functions in regulating the
hypothalamic–pituitary–adrenal (HPA) axis, CRF is widely distributed throughout the
brain. The PVN of the hypothalamus is the major site of CRF-containing cell bodies that
influence anterior pituitary hormone secretion. These neurons originate in the
parvocellular region of the PVN and send axon terminals to the median eminence,
where CRF is released into the portal system in response to stressful stimuli. A small
group of PVN neurons also projects to the brainstem and spinal cord where they
regulate autonomic aspects of the stress response. CRF-containing neurons are also
found in other hypothalamic nuclei, the neocortex, the extended amygdala, brainstem,
and spinal cord. Central CRF infusion into laboratory animals produces physiological
changes and behavioral effects similar to those observed following stress, including
increased locomotor activity, increased responsiveness to an acoustic startle, and
decreased exploratory behavior in an open field.
The physiological and behavioral roles of the urocortins are less understood, but several studies suggest that urocortins 2
and 3 are anxiolytic and may dampen the stress response. This has led to the hypothesis that CRF and the urocortins act in
opposition, but this is likely an oversimplification. Urocortin 1 is primarily synthesized in the Edinger–Westphal nucleus,
lateral olivary nucleus, and supraoptic hypothalamic nucleus. Urocortin 2 is synthesized primarily in the hypothalamus,
while urocortin 3 cell bodies are found more broadly in the extended amygdala, perifornical area, and preoptic area.
Hyperactivity of the HPA axis in major depression remains one of the most consistent
findings in biological psychiatry. The reported HPA axis alterations in major depression
include hypercortisolemia, resistance to dexamethasone suppression of cortisol secretion
(a measure of negative feedback), blunted adrenocorticotropic hormone (ACTH)
responses to intravenous CRF challenge, increased cortisol responses in the combined
dexamethasone/CRF test, and elevated CSF CRF concentrations. The exact pathological
mechanism(s) underlying HPA axis dysregulation in major depression and other
affective disorders remains to be elucidated.
Mechanistically, two hypotheses have been advanced to account for the ACTH
blunting following exogenous CRF administration. The first hypothesis suggests that
pituitary CRF receptor downregulation occurs as a result of hypothalamic CRF
hypersecretion. The second hypothesis postulates altered sensitivity of the pituitary to
glucocorticoid negative feedback. Substantial support has accumulated favoring the first
hypothesis. However, neuroendocrine studies represent a secondary measure of CNS
activity; the pituitary ACTH responses principally reflect the activity of hypothalamic
CRF rather than that of the corticolimbic CRF circuits. The latter of the two are more
likely to be involved in the pathophysiology of depression.
Of particular interest is the demonstration that the elevated CSF CRF concentrations
in drug-free depressed patients are significantly decreased after successful treatment
with electroconvulsive therapy (ECT), indicating that CSF CRF concentrations, like
hypercortisolemia, represent a state rather than a trait marker. Other recent studies
have confirmed this normalization of CSF CRF concentrations following successful
treatment with fluoxetine. One group demonstrated a significant reduction of elevated
CSF CRF concentrations in 15 female patients with major depression who remained
depression free for at least 6 months following antidepressant treatment, as compared
to little significant treatment effect on CSF CRF concentrations in 9 patients who
relapsed in this 6-month period. This suggests that elevated or increasing CSF CRF
concentrations during antidepressant treatment may be the harbinger of a poor
response in major depression despite early symptomatic improvement. Of interest,
treatment of normal subjects with desipramine or, as noted above, of individuals with
depression with fluoxetine is associated with a reduction in CSF CRF concentrations.
If CRF hypersecretion is a factor in the pathophysiology of depression, then reducing
or interfering with CRF neurotransmission might be an effective strategy to alleviate
depressive symptoms. Over the last several years, a number of pharmaceutical
companies have committed considerable effort to the development of small-molecule
CRF1 receptor antagonists that can effectively penetrate the blood–brain barrier. Several
compounds have been produced with reportedly promising characteristics.
Oxytocin (OT) and Vasopressin (AVP).
The vasopressor effects of posterior
pituitary extracts were first described in 1895, and the potent extracts were named AVP.
OT and AVP mRNAs are among the most abundant messages in the hypothalamus, being
heavily concentrated in the magnocellular neurons of the PVN and the supraoptic
nucleus of the hypothalamus, which send axonal projections to the neurohypophysis.
These neurons produce all of the OT and AVP that is released into the bloodstream
where these peptides act as hormones on peripheral targets. OT and AVP are generally
synthesized in separate neurons within the hypothalamus. OT released from the
pituitary is most often associated with functions associated with female reproduction,
such as regulating uterine contractions during parturition and the milk ejection reflex
during lactation. AVP, also known as antidiuretic hormone, regulates water retention in
the kidney and vasoconstriction through interactions with vasopressin V2 and V1a
receptor subtypes, respectively. AVP is released into the bloodstream from the
neurohypophysis following a variety of stimuli including plasma osmolality,
hypovolemia, hypertension, and hypoglycemia. The actions of OT are mediated via a
single receptor subtype (oxytocin receptor, OTR), which is distributed in the periphery
and within the limbic CNS. In contrast to the OTR there are three vasopressin receptor
subtypes, V1a, V1b, and V2 receptors, each of which are G-protein-coupled, seventransmembrane domain receptors. The V2 receptor is localized in the kidney and is not
found in the brain. The V1a receptor is distributed widely in the CNS and is thought to
mediate most of the behavioral effects of AVP. The V1b receptor is concentrated in the
anterior pituitary, and some reports describe V1b receptor mRNA in the brain, although
its function is unknown.
Neurotensin (NT)
Although NT is found in a number of brain regions, it has been most thoroughly
investigated in terms of its association with other neurotransmitter systems, particularly
the mesolimbic dopamine system, and has gained interest in research on the
pathophysiology of schizophrenia. There are several lines of evidence suggesting that
NT and its receptors should be considered as potential targets for pharmacological
intervention in this disorder. First, the NT system is positioned anatomically to modulate
the neural circuits implicated in schizophrenia. Second, peripheral administration of
antipsychotic drugs has been shown to consistently modulate NT systems. Third, there is
evidence that central NT systems are altered in patients with schizophrenia.
NT was first shown to interact with dopamine systems while undergoing characterization of its potent hypothermicpotentiating and sedative-potentiating activities. Subsequent work indicated that NT possessed many properties that were
also shared by antipsychotic drugs, including the ability to inhibit avoidance, but not escape responding in a conditioned
active avoidance task; the ability to block the effects of indirect dopamine agonists or endogenous dopamine in the
production of locomotor behavior; and the ability to elicit increases in dopamine release and turnover. Perhaps most
importantly, both antipsychotic drugs and NT neurotransmission enhance sensorimotor gating. Sensorimotor gating is the
ability to screen or filter relevant sensory input, deficits in which may lead to an involuntary flooding of indifferent
sensory data. Increasing evidence suggests that deficits in sensorimotor gating are a cardinal feature of schizophrenia. Both
dopamine agonists and NT antagonists disrupt performance on tasks designed to gauge sensorimotor gating. Unlike
antipsychotic drugs, NT is not able to displace dopamine from its receptor. As noted earlier, NT is colocalized in certain
subsets of dopamine neurons and is co-released with dopamine in the mesolimbic and medial prefrontal cortex dopamine
terminal regions that are implicated as the sites of dopamine dysregulation in schizophrenia. Antipsychotic drugs that act
at D2 and D4 receptors increase the synthesis, concentration, and release of NT in those dopamine terminal regions but not
in others. That effect of antipsychotic drugs in increasing NT concentrations persists after months of treatment and is
accompanied by the expected increase in NT mRNA concentrations as well as expression of the “immediate early gene” cfos within hours of initial drug treatment. The altered regulation of NT expression by antipsychotic drugs apparently
extends to the peptidases that degrade the peptide, because recent reports have revealed decreased NT metabolism in rat
brain slices 24 hours after the acute administration of haloperidol. When administered directly into the brain, NT
preferentially opposes dopamine transmission in the nucleus accumbens but not the caudate putamen. In the nucleus
accumbens, NT receptors are located predominantly on GABAergic neurons, which release GABA on dopamine terminals,
thereby inhibiting release.
Decreased CSF NT concentrations have been reported in several populations of
patients with schizophrenia when compared to those of controls or other psychiatric
disorders. Although treatment with antipsychotic drugs has been observed to increase NT
concentrations in the CSF, it is not known whether this increase is causal or merely
accompanies the decrease in psychotic symptoms seen with successful treatment.
Postmortem studies have shown an increase in NT concentrations in the dopamine-rich
Brodmann’s area 32 of the frontal cortex, but that result may have been confounded by
premortem antipsychotic treatment. Other researchers have found no postmortem
alterations in NT concentrations of a wide sampling of subcortical regions. Decreases in
NT receptor densities in the entorhinal cortex have been reported in entorhinal cortices
of schizophrenic postmortem samples. A critical test of the hypothesis that NT may act
as an endogenous antipsychotic-like substance awaits the development of an NT
receptor agonist that can penetrate the blood–brain barrier.
Other Neuropeptides
A number of other neuropeptides have been implicated in the pathophysiology of psychiatric disorders. These include,
but are not limited to, cholecystokinin (CCK), substance P, and neuropeptide Y.
CCK, originally discovered in the gastrointestinal tract, and its receptor are found in areas of the brain associated with
emotion, motivation, and sensory processing (e.g., cortex, striatum, hypothalamus, hippocampus, and amygdala). CCK is
often colocalized with dopamine in the VTA neurons that comprise the mesolimbic and mesocortical dopamine circuits.
Like NT, CCK decreases dopamine release. Infusions of a CCK fragment have been reported to induce panic in healthy
individuals, and patients with panic disorder exhibit increased sensitivity to the CCK fragment compared to that of normal
controls. Pentagastrin, a synthetic CCK agonist, dose-dependently produced increased blood pressure, pulse, HPA
activation, and physical symptoms of panic. Recently, a CCK receptor gene polymorphism has been associated with panic
disorder.
The undecapeptide substance P is localized in the amygdala, hypothalamus, periaqueductal gray, LC, and parabrachial
nucleus and is colocalized with norepinephrine and serotonin. Substance P serves as a pain neurotransmitter, and
administration to animals elicits behavioral and cardiovascular effects resembling the stress response. More recent data
suggest a role for substance P in major depression and PTSD. Both depressed and PTSD patients had elevated CSF substance
P concentrations. Furthermore, in PTSD patients, marked increases in CSF substance P concentrations were detected
following precipitation of PTSD symptoms. One study has indicated that a substance P receptor (termed the neurokinin 1
[NK1] receptor) antagonist capable of passing the BBB is more effective than placebo and as effective as paroxetine in
patients with major depression with moderate to severe symptom severity, although subsequent studies have been unable
to confirm these findings.
Neuropeptide Y (NPY) is a 36 amino acid peptide found in the hypothalamus, brainstem, spinal cord, and several limbic
structures and is involved in the regulation of appetite, reward, anxiety, and energy balance. NPY is colocalized with
serotonergic and noradrenergic neurons and is thought to facilitate the containment of negative effects following exposure
to stress. Suicide victims with a diagnosis of major depression are reported to have a pronounced reduction in NPY levels
in the frontal cortex and caudate nucleus. Furthermore, CSF NPY levels are decreased in depressed patients. Chronic
administration of antidepressant drugs increases neuropeptide Y concentrations in the neocortex and hippocampus in rats.
Plasma NPY levels were found to be elevated in soldiers subjected to the “uncontrollable stress” of interrogation, and NPY
levels were correlated with the feelings of dominance and confidence during the stress. In addition, low NPY response to
stress has been associated with increased vulnerability to depression and PTSD.
NOVEL NEUROTRANSMITTERS
Nitric Oxide
The discovery that gases could function as neurotransmitters revealed that highly
atypical modes of signaling existed between neurons. In the early 1990s, nitric oxide
was the first gas to be ascribed a neurotransmitter function and proved to be an atypical
neurotransmitter for several reasons. First, it was not stored in or released from
synaptic vesicles, as it was a small gas it could freely diffuse into the target neuron.
Second, its target was not a specific receptor on the surface of a target neuron, but
intracellular proteins whose activity could directly be modulated by nitric oxide, leading
to neurotransmission. Nitric oxide also lacks a reuptake mechanism to remove it from
the synapse. Although enzymatic inactivation of it is postulated to exist, nitric oxide
appears to have a very short half-life of a few seconds.
Nitric oxide was initially discovered as a bactericidal compound released from
macrophages, and as an endothelial cell it derived relaxation factor allowing for the
dilation of blood vessels. A role for nitric oxide in the brain followed, revealing a role
for the gas in neurotransmission, learning and memory processes, neurogenesis, and
neurodegenerative disease.
Nitric Oxide and Behavior
Nitric oxide neurotransmission can play a role in behavior, as neuronal nitric oxide
synthase (nNOS)–deficient male mice display exaggerated aggressive tendencies and
increased sexual activity. In female mice the contrary is true, as they have reduced
aggression. As manic bipolar patients may show both hypersexuality and aggression, the
nitric oxide pathway may participate in the psychopathology of affective states.
In the periphery, nNOS localizes to neurons that innervate blood vessels of the penis, including the corpus cavernosa.
Stimulation of these nerves releases nitric oxide, leading to cyclic guanosine monophosphate (cGMP) formation, blood
vessel wall relaxation and vasodilation, penile engorgement, and initial erection. The sustained phase of erection also
depends on nitric oxide; turbulent blood flow leads to phosphorylation of eNOS and sustained nitric oxide production.
Drugs used in treatment of erectile dysfunction—sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra)—act to
inhibit phosphodiesterase type 5 (PDE5), an enzyme that degrades cGMP in the penis (Fig. 1.4-12), thereby potentiating
nitric oxide neurotransmission and penile erection.
FIGURE 1.4-12
Neurotransmitter and signaling functions of nitric oxide (NO) via production of cyclic
guanosine monophosphate (cGMP). Gaseous nitric oxide is enzymatically generated and
freely diffuses into an adjacent neuron (upper right). In comparison to traditional
neurotransmitters (upper left), nitric oxide (NO) does not act via a specific
neurotransmitter receptor on the surface membrane of a neuron. In contrast, NO freely
diffuses across the neuronal membrane and activates the enzyme, guanylyl cyclase,
which converts guanosine 5’-triphosphate (GTP) into the second messenger, cGMP.
Nitric oxide effects are mediated, in part, by cGMP activation of neuronal protein
kinases, new gene expression, and effects on neuronal long-term potentiation (LTP) and
long-term depression (LTD). ATP, adenosine triphosphate. (From Sadock BJ, Sadock VA,
Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009:104.)
Numerous lines of evidence have suggested a role for nitric oxide in the regulation of sleep–wake cycles. nNOS
expressing neurons occur in several areas that initiate REM sleep, including the pons, dorsal raphe nucleus, laterodorsal
tegmentum, and pedunculopontine tegmentum. In animal models, microinjection of compounds that release nitric oxide
result in decreased wakefulness and increased slow wave sleep. Consistent with this, NOS inhibitors show a trend toward
decreasing slow wave and REM sleep. Studies of NOS-deficient mice suggest that nitric oxide may serve a more complex
role than merely promoting sleep. nNOS-deficient animals also show reduced REM sleep; however, inducible nitric oxide
synthase (iNOS)–deficient mice demonstrate the reverse, suggesting a complex interplay between NOS enzymatic
isoforms.
Nitric Oxide and Mood Disorders.
NOS-expressing neurons are well
represented in areas implicated in depression, including the dorsal raphe nucleus and
prefrontal cortex. A role for nitric oxide has been suggested in antidepressant response,
as SSRI antidepressants can directly inhibit NOS activity. Moreover, in animal studies
such as the forced swim test, NOS and soluble guanylyl cyclase inhibitors can achieve
antidepressant-like effects. Plasma nitric oxide levels were elevated in patients with
bipolar disorder compared to healthy controls. However, in depressed subjects, studies
have found decreased nitric oxide levels and increased plasma nitrite, a byproduct of
nitric oxide. Reduced NOS has also been described in the paraventricular nucleus of
patients with schizophrenia and depression compared to controls.
Nitric oxide has been questioned as to its ability to regulate neurotransmission at
serotonin, norepinephrine, and dopamine nerve termini. However, there has been no
clear consensus, and nitric oxide appears to be able to increase or decrease activity at
these neurons depending on the timing of its activation and the region of the brain
studied.
Nitric Oxide and Schizophrenia.
Nitric oxide has been investigated as a
candidate molecule contributing to symptoms of schizophrenia. Two genetic studies
have identified schizophrenia-associated single nucleotide polymorphisms (SNPs) in
CAPON, a protein that associates with nNOS. SNPs in nNOS itself have been associated
with schizophrenia, although others have not been able to reproduce such findings.
Changes in NOS levels have been reported in postmortem brain samples of individuals
with schizophrenia. Abnormalities have been noted in the cortex, cerebellum,
hypothalamus, and brainstem, although no specific trend can be discerned. Elevated
NOS activity has been noted in platelets from drug-naive and drug-treated individuals
with schizophrenia. Some investigators find increased nitric oxide activity and others the
reverse. In autopsy samples, schizophrenic patients were found to have abnormally
localized NOS expressing neurons in the prefrontal cortex, hippocampus, and lateral
temporal lobe, consistent with abnormal migration of these neuronal types during
development. In a rat model, prenatal stress led to reduced NOS expressing neurons in
the fascia dentate and hippocampus.
Neuropathological Roles of Nitric Oxide.
Abundant evidence exists that nitric
oxide is a direct participant in a variety of neuropathic events. Superoxide, a byproduct
of cellular metabolism, can react with nitric oxide to form peroxynitrite (chemical
formula ONOO−). This labile and toxic compound forms chemical adducts with protein
tyrosine residues, a process termed protein nitration, and deoxyribonucleic acid (DNA),
leading to cellular dysfunction.
Cell loss resulting from ischemic stroke is mediated in part by overstimulation of the
glutamate NMDA receptor, a process termed excitotoxicity. Nitric oxide produced by
NMDA activation appears to mediate a significant portion of this excitotoxic neuronal
death, and stroke damage is reduced in mice with a genetic deletion of nNOS.
S-Nitrosylation has also been implicated in pathologic processes in the brain. Mutations in the Parkin protein are
associated with early onset Parkinson’s disease. Parkin is an E3 ubiquitin ligase, adding ubiquitin molecules to proteins
and targeting them for destruction in the cell proteasome. In sporadic Parkinson’s disease (i.e., without the early onset
mutation), nitric oxide can nitrosylate the Parkin protein and inhibit its protective E3 ubiquitin ligase function. An
overabundance of nitric oxide signaling may thus predispose to the dysfunction and cell death of dopaminergic neurons in
Parkinson’s disease by interfering with proteins essential for cell functioning. In Alzheimer’s disease excess oxidation of
brain protein, lipids, and carbohydrates has long been appreciated, but nitrosative stress from excess nitric oxide also
appears to participate in the disease. Protein disulfide isomerase (PDI) is a cellular protective protein that may help
combat the accumulation of misfolded proteins such as the amyloid fibrils occurring in the disease. In both Alzheimer’s
and Parkinson’s disease brains, PDI appears to be S-nitrosylated in a harmful way that impedes its cellular protective
function.
The discovery that nitric oxide participates in neurodegenerative processes raises the possibility for improved
diagnostic processes, such as detecting damage to cellular components produced by nitric oxide prior to the onset of fullblown symptoms. In addition, drugs may be designed to attenuate the damage to crucial neuronal proteins that protect
against disease onset. However, completely and nonspecifically inhibiting or stimulating NOS is likely to produce
significant side effects because of its wide-ranging activities throughout the body.
Carbon Monoxide
Although carbon monoxide (CO) is most well known as an air pollutant derived from
combustion reactions, it is produced physiologically in a great variety of organisms
ranging from human to bacterium. Once thought to be a toxic byproduct of metabolic
reactions, carbon monoxide is increasingly recognized to play an important role in
regulating a variety of physiological processes in the brain and other organs. These
varied effects include regulation of olfactory neurotransmission, blood vessel relaxation,
smooth muscle cell proliferation, and platelet aggregation.
Carbon monoxide is far better known for its toxic effects than its activities at
physiologic concentrations. It binds tightly to heme molecules within hemoglobin,
forming carboxyhemoglobin, which can no longer transport oxygen to tissues. One- to
two-pack per day smokers typically have 3 to 8 percent of their hemoglobin as
carboxyhemoglobin, with nonsmokers having less than 2 percent. Following acute
carbon monoxide poisoning, 5 to 10 percent carboxyhemoglobin is associated with
impaired alertness and cognition, and 30 to 50 percent carboxyhemoglobin leads to
significant drops in oxygen transport to tissues.
Carbon Monoxide and Neurotransmission.
Carbon monoxide appears to
participate in the neurotransmission of odorant perception. Odorants lead to carbon
monoxide production and subsequent cGMP synthesis that promotes long-term
adaptation to odor stimuli. Carbon monoxide has the potential to regulate a variety of
perceptual and cognitive processes that are yet untested. Similarly, in the rat retina,
long periods of light exposure led to increased HO1 expression, carbon monoxide
production, and cGMP signaling. Carbon monoxide may also participate in adaptation
to chronic pain. HO2-deficient animals manifest reduced hyperalgesia and allodynia
after exposure to chronic pain stimuli. Carbon monoxide may thus set the threshold for
pain perception, although it is unclear whether the effect occurs in the central or
peripheral nervous system. Aside from its role in promoting cGMP production, carbon
monoxide may also directly bind to and open the calcium-activated big potassium (BKCa)
channel, leading to as yet uncharacterized effects on neurotransmission.
In the gastrointestinal (GI) nervous system, carbon monoxide serves as a
neurotransmitter to relax the internal anal sphincter in response to nonadrenergic
noncholinergic (NANC) nerve stimulation and vasoactive intestinal peptide (VIP).
Carbon monoxide has been implicated in the development of hippocampal LTP,
although lines of evidence are contradictory. Carbon monoxide and tetanic stimulation
of nerves leads to increased excitatory postsynaptic potentials (EPSPs). HO inhibitors
that block carbon monoxide production lead to impaired induction of LTP and reduced
calcium-dependent release of glutamate neurotransmitter. However, HO2-deficient
animals fail to manifest any differences in LTP. These disparate findings may be
explained by a role for HO1 in LTP, or an ability of HO inhibitors to nonspecifically
block some other processes important to LTP induction.
At toxic levels, carbon monoxide is well known to impair oxygen transport by binding
to hemoglobin with a higher affinity than oxygen. Amazingly, carbon monoxide itself
plays a physiological role in the mechanism by which the carotid body senses oxygen.
HO, expressed in glomus cells of the carotid body, uses oxygen as a substrate in the
production of carbon monoxide (Fig. 1.4-13). When oxygen levels drop, so does carbon
monoxide production, leading to a resetting of the threshold in which the carotid body
senses oxygen. The molecular mechanism may occur via carbon monoxide regulation of
the carotid body BK ion channel.
FIGURE 1.4-13
Synthesis of carbon monoxide (CO), an unexpected neurotransmitter. Gaseous carbon
monoxide is enzymatically synthesized in neurons by way of the enzyme heme
oxygenase, also converting heme into the molecule biliverdin and liberating free iron
(Fe). Similar to nitric oxide, CO is not stored in neuronal vesicles and can freely diffuse
across neuronal membranes. CO also similarly activates soluble guanylyl cyclase, and
leads to activation of multiple intracellular signaling molecules such as p38 MAP kinase.
CO exerts its neurotransmitter and signaling functions at concentrations far below that
at which classical CO toxicity occurs. The significance of this pathway in neurons is
underlined by the existence of two distinct heme oxygenase enzymes, one of which is
predominantly expressed in the brain. Biliverdin is converted to bilirubin via the
enzyme biliverdin reductase. Similar to CO, bilirubin is no longer relegated to the status
of toxic byproduct and may be an important antioxidant. (From Sadock BJ, Sadock VA,
Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009:107.)
Endocannabinoids: From Marijuana to Neurotransmission
Whether known as cannabis, hemp, hashish, ma-fen, or a variety of slang terms,
marijuana has been cultivated and utilized by human populations for thousands of
years. Despite long debate as to whether its risks and benefits are evenly matched, it has
only been in recent decades that some of the mystery has been revealed by which
marijuana exerts its effects on the brain. The “high” users experience, euphoria and
tranquility, relates to cannabis acting on a neural pathway involving cannabinoids
endogenous to the human brain, or endocannabinoids.
The first described medicinal use of cannabis dates to approximately 2700 BC in the
pharmacopeia of Chinese Emperor Shen Nung, who recommended its use for a variety of
ailments. At this time, adverse properties were also apparent, and large amounts of the
fruits of hemp could lead to “seeing devils,” or a user might “communicate with spirits
and lightens one’s body.” For centuries, cannabis was employed in India as an appetite
stimulant; habitual marijuana users remain well acquainted with “the munchies.”
For many years the mechanisms by which the active components of marijuana,
cannabinoids, exerted their psychoactive effects remained a mystery. Chemists sought to
isolate the psychoactive components of cannabis from the many components of the
plant oil (Table 1.4-4).
Table 1.4-4
Selected Discoveries in Cannabinoid Research
Discovery of the Brain Endocannabinoid System.
Estimates suggest that 20
to 80 μg of tetrahydrocannabinol (THC) reaches the brain after one smokes a marijuana
cigarette (i.e., “joint”). This is comparable to the 100 to 200 μg of norepinephrine
neurotransmitter present in the entire human brain. Thus the effects of THC might be
explained by the effects on neurotransmitter systems. In the 1960s, there were at least
two schools of thought on how THC exerted its psychoactive effects. One held that THC
worked in a manner similar to that of the inhaled volatile anesthetics (i.e., no specific
receptor existed), and it might have a generalized effect on neuronal membranes or
widespread actions on neurotransmitter receptors. A competing school of thought
speculated that specific receptors for cannabinoids existed in the brain, but they were
difficult to identify due to the lipophilic nature of these chemicals. Novel cannabinoids
were synthesized that were more water soluble, and in the late 1980s, this allowed for
the discovery of a specific cannabinoid receptor, CB1.
Several additional endocannabinoids were soon discovered, 2-arachidonylglycerol (2-AG), N-arachidonyldopamine
(NADA), 2-arachidonoylglycerol ether (noladin ether), and virodhamine (Fig. 1.4-14). The reason for having several
different endocannabinoids may lie with their differing affinities for the cannabinoid receptors, CB1 and CB2. Anandamide
appears to have the greatest selectivity for the CB1 receptor, followed by NADA and noladin ether. In contrast,
virodhamine prefers CB2 receptors and has only partial agonist activity at CB1. 2-AG appears not to discriminate between
CB1 and CB2.
FIGURE 1.4-14
Endogenous cannabinoids. At least five endocannabinoids exist in the mammalian
brain, each differing in affinity for CB1 and CB2 cannabinoid receptors. All are derived
from the essential omega-6 fatty acid, arachidonic acid, which is also a substrate in the
formation of prostaglandins and leukotrienes. (From Sadock BJ, Sadock VA, Ruiz P.
Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott
Williams & Wilkins; 2009:111.)
Biosynthesis of Endocannabinoids.
Arachidonic acid is utilized as a building
block for biosynthesis of endocannabinoids, prostaglandins, and leukotrienes and is
found within cellular phospholipids of the plasma membrane and other intracellular
membranes. Synthesis of anandamide requires the sequential action of two enzymes
(Fig. 1.4-15). In the first reaction the enzyme N-acetyltransferase (NAT) transfers an
arachidonic acid side chain from a phospholipid to phosphatidylethanolamine (PE),
generating NAPE (N-arachidonyl-phosphatidylethanolamine). In the second reaction the
enzyme N-arachidonyl-phosphatidylethanolamine phospholipase (NAPD-PLD) converts
NAPE to anandamide. Because NAPE is already a natural component of mammalian
membranes, it is the second step that generates anandamide, which is most crucial to
neurotransmission.
FIGURE 1.4-15
Retrograde neurotransmission of the endocannabinoids, anandamide, and 2arachidonylglycerol
(2-AG).
Anandamide
is
synthesized
on
demand
for
neurotransmission via a two-step process. The enzyme NAT transfers the arachidonic
acid chain from a phospholipid (APL) to phosphatidylethanolamine (PE), thereby
producing NAPE. A second enzyme, NAPE-PLD, generates anandamide. 2-AG is similarly
synthesized in two steps by the enzymes PLC and DAGL. The endocannabinoids made in
a postsynaptic neuron cross the synapse and activate presynaptic CB1 receptors, and
suppress neurotransmission of the presynaptic neuron (although activation of the
presynaptic neuron occurs in some cases). Enzymes involved in endocannabinoid
synthesis are yellow, those that break them down in red. 2-AG is predominantly
inactivated in the presynaptic neuron by MAGL, whereas anandamide is destroyed in
the postsynaptic neuron by FAAH. PE, phosphatidylethanolamine; APL, arachidonyl
phospholipids; NAT, N-acyltransferase; NAPE, N-arachidonyl-phosphatidylethanolamine;
NAPE-PLD, N-arachidonyl-phosphatidylethanolamine phospholipase D; FAAH, fatty acid
amide hydrolase; MAGL, monoacylglycerol lipase; PLC, phospholipase C; DAG,
diacylglycerol; DAGL, diacylglycerol lipase; R1-R3, various acyl or akyl side chains of
phospholipids; R’, side chain of phospholipid head group. (From Sadock BJ, Sadock VA,
Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009:112.)
Endocannabinoids are not stored in synaptic vesicles for later use, but are synthesized on demand as is done for the
gaseous neurotransmitters. An important criterion for a signaling molecule to be considered a neurotransmitter is that
neuronal depolarization should lead to its release. Depolarization leads to increases in cellular calcium, which in turn
promotes synthesis of the endocannabinoids and their release. The mechanism is explained in part by calcium activation of
NAPE-PLD and DAGL, leading to augmented biosynthesis of anandamide and 2-AG, respectively.
Endocannabinoids generated in a neuron must cross the synaptic cleft to act on cannabinoid receptors. Similar to THC,
endocannabinoids are highly lipophilic and thus poorly soluble in CSF. It is hypothesized that a specific endocannabinoid
transporter exists to allow endocannabinoids to cross the synaptic cleft and allow entry into the target neuron.
Inactivation of Endocannabinoids.
Neurotransmitters are typically inactivated
either by reuptake from the neurons that release them or by degradation by highly
specific enzymes, such as the example of acetylcholine being hydrolyzed by
acetylcholinesterase. At least two enzymes exist to target the destruction of
endocannabinoids and attenuate their neurotransmission. Fatty acid amide hydrolase
(FAAH) converts anandamide to arachidonic acid and ethanolamine (Fig. 1.4-15). FAAH
is found in regions of the brain where CB1 receptors are predominant and localizes to
postsynaptic neurons where anandamide is made. Rapid degradation of anandamide in
part explains its relatively low potency compared to THC. Confirming a role of FAAH in
anandamide inactivation, knockout mice without FAAH exhibit a 15-fold increase of
anandamide, but not 2-AG. These mice have greater behavioral responses to exogenous
anandamide, owing to its decreased degradation. The endocannabinoid 2-AG is
inactivated by FAAH, but also by a monoacylglycerol lipase (MAGL) located in
presynaptic neurons.
Pharmacologic inhibitors of FAAH have analgesic effects and reduce anxiety in animal
models, but do not have the undesirable effects of THC such as immobility, lowered body
temperature, or greater appetite. Such a pharmacological strategy would be analogous
to MAOIs and COMT inhibitors (COMTIs). MAOIs, used to treat depression, slow the
breakdown of serotonin and other monoamines, thereby increasing serotonin, whereas
COMTIs serve an analogous role in blocking destruction of dopamine and other
catecholamines.
Cannabinoid Receptors.
Underscoring their importance in neural functions, CB1
receptors are possibly the most abundant G-protein-coupled receptors in the brain. They
occur at highest density in the basal ganglia, cerebellum, hippocampus, hypothalamus,
anterior cingulate cortex, and cerebral cortex, particularly the frontal cortex. Humans or
animals that receive large doses of THC develop catalepsy, a reduction of spontaneous
movement, and freeze in bizarre and unnatural postures. The action of cannabinoids in
the basal ganglia and cerebellum may be associated with these behaviors, which may
prove relevant in understanding catatonic symptoms in schizophrenia.
CB1 receptors are predominantly found on axons and nerve termini, with little
present on neuronal dendrites and the cell body. CB1 receptors tend to be localized to
the presynaptic rather than postsynaptic side of the neuronal cleft, suggesting a role in
regulation of neurotransmission. A second cannabinoid receptor, CB2, is predominantly
expressed on the surface of white blood cells of the immune system, but small amounts
appear to be present in the brainstem.
EFFECTS ON NEUROTRANSMISSION. The cannabinoid CB1 receptor is associated with G
proteins that mediate its intracellular signaling, in part, through inhibition of adenylyl
cyclase. This leads to a decrease in levels of the important second messenger, cyclic
adenosine monophosphate. Activation of the CB1 receptor also leads to activation of
potassium channels and inhibition of N-type calcium channels. Because calcium is
integral to neurotransmitter release, cannabinoids can block neurotransmission through
this mechanism. Cannabinoid receptors also activate mitogen-activated protein kinases.
With the use of cell culture models and slices of brain, cannabinoids have been shown
to block the release of a variety of neurotransmitters, including GABA, norepinephrine,
and
acetylcholine.
Norepinephrine
and
acetylcholine
tend
to
be
excitatory
neurotransmitters, and cannabinoid inhibition of their release would be expected to
have an overall inhibitory effect. However, GABA is an inhibitory neurotransmitter, and
cannabinoid inhibition of it would lead to overall excitatory effects, demonstrating that
cannabinoids can have complex effects on neurotransmission depending on the specific
context. Cannabinoids also appear to increase the release of brain endorphin
neurotransmitters and increase dopamine release in the nucleus accumbens, a “reward
center” relevant to addiction and learning. The endocannabinoids have been implicated
in a variety of forms of synaptic plasticity, including LTP and long-term depression
(LTD).
Endocannabinoids in Anxiety and Mood.
Endocannabinoid
neurotransmission may be an important regulator of anxiety, and cannabis users
regularly describe a tranquilizing effect of THC. Loss of signaling by the
endocannabinoid system appears to promote anxiety-like states in animal studies. CB1
receptor–deficient animals exhibit more pronounced anxiety behavior when exposed to
stress or new environs.
The endocannabinoid pathway may represent an attractive target in understanding
posttraumatic stress responses and phobias. Although one cannot yet safely measure
endocannabinoid levels in human subjects, this model is supported by clinical trials of
the cannabinoid receptor blocker, rimonabant (Acomplia), which may offer promise as a
strategy for weight loss (see below). A frequent adverse reaction to the drug is increased
anxiety and depression.
ADDICTION. The endocannabinoid system may be an attractive target for understanding
addiction. Mice deficient in CB1 receptors are unsurprisingly resistant to the behavioral
effects of cannabinoids; they also appear to have reduced addiction to and withdrawal
from opiates. Further interaction has also been found between the opioid and
cannabinoid systems, as cannabinoids appear to increase the release of dopamine in the
nucleus accumbens, a key reward area of the brain implicated in addiction. This
dopamine release appears to require μ-opioid receptors, as pharmacological inhibition
of these receptors blocks the ability of cannabinoids to increase dopamine release. Rats
with a preference for alcohol have decreased FAAH activity, suggestive of greater
cannabinoid signaling. CB1 receptor antagonists dampen their alcohol consumption,
whereas inhibiting FAAH increases their alcohol consumption. Furthermore, CB1deficient animals also appear to have reduced alcohol intake. A single amino acid
mutation in human FAAH has been found to be associated with drug abuse, and this
abnormal enzyme appears to be less stable than its wild-type counterpart.
Endocannabinoids in Psychosis.
Heavy use of cannabis can produce psychotic
symptoms in individuals with no prior history of psychiatric disorder, although it is
unclear whether this is solely due to the drug or to an underlying vulnerability to
psychosis in such persons. Cannabis use often worsens psychosis in schizophrenia, and
heavy use has been associated with developing schizophrenia, although some suggest
that this association is an accelerated development of symptoms in those who would
eventually manifest schizophrenia. Nonetheless, the endocannabinoid system has
implications for the pathophysiology of schizophrenia, as cannabinoid signaling appears
to increase the release of dopamine. Medications that act as antagonists of D2 receptors
will likely remain a component of schizophrenia treatment for some time.
FEEDING. Following drug ingestion, THC users develop an increased appetite (“the
munchies”), and cannabis has been utilized as an appetite stimulant for centuries. This
effect may depend on CB1 receptors present in the hypothalamus. Endocannabinoid
levels increase in the hypothalamus and limbic system when animals are deprived of
food. Mice genetically deficient in CB1 receptors become resistant to developing obesity
after being given a high-fat diet. Similarly, the CB1 receptor antagonist, rimonabant,
appears to facilitate weight loss by blocking cannabinoid signaling. In a clinical trial of
more than 3,000 obese patients, those treated with 20 mg per day of rimonabant lost
6.3 kg at 1 year, compared to 1.6 kg in the placebo group. Nausea was the most
common side effect reported. A 2007 meta-analysis of clinical trials reported an overall
4.7 kg weight loss with rimonabant treatment, besting the weight-loss drugs orlistat
(Xenical; 2.9 kg) and sibutramine (Meridia; 4.2 kg).
Effects on Brain Injury and Pain.
In mouse models of traumatic brain injury, 2AG appears neuroprotective, reducing brain edema, infarct size, and cell death, while
improving functional outcomes. Anandamide also protected against brain injury in a
model of multiple sclerosis (MS), and human patients with the disease have increased
production of anandamide. A study of cannabinoid agonist, HU-211, led to more rapid
clinical improvement following head trauma. FAAH inhibitors improved motor
symptoms in a mouse model of Parkinson’s disease, likely via cannabinoids increasing
dopamine neurotransmission.
There is increasing evidence that neurotransmission via the endocannabinoid
pathway regulates pain perception. THC and cannabinoid agonists have proven
effective in animal models of acute and chronic pain, ranging from burn injury to nerve
damage and inflammation. The CB1 receptor plays an important role in these effects, as
the analgesic effects of cannabinoid drugs are lost when CB1 antagonist rimonabant is
given. Similarly, the analgesic effect of THC is lost in mice that are genetically deficient
in the CB1 receptor.
Stress has long been associated with diminished pain perception, such as in cases of
injured military personnel who demonstrate remarkable pain tolerance, a phenomenon
known as stress-induced analgesia. The endocannabinoid system may mediate these
effects. Animal models reveal anandamide and 2-AG production after stress, and stressinduced analgesia is blocked by the CB1 blocker, rimonabant, in these animals.
Endocannabinoid regulation of pain perception appears to be distinct from that of the
endogenous opiate system, but the two pathways may share overlapping neural
pathways. Evidence for this has been provided using CB1 blocker, rimonabant, and
naloxone (Narcan), which blocks opiate receptors. Rimonabant attenuates analgesia
provided by THC and cannabinoids, but only partly blocks the response to morphine.
However, the opposite is true for opiates: Naloxone blocks morphine-induced analgesia
but also partially blocks the analgesia of THC and cannabinoid drugs. Combinations of
cannabinoid and opiate drugs evince synergistic analgesic effects in animal models.
Although it was initially assumed that cannabinoids exert their analgesic effects via the CNS, in animal models it has
been shown that localized administration of cannabinoids may also be effective, including drugs selective for the CB2
receptor, whose expression is minimal in the CNS.
Endocannabinoids may also influence pain sensitivity by mechanisms that do not involve the CB1 and CB2 receptors.
Both anandamide and NADA can also activate a calcium channel known as the vanilloid receptor (also known as transient
receptor potential vanilloid type 1 [TRPV-1]) that is found on sensory nerves. This same receptor is also famous for being
activated by capsaicin, which causes the hot sensation after eating chili peppers. Thus endocannabinoids can exert
opposing functions: Promoting analgesia through the CB1 and CB2 receptors, but potentially increasing pain via TRP
channels. Although CB2 receptors are largely expressed in the periphery, postmortem analyses reveal an upregulation in
brain from those with Alzheimer’s disease.
The rapid development of novel cannabinoid drugs may allow for targeting of specific symptoms, rather than elicit all of
the typical effects of THC. For instance, ajulemic acid demonstrates analgesic and anti-inflammatory properties, but may
offer a benefit of limited psychoactive side effects. In a randomized clinical trial of this compound, Mathias Karst and
colleagues found efficacy in reducing chronic neuropathic pain.
Effects in the Periphery.
Cannabinoids lead to direct relaxation of vascular
smooth muscle by local CB1 receptors. This vasodilation extends to the conjunctiva,
leading to a “bloodshot” appearance in some cannabis users. Relaxation of ocular
arteries by cannabinoids may offer utility as a treatment for glaucoma, a condition of
high intraocular pressure, and activation of CB1 receptors in the kidney can improve
renal blood flow. A role in generalized blood pressure regulation is unproven, and blood
pressure is unaltered in persons treated with rimonabant or animals deficient in CB1
receptors. Cannabinoid signaling may also be relevant to ectopic pregnancy, as CB1deficient mice retain many embryos in the oviduct.
Nonpsychoactive Cannabinoids
Although THC is the principal psychoactive component of cannabis, the many
nonpsychoactive cannabinoids also have intriguing properties and may regulate
neurotransmission.
Cannabidiol may offer potential therapeutic effects and appears to stimulate TRPV-1
receptors and influence endocannabinoid degradation. In addition, cannabidiol
demonstrated a protective effect in a mouse model of inflammatory arthritis. Although
results have been mixed, purified cannabidiol may also exert antipsychotic activity,
although the net effect of plant cannabis use typically exacerbates schizophrenia
symptoms owing to THC. Tetrahydrocannabivarin is a plant cannabinoid that
antagonizes CB1 receptors. It is a candidate marker to distinguish whether a patient has
been using plant-derived cannabis or prescription THC, which contains no
tetrahydrocannabivarin.
Eicosanoids
Overview.
Clinical findings suggest that the dietary supplements omega-3 fatty
acids, eicosapentaenoic acid (EPA), its ester ethyl-eicosapentaenoic (E-EPA), and
docosahexaenoic acid (DHA), help relieve symptoms of depression, bipolar illness,
schizophrenia, and cognitive impairment. DHA and EPA may help reduce behavioral
outbursts and improve attention in children.
Chemistry.
Essential fatty acids are a group of polyunsaturated fats that contain a
carbon–carbon double bond in the third position from the methyl end group in the fatty
acid chain. They are essential because unlike monosaturated and saturated fatty acids,
polyunsaturated fatty acids cannot be synthesized de novo and can be acquired only
through diet from natural fats and oils. Linoleic acid (LA) is the parent compound of
omega-6 fatty acids, and α-linolenic acid (ALA) is the parent compound of omega-3
fatty acids. Both omega-3 and omega-6 groups use the same enzymes for desaturation
and chain elongation. Omega-3 fatty acids are synthesized by algae and plankton. Fish
such as herring, salmon, mackerel, and anchovy feed on these aquatic species and
become a rich dietary source of omega-3. EPA and DHA are highly unsaturated omega-3
fatty acids that contain 6 and 5 double bonds on their long structural chain,
respectively. They are positioned in the cell membrane by phospholipids and play a
crucial role in cell membrane signaling.
Effects on Specific Organs and Systems.
The strongest scientific evidence for
treatment with fatty acid supplements comes from the cardiovascular literature. Several
human trials have demonstrated that omega-3 fatty acids lower blood pressure, reduce
the rate of recurrent myocardial infarction, and lower triglyceride levels. In the nervous
system, fatty acids are essential components of neurons, immune cells, and glial
phospholipid membrane structures. They increase cerebral blood flow, decrease platelet
aggregation, and delay progression of atherosclerosis in the cardiovascular system.
Omega-6 fatty acids appear to reduce inflammation and neuronal apoptosis and
decrease phosphatidylinositol second messenger activity. Omega-3 fatty acids have been
suggested to alter gene expression.
In the CNS, fatty acids are selectively concentrated in neuronal membranes and
involved in cell membrane structure. Omega-6 arachidonic acid has been shown to
enhance glutamate neurotransmission, stimulate stress hormone secretion, and trigger
glial cell activation in the setting of oxidative toxicity and neurodegeneration. The
omega-3 fatty acids DHA and EPA appear to protect neurons from inflammatory and
oxidative toxicities. Increases in serotonin, enhancement of dopamine, and regulation of
CRF have been demonstrated in cell culture models.
In rodent models of depression, chronic EPA treatment normalized behavior in open
field tests. Serotonin and norepinephrine were also increased in the limbic regions. Mice
fed omega-3 poor diets had reduced memory, altered learning patterns, and more
behavioral problems.
Therapeutic Indications.
Clinical research with the use of fish oil for mood
disorders was based on epidemiology studies where there appears to be negative
correlation between fish consumption and depressive symptoms. Countries with lower
per capita fish consumption had up to 60 times increased rates of major depression,
bipolar disorder, and postpartum depression. Observational studies concluded that the
lower incidence of seasonal affective disorder in Iceland and Japan, rather than latitude
predicted, is related to the amount of fatty acid these populations consume in their diet.
A study in Norway showed that use of cod liver oil decreased depressive symptoms.
Depression after a myocardial infarction shows higher arachidonic acid to EPA ratio.
Postmortem studies in brains of patients diagnosed with major depressive disorder show
reduced DHA in the orbitofrontal cortex.
The first randomized, controlled pilot study of omega-3 fatty acids focused on
adjunctive treatment in both bipolar and unipolar patients with depression in addition
to their standard lithium (Eskalith) or valproic acid (Depakene) treatment. The omega-3
fatty acid group had significant improvement on the Hamilton Depression scale and a
longer period of remission than the placebo group. A subsequent larger study supported
a benefit from treatment with E-EPA for bipolar illness. However, a study of a group of
patients with either bipolar disorder or rapid cycling treated with E-EPA showed no
significant difference on any outcome measure between the EPA and placebo groups.
Bleeding time was also increased in the treatment group. There are no current data on
monotherapy in bipolar illness or depression.
The most convincing evidence comes from early brain development and learning
studies. Pregnant mothers who consumed foods rich in DHA gave birth to infants who
had improved problem-solving skills, but not necessarily improved memory. Visual
acuity and eye development are also associated with DHA supplementation during
pregnancy.
Reports of behavioral studies of prisoners in England who consumed higher amounts
of seafood containing omega-3 fatty acids showed a decrease in assault rates. A Finnish
study of violent criminals identified lower levels of omega-3 fatty acids in their system
compared to the nonviolent offenders.
The negative and psychotic symptoms of schizophrenia may be improved with
supplementation with omega-3 fatty acids. Antipsychotic medications like haloperidol
(Haldol) appear to have fewer extrapyramidal side effects when combined with
antioxidants and omega-3 fatty acids.
EPA and DHA have been associated with decreased dementia incidence. After
reviewing the Rotterdam study of a longitudinal cohort of more than 5,300 patients, fish
consumption appeared to be inversely related to development of new cases of dementia.
A later analysis of the study after 6 years demonstrated that low intake of omega-3 fatty
acids was not associated with increased risk of dementia. In contrast, the Zutphen study,
also in the Netherlands, concluded that high fish consumption was inversely related with
cognitive decline at 3-year follow-up and after 5 years. Well-designed clinical trials are
needed before omega-3 fatty acids can be recommended for prevention of cognitive
impairment.
Precautions and Adverse Reactions.
The most adverse complication of
eicosanoid use is increased risk for bleeding. Dietary sources can contain heavy metals,
and there is no standard preparation for capsule formulations. Treatment studies have
yielded a variety of different doses, but evidence for the therapeutic dose and clinical
guidelines are almost nonexistent. The length of treatment still needs to be determined.
Neurosteroids
Background.
Although steroids are critical for the maintenance of body
homeostasis, neurosteroids are synthesized from cholesterol in the brain and
independent of peripheral formation in the adrenals and gonads. Neurosteroids are
produced by a sequence of enzymatic processes governed by cytochrome P450 (CYP)
and non-CYP enzymes, either within or outside the mitochondria of several types of CNS
and peripheral nervous system (PNS) cells.
Recent work has shown that neurosteroids can operate through a nongenomic
pathway to regulate neuronal excitability through their effects on neurotransmittergated ion channels. Receptors are generally located in the nucleus, membrane, or
microtubules of the CNS and PNS. Although steroids and neurosteroids can act on the
same nuclear receptors, neurosteroids differ from steroids in their topological
distribution and regional synthesis. The most well-known effect of neurosteroids is on
the GABA receptor, particularly the GABAA receptor. Neurosteroids acting primarily at
this site include allopregnanolone (3α,5α-tetrahydroprogesterone), pregnanolone
(PREG), and tetrahydrodeoxycorticosterone (THDOC). Dehydroepiandrosterone sulfate
(DHEA-S), the most prevalent neurosteroid, acts as a noncompetitive modulator of
GABA, and its precursor dehydroepiandrosterone (DHEA) has also been shown to exert
inhibitory effects at the GABA receptor. Some neurosteroids may also act at the NMDA,
α-amino-3-hydroxy-5-methyl-4-isoxazole-propanoic acid (AMPA), kainate, glycine,
serotonin, sigma type-1, and nicotinic acetylcholine receptors. Progesterone is also
considered a neurosteroid and has the ability to regulate gene expression at
progesterone receptors.
Neurosteroids in Neurodevelopment and Neuroprotection.
In general,
neurosteroids stimulate axonal growth and promote synaptic transmission. Specific
neuroprotective effects are unique to each neurosteroid. DHEA acts to regulate brain
serotonin and dopamine levels, suppress cortisol, increase hippocampal primed burst
potentiation and cholinergic function, decrease amyloid-β protein, inhibit the
production of proinflammatory cytokines, and prevent free radical scavenging. DHEA
and DHEA-S have both been shown to have a role in glial development and neuronal
growth and to promote their survival in animals; the injection of these substances into
the brains of mice promoted long-term memory while reversing amnesia. Progesterone
is linked to myelinating processes like aiding in the repair of damaged neural
myelination (Color Plate 1.4-16). Allopregnanolone contributes to the reduction of
contacts during axonal regression.
Role of Neurosteroids in Mental Illness.
Neurosteroids have distinct
implications for the maintenance of normal neurologic function and also may contribute
to neuropathology. Neurosteroids are differentially regulated in males and females and
may affect the manifestation of psychological disorders in these two populations.
Specifically, they play a distinct role in depression and anxiety disorders and may be
targeted by psychiatric medications in the near future.
DEPRESSION. When compared with nondepressed controls, studies show that depressed
patients have lower plasma and CSF concentrations of allopregnanolone. In addition,
this research has elucidated an inverse relationship between allopregnanolone
concentrations and severity of depressive illness. However, there are no
allopregnanolone-based therapies available for humans, so its direct efficacy is
unsubstantiated. Antidepressant drugs, specifically fluoxetine (Prozac), have been
shown in multiple studies to increase the levels of certain neurosteroids. Nonetheless,
there is debate over the therapeutic properties of neurosteroids, prompting the
investigation of neurosteroid concentrations in patients undergoing nonpharmacological
therapies. Preliminary results indicate that the lack of modifications in neurosteroid
levels during nonpharmacological treatments supports the validity of the
pharmacological properties of antidepressants, not their therapeutic action, in the
elevation of neurosteroid levels in medicated populations.
ANXIETY DISORDERS. In patients with anxiety disorders, the major mechanism of action is
on the GABA receptor. Homeostasis characterized by normal GABAergic activity is
restored after panic attacks as neurosteroids are released in response to stress.
Allopregnanolone stimulates GABAergic activity with 20 times the strength of
benzodiazepines and 200 times the potency of barbiturates. Both positive and negative
regulation of the GABAA receptor are correlated with anxiolytic and anxiogenic action,
respectively.
PSYCHOTIC DISORDERS. In addition to their primary relevance to the pharmacological
treatment of mood and anxiety disorders, neurosteroids contribute to psychotic,
childhood, substance abuse, eating, and postpartum disorders. The effect of
neurosteroids on psychotic disorders such as schizophrenia is mediated by DHEA and
DHEA-S. DHEA has been dispensed to decrease anxiety in patients with schizophrenia,
as DHEA and DHEA-S suppress GABA inhibition and heighten the neuronal response at
the NMDA and sigma receptors. DHEA and DHEA-S levels are typically elevated in the
initial episode of a patient with schizophrenia, indicating neurosteroids are upregulated
by the onset of psychosis. Because neurosteroid levels are studied across various illness
stages, some questions still exist regarding the role of neurosteroids in psychosis.
CHILDHOOD MENTAL ILLNESS. In children, the clinical symptomology of ADHD is inversely
correlated with DHEA and pregnenolone levels.
SUBSTANCE ABUSE. Alcohol is theorized to regulate the GABA receptor and induce de novo
steroid synthesis in the brain; specifically, pregnenolone, allopregnanolone, and
allotetrahydrodeoxycorticosterone levels are increased in the brain and periphery in
response to increases in peripheral alcohol levels. It is hypothesized that sharp increases
in ethanol concentration may mimic the acute stress response and elevate neurosteroid
concentrations by the HPA axis. To prevent ethanol dependence, researchers are
investigating fluctuations in neurosteroid levels and in vivo neurosteroid
responsiveness. Neurosteroids (increased allopregnanolone levels in particular) are
associated with drug abuse. However, DHEA-S may actually check the acquisition of
morphine tolerance. Past research has shown that DHEA-S levels were also increased in
patients who abstained from cocaine use in a treatment program, and as patients
relapsed DHEA-S concentrations decreased accordingly.
EATING DISORDERS. With regard to eating disorders, DHEA has been shown to diminish
food intake, temper obesity, moderate insulin resistance, and lower lipids in rats with a
model of youth-onset, hyperphagic, and genetic obesity. By regulating the serotonergic
system, DHEA is hypothesized to promote a reduced caloric load. Although hypothetical,
low levels of DHEA and DHEA-S are recorded in young women with anorexia nervosa,
and 3 months of oral DHEA supplementation increased bone density and tempered the
emotional problems associated with the disorder.
POSTPARTUM AND GYNECOLOGICAL DISORDERS. Because estrogen and progesterone levels
fluctuate during the course of pregnancy and drop markedly after delivery,
neurosteroids are thought to contribute to postpartum disorders. Low postpartum DHEA
concentrations have been linked to mood instability. In addition, allopregnanolone
levels correlated with mood disorders during pregnancy and in premenstrual syndrome
(PMS). It has been noted that women with premenstrual dysphoric disorder have higher
allopregnanolone/progesterone ratios than normal controls; women treated for this
disorder reported improvement as allopregnanolone levels decreased.
NEUROSTEROIDS, MEMORY DISORDERS, AND AGING. Neurosteroid levels may be irregular in
neurodegenerative disorders and aging conditions such as Alzheimer’s disease and
Parkinson’s disease. DHEA levels at age 70 are only about 20 percent of their maximum
value recorded in the late 20s, and some researchers believe DHEA supplementation can
prevent or slow the cognitive declines associated with the aging process. However,
conflicting studies have indicated that DHEA administration does not improve cognitive
measures in patients. In addition, in patients with Alzheimer’s disease, DHEA
concentrations have been found to be markedly decreased.
REFERENCES
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Neurobiology of Mental Illness. 3rd ed. New York: Oxford University Press; 2009:321.
Berger M, Honig G, Wade JM, Tecott LH. Monoamine neurotransmitters. In: Sadock BJ, Sadock VA, Ruiz P, eds. Kaplan &
Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009.
Butler JS, Foxe JJ, Fiebelkorn IC, Mercier MR, Molholm S. Multisensory representation of frequency across audition and
touch: High density electrical mapping reveals early sensory-perceptual coupling. J Neurosci. 2012;32:15338.
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05 - 1.5 Psychoneuroendocrinology
1.5 Psychoneuroendocrinology
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mediated depression. Complement Ther Clin Pract. 2014;20:74–80.
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Psychiatry. 2011;168:634.
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Autism. New York: Oxford University Press; 2011:77.
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Drug Targets. 2007;8:651.
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eds. Neurofeedback and Neuromodulation Techniques and Applications. New York: Academic Press; 2011:319.
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Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009.
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BJ, Sadock VA, Ruiz P, eds. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott
Williams & Wilkins; 2009.
1.5 Psychoneuroendocrinology
The term psychoneuroendocrinology encompasses the structural and functional
relationships between hormonal systems and the central nervous system (CNS) and
behaviors that modulate and are derived from both. Classically, hormones have been
defined as the products of endocrine glands transported by the blood to exert their
action at sites distant from their release. Advances in neuroscience have shown,
however, that in the CNS the brain not only serves as a target site for regulatory control
of hormonal release but also has secretory functions of its own and serves as an end
organ for some hormonal actions. These complex interrelationships make classic
distinctions between the origin, structure, and function of neurons and those of
endocrine cells dependent of physiological context.
HORMONE SECRETION
Hormone secretion is stimulated by the action of a neuronal secretory product of
neuroendocrine transducer cells of the hypothalamus. Examples of hormone regulators
(Table 1.5-1) include corticotropin-releasing hormone (CRH), which stimulates
adrenocorticotropin (adrenocorticotropic hormone [ACTH]); thyrotropin-releasing
hormone (TRH), which stimulates release of thyroid-stimulating hormone (TSH);
gonadotropin-releasing hormone (GnRH), which stimulates release of luteinizing
hormone (LH) and follicle-stimulating hormone (FSH); and somatostatin (somatotropin
release-inhibiting factor [SRIF]) and growth-hormone-releasing hormone (GHRH), which
influence growth hormone (GH) release. Chemical signals cause the release of these
neurohormones from the median eminence of the hypothalamus into the portal
hypophyseal bloodstream and subsequent transport to the pituitary to regulate the
release of target hormones. Pituitary hormones in turn act directly on target cells (e.g.,
ACTH on the adrenal gland) or stimulate the release of other hormones from peripheral
endocrine organs. In addition, these hormones have feedback actions that regulate
secretion and exert neuromodulatory effects in the CNS.
Table 1.5-1
Examples of Regulating Hormones
Hormones are divided into two general classes: (1) proteins, polypeptides, and
glycoproteins, and (2) steroids and steroid-like compounds (Table 1.5-2); these are
secreted by an endocrine gland into the bloodstream and are transported to their sites of
action.
Table 1.5-2
Classifications of Hormones
DEVELOPMENTAL PSYCHONEUROENDOCRINOLOGY
Hormones can have both organizational and activational effects. Exposure to gonadal
hormones during critical stages of neural development directs changes in brain
morphology and function (e.g., sex-specific behavior in adulthood). Similarly, thyroid
hormones are essential for the normal development of the CNS, and thyroid deficiency
during critical stages of postnatal life will severely impair growth and development of
the brain, resulting in behavioral disturbances that may be permanent if replacement
therapy is not instituted.
ENDOCRINE ASSESSMENT
Neuroendocrine function can be studied by assessing baseline measures and by
measuring the response of the axis to some neurochemical or hormonal challenge. The
first method has two approaches. One approach is to measure a single time point—for
example, morning levels of growth hormone; this approach is subject to significant error
because of the pulsatile nature of the release of most hormones. The second approach is
to collect blood samples at multiple points or to collect 24-hour urine samples; these
measurements are less susceptible to major errors. The best approach, however, is to
perform a neuroendocrine challenge test, in which the person is given a drug or a
hormone that perturbs the endocrine axis in some standard way. Persons with no
disease show much less variation in their responses to such challenge studies than in
their baseline measurements.
HYPOTHALAMIC–PITUITARY–ADRENAL AXIS
Since the earliest conceptions of the stress response, by Hans Selye and others,
investigation of hypothalamic–pituitary–adrenal (HPA) function has occupied a central
position in psychoendocrine research. CRH, ACTH, and cortisol levels all rise in
response to a variety of physical and psychic stresses and serve as prime factors in
maintaining homeostasis and developing adaptive responses to novel or challenging
stimuli. The hormonal response depends both on the characteristics of the stressor itself
and on how the individual assesses and is able to cope with it. Aside from generalized
effects on arousal, distinct effects on sensory processing, stimulus habituation and
sensitization, pain, sleep, and memory storage and retrieval have been documented. In
primates, social status can influence adrenocortical profiles and, in turn, be affected by
exogenously induced changes in hormone concentration.
Pathological alterations in HPA function have been associated primarily with mood
disorders, posttraumatic stress disorder (PTSD), and dementia of the Alzheimer’s type,
although recent animal evidence points toward a role of this system in substance use
disorders as well. Disturbances of mood are found in more than 50 percent of patients
with Cushing’s syndrome (characterized by elevated cortisol concentrations), with
psychosis or suicidal thought apparent in more than 10 percent of patients studied.
Cognitive impairments similar to those seen in major depressive disorder (principally in
visual memory and higher cortical functions) are common and relate to the severity of
the hypercortisolemia and possible reduction in hippocampal size. In general, reduced
cortisol levels normalize mood and mental status. Conversely, in Addison’s disease
(characterized by adrenal insufficiency), apathy, social withdrawal, impaired sleep, and
decreased concentration frequently accompany prominent fatigue. Replacement of
glucocorticoid (but not of electrolyte) resolves behavioral symptomatology. Similarly,
HPA abnormalities are reversed in persons who are treated successfully with
antidepressant medications. Failure to normalize HPA abnormalities is a poor
prognostic sign. Alterations in HPA function associated with depression include elevated
cortisol concentrations, failure to suppress cortisol in response to dexamethasone,
increased adrenal size and sensitivity to ACTH, a blunted ACTH response to CRH, and,
possibly, elevated CRH concentrations in the brain.
HYPOTHALAMIC–PITUITARY–GONADAL AXIS
The gonadal hormones (progesterone, androstenedione, testosterone, estradiol, and
others) are steroids that are secreted principally by the ovary and testes, but significant
amounts of androgens arise from the adrenal cortex as well. The prostate gland and
adipose tissue, also involved in the synthesis and storage of dihydrotestosterone,
contribute to individual variance in sexual function and behavior.
The timing and presence of gonadal hormones play a critical role in the development
of sexual dimorphisms in the brain. Developmentally, these hormones direct the
organization of many sexually dimorphic CNS structures and functions, such as the size
of the hypothalamic nuclei and corpus callosum, neuronal density in the temporal
cortex, the organization of language ability, and responsivity in Broca’s motor speech
area. Women with congenital adrenal hyperplasia—a deficiency of the enzyme 21hydroxylase, which leads to high exposure to adrenal androgens in prenatal and
postnatal life, in some studies—have been found to be more aggressive and assertive
and less interested in traditional female roles than control female subjects. Sexual
dimorphisms may also reflect acute and reversible actions of relative steroid
concentrations (e.g., higher estrogen levels transiently increase CNS sensitivity to
serotonin).
Testosterone
Testosterone is the primary androgenic steroid, with both androgenic (i.e., facilitating
linear body growth) and somatic growth functions. Testosterone is associated with
increased violence and aggression in animals and in correlation studies in humans, but
anecdotal reports of increased aggression with testosterone treatment have not been
substantiated in investigations in humans. In hypogonadal men, testosterone improves
mood and decreases irritability. Varying effects of anabolic–androgenic steroids on
mood have been noted anecdotally. A prospective, placebo-controlled study of anabolic–
androgenic steroid administration in normal subjects reported positive mood symptoms,
including euphoria, increased energy, and sexual arousal, in addition to increases in the
negative mood symptoms of irritability, mood swings, violent feelings, anger, and
hostility.
Testosterone is important for sexual desire in both men and women. In males, muscle
mass and strength, sexual activity, desire, thoughts, and intensity of sexual feelings
depend on normal testosterone levels, but these functions are not clearly augmented by
supplemental testosterone in those with normal androgen levels. Adding small amounts
of testosterone to normal hormonal replacement in postmenopausal women has proved,
however, to be as beneficial as its use in hypogonadal men.
Dehydroepiandrosterone
DHEA and DHEA sulfate (DHEA-S) are adrenal androgens secreted in response to ACTH
and represent the most abundant circulating steroids. DHEA is also a neurosteroid that is
synthesized in situ in the brain. DHEA has many physiological effects, including
reduction in neuronal damage from glucocorticoid excess and oxidative stress.
Behavioral interest has centered on its possible involvement in memory, mood, and a
number of psychiatric disorders. Adrenarche is the prepubertal onset of adrenal
production of DHEA-S and may play a role in human maturation through increasing the
activity of the amygdala and hippocampus and promoting synaptogenesis in the
cerebral cortex. DHEA has been shown to act as an excitatory neurosteroid and to
enhance memory retention in mice, but studies of DHEA administration to humans have
not consistently shown any improvement in cognition. Several trials of DHEA
administration point to an improvement in well-being, mood, energy, libido, and
functional status in depressed individuals. Administration of DHEA to women with
adrenal insufficiency (e.g., Addison’s disease) has repeatedly been demonstrated to
enhance mood, energy, and sexual function; effects in men remain to be assessed. Mood,
fatigue, and libido improved in human immunodeficiency virus (HIV)–positive patients
treated with DHEA in one study, and DHEA and DHEA-S have been found to be
inversely correlated with severity in attention-deficit/hyperactivity disorder (ADHD).
Women diagnosed with fibromyalgia have significantly decreased DHEA-S levels, but
supplementation does not improve outcome. Several cases of possible DHEA-induced
mania have been reported, and DHEA has been reported to be inversely related to
extrapyramidal symptoms (EPS) in patients with schizophrenia who are treated with
antipsychotics. DHEA administration in these cases improves EPS.
Double-blind treatment studies have shown antidepressant effects of DHEA in patients
with major depression, midlife-onset dysthymia, and schizophrenia, although beneficial
effects on memory have not been reliably demonstrated. A small, double-blind trial of
DHEA treatment of Alzheimer’s disease failed to reveal significant benefit, although a
near-significant improvement in cognitive function was seen after 3 months of
treatment.
Animal studies suggest that DHEA may be involved in eating behavior, aggressiveness, and anxiety as well, with its
effects resulting from its transformation into estrogen, testosterone, or androsterone from its antiglucocorticoid activity, or
from direct effects on GABAA, N-methyl-D-aspartate (NMDA), and σ receptors. Because of the putative antiglucocorticoid
effects, the ratio of cortisol to DHEA levels may be particularly important in understanding adaptive responses to stress.
Both cortisol and DHEA appear to be involved in fear conditioning, with the cortisol/DHEA ratio hypothesized to be an
index of the degree to which an individual is buffered against the negative effects of stress. This ratio has been found to be
related to some measures of psychopathology and response to treatment, predicting the persistence of the first episode
major depression and being related to degree of depression, anxiety, and hostility in patients with schizophrenia and
response to antipsychotic treatment. Patients with PTSD have higher DHEA levels and lower cortisol/DHEA ratios related
to symptom severity, suggesting a role in PTSD recovery. Fear-potentiated startle is larger in individuals with high as
compared to low cortisol/DHEA-S ratios and is positively associated with cortisol and negatively with DHEA-S. Greater
DHEA response to ACTH is related to lower PTSD ratings, and the cortisol/DHEA ratio to negative mood symptoms. A
genetic variation in an ACTH receptor promoter has been found to influence DHEA secretion in response to dexamethasone
and may underlie some individual differences in stress response.
Estrogen and Progesterone
Estrogens can influence neural activity in the hypothalamus and limbic system directly
through modulation of neuronal excitability, and they have complex multiphasic effects
on nigrostriatal dopamine receptor sensitivity. Accordingly, evidence indicates that the
antipsychotic effect of psychiatric drugs can change over the menstrual cycle and that
the risk of tardive dyskinesia depends partly on estrogen concentrations. Several studies
have suggested that gonadal steroids modulate spatial cognition and verbal memory and
are involved in impeding age-related neuronal degeneration. Increasing evidence also
suggests that estrogen administration decreases the risk and severity of dementia of the
Alzheimer’s type in postmenopausal women. Estrogen has mood-enhancing properties
and can also increase sensitivity to serotonin, possibly by inhibiting monoamine
oxidase. In animal studies, long-term estrogen treatment results in a decrease in
serotonin 5-HT1 receptors and an increase in 5-HT2 receptors. In oophorectomized
women, significant reductions in tritiated imipramine binding sites (which indirectly
measures presynaptic serotonin uptake) were restored with estrogen treatment.
The association of these hormones with serotonin is hypothetically relevant to mood
change in premenstrual and postpartum mood disturbances. In premenstrual dysphoric
disorder, a constellation of symptoms resembling major depressive disorder occurs in
most menstrual cycles, appearing in the luteal phase and disappearing within a few
days of the onset of menses. No definitive abnormalities in estrogen or progesterone
levels have been demonstrated in women with premenstrual dysphoric disorder, but
decreased serotonin uptake with premenstrual reductions in steroid levels has been
correlated with the severity of some symptoms.
Most psychological symptoms associated with the menopause are actually reported during perimenopause rather than
after complete cessation of menses. Although studies suggest no increased incidence of major depressive disorder, reported
symptoms include worry, fatigue, crying spells, mood swings, diminished ability to cope, and diminished libido or
intensity of orgasm. Hormone replacement therapy (HRT) is effective in preventing osteoporosis and reinstating energy, a
sense of well-being, and libido; however, its use is extremely controversial. Studies have shown that combined estrogen–
progestin drugs (e.g., Premarin) cause small increases in breast cancer, heart attack, stroke, and blood clots among
menopausal women. Studies of the effects of estrogen alone in women who have had hysterectomies (because estrogen
alone increases the risk for uterine cancer) are ongoing.
HYPOTHALAMIC–PITUITARY–THYROID AXIS
Thyroid hormones are involved in the regulation of nearly every organ system,
particularly those integral to the metabolism of food and the regulation of temperature,
and are responsible for optimal development and function of all body tissues. In
addition to its prime endocrine function, TRH has direct effects on neuronal excitability,
behavior, and neurotransmitter regulation.
Thyroid disorders can induce virtually any psychiatric symptom or syndrome,
although no consistent associations of specific syndromes and thyroid conditions are
found. Hyperthyroidism is commonly associated with fatigue, irritability, insomnia,
anxiety, restlessness, weight loss, and emotional lability; marked impairment in
concentration and memory may also be evident. Such states can progress into delirium
or mania or they can be episodic. On occasion, a true psychosis develops, with paranoia
as a particularly common presenting feature. In some cases, psychomotor retardation,
apathy, and withdrawal are the presenting features rather than agitation and anxiety.
Symptoms of mania have also been reported following rapid normalization of thyroid
status in hypothyroid individuals and may covary with thyroid level in individuals with
episodic endocrine dysfunction. In general, behavioral abnormalities resolve with
normalization of thyroid function and respond symptomatically to traditional
psychopharmacological regimens.
The psychiatric symptoms of chronic hypothyroidism are generally well recognized
(Fig. 1.5-1). Classically, fatigue, decreased libido, memory impairment, and irritability
are noted, but a true secondary psychotic disorder or dementia-like state can also
develop. Suicidal ideation is common, and the lethality of actual attempts is profound.
In milder, subclinical states of hypothyroidism, the absence of gross signs accompanying
endocrine dysfunction can result in its being overlooked as a possible cause of a mental
disorder.
FIGURE 1.5-1
Hands of a patient with hypothyroidism (myxedema), illustrating the swelling of the
soft parts, the broadening of the fingers, and their consequent stumpy or pudgy
appearance. (Reprint from Douthwaite AH, ed. French’s Index of Differential Diagnosis. 7th
ed. Baltimore: Williams & Wilkins; 1954, with permission.)
GROWTH HORMONE
Growth hormone deficiencies interfere with growth and delay the onset of puberty. Low
GH levels can result from a stressful experience. Administration of GH to individuals
with GH deficiency benefits cognitive function in addition to its more obvious somatic
effects, but evidence indicates poor psychosocial adaptation in adulthood for children
who were treated for GH deficiency. A significant percentage of patients with major
depressive disorder and dysthymic disorder may have a GH deficiency. Some
prepubertal and adult patients with diagnoses of major depressive disorder exhibit
hyposecretion of GHRH during an insulin tolerance test, a deficit that has been
interpreted as reflecting alterations in both cholinergic and serotonergic mechanisms. A
number of GH abnormalities have been noted in patients with anorexia nervosa.
Secondary factors, such as weight loss, however, in both major depressive disorder and
eating disorders, may be responsible for alterations in endocrine release. Nonetheless, at
least one study has reported that GHRH stimulates food consumption in patients with
anorexia nervosa and lowers food consumption in patients with bulimia. Administration
of GH to elderly men increases lean body mass and improves vigor. GH is released in
pulses throughout the day, but the pulses are closer together during the first hours of
sleep than at other times.
PROLACTIN
Since its identification in 1970, the anterior pituitary hormone prolactin has been
examined as a potential index of dopamine activity, dopamine receptor sensitivity, and
antipsychotic drug concentration in studies of CNS function in psychiatric patients and
as a correlate of stress responsivity. The secretion of prolactin is under direct inhibitory
regulation by dopamine neurons located in the tuberoinfundibular section of the
hypothalamus and is, therefore, increased by classical antipsychotic medications.
Prolactin also inhibits its own secretion by means of a short-loop feedback circuit to the
hypothalamus. In addition, a great number of prolactin-releasing or prolactin-modifying
factors have been identified, including estrogen, serotonin (particularly through the 5HT2 and 5-HT3 receptors), norepinephrine, opioids, TRH, T4, histamine, glutamate,
cortisol, CRH, and oxytocin, with interaction effects possible. For example, estrogen
may promote the serotonin-stimulated release of prolactin.
Prolactin is primarily involved in reproductive functions. During maturation,
prolactin secretion participates in gonadal development, whereas, in adults, prolactin
contributes to the regulation of the behavioral aspects of reproduction and infant care,
including estrogen-dependent sexual receptivity and breast-feeding. In female rats,
prolactin secretion is strongly stimulated with exposure to pups. In women, basal
prolactin levels are elevated in the postpartum period before weaning, and prolactin
release is stimulated by suckling. Hyperprolactinemia is associated with low testosterone
in men and reduced libido in men and women. In rodents, prolactin level is increased
along with corticosterone in response to such stressful stimuli as immobilization,
hypoglycemia, surgery, and cold exposure and may be specifically associated with the
use of passive coping in the face of a stressor. Prolactin promotes various stress-related
behaviors in rats, depending on the condition, such as increasing object-directed
exploration while decreasing other exploration.
Patients with hyperprolactinemia often complain of depression, decreased libido,
stress intolerance, anxiety, and increased irritability. These behavioral symptoms
usually resolve in parallel with decrements in serum prolactin when surgical or
pharmacological treatments are used. In psychotic patients, prolactin concentrations
and prolactin-related sexual disturbances have been positively correlated with the
severity of tardive dyskinesia. Prolactin levels are also positively correlated with
negative symptoms in schizophrenia.
MELATONIN
Melatonin, a pineal hormone, is derived from the serotonin molecule and it controls
photoperiodically mediated endocrine events (particularly those of the hypothalamic–
pituitary–gonadal axis). It also modulates immune function, mood, and reproductive
performance and is a potent antioxidant and free-radical scavenger. Melatonin has a
depressive effect on CNS excitability, is an analgesic, and has seizure-inhibiting effects
in animal studies. Melatonin can be a useful therapeutic agent in the treatment of
circadian phase disorders such as jet lag. Intake of melatonin increases the speed of
falling asleep, as well as its duration and quality.
OXYTOCIN
Oxytocin, also a posterior pituitary hormone, is involved in osmoregulation, the milk
ejection reflex, food intake, and female maternal and sexual behaviors. Oxytocin is
theorized to be released during orgasm, more so in women than in men, and is
presumed to promote bonding between the sexes. It has been used in autistic children
experimentally in an attempt to increase socialization.
INSULIN
Increasing evidence indicates that insulin may be integrally involved in learning and
memory. Insulin receptors occur in high density in the hippocampus and are thought to
help neurons metabolize glucose. Patients with Alzheimer’s disease have lower insulin
concentrations in the cerebrospinal fluid (CSF) than controls, and both insulin and
glucose dramatically improve verbal memory. Depression is frequent in patients with
diabetes, as are indexes of impaired hormonal response to stress. It is not known
whether these findings represent direct effects of the disease or are secondary,
nonspecific effects. Some antipsychotics are known to dysregulate insulin metabolism.
ENDOCRINE VARIABLES IN PSYCHIATRIC DISORDERS
Although it is clear that alterations in endocrine regulation are involved in the
pathophysiology and treatment responses of many psychiatric disorders, incorporating
these findings into clinical diagnostic assessment and decision-making remains
problematic. Large-scale longitudinal or cost-effectiveness studies are rare, despite
indications that baseline alterations in glucocorticoid regulation and thyroid status (two
of the best studied abnormalities) may actually be useful in subtyping psychiatric
disorders and in prediction of outcome. Alterations in HPA/stress regulation underlie a
number of psychiatric diagnoses and may serve as complementary independent
variables in assigning treatment response and course of illness to the classical
behavioral categories that have thus far defined psychiatric practice. Studying genetic
polymorphisms in factors regulating hormonal response may help us better understand
the influence of hormonal variability on the illness and also possible underlying
differences in the nature of the illness reflected in these genetic subtypes.

06 - 1.6 Immune System and Central Nervous System
1.6 Immune System and Central Nervous System Interactions
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1.6 Immune System and Central Nervous System
Interactions
Interactions between the immune system and the central nervous system (CNS) play a
critical role in the maintenance of bodily homeostasis and the development of diseases,
including psychiatric disease. Alterations in CNS function brought about by a variety of
stressors have been shown to influence both the immune system as well as diseases that
involve the immune system. Moreover, many of the relevant hormonal and
neurotransmitter pathways that mediate these effects have been elucidated. Of
considerable interest is accumulating data that cytokines, which derive from immune
cells and microglia, have profound effects on the CNS. The relative role of cytokines and
their signaling pathways in the various psychiatric diseases is an area of active
investigation, as is the role of infectious and autoimmune diseases in the
pathophysiology of psychiatric disorders. Taken together, these findings highlight the
importance of interdisciplinary efforts involving the neurosciences and immunology for
gaining new insights into the etiology of psychiatric disorders.
OVERVIEW OF THE IMMUNE SYSTEM
The immune system has the capacity to protect the body from the invasion of foreign
pathogens, such as viruses, bacteria, fungi, and parasites. In addition, the immune
system can detect and eliminate cells that have become neoplastically transformed.
These functions are accomplished through highly specific receptors on immune cells for
molecules derived from invading organisms and a rich intercellular communication
network that involves direct cell-to-cell interactions and signaling between cells of the
immune system by soluble factors called cytokines. The body’s absolute dependence on
the efficient functioning of the immune system is illustrated by the less than 1-year
survival rate of untreated infants born with severe combined immunodeficiency disease
and the devastating opportunistic infections and cancers that arise during untreated
acquired immunodeficiency syndrome (AIDS).
BEHAVIORAL CONDITIONING
The fact that learning processes are capable of influencing immunological function is an
example of interactions between the immune system and the nervous system. Several
classical conditioning paradigms have been associated with suppression or enhancement
of the immune response in various experimental designs. The conditioning of
immunological reactivity provides further evidence that the CNS can have significant
immunomodulatory effects.
Some of the first evidence for immunological conditioning was derived from the serendipitous observation that animals
undergoing extinction in a taste-aversion paradigm with cyclophosphamide, an immunosuppressive agent, had
unexpected mortality. In that taste-aversion paradigm, animals were simultaneously exposed to an oral saccharin solution
(the conditioned stimulus) and an intraperitoneal injection of cyclophosphamide (unconditioned stimulus). Because the
animals experienced considerable physical discomfort from the cyclophosphamide injection, through the process of
conditioning they began to associate the ill effects of cyclophosphamide with the taste of the oral saccharin solution. If
given a choice, the animals avoided the saccharin solution (taste aversion). Conditioned avoidance can be eliminated or
extinguished if the saccharin is repeatedly presented in the absence of cyclophosphamide. However, it was observed that
animals undergoing extinction of cyclophosphamide-induced taste aversion unexpectedly died, leading to the speculation
that the oral saccharin solution had a specific conditioned association with the immunosuppressive effects of
cyclophosphamide. Repeated exposure to the saccharin-associated conditioned immunosuppression during extinction
might explain the unexpected death of animals. To test that hypothesis researchers conditioned the animals with saccharin
(conditioned stimulus) and intraperitoneal cyclophosphamide (unconditioned conditioned stimulus) and then immunized
them with sheep red blood cells. At different times after immunization the conditioned animals were re-exposed to
saccharin (conditioned stimulus) and examined. The conditioned animals exhibited a significant decrease in mean antibody
titers to sheep red blood cells when compared to the control animals. Thus, the evidence demonstrated that
immunosuppression of humoral immunity was occurring in response to the conditioned stimulus of saccharin alone.
STRESS AND THE IMMUNE RESPONSE
Interest in the effects of stress on the immune system grew out of a series of animal and
human studies suggesting that stressful stimuli can influence the development of
immune-related disorders, including infectious diseases, cancer, and autoimmune
disorders. Although stress has been historically associated with suppression of immune
function, recent data indicate that such a conclusion oversimplifies the complexities of
the mammalian immune response to environmental perturbation and that stress may
also activate certain aspects of the immune system, particularly the innate immune
response.
Stress and Illness
Experiments conducted on laboratory animals in the late 1950s and the early 1960s indicated that a wide variety of
stressors—including isolation, rotation, crowding, exposure to a predator, and electric shock—increased morbidity and
mortality in response to several types of tumors and infectious diseases caused by viruses and parasites. However, as
research progressed it became increasingly clear that “stress” is too variegated a concept to have singular effects on
immunity and that, in fact, the effects of stress on immunity depend on a number of factors. Chief among these factors is
whether a stressor is acute or chronic. Other critical variables include stressor severity and type, as well as the timing of
stressor application and the type of tumor or infectious agent investigated. For example, mice subjected to electric grid
shock 1 to 3 days before the infection of Maloney murine sarcoma virus-induced tumor cells exhibited decreased tumor
size and incidence. In contrast, mice exposed to grid shock 2 days after tumor cell injection exhibited an increase in tumor
size and number.
The relevance of the effects of stress on immune-related health outcomes in humans
has been demonstrated in studies that have shown an association between chronic stress
and increased susceptibility to the common cold, reduced antibody responses to
vaccination, and delayed wound healing. In addition, stress, as well as depression,
through their effects on inflammation have been linked to increased morbidity and
mortality in infectious diseases, such as HIV infection, autoimmune disorders, neoplastic
diseases, as well as diabetes and cardiovascular disorders, which are increasingly being
recognized as diseases in which the immune system, inflammation in particular, plays a
pivotal role (Fig. 1.6-1).
FIGURE 1.6-1
Inflammation and disease. IL, interleukin; TNF, tumor necrosis factor; NF-κB, nuclear
factor κB; CRP, C-reactive protein. (From Cowles MK, Miller AH. Stress cytokines and
depressive illness. In: Squire LR, ed. The New Encyclopedia of Neuroscience. Academic
Press; 2009:521, with permission.)
Effects of Chronic Stress
When challenged with a medical illness or chronic psychological stressor, complex
interactions between the immune and nervous systems promote a constellation of
immune-induced behavioral changes, alternatively referred to as “sickness syndrome” or
“sickness behavior.” These behavioral changes include dysphoria, anhedonia, fatigue,
social withdrawal, hyperalgesia, anorexia, altered sleep–wake patterns, and cognitive
dysfunction. Although seen in response to infection, the full syndrome can be
reproduced in humans and laboratory animals by administration of innate immune
cytokines. Blocking cytokine activity diminishes or prevents the development of sickness
behavior in laboratory animals, even when such behavior develops as a result of
psychological stress. Evidence that cytokine-induced behavioral toxicity is related to
major depression comes in part from studies showing that in humans and laboratory
animals, antidepressants are able to abolish or attenuate the development of sickness
behavior in response to cytokine administration.
RELEVANCE OF IMMUNE SYSTEM–CNS INTERACTIONS TO PSYCHIATRIC
DISORDERS
Major Depression
The neuropsychiatric disorder that has been best characterized in terms of the influence
of the brain on the immune system and vice versa is major depression. For many years
major depression was seen as the quintessential example of how stress-related disorders
may decrease immunocompetence. More recently, however, it has become evident that
stress also activates inflammatory pathways, even while suppressing measures of
acquired immunity. Not surprisingly, studies now indicate that, in addition to
immunosuppression, major depression is also frequently associated with inflammatory
activation. Recent research showing that proinflammatory cytokines are capable of
suppressing many of the immune measures examined in major depression may provide
a mechanism to account for how chronic stress-induced inflammatory activity may give
rise to depression-related suppression of in vitro functional assays, such as lymphocyte
proliferation.
Bipolar Disorder
Patients with bipolar disorder evince many of the immune alterations frequently
observed in the context of unipolar depression. Several studies have observed that
bipolar patients, especially when manic, demonstrate increased plasma concentrations
of inflammatory cytokines. Other studies indicate that treatments for mania, such as
lithium, lower plasma concentrations of a number of cytokines. Of interest, the
available literature seems to suggest that patients in the manic phase of the disorder
may be more likely than depressed patients to demonstrate increased inflammatory
markers. It should not be surprising that mania—which seems the phenomenological
opposite of depression—should be associated with increased inflammation, given that
mania and depression have also been reported to show identical neuroendocrine and
autonomic abnormalities, such as dexamethasone nonsuppression and increased
sympathetic activity, both of which would be expected to promote inflammatory
activity.
Schizophrenia
There has been growing interest in the idea that infectious agents, particularly viruses,
may underlie at least some cases of schizophrenia. Although it is well established that
viral encephalitis can present clinically as psychosis, the primary focus of the “viral
hypothesis” for schizophrenia has been on infections during neurodevelopment given its
congruence with the emerging consensus that prenatal or early postnatal insult is
implicated in the causality of schizophrenia. Several lines of indirect evidence suggest
that viral infection during CNS development may be involved in the pathogenesis of
schizophrenia. The data include: (1) an excess number of patient births in the late
winter and early spring, suggesting possible exposure to viral infection in utero during
the fall and winter peak of viral illnesses, (2) an association between exposure to viral
epidemics in utero and the later development of schizophrenia, (3) a higher prevalence
of schizophrenia in crowded urban areas, which have conditions that are particularly
conducive to the transmission of viral pathogens, and (4) seroepidemiological studies
indicating a higher infection rate for certain viruses in schizophrenia patients or their
mothers.
In addition, schizophrenia has been associated with indices of immune activation,
including elevations in cytokines. Although these immune findings in patients with
schizophrenia may indicate evidence of immune system activation secondary to
infection, it should be noted that they might also indicate that an autoimmune process is
involved in the disorder. Despite the plethora of studies pointing to abnormalities in
cellular and humoral immunity in schizophrenia, the data have not been uniform or
conclusive, and there is a need for more studies that account for confounding variables
such as medication status and tobacco use. Moreover, attempts to isolate infectious
agents from schizophrenic brain tissue or to detect viral nucleic acids in the CNS or
peripheral blood of patients with schizophrenia have generally yielded negative results.
Because the initial neuronal abnormalities in schizophrenia have been proposed to arise during neurodevelopment, a
perinatal viral infection could insidiously disrupt development and then be cleared by the immune system prior to clinical
diagnosis. In such a scenario, host factors such as cytokines could be responsible for causing the developmental
abnormality by interacting with growth factors or adhesion molecules. Recent animal models have identified that maternal
immune activation with resultant production of interleukin 6 (IL-6) critically affects behavioral and transcriptional
changes in offspring. Behavioral changes, including deficits in prepulse inhibition and latent inhibition, are consistent with
behavioral abnormalities in animal models of both schizophrenia and autism. Various animal models using influenza virus,
Borna disease virus, or lymphocytic choriomeningitis virus in rodents have demonstrated that prenatal or postnatal viral
infections can lead to neuroanatomical or behavioral alterations that are somewhat reminiscent of schizophrenia in
humans. As mentioned earlier, epidemiological studies also support the link between infection with a teratogenic virus and
the development of psychotic disorders later in life. Associations have been observed between maternal infection with
rubella or influenza during gestation and the development of a schizophrenia spectrum disorder in the offspring. Similarly,
maternal antibodies to herpes simplex virus that develop during pregnancy are correlated with increased rates of psychosis
during adulthood in the offspring.
Non-HIV retroviruses might also play a role in the pathogenesis of schizophrenia.
Retroviruses integrate into host deoxyribonucleic acid (DNA) and can disrupt the
function of adjacent genes. Moreover, the genomes of all humans contain sequences of
“endogenous retroviruses” that hold the capacity to alter the transcriptional regulation
of host genes. If genes controlling the development or function of the brain undergo
transcriptional disruption by retroviral effects, then this might lead to a cascade of
biochemical abnormalities eventually giving rise to schizophrenia.
Autism
Although a convincing case can be made for a significant immune component in autism,
the relationship of immune abnormalities to the neurobehavioral symptoms of the
disease remains controversial. The claim that autism is triggered by childhood vaccines
has not been substantiated by recent epidemiological studies, and immune-based
therapies for autism have not been reliably effective. Thus, although it is tempting to
speculate that the immune system holds a clue to a cure for autism, there is currently not
enough data to determine whether immune anomalies cause autism, are caused by
autism, or are just adventitiously associated with the disease.
Alzheimer’s Disease
Although Alzheimer’s disease is not considered primarily an inflammatory disease,
emerging evidence indicates that the immune system may contribute to its pathogenesis.
The discovery that amyloid plaques are associated with acute-phase proteins, such as
complement proteins and C-reactive protein, suggests the possibility of an ongoing
immune response. The idea that inflammatory processes are involved in Alzheimer’s
disease has been bolstered by recent studies showing that long-term use of nonsteroidal
anti-inflammatory drugs (NSAIDs) is negatively correlated with the development of
Alzheimer’s disease.
HIV AIDS
AIDS is an immunological disease associated with a variety of neurological
manifestations, including dementia. HIV encephalitis results in synaptic abnormalities
and loss of neurons in the limbic system, basal ganglia, and neocortex.
Multiple Sclerosis
Multiple sclerosis (MS) is a demyelinating disease characterized by disseminated
inflammatory lesions of white matter. Considerable progress has been made in
elucidating the immunopathology of myelin destruction that occurs in MS and in the
animal model for the disease, experimental allergic encephalomyelitis. Although the
initial step in lesion formation has not been determined, disruption of the blood–brain
barrier and infiltration of T cells, B cells, plasma cells, and macrophages appear to be
associated with lesion formation.
Other Disorders
Finally, several disorders are seen in which neural-immune interactions are suspected
but not well documented. Chronic fatigue syndrome is an illness with a controversial
etiology and pathogenesis. In addition to persistent fatigue, symptoms frequently
include depression and sleep disturbances. Tests of immune function have found
indications of both immune activation and immunosuppression. Neuroendocrine
assessments
indicate
that
patients
with
chronic
fatigue
syndrome
may
be
hypocortisolemic because of impaired activation of the hypothalamic–pituitary–adrenal
axis. Although an acute viral infection frequently precedes the onset of chronic fatigue
syndrome, no infectious agent has been causally associated with it. In contrast, Lyme
disease, in which sleep disturbances and depression are also common, is clearly caused
by infection with the tick-borne spirochete Borrelia burgdorferi, which can invade the CNS
and cause encephalitis and neurological symptoms. Lyme disease is remarkable because
it appears to produce a spectrum of neuropsychiatric disorders, including anxiety,
irritability,
obsessions,
compulsions,
hallucinations,
and
cognitive
deficits.
Immunopathology of the CNS may be involved, because symptoms can persist or
reappear even after a lengthy course of antibiotic treatment, and the spirochete is
frequently difficult to isolate from the brain. Gulf War syndrome is a controversial
condition with inflammatory and neuropsychiatric features. The condition has been
attributed variously to combat stress, chemical weapons (e.g., cholinesterase inhibitors),
infections, and vaccines. Given the impact of stress on neurochemistry and immune
responses, these pathogenic mechanisms are not mutually exclusive.
THERAPEUTIC IMPLICATIONS
The bidirectional nature of CNS–immune system interactions implies the therapeutic
possibility that agents known to positively alter stress system activity might benefit
immune functioning and, conversely, that agents that modulate immune functioning
may be of potential benefit in the treatment of neuropsychiatric disturbance, especially
in the context of medical illness. Increasing evidence supports both hypotheses.
Antidepressants and the Immune System
Emerging data indicate that in animals and humans, antidepressants attenuate or
abolish behavioral symptoms induced by inflammatory cytokine exposure. For example,
pretreatment of rats with either imipramine or fluoxetine (a tricyclic antidepressant and
selective serotonin reuptake inhibitor, respectively) for 5 weeks prior to endotoxin
administration significantly attenuated endotoxin-induced decrements in saccharine
preference (commonly accepted as a measure for anhedonia), as well as weight loss,
anorexia, and reduced exploratory, locomotor, and social behavior. Similarly, several
studies in humans suggest that antidepressants can ameliorate mood disturbances in the
context of chronic cytokine therapies, especially if given prophylactically before
cytokine exposure. For example, the selective serotonin reuptake inhibitor paroxetine
significantly decreased the development of major depression in patients receiving high
doses of interferon-α (IFN-α) for malignant melanoma.
Behavioral Interventions and Immunity
It has been known for years that psychosocial factors can mitigate or worsen the effects
of stress, not only on immune functioning but also on the long-term outcomes of medical
conditions in which the immune system is known to play a role. Therefore, behavioral
interventions aimed at maximizing protective psychosocial factors might be predicted to

07 - 1.7 Neurogenetics
1.7 Neurogenetics
have a beneficial effect, not only in terms of mitigating the effect of stress on immune
functioning but perhaps also on diminishing emotional disturbances that arise in the
context of immune system dysregulation.
Two factors that have been repeatedly identified as protective against stress-induced
immune alterations are social support and the ability to see stressors as being to some
degree under the individual’s control. In this regard, a recent study that conducted a
genome-wide scan to assess gene expression activity in socially isolated versus
nonisolated individuals found that social isolation was associated with increased
activation of a number of proinflammatory, cytokine-related pathways and reduced
activity in anti-inflammatory cytokine pathways, as well as in the glucocorticoid
receptor, which plays an important role in neuroendocrine control of inflammatory
processes. Of interest, the two types of psychotherapy most often examined in illnesses
associated with immune dysregulation are group therapy, which provides social support,
and cognitive behavioral therapy, which provides cognitive reframing techniques aimed
at enhancing one’s sense of agency (and hence control).
REFERENCES
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Capuron L, Miller AH. Immune system to brain signaling: Neuropsychopharmacological implications. Pharmacol Ther.
2011;130(2):226.
Danese A, Moffitt TE, Pariante CM, Ambler A, Poulton R. Elevated inflammation levels in depressed adults with a history of
childhood maltreatment. Arch Gen Psychiatry. 2008;65:409.
Dantzer R, O’Connor JC, Freund GG, Johnson RW, Kelley KW. From inflammation to sickness and depression: When the
immune system subjugates the brain. Nat Rev Neurosci. 2008;9:46.
Raison CL, Borisov AS, Woolwine BJ, Massung B, Vogt G, Miller AH. Interferon-α effects on diurnal hypothalamicpituitary-adrenal axis activity: Relationship with proinflammatory cytokines and behavior. Mol Psychiatry. 2010;15:535.
Raison CL, Cowles MK, Miller AH. Immune system and central nervous system interactions. In: Sadock BJ, Sadock VA, Ruiz
P, eds. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th edition. Philadelphia: Lippincott Williams &
Wilkins; 2009:175.
Ransohoff RM, Brown MA. Innate immunity in the central nervous system. J Clin Invest. 2012;122(4):1164.
Steiner J, Bernstein HG, Schiltz K, Müller UJ, Westphal S, Drexhage HA, Bogerts B. Immune system and glucose
metabolism interaction in schizophrenia: A chicken–egg dilemma. Prog Neuropsychopharmacol Biol Psychiatry.
2014;48:287–294.
Wilson EH, Weninger W, Hunter CA. Trafficking of immune cells in the central nervous system. J Clin Invest.
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Yousef S, Planas R, Chakroun K, Hoffmeister-Ullerich S, Binder TM, Eiermann TH, Martin R, Sospedra M. TCR bias and
HLA cross-restriction are strategies of human brain-infiltrating JC virus-specific CD4+ T cells during viral infection. J
Immunol. 2012;189(7):3618.
1.7 Neurogenetics
Starting from the rediscovery of Gregor Mendel’s basic concepts at the turn of the 20th
century, the field of genetics has matured into an essential cornerstone not only of the
biological sciences but of all of medicine. The discovery of the basic structure and
properties of deoxyribonucleic acid (DNA) in the middle of the century led to an
exponential acceleration in our understanding of all aspects of the life sciences,
including deciphering the complete sequence of the human genome, and those of myriad
other species. Massive databases of such sequences now provide 21st-century biologists
with the task of decoding the functional significance of all this information. In
particular, attention has turned to determining how sequence variations contribute to
the phenotypic variation between species and between individuals within a species; in
humans it is hoped that discoveries about the relationship between genotypes and
phenotypes will revolutionize our understanding of why and how some individuals but
not others develop common diseases. This hope is particularly strong for psychiatry, as
our knowledge of the pathogenic mechanisms of psychiatric disease remains sparse.
Genetic mapping studies aim to identify the genes implicated in heritable diseases,
based on their chromosomal location. These studies are carried out by investigating
affected individuals and their families through two approaches, linkage and association
(Fig. 1.7-1). It is now straightforward to genetically map Mendelian traits (traits for
which a specific genotype at one particular locus is both necessary and sufficient to
cause the trait). Psychiatric diseases, however, do not follow simple Mendelian
inheritance patterns but rather are examples of etiologically complex traits. Etiological
complexity may be due to many factors, including incomplete penetrance (expression of
the phenotype in only some of the individuals carrying the disease-related genotype),
the presence of phenocopies (forms of the disease that are not caused by genetic
factors), locus heterogeneity (different genes associated with the same disease in
different families or populations), or polygenic inheritance (risk for disease increases
only if susceptibility variants at multiple genes act in concert). Mapping a complex
disorder involves several component steps, including definition of the phenotype to be
studied, epidemiological studies to determine the evidence for genetic transmission of
that phenotype, choice of an informative study population, and determination of the
appropriate experimental and statistical approaches.
FIGURE 1.7-1
Comparison of gene-mapping strategies. Genetic mapping approaches can be divided
into those that rely on linkage analysis and those that rely on association analysis.
Linkage studies can be further categorized as either focused on investigation of
pedigrees or focused on investigation of sib pairs. Association studies can be categorized
as either case–control or family-based. Some of the key features as well as advantages
and disadvantages of these different approaches are shown. (From Sadock BJ, Sadock
VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia:
Lippincott Williams & Wilkins; 2009:321.)
GENETIC EPIDEMIOLOGICAL APPROACHES
Genetic epidemiological investigations provide quantitative evidence regarding the
degree to which a given trait aggregates in families and, furthermore, can suggest to
what degree such aggregation reflects a genetic contribution to the etiology of the trait.
Family studies compare the aggregation of disease among the relatives of affected
individuals compared to control samples. Because these studies do not differentiate
between genetic and environmental contributions to such familial aggregation, they
provide only indirect evidence regarding the heritability of a trait. Often these studies
measure the relative risk (λ), defined as the rate of occurrence of a disease among
specified categories of relatives of an affected individual divided by the rate of
occurrence of the disease for the general population. A relative risk of >1 suggests a
genetic etiology, and the magnitude of the measure gives an estimate of the genetic
contribution to the disease. Relative risks can be calculated for sibling pairs, parent–
offspring pairs, and various other types of family relationships. Likely modes of
transmission can be assessed by comparing the degree of relative risk for each type of
relationship. Multiple family studies have been carried out for many of the major
psychiatric disorders, including major depression, bipolar disorder, schizophrenia, and
obsessive-compulsive disorder (OCD). Although these studies have consistently reported
familial aggregation for all of these disorders, the degree of such aggregation has varied
substantially across studies, largely reflecting differences in phenotype definition and
how study samples were ascertained and assessed.
Twin studies examine the concordance rates of a particular disorder (the percentage of twin pairs where both twins
have the disorder) in monozygotic (MZ) and dizygotic (DZ) twins. For a disorder that is strictly determined by genetic
factors, the concordance rate should be 100 percent in MZ twin pairs (who share 100 percent of their genetic material) and
25 or 50 percent in DZ twin pairs (who are no more closely related than any siblings), depending on whether the disease is
recessive or dominant, respectively. For a disorder where genetic factors play a role in disease causation but are not the
exclusive cause of disease, the concordance rates should be greater for MZ twins than those for DZ twins. The higher the
degree of concordance of MZ twins, the higher the trait heritability or the evidence for a genetic contribution to disease
risk. When genetic factors do not play a role, the concordance rates should not differ between the twin pairs, under the
simplifying assumption that the environment for MZ twin pairs is no more similar than that for DZ twin pairs. The several
twin studies that have been conducted for traits such as autism, bipolar disorder, and schizophrenia have consistently
suggested high heritability and have therefore spurred efforts to genetically map loci for each of these conditions. Different
twin studies may however generate varying point estimates for the heritability of any given disorder. When evaluating the
results of twin studies, it is therefore important to scrutinize how the phenotype was ascertained because, as with family
studies, the different heritability estimates are likely due to differences in the mode of assessing and defining phenotypes.
For example, early twin studies of psychiatric disorders often relied for their phenotypes on unstructured interviews by a
single clinician. In contrast, modern studies generally utilize standardized assessments and review of diagnostic material by
a panel of expert clinicians. Similarly, part of the apparent variation in heritability between different twin studies can be
attributed to the fact that some studies employ narrow definitions of affectedness for a given phenotype, while other
studies employ broader phenotype definitions (e.g., considering a twin with major depressive disorder to be
phenotypically concordant with a co-twin diagnosed with bipolar disorder). Because of such differences in approach
across studies it is usually prudent to view such investigations as providing a rough estimate of the genetic contribution to
trait variability. Nevertheless, even such estimates are useful in deciding which traits are likely to be mappable.
BASIC CONCEPTS OF GENE MAPPING
Recombination and Linkage
Once genetic epidemiological studies of particular phenotypes have suggested that these
phenotypes are heritable, genetic mapping studies are conducted to identify the specific
genetic variants that contribute to the risk of the disorder. All genetic mapping methods
aim to identify disease-associated variants based on their chromosomal position and the
principle of genetic linkage. All cells contain two copies of each chromosome (called
homologs), one inherited from the mother and one inherited from the father. During
meiosis, the parental homologs cross over, or recombine, creating unique new
chromosomes that are then passed on to the progeny. Genes that are physically close to
one another on a chromosome are genetically linked, and those that are farther apart or
are on different chromosomes are genetically unlinked. Genes that are unlinked will
recombine at random (i.e., there is a 50 percent chance of recombination with each
meiosis). Genetic loci that are linked will recombine less frequently than expected by
random segregation, with the degree of recombination proportional to the physical
distance between them. The principle of linkage underlies the use of genetic markers,
segments of DNA of known chromosomal location that contain variations or
polymorphisms (described in more detail later). Strategies to map disease genes are
based on identifying genetic marker alleles that are shared—to a greater extent than
expected by chance—by affected individuals. It is presumed that such sharing reflects
linkage between a disease locus and a marker locus, that is, the alleles at both loci are
inherited “identical by descent” (IBD), from a common ancestor, and, furthermore, that
this linkage pinpoints the chromosomal site of the disease locus.
The evidence for linkage between two loci depends on the recombination frequency between them. Recombination
frequency is measured by the recombination fraction (Θ) and is equal to the genetic distance between the two loci (1
percent recombination equals 1 centimorgan [cM] in genetic distance and, on average, covers a physical distance of about 1
megabase [mB] of DNA). A recombination fraction of 0.5 or 50 percent indicates that two loci are not linked but rather
that they are segregating independently. A LOD (logarithm of the odds) score is calculated to determine the likelihood that
two loci are linked at any particular genetic distance. The LOD score is calculated by dividing the likelihood of acquiring
the data if the loci are linked at a given recombination fraction by the likelihood of acquiring the data if the loci are
unlinked (Θ = 0.5). This step gives an odds ratio, and the log (base 10) of this odds ratio is the LOD score. A LOD score
can be obtained for various values of the recombination fraction, from Θ = 0 (completely linked) to Θ = 0.5 (unlinked).
The value of Θ that gives the largest LOD score is considered to be the best estimate of the recombination fraction between
the disease locus and the marker locus. This recombination fraction can then be converted into a genetic map distance
between the two loci.
Linkage Disequilibrium
Linkage disequilibrium (LD) is a phenomenon that is used to evaluate the genetic
distance between loci in populations rather than in families. When alleles at two loci
occur together in the population more often than would be expected given the allele
frequencies at the two loci, those alleles are said to be in LD. When strong LD is
observed between two loci it usually indicates that the two loci are sited in very close
physical proximity to one another on a given chromosome, and is useful in mapping
disease susceptibility loci because one locus can be used to predict the presence of
another locus. This predictability is important because current gene-mapping strategies
are able to sample only a subset of the estimated 10 million common human
polymorphisms. Because of the existence of LD, one can use data from a subset of
genotyped polymorphisms to infer genotypes at nearby loci. Clusters of alleles that are
in LD and inherited as a single unit are termed haplotypes. Thus LD mapping
“consolidates” genomic information by identifying haplotypes in populations that can
then be used to infer IBD sharing among unrelated individuals.
There are several methods to measure the extent of LD. One of the most commonly used measures of LD is r2, a measure
of the difference between observed and expected haplotype probabilities. Unlike D′, another widely used measure of LD, r2
values do not depend on the allele frequencies of the loci being assessed. A large r2 value indicates that the observed
frequency of association between two alleles is greater than that expected by chance; that is, the alleles are in LD. LD
studies have traditionally been used to complement traditional pedigree analyses, for example, to hone in on a locus that
has been mapped by linkage analysis. However, LD-based association analysis has become the method of choice for whole
genome screens, particularly for diseases where traditional linkage studies have been unsuccessful. These studies have one
great advantage over a traditional family analysis: because affected individuals are chosen from an entire population rather
than from one or a few pedigrees, the number of potential subjects is limited only by the size of the population and the
frequency of the disease. Maximizing the potential number of affected individuals that can be included in the analysis is
extremely important for disorders where genetic heterogeneity or incomplete penetrance is likely to be a factor.
Genetic Markers
Mapping studies, regardless of their type, depend on the availability of genetic markers.
The most widely used markers are microsatellite markers (also called simple tandem
repeats [STRs], or simple sequence length polymorphisms [SSLPs]) and single nucleotide
polymorphisms (SNPs). SSLPs are stretches of variable numbers of repeated nucleotides
two to four base pairs in length. These markers are highly polymorphic, as the number
of repeat units at any given STR locus varies substantially between individuals. SNPs, as
the name implies, are single base pair changes at a specific nucleotide; they are the
most common form of sequence variation in the genome. SNPs are widely used for
genetic mapping studies because they are distributed so widely across the genome and
because they can be assessed in a high-throughput, automated fashion. Other forms of
genetic variation that have been investigated for use as genetic markers include small
insertion or deletion polymorphisms, termed indels, that generally range between 1 and
30 base pairs and copy number variations (CNVs), which can refer to either deletions or
duplications. Recent genomewide surveys have revealed that CNVs are common and can
range in length from several base pairs to several million base pairs. CNVs may
contribute to chromosomal recombination and rearrangements, thereby playing an
important role in generating genetic diversity, and also, as many of these variants are
sizable, it is hypothesized that they may significantly influence the expression of genes
that encompass or are adjacent to the variant.
MAPPING STRATEGIES
The genetic variants that contribute to disease susceptibility can be roughly categorized
into those that are highly penetrant and those that are of low penetrance. Highpenetrance variants by definition have a large effect on phenotype, and therefore
identifying these variants usually provides fundamental insights into pathobiology.
Because individuals carrying high-penetrance variants have a high probability of
expressing a disease phenotype, such variants tend to be rare and to segregate in
families and are generally most powerfully mapped using pedigree-based approaches
(see Fig. 1.7-1). In contrast, low-penetrance variants have a relatively weak effect on
phenotype, and therefore identification of individual low-penetrance variants may, at
least initially, provide relatively little new biological knowledge. However, because of
their small effects, such variants are typically common in the population, and therefore
identifying them may add to our understanding of disease risk in the population as a
whole. Because we do not expect these variants to segregate strongly with the disease
phenotype in pedigrees, efforts to identify them focus on population samples.
Pedigree Analysis
A pedigree analysis, which is conducted in multigenerational families, consists of
scanning the genome or a portion of the genome with a series of markers in one or
more affected pedigrees, calculating a LOD score at each marker position, and
identifying the chromosomal regions that show a significant deviation from what would
be expected under independent assortment. The primary goal of pedigree analysis is to
determine if two or more genetic loci (i.e., a genetic marker of known location and the
unknown disease loci) are cosegregating within a pedigree.
Following the successful application of pedigree analysis to map Mendelian disorders
such as Huntington’s disease, many investigators adopted this strategy for mapping
psychiatric disease genes with, at best, mixed success. In the late 1980s and mid-1990s,
several pedigree-based studies reported the mapping of susceptibility loci for Alzheimer’s
disease, bipolar disorder, and schizophrenia. Although the linkage findings for three
Alzheimer’s disease loci were relatively quickly replicated, the findings reported for
bipolar disorder and schizophrenia were ultimately determined to have been false
positives. A number of different explanations have been proposed for the failure of
pedigree-based approaches to map psychiatric loci; however, most investigators now
recognize that these studies were generally drastically underpowered considering the
apparent etiological complexity of psychiatric disorders.
Pedigree analysis in psychiatry has increasingly turned toward an application that is
more appropriately powered, namely, the mapping of quantitative trait loci (QTLs).
QTLs are defined as genetic loci that contribute to the variation in continuously varying
traits (as opposed to categorical traits such as disease diagnoses). QTLs are typically loci
of small effect that only contribute to a portion of the observed variance of a trait in the
population. It is now generally accepted that, using analytical methods developed in the
late 1990s, it may be possible to use pedigree studies to map a wide range of
quantitative traits that are relevant for understanding psychiatric disorders. Several such
studies are now being undertaken, typically with multiple phenotypes being assessed in
each individual in the pedigree.
Sib Pair Analysis
Affected sib pair (ASP) analysis became widely used during the 1990s for the genetic
mapping of complex traits, including many psychiatric disorders. Sib pair analysis
examines the frequency with which sibling pairs concordant for a trait share a particular
region of the genome compared with the frequency that is expected under random
segregation.
Sib pair analysis is based on the fact that siblings share approximately 50 percent of
their genomes IBD. Therefore, if a set of unrelated sib pairs affected with a given trait
shares a particular area of the genome at a frequency significantly greater than 50
percent (the proportion of sharing expected under conditions of random segregation),
then that area of the genome is likely to be linked to the trait in question. In this
method, siblings are genotyped, and population frequencies and parental genotypes are
used to estimate the proportion of genes shared IBD at each site for each sib pair. The
linkage analysis then compares those pairs concordant and discordant for each locus.
Like pedigree studies, ASP studies have more power to locate genes of large effect than genes of small effect. This
limitation can be partially addressed by a two-tiered design that incorporates additional markers or family members after
an initial linkage study in affected siblings or by increased sample size. It generally requires less effort to identify and assess
even large sets of affected sibs than to identify and assess all members of extended pedigrees, particularly when
investigators can take advantage of data repositories that include samples and phenotype data from sib pairs ascertained
from multiple sites. For example, the U.S. National Institute of Mental Health (NIMH) maintains such repositories for
sizable collections of sib pairs affected with schizophrenia, bipolar disorder, autism, and Alzheimer’s disease. An
additional benefit of the ASP design is that it allows for the incorporation of epidemiological information, permitting the
simultaneous examination of environmental and gene–environment interactions.
Association Studies
In the last few years, there has been increasing acceptance of the notion that association
studies are more powerful than linkage approaches for mapping the loci of relatively
small effect that are thought to underlie much of the risk for complex disorders. Whereas
linkage studies attempt to find cosegregation of a genetic marker and a disease locus
within a family or families, association studies examine whether a particular allele
occurs more frequently than expected in affected individuals within a population. As
noted previously in this chapter, mapping of genes using association studies is based on
the idea that certain alleles at markers closely surrounding a disease gene will be in LD
with the gene; that is, these alleles will be carried in affected individuals more often
than expected by random segregation, because they are inherited IBD.
There are two common approaches to association studies (see Fig. 1.7-1), case–control
designs and family-based designs, which typically investigate trios (mother, father, and
an affected offspring). In a case–control study, allele frequencies are compared between
a group of unrelated affected individuals and a matched control sample. This design is
generally more powerful than a family-based design, because large samples of cases and
controls are easier to collect than trios and are less expensive, since they require the
genotyping of fewer individuals. Case–control samples may be the only practical design
for traits with a late age of onset (such as Alzheimer’s disease) for which parents of
affected individuals are typically unavailable. The main drawback of the case–control
approach is the potential problem of population stratification; if the cases and controls
are not carefully matched demographically, then they may display substantial
differences in allele frequency that reflect population differences rather than
associations with the disease.
Family-based association studies are designed to ameliorate the problem of population stratification. In this design, the
nontransmitted chromosomes (the copy of each chromosome that is not passed from parent to child) are used as control
chromosomes, and differences between allele frequencies in the transmitted and nontransmitted chromosomes are
examined, eliminating the problem of stratification, as the comparison group is by definition genetically similar to the case
group. Although more robust to population stratification than a case–control study, family-based studies are only about
two-thirds as powerful using the same number of affected individuals, as noted previously.
Until recently, it was not practical to conduct association studies on a genomewide basis, as relatively few SNPs were
available. Therefore, association studies focused on testing one or a few markers in candidate genes chosen on the basis of
their hypothesized function in relation to a given disease. Recently, however, as a result of international efforts that have
identified millions of SNPs distributed relatively evenly across the genome and that have developed technology for
genotyping them relatively inexpensively, genomewide association (GWA) studies are now a reality. Such studies hold
much promise for the identification of common variants contributing to common diseases. While few GWA studies of
psychiatric disorders have been completed, such studies have already reported remarkable findings for complex traits such
as rheumatoid arthritis, inflammatory bowel disease, and type 2 diabetes. The successful studies of these diseases have
made use of very large samples (in some cases up to several thousand cases and controls), providing further support for
the hypothesis that underpowered study designs bear much of the responsibility for the disappointing results to date of
psychiatric genetic investigations.
Statistical Considerations
Scientists in other biomedical research fields are often surprised by the apparently high
level of statistical evidence that geneticists require to consider a linkage or association
result to be significant. Most simply, this requirement can be thought of in terms of the
very low expectation that any two loci selected from the genome are either linked or
associated with one another. The likelihood that any two given loci are linked (i.e., the
prior probability of linkage) is expected to be approximately 1:50, based on the genetic
length of the genome. To compensate for this low prior probability of linkage and bring
the posterior (or overall) probability of linkage to about 1:20, which corresponds to the
commonly accepted significance level of P = .05, a conditional probability of 1,000:1
odds in favor of linkage is required, corresponding to the traditionally accepted LOD
score threshold of 3. This generally provides an acceptable false-positive rate (Fig. 1.72), but some false-positive findings have exceeded even this threshold.
FIGURE 1.7-2
Number of false positives expected in a whole genome scan for a given threshold of
logarithm of odds (LOD) score. Solid line represents the expectation for a perfect genetic
map. Symbols represent the results for 100 sib pairs using genetic maps with markers
spaced every .1 cM (circles), every 1 cM (squares), and every 10 cM (triangles). The
dotted line indicates the 5 percent genomewide significance level. (Courtesy of Dr. Eric
Lander).
Geneticists generally assume that the expectation that any two loci in the genome are associated with one another is
even lower than that of their being in linkage, and typically a P value of less than about 10−7 is considered to indicate
“genomewide significance.” This standard essentially discounts the prior probability that some investigators assign to
variants in candidate genes chosen on the basis of their hypothesized functional relevance to a given disorder or trait. GWA
studies are now replicating associations with very low P values for a wide range of complex traits, whereas most candidate
gene associations (which usually report as significant much higher P values) remain unreplicated. It is therefore
increasingly apparent that genomewide levels of significance are appropriately applied to all initial association studies for
a given trait.
DEFINING PHENOTYPES FOR MAPPING STUDIES
The generally disappointing results of psychiatric genetic mapping studies have focused
increasing attention on the problem of defining and assessing phenotypes for such
studies. Most psychiatric mapping studies to date have relied on categorical disease
diagnoses, as exemplified by the Diagnostic and Statistical Manual (DSM-5) classification
scheme. Criticisms of this approach rest on two arguments. First, diagnosis of psychiatric
disease depends on subjective clinical evaluation, a fact that underscores the difficulty in
ascertaining individuals who can be considered definitely affected with a given disease.
Second, even when a psychiatric diagnosis can be established unambiguously, the menubased system used for psychiatric classification provides the possibility that any two
individuals affected with a given disorder may display largely nonoverlapping sets of
symptoms, likely reflecting distinct etiologies. Concern that the diagnosis-based
approach to phenotyping may represent one of the chief obstacles to the genetic
mapping of psychiatric phenotypes has generated considerable interest in mapping
heritable traits known to demonstrate continuous variation in the population.
Continuous measures that are hypothesized to be related to psychiatric disorders include
biochemical measures (e.g., serum or CSF levels of neurotransmitter metabolites or
hormones), cognitive measures, personality assessments, structural or functional brain
images, biophysical markers such as responses to evoked potentials, or molecular assays
such as gene expression profiles. Key features of categorical and continuous
phenotyping strategies are shown in Figure 1.7-3, and each is discussed in more detail
below.
FIGURE 1.7-3
Two alternate schemes for conceptualizing psychiatric phenotypes. A. Categorical Traits
as conceptualized by the Diagnostic and Statistical Manual (DSM-5) represent a “menubased” approach to psychiatric disorders. Individuals are assessed for a checklist of signs
and symptoms that are then used to categorize the individual as “affected” according to
a specific diagnosis. Not all symptoms are present in samples of individuals who carry a
particular DSM diagnosis, and many of these symptoms occur across diagnostic
boundaries, as illustrated in this Venn diagram. DSM phenotypes therefore probably
represent etiologically heterogeneous categories, and this fact may help to explain the
limited progress thus far of genetic mapping investigations focused on these
phenotypes. B. Alternatively, in the Continuous Traits model, “affectedness” can be
conceptualized in terms of an expectation that an individual will demonstrate extreme
values on a set of continuous measures that correlate with psychopathology and thus are
hypothesized to underlie the disorder (as illustrated by examples of six different types of
measures shown in the hexagon). Such measures may also be associated with particular
components of categorical phenotypes, such as those depicted in the Venn diagram in
Figure 19-3A. The justification for using continuous measures as the phenotypes for
genetic mapping studies is that they are considered etiologically simpler and more
reliably assessed compared to categorical phenotypes. In addition, mapping such traits
combines information from all members of the study population (affected and
unaffected individuals alike), which adds considerably to power. (From Sadock BJ,
Sadock VA, Ruiz P. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry. 9th ed.
Philadelphia: Lippincott Williams & Wilkins; 2009:325.)
Categorical Phenotypes
The most commonly used categorical phenotypes in psychiatry are DSM diagnoses.
Some studies focus on a single DSM diagnosis, whereas other studies include individuals
with a range of different diagnoses. The latter approach is typically used for disorders
that are hypothesized to represent a single disease spectrum, such as mood disorders.
Using the categorical approach, it is important to be able to classify subjects as
unambiguously as possible. Several strategies are used to accomplish this goal. The first
strategy involves deciding on the appropriate diagnostic criteria for the study in
question and deciding how these criteria will be applied to individuals in the study. One
way of standardizing the procedures used to identify and assess potential study subjects
is to use only experienced clinicians in the diagnostic process and to train them in the
administration of the instruments and the diagnostic criteria to be employed. In
addition, a “best estimate” procedure and/or a consensus diagnosis is frequently used.
The best estimate process involves making use of every piece of available information,
including medical records, interviews, and videotapes, to arrive at a diagnosis. For a
consensus diagnosis, two or more diagnosticians independently review the material and
make a diagnosis for each individual. The diagnoses are then compared, and individuals
for whom an agreement in diagnosis cannot be reached are not entered as “affected”
into the study.
A well-designed study makes use of all available information about the genetic
epidemiology of the disorder to choose a sample of affected individuals to study. It is
often the case that a subset of families carries the disorder in what appears to be a
simple Mendelian pattern, whereas the inheritance pattern is less clear for other
families or groups. In a disorder where there are likely to be multiple genes contributing
to the phenotype, it makes sense to begin with a study sample where there may be
major loci. Redefining the disease phenotype can often simplify the mapping process by
identifying such groups or families. For example, in the search for a genetic defect for
Alzheimer’s disease, the process was advanced enormously by limiting the study
population to those individuals who had early age of onset (before age 65); the early
onset trait segregated in an autosomal dominant fashion. Other ways of redefining the
phenotype include focusing on factors such as ethnic background, age of onset,
treatment response, symptom severity, or the presence of comorbid disorders.
Narrowing the phenotype using the approaches discussed earlier may increase the chances of finding a genetic defect in
complex diseases, but it can also greatly reduce the power of the study by limiting the number of available affected
individuals. For this reason, it has been argued that for some disorders broadening the phenotype is an appropriate
strategy. The suggestion is that for some complex diseases the phenotype of interest may represent the extreme end of a
spectrum and that to have enough power to map genes other phenotypes within the spectrum must also be included. For
example, mapping studies of bipolar disorder might include as affected individuals with major depressive disorder as well
as those individuals diagnosed with bipolar disorder.
Although the two approaches of narrowing the disease phenotype and broadening the disease phenotype may seem to be
mutually exclusive, many groups studying complex disorders have incorporated both approaches into their study designs.
One way to do this is to create stratified diagnostic categories, ranging from a narrow diagnostic category to a broad
diagnostic category, and test for genetic linkage under each of these schemas. Some investigators argue that for complex
diseases that are part of a spectrum, this strategy decreases the rate of false negatives, that is, of missing an existing linkage
because of misspecification. Others argue that using several models and picking the one that gives the highest scores
greatly increases the rates of false positives, that is, of identifying an area of linkage where none exists. One problem that
clearly exists with the use of multiple diagnostic categories is that as more models are used (and therefore more statistical
tests are performed), increasingly stringent levels of evidence are required to consider a result significant.
While categorical phenotypes remain the mainstay of psychiatric genetic studies, the limitations of DSM nosology as the
basis of phenotyping for genetic studies are becoming clear. Genetic investigations are focusing increasingly on traits that
may be components of one or more DSM diagnostic categories. For example, there is growing evidence that genetic
susceptibility to psychosis, broadly defined, contributes to both severe bipolar disorder and schizophrenia, and a number
of investigative approaches are being employed to attempt to identify genes that underlie such susceptibility and even to
explore possible etiological relationships between psychiatric and nonpsychiatric disorders. For example, bioinformatics
models have been employed to investigate medical records databases and have uncovered extensive pairwise correlations
among a diverse list of psychiatric disorders, neurological disorders, autoimmune disorders, and infectious diseases.
Eventually, the results of such model-fitting experiments may provide a framework to design more powerful linkage and
association studies that can search for alleles that contribute to susceptibility to multiple disorders.
Continuous Phenotypes
Because of the difficulties experienced in genetic mapping of categorical diagnoses,
neurobehavioral geneticists are increasingly focused on investigating quantitative traits
that are hypothesized to underlie a particular psychiatric diagnosis and that may be
simpler to genetically map. The rationale for efforts to map such alternative
phenotypes, or endophenotypes, is that the genes identified through such efforts may
provide clues regarding the biological pathways that are relevant to understanding a
particular disorder. Several features characterize useful endophenotypes. First, they
should be state-independent; that is, they should not fluctuate as a function of the
disease course or medication treatment and should show adequate test–retest stability.
Second, they should be heritable; that is, there should be evidence that genetic factors
are responsible for a substantial proportion of the variability of the trait within the
population. Third, the endophenotype should be correlated with the disease under
investigation; that is, different values of the trait measure are observed in patients
compared to unrelated control subjects.
Measures of brain structure and function provide most of the traits now under investigation as endophenotypes for
psychiatric disorders. For example, several features of brain morphometry (as assessed by magnetic resonance imaging
[MRI]) are highly heritable (in the range of 60 to 95 percent) including total brain volume, cerebellar volume, gray and
white matter density, amygdala and hippocampal volume, and regional cortical volume. Several studies show that brain
structural features that are correlated in clinical samples with disorders such as schizophrenia or bipolar disorder are also
abnormal in relatives of affected individuals. Physiological measures of brain activity that have been employed as
candidate endophenotypes for psychiatric disorders include electroencephalography (EEG) patterns. Several “pencil and
paper” assessments have been employed to measure endophenotypes relating to neurocognitive function and temperament.
Animal Models
In contrast to categorical phenotypes, endophenotypes can be more straightforwardly
related to phenotypes that can be assessed in animal models. Studies of genetic
variations that affect circadian rhythms provide a good example. Variations in circadian
rhythms have long been recognized as important features of mood disorders, and
quantitative assessments of activity patterns have been proposed as endophenotypes for
such disorders. Numerous studies in animal models have demonstrated that genetically
controlled biological clocks determine circadian activity and that variations in clock
genes are associated with variations in such activity from bacteria to humans. Genetic
mapping efforts in fruit flies starting in the early 1970s resulted in the identification of
at least seven “clock genes,” beginning with period. Subsequent studies showed that the
homologs of several of these genes play essential roles in regulating mammalian
circadian rhythms. Genetic mapping studies in mice also have identified previously
unknown circadian rhythm genes, beginning with the discovery and characterization in
the early 1990s of clock. These genetic discoveries have not only explicated the cellular
networks and neurophysiological circuits responsible for the control of mammalian
circadian rhythms but have also generated animal models that may shed light on the
pathobiology of psychiatric syndromes such as bipolar disorder. For example, mice
carrying a targeted mutation in clock demonstrate abnormal activity patterns, such as
hyperactivity and decreased sleep, which are apparently modified by administration of
lithium.
PROGRESS IN THE GENETICS OF SPECIFIC DISORDERS
Taken as a whole, the progress in identifying susceptibility genes for psychiatric
disorders has been disappointing compared to that observed for nonpsychiatric
disorders. Alzheimer’s disease represents the most successful application of gene-
mapping strategies to complex neurobehavioral disorders, and the section on this
disease provides an example of how genetic linkage studies add to understanding of the
pathogenesis of a complex trait. An overview section on autism describes genetic
investigations of syndromes that have features of autism but have relatively simple
inheritance patterns and discusses how these studies have provided starting points for
investigations of more complex autism spectrum disorders. Finally, the frustrating
search for unequivocal gene findings for bipolar disorder and schizophrenia is used to
illustrate the challenges that are motivating new approaches in the field of
neurobehavioral genetics.
ALZHEIMER’S DISEASE
Alzheimer’s disease provides an excellent example of the power of genetics to elucidate
the complex biology of a neuropsychiatric disorder. Alzheimer’s disease is a well-defined
form of dementia characterized by progressive impairment of memory and intellectual
functioning. The clinical signs and symptoms, although characteristic, are not limited to
Alzheimer’s disease; they are also found in several other types of dementia. For this
reason, the diagnosis of Alzheimer’s disease can only be confirmed histopathologically at
autopsy. The presence of senile plaques (made up of a core of β-amyloid fibrils
surrounded by dystrophic neurites), tau-rich neurofibrillary tangles, and congophilic
angiopathy in the brain parenchyma and associated blood vessels are pathognomonic
for Alzheimer’s disease.
A variable age of onset has been noted for Alzheimer’s disease, ranging from as early
as age 35 to as late as age 95. The concordance rate for Alzheimer’s disease in MZ twin
pairs is about 50 percent, indicating a moderately strong genetic contribution to disease
risk. It is now evident from a wide range of genetic studies that Alzheimer’s disease can
be divided into two broad categories: familial forms, which account for a tiny minority
of Alzheimer’s disease cases and are characterized by early onset and autosomal
dominant inheritance with high penetrance; and sporadic forms, in which the genetic
contribution is hypothesized to be similar to that characterizing other common
neuropsychiatric diseases.
The search for the genetic basis of familial Alzheimer’s disease began with traditional linkage studies. First, an
investigation of a candidate locus on chromosome 21 in humans identified mutations in the amyloid precursor protein
(APP) gene in a small number of families in which significant linkage had previously been observed to markers from this
region. Transgenic mice with different APP mutations were created and have been shown to produce β-amyloid deposits
and senile plaques as well as to show synapse loss, astrocytosis, and microgliosis, all part of the pathology of Alzheimer’s
disease. Mutations in the genes that encode β-APP all lead to an increase in the extracellular concentration of longer
fragments of β-amyloid (Aβ42). Most of the strains of transgenic mice with mutations in APP exhibit increased rates of
behavioral changes and impairment in several memory tasks, indicating dysfunction in object-recognition memory and
working memory among others. These findings represent striking evidence that mutations in the β-amyloid gene are
indeed responsible for at least some of the histopathological elements of Alzheimer’s disease.
Even as the preceding findings were being reported, it was clear that mutations in the β-amyloid gene could not
completely explain the etiology and pathology of Alzheimer’s disease, not least because it was shown that linkage to
chromosome 21 was excluded in most early onset Alzheimer’s disease families. In addition, no neurofibrillary tangles are
observed in most of the different β-amyloid transgenic mice. The subsequent search for the genetic underpinnings of
Alzheimer’s disease using genomewide linkage analysis of early onset Alzheimer’s disease families resulted in the
identification of two additional Alzheimer’s disease susceptibility genes: presenilin-1 (PS-1) on chromosome 14q24.3 and
presenilin-2 (PS-2) on chromosome 1q. PS-1 and PS-2 are integral transmembrane proteins with at least seven
transmembrane domains. Although their function has not yet been completely elucidated, they are clearly involved in the
pathogenesis of Alzheimer’s disease. Inactivation of presenilins in mice leads to neurodegeneration and behavioral
manifestations of memory loss. Biochemical and cellular studies have implicated presenilins in several important
pathways, including apoptosis (programmed cell death) and protein processing in the endoplasmic reticulum.
These findings emphasize one of the strengths of using family-based linkage analysis.
Pedigree-based studies are especially suited to identify highly penetrant disease genes
that serve important roles in important biological processes. Although mutations in APP
and presenilin are rare, research into the biology of the expressed proteins has provided
key insights into the pathophysiology of dementia. Because these highly penetrant
mutations elucidate important biological functions, they also provide a firm ground to
design therapeutic interventions. For example, amyloid-β “vaccines” designed to induce
an immunogenic response to pathogenic amyloid are now in advanced clinical trials.
Unlike the current psychopharmacological treatments for Alzheimer’s disease that
nonspecifically target cholinergic and glutaminergic neuronal systems, the amyloid-β
vaccines specifically treat the causes of Alzheimer’s disease by generating an immune
response that may actually reverse the deposition of senile plaques.
Sporadic and Late-Onset Alzheimer’s Disease
Mutations in APP, PS-1, or PS-2 are present in a majority of familial cases of early-onset Alzheimer’s disease but do not
account for sporadic or familial late-onset Alzheimer’s disease. For this reason, investigators turned to other approaches to
search for evidence of linkage in a large number of small families with late-onset Alzheimer’s disease. In 1991, the results
of a nonparametric linkage study using 36 markers in late-onset Alzheimer’s disease families provided evidence for a
susceptibility gene on the long arm of chromosome 19. In 1993, association studies revealed that the e4 allele of the
apolipoprotein E gene was strongly associated with late-onset Alzheimer’s disease and that this association almost certainly
was responsible for the previously observed linkage signal on chromosome 19. There are three known alleles of this gene—
e2, e3, and e4. In most populations, the e3 allele is the most common. However, in familial late-onset Alzheimer’s disease
the incidence of e4 is approximately 50 percent, and in sporadic late-onset Alzheimer’s disease it is 40 percent, compared
with about 16 percent in normal controls. Epidemiological studies suggest that between 30 and 60 percent of late-onset
Alzheimer’s disease cases have at least one apoE-e4 allele. The e4 genotype appears to be a more important risk factor for
Alzheimer’s disease in populations of European and Asian origin when compared with populations of African origin.
Overall, the association of apoE-e4 with Alzheimer’s disease remains probably the strongest association yet identified for a
common human disease.
The establishment of apoE-e4 as a susceptibility allele for late-onset Alzheimer’s
disease has led to the search for additional alleles that might interact with apoE-e4 to
modify disease risk. In 2007, investigators used genomewide association strategies (in
histologically confirmed cases and controls) to identify GAB2 (GRB-associated binding
protein 2) as an additional risk allele in apoE-e4 carriers (but not in Alzheimer’s disease
patients who were not e4 carriers). Initial studies suggest that carriers of both apoE-e4
and GAB2 risk alleles have an almost 25-fold greater risk for Alzheimer’s disease than
individuals who do not carry either risk allele. Larger-scale GWA studies of Alzheimer’s
disease are in progress and will likely yield further associations; however, it is unlikely
that any will have as strong an effect as apoE.
AUTISM
Autism is a severe neurodevelopmental disorder that is characterized by three primary
features: impaired language and communication; abnormal or impaired social
interaction; and restricted, repetitive, and stereotyped patterns of behavior.
Understanding of the etiology of autism has proceeded slowly, but there is now
convincing
evidence
that
alterations
in
specific
cellular
and
molecular
neurodevelopmental pathways are important in its etiology. In comparison with other
neuropsychiatric disorders, there is particularly strong evidence for a genetic
contribution to the risk of autism and autism spectrum disorders (ASDs). The sibling
recurrence risk for autism and/or ASD is between 2 and 6 percent. Given a population
prevalence of about 1 in 2,000 (.04 percent), this means that the siblings of autistic
individuals are approximately 50 to 100 times more likely to develop autism than a
person in the general population. Twin studies of autism show an extraordinarily high
heritability (as demonstrated by MZ twin concordance of 80 to 92 percent) but also
demonstrate the genetic complexity of these disorders, with the DZ twin concordance
rate of 1 to 10 percent suggesting a highly multigenic mode of inheritance.
Increasing interest is now focused on the possibility that individuals affected with
autism may display larger numbers of large-scale chromosomal aberrations (5 to 10
percent in some studies) than unaffected individuals. In addition to such gross
abnormalities, several recent studies have suggested that autism is associated with an
unusually high prevalence of submicroscopic CNVs. For example, in 2007, the Autism
Genome Project Consortium applied microarray strategies to almost 8,000 individuals
from about 1,500 families, each with at least two affected family members, and found
that about 10 percent of the ASD families carried CNVs, with an average size of more
than 3 million base pairs, mostly consisting of duplications rather than deletions.
Although the design of this study did not permit assessment of whether the frequency of
CNVs is greater in patients with autism than that in controls, another study found a de
novo CNV incidence of 10 percent in sporadic (no family history) cases of autism
compared to an incidence of 1 percent in controls. These results, while exciting, are still
considered preliminary. Even before the demonstration of high rates of de novo
mutations in autism, epidemiological studies had strongly suggested that the genetic
basis of this disorder is likely complex. For example, although the risk of autism in firstdegree relatives of autistic probands is high, there is a substantial falloff for second-
degree and third-degree relatives of such probands, suggesting that multiple genetic
variants must interact to increase susceptibility to this syndrome. Segregation analyses
of autism also support the hypothesis that it is a heterogeneous disorder that reflects the
actions of multiple genetic variants of small effect. A latent class analysis performed to
study possible modes of transmission suggested an epistatic model with up to about 10
interacting loci, whereas other studies have estimated that as many as 15 such loci may
be involved. Genetic studies of autism have included whole genome screens, candidate
gene studies, chromosome rearrangement studies, mutation analyses, and, most
recently, comparative genomic hybridization studies. Taken together and recognizing
that most findings still await adequate replication, these studies have contributed to an
emerging picture of autism susceptibility that includes genes involved in three major
systems: those involving synapse formation and maintenance, those involving cell
migration, and those involving the excitatory/inhibitory neurotransmitter networks.
Figure 1.7-4 shows a schematic of the currently known potential candidate genes for
autism and their molecular relationships with one another.
FIGURE 1.7-4
Schematic of the cell biology of proteins expressed from genes identified through
mapping studies of autism spectrum disorders. The function of each gene product falls
into three broad functional categories. Proteins involved in synapse formation and
maintenance include FMR1, TSC1, TSC2, MeCP2, NLGN 3 and 4, and SHANK3. Another
set of proteins is involved in neuronal migration and cell fate including REELIN, WNT2,
LAMB1, and NrCAM. Proteins involved in neurotransmitter systems are also altered in
some individuals with autism and include 5-HTT (serotonin transporter encoded by
SLC6A4), GABAR, and the NMDA subunit encoded by GRIN2A. See text for details.
(From Persico AM, Bourgeron T. Searching for ways out of the autism maze: Genetic,
epigenetic and environmental clues. Trends Neurosci. 2006;29:349, with permission.)
Synapse Formation and Maintenance
Perhaps the biggest breakthroughs in identifying susceptibility genes for autism have
come from studies of disorders that display clinical features associated with autism or
ASDs but with simpler inheritance patterns, including fragile X syndrome, tuberous
sclerosis, and Rett syndrome. In general, the genetic defects associated with these
disorders affect synapse formation and maintenance. Fragile X, which accounts for 3 to
4 percent of autism cases, is caused by an unstable trinucleotide repeat in the 5′ region
of the fragile X mental retardation 1 (FMR1) gene at Xq27.3. This repeat expands as it is
transmitted to succeeding generations, resulting in abnormal methylation and inhibition
of expression of FMR1. FMR1 produces a ribonucleic acid (RNA)-binding protein that
acts as a chaperone for the transport of RNA from the nucleus to the cytoplasm and is
involved in messenger RNA (mRNA) translation at the synapse. Abnormalities in
dendritic spine density (increased over normal) and anatomy (longer and thinner than
normal) have been reported in individuals with fragile X as well as in mouse models of
this disorder. Tuberous sclerosis, which accounts for perhaps 2 to 10 percent of autism
cases (the rate of tuberous sclerosis is higher among autistic individuals with seizure
disorders), results from mutations in one of two tumor suppressor genes, TSC1 on 9q34,
and TSC2 on 16p13, both of which are involved in guanosine triphosphatase (GTPase)
inactivation. Loss of a single copy of TSC1 in mice has been shown to disrupt
cytoskeletal dynamics and dendritic spine structure. Although somewhat less well
understood, the genetics of Rett syndrome, an X-linked pervasive developmental
disorder (the first with a known genetic etiology) that occurs only in girls and is
associated with normal early development followed by loss of skills—particularly social
engagement and purposeful hand skills by age 4—also point to abnormalities in
synapse formation and maintenance in ASD and ASD-like disorders. Rett syndrome is
caused by mutations in MeCP2, which makes a methylated-DNA-binding protein that
regulates gene expression and chromatin structure. Although little is known about the
exact role of MeCP2 in the development of Rett syndrome, the pattern of normal early
development and later regression suggests that this gene is more likely to be involved in
synapse maintenance and remodeling than in synapse development.
Neuroligin (NLGN) 3 and 4 and SHANK3, additional genes that appear to play a role in synapse formation, may be
affected by chromosomal rearrangements observed in some individuals affected with autism. The neuroligin genes, sited on
the X chromosome, produce cell adhesion molecules that are located on postsynaptic glutamatergic neurons. When
mutated in rodents, these genes show defective trafficking and synapse induction. In nonmutated form, their expression
induces the formation of normal, presynaptic terminals in axons. SHANK3 is a binding partner of the neuroligins and
regulates the structural organization of dendritic spines. Mutations in SHANK3 have been identified in ASD-affected
members of at least three families to date, and a comparative genomic hybridization study of autistic individuals, their
family members, and controls recently identified a large deletion in chromosome 22q13, the region containing SHANK3, in
at least one individual with autism.
Cell Migration
Of the regions highlighted by a genome screen in autism families, chromosome 7q has
provided the most consistent evidence for linkage, albeit over a very broad region.
Known chromosomal rearrangements in this region in individuals affected with autism
add to its interest. The linkage region on chromosome 7q contains several genes that are
strong candidates for autism, most notably RELN, which maps to chromosome 7q22.
RELN codes for reelin, a signaling protein secreted by Cajal-Retzius cells located in the
marginal zone of the developing brain. It plays an important role in neuronal migration
as well as in the development of neural connections. Reeler mice, which have
spontaneous deletions of RELN, have cytoarchitectonic alterations in their brains during
development that are similar to those that have been described in autistic brains. The
complete absence of RELN in humans leads to a more severe phenotype with
lissencephaly and severe mental retardation but not autism. Individuals with autism
show reduced levels of reelin mRNA and protein in brain and blood serum, suggesting
that mutations leading to reduced expression of RELN rather than its absence may be
important in ASD. Genetic association studies with RELN have been equivocal,
suggesting that if RELN does contribute to the development of autism, then it may play
such a role in a small subset of affected individuals. WNT2 (wingless-type MMTV
integration site family member 2) is another gene identified as a potential candidate for
autism based on linkage studies. WNT2 is located on 7q31 and is part of a family of
genes that encode secreted signaling proteins implicated in several developmental
processes, including the regulation of cell fate and patterning during embryogenesis. At
least two families have been identified in which nonconservative coding sequence
variants in WNT2 segregate with autism. LD between a SNP in the 3′ untranslated
region of WNT2 and autism is also present in families with severe language
abnormalities that accounted for most of the evidence for linkage on chromosome 7q in
one of the original genome screens.
Excitatory/Inhibitory Neurotransmitter Systems
Although there is little current evidence that mutations in genes encoding
neurotransmitter transporters and/or receptors are directly responsible for the
development of autism, there is some evidence that such genes might act as modifiers or
susceptibility factors for an autism spectrum phenotype. The evidence is perhaps
strongest for the role of the γ-aminobutyric acid (GABA) receptors in the development
and expression of autistic disorders. These receptors occur in a cluster on chromosome
15q11–13, and duplications of this region are the most common cytogenetic
abnormalities seen in autism cases (up to 6 percent of cases). GABA is an important
inhibitory neurotransmitter in the central nervous system and is responsible for
controlling excitability in mature brains. Chromosome 15q11–13 is one of the most
complex regions of the genome. It has a high rate of genomic instability, including
frequent duplication and deletion events, and imprinting plays an important role in the
expression of genes in this region. The 15q11–13 region is the critical region for
Angelman and Prader-Willi syndromes, neurological disorders due to deletions or
mutations in this region that occur on maternally and paternally inherited
chromosomes, respectively.
Despite the high rate of duplications of 15q11–13 among autistic individuals, genome
screens have not shown strong support for linkage or association to this region.
Candidate gene studies continue, however, in part because a rate of 6 percent of autistic
individuals with duplications in this region is hard to ignore.
BIPOLAR DISORDER
The search for the genetic basis of bipolar affective disorder has been fraught with
missteps and partial answers. The history of genetic mapping attempts for bipolar
disorder illustrates not only the extreme complexity of psychiatric disorders but also the
evolution of genetic approaches to such diseases. Bipolar disorder is an episodic illness
characterized by recurrent periods of both mania and depression. Psychotic symptoms
are often a part of the clinical picture, particularly in more severely affected
individuals.
Numerous genetic epidemiological investigations conducted over several decades have
strongly supported a genetic contribution to risk for bipolar disorder. As with other
psychiatric disorders, however, the definition of the bipolar disorder phenotype in these
studies has varied substantially, and this in turn has resulted in a wide range in
estimates of its heritability. For example, many early studies into the genetic basis of
mood disorders did not distinguish between unipolar and bipolar mood disorders.
Furthermore, the diagnostic methodology used in such early studies differs substantially
from that employed in current-day genetic studies. For example, a Danish twin study
that suggested a very high heritability for bipolar disorder and thereby had a heavy
influence on the design of initial genetic mapping studies of mood disorders employed
only unstructured diagnostic interviews by a single clinician rather than the structured
assessments used in current studies, which have suggested somewhat lower heritabilities.
Current estimates of concordance for bipolar disorder range between 65 and 100 percent in MZ twins and between 10
and 30 percent in DZ twins, indicating that the disorder is highly heritable (between about 60 and 80 percent). Several
studies have shown that bipolar disorder is substantially more heritable than unipolar major depression, which has an
estimated heritability between 30 and 40 percent.
Early family studies suggested that bipolar disorder segregation patterns were compatible with single gene inheritance of
a locus of major effect. However, although it is possible that some bipolar disorder pedigrees segregate such a locus,
mounting evidence indicates that if such pedigrees exist they must be quite rare. Furthermore, the fact that genetic linkage
studies have failed to uncover such a locus with unequivocal evidence in any pedigrees argues against this possibility. The
observed rapid decrease in recurrence risk for bipolar disorder from monozygotic co-twins to first-degree relatives is also
not consistent with single gene inheritance models but rather suggests models of multiple interacting genes.
Early Linkage Studies
Tremendous excitement followed the first reports of linkage to bipolar disorder on
chromosomes X and 11 in 1987. Investigators noted that in several families, bipolar
disorder and other affective disorders appeared to be inherited in an X-linked fashion.
Likewise, these disorders appeared to cosegregate in several Israeli families with color
blindness and G6PD deficiency, which map to the X chromosome. Linkage studies in
these pedigrees, using color blindness or G6PD deficiency as marker loci, gave LOD
scores between 4 and 9. Early studies of chromosome 11 were similar to those for
chromosome X in that they reported significant linkage after testing only a few markers
in a single region, in this case in an extended Old Order Amish pedigree heavily loaded
for bipolar disorder.
Not surprisingly, these findings generated a great deal of interest. Both studies showed
high LOD scores and seemed to provide clear evidence for linkage. However, replication
studies in other populations failed to produce positive results for either the X
chromosome or chromosome 11, and evidence for linkage essentially disappeared in
both chromosomal regions in the samples in which linkage was originally reported when
the pedigrees were extended to include additional affected individuals and when
additional markers were typed in the putative linkage regions. The most likely
explanation in each case is that the original linkage results were false-positive findings
and may have reflected overoptimistic interpretation of evidence that, in retrospect,
was relatively scanty.
Genomewide Screens
The early linkage studies of bipolar disorder evaluated only a few markers because they
were all that were available. With the construction of genetic linkage maps of the
genome in the 1990s, linkage studies of most complex traits, including bipolar disorder,
began to search genomewide. The advantage of genomewide mapping studies is that
they do not require a priori knowledge of the biological underpinnings of a particular
phenotype. Complete genome screens provide an opportunity to evaluate the evidence
of linkage at all points in the genome without bias (see Color Plate 1.7-5). Although
genomewide studies clearly had greater power to detect true linkage than studies
focused on only a few markers in arbitrary locations or around a few candidate genes,
these investigations have also generally had disappointing results. The challenge of
achieving replicated significant linkage results for bipolar disorder and other complex
traits is apparent when one reviews the many gene-mapping studies that have suggested
—but not demonstrated unequivocally—bipolar disorder susceptibility loci on
chromosome 18.
Chromosome 18
The first report of linkage came from a partial genome screen that examined 11 markers
on chromosome 18 and identified suggestive linkage near the centromere. Because the
inheritance patterns for bipolar disorder are unknown, the results were analyzed using
both recessive and dominant models. Some of the markers were positive under a
recessive model in some families, some were positive under a dominant model in other
families, and some markers gave positive LOD scores in a subset of families under both
models. Attempts to replicate this finding in other populations have been mixed. So far
at least two groups have found no evidence for linkage to the pericentromeric region of
chromosome 18 in their samples, although one other group has found evidence to
support linkage to this region. Other studies have found suggestive evidence for linkage
on chromosome 18, including a complete genome screen in two large Costa Rican
pedigrees that gave evidence for linkage on chromosome 18q22–23 as well as in an area
on 18p. The combined evidence of these several studies, although somewhat
contradictory and confusing, points to at least two different susceptibility loci on
chromosome 18: one on 18p and one on 18q.
Improving Study Power
The equivocal findings represented by the attempts to pinpoint susceptibility loci on
chromosome 18 have led investigators to implement several new strategies to map
bipolar disorder genes. One such strategy is meta-analysis, which involves combining
data across multiple individual investigations to increase statistical power, and in some
cases the combined analysis points to loci not originally found in the individual studies.
Several meta-analytical techniques have been used to explore gene-mapping studies for
bipolar disorder. The multiple scan probability (MSP) and genome scan meta-analysis
(GSMA) methods require only linkage statistics and P-values from each study to examine
combined data. MSP was used to combine chromosomal regions with P-values less than
.01 from 11 independent bipolar disorder studies and provided evidence for
susceptibility loci on chromosomes 13q and 22q. Although the MSP and GSMA methods
have the advantage of requiring only linkage significance data, they are not able to
account for study-specific issues that will limit the extent to which multiple studies can
be compared. Combining original genotype data from multiple studies can circumvent
this problem. With this method, the largest meta-analysis to date combined 11 bipolar
disorder genomewide linkage scans consisting of 5,179 individuals from 1,067 families.
Access to the original genotype data allowed the construction of a standardized genetic
map in which the markers of each respective study were mapped onto one common
gender-averaged map. The results of this meta-analysis identified two susceptibility loci
with genomewide significance on 6q and 8q.
Another strategy that has been used to increase the power of gene-mapping studies is the formation of consortia that
combine data across multiple clinical sites. A consortium combining data from the UK and Ireland led to support for
linkage at 9p21 and 10p14–21. Likewise, combining data from Spanish, Romanian, and Bulgarian families provided
additional support for findings on chromosomes 4q31 and 6q24. Investigators can also increase power by standardizing
marker sets and clinical evaluation protocols between independent studies to permit direct comparisons between such
studies. This approach was used to identify a bipolar disorder susceptibility locus on chromosome 5q31–33. The region
showed suggestive nonparametric linkage results in pedigrees from the Central Valley of Costa Rica. With identical genetic
markers and diagnostic criteria, the same region was highlighted in an independent analysis of a set of Colombian families
who have a genetic background similar to that of the Costa Rican families. A follow-up study using additional markers in
an expanded set of Colombian and Costa Rican families confirmed genomewide significant evidence to a candidate region of
10 cM in 5q31–33. This finding is especially interesting given that the linkage peak in the bipolar studies overlaps with
linkage regions for schizophrenia and psychosis, identified in a previous study of 40 families from the Portuguese Islands.
These results contribute to a growing opinion that there may be substantial genetic overlap between different DSM
disorders.
SCHIZOPHRENIA
As with bipolar disorder, investigations of the genetic basis of schizophrenia exemplify
the frustrations still characteristic of psychiatric genetics, and the field still struggles to
interpret the significance of initially promising linkage and association results that
began to emerge over a decade ago. Unlike with bipolar disorder, however, candidate
genes have emerged from each of the regions highlighted from these studies. Thus,
although none of these findings have been validated unequivocally, they have spawned
a diverse range of basic and clinical investigations aiming to elucidate their functional
significance, for example, using mouse gene targeting and functional MRI. Here we
discuss some of the more extensively investigated loci for purposes of illustration; it
could be argued that roughly equivalent evidence supports schizophrenia candidate loci
that we do not discuss in detail, for example, AKT1 on chromosome 14 or COMT on
chromosome 22.
Chromosome 6p24–22 was among the first regions to be implicated by a complete
genome screen for schizophrenia, in this case from a study of Irish families heavily
loaded for schizophrenia. The linkage results were strongest under a broad diagnostic
definition that included schizophrenia spectrum disorders, such as schizotypal
personality disorder. Six additional linkage studies have shown positive results over
approximately the same region, but at least three studies have found no linkage to the
region. Fine-scale mapping of this region using association analysis in the original Irish
kindreds led to the proposal of Dysbindin (DTNB1) as a candidate gene for
schizophrenia. Additional association studies of Dysbindin have been equivocal. Although
multiple association studies in a variety of populations have shown positive results,
interpretation of the results has been difficult. Different association studies have not
used the same SNP marker sets. Meta-analysis of five “positive” association studies using
a high-resolution haplotype map designed to compare the five studies showed significant
inconsistencies with regard to the identified disease-associated Dysbindin allele. Although
it is possible that several different variants in the same gene could each contribute to
disease susceptibility in different families or populations, this possibility does not
explain the inconsistencies between the several Dysbindin association studies.
Linkage studies subsequently pointed to a region on chromosome 1 containing the candidate genes DISC 1 and DISC 2
(disrupted in schizophrenia 1 and 2) located on chromosome 1q21–22 and 1q32–42. These genes were initially identified in
a large Scottish pedigree in the early 1990s. A balanced translocation between chromosomes 1 and 11 segregated in this
pedigree and was possibly associated with serious mental illness. DISC 1 and 2 were identified in the original Scottish
family because of their location near the chromosomal translocation breakpoint. As with Dysbindin, follow-up studies of
DISC 1 and 2 have been equivocal.
Genome screens, including a screen focused on extended Icelandic kindreds, have identified a schizophrenia candidate
region on chromosome 8p21–22. Fine mapping of the region narrowed the search and eventually led to the proposal of
neuregulin 1 (NRG1) as a schizophrenia candidate gene. Association studies again provided equivocal and difficult-tointerpret results. Meta-analysis of 14 separate studies using the SNP marker that demonstrated an association in the
original study showed significant heterogeneity between the follow-up studies. It also showed that there is no consistent
association between the specific risk allele “tagged” by the marker SNP and schizophrenia in different populations.
However, after taking account of the statistical power of each association study, the meta-analysis showed a positive
association between NRG1 at the level of the gene (as opposed to the SNP or haplotype level).
Despite the equivocal genetic studies, significant resources have been channeled into
molecular and neurophysiological investigations of the functional products of dysbindin,
DISC 1 and 2, and neuregulin. Mutant mice for each of the three genes are now available
and have been used to demonstrate interesting biological findings. For example,
dysbindin is expressed in the hippocampus and dorsolateral prefrontal cortex. The
dysbindin protein binds to B-dystrobrevin and has been implicated in synaptic structure
and signaling. DISC 1 has been shown to influence neurite formation in cellular studies,
and mutant mice for DISC 1 show impairments in a wide variety of tests including
learning, memory, and sociability. Neuregulin belongs to a family of growth factors that
mediate numerous functions including synapse formation, neuronal migration, and
neurotransmission. Targeted disruption of erbB4, the postsynaptic target of neuregulin,
leads to synaptic glutamatergic hypofunction. Despite the interesting biology uncovered,
it remains unclear whether and to what extent any of these genes contribute to the
etiology of schizophrenia in humans, and many geneticists have been cautious in their
endorsement of the legitimacy of the mutant mice generated from the current list of
candidate genes as models of psychiatric disorders.
As with bipolar disorder, the genetic mapping findings for schizophrenia are
promising but equivocal. Unlike for bipolar disorder, these mapping studies have
generated a set of candidate genes that have stimulated a wide range of functional
investigations, many of which have biologically interesting findings. As with bipolar
disorder and other psychiatric disorders, the primary challenge in elucidating the genetic
basis of schizophrenia is assembling adequate richly phenotyped samples for wellpowered genomewide mapping studies.
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08 - 1.8 Applied Electrophysiology
1.8 Applied Electrophysiology
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1.8 Applied Electrophysiology
Electroencephalography (EEG) is the recording of the electrical activity of the brain. It is
used in clinical psychiatry principally to evaluate the presence of seizures, particularly
temporal lobe, frontal lobe, and petit mal seizures (absence seizures), which can
produce complex behaviors. EEG is also used during electroconvulsive therapy (ECT) to
monitor the success of the stimulus in producing seizure activity, and as a key
component of polysomnography used in the evaluation of sleep disorders. Quantitative
electroencephalography (QEEG) and cerebral evoked potentials (EPs) represent newer
EEG-based methods that provide improved research and clinical insights into brain
functioning.
ELECTROENCEPHALOGRAPHY
A brain wave is the transient difference in electrical potential (greatly amplified)
between any two points on the scalp or between some electrode placed on the scalp and
a reference electrode located elsewhere on the head (i.e., ear lobe or nose). The
difference in electrical potential measured between any two EEG electrodes fluctuates or
oscillates rapidly, usually many times per second. It is this oscillation that produces the
characteristic “squiggly line” that is recognized as the appearance of “brain waves.”
Brain waves reflect change by becoming faster or slower in frequency or lower or
higher in voltage, or perhaps some combination of these two responses. A normal EEG
can never constitute positive proof of absence of brain dysfunction. Even in diseases
with established brain pathophysiology, such as multiple sclerosis, deep subcortical
neoplasm, some seizure disorders, and Parkinson’s disease and other movement
disorders, a substantial incidence of patients with normal EEG studies may be
encountered. Nonetheless, a normal EEG can often provide convincing evidence for
excluding certain types of brain pathology that may present with behavioral or
psychiatric symptoms. More often, information from the patient’s symptoms, clinical
course and history, and other laboratory results identifies a probable cause for the EEG
findings. EEG studies are often ordered when a pathophysiological process is already
suspected or a patient experiences a sudden, unexplained change in mental status.
Electrode Placement
The electrodes normally used to record the EEG are attached to the scalp with a
conductive paste. A standard array consists of 21 electrodes. Placement of the electrodes
is based on the 10/20 International System of Electrode Placement (Fig. 1.8-1). This
system measures the distance between readily identifiable landmarks on the head and
then locates electrode positions at 10 percent or 20 percent of that distance in an
anterior–posterior or transverse direction. Electrodes are then designated by an
uppercase letter denoting the brain region beneath that electrode and a number, with
odd numbers used for the left hemisphere and with even numbers signifying the right
hemisphere (the subscript Z denotes midline electrodes). Thus, the O2 electrode is placed
over the right occipital region, and the P3 lead is found over the left parietal area (Fig.
1.8-2).
FIGURE 1.8-1
International 10–20 Electrode Placement System. (Courtesy of Grass, Astro-Med, Inc.
Product Group.)
FIGURE 1.8-2
A left-lateral diagram of the head showing the locations of the routine 10–20 electrodes
(left-side electrode locations F7 and T3 and the new electrode placement [T1]) in
relation to the temporal pole. (Modification of figure reprinted courtesy of Grass, AstroMed, Inc. Product Group.)
In special circumstances, other electrodes may be used. Nasopharyngeal (NP) electrodes can be inserted into the NP
space through the nostrils and can be closer to the temporal lobe than scalp electrodes. No actual penetration of tissue
occurs. These electrodes may be contraindicated with many psychiatric patients displaying behaviors, such as confusion,
agitation, or belligerence, which could pull the leads out, possibly lacerating the nasal passage. Sphenoidal electrodes use a
hollow needle through which a fine electrode that is insulated, except at the tip, is inserted between the zygoma and the
sigmoid notch in the mandible, until it is in contact with the base of the skull lateral to the foramen ovale.
Activated EEG
Certain activating procedures are used to increase the probability that abnormal
discharges, particularly spike or spike-wave seizure discharges, will occur. Strenuous
hyperventilation is one of the most frequently used activation procedures. While
remaining reclined with the eyes closed, the patient is asked to overbreathe through the
open mouth with deep breaths for 1 to 4 minutes, depending on the laboratory (3
minutes is common). In general, hyperventilation is one of the safest EEG-activating
procedures, and, for most of the population, it presents no physical risk. It can pose a
risk for patients with cardiopulmonary disease or risk factors for cerebral vascular
pathophysiology, however. Photic stimulation (PS) generally involves placing an
intense strobe light approximately 12 inches in front of the subject’s closed eyes and
flashing at frequencies that can range from 1 to 50 Hz, depending on how the procedure
is carried out. Retinal damage does not occur, because each strobe flash, although
intense, is extremely brief in duration. When the resting EEG is normal, and a seizure
disorder or behavior that is suspected to be a manifestation of a paroxysmal EEG
dysrhythmia is suspected, PS can be a valuable activation method to use. EEG recording
during sleep, natural, or sedated, is now widely accepted as an essential technique for
eliciting a variety of paroxysmal discharges, when the wake tracing is normal, or for
increasing the number of abnormal discharges to permit a more definitive
interpretation. It has been shown that the central nervous system (CNS) stress produced
by 24 hours of sleep deprivation alone can lead to the activation of paroxysmal EEG
discharges in some cases.
NORMAL EEG TRACING
The normal EEG tracing (Fig. 1.8-3) is composed of a complex mixture of many different
frequencies. Discrete frequency bands within the broad EEG frequency spectrum are
designated with Greek letters.
FIGURE 1.8-3
Normal electroencephalogram (EEG) tracings in an awake 28-year-old man. (Reprinted
from Emerson RG, Walesak TS, Turner CA. EEG and evoked potentials. In: Rowland LP,
ed. Merritt’s Textbook of Neurology. 9th ed. Baltimore: Lippincott Williams & Wilkins;
1995:68, with permission.)
Awake EEG
The four basic wave forms are alpha, beta, delta, and theta. Highly rhythmic alpha waves
with a frequency range of 8 to 13 Hz constitute the dominant brain wave frequency of
the normal eyes-closed awake EEG. Alpha frequency can be increased or decreased by a
wide variety of pharmacological, metabolic, or endocrine variables. Frequencies that are
faster than the upper 13 Hz limit of the alpha rhythm are termed beta waves, and they
are not uncommon in normal adult waking EEG studies, particularly over the frontal–
central regions. Delta waves (≤3.5 Hz) are not present in the normal waking EEG, but
are a prominent feature of deeper stages of sleep. The presence of significant
generalized or focal delta waves in the awake EEG is strongly indicative of a
pathophysiological process. Waves with a frequency of 4.0 to 7.5 Hz are collectively
referred to as theta waves. A small amount of sporadic, arrhythmic, and isolated theta
activity can be seen in many normal waking EEG studies, particularly in frontal–
temporal regions. Although theta activity is limited in the waking EEG, it is a prominent
feature of the drowsy and sleep tracing. Excessive theta in awake EEG, generalized or
focal in nature, suggests the operation of a pathological process.
With maturation, EEG activity gradually goes from a preponderance of irregular
medium- to high-voltage delta activity in the tracing of the infant, to greater frequency
and more rhythmic pattern. Rhythmic activity in the upper theta–lower alpha range (7
to 8 Hz) can be seen in posterior areas by early childhood, and, by mid-adolescence, the
EEG essentially has the appearance of an adult tracing.
Sleep EEG
The EEG patterns that characterize drowsy and sleep states are different from the
patterns seen during the awake state. The rhythmic posterior alpha activity of the
waking state subsides during drowsiness and is replaced by irregular low-voltage theta
activity. As drowsiness deepens, slower frequencies emerge, and sporadic vertex sharp
waves may appear at central electrode sites, particularly among younger persons.
Finally, the progression into sleep is marked by the appearance of 14-Hz sleep spindles
(also called sigma waves), which, in turn, gradually become replaced by high-voltage
delta waves as deep sleep stages are reached.
EEG ABNORMALITIES
Apart from some of the obvious indications for an EEG study (i.e., suspected seizures),
EEG studies are not routinely performed as part of a diagnostic work-up in psychiatry.
EEG, however, is a valuable assessment tool in clinical situations in which the initial
presentation or the clinical course appear to be unusual or atypical (Table 1.8-1). Table
1.8-2 summarizes some common types of EEG abnormalities.
Table 1.8-1
Warning Signs of the Presence of Covert Medical or Organic Factors Causing
or Contributing to Psychiatric Presentation
Table 1.8-2
Common Electroencephalography (EEG) Abnormalities
Some psychotropic medications and recreational or abused drugs produce EEG
changes, yet, with the exception of the benzodiazepines and some compounds with a
propensity to induce paroxysmal EEG discharges, little, if any, clinically relevant effect
is noted when the medication is not causing toxicity. Benzodiazepines, which always
generate a significant amount of diffuse beta activity, have EEG-protective effects, so
that they can mask alterations caused by concomitant medications (Table 1.8-3).
Table 1.8-3
Electroencephalography (EEG) Alterations Associated with Medication and
Drugs
Medical and neurological conditions produce a wide range of abnormal EEG findings.
EEG studies, thus, can contribute to the detection of unsuspected organic
pathophysiology influencing a psychiatric presentation (Fig. 1.8-4). Table 1.8-4 lists
EEG alterations in medical disorders and Table 1.8-5 lists EEG alterations associated
with psychiatric disorders.
FIGURE 1.8-4
Diffuse slowing in a 67-year-old patient with dementia. Six- to seven cycles per second
(cps) activity predominates over the parieto-occipital regions. Although reactive to eye
closure, the frequency of this rhythm is abnormally slow. (Reprinted from Emerson RG,
Walesak TS, Turner CA. EEG and evoked potentials. In: Rowland LP, ed. Merritt’s
Textbook of Neurology. 9th ed. Baltimore: Lippincott Williams & Wilkins; 1995:68, with
permission.)
Table 1.8-4
Electroencephalography (EEG) Alterations Associated with Medical Disorders
Table 1.8-5
Electroencephalography (EEG) Alterations Associated with Psychiatric
Disorders
TOPOGRAPHIC QUANTITATIVE ELECTROENCEPHALOGRAPHY (QEEG)
Unlike standard EEG interpretation, which relies on waveform recognition, QEEG
involves a computer analysis of data extracted from the EEG. Findings are compared
with a large population database of subjects without any known neurological or
psychiatric disorder as well as QEEG profiles that may be characteristic of some defined
diagnostic group. In QEEG, the analog-based electrical signals are processed digitally
and converted to graphic, colored topographical displays. These images are sometimes
called “brain maps.” Color Plate 1.8-5 illustrates topographic QEEG images of a patient
with a closed head injury.
QEEG remains primarily a research method, but it holds considerable clinical potential for psychiatry, mainly in
establishing neurophysiological subtypes of specific disorders and for identifying electrophysiological predictors of
response. Examples of some of the more promising results of QEEG research include the identification of subtypes of
cocaine dependence and the subtype most likely to be associated with sustained abstinence; identification of subtypes of
obsessive–compulsive disorder (OCD) that predict clinical responsiveness or lack of responsiveness to selective serotonin
reuptake inhibitors (SSRIs); and the differentiation between normals, attention–deficit disorder and attentiondeficit/hyperactivity disorder (ADHD), and learning disability subpopulations. QEEG findings in ADHD show that
increased theta abnormality frontally may be a strong predictor of response to methylphenidate and other
psychostimulants and that favorable clinical responses may be associated with a normalization of the EEG abnormality.
CEREBRAL EVOKED POTENTIALS
Cerebral EPs are a series of surface (scalp) recordable waves that result from brain
visual, auditory, somatosensory, and cognitive stimulation. They have been shown to be
abnormal in many psychiatric conditions, including schizophrenia and Alzheimer’s
disease, thus creating difficulty in using cerebral EPs for differential diagnosis purposes.
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Comprehensive Textbook of Psychiatry. 9th ed. Philadelphia: Lippincott Williams & Wilkins; 2009:211.
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schizophrenia. J Abnorm Psychol. 2010;119:18.
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09 - 1.9 Chronobiology
1.9 Chronobiology
treatment outcomes in major depressive disorder. Psychiatr Clin North Am. 2007;30:105.
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Winterer G, McCarley RW. Electrophysiology of schizophrenia. In: Weinberger DR, Harrison PJ. Schizophrenia. 3rd ed.
Hoboken, NJ: Blackwell Publishing Ltd; 2011:311.
1.9 Chronobiology
Chronobiology is the study of biological time. The rotation of the Earth about its axis
imposes a 24-hour cyclicity on the biosphere. Although it is widely accepted that
organisms have evolved to occupy geographical niches that can be defined by the three
spatial dimensions, it is less appreciated that organisms have also evolved to occupy
temporal niches that are defined by the fourth dimension—time. Much like light
represents a small portion of the electromagnetic spectrum, the 24-hour periodicity
represents a small time domain within the spectrum of temporal biology. A broad range
of frequencies exist throughout biology, ranging from millisecond oscillations in ocular
field potentials to the 17-year cycle of emergence seen in the periodic cicada (Magicicada
spp.). Although these different periodicities all fall within the realm of chronobiology,
circadian (Latin: circa, about; dies, day) rhythms that have a period of about one day are
among the most extensively studied and best understood biological rhythms.
A defining feature of circadian rhythms is that they persist in the absence of time cues
and are not simply driven by the 24-hour environmental cycle. Experimental animals
housed for several months under constant darkness, temperature, and humidity continue
to exhibit robust circadian rhythms. Maintenance of rhythmicity in a “timeless”
environment points to the existence of an internal biological timing system that is
responsible for generating these endogenous rhythms.
The site of the primary circadian oscillator in mammals, including humans, is the
suprachiasmatic nucleus (SCN), located in the anterior hypothalamus. The mean
circadian period generated by the human SCN is approximately 24.18 hours. Like a
watch that ticks 10 minutes and 48 seconds too slowly per day, an individual with such
a period gradually comes out of synchrony with the astronomical day. In slightly more
than 3 months, a normally diurnal human would be in antiphase to the day–night cycle
and thus would become transiently nocturnal. Therefore, a circadian clock must be reset
on a regular basis to be effective at maintaining the proper phase relationships of
behavioral and physiological processes within the context of the 24-hour day.
Although factors such as temperature and humidity exhibit daily fluctuations, the
environmental parameter that most reliably corresponds to the period of Earth’s
rotation around its axis is the change in illuminance associated with the day–night cycle.
Accordingly, organisms have evolved to use this daily change in light levels as a time
cue or zeitgeber (German: zeit, time; geber, giver) to reset the endogenous circadian
clock. Regulation of the circadian pacemaker through the detection of changes in
illuminance requires a photoreceptive apparatus that communicates with the central
oscillator. This apparatus is known to reside in the eyes, because surgical removal of the
eyes renders an animal incapable of resetting its clock in response to light.
The circadian clock drives many rhythms, including rhythms in behavior, core body
temperature, sleep, feeding, drinking, and hormonal levels. One such circadianregulated hormone is the indoleamine, melatonin. Melatonin synthesis is controlled
through a multisynaptic pathway from the SCN to the pineal gland. Serum levels of
melatonin become elevated at night and return to baseline during the day. The
nocturnal rise in melatonin is a convenient marker of circadian phase. Exposure to light
elicits two distinct effects on the daily melatonin profile. First, light acutely suppresses
elevated melatonin levels, immediately decreasing them to baseline levels. Second, light
shifts the phase of the circadian rhythm of melatonin synthesis. Because melatonin can
be assayed easily, it provides a convenient window into the state of the circadian
pacemaker. Any perturbation of the clock is reflected in the melatonin profile; thus
melatonin offers an output that can be used to study the regulation of the central
circadian pacemaker.
SLEEP AND CIRCADIAN RHYTHMS
Sleep Regulation
Restful consolidated sleep is most appreciated when sleep disturbances are experienced.
Sleep is the integrated product of two oscillatory processes. The first process, frequently
referred to as the sleep homeostat, is an oscillation that stems from the accumulation and
dissipation of sleep debt. The biological substrates encoding sleep debt are not known,
although adenosine is emerging as a primary candidate neuromodulator of the sleep
homeostat. The second oscillatory process is governed by the circadian clock and
controls a daily rhythm in sleep propensity or, conversely, arousal. These interacting
oscillations can be dissociated by housing subjects in a timeless environment for several
weeks.
The circadian cycle in arousal (wakefulness) steadily increases throughout the day,
reaching a maximum immediately before the circadian increase in plasma melatonin
(Fig. 1.9-1). Arousal subsequently decreases to coincide with the circadian trough in core
body temperature. Experiments imposing forced sleep schedules throughout the
circadian day have shown that an uninterrupted 8-hour bout of sleep can only be
obtained if sleep is initiated approximately 6 hours before the temperature nadir. This
nadir typically occurs at approximately 5:00 AM to 6:00 AM. In healthy individuals,
initiating sleep between 11:00 PM and 12:00 AM affords the highest probability of getting
8 solid hours of sleep.
FIGURE 1.9-1
Relative phase relationship of sleep in young adults to other circadian phase markers.
(From Dijk D-J, Lockley SW. Invited review: Integration of human sleep-wake
regulation and circadian rhythmicity. J Appl Physiol. 2002;92:852, with permission.)
It should be stressed that diurnal preference varies among individuals as a function of
age, endogenous circadian periods, and other factors. This variability is paralleled by
physiology. Clinically, diurnal preference can be quantified using the Horne–Östberg
(HO) Morningness-Eveningness Questionnaire (MEQ). In qualitative terms, morning
people or morning larks tend to awaken earlier and experience the core body temperature
minimum at an earlier clock time relative to night people or night owls. Sleep deprivation
studies have shown that the homeostatic component of sleep is remarkably similar
among individuals of similar age. (It should be noted that there is a well-established agedependent decline in sleep need.) Therefore, diurnal preference is dictated almost
exclusively by the circadian component of sleep regulation.
Circadian Sleep Disorders
Advanced sleep phase syndrome (ASPS) is a pathological extreme of the morning lark
phenotype. An autosomal-dominant familial form of ASPS (FASPS) recently has been
genetically characterized. Afflicted family members exhibit a striking 4-hour advance of
the daily sleep–wake rhythm. They typically fall asleep at approximately 7:30 PM and
spontaneously awaken at approximately 4:30 AM. Affected individuals have a single
nucleotide polymorphism in the gene encoding hPER2, the human homolog of the mouse
Per2 clock gene. This adenine-to-guanine nucleotide polymorphism results in serine-toglycine amino acid substitution that causes the mutant protein to be inefficiently
phosphorylated by casein kinase Iε, an established component of the circadian
molecular clockwork. Similarly, delayed sleep phase syndrome (DSPS) has been shown
to be influenced by genetics. A length polymorphism in a repeat region of the hPER3
gene appears to be associated with diurnal preference in patients with DSPS, the shorter
allele being associated with evening preference.
The advent of the light bulb has extended the human day into the natural night. This
encroachment on the night, although increasing productivity, has affected human sleep
patterns (Fig. 1.9-2). Typical use of artificial lights results in a single, consolidated bout
of sleep lasting approximately 8 hours. This pattern of sleep is uncommon among most
other mammals, which typically experience more fractured sleep. Human sleep under
more natural photoperiods, where the duration of the night is longer, becomes
decompressed. Specifically, a bimodal distribution of sleep is observed; bouts of sleep
occur in early and late night. Periods of quiet wakefulness are interspersed between the
two primary bouts of sleep. This natural sleep pattern is more similar to the sleep
patterns of other mammals.
FIGURE 1.9-2
Change in sleep structure in response to artificial lighting. Total sleep time is reduced,
and periods of quiet wakefulness are abolished by extending daytime into nighttime
through artificial lighting. (From Wehr TA, Moul DE, Barbato G, et al. Conservation of
photoperiod-responsive mechanisms in humans. Am J Physiol. 1993;265:R846, with
permission.)
SEASONALITY
The 24-hour period of the Earth’s rotation around its axis is unchanging. However, the
Earth’s axis is tilted 23.45 degrees from the plane of its own orbit around the sun (the
ecliptic). As a result, the relative proportion of daytime to nighttime within the 24-hour
astronomical day varies as the Earth proceeds through its orbit of the sun. Many
organisms are capable of synchronizing physiology to the seasonal cycle to maximize
survival. For example, precise seasonal cycles in reproduction are seen throughout the
plant and animal kingdoms. Large mammals that typically have long gestation periods,
such as sheep, conceive in the fall when the nights are long and the days are short, so
birth occurs during the relatively mild season of spring. These animals are referred to as
short-day breeders. Conversely, mammals with gestation periods of only a few weeks,
such as hamsters, conceive and give birth during spring and summer, when the days are
long and the nights are short. Hence, these animals are referred to as long-day breeders.
Like circadian rhythms, many of these yearly (circannual) rhythms tend to persist in the
absence of seasonal cues with endogenous periods of approximately 1 year.
Melatonin and Seasonality
The most reliable environmental parameter providing a faithful representation of the
solar day is the day–night cycle. Similarly, the most reliable environmental parameter
reflecting the progression through the seasons is the change in day length, the fraction
of the 24-hour day between sunrise and sunset. In seasonally breeding animals, day
length is physiologically encoded through the melatonin profile. As described previously,
melatonin levels are elevated during the night. A long night, such as that experienced
during the short day lengths of winter, results in an elevated melatonin profile of a
relatively long duration. A short summer night, by contrast, results in a short duration of
elevated melatonin. This seasonal signal is interpreted by the reproductive axis,
resulting in an appropriate reproductive response. Melatonin’s role in transducing day
length was elucidated by pinealectomizing seasonally breeding animals, thereby
removing the primary endogenous source of melatonin. Melatonin was then infused in
profiles mimicking long days or short days. The duration of elevated melatonin was the
primary determinant of seasonal reproductive status, even when the infused profile was
administered under a conflicting day length. Variations in other parameters, such as the
amplitude of the melatonin profile, the amount of total melatonin synthesized, or the
phase relationship of the profile to the light–dark cycle, are of limited importance in
producing a humoral signal that transduces day length.
Reproductive responses to changing day length can be dramatic. A male Siberian
hamster (Phodopus sungorus) maintained in long days is reproductively competent and
typically has a testicular weight of approximately 250 mg per testis. Under short days,
however, the testes regress to approximately 15 mg per testis, representing a 94 percent
decrease in testicular mass. The same degree of regression is observed in response to
melatonin infusions that mimic short days. Communication of the hormonally
transduced day length to the reproductive axis is likely to be mediated, at least partially,
through melatonin receptors in the pars tuberalis of the pituitary gland. The exact
mechanism remains unknown, but activation of these receptors is hypothesized to
indirectly regulate an unidentified factor putatively named tuberalin. Tuberalin, in turn,
controls gene expression and prolactin release from lactotrophs in the adenohypophysis
of the pituitary.
Seasonality in Humans
Whether humans are truly seasonal is still a point of considerable debate. Several lines
of evidence exist that suggest the presence of a residual tendency toward seasonality. A
peak in the rate of suicide occurs in the summer; this peak is cross-cultural. Birth rates
also tend to show a seasonal variation; a small but distinguishable peak in the rate of
births occurs in spring and summer. This pattern, however, is itself variable and is
heavily influenced by unknown cultural and geographic factors. Of interest, the
amplitude of the spring–summer birth rate peak has diminished as societies have become
industrialized.
The decompressed bimodal structure of human sleep during long nights indicates that
the length of natural sleep is related to the length of the night. Potentially, a twooscillator system could function to maintain proper sleep patterns during changing
photoperiods. Such a proposed system would consist of an evening oscillator that tracks
the transition from day to night (dusk) and a morning oscillator that tracks the
transition from night to day (dawn). The relative phase differences between these
oscillators may encode the changing day lengths associated with the passing of the
seasons. Biological evidence for a two-oscillator system exists in rodents and humans.
The melatonin profile of many vertebrates, including some humans, is bimodal, with
evening and morning peaks. In rodents, metabolic and electrophysiological studies of
the SCN typically have been done in brain slices cut in the coronal plane. Results of
electrophysiological studies conducted in brain slices cut in the horizontal plane have
provided new insights. The action potential frequency in SCN neurons from horizontally
cut preparations is bimodal, with peaks in the early and late subjective day.
Furthermore, the interpeak interval varies as a function of the photoperiod in which the
animal was housed. These studies lend credence to long-standing suspicions that the SCN
of seasonally breeding mammals and, perhaps, nonseasonal mammals harbor a morning
and evening oscillator that interact to convey day-length information.
Effect of Aging
In general, as humans age, the circadian period shortens, the circadian phase advances
resulting in earlier waking times and bedtimes, the amplitudes of most circadian
rhythms decrease, and dramatic phase shifts such as those caused by jet-lag are less
tolerated. Again, a mouse model has provided interesting insight into the interaction of
the aging process and the circadian clock. The effect of chronic jet-lag on aged mice has
dramatic consequences on mortality. About half of aged mice forced to phase advance 6
hours once per week survive this treatment compared with an 83 percent survival rate
in unshifted age-matched mice. Aged mice subjected to weekly 6-hour phase delays show
an intermediate survival of 68 percent. These profound effects of phase shifting are not
observed in younger mice. The pathogenesis of chronic jet-lag remains to be determined.
Of interest, these mice did not have an increased rate of tumorigenesis. It is likely that
in humans, as in mice, the internal desynchrony of oscillators that result from a rotating
light schedule may have dire consequences that may be exacerbated by aging.
CIRCADIAN RHYTHMS AND PHARMACOTHERAPY
Circadian rhythmicity can be affected by drugs, and conversely, the circadian clock can
modulate the efficacy of drugs throughout the course of the day. A better understanding
of these interactions will lead to more effective pharmacotherapies. Some of the beststudied interactions between medications and the circadian clock have included the
circadian effects of antidepressants. Elevated nocturnal body temperature is a common
feature among depressed patients. This effect may be due to a reduced amplitude of the
master circadian oscillator in the hypothalamus that drives body temperature. Tricyclic
antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs) reduce
elevated nocturnal body temperature while simultaneously enhancing circadian
amplitude. Similarly, many depressed patients exhibit a dampened amplitude in daily
activity rhythms. Like body temperature, the amplitude in daily activity cycles of
depressed individuals may be augmented by TCA or SSRI therapy.
The use of lithium to treat bipolar disorder has been long established. However,
lithium also affects the circadian system, resulting in a lengthening of circadian period.
The molecular mechanism by which this occurs remains unknown. Glycogen synthase
kinase 3β (GSK3β) has been implicated in participating within the molecular clock
mechanism. Of interest, GSK3β is inhibited by lithium. In cell culture, GSK3β has been
shown to stabilize the negative clockwork regulator REV-ERBα via phosphorylation.
REV-ERBα typically represses the transcription of the BMAL1 gene. In the presence of
lithium, however, GSK3β is inhibited, thereby preventing the phosphorylation and
stabilization of REV-ERBα, which as a consequence is targeted for proteasomal
degradation. The degradation of REV-ERBα results in the de-repression of BMAL1
transcription. Whether lithium’s influence on circadian behavior is attributable to its
inhibitory effect on GSK3β-mediated stabilization of REV-ERBα remains to be
determined.
Short-acting benzodiazepines (e.g., triazolam [Halcion] and brotizolam [Lendormin])
also exert chronobiological effects. In hamsters, triazolam or brotizolam administered
during the middle of the subjective day induces circadian phase advances in activity.
Brotizolam has been shown to reduce the light-induced expression of clock genes Per1
and Per2 in the SCN. Although benzodiazepines are allosteric modulators of γaminobutyric acid A receptors (GABAA), several lines of evidence indicate that the
circadian effects of short-acting benzodiazepines require an intact serotonergic system.
When the 5-HT1A/7 receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT) is injected into hamsters at subjective midday, phase advances in locomotor
behavior and SCN neuronal activity are observed in addition to a reduction in Per1 and
Per2 gene expression in the SCN. Recreational drugs of abuse also affect the circadian
system. 3,4-Methylenedioxymethamphetamine (MDMA), or “ecstasy,” can act as a
serotonin neurotoxin. Hamsters treated with MDMA showed reduced triazolam-induced
phase shifts in circadian locomotor activity and a diminished ability to reentrain
rhythms posttreatment. MDMA-treated animals exhibited a reduction of serotonergic
axonal terminals in the SCN, again emphasizing the importance of an intact
serotonergic system in the regulation of the circadian axis. Recreational use of
methamphetamine
has
increased
dramatically.
Chronic
administration
of
methamphetamine disorganizes rodent activity rhythms. However, administration of
methamphetamine to rodents rendered arrhythmic through ablation of the SCN results in
a reemergence of rhythmicity. The mechanism involved in the rescue of rhythmicity or
site of action remains unknown.
The efficacy and toxicity of many pharmacotherapeutics vary as a function of
circadian phase. Daily variations in fixed-dose lethal toxicity have been appreciated in
rodents for years. Many anticancer drugs, ranging in mechanism from antimetabolites
to deoxyribonucleic acid (DNA) intercalators to mitotic inhibitors, have been shown to
have 2- to 10-fold changes in tolerability in rodents over the course of the day. Much of
this difference is attributed to circadian variations in the body’s ability to absorb,
distribute, metabolize, and eliminate toxic compounds. These four processes are affected
by underlying circadian rhythms in physiological processes such as daily variations in
gastric pH, gastrointestinal mobility, glomerular filtration rate, and membrane
viscosity. The rhythmic intake of food during traditionally timed meals also influences
how therapeutic drugs are handled by the body. It is becoming clear that to maximize
efficacy and minimize toxicity of drugs, the circadian phase of administration must be
considered. Appropriate circadian timing of the administration of multiple drugs can be
a daunting challenge to infirmed individuals or their caretakers. The development of
small implanted programmable pumps that can be directed to administer anticancer
drugs or other therapeutics at particular times of day may provide a limited solution to
this challenge. The emergence of the field of chronotherapy is a reflection of our
increased understanding of the impact of the circadian system on the effectiveness of
pharmacological treatments.
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