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to Preclinical Neuropsychopharmacology
Introduction to Preclinical Neuropsychopharmacology
Floyd E. Bloom
Neuropsychopharmacology links the frontiers of basic neuroscience to the treatment of neurological and psychiatric diseases. On one level, this scientific field seeks to understand how drugs can affect the central nervous system (CNS) selectively to relieve pain, heighten attention, induce sleep, reduce fever or appetite, suppress disordered movement, or prevent seizures. Notably, this is the field that seeks to understand how drugs can treat anxiety, mania, depression, or schizophrenia without altering consciousness. On a more profound level, this field seeks to understand the biological basis for such complex mental states as the disordered cognition of the schizophrenic. On this level, the goal is not only to understand the nature of the alterations in biology which lead to altered emotions and thought processes, but also to develop specific therapeutic molecules to regulate their biological underpinnings—namely, the as yet unspecified sequences of multineuronal interactions by which these behaviors emerge.
Drugs can affect precise molecular targets in discrete cells within selected circuits to influence specific interconnected systems of neurons that mediate or generate behaviors. By illuminating this process, neuropsychopharmacologists stand at the threshold of a profound scientific challenge—namely, to understand the cellular and molecular basis for the enormously complex and varied functions of the human brain. Among the most profound research questions that now pose testable challenges for neuropsychopharmacologists are the detailed understanding of the brain operations that account for normal mental activity and the pathological mental states of emotion, cognition, and perception. In this effort, neuropsychopharmacologists employ drugs in two strategies: (i) to dissect the functional and structural systems that operate in the normal CNS, thereby defining the specificity of these drugs as well as the systems on which they act, and (ii) to provide the means to develop appropriate drugs to correct pathophysiological events in the abnormal CNS.
A BRIEF GUIDE TO THE PRECLINICAL NEUROPSYCHOPHARMACOLOGY CHAPTERS
Comprehending the sites and mechanisms of action of drugs and drug classes requires an understanding of (a) the molecular biology of the cell classes of the brain and (b) the means by which these properties define the anatomy, physiology, and chemistry of the nervous system. This chapter serves to introduce the preclinical portion of this Generation of Progress volume. For scholars with limited prior experience in the neurosciences, it will provide some fundamental principles and concepts for the comprehensive analysis of drug actions on the CNS. As an overview, this chapter provides (a) a summary of the principal methods by which preclinical data are obtained and analyzed (see Basic concepts and Techniques of Molecular Genetics, Cytology and Circuitry, A critical Analysis of Neurochemical Methods for Montoring Transmitter Dynamics in the Brain, Electrophysiology and Behavioral Techniques in Preclinical Neuropsychopharmacology) and (b) a summary of the ensuing chapters (Chapters 7–57), which detail the recent progress on the basic elements of the pertinent brain systems and discuss the clinical implications of this progress. In addition, this chapter will survey some promising future developments that cut across diverse facets of the current research frontier (see Luteinzing Hormone-Releasing Hormone Neuronal, Arachidonic Acid, Nitric Oxide and Related Substance as Neural Messengers, Neuronal Growth and Differentiation Factors and Synaptic Placticity, Proto-Oncogenes: Beyond Second Messengers, Purinoceptors in Central Nervous System Function: Targets for Therapeutic Intervention, Brain Energy Metabolism: An Integrated Cellular Perspective, Molecular and Cellular Mechanisms of Brain Development, The Development of Brain and Behavior, Intracellular Messenger Pathways as Mediators of Neural Plasticity, Neuroendocrine Interactions and Interactions Between the Nervous System and the Immune System: Implications for Psychopharmacology), many of which provide the thresholds to the next generation of progress.
Within these three preclinical sections, certain key goals were held as uniform targets: Section I, the Critical Analyses of Methods (see Basic Concepts and Techniques of Molecular Genetics, Cytology and Circuitry, A Critical Analysis of Neurochemical Methods for Monitoring Transmitter Dynamics in the Brain, Electrophysiology, and Behavioral Techniques in Preclinical Neuropsychopharmacology Research) are intended to (a) provide new scholars of neuropsychopharmacology with a critical assessment of the primary research methods being applied currently, (b) indicate their advantages and disadvantages, and (c) indicate what qualities of data they can or cannot now provide. Section II, The Transmitter Systems, focuses on the well-defined neurotransmitter systems which provide the molecular and cellular substrate by which drugs act to influence behavior and to treat neurological and psychiatric diseases. The transmitter systems are arbitrarily divided into three traditional chemical categories: (i) amino acids (see Excitatory Amino Acid Neurotransmission, and GABA and Glycine); (ii) aminergic neurons, further subdivided into transmitters: acetylcholine (see Neuronal Nicotinic Acetylcholine Receptors: Novel Targets for Central Nervous System Therapeutics, Molecular Biology, Pharmacology, and Brain Distribution of Subtypes of the Muscarinic Receptor, Cholinergic Transduction, Structure and Function of Colonergic Pathways in the Cerebal Cortex, Limbic System, Basal Ganglia, and Thalamus of the Human Brain, and Functional Heterogeneity of Central Cholinergic Systems), dopamine (see Molecular Biology of the Dopamine Receptor Subtypes, Electrophysiological Properties of Midbrain Dopamine Neurons, The Dopamine Transporter: Potential Involvement in Neuropsychiatric Disorders, Long - and - Short - Term Regulation of Tyrosine Hydroxylase, Colocalization in Dopamine Neurons, Dopamine Receptor Expression in the Central Nervous System, Dopamine Autoreceptor Signal Transduction and Regulation, Biochemical Pharmacology of Midbrain Dopamine Neurons, Dopaminergic Neuronal Systems in the Hypothalamus, Electron Microscopy of Central Dopamine Systems, Development of Mesencephalic Dopamine Neurons in the Nonhuman Primate: Relationship to Survival and Growth Following Neural Transplantation, Mesocorticolimbic Dopaminergic Neurons: Functional and Regulatory Roles, and Dopamine Receptor Transcript Localization in Human Brain), noradrenaline (see Dopamine Receptors: Clinical Correlates, Signal Transduction Pathways for Catecholamine Receptors, Norepinephrine and Serotonin Transporters: Molecular Targets of Antidepressant Drugs, Pharmacology and Physiology of Central Noradrenergic Systems, Coexisting Neurotransmitters in Central Noradrenergic Neurons, Modification of Central Catecholaminergic Systems by Stress and Inujury: Functional Significance and Clinical Implications, Central Norepinephrine Neutrons and Behavior, and The Noradrenergic Receptor Subtypes), histamine (see Physiological and Anatomical Determinants of Locus Coeruleus Discharge: Behavior and Clinical implecations), and serotonin (see Noradrenergic Neural Subtrates for Anxiety and Fear: Clinical Associations Based on Preclinical Research, Histamine, Molecular Biology of Serotonin Receptors: A basis for Understanding and Addressing Brain Function, Serotonin Receptor Subtypes and Ligands, Gene Targeting Approaches to Serotonin Receptors, Serotonin Receptors: Signal Transduction Pathways, and Anatomy, Cell Biology, and Maturation of the Serotonergic System: Neurotrophic Implications for the Actions of Psychotropic Drugs); and (iii) neuropeptides (see Electrophysiology of Serotonin Receptor Subtypes and Signal Transduction Pathways, Serotonin and Behavior: A General Hypothesis, Indoleamines: the Role of Serotonin in Clinical Disorders, Monoamine Oxidase: Basic and Clinical Perspectives, General Overview of Neuropeptides, Thyrotropin - Releasing Hormone: Focus on Basic Neurobiology, Corticotropin - Releasing Factor: Physiology, Pharmacology, and Role in Central Nervous System and Immune Disorders, Neuropharmacology of Endogenous Opioid Peptides, Vassopressin and Oxytocin in the Central Nervous System, Neuropeptide Y and Related Peptides). Within each of these transmitter systems, the authors have identified the essential starting premises from the previous Generation of Progress volume (5) and have focused their attention mainly on what are considered to be the most important elements of subsequent progress. Depending on the depth of neuropsychopharmacological relevance of these transmitter systems, each transmitter's coverage may include, to varying degrees, data from each of the principal levels of analysis, namely, molecular, cellular, systems, and behavioral (see below).
These chapters on the traditional transmitters are then followed by preliminary discussions on several signaling molecules that are likely to come into increased recognition as important members of the interneuronal signaling process: arachidonic acid (see Somatostatin in the Central Nervous System), nitric oxide and related substances (see Galanin: A Neuropeptide with important Central Nervous System Actions), purines (see Vasoactive Intestinal Peptide in Central Nervous), and the series of intercellular (see The Neurobiology of Neurotensin) and intracellular (see Cholecystokinin) growth and differentiation molecules which seem critical for normal nervous system development as well as for maintenance of function.
Section III examines integrative concepts which transcend individual transmitter systems, but which enlighten the means by which neuronal activity is normally coordinated to meet the demands of the internal and external environments faced by a given individual. These critical brief reviews examine brain energy metabolism (see Luteinzing Hormone-Releasing Hormone Neuronal), brain development (see Arachidonic Acid), the development of behavior (see Nitric Oxide and Related Substance as Neural Messengers), intracellular mechanisms of adaptive plasticity (see Neuronal Growth and Differentiation Factors and Synaptic Placticity), the interactions of the nervous system with the endocrine (see Proto-Oncogenes: Beyond Second Messengers and Intracellular Messenger Pathways as Mediators of Neural Plasticity) and immune systems (see Purinoceptors in Central Nervous System Function: Targets for Therapeutic Intervention), the means by which animals become tolerant to the behavior altering effects of drugs (see Brain Energy Metabolism: An Integrated Cellular Perspective), and the utility of genetic models (see Interactions Between the Nervous System and the Immune System: Implications for Psychopharmacology), as well as more traditional animal models for abused drugs (see The Development of Brain and Behavior) and for psychiatric disorders (see Neuroendocrine Interactions) including dysfunctional sexual behavior (see Molecular and Cellular Mechanisms of Brain Development).
These preclinical progress reports thus provide the basis for the specific therapeutic approaches to neurological and psychiatric disorders which are presented in the clinical chapters that follow (Sections IV–VI). The relationship is fruitfully bidirectional: Untreatable diseases and unexpected nontherapeutic side effects reveal ill-defined mechanisms of pathophysiology which can drive preclinical research to search for additional mechanisms of cellular regulation to link molecular processes to behavior.
HIERARCHICAL LEVELS OF RESEARCH IN NEUROSCIENCES
Four hierarchical levels of analysis epitomize the research strategies used to analyze the neuroscientific substrates of neuropsychological phenomena: molecular, cellular, multicellular (or systems), and behavioral. These terms constitute the minimal dissection of a complex hierarchical ensemble that we have previously noted to epitomize the principal methods of neuroscience research (see ref. 2). The main underlying concept of neuropsychopharmacology is that drugs which influence behavior and improve the functional status of patients with neurological or psychiatric diseases act by enhancing or blunting the effectiveness of chemical transmission at the sites of principal interneuronal communication, the specialized chemical junctions termed synapses.
The intensively exploited molecular level has been the traditional focus for characterizing drugs that alter behavior. Molecular discoveries provide biochemical probes for identifying the appropriate neuronal sites and their mediative mechanisms. Such mechanisms include the neurotransmitters' receptors as well as the auxiliary molecules that allow these receptors to influence the short-term biology of responsive neurons (through regulation of ion channels) and their longer-term regulation (through alterations in gene expression (see Basic concepts and Techniques of Molecular Genetics), (see Dopamine Receptors: Clinical Correlates), (see Coexisting Neurotransmitters in Central Noradrenergic Neurons), (see The Neurobiology of Neurotensin), (seeCholecystokinin and Neuronal Growth and Differentiation Factors and Synaptic Placticity). Molecular level research also provides the pharmacologic tools to verify the working hypotheses of other molecular, cellular, and behavioral strategies and allows for a means to pursue their genetic basis (see Interactions Between the Nervous System and the Immune System: Implications for Psychopharmacology, Psychopharmacology of Anorexia Nervosa, Bulimia Nervosa, and Binge Eating, and Basic Biological Overview of Eating Disorders).
During the interval since the previous Generation of Progress volume (see ref. 5), some of the most exciting and rigorous discoveries at the molecular level have been made, finally affording scientists a glimpse into the nature of the molecules that serve as ion channels, as neurotransmitter receptors, and as transporters for the reaccumulation of transmitter into some neurons after its release. At the level of the molecules that make up the transmitter storage and release sites (i.e., the small organelles termed synaptic vesicles), virtually every step of the physiological process of loading the transmitter into the vesicles, storing it, moving the vesicle close to the release sites, activating the Ca-dependent process of transmitter release, and then recycling the vesicle for re-use has now been identified (see ref. 4).
The most basic molecular phenomena of neurons are now becoming visualizable in terms of such discrete molecular entities. While it has been known for some time that the basic excitability of neurons was achieved through modifications of the ion channels that all neurons express in abundance in their plasma membranes, it is now possible to understand precisely how the three major cations—Na, K, and Ca—are regulated in their flow through highly discriminated ion channels, and it is also possible to determine how drugs, toxins, and imposed voltages can alter the excitability of a neuron, can allow it to become active spontaneously, or can lead to its death through prolonged opening of such channels. The scope of this work was deemed too detailed for this volume, but several recent compilations of such information may provide the interested reader with starting points (see refs. 1 and 3).
Research at the cellular level determines which specific neurons and which of their most proximate synaptic connections may mediate a behavior or the behavioral effects of a given drug. For example, research at the cellular level into the basis of emotion exploits both molecular and behavioral leads to determine the most likely brain sites at which behavioral changes pertinent to emotion can be analyzed, and it provides the preliminary clues as to the nature of the interactions in terms of interneuronal communication (i.e., excitation, inhibition, or more complex forms of synaptic interaction (see ref. 2); also see Electrophysiology, for references).
At present, the most underilluminated phase of this multilevel strategic conceptualization is the multicellular, or systems, level—namely, the means by which events on the behavioral level can be linked to discrete cells and circuits, and obviously vice versa. Such an understanding of systems levels is obligatory, for example, in order to draw together the descriptive structural and functional properties of the central catecholamine neurons and their possible function at the behavioral level. For example, two decades ago brain catecholamines were implicated as "the" critical chemical mediators of a variety of physiological–behavioral outputs of the brain, ranging from feeding, drinking, thermoregulation, and sexual behavior to such abstract actions as pleasure, reinforcement, attention, motivation, memory, learning–cognition, and the major psychoses and their chemotherapy. While many such hypotheses were proposed, there was no conclusive proof that a monoamine "mediates" any behavior (see Modification of Central Catecholaminergic Systems by Stress and Injury: Functional Significance and Clinical Implications). Furthermore, a unifying hypothesis as to how any catecholamine cellular system could possibly be legitimately involved in so many global actions was difficult to conceive.
From a purely hypothetical view, one could argue that behavioral tests of a central catecholamine circuit do not require a database of molecular and cellular attributes. In contrast, a cellularly based neurobiologist would argue that until hypotheses of behavioral level functions integrate themselves with the known anatomy and cellular physiology of these transmitter systems, the behavioral interpretations cannot be validated at the cellular level. Thus, a behavioral event lacking intrinsically specified operations for particular cellular sequences is a relatively untestable hypothesis in terms of whether the cells and transmitter system are, in fact, essential for the behavior. For example, with the locus coeruleus system (see Norepinephrine and Serotonin Transporters: Molecular Targets of Antidepressants Drugs and Central Norepinephrine Neurons and Behavior), the recent direct electrophysiological observations in the behaviorally responsive animal provide a far more detailed database on which to formulate specific testable hypotheses regarding a noradrenaline cellular role in specific types of behavior (also see The Noradrenergic Receptor Subtypes). These cellular data indicate that the locus coeruleus system fires with the occurrence of novel external sensory events, rather than with learning or extinction per se, and that the neurons are under strong inhibitory influences during vegetative acts such as eating, grooming, or sleeping. If we were to take a "bottoms up" approach to analyzing noradrenergic relevant behavior based on these cellular analyses, the suggestion might be that tasks involving contingencies of sensory discrimination of novel objects or during significant environmental demand (stress) might be the most relevant conditions in which to demonstrate behavioral perturbations (see Modification of Central Catecholaminergic Systems by Stress and Injury: Functional Significance and Clinical Implications).
In regard to the dopaminergic systems (see Brain Imaging in Mood Disorders), the cellular correlative approach to behavioral function has so far been less rewarding. Better clues to be followed in characterizing the dopaminergic neurons may, in contrast, be a "top down" approach, in which the optimal conditions for seeking cellular correlates would be based upon the results of the behavioral observations. The behavioral observations (see Mesocorticolimbic Dopaminergic Neurons: Functional and Regulatory Roles) suggest that a response initiation task would be most critically dependent upon dopaminergic function. If this view were valid, cellular correlates of neuronal firing should then become apparent in such tasks. Clearly, within the functional properties of dopamine neurons lies a robust capacity to maintain and restore their synaptic operations even under conditions in which they have been almost completely ablated (see A Critical Analysis of Neurochemical Methods for Monitoring Transmitter Dynamics in the Brain).
Among the many profound problems awaiting future research is the riddle of how to evolve these tentative behavioral consequences in the normal brain to clarify the possible relevance of the central catecholamine neurons to the behavioral symptoms of the major psychopathologies. Although that extension currently seems unlikely to be realized in the near future, continued attention to a multidisciplinary, multilevel approach to central catecholamine neuron function (or any other specified cellular system) may eventually provide such answers.
Research at the behavioral level (see Behavioral Techniques in Preclinical Neuropsychopharmacology Research, Modification of Central Catecholaminergic Systems by Stress and Injury: Functional Significance and Clinical Implications, Brain Energy Metabolism: An Integrated Cellular Perspective, Molecular and Cellular Mechanisms of Brain Development, The Development of Brain and Behavior, and Neuroendocrine Interactions) centers on the integrative phenomena that link populations of neurons (often through operationally or empirically defined ways) into extended specialized circuits, ensembles, or more pervasively distributed "systems" that integrate the physiological expression of a learned, reflexive, or spontaneously generated behavioral response. The entire concept of "animal models" of human psychiatric diseases rests on the assumption that scientists can appropriately infer from observations of behavior and physiology (heart rate, respiration, locomotion, etc.) that the states experienced by animals are equivalent to the emotional states experienced by humans expressing these same sorts of physiological changes. Such hypotheses as the catecholamine hypothesis of depression or the dopamine hypothesis of schizophrenia are continuously tested by clinical observations on the neurochemical and neuropharmacological correlates of emotional diseases (e.g., major depression; see The Dopamine Transporter: Potential Involvement in Neuropsychiatric Disorders) and the anxiety disorders (see The Noradrenergic Receptor Subtypes) as well as by the emotional consequences of the self-administration of drugs that are addictive in humans (see The Development of Brain and Behavior). Inferences as to the locus of the cells or cell systems central to experimental analysis of emotion have also been provided by lesions or stimulations of specific brain sites (see Behavioral Techniques in Preclinical Neuropsychopharmacology Research), and both approaches figure prominently into attempts to establish animals models for at least certain aspects of these complex human conditions (see Neuroendocrine Interactions).
HOW THE BRAIN IS ORGANIZED
The brain is an assembly of interrelated neural systems that regulate their own and each other's activity in a dynamic, complex fashion (2). The major visible regions of the brain can be linked superficially to coarse definitions of brain functions, and within and between these visible macroscopic regions lie interconnected cellular elements which provide their detailed and interdependent operations. Thus, the central nervous system can be subdivided into the forebrain (cerebral cortex, thalamus, and hypothalamus), the midbrain, the hindbrain (pons, medulla, and cerebellum), and the spinal cord. The largest mass consists of the cerebral hemispheres, comprised of the outer cellular zone or cortex, and a number of well-defined subcortical regions, named on the basis of their appearance or location, including several of immediate pertinence to neuropsychopharmacology: the hippocampal formation, the basal ganglia, the amygdaloid complex, the thalamus, and the hypothalamus.
Cerebral hemispheres, in turn, are typically classified into cortical regions on the basis of one of several characteristics such as the sensory modality subserved there (e.g., somatosensory, visual, auditory, or olfactory, while other regions are concerned with motor operations or are termed "associational" to imply integrations between sensory modalities and motor performance) or the anatomical location (frontal, temporal, parietal, or occipital). An alternative scheme classifies the cortex microscopically in terms of the geometrical relationship between cell types—generally their size, shape, and packing density across the major cortical layers (so-called cytoarchitectonic classifications).
Within any given region of the cerebral cortex, the four or six layers of which it is composed will appear essentially uniform microscopically, and it is thought that ensembles of vertically connected neurons which span the layers comprise the elemental processing modules. The specialized functions of a cortical region arise from the interplay upon this basic module of connections to and from both other regions of the cortex (corticocortical systems) and noncortical areas of the brain (subcortical systems). Varying numbers of adjacent columnar modules may be functionally, but transiently, linked into larger information-processing ensembles. The pathology of Alzheimer's disease (see New Developments in Dopamine and Schizophrenia), for example, destroys the integrity of the columnar modules and the corticocortical connections.
CELLS OF THE BRAIN
At the most elemental level, the cell types found in the nervous system can be divided into neurons and non-neuronal cells. The non-neuronal cells are estimated to outnumber the neuronal cells by at least an order of magnitude. The non-neuronal cells of the central nervous system consist of the macroglia, the microglia, and the cells of the vascular elements, including the intracerebral vessels and the vasculature of the cerebrospinal fluid forming tissues found within the cerebral ventricles, the choroid plexus. The macroglia, like the neurons, arise from the neuroectoderm, but somewhat later during development (see Arachidonic Acid). This cell class can be further divided into (a) the astrocytes, which are interposed between the vasculature and the neurons and which are regarded as serving supportive metabolic roles for the neurons especially within the gray matter of the brain and spinal cord (these metabolic and support functions are described in Luteinzing Hormone-Releasing Hormone Neuronal), and (b) the oligodendroglia, a class of cells that produce the myelin coating that allows some axons (the efferent process of neurons) to conduct bioelectric signals rapidly over long distances. The microglia are a second form of incompletely characterized central nervous system supportive cells; these cells are believed to be derived from mesodermal origin and are thought to be related to the macrophage monocyte cell lineage. This cell class has some members that are permanently resident in the brain, but the general class may be augmented from the peripheral circulation during injury or acute inflammatory responses (progress in the understanding of the interfaces between the neural and immune systems are described in Purinoceptors in Central Nervous System Function: Targets for Terapeutic Intervention).
Neurons are the class of cells whose physical interconnections constitute the circuitry of the brain, spinal cord, and the peripheral nervous system and give rise to the multicellular ensembles of neurons which carry out the functions of the nervous system. Neurons are thus regarded as the information-processing elements. Neurons differ widely in their size, shape, location, and other intrinsic properties. Neurons communicate chemically by releasing (or secreting) and responding to a wide range of chemical substances, referred to in the aggregate as neurotransmitters. The release of the neurotransmitters is tightly coupled temporally to neuronal activity according to rather stringent functional rules. The sets of chemical substances that neurons can secrete when they are active can also influence the non-neuronal cells. The functional activity of a neuron, measured either through changes in its excitability or by changes in its chemical operations, can also be modified by a different range of chemicals released from non-neuronal cells of the central or peripheral nervous system, and the latter substances are often referred to as neuromodulators. Products released by the non-neuronal cells of the immune system that may be present in the brain during acute infections form a major focus of attention in the considerations of neural-immune interactions (see Purinoceptors in Central Nervous System Function: Targets for Terapeutic Intervention).
Current research on neurotransmitters and neuromodulators is devoted to: (a) understanding the genes (see Basic concepts and Techniques of Molecular Genetics) that control the synthesis, storage, release, conservation (see Signal Transduction Pathways for Catecholamine Receptors), and metabolism of known neurotransmitters; (b) identifying new substances that meet the identifying criteria to be recognized as neurotransmitters; (c) understanding the molecular events by which neurons and other cells react to neurotransmitters (a process often termed signal transduction, which cells of the nervous system share with most other cells of the body) in the short-term (see Dopamine Receptors: Clinical Correlates and Norepinephrine and Serotonin Transporters: Molecular Targets of Antidepressants Drugs) and long-term time frame (see Norepinephrine and Serotonin Transporters: Molecular Targets of Antidepressants Drugs and Cholecystokinin); and (d) understanding the operations of neuronal communication in an integrative context of the circuits that release and respond to specific transmitters, and the way in which these neuronal circuits participate in defined types of behavior, either normal or abnormal.
CONCEPTS OF NEURONAL CIRCUITRY AND SIGNAL TRANSDUCTION IN THE NORMAL BRAIN
After two decades of increasingly precise examination, a very large part of the organization of the mammalian brain's circuitry has been resolved, at least in rodents (see Cytology and Circuitry; also see Electron Microscopy of Central Dopamine Systems for dopaminergic considerations, Structure and Function of Colonergic Pathways in the Cerebal Cortex, Limbic System, Basal Ganglia, and Thalamus of the Human Brain for cholinergic considerations, and Serotonin Receptor Subtypes and Ligands for serotonergic considerations), and the major mechanisms and mediators of signal transduction have been determined (see Electrophysiology). From this body of work, three sets of simplifying observations can be extracted for a modern, but hypothetically simplified, view of brain structure and function.
Brain Circuitry Patterns
Three main patterns of neuronal circuitry are recognized:
1. long hierarchical circuits (such as those characterizing the interconnected major pathways of the sensory, motor, and intracortical relay systems in which excitatory amino acids are generally the transmitter—see Excitatory Amino Acid Neurotransmission);
2. local circuit neurons (such as the short axon neurons, both excitatory and inhibitory, that regulate the extent to which afferent signals can spread, a role frequently subsumed by the inhibitory amino acid transmitters, GABA and GLY (see GABA and Glycine), and containing one or more neuropeptide co-transmitters (see Anatomy, Cell Biology, and Maturation of the Serotonergic System: Neurotrophic Implications for the Actions fo Psychotropic Drugs, Electrophysiology of Serotonin Receptor Subtypes and Signal Transduction Pathways, Serotonin and Behavior: A General Hypothesis, Indoleamines: the Role of Serotonin in Clinical Disorders, Monoamine Oxidase: Basic and Clinical Perspectives, General Overview of Neuropeptides, Thyrotropin - Releasing Hormone: Focus on Basic Neurobiology, Corticotropin - Releasing Factor: Physiology, Pharmacology, and Role in Central Nervous System and Immune Disorders, Neuropharmacology of Endogenous Opioid Peptides, Vassopressin and Oxytocin in the Central Nervous System)
3. single-source, divergent neurons (such as those neurons of the brainstem's reticular core nuclei, whose axons diverge to target cells in many parts of the neuraxis, the format exhibited by virtually all of the aminergic neurons (see Structure and Function of Colonergic Pathways in the Cerebal Cortex, Limbic System, Basal Ganglia, and Thalamus of the Human Brain) and by some of the neuropeptide-containing neurons (see Monoamine Oxidase: Basic and Clinical Perspectives and General Overview of Neuropeptides).
Against this simplified view of neuronal circuitry, the major chemical classes of neurotransmitters may be captured in a similar triadic fashion: amino acid transmitters, of which glutamate and aspartate (see Excitatory Amino Acid Neurotransmission) are recognized as the major excitatory transmitting signals, and gamma aminobutyrate (GABA) and glycine (see GABA and Glycine) as the major inhibitory transmitters; the aminergic transmitters (acetylcholine, epinephrine, norepinephrine, dopamine, serotonin, and histamine; each are described in a series of chapters in Section II); and the literally dozens of peptides (see Electrophysiology of Serotonin Receptor Subtypes and Signal Transduction Pathways, and related chapters that follow). A revolutionary finding has emerged here in concepts of brain system interactions: It would now seem that neuropeptides are almost certainly never the sole signal to be secreted by a central neuron that contains such a signaling molecule, but rather a companion signal to one or more potentially secreted signals. In the best-studied cases, neuropeptides are found with either an amino acid or an amine transmitter at intrasynaptic terminal concentrations a thousand- to a millionfold higher (see Colocalization in Dopamine Neurons). The peptide-containing neurons may also contain a second or third peptide as well (see Electrophysiology of Serotonin Receptor Subtypes and Signal Transduction Pathways).
It is also likely, but not yet definitively established, that other kinds of molecules, from purines like adenosine triphosphate (see Vasoactive Intestinal Peptide in Central Nervous), lipids like arachidonic acid and prostaglandins (see Somatostatin in the Central Nervous System), and steroids similar to those made and released by the adrenal cortex and the gonads (see Proto-Oncogenes: Beyond Second Messengers, Purinoceptors in Central Nervous System Function: Targets for Terapeutic Intervention, Intracellular Messenger Pathways as Mediators of Neutral Plasticity, and Molecular Neurobiology of Development), may also be made by neurons to play important auxiliary roles in intercellular transmission in the nervous system. A recent flurry of activity has revealed that under some conditions active neurons may synthesize gaseous signals (such as nitric oxide and carbon monoxide) that can carry rapidly evanescent signals over short distances (see Galanin: A Neuropeptide with important Central Nervous System Actions). Some peptide growth factors can effect trophic actions on cells of the nervous system as well as on non-neuronal cells in many other tissues, while other polypeptide growth factors are selective in their trophic actions (see The Neurobiology of Neurotensin). Interestingly, some "neuropeptides" have considerable growth potential for non-neuronal cells outside of the central nervous system in addition to their effects in communication between neurons (see Vassopressin and Oxytocin in the Central Nervous System).
The bioelectric properties of neurons and their synaptic junctions in the CNS generally follow the general mechanisms of chemical transmission defined for the peripheral somatomotor and autonomic nervous systems (see Electrophysiology and Dopamine Receptors: Clinical Correlates). The series of defined steps in the transmission of chemical messages from neurons to their target cells each provide the potential for pharmacological intervention. Transmitters, or drugs that can either stimulate or antagonize them, act at specific molecular recognition sites, termed receptors. In molecular morphology, the known classes of receptors also conveniently break down into distinct categories.
In the simplest case, the "ionophore receptor," the receptor is oligomeric (i.e., formed as an assembly of four or five highly similar subunit molecules, which in this case are inferred from their chemical properties to exhibit four transmembrane domains, as well as intracytoplasmic loops that provide links to other transductive sequences). Such a macromolecule constitutes both the functional ion channel to be opened or closed by the action of the transmitter and also contains the sites for recognition and binding of the transmitter. Examples include receptors for GABA and glycine in the CNS (see GABA and Glycine), the excitatory amino acid glutamate (see Excitatory Amino Acid Neurotransmission), the nicotinic cholinergic receptor (see Neuronal Nicotinic Acetylcholine Receptors: Novel Targets for Central Nervous System Therapeutics), and one of the several subtypes of serotonin receptors (see Molecular Biology of Serotonin Receptors: A basis for Understanding and Addressing Brain Function). Multiple forms of the mRNAs that encode one or more of the subunits of several ligand-regulated ion channels have been detected, and it is very likely that different forms of these receptors are expressed in various types of neurons (see ref. 6).
The mechanism of action of the other major class of neurotransmitter receptors, termed G-protein-coupled receptors (see Neuronal Growth and Differentiation Factors and Synaptic Placticity), is clearly more complex and involves the concerted function of a series of interacting macromolecules. These include: the plasma-membrane-bound receptor itself, with its extracellularly oriented ligand-binding domain; the membrane-associated "G proteins" [proteins that specifically bind guanosine triphosphate (GTP) and that hydrolyze it to guanosine monophosphate during this process], which act as transducers by coupling activation of the receptor to regulation of the activity of an effector protein; and the actual effector molecule of the transductive pathway, which may be either a membrane-bound enzyme (e.g., adenyl cyclase) or an ion channel. The genes encoding more than two dozen of these G-protein-coupled receptors have been sequenced, providing for further interesting generalizations. These include muscarinic cholinergic receptors (see Molecular Biology, Pharmacology, and Brain Distribution of Subtypes of the Muscarinic Receptor), both a- and b-adrenergic receptors (see Dopamine Receptors: Clinical Correlates), all other types of tryptaminergic receptors (see Histamine and Gene Targeting Approaches to Serotonin Receptors), and the receptors for all known neuropeptides (see Electrophysiology of Serotonin Receptor Subtypes and Signal Transduction Pathways). All G-protein-coupled receptors consist of seven presumptive transmembrane spans, with intervening extracellular and cytoplasmic loops. The ligand-binding sites of these receptors are contributed by several amino acid residues that lie within the transmembrane-spanning segments. The cytoplasmic loops interact with the G protein. Depending on the receptor and the G protein involved, the ultimate result can be activation or inhibition of adenyl cyclase, activation of one or more phospholipases (see Cholinergic Transduction), or regulation of the activity of a variety of ion channels (e.g., for K+ or Ca2+; see Electrophysiology).
DRUGS CAN SELECTIVELY MODIFY CNS FUNCTIONThe structural and functional properties of neurons provide a means to specify the possible sites at which drugs could interact specifically or generally in the CNS. In this scheme, drugs that affect neuronal energy metabolism (see Luteinzing Hormone-Releasing Hormone Neuronal), membrane integrity, or transmembrane ionic equilibria would be generally acting compounds. Similarly general in action would be drugs that affect the two-way intracellular transport systems by which molecules are transported from the perinuclear cytoplasm of a neuron up the dendrites and down the axons and then back again (e.g., colchicine). These general effects can still exhibit different dose–response or time–response relationships among different neurons based, for example, on such neuronal properties as rate of firing, dependence of discharge on external stimuli or internal pacemakers, resting ionic fluxes, or axon length.
In contrast, when drug actions can be related to specific aspects of the metabolism, release, or function of a neurotransmitter, the site, specificity, and mechanism of action of a drug can be defined by systematic studies of dose–response and time–response relationships. From such data the most sensitive, rapid, or persistent neuronal event can be identified. Transmitter-dependent actions of drugs can be organized conveniently into presynaptic and postsynaptic categories. Each of these presynaptic or postsynaptic actions is potentially highly specific and can be envisioned as being restricted to a single, chemically defined subset of CNS cells.
The presynaptic category includes all of the events in the perikaryon and nerve terminal that regulate transmitter synthesis (including the acquisition of adequate substrates and cofactors), storage, release, reuptake, and catabolism. Transmitter concentrations can be lowered by blockade of synthesis, storage, or both. The amount of transmitter released per impulse is generally stable but can also be regulated. The effective concentration of transmitter may be increased by inhibition of reuptake or by blockade of catabolic enzymes. The transmitter that is released at a synapse can also exert actions upon the terminal from which it was released by interacting with receptors at these sites (termed autoreceptors; see Biochemical Pharmacology of Midbrain Dopamine Neurons). Activation of presynaptic autoreceptors can slow the rate of release of transmitter and thereby provide a feedback mechanism that controls the concentration of transmitter in the synaptic cleft. Coexisting peptides may also perform similar functions for other aminergic neurons (see Thyrotropin - Releasing Hormone: Focus on Basic Neurobiology and Neuropharmacology of Endogenous Opioid Peptides).
The postsynaptic category includes all the events that follow release of the transmitter in the vicinity of the postsynaptic receptor—in particular, the molecular mechanisms by which occupation of the receptor by the transmitter produces changes in the properties of the membrane of the postsynaptic cell (shifts in membrane potential) as well as more enduring biochemical actions (changes in intracellular cyclic nucleotides, protein kinase activity, and related substrate proteins; see Structure and Function of Colonergic Pathways in the Cerebal Cortex, Limbic System, Basal Ganglia, and Thalamus of the Human Brain, Dopamine Receptors: Clinical Correlates, Molecular Biology of Serotonin Receptors: A basis for Understanding and Addressing Brain Function, Cholecystokinin, and Neuronal Growth and Differentiation Factors and Synaptic Placticity).
Current efforts in neuropsychopharmacology focus in part on the adaptive changes imposed on the nervous system by chronic treatment with drugs (see Neuronal Growth and Differentiation Factors and Synaptic Placticity) and in part on the nature of the ongoing dynamic regulation of neuronal functions in proportion to the demands placed on the behaving organism. Clearly the metabolic and functional changes observed initially on acute treatments of normally behaved animals do not persist and are replaced at varying intervals by changes that may in fact be opposite to those seen acutely. With the ability to clone, sequence, and express the genes that encode receptor molecules for neurotransmitters, a new era in drug development is approaching. Such studies have permitted the identification of novel receptor subtypes that were undetected by traditional pharmacological approaches; the pace of such discovery will accelerate. It is now possible to create novel mouse transgenic mutants in which specific genes have been knocked out or amplified in a few or many cells (see Basic concepts and Techniques of Molecular Genetics and Interactions Between the Nervous System and the Immune System: Implications for Psychopharmacology). Likewise, in normal experimental animals it is possible to inject nucleic acid probes which can (a) hybridize to normal messenger RNAs (so-called antisense probes) for specific receptor proteins or their transductive intermediate proteins and (b) prevent the expression and hence the execution of a particular transductive pathway (see Thyrotropin - Releasing Hormone: Focus on Basic Neurobiology). Furthermore, the incredible number of subunit forms for every neurotransmitter receptor complex generates a molecular heterogeneity which provides an opportunity for still greater pharmacological selectivity. In situ hybridization with appropriate probes facilitates unambiguous cellular localization of individual forms of a receptor and expression of the receptor (see Dopamine Receptor Expression in the Central Nervous System).
In the future, molecular modeling based on the primary amino acid sequence of a receptor should make it possible to define the precise structure of the ligand-binding site and should permit synthesis of novel compounds tailored to these sites (see Early-Onset Mood Disorder). Future efforts to provide explanations for drug-induced neurological changes will undoubtedly continue to focus on synaptic transmitters and their mechanisms. If estimates of the complexity of brain-specific mRNA are any indication, many more transmitter-important molecules remain to be discovered (7).