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Drugs acting in the central nervous system (CNS) were among the first to be discovered by primitive humans and are still the most widely used group of pharmacologic agents.* These include medications used to treat a wide range of neurologic and psychiatric conditions as well as drugs that relieve pain, suppress nausea, and reduce fever, among other symptoms. In addition, many CNS-acting drugs are used without prescription to increase the sense of well-being.
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Due to their complexity, the mechanisms by which various drugs act in the CNS have not always been clearly understood. In recent decades, however, dramatic advances have been made in the methodology of CNS pharmacology. It is now possible to study the action of a drug on individual neurons and even single receptors within synapses. The information obtained from such studies is the basis for several major developments in studies of the CNS. First, it is clear that nearly all drugs with CNS effects act on specific receptors that modulate synaptic transmission. While a few agents such as general anesthetics and alcohol may have nonspecific actions on membranes (although these exceptions are not fully accepted), even these non–receptor-mediated actions result in demonstrable alterations in synaptic transmission.
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Second, drugs are among the most valuable tools for studying CNS function, from understanding the mechanism of convulsions to the laying down of long-term memory. Both agonists that mimic natural transmitters (and in many cases are more selective than the endogenous substances) and antagonists are extremely useful in such studies. Third, unraveling the actions of drugs with known clinical efficacy has led to some of the most fruitful hypotheses regarding the mechanisms of disease. For example, information about the action of antipsychotic drugs on dopamine receptors has provided the basis for important hypotheses regarding the pathophysiology of schizophrenia. Studies of the effects of a variety of agonists and antagonists on γ-aminobutyric acid (GABA) receptors have resulted in new concepts pertaining to the pathophysiology of several diseases, including anxiety and epilepsy.
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A full appreciation of the effects of a drug on the CNS requires an understanding of the multiple levels of brain organization, from genes to circuits to behavior. This chapter introduces the functional organization of the CNS and its synaptic transmitters as a basis for understanding the actions of the drugs described in the following chapters.
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ORGANIZATION OF THE CNS
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The CNS is composed of the brain and spinal cord and is responsible for integrating sensory information and generating motor output and other behaviors needed to successfully interact with the environment and enhance species survival. The human brain contains about 100 billion interconnected neurons surrounded by various supporting glial cells. Throughout the CNS, neurons are either clustered into groups called nuclei or are present in ...