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Data obtained with experimental animals and with humans concerning the involvement of GABA-ergic systems in the control of hormonal secretion have been discussed and analyzed. The available evidence indicates that GABA-ergic systems might modulate the release of several hypothalamic-hypophyseal hormones that are involved in behavioral regulation, either via their endocrine actions or via their direct actions on the brain. Further studies along this line might lead to the development of GABA-ergic drugs that will be useful for treating certain hormonal or neuropyschiatric disorders.

Central GABA-ergic systems appear to be involved in the regulation of food intake and body weight in animals. Studies performed with systemic or intracerebral administration of GABA, GABA-agonists and GABA-antagonists support this view.

One has only to recall that GABA appears to be a major inhibitory neurotransmitter in the vertebrate central nervous system (CNS), that it exerts a hypotensive action upon systemic administration, that it is present in the blood, that its actions are antagonized by bicuculline and Picrotoxin and facilitated by benzodiazepines, and that receptors for GABA exist in cerebral blood vessels, to consider that GABA is somehow involved in cardiovascular regulation. One might even postulate that influences upon GABA-ergic mechanisms are involved in that dangerous sequence of biological activities: environmental stress → anxiety → atherosclerosis and hypertension, the latter of which represent major health problems of man. Herein, some evidence that appears to support this view will be presented.

Studies of the possible multiplicity of GABA-receptors that have been conducted with ligand-binding techniques and with certain electrophysiological and behavioral methods have been reviewed and analyzed. It seems evident that different populations of GABA binding sites exist in subcellular preparations of CNS tissues, but it is not yet certain that these sites reflect different populations of GABA-receptors that might exist in vivo.

Publisher Summary As γ-aminobutyric acid (GABA) is involved in physiological mechanisms that govern various normal or altered behaviors in vertebrates, it becomes essential to learn more about the membrane receptors that are associated with these actions of GABA. The ligand binding technique has been widely employed to study such GABA receptors in central nervous system (CNS) sub-cellular particles, a major aim of these studies being the separation of GABA binding into components related to its uptake (inactivation) and receptor interaction. This chapter discusses representative protocols for studying GABA binding, either in the presence or absence of added Na + . It has been found that GABA receptors might be considered to consist of several macromolecules: a GABA recognition site, a Cl - -ionophore, and a benzodiazepine receptor; also, certain proteins, gangliosides, phospholipids, and perhaps other membrane constituents might be involved in modulating the interaction of GABA and its recognition site. It is evident that the binding of radio-labeled GABA agonists to CNS membrane sites involves neuronal GABA receptors and that the ligand binding method should be useful for further characterizing and isolating these receptors.

Publisher Summary Muscimol, the enol-betaine of 5-aminomethyl-3-hydroxyisoxazole, has been isolated from the mushrooms Amanita muscaria, Amanita stabiliformis, and Tricholoma muscarium. X-ray crystallographic studies have shown that muscimol is structurally related to the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Muscimol is responsible for some of the central nervous system (CNS) effects of these mushrooms. Administration of muscimol to animals alters CNS functions. Because of its high affinity for GABA receptors, muscimol has been used in ligand-binding studies aimed at characterizing such receptors. This chapter presents some of the data obtained in these studies. [ 3 H]Muscimol binding sites, like [ 3 H]GABA binding sites, exist on synaptic and extrasynaptic membranes. Studies with tissue cultures have revealed the presence of neuronal GABA (muscimol) receptors; therefore, [ 3 H]muscimol and [ 3 H]GABA can serve as neuronal markers because they are not bound to glial membranes in the absence of Na + . Culture models might be useful for isolating and characterizing synaptic GABA receptors.

This chapter discusses a study to analyze GABA binding processes in rat brain. In this study, [ 3 H]GABA was bound to a synaptosomal fraction of rat cerebral cortex in bicarbonate-buffered media. In the presence of Na + , GABA was bound by two high-affinity processes; in Na + -free medium, a single process was detected. All processes were sensitive to bicuculline; hence, this agent might interact with synaptic receptors and transport sites for GABA. In Na + -free Tris-citrate medium, the capacity of high-affinity binding of [ 3 H]-muscimol to a crude membrane fraction was twice that of [ 3 H]GABA. Thus, the density of GABA-receptors might be over-estimated with [ 3 H]muscimol. Using discontinuous sucrose gradients, more specific binding sites existed for [ 3 H]muscimol than for [ 3 H]GABA in five subfractions of a crude membrane fraction; both radio-ligands were bound more to fractions containing heavy membranes and mitochondria than to those enriched in low-density membranes or myelin. [ 3 H]GABA and [ 3 H]muscimol were bound to membranes prepared from a neuron-enriched culture of rat brain but not to particles of cultured astrocytes.