Biochemical identification of pharmacologically and functionally distinct GABA receptors in rat brain

Receptor binding studies were undertaken in an attempt to identify and characterize pharmacologically and functionally distinct receptor sites for gamma-aminobutyric acid (GABA) in rat brain. The results indicated that the potency of bicuculline, a GABA receptor antagonist, to displace membrane-bound [3H]GABA varies significantly among different brain regions, with the greatest potency found in the cerebral cortex and midbrain. In addition, in the presence of 50 mM ammonium thiocyanate, the potency of bicuculline to displace specifically bound [3H]GABA was increased significantly, with the magnitude of this increase being greater in some brain areas than others. The biological relevance of this thiocyanate-induced shift in the potency of bicuculline to inhibit [3H]GABA binding was indicated by the finding that ammonium thiocyanate also increased the potency of bicuculline to inhibit GABA-activated benzodiazepine receptor binding, a biochemical measure of GABA receptor function. Receptor site saturation analysis revealed that ammonium thiocyanate selectively abolished the high affinity GABA binding site without affecting either the low affinity component or GABA-activated benzodiazepine receptor binding. These findings provide further evidence for the existence of pharmacologically distinct GABA receptor sites, with some being more sensitive to the blocking action of bicuculline than others. Furthermore, the data provide direct evidence to support the hypothesis that only low affinity GABA receptor sites are linked to the benzodiazepine receptor, indicating that the kinetically different GABA binding sites are also functionally distinct. The discovery that ammonium thiocyanate selectively destroys high affinity GABA receptor binding may be useful for further defining the pharmacological, biochemical, and functional differences between GABA receptors in brain.