Bicuculline-sensitive γ-aminobutyrate binding processes in a synaptosome-enriched fraction of rat cerebral cortex

Abstract

The binding of low concentrations of [ 3H]y-aminobutyrate (GABA) to a synaptosomal fraction of rat cerebral, cortex was studied in the presence and absence of added Na +; unlabelled GABA and bicuculline methiodide were used to estimate ‘GABA-sensitive’ and ‘bicuculline-sensitive’ binding sites. Significant amounts of [ 3H]GABA were bound to the particles in both the presence and absence of added Na +. In Na +-free medium, a single bicuculline-sensitive, high-affinity [ 3H]GABA binding process was detected, its GABA-sensitive sites having K B ∼- 10 −5 M. In Na +-containing medium, two bicuculline-sensitive, high-affinity [ 3H]GABA binding processes were detected, the lower-affinity process having GABA-sensitive sites with K B ∼- 10 −5 M and the higher-affinity process having GABA-sensitive sites with K B ∼- 4 × 10 −8 M Our findings suggest that bicuculline-sensitive components of [ 3H]GABA binding in the absence of Na + and in the presence of Na + (higher-affinity process) could represent estimates of Na +-indepen dent and Na +-dependent synaptic GABA-receptors. The bicuculline-sensitive component of lower-affinity [ 3H]GABA binding in the presence of Na + might be related to an action of bicuculline on GABA transport. Two GABA-receptor agonists displaced about the same amount of [ +H]GABA as did bicuculline in Na +-free medium, indicating further that a synaptic GABA-receptor was involved in [ 3H]GABA binding under these conditions. Nipecotic acid markedly inhibited [ 3H]GABA binding in both the presence and absence of added Na +, indicating that some transport sites become occupied by GABA in the absence of added Na + and that nipecotic acid might interact to some extent with synaptic GABA-receptors.