GABAA receptor blockers reverse the inhibitory effect of GABA on brain-specific [35S]TBPS binding

Thirteen substances previously reported to antagonize the electrophysiological effects of gamma-aminobutyric acid (GABA) on neurons also reversed the inhibitory effects of GABA on specific [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding to sites on rat brain membranes in vitro with a rank-order of potencies similar to those found in electrophysiological systems (R 5135 greater than pitrazepin greater than bicuculline greater than SR 95103 greater than securinine) confirming the earlier conclusion that GABA inhibits [35S]TBPS binding by acting allosterically on physiologically relevant GABAA receptors. Pitrazepin is the most potent of a series of mono N-aryl piperazines that block GABAA receptors. The new aryl amino pyridazine GABA derivative SR 95531 was about 3-fold more potent than bicuculline and 39-fold more potent than the structurally related SR 95103. Four known GABA antagonists have the same rank orders of potencies as convulsants and as reversers of GABA's inhibitory action on [35S]TBPS binding (bicuculline greater than securinine greater than theophylline greater than caffeine). Reversal of GABA-induced suppression of [35S]TBPS binding provides a simple method for further characterizing GABAA receptors linked to TBPS binding sites, and facilitates identification of convulsants and novel, perhaps selective, GABA antagonists.