AbstractThe molecular mechanisms regulating GABAAreceptor activity in cultured frog melanotrophs were studied using the patch‐clamp technique. In the whole‐cell configuration, application of GABA evoked a dose‐related increase of inward chloride currents. The ED50value, estimated from the sigmoidal dose‐response curve was 2 × 10−6M and the Hill coefficient was 1.55. The amplitude of the GABA‐induced current decayed with time. Kinetics analysis of the desensitization revealed that the time‐course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time‐course of the current did not significantly vary at different holding potentials. In the outside‐out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3ms at ‐60mV). Altogether these data reveal that frog melanotrophs possess a single population of GABAAreceptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABAAreceptor appears to be independent of the accumulation of intrace