Experiments were performed to examine neurochemically the functional diversity of GABAA receptors as measured by muscimol-, 5a-pregnane-3a,21-diol-20-one (THD0C)-, and pentobarbital-stimulated 36Cl-uptake, and region-specific changes in muscimol-, THDOC-and THDOC-induced potentiation of muscimol-stimulated 36Cl-uptake in rats treated acutely or subacutely with a subconvulsive dose of bicuculline. The data for stimulation of 36Cl-uptake by muscimol showed a single binding site interaction in the cerebral cortex, hippocampus and cerebellum. The concentration-response curves for muscimol in the cerebral cortex and hippocampus were steep and indicated an increase of approximately 130% at the maximum concentration. In contrast, the curve for the cerebellum was shallow and exhibited a smaller maximal response (-60%). Apparent affinity for muscimol also differed among these brain regions. The regional differences in 36Cl-uptake induced by THDOC and pentobarbital were not as apparent as those induced by muscimol; however, the maximal modulatory effect of pentobarbital in the hippocampus was significantly higher than that in the cerebellum. In rats treated subacutely with a subconvulsive dose of bicuculline, a significant increase in muscimol-stimulated 36Cl-uptake was observed in the cerebellum but not in the frontal cortex or hippocampus. Analysis of the concentration-response curves for muscimol-stimulated 36Cl-uptake in the cerebellum revealed that the Vmax for muscimol in the subacutely treated group was significantly higher than those for muscimol in the control and acutely treated groups without any differences in the KD value. In addition, THDOC-induced potentiation of muscimol-stimulated 36Cl-uptake in the subacutely treated group was significantly higher than those in the control and acutely treated groups when a lower concentration (10 nM) of THDOC was used. These results suggest that regional variation of 36Cl-uptake stimulated by muscimol is more apparent than those stimulated by THDOC and pentobarbital. The present results, along with our previous findings, also indicate that region-specific bicuculline up-regulation of GAEiAA receptors in the cerebellum involves changes not only in binding characteristics but also in function of GABA receptor-gated chloride channel. o 1995 Wiley-Liss, Inc.