Binding of [3H]ethynylbicycloorthobenzoate ([3H]EBOB) to the gamma-aminobutyric acid type A (GABAA) receptor of cultured cerebellar granule neurons is inhibited by GABA, muscimol and 3-aminopropanesulfonic acid with IC50 values of 69-250 nM. Sensitivity to these GABA mimetics is lower by 3-4-fold for cerebellum and 10-20-fold for cerebral cortex, midbrain, and pons and medulla, a differential sensitivity by brain region and cell type consistent with earlier findings using tert-[35S]butylbicyclophosphorothionate and GABA. In contrast, the inhibitory potencies of two chloride channel blockers, alpha-endosulfan and picrotoxinin, do not differ in these assays. The hypothesis that this pharmacological profile is conferred by the alpha 6 subunit specific to cerebellar granule cells is supported by the finding that forskolin (which downregulates the alpha 6 subunit) but not the inactive dideoxyforskolin markedly decreases the sensitivity of [3H]EBOB binding to GABA without affecting inhibition by alpha-endosulfan.