The α1 and α6 Subunits Can Coexist in the Same Cerebellar GABAA Receptor Maintaining Their Individual Benzodiazepine-Binding Specificities

Abstract: Two GABA_A receptor subunit-specific antibodies anti-α₆ and anti-α₁ have been used for elucidating the relationship between the presence of α₁ and/or α₆ subunits in the cerebellar GABA_A receptors and the benzodiazepine-binding specificity. Receptor immunoprecipitation with the subunit-specific antibodies shows that 39% of the cerebellar GABA_A receptors have α₆, whereas 76% of the receptors have α₁ as determined by ³H]muscimol binding. Results show that 42–45% of the receptors having α₆ also have α₁, whereas 13–15% of the receptors that contain α₁ also have α₆. The immunoprecipitation results as well as immunopurification and immunoblotting experiments reveal the existence of three types of cerebellar GABA_A receptors; i.e., one has both α₁ and α₆ subunits, a second type has α₁ but not α₆, and a third type has α₆ but not α₁ subunits. The results also show that receptors where α₁ and α₆ subunits coexist have two pharmacologically different benzodiazepine-binding properties, each associated with a different α subunit. The α₁ subunit contributes the high-affinity binding of ³H]Ro 15-1788 (flumazenil) and the diazepam-sensitive binding of ³H]Ro 15-4513. The α₆ subunit contributes the diazepam-insensitive binding of ³H]Ro 15-4513, but it does not bind ³H]Ro 15-1788 with high affinity. Thus, in the cerebellar α₁–α₆ GABA_A receptors, there is no dominance of the pharmacology of one α subunit over the other.