Pharmacological and Electrophysiological Properties of Recombinant GABAA Receptors Comprising the alpha3, beta1 and gamma2 Subunits.

To assess the role of subunits for channel function and drug modulation in recombinant GABAA receptors, the alpha3beta1gamma2 subunits and the dual combinations alpha3beta1, beta1gamma2 and alpha3gamma2 were expressed by transfection of human embryonic kidney cells and by RNA injection in Xenopus oocytes (alpha3beta1gamma2 combination). GABA-induced chloride currents were recorded using the whole-cell configuration of the patch-clamp technique (transfected cells) or the voltage-clamp technique (oocytes). The currents recorded from the alpha3beta1gamma2 subunit combination in transfected cells were reduced by bicuculline and picrotoxin, enhanced by flunitrazepam in a flumazenil-sensitive manner and reduced by beta-carboline-3-carboxylic acid methyl ester (beta-CCM). The GABA-induced current was reduced by beta-CCM in all combinations containing the gamma2 subunit, but potentiation by flunitrazepam was only obtained when the gamma2 subunit was coexpressed in the presence of the alpha3 subunit (alpha3beta1gamma2 or alpha3gamma2). The GABA sensitivities of the receptors were similar when the alpha3beta1gamma2 combination was expressed in oocytes (half-maximum effective concentration=240 microM) or in the kidney cell line (270 microM). However, the currents were less potentiated by flunitrazepam in oocytes (129% of controls) than in transfected cells (189%). These results suggest that the alpha3beta1gamma2 subunit combination, which is coexpressed in various brain regions as shown by in situ hybridization histochemistry, may represent a building block of functional GABAA receptors in situ.