PHARMACOLOGICAL AND ELECTROPHYSIOLOGICAL PROPERTIES OF RECOMBINANT GABA-A-RECEPTORS COMPRISING THE ALPHA-3-SUBUNIT, BETA-1-SUBUNIT AND GAMMA-2-SUBUNIT

To assess the role of subunits for channel function and drug modulation in recombinant GABA(A) receptors, the alpha-3-beta-1-gamma-2 subunits and the dual combinations alpha-3-beta-1, beta-1-gamma-2 and alpha-3-gamma-2 were expressed by transfection of human embryonic kidney cells and by RNA injection in Xenopus oocytes (alpha-3-beta-1-gamma-2 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 alpha-3-beta-1-gamma-2 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 gamma-2 subunit, but potentiation by flunitrazepam was only obtained when the gamma-2 subunit was coexpressed in the presence of the alpha-3 subunit (alpha-3-beta-1-gamma-2 or alpha-3-gamma-2). The GABA sensitivities of the receptors were similar when the alpha-3-beta-1-gamma-2 combination was expressed in oocytes (half-maximum effective concentration = 240-mu-M) or in the kidney cell line (270-mu-M). However, the currents were less potentiated by flunitrazepam in oocytes (129% of controls) than in transfected cells (189%). These results suggest that the alpha-3-beta-1-gamma-2 subunit combination, which is coexpressed in various brain regions as shown by in situ hybridization histochemistry, may represent a building block of functional GABA(A) receptors in situ.