FORMATION OF HETEROMERIC GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTORS CONTAINING 2 DIFFERENT ALPHA-SUBUNITS

The functional properties of recombinant gamma-aminobutyric acid (GABA ) receptors expressed transiently in human embryonic kidney 293 cells were examined. Combinations of alpha1beta2gamma2, alpha3beta2gamma2, a nd alpha1alpha3beta2gamma2 subunits were transiently expressed and the properties of the resulting receptors were studied with patch-clamp e lectrophysiology. Each subunit combination produced receptors having a unique set of functional properties. Concentration-response experimen ts showed that receptors composed of alpha1beta2gamma2 subunit combina tions were more sensitive to GABA (EC50 = 17.4 muM) than were either a lpha3beta2gamma2 (EC50 = 103 muM) or alpha1alpha3beta2gamma2 (EC50 = 5 5.8 muM) receptors. Consistent with its action at native GABA-A recept ors, diazepam (1 muM) potentiated the effect of GABA by shifting the G ABA concentration-response curve to the left. The magnitude of the dia zepam shift also differed between subunit combinations. The apparent p otency of GABA was increased 2-fold by diazepam with alpha1beta2gamma2 receptors, 3-fold with alpha3beta2gamma2 receptors, and 5-fold with a lpha1alpha3beta2gamma2 receptors. Brief applications (6-25 msec) of 3 mm GABA to outside-out patches revealed that currents decayed predomin antly with double-exponential time courses. The decay time courses of currents mediated by alpha1beta2gamma2 and alpha1alpha3beta2gamma2 rec eptors were similar, whereas the alpha3beta2gamma2 receptor response d ecayed more slowly. The distinct properties observed in cells expressi ng each of these subunit combinations suggest that the subunits form u nique receptors. The possibility that some neuronal GABA receptors con tain two different alpha subunits is discussed.