Formation of heteromeric gamma-aminobutyric acid type A receptors containing two 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 alpha 1 beta 2 gamma 2, alpha 3 beta 2 gamma 2, and alpha 1 alpha 3 beta 2 gamma 2 subunits were transiently expressed and the properties of the resulting receptors were studied with patch-clamp electrophysiology. Each subunit combination produced receptors having a unique set of functional properties. Concentration-response experiments showed that receptors composed of alpha 1 beta 2 gamma 2 subunit combinations were more sensitive to GABA (EC50 = 17.4 microM) than were either alpha 3 beta 2 gamma 2 (EC50 = 103 microM) or alpha 1 alpha 3 beta 2 gamma 2 (EC50 = 55.8 microM) receptors. Consistent with its action at native GABA-A receptors, diazepam (1 microM) potentiated the effect of GABA by shifting the GABA concentration-response curve to the left. The magnitude of the diazepam shift also differed between subunit combinations. The apparent potency of GABA was increased 2-fold by diazepam with alpha 1 beta 2 gamma 2 receptors, 3-fold with alpha 3 beta 2 gamma 2 receptors, and 5-fold with alpha 1 alpha 3 beta 2 gamma 2 receptors. Brief applications (6-25 msec) of 3 mM GABA to outside-out patches revealed that currents decayed predominantly with double-exponential time courses. The decay time courses of currents mediated by alpha 1 beta 2 gamma 2 and alpha 1 alpha 3 beta 2 gamma 2 receptors were similar, whereas the alpha 3 beta 2 gamma 2 receptor response decayed more slowly. The distinct properties observed in cells expressing each of these subunit combinations suggest that the subunits form unique receptors. The possibility that some neuronal GABA receptors contain two different alpha subunits is discussed.