Proton modulation of alpha 1 beta 3 delta GABAA receptor channel gating and desensitization.

Alphabetagamma GABA(A) receptor currents are phasic and desensitizing, whereas alphabetadelta GABA(A) receptor currents are tonic and have no fast desensitization. alphabetagamma receptors are subsynaptic and mediate phasic inhibition, whereas alphabetadelta receptors are extra- or perisynaptic and mediate tonic inhibition. Given the different roles of these GABA(A) receptor isoforms and the fact that GABA(A) receptors are allosterically regulated by extracellular pH in a subunit-dependent manner, we compared the effects of changing pH on rat delta or gamma2L subunit-containing GABA(A) receptor currents. Human embryonic kidney cells (HEK293T) were transfected with cDNAs encoding rat alpha1, beta3, gamma2L, or delta GABA(A) receptor subunits in several binary and ternary combinations, and whole cell and single channel patch-clamp recordings were obtained. Lowering pH substantially enhanced alpha1beta3 receptor currents. This effect was significantly more pronounced for ternary alpha1beta3delta receptors, whereas ternary alpha1beta3gamma2L receptors were relatively insensitive to lowered pH. Lowering pH did not affect the extent of desensitization of alpha1beta3 and alpha1beta3gamma2L receptor currents, but significantly increased the extent of desensitization of alpha1beta3delta receptor currents. Lowering pH prolonged deactivation of alpha1beta3 and alpha1beta3delta receptor currents and enhanced the "steady-state" currents of alpha1beta3delta receptors evoked by long-duration (28 s) GABA applications. Lowering pH significantly increased mean open duration of alpha1beta3delta steady-state single channel currents due to introduction of a longer-duration open state, suggesting that low pH enhances alpha1beta3delta receptor steady-state currents by modifying GABA(A) receptor gating properties.