Natural and enantiomeric etiocholanolone interact with distinct sites on the rat α1β2γ2L GABA_A receptor

We have studied the ability of the androgen etiocholanolone and its enantiomer (ent-etiocholanolone) to modulate rat α1β2γ2L GABA_A receptor function transiently expressed in human embryonic kidney cells. Studies on steroid enantiomer pairs can yield powerful new information on the pharmacology of steroid interactions with the GABA _A receptor. Both steroids enhance currents elicited by GABA, but ent-etiocholanolone is much more powerful than etiocholanolone at producing potentiation. At a low GABA concentration (0.5 μM, <EC₅), the presence of 10 μM ent-etiocholanolone potentiates whole-cell currents by almost 30-fold, whereas 10 μM etiocholanolone merely doubles the peak response. At higher GABA concentration (5 μM, ∼EC₂₅), the potentiation curve for ent-etiocholanolone is positioned at lower concentrations than that for etiocholanolone. Single-channel kinetic analysis shows that exposure to etiocholanolone has a single effect on currents: the relative frequency of long openings is increased in the presence of steroid. But exposure to ent-etiocholanolone produces two kinetic effects: an increase in the relative frequency of long openings and a decrease in the frequency of long closed times. The presence of etiocholanolone does not inhibit potentiation by ent-etiocholanolone, suggesting that etiocholanolone is unable to interact with the sites through which ent-etiocholanolone modifies receptor function. The double mutation α1(N407A/Y410F) prevents potentiation by etiocholanolone but not by ent-etiocholanolone, and the α1(Q241A) and α1(I238N) point mutations fully abolish potentiation by etiocholanolone but not by ent-etiocholanolone. We conclude that etiocholanolone and its enantiomer interact with distinct sites on the α1β2γ2L GABA_A receptor.