Mechanisms of anabolic androgenic steroid modulation of alpha(1)beta(3)gamma(2L) GABA(A) receptors.

Modulation of GABA(A) receptors induced by both anabolic androgenic steroids (AAS) and the benzodiazepine (BZ) site agonist, zolpidem, show equivalent dependence upon gamma subunit composition suggesting that both compounds may be acting at a shared allosteric site. Here we have characterized modulation induced by the AAS, 17alpha-methyltestosterone (17alpha-MeT), for responses elicited from alpha(1)beta(3)gamma(2L) GABA(A) receptors and compared it to modulation induced by the BZ site agonists, zolpidem and diazepam. For responses elicited by brief pulses of 20 microM GABA, both the AAS and the BZ site compounds significantly increased the peak current amplitudes and total charge transfer, although 17alpha-MeT was an appreciably weaker agonist than either diazepam or zolpidem at alpha(1)beta(3)gamma(2L) receptors. Neither class of modulator enhanced peak current amplitudes for responses elicited by mM concentrations of GABA. BZ site compounds altered time constants of deactivation, desensitization, and recovery from desensitization, however 17alpha-MeT had no overall effect on these parameters. Experiments in which 17alpha-MeT and BZ site ligands were applied concomitantly indicated that potentiation elicited by 17alpha-MeT and zolpidem were additive and that potentiation by 17alpha-MeT could be elicited in the presence of concentrations of flumazenil that blocked BZ potentiation. Finally, kinetic modeling suggests that while effects of 17alpha-MeT can be simulated by altering receptor affinity, the data for these alpha(1)beta(3)gamma(2L) receptors were best fitted by simulations in which 17alpha-MeT increases transitions into the singly liganded open state. Taken together, our results suggest that 17alpha-MeT does not act at the high-affinity BZ site, but may elicit some of its effects at the low affinity BZ site or at a novel site.