Enantioselectivity of steroid-induced γ-aminobutyric acid_A receptor modulation and anesthesia

Neuroactive steroids have been postulated to cause anesthesia by binding to unique steroid recognition sites on γ-aminobutyric acid (GABA) receptors and modulating GABA receptor function. Steroids interact with these sites diastereoselectively, but it is unknown whether steroid sites show enantioselectivity. To address this issue, we synthesized enantiomers to (+)-3α-hydroxy-5α-androstane-17β-carbortitrile and (+)-3α-hydroxy-5α-pregnan-20-one. In this study, we show that potentiation of GABA-mediated currents and gating of the GABA_A channel by steroids, as well as steroid-induced anesthesia in tadpoles and mice, is enantioselective, with the (+)-enantiomers exhibiting significantly greater potency in all assays. The correlation between the effects of steroid enantiomers on channel behavior and their effects as anesthetics provides strong evidence that GABA_A receptors play a predominant role in steroid-induced anesthesia. The enantiomers also provide a tool to probe the relative contributions of direct chloride channel activation versus potentiation of GABA-elicited currents to the induction of anesthesia. Studies examining the effects of combinations of (+)- and (-)-3α-hydroxy-5α-androstane-17β-carbonitrile were consistent with the hypothesis that potentiation of GABA-activated currents contributes to steroid-induced anesthesia but indicated that direct steroid activation of GABA_A receptors is not mechanistically important in producing anesthesia.