Neurosteroid Analogues. 10. The Effect of Methyl Group Substitution at the C-6 and C-7 Positions on the GABA Modulatory and Anesthetic Actions of (3α,5α)- and (3α,5β)-3-Hydroxypregnan-20-one

The planar 5α-reduced steroid (3α,5α)-3-hydroxypregnan-20-one and the nonplanar 5β-reduced steroid (3α,5β)-3-hydroxypregnan-20-one act at GABAA receptors to induce general anesthesia. The structural features of the binding sites for these anesthetic steroids on GABAA receptors have not been determined. To determine how structural modifications at the steroid C-6 and C-7 positions effect the actions of these anesthetic steroids, an axial or equatorial methyl group was introduced at these positions. The analogues were evaluated (1) in [35S]-tert-butylbicyclophosphorothionate binding experiments, (2) in electrophysiological experiments using rat α1β2γ2L GABAA receptors expressed in Xenopus laevis oocytes, and (3) as tadpole anesthetics. The effects of methyl group substitution in the 5α- and 5β-reduced series of compounds were strikingly similar. In both series, a 6β-Me group gave compounds with actions similar to or greater than those of the parent steroids. A 6α-, 7β- or 7α-Me substituent resulted in reduc...