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 GABA_A receptors to induce general anesthesia. The structural features of the binding sites for these anesthetic steroids on GABA_A 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 α₁β ₂γ_2L GABA_A 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 reduced potency for inhibition of radioligand binding, GABA_A receptor modulation and tadpole anesthesia. Because of the similar effects of methyl group substitution in the two series of compounds and previous results from other studies showing that structural modifications in the steroid D ring/side chain region produce similar effects regardless of the stereochemistry of the A,B-ring fusion, we propose that either the 3α-hydroxyl groups of planar and nonplanar anesthetic steroids hydrogen bond to different amino acids on GABA_A receptors or that this critical hydrogen bonding group interacts with membrane lipids instead of the receptor.