NEUROSTEROID ANALOGS .4. THE EFFECT OF METHYL SUBSTITUTION AT THE C-5AND C-10 POSITIONS OF NEUROSTEROIDS ON ELECTROPHYSIOLOGICAL ACTIVITY AT GABA(A) RECEPTORS

A series of analogues of the neuroactive steroids 3 alpha-hydroxy-5 al pha-pregnan-20-one and 3 alpha-hydroxy-5 beta-pregnan-20-one were stud ied to elucidate the mode of binding of 5 alpha- and 5 beta-reduced st eroids to steroid binding sites on GABA(A) receptors. Analogues which were either 3 alpha-hydroxy-20-ketosteroids or 3 alpha-hydroxysteroid- 17 beta-carbonitriles and which contained various methyl group substit ution patterns at C-5 and C-10 were prepared. Evaluations utilized who le-cell patch clamp electrophysiological methods carried out on cultur ed rat hippocampal neurons, and the results obtained with the rigid 17 beta-carbonitrile analogs were analyzed using molecular modeling meth ods. The molecular modeling results provide a rationale for the observ ation that the configuration of the hydroxyl group at C-3 is a greater determinant of anesthetic potency than the configuration of the A,B r ing fusion at C-5. The electrophysiological results identify steric re strictions for the space that can be occupied in 5 alpha- and 5 beta-r educed steriod modulators of GABA(A) receptors in the regions of space proximate to the steroid C-5, C-10, and possibly C-4 positions. This information is useful for the development of nonsteroidal analogues th at can modulate GABA(A) receptors via interactions at steroid binding sites.