Enantioselectivity of pregnanolone-induced gamma-aminobutyric acid(A) receptor modulation and anesthesia.

This study reports the actions of enantiomer pairs of anesthetic steroids 3alpha5alphaP/ent-3alpha5alphaP and 3alpha5betaP/ent-3alpha5betaP as modulators of gamma-aminobutyric acid (GABA)(A) receptors and as anesthetics. The enantiomers of structurally related 17-carbonitrile analogs also are examined. These studies were aimed at 1) determining whether the steroid recognition site could distinguish between molecules differing in shape, but not other physical properties (enantioselectivity); 2) providing further insight into the structure-activity relationships of anesthetic steroids; and 3) determining whether modulation of GABA(A) receptor function correlates with anesthetic potency for anesthetic steroid enantiomers. Stereoselective actions of the compounds were evaluated in four different bioassays: 1) noncompetitive displacement of [(35)S]t-butylbicyclophosphorothionate from the picrotoxin site of GABA(A) receptors present in rat brain membrane preparations; 2) modulation of GABA currents in cultured rat hippocampal neurons; 3) loss of righting reflex in tadpoles; and 4) loss of righting reflex in mice. The data indicate that 5alpha-reduced steroids, but not 5beta-reduced steroids, show a high degree of enantioselectivity/enantiospecificity in their actions as modulators of GABA(A) receptors and as anesthetics. For all compounds studied, the effects on GABA(A) receptor function closely tracked with anesthetic effects. These data show that the anesthetic steroid recognition site is capable of distinguishing enantiomers, suggesting a protein-binding site of specific dimensions and shape. The results are compatible either with a structural model of the binding site that can accommodate 3alpha5alphaP, 3alpha5betaP, and ent-3alpha5betaP, but not ent-3alpha5alphaP, or with two different binding sites for steroid anesthetics.