DIFFERENTIAL ANTAGONISM BY EPIPREGNANOLONE OF ALPHAXALONE AND PREGNANOLONE POTENTIATION OF [H-3] FLUNITRAZEPAM BINDING SUGGESTS MORE THAN ONE CLASS OF BINDING-SITE FOR STEROIDS AT GABA-A RECEPTORS

In rat brain membranes, the 3alpha-hydroxy pregnane steroids, pregnanolone, allopregnanolone, alphaxalone and 5beta-alphaxalone potentiated 1 nM [H-3]flunitrazepam binding at the GABA(A) receptor, with maximal potentiations of 140-150% of control. The potencies of the 5alpha isomers were greater than the 5beta and the presence of an 11-keto group conferred lower potency. The potentiation produced by these steroids was antagonised by the 3beta-OH isomers epipregnanolone, isopregnanolone and betaxalone (60 muM). The dose-effect curves for pregnanolone and allopregnanolone were shifted to the right, with no reduction in the maximal potentiation. In contrast, the maximal effect of alphaxalone and 5beta-alphaxalone was reduced with no change in EC50. Alphaxalone (1 muM) caused an increase in the binding of [H-3]flunitrazepam in the presence of maximal concentrations of pregnanolone or allopregnanolone. These results suggest mulitiple sites of action for neurosteroids in the brain.