Differential antagonism by epipregnanolone of alphaxalone and pregnanolone potentiation of [3H]flunitrazepam binding suggests more than one class of binding site for steroids at GABAA receptors.
In rat brain membranes, the 3 alpha-hydroxy pregnane steroids, pregnanolone, allopregnanolone, alphaxalone and 5 beta-alphaxalone potentiated 1 nM [3H]flunitrazepam binding at the GABAA receptor, with maximal potentiations of 140-150% of control. The potencies of the 5 alpha isomers were greater than the 5 beta and the presence of an 11-keto group conferred lower potency. The potentiation produced by these steroids was antagonised by the 3 beta-OH isomers epipregnanolone, isopregnanolone and betaxalone (60 microM). 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 5 beta-alphaxalone was reduced with no change in EC50. Alphaxalone (1 microM) caused an increase in the binding of [3H]flunitrazepam in the presence of maximal concentrations of pregnanolone or allopregnanolone. These results suggest multiple sites of action for neurosteroids in the brain.
Prince
R J
RJ
Department of Pharmacology, School of Pharmacy, University of London, U.K.
Simmonds
M A
MA
D000777:Anesthetics / Q000494:pharmacology*; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D005445:Flunitrazepam / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D011277:Pregnanediones / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D011280:Pregnanolone / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000187:drug effects / Q000378:metabolism*
D000777:Anesthetics; D011277:Pregnanediones; D011963:Receptors, GABA-A; D005445:Flunitrazepam; C006477:alphaxalone; D011280:Pregnanolone