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.
Department of Pharmacology, School of Pharmacy, University of London, U.K.
R J
RJ
Prince
M A
MA
Simmonds
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