Characterization of steroid interactions with gamma-aminobutyric acid receptor-gated chloride ion channels: evidence for multiple steroid recognition sites.
The potentiation of gamma-aminobutyric acid (GABA) receptor-mediated 36Cl- uptake by various steroids has been characterized in rat cerebral cortical synaptoneurosomes. Several of these steroids, including 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha-OH-DHP) and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (THDOC), increase the potency of muscimol to stimulate 36Cl- uptake in a concentration-dependent and stereospecific manner. Concentration-response curves for 3 alpha-OH-DHP, THDOC, 3 alpha-hydroxy-pregn-4-en-20-one, and pentobarbital enhancement of muscimol-stimulated 36Cl- uptake are biphasic, with Hill coefficients significantly less than 1.0. Computer-modeling (ALLFIT analysis) of these curves suggests that these steroids and pentobarbital interact with multiple binding sites on GABAA receptor(s). In contrast, the concentration-response curve for THDOC 21-mesylate is monophasic, with a smaller maximal response, and yields a Hill coefficients of 1.0. In addition to modulating GABA receptor-mediated 36Cl- uptake, THDOC enhanced the ability of the benzodiazepine clonazepam to potentiate muscimol-stimulated 36Cl- uptake. The central benzodiazepine antagonist Ro15-1788 failed to inhibit THDOC-induced potentiation of muscimol-stimulated 36Cl- uptake, although it has been previously reported to inhibit some of the behavioral actions of THDOC. In contrast to the A ring-reduced metabolites and analogues of progesterone and deoxycorticosterone, glucocorticoids had no effect on muscimol-stimulated 36Cl- uptake in cerebral cortical synaptoneurosomes at concentrations between 20 nM and 5 microM.
Morrow
A L
AL
Section of Molecular Pharmacology, National Institute of Mental Health, Bethesda, Maryland 20892.
Pace
J R
JR
Purdy
R H
RH
Paul
S M
SM
D004092:20-alpha-Dihydroprogesterone / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002540:Cerebral Cortex / Q000187:drug effects; D018118:Chloride Channels; D002712:Chlorides / Q000378:metabolism*; D003900:Desoxycorticosterone / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000187:drug effects*; D008565:Membrane Proteins / Q000378:metabolism*; D011280:Pregnanolone / Q000494:pharmacology; D051381:Rats; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000378:metabolism; D013256:Steroids / Q000378:metabolism / Q000494:pharmacology*; D013329:Structure-Activity Relationship; D013574:Synaptosomes / Q000187:drug effects
D018118:Chloride Channels; D002712:Chlorides; D007473:Ion Channels; D008565:Membrane Proteins; D011963:Receptors, GABA-A; D013256:Steroids; D004092:20-alpha-Dihydroprogesterone; C052610:3-hydroxypregn-4-en-20-one; D003900:Desoxycorticosterone; C009413:tetrahydrodeoxycorticosterone; D011280:Pregnanolone