3β-Hydroxypregnane Steroids Are Pregnenolone Sulfate-Like GABA_AReceptor Antagonists

Abstract

Endogenous neurosteroids have rapid actions on ion channels, particularly GABA(A) receptors, which are potentiated by nanomolar concentrations of 3alpha-hydroxypregnane neurosteroids. Previous evidence suggests that 3beta-hydroxypregnane steroids may competitively antagonize potentiation induced by their 3alpha diastereomers. Because of the potential importance of antagonists as experimental and clinical tools, we characterized the functional effect of 3beta-hydroxysteroids. Although 3beta-hydroxysteroids reduced the potentiation induced by 3alpha-hydroxysteroids, 3beta-hydroxysteroids acted noncompetitively with respect to potentiating steroids and inhibited the largest degrees of potentiation most effectively. Potentiation by high concentrations of barbiturates was also reduced by 3beta-hydroxysteroids. 3beta-Hydroxysteroids are also direct, noncompetitive GABA(A) receptor antagonists. 3beta-Hydroxysteroids coapplied with GABA significantly inhibited responses to > or =15 microm GABA. The profile of block was similar to that exhibited by sulfated steroids, known blockers of GABA(A) receptors. This direct, noncompetitive effect of 3beta-hydroxysteroids was sufficient to account for the apparent antagonism of potentiating steroids. Mutated receptors exhibiting decreased sensitivity to sulfated steroid block were insensitive to both the direct effects of 3beta-hydroxysteroids on GABA(A) responses and the reduction of potentiating steroid effects. At concentrations that had little effect on GABAergic synaptic currents, 3beta-hydroxysteroids and low concentrations of sulfated steroids significantly reversed the potentiation of synaptic currents induced by 3alpha-hydroxysteroids. We conclude that 3beta-hydroxypregnane steroids are not direct antagonists of potentiating steroids but rather are noncompetitive, likely state-dependent, blockers of GABA(A) receptors. Nevertheless, these steroids may be useful functional blockers of potentiating steroids when used at concentrations that do not affect baseline neurotransmission.

Mechanisms of neurosteroid interactions with GABAA receptors

“…The next most potent effects are the non-competitive antagonist actions of steroids sulfated at C3, typified by pregnenolone sulfate and sulfated pregnane steroids, with either α or β stereochemistry at C3 and at C5 (Majewska et al, 1988;Majewska & Schwartz, 1987;ParkChung et al, 1999;Wang et al, 2002). The IC 50 for the action of this class of neuroactive steroids varies with activation state of the receptor Wang et al, 2002), but typical values are in the high nanomolar to micromolar range.…”

“…The IC 50 for the action of this class of neuroactive steroids varies with activation state of the receptor Wang et al, 2002), but typical values are in the high nanomolar to micromolar range. Finally, 3β-OH steroids can also antagonize GABA A receptors in a manner similar to C3 sulfated steroids, but 3β-OH steroid potency, and perhaps efficacy, are weaker than that of sulfated steroids.…”

“…However, when we explored the actions of 3β-OH steroids in detail with electrophysiological techniques, we found that the antagonistic interaction between 3β-OH steroids and 3α-OH steroids is not competitive. Instead, 3β-OH steroids produce direct antagonism of GABA A receptors, with effectiveness of antagonism dependent upon channel activation (Wang et al, 2002). These steroids fail to influence GABA-gated responses at low GABA concentrations.…”

Steroid modulation of GABAA receptor-mediated transmission in the hypothalamus: Effects on reproductive function

“…The best-characterized negative allosteric modulators include the sulfate-derivatives of pregnenolone (5-pregnen-3β-ol-20-one; pregnenolone sulfate) and dehydroepiandrosterone (3α-ol-5-androstene-17-one; DHEAS) and the 3β-hydroxypregnane steroids (Wang et al, 2002; for review, Henderson and Jorge, 2004) (Figure 1). These compounds are believed to act as non-competitive antagonists at site(s) separate from those that bind the positive neurosteroid modulators.…”

The other side of progestins: effects in the brain

“…Furthermore, several effects of allopregnanolone appear to be mediated by PAQR and by the pregnane xenobiotic receptor (PXR) (Cooke et al 2013, Frye et al 2013. In contrast, isopregnanolone does not bind directly to the GABA-A receptor (Bäckström et al 2005, Bitran et al 1991, but it antagonizes the effect of allopregnanolone on the GABA-A receptor (Wang et al 2002). Many reviews have recently discussed the effects of PROG and its metabolites in the nervous system (Melcangi et al 2014, Barros et al 2015.…”