3 -Hydroxypregnane Steroids Are Pregnenolone Sulfate-Like GABAA Receptor Antagonists

Endogenous neurosteroids have rapid actions on ion channels, particularly GABAA receptors, which are potentiated by nanomolar concentrations of 3 -hydroxypregnane neurosteroids. Previous evidence suggests that 3 -hydroxypregnane steroids may competitively antagonize potentiation induced by their 3 diastereomers. Because of the potential importance of antagonists as experimental and clinical tools, we characterized the functional effect of 3 -hydroxysteroids. Although 3 -hydroxysteroids reduced the potentiation induced by 3 -hydroxysteroids, 3 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 3 -hydroxysteroids. 3 -Hydroxysteroids are also direct, noncompetitive GABAA receptor antagonists. 3 -Hydroxysteroids coapplied with GABA significantly inhibited responses to 15 M GABA. The profile of block was similar to that exhibited by sulfated steroids, known blockers of GABAA receptors. This direct, noncompetitive effect of 3 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 3 -hydroxysteroids on GABAA responses and the reduction of potentiating steroid effects. At concentrations that had little effect on GABAergic synaptic currents, 3 hydroxysteroids and low concentrations of sulfated steroids significantly reversed the potentiation of synaptic currents induced by 3 -hydroxysteroids. We conclude that 3 -hydroxypregnane steroids are not direct antagonists of potentiating steroids but rather are noncompetitive, likely state-dependent, blockers of GABAA receptors. Nevertheless, these steroids may be useful functional blockers of potentiating steroids when used at concentrations that do not affect baseline neurotransmission.