The neurosteroid 5β-pregnan-3α-ol-20-one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABAA receptors.

Neurosteroids potentiate responses of the GABAA receptor to the endogenous agonist GABA. Here, we examined the ability of neurosteroids to potentiate responses to the allosteric activators etomidate, pentobarbital and propofol. Electrophysiological assays were conducted on rat α1β2γ2L GABAA receptors expressed in HEK 293 cells. The sedative activity of etomidate was studied in Xenopus tadpoles and mice. Effects of neurosteroids on etomidate-elicited inhibition of cortisol synthesis were determined in human adrenocortical cells. The neurosteroid 5β-pregnan-3α-ol-20-one (3α5βP) potentiated activation of GABAA receptors by GABA and allosteric activators. Co-application of 1 μM 3α5βP induced a leftward shift (almost 100-fold) of the whole-cell macroscopic concentration-response relationship for gating by etomidate. Co-application of 100 nM 3α5βP reduced the EC50 for potentiation by etomidate of currents elicited by 0.5 μM GABA by about three-fold. In vivo, 3α5βP (1mg kg(-1) ) reduced the dose of etomidate required to produce loss of righting in mice (ED50 ) by almost 10-fold. In tadpoles, the presence of 50 or 100 nM 3α5βP shifted the EC50 for loss of righting about three- or ten-fold respectively. Exposure to 3α5βP did not influence inhibition of cortisol synthesis by etomidate. Potentiating neurosteroids act similarly on orthosterically and allosterically activated GABAA receptors. Co-application of neurosteroids with etomidate can significantly reduce dosage requirements for the anaesthetic, and is a potentially beneficial combination to reduce undesired side effects.