The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2 L GABA A receptors

Background and Purpose Neurosteroids potentiate responses of the GABA A receptor to the endogenous agonist GABA . Here, we examined the ability of neurosteroids to potentiate responses to the allosteric activators etomidate, pentobarbital and propofol. Experimental Approach Electrophysiological assays were conducted on rat α1β2γ2 L GABA A 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. Key Results The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one (3α5β P ) potentiated activation of GABA A receptors by GABA and allosteric activators. Co‐application of 1 μM 3α5β P induced a leftward shift (almost 100‐fold) of the whole‐cell macroscopic concentration–response relationship for gating by etomidate. Co‐application of 100 nM 3α5β P reduced the EC 50 for potentiation by etomidate of currents elicited by 0.5 μM GABA by about three‐fold. In vivo , 3α5β P (1mg kg ‐1 ) reduced the dose of etomidate required to produce loss of righting in mice ( ED 50 ) by almost 10‐fold. I n tadpoles, the presence of 50 or 100 nM 3α5β P shifted the EC 50 for loss of righting about three‐ or ten‐fold respectively. Exposure to 3α5β P did not influence inhibition of cortisol synthesis by etomidate. Conclusions and Implications Potentiating neurosteroids act similarly on orthosterically and allosterically activated GABA A 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.