The interaction of anaesthetic steroids with recombinant glycine and GABAA receptors

Background. Anaesthetic steroids are established positive allosteric modulators of GABA(A) receptors, but little is known concerning steroid modulation of strychnine-sensitive glycine receptors, the principal mediators of fast, inhibitory neurotransmission in the brain stem and spinal cord. This study compared the modulatory actions of five anaesthetic pregnane steroids and two non-anaesthetic isomers at human recombinant alpha(1) glycine and alpha(1)beta(2)gamma(2L) GABA(A) receptors. Methods. Recombinant alpha(1) glycine or alpha(1)beta(2)gamma(2L) GABA(A) receptors were expressed in Xenopus laevis oocytes and agonist-evoked currents recorded under voltage-clamp. Steroid modulation of currents evoked by GABA, or glycine, was quantified by determining the potency (EC50) and maximal effect of the compounds. Results. The anaesthetics minaxolone (EC50=1.3 muM), Org20599 (EC50=1.1 muM) and alphaxalone (EC50=2.2 muM) enhanced currents mediated by GABA(A) receptors. The anaesthetics also enhanced currents mediated by glycine receptors, although with higher EC50 values (minaxolone 13.1 muM; Org20599=22.9 muM and alphaxalone=27.8 muM). The maximal enhancement (to 780-950% of control) produced by the three steroids acting at the GABA(A) receptor was similar, but currents evoked by glycine were potentiated with increasing effectiveness by alphaxalone (199%) <Org20599 (525%) <minaxolone (1197%). The anaesthetic isomers, 5alpha-pregnan-3alpha-ol-20-one and 5beta-pregnan-3alpha-ol-20-one (eltanolone) enhanced GABA(A) receptor-mediated currents with similar potency and efficacy, but only the former enhanced glycine, the latter causing inhibition. The non-anaesthetic steroids 5alpha-pregnan-3beta-ol-20-one and 5beta-pregnan-3beta-ol-20-one modulated neither GABA(A), nor glycine, receptors. Conclusions. The data demonstrate that structure-activity relationships for steroid modulation at glycine and GABA(A) receptors differ. Comparing the EC50 values reported here with free plasma concentrations during steroid-induced anaesthesia indicates that a selective modulation of GABA(A) receptor activity is likely to occur in vivo.