Subunit‐dependent interaction of the general anaesthetic etomidate with the γ‐aminobutyric acid type A receptor

The GABA modulating and GABA‐mimetic actions of the general anaesthetic etomidate were examined in voltage‐clamp recordings performed on Xenopus laevis oocytes induced, by cRNA injection, to express human recombinant γ‐aminobutyric acid A (GABA A ) receptor subunits. Currents mediated by recombinant receptors with the ternary subunit composition α x β y γ 2L (where x=1,2,3 or 6 and y=1 or 2), in response to GABA applied at the appropriate EC 10 , were enhanced by etomidate in a manner that was dependent upon the identity of both the α and β subunit isoforms. For the β 2 ‐subunit containing receptors tested, the EC 50 for the potentiation of GABA‐evoked currents by etomidate (range 0.6 to 1.2 μ m ) was little affected by the nature of the α subunit present within the hetero‐oligomeric complex. However, replacement of the β 2 by the β 1 subunit produced a 9–12 fold increase in the etomidate EC 50 (6 to 11 μ m ) for all α‐isoforms tested. For α 1 , α 2 and α 6 , but not α 3 ‐subunit containing receptors, the maximal potentiation of GABA‐evoked currents by etomidate was greater for β 2 ‐ than for β 1 ‐subunit containing receptors. This was most clearly exemplified by receptors composed of α 6 β 1 γ 2L compared to α 6 β 2 γ 2L subunits, where a maximally effective concentration of etomidate potentiated currents evoked by GABA at EC 10 to 28±2% and 169±4% of the maximal GABA response, respectively. For α 1 subunit‐containing receptors, the potency and maximal potentiating effect of either pentobarbitone or propofol was essentially unaffected by the β subunit isoform contained within the receptor complex. The potency of the anaesthetic neurosteroid 5α‐pregnan‐3α‐ol‐20‐one was marginally higher for β 1 rather than the β 2 subunit‐containing receptor, although its maximal effect was similar at the two receptor isoforms. The GABA‐mimetic action of etomidate was supported by β 2 ‐ but not β 1 ‐subunit containing receptors, whereas that of pentobarbitone or propofol was evident with either β isoform. For β 2 ‐subunit containing receptors, both the agonist EC 50 and the maximal current produced by etomidate were additionally influenced by the α isoform. It is concluded that the subtype of β‐subunit influences the potency with which etomidate potentiates GABA‐evoked currents and that the β isoform is a crucial determinant of the GABA‐mimetic activity of this compound. The nature of the α‐subunit also impacts upon the maximal potentiation and activation that the compound may elicit. Such pronounced influences may aid the identification of the site that recognises etomidate. More generally, these results provide a clear example of structural specificity in anaesthetic action. British Journal of Pharmacology (1997) 120 , 749–756; doi: 10.1038/sj.bjp.0700927