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

Abstract

1. 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.

2. Currents mediated by recombinant receptors with the ternary subunit composition α_xβ_yγ_2L (where x=1,2,3 or 6 and y=1 or 2), in response to GABA applied at the appropriate EC₁₀, were enhanced by etomidate in a manner that was dependent upon the identity of both the α and β subunit isoforms.

3. For the β₂-subunit containing receptors tested, the EC₅₀ for the potentiation of GABA-evoked currents by etomidate (range 0.6 to 1.2 μM) was little affected by the nature of the α subunit present within the hetero-oligomeric complex. However, replacement of the β₂ by the β₁ subunit produced a 9–12 fold increase in the etomidate EC₅₀ (6 to 11 μM) for all α-isoforms tested.

4. For α₁, α₂ and α₆, but not α₃-subunit containing receptors, the maximal potentiation of GABA-evoked currents by etomidate was greater for β₂- than for β₁-subunit containing receptors. This was most clearly exemplified by receptors composed of α₆β₁γ_2L compared to α₆β₂γ_2L subunits, where a maximally effective concentration of etomidate potentiated currents evoked by GABA at EC₁₀ to 28±2% and 169±4% of the maximal GABA response, respectively.

5. For α₁ 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 β₁ rather than the β₂ subunit-containing receptor, although its maximal effect was similar at the two receptor isoforms.

6. The GABA-mimetic action of etomidate was supported by β₂- but not β₁-subunit containing receptors, whereas that of pentobarbitone or propofol was evident with either β isoform. For β₂-subunit containing receptors, both the agonist EC₅₀ and the maximal current produced by etomidate were additionally influenced by the α isoform.

7. 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.

Full Text

The Full Text of this article is available as a PDF (544.1 KB).