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 g-aminobutyric acid A (GABA A ) receptor subunits. 2 Currents mediated by recombinant receptors with the ternary subunit composition a x b y g 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 a and b subunit isoforms. 3 For the b 2 -subunit containing receptors tested, the EC 50 for the potentiation of GABA-evoked currents by etomidate (range 0.6 to 1.2 mM) was little aected by the nature of the a subunit present within the hetero-oligomeric complex. However, replacement of the b 2 by the b 1 subunit produced a 9 ± 12 fold increase in the etomidate EC 50 (6 to 11 mM) for all a-isoforms tested. 4 For a 1 , a 2 and a 6 , but not a 3 -subunit containing receptors, the maximal potentiation of GABAevoked currents by etomidate was greater for b 2 -than for b 1 -subunit containing receptors. This was most clearly exempli®ed by receptors composed of a 6 b 1 g 2L compared to a 6 b 2 g 2L subunits, where a maximally eective concentration of etomidate potentiated currents evoked by GABA at EC 10 to 28+2% and 169+4% of the maximal GABA response, respectively. 5 For a 1 subunit-containing receptors, the potency and maximal potentiating eect of either pentobarbitone or propofol was essentially unaected by the b subunit isoform contained within the receptor complex. The potency of the anaesthetic neurosteroid 5a-pregnan-3a-ol-20-one was marginally higher for b 1 rather than the b 2 subunit-containing receptor, although its maximal eect was similar at the two receptor isoforms. 6 The GABA-mimetic action of etomidate was supported by b 2 -but not b 1 -subunit containing receptors, whereas that of pentobarbitone or propofol was evident with either b isoform. For b 2 -subunit containing receptors, both the agonist EC 50 and the maximal current produced by etomidate were additionally in¯uenced by the a isoform. 7 It is concluded that the subtype of b-subunit in¯uences the potency with which etomidate potentiates GABA-evoked currents and that the b isoform is a crucial determinant of the GABA-mimetic activity of this compound. The nature of the a-subunit also impacts upon the maximal potentiation and activation that the compound may elicit. Such pronounced in¯uences may aid the identi®cation of the site that recognises etomidate. More generally, these results provide a clear example of structural speci®city in anaesthetic action.

The interaction of the general anesthetic etomidate with the γ-aminobutyric acid type A receptor is influenced by a single amino acid

“…For the clinically utilized anesthetic etomidate [i.e., (R)-(ϩ)-ethyl-1-(1-phenylethyl)-1H-imidazole-5-carboxylate], the ␣ and ␤ subunit isoforms present within the receptor complex influence these effects (12). In this communication, we report that the actions of etomidate at recombinant GABA A receptors are strongly influenced by a single amino acid residue within the ␤ subunit.…”

“…A direct interaction between general anesthetics and the GABA A receptor gains credence from studies that demonstrate the subunit composition of the pentameric complex (which may be drawn from ␣ 1-6 , ␤ 1-3 , ␥ 1-3 , ␦, or subtypes) to affect either the modulatory and͞or agonist actions of certain agents (10)(11)(12)(13). For the clinically utilized anesthetic etomidate [i.e., (R)-(ϩ)-ethyl-1-(1-phenylethyl)-1H-imidazole-5-carboxylate], the ␣ and ␤ subunit isoforms present within the receptor complex influence these effects (12).…”

Thymol, a constituent of thyme essential oil, is a positive allosteric modulator of human GABAA receptors and a homo‐oligomeric GABA receptor from Drosophila melanogaster

“…However, thymol showed no particular selectivity for a1 or a6 in terms of potency or maximal potentiation, suggesting that it does not act via the benzodiazepine-binding site, nor the ( þ )-ROD188 site. Loreclezole potentiates the actions of GABA at both RDL ac and GABA A receptors, the determinants of potency at the loreclezole site on GABA A receptors being shared by the positive modulatory actions of etomidate and DMCM (Stevenson et al, 1995;Hill-Venning et al, 1997), but no competitive inhibitors have thus far been identified. Ligands binding to the loreclezole site are characterised by a reduced potency on b1-containing receptors in comparison to b3-containing receptors.…”

Salicylidene salicylhydrazide, a selective inhibitor ofβ1‐containing GABAAreceptors

“…A number of allosteric modulators demonstrate b-subunit selectivity, for example, loreclezole, etomidate, furosemide Hill-Venning et al, 1997;Thompson et al, 1999). Regardless of whether the compound inhibits (e.g.…”

“…In recent years, a number of modulators of the GABA A receptor have been reported that demonstrate b2/3 selectivity over b1, for example, loreclezole, etomidate, tracazolate, mefenamic acid, furosemide Hill-Venning et al, 1997;Halliwell et al, 1999;Thompson et al, 1999;. In all cases, the potency of the modulator was reduced or abolished when serine was introduced into b2 or b3 (position 289 in human b2 and 290 in human b3) and increased or conferred when asparagine was introduced into b1 (position 290).…”