The effects of a competitive antagonist on GABA-evoked currents in the presence of sedative-hypnotic agents

Many sedative-hypnotic agents are thought to act by positively modulating γ-aminobutyric acid type A (GABA_A) receptors. However, for many agents, the location(s) of the binding site(s) responsible for such receptor modulation is uncertain. We previously developed a low efficacy ligand (naphthalene-etomidate) that binds within a homologous set of hydrophobic cavities located at GABA_A receptor subunit interfaces in the transmembrane domain, and thus acts as a competitive antagonist for higher efficacy sedative-hypnotics that also bind to these sites. In this report, we describe studies using this compound as a pharmacological screening tool to test whether sedative-hypnotics representing a range of chemical classes can modulate GABA_A receptors by binding within these receptor cavities.

The impact of naphthalene-etomidate on GABA-evoked currents that were mediated by oocyte-expressed α₁β₃γ_2L GABA_A receptors and potentiated by muscimol, alphaxalone, 2,2,2-trichloroethanol, isoflurane, AA29504, loreclezole, or diazepam was quantified using electrophysiological techniques.

Naphthalene-etomidate (300 µM) significantly reduced GABA_A receptor currents potentiated by alphaxalone (by 22 ± 11%), 2,2,2-trichloroethanol (by 23 ± 6%), isoflurane (by 32 ± 10%), AA29504 (by 41 ± 6%), loreclezole (by 43 ± 9%), but significantly increased those potentiated by muscimol (by 26 ± 11%). Naphthalene-etomidate significantly increased currents potentiated by a low (1 µM) diazepam concentration (by 56 ± 14%) while reducing those potentiated by a high (100 µM) diazepam concentration (by 11 ± 7%).

Our results suggest that many (but not all) sedative-hypnotics are capable of positively modulating the GABA_A receptor by binding within a common set of hydrophobic cavities.