Abstract
Etifoxine (EFX) is a non-benzodiazepine psychoactive drug which exhibits anxiolytic effects through a dual mechanism, by directly binding to GABAA receptors (GABAARs) and to the mitochondrial 18-kDa translocator protein, resulting in the potentiation of the GABAergic function. The subunit subtype plays a key role in the EFX-GABAAR interaction, however this does not explain the anxiolytic effects of this drug. Here, we combined behavioral and electrophysiological experiments to challenge the role of the GABAAR subunit in the EFX mode of action. After single administrations of anxiolytic doses (25-50 mg/kg, intraperitoneal), EFX did not induce any neurological nor locomotor impairments, unlike the benzodiazepine bromazepam (0.5-1 mg/kg, intraperitoneal). We established the EFX pharmacological profile on heteropentameric GABAARs constructed with α1 to α6 subunit expressed in Xenopus oocyte. Unlike what is known for benzodiazepines, neither the γ nor δ subunits influenced EFX-mediated potentiation of GABA-evoked currents. EFX acted first as a partial agonist on α23γ2S, α33γ2S, α63γ2S and α63 GABAARs, but not on α13γ2S, α43γ2S, α43 nor α53γ2S GABAARs. Moreover, EFX exhibited much higher positive allosteric modulation towards α23γ2S, α33γ2S and α63γ2S than for α13γ2S, α43γ2S and α53γ2S GABAARs. At 20 µM, corresponding to brain concentration at anxiolytic doses, EFX increased GABA potency to the highest extent for α33γ2S GABAARs. We built a docking model of EFX on α3β3γ2S GABAARs, which is consistent with a binding site located between α and β subunits in the extracellular domain. In conclusion, EFX preferentially potentiates α23γ2S and α33γ2S GABAARs, which might support its advantageous anxiolytic/sedative balance.