Involvement of the GABA receptor α subunit in the mode of action of etifoxine.

Etifoxine (EFX) is a non-benzodiazepine psychoactive drug which exhibits anxiolytic effects through a dual mechanism, by directly binding to GABA receptors (GABARs) and to the mitochondrial 18-kDa translocator protein, resulting in the potentiation of the GABAergic function. The β subunit subtype plays a key role in the EFX-GABAR 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 GABAR α 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 GABARs 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 α2β3γ2S, α3β3γ2S, α6β3γ2S and α6β3δ GABARs, but not on α1β3γ2S, α4β3γ2S, α4β3δ nor α5β3γ2S GABARs. Moreover, EFX exhibited much higher positive allosteric modulation towards α2β3γ2S, α3β3γ2S and α6β3γ2S than for α1β3γ2S, α4β3γ2S and α5β3γ2S GABARs. At 20 μM, corresponding to brain concentration at anxiolytic doses, EFX increased GABA potency to the highest extent for α3β3γ2S GABARs. We built a docking model of EFX on α3β3γ2S GABARs, which is consistent with a binding site located between α and β subunits in the extracellular domain. In conclusion, EFX preferentially potentiates α2β3γ2S and α3β3γ2S GABARs, which might support its advantageous anxiolytic/sedative balance.