The anxiolytic etifoxine activates the peripheral benzodiazepine receptor and increases the neurosteroid levels in rat brain

The peripheral benzodiazepine receptors (PBR) might be involved in certain pathophysiological events, such as anxiety, by stimulating the production of neuroactive steroids in the brain. A recent electrophysiological study has revealed an interaction between PK11195, a PBR ligand and the anxiolytic compound etifoxine at micromolar concentrations. The present work was aimed at further characterizing the etifoxine-PBR interaction. In membrane preparations from intact male rat forebrain, etifoxine uncompetitively inhibited the binding of [3H]PK11195 with an IC50 = 18.3 ± 1.2 μM, a value consistent with etifoxine plasma and brain concentrations measured after an anxiolytic-like dose (50 mg/kg). In vivo, that etifoxine dose was associated with increased concentrations of pregnenolone, progesterone, 5α-dihydroprogesterone and allopregnanolone in plasma and brain of sham-operated animals. In adrenalectomized and castrated rats, etifoxine enhanced the brain levels of these steroids, suggesting a stimulation of their local synthesis and/or a decrease of their disappearance rate, independently of peripheral sources. Finasteride, an inhibitor of 5α-reductase that converts progesterone into its 5α-reduced metabolites like allopregnanolone, attenuated the anti-conflict effect of etifoxine even though brain allopregnanolone contents were drastically reduced. These results indicate that following activation of the PBR in the brain, an increased cerebral production of allopregnanolone, a potent positive modulator of the GABAA receptor function, may partially contribute to the anxiolytic-like effects of etifoxine.