Attaining in vivo selectivity of positive modulation of alpha 3 ss gamma 2 GABA(A) receptors in rats: A hard task!

Abstract

It is unclear whether GABA(A) receptors (GABA(A)Rs) that contain the alpha 3-subunit are substantially involved in the anxiolytic effects of benzodiazepines (BDZs). In the present study, we tested YT-III-31, a newer BDZ ligand with functional preference for alpha 3 ss gamma 2 GABA(A) Rs, in two paradigms of unconditioned anxiety, the open field and elevated plus maze in rats. The effective dose of YT-III-31 (2 mg/kg) displayed a clear anxiolytic-like profile, unhampered by sedative action, in both tests. At a higher dose (10 mg/kg), YT-III-31 induced ataxia in the rotarod and sedation in spontaneous locomotor activity test. The latter effect was preventable by flumazenil and ss CCt, the non-selective and alpha 1 ss gamma 2 GABA(A)R affinity-selective antagonist, respectively, demonstrating that sedative properties of YT-III-31, when attained, are mediated by the alpha 1 gamma 2 site. To elucidate the receptor substrate of subtle behavioral differences between YT-III-31 and diazepam, we approximated in vivo receptor potentiation for both ligands, based on estimated unbound concentrations in rat brains. Far different from diazepam, YT-III-31 has significantly lower affinity for the alpha 1 gamma 2 over other BDZ-sensitive sites, and at lower doses (1-2 mg/kg) was devoid of potentiation at alpha 1 ss gamma 2 GABA(A)Rs. The approximation approach revealed a modest selectivity of YT-III-31 for alpha 3 gamma 2- in comparison to alpha 2 gamma 2 and alpha 5 gamma 2 binding sites, suggesting that its anxiolytic-like activity may not necessarily or predominantly reflect potentiation at alpha 3 ss gamma 2 GABA(A)Rs. Nonetheless, as the anxiolytic effects are achievable at a dose devoid of any sedative potential, and having favorable safety (cytotoxicity) and metabolic stability profile, YT-III-31 represents a valuable candidate for further translational research.