Effect of two novel GABAA receptor positive allosteric modulators on neuropathic and inflammatory pain in mice

Loss of GABAergic inhibition in the spinal dorsal horn (SDH) is implicated in central sensitization and chronic pain. Both agonists and positive allosteric modulators (PAMs) of GABAA receptor are found to be effective in the management of chronic pain. In addition to benzodiazepines, neuroactive steroids (NASs) also act as PAMs through binding to unique sites of GABAA receptors. Thus, it is worth investigating whether these NASs can attenuate chronic pain. This study tested the antinociceptive properties of two novel NAS PAMs, ganaxolone and zuranolone, in segmental spinal nerve ligation (SNL)-induced neuropathic pain and complete Freund's adjuvant (CFA)-induced inflammation pain models. Spinally administered ganaxolone and zuranolone both exhibited dose-dependent analgesic effects but with quite different durations. This antinociceptive effect might be generated from elevated GABAergic inhibition, as the PAMs both enhanced GABA-evoked currents in SDH neurons, and the K⁺-Cl^- cotransporter isoform 2 (KCC2) antagonist reversed the analgesic effect of the PAMs. Different from ganaxolone, zuranolone produced a durable increase in the surface expression of GABAA receptors and of the amplitude of spontaneous inhibitory currents, which may contribute to the long-lasting analgesic effect. Furthermore, the PAMs alleviated SNL-induced mechanical allodynia synergistically with diazepam or GABAA receptor activator muscimol at inactive doses, consistent with the non-competitive activity and distinct binding sites from benzodiazepines. In summary, our findings suggest that NASs may not only acutely modulate GABA receptor activity but also induce sustained metabotropic effects on GABAA receptors and thus exert long-lasting antinociceptive effects.