The involvement of the neurosteroid allopregnanolone in the antihyperalgesic effect of paroxetine in a rat model of neuropathic pain.

Paroxetine increases the levels of neurosteroids, such as allopregnanolone (AP), that influence the excitability of the central nervous system by positive allosteric modulation of γ-aminobutyric acid type A receptors. Here, we investigated the role of AP synthesis on the paroxetine-induced antihyperalgesic effect in a rat model of neuropathic pain induced by lumbar spinal nerve ligation (SNL). Subcutaneous administration of paroxetine in SNL rats, dose-dependently decreased the probability of hyperalgesic response and increased AP levels in the spine but not in either brain or serum. Concomitant treatment with an inhibitor of the AP-synthesizing enzyme, finasteride, attenuated the paroxetine-induced antihyperalgesic effect as well as the paroxetine-induced increase in spinal AP levels. Intrathecal injection of exogenous AP mimicked the analgesic effects of paroxetine in vehicle-treated SNL rats, whereas no additional analgesic effects were observed in paroxetine-treated SNL rats. Our findings suggest that the antihyperalgesic effect of paroxetine in a rat neuropathic pain model is AP-mediated. These results also suggest that pharmacological-based therapies targeting AP synthesis might be a promising treatment for neuropathic pain.