Involvement of nNOS in the antinociceptive activity of melatonin in inflammatory pain at the level of sensory neurons.

OBJECTIVE: The efficacy of melatonin as an analgesic agent has been well documented in animals and humans. However, the underlying mechanisms by which melatonin exerts antinociceptive effects on inflammatory pain are poorly understood. Here, we investigated the potential of melatonin to ameliorate inflammatory pain. MATERIALS AND METHODS: In vitro, ND7/23 neurons were treated with capsaicin. We used PCR and Western blot analyses to detect the expression of neuronal nitric oxide synthase (nNOS) in response to melatonin. Orofacial inflammatory pain was induced by 4% formalin administration on the right whisker pad of Sprague Dawley (SD) rats. The analgesic effect of melatonin was evaluated using mechanical threshold analyses. The expression level of nNOS in the trigeminal ganglion (TG) and trigeminal nucleus caudalis (Vc) neurons was assessed by RNAscope and immunohistochemistry. RESULTS: In vitro, capsaicin upregulated the expression of nNOS, which was dose-dependently reversed by melatonin pretreatment (p < 0.001). In a rat model of orofacial inflammatory pain, melatonin pretreatment significantly attenuated mechanical allodynia in both the acute and chronic phases (p < 0.05). Furthermore, melatonin decreased the formalin-evoked elevated nNOS mRNA and protein levels in the TG and Vc neurons in the acute and chronic phases (p < 0.05). CONCLUSIONS: Taken together, these results suggest that nNOS may play an active role in both peripheral and central processing of nociceptive information following orofacial inflammatory pain induction. The regulatory effect of melatonin on nNOS in inflammatory pain may have potential implications for the development of novel analgesic strategies.