P2X₃ receptor upregulation in trigeminal ganglion neurons through TNFα production in macrophages contributes to trigeminal neuropathic pain in rats

Background: Trigeminal neuralgia is a characteristic disease that manifests as orofacial phasic or continuous severe pain triggered by innocuous orofacial stimulation; its mechanisms are not fully understood. In this study, we established a new animal model of trigeminal neuralgia and investigated the role of P2X₃ receptor (P2X₃R) alteration in the trigeminal ganglion (TG) via tumor necrosis factor alpha (TNFα) signaling in persistent orofacial pain. Methods: Trigeminal nerve root compression (TNC) was performed in male Sprague-Dawley rats. Changes in the mechanical sensitivity of whisker pad skin, amount of TNFα in the TG, and number of P2X₃R and TNF receptor-2 (TNFR2)-positive TG neurons were assessed following TNC. The effects of TNFR2 antagonism in TG and subcutaneous P2X₃R antagonism on mechanical hypersensitivity following TNC were examined. Results: TNC induced unilateral continuous orofacial mechanical allodynia, which was depressed by carbamazepine. The accumulation of macrophages showing amoeboid-like morphological changes and expression of TNFα in the TG was remarkably increased following TNC treatment. The number of P2X₃R- and TNFR2-positive TG neurons innervating the orofacial skin was significantly increased following TNC. TNFα was released from activated macrophages that occurred in the TG following TNC, and TNFR2 antagonism in the TG significantly diminished the TNC-induced increase in P2X₃R-immunoreactive TG neurons. Moreover, subcutaneous P2X₃R antagonism in the whisker pad skin significantly depressed TNC-induced mechanical allodynia. Conclusions: Therefore, it can be concluded that the signaling of TNFα released from activated macrophages in the TG induces the upregulation of P2X₃R expression in TG neurons innervating the orofacial region, resulting in orofacial mechanical allodynia following TNC.