• Asako Kubo, Masamichi Shinoda, Ayano Katagiri, Mamoru Takeda, Tatsuro Suzuki, Junichi Asaka, David C Yeomans, and Koichi Iwata.
• aDepartment of Physiology, Nihon University School of Dentistry, Tokyo, Japan bDivision of Functional Morphology, Dental Research Center, Nihon University School of Dentistry, Tokyo, Japan cLaboratory of Food and Physiological Sciences, Department of Life and Food Sciences, School of Life and Environmental Sciences, Azabu University, Kanagawa, Japan dDivision of Applied Oral Science, Nihon University Graduate School of Dentistry, Tokyo, Japan eFaculty of Anesthesia, Stanford University, School of Medicine, Stanford, CA, USA.
• Pain. 2017 Apr 1; 158 (4): 649-659.

AbstractOxytocin (OXT) is a neuropeptide hormone synthesized and secreted by hypothalamic neurons and has been reported to play a significant role in pain modulation. However, the mechanisms underlying OXT's antinociceptive effect on neuropathic pain are not fully understood. In this study, we examined the peripheral effect of OXT on mechanical hypersensitivity induced by partial ligation of the infraorbital nerve (PNL) in rats. Mechanical hypersensitivity in the whisker pad skin after PNL was attenuated by the direct administration of OXT into the trigeminal ganglion (TG). The proportion of vasopressin-1A receptor (V1A-R)-immunoreactive, but not OXT-receptor-immunoreactive, neurons significantly increased among TG neurons innervating the whisker pad skin after PNL. In a patch-clamp recording from TG neurons isolated from PNL rats, the resting membrane potential of OXT-treated neurons was significantly decreased, and the current thresholds of OXT-treated neurons for spike generation (rheobases) were significantly greater than those of vehicle-treated neurons. In addition, OXT increased voltage-gated K channel currents in PNL animals. Furthermore, intra-TG administration of a selective V1A-R antagonist reversed the OXT-induced alleviation of mechanical hypersensitivity, and coapplication of the antagonist opposed OXT's effects on the resting membrane potential, rheobase, and K current. These findings suggest that OXT is effective at suppressing TG neuronal hyperexcitability after nerve injury, likely by modulation of voltage-gated K channels through V1A-R. This signaling mechanism represents a potential therapeutic target for the treatment of orofacial neuropathic pain.

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