pERK involvement in thermal allodynia in capsaicin-treated face in rats

Objectives: VR1 and VR2 receptors are known to be involved in an increase in an excitability of in C- and A delta-fibers, resulting in the peripheral sensitization. Sensitized primary afferent-fibers affect the excitability of dorsal horn neurons and cause the sensitization of dorsal horn neurons. However, how the dorsal horn neurons are sensitized following capsaicin application is not known. The aim of present study is to clarify the neural mechanisms underlying capsaicin-induced thermal allodynia in face induced by capsaicin application. Methods: Male sprague-Dawley rats were anesthetized with Na pentobarbital (50mg/kg/ i.p.) and capsaicin or vehicle was applied to the facial skin. One hour after that, capsaicin- or vehicle-treated rats received thermal stimulation (cold: 5, 15 or 25 dgree C, heat: 45 or 50 degree C) to the face and phosphorylation of extracellular signal-regulated kinase (pERK) immunohistochemistry were performed in the trigeminal spinal subnucleus caudalis (Vc) and upper cervical cord. We also measured the scratching behavior after acetone application and head withdrawal latency to heating of the face in capsaicin- or vehicle-treated rats. Results: The number of face scratching was significantly increased in capsaicin- treated rats following acetone application to the face as compared with vehicle-treated rats. Head withdrawal latency to heating of the face was significantly shorter in capsaicin treated rats. A large number of pERK-like immunoreactive (LI) cells was observed in the superficial laminae of Vc after heating of the face in vehicle-treated rats, but not after cold stimulation. However, the number of pERK-LI cells was significantly increased after cold stimulation of the face in the rats with capsaicin treatment compared with vehicle-treated rats as well as heat stimulation. Conclusion: The present findings suggest that the sensitization of TRPV1 channels is involved in capsaicin-induced thermal allodynia in the facial skin.