Trigeminal nervous system sensitization by infraorbital nerve injury enhances responses in a migraine model

Background Although the peripheral and central sensitizations of trigeminal nervous system may be one of the important factors of migraine, the precise mechanism is not fully understood. In this study, we examined the influence of the sensitization of the second division of the trigeminal nerve (V2) by chronic constriction injury (CCI) of the infraorbital nerve (ION) on migraine headache, using the capsaicin-induced migraine model. Methods Male Sprague-Dawley rats were assigned to four groups: (a) sham surgery and topical-dural vehicle application (Sham+Vehicle) group, (b) CCI-ION and topical-dural vehicle application (CCI-ION+Vehicle) group, (c) sham surgery and topical-dural capsaicin application (Sham+Capsaicin) group, (d) CCI-ION and topical-dural capsaicin application (CCI-ION+Capsaicin) group. Behavioral testing and immunohistochemical staining were performed. Results In the behavioral test, the Sham+Capsaicin group showed significantly longer duration of immobilization and shorter duration of exploration compared with the Sham+Vehicle group, which is similar to clinical features of migraine patients. Moreover, CCI-ION enhanced these effects in the CCI-ION+Capsaicin group. Immunohistochemical staining for phospho-extracellular signal-related kinase (pERK) in the trigeminal ganglion (TG) containing first and second divisions of the trigeminal nerve and the trigeminocervical complex (TCC) revealed that pERK expression was significantly increased in the CCI-ION+Capsaicin group compared with the other groups. However, comparing between effects of the peripheral and central sensitizations (in the TG and TCC), from our results, peripheral sensitization would play a much less or not significant role. Conclusions These data demonstrate that the sensitization of V2 could influence the activation and the sensitization of the first division of the trigeminal nerve in the TCC, subsequently exacerbating pain sensation and pain-related behaviors. We have shown for the first time that the existence of the central sensitization of V2 can be an exacerbating factor for migraine related nociceptive thresholds/activation.