Application of a pro-inflammatory agent to the orbital portion of the rat infraorbital nerve induces changes indicative of ongoing trigeminal pain

The present experiments investigated the behavioral and immunocytchemical (ICC) effects of applying complete Freund's adjuvant (CFA) to the orbital portion of the infraorbital nerve (IOn). Two control groups, the first had saline applied to the IOn and the second underwent sham operation, were included in the study. In the CFA group, significant hyper-responsiveness to von Frey (analysis of variance <0.05) and to pinprick stimulation (Kruskal Wallis <0.05) in the vibrissal pad was observed on the fourth and the fifth days post-operative (dpo). This was accompanied by a reduced bite force and altered bite patterns of similar duration. Histology of the IOn in CFA rats revealed immune cell infiltration and edema around and in the nerve trunk with only mild axonal damage confirmed by neuropeptide Y immunoreactivity in trigeminal ganglion. Histological areas of inconsistent and mild inflammation were observed in the saline group that were accompanied by similarly attenuated behavioral and ICC changes. This model of inflammation-induced neuropathic pain is highly applicable to the study of neuroinflammatory orofacial pain.