Development of Animal Model of Chronic Orofacial Pain

Objectives: The aim of this study was to develop a rat model of chronic orofacial pain associated with inflammation. Methods: Adult, male, Sprague-Dawley rats received a subcutaneous injection of complete Freund's adjuvant (CFA) or saline into the vibrissal pad, and spontaneous and mechanically evoked behaviors were monitored in awake rats. For the measurement of behavioural responses, withdrawal threshold and withdrawal frequency to mechanical stimulation applied to the vibrissal pad using von Frey filaments as well as facial grooming were assessed daily before (baseline) and after injection of CFA or saline up to 6 weeks after injection. Results: CFA Injection into the vibrissal pad did not produce any significant changes in facial grooming. The significant reduction in withdrawal threshold to mechanical stimulation over the injection site became apparent within 6 hours, peaked at 1-2 weeks, and persisted for 4 weeks after CFA injection as compared to baseline values (Wilcoxon signed rank test, P<0.05). In the whisker pad contralateral to the CFA injection, withdrawal threshold was significantly decreased at day 3 after injection but recovered soon. The withdrawal frequency to suprathreshold mechanical stimulation of the ipsilateral whisker pad was significantly increased and persisted for 4 weeks in CFA-treated rats (Wilcoxon signed rank test, P<0.05). In contrast, saline-treated rats showed no significant changes in facial grooming, withdrawal threshold, and withdrawal frequency (Wilcoxon signed rank test, P>0.05). Conclusions: These results indicate that the CFA-induced inflammation in orofacial cutaneous tissues resulted in significant mechanical allodynia and hyperalgesia persisting for 4 weeks.