Validation of a Novel Rat-Holding Device for Studying Heat- and Mechanical-Evoked Trigeminal Nocifensive Behavioral Responses

Aims: To test the reliability and validity of a novel rat-holding device designed to be used in conjunction with the plantar test apparatus for studying nocifensive behavioral responses in an established model of temporomandibular joint (TMJ) pathology. Methods: Thirty-five young adult male Sprague-Dawley rats were used. Withdrawal latencies in response to infrared 40 heat stimulation of the submandibular region in naïve animals (n = 4) and animals injected with saline or complete Freund's adjuvant (CFA) in the TMJ (n > 9) were measured over a 2-week time period. Nocifensive responses to mechanical stimulation of the cutaneous tissue directly over the TMJ with von Frey filaments were investigated in animals injected with CFA in the TMJ (n = 6). The effect on nocifensive responses to heat and mechanical stimulation of subcutaneous administration of buprenorphine (0.05 mg/kg) into the hindquarter was assessed in CFA and cotreated animals (n = 6). Statistical analysis was performed using a nonparametric Mann-Whitney U test. Results: Under basal conditions, withdrawal latencies to heat stimulation of the orofacial region remained consistently around 15 seconds over 14 days. Unilateral CFA injection in the TMJ significantly decreased heat-withdrawal latencies on days 1, 2, 7, and 14 in the ipsilateral side (P .05), but not contralateral side, when compared with basal values. CFA also significantly decreased the nocifensive threshold to mechanical stimulation on days 1, 2, and 7 postinjection (P .05). CFA-mediated changes in heat withdrawal and mechanical thresholds in the orofacial region were significantly suppressed by subcutaneous administration of buprenorphine into the hindquarter (P .05). Conclusion: Findings from this study provide evidence to validate the use of this holding device for studying nocifensive behaviors in the orofacial region of rats in response to heat or mechanical orofacial stimulation. Keywords: inflammation, plantar test, TMJ, trigeminal, von Frey