The role of peripheral 5HT2A and 5HT1A receptors on the orofacial formalin test in rats with persistent temporomandibular joint inflammation

The role of peripheral serotonin (5HT) 2A and 5HT1A receptors on the orofacial nocifensive behavioral activities evoked by the injection of formalin into the masseter muscle was evaluated in the rats with persistent temporomandibular joint (TMJ) inflammation evoked by Complete Freund's Adjuvant (CFA). The orofacial nocifensive behavioral activities evoked by the injection of formalin into masseter muscle were significantly enhanced at 1 day (CFA day 1 group) or 7 days (CFA day 7 group) during TMJ inflammation. Pretreatment with local administration of 5HT2A receptor antagonist, ketanserin (0.01, 0.1 mg/rat) into the masseter muscle or systemic administration of ketanserin via i.p. injection (1 mg/kg) reduced the orofacial nocifensive behavioral activities of the late phase evoked by formalin injection into masseter muscle on the side of TMJ inflammation (CFA day 7 group). However, local (0.001–0.1 mg/rat) or systemic (1 mg/kg) administration of 5HT1A receptor antagonist, propranolol, into masseter muscle did not produce the antinociceptive effect in CFA day 7 group. Moreover, local administration of ketanserin (0.1 mg) or propranolol (0.1 mg) into masseter muscle did not inhibit nocifensive orofacial behavior in rats without TMJ inflammation. These data suggest that persistent TMJ inflammation causes the elevation of the orofacial nocifensive behavior, and peripheral 5HT2A receptors play an important role in mediating the deep craniofacial tissue nociception in rats with TMJ inflammation.