Development of a behavioral model of TMJ pain in rats: the TMJ formalin test

Temporomandibular joint (TMJ) pain conditions are poorly understood. Since formalin is a noxious stimulus widely used in animal behavioral experiments for studying pain mechanisms, the aim of this study was to develop a behav ioral model to study the TMJ pain conditions by characterizing the nocicept ive behavioral responses induced by the injection of formalin into the TMJ region of rats. NaCl (0.9%) or different concentrations of formalin (0.5, 1 .5, 2.5 or 5%) were administrated into the TMJ region. The form alin-induce d behavioral responses characterized by moving the mandible, rubbing the or ofacial region and flinching the head quickly were quantified for 45 mm. Th e TMJ injection of formalin significantly increased the asymmetrical orofac ial rubbing and head flinching behaviors, but not the movement of the mandi ble with concentrations of 1.5% and above (P < 0.05, Dunn's test) when comp ared with the NaCl (0.9%) injection. These responses were significantly red uced (P < 0.05, Mann-Whitney test) by the co-application of lidocaine N-eth yl bromide quaternary salt, QX-314 (2%), and by the administration of intra peritoneal morphine (4 mg/kg) 30 min prior to the TMJ formalin injection. T his study demonstrates that the injection of formalin into the TMJ region o f rats produces quantitative nociceptive behaviors constituting a novel beh avioral model for TMJ pain. (C) 2001 International Association for the Stud y of Pain. Published by Elsevier Science B.V. All rights reserved.