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.