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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 behavioral model to study the TMJ pain conditions by characterizing the nociceptive 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 formalin-induced behavioral responses characterized by moving the mandible, rubbing the orofacial region and flinching the head quickly were quantified for 45 min. The TMJ injection of formalin significantly increased the asymmetrical orofacial rubbing and head flinching behaviors, but not the movement of the mandible with concentrations of 1.5% and above (P<0.05, Dunn's test) when compared with the NaCl (0.9%) injection. These responses were significantly reduced (P<0.05, Mann-Whitney test) by the co-application of lidocaine N-ethyl bromide quaternary salt, QX-314 (2%), and by the administration of intraperitoneal morphine (4 mg/kg) 30 min prior to the TMJ formalin injection. This study demonstrates that the injection of formalin into the TMJ region of rats produces quantitative nociceptive behaviors constituting a novel behavioral model for TMJ pain.

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Within the limits of this study, it was concluded that type 2 diabetes was associated with increased expression of interleukin-1beta and interleukin-6 in periodontally inflamed tissues of diabetic patients, relative to nondiabetic subjects, and that such overexpression may be involved in the mechanisms by which type 2 diabetes enhances periodontal destruction.

In conclusion, smoking modulates gene expression in the periodontium, and the influence of smoking on periodontal disease may involve effects of interleukin-6:interleukin-10 and RANKL:osteoprotegerin ratios.