Experimental study of central nervous modulation of the temporomandibular joint disorder associated with malocculusion

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Orthodontic tooth movement induces pain and discomfort. When an orthodontic force is applied to teeth, an inflammatory event occurs within the periodontium and substances are released, which could be related to the mechanism of pain sensation. Immediate early gene, c-fos, encodes the nuclear protein Fos. The induction of Fos in the brain following synaptic activation is specific for neurons, and c-fos activation has been reported to be utilized as a metabolic maker for neurosynaptic pathway tracing in the brain. We investigated the effect of experimental tooth movement on the c-fos expression in the rat brain by using Fos immunohistochemistry. An orthodontic elastic module was unilaterally inserted between upper molars of rats. Twenty-four hours after the commencement of the experimental tooth movement, Fos-like immunoreactive neurons appeared ipsilaterally in the trigeminal subnucleus caudalis and bilaterally in the lateral parabrachial nucleus, which is known to be involved in transmission of the nociceptive information. Following experimental tooth movement, the Fos-like immunoreactive neurons appeared in the central nucleus of the amygdala (ACE), paraventricular nucleus of the hypothalamus (PVH) and paraventricular nucleus of the thalamus (PV) of the experimental rat forebrain. The numbers of the labeled neurons were significantly increased by 639% and 644% in the ipsilateral and contralateral side of ACE respectively, by 292% and 307% in the ipsilateral and contralateral side of PVH,and by 264% in the PV with respect to sham control rats. These results suggest that nociceptive information caused by experimental tooth movement might be transmitted and modulated by several regions of the central nervous system.