The effect of analgesics on central nervous system following experimental tooth movement

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Orthodontic tooth movement induces pain and discomfort. When an orthodontic force is applied to teeth, an inflammatory event occurs with 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 in the trigeminal subnucleus caudalis and 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. By the way, the N-methyl-D-asparate (NMDA) receptor is implicated in multiple aspects of pain. We examined the influence of the NMDA receptor antagonist, MK-801 following experimental tooth movement. Pretreatment with MK-801 produced a tendency towards a reduction of the number of the Fos-like immunoreactive neurons appeared in ACE, PVH and PV of the rat forebrain.