The Study On Behaviors & Peripheral And Central Mechanisms Of Masticatory Muscles Mechanical Hyperalgesia Induced By Occlusal Trauma In Rats

Muscle pain is one of the most frequent symptoms of TMD patient, and trauma from occlusal interference was considered to be one important factor. Little is known about the pain mechanisms. In our study, an animal model of occlusal alteration was developed and pain responses of the masseter and the temporalis muscles to mechanical stimulation were investigated. Histologic and substance P immunohistochemical changes of the masseter muscles were studied. C-fos and substance P immunohistochemical changes of brainstem and upper cervical spinal cord were also investigated.The first maxillary left molar from sixteen male Sprague-Dawley rats was selected. The occlusal surface of the 8 rats was raised 0.5-1 mm by bonding a screw in the pulp cavity which was opened and devitalized one week before, the other 8 rats were refilled with methylmetacrylate resin as control. Mechanical pain response was defined in the awake rats without restraint, by stimulating masseter and temporalis using von Frey filaments. The animals were killed in 3 and 7 days. Masseter tissuesfrom all rats were investigated histological and substance P immunohistochemical changes.And then, another sixteen male Sprague-Dawley rats were divided into two groups, eight rats were made occlusal interference as above,and other eight were treated as control . 7 days after occlusal interference, four rats in each group were light anesthesized and nociceptive mechanical stimuli (Von Frey filament) were applied to the ipsilateral masseter. Pain response was recorded and all the animals were killed 2 hours later. Another four rats in each group were deep anesthetized and nociceptive chemical stimulation (5% formalin) were applied to the ipsilateral masseter, 2 hours later, the rats were killed. The brainstem and cervical spinal cord tissue from 4-mm rostral to 8-mm caudal to obex was removed. The frozen tissue was transversely cut into 40 μm thick, processed c-fos immunoreactivity for all animals and processed substance P immunoreactivity only for mechanical stimulated animals. Quntitative analysis: Successive 4 sections at the level of obex and cl-2 level were selected and C-fos expression were quantified under microscope and SP immunoreactivity were image analysed .Occlusal interference induced muscle mechanical hyperalgesia in the ipsilateral side of both the masseter and the temporalis muscles. No inflammatory changes were observed, but muscle-fiber destruction was noted. There were more localized areas of substance P immunoreactivity found in the ipsilateral side of the masseter muscles following occlusal interference. 7 days after occlusal interference the ipsilateral masseter in occlusal interference group responded hyperactivity to mechanical stimulation under light anesthesia. Mechanical nociceptive stimuli and nociceptive chemical stimuli to the ipsilateral masseter induced increasing neuronal c-fos expression in the trigeminal nucleus Vi/Vc zone at obex level and in laminae Ⅰ-Ⅳ of the cl-2 spinal dorsal horn nucleus in the group of occlusal interference (p<0.05). For mechanical stimulated animals, SP expression in the trigeminal nucleus Vi/Vc transition zone was increased in the group of occlusal interference (p<0.01).Though above research,we achieved following conclusions:1. Our pain behavior animal study suggested that occlusal interference could induce masticatory muscle pain.2. The pain appearance mimics delayed-onset muscle pain in clinic, and peripheral pain mechanisms are associated with considerable muscle damage.3. The sensitization of neurons in the brainstem play an important role in the masticatory muscle pain induced by occlusal trauma.