Study On The Mechanism Of Masseter Muscle Hyperalgesia Induced By Psychological Stress In Rats

The pain is the result of the stimulation of special sensory nerve endings by injury or physical harm, by which the nociceptor is triggered, harm information is transducted along the relative input nerve access of the central nerve system to the spinal cord, brain stem, diencephalon, or further to the limbic system and cerebral cortex. Thus, the feeling and reactions of pain formed finally by the central integration at different level of Central Nervous System(CNS). The whole integration process includes four parts of transduction, transmission, modulation and perception. It was manifested that the reactivity of CNS could be modulated by either peripheral sensitization or central sensitization during the outside trauma, which contributes to the formation of hyperalgesia after the physical trauma.Orofacial pain is the most common pain disease, which has seriously affected the daily life and human health. Orofacial pain has many types, including temporomandibular joint disorder syndrome, trigeminal neuralgia, migraine, etc. Psychological factor plays an important role in the occurrence, repetitive reoccurrence and treatment of the oralfacial pain. The current study on roles of psychological stress in the head and facial pain just limited to clinical epidemiological investigation or some biological changes, such as EMG, blood flow, and so on, which have come to an encouraging result and fully confirmed the relevance between psychological stress and orofacial pain. But we also noticed that there were still lack of enough experiments on the metabolism of pain induced by psychological stress, as well as the recovery process after the removal of the psychological stress.Thereby, in this study, a communication box model was adopted to simulate the psychological stress. With the masseter muscle and trigeminal ganglion being chosen as the observation targets, the mechanism of orofacial muscle hyperalgesia induced by psychological stress, including behavioral changes and pain-related nerve peptides expression, were investigated here, which could help to further identify the pathophysiology basis of masticatory muscles functional disorder and orofacial muscle pain induce by psychological stress.In the first part, a communication box was adopted to establish animal model of psychological stress. 100 Sprague-Dawley rats were randomly divided into 5 groups: control group, psychological stress (PS) group, psychological stress plus diazepam injection (PS+DI) group, psychological stress plus saline injection (PS+SI) group and footshock (FS) group, with 20 rats for each group. All the rats were fed under the same conditions. According to Rens'method, von Frey filaments were used to assess the mechanical pain threshold of the muscle, with the head withdrawal, leg raising and crying being identified as pain actions. From the 7th days before psychological stress to 14th day after psychological stress removal, we recorded the mechanical pain threshold of masseter muscle. Then we calculated pain value by the equation as the following. Pain value of each day equals to the mechanical pain threshold of that day minus that of the 7th day before psychological stress. The results showed that during the adaptive period of 7th days before the psychological stress experiment, mechanical pain value of bilateral masseter muscles were in descent and stabilized on the 5th～7th day. During the psychological stress period, hyperalgesia of PS group were observed in bilateral masseter muscles, with the peak time on the 6th-8th day. Then, it alleviated in the following days and was stabilized on the 12th-14th day, while with its pain threshold still being lower than the blank control group. On the other hand, during the psychological stress period, the variation trend of the mechanical pain threshold of PS+DI group resembled that of the psychological stress group, but the threshold values of the former group showed higher than those of the latter. It was suggested that psychological stress could lead to the hyperalgesia of masseter muscles, which could be effectively relieved by the antidepressant drug.After the psychological stress induced by our equipment of communication box was proved to lead to the hyperalgesia of masticatory muscles, we continued to further investigate the molecular mechanism of it. All the specimens of masseter muscle were taken on the 1st day, 3rd day, 7th day, 14th day after psychological stress, 7th day and 14th day after removal of psychological stress. The expression of SP mRNA and CGRP mRNA in masseter muscle was detected by reverse transcription-polymerase chain reaction (RT-PCR). The result showed that the SP mRNA and CGRP mRNA expression of masseter muscle in ES group was stronger than those of control group at most time points during psychological stress. SP mRNA and CGRP mRNA level increased after the 1st day and reached the peak level on the 7th day. Afterwards, they started to decrease gradually but still showed higher than those of the control till the 14th day after psychological stress. SP mRNA and CGRP mRNA level were gradually decreased after removing the psychological stress, and back to rest level in the 14th day after the removal of psychological stress.In order to further investigate SP mRNA and CGRP mRNA expression in the low-level pain central system, which help to understand the mechanism of masseter muscle hyperalgersia after psychological stress, in the 3rd part of this research work, the expression of SP mRNA and CGRP mRNA in trigeminal ganglion was detected by RT-PCR. The results showed that the SP mRNA and CGRP mRNA expression of trigeminal ganglion in ES group was stronger than those of the control at most parts of time points during psychological stress. SP mRNA and CGRP mRNA level increased after 1st day and reached the peak level on 7th day. Afterwards, they started to decrease gradually but still showed higher than those of the control till the 14th day after psychological stress. After removing the psychological stress, the expression of both SP mRNA and CGRP mRNA gradually decreased, and back to normal level till the 14th day.Conclusions1. Psychological stress can lead to the hyperalgesia of masseter muscles, antidepressant drug lower the hyperalgsia resulted of psychological stress.2. Psychological stress can result in the variation of the expression of SP mRNA and CGRP mRNA in rat masseter muscle and trigeminal ganglion, which may be one of the reasons of masseter muscles hyperalgesia under psychological stress.