| Psychological stress is a tense reaction when the organism can't accommodate to the surroundings as the result of the imbalance of objective damand and coping ability under some stimulation. In morden society, the physiological and psychological functinons of human beings are affected by various kinds of stresses such as fierce competition, speedy work pace, increasement of life stress. Although short-term or moderate psycholgical stress could keep people alert and was beneficial to people's health and action, long-term or highly intensified psychological stress will be harmful. People are paying more and more attention to the relationship between the psychological stress and psychophysiological disorder. The previous animal experiment approved that psychological stress were closely related with morphology, metabolism, transformation of muscle fiber and the pain threshold of masticatory muscle. However, the mechanism of the effect of psychological stress on masticatory muscle is still unclear . It calls us to do further study.The maticatory muscles, tempromandibular joint (TMJ) and occlusion support and restrict each other under the control of the central nervous system (CNS). They contribute to the normal occlusive function together. As one kind of maticatory muscles, masseter muscle takes effect in chewing, swallowing, talking and so on. Moreover, the density of proprioceptors such as muscle spindle in elevated jaw muscles is higher than that in the descended jaw muscles, especially the masseter muscle. So masseter muscle is vulnerable to many kinds of stresses. Therefore, the masseter muscle was chosen as the object in our experiment. Communication box was used to establish the psychological stress model in rats and animal's behavior change was observd. The free radical metabolism, the expression of HSP70 in rat's masseter muscle and the relationship between them were investigated in order to explore the harmful effect of psychological stress on masseter muscle. With the experiment, we hope it could help us further study the mechanism of masticatory muscle fatigue, dysfunction or even gnathostomatic system diseases result from psychological stress, and establish the experimental foundation for preventing the gnathostomatic system dysfunction.The experiment is composed of three parts. In the first part, seventy-two SD male rats were randomly divided into 3 groups: psychological stress group (PS group), footshock group (FS group) and control group, with 24 rats in each group. And each group comprise three subgroups according to the time 1 week, 3 weeks and 5 weeks. The PS group rats were placed into the communication box to receive the psychological stress produced by the FS rats during the experimental time, the control group rats received no stimulus at all. Only the PS group rats and the control group rats were studied. All the animals were normally fed under the same condition in the experiment. Elevated plus maze (EPM) test and Open field test were used in order to observe the behavior of the rats. The EPM test indicated that the frequencies in open arms of PS group rats were decreasing with time-dependent as well as the time spent in open arms which was shorter than control group rats 3 weeks and 5 weeks after psychological stress. Also, the frequencies in closed arms of PS group rats were fewer at 3 and 5 weeks. However, the time spent in closed arms of PS group rats was longer compared with the control group rats all the time, and the distance moved of PS group rats after psychological stress of 3 weeks and 5 weeks in EPM was shorter. The results of open field test demonstrated that the velocity and the distance moved of PS group rats were smaller than the control group rats 3 weeks and 5 weeks after psychological stress, and the time spent in open field center of PS group animals was decreasing with time-dependen.The second part was based on the first part which testified the effect of psychological stress on the rat's behavior. PS group rats and control group rats were executed and sampled at 1 week,3 weeks and 5 weeks separately and measured the free radical metabolism in masseter muscles of rats. The results showed that there were no obvious changes in free radical metabolism in masseter muscles of rats between PS group and control group after the psychological stress of 1 week. However, the activity of SOD,GSH-PX and CAT decreasd and the content of MDA increased in PS group 3 weeks after stress . Though the activity of SOD and GSH-PX in PS group of 5 weeks elevated in comparison to the PS group of 3 weeks, they both were lower than the control group as well as the activity of CAT. The content of MDA of PS group with psychological stress of 5 weeks was fewer than that of 3 weeks, although it was still more than the control group. The content of H2O2 of PS group rats increased only after the psychological stress of 3 weeks.In the third part, the expression of heat shock protein 70 (HSP70) was evaluated by immunohistochemistry and Western blot. The expression of HSP70 in masseter muscles showed no differences between the PS group and the control group 1 week or 3 weeks after psychological stress. But the HSP70 expression increased singnificantly in the masseter muscles of PS group 5 weeks after stress, and also expressed more than the other PS groups. Meanwhile, there was negative correlation between the expression of HSP70 and the content of MDA.Conclusions:1,The communication box can establish the psychological stress model in rats effectively, and the psychological stress could lead to the behavior of anxiety in rats.2,Psychological stress can result in the abnormality of the free radical metabolism in rat's masseter muscle, which may be one of the mechanism of dysfunction of masseter muscle induced by psychological stress.3,Long-term psychological stress results in the enhancement of the expression of HSP70 in the masseter muscle of rats, and the increasement of HSP70 may control the free radical metabolism and reduce the injury result from free radical. |
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