[The effect of restraint stress on masseter mechanical hyperalgesia and the activity of neurons and astrocytes in the spinal trigeminal nucleus caudalis in rats].

Objective: To investigate the effect of restraint stress on masseter mechanical hyperalgesia and the activity of neurons and astrocytes in the spinal trigeminal nucleus caudalis (Vc). Methods: The animals were randomly divided into the control group, 1-, 3-, 5-, 7-, 9-, 11-and 14-day stress groups, with 10 rats in each group. The body weight increase and behavior tests were used to testify the animal model. The mechanical sensitivity of masseter of the rat before and after the stress was measured with Von Frey filaments. Histological examinations were used to evaluate the expression of neuronal c-fos and astrocytic glial fibrillary acidic protein (GFAP). Results: Restraint stress resulted in remarkable mechanical allodynia in the masseter muscle. The head withdrawal threshold was significantly lower in the 7-, 9-, 11-and 14-day stress groups ([0.071±0.011], [0.059±0.020], [0.052±0.011], [0.033±0.011] N) than that in the control group ([0.120±0.025] N) (P<0.05). Compared to the control group, the rats in the 1-day stress group showed a significant increase of c-fos in neurons of the Vc and then declined to normal level after 1 week gradually. The GFAP expression in astrocytes of the Vc was significantly increased in the 7-, 9-, 11-and 14-day stress groups (4.3±1.0, 4.5±0.6, 4.6±0.5, 4.8±1.3) compared with the control group (2.0±0.8) (P<0.05). Conclusions: Chronic restraint stress could lower the threshold of mechanical allodynia in the masseter muscle and activate the neurons and astrocytes in Vc. The activation of neurons and astrocytes plays an important role in the masseter hyperalgesia induced by restraint stress in rats.