Abstract: Currently, there exists a substantial body of evidence suggesting that chronic stress can increase susceptibility to diseases such as depression and posttraumatic stress disorder. Chronic stress induces neuronal atrophy and death in the cortex as well as in the hippocampus. Proto-oncogene protein kinase B (PKB), also known as Akt, plays a central role in a signaling pathway whose various components have been linked to cellular survival. In recent years, Akt as well as one of its downstream targets Forkhead transcription factors (forkhead homologue in rhabdomyosarcoma, (FKHR)) and the Fas/FasL system have emerged as cardinal pathways underlying cellular survival and opposing apoptosis in neurons. In our previous experiments, we observed that repeated immobilization stress exposure can change the levels of phosphorylation of Akt and FKHRL1 in the rat hippocampus. However, the activation of the Fas/FasL system following chronic stress in the brain remains poorly defined. We hypothesized that repeated immobilization stress exposure may change the levels of the Fas/FasL system in the brains of rats. The purpose of the present study was to detect the effects of chronic immobilization stress on the expression of Fas/FasL in the prefrontal cortex, entorhinal cortex, and hippocampus in rats.
Twenty four male Sprague-Dawley rats (280 ± 20 g) were purchased from Shanghai Laboratory Animal Center, Chinese Academy Sciences, and they were used for all experiments. These rats were randomly divided into three groups—the immobilization stressed group, apparatus control group, and control group. Each group comprised eight rats. The rats were housed in groups of four, under standard laboratory conditions in temperature-controlled rooms (24℃), and maintained under a 12-h light/dark cycle (lights on at 0800 hours) with food pellets and water available ad libitum. Rats from the immobilization stressed group were immobilized by fixing a board on their backs for an hour each day for 14 consecutive days. The apparatus controls were placed in a novel environment for an hour per day for 14 consecutive days. The controls were free of stress. At the end of the experimental period, the rats were decapitated and their prefrontal cortex, entorhinal cortex, and hippocampi were rapidly removed and maintained at &#8722;80℃ until analysis. The protein levels of Fas and FasL were determined by Western blotting. The intensities of the bands corresponding to the protein of interest were quantified using scanning densitometry and compared using one-way ANOVA. The adrenal gland and thymus gland indices were also calculated on the day following the 14-day-stress modeling. Organ index = wet weight/weight of rat × 100. The statistical significance was determined at p < 0.05.
A total of 24 rats were considered in the analysis of the results. The body weight gain of the stressed group was significantly lower than that of the control group and the apparatus control group. A significant change was observed in the organ indices of the stressed groups as compared to the control group. The Fas and FasL levels of the hippocampus revealed a significant difference among three groups (F = 26.9, p < 0.001; F = 40.29, p < 0.01, respectively). The Fas and FasL levels were significantly increased in the immobilization stressed group than in the control group (p < 0.01). The Fas levels of the prefrontal cortex and entorhinal cortex in the immobilization stressed group were significantly increased than those in the control group (p < 0.01). In contrast, the FasL levels of the prefrontal cortex and entorhinal cortex did not reveal a significant deference between the immobilization stressed group and control group.
The results suggested that chronic immobilization stress can induce a more significant increase in both the Fas and FasL levels in the hippocampus. Changing the expression of Fas/FasL may be an effective biological predictor for the change of structure or function of hippocampus induced by chronic stress in rats

Key words: immobilization stress, prefrontal cortex, entorhinal cortex, hippocampus, Fas, FasL