The Discussion On The Mechanisms Of Hippocampal Neurons Injury In Rats Induced By Different Stress Modes

Backgrounds and objective:Depression is one of the common mental disorders at present, and now theprevalence of depression is increasing every year. Stressful life events, withfeatures of chronic disease, low-intensity, long duration, are the most commonpredisposing factor of depression, which is of unpredictability. The more lifeevents occur and the more serious they are, the higher incidence of depressionwill have.Hippocampus is not only an important cognitive and emotional response center,but also a target organ to which stress is easy to do damage. Patients withlong-term and severe depression have a reduced hippocampal volume. Decreasingnumber of hippocampal neurons or increasing apoptosis were also found in animalmodels of depression. Studies have shown that stress can cause over-activationof the HPA axis and enhancement of neural response, resulting in increasingsynthesis and release of ACTH and plasma Corticosterone, as well as significantincrease of central inflammatory cytokines IL-1βlevels. Our previous studieshave shown that, higher expression of IL-1β(such as intraperitoneal injectionof LPS or intracerebroventricular injection of IL-1β) can cause down-regulationof CREB and BDNF. BDNF, the neurotrophic factor in brain, plays a key role inneuronal survival and development, while the lack of BDNF can also increase cellshrinkage and apoptosis.Therefore, based on preliminary studies, our study is aimed to explore themechanisms of hippocampal injury resulting from predictable and unpredictablestress. According to chronic unpredictable stress and chronic restraint stress(predictable stress), animal models were established to detect behavior of rats,changes in serum corticosterone, changes of neuronal apoptosis and neuron numberin regions of hippocampus and the expression of BDNF and IL-1β. Meanwhile analysis of possible biological mechanisms was done, in order to provide a theoretical andexperimental basis to prevent from depression.Materials and methods:Twenty-four adult male Sprague-Dawley (SD) rats, weighted 240-280g, wererandomly divided into 3 groups after one week of adaptive feeding: normal controlgroup(hereafter referred to as the control group), chronic unpredictable stressgroup(hereinafter referred to as chronic stress group), chronic restraint stressgroup(hereinafter referred to as restraint stress group), with 8 rats in everygroup. Large cage was used for combined feeding in the control group, with 4 ratsper cage. Chronic stress group received 21 days of chronic unpredictable stress,while restraint stress group received 21 days of chronic restraint stress,bothof them were housed singly in their own cage. We weighted the rats and testedspontaneous activity every week during the stress. We collected tail blood of therats on 1st,7th,14th and 21st day of the stress. 1% sucrose consumption weretested after stress for observing changes of depressive behavior of rats. ELISAmethod to measure serum corticosterone levels, and immunohistochemistry methodto measure expression of BDNF and IL-1βin hippocampus of the brain. Nisslstaining was used to observed changes in the number of hippocampal neurons, TUNELmethod to measure neuronal apoptosis. The results were analysed using Excel 2003and SPSS 13.0 statistical software. Data were shown in the forms of mean±standard deviation ((x|-)±s). The single factor analysis of variance (ANOVA) was usedto compare means between groups, nonparametric statistics was used whenheteroscedasticity. P<0.05 means the difference was significant.Results:1. Experiments of locomotor activityAt the 21th day of stress, compared with the control group, the total distance and the central distance in both two stress groups were all reduced (P<0.05).2. 1% Sucrose consumption testAfter 21 days of chronic stress, compared with the control group, sucroseconsumption decreased in both experimental groups (P<0.05). The percentage ofsucrose consumption was both significantly decreased (P<0.01).3. Effects of different stress modes on body weightDuring the chronic stress, the rate of body weight gain significantlydecreased in restraint stress group at the first week (P<0.01),both the two stressgroups showed lower rate of body weight gain at the second and thirdweek(P<0.05,P<0.01). At the first week, the rate of body weight gain in restraintstress group was significantly lower than that in chronic stress group, with anegative growth (P<0.01). At the third week, the rate of body weight gain inrestraint stress group was significantly higher than that in chronic stress group(P<0.01).4. Effects of different stress modes on serum corticosterone levelsIn the stress process, as compared with the control group, corticosterone wassignificantly higher in both chronic stress group and restraint stress group(P<0.01). Compared with the chronic stress group, restraint stress group showedan increasing corticosterone levels at the 1st and 7th day of stress (P<0.05,P<0.01), but a decrease in the level of corticosterone at 14th and 21th day ofstress (P<0.01).5. Effects of different stress modes on expression of BDNF inhippocampusCompared with the control group, the expression of BDNF decreased in CA1 andCA3 regions of hippocampus in chronic stress group (P<0.01,P<0.05) and in theCA3 region in restraint stress group(P<0.05).6. Effects of different stress modes on expression of IL-1βinhippocampus There were no significant differences of the expression of IL-1βinhippocampal regions between the two stress groups and the control group(P>0.05).7. Effects of different stress modes on the number of hippocampalneuronsCompared with the control group, chronic stress group had a reduction in thenumber of neurons in CA1, CA3 regions and dentate gyrus (DG region) (P<0.05),restraint stress group presented a decrease in the number of neurons in CA3 andDG region (P<0.05).8. Effects of different stress modes on hippocampal neuronsapoptosisCompared with the control group, chronic stress group had an increase inapoptosis of neurons in DG region (P<0.05,H-Test), while there was no significantdifference in apoptosis of the hippocampal regions in restraint stress group(P>0.05).Conclusions:1. Chronic unpredictable stress and Chronic restraint stress can bothlead to depression-like behaviors in rats,and both injury the hippocampus.The same mechanism may as follow: Glucocorticoid increases in blood after theexcessive activation of HPA axis and damage the hippocampal neurons.2. The injury of hippocampal neurons induced by chronic unpredictable stressis heavier than the injury induced by chronic restraint stress (predictablestress) that lasts 3 hours per day.