NNOS Regulating Hippocampal Neurogenesis And Depression

Depression is a common mood disorder with the core symptom of anhedonia. About 10-20% individuals undergo depression overall lifetime. Suicide, with the prevalence rate of 15% in depressive patients, is an important problem caused by depression. It is estimated by the World Health Organization that depression will be the most important cause of disability in the world by the year 2020, yet the mechanisms underlying its pathophysiology are still poorly understood.In recent years, more and more attention has been devoted to the relationship between depression and hippocampus, especially the hippocampal neurogenesis. Numerous evidences show that hippcampal neurogenesis correlates with depression closely. For example, almost all types of antidepressants promote hippocampal neurogenesis and many patients suffering from major depression have decreased level of hippocampal neurogenesis. However, it remains unclear whether blockade of hippocampal neurogenesis gives rise to depressive behaviors.It has been shown that neuronal nitric oxide sythase(nNOS), enriched in the hippocampus, is an endogenous repressor of neurogenesis. The hippocampus, one of the components of the limbic system, is structurally and functionally affected in mood disorders. It remains in question whether nNOS regulates behavior of depression by regulating hippocampal neurogenesis, and whether nNOS could be a potential drug target for antidepressants. Stress is a risk factor for depression. Elevation of glucocorticoid is a major and severe response of the organism to stress insult. Whether the elevated glucocorticoid affects hippocampal neurogenesis, glucocorticoid receptor expression and behavior changes of depression via regulating nNOS is unknown.Accordingly, our study focused on items as following:?. Whether reduction of the adult hippocampal neurogenesis induces depressive-like behavior.There is extensive evidence indicating that hippocampal neurogenesis play an important role in the behavioral effects of chronic antidepressants. However, it is not known whether impaired hippocampal neurogenesis leads to the affective symptoms of depression. We used 3'-azido-deoxythymidine (AZT), a telomerase inhibitor, to diminish cell birth in the adult hippocampal dentate gyrus (DG) or the subventricular zone (SVZ) of the lateral ventricle in mice. We measured behavioral modifications in the tail suspension test (TST), forced swimming test (FST) and sucrose preference test (SPT) in the mice. A moderate decrease in the number of newborn cells in the hippocampal dentate gyrus (DG) produced a despair behavior and an impaired sensitivity to reward in adult mice. However, impaired neurogenesis in the SVZ did not influence the behavioral phenotype. Moreover, the treatment with AZT did not cause neuronal degeneration or produce general aversive effects. Our findings suggest that impaired hippocampal neurogenesis gives rise to depressive-like behavior.?. Whether nNOS contributes to chronic stress-induced depression by suppressing hippocampal neurogenesis.Increasing evidence suggests that depression may be associated with a lack of hippocampal neurogenesis. It is well established that neuronal nitric oxide synthase (nNOS)-derived NO exerts a negative control on the hippocampal neurogenesis. Using genetic and pharmacological methods, we investigated the roles of nNOS in depression induced by chronic mild stress (CMS) in mice. Hippocampal nNOS over-expression was first observed 4 days and remained elevated 21 and 56 days after exposure to CMS. The mice exposed to CMS exhibited behavioral changes typical of depression, and impaired neurogenesis in the hippocampus. The CMS-induced behavioral despair and hippocampal neurogenesis impairment were prevented and reversed in the null mutant mice lacking nNOS gene (nNOS-/-) and in the mice receiving nNOS inhibitor. Disrupting hippocampal neurogenesis blocked the antidepressant effect of nNOS inhibition. Moreover, nNOS-/- mice exhibited antidepressant-like properties. Our findings suggest that nNOS over-expression in the hippocampus is essential for chronic stress-induced depression and inhibiting nNOS signaling in brain may represent a novel approach for the treatment of depressive disorders.?. Whether CMS-induced decrease in GR expression in the hippocampus is caused by corticosterone via up-regulating nNOS expression, if so, whether this pathway is involved in the modifications of depressive behaviors.To investigate whether glucocorticoid is involved in CMS-induced over-expression of nNOS, we treated mice with corticosterone (CORT), the glucocorticoid in rodent, for 7, 14 or 21 days (40mg/kg, s.c.) and incubated hippocampal slices with CORT in vitro for 24, 48 or 72 hours (10^-5, 10^-6, 10^-7, 10^-8 and 0M). At the different time points, the mice were killed and the hippocampal slices were harvested. Then, the expression of nNOS and GR was measured by Western blot. We found that corticosterone treatments (7, 14 or 21days) and incubation of corticosterone(10^-5M, 24, 48, 72 hours) in hippocampal slices increased the nNOS expression in the hippocampus, while corticosterone treatments (14, 21days) and incubation of corticosterone in hippocampal slices (10^-5M, 72 hours) began to decrease the expression of GR in the hippocampus. To investigate whether the CMS-induced or elevated CORT-induced over-expression of nNOS results in the decrease in the GR expression, we used two independent approaches, selective nNOS enzyme activity inhibitor-7-NI as a pharmacological approach and nNOS gene knockout mice as a genetic approach. Western blot and RT-PCR were carried out to detect the expression of nNOS and GR in the hippocampus. The data showed that 7-NI treatment (30mg/kg,i.p.,21days) reversed the decrease in GR expression induced by high concentration of corticosterone (40mg/kg, s.c., 21days) or induced by CMS (21 days). Likewise, nNOS-/- mice showed no decrease in GR expression after CORT treatment.To investigate whether nNOS-derived NO can modulate GR expression in the hippocampus, we applied 7-NI to reduce nNOS-derived NO and EDTA/NONOate to provide exogenous NO. Western blot and RT-PCR was performed to detect the expression of GR in the hippocampus. Our results revealed that high concentration of NO transformed from EDTA/NONOate (100?mol, 24hours) decreased the GR expression in cultured hippocampal neurons in vitro and that 7-NI treatment (30mg/kg, 50mg/kg, i.p., 7days) increased the GR expression in the hippocampus.To investigate whether nNOS inhibition can reverse the elevated CORT-induced depressive-like behaviors of mice, we measured the immobility time in tail suspension test and forced swimming test, measured intake of sucrose, percent sucrose preferation, and weight growth. Our results revealed that 7-NI treatment (30mg/kg, i.p., 7days) reversed the corticosterone (40mg/kg, s.c., 21days)-induced weight loss, prolonged immobility time in the tail suspension test, prolonged immobility time in the forced swimming test, decreased intake of sucrose and decreased percent sucrose preferation in mice.In conclusion, results in this part suggested corticosterone impaired the GR expression by up-regulating nNOS expression in the hippocampus, consequently led to development of depression.Overall, our study indicates that blockade of hippocampal neurogenesis induces depressive-like behavior phenotype in adult mice; over-expression of nNOS contributes to depressive behavior changes by suppressing hippocampal neurogenesis in mice exposed to CMS; stress up-regulates nNOS expression by the elevated glucocorticoid in the hippocampus, in turn impairs the expression of GR in the hippocampus and the function of HPA axis. Hence, our study suggests that nNOS may be a novle target for antidepressans development.