Neuronal nitric oxide synthase-derived nitric oxide inhibits neurogenesis in the adult dentate gyrus by down-regulating cyclic AMP response element binding protein phosphorylation

DETA/NONOate, a nitric oxide donor, produces antidepressant effects by promoting hippocampal neurogenesis

RATIONALE

Increasing evidence suggests that depression may be associated with a lack of hippocampal neurogenesis. Our recent study shows that endogenous nitric oxide (NO) contributes to chronic mild stress (CMS)-induced depression by suppressing hippocampal neurogenesis.

OBJECTIVES

The aim of this study was to investigate the effects of exogenous NO in CMS-induced depression in young adult mice.

RESULTS

In normal mice, administration of a pure NO donor (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl) aminio] diazen-1-ium-1,2-diolate (DETA/NONOate; 0.4 mg/kg, i.p., for 7 days) produced an antidepressant-like effect and significantly increased hippocampal neurogenesis. The mice exposed to CMS exhibited behavioral changes typical of depression and impaired neurogenesis in the hippocampus. Treatment with DETA/NONOate (0.4 mg/kg, i.p., for 7 days) reversed CMS-induced behavioral despair and hippocampal neurogenesis impairment. We treated mice with a telomerase inhibitor 3'-azido-deoxythymidine (AZT; 100 mg/kg, i.p., for 14 days) to disrupt neurogenesis. From day 4 to day 11 of AZT treatment, mice were injected with DETA/NONOate (0.4 mg/kg, i.p., for 7 days). Disrupting hippocampal neurogenesis blocked the antidepressant effect of DETA/NONOate.

CONCLUSIONS

Our findings suggest that exogenous NO benefits chronic stress-induced depression by stimulating hippocampal neurogenesis and may represent a novel approach for the treatment of depressive disorders.

Changes in glutamate decarboxylase enzyme activity and tau-protein phosphorylation in the hippocampus of old rats exposed to chronic mild stress: reversal with the neuronal nitric oxide synthase inhibitor 7-nitroindazole

Effects of chronic stress are not completely understood. They may underlie depression and dementia. This study assessed the association between chronic stress, glutamate levels, tau-protein phosphorylation, and nitric-oxide in old rats exposed to chronic mild stress (CMS). Old (>15 months) male Wistar rats were exposed to CMS. Comparison groups included old and young control rats, young CMS-exposed, and old CMS-exposed rats treated with the neuronal nitric-oxide synthase (nNOS) enzyme inhibitor, 7-nitroindazole (20 mg/kg/day i.p.). Hippocampal glutamate levels and glutamate decarboxylase (GAD) activity were determined and tau protein phosphorylation was assessed. Age was a significant (p=0.025) source of variation in glutamate level [811.71+/-218.1, 665.9+/-124.9 micromol/g tissue protein (M+/-SD) in young and old control rats, respectively]. Old rats exposed to CMS were characterized by an increased risk to develop anhedonia. There was significant (p=0.035) decrease in GAD enzyme activity (-60.06%) and increased tau protein hyperphosphorylation in old rats exposed to CMS compared to control. Administration of 7-nitroindazole to CMS-exposed old rats significantly (p=0.002) increased GAD activity, decreased glutamate levels (7.19+/-3.19 vs. 763.9+/-91 micromol/g tissue protein; p=0.0005), and decreased phosphorylation of tau proteins compared to CMS exposed rats.

GM-CSF enhances neural differentiation of bone marrow stromal cells

Recent reports suggest that bone marrow stromal cells may be induced into neural cells both in vivo and in vitro. The factors that regulate the neural differentiation and the mechanism involved, however, remains unclear. Here we demonstrated that granulocyte-macrophage colony-stimulating factor (GM-CSF), a potent hematopoietic factor, was able to enhance the neural differentiation of bone marrow stromal cells. Moreover, we found that GM-CSF receptors are abundantly distributed in the bone marrow stromal cells and GM-CSF significantly upregulated the phosphorylation of cAMP-responsive element binding protein in bone marrow stromal cells. These findings suggest that GM-CSF may activate its receptor and then enhance neural differentiation of bone marrow stromal cells by upregulating phosphorylation of cAMP-responsive element binding protein.

Neuronal nitric oxide synthase 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.

Reduced neuronal nitric oxide synthase is involved in ischemia-induced hippocampal neurogenesis by up-regulating inducible nitric oxide synthase expression

Nitric oxide (NO), a free radical with signaling functions in the CNS, is implicated in some developmental processes, including neuronal survival, precursor proliferation, and differentiation. However, neuronal nitric oxide synthase (nNOS) -derived NO and inducible nitric oxide synthase (iNOS) -derived NO play opposite role in regulating neurogenesis in the dentate gyrus after cerebral ischemia. In this study, we show that focal cerebral ischemia reduced nNOS expression and enzymatic activity in the hippocampus. Ischemia-induced cell proliferation in the dentate gyrus was augmented in the null mutant mice lacking nNOS gene (nNOS-/-) and in the rats receiving 7-nitroindazole, a selective nNOS inhibitor, after stroke. Inhibition of nNOS ameliorated ischemic injury, up-regulated iNOS expression, and enzymatic activity in the ischemic hippocampus. Inhibition of nNOS increased and iNOS inhibitor decreased cAMP response element-binding protein phosphorylation in the ipsilateral hippocampus in the late stage of stroke. Moreover, the effects of 7-nitroindazole on neurogenesis after ischemia disappeared in the null mutant mice lacking iNOS gene (iNOS-/-). These results suggest that reduced nNOS is involved in ischemia-induced hippocampal neurogenesis by up-regulating iNOS expression and cAMP response element-binding protein phosphorylation.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

Migrating neuroblasts of the rostral migratory stream are putative targets for the action of nitric oxide

It has been demonstrated that the gaseous messenger nitric oxide influences cell proliferation and cell migration, and therefore affects adult neurogenesis in mammals. Here, we investigated the putative targets for this action in the rostral migratory stream of the rat. We used immunocytochemical detection of the beta1 subunit of the enzyme soluble guanylyl cyclase, which can be activated by nitric oxide. Our results under light and electron microscopy demonstrated that the migrating neuroblasts (type A cells) were beta1-immunopositive. The astrocytes (type B cells), immature precursors (type C cells) and ependymal cells (type E cells) were beta1-immunonegative. The neurochemical characterization of the soluble guanylyl cyclase-containing cells confirmed these results. In this regard, the beta1-containing cells expressed doublecortin, a protein expressed by type A cells, and did not express glial fibrillary acidic protein, which is a marker for type B cells. Injection of 5-bromo-2'-deoxyuridine 2 h before killing demonstrated that proliferating cells did not contain soluble guanylyl cyclase. Finally, we found that beta1-containing type A cells also expressed the A3 subunit of the cyclic nucleotide-gated ion channels. Altogether, the present results indicate that nitric oxide may influence adult neurogenesis acting on the migrating neuroblasts of the rostral migratory stream. In these cells, nitric oxide may activate the enzyme soluble guanylyl cyclase, triggering the production of the second messenger cGMP. In turn, cGMP might induce the opening of cyclic nucleotide-gated ion channels, which are present in these cells.

Neuronal nitric oxide synthase (NOS-I) knockout increases the survival rate of neural cells in the hippocampus independently of BDNF

Investigations regarding the regulation of adult neurogenesis, i.e., the generation of new neurons from progenitor cells, have revealed a high degree of complexity. Although the pleiotropic messenger molecule nitric oxide (NO) has been suggested to modulate adult neurogenesis, the evidence is inconclusive due to the presence of different NO synthase isoforms in the brain. We therefore investigated whether stem cell proliferation or survival is altered in mice lacking neuronal nitric oxide synthase (NOS-I) or both endothelial and neuronal NOS (NOS-I/-III double knockout). While proliferation of neural stem cells was only numerically, but not significantly increased in NOS-I knockdown animals, the survival of newly formed neurons was substantially higher in NOS-I-deficient mice. In contrast, NOS-I/-III double knockout had significantly decreased survival rates. QRT-PCR in NOS-I-deficient mice revealed neither NOS-III upregulation compensating for the loss of NOS-I, nor alterations in VEGF levels as found in NOS-III-deficient animals. As changes in BDNF expression or protein levels were observed in the cortex, cerebellum and striatum, but not the hippocampus, the increase in stem cell survival appears not to be due to a BDNF mediated mechanism. Finally, NOS-I containing neurons in the dentate gyrus are rare and not localized close to progenitor cells, rendering direct NO effects on these cells unlikely. In conclusion, we suggest that NO predominantly inhibits the survival of new-born cells, by an indirect mechanism not involving BDNF or VEGF. Together, these results emphasize the important role of the different NOS isoforms with respect to adult neurogenesis.

Evidence of postnatal neurogenesis in dorsal root ganglion: role of nitric oxide and neuronal restrictive silencer transcription factor

The various mechanisms underlying postnatal neurogenesis from discrete CNS regions have emerged recently. However, little is known about postnatal neurogenesis in dorsal root ganglion (DRG). BrdU incorporation and subsequent immunostaining for BrdU, neural stem cell marker, nestin and neuronal marker, PGP 9.5 have provided evidence for postnatal neurogenesis in DRG. We further demonstrate, in vivo and in vitro, that nitric oxide (NO) regulates neural stem cells (nestin+) proliferation and, possibly, differentiation into neurons. Surprisingly, nerve growth factor (NGF) had no effect on nestin+ cells proliferation. Axotomy or NGF-deprivation of DRG neurons-satellite glia co-culture increases NO production by neurons and treating with a NO synthase (NOS) inhibitor, N G-nitro-L-arginine methylester (L-NAME) in vitro or 7-nitroindazole (7NI) in vivo, causes a significant increase in nestin+ cell numbers. However, a soluble guanylyl cyclase (sGC) blocker, 1H-[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one (ODQ) treatment of NGF-deprived DRG neurons-satellite glia co-culture had no significant effect on nestin+ cell numbers. This implies NO regulates nestin+ cell proliferation independent of cGMP. We hypothesised that the neuronal-restrictive silencer transcription factor (NRSF, also termed REST), a master regulator of neuronal genes in non-neuronal cells, may be modulated by NO in satellite glia cultures. A NO donor, dimethyl-triamino-benzidine (DETA)-NO treatment of satellite glia cell cultures results in a significant increase in the NRSF/REST mRNA expression. The majority of cultured satellite glia cells express nestin, and also show increased levels of NOS, thus L-NAME treatment of these cultures causes a dramatic reduction in NRSF/REST mRNA. Overall these results suggest that NO inhibits neurogenesis in DRG and this is correlated with modulation of NRSF, a known modulator of differentiation.