Impact of age and caloric restriction on neurogenesis in the dentate gyrus of C57BL/6 mice

Age-related changes in neurogenesis and its modulation by caloric restriction (CR) were studied in C57BL/6 mice. To this end, bromodeoxyuridine (BrdU) labeling was used to assess neuronal and glial precursor proliferation and survival in the granular cell layer (GCL) and the hilus of the dentate gyrus of 2-, 12-, 18-, and 24-month-old mice. For both regions, we found an age-dependent decrease in proliferation but not in survival of newborn cells. Interestingly, the reduction in proliferation occurred between 2 and 18 months of age with no additional decline between 18- and 24-month-old mice. Phenotyping of the newborn cells revealed a decrease in the neuron fraction in the GCL between 2 and 12 months of age but not thereafter. The majority of BrdU cells in the hilus colocalized with astrocytic but none with neuronal markers. CR from 3 to 11 months of age had no effect on neurogenesis in the GCL, but had a survival-promoting effect on newly generated glial cells in the hilus of the dentate gyrus. In conclusion, C57BL/6 mice reveal a substantial reduction in neurogenesis in the dentate gyrus until late adulthood with no further decline with aging. Long-term CR does not counteract this age-related decline in neurogenesis but promotes survival of hilar glial cells.     

Intracerebroventricular infusion of insulin-like growth factor-I ameliorates the age-related decline in hippocampal neurogenesis.

The dentate gyrus of the hippocampus is one of few regions in the adult mammalian brain characterized by ongoing neurogenesis. Significantly, recent studies indicate that the rate of neurogenesis in the hippocampus declines with age, perhaps contributing to age-related cognitive changes. Although a variety of factors may influence the addition of new neurons in the adult dentate gyrus, the mechanisms responsible for the age-related reduction remain to be established. Insulin-like growth factor-I (IGF-I) is one promising candidate to regulate neurogenesis in the adult and aging brain since it influences neuronal production during development and since, like the rate of neurogenesis, it decreases with age. In the current study, we used bromodeoxyuridine labeling and multilabel immunofluorescence to assess age-related changes in neuronal production in the dentate gyrus of adult Brown Norway x Fischer 344 rats. In addition, we investigated the relationship between changes in neurogenesis and the age-dependent reduction in IGF-I by evaluating the effect of i.c.v. infusion of IGF-I on neurogenesis in the senescent dentate gyrus. The analyses revealed an age-dependent reduction in the number of newly generated cells in the adult dentate subgranular proliferative zone and, in addition, a 60% reduction in the differentiation of newborn cells into neurons. Restoration of IGF-I levels in senescent rats significantly restored neurogenesis through an approximately three-fold increase in neuronal production.The results of this study suggest that IGF-I may be an important regulator of neurogenesis in the adult and aging hippocampus and that an age-related decline in IGF-I-dependent neurogenesis could contribute to age-related cognitive changes.     

Differential stroke-induced proliferative response of distinct precursor cell subpopulations in the young and aged dentate gyrus

The capability of the adult brain to generate new hippocampal neurons after brain insults like stroke is decreasing during the aging process. Recent evidence further indicates that the proliferative properties of the precursor cells change in the aged brain. We therefore analyzed the early proliferative response of distinct precursor cell populations in the subgranular zone of the dentate gyrus in 3 and 16 months old transgenic nestin-green-fluorescent protein mice 4 days after ischemic cortical infarcts. A detailed immunocytochemical analysis of proliferating precursors revealed a significant infarct-induced activation of the earliest radial glia-like precursor cells (type 1 cells) and the more differentiated precursor cell subtypes (type 2b cells) in young mice. In contrast the proliferation of early neuronal precursor cells (type 2a cells) was stimulated in the aged brain. Additional long-term experiments further demonstrated that this differential proliferative response of distinct precursor cells is associated with an enhanced number of newborn neurons in the young DG after stroke whereas this increase in neurogenesis was absent in the aged brain. However, our study demonstrates that even precursor cells in the aged hippocampus possess the ability to respond to remote cortical infarcts.     

Effect of neural precursor proliferation level on neurogenesis in rat brain during aging and after focal ischemia

The observed age-related decline in neurogenesis may result from reduced proliferation or increased death rate of neuronal precursor cells (NPCs). We found that caspase-3, but not caspase-6, -7, or -9, was activated in NPCs in neurogenic regions of young, young-adult, middle-aged and aged rat brains. The number of capase-3-immunoreactive cells was highest in young and lowest in aged rats. Surprisingly, intraventricular administration of a caspase-3 inhibitor failed to restore the number of BrdU-positive cells in the aged dentate gyrus, suggesting that the age-related decline in neurogenesis may be attributable primarily to reduced proliferation. Additionally, we also found that NPCs in the subventricular zone of young-adult and aged rat brain were increased after focal cerebral ischemia, suggesting that the increase in neurogenesis induced by ischemia may result from an increase in the rate of NPC proliferation, but not from a decrease in NPC death. Thus, our results suggest that age-related and injury-induced changes in the rate of neurogenesis are controlled at the level of NPC proliferation. Furthermore, our results may imply that the mechanisms that maintain a stable population of NPCs in the normal adult and in the ischemic brain, which account for the observed age-dependent reduction or injury-induced increases in neurogenesis, impinge on the regulation of cell division at the NPC level.     

模拟失重后大鼠海马齿状回神经发生的实验研究

观察模拟失重对大鼠海马齿状回神经发生的影响,为进一步阐明模拟失重对成年大鼠海马齿状回神经发生影响的规律及其相关生物学机制提供基本的实验依据。采用尾部悬吊法建立大鼠模拟失重模型,通过5-溴-2’-脱氧尿苷(5-bromo-2’-de-oxyuridine,BrdU)标记分裂细胞、微管相关蛋白(doublecortin,DCX)标记神经干细胞、神经元核蛋白(NeuN)标记神经元及胶质原纤维酸性蛋白(GFAP)标记神经胶质细胞的单、双重免疫组织化学染色方法比较尾部悬吊后7、14、28d模拟失重组大鼠与相应时间对照组大鼠之间海马齿状回神经前体细胞增殖、迁移和分化的情况。结果显示:模拟失重后7、14d尾部悬吊法模拟失重大鼠齿状回的BrdU免疫阳性细胞数目较相应对照组明显减少(P<0.01),而模拟失重后28d时两组大鼠齿状回BrdU免疫阳性细胞数目无显著差异(P>0.05)。本研究结果提示,模拟失重可抑制海马齿状回神经发生的水平。     

Antisense Noggin oligodeoxynucleotide administration decreases cell proliferation in the dentate gyrus of adult rats

The dentate gyrus of the hippocampus is one of few regions in the adult mammalian brain characterized by ongoing neurogenesis. It has been demonstrated that Noggin antagonizes bone morphogenetic protein-4 (BMP4) to create a niche for subventricular zone neurogenesis. We previously demonstrated that Noggin and BMP4 showed strong expression in the proliferative subgranular layer of the dentate gyrus in adult rats. To examine the action of Noggin on cell proliferation in the dentate gyrus of adult rats, we administered antisense oligodeoxynucleotide (ASODN) to Noggin by continuous infusion into the lateral ventricle of rats. Antisense-infused rats displayed significant reduction in number of bromodeoxyuridine (BrdU) labeled cells in the dentate gyrus. This indicated that endogenous Noggin activity is important for naturally occurring cell proliferation in the dentate gyrus, and perhaps neurogenesis, and is one of the many factors involved in its regulation.     

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

Neuronal nitric oxide synthase, the major nitric oxide synthase isoform in the mammalian brain, is implicated in some developmental processes, including neuronal survival, precursor proliferation and differentiation. However, reports about the role of neuronal nitric oxide synthase in neurogenesis in the adult dentate gyrus are conflicting. Here we show that 5-bromodeoxyuridine-labeled dividing progenitor cells in the dentate gyrus were significantly increased in mice receiving 7-nitroindazole, a selective neuronal nitric oxide synthase inhibitor, and in null mutant mice lacking neuronal nitric oxide synthase gene (nNOS-/-) 6 h and 4 weeks after 5-bromodeoxyuridine incorporation. The increase in 5-bromodeoxyuridine positive cells in 7-nitroindazole-treated mice was accompanied by activation of cyclic AMP response element binding protein phosphorylation in the dentate gyrus. Pretreatment with N-methyl-D-aspartate receptor antagonist MK-801 fully abolished the effects of 7-nitroindazole on neurogenesis and cyclic AMP response element binding protein phosphorylation. Furthermore, neuronal nitric oxide synthase inhibition significantly enhanced the survival of newborn cells and the number of 5-bromodeoxyuridine positive/NeuN positive cells in the dentate gyrus. These results indicate that neuronal nitric oxide synthase-derived nitric oxide suppresses neurogenesis in the adult dentate gyrus, in which N-methyl-D-aspartate receptor functions and cyclic AMP response element binding protein phosphorylation may be involved.     

Influence of mitochondrial enzyme deficiency on adult neurogenesis in mouse models of neurodegenerative diseases

Mitochondrial defects including reduction of a key mitochondrial tricarboxylic acid cycle enzyme alpha-ketoglutarate-dehydrogenase complex (KGDHC) are characteristic of many neurodegenerative diseases. KGDHC consists of alpha-ketoglutarate dehydrogenase, dihydrolipoyl succinyltransferase (E2k), and dihydrolipoamide dehydrogenase (Dld) subunits. We investigated whether Dld or E2k deficiency influences adult brain neurogenesis using immunohistochemistry for the immature neuron markers, doublecortin (Dcx) and polysialic acid-neural cell adhesion molecule, as well as a marker for proliferation, proliferating cell nuclear antigen (PCNA). Both Dld- and E2k-deficient mice showed reduced Dcx-positive neuroblasts in the subgranular zone (SGZ) of the hippocampal dentate gyrus compared with wild-type mice. In the E2k knockout mice, increased immunoreactivity for the lipid peroxidation marker, malondialdehyde occurred in the SGZ. These alterations did not occur in the subventricular zone (SVZ). PCNA staining revealed decreased proliferation in the SGZ of E2k-deficient mice. In a transgenic mouse model of Alzheimer's disease, Dcx-positive cells in the SGZ were also reduced compared with wild type, but Dld deficiency did not exacerbate the reduction. In the malonate lesion model of Huntington's disease, Dld deficiency did not alter the lesion-induced increase and migration of Dcx-positive cells from the SVZ into the ipsilateral striatum. Thus, the KGDHC subunit deficiencies associated with elevated lipid peroxidation selectively reduced the number of neuroblasts and proliferating cells in the hippocampal neurogenic zone. However, these mitochondrial defects neither exacerbated certain pathological conditions, such as amyloid precursor protein (APP) mutation-induced reduction of SGZ neuroblasts, nor inhibited malonate-induced migration of SVZ neuroblasts. Our findings support the view that mitochondrial dysfunction can influence the number of neural progenitor cells in the hippocampus of adult mice.     

Restoring production of hippocampal neurons in old age.

The production of hippocampal granule neurons continues throughout adulthood but dramatically decreases in old age. Here we show that reducing corticosteroid levels in aged rats restored the rate of cell proliferation, resulting in increased numbers of new granule neurons. This result indicates that the neuronal precursor population in the dentate gyrus remains stable into old age, but that neurogenesis is normally slowed by high levels of corticosteroids. The findings further suggest that decreased neurogenesis may contribute to age-related memory deficits associated with high corticosteroids, and that these deficits may be reversible.     

营养不良可增加幼鼠齿状回颗粒下层的细胞增殖和神经生发(英文)

为了观察营养不良对幼鼠海马齿状回 (DG)和脑室下层 (SVZ)的细胞增殖和神经发生的影响 ,采用 5 -溴 -2 -脱氧尿苷(Brd U)标记结合免疫组织化学方法对脑切片分别进行 Brd U、Tu J1(β tubulin,β微管蛋白 )及 GFAP(胶质纤维酸性蛋白 )反应或双重反应。结果表明 ,营养不良幼鼠齿状回的细胞增殖和神经生发明显高于营养良好的幼鼠而脑室下层的细胞增殖数量在两者却无明显差异。在齿状回 ,新生的细胞中大约有 5 0 %为新生的神经元 ,10～ 2 0 %为神经胶质细胞。本文结果提示 ,幼鼠海马齿状回的细胞增殖和神经生发可能因营养不良而增加 ,这些新生的细胞可能对日后某些海马依赖性行为产生一定的影响     

Hippocampal neuron and synaptophysin-positive bouton number in aging C57BL/6 mice

A loss of hippocampal neurons and synapses had been considered a hallmark of normal aging and, furthermore, to be a substrate of age-related learning and memory deficits. Recent stereological studies in humans have shown that only a relatively minor neuron loss occurs with aging and that this loss is restricted to specific brain regions, including hippocampal subregions. Here, we investigate these age-related changes in C57BL/6J mice, one of the most commonly used laboratory mouse strains. Twenty-five mice (groups at 2, 14, and 28–31 months of age) were assessed for Morris water-maze performance, and modern stereological techniques were used to estimate total neuron and synaptophysin-positive bouton number in hippocampal subregions at the light microscopic level. Results revealed that performance in the water maze was largely maintained with aging. No age-related decline was observed in number of dentate gyrus granule cells or CA1 pyramidal cells. In addition, no age-related change in number of synaptophysin-positive boutons was observed in the molecular layer of the dentate gyrus or CA1 region of hippocampus. We observed a significant correlation between dentate gyrus synaptophysin-positive bouton number and water-maze performance. These results demonstrate that C57BL/6J mice do not exhibit major age-related deficits in spatial learning or hippocampal structure, providing a baseline for further study of mouse brain aging.     

Cell proliferation and total granule cell number in dentate gyrus of transgenic Tg2576 mouse

This report provides in vivo evidence of adult neurogenesis and the total granule cell count in the dentate gyrus of the Tg2576 mouse model of Alzheimer's disease. Mice were deeply anaesthetized and perfused with 4 percent buffered paraformaldehyde. Brains were removed and post-fixed in the same fixative overnight. Following equilibration in 30 percent sucrose, 30 micrometer sections were cut in sagittal plane in freezing microtome for immunohistochemistry and 20 micrometer from plastic embedded brains. Thioflavin-S confirmed the presence of amyloid plaques in the Tg2576 mice. Cell proliferation in the subventricular zone and dentate gyrus of hippocampus were observed with Ki-67 and doublecortin markers. Using optical fractionator, total granule number was estimated to be 445,280 per hemisphere in the 18-month-old Tg2576 mouse. Cell proliferation tends to end in the dentate gyrus but continues in the SVZ and the total granule cell number was less compared to normal laboratory and wild rodents.     

Newborn granule cells in the ageing dentate gyrus

The dentate gyrus of the hippocampus generates neurons throughout life, but adult neurogenesis exhibits a marked age-dependent decline. Although the decrease in the rate of neurogenesis has been extensively documented in the ageing hippocampus, the specific characteristics of dentate granule cells born in such a continuously changing environment have received little attention. We have used retroviral labelling of neural progenitor cells of the adult mouse dentate gyrus to study morphological properties of neurons born at different ages. Dendritic spine density was measured to estimate glutamatergic afferent connectivity. Fully mature neurons born at the age of 2 months display ∼2.3 spines μm⁻¹ and maintain their overall morphology and spine density in 1-year-old mice. Surprisingly, granule cells born in 10-month-old mice, at which time the rate of neurogenesis has decreased by ∼40-fold, reach a density of dendritic spines similar to that of neurons born in young adulthood. Therefore, in spite of the sharp decline in cell proliferation, differentiation and overall neuronal number, the ageing hippocampus presents a suitable environment for new surviving neurons to reach a high level of complexity, comparable to that of all other dentate granule cells.     

Neural precursor proliferation and newborn cell survival in the adult rat dentate gyrus are affected by vitamin E deficiency

The adult hippocampal neurogenesis is affected by vitamin E deficiency. In the present investigation we examined if neural precursor proliferation, newborn cell survival or both are altered by vitamin E deficiency. 5-Bromo-2'-deoxyuridine (BrdU) was employed as a marker of proliferating cells. BrdU-labelled cells were revealed 1 and 30 days after BrdU administration in order to evaluate proliferation and newborn cell survival, respectively. Cell proliferation decreased in controls from juvenile to adult age, and the decrease was lesser in vitamin E deficiency. Thus we found a higher number of proliferating cells in vitamin E-deficient rats than in age-matched controls at 5 months of age. Comparing the number of BrdU-positive cells between 1 and 30 days after the last BrdU injection revealed a remarkable decrease in all groups; this is the greatest in vitamin E-deficient rats and the lowest in control rats. Consistently cell death in the dentate gyrus, assessed by TUNEL technique, was found to decrease from 1 to 5 months of age, but at 5 months it was significantly higher in vitamin E-deficient rats than in age-matched controls. These data show that vitamin E deficiency enhances neural precursor proliferation and cell death during adult neurogenesis.     

Exercise-induced changes in dendritic structure and complexity in the adult hippocampal dentate gyrus

Neurogenesis is a constitutive activity in the adult dentate gyrus whereby new cells are created in the subgranular zone, before becoming neurons in the dentate gyrus granule cell layer. New granule cells are thought to migrate from the subgranular zone outwards to the edge of the cell layer as they mature. In these experiments we examined the dendritic morphology of granule cells in the subgranular zone, and the inner and outer regions of the granule cell zone in Sprague-Dawley rats with low and high rates of neurogenesis. In animals with lower rates of neurogenesis, the number of primary dendrites, degree of dendritic complexity and total dendritic length was lowest in cells located in the subgranular zone, higher in inner granule cell zone neurons, and highest in outer granule cell zone granule cells. Subgranular zone granule cells typically extended one primary dendrite and had a simple, immature dendritic tree, while granule cells in the outer granule cell zone had an increased number of primary dendrites, greater dendritic complexity, and greater total dendritic length. Animals that engaged in voluntary exercise showed increased neurogenesis, and the proportion of cells with one or two primary dendrites was increased in all of the granule cell zones. Despite having fewer primary processes, these cells showed enhanced dendritic complexity and an overall increase in their total dendritic length. These results indicate that granule cell dendritic morphology may be indicative of the age and position of a cell in the granule cell layer, but that in animals with increased rates of neurogenesis, the proportion of cells exhibiting what is considered an immature phenotype is increased throughout the all regions of the dentate gyrus cell layer.     

Melatonin supplementation delays the decline of adult hippocampal neurogenesis during normal aging of mice

Melatonin modulates adult hippocampal neurogenesis in adult mice. Also, plasma melatonin levels and new neuron formation decline during aging probably causing cognitive alterations. In this study, we analyzed the impact of exogenous supplementation with melatonin in three key events of hippocampal neurogenesis during normal aging of mice. The analysis was performed in rodents treated with melatonin during 3, 6, 9 or 12 months. We found an increase in cell proliferation in the dentate gyrus of the hippocampus after 3, 6 and 9 months of treatment (>90%). Additionally, exogenous melatonin promoted survival of new cells in the dentate gyrus (>50%). Moreover, melatonin increased the number of doublecortin-labeled cells after 6 and 9 months of treatment (>150%). In contrast, melatonin administered during 12 months did not induce changes in hippocampal neurogenesis. Our results indicate that melatonin also modulates the neurogenic process in the hippocampus during normal aging of mice. Together, the data support melatonin as one of the positive endogenous regulators of neurogenesis during aging.     

Involvement of cyclooxygenase-2 in lipopolysaccharide-induced impairment of the newborn cell survival in the adult mouse dentate gyrus

There is growing evidence indicating that neurogenesis in adulthood is influenced by certain types of the central diseases such as neuroinflammation, however, its mechanism is not fully understood. This study was, therefore, designed to examine the effects of lipopolysaccharide (LPS), a bacterial endotoxin known to cause the neuroinflammation, on the neurogenesis in the dentate gyrus of adult mice using the bromodeoxyuridine (BrdU) -pulse chase method. LPS failed to affect the number of BrdU-labeled cells in the dentate gyrus 2 h after BrdU injection, indicating no effects of LPS on the proliferation of the neural stem cells (NSCs). On the other hand, we found that LPS dose-dependently (0.1, 0.5, 1 mg/kg) decreased the number of BrdU-labeled cells 7 and 21 days after BrdU injection. We also observed that LPS increased cell death in the dentate gyrus using terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining, suggesting that LPS impaired the survival of newborn cells derived from the NSCs. The double-immunostaining for BrdU and specific cell type markers revealed that LPS did not alter the commitment of the NSCs to the neurons and astrocytes. The systemic injection of indomethacin, a non-selective cyclooxygenase (COX) inhibitor, and NS398, a selective COX-2 inhibitor, but not SC560, a selective COX-1 inhibitor, did not only ameliorate LPS-induced suppression of the newborn cell survival, they fully protected against the LPS effect. Furthermore, the central injection of NS398 also ameliorated LPS-induced suppression of the newborn cell survival in the dentate gyrus. The treatment with LPS increased the expression of COX-2 protein 7 h and 7 days after the injection in the dentate gyrus. These results suggest that LPS impairs the survival of newly generated cells derived from the NSCs in the dentate gyrus without affecting the differentiation fate, and these effects of LPS were mediated presumably by COX-2 expression in the dentate gyrus.     

Estrogen receptor alpha and beta mRNA expressions by proliferating and differentiating cells in the adult rat dentate gyrus and subventricular zone

Numerous factors modulate neurogenesis in the adult dentate gyrus and subventricular zone, but it is often not clear if the modulation is mediated by direct effects on the proliferating and differentiating cells or secondary to effects on other cells. Also, while some factors selectively affect neurogenesis in one of the neurogenetic zones, it is not clear how selectivity is achieved. Estrogen is a hormonal modulator of neurogenesis. To address the issues of direct versus indirect control and regional specificity we investigated the colocalization of immunoreactivity for a proliferating cell marker, Ki-67, and a marker for migrating and differentiating cells with a neuronal phenotype, doublecortin, with the expressions of mRNA for estrogen receptors alpha and beta. We found an extensive colocalization of estrogen receptor alpha with both markers in the dentate gyrus and only with Ki-67 in the subventricular zone. An extensive colocalization of estrogen receptor beta with both markers was found in the dentate gyrus, but only a few Ki-67-immunoreactive and no doublecortin-immunoreactive cells of the subventricular zone expressed estrogen receptor beta mRNA. Estrogen receptor alpha and beta mRNAs were not expressed in other telencephalic Ki-67-immunoreactive cells or in constitutively doublecortin-immunoreactive cells of the piriform cortex. The extensive colocalization of immunoreactive markers for cell proliferation and differentiation with mRNAs for estrogen receptor alpha and estrogen receptor beta points to the direct modulation of dentate cell proliferation, differentiation and survival by estrogen, while direct effects of estrogen in the subventricular zone appear restricted to estrogen receptor alpha-mediated effects operating at the time of cell proliferation.     

Short term treatment with estradiol decreases the rate of newly generated cells in the subventricular zone and main olfactory bulb of adult female mice

Adult neurogenesis occurs most notably in the subgranular zone (SGZ) of the hippocampal dentate gyrus and in the olfactory bulb (OB) where new neurons are generated from neural progenitors cells produced in the subventricular zone (SVZ) of the forebrain. As it is well known that gonadal steroid hormones, primarily estradiol, modulate neurogenesis in the hippocampus of adult female rodents, we wanted to determine whether estradiol would also affect the proliferation of progenitor cells in the SVZ and by consequence the rate of newly generated cells in the main OB. Thus a first group of adult female C57Bl6/J mice was ovariectomized and received a short term treatment with estradiol (single injection of 1 or 10 μg 17β-estradiol or Silastic capsule of estradiol during 2 days) before receiving a single injection with BrdU to determine whether estradiol would modulate the cell proliferation in the SVZ. A second group of adult ovariectomized female mice was submitted to the same estradiol treatment before receiving four BrdU injections, and was sacrificed 21 days later to determine whether a modulation in cell proliferation actually leads to a modulation in the number of newborn cells in the main OB. We observed a decrease in cell proliferation in the SVZ following either dose of estradiol compared to the controls. Furthermore, 21 days after their generation in the SVZ, the number of BrdU labeled cells was also lower in the main OB, both in the granular and periglomerular cell layers of estradiol-treated animals. These results show that a short term treatment with estradiol actually downregulates cell proliferation leading to a decreased number of newborn cells in the OB.     

I.c.v. administration of orexin-A induces an antidepressive-like effect through hippocampal cell proliferation

A decrease in orexin-A (OX-A) levels has been reported to be associated with depression. It is also well known that stress and depression can disrupt neurogenesis in the dentate gyrus of the hippocampus; however, it is unclear how OX-A is involved in depression and/or neurogenesis. In the present study, we investigated the effect of i.c.v. administration of OX-A on the forced swimming test (FST), an accepted behavioral screen of antidepressant-like activity, and on the cell proliferation with bromodeoxyuridine (BrdU) in the dentate gyrus at 4 days after i.c.v. administration of OX-A. OX-A administration (140 pmol/mouse) led to a significant reduction in animal immobility in the FST, without affecting spontaneous locomotor activities or serum corticosterone levels. In addition, the number of BrdU-positive cells in the dentate gyrus was significantly increased in OX-A-treated mice in vivo; however, OX-A did not affect the percentage of doublecortin-positive cells in the dentate gyrus. The proliferation of neural progenitor cells derived from rat fetal brain was not affected by OX-A treatment in vitro, and the orexin receptor 1 (OXR1) protein was not expressed in these cells. Treatment with the OXR1 antagonist SB-334867 (30 mg/kg, i.p.) blocked both the OX-A-induced decrease in the immobility of FST and increase in BrdU-positive. Moreover, the OX-A-induced increase in neuropeptide Y (NPY)-positive cells in the hilus of the dentate gyrus was blocked by SB-334867. These results suggest that OX-A induces an antidepressive-like effect, at least in part, via the enhancement of cell proliferation in the dentate gyrus. These effects of OX-A also may be partly relevant to the regulation of the NPY system in the hilus of the dentate gyrus.