Neurons born in the adult dentate gyrus form functional synapses with target cells

Adult neurogenesis occurs in the hippocampus and the olfactory bulb of the mammalian CNS. Recent studies have demonstrated that newborn granule cells of the adult hippocampus are postsynaptic targets of excitatory and inhibitory neurons, but evidence of synapse formation by the axons of these cells is still lacking. By combining retroviral expression of green fluorescent protein in adult-born neurons of the mouse dentate gyrus with immuno-electron microscopy, we found output synapses that were formed by labeled terminals on appropriate target cells in the CA3 area and the hilus. Furthermore, retroviral expression of channelrhodopsin-2 allowed us to light-stimulate newborn granule cells and identify postsynaptic target neurons by whole-cell recordings in acute slices. Our structural and functional evidence indicates that axons of adult-born granule cells establish synapses with hilar interneurons, mossy cells and CA3 pyramidal cells and release glutamate as their main neurotransmitter.     

Biphasic effects of the cholinergic agonist carbachol on long-term potentiation in the dentate gyrus of the mammalian hippocampus

The dentate gyrus, an integral part of the hippocampal circuit, is capable of producing new neurons in adulthood, some of which become integrated into neuronal circuits that participate in processes underlying learning and memory. Acetylcholine (Ach) is an important neuromodulator of synaptic activity in the hippocampus but its action on activity-dependent plasticity of mature and young neurons has not been studied. Using standard hippocampal slice preparations and a functional assay for distinguishing young and mature neuronal populations, we found that Ach has a preferential stimulatory effect on long-term synaptic plasticity of mature neurons. This is in contrast to its inhibitory effect on synaptic plasticity of immature, adult-born neurons. This differential effect of Ach may contribute to differences in learning and memory in young and old brains, particularly in tasks that are sensitive to adult neurogenesis.     

吗啡依赖小鼠海马神经元c-fos的表达

目的:探讨吗啡依赖小鼠海马不同亚区神经元c-fos表达的差异。方法:以剂量递增法皮下注射吗啡建立吗啡依赖小鼠模型,腹腔注射纳洛酮诱发戒断症状。根据小鼠戒断反应中出现的跳跃次数、体重下降等指标评定戒断反应强度。采用免疫组织化学法显示吗啡依赖小鼠和正常对照组小鼠海马神经元c-fos的表达。结果:吗啡依赖组海马CA1区和齿状回Fos阳性神经元数目明显增加(P<0.05),CA3区无明显改变(P>0.05)。纳洛酮催促戒断组海马CA1和CA3区Fos阳性神经元数目明显增加(P<0.05),齿状回Fos阳性神经元数目增加更为明显(P<0.01)。吗啡依赖组与纳洛酮催促戒断组CA3区阳性神经元数目有显著性差异(P<0.05)。结论:海马神经元c-fos的表达增强可能与吗啡依赖对神经元的损伤有关。     

Hippocampal heterogeneity in spatial memory revealed by cytochrome oxidase

The oxidative metabolism was assessed in the septal, intermediate and temporal hippocampus in Wistar rats that were trained following a working memory schedule in the Morris water maze. The cytochrome oxidase histochemistry was measured at 90 min, 6, 24 and 48 h post-training. We found an increase in the septal dentate gyrus at 90 min, at 6 h the increase was also found in CA3 and CA1 regions and returned to basal levels at 24 h. In contrast, the intermediate region showed lower increase, limited to the dentate gyrus and CA3 at 24 h post-training. No changes were found in the temporal hippocampus. These findings suggest that septal and intermediate hippocampal zones participate in this spatial learning and contribute at different moments to process this information.     

慢性复合应激增强大鼠海马Doublecortin的表达

目的 探讨慢性复合应激性学习记忆增强大鼠海马齿状回(DG)新生神经元数量变化以及Doublecortin(DCX)在海马组织中表达的变化及其意义.方法 成年雄性大鼠随机分为复合应激组和正常对照组.复合应激组动物每天交替暴露于复合应激原中达6周.实验结束后,所有动物分别进行3d的Morris水迷宫测试,记录其学习和记忆成绩.运用免疫细胞化学方法观察海马DG新生神经元数量的变化,同时运用Western blotting和RT-PCR技术分别检测DCX在海马的表达及其mRNA水平的变化.结果 与对照组相比,复合应激组动物的学习与记忆成绩优于对照组(P＜0.05);其海马DG新生神经元数明显增多(P＜0.05);海马DCX蛋白的表达明显增加(P＜0.05);海马DCX mRNA水平明显上调(P＜0.05).结论 慢性复合应激致大鼠的学习与记忆能力增强,海马DG内DCX阳性细胞数增多,提示新生神经元数量增加是导致大鼠学习记忆能力增强的原因之一.     

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.     

Chronic non-invasive glucocorticoid administration decreases polysialylated neural cell adhesion molecule expression in the adult rat dentate gyrus

The expression of the polysialylated neural cell adhesion molecule (PSA-NCAM) is increased in the hippocampus after chronic restraint stress (CRS) and may play a permissive role in structural changes that include dendrite reorganization in dentate gyrus (DG) and CA3 pyramidal neurons and suppression of neurogenesis in DG. We report that chronic oral corticosterone (CORT) administration decreases the number of PSA-NCAM immunoreactive granule neurons in the adult rat dentate gyrus, and the available evidence suggests that this is an indirect effect of CORT, possibly involving excitatory amino acids, that may not be directly related to neurogenesis. Because CORT treatment reduces but does not eliminate PSA-NCAM expression, the present results do not exclude a permissive role for PSA-NCAM in CORT or CRS-induced structural plasticity in hippocampus.     

听源性惊厥易感大鼠惊厥和点燃后前脑结构内的c—fos表达

为探讨听源性惊厥点燃和前脑结构的关系，用免疫细胞化学方法结合体视学分析，研究Ｗｉｓｔａｒ种系的听源性惊厥易感大鼠惊厥和点燃后，前脑结构内ｃ－ｆｏｓ表达的差异。结果显示，１．正常Ｗｉｓｔａｒ大鼠接受一次强音刺激后，海马，齿状回，杏仁核，内嗅皮质，嗅周皮质和额－顶皮质内未见Ｆｏｘ阳性神经元；２．Ｐ７７ＰＭＣ大鼠一次惊厥后，除海马，齿状回外，上述被检各区内可见广泛的Ｆｏｓ阳性神经元，其分布具有区域差异．     

Roles of continuous neurogenesis in the structural and functional integrity of the adult forebrain

Neurogenesis occurs continuously in the forebrain of adult mammals, but the functional importance of adult neurogenesis is still unclear. Here, using a genetic labeling method in adult mice, we found that continuous neurogenesis results in the replacement of the majority of granule neurons in the olfactory bulb and a substantial addition of granule neurons to the hippocampal dentate gyrus. Genetic ablation of newly formed neurons in adult mice led to a gradual decrease in the number of granule cells in the olfactory bulb, inhibition of increases in the granule cell number in the dentate gyrus and impairment of behaviors in contextual and spatial memory, which are known to depend on hippocampus. These results suggest that continuous neurogenesis is required for the maintenance and reorganization of the whole interneuron system in the olfactory bulb, the modulation and refinement of the existing neuronal circuits in the dentate gyrus and the normal behaviors involved in hippocampal-dependent memory.     

The impact of maternal separation on adult mouse behaviour and on the total neuron number in the mouse hippocampus

The maternal separation paradigm has been applied to C57BL/6J mice as an animal developmental model for understanding structural deficits leading to abnormal behaviour. A maternal separation (MS) model was used on postnatal day (PND) 9, where the pups were removed from their mother for 24 h (MS24). When the pups were 10 weeks old, the level of anxiety and fear was measured with two behavioural tests; an open field test and an elevated plus maze test. The Barnes platform maze was used to test spatial learning, and memory by using acquisition trials followed by reverse trial sessions. The MS24 mice spent more time in the open arms of the elevated plus maze compared to controls, but no other treatment differences were found in the emotional behavioural tests. However, in the reverse trial for the Barnes maze test there was a significant difference in the frequency of visits to the old goal, the number of errors made by the MS24 mice compared to controls and in total distance moved. The mice were subsequently sacrificed and the total number of neurons estimated in the hippocampus using the optical fractionator. We found a significant loss of neurons in the dentate gyrus in MS mice compared to controls. Apparently a single maternal separation can impact the number of neurons in mouse hippocampus either by a decrease of neurogenesis or as an increase in neuron apoptosis. This study is the first to assess the result of maternal separation combining behaviour and stereology.     

Potential role for adult neurogenesis in the encoding of time in new memories

The dentate gyrus in the hippocampus is one of two brain regions with lifelong neurogenesis in mammals. Despite an increasing amount of information about the characteristics of the newborn granule cells, the specific contribution of their robust generation to memory formation by the hippocampus remains unclear. We describe here a possible role that this population of young granule cells may have in the formation of temporal associations in memory. Neurogenesis is a continuous process; the newborn population is only composed of the same cells for a short period of time. As time passes, the young neurons mature or die and others are born, gradually changing the identity of this young population. We discuss the possibility that one cognitive impact of this gradually changing population on hippocampal memory formation is the formation of the temporal clusters of long-term episodic memories seen in some human psychological studies.     

老年学习记忆减退大鼠的海马GABA能神经元数量变化

为探讨γ-氨基丁酸 (GABA)能神经元在老年脑的改变及其与老年性学习记忆减退间的关系。本研究通过 Morris水迷宫测试将老年大鼠分为学习记忆正常和损害组 ,用免疫细胞化学和体现视学方法定量描述 GABA能神经元的改变。结果显示 ,老年记忆正常和损害组海马本部各区的 GABA能神经元均较年轻组明显减少 ,但正常组和损害组海马之间无显著性差异。齿状回分子层的 GABA阳性神经元的数量仅在老年学习记忆损害组明显减少 ,且 GABA神经元数量与搜寻平台的游泳时间呈负相关 (r=- 0 .91,P<0 .0 1)。提示老年学习记忆能力减退可能与齿状回分子层的 GABA能神经元丢失有关     

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.     

Effect of long-lasting serotonin depletion on environmental enrichment-induced neurogenesis in adult rat hippocampus and spatial learning

The dentate gyrus of the hippocampal formation produces new neurons throughout adulthood in mammalian species. Several experimental statuses and factors regulating to neurogenesis have been identified in the adult dentate gyrus. For example, exposure to an enriched environment enhances neurogenesis in the dentate gyrus and improves hippocampus-dependent spatial learning. Furthermore, serotonin is known to influence adult neurogenesis, and learning and memory. However, the effects of long-lasting depletion of serotonin over the developing period on neurogenesis have not been investigated. Thus, we examined the influence of long-lasting serotonin depletion on environmental enrichment-induced neurogenesis and spatial memory performance. As reported previously, environmental enrichment significantly increased new neurons in the dentate gyrus. However, there was no improvement of the spatial learning test in adult rats in standard and in environmental enrichment housings. Intracisternal administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine, on postnatal day 3 apparently reduced serotonin content in the adult hippocampus without regeneration. This experimental depletion of serotonin in the hippocampus of rats housed in an enriched environment had no effect on spatial memory performance, but produced significant decreases in the number of bromodeoxyuridine-labeled new cells in the dentate gyrus. These findings indicate that newly generated cells stimulated by environmental enrichment are not critical for improvements in hippocampus-dependent learning. Furthermore, numbers of bromodeoxyuridine-labeled cells in the dentate gyrus of 5,7-dihydroxytryptamine-injected rats did not differ between 1 day and 4 weeks after bromodeoxyuridine injection. These data suggest that survival of newly generated dentate gyrus cells remains relatively constant under long-lasting serotonin depletion.     

Topographical gradient in expression of R2D5 antigen in superior olivary nuclei and hippocampal dentate gyrus of the cat.

An immunohistochemical analysis of the cat central nervous system revealed that a monoclonal antibody which recognizes a soluble cytosolic protein, R2D5, bound two regions in a prominent spatial gradient. In the medial and lateral superior olivary nuclei of the brainstem, R2D5 immunoreactivity appeared as a gradient across a population of topographically ordered principal neurons. The spatial gradient corresponded to the tonotopic organization in the superior olivary nuclei: i.e., R2D5 immunoreactivity tended to occur more frequently and intensely in low-frequency neurons than in high-frequency neurons. Granule cells in the hippocampal dentate gyrus also had a pronounced spatial gradient in R2D5 immunoreactivity expression, and this gradient corresponded to the septotemporal axis of the hippocampus. Granule cells of the temporal (ventral) portions of the hippocampus were labeled intensely with R2D5 antibody, while those located in progressively more septal (dorsal) portions had gradually less immunoreactivity. These results suggest that in both the superior olivary nuclei and the hippocampal dentate gyrus, neurons differ in intrinsic properties by their position along specific axes. They suggest also that the hippocampus has an intrinsic functional organization related to the spatial gradient along its septotemporal axis.     

Pharmacological evidence of cholinergic involvement in adult hippocampal neurogenesis in rats

In adult hippocampus, neural progenitor cells give rise to neurons throughout life, and the neurogenesis is modulated by various intrinsic and extrinsic factors. Recent reports showed that lesion of septal cholinergic nuclei projecting to hippocampus suppressed the survival of newborn cells in the dentate gyrus (DG) of hippocampus. Here, we studied whether pharmacological treatment to activate or inhibit the cholinergic system could modulate adult hippocampal neurogenesis. 5'-Bromo-2'-deoxyuridine (BrdU) was injected to label dividing cells before the drug treatment. Immunohistochemical analysis was performed in normal rats chronically treated with an acetylcholinesterase inhibitor donepezil or a muscarinic acetylcholine receptor blocker scopolamine for four weeks. Donepezil increased, but scopolamine decreased, the number of BrdU-positive cells in the DG as compared with the control. Neither drug altered the percentage of BrdU-positive cells that were also positive for a neuronal marker neuronal nuclei, nor net population of proliferative cells labeled with proliferating cell nuclear antigen. We also found that donepezil enhanced, and scopolamine suppressed, the expression level of phosphorylated cAMP response element binding protein (CREB), which is related to cell survival, in the DG. These results indicate that donepezil enhances and scopolamine suppresses the survival of newborn cells in the DG via CREB signaling without affecting neural progenitor cell proliferation and the neuronal differentiation. This is the first evidence that pharmacological manipulation of the cholinergic system can modulate adult hippocampal neurogenesis.     

Morris water maze learning in two rat strains increases the expression of the polysialylated form of the neural cell adhesion molecule in the dentate gyrus but has no effect on hippocampal neurogenesis

In the current study, the authors investigated whether Morris water maze learning induces alterations in hippocampal neurogenesis or neural cell adhesion molecule (NCAM) polysialylation in the dentate gyrus. Two frequently used rat strains, Wistar and Sprague-Dawley, were trained in the spatial or the nonspatial version of the water maze. Both training paradigms did not have an effect on survival of newly formed cells that were labeled 7-9 days prior to the training or on progenitor proliferation in the subgranular zone. However, the granule cell layer of the spatially trained rats contained significantly more positive cells of the polysialylated form of the NCAM. These data demonstrate that Morris water maze learning causes plastic change in the dentate gyrus without affecting hippocampal neurogenesis.     

Effects of age and insulin-like growth factor-1 on neuron and synapse numbers in area CA3 of hippocampus.

Age-related effects associated with the hippocampus include declines in numbers of neurons and synapses in the dentate gyrus and area CA1, and decreased cognitive ability as assessed with the Morris water maze. The present study quantified both neuron and synapse number in the same tissue block of area CA3 of the hippocampus. No investigations of both density of neurons and synapses together in area CA3 of hippocampus have been performed previously, despite its importance as the terminal field of dentate gyrus mossy fibers, the second synapse in the trisynaptic circuit in the hippocampus. Numerical density of neurons and synapses were assessed in 4-, 18-, and 29-month-old rats receiving infusions of saline into the lateral ventricle and in 29-month-old rats receiving infusions of insulin-like growth factor-1 (IGF-1). Numerical density of neurons of the stratum pyramidale of CA3 of hippocampus remained constant across the life span as did the numerical density of synapses in stratum lucidum of area CA3. Despite the reported role of IGF-1 in synaptogenesis and improvements in behavior with age, ventricular infusion of this growth factor did not affect the numerical density of neurons or synapses in 29-month-old rats when compared to saline-infused old rats. Further, reported effects of IGF-1 on adult neurogenesis in the dentate gyrus are not reflected in an IGF-1-related increase in synapse density in this region.     

Chronic high corticosterone reduces neurogenesis in the dentate gyrus of adult male and female rats

Adult neurogenesis in the dentate gyrus of the hippocampus is altered with stress exposure and has been implicated in depression. High levels of corticosterone (CORT) suppress neurogenesis in the dentate gyrus of male rats. However both acute and chronic stress do not consistently reduce adult hippocampal neurogenesis in female rats. Therefore, this study was conducted to investigate the effect of different doses of corticosterone on hippocampal neurogenesis in male and female rats. Rats received 21 days of s.c. injections of either oil, 10 or 40 mg/kg CORT. Subjects were perfused 24 h after the last CORT injection and brains were analyzed for cell proliferation (Ki67-labeling) or immature neurons (doublecortin-labeling). Results show that in both males and females high CORT, but not low CORT, reduced both cell proliferation and the density of immature neurons in the dentate gyrus. Furthermore, high CORT males had reduced density in immature neurons in both the ventral and dorsal regions while high CORT females only showed the reduced density of immature neurons in the ventral hippocampus. The high dose of CORT disrupted the estrous cycle of females. Further, the low dose of CORT significantly reduced weight gain and increased basal CORT levels in males but not females, suggesting a greater vulnerability in males with the lower dose of CORT. Thus we find subtle sex differences in the response to chronic CORT on both body weight and on neurogenesis in the dorsal dentate gyrus that may play a role in understanding different vulnerabilities to stress-related neuropsychiatric disorders between the sexes.