Ginseng enhances contextual fear conditioning and neurogenesis in rats

Panax Ginseng is a commonly used galenical known to have an enhancing effect on learning. Neurogenesis in the hippocampus has been shown to be necessary for hippocampus/amygdala-dependent learning tasks. To investigate the role of Ginseng in neurogenesis and learning of rats, we administered both Ginseng and BrdU for five consecutive days. As a result, Ginseng increased the number of BrdU-positive cells in the dentate gyrus in a dose-dependent manner. Further, we administered one dose of BrdU after Ginseng treatment for five consecutive days, and the number of BrdU-positive cells did not increase significantly. However, when one dose of BrdU was given 1 day before the following five consecutive days of Ginseng treatment, the number of BrdU-positive cells markedly increased in the hippocampus. Therefore, it is likely that Ginseng enhances not proliferation but survival of newly generated neurons in the hippocampus. Second, we administered both Ginseng and BrdU to rats for five consecutive days. One day after the last Ginseng and BrdU co-administration, contextual fear conditioning (CFC) was conducted. Ginseng in a dose-dependent manner increased the % freezing time and the number of BrdU-positive cells in the dentate gyrus of rats that received CFC. Thus, an increase in CFC-related neurogenesis may be one mechanism of Ginseng's properties to enhance learning ability.     

Estradiol enhances neurogenesis in the dentate gyri of adult male meadow voles by increasing the survival of young granule neurons

This study investigated whether estradiol influenced the survival of new granule neurons, independent of altering cell proliferation, in the adult rodent dentate gyrus and whether estradiol-induced changes in new neuron number relate to any observed changes in hippocampus-dependent behavior. To test whether estradiol specifically promotes the survival of new neurons we injected castrated adult male meadow voles with the cell synthesis marker bromodeoxyuridine (BrdU; 50 mg/kg) twice on day 0 and then injected either estradiol (10 microg) or vehicle for 5 consecutive days either over days 1-5, days 6-10 or days 11-15 and perfused them on day 16. Estradiol doubled the number of hippocampal BrdU-labeled neurons but only when administered during a discrete period (days 6-10; P< or =0.01) when most new neurons extend their axons [J Comp Neurol 413 (1999) 146]. To test whether the estradiol-induced increase in new neuron number was related to hippocampus-dependent behavior, males were injected with BrdU twice on day 0 and with estradiol or vehicle over days 6-10 before standard Morris water maze training commenced on day 16, 5 days after the final hormone injection. As in the first study, estradiol-treated males had more BrdU-labeled cells than vehicle-treated males. On a probe trial, estradiol-treated males spent significantly more time in the training quadrant than vehicle-treated males despite similar performance between groups during acquisition and reversal training trials. Thus estradiol enhanced the survival of young neurons but only when administered during their 'axon extension' phase and this effect was related to better spatial memory in male voles.     

A decreased survival of proliferated cells in the hippocampus is associated with a decline in spatial memory in aged rats

In aged rats, although learning and memory impairment is prominent, both the number of granular cells and the degree of neuronal progenitor proliferation in the hippocampus are known to be preserved. We examined the association between the survival of newly generated neurons in the hippocampus and the learning ability in aged rats. By using BrdU, a cell proliferation marker to determine neurogenesis and contextual fear conditioning to determine learning ability, we found that in aged rats, along with memory impairment, the survival of both the proliferated cells at baseline and those enhanced by contextual fear conditioning decreased remarkably. These results suggest that the integration of newly generated neurons into hippocampal circuitry is decreased with aging, this phenomenon may, in part, explain the decline in learning and memory in aged rats.     

Forebrain acetylcholine regulates adult hippocampal neurogenesis and learning

Hippocampus-mediated learning enhances neurogenesis in the adult dentate gyrus (DG), and this process has been suggested to be involved in memory formation. The hippocampus receives abundant cholinergic innervation and acetylcholine (ACh) plays an important role in learning and Alzheimer's disease (AD) pathophysiology. Here, we show that a selective neurotoxic lesion of forebrain cholinergic input with 192 IgG-saporin reduces DG neurogenesis with a concurrent impairment in spatial memory. Conversely, systemic administration of the cholinergic agonist physostigmine increases DG neurogenesis. We find that changes of forebrain ACh levels primarily influence the proliferation and/or the short-term survival rather than the long-term survival or differentiation of the new neurons. We further demonstrate that these newly born cells express the muscarinic receptor subtypes M1 and M4. Our data provide evidence that forebrain ACh promotes neurogenesis, and suggest that the impaired cholinergic function in AD may in part contribute to deficits in learning and memory through reductions in the formation of new hippocampal neurons.     

A role for adult neurogenesis in spatial long-term memory

Adult hippocampal neurogenesis has been linked to learning but details of the relationship between neuronal production and memory formation remain unknown. Using low dose irradiation to inhibit adult hippocampal neurogenesis we show that new neurons aged 4–28 days old at the time of training are required for long-term memory in a spatial version of the water maze. This effect of irradiation was specific since long-term memory for a visibly cued platform remained intact. Furthermore, irradiation just before or after water maze training had no effect on learning or long-term memory. Relationships between learning and new neuron survival, as well as proliferation, were investigated but found non-significant. These results suggest a new role for adult neurogenesis in the formation and/or consolidation of long-term, hippocampus-dependent, spatial memories.     

Entorhinal cortex lesioning promotes neurogenesis in the hippocampus of adult mice

Hippocampal neurogenesis in adult mammals is influenced by many factors. Lesioning of the entorhinal cortex is a standard model used to study injury and repair in the hippocampus. Here we use bromodeoxyuridine (BrdU) labeling combined with immunohistochemical identification using cell type specific markers to follow the fate of neural progenitors in the hippocampus following entorhinal cortex lesioning in mice. We show that unilateral entorhinal cortex lesioning does not alter the rate of neural progenitor proliferation in the ipsilateral dentate gyrus during the first 3 days after lesioning. However it enhances cell survival at 42 days post-lesioning leading to an increased number of beta-III tubulin and calbindin-immunoreactive neurons being produced. By contrast, when BrdU was administered 21 days post-lesioning, the number of surviving cells 21 days later was similar on the lesioned and non-lesioned sides. Thus, acutely entorhinal cortex lesioning promotes neurogenesis by enhancing survival of either neural progenitors or their progeny. However, this stimulus to neurogenesis is not sustained into the recovery period.     

Young hippocampal neurons are critical for recent and remote spatial memory in adult mice

New granule cells are continuously generated throughout adulthood in the mammalian hippocampus. These newly generated neurons become functionally integrated into existing hippocampal neuronal networks, such as those that support retrieval of remote spatial memory. Here, we sought to examine whether the contribution of newly born neurons depends on the type of learning and memory task in mice. To do so, we reduced neurogenesis with a cytostatic agent and examined whether depletion of young hippocampal neurons affects learning and/or memory in two hippocampal-dependent tasks (spatial navigation in the Morris water maze and object location test) and two hippocampal-independent tasks (cued navigation in the Morris water maze and novel object recognition). Double immunohistofluorescent labeling of the birth dating marker 5-bromo-2'deoxyuridine (BrdU) together with NeuN, a neuron specific marker, was employed to quantify reduction of hippocampal neurogenesis. We found that depletion of young adult-generated neurons alters recent and remote memory in spatial tasks but spares non-spatial tasks. Our findings provide additional evidence that generation of new cells in the adult brain is crucial for hippocampal-dependent cognitive functions.     

Intact neurogenesis is required for benefits of exercise on spatial memory but not motor performance or contextual fear conditioning in C57BL/6J mice

The mammalian hippocampus continues to generate new neurons throughout life. Experiences such as exercise, anti-depressants, and stress regulate levels of neurogenesis. Exercise increases adult hippocampal neurogenesis and enhances behavioral performance on rotarod, contextual fear and water maze in rodents. To directly test whether intact neurogenesis is required for gains in behavioral performance from exercise in C57BL/6J mice, neurogenesis was reduced using focal gamma irradiation (3 sessions of 5 Gy). Two months after treatment, mice (total n=42 males and 42 females) (Irradiated or Sham), were placed with or without running wheels (Runner or Sedentary) for 54 days. The first 10 days mice received daily injections of bromodeoxyuridine (BrdU) to label dividing cells. The last 14 days mice were tested on water maze (two trials per day for 5 days, then 1 h later probe test), rotarod (four trials per day for 3 days), and contextual fear conditioning (2 days), then measured for neurogenesis using immunohistochemical detection of BrdU and neuronal nuclear protein (NeuN) mature neuronal marker. Consistent with previous studies, in Sham animals, running increased neurogenesis fourfold and gains in performance were observed for the water maze (spatial learning and memory), rotarod (motor performance), and contextual fear (conditioning). These positive results provided the reference to determine whether gains in performance were blocked by irradiation. Irradiation reduced neurogenesis by 50% in both groups, Runner and Sedentary. Irradiation did not affect running or baseline performance on any task. Minimal changes in microglia associated with inflammation (using immunohistochemical detection of cd68) were detected at the time of behavioral testing. Irradiation did not reduce gains in performance on rotarod or contextual fear, however it eliminated gain in performance on the water maze. Results support the hypothesis that intact exercise-induced hippocampal neurogenesis is required for improved spatial memory, but not motor performance or contextual fear in C57BL/6J mice.     

Adult neurogenesis of epidermal neural crest stem cells (EPI-NCSC) in hippocampus of Alzheimer's rat model

Alzheimer’s disease (AD) is characterised by progressive neuronal loss in the hippocampus. Our aim was to evaluate the effects of transplanting epidermal neural crest stem cells (EPI-NCSC) into the hippocampus in vivo and to assess adult neurogenesis and total granule cell number in the hippocampus of an Alzheimer’s rat model after a single injection of EPI-NCSCs. Fourteen days after a bilateral injection of β-amyloid 1–40 into the hippocampus, 10 AD model rats received an intra-hippocampal injection of EPI-NCSCs; the cells were obtained from the vibrissa hair follicle of the rat, cultured, labelled with bromodeoxyuridine (BrdU) and suspended in normal saline. Y-Maze and single trial passive avoidance tests were used to show any learning and memory deficit. Nestin staining was performed in vitro. Double staining of BrdU–GFAP and BrdU–β??? was undertaken to study survival and differentiation of the grafted cells. Cell proliferation and differentiation were observed in all part of hippocampus in the double-stained histological sections. Total granular cell number was estimated to be more per hippocampus in the rats receiving the transplanted cells compared to the AD control group. We observed that rats with hippocampal damage made significantly more errors than control rats on the Y-maze. We showed that transplanted EPI-NCSCs survived and differentiated into neurons and glial cells. Total granule cell number in the treatment group was equal to the control group. Cell proliferation and migration tends to end in the dentate gyrus and the other part of hippocampus. Transplantation of EPI-NCSCs into the hippocampus might differentiate into neurons or induce neurogenesis.     

Working memory training decreases hippocampal neurogenesis

The relationship between adult hippocampal neurogenesis and cognition appears more complex than suggested by early reports. We aimed to determine if the duration and task demands of spatial memory training differentially affect hippocampal neurogenesis. Adult male rats were trained in the Morris water maze in a reference memory task for 4 days, or alternatively working memory for either 4 or 14 days. Four days of maze training did not impact neurogenesis regardless of whether reference or working memory paradigms were used. Interestingly, 2 weeks of working memory training using a hidden platform resulted in fewer newborn hippocampal neurons compared with controls that received either cue training or no maze exposure. Stress is a well-established negative regulator of hippocampal neurogenesis. We found that maze training in general, and a working memory task in particular, increased levels of circulating corticosterone after 4 days of training. Our study indicates that working memory training over a prolonged period of time reduces neurogenesis, and this reduction may partially be mediated by increased stress.     

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.     

Reproductive experience alters hippocampal neurogenesis during the postpartum period in the dam

Pregnancy and the postpartum period are a time of maximal neural and behavioral plasticity. Recent work has shown that hippocampus-dependent learning and memory performance and hippocampus morphology are affected by motherhood and reproductive experience (number of times pregnant and given birth). Adult neurogenesis in the dentate gyrus of the hippocampus is influenced by steroid hormones such as estradiol and corticosterone, which fluctuate during pregnancy and the postpartum period. Thus, it is possible that hippocampal neurogenesis may be affected by motherhood and reproductive experience. The present study aimed to investigate the role of reproductive experience on hippocampal neurogenesis via cell proliferation and cell survival and to determine whether differences were due to the effect of pregnancy and/or pup-exposure alone. Four groups of female Sprague-Dawley rats were used; multiparous, primiparous, nulliparous, and nulliparous rats exposed to pups. All rats were injected with 5-bromo-2-deoxyuridine (BrdU) (200 mg/kg) approximately 24 h after birth/pup-exposure with age-matched controls. Rats were perfused either 24 h (Expt. 1: Cell proliferation) or 21 days (Expt. 2: Cell survival) after BrdU injection. Results show there is a significant decrease in cell proliferation in the dentate gyrus of primiparous and multiparous rats during the early postpartum period, and a decrease in cell survival in the dentate gyrus during the postpartum in primiparous rats, regardless of pup-exposure, compared with all other groups. In addition, brief pup exposure to nulliparous rats significantly increased cell proliferation and cell death in the dentate gyrus, while 22 days of pup exposure to nulliparous rats (sensitized rats) resulted in increased cell survival and cell death in the dentate gyrus. Collectively these results indicate that reproductive experience significantly affects hippocampal neurogenesis and that these effects are not due to the effect of pregnancy or pup-exposure alone.     

Neurogenesis in the dentate gyrus of the rat hippocampus enhanced by tickling stimulation with positive emotion

Hippocampal neurogenesis is influenced by many factors. In this study, we examined the effect of tactile stimulation (tickling), which induced positive emotion, on neurogenesis in the dentate gyrus (DG) of the hippocampus. Four week-old rats were tickled for 5 min/day on 5 consecutive days and received 5-bromo-2′-deoxyuridine (BrdU) administration for 4 days from the second tickling day. Then they were allowed to survive for 18 h or 3 weeks after the end of BrdU treatment. Neurogenesis in the DG was compared between the tickled and untickled rats by using immunohistochemistry with anti-BrdU antibody. The result showed that the number of BrdU- and NeuN (neural cell marker)-double positive neurons on 18 h as well as 3 weeks of the survival periods was significantly increased in the tickled group as compared with the untickled group. The expression of mRNA of brain-derived neurotrophic factor (BDNF) in the hippocampus of the tickled rats was not altered when compared with the control rats. In conclusion, tickling stimulation which induces positive emotion may affect the generation and survival of new neurons of the DG through the BDNF-independent pathway.     

Effects of a constant light environment on hippocampal neurogenesis and memory in mice

Because the environmental light–dark cycle is a key factor involved in modulating circadian rhythm in mammals, disruption of cyclic light conditions has a variety of effects on physiology and behavior. In the hippocampus, neurogenesis, which continues to occur throughout life, has been reported to exhibit circadian variation under cyclic light–dark conditions. In the present study, we examined whether a constant light environment affected hippocampal neurogenesis in mice. Half of the animals were exposed to continuous light conditions (L/L group), while the other half remained under normal cyclic light–dark conditions (L/D group). In the L/L group, the number of BrdU-labeled cells (proliferating cells) and that of BrdU and class III β-tubulin double-labeled cells (newborn neurons) in the granule cell layer were significantly decreased compared with the L/D group. Because hippocampal neurogenesis is involved in memory and learning, we also investigated the effects on performance in water maze tasks to assess spatial learning. Exposure to L/L treatment for 3 weeks impaired spatial learning task performance, although there was no difference in the open field behaviors between the groups. These findings demonstrate that the constant light conditions impaired hippocampal neurogenesis as well as cognitive performance, and suggest an important role for the cyclic light–dark environment in appropriate maintenance of the hippocampal system.     

Repeated estradiol administration alters different aspects of neurogenesis and cell death in the hippocampus of female, but not male, rats

Estradiol has been shown to have neuroprotective effects, and acute estradiol treatment enhances hippocampal neurogenesis in the female brain. However, little is known about the effects of repeated administration of estradiol on the female brain, or about the effects of estradiol on the male brain. Gonadectomized male and female adult rats were injected with 5-bromo-2-deoxyuridine (BrdU) (200 mg/kg), and then 24 h later were given subcutaneous injections of either estradiol benzoate (33 mug/kg) or vehicle daily for 15 days. On day 16, animals were perfused and the brains processed to examine cells expressing Ki-67 (cell proliferation), BrdU (cell survival), doublecortin (young neuron production), pyknotic morphology (cell death), activated caspase-3 (apoptosis), and Fluoro-Jade B (degenerating neurons) in the dentate gyrus. In female rats, repeated administration of estradiol decreased the survival of new neurons (independent of any effects on initial cell proliferation), slightly increased cell proliferation, and decreased overall cell death in the dentate gyrus. In male rats, repeated administration of estradiol had no significant effect on neurogenesis or cell death. We therefore demonstrate a clear sex difference in the response to estradiol of hippocampal neurogenesis and apoptosis in adult rats, with adult females being more responsive to the effects of estradiol than males.     

Decreased cytogenesis in the granule cell layer of the hippocampus and impaired place learning after irradiation of the young mouse brain evaluated using the IntelliCage platform

Radiation therapy is used to treat malignant tumors in the brain and central nervous system involvement of leukemia and lymphomas in children. However, ionizing radiation causes a number of adverse long-term side effects in the brain, including cognitive impairment. Hippocampal neurogenesis is important for place learning and has been shown to be decreased by irradiation (IR) in rats and mice. In the present study, 10-day-old male mice received 6-Gy IR to the brain on postnatal day 10. We used BrdU labeling of the granule cell layer (GCL) of the hippocampus to evaluate cell proliferation and survival. An unbiased, automated platform for monitoring of behavior in a group housing environment (IntelliCage) was used to evaluate place learning 2 months after IR. We show that cranial IR impaired place learning and reduced BrdU labeling by 50% in the GCL. Cranial IR also reduced whole body weight gain 5%. We conclude that this experimental paradigm provides a novel and time-saving model to detect differences in place learning in mice subjected to IR. This method of detecting behavioral differences can be used for further studies of adverse effects of IR on hippocampal neurogenesis and possible new strategies to ameliorate the negative effects of IR on cognition.     

Rats with hippocampal lesions can learn a place response, but how long can they retain it?

Previous research has shown that electrolytic hippocampal lesions do not affect the acquisition of a place response if a special training procedure is used. However, 24 days later, the hippocampal rats manifest a profound deficit in the retention of the spatial information (J. M. J. Ramos, 2000). The goal of the present study was, therefore, to investigate how long the hippocampal rats can retain a place response. Results showed that, 3 days after the end of the training, lesioned rats remembered as well as the control rats, but this was no longer true 6 or 12 days after the training. This retention deficit was not observed when the spatial information was acquired by means of a guidance strategy. These results suggest that, when a special training procedure is used, the hippocampus is not necessary for the learning of a place task but is required for the formation of long-term spatial memory.     

Wnt/β-catenin信号参与慢性痛引起的小鼠海马神经元发生减少

目的:研究小鼠慢性痛模型中海马神经元发生减弱的现象,并从Wnt/β-catenin信号的角度探索相关机制,以及过表达β-catenin对慢性痛小鼠学习记忆能力的影响。方法:采用选择性坐骨神经损伤(spared nerve injury,SNI)模型,利用Brd U/DCX免疫荧光双标研究神经元发生;采用Wnt信号报告基因Topgal小鼠研究Wnt信号的改变;利用条件性过表达β-catenin的Nestin-Cre ER:β-catenin EX3~(loxp/+)小鼠研究过度激活Wnt信号对慢性痛小鼠海马神经元发生及学习记忆的影响;利用Morris水迷宫评估小鼠的空间学习记忆能力。结果:SNI后小鼠的机械和热痛阈显著下降,持续至少3周。Brd U/DCX免疫荧光双标显示SNI小鼠海马神经元发生明显减弱。Wnt信号报告基因β-gal在海马神经干细胞的表达降低。在SNI小鼠的成年神经干细胞中过表达β-catenin可显著促进海马神经元发生,并增强小鼠的空间记忆能力。结论:Wnt/β-catenin信号参与介导慢性痛引起的海马神经元发生减弱,增强Wnt信号可改善SNI小鼠海马的神经元发生及其学习记忆能力。     

The role of adult hippocampal neurogenesis in reducing interference

Adult neurogenesis in the hippocampal dentate gyrus plays an important role in learning and memory. However, the precise contribution of the new neurons to hippocampal function remains controversial. Emerging evidence suggests that neurogenesis is important for pattern separation and for mitigating interference when similar items must be learned at different times. In the present study, we directly test this prediction using a recently developed olfactory memory task that has those specific features. In this task, rats learn two highly interfering lists of odor pairs, one after the other, in either the same or in different contexts. Consistent with our hypothesis, focal cranial irradiation, resulting in selective reduction of neurogenesis within the dentate gyrus, significantly impaired the ability to overcome interference during learning of the second list. The ability to learn a single odor list was unimpaired. We also show that irradiation had no effect on learning in a hippocampal-dependent spatial alternation task. Although both tasks involved learning interfering responses, the time course for learning the interfering items differed. Learning the interfering odor lists took place sequentially, over the course of several sessions, whereas learning the interfering spatial locations took place concurrently, within each session. Thus, the gradual addition of new neurons may have provided a pattern separation mechanism for the olfactory task but not for the maze task. These findings demonstrate a role for neurogenesis in resolving interference and they are consistent with models suggesting a critical role for neurogenesis in pattern separation.     

Environmental lead exposure during early life alters granule cell neurogenesis and morphology in the hippocampus of young adult rats

Exposure to environmentally relevant levels of lead (Pb(2+)) during early life produces deficits in hippocampal synaptic plasticity in the form of long-term potentiation (LTP) and spatial learning in young adult rats [Nihei MK, Desmond NL, McGlothan JL, Kuhlmann AC, Guilarte TR (2000) N-methyl-D-aspartate receptor subunit changes are associated with lead-induced deficits of long-term potentiation and spatial learning. Neuroscience 99:233-242; Guilarte TR, Toscano CD, McGlothan JL, Weaver SA (2003) Environmental enrichment reverses cognitive and molecular deficits induced by developmental lead exposure. Ann Neurol 53:50-56]. Other evidence suggests that the performance of rats in the Morris water maze spatial learning tasks is associated with the level of granule cell neurogenesis in the dentate gyrus (DG) [Drapeau E, Mayo W, Aurousseau C, Le Moal M, Piazza P-V, Abrous DN (2003) Spatial memory performance of aged rats in the water maze predicts level of hippocampal neurogenesis. Proc Natl Acad Sci U S A 100:14385-14390]. In this study, we examined whether continuous exposure to environmentally relevant levels of Pb(2+) during early life altered granule cell neurogenesis and morphology in the rat hippocampus. Control and Pb(2+)-exposed rats received bromodeoxyuridine (BrdU) injections (100 mg/kg; i.p.) for five consecutive days starting at postnatal day 45 and were killed either 1 day or 4 weeks after the last injection. The total number of newborn cells in the DG of Pb(2+)-exposed rats was significantly decreased (13%; P<0.001) 1 day after BrdU injections relative to controls. Further, the survival of newborn cells in Pb(2+)-exposed rats was significantly decreased by 22.7% (P<0.001) relative to control animals. Co-localization of BrdU with neuronal or astrocytic markers did not reveal a significant effect of Pb(2+) exposure on cellular fate. In Pb(2+)-exposed rats, immature granule cells immunolabeled with doublecortin (DCX) displayed aberrant dendritic morphology. That is, the overall length-density of the DCX-positive apical dendrites in the outer portion of the DG molecular layer was significantly reduced up to 36% in the suprapyramidal blade only. We also found that the area of Timm's-positive staining representative of the mossy fibers terminal fields in the CA3 stratum oriens (SO) was reduced by 26% in Pb(2+)-exposed rats. These findings demonstrate that exposure to environmentally relevant levels of Pb(2+) during early life alters granule cell neurogenesis and morphology in the rat hippocampus. They provide a cellular and morphological basis for the deficits in synaptic plasticity and spatial learning documented in Pb(2+)-exposed animals.