Extensive training in a maze task reduces neurogenesis in the adult rat dentate gyrus probably as a result of stress

It has recently been shown that hippocampal neurogenesis can be modulated either directly or indirectly by ascending cholinergic inputs from the basal forebrain. In the present work, we sought to address whether extended training in a spatial navigation task would affect hippocampal neurogenesis in the presence of a severe and selective cholinergic depletion. Young female rats received stereotaxic injections of the immunotoxin 192 IgG-saporin into the basal forebrain nuclei and/or the cerebellar cortex. Starting from 4 to 5 weeks post-lesion, and for the subsequent 2 weeks, the animals were trained on paradigms of reference and working memory in the water maze and received single daily i.p. injections of bromodeoxyuridine (BrdU) at the end of each testing session. In line with previous observations, a dramatic 80% decrease in neuron proliferation was seen in the dentate gyrus of lesioned animals, as compared to vehicle-injected or intact controls. Interestingly, however, rats subjected to maze training over 2 weeks, irrespective of their learning success, exhibited significantly fewer newborn neurons than matched controls with no maze exposure. Thus, at least for the type of task used here, which has previously been shown to impose a certain degree of stress, extended training and learning does not appear to affect proliferation in the dentate gyrus.     

Effects of morphine dependence on the performance of rats in reference and working versions of the water maze

Numerous studies have dealt with the role of opiate system in tasks aimed at measurement of cognitive behavior, but the role of morphine dependence on learning and memory is still controversial. In this study chronic exposure to morphine was employed to evaluate learning ability and spatial short-term memory (working memory) and long-term memory (reference memory) in the water maze task. Male albino rats were made dependent by chronic administration of morphine in drinking water that lasted at least 21 days. In Experiment 1, the performance of animals was evaluated in reference memory version of the water maze. Rats were submitted to a session of 6 trials for 6 consecutive days to find the submerged platform that was located in the center of a quadrant. Latency and traveled distance to find the platform were measured as indexes of learning. Memory retention was tested 24 h after the last training session in a probe trial (60 s) in which there was no platform and the time spent in each quadrant of the water maze was recorded. Results indicated that latency and traveled distance to find the platform were same in control and dependent rats during training days, but during the probe test morphine-dependent group spent significantly less time in the target quadrant. In Experiment 2, training on working memory version of the water maze task was started. Only two trials per day were given until the performance of animals was stabilized (at least 5 days). Final test was done at day 6. Acquisition-retention interval was 75 min. No significant differences were found on acquisition and retention trials between morphine and control groups. Our findings indicate that chronic exposure to morphine did not impair learning ability, but partially impaired retention of spatial long-term (reference) memory. Moreover, dependence on morphine did not affect either acquisition or retention of spatial short (working) memory.     

The hormone therapy,Premarin, impairs hippocampus-dependent spatial learning and memory and reduces activation of new granule neurons in response to memory in female rats

Estrogens have been implicated as possible therapeutic agents for improving cognition in postmenopausal women and have been linked to neurodegenerative disorders such as Alzheimer's disease. However, the utility of Premarin (Wyeth Pharmaceuticals, Markham, ON, Canada), a conjugated equine estrogen and the most commonly prescribed hormone therapy, has recently been questioned. The purpose of this study was to investigate the effects of Premarin at 2 different doses (10 or 20 μg) on hippocampus-dependent spatial learning and memory, hippocampal neurogenesis, and new neuronal activation using a rodent model of surgical menopause. Rats were treated daily with subcutaneous injections of Premarin and trained on the spatial working/reference memory version of the radial arm maze. Premarin impaired spatial reference and working learning and memory, increased hippocampal neurogenesis, but either decreased or increased activation of new neurons in response to memory retrieval as indexed by the expression of the immediate early gene product zif268, depending on the maturity of cells examined. This activation of new neurons was related to impaired performance in Premarin-treated but not control-treated female rats. These results indicate that Premarin may be impairing hippocampus-dependent learning and memory by negatively altering the neurogenic environment in the dentate gyrus thus disrupting normal activity of new 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.     

Temporary inactivation of the supramammillary area impairs spatial working memory and spatial reference memory retrieval

The aim of our study was to examine the supramammillary (SuM) area involvement in spatial memory. Sprague-Dawley rats with chronically implanted cannula in the supramammillary area were trained in two spatial memory tasks with different memory demands: reference and working memory. In the spatial reference memory task, the rats received microinjections in the SuM area of tetrodotoxin (TTX) (0.5 ng diluted in 0.5 microL of saline) or saline (0.5 microL). The microinjections were administered 30 min before the spatial training (day 4) (to assess the effect on acquisition) and on the following two days (days 5 and 6) the training was conducted without microinjections (to study the effect on consolidation). On the last training day (day 7), in order to assess the retrieval of spatial information, the rats received the microinjections 30 min before the spatial training. The spatial working memory used was a delayed-matching-to-position (DMTP) task. Spatial training was performed for seven days. During the first three days of the spatial training, the rats achieved a good spatial knowledge and learnt the working memory rule necessary to solve the spatial task. On days 4 and 6, the rats received microinjections to study involvement of the SuM area in working memory. The results showed that temporary inactivation of SuM area impairs both the rat's ability to solve a spatial working memory task with DMTP demands and the recovery of spatial information in a spatial reference memory task. We suggest that SuM area is involved in the rearrangement of spatial information during spatial working memory tasks with DMTP memory demands.     

Effect of reversible inactivation of locus ceruleus on spatial reference and working memory

The locus coeruleus (LC) is the largest source of norepinephrine (NE) in the prefrontal cortex and the hippocampus, influencing the cognitive functions of these areas. All previous studies have studied the role of the LC–NE system on learning and memory using the irreversible lesion technique, employing either electrocoagulation or excitotoxins. However, the reversible functional inactivation of LC by means of stereotaxic local microinjection of lidocaine could measure the phases of memory processing (acquisition, consolidation and retention) without any interference with the other cognitive functions of the same structure either during earlier or later phases of the same process. The aim of this study is to investigate LC involvement in spatial reference and working memory by inducing bilateral pre-training, post-training and pre-retrieval lidocaine functional inactivation using the Morris water maze task. The reversible inactivation of LC was applied at different stages of spatial memory formation: (1) immediately before the training sessions to determine the effects on acquisition of the both reference and working memory; (2) immediately after the training session to evaluate effects on both spatial memory consolidation and retention of working memory; and (3) immediately before the 24 h retention session to analyze the effects on the retrieval process of reference memory. Our results indicate that the bilateral reversible inactivation of LC significantly impaired the acquisition of reference and working memory, while it had no effect on consolidation and/or retention of such memories in the Morris water maze (MWM) task. Therefore, the noradrenergic system of the LC may play a more important role in acquisition than in consolidation and retrieval of spatial memory in wistar rats.     

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.     

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.     

Hippocampal seizures disrupt working memory performance but not reference memory acquisition

The effect of hippocampal seizures in rats was assessed in two spatial memory tasks: The reference memory task was a simultaneous two-choice discrimination in a T-maze. The working memory task was a delayed conditional discrimination in a radial arm maze. In each task the hippocampus of each rat was stimulated to seizure after the presentation of the information to be remembered. In the reference memory task, hippocampal seizures did not impair acquisition, whether the stimulation was given immediately after or 4 hr after the presentation of the stimuli to be remembered. In the working memory task, hippocampal seizures did impair performance in a group of the same rats. These results support the distinction between a trial-dependent working memory system that requires hippocampal function and a trial-independent memory system that does not depend on hippocampal function.     

Spatial memory processing during hippocampal long-term potentiation in rats.

It was investigated in rats whether hippocampal long-term potentiation (LTP) influences working and/or reference memory processing in the radial maze. After preliminary training to an intermediate level of performance, experimental subjects received a series of high-frequency trains of electrical pulses applied to the right perforant path. Two control groups were adopted in order to control for possible effects of stimulation plus operation, and operation alone, respectively. Twenty-four hours after the experimental treatment, animals were administered one trial of radial maze training. This sequence of hippocampal stimulation and radial maze training was replicated 15 times. After a retention interval of 2 months, one radial maze trial was presented on each of 3 consecutive days. The analysis of field potential data showed that periodic LTP stimulation produced a state of hippocampal LTP confined to the initial portion of the acquisition phase. Evaluation of radial maze data revealed a marginal improvement of working memory performance in the experimental group during the rising phase of hippocampal LTP.     

Cholinergic mediation of spatial memory in the preweanling rat: application of the radial arm maze paradigm

These experiments assessed spatial memory capabilities in the developing rat by the radial arm maze paradigm. Subjects were trained and tested in the maze beginning at 16 days until they were 25 days of age. Results showed that animals that received training performed significantly better than those naive to the task, and better than chance, which suggests an early capacity for this type of learning. The second experiment investigated the neural mechanism that underlies spatial memory at this age as measured by the radial arm maze. In order to distinguish between the working and the reference memory components of the task, a modification of the basic radial arm maze paradigm was used. Subjects trained from 16 days received drug injections of saline, methylscopolamine, scopolamine, or arecoline prior to testing at age 25 days. Results indicated that central cholinergic antagonism severely impairs working memory while sparing reference memory. This finding is consistent with the existing literature that suggests a role for acetylcholine in adult learning and memory, specifically in working memory. Most important, these experiments document that (a) the radial arm maze paradigm can be used effectively for the developmental study of learning and memory in the rat and (b) cholinergic system(s) mediate working memory at an early age.     

miR-30c通过调节海马神经发生对APP/PS1转基因小鼠学习记忆能力的影响*

目的：探讨miR-30c 通过调控海马神经发生对APP/PS1 转基因小鼠的学习记忆的影响。方法：用免疫荧光 检测Ki67 的表达和Morris 行为学检测APP/PS1 转基因小鼠海马的神经发生及学习记忆能力,并通过脑立体定位 技术显微注射miR-30c 过表达和miR-30c 敲减慢病毒载体以干扰海马齿状回区域的神经发生,检测海马神经发生 对学习记忆的影响。结果：APP/PS1 转基因小鼠的海马神经发生和学习记忆能力明显低于野生小鼠。然而,当 APP/PS1 转基因小鼠海马的miR-30c 过表达后,海马神经发生明显增加（14.2 倍）;相反,当海马miR-30c 敲减后, 海马神经发生明显降低（0.25 倍）,并且Morris 行为学结果显示,miR-30c 敲减后小鼠的学习记忆能力明显降低, 而miR-30c 过表达后小鼠的学习记忆能力明显增强。结论：miR-30c 可能通过调控海马神经发生影响学习记忆功能, 提示miR-30c 可作为干预阿尔茨海默症神经发生与学习记忆能力的潜在靶点。     

Hippocampus-dependent learning promotes survival of new neurons in the dentate gyrus at a specific time during cell maturation

Adult neurogenesis in the hippocampus continues throughout life and may play an important role in hippocampus-dependent learning and memory. Previous research has been equivocal, demonstrating that spatial learning may enhance, decrease or not significantly affect the survival of new neurons. A potential cause of these varying results may be differences in when bromodeoxyuridine (BrdU) was administered relative to spatial training. We examined whether the time elapsed between BrdU administration and spatial learning would alter the survival of the labeled cells. We injected rats with BrdU once on day 0 and then trained in the standard place version of the Morris water task on days 1-5, 6-10 or 11-15 after BrdU injection. We found an enhancement of neurogenesis in the hippocampus only when BrdU was administered 6 days prior to the beginning of spatial training. There was no significant change in hippocampal neurogenesis for groups that started training either 1 or 11 days following BrdU administration. This suggests that a critical period exists in the development of new neurons during which time their survival may be altered by activation of the hippocampus. Furthermore, when dividing rats into poor versus good learners based on overall performance using a median split, only poor place learners and not good place learners exhibit increased hippocampal neurogenesis compared with cue learning, collapsed across time of training. These findings provide further evidence of a link between learning and adult neurogenesis.     

铅暴露对大鼠学习和记忆的影响

目的:研究低水平铅暴露对大鼠学习和记忆的影响。方法:0.05%水平醋酸铅污染大鼠饮用水28天,用Morris水迷宫试验测定大鼠的学习记忆功能。结果:在定位导航任务两组之间没有差异(P>0.05);在探索试验和工作记忆任务中,铅暴露组和正常对照组大鼠成绩显著差异(P<0.01)。结论:铅暴露对大鼠空间参考记忆和工作记忆有明显损害作用,对空间学习未见明显影响。     

Agmatine protects against β-amyloid25-35-induced memory impairments in the rat

Amyloid beta fragment 25-35 (Aβ_25-35) is the neurotoxic domain of the full-length Aβ_1-42 and causes memory impairments in rodents. Recent research suggests that agmatine, decarboxylated arginine, has a neuroprotective role. This study investigated the effects of a single bilateral i.c.v. infusion of aggregated Aβ_25-35 (30 nmol) in a battery of behavioural tests conducted during the period 4–6 (Experiment 1) and 4–14 (Experiment 2) weeks post-Aβ_25-35 infusion, and evaluated the protective effect of agmatine (40 mg/kg) administered i.p. 30 min prior to Aβ_25-35 infusion and once daily for a further nine consecutive days. In Experiment 1, Aβ_25-35 rats with saline treatment were not impaired in the elevated plus maze and open field and mildly impaired in the reference memory version of the water maze task, but performed poorly in the working memory version of the water maze task and the object recognition memory task, relative to the control rats that received the i.c.v. infusion of Aβ_35-25 (inactive peptide) and saline treatment. By contrast, Aβ_25-35 rats with agmatine treatment did not show performance impairments in the working memory version of the water maze task and the object recognition memory task. In Experiment 2, Aβ_25-35 rats with saline treatment were significantly impaired in the standard radial arm maze task, but only displayed no or very mild impairments in the delayed non-match to position and reference memory versions of the radial arm maze task, T-maze, object recognition memory task, both the reference and working memory versions of the water maze task, elevated plus maze and open field. By contrast, Aβ_25-35 rats with agmatine treatment were not impaired in the standard radial arm maze and performed even better than the controls in the reference memory version of the task. These results demonstrate that agmatine is able to protect against Aβ_25-35-induced memory deficits.     

Sleep restriction suppresses neurogenesis induced by hippocampus-dependent learning

Sleep deprivation impairs hippocampal-dependent learning, which, in turn, is associated with increased survival of newborn cells in the hippocampus. We tested whether the deleterious effects of sleep restriction on hippocampus-dependent memory were associated with reduced cell survival in the hippocampus. We show that sleep restriction impaired hippocampus-dependent learning and abolished learning-induced neurogenesis. Animals were trained in a water maze on either a spatial learning (hippocampus-dependent) task or a nonspatial (hippocampus-independent) task for 4 days. Sleep-restricted animals were kept awake for one-half of their rest phase on each of the training days. Consistent with previous reports, animals trained on the hippocampus-dependent task expressed increased survival of newborn cells in comparison with animals trained on the hippocampus-independent task. This increase was abolished by sleep restriction that caused overall reduced cell survival in all animals. Sleep restriction also selectively impaired spatial learning while performance in the nonspatial task was, surprisingly, improved. Further analysis showed that in both training groups fully rested animals applied a spatial strategy irrespective of task requirements; this strategy interfered with performance in the nonspatial task. Conversely, in sleep-restricted animals, this preferred spatial strategy was eliminated, favoring the use of nonspatial information, and hence improving performance in the nonspatial task. These findings suggest that sleep loss altered behavioral strategies to those that do not depend on the hippocampus, concomitantly reversing the neurogenic effects of hippocampus-dependent learning.     

Inhibition of adult hippocampal neurogenesis disrupts contextual learning but spares spatial working memory,long-term conditional rule retention and spatial reversal

Neurogenesis in the adult dentate gyrus (DG) of the hippocampus has been implicated in neural plasticity and cognition but the specific functions contributed by adult-born neurons remain controversial. Here, we have explored the relationship between adult hippocampal neurogenesis and memory function using tasks which specifically require the participation of the DG. In two separate experiments several groups of rats were exposed to fractionated ionizing radiation (two sessions of 7 Gy each on consecutive days) applied either to the whole brain or focally, aiming at a region overlying the hippocampus. The immunocytochemical assays showed that the radiation significantly reduced the expression of doublecortin (DCX), a marker for immature neurons, in the dorsal DG. Ultrastructural examination of the DG region revealed disruption of progenitor cell niches several weeks after the radiation. In the first experiment, whole-brain and focal irradiation reduced DCX expression by 68% and 43%, respectively. Whole-brain and focally-irradiated rats were unimpaired compared with control rats in a matching-to-place (MTP) working memory task performed in the T-maze and in the long-term retention of the no-alternation rule. In the second experiment, focal irradiation reduced DCX expression by 36% but did not impair performance on (1) a standard non-matching-to-place (NMTP) task, (2) a more demanding NMTP task with increasingly longer within-trial delays, (3) a long-term retention test of the alternation rule and (4) a spatial reversal task. However, rats irradiated focally showed clear deficits in a “purely” contextual fear-conditioning task at short and long retention intervals. These data demonstrate that reduced adult hippocampal neurogenesis produces marked deficits in the rapid acquisition of emotionally relevant contextual information but spares spatial working memory function, the long-term retention of acquired spatial rules and the ability to flexibly modify learned spatial strategies.     

Nerve growth factor improves spatial learning and restores hippocampal cholinergic fibers in rats withdrawn from chronic treatment with ethanol

The cholinergic septohippocampal pathway has long been known to be important for learning and memory. Prolonged intake of ethanol causes enduring memory deficits, which are paralleled by partial depletion of hippocampal cholinergic afferents. We hypothesized that exogenous supply of nerve growth factor (NGF), known to serve as a trophic substance for septal cholinergic neurons, can revert the ethanol-induced changes in the septohippocampal cholinergic system. Adult rats were given a 20% ethanol solution as their only source of fluid for 6 months. During the first 4 weeks after the animals were withdrawn from ethanol, they were intraventricularly infused with either NGF or vehicle alone via implanted osmotic minipumps. The vehicle-infused withdrawn animals showed impaired performance on a spatial reference memory version of the Morris water maze task, both during the task acquisition and on the retention test. In contrast, NGF-treated withdrawn rats were able to learn the task as well as controls, and significantly outperformed the vehicle-infused withdrawn rats. The histological analysis revealed that, in the latter group, the length density of fibers immunoreactive to choline acetyltransferase was reduced relative to control values by approximately 25%, as measured in the dentate gyrus and regio superior of the hippocampal formation. However, in NGF-treated withdrawn rats, the length density of these fibers was identical to that of control rats. These data provide support to the notion that NGF is capable of ameliorating memory deficits and restoring septohippocampal cholinergic projections following chronic treatment with ethanol. Electronic Publication     

Hippocampal dysfunction during aging II: deficits on the radial-arm maze

Middle-aged and aged rats received dorsal hippocampal lesions before performance was evaluated on the radial-arm maze. The maze task contained simultaneous spatial working memory and visually cued reference memory components. Both middle-aged and aged rats that received lesions committed more errors of both types than sham-operated rats. Moreover, an age-related deficit was found for working and reference memory errors. After 14 sessions of training, a probe session revealed that: (a) middle-aged sham rats relied on spatial cues, (b) middle-aged lesioned rats employed the visual cues at the ends of the maze arms, (c) aged sham rats relied predominately on spatial information, (d) aged lesioned rats could not use spatial information or the visual cues at the ends of the maze arms. The additive effect of lesion and age suggests continued reliance on the hippocampus despite age-related deficits in its functioning. These data are suggestive of reduction in flexible cue utilization during aging, resulting paradoxically in more dependence on the hippocampus for aged rats than younger animals.     

Role of hippocampal M2 muscarinic receptors in the estrogen-induced enhancement of working memory

We have previously demonstrated that acetylcholine, acting at M2 muscarinic receptors, mediates the estradiol-induced increase in hippocampal N-methyl-d-aspartate receptor binding and the associated enhancement in working memory. The goal of present experiment was to investigate the role of hippocampal M2 receptors in the behavioral aspects of these effects. Ovariectomized rats were trained to locate a hidden escape platform on a matching-to-place version of the water maze in which the platform was moved to a new location for each session of four daily trials. Following 18 days of training, rats were randomly assigned to receive one of the following treatments: 1) injections of oil vehicle delivered 72 and 48 h before testing and continuous delivery of vehicle into the dorsal hippocampus via bilateral cannulae implants connected to osmotic minipumps; 2) injections of estradiol benzoate (EB) delivered 72 and 48 h before testing and continuous delivery of vehicle into the hippocampus; 3) injections of EB delivered 72 and 48 h before testing and continuous delivery of the M2 muscarinic receptor antagonist, AFDX 116, into the hippocampus; and 4) injections of EB delivered 72 and 48 h before testing and continuous delivery of AFDX 116 into a control site in the cortex. Chronic administration of AFDX 116 into the hippocampus, but not the cortex, significantly attenuated an estrogen-induced enhancement in performance on a working memory task in the water maze as indicated by increased latency and increased path length to locate an escape platform during a test trial when a 90 min delay was imposed between the first and second trials. These results indicate that acetylcholine acts at M2 muscarinic receptors located in the hippocampus to mediate the positive effects exerted by estrogen on working memory.