The effects of clozapine on delayed spatial alternation deficits in rats with hippocampal damage

Clozapine is an atypical antipsychotic drug that has been shown to improve spatial memory in some animal models; however its efficacy in reversing spatial memory impairment in rats with hippocampal lesions is unknown. To address this issue, we tested the effects of clozapine on delayed spatial alternation deficits in rats with hippocampal damage in three separate experiments. In each experiment, adult male rats received sham surgery or direct stereotaxic infusions of the excitotoxin, NMDA, into the hippocampus. In the first study, seven days after surgery, the sham control animals received daily saline injections while the lesioned animals were split into two groups that received daily saline or clozapine (2.0 mg/kg, sc) injections. During the fifth week of injections, all animals were tested in a food-motivated delayed spatial alternation task. Saline-treated rats with excitotoxic hippocampal damage displayed significant deficits in delayed spatial alternation. Daily clozapine injections completely reversed this deficit. In a second experiment, it was found that clozapine treatment limited to the testing days only did not improve alternation performance in lesioned rats. Finally, in a third experiment, chronic clozapine treatment did not improve alternation performance in lesioned rats that were pre-trained in the alternation task prior to surgery. These results suggest that chronic, but not acute, clozapine treatment enables rats with hippocampal damage to develop new spatial learning, but can not rescue old spatial learning established prior to damage. These results may have implications for the treatment of cognitive deficits caused by hippocampal dysfunction in disorders such as schizophrenia, Alzheimer's disease, and others.     

阻断内侧前额叶皮质TrkB受体对大鼠认知和海马BDNF表达的影响

脑源性神经营养因子(brain-derived neurotrophic factor, BDNF)广泛参与了个体学习和记忆等认知功能, 通过与其酪氨酸激酶受体(tyrosine kinase, TrkB)特异性结合, 实现其多种神经生化功能。本研究观察了TrkB受体阻断剂ANA-12的慢性内侧前额叶皮质(medial prefrontal cortex, mPFC)注射对大鼠旷场行为、Morris水迷宫空间学习和逆反学习的影响。研究结果表明, mPFC的慢性BDNF阻断显著降低了大鼠在逆反学习测试中的逃离潜伏期和运动距离即增强了大鼠的逆反学习能力, 但不影响其旷场行为和水迷宫空间学习能力。同时, 慢性阻断mPFC-TrkB受体也并未导致大鼠海马BDNF蛋白含量的显著改变。这些结果提示, 对于大鼠的Morris水迷宫空间学习和逆反学习, mPFC-BDNF主要在逆反学习调节中发挥重要作用。这对于进一步探索海马和mPFC在调节个体认知功能中各自的作用及其潜在的相互关系提供了有力的证据和支持。     

Enriched environment treatment restores impaired hippocampal synaptic plasticity and cognitive deficits induced by prenatal chronic stress

Prenatal stress can cause long-term effects on cognitive functions in offspring. Hippocampal synaptic plasticity, believed to be the mechanism underlying certain types of learning and memory, and known to be sensitive to behavioral stress, can be changed by prenatal stress. Whether enriched environment treatment (EE) in early postnatal periods can cause a recovery from these deficits is unknown. Experimental animals were Wistar rats. Prenatal stress was evoked by 10 foot shocks (0.8 mA for 1s, 2-3 min apart) in 30 min per day at gestational day 13-19. After weaning at postnatal day 22, experimental offspring were given the enriched environment treatment through all experiments until tested (older than 52 days age). Electrophysiological and Morris water maze testing was performed at 8 weeks of age. The results showed that prenatal stress impaired long-term potentiation (LTP) but facilitated long-term depression (LTD) in the hippocampal CA1 region in the slices. Furthermore, prenatal stress exacerbated the effects of acute stress on hippocampal LTP and LTD, and also impaired spatial learning and memory in the Morris water maze. However, all these deficits induced by prenatal stress were recovered by enriched environment treatment. This work observes a phenomenon that may contribute to the understanding of clinically important interactions among cognitive deficit, prenatal stress and enriched environment treatment. Enriched environment treatment on early postnatal periods may be one potentially important target for therapeutic interventions in preventing the prenatal stress-induced cognitive disorders.     

Chronic oral methylphenidate induces post-treatment impairment in recognition and spatial memory in adult rats

Recent pre-clinical research has indicated that chronic treatment with methylphenidate (Ritalin^®) in young animals can result in lasting and potentially detrimental alterations in brain function that can persist into adulthood. Chronic methylphenidate-induced neuronal alterations may result in behavior and cognitive deficits that include increases in behavioral responses and impairment in recognition memory. This study compared the cognitive consequences following chronic treatment with two doses (5 and 10 mg/kg) of methylphenidate on recognition and spatial memory in adult male Long-Evans rats using an established oral dosing procedure. The animals were then tested in the Object Recognition test at 14 days post treatment and the Object Placement test at 21 days post treatment. The results indicate that repeated exposure to oral methylphenidate impaired the performance of rats in these tests. The current findings add to recent research demonstrating negative consequences in rats pre-treated with methylphenidate, and extend previous findings to include deficits in spatial recognition memory.     

Involvement of BDNF receptor TrkB in spatial memory formation

The N-methyl-D-aspartate (NMDA) receptors are involved in long-term potentiation (LTP), and are phosphorylated by several tyrosine kinases including a Src-family tyrosine kinase Fyn. Brain-derived neurotrophic factor (BDNF) is a neurotrophin, which also enhances hippocampal synaptic transmission and efficacy by increasing NMDA receptor activity. Here, we show that Fyn is a key molecule linking the BDNF receptor TrkB with NMDA receptors, which play an important role in spatial memory formation in a radial arm maze. Spatial learning induced phosphorylation of TrkB, Fyn, and NR2B, but not NR2A, in the hippocampus. Fyn was coimmunoprecipitated with TrkB and NR2B, and this association was increased in well-trained rats compared with control animals. Continuous intracerebroventricular infusion of PP2, a tyrosine kinase inhibitor, in rats delayed memory acquisition in the radial arm maze, but PP2-treated animals reached the same level of learning as the controls. The phosphorylation of Fyn and NR2B, but not TrkB, was diminished by PP2 treatment. Our findings suggest the importance of interaction between BDNF/TrkB signaling and NMDA receptors for spatial memory in the hippocampus.     

Combined uridine and choline administration improves cognitive deficits in spontaneously hypertensive rats

Rationale. Hypertension is considered a risk factor for the development of cognitive disorders, because of its negative effects on cerebral vasculature and blood flow. Genetically induced hypertension in rats has been associated with a range of cognitive impairments. Therefore, spontaneously hypertensive rats (SHR) can potentially be used as a model for cognitive deficits in human subjects. Consecutively, it can be determined whether certain food components can improve cognition in these rats. Objective. The present study aimed to determine whether SHR display specific deficits in attention, learning, and memory function. Additionally, effects of chronic uridine and choline administration were studied. Methods. 5-7 months old SHR were compared with normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats. (a) The operant delayed non-matching-to-position (DNMTP) test was used to study short-term memory function. (b) The five-choice serial reaction time (5-CSRT) task was used to assess selective visual attention processes. (c) Finally, the Morris water maze (MWM) acquisition was used as a measure for spatial learning and mnemonic capabilities. Results. (1) SHR exhibited significantly impaired performance in the 5-CSRT test in comparison with the two other rat strains. Both the SHR and WKY showed deficits in spatial learning when compared with the SD rats. (2) Uridine and choline supplementation normalized performance of SHR in the 5-CSRT test. (3) In addition, uridine and choline treatment improved MWM acquisition in both WKY and SHR rats. Conclusion. The present results show that the SHR have a deficiency in visual selective attention and spatial learning. Therefore, the SHR may provide an interesting model in the screening of substances with therapeutic potential for treatment of cognitive disorders. A combination of uridine and choline administration improved selective attention and spatial learning in SHR.     

Cognitive performances and locomotor activity following dentate granule cell damage in rats: role of lesion extent and type of memory tested

Intradentate injection of colchicine is one of the techniques used to destroy granule cells. This study compared the behavioral effects of various amounts of colchicine (1.0, 3.0, and 6.0 microg; Col 1, Col 3, and Col 6, respectively) injected into the dentate gyrus of adult Long-Evans male rats. Starting 10 days after lesion surgery, behavioral testing assessed home-cage and open-field locomotion, alternation in a T-maze, water-maze, and radial-maze learning according to protocols placing emphasis on reference, and working memory. All of these tasks are sensitive to hippocampal disruption. Histological verifications showed that the extent of the lesions depends on the dose of colchicine (index of dentate gyrus shrinkage: -33% in Col 1, -54% in Col 3, and -67% in Col 6 rats). Colchicine dose-dependently increased nocturnal home cage activity (an effect found 10 days but not 5 months after surgery), but had no significant effect on open-field locomotion or T-maze alternation. A dose-dependent reference memory impairment was found during the acquisition of spatial navigation in the water maze; Col 3 and Col 6 rats were more impaired than Col 1 rats. During the probe trial (platform removed), control rats spent a longer distance swimming over the platform area than all rats with colchicine lesions. In the working memory version of the test, all rats with colchicine lesions showed significant deficits. The deficits were larger in Col 3 and Col 6 rats compared to Col 1 rats. The lesions had no effect on swimming speed. In the radial-maze test, there was also a dose-dependent working memory impairment. However, reference memory was disrupted in a manner that did not differ among the three groups of lesioned rats. Our data are in line with the view that the dentate gyrus plays an important role in the acquisition of new information and is an integral neural substrate for spatial reference and spatial working memory. They also suggest that damage to granule cells might have more pronounced effects on reference than on working memory in the radial maze. Finally, they demonstrate that part of the variability in the conclusions from previous experiments concerning the role of granule cells in cognitive processes, particularly in spatial learning and memory, may be due to the type of tests used and/or the extent of the damage produced.     

Longitudinal attentional engagement rescues mice from age-related cognitive declines and cognitive inflexibility

Learning, attentional, and perseverative deficits are characteristic of cognitive aging. In this study, genetically diverse CD-1 mice underwent longitudinal training in a task asserted to tax working memory capacity and its dependence on selective attention. Beginning at 3 mo of age, animals were trained for 12 d to perform in a dual radial-arm maze task that required the mice to remember and operate on two sets of overlapping guidance (spatial) cues. As previously reported, this training resulted in an immediate (at 4 mo of age) improvement in the animals' aggregate performance across a battery of five learning tasks. Subsequently, these animals received an additional 3 d of working memory training at 3-wk intervals for 15 mo (totaling 66 training sessions), and at 18 mo of age were assessed on a selective attention task, a second set of learning tasks, and variations of those tasks that required the animals to modify the previously learned response. Both attentional and learning abilities (on passive avoidance, active avoidance, and reinforced alternation tasks) were impaired in aged animals that had not received working memory training. Likewise, these aged animals exhibited consistent deficits when required to modify a previously instantiated learned response (in reinforced alternation, active avoidance, and spatial water maze). In contrast, these attentional, learning, and perseverative deficits were attenuated in aged animals that had undergone lifelong working memory exercise. These results suggest that general impairments of learning, attention, and cognitive flexibility may be mitigated by a cognitive exercise regimen that requires chronic attentional engagement.     

Corticosterone effects on BDNF mRNA expression in the rat hippocampus during morris water maze training

Corticosterone and Brain-Derived Neurotrophic Factor (BDNF) have both been shown to be involved in spatial memory formation in rats. In the present study we have investigated the effect of corticosterone on hippocampal BDNF mRNA expression after training in the Morris water maze in young adult Wistar rats. Therefore, we first studied BDNF mRNA levels in the hippocampus in relation to corticosterone levels at several time points after 4 training trials in the Morris water maze. Corticosterone levels were significantly increased after this procedure, and hippocampal BDNF mRNA levels only displayed a minor change: an increase in CA1 at 1 hr after training. However, in a previous study we observed dramatically decreased hippocampal BDNF mRNA levels in dentate gyrus and CA1 at 3 hr after injection of corticosterone. In order to analyze this discrepancy, we subsequently investigated if hippocampal BDNF mRNA expression is affected by corticosterone at 3 hr after water maze training. Therefore, we incorporated ADX animals and ADX animals which were injected with corticosterone in our study. ADX animals which were subjected to water maze training displayed similar hippocampal BDNF mRNA levels 3 hr after training compared to control ADX animals. Furthermore, ADX animals which were injected with corticosterone showed decreased BDNF mRNA levels in all hippocampal regions compared to control ADX animals. Water maze training did not alter this effect. Thus, the increased corticosterone levels during water maze training do not affect hippocampal BDNF mRNA expression, although exogenous corticosterone is effective under these conditions. Hence, our results suggest that in this situation BDNF is resistant to regulation by endogenous corticosterone, which may be important for learning and memory processes.     

Dietary restriction inhibits spatial learning ability and hippocampal cell proliferation in rats1

Abstract: We investigated the effect of dietary restriction on spatial learning ability and hippocampal cell proliferation in adult rats using two spatial learning tasks and immunohistochemical staining with 5‐bromo‐2′‐deoxyuridine (BrdU). Sixteen rats were divided into restricted or ad lib feeding groups at 70 days of age, and were trained in the delayed‐matching‐to‐place (DMTP) task (a working memory task) from 93 days of age, and then the Morris water maze task (a reference memory task). Dietary restriction had no effect on the DMTP task with 30 s delay and on the water maze task. However, in the DMTP task with 30 min delay, restricted rats performed significantly more poorly than ad lib rats. Quantitative analysis of hippocampal cell proliferation revealed that the density of newborn cells in restricted rats was significantly lower than that in ad lib rats. These results suggest that a loss of proliferating capacity in the hippocampus may be a candidate for an anatomical and biological basis for the cognitive decline caused by dietary restriction.     

Effects of chronic cocaine administration on spatial learning and hippocampal spine density in two genetically different strains of rats

Lewis and Fischer-344 rats have been proposed as an addiction model because of their differences in addiction behaviour. It has been suggested that drug addiction is related to learning and memory processes and depends on individual genetic background. We have evaluated learning performance using the eight-arm radial maze (RAM) in Lewis and Fischer-344 adult rats undergoing a chronic treatment with cocaine. In order to study whether morphological alterations were involved in the possible changes in learning after chronic cocaine treatment, we counted the spine density in hippocampal CA1 neurons from animals after the RAM protocol. Our results showed that Fischer-344 rats significantly took more time to carry out test acquisition and made a greater number of errors than Lewis animals. Nevertheless, cocaine treatment did not induce changes in learning and memory processes in both strains of rats. These facts indicate that there are genetic differences in spatial learning and memory that are not modified by the chronic treatment with cocaine. Moreover, hippocampal spine density is cocaine-modulated in both strains of rats. In conclusion, cocaine induces similar changes in hippocampal neurons morphology that are not related to genetic differences in spatial learning in the RAM protocol used here.     

Sex differences in spatial and non-spatial Y-maze performance after chronic stress

Chronic restraint is known to alter hippocampal CA3 dendritic morphology and spatial memory in male rats. The present study examined whether female rats, which exhibit different anatomical adaptations to chronic stress than those of males, would also show spatial memory impairments. Male and female Sprague-Dawley rats were restrained for 6 h/day for 21 days, a time frame previously demonstrated to cause hippocampal CA3 dendritic atrophy. The day after the last restraint session, rats were tested on a Y-maze, a habituation task that can be used to assess spatial memory. Chronic stress impaired Y-maze performance in both sexes without affecting levels of locomotion as measured by total arm entries in the first minute. However, Y-maze performance of stressed females improved in 2-5 min when chronically stressed males continued to show poor Y-maze performance. The enhanced Y-maze performance of chronically stressed females occurred when total arm entries were higher compared to the entries made by males. Therefore, correlations were performed between total arm entries and spatial memory in 1 and 2-5 min. In the first minute when control females demonstrated functional spatial memory, female controls with the lowest locomotor levels exhibited the best performance. The correlations for stressed females were not significant, and neither were the correlations for any group in 2-5 min. Overall, these results show important sex differences in response to chronic stress with females exhibiting an ability to recover quickly from deficits in Y-maze performance.     

Neurotoxic dorsal CA1 lesions versus 4 VO ischaemic lesions: behavioural comparisons

Anterograde amnesia, a common consequence of transient cerebral ischaemia, has been attributed to cell loss in the hippocampal CA1 subfield. However, variable, widespread damage outside hippocampal CA1 can also occur following ischaemia. We compared the functional consequences of ischaemia and ibotenate acid CA1 lesions on 2 spatial memory tasks (water maze 'place' and 'matching-to-position') to address the possibility that extra-CA1 loss contributes to ischaemia-induced memory deficits in the rat. During place task acquisition, ischaemic rats showed deficits on more measures than ibotenic rats, and during a 1 min probe trial, only ischaemic rats were impaired. On the matching-to-position task, ibotenic rats showed greater impairment than ischaemic rats in terms of one-trial learning, whereas ischaemic rats were more impaired after Trial 2. Ischaemia and ibotenic acid lesions resulted in equivalent CA1 loss, but silver impregnation revealed additional extra-CA1 cell loss in ischaemic rats. Together with the greater behavioural deficits of ischaemic rats, these data indicate a role for extra-CA1 cell loss in ischaemia-induced memory impairments in both animals and humans.     

The enhancing effects of hippocampal infusions of glucose are not restricted to spatial working memory

Extensive evidence shows that hippocampal infusions of glucose enhance spontaneous alternation (SA) performance or reverse deficits in this task. The current experiments determined whether the enhancing effects of hippocampal infusions of glucose are restricted to spatial working memory. Specifically we tested whether hippocampal infusions of glucose would reverse deficits in an emotional reference memory task (continuous multiple trial inhibitory avoidance [CMIA]) produced by septal infusions of the gamma-aminobutyric acid agonist muscimol. Male Sprague-Dawley rats were given septal infusions of vehicle or muscimol (0.15 nmol: SA; 5 nmol: CMIA) combined with hippocampal infusions of vehicle or glucose (50 nmol) 15 min prior to assessing SA or CMIA training. CMIA retention was tested 48 h later. Muscimol infusions decreased percent alternation scores and avoidance retention latencies. Importantly, hippocampal infusions of glucose reversed the deficits produced by the septal muscimol infusions on both tasks. These findings show for the first time that hippocampal glucose infusions also influence emotional memory, indicating that the enhancing effects of glucose generalize to memory tasks that vary in motivational and cognitive demand.     

Transgenic mice expressing a truncated form of CREB-binding protein (CBP) exhibit deficits in hippocampal synaptic plasticity and memory storage

Deletions, translocations, or point mutations in the CREB-binding protein (CBP) gene have been associated with Rubinstein-Taybi Syndrome; a human developmental disorder characterized by retarded growth and reduced mental function. To examine the role of CBP in memory, transgenic mice were generated in which the CaMKII alpha promoter drives expression of an inhibitory truncated CBP protein in forebrain neurons. Examination of hippocampal long-term potentiation (LTP), a form of synaptic plasticity thought to underlie memory storage, revealed significantly reduced late-phase LTP induced by dopamine-regulated potentiation in hippocampal slices from CBP transgenic mice. However, four-train induced late-phase LTP is normal. Behaviorally, CBP transgenic mice exhibited memory deficits in spatial learning in the Morris water maze and deficits in long-term memory for contextual fear conditioning, two hippocampus-dependent tasks. Together, these results demonstrate that CBP is involved in specific forms of hippocampal synaptic plasticity and hippocampus-dependent long-term memory formation.     

Long-term enrichment enhances the cognitive behavior of the aging neurogranin null mice without affecting their hippocampal LTP

Neurogranin (Ng), a PKC substrate, is abundantly expressed in brain regions important for cognitive functions. Deletion of Ng caused severe deficits in spatial learning and LTP in the hippocampal CA1 region of mice. These Ng-/- mice also exhibit deficits in the amplification of their hippocampal signaling pathways critical for learning and memory. A short-term exposure to an enriched environment failed to improve their behavioral performances. Here, we showed that a long-term enrichment protocol for the aging mice was beneficial to the Ng-/- as well as Ng+/+ and Ng+/- mice in preventing age-related cognitive decline. Enrichment also caused an increase in the hippocampal CREB level of all three genotypes and Ng level of Ng+/+ and Ng+/- mice, but not that of alphaCaMKII or ERK. Interestingly, hippocampal slices of these enriched aging Ng-/- mice, unlike those of Ng+/+ and Ng+/- mice, did not show enhancement in the high frequency stimulation (HFS)-induced LTP in the CA1 region. It appears that the learning and memory processes in these enriched aging Ng-/- mice do not correlate with the HFS-induced LTP, which is facilitated by Ng. These results demonstrated that long-term enrichment for the aging Ng-/- mice may improve their cognitive function through an Ng-independent plasticity pathway.     

Differential Effects of Global Ischemia on Delayed Matching- and Non-Matching-to-Position Tasks in the Water Maze and Skinner Box

In order to assess effects of global ischemia in tasks of spatial learning and working memory, male Wistar rats were subjected to four vessel occlusion (4 VO) for periods of 5, 10, and 20 min and compared with sham-operated controls over four test phases, from 6 to 54 weeks after surgery. Rats were assessed on acquisition in the water maze, a task that is sensitive to ischemic impairments, before testing in Skinner box and water maze working memory tasks, which both require the short-term storage of information, but make different demands on spatial information processing. Phases 1 and 3 assessed spatial learning in a standard water maze procedure (12 and 10 training days, 2 trials/day with a 10-min intertrial interval: ITI). Phase 2 involved training and testing in delayed non-matching-to-position task in the Skinner box, with delays of 2–10 s between the information and choice stages. Phase 4 examined working memory in a water maze delayed matching-to-position task with 4 trials/day, an ITI of 30 s, and a novel platform position on each day. Ischemic rats showed duration-related impairments in water maze acquisition and working memory, but not in the less spatially demanding Skinner box task. Since water maze acquisition deficits were seen both before and after testing in the Skinner box the lack of effect cannot be attributed to time or to prior training. Ischemic deficits were more marked in Phase 3 than in Phase 1 of acquisition, suggesting that impairment may be progressive. Histological assessment showed that cell loss was largely confined to the hippocampal CA1 field and was linearly related to duration of occlusion. At the maximal level of loss (5.7 mm before the interaural line) the 20-min group showed 90% loss, the 10-min group 60% loss, and the 5-min group, which did not differ from controls, less than 10% loss. Only the 20-min group showed significant damage beyond the CA1 field, ranging from 30–40% loss in the CA3 field to 5% loss in one striatal area. No cortical damage was seen. The extent of CA1 cell loss correlated modestly with water maze acquisition (Phase 3) and working memory scores, but not with trials to criterion in the Skinner box task. There were significant correlations between different measures both within and between water maze tasks, but not Skinner box tasks, suggesting that the two types of procedure engaged different cognitive processes. The results indicate that the intrahippocampal damage induced by 4 VO impaired tasks which required processing of allocentric spatial information, but did not impair the storage of limited spatial information in working memory.     

The Neuropsychology of Amphetamine and Opiate Dependence: Implications for Treatment

Chronic use of amphetamines and/or opiates has been associated with a wide range of cognitive deficits, involving domains of attention, inhibitory control, planning, decision-making, learning and memory. Although both amphetamine and opiate users show marked impairment in various aspects of cognitive function, the impairment profile is distinctly different according to the substance of abuse. In light of evidence showing that cognitive impairment in drug users has a negative impact on treatment engagement and efficacy, we review substance-specific deficits on executive and memory function, and discuss possibilities to address these during treatment intervention.     

Short forms of the "reference-" and "working-memory" Morris water maze for assessing age-related deficits.

Short forms of the reference- and working-memory versions of the Morris water maze, each limited to 10 trials, were examined for their reliability and sensitivity to age-related deficits in 16- and 24-month F-344 rats, relative to 2- to 2.5-month young controls. The reference-memory task used long intertrial intervals of 23 h, but required learning only one target location, while the working-memory task used shorter intertrial intervals of 60 min but required learning many different target locations. The reference-memory task was very reliable, revealed large age-related deficits, and correctly identified almost all aged rats as impaired relative to young controls. The working-memory task was less reliable, revealed smaller deficits than the reference memory task at 24 months, and did not discriminate as well between 2.5- and 24-month rats. Furthermore, in the working-memory task 16- and 24-month rats had longer swim paths than 2- to 2.5-month rats on the first trial of each trial pair, which is suggestive of a deficit in processing spatial information and raises questions about the validity of this test as a specific test of working memory. Although the working-memory procedures may be preferable under certain conditions, perhaps as a measure specific to hippocampal dysfunction, the reference-memory task seems more sensitive to age-related deficits and more accurately identifies older rats as impaired. These results are consistent with previous reports that age-related deficits in acquiring spatial learning tasks are common and that the magnitude of the deficit increases as the length of the retention interval increases.