Enrichment enhances spatial memory and increases synaptophysin levels in aged female mice

The present study tested whether environmental enrichment can reduce age-related spatial reference memory deficits and alter synaptic protein levels in aged female mice. Female C57BL/6 mice, (4 or 27-28 months), were tested in spatial and cued Morris water maze tasks. Prior to (14 days) and during testing, a subset of aged females was exposed to rodent toys and running wheels for 3h per day. The remaining aged females were group housed but were not exposed to enriching objects. At the conclusion of testing, levels of the presynaptic protein synaptophysin were measured in hippocampus and frontoparietal cortex. Enrichment improved spatial memory acquisition; relative to young controls, aged enriched females performed similarly, whereas aged control females were impaired. Enrichment also accelerated the development of a spatial bias in spatial probe trials. In contrast, the cued task was not significantly affected by enrichment. Hippocampal and cortical synaptophysin levels were increased in aged enriched females relative to young and aged controls. These data suggest that environmental enrichment can be a potent cognitive enhancer for aged females and suggests a potential neurobiological mechanism of this effect.     

Early postnatal hyperthyroidism improves both working and reference memory in a spatial radial-maze task in adult mice

Newborn male pups of the inbred mouse strain DBA/2J were injected with a buffered solution of L-thyroxine, known to induce a hyperplasia of the intra- and infrapyramidal mossy fiber terminal fields (iip-MF). Because previous studies have shown that the size of the iip-MF correlates positively with both reference memory and working memory in a spatial radial maze task, we expected improved performance on such a task in the treated animals. Such an improvement was in fact found for both reference and working memory. We conclude that the iip-MF are functionally involved in the regulation of spatial learning and memory in mice.     

Degeneration of hippocampal fibers and spatial memory deficit in the aged rat

Old and young Fisher 344 rats were compared for their ability to learn a delayed alternation task. The old animals displayed significant impairment of alternation learning, and were slower than the young animals. The brains of these animals were examined using a silver degeneration stain, and among old rats there was conspicuous degeneration. The greatest density of degenerating fibers was seen in the hippocampus and in anatomically related tracts, but there was substantial fiber staining in the corpus callosum, anterior commissure, and internal capsule. Examination of the young brains revealed only an occasional fiber. There were no signs of cortical atrophy in the old animals. The histopathology of the aged animals' hippocampus and fiber tracts supports the possibility that the delayed alternation impairment shown by these animals was a result of age related degenerative changes.     

Sulindac improves memory and increases NMDA receptor subunits in aged Fischer 344 rats

Inflammatory processes in the central nervous system are thought to contribute to Alzheimer's disease (AD). Chronic administration of nonsteroidal anti-inflammatory drugs (NSAIDs) decreases the incidence of Alzheimer's disease. There are very few studies, however, on the cognitive impact of chronic NSAID administration. The N-methyl-d-aspartate (NMDA) receptor is implicated in learning and memory, and age-related decreases in the NMDA NR2B subunit correlate with memory deficits. Sulindac, an NSAID that is a nonselective cyclooxygenase (COX) inhibitor was chronically administered to aged Fischer 344 rats for 2 months. Sulindac, but not its non-COX active metabolite, attenuated age-related deficits in learning and memory as assessed in the radial arm water maze and contextual fear conditioning tasks. Sulindac treatment also attenuated an age-related decrease in the NR1 and NR2B NMDA receptor subunits and prevented an age-related increase in the pro-inflammatory cytokine, interleukin 1beta (IL-1beta), in the hippocampus. These findings support the inflammation hypothesis of aging and have important implications for potential cognitive enhancing effects of NSAIDs in the elderly.     

Partial hippocampal inactivation: effects on spatial memory performance in aged and young rats

Changes in anatomical or functional connectivity during normal aging are thought to contribute to cognitive alterations over the lifespan. Neural network theories predict that synaptic loss in an aging brain could place the organism near the point of dysfunction in the nonlinear curve defining neural compromise versus performance. The present experiments examined whether aged rats are closer to this point of behavioral dysfunction by reversibly inactivating one or both hippocampal hemispheres. As expected, bilateral tetracaine inactivation of the hippocampus disrupted spatial memory in both age groups. Unilateral left hippocampal inactivation significantly increased errors only in aged rats; however, unilateral inactivation of the right hippocampus had no effect. The present outcome could reflect more extensive synaptic dysfunction in the aged right hippocampus or a greater involvement of the left hippocampus in spatial working memory problems.     

苯丙胺对大鼠行为、学习能力及海马CA3区突触素表达的影响

目的观察苯丙胺对大鼠行为、空间辨别性学习能力和海马CA3区突触素表达的影响。方法将45只健康雄性SD大鼠随机分为:正常对照组、生理盐水组和苯丙胺组。①苯丙胺组每天肌注0.5mg/kg苯丙胺1次;②生理盐水组即注射等体积生理盐水;③正常对照组不予任何处理。每组大鼠分14d、28d和42d三个时间段进行一般行为学、学习记忆能力及海马CA3区突触素表达的检测。结果①苯丙胺组大鼠在注射苯丙胺10d后出现直立、点头、狂躁等行为改变。正常对照组、生理盐水组均无此现象出现;②苯丙胺组大鼠空间辨别性学习记忆能力的影响,在第1、2个时间段与对照组的比未见组间差异,在第3时间段的平均运行时间和正确率比对照组的延长和降低(P<0.05)。③苯丙胺组大鼠海马CA3区突触素表达在第一阶段比对照组的减少,并随着用药时间的延长减少的更明显(P<0.05)。结论苯丙胺对空间辨别性学习记忆时大鼠行为及海马CA3区突触素表达有影响。     

Estrogen effects on object memory and cholinergic receptors in young and old female mice.

We investigated whether object recognition memory is modulated by estrogen in young (5 month) and aged (24 month) female C57Bl/6J mice, and if cholinergic muscarinic receptors might contribute to this response. Mice that were ovariectomized, or ovariectomized plus estradiol-treated three weeks before behavioral testing or quantitative autoradiography were compared to intact mice. Memory for a previously encountered object deteriorated significantly between 3 and 6h after initial exposure, regardless of animal age. In both young and aged mice, estradiol-treated mice showed significantly greater recall than did ovariectomized mice. In both age groups, the apparent number of [(3)H]pirenzepine/M(1)-like and [(3)H]AFDX384/M(2)-like muscarinic receptor binding sites was reduced in the basal forebrain as well as its projection areas following ovariectomy, but this decrease was not alleviated by estrogen. Aging poorly affected object memory, but reduced muscarinic binding in some cortical subregions and in the caudate nucleus. These findings suggest that estrogen effects on memory in C57Bl/6J mice are not due to changes in the number of muscarinic receptors.     

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.     

Aged SOD overexpressing mice exhibit enhanced spatial memory while lacking hippocampal neurogenesis

The recent finding that hippocampal slices from aged mice overexpressing the gene for superoxide dismutase (SOD) exhibit long-term potentiation (LTP) of reactivity to afferent stimulation that is significantly larger than that produced in aged wild-type (wt) mice has encouraged the exploration of the effects of reactive oxygen species (ROS) on learning in aged mice. In addition, young-adult and aged wt and SOD transgenic mice were used in an attempt to correlate adult neurogenesis with spatial learning. Aged wt and SOD mice exhibited a 90% reduction in doublecortin-labeled new dentate gyrus neurons as compared to young mice, with no significant difference between genotypes. In addition, aged SOD mice exhibited better performance than wt controls in both reference and working memory tasks in a water maze. These findings provide a behavioral measure to demonstrate that excessive production of H2O2 is beneficial in aged mice.     

Chronic dietary alpha-lipoic acid reduces deficits in hippocampal memory of aged Tg2576 mice

Oxidative stress may play a key role in Alzheimer's disease (AD) neuropathology. Here, the effects of the antioxidant, alpha-lipoic acid (ALA) were tested on the Tg2576 mouse, a transgenic model of cerebral amyloidosis associated with AD. Ten-month old Tg2576 and wild type mice were fed an ALA-containing diet (0.1%) or control diet for 6 months and then assessed for the influence of diet on memory and neuropathology. ALA-treated Tg2576 mice exhibited significantly improved learning, and memory retention in the Morris water maze task compared to untreated Tg2576 mice. Twenty-four hours after contextual fear conditioning, untreated Tg2576 mice exhibited significantly impaired context-dependent freezing. ALA-treated Tg2576 mice exhibited significantly more context freezing than the untreated Tg2576 mice. Assessment of brain soluble and insoluble beta-amyloid levels revealed no differences between ALA-treated and untreated Tg2576 mice. Brain levels of nitrotyrosine, a marker of nitrative stress, were elevated in Tg2576 mice, while F2 isoprostanes and neuroprostanes, oxidative stress markers, were not elevated in the Tg2576 mice relative to wild type. These data indicate that chronic dietary ALA can reduce hippocampal-dependent memory deficits of Tg2576 mice without affecting beta-amyloid levels or plaque deposition.     

Taurine improves learning and retention in aged mice

Aging of the brain is characterized by several neurochemical modifications involving structural proteins, neurotransmitters, neuropeptides and related receptors. Alterations of neurochemical indices of synaptic function have been considered as indicators of age-related impairment of central functions, such as locomotion, memory and sensory performances. Several studies demonstrated that GABA receptors, glutamic acid decarboxylase (GAD65&67), and different subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in GABA inhibitory neurotransmission. In this study, using a passive avoidance test, we show that chronic supplementation of taurine to aged mice significantly ameliorates the age-dependent decline in memory acquisition and retention. We have previously shown that taurine supplementation caused changes in the GABAergic system. These changes include increased levels of the neurotransmitters GABA and glutamate, increased expression of glutamic acid decarboxylase and the neuropeptide somatostatin and increase in the number of somatostatin-positive neurons. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally occurring in aging, and suggest a protective role of taurine against the normal aging process. Increased understanding of age-related neurochemical changes in the GABAergic system will be important in elucidating the underpinnings of the functional changes of aging. Taurine might help forestall the age-related decline in cognitive functions through alterations of the GABAergic system.     

Positive effects of deprenyl and estradiol on spatial memory and oxidant stress in aged female rat brains

Increasing age decreases spatial learning and memory. Spatial learning is coordinated with different brain regions. Since the oxidative damage may play a role in the aging process, including the associated cognitive decline, age-related impairment in spatial learning and memory may be alleviated by antioxidant treatment. The present study examined the effects of the monoamine oxidase B inhibitor L-deprenyl, alone and in combination with estradiol, on spatial memory using the Morris water maze and oxidant stress in aged female rat brains. We demonstrated that co-administration of deprenyl and estradiol caused a synergistic effect on spatial memory. However, use of either deprenyl or estradiol alone increased antioxidant enzyme activities in brain and reduced lipid peroxidation. Therefore, positive effects of deprenyl and estradiol on spatial memory may occur due not only to their antioxidant activities but also to the different actions.     

Occlusal disharmony induces spatial memory impairment and hippocampal neuron degeneration via stress in SAMP8 mice

We examined the effect of occlusal disharmony in senescence-accelerated (SAMP8) mice on plasma corticosterone levels, hippocampal neuron number, and spatial performance in the water maze. The bite-raised condition was associated with an accelerated age-related decline in spatial memory, increased plasma corticosterone levels, and a decreased number of neurons in the hippocampal CA3 region. The findings suggest that the bite-raised condition in aged SAMP8 mice induces hippocampal neuron loss, thereby leading to senile memory deficits.     

脑室注射链脲佐菌素对大鼠空间学习记忆能力及海马CA1区神经元数量和海马突触素表达的影响

为了观察脑室内注射链脲佐菌素(streptozotocin,STZ)对大鼠空间学习记忆能力的影响,分析其可能的内在机制,本实验选用40只SD大鼠随机分成对照组和模型组,模型组于第1 d和第3 d脑室注射STZ(总量3 mg/kg)建立Alzheimer病(AD)模型,15 d后进行Morris水迷宫试验,利用RT-PCR方法检测突触素(synaptophysin,SYP)mRNA的表达变化;Nissl染色观察海马结构的改变。结果显示:与对照组相比,脑室注射STZ后大鼠平均上台潜伏期明显延长,穿越平台次数明显减少,靶象限游泳时间的百分比降低,SYP mRNA的表达明显减少,Nissl染色见海马CA1区神经元减少。以上结果提示脑室注射STZ可损伤大鼠的空间学习记忆能力,这可能与海马CA1区神经元和突触结构的损伤有关。     

Inhibition of nitric oxide synthase increases synaptophysin mRNA expression in the hippocampal formation of rats

Synaptophysin is a protein involved in the biogenesis of synaptic vesicles and budding. It has been used as an important tool to investigate plastic effects on synaptic transmission. Nitric oxide (NO) can influence plastic changes in specific brain regions related to cognition and emotion. Experimental evidence suggests that NO and synaptophysin are co-localized in several brain regions and that NO may change synaptophysin expression. Therefore, the aim of the present work was to investigate if inhibition of NO formation would change synaptophysin mRNA expression in the hippocampal formation. Male Wistar rats received single or repeated (once a day for 4 days) i.p. injections of saline or l-nitro-arginine (l-NOARG, 40 mg/kg), a non-selective inhibitor of nitric oxide synthase (NOS). Twenty-four hours after the last injection the animals were sacrificed and their brains removed for ‘in situ’ hybridization study using ³⁵S-labeled oligonucleotide probe complementary to synaptophysin mRNA. The results were analyzed by computerized densitometry. Acute administration of l-NOARG induced a significant (p < 0.05, ANOVA) increase in synaptophysin mRNA expression in the dentate gyrus, CA1 and CA3. The effect disappeared after repeated drug administration. No change was found in the striatum, cingulated cortex, substantia nigra or nucleus accumbens. These results reinforce the proposal that nitric oxide is involved in plastic events in the hippocampus.     

Estradiol,but not raloxifene,improves aspects of spatial working memory in aged ovariectomized rhesus monkeys

Estrogen replacement therapy (ERT) alleviates many postmenopausal symptoms but whether it also benefits cognitive function remains controversial. Further, since estrogen increases the risk of breast and uterine cancers, a new class of compounds, called selective estrogen receptor modulators (SERMs) is being considered as possible alternative to ERT. The SERM raloxifene is particularly interesting because, like estrogen, it improves lipid metabolism and reduces bone loss, without adverse effects on the breast or uterus. Little is known, however, about its effect upon cognitive function.We used a rhesus monkey model of human menopause to examine the effects of ERT and raloxifene on cognitive function. We tested 5 aged females (21-24 years old) ovariectomized long-term (10-16 years) on a battery of age-sensitive tasks, including the Delayed Response (DR), the Delayed Non-Matching-to-Sample-10 min (DNMS-10 min) and the spatial-Delayed Recognition Span Test (DRST). Monkeys were tested 5 days a week on each task for 9 consecutive months, while undergoing treatments with placebo, ethinyl estradiol (EE(2)), and raloxifene in alternating 28-days blocks. EE(2) transiently enhanced the working memory component of the spatial-DRST, but did not affect performance on the other tasks of the battery. Raloxifene had no effect on cognitive performance. These findings indicate that estradiol is able to enhance some aspects of spatial working memory in aged monkeys despite many years of estrogenic deprivation. Further, they suggest that raloxifene does not affect cognitive function after long-term ovarian hormone deprivation.     

Preservation of accurate spatial memory in aged rats

Male rats were tested in an 8 arm radial maze from 6–26 months of age with 5 hr delay imposed between choices 4 and 5. At 26 months their spatial memory was more accurate than when they were first tested at 6 months and also more accurate than that exhibited by another 5 month old group tested concurrently. However, these old rats acquired a noval spatial habit more slowly than the younger animals. In a subsequent study, we compared the acquisition of accurate spatial memory by rats that were 3 or 21.5 months old at the start of training. Older rats adapted to the maze more slowly and required more sessions to achieve criterion with no delay imposed during the test. There was no reliable difference in acquisition when a 1 hr delay was imposed between choices 4 and 5, but the old rats learned more slowly with a 5 hr delay. On memory tests after criterion performance had been achieved, the older rats performed as well as the younger animals at all delay intervals. Aged rats are deficient in acquiring the skills required for accurate spatial memory, but once acquired these skills do not deteriorate. The possibility that other “memory” deficits associated with aging might be alleviated by overtraining is discussed.     

Alteration of CREB phosphorylation and spatial memory deficits in aged 129T2/Sv mice

Phosphorylation of cAMP-response element binding protein (CREB) is required for hippocampus-dependent long-term memory formation. The present study was designed to determine whether spatial memory deficits in aged mice were associated with alteration of hippocampal CREB phosphorylation. We examined the temporal pattern of CREB activation in 5–6 and 23–24-month-old 129T2/Sv mice trained on a spatial reference memory task in the water maze. Phosphorylated CREB (pCREB), total CREB (t-CREB) and c-Fos immunoreactivity (ir) were measured at four time points after the end of training. In young mice, pCREB-ir was significantly increased 15 and 60 min after training in the CA1 region and dentate gyrus. In aged mice sacrificed 15 min after training, pCREB-ir in these structures was reduced whereas t-CREB-ir remained unchanged compared to respective young-adults. An age-related reduction of c-Fos-ir also occurred selectively in hippocampal CA1 region. Since reduced pCREB-ir in CA1 from the 15 min-aged group strongly correlated with individual learning performance, we suggest that altered CREB phosphorylation in CA1 may account for spatial memory impairments during normal aging.