Reduced serine racemase expression contributes to age-related deficits in hippocampal cognitive function

To gain insight into the contribution of d-serine to impaired cognitive aging, we compared the metabolic pathway and content of the amino acid as well as d-serine-dependent synaptic transmission and plasticity in the hippocampus of young and old rats of the Wistar and Lou/C/Jall strains. Wistar rats display cognitive impairments with aging that are not found in the latter strain, which is therefore considered a model of healthy aging. Both mRNA and protein levels of serine racemase, the d-serine synthesizing enzyme, were decreased in the hippocampus but not in the cerebral cortex or cerebellum of aged Wistar rats, whereas the expression of d-amino acid oxidase, which degrades the amino acid, was not affected. Consequently, hippocampal levels of endogenous d-serine were significantly lower. In contrast, serine racemase expression and d-serine levels were not altered in the hippocampus of aged Lou/C/Jall rats. Ex vivo electrophysiological recordings in hippocampal slices showed a marked reduction in N-methyl-d-aspartate-receptor (NMDA-R)-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) in the CA1 area of aged Wistar rats, which were restored by exogenous d-serine. In contrast, NMDA-R activation, LTP induction and responses to d-serine were not altered in aged Lou/C/Jall rats.These results further strengthen the notion that the serine racemase-dependent pathway is a prime target of hippocampus-dependent cognitive deficits with aging. Understanding the processes that specifically affect serine racemase during aging could thus provide key insights into the treatment of memory deficits in the elderly.     

Preserved memory capacities in aged Lou/C/Jall rats

Although memory impairments are a hallmark of aging, the degree of deficit varies across animal models, and is likely to reflect different states of deterioration in metabolic and endocrinological properties. This study investigated memory-related processes in young (3-4 months) and old (24 months) Sprague-Dawley rats (SD), which develop age-linked pathologies such as obesity or insulin-resistance and Lou/C/Jall rats, which do not develop such impairments. In short- and long-term memory recognition tasks, old Lou/C/Jall rats were never impaired whereas old SD rats were deficient at 1 and 24h latencies. The expression of N-methyl-d-aspartate receptors (NMDAR)-mediated synaptic plasticity in CA1 hippocampal networks shifted towards lower activity values in old Lou/C/Jall rats whereas long-term potentiation was impaired in age-matched SD rats. Age-related decrease in NR2A subunits occurred in both strains, extended to NR2B, NR1 and GluR1 subunits in older animals (28 months) but only in SD rats. Therefore, the Lou/C/Jall rats can be considered as a model of healthy aging, not only in terms of its preserved metabolism, but also in terms of cognition and synaptic plasticity.     

大鼠海马微血管构筑的定量分析及衰老变化

目的:观察并定量分析大鼠海马的微血管构筑及衰老变化。方法:采用单宁酸-氯化铁媒染微血管的方法分别观察青龄、老龄大鼠海马与额叶皮质的微血管构筑,并采用MiVnt图像分析系统对海马与额叶皮质的微血管密度(MVD)及微血管面积密度(MVA)进行定量分析。结果:老龄大鼠海马、皮质的微血管数量显著减少,分布杂乱、扭曲缠结。定量分析显示老龄大鼠海马、皮质的MVD值和MVA值均明显低于青龄大鼠,差异具有统计学意义。结论:老龄大鼠海马及额叶皮质的MVD值、MVA值均明显低于青龄大鼠,这是老年血管性痴呆发生机制的主要形态学依据。     

Muscarinic receptor-mediated GTP-Eu binding in the hippocampus and prefrontal cortex is correlated with spatial memory impairment in aged rats

The present study examined muscarinic receptor/G-protein coupling in the hippocampus and the prefrontal cortex of young and aged Long-Evans rats characterized for spatial learning ability in the Morris water maze. In a highly sensitive time-resolved fluorometry GTP-Eu binding assay, muscarinic-mediated GTP-Eu binding was severely blunted in hippocampus (-32%) and prefrontal cortex (-34%) as a consequence of aging. Furthermore, the magnitude of decreased muscarinic-mediated GTP-Eu binding was significantly correlated with the severity of spatial learning impairment in hippocampus and prefrontal cortex of aged rats and was specifically decreased in the subset of aged rats that were spatial learning impaired when compared to the aged unimpaired and the young rats. Western blot data indicated a preservation of the membrane-bound M1 receptor and the Galphaq/11 protein in both brain regions. These data demonstrate that muscarinic signaling is severely impaired as a consequence of normal aging in a manner that is closely associated with age-related cognitive decline.     

Prominent hippocampal CA3 gene expression profile in neurocognitive aging

Research in aging laboratory animals has characterized physiological and cellular alterations in medial temporal lobe structures, particularly the hippocampus, that are central to age-related memory deficits. The current study compares molecular alterations across hippocampal subregions in a rat model that closely mirrors individual differences in neurocognitive features of aging humans, including both impaired memory and preserved function. Using mRNA profiling of the CA1, CA3 and dentate gyrus subregions, we have distinguished between genes and pathways related to chronological age and those associated with impaired or preserved cognitive outcomes in healthy aged Long-Evans rats. The CA3 profile exhibited the most prominent gene expression differences related to cognitive status and of the three subregions, best distinguished preserved from impaired function among the aged animals. Within this profile differential expression of synaptic plasticity and neurodegenerative disease-related genes suggests recruitment of adaptive mechanisms to maintain function and structural integrity in aged unimpaired rats that does not occur in aged impaired animals.     

Naltrexone reverses age-induced cognitive deficits in rats

We evaluated young (3-4 months) and aged (22-24 months) male Sprague-Dawley rats in an attentional set-shifting procedure that assessed reversal, intradimensional shift (IDS), and extradimensional shift (EDS) discrimination learning tasks within one test session. These aspects of discrimination learning are sensitive to damage to distinct regions of frontal cortex. Compared to young animals, aged rats were significantly impaired on the EDS task and did not demonstrate significant impairment on the reversal or IDS tasks. The opioid antagonist naltrexone (2mg/kg, ip) was administered to young and aged rats prior to testing to assess possible improvements in aged-related cognitive impairments. Naltrexone (2mg/kg) attenuated the impairments in cognitive function in the EDS task for aged animals, but did not alter any task performance in the younger group. These results suggest that normal aging in rats is associated with impaired medial frontal cortex function as assessed by this attentional set-shifting procedure and opioid mediated mechanisms may represent a therapeutic target for drugs to improve cognitive deficits associated with aging.     

Selective influence of rat genotype on age-related regional changes in forebrain muscarinic binding

Muscarinic binding was analyzed in the hippocampus and frontal cerebral cortex of 3 (young) and 24-month-old (aged) inbred Wistar-Kyoto (WKY), Brown-Norway (BN) and Lewis rats, by the specific binding of the antagonist quinuclidinylbenzilate (QNB). While no age-related changes were detected in the hippocampus of all strains, complex changes were found in the cortex. Maximal binding capacity (Bmax) of [³H]QNB was decreased in the cortex of aged WKY rats, remained unchanged in the BN strain and was elevated in Lewis rats. We conclude that regional differences exist in age-related changes in forebrain muscarinic binding which are probably due to differences in the types and connections of cholinoceptive neurons in these regions. The results suggest that it is the rate of these changes which is affected by the rat genotype.     

老龄大鼠和年轻大鼠大脑皮层胆碱能系统功能的比较研究

用脑内微透析技术结合 HPL C-柱后固定化酶反应器 -电化学检测器方法分析比较了老龄大鼠和年轻成年大鼠额叶皮层乙酰胆碱的释放 ,并探讨了它和学习记忆功能减退的相关性。老龄大鼠额叶皮层乙酰胆碱的基础水平为 ( 0 .15 5 9± 0 .18)pmol/5μl,远较年轻大鼠的 ( 0 .2 93 9± 0 .14 ) pmol/5μl为低。老龄大鼠额叶胆碱能神经元对高 [K+ ]溶液的反应较弱 ,乙酰胆碱的释放量仅增加 3 0 .18% ,而年轻大鼠可增加 96%。老龄大鼠对新异环境刺激的反应也较年轻大鼠为弱 ,但电刺激 Meynert基底核仍能明显提高老龄大鼠额叶皮层乙酰胆碱的释放。实验结果还表明 ,老龄大鼠额叶皮层的乙酰胆碱水平有较大的个体差异 ,9只老龄大鼠额叶皮层乙酰胆碱释放量的变化范围为 0 .0 0 67pmol/5 μl～ 0 .41pmol/5 μl。这种差异也反映在老龄大鼠对被动回避反应的记忆保持能力上 ,2 4h步下潜伏期和乙酰胆碱释放量有较好的相关性 ,相关系数为 0 .86。本研究所获得的实验结果对进一步阐明老龄大鼠认知性功能缺损的机制具有重要意义     

大白鼠脑与松果体β肾上腺素能受体的老年性变化

目的 研究老年大鼠大脑皮质、海马、小脑皮质和松果体β-肾上腺素能受体的变化.方法 选用老年(20个月)和青年(3个月)SD大白鼠,以放射配基受体结合分析法测定大鼠大脑皮质、海马、小脑皮质和松果体β受体密度、比较2个年龄组上述脑区β受体含量变化及松果体β受体日夜周期变化.结果 与青年组比较,老年组大脑皮质、海马β受体含量明显降低(P<0.01),小脑皮质β受体含量变化不明显(P>0.05),松果体β受体含量在青年组白天明显高于夜间,而老年组变化不明显.结论 衰老时大鼠大脑皮质和海马β受体含量减少,松果体β受体含量的日夜周期改变消失.     

In vivo molecular imaging of the GABA/benzodiazepine receptor complex in the aged rat brain

The GABA-ergic system, known to regulate neural tissue genesis during cortical development, has been postulated to play a role in cerebral aging processes. Using in vivo molecular imaging and voxel-wise quantification, we aimed to assess the effects of aging on the benzodiazepine (BDZ) recognition site of the GABA(A) receptor. To visualize BDZ site availability, [(11)C]-flumazenil microPET acquisitions were conducted in young and old rats. The data were analyzed and region of interest analyses were applied to validate the voxel-wise approach. We observed decreased [(11)C]-flumazenil binding in the aged rat brains in comparison with the young control group. More specifically, clusters of reduced radioligand uptake were detected in the bilateral hippocampus, cerebellum, midbrain, and bilateral frontal and parieto-occipital cortex. Our results support the pertinence of voxel-wise quantification in the analysis of microPET data. Moreover, these findings indicate that the aging process involves declines in neural BDZ recognition site availability, proposed to reflect alterations in GABA(A) receptor subunit polypeptide expression.     

Rescue of cognitive-aging by administration of a neurogenic and/or neurotrophic compound

Aging is characterized by a progressive decline of cognitive performance, which has been partially attributed to structural and functional alterations of hippocampus. Importantly, aging is the major risk factor for the development of neurodegenerative diseases, especially Alzheimer's disease. An important therapeutic approach to counteract the age-associated memory dysfunctions is to maintain an appropriate microenvironment for successful neurogenesis and synaptic plasticity. In this study, we show that chronic oral administration of peptide 021 (P021), a small peptidergic neurotrophic compound derived from the ciliary neurotrophic factor, significantly reduced the age-dependent decline in learning and memory in 22 to 24-month-old Fisher rats. Treatment with P021 inhibited the deficit in neurogenesis in the aged rats and increased the expression of brain derived neurotrophic factor. Furthermore, P021 restored synaptic deficits both in the cortex and the hippocampus. In vivo magnetic resonance spectroscopy revealed age-dependent alterations in hippocampal content of several metabolites. Remarkably, P021 was effective in significantly reducing myoinositol (INS) concentration, which was increased in aged compared with young rats. These findings suggest that stimulating endogenous neuroprotective mechanisms is a potential therapeutic approach to cognitive aging, Alzheimer's disease, and associated neurodegenerative disorders and P021 is a promising compound for this purpose.     

Gene delivery of Homer1c rescues spatial learning in a rodent model of cognitive aging

Homer1c has been shown to play a role in learning and memory. Overexpression of Homer1c in the hippocampus can improve memory in normal rats and can also rescue spatial learning deficits in Homer1 knockout mice. In a previous study, we found that Homer1c mRNA is upregulated after a spatial learning paradigm in aged rats that successfully learn the task, when compared to aged rats that are learning-impaired (AI). This study was designed to validate the role of Homer1c in successful cognitive aging. In this article, we report that gene delivery of Homer1c into the hippocampus of aged learning-impaired rats significantly improves individual performance on an object location memory task. The learning ability of these rats on the Morris Water Maze was also superior to that of AI control rats. In summary, using 2 independent spatial memory tasks, we demonstrate that Homer1c is sufficient to improve the spatial learning deficits in a rodent model of cognitive aging. These results point to Homer1c as a potential therapeutic target for improving age-related cognitive impairment.     

The effects of aging on N-methyl-d-aspartate receptor subunits in the synaptic membrane and relationships to long-term spatial memory

There are declines in the protein expression of the NR2B (mouse ε2) and NR1 (mouse ζ1) subunits of the N-methyl-d-aspartate (NMDA) receptor in the cerebral cortex and hippocampus during aging in C57BL/6 mice. This study was designed to determine if there is a greater effect of aging on subunit expression and a stronger relationship between long-term spatial memory and subunit expression within the synaptic membrane than in the cell as a whole. Male, C57BL/6JNIA mice (4, 11 and 26 months old) were tested for long-term spatial memory in the Morris water maze. Frontal cortex, including prefrontal regions, and hippocampus were homogenized and fractionated into light and synaptosomal membrane fractions. Western blots were used to analyze protein expression of NR2B and NR1 subunits of the NMDA receptor. Old mice performed significantly worse than other ages in the spatial task. In the frontal cortex, the protein levels of the NR2B subunit showed a greater decline with aging in the synaptic membrane fraction than in the whole homogenate, while in the hippocampus a similar age-related decline was observed in both fractions. There were no significant effects of aging on the expression of the NR1 subunit. Within the middle-aged mouse group, higher expression of both NR2B and NR1 subunits in the synaptic membrane of the hippocampus was associated with better memory. In the aged mice, however, higher expression of both subunits was associated with poorer memory. These results indicate that aging could be altering the localization of the NR2B subunit to the synaptic membrane within the frontal cortex. The correlational results suggest that NMDA receptor functions, receptor subunit composition, and/or the environment in which the receptor interacted in the hippocampus were not the same in the old animals as in younger mice and this may have contributed to memory declines during aging.     

Age-related changes in ethanolamine glycerophospholipid fatty acid levels in rat frontal cortex and hippocampus

Morphological and biochemical alterations are associated with a progressive age-related cognitive deficit. Plasmenylethanolamine, the major brain plasmalogen, may be modified during aging because of a possible antioxidant role and involvement in synaptic transmission. Two- and 18-month-old rats were used to study the effect of aging on the levels and acyl composition of plasmenylethanolamine (PmE), phosphatidylethanolamine (PE), and phosphatidylserine (PS) in the frontal cortex and hippocampus. Aging only reduced significantly the PE levels in the frontal cortex. In 18-month-old rats, the fatty acid composition of the three phospholipid classes studied showed an increase of monounsaturated fatty acid (18:1 n-9 and 20:1 n-9) and a decrease in polyunsaturated fatty acid (PUFAs), essentially docosahexaenoic acid (DHA). DHA was markedly decreased in hippocampus PE. DHA, but also arachidonic acid, were considerably lower in frontal cortex PmE. PS modifications were similar in both regions. Hippocampus and frontal cortex underwent specific age-induced modifications in PmE and PE acyl composition. This could produce different effects on the functional ability of these two structures involved in the processes of specific memorization.     

A comparison of the steroidogenic acute regulatory protein-related lipid transfer (START) domain-containing 6 on the brain and testes between young and aged rats

The START domain-containing 6 (StarD6) was originally reported to play a role during male germ cell maturation. We have since reported on StarD6 in the developing hypothyroid rat brain. Therefore, we investigated qualitative and quantitative changes of StarD6 in the aging rat brain and testes of male Sprague-Dawley rats. Serum testosterone levels decreased with aging and total protein levels of StarD6 in the testes decreased. While the immunolocalization of StarD6 in the spermatocytes decreased, cytoplasmic localization appeared in the aged testes. Compared with young rats, aged rats showed decreased StarD6 in the cerebrum and cerebellum without changes in immunolocalization in the cortical neurons of the cerebral cortex and Purkinje cells of the cerebellar cortex. Aged rats also showed increases in StarD6 in the hippocampus with changes in its immunolocalization from the Stratum pyramidale to the Stratum radiatum and Stratum lacunosum-moleculare. Taken together, StarD6 decreased with aging in the testes, which implies that StarD6 might play a role in impaired spermatogenesis in the aged rat. StarD6 decreased in the cerebrum and the cerebellum, but slightly increased in the hippocampus, which suggests that StarD6 might also play a role for neurosteroidogenesis in the hippocampus of aged rats.     

Age-related differences in brain choline acetyltransferase, cholinesterases and muscarinic receptor sites in two strains of rats

The age-related changes in choline acetyltransferase (ChAT), cholinesterases (ChE) and muscarinic receptor sites (measured as Bmax of 3H-QNB binding) were evaluated in the cerebral cortex, hippocampus and striatum of Fischer 344 and Wistar male rats at the ages of 3 and 24 months. In the aged Fischer rats there was a significant decline of ChAT (except the hippocampus), ChE and muscarinic receptor densities in the regions analyzed. In the aged Wistar rats cortical and hippocampal ChAT as well as cortical muscarinic receptors remained constant while striatal ChAT, hippocampal and striatal muscarinic receptors decreased significantly; ChE were reduced in all regions analyzed. Factorial analysis of variance (2 strains x 2 ages ANOVA) showed significant strain-related differences in ChAT and muscarinic receptor sites in the three brain areas (about 1.5 times higher levels in the Fischer rats). The same analysis showed significant interactions between strain and age for ChAT and muscarinic receptors in the cerebral cortex, but not in the hippocampus and striatum; no interactions were found for ChE in the regions analyzed. This means that cortical ChAT and muscarinic receptors behave differently in aging in the two strains of rats, i.e., their alterations are strain-specific. Conversely, all other age-related changes (or lack of them for hippocampal ChAT) cannot be considered strain-specific. Moreover, an additional group of 33-month Wistar rats showed a significant decline of cortical muscarinic receptors with respect to 24 month rats but not of other markers in any area. The data underscore the need to consider genotype in the assessment of age-related cholinergic deficits in animal models.     

老年大鼠血液白细胞基因表达谱的生物信息学分析

目的了解老年大鼠血液白细胞基因表达水平的变化。方法将雄性Wister大鼠分为老年组（18月龄）和年轻组（4月龄），用illumina大鼠全基因组芯片研究其分子水平的变化。结果生物信息学分析显示．造成老年组和年轻组显著生物学差异的基因功能及所属基因是：28条凋亡基因和22条炎性基因上调，9条涉及氧与活性氧中间物代谢的基因下调。结论差异基因功能导致老年大鼠和年轻大鼠的生物学差异。     

Expression of apoptosis-inducing factor (AIF) in the aged rat brain

The mitochondrial flavoprotein apoptosis-inducing factor (AIF) promotes cell death upon nuclear translocation or by impinging on mitochondrial respiratory complex-I activity. Because decreased complex-I activity is associated with brain aging, we investigated the expression and distribution of AIF in frontal cortex, hippocampus and striatum of aged Long-Evans rat brains. We found that AIF was: (i) more abundantly expressed in striatum than in the other two brain regions, (ii) enriched in deep layers of frontal cortex and in the pyramidal cell layer of hippocampus, and (iii) overall mainly localized to mitochondria, but significantly more translocated to the nucleus in the deep layers of frontal cortex. Altogether, our data point to a difference in regionl AIF expression patterns, and provide evidence for the involvement of AIF in the cell death of a subpopulation of cortical neurons in aged animals.     

Microvascular changes in aged rat forebrain. Effects of chronic nimodipine treatment

In the present study the effects of long-term treatment with the 1,4-dihydropyridine calcium antagonist nimodipine on ultrastructural alterations of the microvascular morphology were examined in the frontoparietal cortex, entorhinal cortex and CA1 of the hippocampus in the aged rat. Qualitative observations of cerebral microvasculature of aged (30 months) Wistar rats revealed the presence of microvascular fibrosis, membranous inclusions within the basement membrane and basement membrane thickenings. In several cortical regions the percentage of aberrant microvessels was significantly reduced in the nimodipine-treated rats. The observed microvascular anomalies were classified into five distinct categories of which microvascular fibrosis type II, defined as collagen deposits up to 1 micron within the microvascular basement membrane, showed the strongest reduction in the nimodipine-treated cases. The decrement of the percentage of aberrant microvessels and the relative occurrence of several classes of microvascular deviations showed some variation in the various brain regions examined and was most pronounced in frontoparietal cortex layer III. These results may provide a morphological basis for the improved motor and cognitive performance in aged rats after long-term oral nimodipine administration.