Accelerated senescence prone mouse-8 shows early onset of deficits in spatial learning and memory in the radial six-arm water maze

Available data indicate that the senescence-accelerated prone mouse 8 (SAMP8) is an appropriate model of brain aging, with impairments in nonspatial learning and memory beginning as early as 2 months of age, and spatial learning and memory deficiencies not becoming apparent until after 4 months of age. However, with other strains (e.g., C57BL mice), the impairment in spatial memory was found earlier than that in nonspatial memory. We considered the possibility that the observed differences could be due to strain-specific differences in the training equipment. In the present study, a new optimized testing apparatus-the radial six-arm water maze (RAWM)-for detecting spatial learning and memory in mice, was employed, to determine whether there is impairment of spatial learning and memory in young SAMP8. The relationship between the spatial learning measures observed with the RAWM and the Morris maze, a classic spatial learning and memory testing apparatus, was also explored. It was found that, in the RAWM, rather than in the Morris maze, the impairment in spatial learning could be measured in SAMP8 mice as early as 3 months old, and the impairment in spatial memory in SAMP8 mice aged 5 months. These results suggested that the spatial learning and memory deficiencies could be found in early life of SAMP8 mice, and that RAWM and Morris maze each detect different aspects of spatial learning and memory.     

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.     

Impairments in acquisition and reversals of two-choice discriminations by aged rhesus monkeys

The ability to learn and perform reversals of two object, two patterns, and one spatial discrimination was examined in eight aged (28–34 years), and four adult (8–13 years) behaviorally naive monkeys. As a group, the aged monkeys demonstrated significant difficulties in learning and reversing some of the visual discrimination problems, but had no difficulty learning or reversing the spatial discrimination. Additional analyses revealed that an impairment in learning an object discrimination by the aged monkeys was characterized by a prolonged period of chance performance, and the impairments in performing visual discrimination reversals was related to difficulties in two distinct stages of reversal learning. Despite age-related differences, there was considerable variability in performance among the aged monkeys. These experiments provide the first evidence of significant impairments in learning and reversing visual discriminations by aged monkeys that have not had prior exposure to complex behavioral tasks.     

Lingual deficits in neurotrophin double knockout mice

Brain-derived neurotrophic factor (BDNF) and Neurotrophin 3 (NT-3) are members of the neurotrophin family and are expressed in the developing and adult tongue papillae. BDNF null-mutated mice exhibit specific impairments related to innervation and development of the gustatory system while NT-3 null mice have deficits in their lingual somatosensory innervation. To further evaluate the functional specificity of these neurotrophins in the peripheral gustatory system, we generated double BDNF/NT-3 knockout mice and compared the phenotype to BDNF^−/− and wild-type mice. Taste papillae morphology was severely distorted in BDNF^−/− xNT-3^−/− mice compared to single BDNF^−/− and wild-type mice. The deficits were found throughout the tongue and all gustatory papillae. There was a significant loss of fungiform papillae and the papillae were smaller in size compared to BDNF^−/− and wild-type mice. Circumvallate papillae in the double knockouts were smaller and did not contain any intraepithelial nerve fibers. BDNF^−/− xNT-3^−/− mice exhibited additive losses in both somatosensory and gustatory innervation indicating that BDNF and NT-3 exert specific roles in the innervation of the tongue. However, the additional loss of fungiform papillae and taste buds in BDNF^−/− xNT-3^−/− mice compared to single BDNF knockout mice indicate a synergistic functional role for both BDNF-dependent gustatory and NT-3-dependent somatosensory innervations in taste bud and taste papillae innervation and development.     

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.     

Correlation of increased hippocampal Sumo3 with spatial learning ability in old C57BL/6 mice

Age-related impairment of learning and memory is a common phenomenon in humans and animals, yet the underlying mechanism remains unclear. We hypothesize that a small ubiquitin-related modifier (Sumo) might correlate with age-related loss of learning and memory. To test this hypothesis, the present study evaluated age-related spatial learning and memory in C57BL/6 mice (25 aged 7 months and 21 aged 25 months) using a radial six-arm water maze (RAWM). After the behavioral test, the protein expression of Sumo3 was determined in different brain regions using Western blotting. The results showed that the 25-month-old mice had longer latency and a higher number of errors in both learning and memory phases in the RAWM task than the 7-month-old mice. Compared to the latter, the former's level of Sumo3 protein was significantly increased in the dorsal and ventral hippocampus. For the 25-month-old mice, the number of errors and the latency in the learning phase negatively correlated with the Sumo3 level in the dorsal hippocampus. These results suggest that increased Sumo3 in the hippocampus may be correlated with spatial learning ability in old C57BL/6 mice.     

TrkB but not trkC receptors are necessary for postnatal maintenance of hippocampal spines

Dendritic spines are major sites of excitatory synaptic transmission and changes in their densities have been linked to alterations in learning and memory. The neurotrophins brain-derived neurotrophic factor and neurotrophin-3 and their receptors, trkB and trkC, are thought to be involved in learning, memory and long-term potentiation (LTP). LTP is known to induce trkB and trkC gene expression as well as spinogenesis in the hippocampus. In the aging hippocampus, declines in trkB and trkC mRNA levels may underlie, at least in part, impairments in spatial memory and reductions in spine densities. To determine the significance of trkB and trkC for the maintenance of dendritic spines, we have analyzed Golgi-impregnated hippocampi of adult and aged mice heterozygous for trkB, trkC, or both along with respective wildtype littermates. Deletion of one allele of trkB, but not trkC, significantly reduces spine densities of CA1 pyramidal neurons in both adult and aged mice, as compared to age-matched controls. This indicates that trkB, but not trkC, receptors are necessary for the maintenance of hippocampal spines during postnatal life.     

A role for neurotrophin-3 in targeting developing cholinergic axon projections to cerebral cortex

This study examined the relationship between expression of neurotrophin-3 (NT-3) and the ingrowth of cholinergic axonal projections in cerebral cortex. Patterns of expression of NT-3 (defined by beta-galactosidase reporter expression in heterozygous offspring of transgenic NT-3(lacZneo/+) mice) revealed that limbic cortical regions (including frontal, cingulate, and insular cortex, as well as the dentate gyrus) express NT-3 and that these cortical regions receive early and relatively dense cholinergic axons (stained for acetylcholinesterase, AChE). Using the dentate gyrus as a model system, studies revealed that expression of the NT-3 reporter parallels, and precedes by approximately 2 days, the ingrowth of AChE positive cholinergic axons. Studies of forebrain organotypic slice cultures demonstrate that basal forebrain-derived cholinergic axons extend into cortical regions in a pattern that mimics the pattern of expression of the NT-3 reporter. Similarly, chimeric co-cultures, combining wild type septum with a slice of hippocampus from heterozygous NT-3(lacZneo/+) mice, demonstrate that cholinergic axons grow into regions of the dentate gyrus that express the NT-3 reporter. Hemisphere slice cultures made from NT-3 knockout mice reveal cholinergic axonal growth into cortex, but these axons do not form the regional pattern characteristic of slice cultures made from wild type or heterozygous NT-3(lacZneo/+) mice. Further, chimeric co-cultures made using slices of wild type septum combined with slices of hippocampus from NT-3 knockout mice demonstrate robust cholinergic axonal growth into the hippocampus, but the cholinergic axons do not form the characteristic preterminal pattern associated with the dentate gyrus. Slice cultures from limbic cortical tissue from the NT-3 null mice do not display exaggerated levels of cell death. In aggregate, these data support the hypothesis that expression of NT-3 by cortical neurons serves to attract basal forebrain cholinergic projections to their target cells in cerebral cortex.     

Effects of age and dopamine D2L receptor-deficiency on motor and learning functions

Decreases in the activity or density of dopamine D2 receptor (D2R) have been associated with age-related changes and neurodegenerative diseases such as Parkinson's disease. There are two isoforms of the D2R, termed the D2 long receptor (D2LR) and D2 short receptor (D2SR). To study the function of these two isoforms and their role in aging, we generated mice selectively lacking D2LR (D2L-/-). Here, we showed that middle-aged (12 months) to aged wild-type (WT) mice (22-24 months) displayed significantly lower levels of motor and learning functions than young WT mice (3 months). Interestingly, young D2L-/- mice (which still express D2SR) showed behavioral deficits similar to aged WT mice. It is possible that deletion of the D2LR might facilitate the aging process in mice. Our results also suggest that a deterioration of the D2LR (but not D2SR) system during aging may account, at least in part, for the motor and learning deficits exhibited in aged WT mice. We also showed that the critical age at which significant reduction in behavior occurred varied among different behaviors. Defining the age-related critical periods and understanding the role of the two D2R isoforms in aging may facilitate the development of new strategies for delaying or ameliorating age-related motor and learning impairments.     

Mitochondrial structure and function in the untreated Jackson toxic milk (tx-j) mouse, a model for Wilson disease

Structural changes in hepatocellular mitochondria are characteristic of Wilson disease (WD). Features include variability in size and shape, increased density of matrix, discreet inclusions, and cystic dilatation of the cristae. We examined the functional basis for these mitochondrial changes in the toxic milk (tx-j) mouse model for WD. Its normal syngeic strain, C3H, served as control. Hepatic histology was near-normal in tx-j mice at 3-4-months-old and showed mild inflammation and steatosis at 6-months-old. Transmission electron microscopy showed typical mitochondrial abnormalities, specifically cystic dilatation of tips of cristae, in 3, 4, and 6-month-old tx-j mice and none in normal 3-month-old C3H mice. Citrate synthase (CS) activity was initially lower in tx-j mice than age-matched controls but increased over the first 6 months such that it was significantly greater at 5 and 6-months-old (p<0.003). No evidence for hepatic mtDNA depletion was found by long-PCR analysis. NB-PAGE showed preservation of all complexes in the oxidative-phosphorylation chain except complex IV which declined markedly from 5-months-old onwards. Hepatic complex IV activity was significantly decreased in 5-month-old tx-j mice (p<0.04). Expression of mitochondrial transfer factor A (TFAM) mRNA declined progressively in 6-8-month-old tx-j mice; immunodetectable protein levels declined in parallel. Expression of mtSSB mRNA was uniformly low in tx-j mice from 1-8-months-old. Levels of two mitochondrial antioxidant proteins capable of binding copper, thioredoxin-2 and peroxiredoxin-3, rose over the first 6 months of life. Mitochondrial changes occur early in WD and reflect complex, probably oxidative, injury.     

Malocclusion induces chronic stress

We examined the effect of occlusal disharmony in senescence-accelerated (SAMP8) mice on plasma corticosterone levels, spatial learning in the water maze, fos induction, hippocampal neuron number, expression of glucocorticoid receptors (GR) and glucocorticoid receptor messenger ribonucleic acid (GRmRNA) in hippocampus and inhibitor of glucocorticoid (metyrapone). Bite-raised aged mice had significantly greater plasma corticosterone levels than age-matched control mice as well as impaired spatial memory and decreased Fos induction and a number of neurons in hippocampus. GR and GRmRNA expressions were significantly decreased in aged bite-raised mice compared with age-matched control mice. Pretreatment with metyrapone inhibited not only the bite-raised induced increase in plasma corticosterone levels, but also the reduction in the number of hippocampal neurons and impaired spatial learning. These datas suggest that the bite-raised condition may enhance the aging process in hippocampus, thereby leading to impairment of spatial memory by stress.     

Spatial learning deficits in old rats: a model for memory decline in the aged

Spatial learning tasks are sensitive to functional decline in aged laboratory rodents. This is a review of recent work that has examined both the nature of age-related impairments on spatial tasks, and the relation of such deficits to underlying neurobiological mechanisms. The review supports the notion that hippocampal dysfunction underlies the mild/moderate cognitive decline that often accompanies normal aging. Thus the spatial learning deficit in aged rodents is a promising model for understanding the effect of age on brain systems that serve a memory function in humans.     

Spatial learning in transgenic mice expressing human presenilin 1 (PS1) transgenes

Dominant mutations in the Presenilin 1 gene are linked to an aggressive, early-onset form of familial Alzheimer's Disease (FAD). Spatial memory of transgenic (Tg) mice expressing either mutant (lines Tg(M146L)1, Tg(M146L)76, Tg(L286V)198) or wild type (line Tg(PS1wt)195) human PS1 transgenes was investigated in the Morris water maze (WM) test at 6 and 9 months of age. The results showed that the mutated Tg mice had increased swim speed when compared to non-Tg littermates or Tg PS1 wild type mice. The swim speed difference did not, however, significantly affect the spatial learning in the WM test and all groups showed comparable search paths during training and similar spatial bias during probe trials. When re-tested at 9 months, all mice showed significantly improved learning acquisition of spatial information. The lack of progressive spatial learning impairment in mice expressing the mutated human PS1 transgene in the WM does not preclude impairments in other cognitive tasks but suggests that full phenotypic expression of mutant PS1 alleles may require co-expression of human versions of other AD-associated genes.     

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.     

Insulin-like growth factor 1 reduces age-related disorders induced by prenatal stress in female rats

Stress during the prenatal period can induce permanent abnormalities in adult life such as increased anxiety-like behavior and hyperactivity of hypothalamo-pituitary-adrenal (HPA) axis system. The present study was designed to investigate whether prenatal stress could induce spatial learning impairment in aged female rats. Furthermore, since it has been recently reported that insulin-like growth factor 1 (IGF-1) attenuates spatial learning deficits in aged rats and promotes neurogenesis in the hippocampus, we assessed the impact of a chronic infusion of IGF-1 on age-related disorders. Our results show that females stressed during prenatal life exhibit learning impairments in the water maze task. Chronic IGF-1 treatment restores their spatial abilities, reduces their HPA axis dysfunction and increases plasma estradiol levels. Parallel to these effects, chronic IGF-1 up-regulates neural proliferation in the dentate gyrus of the hippocampus. These findings support the hypothesis of an early programming of the vulnerability to some neurological diseases during senescence and reinforce the potential therapeutic interest of IGF-1 during brain aging.     

Mice heterozygous for neurotrophin-3 display enhanced vulnerability to excitotoxicity in the striatum through increased expression of N-methyl-D-aspartate receptors

The striatum is one of the brain areas most vulnerable to excitotoxicity, a lesion that can be prevented by neurotrophins. In the present study, intrastriatal injection of the N-methyl-d-aspartate receptor (NMDAR) agonist quinolinate (QUIN) was performed in mice heterozygous for neurotrophin-3 (NT3 +/-) or brain-derived neurotrophic factor (BDNF +/-) to analyze the role of endogenous neurotrophins on the regulation of striatal neurons susceptibility to excitotoxic injury. QUIN injection induced a decrease in dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa (DARPP-32) protein levels that was higher in NT-3 +/- than in BDNF+/- or wild type animals. This enhanced susceptibility was specific for enkephalin- and tachykinin-positive projection neurons, and also for parvalbumin-positive interneurons. However the excitotoxic damage in large interneurons was not modified in NT-3 +/- mice compared with wild type animals. This effect can be related to the regulation of NMDARs by endogenous NT-3. Thus, our results show that there is an age-dependent regulation of NMDAR subunits NR1 and NR2A, but not NR2B, in NT-3 +/- mice. The deficit of endogenous NT-3 induced a decrease in NR1 and NR2A subunits at postnatal day (P) 0 and P3 mice respectively, whereas an upregulation was observed in 12 week old NT-3 +/- mice. This differential effect was also observed after administration of exogenous NT-3. In primary striatal cultures, NT-3 treatment induced an enhancement in NR2A, but not NR2B, protein levels. However, intrastriatal grafting of NT-3 secreting-cells in adult wild type mice produced a down-regulation of NR2A subunit. In conclusion, NT-3 regulates the expression of NMDAR subunits modifying striatal neuronal properties that confers the differential vulnerability to excitotoxicity in projection neurons and interneurons in the striatum.     

Long-term impaired memory following predatory stress in mice

Brief exposure of mice to unavoidable predatory stimuli is associated with behavioral and neurochemical changes consistent with increased anxiety and produces short-term impaired learning restored by acute fluoxetine treatment, but not by diazepam. The present study investigated long-lasting changes induced by a unique unavoidable cat exposure in BALB/c mice on learning abilities using learning tests (radial maze, spatial configuration of objects recognition test). Results from the group comparison showed that predatory exposure induced significant learning disabilities in the radial maze (16 to 22 days poststressor) and in the spatial configuration of objects recognition test (26 to 28 days poststressor). These findings indicate that memory impairments may persist for extended periods beyond a predatory stress. This animal model of unique exposure of mice to unavoidable predatory stimuli has proven to be a useful model for the study of reactions to traumatic stress.     

"I've seen it all before": explaining age-related impairments in object recognition. Theoretical comment on Burke et al. (2010)

Animal models are useful in elucidating the neural basis of age-related impairments in cognition. Burke, Wallace, Nematollahi, Uprety, and Barnes (2010) tested young and aged rats in several different protocols to measure object recognition memory and found that object recognition deficits in aged rats were consistent with these rats behaving as if novel objects were familiar, rather than familiar objects being treated as novel (that is, forgotten). A similar pattern of behavior has been observed in young rats with perirhinal cortex lesions. Moreover, age-related impairments in object recognition were uncorrelated with deficits in spatial learning in the water maze, a task that requires the integrity of the hippocampus and is also reliably impaired in aged rats. Taken together, these findings support functional specialization of structures within the medial temporal lobe "memory system," as well as the independence of age-related deficits in different cognitive domains. They also potentially form a foundation for neurobiological study of age-related impairments in perirhinal cortex function.