Aging and corticosterone injections affect spatial learning in Fischer-344 X Brown norway rats

Characterization of Spatial Learning and Sickness Responses in Aging Rats Following Recurrent Lipopolysaccharide Administration

Infections in older individuals can result in cognitive function decline, yet research is limited on how recurrent infections affect cognitive responses. Activation of the immune system results in sickness responses mediated by cytokines. This pilot study examined effects of a model of recurrent infection in aged, male Brown Norway rats on sickness responses, including spatial learning, and cytokine levels. To model initial and recurrent infection, 300 μg/kg lipopolysaccharide (LPS) or saline was administered 1/day for 2 consecutive days during 2 weeks separated by 16 days. Testing occurred for 6 days during each LPS injection week using the Morris water maze, a measure used to evaluate spatial learning. Directional heading error (DHE) and swim time latency served as spatial learning indices. Retention tests and probe trials assessed memory. Plasma cytokine levels were assessed 5 and 24 hr after each LPS injection during Week 2. While food intake and weight decreased significantly in LPS-injected rats compared to controls during Week 1, both displayed increased DHE. Despite exhibiting lessened sickness behaviors during Week 2, experimental animals still displayed spatial learning deficits. Probe trials revealed memory deficits in LPS-injected animals. Interleukin 6 level was higher in the experimental group 5 and 24 hr after LPS injection on Day 1 compared to Day 2 and higher in the experimental compared to the control group at 5 hr on Day 1. Cognitive effects were dissociated from metabolic effects in aged rats, with recurring LPS exposure resulting in persistent cognitive impairment despite decreased sickness responses. Further research with older individuals is warranted.

Effects of low-dose lipopolysaccharide and age on spatial learning in different Morris water maze protocols

Objectives:

Animals administered lipopolysaccharide exhibit dose-related sickness behaviors (decreased food intake, weight loss, and cognitive changes). While research has demonstrated that spatial learning is impaired following a lipopolysaccharide immune challenge, the results differ depending on the methodology used to evaluate spatial learning. Additionally, few studies have evaluated the effects of low-dose lipopolysaccharide on spatial learning. Therefore, we assessed spatial learning, food intake, and weight changes in adult and aged rats after a low-dose lipopolysaccharide immune challenge in the Morris water maze using two water temperatures.

Methods:

Adult (5–6 months) and aged (22 months) male Brown-Norway rats were administered either 50 or 100 μg/kg lipopolysaccharide or saline, and then tested in the Morris water maze for 5 days, rested for 7 days, and later underwent 2 days of retention tests. Probe trials were conducted at the end of initial and retention testing.

Results:

Low-dose lipopolysaccharide administration did not result in food intake or weight changes. While the aged experimental group took longest to improve directional heading error in both cold and warm water, heading error was greater in cold water. Behavioral testing revealed an apparent age and water temperature effect on swim time. Retention and probe trial results showed that aged experimental animals had the worst performance in cold water.

Conclusion:

We conclude that while low-dose lipopolysaccharide did not result in typical sickness behaviors (decreased food intake or weight), spatial learning and memory were impaired in the aged experimental group. These results have important implications for the care of elderly individuals experiencing mild to moderate infections.

Spatial Learning Responses to Lipopolysaccharide in Adult and Aged Rats

Sickness behaviors, adaptive responses to infections, include lethargy, depression, reduced eating and grooming, and concentration problems resulting from interactions between the immune and neuroendocrine systems. Detecting these responses is especially critical in the elderly, as the infections that cause them can lead to cognitive impairment. While deficits in spatial learning, a hippocampal-dependent form of learning, are part of the sickness response, directional heading errors (DHEs; an indicator of spatial-learning deficits) and their time trajectories need further examination. Therefore, we simultaneously investigated the time trajectory of age-dependent sickness responses and spatial learning over 5 days in adult (5-6 months) and aged (22 months) male Brown-Norway rats injected with 250 μg/kg lipopolysaccharide (LPS; experimental group) or 0.9% sodium chloride (control group). LPS administration resulted in pronounced, age-dependent weight loss and decreased food intake that persisted in the aged group. Animals were tested for 5 days (trial) in the Morris water maze. After 7 days of rest, animals were retested for 2 days (retention). Adult and aged LPS-treated animals displayed greater differences in mean DHE than the control groups, indicating that they exhibited more DHE over the trial days. Experimental groups did not show consistent DHE improvement until Day 4 (adult) or 5 (aged). LPS had no effect on probe or retention trials. We conclude that LPS activation of the immune system results in a selective, age-dependent impairment in spatial learning, decreased food intake, and weight loss. All of these results are prolonged in aged animals.

Age-related impairments in object-place associations are not due to hippocampal dysfunction

Age-associated cognitive decline can reduce an individual's quality of life. As no single neurobiological deficit can account for the wide spectrum of behavioral impairments observed in old age, it is critical to develop an understanding of how interactions between different brain regions change over the life span. The performance of young and aged animals on behaviors that require the hippocampus and cortical regions to interact, however, has not been well characterized. Specifically, the ability to link a spatial location with specific features of a stimulus, such as object identity, relies on the hippocampus, perirhinal and prefrontal cortices. Although aging is associated with dysfunction in each of these brain regions, behavioral measures of functional change within the hippocampus, perirhinal and prefrontal cortices in individual animals are often not correlated. Thus, how dysfunction of a single brain region within this circuit, such as the hippocampus, impacts behaviors that require communication with the perirhinal and prefrontal cortices remains unknown. To address this question, young and aged rats were tested on the interregion dependent object-place paired association task, as well as a hippocampal-dependent test of spatial reference memory. This particular cohort of aged rats did not show deficits on the hippocampal-dependent task, but were significantly impaired at acquiring object-place associations relative to young. These data suggest that behaviors requiring functional connectivity across different regions of the memory network may be particularly sensitive to aging, and can be used to develop models that will clarify the impact of systems-level dysfunction in the elderly.

Spatial reference memory in normal aging Fischer 344 × Brown Norway F1 hybrid rats

Fischer 344 × Brown Norway F1 (F344 × BN-F1) hybrid rats express greater longevity with improved health relative to aging rodents of other strains; however, few behavioral reports have thoroughly evaluated cognition across the F344 × BN-F1 lifespan. Consequently, this study evaluated spatial reference memory in F344 × BN-F1 rats at 6, 18, 24, or 28 months of age in the Morris water maze. Reference memory decrements were observed between 6 and 18 months and 18 and 24 months. At 28 months, spatial learning was not worse than 24 months, but swim speed was significantly slower. Reliable individual differences revealed that ∼50% of 24- to 28-month-old rats performed similarly to 6 months, whereas others were spatial learning impaired. Aged rats were impaired at learning within daily training sessions but not impaired at retaining information between days of training. Aged rats were also slower to learn to escape onto the platform, regardless of strategy. In summary, these data clarify the trajectory of cognitive decline in aging F344 × BN-F1 rats and elucidate relevant behavioral parameters.

Dissecting the age-related decline on spatial learning and memory tasks in rodent models: N-methyl-D-aspartate receptors and voltage-dependent Ca2+ channels in senescent synaptic plasticity

In humans, heterogeneity in the decline of hippocampal-dependent episodic memory is observed during aging. Rodents have been employed as models of age-related cognitive decline and the spatial water maze has been used to show variability in the emergence and extent of impaired hippocampal-dependent memory. Impairment in the consolidation of intermediate-term memory for rapidly acquired and flexible spatial information emerges early, in middle-age. As aging proceeds, deficits may broaden to include impaired incremental learning of a spatial reference memory. The extent and time course of impairment has been be linked to senescence of calcium (Ca²⁺) regulation and Ca²⁺-dependent synaptic plasticity mechanisms in region CA1. Specifically, aging is associated with altered function of N-methyl-D-aspartate receptors (NMDARs), voltage-dependent Ca²⁺ channels (VDCCs), and ryanodine receptors (RyRs) linked to intracellular Ca²⁺ stores (ICS). In young animals, NMDAR activation induces long-term potentiation of synaptic transmission (NMDAR-LTP), which is thought to mediate the rapid consolidation of intermediate-term memory. Oxidative stress, starting in middle-age, reduces NMDAR function. In addition, VDCCs and ICS can actively inhibit NMDAR-dependent LTP and oxidative stress enhances the role of VDCC and RyR-ICS in regulating synaptic plasticity. Blockade of L-type VDCCs promotes NMDAR-LTP and memory in older animals. Interestingly, pharmacological or genetic manipulations to reduce hippocampal NMDAR function readily impair memory consolidation or rapid learning, generally leaving incremental learning intact. Finally, evidence is mounting to indicate a role for VDCC-dependent synaptic plasticity in associative learning and the consolidation of remote memories. Thus, VDCC-dependent synaptic plasticity and extrahippocampal systems may contribute to incremental learning deficits observed with advanced aging.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

Behavioral and neurobiological consequences of prolonged glucocorticoid exposure in rats: relevance to depression

Stress is a critical environmental trigger for the development of clinical depression, yet little is known about the specific neurobiological mechanisms by which stress influences the development of depressive symptomatology. Animal models provide an efficient way to study the etiology of human disorders such as depression, and a number of preclinical models have been developed to assess the link between stress, glucocorticoids, and depressive behavior. These mode ls typically make use of repeated exposure to physical or psychological stressors in rodents or other small laboratory animals. This review focuses primarily on a recently developed preclinical model of depression that uses exogenous administration of the stress hormone corticosterone (CORT) in rodents instead of exposure to physical or psychological stressors. Repeated CORT administration in rats or mice produces reliable behavioral and neurobiological alterations that parallel many of the core symptoms and neurobiological changes associated with human depression. This provides an opportunity to study behavior and neurobiology in the same animal, so that the neurobiological factors that underlie specific symptoms can be identified. Taken together, these findings suggest that exogenous CORT administration is a useful method for studying the relationship between stress, glucocorticoids, and depression. Further study with this model may provide important new data regarding the neurobiological bases of depression.

Glial glucocorticoid receptors in aged Fisher 344 (F344) and F344/Brown Norway rats

Glucocorticoid receptors (GR) regulate glial function, and changes in astrocyte gene expression are implicated in age-related pathology. We evaluated changes in astroglial GR expression in two strains of rats--Fisher 344 (F344; 4, 12 and 24 months) and F344/Brown Norway strain (F344/BN; 4, 12 and 30 months). In both strains basal levels of corticosterone were higher in the oldest groups of rats. Age-related increases in GR (+) astrocytes but not the percent of astrocytes expressing GR were observed in the hippocampus CA1 region in F344 rats. Age-related decreases in CA1 GR (+) astrocytes and the percentage of GR (+) astrocytes were observed in the F344/BN strain only. Similar strain-specific changes were observed in the dentate gyrus. In the hypothalamic paraventricular nucleus: (1) F344 rats exhibited significant decreases in the overall number of glial profiles with age, (2) F344/BN rats exhibited decreases in the numbers of GR (+) astrocytes with aging and (3) the proportion of GR (+) astrocytes decreased in older F344/BN, but not F344 rats. Overall, the data demonstrate age- and strain-related alterations in GR astrocytic expression that may explain unique phenotypic differences in brain function observed in both strains.

Abnormal differentiation of newborn granule cells in age-related working memory impairments

Age-related declines in spatial memory have been linked to abnormal functional properties and connectivity of newborn granule cells. However, the relationship between adult neurogenesis, aging, and cognitive performance seems more complex than previously anticipated, likely due to the difficulty of disentangling alterations related to training as such and those associated with cognitive performance. Here, we investigated how different aspects of adult neurogenesis might be related to training, age and cognitive performance amongst aged subjects by comparing behaviourally naïve and tested rats of 3, 6, 24mo of age. We separated aged rats into learning-impaired and -unimpaired groups based on their performance in the Morris water maze to investigate neurogenesis-related morphological and neurochemical changes. We report an age-related decline in cell proliferation and maturation independent of cognitive performance and testing. We confirm an age-related altered differentiation of newborn neurons which was particularly prominent in learning-impaired rats. This was associated with an abnormally prolonged expression of the early progenitor marker Nestin, potentially also affecting maturation, survival/integration of newborn neurons into existing neuronal networks, which might underlie the individual differences in cognitive performance during aging.

Female CREBalphadelta- deficient mice show earlier age-related cognitive deficits than males

Age-related changes in the hippocampus increase vulnerability to impaired learning and memory. Our goal is to understand how a genetic vulnerability to cognitive impairment can be modified by aging and sex. Mice with a mutation in the cAMP response element binding (CREB) protein gene (CREB(alphadelta-) deficient mice) have a mild cognitive impairment and show test condition-dependent learning and memory deficits. We tested three ages of CREB(alphadelta-) deficient and wild-type (WT) mice in two Morris water maze (MWM) protocols: four trials per day with a 3-5 min inter-trial interval (ITI) (MWM4) and two trials per day with a 1 min ITI (MWM2). All CREB(alphadelta-) deficient mice performed well in the easier MWM4, except for the aged females that performed poorly. In the harder MWM2, young male and female and middle-aged male CREB(alphadelta-) deficient mice performed well, but aged male and all middle-aged and aged female CREB(alphadelta-) deficient mice were impaired. These results show that mice with a genetic vulnerability to impaired learning and memory exhibit increased vulnerability with age that is most apparent among females. Thus, a genetic predisposition to cognitive impairment may render females more vulnerable than males to such deficits with age.

Two months makes a difference in spatial orientation learning in very old FBNF1 rats

Age-related changes in cognitive performance may be more pronounced in the period near or exceeding the median life span. Therefore, we compared the acquisition of a Morris water escape task by two groups of very old Fischer344 x Brown Norway hybrid rats. The mean age difference between the two groups of rats (a 33- to 34-month-old group versus a 35- to 36-month-old group) was about 2 months. Both groups of rats initially had the same level of performance, but then the younger group learned to escape onto the submerged platform faster, swimming a shorter distance, than the older group. By the fifth acquisition session, the younger rats needed only half the time and swam a shorter distance before they reached the platform than the older rats. These differences in learning were not due to different locomotor abilities as both groups had a similar swimming speed. These results suggest that age-related changes in cognitive performance are indeed more pronounced in the period around the median life span. We also discussed different set-ups to perform cross-sectional age-comparison studies. If there are not sufficient animals from one batch, it may be worthwhile to combine animals from different batches per age group, provided that breeding, rearing, housing, and testing conditions are highly standardized.

What is the functional significance of chronic stress-induced CA3 dendritic retraction within the hippocampus?

Chronic stress produces consistent and reversible changes within the dendritic arbors of CA3 hippocampal neurons, characterized by decreased dendritic length and reduced branch number. This chronic stress-induced dendritic retraction has traditionally corresponded to hippocampus-dependent spatial memory deficits. However, anomalous findings have raised doubts as to whether a CA3 dendritic retraction is sufficient to compromise hippocampal function. The purpose of this review is to outline the mechanism underlying chronic stress-induced CA3 dendritic retraction and to explain why CA3 dendritic retraction has been thought to mediate spatial memory. The anomalous findings provide support for a modified hypothesis, in which chronic stress is proposed to induce CA3 dendritic retraction, which then disrupts hypothalamic-pituitary-adrenal axis activity, leading to dysregulated glucocorticoid release. The combination of hippocampal CA3 dendritic retraction and elevated glucocorticoid release contributes to impaired spatial memory. These findings are presented in the context of clinical conditions associated with elevated glucocorticoids.

Delayed non-matching to position performance in aged hybrid Fischer 344 x brown Norway rats: a longitudinal study

In this study, the effects of aging on the performance in a delayed non-matching to position (DNMTP) task were investigated longitudinally in hybrid Fischer 344 x Brown Norway rats. The rats were first trained to perform the task. Subsequently, their performance was assessed monthly from 28 to 34 months of age. The measures of responding on the DNMTP schedule did not decrease in the course of the study. After the last DNMTP test, choline acetyltransferase (ChAT) activity and glial fibrillary acidic protein (GFAP) content were measured in frontal cortex and hippocampus. We found that higher levels of GFAP in the frontal cortex, but not hippocampus, were associated with a poorer performance in the DNMTP task. Our findings support the notion that repeated testing prevents the age-related decline in cognitive functions that has been reported in cross-sectional studies. Pathology of the frontal cortex seems to predict a faster rate of forgetting in aging rats.

The effects of rAAV2-mediated NGF gene delivery in adult and aged rats

Nerve growth factor (NGF) therapy has been proposed to treat patients with age-related cognitive deficits, including those with Alzheimer's disease. One promising approach to delivering this protein into brain involves viral vectors. However, little is known about the effects of aging on gene transfer in brain generally and in particular its effect on transgenic NGF expression. To examine the transgene expression and biological effects of NGF gene transfer in adult and aged rats, we delivered mouse NGF with C-terminal myc-tag, using a recombinant adeno-associated virus serotype 2 (rAAV2) vector, into the septum of 6- and 21-month-old Fischer 344/Brown Norway hybrid rats. Other animals received a control vector encoding green fluorescent protein. As expected, this strain of rat demonstrated very few age-related deficits in spatial memory-related behavior in the Morris water task either before gene transfer (6 vs 21 months) or afterward (up to 11 vs 26 months). We found that rAAV2 vectors drove transgene expression in aged rats up to 5 months, although the level of transgene expression was lower than that of adult animals. We also showed that NGF gene transfer into the septum of aged animals induced local trophic effects by increasing the number and soma area of septal cholinergic neurons and improved distal synaptic activity by increasing the level of depolarization-induced acetylcholine (ACh) release from hippocampal synaptic terminals. Interestingly, NGF gene transfer suppressed depolarization-induced ACh release in adult rats. These findings show for the first time, to our knowledge, that septal NGF gene transfer modulates hippocampal nerve terminal function. These results are relevant for the potential clinical application of NGF gene therapy.

Omega-3 fatty acid ethyl-eicosapentaenoate, but not soybean oil, attenuates memory impairment induced by central IL-1β administration

Proinflammatory cytokine interleukin (IL)-1beta can cause cognitive impairment, activate the hypothalamic-pituitary-adrenal axis and impair monoaminergic neurotransmission in the rat. IL-1beta has also been shown to increase the concentration of the inflammatory mediator prostaglandin E2 (PGE2) in the blood. Omega (n)-3 fatty acids, such as eicosapentaenoic acid (EPA), which are components of fish oil, have been shown to reduce both the proinflammatory cytokines and the synthesis of PGE2. The purpose of this study was to determine whether dietary supplements of EPA would attenuate the inflammation-induced impairment of spatial memory by centrally administered IL-1beta. Rats were fed with a diet of coconut oil (contained a negligible quantity of fatty acids), soybean oil (contained mainly n-6 fatty acids), or a diet of coconut oil enriched with ethyl-EPA (E-EPA). The rats were then injected intracerebroventricularly with IL-1beta or saline. The results of this study demonstrated that the IL-1-induced deficit in spatial memory was correlated with an impairment of central noradrenergic and serotonergic (but not dopaminergic) function and an increase in the serum corticosterone concentration. IL-1beta also caused an increase in the hippocampal PGE2 concentration. These effects of IL-1 were attenuated by the chronic administration of E-EPA. By contrast, rats fed with the soybean oil diet showed no effect on the changes induced by the IL-1 administration.

Chronic administration of corticosterone impairs spatial reference memory before spatial working memory in rats

Corticosterone (CORT), the predominant glucocorticoid in rodents, elevated for 21 days damages hippocampal subregion CA3. We tested the hypothesis that CORT would impair spatial memory, a hippocampal function. In each of the three experiments, rats received daily, subcutaneous injections of either CORT (26.8 mg/kg body weight in sesame oil) or sesame oil vehicle alone (VEH). CORT given for 21 or 56 days effectively attenuated body weight gain and reduced selective organ and muscle weights. All behavioral testing was done on tasks that are minimally stressful and avoid deprivation. For each experiment, testing commenced 24h after the last injection. CORT given for 21 days did not impair spatial working memory in the Y-maze (Experiments 1 and 2). After 56-day administration of CORT, spatial working memory was impaired in the Y-maze (Experiment 2). CORT given for 21 days also failed to impair spatial working memory in the Barnes maze (Experiment 3). However, in trials that depended solely on reference memory, the VEH group improved in performance, whereas the CORT group did not. In conclusion, CORT elevated over a period of 21 days did not impair spatial working memory, but impaired the formation of a longer-term form of memory, most likely reference memory. Impairments in spatial working memory are seen only after longer durations of CORT administration.

Assessment of age-associated cognitive deficits in rats: a tricky business

Every living organism is affected by changes as a consequence of aging. Perhaps the most appropriate concept to describe age-related changes is that of 'functional age'. Laboratory rodents are especially suited as models of cognitive aging in humans, because they have a relatively short life-span and because many tests have been developed to investigate their cognitive performance. Examples from studies using the Morris water escape task were chosen to describe and discuss factors which must be considered before drawing conclusions about age-related cognitive deficits. In particular, the roles of rearing and housing conditions, of sensorimotor impairments, and of motivational differences between young and old rats are discussed. Conclusions are drawn about how aging studies should be conceived and performed.

Prenatal ethanol exposure and spatial navigation: effects of postnatal handling and aging

Prenatal ethanol exposure results in spatial navigation deficits in young and mid-aged animals. In contrast, postnatal handling attenuates spatial deficits that emerge with age in animals that are not handled. Therefore, we investigated the ability of handling to attenuate spatial deficits in animals prenatally exposed to ethanol (E). Sprague-Dawley male offspring from E, pair-fed (PF), and control (C) groups were handled (H) or nonhandled (NH) from 1 to 15 days of age and tested on the Morris water maze at 2 or 13 to 14 months of age. In young animals, H-E males had longer latencies to locate the submerged platform, and E animals, across handling conditions, showed altered search patterns compared to their PF and C counterparts. Mid-aged animals had longer latencies than young animals, with no differences among E, PF, and C animals. However, corticosterone levels were higher in mid-aged E than in C males. Handling did not attenuate impairments associated with either prenatal ethanol exposure or aging.

Aging facilitates long-trace taste-aversion conditioning in rats

In order to examine age-related changes in long-trace conditioning, five age groups (0.25, 1, 1.5, 2, and 2.5 years) of Wistar-derived female albino rats were subjected to taste-aversion conditioning at one of five conditioned stimulus-conditioned stimulus (CS-US) intervals (0, 45, 90, 180, and 360 min). Age differences in the strength of the aversion were evident at CS-US intervals greater than 0 min and the strength of the aversion was directly related to age. An aversion was conditioned in only the two oldest age groups when the CS-US interval was 360 min. The age differences in taste-aversion and the superior long-trace conditioning in old-age rats were attributed to factors that accompany aging, for example, the gradual slowing down of a metabolic pacemaker.