Extensive enriched environments protect old rats from the aging dependent impairment of spatial cognition, synaptic plasticity and nitric oxide production

MSK1 is required for the beneficial synaptic and cognitive effects of enriched experience across the lifespan

Positive experiences, such as social interaction, cognitive training and physical exercise, have been shown to ameliorate some of the harms to cognition associated with ageing. Animal models of positive interventions, commonly known as environmental enrichment, strongly influence neuronal morphology and synaptic function and enhance cognitive performance. While the profound structural and functional benefits of enrichment have been appreciated for decades, little is known as to how the environment influences neurons to respond and adapt to these positive sensory experiences. We show that adult and aged male wild-type mice that underwent a 10-week environmental enrichment protocol demonstrated improved performance in a variety of behavioural tasks, including those testing spatial working and spatial reference memory, and an enhancement in hippocampal LTP. Aged animals in particular benefitted from enrichment, performing spatial memory tasks at levels similar to healthy adult mice. Many of these benefits, including in gene expression, were absent in mice with a mutation in an enzyme, MSK1, which is activated by BDNF, a growth factor implicated in rodent and human cognition. We conclude that enrichment is beneficial across the lifespan and that MSK1 is required for the full extent of these experience-induced improvements of cognitive abilities, synaptic plasticity and gene expression.

Long-term environmental enrichment overcomes depression, learning, and memory impairment in elderly CD-1 mice with maternal sleep deprivation exposure

Early-life stress disrupts central nervous system development and increases the risk of neuropsychiatric disorder in offspring based on rodent studies. Maternal sleep deprivation (MSD) in rodents has also been associated with depression and cognitive decline in adult offspring. However, it is not known whether these issues persist into old age. Environmental enrichment is a non-pharmacological intervention with proven benefits in improving depression and cognitive impairment; however, it is unclear whether these benefits hold for aging mice following MSD exposure. The aim of this study was to explore the effects of MSD on depression and cognition in elderly offspring CD-1 mice and to determine whether long-term environmental enrichment could alleviate these effects by improving neuroinflammation and synaptic plasticity. The offspring mice subjected to MSD were randomly assigned to either a standard environment or an enriched environment. At 18 months of age, the forced swimming and tail suspension tests were used to evaluated depression-like behaviors, and the Morris water maze test was used to evaluate cognitive function. The expression levels of hippocampal proinflammatory cytokines and synaptic plasticity-associated proteins were also measured. MSD increased depression-like behaviors and impaired cognition function in aging CD-1 offspring mice. These effects were accompanied by upregulated interleukin (IL)-1β, IL-6, and tumor necrosis factor-α expression, and downregulated brain-derived neurotrophic factor, tyrosine kinase receptor B, postsynaptic density-95, and synaptophysin expression in the hippocampus. All of these changes were reversed by long-term exposure to an enriched environment. These findings suggest that MSD exerts long-term effects on the behaviors of offspring in mice, leading to depression and cognitive impairment in older age. Importantly, long-term environmental enrichment could counteract the behavior difficulties induced by MSD through improving hippocampal proinflammatory cytokines and synaptic plasticity-associated proteins.

Environmental enrichment and the aging brain: is it time for standardization?

Aging entails a progressive decline of cognitive abilities. However, since the brain is endowed with considerable plasticity, adequate stimulation can delay or partially compensate for age-related structural and functional impairment. Environmental enrichment (EE) has been reported to determine a wide range of cerebral changes. Although most findings have been obtained in young and adult animals, research has recently turned to aged individuals. Notably, EE can contribute identifying key lifestyle factors whose change can help extend the "mind-span", i.e., the time an individual lives in a healthy cognitive condition. Here we discuss specific methodological issues that can affect the outcomes of EE interventions applied to aged rodents, summarize the main variables that would need standardization (e.g., timing and duration, enrichment items, control animals and setting), and offer some suggestions on how this goal may be achieved. Reaching a consensus on EE experiment design would significantly reduce differences between and within laboratories, enable constructive discussions among researchers, and improve data interpretation.

Effects of early noise exposure on hippocampal-dependent behaviors during adolescence in male rats: influence of different housing conditions

Central nervous system (CNS) development is a very complex process that can be altered by environmental stimuli such as noise, which can generate long-term auditory and/or extra-auditory impairments. We have previously reported that early noise exposure can induce hippocampus-related behavioral alterations in postnatal day (PND) 28 adolescent rats. Furthermore, we recently found biochemical modifications in the hippocampus (HC) of these animals that seemed to endure even in more mature animals (i.e. PND35) and that have not been studied along with behavioral correlates. Thus, the aim of this work was to reveal novel data about the effects of early noise exposure on hippocampal-dependent behaviors in more mature animals. Additionally, extended enriched environment (EE) housing was evaluated to determine its capacity to induce behavioral modifications, either by its neuroprotective ability or the greater stimulation that it generates. Male Wistar rats were exposed to different noise schemes at PND7 or PND15. Upon weaning, some animals were transferred to EE whereas others were kept in standard cages. At PND35, different hippocampal-dependent behavioral assessments were performed. Results showed noise-induced behavioral changes that differed according to the scheme and age of exposure used. In addition, housing in an EE was effective either in preventing some of these changes or in inducing the appearance of new behavioral modifications. These findings suggest that CNS development would be sensitive to the effects of different type of environmental stimuli such as noise or enriched housing, leading to maladaptive behavioral changes that last even until adolescence.

Noise-induced hippocampal oxidative imbalance and aminoacidergic neurotransmitters alterations in developing male rats: Influence of enriched environment during adolescence

Living in big cities might involuntarily expose people to high levels of noise causing auditory and/or extra-auditory impairments, including adverse effects on central nervous system (CNS) areas such as the hippocampus. In particular, CNS development is a very complex process that can be altered by environmental stimuli. We have previously shown that noise exposure of developing rats can induce hippocampal-related behavioral alterations. However, noise-induced biochemical alterations had not been studied yet. Thus, the aim of this work was to assess whether early noise exposure can affect rat hippocampal oxidative state and aminoacidergic neurotransmission tone. Additionally, the effectiveness of an enriched environment (EE) as a neuroprotective strategy was evaluated. Male Wistar rats were exposed to different noise schemes at 7 or 15 days after birth. Upon weaning, some animals were transferred to an EE whereas others were kept in standard cages. Short- and long-term measurements were performed to evaluate reactive oxygen species, thioredoxins levels and catalase activity as indicators of hippocampal oxidative status as well as glutamic acid decarboxylase and a subtype of glutamate transporter to evaluate aminoacidergic neurotransmission tone. Results showed noise-induced changes in hippocampal oxidative state and aminoacidergic neurotransmission markers that lasted until adolescence and differed according to the scheme and the age of exposure. Finally, EE housing was effective in preventing some of these changes. These findings suggest that CNS development seems to be sensitive to the effects of stressors such as noise, as well as those of an environmental stimulation, favoring prompt and lasting molecular changes.

Exposure of Developing Male Rats to One or Multiple Noise Sessions and Different Housing Conditions: Hippocampal Thioredoxin Changes and Behavioral Alterations

Exposure of developing rats to noise has shown to induce hippocampal-related behavioral alterations that were prevented after a week of housing in an enriched environment. However, neither the effect of repeated exposures nor its impact on key endogenous antioxidants had been studied yet. Thus, the aim of the present work was to reveal novel data about hippocampal oxidative state through the measurement of possible age-related differences in the levels of hippocampal thioredoxins in rats exposed to noise at different developmental ages and subjected to different schemes and housing conditions. In addition, the possibility that oxidative changes could underlie hippocampal-related behavioral changes was also analyzed. Developing male Wistar rats were exposed to noise for 2 h, either once or for 5 days. Upon weaning, some animals were transferred to an enriched cage for 1 week, whereas others were kept in standard cages. One week later, auditory and behavioral assessments, as well as measurement of hippocampal thioredoxin, were performed. Whereas no changes in the auditory function were observed, significant behavioral differences were found, that varied according to the age, scheme of exposure and housing condition. In addition, a significant increase in Trx-1 levels was found in all noise-exposed groups housed in standard cages. Housing animals in an enriched environment for 1 week was effective in preventing most of these changes. These findings suggest that animals become less susceptible to undergo behavioral alterations after repeated exposure to an environmental challenge, probably due to the ability of adaptation to an unfavorable condition. Moreover, it could be hypothesized that damage to younger individuals could be more easily prevented by a housing manipulation.

The low affinity neurotensin receptor antagonist levocabastine impairs brain nitric oxide synthesis and mitochondrial function by independent mechanisms

Neurotensin is known to inhibit neuronal Na⁺ , K⁺ -ATPase, an effect that is rescued by nitric oxide (NO) synthase inhibition. However, whether the neurotensinergic and the nitrergic systems are independent pathways, or are mechanistically linked, remains unknown. Here, we addressed this issue and found that the administration of low affinity neurotensin receptor (NTS2) antagonist, levocabastine (50 μg/kg, i.p.) inhibited NO synthase (NOS) activity by 74 and 42% after 18 h in synaptosomal and mitochondrial fractions isolated from the Wistar rat cerebral cortex, respectively; these effects disappeared 36 h after levocabastine treatment. Intriguingly, whereas neuronal NOS protein abundance decreased (by 56%) in synaptosomes membranes, it was enhanced (by 86%) in mitochondria 18 h after levocabastine administration. Levocabastine enhanced the respiratory rate of synaptosomes in the presence of oligomycin, but it failed to alter the spare respiratory capacity; furthermore, the mitochondrial respiratory chain (MRC) complexes I-IV activities were severely diminished by levocabastine administration. The inhibition of NOS and MRC complexes activities were also observed after incubation of synaptosomes and mitochondria with levocabastine (1 μM) in vitro. These data indicate that the NTS2 antagonist levocabastine regulates NOS expression and activity at the synapse, suggesting an interrelationship between the neurotensinergic and the nitrergic systems. However, the bioenergetics effects of NTS2 activity inhibition are likely to be independent from the regulation of NO synthesis.

Role of early environmental enrichment on the social dominance tube test at adulthood in the rat

RATIONALE

Environmental enrichment (EE) could influence brain plasticity and behavior in rodents. Whether the early EE may predispose individuals to a particular social hierarchy in the social dominance tube test (SDTT) at adulthood is still unknown.

OBJECTIVE

The present study directly investigated the influence of EE on competitive success in the SDTT among adult rats.

METHODS

Male rats were maintained in EE from postnatal days 21 to 35. Social dominance behavior was determined by SDTT, competitive food foraging test, and mate preference test at adulthood. IBA-1 expression in the hypothalamus was examined using immunohistochemistry and western blot.

RESULTS

EE rats were prone to become submissive during a social encounter with standard environment (SE) rats in the SDTT. No difference was found in food foraging in the competitive food foraging test between SE and EE rats. Male EE rats were more attractive than the SE to the female rats in the mate preference test. IBA-1 expression was found to be decreased in the hypothalamus of EE rats compared to SE group. Infusion of a microglia inhibitor reduced percentage of forward in SE rats in the SDTT. Infusion of DNA methyltransferase inhibitor prevented the development of subordinate status in EE rats and restored the expression of IBA-1 in the hypothalamus.

CONCLUSIONS

The results suggest that early EE did not lead to reduced social hierarchy in the male rat. However, EE caused a reduction in the percentage of forward in the SDTT, which might be associated with reduced number of microglia in the hypothalamus.

What success can teach us about failure: the plasma metabolome of older adults with superior memory and lessons for Alzheimer's disease

As the world population ages, primary prevention of age-related cognitive decline and disability will become increasingly important. Prevention strategies are often developed from an understanding of disease pathobiology, but models of biological success may provide additional useful insights. Here, we studied 224 older adults, some with superior memory performance (n = 41), some with normal memory performance (n = 109), and some with mild cognitive impairment or Alzheimer's disease (AD; n = 74) to understand metabolomic differences which might inform future interventions to promote cognitive health. Plasma metabolomics revealed significant differential abundance of 12 metabolites in those with superior memory relative to controls (receiver operating characteristic area under the curve [AUC] = 0.89) and the inverse abundance pattern in the mild cognitive impairment, AD (AUC = 1.0) and even preclinical AD groups relative to controls (AUC = 0.97). The 12 metabolites are components of key metabolic pathways regulating oxidative stress, inflammation, and nitric oxide bioavailability. These findings from opposite ends of the cognitive continuum highlight the role of these pathways in superior memory abilities and whose failure may contribute to age-related memory impairment. These pathways may be targeted to promote successful cognitive aging.

Noise exposure of immature rats can induce different age-dependent extra-auditory alterations that can be partially restored by rearing animals in an enriched environment

It has been previously shown that different extra-auditory alterations can be induced in animals exposed to noise at 15 days. However, data regarding exposure of younger animals, that do not have a functional auditory system, have not been obtained yet. Besides, the possibility to find a helpful strategy to restore these changes has not been explored so far. Therefore, the aims of the present work were to test age-related differences in diverse hippocampal-dependent behavioral measurements that might be affected in noise-exposed rats, as well as to evaluate the effectiveness of a potential neuroprotective strategy, the enriched environment (EE), on noise-induced behavioral alterations. Male Wistar rats of 7 and 15 days were exposed to moderate levels of noise for two hours. At weaning, animals were separated and reared either in standard or in EE cages for one week. At 28 days of age, different hippocampal-dependent behavioral assessments were performed. Results show that rats exposed to noise at 7 and 15 days were differentially affected. Moreover, EE was effective in restoring all altered variables when animals were exposed at 7 days, while a few were restored in rats exposed at 15 days. The present findings suggest that noise exposure was capable to trigger significant hippocampal-related behavioral alterations that were differentially affected, depending on the age of exposure. In addition, it could be proposed that hearing structures did not seem to be necessarily involved in the generation of noise-induced hippocampal-related behaviors, as they were observed even in animals with an immature auditory pathway. Finally, it could be hypothesized that the differential restoration achieved by EE rearing might also depend on the degree of maturation at the time of exposure and the variable evaluated, being younger animals more susceptible to environmental manipulations.

Alcohol hangover induces mitochondrial dysfunction and free radical production in mouse cerebellum

Alcohol hangover (AH) is defined as the temporary state after alcohol binge-like drinking, starting when ethanol (EtOH) is absent in plasma. Previous data indicate that AH induces mitochondrial dysfunction and free radical production in mouse brain cortex. The aim of this work was to study mitochondrial function and reactive oxygen species production in mouse cerebellum at the onset of AH. Male mice received a single i.p. injection of EtOH (3.8g/kg BW) or saline solution. Mitochondrial function was evaluated 6h after injection (AH onset). At the onset of AH, malate-glutamate and succinate-supported state 4 oxygen uptake was 2.3 and 1.9-fold increased leading to a reduction in respiratory control of 55% and 48% respectively, as compared with controls. Decreases of 38% and 16% were found in Complex I-III and IV activities. Complex II-III activity was not affected by AH. Mitochondrial membrane potential and mitochondrial permeability changes were evaluated by flow cytometry. Mitochondrial membrane potential and permeability were decreased by AH in cerebellum mitochondria. Together with this, AH induced a 25% increase in superoxide anion and a 92% increase in hydrogen peroxide production in cerebellum mitochondria. Related to nitric oxide (NO) metabolism, neuronal nitric oxide synthase (nNOS) protein expression was 52% decreased by the hangover condition compared with control group. No differences were found in cerebellum NO production between control and treated mice. The present work demonstrates that the physiopathological state of AH involves mitochondrial dysfunction in mouse cerebellum showing the long-lasting effects of acute EtOH exposure in the central nervous system.

Life-long environmental enrichment counteracts spatial learning, reference and working memory deficits in middle-aged rats subjected to perinatal asphyxia

Continuous environmental stimulation induced by exposure to enriched environment (EE) has yielded cognitive benefits in different models of brain injury. Perinatal asphyxia results from a lack of oxygen supply to the fetus and is associated with long-lasting neurological deficits. However, the effects of EE in middle-aged rats suffering perinatal asphyxia are unknown. Therefore, the aim of the present study was to assess whether life-long exposure to EE could counteract the cognitive and behavioral alterations in middle-aged asphyctic rats. Experimental groups consisted of rats born vaginally (CTL), by cesarean section (C+), or by C+ following 19 min of asphyxia at birth (PA). At weaning, rats were assigned to standard (SE) or enriched environment (EE) for 18 months. During the last month of housing, animals were submitted to a behavioral test battery including Elevated Plus Maze, Open Field, Novel Object Recognition and Morris water maze (MWM). Results showed that middle-aged asphyctic rats, reared in SE, exhibited an impaired performance in the spatial reference and working memory versions of the MWM. EE was able to counteract these cognitive impairments. Moreover, EE improved the spatial learning performance of middle-aged CTL and C+ rats. On the other hand, all groups reared in SE did not differ in locomotor activity and anxiety levels, while EE reduced locomotion and anxiety, regardless of birth condition. Recognition memory was altered neither by birth condition nor by housing environment. These results support the importance of environmental stimulation across the lifespan to prevent cognitive deficits induced by perinatal asphyxia.

Promoting our understanding of neural plasticity by exploring developmental plasticity in early and adult life

Developmental plasticity (DP) is widely considered to be a property of early life stages, but evidence suggests it can be reactivated in mature brains. For example, recent developments on animal models suggest that experience in enriched environments (EE) can induce DP and enable adult recovery from amblyopia; even when the typical critical period for that recovery has closed. An interesting body of evidence suggests that extrapolation of the rejuvenatory power of that paradigm in mature human brains is feasible. These studies show that exposure to EE throughout life is associated with a delay, or even prevention, of age-related cognitive deficits. Consequently, it can be concluded that DP might underlie the neuroprotective effects against a neurocognitive breakdown that have been observed, and that EE exposure later in life might induce DP in a similar way to early EE exposure. Thus, the DP might exert its influence beyond the typical developing age ranges: childhood and adolescence. Although further research is still required, the observation of EE related neuroprotective effects are a breakthrough in the study of DP in humans and new advances in our understanding of neural plasticity have thus been reached.

Effects of intrahippocampal L-NAME treatment on the behavioral long-term potentiation in dentate gyrus

Using a combination of electrophysiological recordings, behavioral tests and local pharmacological administration in hippocampus, we investigated in the present study the effects of nitric oxide (NO) synthase inhibitor N-nitro-l-arginine methyl ester (l-NAME) on the behavioral long-term potentiation (LTP) and maze learning performance in freely moving rats. The results showed as follows: (1) intrahippocampal l-NAME administration led to a defect in maze learning performance of the animals; (2) l-NAME treatment also substantially impaired the induction of the behavioral LTP in perforant pathway to dentate gyrus (PP-DG) pathway induced by maze learning task, while it had no significant effects on basic synaptic transmission in PP-DG pathway; Collectively, these results indicate that NO synthesis may be critical for the behavioral LTP in PP-DG pathway and maze learning performance.

Environmental enrichment restores neurogenesis and rapid acquisition in aged rats

Strategies combatting cognitive decline among the growing aging population are vital. We tested whether environmental enrichment could reverse age-impaired rapid spatial search strategy acquisition concomitantly with hippocampal neurogenesis in rats. Young (5-8 months) and aged (20-22 months) male Fischer 344 rats were pair-housed and exposed to environmental enrichment (n = 7 young, 9 aged) or housed individually (n = 7 young, 7 aged) for 10 weeks. After 5 weeks, hidden platform trials (5 blocks of 3 trials; 15 m inter-block interval), a probe trial, and then visible platform trials (5 blocks of 3 trials; 15 m inter-block interval) commenced in the water maze. One week after testing, rats were given 5 daily intraperitoneal bromodeoxyuridine (50 mg/kg) injections and perfused 4 weeks later to quantify neurogenesis. Although young rats outperformed aged rats, aged enriched rats outperformed aged individually housed rats on all behavioral measures. Neurogenesis decreased with age but enrichment enhanced new cell survival, regardless of age. The novel correlation between new neuron number and behavioral measures obtained in a rapid water maze task among aged rats, suggests that environmental enrichment increases their ability to rapidly acquire and flexibly use spatial information along with neurogenesis.

Enriched environment and white matter in aging brain

Normal aging is commonly associated with decreased cognitive functions, which could be conspicuously alleviated by enriched environment (EE) with physical, social, and sensory stimuli, suggesting that aging brain still has intriguing plasticity. Multiple researches have been carried out to explore the structural and the molecular changes in aging brain, which would be considered for evidences that EE regulated brain plasticity. Because there is no significant neuron loss in aging cerebral cortex and the white matter is crucial for cognitive functions, this review focused on the age-related white matter changes and the effects of EE on aged white matter. Data from our stereology laboratory revealed that age-related spatial memory declines had more to do with white matter alterations, which were due to marked demyelination and loss of oligodendrocytes in the white matter. We also demonstrated that EE recovered spatial memory impairment and increased white matter volume by promoting marked remyelination in aged brain. This review approached the issue that EE might contribute to normal aging and be beneficial for those suffering from demyelinated diseases.

Short-term enrichment makes male rats more attractive, more defensive and alters hypothalamic neurons

Innate behaviors are shaped by contingencies built during evolutionary history. On the other hand, environmental stimuli play a significant role in shaping behavior. In particular, a short period of environmental enrichment can enhance cognitive behavior, modify effects of stress on learned behaviors and induce brain plasticity. It is unclear if modulation by environment can extend to innate behaviors which are preserved by intense selection pressure. In the present report we investigate this issue by studying effects of relatively short (14-days) environmental enrichment on two prominent innate behaviors in rats, avoidance of predator odors and ability of males to attract mates. We show that enrichment has strong effects on both the innate behaviors: a) enriched males were more avoidant of a predator odor than non-enriched controls, and had a greater rise in corticosterone levels in response to the odor; and b) had higher testosterone levels and were more attractive to females. Additionally, we demonstrate decrease in dendritic length of neurons of ventrolateral nucleus of hypothalamus, important for reproductive mate-choice and increase in the same in dorsomedial nucleus, important for defensive behavior. Thus, behavioral and hormonal observations provide evidence that a short period of environmental manipulation can alter innate behaviors, providing a good example of gene-environment interaction.

Aging well and the environment: toward an integrative model and research agenda for the future

PURPOSE OF THE STUDY

The effects of the physical-spatial-technical environment on aging well have been overlooked both conceptually and empirically. In the spirit of M. Powell Lawton's seminal work on aging and environment, this article attempts to rectify this situation by suggesting a new model of how older people interact with their environment.

DESIGN AND METHODS

Goals of the paper include (a) integration of the essential elements of the ecology and aging literature, particularly in regard to Lawton's research, (b) development of connections between traditional theories of ecology of aging and life span developmental models of aging well, (c) acknowledgment of the pronounced historical and cohort-related changes affecting the interactions of older people with their environment, and (d) discussion of the implications of this analysis for concepts and theories of aging well.

RESULTS

The model builds on a pair of concepts: environment as related to agency and belonging, founded in motivational psychology, and developmental science.

IMPLICATIONS

After describing the model's key components, we discuss its heuristic potential in four propositions for future gerontological research and identify implications of the model for future empirical research.

Age-related deficits in spatial memory and hippocampal spines in virgin, female Fischer 344 rats

Effects of aging on memory and brain morphology were examined in aged, 21-month-old, and young, 4-month-old, Fischer 344 female rats. Spatial memory was assessed using the object placement task, and dendritic spine density was determined on pyramidal neurons in the hippocampus following Golgi impregnation. Consistent with previous studies, aged females showed poorer object placement performance than young subjects. Young subjects significantly discriminated the location of objects with a 1.5-hour intertrial delay while aged subjects did not. Spine density of basal dendrites on CA1 pyramidal cells was 16% lower in the aged subjects as compared to the young subjects. No differences in spine density were found between young and aged subjects in basal dendrites of CA1 or in either dendritic field of CA3 pyramidal neurons. Thus, decreased hippocampal CA1 dendritic spine density in aged rats may contribute to poorer spatial memory as compared to young rats. The possibility that the neuroplastic changes observed in this study may pertain only to female subjects having had a specific set of life experiences is discussed. Different factors, such as reproductive status, diet, and handling may contribute to neuroplasticity of the brain during aging; however, this view requires further examination.

Influence of late-life exposure to environmental enrichment or exercise on hippocampal function and CA1 senescent physiology

Aged (20-22 months) male Fischer 344 rats were randomly assigned to sedentary (A-SED), environmentally-enriched (A-ENR), or exercise (A-EX) conditions. After 10-12 weeks of differential experience, the 3 groups of aged rats and young sedentary controls were tested for physical and cognitive function. Spatial discrimination learning and memory consolidation, tested on the water maze, were enhanced in environmentally-enriched compared with sedentary. A-EX exhibited improved and impaired performance on the cue and spatial task, respectively. Impaired spatial learning in A-EX was likely due to a bias in response selection associated with exercise training, as object recognition memory improved for A-EX rats. An examination of senescent hippocampal physiology revealed that enrichment and exercise reversed age-related changes in long-term depression (LTD) and long-term potentiation (LTP). Rats in the enrichment group exhibited an increase in cell excitability compared with the other 2 groups of aged animals. The results indicate that differential experience biased the selection of a spatial or a response strategy and factors common across the 2 conditions, such as increased hippocampal activity associated with locomotion, contribute to reversal of senescent synaptic plasticity.

Age-related alterations in mitochondrial physiological parameters and nitric oxide production in synaptic and non-synaptic brain cortex mitochondria

Brain aging has been associated with mitochondrial dysfunction and changes in nitric oxide levels. The aim of this study was to evaluate the susceptibility of synaptic and non-synaptic mitochondria to aging-dependent dysfunction. State 3 respiratory rate and respiratory control were 43% and 33% decreased, respectively in brain cortex synaptosomes from 14-month-old animals, as compared with synaptosomes from 3-month-old mice. Respiratory rates were not significantly affected by aging in non-synaptic mitochondrial fractions. Mitochondrial dysfunction was associated with increases of 84% and 38% in H₂O₂ production rates in brain cortex synaptosomes and non-synaptic mitochondria, respectively, from 14-month-old mice, as compared with young animals. Synaptic mitochondria seem to be more susceptible to calcium insult in 14-month-old mice, as compared with non-synaptic mitochondria, as measured by response of both types of fractions to calcium-induced depolarization. With aging, nitric oxide (NO) production was 44% and 27% decreased both in synaptosomal and non-synaptic mitochondrial fractions, respectively. The results of this study suggest that with aging, mitochondrial function at the nerve terminals would be more susceptible to suffer alterations by the constant calcium changes occurring as a consequence of synaptic activity. Non-synaptic mitochondria would be more resistant to age-related dysfunction and oxidative damage.

Continuous enriched environment improves learning and memory in adult NMRI mice through theta burst-related-LTP independent mechanisms but is not efficient in advanced aged animals

INTRODUCTION

Effects of 3-month continuous environmental enrichment (EE) on cognitive abilities and on theta burst-related synaptic plasticity of CA1 hippocampal neuronal networks have been assessed in 6- and 20-month old NMRI female mice.

RESULTS

EE decreased anxiety-like behavior and improved learning and memory performances in adult but not in aged mice. Electrophysiological results in CA1 hippocampal slices showed that basal synaptic transmission was not affected by EE in adult mice whereas it was partially improved in aged animals, even though not sufficient to rescue the decrease related to aging. Besides, no effect of EE on N-methyl-d-aspartate receptor activation and theta-burst-induced long-term potentiation was found in adult or aged animals.

DISCUSSION

These results indicate that continuous EE is able to improve cognitive abilities in adult NMRI female mice, that does not correlate with changes in theta burst-related synaptic plasticity within neuronal networks. In addition, the lack of effects in aged animals suggests the existence of a critical delay for the beneficial effects of EE on cognitive aging.

The impact of environmental enrichment in laboratory rats--behavioural and neurochemical aspects

The provision of environmental enrichment (EE) for laboratory rats is recommended in European guidelines governing laboratory animal welfare. It is believed the EE implementation can improve animals' well-being and EE has been used to demonstrate learning and plasticity of the brain in response to the environment. This review suggests that the definition and duration of EE varies considerably across laboratories. Notwithstanding this, some EE protocols have revealed profound effects on brain neurochemistry and resulting behaviour, suggesting that EE can have the potential to significantly modify these parameters in rats. For this review, a literature search was conducted using PubMed and the search terms "Environmental Enrichment" and "rats". From the results of this search the most important variables for consideration in the implementation of EE are identified and summarised, and include cage size and housing density; rat age, sex and strain; duration of EE; the EE protocol and enrichment items employed; and the use of appropriate controls. The effects of EE in a number of behavioural tests and its effects on neurotransmitters, neurotrophic factors, stress hormones and neurogenesis and proliferation are outlined. The findings summarised in the present review show the range of EE protocols employed and their effects in tests of activity, learning and affect, as well neurochemical effects which mediate enhanced plasticity in the brain. EE, as is provided in many laboratories, may be of benefit to the animals, however it is important that future work aims to provide a better understanding of EE effects on research outcomes.

Age-related declines in exploratory behavior and markers of hippocampal plasticity are attenuated by prenatal choline supplementation in rats

Supplemental choline in the maternal diet produces a lasting enhancement in memory in offspring that resists age-related decline and is accompanied by neuroanatomical, neurophysiological and neurochemical changes in the hippocampus. The present study was designed to examine: 1) if prenatal choline supplementation alters behaviors that contribute to risk or resilience in cognitive aging, and 2) whether, at old age (25 months), prenatally choline-supplemented rats show evidence of preserved hippocampal plasticity. A longitudinal design was used to look at exploration of an open field, with and without objects, at 1 and 24 months of age in male and female rats whose mothers were fed a diet supplemented with choline (SUP; 5 mg/kg choline chloride) or not supplemented (CON; 1.1 mg/kg choline chloride) on embryonic days 12-17. Aging caused a significant decline in open field exploration that was more pronounced in males but interest in novel objects was maintained in both sexes. Prenatal choline supplementation attenuated, but did not prevent age-related decline in exploration in males and increased object exploration in young females. Following behavioral assessment, rats were euthanized to assess markers of hippocampal plasticity. Aged SUP males and females had more newly proliferated cells in the hippocampal dentate gyrus and protein levels of vascular endothelial growth factor (VEGF) and neurotrophin-3 (NT-3) were significantly elevated in female SUP rats in comparison to all other groups. Taken together, these findings provide the first evidence that prenatal choline supplementation causes changes in exploratory behaviors over the lifespan and preserves some features of hippocampal plasticity that can be seen even at 2 years of age.

Exposure to enriched environments increases brain nitric oxide synthase and improves cognitive performance in prepubertal but not in young rats

Rats were randomly assigned to enriched (EE) or standard environments (SE) at 21 or 73 days of age, for 17 days. Half of the rats of each rearing condition were trained in a radial maze (RM). At 38 days (pre-pubertal) or 90 days (young), rats were sacrificed and brain cytosolic and mitochondrial nitric oxide synthase (mtNOS) activity was assayed. Western blot analysis of brain mtNOS was conducted. In the pre-pubertal group, EE rats improved their performance in the RM while SE rats did not. In the young group, SE and EE rats showed a random performance in the RM. In SE pre-pubertal rats, training increased brain cytosolic NOS and mtNOS activity by 68% and 82%. In EE non-trained pre-pubertal rats, brain cytosolic NOS and mtNOS activity increased by 80% and 60%, as compared with SE non-trained pre-pubertal rats. In EE pre-pubertal rats that were trained, brain cytosolic NOS and mtNOS activity increased by 70% and 90%, as compared with SE pre-pubertal rats that were not trained. A higher protein expression of brain mtNOS was found in EE rats, as compared with SE animals. Mitochondrial complex I activity was higher in EE than in SE rats. Training had no effect on complex I activity neither in SE nor in EE rats. In young rats, no significant differences in enzyme activities were found between EE and SE rats. These results support the hypothesis that brief exposure to EE and training produce effects on behavioral performance and on biochemical parameters in an age-dependent manner.

Behavioral impairments of the aging rat

Several disturbances occurring during aging of humans and rodents alike stem from changes in sensory and motor functions. Using a battery of behavioral tests we have studied alterations in performance with advancing age in female and male rats of some frequently used strains. In parallel, we collected survival and body weight data. The median survival age was similar for female and male Sprague-Dawley rats, inbred female Lewis and outbred male Wistar rats (29-30 months). In contrast, male Fisher 344 had a significantly shorter median life span. During aging there is a gradual decline in locomotor activity and explorative behavior while disturbances of coordination and balance first became evident at more advanced age. In old age, also weight carrying capacity, limb movement and temperature threshold were impaired. While whole body weight continues to increase over the better part of a rats' life span, the behavioral changes in old age associated with a decrease in both total body weight and muscle mass. Dietary restriction increases median life span expectancy; retards the pace of behavioral aging and impedes sarcopenia. Housing in enriched environment did not improve the scoring in the behavioral tests but tended to increase median life span. Finally, there was an agreement between behavioral data collected from longitudinal age-cohorts and those obtained from multiple age-cohorts.

Mitochondrial regulation of neuronal plasticity

The structure and function of neurons is dynamic during development and in adaptive responses of the adult nervous system to environmental demands. The mechanisms that regulate neuronal plasticity are poorly understood, but are believed to involve neurotransmitter and neurotrophic factor signaling pathways. In the present article, I review emerging evidence that mitochondria play important roles in regulating developmental and adult neuroplasticity. In neurons, mitochondria are located in axons, dendrites, growth cones and pre- and post-synaptic terminals where their movements and functions are regulated by local signals such as neurotrophic factors and calcium influx. Mitochondria play important roles in fundamental developmental processes including the establishment of axonal polarity and the regulation of neurite outgrowth, and are also involved in synaptic plasticity in the mature nervous system. Abnormalities in mitochondria are associated with neurodegenerative and psychiatric disorders, suggesting a therapeutic potential for approaches that target mitochondrial mechanisms.

Changes of brain activity in the aged SAMP mouse

This study investigates characteristics of aging in the central nervous system of the senescence accelerated prone mice (SAMP8). We examined 3 and 10-months old senescence-accelerated-prone mice (SAMP8) for functional and molecular changes in their brains, specifically in the hippocampus and somatosensory cortex. There was no statistically significant increase in the apoptosis indicators as revealed by Western Blotting for BAD and TUNEL experiments. However, the functional magnetic resonance imaging showed an increase in the area of BOLD images from the 3-month old to the 10-months old SAMP mice upon the application of tail stimulus. These results demonstrated a lack of neuronal deaths but an increase in the activated brain area with age.

Neuroprotective effects of behavioural training and nicotine on age-related deficits in spatial learning

Studies in humans and animals show a clear decline in spatial memory with age and several approaches have been adopted to alleviate this impairment. The purpose of our review is to assess the studies that have suggested the possible neuroprotective actions of behavioural training and nicotine-applied both independently and in conjunction-on age-related deficits in spatial learning. Both spatial pretraining and nonspatial experiences influence an animal's performance in spatial tasks. In aged rats, the experience of training in the water maze task increases the number of newly generated neurons in the hippocampus. The neuroprotective effects of nicotine have been demonstrated in both in-vitro and in-vivo models, although the molecular mechanisms underlying these actions are not yet fully understood. It had been concluded in different studies that nicotine can improve, impair or have no effect on performance in the water maze. Neurobiological data also suggest an interaction between nicotine and prior experience in complex tasks, although few studies have raised the question of whether nicotine treatment and training in spatial tasks may contribute in an interactive manner to alleviate spatial cognition impairment associated with the ageing process. Different findings suggest that past experience could be a confounding variable in longitudinal studies that aim to evaluate the neuroprotective effects of nicotine on age-related deficits in spatial learning.