Environmental enrichment influences survival rate and enhances exploration and learning but produces variable responses to the radial maze in old rats

Failure of senolytic treatment to prevent cognitive decline in a female rodent model of aging

There are sex differences in vulnerability and resilience to the stressors of aging and subsequent age-related cognitive decline. Cellular senescence occurs as a response to damaging or stress-inducing stimuli. The response includes a state of irreversible growth arrest, the development of a senescence-associated secretory phenotype, and the release of pro-inflammatory cytokines associated with aging and age-related diseases. Senolytics are compounds designed to eliminate senescent cells. Our recent work indicates that senolytic treatment preserves cognitive function in aging male F344 rats. The current study examined the effect of senolytic treatment on cognitive function in aging female rats. Female F344 rats (12 months) were treated with dasatinib (1.2 mg/kg) + quercetin (12 mg/kg) or ABT-263 (12 mg/kg) or vehicle for 7 months. Examination of the estrus cycle indicated that females had undergone estropause during treatment. Senolytic treatment may have increased sex differences in behavioral stress responsivity, particularly for the initial training on the cued version of the watermaze. However, pre-training on the cue task reduced stress responsivity for subsequent spatial training and all groups learned the spatial discrimination. In contrast to preserved memory observed in senolytic-treated males, all older females exhibited impaired episodic memory relative to young (6-month) females. We suggest that the senolytic treatment may not have been able to compensate for the loss of estradiol, which can act on aging mechanisms for anxiety and memory independent of cellular senescence.

Use of running plates by floor housed rats: A pilot study

The outfit of husbandry facilities of, and the enrichment provided for, experimental rodents plays an important role in the animals' welfare, and hence also for the societal acceptance of animal experiments. Whether rats and mice benefit from being provided with running wheels or plates is discussed controversially. Here we present observations from a feeding experiment, where rats were provided a running plate. As a pilot study, six identical cages, with three animals per cage, were filmed for six days, and the resulting footage was screened for the number of bouts and the time the animals spent on the plates. The main activities observed on the plate in descending order were sitting (18.5 ± 13.8 bouts or 8.0 ± 13.7 min/animal per day), standing (10.2 ± 11.6 bouts, 3.8 ± 4.2 min), running (8.2 ± 13.3 bouts, 10.1 ± 21.4 min), grooming (2.0 ± 2.8 bouts, 6.7 ± 25.7 min), sleeping (1.0 ± 2.6 bouts, 24.0 ± 61.8 min) and playing (0.5 ± 0.9 bouts, 0.1 ± 0.5 min). Most of these activities (91% of all bouts, 90% of total time) occurred at night, similar to previous studies on running wheel usage. The running plate seems well-accepted as cage enrichment, even though in further studies, the motivating triggers and the effects of long-term use could be evaluated more in-depth.

The impact of environmental and social factors on learning abilities: a meta-analysis

Since the 1950s, researchers have examined how differences in the social and asocial environment affect learning in rats, mice, and, more recently, a variety of other species. Despite this large body of research, little has been done to synthesize these findings and to examine if social and asocial environmental factors have consistent effects on cognitive abilities, and if so, what aspects of these factors have greater or lesser impact. Here, we conducted a systematic review and meta-analysis examining how different external environmental features, including the social environment, impact learning (both speed of acquisition and performance). Using 531 mean-differences from 176 published articles across 27 species (with studies on rats and mice being most prominent) we conducted phylogenetically corrected mixed-effects models that reveal: (i) an average absolute effect size |d| = 0.55 and directional effect size d = 0.34; (ii) interventions manipulating the asocial environment result in larger effects than social interventions alone; and (iii) the length of the intervention is a significant predictor of effect size, with longer interventions resulting in larger effects. Additionally, much of the variation in effect size remained unexplained, possibly suggesting that species differ widely in how they are affected by environmental interventions due to varying ecological and evolutionary histories. Overall our results suggest that social and asocial environmental factors do significantly affect learning, but these effects are highly variable and perhaps not always as predicted. Most notably, the type (social or asocial) and length of interventions are important in determining the strength of the effect.

Cognitive Reserve in Model Systems for Mechanistic Discovery: The Importance of Longitudinal Studies

The goal of this review article is to provide a resource for longitudinal studies, using animal models, directed at understanding and modifying the relationship between cognition and brain structure and function throughout life. We propose that forthcoming longitudinal studies will build upon a wealth of knowledge gleaned from prior cross-sectional designs to identify early predictors of variability in cognitive function during aging, and characterize fundamental neurobiological mechanisms that underlie the vulnerability to, and the trajectory of, cognitive decline. Finally, we present examples of biological measures that may differentiate mechanisms of the cognitive reserve at the molecular, cellular, and network level.

The gains of a 4-week cognitive training are not modulated by novelty

Cognitive training should not only improve performance of the trained task, but also untrained abilities. Exposure to novelty can improve subsequent memory performance, suggesting that novelty exposure might be a critical factor to promote the effects of cognitive training. Therefore, we combined a 4‐week working memory training with novelty exposure. Neuropsychological tests and MRI data were acquired before and after training to analyze behavior and changes in gray matter volume, myelination, and iron levels. In total, 83 healthy older humans participated in one of three groups: Two groups completed a 4‐week computerized cognitive training of a two‐back working memory task, either in combination with novel or with familiarized nature movies. A third group did not receive any training. As expected, both training groups showed improvements in task specific working memory performance and reaction times. However, there were no transfer or novelty effects on fluid intelligence, verbal memory, digit‐span, and executive functions. At the neural level, no significant micro‐ or macrostructural changes emerged in either group. Our findings suggest that working memory training in healthy older adults is associated with task‐specific improvements, but these gains do not transfer to other cognitive domains, and it does not lead to structural brain changes.

Longitudinal Characterization and Biomarkers of Age and Sex Differences in the Decline of Spatial Memory

The current longitudinal study examined factors (sex, physical function, response to novelty, ability to adapt to a shift in light/dark cycle, brain connectivity), which might predict the emergence of impaired memory during aging. Male and female Fisher 344 rats were tested at 6, 12, and 18 months of age. Impaired spatial memory developed in middle-age (12 months), particularly in males, and the propensity for impairment increased with advanced age. A reduced response to novelty was observed over the course of aging, which is inconsistent with cross-sectional studies. This divergence likely resulted from differences in the history of environmental enrichment/impoverishment for cross-sectional and longitudinal studies. Animals that exhibited lower level exploration of the inner region on the open field test exhibited better memory at 12 months. Furthermore, males that exhibited a longer latency to enter a novel environment at 6 months, exhibited better memory at 12 months. For females, memory at 12 months was correlated with the ability to behaviorally adapt to a shift in light/dark cycle. Functional magnetic resonance imaging of the brain, conducted at 12 months, indicated that the decline in memory was associated with altered functional connectivity within different memory systems, most notably between the hippocampus and multiple regions such as the retrosplenial cortex, thalamus, striatum, and amygdala. Overall, some factors, specifically response to novelty at an early age and the capacity to adapt to shifts in light cycle, predicted spatial memory in middle-age, and spatial memory is associated with corresponding changes in brain connectivity. We discuss similarities and differences related to previous longitudinal and cross-sectional studies, as well as the role of sex differences in providing a theoretical framework to guide future longitudinal research on the trajectory of cognitive decline. In addition to demonstrating the power of longitudinal studies, these data highlight the importance of middle-age for identifying potential predictive indicators of sexual dimorphism in the trajectory in brain and cognitive aging.

Age, environment, object recognition and morphological diversity of GFAP-immunolabeled astrocytes

Background

Few studies have explored the glial response to a standard environment and how the response may be associated with age-related cognitive decline in learning and memory. Here we investigated aging and environmental influences on hippocampal-dependent tasks and on the morphology of an unbiased selected population of astrocytes from the molecular layer of dentate gyrus, which is the main target of perforant pathway.

Results

Six and twenty-month-old female, albino Swiss mice were housed, from weaning, in a standard or enriched environment, including running wheels for exercise and tested for object recognition and contextual memories. Young adult and aged subjects, independent of environment, were able to distinguish familiar from novel objects. All experimental groups, except aged mice from standard environment, distinguish stationary from displaced objects. Young adult but not aged mice, independent of environment, were able to distinguish older from recent objects. Only young mice from an enriched environment were able to distinguish novel from familiar contexts. Unbiased selected astrocytes from the molecular layer of the dentate gyrus were reconstructed in three-dimensions and classified using hierarchical cluster analysis of bimodal or multimodal morphological features. We found two morphological phenotypes of astrocytes and we designated type I the astrocytes that exhibited significantly higher values of morphological complexity as compared with type II. Complexity = [Sum of the terminal orders + Number of terminals] × [Total branch length/Number of primary branches]. On average, type I morphological complexity seems to be much more sensitive to age and environmental influences than that of type II. Indeed, aging and environmental impoverishment interact and reduce the morphological complexity of type I astrocytes at a point that they could not be distinguished anymore from type II.

Conclusions

We suggest these two types of astrocytes may have different physiological roles and that the detrimental effects of aging on memory in mice from a standard environment may be associated with a reduction of astrocytes morphological diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12993-016-0111-2) contains supplementary material, which is available to authorized users.

Age-dependent effects of environmental enrichment on spatial memory and neurochemistry

Although aging and environmental stimulation are well-known to affect cognitive abilities, the question of whether aging effects can be distinguished in already-mature adult rats has not been fully addressed. In the present study, therefore, young and mature adult rats were housed in either enriched or standard conditions (EE or SC) for three months. Open-field (OFT) and radial-maze (RM) behavior, and ex-vivo contents of GABA and glutamate in hippocampus, and of dopamine and DOPAC in ventral striatum (VS) were analyzed and compared between the four groups. In OFT, young rats were more active than mature adults irrespective of the housing condition. Surprisingly, in the RM test, mature adults outperformed young counterparts except for the young-enriched rats, which showed a progressive improvement in RM performance. At the neurochemical level, young EE rats showed higher hippocampal glutamate and GABA concentrations, and DA turnover in VS, which correlated with RM performance. Altogether, the behavioral and cognitive strategies underlying habituation learning and spatial memory seem to be qualitatively different between the two ages analyzed. These results challenge the assumption that mature adult animals are always worse in learning and memory tasks. However, young rats benefited more from the social and physical stimulation provided by the enrichment than mature adult counterparts. The latter effect was evident not just on behavior, but also on brain neurochemistry.

What is the impact of low testosterone levels on the anatomical and behavioral repertoire of long-term enriched housing of male mice?

Environmental enrichment is a strategy to improve animal welfare, providing brain plasticity with changes at cellular, molecular and behavioral levels. In order to test the long-term effects of enriched housing and the importance of testosterone levels for the expression of behavioral plasticity, 28 categories were assessed in 45 adult Swiss mice, subdivided in prepubertal castrated and non-castrated groups, maintained for seven months as three non-sibling mates. Enrichment consisted of introducing insets for gnawing, climbing and hiding. Tests of spontaneous exploration (barrier), territoriality (intruder) and hierarchical organization (group) were applied at once. Measurements of body weight and the relative weight of key organs were done at the end of the experiment. Mice kept in enriched cages, either castrated or non-castrated, showed more spontaneous exploration than those raised in standard cages. Non-castrated mice housed in structured cages had a lower frequency of attack in the resident-intruder test than the non-castrated standard caged mice, indicating a decrease in territoriality in the first group. Independent of the housing conditions, castrated mice showed reduction of offensive, defensive, and social contacts, as well as low frequency of attack in both agonistic tests. The well-known importance of testes to ensure the expression of aggressive and social contact behaviors was therefore not challenged by the enrichment condition. Behavioral repertoire at the home cage, performance in the group-test, and organometric measurements were not significantly different between the groups kept in enriched and non-enriched cages. Our results suggest that the experience in enriched environment does not increase aggressiveness in their routine in the home-cage nor negatively influence physiological parameters, independently of the testosterone level.

Age-dependent effects of environmental enrichment on brain networks and spatial memory in Wistar rats

We assessed the effect of 3h of environmental enrichment (EE) exposure per day started at different ages (3 and 18months old) on the performance in a spatial memory task and on brain regions involved in the spatial learning (SPL) process using the principal component analysis (PCA). The animals were tested in the four-arm radial water maze (4-RAWM) for 4days, with six daily trials. We used cytochrome c oxidase (COx) histochemistry to determine the brain oxidative metabolic changes related to age, SPL and EE. Behavioural results showed that the enriched groups, regardless of their age, achieved better performance in the spatial task. Interestingly, in the case of the distance travelled in the 4-RAWM, the effect of the EE was dependent on the age, so the young enriched group travelled a shorter distance compared to the aged enriched group. Respect to COx histochemistry results, we found that different brain mechanisms are triggered in aged rats to solve the spatial task, compared to young rats. PCA revealed the same brain functional network in both age groups, but the contribution of the brain regions involved in this network was slightly different depending on the age of the rats. Thus, in the aged group, brain regions involved in anxiety-like behaviour, such as the amygdala or the bed nucleus of the stria terminalis had more relevance; whereas in the young enriched group the frontal and the hippocampal subregions had more contribution.

Loss of hippocampal neurogenesis, increased novelty-induced activity, decreased home cage activity, and impaired reversal learning one year after irradiation of the young mouse brain

Radiotherapy is a major cause of long-term complications in survivors of pediatric brain tumors. These complications include intellectual and memory impairments as well as perturbed growth and puberty. We investigated the long-term effects of a single 8 Gy irradiation dose to the brains of 14-day-old mice. Behavior was assessed one year after irradiation using IntelliCage and open field, followed by immunohistochemical investigation of proliferation and neurogenesis in the dentate gyrus of the hippocampus. We found a 61% reduction in proliferation and survival (BrdU incorporation 4 weeks prior to sacrifice), 99% decrease in neurogenesis (number of doublecortin-positive cells) and gliosis (12% higher astrocyte density) one year following irradiation. Irradiated animals displayed increased activity in a novel environment but decreased activity in their home cage. Place learning in the IntelliCage was unaffected by irradiation but reversal learning was impaired. Irradiated animals persevered in visiting previously correct corners to a higher extent compared to control animals. Hence, despite the virtual absence of neurogenesis in these old mice, spatial learning could take place. Reversal learning however, where a previous memory was replaced with a new one, was partly impaired. This model is useful to study the so called late effects of radiotherapy to the young brain and to evaluate possible interventions.

Enhancement of cognitive function in models of brain disease through environmental enrichment and physical activity

This review will provide an overview of the non-drug based approaches that have been demonstrated to enhance cognitive function of the compromised brain, primarily focussed on the two most widely adopted paradigms of environmental enrichment and enhanced physical exercise. Environmental enrichment involves the generation of novelty and complexity in animal housing conditions which facilitates enhanced sensory and cognitive stimulation as well as physical activity. In a wide variety of animal models of brain disorders, environmental enrichment and exercise have been found to have beneficial effects, including cognitive enhancement, delayed disease onset, enhanced cellular plasticity and associated molecular processes. Potential cellular and molecular mechanisms will also be discussed, which have relevance for the future development of 'enviromimetics', drugs which could mimic or enhance the beneficial effects of environmental stimulation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Influence of environmental enrichment on hypothalamic-pituitary-adrenal (HPA) responses to single-dose nicotine, continuous nicotine by osmotic mini-pumps, and nicotine withdrawal by mecamylamine in male and female rats

In the present study, we determined the effects of environmental enrichment (EE; Kong Toys and Nestlets) on sexually diergic HPA axis responses to single-dose nicotine (NIC), single-dose NIC following continuous NIC administration for two weeks, and NIC withdrawal by single-dose mecamylamine (MEC) in male and female rats. Blood sampling occurred before and after MEC and NIC administrations for the determination of adrenocorticotropic hormone (ACTH) and corticosterone (CORT). Supporting and extending our previous findings, EE appeared to produce anxiolytic effects by reducing hormone responses: Male and female rats housed with EE had lower baseline ACTH and significantly lower HPA axis responses to the mild stress of saline (SAL) injection than did those housed without EE. The sexually diergic responses to single dose NIC, continuous NIC, and MEC-induced NIC withdrawal were reduced by EE in many male and female groups. ACTH responses to continuous NIC and MEC-induced NIC withdrawal were blunted to a greater extent in female EE groups than in male EE groups, suggesting that females are more sensitive to the anxiolytic effects of EE. Because EE lowered stress-responsive hormones of the HPA axis in most groups, EE may be a useful intervention for stress reduction in animal models of NIC addiction. As well, the effectiveness of EE in animal studies of NIC withdrawal may enlighten human studies addressing coping styles and tobacco cessation in men and women.

Impoverished environment, cognition, aging and dementia

Animals living in an impoverished environment, i.e., without the possibility of physical and social activity, perform worse on cognitive tests compared to animals in an enriched environment. The same cognitive difference is also observed in humans. However, it is not clear whether this difference is caused by a decrease in cognition due to an impoverished environment or an increase due to an enriched environment. This review discusses the impact of an impoverished environment on cognition in animal experimental studies and human experimental studies with community-dwelling and institutionalized older people. Results show that the cognitive functioning of old rats is more affected by an impoverished environment than young rats. Similarly, sedentary and lonely people (impoverished environment) have worse cognitive functioning and show a faster cognitive decline than physically and socially active people. Institutionalization further aggravates cognitive decline, probably due to the impoverished environment of nursing homes. In institutions, residents spend an unnecessary and excessive amount of time in bed; out of bed they show mainly sedentary or completely passive behavior. In conclusion, older people, especially those that have been institutionalized, have poor levels of physical and social activity, which in turn has a negative impact on cognitive functioning.

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.