Animals In Enriched Environment Research Articles

Sex-Specific Differences and the Role of Environmental Enrichment in the Expression of Hippocampal CB1 Receptors following Chronic Unpredictable Stress.

Stress-related mental disorders have become increasingly prevalent, thus endangering mental health worldwide. Exploring stress-associated brain alterations is vital for understanding the possible neurobiological mechanisms underlying these changes. Based on existing evidence, the brain endogenous cannabinoid system (ECS) plays a significant role in the stress response, and disruptions in its function are associated with the neurobiology of various stress-related disorders. This study primarily focuses on investigating the impact of chronic unpredictable stress (CUS) on the expression of hippocampal cannabinoid type 1 (CB1) receptors, part of the ECS, in adult male and female Wistar rats. Additionally, it explores whether environmental enrichment (EE) initiated during adolescence could mitigate the CUS-associated alterations in CB1 expression. Wistar rats, shortly after weaning, were placed in either standard housing (SH) or EE conditions for a duration of 10 weeks. On postnatal day 66, specific subgroups of SH or EE animals underwent a 4-week CUS protocol. Western blot (WB) analysis was conducted in the whole hippocampus of the left brain hemisphere to assess total CB1 protein expression, while immunohistochemistry (IHC) was performed on the right hemisphere to estimate the expression of CB1 receptors in certain hippocampal areas (i.e., CA1, CA3 and dentate gyrus-DG). The WB analysis revealed no statistically significant differences in total CB1 protein levels among the groups; however, reduced CB1 expression was found in specific hippocampal sub-regions using IHC. Specifically, CUS significantly decreased CB1 receptor expression in the CA1 and DG of both sexes, whereas in CA3 the CUS-associated decrease was limited to SH males. Interestingly, EE housing proved protective against these reductions. These findings suggest a region and sex-specific endocannabinoid response to chronic stress, emphasizing the role of positive early experiences in the protection of the adolescent brain against adverse conditions later in life.

The differential effects of brief environmental enrichment following social isolation in rats.

Environmental enrichment (EE) in rodents is associated with a wide range of physiological, affective, and cognitive benefits. A seemingly opposite housing condition, social isolation (SI), is used as a rodent model of stress, negatively affecting several neurobiological mechanisms and hampering cognitive performance. Experimental designs that involve switching between these housing conditions produced mixed results. We evaluated different behavioral and cognitive effects of brief EE following long-term, SI-induced stress. We revealed the influence of enrichment after 30 days of isolation on behavioral despair, anxiety-like behavior, and spatial working memory in adult male Wistar rats and found a substantial anxiolytic effect in the experimental (SI to EE) group. Interestingly, rats exposed to EE also showed increased behavioral despair compared with the control (continuous SI) group. There was no difference in spatial working memory performance at the end of a 5-day water Y-maze (WYM) test. However, the SI to EE animals displayed better memory performance in the first 2 days of the WYM, indicating faster learning. In line with this difference, we recorded significantly more c-Fos-immunopositive (c-Fos+) cells in the retrosplenial and perirhinal cortices of the SI to EE animals. The lateral and basolateral nuclei of the amygdala showed no such difference. These results suggest that brief enrichment following isolation stress leads to differential results in affective and cognitive systems.

Lower ΔFosB expression in the dopaminergic system after stevia consumption in rats housed under environmental enrichment conditions

Environmental enrichment (EE) has been proven to reduce drug seeking and the development of addiction-related behaviors in rodent models, but the effects of EE on natural reward acquisition in the form of sweet beverages are poorly understood. Accumulating evidence shows that the intake of sugar, the main ingredient of sweet beverages, alters the dopaminergic system, leading to addiction-related physiological and molecular changes. Sugar in sweet beverages has been replaced with natural sweeteners, such as stevia extract, which has greater sweetener potential but no energy content. Our research group found that sucralose consumption increased the expression of ΔFosB in reward-related nuclei, suggesting activation of the dopaminergic system. The present study assessed the effects of EE on stevia consumption and the expression of ΔFosB in the nucleus accumbens, caudate putamen, and prefrontal cortex. Sixteen male Wistar rats, 21 days old, were randomly assigned to an EE group (n = 8) or standard environment (SE) group (n = 8) and reared for 30 days. On postnatal day 52 (PND52), the brains of four animals in each housing condition were extracted to determine basal ΔFosB levels. Stevia consumption with intermittent access and ΔFosB immunoreactivity were measured for 21 days in the remainder of the rats. Compared with SE animals, EE animals exhibited a reduction of stevia consumption and alterations of ΔFosB immunoreactivity in the reward system. These results indicate that EE reduces stevia consumption and the stevia-induced ΔFosB expression, suggesting addiction-related changes in dopaminergic nuclei, which may be interpreted as a neuroprotective effect.

Combination effect of exercise and environmental enrichment on cognitive functions and hippocampal neurogenesis markers of rat.

Cognitive decline is one of the most prevalent health problems and is associated with increased healthcare utilization and economic burden. Physical and cognitive training both have positive effects on cognition but have been less applied in combination. We hypothesized that simultaneous cognitive-physical components would yield greater cognitive benefits than single-domain interventions in rats. A total of 40 male Wistar rats were divided into four treatment groups: the control, enriched environment (EE), exercise (EX), and EE + EX. Animals in EE groups housed in the large cages (50 × 50 × 50 cm) contained differently shaped objects for 3 weeks. EX animals were forced to run on a treadmill once daily for 3 consecutive weeks. Morris water maze test was used for the assessment of spatial learning and memory. Real-time PCR was performed to assess the expression of nestin, and Sox2 in the hippocampus. EX and EE animals separately did not show a significantly enhanced function in spatial memory in comparison with the control group. When animals were treated with EE and EX simultaneously, they exhibited significantly superior performance in spatial memory than control, EX, or EE groups separately. The hippocampal expression of Sox2 was significantly higher in EE + EX group than in the control, EX, and EE alone. These results may have clinical implications for behavioral interventions in conditions with cognitive deficiencies.

Maternal Morphine Exposure and Post-Weaning Social Isolation Impair Memory and Ventral Striatum Dopamine System in Male Offspring: Is an Enriched Environment Beneficial?

Maternal opioids abuse has some deleterious consequences on next generations. Besides, children’s rearing conditions can affect the behavioral states and brain plasticity in their later life. In the present study, we investigated the effects of maternal morphine (MOR) treatment and post-weaning rearing conditions on memory, pain threshold, and the ventral striatum dopaminergic activity in male offspring. Female Wistar rats were treated twice daily either with escalating doses of MOR or with normal saline (NS) one week before mating, during pregnancy and lactation. After weaning, the male pups were assigned to six groups and then raised for an 8-week period under three different conditions: standard (STD), isolated (ISO) or enriched environment (EE). The behavioral tests, including passive avoidance task, novel object recognition, and tail-flick test, were also performed. Moreover, the ventral striatum dopamine’s content (DA), mRNA expressions of dopamine receptor 1(D1R) and dopamine receptor 2 (D2R), and dopamine transporter (DAT) were evaluated. The obtained data showed that maternal MOR exposure and post-weaning social isolation could dramatically impair memory in offspring, while EE could reverse these adverse outcomes. Moreover, results of tail flick latency indicated the increased pain threshold in EE animals. At molecular level, maternal MOR injections and social isolation reduced DA levels and altered expressions of D1R, D2R, and DAT within the ventral striatum of these male offspring. However, post-weaning EE partially buffered these changes. Our finding signified the effects of maternal MOR exposure and social isolation on the behaviors and neurochemistry of brain in next generation, and it also provided evidence on reversibility of these alterations following EE.

The downstream effects of forced exercise training and voluntary physical activity in an enriched environment on hippocampal plasticity in preadolescent rats

During critical periods of brain development, exercise-induced physical fitness may greatly impact the brain structure and function. Nevertheless, forced and intensive physical activities may display negative effects, particularly in the pre-pubertal period. Preadolescent rats were exposed to an enriched environment and combined exercise training for three consecutive weeks in the present study. There was a large cage with enriching stimuli and voluntary physical activity opportunities as an enriched environment (EE). The combined exercise training (CET) consisted of aerobic and resistance training programs. The protein levels of corticosterone (CORT), glucocorticoid receptors (GRs), insulin-like growth factor-1 (IGF-1), brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) were assessed using Enzyme-linked immunosorbent assay and western blotting. Cresyl violet staining was also used to evaluate the number of cells in the hippocampus. While GRs levels were significantly increased in both EE and CET groups (P < 0.001), decreased CORT levels were found in enriched rats (P < 0.05). Moreover, elevated BDNF levels were found in the EE (P < 0.01) and CET (P < 0.05) groups. Similarly, VEGF significantly increased in the EE (P < 0.01) and CET (P < 0.05) animals. However, IGF-1 levels were high only in trained rats (P < 0.05). The number of cells also significantly increased in the DG and CA1 region of the hippocampus after each intervention (P < 0.001). These findings clarified that combined exercise training and voluntary physical activity in an enriched environment during the preadolescent period might promote the downstream plasticity effects on the hippocampus.

Early exposure to environmental enrichment protects male rats against neuropathic pain development after nerve injury

Because environmental elements modify chronic pain development and endogenous mechanisms of pain control are still a great therapeutic source, we investigated the effects of an early exposure to environmental enrichment (EE) in a translational model of neuropathic pain. Young male rats born and bred in an enriched environment, which did not count on running wheel, underwent chronic constriction injury (CCI) of sciatic nerve. EE abolished neuropathic pain behavior 14 days after CCI. Opioid receptors' antagonism reversed EE-analgesic effect. β-endorphin and met-enkephalin serum levels were increased only in EE-CCI group. Blockade of glucocorticoid receptors did not alter EE-analgesic effect, although corticosterone circulating levels were increased in EE animals. In the spinal cord, EE controlled CCI-induced serotonin increase. In DRG, EE blunted the expression of ATF-3 after CCI. Surprisingly, EE-CCI group showed a remarkable preservation of sciatic nerve fibers compared to NE-CCI group. This work demonstrated global effects induced by an EE protocol that explain, in part, the protective role of EE upon chronic noxious stimulation, reinforcing the importance of endogenous mechanisms in the prevention of chronic pain development.

Aged rats with different performances at environmental enrichment onset display different modulation of habituation and aversive memory.

A wide agreement exists that environmental enrichment (EE) is most beneficial if introduced early in life, but numerous studies reported that also aged animals remain responsive. As age-related memory and cognition impairments are not uniform, an open question is whether EE might exert different effects in animals with different age-related deficits. A 12-week EE protocol was applied to late adult rats pretested for habituation and aversive memory. Animals were classified as low (LP) and high (HP) performers according to percent exploration change in Open Field test (OF) and as impaired (I) and not impaired (NI) according to latency in Step-through Passive Avoidance test (PA). Standard housing (SH) animals pretested by OF and PA, and naïve (non-pretested) EE and SH rats were used as controls. In comparison to pretest, after the housing protocol, EE LP ameliorated while EE HP and both SH HP and LP worsened their habituation pattern. The positive influence of EE on LP was probably due to the more active interaction with and the faster adaptation to surroundings promoted by continuous, multiple stimuli provided during the enriched housing. Regarding HP, EE did not boost the basal behavior, which likely represented the maximum achievable for that age, and the post housing exploration change dropped, as in SH animals, because of the retesting. After EE, a significant percentage of NI animals became I and a significant percentage of I animals became NI. The changes evidenced in the NI group likely depended on EE-related reduction of anxiety and the consequent more efficient coping with fearful situations. This hypothesis was strengthened by the observation that naïve EE animals were almost all I. Pretested EE I rats were not influenced by the rearing condition: their behavior was comparable to SH animals' behavior and determined by retesting. In conclusion, these results demonstrated that, when applied to aging rats, EE produces different effects based on pre-housing cognitive performances. The issue needs further analyses, but the observation that not all animals are able to take advantage of EE to the same extent suggests the opportunity to design individually tailored approaches to optimize their efficacy and minimize possible unwanted consequences.

Environmental enrichment cognitive neuroprotection in an experimental model of cerebral ischemia: biochemical and molecular aspects

Stroke is considered a major cause of global morbidity. Currently, there are no effective treatments for post-stroke cognitive impairment. Enriched environment (EE) has been brought forward as a preconditioning method to induce cerebral tolerance in an ischemic event. However, the subjacent mechanisms involved in this tolerance are not yet clear. Herein we aimed to identify the mechanisms of neuroprotection triggered by EE preconditioning in a murine model of ischemic stroke. In order to do so, C57Bl/6 mice were kept for five weeks either in EE or in standard environment (SC) prior to ischemic injury through bilateral carotid occlusion (BCCAo) or sham surgery. To evaluate cognitive deficits resulting from ischemia, animals were subjected to a set of behavioral test to assess short-term (STM), long-term (LTM) and working memory (WM) performance. Despite no effect of EE having been observed in LTM and WM, EE preconditioning was able to prevent short-term deficits in response to ischemia. This improvement was accompanied by a reduction in the infarct volume in animals following EE pre-exposure. Next, we aimed to analyze the expression of genes involved in cholinergic (M1 and alpha 7 receptors) and glutamatergic (NMDA subunits GluN1, GluN2A, GluN2B and GluN2C) neurotransmission, inflammatory mediators (GFAP and IL-1β) and of the neurotrophin BDNF. Animals tested for STM did not present alterations in the expression of glutamatergic or cholinergic receptors; however, EE was shown to prevent increased expression of IL1-β. On the other hand, in animals submitted to LTM task, EE exposure lead to increased GFAP expression in EE animals that underwent ischemic injury, affecting also the expression of NMDA subunits. In spite of that, no alterations in glutamate content were observed in either group. Altogether, this study suggests that the changes observed in the expression of glutamatergic receptors, the reduction of the inflammatory cytokine IL1-β expression and the increased expression of GFAP in ischemic animals might contribute to the cognitive improvement induced by the EE paradigm.

Experiential learning in rodents: past experience enables rapid learning and localized encoding in hippocampus.

Humans routinely use past experience with complexity to deal with novel, challenging circumstances. This fundamental aspect of real-world behavior has received surprisingly little attention in animal studies, and the underlying brain mechanisms are unknown. The present experiments tested for transfer from past experience in rats and then used quantitative imaging to localize synaptic modifications in hippocampus. Six daily exposures to an enriched environment (EE) caused a marked enhancement of short- and long-term memory encoded during a 30-min session in a different and complex environment relative to rats given extensive handling or access to running wheels. Relatedly, the EE animals investigated the novel environment in a different manner than the other groups, suggesting transfer of exploration strategies acquired in earlier interactions with complexity. This effect was not associated with changes in the number or size of excitatory synapses in hippocampus. Maps of synapses expressing a marker for long-term potentiation indicated that encoding in the EE group, relative to control animals, was concentrated in hippocampal field CA1. Importantly, <1% of the total population of synapses was involved in production of the regional map. These results constitute the first evidence that the transfer of experience profoundly affects the manner in which hippocampus encodes complex information.

The impact of environmental enrichment on the murine inflammatory immune response.

Living in a mentally and physically stimulating environment has been suggested to have a beneficial effect on the immune response. This study investigates these effects, utilizing a 2-week program of environmental enrichment (EE) and 2 models of acute inflammation: zymosan-induced peritonitis (ZIP) and the cecal ligation and puncture (CLP) model of sepsis. Our results revealed that following exposure to EE, mice possessed a significantly higher circulating neutrophil to lymphocyte ratio compared with control animals. When subject to ZIP, EE animals exhibit enhanced neutrophil and macrophage influx into their peritoneal cavity. Corresponding results were found in CLP, where we observed an improved capacity for enriched animals to clear systemic microbial infection. Ex vivo investigation of leukocyte activity also revealed that macrophages from EE mice presented an enhanced phagocytic capacity. Supporting these findings, microarray analysis of EE animals revealed the increased expression of immunomodulatory genes associated with a heightened and immunoprotective status. Taken together, these results provide potentially novel mechanisms by which EE influences the development and dynamics of the immune response.

Corticosterone levels and behavioral changes induced by simultaneous exposure to chronic social stress and enriched environments in NMRI male mice

Environmental enrichment (EE) is an experimental model which is believed to counteract some of the effects induced by stressors, although few studies have exposed rodents simultaneously to EE and stress. Our aim was to compare the short- and long-term effects of different housing conditions in mice submitted to chronic stress. 128 NMRI male mice arrived at our laboratory on postnatal day (PND) 21. During Phase I (PND 28), animals were randomly assigned to four experimental conditions: 1) EE+STRESS: mice housed in EE and submitted to social stress (n=32); 2) EE+NO STRESS: mice housed in EE without stress (n=32); 3) SE+STRESS: mice maintained in standard conditions (SE) and submitted to social stress (n=32); and 4) SE+NO STRESS (n=32). At the end of Phase I (PND 77), one cohort of 32 animals was used for behavioral assessment whereas another cohort of 32 was sacrificed for corticosterone analysis. Results indicated that EE animals showed less body weight, higher water and food intake, diminished anxiety response and decreased motor and exploratory behavior than SE mice. Mice exposed to stress gained less body weight, showed higher food and fluid intake and displayed decreased exploratory behavior than non-stressed mice. Furthermore, EE+STRESS group displayed significantly higher corticosterone levels than EE+NO STRESS group whereas EE+NO STRESS group showed lower levels than SE+NO STRESS. On PND 83, Phase II of the study began. Animals (n=96) were assigned to two different housing conditions: EE (n=48) and SE (n=48). On PND 112, corticosterone analysis (n=32) and behavioral study (n=64) were done. The factor “Housing Phase II” reached statistical significance. Results indicated that EE animals showed lower body weight and higher fluid intake than SE group, as well as decreased anxiety. No clear effects on motor and exploratory behavior or learning were observed. When long-term effects were analyzed, results indicated that “Initial Housing” condition was significant: animals allocated in EE during Phase I of the study showed higher corticosterone levels, lower body weight and higher fluid intake than SE mice. “Initial Stress” had significant long-term actions on food intake and exploratory behavior: animals initially reared under stress conditions displayed higher food intake and lower exploration levels on the hole-board test than non-stressed mice. In the elevated plus-maze, there were significant interactions between factors “Initial Housing” and “Initial Stress”. These factors did not reach statistical significance for motor activity or learning task. We can conclude that both short- and long-term effects of housing conditions are evident for corticosterone levels, body weight and fluid intake. Social stress induced short-term effects on body weight, food and fluid intake and exploratory behavior whereas long-acting effects were reflected on food intake and exploratory behavior. Further studies are needed in order to explore more in depth behavioral and physiological consequences of social stress and environmental enrichment.

Environmental Enrichment Increases Glucocorticoid Receptors and Decreases GluA2 and Protein Kinase M Zeta (PKMζ) Trafficking During Chronic Stress: A Protective Mechanism?

Environmental enrichment (EE) housing paradigms have long been shown beneficial for brain function involving neural growth and activity, learning and memory capacity, and for developing stress resiliency. The expression of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA2, which is important for synaptic plasticity and memory, is increased with corticosterone (CORT), undermining synaptic plasticity and memory. Thus, we determined the effect of EE and stress on modulating GluA2 expression in Sprague-Dawley male rats. Several markers were evaluated which include: plasma CORT, the glucocorticoid receptor (GR), GluA2, and the atypical protein kinase M zeta (PKMζ). For 1 week standard-(ST) or EE-housed animals were treated with one of the following four conditions: (1) no stress; (2) acute stress (forced swim test, FST; on day 7); (3) chronic restraint stress (6 h/day for 7 days); and (4) chronic + acute stress (restraint stress 6 h/day for 7 days + FST on day 7). Hippocampi were collected on day 7. Our results show that EE animals had reduced time immobile on the FST across all conditions. After chronic + acute stress EE animals showed increased GR levels with no change in synaptic GluA2/PKMζ. ST-housed animals showed the reverse pattern with decreased GR levels and a significant increase in synaptic GluA2/PKMζ. These results suggest that EE produces an adaptive response to chronic stress allowing for increased GR levels, which lowers neuronal excitability reducing GluA2/PKMζ trafficking. We discuss this EE adaptive response to stress as a potential underlying mechanism that is protective for retaining synaptic plasticity and memory function.

Infralimbic cortex controls the activity of the hypothalamus–pituitary–adrenal axis and the formation of aversive memory: Effects of environmental enrichment

The aim of the present study was to investigate the effects of the stimulation and inhibition of the ventral part of the medial prefrontal cortex (infralimbic cortex) on basal and stress-induced plasma levels of corticosterone and on the acquisition of aversive memory in animals maintained in control and environmental enrichment (EE) conditions. Intracortical microinjections of the GABAA antagonist picrotoxin and agonist muscimol were performed in male Wistar rats to stimulate and inhibit, respectively, the activity of the infralimbic cortex. Injections were performed 60min before foot shock stress and training in the inhibitory avoidance task. Picrotoxin injections into the infralimbic cortex increased basal plasma levels of corticosterone. These increases were higher in EE rats which suggest that EE enhances the control exerted by infralimbic cortex over the hypothalamus–pituitary–adrenal (HPA) axis and corticosterone release. Muscimol injections into the infralimbic cortex reduced the stress-induced plasma levels of corticosterone and the retention latency 24h after training in the inhibitory avoidance performance in control and EE animals, respectively. These results further suggest that the infralimbic cortex is required for the activation of the HPA axis during stress and for the acquisition of contextual aversive memories.

Influence of environmental enrichment vs. time-of-day on behavioral repertoire of male albino Swiss mice

Environmental enrichment (EE) is a non-pharmacological manipulation that promotes diverse forms of benefits in the central nervous system of captive animals. It is thought that EE influences animal behavior in a specie-(strain)-specific manner. Since rodents in general present different behaviors during distinct periods of the day, in this study we aimed to investigate the influence of time-of-day on behavioral repertoire of Swiss mice that reared in EE. Forty male Swiss mice (21days old) were housed in standard (SC) or enriched conditions (EC) for 60days. Behavioral assessments were conducted during the light phase (in presence of light) or dark phase (in absence of light) in the following tasks: open field, object recognition and elevated plus maze. First, we observed that the locomotor and exploratory activities are distinct between SC and EC groups only during the light phase. Second, we observed that “self-protective behaviors” were increased in EC group only when mice were tested during the light phase. However, “less defensive behaviors” were not affected by both housing conditions and time-of-day. Third, we showed that the performance of EE animals in object recognition task was improved in both light and dark conditions. Our findings highlight that EE-induced alterations in exploratory and emotional behaviors are just evident during light conditions. However, EE-induced cognitive benefits are remarkable even during dark conditions, when exploratory and emotional behaviors were similar between groups.

Long-term effects of enriched environment on neurofunctional outcome and CNS lesion volume after traumatic brain injury in rats.

To determine whether the exposure to long term enriched environment (EE) would result in a continuous improvement of neurological recovery and ameliorate the loss of brain tissue after traumatic brain injury (TBI) vs. standard housing (SH). Male Sprague-Dawley rats (300-350 g, n=28) underwent lateral fluid percussion brain injury or SHAM operation. One TBI group was held under complex EE for 90 days, the other under SH. Neuromotor and sensorimotor dysfunction and recovery were assessed after injury and at days 7, 15, and 90 via Composite Neuroscore (NS), RotaRod test, and Barnes Circular Maze (BCM). Cortical tissue loss was assessed using serial brain sections. After day 7 EE animals showed similar latencies and errors as SHAM in the BCM. SH animals performed notably worse with differences still significant on day 90 (p<0.001). RotaRod test and NS revealed superior results for EE animals after day 7. The mean cortical volume was significantly higher in EE vs. SH animals (p=0.003). In summary, EE animals after lateral fluid percussion (LFP) brain injury performed significantly better than SH animals after 90 days of recovery. The window of opportunity may be wide and also lends further credibility to the importance of long term interventions in patients suffering from TBI.

English

The enriched environment (EE) is a promoter of physical activity, by its characteristics such as ample room for movement and exploration, presence of wheels, tunnels and toys. The maintenance of animals in enriched environment can bring a range of benefits, but the majority of the researches investigate cognitive parameters and changes related to the nervous system. The aim of this study was to examine the effects of the maintenance of aged rats in enriched cages on biochemical and metric parameters. Wistar rats were randomly distributed (n=6) into two groups during 6 weeks: control (C) in a conventional cage and enriched environment (EE). The body mass were recorded weekly and the body length at the end of the study. After euthanasia, blood was collected for analysis of glucose, triglycerides and the brain was collected for analysis of mass. The EE group had higher brain mass and lower gain of body weight compared to control group. The control group animals had normal values of blood glucose and triglyceride levels, and the maintenance in an EE did not promote changes in these parameters. Therefore, it can be concluded that the EE group increases brain mass and reduces the gain of body weight without changing the blood glucose and triglycerides in aged animals.

PDE5 inhibition improves object memory in standard housed rats but not in rats housed in an enriched environment: implications for memory models?

Drug effects are usually evaluated in animals housed under maximally standardized conditions. However, it is assumed that an enriched environment (EE) more closely resembles human conditions as compared to maximally standardized laboratory conditions. In the present study, we examined the acute cognition enhancing effects of vardenafil, a PDE5 inhibitor, which stimulates protein kinase G/CREB signaling in cells, in three different groups of male Wistar rats tested in an object recognition task (ORT). Rats were either housed solitarily (SOL) or socially (SOC) under standard conditions, or socially in an EE. Although EE animals remembered object information longer in the vehicle condition, vardenafil only improved object memory in SOL and SOC animals. While EE animals had a heavier dorsal hippocampus, we found no differences between experimental groups in total cell numbers in the dentate gyrus, CA2–3 or CA1. Neither were there any differences in markers for pre- and postsynaptic density. No changes in PDE5 mRNA- and protein expression levels were observed. Basal pCREB levels were increased in EE rats only, whereas β-catenin was not affected, suggesting specific activation of the MAP kinase signaling pathway and not the AKT pathway. A possible explanation for the inefficacy of vardenafil could be that CREB signaling is already optimally stimulated in the hippocampus of EE rats. Since previous data has shown that acute PDE5 inhibition does not improve memory performance in humans, the use of EE animals could be considered as a more valid model for testing cognition enhancing drugs.

Enriched environment attenuates nicotine self-administration and induces changes in ΔFosB expression in the rat prefrontal cortex and nucleus accumbens

Environment enrichment conditions have important consequences on subsequent vulnerability to drugs of abuse. The present work examined whether exposure to an enriched environment (EE) decreases oral self-consumption of nicotine. Wistar rats were housed either in a standard environment (SE, four rats per standard cage) or in an EE during 60 days after weaning. EE consisted of eight animals housed in larger cages containing a variety of objects such as boxes, toys, and burrowing material that were changed three times a week. After this period, animals were exposed to nicotine for 3 weeks, where animals chose freely between water and a nicotine solution (0.006% in water). Fluid consumption was evaluated on a daily basis. ΔFosB immunohistochemistry in the prefrontal cortex and nucleus accumbens was also performed. Rats of the EE group consumed less nicotine solution (0.25±0.04 mg/kg/day) than SE rats (0.54±0.05 mg/kg/day). EE increased the number of ΔFos-immunoreactive (ΔFos-ir) cells in the nucleus accumbens core and shell and in the prefrontal cortex, compared with animals in the standard condition. However, rats exposed to nicotine in the SE showed higher ΔFos-ir cells in the nucleus accumbens core and shell than nonexposed rats. Nicotine consumption did not modify ΔFos-ir cells in these brain areas in EE animals. These results support the idea of a possible protective effect of the EE on reward sensitivity and the development of an addictive behavior to nicotine.

Enriched environment induces beneficial effects on memory deficits and microglial activation in the hippocampus of type 1 diabetic rats

Type 1 diabetes mellitus (T1DM) has been associated with long-term complications in the central nervous system, causing brain cellular dysfunctions and cognitive deficits. On the other hand, enriched environment (EE) induces experience-dependent plasticity, especially in the hippocampus, improving the performance of animals in learning and memory tasks. Thus, our objective was to investigate the influence of the EE on memory deficits, locomotion, corticosterone levels, synaptophysin (SYP) protein immunoreactivity, cell survival and microglial activation in the dentate gyrus (DG) of T1DM rat hippocampus. Male Wistar rats (21-day-old) were exposed to EE or maintained in standard housing (controls, C) for 3 months. At adulthood, the C and EE animals were randomly divided and diabetes was induced in half of them. All the animals received 4 doses of BrdU, 24 h apart. Hippocampus-dependent spatial memory, general locomotion and serum corticosterone levels were evaluated at the end of the experiment. The animals were transcardially perfused 30 days post-BrdU administration. Our results showed that EE was able to prevent/delay the development of memory deficits caused by diabetes in rats, however it did not revert the motor impairment observed in the diabetic group. SYP immunoreactivity was increased in the enriched healthy group. The EE decreased the serum corticosterone levels in diabetic adult rats and attenuated the injurious microglial activation, though without altering the decrease of the survival cell. Thus, EE was shown to help to ameliorate cognitive comorbidities associated with T1DM, possibly by reducing hyperactivity in the hypothalamic-pituitary-adrenal axis and microglial activation in diabetic animals.