Enriched environment effects on remote object recognition memory

Environmental Enrichment Improves the Recognition Memory in Adult Mice Following Social Isolation via Downregulation of Kv4.2 Potassium Channels

Recognition memory is a cognitive process that enables us to distinguish familiar objects and situations from new items, which is essential for mammalian survival and adaptation to a changing environment. Social isolation (SI) has been implicated as a detrimental factor for recognition memory. The medial prefrontal cortex (mPFC) has been shown to carry information concerning the relative familiarity of individual stimuli, and modulating neuronal function in this region may contribute to recognition memory. The present study aimed to investigate the neuronal mechanisms in the mPFC of environmental enrichment (EE) on recognition memory in adult mice following SI. Mice were assigned into three groups: control, SI, and SI + EE groups. Novel location recognition (NLR) and novel object recognition (NOR) tests were performed to evaluate the recognition memory. The levels of Kv4 channels were assessed by qRT-PCR and western blotting. The effects of SI and SI + EE on the excitability of pyramidal neurons in the mPFC were measured using whole-cell recording. We found that SI led to a reduction in the excitability of pyramidal neurons. Specifically, we have identified that the reduction in the firing activity of pyramidal neurons resulted from alterations in the function and expression of Kv4.2 channels. Furthermore, EE regulated Kv4.2 channels, normalized the activity of pyramidal neurons, and restored the behavioral deficits following SI. Thus, the roles of Kv4.2 channels in excitability of pyramidal neurons suggest that the Kv4.2 channels present a promising therapeutic target for recognition memory impairment.

Maintaining a Dynamic Brain: A Review of Empirical Findings Describing the Roles of Exercise, Learning, and Environmental Enrichment in Neuroplasticity from 2017-2023

Brain plasticity, also termed neuroplasticity, refers to the brain's life-long ability to reorganize itself in response to various changes in the environment, experiences, and learning. The brain is a dynamic organ capable of responding to stimulating or depriving environments, activities, and circumstances from changes in gene expression, release of neurotransmitters and neurotrophic factors, to cellular reorganization and reprogrammed functional connectivity. The rate of neuroplastic alteration varies across the lifespan, creating further challenges for understanding and manipulating these processes to benefit motor control, learning, memory, and neural remodeling after injury. Neuroplasticity-related research spans several decades, and hundreds of reviews have been written and published since its inception. Here we present an overview of the empirical papers published between 2017 and 2023 that address the unique effects of exercise, plasticity-stimulating activities, and the depriving effect of social isolation on brain plasticity and behavior.

Enriched Environment Enhances Sociability Through the Promotion of ESyt1-Related Synaptic Formation in the Medial Prefrontal Cortex

Sociability stands as a crucial factor in the evolutionary success of all mammalian species. Notably, enriched environment (EE) housing has been shown to enhance sociability in mice. However, the precise underlying molecular mechanism remains elusive. In this study, we established an EE paradigm, housing mice for a 14-day period. Both enhanced sociability and an increased spine density in the medial prefrontal cortex (mPFC) of mice subjected to EE were detected. To elucidate the potential molecular pathway, we conducted high-performance liquid chromatography tandem mass spectrometry (HPLC-MS) analysis of the entire mPFC from both EE and home-caged (HC) housed mice. Our analysis identified 16 upregulated and 20 downregulated proteins in the EE group. Among them, Extended Synaptotagmin 1 (ESyt1), an activity-dependent endoplasmic reticulum (ER)-plasma membrane (PM) tethering protein associated with synaptic function and growth, emerged as a potentially key player in the increased synapse formation and enhanced sociability observed in EE-housed mice. Further investigation, involving the knockdown of ESyt1 expression via sh ESyt1 lentivirus in the mPFC, revealed that ESyt1 is crucial for increased spine density of mPFC and enhanced sociability of mice in an enriched environment but not in normal condition. Overall, our findings uncover a novel mechanistic insight into the positive influence of environmental enrichment on social behavior via ESyt1-mediated pathways.

Memory persistence induced by environmental enrichment is dependent on different brain structures

Environmental enrichment (EE) has been demonstrated to have a beneficial effect on different functions of the central nervous system in several mammal species, being used to improve behavior and cell damage in various neurological and psychiatric diseases. However, little has been investigated on the effect of EE in healthy animals, particularly regarding its impact on memory persistence and the brain structures involved. Therefore, here we verified in male Wistar rats that contextual fear conditioning (CFC) memory persistence, tested 28 days after the CFC training session, was facilitated by 5 weeks of exposure to EE, with no effect in groups tested 7 or 14 days after CFC training. However, a two-week exposure to EE did not affect memory persistence. Moreover, we investigated the role of specific brain regions in mediating the effect of EE on memory persistence. We conducted inactivation experiments using the GABAergic agonist Muscimol to target the basolateral amygdala (BLA), medial prefrontal cortex (mPFC), and CA1 region of the hippocampus (CA1). Inactivation of the BLA immediately and 12 h after CFC training impaired the effect of EE on memory persistence. Similarly, inactivation of the CA1 region and mPFC 12 h after training, but not immediately, also impaired the effect of EE on memory persistence. These results have important scientific implications as they shed new light on the effect of an enriched environment on memory persistence and the brain structures involved, thereby helping elucidate how an environment rich in experiences can modify the persistence of learned information.

Performing Enriched Environment Studies to Improve Functional Recovery

Physical therapy and social interactions between the stroke patient and healthcare professionals or relatives facilitate the process of recovery and promote improvement of lost neurological function after stroke. These observations can be mimicked in an experimental setting by multimodal stimulation provided in the concept of enriched environment. The enriched environment is a housing condition combining social interactions and sensorimotor stimulation that improves lost neurological function without affecting the extent of brain damage after experimental stroke. This chapter provides a detailed protocol on how to perform enriched housing experiments including conceptual and technical considerations as a tool to investigate mechanisms of recovery after brain injury.

Combined long-term enriched environment and caffeine supplementation improve memory function in C57Bl6 mice

Regular physical activity has been associated with healthy brain aging, reflected by beneficial effects on cognition and learning and memory. Nutritional supplements such as caffeine have been shown to act as cognitive enhancers and may possess neuroprotective properties. Interestingly, caffeine also improves athletic capabilities and is widely used by athletes because of its performance-enhancing effect, while information on potential additive beneficial effects of physical activity and caffeine on cognitive performance is scarce. In the present study, the effects of caffeine supplementation in combination with prolonged physical and cognitive stimulation in the form of the enriched environment (EE) housing for a duration of 4 months were analyzed. We demonstrate that caffeine supplementation together with prolonged environmental enrichment led to enhanced memory function, resulting in improved recognition and spatial working memory in behavioral paradigms such as the novel object recognition task or the Morris water maze in C57Bl6 wild-type mice. Mice housed under EE conditions showed increased gene expression levels of brain-derived neurotrophic factor (BDNF) in the hippocampus. The present findings underscore the potential impact of continuous physical activity in the prevention of age-related cognitive decline and may offer new options for combinatorial approaches.

Moderate effect of early-life experience on dentate gyrus function

The development, maturation, and plasticity of neural circuits are strongly influenced by experience and the interaction of an individual with their environment can have a long-lasting effect on cognitive function. Using an enriched environment (EE) paradigm, we have recently demonstrated that enhancing social, physical, and sensory activity during the pre-weaning time in mice led to an increase of inhibitory and excitatory synapses in the dentate gyrus (DG) of the hippocampus. The structural plasticity induced by experience may affect information processing in the circuit. The DG performs pattern separation, a computation that enables the encoding of very similar and overlapping inputs into dissimilar outputs. In the presented study, we have tested the hypothesis that an EE in juvenile mice will affect DG's functions that are relevant for pattern separation: the decorrelation of the inputs from the entorhinal cortex (EC) and the recruitment of the principal excitatory granule cell (GC) during behavior. First, using a novel slice electrophysiology protocol, we found that the transformation of the incoming signal from the EC afferents by individual GC is moderately affected by EE. We further show that EE does not affect behaviorally induced recruitment of principal excitatory GC. Lastly, using the novel object recognition task, a hippocampus-dependent memory test, we show that the ontogeny of this discrimination task was similar among the EE mice and the controls. Taken together, our work demonstrates that pre-weaning enrichment moderately affects DG function.

Bored at home?—A systematic review on the effect of environmental enrichment on the welfare of laboratory rats and mice

Boredom is an emotional state that occurs when an individual has nothing to do, is not interested in the surrounding, and feels dreary and in a monotony. While this condition is usually defined for humans, it may very well describe the lives of many laboratory animals housed in small, barren cages. To make the cages less monotonous, environmental enrichment is often proposed. Although housing in a stimulating environment is still used predominantly as a luxury good and for treatment in preclinical research, enrichment is increasingly recognized to improve animal welfare. To gain insight into how stimulating environments influence the welfare of laboratory rodents, we conducted a systematic review of studies that analyzed the effect of enriched environment on behavioral parameters of animal well–being. Remarkably, a considerable number of these parameters can be associated with symptoms of boredom. Our findings show that a stimulating living environment is essential for the development of natural behavior and animal welfare of laboratory rats and mice alike, regardless of age and sex. Conversely, confinement and under-stimulation has potentially detrimental effects on the mental and physical health of laboratory rodents. We show that boredom in experimental animals is measurable and does not have to be accepted as inevitable.

The Influence of an Enriched Environment in Enhancing Recognition Memory in Zebrafish (Danio rerio)

Environmental enrichment is used to increase social and physical stimulation for animals in captivity which can lead to enhanced cognition. Fundamental to the positive effect enrichment has on the brain is that it provides opportunities for captive animals to recognize and discriminate between different stimuli in the environment. In the wild, being able to discriminate between novel or familiar stimuli has implications for survival, for example finding food, hiding from predators, or even choosing a mate. The novel object recognition (NOR) test is a cognitive task that is used extensively in the rodent literature to assess object recognition and memory, where the amount of time an animal spends exploring a novel vs. familiar object is quantified. Enrichment has been shown to enhance object recognition in rodents. More recently, the use of the NOR test has been applied to another animal model, zebrafish (Danio rerio), however, the effects of enrichment have not yet been explored. In the current study we looked at the effects of enrichment on object recognition in zebrafish using the NOR test. Adult zebrafish were housed in either enriched conditions (gravel substrate, plastic plants, shelter, heater and a filter) or plain conditions (heater and filter only) for 6 months before behavioral NOR tests were conducted. Enriched fish showed a preference for a novel object over a familiar one at a distance but did not show a preference during close inspection. Control fish did not show a preference at either distance. Our results suggest that enrichment can enhance zebrafish ability to discriminate between novel and familiar objects, but distance from the object may be an important factor. Future research is needed to determine whether any enhancements in object recognition are a result of an increase in sensory stimulation from being reared with enrichment, or whether it is due to a reduction in stress reactivity.

Hippocampus-Prefrontal Coupling Regulates Recognition Memory for Novelty Discrimination

Recognition memory provides the ability to distinguish familiar from novel objects and places, and is important for recording and updating events to guide appropriate behavior. The hippocampus (HPC) and medial prefrontal cortex (mPFC) have both been implicated in recognition memory, but the nature of HPC-mPFC interactions, and its impact on local circuits in mediating this process is not known. Here we show that novelty discrimination is accompanied with higher theta activity (4-10 Hz) and increased c-Fos expression in both these regions. Moreover, theta oscillations were highly coupled between the HPC and mPFC during recognition memory retrieval for novelty discrimination, with the HPC leading the mPFC, but not during initial learning. Principal neurons and interneurons in the mPFC responded more strongly during recognition memory retrieval compared with learning. Optogenetic silencing of HPC input to the mPFC disrupted coupled theta activity between these two structures, as well as the animals' (male Sprague Dawley rats) ability to differentiate novel from familiar objects. These results reveal a key role of monosynaptic connections between the HPC and mPFC in novelty discrimination via theta coupling and identify neural populations that underlie this recognition memory-guided behavior.SIGNIFICANCE STATEMENT Many memory processes are highly dependent on the interregional communication between the HPC and mPFC via neural oscillations. However, how these two brain regions coordinate their oscillatory activity to engage local neural populations to mediate recognition memory for novelty discrimination is poorly understood. This study revealed that the HPC and mPFC theta oscillations and their temporal coupling is correlated with recognition memory-guided behavior. During novel object recognition, the HPC drives mPFC interneurons to effectively reduce the activity of principal neurons. This study provides the first evidence for the requirement of the HPC-mPFC pathway to mediate recognition memory for novelty discrimination and describes a mechanism for how this memory is regulated.

Enriched environment ameliorates memory impairments in rats after postsurgery sleep deprivation

Postsurgery sleep deprivation is a common complication that severely deteriorates the quality of life of patients. Here we aim to investigate the effects and mechanism of enriched environment in ameliorating sleep deprivation and memory impairments. Hernia repair surgery was performed on rats to induce sleep deprivation. Enriched environment (EE) was used to treat rats with sleep deprivation, and open field and Y-maze tests were performed to compare behavioral parameters of sleep deprivation rats with or without EE treatments to those of normal rats. To understand the mechanism, neurotrophic and growth factors including BDNF, NGF, NT-3 and GDNF were analyzed using enzyme-linked immunosorbent assay (ELISA). AMPAR subunits, including GluA1-A3, and GABA_A receptor α1 subunit expression in hippocampus tissues were assessed using western blot. EE restored normal levels of anxiety index and freezing behavior in open field test and level of alternation in Y-maze test, suggesting the reduction of anxiolytic effects and spatial memory impairment induced by sleep deprivation. EE increased BDNF levels and reduced NT-3 levels in sleep deprivation rats. GluA1/GluA2 ratio was increased by EE. GABA_A receptor α1 subunit expression was decreased by EE. EE is effective in ameliorating the detrimental effects of sleep deprivation in spatial memory impairment, and restoring normal levels of neurotrophic factors, which are potentially mediated by attenuating the changes in AMPAR subunit expression and reducing GABA_A receptor α1 subunit expression. These data provide supporting evidences for the use of EE to treat adverse outcomes of sleep deprivation induced by surgery.

The cannabinoid agonist WIN-2 affects acquisition but not consolidation of a spatial information in training and retraining processes: Relation with transcriptional regulation of the endocannabinoid system?

The endocannabinoid system is capable of modulating multiple physiological brain functions including learning and memory. Moreover, there is evidence that the processes of acquisition and consolidation have distinct biological basis. We used the cannabinoid agonist WIN 55,212-2 (WIN-2) to investigate whether chronic CB1 activation affects acquisition and consolidation differently by evaluating gene expression in the hippocampus (HIP) and prefrontal cortex (PFC). Swiss mice were treated with WIN-2 (2 mg/kg) and submitted to the Morris water maze to evaluate different aspects of memory. We observed short-term memory impairment in acquisition of the spatial task while consolidation remained unchanged. In the PFC, animals that received WIN-2 prior to the task exhibited increased expression of the 2-AG synthesis enzyme diacylglycerol lipase and decreased levels of the degradation enzyme monoacylglycerol lipase, while mice that were treated after the task for the evaluation of consolidation exhibited the opposite profile. With respect to genes related to AEA metabolism, no correlation between the molecular and behavioral data could be established. In this sense, the cognitive impairment in the acquisition promoted by WIN-2 treatment may be related to a possible increase in the concentration of 2-AG in the PFC. Overall, this study confirms the relevance of the endocannabinoid system in the modulation of cognitive processes. A better understanding of the mechanisms underlying endocannabinoids roles in cognition could provide guidance for the development of treatments to reduce the cognitive deficits caused by drug abuse.

Environmental enrichment, new neurons and the neurobiology of individuality

'Enriched environments' are a key experimental paradigm to decipher how interactions between genes and environment change the structure and function of the brain across the lifespan of an animal. The regulation of adult hippocampal neurogenesis by environmental enrichment is a prime example of this complex interaction. As each animal in an enriched environment will have a slightly different set of experiences that results in downstream differences between individuals, enrichment can be considered not only as an external source of rich stimuli but also to provide the room for individual behaviour that shapes individual patterns of brain plasticity and thus function.

Can the effects of environmental enrichment modulate BDNF expression in hippocampal plasticity? A systematic review of animal studies

BACKGROUND AND AIM

Environmental enrichment (EE) can be related to changes in the expression of brain-derived neurotrophic factor (BDNF) in the hippocampus of adult rodents. Exposure to EE may also induce neurogenesis in the dentate gyrus (DG). The aim of this systematic review was to analyze the current literature on the correlation between neurogenesis and BDNF expression in the hippocampal DG region resulting from exposure to EE, which is associated with changes in memory, in rodents.

METHODS

Bibliographic searches of the Medline/PubMed and ScienceDirect databases were carried out, and 334 studies were found. A predefined protocol was used and registered on PROSPERO, and 32 studies were included for qualitative synthesis. The PRISMA was used to report this systematic review.

RESULTS

Most of the included studies showed that there is little evidence in the literature demonstrating that memory changes resulting from EE are dependent on BDNF expression and that there is an induction of neurogenesis in the hippocampal DG. However, the observed increase in molecular expression levels and cell proliferation is dependent on the age, the timing and duration of exposure to EE. Regarding the methodological quality of the studies, the majority presented a risk of bias due to the high variability in the age of the animals.

CONCLUSION

There are few studies in the literature that correlate the molecular and cellular mechanisms involved in neurogenesis in the hippocampal DG with BDNF expression in this region in rodents exposed to EE; however, there are other factors that can modulate this neurogenesis.

Selective increases in inter-individual variability in response to environmental enrichment in female mice

One manifestation of individualization is a progressively differential response of individuals to the non-shared components of the same environment. Individualization has practical implications in the clinical setting, where subtle differences between patients are often decisive for the success of an intervention, yet there has been no suitable animal model to study its underlying biological mechanisms. Here we show that enriched environment (ENR) can serve as a model of brain individualization. We kept 40 isogenic female C57BL/6JRj mice for 3 months in ENR and compared these mice to an equally sized group of standard-housed control animals, looking at the effects on a wide range of phenotypes in terms of both means and variances. Although ENR influenced multiple parameters and restructured correlation patterns between them, it only increased differences among individuals in traits related to brain and behavior (adult hippocampal neurogenesis, motor cortex thickness, open field and object exploration), in agreement with the hypothesis of a specific activity-dependent development of brain individuality.

Environmental Enrichment Induces Changes in Long-Term Memory for Social Transmission of Food Preference in Aged Mice through a Mechanism Associated with Epigenetic Processes

Decline in declarative learning and memory performance is a typical feature of normal aging processes. Exposure of aged animals to an enriched environment (EE) counteracts this decline, an effect correlated with reduction of age-related changes in hippocampal dendritic branching, spine density, neurogenesis, gliogenesis, and neural plasticity, including its epigenetic underpinnings. Declarative memories depend on the medial temporal lobe system, including the hippocampus, for their formation, but, over days to weeks, they become increasingly dependent on other brain regions such as the neocortex and in particular the prefrontal cortex (PFC), a process known as system consolidation. Recently, it has been shown that early tagging of cortical networks is a crucial neurobiological process for remote memory formation and that this tagging involves epigenetic mechanisms in the recipient orbitofrontal (OFC) areas. Whether EE can enhance system consolidation in aged animals has not been tested; in particular, whether the early tagging mechanisms in OFC areas are deficient in aged animals and whether EE can ameliorate them is not known. This study aimed at testing whether EE could affect system consolidation in aged mice using the social transmission of food preference paradigm, which involves an ethologically based form of associative olfactory memory. We found that only EE mice successfully performed the remote memory recall task, showed neuronal activation in OFC, assessed with c-fos immunohistochemistry and early tagging of OFC, assessed with histone H3 acetylation, suggesting a defective system consolidation and early OFC tagging in aged mice which are ameliorated by EE.

Hippocampal β2‑microglobulin mediates sepsis‑induced cognitive impairment

Acute brain dysfunction is a frequent complication in sepsis patients and is associated with long‑term neurocognitive consequences and increased mortality, yet the underlying mechanism remains unclear. Emerging evidence has suggested that β2‑microglobulin [a component of major histocompatibility complex (MHC) class I molecules] is involved in cognitive dysfunction in various neurological diseases. Therefore, the present study tested the hypothesis that β2‑microglobulin in the brain also mediates sepsis‑induced cognitive impairment. In the present study, wild‑type and antigen processing 1 (Tap1)‑deficient mice (Tap1‑/‑) were subjected to cecal ligation and puncture (CLP). Survival rate, cognitive function, and biochemical analysis were performed at the indicated time points. The data revealed that CLP induced anxiety‑like behavior and impaired hippocampal‑dependent contextual memory in wild‑type mice, which was accompanied by hippocampal microglial activation, increased level of interleukin‑1β, and decreased concentrations of brain derived neurotrophic factor and postsynaptic density protein 95. Notably, it was demonstrated that Tap1‑/‑ mice with reduced cell surface expression of MHC I protected mice from anxiety‑like behavior and impaired hippocampal‑dependent contextual memory and reversed most of these biochemical parameters following sepsis development. In summary, the results of the present study suggest that β2‑microglobulin negatively regulates cognitive impairment in an animal model of sepsis induced by CLP.

Differential effects of MDMA and cocaine on inhibitory avoidance and object recognition tests in rodents

INTRODUCTION

Drug addiction continues being a major public problem faced by modern societies with different social, health and legal consequences for the consumers. Consumption of psychostimulants, like cocaine or MDMA (known as ecstasy) are highly prevalent and cognitive and memory impairments have been related with the abuse of these drugs.

AIM

The aim of this work was to review the most important data of the literature in the last 10 years about the effects of cocaine and MDMA on inhibitory avoidance and object recognition tests in rodents.

DEVELOPMENT

The object recognition and the inhibitory avoidance tests are popular procedures used to assess different types of memory. We compare the effects of cocaine and MDMA administration in these tests, taking in consideration different factors such as the period of life development of the animals (prenatal, adolescence and adult age), the presence of polydrug consumption or the role of environmental variables. Brain structures involved in the effects of cocaine and MDMA on memory are also described.

CONCLUSIONS

Cocaine and MDMA induced similar impairing effects on the object recognition test during critical periods of lifetime or after abstinence of prolonged consumption in adulthood. Deficits of inhibitory avoidance memory are observed only in adult rodents exposed to MDMA. Psychostimulant abuse is a potential factor to induce memory impairments and could facilitate the development of future neurodegenerative disorders.