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

A bibliometric analysis of studies on environmental enrichment spanning 1967-2024: patterns and trends over the years

Environmental Enrichment (EE) has received considerable attention for its potential to enhance cognitive and neurobiological outcomes in animal models. This bibliometric analysis offers a comprehensive evaluation of the EE research spanning from 1967 to 2024, utilizing data extracted from Scopus and analyzed through R and VOSviewer. The volume of publications, citation patterns, and collaborations were systematically reviewed, highlighting important contributions and emerging trends within the field of animal research. Core concepts of EE research are mapped, revealing key themes such as neuroplasticity, cognitive function, and behavioral outcomes. A significant increase in EE research is demonstrated, particularly after the year 2000, reflecting growing scientific and public interest in EE paradigms. This analysis provides insights into the global contributions and collaborative networks that have shaped EE studies over time. The role of EE in advancing the understanding of neurobiological, neurodevelopmental, and neurodegenerative processes is underscored. Influential contributors, leading countries, and high-impact journals in the field of EE are identified, offering a valuable resource for researchers seeking to understand or extend the current knowledge base. The strategic selection of keywords and rigorous data curation methods ensure that the findings accurately reflect the most impactful aspects of EE research in animals. This study serves as an essential reference for future explorations and applications of EE across disciplines. By providing a clear and structured overview of the field, this paper aims to serve as a foundation for ongoing and future research initiatives, encouraging more robust investigations and applications of EE to enhance cognitive and neurological health globally.

Environmental enrichment in middle age rats improves spatial and object memory discrimination deficits

Changes in memory performance are one of the main symptoms of normal aging. The storage of similar experiences as different memories (ie. behavioral pattern separation), becomes less efficient as aging progresses. Studies have focused on hippocampus dependent spatial memories and their role in the aging related deficits in behavioral pattern separation (BPS) by targeting high similarity interference conditions. However, parahippocampal cortices such as the perirhinal cortex are also particularly vulnerable to aging. Middle age is thought to be the stage where mild mnemonic deficits begin to emerge. Therefore, a better understanding of the timing of the spatial and object domain memory impairment could shed light over how plasticity changes in the parahipocampal-hippocampal system affects mnemonic function in early aging. In the present work, we compared the performance of young and middle-aged rats in both spatial (spontaneous location recognition) and non-spatial (spontaneous object recognition) behavioral pattern separation tasks to understand the comparative progression of these deficits from early stages of aging. Moreover, we explored the impact of environmental enrichment (EE) as an intervention with important translational value. Although a bulk of studies have examined the contribution of EE for preventing age related memory decline in diverse cognitive domains, there is limited knowledge of how this intervention could specifically impact on BPS function in middle-aged animals. Here we evaluate the effects of EE as modulator of BPS, and its ability to revert the deficits caused by normal aging at early stages. We reveal a domain-dependent impairment in behavioral pattern separation in middle-aged rats, with spatial memories affected independently of the similarity of the experiences and object memories only affected when the stimuli are similar, an effect that could be linked to the higher interference seen in this group. Moreover, we found that EE significantly enhanced behavioral performance in middle-aged rats in the spatial and object domain, and this improvement is specific of the high similarity load condition. In conclusion, these results suggest that memory is differentially affected by aging in the object and spatial domains, but that BPS function is responsive to an EE intervention in a multidomain manner.

The Hidden Dangers of Sedentary Living: Insights into Molecular, Cellular, and Systemic Mechanisms

With the aging of the global population, neurodegenerative diseases are emerging as a major public health issue. The adoption of a less sedentary lifestyle has been shown to have a beneficial effect on cognitive decline, but the molecular mechanisms responsible are less clear. Here we provide a detailed analysis of the complex molecular, cellular, and systemic mechanisms underlying age-related cognitive decline and how lifestyle choices influence these processes. A review of the evidence from animal models, human studies, and postmortem analyses emphasizes the importance of integrating physical exercise with cognitive, multisensory, and motor stimulation as part of a multifaceted approach to mitigating cognitive decline. We highlight the potential of these non-pharmacological interventions to address key aging hallmarks, such as genomic instability, telomere attrition, and neuroinflammation, and underscore the need for comprehensive and personalized strategies to promote cognitive resilience and healthy aging.

Mitigation of aging-related plasticity decline through taurine supplementation and environmental enrichment

Aging-related biochemical changes in nerve cells lead to dysfunctional synapses and disrupted neuronal circuits, ultimately affecting vital processes such as brain plasticity, learning, and memory. The imbalance between excitation and inhibition in synaptic function during aging contributes to cognitive impairment, emphasizing the importance of compensatory mechanisms. Fear conditioning-related plasticity of the somatosensory barrel cortex, relying on the proper functioning and extensive up regulation of the GABAergic system, in particular interneurons containing somatostatin, is compromised in aging (one-year-old) mice. The present research explores two potential interventions, taurine supplementation, and environmental enrichment, revealing their effectiveness in supporting learning-induced plasticity in the aging mouse brain. They do not act through a mechanism normalizing the Glutamate/GABA balance that is disrupted in aging. Still, they allow for increased somatostatin levels, an effect observed in young animals after learning. These findings highlight the potential of lifestyle interventions and diet supplementation to mitigate age-related cognitive decline by promoting experience-dependent plasticity.

Neural ageing and synaptic plasticity: prioritizing brain health in healthy longevity

Ageing is characterized by a gradual decline in the efficiency of physiological functions and increased vulnerability to diseases. Ageing affects the entire body, including physical, mental, and social well-being, but its impact on the brain and cognition can have a particularly significant effect on an individual's overall quality of life. Therefore, enhancing lifespan and physical health in longevity studies will be incomplete if cognitive ageing is over looked. Promoting successful cognitive ageing encompasses the objectives of mitigating cognitive decline, as well as simultaneously enhancing brain function and cognitive reserve. Studies in both humans and animal models indicate that cognitive decline related to normal ageing and age-associated brain disorders are more likely linked to changes in synaptic connections that form the basis of learning and memory. This activity-dependent synaptic plasticity reorganises the structure and function of neurons not only to adapt to new environments, but also to remain robust and stable over time. Therefore, understanding the neural mechanisms that are responsible for age-related cognitive decline becomes increasingly important. In this review, we explore the multifaceted aspects of healthy brain ageing with emphasis on synaptic plasticity, its adaptive mechanisms and the various factors affecting the decline in cognitive functions during ageing. We will also explore the dynamic brain and neuroplasticity, and the role of lifestyle in shaping neuronal plasticity.

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.

Senolytic treatment alleviates doxorubicin-induced chemobrain

Doxorubicin (Dox), a widely used treatment for cancer, can result in chemotherapy-induced cognitive impairments (chemobrain). Chemobrain is associated with inflammation and oxidative stress similar to aging. As such, Dox treatment has also been used as a model of aging. However, it is unclear if Dox induces brain changes similar to that observed during aging since Dox does not readily enter the brain. Rather, the mechanism for chemobrain likely involves the induction of peripheral cellular senescence and the release of senescence-associated secretory phenotype (SASP) factors and these SASP factors can enter the brain to disrupt cognition. We examined the effect of Dox on peripheral and brain markers of aging and cognition. In addition, we employed the senolytic, ABT-263, which also has limited access to the brain. The results indicate that plasma SASP factors enter the brain, activating microglia, increasing oxidative stress, and altering gene transcription. In turn, the synaptic function required for memory was reduced in response to altered redox signaling. ABT-263 prevented or limited most of the Dox-induced effects. The results emphasize a link between cognitive decline and the release of SASP factors from peripheral senescent cells and indicate some differences as well as similarities between advanced age and Dox treatment.

Recent Advancement in Elimination Strategies and Potential Rejuvenation Targets of Senescence

Cellular senescence is a state of exiting the cell cycle, resisting apoptosis, and changing phenotype. Senescent cells (SCs) can be identified by large, distorted morphology and irreversible inability to replicate. In early development, senescence has beneficial roles like tissue patterning and wound healing, where SCs are cleared by the immune system. However, there is a steep rise in SC number as organisms age. The issue with SC accumulation stems from the loss of cellular function, alterations of the microenvironment, and secretions of pro-inflammatory molecules, consisting of cytokines, chemokines, matrix metalloproteinases (MMPs), interleukins, and extracellular matrix (ECM)-associated molecules. This secreted cocktail is referred to as the senescence-associated secretory phenotype (SASP), a hallmark of cellular senescence. The SASP promotes inflammation and displays a bystander effect where paracrine signaling turns proliferating cells into senescent states. To alleviate age-associated diseases, researchers have developed novel methods and techniques to selectively eliminate SCs in aged individuals. Although studies demonstrated that selectively killing SCs improves age-related disorders, there are drawbacks to SC removal. Considering favorable aspects of senescence in the body, this paper reviews recent advancements in elimination strategies and potential rejuvenation targets of senescence to bring researchers in the field up to date.

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.

Age-dependent Effects of Dopamine on Working Memory and Synaptic Plasticity in Hippocampal CA3-CA1 Synapses in Mice

Normal aging in mammals is accompanied by a decline in learning and memory. Dopamine plays a vital role in regulating cognitive functions, but it declines with age: During non-pathological aging, dopamine levels, receptors, and transporters decrease. Regarding the role of the dopaminergic system's changes in old age, we examined the effect of age and applied dopamine on working memory, synaptic transmission, and long-term potentiation (LTP) induction and maintenance in young adult and mature adult mice. We employed the Y-maze spontaneous alteration test to evaluate working memory. Maturation had no observed effect on working memory performance. Interestingly, working memory performance increased following intracerebroventricular administration of dopamine only in mature adult mice. We employed evoked field potential recording (in vitro) to assess the effects of age and maturation on the long-term potentiation (LTP) induction and maintenance. There was no difference in LTP induction and maintenance between young and mature adult mice before dopamine application. However, the application of dopamine on mature adult murine slices increased LTP magnitude compared to slices from young adults. According to the obtained results, it may be concluded that hippocampal neural excitability increased in mature adult subjects, and application of dopamine abolished the difference in neural excitability among young mature and adult mature groups; which was accompanied with increment of working memory and synaptic potentiation in mature adult animals.

Effect of peripheral cellular senescence on brain aging and cognitive decline

We examine similar and differential effects of two senolytic treatments, ABT-263 and dasatinib + quercetin (D + Q), in preserving cognition, markers of peripheral senescence, and markers of brain aging thought to underlie cognitive decline. Male F344 rats were treated from 12 to 18 months of age with D + Q, ABT-263, or vehicle, and were compared to young (6 months). Both senolytic treatments rescued memory, preserved the blood-brain barrier (BBB) integrity, and prevented the age-related decline in hippocampal N-methyl-D-aspartate receptor (NMDAR) function associated with impaired cognition. Senolytic treatments decreased senescence-associated secretory phenotype (SASP) and inflammatory cytokines/chemokines in the plasma (IL-1β, IP-10, and RANTES), with some markers more responsive to D + Q (TNFα) or ABT-263 (IFNγ, leptin, EGF). ABT-263 was more effective in decreasing senescence genes in the spleen. Both senolytic treatments decreased the expression of immune response and oxidative stress genes and increased the expression of synaptic genes in the dentate gyrus (DG). However, D + Q influenced twice as many genes as ABT-263. Relative to D + Q, the ABT-263 group exhibited increased expression of DG genes linked to cell death and negative regulation of apoptosis and microglial cell activation. Furthermore, D + Q was more effective at decreasing morphological markers of microglial activation. The results indicate that preserved cognition was associated with the removal of peripheral senescent cells, decreasing systemic inflammation that normally drives neuroinflammation, BBB breakdown, and impaired synaptic function. Dissimilarities associated with brain transcription indicate divergence in central mechanisms, possibly due to differential access.

Viral vector-mediated upregulation of serine racemase expression in medial prefrontal cortex improves learning and synaptic function in middle age rats

An age-associated decrease in N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic function contributes to impaired synaptic plasticity and is associated with cognitive impairments. Levels of serine racemase (SR), an enzyme that synthesizes D-serine, an NMDAR co-agonist, decline with age. Thus, enhancing NMDAR function via increased SR expression in middle age, when subtle declines in cognition emerge, was predicted to enhance performance on a prefrontal cortex-mediated task sensitive to aging. Middle-aged (~12 mo) male Fischer-344 rats were injected bilaterally in the medial prefrontal cortex (mPFC) with viral vector (LV), SR (LV-SR) or control (LV-GFP). Rats were trained on the operant attentional set-shift task (AST) to examine cognitive flexibility and attentional function. LV-SR rats exhibited a faster rate of learning compared to controls during visual discrimination of the AST. Extradimensional set shifting and reversal were not impacted. Immunohistochemical analyses demonstrated that LV-SR significantly increased SR expression in the mPFC. Electrophysiological characterization of synaptic transmission in the mPFC slices obtained from LV-GFP and LV-SR animals indicated a significant increase in isolated NMDAR-mediated synaptic responses in LV-SR slices. Thus, results of the current study demonstrated that prefrontal SR upregulation in middle age rats can improve learning of task contingencies for visual discrimination and increase glutamatergic synaptic transmission, including NMDAR activity.

Intense long-term training impairs brain health compared with moderate exercise: Experimental evidence and mechanisms

The consequences of extremely intense long-term exercise for brain health remain unknown. We studied the effects of strenuous exercise on brain structure and function, its dose-response relationship, and mechanisms in a rat model of endurance training. Five-week-old male Wistar rats were assigned to moderate (MOD) or intense (INT) exercise or a sedentary (SED) group for 16 weeks. MOD rats showed the highest motivation and learning capacity in operant conditioning experiments; SED and INT presented similar results. In vivo MRI demonstrated enhanced global and regional connectivity efficiency and clustering as well as a higher cerebral blood flow (CBF) in MOD but not INT rats compared with SED. In the cortex, downregulation of oxidative phosphorylation complex IV and AMPK activation denoted mitochondrial dysfunction in INT rats. An imbalance in cortical antioxidant capacity was found between MOD and INT rats. The MOD group showed the lowest hippocampal brain-derived neurotrophic factor levels. The mRNA and protein levels of inflammatory markers were similar in all groups. In conclusion, strenuous long-term exercise yields a lesser improvement in learning ability than moderate exercise. Blunting of MOD-induced improvements in CBF and connectivity efficiency, accompanied by impaired mitochondrial energetics and, possibly, transient local oxidative stress, may underlie the findings in intensively trained rats.

Older people with severe loneliness have an atrophied thalamus, hippocampus, and entorhinal cortex

OBJECTIVES

Loneliness has been shown to increase the risk of dementia. However, it is unclear why greater loneliness is associated with greater susceptibility to dementia. Herein, we aimed to examine the morphological characteristics of the brain associated with loneliness in older people concerned about cognitive dysfunction.

METHODS

In this retrospective study, 110 participants (80 with amnestic mild cognitive impairment, and 30 cognitively healthy individuals) were included. Participants were assessed using the revised University of California at Los Angeles (UCLA) loneliness scale and had undergone magnetic resonance imaging. Spearman correlation analysis and Mann-Whitney U tests were used to examine the clinical factors associated with loneliness. Multiple regression was performed to examine the relationship between the revised UCLA loneliness scale score and regional gray matter (GM) volume on voxel-based morphometry.

RESULTS

The revised UCLA loneliness scale scores were not significantly correlated with age, sex, or mini-mental state examination (MMSE) scores. Multiple regression using age, sex, MMSE score, and total brain volume as covariates showed negative correlations of the revised UCLA loneliness scale scores with the grey matter volume in regions centered on the bilateral thalamus, left hippocampus and left parahippocampal gyrus, and left entorhinal area.

CONCLUSIONS

Subjective loneliness was associated with decreased GM volume in the bilateral thalamus, left hippocampus, and left entorhinal cortex of the brain in the older people, thereby providing a morphological basis for the increased risk of dementia associated with greater loneliness.

Restoring Age-Related Cognitive Decline through Environmental Enrichment: A Transcriptomic Approach

Cognitive decline is one of the greatest health threats of old age and the maintenance of optimal brain function across a lifespan remains a big challenge. The hippocampus is considered particularly vulnerable but there is cross-species consensus that its functional integrity benefits from the early and continuous exercise of demanding physical, social and mental activities, also referred to as environmental enrichment (EE). Here, we investigated the extent to which late-onset EE can improve the already-impaired cognitive abilities of lifelong deprived C57BL/6 mice and how it affects gene expression in the hippocampus. To this end, 5- and 24-month-old mice housed in standard cages (5mSC and 24mSC) and 24-month-old mice exposed to EE in the last 2 months of their life (24mEE) were subjected to a Barnes maze task followed by next-generation RNA sequencing of the hippocampal tissue. Our analyses showed that late-onset EE was able to restore deficits in spatial learning and short-term memory in 24-month-old mice. These positive cognitive effects were reflected by specific changes in the hippocampal transcriptome, where late-onset EE affected transcription much more than age (24mSC vs. 24mEE: 1311 DEGs, 24mSC vs. 5mSC: 860 DEGs). Remarkably, a small intersection of 72 age-related DEGs was counter-regulated by late-onset EE. Of these, Bcl3, Cttnbp2, Diexf, Esr2, Grb10, Il4ra, Inhba, Rras2, Rps6ka1 and Socs3 appear to be particularly relevant as key regulators involved in dendritic spine plasticity and in age-relevant molecular signaling cascades mediating senescence, insulin resistance, apoptosis and tissue regeneration. In summary, our observations suggest that the brains of aged mice in standard cage housing preserve a considerable degree of plasticity. Switching them to EE proved to be a promising and non-pharmacological intervention against cognitive decline.

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.

Exercise ameliorates aberrant synaptic plasticity without enhancing adult-born cell survival in the hippocampus of serotonin transporter knockout mice

Deficits in hippocampal cellular and synaptic plasticity are frequently associated with cognitive and mood disorders, and indeed common mechanisms of antidepressants are thought to involve neuroplastic processes. Here, we investigate hippocampal adult-born cell survival and synaptic plasticity (long-term potentiation, LTP, and long-term depression, LTD) in serotonin transporter (5-HTT) knockout (KO) mice. From 8 weeks of age, mice either continued in standard-housing conditions or were given access to voluntary running wheels for 1 month. Electrophysiology was performed on hippocampal slices to measure LTP and LTD, and immunohistochemistry was used to assess cell proliferation and subsequent survival in the dentate gyrus. The results revealed a reduced LTP in 5-HTT KO mice that was restored to wild-type (WT) levels after chronic exercise. While LTD appeared normal in 5-HTT KO, exercise decreased the magnitude of LTD in both WT and 5-HTT KO mice. Furthermore, although 5-HTT KO mice had normal hippocampal adult-born cell survival, they did not benefit from the pro-proliferative effects of exercise observed in WT animals. Taken together, these findings suggest that reduced 5-HTT expression is associated with significant alterations to functional neuroplasticity. Interestingly, 5-HTT appeared necessary for exercise-induced augmentation of adult-born hippocampal cell survival, yet exercise corrected the LTP impairment displayed by 5-HTT KO mice. Together, our findings further highlight the salience of serotonergic signalling in mediating the neurophysiological benefits of exercise.

The influence of sensory experience on the glutamatergic synapse

The ability of glutamatergic synaptic strength to change in response to prevailing neuronal activity is believed to underlie the capacity of animals, including humans, to learn from experience. This learning better equips animals to safely navigate challenging and potentially harmful environments, while reinforcing behaviours that are conducive to survival. Early descriptions of the influence of experience on behaviour were provided by Donald Hebb who showed that an enriched environment improved performance of rats in a variety of behavioural tasks, challenging the widely-held view at the time that psychological development and intelligence were largely predetermined through genetic inheritance. Subsequent studies in a variety of species provided detailed cellular and molecular insights into the neurobiological adaptations associated with enrichment and its counterparts, isolation and deprivation. Here we review those experience-dependent changes that occur at the glutamatergic synapse, and which likely underlie the enhanced cognition associated with enrichment. We focus on the importance of signalling initiated by the release of BDNF and a prime downstream effector, MSK1, in orchestrating the many structural and functional neuronal adaptations associated with enrichment. In particular we discuss the MSK1-dependent expansion of the dynamic range of the glutamatergic synapse, which may allow enhanced information storage or processing, and the establishment of a genomic homeostasis that may both stabilise the enriched brain, and may make it better able to respond to novel experiences.

Partial microglial depletion is associated with impaired hippocampal synaptic and cognitive function in young and aged rats

The role of microglia in mediating age-related changes in cognition and hippocampal synaptic function was examined by microglial depletion and replenishment using PLX3397. We observed age-related differences in microglial number and morphology, as well as increased Iba-1 expression, indicating microglial activation. PLX3397 treatment decreased microglial number, with aged rats exhibiting the lowest density. Young rats exhibited increased expression of pro-inflammatory cytokines during depletion and repopulation and maintenance of Iba-1 levels despite reduced microglial number. For aged rats, several cytokines increased with depletion and recovered during repopulation; however, aged rats did not fully recover microglial cell number or Iba-1 expression during repopulation, with a recovery comparable to young control levels rather than aged controls. Hippocampal CA3-CA1 synaptic transmission was impaired with age, and microglial depletion was associated with decreased total synaptic transmission in young and aged rats. A robust decline in N-methyl-d-aspartate-receptor-mediated synaptic transmission arose in young depleted rats specifically. Microglial replenishment normalized depletion-induced synaptic function to control levels; however, recovery of aged animals did not mirror young. Microglial depletion was associated with decreased context-object discrimination memory in both age groups, which recovered with microglial repopulation. Aged rats displayed impaired contextual and cued fear memory, and microglial replenishment did not recover their memory to the level of young. The current study indicates that cognitive function and synaptic transmission benefit from the support of aged microglia and are hindered by removal of these cells. Replenishment of microglia in aging did not ameliorate age-related cognitive impairments or senescent synaptic function.

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 Quest for the Hippocampal Memory Engram: From Theories to Experimental Evidence

More than a century after Richard Semon's theoretical proposal of the memory engram, technological advancements have finally enabled experimental access to engram cells and their functional contents. In this review, we summarize theories and their experimental support regarding hippocampal memory engram formation and function. Specifically, we discuss recent advances in the engram field which help to reconcile two main theories for how the hippocampus supports memory formation: The Memory Indexing and Cognitive Map theories. We also highlight the latest evidence for engram allocation mechanisms through which memories can be linked or separately encoded. Finally, we identify unanswered questions for future investigations, through which a more comprehensive understanding of memory formation and retrieval may be achieved.

In aged rats, differences in spatial learning and memory influence the response to late-life Environmental Enrichment

It has clearly been demonstrated that cognitive stimulation, physical exercise, and social engagement help counteract age-related cognitive decline. However, several important issues remain to be addressed. Given the wide differences in cognitive impairment found among individuals of the same age, identifying the subjects who will benefit most from late-life interventions is one such issue. Environmental Enrichment (EE) is a particularly valuable approach to do this. In this study, aged (21-month-old) rats were assigned to a better (BL) or a worse (WL) learner group (training phase) and to a non-impaired (NI) or an impaired (I) group (probe phase) by their performance on the Morris Water Maze, using the test performances of adult (12-month-old) rats as the cut-offs. The aged rats were retested after a 12-week EE or standard housing (SH) protocol. After 12 weeks, the performances of SH rats had deteriorated, whereas all rats benefited from EE, albeit in different ways. In particular, the animals assigned to the BL and the NI groups prior to EE still performed as well as the adult rats (performance preservation) whereas, critically, the animals assigned to the WL and the I groups before EE showed such improved performances that they reached the level of the adult rats (performance improvement), despite having aged further. EE seems to induce the preservation in BLs and the improvement in WLs of spatial search strategies and the preservation in NIs and the increase in Is of a focused and protract research of the escape point. Our findings suggest that late-life EE prevents spatial learning and memory decline in still cognitively preserved animals and stimulates residual functional reserve in already cognitively compromised animals. Future research should focus on individually tailored stimulation protocols to improve their effect and afford a better understanding of the underlying processes.

Effects of Gestational Inflammation with Postpartum Enriched Environment on Age-Related Changes in Cognition and Hippocampal Synaptic Plasticity-Related Proteins

Increasing evidence indicates that exposure to inflammation during pregnancy intensifies the offspring’s cognitive impairment during aging, which might be correlated with changes in some synaptic plasticity-related proteins. In addition, an enriched environment (EE) can significantly exert a beneficial impact on cognition and synaptic plasticity. However, it is unclear whether gestational inflammation combined with postnatal EE affects the changes in cognition and synaptic plasticity-related proteins during aging. In this study, pregnant mice were intraperitoneally injected with lipopolysaccharides (LPS, 50 μg/kg) or normal saline at days 15–17 of pregnancy. At 21 days after delivery, some LPS-treated mice were randomly selected for EE treatment. At the age of 6 and 18 months, Morris water maze (MWM) and western blotting were, respectively, used to evaluate or measure the ability of spatial learning and memory and the levels of postsynaptic plasticity-related proteins in the hippocampus, including postsynaptic density protein 95 (PSD-95), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) GluA1 subunit, and Homer-1b/c. The results showed that 18-month-old control mice had worse spatial learning and memory and lower levels of these synaptic plasticity-related proteins (PSD-95, GluA1, and Homer-1b/c) than the 6-month-old controls. Gestational LPS exposure exacerbated these age-related changes of cognition and synaptic proteins, but EE could alleviate the treatment effect of LPS. In addition, the performance during learning and memory periods in the MWM correlated with the hippocampal levels of PSD-95, GluA1, and Homer-1b/c. Our results suggested that gestational inflammation accelerated age-related cognitive impairment and the decline of PSD-95, GluA1, and Homer-1b/c protein expression, and postpartum EE could alleviate these changes.

Age-related memory decline, dysfunction of the hippocampus and therapeutic opportunities

While the aging of the population is a sign of progress for societies, it also carries its load of negative aspects. Among them, cognitive decline and in particular memory loss is a common feature of non-pathological aging. Autobiographical memories, which rely on the hippocampus, are a primary target of age-related cognitive decline. Here, focusing on the neurobiological mechanisms of memory formation and storage, we describe how hippocampal functions are altered across time in non-pathological mammalian brains. Several hallmarks of aging have been well described over the last decades; among them, we consider altered synaptic communication and plasticity, reduction of adult neurogenesis and epigenetic alterations. Building on the neurobiological processes of cognitive aging that have been identified to date, we review some of the strategies based on lifestyle manupulation allowing to address age-related cognitive deficits.

Enriching hippocampal memory function in older adults through video games

Healthy aging is accompanied by a steady cognitive decline with clear losses in memory. Animal studies have consistently demonstrated that simply modifying an animal's living environment (known as environmental enrichment) can have a positive influence on age-related cognitive decline in the hippocampus. Previously, we showed that playing immersive 3D video games can improve hippocampal-based memory in young healthy adults, suggesting that the exploration of the large open worlds of modern-day video games may act as proxy for environmental enrichment in humans. Here, we replicated our previous video game study in healthy older adults, showing that playing video games for four weeks can improve hippocampal-based memory in a population that is already experiencing age-related decline in memory. Furthermore, we showed that the improvements last for up to four weeks past the intervention, highlighting the potential of video games as intervention for age-related cognitive decline.

Blood factors transfer beneficial effects of exercise on neurogenesis and cognition to the aged brain

Reversing brain aging may be possible through systemic interventions such as exercise. We found that administration of circulating blood factors in plasma from exercised aged mice transferred the effects of exercise on adult neurogenesis and cognition to sedentary aged mice. Plasma concentrations of glycosylphosphatidylinositol (GPI)-specific phospholipase D1 (Gpld1), a GPI-degrading enzyme derived from liver, were found to increase after exercise and to correlate with improved cognitive function in aged mice, and concentrations of Gpld1 in blood were increased in active, healthy elderly humans. Increasing systemic concentrations of Gpld1 in aged mice ameliorated age-related regenerative and cognitive impairments by altering signaling cascades downstream of GPI-anchored substrate cleavage. We thus identify a liver-to-brain axis by which blood factors can transfer the benefits of exercise in old age.

Differential Effect of Repeated Lipopolysaccharide Treatment and Aging on Hippocampal Function and Biomarkers of Hippocampal Senescence

Markers of brain aging and cognitive decline are thought to be influenced by peripheral inflammation. This study compared the effects of repeated lipopolysaccharide (LPS) treatment in young rats to age-related changes in hippocampal-dependent cognition and transcription. Young Fischer 344 X Brown Norway hybrid rats were given intraperitoneal injections once a week for 7 weeks with either LPS or vehicle. Older rats received a similar injection schedule of vehicle. Old vehicle and young LPS rats exhibited a delay-dependent impairment in spatial memory. Further, LPS treatment reduced the hippocampal CA3-CA1 synaptic response. RNA sequencing, performed on CA1, indicated an increase in genes linked to neuroinflammation in old vehicle and young LPS animals. In contrast to an age-related decrease in transcription of synaptic genes, young LPS animals exhibited increased expression of genes that support the growth and maintenance of synapses. We suggest that the increased expression of genes for growth and maintenance of synapses in young animals represents neuronal resilience/recovery in response to acute systemic inflammation. Thus, the results indicate that repeated LPS treatment does not completely recapitulate the aging phenotype for synaptic function, possibly due to the chronic nature of systemic inflammation in aging and resilience of young animals to acute treatments.

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.

Calcium Signaling During Brain Aging and Its Influence on the Hippocampal Synaptic Plasticity

Calcium (Ca²⁺) ions are highly versatile intracellular signaling molecules and are universal second messenger for regulating a variety of cellular and physiological functions including synaptic plasticity. Ca²⁺ homeostasis in the central nervous system endures subtle dysregulation with advancing age. Research has provided abundant evidence that brain aging is associated with altered neuronal Ca²⁺ regulation and synaptic plasticity mechanisms. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during aging. The current chapter takes a specific perspective, assessing various Ca²⁺ sources and the influence of aging on Ca²⁺ sources and synaptic plasticity in the hippocampus. Integrating the knowledge of the complexity of age-related alterations in neuronal Ca²⁺ signaling and synaptic plasticity mechanisms will positively shape the development of highly effective therapeutics to treat brain disorders including cognitive impairment associated with aging and neurodegenerative disease.

Learning and aging affect neuronal excitability and learning

The first study that demonstrated a change in intrinsic neuronal excitability after learning in ex vivo brain tissue slices from a mammal was published over thirty years ago. Numerous other manuscripts describing similar learning-related changes have followed over the years since the original paper demonstrating the postburst afterhyperpolarization (AHP) reduction in CA1 pyramidal neurons from rabbits that learned delay eyeblink conditioning was published. In addition to the learning-related changes, aging-related enlargement of the postburst AHP in CA1 pyramidal neurons have been reported. Extensive work has been done relating slow afterhyperpolarization enhancement in CA1 hippocampus to slowed learning in some aging animals. These reproducible findings strongly implicate modulation of the postburst AHP as an essential cellular mechanism necessary for successful learning, at least in learning tasks that engage CA1 hippocampal pyramidal neurons.

Environmental Training and Synaptic Functions in Young and Old Brain: A Presynaptic Perspective

BACKGROUND

Aging is an unavoidable, physiological process that reduces the complexity and the plasticity of the synaptic contacts in Central Nervous System (CNS), having profound implications for human well-being. The term "cognitive reserve" refers to central cellular adaptations that augment the resilience of human brain to damage and aging. The term "Cognitive training" indicates the cultural, social and physical stimulations proposed as add-on therapy for the cure of central neurological diseases. "Cognitive training" reinforces the "cognitive reserve" permitting to counteract brain impairments and rejuvenating synaptic complexity. The research has begun investigating the clinical impact of the "cognitive training" in aged people, but additional work is needed to definitively assess its effectiveness. In particular, there is a need to understand, from a preclinical point of view, whether "cognitive training" promotes compensatory effects or, alternatively, if it elicits genuine recovery of neuronal defects. Although the translation from rodent studies to the clinical situation could be difficult, the results from pre-clinical models are of high clinical relevance, since they should allow a better understanding of the effects of environmental interventions in aging-associated chronic derangements in mammals.

CONCLUSION

Data in literature and the recent results obtained in our laboratory concerning the impact of environmental stimulation on the presynaptic release of noradrenaline, glutamate and gamma amino butyric acid (GABA) suggest that these neurotransmitters undergo different adaptations during aging and that they are differently tuned by "cognitive training". The impact of "cognitive training" on neurotransmitter exocytosis might account for the cellular events involved in reinforcement of "cognitive reserve" in young and old animals.

Structural Brain Correlates of Loneliness among Older Adults

Ample evidence indicates that loneliness in old age is associated with poor bodily and mental health. However, little is known about structural cerebral correlates of loneliness in healthy older adults. We examined such correlates in a magnetic resonance imaging (MRI) subsample of 319 older adults aged 61 to 82 years drawn from the Berlin Aging Study II. Using voxel-based morphometry (VBM) and structural equation modeling (SEM), latent hierarchical regression analyses were performed to examine associations of (i) loneliness, (ii) a range of covariates, and (iii) loneliness by covariate interactions with latent brain volume estimates of brain structures known to be involved in processing, expressing, and regulating emotions. Results from whole-brain VBM analyses showed that individuals with higher loneliness scores tended to have smaller gray matter volumes in three clusters comprising (i) the left amygdala/anterior hippocampus, (ii) the left posterior parahippocampus and (iii) the left cerebellum. Significant associations and interactions between loneliness and latent factors for the amygdala and the hippocampus were confirmed with a region-of-interest (ROI)-based approach. These findings suggest that individual differences in loneliness among older adults are correlated with individual differences in the volumes of brain regions that are central to cognitive processing and emotional regulation, also after correcting for confounders such as social network size. We discuss possible mechanisms underlying these associations and their implications.

Senescent neurophysiology: Ca2+ signaling from the membrane to the nucleus

The current review provides a historical perspective on the evolution of hypothesized mechanisms for senescent neurophysiology, focused on the CA1 region of the hippocampus, and the relationship of senescent neurophysiology to impaired hippocampal-dependent memory. Senescent neurophysiology involves processes linked to calcium (Ca²⁺) signaling including an increase in the Ca²⁺-dependent afterhyperpolarization (AHP), decreasing pyramidal cell excitability, hyporesponsiveness of N-methyl-D-aspartate (NMDA) receptor function, and a shift in Ca²⁺-dependent synaptic plasticity. Dysregulation of intracellular Ca²⁺ and downstream signaling of kinase and phosphatase activity lies at the core of senescent neurophysiology. Ca²⁺-dysregulation involves a decrease in Ca²⁺ influx through NMDA receptors and an increase release of Ca²⁺ from internal Ca²⁺ stores. Recent work has identified changes in redox signaling, arising in middle-age, as an initiating factor for senescent neurophysiology. The shift in redox state links processes of aging, oxidative stress and inflammation, with functional changes in mechanisms required for episodic memory. The link between age-related changes in Ca²⁺ signaling, epigenetics and gene expression is an exciting area of research. Pharmacological and behavioral intervention, initiated in middle-age, can promote memory function by initiating transcription of neuroprotective genes and rejuvenating neurophysiology. However, with more advanced age, or under conditions of neurodegenerative disease, epigenetic changes may weaken the link between environmental influences and transcription, decreasing resilience of memory function.

Exercise for Brain Health: An Investigation into the Underlying Mechanisms Guided by Dose

There is a strong link between the practice of regular physical exercise and maintenance of cognitive brain health. Animal and human studies have shown that exercise exerts positive effects on cognition through a variety of mechanisms, such as changes in brain volume and connectivity, cerebral perfusion, synaptic plasticity, neurogenesis, and regulation of trophic factors. However, much of this data has been conducted in young humans and animals, raising questions regarding the generalizability of these findings to aging adults. Furthermore, it is not clear at which doses these effects might take place, and if effects would differ with varying exercise modes (such as aerobic, resistance training, combinations, or other). The purpose of this review is to summarize the evidence on the effects of exercise interventions on various mechanisms believed to support cognitive improvements: cerebral perfusion, synaptic neuroplasticity, brain volume and connectivity, neurogenesis, and regulation of trophic factors. We synthesized the findings according to exposure to exercise (short- [1 day-16 weeks], medium- [24-40 weeks], and long-term exercise [52 weeks and beyond]) and have limited our discussion of dose effects to studies in aging adults and aged animals (when human data was not available).

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.

Physical Activity Alleviates Cognitive Dysfunction of Alzheimer's Disease through Regulating the mTOR Signaling Pathway

Alzheimer's disease (AD) is one of the most common aging-related progressive neurodegenerative disorders, and can result in great suffering for a large portion of the aged population. Although the pathogenesis of AD is being elucidated, the exact mechanisms are still unclear, thereby impeding the development of effective drugs, supplements, and other interventional strategies for AD. In recent years, impaired autophagy associated with microRNA (miRNA) dysfunction has been reported to be involved in aging and aging-related neurodegenerative diseases. Therefore, miRNA-mediated regulation for the functional status of autophagy may become one of the potent interventional strategies for AD. Mounting evidence from in vivo AD models has demonstrated that physical activity can exert a neuroprotective role in AD. In addition, autophagy is strictly regulated by the mTOR signaling pathway. In this article, the regulation of the functional status of autophagy through the mTOR signaling pathway during physical activity is systematically discussed for the prevention and treatment of AD. This concept will be beneficial to developing novel and effective targets that can create a direct link between pharmacological intervention and AD in the future.

Systemic administration of a fibroblast growth factor receptor 1 agonist rescues the cognitive deficit in aged socially isolated rats

Social isolation predominantly occurs in elderly people and it is strongly associated with cognitive decline. However, the mechanisms that produce isolation-related cognitive dysfunction during aging remain unclear. Here, we evaluated the cognitive, electrophysiological, and morphological effects of short- (4 weeks) and long-term (12 weeks) social isolation in aged male Wistar rats. Long-term but not short-term social isolation increased the plasma corticosterone levels and impaired spatial memory in the Morris water maze. Moreover, isolated animals displayed dampened hippocampal long-term potentiation in vivo, both in the dentate gyrus (DG) and CA1, as well as a specific reduction in the volume of the stratum oriens and spine density in CA1. Interestingly, social isolation induced a transient increase in hippocampal basic fibroblast growth factor (FGF2), whereas fibroblast growth factor receptor 1 (FGFR1) levels only increased after long-term isolation. Importantly, subchronic systemic administration of FGL, a synthetic peptide that activates FGFR1, rescued spatial memory in long-term isolated rats. These findings provide new insights into the neurobiological mechanisms underlying the detrimental effects on memory of chronic social isolation in the aged.

Exercising New Neurons to Vanquish Alzheimer Disease

Alzheimer disease (AD) is the most common type of dementia in individuals over 65 years of age. The neuropathological hallmarks of the condition are Tau neurofibrillary tangles and Amyloid-β senile plaques. Moreover, certain susceptible regions of the brain experience a generalized lack of neural plasticity and marked synaptic alterations during the progression of this as yet incurable disease. One of these regions, the hippocampus, is characterized by the continuous addition of new neurons throughout life. This phenomenon, named adult hippocampal neurogenesis (AHN), provides a potentially endless source of new synaptic elements that increase the complexity and plasticity of the hippocampal circuitry. Numerous lines of evidence show that physical activity and environmental enrichment (EE) are among the most potent positive regulators of AHN. Given that neural plasticity is markedly decreased in many neurodegenerative diseases, the therapeutic potential of making certain lifestyle changes, such as increasing physical activity, is being recognised in several non-pharmacologic strategies seeking to slow down or prevent the progression of these diseases. This review article summarizes current evidence supporting the putative therapeutic potential of EE and physical exercise to increase AHN and hippocampal plasticity both under physiological and pathological circumstances, with a special emphasis on neurodegenerative diseases and AD.

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.

Lifelong environmental enrichment in the absence of exercise protects the brain from age-related cognitive decline

Environmental manipulations enhance neuroplasticity, with enrichment-induced cognitive improvements linked to increased expression of growth factors and enhanced hippocampal neurogenesis. Environmental enrichment (EE) is defined as the addition of social, physical and somatosensory stimulation into an animal's environment via larger group housing, extra objects and, often, running wheels. Previous studies from our laboratory report that physical activity is a potent memory enhancer but that long-term environmental stimulation can be as effective as exercise at ameliorating age-related memory decline. To assess the effects of EE, in the absence of exercise, rats were housed in continuous enriched conditions for 20 months and memory assessed at young, middle aged and aged timepoints. MRI scans were also performed at these timepoints to assess regional changes in grey matter and blood flow with age, and effects of EE upon these measures. Results show an age-related decline in recognition, spatial and working memory that was prevented by EE. A parallel reduction in βNGF in hippocampus, and cell proliferation in the dentate gyrus, was prevented by EE. Furthermore, EE attenuated an age-related increase in apoptosis and expression of pro-inflammatory markers IL-1β and CD68. Long-term EE induced region-specific changes in grey matter intensity and partially rescued age-related reductions in cerebral blood flow. This study demonstrates that sensory enrichment alone can ameliorate many features typical of the ageing brain, such as increases in apoptosis and pro-inflammatory markers. Furthermore, we provide novel data on enrichment-induced regional grey matter alterations and age-related changes in blood flow in the rat. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Heart rate variability and baroreflex sensitivity in bilateral lung transplant recipients

The effects of lung afferents denervation on cardiovascular regulation can be assessed on bilateral lung transplantation patients. The high-frequency component of heart rate variability is known to be synchronous with breathing frequency. Then, if heart beat is neurally modulated by breathing frequency, we may expect disappearance of high frequency of heart rate variability in bilateral lung transplantation patients. On 11 patients and 11 matching healthy controls, we measured R-R interval (electrocardiography), blood pressure (Portapres^® ) and breathing frequency (ultrasonic device) in supine rest, during 10-min free breathing, 10-min cadenced breathing (0·25 Hz) and 5-min handgrip. We analysed heart rate variability and spontaneous variability of arterial blood pressure, by power spectral analysis, and baroreflex sensitivity, by the sequence method. Concerning heart rate variability, with respect to controls, transplant recipients had lower total power and lower low- and high-frequency power. The low-frequency/high-frequency ratio was higher. Concerning systolic, diastolic and mean arterial pressure variability, transplant recipients had lower total power (only for cadenced breathing), low frequency and low-frequency/high-frequency ratio during free and cadenced breathing. Baroreflex sensitivity was decreased. Denervated lungs induced strong heart rate variability reduction. The higher low-frequency/high-frequency ratio suggested that the total power drop was mostly due to high frequency. These results support the hypothesis that neural modulation from lung afferents contributes to the high frequency of heart rate variability.

Environmental enrichment improves hippocampal function in aged rats by enhancing learning and memory, LTP, and mGluR5-Homer1c activity

Previous studies from our laboratory have shown that environmental enrichment (EE) in young rats results in improved learning ability and enhanced metabotropic glutamate receptor-dependent long-term potentiation (mGluR-dependent LTP) resulting from sustained activation of p70S6 kinase. Here, we investigated whether 1-month EE is sufficient to improve hippocampus-dependent learning and memory and enhance hippocampal LTP in 23-24 month-old Fischer 344 male rats. Aged rats were housed in environmentally enriched, socially enriched, or standard housing conditions. We find that aged rats exposed to 1-month of EE demonstrate enhanced learning and memory relative to standard housed controls when tested in the Morris water maze and novel object recognition behavioral tasks. Furthermore, we find that environmentally enriched rats perform significantly better than socially enriched or standard housed rats in the radial-arm water maze and display enhanced mGluR5-dependent hippocampal LTP. Enhanced hippocampal function results from activity-dependent increases in the levels of mGluR5, Homer1c, and phospho-p70S6 kinase. These findings demonstrate that a short exposure of EE to aged rats can have significant effects on hippocampal function.

miRNA in Circulating Microvesicles as Biomarkers for Age-Related Cognitive Decline

Community dwelling older individuals from the North Florida region were examined for health status and a comprehensive neuropsychological battery, including the Montreal Cognitive Assessment (MoCA), was performed on each participant. A subpopulation (58 females and 39 males) met the criteria for age (60–89) and no evidence of mild cognitive impairment, with a MoCA score ≥23. Despite the stringent criteria for participation, MoCA scores were negatively correlated within the limited age range. Extracellular microvesicles were isolated from the plasma and samples were found to be positive for the exosome marker CD63, with an enrichment of particles within the size range for exosomes. miRNA was extracted and examined using next generation sequencing with a stringent criterion (average of ≥10 counts per million reads) resulting in 117 miRNA for subsequent analysis. Characterization of expression confirmed pervious work concerning the relative abundance and overall pattern of expression of miRNA in plasma. Correlation analysis indicated that most of the miRNAs (74 miRNAs) were positively correlated with age (p <0.01). Multiple regression was employed to identify the relationship of miRNA expression and MoCA score, accounting for age. MoCA scores were negatively correlated with 13 miRNAs. The pattern of expression for cognition-related miRNA did not match that previously described for Alzheimer’s disease. Enrichment analysis was employed to identify miRNA–gene interactions to reveal possible links to brain function.

Indomethacin Increases Neurogenesis across Age Groups and Improves Delayed Probe Trial Difference Scores in Middle-Aged Rats

We tested whether indomethacin or rosiglitazone treatment could rejuvenate spatial ability and hippocampal neurogenesis in aging rats. Young (4 mo; n = 30), middle-aged (12 mo; n = 31), and aged (18 mo; n = 31) male Fischer 344 rats were trained and then tested in a rapid acquisition water maze task and then fed vehicle (500 μl strawberry milk), indomethacin (2.0 mg/ml), or rosiglitazone (8.0 mg/ml) twice daily for the remainder of the experiment. A week after drug treatment commenced, the rats were given 3 daily BrdU (50 mg/kg) injections to test whether age-related declines in neurogenesis were reversed. One week after the final BrdU injection (~2.5 weeks after the 1st water maze session), the rats were trained to a find novel hidden water maze platform location, tested on 15 min and 24 h probe trials and then killed 24 h later. During the first water maze session, young rats outperformed aged rats but all rats learned information about the hidden platform location. Middle-aged and aged rats exhibited better memory probe trial performances than young rats in the 2nd water maze session and indomethacin improved memory probe trial performances on the 2nd vs. 1st water maze session in middle-aged rats. Middle-aged rats with more new neurons had fewer phagocytic microglia and exhibited better hidden platform training trial performances on the 2nd water maze session. Regardless of age, indomethacin increased new hippocampal neuron numbers and both rosiglitazone and indomethacin increased subependymal neuroblasts/neuron densities. Taken together, our results suggest the feasibility of studying the effects of longer-term immunomodulation on age-related declines in cognition and neurogenesis.

Sex differences in aerobic exercise efficacy to improve cognition: A systematic review and meta-analysis of studies in older rodents

Research in humans indicates that women may show greater cognitive benefits from aerobic training (AT) than men. To determine whether this sex difference extends to rodents, we conducted a systematic review and meta-analysis of studies in healthy, older rodents. Results indicate that compared to controls, AT improved hippocampus-dependent and -independent learning and memory. A sex difference was found with males showing larger benefits from AT on conditioned-avoidance and non-spatial memory tasks. AT also increased brain-derived neurotrophic factor compared to controls, with larger effects in females. As an exploratory analysis, sex differences in voluntary AT were examined separately from forced AT. Voluntary AT enhanced non-spatial memory to a greater extent in males. Forced AT enhanced hippocampus-dependent learning and memory more so in females. These findings suggest that sex is an important factor to consider, and studies directly assessing sex differences in the ability of exercise to improve brain function are needed.

Mechanisms of Hippocampal Aging and the Potential for Rejuvenation

The past two decades have seen remarkable progress in our understanding of the multifactorial drivers of hippocampal aging and cognitive decline. Recent findings have also raised the possibility of functional rejuvenation in the aged hippocampus. In this review, we aim to synthesize the mechanisms that drive hippocampal aging and evaluate critically the potential for rejuvenation. We discuss the functional changes in synaptic plasticity and regenerative potential of the aged hippocampus, followed by mechanisms of microglia aging, and assess the cross talk between these proaging processes. We then examine proyouth interventions that demonstrate significant promise in reversing age-related impairments in the hippocampus and, finally, attempt to look ahead toward novel therapeutics for brain aging.

Contribution of neuroinflammation and immunity to brain aging and the mitigating effects of physical and cognitive interventions

It is widely accepted that the brain and the immune system continuously interact during normal as well as pathological functioning. Human aging is commonly accompanied by low-grade inflammation in both the immune and central nervous systems, thought to contribute to many age-related diseases. This review of the current literature focuses first on the normal neuroimmune interactions occurring in the brain, which promote learning, memory and neuroplasticity. Further, we discuss the protective and dynamic role of barriers to neuroimmune interactions, which have become clearer with the recent discovery of the meningeal lymphatic system. Next, we consider age-related changes of the immune system and possible deleterious influences of immunosenescence and low-grade inflammation (inflammaging) on neurodegenerative processes in the normally aging brain. We survey the major immunomodulators and neuroregulators in the aging brain and their highly tuned dynamic and reciprocal interactions. Finally, we consider our current understanding of how physical activity, as well as a combination of physical and cognitive interventions, may mediate anti-inflammatory effects and thus positively impact brain aging.