Aging impairs the late phase of long-term potentiation at the medial perforant path-CA3 synapse in awake rats

BDNF and Lactate as Modulators of Hippocampal CA3 Network Physiology

Growing evidence supports the notion that brain-derived neurotrophic factor (BDNF) and lactate are potent modulators of mammalian brain function. The modulatory actions of those biomolecules influence a wide range of neuronal responses, from the shaping of neuronal excitability to the induction and expression of structural and synaptic plasticity. The biological actions of BDNF and lactate are mediated by their cognate receptors and specific transporters located in the neuronal membrane. Canonical functions of BDNF occur via the tropomyosin-related kinase B receptor (TrkB), whereas lactate acts via monocarboxylate transporters or the hydroxycarboxylic acid receptor 1 (HCAR1). Both receptors are highly expressed in the central nervous system, and some of their physiological actions are particularly well characterized in the hippocampus, a brain structure involved in the neurophysiology of learning and memory. The multifarious neuronal circuitry between the axons of the dentate gyrus granule cells, mossy fibers (MF), and pyramidal neurons of area CA3 is of great interest given its role in specific mnemonic processes and involvement in a growing number of brain disorders. Whereas the modulation exerted by BDNF via TrkB has been extensively studied, the influence of lactate via HCAR1 on the properties of the MF-CA3 circuit is an emerging field. In this review, we discuss the role of both systems in the modulation of brain physiology, with emphasis on the hippocampal CA3 network. We complement this review with original data that suggest cross-modulation is exerted by these two independent neuromodulatory systems.

Extracellular matrix proteoglycans support aged hippocampus networks: a potential cellular-level mechanism of brain reserve

One hallmark of normative brain aging is vast heterogeneity in whether older people succumb to or resist cognitive decline. Resilience describes a brain's capacity to maintain cognition in the face of aging and disease. One factor influencing resilience is brain reserve-the status of neurobiological resources available to support neuronal circuits as dysfunction accumulates. This study uses a cohort of behaviorally characterized adult, middle-aged, and aged rats to test whether neurobiological factors that protect inhibitory neurotransmission and synapse function represent key components of brain reserve. Histochemical analysis of extracellular matrix proteoglycans, which play critical roles in stabilizing synapses and modulating inhibitory neuron excitability, was conducted alongside analyses of lipofuscin-associated autofluorescence. The findings indicate that aging results in lower proteoglycan density and more lipofuscin in CA3. Aged rats with higher proteoglycan density exhibited better performance on the Morris watermaze, whereas lipofuscin abundance was not related to spatial memory. These data suggest that the local environment around neurons may protect against synapse dysfunction or hyperexcitability and could contribute to brain reserve mechanisms.

The times they are a-changin': a proposal on how brain flexibility goes beyond the obvious to include the concepts of "upward" and "downward" to neuroplasticity

Since the brain was found to be somehow flexible, plastic, researchers worldwide have been trying to comprehend its fundamentals to better understand the brain itself, make predictions, disentangle the neurobiology of brain diseases, and finally propose up-to-date treatments. Neuroplasticity is simple as a concept, but extremely complex when it comes to its mechanisms. This review aims to bring to light an aspect about neuroplasticity that is often not given enough attention as it should, the fact that the brain's ability to change would include its ability to disconnect synapses. So, neuronal shrinkage, decrease in spine density or dendritic complexity should be included within the concept of neuroplasticity as part of its mechanisms, not as an impairment of it. To that end, we extensively describe a variety of studies involving topics such as neurodevelopment, aging, stress, memory and homeostatic plasticity to highlight how the weakening and disconnection of synapses organically permeate the brain in so many ways as a good practice of its intrinsic physiology. Therefore, we propose to break down neuroplasticity into two sub-concepts, "upward neuroplasticity" for changes related to synaptic construction and "downward neuroplasticity" for changes related to synaptic deconstruction. With these sub-concepts, neuroplasticity could be better understood from a bigger landscape as a vector in which both directions could be taken for the brain to flexibly adapt to certain demands. Such a paradigm shift would allow a better understanding of the concept of neuroplasticity to avoid any data interpretation bias, once it makes clear that there is no morality with regard to the organic and physiological changes that involve dynamic biological systems as seen in the brain.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Activation of NLRP3-Caspase-1 pathway contributes to age-related impairments in cognitive function and synaptic plasticity

Aging is characterized by a progressive deterioration in physiological functions that is associated with cognitive decline as well as other physical functional impairments. Microglia activation leading to neuroinflammation has been generally recognized as playing a critical role in the development of age-related cognitive decline. NLRP3 inflammasome in microglia is fundamental for IL-1β maturation and subsequent inflammatory events. However, it remains unknown whether NLRP3 activation contributes to aging-induced cognitive decline in vivo. Here, our study demonstrated that aging rats showed declined cognitive function and impaired synaptic plasticity as well as decreased density of dendritic spines. Importantly, our data demonstrated strongly enhanced expression of NLRP3, ASC and Caspase-1 in the hippocampus of aged rats as well as decreased AMPA receptor and phosphorylated levels of CaMKII and CREB in the hippocampus of natural aging rats. Furthermore, NLRP3 inflammasome inhibitor elevated the surface expression of AMPA receptor and the phosphorylated levels of CaMKII, CREB in hippocampus, and finally contributed to the attenuation of hippocampal long-term potentiation (LTP) deficits and the improvement of cognitive decline of natural aging rats. These results revealed an important role for the NLRP3-Caspase-1 pathway in aging-induced cognitive decline and suggested that inhibition of NLRP3 inflammasome represented a novel therapeutic intervention for aging-related cognitive impairment.

Synaptic plasticity in Alzheimer's disease and healthy aging

The strength and efficiency of synaptic connections are affected by the environment or the experience of the individual. This property, called synaptic plasticity, is directly related to memory and learning processes and has been modeled at the cellular level. These types of cellular memory and learning models include specific stimulation protocols that generate a long-term strengthening of the synapses, called long-term potentiation, or a weakening of the said long-term synapses, called long-term depression. Although, for decades, researchers have believed that the main cause of the cognitive deficit that characterizes Alzheimer's disease (AD) and aging was the loss of neurons, the hypothesis of an imbalance in the cellular and molecular mechanisms of synaptic plasticity underlying this deficit is currently widely accepted. An understanding of the molecular and cellular changes underlying the process of synaptic plasticity during the development of AD and aging will direct future studies to specific targets, resulting in the development of much more efficient and specific therapeutic strategies. In this review, we classify, discuss, and describe the main findings related to changes in the neurophysiological mechanisms of synaptic plasticity in excitatory synapses underlying AD and aging. In addition, we suggest possible mechanisms in which aging can become a high-risk factor for the development of AD and how its development could be prevented or slowed.

APOE moderates the effect of hippocampal blood flow on memory pattern separation in clinically normal older adults

Pattern separation, the ability to differentiate new information from previously experienced similar information, is highly sensitive to hippocampal structure and function and declines with age. Functional MRI studies have demonstrated hippocampal hyperactivation in older adults compared to young, with greater task-related activation associated with worse pattern separation performance. The current study was designed to determine whether pattern separation was sensitive to differences in task-free hippocampal cerebral blood flow (CBF) in 130 functionally intact older adults. Given prior evidence that apolipoprotein E e4 (APOE e4) status moderates the relationship between CBF and episodic memory, we predicted a stronger negative relationship between hippocampal CBF and pattern separation in APOE e4 carriers. An interaction between APOE group and right hippocampal CBF was present, such that greater right hippocampal CBF was related to better lure discrimination in noncarriers, whereas the effect reversed directionality in e4 carriers. These findings suggest that neurovascular changes in the medial temporal lobe may underlie memory deficits in cognitively normal older adults who are APOE e4 carriers.

Comorbidity rates of Posttraumatic Stress Disorder in dementia: a systematic literature review

Background: Post Traumatic Stress Disorder (PTSD) has been described as an independent risk factor for cognitive decline and dementia. At the same time, cognitive deterioration and increased loss experiences in dementia may increase liability for the reactivation of traumatic memories and thereby PTSD symptoms. Objective: In order to investigate co-occurrence of PTSD in dementia this systematic literature review summarizes all the available evidence on reported comorbidity rates of PTSD in patients with dementia. Method: PubMed, Embase, PsycINFO and CINAHL were searched for potential publications investigating the co-occurrence of PTSD in dementia until 25 November 2019. Cohort and cross-sectional studies were included. To assure current comorbidity of PTSD in dementia, only publications with a recent PTSD diagnosis (<2 years before onset of dementia) were selected. Results: Of the 860 identified abstracts, three studies (0.35%) met the eligibility criteria and were included. These three studies concerned only military veteran populations, and they comprised two cross-sectional cohort studies and one prospective cohort study. The estimated comorbidity rate of PTSD in veterans with dementia varied between 4.7% and 7.8%. Conclusions: The limited research available shows comorbidity rates only in military veterans, which were possibly dependent on investigated population with respect to dementia severity and possibly associated behavioural and psychiatric symptoms of dementia (BPSD). In dementia patients the comorbidity with PTSD may be high and we suggest that worldwide the impact of PTSD in dementia is high and probably underestimated. Research and care on this topic should improve urgently with the current expanding prevalence of dementia. A first step to improve quality of dementia research and care would be to develop a structured tool to diagnose PTSD in these patients.

Decoding Neurotransmitter Switching: The Road Forward

Neurotransmitter switching is a form of brain plasticity in which an environmental stimulus causes neurons to replace one neurotransmitter with another, often resulting in changes in behavior. This raises the possibility of applying a specific environmental stimulus to induce a switch that can enhance a desirable behavior or ameliorate symptoms of a specific pathology. For example, a stimulus inducing an increase in the number of neurons expressing dopamine could treat Parkinson's disease, or one affecting the number expressing serotonin could alleviate depression. This may already be producing successful treatment outcomes without our knowing that transmitter switching is involved, with improvement of motor function through physical activity and cure of seasonal depression with phototherapy. This review presents prospects for future investigation of neurotransmitter switching, considering opportunities and challenges for future research and describing how the investigation of transmitter switching is likely to evolve with new tools, thus reshaping our understanding of both normal brain function and mental illness.

Altered Hippocampal Place Cell Representation and Theta Rhythmicity following Moderate Prenatal Alcohol Exposure

Prenatal alcohol exposure (PAE) leads to profound deficits in spatial memory and synaptic and cellular alterations to the hippocampus that last into adulthood. Neurons in the hippocampus called place cells discharge as an animal enters specific places in an environment, establish distinct ensemble codes for familiar and novel places, and are modulated by local theta rhythms. Spatial memory is thought to critically depend on the integrity of hippocampal place cell firing. Therefore, we tested the hypothesis that hippocampal place cell firing is impaired after PAE by performing in vivo recordings from the hippocampi (CA1 and CA3) of moderate PAE and control adult rats. Our results show that hippocampal CA3 neurons from PAE rats have reduced spatial tuning. Second, CA1 and CA3 neurons from PAE rats are less likely to orthogonalize their firing between directions of travel on a linear track and between changes in contextual stimuli in an open arena compared to control neurons. Lastly, reductions in the number of hippocampal place cells exhibiting significant theta rhythmicity and phase precession were observed, which may suggest changes to hippocampal microcircuit function. Together, the reduced spatial tuning and sensitivity to contextual changes provide a neural systems-level mechanism to explain spatial memory impairment after moderate PAE.

Hippocampal Subregion Transcriptomic Profiles Reflect Strategy Selection during Cognitive Aging

Age-related cognitive impairments are associated with differentially expressed genes (DEGs) linked to defined neural systems; however, studies examining multiple regions of the hippocampus fail to find links between behavior and transcription in the dentate gyrus (DG). We hypothesized that use of a task requiring intact DG function would emphasize molecular signals in the DG associated with a decline in performance. We used a water maze beacon discrimination task to characterize young and middle-age male F344 rats, followed by a spatial reference memory probe trial test. Middle-age rats showed increased variability in discriminating two identical beacons. Use of an allocentric strategy and formation of a spatial reference memory were not different between age groups; however, older animals compensated for impaired beacon discrimination through greater reliance on spatial reference memory. mRNA sequencing of hippocampal subregions indicated DEGs in the DG of middle-age rats, linked to synaptic function and neurogenesis, correlated with beacon discrimination performance, suggesting that senescence of the DG underlies the impairment. Few genes correlated with spatial memory across age groups, with a greater number in region CA1. Age-related CA1 DEGs, correlated with spatial memory, were linked to regulation of neural activity. These results indicate that the beacon task is sensitive to impairment in middle age, and distinct gene profiles are observed in neural circuits that underlie beacon discrimination performance and allocentric memory. The use of different strategies in older animals and associated transcriptional profiles could provide an animal model for examining cognitive reserve and neural compensation of aging.SIGNIFICANCE STATEMENT Hippocampal subregions are thought to differentially contribute to memory. We took advantage of age-related variability in performance on a water maze beacon task and next-generation sequencing to test the hypothesis that aging of the dentate gyrus is linked to impaired beacon discrimination and compensatory use of allocentric memory. The dentate gyrus expressed synaptic function and neurogenesis genes correlated with beacon discrimination in middle-age animals. Spatial reference memory was associated with CA1 transcriptional correlates linked to regulation of neural activity and use of an allocentric strategy. This is the first study examining transcriptomes of multiple hippocampal subregions to link age-related impairments associated with discrimination of feature overlap and alternate response strategies to gene expression in specific hippocampal subregions.

Functional Connectivity of Hippocampal CA3 Predicts Neurocognitive Aging via CA1-Frontal Circuit

The CA3 and CA1 principal cell fields of the hippocampus are vulnerable to aging, and age-related dysfunction in CA3 may be an early seed event closely linked to individual differences in memory decline. However, whether the differential vulnerability of CA3 and CA1 is associated with broader disruption in network-level functional interactions in relation to age-related memory impairment, and more specifically, whether CA3 dysconnectivity contributes to the effects of aging via CA1 network connectivity, has been difficult to test. Here, using resting-state fMRI in a group of aged rats uncontaminated by neurodegenerative disease, aged rats displayed widespread reductions in functional connectivity of CA3 and CA1 fields. Age-related memory deficits were predicted by connectivity between left CA3 and hippocampal circuitry along with connectivity between left CA1 and infralimbic prefrontal cortex. Notably, the effects of CA3 connectivity on memory performance were mediated by CA1 connectivity with prefrontal cortex. We additionally found that spatial learning and memory were associated with functional connectivity changes lateralized to the left CA3 and CA1 divisions. These results provide novel evidence that network-level dysfunction involving interactions of CA3 with CA1 is an early marker of poor cognitive outcome in aging.

The effects of developmental alcohol exposure on the neurobiology of spatial processing

The consumption of alcohol during gestation is detrimental to the developing central nervous system. One functional outcome of this exposure is impaired spatial processing, defined as sensing and integrating information pertaining to spatial navigation and spatial memory. The hippocampus, entorhinal cortex, and anterior thalamus are brain regions implicated in spatial processing and are highly susceptible to the effects of developmental alcohol exposure. Some of the observed effects of alcohol on spatial processing may be attributed to changes at the synaptic to circuit level. In this review, we first describe the impact of developmental alcohol exposure on spatial behavior followed by a summary of the development of brain areas involved in spatial processing. We then provide an examination of the consequences of prenatal and early postnatal alcohol exposure in rodents on hippocampal, anterior thalamus, and entorhinal cortex-dependent spatial processing from the cellular to behavioral level. We conclude by highlighting several unanswered questions which may provide a framework for future investigation.

Honey as an antioxidant therapy to reduce cognitive ageing

This paper reviews the potential role of honey as a therapeutic antioxidant to reduce oxidative stress and improve cognitive ageing. All articles indexed to PubMed Central (PMC) were searched using the following key words: honey, antioxidant, memory and ageing. Honey is a natural insect-derived product with therapeutic, medicinal and nutritional values. Antioxidant properties of honey quench biologically-circulating reactive oxygen species (ROS) and counter oxidative stress while restoring the cellular antioxidant defense system. Antioxidant properties of honey may complement its nootropic effects to reduce cognitive ageing.

Potential and Challenges for the Clinical Use of d-Serine As a Cognitive Enhancer

After 25 years of its discovery in the rat brain, d-serine is a recognized modulator of synaptic plasticity and cognitive processes through its actions on the NMDA-glutamate receptor. Importantly, cognitive impairment is a core feature of conditions, such as schizophrenia, Alzheimer's disease, depression, and aging, and is associated to disturbances in NMDA-glutamate receptors. The d-serine pathway has been associated with cognitive deficits and these conditions, and, for this reason, d-serine signaling is subject of intense research to probe its role in aiding diagnosis and therapy. Nevertheless, this has not resulted in new therapies being incorporated into clinical practice. Therefore, in this review we will address many questions that need to be solved by future studies, regarding d-serine pharmacokinetics, possible side effects, other strategies to modulate its levels, and combination with other therapies to increase its efficacy.

The Aging Navigational System

The discovery of neuronal systems dedicated to computing spatial information, composed of functionally distinct cell types such as place and grid cells, combined with an extensive body of human-based behavioral and neuroimaging research has provided us with a detailed understanding of the brain's navigation circuit. In this review, we discuss emerging evidence from rodents, non-human primates, and humans that demonstrates how cognitive aging affects the navigational computations supported by these systems. Critically, we show 1) that navigational deficits cannot solely be explained by general deficits in learning and memory, 2) that there is no uniform decline across different navigational computations, and 3) that navigational deficits might be sensitive markers for impending pathological decline. Following an introduction to the mechanisms underlying spatial navigation and how they relate to general processes of learning and memory, the review discusses how aging affects the perception and integration of spatial information, the creation and storage of memory traces for spatial information, and the use of spatial information during navigational behavior. The closing section highlights the clinical potential of behavioral and neural markers of spatial navigation, with a particular emphasis on neurodegenerative disorders.

Age-related Changes in Lateral Entorhinal and CA3 Neuron Allocation Predict Poor Performance on Object Discrimination

Age-related memory deficits correlate with dysfunction in the CA3 subregion of the hippocampus, which includes both hyperactivity and overly rigid activity patterns. While changes in intrinsic membrane currents and interneuron alterations are involved in this process, it is not known whether alterations in afferent input to CA3 also contribute. Neurons in layer II of the lateral entorhinal cortex (LEC) project directly to CA3 through the perforant path, but no data are available regarding the effects of advanced age on LEC activity and whether these activity patterns update in response to environmental change. Furthermore, it is not known the extent to which age-related deficits in sensory discrimination relate to the inability of aged CA3 neurons to update in response to new environments. Young and aged rats were pre-characterized on a LEGO^© object discrimination task, comparable to behavioral tests in humans in which CA3 hyperactivity has been linked to impairments. The cellular compartment analysis of temporal activity with fluorescence in situ hybridization for the immediate-early gene Arc was then used to identify the principal cell populations that were active during two distinct epochs of random foraging in different environments. This approach enabled the extent to which rats could discriminate two similar objects to be related to the ability of CA3 neurons to update across different environments. In both young and aged rats, there were animals that performed poorly on the LEGO object discrimination task. In the aged rats only, however, the poor performers had a higher percent of CA3 neurons that were active during random foraging in a novel environment, but this is not related to the ability of CA3 neurons to remap when the environment changed. Afferent neurons to CA3 in LEC, as identified with the retrograde tracer choleratoxin B (CTB), also showed a higher percentage of cells that were positive for Arc mRNA in aged poor performing rats. This suggests that LEC contributes to the hyperactivity seen in CA3 of aged animals with object discrimination deficits and age-related cognitive decline may be the consequence of dysfunction endemic to the larger network.

Increased hippocampal activation in ApoE-4 carriers and non-carriers with amnestic mild cognitive impairment

Increased fMRI activation in the hippocampus is recognized as a signature characteristic of the amnestic mild cognitive impairment (aMCI) stage of Alzheimer's disease (AD). Previous work has localized this increased activation to the dentate gyrus/CA3 subregion of the hippocampus and showed a correlation with memory impairments in those patients. Increased hippocampal activation has also been reported in carriers of the ApoE-4 allelic variation independently of mild cognitive impairment although these findings were not localized to a hippocampal subregion. To assess the ApoE-4 contribution to increased hippocampal fMRI activation, patients with aMCI genotyped for ApoE-4 status and healthy age-matched control participants completed a high-resolution fMRI scan while performing a memory task designed to tax hippocampal subregion specific functions. Consistent with previous reports, patients with aMCI showed increased hippocampal activation in the left dentate gyrus/CA3 region of the hippocampus as well as memory task errors attributable to this subregion. However, this increased fMRI activation in the hippocampus did not differ between ApoE-4 carriers and ApoE-4 non-carriers and the proportion of memory errors attributable to dentate gyrus/CA3 function did not differ between ApoE-4 carriers and ApoE-4 non-carriers. These results indicate that increased fMRI activation of the hippocampus observed in patients with aMCI is independent of ApoE-4 status and that ApoE-4 does not contribute to the dysfunctional hippocampal activation or the memory errors attributable to this subregion in these patients.

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Patients with aMCI show increased fMRI activation in DG/CA3 relative to controls.

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Increased DG/CA3 activation is observed equally in ApoE-4 carriers and non-carriers.

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Hippocampal dysfunction in aMCI is observed independent of ApoE-4 carrier status.

Aging-related impairments of hippocampal mossy fibers synapses on CA3 pyramidal cells

The network interaction between the dentate gyrus and area CA3 of the hippocampus is responsible for pattern separation, a process that underlies the formation of new memories, and which is naturally diminished in the aged brain. At the cellular level, aging is accompanied by a progression of biochemical modifications that ultimately affects its ability to generate and consolidate long-term potentiation. Although the synapse between dentate gyrus via the mossy fibers (MFs) onto CA3 neurons has been subject of extensive studies, the question of how aging affects the MF-CA3 synapse is still unsolved. Extracellular and whole-cell recordings from acute hippocampal slices of aged Wistar rats (34 ± 2 months old) show that aging is accompanied by a reduction in the interneuron-mediated inhibitory mechanisms of area CA3. Several MF-mediated forms of short-term plasticity, MF long-term potentiation and at least one of the critical signaling cascades necessary for potentiation are also compromised in the aged brain. An analysis of the spontaneous glutamatergic and gamma-aminobutyric acid-mediated currents on CA3 cells reveal a dramatic alteration in amplitude and frequency of the nonevoked events. CA3 cells also exhibited increased intrinsic excitability. Together, these results demonstrate that aging is accompanied by a decrease in the GABAergic inhibition, reduced expression of short- and long-term forms of synaptic plasticity, and increased intrinsic excitability.

Loss of Catecholaminergic Neuromodulation of Persistent Forms of Hippocampal Synaptic Plasticity with Increasing Age

Neuromodulation by means of the catecholaminergic system is a key component of motivation-driven learning and behaviorally modulated hippocampal synaptic plasticity. In particular, dopamine acting on D1/D5 receptors and noradrenaline acting on beta-adrenergic receptors exert a very potent regulation of forms of hippocampal synaptic plasticity that last for very long-periods of time (>24 h), and occur in conjunction with novel spatial learning. Antagonism of these receptors not only prevents long-term potentiation (LTP) and long-term depression (LTD), but prevents the memory of the spatial event that, under normal circumstances, leads to the perpetuation of these plasticity forms. Spatial learning behavior that normally comes easily to rats, such as object-place learning and spatial reference learning, becomes increasingly impaired with aging. Middle-aged animals display aging-related deficits of specific, but not all, components of spatial learning, and one possibility is that this initial manifestation of decrements in learning ability that become apparent in middle-age relate to changes in motivation, attention and/or the regulation by neuromodulatory systems of these behavioral states. Here, we compared the regulation by dopaminergic D1/D5 and beta-adrenergic receptors of persistent LTP in young (2-4 month old) and middle-aged (8-14 month old) rats. We observed in young rats, that weak potentiation that typically lasts for ca. 2 h could be strengthened into persistent (>24 h) LTP by pharmacological activation of either D1/D5 or beta-adrenergic receptors. By contrast, no such facilitation occurred in middle-aged rats. This difference was not related to an ostensible learning deficit: a facilitation of weak potentiation into LTP by spatial learning was possible both in young and middle-aged rats. It was also not directly linked to deficits in LTP: strong afferent stimulation resulted in equivalent LTP in both age groups. We postulate that this change in catecholaminergic control of synaptic plasticity that emerges with aging, does not relate to a learning deficit per se, rather it derives from an increase in behavioral thresholds for novelty and motivation that emerge with increasing age that impact, in turn, on learning efficacy.

Epigenetic alterations in the suprachiasmatic nucleus and hippocampus contribute to age-related cognitive decline

Circadian rhythm dysfunction and cognitive decline, specifically memory loss, frequently accompany natural aging. Circadian rhythms and memory are intertwined, as circadian rhythms influence memory formation and recall in young and old rodents. Although, the precise relationship between circadian rhythms and memory is still largely unknown, it is hypothesized that circadian rhythm disruption, which occurs during aging, contributes to age-associated cognitive decline, specifically memory loss. While there are a variety of mechanisms that could mediate this effect, changes in the epigenome that occur during aging has been proposed as a potential candidate. Interestingly, epigenetic mechanisms, such as DNA methylation and sirtuin1 (SIRT1) are necessary for both circadian rhythms and memory. During aging, similar alterations of epigenetic mechanisms occur in the suprachiasmatic nucleus (SCN) and hippocampus, which are necessary for circadian rhythm generation and memory, respectively. Recently, circadian rhythms have been linked to epigenetic function in the hippocampus, as some of these epigenetic mechanisms oscillate in the hippocampus and are disrupted by clock gene deletion. The current paper will review how circadian rhythms and memory change with age, and will suggest how epigenetic changes in these processes might contribute to age-related cognitive decline.

Adenosine A1 Receptor-Mediated Endocytosis of AMPA Receptors Contributes to Impairments in Long-Term Potentiation (LTP) in the Middle-Aged Rat Hippocampus

Aging causes multiple changes in the mammalian brain, including changes in synaptic signaling. Previous reports have shown increased extracellular adenosine in the aging brain, and we recently reported that activation of adenosine A1 receptors (A1Rs) induces AMPA receptor (AMPAR) internalization in rat hippocampus. This study investigated whether aging-related changes in the rat hippocampus include altered surface expression of adenosine A1 and A2A receptors, and whether these changes correspond to changes in AMPAR surface expression and altered synaptic plasticity. We found reduced A1R surface expression in middle-aged rat hippocampus, and also reduced GluA1 and GluA2 AMPAR subunit surface expression. Using a chemically-induced LTP (cLTP) experimental protocol, we recorded fEPSPs in young (1 month old) and middle-aged (7-12 month old) rat hippocampal slices. There were significant impairments in cLTP in middle-aged slices, suggesting impaired synaptic plasticity. Since we previously showed that the A1R agonist N(6)-cyclopentyladenosine (CPA) reduced both A1Rs and GluA2/GluA1 AMPARs, we hypothesized that the observed impaired synaptic plasticity in middle-aged brains is regulated by A1R-mediated AMPAR internalization by clathrin-mediated endocytosis. Following cLTP, we found a significant increase in GluA1 and GluA2 surface expression in young rats, which was blunted in middle-aged brains or in young brains pretreated with CPA. Blocking A1Rs with 8-cyclopentyl-1,3-dipropylxanthine or AMPAR endocytosis with either Tat-GluA2-3Y peptide or dynasore (dynamin inhibitor) similarly enhanced AMPAR surface expression following cLTP. These data suggest that age-dependent alteration in adenosine receptor expression contributes to increased AMPAR endocytosis and impaired synaptic plasticity in aged brains.

Memory for sequences of events impaired in typical aging

Typical aging is associated with diminished episodic memory performance. To improve our understanding of the fundamental mechanisms underlying this age-related memory deficit, we previously developed an integrated, cross-species approach to link converging evidence from human and animal research. This novel approach focuses on the ability to remember sequences of events, an important feature of episodic memory. Unlike existing paradigms, this task is nonspatial, nonverbal, and can be used to isolate different cognitive processes that may be differentially affected in aging. Here, we used this task to make a comprehensive comparison of sequence memory performance between younger (18-22 yr) and older adults (62-86 yr). Specifically, participants viewed repeated sequences of six colored, fractal images and indicated whether each item was presented "in sequence" or "out of sequence." Several out of sequence probe trials were used to provide a detailed assessment of sequence memory, including: (i) repeating an item from earlier in the sequence ("Repeats"; e.g., AB A: DEF), (ii) skipping ahead in the sequence ("Skips"; e.g., AB D: DEF), and (iii) inserting an item from a different sequence into the same ordinal position ("Ordinal Transfers"; e.g., AB 3: DEF). We found that older adults performed as well as younger controls when tested on well-known and predictable sequences, but were severely impaired when tested using novel sequences. Importantly, overall sequence memory performance in older adults steadily declined with age, a decline not detected with other measures (RAVLT or BPS-O). We further characterized this deficit by showing that performance of older adults was severely impaired on specific probe trials that required detailed knowledge of the sequence (Skips and Ordinal Transfers), and was associated with a shift in their underlying mnemonic representation of the sequences. Collectively, these findings provide unambiguous evidence that the capacity to remember sequences of events is fundamentally affected by typical aging.

Circuit-specific changes in D-serine-dependent activation of the N-methyl-D-aspartate receptor in the aging hippocampus

Age-related memory deficits have recently been associated with the impaired expression of D-serine-dependent synaptic plasticity in neuronal networks of the hippocampal CA1 area. However, whether such functional alterations are common to the entire hippocampus during aging remains unknown. Here, we found that D-serine was also required for the induction of N-methyl-D-aspartate receptor (NMDA-R)-dependent long-term potentiation (LTP) at perforant path-granule cell synapses of the dentate gyrus. LTP as well as isolated NMDA-R synaptic potentials were impaired in slices from aged rats, but in contrast to the CA1, this defect was not reversed by exogenous D-serine. The lower activation of the glycine-binding site by the endogenous co-agonist does not therefore appear to be a critical mechanism underlying age-related deficits in NMDA-R activation in the dentate gyrus. Instead, our data highlight the role of changes in presynaptic inputs as illustrated by the weaker responsiveness of afferent glutamatergic fibers, as well as changes in postsynaptic NMDA-R density. Thus, our study indicates that although NMDA-R-dependent mechanisms driving synaptic plasticity are quite similar between hippocampal circuits, they show regional differences in their susceptibility to aging, which could hamper the development of effective therapeutic strategies aimed at reducing cognitive aging.

Role of stress, depression, and aging in cognitive decline and Alzheimer's disease

Late-onset Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the most common cause of progressive cognitive dysfunction and dementia. Despite considerable progress in elucidating the molecular pathology of this disease, we are not yet close to unraveling its etiopathogenesis. A battery of neurotoxic modifiers may underpin neurocognitive pathology via deleterious heterogeneous pathologic impact in brain regions, including the hippocampus. Three important neurotoxic factors being addressed here include aging, stress, and depression. Unraveling "upstream pathologies" due to these disparate neurotoxic entities, vis-à-vis cognitive impairment involving hippocampal dysfunction, is of paramount importance. Persistent systemic inflammation triggers and sustains neuroinflammation. The latter targets several brain regions including the hippocampus causing upregulation of amyloid beta and neurofibrillary tangles, synaptic and neuronal degeneration, gray matter volume atrophy, and progressive cognitive decline. However, what is the fundamental source of this peripheral inflammation in aging, stress, and depression? This chapter highlights and delineates the inflammatory involvement-i.e., from its inception from gut to systemic inflammation to neuroinflammation. It highlights an upregulated cascade in which gut-microbiota-related dysbiosis generates lipopolysaccharides (LPS), which enhances inflammation and gut's leakiness, and through a Web of interactions, it induces stress and depression. This may increase neuronal dysfunction and apoptosis, promote learning and memory impairment, and enhance vulnerability to cognitive decline.

AMPA receptor trafficking and the mechanisms underlying synaptic plasticity and cognitive aging

Even in healthy individuals there is an inexorable agerelated decline in cognitive function. This is due, in large part, to reduced synaptic plasticity caused by changes in the molecular composition of the postsynaptic membrane. AMPA receptors (AMPARs) are glutamate-gated cation channels that mediate the overwhelming majority of fast excitatory transmission in the brain. Changes in AMPAR number and/or function are a core feature of synaptic plasticity and age-related cognitive decline, AMPARs are highly dynamic proteins that are subject to highly controlled trafficking, recycling, and/or degradation and replacement. This active regulation of AMPAR synthesis, targeting, synaptic dwell time, and degradation is fundamentally important for memory formation and storage. Further, aberrant AMPAR trafficking and consequent detrimental changes in synapses are strongly implicated in many brain diseases, which represent a vast social and economic burden. The purpose of this article is to provide an overview of the molecular and cellular AMPA receptor trafficking events that control synaptic responsiveness and plasticity, and highlight what is known currently known about how these processes change with age and disease.

Age-related cognitive impairments in mice with a conditional ablation of the neural cell adhesion molecule

Most of the mechanisms involved in neural plasticity support cognition, and aging has a considerable effect on some of these processes. The neural cell adhesion molecule (NCAM) of the immunoglobulin superfamily plays a pivotal role in structural and functional plasticity and is required to modulate cognitive and emotional behaviors. However, whether aging is associated with NCAM alterations that might contribute to age-related cognitive decline is not currently known. In this study, we determined whether conditional NCAM-deficient mice display increased vulnerability to age-related cognitive and emotional alterations. We assessed the NCAM expression levels in the hippocampus and medial prefrontal cortex (mPFC) and characterized the performance of adult and aged conditional NCAM-deficient mice and their age-matched wild-type littermates in a delayed matching-to-place test in the Morris water maze and a delayed reinforced alternation test in the T-maze. Although aging in wild-type mice is associated with an isoform-specific reduction of NCAM expression levels in the hippocampus and mPFC, these mice exhibited only mild impairments in working/episodic-like memory performance. However, aged conditional NCAM-deficient mice displayed pronounced impairments in both the delayed matching-to-place and the delayed reinforced alternation tests. Importantly, the deficits of aged NCAM-deficient mice in these working/episodic-like memory tasks could not be attributed to increased anxiety-like behaviors or to differences in locomotor activity. Taken together, these data indicate that reduced NCAM expression in the forebrain might be a critical factor for the occurrence of cognitive impairments during aging.

AMPA receptor trafficking and the mechanisms underlying synaptic plasticity and cognitive aging

Even in healthy individuals there is an inexorable agerelated decline in cognitive function. This is due, in large part, to reduced synaptic plasticity caused by changes in the molecular composition of the postsynaptic membrane. AMPA receptors (AMPARs) are glutamate-gated cation channels that mediate the overwhelming majority of fast excitatory transmission in the brain. Changes in AMPAR number and/or function are a core feature of synaptic plasticity and age-related cognitive decline, AMPARs are highly dynamic proteins that are subject to highly controlled trafficking, recycling, and/or degradation and replacement. This active regulation of AMPAR synthesis, targeting, synaptic dwell time, and degradation is fundamentally important for memory formation and storage. Further, aberrant AMPAR trafficking and consequent detrimental changes in synapses are strongly implicated in many brain diseases, which represent a vast social and economic burden. The purpose of this article is to provide an overview of the molecular and cellular AMPA receptor trafficking events that control synaptic responsiveness and plasticity, and highlight what is known currently known about how these processes change with age and disease.

Spatial pattern completion deficits in older adults

Aging may have an impact on the CA3 autoassociative network of the hippocampus, posited by computational models as supporting pattern completion. Twenty-five young (YAs) and 25 older adults (OAs) performed a spatial pattern completion task using a computerized navigational paradigm analogous to a rodent pattern completion task reliant on the CA3. Participants identified a previously seen goal location, and the availability of distal cues in the environment was manipulated such that 0, 2, or 4 cues were missing. Performance in both groups declined as a function of decreased cue availability. However, controlling for age differences in task performance during a pre-experimental baseline task, OAs performed equivalently to YAs when all cues were available, but worse than YAs as the number of cues decreased. These findings suggest spatial pattern completion may be impaired in OAs. We discuss these findings in the context of a growing body of literature suggesting age-related imbalances in pattern separation vs. pattern completion.

Age-specific peculiarities of formation of long-term posttetanic potentiation in OXYS rats

OXYS rats with hereditary hyperproduction of active oxidative radicals and early disorders in the mitochondrial structure and functions are an interesting model for studies of age-specific features of synaptic plasticity. The formation of long-term posttetanic potentiation in the mossy fibers-CA3 pyramidal neuron system were studied in hippocampal slices from Wistar and OXYS rats aged 3 and 4.5 months (young), 11 (middle-aged), and 18 months (old). No appreciable age-related differences were detected in the amplitudes and latencies of stimulatory postsynaptic summary potentials of the mossy synapses evoked by test stimuli in Wistar and OXYS rat groups of different age and between the two strains. The capacity to induction and formation of long-term posttetanic potentiation and its value decreased in 18-month-old Wistar rats, which attested to disorders in synaptic plasticity of old animals. The capacity to induction and formation of long-term posttetanic potentiation and its value in OXYS were lower than Wistar rats of the same age in all the studied groups.

The hippocampal physiology of approaching middle-age: early indicators of change

Age-related cognitive decline presents serious lifestyle challenges, and anatomical changes to the hippocampus are often implicated in clinical conditions later in life. However, relatively little is known about how hippocampal physiology is altered in the transition to middle-age, when early detection may offer the best opportunity for successful treatment. High-yield extracellular recording is a powerful tool for understanding brain function in freely moving animals at single-cell resolution and with millisecond precision. We used this technique to characterize changes to hippocampal physiology associated with maturation in 35-week-old rats. Combining a series of behavioral tasks with recordings of large numbers of neurons, local field potentials (LFP), and network patterns of activation, we were able to generate a comprehensive picture based on more than 25 different assays for each subject. Notable changes associated with aging included increased firing rates in interneurons, reduced LFP power but increased frequency in the 4-12 Hz theta band, and impairment in hippocampal pattern-separation for different environments. General properties of pyramidal cell firing and spatial map integrity were preserved. There was no impairment in theta phase-precession, experience-dependent place field expansion, or sleep reactivation of waking network patterns. There were however changes in foraging strategy and behavioral responses to the introduction of a novel environment. Taken together the results reveal a diverse pattern of changes which are of increasing relevance in an aging population. They also highlight areas where high-yield electrophysiological assays can be used to provide the sensitivity and throughput required for pre-clinical drug-discovery programs.

Pattern separation in the hippocampus

The ability to discriminate among similar experiences is a crucial feature of episodic memory. This ability has long been hypothesized to require the hippocampus, and computational models suggest that it is dependent on pattern separation. However, empirical data for the role of the hippocampus in pattern separation have not been available until recently. This review summarizes data from electrophysiological recordings, lesion studies, immediate-early gene imaging, transgenic mouse models, as well as human functional neuroimaging, that provide convergent evidence for the involvement of particular hippocampal subfields in this key process. We discuss the impact of aging and adult neurogenesis on pattern separation, and also highlight several challenges to linking across species and approaches, and suggest future directions for investigation.

Breeder and batch-dependent variability in the acquisition and performance of a motor skill in adult Long-Evans rats

Reaching tasks are popular tools for investigating the neural mechanisms of motor skill learning and recovery from brain damage in rodents, but there is considerable unexplained variability across studies using these tasks. We investigated whether breeder, batch effects, experimenter, time of year, weight and other factors contribute to differences in the acquisition and performance of a skilled reaching task, the single pellet retrieval task, in adult male Long-Evans hooded rats. First, we retrospectively analyzed task acquisition and performance in rats from different breeding colonies that were used in several studies spanning a 3 year period in our laboratory. Second, we compared reaching variables in age-matched rats from different breeders that were trained together as a batch by the same experimenters. All rats had received daily training on the reaching task until they reached a criterion of successful reaches per attempt. We found significant breeder-dependent differences in learning rate and final performance level. This was found even when age-matched rats from different breeders were trained together by the same experimenters. There was also significant batch-to-batch variability within rats from the same breeder trained by the same experimenter. Other factors, including weight, paw preference and the experimenter, were not as strong or consistent in their contributions to differences across studies. The breeder and batch effects found within the same rat strain may reflect genetic and environmental influences on the neural substrates of motor skill learning. This is an important consideration when comparing baseline performance across studies and for controlling variability within studies.

Age-related memory deficits linked to circuit-specific disruptions in the hippocampus

Converging data from rodents and humans have demonstrated an age-related decline in pattern separation abilities (the ability to discriminate among similar experiences). Several studies have proposed the dentate and CA3 subfields of the hippocampus as the potential locus of this change. Specifically, these studies identified rigidity in place cell remapping in similar environments in the CA3. We used high-resolution fMRI to examine activity profiles in the dentate gyrus and CA3 in young and older adults as stimulus similarity was incrementally varied. We report evidence for "representational rigidity" in older adults' dentate/CA3 that is linked to behavioral discrimination deficits. Using ultrahigh-resolution diffusion imaging, we quantified both the integrity of the perforant path as well as dentate/CA3 dendritic changes and found that both were correlated with dentate/CA3 functional rigidity. These results highlight structural and functional alterations in the hippocampal network that predict age-related changes in memory function and present potential targets for intervention.

Senescent synapses and hippocampal circuit dynamics

Excitatory synaptic transmission is altered during aging in hippocampal granule cells, and in CA3 and CA1 pyramidal cells. These functional changes contribute to age-associated impairments in experimentally-induced plasticity in each of these primary hippocampal subregions. In CA1, plasticity evoked by stimulation shares common mechanisms with the synaptic modification observed following natural behavior. Aging results in deficits in both artificially- and behaviorally-induced plasticity, and this could in part reflect age-related changes in Ca2+ homeostasis. Other observations, however, suggest that increased intracellular Ca2+ levels are beneficial under some circumstances. This review focuses on age-associated changes in synaptic function, how these alterations might contribute to cognitive decline, and the extent to which altered hippocampal circuit properties are detrimental or reflect compensatory processes.

Stem-cell-associated structural and functional plasticity in the aging hippocampus

Aging frequently leads to a functional decline across multiple cognitive domains, often resulting in a severe reduction in life quality and also causing substantial care-related costs. Understanding age-associated structural and functional changes of neural circuitries within the brain is required to improve successful aging. In this review, the authors focus on age-dependent alterations of the hippocampus and the decline of hippocampal function, which are critically involved in processes underlying certain forms of learning and memory. Despite the dramatic reductions in hippocampus-dependent function that accompany advancing age, there is also striking evidence that even the aged brain retains a high level of plasticity. Thus, one promising avenue to reach the goal of successful aging might be to boost and recruit this plasticity, which is the interplay between neural structure, function, and experience, to prevent age-related cognitive decline and age-associated comorbidities.

Brief novelty exposure facilitates dentate gyrus LTP in aged rats

Aging is associated with a decreased capacity for dentate gyrus (DG) granule cell depolarization as well as reduced perforant path activation. Although it is well established that the maintenance of DG long-term potentiation (LTP) over days is impaired in aged, as compared to young animals, the threshold for inducing this LTP has never been investigated in aged, awake animals. In addition, although exposure to novelty prior to theta-burst stimulation (TBS) increases both the induction and longevity of DG LTP in adult rats, the effects of exposure to novelty on LTP in aged rats have never been investigated. Here, we report that although TBS delivered in the home cage induces robust and long-lasting DG LTP in young rats, TBS fails to induce DG LTP in aged rats. Interestingly, delivery of TBS to aged rats exploring novel environments induces robust and long-lasting LTP, with the induction, but not the longevity, of this LTP being similar in magnitude to that observed in young rats delivered TBS in the home cage. These results indicate that although TBS-induced DG LTP is impaired in aged, as compared to young rats, TBS during exploration of novel environments is sufficient to rescue age-related deficits in DG LTP. We discuss these observations in the context of previous findings suggesting that the facilitation of LTP by exposure to novel environments results as a consequence of reduced network inhibition in the DG and we suggest that, in spite of age-related changes in the DG, this capacity persists in aged rats and represents a nondietary and nonpharmacological way to facilitate DG LTP during aging.

Neurocognitive aging: prior memories hinder new hippocampal encoding

Normal aging is often accompanied by impairments in forming new memories, and studies of aging rodents have revealed structural and functional changes to the hippocampus that might point to the mechanisms behind such memory loss. In this article, we synthesize recent neurobiological and neurophysiological findings into a model of the information-processing circuit of the aging hippocampus. The key point of the model is that small concurrent changes during aging strengthen the auto-associative network of the CA3 subregion at the cost of processing new information coming in from the entorhinal cortex. As a result of such reorganization in aged memory-impaired individuals, information that is already stored would become the dominant pattern of the hippocampus to the detriment of the ability to encode new information.

Neural plasticity in the ageing brain

The mechanisms involved in plasticity in the nervous system are thought to support cognition, and some of these processes are affected during normal ageing. Notably, cognitive functions that rely on the medial temporal lobe and prefrontal cortex, such as learning, memory and executive function, show considerable age-related decline. It is therefore not surprising that several neural mechanisms in these brain areas also seem to be particularly vulnerable during the ageing process. In this review, we discuss major advances in our understanding of age-related changes in the medial temporal lobe and prefrontal cortex and how these changes in functional plasticity contribute to behavioural impairments in the absence of significant pathology.

Age-related alterations in hippocampal spines and deficiencies in spatial memory in mice

Alterations in neuronal morphology occur in the brain during normal aging, but vary depending on neuronal cell types and brain regions. Such alterations have been related to memory and cognitive impairment. Changes in hippocampal spine densities are thought to represent a morphological correlate of altered brain functions associated with hippocampal-dependent learning and memory. We therefore have analyzed the impact of aging on different hippocampal-dependent learning tasks and on changes in dendritic spines of CA1 hippocampal and dentate gyrus neurons by analyzing adult (6-7 months) and aged (21-22 months) C57/Bl6 mice. We found a significant decrease in spine numbers of basal CA1 dendrites and decreases in spine length of apical dendrites of CA1 and dentate gyrus neurons. Furthermore, aged mice exhibited significant deficits in hippocampus-dependent learning tasks, such as the probe trial of the Morris water maze and T maze learning. Given the fact that there is no neuronal loss in the hippocampus in aged mice (von Bohlen und Halbach and Unsicker [2002] Eur. J. Neurosci. 16:2434-2440), we suggest that the memory and cognitive decline in the context of aging may be accompanied by rather subtle anatomical changes, such as numbers and morphology of dendritic spines.