Theta-frequency synaptic potentiation in CA1 in vitro distinguishes cognitively impaired from unimpaired aged Fischer 344 rats

Arbovirus infection increases the risk for the development of neurodegenerative disease pathology in the murine model

Alzheimer's disease is classified as a progressive disorder resulting from protein misfolding, also known as proteinopathies. Proteinopathies include synucleinopathies triggered by misfolded amyloid α-synuclein, tauopathies triggered by misfolded tau, and amyloidopathies triggered by misfolded amyloid of which Alzheimer's disease (β-amyloid) is most prevalent. Most neurodegenerative diseases (>90%) are not due to dominantly inherited genetic causes. Instead, it is thought that the risk for disease is a complicated interaction between inherited and environmental risk factors that, with age, drive pathology that ultimately results in neurodegeneration and disease onset. Since it is increasingly appreciated that encephalitic viral infections can have profoundly detrimental neurological consequences long after the acute infection has resolved, we tested the hypothesis that viral encephalitis exacerbates the pathological profile of protein-misfolding diseases. Using a robust, reproducible, and well-characterized mouse model for β-amyloidosis, Tg2576, we studied the contribution of alphavirus-induced encephalitis (TC-83 strain of VEEV to model alphavirus encephalitis viruses) on the progression of neurodegenerative pathology. We longitudinally evaluated neurological, neurobehavioral, and cognitive levels, followed by a post-mortem analysis of brain pathology focusing on neuroinflammation. We found more severe cognitive deficits and brain pathology in Tg2576 mice inoculated with TC-83 than in their mock controls. These data set the groundwork to investigate sporadic Alzheimer's disease and treatment interventions for this infectious disease risk factor.

SVHRSP Alleviates Age-Related Cognitive Deficiency by Reducing Oxidative Stress and Neuroinflammation

BACKGROUND

Our previous studies have shown that scorpion venom heat-resistant synthesized peptide (SVHRSP) induces a significant extension in lifespan and improvements in age-related physiological functions in worms. However, the mechanism underlying the potential anti-aging effects of SVHRSP in mammals remains elusive.

METHODS

Following SVHRSP treatment in senescence-accelerated mouse resistant 1 (SAMR1) or senescence-accelerated mouse prone 8 (SAMP8) mice, behavioral tests were conducted and brain tissues were collected for morphological analysis, electrophysiology experiments, flow cytometry, and protein or gene expression. The human neuroblastoma cell line (SH-SY5Y) was subjected to H2O2 treatment in cell experiments, aiming to establish a cytotoxic model that mimics cellular senescence. This model was utilized to investigate the regulatory mechanisms underlying oxidative stress and neuroinflammation associated with age-related cognitive impairment mediated by SVHRSP.

RESULTS

SVHRSP significantly ameliorated age-related cognitive decline, enhanced long-term potentiation, restored synaptic loss, and upregulated the expression of synaptic proteins, therefore indicating an improvement in synaptic plasticity. Moreover, SVHRSP demonstrated a decline in senescent markers, including SA-β-gal enzyme activity, P16, P21, SIRT1, and cell cycle arrest. The underlying mechanisms involve an upregulation of antioxidant enzyme activity and a reduction in oxidative stress-induced damage. Furthermore, SVHRSP regulated the nucleoplasmic distribution of NRF2 through the SIRT1-P53 pathway. Further investigation indicated a reduction in the expression of proinflammatory factors in the brain after SVHRSP treatment. SVHRSP attenuated neuroinflammation by regulating the NF-κB nucleoplasmic distribution and inhibiting microglial and astrocytic activation through the SIRT1-NF-κB pathway. Additionally, SVHRSP significantly augmented Nissl body count while suppressing neuronal loss.

CONCLUSION

SVHRSP could remarkably improve cognitive deficiency by inhibiting oxidative stress and neuroinflammation, thus representing an effective strategy to improve brain health.

Dual Role of NMDAR Containing NR2A and NR2B Subunits in Alzheimer's Disease

Alzheimer's disease (AD) is the main cause of dementia worldwide. Given that learning and memory are impaired in this pathology, NMDA receptors (NMDARs) appear as key players in the onset and progression of the disease. NMDARs are glutamate receptors, mainly located at the post-synapse, which regulate voltage-dependent influx of calcium into the neurons. They are heterotetramers, and there are different subunits that can be part of the receptors, which are usually composed of two obligatory GluN1 subunits plus either two NR2A or two NR2B subunits. NR2A are mostly located at the synapse, and their activation is involved in the expression of pro-survival genes. Conversely, NR2B are mainly extrasynaptic, and their activation has been related to cell death and neurodegeneration. Thus, activation of NR2A and/or inactivation of NR2B-containing NMDARS has been proposed as a therapeutic strategy to treat AD. Here, we wanted to investigate the main differences between both subunits signalling in neuronal primary cultures of the cortex and hippocampus. It has been observed that Aβ induces a significant increase in calcium release and also in MAPK phosphorylation signalling in NR2B-containing NMDAR in cortical and hippocampal neurons. However, while NR2A-containing NMDAR decreases neuronal death and favours cell viability after Aβ treatment, NR2B-containing NMDAR shows higher levels of cytotoxicity and low levels of neuronal survival. Finally, it has been detected that NMDAR has no effect on pTau axonal transport. The present results demonstrate a different role between GluNA and GluNB subunits in neurodegenerative diseases such as Alzheimer's.

Spatial Coding Dysfunction and Network Instability in the Aging Medial Entorhinal Cortex

Across species, spatial memory declines with age, possibly reflecting altered hippocampal and medial entorhinal cortex (MEC) function. However, the integrity of cellular and network-level spatial coding in aged MEC is unknown. Here, we leveraged in vivo electrophysiology to assess MEC function in young, middle-aged, and aged mice navigating virtual environments. In aged grid cells, we observed impaired stabilization of context-specific spatial firing, correlated with spatial memory deficits. Additionally, aged grid networks shifted firing patterns often but with poor alignment to context changes. Aged spatial firing was also unstable in an unchanging environment. In these same mice, we identified 458 genes differentially expressed with age in MEC, 61 of which had expression correlated with spatial firing stability. These genes were enriched among interneurons and related to synaptic transmission. Together, these findings identify coordinated transcriptomic, cellular, and network changes in MEC implicated in impaired spatial memory in aging.

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.

Decreased Hippocampal Neurogenesis in Aged Male Wistar Rats Is Not Associated with Memory Acquisition in a Water Maze

Brain aging is associated with a progressive decrease in learning abilities, memory, attention, decision making, and sensory perception. Age-related cognitive disturbances may be related to a decrease in the functional capacities of the hippocampus. This brain region is essential for learning and memory, and the lifelong neurogenesis occurring in the subgranular zone of the dentate gyrus may be a key event mediating the mnemonic functions of the hippocampus. In the present study, we investigated whether age-related changes in hippocampal neurogenesis are associated with learning and memory disturbances. Four- and 24-month-old rats were trained to find a hidden platform in a water maze. Though the older group showed higher latency to search the platform as compared to the younger group, both groups learned the task. However, the density of proliferating (PCNA-positive), differentiating (Dcx-positive), and new neurons (pre-labeled BrdU-positive) was significantly lower in the hippocampus of aged rats as compared to young ones. This inhibition of neurogenesis could be related to increased local production of nitric oxide since the density of neurons expressing neuronal NO-synthase was higher in the aged hippocampus. Thus, we can suggest that an age-related decrease in neurogenesis is not directly associated with place learning in aged rats.

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.

Neurobiological Mechanisms of Cognitive Decline Correlated with Brain Aging

Cognitive decline has emerged as one of the greatest health threats of old age. Meanwhile, aging is the primary risk factor for Alzheimer's disease (AD) and other prevalent neurodegenerative disorders. Developing therapeutic interventions for such conditions demands a greater understanding of the processes underlying normal and pathological brain aging. Despite playing an important role in the pathogenesis and incidence of disease, brain aging has not been well understood at a molecular level. Recent advances in the biology of aging in model organisms, together with molecular- and systems-level studies of the brain, are beginning to shed light on these mechanisms and their potential roles in cognitive decline. This chapter seeks to integrate the knowledge about the neurological mechanisms of age-related cognitive changes that underlie aging.

Advanced age has dissociable effects on hippocampal CA1 ripples and CA3 high frequency events in male rats

Sharp wave/ripples/high frequency events (HFEs) are transient bursts of depolarization in hippocampal subregions CA3 and CA1 that occur during rest and pauses in behavior. Previous studies have reported that CA1 ripples in aged rats have lower frequency than those detected in young animals. While CA1 ripples are thought to be driven by CA3, HFEs in CA3 have not been examined in aged animals. The current study obtained simultaneous recordings from CA1 and CA3 in young and aged rats to examine sharp wave/ripples/HFEs in relation to age. While CA1 ripple frequency was reduced with age, there were no age differences in the frequency of CA3 HFEs, although power and length were lower in old animals. While there was a proportion of CA1 ripples that co-occurred with a CA3 HFE, none of the age-related differences in CA1 ripples could be explained by alterations in CA3 HFE characteristics. These findings suggest that age differences in CA1 are not due to altered CA3 activity, but instead reflect distinct mechanisms of ripple generation with age.

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 tau: A pathological or physiological phenomenon?

In this review, we discuss the synaptic aspects of Tau pathology occurring during Alzheimer's disease (AD) and how this may relate to memory impairment, a major hallmark of AD. Whilst the clinical diagnosis of AD patients is a loss of working memory and long-term declarative memory, the histological diagnosis is the presence of neurofibrillary tangles of hyperphosphorylated Tau and Amyloid-beta plaques. Tau pathology spreads through synaptically connected neurons to impair synaptic function preceding the formation of neurofibrillary tangles, synaptic loss, axonal retraction and cell death. Alongside synaptic pathology, recent data suggest that Tau has physiological roles in the pre- or post- synaptic compartments. Thus, we have seen a shift in the research focus from Tau as a microtubule-stabilising protein in axons, to Tau as a synaptic protein with roles in accelerating spine formation, dendritic elongation, and in synaptic plasticity coordinating memory pathways. We collate here the myriad of emerging interactions and physiological roles of synaptic Tau, and discuss the current evidence that synaptic Tau contributes to pathology in AD.

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.

Long-term Memory Upscales Volume of Postsynaptic Densities in the Process that Requires Autophosphorylation of αCaMKII

It is generally accepted that formation and storage of memory relies on alterations of the structure and function of brain circuits. However, the structural data, which show learning-induced and long-lasting remodeling of synapses, are still very sparse. Here, we reconstruct 1927 dendritic spines and their postsynaptic densities (PSDs), representing a postsynaptic part of the glutamatergic synapse, in the hippocampal area CA1 of the mice that underwent spatial training. We observe that in young adult (5 months), mice volume of PSDs, but not the volume of the spines, is increased 26 h after the training. The training-induced growth of PSDs is specific for the dendritic spines that lack smooth endoplasmic reticulum and spine apparatuses, and requires autophosphorylation of αCaMKII. Interestingly, aging alters training-induced ultrastructural remodeling of dendritic spines. In old mice, both the median volumes of dendritic spines and PSDs shift after training toward bigger values. Overall, our data support the hypothesis that formation of memory leaves long-lasting footprint on the ultrastructure of brain circuits; however, the form of circuit remodeling changes with age.

Dose-Response Effect of Vibratory Stimulus on Synaptic and Muscle Plasticity in a Middle-Aged Murine Model

Whole body vibration plays a central role in many work categories and can represent a health risk to the musculoskeletal system and peripheral nervous system. However, studies in animal and human models have shown that vibratory training, experimentally and/or therapeutically induced, can exert beneficial effects on the whole body, as well as improve brain functioning and reduce cognitive decline related to the aging process. Since the effects of vibratory training depend on several factors, such as vibration frequency and vibration exposure time, in this work, we investigated whether the application of three different vibratory protocols could modulate synaptic and muscle plasticity in a middle-aged murine model, counteracting the onset of early symptoms linked to the aging process. To this end, we performed in vitro electrophysiological recordings of the field potential in the CA1 region of mouse hippocampal slices, as well as histomorphometric and ultrastructural analysis of muscle tissue by optic and transmission electron microscopy, respectively. Our results showed that protocols characterized by a low vibration frequency and/or a longer recovery time exert positive effects at both hippocampal and muscular level, and that these effects improve significantly by varying both parameters, with an action comparable with a dose-response effect. Thus, we suggested that vibratory training may be an effective strategy to counteract cognitive impairment, which is already present in the early stages of the aging process, and the onset of sarcopenia, which is closely related to a sedentary lifestyle. Future studies are needed to understand the underlying molecular mechanisms and to determine an optimal vibratory training protocol.

Learning and memory disorders related to hippocampal inflammation following exposure to air pollution

It has been demonstrated that sub-chronic exposure to air pollution containing nanoscale (˂100 nm) diesel exhaust particles (DEPs) may lead to excessive oxidative stress and neuro-inflammation in adult male mice. Hereby, we investigated the effects of DEPs on hippocampus-dependent spatial learning and neuro-inflammation and memory-related gene expression in male mice. In this study, we divided 48 adult NMRI male mice into control group VS. three exposure groups. Mice were exposed to 300-350 μg/m³ DEPs for 2, 5, and 7 h daily for 12 weeks. The Morris Water Maze (MWM) and Elevated Plus Maze device were used to examine anxiety, spatial memory and learning, respectively. The mRNAs expression of pro-inflammatory cytokines, N-methyl-D-aspartate (NMDA) receptor subunits, and glutaminase were studied in hippocampus (HI) by real-time RT-PCR. Besides, malondialdehyde (MDA) tests were used to determine the state of oxidative stress. After 5 and 7 h. of DEPs exposure, mRNA expression of NR2A and NR3B IL1α, IL1β, TNFα, NMDA receptor subunits and MDA levels increased significantly (P < 0.05). Also, DEPs exposed mice for 2, 5, and 7 h. showed diminished entrance into open arms with short time spent there. Indeed, 5 and 7 h/day exposed mice required a longer time to reach the hidden platform. Sub-chronic exposure to DEPs increased oxidative stress markers, neuroinflammation, anxiety, impaired spatial learning and memory.

Altered hippocampal astroglial metabolism is associated with aging and preserved spatial learning and memory

An age-related decrease in hippocampal metabolism correlates with cognitive decline. Hippocampus-dependent learning and memory requires glutamatergic neurotransmission supported by glutamate-glutamine (GLU-GLN) cycling between neurons and astrocytes. We examined whether GLU-GLN cycling in hippocampal subregions (dentate gyrus and CA1) in Fischer 344 rats was altered with age and cognitive status. Hippocampal slices from young adult, aged cognitively-unimpaired (AU) and aged cognitively-impaired (AI) rats were incubated in artificial cerebrospinal fluid (aCSF) containing 1-13C-glucose to assess neural metabolism. Incorporation of 13C-glucose into glutamate and glutamine, measured by mass spectroscopy/liquid chromatography tandem mass spectroscopy, did not significantly differ between groups. However, when 13C-acetate, a preferential astrocytic metabolite, was used, a significant increase in 13C-labeled glutamate was observed in slices from AU rats. Taken together, the data suggest that resting state neural metabolism and GLU-GLN cycling may be preserved during aging when sufficient extracellular glucose is available, but that enhanced astroglial metabolism can occur under resting state conditions. This may be an aging-related compensatory change to maintain hippocampus-dependent cognitive function.

Axonal Conduction Velocity in CA1 Area of Hippocampus is Reduced in Mouse Models of Alzheimer's Disease

The timing of action potentials arrival at synaptic terminals partially determines integration of synaptic inputs and is important for information processing in the CNS. Therefore, axonal conduction velocity (VC) is a salient parameter, influencing the timing of synaptic inputs. Even small changes in VC may disrupt information coding in networks requiring accurate timing. We recorded compound action potentials in hippocampal slices to measure VC in three mouse models of Alzheimer's disease. We report an age-dependent reduction in VC in area CA1 in two amyloid-β precursor protein transgenic mouse models, line 41 and APP/PS1, and in a tauopathy model, rTg4510.

Novel Players in the Aging Synapse: Impact on Cognition

While neuronal loss has long been considered as the main contributor to age-related cognitive decline, these alterations are currently attributed to gradual synaptic dysfunction driven by calcium dyshomeostasis and alterations in ionotropic/metabotropic receptors. Given the key role of the hippocampus in encoding, storage, and retrieval of memory, the morpho- and electrophysiological alterations that occur in the major synapse of this network-the glutamatergic-deserve special attention. We guide you through the hippocampal anatomy, circuitry, and function in physiological context and focus on alterations in neuronal morphology, calcium dynamics, and plasticity induced by aging and Alzheimer's disease (AD). We provide state-of-the art knowledge on glutamatergic transmission and discuss implications of these novel players for intervention. A link between regular consumption of caffeine-an adenosine receptor blocker-to decreased risk of AD in humans is well established, while the mechanisms responsible have only now been uncovered. We review compelling evidence from humans and animal models that implicate adenosine A2A receptors (A2AR) upsurge as a crucial mediator of age-related synaptic dysfunction. The relevance of this mechanism in patients was very recently demonstrated in the form of a significant association of the A2AR-encoding gene with hippocampal volume (synaptic loss) in mild cognitive impairment and AD. Novel pathways implicate A2AR in the control of mGluR5-dependent NMDAR activation and subsequent Ca2+ dysfunction upon aging. The nature of this receptor makes it particularly suited for long-term therapies, as an alternative for regulating aberrant mGluR5/NMDAR signaling in aging and disease, without disrupting their crucial constitutive activity.

Prenatal exposure to diesel exhaust particles causes anxiety, spatial memory disorders with alters expression of hippocampal pro-inflammatory cytokines and NMDA receptor subunits in adult male mice offspring

Air pollution by Diesel exhaust (DE) consists of gaseous compounds and diesel exhaust particles (DEPs). Previous studies show associations between prenatal exposure to diesel exhaust affects the central nervous system (CNS). However, there was not reported that these effects were caused by gaseous compounds, diesel exhaust particles, or both. A limited number of studies in rodent models have shown that exposure to DEPs can result in CNS. Here, we explored the effects of prenatal exposure to DEPs on anxiety and learning and memory in NMRI mice male offspring. Three groups of pregnant mice were exposed to 350-400 μg DEPs/m³ for 2, 4 and 6 h daily in a closed system room. We examined anxiety and learning and memory in 8-to-9-week-old male offspring using the Elevated plus maze and Morris water maze (MWM) test. Hippocampi were isolated after the behavioral tests and measured pro-inflammatory cytokines and N-methyl-D-aspartate (NMDA) receptor expression by quantitative RT-PCR analysis. Mice exposed to DEPs in utero showed deficits in the Elevated plus maze and Morris water maze test. In addition, DEPs exposed mice exhibited decreased hippocampal NR2A and NR3B expression. Taken together, our data suggest that maternal DEP exposure is associated with anxiety, disrupts learning and memory and reduction hippocampal NR2A and NR3B expression in male offspring.

Physical Exercise Enhances Neuroplasticity and Delays Alzheimer's Disease

Accumulating evidence indicates that exercise can improve learning and memory as well as attenuate neurodegeneration, including Alzheimer's disease (AD). In addition to improving neuroplasticity by altering the synaptic structure and function in various brain regions, exercise also modulates systems like angiogenesis and glial activation that are known to support neuroplasticity. Moreover, exercise helps to maintain a cerebral microenvironment that facilitates synaptic plasticity by enhancing the clearance of Aβ, one of the main culprits of AD pathogenesis. The purpose of this review is to highlight the positive impacts of exercise on promoting neuroplasticity. Possible mechanisms involved in exercise-modulated neuroplasticity are also discussed. Undoubtedly, more studies are needed to design an optimal personalized exercise protocol for enhancing brain function.

Cellular Redox Imbalance and Neurochemical Effect in Cognitive-Deficient Old Rats

The purpose of the present study is to access the linkage between dysregulation of glutamatergic neurotransmission, oxidative metabolism, and serine signaling in age-related cognitive decline. In this work, we evaluated the effect of natural aging in rats on the cognitive abilities for hippocampal-dependent tasks. Oxidative metabolism indicators are glutathione (GSH), malondialdehyde (MDA) concentrations, and cytosolic phospholipase A₂ (PLA₂) activity. In addition, neurotransmitter amino acid (L-Glutamic acid, γ-aminobutyric acid (GABA), DL-Serine and DL-Aspartic acid) concentrations were studied in brain areas such as the frontal cortex (FC) and hippocampus (HPC). The spatial long-term memory revealed significant differences among experimental groups: the aged rats showed an increase in escape latency to the platform associated with a reduction of crossings and spent less time on the target quadrant than young rats. Glutathione levels decreased for analyzed brain areas linked with a significant increase in MDA concentrations and PLA₂ activity in cognitive-deficient old rats. We found glutamate levels only increased in the HPC, whereas a reduced level of serine was found in both regions of interest in cognitive-deficient old rats. We demonstrated that age-related changes in redox metabolism contributed with alterations in synaptic signaling and cognitive impairment.

Long-Term Moderate Exercise Rescues Age-Related Decline in Hippocampal Neuronal Complexity and Memory

Background: Aging impairs hippocampal neuroplasticity and hippocampus-related learning and memory. In contrast, exercise training is known to improve hippocampal neuronal function. However, whether exercise is capable of restoring memory function in old animals is less clear. Objective: Here, we investigated the effects of exercise on the hippocampal neuroplasticity and memory functions during aging. Methods: Young (3 months), middle-aged (9–12 months), and old (18 months) mice underwent moderate-intensity treadmill running training for 6 weeks, and their hippocampus-related learning and memory, and the plasticity of their CA1 neurons was evaluated. Results: The memory performance (Morris water maze and novel object recognition tests), and dendritic complexity (branch and length) and spine density of their hippocampal CA1 neurons decreased as their age increased. The induction and maintenance of high-frequency stimulation-induced long-term potentiation in the CA1 area and the expressions of neuroplasticity-related proteins were not affected by age. Treadmill running increased CA1 neuron long-term potentiation and dendritic complexity in all three age groups, and it restored the learning and memory ability in middle-aged and old mice. Furthermore, treadmill running upregulated the hippocampal expressions of brain-derived neurotrophic factor and monocarboxylate transporter-4 in middle-aged mice, glutamine synthetase in old mice, and full-length TrkB in middle-aged and old mice. Conclusion: The hippocampus-related memory function declines from middle age, but long-term moderate-intensity running effectively increased hippocampal neuroplasticity and memory in mice of different ages, even when the memory impairment had progressed to an advanced stage. Thus, long-term, moderate intensity exercise training might be a way of delaying and treating aging-related memory decline.

AMPA Receptor Trafficking in Natural and Pathological Aging

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) enable most excitatory transmission in the brain and are crucial for mediating basal synaptic strength and plasticity. Because of the importance of their function, AMPAR dynamics, activity and subunit composition undergo a tight regulation which begins as early as prenatal development and continues through adulthood. Accumulating evidence suggests that the precise regulatory mechanisms involved in orchestrating AMPAR trafficking are challenged in the aging brain. In turn dysregulation of AMPARs can be linked to most neurological and neurodegenerative disorders. Understanding the mechanisms that govern AMPAR signaling during natural and pathological cognitive decline will guide the efforts to develop most effective ways to tackle neurodegenerative diseases which are one of the primary burdens afflicting an increasingly aging population. In this review, I provide a brief overview of the molecular mechanisms involved in AMPAR trafficking highlighting what is currently known about how these processes change with age and disease. As a particularly well-studied example of AMPAR dysfunction in pathological aging I focus in Alzheimer’s disease (AD) with special emphasis in how the production of neurofibrillary tangles (NFTs) and amyloid-β plaques may contribute to disruption in AMPAR function.

Differences in Synaptic Dysfunction Between rTg4510 and APP/PS1 Mouse Models of Alzheimer's Disease

Genetically modified mice have provided insights into the progression and pathology of Alzheimer's disease (AD). Here, we have examined two mouse models of AD: the rTg4510 mouse, which overexpresses mutant human Tau gene, and the APP/PS1 mouse, which overexpresses mutant human genes for amyloid precursor protein and presenilin 1. Both models exhibit deficits in hippocampal function, but comparative analyses of these deficits are sparse. We used extracellular field potential recordings in hippocampal slices to study basal synaptic transmission (BST), paired-pulse facilitation (PPF), and long-term potentiation (LTP) at the Schaffer collateral-CA1 pyramidal cell synapses in both models. We found that 6-7, but not 2-3-month-old rTg4510 mice exhibited reduced pre-synaptic activation (fiber volley (FV) amplitude, ∼50%) and field excitatory post-synaptic potential (fEPSP) slope (∼40%) compared to wild-type controls. In contrast to previous reports, BST, when controlled for FV amplitude, was not altered in rTg4510. APP/PS1 mice (2-3 mo and 8-10 mo) had unchanged FV amplitude compared to wild-type controls, while fEPSP slope was reduced by ∼34% in older mice, indicating a deficit in BST. PPF was unchanged in 8-10-month-old APP/PS1 mice, but was reduced in 6-7-month-old rTg4510 mice. LTP was reduced only in older rTg4510 and APP/PS1 mice. Our data suggest that BST deficits appear earlier in APP/PS1 than in rTg4510, which exhibited no BST deficits at the ages tested. However, FV and synaptic plasticity deficits developed earlier in rTg4510. These findings highlight fundamental differences in the progression of synaptic pathology in two genetically distinct models of AD.

Dopamine D4 receptor activation restores CA1 LTP in hippocampal slices from aged mice

Normal aging is characterized with a decline in hippocampal memory functions that is associated with changes in long-term potentiation (LTP) of the CA3-to-CA1 synapse. Age-related deficit of the dopaminergic system may contribute to impairment of CA1 LTP. Here we assessed how the modulation of CA1 LTP by dopamine is affected by aging and how it is dependent on the Ca2+ source. In slices from adult mice, the initial slope of the field potential showed strong LTP, but in slices from aged mice LTP was impaired. Dopamine did not affect LTP in adult slices, but enhanced LTP in aged slices. The dopamine D1/D5 receptor (D1R/D5R) agonist SKF-81297 did not affect LTP in adult but caused a relative small increase in LTP in aged slices; however, although there was no difference in dopamine D4 receptor (D4R) expression, the D4R agonist PD168077 increased LTP in aged slices to a magnitude similar to that in adult slices. The N-Methyl-D-aspartate receptor antagonist D-AP5 reduced LTP in adult slices, but not in aged slices. However, in the presence of D-AP5, PD168077 completely blocked LTP in aged slices. The voltage-dependent calcium channel (VDCC) blocker nifedipine reduced LTP in adult slices, but surprisingly enhanced LTP in aged slices. Furthermore, in the presence of nifedipine, PD168077 caused a strong enhancement of LTP in aged slices to a magnitude exceeding LTP in adult slices. Our results indicate that the full rescue of impaired LTP in aging by the selective D4R activation and that a large potentiation role on LTP by co-application of D4R agonist and VDCC blocker may provide novel strategies for the intervention of cognitive decline of aging and age-related diseases.

CA1 pyramidal cells have diverse biophysical properties, affected by development, experience, and aging

Neuron types (e.g., pyramidal cells) within one area of the brain are often considered homogeneous, despite variability in their biophysical properties. Here we review literature demonstrating variability in the electrical activity of CA1 hippocampal pyramidal cells (PCs), including responses to somatic current injection, synaptic stimulation, and spontaneous network-related activity. In addition, we describe how responses of CA1 PCs vary with development, experience, and aging, and some of the underlying ionic currents responsible. Finally, we suggest directions that may be the most impactful in expanding this knowledge, including the use of text and data mining to systematically study cellular heterogeneity in more depth; dynamical systems theory to understand and potentially classify neuron firing patterns; and mathematical modeling to study the interaction between cellular properties and network output. Our goals are to provide a synthesis of the literature for experimentalists studying CA1 PCs, to give theorists an idea of the rich diversity of behaviors models may need to reproduce to accurately represent these cells, and to provide suggestions for future research.

Modulation of GDF11 expression and synaptic plasticity by age and training

The Growth Differentiation Factor 11 (GDF11) has been controversially involved in the aging/rejuvenation process. To clarify whether GDF11 is differently expressed during aging, we have evaluated GDF11 levels in skeletal muscles and hippocampi of young and old mice, sedentary or subjected to a 12-weeks triweekly training protocol. The results of real-time PCR and Western blot analyses indicate that skeletal muscles of sedentary old mice express higher levels of GDF11 compared to young animals (p < 0.05). Conversely, in hippocampi no significant differences of GDF11 expression are detected. Analysis of long-term potentiation, a synaptic plasticity phenomenon, reveals that population spikes in response to a tetanic stimulus are significantly higher in sedentary young mice than in old animals (p < 0.01). Training induces a significant improvement of long-term potentiation in both young and old animals (p < 0.05), an increase (p < 0.05) of skeletal muscle GDF11 levels in young mice and a reduction of GDF11 expression in hippocampi of old mice (p < 0.05). Overall, data suggest that GDF11 can be considered an aging biomarker for skeletal muscles. Moreover, physical exercise has a positive impact on long-term potentiation in both young and old mice, while it has variable effects on GDF11 expression depending on age and on the tissue analyzed.

'Tagging' along memories in aging: Synaptic tagging and capture mechanisms in the aged hippocampus

Aging is accompanied by a general decline in the physiological functions of the body with the deteriorating organ systems. Brain is no exception to this and deficits in cognitive functions are quite common in advanced aging. Though a variety of age-related alterations are observed in the structure and function throughout the brain, certain regions show selective vulnerability. Medial temporal lobe, especially the hippocampus, is one such preferentially vulnerable region and is a crucial structure involved in the learning and long-term memory functions. Hippocampal synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), are candidate cellular correlates of learning and memory and alterations in these properties have been well documented in aging. A related phenomenon called synaptic tagging and capture (STC) has been proposed as a mechanism for cellular memory consolidation and to account for temporal association of memories. Mounting evidences from behavioral settings suggest that STC could be a physiological phenomenon. In this article, we review the recent data concerning STC and provide a framework for how alterations in STC-related mechanisms could contribute to the age-associated memory impairments. The enormity of impairment in learning and memory functions demands an understanding of age-associated memory deficits at the fundamental level given its impact in the everyday tasks, thereby in the quality of life. Such an understanding is also crucial for designing interventions and preventive measures for successful brain aging.

Neuroprotective effects of phytosterol esters against high cholesterol-induced cognitive deficits in aged rat

Accumulating epidemiological and experimental studies have confirmed that a high-cholesterol diet is detrimental to cognitive performance in animal models.

Short-term environmental enrichment enhances synaptic plasticity in hippocampal slices from aged rats

Age-associated changes in cognition are mirrored by impairments in cellular models of memory and learning, such as long-term potentiation (LTP) and long-term depression (LTD). In young rodents, environmental enrichment (EE) can enhance memory, alter LTP and LTD, as well as reverse cognitive deficits induced by aging. Whether short-term EE can benefit cognition and synaptic plasticity in aged rodents is unclear. Here, we tested if short-term EE could overcome age-associated impairments in induction of LTP and LTD. LTP and LTD could not be induced in the CA1 region of hippocampal slices in control, aged rats using standard stimuli that are highly effective in young rats. However, exposure of aged littermates to EE for three weeks enabled successful induction of LTP and LTD. EE-facilitated LTP was dependent upon N-methyl-d-aspartate receptors (NMDARs). These alterations in synaptic plasticity occurred with elevated levels of phosphorylated cAMP response element-binding protein and vascular endothelial growth factor, but in the absence of changes in several other synaptic and cellular markers. Importantly, our study suggests that even a relatively short period of EE is sufficient to alter synaptic plasticity and molecular markers linked to cognitive function in aged animals.

Molecular and cellular aspects of age-related cognitive decline and Alzheimer's disease

As the population of people aged 60 or older continues to rise, it has become increasingly important to understand the molecular basis underlying age-related cognitive decline. In fact, a better understanding of aging biology will help us identify ways to maintain high levels of cognitive functioning throughout the aging process. Many cellular and molecular aspects of brain aging are shared with other organ systems; however, certain age-related changes are unique to the nervous system due to its structural, cellular and molecular complexity. Importantly, the brain appears to show differential changes throughout the aging process, with certain regions (e.g. frontal and temporal regions) being more vulnerable than others (e.g. brain stem). Within the medial temporal lobe, the hippocampus is especially susceptible to age-related changes. The important role of the hippocampus in age-related cognitive decline and in vulnerability to disease processes such as Alzheimer's disease has prompted this review, which will focus on the complexity of changes that characterize aging, and on the molecular connections that exist between normal aging and Alzheimer's disease. Finally, it will discuss behavioral interventions and emerging insights for promoting healthy cognitive aging.

Exercise Counteracts Aging-Related Memory Impairment: A Potential Role for the Astrocytic Metabolic Shuttle

Age-related cognitive impairment has become one of the most common health threats in many countries. The biological substrate of cognition is the interconnection of neurons to form complex information processing networks. Experience-based alterations in the activities of these information processing networks lead to neuroadaptation, which is physically represented at the cellular level as synaptic plasticity. Although synaptic plasticity is known to be affected by aging, the underlying molecular mechanisms are not well described. Astrocytes, a glial cell type that is infrequently investigated in cognitive science, have emerged as energy suppliers which are necessary for meeting the abundant energy demand resulting from glutamatergic synaptic activity. Moreover, the concerted action of an astrocyte-neuron metabolic shuttle is essential for cognitive function; whereas, energetic incoordination between astrocytes and neurons may contribute to cognitive impairment. Whether altered function of the astrocyte-neuron metabolic shuttle links aging to reduced synaptic plasticity is unexplored. However, accumulated evidence documents significant beneficial effects of long-term, regular exercise on cognition and synaptic plasticity. Furthermore, exercise increases the effectiveness of astrocyte-neuron metabolic shuttle by upregulation of astrocytic lactate transporter levels. This review summarizes previous findings related to the neuronal activity-dependent astrocyte-neuron metabolic shuttle. Moreover, we discuss how aging and exercise may shape the astrocyte-neuron metabolic shuttle in cognition-associated brain areas.

Neuroinflammatory TNFα Impairs Memory via Astrocyte Signaling

The occurrence of cognitive disturbances upon CNS inflammation or infection has been correlated with increased levels of the cytokine tumor necrosis factor-α (TNFα). To date, however, no specific mechanism via which this cytokine could alter cognitive circuits has been demonstrated. Here, we show that local increase of TNFα in the hippocampal dentate gyrus activates astrocyte TNF receptor type 1 (TNFR1), which in turn triggers an astrocyte-neuron signaling cascade that results in persistent functional modification of hippocampal excitatory synapses. Astrocytic TNFR1 signaling is necessary for the hippocampal synaptic alteration and contextual learning-memory impairment observed in experimental autoimmune encephalitis (EAE), an animal model of multiple sclerosis (MS). This process may contribute to the pathogenesis of cognitive disturbances in MS, as well as in other CNS conditions accompanied by inflammatory states or infections.

Nampt is required for long-term depression and the function of GluN2B subunit-containing NMDA receptors

Nicotinamide adenine dinucleotide (NAD(+)) is an essential coenzyme/cosubstrate for many biological processes in cellular metabolism. The rate-limiting step in the major pathway of mammalian NAD(+) biosynthesis is mediated by nicotinamide phosphoribosyltransferase (Nampt). Previously, we showed that mice lacking Nampt in forebrain excitatory neurons (CamKIIαNampt(-/-) mice) exhibited hyperactivity, impaired learning and memory, and reduced anxiety-like behaviors. However, it remained unclear if these functional effects were accompanied by synaptic changes. Here, we show that CamKIIαNampt(-/-) mice have impaired induction of long-term depression (LTD) in the Schaffer collateral pathway, but normal induction of long-term potentiation (LTP), at postnatal day 30. Pharmacological assessments demonstrated that CamKIIαNampt(-/-) mice also display dysfunction of synaptic GluN2B (NR2B)-containing N-methyl-d-aspartate receptors (NMDARs) prior to changes in NMDAR subunit expression. These results support a novel, important role for Nampt-mediated NAD(+) biosynthesis in LTD and in the function of GluN2B-containing NMDARs.

Learning impairments identified early in life are predictive of future impairments associated with aging

The Morris water maze (MWM) behavioral paradigm is commonly used to measure spatial learning and memory in rodents. It is widely accepted that performance in the MWM declines with age. However, young rats ubiquitously perform very well on established versions of the water maze, suggesting that more challenging tasks may be required to reveal subtle differences in young animals. Therefore, we have used a one-day water maze and novel object recognition to test whether more sensitive paradigms of memory in young animals could identify subtle cognitive impairments early in life that might become accentuated later with senescence. We have found that these two tasks reliably separate young rats into inferior and superior learners, are highly correlated, and that performance on these tasks early in life is predictive of performance at 12 months of age. Furthermore, we have found that repeated training in this task selectively improves the performance of inferior learners, suggesting that behavioral training from an early age may provide a buffer against age-related cognitive decline.

Diverse impact of neuronal activity at θ frequency on hippocampal long-term plasticity

Brain oscillatory activity is considered an essential aspect of brain function, and its frequency can vary from <1 Hz to >200 Hz, depending on the brain states and projection. Episodes of rhythmic activity accompany hippocampus-dependent learning and memory in vivo. Therefore, long-term synaptic potentiation (LTP) and long-term depression, which are considered viable substrates of learning and memory, are often experimentally studied in paradigms of patterned high-frequency (>50 Hz) and low-frequency (<5 Hz) stimulation. However, the impact of intermediate frequencies on neuronal plasticity remains less well understood. In particular, hippocampal neurons are specifically tuned for activity at θ frequency (4-8 Hz); this band contributes significantly to electroencephalographic signals, and it is likely to be involved in shaping synaptic strength in hippocampal circuits. Here, we review in vitro and in vivo studies showing that variation of θ-activity duration may affect long-term modification of synaptic strength and neuronal excitability in the hippocampus. Such θ-pulse-induced neuronal plasticity 1) is long-lasting, 2) may be built on previously stabilized potentiation in the synapse, 3) may produce opposite changes in synaptic strength, and 4) requires complex molecular machinery. Apparently innocuous episodes of low-frequency synaptic activity may have a profound impact on network signaling, thereby contributing to information processing in the hippocampus and beyond. In addition, θ-pulse-induced LTP might be an advantageous protocol in studies of specific molecular mechanisms of synaptic plasticity.

D-Serine in the aging hippocampus

Experimental evidences now indicate that memory formation relies on the capacity of neuronal networks to manage long-term changes in synaptic communication. This property is driven by N-methyl-D-aspartate receptors (NMDAR), which requires the binding of glutamate but also the presence of the co-agonist D-serine at the glycine site. Defective memory function and impaired brain synaptic plasticity observed in aging are rescued by partial agonist acting at this site suggesting that this gating process is targeted to induce age-related cognitive defects. This review aims at compelling recent studies characterizing the role of D-serine in changes in functional plasticity that occur in the aging hippocampus since deficits are rescued by D-serine supplementation. The impaired efficacy of endogenous D-serine is not due to changes in the affinity to glycine-binding site but to a decrease in tissue levels of the amino acid resulting from a weaker expression of the producing enzyme serine racemase (SR). Interestingly, neither SR expression, D-serine levels, nor NMDAR activation is affected in aged LOU/C rats, a model of healthy aging in which memory deficits do not occur. These old animals do not develop oxidative stress suggesting that the D-serine-related pathway could be targeted by the age-related accumulation of reactive oxygen species. Accordingly, senescent rats chronically treated with the reducing agent N-acetyl-cysteine to prevent oxidative damage, show intact NMDAR activation linked to preserved D-serine levels and SR expression. These results point to a significant role of D-serine in age-related functional alterations underlying hippocampus-dependent memory deficits, at least within the CA1 area since the amino acid does not appear as critical in changes affecting the dentate gyrus.

Signaling pathways relevant to cognition-enhancing drug targets

Aging is generally associated with a certain cognitive decline. However, individual differences exist. While age-related memory deficits can be observed in humans and rodents in the absence of pathological conditions, some individuals maintain intact cognitive functions up to an advanced age. The mechanisms underlying learning and memory processes involve the recruitment of multiple signaling pathways and gene expression, leading to adaptative neuronal plasticity and long-lasting changes in brain circuitry. This chapter summarizes the current understanding of how these signaling cascades could be modulated by cognition-enhancing agents favoring memory formation and successful aging. It focuses on data obtained in rodents, particularly in the rat as it is the most common animal model studied in this field. First, we will discuss the role of the excitatory neurotransmitter glutamate and its receptors, downstream signaling effectors [e.g., calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), extracellular signal-regulated kinases (ERK), mammalian target of rapamycin (mTOR), cAMP response element-binding protein (CREB)], associated immediate early gene (e.g., Homer 1a, Arc and Zif268), and growth factors [insulin-like growth factors (IGFs) and brain-derived neurotrophic factor (BDNF)] in synaptic plasticity and memory formation. Second, the impact of the cholinergic system and related modulators on memory will be briefly reviewed. Finally, since dynorphin neuropeptides have recently been associated with memory impairments in aging, it is proposed as an attractive target to develop novel cognition-enhancing agents.

Immune dysregulation and cognitive vulnerability in the aging brain: Interactions of microglia, IL-1β, BDNF and synaptic plasticity

Older individuals often experience declines in cognitive function after events (e.g. infection, or injury) that trigger activation of the immune system. This occurs at least in part because aging sensitizes the response of microglia (the brain's resident immune cells) to signals triggered by an immune challenge. In the aging brain, microglia respond to these signals by producing more pro-inflammatory cytokines (e.g. interleukin-1beta or IL-1β) and producing them for longer than microglia in younger brains. This exaggerated inflammatory response can compromise processes critical for optimal cognitive functioning. Interleukin-1β is central to the inflammatory response and is a key mediator and modulator of an array of associated biological functions; thus its production and release is usually very tightly regulated. This review will focus on the impact of dysregulated production of IL-1β on hippocampus dependent-memory systems and associated synaptic plasticity processes. The neurotrophin brain-derived neurotrophic factor (BNDF) helps to protect neurons from damage caused by infection or injury, and it plays a critical role in many of the same memory and hippocampal plasticity processes compromised by dysregulated production of IL-1β. This suggests that an exaggerated brain inflammatory response, arising from aging and a secondary immune challenge, may erode the capacity to provide the BDNF needed for memory-related plasticity processes at hippocampal synapses. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Spatial reference memory in normal aging Fischer 344 × Brown Norway F1 hybrid rats

Fischer 344 × Brown Norway F1 (F344 × BN-F1) hybrid rats express greater longevity with improved health relative to aging rodents of other strains; however, few behavioral reports have thoroughly evaluated cognition across the F344 × BN-F1 lifespan. Consequently, this study evaluated spatial reference memory in F344 × BN-F1 rats at 6, 18, 24, or 28 months of age in the Morris water maze. Reference memory decrements were observed between 6 and 18 months and 18 and 24 months. At 28 months, spatial learning was not worse than 24 months, but swim speed was significantly slower. Reliable individual differences revealed that ∼50% of 24- to 28-month-old rats performed similarly to 6 months, whereas others were spatial learning impaired. Aged rats were impaired at learning within daily training sessions but not impaired at retaining information between days of training. Aged rats were also slower to learn to escape onto the platform, regardless of strategy. In summary, these data clarify the trajectory of cognitive decline in aging F344 × BN-F1 rats and elucidate relevant behavioral parameters.

Decreased response to acetylcholine during aging of aplysia neuron R15

How aging affects the communication between neurons is poorly understood. To address this question, we have studied the electrophysiological properties of identified neuron R15 of the marine mollusk Aplysia californica. R15 is a bursting neuron in the abdominal ganglia of the central nervous system and is implicated in reproduction, water balance, and heart function. Exposure to acetylcholine (ACh) causes an increase in R15 burst firing. Whole-cell recordings of R15 in the intact ganglia dissected from mature and old Aplysia showed specific changes in burst firing and properties of action potentials induced by ACh. We found that while there were no significant changes in resting membrane potential and latency in response to ACh, the burst number and burst duration is altered during aging. The action potential waveform analysis showed that unlike mature neurons, the duration of depolarization and the repolarization amplitude and duration did not change in old neurons in response to ACh. Furthermore, single neuron quantitative analysis of acetylcholine receptors (AChRs) suggested alteration of expression of specific AChRs in R15 neurons during aging. These results suggest a defect in cholinergic transmission during aging of the R15 neuron.

Leptin regulation of hippocampal synaptic function in health and disease

The endocrine hormone leptin plays a key role in regulating food intake and body weight via its actions in the hypothalamus. However, leptin receptors are highly expressed in many extra-hypothalamic brain regions and evidence is growing that leptin influences many central processes including cognition. Indeed, recent studies indicate that leptin is a potential cognitive enhancer as it markedly facilitates the cellular events underlying hippocampal-dependent learning and memory, including effects on glutamate receptor trafficking, neuronal morphology and activity-dependent synaptic plasticity. However, the ability of leptin to regulate hippocampal synaptic function markedly declines with age and aberrant leptin function has been linked to neurodegenerative disorders such as Alzheimer's disease (AD). Here, we review the evidence supporting a cognitive enhancing role for the hormone leptin and discuss the therapeutic potential of using leptin-based agents to treat AD.

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.

Water maze testing to identify compounds for cognitive enhancement

The water maze task is widely used to evaluate spatial learning and memory in rodents. The basic paradigm requires an animal to swim in a pool until it finds a hidden escape platform. The animals learn to find the platform using extra-maze cues and, after several training trials, are able to swim directly to it from any starting location. Memory for the platform location is assessed by examining swimming behavior with the platform removed from the maze, while sensory, motor and motivational aspects of the task can be examined by making the platform visible to the animals. Described in this unit is the use of the water maze to identify rats with age-related spatial learning and memory impairments. The efficacy of potential pharmacological treatments for alleviating these deficits is then evaluated. This assay provides a means for studying the neurobiology of spatial learning and memory, and to identify potential pharmacotherapies for treating memory-impaired humans. While the use of aged rats is described in this unit, the protocol can also be employed for compound screening with other rodent models that have spatial learning and memory impairments, such as transgenic mouse models of Alzheimer's disease.

Positive allosteric modulator of α7 nicotinic-acetylcholine receptors, PNU-120596 augments the effects of donepezil on learning and memory in aged rodents and non-human primates

The development of novel therapeutic agents for disorders of cognition such as Alzheimer's disease (AD) is of paramount importance given the ever-increasing elderly population, however; there is also considerable interest in any strategy that might enhance the clinical efficacy of currently available treatments. The purpose of this study was to evaluate an adjunctive treatment strategy to memory enhancement, namely combining the commonly prescribed acetylcholinesterase inhibitor (AChEI) donepezil, with a positive allosteric modulator (PAM) of α7 nicotinic-acetylcholine receptors (α7-nAChRs), PNU-120596. The treatment strategy was evaluated in a (non-spatial) spontaneous novel object recognition (NOR) task in young rats; a water maze spatial learning and recall procedure in aged, cognitively-impaired rats, and a delayed match to sample (working/short term memory) task in aged rhesus monkeys. In all three experiments a similar drug response was observed, namely that donepezil administered alone improved task performance in a dose-dependent manner; that PNU-120596 administered alone was without significant effect, but that the combination of PNU-120596 with a subthreshold dose of donepezil was effective. The positive effect of the drug combination appeared to be α7-nAChR mediated given that it was blocked in the NOR task by the selective α7-nAChR antagonist methyllycaconitine (MLA). Collectively, these data indicate that PNU-120596 increases the effective dose range of donepezil in learning/memory-related tasks in young and age-impaired animal models. The results suggest that α7-nAChR-selective PAMs like PNU-120596 have potential as adjunctive treatments with acetylcholinesterase inhibitors (e.g., donepezil) for age-related illnesses such as AD as well memory disorders not necessarily associated with advanced age.