Intracellular redox state alters NMDA receptor response during aging through Ca2+/calmodulin-dependent protein kinase II

Age-dependent alterations in cAMP signaling contribute to synaptic plasticity deficits following traumatic brain injury

The elderly have comparatively worse cognitive impairments from traumatic brain injury (TBI) relative to younger adults, but the molecular mechanisms that underlie this exacerbation of cognitive deficits are unknown. Experimental models of TBI have demonstrated that the cyclic AMP-protein kinase A (cAMP-PKA) signaling pathway is downregulated after brain trauma. Since the cAMP-PKA signaling pathway is a key mediator of long-term memory formation, we investigated whether the TBI-induced decrease in cAMP levels is exacerbated in aged animals. Aged (19 months) and young adult (3 months) male Fischer 344 rats received sham surgery or mild (1.4-1.6 atmospheres, atm) or moderate (1.7-2.1 atm) parasagittal fluid-percussion brain injury. At various time points after surgery, the ipsilateral parietal cortex, hippocampus, and thalamus were assayed for cAMP levels. Mild TBI lowered cAMP levels in the hippocampus of aged, but not young adult animals. Moderate TBI lowered cAMP levels in the hippocampus and parietal cortex of both age groups. In the thalamus, cAMP levels were significantly lowered after moderate, but not mild TBI. To determine if the TBI-induced decreases in cAMP had physiological consequences in aged animals, hippocampal long-term potentiation (LTP) in the Schaffer collateral pathway of the CA1 region was assessed. LTP was significantly decreased in both young adult and aged animals after mild and moderate TBI as compared to sham surgery animals. Rolipram rescued the LTP deficits after mild TBI for young adult animals and caused a partial recovery for aged animals. However, rolipram did not rescue LTP deficits after moderate TBI in either young adult or aged animals. These results indicate that the exacerbation of cognitive impairments in aged animals with TBI may be due to decreased cAMP levels and deficits in hippocampal LTP.

Memory in aged mice is rescued by enhanced expression of the GluN2B subunit of the NMDA receptor

The GluN2B subunit of the N-methyl-d-aspartate (NMDA) receptor shows age-related declines in expression across the frontal cortex and hippocampus. This decline is strongly correlated to age-related memory declines. This study was designed to determine if increasing GluN2B subunit expression in the frontal lobe or hippocampus would improve memory in aged mice. Mice were injected bilaterally with either the GluN2B vector, containing cDNA specific for the GluN2B subunit and enhanced green fluorescent protein (eGFP); a control vector or vehicle. Spatial memory, cognitive flexibility, and associative memory were assessed using the Morris water maze. Aged mice, with increased GluN2B subunit expression, exhibited improved long-term spatial memory, comparable to young mice. However, memory was rescued on different days in the Morris water maze; early for hippocampal GluN2B subunit enrichment and later for the frontal lobe. A higher concentration of the GluN2B antagonist, Ro 25-6981, was required to impair long-term spatial memory in aged mice with enhanced GluN2B expression, as compared to aged controls, suggesting there was an increase in the number of GluN2B-containing NMDA receptors. In addition, hippocampal slices from aged mice with increased GluN2B subunit expression exhibited enhanced NMDA receptor-mediated excitatory post-synaptic potentials (EPSP). Treatment with Ro 25-6981 showed that a greater proportion of the NMDA receptor-mediated EPSP was due to the GluN2B subunit in these animals, as compared to aged controls. These results suggest that increasing the production of the GluN2B subunit in aged animals enhances memory and synaptic transmission. Therapies that enhance GluN2B subunit expression within the aged brain may be useful for ameliorating age-related memory declines.

Curcumin rescues aging-related loss of hippocampal synapse input specificity of long term potentiation in mice

Curcumin has neuroprotective effect and could enhance memory. However, the mechanisms underlying the protection of curcumin on aging-related memory decline are not well understood. In this study, high frequency stimulation (HFS)-induced long term potentiation (LTP) was evaluated by a cellular model of memory formation. A two-input stimulation paradigm was used to record the potentiation as well as synapse input specificity. The data suggested that an N-Methyl-D-aspartate receptors (NMDAR) -dependent LTP was inducible in adult hippocampal slices with a characteristic of synapse input specificity. It also indicated that aging resulted in a reduction in LTP but more importantly a loss of synaptic input specificity. The reason behind the above conclusions is that LTP induction is more dependent on the calcium channel. This is due to a switch of the dependence of LTP induction to voltage-dependent calcium channel (VDCC) compared to NMDA receptors. Curcumin administration recovers input specificity by re-establishing NMDA receptor dependence of induction. In addition, curcumin administration ameliorated aging-related increase of brain thiobarbituric acid-reactive substances and elevated aging-related decrease of glutathione in hippocampus. It is then concluded that curcumin modulates hippocampal redox status and restores aging-related loss of synapse input specificity of HFS-induced LTP by switching VDCC calcium source into NMDA receptor-dependent one.

Impairments of synaptic plasticity in aged animals and in animal models of Alzheimer's disease

Aging is associated with a gradual decline in cognitive functions, and more dramatic cognitive impairments occur in patients affected by Alzheimer's disease (AD). Electrophysiological and molecular studies performed in aged animals and in animal models of AD have shown that cognitive decline is associated with significant modifications in synaptic plasticity (i.e., activity-dependent changes in synaptic strength) and have elucidated some of the cellular mechanisms underlying this process. Morphological studies have revealed a correlation between the quality of memory performance and the extent of structural changes of synaptic contacts occurring during memory consolidation. We briefly review recent experimental evidence here.

Estradiol acts directly and indirectly on multiple signaling pathways to phosphorylate cAMP-response element binding protein in GnRH neurons

Rapid, nonclassical 17β-estradiol (E2) actions are thought to play an important role in the modulation of neuronal function. The present study addresses the intracellular signaling cascades involved in the rapid E2-induced phosphorylation of cAMP response element binding protein (CREB) in GnRH neurons. Administration of E2 to adult female mice resulted in the activation of ERK1/2 in GnRH neurons within 15 min. In vitro studies using pharmacological antagonists showed that ERK1/2 was essential for E2-induced CREB phosphorylation in GnRH neurons. Upstream to this, protein kinase A and calcium/calmodulin-dependent protein kinase type II, but not protein kinase C, were found to be necessary for E2-induced phosphorylation of ERK1/2. This rapid E2 signaling cascade in GnRH neurons was found to require both direct and indirect E2 actions. E2 failed to phosphorylate ERK1/2 and CREB in GnRH neuron-specific estrogen receptor β knockout mice in vivo. Equally, however, a cocktail of tetrodotoxin and γ-aminobutyric acid(A)/glutamate receptor antagonists also blocked E2-induced ERK1/2 phosphorylation in GnRH neurons in wild-type mice in vitro. Together, these observations indicate that E2 acts through calcium/calmodulin-dependent protein kinase type II and protein kinase A to rapidly phosphorylate ERK1/2, which then acts to phosphorylate CREB in adult female GnRH neurons. Intriguingly, these effects of E2 are dependent upon both direct ERβ mechanisms as well as indirect actions mediated by afferent inputs to GnRH neurons.

Dietary phytoestrogen intakes and cognitive function during the menopausal transition: results from the Study of Women's Health Across the Nation Phytoestrogen Study

OBJECTIVE

Phytoestrogens, which consist mainly of isoflavones, lignans, and coumestans have estrogenic and anti-inflammatory properties. Previous research suggests that higher dietary or supplemental intakes of isoflavones and lignans are related to better cognitive performance in middle-aged and older women.

METHODS

We conducted longitudinal analysis of dietary phytoestrogens and cognitive performance in a cohort of African American, white, Chinese, and Japanese women undergoing the menopausal transition. The tests were Symbol Digit Modalities, East Boston Memory, and Digits Span Backward. Phytoestrogens were assessed using the Food Frequency Questionnaire. We modeled each cognitive score as a function of concurrent value of the primary predictors (highest tertile of isoflavones, lignans, or coumestrol) and covariates including the menopausal transition stage.

RESULTS

Coumestrol and isoflavone intakes were 10 and 25 times greater, respectively, in Asian than in non-Asian participants. During late perimenopause and postmenopause, Asian women with high isoflavone intakes did better on processing speed, but during early perimenopause and postmenopause, high-isoflavone Asian consumers performed worse on verbal memory. The highest isoflavone consumers among non-Asians likewise posted lower verbal memory scores during early perimenopause. A verbal memory benefit of higher dietary lignan consumption was apparent only during late perimenopause, when women from all ethnic/racial groups who were in the highest tertile of intake demonstrated a small advantage. Coumestrol was unrelated to cognitive performance.

CONCLUSIONS

The cognitive effects of dietary phytoestrogens are small, seem to be class-specific, vary by menopause stage and cognitive domain, and differ among ethnic/racial groups (but whether this is related to dose or to host factors cannot be discerned).

Aging, synaptic dysfunction, and insulin-like growth factor (IGF)-1

Insulin-like growth factor (IGF)-1 is an important neurotrophic hormone. Deficiency of this hormone has been reported to influence the genesis of cognitive impairment and dementia in the elderly patients. Nevertheless, there are studies indicating that cognitive function can be maintained into old age even in the absence of circulating IGF-1 and studies that link IGF-1 to an acceleration of neurological diseases. Although IGF-1 has a complex role in brain function, synaptic effects appear to be central to the IGF-1-induced improvement in learning and memory. In this review, synaptic mechanisms of learning and memory and the effects of IGF-1 on synaptic communication are discussed. The emerging data indicate that synaptic function decreases with age and that IGF-1 contributes to information processing in the brain. Further studies that detail the specific actions of this important neurotrophic hormone will likely lead to therapies that result in improved cognitive function for the elderly patients.

Session III: Mechanisms of age-related cognitive change and targets for intervention: inflammatory, oxidative, and metabolic processes

There is increasing evidence from basic science and human epidemiological studies that inflammation, oxidative stress, and metabolic abnormalities are associated with age-related cognitive decline and impairment. This article summarizes selected research on these topics presented at the Cognitive Aging Summit II. Speakers in this session presented evidence highlighting the roles of these processes and pathways on age-related cognitive decline, pointing to possible targets for intervention in nondemented older adults. Specific areas discussed included age differences in the production of cytokines following injury or infection, mechanisms underlying oxidative stress-induced changes in memory consolidation, insulin effects on brain signaling and memory, and the association between metabolic syndrome and cognitive decline in older adults. These presentations emphasize advances in our understanding of mechanisms and modifiers of age-related cognitive decline and provide insights into potential targets to promote cognitive health in older adults.

Loss of glutathione homeostasis associated with neuronal senescence facilitates TRPM2 channel activation in cultured hippocampal pyramidal neurons

Background

Glutathione (GSH) plays an important role in neuronal oxidant defence. Depletion of cellular GSH is observed in neurodegenerative diseases and thereby contributes to the associated oxidative stress and Ca²⁺ dysregulation. Whether depletion of cellular GSH, associated with neuronal senescence, directly influences Ca²⁺ permeation pathways is not known. Transient receptor potential melastatin type 2 (TRPM2) is a Ca²⁺ permeable non-selective cation channel expressed in several cell types including hippocampal pyramidal neurons. Moreover, activation of TRPM2 during oxidative stress has been linked to cell death. Importantly, GSH has been reported to inhibit TRPM2 channels, suggesting they may directly contribute to Ca²⁺ dysregulation associated with neuronal senescence. Herein, we explore the relation between cellular GSH and TRPM2 channel activity in long-term cultures of hippocampal neurons.

Results

In whole-cell voltage-clamp recordings, we observe that TRPM2 current density increases in cultured pyramidal neurons over time in vitro. The observed increase in current density was prevented by treatment with NAC, a precursor to GSH synthesis. Conversely, treatment of cultures maintained for 2 weeks in vitro with L-BSO, which depletes GSH by inhibiting its synthesis, augments TRPM2 currents. Additionally, we demonstrate that GSH inhibits TRPM2 currents through a thiol-independent mechanism, and produces a 3.5-fold shift in the dose-response curve generated by ADPR, the intracellular agonist for TRPM2.

Conclusion

These results indicate that GSH plays a physiologically relevant role in the regulation of TRPM2 currents in hippocampal pyramidal neurons. This interaction may play an important role in aging and neurological diseases associated with depletion of GSH.

Reversal of age-related oxidative stress prevents hippocampal synaptic plasticity deficits by protecting D-serine-dependent NMDA receptor activation

Oxidative stress (OS) resulting from an imbalance between antioxidant defenses and the intracellular accumulation of reactive oxygen species (ROS) contributes to age-related memory deficits. While impaired synaptic plasticity in neuronal networks is thought to underlie cognitive deficits during aging, whether this process is targeted by OS and what the mechanisms involved are still remain open questions. In this study, we investigated the age-related effects of the reducing agent N-acetyl-L-cysteine (L-NAC) on the activation of the N-methyl-D-aspartate receptor (NMDA-R) by its co-agonist D-serine, because alterations in this mechanism contribute greatly to synaptic plasticity deficits in aged animals. Long-term dietary supplementation with L-NAC prevented oxidative damage in the hippocampus of aged rats. Electrophysiological recordings in the CA1 of hippocampal slices indicated that NMDA-R-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) were depressed in aged animals, deficits that could be reversed by exogenous D-serine. Chronic treatment with L-NAC, but not acute application of the reducing agent, restored potent D-serine-dependent NMDA-R activation and LTP induction in aged rats. In addition, it is also revealed that the age-related decrease in D-serine levels and in the expression of the synthesizing enzyme serine racemase, which underlies the decrease in NMDA-R activation by the amino acid, was rescued by long-term dietary treatment with L-NAC. Our results indicate that protecting redox status in aged animals could prevent injury to the cellular mechanisms underlying cognitive aging, in part by maintaining potent NMDA-R activation through the D-serine-dependent pathway.

Influence of viral vector-mediated delivery of superoxide dismutase and catalase to the hippocampus on spatial learning and memory during aging

AIMS

Studies employing transgenic mice indicate that overexpression of superoxide dismutase 1 (SOD1) improves memory during aging. It is unclear whether the improvement is due to a lifetime of overexpression, decreasing the accumulation of oxidized molecules, or if increasing antioxidant enzymes in older animals could reduce oxidative damage and improve cognitive function. We used adeno-associated virus to deliver antioxidant enzymes (SOD1, SOD2, catalase [CAT], and SOD1+CAT) to the hippocampus of young (4 months) and aged (19 months) F344/BN F1 male rats and examined memory-related behavioral performance 1 month and 4 months postinjection.

RESULTS

Overexpression of antioxidant enzymes reduced oxidative damage; however, memory function was not related to the level of oxidative damage. Increased expression of SOD1, initiated in advanced age, impaired learning. Increased expression of SOD1+CAT provided protection from impairments associated with overexpression of SOD1 alone and appears to guard against cognitive impairments in advanced age.

INNOVATION

Viral vector gene delivery provides a novel approach to test the hypothesis that increased expression of antioxidant enzymes, specifically in hippocampal neurons, will provide protection from age-related cognitive decline. Further, expression of multiple vectors permits more detailed investigation of mechanistic pathways.

CONCLUSION

Oxidative stress is a likely component of aging; however, it is unclear whether increased production of reactive oxygen species or the accumulation of oxidative damage is the primary cause of functional decline. The results provide support for the idea that altered redox-sensitive signaling rather than the accumulation of damage may be of greater significance in the emergence of age-related learning and memory deficits.

Age-dependent targeting of protein phosphatase 1 to Ca2+/calmodulin-dependent protein kinase II by spinophilin in mouse striatum

Mechanisms underlying age-dependent changes of dendritic spines on striatal medium spiny neurons are poorly understood. Spinophilin is an F-actin- and protein phosphatase 1 (PP1)-binding protein that targets PP1 to multiple downstream effectors to modulate dendritic spine morphology and function. We found that calcium/calmodulin-dependent protein kinase II (CaMKII) directly and indirectly associates with N- and C-terminal domains of spinophilin, but F-actin can displace CaMKII from the N-terminal domain. Spinophilin co-localizes PP1 with CaMKII on the F-actin cytoskeleton in heterologous cells, and spinophilin co-localizes with synaptic CaMKII in neuronal cultures. Thr286 autophosphorylation enhances the binding of CaMKII to spinophilin in vitro and in vivo. Although there is no change in total levels of Thr286 autophosphorylation, maturation from postnatal day 21 into adulthood robustly enhances the levels of CaMKII that co-immunoprecipitate with spinophilin from mouse striatal extracts. Moreover, N- and C-terminal domain fragments of spinophilin bind more CaMKII from adult vs. postnatal day 21 striatal lysates. Total levels of other proteins that interact with C-terminal domains of spinophilin decrease during maturation, perhaps reducing competition for CaMKII binding to the C-terminal domain. In contrast, total levels of α-internexin and binding of α-internexin to the spinophilin N-terminal domain increases with maturation, perhaps bridging an indirect interaction with CaMKII. Moreover, there is an increase in the levels of myosin Va, α-internexin, spinophilin, and PP1 in striatal CaMKII immune complexes isolated from adult and aged mice compared to those from postnatal day 21. These changes in spinophilin/CaMKII interactomes may contribute to changes in striatal dendritic spine density, morphology, and function during normal postnatal maturation and aging.

Dissecting the age-related decline on spatial learning and memory tasks in rodent models: N-methyl-D-aspartate receptors and voltage-dependent Ca2+ channels in senescent synaptic plasticity

In humans, heterogeneity in the decline of hippocampal-dependent episodic memory is observed during aging. Rodents have been employed as models of age-related cognitive decline and the spatial water maze has been used to show variability in the emergence and extent of impaired hippocampal-dependent memory. Impairment in the consolidation of intermediate-term memory for rapidly acquired and flexible spatial information emerges early, in middle-age. As aging proceeds, deficits may broaden to include impaired incremental learning of a spatial reference memory. The extent and time course of impairment has been be linked to senescence of calcium (Ca²⁺) regulation and Ca²⁺-dependent synaptic plasticity mechanisms in region CA1. Specifically, aging is associated with altered function of N-methyl-D-aspartate receptors (NMDARs), voltage-dependent Ca²⁺ channels (VDCCs), and ryanodine receptors (RyRs) linked to intracellular Ca²⁺ stores (ICS). In young animals, NMDAR activation induces long-term potentiation of synaptic transmission (NMDAR-LTP), which is thought to mediate the rapid consolidation of intermediate-term memory. Oxidative stress, starting in middle-age, reduces NMDAR function. In addition, VDCCs and ICS can actively inhibit NMDAR-dependent LTP and oxidative stress enhances the role of VDCC and RyR-ICS in regulating synaptic plasticity. Blockade of L-type VDCCs promotes NMDAR-LTP and memory in older animals. Interestingly, pharmacological or genetic manipulations to reduce hippocampal NMDAR function readily impair memory consolidation or rapid learning, generally leaving incremental learning intact. Finally, evidence is mounting to indicate a role for VDCC-dependent synaptic plasticity in associative learning and the consolidation of remote memories. Thus, VDCC-dependent synaptic plasticity and extrahippocampal systems may contribute to incremental learning deficits observed with advanced aging.

Protein kinases as targets for interventive biogerontology: overview and perspectives

Protein kinases are enzymes that catalyze the transfer of γ phosphate from adenosine triphosphate to substrate proteins, and are important signal transduction mediators in a diversity of biological processes, ranging from apoptosis to energy metabolism. In this article, we will take this prominent class of proteins as an example to illustrate the involvement of proteins in modulation of aging and to highlight the prospects and challenges of protein-targeted interventions for anti-aging purposes. It is hoped that through this article, more empirical work on interventive gerontology will follow, and with collaborative endeavors among researchers, hurdles in anti-aging intervention development can be overcome in the near future.

Dysregulated phosphorylation of Ca(2+) /calmodulin-dependent protein kinase II-α in the hippocampus of subjects with mild cognitive impairment and Alzheimer's disease

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder and the most prevalent senile dementia. The early symptom of memory dysfunction involves synaptic loss, thought to be mediated by soluble amyloid-beta (Aβ) oligomers. These aggregate species target excitatory synapses and their levels correlate with disease severity. Studies in cell culture and rodents have shown that oligomers increase intracellular calcium (Ca(2+)), impairing synaptic plasticity. Yet, the molecular mechanism mediating Aβ oligomers' toxicity in the aged brain remains unclear. Here, we apply quantitative immunofluorescence in human brain tissue from clinically diagnosed mild cognitive impaired (MCI) and AD patients to investigate the distribution of phosphorylated (active) Ca(2+) /calmodulin-dependent protein kinase-α (p(Thr286)CaMKII), a critical enzyme for activity-dependent synaptic remodeling associated with cognitive function. We show that p(Thr286)CaMKII immunoreactivity is redistributed from dendritic arborizations to neural perikarya of both MCI and AD hippocampi. This finding correlates with cognitive assessment scores, suggesting that it may be a molecular read-out of the functional deficits in early AD. Treatment with oligomeric Aβ replicated the observed phenotype in mice and resulted in a loss of p(Thr286)CaMKII from synaptic spines of primary hippocampal neurons. Both outcomes were prevented by inhibiting the phosphatase calcineurin (CaN). Collectively, our results support a model in which the synaptotoxicity of Aβ oligomers in human brain involves the CaN-dependent subcellular redistribution of p(Thr286)CaMKII. Therapies designed to normalize the homeostatic imbalance of neuronal phosphatases and downstream dephosphorylation of synaptic p(Thr286)CaMKII should be considered to prevent and treat early AD.

Enhanced expression of Pctk1, Tcf12 and Ccnd1 in hippocampus of rats: Impact on cognitive function, synaptic plasticity and pathology

We previously identified a set of 50 genes that were differentially transcribed in the hippocampal CA1 region of aged, learning-impaired rats compared to aged, superior learning animals during a Morris water maze paradigm. In the current study, we expressed three of these genes (Pctk1, Tcf12 and Ccnd1), which had shown increased transcription in aged, learning impaired rats, in the hippocampus of young rats using viral gene transfer and tested for learning and memory deficits at age 7-14months. Pctk1 injected animals displayed a modest deficit in acquiring latency in both the Morris water maze and the reverse Morris maze. In the radial arm water maze paradigm, Pctk1, Tcf12 and Ccnd1 expressing animals all showed significant deficits in spatial working memory compared to controls. Rats injected with Ccnd1 and Tcf12, but not Pctk1, also showed a significant deficit in spatial reference memory in the radial arm water maze. Electrophysiological experiments revealed no difference in LTP in Ccnd1 and Pctk1 animals. However, LTD induced by low frequency stimulation was observed in control and Ccnd1 animals, but not in Pctk1 treated animals. In addition, neither Ccnd1 nor Pctk1 expression produced any detectable neuropathology. In contrast Tcf12 expressing animals displayed significant neurodegeneration in both CA1 and dentate gyrus. Several Tcf12 animals also developed tumors that appeared to be glioblastomas, suggesting that aberrant Tcf12 expression in the hippocampus is tumorigenic. Thus, behavioral experiments suggested that overexpression of Pctk1 and Ccnd1 produce a deficit in learning and memory, but electrophysiological experiments do not point to a simple mechanism. In contrast, the learning and memory deficits in Tcf12 animals are likely due to neuropathology associated with Tcf12 gene expression.

Single-cell redox imaging demonstrates a distinctive response of dopaminergic neurons to oxidative insults

AIMS

The study of the intracellular oxido-reductive (redox) state is of extreme relevance to the dopamine (DA) neurons of the substantia nigra pars compacta. These cells possess a distinct physiology intrinsically associated with elevated reactive oxygen species production, and they selectively degenerate in Parkinson's disease under oxidative stress conditions. To test the hypothesis that these cells display a unique redox response to mild, physiologically relevant oxidative insults when compared with other neuronal populations, we sought to develop a novel method for quantitatively assessing mild variations in intracellular redox state.

RESULTS

We have developed a new imaging strategy to study redox variations in single cells, which is sensitive enough to detect changes within the physiological range. We studied DA neurons' physiological redox response in biological systems of increasing complexity--from primary cultures to zebrafish larvae, to mammalian brains-and identified a redox response that is distinctive for substantia nigra pars compacta DA neurons. We studied simultaneously, and in the same cells, redox state and signaling activation and found that these phenomena are synchronized.

INNOVATION

The redox histochemistry method we have developed allows for sensitive quantification of intracellular redox state in situ. As this method is compatible with traditional immunohistochemical techniques, it can be applied to diverse settings to investigate, in theory, any cell type of interest.

CONCLUSION

Although the technique we have developed is of general interest, these findings provide insights into the biology of DA neurons in health and disease and may have implications for therapeutic intervention.

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

Aged (20-22 months) male Fischer 344 rats were randomly assigned to sedentary (A-SED), environmentally-enriched (A-ENR), or exercise (A-EX) conditions. After 10-12 weeks of differential experience, the 3 groups of aged rats and young sedentary controls were tested for physical and cognitive function. Spatial discrimination learning and memory consolidation, tested on the water maze, were enhanced in environmentally-enriched compared with sedentary. A-EX exhibited improved and impaired performance on the cue and spatial task, respectively. Impaired spatial learning in A-EX was likely due to a bias in response selection associated with exercise training, as object recognition memory improved for A-EX rats. An examination of senescent hippocampal physiology revealed that enrichment and exercise reversed age-related changes in long-term depression (LTD) and long-term potentiation (LTP). Rats in the enrichment group exhibited an increase in cell excitability compared with the other 2 groups of aged animals. The results indicate that differential experience biased the selection of a spatial or a response strategy and factors common across the 2 conditions, such as increased hippocampal activity associated with locomotion, contribute to reversal of senescent synaptic plasticity.

Glutamatergic neurons in rodent models respond to nanoscale particulate urban air pollutants in vivo and in vitro

BACKGROUND

Inhalation of airborne particulate matter (PM) derived from urban traffic is associated with pathology in the arteries, heart, and lung; effects on brain are also indicated but are less documented.

OBJECTIVE

We evaluated rodent brain responses to urban nanoscale (< 200 nm) PM (nPM).

METHODS

Ambient nPM collected near an urban freeway was transferred to aqueous suspension and reaerosolized for 10-week inhalation exposure of mice or directly applied to rat brain cell cultures.

RESULTS

Free radicals were detected by electron paramagnetic resonance in the nPM 30 days after initial collection. Chronic inhalation of reaerosolized nPM altered selected neuronal and glial activities in mice. The neuronal glutamate receptor subunit (GluA1) was decreased in hippocampus, whereas glia were activated and inflammatory cytokines were induced [interleukin-1α (IL-1α), tumor necrosis factor-α (TNFα)] in cerebral cortex. Two in vitro models showed effects of nPM suspensions within 24-48 hr of exposure that involved glutamatergic functions. In hippocampal slice cultures, nPM increased the neurotoxicity of NMDA (N-methyl-d-aspartic acid), a glutamatergic agonist, which was in turn blocked by the NMDA antagonist AP5 [(2R)-amino-5-phosphonopentanoate]. In embryonic neuron cultures, nPM impaired neurite outgrowth, also blocked by AP5. Induction of IL-1α and TNFα in mixed glia cultures required higher nPM concentrations than did neuronal effects. Because conditioned media from nPM-exposed glia also impaired outgrowth of embryonic neurites, nPM can act indirectly, as well as directly, on neurons in vitro.

CONCLUSIONS

nPM can affect embryonic and adult neurons through glutamatergic mechanisms. The interactions of nPM with glutamatergic neuronal functions suggest that cerebral ischemia, which involves glutamatergic excitotoxicity, could be exacerbated by nPM.

Age-Induced Alterations in Hippocampal Function and Metabolism

As the nervous system ages, a variety of changes occur in metabolism supporting glial and neuronal function, resulting in greater susceptibility to disease conditions. Changes with aging in the metabolic unit (i.e., neurons, glial cells and blood vessels) have been reported to include alterations of vascular reactivity, impaired transport of critical substrates underlying metabolism, enhanced reactive oxygen species production and alterations in calcium signaling. Some diseases are focused on the elderly, particularly cerebral ischemia, cognitive limitations, iatrogenic hypoglycemia, malignant brain tumors (i.e., glioblastoma), and Alzheimer's disease, partly due to metabolic alterations with aging. These metabolic changes with aging are discussed in light of primary theories of aging of the brain, which include mitochondrial, calcium dysfunction and enhanced oxidative damage. Here we focus on metabolic changes with aging which can influence the susceptibility of the brain to ischemia and cognitive function. Lastly, we describe treatment possibilities for these abnormal responses to aging, particularly the topic of caloric/dietary restriction, and possible mechanisms underlying this treatment direction.

Glutathione restores the mechanism of synaptic plasticity in aged mice to that of the adult

Glutathione (GSH), the major endogenous antioxidant produced by cells, can modulate the activity of N-methyl-D-aspartate receptors (NMDARs) through its reducing functions. During aging, an increase in oxidative stress leads to decreased levels of GSH in the brain. Concurrently, aging is characterized by calcium dysregulation, thought to underlie impairments in hippocampal NMDAR-dependent long-term potentiation (LTP), a form of synaptic plasticity thought to represent a cellular model for memory.

Here we show that orally supplementing aged mice with N-acetylcysteine, a precursor for the formation of glutathione, reverses the L-type calcium channel-dependent LTP seen in aged animals to NMDAR-dependent LTP. In addition, introducing glutathione in the intrapipette solution during whole-cell recordings restores LTP obtained in whole-cell conditions in the aged hippocampus. We conclude that aging leads to a reduced redox potential in hippocampal neurons, triggering impairments in LTP.

Tetrahydroxystilbene glucoside, a plant-derived cognitive enhancer, promotes hippocampal synaptic plasticity

Plant or food derived polyphenols have received a great deal of attention due to their biological properties including anti-oxidative effects, neuroprotection and memory enhancement. Here, we investigated the roles of 2,3,5,4'-tetrahydroxystilbene-2-O-β-d-glucoside (TSG), an active component of the rhizome extracted from Polygonum multiflorum, in the regulation of hippocampal synaptic plasticity in normal mice as well as the underlying mechanisms. It was shown that TSG promoted the differentiation of PC12 cells and increased the intracellular calcium level in hippocampal neurons. TSG facilitated high-frequency stimulation (HFS)-induced hippocampal long-term potentiation (LTP) in a bell-shaped manner. The facilitation of LTP induction by TSG required calcium/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) activation. Taken together, our data demonstrate that TSG promotes LTP induction and this effect may contribute to the enhancement of learning and memory seen in animal models.

Redox sensitive calcium stores underlie enhanced after hyperpolarization of aged neurons: role for ryanodine receptor mediated calcium signaling

A decrease in the excitability of CA1 pyramidal neurons contributes to the age related decrease in hippocampal function and memory decline. Decreased neuronal excitability in aged neurons can be observed as an increase in the Ca(2+)- activated K(+)- mediated post burst afterhyperpolarization (AHP). In this study, we demonstrate that the slow component of AHP (sAHP) in aged CA1 neurons (aged-sAHP) is decreased ∼50% by application of the reducing agent dithiothreitol (DTT). The DTT-mediated decrease in the sAHP was age specific, such that it was observed in CA1 pyramidal neurons of aged (20-25 mo), but not young (6-9 mo) F344 rats. The effect of DTT on the aged-sAHP was blocked following depletion of intracellular Ca(2+) stores (ICS) by thapsigargin or blockade of ryanodine receptors (RyRs) by ryanodine, suggesting that the age-related increase in the sAHP was due to release of Ca(2+) from ICS through redox sensitive RyRs. The DTT-mediated decrease in the aged-sAHP was not blocked by inhibition of L-type voltage gated Ca(2+) channels (L-type VGCC), inhibition of Ser/Thr kinases, or inhibition of the large conductance BK potassium channels. The results add support to the idea that a shift in the intracellular redox state contributes to Ca(2+) dysregulation during aging.

Carbachol-induced long-term synaptic depression is enhanced during senescence at hippocampal CA3-CA1 synapses

Dysregulation of the cholinergic transmitter system is a hallmark of Alzheimer's disease and contributes to an age-associated decline in memory performance. The current study examined the influence of carbachol, a cholinergic receptor agonist, on synaptic transmission over the course of aging. Extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synapses in acute hippocampal slices obtained from young adult (5-8 mo) and aged (22-24 mo) male Fischer 344 rats. Bath application of carbachol elicited a transient depression of synaptic transmission, which was followed by a long-lasting depression (CCh-LTD) observed 90 min after carbachol cessation in both age groups. However, the magnitude of CCh-LTD was significantly larger in senescent animals and was attenuated by N-methyl-D-aspartate receptor blockade in aged animals. Blockade of L-type Ca(2+) channels inhibited CCh-LTD to a greater extent in aged animals compared to young adults. Finally, the expression of CCh-LTD was dependent on protein synthesis. The results indicate that altered Ca(2+) homeostasis or muscarinic activation of Ca(2+) signaling contribute to the enhanced CCh-LTD during senescence.