Signatures of hippocampal oxidative stress in aged spatial learning-impaired rodents

Contributions of impaired hippocampal plasticity and neurodegeneration to age-related deficits in hormonal pulsatility

Connectivity between the hippocampus and hypothalamus plays an essential role in circadian rhythmicity and stress responsiveness. Both hippocampal dysfunction and loss of hormonal pulsatility have been demonstrated in aged animals, but the possibility of a functional interaction between these two processes remains unexplored. Correlated hippocampal neuropathology and flattening of the circadian rhythms occur in the elderly, and we propose that these processes are causally linked. In this review, we discuss the anatomical and functional nature of hippocampal interconnections with the hypothalamus. We also discuss the results of studies exploring the relationship between circadian phase and hippocampal plasticity in young animals, with the goal of understanding how these mechanisms might be restored in the aging brain.

Reversion of age-related recognition memory impairment by iron chelation in rats

It is now generally accepted that iron accumulates in the brain during the ageing process. Increasing evidence demonstrate that iron accumulation in selective regions of the brain may generate free radicals, thereby possessing implications for the etiology of neurodegenerative disorders. In a previous study we have reported that aged rats present recognition memory deficits. The aim of the present study was to evaluate the effect of desferoxamine (DFO), an iron chelator agent, on age-induced memory impairment. Aged Wistar rats received intraperitoneal injections of saline or DFO (300mg/kg) for 2 weeks. The animals were submitted to a novel object recognition task 24h after the last injection. DFO-treated rats showed normal recognition memory while the saline group showed long-term recognition memory deficits. The results show that DFO is able to reverse age-induced recognition memory deficits. We also demonstrated that DFO reduced the oxidative damage to proteins in cortex and hippocampus. Thus, the present findings provide the first evidence that iron chelators might prevent age-related memory dysfunction.

An examination of the effects of the antioxidant Pycnogenol on cognitive performance, serum lipid profile, endocrinological and oxidative stress biomarkers in an elderly population

The study examines the effects of the antioxidant flavonoid Pycnogenol on a range of cognitive and biochemical measures in healthy elderly individuals. The study used a double-blind, placebo-controlled, matched-pair design, with 101 elderly participants (60-85 years) consuming a daily dose of 150 mg of Pycnogenol for a three-month treatment period. Participants were assessed at baseline, then at 1, 2, and 3 months of the treatment. The control (placebo) and Pycnogenol groups were matched by age, sex, body mass index, micronutrient intake, and intelligence. The cognitive tasks comprised measures of attention, working memory, episodic memory, and psychomotor performance. The biological measures comprised levels of clinical hepatic enzymes, serum lipid profile, human growth hormone, and lipid peroxidation products. Statistically significant interactions were found for memory-based cognitive variables and lipid peroxidation products, with the Pycnogenol group displaying improved working memory and decreased concentrations of F2-isoprostanes relative to the control group.

Prevention of cognitive deficits and brain oxidative stress with superoxide dismutase/catalase mimetics in aged mice

Continuous decline in cognitive performance accompanies the natural aging process in humans, and multiple studies in both humans and animal models have indicated that this decrease in cognitive function is associated with an age-related increase in oxidative stress. Treating aging mammals with exogenous free radical scavengers has generally been shown to attenuate age-related cognitive decline and oxidative stress. The present study assessed the effectiveness of the superoxide dismutase/catalase mimetics EUK-189 and EUK-207 on age-related decline in cognitive function and increase in oxidative stress. C57/BL6 mice received continuous treatment via osmotic minipumps with either EUK-189 or EUK-207 for 6 months starting at 17 months of age. At the end of treatment, markers for oxidative stress were evaluated by analyzing levels of free radicals, lipid peroxidation and oxidized nucleic acids in brain tissue. In addition, cognitive performance was assessed after 3 and 6 months of treatment with fear conditioning. Both EUK-189 and EUK-207 treatments resulted in significantly decreased lipid peroxidation, nucleic acid oxidation, and reactive oxygen species (ROS) levels. In addition, the treatments also significantly improved age-related decline in performance in the fear-conditioning task. Our results thus confirm a critical role for oxidative stress in age-related decline in learning and memory and strongly suggest a potential usefulness for salen-manganese complexes in reversing age-related declines in cognitive function and oxidative load.

Minocycline attenuates lipopolysaccharide (LPS)-induced neuroinflammation, sickness behavior, and anhedonia

Background

Activation of the peripheral innate immune system stimulates the secretion of CNS cytokines that modulate the behavioral symptoms of sickness. Excessive production of cytokines by microglia, however, may cause long-lasting behavioral and cognitive complications. The purpose of this study was to determine if minocycline, an anti-inflammatory agent and purported microglial inhibitor, attenuates lipopolysaccharide (LPS)-induced neuroinflammation, sickness behavior, and anhedonia.

Methods

In the first set of experiments the effect of minocycline pretreatment on LPS-induced microglia activation was assessed in BV-2 microglia cell cultures. In the second study, adult (3–6 m) BALB/c mice received an intraperitoneal (i.p.) injection of vehicle or minocycline (50 mg/kg) for three consecutive days. On the third day, mice were also injected (i.p.) with saline or Escherichia coli LPS (0.33 mg/kg) and behavior (i.e., sickness and anhedonia) and markers of neuroinflammation (i.e., microglia activation and inflammatory cytokines) were determined. In the final study, adult and aged BALB/c mice were treated with the same minocycline and LPS injection regimen and markers of neuroinflammation were determined. All data were analyzed using Statistical Analysis Systems General Linear Model procedures and were subjected to one-, two-, or three-way ANOVA to determine significant main effects and interactions.

Results

Minocycline blocked LPS-stimulated inflammatory cytokine secretion in the BV-2 microglia-derived cell line and reduced LPS-induced Toll-like-receptor-2 (TLR2) surface expression on brain microglia. Moreover, minocycline facilitated the recovery from sickness behavior (i.e., anorexia, weight loss, and social withdrawal) and prevented anhedonia in adult mice challenged with LPS. Furthermore, the minocycline associated recovery from LPS-induced sickness behavior was paralleled by reduced mRNA levels of Interleukin (IL)-1β, IL-6, and indoleamine 2, 3 dioxygenase (IDO) in the cortex and hippocampus. Finally, in aged mice, where exaggerated neuroinflammation was elicited by LPS, minocycline pretreatment was still effective in markedly reducing mRNA levels of IL-1β, TLR2 and IDO in the hippocampus.

Conclusion

These data indicate that minocycline mitigates neuroinflammation in the adult and aged brain and modulates the cytokine-associated changes in motivation and behavior.

Comparison of ionized calcium-binding adapter molecule 1 immunoreactivity of the hippocampal dentate gyrus and CA1 region in adult and aged dogs

Similarities between age-related changes in the canine and human brain have resulted in the general acceptance of the canine brain as a model of human brain aging. The hippocampus is essentially required for intact cognitive ability and appears to be particularly vulnerable to the aging process. We observed changes in ionized calcium-binding adapter molecule 1 (Iba-1, a microglial marker) immunoreactivity and protein levels in the hippocampal dentate gyrus and CA1 region of adult (2-3 years) and aged (10-12 years) dogs. We also observed the interferon-gamma (IFN-gamma), a pro-inflammatory cytokine, protein levels in these groups. In the dentate gyrus and CA1 region of the adult dog, Iba-1 immunoreactive microglia were well distributed and their processes were highly ramified. However, in the aged dog, the processes of Iba-1 immunoreactive microglia were hypertrophied in the dentate gyrus. Moreover, Iba-1 protein level in the dentate gyrus in the aged dog was higher than in the adult dog. IFN-gamma expression was increased in the dentate gyrus homogenates of aged dogs than adult dogs. In addition, we found that some neurons were positive to Fluoro-Jade B (a marker for neuronal degeneration) in the dentate polymorphic layer, but not in the hippocampal CA1 region in the aged dog. These results suggest that Iba-1 immunoreactive microglia are hypertrophied in the dentate gyrus in the aged dog.

N-acetyl cysteine supplementation prevents impairment of spatial working memory functions in rats following exposure to hypobaric hypoxia

Exposure to high altitude (HA), especially extreme altitude, is associated with impairment of cognitive functions including memory and increased oxidative stress. However, the underlying mechanisms involved are not well understood. It is hypothesized that HA induced oxidative stress may be one of the factors underlying hypoxia induced memory impairment. The aim of the present study was to investigate the effect of hypobaric hypoxia (HH) on spatial working and reference memory functions, oxidative stress markers in rats and effect of supplementation of N-acetyl cysteine (NAC). The rats were divided into four groups. Group I served as normoxic (n=6), Group II served as hypoxic (n=6), Group III as hypoxia group treated with NAC (n=6) and Group IV served as normoxic group treated with NAC (n=6). Group II & III were exposed to HH for 3 days equivalent to 6100 m and received oral NAC supplementation (750 mg/kg) daily. Rats from all the groups were trained in Morris Water Maze (MWM) task for 8 consecutive days. Spatial working and reference memory were tested immediately after the termination of HH and then the rats were sacrificed for estimation of oxidative stress markers in hippocampus. Rats displayed significant deficits in spatial working memory, and increased oxidative stress along with decrease in antioxidant status on hypoxic exposure. Supplementation with NAC in hypoxia-exposed group improved spatial memory performance, and decreased oxidative stress. These findings indicate that hypoxic exposure is associated with increased oxidative stress, which may have caused memory deficit in rats exposed to simulated HA.

Benzo(a)pyrene-induced acute neurotoxicity in the F-344 rat: role of oxidative stress

Given the link between neurotoxicity and exposure to pollutants, the potential behavioral neurotoxicity of benzo(a)pyrene [B(a)P] was investigated. Studies have established that B(a)P requires metabolic activation to highly reactive species to elicit many of its adverse effects. This study investigated the perturbation of nervous system function by correlating behavioral changes with the metabolism of B(a)P, antioxidant enzyme levels and lipid peroxidation in selected brain regions. The neurobehavioral effects of single oral doses of B(a)P (25-200 mg kg(-1) body weight) on motor activity were examined in male F-344 rats at 2, 4, 6, 12, 24, 48, 72 and 96 h post treatment. Parent B(a)P and metabolites were measured at the above mentioned time points by reverse phase HPLC. The activity of several antioxidant enzymes (superoxide dismutase, catalase and glutathione peroxidase) and levels of malondialdehyde were determined at 6 and 96 h in both the striatum and hippocampus of B(a)P exposed rats. Suppression of motor activity (up to 70%) reached a maximum at 6 h, but was reversible at 96 h in all dose groups. The kinetics of disposition data show a strong link between B(a)P metabolism and the onset and duration of behavioral effects. Benzo(a)pyrene caused a 15-70% inhibition in the activity of superoxide dismutase and glutathione peroxidase and an enhancement in catalase and lipid peroxidation (up to 68%) in the striatum and hippocampus at 6 and 96 h post treatment, respectively. These findings suggest that B(a)P-induced acute neurobehavioral toxicity may occur through oxidative stress due to inhibition of the brain antioxidant scavenging system.

Memantine reduces oxidative damage and enhances long-term recognition memory in aged rats

Many neurodegenerative diseases, including Alzheimer's (AD), Parkinson's (PD) and Huntington's diseases (HD), are caused by different mechanisms but may share a common pathway to neuronal injury as a result of the overstimulation of glutamate receptors. It has been suggested that this pathway can be involved in generation of cognitive deficits associated with normal aging. Previous studies performed in our laboratory have demonstrated that aged rats presented recognition memory deficits. The aim of the present study was to evaluate the effect of memantine, a low-affinity N-methyl-D-aspartate (NMDA) receptor antagonist, on age-induced recognition memory deficits. Additionally, parameters of oxidative damage in cerebral regions related to memory formation were evaluated. In order to do that, male Wistar rats (24 months old) received daily injections of saline solution or memantine (20 mg/kg i.p.) during 21 days. The animals were submitted to a novel object recognition task 1 week after the last injection. Memantine-treated rats showed normal recognition memory while the saline group showed long-term recognition memory deficits. The results show that memantine is able to reverse age-induced recognition memory deficits. We also demonstrated that memantine reduced the oxidative damage to proteins in cortex and hippocampus, two important brain regions involved in memory formation. Thus, the present findings suggest that, at least in part, age-induced cognitive deficits are related to oxidative damage promoted by NMDA receptor overactivation.

Exaggerated sickness behavior and brain proinflammatory cytokine expression in aged mice in response to intracerebroventricular lipopolysaccharide

Age-associated changes in glial reactivity may predispose individuals to exacerbated neuroinflammatory cytokine responses that are permissive to cognitive and behavioral complications. The purpose of this study was to determine if aging is associated with an exaggerated sickness response to central innate immune activation. Our results show that intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) caused a heightened proinflammatory cytokine response (IL-1beta, IL-6, and TNFalpha) in the cerebellum 2h post i.c.v. injection in aged mice compared to adults. This amplified inflammatory profile was consistent with a brain region-dependent increase in reactive glial markers (MHC class II, TLR2 and TLR4). Moreover, LPS caused a prolonged sickness behavior response in aged mice that was paralleled by a protracted expression of brain cytokines in the cerebellum and hippocampus. Finally, central LPS injection caused amplified and prolonged IL-6 levels at the periphery of aged mice. Collectively, these data establish that activation of the central innate immune system leads to exacerbated neuroinflammation and prolonged sickness behavior in aged as compared to adult mice.

Aging causes partial loss of basal forebrain but no loss of pontine reticular cholinergic neurons

Cholinergic degeneration occurs in several neurodegenerative diseases. To investigate whether normal aging causes selective neurodegeneration, we compared counts of cholinergic neurons in the medial septum/vertical limb of the diagonal band and pedunculopontine and laterodorsal tegmental nuclei of the brainstem in young and aged Long-Evans rats characterized for their spatial learning ability in the Morris water maze. A subset of aged rats (aged-unimpaired) learned the spatial learning task as young rats, whereas another group (age-impaired) showed poorer learning than young animals. In the medial septum/diagonal band, there was a significant loss (-23%, P < 0.02) of cholinergic neurons in aged-impaired animals compared with young subjects. In the brainstem, there were no significant differences in cholinergic cell number in any group. This selective loss of cholinergic neurons may, in part, account for the cognitive deficits observed in aging and, considering previous findings in this model, may be related to oxidative stress.

Effects of age and caloric intake on glutathione redox state in different brain regions of C57BL/6 and DBA/2 mice

The main purpose of the present study was to determine whether specific regions of the mouse brain exhibit different age-related changes in oxidative stress, as indicated by glutathione redox state and the level of protein-glutathionyl mixed disulfides. Comparison of 3- and 21-month-old mice indicated an age-related decrease in the ratio of reduced to oxidized glutathione (GSH/GSSG) as well as a pro-oxidizing shift in the calculated redox potential (ranging from 6 to 15 mV) in the cortex, hippocampus, striatum and cerebellum, whereas there was little change in the brainstem. This pro-oxidizing shift in redox state was due to a modest decrease in GSH content occurring in all the brain regions examined, and elevations in GSSG amount that were most pronounced in the striatum and cerebellum. The regional changes in glutathione redox state were paralleled by increases in the amounts of protein-mixed disulfides. A reduction of caloric intake by 40% for a short period (7 weeks), implemented in relatively old mice (17 months), increased the GSH/GSSG ratio and redox potential at 19 months in the same brain regions that exhibited age-related decreases. The effects of age and caloric restriction were qualitatively similar in C57BL/6 and DBA/2 mice. However, young DBA/2 mice, which do not show extension of life span in response to long-term caloric restriction, had lower GSH/GSSG ratios and higher protein-mixed disulfides than age-matched C57BL/6 mice. The current findings demonstrate that oxidative stress, as reflected by glutathione redox state, increases in the aging brain in regions linked to age-associated losses of function and neurodegenerative diseases.

Different organization of base excision repair of uracil in DNA in nuclei and mitochondria and selective upregulation of mitochondrial uracil-DNA glycosylase after oxidative stress

Oxidative stress in the brain may cause neuro-degeneration, possibly due to DNA damage. Oxidative base lesions in DNA are mainly repaired by base excision repair (BER). The DNA glycosylases Nei-like DNA glycosylase 1 (NEIL1), Nei-like DNA glycosylase 2 (NEIL2), mitochondrial uracil-DNA glycosylase 1 (UNG1), nuclear uracil-DNA glycosylase 2 (UNG2) and endonuclease III-like 1 protein (NTH1) collectively remove most oxidized pyrimidines, while 8-oxoguanine-DNA glycosylase 1 (OGG1) removes oxidized purines. Although uracil is the main substrate of uracil-DNA glycosylases UNG1 and UNG2, these proteins also remove the oxidized cytosine derivatives isodialuric acid, alloxan and 5-hydroxyuracil. UNG1 and UNG2 have identical catalytic domain, but different N-terminal regions required for subcellular sorting. We demonstrate that mRNA for UNG1, but not UNG2, is increased after hydrogen peroxide, indicating regulatory effects of oxidative stress on mitochondrial BER. To examine the overall organization of uracil-BER in nuclei and mitochondria, we constructed cell lines expressing EYFP (enhanced yellow fluorescent protein) fused to UNG1 or UNG2. These were used to investigate the possible presence of multi-protein BER complexes in nuclei and mitochondria. Extracts from nuclei and mitochondria were both proficient in complete uracil-BER in vitro. BER assays with immunoprecipitates demonstrated that UNG2-EYFP, but not UNG1-EYFP, formed complexes that carried out complete BER. Although apurinic/apyrimidinic site endonuclease 1 (APE1) is highly enriched in nuclei relative to mitochondria, it was apparently the major AP-endonuclease required for BER in both organelles. APE2 is enriched in mitochondria, but its possible role in BER remains uncertain. These results demonstrate that nuclear and mitochondrial BER processes are differently organized. Furthermore, the upregulation of mRNA for mitochondrial UNG1 after oxidative stress indicates that it may have an important role in repair of oxidized pyrimidines.

Differential aging of the brain: patterns, cognitive correlates and modifiers

Deciphering the secret of successful aging depends on understanding the patterns and biological underpinnings of cognitive and behavioral changes throughout adulthood. That task is inseparable from comprehending the workings of the brain, the physical substrate of behavior. In this review, we summarize the extant literature on age-related differences and changes in brain structure, including postmortem and noninvasive magnetic resonance imaging (MRI) studies. Among the latter, we survey the evidence from volumetry, diffusion-tensor imaging, and evaluations of white matter hyperintensities (WMH). Further, we review the attempts to elucidate the mechanisms of age-related structural changes by measuring metabolic markers of aging through magnetic resonance spectroscopy (MRS). We discuss the putative links between the pattern of brain aging and the pattern of cognitive decline and stability. We then present examples of activities and conditions (hypertension, hormone deficiency, aerobic fitness) that may influence the course of normal aging in a positive or negative fashion. Lastly, we speculate on several proposed mechanisms of differential brain aging, including neurotransmitter systems, stress and corticosteroids, microvascular changes, calcium homeostasis, and demyelination.

Age and Neuroinflammation: A Lifetime of Psychoneuroimmune Consequences

This article reviews the literature indicating that the innate immune cells of the brain become more reactive with age. Although it is unclear how glia reactivity increases, emerging evidence suggests these alterations allow exacerbated neuroinflammation and sickness behavior following peripheral immune activation. This amplified or prolonged exposure to inflammatory cytokines in the brain may impair neuronal plasticity and underlie a heightened neuroinflammatory response in the aged that also may lead to other neurobehavioral impairments such as delirium, depression, and, potentially, the onset of neurologic disease. Therefore pharmacologic strategies to decrease neuroinflammation associated with infection may be important for improving recovery from sickness and reducing neurobehavioral deficits in the elderly.

mRNA up-regulation of MHC II and pivotal pro-inflammatory genes in normal brain aging

In normal brain aging, CNS resident macrophages exhibit increased expression of major histocompatibility complex (MHC) II expression. However, the transcriptional basis for this observation has not been clarified nor have age-related alterations in pivotal pro-inflammatory genes been characterized. Age-related mRNA alterations in MHC II, MHC II accessory molecules and several pro-inflammatory mediators were measured in older (24 months) and younger (3 months) male F344xBN F1 rats. Real time RT-PCR was utilized to measure steady state mRNA levels in hippocampus. Older as compared to younger animals exhibited increased mRNA levels of MHC II, CD86, CIITA and IFN-gamma. Furthermore, IL-10 and CD200 mRNA, molecules that down-regulate macrophage activation, was decreased in older animals. The present results indicate that normal brain aging is characterized by a shift towards a pro-inflammatory microenvironment in the CNS.

Aging-dependent alterations in synaptic plasticity and memory in mice that overexpress extracellular superoxide dismutase

Oxidative damage caused by reactive oxygen species (ROS) has been proposed to be critically involved in several pathological manifestations of aging, including cognitive dysfunction. ROS, including superoxide, are generally considered as neurotoxic molecules whose effects can be alleviated by antioxidant enzymes. However, ROS also are known to be necessary components of the signal transduction cascades underlying normal synaptic plasticity. Therefore, we reasoned that the role that ROS and antioxidant enzymes play in modulating neuronal processes varies over the lifespan of an animal. We examined hippocampal long-term potentiation (LTP) and memory-related behavioral performance in transgenic mice overexpressing extracellular superoxide dismutase (EC-SOD) and their wild-type littermates at different ages. We found that aged EC-SOD transgenic mice exhibited enhanced hippocampal LTP, better cerebellum-dependent motor learning, and better hippocampus-dependent spatial learning compared with their wild-type littermates. We also found that EC-SOD overexpression impaired contextual learning, but the impairment was decreased in the aged transgenic mice. At the molecular level, aged EC-SOD transgenic mice had lower superoxide levels, a decrease in protein carbonyl levels, and a decrease in p38 and extracellular signal-regulated kinase 2 phosphorylation compared with aged wild-type mice. Our findings suggest that elevated levels of superoxide contribute to aging-related impairments in hippocampal LTP and memory, and that these impairments can be alleviated by overexpression of EC-SOD. We conclude that there is an age-dependent alteration in the role of superoxide in modulating synaptic plasticity and learning and memory.

Muscarinic receptor-mediated GTP-Eu binding in the hippocampus and prefrontal cortex is correlated with spatial memory impairment in aged rats

The present study examined muscarinic receptor/G-protein coupling in the hippocampus and the prefrontal cortex of young and aged Long-Evans rats characterized for spatial learning ability in the Morris water maze. In a highly sensitive time-resolved fluorometry GTP-Eu binding assay, muscarinic-mediated GTP-Eu binding was severely blunted in hippocampus (-32%) and prefrontal cortex (-34%) as a consequence of aging. Furthermore, the magnitude of decreased muscarinic-mediated GTP-Eu binding was significantly correlated with the severity of spatial learning impairment in hippocampus and prefrontal cortex of aged rats and was specifically decreased in the subset of aged rats that were spatial learning impaired when compared to the aged unimpaired and the young rats. Western blot data indicated a preservation of the membrane-bound M1 receptor and the Galphaq/11 protein in both brain regions. These data demonstrate that muscarinic signaling is severely impaired as a consequence of normal aging in a manner that is closely associated with age-related cognitive decline.

Secrets of the lac operon. Glucose hysteresis as a mechanism in dietary restriction, aging and disease

Elevated blood glucose associated with diabetes produces progressive and apparently irreversible damage to many cell types. Conversely, reduction of glucose extends life span in yeast, and dietary restriction reduces blood glucose. Therefore it has been hypothesized that cumulative toxic effects of glucose drive at least some aspects of the aging process and, conversely, that protective effects of dietary restriction are mediated by a reduction in exposure to glucose. The mechanisms mediating cumulative toxic effects of glucose are suggested by two general principles of metabolic processes, illustrated by the lac operon but also observed with glucose-induced gene expression. First, metabolites induce the machinery of their own metabolism. Second, induction of gene expression by metabolites can entail a form of molecular memory called hysteresis. When applied to glucose-regulated gene expression, these two principles suggest a mechanism whereby repetitive exposure to postprandial excursions of glucose leads to an age-related increase in glycolytic capacity (and reduction in beta-oxidation of free fatty acids), which in turn leads to an increased generation of oxidative damage and a decreased capacity to respond to oxidative damage, independent of metabolic rate. According to this mechanism, dietary restriction increases life span and reduces pathology by reducing exposure to glucose and therefore delaying the development of glucose-induced glycolytic capacity.

Harnessing the power of gene microarrays for the study of brain aging and Alzheimer's disease: statistical reliability and functional correlation

During normal brain aging, numerous alterations develop in the physiology, biochemistry and structure of neurons and glia. Aging changes occur in most brain regions and, in the hippocampus, have been linked to declining cognitive performance in both humans and animals. Age-related changes in hippocampal regions also may be harbingers of more severe decrements to come from neurodegenerative disorders such as Alzheimer's disease (AD). However, unraveling the mechanisms underlying brain aging, AD and impaired function has been difficult because of the complexity of the networks that drive these aging-related changes. Gene microarray technology allows massively parallel analysis of most genes expressed in a tissue, and therefore is an important new research tool that potentially can provide the investigative power needed to address the complexity of brain aging/neurodegenerative processes. However, along with this new analytic power, microarrays bring several major bioinformatics and resource problems that frequently hinder the optimal application of this technology. In particular, microarray analyses generate extremely large and unwieldy data sets and are subject to high false positive and false negative rates. Concerns also have been raised regarding their accuracy and uniformity. Furthermore, microarray analyses can result in long lists of altered genes, most of which may be difficult to evaluate for functional relevance. These and other problems have led to some skepticism regarding the reliability and functional usefulness of microarray data and to a general view that microarray data should be validated by an independent method. Given recent progress, however, we suggest that the major problem for current microarray research is no longer validity of expression measurements, but rather, the reliability of inferences from the data, an issue more appropriately redressed by statistical approaches than by validation with a separate method. If tested using statistically defined criteria for reliability/significance, microarray data do not appear a priori to require more independent validation than data obtained by any other method. In fact, because of added confidence from co-regulation, they may require less. In this article we also discuss our strategy of statistically correlating individual gene expression with biologically important endpoints designed to address the problem of evaluating functional relevance. We also review how work by ourselves and others with this powerful technology is leading to new insights into the complex processes of brain aging and AD, and to novel, more comprehensive models of aging-related brain change.

Growth hormone administration to aged animals reduces disulfide glutathione levels in hippocampus

Systemic growth hormone (GH) and insulin-like growth factor-1 (IGF-1), potent anabolic hormones, decrease with age. In humans and animal models, administration of growth hormone or IGF-1 to aged subjects improves learning and memory, suggesting that the age-related decline in cognitive performance results, in part, from peripheral GH/IGF-1 deficiency. However, the cellular mechanisms by which GH/IGF-1 effect cognitive function are unknown. We propose that the effects of these hormones may be mediated by increasing cellular redox potential resulting in reduced oxidative stress. Because the most abundant endogenous antioxidant is glutathione (GSH), we assessed GSH and disulfide glutathione (GSSH) levels in hippocampus and frontal cortex of young (4-month-old) and aged (30-month-old) male Fisher 344xBrown Norway rats treated with porcine growth hormone (200microg/animal, twice/daily) or vehicle. We report that hippocampal levels of GSSG increase with age (0.54+/-0.08 to 1.55+/-0.24nmolGSSG/mgprotein, p<0.05) and growth hormone treatment ameliorates both the age-related rise in GSSG (1.55+/-0.24 to 0.87+/-0.24nmolGSSG/mgprotein, p<0.05) and the decline in GSH/GSSG ratios. Analysis of GSSG reductase activity in aged animals indicated no effect of either age or growth hormone treatment (p=0.81). Although similar age-related increases in GSSG and decreases in GSH/GSSG ratios were evident in frontal cortex, growth hormone had no effect. Subsequently, we assessed whether the effects of age and growth hormone treatment result from modulating trace metal accumulation. Thirteen metals were analyzed in hippocampus and frontal cortex by inductive coupled plasma mass spectrometry. Aluminum, copper, iron, manganese and zinc levels increased with age (p<0.05 each) but growth hormone replacement had no effect on metal accumulation. Our results indicate that growth hormone replacement attenuates the age-related increase in oxidative stress in hippocampus without effects on glutathione reductase or trace metal accumulation. We conclude that the age-related decline in circulating growth hormone and IGF-1 contribute to increased oxidative stress in hippocampus with age.

Brain cholinergic vulnerability: Relevance to behavior and disease

The major populations of cholinergic neurons in the brain include two "projection" systems, located in the pontine reticular formation and in the basal forebrain. These two complexes comprise, in part, the anatomical substrates for the "ascending reticular activating system" (ARAS). The pontine cholinergic system relays its rostral influences mainly through thalamic intralaminar nuclei, but it also connects to the basal forebrain and provides a minor innervation of cortex. The basal forebrain cholinergic complex (BFCC) projects directly to cortex and hippocampus, and has a minor connection with the thalamus. Recent data reveal that a parallel system of basal forebrain GABAergic projection neurons innervates cortex/hippocampus in a way that seems to complement the BFCC. Generally, the picture developed from more than 50 years of research is consistent with a "global" influence of these two ascending cholinergic projections on cortical and hippocampal regions. Seemingly, the BFCC acts in tandem or in parallel with the pontine cholinergic projection to activate the electro-encephalogram, increase cerebral blood flow, regulate sleep-wake cycling, and modulate cognitive function. There are quite a number and variety of human brain conditions, notably including Alzheimer's disease, in which degeneration of basal forebrain cholinergic neurons has been documented. Whether the corticopetal GABA system is affected by disease has not been established. Studies of degeneration of the pontine projection are limited, but the available data suggest that it is relatively preserved in Alzheimer's disease. Hypotheses of BFCC degeneration include growth factor deprivation, intracellular calcium dysfunction, amyloid excess, inflammation, and mitochondrial abnormalities/oxidative stress. But, despite considerable research conducted over several decades, the exact mechanisms underlying brain cholinergic vulnerability in human disease remain unclear.

Exaggerated neuroinflammation and sickness behavior in aged mice following activation of the peripheral innate immune system

Acute cognitive impairment (i.e., delirium) is common in elderly emergency department patients and frequently results from infections that are unrelated to the central nervous system. Since activation of the peripheral innate immune system induces brain microglia to produce inflammatory cytokines that are responsible for behavioral deficits, we investigated if aging exacerbated neuroinflammation and sickness behavior after peripheral injection of lipopolysaccharide (LPS). Microarray analysis revealed a transcriptional profile indicating the presence of primed or activated microglia and increased inflammation in the aged brain. Furthermore, aged mice had a unique gene expression profile in the brain after an intraperitoneal injection of LPS, and the LPS-induced elevation in the brain inflammatory cytokines and oxidative stress was both exaggerated and prolonged compared with adults. Aged mice were anorectic longer and lost more weight than adults after peripheral LPS administration. Moreover, reductions in both locomotor and social behavior remained 24 h later in aged mice, when adults had fully recovered, and the exaggerated neuroinflammatory response in aged mice was not reliably paralleled by increased circulating cytokines in the periphery. Taken together, these data establish that activation of the peripheral innate immune system leads to exacerbated neuroinflammation in the aged as compared with adult mice. This dysregulated link between the peripheral and central innate immune system is likely to be involved in the severe behavioral deficits that frequently occur in older adults with systemic infections.

Regulation of cytochrome oxidase activity in the rat forebrain throughout adulthood

Measures of metabolic activity can provide useful indices of the effects of aging on neural function, since sustained changes in neural activity alter metabolic demand and the activity of metabolic enzymes. Previous reports of effects of aging on key enzymes for oxidative metabolism are mixed, however, with some reports that activity declines in the aging brain and others that activity remains stable or increases. We used high-resolution, quantitative histochemistry to test whether cytochrome oxidase (CO) activity changes in the forebrain during adulthood and senescence, measuring activity in each layer of the hippocampus and several cerebral cortical areas. In most forebrain regions, average cytochrome oxidase activity was slightly higher in middle-aged than in young adult rats but did not differ between middle-aged and old rats. Thus, there was no significant change in cytochrome oxidase activity with senescence. Additional analyses indicated that cytochrome oxidase activity is regulated regionally in the brain, as well as focally, and that differences in regional regulation may contribute to variation in CO activity among individuals, which was greater in young and old rats than in middle-aged animals.

Lifelong vitamin E intake retards age-associated decline of spatial learning ability in apoE-deficient mice

The potential for lifelong vitamin E supplementation to delay age-associated cognitive decline was tested in apoE-deficient and wild-type C57BL/6 mice. Beginning at eight weeks of age, the mice were maintained on a control diet or diets supplemented with dl-α-tocopheryl acetate yielding approximate daily intakes of either 20 or 200 mg/kg body weight. When 6 or 18 months of age, cognitive functioning of the mice was assessed using swim maze and discriminated avoidance testing procedures. For the mice maintained on control diets, the age-related declines in swim maze performance were relatively larger in apoE-deficient mice when compared with wild-type. On the other hand, age-associated declines in learning and working memory for discriminated avoidance were similar in the two genotypes. The 200-mg/kg dose of vitamin E prevented the accelerated decline in spatial learning apparent in 18-month-old apoE-deficient mice, but had no equivalent effect on performance declines attributable to normal aging in the wild-type mice. Vitamin E supplementation failed to prevent age-related impairments in learning and memory for discriminated avoidance observed in both the wild-type and apoE-deficient mice. The current findings are consistent with the hypothesis that apoE deficiency confers an accelerated, though probably selective, loss of brain function with age. This loss of function would appear to involve pathogenic oxidative mechanisms that can be prevented or offset by antioxidant supplementation.

More is less: neurogenesis and age-related cognitive decline in Long-Evans rats

A reduction in the ability to generate new neurons in the brain has been suggested to contribute to cognitive decline with advanced age. In an outbred model strain of Long-Evans rats, cognitive performance as a function of age is variable in assessments of hippocampal-dependent spatial memory. Recent research indicates that greater hippocampal neurogenesis accompanies diminished cognitive abilities in older Long-Evans rats. These findings imply that the role of neurogenesis might change between youth and old age, and that further work is needed to understand the potential benefits and liabilities that new neurons may afford an aging brain.

Reactive oxygen species and synaptic plasticity in the aging hippocampus

Aging is associated with a general decline in physiological functions including cognitive functions. Given that the hippocampus is known to be critical for certain forms of learning and memory, it is not surprising that a number of neuronal processes in this brain area appear to be particularly vulnerable to the aging process. Long-term potentiation (LTP), a form of synaptic plasticity that has been proposed as a biological substrate for learning and memory, has been used to examine age-related changes in hippocampal synaptic plasticity. A current hypothesis states that oxidative stress contributes to age-related impairment in learning and memory. This is supported by a correlation between age, memory impairment, and the accumulation of oxidative damage to cellular macromolecules. However, it also has been demonstrated that ROS are necessary components of signal transduction cascades during normal physiological processes. This review discusses the evidence supporting the dual role of reactive oxygen species (ROS) as cellular messenger molecules in normal LTP, as well their role as damaging toxic molecules in the age-related impairment of LTP. In addition, we will discuss parallel analyses of LTP and behavioral tests in mice that overexpress antioxidant enzymes and how the role of antioxidant enzymes and ROS in modulating these processes may vary over the lifespan of an animal.

Neurogenesis in a rat model of age-related cognitive decline

Age-related decrements in hippocampal neurogenesis have been suggested as a basis for learning impairment during aging. In the current study, a rodent model of age-related cognitive decline was used to evaluate neurogenesis in relation to hippocampal function. New hippocampal cell survival was assessed approximately 1 month after a series of intraperitoneal injections of 5-bromo-2'-deoxyuridine (BrdU). Correlational analyses between individual measures of BrdU-positive cells and performance on the Morris water maze task provided no indication that this measure of neurogenesis was more preserved in aged rats with intact cognitive abilities. On the contrary, among aged rats, higher numbers of BrdU-positive cells in the granule cell layer were associated with a greater degree of impairment on the learning task. Double-labelling studies confirmed that the majority of the BrdU+ cells were of the neuronal phenotype; the proportion of differentiated neurons was not different across a broad range of cognitive abilities. These data demonstrate that aged rats that maintain cognitive function do so despite pronounced reductions in hippocampal neurogenesis. In addition, these findings suggest the interesting possibility that impaired hippocampal function is associated with greater survival of newly generated hippocampal neurons at advanced ages.

Short-term vitamin E intake fails to improve cognitive or psychomotor performance of aged mice

The purpose of this study was to determine if relatively short-term vitamin E supplementation could reverse age-associated impairments in cognitive or motor function and the accumulated oxidative damage in the brain of aged mice. Separate groups of 5- or 20-month-old C57BL6 mice were placed on either a control diet or the same diet supplemented with alpha-tocopheryl acetate (1.65 g/kg). After 4 weeks on the diets, mice were tested for cognitive and motor functions over the next 8 weeks, during which the supplementation was maintained. Vitamin E supplementation increased the concentration of alpha-tocopherol in the cerebral cortex of both the young and old mice, but did not significantly affect oxidative damage to proteins and lipids in the brain cortex. When compared with young controls, the old control mice showed slower learning of a swim maze, longer reaction times, diminished auditory and shock-startle responsiveness, and diminished motor performance on tests of coordinated running and bridge walking. The vitamin E-administered old mice failed to show improvement of function relative to age-matched controls on any of the tests, but did show altered retention performance on the swim maze task and impaired performance in the test of coordinated running. The latter effects were not evident in young mice on the supplemented diet. Results of this study suggest that, when implemented in relatively old mice, supplementation of vitamin E is ineffective in reversing preexisting age-related impairments of cognitive or motor function, and has little effect on common measures of protein or lipid oxidative damage in the mouse brain. Moreover, the current findings indicate that vitamin E could have detrimental effects on some brain functions when implemented in older animals.

Free radicals and brain aging

We reviewed here the formation of free radicals and its effect physiologically. Studies mentioned above have indicated that free radical/ROS/RNS involvement in brain aging is direct as well as correlative. Increasing evidence demonstrates that accumulation of oxidation of DNA, proteins, and lipids by free radicals are responsible for the functional decline in aged brains. Also, lipid peroxidation products, such as MDA, HNE, and acrolein, were reported to react with DNA and proteins to produce further damage in aged brains. Therefore, the impact of free radicals on brain aging is pronounced. It has been estimated that 10,000 oxidative interactions occur between DNA and endogenously generated free radicals per human cell per day, and at least one of every three proteins in the cell of older animals is dysfunctional as an enzyme or structural protein, due to oxidative modification. Although these estimated numbers reveal that free radical-mediated protein and DNA modification play significant roles in the deterioration of aging brain, they do not imply that free radical damages are the only cause of functional decline in aged brain. Nevertheless,although other factors may be involved in the cascade of damaging effects in the brain, the key role of free radicals in this process cannot be underestimated. This article has examined the role and formation of free radicals in brain aging. We propose that free radicals are critical to cell damage in aged brain and endogenous, and that exogenous antioxidants, therefore, may play effective roles in therapeutic strategies for age-related neurodegenerative disorders.

Gene microarrays in hippocampal aging: statistical profiling identifies novel processes correlated with cognitive impairment

Gene expression microarrays provide a powerful new tool for studying complex processes such as brain aging. However, inferences from microarray data are often hindered by multiple comparisons, small sample sizes, and uncertain relationships to functional endpoints. Here we sought gene expression correlates of aging-dependent cognitive decline, using statistical profiling of gene microarrays in well powered groups of young, mid-aged, and aged rats (n = 10 per group). Animals were trained on two memory tasks, and the hippocampal CA1 region of each was analyzed on an individual microarray (one chip per animal). Aging- and cognition-related genes were identified by testing each gene by ANOVA (for aging effects) and then by Pearson's test (correlating expression with memory). Genes identified by this algorithm were associated with several phenomena known to be aging-dependent, including inflammation, oxidative stress, altered protein processing, and decreased mitochondrial function, but also with multiple processes not previously linked to functional brain aging. These novel processes included downregulated early response signaling, biosynthesis and activity-regulated synaptogenesis, and upregulated myelin turnover, cholesterol synthesis, lipid and monoamine metabolism, iron utilization, structural reorganization, and intracellular Ca2+ release pathways. Multiple transcriptional regulators and cytokines also were identified. Although most gene expression changes began by mid-life, cognition was not clearly impaired until late life. Collectively, these results suggest a new integrative model of brain aging in which genomic alterations in early adulthood initiate interacting cascades of decreased signaling and synaptic plasticity in neurons, extracellular changes, and increased myelin turnover-fueled inflammation in glia that cumulatively induce aging-related cognitive impairment.

Intermittent hypoxia is associated with oxidative stress and spatial learning deficits in the rat

In the adult rat, exposure to intermittent hypoxia (IH), such as occurs in sleep-disordered breathing, is associated with neurobehavioral impairments and increased apoptosis in the hippocampal CA1 region and cortex. We hypothesized that the episodic hypoxic-reoxygenation cycles of IH would induce oxidant stress, and the latter may underlie the IH-associated spatial learning and retention deficits. Adult male rats were therefore exposed to IH (90-second alternations of 10% oxygen and 21% oxygen) or room air (RA) for 7 days, and received twice-daily injections of either 3 mg/kg of the antioxidant PNU-101033E (PNU) or vehicle (V). Rats were then trained in a standard place-training task in the water maze. V-IH displayed significant impairments of spatial learning in the water maze, which were attenuated by PNU-101033E. Post hoc analyses further revealed that V-IH had significantly longer latencies and pathlengths to locate the hidden platform than PNU-IH, V-RA, or PNU-RA, indicating that PNU-101033E treatment reduced the behavioral impairments associated with IH. In addition, treatment with PNU-101033E markedly attenuated the increase in lipid peroxidation, and isoprostane concentrations associated with exposure to IH. Collectively, these findings indicate that the IH exposure is associated with increased oxidative stress, which is likely to play an important role in the behavioral impairments observed in a rodent model of sleep-disordered breathing.

Regional rat brain distribution of heme oxygenase-1 and manganese superoxide dismutase mRNA: relevance of redox homeostasis in the aging processes

Increasing evidence supports the notion that reduction of cellular expression and activity of antioxidant proteins and the resulting increase of oxidative stress are fundamental causes in the aging processes and neurodegenerative diseases. In the present study, we evaluated, in the brains of young and aged rats, the gene expression profiles of two inducible proteins critically involved in the cellular defense against endogenous or exogenous oxidants: heme oxygenase-1 (HO-1) and manganese superoxide dismutase-2 (SOD-2). SOD-2 is an essential antioxidant and HO-1 has been reported to be very active in regulating cellular redox homeostasis. Deregulation of these enzymes has been extensively reported to play a crucial role in the pathogenesis of neurodegenerative disorders. To measure the regional distribution of HO-1 and SOD-2 transcript levels in the rat brain, we have developed a real time quantitative reverse transcription-polymerase chain reaction protocol. Although these two genes presented a highly dissimilar range of expression, with SOD-2 >HO-1, both transcripts were highly expressed in the cerebellum and the hippocampus, showing in a different scale a strikingly parallel distribution gradient. To further investigate the regional brain expression of these mRNAs, we performed in situ hybridization using specific riboprobes. In situ hybridization results showed that both transcripts were highly concentrated in the hippocampus, the cerebellum and some specific regions of the brain cortex. We have also quantified, by reverse transcription-polymerase chain reaction, the brain expression of HO-1 and SOD-2 mRNAs in middle aged (12 months) and aged (28 months) rats. We found that the hippocampus of aged rats presents a significant down regulation of SOD2 mRNA expression and a parallel upregulation of HO-1 mRNA compared with young (6 months) and middle-aged rats. Furthermore, in the cerebellum of the aged rats, we detected a parallel significant upregulation of both HO-1 and SOD-2 transcripts. These regional age-dependent differences may help to explain the increased susceptibility to oxidative damage in these two brain areas during aging.

[Glial activation and brain aging]

While basal forebrain cholinergic neurons degenerate in aging and Alzheimer's disease, the cholinergic groups of the upper brainstem are preserved. Since the brainstem reticular-like cholinergic neurons differ from the rostral cholinergic phenotype by their high expression of nitric oxide synthase (NOS) mRNA, we hypothesized that they contain biochemical mechanisms to protect themselves against self-induced damage by nitric oxide (NO). Our initial question was a source of the NO during the aging process. We found a significant correlation between cognitive function and markers for glial activation and oxidative stress using aged rats. This result indicates that oxidative stress accompanied by glial activation may be occurred in the cognitively impaired animals. We also found mitochondrial DNA (mDNA) was significantly damaged in these animals, while accumulation of oxidative damage was not evident in other molecules. Therefore, oxidative damage to the mDNA by glial activation may occur in the cells having poor protection against oxidative stress during aging. Then the dysfunction of mitochondria, induced by the mDNA damage, may induce cell death as well as produce another oxidative stress to cause neuronal damage. The damaged neurons induce further glial activation and such self-accelerated immune-like response results in progressive neurodegeneration.