Aging is associated with a decrease in synaptosomal glutamate uptake and an increase in the susceptibility of synaptosomal vitamin E to oxidative stress

Age-Associated Upregulation of Glutamate Transporters and Glutamine Synthetase in Senescent Astrocytes In Vitro and in the Mouse and Human Hippocampus

Aging is marked by complex and progressive physiological changes, including in the glutamatergic system, that lead to a decline of brain function. Increased content of senescent cells in the brain, such as glial cells, has been reported to impact cognition both in animal models and human tissue during normal aging and in the context of neurodegenerative disease. Changes in the glutamatergic synaptic activity rely on the glutamate-glutamine cycle, in which astrocytes handle glutamate taken up from synapses and provide glutamine for neurons, thus maintaining excitatory neurotransmission. However, the mechanisms of glutamate homeostasis in brain aging are still poorly understood. Herein, we showed that mouse senescent astrocytes in vitro undergo upregulation of GLT-1, GLAST, and glutamine synthetase (GS), along with the increased enzymatic activity of GS and [³H]-D-aspartate uptake. Furthermore, we observed higher levels of GS and increased [³H]-D-aspartate uptake in the hippocampus of aged mice, although the activity of GS was similar between young and old mice. Analysis of a previously available RNAseq dataset of mice at different ages revealed upregulation of GLAST and GS mRNA levels in hippocampal astrocytes during aging. Corroborating these rodent data, we showed an increased number of GS + cells, and GS and GLT-1 levels/intensity in the hippocampus of elderly humans. Our data suggest that aged astrocytes undergo molecular and functional changes that control glutamate-glutamine homeostasis upon brain aging.

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Susceptibility to Aβo and TBOA of LTD and Extrasynaptic NMDAR-Dependent Tonic Current in the Aged Rat Hippocampus

Aging, as the major risk factor of Alzheimer's disease (AD), may increase susceptibility to neurodegenerative diseases through many gradual molecular and biochemical changes. Extracellular glutamate homeostasis and extrasynaptic glutamate N-methyl-D-aspartate receptors (NMDAR) are among early synaptic targets of oligomeric amyloid β (Aβo), one of the AD related synaptotoxic protein species. In this study, we asked for the effects of Aβo on long-term depression (LTD), a form of synaptic plasticity dependent on extrasynaptic NMDAR activation, and on a tonic current (TC) resulting from the activation of extrasynaptic NMDAR by ambient glutamate in hippocampal slices from young (3-6-month-old) and aged (24-28-month-old) Sprague-Dawley rats. Aβo significantly enhanced the magnitude of LTD and the amplitude of TC in aged slices compared to young ones. TBOA, a glutamate transporter inhibitor, also significantly increased LTD magnitude and TC amplitude in slices from aged rats, suggesting either an age-related weakness of the glutamate clearance system and/or a facilitated extrasynaptic NMDAR activation. From our present data, we hypothesize that senescence-related impairment of the extrasynaptic environment may be a vector of vulnerability of the aged hippocampus to neurodegenerative promotors such as Aβo.

Astrocyte elevated gene-1 (AEG-1) and the A(E)Ging HIV/AIDS-HAND

Recent attempts to analyze human immunodeficiency virus (HIV)-1-induced gene expression changes in astrocytes uncovered a multifunctional oncogene, astrocyte elevated gene-1 (AEG-1). Our previous studies revealed that AEG-1 regulates reactive astrocytes proliferation, migration and inflammation, hallmarks of aging and CNS injury. Moreover, the involvement of AEG-1 in neurodegenerative disorders, such as Huntington's disease and migraine, and its induction in the aged brain suggest a plausible role in regulating overall CNS homeostasis and aging. Therefore, it is important to investigate AEG-1 specifically in aging-associated cognitive decline. In this study, we decipher the common mechanistic links in cancer, aging and HIV-1-associated neurocognitive disorders that likely contribute to AEG-1-based regulation of astrocyte responses and function. Despite AEG-1 incorporation into HIV-1 virions and its induction by HIV-1, tumor necrosis factor-α and interleukin-1β, the specific role(s) of AEG-1 in astrocyte-driven HIV-1 neuropathogenesis are incompletely defined. We propose that AEG-1 plays a central role in a multitude of cellular stress responses involving mitochondria, endoplasmic reticulum and the nucleolus. It is thus important to further investigate AEG-1-based cellular and molecular regulation in order to successfully develop better therapeutic approaches that target AEG-1 to combat cancer, HIV-1 and aging.

Glutamatergic regulation prevents hippocampal-dependent age-related cognitive decline through dendritic spine clustering

The dementia of Alzheimer's disease (AD) results primarily from degeneration of neurons that furnish glutamatergic corticocortical connections that subserve cognition. Although neuron death is minimal in the absence of AD, age-related cognitive decline does occur in animals as well as humans, and it decreases quality of life for elderly people. Age-related cognitive decline has been linked to synapse loss and/or alterations of synaptic proteins that impair function in regions such as the hippocampus and prefrontal cortex. These synaptic alterations are likely reversible, such that maintenance of synaptic health in the face of aging is a critically important therapeutic goal. Here, we show that riluzole can protect against some of the synaptic alterations in hippocampus that are linked to age-related memory loss in rats. Riluzole increases glutamate uptake through glial transporters and is thought to decrease glutamate spillover to extrasynaptic NMDA receptors while increasing synaptic glutamatergic activity. Treated aged rats were protected against age-related cognitive decline displayed in nontreated aged animals. Memory performance correlated with density of thin spines on apical dendrites in CA1, although not with mushroom spines. Furthermore, riluzole-treated rats had an increase in clustering of thin spines that correlated with memory performance and was specific to the apical, but not the basilar, dendrites of CA1. Clustering of synaptic inputs is thought to allow nonlinear summation of synaptic strength. These findings further elucidate neuroplastic changes in glutamatergic circuits with aging and advance therapeutic development to prevent and treat age-related cognitive decline.

Neuroanatomical changes associated with cognitive aging

The literature on the neuroanatomical changes that occur during normal, non-demented aging is reviewed here with an emphasis on the improved accuracy of studies that use stereological techniques. Loss of neural tissue involved in cognition occurs during aging of humans as well as the other mammals that have been examined. There is considerable regional specificity within the cerebral cortex and the hippocampus in both the degree and cellular basis for loss. The anatomy of the prefrontal cortex is especially vulnerable to the effects of aging while the major subfields of the hippocampus are not. A loss of neurons, dendrites and synapses has been documented, as well as changes in neurotransmitter systems, in some regions of the cortex and hippocampus but not others. Species differences are also apparent in the cortical white matter and the corpus callosum where there are indications of loss of myelin in humans, but most evidence favors preservation in rats. The examination of whether the course of neuroanatomical aging is altered by hormone replacement in females is just beginning. When hormone replacement is started close to the time of cycle cessation, there are indications in humans and rats that replacement can preserve neural tissue but there is some variability due to the type of hormones and regimen of administration.

Comparison of metabolic profiling of cyanidin-3-O-galactoside and extracts from blueberry in aged mice

The metabonomics changes of plasma and brain tissue after dietary supplementation with blueberry extracts (BBE) and cyanidin-3-O-galactoside from blueberry (BBM) in aged mice were investigated by (1)H NMR technique. The mice received intragastric administration of BBE (200 mg/kg/day), BBM (50 mg/kg/day), and saline water (0.9%) for 6 weeks, respectively, in the BBE, BBM, and control groups. At the end of the experiment, plasma and brain samples were collected for NMR analysis. The results demonstrated that the level of choline in plasma from BBE and BBM groups were obviously elevated relative to the control group, whereas the levels of lactate and phosphocholine in plasma were remarkably reduced. Compared with those in the control group, the levels of choline and GABA in the brain from the BBE group were obviously increased, whereas glutamate and phosphocholine in the BBE group were significantly decreased. The level of taurine in the brain from the BBM group was particularly higher than that in the control group. These results indicated supplementation with BBE or BBM might induce similar changes of endogenous plasma and brain metabolic profiles in aged mice.

Reduction in glutamate uptake is associated with extrasynaptic NMDA and metabotropic glutamate receptor activation at the hippocampal CA1 synapse of aged rats

This study aims to determine whether the regulation of extracellular glutamate is altered during aging and its possible consequences on synaptic transmission and plasticity. A decrease in the expression of the glial glutamate transporters GLAST and GLT-1 and reduced glutamate uptake occur in the aged (24-27 months) Sprague-Dawley rat hippocampus. Glutamatergic excitatory postsynaptic potentials recorded extracellularly in ex vivo hippocampal slices from adult (3-5 months) and aged rats are depressed by DL-TBOA, an inhibitor of glutamate transporter activity, in an N-Methyl-d-Aspartate (NMDA)-receptor-dependent manner. In aged but not in young rats, part of the depressing effect of DL-TBOA also involves metabotropic glutamate receptor (mGluRs) activation as it is significantly reduced by the specific mGluR antagonist d-methyl-4-carboxy-phenylglycine (MCPG). The paired-pulse facilitation ratio, a functional index of glutamate release, is reduced by MCPG in aged slices to a level comparable to that in young rats both under control conditions and after being enhanced by DL-TBOA. These results suggest that the age-associated glutamate uptake deficiency favors presynaptic mGluR activation that lowers glutamate release. In parallel, 2 Hz-induced long-term depression is significantly decreased in aged animals and is fully restored by MCPG. All these data indicate a facilitated activation of extrasynaptic NMDAR and mGluRs in aged rats, possibly because of an altered distribution of glutamate in the extrasynaptic space. This in turn affects synaptic transmission and plasticity within the aged hippocampal CA1 network.

Transcriptomic responses in mouse brain exposed to chronic excess of the neurotransmitter glutamate

Background

Increases during aging in extracellular levels of glutamate (Glu), the major excitatory neurotransmitter in the brain, may be linked to chronic neurodegenerative diseases. Little is known about the molecular responses of neurons to chronic, moderate increases in Glu levels. Genome-wide gene expression in brain hippocampus was examined in a unique transgenic (Tg) mouse model that exhibits moderate Glu hyperactivity throughout the lifespan, the neuronal Glutamate dehydrogenase (Glud1) mouse, and littermate 9 month-old wild type mice.

Results

Integrated bioinformatic analyses on transcriptomic data were used to identify bio-functions, pathways and gene networks underlying neuronal responses to increased Glu synaptic release. Bio-functions and pathways up-regulated in Tg mice were those associated with oxidative stress, cell injury, inflammation, nervous system development, neuronal growth, and synaptic transmission. Increased gene expression in these functions and pathways indicated apparent compensatory responses offering protection against stress, promoting growth of neuronal processes (neurites) and re-establishment of synapses. The transcription of a key gene in the neurite growth network, the kinase Ptk2b, was significantly up-regulated in Tg mice as was the activated (phosphorylated) form of the protein. In addition to genes related to neurite growth and synaptic development, those associated with neuronal vesicle trafficking in the Huntington's disease signalling pathway, were also up-regulated.

Conclusions

This is the first study attempting to define neuronal gene expression patterns in response to chronic, endogenous Glu hyperactivity at brain synapses. The patterns observed were characterized by a combination of responses to stress and stimulation of nerve growth, intracellular transport and recovery.

Synaptosomal glutamate release and uptake in mice lacking the cellular prion protein

Glutamate plays a central role in the fast excitatory synaptic transmission and is a key neurotransmitter involved in several neurophysiological processes. Glutamate levels on the synaptic cleft are related to neural excitability, neuroplasticity, and neuronal damage associated with excitotoxicity. Mice lacking the cellular prion protein (PrP(c)) gene (Prnp) present a decreased astrocytic glutamate uptake in cultures, higher neuronal excitability in vitro and sensitivity to pro-convulsant drugs in vivo, and age-dependent memory impairment. Here, we investigate if PrP(c) might be involved in neuronal uptake and release of glutamate. For this purpose, we compared synaptosomal preparations from the cerebral cortex, entorhinal cortex, hippocampus, cerebellum, and olfactory bulb of 3- or 9-month-old PrP(c) null mice and with respective wild-type controls. Although we observed differences in synaptosomal glutamate release and uptake regarding the age of mice and the brain structure studied, these differences were similar for PrP(c) null mice and their respective wild-type controls. Therefore, despite a possible correlation between neuronal glutamate transporters, excitability, and neuronal damage, our results suggest that PrP(c) expression is not critical for neuronal glutamate transport.

Aging, peripheral nerve injury and nociception: effects of the antioxidant 16-desmethyltirilazad

Peripheral neuropathies increase with aging, and reactive oxygen species contribute to the symptomatology of neuropathic pain disorders. In this study, we examined age-related differences in the therapeutic efficacy of pre- or post-treatments of the amino-steroidal antioxidant 16-desmethyltirilazad in delaying the onset and/or limiting the duration of tactile-evoked allodynia following the induction of peripheral mononeuropathies in rats. Two different models of nerve injury were utilized to induce allodynia in young and aged rats: (1) the chronic constriction injury (CCI) model of Bennett and Xie [Bennett GJ, Xie Y-K. A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain 1988;33:87-107]; (2) the partial sciatic nerve ligation (PSNL) model of Seltzer et al. [Seltzer Z, Dubner R, Shir YA. Novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury. Pain 1990;43:205-18]. Calibrated von Frey filaments were used to examine changes in paw withdrawal threshold values. The results demonstrated that pre-treating young and aged rats with 16-desmethyltirilazad prior to the induction of peripheral mononeuropathies prevented the onset of neuropathic pain. However, once post-operative tactile allodynia was established, post-treatment therapy was ineffective at reversing the symptoms. These findings support the mediatory role of reactive oxygen species in neuropathic pain disorders, and suggest that the antiallodynic efficacy of antioxidant intervention is dependent on the time course of administration.

Fibroblast glutamate transport in aging and in AD: correlations with disease severity

Altered glutamate transport and aberrant EAAT1 expression were shown in Alzheimer's disease (AD) brains. It is presently unknown whether these modifications are a consequence of neurodegeneration or play a pathogenetic role. However, recent findings of decreased glutamate uptake, EAAT1 protein and mRNA in AD platelets suggest that glutamate transporter modifications may be systemic and might explain the decreased glutamate uptake. We now used primary fibroblast cultures from 10 AD patients to further investigate the specific involvement of glutamate transporters in this disorder and in normal aging. Decreased glutamate uptake (p<0.001), EAAT1 expression (p<0.05) and mRNA (p<0.01) were observed in aged people, compared to younger controls. In AD fibroblasts, compared to age-matched controls, we observed further reductions of glutamate uptake (p<0.0005) and EAAT1 expression (p<0.005), while EAAT1 mRNA increase (p<0.001) was shown. EAAT1 parameters were mutually correlated (p<0.01) and correlations were shown with dementia severity (p<0.05 MMSE-expression, p<0.005 MMSE-mRNA). We suggest fibroblast cultures as possible ex vivo peripheral model to study the glutamate involvement and possible molecular and therapeutic targets in AD.

Age-related susceptibility to oxygen and glucose deprivation damage in rat hippocampal slices

Aging is an important risk factor for stroke. We evaluated the effects of aging on cell susceptibility to oxygen and glucose deprivation (OGD) in slices of the hippocampus from Wistar rats aged 2, 11 and 24 months. Lactate dehydrogenase (LDH) released to the incubation media and free radical content were markedly increased in the 24-month group submitted to OGD. These results confirm that hippocampal tissue from old animals is more susceptible to ischemia-reoxygenation injury.

Intervention against the Maillard reaction in vivo

The field of Maillard/glycation reactions in vivo has grown enormously during the past 20 years, going from 25 to 500 publications per year. It is now well recognized that many of the "advanced" products form oxidatively or anaerobically and can have deleterious effects on macromolecular and biological function. The feasibility of developing pharmacological agents with beneficial in vivo properties, based on in vitro inhibition of glycation, has been surprisingly successful. This Editorial sets the stage for a series of articles by experts in the field, who have made key contributions to our understanding of the Maillard reaction in vivo.

Glutamate uptake in cultured astrocytes depends on age: a study about the effect of guanosine and the sensitivity to oxidative stress induced by H(2)O(2)

Relatively few studies have been conducted to investigate the relationship between glutamate and development and/or aging. Rat cortical astrocyte cultures were used as a model to investigate glutamate uptake during development. The immunocontent of the markers glial fibrillary acidic protein (GFAP) and S100B increased, while basal secretion of S100B decreased, in astrocytes from 10 to 40 days in vitro (DIV). Basal glutamate uptake increased with age. Exposure to hydrogen peroxide decreased glutamate uptake more potently at 40 than 10 DIV. Moreover, 40 DIV astrocytes showed earlier loss of integrity (at 6 h) than 10 DIV astrocytes (at 24 h) after H(2)O(2) exposure. Addition of guanosine stimulated glutamate uptake only in 10 DIV astrocytes. The present work shows that mature astrocytes in culture present some neurochemical alterations also observed in astrocytes of aged animals. These results can contribute to the understanding of some consequences of the excitotoxicity and oxidative stress during brain aging.

Plasminogen activator inhibitor-1 is a major stress-regulated gene: implications for stress-induced thrombosis in aged individuals

Plasminogen activator inhibitor-1 (PAI-1) is one of the primary inhibitors of the fibrinolytic system and has been implicated in a variety of thrombotic disorders. In this report, stress-induced changes in murine PAI-1 gene expression were investigated to study the role of this inhibitor in the development of stress-induced hypercoagulability. Restraint stress led to a dramatic induction of plasma PAI-1 antigen and of tissue PAI-1 mRNA with maximum induction in adipose tissues. In situ hybridization analysis of the stressed mice revealed that strong signals for PAI-1 mRNA were localized to hepatocytes, renal tubular epithelial cells, adrenomedullar chromaffin cells, neural cells in the paraaortic sympathetic ganglion, vascular smooth muscle cells, and adipocytes, but not to endothelial cells. These observations indicate that the stress induces the PAI-1 gene expression in a tissue-specific and cell type-specific manner. The induction of PAI-1 mRNA by restraint stress was greater than that observed for heat shock protein, a typical stress protein, suggesting that PAI-1 is one of the most highly induced stress proteins. Importantly, the magnitude of induction of PAI-1 mRNA by stress increased markedly with age, and this increase in PAI-1 correlated with tissue thrombosis in the older stressed mice. Moreover, much less tissue thrombosis was induced by restraint stress in young and aged PAI-1-deficient mice compared with age-matched wild-type mice. These results suggest that the large induction of PAI-1 by stress increases the risk for thrombosis in the older populations, and that the adipose tissue may be involved.

5-Aminosalicylic acid protection against oxidative damage to synaptosomal membranes by alkoxyl radicals in vitro

The antioxidant properties of 5-aminosalicylic acid in vitro were evaluated in a synaptosomal membrane system prepared from gerbil cortical synaptosomes using EPR spin labeling and spectroscopic techniques. MAL-6 (2,2,6,6-tetramethyl-4-maleimidopiperidin-1-oxyl) and 5-NS (5-nitroxide stearate) spin labels were used to assess changes in protein oxidation and membrane lipid fluidity, respectively. Synaptosomal membranes were subjected to oxidative stress by incubation with 1 mM azo-bis(isobutyronitrile) (AIBN) or 1 mM 2,2'-azobis(amidino propane) dihydrochloride (AAPH) at 37 degrees C for 30 minutes. The EPR analyses of the samples showed significant oxidation of synaptosomal proteins and a decrease in membrane fluidity. 5-Aminosalicylic acid also was evaluated by means of FRAP (the ferric reducing ability of plasma) test as a potential antioxidant. 5-Aminosalicylic acid also showed protection against the oxidation in gerbil cortical synaptosomes system caused by AIBN and AAPH. These results are consistent with the notion of antioxidant protection against free radical induced oxidative stress in synaptosomal membrane system by this agent.

Drastic neuronal loss in vivo by beta-amyloid racemized at Ser(26) residue: conversion of non-toxic [D-Ser(26)]beta-amyloid 1-40 to toxic and proteinase-resistant fragments

It is unclear how and when insoluble beta-amyloid in senile plaques exerts degenerative effects on distant hippocampal neurons in Alzheimer's disease. Racemization of Ser and Asp residues of insoluble beta-amyloid is a typical age-dependent process. In this study, we investigated the fibril formation activity and cytotoxic activity of beta-amyloid 1-40 racemized at the Asp or Ser residue. In contrast to beta-amyloid 1-40 and its derivative substituted with the D-Asp(1, 7 or 23) or D-Ser(8) residue, [D-Ser(26)]beta-amyloid 1-40 was non-toxic to PC12 cells, and did not exhibit significant fibril formation activity making it soluble. However, [D-Ser(26)]beta-amyloid 1-40, but not beta-amyloid 1-40, was converted in vitro to a potent neurotoxic and truncated peptide, [D-Ser(26)]beta-amyloid 25-35 or [D-Ser(26)]beta-amyloid 25-40, by chymotrypsin-like enzymes and aminopeptidase M. Soluble [D-Ser(26)]beta-amyloid 1-40 was injected into rat hippocampus with a non-toxic dose of ibotenic acid, an excitatory amino acid. Nissl staining and microtubule-associated protein-2 immunostaining revealed that [D-Ser(26)]beta-amyloid 1-40, as well as [D-Ser(26)]beta-amyloid 25-35, produced a drastic degeneration of the CA1 neurons with ibotenic acid although [D-Ser(26)]beta-amyloid 1-40 alone or ibotenic acid alone did not exert neuronal damage. This suggests the in vivo conversion of non-toxic [D-Ser(26)]beta-amyloid 1-40 to the toxic and truncated peptides which enhance the susceptibility of neurons to the excitatory amino acid.These results and the presence of [D-Ser(26)]beta-amyloid 25-35-like antigens in Alzheimer's disease brains suggest that soluble [D-Ser(26)]beta-amyloid 1-40, possibly formed during the aging process, is released from senile plaques, and converted by brain proteinases to truncated [D-Ser(26)]beta-amyloid 25-35(40)-like peptides, which degenerate hippocampal neurons by enhancing the susceptibility to excitatory amino acids in Alzheimer's disease brains. These findings may provide the basis for a new therapeutic approach to prevent the neurodegeneration in Alzheimer's disease.

Glutamatergic neurotransmission in aging: a critical perspective

The effects of aging on glutamate neurotransmission in the brain is reviewed and evaluated. Glutamate is the neurotransmitter in most of the excitatory synapses and appears to be involved in functions such as motor behaviour, cognition and emotion, which alter with age. However, relatively few studies have been conducted to study the relationship between glutamate and aging of the brain. The studies presented here indicate the existence of a number of changes in the glutamatergic system during the normal process of aging. First, an age-related decrease of glutamate content in tissue from cerebral cortex and hippocampus has been reported, although it may be mainly a consequence of changes in metabolic activity rather than glutamatergic neurotransmission. On the other hand, studies in vitro and in vivo have shown no changes in glutamate release during aging. Since glutamate sampled in most of these studies is the result of a balance between release and uptake processes, the lack of changes in glutamate release may be due to compensatory changes in glutamate uptake. In fact, a reduced glutamate uptake capacity, as well as a loss in the number of high affinity glutamate transporters in glutamatergic terminals of aged rats, have been described. However, the most significant and consistent finding is the decrease in the density of glutamatergic NMDA receptors with age. A new perspective, in which glutamate interacts with other neurotransmitters to conform the substrates of specific circuits of the brain and its relevance to aging, is included in this review. In particular, studies from our laboratory suggest the existence of age-related changes in the interaction between glutamate and other neurotransmitters, e.g. dopamine and GABA, which are regionally specific.

Changes in spinal serotonin turnover mediate age-related differences in the behavioral manifestations of peripheral nerve injury

The Bennett and Xie model of peripheral nerve injury was used to study the effects of aging on the onset and progression of sciatic nerve ligation (SNL)-induced thermal hyperalgesia and tactile-evoked allodynia in young, mature, and aged Fischer 344 FBNF1 male rats (4-6, 14-16, and 24-26 months old, respectively). A plantar analgesia meter and calibrated von Frey pressure filaments were employed as the analgesiometric assays. In the absence of nerve injury, aged rats were found to be more sensitive than younger animals to noxious thermal stimuli. Following the SNL surgery, thermal hyperalgesia was observed in all three age groups within 3 days. On post-SNL day 35, the paw-withdrawal latency values of the young and mature animals returned to presurgical baseline levels, while the aged rats continued to exhibit thermal hyperalgesia. Tactile-evoked allodynia was apparent within 3 days following peripheral nerve injury in the oldest cohort, but was delayed in the younger animals. On post-SNL days 0 (control), 3, 21, and 35, young, mature, and aged rats were sacrificed and high-performance liquid chromatography and electrochemical detection (HPLC/ECD) methods were used for neurochemical analyses of spinal serotonin (5-HT), norepinephrine (NE), and 5-hydroxyindoleacetic acid (5-HIAA). Spinal 5-HT and NE levels were not significantly altered by the aging process, nor were they affected by peripheral nerve injury. However, spinal 5-HT turnover from the aged animals was greater than that detected in spinal tissue from the younger counterparts. Differences in spinal 5-HT turnover may contribute to age-related variability in spinal nociceptive processing.

Aging and neuropathic pain

Although chronic neuropathic pain disorders are more prevalent in the senescent population, little is known about how the aging process alters the thermal hyperalgesic sensitivity to peripheral nerve injury. In this study, neuropathic pain was induced in young, mature and aged FBNF1 hybrid rats via unilateral ligation of the left sciatic nerve. The extent to which the aging process affects the thermal hyperalgesic responsiveness of these animals was investigated. The results demonstrate that the aging process differentially alters nociceptive processing.

Basal and IL-1beta-stimulated cytokine and neuropeptide mRNA expression in brain regions of young and old Long-Evans rats

Young and old Long-Evans rats respond with fevers of equal magnitude and duration to the brain administration of interleukin-1beta (IL-1beta). Here, we characterized brain regional mRNA expression of cytokine and neuropeptide components in response to the brain administration of IL-1beta. We used specific and highly sensitive RNase protection assays to determine mRNA changes for IL-1beta, IL-1 receptor type I (IL-1RI), IL-1R accessory proteins I and II (IL-1R AcP I and II), IL-1 receptor antagonist (IL-1Ra), transforming growth factor-beta1 (TGF-beta1), glycoprotein 130 (gp 130), leptin receptor (OB-R), neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) in the cerebellum, parieto-frontal cortex, hippocampus, hypothalamus, and midbrain of male young (3-5 months) and old (24-26 months) Long-Evans rats. In both young and old rats, IL-1beta induced a significant up-regulation of cerebellar IL-1Ra, IL-1RI, and TGF-beta1 mRNAs; hippocampal TGF-beta1 mRNA; hypothalamic IL-1beta, IL-1Ra, TGF-beta1, and gp 130 mRNAs; and midbrain IL-1beta and TGF-beta1 mRNAs. There were no age-related differences in any cytokine mRNA levels under basal or IL-1beta-stimulated conditions. Levels of hypothalamic POMC mRNA were different between age groups under basal and stimulated conditions. IL-1R AcP I and leptin receptor did not change in any brain region from either young or old rats, suggesting specificity of transcriptional changes. The data show that old Long-Evans rats are not defective in their capacity to develop an appropriate cytokine response to the brain administration of IL-1beta. The implications of these findings for neuroimmunological-neuroinflammatory and neurotoxic/neurodegenerative processes are discussed.