In vivo amino acid release from the striatum of aging rats: adenosine modulation

Glutamatergic Neurotransmission in Aging and Neurodegenerative Diseases: A Potential Target to Improve Cognitive Impairment in Aging

Aging is characterized by the decline in many of the individual's capabilities. It has been recognized that the brain undergoes structural and functional changes during aging that are occasionally associated with the development of neurodegenerative diseases. In this sense, altered glutamatergic neurotransmission, which involves the release, binding, reuptake, and degradation of glutamate (Glu) in the brain, has been widely studied in physiological and pathophysiological aging. In particular, changes in glutamatergic neurotransmission are exacerbated during neurodegenerative diseases and are associated with cognitive impairment, characterized by difficulties in memory, learning, concentration, and decision-making. Thus, in the present manuscript, we aim to highlight the relevance of glutamatergic neurotransmission during cognitive impairment to develop novel strategies to prevent, ameliorate, or delay cognitive decline. To achieve this goal, we provide a comprehensive review of the changes reported in glutamatergic neurotransmission components, such as Glu transporters and receptors during physiological aging and in the most studied neurodegenerative diseases. Finally, we describe the current therapeutic strategies developed to target glutamatergic neurotransmission.

Adenosine receptor stimulation inhibits methamphetamine-associated cue seeking

BACKGROUND

Methamphetamine (METH) is a psychostimulant drug that remains a popular and threatening drug of abuse with high abuse liability. There is no established pharmacotherapy to treat METH dependence, but evidence suggests that stimulation of adenosine receptors reduces the reinforcing properties of METH and could be a potential pharmacological target. This study examines the effects of adenosine receptor subtype stimulation on METH seeking using both a cue-induced reinstatement and cue-craving model of relapse.

METHODS

Male and female rats were trained to self-administer METH during daily 2-h sessions. Cue-induced reinstatement of METH seeking was evaluated after extinction training. A systemic pretreatment of an adenosine A1 receptor (A1R) or A2A receptor (A2AR) agonist was administered prior to an extinction or cue session to evaluate the effects of adenosine receptor subtype stimulation on METH seeking. The effects of a systemic pretreatment of A1R or A2AR agonists were also evaluated in a cue-craving model where the cued-seeking test was conducted after 21 days of forced home-cage abstinence without extinction training.

RESULTS

Cue-induced reinstatement was reduced in both male and female rats that received A1R or A2AR agonist pretreatments. Similarly, an A1R or A2AR agonist pretreatment also inhibited cue craving in both male and female rats.

CONCLUSION

Stimulation of either adenosine A1R or A2AR subtypes inhibits METH-seeking behavior elicited by METH-associated cues. These effects may be attributed to the ability of A1R and A2AR stimulation to disrupt cue-induced dopamine and glutamate signaling throughout the brain.

The Neuroprotective Effects of Moderate and Regular Caffeine Consumption in Alzheimer's Disease

The increasing numbers of elderly Alzheimer's disease (AD) patients because of a steady increase in the average lifespan and aging society attract great scientific concerns, while there were fewer effective treatments on AD progression due to unclear exact causes and pathogenesis of AD. Moderate (200-500 mg/d) and regular caffeine consumption from coffee and tea are considered to alleviate the risk of AD and have therapeutic potential. This paper reviewed epidemiological studies about the relationship of caffeine intake from coffee or/and tea with the risk of AD and summarized the caffeine-related AD therapies based on experimental models. And further well-designed and well-conducted studies are suggested to investigate the optimal dosages, frequencies, and durations of caffeine consumption to slow down AD progression and treat AD.

Sleep is more sensitive to high doses of caffeine in the middle years of life

During the middle years of life, sleep becomes more fragile and its sensitivity to psychostimulants may increase. This study evaluated the effects of 200 mg and 400 mg of caffeine on sleep in young and middle-aged adults. The sleep of 22 young (23.5 ± 1.9 years) and 24 middle-aged (51.7 ± 11.5 years) adults was recorded using polysomnography in two conditions (placebo and caffeine) in a double-blind cross-over design. Compared to placebo, caffeine increased sleep latency, shortened total sleep duration and reduced sleep efficiency. At the higher dose, these effects were more pronounced in middle-aged than in young adults. Furthermore, the higher dose of caffeine increased absolute stage 1 sleep in young adults, whereas it decreased absolute stage 2 sleep in middle-aged adults. Caffeine also induced dose-dependent increases in relative stage 1 sleep and reductions in absolute and relative slow wave sleep and absolute rapid eye movement sleep in both age groups. There was no dose- or age-related modulation of the effects of caffeine on quantified electroencephalographic measures. These results indicate that, compared to young adults, middle-aged adults are generally more sensitive to the effects of a high dose of caffeine on sleep quantity and quality.

Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A 2A receptors during extinction training in rats

RATIONALE

Adenosine receptor stimulation and blockade have been shown to modulate a variety of cocaine-related behaviors.

OBJECTIVES

These studies identify the direct effects of adenosine receptor stimulation on cocaine seeking during extinction training and the persistent effects on subsequent reinstatement to cocaine seeking.

METHODS

Rats self-administered cocaine on a fixed ratio one schedule in daily sessions over 3 weeks. Following a 1-week withdrawal, the direct effects of adenosine receptor modulation were tested by administering the adenosine A1 receptor agonist, N(6)-cyclopentyladenosine (CPA, 0.03 and 0.1 mg/kg), the adenosine A2A agonist, CGS 21680 (0.03 and 0.1 mg/kg), the presynaptic adenosine A2A receptor antagonist, SCH 442416 (0.3, 1, and 3 mg/kg), or vehicle prior to each of six daily extinction sessions. The persistent effects of adenosine receptor modulation during extinction training were subsequently tested on reinstatement to cocaine seeking induced by cues, cocaine, and the dopamine D2 receptor agonist, quinpirole.

RESULTS

All doses of CPA and CGS 21680 impaired initial extinction responding; however, only CPA treatment during extinction produced persistent impairment in subsequent cocaine- and quinpirole-induced seeking. Dissociating CPA treatment from extinction did not alter extinction responding or subsequent reinstatement. Administration of SCH 442416 had no direct effects on extinction responding but produced dose-dependent persistent impairment of cocaine- and quinpirole-induced seeking.

CONCLUSIONS

These findings demonstrate that adenosine A1 or A2A receptor stimulation directly impair extinction responding. Interestingly, adenosine A1 receptor stimulation or presynaptic adenosine A2A receptor blockade during extinction produces lasting changes in relapse susceptibility.

Binge drinking +/- chronic nicotine administration alters extracellular glutamate and arginine levels in the nucleus accumbens of adult male and female Wistar rats

AIMS

The effect of 'binge drinking' coupled or not with chronic nicotine administration on nucleus accumbens (NAc) glutamate, arginine, taurine and hydroxyl radical levels has been investigated in these present studies.

METHODS AND RESULTS

Ethanol, 2 or 3 g/kg, has been administered to male or female adult rats in a 'binge-type' regime for 3 weeks, +/- nicotine, and changes in glutamate, arginine and taurine content in the NAc, assayed by microdialysis after a further dose of ethanol. The basal concentration of NAc glutamate increased 8-fold in the female adult rats but did not change significantly after further doses of ethanol. In contrast, the male adult rats showed no changes in basal glutamate content but exhibited a dose-dependent increase in NAc glutamate after further doses of ethanol. NAc arginine basal levels decreased significantly in both male and female adult rats after further doses of ethanol. Co-administration of nicotine modified the toxicity of ethanol as exemplified by diminishment of both the basal NAc glutamate release as well as modifying the release of this excitatory amino acid after further ethanol doses, particularly in female rats. In addition, the marked changes in arginine release after further ethanol doses were less evident. There was no evidence for increased hydroxyl radical production in the NAc after 'binge drinking' +/- nicotine.

CONCLUSION

There appeared to be a greater vulnerability to ethanol toxicity in female adult rats after 'binge drinking'. It remains unclear whether the increased release of glutamate during the microdialysis evokes activation of inducible nitric oxide synthase (iNOS), which would utilize arginine in the formation of nitric oxide.

The role of adenosine in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder of the central nervous system manifested by cognitive and memory deterioration, a variety of neuropsychiatric symptoms, behavioral disturbances, and progressive impairment of daily life activities. Current pharmacotherapies are restricted to symptomatic interventions but do not prevent progressive neuronal degeneration. Therefore, new therapeutic strategies are needed to intervene with these progressive pathological processes. In the past several years adenosine, a ubiquitously released purine ribonucleoside, has become important for its neuromodulating capability and its emerging positive experimental effects in neurodegenerative diseases. Recent research suggests that adenosine receptors play important roles in the modulation of cognitive function. The present paper attempts to review published reports and data from different studies showing the evidence of a relationship between adenosinergic function and AD-related cognitive deficits. Epidemiological studies have found an association between coffee (a nonselective adenosine receptor antagonist) consumption and improved cognitive function in AD patients and in the elderly. Long-term administration of caffeine in transgenic animal models showed a reduced amyloid burden in brain with better cognitive performance. Antagonists of adenosine A2A receptors mimic these beneficial effects of caffeine on cognitive function. Neuronal cell cultures with amyloid beta in the presence of an A2A receptor antagonist completely prevented amyloid beta-induced neurotoxicity. These findings suggest that the adenosinergic system constitutes a new therapeutic target for AD, and caffeine and A2A receptor antagonists may have promise to manage cognitive dysfunction in AD.

Spatial reference- (not working- or procedural-) memory performance of aged rats in the water maze predicts the magnitude of sulpiride-induced facilitation of acetylcholine release by striatal slices

Cluster analysis of water-maze reference-memory performance distinguished subpopulations of young adult (3-5 months), aged (25-27 months) unimpaired (AU) and aged impaired (AI) rats. Working-memory performances of AU and AI rats were close to normal (though young and aged rats differed in exploration strategies). All aged rats showed impaired procedural-memory. Electrically evoked release of tritium was assessed in striatal slices (preloaded with [(3)H]choline) in the presence of oxotremorine, physostigmine, atropine+physostigmine, quinpirole, nomifensine or sulpiride. Aged rats exhibited reduced accumulation of [(3)H]choline (-30%) and weaker transmitter release. Drug effects (highest concentration) were reductions of release by 44% (oxotremorine), 72% (physostigmine), 84% (quinpirole) and 65% (nomifensine) regardless of age. Sulpiride and atropine+physostigmine facilitated the release more efficiently in young rats versus aged rats. The sulpiride-induced facilitation was weaker in AI rats versus AU rats; it significantly correlated with reference-memory performance. The results confirm age-related alterations of cholinergic and dopaminergic striatal functions, and point to the possibility that alterations in the D(2)-mediated dopaminergic regulation of these functions contribute to age-related reference-memory deficits.

Effect of aging on cerebral A1 adenosine receptors: A [18F]CPFPX PET study in humans

Cerebral A(1) adenosine receptors (A(1)AR) fulfill important neuromodulatory and homeostatic functions. The present study examines possible age-related A(1)AR changes in living humans by positron emission tomography (PET) and the A(1)AR ligand [(18)F]CPFPX. Thirty-six healthy volunteers aged 22-74 years were included. The apparent binding potential (BP'2) of [(18)F]CPFPX in various cerebral regions was calculated non-invasively using the cerebellum as reference region. In addition, the total distribution volume (DV't) was assessed in 10 subjects undergoing arterial blood sampling. There was no significant association between regional DV't and age, gender, caffeine consumption or sleep duration. BP'2 showed a significant age-dependent decrease in all regions except cingulate gyrus (p=0.062). The BP'2 decline ranged from -17% (striatum) to -34% (postcentral gyrus), the average cortical decline being -23%. There was no significant effect of gender, caffeine consumption and sleep duration on BP'2. In line with in vitro animal studies, the present in vivo PET study detected an age-dependent A(1)AR loss in humans that may be of pathophysiological importance in various neurological diseases associated with aging. Because of the discrepant results of the invasive (DV't) and the non-invasive (BP'2) analyses the present study needs further validation.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.

Combining Insulin-Like Growth Factor Derivatives Plus Caffeinol Produces Robust Neuroprotection After Stroke in Rats

Background and Purpose— Insulin-like growth factor-1 (IGF-1) and caffeinol are both neuroprotective and probably have different mechanisms of action; therefore, they may be more effective in combination.

Methods— We tested the N-terminal tripeptide of IGF-1, Gly-Pro-Glu (GPE), and its analogue, G2MePE, alone and with caffeinol in a rat middle cerebral artery (MCA) suture occlusion model. We randomly assigned rats to 6 groups of 8 to 12 animals: (1) control; (2) GPE, 3 mg/kg per hour; (3) G2MePE, 0.3 mg/kg per hour; (4) caffeinol, a mixture of caffeine (10 mg/kg) with ethanol (0.32 g/kg); (5) GPE with caffeinol (combination of group 2 with 4); and (6) G2MePE with caffeinol (combination of group 3 with 4). Drugs were started 75 minutes after suture occlusion, at the start of reperfusion. Three days after MCA occlusion, neurological deficit (Neurological Deficit Score [NDS]) and lesion volume were measured.

Results— GPE and caffeinol improved NDS by 34% and 36%, respectively ( P <0.01), and also decreased cortical but not striatal lesion volume compared with control (GPE cortex, 121 mm 3 ; caffeinol cortex, 134 mm 3 ; and control, 221 mm 3 ; P <0.01). GPE plus caffeinol did not have more efficacy than either GPE or caffeinol alone. G2MePE slightly improved NDS (19.7%, P =0.05) but not lesion volume. However, G2MePE plus caffeinol very significantly improved NDS (64%) and lesion volume in both cortex (combination 95 mm 3 versus control 221 mm 3 ) and striatum (combination 74 mm 3 versus control 110 mm 3 ) ( P <0.001), and was significantly more effective than either caffeinol or G2MePE alone.

Conclusion— Both GPE and caffeinol significantly protect cortex after MCA occlusion. At the doses used in this study, the GPE analogue G2MePE by itself had minimal protective effects, but when combined with caffeinol, it demonstrated robust beneficial effects on cortical and subcortical lesion size and behavioral deficit. Further study of this combination appears justified.

Adenosine and glutamate extracellular concentrations and mitogen-activated protein kinases in the striatum of Huntington transgenic mice. Selective antagonism of adenosine A2A receptors reduces transmitter outflow

The basal ganglia and deep layers of cerebral cortex neurodegeneration typically characterize the postmortem brain of Huntington disease (HD) patients. In this study, we employed 10- to 11-week-old transgenic HD mice (R6/2 line), in which the striatal adenosine extracellular levels, measured using the microdialysis technique, are significantly increased in comparison to wild-type mice. An increase in striatal adenosine is probably a precocious index of mitochondrial dysfunction that is described in both the postmortem brain of HD patients and transgenic mice striatal cells. The adenosine increase is matched by activation of the p38 mitogen-activated protein kinase (MAPK) in the striatal neurons of R6/2 mouse but not in the cortex. This result indicates that p38 MAPK is a correlate of striatal damage and suggests a role for p38 in the striatal neuron suffering and apoptosis described in this disease. The selective adenosine A(2A) receptor antagonist SCH 58261, administered through microdialysis fiber into the striatum, significantly decreases the outflow of glutamate in R6/2 mice. Antagonism of adenosine A(2A) receptors might be regarded as potentially useful in the treatment of this disease to control striatal excitotoxicity.

Adenosinergic protection of dopaminergic and GABAergic neurons against mitochondrial inhibition through receptors located in the substantia nigra and striatum, respectively

Mitochondrial dysfunction may contribute to dopaminergic (DAergic) cell death in Parkinson's disease and GABAergic cell death in Huntington's disease. In the present work, we tested whether blocking A1 receptors could enhance the damage to DAergic and GABAergic neurons caused by mitochondrial inhibition, and whether blocking A2a receptors could protect against damage in this model. Animals received an intraperitoneal injection of 8-cyclopentyl-1,3-dipropylxanthine (CPX) (A1 antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX) (A2a antagonist) 30 min before intrastriatal infusion of malonate (mitochondrial complex II inhibitor). Damage was assessed 1 week later by measuring striatal dopamine, tyrosine hydroxylase (TH), and GABA content. In mice and rats, malonate-induced depletion of striatal dopamine, TH, or GABA was potentiated by pretreatment with 1 mg/kg CPX and attenuated by pretreatment with 5 mg/kg DMPX. To determine the location of the A1 and A2a receptors mediating these effects, CPX or DMPX was infused directly into the striatum or substantia nigra of rats 30 min before intrastriatal infusion of malonate. When infused into the striatum, CPX (20 ng) potentiated, whereas DMPX (50 ng) prevented malonate-induced GABA loss, but up to 100 ng of CPX or 500 ng of DMPX did not alter malonate-induced striatal dopamine loss. Intranigral infusion of CPX (100 ng) or DMPX (500 ng), however, did exacerbate and protect, respectively, against malonate-induced striatal dopamine loss. Thus, A1 receptor blockade enhances and A2a receptor blockade protects against damage to DAergic and GABAergic neurons caused by mitochondrial inhibition. Interestingly, these effects are mediated by A1 and A2a receptors located in the substantia nigra for DAergic neurons and in the striatum for GABAergic neurons.

Age-related changes in the antinociception induced by taurine in mice

Taurine is a nonessential amino acid that is of medical interest for the nutrition of infants. Taurine has been found in the central nervous system of rodents and humans, and among its potential therapeutic uses, it is interesting to remark its analgesic actions. It is also well known that concentration levels during the fetal and prenatal periods are higher than in adulthood. The data obtained so far indicate that taurine is involved in the development process of the brain and possibly other organs. The taurine levels in old age are still unknown, but it is presumed that they will be different from those of younger animals. Data about age-related alterations and functional modifications of this and other amino acids are still scarce. The aim of the present work was to study the antinociceptive effect of taurine and its relationship with aging in mice. No differences were found between prepubertal and young adult animals; on the contrary, old animals showed significantly reduced sensitivity to the antinociception induced by taurine; in fact, at the tested doses, taurine did not induce antinociception in this group of mice. The mechanism underlying this effect has not been clarified because there are several mechanisms and neurotransmitter systems involved in the antinociception induced by taurine.

Up-regulation of A(2A) adenosine receptors by proinflammatory cytokines in rat PC12 cells

The purpose of this study was to examine the regulation of A(2A) adenosine receptor (A(2A) AR) gene expression induced by proinflammatory cytokines in PC12 cells. The A(2A) AR mRNA levels were substantially increased following 3-48 hr PC12 cell treatment with interleukin 1 beta (500 unit/mL) or tumor necrosis factor alpha (1000 unit/mL), as revealed by RT-PCR analysis. In parallel, cell cytokine treatment induced an up-regulation of A(2A) receptor protein. Equilibrium radioligand binding studies on treated-cells showed a significant increase in maximum density of [3H] 2-(carboxyethylphenylethylamino) adenosine-5'-carboxamide binding sites, with no significant changes in the affinity constant value. The increase in A(2A) receptor density was also demonstrated by Western blot analysis. Interleukin 1 beta and tumor necrosis factor alpha effects on A(2A) AR mRNA and protein levels were detectable after 3 hr cytokine treatment and reached a maximum within 24 and 48 hr, respectively. These results demonstrated the existence of heterologous regulation of A(2A) ARs by proinflammatory cytokines. The biological significance of this regulation might be associated with modulating cellular activity in response to tissue damage associated with inflammatory mediator production.

Functional striatal hypodopaminergic activity in mice lacking adenosine A(2A) receptors

Adenosine and caffeine modulate locomotor activity and striatal gene expression, partially through the activation and blockade of striatal A(2A) receptors, respectively. The elucidation of the roles of these receptors benefits from the construction of A(2A) receptor-deficient mice (A(2A)-R(-/-)). These mice presented alterations in locomotor behaviour and striatal expression of genes studied so far, which are unexpected regarding the specific expression of A(2A) receptor by striatopallidal neurones. To clarify the functions of A(2A) receptors in the striatum and to identify the mechanisms leading to these unexpected modifications, we studied the basal expression of immediate early and constitutive genes as well as dopamine and glutamate neurotransmission in the striatum. Basal zif268 and arc mRNAs expression was reduced in mutant mice by 60-80%, not only in the striatum but also widespread in the cerebral cortex and hippocampus. Striatal expression of substance P and enkephalin mRNAs was reduced by about 50% and 30%, respectively, whereas the expression of GAD67 and GAD65 mRNAs was slightly increased and unaltered, respectively. In vivo microdialysis in the striatum revealed a 45% decrease in the extracellular dopamine concentration and three-fold increase in extracellular glutamate concentration. This was associated with an up-regulation of D(1) and D(2) dopamine receptors expression but not with changes in ionotropic glutamate receptors. The levels of tyrosine hydroxylase and of striatal and cortical glial glutamate transporters as well as adenosine A(1) receptors expression were indistinguishable between A(2A)-R(-/-) and wild-type mice. Altogether these results pointed out that the lack of A(2A) receptors leads to a functional hypodopaminergic state and demonstrated that A(2A) receptors are necessary to maintain a basal level in immediate early and constitutive genes expression in the striatum and cerebral cortex, possibly via their control of dopamine pathways.

Endogenous glutamate-taurine interaction in striatum and nucleus accumbens of the freely moving rat: studies during the normal process of aging

Using microdialysis, the effects of endogenous glutamate on extracellular concentrations of taurine, in striatum and nucleus accumbens of the awake rat during the normal process of aging were investigated. The glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) was perfused through the microdialysis probe to increase the endogenous concentration of glutamate. Young (2-4 months), middle aged (12-14 months), aged (27-32 months) and very aged (37 months) male Wistar rats were used. PDC (1, 2 and 4 mM) produced a dose-related increase of extracellular concentrations of glutamate and taurine in striatum and nucleus accumbens in all groups of age. Increases of glutamate, but not of taurine, decreased during aging in striatum. In nucleus accumbens there were no age-related changes in the increases of glutamate and taurine induced by PDC. In all age groups, increases of extracellular taurine were significantly correlated with increases of extracellular glutamate. However, when these taurine-glutamate correlations for all groups of age were compared, no statistical differences were found. These results show first that a decrease in the increases of glutamate produced by the glutamate reuptake inhibitor in striatum, but not in nucleus accumbens, is produced by age; second that glutamate-taurine interaction in striatum and nucleus accumbens does not change during the normal process of aging.

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.

Parallel modification of adenosine extracellular metabolism and modulatory action in the hippocampus of aged rats

The neuromodulator adenosine can be released as such, mainly activating inhibitory A1 receptors, or formed from released ATP, preferentially activating facilitatory A2A receptors. We tested if changes in extracellular adenosine metabolism paralleled changes in A1/A2A receptor neuromodulation in the aged rat hippocampus. The evoked release and extracellular catabolism of ATP were 49-55% lower in aged rats, but ecto-5'-nucleotidase activity, which forms adenosine, was 5-fold higher whereas adenosine uptake was decreased by 50% in aged rats. The evoked extracellular adenosine accumulation was 30% greater in aged rats and there was a greater contribution of the ecto-nucleotidase pathway and a lower contribution of adenosine transporters for extracellular adenosine formation in nerve terminals. Interestingly, a supramaximal concentration of an A1 receptor agonist, N6-cyclopentyladenosine (250 nM) was less efficient in inhibiting (17% in old versus 34% in young) and A2A receptor activation with 30 nM CGS21680 was more efficient in facilitating (63% in old versus no effect in young) acetylcholine release from hippocampal slices of aged compared with young rats. The parallel changes in the metabolic sources of extracellular adenosine and A1/A2A receptor neuromodulation in aged rats further strengthens the idea that different metabolic sources of extracellular adenosine are designed to preferentially activate different adenosine receptor subtypes.

Modification of adenosine modulation of synaptic transmission in the hippocampus of aged rats

We compared the modulation of synaptic transmission by adenosine A(1) receptors in the hippocampus of aged (24 months) and young adult rats (6 weeks). The adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine, was less potent (P:<0.05) to inhibit synaptic transmission in aged (EC(50)=53 nM) than young adult (EC(50)=14 nM) hippocampal slices, these effects being prevented by the A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). In contrast with the lower effect of the A(1) receptor agonist, it was observed that blockade of A(1) receptors with DPCPX (50 nM), or removal of endogenous extracellular adenosine with adenosine deaminase (2 u ml(-1)), caused a more pronounced disinhibition of synaptic transmission in aged rats. Also consistent with a more intense A(1) receptor-mediated inhibitory tonus by endogenous adenosine in aged rats was the finding that to fully prevent the depression of synaptic transmission induced by 3 min hypoxia, a higher concentration of DPCPX was required in slices from aged (100 nM) than from young (50 nM) rats. It is concluded that in hippocampal slices of aged rats the efficiency of A(1) receptors to modulate synaptic transmission is reduced, but this may be compensated by an enhanced inhibitory tonus by endogenous adenosine.

Modulation of the ischemia-induced taurine release by adenosine receptors in the developing and adult mouse hippocampus

The release of the inhibitory amino acid taurine is markedly enhanced under ischemic conditions in both adult and developing hippocampus, together with a pronounced increase in the release of excitatory amino acids and the neuromodulator adenosine. We studied the effects of adenosine receptor agonists and antagonists as well as adenosine transport inhibitors on hippocampal [(3)H]taurine release in normoxia and ischemia, using a superfusion system. Under standard conditions the adenosine A(1) receptor agonists N(6)-cyclohexyladenosine and R(-)N(6)-(2-phenylisopropyl)adenosine potentiated basal taurine release in developing mice and depressed the release in adults in a receptor-mediated manner. Adenosine A(2) receptor compounds had only minor effects on the basal release and the K(+)-stimulated release was not affected by these drugs. The adenosine uptake inhibitor dipyridamole enhanced basal taurine release in the developing hippocampus and reduced it in the adult. In ischemia the adenosine compounds had no marked effects on taurine release in immature animals, whereas A(1) receptor activation was still able to evoke taurine release in adults by a receptor-mediated mechanism. The results show that the basal release of taurine is modulated by A(1) receptors in both mature and immature hippocampus, whereas in ischemia these receptors potentiate taurine release only in adults. The elevated taurine levels together with the depression of excitatory amino acid release by adenosine receptor activation could be beneficial under ischemic conditions, protecting neural cells against excitotoxicity and hyperexcitation.

Adenosine modulation of D-[3H]aspartate release in cultured retina cells exposed to oxidative stress

In this study we evaluated the role of adenosine receptor activation on the K+-evoked D-[3H]aspartate release in cultured chick retina cells exposed to oxidant conditions. Oxidative stress, induced by ascorbate (3.5 mM)/Fe2+ (100 microM), increased by about fourfold the release of D-[3H]aspartate, evoked by KCl 35 mM in the presence and in the absence of Ca2+. The agonist of A1 adenosine receptors, N6-cyclopentyladenosine (CPA; 10 nM), inhibited the K+-evoked D-[3H]aspartate release in control in oxidized cells. The antagonist of A1 adenosine receptor, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 50 nM), potentiated the release of D-[3H]aspartate in oxidized cells, and reverted the effect observed in the presence of CPA 10 nM. However, in oxidized cells, when DPCPX was tested together with CPA 100 nM the total release of D-[3H]aspartate increased from 5.1 +/- 0.4% to 11.4 +/- 1.0%, this increase being reverted by 3,7-dimethyl-1-propargylxanthine (DMPX; 100 nM), an antagonist of A2A adenosine receptors. In cells of both experimental conditions, the K+-evoked release of D-[3H]aspartate was potentiated by the selective agonist of A2A adenosine receptors, 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosin e (CGS 21680; 10 nM), whereas the antagonist of these receptors, DMPX (100 nM), inhibited the release of D-[3H]aspartate in oxidized cells, but not in control cells. Adenosine deaminase (ADA; 1 U/ml), which is able to remove adenosine from the synaptic space, reduced the K+-evoked D-[3H]aspartate release, from 5.1 +/- 0.4% to 3.1 +/- 0.3% in oxidized cells, and had no significant effect in control cells. The extracellular accumulation of endogenous adenosine, upon K+-depolarization, was higher in oxidized cells than in control cells, and was reduced by the inhibitors of adenosine transporter (NBTI) and of ecto-5'-nucleotidase (AOPCP). This suggests that adenosine accumulation resulted from the outflow of adenosine mediated by the transporter, and from extracellular degradation of adenine nucleotide. Our data show that both inhibitory A1 and excitatory A2A adenosine receptors are present in cultured retina cells, and that the K+-evoked D-[3H]aspartate release is modulated by the balance between inhibitory and excitatory responses. Under oxidative stress conditions, the extracellular accumulation of endogenous adenosine seems to reach levels enough to potentiate the release of D-[3H]aspartate by the tonic activation of A2A adenosine receptors.

A cautionary note: the actions of adenosine agonists and antagonists may be reversed under certain conditions in primary cultures

It is now generally accepted that adenosine has a neuroprotective role in the central nervous system. Agonists of adenosine such as 2-chloroadenosine (2-ClA) have been shown to be neuroprotective, while antagonists such as 8-phenyltheophylline (8-PT) increase neurotoxicity. However, paradoxical results have been reported with adenosine analogues, especially with respect to length of time of administration. We observe similarly contradictory findings with respect to 2-ClA and 8-PT actions in primary hippocampal cultures exposed to glutamate or kainic acid. We found 8-PT and 2-ClA had antagonist and agonist actions, respectively, only with acute (1 h) treatment; with chronic treatment (24 h), 2-ClA had no effects, while 8-PT had significant agonist actions. We also show that with variations in the type of culturing system, concentration, and pH that 8-PT's neurotoxic antagonist actions could be dramatically changed. We, therefore, present this paper as a cautionary note in experimenting with adenosine analogues.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

A(2A) adenosine receptor deficiency attenuates brain injury induced by transient focal ischemia in mice

Extracellular adenosine critically modulates ischemic brain injury, at least in part through activation of the A(1) adenosine receptor. However, the role played by the A(2A) receptor has been obscured by intrinsic limitations of A(2A) adenosinergic agents. To overcome these pharmacological limitations, we explored the consequences of deleting the A(2A) adenosine receptor on brain damage after transient focal ischemia. Cerebral morphology, as well as vascular and physiological measures (before, during, and after ischemia) did not differ between A(2A) receptor knock-out and wild-type littermates. The volume of cerebral infarction, as well as the associated neurological deficit induced by transient filament occlusion of the middle cerebral artery, were significantly attenuated in A(2A) receptor knock-out mice. This neuroprotective phenotype of A(2A) receptor-deficient mice was observed in different genetic backgrounds, confirming A(2A) receptor disruption as its cause. Together with complimentary pharmacological studies, these data suggest that A(2A) receptors play a prominent role in the development of ischemic injury within brain and demonstrate the potential for anatomical and functional neuroprotection against stroke by A(2A) receptor antagonists.

Striatal outflow of adenosine, excitatory amino acids, gamma-aminobutyric acid, and taurine in awake freely moving rats after middle cerebral artery occlusion: correlations with neurological deficit and histopathological damage

BACKGROUND AND PURPOSE

While a number of studies have investigated transmitter outflow in anesthetized animals after middle cerebral artery occlusion (MCAO) performed by craniectomy, studies have never been performed after MCAO induced by intraluminal filament. In addition, it has been reported that after MCAO, infarct volume correlates with functional outcome and with transmitter outflow, although there are no studies that demonstrate a direct correlation between transmitter outflow and functional outcome. The purpose of the present study was to assess excitatory amino acids, gamma-aminobutyric acid, taurine, and adenosine outflow in awake rats after intraluminal MCAO and to determine whether, in the same animal, outflow was correlated with neurological outcome and histological damage.

METHODS

Vertical microdialysis probes were placed in the striatum of male Wistar rats. After 24 hours, permanent MCAO was induced by the intraluminal suture technique. The transmitter concentrations in the dialysate were determined by high-performance liquid chromatography. Twenty-four hours after MCAO, neurological deficit and histological outcome were evaluated.

RESULTS

All transmitters significantly increased after MCAO. Twenty-four hours after MCAO, the rats showed a severe sensorimotor deficit and massive ischemic damage in the striatum and in the cortex (9+/-2% and 25+/-6% of hemispheric volume, respectively). Significant correlations were found between the efflux of all transmitters, neurological score, and striatal infarct volume.

CONCLUSIONS

In this study, for the first time, amino acid and adenosine extracellular concentrations during MCAO by the intraluminal suture technique were determined in awake and freely moving rats, and a significant correlation was found between transmitter outflow and neurological deficit. The evaluation of neurological deficit, histological damage, and transmitter outflow in the same animal may represent a useful approach for studying neuroprotective properties of new drugs/agents against focal ischemia.