Blockade of adenosine A2A receptors by SCH 58261 results in neuroprotective effects in cerebral ischaemia in rats

Selective inactivation or reconstitution of adenosine A2A receptors in bone marrow cells reveals their significant contribution to the development of ischemic brain injury

Inactivation of the adenosine A(2A) receptor (A(2A)R) consistently protects against ischemic brain injury and other neural insults, but the relative contribution of A(2A)Rs on peripheral inflammatory cells versus A(2A)Rs expressed on neurons and glia is unknown. We created a chimeric mouse model in which A(2A)Rs on bone marrow-derived cells (BMDCs) were selectively inactivated or reconstituted by bone marrow transplantation. Selective reconstitution of A(2A)Rs on BMDCs (A(2A)R knockout mice transplanted with wild-type bone marrow cells) largely reinstates ischemic brain injury in global A(2A)R knockout mice. Conversely, selective inactivation of A(2A)Rs on BMDCs (wild-type mice transplanted with A(2A)R knockout bone marrow cells) attenuates infarct volumes and ischemia-induced expression of several proinflammatory cytokines in the brain, but exacerbates ischemic liver injury. These results indicate that the A(2A)R-stimulated cascade in BMDCs is an important modulator of ischemic brain injury and that ischemic brain and liver injuries are regulated distinctly by A(2A)Rs on BMDCs.

Controversies surrounding xanthine therapy

The methylxanthines aminophylline, theophylline and caffeine have been used for more than 30 years to treat apnoea of prematurity. Today, they are among the most commonly prescribed drugs in neonatal medicine. Methylxanthines reduce the frequency of idiopathic apnoea and the need for mechanical ventilation by acting as non-specific inhibitors of adenosine A(1) and A(2a) receptors. However, recent and rapidly growing research into the actions of adenosine and its receptors raises concerns about the safety of methylxanthine therapy in very preterm infants. Possible adverse effects include impaired growth, lack of neuroprotection during acute hypoxic-ischaemic episodes and abnormal behaviour. An international controlled clinical trial is underway to examine the long-term efficacy and safety of methylxanthine therapy in very low birth weight babies.

Caffeinol confers cortical but not subcortical neuroprotection after transient focal cerebral ischemia in rats

The combination of low-dose ethanol and caffeine (caffeinol) protects cortical areas of the brain from damage produced by distal focal ischemia in rats. There are no data, however, as to whether caffeinol influences injury in subcortical brain regions. Rats were anesthetized with halothane and subjected to 2 h of MCAo by poly-l-lysine-coated intraluminal suture. Caffeinol [a combination of ethanol, 0.33 g/kg, and caffeine, 10 mg/kg (n=5)] or vehicle (0.9% NaCl; n=7) was administered by i.v. infusion over a 2.5-h period beginning 15 min after reperfusion. Neurological status was evaluated daily, and histopathology was quantified at 3 days. Caffeinol therapy significantly improved the neurological score, reduced the total infarct volume (by 52%) and cortical infarct areas at multiple coronal levels, but subcortical infarction and brain swelling were not affected.

Binding of the prototypical adenosine A(2A) receptor agonist CGS 21680 to the cerebral cortex of adenosine A(1) and A(2A) receptor knockout mice

1. 2-p-(2-carboxyethylphenethylamino-5'-ethylcarboxamidoadenosine) (CGS 21680) is considered the reference compound to study adenosine A(2A) receptors. However, CGS 21680 binding in the cerebral cortex, where adenosine A(1) receptors are predominant, displays a mixed A(2A)/A(1) receptor pharmacology. We now use adenosine A(1) and A(2A) receptor knockout mice to investigate the characteristics of cortical [(3)H]CGS 21680 binding. 2. [(3)H]CGS 21680 binding to the cerebral cortex was strongly reduced in adenosine A(1) receptor knockout mice, but only slightly reduced in A(2A) receptor knockout mice compared with the corresponding wild-type littermates. 3. Another selective A(2A) receptor ligand, [(3)H]-5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine ([(3)H]SCH 58261), displayed a saturable binding to mouse cortical membranes, albeit with a binding density 20 times lower than that of striatal membranes, and this [(3)H]SCH58261 binding was abolished in both striatal and cortical membranes of A(2A) receptor knockout mice and unchanged in A(1) receptor knockout mice. 4. The presence of A(2A) receptors in cortical neurons was further confirmed by Western blot in mouse cortical nerve terminal membranes. 5. It is concluded that, although A(2A) receptors are present in the cerebral cortex, the purportedly selective A(2A) receptor agonist [(3)H]CGS 21680 binds in the cerebral cortex to an entity that requires the presence of adenosine A(1) receptors. Thus, CGS 21680 should be used with care in all preparations where adenosine A(1) receptors out-number A(2A) receptors.

Animal models for the study of adenosine receptor function

Adenosine receptors represent a family of G-protein coupled receptors that are ubiquitously expressed in a wide variety of tissues. This family contains four receptor subtypes: A1 and A3, which mediate inhibition of adenylyl cyclase; and A2a and A2b, which mediate stimulation of this enzyme. Currently, all receptor subtypes have been genetically deleted in mouse models except for the A2b adenosine receptor, and some have been overexpressed in selective tissues of transgenic mice. Studies involving these transgenic mice indicated that receptor levels are rate limiting, as effects were amplified upon increases in receptor level. The knockout models pointed to clusters of activities related to the physiologies of the cardiovascular and the nervous systems, which are either reduced or enhanced upon specific receptor deletion. Interestingly, the trend of effects on these systems is similar in the A1 and A3 adenosine receptor knockout mice and opposite to the effects observed in the A2a adenosine receptor knockout model. This review summarizes in vitro studies on pathways affected by each adenosine receptor, and primarily focuses on the above in vivo models generated to investigate the physiologic role of adenosine receptors. Furthermore, it illustrates the need for multiple adenosine receptor subtype deficiency studies in mice and the deletion of the A2b subtype.

Adenosine A2A receptor blockade differentially influences excitotoxic mechanisms at pre- and postsynaptic sites in the rat striatum

Adenosine A(2A) receptor antagonists are being regarded as potential neuroprotective drugs, although the mechanisms underlying their effects need to be better studied. The aim of this work was to investigate further the mechanism of the neuroprotective action of A(2A) receptor antagonists in models of pre- and postsynaptic excitotoxicity. In microdialysis studies, the intrastriatal perfusion of the A(2A) receptor antagonist ZM 241385 (5 and 50 nM) significantly reduced, in an inversely dose-dependent way, the raise in glutamate outflow induced by 5 mM quinolinic acid (QA). In rat corticostriatal slices, ZM 241385 (30-100 nM) significantly reduced 4-aminopyridine (4-AP)-induced paired-pulse inhibition (PPI; an index of neurotransmitter release), whereas it worsened the depression of field potential amplitude elicited by N-methyl-D-aspartate (NMDA; 12.5 and 50 microM). The A(2A) antagonist SCH 58261 (30 nM) mimicked the effects of ZM 241385, whereas the A(2A) agonist CGS 21680 (100 nM) showed a protective influence toward 50 microM NMDA. In rat striatal neurons, 50 nM ZM 241385 did not affect the increase in [Ca(2+)](i) or the release of lactate dehydrogenase (LDH) induced by 100 and 300 microM NMDA, respectively. The ability of ZM 241385 to prevent QA-induced glutamate outflow and 4-AP-induced effects confirms that A(2A) receptor antagonists have inhibitory effects on neurotransmitter release, whereas the results obtained toward NMDA-induced effects suggest that A(2A) receptor blockade does not reduce, or even amplifies, excitotoxic mechanisms due to direct NMDA receptor stimulation. This indicates that the neuroprotective potential of A(2A) antagonists may be evident mainly in models of neurodegeneration in which presynaptic mechanisms play a major role.

Genetic and pharmacological inactivation of the adenosine A2A receptor attenuates 3-nitropropionic acid-induced striatal damage

Adenosine A2A receptor (A2AR) antagonism attenuates 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic neurodegeneration and quinolinic acid-induced excitotoxicity in the neostriatum. As A2ARs are enriched in striatum, we investigated the effect of genetic and pharmacological A2A inactivation on striatal damage produced by the mitochondrial complex II inhibitor 3-nitropriopionic acid (3-NP). 3-NP was administered to A2AR knockout (KO) and wild-type (WT) littermate mice over 5 days. Bilateral striatal lesions were analyzed from serial brain tissue sections. Whereas all of the 3-NP-treated WT mice (C57BL/6 genetic background) had bilateral striatal lesions, only one of eight of the 3-NP-treated A2AR KO mice had detectable striatal lesions. Similar attenuation of 3-NP-induced striatal damage was observed in A2AR KO mice in a 129-Steel background. In addition, the effect of pharmacological antagonism on 3-NP-induced striatal neurotoxicity was tested by pre-treatment of C57Bl/6 mice with the A2AR antagonist 8-(3-chlorostyryl) caffeine (CSC). Although bilateral striatal lesions were observed in all mice treated either with 3-NP alone or 3-NP plus vehicle, there were no demonstrable striatal lesions in mice treated with CSC (5 mg/kg) plus 3-NP and in five of six mice treated with CSC (20 mg/kg) plus 3-NP. We conclude that both genetic and pharmacological inactivation of the A2AR attenuates striatal neurotoxicity produced by 3-NP. Since the clinical and neuropathological features of 3-NP-induced striatal damage resemble those observed in Huntington's disease, the results suggest that A2AR antagonism may be a potential therapeutic strategy in Huntington's disease patients.

A(2A) adenosine receptor ligands and proinflammatory cytokines induce PC 12 cell death through apoptosis

A(2A) adenosine receptor-mediated signaling affects a variety of important processes in the central nervous system both in physiological and pathological conditions, and has been indicated as possible novel therapeutic target in several nervous system diseases. In the present work, cell death induction was investigated after neuronal PC 12 cell treatment with proinflammatory cytokines and adenosine receptor ligands. Interleukin-1-beta (IL-1-beta, 500 U/mL), tumor necrosis factor-alpha (TNF-alpha, 1000 U/mL) and the non selective adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), caused a significant reduction of cell viability with a maximal effect within 3-48 hr. Moreover, an addictive effect was detected when the cells were simultaneously treated with Interleukin-1-beta and NECA for 3 hr. To investigate the adenosine receptor subtypes involved in PC 12 cell death, the effects of several adenosine receptor agonists/antagonists were evaluated. The endogenous nucleoside, adenosine, and the selective A(2A) adenosine receptor agonist, 2-(carboxyethylphenylethylamino)adenosine-5'-carboxamide (CGS21680) reduced PC 12 cell viability. This effect was counteracted by the selective A(2A) adenosine receptor antagonist, 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5c]pyrimidine (SCH58261), but not by selective A(2B) adenosine receptor antagonist N-(4-acethylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide (MRS1706), suggesting the specific involvement of A(2A) adenosine receptor subtype in adenosine-mediated cytotoxicity. Moreover, the selective A(1) adenosine receptor agonist, N(6)-cyclohexyladenosine (CHA), did not induce any significant effect on cell viability. By ELISA immunoassay cell death detection and transmission electron microscopy (TEM) we demonstrated that A(2A) adenosine receptor ligands and cytokines induced cell death through an apoptotic pathway. In conclusion, our results showed that A(2A) adenosine receptors are involved in the control of PC 12 cell survival/death and may contribute to modulate cellular activity in response to tissue damage associated with inflammatory mediator production.

Presynaptic modulation controlling neuronal excitability and epileptogenesis: role of kainate, adenosine and neuropeptide Y receptors

Based on the idea that seizures may arise from an overshoot of excitation over inhibition, all substances that may decrease glutamatergic function while having no effect or even increasing GABAergic neurotransmission are likely to be effective anticonvulsants. We now review the possible role of three such neuromodulators, kainate, adenosine, and neuropeptide Y receptors in controlling hyperexcitability and epileptogenesis. Particular emphasis is given on the robust neuromodulatory role of these three groups of receptors on the release of glutamate in the hippocampus, a main focus of epilepsy. Moreover, we also give special attention to the mechanisms of receptor activation and coupled signaling events that can be explored as attractive targets for the treatment of epilepsy and excitotoxicity. The present paper is a tribute to Arsélio Pato de Carvalho who has been the main driving force for the development of Neuroscience in Portugal, notably with a particular emphasis on the presynaptic mechanisms of modulation of neurotransmitter release.

A dual role of adenosine A2A receptors in 3-nitropropionic acid-induced striatal lesions: implications for the neuroprotective potential of A2A antagonists

Reduction of A2A receptor expression is one of the earliest events occurring in both Huntington's disease (HD) patients and mice overexpressing the N-terminal part of mutated huntingtin. Interestingly, increased activity of A2A receptors has been found in striatal cells prone to degenerate in experimental models of this neurodegenerative disease. However, the role of A2A receptors in the pathogenesis of HD remains obscure. In the present study, using A2A-/- mice and pharmacological compounds in rat, we demonstrate that striatal neurodegeneration induced by the mitochondrial toxin 3-nitropropionic acid (3NP) is regulated by A2A receptors. Our results show that the striatal outcome induced by 3NP depends on a balance between the deleterious activity of presynaptic A2A receptors and the protective activity of postsynaptic A2A receptors. Moreover, microdialysis data demonstrate that this balance is anatomically determined, because the A2A presynaptic control on striatal glutamate release is absent within the posterior striatum. Therefore, because blockade of A2A receptors has differential effects on striatal cell death in vivo depending on its ability to modulate presynaptic over postsynaptic receptor activity, therapeutic use of A2A antagonists in Huntington's as well as in other neurodegenerative diseases could exhibit undesirable biphasic neuroprotective-neurotoxic effects.

Sensitivity to selective adenosine A1 and A2A receptor antagonists of the release of glutamate induced by ischemia in rat cerebrocortical slices

Adenosine released during cerebral ischemia is considered to act as a neuroprotectant, possibly through the inhibition of glutamate release. The involvement of A(1) and A(2A) receptors in the control of the rise of extracellular glutamate during ischemia was investigated by monitoring the effects of selective A(1) and A(2A) receptor antagonists on ischemia-evoked glutamate release in rat cerebrocortical slices.Slices were superfused with oxygen- and glucose-deprived medium and [(3)H]D-aspartate or endogenous glutamate was measured in the superfusate fractions. Withdrawal of Ca(2+) ions or addition of tetrodotoxin more than halved the ischemia-evoked efflux of [(3)H]D-aspartate or glutamate, compatible with a vesicular-like release. The glutamate transporter inhibitor DL-TBOA prevented the ischemia-evoked efflux of [(3)H]D-aspartate by about 40%, indicating a carrier-mediated efflux. The ischemia-evoked efflux of [(3)H]D-aspartate or glutamate was increased by the A(1) receptor antagonist DPCPX. The A(2A) antagonist SCH 58261 decreased [(3)H]D-aspartate or endogenous glutamate efflux (50 and 55% maximal inhibitions; EC(50): 14.9 and 7.6 nM, respectively); the drug was effective also if added during ischemia. No effect of either the A(1) or the A(2A) receptor antagonist was found on the ischemia-evoked efflux of [(3)H]D-aspartate in Ca(2+)-free medium. Our data suggest that adenosine released during cerebral ischemia can activate inhibitory A(1) and stimulatory A(2A) receptors that down- or up-regulate the vesicular-like component of glutamate release.

Purines and neuroprotection

The activation of adenosine A1, A2 andA3 receptors can protect neurones against damage generated by mechanical or hypoxic/ischaemic insults as well as excitotoxins. A1 receptors are probably effective by suppressing transmitter release and producing neuronal hyperpolarisation. They are less likely to be of therapeutic importance due to the plethora of side effects resulting from A1 agonism, although the existence of receptor subtypes and recent synthetic chemistry efforts to increase ligand selectivity, may yet yield clinically viable compounds. Activation of A2A receptors can protect neurons, although there is much uncertainty as to whether agonists are acting centrally or via a peripheral mechanism such as altering blood flow or immune cell function. Selective antagonists at the A2A receptor, such as 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-yl-amino]ethyl)phenol (ZM 241385) and 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261), can also protect against neuronal death produced by ischaemia or excitotoxicity. In addition, A2A receptor antagonists can reduce damage produced by combinations of subthreshold doses of the endogenous excitotoxin quinolinic acid and free radicals. Since the A2A receptors do not seem to be activated by normal endogenous levels of adenosine, their blockade should not generate significant side effects, so that A2A receptor antagonists appear to be promising candidates as new drugs for the prevention of neuronal damage. Adenosine A3 receptors have received less attention to date, but agonists are clearly able to afford protection against damage when administered chronically. Given the disappointing lack of success of NMDA receptor antagonists in human stroke patients, despite their early promise in animal models, it is possible that A2A receptor antagonists could have a far greater clinical utility.

Adenosine receptors and Huntington's disease: implications for pathogenesis and therapeutics

Huntington's disease (HD) is a devastating hereditary neurodegenerative disorder, the progression of which cannot be prevented by any neuroprotective approach, despite major advances in the understanding of its pathogenesis. The study of several animal models of the disease has led to the discovery of both loss-of-normal and gain-of-toxic functions of the mutated huntingtin protein and the elucidation of the mechanisms that underlie the formation of huntingtin aggregates and nuclear inclusions. Moreover, these models also provide good evidence of a role for excitotoxicity and mitochondrial metabolic impairments in striatal neuronal death. Adenosine has neuroprotective potential in both acute and chronic neurological disorders such as stroke or Parkinson's disease. Here we review experimental data on the role of A1 and A2A adenosine receptors in HD that warrant further investigation of the beneficial effects of A1 agonists and A2A antagonists in animal models of HD. Future pharmacological analysis of adenosine receptors could justify the use of A1 agonists and A2A antagonists for the treatment of HDin clinical trials.

Recent developments in the field of A2A and A3 adenosine receptor antagonists

In the last years adenosine receptors have been extensively studied, and mainly at present we understand the importance of A(2A) and A(3) adenosine receptors. A(2A) selective adenosine receptors antagonists are promising new drugs for the treatment of Parkinson's disease, while A(3) selective adenosine receptors antagonists have been postulated as novel anti-inflammatory and antiallergic agents; recent studies also indicated a possible employment of these derivatives as antitumour agents. Lately different classes of compounds have been identified as potent A(2A) and A(3) antagonists. In this article we report the past and present efforts which led to development of more potent and selective A(2A) and A(3) antagonists. Our group has mainly worked on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine nucleus both as A(2A) and A(3) antagonists, aiming to improve the affinity, selectivity and the hydrophilic profile. In fact, we have synthesised several compounds endowed with high affinity and selectivity versus A(2A) adenosine receptors, as 2, 2a-c (K(i)A(2A)=0.12-0.19 nM), or A(3) adenosine receptors, as 4p (K(i)A(3)=0.01 nM) and 4q (K(i)A(3)=0.04 nM).

Neuroprotection by caffeine and adenosine A2A receptor blockade of beta-amyloid neurotoxicity

Adenosine is a neuromodulator in the nervous system and it has recently been observed that pharmacological blockade or gene disruption of adenosine A(2A) receptors confers neuroprotection under different neurotoxic situations in the brain. We now observed that coapplication of either caffeine (1-25 micro M) or the selective A(2A) receptor antagonist, 4-(2-[7-amino-2(2-furyl)(1,2,4)triazolo (2,3-a)(1,3,5)triazin-5-ylamino]ethyl)phenol (ZM 241385, 50 nM), but not the A receptor antagonist, 8-cyclopentyltheophylline (200 nM), prevented the neuronal cell death caused by exposure of rat cultured cerebellar granule neurons to fragment 25-35 of beta-amyloid protein (25 micro M for 48 h), that by itself caused a near three-fold increase of propidium iodide-labeled cells. This constitutes the first in vitro evidence to suggest that adenosine A(2A) receptors may be the molecular target responsible for the observed beneficial effects of caffeine consumption in the development of Alzheimer's disease.

Aggravated brain damage after hypoxic ischemia in immature adenosine A2A knockout mice

BACKGROUND AND PURPOSE

Cerebral hypoxic ischemia (HI) is an important cause of brain injury in the newborn infant. Adenosine is believed to protect against HI brain damage. However, the roles of the different adenosine receptors are unclear, particularly in young animals. We examined the role of adenosine A2A receptors (A2AR) using 7-day-old A2A knockout (A2AR(-/-)) mice in a model of HI.

METHODS

HI was induced in 7-day-old CD1 mice by exposure to 8% oxygen for 30 minutes after occlusion of the left common carotid artery. The resulting unilateral focal lesion was evaluated with the use of histopathological scoring and measurements of residual brain areas at 5 days, 3 weeks, and 3 months after HI. Behavioral evaluation of brain injury by locomotor activity, rotarod, and beam-walking test was made 3 weeks and 3 months after HI. Cortical cerebral blood flow, assessed by laser-Doppler flowmetry, and rectal temperature were measured during HI.

RESULTS

Reduction in cortical cerebral blood flow during HI and rectal temperature did not differ between wild-type (A2AR(+/+)) and knockout mice. In the A2AR(-/-) animals, brain injury was aggravated compared with wild-type mice. The A2AR(-/-) mice subjected to HI displayed increased forward locomotion and impaired rotarod performance in adulthood compared with A2AR(+/+) mice subjected to HI, whereas beam-walking performance was similarly defective in both groups.

CONCLUSIONS

These results suggest that, in contrast to the situation in adult animals, A2AR play an important protective role in neonatal HI brain injury.

DARPP chocolate: a caffeinated morsel of striatal signaling

The psychomotor stimulant effects of caffeine, the most widely consumed psychoactive substance, are mediated through its antagonism of extracellular adenosine receptors in the basal ganglia. In the absence of caffeine, adenosine stimulates inhibitory striatopallidal neurons that suppress motor activity by binding to A2A receptors, thereby activating a cyclic adenosine 3',5'-monophosphate (cAMP) and protein kinase A signaling pathway. Bastia and Schwarzschild discuss recent research implicating DARRP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kilodaltons) as an attractive mediator of the sustained psychomotor stimulant effect seen with low doses of caffeine. They highlight the role of postsynaptic A2A receptor blockade, but leave open the possibility that antagonism of presynaptic or postsynaptic A1 receptors also contributes to DARPP-32-dependent psychomotor stimulation by caffeine.

The selective A2A receptor antagonist SCH 58261 reduces striatal transmitter outflow, turning behavior and ischemic brain damage induced by permanent focal ischemia in the rat

Adenosine A(2A) receptor antagonists have been proved protective in different ischemia models. In this study we verified if the protective effect of the selective A(2A) antagonist, SCH 58261, could be attributed to the reduction of the excitatory amino acid outflow induced by cerebral focal ischemia. A vertical microdialysis probe was inserted into the striatum of male Wistar rats and, after 24 h, permanent right intraluminal middle cerebral artery occlusion (MCAo) was induced. Soon after waking, rats showed a definite contralateral turning behavior, which persisted up to 7 h after MCAo. During 4 h after MCAo, glutamate, aspartate, GABA, adenosine and taurine outflow increased. SCH 58261 (0.01 mg/kg, i.p.), administered 5 min after MCAo, suppressed turning behavior and significantly reduced the outflow of glutamate, aspartate, GABA and adenosine. At 24 h after MCAo, the rats showed severe sensorimotor deficit and damage in both the striatum and cortex. SCH 58261 significantly reduced cortical damage but did not protect against the sensorimotor deficit. The protective effect of SCH 58261 against turning behavior and increased outflow of excitatory amino acids in the first hours after MCAo suggests the potential utility of selective adenosine A(2A) antagonists when administered in the first hours after ischemia. Furthermore, this study, for the first time, proposes that turning behavior after permanent intraluminal MCAo, be used as a precocious index of neurological deficit and neuronal damage.

Multiple effects of adenosine in the arterially perfused mammalian eye. Possible mechanisms for the neuroprotective function of adenosine in the retina

It has been postulated that the major physiological role of adenosine is protection of the central nervous system in conditions such as ischemia, hypoxia, or prolonged neuronal excitation. Under these conditions adenosine is released, and exerts multiple effects, including vasodilation, inhibition of neuronal activity, and enhancement of glycogenolysis, resulting in neuroprotection. In this article, published as well as unpublished data on the multiple effects of exogenous adenosine and application of adenosine-related agents, performed using the arterially perfused cat eye, will be reviewed and discussed within the framework of the neuroprotective role of adenosine. The isolated, arterially perfused eye preparation has the advantage of combining integrity of the eye structure, exact control of arterial concentration and timing of applied pharmacological agents, and access to electrophysiological parameters of both retina and optic nerve, as well as the ability to control and monitor perfusate flow. The absence of red blood cells in the perfusate prevents adenosine from being metabolized prior to reaching the eye.

SCH 58261 differentially influences quinolinic acid-induced effects in striatal and in hippocampal slices

The influence of the adenosine A(2A) receptor antagonist SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-trizolo[1,5-c] pyrimidine) (50, 200 nM, 1 microM) on quinolinic acid effects has been studied in rat striatal and hippocampal slices. Quinolinic acid induced disappearance of field potentials at concentrations of 500 microM and 2 mM in hippocampal and corticostriatal slices, respectively. We found that 1 microM SCH 58261 prevented quinolinic acid-induced field potential disappearance in corticostriatal but not in hippocampal slices. This finding demonstrates that the peculiar binding profile of SCH 58261 and the predominance in the hippocampus of "atypical" adenosine A(2A) receptor population (not recognized by SCH 58261) could have a functional relevance in the occurrence of region-specific neuroprotective effects.

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.

The pharmacological manipulation of glutamate receptors and neuroprotection

The overactivation of glutamate receptors is a major cause of Ca(2+) overload in cells, potentially leading to cell damage and death. There is an abundance of agents and mechanisms by which glutamate receptor activation can be prevented or modulated in order to control these effects. They include the well-established, competitive and non-competitive antagonists at the N-methyl-D-aspartate (NMDA) receptors and modulators of desensitisation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. More recently, it has emerged that some compounds can act selectively at different subunits of glutamate receptors, allowing a differential blockade of subtypes. It is also becoming clear that a number of endogenous compounds, including purines, can modify glutamate receptor sensitivity. The kynurenine pathway is an alternative but distinct pathway to the generation of glutamate receptor ligands. The products of tryptophan metabolism via the kynurenine pathway include both quinolinic acid, a selective agonist at NMDA receptors, and kynurenic acid, an antagonist at several glutamate receptor subtypes. The levels of these metabolites change as a result of the activation of inflammatory processes and immune-competent cells, and may have a significant impact on Ca(2+) fluxes and neuronal damage. Drugs which target some of these various sites and processes, or which change the balance between the excitotoxin quinolinic acid and the neuroprotective kynurenic acid, could also have potential as neuroprotective drugs.

Effects of adenosine A1 and A2A receptor activation on the evoked release of glutamate from rat cerebrocortical synaptosomes

1. The effects of adenosine A2A and A1 receptor activation on the release of glutamate were studied in rat cerebral cortex synaptosomes exposed in superfusion to adenosine receptor ligands. 2. Adenosine (0.1 microM) produced a significant potentiation of the Ca2+-dependent K+ (15 mM)-evoked [3H]-D-aspartate overflow (20.4+/-3.5%), which was blocked by A2A blocker SCH58261 (0.1 microM). At higher concentrations (10 - 1000 microM) adenosine inhibited in a DPCPX-sensitive manner the Ca2+-dependent K+-evoked [3H]-D-aspartate overflow. The inhibitory effect of adenosine at 1000 microM was significantly increased by SCH58261. This inhibition was antagonized by 1 microM DPCPX. Adenosine did not produce any effect on basal release. 3. The A2A receptor agonist CGS 21680 was ineffective on basal release, but stimulated the Ca2+-dependent K+-evoked overflow of [3H]-D-aspartate (EC50 approximately 1 pM). The effect of 0.01 nM CGS 21680 was totally sensitive to the A2A receptor antagonist SCH58261 (IC50 approximately 5 nM). 4. The A1 receptor agonist CCPA inhibited the Ca2+-dependent K+-evoked [3H]-D-aspartate overflow (EC50 approximately 20 nM). The effect of 100 nM CCPA was abolished by 100 nM of the A1 receptor antagonist DPCPX. 5. The K+ (15 mM)-evoked overflow of endogenous glutamate was enhanced by CGS 21680 (0.01 nM) and inhibited by CCPA (0.1 microM). The effect of CGS 21680 was abolished by SCH58261 (0.1 microM) and that of CCPA by DPCPX (0.1 microM). 6. It is concluded that adenosine and adenosine receptor agonists modulate glutamate release by activating inhibitory A1 and excitatory A2A receptors present on glutamatergic terminals of the rat cerebral cortex.

Caffeinated clues and the promise of adenosine A(2A) antagonists in PD

Large prospective epidemiologic studies have linked the consumption of coffee and other caffeinated beverages to a reduced risk of subsequently developing PD. Caffeine as well as more specific antagonists of the adenosine A(2A) receptor have also now been found to attenuate neurotoxicity in a mouse model of PD. The convergence of these epidemiologic and laboratory data supports the possibility that caffeine may reduce the risk of developing PD. However, a neuroprotective effect of caffeine in PD remains unproven; current evidence does not provide a rational basis for recommending caffeine consumption to modify the risk or progression of PD. In addition to possessing neuroprotective potential, caffeine and other A(2A) antagonists have long been known to acutely reverse motor deficits in a variety of PD models. This symptomatic antiparkinsonian benefit of blocking A(2A) receptors, coupled with their remarkably restricted expression in the basal ganglia, have made A(2A) antagonists attractive targets for drug development. Now, with the prospect of a neuroprotective bonus, the novel therapeutic potential of A(2A) antagonists appears all the more promising just as they are entering clinical trials for PD.

Intracerebral adenosine infusion improves neurological outcome after transient focal ischemia in rats

Second Institute of New Drug Research, Otsuka Pharmaceutical Co., Ltd., Tokushima, Japan In order to elucidate the role of adenosine in brain ischemia, the possible protective effects of adenosine on ischemic brain injury were investigated in a rat model of brain ischemia both in vitro and in vivo. Exogenous adenosine dose-dependently rescued cortical neuronal cells from injury after glucose deprivation in vitro. Adenosine (1 mM) also significantly reduced hypoglycemia/hypoxia-induced glutamate release from the hippocampal slice. In a rat model of transient middle cerebral artery occlusion (MCAO), extracellular adenosine concentration was increased immediately after occlusion, and then returned to the baseline by 30 min after reperfusion. Adenosine infusion through a microdialysis probe into the ipsilateral striatum (1 mM adenosine, 2 microl min(-1), total 4.5 h from the occlusion to 3 h after reperfusion) showed a significant improvement in the neurological outcome, and about 25% reduction of infarct volume, although the effect did not reach statistical significance, compared with the vehicle-treated group at 20 h after 90 min of MCAO. These results demonstrated the neuroprotective effect of adenosine against ischemic brain injury both in vitro and in vivo, suggesting the possible therapeutic application of adenosine regulating agents, which inhibit adenosine uptake or metabolism to enhance or maintain extracellular endogenous adenosine levels, for stroke treatment.

Blockade of striatal adenosine A2A receptor reduces, through a presynaptic mechanism, quinolinic acid-induced excitotoxicity: possible relevance to neuroprotective interventions in neurodegenerative diseases of the striatum

The aim of the present study was to evaluate whether, and by means of which mechanisms, the adenosine A2A receptor antagonist SCH 58261 [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine] exerted neuroprotective effects in a rat model of Huntington's disease. In a first set of experiments, SCH 58261 (0.01 and 1 mg/kg) was administered intraperitoneally to Wistar rats 20 min before the bilateral striatal injection of quinolinic acid (QA) (300 nmol/1 microl). SCH 58261 (0.01 but not 1 mg/kg, i.p.) did reduce significantly the effects of QA on motor activity, electroencephalographic changes, and striatal gliosis. Because QA acts by both increasing glutamate outflow and directly stimulating NMDA receptors, a second set of experiments was performed to evaluate whether SCH 58261 acted by preventing the presynaptic and/or the postsynaptic effects of QA. In microdialysis experiments in naive rats, striatal perfusion with QA (5 mm) enhanced glutamate levels by approximately 500%. Such an effect of QA was completely antagonized by pretreatment with SCH 58261 (0.01 but not 1 mg/kg, i.p.). In primary striatal cultures, bath application of QA (900 microm) significantly increased intracellular calcium levels, an effect prevented by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. In this model, bath application of SCH 58261 (15-200 nm) tended to potentiate QA-induced calcium increase. We conclude the following: (1) the adenosine A2A receptor antagonist SCH 58261 has neuroprotective effects, although only at low doses, in an excitotoxic rat model of HD, and (2) the inhibition of QA-evoked glutamate outflow seems to be the major mechanism underlying the neuroprotective effects of SCH 58261.

Enhanced neuronal damage by co-administration of quinolinic acid and free radicals, and protection by adenosine A2A receptor antagonists

1. Quinolinic acid may be an important endogenous excitotoxin, but its concentrations in brain are low. We have therefore attempted to determine whether its neurotoxicity can be increased by the simultaneous presence of free radicals. 2. Quinolinic acid was injected into the hippocampus of anaesthetized rats at doses of 40 and 80 nmols which produced little neuronal loss, and 120 nmols which produced over 90% neuronal loss. 3. A mixture of xanthine and xanthine oxidase, a known source of free radical reactive oxygen species, also generated little damage alone, but killed over 80% of CA1 neurons when combined with 80 nmols of quinolinic acid. Similarly, the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) potentiated the damage produced by quinolinic acid. 4. The glutamate antagonist 5,7-dichlorokynurenic acid prevented the damage produced by 120 nmols of quinolinic acid, but not that produced by quinolinic acid plus xanthine/xanthine oxidase, indicating that damage was not simply the result of free radical enhancement of NMDA receptor activation. 5. Three chemically dissimilar antagonists at adenosine A(2A) receptors prevented the damage caused by quinolinic acid and xanthine/xanthine oxidase or by quinolinic acid plus SNAP. 6. It is concluded that reactive oxygen species can potentiate the neurotoxicity of quinolinic acid. The site of interaction is probably distal to the NMDA receptor. Blockade of adenosine A(2A) receptors can protect against this combined damage, suggesting potential value in the prevention of brain damage.

Persistent behavioral sensitization to chronic L-DOPA requires A2A adenosine receptors

To investigate the role of A(2A) adenosine receptors in adaptive responses to chronic intermittent dopamine receptor stimulation, we compared the behavioral sensitization elicited by repeated l-DOPA treatment in hemiparkinsonian wild-type (WT) and A(2A) adenosine receptor knock-out (A(2A) KO) mice. Although the unilateral nigrostriatal lesion produced by intrastriatal injection of 6-hydroxydopamine was indistinguishable between WT and A(2A) KO mice, they developed strikingly different patterns of behavioral sensitization after daily treatment with low doses of l-DOPA for 3 weeks. WT mice initially displayed modest contralateral rotational responses and then developed progressively greater responses that reached a maximum within 1 week and persisted for the duration of the treatment. In contrast, any rotational behavioral sensitization in A(2A) KO mice was transient and completely reversed within 2 weeks. Similarly, the time to reach the peak rotation was progressively shortened in WT mice but remained unchanged in A(2A) KO mice. Furthermore, daily l-DOPA treatment produced gradually sensitized grooming in WT mice but failed to induce any sensitized grooming in A(2A) KO mice. Finally, repeated l-DOPA treatment reversed the 6-OHDA-induced reduction of striatal dynorphin mRNA in WT but not A(2A) KO mice, raising the possibility that the A(2A) receptor may contribute to l-DOPA-induced behavioral sensitization by facilitating adaptations within the dynorphin-expressing striatonigral pathway. Together these results demonstrate that the A(2A) receptor plays a critical role in the development and particularly the persistence of behavioral sensitization to repeated l-DOPA treatment. Furthermore, they raise the possibility that the maladaptive dyskinetic responses to chronic l-DOPA treatment in Parkinson's disease may be attenuated by A(2A) receptor inactivation.

Blockade of A(2A) adenosine receptors leads to c-fos inhibition in a rat model of brain ischemia

Adenosine plays an important role in cerebral ischemia by acting on its own receptors, in particular the A(2A)receptor. Its activation leads to excitatory amino acid release thus contributing to the ischemic damage. Blockade by specific antagonists may protect against cytotoxic injury. Our study was aimed to investigate the effect of the blockade of A(2A)receptors, by Sch 58261, on the expression of the early gene c-fos, in a model of permanent middle cerebral artery occlusion (pMCAo), in rats. In the pMCAo model, ischemia was induced in the right hemisphere whereas the contralateral one was considered the control. In our study, we have compared pMCAo rats, pMCAo rats treated with Sch 58261 and sham operated ones.C-fos was markedly expressed in the ischemic hemispheres, whereas lower levels were detected in the contralateral ones of the ischemic animals. The lowest bands were observed in sham operated rats. After treatment with Sch 58261, a considerable reduction in c-fos expression was observed in the ischemic hemispheres, whereas a limited effect was detected in the others. Our results suggest that inhibition of immediate-early gene expression by the A(2A)receptors antagonist Sch 58261 may contribute to its neuroprotective activity.

Hippocampal injury and neurobehavioral deficits following hyperglycemic cerebral ischemia: effect of theophylline and ZM 241385

OBJECT

The effects of the adenosine receptor antagonists theophylline (for A1 and A2) and ZM 241385 (for A2A) on hippocampal injury and Morris water maze (MWM) performance in rats were investigated following normoglycemic and hyperglycemic cerebral ischemia (induced by four vessel occlusion for 10 minutes).

METHODS

Theophylline (36 mg/kg), ZM 241385 (1 mg/kg), or an equivalent volume of saline was administered to rats intraperitoneally 30 minutes before ischemia was induced. Moderate hyperglycemia was achieved by intraperitoneal administration of D-glucose (3 g/kg, 15 minutes before induction of ischemia). Morris water maze trials were performed on the 6th. 7th, and 8th days after ischemic insult. After the conclusion of the performance tests, the rat brains were cut into 8-microm sections, stained with cresyl violet and acid fuchsin, and evaluated in a blinded fashion to determine the extent of injury. Theophylline worsened injury in the hippocampus following normoglycemic and hyperglycemic ischemia. Moreover, theophylline significantly (p < 0.05, six animals) worsened latency and learning index (LI) scores during the MWM trials in both normoglycemic and hyperglycemic animals. On the other hand, ZM 241385 had no effect on either ischemic injury or MWM performance in normoglycemic animals. In the animals in the hyperglycemic ischemia group, however, ZM 241385 significantly (p < 0.05, five animals) reduced injury in the CA1 (94.6 +/- 1.7% compared with 79.2 +/- 10.9%), CA3 (26 +/- 12.5% compared with 11.2 +/- 4.3%), and hilum (22.4 +/- 8.1% compared with 11 +/- 5.5%) regions. In addition, ZM 241385 significantly improved latency (52 +/- 29.7 seconds compared with 24.8 +/- 11.2 seconds, p < 0.05) and LI scores (203.2 +/- 33.3 compared with 152.1 +/- 31.8, p < 0.05) in the MWM trials. A statistically significant correlation was also found between hippocampal injury (CA1, CA3, and hilum) and MWM performance.

CONCLUSIONS

The results of this study provide further evidence for a neuromodulatory role of adenosine during normoglycemic and hyperglycemic ischemia.

Increased adenosine in cerebrospinal fluid after severe traumatic brain injury in infants and children: association with severity of injury and excitotoxicity

OBJECTIVES

To measure adenosine concentration in the cerebrospinal fluid of infants and children after severe traumatic brain injury and to evaluate the contribution of patient age, Glasgow Coma Scale score, mechanism of injury, Glasgow Outcome Score, and time after injury to cerebrospinal fluid adenosine concentrations. To evaluate the relationship between cerebrospinal fluid adenosine and glutamate concentrations in this population.

DESIGN

Prospective survey.

SETTING

Pediatric intensive care unit in a university-based children's hospital.

PATIENTS

Twenty-seven critically ill infants and children who had severe traumatic brain injury (Glasgow Coma Scale < 8), who required placement of an intraventricular catheter and drainage of cerebrospinal fluid as part of their neurointensive care.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Patients ranged in age from 2 months to 14 yrs. Cerebrospinal fluid samples (n = 304) were collected from 27 patients during the first 7 days after traumatic brain injury. Control cerebrospinal fluid samples were obtained from lumbar puncture on 21 infants and children without traumatic brain injury or meningitis. Adenosine concentration was measured by using high-pressure liquid chromatography. Adenosine concentration was increased markedly in cerebrospinal fluid of children after traumatic brain injury vs. controls (p < .001). The increase in cerebrospinal fluid adenosine was independently associated with Glasgow Coma Scale < or = 4 vs. > 4 and time after injury (both p < .005). Cerebrospinal fluid adenosine concentration was not independently associated with either age (< or = 4 vs. > 4 yrs), mechanism of injury (abuse vs. other), or Glasgow Outcome Score (good/moderately disabled vs. severely disabled, vegetative, or dead). Of the 27 patients studied, 18 had cerebrospinal fluid glutamate concentration previously quantified by high-pressure liquid chromatography. There was a strong association between increases in cerebrospinal fluid adenosine and glutamate concentrations (p < .005) after injury.

CONCLUSIONS

Cerebrospinal fluid adenosine concentration is increased in a time- and severity-dependent manner in infants and children after severe head injury. The association between cerebrospinal fluid adenosine and glutamate concentrations may reflect an endogenous attempt at neuroprotection against excitotoxicity after severe traumatic brain injury.

Adenosine A(2a) receptor antagonists: potential therapeutic and neuroprotective effects in Parkinson's disease

The most effective treatment of Parkinson's disease (PD) is, at present, the dopamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA), however a number of disadvantages such as a loss of drug efficacy and severe side-effects (psychoses, dyskinesias and on-off phenomena) limit long-term effective utilisation of this drug. Recent experimental studies in which selective antagonists of adenosine A(2A) receptors were used, have shown an improvement in motor disabilities in animal models of PD. The A(2A) antagonist [7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-(4,3-e)-1,2,4-triazolo(1,5-c) pyrimidine] (SCH 58261) potentiated the contralateral turning behavior induced by a threshold dose of L-DOPA or direct dopamine receptor agonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, an effect accompanied by an increase in Fos-like-immunoreactivity in neurons of the lesioned striatum. Likewise, other A(2A) receptor antagonists such as (3,7-dimethyl-1-propargylxanthine) (DMPX), [E-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine] (KF 17837) and [E-1,3-diethyl-8(3,4-dimethoxystyryl-7-methyl-3,7-dihydro-1H-purine-2,6-dione] (KW 6002) antagonized catalepsy induced by haloperidol or reserpine in the rat, whereas in non-human primate models of PD, KW 6002 reduced the rigidity and improved the disability score of MPTP-treated marmosets and cynomolgus monkeys. Moreover, in contrast to L-DOPA, selective A(2A) receptor antagonists administered chronically did not produce dyskinesias and did not evoke tolerance in 6-OHDA and MPTP models of PD. An additional therapeutic potential of adenosine A(2A) antagonists emerged from studies showing neuroprotective properties of these compounds in animal models of cerebral ischemia and excitotoxicity, as well as in the MPTP model of PD. Adenosine A(2A) receptor antagonists by reversing motor impairments in animal models of PD and by contrasting cell degeneration are some of the most promising compounds for the treatment of PD.

Selective adenosine A2A receptor antagonists

In the early 1990s it became clear that the A2A adenosine receptor had characteristics that made it distinct from the other A1, A2B and A3 adenosine receptors. Great progress has been made with the discovery of selective A2A receptor antagonists. A variety of synthetic substitutions on the xanthine moiety led the chemists of Kyowa-Hakko to discover that introduction of the styryl group in the 8 position of xanthines was critical in achieving compounds endowed with selective A2A receptor antagonistic properties. One compound, KW 6002, (E)1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methylxanthine, is currently being developed for treatment of Parkinson's disease. A number of non-xanthine heterocycles have also been synthesized starting from the non-selective adenosine antagonist CGS 15943, a triazoloquinazoline. Thus, replacement of the phenyl ring of CGS 15943 with a heterocyclic ring such as pyrazole or imidazole, led to a series of interesting compounds whose prototype, SCH 58261, 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine, has become a reference A2A receptor antagonist. Modification of N7 substituents has progressed to optimize A2A receptor selectivity and pharmacokinetic characteristics. A related class of compounds having a bicyclic instead of the tricyclic ring structure is also of interest. The prototype of these triazintriazolo derivatives, ZM 241385, is a potent A2A receptor antagonist; however, it also shows interactions with A2B receptors. The relevance of the A2A receptors in specific disease states, especially in the central nervous system, makes this class of adenosine receptor blockers of interest for treatment of neurodegenerative disorders such as Parkinson's disease.

Production of leukotrienes in a model of focal cerebral ischaemia in the rat

1. The aim of this work was to evaluate the role of leukotrienes in brain damage in vivo in a model of focal cerebral ischaemia in the rat, obtained by permanent occlusion of middle cerebral artery. 2. A significant (P < 0.01) elevation of LTC(4), LTD(4) and LTE(4) (cysteinyl-leukotrienes) levels occurred 4 h after ischaemia induction in the ipsilateral cortices of ischaemic compared to sham-operated animals (3998 +/- 475 and 897 +/- 170 fmol g(-1) tissue, respectively, P < 0.01). 3. The NMDA receptor antagonist MK-801 and the adenosine A(2A) receptor antagonist SCH 58261 were administered in vivo at doses known to reduce infarct size and compared with the leukotriene biosynthesis inhibitor MK-886. 4. MK-886 (0.3 and 2 mg kg(-1) i.v.) and MK-801 (3 mg kg(-1) i.p.) decreased cysteinyl-leukotriene levels (-78%, P < 0.05; -100%, P < 0.01; -92%, P < 0.01, respectively) 4 h after permanent occlusion of the middle cerebral artery, whereas SCH 58261 (0.01 mg kg(-1) i.v.) had no significant effects. 5. MK-886 (2 mg kg(-1) i.v.) was also able to significantly reduce the cortical infarct size by 30% (P < 0.05). 6. We conclude that cysteinyl-leukotriene formation is associated with NMDA receptor activation, and that it represents a neurotoxic event, the inhibition of which is able to reduce brain infarct area in a focal ischaemic event.

SCH 58261 (an adenosine A(2A) receptor antagonist) reduces, only at low doses, K(+)-evoked glutamate release in the striatum

The aim of the present work was to determine whether systemic administration of the adenosine A(2A) receptor antagonist, SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4,triazolo[1,5-c]pyrimidine), could modulate striatal glutamate outflow in the rat. Microdialysis experiments were performed in male Wistar rats implanted with microdialysis probes in the striatum. Pretreatment (15 min before) with SCH 58261 (0.01 and 0.1, but not 1 mg/kg intraperitoneally) significantly prevented K(+)-stimulated glutamate release. These results suggest that SCH 58261 could possess neuroprotective effects in the low dose range, while, at higher doses, the occurrence of additional mechanisms may limit the neuroprotective potential of this drug.

Adenosine extracellular brain concentrations and role of A2A receptors in ischemia

Various experimental approaches have been used to determine the concentration of adenosine in extracellular brain fluid. The cortical cup technique or the microdialysis technique, when adenosine concentrations are evaluated 24 hours after implantation of the microdialysis probe, are able to measure adenosine in the nM range under normoxic conditions and in the microM range under ischemia. In vitro estimation of adenosine show that it can reach 30 microM at the receptor level during ischemia, a concentration able to stimulate all adenosine receptor subtypes so far identified. Although the protective role of A1 receptors in ischemia seems consistent, the protective role of A2A receptors appears to be controversial. Both A2A agonists and antagonists have been shown to be neuroprotective in various in vivo ischemia models. Although A2A agonists may be protective, mainly through peripherally mediated effects, A2A antagonists may be protective through local brain mediated effects. It is possible that A2A receptors are tonically activated following a prolonged increase of adenosine concentration, such as occurs during ischemia. A2A receptor activation desensitizes A1 receptors and reduces A1 mediated effects. Under these conditions A2A receptor antagonists may be protective by potentiating all the neuroprotective A1 mediated effects, including decreased neurotoxicity due to reduced ischemia induced glutamate outflow.

Absence of the adenosine A(2A) receptor or its chronic blockade decrease ethanol withdrawal-induced seizures in mice

Considering the existing interactions between ethanol and adenosine, the influence of the genetic impairment of the adenosine A(2A) receptor has been examined upon the seizures occurring at the cessation of chronic ethanol intake or 'ethanol withdrawal' in male mice. Acute clearance of ethanol did not differ between adenosine A(2A) receptor knockout and wild-type mice. Mice were exposed for 10 days to a diet consisting of a milky chocolate drink that contained increasing concentrations (1.8, 3.6 and 6.3% v/v) of ethanol. Adenosine A(2A) receptor knockout mice ingested similar amounts of the fluid, either containing alcohol or not, as did the controls. The severity of handling-induced convulsions during withdrawal was significantly reduced in the adenosine A(2A) receptor knockout mice as compared with their wild-type controls. The selective adenosine A(2A) receptor antagonist ZM 241385 (20 mg/kg) also significantly attenuated the intensity of withdrawal-induced seizures occurring in wild-type male mice when intraperitoneally administered twice daily during the last 5 days of the forced alcohol intake. These results suggest that selective adenosine A(2A) receptor antagonists may be useful in the treatment of alcohol withdrawal.

Adenosine and neurotrauma: therapeutic perspectives

Cerebral ischemia studies demonstrating that stimulation of adenosine A1 receptors by either endogenously released adenosine or the administration of selective receptor agonists causes significant reductions in the morbidity and mortality associated with focal or global brain ischemias have triggered interest in the potential of purinergic therapies for the treatment of traumatic injuries to the brain and spinal cord. Preliminary findings indicate that activation of A1 adenosine receptors can ameliorate trauma-induced death of central neurons. Other avenues of approach include the administration of agents which elevate local concentrations of adenosine at injury sites by inhibiting its metabolism to inosine by adenosine deaminase, rephosphorylation to adenosine triphosphate by adenosine kinase; or re-uptake into adjacent cells. Amplification of the levels of endogenously released adenosine in such a 'site and event specific' fashion has the advantage of largely restricting the effect of such inhibitors to areas of injury-induced adenosine release. Another approach involving purinergic therapy has been applied to the problem of respiratory paralysis following high spinal cord injuries. In this instance, the adenosine antagonist theophylline has been used to enhance residual synaptic drive to spinal respiratory neurons by blocking adenosine A1 receptors. Theophylline induced, and maintained, hemidiaphragmatic recovery for prolonged periods after C2 spinal cord hemisection in rats and may prove to be beneficial in assisting respiration in spinal cord injury patients.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

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.

Lack of tolerance to motor stimulant effects of a selective adenosine A(2A) receptor antagonist

It is well known that tolerance develops to the actions of caffeine, which acts as an antagonist on adenosine A(1) and A(2A) receptors. Since selective adenosine A(2A) antagonists have been proposed as adjuncts to 3,4-dihydroxyphenylalanine (L-DOPA) therapy in Parkinson's disease we wanted to examine if tolerance also develops to the selective A(2A) receptor antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2, 4-triazolo [1,5-c]pyrimidine (SCH 58261). SCH 58261 (0.1 and 7.5 mg/kg) increased basal locomotion and the motor stimulation afforded by apomorphine. Neither effect was subject to tolerance following long-term treatment with the same doses given intraperitoneally twice daily. There were no adaptive changes in A(1) and A(2A) adenosine receptors or their corresponding messenger RNA or in dopamine D(1) or D(2) receptors. These results demonstrate that the tolerance that develops to caffeine is not secondary to its inhibition of adenosine A(2A) receptors. The results also offer hope that long-term treatment with an adenosine A(2A) receptor antagonist may be possible in man.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

SCH 58261 and ZM 241385 differentially prevent the motor effects of CGS 21680 in mice: evidence for a functional 'atypical' adenosine A(2A) receptor

The acute motor effects elicited by drugs acting upon adenosine A(2A) receptors, namely the highly selective agonist CGS 21680 or the antagonists SCH 58261 and ZM 241385, were investigated in mice. CGS 21680 dose-dependently (0.1-2.5 mg/kg i.p.) decreased horizontal and vertical motor activities. The depressant effect of CGS 21680 (0. 5 mg/kg i.p.) was maintained in mice pretreated by the adenosine receptor antagonist 8-(p-sulfophenyl)-theophylline (10-30 mg/kg i.p. ), which poorly penetrates the blood-brain barrier, but was completely lost in adenosine A(2A) receptor knockout mice. Thus, the adenosine A(2A) receptor is critically involved in motor activity. SCH 58261 (1-10 mg/kg i.p.) increased locomotion and rearing with a quick onset, but for a shorter period in mice habituated to the environment than in mice unfamiliar to it. ZM 241385 (7.5-60 mg/kg i. p.) stimulated horizontal and vertical activities with a slow onset at the two highest tested doses, similarly in naive and in habituated mice. The increase in locomotion elicited by ZM 241385 (15-30 mg/kg i.p. and 10-20 nM i.c.v.) was retained in mice treated by CGS 21680 (0.5 mg/kg i.p.) but that elicited by SCH 58261 (1-3-10 mg/kg i.p. and 10-20 nM i.c.v.) partially subsided. In conclusion, both 'striatal-like'/'SCH 58261-sensitive' adenosine A(2A) receptors and 'ZM 241385-sensitive'/'atypical' CGS 21680 binding sites may mediate CGS 21680-induced motor effects. Moreover, our results suggest that 'atypical' CGS 21680 binding sites could be adenosine A(2A) receptors with a peculiar pharmacological profile.

Brain adenosine receptors as targets for therapeutic intervention in neurodegenerative diseases

Adenosine acts as a neurotransmitter in the brain through the activation of four specific G-protein-coupled receptors (the A1, A2A, A2B, and A3 receptors). The A1 receptor has long been known to mediate neuroprotection, mostly by blockade of Ca2+ influx, which results in inhibition of glutamate release and reduction of its excitatory effects at a postsynaptic level. However, the development of selective A1 receptor agonists as antiischemic agents has been hampered by their major cardiovascular side effects. More recently, apparently deleterious effects have been reported following the activation of other adenosine receptor subtypes, namely, the A2A and the A3 receptors. In particular, selective A2A receptor antagonists have been demonstrated to markedly reduce cell death associated with brain ischemia in the rat, suggesting that the cerebral A2A receptor may indeed contribute to the development of ischemic damage. The beneficial effects evoked by A2A antagonists may be due to blockade of presynaptic A2A receptors (which are stimulatory on glutamate release) and/or to inhibition of A2A receptor-mediated activation of microglial cells. Even more puzzling data have been reported for the A3 receptor subtype, which can indeed mediate both cell protection and cell death, simply depending upon the degree of receptor activation and/or specific pathophysiological conditions. In particular, a mild subthreshold activation of this receptor has been associated with a reinforcement of the cytoskeleton and reduction of spontaneous apoptosis, which may play a role in "ischemic preconditioning" of the brain, according to which a short ischemic period may protect the brain from a subsequent, sustained ischemic insult that would be lethal. In contrast, a robust and prolonged activation of the A3 receptor has been shown to trigger cell death by either necrosis or apoptosis. Such apparently opposing actions may be reconciled by hypothesizing that adenosine-mediated cell killing during ischemia may be aimed at isolating the most damaged areas to favor those parts of the brain that still retain a chance for functional recovery. In fact, both A3 receptor-mediated cell death and A2A receptor-mediated actions may be viewed as an attempt to selectively kill irreversibly damaged cells in the "core" ischemic area, in order to save space and energy for the surrounding live cells in the "pneumbra" area. Hence, the pharmacological modulation of the A2A and A3 receptors via selective ligands may represent a novel strategy in the therapeutic approach to pathologies characterized by acute or chronic neurodegenerative events.