Adenosine and glutamate modulate each other's release from rat hippocampal synaptosomes

Central adenosine A1 receptors inhibit cough via suppression of excitatory glutamatergic and tachykininergic neurotransmission

BACKGROUND AND PURPOSE

The adenosine A1 receptor is reported to mediate several excitatory effects in the airways and has inhibitory effects in the CNS. In this study, we investigated the role of peripheral and central A1 receptors in regulating cough and airway obstruction.

EXPERIMENTAL APPROACH

Drugs were administered to guinea pigs via inhalation or i.c.v. infusion. Following the administration of different drugs, cough was induced by exposing guinea pigs to aerosolized 0.4 M citric acid. An automated analyser recorded both cough and airway obstruction simultaneously using whole-body plethysmography.

KEY RESULTS

The A1 receptor agonist, cyclopentyladenosine (CPA, administered by inhalation), dose-dependently inhibited cough and also inhibited airway obstruction. Similarly, CPA, administered i.c.v., inhibited both the citric acid-induced cough and airway obstruction; this was prevented by pretreatment with the A1 receptor antagonist DPCPX (i.c.v.). Treatment with DPCPX alone dose-dependently enhanced the citric acid-induced cough and airway obstruction. This effect was reversed following treatment with either the glutamate GluN1 receptor antagonist D-AP5 or the neurokinin NK1 receptor antagonist FK-888.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that activation of either peripheral or central adenosine A1 receptors inhibits citric acid-induced cough and airway obstruction. The data also suggest that tonic activation of central adenosine A1 receptors serves as a negative regulator of cough and airway obstruction, secondary to inhibition of excitatory glutamatergic and tachykininergic neurotransmission.

Neuromodulation and metamodulation by adenosine: Impact and subtleties upon synaptic plasticity regulation

Synaptic plasticity mechanisms, i.e. the sequence of events that underlies persistent changes in synaptic strength as a consequence of transient alteration in neuronal firing, are greatly influenced by the 'chemical atmosphere' of the synapses, that is to say by the presence of molecules at the synaptic cleft able to fine-tune the activity of other molecules more directly related to plasticity. One of those fine tuners is adenosine, known for a long time as an ubiquitous neuromodulator and metamodulator and recognized early as influencing synaptic plasticity. In this review we will refer to the mechanisms that adenosine can use to affect plasticity, emphasizing aspects of the neurobiology of adenosine relevant to its ability to control synaptic functioning. This article is part of a Special Issue entitled Brain and Memory.

NMDA preconditioning attenuates cortical and hippocampal seizures induced by intracerebroventricular quinolinic acid infusion

Searching for new therapeutic strategies through modulation of glutamatergic transmission using effective neuroprotective agents is essential. Glutamatergic excitotoxicity is a common factor to neurodegenerative diseases and acute events such as cerebral ischemia, traumatic brain injury, and epilepsy. This study aimed to evaluate behavioral and electroencephalographic (EEG) responses of mice cerebral cortex and hippocampus to subconvulsant and convulsant application of NMDA and quinolinic acid (QA), respectively. Moreover, it aimed to evaluate if EEG responses may be related to the neuroprotective effects of NMDA. Mice were preconditioned with NMDA (75 mg/kg, i.p.) and EEG recordings were performed for 30 min. One day later, QA was injected (36.8 nmol/site) and EEG recordings were performed during 10 min. EEG analysis demonstrated NMDA preconditioning promotes spike-wave discharges (SWDs), but it does not display behavioral manifestation of seizures. Animals that were protected by NMDA preconditioning against QA-induced behavioral seizures, presented higher number of SWD after NMDA administration, in comparison to animals preconditioned with NMDA that did display behavioral seizures after QA infusion. No differences were observed in latency for the first seizure or duration of seizures. EEG recordings after QA infusion demonstrated there were no differences in the number of SWD, latency for the first seizure or duration of seizures in animals pretreated with saline or in animals preconditioned by NMDA that received QA. A negative correlation was identified between the number of NMDA-induced SWD and QA-induced seizures severity. These results suggest a higher activation during NMDA preconditioning diminishes mice probability to display behavioral seizures after QA infusion.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Effect of the L- or D-aspartate on ecto-5'nucleotidase activity and on cellular viability in cultured neurons: participation of the adenosine A(2A) receptors

Glutamate increases the extracellular adenosine levels, an important endogenous neuromodulator. The neurotoxicity induced by glutamate increases the ecto-5'-nucleotidase activity in neurons, which produces adenosine from AMP. L- and D-aspartate (Asp) mimic most of the actions of glutamate in the N-methyl-D-aspartate (NMDA) receptors. In the present study, both amino acids stimulated the ecto-5'-nucleotidase activity in cerebellar granule cells. MK-801 and AP-5 prevented the L- and D-Asp-evoked activation of ecto-5'-nucleotidase. Both NMDA receptor antagonists prevented completely the damage induced by L-Asp, but partially the D-Asp-induced damage. The antagonist of adenosine A(2A) receptors (ZM 241385) prevented totally the L- Asp-induced cellular death, but partially the neurotoxicity induced by D-Asp and the antagonist of adenosine A(1) receptors (CPT) had no effect. The results indicated a different involvement of NMDA receptors on the L- or D-Asp-evoked activation of ecto-5'-nucleotidase and on cellular damage. The adenosine formed from ecto-5'-nucleotidase stimulation preferentially acted on adenosine A(2A) receptor which is probably co-operating with the neurotoxicity induced by amino acids.

Effect of allopurinol on brain adenosine levels during hypoxia in newborn piglets

Adenosine, a purine nucleoside, is a potent inhibitory neuromodulator in the brain which may provide an important endogenous neuroprotective role during hypoxia-ischemia. Allopurinol, a xanthine oxidase inhibitor, blocks purine degradation and may result in the accumulation of purine metabolites, including adenosine, during hypoxia. The present study determines the effect of allopurinol administration prior to hypoxia on brain levels of adenosine and purine metabolites in the newborn piglet. Twenty-two newborn piglets (age 3-7 days) were studied: 5 untreated normoxic and 6 allopurinol-treated normoxic controls were compared to 5 untreated hypoxic and 6 allopurinol-treated hypoxic animals. Brain tissue energy metabolism was continuously monitored during hypoxia by (31)P NMR spectroscopy. Brain tissue levels of purines increased in both hypoxic groups during hypoxia, however, there were significantly higher increases in brain tissue levels of adenosine (66.5 +/- 30.5 vs. 19.4 +/- 10.7 nmol/gm), P < 0.01 and inosine (265 +/- 97.6 vs. 162.8 +/- 38.3 nmol/gm), P = 0.05 in the allopurinol-treated hypoxic group. Allopurinol inhibits purine degradation under severe hypoxic conditions and results in a significant increase in brain tissue levels of adenosine and inosine. The increased accumulation of CNS adenosine during hypoxia which is seen in the allopurinol-treated animals may potentiate adenosine's intrinsic neuroprotective mechanisms.

Adenosine mediation of presynaptic feedback inhibition of glutamate release

Conditions of increased metabolic demand relative to metabolite availability are associated with increased extracellular adenosine in CNS tissue. Synaptic activation of postsynaptic NMDA receptors on neurons of the cholinergic brainstem arousal center can increase sufficient extracellular adenosine to act on presynaptic A1 adenosine receptors (A1ADRs) of glutamate terminals, reducing release from the readily releasable pool. The time course of the adenosine response to an increase in glutamate release is slow (tau > 10 min), consistent with the role of adenosine as a fatigue factor that inhibits the activity of cholinergic arousal centers to reduce arousal.

Adenosine receptors co-operate with NMDA preconditioning to protect cerebellar granule cells against glutamate neurotoxicity

N-Methyl-D-aspartate (NMDA) preconditioning is evoked by subtoxic concentrations of NMDA (50 microM), which has been shown previously to lead to transient resistance to subsequent lethal dose of glutamate or NMDA in cultured neurons. The purpose of this study was to investigate the participation of adenosine A1 and A2A receptors on NMDA preconditioning against glutamate-induced cellular damage in cerebellar granule cells. NMDA preconditioning prevented the stimulatory effect induced by glutamate on AMP hydrolysis, but not on ADP hydrolysis. The neuroprotection evoked by NMDA preconditioning against glutamate-induced cellular damage was prevented by the presence of adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT, 100 nM), but not by the adenosine A2A receptors antagonist, (4-(2[7-amino-2-(2-furyl {1,2,4}-triazolo{2,3-a{1,3,5}triazian-5-yl-aminoethyl)phenol (ZM 241385, 50 nM). Interestingly, a long-term treatment with CPT or ZM 241385 alone protected cells against glutamate-induced neurotoxicity. Moreover, the functionality of adenosine A1 receptor was not affected by NMDA preconditioning, but this treatment promoted adenosine A2A receptor desensitization, measured by cAMP accumulation. Taken together, the results described herein suggest that the neuroprotection evoked by NMDA preconditioning against cellular damage elicited by glutamate occurs through mechanisms involving adenosine A2A receptors desensitization co-operating with adenosine A1 receptors activation in cerebellar granule cells.

Effect of the postictal state on visual-spatial memory in immature rats

Postictal cognitive impairment following seizures can last from minutes to days and be disabling to the patient. The purpose of this study was to compare the behavioral features of seizures with postictal memory impairment in young seizure-naive rats and rats with a prior history of status epilepticus (SE) and examine the relationship between postictal EEG changes and cognitive recovery. Following training in the water maze to asymptote levels of learning, rats with a prior history of SE and seizure-naive rats had flurothyl-induced generalized seizures and time to recovery to baseline was then measured. Following generalized seizures rats had impaired performance in the water maze with the duration of the cognitive deficits exceeding the length of the seizure. There was not a close relationship between duration of cognitive impairment and either latency to onset of seizure or duration. The animal's neurological status was a factor in the duration of cognitive impairment following seizures; while there were no differences between SE and seizure-naive rats in latency to seizure onset or duration of the seizures, animals with a prior history of SE had a longer period of impairment following a seizure than animals without such a history. Postictal cognitive impairment was associated with changes in theta activity in animals with a prior history of SEs but not in seizure-naive animals. Caffeine, when administered following the seizure, reduced postictal cognitive impairment in a dose-dependent manner. This study demonstrates that duration of postictal cognitive impairment is not directly related to duration of the seizure. The neurological status of the animal is a determining factor in duration of postictal impairment.

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.

Modulatory role of adenosine receptors in insect motor nerve terminals

The effects of adenosine and ATP were studied on blowfly larvae Calliphora vicina neuromuscular preparation. Adenosine diminished (IC50 = 40 +/- 3 microM) the amplitude of nerve-evoked postsynaptic currents (EPSCs) and slightly decreased the frequency of spontaneous currents without affecting their amplitude. EPSCs were slightly reduced by ATP, and this effect was prevented by concanavalin A. Presynaptic inhibition by adenosine was temperature-dependent and insensitive to pertussis toxin. A1 agonists of vertebrate adenosine receptor CPA and NECA failed to reproduce the effect of adenosine, and 2-CADO enhanced the EPSCs. A1 antagonist DPCPX competitively inhibited adenosine action. A2 agonist DPMA potentiated EPSCs, and its effect was abolished by A2 antagonist DMPX. Adenosine and ATP failed to affect the nonquantal release of glutamate. The results show for the first time the presence of presynaptic adenosine receptors regulating transmitter release at insect motor nerve terminals and point to differences in pharmacological properties of adenosine receptor subtypes in insects and vertebrates.

Coexistence and function of different neurotransmitter transporters in the plasma membrane of CNS neurons

Transporters able to recapture released neurotransmitters into neurons can no longer be considered as cell-specific neuronal markers. In fact, colocalization on one nerve terminal of transporters able to selectively recapture the released endogenously synthesized transmitter (homotransporters) and of transporters that can selectively take up transmitters/modulators originating from neighboring structures (heterotransporters) has been demonstrated to occur on several families of nerve terminals. Activation of heterotransporters often increases the release of the transmitter stored in the terminals on which the heterotransporters are localized. The release caused by heterotransporter activation takes place through multiple mechanisms including exocytosis, either dependent on external Ca(2+) or on Ca(2+) mobilized from intraterminal stores, and homotransporter reversal. Homocarrier-mediated release elicited by heterocarrier activation represents a clear case of transporter-transporter interaction. Although the functional significance of transporter coexpression on one nerve terminal remains to be established, it may in some instances reflect cotransmission. In other cases, heterotransporters may mediate modulation of basal transmitter release in addition to the modulation of the evoked release brought about by presynaptic heteroreceptors. Heterotransporters are also increasingly reported to exist on neuronal soma/dendrites. With the exception of EAAT4, the glutamate transporter/chloride channel situated on GABAergic Purkinje cells in the cerebellum, the functions of somatodendritic heterocarriers is not understood.

Effect of chronic and acute stress on ectonucleotidase activities in spinal cord

We have previously observed that, while acute stress induces analgesia, chronic stress causes a hyperalgesic response in male rats. No effect was observed in females. There is increasing evidence that both ATP and adenosine can modulate pain. Extracellular ATP and ADP are hydrolyzed by an apyrase in synaptosomes from the peripheral and central nervous systems. In the present study, we investigated the effect of chronic and acute stress on ATPase-ADPase and 5'-nucleotidase activities in spinal cord of male and female rats. Adult male and female Wistar rats were submitted to 1 h restraint stress/day for 1 day (acute) or 40 days (chronic) and were sacrificed 24 h later. ATPase-ADPase activities were assayed in the synaptosomal fraction obtained from the spinal cord of control and stressed animals. ADP hydrolysis was decreased 25% in chronically stressed males, while no change was observed on ATPase activity. There was an increase in the 5'-nucleotidase activity in the same group. No effect on ADPase, ATPase or on 5'-nucleotidase activity was observed in females with chronic stress, or after acute stress neither in males or females. Chronic stress reduced ADP hydrolysis and increased 5'-nucleotidase activity in the spinal cord in male rats.

In vivo effects of adenosine A1 receptor agonist and antagonist on neuronal and astrocytic intermediary metabolism studied with ex vivo 13C NMR spectroscopy

Adenosine is a neuromodulator, and it has been suggested that cerebral acetate metabolism induces adenosine formation. In the present study the effects that acetate has on cerebral intermediary metabolism, compared with those of glucose, were studied using the adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Fasted rats received an intravenous injection of CCPA, DPCPX, or vehicle. Fifteen minutes later either [1,2-13C]acetate or [1-13C]glucose was given intraperitoneally; after another 30 min the rats were decapitated. Cortical extracts were analyzed with 13C NMR spectroscopy and HPLC analysis. DPCPX affected neuronal and astrocytic metabolism. De novo synthesis of GABA from neuronal and astrocytic precursors was significantly reduced. De novo syntheses of glutamate and aspartate were at control levels, but their degradation was significantly elevated. In glutamine the anaplerotic activity and the amount of label in the position representing the second turn in the tricarboxylic acid cycle were significantly increased, suggesting elevated metabolic activity in astrocytes. CCPA did not influence GABA, aspartate, or glutamine synthesis. In glutamate the contribution from the astrocytic anaplerotic pathway was significantly decreased. In the present study the findings in the [1,2-13C]acetate and [1-13C]glucose control, CCPA, and DPCPX groups were complementary, and no adenosine A1 agonist effects arising from cerebral acetate metabolism were detected.

Hemodynamic and metabolic responses to moderate asphyxia in brain and skeletal muscle of late-gestation fetal sheep

The purpose of this study was to investigate metabolic and hemodynamic responses in two fetal tissues, hindlimb muscle and brain, to an episode of acute moderate asphyxia. Near-infrared spectroscopy was used to measure changes in total hemoglobin concentration ([tHb]) and the redox state of cytochrome oxidase (COX) simultaneously in the brain and hindlimb of near-term unanesthetized fetal sheep in utero. Oxygen delivery (DO(2)) to, and consumption (VO(2)) by, each tissue was derived from the arteriovenous difference in oxygen content and blood flow, measured by implanted flow probes. One hour of moderate asphyxia (n = 11), caused by occlusion of the maternal common internal iliac artery, led to a significant fall in DO(2) to both tissues and to a significant drop in VO(2) by the head. This was associated with an initial fall in redox state COX in the leg but an increase in the brain. [tHb], and therefore blood volume, fell in the leg and increased in the brain. These data suggest the presence of a fetal metabolic response to hypoxia, which, in the brain, occurs rapidly and could be neuroprotective.

Pharmacological characterization of adenosine A1 receptors and its functional role in brown trout (Salmo trutta) brain

The adenosine receptor agonist N(6)-cyclohexyl[(3)H]adenosine ([(3)H]CHA) was used to identify and pharmacologically characterize adenosine A1 receptors in brown trout (Salmo trutta) brain. In membranes prepared from trout whole brain, the A1 receptor agonist [(3)H]CHA bound saturably, reversibly and with high affinity (K(d)=0. 69+/-0.04 nM; B(max)=0.624+/-0.012 pmol/mg protein) to a single class of binding sites. In equilibrium competition experiments, the adenosine agonists and antagonists all displaced [(3)H]CHA from high-affinity binding sites with the rank order of potency characteristic for an adenosine A1 receptors. A1 receptor density appeared not age-related (from 3 months until 4 years), and was similar in different brain areas. The specific binding was inhibited by guanosine 5'-triphosphate (IC(50)=0.778+/-0.067 microM). GTP (5 microM) induced a low affinity state of A1 receptors. In superfused trout cerebral synaptosomes, 30 mM K(+) stimulated the release of glutamate in a calcium dependent manner. Glutamate-evoked release was dose-dependently reduced by CHA, and the inhibition was reversed by the A1 antagonist 8-cyclopentyltheophylline (CPT). In the same synaptosomal preparation, 30 mM K(+) as well as 1 mM glutamate stimulated the release of adenosine in a Ca(2+)-independent manner and tetrodotoxin insensitive. These findings show that in trout brain adenosine A1 receptors are present which are involved in the modulation of glutamate transmitter release. Moreover, the stimulation of adenosine release by K(+) depolarisation or glutamate support the hypothesis that, as in mammalian brain, a cross-talk between adenosine and glutamate systems exists also in trout brain.

Protection against hippocampal kainate excitotoxicity by intracerebral administration of an adenosine A2A receptor antagonist

We have previously shown that the peripheral administration of an A2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680) protected the hippocampus against kainate-induced excitotoxicity. The present study utilises the intrahippocampal route to further investigate CGS 21680-mediated protection as well as examining the role of adenosine and both A1 and A2A receptors in kainate-induced excitotoxicity. Injections were made directly into the hippocampus of anaesthetised male Wistar rats. Following surgery and the administration of 0.25 nmol kainate in 1 microl of solution, the animals were left to recover for seven days before perfusion and brain slicing. Haematoxylin and eosin staining revealed substantial damage to the CA3 region. Co-administration of the A2A receptor agonist CGS 21680 over a range of doses did not protect the region to any degree. Similarly neither the A1 receptor agonist R-phenylisopropyladenosine (R-PIA), nor adenosine itself reduced kainate-induced damage. The intrahippocampal injection of the selective A2A receptor antagonist, 4-(2-[7-amino-2-¿2-furyl¿¿1,2, 4¿triazolo¿2,3-a¿¿1,3,5¿triazin-5-yl-amino]ethyl)phenol (ZM241385) however, significantly decreased kainate damage to the CA3 region. These results show that adenosine A2A receptor-induced protection is most likely to be mediated peripherally and is probably not due to activation of A2A receptors within the hippocampus. The lack of protection observed with either R-PIA or adenosine may be due to an inhibitory action of the A2A receptor on the neuroprotective A1 receptor. Importantly, this study also questions the role of endogenously released adenosine in protecting the hippocampus from excitotoxic damage.

Protection against kainate-induced excitotoxicity by adenosine A2A receptor agonists and antagonists

The neuroprotective role of adenosine receptor agonists in various models of ischaemia and neuronal excitotoxicity has been attributed to adenosine A1 receptor activation. In this study we examine the role of the A2A receptor in the kainate model of excitotoxicity. Kainate (10 mg/kg) was administered systemically 10 min after the intraperitoneal injection of adenosine analogues. The A2A agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS21680) protected the hippocampus at concentrations of 0.1 and 0.01 mg/kg, but not at 2 microg/kg. The addition of the centrally acting adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine partially reduced protection only in the CA3a region, suggesting that only a small proportion of the protection was attributable to the A1 receptor. A less potent A2A agonist, N6-[2-(3,5-dimethyoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (1 mg/kg), provided only partial protection against kainate. 4-(2-[7-Amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl -amino]ethyl)phenol, a selective A2A antagonist, also showed protection against kainate-induced neuronal death, when administered alone or in combination with CGS21680. These results show that adenosine A2A receptor activation is protective against excitotoxicity. The protection is largely independent of A, receptor activation or blockade.

Evidence for physiologically active axonal adenosine receptors in the rat corpus callosum

Several neurotransmitter receptors have been identified on axons, and emerging evidence suggests that central axonal conduction may be modulated by neurotransmitters. We have recently demonstrated the presence of extra-synaptic adenosine Al receptors along rat hippocampal axons. We now present immunocytochemical evidence for Al receptors on rat corpus callosum axons and show that these receptors actively modulate axon physiology. Using rat brain coronal slices, we stimulated the corpus callosum and recorded the evoked extracellular compound action potential. The lipid-soluble, Al-specific adenosine receptor agonist cyclopentyladenosine, dose-dependently decreased the compound action potential amplitude, an effect reversed by the specific Al antagonist 8-cyclopentyl-1, 3-dipropylxanthine. These data provide the first direct evidence that axonal Al adenosine receptors modulate axon physiology in the adult mammalian brain. Influencing axonal transmission is a potentially powerful mechanism of altering information processing in the nervous system.

Inhibition of N-,P/Q- and other types of Ca2+ channels in rat hippocampal nerve terminals by the adenosine A1 receptor

The effects of the adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA), on both the increase in intracellular free Ca2+ concentration ([Ca2+]i) and on the release of endogenous glutamate in rat hippocampal synaptosomes were studied. The inhibitory effect of CPA on the increase in [Ca2+]i stimulated with 4-aminopyridine was neutralized by the adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The inhibitory effect of CPA was greater in synaptosomes from the CA1 subregion than in whole hippocampal synaptosomes. The inhibitory effects of both CPA and of the Ca2+ channel blockers, omega-conotoxin GVIA, omega-conotoxin MVIIC or omega-conotoxin GVIA plus omega-conotoxin MVIIC, were greater than those caused by the Ca2+ channel blockers. The release of endogenous glutamate was inhibited by 41% by CPA. The inhibition observed when CPA and omega-conotoxin GVIA or CPA and omega-conotoxin MVIIC were present was also greater than the inhibition by the Ca2+ channel blockers alone. The presence of both omega-conotoxin GVIA and omega-conotoxin MVIIC did not completely inhibit the release of glutamate, and CPA significantly enhanced this inhibition. The membrane potential and the accumulation of [3H]tetraphenylphosphonium of polarized or depolarized synaptosomes was not affected by CPA, suggesting that adenosine did not increase potassium conductances. The present results suggest that, in hippocampal glutamatergic nerve terminals, adenosine A1 receptor activation partly inhibits P/Q- and other non-identified types of Ca2+ channels.

Neurotoxicity in organotypic hippocampal slices mediated by adenosine analogues and nitric oxide

Adenosine (ADO) and nitric oxide (NO) have been implicated in a variety of neurophysiological actions, including induction of long-term potentiation, regulation of cerebral blood flow, and neurotoxicity/neuroprotection. ADO has been shown to promote NO release from astrocytes by a direct effect on A1 and A2 receptors, thus providing a link between actions of NO and adenosine in the brain. However, while adenosine acts as an endogenous neuroprotectant, NO is believed to be the effector of glutamate neurotoxicity. To resolve this apparent paradox, we have further investigated the effects of adenosine and NO on neuronal viability in cultured organotypic hippocampal slices exposed to sub-lethal (20') in vitro ischemia. Up to a concentration of 500 microM ADO did not cause toxicity while exposures to 100 microM of the stable ADO analogue chloroadenosine (CADO) caused widespread neuronal damage when paired to anoxia/hypoglycemia. CADO effects were significantly prevented by the ADO receptor antagonist theophylline and blockade of NO production by L-NA (100 microM). Moreover, CADO effects were mimicked by the NO donor SIN-1 (100 microM). Application of 100 microM ADO following blockade of adenosine deaminase (with 10 microM EHNA) replicated the effects of CADO. CADO, ADO + EHNA but not ADO alone caused a prolonged and sustained release of nitric oxide as measured by direct amperometric detection. We conclude that at high concentrations and/or following blockade of its enzymatic catabolism, ADO may cause neurotoxicity by triggering NO release from astrocytes. These results demonstrate for the first time that activation of pathways other than those involving neuronal glutamate receptors can trigger NO-mediated neuronal cell death in the hippocampus.

Adenosine A1 receptors are located predominantly on axons in the rat hippocampal formation

The nucleoside adenosine exerts potent biological effects via specific receptors, including the inhibitory A1 adenosine receptor (A1AR). In the hippocampus A1ARs play an important role in regulating neuronal activity. However, the cellular sites of hippocampal A1ARs are undefined. Using in situ hybridization, receptor autoradiography, and single- and double-label immunocytochemistry techniques, we have characterized the cellular sites of A1AR expression in the rat hippocampus. In situ hybridization and receptor autoradiography studies revealed strikingly different patterns of labeling. In situ hybridization studies revealed heaviest labeling of cell bodies in the granular layer of the dentate gyrus and the pyramidal layers of Ammon's horn. In contrast, using [3H]DPCPX, we observed heavy specific labeling over the neuropil in the dentate hilus stratum moleculare, stratum lacunosum-moleculare, stratum radiatum, and stratum oriens, and little labeling over cell bodies. Using single-label immunocytochemistry, A1AR immunoreactivity was found to be heaviest over fibers in regions corresponding with heavy [3H]DPCPX labeling. Double-label florescent confocal microscopy was then used to determine the identity of labeled fibers. A1AR immunoreactivity was found to co-localize with SMI-31 that labels axons, but not with MAP2a,b that labels cell bodies and dendrites, or with synaptophysin that labels synapses. These data identify axons as the predominant site of A1AR expression in hippocampus. Activation of A1ARs may be a powerful mechanism by which adenosine alters axonal transmission to inhibit neurotransmitter release.

Purinergic inhibition of neurotransmitter release in the central nervous system

Neurotransmitter release and the role of adenosine in its regulation has been investigated for more than twenty years, and it is now widely accepted that adenosine tonically inhibits the release of excitatory neurotransmitters. This effect of adenosine is operated by an A1 adenosine receptor. Since activation of this receptor could inhibit Ca2+ conductance, increase K+ conductance, inhibit adenylate cyclase or phospholipase C, it is not clear if there is only one mechanism or several mechanisms operated by adenosine to inhibit neurotransmitter release, and in that case, what is the relative importance of each mechanism. The mechanism by which adenosine inhibits evoked synchronous transmitter release might be different from that used by the nucleoside to inhibit spontaneous asynchronous release. In some systems adenosine triphosphate per se acts like adenosine and inhibits neurotransmitter release. However, in most cases the inhibitory effect of this adenine nucleotide depends upon its hydrolysis into adenosine by a cascade of ectoenzymes, the last step being mediated by ecto-5'-nucleotidase.

Adenosine modulates N-methyl-D-aspartate-stimulated hippocampal nitric oxide production in vivo

BACKGROUND AND PURPOSE

Adenosine acts presynaptically to inhibit release of excitatory amino acids (EAAs) and is thus considered to be neuroprotective. Because EAA-stimulated synthesis of nitric oxide (NO) may play an important role in long-term potentiation and excitotoxic-mediated injury, we tested the hypotheses that adenosine agonists attenuate basal and EAA-induced NO production in the hippocampus in vivo and that adenosine A1 receptors mediate this response.

METHODS

Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 hours with artificial cerebrospinal fluid containing 3 mumol/L [14C]L-arginine. Recovery of [14C]L-citrulline in the effluent was used as a marker of NO production. In 10 groups of rats, time-dependent increases in [14C]L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of N-methyl-D-aspartate (NMDA), adenosine agonists, adenosine antagonists, and the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME).

RESULTS

Recovery of [14C]L-citrulline during perfusion with artificial cerebrospinal fluid progressively increased to 141 +/- 27 fmol/min (+/- SEM) over 5 hours. Contralateral perfusion with 1 mmol/L NMDA augmented [14C]L-citrulline recovery to 317 +/- 62 fmol/min. Perfusion of 1 mmol/L L-NAME with NMDA inhibited [14C]L-citrulline recovery compared with NMDA alone. Perfusion with 0.1 mmol/L 2-chloroadenosine attenuated basal as well as NMDA-enhanced [14C]L-citrulline recovery. This action of 2-chloroadenosine was reversed by infusion of 0.1 mmol/L 8-cyclopentyl-1,3-dipropylxanthine, a specific A1 receptor antagonist. Infusion of 0.1 mmol/L (2S)-N6-[2-endo-norboryl]adenosine, a specific A1 receptor agonist, also attenuated the 0.1 mmol/L and 1 mmol/L NMDA-enhanced [14C]L-citrulline recovery.

CONCLUSIONS

Using an indirect method of assessing NO production in vivo, these data are consistent with in vitro results showing that NMDA receptor stimulation enhances NO production. Furthermore, we conclude that stimulation of A1 receptors can attenuate the basal as well as NMDA-induced production of NO. Because NMDA receptor stimulation amplifies glutamate release, our data are consistent with presynaptic A1 receptor-mediated inhibition of EAA release and consequent downregulation of NO production.

Co-existence and interaction between facilitatory and inhibitory metabotropic glutamate receptors and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals

We have investigated the interaction between facilitatory and inhibitory metabotropic glutamate receptors (mGluRs) and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals from young (3 weeks postnatal) rats. The adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) (1 microM) and the mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) (100 microM) inhibited Ca(2+)-dependent release of glutamate evoked by depolarization of synaptosomes with 30 mM KCl to 33 +/- 6 and 30 +/- 4% of control values, respectively. The CHA and L-AP4 inhibition of release was consistent with the reduction of a component of Ca2+ entry in nerve terminals which was also sensitive to omega-Aga-IVA. When the inhibitory agonists were co-applied at optimal concentrations, no additivity of the inhibitory effects on either glutamate release or [Ca2+]c was observed. The nerve terminals from young rats also exhibit the facilitatory pathway for glutamate release that is observed during 4-aminopyridine-evoked depolarization after stimulation of mGluRs with the agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) in the presence of arachidonic acid (AA). The addition of ACPD or AA alone did not alter the ability of CHA and L-AP4 to reduce the release, however the co-application of AA and ACPD abolished the inhibitory effect induced by CHA and L-AP4 whether alone or in combination. These results indicate the co-existence of the three modulatory pathways of glutamate release and the dominant role of the ACPD/AA activated facilitatory pathway in its interaction with the inhibitory pathways activated by L-AP4 and CHA.

GABAA receptor activation induces GABA and glutamate release from preoptic area

The effect of GABA receptor agonists on release in vitro of radiolabeled GABA and glutamate was studied using a crude preparation of isolated nerve terminals (neurosomes). GABA agonists were incubated (2 min, 37 degrees C) with neurosomes prepared from hypothalamus, preoptic area (POA) and frontal cortex tissues. Under these conditions, GABA and the GABAA receptor agonist muscimol, but not the GABAB receptor agonist baclofen, stimulated 3H-GABA and 3H-glutamate release from POA but not hypothalamic or cortical neurosomes of gonadally intact male rats. These effects were inhibited by the GABAA receptor antagonists picrotoxin, bicuculline and SR-95531. Significant efflux of 3H-glutamate could be elicited from cortical neurosomes following longer (5 min) incubations with 500 microM GABA and 400 microM muscimol. Muscimol-induced release of 3H-glutamate and 3H-GABA was dependent on extracellular calcium. Muscimol and GABA failed to release 3H-GABA or 3H-glutamate from POA neurosomes of ovariectomized female rats. However, administration of estradiol and progesterone to ovariectomized females prior to sacrifice caused the appearance of muscimol induced-release of amino acids from POA neurosomes comparable to that obtained in male rats. GABA-induced release of 3H-glutamate was similarly dependent on pretreatment of ovariectomized rats with ovarian steroids. GABAA receptor-induced release of amino acids is therefore brain region-specific and modified by hormonal status.

Opposite effects of cyclohexyladenosine and theophylline on hypoxic damage in cultured neurons

To study the central effects of adenosine on hypoxia, the influence of treatment by the A1 receptor agonist cyclohexyladenosine (1 microM) or by the antagonist theophylline (10 microM) was tested on cell damage in a model of neuronal culture. Whereas theophylline enhanced cell injury induced by 8 h hypoxia, cyclohexyladenosine decreased lactate dehydrogenase leakage, abolished the transient increase in 2-D-deoxyglucose transport and improved cell morphology. Such actions might involve regulation of excitatory amino acid release and maintenance of calcium homeostasis.

Excitatory effects of adenosine receptor agonists and antagonists on neurotransmission in guinea pig superior collicular slices

Adenosine has excitatory actions on neurotransmission in the superior colliculus. To investigate whether adenosine A1 or A2 receptors are involved in mediating these excitatory actions, the effect of A1 and A2 receptor agonists and antagonists on the evoked postsynaptic potentials (PSP) in the superficial grey layer were tested using slices of the superior colliculus. Application of both A1 agonists, such as CHA, R-PIA, and the A2 agonist, CGS-21680 increased the amplitude of the PSP. The increase in PSP amplitude occurred gradually over 20-30 min after application of these adenosine agonists. Application of the A1 antagonist 8-CPT, and the A2 antagonists, DMPX and CGS-15943, increased the amplitude of the PSP and could not antagonize the excitatory effect of adenosine. These results suggest that the mechanism of the excitatory effect of adenosine cannot be explained by the classical concept of A1 and A2 adenosine receptor subtypes which were identified by their effect on adenylate cyclase activity.

In vivo regulation of extracellular adenosine levels in the cerebral cortex by NMDA and muscarinic receptors

The adenosine concentration in samples of perfusate was determined 24 h after implantation of microdialysis fibre in the cortex. High performance liquid chromatography coupled with a fluorometric detector was used. K+ (100 mM) depolarization was followed by a 2- to 4-fold increase in adenosine efflux. The addition of tetrodotoxin (1 microM) to the perfusate was followed by a decrease in spontaneous and K(+)-evoked adenosine efflux. The increase induced by high K+ was markedly inhibited by the NMDA receptor antagonist, D(-)-2-amino-7-phosphonoheptanoic acid (1 mM, D-AP7), but not by the muscarinic receptor antagonist, atropine (1.5 microM). The acetylcholine esterase inhibitor, physostigmine (7 microM), and the muscarinic receptor agonist, oxotremorine (100 microM), significantly enhanced the K(+)-evoked increase in adenosine. The spontaneous efflux of adenosine was not modified by any of the drugs tested. A neurotoxic lesion of the cholinergic pathway innervating the cortex, although inducing a marked decrease in cortical choline acetyltransferase activity, did not significantly modify the cortical adenosine efflux. It is concluded that, under K(+)-depolarizing conditions, adenosine efflux is triggered by excitatory amino acids and enhanced by muscarinic activation.

Heterocarrier-mediated reciprocal modulation of glutamate and glycine release in rat cerebral cortex and spinal cord synaptosomes

The effects of glutamic acid (Glu) and glycine (Gly) on each others release were studied using rat brain cortex and spinal cord synaptosomes. Previously taken up [3H]Gly and [3H]D-aspartic acid ([3H]D-Asp) was employed as markers for Gly and Glu/Asp release, respectively. Glu enhanced the release of [3H]Gly (EC50 = 8.4 microM) from cortical synaptosomes. The effect of Glu was not mimicked by the glutamate receptor agonists N-methyl-D-aspartic acid (NMDA), kainic or quisqualic acid. The Glu effect was abolished by the Glu/Asp uptake inhibitor D-threo-hydroxy-aspartic acid and it was Na+ sensitive. D-Asp also increased [3H]Gly release (EC50 = 9.9 microM) and the effect was blocked by the Glu/Asp uptake inhibitor. In contrast to its effect in the cortex, Glu failed to increase the release of [3H]Gly from spinal cord synaptosomes. Gly enhanced the outflow of [3H]D-Asp from rat cerebral cortex and spinal cord synaptosomes (EC50 = 75.0 and 99.5 microM, respectively). Gly was much more potent a releaser of [3H]D-Asp in the spinal cord than in the cortex. The Gly effects were insensitive to strychnine or to 7-Cl-kynurenic acid, antagonists at the two known Gly receptors, but they were strongly Na+ dependent. Our results are compatible with the idea that high-affinity uptake systems specific for Glu/Asp or Gly are colocalized on the same nerve terminal in rat spinal cord and cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional adenosine A1 receptors in goldfish brain: regional distribution and inhibition of K(+)-evoked glutamate release from cerebellar slices

In goldfish brain, [3H]cyclohexyladenosine binding sites are ubiquitously distributed with a maximum in the hypothalamus and a minimum in the spinal cord. The binding parameters measured in cerebellar membranes (Kd = 0.88 +/- 0.08 nM; Bmax = 59.65 +/- 2.62 fmol/mg protein) are not significantly different from those of the whole brain. In perfused goldfish cerebellar slices, stimulation of cyclic AMP accumulation by 10(-5) M forskolin was markedly reduced (58.7%) by treatment with 10(-4) M cyclohexyladenosine, an adenosine A1 receptor agonist, and the reduction was reversed in the presence of 10(-4) M 8-cyclopentyltheophylline, a selective A1 receptor antagonist. In the same brain preparation, 30 mM K+ stimulated the release of glutamate, glutamine, glycine and GABA in a Ca(2+)-dependent manner, whereas the aspartate and taurine release was Ca(2+)-independent. Cyclohexyladenosine inhibited the 30 mM K(+)-evoked release of glutamate in a dose-related manner. This effect was reversed by 8-cyclopentyltheophylline. These results support the hypothesis that adenosine A1 receptors present in goldfish cerebellum are involved in the modulation of glutamate transmitter release.

Status epilepticus may be caused by loss of adenosine anticonvulsant mechanisms

The inhibitory neuromodulator adenosine is an endogenous anticonvulsant that terminates brief seizures in the brain and it has been proposed that loss of adenosine or adenosine-mediating systems may play a major role in the development of status epilepticus, a seizure condition characterized by prolonged and/or recurrent seizures that last by definition, at least 20 min. In this study, the effect of specific A1-adenosine agonists and antagonists were tested for their ability to prevent and cause status epilepticus in two electrical stimulation models in rats. In a recurrent electrical stimulation model, whereas no vehicle-treated animals developed status epilepticus after 20 recurrent electrical stimulations, rats injected with 10 mg/kg of the specific A1-adenosine antagonist 8-cyclopentyl-1,3-dimethylxanthine intraperitoneally developed status epilepticus after stimulation. 8-(p-Sulphophenyl)-theophylline, which has limited penetrability into the brain when administered peripherally, did not cause status epilepticus when injected intraperitoneally. However, when 200 micrograms of 8-(p-sulphophenyl)-theophylline were administered intracerebroventricularly, status epilepticus developed in all animals, suggesting status epilepticus developed as a result of central adenosine receptor antagonism. In the second study, whereas all vehicle-treated animals developed status epilepticus after constant electrical stimulation, administration of N6-cyclohexyladenosine and N6-cyclopentyladenosine prior to stimulation suppressed the development of status epilepticus. N6-Cyclohexyladenosine was also effective in terminating status epilepticus after it had progressed for 20 min. The effects of a selective A2-agonist was also tested on both stimulation models and had no anticonvulsant effects. An electrical stimulus given to rats pretreated three days prior to stimulation with pertussis toxin, a compound which inactivates Gi-proteins, also resulted in generalized status epilepticus, suggesting that impairment of G-protein-linked receptors is involved in the development of status epilepticus. The effects of a GABAB antagonist, phaclofen, and a GABAB agonist, baclofen, were also tested in the recurrent stimulation model, as GABAB receptors are also coupled to the same subset of K+ channels as the A1-receptor. Rats given phaclofen did not develop status epilepticus after recurrent electrical stimulation, although baclofen was effective at preventing the induction of status epilepticus in the constant stimulation model. These results, together with some preliminary data obtained showing that the GABAA antagonist picrotoxin did not cause status epilepticus after recurrent stimulation, suggest that loss of GABAergic inhibition only has a minor role in status epilepticus development in our models. Brains from all animals were also assessed for brain injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Nicotinamide adenine dinucleotides mimic adenosine inhibition on synaptic transmission by decreasing glutamate release in rat hippocampal slices

To assess the possible inhibitory action of nicotinamide adenine dinucleotides on the synaptic release of glutamate, electrophysiological and biochemical experiments were performed on rat hippocampal slices. Perfusion of adenosine, beta-nicotinamide adenine dinucleotide (NAD) or beta-nicotinamide adenine dinucleotide phosphate (NADP), reversibly inhibited the field excitatory postsynaptic potentials (fEPSP). Dose-response curves for their inhibitory action showed that these three substances had a similar potency in the range of concentrations from 0.1 microM to 100 microM. NADP and adenosine (100 microM) halved the K(+)-induced release of endogenous glutamate and aspartate, leaving gamma-amino-butyric acid (GABA) levels unchanged. 3-Isobutyl-1-methylxanthine (IBMX) 200 microM, an antagonist of the P1-purinoreceptors, antagonized the depressant effects of these coenzymes on both fEPSP and also on amino acid release. Based on these results we propose that nicotinamide adenine dinucleotides, similar to adenosine, inhibit excitatory synaptic transmission in the rat hippocampus by decreasing glutamate release from synaptic terminals.

Adenosine A1 receptor-mediated inhibition of evoked glutamate release is coupled to calcium influx decrease in goldfish brain synaptosomes

Binding of [3H]cyclohexyladenosine (CHA) to the cellular fractions and P2 subfractions of the goldfish brain was studied. The A1 receptor density was predominantly in synaptosomal membranes. In goldfish brain synaptosomes (P2), 30 mM K+ stimulated glutamate, taurine and GABA release in a Ca(2+)-dependent fashion, whereas the aspartate release was Ca(2+)-independent. Adenosine, R-phenylisopropyladenosine (R-PIA) and CHA (100 microM) inhibited K(+)-stimulated glutamate release (31%, 34% and 45%, respectively). All of these effects were reversed by the selective adenosine A1 receptor antagonist, 8-cyclopentyltheophylline (CPT). In the same synaptosomal preparation, K+ (30 mM) stimulated Ca2+ influx (46.8 +/- 6.8%) and this increase was completely abolished by pretreatment with 100 nM omega-conotoxin. Pretreatment with 100 microM R-PIA or 100 microM CHA, reduced the evoked increase of intra-synaptosomal Ca2+ concentration, respectively by 37.7 +/- 4.3% and 39.7 +/- 9.0%. A possible correlation between presynaptic A1 receptor inhibition of glutamate release and inhibition of calcium influx is discussed.

Glutamic acid and gamma-aminobutyric acid modulate each other's release through heterocarriers sited on the axon terminals of rat brain

The effects of gamma-aminobutyric acid (GABA) on the spontaneous release of endogenous glutamic acid (Glu) or aspartic acid (Asp) and the effects of Glu on the release of endogenous GABA or [3H]GABA were studied in superfused rat cerebral cortex synaptosomes. GABA increased the outflow of Glu (EC50 17.2 microM) and Asp (EC50 18.4 microM). GABA was not antagonized by bicuculline or picrotoxin. Neither muscimol nor (-)-baclofen mimicked GABA. The effects of GABA were prevented by GABA uptake inhibitors and were Na+ dependent. Glu enhanced the release of [3H]GABA (EC50 11.5 microM) from cortical synaptosomes. Glu was not mimicked by the glutamate receptor agonists N-methyl-D-aspartic, kainic, or quisqualic acid. The Glu effect was decreased by the Glu uptake inhibitor D-threo-hydroxyaspartic acid (THA) and it was Na+ sensitive. Similarly to Glu, D-Asp increased [3H]GABA release (EC50 9.9 microM), an effect blocked by THA. Glu also increased the release of endogenous GABA from cortex synaptosomes. In this case the effect was in part blocked by the (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, whereas the 6-cyano-7-nitroquinoxaline-2,3-dione-insensitive portion of the effect was prevented by THA. GABA increased the [3H]D-Asp outflow (EC50 13.7 microM) from hippocampal synaptosomes in a muscimol-, (-)-baclofen-, bicuculline-, and picrotoxin-insensitive manner. The GABA effect was abolished by blocking GABA uptake and was Na+ dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine modulation of amino acid release in rat hippocampus during ischemia and veratridine depolarization

This study was undertaken to determine whether endogenous adenosine modulates 'in vivo' neurotransmitter amino acid release via its presynaptic receptors. Two conditions were compared: neuronal depolarization by local infusion of veratridine (600 microM), and transient global ischemia by four-vessel occlusion. Both stimuli were applied for 20 min. Extracellular amino acid (glutamate, taurine/GABA, glycine) variations in concentration were determined in the rat hippocampus by microdialysis and HPLC. Modulation of adenosine receptor activity was objectified by continuous local infusion of an adenosine agonist (R-phenylisopropyladenosine R-PIA) or an antagonist (theophylline), starting one hour before stimulation of amino acid release. R-PIA (100 microM) significantly decreased the glutamate release (50%) evoked by veratridine, whereas it did not significantly modify the ischemia-induced glutamate release. In contrast, theophylline did not significantly affect veratridine-induced glutamate release, but it significantly potentiated glutamate efflux (400%) under ischemic conditions. Neither treatment altered the release of the other amino acids. These data suggest that endogenous adenosine appearing in the extracellular space during veratridine-induced depolarization cannot control glutamate release. In contrast, ischemia-induced glutamate release was strongly inhibited by the concomitant increase in extracellular adenosine.

Adenosine inhibits the NMDA receptor-mediated excitatory postsynaptic potential in the hippocampus

The effect of the stable adenosine analogue 2-chloroadenosine (CADO) on the component of the field excitatory postsynaptic potential (fepsp) mediated by the N-methyl-D-aspartate (NMDA) type of glutamate receptor was studied in the hippocampal CA1 area of the rat. CADO inhibited the NMDA receptor-mediated component of the fepsp (EC50 = 0.10 +/- 0.02 microM), more efficiently than it inhibited the fepsp (EC50 = 0.40 +/- 0.08 microM). The results suggest that adenosine may modulate phenomena associated with the NMDA receptor, such as synaptic plasticity and excitotoxicity.

Adenosine A1 receptor inhibition of glutamate exocytosis and protein kinase C-mediated decoupling

The adenosine modulation of glutamate exocytosis from guinea pig cerebrocortical synaptosomes is investigated. Endogenously leaked adenosine is sufficient to cause a partial tonic inhibition of 4-aminopyridine-evoked glutamate release, which can be relieved by adenosine deaminase. The adenosine A1 receptor is equally effective in mediating inhibition of glutamate exocytosis evoked by 4-aminopyridine (where K(+)-channel activation would inhibit release) and by elevated KCl (where K(+)-channel activation would have no effect), arguing for a central role of Ca(2+)-channel modulation. In support of this, the plateau phase of depolarization-evoked free Ca2+ elevation is decreased by adenosine with both depolarization protocols. No effect of adenosine agonists is seen on membrane potential in polarized or KCl- or 4-aminopyridine-stimulated synaptosomes. The interaction of protein kinase C with the A1 receptor-mediated inhibition is examined. Activation of protein kinase C by 4 beta-phorbol dibutyrate has been shown previously by this laboratory to modulate glutamate release via K(+)-channel inhibition, and is shown here to have an additional action of decoupling the adenosine inhibition of glutamate exocytosis.

Reduced density of adenosine A1 receptors and preserved coupling of adenosine A1 receptors to G proteins in Alzheimer hippocampus: a quantitative autoradiographic study

Binding to adenosine A1 receptors and the status of their coupling to G proteins were studied in the hippocampus and parahippocampal gyrus of Alzheimer individuals and age-matched controls. The binding to A1 receptors was compared with binding to the N-methyl-D-aspartate receptor complex channel-associated sites (labeled with (+)-[3H]5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate). In vitro quantitative autoradiography demonstrated a similar anatomical distribution of A1 receptors labeled either with an agonist ((-)-[3H]phenylisopropyladenosine) or antagonist ([3H]8-cyclopentyl-1,3-dipropylxanthine) in the brains of elderly controls. In Alzheimer patients, significant decreases in the density of both agonist and antagonist binding sites were found in the molecular layer of the dentate gyrus. Decreased A1 agonist binding was also observed in the CA1 stratum oriens and outer layers of the parahippocampal gyrus, while reduced antagonist binding was found in the subiculum and CA3 region. Reduced density of the N-methyl-D-aspartate receptor channel sites was found in the CA1 region and parahippocampal gyrus. The reductions in binding to adenosine A1 and N-methyl-D-aspartate receptors were due to a decrease in the density of binding sites (Bmax), and not changes in receptor affinity (KD). In both elderly control and Alzheimer subjects, GTP substantially reduced the density of A1 agonist binding sites with a concomitant increase in the KD values, whereas antagonist binding was unaffected by GTP. The results suggest that adenosine A1 receptor agonists and antagonists recognize overlapping populations of binding sites. Reduced density of A1 receptors in the molecular layer of the dentate gyrus most probably reflects damage of the perforant path input in Alzheimer's disease, while altered binding in the CA1 and CA3 regions is probably due to loss of intrinsic neurons. Similar effects of GTP on binding to A1 receptors in control and Alzheimer individuals suggest lack of alterations in coupling of A1 receptors to G proteins in Alzheimer's disease, thus supporting the notion of normal receptor coupling to their effector systems in Alzheimer's disease.

The effects of the adenosine reuptake inhibitor soluflazine on synaptic potentials and population hypoxic depolarizations in area CA1 of rat hippocampus in vitro

Adenosine has recently been shown to play a potentially important role in the regulation of synaptic excitability during experimental hypoxia in the hippocampus of the rat. Endogenous adenosine, rapidly released at the initiation of a hypoxic episode, produced synaptic depression, which could protect sensitive neurons. In the present experiments, an inhibitor of the reuptake of adenosine, soluflazine (R64719) was employed to increase the levels of endogenous adenosine under normoxic and hypoxic conditions in slices of the hippocampus of the rat. Soluflazine produced a slow-onset, concentration-dependent depression of population excitatory postsynaptic potentials, which was reversed by the specific A1 adenosine receptor antagonist, 8-cyclopentyltheophylline. During severe N2-induced hypoxia, soluflazine significantly delayed hypoxic depolarization. These results suggest that inhibition of the reuptake of adenosine may have therapeutic potential in the amelioration of hypoxic/ischemic neuronal damage, particularly in the hippocampus.

Mechanisms of drug actions against neuronal damage caused by ischemia--an overview

1. Oxygen and energy deficits induces a cascade of pathological processes which lead to neuronal dysfunction and cell death. 2. The pathogenesis of ischemia-induced neuronal damage includes a disturbed calcium homeostasis, an excessive release of EAA and an enhanced formation of free oxygen radicals. 3. Calcium antagonists inhibit Ca2+ influx into the neuronal cell via VSCC. 4. Glutamate antagonists reduce intracellular Ca2+ concentration by inactivation of NMDA receptor-associated calcium channels (NMDA antagonists) or AMPA/quisqualate receptor-linked sodium channels (non-NMDA antagonists). 5. Furthermore, oxygen radical scavengers can avoid neuronal damage. 6. Agonists of the adenosinergic and serotonergic transmitter systems contribute to neuroprotection by hyperpolarization of the neuronal membrane due to an increase of K+ permeability.