N-methyl-D-aspartate, kainate and quisqualate release endogenous adenosine from rat cortical slices

Sex-dependent worsening of NMDA-induced responses, anxiety, hypercortisolemia, and organometry of early peripheral immunoendocrine impairment in adult 3xTg-AD mice and their long-lasting ontogenic modulation by neonatal handling

The neuroimmunomodulation hypothesis for Alzheimer's disease (AD) postulates that alterations in the innate immune system triggered by damage signals result in adverse effects on neuronal functions. The peripheral immune system and neuroimmunoendocrine communication are also impaired. Here we provide further evidence using a longitudinal design that also studied the long-lasting effects of an early life sensorial intervention (neonatal handling, from postnatal day 1-21) in 6-month-old (early stages of the disease) male and female 3xTg-AD mice compared to age- and sex-matched non-transgenic (NTg) mice with normal aging. The behavioral patterns elicited by the direct exposure to an open field, and the motor depression response evoked by NMDA (25 mg/kg, i.p) were found correlated to the organometry of peripheral immune-endocrine organs (thymus involution, splenomegaly, and adrenal glands' hypertrophy) and increased corticosterone levels, suggesting their potential value for diagnostic and biomonitoring.The NMDA-induced immediate and depressant motor activity and endocrine (corticosterone) responses were sensitive to sex and AD-genotype, suggesting worse endogenous susceptibility/neuroprotective response to glutamatergic excitotoxicity in males and in the AD-genotype. 3xTg-AD females showed a reduced immediate response, whereas the NTg showed higher responsiveness to subsequent NMDA-induced depressant effect than their male counterparts. The long-lasting ontogenic modulation by handling was shown as a potentiation of NMDA-depressant effect in NTg males and females, while sex × treatment effects were found in 3xTg-AD mice. Finally, NMDA-induced corticosterone showed sex, genotype and interaction effects with sexual dimorphism enhanced in the AD-genotype, suggesting different endogenous vulnerability/neuroprotective capacities and modulation of the neuroimmunoendocrine system.

Major dorsoventral differences in the modulation of the local CA1 hippocampal network by NMDA, mGlu5, adenosine A2A and cannabinoid CB1 receptors

Recent research points to diversification in the local neuronal circuitry between dorsal (DH) and ventral (VH) hippocampus that may be involved in the large-scale functional segregation along the long axis of the hippocampus. Here, using CA1 field recordings from rat hippocampal slices, we show that activation of N-methyl-d-aspartate receptors (NMDARs) reduced excitatory transmission more in VH than in DH, with an adenosine A1 receptor-independent mechanism, and reduced inhibition and enhanced postsynaptic excitability only in DH. Strikingly, co-activation of metabotropic glutamate receptor-5 (mGluR5) with NMDAR, by CHPG and NMDA respectively, strongly potentiated the effects of NMDAR in DH but had not any potentiating effect in VH. Furthermore, the synergistic actions in DH were occluded by blockade of adenosine A2A receptors (A2ARs) by their antagonist ZM 241385 demonstrating a tonic action of these receptors in DH. Exogenous activation of A2ARs by 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680) did not change the effects of mGluR5-NMDAR co-activation in either hippocampal pole. Importantly, blockade of cannabinoid CB1 receptors (CB1Rs) by their antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM 281) restricted the synergistic actions of mGluR5-NMDARs on excitatory synaptic transmission and postsynaptic excitability and abolished their effect on inhibition. Furthermore, AM 281 increased the excitatory transmission only in DH indicating that CB1Rs were tonically active in DH but not VH. Removing the magnesium ions from the perfusion medium neither stimulated the interaction between mGluR5 and NMDAR in VH nor augmented the synergy of the two receptors in DH. These findings show that the NMDAR-dependent modulation of fundamental parameters of the local neuronal network, by mGluR5, A2AR and CB1R, markedly differs between DH and VH. We propose that the higher modulatory role of A2AR and mGluR5, in combination with the role of CB1Rs, provide DH with higher functional flexibility of its NMDARs, compared with VH.

Neuronal transporter and astrocytic ATP exocytosis underlie activity-dependent adenosine release in the hippocampus

The neuromodulator adenosine plays an important role in many physiological and pathological processes within the mammalian CNS. However, the precise mechanisms of how the concentration of extracellular adenosine increases following neural activity remain contentious. Here we have used microelectrode biosensors to directly measure adenosine release induced by focal stimulation in stratum radiatum of area CA1 in mouse hippocampal slices. Adenosine release was both action potential and Ca²⁺ dependent and could be evoked with low stimulation frequencies and small numbers of stimuli. Adenosine release required the activation of ionotropic glutamate receptors and could be evoked by local application of glutamate receptor agonists. Approximately 40% of stimulated-adenosine release occurred by translocation of adenosine via equilibrative nucleoside transporters (ENTs). This component of release persisted in the presence of the gliotoxin fluoroacetate and thus results from the direct release of adenosine from neurons. A reduction of adenosine release in the presence of NTPDase blockers, in slices from CD73(-/-) and dn-SNARE mice, provides evidence that a component of adenosine release arises from the extracellular metabolism of ATP released from astrocytes. This component of release appeared to have slower kinetics than the direct ENT-mediated release of adenosine. These data suggest that activity-dependent adenosine release is surprisingly complex and, in the hippocampus, arises from at least two distinct mechanisms with different cellular sources.

Cortical spreading depression-induced preconditioning in mouse neocortex is lamina specific

Cortical spreading depression (CSD) is able to confer neuroprotection when delivered at least 1 day in advance of an ischemic event. However, its ability to confer neuroprotection in a more immediate time frame has not previously been investigated. Here we have used mouse neocortical brain slices to study the effects of repeated episodes of CSD in layer V and layer II/III pyramidal neurons. In layer V, CSD evoked at 15-min intervals caused successively smaller membrane depolarizations and increases in intracellular calcium compared with the response to the first CSD. With an inter-CSD interval of 30 min this preconditioning effect was much less marked, indicating that preconditioning lasts between 15 and 30 min. A single episode of CSD also provided a degree of protection in oxygen-glucose deprivation (OGD) by significantly lengthening the time a cell could withstand OGD before anoxic depolarization occurred. In layer II/III pyramidal neurons no preconditioning by CSD on subsequent episodes of CSD was observed, demonstrating that the response of pyramidal neurons to repeated CSD is lamina specific. The A1 receptor antagonist 8-cyclopentyl theophylline (8-CPT) reduced the layer V preconditioning in a concentration-related manner. Inhibition of extracellular formation of adenosine by blocking ecto-5'-nucleotidase with α,β-methyleneadenosine 5'-diphosphate prevented preconditioning in most but not all cells. Block of equilibrative nucleoside transporters 1 and 2 with dipyramidole alone or in combination with 6-[(4-nitrobenzyl)thio]-9-β-d-ribofuranosylpurine also prevented preconditioning in some but not all cells. These data provide evidence that rapid preconditioning of one CSD by another is primarily mediated by adenosine.

Glutamate-induced depression of EPSP-spike coupling in rat hippocampal CA1 neurons and modulation by adenosine receptors

The presence of high concentrations of glutamate in the extracellular fluid following brain trauma or ischaemia may contribute substantially to subsequent impairments of neuronal function. In this study, glutamate was applied to hippocampal slices for several minutes, producing over-depolarization, which was reflected in an initial loss of evoked population potential size in the CA1 region. Orthodromic population spikes recovered only partially over the following 60 min, whereas antidromic spikes and excitatory postsynaptic potentials (EPSPs) showed greater recovery, implying a change in EPSP-spike coupling (E-S coupling), which was confirmed by intracellular recording from CA1 pyramidal cells. The recovery of EPSPs was enhanced further by dizocilpine, suggesting that the long-lasting glutamate-induced change in E-S coupling involves NMDA receptors. This was supported by experiments showing that when isolated NMDA-receptor-mediated EPSPs were studied in isolation, there was only partial recovery following glutamate, unlike the composite EPSPs. The recovery of orthodromic population spikes and NMDA-receptor-mediated EPSPs following glutamate was enhanced by the adenosine A1 receptor blocker DPCPX, the A2A receptor antagonist SCH58261 or adenosine deaminase, associated with a loss of restoration to normal of the glutamate-induced E-S depression. The results indicate that the long-lasting depression of neuronal excitability following recovery from glutamate is associated with a depression of E-S coupling. This effect is partly dependent on activation of NMDA receptors, which modify adenosine release or the sensitivity of adenosine receptors. The results may have implications for the use of A1 and A2A receptor ligands as cognitive enhancers or neuroprotectants.

Purine level regulation during energy depletion associated with graded excitatory stimulation in brain

OBJECTIVES

The formation and release of adenosine following graded excitatory stimulation of the brain may serve important physiological functions such as sleep regulation, as well as an early resistance mechanism against excitotoxicity. However, adenosine at high levels may reflect merely the results of obstructed energy metabolism.

METHODS

We examined the extent to which levels of adenosine and adenylate energy charge are affected in vivo by graded excitatory stimulations of brain using unilateral intrastriatal injections of glutamatergic agents and head-focused high energy microwaving for accurate and precise measures of purines.

RESULTS

Our results confirmed that adenosine levels rise when adenylate energy charge decreases and showed that these increases occurred in three distinct phases with the rate of adenosine formation in each phase increasing as tissue adenylate energy charge was further depleted. In addition, we observed that, in most cases, the effects of focal excitatory stimulation on changes in tissue purine levels were restricted spatially within the immediate vicinity of the injection site; however, when strongly depolarizing stimuli were used, changes in purine levels could be observed in adjacent and, occasionally, even in contralateral brain regions.

DISCUSSION

These results provide new insight into purine regulation that occurs under physiologically relevant conditions, such as sleep and during the early stages of brain insults that induce excitotoxicity.

NMDA preconditioning protects against seizures and hippocampal neurotoxicity induced by quinolinic acid in mice

PURPOSE

N-methyl D-aspartate (NMDA) preconditioning has been used to prevent cellular death induced by glutamate or NMDA in cultured neurons. Quinolinic acid (QA)-induced seizures are used to average NMDA receptors-evoked neurotoxicity in animal models. The purpose of this study was to investigate the potential neuroprotective effects of NMDA preconditioning against QA-induced seizures and hippocampal damage in vivo.

METHODS

Mice were pretreated with nonconvulsant doses of NMDA for different times before i.c.v. QA infusion and observed for the occurrence of seizures. Hippocampal slices from mice were assayed to measure cellular viability.

RESULTS

NMDA preconditioning presented 53% protection against QA-induced seizures, as well as QA-induced cellular death in the hippocampus. The NMDA receptor antagonist, MK-801, prevented the protection evoked by NMDA preconditioning. The adenosine A1 receptor antagonist, CPT, prevented the protection evoked by NMDA preconditioning against QA-induced seizures, but not against QA-induced hippocampal cellular damage. The adenosine A1 receptor agonist, CPA, did not mimic the NMDA preconditioning-evoked protective effects.

CONCLUSIONS

These results suggest that in vivo preconditioning with subtoxic doses of NMDA protected mice against seizures and cellular hippocampal death elicited by QA, probably through mechanisms involving NMDA receptors operating with adenosine A1 receptors.

In vivo neuroprotective role of NMDA receptors against kainate-induced excitotoxicity in murine hippocampal pyramidal neurons

Activation of NMDA receptors has been shown to induce either neuronal cell death or neuroprotection against excitotoxicity in cultured cerebellar granule neurons in vitro. We have investigated the effects of pretreatment with NMDA on kainate-induced neuronal cell death in mouse hippocampus in vivo. The systemic administration of kainate (30 mg/kg), but not NMDA (100 mg/kg), induced severe damage in pyramidal neurons of the hippocampal CA1 and CA3 subfields 3-7 days later, without affecting granule neurons in the dentate gyrus. An immunohistochemical study using an anti-single-stranded DNA antibody and TdT-mediated dUTP nick end labeling analysis both revealed that kainate, but not NMDA, induced DNA fragmentation in the CA1 and CA3 pyramidal neurons 1-3 days after administration. Kainate-induced neuronal loss was completely prevented by the systemic administration of NMDA (100 mg/kg) 1 h to 1 day previously. No pyramidal neuron was seen with fragmented DNA in the hippocampus of animals injected with kainate 1 day after NMDA treatment. The neuroprotection mediated by NMDA was prevented by the non-competitive NMDA receptor antagonist MK-801. Taken together these results indicate that in vivo activation of NMDA receptors is capable of protecting against kainate-induced neuronal damage through blockade of DNA fragmentation in murine hippocampus.

Activation of cholinergic and adrenergic receptors increases the concentration of extracellular adenosine in the cerebral cortex of unanesthetized rat

Adenosine is an inhibitory neuromodulator in the CNS. For extracellular adenosine to play a physiological role in the brain, it must be present at effective concentrations. Acetylcholine and noradrenaline are known to play an important role in modulating the activity of cortical neurons, and they might have a role also in the release of adenosine in the cerebral cortex in vivo. We examined whether activation of cholinergic and adrenergic receptors affects extracellular adenosine levels in the cerebral cortex of unanesthetized rats using in vivo microdialysis. All drugs were administered locally within the cortex by reverse dialysis. Both acetylcholine and the acetylcholinesterase inhibitor neostigmine increased extracellular adenosine levels, and the effect of neostigmine was blocked by the nicotinic receptor antagonist mecamylamine. Both nicotine and the nicotinic receptor agonist epibatidine increased the concentration of extracellular adenosine. Activation of muscarinic receptors using the nonselective agonist oxotremorine and a selective M1 receptor agonist also increased extracellular adenosine levels. Noradrenaline and the noradrenergic reuptake inhibitor desipramine increased extracellular adenosine levels. The alpha(1)-adrenergic receptor agonist phenylephrine and the beta-adrenergic agonist isoproterenol increased extracellular adenosine levels, whereas the alpha(2)-adrenergic receptor agonist clonidine did not have an effect. These findings indicate that activation of specific cholinergic and adrenergic receptors can increase extracellular levels of adenosine in the cortex, and suggest that cholinergic and adrenergic receptor-mediated regulation of adenosine levels may represent a mechanism for controlling the excitability of cortical neurons.

Adenosine receptors mediate glutamate-evoked arteriolar dilation in the rat cerebral cortex

We tested the hypothesis that adenosine (Ado) mediates glutamate-induced vasodilation in the cerebral cortex by monitoring pial arteriole diameter in chloralose-anesthetized rats equipped with closed cranial windows. Topical application of 100 microM glutamate and 100 microM N-methyl-d-aspartate (NMDA) dilated pial arterioles (baseline diameter 25 +/- 2 microm) by 17 +/- 1% and 18 +/- 4%, respectively. Coapplication of the nonselective Ado receptor antagonist theophylline (Theo; 10 microM) significantly reduced glutamate- and NMDA-induced vasodilation to 4 +/- 2% (P < 0.01) and 6 +/- 2% (P < 0.05), whereas the Ado A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM) had no effect. Moreover, application of the Ado A(2A) receptor-selective 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), either by superfusion (0.1 microM, 1 microM) or intravenously (1 mg/kg), significantly inhibited the pial arteriole dilation response to glutamate. Neither Theo nor ZM-241385 affected vascular reactivity to mild hypercapnia induced by 5% CO(2) inhalation. These results suggest that Ado contributes to the dilation of rat cerebral arterioles induced by exogenous glutamate, and that the Ado A(2A) receptor subtype may be involved in this dilation response.

NMDA receptor-mediated extracellular adenosine accumulation in rat forebrain neurons in culture is associated with inhibition of adenosine kinase

The effect of N-methyl-d-aspartate (NMDA) on regulation of extracellular adenosine was investigated in rat forebrain neurons in culture. NMDA evoked accumulation of extracellular adenosine with an EC50 value of 4.8 +/- 1.2 microM. The effect of NMDA was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d] cyclohepten-5,10-imine hydrogen maleate indicating that NMDA receptor activation was involved. The NMDA effect was also blocked by chelation of extracellular Ca2+ indicating that influx of calcium was required. The nitric oxide-cyclic GMP signalling pathway was not involved, as nitric oxide synthase inhibitors were unable to block, and cGMP analogs were unable to mimic, the effect of NMDA. The source for extracellular adenosine was likely to be intracellular adenosine as the ecto-5'-nucleotidase inhibitor alpha beta-methylene-ADP was unable to block the effect of NMDA. One possible cause of intracellular adenosine accumulation might be NMDA receptor-mediated inhibition of mitochondrial function and ATP hydrolysis. We found that NMDA caused a concentration dependent depletion of intracellular ATP with an EC50 value of 21 +/- 8 microM. NMDA also caused a significant decrease in adenosine kinase activity, assayed by two different methods. Consistent with the hypothesis that inhibition of adenosine kinase is sufficient to cause an increase in extracellular adenosine, inhibition of adenosine kinase by 5'-iodotubercidin resulted in elevation of extracellular adenosine. However, in the presence of a concentration of 5'-iodotubercidin that inhibited over 90% of adenosine kinase activity, exposure to NMDA still caused adenosine accumulation. These studies suggest that several possible mechanisms are likely to be involved in NMDA-evoked extracellular adenosine accumulation.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

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.

Effect of N-methyl-D-aspartate on motor activity and in vivo adenosine striatal outflow in the rat

It has been previously found that the systemic administration of low doses of N-methyl-D-aspartate (NMDA) in mice induces motor depression. The effects of the systemic administration of different doses of NMDA (10, 30 and 60 mg/kg s.c.) on the motor activity and on the in vivo extracellular levels of adenosine in the striatum was studied in Sprague-Dawley rats. The adenosine concentration in samples of perfusate was determined 24 h after implantation of a transverse microdialysis probe. At 30 and 60 mg/kg, but not 10 mg/kg, NMDA induced both a significant motor depression (motility and rearing) and a significant increase in the striatal extracellular levels of adenosine. Both the motor depression and the changes in the extracellular levels of adenosine were only evident during the first 30 min after NMDA administration. The non-competitive NMDA receptor antagonist MK-801 (0.1 mg/kg s.c.) completely counteracted the effects of NMDA (30 mg/kg s.c.) on motor activity (motility) and on the striatal extracellular levels of adenosine. The correlation between the behavioural and the biochemical data strongly support the hypothesis that adenosine release in the striatum is a main mechanism responsible for the motor depressant effects produced by the systemic administration of NMDA.

The bacterial endotoxin lipopolysaccharide causes rapid inappropriate excitation in rat cortex

There is mounting evidence that inflammation and associated excitotoxicity may play important roles in various neurodegenerative disorders, such as bacterial infections, Alzheimer's disease, AIDS dementia, and multiple sclerosis. The immunogen E. coli lipopolysaccharide (LPS, endotoxin) has been widely used to stimulate immune/inflammatory responses both systemically and in the CNS. Here, we show that exposure of parietal cortical slices from adult rats to LPS triggered very rapid (<2.5 min) and sustained releases of the neurotransmitters glutamate and noradrenaline, and of the neuromodulator adenosine. The responses to LPS declined rapidly following removal of the LPS and exhibited no tachyphylaxis to repeated exposures to LPS. The detoxified form of LPS had no effect. LPS-evoked release of [3H]noradrenaline, but not of glutamate or adenosine, appears to be partly due to the released glutamate acting at ionotropic receptors on the noradrenergic axons present in the cortical slices. LPS appears to release glutamate, which then acts at non-NMDA receptors to remove the voltage-sensitive Mg2+ block of NMDA receptors, thus permitting NMDA receptors to be activated and noradrenaline release to proceed. It seems possible that rapid, inappropriate excitation may occur in the immediate vicinity of gram-negative bacterial infections in the brain. If similar inappropriate excitations are also triggered by those immunogens specifically associated with Alzheimer's disease (beta-amyloid), AIDS dementia (gp120 and gp41), or multiple sclerosis (myelin basic protein), they might explain some of the acute, transient neurological and psychiatric symptoms associated with these disorders.

Electrically-evoked dopamine and acetylcholine release from rat striatal slices perfused without magnesium: regulation by glutamate acting on NMDA receptors

1. Rat striatal slices, preincubated with [3H]-dopamine and [14C]-choline, were continuously superfused and electrically stimulated. Electrically evoked release of [3H]-dopamine and [14C]-acetylcholine (ACh) was not significantly changed by elimination of Mg2+ from superfusion buffer, but the basal release of [3H]-dopamine was doubled. 2. Kynurenic acid (100-800 microM) caused, in the absence but not presence of Mg2+, a concentration-dependent decrease in the evoked release of these two transmitters. The addition of glycine reversed the inhibition of the evoked release of both transmitters caused by kynurenic acid (400 microM) in a concentration-dependent manner. In addition, glycine increased the evoked release of [3H]-dopamine via a site inhibitable by strychnine (1 microM). 3. Another two antagonists at N-methyl-D-aspartate (NMDA) receptors, 2-amino-5-phosphonovaleric acid and dizocilpine, also decreased significantly the evoked release of the two transmitters in a concentration-dependent manner in the absence, but not presence of Mg2+. By contrast, an antagonist of non-NMDA receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) significantly decreased the evoked release of the two transmitters in the presence, but not in the absence of Mg2+. 4. Electrical field stimulation evoked release of endogenous adenosine, and this release tended to be higher in the absence of Mg2+. However, the addition of a selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (200 nM) did not influence the evoked release of the two transmitters, showing that the released adenosine is of little importance in controlling ACh and dopamine release from striatal slices. Non-NMDA receptors may play a similar role when Mg2+ ions are present. 5. The results indicate that NMDA receptors activated in the absence of Mg2+ participate in the electrically-evoked release of [3H]-dopamine and [14C]-ACh from the striatum.

cAMP production in rabbit carotid body: role of adenosine

In the present study, we have investigated the possible role of adenosine in the hypoxia-mediated increase in adenosine 3',5'-cyclic monophosphate (cAMP) in the carotid body. cAMP levels in rabbit carotid bodies superfused in vitro for 10 min were increased in the presence of adenosine (100 microM and 1.0 mM; maximum increase = 127%, P < 0.01). These effects were reduced by the nonspecific adenosine-receptor antagonist 1,3-dipropyl-8[p-sulfophenyl]xanthine (DPSPX; 10 microM). The specific A2-receptor agonist 2-[4'(2-carboxymethyl)phenylethylamino]-5'-N-ethylcarboxamido adenosine (CGS-21680; 100 nM) also elevated carotid body cAMP levels, an effect that was blocked by the specific A2-antagonist 3,7-dimethyl-L-propargyl-xanthine (DMPX; 50 microM). Hypoxia-evoked elevations in cAMP were potentiated in the presence of the adenosine-uptake inhibitor dipyridamole (100 nM) and blocked by exposure to adenosine-receptor antagonists. Our data suggest that the rabbit carotid body contains specific adenosine receptors (A2 subtype) that are positively coupled to adenylate cyclase and that increases in cAMP associated with hypoxia are mediated by the release of endogenous adenosine.

Release of ATP from cultured rat astrocytes elicited by glutamate receptor activation

The release of ATP was studied in cultures of astrocytes derived from the brain hemispheres of newborn rats. There was a basal efflux of ATP, which was increased up to 19-fold by glutamate (300-1000 microM). N-methyl-D-aspartate (20-500 microM), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 30-100 microM) and kainate (20 microM). The N-methyl-D-aspartate receptor-selective antagonist 2-amino-5-phosphonopentanoate (100 microM) blocked the effect of N-methyl-D-aspartate but not the effects of AMPA, kainate and glutamate. The AMPA receptor-selective antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (30 microM) blocked the effect of AMPA and also of glutamate and N-methyl-D-aspartate, but not the effect of kainate. The kainate receptor-selective antagonist D-glutamyl-amino-methanesulfonate (30 microM) blocked the effect of kainate but not of glutamate. Glutamate (1000 microM) did not increase the release of lactate dehydrogenase from astrocytes. Excitatory amino acids are known to release adenyl compounds in the brain. The present results identify one adenyl compound thus released, namely ATP, and identify astrocytes as one source. The release is brought about by activation of any of the three ionotropic glutamate receptor types-N-methyl-D-aspartate, AMPA and kainate receptors. AMPA receptors seem to mediate at least a part of the effect of glutamate itself, but the involvement of other receptors cannot be ruled out. ATP and its degradation products, such as adenosine, once released, may exert acute as well as trophic effects on neurons and glial cells.

Regulation of cerebral microvessels by glutamatergic mechanisms

This study examined the role of glutamate receptor activation in the regulation of microvascular tone in the hippocampus and neocortex of the rat. Microvascular and neuronal responses were simultaneously recorded in brain slices using videomicroscopic analysis in conjunction with electrophysiological recording. Glutamate and other glutamate receptor agonists, including NMDA, kainic acid, and ACPD elicited dose-dependent dilation in preconstricted hippocampal microvessels. The lower concentrations of NMDA elicited dilation with an increase in neuronal excitability while dilatory responses to other agonists were associated with substantial depolarization. NMDA-mediated dilation was inhibited completely with a sodium channel blocker (TTX), an NOS inhibitor (L-NNA), or a specific inhibitor of neuronal NOS (7-NI). Inhibition of the GABA(A) or the A2 adenosine receptor did not attenuate the NMDA-induced dilation. The role of spontaneous glutamate receptor activation by endogenous glutamate in the regulation of resting dilatory tone was also examined. Blocking AMPA or metabotropic glutamate receptors did not induce significant responses in resting hippocampal vessels. However, the NMDA receptor antagonist, APV, elicited a dose-dependent constriction. In surface vessels of the neocortex, NMDA elicited a comparable dose-dependent dilation, and APV elicited a significantly smaller dose-dependent constriction. A 60 min period of hypoxia elicited a significant dilation of preconstricted hippocampal microvessels. APV did not significantly influence this dilatory response indicating that hypoxia-induced dilation is not mediated by NMDA receptor activation. Taken together, these results indicate that glutamate contributes to the dilatory tone of cerebral microvessels under physiologic conditions and that this effect is mediated by NMDA receptors. Glutamatergic vasodilation is dependent on neuronal discharge activity and the neuronal production of NO. The tonic influence is more pronounced in hippocampal microvessels than in neocortical vessels suggesting that the contribution of NMDA receptor activation to resting dilatory tone is dependent on the location of vessels within the brain.

Effects of glutamate application on the rhythm of low magnesium-induced epileptiform activity in hippocampal slices of guinea-pigs

The extracellular concentration of glutamate has previously been reported to increase to more than 10-fold the basal level during seizure activity. In the present study, we tested whether localized increases in extracellular glutamate concentration influence the rhythm of epileptiform discharges in the low-magnesium epilepsy model. In hippocampal slices of guinea-pigs, epileptiform activity was induced by omission of magnesium from the bath fluid. Glutamate and its subreceptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were ejected into different strata of the CA3 and CA1 regions using microiontophoretic and micropressure application. Glutamate, NMDA and AMPA applied to the CA3 region, but not to the CA1 region, induced a short-lasting increase in epileptiform discharge frequency, often followed by a transient reduction. The effect was most pronounced with application into the stratum lacunosum-moleculare of the CA3 region and could only be evoked in slices exceeding 400 microns in thickness. The effects on the rhythm of epileptiform discharges induced by NMDA and AMPA were blocked by their specific receptor antagonists. They were not influenced by application of GABAA and GABAB receptor antagonists. Changes in somatic membrane potential of CA3 pyramidal neurons did not correlate with changes in the rhythm of epileptiform discharges elicited in this region. The transient suppression of epileptiform discharges that followed the increase in discharge frequency was abolished by an adenosine A1 receptor antagonist. We propose that localized increases in extracellular glutamate concentration modify the rhythm of epileptiform discharges due to changes in neuronal network activity.

Differentiation of sigma ligand-activated receptor subtypes that modulate NMDA-evoked [3H]-noradrenaline release in rat hippocampal slices

1. It is now widely accepted that there are two classes of sigma (sigma) binding sites, denoted sigma(1) and sigma(2), and recently sigma(3) subtype has been proposed. Selective sigma(1) and sigma(2) receptor agonists are known to modulate the neuronal response to N-methyl-D-aspartate (NMDA) in vivo and in vitro. To identify the site of action of a series of recently synthesised high affinity sigma ligands, the present in vitro series of experiments was carried out on NMDA-evoked [3H]-noradrenaline ([3H]-NA) overflow from preloaded hippocampal slices of the rat. 2. The ligands (+)-cis-N-methyl-N-[2,(3,4-dichlorophenyl) ethyl]-2-(1-pyrrolidinyl) cyclohexylamine (BD-737) and (+)-pentazocine, considered as the prototypic sigma(1) agonists, potentiated the NMDA response from 10 nM to 100 nM. This potentiation faded between 100 nM and 1 microM ligand concentrations. On the other hand, 1,3-di(2-tolyl)guanidine (DTG), a mixed sigma(1)/sigma(2) agonist, at concentrations greater than 100 nM inhibited the NMDA-evoked [3H]-NA release. Spiperone, considered as active on putative sigma(3) receptors, was without effect on the NMDA response, or on the potentiating effect of BD-737. 3. The high affinity sigma antagonists haloperidol and 1[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine (BD-1063), inactive by themselves on the NMDA-induced response, at concentrations above 30 nM totally prevented the potentiating effect of (+)-pentazocine (100 nM) as well as the inhibitory effect of DTG (300 nM) on NMDA-evoked [3H]-NA release. Whereas haloperidol and BD-1063, at concentrations < 1 microM, were inactive on the potentiating effect of BD-737 (100 nM). 4. 4-(4-Chlorophenyl)-alpha-4-fluorophenyl-4-hydroxy-1-piperidinebutanol (reduced haloperidol), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD-1008), inactive by themselves on the NMDA-evoked [3H]-NA release, failed to reverse the effects of (+)-pentazocine and DTG, but at concentrations of 30 nM to 1 microM antagonised the BD-737-induced potentiation of the NMDA response. Conversely, N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) blocked the effects of (+)-pentazocine as well as those of BD-737, but not those of DTG. 5. The present results provide in vitro functional evidence for a sigma receptor type preferentially sensitive to BD-737, reduced haloperidol, BD-1008 and also to NE-100, that differs from the already identified sigma(1), sigma(2) and sigma(3) sites.

Potentiation of excitatory amino acid-evoked adenosine release from rat cortex by inhibitors of adenosine kinase and adenosine deaminase and by acadesine

Endogenous extracellular adenosine provides some protection against excitotoxicity in the central nervous system, but it appears to be incomplete. Potentiating the formation of extracellular adenosine that occurs when excitatory amino acid receptors are activated might provide additional protection. We studied the effects of AICAR (AICA riboside, acadesine) and of inhibitors of adenosine metabolism on the release of adenosine from rat cortical slices. AICAR had no effects on basal N-methyl-D-aspartate (NMDA)- or (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxasole propionic acid (AMPA)-evoked adenosine release, but it increased kainate-evoked adenosine release 1.4-fold. This selective action of AICAR may make it useful for treating kainate receptor-mediated excitotoxicity. Inhibition of adenosine kinase with either 20 microM 5'-amino-5'-deoxyadenosine or 5'-iodotubercidin had a much greater effect on excitatory amino acid-evoked adenosine release than on basal adenosine release. Inhibition of adenosine kinase increased excitatory amino acid-evoked adenosine release 3-7-fold whereas inhibition of adenosine deaminase only increased evoked adenosine release 2-2.5-fold. Finally, 0.2 microM 5'-iodotubercidin and 200 microM 2'-deoxycoformycin caused similar increases in the basal rates of extracellular adenosine formation, but 5'-iodotubercidin produced over twice as much potentiation of the rate of NMDA-evoked adenosine formation than did 2'-deoxycoformycin. These findings suggest that adenosine kinase inhibitors may produce an event-specific potentiation of evoked adenosine formation, i.e. more effect on evoked formation than on basal formation. If so, adenosine kinase inhibitors may prove useful for preventing/treating diseases associated with excessive excitation in the brain, such as seizures, excitotoxicity and neurodegeneration.

Enhancement of NMDA-induced increases in levels of endogenous adenosine by adenosine deaminase and adenosine transport inhibition in rat striatum

Unilateral microinjection of N-methyl-D-aspartate (NMDA) into striatum of rats subsequently killed by high-energy focused microwave irradiation significantly increased in vivo levels of endogenous adenosine. At a dose of 25 nmol NMDA, levels of adenosine in injected striata were 263% of levels in uninjected contralateral striata. An inhibitor of adenosine deaminase (deoxycoformycin, DCF) in combination with an inhibitor of adenosine transport (dilazep, DLZP) at a dose that did not affect levels of endogenous adenosine, potentiated NMDA-induced increases in adenosine levels to 426% of contralateral striata. In the presence of DCF and DLZP, NMDA dose-dependently increased levels of adenosine (% of contralateral striatum) from 166% at 10 nmol to 622% at 100 nmol. NMDA-induced increases in levels of endogenous adenosine were completely blocked by prior administration of the NMDA receptor antagonist MK 801 (dizocilpine). Inhibitors of adenosine metabolism and transport may provide therapeutic benefit by potentiating excitatory amino acid-induced increases in levels of endogenous adenosine in vivo.

Chronic NMDA receptor stimulation: therapeutic implications of its effect on adenosine A1 receptors

It is known that stimulation of adenosine A1 receptors has a modulatory effect on the excitability of postsynaptic NMDA receptors. Conversely, acute stimulation of NMDA receptors results in release of adenosine via calcium-independent mechanisms. These findings indicate a close functional relationship between these receptors. It is, therefore, possible that chronic, low level stimulation of the NMDA receptor may have a negative impact on these modulatory processes. To investigate this possibility, we have subjected C57BL mice either to an acute injection of a N6-cyclopentyladenosine (CPA, 0.01 mg/kg) or deoxycoformycin (1 mg/kg) followed by a convulsant dose of N-methyl-D-aspartate (NMDA) (60 mg/kg) or to chronic, low level (20 mg/kg i.p. daily) exposure to NMDA for 8 weeks. One day after the last injection of NMDA, animals were injected either with a convulsant dose of NMDA alone, or with either CPA at 0.001 or 0.01 mg/kg, or with 1 mg/kg deoxycoformycin followed 15 min later by 60 mg/kg NMDA. Neither CPA nor deoxycoformycin were protective when NMDA was given acutely at 60 mg/kg. Chronic treatment with NMDA alone or chronic administration of NMDA followed by 0.001 mg/kg CPA had no significant effect on mortality following a convulsant dose of NMDA. However, when the chronic regimen of NMDA was followed by either 0.01 mg/kg CPA or 1 mg/kg deoxycoformycin, mortality was reduced to 10% (CPA), or eliminated completely (deoxycoformycin). Moreover, combination of chronic NMDA treatment with either CPA (both doses) or deoxycoformycin produced a significant improvement in other measures, i.e., seizure onset, intensity of neurological impairment, and extension of time to death.(ABSTRACT TRUNCATED AT 250 WORDS)

Combining laser Doppler flowmetry with microdialysis: a novel approach to investigate the coupling of regional cerebral blood flow to neuronal activity

Regional cerebral blood flow (rCBF) is directly coupled to neuronal activity; however, the mediators of this coupling have not been established. The characterization of these vasoactive substances requires a technique which enables sampling of locally released mediators together with the simultaneous monitoring of rCBF. The goal of this study was to establish such a technique by combining microdialysis and laser doppler flowmetry. Laser doppler and microdialysis probes were inserted into the dorsal hippocampal, CA1-dentate hilus, of rats. Animals received sequentially increasing concentrations of N-methyl-D-aspartate (NMDA). rCBF responded in a dose-dependent manner, increasing to 106.8 +/- 2.3%, 119.7 +/- 6.3%, 148.0 +/- 16.6%, and 191.4 +/- 20.4% of baseline at 50 microM, 100 microM, 200 microM, and 500 microM NMDA, respectively. All doses of NMDA produced an increase in extracellular concentrations of adenosine and citrulline, an indirect measure of nitric oxide generation. These results indicate that the combination of microdialysis and laser doppler flowmetry is a valuable tool to investigate the coupling of rCBF to neuronal activity. Moreover our data suggest two possible mediators of this coupling, nitric oxide and adenosine, which require further investigation.

Effects of the systemic administration of kainic acid and NMDA on exploratory activity in rats

In spite of growing evidence for the involvement of the glutamatergic system in mammal's locomotion, studies on behavioral effects induced by the systemic administration of excitatory amino acids not associated to convulsions are lacking. In the present work, the effect of one single systemic administration of kainic acid (KA) (9 mg/kg, IP) or NMDA (100 mg/kg, IP) on exploratory activity in the rat during 6 consecutive days was studied. Separation of exploratory activity in fast (FM) and slow movements (SM) and rearings (R), together with the analysis of those variables during both the light and dark periods of the light-dark cycle, allowed finding specific drug-induced effects. KA produced an acute short-lasting increase in exploratory activity, only significant for FM. On the other hand, NMDA produced an acute short-lasting depressant effect on FM, SM, and R, followed during the next 2 days by a long-lasting increase in exploratory activity, only significant for FM during the dark period. These results underline the importance of using repeated testing during both light and dark periods of the light-dark cycle when analyzing drug-induced changes on exploratory activity.

Inhibition of NMDA receptor-mediated currents in isolated rat hippocampal neurones by adenosine A1 receptor activation

The effect of the stable adenosine analogue, 2-chloro-adenosine (CADO), on the currents elicited by iontophoretic application of N-methyl-D-aspartate (NMDA) to pyramidal cells acutely dissociated from the CA1 area of the rat hippocampus was studied using the patch-clamp technique in the whole-cell configuration. CADO (3-300 nM) reversibly inhibited NMDA receptor-mediated currents (maximal effect: 54.2 +/- 6.6% decrease, EC50 = 10.3 nM). This effect was prevented by the adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (50 nM). CADO (100 nM inhibited the conductance induced by iontophoretic application of NMDA, without changing its reversal potential, in both the absence and the presence of Mg2+ (30 microM). Adenosine may contribute to the regulation of the NMDA receptor function, particularly under conditions, like hypoxia and ischaemia, leading to excessive NMDA receptor activation.

Neurosteroids, via sigma receptors, modulate the [3H]norepinephrine release evoked by N-methyl-D-aspartate in the rat hippocampus

N-Methyl-D-aspartate (NMDA, 200 microM) evokes the release of [3H]norepinephrine ([3H]NE) from preloaded hippocampal slices. This effect is potentiated by dehydroepiandrosterone sulfate (DHEA S), whereas it is inhibited by pregnenolone sulfate (PREG S) and the high-affinity sigma inverse agonist 1,3-di(2-tolyl)guanidine, at concentrations of > or = 100 nM. Neither 3 alpha-hydroxy-5 alpha-pregnan-20-one nor its sulfate ester modified NMDA-evoked [3H]NE overflow. The sigma antagonists haloperidol and 1-[2-(3,4-dichlorophenyl)-ethyl]-4-methylpiperazine, although inactive by themselves, completely prevented the effects of DHEA S, PREG S, and 1,3-di(2-tolyl)guanidine on NMDA-evoked [3H]NE release. Progesterone (100 nM) mimicked the antagonistic effect of haloperidol and 1-[2-(3,4-dichlorophenyl)ethyl]-4-methyl-piperazine. These results indicate that the tested steroid sulfate esters differentially affected the NMDA response in vitro and suggest that DHEA S acts as a sigma agonist, that PREG S acts as a sigma inverse agonist, and that progesterone may act as a sigma antagonist. Pertussis toxin, which inactivates the Gi/o types of guanine nucleotide-binding protein (Gi/o protein) function, suppresses both effects of DHEA S and PREG S. Since sigma 1 but not sigma 2 receptors are coupled to Gi/o proteins, the present results suggest that DHEA S and PREG S control the NMDA response via sigma 1 receptors.

Dopamine-independent and adenosine-dependent mechanisms involved in the effects of N-methyl-D-aspartate on motor activity in mice

The involvement of dopamine and adenosine mechanisms in the motor effects of systemically administered N-methyl-D-aspartate (NMDA) was studied in non-reserpinized and in reserpinized mice. In non-reserpinized mice NMDA induced motor depression (with 8, 25 and 75 mg/kg i.p.) during the first hour and motor activation (with 25 and 75 mg/kg i.p.) during the second hour after its administration. The non-selective adenosine antagonist, theophylline (3, 10 and 30 mg/kg i.p) induced motor activation during both 1-h periods of observation. NMDA-induced motor depression in non-reserpinized mice was antagonized by theophylline. Higher doses of theophylline were needed to counteract the motor depressant effect induced by higher doses of NMDA. The motor activation induced by NMDA and theophylline in non-reserpinized mice was not additive and theophylline did not enhance the motor activation induced by high doses of NMDA. Both NMDA (25 and 75 mg/kg i.p.) and theophylline (10 and 30 mg/kg) induced motor activation in reserpinized mice and, when coadministered, NMDA counteracted the effect of theophylline. NMDA (8 and 25 mg/kg i.p.) antagonized and theophylline (3, 10 and 30 mg/kg i.p.) potentiated the motor activation induced by the non-selective dopamine agonist, apomorphine (0.1 mg/kg s.c.), in reserpinized mice. In reserpinized mice, the non-selective dopamine antagonist, haloperidol (0.5 mg/kg s.c.) antagonized the motor activation induced by apomorphine (0.1 mg/kg s.c.) and the induced by theophylline (10 mg/kg i.p.) and did not modify NMDA-induced motor activation.(ABSTRACT TRUNCATED AT 250 WORDS)

N-methyl-D-aspartate receptors modulate neurotransmitter release and peristalsis in the guinea pig isolated colon

To assess the role of NMDA receptors in modulating neurotransmitter release in the myenteric plexus, we studied the effects of L-glutamic acid and NMDA on endogenous acetylcholine and noradrenaline overflow (assayed by HPLC) from the guinea pig isolated distal colon. L-Glutamic acid and NMDA enhanced electrically evoked acetylcholine and noradrenaline overflow and these effects were reversed by selective NMDA receptor antagonists. The possible functional significance of these findings was studied by measuring the efficiency of the colonic peristaltic reflex in the presence of NMDA receptor agonists. NMDA inhibited propulsion velocity at all concentrations tested, this effect being antagonized by (+/-)-2-amino-5-phosphonopentanoic acid and virtually abolished in sympathetically denervated animals. In conclusion, the inhibitory effect of NMDA on peristalsis, being almost entirely dependent on the integrity of sympathetic pathways, could be, at least in part, due to NMDA-induced noradrenaline release.

Modulation of nerve and glial function by adenosine--role in the development of ischemic damage

Adenosine is released during brain ischemia and provides neuroprotection by actions on nerve and glial cells. Activation of the adenosine A1 receptor enhances the K+ and Cl- conductance in neurons, leading to membrane hyperpolarization and postsynaptic reduction of neuronal Ca2+ influx through voltage- and NMDA receptor-dependent channels. In addition adenosine A1 receptor activation decreases excitatory amino acid release, possibly via inhibition of N- and P-type Ca2+ channels. The A1 and A2 receptors, coupled to Gi/G(o) and Gs proteins respectively, often co-exist and interact with the phospholipase C-dependent activation of the protein kinase C and the adenylyl cyclase. Activation of the A1 receptor may mimic metabotropic receptor stimulation in activating intracellular Ca2+ mobilization and PKC. A2 receptor mediated cAMP formation is depressed by high intracellular Ca2+ but enhanced by PKC activation. By modulating these metabolic signaling events, adenosine may influence acute cell functions, gene transcription and sustained changes of nerve and glial cells relevant for the development of ischemic damage. The neuroprotective adenosine effect seems to be amplified by treatment with propentofylline, which enhances adenosine release, influences the balance between A1 and A2 receptor mediated actions, depresses the free radical formation in activated microglia and influences astrocyte reactions.

Is cyclic AMP involved in excitatory amino acid-evoked adenosine release from rat cortical slices?

Activation of both N-methyl-D-aspartate (NMDA) and non-NMDA receptors releases endogenous adenosine from superfused rat cortical slices. NMDA-evoked adenosine release is Ca(2+)-dependent and results from the extracellular degradation of a released nucleotide, whereas non-NMDA receptor activation releases adenosine per se in a Ca(2+)-independent manner. IBMX selectively inhibits NMDA- but not non-NMDA-evoked adenosine release. Forskolin, but not 1,9-dideoxy-forskolin, produced a slight but significant increase in NMDA-evoked adenosine release, suggesting that the formation of cyclic AMP may somehow be involved. The inhibition of NMDA-evoked adenosine release by IBMX is not accompanied by enhanced cyclic AMP recovery in superfusates, nor is release diminished when cyclic AMP transport is inhibited by probenecid, suggesting that the adenosine is not derived from the extracellular metabolism of released cyclic AMP. It is possible that 5'AMP, derived from the intracellular conversion of cyclic AMP by phosphodiesterase, might be released during NMDA receptor activation. However, more selective inhibitors of the specific phosphodiesterase isozymes known to be located in the cortex failed to diminish NMDA-evoked adenosine release. Therefore, the effects of both forskolin and IBMX on NMDA-evoked adenosine release could be nonspecific, coincidental and unrelated to their actions on cyclic AMP levels in the cortex. However, it is also possible that a novel IBMX-sensitive phosphodiesterase plays a primary role in converting cyclic AMP to 5'AMP intracellularly during NMDA receptor activation; the 5'AMP could then exit the cells and be converted to adenosine extracellularly.

Blockade by 1,3-dipropyl-8-cyclopentylxanthine (CPX) of purine protection against kainate neurotoxicity

The adenosine A1 receptor selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (CPX) has been administered systemically to rats together with the neurotoxin kainic acid. At the lower doses of CPX tested, 10 and 50 micrograms/kg, which were sufficient to prevent the neuroprotective activity of exogenous agonists, there was no exacerbation of the neuronal damage. At 250 micrograms/kg, some enhancement of damage was found, which was also produced by 8-(p-sulphophenyl)theophylline, a non-selective xanthine which does not cross the blood-brain barrier. The results are consistent with the involvement of a central A1 receptor in the neuroprotective activity of purines, and suggest that blockade of a peripheral adenosine receptor, possibly of the A2 type, may increase neuronal damage.

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.

Chronic administration of selective adenosine A1 receptor agonist or antagonist in cerebral ischemia

The effect of chronic administration of selective adenosine A1 receptor agonists and antagonists on the outcome of cerebral ischemia is entirely unknown. Therefore, we have investigated the impact of such regimens on the hippocampal adenosine A1 receptor density, and on the recovery from 10 min forebrain ischemia in gerbils. While acutely administered N6-cyclopentyladenosine (CPA) given at 0.02 mg/kg resulted only in a significant reduction of mortality, at 1 mg/kg it improved both survival and neuronal preservation in the hippocampal CA1 region. Acute treatment with 1,3-dipropyl-8-cyclopentylxanthine (CPX) significantly worsened the outcome and enhanced neuronal destruction. The effects of chronic administration of these drugs (15 days followed by 1 drug-free day) were opposite. Thus, although chronic CPA at 0.02 mg/kg did not have any effect at all, at 1 mg/kg both survival and neuronal preservation were significantly poorer than in controls, while chronic CPX resulted in a significant improvement of both measures. These results were not accompanied by adenosine A1 receptor up- or downregulation. Our study indicates that highly selective adenosine analogues may have therapeutic potential in treatment of cerebral ischemia/stroke and possibly other neurodegenerative disorders as well.

Release and actions of adenosine in the central nervous system

Adenosine is released from active neurons into the extracellular fluid at a concentration of about 1 mumol/l. Neither the precise cellular origin nor the biochemical form of release has been firmly established, though the nucleotide is probably released partly directly, as a result of raised intracellular levels, and partly as nucleotides, which are subsequently hydrolysed. Once in the extracellular medium, adenosine markedly inhibits the release of excitatory neurotransmitters and modulatory peptides and has direct inhibitory effects on postsynaptic excitability via A1 receptors. A population of A2 receptors may mediate depolarization and enhanced transmitter release. Adenosine also modulates neuronal sensitivity to acetylcholine and catecholamines, all these effects probably contributing to the behavioural changes observed in conscious animals. As a result of their many actions, adenosine analogues are being intensively investigated for use as anticonvulsant, anxiolytic, and neuroprotective agents.

Ionotropic glutamate receptor types leading to adenosine-mediated inhibition of electrically evoked [3H]-noradrenaline release in rabbit brain cortex slices

1. Glutamate inhibits the electrically evoked release of noradrenaline in rabbit brain cortex slices; the inhibition is mediated by adenyl compounds, presumably adenosine. The aim of the present study was to identify the receptors involved in this indirect inhibitory effect of glutamate. Slices of the occipitoparietal cortex were preincubated with [3H]-noradrenaline and then superfused and stimulated by trains of 6 pulses, 100 Hz. 2. The ionotropic glutamate receptor agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AM-PA; 10-100 microM), kainate (10-100 microM) and N-methyl-D-aspartate (NMDA; 30-300 microM) but not the metabotropic glutamate receptor agonist, 1-amino-1,3-cyclopentanedicarboxylate (ACPD; 10-100 microM) reduced the electrically evoked overflow of tritium. 3. The effects of AMPA, kainate and NMDA were attenuated or abolished by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as by adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as by adenosine deaminase but not by the alpha 2-adrenoceptor antagonist yohimbine, the gamma-aminobutyric acid (GABA) receptor antagonists, bicuculline and 2-hydroxysaclofen and the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). 4. The NMDA receptor antagonist, 2-amino-5-phosphonopentanoate (AP5) blocked the inhibitory effect of NMDA but not that of AMPA and kainate. The non-NMDA-receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the effect of AMPA but not of kainate and NMDA. 5. In addition to decreasing the electrically evoked overflow of tritium, AMPA, kainate and NMDA but not ACPD caused a steep but transient rise of basal tritium efflux. This immediate releasing effect was not significantly changed by DPCPX, adenosine deaminase, yohimbine, bicuculline, 2-hydroxysaclofen and L-NAME (except that L-NAME enhanced the effect of kainate). AP5 and CNQX antagonized the immediate releasing effects in the same way that they antagonized the inhibition by AMPA, kainate and NMDA of the electrically evoked overflow of tritium.6. It is concluded that AMPA, kainate and NMDA, like glutamate, reduce the electrically evoked release of noradrenaline by releasing adenosine or an adenine nucleotide which is then degraded to adenosine. Activation of each of the three ionotropic glutamate receptors, AMPA, kainate and NMDA receptors, but not activation of metabotropic glutamate receptors can initiate this indirect inhibitory effect on the release of noradrenaline (as well as the known noradrenaline releasing effect).

Metabotropic glutamate receptor modulation of cAMP accumulation in the neonatal rat hippocampus

The pharmacology and cellular mechanism by which metabotropic glutamate receptor (mGluR) activation modulates cAMP formation was studied in cross-chopped hippocampal slices from neonatal (7 day old) rats. The selective mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), and other non-selective mGluR agonists produced concentration-related stimulation of basal cAMP formation in this tissue. The relative agonist potency order was 1S,3R-ACPD = quisqualate > ibotenate >> 1R,3S-ACPD. 1S,3R-ACPD stimulated cAMP accumulation was antagonized in a stereoselective manner by L-2-amino-3-phosphonopropionate (L-AP3), but not by higher chain homologues such as L-2-amino-4-phosphonobutyrate (L-AP4) and 2-amino-5-phosphonopentanoate (AP5). 1S,3R-ACPD-enhanced cAMP formation was greatly inhibited by incubation with adenosine deaminase. In the adult rat hippocampus, 1S,3R-ACPD did not appreciably increase basal cAMP, but inhibited forskolin-stimulated cAMP formation, and this effect was observed with or without adenosine deaminase. In the presence of the adenosine receptor antagonist and cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX), 1S,3R-ACPD did not enhance cAMP formation in the neonatal hippocampus, but inhibited forskolin-stimulated cAMP (like in the adult tissue). These results demonstrate that mGluRs that increase cAMP in the neonatal hippocampus have a unique pharmacology when compared to mGluRs that decrease cAMP accumulation and increase phosphoinositide hydrolysis. 1S,3R-ACPD stimulation of cAMP in the neonatal rat hippocampal slice involves potentiation of responses to endogenous adenosine. Negatively coupled cAMP linked mGluRs are also present in the neonatal tissue, but are masked by the predominance of the positively coupled mGluR cAMP response.

Effects of N6-cyclopentyl adenosine and 8-cyclopentyl-1,3-dipropylxanthine on N-methyl-D-aspartate induced seizures in mice

The effect of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX) on N-methyl-D-aspartate (NMDA)-evoked seizures was studied in C57BL/6 mice (20/group). Animals were injected i.p. either with CPA (0.5, 1, 2 mg/kg) or CPX (1, 2 mg/kg) 15 min prior to administration of NMDA (30, 60, 125 mg/kg). Administration of NMDA alone resulted in a complete locomotor arrest at 30 mg/kg, while clonic/tonic seizures and progressively increasing mortality were seen at higher doses. Prior administration of CPA resulted either in a delay of seizure onset and unchanged mortality (0.5 mg/kg CPA, 60 mg/kg NMDA) or in elimination of tonic episodes and a significant reduction in postictal mortality (1, 2 mg/kg CPA; 60, 125 mg/kg NMDA). Pretreatment with CPX at either 1 or 2 mg/kg eliminated locomotor depression in animals injected with NMDA at 30 mg/kg. At 60 mg/kg NMDA, the effect of CPX administration resulted in mortality equivalent to that seen with 125 mg/kg NMDA administered alone. The results indicate that A1 receptor agonists may protect against NMDA-evoked seizures and that the adenosine A1 receptor may be directly involved in these actions.

NMDA-evoked adenosine release from rat cortex does not require the intermediate formation of nitric oxide

Excitatory amino acids (EAAs) such as glutamate release the inhibitory neuromodulator adenosine from superfused rat cortical slices through the activation of both NMDA and non-NMDA EAA receptors. This study investigated the possibility that NMDA-evoked adenosine release may involve the intermediate formation of nitric oxide (NO). However, sodium nitroprusside did not evoke the release of adenosine, L-arginine did not augment and L-Nv-nitroarginine did not diminish NMDA-evoked adenosine release. It appears, therefore, that NMDA-evoked NO formation does not play a role in NMDA-evoked adenosine release in the cortex.

The adenosine binding enhancer, PD 81,723, inhibits epileptiform bursting in the hippocampal brain slice

The effects of adenosine and the adenosine binding enhancer, PD 81,723, on low magnesium-induced bursting in the in vitro hippocampal slice were examined. Extracellular recordings were obtained from the CA3 pyramidal cell layer while electrically stimulating in the stratum radiatum under low magnesium perfusion. Adenosine (6-100 microM) reduced the duration of epileptiform bursting in a dose-related manner, which was reversible upon washout of adenosine. Application of PD 81,723 (50-100 microM) also resulted in a dose-dependent reduction in the duration of the triggered burst, which was irreversible. These results demonstrate anticonvulsant activity of adenosine and the adenosine binding enhancer, PD 81,723, in the low magnesium model of epilepsy.

Purine release and inhibition of synaptic transmission during hypoxia and hypoglycemia in rat hippocampal slices

Evoked synaptic potentials and purine efflux were measured simultaneously from rat hippocampal slices. Slices were exposed to hypoxia, to glucose-free medium, and to in vitro ischemia consisting of glucose-free, hypoxic medium. During exposure to hypoxia or the glucose-free condition, radiolabelled purine efflux increased and the evoked population spike declined. Synaptic potentials and purine efflux returned to baseline values after reintroduction of normoxic and normoglycemic medium. During exposure to in vitro ischemia, purine and adenosine efflux were greatly increased with the appearance of the anoxic depolarization.

Investigations into the adenosine outflow from hippocampal slices evoked by ischemia-like conditions

The characteristics of adenosine and inosine outflow evoked by 5 min of ischemia-like conditions in vitro (superfusion with glucose-free Krebs solution gassed with 95% N2/5% CO2) were investigated on rat hippocampal slices. The viability of the slices after "ischemia" was evaluated by extracellular recording of the evoked synaptic responses in the CA1 region. The evoked dendritic field potentials were abolished after 5 min of superfusion under "ischemia" but a complete recovery occurred after 5 min of reperfusion with normal oxygenated Krebs solution. No recovery took place after 10 min of "ischemia." The addition of the adenosine A1 receptor antagonist 8-phenyltheophylline to the superfusate antagonized the depression of the evoked field potentials caused by 5 min of "ischemia." Five minutes of "ischemia" brought about a six- and fivefold increase in adenosine and inosine outflow, respectively, within 10 min. Tetrodotoxin reduced the outflow of adenosine and inosine by 42 and 33%, respectively, whereas the removal of Ca2+ caused a further increase. The NMDA receptor antagonist D(-)-2-amino-7-phosphonoheptanoic acid and the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione brought about small, not statistically significant decreases of adenosine and inosine outflow. The glutamate uptake inhibitor dihydrokainate did not affect the outflow of adenosine and inosine. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP did not affect basal adenosine outflow but potentiated "ischemia"-evoked adenosine outflow. It is concluded that ischemia-like conditions in vitro evoke a Ca(2+)-independent adenosine and inosine outflow, through a mechanism that partly depends on propagated nervous activity but does not involve excitatory amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

N-methyl-D-aspartate- and non-N-methyl-D-aspartate-evoked adenosine release from rat cortical slices: distinct purinergic sources and mechanisms of release

Excitatory amino acids, acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors, release the inhibitory neuromodulator adenosine from superfused rat cortical slices. This study was initiated to investigate the possible purinergic sources and mechanisms of release for the adenosine release evoked by NMDA and non-NMDA receptor activation. Inhibition of the bidirectional nucleoside transporter with dipyridamole greatly enhanced adenosine release evoked by glutamate. NMDA, kainate, and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Inhibition of ecto-5'-nucleotidase with alpha, beta-methylene ADP and GMP had no effect on either kainate- or AMPA-evoked adenosine release, but it decreased glutamate- and NMDA-evoked adenosine release by 23 and 68%, respectively. A similar inhibition of NMDA-evoked adenosine release was observed with alpha, beta-methylene ADP alone, indicating that the inhibitory effect was not due to the reported competitive inhibition of NMDA receptors by GMP. Finally, NMDA-evoked adenosine release, but not kainate- or AMPA-evoked release, was Ca2+ dependent. These results indicate that activation of non-NMDA receptors releases adenosine per se in a Ca(2+)-independent manner. In contrast, NMDA receptor activation releases primarily a nucleotide that is subsequently converted extracellularly to adenosine; in this case, release is Ca2+ dependent. Although neither NMDA- nor non-NMDA-evoked adenosine release occurs via the nucleoside transporter, this transporter does appear to be a major route for removal of adenosine from the extracellular space.