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

P2 receptors are involved in the mediation of motivation-related behavior

The importance of purinergic signaling in the intact mesolimbic–mesocortical circuit of the brain of freely moving rats is reviewed. In the rat, an endogenous ADP/ATPergic tone reinforces the release of dopamine from the axon terminals in the nucleus accumbens as well as from the somatodendritic region of these neurons in the ventral tegmental area, as well as the release of glutamate, probably via P2Y₁ receptor stimulation. Similar mechanisms may regulate the release of glutamate in both areas of the brain. Dopamine and glutamate determine in concert the activity of the accumbal GABAergic, medium-size spiny neurons thought to act as an interface between the limbic cortex and the extrapyramidal motor system. These neurons project to the pallidal and mesencephalic areas, thereby mediating the behavioral reaction of the animal in response to a motivation-related stimulus. There is evidence that extracellular ADP/ATP promotes goal-directed behavior, e.g., intention and feeding, via dopamine, probably via P2Y₁ receptor stimulation. Accumbal P2 receptor-mediated glutamatergic mechanisms seem to counteract the dopaminergic effects on behavior. Furthermore, adaptive changes of motivation-related behavior, e.g., by chronic succession of starvation and feeding or by repeated amphetamine administration, are accompanied by changes in the expression of the P2Y₁ receptor, thought to modulate the sensitivity of the animal to respond to certain stimuli.

Region-specific neuroprotective effect of ZM 241385 towards glutamate uptake inhibition in cultured neurons

Active uptake by neurons and glial cells is the main mechanism for maintaining extracellular glutamate at low, non-toxic concentrations. Adenosine A(2A) receptors regulate extracellular glutamate levels by acting on both the release and the uptake of glutamate. The aim of this study was to evaluate whether the inhibition of the effects of glutamate uptake blockers by adenosine A(2A) receptor antagonists resulted in neuroprotection. In cortical and striatal neuronal cultures, the application of l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, a transportable competitive inhibitor of glutamate uptake), induced a dose-dependent increase in lactate dehydrogenase (LDH) levels, an index of cytotoxicity. Such an effect of PDC was significantly reduced by pre-treatment with the adenosine A(2A) receptor antagonist ZM 241385 (50 nM) in striatal, but not cortical, cultures. The protective effects of ZM 241385 were specifically due to a counteraction of PDC effects, since ZM 241385 was totally ineffective in preventing the cytotoxicity induced by direct application of glutamate to cultures. These results indicate that adenosine A(2A) receptor antagonists prevent the toxic effects induced by a transportable competitive inhibitor of glutamate uptake, that such an effect specifically occurs in the striatum and that it does not depend on a direct blockade of glutamate-induced toxicity.

Astrocytes affect the profile of purines released from cultured cortical neurons

Adenosine (ADO) is produced by cultured neurons and astrocytes, albeit by different pathways, during in vitro stroke models (Parkinson and Xiong [2004] J. Neurochem. 88:1305-1312). Expression of ecto-5' nucleotidase (e-N), the enzyme responsible for extracellular dephosphorylation of AMP to ADO, is more abundant in astrocytes than neurons. Therefore, we tested the hypothesis that N-methyl-D-aspartate (NMDA) evokes ADO release per se from neurons, whereas dephosphorylation of extracellular adenine nucleotides contributes to NMDA-evoked ADO production in the presence of astrocytes. We used four different cell preparations-cortical rat neurons, cortical rat astrocytes, cocultures of neurons and astrocytes, and transient cocultures of neurons with astrocytes on transwell filters-to show that astrocytes contribute to NMDA-evoked increases in extracellular ADO. NMDA significantly increased ADO and inosine (INO) production from cultured cortical neurons but only increased extracellular INO production from cocultures. In neurons, the equilibrative nucleoside transport (ENT) inhibitor dipyridamole (DPR) prevented NMDA-evoked ADO and INO production, whereas the e-N inhibitor alpha,beta-methylene ADP (AOPCP) had no effect. Conversely, from both cocultures and transient cocultures DPR significantly decreased NMDA-evoked INO but not ADO generation. AOPCP inhibited NMDA-evoked production of both ADO and INO from transient cocultures. In the absence of astrocytes, NMDA evoked release of intracellular ADO and INO from cultured cortical neurons through ENT. However, in the presence of astrocytes, extracellular conversion of adenine nucleotides to ADO contributed significantly to NMDA-evoked production of this purine.

Adenosine as a signal for ion channel arrest in anoxia-tolerant organisms

Certain freshwater turtles and fish are extremely anoxia-tolerant, capable of surviving hours of anoxia at high temperatures and weeks to months at low temperatures. There is great interest in understanding the cellular mechanisms underlying anoxia-tolerance in these groups because they are anoxia-tolerant vertebrates and because of the far-reaching medical benefits that would be gained. It has become clear that a pre-condition of prolonged anoxic survival must involve the matching of ATP production with ATP utilization to maintain stable ATP levels during anoxia. In most vertebrates, anoxia leads to a severe decrease in ATP production without a concomitant reduction in utilization, which inevitably leads to the catastrophic events associated with cell death or necrosis. Anoxia-tolerant organisms do not increase ATP production when faced with anoxia, but rather decrease utilization to a level that can be met by anaerobic glycolysis alone. Protein synthesis and ion movement across the plasma membrane are the two main targets of regulatory processes that reduce ATP utilization and promote anoxic survival. However, the oxygen sensing and biochemical signaling mechanisms that achieve a coordinated reduction in ATP production and utilization remain unclear. One candidate-signaling compound whose extracellular concentration increases in concert with decreasing oxygen availability is adenosine. Adenosine is known to have profound effects on various aspects of tissue metabolism, including protein synthesis, ion pumping and permeability of ion channels. In this review, I will investigate the role of adenosine in the naturally anoxia-tolerant freshwater turtle and goldfish and give an overview of pathways by which adenosine concentrations are regulated.

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.

Mechanisms of adenosine release in the developing and adult mouse hippocampus

Adenosine is a neuromodulator known to inhibit the synaptic release of neurotransmitters, e.g., glutamate, and to hyperpolarize postsynaptic neurons. The release of adenosine is markedly enhanced under ischemic conditions. It may then act as an endogenous neuroprotectant against cerebral ischemia and excitotoxic neuronal damage. The mechanisms by which adenosine is released from nervous tissue are not fully known, particularly in the immature brain. We now characterized the release of [3H]adenosine from hippocampal slices from developing (7-day-old) and adult (3-month-old) mice using a superfusion system. The properties of the release differed only partially in the immature and mature hippocampus. The K(+)-evoked release was Ca2+ and Na+ dependent. Anion channels were also involved. Ionotropic glutamate receptor agonists potentiated the release in a receptor-mediated manner. Activation of metabotropic glutamate receptors enhanced the release in developing mice, with group II receptors alone being effective. The evoked adenosine release apparently provides neuroprotective effects against excitotoxicity under cell-damaging conditions. Taurine had no effect on adenosine release in adult mice, but depressed the release concentration dependently in the immature hippocampus.

Effects of salient environmental stimuli on extracellular adenosine levels in the rat nucleus accumbens measured by in vivo microdialysis

In the nucleus accumbens (NAc), the neuromodulator adenosine plays a major role in control of behaviour. The NAc subserves behaviour governed by salient stimuli in the environment, however, it is unknown whether such stimuli and the behavioural effects elicited are associated with changes in NAc extracellular adenosine levels. In order to further characterise the neuromodulatory actions of adenosine, the present study investigated for the first time the effects of four prototypical stimuli known to involve NAc processing on extracellular levels of adenosine in the NAc. Using in vivo microdialysis, the following stimuli were examined: (1) an appetitive, unfamiliar stimulus (palatable food), (2) an appetitive, familiar stimulus (standard laboratory food), (3) an aversive stimulus (handling) and (4) a novelty stimulus (cage change). Results revealed that neither of these stimuli significantly changed extracellular adenosine levels in the NAc. These findings demonstrate that NAc extracellular adenosine is not responsive to a number of prototypical salient stimuli in the environment. Thus the data provide no clues to suggest that transient changes of extracellular adenosine in the NAc modulate behavioural responses governed by these stimuli.

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.

A common signaling pathway for striatal NMDA and adenosine A2a receptors: implications for the treatment of Parkinson's disease

The striatum is the major input region of the basal ganglia, playing a pivotal role in the selection, initiation, and coordination of movement both physiologically and in pathophysiological situations such as Parkinson's disease. In the present study, we characterize interactions between NMDA receptors, adenosine receptors, and cAMP signaling within the striatum. Both NMDA (100 micrometer) and the adenosine A(2a) receptor agonist CPCA (3 micrometer) increased cAMP levels (218.9 +/- 19.9% and 395.7 +/- 67.2%, respectively; cf. basal). The NMDA-induced increase in cAMP was completely blocked when slices were preincubated with either the NMDA receptor antagonist 7-chlorokynurenate or the adenosine A(2) receptor antagonist DMPX (100 micrometer), suggesting that striatal NMDA receptors increase cAMP indirectly via stimulation of adenosine A(2a) receptors. Thus, NMDA receptors and adenosine A(2a) receptors might share a common signaling pathway within the striatum. In striatal slices prepared from the 6-hydroxydopamine-lesioned rat model of Parkinson's disease, NMDA receptor-mediated increases in cAMP were greater on the lesioned side compared with the unlesioned side (349.6 +/- 40.2% compared with 200.9 +/- 21.9% of basal levels, respectively). This finding substantiates previous evidence implicating overactivity of striatal NMDA receptors in parkinsonism and suggests that a common NMDA receptor-adenosine A(2a) receptor-cAMP signaling cascade might be an important mechanism responsible for mediating parkinsonian symptoms.

Nitric oxide as modulator of neuronal function

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Sodium nitroprusside-induced seizures and adenosine release in rat hippocampus

In the present study, we examined the effects of nitric oxide (NO)-related compounds, i.e. sodium nitroprusside (NO donor), diethyldithiocarbamate (NO trapper) and dithiothreitol (superoxide radical scavenger) on release of aspartate and adenosine from rat hippocampus using electrophysiological and microdialysis methods. Perfusion with 0.05 or 0.5 mM sodium nitroprusside significantly reduced high K(+)-evoked release of aspartate during high K(+) perfusion. Perfusion with 0.5 mM sodium nitroprusside always induced seizures and significantly increased release of aspartate and adenosine during washout of sodium nitroprusside. Diethyldithiocarbamate (5 mM) reversed the effects of sodium nitroprusside. Dithiothreitol (1 mM) significantly reduced the increase in adenosine release by sodium nitroprusside. These findings indicate that adenosine release is closely related to development of seizures, which are triggered by an increase in both NO itself and in part peroxynitrite, which results in reaction with superoxide radicals.

Nitric oxide-stimulated increase in extracellular adenosine accumulation in rat forebrain neurons in culture is associated with ATP hydrolysis and inhibition of adenosine kinase activity

Adenosine is a putative endogenous sleep-inducing substance, and nitric oxide has been implicated in arousal and sleep mechanisms. We found that various nitric oxide donors, including diethylamine NONOate (DEA/NO), stimulated large increases in extracellular adenosine in nearly pure cultures of forebrain neurons. The effect of DEA/NO could be blocked by 2-phenyl-4,4,5, 5-tetramethyl-imidazoline-1-oxyl-oxide and could not be mimicked by degraded solutions of DEA/NO or by DEA itself; therefore, it was caused by nitric oxide release on hydrolysis of the parent compound. The accumulation of adenosine was not blocked by probenecid or GMP, suggesting that neither extracellular cAMP nor extracellular AMP was the source, and that adenosine was therefore the most likely species transported across the plasma membrane. To pursue this further, we tested the effect of DEA/NO on cellular ATP and found a significant fall in ATP associated with exposure to nitric oxide. In addition, exposure to DEA/NO nearly completely inhibited adenosine kinase activity. It has been found previously that adenosine kinase is inhibited by its substrate, adenosine. We found that exposure to nitric oxide increased intracellular adenosine to 125 +/- 18% of control values (p < 0.01), consistent with the possibility that in our system the inhibition of adenosine kinase is related to an increase in intracellular adenosine, and that the effect of nitric oxide on extracellular adenosine is significantly potentiated by substrate inhibition of adenosine kinase. Furthermore, nitric oxide-stimulated adenosine accumulation may be important in the regulation of behavioral state.

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.

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.

Adenosine release in the ventral striatum of the rat is modulated by endogenous nitric oxide

The influence of nitric oxide (NO) on adenosine release was investigated by the push-pull technique in the ventral striatum of the urethane-anaesthetized rat. Superfusion with the NO donor diethylamine-NO enhanced, whereas superfusion with the NO synthase inhibitor L-NG-nitroarginine methyl ester decreased the output of adenosine. The effect of L-NG-nitroarginine methyl ester was abolished by L-arginine methyl ester. These findings indicate that, in the ventral striatum of the rat, NO modulates adenosine release.

Nitric oxide (NO) is an endogenous anticonvulsant but not a mediator of the increase in cerebral blood flow accompanying bicuculline-induced seizures in rats

Neurons synthesize NO, which may act as a retrograde messenger, involved in either potentiating or depressing neuronal excitability. NO may also play a role in the cerebral vasodilatory response to increased neuronal activity (i.e., seizures). In this study, two questions were asked: (1) is NO an endogenous anticonvulsant or proconvulsant substance? and (2) is the cerebral blood flow (CBF) increase accompanying bicuculline (BC)-induced seizures mediated by NO? The experiments were performed in 300-400-g Wistar rats anesthetized with 0.6% halothane and 70% N2O/30% O2. CBF was measured using the intracarotid 133Xe clearance method or laser-Doppler flowmetry. EEG activity was recorded. Chronic treatment (4 days) with nitro-L-arginine (L-NA), a potent NO synthase (NOS) inhibitor (400 mg/kg total), suppressed brain NOS by > 97% and prolonged seizure duration from 6 +/- 1 (saline-treated controls) to 12 +/- 2 min. In the L-NA-treated group, the CBF increase was sustained as long as seizure activity remained, indicating that CBF was still tightly coupled to seizure activity. Interestingly, the supposed inactive enantiomer of L-NA, D-NA, also showed an inhibition of brain NOS activity, ranging from 87 to 100%. The duration of seizures in this group (average 8 +/- 2 min) corresponded directly to the magnitude of reduction in NOS activity (r = 0.83, P < 0.05). Specifically, the D-NA results indicated that NOS inhibition had to exceed 95% before any effect on seizure duration could be seen.(ABSTRACT TRUNCATED AT 250 WORDS)

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.