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

The Possible Role of Nitric Oxide signaling and NMDA Receptors in Allopurinol effect on Maximal Electroshock- and Pentylenetetrazol-Induced Seizures in Mice

Allopurinol, a uric-acid-lowering medication, has shown its efficacy in several studies suggesting that allopurinol can be prescribed as adjunctive cure meant for intractable epilepsy. The exact mechanism of allopurinol is still unknown. This study evaluates allopurinol's effect on seizure threshold, seizure incidence, and mortality rate in mice models. Moreover, the possible involvement of nitric oxide (NO) pathway and N-methyl-D-aspartate (NMDA) receptors are investigated. To evaluate the effect of allopurinol on seizure, we used the pentylenetetrazole (PTZ)-induced seizure along with maximal electroshock (MES)-induced seizure. To assess the underlying mechanism behind the allopurinol activity, we used nitric oxide synthase (NOS) substrate (L-arginine), NOS inhibitors (L-NAME, aminoguanidine, 7-nitroindazole), and NMDA receptor antagonist (MK-801). Intraperitoneal allopurinol administration at a dose of 50 mg/kg in mice showed a significant (p<0.001) anti-convulsant activity in the PTZ-induced seizure. Even though pre-treatment with L-Arginine (60 mg/kg) potentiates allopurinol's anti-convulsant effect in the PTZ-induced seizure, pre-treatment with L-NAME (10 mg/kg), aminoguanidine (100 mg/kg), and 7-nitroindazole (30 mg/kg) reversed the anti-convulsant effect of allopurinol in the PTZ-induced seizure. In addition, pre-treatment with MK-801 also decreased the anti-convulsant effect of allopurinol in the PTZ-induced seizure. While allopurinol at a dose of 50 mg/kg and 100 mg/kg did not induce protection against seizure incidence in the MES-induced seizure, it revealed a remarkable effect in reducing the mortality rate in the MES-induced seizure. Allopurinol increases the seizure threshold in PTZ-induced seizure and enhances the survival rate in MES-induced seizure. Allopurinol exerts its anti-convulsant effect, possibly through targeting NO pathway and NMDA receptors.

Anti-nociceptive properties of the xanthine oxidase inhibitor allopurinol in mice: role of A1 adenosine receptors

BACKGROUND AND PURPOSE

Allopurinol is a potent inhibitor of the enzyme xanthine oxidase, used primarily in the treatment of hyperuricemia and gout. It is well known that purines exert multiple effects on pain transmission. We hypothesized that the inhibition of xanthine oxidase by allopurinol, thereby reducing purine degradation, could be a valid strategy to enhance purinergic activity. The aim of this study was to investigate the anti-nociceptive profile of allopurinol on chemical and thermal pain models in mice.

EXPERIMENTAL APPROACH

Mice received an intraperitoneal (i.p.) injection of vehicle (Tween 10%) or allopurinol (10-400 mg kg(-1)). Anti-nociceptive effects were measured with intraplantar capsaicin, intraplantar glutamate, tail-flick or hot-plate tests.

KEY RESULTS

Allopurinol presented dose-dependent anti-nociceptive effects in all models. The opioid antagonist naloxone did not affect these anti-nociceptive effects. The non-selective adenosine-receptor antagonist caffeine and the selective A(1) adenosine-receptor antagonist, DPCPX, but not the selective A(2A) adenosine-receptor antagonist, SCH58261, completely prevented allopurinol-induced anti-nociception. No obvious motor deficits were produced by allopurinol, at doses up to 200 mg kg(-1). Allopurinol also caused an increase in cerebrospinal fluid levels of purines, including the nucleosides adenosine and guanosine, and decreased cerebrospinal fluid concentration of uric acid.

CONCLUSIONS AND IMPLICATIONS

Allopurinol-induced anti-nociception may be related to adenosine accumulation. Allopurinol is an old and extensively used compound and seems to be well tolerated with no obvious central nervous system toxic effects at high doses. This drug may be useful to treat pain syndromes in humans.

Neuroprotective Effect of Chuk-Me-Sun-Dan on NMDA- and AMPA-Evoked Nitric Oxide Synthase Activity in Mouse Brain

Chukmesundan (CMSD) is composed of 8 medicinal herbs including Panex ginseng C.A. MEYER, Atractylodes macrocephala KOID, Poria cocos WOLF, Pinellia ternata BREIT, Brassica alba BOISS, Aconitum carmichaeli DEBX, Cynanchum atratum BGE, and Cuscuta chinensis LAM and used for the treatment of various symptoms accompanying hypertension and cerebrovascular disorders. This study was carried out to examine the effects of CMSD on N-methyl-D-aspartate (NMDA)-evoked, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-evoked nitric oxide synthase (NOS) activity in mouse brain. In adult forebrain, CMSD influences neuronal maintenance and is neuroprotective in several injury models through mechanisms that are incompletely understood. Interaction is observed between CMSD and nitric oxide (NO). Because NO affects both neural plasticity and degeneration, we hypothesized that CMSD might rapidly modulate NO production. Using in vivo microdialysis we measured conversion of L-[14C] arginine to L-[14C] citrulline as an accurate reflection of NOS activity in adult mouse hippocampus. CMSD significantly reduced NOS activities to 62% of basal levels within 2 days of onset of delivery and maintained NOS activity at less than 45% of baseline throughout 3 days of delivery. These effects did not occur with control (distilled water) and were not mediated by effect of CMSD on glutamate levels. In addition, simultaneous delivery of CMSD treatment prevented significant increases in NOS activity triggered by the glutamate receptor agonists NMDA and AMPA. Rapid suppression by CMSD of basal and glutamate-stimulated NOS activity may regulate neuromodulatory functions of NO or protect neurons from NO toxicity and suggests a novel mechanism for rapidly mediating functions of CMSD. It is shown that NMDA receptor stimulation leads to activation of p21ras (Ras) through generation of NO via neuronal NOS. The competitive NOS inhibitor, L-nitroarginine methyl ester, and CMSD prevents Ras activation elicited by NMDA, thus supporting the physiologic relevance of endogenous NO regulation of Ras. These results suggest that Ras is a physiologic target of endogenously produced NO and indicates a signaling pathway for NMDA receptor activation that may be important for long-lasting neuronal responses.

Paracetamol inhibits nitric oxide synthesis in murine spinal cord slices

Paracetamol is an effective analgesic but its mechanism of action is unclear. We investigated the effect of paracetamol and the analgesic adjuvant caffeine on the activity of NO synthase in mouse spinal cord and cerebellar slices in vitro, by measuring the conversion of [(3)H]arginine to [(3)H]citrulline. Paracetamol (100 microM) had no effect on NO synthase activity in cerebellum, but in the spinal cord both paracetamol (100 microM) and caffeine (30 microM) attenuated glutamate (5 mM)-induced [(3)H]citrulline production and in combination they abolished it. In conclusion paracetamol inhibits spinal cord NO synthesis and this may be related to its analgesic effects.

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

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

The physiology and pathophysiology of nitric oxide in the brain

Nitric oxide (NO) is a molecule with pleiotropic effects in different tissues. NO is synthesized by NO synthases (NOS), a family with four major types: endothelial, neuronal, inducible and mitochondrial. They can be found in almost all the tissues and they can even co-exist in the same tissue. NO is a well-known vasorelaxant agent, but it works as a neurotransmitter when produced by neurons and is also involved in defense functions when it is produced by immune and glial cells. NO is thermodynamically unstable and tends to react with other molecules, resulting in the oxidation, nitrosylation or nitration of proteins, with the concomitant effects on many cellular mechanisms. NO intracellular signaling involves the activation of guanylate cyclase but it also interacts with MAPKs, apoptosis-related proteins, and mitochondrial respiratory chain or anti-proliferative molecules. It also plays a role in post-translational modification of proteins and protein degradation by the proteasome. However, under pathophysiological conditions NO has damaging effects. In disorders involving oxidative stress, such as Alzheimer's disease, stroke and Parkinson's disease, NO increases cell damage through the formation of highly reactive peroxynitrite. The paradox of beneficial and damaging effects of NO will be discussed in this review.

Adenosine protects cultured retinal neurons against NMDA-induced cell death through A1 receptors

PURPOSE

To determine whether adenosine can protect cultured retinal neurons, consisting mainly of amacrine cells, from N-methyl-D-aspartate (NMDA)-induced neurotoxicity, and to determine whether agonists and antagonists of adenosine receptors also have a protective effect.

METHODS

Cultured retinal neurons obtained from fetal Wistar rats (gestational age 18-19 days) were maintained for 10-11 days. Neurons were exposed to NMDA (1.0 mM) for 10 min with or without adenosine or to NMDA with adenosine receptor agonists or antagonists. Neuronal death was assessed by the trypan-blue exclusion method 24 hr after the exposure.

RESULTS

Adenosine at doses of 0.01 microM and higher significantly protected (p < 0.05, Dunnett) primary cultured fetal rat retinal neurons from apoptotic and/or necrotic death induced by NMDA (1.0 mM). The protective effect of adenosine (10 microM) against NMDA-induced neuronal death was lost by simultaneous exposure to selective A1 receptor antagonist but not to A2a receptor antagonist. Selective A1 receptor agonists had similar effects as adenosine, but A2a receptor agonists and 8-Br-cyclic AMP had no effect on cell viability.

CONCLUSIONS

Adenosine can protect cultured retinal neurons against NMDA-induced cell death via the A1 receptor.

NO inhibition of the magnocellular neuroendocrine system in rats is independent of cGMP signaling pathway

Our objective was to test the hypothesis that the cGMP signal-transduction mechanism mediates nitric oxide's (NO) modulation of oxytocin (OT) and vasopressin (VP) secretion from the hypothalamo-neurohypophysial system. Three studies were conducted in adult male Sprague-Dawley rats: (1a) Euhydrated rats received an intracerebroventricular (icv) infusion (1 microl/min for 30 min) of artificial cerebrospinal fluid (aCSF), vehicle (2.6% dimethyl sulfoxide [DMSO]) or 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (0.05 microg/microl), an inhibitor of soluble guanylyl cyclase (sGC). ODQ did not affect basal levels of plasma VP or OT; (1b) Rats dehydrated for 24 h received aCSF or 8-Br-cGMP (icv), a membrane-permeable analog of cGMP, and plasma hormones were measured 2 min later. 8-Br-cGMP did not significantly change VP or OT levels; (2) Rats ingested water or 2% NaCl for 4 days, and NO synthase (NOS) and sGC activities were measured in posterior pituitaries, the anatomical site of hormone secretion. Salt loading enhanced (P < 0.001) production of [(14)C]citrulline, the coproduct of NO synthesis, without altering cGMP; (3) One SON was microdialyzed with [(14)C]arginine and NOS and sGC activities were quantified in microdialysates during intravenous (iv) infusion of isotonic or hypertonic saline in awake and anesthetized rats. In awake rats, [(14)C]citrulline recovery, but not cGMP, increased (P < 0.05) during intravenous infusion of both isotonic and hypertonic solutions, and after insertion of microdialysis probe itself. In anesthetized rats, however, where basal NOS activity is low, intravenous infusion of hypertonic, but not isotonic solution, increased [(14)C]citrulline recovery without affecting cGMP. Thus, in the forebrain, neither NO produced basally nor during osmotic stimulation depends on cGMP to modulate plasma vasopressin and oxytocin secretion.

Evidence for the role of nitric oxide in caffeine-induced locomotor activity in mice

RATIONALE

Nitric oxide (NO) is implicated in the acute locomotor activating effects of some addictive drugs such as amphetamine and cocaine, but has not been investigated in the case of caffeine.

OBJECTIVES

We investigated the effects of a nitric oxide synthase (NOS) inhibitor Nomega-Nitro- l-arginine methyl ester ( l-NAME) and a combination of l-arginine, a NO precursor, and l-NAME on caffeine induced locomotor activity in Swiss Webster mice.

METHODS

Locomotor activity was recorded for 30 min immediately following caffeine (0.25-128 mg/kg) or saline administration. In a further study, l-NAME (15 and 30 mg/kg) was administered to another group of mice 30 min before caffeine (1 and 16 mg/kg) injections. Finally, l-arginine (1000 mg/kg), a nitric oxide precursor, was administered 20 min before the l-NAME (15 and 30 mg/kg) treatments and locomotor activities were again recorded immediately after caffeine (1 and 16 mg/kg) injections.

RESULTS

Caffeine (0.5-16 mg/kg) significantly increased locomotor activity, while l-NAME (30 mg/kg) blocked caffeine (1 and 16 mg/kg)-induced locomotor activity. The low dose of l-NAME blocked only caffeine (1 mg/kg)-induced locomotor activity. l-Arginine reversed the inhibitory effects of l-NAME on caffeine-induced locomotor activity. l-NAME and l-arginine had no effect on the locomotor activity of the mice when given by themselves.

CONCLUSIONS

The results suggest that caffeine-induced locomotor activity might be modulated by NO in mice.

Positive N-methyl-d-aspartate receptor modulation by selective glycine transporter-1 inhibition in the rat dorsal spinal cord in vivo

In this study we have employed the selective glycine transporter-1 (GlyT-1) and GlyT-2 transporter inhibitors R-(-)-N-methyl-N-[3-[(4-trifluoromethyl)phenoxy]-3-phenyl-propyl]glycine (1:1) lithium salt (Org 24598) and 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (Org 25543), respectively, and microdialysis perfusion to determine the effect of GlyT transporter inhibition on extracellular amino acid concentrations in the lumbar dorsal spinal cord of the halothane-anaesthetised rat. Reverse dialysis of Org 24598 (0.1-10 microM) induced a concentration-related increase in extracellular glycine accompanied by a progressive increase in citrulline, but not aspartate, glutamate or GABA, efflux. Org 25543 (10 microM) by the same route induced a similar increase in glycine levels without affecting the efflux of other amino acids quantified. To test the hypothesis that the increase in citrulline efflux resulted from activation of the N-methyl-D-aspartate receptor (NMDA-R)/nitric oxide synthase (NOS) signalling cascade, the sensitivity was determined of GlyT-1 inhibition-induced effects to NMDA-R antagonism or NOS inhibition. Co-administration by reverse dialysis of the selective NMDA-R channel blocker MK-801 (0.5 mM) or the selective antagonist of the strychnine-insensitive glycine site, 7-chlorokynurenic acid (1 mM), with Org 24598 (10 microM) did not affect the uptake inhibition-induced increase in glycine efflux, but did significantly attenuate the increase in extracellular citrulline. Similarly, co-administration with Org 24598 of the isoform non-selective and selective neuronal NOS inhibitors Nomega-nitro-L-arginine methyl ester (1 mM) or 1-(2-trifluoromethylphenyl)imidazole (0.2 mM), respectively, prevented Org 24598-induced citrulline efflux with no effect on increased glycine efflux. These data provide evidence that the observed increased in extracellular citrulline is a consequence of positive modulation of NMDA-R, secondary to increased extracellular glycine and support a protective role for GlyT-1 against fluctuations in extracellular glycine uptake at glutamatergic synapses in the dorsal spinal cord. Such a mechanism could be important to NMDA-R-mediated synaptic plasticity in the spinal cord and be of relevance to the clinical usage of GlyT-1 inhibitors.

Non-synaptic release of [3H]noradrenaline in response to oxidative stress combined with mitochondrial dysfunction in rat hippocampal slices

Brain ischemia is frequently associated with oxidative stress in the reperfusion period. It is known that noradrenaline (NA) is released in excess under energy deprivation by the sodium-dependent reversal of the monoamine carrier. However, it is not known how oxidative stress affects NA release in the brain alone or in combination with energy deprivation. As a model of oxidative stress, the effect of H(2)O(2) (0.1-1.5 mM) perfusion was investigated in superfused rat hippocampal slices. It elicited a dose-dependent elevation of the release of [(3)H]NA and its tritiated metabolites as well as a simultaneous drop in the tissue energy charge. Mitochondrial inhibitors, i.e. rotenone (10 microM), and oligomycin (10 microM) in combination, also decreased the energy charge, but they had only a mild effect on [(3)H]NA release. However, when H(2)O(2) was added together with oligomycin and rotenone their effect on [(3)H]NA release was greatly exacerbated. H(2)O(2) and mitochondrial inhibitors also induced an increase in [Na(+)](i) in isolated nerve terminals, and their effect was additive. The effect of H(2)O(2) on tritium release was temperature-dependent. It was also attenuated by the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (30 microM) and (+/-)-2-amino-5-phosphonopentanoic acid (10 microM), by the nitric oxide synthase inhibitors, N omega-nitro-L-arginine methyl ester (100 microM), or 7-nitroindazole (50 microM) and by the vesicular uptake inhibitor tetrabenazine (1 microM). Our results suggest that oxidative stress releases glutamate followed by activation of postsynaptic ionotropic glutamate receptors that trigger nitric oxide production and results in a flood of NA from cytoplasmic stores. The massive elevation of extracellular NA under conditions of oxidative stress combined with mitochondrial dysfunction may provide an additional source of highly reactive free radicals thus initiating a self-amplifying cycle leading to neuronal degeneration.

Potent sigma 1-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine provides ischemic neuroprotection without altering dopamine accumulation in vivo in rats

The in vivo signaling of ischemic neuroprotection provided by sigma-receptor ligands remains unclear. Catecholamines have been implicated in the propagation of ischemic neuronal injury, and previous in vitro studies suggest that sigma ligands modulate dopaminergic neurotransmission. In this study, we tested the hypothesis that the potent sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) attenuates the increase of extracellular dopamine in ischemic striatum. Under controlled physiological conditions, a microdialysis probe was implanted in right caudoputamen (CP) complex of adult male Wistar rats. Rats were subjected to 2 h of transient middle cerebral artery occlusion (MCAO) by the intraluminal suture technique. In a blinded, randomized fashion, rats were divided into five treatment groups: Group 1 (n = 8; saline-saline) continuous i.v. infusion of saline vehicle 30 min before MCAO followed by saline at reperfusion until the end of the experiment; Group 2 (n = 8; PPBP-PPBP) i.v. PPBP 30 min before MCAO followed by 1 micromol x kg(-1) x h(-1) of PPBP; Group 3 (n = 8; saline-PPBP) i.v. saline before MCAO followed by PPBP; Group 4 (n = 4) surgical shams (saline-saline); and Group 5 (n = 4) surgical shams (PPBP-PPBP). Infarction volume at 22 h of reperfusion in the CP complex (percentage of ipsilateral structure) was significantly attenuated in rats treated with PPBP-PPBP (27.3% +/- 9.1%) and saline-PPBP (27.8% +/- 12.7%) compared with saline-saline (59.3% +/- 7.3%) treatment. There was a three- to fourfold increase in dopamine concentrations in the microdialysates within 40 min of the onset of MCAO. Dopamine and its metabolites dihydroxy phenylacetic acid and homovallinic acid levels were similar among the three groups subjected to MCAO. Therefore, PPBP provides significant ischemic neuroprotection in the CP complex without altering the acute accumulation of dopamine in vivo during transient focal ischemia in the rat.

P-450 epoxygenase and NO synthase inhibitors reduce cerebral blood flow response to N-methyl-D-aspartate

Epoxyeicosatrienoic acids are cerebral vasodilators produced in astrocytes by cytochrome P-450 epoxygenase activity. The P-450 inhibitor miconazole attenuates the increase in cerebral blood flow (CBF) elicited by glutamate. We evaluated whether epoxygenase activity is involved in the CBF response to activation of the N-methyl-D-aspartate (NMDA) receptor subtype by using two structurally distinct inhibitors, miconazole and N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH), a selective epoxygenase substrate inhibitor. Drugs were delivered locally through microdialysis probes in striata of anesthetized rats. Local CBF was measured by hydrogen clearance and compared with CBF in contralateral striatum receiving vehicle. Microdialysis perfusion of NMDA doubled CBF and increased nitric oxide (NO) production estimated by recovery of labeled citrulline in the dialysate during labeled arginine infusion. Perfusion of miconazole or MS-PPOH blocked the increase in CBF without decreasing citrulline recovery. Perfusion of N(omega)-nitro-L-arginine decreased baseline CBF and inhibited the CBF response to NMDA. Perfusion of MS-PPOH did not inhibit the CBF response to sodium nitroprusside. We conclude that both the P-450 epoxygenase and NO synthase pathways are involved in the local CBF response to NMDA receptor activation, and that the signaling pathway may be more complex than simply NO diffusion from neurons to vascular smooth muscle.

Adenosine-amino acid interactions in the chick brain: a role in passive avoidance learning

The present work describes interactions between adenosine and the amino acids glutamate and GABA in slices of intermediate medial hyperstriatum ventrale (IMHV), an area of the chick brain known to be involved in learning and memory events associated with a one-trial passive avoidance task. In slices derived from the IMHV of untrained chicks, the A(1) receptor agonist N(6)-cyclohexyladenosine (CHA; 10 microM) specifically inhibited glutamate release. Conversely, cyclopentyltheophylline (CPT; 100 microM an A(1) antagonist) increased glutamate release from the slices and blocked the CHA-induced inhibition of glutamate. The A(2) receptor agonist 2-p-(2-carboxylethyl)-phenylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) selectively increased glutamate release when applied at 5 microM while it selectively inhibited GABA release at a lower concentration (10 nM). The addition of NMDA to the medium, resulted in increased adenosine release equivalent to that found following stimulation with 50 mM KCl. Both the NMDA and the KCl-induced increases were eliminated by addition of D-2-amino-5 phosphopentanoic acid (D-AP5), an NMDA-receptor antagonist. Slices prepared from the IMHV of chicks following successful training on the task showed enhanced adenosine release 30 min, 1, 3 and 6.5 h after training compared to chicks trained to peck a water-coated bead. The results show that changes in adenosine release from the IMHV accompany memory formation in the chick. We suggest that adenosine-amino acid transmitter interactions potentially via the activation of NMDA receptors, a necessary step in long-term memory formation for the task, may modulate the formation of memory for the one-trial passive avoidance task.

Methylene blue inhibits hippocampal nitric oxide synthase activity in vivo

The aim of the present study was to investigate the effect of methylene blue, a guanylate cyclase inhibitor, on the hippocampal nitric oxide synthase activity in vivo. We used a microdialysis-based technique of measuring conversion of [3H]l-arginine to [3H]l-citrulline in freely moving rats. The administration of methylene blue (0.1 and 1 mM) via the microdialysis probe caused a dose-dependent decrease in [3H]l-citrulline efflux comparable with the effect of unselective NOS inhibitor NG-nitro-L-arginine (2 mM). We conclude that methylene blue inhibits brain NOS activity in vivo and thus interferes with NO-cGMP cascade in different levels.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A1 receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.

Nitric oxide (NO) release by glutamate and NMDA in the dorsal horn of the spinal cord: an in vivo electrochemical approach in the rat

Glutamate acts as a neurotransmitter of primary afferent messages in the spinal cord. Through glutamatergic mechanisms nitric oxide (NO) is also a potential intermediary in the transmission of sensory messages, particularly nociceptive, at the spinal level. The aim of the present study was, by using electrochemical monitoring of NO, to determine if the activation of glutamatergic transmission, particularly through NMDA receptors, could increase NO production within the dorsal horn of the lumbar spinal cord in the rat. 30 micrometers diameter treated carbon fiber electrodes coated with nickel-porphyrine and Nafion(R), and associated with differential normal pulse voltammetry, have been used in vivo to monitor NO within the dorsal horn of the lumbar spinal cord of decerebrated-spinalized rats. A NO-dependent peak of oxidation current (650 mV vs. Ag-AgCl), remaining stable for up to 3 h (+/-5%) could be detected under basal conditions, which indicates that significant amounts of NO are produced continuously. The non-competitive N-methyl-d-aspartate (NMDA) channel blockers, Ketamine (100 mg kg-1 i.p.) and MK-801 (10 mg kg-1 i.p.), decreased the voltammograms to 70+/-5% and 69+/-2% of controls at 120 min, respectively. Glutamate (10 mM), when directly superfused upon the spinal cord (20 min at 50 microliters min-1) induced a rapid and significant increase of the 650 mV peak, with a maximum at around 90 min (148+/-6% of control) followed by a slow decay (138+/-4% of control at 150 min). This increase could be totally reversed or blocked by i.p. injection of 100 mg kg-1 of Ketamine. NMDA (30 mg kg-1 i.p.) induced a long-lasting increase in the peaks (149+/-11% at 90 min and 162+/-20% at 120 min), which was also fully reversed by Ketamine or MK-801. These results provide in vivo direct evidence of a glutamate- and/or NMDA-induced release of NO at the spinal level, and is discussed in relation to the glutamatergic transmission of primary afferent messages.

Photolytically released nitric oxide produces a delayed but persistent suppression of LTP in area CA1 of the rat hippocampal slice

1. We have used flash photolysis of a caged form of nitric oxide (NO), potassium pentachloronitrosylruthenate (K2Ru(NO)Cl5), to apply known concentrations of NO, with a high degree of temporal resolution, to hippocampal slices prepared from juvenile male rats maintained in an interface recording chamber. 2. Photolytically released NO (1-4.5 microM) from bath applied caged NO reduced the magnitude of long-term potentiation (LTP) in a concentration-dependent manner. This effect was abolished in the presence of the NO scavenger haemoglobin. NO had no effect on pre-established LTP. 3. Exposure to photolytically released NO had no effect on normal fast synaptic transmission, but did result in depression of N-methyl-D-aspartate (NMDA) receptor-mediated transmission recorded using extracellular electrodes. The onset of NO-induced depression was relatively slow, taking >40 s to manifest itself, and several minutes to achieve maximum depression (t approximately 70 s). NO-induced depression persisted for more than 2 h after photolysis. The time courses of the action of NO on NMDA receptor-mediated responses and its action on the induction of LTP were similar. 4. These results suggest that released NO may play a role in determining the subsequent threshold for the induction of LTP at Schaffer-commissural synapses through a reduction in the efficacy of NMDA receptor function when repeated conditioning trains are used.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.

Nerve growth factor rapidly suppresses basal, NMDA-evoked, and AMPA-evoked nitric oxide synthase activity in rat hippocampus in vivo

In adult forebrain, nerve growth factor (NGF) influences neuronal maintenance and axon sprouting and is neuroprotective in several injury models through mechanisms that are incompletely understood. Most NGF signaling is thought to occur after internalization and retrograde transport of trkA receptor and be mediated through the nucleus. However, NGF expression in hippocampus is rapidly and sensitively regulated by synaptic activity, suggesting that NGF exerts local effects more dynamically than possible through signaling requiring retrograde transport to distant afferent neurons. Interactions have been reported between NGF and nitric oxide (NO). Because NO affects both neural plasticity and degeneration, and trk receptors can mediate signaling within minutes, we hypothesized that NGF might rapidly modulate NO production. Using in vivo microdialysis we measured conversion of L-[14C]arginine to L-[14C]citrulline as an accurate reflection of NO synthase (NOS) activity in adult rat hippocampus. NGF significantly reduced NOS activity to 61% of basal levels within 20 min of onset of delivery and maintained NOS activity at less than 50% of baseline throughout 3 hr of delivery. This effect did not occur with control protein (cytochrome c) and was not mediated by an effect of NGF on glutamate levels. In addition, simultaneous delivery of NGF prevented significant increases in NOS activity triggered by the glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Rapid suppression by NGF of basal and glutamate-stimulated NOS activity may regulate neuromodulatory functions of NO or protect neurons from NO toxicity and suggests a novel mechanism for rapidly mediating functions of NGF and other neurotrophins.

Inhibitory effects of hypoxia and adenosine on N-methyl-D-aspartate-induced pial arteriolar dilation in piglets

Our previous studies have indicated that oxygen radicals, produced during reoxygenation following short-term arterial hypoxia, lead to sustained suppression of cerebral arteriolar responses to N-methyl-D-aspartate (NMDA). However, whether arteriolar dilator responses to NMDA are reduced during arterial hypoxia has never been examined. In this study, we determined whether hypoxia or hypoxia-related metabolites such as adenosine or nitric oxide (NO) will reduce NMDA-induced arteriolar dilation. We have also determined the location of NMDA receptor- and brain nitric oxide synthase (bNOS)-positive neurons in the cerebral cortex. In anesthetized piglets, pial arteriolar diameters were determined using intravital microscopy. Baseline arteriolar diameters were approximately 100 microns. Topical application of NMDA at concentrations of 10(-5), 5 x 10(-5) and 10(-4) M resulted in dose-dependent vasodilation (9 +/- 2, 18 +/- 2 and 29 +/- 2% above baseline, respectively, n = 21). Administration of theophylline (20 mg/kg, i.v.) had no effect on NMDA-dependent vasodilation, but it did block dilation to hypoxia (inhalation of 8.5% O2). In theophylline-treated animals, NMDA responses were completely abolished during hypoxia (28 +/- 2 vs. 2 +/- 1%, respectively to 10(-4) M, n = 7) while sodium nitroprusside (SNP, 10(-4) M) still dilated pial arterioles normally. NMDA-induced vasodilation was not modified after application and removal of adenosine (10(-4) M; n = 5) or SNP (10(-5) M; n = 4), or when SNP (10(-7) M) was coapplied with NMDA (n = 6). Conversely, coapplication of adenosine (10(-6) M) attenuated NMDA responses (31 +/- 5 vs. 20 +/- 3%, n = 7). We also found that NMDA receptor- and bNOS-containing neurons were located predominantly in layers II/III of the cortex. Proximity of these neurons to the cortical surface is consistent with diffusion of NO to pial arterioles as the mechanism of dilation to NMDA. We conclude that NMDA-induced cerebral arteriolar dilation is inhibited by hypoxia alone and by exogenous adenosine, but not by NO.

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

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

The protective effect of 2-chloroadenosine against the development of amygdala kindling and on amygdala-kindled seizures

The influence of 2-chloroadenosine, a non-metabolizable adenosine A1 receptor agonist, was tested on the development of electrically kindled amygdala and on the seizure responses of fully kindled rats. Focal intra-amygdaloid injection of 2-chloroadenosine (1-10 nmol/0.5 microl) 20 min before applying the daily kindling stimulus prevented the development of the kindling process. The behavioural seizure score and the afterdischarge duration were reduced below their initial values. The antiepileptogenic effects of 1 and 10 nmol of 2-chloroadenosine were reversible 8-10 days after withdrawal of the drug. When 2-chloroadenosine was tested on fully developed stage 5 amygdala-kindled seizures, it increased the generalised seizure threshold in a dose-dependent manner. A maximum efficiency of 125% (P < 0.001) was achieved with 5 nmol and the median effective dose was 0.55 nmol. Higher doses resulted in the reduced anticonvulsant effect (P < 0.05). With the same daily stimulation, 2-chloroadenosine 5 nmol in 0.5 microl vehicle, significantly reduced the maximum seizure score by 90%, the afterdischarge duration by 88% and completely blocked the generalised seizure duration. The antiseizure activity of the drug lasted for 3 days. In conclusion, 2-chloroadenosine not only acts as an anticonvulsant against electrically induced kindled seizures as described here, and against audiogenic seizures, electroshock and a variety of chemical convulsants as described by others, it prevents the development of the epileptic state by kindling-stimulation, i.e., it is antiepileptogenic. We theorise here that this is due to its blockade of presynaptic glutamate release.

Characterization of metabotropic glutamate receptor-mediated nitric oxide production in vivo

We tested the hypothesis that stimulation of metabotropic glutamate receptors (mGluRs) increases nitric oxide (NO) production in the hippocampus in vivo. Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 h with artificial cerebrospinal fluid (CSF) containing 3 microM [14C]-L-arginine. Recovery of [14C]-L-citrulline in the effluent was used as a marker of NO production. In nine groups of rats, increases in [14C]-L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of the selective mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD); the mGluR antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG); the NO synthase inhibitor, N-nitro-L-arginine (LNNA); the ryanodine sensitive calcium-release channel inhibitor dantrolene, the non-N-methyl-D-aspartate (NMDA); receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801); and the Na+ channel blocker, tetrodotoxin. Recovery of [14C]-L-citrulline during perfusion with artificial CSF progressively increased to 90 +/- 21 fmol/min (+/-SD) over 5 h. Perfusion in the contralateral hippocampus with 1 mM ACPD augmented [14C]-L-citrulline recovery to 250 +/- 81 fmol/min. Perfusion of 1 mM nitroarginine + ACPD inhibited [14C]-L-citrulline recovery compared to that with ACPD alone. Perfusion with 1 mM MCPG + ACPD attenuated ACPD enhanced [14C]-L-citrulline recovery. Perfusion of 1 mM dantrolene + ACPD inhibited the ACPD-evoked increase in [14C]-L-citrulline recovery. Perfusion of 1 mM MCPG or dantrolene without ACPD did not decrease [14C]-L-citrulline recovery as compared to CSF alone. ACPD-enhanced [14C]-L-citrulline recovery was not attenuated by CNQX, MK-801, or tetrodotoxin (TTX). Using an indirect method of assessing NO production in vivo, these data demonstrate that mGluR stimulation enhances NO production in rat hippocampus. Inhibition with dantrolene suggests that calcium-induced calcium release amplifies the inositol triphosphate-mediated calcium signal associated with mGluR stimulation, thereby resulting in augmented calcium-dependent NO production.