Non-selective Adenosine Receptor Antagonist Research Articles

Caffeine reduces hypnotic effects of alcohol through adenosine A 2A receptor blockade

The role of the adenosine A 2A receptor in the hypnotic effects of ethanol was assessed in mice. The duration of the loss of righting reflex following acute ethanol administration was shorter for A 2A receptor-deficient mice (A 2AR KO) than for wild-type mice (A 2AR WT), whereas the fall in body temperature was not different between the two phenotypes. In contrast, the duration of the loss of righting reflex was increased in A 2AR KO mice versus controls after administration of pentobarbital. Dipyridamole, an inhibitor of adenosine uptake, increased the sleep time observed following administration of ethanol in CD1 mice and in A 2AR WT but not in A 2AR KO mice. SCH 58261, a selective A 2A receptor antagonist, unlike DPCPX, a selective A 1 receptor antagonist, shortened the duration of the loss of righting reflex induced by ethanol, thus mimicking the lack of receptor in deficient mice. Finally, the non-selective adenosine receptor antagonist caffeine (25 mg/kg) reduced ethanol-induced hypnotic effects. These results indicate that the activation of A 2A receptors that follows an increase in extracellular adenosine levels caused by the administration of high doses of ethanol plays a role in its hypnotic effects. Thus, A 2A receptor antagonists may be useful therapeutic agents for alleviating ethylic coma.

Effects of adenosine receptor antagonists on pial arteriolar dilation during carbon dioxide inhalation

The role of adenosine in the cerebrovascular response to carbon dioxide inhalation was evaluated in two sets of experiments. The pial circulation was recorded by a Laser-Doppler flow probe placed over a closed cranial window in methoxyflurane anesthetized rats. Topical application of the nonselective adenosine receptor antagonist caffeine (1 mM), the selective A 1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX,1 μM), or the selective A 2A receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3- a]triazin-5-yl amino]ethyl) phenol (ZM 241385, 1 μM) all failed to affect mean arterial blood pressure, basal cerebral blood flow, or the carbon dioxide-evoked hyperemia. Systemically administered caffeine (20 mg/kg) also had no significant effects. However, following the systemic administration of the nonselective nitric oxide synthase inhibitor, N G-nitro- l-arginine methyl ester ( l-NAME, 20 mg/kg), the topical application of both caffeine and ZM 241385 (but not DPCPX) significantly reduced the carbon dioxide-evoked hyperemia. l-NAME (20 mg/kg) administered intravenously, evoked a significant increase in mean arterial blood pressure, a slow progressive decline in cerebral blood flow and, during brief (60–90 s) periods of 7.5% carbon dioxide inhalation, a significant decrease in arterial blood pressure. l-NAME failed to reduce the carbon dioxide-evoked increase in cerebral blood flow as measured by the area under the curve (AUC), although it did reduce the peak flow response. Topically applied l-NAME (1 mM) failed to alter mean arterial blood pressure, basal cerebral blood flow, or the carbon dioxide-evoked increases in cerebral blood flow. In a second series of experiments, we evaluated the ability of 10% carbon dioxide inhalation for 8 min to elicit a release of adenosine from the cerebral cortex. Adenosine levels in the cortical superfusates rose significantly during periods of carbon dioxide inhalation. The data suggest that following the removal of the confounding effects of nitric oxide, which are unlikely to be mediated locally, a significant contribution by adenosine A 2A receptor activation to the carbon dioxide-evoked cortical hyperemia was evident.

A 1-receptor blockade: a novel approach for assessing myocardial viability in chronic ischemic cardiomyopathy

Diminished myocardial function can be seen in chronic coronary stenosis (CS) even in the presence of normal resting myocardial blood flow. We hypothesized that adenosine contributes to myocardial depression in this setting, predominantly through activation of the A 1 adenosine receptor. To test this hypothesis we used aminophylline, a nonselective adenosine receptor antagonist, and 8-cyclopentyl 1,3 dipropylxanthine, a selective A 1 adenosine receptor antagonist, in a canine model of chronic CS. Chronic CS was produced by placement of ameroid constrictors on the left anterior descending and left circumflex coronary arteries in 17 adult mongrel dogs, which resulted in severe left ventricular dysfunction 6 weeks later. Eight dogs without ameroid placement were used as controls (C). Closed-chest echocardiographic short-axis images at the low midpapillary level, hemodynamics, and radiolabeled microsphere-derived myocardial blood flow were obtained before and immediately after injection of either 5 mg/kg −1 of aminophylline (7 left ventricular dysfunction and 4 C dogs) or 1 mg/kg −1 of 8-cyclopentyl 1,3-dipropylxanthine (10 left ventricular dysfunction and 4 C dogs). Both 8-cyclopentyl 1,3-dipropylxanthine and aminophylline had no effect in C animals but resulted in a significant transient increase in regional percent wall thickening ( P < .05) with a concomitant decrease in end-systolic wall stress ( P < .05) in CS animals. There was no change in transmural myocardial blood flow or systemic hemodynamics to explain these results. Thus, adenosine plays a significant role in myocardial dysfunction in chronic ischemia by activation of the A 1 receptor. Aminophylline or a selective A 1 adenosine receptor antagonist can be used to detect viable myocardium and may be safer than dobutamine in severe chronic ischemic heart disease.

Losartan and atenolol on hypertension induced by adenosine receptor blockade.

1. The prolonged infusion of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, induces hypertension, an increase in plasma renin activity and morphological cardiovascular changes. 2. The aim of this work was to evaluate the effects of losartan, a selective AT1 receptor antagonist, and atenolol, a beta-adrenoceptor antagonist, on DPSPX-induced hypertension. 3. Male Wistar rats (250-300 g, n = 4-6) were treated for 1 or 4 weeks with: saline i.p.; DPSPX (90 microg kg(-1) h(-1)) i.p.; losartan (15 mg kg(-1) day(-1)) p.o.; atenolol (25 mg kg(-1) day(-1)) p.o.; DPSPX (90 microg kg(-1) h(-1)) i.p. + losartan (15 mg kg(-1) day(-1)) p.o.; DPSPX (90 microg kg(-1) h(-1)) i.p. + atenolol (25 mg kg(-1) day(-1)) p.o. Blood pressure was measured by the 'tail-cuff' method in conscious animals. Fragments of the mesenteric and tail arteries were processed for morphological study and the mean diameter of the vascular smooth muscle cells was determined. 4. DPSPX increased blood pressure. Losartan and atenolol prevented this rise but had no effect on blood pressure of control rats. DPSPX-treated groups showed hypertrophy of the vascular smooth muscle cells and proliferation of subintimal cells. Losartan but not atenolol prevented these changes. Losartan had no effect on the vascular morphology of control rats, while treatment with atenolol for 4 weeks induced hypertrophy of the vascular smooth muscle cells. 5. Both losartan and atenolol counteract the development of DPSPX-induced hypertension but only losartan prevents the alterations in vascular morphology.

ATP stimulates the release of prostacyclin from perfused veins isolated from the hamster hindlimb.

ATP-stimulated prostacyclin release from veins was investigated using epigastric veins isolated from hamsters. Veins were perfused with MOPS-buffered physiological salt solution (PSS). ATP was administered into the perfusate, and the bath solution (MOPS-PSS) was collected and assayed for the presence of the stable prostacyclin metabolite 6-keto-PGF1alpha. ATP (100 microM) resulted in reproducible increases in bath concentration from 73 +/- 22 to 279 +/- 50 pg/ml (P < 0.05, n = 5). This response was abolished by indomethacin (10 microM, P < 0.05). To ascertain whether the endothelium was the source of prostacyclin, endothelium was disrupted using air (n = 10) or deoxycholic acid (n = 6). Perfusion with air significantly reduced (P < 0.05) but did not completely abolish ATP-stimulated release of prostacyclin, while deoxycholic acid totally abolished the response (P < 0.05). The nonselective P2 receptor antagonist reactive blue 2 (100 microM) attenuated ATP-mediated release of prostacyclin but did not significantly alter ACh-stimulated release of prostacyclin. The nonselective adenosine receptor antagonist xanthine amine congener (1 microM) had no effect on ATP-stimulated release, and adenosine did not stimulate the release of prostacyclin. These results show that increases in intraluminal concentration of ATP stimulate abluminal release of prostacyclin from the venous endothelium. This effect is mediated by P2 receptors while adenosine and its receptors are not involved in this response.

Adenosine receptor blockade reverses hypophagia and enhances locomotor activity of dopamine-deficient mice.

Adenosine receptors modulate dopaminergic function by regulating dopamine release in presynaptic neurons and intracellular signaling in postsynaptic striatal neurons. To investigate how adenosine impinges on the action of dopamine in feeding and locomotion, genetically altered, dopamine-deficient mice were treated with adenosine receptor antagonists. Acute administration of the nonselective adenosine receptor antagonist, caffeine (5-25 mgkg i.p.), reversed the hypophagia of mutant mice and induced hyperactivity in both control and mutant animals. However, caffeine treatment elicited much less hyperactivity in dopamine-deficient mice than did l-3,4-dihydroxyphenylalanine (l-dopa) administration, which partially restores dopamine content. Caffeine treatment enhanced feeding of l-dopa-treated mutants but, unexpectedly, it reduced their hyperlocomotion. Caffeine administration induced c-Fos expression in the cortex of dopamine-deficient mice but had no effect in the striatum by itself. Caffeine attenuated dopamine agonist-induced striatal c-Fos expression. An antagonist selective for adenosine A(2A) receptors induced feeding and locomotion in mutants much more effectively than an A(1) receptor antagonist. l-dopa-elicited feeding and hyperlocomotion were reduced in mutants treated with an A(1) receptor agonist, whereas an A(2A) receptor agonist decreased l-dopa-induced feeding without affecting locomotion. The observations suggest that the hypophagia and hypoactivity of mutants result not only because of the absence of dopamine but also because of the presence of A(2A) receptor signaling. This study of a genetic model of dopamine depletion provides evidence that A(2A) receptor antagonists could ameliorate the hypokinetic symptoms of advanced Parkinson's disease patients without inducing excessive motor activity.

Involvement of adenosine A1 receptors in forced walking stress-induced analgesia in mice.

We investigated the involvement of adenosine receptors on forced walking stress-induced analgesia using a formalin-induced paw-licking test in male mice. Exposure to forced walking stress for 6 h showed stress-induced analgesia in the second phase (10-30 min), but not in the first phase (0-10 min). In the second phase, forced walking stress-induced analgesia was blocked by theophylline, a nonselective adenosine-receptor antagonist and DPCPX, an adenosine A1-receptor antagonist, but not ZM 241385, an adenosine A2A-receptor antagonist. These findings suggest that adenosine A1 receptors are involved in the analgesic mechanism activated by the forced walking stress.

Effect of hypoxia on excitatory transmission in the rat substantia gelatinosa neurons

We have investigated the effect of hypoxia on the excitatory synaptic transmission in the substantia gelatinosa neurons using perforated-patch-clamp configuration. Brief periods of hypoxia induced a depression in the evoked excitatory postsynaptic current (eEPSC) amplitude. The hypoxia-induced depression of eEPSC was not observed in the presence of theophylline, a nonselective adenosine receptor antagonist, and DPCPX, a selective adenosine receptor A 1 antagonist. Application of adenosine (100 μM) also depressed eEPSC in a similar way as with hypoxia. This adenosine-induced depression of eEPSC was inhibited by DPCPX. Hypoxia and exogenous adenosine decreased the frequency of the spontaneous excitatory postsynaptic current (sEPSC) but not the amplitude of sEPSC and increased the paired-pulse ratio. From these results, it is suggested that acute hypoxia depresses the excitatory synaptic transmission by activating the presynaptic adenosine A 1 receptor.

Effects of adenosine analogs on inositol 1,4,5-trisphosphate production in porcine coronary artery

Using various pharmacological methods, we previously demonstrated that the smooth muscle and endothelium of porcine coronary artery contain vasorelaxant adenosine A 2 receptors, which are predominantly the A 2A subtype. The present study was intended to investigate the effect of adenosine receptor stimulation on agonist-induced inositol 1,4,5-trisphosphate (IP 3) generation in porcine coronary artery using the nonselective adenosine analogs, 2-chloroadenosine (CAD) and 5′-( N-ethylcarboxamido)adenosine (NECA), and the A 2A selective analog 2- p-(2-carboxyethyl)-phenethylamino-5′- N-ethylcarboxamidoadenosine (CGS). In both endothelium-intact and denuded coronary artery rings, CAD, NECA and CGS elicited a dose-dependent inhibition of prostaglandin F 2α (PG)-induced IP 3 production. However, the inhibitory effect of NECA was relatively less in endothelium-denuded preparations. The nonselective xanthine adenosine receptor antagonist, 8-sulfophenyltheophylline (8-SPT), significantly attenuated the IP 3-inhibitory effect of CAD and, to a lesser extent, that of NECA. Further, the A 2A selective nonxanthine antagonist, 5-amino-7-(2-phenylethyl)-2-(-furyl)-pyrazolo[4,3]-1,2,4-triazolo[1,5- c]pyrimidine (SCH), markedly decreased the effects of all CAD, NECA and CGS on PG-induced IP 3 generation. These results provide evidence that activation of adenosine A 2 receptors by CAD, NECA and CGS in porcine coronary artery causes inhibition of agonist-induced IP 3 production, and these receptors involve at least the A 2A subtype.

Angiotensin converting enzyme inhibition prevents trophic and hypertensive effects of an antagonist of adenosine receptors

The continuous infusion of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, causes hypertension and marked cardiovascular structural changes in Wistar rats. Adenosine inhibits noradrenaline and renin release. We investigated the effects of sympathetic denervation, evaluated renin activity and the influence of angiotensin converting enzyme inhibition in DPSPX-treated rats. Captopril was given (30 or 100 mg kg −l day −l; p.o.) from day −l to day 28. On day 0, constant infusions of DPSPX (90 μg kg −l h −l; i.p.) or vehicle were started. On day 28, fragments of the left ventricle, mesenteric and tail arteries were processed for morphological studies. Plasma renin activity was increased in DPSPX-treated animals. Sympathetic denervation delayed and partially prevented blood pressure rise. Angiotensin converting enzyme inhibition prevented DPSPX-induced hypertension and morphological changes. Our results, although pointing to the involvement of the sympathetic system, suggest that other mechanisms are involved. We could not differentiate between the trophic and anti-hypertensive effects of angiotensin converting enzyme inhibition.

2-Chloroadenosine, a preferential agonist of adenosine A 1 receptors, enhances the anticonvulsant activity of carbamazepine and clonazepam in mice

2-Chloroadenosine (0.25–1 mg/kg) significantly raised the threshold for electroconvulsions in mice. This preferential adenosine A 1 receptor agonist (at 0.125 mg/kg) significantly potentiated the protective activity of carbamazepine against maximal electroshock-induced seizures in mice. 2-Chloroadenosine (1 mg/kg) showed also anticonvulsive efficacy against pentylenetetrazol-evoked seizures, raising the CD 50 value for pentylenetetrazol from 77.2 to 93.7 mg/kg. The drug (at 0.5 mg/kg) significantly enhanced the protective action of clonazepam in this test, decreasing its ED 50 value from 0.033 to 0.011 mg/kg. Moreover, aminophylline, a non-selective adenosine receptor antagonist (5 mg/kg), and 8-cyclopentyl-1,3-dimethylxanthine (8-CPX), a selective A 1 adenosine receptor antagonist (5 mg/kg) reversed the 2-chloroadenosine (0.125 mg/kg)-induced enhancement of the protective activity of carbamazepine and clonazepam. 2-Chloroadenosine administered alone or combined with antiepileptic drugs, caused neither motor nor long-term memory impairment. Finally, the adenosine A 1 agonist did not change the free plasma concentration of antiepileptics, so a pharmacokinetic factor is not probable. Summing up, 2-chloroadenosine potentiated the protective activity of both carbamazepine and clonazepam, which seems to be associated with the enhancement of purinergic transmission mediated through adenosine A 1 receptors.

Effects of nitrobenzylthioinosine on neuronal injury, adenosine levels, and adenosine receptor activity in rat forebrain ischemia.

Adenosine levels increase in brain during cerebral ischemia, and adenosine has receptor-mediated neuroprotective effects. This study was performed to test the hypothesis that nitrobenzylthioinosine (NBMPR), a selective and potent inhibitor of one adenosine transporter subtype termed ENT1, or es, can protect against ischemic neuronal injury by enhancing adenosine levels and potentiating adenosine receptor-mediated effects, including attenuation of the cellular production and release of tumor necrosis factor-alpha (TNF-alpha). In rats, the phosphorylated prodrug form of NBMPR, NBMPR-phosphate, or saline was administered by intracerebroventricular injection 30 min before forebrain ischemia. Seven days following the ischemic episode, rats were killed, and neuronal damage in the CA1 region of the hippocampus was assessed. The number of pyramidal neurons was significantly (p < 0.001) greater in the NBMPR-P treatment group. A trend toward protection was still evident at 28 days postreperfusion. Adenosine increased significantly during ischemia to levels eight- to 85-fold above basal. NBMPR-P treatment did not cause statistically significant increases in ischemic adenosine levels; however, this treatment tended to increase adenosine levels in all brain regions at 7 min postreperfusion. Ischemia-induced expression of TNF-alpha was not altered by NBMPR-P treatment, and the nonselective adenosine receptor antagonist 8-(p-sulfophenyl) theophylline did not abolish the neuroprotective effects of NBMPR-P treatment. These data indicate that NBMPR can protect CA1 pyramidal neurons from ischemic death without statistically significant effects on adenosine levels or adenosine receptor-mediated inhibition of the proinflammatory cytokine TNF-alpha.

KF24345, an adenosine uptake inhibitor, suppresses lipopolysaccharide-induced tumor necrosis factor-alpha production and leukopenia via endogenous adenosine in mice.

3-[1-(6,7-Diethoxy-2-morpholinoquinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione hydrochloride (KF24345) is a novel potent adenosine uptake inhibitor. KF24345 inhibited [(3)H]adenosine uptake into erythrocytes from human, mouse, rabbit, and hamster with IC(50) values of 59.5, 130.1, 104.2, and 30.9 nM, respectively. In mice, oral administration of KF24345 at 10 mg/kg almost completely inhibited the [(3)H]adenosine uptake into sampled blood cells at least up to 10 h of the administration. In this study, to examine whether the adenosine uptake inhibition exhibits anti-inflammatory effects, we determined the effects of KF24345 on lipopolysaccharide (LPS)-induced tumor necrosis factor-alpha (TNF-alpha) production and leukopenia in mice. KF24345 (10 mg/kg p.o.) significantly suppressed the elevation of serum TNF-alpha concentration after the LPS injection, and the suppressing effect of KF24345 was abolished by the treatment with 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol, a selective adenosine A(2) receptor antagonist, but not with 8-(noradamantan-3-yl)-1,3-dipropylxanthine, a selective adenosine A(1) receptor antagonist. KF24345 (10 mg/kg p.o.) also inhibited the decrease of leukocytes after the LPS injection, and 8-(p-sulfophenyl)theophylline, a nonselective adenosine receptor antagonist, completely reversed the inhibitory effect of KF24345. These results demonstrate that KF24345 inhibits LPS-induced TNF-alpha production and leukopenia via enhancing the effect of endogenous adenosine. It is thus suggested that the adenosine uptake inhibitor has anti-inflammatory effects in vivo and represents a novel therapeutic approach to the treatment of various inflammatory diseases.

Functional characterization of an endogenous Xenopus oocyte adenosine receptor

To investigate the effects of adenosine on endogenous Xenopus oocyte receptors, we analysed defolliculated oocytes injected with mRNAs for the G protein-activated inwardly rectifying K(+) (GIRK) channels. In oocytes injected with mRNAs for either GIRK1/GIRK2 or GIRK1/GIRK4 subunits, application of adenosine or ATP reversibly induced inward K(+) currents, although ATP was less potent than adenosine. The responses were attenuated by caffeine, a non-selective adenosine receptor antagonist. Furthermore, in uninjected oocytes from the same donor, adenosine produced no significant current. The endogenous receptor was activated by two selective A(1) adenosine receptor agonists, N(6)-cyclopentyladenosine (CPA) and N(6)-cyclohexyladenosine (CHA), and antagonized by a selective A(1) adenosine receptor antagonist, 1,3-dipropyl-8-cyclopenylxanthine (DPCPX) at moderate nanomolar concentrations, but insensitive to micromolar concentrations of selective A(2A) and A(3) adenosine receptor agonists, 2-[p-(2-carbonyl-ethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) and N(6)-(3-iodobenzyl)-5'-(N-methylcarbamoyl)adenosine (IB-MECA), respectively. However, the pharmacological characteristics of the receptor were different from those of the cloned Xenopus A(1) adenosine receptor and previously proposed adenosine receptors. The adenosine-induced GIRK currents were abolished by injection of pertussis toxin and CPA inhibited forskolin-stimulated cyclic AMP accumulation. We conclude that an adenosine receptor on the Xenopus oocyte membrane can activate GIRK channels and inhibit adenylyl cyclase via G(i/o) proteins. Moreover, our results suggest the existence of an endogenous adenosine receptor with the unique pharmacological characteristics. As the receptor was activated by nanomolar concentrations of adenosine, which is a normal constituent of extracellular fluid, the receptor may be involved in some effects through the G(i/o) protein signalling pathways in ovarian physiology.

Regulation of Renal A1 Adenosine Receptors in vivo

We have compared renal A₁ adenosine receptor (AR) regulations in rats after chronic agonist and antagonist treatments. In one group, R-phenylisopropyladenosine (R-PIA), a selective A₁ AR agonist, was infused subcutaneously for 7 days. Another group was fed theophylline, a non-selective AR antagonist, for 2 weeks. Renal cortex membrane A₁ AR binding with 1,3-[³H]-dipropyl-8-cyclopentylxanthine demonstrated ∼40% reduction in the B_max for the R-PIA group without any changes in the K_d values. Neither the B_max nor the K_d changed following chronic theophylline treatment. Renal cortex G_iα-proteins from the R-PIA treated rats decreased by ∼30%. Renal G_iα levels did not change in theophylline-treated rats. Consistent with the A₁ AR desensitization, R-PIA-treated rats had significantly higher basal renin release and showed attenuated A₁ AR-mediated inhibition of renin release. These data suggest that prolonged A₁ AR stimulation results in downregulation of renal A₁ ARs and G_iα, accompanied by desensitization of A₁ AR-mediated inhibitory effects on renin release. Unlike cardiac and brain A₁ ARs, renal A₁ receptors are not subject to up-regulation following chronic antagonist treatment.

In vivo labelling of the adenosine A2A receptor in mouse brain using the selective antagonist [3H]SCH 58261.

The selective A2A receptor antagonist [3H]SCH 58261 was injected intravenously in mice and the radioactivity accumulating in various brain regions was determined by tissue sampling. Radioactivity levels in regions of interest such as the striatum were highest 15 min after injection and quickly declined thereafter (30 min and 1 h postinjection) in a time-dependent manner. The amount of labelling was ranked as follows: striatum (4.6 +/- 0.3 fmol/mg protein) >> cortex > hippocampus > pons = hypothalamus > cerebellum (0.5 +/- 0.05 fmol/mg protein). Specific labelling of the A2A receptor occurred in striatum and cortex because significantly less radioactivity accumulated in these areas from adenosine A2A receptor knockout mice as compared to wild-type littermates. In control outbred CD1 mice, a striatum-to-cerebellum ratio of 7.6 +/- 0.6 was found. At 30 min postinjection, the nonselective adenosine receptor antagonist caffeine reduced the radioactivity due to [3H]SCH 58261 in the striatum by 32% at 1 mg/kg i.p. and by 66% at the stimulant dose of 6.25 mg/kg i.p. Radioactivity in the striatum was lowered, respectively, by 66 and 86% 30 min after injection of 3 or 10 mg/kg i.p. doses of unlabelled SCH 58261. The present results indicate that [3H]SCH 58261 directly labels striatal A2A receptors in vivo. Thus [3H]SCH 58261 is an excellent tool for studying brain distribution and A2A receptor occupancy of various compounds ranging from xanthines, such as caffeine, to other A2A antagonists.

Catalepsy and hypolocomotion induced by a nitric oxide donor: attenuation by theophylline

Nitric oxide (NO) promotes adenosine release in the striatum and hippocampus. Behavioral effects of the nitric oxide donor sodium nitroprusside were studied in mice and included an examination of spontaneous locomotion and catalepsy, which are behaviors modulated by adenosine. Sodium nitroprusside caused a dose-dependent (2, 4 and 6 mg/kg) decrease in locomotor activity and catalepsy at the dose of 6 mg/kg. These effects were substantially attenuated by pretreatment with the non-selective adenosine receptors antagonist theophylline (10 and 30 mg/kg). Moreover, combined treatment with theophylline (30 mg/kg) and sodium nitroprusside (6 mg/kg) induced limbic seizures in 23% of animals. The pretreatment with the selective adenosine A 1 receptor antagonist 8-cyclopentyl-1, 3-dimethylxanthine (CPT) (1.2 mg/kg) caused no effect on the spontaneous or sodium nitroprusside-induced behavior. These data suggest that these behavioral effects of sodium nitroprusside are at least partially mediated by adenosine in the striatum and hippocampus, probably via adenosine A 2A receptors.

Inhibition of adenosine deaminase attenuates endotoxin-induced release of cytokines in vivo in rats.

The purpose of this study was to investigate in vivo the effects of modulating the adenosine system on endotoxin-induced release of cytokines and changes in heart performance and neurohumoral status in early, profound endotoxemia in rats. Time/pressure variables of heart performance and blood pressure were recorded continuously, and plasma levels of tumor necrosis factor alpha (TNFalpha), interleukin 1-beta (IL-1beta), plasma renin activity (PRA), and catecholamines were determined before and 90 min after administration of endotoxin (30 mg/kg of lipopolysaccharide, i.v.). Erythro-9[2-hydroxyl-3-nonyl] adenine (EHNA; an adenosine deaminase inhibitor) had no effects on measured time-pressure variables of heart performance under baseline conditions and during endotoxemia, yet significantly attenuated endotoxin-induced release of cytokines and PRA. Pretreatment with the non-selective adenosine receptor antagonist DPSPX not only prevented the effects of EHNA but also increased the basal release of cytokines and augmented PRA. At baseline, caffeine (a non-selective adenosine receptor antagonist) increased HR, +dP/dtmax, heart rate x ventricular pressure product (HR x VPSP) and +dP/dtmax normalized by pressure (+dP/dtmax/VPSP), and these changes persisted during endotoxemia. Caffeine attenuated endotoxin-induced release of cytokines and augmented endotoxin-induced increases in plasma catecholamines and PRA. Pretreatment with propranolol abolished the effects of caffeine on heart performance and neurohumoral activation during the early phase of endotoxemia. 6N-cyclopentyladenosine (CPA; selective A1 adenosine receptor agonist) induced bradicardia and negative inotropic effects, reduced work load (i.e., decreased HR, VPSP, +dP/dtmax, +dP/dtmax/VPSP and HR x VPSP) and inhibited endotoxin-induced tachycardia and renin release. CGS 21680 (selective A2A adenosine receptor agonist) decreased blood pressure under basal condition but did not potentiate decreases in blood pressure during endotoxemia. CGS 21680 completely inhibited endotoxin-induced release of TNFalpha, augmented sympathetic activity and PRA, and increased +dP/dtmax and +dP/dtmax/VPSP in the absence and presence of endotoxin. The present study provides strong evidence that inhibition of adenosine deaminase reduces cytokine release in vivo without producing significant hemodynamic and cardiac effects during the early phase of profound endotoxemia in rats. The augmented neurohumoral activation induced by caffeine is associated with decreased cytokine release induced by endotoxin. Further studies are warranted to determine the impact of these effects on cardiac function and hemodynamics in the late phase of endotoxemia.

Role of adenosine in ischemic preconditioning in rats depends critically on the duration of the stimulus and involves both A(1) and A(3) receptors.

There is currently general agreement that adenosine is not involved in ischemic preconditioning (IP) in rat hearts. We hypothesized that the failure to show a role for adenosine is due to the use of brief preconditioning stimuli, and therefore investigated whether adenosine is involved when longer stimuli are employed and which receptor subtypes are involved. Infarct size (IS) was determined in anesthetized rats after 180 min of reperfusion (REP) following a 60-min coronary artery occlusion (CAO). IS was 69+/-2% (n=15) of the risk area in control rats and 45+/-2% (n=19; P<0.05) following IP by a single 15-min CAO. The non-selective adenosine receptor antagonist SPT, which itself had no effect on IS (74+/-1%), blunted the protection by IP (IS=57+/-2%, P<0.05) in a dose of 2 x 5 mg/kg i.v., and abolished the protection (IS=70+/-1%) at 2 x 25 mg/kg i.v. Following IP by three cycles of 3-min CAO and 3-min REP, IS was 24+/-6% (P<0.05), which was not affected by SPT in doses of 2 x 10 and 2 x 25 mg/kg i.v. The A(3) antagonist MRS-1191 (3.3 mg/kg, i.p.), which itself did not affect IS (70+/-2%), blunted the protection by IP with a 15-min CAO (IS=54+/-2%, P<0.05). When 2 x 5 mg/kg SPT (a dose selective for A(1)-receptors, as it did not affect the protection by the A(3) selective agonist IB-MECA, 51+/-3%) and MRS 1191 were combined the protection by IP was abolished (IS=67+/-2%). Involvement of adenosine in IP in rats depends critically on the duration of the stimulus. Thus, whereas adenosine was not involved when stimuli of 3-min duration were employed, activation of both A(1) and A(3) receptors contributed when a stimulus of 15 min was used.

The plasma protein extravasation induced by adenosine and its analogues in the rat dorsal skin: evidence for the involvement of capsaicin sensitive primary afferent neurones and mast cells.

1. The contribution of sensory neurons and mast cells to the oedema evoked by adenosine A1 (N(6)-cyclopentyladenosine, CPA, 3 - 30 nmol site(-1)), A2 (5'N-ethylcarboxamidoadenosine, NECA, 1 - 10 nmol site(-1)) and A3 receptor agonists (N6-[3-iodobenzyl]-N-methyl-5'-carboxiamidoadenosine, IB-MECA, 0.01 - 3 nmol site(-1)) was investigated in the rat skin microvasculature, by the extravascular accumulation of intravenously-injected (i.v.) 125I-albumin. 2. Intradermal (i.d.) injection of adenosine and analogues induced increased microvascular permeability in a dose-dependent manner (IB-MECA > NECA > CPA > adenosine). The non-selective adenosine receptor antagonist theophylline (5 - 50 nmol site(-1)) markedly inhibited adenosine, CPA or NECA but not IB-MECA-induced plasma extravasation. The A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.3 - 3 micromol kg(-1), i.v.) significantly reduced CPA-induced plasma extravasation whereas responses to adenosine, NECA or IB-MECA were unchanged. The A2 receptor antagonist 3,7-dymethyl-1-proprargylxanthine (DMPX, 0.5 - 50 nmol site(-1)) significantly reduced NECA-induced plasma extravasation without affecting responses to adenosine, CPA and IB-MECA. 3. The tachykinin NK1 receptor antagonist (S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl) piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane chloride (SR140333), but not the NK2 receptor antagonist (S)-N-methyl-N[4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl]-benzamide (SR48968), significantly inhibited the plasma extravasation evoked by higher doses of adenosine (100 nmol site(-1)), CPA (100 nmol site(-1)), NECA (1 nmol site(-1)) and IB-MECA (0.1 - 1 nmol site(-1)). In rats treated with capsaicin to destroy sensory neurons, the response to higher doses of adenosine, CPA and NECA, but not IB-MECA, was significantly inhibited. 4. The effects of adenosine and analogues were largely inhibited by histamine and 5-hydroxytryptamine (5-HT) antagonists and by compound 48/80 pretreatment. 5. In conclusion, our results provide evidence that adenosine A1 and A2, but not A3, receptor agonists may function as cutaneous neurogenic pro-inflammatory mediators; acting via microvascular permeability-increasing mechanisms that can, depending on dose of agonist and purine receptor under study, involve the tachykinin NK1 receptor and mast cell amines.