Adenosine, the imperfect endogenous anti-ischemic cardio-neuroprotector

Serum adenosine deaminase activity and acute cerebral infarction: a retrospective case-control study based on 7913 participants

BACKGROUND

Adenosine deaminase (ADA) is a key enzyme that catalyzes the deamination of adenosine into inosine, which eventually decomposes into uric acid (UA). A body of papers have reported that adenosine and UA are closely related to cerebrovascular events. However, the association between serum ADA activity and acute cerebral infarction (ACI) remains unclear.

METHODS

7913 subjects were enrolled, including 3968 ACI patients and 3945 controls, in this study. An automatic biochemistry analyzer was used to determine serum activity.

RESULTS

Serum ADA activity was found that was significantly decreased in patients with ACI (10.10 ± 3.72 U/L) compared to those without ACI (11.07 ± 2.85 U/L, p < 0.001). After Logistic regression analysis, ADA concentrations were negatively correlated with ACI (OR = 1.161, 95% CI: 1.140-1.183, p < 0.001). Smoking and alcohol consumption decreased serum ADA concentrations in patients with ACI, whereas diabetes and hypertension had the opposite effect.

CONCLUSIONS

Serum ADA concentrations in patients with ACI are markedly decreased, suggesting that the decreased ADA concentrations may be involved in the pathogenesis of ACI. We hypothesized that decreased ADA activity may be an adaptive mechanism to maintain adenosine levels and protect against ischemic brain injury.

Expressions of adenosine A2A receptors in coronary arteries and peripheral blood mononuclear cells are correlated in coronary artery disease patients

BACKGROUND

Altered coronary blood flow occurs in patients with coronary artery disease (CAD). Adenosine strongly impacts blood flow mostly via adenosine A_2A receptor (A_2AR) expressed in coronary tissues. As part of a systemic regulation of the adenosinergic system, we compared A_2AR expression in situ, and on peripheral blood mononuclear cells (PBMC) in CAD patients.

METHODS AND RESULTS

Aortic and coronary tissues, and PBMC were sampled in 20 CAD patients undergoing coronary artery bypass surgery and consecutively included. Controls were PBMC obtained from 15 healthy subjects. Expression and activity of A_2AR were studied by Western blotting and cAMP measurement, respectively. A_2AR expression on PBMC was lower in patients than in controls (0.83±0.31 vs 1.2±0.35 arbitrary units; p<0.01), and correlated with A_2AR expression in coronary and aortic tissues (Pearson's r: 0.77 and 0.59, p<0.01, respectively). Basal and maximal cAMP productions following agonist stimulation of PBMC were significantly lower in patients than in controls (120±42 vs 191±65 and 360±113 vs 560±215pg/10⁶ cells, p<0.05, respectively). In CAD patients, the increase from basal to maximal cAMP production in PBMC and aortic tissues was similar (+300% and +246%, respectively).

CONCLUSION

Expression of A_2AR on PBMC correlated with those measured in coronary artery and aortic tissues in CAD patients, A_2AR activity of PBMC matched that observed in aorta, and A_2AR expression and activity in PBMC were found reduced as compared to controls. Measuring the expression level of A_2AR on PBMC represents a good tool to address in situ expression in coronary tissues of CAD patients.

Cytotoxic purine nucleoside analogues bind to A1, A2A, and A3 adenosine receptors

Fludarabine, clofarabine, and cladribine are anticancer agents which are analogues of the purine nucleoside adenosine. These agents have been associated with cardiac and neurological toxicities. Because these agents are analogues of adenosine, they may act through adenosine receptors to elicit their toxic effects. The objective of this study was to evaluate the ability of cytotoxic nucleoside analogues to bind and activate adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)). Radioligand binding studies utilizing Chinese hamster ovary cells, stably transfected with adenosine A(1), A(2A), or A(3) receptor subtype, were used to assess the binding affinities of these compounds, whereas adenylyl cyclase activity was used to assess the binding to A(2B) receptors. Clofarabine and cladribine both bound to the A(2A) receptor with a K (i) of 17 and 15 μM, respectively. Clofarabine was the only adenosine analogue to bind to the A(3) receptor with a K (i) of 10 μM, and none of these compounds bound to the A(2B) receptor. Results show that clofarabine, cladribine, and fludarabine bind to the A(1) receptor. In addition, clofarabine, cladribine, and fludarabine were A(1) agonists (IC(50) 3.1, 30, and 30 μM, respectively). Neither pyrimidine nucleoside analogues gemcitabine nor cytarabine associated with any of the adenosine receptor subtypes (K (i) > 100μM). This is the first report of an interaction between all adenosine receptor subtypes and chemotherapeutic nucleoside analogues commonly used in the treatment of cancer. Therefore, activation of these receptors may be at least one mechanism through which fludarabine-associated toxicity occurs.

Adenosine and stroke: maximizing the therapeutic potential of adenosine as a prophylactic and acute neuroprotectant

Stroke is a leading cause of morbidity and mortality in the United States. Despite intensive research into the development of treatments that lessen the severity of cerebrovascular injury, no major therapies exist. Though the potential use of adenosine as a neuroprotective agent in the context of stroke has long been realized, there are currently no adenosine-based therapies for the treatment of cerebral ischemia and reperfusion. One of the major obstacles to developing adenosine-based therapies for the treatment of stroke is the prevalence of functional adenosine receptors outside the central nervous system. The activities of peripheral immune and vascular endothelial cells are particularly vulnerable to modulation via adenosine receptors. Many of the pathophysiological processes in stroke are a direct result of peripheral immune infiltration into the brain. Ischemic preconditioning, which can be induced by a number of stimuli, has emerged as a promising area of focus in the development of stroke therapeutics. Reprogramming of the brain and immune responses to adenosine signaling may be an underlying principle of tolerance to cerebral ischemia. Insight into the role of adenosine in various preconditioning paradigms may lead to new uses for adenosine as both an acute and prophylactic neuroprotectant.

Opposite effects of uracil and adenine nucleotides on the survival of murine cardiomyocytes

We previously showed that the human heart expresses all known P2X and P2Y receptors activated by extra-cellular adenine or uracil nucleotides. Despite evidence that, both in humans and rodents, plasma levels of ATP and UTP markedly increase during myocardial infarction, the differential effects mediated by the various adenine- and uracil-preferring myocardial P2 receptors are still largely unknown. Here, we studied the effects of adenine and uracil nucleotides on murine HL-1 cardiomyocytes. RT-PCR analysis showed that HL-1 cardiomyocytes express all known P2X receptors (except for P2X(2)), as well as the P2Y(2,4,6,14) subtypes. Exposure of cardiomyocytes to adenine nucleotides (ATP, ADP or BzATP) induced apoptosis and necrosis, as determined by flow-cytometry. Cell death was exacerbated by tumour necrosis factor (TNF)-alpha, a cytokine implicated in chronic heart failure progression. Conversely, uracil nucleotides (UTP, UDP and UDPglucose) had no effect 'per se', but fully counteracted the deleterious effects induced by adenine nucleotides and TNF-alpha, even if added to cardiomyocytes after beginning exposure to these cell death-inducing agents. Thus, exposure of cardiomyocytes to elevated concentrations of ATP or ADP in the presence of TNF-alpha contributes to cell death, an effect which is counteracted by uracil-preferring P2 receptors. Cardiomyocytes do not need to be 'primed' by uracil nucleotides to become insensitive to adenine nucleotides-induced death, suggesting the existence of a possible 'therapeutic' window for uracil nucleotides-mediated protection. Thus, release of UTP during cardiac ischaemia and in chronic heart failure may protect against myocardial damage, setting the basis for developing novel cardioprotective agents that specifically target uracil-preferring P2Y receptors.

Increased expression of adenosine A2A receptors in patients with spontaneous and head-up-tilt-induced syncope

BACKGROUND

Adenosine may play a role in the triggering of neurocardiogenic syncope, but no information on adenosine receptors is available at the present time.

OBJECTIVE

The purpose of this study was to investigate whether adenosine A2A receptors expression is altered in patients with neurocardiogenic syncope.

METHODS

Adenosine plasma levels (APLs), the expression of A2A receptors, were measured (mean +/- standard error of the mean) during tilt testing. Expression of receptors was assessed on mononuclear cells using a selective receptor ligand.

RESULTS

At baseline, the APLs of 16 patients with a positive test were higher than those of 17 patients with a negative test and of those of a control group (2.10 +/- 0.30 vs. 0.40 +/- 0.05 and 0.41 +/- 0.06 muM, respectively; P <.0001). The number of receptors was higher in patients tested positive than in patients tested negative or in the control group (122 +/- 10 vs. 38 +/- 4 and 44 +/- 4 fmol/g of proteins, respectively; P <.0001). No difference was found in the affinity or synthesis among the three groups.

CONCLUSION

This study showed an increased number and an up-regulation of adenosine A2A receptors in patients with spontaneous syncope and a positive head-up tilt, which in the context of high APLs may play a role in the recurrence of syncopal episodes.

P1 receptors and cytokine secretion

Evidence has accumulated in the last three decades to suggest tissue protection and regeneration by adenosine in multiple different cell types. Adenosine produced in hypoxic or inflamed environments reduces tissue injury and promotes repair by receptor-mediated mechanisms. Among other actions, regulation of cytokine production and secretion by immune cells, astrocytes and microglia (the brain immunocytes) has emerged as a main mechanism at the basis of adenosine effects in diseases characterized by a marked inflammatory component. Many recent studies have highlighted that signalling through A₁ and A_2A adenosine receptors can powerfully prevent the release of pro-inflammatory cytokines, thus inhibiting inflammation and reperfusion injury. However, the activation of adenosine receptors is not invariably protective of tissues, as signalling through the A_2B adenosine receptor has been linked to pro-inflammatory actions which are, at least in part, mediated by increased release of pro-inflammatory cytokines from epithelial cells, astrocytes and fibroblasts. Here, we discuss the multiple actions of P1 receptors on cytokine secretion, by analyzing, in particular, the role of the various adenosine receptor subtypes, the complex reciprocal interplay between the adenosine and the cytokine systems, their pathophysiological significance and the potential of adenosine receptor ligands as new anti-inflammatory agents.

Adenosine receptor signaling in the brain immune system

The brain immune system, which consists mainly of astrocytes, microglia and infiltrating immune cells, is quiescent normally, but it is activated in response to pathophysiological events such as ischemia, trauma, inflammation and infection. Adenosine is an endogenous purine nucleoside that is generated at sites that are subjected to these "stressful" conditions. Adenosine interacts with specific G-protein-coupled receptors on astrocytes, microglia and infiltrating immune cells to regulate the function of the immune system in the brain. Although many of the effects of adenosine on immune-competent cells in the brain protect neuronal integrity, adenosine might also aggravate neuronal injury by promoting inflammatory processes. A more complete understanding of adenosine receptor function in the brain immune system should help develop novel therapeutic ways to treat brain disorders that are associated with a dysfunctional immune response.

Cyclosporin and tacrolimus increase plasma levels of adenosine in kidney transplanted patients

The immunosuppressive agents, cyclosporin (CsA) and tacrolimus (FK506), display cardioprotective activities. The mechanism would consist on the inhibition of the enzyme, adenosine kinase (AK), leading to an increase in adenosine (ADO) levels. ADO, inosine (INO) and nucleotide plasma levels were measured in kidney transplant recipients before and 1, 2, 4, 6 and 8 h after the administration of CsA or FK506. After CsA and FK506 administration, ADO plasma levels significantly increased, reaching a peak level after 2 h (483 +/- 124 and 429 +/- 96 nm, respectively), and then progressively declined. Calculated peak values (t(max)) of ADO were slightly delayed with respect to those of CsA and FK506. Treatment with rapamycin did not influence the phenomenon. The dynamic profile of plasma changes of ADO, nucleotides and INO were consistent with the inhibition of the enzyme, AK. ADO increase may be clinically relevant in terms of anti-ischaemic, tissue protecting, and immunosuppressive activities as well as in terms of nephrotoxicity.

AMPD1 (C34T) polymorphism and clinical outcomes in patients undergoing myocardial revascularization

BACKGROUND

C34T variant of adenosine monophosphate deaminase 1 (AMPD1) gene has been associated with a prolonged survival in heart failure and coronary artery disease, hypothetically linked to an enhanced production of adenosine.

DESIGN

Since adenosine administration is a promising approach for the prevention of the ischemia-reperfusion in myocardial revascularization, the aim of this study was to investigate whether the AMPD1 (-) allele is associated with a favorable prognosis after coronary revascularization. In addition, we assessed the association between AMPD1 polymorphism and plasma adenosine levels.

METHODS

We investigated a total of 161 patients receiving coronary revascularization (70 percutaneous transluminal coronary angioplasty and 91 coronary artery bypass graft). They were investigated for a composite endpoint including recurrent angina, non-fatal MI, target vessel revascularization, heart failure and cardiac death. Plasma adenosine was also measured by high-performance liquid chromatography methods on a subset of 25 patients.

RESULTS

During the follow-up period (7.0+/-0.3 months), the overall combined endpoint accounted for 17 events (10 cardiac-related deaths, 6 revascularization procedures and 1 congestive heart failure). The composite endpoint was 9.8% for AMPD1 (-) allele carriers vs. 11.5% for non-carriers (log-rank statistic, p=n.s.). In the logistic analysis only low (</=40%) ejection fraction was an independent predictor of adverse events (p=0.01, OR=3.8, 95% CI 1.3-11.4). Plasma adenosine levels were similar for AMPD1 (-) allele patients (n=7) as compared for AMPD1 (+) allele (n=18) subjects (290.5+/-31.0 vs. 303.3+/-28.5 nM, p=n.s.).

CONCLUSIONS

Our results indicate that AMPD1 (-) allele is not associated with a more favorable outcome after coronary revascularization. Alternative cardioprotective pathways of the AMPD1 gene-involving an enhanced chronic long-term production of adenosine-might be responsible for survival.

Modulation of A2A adenosine receptors and associated Galphas proteins by ZM 241385 treatment of porcine coronary artery

Functional regulation and expression of the adenosine A2A receptor and associated G-protein were investigated in porcine coronary artery exposed to an A2A receptor antagonist, ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol). The arteries were incubated for 3 days in culture medium in the absence (control) and presence (treated) of 10 microM ZM 241385. Changes in isometric tension by adenosine receptor agonists were evaluated in endothelium-free tissues. ZM 241385-treatment produced a statistically significant rightward displacement of CGS-21680, NECA, and CAD concentration-response curves compared with the respective controls (P < 0.05). The EC50, expressed in nM, values in treated and control tissues were: 617.3 +/- 23 versus 24.9 +/- 1.5 for CGS-21680 (2-(p-(2-carboxyethyl)phenethylamino)-5'N-ethylcarboxamidoadenosine), 27.4 +/- 6.3 versus 3.06 +/- 0.8 for NECA (5'-N-ethylcarboxamidoadenosine), and 5786.2 +/- 160 versus 89.2 +/- 24.1 for CAD (chloroadenosine). However, the relaxing effect of forskolin remained unchanged in treated and control tissues. The concentration-response curves for NECA, CAD, and CGS-21680 were also displaced to the right when cAMP levels were measured in treated and control smooth muscle cells while no differences were observed with forskolin. Quantitative Western blot analysis demonstrated that the density of A2A receptors increased in ZM 241385-treated artery. We also showed a significant decrease in Galphas protein levels after ZM 241385 treatment compared with control. Taken together, these data indicate that prolonged blockade of A2A receptors in the coronary artery leads to desensitization of the functional effects of adenosine agonists by a mechanism that involves decreases in cAMP production. This was associated with an up-regulation of A2A receptors and a decrease in Galphas protein expression.

Blunted increase in plasma adenosine levels following dipyridamole stress in dilated cardiomyopathy patients

BACKGROUND

Heart failure is characterized by chronically increased adenosine levels, which are thought to express a protective anti-heart failure activation of the adenosinergic system. The aim of the study was to assess whether the activation of adenosinergic system in idiopathic dilated cardiomyopathy (IDC) can be mirrored by a blunted increase in plasma adenosine concentration following dipyridamole stress, which accumulates endogenous adenosine.

METHODS

Two groups were studied: IDC patients (n = 19, seven women, mean age 60 +/- 12 years) with angiographically confirmed normal coronary arteries and left ventricular ejection fraction <35%; and normal controls (n = 15, six women, mean age 68 +/- 5 years). Plasma adenosine was assessed by high-performance liquid chromatography methods in blood samples from peripheral vein at baseline and 12 min after dipyridamole infusion (0.84 mg kg-1 in 10 min).

RESULTS

At baseline, IDC patients showed higher plasma adenosine levels than controls (276 +/- 27 nM L-1 vs. 208 +/- 48 nM L-1, P < 0.001). Following dipyridamole, IDC patients showed lower plasma adenosine levels than controls (322 +/- 56 nM L-1 vs. 732 +/- 250 nM L-1, P < 0.001). The dipyridamole-induced percentage increase in plasma adenosine over baseline was 17% in IDC and 251% in controls (P < 0.001). By individual patient analysis, 18 IDC patients exceeded (over the upper limit) the 95% confidence limits for normal plasma adenosine levels at baseline, and all 19 exceeded (below the lower limit) the 95% confidence limits for postdipyridamole plasma adenosine levels found in normal subjects.

CONCLUSION

Patients with IDC have abnormally high baseline adenosine levels and--even more strikingly--blunted plasma adenosine increase following dipyridamole infusion. This is consistent with a chronic activation of the adenosinergic system present in IDC, which can be measured noninvasively in the clinical theatre.

Microglial ecto-Ca(2+)-ATPase activity in a rat model of focal homologous blood clot embolic cerebral ischemia: an enzyme histochemical study

Post-ischemic changes in ecto-Ca(2+)-ATPase activity in microglia and the infarcted tissue were studied in a rat model of focal embolic cerebral ischemia using an enzyme histochemical method. Ecto-Ca(2+)-ATPase activity was observed in whole brains in non-operated and sham-operated control animals. In addition, this enzyme activity was determined to be localized in ramified microglia. At 30 min after ischemia, non-microglial ecto-Ca(2+)-ATPase activity in the infarcted tissue slightly decreased and continued to decrease thereafter. The ecto-Ca(2+)-ATPase activity in microglia did not appear changed at this time. The decrease of enzyme activity in the infarcted tissue made it much easier to clearly observe ecto-Ca(2+)-ATPase-positive microglia. The enzyme activity of microglia in the ischemic area began to decrease 2 or 4h after embolization and remarkably decreased, except in the perinuclear cytoplasm, apical parts of the processes, and several parts along the processes, 8h after ischemia. By 12h after onset of embolization, the enzyme activity of microglia and infarcted tissue had almost completely disappeared. Ecto-Ca(2+)-ATPase of microglia is likely to play an important role in the metabolism of extracellular nucleotides in the ischemic area immediately after the onset of embolization by means of ecto-enzymes. Thus, the findings of the present study suggest that microglia might serve to protect the infarcted tissue in the ischemic brain.

Changes of peripheral A2A adenosine receptors in chronic heart failure and cardiac transplantation

Peripheral blood mononuclear cells of chronic heart failure (CHF) patients produce great amounts of pro-inflammatory cytokines, indicating that circulating cells are activated and could mirror changes occurring in inflammatory cells infiltrating the failing heart. Adenosine is a regulatory metabolite acting through four membrane receptors that are linked to adenylyl cyclase: activation of the A2A receptor subtype has been reported to inhibit cytokine release. Changes of the adenosinergic system may play a role in CHF development. Here we report an increase of A2A receptor expression, density, and coupling to adenylyl cyclase in blood circulating cells of CHF patients. A2A receptor up-regulation was also found in the explanted hearts of these patients, suggesting that changes of peripheral adenosine receptors mirror changes occurring in the disease target organ. In a cohort of patients followed longitudinally after heart transplantation, alterations of peripheral A2A adenosine receptor progressively normalized to control values within 6 months, suggesting that improvement of cardiac performance is accompanied by progressive restoration of a normal adenosinergic system. These results validate the importance of the A2A receptor in human diseases characterized by a marked inflammatory/immune component and suggest that the evaluation of this receptor in peripheral blood cells may be useful for monitoring hemodynamic changes and the efficacy of pharmacological and non-pharmacological treatments in CHF patients.

Physiologic pH changes modulate calcium ion dependence of brain nitric oxide synthase in Carassius auratus

Species of the fish genus Carassius survive prolonged anoxia. Nitric oxide (NO) regulates cerebral blood flow in these fish during normoxic conditions whereas adenosine is the main vasoregulating molecule during anoxia. We investigated the calcium ion dependence of Carassius auratus brain NO synthase (NOS) as a function of pH. The physiological pH decrease from 7.2 to 6.8, which takes place during anoxia, greatly decreases NOS activity. This strong pH dependence is mainly due to variation of the calcium sensitivity of the enzyme. The EC(50) is 0.15 microM at pH 7.2 and 2.1 microM at pH 6.8 for the soluble enzyme. The particulate enzyme is also dependent on pH variations. The reduced sensitivity to calcium ions at acidic pH decreases both NO and H(2)O(2) production, saving the cells by suppression of the formation of potentially toxic nitrogen and oxygen species. Modulation of NOS activity by variation of its calcium affinity within the range of physiological pH constitutes an important and rapid mechanism to control the formation of NO and H(2)O(2) during normoxia-anoxia and anoxia-normoxia transitions.

Extracellular ATP and nerve growth factor intensify hypoglycemia-induced cell death in primary neurons: role of P2 and NGFRp75 receptors

In this study, we monitored the direct expression of P2 receptors for extracellular ATP in cerebellar granule neurons undergoing metabolism impairment. Glucose deprivation for 30-60 min inhibited P2Y1 receptor protein, only weakly modulated P2X1, P2X2 and P2X3, and up-regulated by about two-fold P2X4, P2X7 and P2Y4. The P2X/Y antagonist basilen blue, protecting cerebellar neurons from hypoglycemic cell death, maintained within basal levels only the expression of P2X7 and P2Y4 proteins, but not P2X4 or P2Y1. Glucose starvation transiently increased (up to three-fold) the expression of NGFRp75 receptor protein and strongly stimulated the extracellular release of nerve growth factor (NGF; about 10-fold). Exogenously added NGF then augmented hypoglycemic neuronal death by about 60%, increasing the percentage of Höechst-positive nuclei (from approximately 62 to 95%), reducing lactate dehydrogenase (LDH) release (from about 50 to 14%) and significantly overstimulating the hypoglycemia-induced expression of P2X7 and P2Y4. Conversely, extracellular ATP augmented hypoglycemic neuronal death by about 80%, reducing the number of Höechst-positive nuclei (from approximately 62% to 14%), augmenting LDH outflow (by about 30%) and further increasing the hypoglycemia-induced expression of NGFRp75. Our results indicate that P2 and NGFRp75 receptors are modulated during glucose starvation and that extracellular ATP and NGF drive features of, respectively, necrotic and apoptotic hypoglycemic cell death, aggravating the consequences of metabolism impairment in cerebellar primary neurons.

Effects of doxazosin and propranolol on A2A adenosine receptors in essential hypertension

A2A adenosine receptors inhibit neutrophil adhesion and superoxide anion generation. The aim of the present study was to evaluate the effect of antihypertensive treatment with doxazosin or propranolol on the binding and functional parameters of A2A adenosine receptors of lymphocytes and neutrophils in essential hypertensive patients. Two groups of previously untreated, essential hypertensive patients were studied. The mean affinity (K(d)) and density (B(max)) of adenosine receptors, by the A2A selective radioligand [3H]-ZM-241385 binding assays, and EC50, by cAMP assays, were obtained first on no medication and a second time after treatment for up to 13 weeks with doxazosin (13 patients) or propranolol (8 patients). A third group of 15 healthy normotensive volunteers matched by age, sex, and body mass index was used as a control. Binding and functional parameters of the A2A adenosine receptors were significantly higher in the 2 hypertensive groups than in controls (P always <0.0001), both in lymphocyte and neutrophil membranes. After treatment with propranolol, the binding parameters did not change significantly, whereas after treatment with doxazosin, K(d), B(max), and EC50 values returned to control levels. In never-treated essential hypertensive patients, lower affinity, higher density, and impaired function of A2A adenosine receptors are present. The binding and functional parameters of A2A adenosine receptors appear to be normalized after treatment with doxazosin but not with propranolol.

Blockade of striatal adenosine A2A receptor reduces, through a presynaptic mechanism, quinolinic acid-induced excitotoxicity: possible relevance to neuroprotective interventions in neurodegenerative diseases of the striatum

The aim of the present study was to evaluate whether, and by means of which mechanisms, the adenosine A2A receptor antagonist SCH 58261 [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine] exerted neuroprotective effects in a rat model of Huntington's disease. In a first set of experiments, SCH 58261 (0.01 and 1 mg/kg) was administered intraperitoneally to Wistar rats 20 min before the bilateral striatal injection of quinolinic acid (QA) (300 nmol/1 microl). SCH 58261 (0.01 but not 1 mg/kg, i.p.) did reduce significantly the effects of QA on motor activity, electroencephalographic changes, and striatal gliosis. Because QA acts by both increasing glutamate outflow and directly stimulating NMDA receptors, a second set of experiments was performed to evaluate whether SCH 58261 acted by preventing the presynaptic and/or the postsynaptic effects of QA. In microdialysis experiments in naive rats, striatal perfusion with QA (5 mm) enhanced glutamate levels by approximately 500%. Such an effect of QA was completely antagonized by pretreatment with SCH 58261 (0.01 but not 1 mg/kg, i.p.). In primary striatal cultures, bath application of QA (900 microm) significantly increased intracellular calcium levels, an effect prevented by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. In this model, bath application of SCH 58261 (15-200 nm) tended to potentiate QA-induced calcium increase. We conclude the following: (1) the adenosine A2A receptor antagonist SCH 58261 has neuroprotective effects, although only at low doses, in an excitotoxic rat model of HD, and (2) the inhibition of QA-evoked glutamate outflow seems to be the major mechanism underlying the neuroprotective effects of SCH 58261.

Exogenous anandamide protects rat brain against acute neuronal injury in vivo

The endocannabinoid anandamide [N-arachidonoylethanolamine (AEA)] is thought to function as an endogenous protective factor of the brain against acute neuronal damage. However, this has never been tested in an in vivo model of acute brain injury. Here, we show in a longitudinal pharmacological magnetic resonance imaging study that exogenously administered AEA dose-dependently reduced neuronal damage in neonatal rats injected intracerebrally with the Na(+)/K(+)-ATPase inhibitor ouabain. At 15 min after injury, AEA (10 mg/kg) administered 30 min before ouabain injection reduced the volume of cytotoxic edema by 43 +/- 15% in a manner insensitive to the cannabinoid CB(1) receptor antagonist SR141716A. At 7 d after ouabain treatment, 64 +/- 24% less neuronal damage was observed in AEA-treated (10 mg/kg) rats compared with control animals. Coadministration of SR141716A prevented the neuroprotective actions of AEA at this end point. In addition, (1) no increase in AEA and 2-arachidonoylglycerol levels was detected at 2, 8, or 24 hr after ouabain injection; (2) application of SR141716A alone did not increase the lesion volume at days 0 and 7; and (3) the AEA-uptake inhibitor, VDM11, did not affect the lesion volume. These data indicate that there was no endogenous endocannabinoid tone controlling the acute neuronal damage induced by ouabain. Although our data seem to question a possible role of the endogenous cannabinoid system in establishing a brain defense system in our model, AEA may be used as a structural template to develop neuroprotective agents.

Role of pH on the calcium ion dependence of the nitric oxide synthase in the carp brain

The role of pH on the calcium dependence of nitric oxide synthase (NOS) of Cyprinus carpio brain was investigated. This fish is known to survive prolonged periods of hypoxia. Under this condition, cerebral blood flow is no longer regulated by nitric oxide (NO). Nitric oxide synthase activity is pH dependent in the range of pH between 7.4 and 6.2 with a decrease when tissue acidifies. At acidic pH, the dependence of the NOS activity on the free Ca(2+) concentrations changes considerably and shows an EC(50) of 0.13 microM at pH 7.1 and of 5.1 microM at pH 6.2 for the soluble enzyme. The variation in the Ca(2+) dependence with acidification is greater for the soluble than for the particulate enzyme. This may be the main factor protecting sudden NO formation mainly during anoxic-normoxic transitions.

K(ATP) channel blockers selectively interact with A(1)-adenosine receptor mediated modulation of acetylcholine release in the rat hippocampus

In this study the role of ATP-sensitive K(+) channels (K(ATP) channels) in the A(1) receptor mediated presynaptic inhibitory modulation of acetylcholine release was investigated in the rat hippocampus. N(6)-Cyclohexyladenosine (CHA), the selective A(1)-adenosine receptor agonist, reduced concentration-dependently the stimulation-evoked (2 Hz, 1 ms, 240 shocks) [3H]acetylcholine ([3H]ACh) release, from in vitro superfused hippocampal slices preloaded with [3H]choline, an effect prevented by the selective A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). By themselves, neither K(ATP) channel openers, i.e. diazoxide, pinacidil and cromakalim, nor glibenclamide and glipizide, the inhibitors of K(ATP) channels, exerted a significant effect on the resting and evoked release of [3H]ACh. Glibenclamide and glipizide (10-100 microM) completely prevented the inhibitory effect of 0.1 microM CHA and shifted the concentration response curve of CHA to the right. 4-Aminopyridine (10-100 microM), the non-selective potassium channel blocker, increased the evoked release of [3H]ACh, but in the presence of 4-aminopyridine, the inhibitory effect of CHA (0.1 microM) still persisted. Oxotremorine, the M(2) muscarinic receptor agonist, decreased the stimulation-evoked release of [3H]ACh, but its effect was not reversed by glibenclamide. 1,3-Diethyl-8-phenylxanthine (DPX), the selective A(1)-antagonist, effectively displaced [3H]DPCPX in binding experiments, while in the case of glibenclamide and glipizide, only slight displacement was observed. In summary, our results suggest that K(ATP) channels are functionally coupled to A(1) receptors present on cholinergic terminals of the hippocampus, and glibenclamide and glipizide, by interacting with K(ATP) channels, relieve this inhibitory neuromodulation.