Adenosine A(2A) receptors are necessary and sufficient to trigger memory impairment in adult mice.

BACKGROUND AND PURPOSE: Caffeine (a non-selective adenosine receptor antagonist) prevents memory deficits in aging and Alzheimer's disease, an effect mimicked by adenosine A2 A receptor, but not A1 receptor, antagonists. Hence, we investigated the effects of adenosine receptor agonists and antagonists on memory performance and scopolamine-induced memory impairment in mice. EXPERIMENTAL APPROACH: We determined whether A2 A receptors are necessary for the emergence of memory impairments induced by scopolamine and whether A2 A receptor activation triggers memory deficits in naïve mice, using three tests to assess short-term memory, namely the object recognition task, inhibitory avoidance and modified Y-maze. KEY RESULTS: Scopolamine (1.0 mg·kg(-1) , i.p.) impaired short-term memory performance in all three tests and this scopolamine-induced amnesia was prevented by the A2 A receptor antagonist (SCH 58261, 0.1-1.0 mg·kg(-1) , i.p.) and by the A1 receptor antagonist (DPCPX, 0.2-5.0 mg·kg(-1) , i.p.), except in the modified Y-maze where only SCH58261 was effective. Both antagonists were devoid of effects on memory or locomotion in naïve rats. Notably, the activation of A2 A receptors with CGS 21680 (0.1-0.5 mg·kg(-1) , i.p.) before the training session was sufficient to trigger memory impairment in the three tests in naïve mice, and this effect was prevented by SCH 58261 (1.0 mg·kg(-1) , i.p.). Furthermore, i.c.v. administration of CGS 21680 (50 nmol) also impaired recognition memory in the object recognition task. CONCLUSIONS AND IMPLICATIONS: These results show that A2 A receptors are necessary and sufficient to trigger memory impairment and further suggest that A1 receptors might also be selectively engaged to control the cholinergic-driven memory impairment.

Interações medicamentosas em cardiologia: Fármacos antiárritmicos / Drugs interactions in cardiology: Antiarrhythmics drugs

As arritmias cardíacas são geradas por diferentes mecanismoseletrofisiológicos que atuam isoladamente ou interagem entresi para a formação e condução do impulso anormal. Com baseno conhecimento da eletrofisiologia celular e dos mecanismosgeradores de arritmias, diversos fármacos antiarrítmicos foramdesenvolvidos com objetivo de propiciar terapias cada vez maiseficazes e seguras. A necessidade de se agrupar os antiarrítmicos deacordo com seu mecanismo de ação e efeitos no impulsocardíaco, resultou na classificação de Vaughan- Williams que,apesar de amplamente difundida, não contempla algumasmedicações classicamente utilizadas como antiarrítmicos, taiscomo a adenosina e os digitálicos. Os antiarrítmicos são, emgeral, metabolizados pelo fígado por meio dos citocromos.Fármacos que interagem no mesmo sítio de ação em que sãometabolizados podem resultar em potencialização ou inibiçãodos efeitos antiarrítmicos. A redução ou o aumento do nívelsérico do antiarrítmico causado pelo fármaco utilizado concomitantemente,em decorrência da alteração na velocidade demetabolização, da redução na absorção ou somatório de efeitos,pode aumentar o potencial para efeitos colaterais deletérios eefeitos pró-arrítmicos e resultar em efeitos tóxicos potencialmentegraves. O objetivo deste capítulo é revisar os diversosmecanismos de interação medicamentosa que podem ocorrerenvolvendo as classes de antiarrítmicos...

Cardiac arrhythrnias are generated by different electrophysiologicalmechanisms that act alone or interact for the formation andconduction of the abnormal impulse. Based on the knowledge ofcellular electrophysiology and arrhythmia mechanisms, severalantiarrhythrnics were developed in order to provide therapiesincreasingly effective and safe. The need of grouping the antiarrhythmicagents according to their mechanism of action andeffects on cardiac impulse have led to the development of theVaughan- Williams c1assification. Although widespread used,this c1assification does not include some drugs c1assically usedas antiarrhythmics such as adenosine and digitalis.Antiarrhythmic agents are generaUy metabolized by the livervia the cytochrome. Drugs that interact at the same site of actionthat are metabolized may result in potentiation or inhibition ofantiarrhythmic effects. The reduction or increase in serum levelscaused by antiarrhythmic drug used concomitantly, due to thechange in the metabolism, reduction in absorption or summationeffects may increase the potential for deleterious side effects andproarrhythmic effects and result in potentiaUy serious toxic effects.The purpose of this chapter is to review the variousmechanisms of drug interactions that may occur involving theantiarrhythmic drugs...

Differential adenosine uptake in mixed neuronal/glial or purified glial cultures of avian retinal cells: modulation by adenosine metabolism and the ERK cascade.

Adenosine is an important modulator of neuronal survival and differentiation in the CNS. Our previous work showed that nucleoside transporters (NTs) are present in cultures of chick retinal cells, but little is known about the mechanisms regulating adenosine transport in these cultures. Our aim in the present work was to study the participation of the adenosine metabolism as well as the ERK pathway on adenosine uptake in different types of retinal cultures (mixed and purified glial cultures). Kinetic analysis in both cultures revealed that the uptake reached equilibrium after 30 min and presented two components. Incubation of cultures with S-(p-nitrobenzyl)-6-thioinosine (NBTI) or dipyridamole, different inhibitors of equilibrative nucleoside transporters (ENTs), produced a significant and concentration-dependent uptake reduction in both cultures. However, while dipyridamole presented similar maximal inhibitory effects in both cultures (although in different concentrations), the inhibition by NBTI was smaller in glial cultures than in mixed cultures, suggesting the presence of different transporters. Moreover, pre-incubation of [(3)H]-adenosine with adenosine deaminase (ADA) or adenosine kinase (ADK) inhibition with iodotubercidin promoted significant uptake inhibition in both cultures, indicating that the uptake is predominantly for adenosine and not inosine, and that taken up adenosine is preferentially directed to the synthesis of adenine nucleotides. In both cultures, the MEK inhibitors PD98059 or UO126, but not the inactive analog U0124, induced a significant and concentration-dependent uptake decrease. We have not observed any change in adenosine metabolism induced by MEK inhibitors, suggesting that this pathway is mediating a direct effect on NTs. Our results show the expression of different NTs in retinal cells in culture and that the activity of these transporters can be regulated by the ERK pathway or metabolic enzymes such as ADK which are then potential targets for regulation of Ado levels in normal or pathological conditions.

Adenosine drives recycled vesicles to a slow-release pool at the mouse neuromuscular junction.

The effects of adenosine on neurotransmission have been widely studied by monitoring transmitter release. However, the effects of adenosine on vesicle recycling are still unknown. We used fluorescence microscopy of FM2-10-labeled synaptic vesicles in combination with intracellular recordings to examine whether adenosine regulates vesicle recycling during high-frequency stimulation at mouse neuromuscular junctions. The A(1) adenosine receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine) increased the quantal content released during the first endplate potential, suggesting that vesicle exocytosis can be restricted by endogenous adenosine, which accordingly decreases the size of the recycling vesicle pool. Staining protocols designed to label specific vesicle pools that differ in their kinetics of release showed that all vesicles retrieved in the presence of 8-cyclopentyl-1,3-dipropylxanthine were recycled towards the fast-release pool, favoring its loading with FM2-10 and suggesting that endogenous adenosine promotes vesicle recycling towards the slow-release pool. In accordance with this effect, exogenous applied adenosine prevented the replenishment of the fast-release vesicle pool and, thus, hindered its loading with the dye. We had found that, during high-frequency stimulation, Ca(2+) influx through L-type channels directs newly formed vesicles to a fast-release pool (Perissinotti et al., 2008). We demonstrated that adenosine did not prevent the effect of the L-type blocker on transmitter release. Therefore, activation of the A(1) receptor promotes vesicle recycling towards the slow-release pool without a direct effect on the L-type channel. Further studies are necessary to elucidate the molecular mechanisms involved in the regulation of vesicle recycling by adenosine.

Adenosine receptor antagonists for cognitive dysfunction: a review of animal studies.

Over the last decade, adenosine receptors in the central nervous system have been implicated in the modulation of cognitive functions. Despite the general view that endogenous adenosine modulates cognition through the activation of adenosine A1 receptors, evidence is now emerging on a possible role of A2A receptors in learning and memory. The present review attempts to examine results reported in different studies using diverse animal models, to provide a comprehensive picture of the recent evidence of a relationship between adenosinergic function and memory deficits. The present data suggest that caffeine (a nonselective adenosine receptor antagonist) and selective adenosine A2A receptor antagonists can improve memory performance in rodents evaluated through different tasks. They might also afford protection against memory dysfunction elicited in experimental models of aging, Alzheimer's disease, Parkinson's disease and, in spontaneously hypertensive rats (SHR), a putative genetic model of attention deficit hyperactivity disorder (ADHD).

Aspectos relevantes de la S-adenosilhomocisteína hidrolasa relacionados con el metabolismo de la S-adenosilhomocisteína / Relevant aspects of S-adenosylhomocysteine hydrolase related to the metabolism of S-adenosylhomocysteine

Se realizó una revisión de la S-adenosil metionina, considerado como el donante fundamental de grupos metilo en el organismo. Por una reacción de transmetilación es obtenida la S-adenosil homocisteína, que es un potente inhibidor de transmetilasas dependientes de S-adenosil metionina. La inhibición de las reacciones de transmetilación dependientes de esta última, es revelada por la conversión metabólica de S-adenosil homocisteína en adenosina y L-homocisteína mediante una reacción reversible, catalizada por la enzima S-adenosil homocisteína hidrolasa. Esta enzima se encuentra fundamentalmente localizada en hígado, páncreas y riñón, en el organismo. Está compuesta por 2 cadenas alfa y 2 beta, unidas por 4 puentes disulfuro y contiene residuos sulfidrilos. Se encuentra regulada por sus sustratos, adenosina y L-homocisteína, además, por purinas y otros metabolitos in vitro. La S-adenosil homocisteína hidrolasa se encuentra inhibida de forma irreversible por 5-desoí-5-difluorometil tioadenosina y 5-deoxy-5-trifluorometil tioadenosina(AU)

Reduced oxygen supply explains the negative force-frequency relation and the positive inotropic effect of adenosine in buffer-perfused hearts.

In isolated rat hearts perfused with HEPES and red blood cell-enriched buffers, we examined changes in left ventricular pressure induced by increases in heart rate or infusion of adenosine to investigate whether the negative force-frequency relation and the positive inotropic effect of adenosine are related to an inadequate oxygen supply provided by crystalloid perfusates. Hearts perfused with HEPES buffer at a constant flow demonstrated a negative force-frequency relation, whereas hearts perfused with red blood cell-enriched buffer exhibited a positive force-frequency relation. In contrast, HEPES buffer-perfused hearts showed a concentration-dependent increase in left ventricular systolic pressure [EC50 = 7.0 +/- 1.2 nM, maximal effect (Emax) = 104 +/- 2 and 84 +/- 2 mmHg at 0.1 microM and baseline, respectively] in response to adenosine, whereas hearts perfused with red blood cell-enriched buffer showed no change in left ventricular pressure. The positive inotropic effect of adenosine correlated with the simultaneous reduction in heart rate (r = 0.67, P < 0.01; EC50 = 3.8 +/- 1.4 nM, baseline 228 +/- 21 beats/min to a minimum of 183 +/- 22 beats/min at 0.1 microM) and was abolished in isolated hearts paced to suppress the adenosine-induced bradycardia. In conclusion, these results indicate that the negative force-frequency relation and the positive inotropic effect of adenosine in the isolated rat heart are related to myocardial hypoxia, rather than functional peculiarities of the rat heart.

The adenosine antagonist theophylline impairs p50 auditory sensory gating in normal subjects.

In the p50 suppression paradigm, when two auditory stimuli are presented 500 ms apart, the amplitude of the second response (S2), compared with the first (S1), is markedly attenuated in healthy subjects. This is an index of sensory gating. Most schizophrenic patients fail to inhibit the p50 response to the second stimulus, which is assumed to reflect an inhibitory deficit. Adenosine is a neuromodulator with mostly inhibitory activity which is released by physiological stimuli. Since this inhibitory pattern resembles the phenomenon of sensory gating, the contribution of adenosine to p50 suppression was investigated in normal volunteers after treatment with the adenosine antagonist theophylline or placebo. P50 recordings were conducted in thirteen healthy subjects at baseline and 5, 30, 60, and 90 min after oral administration of theophylline (0.66 mg/kg, maximum dose of 500 mg) or placebo in a cross-over design. Baseline results from 17 drug-treated schizophrenic patients were included for comparison. Compared with placebo, theophylline treatment significantly increased P50 ratio (S2/S1) from 0.28 +/- 0.03 to 0.82 +/- 0.11 at 30 min and 0.61 +/- 0.07 at 60 min (mean +/- SEM), which were not significantly different from the schizophrenia group (0.74 +/- 0.05). The increased p50 ratio by theophylline was due to a combined decrease in S1 and increase in S2 amplitude. The impairment of p50 suppression by theophylline in normal subjects suggests a modulatory role of adenosine in sensory gating, which may be related to p50 suppression deficit in schizophrenia and is in agreement with a hypoadenosinergic model of schizophrenia.

Inhibition of vaccinia virus replication by adenosine in BSC-40 cells: involvement of A(2) receptor-mediated PKA activation.

In the present study, we show that adenosine (Ado) affects vaccinia virus (VV) replication in BSC-40 cells. In order to identify its effects on VV replicative cycle we analyzed the synthesis of virus macromolecules in cells incubated with 0.5 mM Ado. A 50% reduction in the steady-state level of virus DNA synthesis was observed. Consequently, virus post-replicative gene expression was also affected. A prolonged synthesis of the F11L early virus protein was also observed and it is likely related to a slow decline in the steady-state level of early mRNAs, as suggested by northern blot analysis of the VGF early transcript. The involvement of cAMP-signaling pathway as mediator of Ado response was also evaluated. Ado stimulated a three-fold increase in cAMP levels in BSC-40 cells and cAMP-mimetic agents reduced virus yield in a dose-dependent manner. Co-incubation of infected cells with H-89 reduced the inhibitory effects of 8-Br-cAMP and Ado on VV yields suggesting PKA involvement. A(2) receptor-mediated activation of PKA was indicated by antagonism of Ado response by theophylline and DMPX. Together, these results indicate that virus DNA replication is the main target of Ado. The mechanism involved is not related to reduction of the pyrimidine nucleotide synthesis. Furthermore, Ado-induced PKA activation modulates negatively an unidentified step of the virus replicative cycle.

Regulation of adenosine transport by D-glucose in human fetal endothelial cells: involvement of nitric oxide, protein kinase C and mitogen-activated protein kinase.

The effects of elevated D-glucose on adenosine transport were investigated in human cultured umbilical vein endothelial cells isolated from normal pregnancies. Elevated D-glucose resulted in a time- (8-12 h) and concentration-dependent (half-maximal at 10+/-2 mM) inhibition of adenosine transport, which was associated with a reduction in the Vmax for nitrobenzylthioinosine (NBMPR)-sensitive (es) saturable nucleoside with no significant change in Km. d-Fructose (25 mM), 2-deoxy-D-glucose (25 mM) or D-mannitol (20 mM) had no effect on adenosine transport. Adenosine transport was inhibited following incubation of cells with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA; 100 nM, 30 min to 24 h). D-Glucose-induced inhibition of transport was abolished by calphostin C (100 nM, an inhibitor of PKC), and was not further reduced by PMA. Increased PKC activity in the membrane (particulate) fraction of endothelial cells exposed to D-glucose or PMA was blocked by calphostin C but was unaffected by NG-nitro-L-arginine methyl ester (L-NAME; 100 microM, an inhibitor of nitric oxide synthase (NOS)) or PD-98059 (10 microM, an inhibitor of mitogen-activated protein kinase kinase 1). D-Glucose and PMA increased endothelial NOS (eNOS) activity, which was prevented by calphostin C or omission of extracellular Ca2+ and unaffected by PD-98059. Adenosine transport was inhibited by S-nitroso-N-acetyl-l, d-penicillamine (SNAP; 100 microM, an NO donor) but was increased in cells incubated with L-NAME. The effect of SNAP on adenosine transport was abolished by PD-98059. Phosphorylation of mitogen-activated protein kinases p44mapk (ERK1) and p42mapk (ERK2) was increased in endothelial cells exposed to elevated D-glucose (25 mM for 30 min to 24 h) and the NO donor SNAP (100 microM, 30 min). The effect of D-glucose was blocked by PD-98059 or L-NAME, which also prevented the inhibition of adenosine transport mediated by elevated D-glucose. Our findings provide evidence that D-glucose inhibits adenosine transport in human fetal endothelial cells by a mechanism that involves activation of PKC, leading to increased NO levels and p42-p44mapk phosphorylation. Thus, the biological actions of adenosine appear to be altered under conditions of sustained hyperglycaemia.

Role of central adenosine in the respiratory and thermoregulatory responses to hypoxia.

No reports are available about the role of central adenosine in the respiratory and thermoregulatory responses to hypoxia in conscious rats. We therefore measured ventilation (VE) and body temperature (Tb) before and after intracerebroventricular injection of saline or aminophylline (adenosine antagonist), followed by a 30-min period of hypoxia exposure. Aminophylline did not change VE or Tb during normoxia; however, during hypoxia, it caused a significant increase in VE, and significantly attenuated hypoxic hypothermia. The present data indicate that central adenosine has an inhibitory effect on hypoxic hyperventilation and partially causes hypoxic hypothermia, suggesting that the ventilatory and metabolic interaction during hypoxia does not involve opposing mechanisms.

Adenosine regulates renal nitric oxide production in hypothyroid rats.

In the hypothyroid kidney, exogenous adenosine (ADO) produces vasodilation and restores renal function to near-normal values. This study evaluates whether this response is mediated by nitric oxide synthesis stimulated by adenosine. GFR and urinary excretion of NO2-/NO3- (UNO2-/NO3-) were measured in normal (NL) and hypothyroid (HTX) rats under basal conditions and during infusion of: intra-aortic ADO, intravenously, 1,3-dipropyl-8p-sulfophenylxanthine (DPSPX), 8-cyclopentyl-1,3-dipropyl xanthine (DPCPX), N(omega)-nitro-L-arginine methylester (L-NAME) + ADO, L-NAME + PSPX, L-NAME + DPCPX, and intrarenal (IR) ADO or DPCPX + IR ADO. Intra-aortic ADO induced a fall in GFR and increased UNO2-/NO3- slightly in NL rats; in HTX rats, both GFR and UNO2-/NO3- increased significantly. DPSPX and DPCPX increased UNO2-/NO3- excretion in NL animals with minor changes in GFR; the blockers increased both GFR and UNO2-/ NO3- in HTX rats. L-NAME completely blocked the increase in NO2-/NO3- induced by ADO, DPSPX, and DPCPX. The intrarenal infusion of ADO at 1, 10, and 35 nmol/kg per min progressively decreased GFR with a slight increase in UNO2-/ NO3- in NL rats; in the HTX, GFR increased with the highest dose and UNO2-/NO3- progressively increased. DPCPX prevented the fall in GFR induced by intrarenal ADO in NL rats, with no further changes in UNO2-/NO3-; in HTX rats, intrarenal ADO under A1 blockade further increased GFR and UNO2-/NO3-. Arterial and venous ADO concentrations were lower in the HTX rats. In the HTX kidney, NO production was stimulated by ADO, most likely through activation of A2 or A3 receptors, whereas A1 receptors had an inhibitory effect. Thus, ADO receptors are involved in the regulation of kidney function in pathophysiologic conditions.

Use of aminophylline and enalapril in posttransplant polycythemia.

BACKGROUND: Posttransplant polycythemia (PTP) affects 6-30% of renal transplant recipients and can result in thromboembolic disease. The pathogenesis of PTP remains unknown and may be multifactorial. Although phlebotomy has previously been the treatment for PTP, drugs such as adenosine receptor antagonists or angiotensin-converting enzyme inhibitors can be used to control PTP. METHODS: The authors performed a prospective study of two different drugs to treat PTP: aminophylline and enalapril. Twenty-seven patients with PTP lasting more than 6 months were evaluated. During phase 1, aminophylline was compared with enalapril. The patients sequentially received aminophylline and enalapril during 12-week periods, intercalated by 12-week periods of no drugs. During phase 2, enalapril was administered for 12 weeks. RESULTS: From January 1984 to December 1993, 110 of 333 patients with PTP lasting more than 6 months (33%) developed polycythemia, and 27 patients were included in the present study. In phase 1, aminophylline had no effect on PTP. Enalapril promoted an erythropoiesis inhibition, characterized by a decrease in hematocrit and an increase in iron stores and ferritin levels. After withdrawal of enalapril, the hematocrit increased and the iron stores decreased. In phase 2, there was a progressive reduction in hematocrit after the 4th week of therapy. The lowest hematocrit was observed in the 12th week and then enalapril was stopped, leading to a subsequent rise in hematocrit. Erythropoietin levels and renal function remained constant during all periods of both phases of the study. CONCLUSION: The use of adenosine antagonists was ineffective to treat PTP in our series. However, treatment with enalapril promoted an erythropoiesis inhibition, demonstrated by a reduction in hematocrit, hemoglobin, red blood cell count, and reticulocyte count, associated with an increase in iron stores. This response occurred independently from erythropoietin levels or hemodynamic graft changes.

[Role of adenosine in the digitalis action on atrio-ventricular conduction in the dog heart]. / Participación de la adenosina en la acción digitalica sobre la conducción auriculo-ventricular del corazón canino.

In order to explore the hypothesis about the existence of an adenylic component on ouabain effects on the atrioventricular node, we perform experiments on anesthetized and vagotomized dogs. Decremental propagation of impulses through atrioventricular conduction system was evaluated stimulating the right atria at frequencies from 3.5 to 5.0 per second. Aminophylline antagonized and dipyridamole synergized the atrioventricular decremental conduction induced by digitalis. The blockade of purinergic receptors produced by aminophylline and the inhibition of adenosine endocytosis by dipyridamole could explain these antagonic and synergistic interaction with ouabain, and constitute an experimental evidence favorable to the possibility of the existence of a purinergic component on the digitalis mechanism of action.

Further evidence for the purinergic inhibition of adrenergic neurotransmission in the rat portal vein.

The inhibition of 3H-noradrenaline (3H-NA) release by adenosine, and the possible involvement of purine receptors in the regulation of transmitter release in the portal vein were studied. The inhibitory effect of different concentrations of adenosine (10, 30, 100 and 300 microM) decreased with frequency of stimulation, but there was no marked concentration-dependence. Tetraethylammonium (TEA) enhanced the 3H-NA overflow induced by transmural stimulation. The adenosine-induced inhibition of 3H-NA overflow was antagonized by TEA. Transmural stimulation induced release of tritium from tissues prelabelled with either 3H-NA or 3H-adenine had a similar pattern of distribution. In contrast, when the rat portal vein was stimulated with (-) NA, the overflow of purine derivates was delayed and the maximum release was achieved 5 min later than the maximum induced by transmural stimulation. Phenoxybenzamine (PBA) increased 3H-NA overflow two-fold, but had no effect on the 3H-purine release induced by transmural stimulation. PBA reduced the 3H-purine release by exogenous (-) NA. These results indicate that in rat portal vein, the purine compounds have pre- and postjunctional origins and that the purine that modulates adrenergic neurotransmission might be of neuronal origin, possibly independent of adrenergic innervation.