Pharmacological characterisation and inhibitory effects of (2R,3R,4S,5R)-2-(6-amino-2-{[(1S)-2-hydroxy-1-(phenylmethyl)ethyl]amino}-9H-purin-9-yl)-5-(2-ethyl-2H-tetrazol-5-yl)tetrahydro-3,4-furandiol, a novel ligand that demonstrates both adenosine A(2A) receptor agonist and adenosine A(3) receptor antagonist activity

The pharmacological properties of the novel ligand, (2R,3R,4S,5R)-2-(6-amino-2-{[(1S)-2-hydroxy-1-(phenylmethyl)ethyl]amino}-9H-purin-9-yl)-5-(2-ethyl-2H-tetrazol-5-yl)tetrahydro-3,4-furandiol (I), at the human adenosine receptors were investigated using Chinese hamster ovary cell lines recombinantly expressing these receptors. Functional studies were performed using a cyclic AMP-coupled reporter gene system. Binding studies were performed using membranes from these cells. The effects of ligand (I) were also determined on functional responses of human neutrophils and eosinophils. Ligand (I) had a high affinity for the adenosine A(2A) receptor (pKi 7.8+/-0.2) and was a potent agonist at this receptor (pEC(50) 9.0+/-0.2). Ligand (I) had a similar affinity for the adenosine A(3) receptor (pKi 7.8+/-0.1) but displayed no agonist activity, acting instead as a competitive antagonist (pA(2) 8.3+/-0.04). Ligand (I) had lower affinity for adenosine A(1) and A(2B) receptors (pKi相似文献   

A new chemical tool (C0036E08) supports the role of adenosine A(2B) receptors in mediating human mast cell activation

Asthma is a chronic inflammatory disease of the airways that involves many cell types, amongst which mast cells are known to be important. Adenosine, a potent bronchoconstricting agent, exerts its ability to modulate adenosine receptors of mast cells thereby potentiating derived mediator release, histamine being one of the first mediators to be released. The heterogeneity of sources of mast cells and the lack of highly potent ligands selective for the different adenosine receptor subtypes have been important hurdles in this area of research. In the present study we describe compound C0036E08, a novel ligand that has high affinity (pK(i) 8.46) for adenosine A(2B) receptors, being 9 times, 1412 times and 3090 times more selective for A(2B) receptors than for A(1), A(2A) and A(3) receptors, respectively. Compound C0036E08 showed antagonist activity at recombinant and native adenosine receptors, and it was able to fully block NECA-induced histamine release in freshly isolated mast cells from human bronchoalveolar fluid. C0036E08 has been shown to be a valuable tool for the identification of adenosine A(2B) receptors as the adenosine receptors responsible for the NECA-induced response in human mast cells. Considering the increasing interest of A(2B) receptors as a therapeutic target in asthma, this chemical tool might provide a base for the development of new anti-asthmatic drugs.     

Effects of some organophosphorus compounds on the binding of a radioligand (8-cyclopentyl 1, 3-[3H]dipropylxanthine) to adenosine receptors in ovine cardiac membranes.

8-Cyclopentyl-1,3-[3H] dipropylxanthine [( 3H]CPX) is a potent radioligand that specifically binds to the A1 adenosine receptors. Its high specificity makes it a suitable ligand for the characterization of A1 adenosine receptors in tissues with low receptor densities. We have demonstrated that the organophosphorus compounds soman, tabun and sarin, at relatively high concentrations, all bind to the A1 adenosine receptors in ovine cardiac membranes with Ki values of 36.7, 328 and 175 microM, respectively. The binding of soman to the receptor site was found to be totally reversible. We suggested that these organophosphorus compounds affect the mechanical responses of the heart through interaction with a potassium channel that does not seem to be closely linked to the A1 adenosine receptors.     

Facilitation of noradrenaline release by adenosine A(2A) receptors in the epididymal portion and adenosine A(2B) receptors in the prostatic portion of the rat vas deferens

The adenosine-receptor modulation of noradrenaline release was compared in prostatic and epididymal portions of rat vas deferens. In both portions, tritium overflow elicited by electrical stimulation (100 pulses/8 Hz) was reduced by the adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine, and enhanced by the nonselective receptor agonist, 5'-N-ethylcarboxamidoadenosine, in the presence of the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 20 and 100 nM). The adenosine A(2A) receptor agonist, 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine, increased tritium overflow, but only in the epididymal portion. The enhancement caused by NECA was prevented by the adenosine A(2A) receptor antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385; 20 nM), in the epididymal and by the adenosine A(2B) receptor antagonist, alloxazine (1 microM), in the prostatic portion. Inhibition of adenosine uptake enhanced tritium overflow in both portions, an effect blocked by ZM 241385 in the epididymal and by alloxazine in the prostatic portion. The results indicate that adenosine exerts an adenosine A(1) receptor-mediated inhibition, in both portions, and facilitation mediated by adenosine A(2A) receptors in the epididymal and by A(2B) receptors in the prostatic portion.     

Treating lung inflammation with agonists of the adenosine A2A receptor: promises, problems and potential solutions

Adenosine A(2A) receptor agonists may be important regulators of inflammation. Such conclusions have come from studies demonstrating that, (i) adenosine A(2A) agonists exhibit anti-inflammatory properties in vitro and in vivo, (ii) selective A(2A) antagonists enhance inflammation in vivo and, (iii) knock outs of this receptor aggravate inflammation in a wide variety of in vivo models. Inflammation is a hallmark of asthma and COPD and adenosine has long been suggested to be involved in disease pathology. Two recent publications, however, suggested that an inhaled adenosine A(2A) receptor agonist (GW328267X) did not affect either the early and late asthmatic response or symptoms associated with allergic rhinitis suggesting that the rationale for treating inflammation with an adenosine A(2A) receptor agonist may be incorrect. A barrier to fully investigating the role of adenosine A(2A) receptor agonists as anti-inflammatory agents in the lung is the side effect profile due to systemic exposure, even with inhalation. Unless strategies can be evolved to limit the systemic exposure of inhaled adenosine A(2A) receptor agonists, the promise of treating lung inflammation with such agents may never be fully explored. Using strategies similar to that devised to improve the therapeutic index of inhaled corticosteroids, UK371,104 was identified as a selective agonist of the adenosine A(2A) receptor that has a lung focus of pharmacological activity following delivery to the lung in a pre clinical in vivo model of lung function. Lung-focussed agents such as UK371,104 may be suitable for assessing the anti-inflammatory potential of inhaled adenosine A(2A) receptor agonists.     

Medicinal chemistry of adenosine A1 receptor ligands

In this review the latest developments in ligand design for the adenosine A(1) receptor are summarized. Novel series of agonists and antagonists are discussed, leading to the conclusion that ligands truly selective for the human adenosine A(1) receptor are still scarce.     

Reduced appetite for caffeine in adenosine A(2A) receptor knockout mice

Adenosine A(2A) receptor knockout mice (A(2A)R KO) were compared to wild-type controls (A(2A)R WT) in a caffeine intake paradigm. When mice had ad libitum access to caffeine (0.3 g/l) and water in a two-bottle paradigm for 12 consecutive days, adenosine A(2A)R KO mice drank less caffeinated solution, demonstrating a reduced appetite for caffeine as compared to adenosine A(2A)R WT mice. These data reveal an important role for the adenosine A(2A) receptor in the appetitive properties of caffeine.     

Allosteric modulation, thermodynamics and binding to wild-type and mutant (T277A) adenosine A1 receptors of LUF5831, a novel nonadenosine-like agonist

The interaction of a new nonribose ligand (LUF5831) with the human adenosine A1 receptor was investigated in the present study. Radioligand binding experiments were performed in the absence and presence of diverse allosteric modulators on both wild-type (wt) and mutant (T277A) adenosine A1 receptors. Thermodynamic data were obtained by performing these assays at different temperatures. In addition, cyclic adenosine monophosphate (cAMP) assays were performed. The presence of allosteric modulators had diverse effects on the affinity of LUF5831, N6-cyclopentyladenosine (CPA), a full agonist, and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an inverse agonist/antagonist, for the adenosine A1 receptor. PD81,723, for example, increased the affinity of CPA, while the affinity of LUF5831 was decreased. However, the affinity of DPCPX was decreased even more. In addition, LUF5831 was shown to have an affinity for the mutant (T277A) adenosine A1 receptor (Ki=122+/-22 nM), whereas CPA's affinity was negligible. The results of temperature-dependent binding assays showed that the binding of LUF5831 was entropy driven, in between the behaviour of CPA binding to the high- and low-affinity states of the receptor, respectively.The inhibition of the forskolin-induced production of cAMP through activation of the wt adenosine A1 receptor showed that LUF5831 had a submaximal effect (37+/-1%) in comparison to CPA (66+/-5%). On the mutant receptor, however, neither CPA nor LUF5831 inhibited cAMP production. This study indicates that the nonribose ligand, LUF5831, is a partial agonist for the adenosine A1 receptor.     

Adenosine receptor agonists

Adenosine, a naturally-occurring nucleoside, modulates a variety of physiological and pathophysiological processes. The effects of adenosine are mediated via a family of cell surface G-protein-coupled receptors designated into four subtypes, A₁, A_2A, A_2B and A₃. The adenosine receptors have widespread tissue distribution and are often co-expressed in the same cell type. Research on adenosine receptors over the past few decades has resulted in the molecular cloning of the four subtypes from multiple species, significant progress in identifying selective agonists and antagonists and an increased understanding of the particular roles adenosine receptor subtypes play in physiological processes. This knowledge has continued to fuel considerable interest in pursuing adenosine receptors as therapeutic targets. For example, adenosine receptor agonists have been proposed for the treatment of heart arrhythmias, inflammatory diseases and in diagnosing coronary artery disease. In general, adenosine receptor agonists are derivatives of the physiological agonist, adenosine. The development of adenosine receptor agonists has been limited by an essential requirement for retention of the ribose moiety for agonist activity. Despite this restriction, significant progress has been made in the identification of potent and selective adenosine receptor agonists, some of which have entered clinical trials.     

Effects of 2-alkynyladenosine derivatives on intraocular pressure in rabbits

We evaluated the activities of 2-alkynyladenosine derivatives, relatively selective adenosine A2 receptor agonists, in the intraocular pressure regulation in rabbits. An adenosine A2 receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS-21680) decreased intraocular pressure, while another A2 receptor agonist 2-(phenylamino)adenosine transiently increased it. The first group of 2-alkynyladenosine derivatives (1-hexyn-1-yl derivatives) caused a transient increase followed by decrease in intraocular pressure, while the second group (1-octyn-1-yl and 6-cyano-1-hexyn-1-yl derivatives) only decreased it. The second group is also effective in the ocular hypertensive models induced by water-loading and alpha-chymotrypsin. The outflow facility was increased by a 1-octyn-1-yl derivative. Both increase and decrease in intraocular pressure induced by 2-alkynyladenosine derivatives were inhibited by an adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine, but not by an adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropyl xanthine. These findings suggest that 2-alkynyladenosine derivatives may affect intraocular pressure via adenosine A2 receptor, and 2-alkynyladenosine derivative-induced ocular hypotension is due to the increase of outflow facility.     

Hypotensive effects of intravenously administered uridine and cytidine in conscious rats: involvement of adenosine receptors

In the present study, we investigated the cardiovascular effects of intravenously injected uridine or cytidine, and the role of adenosine receptors in mediating these effects, in conscious normotensive rats. Intravenous (i.v.) administration of uridine (124, 250, 500 mg/kg) dose-dependently decreased arterial pressure and heart rate. Cytidine (124, 250, 500 mg/kg; i.v.) produced slight dose-related hypotension without changing heart rate. Plasma uridine and cytidine concentrations increased time- and dose-dependently while plasma adenosine levels did not change after injection of the respective nucleosides. Pretreatment with intravenous caffeine (20 mg/kg), 8-phenyltheophylline (8-PT) (1 mg/kg), nonselective adenosine receptor antagonists, or 8-p-sulfophenyltheophylline (8-SPT) (20 mg/kg), a nonselective adenosine receptor antagonist which does not cross the blood-brain barrier, abolished the cardiovascular effects of uridine (250 mg/kg; i.v.) or cytidine (250 mg/kg; i.v.). Intracerebroventricular (i.c.v.) caffeine (200 microg) or 8-SPT (50 microg) pretreatment did not change the magnitude of the cardiovascular responses induced by nucleosides. Intravenous 8-cyclopenthyl-1,3-dipropylxanthine (DPCPX) (5 mg/kg), a selective adenosine A(1) receptor antagonist, greatly attenuated the cardiovascular responses to uridine and cytidine. Pretreatment with 3,7,-dimethyl-1-propargylxanthine (DMPX) (2 mg/kg), an adenosine A(1)/A(2) receptor antagonist, attenuated hypotension induced by uridine and blocked the arterial pressure decrease in response to cytidine. Uridine-induced bradycardia was blocked by DMPX. 4-(2-[7-amino-2-(2-furyl[1,2,4]-triazolo[2,3-a[1,3,5]triazin-5-yl-aminoethyl)phenol (ZM241385) (1 mg/kg; i.v.), a selective adenosine A(2A) receptor antagonist, pretreatment produced an only very small blockade in the first minute of the hypotensive effects of uridine without affecting the bradycardia. ZM241385 pretreatment completely blocked cytidine's hypotensive effect. In Langendorff-perfused rat heart preparation, uridine (10(-3) M), but not cytidine, decreased the heart rate. Our results show that intravenously injected uridine or cytidine is able to decrease arterial pressure by activating peripheral adenosine receptors. The data also implicates that the mainly adenosine A(1) receptor activation is involved in the uridine-induced cardiovascular effects, while both adenosine A(1) and A(2A) receptor activations mediate the cytidine's effects.     

Functional expression of adenosine A2A and A3 receptors in the mouse dendritic cell line XS-106

There is increasing evidence to suggest that adenosine receptors can modulate the function of cells involved in the immune system. For example, human dendritic cells derived from blood monocytes have recently been described to express functional adenosine A1, A2A and A3 receptors. Therefore, in the present study, we have investigated whether the recently established murine dendritic cell line XS-106 expresses functional adenosine receptors. The selective adenosine A3 receptor agonist 1-[2-chloro-6[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-D-ribofuranuronamide (2-Cl-IB-MECA) inhibited forskolin-mediated [3H]cyclic AMP accumulation and stimulated concentration-dependent increases in p42/p44 mitogen-activated protein kinase (MAPK) phosphorylation. The selective adenosine A2A receptor agonist 4-[2-[[-6-amino-9-(N-ethyl-beta-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzene-propanoic acid (CGS 21680) stimulated a robust increase in [3H]cyclic AMP accumulation and p42/p44 MAPK phosphorylation. In contrast, the selective adenosine A1 receptor agonist CPA (N6-cyclopentyladenosine) did not inhibit forskolin-mediated [3H]cyclic AMP accumulation or stimulate increases in p42/p44 MAPK phosphorylation. These observations suggest that XS-106 cells express functional adenosine A2A and A3 receptors. The non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) inhibited lipopolysaccharide-induced tumour necrosis factor-alpha (TNF-alpha) release from XS-106 cells in a concentration-dependent fashion. Furthermore, treatment with Cl-IB-MECA (1 microM) or CGS 21680 (1 microM) alone produced a partial inhibition of lipopolysaccharide-induced TNF-alpha release (when compared to NECA), whereas a combination of both agonists resulted in the inhibition of TNF-alpha release comparable to that observed with NECA alone. Treatment of cells with the adenosine A2A receptor selective antagonists 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5ylamino]ethyl)phenol (ZM 241385; 100 nM) and 5-amino-2-(2-furyl)-7-phenylethyl-pyrazolo[4,3-e]-1,2,4-triazolo[1,5c]pyrimidine (SCH 58261; 100 nM) and the adenosine A3 receptor selective antagonist N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide (MRS 1220; 100 nM) partially blocked the inhibitory effects of NECA on lipopolysaccharide-induced TNF-alpha release. Combined addition of MRS 1220 and SCH 58261 completely blocked the inhibitory effects of NECA on lipopolysaccharide-induced TNF-alpha release. In conclusion, we have shown that the mouse dendritic cell line XS-106 expresses functional adenosine A2A and A3 receptors, which are capable of modulating TNF-alpha release.     

Neonatal caffeine exposure alters developmental sensitivity to adenosine receptor ligands

Studies were done to determine whether the apparent changes in behavioral sensitivity to adenosine receptor ligands that occur with age and with neonatal caffeine exposure were due to a change in sensitivity of the receptor for the ligand or to a more fundamental change in the receptor. Using an animal model that mimics the brain developmental period and level of caffeine exposure in human premature neonates treated with caffeine for apnea of prematurity, behavioral and neurochemical investigations were undertaken. The locomotor responses to acute challenge with caffeine (15, 30 and 60 mg/kg) and with D-phenylisopropyladenosine (D-PIA) (0.038 and 0.38 mg/kg), an adenosine receptor agonist, were measured in control and neonatally caffeine-exposed rats at 12, 15, 18, and 28 days of age. The dissociation constants (K_d) and maximal binding densities (B_max) for agonist binding at the adenosine A1 receptor site were determined over a similar time period. Caffeine displacement of an adenosine A1 agonist was also measured to examine in vitro sensitivity to caffeine as a function of age and neonatal caffeine exposure. Our studies demonstrated that the differential responses to adenosine receptor ligands seen as a function of both age and neonatal caffeine exposure could not be overcome by merely increasing the doses of ligand administered. In addition, the results of the binding studies indicated that changes in the adenosine receptor are occuring as a function of age in different regions of the brain of control animals and that this development is influenced by neonatal caffeine exposure.     

Molecular modeling of adenosine receptors. I. The ligand binding site on the A1 receptor.

The amino acid sequence of the canine adenosine A1 receptor and the atomic coordinates of a structurally related protein, bacteriorhodopsin, were combined to generate a three-dimensional model for the adenosine A1 receptor. This model consists of seven amphipathic alpha-helices, forming a pore that has a rather distinct partition between hydrophobic and hydrophilic regions. Subsequently, a highly potent and selective ligand, N6-cyclopentyladenosine, was docked into this cavity. A binding site is proposed that takes into account the conformational characteristics of the ligand, obtained from indirect modeling studies by the 'active analog approach'. Moreover, it involves two histidine residues that were shown to be important for ligand coordination from chemical modification studies. Finally, the deduced binding site was used to model other potent ligands that could all be accommodated consistent with earlier biochemical and pharmacological findings.     

The receptor mechanism mediating the contractile response to adenosine on lung parenchymal strips from actively sensitised,allergen-challenged Brown Norway rats

Parenchymal strips prepared from lungs removed from actively sensitised Brown Norway rats challenged with allergen show hyperresponsiveness to adenosine. The response is mast cell mediated and a preliminary pharmacological analysis suggested the involvement of a receptor (or receptors) that could not be classified as any of the known adenosine receptor subtypes. We present a further analysis of the response. Male Brown Norway (BN) rats, actively sensitised to ovalbumin (OA), were challenged intratracheally with OA and killed 3 h later to provide parenchymal strip preparations. The augmented contractile responses to adenosine were partially blocked by the 5-HT receptor antagonist, methysergide, or the A₁ receptor antagonist, DPCPX, and abolished in the presence of both antagonists. Responses to high concentrations of the A₁ receptor agonist, CPA were, like those to adenosine, augmented on tissues from allergen-challenged animals and blocked by a combination of methysergide and DPCPX. The A₃ receptor agonist, Cl-IB-MECA, did not contract the tissue, but partially blocked the response to adenosine. A combination of Cl-IB-MECA and methysergide induced a similar degree of blockade to that seen with either drug given alone. Combination of Cl-IB-MECA and/or methysergide with DPCPX abolished the response to adenosine. The effects of the A₃ receptor agonist, inosine, were augmented on tissues from allergen-challenged animals and markedly inhibited by disodium cromoglycate, methysergide or Cl-IB-MECA. Responses to adenosine were abolished when parenchymal strips were taken from rats pretreated 48 h previously with pertussis toxin. 8-SPT, CGS 15943, XAC, MRS 1754, DPCPX and theophylline, at concentrations which inhibit the A₁ A_2A and/or A_2B receptors but have negligible affinity for the rat A₃ receptor, inhibited responses to adenosine, but high concentrations were required and blockade was incomplete. MRS 1523 and MRS 1191, which are antagonists at the rat A₃ receptor, had no effect on the response to adenosine. The present results support and clarify our earlier conclusion that an atypical receptor mechanism mediates contraction of the parenchymal strip prepared from the lungs of actively sensitised BN rats challenged with allergen to adenosine. The response arises from a combined effect of adenosine on the A₁ receptor and a receptor with similarities to the A₃ receptor, but where Cl-IB-MECA behaves as an antagonist and MRS 1523 and MRS 1191 are inactive at concentrations that substantially exceed their affinities for the rat A₃ receptor.A part of this work was presented to the British Pharmacological Society in January 2003     

KF26777 (2-(4-bromophenyl)-7,8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one dihydrochloride), a new potent and selective adenosine A3 receptor antagonist

We investigated the biochemical and pharmacological properties of a new adenosine A(3) receptor antagonist, KF26777 (2-(4-bromophenyl)-7,8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one dihydrochloride). This compound was characterized using N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]AB-MECA) or [35S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding to membranes from human embryonic kidney 293 (HEK293) cells expressing human adenosine A(3) receptors. KF26777 showed a K(i) value of 0.20+/-0.038 nM for human adenosine A(3) receptors labeled with [125I]AB-MECA and possessed 9000-, 2350- and 3100-fold selectivity vs. human adenosine A(1), A(2A) and A(2B) receptors, respectively. The inhibitory mode of binding was competitive. KF26777 inhibited the binding of [35S]GTPgammaS stimulated by 1 microM 2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA). The IC(50) value was 270+/-85 nM; the compound had no effect on basal activity. Dexamethasone treatment for HL-60 cells, human promyelocytic leukemia, up-regulated functional adenosine A(3) receptors expression, and resulted in the enhanced elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) via the adenosine A(3) receptor. KF26777 antagonized this [Ca(2+)](i) mobilization induced by Cl-IB-MECA, with a K(B) value of 0.42+/-0.14 nM. These results indicate that KF26777 is a highly potent and selective antagonist of the human adenosine A(3) receptor.     

Tonic adenosine A1 and A2A receptor activation is required for the excitatory action of VIP on synaptic transmission in the CA1 area of the hippocampus

Adenosine can regulate synaptic transmission through modulation of the action of other neurotransmitters. The influence of adenosine on VIP enhancement of synaptic transmission in hippocampal slices was investigated. Facilitation of fEPSP slope by 1 nM VIP (23.3+/-1.3%) was turned into an inhibition (-12.1+/-3.4%) when extracellular endogenous adenosine was removed using adenosine deaminase (ADA, 1U/ml). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM) or of A(2A) receptors with ZM241385 (20 nM) attenuated the effect of VIP. When both DPCPX and ZM241385 were present the effect of VIP was abolished. In the presence of ADA, selective A(1) receptor activation with N(6)-cyclopentyladenosine (CPA, 15 nM) or A(2A) receptor-activation with CGS21680 (10 nM) partially readmitted the excitatory effect of VIP on fEPSPs. In contrast, facilitation of PS amplitude by 1 nM VIP (19.1+/-1.2%) was attenuated in the presence of ADA or DPCPX but was not changed by ZM241385. CPA, in the presence of ADA, fully restored the effect of VIP on PS amplitude. In conclusion, VIP facilitation of synaptic transmission to hippocampal pyramidal cell dendrites is dependent on both A(1) and A(2A) receptor activation by endogenous adenosine. VIP effects on PS amplitude are only dependent on A(1) adenosine receptor activation. This differential sensitivity to adenosine modulation might be due to the different VIP circuits contributing to VIP effects on pyramidal cell dendrites and pyramidal cell bodies.     

The adenosine A1 receptor contributes to the stimulatory, but not the inhibitory effect of caffeine on locomotion: a study in mice lacking adenosine A1 and/or A2A receptors

Caffeine has biphasic effects on locomotion, and blockade of the adenosine A(2A) receptor (A2AR) is necessary for the stimulatory effect of low doses of caffeine, but not for the locomotor depressant effect observed at high doses. We wanted to elucidate the role of the adenosine A(1) receptor (A1R) in mediating the locomotor effects of increasing doses of caffeine using wild-type mice (A1R(WT)), mice heterozygous for (A1R(HET)), and mice lacking the adenosine A(1) receptor (A1R(KO)). Caffeine had the typical biphasic dose-effect relationship in all three genotypes, but the stimulatory action of caffeine was facilitated in the A1R(KO) mice. In order to investigate the interaction between blockade of A1Rs and A2ARs, mice lacking both receptors (A1R(KO)/A2AR(KO)) were tested. Regardless of A1R genotype, animals lacking A2AR were not stimulated by caffeine, whereas animals heterozygous for A2AR were. As expected, the A1R is not crucial for the stimulatory effect of caffeine, but seems to modulate the effect of caffeine exerted via A2AR blockade. Furthermore, these results suggest that the inhibitory effect of high doses of caffeine is due neither to blockade of the A1R, nor of the A2AR, and an effect independent of these adenosine receptors is likely.     

Enhancement of long-term potentiation by brain-derived neurotrophic factor requires adenosine A2A receptor activation by endogenous adenosine

The excitatory action of brain-derived neurotrophic factor (BDNF) on synaptic transmission is triggered by adenosine A2A receptor activation. Since high-frequency neuronal firing, such as that inducing long-term potentiation (LTP), favours both A2A receptor activation and BDNF effects on transmission, we now evaluated the influence of adenosine on the facilitatory action of BDNF upon CA1 hippocampal LTP. theta-Burst stimulation of the pyramidal inputs induced a significant and persistent increase in field EPSP slopes, and this potentiation was augmented in the presence of BDNF (20 ng/ml), an action prevented by the inhibitor of Trk receptor autophosphorylation, K252a (200 nM). Removal of endogenous extracellular adenosine with adenosine deaminase (ADA, 1 U/ml), as well as the antagonism of adenosine A2A receptors with SCH58261 (100 nM), prevented the excitatory action of BDNF upon LTP. In an adenosine depleted background (with ADA), activation of adenosine A2A receptors (with 10nM CGS21680) restored the facilitatory effect of BDNF on LTP; this was fully prevented by the protein kinase A inhibitor, H-89 (1 microM) and mimicked by the adenylate cyclase activator, forskolin (10 microM). In similar experiments, activation of adenosine inhibitory A1 receptors (with 5 nM CPA) did not affect the facilitatory effect of BDNF. In conclusion, the facilitatory action of BDNF upon hippocampal LTP is critically dependent on the presence of extracellular adenosine and A2A receptor activation through a cAMP/PKA-dependent mechanism. Since extracellular adenosine accumulates upon high-frequency neuronal firing, the present results reveal a key process to allow the influence of BDNF upon synaptic plasticity.     

Adenosine A2A receptors inhibit the N-methyl-D-aspartate component of excitatory synaptic currents in rat striatal neurons

The effects of the adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS 21680) on currents mediated by excitatory amino acid receptors were examined in rat striatal brain slices. In a Mg(2+)-free superfusion medium, CGS 21680 decreased the amplitude of excitatory postsynaptic currents (EPSCs) in about 70% of striatal neurons. The inhibitory effect of CGS 21680 disappeared both in the presence of the adenosine A(2A) receptor antagonist 8-(3-chlorostyryl) caffeine and the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP-5). NMDA-induced currents were also depressed by CGS 21680 in a subset of striatal cells, whereas alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-induced currents were not affected. The results suggest that adenosine A(2A) receptor agonists inhibit the NMDA component of the EPSC.