Involvement of adenosine A1 and A2A receptors in the adenosinergic modulation of the discriminative-stimulus effects of cocaine and methamphetamine in rats.
Adenosine, by acting on adenosine A1 and A2A receptors, is known to antagonistically modulate dopaminergic neurotransmission. We have recently reported that nonselective adenosine receptor antagonists (caffeine and 3,7-dimethyl-1-propargylxanthine) can partially substitute for the discriminative-stimulus effects of methamphetamine. In the present study, by using more selective compounds, we investigated the involvement of A1 and A2A receptors in the adenosinergic modulation of the discriminative-stimulus effects of both cocaine and methamphetamine. The effects of the A1 receptor agonist N6-cyclopentyladenosine (CPA; 0.01-0.1 mg/kg) and antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 1.3-23.7 mg/kg) and the A2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680; 0.03-0.18 mg/kg) and antagonist 3-(3-hydroxypropyl)-8-(3-methoxystyryl)-7-methyl-1-propargylxanthin phosphate disodium salt (MSX-3; 1-56 mg/kg) were evaluated in rats trained to discriminate either 1 mg/kg methamphetamine or 10 mg/kg cocaine from saline under a fixed-ratio 10 schedule of food presentation. The A1 and A2A receptor antagonists (CPT and MSX-3) both produced high levels of drug-lever selection when substituted for either methamphetamine or cocaine and significantly shifted dose-response curves of both psychostimulants to the left. Unexpectedly, the A2A receptor agonist CGS 21680 also produced drug-appropriate responding (although at lower levels) when substituted for the cocaine-training stimulus, and both CGS 21680 and the A1 receptor agonist CPA significantly shifted the cocaine dose-response curve to the left. In contrast, both agonists did not produce significant levels of drug-lever selection when substituted for the methamphetamine-training stimulus and failed to shift the methamphetamine dose-response curve. Therefore, adenosine A1 and A2A receptors appear to play important but differential roles in the modulation of the discriminative-stimulus effects of methamphetamine and cocaine. (+info
Comparative pharmacological studies on the A2 adenosine receptor agonist 5'-n-ethyl-carboxamidoadenosine and its F19 isotope labelled derivative.
Adenosine receptors are expressed in various mammalian tissues where they mediate the effects of adenosine on cellular functions through a number of signalling mechanisms. 18F-NECA is the positron-emitting derivative of the A(2)-receptor agonist NECA (5'-n-ethyl-carboxamidoadenosine) and is a radioligand for PET imaging of adenosine receptors. Contractility and relaxation studies were performed on guinea pig atrial myocardium, pulmonary artery, and thoracic aorta to compare the pharmacological effects of NECA and F-NECA (a non-emitting derivative) on tissues. Furthermore, the effect of NECA and F-NECA on the potassium conductance was investigated in DDT1 MF-2 smooth muscle cells with the patch-clamp technique. Both NECA and F-NECA reduced the contractile force in atrial myocardium and evoked phasic contraction in pulmonary artery (A(1) adenosine-receptor-mediated actions) in a dose dependent manner; however, the apparent affinity was lower for F-NECA. No difference was found in relaxation induced by these compounds in 1 microM noradrenaline-precontracted aorta and pulmonary artery (in the presence of DPCPX, an A(1) adenosine receptor antagonist, tissue containing A(2B) adenosine receptors). NECA (5 microM) and F-NECA (5 microM) also decreased the peak current and accelerated activation and inactivation properties of the potassium channels, but F-NECA was less effective. These results suggest that while NECA and F-NECA are equivalent agonists of vascular A(2B) receptors, they mediate different changes of some parameters. When evaluating the data obtained by the use of radiolabelled ligands, one has to take into consideration the possible physiological effects of the ligands besides its binding properties to tissues. (+info
Protection from ischemic liver injury by activation of A2A adenosine receptors during reperfusion: inhibition of chemokine induction.
Ischemia-reperfusion (I/R) injury occurs as a result of restoring blood flow to previously hypoperfused vessels or after tissue transplantation and is characterized by inflammation and microvascular occlusion. We report here that 4-[3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2 -yl]-prop-2-ynyl]-cyclohexanecarboxylic acid methyl ester (ATL146e), a selective agonist of the A(2A) adenosine receptor (A(2A)AR), profoundly protects mouse liver from I/R injury when administered at the time of reperfusion, and protection is blocked by the antagonist ZM241385. ATL146e lowers liver damage by 90% as assessed by serum glutamyl pyruvic transaminase and reduces hepatic edema and MPO. Most protection remains if ATL146e treatment is delayed for 1 h but disappears when delayed for 4 h after the start of reperfusion. In mice lacking the A(2A)AR gene, protection by ATL1465e is lost and ischemic injury of short duration is exacerbated compared with wild-type mice, suggesting a protective role for endogenous adenosine. I/R injury causes induction of hepatic transcripts for IL-1alpha, IL-1beta, IL-1Ra, IL-6, IL-10, IL-18, INF-beta, INF-gamma, regulated on activation, normal T cell expressed, and presumably secreted (RANTES), major intrinsic protein (MIP)-1alpha, MIP-2, IFN-gamma-inducible protein (IP)-10, and monocyte chemotactic protein (MCP)-1 that are suppressed by administering ATL146e to wild-type but not to A(2A)AR knockout mice. RANTES, MCP-1, and IP-10 are notable as induced chemokines that are chemotactic to T lymphocytes. The induction of cytokines may contribute to transient lymphopenia and neutrophilia that occur after liver I/R injury. We conclude that most damage after hepatic ischemia occurs during reperfusion and can be blocked by A(2A)AR activation. We speculate that inhibition of chemokine and cytokine production limits inflammation and contributes to tissue protection by the A(2A)AR agonist ATL146e. (+info
Input-specific modulation of neurotransmitter release in the lateral horn of the spinal cord via adenosine receptors.
Activation of adenosine A2A receptors (A2ARs) in the CNS produces a variety of neuromodulatory actions dependent on the region and preparation examined. In autonomic regions of the spinal cord, A1R activation decreases excitatory synaptic transmission, but the effects of A2AR stimulation are unknown. We sought to determine the location and function of the A2ARs in the thoracic spinal cord, focusing on the intermediolateral cell column (IML). A2AR immunoreactivity was observed throughout the gray matter, with particularly dense immunostaining in regions containing sympathetic preganglionic neurons (SPNs), namely, the IML and intercalated nucleus. Electron microscopy revealed A2AR immunoreactivity within presynaptic terminals and in postsynaptic structures in the IML. To study the functional relevance of these A2ARs, visualized whole-cell patch-clamp recordings were made from electrophysiologically identified SPNs and interneurons within the IML. The A2AR agonist c2-[p-(carboxyethyl)phenethylamino]-5'-N-ethylcarboxyamidoadenosine (CGS 21680) had no significant effect on EPSPs but increased the amplitude of IPSPs elicited by stimulation of the lateral funiculus. These effects were attributable to activation of presynaptic A2ARs because CGS 21680 application altered the paired pulse ratio. Furthermore, neurons in the IML that have IPSPs increased via A2AR activation also receive excitatory inputs that are inhibited by A1R activation. These data show that activating A2ARs increase inhibitory but not excitatory transmission onto neurons in the IML. Simultaneous activation of A1Rs and A2ARs therefore could facilitate inhibition of the postsynaptic neuron, leading to an overall reduction of sympathetic nervous activity. (+info
Epoxyeicosatrienoic acids mediate adenosine-induced vasodilation in rat preglomerular microvessels (PGMV) via A2A receptors.
Activation of rat adenosine2A receptors (A2A R) dilates preglomerular microvessels (PGMV), an effect mediated by epoxyeicosatrienoic acids (EETs). Incubation of PGMV with a selective A2A R agonist, 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 microM), increased isolated PGMV EET levels to 7.57+/-1.53 ng mg-1 protein from 1.06+/-0.22 ng mg-1 protein in controls (P<0.05), without affecting hydroxyeicosatetraenoic acid (HETE) levels (10.8+/-0.69 vs 11.02+/-0.74 ng mg-1 protein). CGS 21680-stimulated EETs was abolished by preincubation with an A2A R antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phe nol (ZM241385) (100 microM). A selective epoxygenase inhibitor, methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH; 12 microM) prevented CGS 21680-induced increase in EETs, indicating inhibition of de novo synthesis of EETs. In pressurized (80 mmHg) renal arcuate arteries (110-130 microm) preconstricted with phenylephrine (20 nM), superfusion with CGS 21680 (0.01-10 microM) increased the internal diameter (i.d.) concentration-dependently; vasodilation was independent of nitric oxide and cyclooxygenase activity. CGS 21680 (10 microM) increased i.d. by 32+/-6 microm; vasodilation was prevented by inhibition of EET synthesis with MS-PPOH. Addition of 3 nM 5,6-EET, 8,9-EET and 11,12-EET increased i.d. by 53+/-9, 17+/-4 and 53+/-5 microm, respectively, whereas 14,15-EET was inactive. The responses to 5,6-EET were, however, significantly inhibited by indomethacin. We conclude that 11,12-EET is the likely mediator of A2A R-induced dilation of rat PGMV. Activation of A2A R coupled to de novo EET stimulation may represent an important mechanism in regulating preglomerular microvascular tone. British Journal of Pharmacology (2004) 141, 441-448. doi:10.1038/sj.bjp.0705640 (+info
Randomized, controlled dose-ranging study of the selective adenosine A2A receptor agonist binodenoson for pharmacological stress as an adjunct to myocardial perfusion imaging.
BACKGROUND: Dipyridamole and adenosine cause frequent side effects as a result of nonspecific adenosine receptor stimulation. Selective agonism of the adenosine A2A receptor should result in a similar degree of coronary vasodilation (and thus similar perfusion images) with fewer side effects. METHODS AND RESULTS: In a multicenter, randomized, single-blind, 2-arm crossover trial, 240 patients underwent 2 single photon emission computed tomographic (SPECT) imaging studies in random order, first after pharmacological stress with adenosine and a second study with the selective adenosine A2A receptor agonist binodenoson, using 1 of 4 dosing regimens. Safety, tolerability, and SPECT image concordance between the 2 agents were examined. Exact categorical agreement in the extent and severity of reversible perfusion defects ranged from 79% to 87%, with kappa values from 0.69 to 0.85, indicating very good to excellent agreement between binodenoson and adenosine. The risk of any safety event/side effect was significantly lower with any dose of binodenoson than with adenosine (P< or =0.01) because of a dose-related reduction in subjective side effects, as objective events were infrequent. There was a reduction in the severity of chest pain, dyspnea, and flushing in all binodenoson doses compared with adenosine (P<0.01), and the magnitude of severity reduction was dose-related. CONCLUSIONS: The selective adenosine A2A receptor agonist binodenoson results in an extent and severity of reversible perfusion defects on SPECT imaging similar to nonselective adenosine receptor stimulation, accompanied by a dose-related reduction in the incidence and severity of side effects. (+info
Role of adenosine A2A receptor in the regulation of gastric somatostatin release.
Adenosine has been demonstrated to inhibit gastric acid secretion. In the rat stomach, this inhibitory effect may be mediated indirectly by increasing the release of somatostatin-like immunoreactivity (SLI). Results show that adenosine analogs augmented SLI release in the isolated vascularly perfused rat stomach. The rank order of potency of the analogs in stimulating SLI release was 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) approximately 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > R-(-)-N(6)-(2-phenylisopropyl)adenosine >1-deoxy-1-[6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-N-methyl-beta-d-ribofu ranuronamide > N(6)-cyclopentyladenosine approximately N(6)-cyclohexyladenosine > S-(+)-N(6)-(2-phenylisopropyl) adenosine, suggesting the involvement of the A(2A) receptor. In agreement, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a] [1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385), an A(2A) receptor antagonist, was shown to abolish the adenosine- and CGS 21680-stimulated SLI release. Immunohistochemical studies reveal the presence of A(2A) receptor immunoreactivity on the gastric plexi and mucosal D-cells, but not on parietal cells and G-cells, suggesting that adenosine may act directly on D-cells or indirectly on the gastric plexi to augment SLI release. The present study also demonstrates that the structure of the mucosal A(2A) receptor is identical to that in the rat brain, and that alternative splicing of this gene does not occur. A real-time reverse transcription-polymerase chain reaction assay has also been established to quantify the levels of A(2A) receptor mRNA. Results show that gastric tissues contained significantly lower levels of A(2A) receptor mRNA compared with the striatum. The lowest level was detected in the mucosa. In conclusion, adenosine may act on A(2A) receptors to augment SLI release and consequently control gastric acid secretion. (+info
A1 and A2A adenosine receptor modulation of alpha 1-adrenoceptor-mediated contractility in human cultured prostatic stromal cells.
1. This study investigated the possibility that adenosine receptors modulate the alpha(1)-adrenoceptor-mediated contractility of human cultured prostatic stromal cells (HCPSC). 2. The nonselective adenosine receptor agonist, 5'-N-ethylcarboxamido-adenosine (NECA; 10 nm-10 microm), and the A(1) adenosine receptor selective agonist, cyclopentyladenosine (CPA; 10 nm-10 microm), elicited significant contractions in HCPSC, with maximum contractile responses of 18+/-3% and 17+/-2% reduction in initial cell length, respectively. 3. In the presence of a threshold concentration of phenylephrine (PE) (100 nm), CPA (1 nm-10 microm) caused contractions, with an EC(50) of 124+/-12 nm and maximum contractile response of 37+/-4%. The A(1) adenosine receptor-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 100 nm) blocked this effect. In the presence of DPCPX (100 nm), NECA (1 nm-10 microm) inhibited contractions elicited by a submaximal concentration of PE (10 microm), with an IC(50) of 48+/-2 nm. The A(2A) adenosine receptor-selective antagonist 4-(2-[7-amino-2-[furyl][1,2,4]triazolo[2,3-alpha][1,3,5,]triazin-5-yl amino]ethyl)phenol (Zm241385 100 nm) blocked this effect. 4. In BCECF-AM (10 microm)-loaded cells, both CPA (100 pM-1 microm) and NECA (100 pm-10 microm) elicited concentration-dependent decreases in intracellular pH (pH(i)), with EC(50) values of 3.1+/-0.3 and 6.0+/-0.3 nm, respectively. The response to NECA was blocked by Zm241385 (100 nm; apparent pK(B) of 9.4+/-0.4), but not by DPCPX (100 nm). The maximum response to CPA was blocked by DPCPX (100 nm), and unaffected by Zm241385 (100 nm). 5. NECA (10 nm-10 microm) alone did not increase [(3)H]-cAMP in HCPSC. In the presence of DPCPX (100 nm), NECA (10 nm-10 microm) caused a concentration dependent increase in [(3)H]-cAMP, with an EC(50) of 1.2+/-0.1 microm. This response was inhibited by Zm241385 (100 nm). CPA (10 nm-10 microm) had no effect on cAMP, in the presence or absence of forskolin (1 microm). 6. These findings are consistent with a role for adenosine receptors in the modulation of adrenoceptor-mediated contractility in human prostate-derived cells. (+info