Involvement Of Adenosine Receptors Research Articles

Heukharang (Lactuca sativa L.) extracts enhanced the sleep behavior of mice: potential involvement of adenosine A1 and A2A receptors.

A significant proportion of the world's population suffers from insomnia, a disorder characterized by complications in initiating and maintaining sleep. Many medications used to treat insomnia target the γ-aminobutyric acid (GABA) neurotransmitter system. However, these substances, such as benzodiazepines, induce significant adverse consequences, including dependence and memory impairment, after prolonged use. Thus, current studies are aimed at developing therapeutic hypnotics derived from natural sources that may cause less severe side effects. Heukharang is a variety of lettuce from Korea that was discovered to contain sleep-promoting compounds. Therefore, we investigated the potential effects of sub-chronic administration of Heukharang extract (FSD-LS) on sleep behavior (pentobarbital-induced sleeping test), brain wave activity and sleep architecture (electroencephalography), and physiological behavior (open-field test and rota-rod) in mice, along with radioligand binding assays (GABAA, adenosine A1 and A2A receptors). We found that FSD-LS prolonged the total sleep duration and reduced the onset time of sleep, and enhanced delta wave power and non-rapid eye movement (NREM) sleep duration, all indicating persistent sleep-enhancing effects. FSD-LS lacked adverse effects on the spontaneous locomotor activity and motor coordination of mice, unlike diazepam. Pharmacological blocking using caffeine and bicuculline supported the possible involvement of adenosine receptors in the sleep-promoting effects of FSD-LS, with partial contribution from GABA receptor activity. Overall, our study recommends FSD-LS as a potential source for the development of sleep-aiding therapeutics.

Guanosine modulates K+ membrane currents in SH-SY5Y cells: involvement of adenosine receptors

Guanosine (GUO), widely considered a key signaling mediator, is implicated in the regulation of several cellular processes. While its interaction with neural membranes has been described, GUO still is an orphan neuromodulator. It has been postulated that GUO may eventually interact with potassium channels and adenosine (ADO) receptors (ARs), both particularly important for the control of cellular excitability. Accordingly, here, we investigated the effects of GUO on the bioelectric activity of human neuroblastoma SH-SY5Y cells by whole-cell patch-clamp recordings. We first explored the contribution of voltage-dependent K+ channels and, besides this, the role of ARs in the regulation of GUO-dependent cellular electrophysiology. Our data support that GUO is able to specifically modulate K+-dependent outward currents over cell membranes. Importantly, administering ADO along with GUO potentiates its effects. Overall, these results suggested that K+ outward membrane channels may be targeted by GUO with an implication of ADO receptors in SH-SY5Y cells, but also support the hypothesis of a functional interaction of the two ligands. The present research runs through the leitmotif of the deorphanization of GUO, adding insight on the interplay with adenosinergic signaling and suggesting GUO as a powerful modulator of SH-SY5Y excitability.

Caffeine modulates voluntary alcohol intake in mice depending on the access conditions: Involvement of adenosine receptors and the role of individual differences

Caffeine is the most consumed psychoactive stimulant and the main active ingredient of energy drinks. Epidemiology studies have shown a positive correlation between the consumption of energy drinks and that of ethanol. The popular belief is that caffeine antagonizes the intoxicating effects of alcohol. Both drugs act on the adenosine system but have opposite effects. Caffeine is a methylxanthine that acts as a nonselective adenosine receptor antagonist, binding to A1 and A2A receptor subtypes. In contrast, ethanol increases extracellular adenosinergic tone. The purpose of this study was to examine the impact of a broad range of doses of caffeine and of selective adenosine A1 and A2A receptor antagonists on voluntary ethanol intake under different ethanol access conditions. C57BL/6 J male mice had access to ethanol (10% w/v) under different conditions: restricted (2 h in the dark), unrestricted (24 h access), or after 4 days of alcohol removal following several periods of unrestricted access. Mice reduced ethanol intake in the restricted access condition after receiving caffeine (20.0 mg/kg), or theophylline (20.0 mg/kg), another methylxanthine. Selective A1 and A2A adenosine receptor antagonists, or their combination, did not have any effect. However, under unrestricted access conditions caffeine and the adenosine A2A receptor antagonist increased ethanol intake. After splitting animals into high, moderate and low ethanol consumers, caffeine (2.5–20.0 mg/kg) significantly increased ethanol consumption in moderate consumers with no effect on low or high consumers. In addition, after reintroducing ethanol access, caffeine (5.0 mg/kg) decreased ethanol consumption among moderate consumers. Thus, caffeine produced different effects on ethanol intake depending on the access condition and the baseline consumption of ethanol.

Effect of acute caffeine administration on PTZ-induced seizure threshold in mice: Involvement of adenosine receptors and NO-cGMP signaling pathway

PurposeCaffeine is a non-selective antagonist of A1 and A2A adenosine receptors (ARs). In this regard, nitric oxide (NO) is partly involved in the central effects of caffeine. In this study, we examined the effect of acute caffeine administration on pentylenetetrazole (PTZ)-induced seizure threshold by focusing on A1Rs, A2ARs, and NO-cGMP signaling pathway. MethodsNMRI male mice (25–30 g) received caffeine (5, 50, and 100 mg/kg) alone, whereas 8-CPT (1 and 5 mg/kg, a selective A1Rs antagonist), SCH-442416 (5 and 10 mg/kg, a selective A2ARs antagonist) or sildenafil (5 and 10 mg/kg, a phosphodiesterase 5 inhibitor) were administrated alone or as pre-treatment before caffeine. Seizure threshold was assessed by intravenous infusion of PTZ. Nitric oxide metabolites (NOx) were measured with the Griess method. ResultsWhen administrated alone, caffeine (5 and 50 mg/kg) and 8-CPT (1 and 5 mg/kg) significantly decreased seizure threshold, while 100 mg/kg of caffeine, SCH-442416 or sildenafil did not change it. Only pre-treatment with SCH-442416 (5 and 10 mg/kg) or sildenafil (5 and 10 mg/kg) before 100 mg/kg of caffeine significantly decreased seizure threshold. Moreover, NOx levels significantly decreased following alone administration of caffeine (100 mg/kg) or 8-CPT (5 mg/kg). ConclusionThe results of present study showed that 5 and 50 mg/kg of caffeine had a proconvulsant effect but caffeine at a dose of 100 mg/kg had no effect on seizure threshold. In addition, it seems that the effect caffeine on seizure threshold is partly mediated through ARs or modulation of the NO-cGMP signaling pathway.

Pharmacologically induced reversible hypometabolic state mitigates radiation induced lethality in mice

Therapeutic hypothermia has proven benefits in critical care of a number of diseased states, where inflammation and oxidative stress are the key players. Here, we report that adenosine monophosphate (AMP) triggered hypometabolic state (HMS), 1–3 hours after lethal total body irradiation (TBI) for a duration of 6 hours, rescue mice from radiation-induced lethality and this effect is mediated by the persistent hypothermia. Studies with caffeine and 6N-cyclohexyladenosine, a non-selective antagonist and a selective agonist of adenosine A1 receptor (A1AR) respectively, indicated the involvement of adenosine receptor (AR) signaling. Intracerebroventricular injection of AMP also suggested possible involvement of central activation of AR signaling. AMP, induced HMS in a strain and age independent fashion and did not affect the behavioural and reproductive capacities. AMP induced HMS, mitigated radiation-induced oxidative DNA damage and loss of HSPCs. The increase in IL-6 and IL-10 levels and a shift towards anti-inflammatory milieu during the first 3–4 hours seems to be responsible for the augmented survival of HSPCs. The syngeneic bone marrow transplantation (BMT) studies further supported the role of radiation-induced inflammation in loss of bone marrow cellularity after TBI. We also showed that the clinically plausible mild hypothermia effectively mitigates TBI induced lethality in mice.

Adenosine Receptors as a Biological Pathway for the Anti-Inflammatory and Beneficial Effects of Low Frequency Low Energy Pulsed Electromagnetic Fields.

Several studies explored the biological effects of low frequency low energy pulsed electromagnetic fields (PEMFs) on human body reporting different functional changes. Much research activity has focused on the mechanisms of interaction between PEMFs and membrane receptors such as the involvement of adenosine receptors (ARs). In particular, PEMF exposure mediates a significant upregulation of A2A and A3ARs expressed in various cells or tissues involving a reduction in most of the proinflammatory cytokines. Of particular interest is the observation that PEMFs, acting as modulators of adenosine, are able to increase the functionality of the endogenous agonist. By reviewing the scientific literature on joint cells, a double role for PEMFs could be hypothesized in vitro by stimulating cell proliferation, colonization of the scaffold, and production of tissue matrix. Another effect could be obtained in vivo after surgical implantation of the construct by favoring the anabolic activities of the implanted cells and surrounding tissues and protecting the construct from the catabolic effects of the inflammatory status. Moreover, a protective involvement of PEMFs on hypoxia damage in neuron-like cells and an anti-inflammatory effect in microglial cells have suggested the hypothesis of a positive impact of this noninvasive biophysical stimulus.

Differential involvement of adenosine receptors in rodent models of parkinsonism and essential tremors

Differential involvement of adenosine receptors in rodent models of parkinsonism and essential tremors

Abstract 10: Endothelial Intracellular Adenosine Epigenetically Regulates Angiogenesis

Background and Objectives: It is well known that extracellular adenosine regulates angiogenesis. However, it is unclear whether intracellular adenosine is able to modulate endothelial angiogenesis. This study is to investigate the effect of elevated intracellular adenosine caused by adenosine kinase (ADK) deletion or knockdown on endothelial angiogenesis and further to determine the involvement of adenosine receptors and methylation pathways in this effect. Methods and results: Human Umbilical Vein Endothelial Cells (HUVECs) were transfected with adenoviral ADK shRNA. ADK knockdown elevated the level of intracellular adenosine, and consequently increased endothelial migration and proliferation. The latter was accompanied with an increase in VEGFR2 expression and PI3K-Akt activation and a decrease of global DNA methylation. Treatment of HUVEC with S-adenosylmethionine or DNA transmethylase inhibitor 5-aza-2-deoxycytodine abolished ADK-knockdown induced VEGFR2 expression and angiogenesis. VEGFR2 expression and global DNA methylation in ADK-knockdown HUVECs were not interfered with adenosine receptor antagonists. Methylation-specific PCR assay showed the decreased methylation of the VEGFR2 promoter, which is the cause for an upregulation of VEGFR2 in ADK deficient HUVECs. Endothelial-specific ADK knockout mice had increased retinal angiogenesis, accelerated wound healing and were protected against hind-limb ischemia. Conclusion: ADK knockdown-mediated elevation of intracellular adenosine in endothelial cells reduces DNA methylation, leading to an upregulation of VEGFR2 and enhancement of angiogenesis.

Role of adenosine receptors in resveratrol-induced intraocular pressure lowering in rats with steroid-induced ocular hypertension.

Steroid-induced ocular hypertension is currently treated in the same way as primary open-angle glaucoma. However, the treatment is often suboptimal and is associated with adverse effects. We evaluated the oculohypotensive effects of topical trans-resveratrol in rats with steroid-induced ocular hypertension and involvement of adenosine receptors (AR) in intraocular pressure (IOP) lowering effect of trans-resveratrol. The oculohypotensive effect of unilateral single-drop application of various concentrations of trans-resveratrol was first studied in oculonormotensive rats. Concentration with maximum effect was similarly studied in rats with steroid-induced ocular hypertension. Involvement of AR was studied by observing the alterations of IOP in response to trans-resveratrol after pretreating animals with AR subtype-specific antagonists. Additionally, we used computational methods, including 3D modelling, 3D structure generation and protein-ligand interaction, to determine the AR-trans-resveratrol interaction. All concentrations of trans-resveratrol produced significant IOP reduction in normotensive rat eyes. Maximum mean IOP reduction of 15.1% was achieved with trans-resveratrol 0.2%. In oculohypertensive rats, trans-resveratrol 0.2% produced peak IOP reduction of 25.2%. Pretreatment with A₁ antagonist abolished the oculohypotensive effect of trans-resveratrol. Pretreatment with A₃ and A₂A AR antagonists produced significant IOP reduction in both treated and control eyes, which was further augmented by trans-resveratrol application in treated eyes. Computational studies showed that trans-resveratrol has highest affinity for A₂B and A₁, followed by A2A and A₃ AR. Topically applied trans-resveratrol reduces IOP in rats with steroid-induced ocular hypertension. Trans-resveratrol-induced oculohypotension involves its agonistic activity at the A₁ AR.

P.1.g.096 Involvement of adenosine receptors in the anticonvulsant effect of agmatine in mice

P.1.g.096 Involvement of adenosine receptors in the anticonvulsant effect of agmatine in mice

Effect of Curcumin Against Pentylenetetrazol-Induced Seizure Threshold in Mice: Possible Involvement of Adenosine A1 Receptors

Curcumin, obtained from Curcuma longa, has been in use for manifold human disorders. The present study explores the effect of curcumin against pentylenetetrazol (PTZ) seizure threshold in mice. The possible involvement of adenosine receptor(s) mechanism was also investigated. Minimal dose of PTZ (i.v., mg/kg) needed to induce different phases of convulsions were recorded as an index of seizure threshold. Curcumin (20-120 mg/kg, p.o.) produced an increase in seizure threshold for convulsions induced by PTZ i.v. infusion. The anticonvulsant effect of curcumin (80 mg/kg) was prevented by 8-phenyltheophylline (0.5 mg/kg, i.p., non-selective adenosine receptor antagonist) and 8-cyclopentyl-1,3-dipropylxanthine (5 mg/kg, i.p., adenosine A1 receptor antagonist) but not by 8-(3-cholorostryl)caffeine (4 mg/kg, i.p., adenosine A2A receptor antagonist). Further, 5'-N-ethylcarboxamidoadenosine (0.005 mg/kg, i.p., non-selective A1 /A2 receptor agonist), or N(6) -cyclohexyladenosine (0.2 mg/kg, i.p., adenosine A1 receptor agonist), was able to potentiate the anticonvulsant action of curcumin. In contrast, 5'-(N-cyclopropyl) carboxamidoadenosine (0.1 mg/kg, i.p., adenosine A2A receptor agonist) failed to potentiate the effect of curcumin. This study demonstrated the anticonvulsant effect of curcumin against PTZ i.v. seizure threshold via a direct or indirect activation of adenosine A1 but not A2A receptors in mice. Thus, curcumin may prove to be an effective adjunct in treatment of convulsions.

Adenosine receptor involvement in both the initiation and resolution of inflammation in the central nervous system during experimental autoimmune encephalomyelitis (54.6)

Abstract Extracellular adenosine is a key regulator of inflammatory immune responses. Utilizing the experimental autoimmune encephalomyelitis (EAE) animal model of multiple sclerosis, we have shown that both the lack of CD73 (catalyzes extracellular adenosine) and antagonist blockade of the A2A adenosine receptor (AR) can protect mice from EAE development. To fully define the importance of A2AAR signaling during EAE progression, we induced disease in A2AAR-/- mice. Surprisingly, A2AAR-/- mice were not only susceptible to EAE, but developed more severe disease associated with an increased number of brain and spinal cord lymphocytes and activated microglial cells. Severe EAE was also found in radiation bone marrow chimeric mice given stem cells from A2AAR-/- donor mice. However, irradiated A2AAR-/- mice given wild type donor cells were protected from disease progression. Our data strongly suggests that A2AAR expression on lymphocytes is important in controlling the magnitude of the inflammatory response during EAE, as A2AAR-/- lymphocytes had a higher proliferative potential and produced more IFNγ following MOG stimulation compared to those from wild type mice. However, A2AAR signaling on non-hematopoietic cells also has an important role in promoting disease progression as A2AAR blockade prevented EAE development in T cell deficient mice given A2AAR-/- donor cells. Overall, our results indicate that A2AAR signaling is involved in both the progression and resolution of EAE.

Electromagnetic fields (EMFs) and adenosine receptors modulate prostaglandin E2 and cytokine release in human osteoarthritic synovial fibroblasts

Synovial fibroblasts (SFs) contribute to the development of osteoarthritis (OA) by the secretion of a wide range of pro-inflammatory mediators, including cytokines and lipid mediators of inflammation. Previous studies suggest that electromagnetic fields (EMFs) may represent a potential therapeutic approach to limit cartilage degradation and control inflammation associated to OA, and that they may act through the adenosine pathway. Therefore, we investigated whether EMFs might modulate inflammatory activities of human SFs from OA patients (OASFs) treated with interleukin-1β (IL-1β), and the possible involvement of adenosine receptors (ARs) in mediating EMF effects. EMF exposure induced a selective increase in A(2A) and A(3) ARs. These increases were associated to changes in cAMP levels, indicating that ARs were functionally active also in EMF-exposed cells. Functional data obtained in the presence of selective A(2A) and A(3) adenosine agonists and antagonists showed that EMFs inhibit the release of prostaglandin E(2) (PGE(2)) and the proinflammatory cytokines interleukin-6 (IL-6) and interleukin-8 (IL-8), while stimulating the release of interleukin-10 (IL-10), an antinflammatory cytokine. These effects seem to be mediated by the EMF-induced upregulation of A(2A) and A(3) ARs. No effects of EMFs or ARs have been observed on matrix degrading enzyme production. In conclusion, this study shows that EMFs display anti-inflammatory effects in human OASFs, and that these EMF-induced effects are in part mediated by the adenosine pathway, specifically by the A(2A) and A(3) AR activation. Taken together, these results open new clinical perspectives to the control of inflammation associated to joint diseases.

Electromagnetic fields (EMFs) and adenosine receptors modulate prostaglandin E2 and cytokine production in human osteoarthritic synovial fibroblasts

Objective. Synovial fibroblasts (SFs) contribute to the development of osteoarthritis (OA) by the secretion of a wide range of pro-inflammatory mediators, including cytokines and lipid mediators of inflammation (1). Previous studies show that electromagnetic fields (EMFs) may represent a potential therapeutical approach to limit cartilage degradation and to control inflammation associated to OA, and that they may act through the adenosine pathway (2). On this basis the aim of this study was to investigate if EMFs might modulate inflammatory activities of human SFs derived from OA patients (OASFs) and the possible involvement of adenosine receptors (ARs) in mediating EMF effects. Design. SFs obtained from OA patients, undergoing total hip joint replacement surgery, were exposed to EMFs (1.5 mT; 75 Hz) for 24 hours. In control and EMF-exposed cells, ARs were evaluated by western blotting, quantitative real-time RT-PCR and saturation binding experiments and cAMP levels were measured by a specific assay. In the absence and in the presence of interleukin-1β (IL-1β), used as a pro-inflammatory stimulus, prostaglandin E 2 (PGE 2 ), cytokine and matrix degrading enzyme production was evaluated in OASFs exposed to EMFs and treated with selective adenosine receptor agonists and antagonists. Results. EMF exposure induced a selective increase in A 2A and A 3 ARs. These increases were associated to changes in cAMP levels, indicating that ARs were functionally active in EMF-exposed cells. In IL-1β-treated OASFs, functional data obtained in the presence of A 2A and A 3 adenosine agonists and antagonists showed that EMFs inhibit the release of (PGE 2 ) and of the proinflammatory cytokines interleukin-6 (IL-6) and interleukin-8 (IL-8), whilst stimulate the release of interleukin-10 (IL-10), an antinflammatory cytokine. Further, results show that these effects appear to be mediated by the EMF-induced upregulation of A 2A and A 3 ARs. No effects of EMFs or ARs have been observed on matrix degrading enzymes production. Conclusions: EMFs display anti-inflammatory effects in human OASFs and these EMF-induced .ffects are in part mediated by the adenosine pathway, specifically by the A 2A and A 3 ARs activation. Taken together, these results suggest that SFs could represent potential therapeutic targets cells for EMF treatment and open new clinical perspectives to the control of inflammation associated to joint diseases. 1. Martel-Pelletier J et al. Eklem Hastalik Cerrahisi. 2010; 21(1):2-14. 2. De Mattei M et al. Osteoarthritis Cartilage. 2009; 17(2):252-262.

Adenosine elicits an eNOS-independent reduction in arterial blood pressure in conscious mice that involves adenosine A2A receptors

Adenosine plays an important role in the regulation of heart rate (HR) and vascular reactivity. However, the mechanisms underlying the acute effect of adenosine on arterial blood pressure in conscious mice are unclear. Therefore, this study investigated the effect of the nucleoside on mean arterial blood pressure (MAP) and HR in conscious mice. Chronic indwelling catheters were placed in C57Bl/6J (WT) and endothelial nitric oxide synthase knockout (eNOS(-/-)) mice for continuous measurements of MAP and HR. Using PCR and myograph analysis, involvement of adenosine receptors was investigated in human and mouse renal blood vessels. Bolus infusion of 0.5 mg kg(-1) adenosine elicited significant transient decreases in MAP (99.3 ± 2.3 to 70.4 ± 4.5 mmHg) and HR (603.2 ± 18.3 to 364.3 ± 49.2 min(-1)), which were inhibited by the A(2A) receptor antagonist ZM 241385. Activation of adenosine A(2A) receptors with CGS 21680 (0.02 mg kg(-1)) caused a significant reduction in MAP from 99.6 ± 1.2 to 73.1 ± 3.6 mmHg accompanied by tachycardia (610.5 ± 9.3 to 677.5 ± 9.5 min(-1)). The reduction in MAP observed after adenosine or CGS 21680 administrations was not significantly different in WT and eNOS(-/-) mice. In isolated human and mouse intrarenal arteries, adenosine caused a relaxation dependent on A(2A) adenosine receptor activation. A(2A) receptors were present in both human and mouse arteries whereas A(1) and A(2B) receptors were only present in mouse arteries. In conclusion, acute adenosine administration and selective stimulation of adenosine A(2A) receptors results in an immediate, transient eNOS-independent reduction in MAP. A(2A) receptor activation causes relaxation of human and mouse arteries.

What Knock-Out Animals Tell Us About the Effects of Caffeine

Caffeine is well known for its complex pharmacological actions, in part reflecting the multiple molecular targets of caffeine. The adenosine receptors are the primary extracellular targets of caffeine. Since caffeine has similar affinity for several adenosine receptors, it has been difficult to determine which receptor subtypes mediate caffeine's effects using pharmacological tools. The development of genetic mutant mice deficient in adenosine receptors and other signaling molecules has allowed targeted inquiry into the molecular targets by which caffeine elicits its biological effects on behavior and gene expression. This review summarizes recent work using genetic knockout models to elucidate the mechanisms of caffeine action in the brain. This review focuses on insights into caffeine action from genetic knockout models on: (1) the molecular basis for caffeine's effects on psychomotor activity; (2) the involvement of adenosine receptors in caffeine-mediated arousal and cognitive effects; and (3) a novel approach using knockout animals coupled with microarray profiling to validate multiple molecular targets of caffeine in striatal gene expression.

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 μg) or 8-SPT (50 μg) 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.

Modulatory effect of adenosine receptors on the ascending and descending neural reflex responses of rat ileum

BackgroundAdenosine is known to act as a neuromodulator by suppressing synaptic transmission in the central and peripheral nervous system. Both the release of adenosine within the small intestine and the presence of adenosine receptors on enteric neurons have been demonstrated. The aim of the present study was to characterize a possible involvement of adenosine receptors in the modulation of the myenteric reflex. The experiments were carried out on ileum segments 10 cm in length incubated in an single chambered organ bath, and the reflex response was initiated by electrical stimulation (ES).ResultsES caused an ascending contraction and a descending relaxation followed by a contraction. All motility responses to ES were completely blocked by tetrodotoxin, indicating that they are mediated by neural mechanisms. Atropine blocked the contractile effects, whereas the descending relaxation was significantly increased. The A1 receptor agonist N6-cyclopentyladenosine increased the ascending contraction, whereas the ascending contraction was reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. Activation of the A1 receptor further reduced the descending relaxation and the latency of the peristaltic reflex. The A2B receptor antagonist alloxazine increased ascending contraction, whereas descending relaxation remained unchanged. For A2A and A3 receptors, we found contradictory effects of the agonists and antagonists, thus there is no clear physiological role for these receptors at this time.ConclusionsThis study suggests that the myenteric ascending and descending reflex response of the rat small intestine is modulated by release of endogenous adenosine via A1 receptors.

Alkylxanthine-Induced Recovery of Respiratory Function Following Cervical Spinal Cord Injury in Adult Rats

Previous investigations from our laboratory have demonstrated qualitatively that a latent respiratory pathway can be activated by systemic theophylline administration to restore function to a hemidiaphragm paralyzed by an upper (C2) cervical spinal cord hemisection in adult rats. The present study seeks to extend the previous investigations by contrasting and quantitating the actions of theophylline, 8-phenyltheophylline, enprofylline, and 8(p-Sulfophenyl)theophylline in restoring function 24 h after hemidiaphragm paralysis. The alkylxanthines were selected based on their diverse pharmacologic profiles to elucidate the mechanisms that underlie functional recovery after spinal cord injury. To quantitatively assess the magnitude of recovery, electrophysiological experiments were conducted on pancuronium-paralyzed, hemisected animals under standardized recording conditions. The total absence of respiratory-related activity in the phrenic nerve ipsilateral to the hemisection and paralyzed hemidiaphragm was used as the index of a functionally complete hemisection. Thereafter, drug-induced recovered activity in the phrenic nerve ipsilateral to hemisection was quantified and expressed either as a percentage of contralateral phrenic nerve activity in the same animal prior to drug administration or as a percentage of predrug activity in the homolateral nerve in noninjured animals. With either approach, theophylline (5–15 mg/kg) and 8-phenyltheophylline (5–10 mg/kg) dose-dependently induced respiratory-related recovered activity. Enprofylline, a potent bronchodilator, and 8(p-Sulfophenyl)theophylline, an adenosine receptor antagonist with limited access to the central nervous system, were ineffective. Maximal recovery was attained with theophylline (15 mg/kg) and 8-phenyltheophylline (10 mg/kg). At these doses, theophylline and 8-phenyltheophylline induced recovery that was 70.0 ± 2.5 and 69.3 ± 4.1% of predrug contralateral nerve activity respectively. When expressed as a percentage of activity in the homolateral nerve in noninjured animals, the magnitude changed to 32.9 ± 4.9 and 35.7 ± 6.9%, respectively. Involvement of adenosine receptors in the alkylxanthine-induced actions was confirmed in experiments with the adenosine analog, N6 (l-2-phenylisopropyl) adenosine (L-PIA). It is concluded that central adenosine receptor-mediated mechanisms are implicated in the recovery of respiratory-related activity after spinal cord injury. Furthermore, our results suggest a potential for a new therapeutic approach in the rehabilitation of spinal cord patients with respiratory deficits.

Activation of glutamate uptake by guanosine in primary astrocyte cultures.

Guanine-based purines have been shown to modulate the effects of glutamate, which is essential for brain function and mediates excitotoxicity. In the search for a mechanism involving the interaction between purine nucleoside guanosine and glutamate, we found that guanosine dose-dependently, significantly (63%) and potently (EC50 =2.47 microM) enhanced glutamate uptake in cultured astrocytes. This effect was not inhibited by the blocker of nucleoside transporter dipyridamole nor by the adenosine antagonist theophylline, suggesting an extracellular site of action without the involvement of adenosine receptors. These results indicate a regulatory role of guanosine on extracellular levels of glutamate, possibly contributing for protecting neural cells against glutamate-induced excitotoxicity.