Evidence for the presence of A1 adenosine receptors in the aorta of spontaneously hypertensive rats

1 Isolated aortic rings (endothelium-intact and -denuded) from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats were used in this study to examine the vasoactive e ects of various adenosine analogues. 2 In phenylephrine contracted aortic rings, concentration-response curves were constructed by cumulative additions (10711 ±1075 M) of (2S)-N6-[2-endo-Norbornyl] adenosine (ENBA), N6- cyclopentyladenosine (CPA), R-N6-(2-phenylisopropyl) adenosine (R-PIA), 2-p-(-2-carboxyethyl) phenethylamino-5'-N-thylcarboxamido adenosine (CGS-21680). 3 A non-speci®c adenosine receptor agonist 2-chloroadenosine (CAD) resulted in biphasic response with a small contraction at lower concentrations (1079 ±1078 M) followed by a signi®cant relaxation at higher concentration in endothelium-intact SHR tissues, suggesting presence of both A1 and A2 adenosine receptors in SHR aorta. However, only relaxation was observed in WKY. 4 Contractile response in SHR had the following rank order of potency: ENBA4CPA4R- PIA4CAD. The relaxation response in SHR and WKY had the following rank order of potency: CGS 216804CAD4R-PIA4CPA4ENBA. 5 Removal of endothelium abolished the adenosine analogue induced contractions in SHR aorta and attenuated the vasorelaxation responses in the WKY and SHR. 6 The contractile response in SHR was abolished by A1 adenosine receptor antagonist N6- endonorbornan-2-yl-9-methyladenine (N-0861). A2 adenosine receptor antagonist, 3,7-dimethyl-1-proparglyxanthine (DMPX) did not a ect the contraction response of adenosine analogues. 7 Endothelium-dependent contractions elicited by A1 receptor agonists were blocked by indomethacin and by free radical scavengers. 8 These data suggest that the contractile response to adenosine analogues in SHR aorta is probably mediated by free radicals which are generated through the increased cyclo-oxygenase activity occurring in the vascular endothelium of SHR but not the WKY rats. British Journal of Pharmacology (2001) 134, 1760 ± 1766. Originally published British Journal of Pharmacology, Vol. 134, No. 8, Dec 200

Upregulation of inducible NO synthase by exogenous adenosine in vascular smooth muscle cells activated by inflammatory stimuli in experimental diabetes

BACKGROUND: Adenosine has been shown to induce nitric oxide (NO) production via inducible NO synthase (iNOS) activation in vascular smooth muscle cells (VSMCs). Although this is interpreted as a beneficial vasodilating pathway in vaso-occlusive disorders, iNOS is also involved in diabetic vascular dysfunction. Because the turnover of and the potential to modulate iNOS by adenosine in experimental diabetes have not been explored, we hypothesized that both the adenosine system and control of iNOS function are impaired in VSMCs from streptozotocin-diabetic rats. METHODS: Male Sprague-Dawley rats were injected with streptozotocin once to induce diabetes. Aortic VSMCs from diabetic and nondiabetic rats were isolated, cultured and exposed to lipopolysaccharide (LPS) plus a cytokine mix for 24 h in the presence or absence of (1) exogenous adenosine and related compounds, and/or (2) pharmacological agents affecting adenosine turnover. iNOS functional expression was determined by immunoblotting and NO metabolite assays. Concentrations of adenosine, related compounds and metabolites thereof were assayed by HPLC. Vasomotor responses to adenosine were determined in endothelium-deprived aortic rings. RESULTS: Treatment with adenosine-degrading enzymes or receptor antagonists increased iNOS formation in activated VSMCs from nondiabetic and diabetic rats. Following treatment with the adenosine transport inhibitor NBTI, iNOS levels increased in nondiabetic but decreased in diabetic VSMCs. The amount of secreted NO metabolites was uncoupled from iNOS levels in diabetic VSMCs. Addition of high concentrations of adenosine and its precursors or analogues enhanced iNOS formation solely in diabetic VSMCs. Exogenous adenosine and AMP were completely removed from the culture medium and converted into metabolites. A tendency towards elevated inosine generation was observed in diabetic VSMCs, which were also less sensitive to CD73 inhibition, but inosine supplementation did not affect iNOS levels. Pharmacological inhibition of NOS abolished adenosine-induced vasorelaxation in aortic tissues from diabetic but not nondiabetic animals. CONCLUSIONS: Endogenous adenosine prevented cytokine- and LPS-induced iNOS activation in VSMCs. By contrast, supplementation with adenosine and its precursors or analogues enhanced iNOS levels in diabetic VSMCs. This effect was associated with alterations in exogenous adenosine turnover. Thus, overactivation of the adenosine system may foster iNOS-mediated diabetic vascular dysfunction

The antiepileptic potential of nucleosides

Despite newly developed antiepileptic drugs to suppress epileptic symptoms, approximately one third of patients remain drug refractory. Consequently, there is an urgent need to develop more effective therapeutic approaches to treat epilepsy. A great deal of evidence suggests that endogenous nucleosides, such as adenosine (Ado), guanosine (Guo), inosine (Ino) and uridine (Urd), participate in the regulation of pathomechanisms of epilepsy. Adenosine and its analogues, together with non-adenosine (non-Ado) nucleosides (e.g., Guo, Ino and Urd), have shown antiseizure activity. Adenosine kinase (ADK) inhibitors, Ado uptake inhibitors and Ado-releasing implants also have beneficial effects on epileptic seizures. These results suggest that nucleosides and their analogues, in addition to other modulators of the nucleoside system, could provide a new opportunity for the treatment of different types of epilepsies. Therefore, the aim of this review article is to summarize our present knowledge about the nucleoside system as a promising target in the treatment of epilepsy

Adenosine receptor expression and function in rat striatal cholinergic interneurons

1. Cholinergic neurons were identified in rat striatal slices by their size, membrane properties, sensitivity to the NK(1) receptor agonist (Sar(9), Met(O(2))(11)) Substance P, and expression of choline acetyltransferase mRNA. A(1) receptor mRNA was detected in 60% of the neurons analysed, and A(2A) receptor mRNA in 67% (n=15). 2. The A(1) receptor agonist R-N(6)-(2-phenylisopropyl)adenosine (R-PIA) hyperpolarized cholinergic neurons in a concentration dependent manner sensitive to the A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM). 3. In dual stimulus experiments, the A(2A) receptor antagonist 8-(3-chlorostyryl)caffeine (CSC, 500 nM) decreased release of [(3)H]-acetylcholine from striatal slices (S2/S1 0.78±0.07 versus 0.95±0.05 in control), as did adenosine deaminase (S2/S1 ratio 0.69±0.05), whereas the A(1) receptor antagonist DPCPX (100 nM) had no effect (S2/S1 1.05±0.14). 4. In the presence of adenosine deaminase the adenosine A(2A) receptor agonist 2-p-((carboxyethyl)phenylethylamino)-5′-N-ethylcarboxamidoadenosine (CGS21680, 10 nM) increased release (S2/S1 ratio 1.03±0.05 versus 0.88±0.05 in control), an effect blocked by the antagonist CSC (500 nM, S2/S1 0.68±0.05, versus 0.73±0.08 with CSC alone). The combined superfusion of bicuculline (10 μM), saclofen (1 μM) and naloxone (10 μM) had no effect on the stimulation by CGS21680 (S2/S1 ratio 0.99±0.04). 5. The A(1) receptor agonist R-PIA (100 nM) inhibited the release of [(3)H]-acetylcholine (S2/S1 ratio 0.70±0.03), an effect blocked by DPCPX (S2/S1 ratio 1.06±0.07). 6. It is concluded that both A(1) and A(2A) receptors are expressed on striatal cholinergic neurons where they are functionally active

Adenosine A2A receptor modulation of hippocampal CA3-CA1 synapse plasticity during associative learning in behaving mice

© 2009 Nature Publishing Group All rights reservedPrevious in vitro studies have characterized the electrophysiological and molecular signaling pathways of adenosine tonic modulation on long-lasting synaptic plasticity events, particularly for hippocampal long-term potentiation(LTP). However, it remains to be elucidated whether the long-term changes produced by endogenous adenosine in the efficiency of synapses are related to those required for learning and memory formation. Our goal was to understand how endogenous activation of adenosine excitatory A2A receptors modulates the associative learning evolution in conscious behaving mice. We have studied here the effects of the application of a highly selective A2A receptor antagonist, SCH58261, upon a well-known associative learning paradigm - classical eyeblink conditioning. We used a trace paradigm, with a tone as the conditioned stimulus (CS) and an electric shock presented to the supraorbital nerve as the unconditioned stimulus(US). A single electrical pulse was presented to the Schaffer collateral–commissural pathway to evoke field EPSPs (fEPSPs) in the pyramidal CA1 area during the CS–US interval. In vehicle-injected animals, there was a progressive increase in the percentage of conditioning responses (CRs) and in the slope of fEPSPs through conditioning sessions, an effect that was completely prevented (and lost) in SCH58261 (0.5 mg/kg, i.p.)-injected animals. Moreover, experimentally evoked LTP was impaired in SCH58261- injected mice. In conclusion, the endogenous activation of adenosine A2A receptors plays a pivotal effect on the associative learning process and its relevant hippocampal circuits, including activity-dependent changes at the CA3-CA1 synapse.This study was supported by grants from the Spanish Ministry of Education and Research (BFU2005-01024 and BFU2005-02512), Spanish Junta de Andalucía (BIO-122 and CVI-02487), and the Fundación Conocimiento y Cultura of the Pablo de Olavide University (Seville, Spain).B. Fontinha was in receipt of a studentship from a project grant (POCI/SAU-NEU/56332/2004) supported by Fundação para a Ciência e Tecnologia (FCT, Portugal), and of an STSM from Cost B30 concerted action of the EU

The Interaction of Gabapentin and N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) on Mechanical Allodynia in Rats with a Spinal Nerve Ligation

We examined the antiallodynic interaction between gabapentin and adenosine A1 receptor agonist, N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation for drug administration. Mechanical allodynia was measured by applying von Frey filaments. Gabapentin and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED50). Fractions of ED50s were administered concurrently to establish the ED50 of the drug combination. The drug interaction between gabapentin and R-PIA was analyzed using the isobolographic method. Adenosine A1 receptor antagonist was administered intrathecally to examine the reversal of the antiallodynic effect. Locomotor function changes were evaluated by rotarod testing. Intrathecal gabapentin and R-PIA and their combination produced a dose-dependent antagonism against mechanical allodynia without severe side effects. Intrathecal gabapentin synergistically enhanced the antiallodynic effect of R-PIA when coadministered. There were no significant changes in rotarod performance time, except gabapentin 300 μg. In the combination group, the maximal antiallodynic effect was reversed by A1 adenosine receptor antagonist. These results suggest that activation of adenosine A1 receptors at the spinal level is required for the synergistic interaction on the mechanical allodynia

Role of adenosine signaling in penile erection and erectile disorders.

INTRODUCTION: Penile erection is a hemodynamic process, which results from increased flow and retention of blood in the penile organ due to the relaxation of smooth muscle cells. Adenosine, a physiological vasorelaxant, has been shown to be a modulator of penile erection. AIM: To summarize the research on the role of adenosine signaling in normal penile erection and erectile disorders. MAIN OUTCOME MEASURES: Evidence in the literature on the association between adenosine signaling and normal and abnormal penile erection, i.e., erectile dysfunction (ED) and priapism. METHODS: The article reviews the literature on the role of endogenous and exogenous adenosine in normal penile erection, as well as in erectile disorders namely, ED and priapism. RESULTS: Adenosine has been shown to relax corpus cavernosum from various species including human in both in vivo and in vitro studies. Neuromodulatory role of adenosine in corpus cavernosum has also been demonstrated. Impaired adenosine signaling through A(2B) receptor causes partial resistance of corpus cavernosum, from men with organic ED, to adenosine-mediated relaxation. Increased level of adenosine has been shown to be a causative factor for priapism. CONCLUSION: Overall, the research reviewed here suggests a general role of exogenous and endogenous adenosine signaling in normal penile erection. From this perspective, it is not surprising that impaired adenosine signaling is associated with ED, and excessive adenosine signaling is associated with priapism. Adenosine signaling represents a potentially important diagnostic and therapeutic target for the treatment of ED and priapism

Current and Future Issues in the Development of Spinal Agents for the Management of Pain.

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety