Cordycepin suppresses glutamatergic and GABAergic synaptic transmission through activation of A1 adenosine receptor in rat hippocampal CA1 pyramidal neurons

Cordycepin (known as 3-deoxyadenosine, CRD), a natural product from the valuable traditional Chinese medicine Cordyceps militaris, has been reported to improve cognitive function and modulate neuroprotective effects on the central nervous system (CNS). However, the modulating mechanisms of cordycepin on information processing in hippocampal CA1 pyramidal neurons are not fully understood. To clarify how cordycepin modulates synaptic responses of pyramidal neurons in rat hippocampal CA1 region, we conducted an electrophysiological experiment using whole-cell patch-clamp technique. The spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs, respectively) and the spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs, respectively) recorded by this technique evaluated pure single or multi-synapse responses and enabled us to accurately quantify how cordycepin influenced the pre and postsynaptic aspects of synaptic transmission. The present results showed that cordycepin significantly decreased the frequency of both glutamatergic and GABAergic postsynaptic currents without affecting the amplitude, while these inhibitory effects were antagonized by the A₁ adenosine receptor antagonist (DPCPX), but not the A_2A (ZM 241385), A_2B (MRS1754) and A₃ (MRS1191) adenosine receptor antagonists. Taken together, our results suggested that cordycepin had a clear presynaptic effect on glutamatergic and GABAergic transmission, and provided novel evidence that cordycepin suppresses the synaptic transmission through the activation of A₁AR.

Mediation of the Cardioprotective Effects of Mannitol Discovered, with Refutation of Common Protein Kinases

The osmodiuretic agent Mannitol exerts cardioprotection against ischemia and reperfusion (I/R) injury when applied as a pre- and/or postconditioning stimulus. Previously, we demonstrated that these properties are mediated via the activation of mitochondrial ATP-sensitive potassium (mK_ATP) channels. However, considering Mannitol remains in the extracellular compartment, the question arises as to which receptor and intracellular signaling cascades are involved in myocardial protection by the osmodiuretic substance. Protein kinase B (Akt) and G (PKG), as part of the reperfusion injury salvage kinase (RISK) and/or endothelial nitric oxide (eNOS)/PKG pathway, are two well-investigated intracellular targets conferring myocardial protection upstream of mitochondrial potassium channels. Adenosine receptor subtypes have been shown to trigger different cardioprotective pathways, for example, the reperfusion injury. Further, Mannitol induces an increased activation of the adenosine 1 receptor (A1R) in renal cells conferring its nephroprotective properties. Therefore, we investigated whether (1) Akt and PKG are possible signaling targets involved in Mannitol-induced conditioning upstream of the mK_ATP channel and/or whether (2) cardioprotection by Mannitol is mediated via activation of the A1R. All experiments were performed on male Wistar rats in vitro employing the Langendorff isolated heart perfusion technique with infarct size determination as the primary endpoint. To unravel possible protein kinase activation, Mannitol was applied in combination with the Akt (MK2206) or PKG (KT5823) inhibitor. In further groups, an A1R blocker (DPCPX) was given with or without Mannitol. Preconditioning with Mannitol (Man) significantly reduced the infarct size compared to the control group. Co-administration of the A1R blocker DPXPC fully abolished myocardial protection of Mannitol. Interestingly and in contrast to the initial hypothesis, neither administration of the Akt nor the PKG blocker had any impact on the cardioprotective properties of Mannitol-induced preconditioning. These results are quite unexpected and show that the protein kinases Akt and PKG—as possible targets of known protective signaling cascades—are not involved in Mannitol-induced preconditioning. However, the cardioprotective effects of Mannitol are mediated via the A1R.

The metaplastic effects of cordycepin in hippocampal CA1 area of rats

Metaplasticity is referred to adjustment in the requirements for induction of synaptic plasticity based on the prior history of activity. Synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), has been considered to be the neural processes underlying learning and memory. Previous observations that cordycepin (an adenosine derivative) improved learning and memory seemed to be contradictory to the findings that cordycepin inhibited LTP. Therefore, we speculated that the conflicting reports of cordycepin might be related to metaplasticity. In the current study, population spike (PS) in hippocampal CA1 area of rats was recorded by using electrophysiological method in vivo. The results showed that cordycepin reduced PS amplitude in hippocampal CA1 with a concentration-dependent relationship, and high frequency stimulation (HFS) failed to induce LTP when cordycepin was intrahippocampally administrated but improved LTP magnitude when cordycepin was pre-treated. Cordycepin increased LTD induced by activating N-Methyl-D-aspartate (NMDA) receptors and subsequently facilitated LTP induced by HFS. Furthermore, we found that 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an adenosine A1 receptors antagonist, could block the roles of cordycepin on LTD and LTP. Collectively, cordycepin was able to modulate metaplasticity in hippocampal CA1 area of rats through adenosine A1 receptors. These findings would be helpful to reconcile the conflicting reports in the literatures and provided new insights into the mechanisms underlying cognitive function promotions with cordycepin treatment.

Evaluation of adenosine A1 receptor agonists as neuroprotective countermeasures against Soman intoxication in rats

Soman, an organophosphorus (OP) compound, disrupts nervous system function through inactivation of acetylcholinesterase (AChE), the enzyme that breaks down acetylcholine at synapses. Left untreated, a state of prolonged seizure activity (status epilepticus, SE) is induced, causing widespread neuronal damage and associated cognitive and behavioral impairments. Previous research demonstrated that therapeutic stimulation of A1 adenosine receptors (A1ARs) can prevent or terminate soman-induced seizure. This study examined the ability of three potent A1AR agonists to provide neuroprotection and, ultimately, prevent observable cognitive and behavioral deficits following exposure to soman. Sprague Dawley rats were challenged with a seizure-inducing dose of soman (1.2 x LD50) and treated 1 min later with one of the following A1AR agonists: (6)-Cyclopentyladenosine (CPA), 2-Chloro-N6-cyclopentyladenosine (CCPA) or N-bicyclo(2.2.1)hept-2-yl-5'-chloro-5'-deoxyadenosine (cdENBA). An active avoidance shuttle box task was used to evaluate locomotor responses to aversive stimuli at 3, 7 and 14 days post-exposure. Animals treated with CPA, CCPA or cdENBA demonstrated a higher number of avoidance responses and a faster reaction to the aversive stimulus than the soman/saline control group across all three sessions. Findings suggest that A1AR agonism is a promising neuroprotective countermeasure, capable of preventing the long-term deficits in learning and memory that are characteristic of soman intoxication.

Sensory satellite glial Gq-GPCR activation alleviates inflammatory pain via peripheral adenosine 1 receptor activation

Glial fibrillary acidic protein expressing (GFAP+) glia modulate nociceptive neuronal activity in both the peripheral nervous system (PNS) and the central nervous system (CNS). Resident GFAP+ glia in dorsal root ganglia (DRG) known as satellite glial cells (SGCs) potentiate neuronal activity by releasing pro-inflammatory cytokines and neuroactive compounds. In this study, we tested the hypothesis that SGC Gq-coupled receptor (Gq-GPCR) signaling modulates pain sensitivity in vivo using Gfap-hM3Dq mice. Complete Freund's adjuvant (CFA) was used to induce inflammatory pain, and mechanical sensitivity and thermal sensitivity were used to assess the neuromodulatory effect of glial Gq-GPCR activation in awake mice. Pharmacogenetic activation of Gq-GPCR signaling in sensory SGCs decreased heat-induced nociceptive responses and reversed inflammation-induced mechanical allodynia via peripheral adenosine A1 receptor activation. These data reveal a previously unexplored role of sensory SGCs in decreasing afferent excitability. The identified molecular mechanism underlying the analgesic role of SGCs offers new approaches for reversing peripheral nociceptive sensitization.

Adenosine receptors regulate exosome production

Exosomes, small-sized extracellular vesicles, carry components of the purinergic pathway. The production by cells of exosomes carrying this pathway remains poorly understood. Here, we asked whether type 1, 2A, or 2B adenosine receptors (A1Rs, A2ARs, and A2BRs, respectively) expressed by producer cells are involved in regulating exosome production. Preglomerular vascular smooth muscle cells (PGVSMCs) were isolated from wildtype, A1R-/-, A2AR-/-, and A2BR-/- rats, and exosome production was quantified under normal or metabolic stress conditions. Exosome production was also measured in various cancer cells treated with pharmacologic agonists/antagonists of A1Rs, A2ARs, and A2BRs in the presence or absence of metabolic stress or cisplatin. Functional activity of exosomes was determined in Jurkat cell apoptosis assays. In PGVSMCs, A1R and A2AR constrained exosome production under normal conditions (p = 0.0297; p = 0.0409, respectively), and A1R, A2AR, and A2BR constrained exosome production under metabolic stress conditions. Exosome production from cancer cells was reduced (p = 0.0028) by the selective A2AR agonist CGS 21680. These exosomes induced higher levels of Jurkat apoptosis than exosomes from untreated cells or cells treated with A1R and A2BR agonists (p = 0.0474). The selective A2AR antagonist SCH 442416 stimulated exosome production under metabolic stress or cisplatin treatment, whereas the selective A2BR antagonist MRS 1754 reduced exosome production. Our findings indicate that A2ARs suppress exosome release in all cell types examined, whereas effects of A1Rs and A2BRs are dependent on cell type and conditions. Pharmacologic targeting of cancer with A2AR antagonists may inadvertently increase exosome production from tumor cells.

Effect of adenosine on short-term synaptic plasticity in mouse piriform cortex in vitro: adenosine acts as a high-pass filter

We examined the effect of adenosine and of adenosine A1 receptor blockage on short-term synaptic plasticity in slices of adult mouse anterior piriform cortex maintained in vitro in an in vivo-like ACSF. Extracellular recording of postsynaptic responses was performed in layer 1a while repeated electrical stimulation (5-pulse-trains, frequency between 3.125 and 100 Hz) was applied to the lateral olfactory tract. Our stimulation protocol was aimed at covering the frequency range of oscillatory activities observed in the olfactory bulb in vivo. In control condition, postsynaptic response amplitude showed a large enhancement for stimulation frequencies in the beta and gamma frequency range. A phenomenological model of short-term synaptic plasticity fitted to the data suggests that this frequency-dependent enhancement can be explained by the interplay between a short-term facilitation mechanism and two short-term depression mechanisms, with fast and slow recovery time constants. In the presence of adenosine, response amplitude evoked by low-frequency stimulation decreased in a dose-dependent manner (IC50  = 70 μmol/L). Yet short-term plasticity became more dominated by facilitation and less influenced by depression. Both changes compensated for the initial decrease in response amplitude in a way that depended on stimulation frequency: compensation was strongest at high frequency, up to restoring response amplitudes to values similar to those measured in control condition. The model suggested that the main effects of adenosine were to decrease neurotransmitter release probability and to attenuate short-term depression mechanisms. Overall, these results suggest that adenosine does not merely inhibit neuronal activity but acts in a more subtle, frequency-dependent manner.

Adenosine A1 Receptor Protects Against Cisplatin Ototoxicity by Suppressing the NOX3/STAT1 Inflammatory Pathway in the Cochlea

Cisplatin is a commonly used antineoplastic agent that produces ototoxicity that is mediated in part by increasing levels of reactive oxygen species (ROS) via the NOX3 NADPH oxidase pathway in the cochlea. Recent studies implicate ROS generation in mediating inflammatory and apoptotic processes and hearing loss by activating signal transducer and activator of transcription (STAT1). In this study, we show that the adenosine A1 receptor (A1AR) protects against cisplatin ototoxicity by suppressing an inflammatory response initiated by ROS generation via NOX3 NADPH oxidase, leading to inhibition of STAT1. Trans-tympanic administration of the A1AR agonist R-phenylisopropyladenosine (R-PIA) inhibited cisplatin-induced ototoxicity, as measured by auditory brainstem responses and scanning electron microscopy in male Wistar rats. This was associated with reduced NOX3 expression, STAT1 activation, tumor necrosis factor-α (TNF-α) levels, and apoptosis in the cochlea. In vitro studies in UB/OC-1 cells, an organ of Corti immortalized cell line, showed that R-PIA reduced cisplatin-induced phosphorylation of STAT1 Ser(727) (but not Tyr(701)) and STAT1 luciferase activity by suppressing the ERK1/2, p38, and JNK mitogen-activated protein kinase (MAPK) pathways.R-PIA also decreased the expression of STAT1 target genes, such as TNF-α, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) and reduced cisplatin-mediated apoptosis. These data suggest that the A1AR provides otoprotection by suppressing NOX3 and inflammation in the cochlea and could serve as an ideal target for otoprotective drug therapy.

SIGNIFICANCE STATEMENT

Cisplatin is a widely used chemotherapeutic agent for the treatment of solid tumors. Its use results in significant and permanent hearing loss, for which no US Food and Drug Administration-approved treatment is currently available. In this study, we targeted the cochlear adenosine A1 receptor (A1AR) by trans-tympanic injections of the agonist R-phenylisopropyladenosine (R-PIA) and showed that it reduced cisplatin-induced inflammation and apoptosis in the rat cochlea and preserved hearing. The mechanism of protection involves suppression of the NOX3 NADPH oxidase enzyme, a major target of cisplatin-induced reactive oxygen species (ROS) generation in the cochlea. ROS initiates an inflammatory and apoptotic cascade in the cochlea by activating STAT1 transcription factor, which is attenuated byR-PIA. Therefore, trans-tympanic delivery of A1AR agonists could effectively treat cisplatin ototoxicity.

Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A 2A receptors during extinction training in rats

RATIONALE

Adenosine receptor stimulation and blockade have been shown to modulate a variety of cocaine-related behaviors.

OBJECTIVES

These studies identify the direct effects of adenosine receptor stimulation on cocaine seeking during extinction training and the persistent effects on subsequent reinstatement to cocaine seeking.

METHODS

Rats self-administered cocaine on a fixed ratio one schedule in daily sessions over 3 weeks. Following a 1-week withdrawal, the direct effects of adenosine receptor modulation were tested by administering the adenosine A1 receptor agonist, N(6)-cyclopentyladenosine (CPA, 0.03 and 0.1 mg/kg), the adenosine A2A agonist, CGS 21680 (0.03 and 0.1 mg/kg), the presynaptic adenosine A2A receptor antagonist, SCH 442416 (0.3, 1, and 3 mg/kg), or vehicle prior to each of six daily extinction sessions. The persistent effects of adenosine receptor modulation during extinction training were subsequently tested on reinstatement to cocaine seeking induced by cues, cocaine, and the dopamine D2 receptor agonist, quinpirole.

RESULTS

All doses of CPA and CGS 21680 impaired initial extinction responding; however, only CPA treatment during extinction produced persistent impairment in subsequent cocaine- and quinpirole-induced seeking. Dissociating CPA treatment from extinction did not alter extinction responding or subsequent reinstatement. Administration of SCH 442416 had no direct effects on extinction responding but produced dose-dependent persistent impairment of cocaine- and quinpirole-induced seeking.

CONCLUSIONS

These findings demonstrate that adenosine A1 or A2A receptor stimulation directly impair extinction responding. Interestingly, adenosine A1 receptor stimulation or presynaptic adenosine A2A receptor blockade during extinction produces lasting changes in relapse susceptibility.

Adenosine signaling in striatal circuits and alcohol use disorders

Adenosine signaling has been implicated in the pathophysiology of alcohol use disorders and other psychiatric disorders such as anxiety and depression. Numerous studies have indicated a role for A1 receptors (A1R) in acute ethanol-induced motor incoordination, while A2A receptors (A2AR) mainly regulate the rewarding effect of ethanol in mice. Recent findings have demonstrated that dampened A2AR-mediated signaling in the dorsomedial striatum (DMS) promotes ethanol-seeking behaviors. Moreover, decreased A2AR function is associated with decreased CREB activity in the DMS, which enhances goal-oriented behaviors and contributes to excessive ethanol drinking in mice. Interestingly, caffeine, the most commonly used psychoactive substance, is known to inhibit both the A1R and A2AR. This dampened adenosine receptor function may mask some of the acute intoxicating effects of ethanol. Furthermore, based on the fact that A2AR activity plays a role in goal-directed behavior, caffeine may also promote ethanol-seeking behavior. The A2AR is enriched in the striatum and exclusively expressed in striatopallidal neurons, which may be responsible for the regulation of inhibitory behavioral control over drug rewarding processes through the indirect pathway of the basal ganglia circuit. Furthermore, the antagonistic interactions between adenosine and dopamine receptors in the striatum also play an integral role in alcoholism and addiction-related disorders. This review focuses on regulation of adenosine signaling in striatal circuits and the possible implication of caffeine in goal-directed behaviors and addiction.

[Stimulation of adenosine A1 receptor attenuates angiotensin II induced myocardial hypertrophy in neonatal rats via the extracellular signal-regulated kinase signal pathways]

OBJECTIVE

To observe the impact of adenosine A1 receptor stimulation on extracellular signal-regulated kinase 1/2 (ERK1/2) signal pathways on angiotensin II (AngII) stimulated cardiomyocytes of neonatal rats in vitro.

METHODS

Cardiomyocytes of neonatal rats were cultured in vitro. Cardiomyocytes hypertrophy was induced by AngII (0.1 µmol/L). The antihypertrophic effect of adenosine A1 receptor stimulation via adenosine A1 receptor agonist R-PIA (1 µmol/L) was observed in the presence or absence of ERK1/2 inhibitor 1, 4-Diamino-2, 3-dicyano-1, 4-bis(o-aminophenylmercapto) butadiene (U0126) 1 µmol/L, PKC inhibitor Ro-31-8220 (50 nmol/L), and pertussis toxin (PTX, 5 mg/L). The total protein content was assayed by the method of Lowry. The expression of mRNA of atrial natriuretic peptide (ANP) was determined by RT-PCR. [Ca(2+)]i was measured by confocal microscope using Fluo-3/AM as fluorescent indicator. The relative expression of ERK1/2 was determined by Western blot.

RESULTS

Compared with normal control group, AngII induced significant cardiomyocyte hypertrophy. Compared with AngII group, R-PIA significantly inhibited AngII-induced increase of the protein content, cardiomyocytes volume and expression of ERK1/2, calcium ion fluorescence intensity, similar as U0126 and Ro-31-8220. The inhibitory effects on AngII induced cardiomyocytes hypertrophy of R-PIA were lost when preincubated with PTX.

CONCLUSION

Adenosine A1 receptor can inhibit AngII induced cardiomyocyte hypertrophy through downregulating ERK signal pathways and reducing intracellular Ca(2+).

[Brain injury in adenosine A1 receptor knock-out mice after pentetrazole kindling and its correlation with COX-2]

OBJECTIVE

To investigate the brain injury of neurocytes in hippocampus of adenosine A1 receptor knock-out mice during pentetrazole kindling and detect the correlation of brain injury and the expression of COX-2 so as to evaluate the neuroprotective function of adenosine A1 receptor and its mechanism.

METHODS

The animals were divided into two groups: wild type group and KO group. The kindling model was established by injection of pentetrazole into abdominal cavity. The expression of brain injury related protein, caspase-3 and COX-2 in hippocampus was investigated separately at different time points (24 hour, 1 month) post-kindling.

RESULTS

The expression of Caspase-3 and COX-2 increased in KO group both during acute and chronic phases post-kindling compared with normal mice. There was statistical difference (P < 0.05). Furthermore, the expression levels of two proteins were higher at 1 month compared with 24 hour (P < 0.01). These results indicated that the increased expression of Caspase-3 and COX-2 post-kindling was earlier and much more in KO mice as compared with wild-type ones.

CONCLUSION

Adenosine exerts strong neuroprotective functions through the activation of adenosine A1 receptor. This receptor reduces cell apoptosis during pentetrazole kindling. Cyclooxygenase-2 (COX-2) reflects the extent of brain injury. The neuroprotective function mediated by adenosine A1 receptor might be related with COX-2.

Interactions of Magnolia and Ziziphus extracts with selected central nervous system receptors

ETHNOPHARMACOLOGICAL RELEVANCE

Magnolia officinalis Rehder and Wilson [Magnoliaceae] bark and Ziziphus spinosa (Buhge) Hu ex. Chen. [Fam. Rhamnaceae] seed have a history of use in traditional Asian medicine for mild anxiety, nervousness and sleep-related problems.

AIM OF THE STUDY

To identify pharmacological targets, extracts of Magnolia officinalis (ME), Ziziphus spinosa (ZE), and a proprietary fixed combination (MZE) were tested for affinity with central nervous system receptors associated with relaxation and sleep.

METHODS

In vitro radioligand binding and cellular functional assays were conducted on: adenosine A(1), dopamine (transporter, D(1), D(2S), D(3), D(4.4) and D(5)), serotonin (transporter, 5-HT(1A), 5-HT(1B), 5-HT(4e), 5-HT(6) and 5-HT(7)) and the GABA benzodiazepine receptor.

RESULTS

Interactions were demonstrated with the adenosine A(1) receptor, dopamine transporter and dopamine D(5) receptor (antagonist activity), serotonin receptors (5-HT(1B) and 5-HT(6) antagonist activity) and the GABA benzodiazepine receptor at a concentration of 100 microg/ml or lower. ME had an affinity with adenosine A(1) (K(i) of 9.2+/-1.1 microg/ml) and potentiated the GABA activated chloride current at the benzodiazepine subunits of the GABA receptor (maximum effect at 50 microg/ml). ME had a modest antagonist action with 5-HT(6) and ZE with the 5-HT(1B) receptor.

CONCLUSION

The interactions in the receptor binding models are consistent with the traditional anxiolytic and sleep-inducing activities of Magnolia officinalis bark and Ziziphus spinosa seed.

A1 adenosine receptor antagonists, agonists, and allosteric enhancers

Intense efforts of many pharmaceutical companies and academicians in the A(1) adenosine receptor (AR) field have led to the discovery of clinical candidates that are antagonists, agonists, and allosteric enhancers. The A(1)AR antagonists currently in clinical development are KW3902, BG9928, and SLV320. All three have high affinity for the human (h) A(1)AR subtype (hA(1) K (i) < 10 nM), > 200-fold selectivity over the hA(2A) subtype, and demonstrate renal protective effects in multiple animal models of disease and pharmacologic effects in human subjects. In the A(1)AR agonist area, clinical candidates have been discovered for the following conditions: atrial arrhythmias (tecadenoson, selodenoson and PJ-875); Type II diabetes and insulin sensitizing agents (GR79236, ARA, RPR-749, and CVT-3619); and angina (BAY 68-4986). The challenges associated with the development of any A(1)AR agonist are to obtain tissue-specific effects but avoid off-target tissue side effects and A(1)AR desensitization leading to tachyphylaxis. For the IV antiarrhythmic agents that act as ventricular rate control agents, a selective response can be accomplished by careful IV dosing paradigms. The treatment of type II diabetes using A(1)AR agonists in the clinic has met with limited success due to cardiovascular side effects and a well-defined desensitization of full agonists in human trials (GR79236, ARA, and RPR 749). However, new partial A(1)AR agonists are in development, including CVT-3619 hA(1) AR K(i) = 55nM, hA(2A:hA2B:hA(3))1,000:20, CV Therapeutics), which have the potential to provide enhanced insulin sensitivity without cardiovascular side effects and tachyphylaxis. The nonnucleosidic A(1)AR agonist BAY 68-4986 (capadenoson) represents a novel approach to angina wherein both animal studies and early human studies are promising. T-62 is an A(1)AR allosteric enhancer that is currently being evaluated in clinical trials as a potential treatment for neuropathic pain. The challenges associated with developing A(1)AR antagonists, agonists, or allosteric enhancers for therapeutic intervention are now well defined in humans. Significant progress has been made in identifying A(1)AR antagonists for the treatment of edema associated with congestive heart failure (CHF), A(1)AR agonists for the treatment of atrial arrhythmias, type II diabetes and angina, and A(1)AR allosteric enhancers for the treatment of neuropathic pain.

The potential role of the nodose ganglion adenosine A1 receptor in regulation of breathing in anaesthetized rats

The respiratory effects of stimulation of adenosine A(1) receptors were studied in spontaneously breathing rats that were either (1) neurally intact and subsequently bilaterally vagotomized in the neck, or (2) neurally intact and subjected to supranodosal vagotomy or (3) midcervically vagotomized before and after pharmacological blockade of A(1) receptors. Before neural interventions an intravenous bolus of the A(1) receptor agonist N6-cyclopentyladenosine (CPA, 5 microg kg(-1)) decreased breathing rate, tidal volume, mean arterial blood pressure (MAP) and heart rate. After section of the midcervical vagi, CPA still decreased respiratory rate and tidal volume. Supranodose vagotomy abolished the fall in respiratory rate but did not affect the depression of tidal volume. Blockade of A(1) receptors with intravenous doses of DPCPX (100 microg kg(-1)) eliminated all respiratory effects of CPA challenge. In all the neural states, CPA caused significant falls in mean arterial blood pressure and heart rate. DPCPX pre-treatment prevented these cardiovascular effects. The present data suggest that: (1) CPA-evoked activation of A(1) receptors decreases breathing rate and tidal volume and this occurs central to the cervical vagi; (2) supranodosal vagotomy prevents the decrease in breathing rate, which is presumably due to stimulation of nodosal A(1) receptor; and (3) depression of tidal volume and the hypotensive response result from the excitation of central nervous A(1) expressing neurones.

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, N(6)-(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 (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) 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 microg. 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.

Inhibition of Ca2+ signaling and glucagon secretion in mouse pancreatic alpha-cells by extracellular ATP and purinergic receptors

Glucagon secreted from pancreatic alpha-cells plays a critical role in glycemia, mainly by hepatic glucose mobilization. In diabetic patients, an impaired control of glucagon release can worsen glucose homeostasis. Despite its importance, the mechanisms that regulate its secretion are still poorly understood. Since alpha-cells are particularly sensitive to neural and paracrine factors, in this report we studied the role of purinergic receptors and extracellular ATP, which can be released from nerve terminals and beta-cell secretory granules. Using immunocytochemistry, we identified in alpha-cells the P2 receptor subtype P2Y1, as well as the P1 receptors A1 and A2A. In contrast, only P2Y1 and A1 receptors were localized in beta-cells. To analyze the role of purinergic receptors in alpha-cell function, we studied their participation in Ca2+ signaling. At low glucose concentrations, mouse alpha-cells exhibited the characteristic oscillatory Ca2+ signals that lead to secretion. Application of ATP (1-10 microM) abolished these oscillations or reduced their frequency in alpha-cells within intact islets and isolated in culture. ATPgammaS, a nonhydrolyzable ATP derivative, indicated that the ATP effect was mainly direct rather than through ATP-hydrolytic products. Additionally, adenosine (1-10 microM) was also found to reduce Ca2+ signals. ATP-mediated inhibition of Ca2+ signaling was accompanied by a decrease in glucagon release from intact islets in contrast to the adenosine effect. Using pharmacological agonists, we found that only P2Y1 and A2A were likely involved in the inhibitory effect on Ca2+ signaling. All these findings indicate that extracellular ATP and purinergic stimulation are effective regulators of the alpha-cell function.

Adenosine receptors and asthma

The accumulation of evidence implicating a role for adenosine in the pathogenesis of asthma has led to investigations into all adenosine receptor subtypes as potential therapeutic targets for the treatment of asthma. Selective A(1) receptor antagonists are currently in preclinical development since adenosine has been shown experimentally to mediate various features of asthma through this receptor such as bronchoconstriction, mucus secretion and inflammation. The A(2A) receptor is expressed on most inflammatory cells implicated in asthma, and as A(2A) stimulation activates adenylate cyclase and consequently elevates cAMP, selective A(2A) receptor agonists have now reached clinical development. However, initial reports concerning their efficacy are inconclusive. A(2B) receptor antagonists are also under investigation based on the rationale that inhibiting the effects of adenosine on mast cells would be beneficial, in addition to other reported pro-inflammatory effects mediated by the A(2B) receptor on cells such as airway smooth muscle, epithelial cells and fibroblasts. Whilst the effects in pre-clinical models are promising, their efficacy in the clinical setting has also yet to be reported. Finally, adenosine A(3) receptor stimulation has been demonstrated to mediate inhibitory effects on eosinophils since it also elevates cAMP. However, some experimental reports suggest that A(3) antagonists mediate anti-inflammatory effects, thus the rationale for A(3) receptor ligands as therapeutic agents remains to be determined. In conclusion, establishing the precise role of adenosine in the pathogenesis of asthma and developing appropriate subtype selective agonists/antagonists represents an exciting opportunity for the development of novel therapeutics for the treatment of asthma.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

Brief ischemia causes long-term depression in midbrain dopamine neurons

Degeneration of dopamine neurons in the substantia nigra pars compacta (SNc) plays an important role in the pathophysiology of neurodegenerative diseases like Parkinsonism and vascular dementia. SNc dopamine neurons both in vitro and in vivo show sensitivity to hypoxic/ischemic conditions and undergo degeneration. In acute brain slices, these dopamine neurons undergo hyperpolarization during hypoxia and hypoglycemia, which results in silencing of the neurons. However, the role that SNc excitatory synapses play in this process is poorly understood. Here we examined the effect of oxygen/glucose deprivation (OGD) on glutamatergic synaptic transmission in the SNc in a rat midbrain slice preparation. OGD for 5 min caused pre-synaptic ischemic long-term depression (iLTD) of glutamate transmission, as both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid- and N-methyl-D-aspartate receptor-mediated synaptic currents in SNc dopamine neurons were depressed to a similar extent. This depression began immediately after exposure to OGD and was not recovered upon washout of OGD. Pharmacological studies revealed that the iLTD was triggered by a rise in post-synaptic intracellular calcium and mediated by activation of pre-synaptic adenosine A(1) receptors, which reduced glutamate-dependent synaptic transmission by activating ATP-dependent potassium channels. Furthermore, we observed that iLTD did not occlude tetanic long-term depression (LTD) at the SNc excitatory synapses, suggesting that these two forms of LTD involve different pathways. Taken together, our results showed that brief exposure to hypoxia and hypoglycemia results in LTD of synaptic activity at glutamatergic synapses onto SNc neurons and this phenomenon could represent a protective mechanism by reducing ischemia-induced excitotoxic injury to dopamine neurons.

Phenylethyl-substituted pyrimido[2,1-f]purinediones and related compounds: structure-activity relationships as adenosine A(1) and A(2A) receptor ligands

The synthesis of N-(un)substituted-phenylalkylpyrimido[2,1-f]purinediones was performed starting with 7-(3-chloropropyl)-8-bromotheophylline and 7-(3-chloropropyl)-8-bromo-1,3-dipropylxanthine. Compounds with unsubstituted or substituted ethylene spacer to an aromatic ring were synthesized. Additionally variations in the spacer-elongation of the linker containing more than two atoms, introduction of a double bond or heteroatoms were performed. Physicochemical properties of the synthesized compounds were described. The obtained compounds envisaged as sterically fixed and configurationally stable analogs of 8-styrylxanthines, were evaluated for their affinity to adenosine A(1) and A(2A) receptors, the receptor subtypes that are predominant in the brain. Selected compounds were also investigated for the affinity to the A(2B) and A(3) receptor subtypes. It was stated that phenylethyl pyrimido[2,1-f]purinediones and their analogs with variations of the ethylene spacer (substituted or extended) exhibit micromolar or submicromolar affinity for A(2A) ARs (adenosine receptors); for example compound 2Ac with p-hydroxy substituent displayed a K(i) value of 0.23 microM at the rat A(2A) receptor. In comparison to the previously obtained phenyl and benzyl pyrimido[2,1-f]purinediones compounds with a shorter spacer, phenethyl derivatives were optimal for A(2A) AR. The kind of substituent at the aromatic ring was important for the affinity. Oxygen and nitrogen atoms in the spacer resulted frequently in a slight decrease of the A(2A) AR affinity, introduction of more heteroatoms into the spacer-in carbamates-caused distinctly negative effect on the activity. In this series of compounds more frequently the adenosine A(1) activity was observed, also in submicromolar range as for dipropyl derivative 2Ba with K(i) value of 0.62 microM at the rat A(2A) AR. 3D-QSAR models were developed for the compounds presented in this paper as well as in the previous publications showing activity at adenosine A(1) and A(2A) ARs. It was concluded that for the activity at adenosine A(1) and A(2A) receptors lipophilicity, steric effects along with the molecule's electrostatic surface properties had greatest value. Chosen compounds were evaluated in vivo as anticonvulsants in MES, scMet tests and examined for neurotoxicity. Contrary to previously obtained phenyl and benzyl pyrimido[2,1-f]purinediones, all tested compounds were inactive as anticonvulsants.

ATP modulates the release of noradrenaline through two different prejunctional receptors on the adrenergic nerves of rat prostate

1. The effects of adenosine and ATP receptor agonists on the release of endogenous noradrenaline from electrically stimulated (2 Hz, 0.1 msec) rat prostate were examined in order to clarify the pharmacological properties of prejunctional receptors for adenosine and ATP on the adrenergic nerve varicosities in the prostate. Noradrenaline was quantified by HPLC coupled with electrochemical detection techniques. 2. Both adenosine and ATP receptor agonists (1 micromol/L) inhibited noradrenaline release and the relative order of inhibitory effect was N(6)-cyclopentyl-adenosine (CPA) > 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > adenosine > 2-methylthio-ATP (2mSATP) > AMP > ATP. 3. The adenosine receptor agonist CPA (1 nmol/L-1 micromol/L) and the ATP receptor agonist 2mSATP (100 nmol/L-100 micromol/L) inhibited the stimulation-induced release of noradrenaline in a concentration-dependent manner. The concentrations of CPA and 2mSATP that produced 50% inhibition of noradrenaline release were 9.6 nmol/L and 1.4 micromol/L, respectively. 4. 1,3-Dipropyl-8-cyclopentylxanthine, an adenosine A(1) receptor antagonist, significantly reduced the inhibitory effects of not only CPA, but also 2mSATP. 5. Suramin, an ATP receptor antagonist, significantly reduced the inhibitory effects of 2mSATP, but not those of CPA. 6. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, another ATP receptor antagonist, had no effect on the inhibitory action of either agonist. 7. These results suggest that, in the sympathetic nerve terminals of rat prostate, adenosine and ATP induce inhibition of noradrenaline release via the activation of adenosine A(1) and/or xanthine-sensitive ATP receptors, which play an inhibitory regulatory role in adrenergic neurotransmission in the prostate.

The role of adenosine A1 and A2A receptors of entorhinal cortex on piriform cortex kindled seizures in rats

In this research the role of adenosine A1 and A2A receptors of the entorhinal cortex on piriform cortex kindled seizures was investigated. In piriform cortex kindled rats, N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, 1,3-dimethyl-8 cyclopenthylxanthine (CPT), a selective A1 receptor antagonist, CGS21680 hydrochloride (CGS), a selective A2A receptor agonist and ZM241385 (ZM), a selective A2A receptor antagonist were injected into the entorhinal cortex bilaterally. Five minutes later, animals were stimulated and seizure parameters were recorded. CHA (10 and 100microM) decreased the afterdischarge duration (ADD), stage 5 seizure duration (S5D), and seizure duration (SD), and increased the latency to stage 4 of the seizure (S4L) significantly. Bilateral microinjection of CPT (100microM) increased ADD, S(5)D, and SD, and reduced S(4)L significantly. Pretreatment of animals with CPT (50microM) before CHA (100microM), reduced the effect of CHA on seizure parameters. On the other hand, CGS (1mM) increased only ADD. Bilateral microinjection of ZM had no effect on seizure parameters. However, pretreatment of animals with ZM (200microM) before CGS (1mM), eliminated the excitatory effect of CGS on seizure parameters. These results suggest that activation of A1 receptors of the entorhinal cortex has an anticonvulsant, but activation of A2A receptors of this region has a proconvulsive effect on piriform cortex kindled seizures.

Adenosine A2 receptor activation attenuates afferent arteriolar autoregulation during adenosine receptor saturation in rats

Adenosine is an important paracrine agent regulating renal hemodynamics via adenosine A1 and A2 receptors. To determine the interactions between adenosine A1 and A2 receptors and the possible role of adenosine as a modulator of afferent arteriolar autoregulatory responses, videomicroscopic measurements of afferent arteriolar dimensions were performed at different perfusion pressures (from 100 to 125 and 150 mm Hg) using the isolated-blood-perfused rat juxtamedullary nephron preparation. Single afferent arterioles were visualized and superfused with low or high concentrations of adenosine, either alone or with the adenosine A1 receptor antagonist 8-noradamantan-3-yl-1,3-dipropylxanthine (10 micromol/L) or the adenosine A2 receptor antagonist dimethyl-1-propargylxanthine (10 micromol/L). Adenosine (20 micromol/L) decreased afferent arteriolar diameter by -9.0+/-0.9%, and this effect was enhanced by dimethyl-1-propargylxanthine (10 micromol/L) to -16.1+/-1.2%. However, autoregulatory capability was maintained. Adenosine-induced vasoconstriction was prevented by 8-noradamantan-3-yl-1,3-dipropylxanthine (10 micromol/L) with diameter increasing by 9.6+/-1.2%. Adenosine receptor saturation with a high concentration of adenosine (120 micromol/L) or blocking A1 receptors with 8-noradamantan-3-yl-1,3-dipropylxanthine in the presence of adenosine resulted in marked vasodilation and marked impairment of autoregulatory responses to increases in perfusion pressure (-1.5+/-1.1% and -3.5+/-0.9%). However, afferent arteriolar autoregulatory responses to elevations in perfusion pressure were restored after blockade of A2 receptors alone or in combination with A1 receptor blockade. During treatment with dimethyl-1-propargylxanthine in the presence of adenosine receptor saturation (120 micromol/L), afferent arteriolar autoregulatory responses were intact (-16.5+/-1.6% and -26.4+/-2.1%). These results indicate that the interactions between adenosine A1 and A2 receptors exert important modulatory influences on afferent arteriolar tone and autoregulatory capability. Activation of A2 receptors abrogates the counteracting influences of A1 receptors leading to marked vasodilation and decreased afferent arteriolar autoregulatory efficiency.

Influence of adenosine A(1)-receptor blockade and vagotomy on the gasping and heart rate response to hypoxia in rats during early postnatal maturation

Failure to autoresuscitate from apnea has been suggested to play a role in sudden infant death. Little is known, however, about factors that influence the gasping and heart rate response to severe hypoxia that are fundamental to successful autoresuscitation in the newborn. The present experiments were carried out on 184 rat pups to investigate the influence of the parasympathetic nervous system, as well as adenosine, in mediating the profound bradycardia that occurs with the onset of hypoxic-induced primary apnea and in modulating hypoxic gasping. On days 1 to 2, days 5 to 6, and days 10 to 11 postpartum and following bilateral cervical vagotomy (VAG) or administration of a selective adenosine A(1) receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine; DPCPX), each pup was exposed to a single period of severe hypoxia produced by breathing an anoxic gas mixture (97% N(2)-3% CO(2)). Exposure to severe hypoxia resulted in an age-dependent decrease in heart rate (P < 0.001), accentuated with increasing postnatal age, that was attenuated in all age groups by DPCPX but not by VAG. Furthermore, DPCPX but not VAG decreased the time to last gasp but increased the total number of gasps in the 1- to 2-day-old and 5- to 6-day-old pups but not in the 10- to 11-day-old pups during exposure to severe hypoxia. Thus our data provide evidence that adenosine acting via adenosine A(1) receptors plays a role in modulating hypoxic gasping and in mediating the profound bradycardia that occurs coincident with hypoxic-induced primary apnea in rats during early postnatal life.

Caffeine facilitation of glutamate release from rat cerebral cortex nerve terminals (synaptosomes) through activation protein kinase C pathway: an interaction with presynaptic adenosine A1 receptors

The present study used nerve terminals (synaptosomes) isolated from rat cerebral cortex to investigate the relationship between caffeine and 4-aminopyridine (4AP)-evoked endogenous excitatory neurotransmitter glutamate release. Micromolar concentrations of caffeine facilitated 4AP, but not KCl or ionomycin-evoked glutamate release from synaptosomes. This release facilitation resulted from an enhancement of vesicular and nonvesicular release and associated with an increase both in 4AP-evoked depolarization of the synaptosomal plasma membrane potential and in 4AP-evoked increase in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](C)). In addition, the release facilitation by caffeine was significantly reduced in synaptosomes pretreated with a wide spectrum blocker of N- and P/Q-type Ca(2+) channels, omega-conotoxin MVIIC. Furthermore, protein kinase C (PKC) activator and inhibitor, respectively, superseding or suppressing the caffeine-mediated facilitation of glutamate release. These results concluded that caffeine exerts their presynaptic facilitatory effect, likely through the activation of PKC pathway, which subsequently enhances terminal excitability and Ca(2+) entry to cause an increase in evoked glutamate release from rat cerebrocortical nerve terminals. Additionally, this release facilitation may involve an interaction of caffeine with presynaptic adenosine A1 receptors as adenosine A1 receptor inhibition abolished the caffeine-mediated facilitation of evoked glutamate release.

Effect of adenosine receptor subtypes stimulation on mixed lymphocyte reaction

The cell-to-cell interaction through binding of intercellular adhesion molecule (ICAM)-1 on monocytes to their ligands lymphocyte function-associated antigen (LFA)-1 on T-cells plays important roles in cytokine production and T-cell proliferation. Interleukin (IL)-18, which plasma levels are elevated in patients during acute rejection following organ transplantation, induces the expression of ICAM-1 on monocytes, production of interferon (IFN)-gamma and IL-12 and lymphocyte proliferation during human mixed lymphocyte reaction. Activation of the adenosine A(2A) receptor on during reperfusion of various tissues has been found to markedly reduce ischemia-reperfusion injury. In the present study, we examined the effect of adenosine at increasing concentrations ranging from 0.1 to 100 microM on the IL-18-enhanced expression of ICAM-1, production of IFN-gamma and IL-12 and lymphocyte proliferation during human mixed lymphocyte reaction. Adenosine inhibited the IL-18-initiated immune responses. The IC(50) values of adenosine for inhibition of the IL-18-enhanced ICAM-1 expression, IFN-gamma production and lymphocyte proliferation were 20 microM, respectively. The actions of adenosine depended on the stimulation of adenosine A(2A) receptor. An inhibitor of protein kinase A (PKA) at 100 microM inhibited the actions of adenosine, suggesting that PKA might be involved in the actions of adenosine. On the other hand, the stimulation of adenosine A(1) and A(3) receptor blocked the actions of adenosine A(2A) receptor stimulation. These results suggest that adenosine inhibits the immune responses during mixed lymphocyte reaction via adenosine A(2A) receptor.

Sex differences in mouse heart rate and body temperature and in their regulation by adenosine A1 receptors

AIM

To examine cardiac function, body temperature and locomotor behaviour in the awake adenosine A(1) receptor knock out mouse of both sexes.

METHODS

Male and female A(1)R (+/+) and (-/-) mice, instrumented with telemetric devices, were recorded during basal conditions and after drug administration.

RESULTS

Female mice had higher heart rate, body temperature and locomotion, both during daytime and during the night. Awake A(1)R (-/-) mice had a slightly elevated heart rate, and this was more clear-cut in males. Heart rate was also higher in Langendorff-perfused denervated A(1)R (-/-) hearts. Body temperature was higher in A(1)R (-/-) males and females; locomotor activity was higher in A(1)R (-/-) females, but not in males. The adenosine receptor agonist R-PIA (0.2 mg kg(-1)) decreased heart rate and body temperature, but less in A(1)R (-/-) animals than in A(1)R (+/+) mice (P < 0.001 in both parameters). The unselective adenosine receptor antagonist caffeine had a minor stimulatory effect on heart rate in lower doses, but depressed it at a dose of 75 mg kg(-1). Body temperature was increased after a low dose (7.5 mg kg(-1)) of caffeine in both sexes and genotypes, and markedly reduced after a high dose (75 mg kg(-1)) of caffeine. An intermediary dose of caffeine 30 mg kg(-1) increased or decreased body temperature depending on genotype and sex. Locomotor responses to caffeine were variable depending both on genotype and sex.

CONCLUSION

Thus, the adenosine A(1) receptor is involved in the regulation of heart rate, body temperature and locomotor activity, but the magnitude of the involvement is different in males and females.

L-citrulline inhibits [3H]acetylcholine release from rat motor nerve terminals by increasing adenosine outflow and activation of A1 receptors

BACKGROUND AND PURPOSE

Nitric oxide (NO) production and depression of neuromuscular transmission are closely related, but little is known about the role of L-citrulline, a co-product of NO biosynthesis, on neurotransmitter release.

EXPERIMENTAL APPROACH

Muscle tension recordings and outflow experiments were performed on rat phrenic nerve-hemidiaphragm preparations stimulated electrically.

KEY RESULTS

L-citrulline concentration-dependently inhibited evoked [(3)H]ACh release from motor nerve terminals and depressed nerve-evoked muscle contractions. The NO synthase (NOS) substrate, L-arginine, and the NO donor, 3-morpholinosydnonimine chloride (SIN-1), also inhibited [(3)H]ACh release with a potency order of SIN-1>L-arginine>L-citrulline. Co-application of L-citrulline and SIN-1 caused additive effects. NOS inactivation with N(omega)-nitro-L-arginine prevented L-arginine inhibition, but not that of L-citrulline. The NO scavenger, haemoglobin, abolished inhibition of [(3)H]ACh release caused by SIN-1, but not that caused by L-arginine. Inactivation of guanylyl cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) fully blocked SIN-1 inhibition, but only partially attenuated the effects of L-arginine. Reduction of extracellular adenosine accumulation with adenosine deaminase or with the nucleoside transport inhibitor, S-(p-nitrobenzyl)-6-thioinosine, attenuated the effects of L-arginine and L-citrulline, while not affecting inhibition by SIN-1. Similar results were obtained with the selective adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. L-citrulline increased the resting extracellular concentration of adenosine, without changing that of the adenine nucleotides.

CONCLUSIONS AND IMPLICATIONS

NOS catalyses the formation of two neuronally active products, NO and L-citrulline. While, NO may directly reduce transmitter release through stimulation of soluble guanylyl cyclase, the inhibitory action of L-citrulline may be indirect through increasing adenosine outflow and subsequently activating inhibitory A(1) receptors.

On the stability of 2-aminoselenophene-3-carboxylates: potential dual-acting selenium-containing allosteric enhancers of A1 adenosine receptor binding

Ethyl-2-amino-4,5,6,7-tetrahydro-1-benzoselenophene-3-carboxylate (4), has been prepared as a potential dual-acting selenium-containing allosteric enhancer of adenosine A(1)A receptor binding utilising a modified Gewald reaction. While preliminary testing indicated that 4 is a superior enhancer of A(1)AR binding than its thiophene counterpart, its instability under mildly acidic conditions is cause for concern. X-Ray crystallography, together with DFT calculations, provide evidence that the decomposition of 4 involves the ring-opening of selenophenium ion (12b) followed by the loss of elemental selenium through a radical chain process.

Staurosporine-induced apoptosis in astrocytes is prevented by A1 adenosine receptor activation

Astrocyte apoptosis occurs in acute and chronic pathological processes at the central nervous system and the prevention of astrocyte death may represent an efficacious intervention in protecting neurons against degeneration. Our research shows that rat astrocyte exposure to 100 nM staurosporine for 3h caused apoptotic death accompanied by caspase-3, p38 mitogen-ed protein kinase (MAPK) and glycogen synthase kinase-3beta (GSK3beta) activation. N(6)-chlorocyclopentyladenosine (CCPA, 2.5-75 nM), a selective agonist of A(1) adenosine receptors, added to the cultures 1h prior to staurosporine, induced a dose-dependent anti-apoptotic effect, which was inhibited by the A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine. CCPA also caused a dose- and time-dependent phosphorylation/activation of Akt, a downstream effector of cell survival promoting phosphatidylinositol 3-kinase (PI3K) pathway, which in turn led to inhibition of staurosporine-induced GSK3beta and p38 MAPK activity. Accordingly, the anti-apoptotic effect of CCPA was abolished by culture pre-treatment with LY294002, a selective PI3K inhibitor, pointing out the prevailing role played by PI3K pathway in the protective effect exerted by A(1) receptor activation. Since an abnormal p38 and GSK3beta activity is implicated in acute (stroke) and chronic (Alzheimer's disease) neurodegenerative diseases, the results of the present study provide a hint to better understand adenosine relevance in these disorders.

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</=6) and showed relatively weak agonist activity at these receptors (pEC(50) 7.1 at both receptors). Ligand (I) was a potent inhibitor of the generation of reactive oxygen species from human neutrophils and eosinophils (pEC(50) 9.7+/-0.1 and 9.4+/-0.2 respectively). The inhibitory effect of ligand (I) on the release of reactive oxygen species from neutrophils was antagonised competitively by the adenosine A(2A) receptor antagonist 9-chloro-2-(2-furanyl)-[1,2,4]triazolo[1,5-c]quinazolin-5-amine (CGS15943) with a pA(2) value (10.03+/-0.44) consistent with an effect on adenosine A(2A) receptors. Ligand (I) also inhibited the release of granule proteins from neutrophils and eosinophils (pEC(50) 8.7 and 8.9 respectively), albeit less potently than as an inhibitor of reactive oxygen species generation. In summary, ligand (I) is a potent and selective agonist for the adenosine A(2A) receptor and a competitive antagonist at the adenosine A(3) receptor. Ligand (I) has potent anti-inflammatory effects on human granulocytes in vitro.

Paeoniflorin attenuates neuroinflammation and dopaminergic neurodegeneration in the MPTP model of Parkinson's disease by activation of adenosine A1 receptor

1. This study examined whether Paeoniflorin (PF), the major active components of Chinese herb Paeoniae alba Radix, has neuroprotective effect in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease (PD). 2. Subcutaneous administration of PF (2.5 and 5 mg kg(-1)) for 11 days could protect tyrosine hydroxylase (TH)-positive substantia nigra neurons and striatal nerve fibers from death and bradykinesia induced by four-dose injection of MPTP (20 mg kg(-1)) on day 8. 3. When given at 1 h after the last dose of MPTP, and then administered once a day for the following 3 days, PF (2.5 and 5 mg kg(-1)) also significantly attenuated the dopaminergic neurodegeneration in a dose-dependent manner. Post-treatment with PF (5 mg kg(-1)) significantly attenuated MPTP-induced proinflammatory gene upregulation and microglial and astrocytic activation. 4. Pretreatment with 0.3 mg kg(-1) 8-cyclopentyl-1,3-dipropylxanthine, an adenosine A1 receptor (A1AR) antagonist, 15 min before each dose of PF, reversed the neuroprotective and antineuroinflammatory effects of PF. 5. In conclusion, this study demonstrated that PF could reduce the MPTP-induced toxicity by inhibition of neuroinflammation by activation of the A1AR, and suggested that PF might be a valuable neuroprotective agent for the treatment of PD.

Co-modulation of acute ethanol-induced motor impairment by mouse cerebellar adenosinergic A1 and GABAA receptor systems

We have previously demonstrated that cerebellar adenosine modulates ethanol ataxia. Using Rotorod method, we investigated the role of cerebellar GABA(A) receptors in the adenosinergic modulation of ethanol ataxia in mice. Direct cerebellar microinfusion of GABA(A) agonist, muscimol (2.5, 5 and 10 ng) and antagonist, bicuculline (50, 100 and 200 ng), via permanently implanted guide cannulas, produced a marked and dose-dependent accentuation and attenuation, respectively, of ethanol (2g/kg; IP) ataxia. The accentuation of ethanol ataxia by intracerebellar muscimol was through GABA(A) receptor because intracerebellar pretreatment with bicuculline virtually abolished muscimol effect. Intracerebellar microinfusion of adenosine A(1) agonist, N(6)-cyclohexyladenosine (CHA: 4 ng), and antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX: 100 ng) markedly accentuated and attenuated, respectively, ethanol ataxia consistent with our previously published data. Intracerebellar microinfusion of CHA (4 ng) or DPCPX (100 ng) markedly enhanced and reduced, respectively, muscimol (10 ng)-induced accentuation of ethanol ataxia suggesting co-modulation of ethanol ataxia by cerebellar adenosinergic A(1) and GABA(A) receptors. Similarly, intracerebellar bicuculline (200 ng) pretreatment not only prevented CHA-induced accentuation of ethanol ataxia, but caused further decrease in ethanol ataxia. No change in the normal coordination was observed when microinfusion of the highest dose of muscimol, bicuculline, DPCPX or CHA alone or in combination was followed by saline injection instead of ethanol. The results of the present study suggest a functional similarity between GABA(A) and adenosine A(1) receptors even though both receptor types are known to couple to different signaling system and their location is on the opposite ends of the cerebellar granule cells, axons and axonal terminals (i.e., GABA(A) at the granule cells and adenosine A(1) on axons and axonal terminals of the granule cells) and act as co-modulators of ethanol ataxia.

Neuroprotective effect of paeoniflorin on cerebral ischemic rat by activating adenosine A1 receptor in a manner different from its classical agonists

The effects of paeoniflorin (PF), a compound isolated from Paeony radix, on neurological impairment and histologically measured infarction volume following transient and permanent focal ischemia were examined in Sprague-Dawley rats. In transient ischemia model, rats were subjected to a 1.5-h occlusion of the middle cerebral artery (MCA). The administration of PF (2.5 and 5 mg kg(-1), s.c.) produced a dose-dependent decrease in both neurological impairment and the histologically measured infarction volume. Similar results were also obtained when PF (2.5, 5, and 10 mg kg(-1), s.c.) was given in permanent ischemia model. The neuroprotective effect of PF (10 mg kg(-1), s.c.) was abolished by pretreatment of DPCPX (0.25 mg kg(-1), s.c.), a selective adenosine A1 receptor (A1R) antagonist. PF (10, 40, and 160 mg kg(-1), i.v.) had no effect on mean arterial pressure (MAP) and heart rates (HR) in the conscious rat. Additionally, PF (10(-3) mol l(-1)) had no effect on noradrenaline- (NA-) or high K+ concentration-induced contractions of isolated rabbit primary artery. In competitive binding experiments, PF did not compete with the binding of [3H]DPCPX, but displaced the binding of [3H]NECA to the membrane preparation of rat cerebral cortex. This binding manner was distinguished from the classical A1R agonists. The results demonstrated that activation of A1R might be involved in PF-induced neuroprotection in cerebral ischemia in rat. However, PF had no 'well-known' cardiovascular side effects of classical A1R agonists. The results suggest that PF might have the potential therapeutic value as an anti-stroke drug.

Modulation of ischaemic contracture in mouse hearts: a 'supraphysiological' response to adenosine

While inhibition of ischaemic contracture was one of the first documented cardioprotective actions of exogenously applied adenosine, it is not known whether this is a normal function of endogenous adenosine generated during ischaemic stress. Additionally, the relevance of delayed contracture to postischaemic outcome is unclear. We tested the ability of endogenous versus exogenous adenosine to modify contracture (and postischaemic outcomes) in C57/Bl6 mouse hearts. During ischaemia, untreated hearts developed peak contracture (PC) of 85 +/- 5 mmHg at 8.9 +/- 0.8 min, with time to reach 20 mmHg (time to onset of contracture; TOC) of 4.4 +/- 0.3 min. Adenosine (50 microm) delayed TOC to 6.7 +/- 0.6 min, as did pretreatment with 10 microm 2-chloroadenosine (7.2 +/- 0.5 min) or 50 nm of A(1) adenosine receptor (AR) agonist N(6)-cyclohexyladenosine (CHA) (6.7 +/- 0.3 min), but not A(2A)AR or A(3)AR agonists (20 nm 2-[4-(2-carboxyethyl) phenethylamino]-5' N-methylcarboxamidoadenosine (CGS21680) or 150 nm 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA), respectively). Adenosinergic contracture inhibition was eliminated by A(1)AR gene knockout (KO), mimicked by A(1)AR overexpression, and was associated with preservation of myocardial [ATP]. This adenosine-mediated inhibition of contracture was, however, only evident after prolonged (10 or 15 min) and not brief (3 min) pretreatment. Ischaemic contracture was also insensitive to endogenously generated adenosine, since A(1)AR KO, and non-selective and A(1)AR-selective antagonists (50 microm 8-sulphophenyltheophylline and 150 nm 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), respectively), all failed to alter intrinsic contracture development. Finally, delayed contracture with A(1)AR agonism/overexpression or ischaemic 2,3-butanedione monoxime (BDM; 5 microm to target Ca(2+) cross-bridge formation) was linked to enhanced postischaemic outcomes. In summary, adenosinergic inhibition of contracture is solely A(1)AR mediated; the response is 'supraphysiological', evident only with significant periods of pre-ischaemic AR agonism (or increased A(1)AR density); and ischaemic contracture appears insensitive to locally generated adenosine, potentially owing to the rapidity of contracture development versus the finite time necessary for expression of AR-mediated cardioprotection.

Effects of urea pretreatment on the binding properties of adenosine A1 receptors

The effect of denaturation and/or extraction of nonintegral membrane proteins by 7 M urea on the binding of the antagonist [3H]cyclopentyl-1,3-dipropylxanthine 8 dipropyl-2,3 ([3H]DPCPX), and the agonists adenosine, (-)-N6-(2-phenylisopropyl)-adenosine (R-PIA) and N6-cyclohexyladenosine (CHA), was investigated at human A1 adenosine receptors stably expressed in CHO cells. Pretreatment with urea caused a 56% reduction in membrane proteins. Compared to controls, the use of adenosine deaminase (ADA), 100 microM 5'-guanylylimidodiphosphate (Gpp(NH)p) or urea each caused equivalent increases in specific [3H]DPCPX binding. Neither the binding kinetics nor the affinity of [3H]DPCPX were significantly different in urea-pretreated compared to ADA-pretreated membranes. At 25 degrees C in ADA-pretreated membranes, the competition isotherms for R-PIA and CHA were characterized by two affinity states. Gpp(NH)p (100 microM) reduced, but did not abolish, the value of the high-affinity dissociation constant. Similar results were obtained after treatment with urea for R-PIA, whereas the high-affinity state for CHA was abolished. At 37 degrees C, urea pretreatment, but not 100 microM Gpp(NH)p, abolished high-affinity agonist competition binding. There was no significant effect of any of the treatments on the low-affinity agonist binding state. In urea-pretreated membranes, exogenously added adenosine competed according to a simple mass-action model with a pK(L) of 5.66+/-0.05 (n=3). Compared to the more common approaches of ADA treatment and/or use of guanine nucleotides, our findings suggest that urea pretreatment represents an inexpensive and useful approach for investigating the binding properties of adenosine A1 ligands (including adenosine) to the G protein-uncoupled form of the receptor.

Contribution of adenosine receptors in the control of arteriolar tone and adenosine–angiotensin II interaction

Adenosine (Ado) mediates vasoconstriction via A(1)-Ado receptors and vasodilation via A(2)-Ado receptors in the kidney. It interacts with angiotensin II (Ang II), which is important for renal hemodynamics and tubuloglomerular feedback (TGF). The aim was to investigate the function of Ado receptors in the Ado-Ang II interaction in mouse microperfused, afferent arterioles. Ado (10(-11)-10(-4) mol/l) caused a biphasic response: arteriolar diameters were reduced (-7%) at Ado 10(-11)-10(-9) mol/l and returned to control values at higher concentrations. Treatment with Ang II (10(-10) mol/l) transformed the response into a concentration-dependent constriction. N(6)-cyclopentyladenosine (A(1)-Ado receptor agonist) reduced diameters (12% at 10(-6) mol/l). Application of CGS21680 (10(-12)-10(-4) mol/l, A(2A) receptor agonist) increased the diameter by 13%. Pretreatment with ZM241385 (A(2A)-Ado receptor antagonist) alone or in combination with MRS1706 (A(2B)-Ado receptor antagonist) resulted in a pure constriction upon Ado, whereas 8-cyclopentyltheophylline (CPT) (A(1)-Ado receptor antagonist) inhibited the constrictor response. Afferent arterioles of mice lacking A(1)-Ado receptor did not show constriction upon Ado. Treatment with Ado (10(-8) mol/l) increased the response upon Ang II, which was blocked by CPT. Ado (10(-5) mol/l) did not influence the Ang II response, but an additional blockade of A(2)-Ado receptors enhanced it. The action of Ado on constrictor A(1)-Ado receptors and dilatory A(2)-Ado receptors modulates the interaction with Ang II. Both directions of Ado-Ang II interaction, which predominantly leads to an amplification of the contractile response, are important for the operation of the TGF.

Nitric oxide mediates interactions between GABAA receptors and adenosine A1 receptors in the rat hippocampus

Adenosine and gamma-aminobutyric acid (GABA) are both major inhibitory neuromodulators/neurotransmitters in the CNS. We now investigated if endogenous GABA modulates adenosine A(1)-mediated action on synaptic transmission in the hippocampus. Field excitatory postsynaptic potentials (fEPSP) were recorded from the CA(1) area of rat hippocampal slices. The adenosine analogue 2-chloroadenosine (0.15-1 microM) inhibited synaptic transmission with an EC(50) of 398 nM. Blocking GABA(A) receptors with the specific antagonists, bicuculline (10 microM) or picrotoxin (10 microM) potentiated the inhibitory effect of 2-chloroadenosine. The concentration-response curve for 2-chloroadenosine was displaced to the left by a factor of 2 (EC(50)=210 nM) in the presence of bicuculline (10 microM). GABA(A) receptor blockade also potentiated the action of N(6)-cyclopentyladenosine (CPA, 10 nM), a specific adenosine A(1) receptor agonist. Prevention of adenosine accumulation with adenosine deaminase (1 U/ml) did not influence bicuculline-induced potentiation of the effect of 2-chloroadenosine. The potentiation of adenosine A(1)-mediated response by bicuculline was abolished when nitric oxide (NO) synthase was inhibited with nitroarginine (100 microM), and when guanylyl cyclase was inhibited with 1H-[1,2,4]Oxadiazolo[4,3-a] quinoxalin-1-one (ODQ, 20 microM). The NO donors, (+/-)-S-nitroso-N-acetylpencillamine (SNAP, 300 microM) and diethylamine NONate diethylammonium salt (DEA/NO, 100 microM), significantly enhanced the inhibitory action of 2-chloroadenosine (150 nM). It is concluded that the blockade of GABA(A) receptors induces a potentiation of adenosine A(1) receptor-mediated inhibitory action, an effect that involves NO acting through guanylyl cyclase. Therefore, endogenous GABA might exert an inhibitory effect over adenosine A(1)-mediated responses in the hippocampus, which may represent a physiologic regulatory mechanism between the two inhibitory mediators.

Loss of vascular adenosine A1 receptors with age in the rat heart

OBJECTIVE

To investigate the effects of age on adenosine A1 receptor (ADORA1) mediated vascular, inotropic and chronotropic functional responses in isolated rat hearts.

METHODS

NECA (5'-(N-ethylcarboxamido)adenosine) and R-PIA (R-N6-(1-methyl-2-phenylethyl)adenosine) concentration-response curves were produced in Langendorff prepared hearts isolated from immature (6 weeks), young (16 weeks) and mature (52 weeks) male Wistar rats and the effects of DPCPX (ADORA1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine, 30 nM) and pertussis toxin pre-treatment (PTX, 48 h, 10 microg/kg i.p., inhibits G(i/o)-protein) were observed.

RESULTS

NECA mediated coronary vasodilation and induced biphasic concentration-response curves in hearts from immature rats (pEC50 8.5 (8.1-8.9) and 11.3 (10.3-12.3)). At the low sensitivity site, the potency of NECA increased in young but not mature rats and remained unchanged at the high sensitivity site. Both DPCPX and PTX each blocked NECA at the high sensitivity site in immature rats, producing monophasic concentration-response curves (pEC50 8.6 (8.5-9.9) for DPCPX and pEC50 8.7 (8.3-9.0) for PTX), but not in young and mature rats. A vasoconstrictor response was observed at low NECA concentrations in hearts from PTX pre-treated immature rats, but not in hearts from young and mature rats, and the response was inhibited by DPCPX. No age related changes were observed in R-PIA mediated negative inotropic and chronotropic responses (P>0.05).

CONCLUSION

ADORA1 mediates a vasodilator response as well as a vasoconstrictor response in the coronary resistance vessels; the latter occurs via a PTX-insensitive pathway and declines with age.

A Novel Role for Protein Phosphatase 2A in Receptor-mediated Regulation of the Cardiac Sarcolemmal Na+/H+ Exchanger NHE1

G(q) protein-coupled receptor stimulation increases sarcolemmal Na(+)/H(+) exchanger (NHE1) activity in cardiac myocytes by an ERK/RSK-dependent mechanism, most likely via RSK-mediated phosphorylation of the NHE1 regulatory domain. Adenosine A(1) receptor stimulation inhibits this response through a G(i) protein-mediated pathway, but the distal inhibitory signaling mechanisms are unknown. In cultured adult rat ventricular myocytes (ARVM), the A(1) receptor agonist cyclopentyladenosine (CPA) inhibited the increase in NHE1 phosphorylation induced by the alpha(1)-adrenoreceptor agonist phenylephrine, without affecting activation of the ERK/RSK pathway. CPA also induced significant accumulation of the catalytic subunit of type 2A protein phosphatase (PP2A(c)) in the particulate fraction, which contained the cellular NHE1 complement; this effect was abolished by pretreatment with pertussis toxin to inactivate G(i) proteins. Confocal immunofluorescence microscopic imaging of CPA-treated ARVM revealed significant co-localization of PP2A(c) and NHE1, in intercalated disc regions. In an in vitro assay, purified PP2A(c) dephosphorylated a GST-NHE1 fusion protein containing aa 625-747 of the NHE1 regulatory domain, which had been pre-phosphorylated by recombinant RSK; such dephosphorylation was inhibited by the PP2A-selective phosphatase inhibitor endothall. In intact ARVM, the ability of CPA to attenuate the phenylephrine-induced increase in NHE1 phosphorylation and activity was lost in the presence of endothall. These studies reveal a novel role for the PP2A holoenzyme in adenosine A(1) receptor-mediated regulation of NHE1 activity in ARVM, the mechanism of which appears to involve G(i) protein-mediated translocation of PP2A(c) and NHE1 dephosphorylation.

Peripheral anti-hyperalgesia by oxcarbazepine: involvement of adenosine A1 receptors

In this study we determined whether oxcarbazepine (OXC) could produce local peripheral antinociceptive effects in a rat model of inflammatory hyperalgesia, and whether adenosine receptors were involved. When coadministered with the pro-inflammatory compound concanavalin A, OXC (1000-3000 nmol/paw) caused a significant dose- and time-dependent anti-hyperalgesia. Caffeine (1000-1500 nmol/paw), a nonselective adenosine receptor antagonist, as well as 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (10-30 nmol/paw), a selective A1 receptor antagonist, coadministered with OXC, significantly depressed its anti-hyperalgesic effect. Drugs injected into the contralateral hind paw did not produce significant effects. These results indicate that OXC produces local peripheral anti-hyperalgesic effects, which is mediated via peripheral A1 receptors.

Valerian extract Ze 911 inhibits postsynaptic potentials by activation of adenosine A1 receptors in rat cortical neurons

In this study we evaluated the adenosine A1 receptor-mediated effect of valerian extract (Ze 911) on postsynaptic potentials (PSPs) in pyramidal cells of the rat cingulate cortex in a slice preparation. We first observed that N6-cyclopentyladenosine (CPA, 0.01 - 10 microM), an adenosine A1 receptor agonist, inhibited PSPs in a concentration-dependent manner. The CPA (10 microM)-induced inhibition was antagonized by 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.1 microM), an adenosine A1 receptor antagonist. Ze 911 concentration dependently (0.1 - 15 mg/mL) inhibited PSPs in the presence of the adenosine A2A receptor antagonist 1,3,7-trimethyl-8-(3-chlorostyryl)xanthine (CSC, 0.2 microM) and adenosine deaminase (1 U/mL). The maximal inhibition induced by 10 mg/mL was completely antagonised by DPCPX (0.1 microM), an A1 receptor blocker. The data suggest that activation of adenosine A1 receptors is involved in the pharmacological effects of the valerian extract Ze 911.

The allosteric enhancer PD81,723 increases chimaeric A1/A2A adenosine receptor coupling with Gs

PD81,723 {(2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluromethyl)-phenyl]methanone} is a selective allosteric enhancer of the G(i)-coupled A1 AR (adenosine receptor) that is without effect on G(s)-coupled A2A ARs. PD81,723 elicits a decrease in the dissociation kinetics of A1 AR agonist radioligands and an increase in functional agonist potency. In the present study, we sought to determine whether enhancer sensitivity is dependent on coupling domains or G-protein specificity of the A1 AR. Using six chimaeric A1/A2A ARs, we show that the allosteric effect of PD81,723 is maintained in a chimaera in which the predominant G-protein-coupling domain of the A1 receptor, the 3ICL (third intracellular loop), is replaced with A2A sequence. These chimaeric receptors are dually coupled with G(s) and G(i), and PD81,723 increases the potency of N6-cyclopentyladenosine to augment cAMP accumulation with or without pretreatment of cells with pertussis toxin. Thus PD81,723 has similar functional effects on chimaeric receptors with A1 transmembrane sequences that couple with G(i) or G(s). This is the first demonstration that an allosteric regulator can function in the context of a switch in G-protein-coupling specificity. There is no enhancement by PD81,723 of G(i)-coupled A2A chimaeric receptors with A1 sequence replacing A2A sequence in the 3ICL. The results suggest that the recognition site for PD81,723 is on the A1 receptor and that the enhancer acts to directly stabilize the receptor to a conformational state capable of coupling with G(i) or G(s).

The role of adenosine in the early respiratory and cardiovascular changes evoked by chronic hypoxia in the rat

Experiments were performed on anaesthetized normoxic (N) rats and chronically hypoxic rats that had been exposed to 12% O2 for 1, 3 or 7 days (1, 3 or 7CH rats). The adenosine A1 receptor antagonist DPCPX did not affect the resting hyperventilation of 1-7CH rats breathing 12% O2 and increased resting heart rate (HR) in 1CH rats only. DPCPX partially restored the decreased baseline arterial pressure (ABP) and increased femoral vascular conductance (FVC) of 1 and 3CH rats, but had no effect in N or 7CH rats. DPCPX also attenuated the decrease in arterial blood pressure (ABP) and increase in FVC evoked by acute hypoxia in N and 1-7CH rats. The non-selective adenosine receptor antagonist 8-SPT had no further effect on baselines or cardiovascular responses to acute hypoxia, but attenuated the hypoxia-evoked increase in respiratory frequency in 1-7CH rats. In N, and 1 and 3CH rats, the inducible nitric oxide synthase (iNOS) inhibitor aminoguanidine had no effect on baselines or increases in FVC evoked by acetylcholine. We propose: (i) that tonically released adenosine acting on A1 receptors reduces HR in 1CH rats and stimulates endothelial NOS in 1 and 3CH rats to decrease ABP and increase FVC, the remaining NO-dependent tonic vasodilatation being independent of iNOS activity; (ii) that in 7CH rats, tonic adenosine release has waned; (iii) that in 1-7CH rats, adenosine released by acute hypoxia stimulates A1 but not A2 receptors to produce muscle vasodilatation, and stimulates carotid body A2 receptors to increase respiration.

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 inhibits ENaC via cytochromeP-450 epoxygenase-dependent metabolites of arachidonic acid

We used the patch-clamp technique to examine the effect of adenosine on epithelial sodium channel (ENaC) activity in rat cortical collecting duct (CCD). Application of adenosine inhibits ENaC activity, and the effect of adenosine was mimicked by cyclohexyladenosine (CHA), an A1adenosine-receptor agonist that reduced channel activity from 1.32 to 0.64. The inhibitory effect of CHA on ENaC was mimicked by cyclopentyladenosine (CPA), which reduced channel activity from 1.1 to 0.55. In contrast, application of CGS-21680, an A2aadenosine-receptor agonist, had no effect on ENaC and increased channel activity from 0.96 to 1.22. This suggests that the inhibitory effect of adenosine analogs resulted from stimulation of the A1adenosine receptor. Inhibition of PLC with U-73122 failed to abolish the effect of CHA on ENaC. In contrast, the inhibitory effect of CHA on ENaC was absent in the presence of the PLA2inhibitor arachidonyl trifluoromethyl ketone (AACOCF3). This suggests a role of arachidonic acid (AA) in mediating the effect of adenosine on ENaC. To determine the metabolic pathway of AA responsible for the effect of adenosine, we examined the effect of CHA in the presence of indomethacin or N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH). Inhibition of cytochrome P-450 (CYP) epoxygenase with MS-PPOH blocked the effect of CHA on ENaC. In contrast, CHA reduced ENaC activity in the presence of indomethacin. This suggests that CYP epoxygenase-dependent metabolites of AA mediate the effect of adenosine. Because 11,12-epoxyeicosatrienoic acid (11,12-EET) inhibits ENaC activity in the CCD (Wei Y, Lin DH, Kemp R, Yaddanapudi GSS, Nasjletti A, Falck JR, and Wang WH. J Gen Physiol 124: 719–727, 2004), we examined the role of 11,12-EET in mediating the effect of adenosine on ENaC. Addition of 11,12-EET inhibited ENaC channels in the CCD in which adenosine-induced inhibition was blocked by AACOCF3. We conclude that adenosine inhibits ENaC activity by stimulation of the A1adenosine receptor in the CCD and that the effect of adenosine is mediated by 11,12-EET.

Activation of receptors negatively coupled to adenylate cyclase is required for induction of long-term synaptic depression at Schaffer collateral-CA1 synapses

Chemical LTD (CLTD) of synaptic transmission is triggered by simultaneously increasing presynaptic [cGMP] while inhibiting PKA. Here, we supply evidence that class II, but not III, metabotropic glutamate receptors (mGluRs), and A1 adenosine receptors, both negatively coupled to adenylate cyclase, play physiologic roles in providing PKA inhibition necessary to promote the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Simultaneous activation of group II mGluRs with the selective agonist (2S,2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl) glycine (DCGIV; 5 microM), while raising [cGMP] with the type V phosphodiesterase inhibitor, zaprinast (20 microM), resulted in a long-lasting depression of synaptic strength. When zaprinast (20 microM) was combined with a cell-permeant PKA inhibitor H-89 (10 microM), the need for mGluR IIs was bypassed. DCGIV, when combined with a "submaximal" low frequency stimulation (1 Hz/400 s), produced a saturating LTD. The mGluR II selective antagonist, (2S)-alpha-ethylglutamic acid (EGLU; 5 microM), blocked induction of LTD by prolonged low frequency stimulation (1 Hz/900 s). In contrast, the mGluR III selective receptor blocker, (RS)-a-Cyclopropyl-[3- 3H]-4-phosphonophenylglycine (CPPG; 10 microM), did not impair LTD. The selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM), also blocked induction of LTD, while the adenosine A1 receptor agonist N6-cyclohexyl adenosine (CHA; 50 nM) significantly enhanced the magnitude of LTD induced by submaximal LFS and, when paired with zaprinast (20 microM), was sufficient to elicit CLTD. Inhibition of PKA with H-89 rescued the expression of LTD in the presence of either EGLU or DPCPX, confirming the hypothesis that both group II mGluRs and A1 adenosine receptors enhance the induction of LTD by inhibiting adenylate cyclase and reducing PKA activity.

2-Aminothiophene-3-carboxylates and carboxamides as adenosine A1 receptor allosteric enhancers

Three series of 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene and 2-amino-5,6,7,8-tetrahydrocyclohepta[b]thiophenes with 3-carboxylates and carboxamides have been prepared using the Gewald synthesis and evaluated as A(1)AR allosteric enhancers. The structure-activity relationships of these classes of compound are described. A number of compounds, notably 7b, are more potent and efficacious than PD81,723 (1).

Adenosine in the airways: implications and applications

Adenosine in a signaling nucleoside eliciting many physiological responses. Elevated levels of adenosine have been found in bronchoalveolar lavage, blood and exhaled breath condensate of patients with asthma a condition characterized by chronic airway inflammation. In addition, inhaled adenosine-5'-monophosphate induces bronchoconstriction in asthmatics but not in normal subjects. Studies on animals and humans have shown that bronchoconstriction is most likely due to the release of inflammatory mediators from mast cells. However a number of evidences suggest that adenosine modulates the function of many other cells involved in airway inflammation such as neutrophils, eosinophils, lymphocytes and macrophages. Although this clear pro-inflammatory role in the airways, adenosine may activate also protective mechanisms particularly against lung injury. For many years this dual role of adenosine in the respiratory system has represented an enigma, and only recently it has become clear that biological functions of adenosine are mediated by four distinct subtypes of receptors (A1, A2A, A2B, and A3) and that biological responses are determined by the different pattern of receptors distribution in specific cells. Therefore, pharmacological modulation of adenosine receptors, particularly A2B, may represent a novel therapeutic approach for inflammatory diseases. Moreover, as bronchial response to adenosine strictly reflects airway inflammation in asthma, bronchial challenge with adenosine is considered a valuable clinical tool to monitor airway inflammation, to follow the response to anti-inflammatory treatments and to help in the diagnostic discrimination between asthma and chronic obstructive lung disease.