Critical View on the Usage of Ribavirin in Already Existing Psychostimulant-Use Disorder

Substance-use disorder represents a frequently hidden non-communicable chronic disease. Patients with intravenous drug addiction are at high risk of direct exposure to a variety of viral infections and are considered to be the largest subpopulation infected with the hepatitis C virus. Ribavirin is a synthetic nucleoside analog that has been used as an integral component of hepatitis C therapy. However, ribavirin medication is quite often associated with pronounced psychiatric adverse effects. It is not well understood to what extent ribavirin per se contributes to changes in drug-related neurobehavioral disturbances, especially in the case of psychostimulant drugs, such as amphetamine. It is now well-known that repeated amphetamine usage produces psychosis in humans and behavioral sensitization in animals. On the other hand, ribavirin has an affinity for adenosine A1 receptors that antagonistically modulate the activity of dopamine D1 receptors, which play a critical role in the development of behavioral sensitization. This review will focus on the current knowledge of neurochemical/ neurobiological changes that exist in the psychostimulant drug-addicted brain itself and the antipsychotic-like efficiency of adenosine agonists. Particular attention will be paid to the potential side effects of ribavirin therapy, and the opportunities and challenges related to its application in already existing psychostimulant-use disorder.

Glaucoma Drugs in the Pipeline

Glaucoma is a chronic disease that can be challenging to treat for both patients and physicians. Most patients will require more than 1 medication over time to maintain their intraocular pressure (IOP) at a physiologically benign level. Patients may become refractory to existing compounds and many struggle with adherence to multiple topical drop regimens. The field of glaucoma therapeutics has been advancing rapidly with an emphasis on compounds comprising multiple molecules/mechanisms of action that offer additivity and are complementary to current therapeutics. Several new topical drop compounds directly targeting the trabecular meshwork (TM)/Schlemm canal/conventional outflow pathway to reduce outflow resistance have obtained US Food and Drug Administration approval in the past year. These include rho kinase inhibitors and nitric oxide donating compounds. Alternative therapies that offer long-term IOP lowering while removing the patient from the drug delivery system are moving forward in development. These include gene therapy and stem cell strategies, which could ease or eliminate the burden of topical drop self-administration for several years. Additionally, a variety of novel formulations and devices are in development that aim for controlled, steady state delivery of therapeutics over periods of months. The future of glaucoma therapy is focusing on an increase in specificity for the individual patient: their type of glaucoma; underlying mechanisms; genetic make-up; comorbid conditions; and rate of progression. Maintaining functional vision and improving patient outcomes remains the goal in glaucoma therapeutics. The current collection of novel therapeutics offers an expanded set of tools to achieve that goal.

[The receptorial responsiveness method (RRM): a new possibility to estimate the concentration of pharmacologic agonists at their receptors]

Cardiovascular disease is the biggest challenge in terms of life expectancy in developed countries. Adenosine contributes to the adaptation of the heart to ischemia and hypoxia, because adenosine, in addition to its metabolite role in the nucleic acid metabolism, is the endogenous agonist of the ubiquitous adenosine receptor family. Adenosine receptor activation is beneficial in most cases, it improves the balance between energy supply and consumption, reduces injury caused by stressors and inhibits the unfavorable tissue remodeling. Pharmacological manipulation of cardioprotective effects evoked by adenosine is an important, although to date not sufficiently utilized endeavor that may have therapeutic and preventive implications in cardiovascular diseases. As the ligand binding site of adenosine receptors is accessible from the extracellular space, it is especially important to know the adenosine concentration of the interstitial fluid ([Ado](ISF)). However, in the functioning heart, [Ado](ISF) values range in an extremely wide interval, spanning from nano- to micromolar concentrations, as estimated by the commonly used methods. Our recently developed procedure, the receptorial responsiveness method (RRM), may resolve this problem in certain cases. RRM enables quantification of an acute increase in the concentration of a pharmacological agonist, uniquely in the microenvironment of the receptors of the given agonist. As a limitation, concentration of agonists with short half-life (just like adenosine) at their receptors can only be quantified with the equieffective concentration of a stable agonist exerting the same action. In a previous study using RRM, inhibition of the transmembrane nucleoside transport in the euthyroid guinea pig atrium produced an increase in [Ado](ISF) that was equieffective with 18.8 +/- 3 nM CPA (N6-cyclopentyladenosine, a stable, selective A1 adenosine receptor agonist). This finding is consistent with observations of others, i.e., in the normoxic heart, adenosine flow is directed into the cell interior, and thus transport blockade elevates the extracellular adenosine level. In turn, nucleoside transport inhibition in the hyperthyroid guinea pig atrium caused a rise in [Ado](ISF) equieffective with 46.5 +/- 13.7 nM CPA. In sum, our work team was the first to demonstrate that adenosine transport in the hyperthyroid atrium has the same direction but is more intense as/than that in the euthyroid one.

Current antiplatelet options for NSTE-ACS patients

Non-ST elevation (NSTE) myocardial infarction and unstable angina are the most common clinical presentations of acute coronary syndrome (ACS). Platelet activation is central to the pathogenesis of NSTE-ACS and consensus guidelines that advocate early revascularization supported by intensive antiplatelet therapy. This review examines the drugs used concurrently with aspirin as dual antiplatelet therapy in the NSTE-ACS setting. Clopidogrel represented an important therapeutic advance. However, variations in platelet response and a relatively slow onset of action compromise outcomes with clopidogrel. Evidence reviewed in this article shows that in NSTE-ACS patients, ticagrelor and prasugrel are more effective than clopidogrel and are relatively well tolerated, with an acceptable and manageable bleeding risk. The literature suggests several differences between ticagrelor and prasugrel that should allow clinicians to better tailor treatment to the patient. Head-to-head comparisons are now needed to compare directly the risks and benefits of ticagrelor and prasugrel in NSTE-ACS. Further studies also need to address other outstanding issues such as the benefits and risks of prasugrel pre-treatment and to stratify efficacy and tolerability according to diabetes mellitus (DM) and other co-morbidities. In the meantime, the issues discussed in this review should enhance clinicians' ability to optimize and individualize NSTE-ACS treatment, thereby further reducing the morbidity and mortality associated with this common cardiovascular condition.

[Adenosine alleviates hypoxia-induced rat right ventricular hypertrophy through the NHE-1/CaN signal pathway]

OBJECTIVE

To investigate the effect of adenosine and its agonist on hypoxia-induced right ventricular hypertrophy (RVH) and explore the underlying mechanism.

METHODS

Fifty-six rats were randomly divided into normoxia group, hypoxia group, and treated hypoxia groups (with different treatments with adenosine, A1 receptor agonist CPA, A2 receptor agonist NECA, CPA plus A1 receptor inhibitor DPCPX, or NECA plus A2B receptor inhibitor MRS1754). The rats except for those in normoxia group were exposed to normobaric chronic hypoxia (9.5%-10.5% oxygen) for 21 days, and the corresponding treatments were administered since the 7th day of hypoxia till day 21 via implantable capsule with a pressure pump. After the treatments, the right ventricles were then removed and weighed for evaluation of hypertrophy, and the expressions of NHE-1 and CnAβ mRNA in the myocardial tissue were detected using RT-PCR.

RESULTS

After a 21-day hypoxia, the rats showed significantly increased RV/(LV+S) ratio (0.369∓0.033) and RV/BW ratio (0.75∓0.095) compared to those in normoxia group (0.271∓0.010 and 0.59∓0.039, respectively; P<0.001), adenosine treatment group (0.281∓0.022 and 0.65∓0.077, respectively; P<0.001, P=0.025), hypoxia with CPA group (0.313∓0.021 and 0.66∓0.067, respectively P<0.001), and hypoxia with NECA group(0.333∓0.019, and 0.68∓0.074, respectively P<0.001). The NHE-1 and CnAβ mRNA levels in hypoxia group were significantly higher than those in normoxia group, adenosine treatment group, hypoxia with CPA group, and hypoxia with NECA group(P<0.001).

CONCLUSION

Adenosine and its agonist can inhibit hypoxia-induced RVH in rats through the NHE-1/CaN signal pathway.

Estimation of endogenous adenosine activity at adenosine receptors in guinea-pig ileum using a new pharmacological method

AIM

Adenosine modulates neurotransmission and in the intestine adenosine is continuously released both from nerves and from smooth muscle. The main effect is modulation of contractile activity by inhibition of neurotransmitter release and by direct smooth muscle relaxation. Estimation of adenosine concentration at the receptors is difficult due to metabolic inactivation. We hypothesized that endogenous adenosine concentrations can be calculated by using adenosine receptor antagonist and agonist and dose ratio (DR) equations.

METHODS

Plexus-containing guinea-pig ileum longitudinal smooth muscle preparations were made to contract intermittently by electrical field stimulation in organ baths. Schild plot regressions were constructed with 2-chloroadenosine (agonist) and 8-(p-sulfophenyl)theophylline (8-PST; antagonist). In separate experiments the reversing or enhancing effect of 8-PST and the inhibiting effect of 2-chloroadenosine (CADO) were analysed in the absence or presence of an adenosine uptake inhibitor (dilazep), and nucleoside overflow was measured by HPLC.

RESULTS

Using the obtained DR, baseline adenosine concentration was calculated to 28 nm expressed as CADO activity, which increased dose dependently after addition of 10(-6) m dilazep to 150 nm (P < 0.05). HPLC measurements yielded a lower fractional increment (80%) in adenosine during dilazep, than found in the pharmacological determination (440%).

CONCLUSION

Endogenous adenosine is an important modulator of intestinal neuro-effector activity, operating in the linear part of the dose-response curve. Other adenosine-like agonists might contribute to neuromodulation and the derived formulas can be used to calculate endogenous agonist activity, which is markedly affected by nucleoside uptake inhibition. The method described should be suitable for other endogenous signalling molecules in many biological systems.

Na-Tosyl-Phe chloromethyl ketone prevents granule movement and mast cell synergistic degranulation elicited by costimulation of antigen and adenosine

Adenosine has been shown to enhance mast cell degranulation when added together with an antigen. Such augmentation of mast cell activation is relevant to exacerbation of allergic asthma symptoms. Na-Tosyl-Phe chloromethyl ketone (TPCK) is a chymotrypsine-like chymase inhibitor, which has anti-inflammatory properties. In this study, we investigated the effects of TPCK on mast cell synergistic degranulation induced by antigen and adenosine. Here, we report that TPCK almost completely suppressed enhanced degranulation by inhibiting granule movement. Consistent with this, intraperitoneal administration of TPCK resulted in significant amelioration of passive cutaneous anaphylaxis in mice. Furthermore, we demonstrated that TPCK completely inhibited Thr308 phosphorylation of protein kinase B in mast cells stimulated with antigen and adenosine. These results provide a novel action of TPCK for the prevention of mast cell degranulation induced by antigen and adenosine.

Adenosine and Catecholamine Agonists Speed the Flagellar Beat of Mammalian Sperm by a Non-Receptor-Mediated Mechanism1

Activation of rapid motility apparently is one of the first steps of sperm capacitation and can be studied in vitro. Previously we found that 2-chloro-2'-deoxyadenosine or the catecholamine isoproterenol activates mouse sperm motility in vitro via a pathway mediated by cAMP that requires extracellular Ca2+, the atypical sperm adenylyl cyclase, and sperm-specific protein kinase A. We now show that several other adenosine analogs and catecholamines accelerate the flagellar beat of mouse and human sperm. Unexpectedly, the potent adenosine receptor agonist CGS21680 does not accelerate the beat, and the adenosine receptor antagonist DPCPX does not diminish the accelerating action of 2-chloro-2'-deoxyadenosine. The pharmacological profile for activation by catecholamines is also unusual. Both agonists and antagonists of beta-adrenergic receptors elevate the beat frequency. Moreover, both l-(-) and d-+ isomers of epinephrine, norepinephrine, and isoproterenol produce similar acceleration of the beat. In contrast, inhibitors of equilibrative nucleoside transporters effectively slow the onset of the accelerating action of adenosine analogs. Replacement of external Na+ with Li+ also diminishes the accumulation of cAMP and slows the resultant accelerating action of 2-chloro-2'-deoxyadenosine, suggesting the involvement of a Na+-dependent concentrative nucleoside transporter. Our results show that adenosine and catecholamine agonists act in a novel signaling pathway that does not involve G protein-coupled cell-surface receptors that link to conventional adenylyl cyclases. Instead, adenosine and analogs may be transported into sperm via equilibrative and concentrative nucleoside transporters to act on unknown intracellular targets.

Emerging drugs in neuropathic pain

Neuropathic pain is a personally devastating and costly condition affecting 3-8% of the population. Existing treatments have limited effectiveness and produce relatively frequent adverse effects. Preclinical research has identified many promising pharmacological targets; however, reliable predictors of success in humans remain elusive. At least 50 new molecular entities have reached clinical development including: glutamate antagonists, cytokine inhibitors, vanilloid-receptor agonists, catecholamine modulators, ion-channel blockers, anticonvulsants, opioids, cannabinoids, COX inhibitors, acteylcholine modulators, adenosine receptor agonists and several miscellaneous drugs. Eight drugs are in Phase III trials at present. Strategies that may show promise over existing treatments include topical therapies, analgesic combinations and, in future, gene-related therapies. Recent years have heralded an explosion of pharmaceutical development in neuropathic pain, reflecting advanced knowledge of neurobiology and a heightened perception of the commercial value of neuropathic pain therapeutics. In the interest of improving patient care, the authors recommend implementing comparative studies throughout the development process in order to demonstrate the increased value of novel agents.

Effect of a specific and selective A(2B) adenosine receptor antagonist on adenosine agonist AMP and allergen-induced airway responsiveness and cellular influx in a mouse model of asthma

It has been previously proposed that adenosine plays an important role in the pathogenesis of asthma. The proposed mechanism of action for nucleoside adenosine is to activate A(2B) adenosine receptors (AR) and to indirectly modulate levels of mediators in the lung. In vivo data supporting the role of A(2B) AR in airway reactivity and inflammation in allergic animal models are lacking. The present study describes the effects of a selective A(2B) AR antagonist, CVT-6883 [3-ethyl-1-propyl-8-[1-(3-trifluoromethylbenzyl)-1H-pyrazol-4-yl]-3,7-dihydropurine-2,6-dione], on airway reactivity and inflammation in an allergic mouse model of asthma. Mice were sensitized with ragweed (i.p.) on days 1 and 6 and challenged with 0.5% ragweed on days 11, 12, and 13. On day 14, airway reactivity to 5'-N-ethylcarboxamidoadenosine (NECA), AMP, or allergen challenge was measured in terms of enhanced pause (Penh). Aerosolized NECA elicited concentration-dependent increases in Penh, which were significantly attenuated by CVT-6883 (0.4, 1.0, or 2.5 mg/kg i.p.). Aerosolized AMP elicited significant increases in Penh in sensitized mice, and the effect was significantly attenuated by either CVT-6883 (1 mg/kg i.p.) or montelukast (1 mg/kg i.p.). Allergen challenge induced late allergic response in sensitized mice, which was inhibited by CVT-6883 (1 mg/kg i.p.). Allergen challenge also increased the number of cells in bronchoalveolar lavage fluid obtained from sensitized mice, and that was reduced by either CVT-6883 (6 mg/ml aerosolization for 5 min) or theophylline (36 mg/ml aerosolization for 5 min). These results suggest that A(2B)AR antagonism plays an important role in inhibition of airway reactivity and inflammation in this model of allergic asthma.

Cloning and pharmacological characterization of the equine adenosine A2A receptor: a potential therapeutic target for the treatment of equine endotoxemia

The aim of the current study was to clone the equine adenosine A(2A) receptor gene and to establish a heterologous expression system to ascertain its pharmacologic profile via radioligand binding and functional assays. An eA(2A)-R expression construct was generated by ligation of the eA(2A) cDNA into the pcDNA3.1 expression vector, and stably transfected into human embryonic kidney cells (HEK). Binding assays identified those clones expressing the eA(2A)-R, and equilibrium saturation isotherm experiments were utilized to determine dissociation constants (K(D)), and receptor densities (B(max)) of selected clones. Equilibrium competition binding revealed a rank order of agonist potency of ATL > CV-1808 > NECA > 2-CADO > CGS21680, and a rank order of antagonist potency as ZM241385 > 8-phenyltheophylline > p-sulfophenyltheophylline > caffeine. Furthermore, adenylate cyclase assays using selective A(2A)-R agonists revealed that the eA(2A)-R functionally coupled to Galpha(s) as indicated by an increase in intracellular [(3)H]cAMP upon receptor activation. Finally, NF-kappaB reporter gene assays revealed a CGS21680 concentration-dependent inhibition of NF-kappaB activity. These results indicate that the heterologously expressed eA(2A)-R has a pharmacological profile similar to that of other mammalian A(2A) receptors and thus can be utilized for further characterization of the eA(2A)-R to ascertain whether it can serve as a suitable pharmacological target for equine inflammatory disease.

Cloning and pharmacological characterization of the equine adenosine A3 receptor

The aim of this study was to establish a heterologous expression system for the equine adenosine A(3) receptor (eA(3)-R) in an effort to ascertain its pharmacologic profile. Initially, radioligand binding assays identified clones expressing the eA(3)-R in human embryonic kidney cells (HEK) based on the specific binding of [(125)I]AB-MECA. Subsequently, adenylate cyclase assays were utilized to demonstrate functional coupling of the eA(3)-R to the G-protein/adenylate cyclase system. Equilibrium competition binding assays were then performed using selective and non-selective A(3) agonists and antagonists. Results from these experiments revealed a rank order of agonist potency to be IB-MECA > NECA > CGS21680, and an antagonist potency of MRS1220 > ZM241385 > 8-p-sulphophenyltheophylline; these rank orders were in agreement with that of other mammalian A(3)-R's. Lastly, NF-kappaB reporter gene assays revealed an IB-MECA concentration-dependent inhibition of TNFalpha-stimulated NF-kappaB activity. These results indicate that the heterologously expressed eA(3)-R is functional, has a pharmacological profile similar to that of other mammalian A(3) receptors, and its activation has an inhibitory effect on a key regulatory pathway in the inflammatory response. Thus, the eA(3)-R may serve as a pharmacological target in the treatment of equine inflammatory disease.

Postconditioning protects rabbit hearts through a protein kinase C-adenosine A2b receptor cascade

OBJECTIVE

Ischemic postconditioning protects the reperfused heart from infarction, and this protection is dependent on the occupancy of adenosine receptors. We further explored the role of adenosine receptors in this salvage.

METHODS

In situ rabbit hearts underwent 30 min of regional ischemia and 3 h of reperfusion, and postconditioning was effected with four cycles of 30-s reperfusion/30-s coronary artery occlusion at the end of ischemia.

RESULTS

Postconditioning reduced infarct size from 40.2+/-3.4% of the risk zone in untreated hearts to 15.5+/-2.5%. Protection by postconditioning was blocked by either the non-selective adenosine receptor blocker 8-p-(sulfophenyl)theophylline or the A2b-selective antagonist MRS 1754, injected intravenously 5 min before reperfusion. The protein kinase C (PKC) antagonist chelerythrine also aborted postconditioning's salvage, indicating a PKC-dependent mechanism. Neither the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine nor the A2a-selective antagonist 8-(13-chlorostyryl)caffeine had an effect on protection. The non-selective but A2b-potent adenosine agonist 5'-(N-ethylcarboxamido)adenosine (NECA) infused from 5 min before to 1h after reperfusion mimicked postconditioning's effect on infarct size (17.2+/-2.7% infarction) and MRS 1754 blocked the NECA-induced cardioprotection, confirming that A2b activation was protective. The PKC activator phorbol 12-myristate 13-acetate delivered just before reperfusion also duplicated the protective effect of postconditioning (16.3+/-4.1% infarction), and co-administration of the PKC antagonist chelerythrine aborted PMA's protection, confirming that the protection was the result of PKC activation. NECA's protective effect was not affected by chelerythrine, but rather MRS 1754 blocked PMA's salutary effect (42.8+/-1.0% infarction), suggesting that the A2b receptor's effect is under control of PKC. Finally, wortmannin, a blocker of phosphatidylinositol 3-kinase, also abrogated protection by PMA.

CONCLUSIONS

Salvage of ischemic myocardium by postconditioning is dependent on activation of A2b receptors, which in turn depends on activation of PKC. It is still unclear why PKC activation is required to make the heart's adenosine become protective.

Human osteoblast precursors produce extracellular adenosine, which modulates their secretion of IL-6 and osteoprotegerin

We showed that human osteoprogenitor cells produced adenosine and expressed ecto-5'-nucleotidase and all four adenosine receptor subtypes. Adenosine stimulated IL-6 but inhibited osteoprotegerin secretion, suggesting that adenosine is a newly described regulator of progenitor cell function.

INTRODUCTION

Maintaining skeletal homeostasis relies on there being a balance between bone formation and resorption; an imbalance between these processes can lead to diseases such as osteoporosis and rheumatoid arthritis. Recent reports showed that locally produced ATP, acting through P2 receptors, has pronounced effects on bone formation. However, ATP can be enzymatically cleaved to adenosine that has little or no activity at P2 receptors but mediates its action through the P1 family of receptors. We studied whether adenosine may also have an important role in controlling bone cell differentiation and function.

MATERIALS AND METHODS

Extracellular adenosine levels were analyzed by high-performance liquid chromatography in HCC1 and bone marrow stromal (BMS) cells. Ecto-5'-nucleotidase (CD73) expression and activity was determined by RT-PCR, immunocytochemistry, and the cleavage of etheno-AMP to ethenoadenosine. Adenosine receptor expression and activity were determined by RT-PCR and cAMP measurements. The effects of adenosine receptor agonists on IL-6, osteoprotegerin (OPG), and RANKL expression were determined by ELISA and QRT-PCR.

RESULTS

HCC1 and BMS cells produce adenosine and express CD73 and all four adenosine receptor subtypes. The A2b receptor was shown to be functionally dominant in HCC1 cells, as determined by cAMP production and in its stimulation of IL-6 secretion. Adenosine receptor agonism also inhibited OPG secretion and OPG but not RANKL mRNA expression.

CONCLUSIONS

Our findings show that HCC1 and primary BMS cells produce adenosine, express CD73 and all four adenosine receptor subtypes. In HCC1 cells, adenosine has a potent stimulatory action on IL-6 secretion but an inhibitory action on OPG expression. These data show for the first time that adenosine may be an important regulator of progenitor cell differentiation and hence an important local contributor to the regulation of bone formation and resorption.

Signaling pathways for modulation of mouse sperm motility by adenosine and catecholamine agonists

Capacitation of mammalian sperm, including alterations in flagellar motility, is presumably modulated by chemical signals encountered in the female reproductive tract. This work investigates signaling pathways for adenosine and catecholamine agonists that stimulate sperm kinetic activity. We show that 2-chloro-2'-deoxyadenosine and isoproterenol robustly accelerate flagellar beat frequency with EC50s near 10 and 0.05 microM, respectively. The several-fold acceleration is maximal by 60 sec. Although extracellular Ca2+ is required for agonist action on the flagellar beat, agonist treatment does not elevate sperm cytosolic [Ca2+] but does increase cAMP content. Acceleration does not require the conventional transmembrane adenylyl cyclase ADCY3, since it persists in sperm of ADCY3 knockout mice and in wild-type sperm in the presence of the inhibitors of conventional adenylyl cyclases SQ-22536, MDL-12330A, or 2', 5'-dideoxyadenosine. In contrast, the acceleration by these agents is absent in sperm that lack the predominant atypical adenylyl cyclase, SACY. Responses to these agonists are also absent in sperm from mice lacking the sperm-specific Calpha2 catalytic subunit of protein kinase A (PRKACA). Agonist responses also are strongly suppressed in wild-type sperm by the protein kinase inhibitor H-89. These results show that adenosine and catecholamine analogs activate sperm motility by mechanisms that require extracellular Ca2+, the atypical sperm adenylyl cyclase, cAMP, and protein kinase A.

A neoceptor approach to unraveling microscopic interactions between the human A2A adenosine receptor and its agonists

Strategically mutated neoceptors, e.g., with anionic residues in TMs 3 and 7 intended for pairing with positively charged amine-modified nucleosides, were derived from the antiinflammatory A(2A) adenosine receptor (AR). Adenosine derivatives functionalized at the 5', 2, and N(6) positions were synthesized. The T88D mutation selectively enhanced the binding of the chain-length-optimized 5'-(2-aminoethyl)uronamide but not 5'-(2-hydroxyethyl)uronamide, suggesting a critical role of the positively charged amine. Combination of this modification with the N(6)-(2-methylbenzyl) group enhanced affinity at the Q89D- and N181D- but not the T88D-A(2A)AR. Amino groups placed near the 2- or N(6)-position only slightly affected the binding to mutant receptors. The 5'-hydrazide MRS3412 was 670- and 161-fold enhanced, in binding and functionally, respectively, at the Q89D-A(2A)AR compared to the wild-type. Thus, we identified and modeled pairs of A(2A)AR-derived neoceptor-neoligand, which are pharmacologically orthogonal with respect to the native species.

Effects of adenosine agonist R-phenylisopropyl-adenosine on halothane anesthesia and antinociception in rats

AIM

To investigate the antinociceptive effect of adenosine agonist R-phenylisopropyl-adenosine (R-PIA) given to conscious rats by intracerebroventricular (ICV) and intrathecal (IT), and identify the effect of R-PIA on minimum alveolar concentration (MAC) of halothane with pretreatment of A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or K+ channel blocker 4-aminopyridine (4-AP).

METHODS

Sprague-Dawley rats were implanted with 24-gauge stainless steel guide cannula using stereotaxic apparatus and ICV method, and an IT catheter (PE-10, 8.5 cm) was inserted into the lumbar subarachnoid space, while the rats were under pentobarbital anesthesia. After one week of recovery from surgery, rats were randomly assigned to one of the following protocols: MAC of halothane, or tail-flick latency. All measurements were performed after R-PIA (0.8-2.0 microg) microinjection into ICV and IT with or without pretreatment of DPCPX or 4-AP.

RESULTS

Microinjection of adenosine agonist R-PIA in doses of 0.8-2.0 microg into ICV and IT produced a significant dose- and time-dependent antinociceptive action as reflected by increasing latency times and ICV administration of adenosine agonist R-PIA (0.8 microg) reducing halothane anesthetic requirements (by 29%). The antinociception and reducing halothane requirements effected by adenosine agonist R-PIA was abolished by DPCPX and 4-AP.

CONCLUSION

ICV and IT administration of adenosine agonist R-PIA produced an antinociceptive effect in a dose-dependent manner and decreased halothane MAC with painful stimulation through activation of A1 receptor subtype, and the underlying mechanism involves K+ channel activation.

ATP-sensitive potassium channels and endogenous adenosine are involved in spinal antinociception produced by locus coeruleus stimulation

The effects of locus coeruleus stimulation on nociceptive evoked discharges of thalamic parafascicular (PF) neurons were investigated in lightly urethane-anesthetized rats, aiming to study the mechanisms underlying these effects. Intrathecal (i.t.) administration of aminophylline (an adenosine antagonist), glibenclamide (an ATP-sensitive potassium [K+(ATP)] channels blocker), nicrorandil (Nico; an agonist of K+(ATP) channel and a K+(ATP) channel opener), and 5'-N-ethylcarboxamido-adenosine (NECA; an adenosine agonist) were used. The results showed that (1) locus coeruleus stimulation significantly inhibited the nociceptive evoked discharges of parafascicular neurons, (2) locus coeruleus stimulation-produced antinociception in PF neurons was blocked by both it. glibenclamide and i.t. aminophylline, (3) nociceptive discharges of PF neurons were also suppressed by both i.t. NECA and i.t. nicorandil, and (4) i.t. glibenclamide showed no effect on the suppression of nociceptive discharges induced by NECA, whereas aminophylline blocked the suppression of nociceptive discharges induced by nicorandil. These results suggest that (a) K+(ATP) channels and endogenous adenosine may be involved in the mediation of antinociception induced by norepinephrine, which is released in the dorsal horn by descending fibers originating from the locus coeruleus and (b) the opening of K+(ATP) channels may precede the release of endogenous adenosine in the process of suppressing nociceptive transmission at the spinal level.

Kinetic and functional properties of [3H]ZM241385, a high affinity antagonist for adenosine A2A receptors

We have characterized the binding of [2-(3)H]-4-(2-[7-Amino-2-(2-furyl)-[1,2,4]-triazolo-[2,3-a]-[1,3,5]-triazin-5-ylamino]ethyl)phenol ([(3)H]ZM241385) to adenosine A(2A) receptors in membranes of rat striatum and transfected CHO cells. Saturation experiments showed that [(3)H]ZM241385 binds to a single class of binding sites with high affinity (K(d) = 0.23 nM and 0.14 nM in CHO cell and striatal membranes, respectively). The membranes of CHO cells required pretreatment with adenosine deaminase (ADA) to achieve high-affinity binding, while ADA had no influence on the ligand binding properties in striatal membranes. The binding of [(3)H]ZM241385 was fast and reversible, achieving equilibrium within 20 minutes at all radioligand concentrations. The kinetic analysis of the [(3)H]ZM241385 interaction with A(2A) receptors indicated that the reaction had at least two subsequent steps. The first step corresponds to a fast equilibrium, which also determines the antagonist potency to competitively inhibit CGS21680-induced accumulation of cAMP (first equilibrium constant K(A) = 6.6 nM). The second step corresponds to a slow process of conformational isomerization (equilibrium constant K(i) = 0.03). The combination of the two steps gives the dissociation constant K(d) = 0.20 nM based on the kinetic data, which is in good agreement with the directly measured value. The data obtained shed light on the mechanism of the [(3)H]ZM241385 interaction with adenosine A(2A) receptors from different sources in vitro. The isomerization step of the A(2A) antagonist radioligand binding has to be taken into account for the interpretation of the binding parameters obtained from the various competition assays and explain the discrepancy between antagonist affinity in saturation experiments versus its potency in functional assays.

Adenosine A1-receptor modulation of glutamate-induced calcium influx in rat retinal ganglion cells

PURPOSE

Although adenosine receptors (A(1)-Rs and A(2)-Rs) have been identified in the mammalian retina, the role of adenosine in this tissue is not fully understood. The purpose of this work was to investigate the action of adenosine on glutamate-induced calcium influx in rat retinal ganglion cells (RGCs) and to determine whether adenosine modulates RGC voltage-gated calcium channels.

METHODS

Purified RGC cultures were generated from neonatal rats with a two-step panning procedure. Isolated RGCs were loaded with the ratiometric calcium-indicator dye fura-2, and the effect of adenosine (and related agonists and antagonists) on intracellular calcium levels ([Ca(2+)](i)) during exposure to glutamate (10 microM with 10 microM glycine) was assessed. The effect of adenosine on calcium channel currents was also studied in isolated RGCs with whole-cell patch-clamp techniques. In addition, the effect of adenosine on [Ca(2+)](i) was investigated in fura dextran-loaded RGCs in an intact adult rat retina preparation.

RESULTS

In isolated RGCs, adenosine (10 and 100 microM) significantly reduced the glutamate-induced increase in [Ca(2+)](i) ( approximately 30%). The effect of adenosine was blocked by the A(1)-R antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not by the A(2)-R antagonist 3,7-dimethyl-1-propargylxanthine (DMPX). Adenosine (10 microM) inhibited calcium channel currents by 43%, and again this effect was blocked by DPCPX, but not DMPX. Adenosine (100 microM) also significantly reduced the elevation of [Ca(2+)](i) in RGCs in the intact retina during exposure to N-methyl-d-aspartate (NMDA; 100 microM).

CONCLUSIONS

Adenosine can inhibit glutamate-induced calcium influx and voltage-gated calcium currents in rat RGCs through A(1)-R activation. This work supports a role for adenosine as a neuromodulator of mammalian RGCs.

Modulation of adenosine receptor affinity and intrinsic efficacy in adenine nucleosides substituted at the 2-position

We studied the structural determinants of binding affinity and efficacy of adenosine receptor (AR) agonists. Substituents at the 2-position of adenosine were combined with N(6)-substitutions known to enhance human A(3)AR affinity. Selectivity of binding of the analogues and their functional effects on cAMP production were studied using recombinant human A(1), A(2A), A(2B), and A(3)ARs. Mainly sterically small substituents at the 2-position modulated both the affinity and intrinsic efficacy at all subtypes. The 2-cyano group decreased hA(3)AR affinity and efficacy in the cases of N(6)-(3-iodobenzyl) and N(6)-(trans-2-phenyl-1-cyclopropyl), for which a full A(3)AR agonist was converted into a selective antagonist; the 2-cyano-N(6)-methyl analogue was a full A(3)AR agonist. The combination of N(6)-benzyl and various 2-substitutions (chloro, trifluoromethyl, and cyano) resulted in reduced efficacy at the A(1)AR. The environment surrounding the 2-position within the putative A(3)AR binding site was explored using rhodopsin-based homology modeling and ligand docking.

A TOPS-MODE approach to predict affinity for A1 adenosine receptors. 2-(Arylamino)adenosine analogues

The TOPological Sub-Structural Molecular Design (TOPS-MODE) approach has been applied to the study of the affinity of A(1) adenosine receptor of different 2-(arylamino)adenosine analogues. A model able to describe closed to 79% of the variance in the values for binding experiments of 32 analogues of these compounds through multilinear regression analysis (MRA) was developed with the use of the mentioned approach. In contrast, no one of seven different approaches, including the use of Constitutional, Topological, Molecular walk counts, BCUT, Randic Molecular profiles, Geometrical, and RDF descriptors was able to explain more than 70% of the variance in the mentioned property with the same number of descriptors. In addition, the TOPS-MODE approach permitted to find the contribution of different fragments to the biological property giving to the model a straightforward structural interpretability.

An agonist to the A3 adenosine receptor inhibits colon carcinoma growth in mice via modulation of GSK-3 beta and NF-kappa B

A(3) adenosine receptor (A(3)AR) activation with the specific agonist CF101 has been shown to inhibit the development of colon carcinoma growth in syngeneic and xenograft murine models. In the present study, we looked into the effect of CF101 on the molecular mechanisms involved in the inhibition of HCT-116 colon carcinoma in mice. In tumor lesions derived from CF101-treated mice, a decrease in the expression level of protein kinase A (PKA) and an increase in glycogen synthase kinase-3 beta (GSK-3 beta) was observed. This gave rise to downregulation of beta-catenin and its transcriptional gene products cyclin D1 and c-Myc. Further mechanistic studies in vitro revealed that these responses were counteracted by the selective A(3)AR antagonist MRS 1523 and by the GSK-3 beta inhibitors lithium and SB216763, confirming that the observed effects were A(3)AR and GSK-3 beta mediated. CF101 downregulated PKB/Akt expression level, resulting in a decrease in the level and DNA-binding capacity of NF-kappa B, both in vivo and in vitro. Furthermore, the PKA and PKB/Akt inhibitors H89 and Worthmannin mimicked the effect of CF101, supporting their involvement in mediating the response to the agonist. This is the first demonstration that A(3)AR activation induces colon carcinoma growth inhibition via the modulation of the key proteins GSK-3 beta and NF-kappa B.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

Adenosine suppresses the response of neurons to gaba in the superficial laminae of the rat spinal dorsal horn

With the nystatin-perforated whole-cell patch-clamp recording technique, the modulatory effects of adenosine on GABA-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (laminae I and II) of the rat spinal dorsal horn. The results showed that: (1) GABA acted on GABA(A) receptor and elicited inward Cl(-) currents (I(GABA)) at a holding potential (V(H)) of -40 mV; (2) adenosine suppressed GABA-induced Cl(-) current with affecting neither the reversal potential of I(GABA) nor the apparent affinity of GABA to its receptor; (3) N6-cyclo-hexyladenosine, a selective A(1) adenosine receptor agonist, mimicked the suppressing effect of adenosine on I(GABA), whereas 8-cyclopentyl-1,3-dipropylxanthine, a selective A(1) adenosine receptor antagonist, blocked the suppressing effect of adenosine; (4) chelerythrine, an inhibitor of protein kinase C, reduced the suppressing effect of adenosine on I(GABA); (5) pretreatment with 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester, a Ca(2+) chelator, did not affect adenosine-induced suppression of I(GABA). The results indicate that: (1) the suppression of adenosine on I(GABA) is mediated by adenosine A(1) receptor and through a Ca(2+)-independent protein kinase C transduction pathway; (2) the interactions between adenosine and GABA might be involved in the modulation of nociceptive information transmission at spinal cord level.

Role of endogenous adenosine in the acute and late response to allergen challenge in actively sensitized Brown Norway rats

1. Endogenous adenosine has been suggested to amplify the response of airway mast cells to allergen in vivo. We have sought evidence for this by monitoring the acute and late-phase response to allergen in Brown Norway (BN) rats actively sensitised to ovalbumin (OA) and treated either with adenosine deaminase (ADA) linked covalently to polyethylene glycol (PEG-ADA; Adagen) to decrease adenosine availability or with erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), an inhibitor of ADA, plus S-(4-nitrobenzyl)-6-thioinosine (NBTI), an inhibitor of facilitated adenosine transport, to increase adenosine availability. 2. The cardiovascular effects of adenosine (0.01-3 mg kg(-1) i.v.) were significantly reduced in PEG-ADA-treated animals and augmented in EHNA/NBTI-treated animals. The difference in sensitivity to adenosine in the treated groups was 33- and 15-fold, at the level of 30% reduction in blood pressure and heart rate, respectively. 3. The acute response to allergen, given either intravenously or intratracheally, was quantified as bronchoconstriction. The late phase to allergen was measured as the influx and activation of immunoinflammatory cells into the bronchoalveolar lavage fluid 24 h after challenge. 4. Despite evidence of a substantial difference in adenosine availability following pretreatment with PEG-ADA or EHNA/NBTI, there were no differences in either the acute or late response to allergen in the actively sensitised BN rat. 5. Our data suggest no role for endogenous adenosine in determining the response to allergen under our experimental conditions.

A randomized, double-blinded, placebo-controlled, dose-ranging study measuring the effect of an adenosine agonist on infarct size reduction in patients undergoing primary percutaneous transluminal coronary angioplasty: the ADMIRE (AmP579 Delivery for Myocardial Infarction REduction) study

BACKGROUND

Evidence suggests that myocardial ischemic preconditioning and reperfusion injury may be mediated by adenosine A1 and A2 receptors. AMP579 is a mixed adenosine agonist with both A1 and A2 effects. In animal models of acute myocardial infarction (MI), AMP579 reduced infarct size at serum levels of 15 to 24 ng/mL.

METHODS

The AMP579 Delivery for Myocardial Infarction REduction study evaluated AMP579 in a double-blind, multicenter, placebo-controlled trial of 311 patients undergoing primary percutaneous transluminal coronary angioplasty (PTCA) after acute ST-segment elevation MI. Patients were randomly assigned to placebo or to 3 different doses of AMP579 continuously infused over 6 hours. The primary end point was final MI size measured by technetium Tc-99m sestamibi scanning at 120 to 216 hours after PTCA. Secondary end points included myocardial salvage and salvage index at the same time interval (in a subset of patients who underwent baseline technetium Tc-99m sestamibi scan), left ventricular ejection fraction and heart failure at 4 to 6 weeks, duration of hospitalization, and cardiac events at 4 weeks and 6 months.

RESULTS

Final infarct size did not differ among the placebo group and the active treatment groups for either anterior MI or nonanterior MI. In patients with anterior MI, median myocardial salvage was increasingly higher in the groups receiving ascending dosages of AMP579 plus PTCA. Serum levels approaching levels shown to reduce infarct size in animal models were achieved only in the 60-mcg/kg treatment group.

CONCLUSION

AMP579 was safe at the doses tested, but it did not reduce infarct size. There was a trend toward greater myocardial salvage in treated patients with anterior MI.

Adenosine receptors and Huntington's disease: implications for pathogenesis and therapeutics

Huntington's disease (HD) is a devastating hereditary neurodegenerative disorder, the progression of which cannot be prevented by any neuroprotective approach, despite major advances in the understanding of its pathogenesis. The study of several animal models of the disease has led to the discovery of both loss-of-normal and gain-of-toxic functions of the mutated huntingtin protein and the elucidation of the mechanisms that underlie the formation of huntingtin aggregates and nuclear inclusions. Moreover, these models also provide good evidence of a role for excitotoxicity and mitochondrial metabolic impairments in striatal neuronal death. Adenosine has neuroprotective potential in both acute and chronic neurological disorders such as stroke or Parkinson's disease. Here we review experimental data on the role of A1 and A2A adenosine receptors in HD that warrant further investigation of the beneficial effects of A1 agonists and A2A antagonists in animal models of HD. Future pharmacological analysis of adenosine receptors could justify the use of A1 agonists and A2A antagonists for the treatment of HDin clinical trials.

A2B receptor activation promotes glycogen synthesis in astrocytes through modulation of gene expression

Adenosine has been proposed as a key factor regulating the metabolic balance between energy supply and demand in the central nervous system. Because astrocytes represent an important cellular element in the control of brain energy metabolism, we investigated whether adenosine could induce long-term changes of glycogen levels in primary cultures of mouse cortical astrocytes. We observed that adenosine increased glycogen content, up to 300%, in a time- (maximum at 8 h) and concentration-dependent manner with an EC(50) of 9.69 microM. Pharmacological experiments using the broad-spectrum agonist 5'-(N-ethylcarboxamido)adenosine (NECA) and specific agonists for the A(1), A(2A), and A(3) receptors [N(6)-cyclopentyladenosine (CPA), CGS-21680, and IB-MECA, respectively] suggest that the effect of adenosine is mediated through activation of the low-affinity A(2B) adenosine receptor subtype. Interestingly, adenosine induces in parallel the expression of the protein targeting to glycogen (PTG), one of the protein phosphatase-1 glycogen-targeting subunits that has been implicated in the control of glycogen levels in various tissues. These results indicate that adenosine can exert long-term control over glycogen levels in astrocytes and might therefore play a significant role in physiological and/or pathological processes involving long-term modulation of brain energy metabolism.

Adenosine stimulates anion secretion across cultured and native adult human vas deferens epithelia

Experiments were conducted to determine the responsiveness of human vas deferens epithelial cell monolayers to adenosine and related agonists. Human abdominal vas deferens epithelial cells have been isolated from adult tissues and grown to confluence on permeable supports. All cells exhibit intense ZO-1 and cytokeratin immunoreactivity. Cultured cell monolayers exhibit high electrical resistance with a lumen-negative potential difference and short circuit current (I(sc)) indicative of anion secretion and/or cation absorption. A portion of the basal I(sc) is inhibited by amiloride. Amiloride-sensitive I(sc) is enhanced by exposure to glucocorticoids and is Na(+) dependent, indicating the presence of epithelial sodium channel-mediated Na(+) absorption. Epithelial anion secretion and intracellular generation of cAMP are acutely stimulated by adenosine and the adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine (NECA), with these effects being fully blocked by 8-phenyltheophylline. Adenosine receptors are localized to the apical membrane of the epithelial cells, as basolateral adenosine is without effect. Freshly excised human vas deferens recapitulate observations made on cultured epithelia when evaluated with the self-referencing vibrating probe: amiloride inhibition of basal ion transport, stimulation by adenosine, and inhibition by 8-phenyltheophyline. These results demonstrate that adult human vas deferens epithelium actively transports ions to generate the luminal environment of the deferent duct. Thus, vas deferens epithelium likely plays an active role in male fertility, and interventions that modulate epithelial function might be exploited to treat male-factor infertility or in contraception.

The presynaptic modulation of glutamate release and the membrane dysfunction induced by in vitro ischemia in rat hippocampal CA1 neurons

Superfusion with an oxygen and glucose deprived medium (in vitro ischemia) of rat hippocampal CA1 pyramidal neurons in tissue slices produced a rapid depolarization within 5 min and thereafter showed no functional recovery (irreversible membrane dysfunction), even if oxygen and glucose were reintroduced. We previously suggested that such a rapid depolarization is triggered by the accumulation of extracellular glutamate (Glu). As a result, we examined the effects of either the activation or inhibition of presynaptic receptors, which modulate Glu release from the nerve terminal, on the potential change produced by in vitro ischemia. The adenosine A1 receptor antagonist, 8-cyclopenthyl theophylline, A2a receptor antagonist, ZM241385, and A2b receptor antagonist, alloxazine, did not significantly alter either the latency or the maximal slope of the rapid depolarization. In addition, the GABAB receptor antagonist, 2-hydroxysaclofen, or the metabotropic Glu receptor type 4 antagonist, alpha-methylserine-O-phosphate, did not change either the latency or the maximal slope. The adenosine A(1) receptor agonist, 2-chloro-N6-cyclopentyladenosine, A2a receptor agonist, CGS2168, or A2b receptor agonist, 5'-(N-ethylcarboxamido)-adenosine, did not affect these parameters either. None of these drugs restored the membrane potential to the pre-exposure level after the reintroduction of oxygen and glucose. Simultaneous intracellular recordings from CA1 and CA3 pyramidal neurons in the same slices revealed the membrane of the CA3 neurons to be hyperpolarized when a rapid depolarization occurred in the CA1 neurons. These results suggest that presynaptic Glu release does not accelerate during the generation of the rapid depolarization induced by in vitro ischemia.

Basolateral and apical A1 adenosine receptors mediate sodium transport in cultured renal epithelial (A6) cells

There are conflicting reports in the literature regarding the adenosine receptor that mediates the increase in sodium transport in the A6 cell. In this study we used specific A1 and A2 adenosine receptor agonists and antagonists, as well as two different subclones of the A6 cell, to determine which adenosine receptor mediates the increase in sodium transport. In the A6S2 subclone, basolateral and apical N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, stimulated sodium transport at a threshold concentration <10(-7) M, whereas CGS-21680, a selective A2 receptor agonist, had a threshold concentration that was at least 10(-5) M. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was found to have a nonspecific effect on CHA-stimulated sodium transport, whereas the A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) had no effect. As with the A6S2 subclone, basolateral and apical CHA stimulated sodium transport at a nanomolar concentration in the A6C1 subclone and the threshold concentration for CGS-21680 was in the high micromolar range. Concurrent with the increase in 1 receptor in different subclones of the A6 cell, including a subclone capable of anion secretion.

Lack of central effects of peripherally administered adenosine A(1) agonists on synaptic transmission in the rat hippocampus

Peripheral administration of adenosine A(1) receptor selective agonists is generally thought to protect the hippocampus against ischemic damage via central actions. We examined the effects of two peripherally administered A(1) agonists, cyclohexyladenosine (CHA) and adenosine amine congener (ADAC), on synaptic transmission in the hippocampus and on indices of cardiovascular function. We conclude that the permeability of these agonists is not sufficient to result in concentrations necessary to activate central adenosine A(1) receptors within the hippocampus.

Adenosine-mediated activation of Akt/protein kinase B in the rat hippocampus in vitro and in vivo

Adenosine is considered an endogenous neuroprotective metabolite that through activation of the A(1) receptor results in reduction of neuronal damage following cerebral ischemia. Protein kinase B, also known as Akt/PKB, is part of an endogenous pathway that exerts effective neuroprotection from both necrotic and apoptotic cell death. Using a rat model of unilateral common carotid artery occlusion coupled with hypoxia, and using in vitro rat hippocampal slices, we examined the ability of adenosine to directly activate Akt/PKB. Western blot analysis revealed that levels of phosphorylated Akt/PKB were elevated in vivo under ischemic conditions in an adenosine A(1)-dependent manner and elevated in hippocampal slices treated with an adenosine A(1) agonist. We conclude from these studies that the activation of an adenosine A(1) receptor-mediated signal transduction pathway, either by endogenous adenosine (in vivo) or by an adenosine A(1) agonist (in vitro), results in the activation of the neurotrophic kinase Akt/PKB.

A1 adenosine receptor activation induces ventriculomegaly and white matter loss

A1 adenosine receptors (A1ARs) are widely expressed in the brain during development. To examine whether A1AR activation can alter postnatal brain formation, neonatal rats from postnatal days 3 to 14 were treated with the A1AR agonist N6-cyclopentyladenosine (CPA) in the presence or absence of the peripheral A1AR antagonist 8-(p-sulfophenyl)-theophylline (8SPT). CPA or CPA + 8SPT treatment resulted in reductions in white matter volume, ventriculomegaly, and neuronal loss. Quantitative electron microscopy revealed reductions in total axon volume following A1AR agonist treatment. We also observed reduced expression of myelin basic protein in treated animals. Showing that functional A1ARs were present over the ranges of ages studies, high levels of specific [3H]CCPA binding were observed at PD 4, 7 and 14, and receptor-G protein coupling was present at each age. These observations show that activation of A1ARs with doses of CPA that mimic the effects of high adenosine levels results in damage to the developing brain.

Pharmacological possibilities for protection against myocardial reperfusion injury

Reperfusion through thrombolysis or percutananeous coronary angioplasty is standard treatment in impending acute myocardial infarction. Although restoration of blood flow to the jeopardised myocardial area is a perquisite for myocardial salvage, reperfusion itself may lead to accelerated and additional myocardial injury beyond that generated by ischemia alone. This is referred to as the "reperfusion injury". Since the reperfusion injury is initiated by the treatment of myocardial infarction, it is of importance to limit the extent of the injury. Several studies aimed at preventing reperfusion injury by means of pharmacological agents have therefore been conducted. The design of such studies is crucial for the results. Factors of importance are the timing of drug administration, animal species used, the degree of collateral flow and the duration of ischemia. A variety of pharmacological compounds have been investigated in different experimental models of myocardial ischemia and reperfusion. These include oxygen free radical scavengers, antioxidants, calcium channel blockers, inhibitors of neutrophils, nitric oxide, adenosine-related agents, inhibitors of the renin-angiotensin system, endothelin receptor antagonists, Na(+)/H(+) exchange inhibitors, and anti-apoptotic agents. All these groups of pharmacological agents have been demonstrated to protect from reperfusion injury determined as limitation of infarct size, improved myocardial and endothelial function, and reduced incidence of arrhythmias. The mechanism behind the protective effect may differ between different groups of compounds, but some compounds may exert cardioprotection via common pathways. Such a pathway may be via maintained bioavailability of nitric oxide.

Endothelium-targeted pharmacotherapeutics for the treatment of stroke

The pathophysiology of stroke in humans is much more complex than what is typically studied in animal models. Embolic stroke models are more complex than pure ischemia models, but are more representative of human disease and may be particularly useful in the study of new therapeutic strategies. Vascular damage is a prominent feature of embolic stroke, and may be a useful therapeutic target. Serotonin antagonists, adenosine-regulating agents, free radical scavengers, matrix metalloproteinase inhibitors, and HMG-CoA reductase inhibitors are all potentially valuable agents in treating vascular damage after stroke. These agents facilitate decreased infarction volume, hemorrhage, and improved cerebral bloodflow.

Adenosine exerts multiple effects in dorsal horn neurones of the adult rat spinal cord

In the present study, we have examined the effects of adenosine and its analogues on the electrophysiological properties of dorsal horn neurones in the rat adult spinal cord. Adenosine and the A1 receptor agonist R-phenylisopropyl adenosine (RPIA) reversibly hyperpolarised these neurones via the generation of an outward current at -60 mV that was inhibited by pre-application of barium or Rp-adenosine 3', 5'-cyclic monophosphothioate triethylamine. In contrast, the A2a receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) had no effect on the resting membrane properties of these neurones. Stimulation of the dorsal root evoked non-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) at -60 mV that were completely abolished by 2,3-dihydroxy-6-nitro-7-sulophamoyl-benzo(F)quinoxalone (NBQX). Bath application of adenosine or RPIA reversibly inhibited these EPSCs in a concentration-dependent manner via a presynaptic action. In contrast, CGS21680 increased the amplitude of the EPSC in 20% of neurones tested and decreased the EPSC amplitude in 30% of neurones tested. It is concluded that adenosine exerts multiple effects upon the electrophysiological properties of dorsal horn neurones in the adult spinal cord via interaction with multiple receptors. These findings have important implications in the understanding of adenosine action in preclinical models of pain.

Adenosine and caffeine modulate circadian rhythms in the Syrian hamster

Extracellular adenosine accumulates in some brain areas during sleep deprivation. In Syrian hamsters, both sleep deprivation and adenosine A1 agonists can inhibit phase shifts of circadian rhythms to light at night. Sleep deprivation in the day (sleep period) can shift circadian phase. We examined whether the A1 agonist N-CHA mimics this effect. N-CHA (i.p. or i.c.) in the mid-sleep period induced dose-dependent shifts similar to those induced by 3 h sleep deprivation. The adenosine antagonist caffeine administered systemically at the mid-sleep period induced arousal without shifts, and dose-dependently attenuated shifts to a 3 h sleep deprivation procedure (running in a novel wheel). Adenosine may participate in resetting of the circadian clock by manipulations of behavioral state.

Effects of angiotensin III and angiotensin IV on pentylenetetrazol seizure susceptibility (threshold and kindling): interaction with adenosine A(1) receptors

The effects of angiotensin (ANG) III and ANG IV on pentylenetetrazol (PTZ) seizure susceptibility--threshold and kindling in mice--as well as the influence of adenosine A(1) receptor agents (agonist and antagonist) on these effects were studied. It was found that ANG III and ANG IV increased dose-dependently the PTZ seizure threshold and decreased the seizure intensity in PTZ kindled mice. Cyclohexyladenosine (CHA), an adenosine A(1) receptor agonist, potentiated the effects of ANG III and ANG IV on the seizure threshold and kindling, whereas DPCPX (an A(1) receptor antagonist) reversed peptide-induced effects on the PTZ kindling. Taken together, ANG III and ANG IV decrease the PTZ seizure susceptibility. We could suggest that these effects are realized in part through interaction with adenosine A(1) receptors.

Intraocular pressure responses to the adenosine agonist cyclohexyladenosine: evidence for a dual mechanism of action

PURPOSE

Previous studies have shown that adenosine agonists are effective in reducing intraocular pressure (IOP). However, the mechanism(s) responsible for this ocular hypotensive effect has not been established. This study evaluates the relative contribution of changes in aqueous flow and outflow facility associated with the ocular hypotensive response to the adenosine agonist cyclohexyladenosine (CHA).

METHODS

New Zealand White rabbits were treated topically in one eye with the adenosine A(1) agonist CHA. Changes in IOP, aqueous flow, and total outflow facility at various times after CHA administration were then determined.

RESULTS

These studies demonstrated that CHA produces a dose-related reduction in IOP. Analysis of the dose-response curve revealed an ED(50) and a Hill coefficient of 87 microg and 1.9, respectively. Aqueous flow measurements demonstrated that 1.5 hours after CHA administration, aqueous flow was reduced by 35%. However, by 3.5 hours postdrug, no significant change in aqueous flow was observed. Measurement of the outflow facility found no significant change in facility 1.5 hours after CHA administration. However, by 3.5 hours after CHA administration, outflow facility was significantly increased by 85%.

CONCLUSIONS

These data demonstrate that the adenosine agonist CHA lowers IOP in a dose-related fashion. This hypotensive action results from an early reduction in aqueous flow followed by a subsequent increase in outflow facility. This dual mechanism of action is consistent with analysis of CHA dose-response curve, which indicates that the reduction in IOP induced this agonist's results from multiple mechanisms of action.

Long-term activation of adenosine A(2a) receptors blocks glutamate excitotoxicity in cultures of avian retinal neurons

Previous work showed the presence of adenosine receptors as well as adenosine uptake and release mechanisms in developing chick retinal neurons in culture. In the present work we show that exogenous glutamate or kainate promotes extensive cell death in these cultures which is blocked when the cultures are previously incubated with adenosine. Addition of glutamate or kainate to purified cultures of retinal neurons and photoreceptors induced massive death of cultured cells which was inhibited in both cases by preincubation with MK801, an NMDA antagonist, or DNQX, an AMPA/kainate antagonist. Cell death was also greatly attenuated by preincubation with adenosine plus EHNA, an adenosine deaminase inhibitor, NBI, an adenosine uptake blocker, the permeable cAMP analogs 8-Br cAMP and Sp cAMP and the A(2a) agonists CGS 21680 and DPMA, but not with the A(1) receptor agonist CHA. Kinetic studies performed determining the intracellular LDH activity showed that maximal death was observed after 8 h and in concentrations of glutamate as low as 50 microM. We also observed a time-dependent protective effect of adenosine beginning after 1 h of preincubation and reaching a maximal effect after 24 h, indicating its association with changes in cellular metabolism induced by long-term exposure of cells to the nucleoside. The results show that adenosine inhibits glutamate toxicity in retinal neurons through a long-term activation of A(2a) receptors and elevation of intracellular cyclic AMP levels.

A3 adenosine receptor stimulation modulates sarcoplasmic reticulum Ca(2+) release in rat heart

OBJECTIVE

Stimulation of A3 adenosine receptors has been shown to protect cardiac myocytes from ischemic injury, but the mechanism of this action is unknown. We evaluated the effect of adenosine agonists and antagonists on the sarcoplasmic reticulum (SR) Ca(2+) channels.

METHODS

Isolated rat hearts were perfused with control buffer or different adenosine agonists and antagonists. Hearts were then homogenized and used to determine SR Ca(2+)-induced Ca(2+) release, assayed by quick filtration technique after loading with 45Ca(2+), and the binding of [3H]ryanodine, a specific ligand of the SR Ca(2+) release channel. In parallel experiments, hearts were challenged with 30 min of global ischemia and 120 min of reperfusion, and the extent of tissue necrosis was evaluated by triphenyltetrazolium chloride staining.

RESULTS

Perfusion with the A1>A3 agonist R-PIA and the A3>A1 agonist IB-MECA was associated with reduced [3H]ryanodine binding, due to reduced B(max) (by about 20%), whereas K(d) and Ca(2+)-dependence of the binding reaction were unaffected. These actions were abolished by the A3 antagonist MRS 1191, while they were not affected by A1 and A2 antagonists. The rate constant of SR Ca(2+) release decreased by 25-30% in hearts perfused with R-PIA or IB-MECA. Tissue necrosis was significantly reduced in the presence of R-PIA or IB-MECA. Protection was removed by MRS 1191, and it was not affected by A1 and A2 antagonists. Hearts were also protected by administration of dantrolene, a ryanodine receptor antagonist. In the presence of dantrolene, no further protection was provided by IB-MECA.

CONCLUSION

A3 adenosine receptor stimulation modulates the SR Ca(2+) channel. This action might account for the protective effect of adenosine.

Cyclic AMP-dependent inhibition of human neutrophil oxidative activity by substituted 2-propynylcyclohexyl adenosine A(2A) receptor agonists

Novel 2-propynylcyclohexyl-5'-N:-ehtylcarboxamidoadenosines, trans-substituted in the 4-position of the cyclohexyl ring, were evaluated in binding assays to the four subtypes of adenosine receptors (ARs). Two esters, 4-(3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl)-cyclohexanecarboxylic acid methyl ester (ATL146e) and acetic acid 4-(3-[6-amino-9-(5-ethylcarbamoyl-3, 4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl] -prop-2-ynyl)-cyclohexylmethyl ester (ATL193) were >50 x more potent than 2-[4-(2-carboxyethyl)phenethylamino]-5'-N:-ethylcarboxamidoadenosine (CGS21680) for human A(2A) AR binding. Human A(2A) AR affinity for substituted cyclohexyl-propynyladenosine analogues was inversely correlated with the polarity of the cyclohexyl side chain. There was a comparable order of potency for A(2A) AR agonist stimulation of human neutrophil [cyclic AMP](i), and inhibition of the neutrophil oxidative burst. ATL146e and CGS21680 were approximately equipotent agonists of human A(3) ARs. We measured the effects of selective AR antagonists on agonist stimulated neutrophil [cyclic AMP](i) and the effect of PKA inhibition on A(2A) AR agonist activity. ATL193-stimulated neutrophil [cyclic AMP](i) was blocked by antagonists with the potency order: ZM241385 (A(2A)-selective)>MRS1220 (A(3)-selective)>>N-(4-Cyano-phenyl)-2-[4-(2,6-dioxo-1,3-dipropyl-2,3,4,5,6,7-hexahydro-1H-purin-8-yl)-phenoxy]-acetamide (MRS1754; A(2B)-selective) approximately 8-(N-methylisopropyl)amino-N(6)-(5'-endohydroxy-endonorbornyl)-9-methyladenine (WRC0571; A(1)-selective). The type IV phosphodiesterase inhibitor, rolipram (100 nM) potentiated ATL193 inhibition of the oxidative burst, and inhibition by ATL193 was counteracted by the PKA inhibitor H-89. The data indicate that activation of A(2A)ARs inhibits neutrophil oxidative activity by activating [cyclic AMP](i)/PKA.

K(ATP) channel openers, adenosine agonists and epileptic preconditioning are stress signals inducing hippocampal neuroprotection

Many models of induced ischemic and epileptic tolerance have now been described in the brain. Although detailed mechanisms underlying such protections still remain largely unknown, induction of heat shock proteins is amongst the endogenous responses believed to play an important role in cellular defense mechanisms. This study reveals that the development of epileptic tolerance also coincides with the induction of the 70,000 mol. wt heat shock protein expression within the time window of protection. Adenosine agonists or ATP-sensitive potassium channel openers have also been shown to exert strong neuroprotective effects when injected shortly prior to a severe ischemic or epileptic insult. The present work shows that adenosine receptor activation and ATP-sensitive potassium channel opening induce 70,000 mol. wt heat shock protein expression in the rat hippocampus and are able to mimic neuroprotection driven by preconditioning. R-phenylisopropyladenosine, a purine agonist, or (-)cromakalim, an ATP-sensitive potassium channel opener, was administered three days prior to a lethal ischemic or epileptic episode to mimic preconditioning. Neurodegeneration was assessed using Cresyl Violet staining and cellular DNA fragmentation visualized by the terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling method. 70, 000 mol. wt heat shock protein expression was analysed by western blotting and immunohistochemistry. The results show a long-lasting neuroprotection induced by activation of adenosine receptors or ATP-sensitive K(+) channels as early as three days prior to induction of a severe ischemic or epileptic challenge. This protective effect is associated with enhanced 70,000 mol. wt heat shock protein expression also occurring three days following administration of R-phenylisopropyladenosine or (-)cromakalim. These findings support the idea that preconditioning doses of R-phenylisopropyladenosine and (-)cromakalim act as mild cellular stresses inducing neuroprotection in a manner similar to a mild kainate treatment prior to a lethal ischemic or severe epileptic insult three days later. They also suggest that a delayed 70,000 mol. wt heat shock protein expression induced by excitatory neuronal stresses such as short ischemia, mild kainic acid treatment or activation of adenosine receptors and ATP-sensitive potassium channels is predictive of neuronal survival against a subsequent lethal injury.

Effect of adenosine on adenosine triphosphate-sensitive potassium channel during hypoxia in rat hippocampal neurons

Using the whole-cell patch clamp method, we explored the effect of adenosine on the K(ATP) current and its regulatory mechanisms in acutely dissociated rat hippocampal neurons. A chemical hypoxia model was made using 0.2 mmol/l 2,4dinitrophenol (2,4DNP). During hypoxia, the K(ATP) current was not raised significantly by adenosine alone, but was accelerated significantly by adenosine in combination with the selective A(2) receptor blocker 3, 7-dimethl-1-propargylxanth-ine. The selective A(1) receptor agonist N6-cyclopentyladenosine also accelerated the K(ATP) current. These results suggest that activation of the adenosine A(1) receptor can accelerate opening of the K(ATP) channel during hypoxia, and that the A(2) receptor may have an opposing effect to the A(1) receptor.

Long-term theophylline treatment changes the effects of angiotensin II and adenosinergic agents on the seizure threshold

The effects of angiotensin II (ANG II), sarmesin, losartan, PD 123319, and adenosine A (1) receptor agonist N(6)-cyclopentyladenosine (CPA) administered i.c.v. in untreated and in theophylline-treated male mice (50 mg/kg i.p. twice daily for 14 days) were studied on the pentylenetetrazol (PTZ) seizure threshold. The threshold was increased after long-term theophylline treatment. ANG II, sarmesin, and CPA increased the threshold in theophylline-untreated mice, whereas it decreased the threshold in theophylline-treated animals. Losartan did not change the threshold in theophylline-untreated mice but decreased it in theophylline-treated animals. PD 123319 did not change the seizure threshold both in theophylline-untreated and -treated mice. Taken together, the data demonstrated that repeated exposure to theophylline selectively changes the effects of ANG II and adenosinergic agents on the PTZ seizure threshold. The results indicate that both angiotensin AT(1) and adenosine A(1) receptor subtypes could possess interactive mechanisms of adaptation to chronic theophylline treatment.