Neuroprotective Effects Of Adenosine Research Articles

Adenosine Protects Zebrafish Photoreceptors from Apoptosis and Damage in a Dose‐Dependent Manner in a Light‐Induced Retinal Degeneration Model of Age‐Related Macular Degeneration

PurposeAdenosine has been shown to be neuroprotective in the CNS and in retinal ganglion cells in the inner retina. However, the neuroprotective effects of adenosine on outer retinal neurons, in particular photoreceptors, is not known. Age‐related macular degeneration (AMD) is a multifactorial disease characterized by the impairment or loss of central vision due to macular dropout and loss of photoreceptors. Adenosine has been shown to inhibit photoreceptor excitability by suppressing Ca2+ influx through voltage‐gated Ca2+ channels, which could lead to cell death. The purpose of this study was to see if adenosine would reduce photoreceptor damage and death in a light‐induced retinal degeneration (LIRD) model in pigmented zebrafish.MethodsAdult wild‐type and Tg(nrd:GFP) zebrafish (used to GFP label rod photoreceptors) were dark‐adapted for 24 hours, followed by intravitreal injections of ~ 0.5 ml of 0.1 mM, 1 mM, or 5 mM (final conc: 1.67 to 167 mM) of adenosine in both eyes. Controls were injected with saline in both eyes. All zebrafish were exposed to full spectrum high‐intensity light (≥ 20,000 lux) for four days and collected at 0 and 96 hours post dark‐adaptation. Immunohistochemical analysis was performed on vertical retinal sections in order to visualize cones, rods, and the outer nuclear layer of the retina. Retinal damage was assessed using confocal microscopy with a TUNEL assay and cell specific markers.ResultsPhotoreceptors in control retinas collected at 96 hours exhibited severe truncation or loss of the outer segments, and swelling of the cell body. Additionally, there was an increase in TUNEL‐positive cells and a significant decrease in rod and cone photoreceptors in the control retinas. Photoreceptors from retinas treated with higher concentrations of adenosine (1 mM and 5 mM) lacked swelling, exhibited less cell death, reduced truncation of the outer segments, and had cell morphology similar to retinas collected prior to light exposure. Photoreceptors from retinas treated with 0.1 mM adenosine exhibited no significant differences in cell death in comparison to control retinas at 96 hours, and had similar levels of outer segment truncation and cell body morphology. However, there was a significant reduction in the number of cones and rods lost in the 0.1 mM adenosine‐treated retinas at 96 hours in comparison to control retinas at 96 hours.ConclusionThese findings show that adenosine can prevent light‐induced structural damage to rod and cone photoreceptors, prevent photoreceptor death and loss, and provide evidence of a potential novel therapeutic target for the treatment of the “dry” form of AMD.Support or Funding InformationHMC CURE Zebrafish Pilot Grant from Penn State University College of Medicine, Pennsylvania Department of Health Tobacco CURE Fund

Adenosine Triphosphate–Sensitive Potassium Channel Blockers Attenuate the Antiallodynic Effect of R-PIA in Neuropathic Rats

Nerve injury can generate neuropathic pain. The accompanying mechanical allodynia may be reduced by the intrathecal administration of adenosine. The neuroprotective effects of adenosine are mediated by the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel. We assessed the relationship between the adenosine A1 receptor agonist, N⁶-(R)-phenylisopropyl adenosine (R-PIA), and K(ATP) channels to determine whether the antiallodynic effects of R-PIA are also mediated through K(ATP) channels in a rat nerve ligation injury model of neuropathic pain. Mechanical allodynia was induced by tight ligation of the left lumbar fifth and sixth spinal nerves. Mechanical allodynia in the left hindpaw was evaluated using von Frey filaments to measure withdrawal thresholds. R-PIA (0.5, 1, or 2 μg) was administered intrathecally to induce antiallodynia. We assessed whether pretreatment with the K(ATP) channel blockers glibenclamide or 5-hydroxydecanoate reversed the antiallodynic effect of R-PIA. Also, we evaluated whether diazoxide, a K(ATP) channel opener, had an antiallodynic effect and promoted the antiallodynic effect of R-PIA. Lastly, we investigated whether the voltage-activated K channel blocker 4-aminopyridine attenuated the effect of R-PIA. Intrathecal R-PIA produced maximal antiallodynia at 2 μg (P < 0.05). Intrathecal pretreatment with glibenclamide and intraperitoneal pretreatment 5-hydroxydecanoate significantly reduced the antiallodynic effect of R-PIA. Diazoxide produced an antiallodynic effect and also enhanced the antiallodynic action of R-PIA. 4-Aminopyridine had no effect on the antiallodynic action of R-PIA. The antiallodynic effects of adenosine A1 receptor stimulation may be related to K(ATP) channel activity in a rat model of nerve ligation injury.

Adenosine A2A receptor antagonists exert motor and neuroprotective effects by distinct cellular mechanisms

To investigate whether the motor and neuroprotective effects of adenosine A(2A) receptor (A(2A)R) antagonists are mediated by distinct cell types in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease. We used the forebrain A(2A)R knock-out mice coupled with flow cytometric analyses and intracerebroventricular injection to determine the contribution of A(2A)Rs in forebrain neurons and glial cells to A(2A)R antagonist-mediated motor and neuroprotective effects. The selective deletion of A(2A)Rs in forebrain neurons abolished the motor stimulant effects of the A(2A)R antagonist KW-6002 but did not affect acute MPTP neurotoxicity. Intracerebroventricular administration of KW-6002 into forebrain A(2A)R knock-out mice reinstated protection against acute MPTP-induced dopaminergic neurotoxicity and attenuated MPTP-induced striatal microglial and astroglial activation. A(2A)R activity in forebrain neurons is critical to the control of motor activity, whereas brain cells other than forebrain neurons (likely glial cells) are important components for protection against acute MPTP toxicity.

The effects of methylmercury on motor activity are sex- and age-dependent, and modulated by genetic deletion of adenosine receptors and caffeine administration

Adenosine and its receptors are, as part of the brain stress response, potential targets for neuroprotective drugs. We have investigated if the adenosine receptor system affects the developmental neurotoxicity caused by the fish pollutant methylmercury (MeHg). Behavioral outcomes of low dose perinatal MeHg exposure were studied in mice where the A 1 and A 2A adenosine receptors were either partially blocked by caffeine treatment or eliminated by genetic modification (A 1R and A 2AR knock-out mice). From gestational day 7 to day 7 of lactation dams were administered doses that mimic human intake via normal diet, i.e. 1 μM MeHg and/or 0.3 g/l caffeine in the drinking water. This exposure to MeHg resulted in a doubling of brain Hg levels in wild type females and males at postnatal day 21 (PND21). Open field analysis was performed at PND21 and 2 months of age. MeHg caused time-dependent behavioral alterations preferentially in male mice. A decreased response to amphetamine in 2-month-old males pointed to disturbances in dopaminergic functions. Maternal caffeine intake induced long-lasting changes in the offspring evidenced by an increased motor activity and a modified response to psychostimulants in adult age, irrespectively of sex. Similar alterations were observed in A 1R knock-out mice, suggesting that adenosine A 1 receptors are involved in the alterations triggered by caffeine exposure during development. Perinatal caffeine treatment and, to some extent, genetic elimination of adenosine A 1 receptors, attenuated the behavioral consequences of MeHg in males. Importantly, also deletion of the A 2A adenosine receptor reduced the vulnerability to MeHg, consistent with the neuroprotective effects of adenosine A 2A receptor inactivation observed in hypoxia and Parkinson's disease. Thus, the consequences of MeHg toxicity during gestation and lactation can be reduced by adenosine A 1 and A 2A receptor inactivation, either via their genetic deletion or by treatment with their antagonist caffeine.

Neuroprotective effects of hibernation-regulating substances against low-temperature-induced cell death in cultured hamster hippocampal neurons

The neuroprotective effects of hibernation-regulating substances (HRS) such as adenosine (ADO), opioids, histamine and thyrotropin-releasing hormone (TRH) on low-temperature-induced cell death (LTCD) were examined using primary cultured hamster hippocampal neurons. LTCD was induced when cultures were maintained at <22 °C for 7 days. ADO (10–100 μM) protected cultured neurons from LTCD in a dose-dependent manner. The neuroprotective effects of ADO were reversed by both 8-cyclopenthyltheophilline (CPT; A 1 receptor antagonist) and 3,7-dimethyl-1-propargylxanthine (DMPX; A 2 receptor antagonist). Morphine (a non-selective opioid receptor agonist) was also effective in attenuating LTCD at an in vitro dose range of 10–100 μM. The neuroprotective effects of morphine were antagonized by naloxone (a non-selective opioid receptor antagonist). In addition, although [D-Ala 2, N-Me-Phe 4, Gly-ol 5]-enkephalin (DAMGO; μ-opioid receptor agonist), [D-Pen 2,5]-enkephalin (DPDPE; δ-opioid receptor agonist) and U-69593 (κ-opioid receptor agonist) were also effective, LTCD of cultured hippocampal neurons was not affected by TRH. Furthermore, histamine produced hypothermia in Syrian hamsters and protected hippocampal neurons in vitro at 100 μM. The neuroprotective effect of histamine was reversed by pyrilamine (H 1 receptor antagonist). Apoptosis was probably involved in LTCD. These results suggest that ADO protected hippocampal neurons in vitro via its agonistic actions on both A 1 and A 2 receptors, whereas morphine probably elicited its neuroprotective effects via agonistic effects on the μ-, δ- and κ-opioid receptors. In addition, histamine also protected hippocampal neurons via its agonistic action on the H 1 receptor. Thus, HRS-like adenosine-, opioid- and histamine-like hypothermic actions would most likely induce neuroprotective effects against LTCD in vitro.

Adenosine treatment delays postischemic hippocampal CA1 loss after cardiac arrest and resuscitation in rats

Resuscitation from cardiac arrest results in reperfusion injury that leads to increased postresuscitation mortality and delayed neuronal death. One of the many consequences of resuscitation from cardiac arrest is a derangement of energy metabolism and the loss of adenylates, impairing the tissue's ability to regain proper energy balance. In this study, we investigated the effects of adenosine (ADO) on the recovery of the brain from 12 min of ischemia using a rat model of cardiac arrest and resuscitation. Compared to the untreated group, treatment with adenosine (7.2 mg/kg) initiated immediately after resuscitation increased the proportion of rats surviving to 4 days and significantly delayed hippocampal CA1 neuronal loss. Brain blood flow was increased significantly in the adenosine-treated rats 1 h after cardiac arrest and resuscitation. Adenosine-treated rats exhibited less edema in cortex, brainstem and hippocampus during the first 48 h of recovery. Adenosine treatment significantly lowered brain temperature during recovery, and a part of the neuroprotective effects of adenosine treatment could be ascribed to adenosine-induced hypothermia. With this dose, adenosine may have a delayed transient effect on the restoration of the adenylate pool (AXP = ATP + ADP + AMP) 24 h after cardiac arrest and resuscitation. Our findings suggested that improved postischemic brain blood flow and ADO-induced hypothermia, rather than adenylate supplementation, may be the two major contributors to the neuroprotective effects of adenosine following cardiac arrest and resuscitation. Although adenosine did not prevent eventual CA1 neuronal loss in the long term, it did delay neuronal loss and promoted long-term survival. Thus, adenosine or specific agonists of adenosine receptors should be evaluated as adjuncts to broaden the window of opportunity in the treatment of the reperfusion injury following cardiac arrest and resuscitation.

Hypoxia-induced desensitization and internalization of adenosine A1 receptors in the rat hippocampus

Activation of A1 adenosine receptors is important for both the neuromodulatory and neuroprotective effects of adenosine. However, short periods of global ischemia decrease A1 adenosine receptor density in the brain and it is not known if a parallel loss of functional efficiency of A1 adenosine receptors occurs. We now tested if hypoxia leads to changes in the density and efficiency of A1 adenosine receptors to inhibit excitatory synaptic transmission in rat hippocampal slices. In control conditions, the adenosine analog 2-chloroadenosine, inhibited field excitatory post-synaptic potentials with an EC50 of 0.23μM. After hypoxia (95% N2 and 5% CO2, for 60 min) and reoxygenation (30 min), the EC50 increased to 0.73μM. This EC50 shift was prevented by the presence of the A1 adenosine receptor antagonist 8-phenyltheophyline, but not by the A2AR antagonist 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine, during the hypoxic period. This decreased efficiency of A1 adenosine receptors was not paralleled by a global change of A1 adenosine receptor density or affinity (as evaluated by the binding parameters obtained in nerve terminal membranes). However, the density of biotinylated A1 adenosine receptors at the plasma membrane of nerve terminals was reduced by 30% upon hypoxia/reoxygenation, in a manner prevented by the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine and mimicked by prolonged (60 min) supra-maximal activation of A1 adenosine receptors with 2-chloroadenosine (10μM). These results indicate that hypoxia leads to a rapid (<90 min) homologous desensitization of A1 adenosine receptor-mediated inhibition of synaptic transmission that is likely due to an internalization of A1 adenosine receptors in nerve terminals.

The A 3 adenosine receptor agonist 2-Cl-IB-MECA facilitates epileptiform discharges in the CA3 area of immature rat hippocampal slices

The effects of the A 3 adenosine receptor agonist 2-Cl-IB-MECA were tested on epileptiform field potentials recorded in the CA3 area of postnatal days 10–20 immature hippocampal slices, during perfusion with the GABA A receptor antagonist bicuculline (10 μM). Evoked potentials: 2-Cl-IB-MECA (1–50 μM, n=17) had consistently excitatory effects, blocked by the A 3 receptor antagonist MRS 1220 (1 μM, n=7), but not occluded in the presence of the A 1 antagonist DPCPX (1 μM, n=12) or the A 2A antagonist ZM-241385 (0.1 μM, n=12). 2-Cl-IB-MECA reversed the inhibitory effects ( n=5) of the adenosine uptake blocker nitrobenzylthioinosine (NBTI, 50 μM), but did not increase its excitatory effects ( n=19). Spontaneous discharges: 2-Cl-IB-MECA (1 μM) induced them or increased their frequency in 14/30 slices, an effect reversed by MRS 1220 ( n=3), and observed also following pre-perfusion with DPCPX ( n=11), ZM-241385 ( n=11) or both ( n=10). In the presence of the A 1 antagonist DPCPX, NBTI increased the frequency of spontaneous discharges, an effect partially reversed by MRS 1220 ( n=8), thus suggesting that a rise in endogenous adenosine during disinhibition may activate A 3 receptors. In conclusion, these findings suggest strongly that activation of A 3 receptors, following a rise in endogenous adenosine (i.e. during seizures, hypoxia), facilitates excitation, thus limiting the known inhibitory and/or neuroprotective effects of adenosine in immature brain.

Caffeine impairs short-term neurological outcome after concussive head injury in rats.

Adenosine is an endogenous neuroprotective agent that is released during ischemia, hypoxia, epilepsy, and ischemic brain injury. Caffeine is a receptor antagonist for adenosine that might interfere with the neuroprotective effect of adenosine in ischemic-hypoxic conditions. An investigation was undertaken to study the effect of caffeine on neurological function, edema formation, and blood-brain barrier permeability after experimental head injury in rats. Adult female Wistar rats classified into different groups received caffeine intraperitoneally at doses of 0, 50, 100, and 150 mg/kg body weight. Thirty minutes after the caffeine treatment, the animals were subjected to concussive head injury (CHI) administered by a controlled cortical impact device. Neurological severity score was recorded in each rat at 2 hours after CHI. Specific gravity, water content (as an indicator of edema), and blood-brain barrier impairment were analyzed in the cortical tissue surrounding the injury site. The levels of myeloperoxidase and malondialdehyde in the cortical region were measured as indicators of neutrophil infiltration and lipid peroxidation, respectively. A significant increase in righting latency and neurological deficiency after CHI was observed in caffeine-treated rats as compared with untreated animals. Although no deaths occurred in the rats exposed to CHI after pretreatment with saline, pretreatment with caffeine caused significant mortality of animals after trauma in a dose-dependent manner. Caffeine also exacerbated neutrophil infiltration, edema, and disruption of blood-brain barrier in the traumatic cortex. Light microscopy of brain revealed more severe hemorrhage and neuronal degeneration in the injured hemisphere of caffeine-treated rats as compared with rats in the injury-alone group. A significant increase in malondialdehyde in the brain of injured rats treated with caffeine before CHI clearly indicated the role of oxidative stress. Caffeine adversely affects outcome after CHI, possibly as a result of blockade of adenosine receptors. The findings also point toward the involvement of free radical-mediated neuronal damage in caffeine-induced exacerbation of neurotrauma.

A3 receptors in cortical neurons: Pharmacological aspects and neuroprotection during hypoxia

AbstractAdenosine, which is released under pathophysiological conditions such as hypoxia or ischemia, is involved in a variety of regulatory processes related to neuroprotection. Major advances in the research on neuronal adenosine receptors have provided a better understanding of the mechanisms underlying the neuroprotective effects during hypoxia. The therapeutic potential of adenosine was recognized several years ago, but only the development of stable and selective adenosine receptor agonists and antagonists has offered novel approaches for the pharmacological manipulation of the receptor activity. To date, four G protein coupled receptors have been cloned and identified in the central nervous system: A1, A2A, A2B, and A3. The neuroprotective effects of adenosine are mediated primarily by the activation of A1 receptors. Knowledge of the physiological role of A3 receptors in the brain is still limited. This article focuses on new evidences referring to possible functions of A3 receptors in the CNS, especially the role during hypoxia. Electrophysiological investigations on brain slices give us new insights concerning mechanisms of synaptic modulation. The data from pyramidal cells of the rat cingulate cortex show that a high level of endogenous adenosine (occuring during hypoxia) activates A3 receptors, which mediates a depression of the synaptic transmission. The A3 receptors inhibit the glutamate release additionally to and independently from the A1 receptors. The two distinct mechanisms of synaptic modulation contribute to the neuroprotective action of adenosine during hypoxia and offer new approaches for therapeutic strategies. Drug Dev. Res. 58:420–427, 2003. © 2003 Wiley‐Liss, Inc. © 2003 Wiley‐Liss, Inc.

Intracerebral adenosine infusion improves neurological outcome after transient focal ischemia in rats

In order to elucidate the role of adenosine in brain ischemia, the possible protective effects of adenosine on ischemic brain injury were investigated in a rat model of brain ischemia both in vitro and in vivo. Exogenous adenosine dose-dependently rescued cortical neuronal cells from injury after glucose deprivation in vitro. Adenosine (1 mM) also significantly reduced hypoglycemia/hypoxia-induced glutamate release from the hippocampal slice. In a rat model of transient middle cerebral artery occlusion (MCAO), extracellular adenosine concentration was increased immediately after occlusion, and then returned to the baseline by 30 min after reperfusion. Adenosine infusion through a microdialysis probe into the ipsilateral striatum (1 mM adenosine, 2 µl min-1, total 4.5 h from the occlusion to 3 h after reperfusion) showed a significant improvement in the neurological outcome, and about 25% reduction of infarct volume, although the effect did not reach statistical significance, compared with the vehicle-treated group at 20 h after 90 min of MCAO. These results demonstrated the neuroprotective effect of adenosine against ischemic brain injury both in vitro and in vivo, suggesting the possible therapeutic application of adenosine regulating agents, which inhibit adenosine uptake or metabolism to enhance or maintain extracellular endogenous adenosine levels, for stroke treatment. [Neurol Res 2002; 24: 317-323]

Stimulation of A(2A) adenosine receptor phosphorylation by protein kinase C activation: evidence for regulation by multiple protein kinase C isoforms.

Activation of the A(2A) adenosine receptor (A(2A)AR) contributes to the neuromodulatory and neuroprotective effects of adenosine in the central nervous system. Here we demonstrate that, in rat C6 glioma cells stably expressing an epitope-tagged canine A(2A)AR, receptor phosphorylation on serine and threonine residues can be increased by pretreatment with either the synthetic protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) or endothelin 1, which increases PKC activity via binding to endogenous endothelin(A) receptors. Under conditions in which PMA was maximally effective, activation of other second messenger-regulated kinases was without effect. While basal and PMA-stimulated phosphorylation were unaffected by the A(2A)AR-selective antagonist ZM241385, they were both blocked by GF109203X (a selective inhibitor of conventional and novel PKC isoforms) and rottlerin (a PKCdelta-selective inhibitor) but not Go6976 (selective for conventional PKC isoforms). However, coexpression of the A(2A)AR with each of the alpha, betaI, and betaII isoforms of PKC increased basal and PMA-stimulated phosphorylation. Mutation of the three consensus PKC phosphorylation sites within the receptor (Thr298, Ser320, and Ser335) to Ala failed to inhibit either basal or PMA-stimulated phosphorylation. In addition, phosphorylation of the receptor was not associated with detectable changes in either its signaling capacity or cell surface expression. These observations suggest that multiple PKC isoforms can stimulate A(2A)AR phosphorylation via activation of one or more downstream kinases which then phosphorylate the receptor directly. In addition, it is likely that phosphorylation controls interactions with regulatory proteins distinct from those involved in the classical cAMP signaling pathway utilized by this receptor.

Adenosine improves cerebral recovery in rat after cardiac arrest and resuscitation.

Cardiac arrest and resuscitation produce global cerebral ischemia and reperfusion damage to the brain, which lead to high mortality and delayed neuronal death. Adenosine (ADO) has been suggested as an endogenous neuroprotective molecule, acting through multiple potential mechanisms (Sweeney, 1997). In this study we investigated the possible neuroprotective effects of adenosine on cerebral recovery following global ischemia induced by cardiac arrest and resuscitation.KeywordsCardiac ArrestGlobal Cerebral IschemiaAdenosine TreatmentTotal Cerebral IschemiaEnergy Charge PotentialThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Glutamate neurotoxicity during spinal cord ischemia--the neuroprotective effects of adenosine

Evidence is accumulating that glutamate, a major neurotransmitter, exerts potent neurotoxic activity during ischemia. In our laboratory, a delayed-onset paraplegia model using rabbits has been developed and described. The severity of the ischemic event in this model, i.e., extracellular glutamate overload, is believed to influence the etiology of this delayed neuronal dysfunction. Adenosine, an endogenous neuromodulator, is released after acute ischemic insult and provides neuroprotection by actions on neuronal and glial cells in the still viable border zone of the ischemic focus. We hypothesized that the neuroprotective action of adenosine is associated with inhibition of glutamate neurotoxicity following ischemia. Infrarenal aortic segments from 11 New Zealand white rabbits were isolated for 5 minutes and infused at a rate of 2 ml/min. Group I (n = 6) received normothermic L-glutamate (20 mM). Group II (n = 5) received 75 mg of adenosine and normothermic L-glutamate (20 mM). Neurologic function was assessed at 6, 24, and 48 hours after surgery according to the modified Tarlov scale, After 48 hours, the rabbits were euthanized and their spinal cords were harvested for histologic examination. The neurologic function of three rabbits in group I showed acute paraplegia and the other three showed delayed-onset paraplegia, whereas all group II animals and nearly intact neurologic function. Histologic examination of spinal cords from rabbits in group I showed evidence of moderate spinal cord injury with central gray matter and adjacent white matter necrosis and axonal swelling, whereas spinal cords from group II revealed no evidence of cord injury. Adenosine A1-receptor activation is suspected to reduce excitatory amino acids by controlling the activation of the voltage-dependent NMDA receptor. These results indicate that the neuroprotective effect of adenosine is associated with inhibition of glutamate neurotoxicity, which initiates a deleterious cascade of biochemical events that ultimately result in delayed-onset paraplegia.

Neuroprotective Effects of Adenosine in Cerebral Ischemia: Window of Opportunity

SWEENEY, M.I. Neuroprotective effects of adenosine in cerebral ischemia: window of opportunity. NEUROSCI BIOBEHAV REV 21 [2]207–217, 1997.—The inhibitory neuromodulator adenosine is neuroprotective against damage induced by cerebral ischemia. Its vasodilator effects add to its suitability as a possible anti-stroke agent, but also account for unwanted side effects following systemic administration of adenosine receptor agonists. ATP breakdown during ischemia produces adenosine which effluxes out of the neuron. This review will focus on endogenously produced adenosine and its subsequent protection against ischemia-induced neuronal damage in some stroke models, but will also highlight possible disadvantages to increasing adenosine concentrations. In the advantages column, therapeutic benefits have been obtained by enhancing synaptic concentrations of endogenous adenosine using the adenosine uptake inhibitor propentofylline, but not dipyridamole. There is an emerging role for endogenous adenosine in preventing delayed cell death, e.g. following hypoxic pre-conditioning. One of the cons associated with enhancing the synaptic concentration of adenosine is the appearance of adenosine receptor desensitization over time. Thus, there is a therapeutic window of opportunity during which activation of an adenosine A 1 receptor is beneficial to an ischemic neuron. © 1997 Elsevier Science Ltd. All rights reserved.

Inhibitory effects of propentofylline on [ 3H]adenosine influx : A study of three nucleoside transport systems

The neuroprotective effects of adenosine are well-recognized. Recently, propentofylline, a xanthine derivative, has been shown to increase extracellular concentrations of adenosine in ischemie brain and to limit the extent of neuronal damage in experimental models of cerebral ischemia. Since the concentration of adenosine in brain is controlled, in part, by nucleoside transporter proteins, the action of propentofylline was proposed to be due to inhibition of mediated transfer of adenosine across cell membranes. To determine the likelihood of this mechanism, we examined the inhibitory effects of propentofylline on [ 3H]adenosine transport by the three best-characterized nucleoside transport processes, es, ei, and cif in cultured cell lines under conditions where only a single transporter type was operative. Propentofylline inhibited [ 3H]adenosine uptake by each of the three transport processes in a concentration-dependent manner. The greatest inhibitory potency was for es transporters (L1210/B23.1 cells), with an ic 50 value of 9 μM, followed by ei transporters, with ic 50 values of 170 μM (L1210/C2 cells) and 166 μM (Walker 256 cells). Propentofylline was a weak inhibitor of cif transporter, with an ic 50 value of 6 mM. These results demonstrate that propentofylline is an inhibitor of adenosine transport processes and suggest that its neuroprotective effects may be due to an increase in extracellular concentrations of adenosine by virtue of inhibition of es transporter function.

Focal cerebral ischemia in chronic hypertension: no protection by(R)-phenylisopropyladenosine

The effects of the adenosine agonist, (R)-phenylisopropyladenosine on focal cerebral ischemia induced by middle cerebral artery occlusion were investigated in spontaneously hypertensive rats. The drug was given 30 min before occlusion and each hour thereafter for 6 h. The neurological status of the rats was estimated 2, 24 and 48 h after occlusion. Infarct volumes were measured 48 h after occlusion (Cresyl violet-stained sections). (R)-Phenylisopropyladenosine did not significantly reduce infarct size, nor did it modify the neurological score. As there is considerable evidence of the neuroprotective effects of adenosine in normotensive rats, the present results may be due to a more abrupt reduction in cerebral blood flow in the territory surrounding the ischemic core, where neuroprotection could be expected, in the spontaneously hypertensive rat strain. Consequently, neuroprotection may be more difficult when focal cerebral ischemia is associated with hypertension.

Neuroprotective effects of adenosine

Neuroprotective effects of adenosine