Adenosine A1 receptor agonists as clinically viable agents for treatment of ischemic brain disorders

Adenosine A1R/A3R (Adenosine A1 and A3 Receptor) Agonist AST-004 Reduces Brain Infarction in a Nonhuman Primate Model of Stroke

BACKGROUND AND PURPOSE

Treatment with A1R/A3R (adenosine A1 and A3 receptor) agonists in rodent models of acute ischemic stroke results in significantly reduced lesion volume, indicating activation of adenosine A1R or A3R is cerebroprotective. However, dosing and timing required for cerebroprotection has yet to be established, and whether adenosine A1R/A3R activation will lead to cerebroprotection in a gyrencephalic species has yet to be determined.

METHODS

The current study used clinical study intervention timelines in a nonhuman primate model of transient, 4-hour middle cerebral artery occlusion to investigate a potential cerebroprotective effect of the dual adenosine A1R/A3R agonist AST-004. Bolus and then 22 hours intravenous infusion of AST-004 was initiated 2 hours after transient middle cerebral artery occlusion. Primary outcome measures included lesion volume, lesion growth kinetics, penumbra volume as well as initial pharmacokinetic-pharmacodynamic relationships measured up to 5 days after transient middle cerebral artery occlusion. Secondary outcome measures included physiological parameters and neurological function.

RESULTS

Administration of AST-004 resulted in rapid and statistically significant decreases in lesion growth rate and total lesion volume. In addition, penumbra volume decline over time was significantly less under AST-004 treatment compared with vehicle treatment. These changes correlated with unbound AST-004 concentrations in the plasma and cerebrospinal fluid as well as estimated brain A1R and A3R occupancy. No relevant changes in physiological parameters were observed during AST-004 treatment.

CONCLUSIONS

These findings suggest that administration of AST-004 and combined A1R/A3R agonism in the brain are efficacious pharmacological interventions in acute ischemic stroke and warrant further clinical evaluation.

Activation of Adenosine A1 Receptor in Ischemic Stroke: Neuroprotection by Tetrahydroxy Stilbene Glycoside as an Agonist

Ischemic stroke is the main cause of death/disability, posing a great menace to human health. Though efforts to search for therapeutic drugs are ongoing, few of them have succeeded. Adenosine A₁ receptor (A1R) activation could ameliorate ischemic injury, representing a very tempting target for stroke treatment. Tetrahydroxy stilbene glycoside (TSG), a potent antioxidant from the well-known Chinese herb Polygonum multiflorum Thunb., has been reported to have notable neuroprotective activities but the underlying mechanisms are elusive. This study investigated the mechanism of TSG focusing on A1R. TSG markedly decreased mortality, neurological deficit score, cerebral infarct size and brain water content of MCAO rats, and ameliorated the disorders in purine metabolism, energy metabolism and antioxidative defense system. TSG helped the survival of SH-SY5Y cells in OGD/R by alleviating oxidative stress and glutamate release, and by maintaining calcium homeostasis. TSG effects were abolished by A1R antagonist DPCPX. Docking and binding assays confirmed the binding of TSG with A1R. In addition, TSG upregulated the A1R level lowered by MCAO and OGD/R. The downstream signals of A1R activation, ERK1/2, HIF-1α and NF-κB contributed to the neuroprotection of TSG. Moreover, void of “well-known” cardiovascular side effects of classical A1R agonists, TSG showcased its great potential for stroke treatment.

Hypothalamic or Extrahypothalamic Modulation and Targeted Temperature Management After Brain Injury

Targeted temperature management (TTM) has been recognized to protect tissue function and positively influence neurological outcomes after brain injury. While shivering during hypothermia nullifies the beneficial effect of TTM, traditionally, antishivering drugs or paralyzing agents have been used to reduce the shivering. The hypothalamic area of the brain helps in controlling cerebral temperature and body temperature through interactions between different brain areas. Thus, modulation of different brain areas either pharmacologically or by electrical stimulation may contribute in TTM; although, very few studies have shown that TTM might be achieved by activation and inhibition of neurons in the hypothalamic region. Recent studies have investigated potential pharmacological methods of inducing hypothermia for TTM by aiming to maintain the TTM and reduce the shivering effect without using antiparalytic drugs. Better survival and neurological outcome after brain injury have been reported after pharmacologically induced TTM. This review discusses the mechanisms and modulation of the hypothalamus with other brain areas that are involved in inducing hypothermia through which TTM may be achieved and provides therapeutic strategies for TTM after brain injury.

Peripheral adenosine levels in euthymic patients with bipolar disorder

Recent evidence points to the involvement of the purinergic signaling in the pathophysiology of bipolar disorder. The aim of this study was to assess the serum levels of adenosine and to evaluate its relation to functioning in 24 euthymic patients with bipolar disorder type I and in 25 matched healthy controls. Subjects were evaluated using the functioning assessment short test. Serum purine levels were measured by high pressure liquid chromatography. Our results show a decrease in serum adenosine levels in bipolar disorder patients compared with controls (t= -4.8, df= 43.96, p<0.001). Moreover, a significant negative correlation was found between patient adenosine levels and depression scale scores (r= -0.642, p= 0.001). Higher functional impairment was linked to lower levels of adenosine in patients (rho= -0.551, p= 0.008). Taken together, our results provide evidence for a purinergic imbalance in bipolar disorder, specifically an adenosinergic dysfunction. Our results also indicate a relation between adenosine levels and the functional impairment caused by the disorder, which could demonstrate a potential relation of adenosine levels in worsening of symptoms.

Translating drug-induced hibernation to therapeutic hypothermia

Therapeutic hypothermia (TH) improves prognosis after cardiac arrest; however, thermoregulatory responses such as shivering complicate cooling. Hibernators exhibit a profound and safe reversible hypothermia without any cardiovascular side effects by lowering the shivering threshold at low ambient temperatures (Ta). Activation of adenosine A1 receptors (A1ARs) in the central nervous system (CNS) induces hibernation in hibernating species and a hibernation-like state in rats, principally by attenuating thermogenesis. Thus, we tested the hypothesis that targeted activation of the central A1AR combined with a lower Ta would provide a means of managing core body temperature (Tb) below 37 °C for therapeutic purposes. We targeted the A1AR within the CNS by combining systemic delivery of the A1AR agonist (6)N-cyclohexyladenosine (CHA) with 8-(p-sulfophenyl)theophylline (8-SPT), a nonspecific adenosine receptor antagonist that does not readily cross the blood-brain barrier. Results show that CHA (1 mg/kg) and 8-SPT (25 mg/kg), administered intraperitoneally every 4 h for 20 h at a Ta of 16 °C, induce and maintain the Tb between 29 and 31 °C for 24 h in both naïve rats and rats subjected to asphyxial cardiac arrest for 8 min. Faster and more stable hypothermia was achieved by continuous infusion of CHA delivered subcutaneously via minipumps. Animals subjected to cardiac arrest and cooled by CHA survived better and showed less neuronal cell death than normothermic control animals. Central A1AR activation in combination with a thermal gradient shows promise as a novel and effective pharmacological adjunct for inducing safe and reversible targeted temperature management.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Altered thermoregulation via sensitization of A1 adenosine receptors in dietary-restricted rats

RATIONALE

Evidence links longevity to dietary restriction (DR). A decrease in body temperature (T(b)) is thought to contribute to enhanced longevity because lower T(b) reduces oxidative metabolism and oxidative stress. It is as yet unclear how DR decreases T(b).

OBJECTIVE

Here, we test the hypothesis that prolonged DR decreases T(b) by sensitizing adenosine A(1) receptors (A(1)AR) and adenosine-induced cooling.

METHODS AND RESULTS

Sprague-Dawley rats were dietary restricted using an every-other-day feeding protocol. Rats were fed every other day for 27 days and then administered the A(1)AR agonist, N(6)-cyclohexyladenosine (CHA; 0.5 mg/kg, i.p.). Respiratory rate (RR) and subcutaneous T(b) measured using IPTT-300 transponders were monitored every day and after drug administration. DR animals displayed lower RR on day 20 and lower T(b) on day 22 compared to animals fed ad libitum and displayed a larger response to CHA. In all cases, RR declined before T(b). Contrary to previous reports, a higher dose of CHA (5 mg/kg, i.p.) was lethal in both dietary groups. We next tested the hypothesis that sensitization to the effects of CHA was due to increased surface expression of A(1)AR within the hypothalamus. We report that the abundance of A(1)AR in the membrane fraction increases in hypothalamus, but not cortex of DR rats.

CONCLUSION

These results suggest that every-other-day feeding lowers T(b) via sensitization of thermoregulatory effects of endogenous adenosine by increasing surface expression of A(1)AR.

DISCUSSION

Evidence that diet can modulate purinergic signaling has implications for the treatment of stroke, brain injury, epilepsy, and aging.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Brief ischemia causes long-term depression in midbrain dopamine neurons

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

Neuroprotection (including hypothermia)

There have been over 2000 publications in the last year addressing the topic of neuroprotection. Novel and emerging therapeutic targets that have been explored include cerebral inflammation, hypothermia, neural transplantation and repair and gene therapy. Unfortunately, with few exceptions, the successes of experimental neuroprotection have not been translated into clinical practice. The possible reasons for the discrepancy between experimental success and clinical benefit are explored.

NMDA receptor-mediated extracellular adenosine accumulation is blocked by phosphatase 1/2A inhibitors

We have previously demonstrated that NMDA receptor-mediated extracellular adenosine accumulation in neuronal cultures is receptor-mediated and requires calcium influx. Because protein kinase C (PKC) is a calcium-dependent enzyme, we hypothesized that activation of PKC might be involved in NMDA-mediated adenosine accumulation. PKC inhibitors, however, did not block NMDA-evoked adenosine accumulation, but rather, stimulated basal adenosine accumulation. These data suggested the possibility that NMDA receptor-mediated adenosine accumulation involves net dephosphorylation rather than phosphorylation of one or more substrates. Thus, inhibition of kinases would be expected to increase adenosine accumulation and inhibition of phosphatases would be expected to block adenosine accumulation. To test this hypothesis, we used the phosphatase 1/2A inhibitors calyculin A and okadaic acid. Both inhibitors significantly reduced NMDA-evoked adenosine accumulation. In contrast phosphatase 2B inhibitors did not block NMDA-evoked adenosine accumulation. These data suggest that NMDA-evoked adenosine accumulation is mediated by activation of phosphatase 1/2A. We have established previously that NMDA-mediated adenosine accumulation is associated with adenosine kinase inhibition. However, adenosine kinase is not a direct substrate for phosphatase 1/2A because inhibition of phosphatase 1/2A did not abolish NMDA-evoked adenosine kinase inhibition. Okadaic acid also had no effect on NO donor-evoked adenosine accumulation, which previously has been shown to be associated with adenosine kinase inhibition. Dephosphorylation of one or more proteins other than adenosine kinase as a consequence of NMDA receptor activation might play an important role in extracellular adenosine regulation, with important consequences for the regulation of excitatory synaptic transmission, plasticity, epileptogenesis, and excitotoxicity.

Diadenosine polyphosphate analog controls postsynaptic excitation in CA3-CA1 synapses via a nitric oxide-dependent mechanism

Previously, we have described the modulatory effect of diadenosine polyphosphates Ap4A and Ap5A on synaptic transmission in the rat hippocampal slices mediated by presynaptic receptors (Klishin et al., 1994). In contrast, we now describe how nonhydrolyzable Ap4A analog diadenosine-5',5'''-P1,P4-[beta,beta'-methylene]tetraphosphate (AppCH2ppA) at low micromolar concentrations exerts strong nondesensitizing inhibition of orthodromically evoked field potentials (OFPs) without affecting the amplitude of excitatory postsynaptic currents and antidromically evoked field potentials, as recorded in hippocampal CA1 zone. The effects of AppCH2ppA on OFPs are eliminated by a P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) but not mimicked by purinoceptor agonists alpha,beta-methylene-ATP and adenosine 5'-O-(3-thio)-triphosphate, indicating that a P2-like receptor is involved but not one belonging to the conventional P2X/P2Y receptor classes. Diadenosine polyphosphate receptor (P4) antagonist Ip4I (diinosine tetraphosphate) was unable to modulate AppCH2ppA effects. Thus, the PPADS-sensitive P2-like receptor for AppCH2ppA seems to control selectively dendritic excitation of the CA1 neurons. The specific nitric oxide (NO)-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide is shown to significantly attenuate AppCH2ppA-mediated inhibitory effects, indicating that NO is involved in the cascade of events initiated by AppCH2ppA. Further downstream mediation by adenosine A1 receptors is also demonstrated. Hence, AppCH2ppA-mediated effects involve PPADS-sensitive P2-like receptor activation leading to the production of NO that stimulates intracellular synthesis of adenosine, causing in turn postsynaptic A1 receptor activation and subsequent postsynaptic CA1 dendritic inhibition. Such spatially selective postsynaptic dendritic inhibition may influence dendritic electrogenesis in pyramidal neurons and consequently mediate control of neuronal network activity.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.

Development and characterization of biodegradable nanospheres as delivery systems of anti-ischemic adenosine derivatives

We report a preliminary study concerning the encapsulation modalities in nanoparticles of the anti-ischemic drug N6-cyclopentyladenosine (CPA) and its pro-drug 5'-octanoyl-CPA (Oct-CPA). The release of these compounds and the related pro-drug stability effects in human whole blood have been tested. Moreover, the influence of the delivery systems on CPA interaction toward human adenosine A1 receptor has been analysed. The nanospheres were prepared by nanoprecipitation or double emulsion solvent evaporation method using poly(lactic acid) and recovered by gel filtration or ultracentrifugation or dialysis. Free and encapsulated Oct-CPA was incubated in fresh blood and its stability was analysed with HPLC. Quite spherical nanoparticles with mean diameters ranging between 210+/-50 and 390+/-90 nm were obtained. No encapsulation occurred when CPA was used. Satisfactory results concerning drug content (0.1-1.1% w/w) and encapsulation efficiency (6-56%) were achieved when Oct-CPA was employed. The controlled release of the pro-drug was achieved, being released within a range of 1-4 h, or very slowly, depending on nanoparticle preparations. The hydrolysis rate of Oct-CPA in human whole blood appeared stabilized in human whole blood with modalities related to the release patterns. The presence of all nanoparticle preparations did not interfere with CPA interaction at its action site.

Deletion of the adenosine A1 receptor gene does not alter neuronal damage following ischaemia in vivo or in vitro

Extracellular adenosine is dramatically increased during cerebral ischaemia and is considered to be neuroprotective due to its inhibitory effect on synaptic transmission mediated by the adenosine A1 receptor (A1R). We investigated the importance of the A1R in a mouse model of global ischaemia and in a murine hippocampal slice culture model of in vitro ischaemia, using mice with the A1R gene deleted. In brains from mice lacking the A1R, damage induced by global ischaemia was similar to that in wild-type animals. In contrast, treatment with a selective A1R antagonist [8-cyclo-pentyl theophylline (8-CPT)], administered before the ischaemic insult in naive wild-type mice, exacerbated the neuronal damage following global ischaemia. Although the inhibitory action of adenosine on excitatory neurotransmission in hippocampal slices was lost in A1R knockout mice, there was no difference in damage between slices from wild-type and knockout mice after in vitro ischaemia. The results suggest that some effects of the A1R are compensated for in knockout animals.

Purines and neuroprotection

The activation of adenosine A1, A2 andA3 receptors can protect neurones against damage generated by mechanical or hypoxic/ischaemic insults as well as excitotoxins. A1 receptors are probably effective by suppressing transmitter release and producing neuronal hyperpolarisation. They are less likely to be of therapeutic importance due to the plethora of side effects resulting from A1 agonism, although the existence of receptor subtypes and recent synthetic chemistry efforts to increase ligand selectivity, may yet yield clinically viable compounds. Activation of A2A receptors can protect neurons, although there is much uncertainty as to whether agonists are acting centrally or via a peripheral mechanism such as altering blood flow or immune cell function. Selective antagonists at the A2A receptor, such as 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-yl-amino]ethyl)phenol (ZM 241385) and 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261), can also protect against neuronal death produced by ischaemia or excitotoxicity. In addition, A2A receptor antagonists can reduce damage produced by combinations of subthreshold doses of the endogenous excitotoxin quinolinic acid and free radicals. Since the A2A receptors do not seem to be activated by normal endogenous levels of adenosine, their blockade should not generate significant side effects, so that A2A receptor antagonists appear to be promising candidates as new drugs for the prevention of neuronal damage. Adenosine A3 receptors have received less attention to date, but agonists are clearly able to afford protection against damage when administered chronically. Given the disappointing lack of success of NMDA receptor antagonists in human stroke patients, despite their early promise in animal models, it is possible that A2A receptor antagonists could have a far greater clinical utility.

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.

NMDA receptor-mediated extracellular adenosine accumulation in rat forebrain neurons in culture is associated with inhibition of adenosine kinase

The effect of N-methyl-d-aspartate (NMDA) on regulation of extracellular adenosine was investigated in rat forebrain neurons in culture. NMDA evoked accumulation of extracellular adenosine with an EC50 value of 4.8 +/- 1.2 microM. The effect of NMDA was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d] cyclohepten-5,10-imine hydrogen maleate indicating that NMDA receptor activation was involved. The NMDA effect was also blocked by chelation of extracellular Ca2+ indicating that influx of calcium was required. The nitric oxide-cyclic GMP signalling pathway was not involved, as nitric oxide synthase inhibitors were unable to block, and cGMP analogs were unable to mimic, the effect of NMDA. The source for extracellular adenosine was likely to be intracellular adenosine as the ecto-5'-nucleotidase inhibitor alpha beta-methylene-ADP was unable to block the effect of NMDA. One possible cause of intracellular adenosine accumulation might be NMDA receptor-mediated inhibition of mitochondrial function and ATP hydrolysis. We found that NMDA caused a concentration dependent depletion of intracellular ATP with an EC50 value of 21 +/- 8 microM. NMDA also caused a significant decrease in adenosine kinase activity, assayed by two different methods. Consistent with the hypothesis that inhibition of adenosine kinase is sufficient to cause an increase in extracellular adenosine, inhibition of adenosine kinase by 5'-iodotubercidin resulted in elevation of extracellular adenosine. However, in the presence of a concentration of 5'-iodotubercidin that inhibited over 90% of adenosine kinase activity, exposure to NMDA still caused adenosine accumulation. These studies suggest that several possible mechanisms are likely to be involved in NMDA-evoked extracellular adenosine accumulation.

The adenosine A1 receptor agonist adenosine amine congener exerts a neuroprotective effect against the development of striatal lesions and motor impairments in the 3-nitropropionic acid model of neurotoxicity

Huntington's disease is a genetic neurodegenerative disorder characterized clinically by both motor and cognitive impairments and striatal lesions. At present, there are no pharmacological treatments able to prevent or slow its development. In the present study, we report the neuroprotective effect of adenosine amine congener (ADAC), a specific A1 receptor agonist known to be devoid of any of the side effects that usually impair the clinical use of such compounds. Remarkably, in a rat model of Huntington's disease generated by subcutaneous infusion of the mitochondrial inhibitor 3-nitropropionic acid (3NP), we have observed that an acute treatment with ADAC (100 microg x kg(-1) x d(-1)) not only strongly reduces the size of the striatal lesion (-40%) and the remaining ongoing striatal degeneration (-30%), but also prevents the development of severe dystonia of hindlimbs. Electrophysiological recording on corticostriatal brain slices demonstrated that ADAC strongly decreases the field EPSP amplitude by 70%, whereas it has no protective effect up to 1 microm against the 3NP-induced neuronal death in primary striatal cultures. This suggests that ADAC protective effects may be mediated presynaptically by the modulation of the energetic impairment-induced striatal excitotoxicity. Altogether, our results indicate that A1 receptor agonists deserve further experimental evaluation in animal models of Huntington's disease.

The adenosine A1 receptor agonist adenosine amine congener exerts a neuroprotective effect against the development of striatal lesions and motor impairments in the 3-nitropropionic acid model of neurotoxicity

Huntington's disease is a genetic neurodegenerative disorder characterized clinically by both motor and cognitive impairments and striatal lesions. At present, there are no pharmacological treatments able to prevent or slow its development. In the present study, we report the neuroprotective effect of adenosine amine congener (ADAC), a specific A1 receptor agonist known to be devoid of any of the side effects that usually impair the clinical use of such compounds. Remarkably, in a rat model of Huntington's disease generated by subcutaneous infusion of the mitochondrial inhibitor 3-nitropropionic acid (3NP), we have observed that an acute treatment with ADAC (100 microg x kg(-1) x d(-1)) not only strongly reduces the size of the striatal lesion (-40%) and the remaining ongoing striatal degeneration (-30%), but also prevents the development of severe dystonia of hindlimbs. Electrophysiological recording on corticostriatal brain slices demonstrated that ADAC strongly decreases the field EPSP amplitude by 70%, whereas it has no protective effect up to 1 microm against the 3NP-induced neuronal death in primary striatal cultures. This suggests that ADAC protective effects may be mediated presynaptically by the modulation of the energetic impairment-induced striatal excitotoxicity. Altogether, our results indicate that A1 receptor agonists deserve further experimental evaluation in animal models of Huntington's disease.

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.

Rapid tolerance to focal cerebral ischemia in rats is attenuated by adenosine A1 receptor antagonist

Two types of ischemic tolerance in the brain, rapid and delayed, have been reported in terms of the interval between the conditioning and test insults. Although many reports showed that delayed-phase neuroprotection evoked by preconditioning is evident after 1 week or longer, there have been a few investigations about rapidly induced tolerance, and the reported neuroprotective effects become ambiguous 7 days after the insults. The authors examined whether this rapid ischemic tolerance exists after 7 days of reperfusion in a rat focal ischemic model, and investigated modulating effects of the adenosine A 1 receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine). Preconditioning with 30 minutes of middle cerebral artery occlusion reduced infarct volume 7 days after 180 minutes of subsequent focal ischemia given after 1-hour reperfusion. The rapid preconditioning also improved neurologic outcome. These beneficial effects were attenuated by pretreatment of 0.1 mg/kg DPCPX, which did not influence the infarct volume after conditioning (30 minutes) or test (180 minutes) ischemia when given alone. The results show that preconditioning with a brief focal ischemia induces rapid tolerance to a subsequent severe ischemic insult, the effect of which is still present after 7 days of reperfusion, and that the rapid ischemic tolerance is possibly mediated through an adenosine A 1 receptor-related mechanism.

Adenosine in the treatment of stroke: yes, maybe, or absolutely not?

Agonist stimulation of adenosine A(1) receptors has been consistently shown to result in reduction of brain damage following experimentally induced global and focal brain ischaemia in animals. Unsurprisingly, the use of adenosine A(1) receptors as targets for the development of clinical therapeutics suitable for treatment of ischaemic brain disorders has been suggested by many authors. The latest studies of adenosine and its receptors indicate that adenosine-mediated actions might be far more complex than originally anticipated, casting some doubt about the rapid development of stroke treatment based on adenosine. This review discusses the possible role of adenosine receptor subtypes (A(1), A(2) and A(3)) in the context of their potential as therapeutics in stroke.

Seizure activity produces differential changes in adenosine A1 receptors within rat hippocampus

Specific ligand binding to rat hippocampal adenosine A1 receptor after administration of the convulsant drug 3-mercaptopropionic acid (MP) was studied by means of a quantitative autoradiographic method. 2-Chloro-N6-[cyclopentyl-2,3,4,5-3H adenosine] ([3H]CCPA), a potent and selective A1 receptor ligand, was selected for binding studies. MP administration (150 mg/kg, i.p.), at seizure, caused significant increases in the following CA1 layers: pyramidal (45%), radiatum (18%) and lacunosum molecular (35%); in CA2 area, a significant decrease in stratum oriens (36%) and an increase in stratum radiatum (14%) and lacunosum molecular (33%) layers was observed. In CA3 area a rise in pyramidal (40%) and radiatum layers (26%), as well as in hillus (97%) was found. At postseizure, changes were restricted to CA1, CA2 and CA3 pyramidal layers and to CA1 lacunosum molecular layer, with increases ranging from 22 to 50%. These results show that [3H]CCPA binding is modified diversely in intrahippocampal layers and areas, thus indicating their dissimilar role in seizure activity.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

Direct measurement of adenosine release during hypoxia in the CA1 region of the rat hippocampal slice

We have used an enzyme-based, twin-barrelled sensor to measure adenosine release during hypoxia in the CA1 region of rat hippocampal slices in conjunction with simultaneous extracellular field recordings of excitatory synaptic transmission. When loaded with a combination of adenosine deaminase, nucleoside phosphorylase and xanthine oxidase, the sensor responded linearly to exogenous adenosine over the concentration range 10 nM to 20 microM. Without enzymes, the sensor when placed on the surface of hippocampal slices recorded a very small net signal during hypoxia of 40 +/- 43 pA (mean +/- s.e.m.; n = 7). Only when one barrel was loaded with the complete sequence of enzymes and the other with the last two in the cascade did the sensor record a large net difference signal during hypoxia (1226 +/- 423 pA; n = 7). This signal increased progressively during the hypoxic episode, scaled with the hypoxic depression of the simultaneously recorded field excitatory postsynaptic potential and was greatly reduced (67 +/- 6.5 %; n = 9) by coformycin (0.5-2 microM), a selective inhibitor of adenosine deaminase, the first enzyme in the enzymic cascade within the sensor. For 5 min hypoxic episodes, the sensor recorded a peak concentration of adenosine of 5.6 +/- 1.2 microM (n = 16) with an IC(50) for the depression of transmission of approximately 3 microM. In slices pre-incubated for 3-6 h in nominally Ca(2+)-free artificial cerebrospinal fluid, 5 min of hypoxia resulted in an approximately 9-fold greater release of adenosine (48.9 +/- 17.7 microM; n = 6). High extracellular Ca(2+) (4 mM) both reduced the adenosine signal recorded by the sensor during hypoxia (3.5 +/- 0.6 microM; n = 4) and delayed the hypoxic depression of excitatory synaptic transmission.

Cyclopentyladenosine improves cell proliferation, wound healing, and hair growth

BACKGROUND

N(6)-Cyclopentyladenosine (CPA), a structural analog of adenosine, is a vasodilator with extensive pharmacological effects. However, little is known about the effect of CPA on wound healing and hair growth.

METHODS

Cellular responses to CPA were measured in vitro by tetrazolium dye reduction and in vivo by bromodeoxyuridine (BrdU) uptake. The effect of CPA on healing of incisional and excisional wounds on the dorsum of diabetic (db/db, n = 94) and nondiabetic (db/+, n = 20) mice and hair growth along the wound margin was evaluated with wound breaking strength, wound closure rate, and quantitative histology.

RESULTS

CPA stimulated proliferation of BALB/3T3 fibroblasts and human dermal microvascular endothelial cells in both quiescent and nonquiescent phases. Wounds treated with CPA at 10 microM showed a significant increase in the number of BrdU-labeled cells, including keratinocytes, fibroblasts, endothelial cells, and cells in sebaceous glands and the outer root sheath of hair follicles, compared with controls (P < 0.05). CPA application (5.1 microg/daily for 12 days) significantly increased the breaking strength of incisional wounds at day 24 postwound (P < 0.05). Excisional wound closure rate in the CPA-treated group (3.4 microg/daily for 15 days) was accelerated starting at day 10 postwound compared with controls (P < 0.01). Tissue sections from CPA-treated wounds showed a sevenfold increase in hair follicle number, compared with controls (P < 0.01). Enhanced hair growth along the wound margin was revealed in CPA-treated groups.

CONCLUSION

CPA stimulated proliferation of many cell types in vivo and in vitro and enhanced wound healing and hair growth. Therefore, CPA could be an interesting candidate for clinical application.

The intrastratial injection of an adenosine A(2) receptor antagonist prevents frontal cortex EEG abnormalities in a rat model of Huntington's disease

The influence of 3,7-dimethyl-1-propargylxanthine (DMPX) an adenosine A(2) receptor antagonist, was studied in the quinolinic acid (QA) model of Huntington's disease. Male Wistar rats received bilateral intrastriatal injections of QA (300 nmol) alone or plus DMPX (0.02, 0.2 and 2 microg). At the dose of 0.2 microg, DMPX completely prevented QA-induced EEG abnormalities at the level of frontal cortex. The results support the hypothesis of a neuroprotective role of adenosine A(2) receptor antagonists.

Early changes in adenosine A1 receptors in cerebral cortex slices submitted to in vitro ischemia

The effects of brain ischemia on the maximum binding capacity (Bmax) and affinity (Kd) of A1 receptors were studied in the rat cerebral cortex, with an in vitro approach. The results were correlated with changes in 3H-adenosine release, studied under identical experimental conditions. Fifteen minutes of in vitro 'ischemia' (hypoxic, glucose-free medium) induced a significant increase in both Bmax (2398+/-132 fmol/mg protein, 151% of the control, P < 0.05) and in Kd (2.43+/-0.12 nM, 161% of the control, P < 0.01). At the same time, an increase in tritium efflux from [3H]-adenosine labeled cerebral cortex slices to 324% of the control was observed. A trend toward normalization was evident 5-15 min after 'reoxygenation' (restoring normal medium), but the binding parameters were still altered after 60 min (Bmax 2110+/-82 fmol/mg protein, Kd 2.26+/-0.14 nM, P < 0.01 vs the corresponding control) as was adenosine release (196% of the control). These findings suggest that the increased availability of adenosine and its receptors may be a defense mechanism against ischemic injury, while the reduced affinity of A1 receptors, possibly due to desensitization, may be a sign of ischemia-induced cellular damage.

Dysregulation of adenosine A1 receptor-mediated cytokine expression in peripheral blood mononuclear cells from multiple sclerosis patients

Cytokines, including tumor necrosis factor-alpha (TNF alpha) and interleukin-6 (IL-6), have been implicated in the pathogenesis of multiple sclerosis (MS). The production and release of these cytokines are regulated in part by specific purinergic (adenosine) cell surface receptors. To determine the extent to which the adenosine A1 receptor influenced cytokine expression in peripheral blood mononuclear cells (PBMCs) from MS and control patients, we measured plasma adenosine and TNF alpha levels, A1 receptor messenger RNA (mRNA) and protein amounts, and the effects of activation of A1 receptors on TNF alpha and IL-6 production by PBMCs. Plasma levels of TNF alpha were significantly higher and adenosine levels were significantly lower in MS patients compared with control subjects. Levels of TNF alpha and IL-6 in mitogen-stimulated PBMC culture supernatants from MS patients or control patients were similar. Conversely, treatment of PBMCs with the adenosine A1 receptor agonist R-phenylisopropyladenosine (R-PIA) (1 microM) significantly inhibited mitogen-stimulated production of TNF alpha but not IL-6 in control subjects and significantly inhibited production of IL-6 but not TNF alpha in MS patients. The effects of R-PIA were selectively blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). A1 receptor protein levels were decreased significantly in PBMCs from MS patients. Taken together, these results suggest that decreased levels of adenosine and its A1 receptor modulate TNF alpha and IL-6 levels and may contribute to the pathogenesis of MS.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A1 receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.

Protection against ischemic damage by adenosine amine congener, a potent and selective adenosine A1 receptor agonist

Although the selectivity and potency of adenosine amine congener (ADAC) at adenosine A1 receptors are similar to other highly selective agonists at this receptor type, the chemical structure of the N6 substituent is completely different. We now demonstrate that the characteristics of the therapeutic profile of ADAC are distinct from those observed during our previous studies of adenosine A1 receptor agonist-mediated neuroprotection. Most significantly, chronic treatment with low microgram doses of ADAC (25-100 microg/kg) protects against both mortality and neuronal damage induced by 10 min bilateral carotid occlusion in gerbils. At higher chronic doses, the statistical significance of the protective effect is lost. Acute preischemic administration of the drug at 75-200 microg/kg also results in a statistically significant reduction of postischemic mortality and morbidity. These data indicate that, contrary to other adenosine A1 receptor agonists whose chronic administration enhances postocclusive brain damage, ADAC may be a promising agent in treatment of both acute (e.g., cerebral ischemia) and chronic (seizures) disorders of the central nervous system in which adenosine A receptors appear to be involved.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.