Adenosine A1 receptor activation preferentially protects cultured cerebellar neurons versus astrocytes against hypoxia-induced death

Caffeine and Its Neuroprotective Role in Ischemic Events: A Mechanism Dependent on Adenosine Receptors

Ischemia is characterized by a transient, insufficient, or permanent interruption of blood flow to a tissue, which leads to an inadequate glucose and oxygen supply. The nervous tissue is highly active, and it closely depends on glucose and oxygen to satisfy its metabolic demand. Therefore, ischemic conditions promote cell death and lead to a secondary wave of cell damage that progressively spreads to the neighborhood areas, called penumbra. Brain ischemia is one of the main causes of deaths and summed with retinal ischemia comprises one of the principal reasons of disability. Although several studies have been performed to investigate the mechanisms of damage to find protective/preventive interventions, an effective treatment does not exist yet. Adenosine is a well-described neuromodulator in the central nervous system (CNS), and acts through four subtypes of G-protein-coupled receptors. Adenosine receptors, especially A₁ and A_2A receptors, are the main targets of caffeine in daily consumption doses. Accordingly, caffeine has been greatly studied in the context of CNS pathologies. In fact, adenosine system, as well as caffeine, is involved in neuroprotection effects in different pathological situations. Therefore, the present review focuses on the role of adenosine/caffeine in CNS, brain and retina, ischemic events.

Cyclic adenosine monophosphate in acute ischemic stroke: some to update, more to explore

The development of effective treatment for ischemic stroke, which is a common cause of morbidity and mortality worldwide, remains an unmet goal because the current first-line treatment management interventional therapy has a strict time window and serious complications. In recent years, a growing body of evidence has shown that the elevation of intracellular and extracellular cyclic adenosine monophosphate (cAMP) alleviates brain damage after ischemic stroke by attenuating neuroinflammation in the central nervous system and peripheral immune system. In the central nervous system, upregulated intracellular cAMP signaling can alleviate immune-mediated damage by restoring neuronal morphology and function, inhibiting microglia migration and activation, stabilizing the membrane potential of astrocytes and improving the cellular functions of endothelial cells and oligodendrocytes. Enhancement of the extracellular cAMP signaling pathway can improve neurological function by activating the cAMP-adenosine pathway to reduce immune-mediated damage. In the peripheral immune system, cAMP can act on various immune cells to suppress peripheral immune function, which can alleviate the inflammatory response in the central nervous system and improve the prognosis of acute cerebral ischemic injury. Therefore, cAMP may play key roles in reducing post-stroke neuroinflammatory damage. The protective roles of the cAMP indicate that the cAMP enhancing drugs such as cAMP supplements, phosphodiesterase inhibitors, adenylate cyclase agonists, which are currently used in the treatment of heart and lung diseases. They are potentially able to be applied as a new therapeutic strategy in ischemic stroke. This review focuses on the immune-regulating roles and the clinical implication of cAMP in acute ischemic stroke.

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Attenuation of cerebral vasospasm and secondary injury by 17beta-estradiol following experimental subarachnoid hemorrhage

OBJECT

Cerebral vasospasm remains a major complication in patients who have suffered a subarachnoid hemorrhage (SAH). Previous studies have shown that 17beta-estradiol (E2) attenuates experimental SAH-induced cerebral vasospasm. Moreover, E2 has been shown to reduce neuronal apoptosis and secondary injury following cerebral ischemia. Adenosine A1 receptor (AR-A1) expression is increased following ischemia and may represent an endogenous neuroprotective effect. This study was designed to evaluate the efficacy of E2 in preventing cerebral vasospasm and reducing secondary injury, as evidenced by DNA fragmentation and AR-A1 expression, following SAH.

METHODS

A double-hemorrhage model of SAH in rats was used, and the degree of vasospasm was determined by averaging the cross-sectional areas of the basilar artery 7 days after the first SAH. A cell death assay was used to detect apoptosis. Changes in the protein expression of AR-A1 in the cerebral cortex, hippocampus, and dentate gyrus were compared with levels in normal controls and E2-treated groups (subcutaneous E2, 0.3 mg/ml).

RESULTS

The administration of E2 prevented vasospasm (p < 0.05). Seven days after the first SAH, DNA fragmentation and protein levels of AR-A1 were significantly increased in the dentate gyrus. The E2 treatment decreased DNA fragmentation and prevented the increase in AR-A1 expression in the dentate gyrus. There were no significant changes in DNA fragmentation and the expression of AR-A1 after SAH in the cerebral cortex and hippocampus in the animals in the control and E2-treated groups.

CONCLUSIONS

The E2 was effective in attenuating SAH-induced cerebral vasospasm, decreasing apoptosis in the dentate gyrus, and reducing the expression of AR-A1 in the dentate gyrus after SAH. Interestingly, E2 appears to effectively prevent cerebral vasospasm subsequent to SAH as well as attenuate secondary injury by reducing both apoptosis and a compensatory increase in AR-A1 expression in the dentate gyrus.

Increase of adenosine A1 receptor gene expression in cerebral ischemia of Wistar rats

In an attempt to know the role of adenosine A1 receptor in cerebral ischemia, the present study employed the ligation of bilateral carotid arteries to induce ischemia in Wistar rats. Changes of gene expression of adenosine A1 receptor in cerebral cortex of ischemic rats were compared with normal sham control and reperfusion group that received regular blood flow after a transient ischemia. The mRNA level of adenosine A1 receptor in cerebral cortex was markedly raised by this artificial ischemia. Also, reperfusion reversed this elevation to a level near the control. This change was also observed at the protein level using Western blot analysis of adenosine A1 receptor. The raised protein level of adenosine A1 receptor by ischemia was reversed to normal level after reperfusion. These data suggest that the gene expression of adenosine A1 receptor was increased by ischemia probably due to the compensative response of brain. The raised adenosine A1 receptor may play a protective role in these damaged tissues.

Different roles of adenosine A1, A2A and A3 receptors in controlling kainate-induced toxicity in cortical cultured neurons

Adenosine is a neuromodulator that can control brain damage through activation of A(1), A(2A) and A(3) receptors, which are located in both neurons and other brain cells. We took advantage of cultured neurons to investigate the role of neuronal adenosine receptors in the control of neurotoxicity caused by kainate and cyclothiazide. Both A(1), A(2A) and A(3) receptors were immunocytochemically identified in cortical neurons. Activation of A(1) receptors with 100 nM CPA did not modify the extent of neuronal death whereas the A(1) receptor antagonist, DPCPX (50 nM), attenuated neurotoxicity by 28 +/- 5%, and effect similar to that resulting from the removal of endogenous adenosine with 2U/ml of adenosine deaminase (27 +/- 3% attenuation of neurotoxicity). In the presence of adenosine deaminase, DPCPX had no further effect and CPA now exacerbated neurotoxicity by 42 +/- 4%. Activation of A(2A) receptor with 30 nM CGS21680 attenuated neurotoxicity by 40 +/- 8%, an effect prevented by the A(2A) receptor antagonists, SCH58261 (50 nM) or ZM241385 (50 nM), which by themselves were devoid of effect. Finally, neither A(3) receptor activation with Cl-IB-MECA (100-500 nM) nor blockade with MRS1191 (5 microM) modified neurotoxicity. These results show that A(1) receptor activation enhances and A(2A) receptor activation attenuates neurotoxicity in cultured cortical neurons, indicating that these two neuronal adenosine receptors directly control neurodegeneration. Interestingly, the control by adenosine of neurotoxicity in cultured neurons is similar to that observed in vivo in newborn animals and is the opposite of what is observed in adult brain preparations where A(1) receptor activation and A(2A) receptor blockade are neuroprotective.

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.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Neuroprotective effect of adenine on purkinje cell survival in rat cerebellar primary cultures

Although adenosine or ATP is known to control various physiological functions in the brain, including synaptic transmission, neuronal cell death, and neurite outgrowth via P1 or P2 purinergic receptors in the nervous system, little is known about the functions of many other purine derivatives. We examined the effects of various purines on survival in the cerebellar cortex of Purkinje cells with large cell bodies and highly branched dendrites, and it was found that some purine and pyrimidine derivatives influence Purkinje cell survival. Treatment with adenine, guanine, guanosine, guanine nucleotides, and uracil nucleotides protected Purkinje cells from cell death in the cerebellar primary cultures. Among the effective compounds, adenine had the most potent survival activities on Purkinje cells. Other adenine-based purines such as adenosine, AMP, ADP, and ATP did not promote Purkinje cell survival. Furthermore, metabolic inhibitors of adenine had no effect on the protective ability of adenine for Purkinje cells, suggesting that adenine itself, not adenine metabolites, maintains Purkinje cell survival. These results suggest that adenine is involved in the control of Purkinje cell survival in cerebellar primary cultures via a novel adenine-dependent mechanism.

Differential NF-kappa B regulation of bcl-x gene expression in hippocampus and basal forebrain in response to hypoxia

Cell death often occurs after hypoxic/ischemic injury to the central nervous system. Changes in levels of the anti-apoptotic Bcl-X(L) protein may be a determining factor in hypoxia-induced neuronal apoptosis. The transcription factor NF-kappa B regulates bcl-x gene expression. In this study, we examined the role of NF-kappa B in the regulation of bcl-x in hypoxia-induced cell death. Rat hippocampus and basal forebrain tissues were collected at different time points after hypoxia (7%O(2), 93% N(2) for 10 or 20 min). We found that 1) hypoxia induced apoptosis in the hippocampus and basal forebrain; 2) the NF-kappa B dimers c-Rel/p50 and p50/p50 bound to the bcl-x promoter NF-kappa B sequence (CS4) in the hippocampus, but only p50/p50 bound to the CS4 sequence in the basal forebrain and hypoxia-induced differential binding patterns of c-Rel/p50 and p50/p50 correlated with the bcl-x expression pattern in the hippocampus; 3) the hypoxia-induced patterns of binding of c-Rel/p50 to the bcl-x promoter CS4 sequence were different from those to the IgG-kappa B enhancer sequence, whereas those of p50/p50 were similar to both sequences; 4) nuclear protein levels of c-Rel, but not p50, correlated with the c-Rel/p50 DNA binding patterns to the bcl-x CS4 site; and 5) there were differential responses to hypoxia among the different NF-kappa B protein subunits. These results suggest that there is a tissue-specific regulation of bcl-x gene expression by NF-kappa B in hypoxia-induced cell death in the hippocampus. The absence of these regulating features in the basal forebrain may account for the early appearance of apoptosis in response to hypoxia as compared with that in hippocampus.

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.

Neuroprotection by metabotropic glutamate receptor glutamate receptor agonists: LY354740, LY379268 and LY389795

In rat cortical neuronal cultures, metabotropic glutamate (mGlu) receptor agonists: LY354740 (+)-2-aminobicyclo[3.1.0]hexane-2,6dicarboxylate); LY379268 (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate, and LY389795 (-)-2-thia-4-aminobicyclo[3.1.0]-hexane-4,6-dicarboxylate, were neuroprotective against toxicity induced by N-methyl-D-aspartic acid (NMDA), kainic acid and staurosporine as measured by release of lactate dehydrogenase (LDH) activity into culture supernatants and DNA fragmentation by oligonucleosome formation. The potencies of the agonists were at least 100 times greater in reducing nucleosome formation than LDH release indicating a differential effect on neurons dying by apoptosis than by necrosis. In vivo studies showed that LY354740 was able to mediate a partial protection against apoptosis in CA1 hippocampal cells under ischaemic conditions where substantial CA1 cell loss occurred. The effects of the agonists in vitro were: (a) reversed by mGlu receptor antagonist LY341495, (b) enhanced by the presence of glial cells, (c) abrogated by RNA and protein synthesis inhibitors, and (d) unaltered by inhibition of endogenous adenosine activity. These results suggest that group II mGlu receptor agonists may represent a novel therapeutic strategy for the treatment of neurodegenerative diseases.