Flow thresholds for extracellular purine catabolite elevation in cat focal ischemia

Chronic treatment with adenosine A1 receptor antagonist promotes neurogenesis and improves outcome after cerebral ischemia

Adenosine A1 receptors (A1ARs) are promising targets for stroke treatment, potentially due to their relatively unexplored effects on proliferation and differentiation of newborn neurons. In this study, we investigated the impact of chronic treatment with the A1ARs antagonist DPCPX on neurogenesis following MCAO in rodents, using PET with [18F]FLT in rats and immunohistochemistry in mice. In addition, we assessed the therapeutic properties of DPCPX on stroke recovery with a comprehensive battery of neurological and behavioral tests. The outcome shows that blocking A1ARs signaling with DPCPX improved immunohistochemical results in 8 to 28 days after MCAO in mice. PET imaging with [18F]FLT revealed an increase in cellular proliferation following DPCPX treatment in the subventricular zone at day 8 post-ischemia in rats, a result further supported by IHC in SVZ of ischemic animals. Furthermore, DPCPX enhanced the production and integration of newborn neurons while reducing astrocytic differentiation in the ischemic areas. Finally, behavioral tests showed that chronic treatment with DPCPX ameliorated motor and memory deficits after brain ischemia. All taken in consideration, our results provide novel and compelling evidence of the therapeutic potential of the A1AR antagonist DPCPX for ischemic stroke recovery.

Disruption of CD73-Derived and Equilibrative Nucleoside Transporter 1-Mediated Adenosine Signaling Exacerbates Oxygen-Induced Retinopathy

Retinopathy of prematurity (ROP) is characterized by pathologic angiogenesis in retina, and remains a leading cause of blindness in children. Although enhanced extracellular adenosine is markedly increased in response to retinal hypoxia, adenosine acting at the A1 and A2A receptors has the opposite effect on pathologic angiogenesis. Herein, the oxygen-induced retinopathy (OIR) model of ROP was used to demonstrate that pharmacologic and genetic inactivation of CD73 (the key 5'-ectonucleotidase for extracellular generation of adenosine) did not affect normal retinal vasculature development but exacerbated intravitreal neovascularization at postnatal day (P) 17 and delayed revascularization at P21 of OIR. This exacerbated damage to retinal vessels by CD73 inactivation was associated with increased cellular apoptosis and microglial activation but decreased astrocyte function at P17 of OIR. Furthermore, pharmacologic blockade of equilibrative nucleoside transporter 1/2 (ENT1/2; bidirectional transport for controlling the balance of intracellular and extracellular adenosine) by 6-nitrobenzylthioinosine aggravated pathologic angiogenesis at P17 of OIR. Pharmacologic blockade of ENT1/2 and genetic inactivation of CD73 also aggravated avascular areas at the hyperoxia phase (P12) of OIR. Thus, disruption of CD73-derived extracellular adenosine or ENT1/2-mediated transport of adenosine flux across membrane aggravated the damage to retinal vessels. These findings support the role of adenosine as an endogenous protective regulator that limits oxygen-induced retinopathy. Thus, enhancing extracellular adenosine signaling represents a novel neuroprotection strategy for ROP by targeting CD73 and ENT1/2 activities.

Influence of BMI on adenosine deaminase and stroke outcomes in mechanical thrombectomy subjects

BACKGROUND

Emergent Large Vessel Occlusion (ELVO) strokes are ischemic vascular events for which novel biomarkers and therapies are needed. The purpose of this study is to investigate the role of Body Mass Index (BMI) on protein expression and signaling at the time of ELVO intervention. Additionally, we highlight the protein adenosine deaminase (ADA), which is a deaminating enzyme that degrades adenosine, which has been shown to be neuroprotective in ischemia. We investigate the relationship between ADA and BMI, stroke outcomes, and associated proteomic networks which might aid in personalizing prognosis and future treatment of ELVO stroke.

METHODS

The Blood And Clot Thrombectomy And Collaboration (BACTRAC) study is a continually enrolling tissue bank (clinicaltrials.gov NCT03153683) and registry from stroke patients undergoing mechanical thrombectomy (MT). N ​= ​61 human carotid plasma samples were analyzed for inflammatory and cardiometabolic protein expression by Olink Proteomics. Statistical analyses used t-tests, linear, logistic, and robust regressions, to assess the relationship between BMI, proteomic expression, and stroke-related outcomes.

RESULTS

The 61 subjects studied were broken into three categories: normal weight (BMI 18.5-24.9) which contained 19 subjects, overweight (BMI 25-30) which contained 25 subjects, and obese (BMI ≥30) which contained 17 subjects. Normal BMI group was a significantly older population (mean 76 years) when compared to overweight (mean 66 years) and obese (mean 61 years) with significance of p ​= ​0.041 and p ​= ​0.005, respectively. When compared to normal weight and overweight categories, the obese category had significantly higher levels of adenosine deaminase (ADA) expression (p ​= ​0.01 and p ​= ​0.039, respectively). Elevated levels of ADA were found to have a significant positive correlation with both infarct volume and edema volume (p ​= ​0.013 and p ​= ​0.041, respectively), and were associated with a more severe stroke (NIHSS on discharge) and greater stroke related disability (mRS on discharge) with significance of p ​= ​0.053 and p ​= ​0.032, respectively.

CONCLUSIONS

When examined according to BMI, subjects undergoing MT for ELVO demonstrate significant differences in the expression of certain plasma proteins, including ADA. Levels of ADA were found to be significantly higher in the obese population when compared to normal or overweight groups. Increased levels of ADA in the obese group were predictive of increased infarct volume, edema volume, and worse NIHSS scores and mRS at discharge. These data provide novel biomarker candidates as well as treatment targets while increasing the personalization of stroke prognosis and treatment.

Purines: From Diagnostic Biomarkers to Therapeutic Agents in Brain Injury

The purines constitute a family of inter-related compounds that serve a broad range of important intracellular and extracellular biological functions. In particular, adenosine triphosphate (ATP) and its metabolite and precursor, adenosine, regulate a wide variety of cellular and systems-level physiological processes extending from ATP acting as the cellular energy currency, to the adenosine arising from the depletion of cellular ATP and responding to reduce energy demand and hence to preserve ATP during times of metabolic stress. This inter-relationship provides opportunities for both the diagnosis of energy depletion during conditions such as stroke, and the replenishment of ATP after such events. In this review we address these opportunities and the broad potential of purines as diagnostics and restorative agents.

Spatiotemporal analysis of impaired microglia process movement at sites of secondary neurodegeneration post-stroke

It has recently been identified that after motor cortex stroke, the ability of microglia processes to respond to local damage cues is lost from the thalamus, a major site of secondary neurodegeneration (SND). In this study, we combine a photothrombotic stroke model in mice, acute slice and fluorescent imaging to analyse the loss of microglia process responsiveness. The peri-infarct territories and thalamic areas of SND were investigated at time-points 3, 7, 14, 28 and 56 days after stroke. We confirmed the highly specific nature of non-responsive microglia processes to sites of SND. Non-responsiveness was at no time observed at the peri-infarct but started in the thalamus seven days post-stroke and persisted for 56 days. Loss of directed process extension is not a reflection of general functional paralysis as phagocytic function continued to increase over time. Additionally, we identified that somal P₂Y₁₂ was present on non-responsive microglia in the first two weeks after stroke but not at later time points. Finally, both classical microglia activation and loss of process extension are highly correlated with neuronal damage. Our findings highlight the importance of microglia, specifically microglia dynamic functions, to the progression of SND post-stroke, and their potential relevance as modulators or therapeutic targets during stroke recovery.

Intracerebral microdialysis of adenosine and adenosine monophosphate - a systematic review and meta-regression analysis of baseline concentrations

Microdialysis is a method to study the extracellular space in vivo, based on the principle of diffusion. It can be used to measure various small molecules including the neuroregulator adenosine. Baseline levels of the compounds measured with microdialysis vary over studies. We systematically reviewed the literature to investigate the full range of reported baseline concentrations of adenosine and adenosine monophosphate in microdialysates. We performed a meta‐regression analysis to study the influence of flow rate, probe membrane surface area, species, brain area and anaesthesia versus freely behaving, on the adenosine concentration. Baseline adenosine concentrations in microdialysates ranged from 0.8 to 2100 nM. There was limited evidence on baseline adenosine monophosphate concentrations in microdialysates. Across studies, we found effects of flow rate and anaesthesia versus freely behaving on dialysate adenosine concentrations (p ≤ 0.001), but not of probe membrane surface, species, or brain area (p ≥ 0.14). With increasing flow rate, adenosine concentrations decreased. With anaesthesia, adenosine concentrations increased. The effect of other predictor variables on baseline adenosine concentrations, for example, post‐surgical recovery time, could not be analysed because of a lack of reported data. This study shows that meta‐regression can be used as an alternative to new animal experiments to answer research questions in the field of neurochemistry. However, current levels of reporting of primary studies are insufficient to reach the full potential of this approach; 63 out of 133 studies could not be included in the analysis because of insufficient reporting, and several potentially relevant factors had to be excluded from the analyses. The level of reporting of experimental detail needs to improve.

Acute hypoxic exposure and prolyl-hydroxylase inhibition improves synaptic transmission recovery time from a subsequent hypoxic insult in rat hippocampus

In the CNS short episodes of acute hypoxia can result in a decrease in synaptic transmission which may be fully reversible upon re-oxygenation. Stabilization of hypoxia-inducible factor (HIF) by inhibition of prolyl hydroxylase domain (PHD) enzymes has been shown to regulate the cellular response to hypoxia and confer neuroprotection both in vivo and in vitro. Hypoxic preconditioning has become a novel therapeutic target to induce neuroprotection during hypoxic insults. However, there is little understanding of the effects of repeated hypoxic insults or pharmacological PHD inhibition on synaptic signaling. In this study we have assessed the effects of hypoxic exposure and PHD inhibition on synaptic transmission in the rat CA1 hippocampus. Field excitatory postsynaptic potentials (fEPSPs) were elicited by stimulation of the Schaffer collateral pathway. 30 min hypoxia (gas mixture 95% N₂/5% CO₂) resulted in a significant and fully reversible decrease in fEPSP slope associated with decreases in partial pressures of tissue oxygen. 15-30 min of hypoxia was sufficient to induce stabilization of HIF in hippocampal slices. Exposure to a second hypoxic insult after 60 min resulted in a similar depression of fEPSP slope but with a significantly greater rate of recovery of the fEPSP. Prior single treatment of slices with the PHD inhibitor, dimethyloxalylglycine (DMOG) also resulted in a significantly greater rate of recovery of fEPSP post hypoxia. These results suggest that hypoxia and 'pseudohypoxia' preconditioning may improve the rate of recovery of hippocampal neurons to a subsequent acute hypoxia.

Effects of ticagrelor in a mouse model of ischemic stroke

Ticagrelor is a direct acting and reversibly binding P2Y12 antagonist approved for the prevention of thromboembolic events. Its potential benefits in ischemic stroke have not been investigated sufficiently. Mice were subjected to 2 hours of transient middle cerebral artery occlusion (MCAO). Mice were orally treated with ticagrelor (10 or 30 mg/kg), aspirin (60 mg/kg), or vehicle at 3 and 24 hours before MCAO and 0 and 6 hours after reperfusion. The infarct volume and neurological deficits 22 hours after reperfusion were evaluated. Cerebral blood flow (CBF) within 24 hours after MCAO was monitored. We performed western blotting and in vitro analysis using oxygen-glucose deprivation (OGD) stress in human brain microvessel endothelial cells (HBMVECs) to investigate the protective effects of ticagrelor. Ticagrelor (30 mg/kg) improved neurological deficits, reduced the infarct volume, and improved CBF. It promoted the phosphorylation of endothelial nitric oxide synthase (eNOS) and extracellular signal-regulated kinase 1/2 (ERK1/2) during the early phase after reperfusion. Increased phosphorylation of eNOS and ERK1/2 were also observed in HBMVECs after OGD stress. Ticagrelor attenuate ischemia reperfusion injury possibly via phosphorylation of eNOS and ERK1/2 in endothelial cells. This suggests that ticagrelor has neuroprotective effects via mechanisms other than its antiplatelet action.

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'.

Prolonged adenosine A1 receptor activation in hypoxia and pial vessel disruption focal cortical ischemia facilitates clathrin-mediated AMPA receptor endocytosis and long-lasting synaptic inhibition in rat hippocampal CA3-CA1 synapses: differential regulation of GluA2 and GluA1 subunits by p38 MAPK and JNK

Activation of presynaptic adenosine A1 receptors (A1Rs) causes substantial synaptic depression during hypoxia/cerebral ischemia, but postsynaptic actions of A1Rs are less clear. We found that A1Rs and GluA2-containing AMPA receptors (AMPARs) form stable protein complexes from hippocampal brain homogenates and cultured hippocampal neurons from Sprague Dawley rats. In contrast, adenosine A2A receptors (A2ARs) did not coprecipitate or colocalize with GluA2-containing AMPARs. Prolonged stimulation of A1Rs with the agonist N(6)-cyclopentyladenosine (CPA) caused adenosine-induced persistent synaptic depression (APSD) in hippocampal brain slices, and APSD levels were blunted by inhibiting clathrin-mediated endocytosis of GluA2 subunits with the Tat-GluA2-3Y peptide. Using biotinylation and membrane fractionation assays, prolonged CPA incubation showed significant depletion of GluA2/GluA1 surface expression from hippocampal brain slices and cultured neurons. Tat-GluA2-3Y peptide or dynamin inhibitor Dynasore prevented CPA-induced GluA2/GluA1 internalization. Confocal imaging analysis confirmed that functional A1Rs, but not A2ARs, are required for clathrin-mediated AMPAR endocytosis in hippocampal neurons. Pharmacological inhibitors or shRNA knockdown of p38 MAPK and JNK prevented A1R-mediated internalization of GluA2 but not GluA1 subunits. Tat-GluA2-3Y peptide or A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine also prevented hypoxia-mediated GluA2/GluA1 internalization. Finally, in a pial vessel disruption cortical stroke model, a unilateral cortical lesion compared with sham surgery reduced hippocampal GluA2, GluA1, and A1R surface expression and also caused synaptic depression in hippocampal slices that was consistent with AMPAR downregulation and decreased probability of transmitter release. Together, these results indicate a previously unknown mechanism for A1R-induced persistent synaptic depression involving clathrin-mediated GluA2 and GluA1 internalization that leads to hippocampal neurodegeneration after hypoxia/cerebral ischemia.

Adenosine A2A receptors modulate acute injury and neuroinflammation in brain ischemia

The extracellular concentration of adenosine in the brain increases dramatically during ischemia. Adenosine A_2A receptor is expressed in neurons and glial cells and in inflammatory cells (lymphocytes and granulocytes). Recently, adenosine A_2A receptor emerged as a potential therapeutic attractive target in ischemia. Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia the early massive increase of extracellular glutamate is followed by activation of resident immune cells, that is, microglia, and production or activation of inflammation mediators. Proinflammatory cytokines, which upregulate cell adhesion molecules, exert an important role in promoting recruitment of leukocytes that in turn promote expansion of the inflammatory response in ischemic tissue. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. A_2A receptors present on central cells and on blood cells account for important effects depending on the time-related evolution of the pathological condition. Evidence suggests that A_2A receptor antagonists provide early protection via centrally mediated control of excessive excitotoxicity, while A_2A receptor agonists provide protracted protection by controlling massive blood cell infiltration in the hours and days after ischemia. Focus on inflammatory responses provides for adenosine A_2A receptor agonists a wide therapeutic time-window of hours and even days after stroke.

Continuous adenosine A2A receptor antagonism after focal cerebral ischemia in spontaneously hypertensive rats

Antagonism of the adenosine A2A receptor (A2AR) has been shown to elicit substantial neuroprotective properties when given immediately after cerebral ischemia. We asked whether the continuous application of a selective A2AR antagonist within a clinically relevant time window will be a feasible and effective approach to treat focal cerebral ischemia. To answer this question, we subjected 20 male spontaneously hypertensive rats to permanent middle cerebral artery occlusion and randomized them equally to a verum and a control group. Two hours after stroke onset, the animals received a subcutaneous implantation of an osmotic minipump filled with 5 mg kg(-1) day(-1) 8-(3-chlorostyryl) caffeine (CSC) or vehicle solution. The serum level of CSC was measured twice a day for three consecutive days. The infarct volume was determined at days 1 and 3 using magnetic resonance imaging. We found the serum level of CSC showing a bell-shaped curve with its maximum at 36 h. The infarct volume was not affected by continuous CSC treatment. These results suggest that delayed and continuous CSC application was not sufficient to treat acute ischemic stroke, potentially due to unfavorable hepatic elimination and metabolization of the pharmaceutical.

Hypoxia-inducible factor signalling mechanisms in the central nervous system

In the CNS, neurones are highly sensitive to the availability of oxygen. In conditions where oxygen availability is decreased, neuronal function can be altered, leading to injury and cell death. Hypoxia has been implicated in a number of central nervous system pathologies including stroke, head trauma and neurodegenerative diseases. Cellular responses to oxygen deprivation are complex and result in activation of short- and long-term mechanisms to conserve energy and protect cells. Failure of synaptic transmission can be observed within minutes following this hypoxia. The acute effects of hypoxia on synaptic transmission are primarily mediated by altering ion fluxes across membranes, pre-synaptic effects of adenosine and other actions at glutamatergic receptors. A more long-term feature of the response of neurones to hypoxia is the activation of transcription factors such as hypoxia-inducible factor. The activation of hypoxia-inducible factor is governed by a family of dioxygenases called hypoxia-inducible factor prolyl 4 hydroxylases (PHDs). Under hypoxic conditions, PHD activity is inhibited, thereby allowing hypoxia-inducible factor to accumulate and translocate to the nucleus, where it binds to the hypoxia-responsive element sequences of target gene promoters. Inhibition of PHD activity stabilizes hypoxia-inducible factor and other proteins thus acting as a neuroprotective agent. This review will focus on the response of neuronal cells to hypoxia-inducible factor and its targets, including the prolyl hydroxylases. We also present evidence for acute effects of PHD inhibition on synaptic transmission and plasticity in the hippocampus.

Basic physiology of hyperbaric oxygen in brain

Oxygen is the proverbial 'double-edged sword' in that it is a necessity for life in moderation and toxic and detrimental to life in excess. This too is the dilemma in hyperbaric oxygen (HBO) treatment in cerebral ischemic-anoxic insults such as stroke, head injury, near drowning, asphyxia, cardiac arrest, etc., i.e. the brain at risk, where regions of ischemia are beside regions of marked hyperemia. The natural heterogeneity of normal brain tissue oxygenation compounds the problem with different microvascular brain regions living at various levels of oxygenation from 0 to arterial PO(2) as an added complication. The application of HBO, whether normobaric or hyperbaric, will result in brain tissue oxygenation ranging from normoxic to highly hyperoxic with the latter possibly exacerbating the injury sustained. On this basis, the application of multiple therapeutic interventions may be considered, for example, HBO in combination with free radical scavengers or inhibitors of free radical generating enzymes. Despite these difficulties in moderating oxygen delivery to treat cerebral ischemic-anoxic insults, overwhelming preclinical evidence indicates that HBO administered during or within 2 hours post-insult effectively attenuates the severity of brain damage sustained. The primary disconnection between pre-clinical and clinical efficacy of HBO then appears to be the time of application. Clinically, HBO therapy is applied at the earliest 6 hours post-insult but usually between 12 hours or longer post-insult. Pre-hospital application of HBO may be required for clear-cut demonstration of clinical efficacy.

Cortical spreading depression-induced preconditioning in mouse neocortex is lamina specific

Cortical spreading depression (CSD) is able to confer neuroprotection when delivered at least 1 day in advance of an ischemic event. However, its ability to confer neuroprotection in a more immediate time frame has not previously been investigated. Here we have used mouse neocortical brain slices to study the effects of repeated episodes of CSD in layer V and layer II/III pyramidal neurons. In layer V, CSD evoked at 15-min intervals caused successively smaller membrane depolarizations and increases in intracellular calcium compared with the response to the first CSD. With an inter-CSD interval of 30 min this preconditioning effect was much less marked, indicating that preconditioning lasts between 15 and 30 min. A single episode of CSD also provided a degree of protection in oxygen-glucose deprivation (OGD) by significantly lengthening the time a cell could withstand OGD before anoxic depolarization occurred. In layer II/III pyramidal neurons no preconditioning by CSD on subsequent episodes of CSD was observed, demonstrating that the response of pyramidal neurons to repeated CSD is lamina specific. The A1 receptor antagonist 8-cyclopentyl theophylline (8-CPT) reduced the layer V preconditioning in a concentration-related manner. Inhibition of extracellular formation of adenosine by blocking ecto-5'-nucleotidase with α,β-methyleneadenosine 5'-diphosphate prevented preconditioning in most but not all cells. Block of equilibrative nucleoside transporters 1 and 2 with dipyramidole alone or in combination with 6-[(4-nitrobenzyl)thio]-9-β-d-ribofuranosylpurine also prevented preconditioning in some but not all cells. These data provide evidence that rapid preconditioning of one CSD by another is primarily mediated by adenosine.

Neuroadaptations in adenosine receptor signaling following long-term ethanol exposure and withdrawal

Ethanol affects the function of neurotransmitter systems, resulting in neuroadaptations that alter neural excitability. Adenosine is one such receptor system that is changed by ethanol exposure. The current review is focused on the A(1) and the A(2A) receptor subtypes in the context of ethanol-related neuroadaptations and ethanol withdrawal because these subtypes (i) are activated by basal levels of adenosine, (ii) have been most well-studied for their role in neuroprotection and ethanol-related phenomena, and (iii) are the primary site of action for caffeine in the brain, a substance commonly ingested with ethanol. It is clear that alterations in adenosinergic signaling mediate many of the effects of acute ethanol administration, particularly with regard to motor function and sedation. Further, prolonged ethanol exposure has been shown to produce adaptations in the cell surface expression or function of both A(1) and the A(2A) receptor subtypes, effects that likely promote neuronal excitability during ethanol withdrawal. As a whole, these findings demonstrate a significant role for ethanol-induced adaptations in adenosine receptor signaling that likely influence neuronal function, viability, and relapse to ethanol intake following abstinence.

Adenosine 2A receptor: a crucial neuromodulator with bidirectional effect in neuroinflammation and brain injury

This review summarizes recent developments that have contributed to our understanding of how adenosine 2A receptors (A2ARs) modulate brain damage in various animal models of acute neurological injuries, including brain ischemia, traumatic brain injury, spinal cord injury and hemorrhage stroke. The main conclusions are: (1) pharmacological, neurochemical and molecular/genetic approaches to the complex actions of A2AR in different cellular elements suggest that A2AR activation exerts bidirectional effect (detrimental or protective) after brain insults; (2) modulation of glutamate excitotoxicity and neuroinflammation are involved in the protection of A2AR agonists or antagonists, but the bidirectional effect of A2AR is largely due to the bidirectional regulation of neuroinflammation (anti-inflammation or proinflammation) by A2AR on immune cells such as microglia cells and peripheral bone marrow cells; and (3) the bidirectional effect of A2AR on neuroinflammation and brain injury depends on the distinct and sometimes opposite actions of A2AR in various cellular elements and on different injury models and associated pathological conditions. The local glutamate level in the brain injury is one of the crucial factors that contribute to the direction of A2AR effect on neuroinflammation and brain injury outcome. These developments presented here clearly highlight the complexity of using A2AR agents therapeutically in acute neuronal injuries and confirm that A2AR ligands have many promising characteristics that encourage the pursuit of their full therapeutic potential.

Nuclear Factor κB and Adenosine Receptors: Biochemical and Behavioral Profiling

Adenosine is produced primarily by the metabolism of ATP and mediates its physiological actions by interacting primarily with adenosine receptors (ARs) on the plasma membranes of different cell types in the body. Activation of these G protein-coupled receptors promotes activation of diverse cellular signaling pathways that define their tissue-specific functions. One of the major actions of adenosine is cytoprotection, mediated primarily via two ARs - A(1) (A(1)AR) and A(3) (A(3)AR). These ARs protect cells exposed to oxidative stress and are also regulated by oxidative stress. Stress-mediated regulation of ARs involves two prominent transcription factors - activator protein-1 (AP-1) and nuclear factor (NF)-κB - that mediate the induction of genes important in cell survival. Mice that are genetically deficient in the p50 subunit of NF-κB (i.e., p50 knock-out mice) exhibit altered expression of A(1)AR and A(2A)AR and demonstrate distinct behavioral phenotypes under normal conditions or after drug challenges. These effects suggest an important role for NF-κB in dictating the level of expression of ARs in vivo, in regulating the cellular responses to stress, and in modifying behavior.

Impacts of methylxanthines and adenosine receptors on neurodegeneration: human and experimental studies

Neurodegenerative disorders are some of the most feared illnesses in modern society, with no effective treatments to slow or halt this neurodegeneration. Several decades after the earliest attempt to treat Parkinson's disease using caffeine, tremendous amounts of information regarding the potential beneficial effect of caffeine as well as adenosine drugs on major neurodegenerative disorders have accumulated. In the first part of this review, we provide general background on the adenosine receptor signaling systems by which caffeine and methylxanthine modulate brain activity and their role in relationship to the development and treatment of neurodegenerative disorders. The demonstration of close interaction between adenosine receptor and other G protein coupled receptors and accessory proteins might offer distinct pharmacological properties from adenosine receptor monomers. This is followed by an outline of the major mechanism underlying neuroprotection against neurodegeneration offered by caffeine and adenosine receptor agents. In the second part, we discuss the current understanding of caffeine/methylxantheine and its major target adenosine receptors in development of individual neurodegenerative disorders, including stroke, traumatic brain injury Alzheimer's disease, Parkinson's disease, Huntington's disease and multiple sclerosis. The exciting findings to date include the specific in vivo functions of adenosine receptors revealed by genetic mouse models, the demonstration of a broad spectrum of neuroprotection by chronic treatment of caffeine and adenosine receptor ligands in animal models of neurodegenerative disorders, the encouraging development of several A(2A) receptor selective antagonists which are now in advanced clinical phase III trials for Parkinson's disease. Importantly, increasing body of the human and experimental studies reveals encouraging evidence that regular human consumption of caffeine in fact may have several beneficial effects on neurodegenerative disorders, from motor stimulation to cognitive enhancement to potential neuroprotection. Thus, with regard to neurodegenerative disorders, these potential benefits of methylxanthines, caffeine in particular, strongly argue against the common practice by clinicians to discourage regular human consumption of caffeine in aging populations.

Brain hypoxanthine concentration correlates to lactate/pyruvate ratio but not intracranial pressure in patients with acute liver failure

BACKGROUND & AIMS

The pathogenesis of cerebral edema in acute liver failure is suggested, in in vitro and animal studies, to involve a compromised oxidative metabolism with a decrease in cerebral ATP levels and an increase in purine concentrations. In this study we hypothesize that the cerebral concentrations of hypoxanthine, inosine, and lactate/pyruvate (LP) ratio are increased and correlated in patients with acute liver failure. Furthermore, we expect the purines and L/P ratio to correlate with intracranial pressure (ICP) (positively), and cerebral perfusion pressure (CPP) (negatively).

METHODS

In 17 patients (aged 18-60 years) with acute liver failure and severe hyperammonemia (182 ± 36 μM (mean ± SD)), cerebral microdialysis was performed, and ICP and CPP were monitored. Microdialysate concentrations of hypoxanthine, inosine, lactate, and pyruvate were measured.

RESULTS

The hypoxanthine concentration was 23.0 ± 12 μM in early samples and 11.7 ± 6.8 μM in late samples (normal level ~2.0 μM). The inosine concentration was 7.2 ± 7.1 μM and 2.8 ± 1.6 μM, and the LP ratio was 55.8 ± 21.6 and 45.6 ± 20.8, respectively (normal level ~18). Hypoxanthine correlated significantly to LP ratio (r(2)=0.40, p<0.01) while inosine did not. The purine levels and L/P ratio did not correlate to ICP or CPP, nor did they differ between patients with high ICP (>20 mmHg, n=9) and patients without (n=8).

CONCLUSIONS

This study shows that the high cerebral LP ratio correlates to the hypoxanthine level in patients with acute liver failure. However, these metabolic alterations were not related to the development of intracranial hypertension.

Adenosine signaling and function in glial cells

Despite major advances in a variety of neuroscientific research fields, the majority of neurodegenerative and neurological diseases are poorly controlled by currently available drugs, which are largely based on a neurocentric drug design. Research from the past 5 years has established a central role of glia to determine how neurons function and, consequently, glial dysfunction is implicated in almost every neurodegenerative and neurological disease. Glial cells are key regulators of the brain's endogenous neuroprotectant and anticonvulsant adenosine. This review will summarize how glial cells contribute to adenosine homeostasis and how glial adenosine receptors affect glial function. We will then move on to discuss how glial cells interact with neurons and the vasculature, and outline new methods to study glial function. We will discuss how glial control of adenosine function affects neuronal cell death, and its implications for epilepsy, traumatic brain injury, ischemia, and Parkinson's disease. Eventually, glial adenosine-modulating drug targets might be an attractive alternative for the treatment of neurodegenerative diseases. There are, however, several major open questions that remain to be tackled.

Release of adenosine and ATP during ischemia and epilepsy

Eighty years ago Drury & Szent-Györgyi described the actions of adenosine, AMP (adenylic acid) and ATP (pyrophosphoric or diphosphoric ester of adenylic acid) on the mammalian cardiovascular system, skeletal muscle, intestinal and urinary systems. Since then considerable insight has been gleaned on the means by which these compounds act, not least of which in the distinction between the two broad classes of their respective receptors, with their many subtypes, and the ensuing diversity in cellular consequences their activation invokes. These myriad actions are of course predicated on the release of the purines into the extracellular milieu, but, surprisingly, there is still considerable ambiguity as to how this occurs in various physiological and pathophysiological conditions. In this review we summarise the release of ATP and adenosine during seizures and cerebral ischemia and discuss mechanisms by which the purines adenosine and ATP may be released from cells in the CNS under these conditions.

A central role of connexin 43 in hypoxic preconditioning

Preconditioning is an endogenous mechanism in which a nonlethal exposure increases cellular resistance to subsequent additional severe injury. Here we show that connexin 43 (Cx43) plays a key role in protection afforded by preconditioning. Cx43 null mice were insensitive to hypoxic preconditioning, whereas wild-type littermate mice exhibited a significant reduction in infarct volume after occlusion of the middle cerebral artery. In cultures, Cx43-deficient cells responded to preconditioning only after exogenous expression of Cx43, and protection was attenuated by small interference RNA or by channel blockers. Our observations indicate that preconditioning reduced degradation of Cx43, resulting in a marked increase in the number of plasma membrane Cx43 hemichannels. Consequently, efflux of ATP through hemichannels led to accumulation of its catabolic product adenosine, a potent neuroprotective agent. Thus, adaptive modulation of Cx43 can offset environmental stress by adenosine-mediated elevation of cellular resistance.

Temporal and mechanistic dissociation of ATP and adenosine release during ischaemia in the mammalian hippocampus

Adenosine is well known to be released during cerebral metabolic stress and is believed to be neuroprotective. ATP release under similar circumstances has been much less studied. We have now used biosensors to measure and compare in real time the release of ATP and adenosine during in vitro ischaemia in hippocampal slices. ATP release only occurred following the anoxic depolarisation, whereas adenosine release was apparent almost immediately after the onset of ischaemia. ATP release required extracellular Ca²⁺. By contrast adenosine release was enhanced by removal of extracellular Ca²⁺, whilst TTX had no effect on either ATP release or adenosine release. Blockade of ionotropic glutamate receptors substantially enhanced ATP release, but had only a modest effect on adenosine release. Carbenoxolone, an inhibitor of gap junction hemichannels, also greatly enhanced ischaemic ATP release, but had little effect on adenosine release. The ecto-ATPase inhibitor ARL 67156, whilst modestly enhancing the ATP signal detected during ischaemia, had no effect on adenosine release. Adenosine release during ischaemia was reduced by pre-treament with homosysteine thiolactone suggesting an intracellular origin. Adenosine transport inhibitors did not inhibit adenosine release, but instead they caused a twofold increase of release. Our data suggest that ATP and adenosine release during ischaemia are for the most part independent processes with distinct underlying mechanisms. These two purines will consequently confer temporally distinct influences on neuronal and glial function in the ischaemic brain.

Adenosine A2A receptors are expressed by GABAergic neurons of medulla oblongata in developing rat

During early development, adenosine contributes to the occurrence of respiratory depression and recurrent apneas. Recent physiological studies indicate that GABAergic mechanisms may be involved in this inhibitory action of adenosine, via their A(2A) receptors. In the present study, in situ hybridization with ribonucleotide probes for A(2A) receptor (A(2A)R) mRNA was combined with the immunolabeling technique for parvalbumin and transneuronal retrograde tracing method using green fluorescent protein expressing pseudorabies virus (GFP-PRV) to (1) characterize age-dependent changes in the expression of adenosine A(2A)Rs mRNA in brain stem regions where GABAergic neurons are located; (2) determine whether GABA-containing neurons express A(2A)R mRNA traits, and (3) identify whether bulbospinal GABAergic neurons projecting to phrenic nuclei contain A(2A)R mRNA. Results revealed expression of A(2A) receptors in regions of medulla oblongata containing GABAergic neurons, namely in the ventral aspect of the medulla, within the Bötzinger region and caudal to it, the gigantocellular reticular nucleus, midline neurons and the caudal ventrolateral medulla oblongata. Furthermore, a subpopulation of identified GABAergic neurons, projecting to the phrenic motor nuclei, possess A(2A)R mRNA. It is concluded that adenosine A(2A)Rs expressed by GABAergic neurons are likely to play a role in mediating adenosine-induced respiratory depression.

Extracellular correlates of glutamate toxicity in short-term cerebral ischemia and reperfusion: A direct in vivo comparison between white and gray matter

Glutamate toxicity and cellular calcium overload are thought to be pathophysiological key factors not only in gray matter (GM) but also in white matter (WM) ischemia. Correlates of excitotoxicity have never been directly investigated in vivo in GM and WM ischemia and reperfusion. We measured simultaneously amino acids, purines, and calcium in relation to tissue depolarization using microdialysis and ion-selective electrodes and regional CBF using hydrogen clearance in GM and WM of cats during 10 min of global ischemia and 120 min of reperfusion. CBF ceased during ischemia. Reperfusion was followed by hyperperfusion that turned into hypoperfusion within 60 min in both GM and WM. Direct current potential decreased in ischemia to around -15 mV in GM and -10 mV in WM and shifted back after reperfusion towards control levels in both compartments. Extracellular calcium decreased in GM during ischemia, whereas it increased in WM. After reperfusion, calcium returned to control levels in both GM and WM. Glutamate, aspartate, GABA, and taurine increased in GM but not in WM during ischemia and reperfusion. Adenosine increased transiently in both compartments peaking during the first minutes of reperfusion, and returned thereafter to control levels. Contrasting with GM, deleterious processes such as glutamate accumulation and cellular Ca(2+) influx do not occur in WM during short-term ischemia and reperfusion. Rather, an intrinsic neuroprotective role of adenosine may be discussed. In our view, therefore, therapeutic strategies against glutamate toxicity in short-term ischemia and reperfusion should be mainly focused on GM.

Peri-infarct depolarizations reveal penumbra-like conditions in striatum

Spreading depression-like peri-infarct depolarizations not only characterize but also worsen penumbra conditions in cortical border zones of experimental focal ischemia. We intended to investigate the relevance of ischemic depolarization in subcortical regions of ischemic territories. Calomel electrodes measured DC potentials simultaneously in the lateral and medial portions of the caudate nucleus (CN) of 11 anesthetized cats after permanent occlusion of the middle cerebral artery. Additionally, platinum electrodes measured cerebral blood flow (CBF) in the CN, and laser Doppler probes CBF in the cortex. Depolarizations (negative DC shifts >10 mV) were obtained in 10 of 11 cats. Further differentiation revealed that short-lasting spreading depression-like depolarizations (SDs; 5 of 10 cats: 5.24 +/- 1.22 min total duration; 23.3 +/- 4.2 mV amplitude) were predominantly found in medial and longer depolarizations (LDs; 4 of 10 cats: 64.7 +/- 47.5 min; 25.0 +/- 11.3 mV) in the lateral CN. Terminal depolarizations (TDs; 6 of 10 cats; without repolarization) occurred immediately after occlusion or at later stages, being then accompanied by elevations of intracranial pressure presumably inducing secondary CBF reduction. CBF tended to be lower in regions with TDs (33.3 +/- 29.9% of control) and LDs (37.3 +/- 22.8%) than in regions with SDs (51.5 +/- 48.0%). We conclude that in focal ischemia, transient peri-infarct depolarizations emerge not only in cortical but also in striatal gray matter, thereby demonstrating the existence of subcortical zones of ischemic penumbra. The generation of these ischemic depolarizations is a multifocal process possibly linked to brain swelling and intracranial pressure rise in the later course of focal ischemia, and therefore a relevant correlate of progressively worsening conditions.

Noise induces A1 adenosine receptor expression in the chinchilla cochlea

Adenosine plays a major cytoprotective role during ischemia and conditions of oxidative stress. Previous studies in our laboratory indicate that oxidative stress induces expression of the A1 adenosine receptor (A1AR) via activation of nuclear factor (NF)-kappaB. In this study, we tested whether noise exposure could induce oxidative stress and determine whether this induces expression of the A1AR in the chinchilla cochlea. Chinchillas were exposed to a 96 dB 4 kHz octave band of noise for 6 h of daily exposure, followed by an 18 h noise-free period. This noise paradigm resulted in threshold shifts of 10-60 dB over the frequency range (1-16 kHz) tested. Radioligand binding studies for the A1AR indicate a significant increase in receptor ( approximately 2-fold) expression soon after the first noise exposure period (usually within approximately 8 h of the initiation of noise), which gradually returned to basal levels by day 7. The rise in A1AR levels was followed by a significant increase in malondialdehyde levels by day 3, which also recovered by day 7. Assessment of the activity of NADPH oxidase in the cochlea indicates a significant increase in enzyme activity which was evident by approximately 8 h following initiation of noise exposure, and which persisted for at least up to day 3. Electrophoretic mobility shift assays indicate that the increase in A1AR was associated with a significant increase in NF-kappaB activity following noise exposure. We conclude that noise exposure induces A1AR expression, which might be mediated, in part, through generation of reactive oxygen species and activation of NF-kappaB.

Adrenoceptor subtype-specific acceleration of the hypoxic depression of excitatory synaptic transmission in area CA1 of the rat hippocampus

The depression of excitatory synaptic transmission by hypoxia in area CA1 of the hippocampus is largely dependent upon the activation of adenosine A(1) receptors on presynaptic glutamatergic terminals. As well as adenosine, norepinephrine levels increase in the hypoxic/ischemic hippocampus. We sought to determine the influence of alpha- and beta-adrenoceptor (AR) activation on the hypoxic depression of synaptic transmission utilizing electrophysiological, pharmacological and adenosine sensor techniques. Norepinephrine depressed synaptic transmission and significantly accelerated the hypoxic depression of synaptic transmission. The alpha-AR agonist 6-fluoronorepinephrine mimicked both of these effects whilst the alpha(2)-AR antagonist yohimbine, but not the alpha(1)-AR antagonist urapidil, prevented the actions of 6-fluoronorepinephrine. In contrast, the beta-AR agonist isoproterenol enhanced synaptic transmission and only accelerated the hypoxic depression of transmission in hypoxia-conditioned slices in which the hypoxic release of adenosine is reduced. The effects of isoproterenol were blocked by the non-selective beta-AR antagonist propranolol and the selective beta(1)-AR antagonist betaxolol. Using an enzyme-based adenosine sensor we observed that the application of the beta-AR agonist resulted in increased extracellular adenosine during repeated hypoxia. Our results suggest that alpha(2)-AR activation facilitates the hypoxic depression of synaptic transmission probably via the known alpha(2)-AR-mediated inhibition of presynaptic calcium channels whereas beta(1)-AR activation does so via increased extracellular adenosine and greater activation of inhibitory adenosine A(1) receptors.

Plasticity of purine release during cerebral ischemia: clinical implications?

Adenosine is a powerful modulator of neuronal function in the mammalian central nervous system. During a variety of insults to the brain, adenosine is released in large quantities and exerts a neuroprotective influence largely via the A(1) receptor, which inhibits glutamate release and neuronal activity. Using novel enzyme-based adenosine sensors, which allow high spatial and temporal resolution recordings of adenosine release in real time, we have investigated the release of adenosine during hypoxia/ischemia in the in vitro hippocampus. Our data reveal that during the early stages of hypoxia adenosine is likely released per se and not as a precursor such as cAMP or an adenine nucleotide. In addition, repeated hypoxia results in reduced production of extracellular adenosine and this may underlie the increased vulnerability of the mammalian brain to repetitive or secondary hypoxia/ischemia.

PET neuroreceptor imaging as predictor of severe cerebral ischemic insult

Measurement of the adenosine A1 receptor (A1-R) with positron emission tomography (PET) using a newly developed positron ligand, [1-methyl-11C]8-dicyclopropylmethyl-1-methyl-3-propylxanthine (MPDX). were performed in a cat middle cerebral artery (MCA) occlusion and reperfusion. Eighteen adult cats underwent PET measurement of; 1) cerebral blood flow (CBF). 2) A1-R, 3) central benzodiazepine receptor (BDZ-R) and 4) glucose metabolism with 15O labeled water, MPDX, 11C-flumazenil (FMZ) and 18F-fluorodeoxyglucose (FDG), respectively. The CBF, A1-R, BDZ-R and FDG uptake were serially measured after 60 min occlusion of MCA in this order. MPDX binding and FMZ binding, but not CBF and FDG uptake, were significantly reduced in the groups with severer ischemic insult than in the groups with no or milder insults. Of the two receptor ligands, the reduction rate of the MPDX binding to A1-Rs was larger in a group that caused fatal ischemic insult. The newly developed PET in vivo imaging technique using MPDX was suitable in evaluating the function of adenosine and A1-Rs in relation to cerebral ischemia.

Metabolic changes of arachidonic acid after cerebral ischemia–reperfusion in diabetic rats

The purpose of this study is to discuss an important component-arachidonic acid (AA) cascade of inflammatory reaction in diabetic rats with cerebral ischemia. Using the model of middle cerebral artery occlusion (MCAO), we have compared the expression of cyclooxygenase-2 (COX-2) and 5-lipoxygenase (5-LOX), and measured the levels of their products prostaglandin E2 (PGE(2)) and cysteine-containing leukotrienes (cys-LTs) after different reperfusion periods in diabetic and normal rats. Cerebral ischemia-reperfusion was accompanied by increased expression of COX-2 and release of PGE(2), peaking at 12 h after reperfusion. The expression of COX-2 was maintained at a high level until 24 h after reperfusion, while the levels of PGE(2) were declined rapidly to baseline. The expression of 5-LOX and levels of cys-LTs reached a peak at 6 and 12 h after reperfusion, respectively, and was returned to baseline at 24 h after reperfusion. Compared with normal rats, the expression of COX-2 and 5-LOX as well as release of PGE(2) and cys-LTs was elevated in the brains of diabetic rats, revealing a possible mechanism for hyperglycemia-mediated aggravation of cerebral ischemic injury. A reduction of arachidonic acid metabolites mediated by inhibitors of its metabolites could be helpful in preventing ischemic brain injury in diabetic rats.

Microglial ecto-Ca(2+)-ATPase activity in a rat model of focal homologous blood clot embolic cerebral ischemia: an enzyme histochemical study

Post-ischemic changes in ecto-Ca(2+)-ATPase activity in microglia and the infarcted tissue were studied in a rat model of focal embolic cerebral ischemia using an enzyme histochemical method. Ecto-Ca(2+)-ATPase activity was observed in whole brains in non-operated and sham-operated control animals. In addition, this enzyme activity was determined to be localized in ramified microglia. At 30 min after ischemia, non-microglial ecto-Ca(2+)-ATPase activity in the infarcted tissue slightly decreased and continued to decrease thereafter. The ecto-Ca(2+)-ATPase activity in microglia did not appear changed at this time. The decrease of enzyme activity in the infarcted tissue made it much easier to clearly observe ecto-Ca(2+)-ATPase-positive microglia. The enzyme activity of microglia in the ischemic area began to decrease 2 or 4h after embolization and remarkably decreased, except in the perinuclear cytoplasm, apical parts of the processes, and several parts along the processes, 8h after ischemia. By 12h after onset of embolization, the enzyme activity of microglia and infarcted tissue had almost completely disappeared. Ecto-Ca(2+)-ATPase of microglia is likely to play an important role in the metabolism of extracellular nucleotides in the ischemic area immediately after the onset of embolization by means of ecto-enzymes. Thus, the findings of the present study suggest that microglia might serve to protect the infarcted tissue in the ischemic brain.

The selective A2A receptor antagonist SCH 58261 reduces striatal transmitter outflow, turning behavior and ischemic brain damage induced by permanent focal ischemia in the rat

Adenosine A(2A) receptor antagonists have been proved protective in different ischemia models. In this study we verified if the protective effect of the selective A(2A) antagonist, SCH 58261, could be attributed to the reduction of the excitatory amino acid outflow induced by cerebral focal ischemia. A vertical microdialysis probe was inserted into the striatum of male Wistar rats and, after 24 h, permanent right intraluminal middle cerebral artery occlusion (MCAo) was induced. Soon after waking, rats showed a definite contralateral turning behavior, which persisted up to 7 h after MCAo. During 4 h after MCAo, glutamate, aspartate, GABA, adenosine and taurine outflow increased. SCH 58261 (0.01 mg/kg, i.p.), administered 5 min after MCAo, suppressed turning behavior and significantly reduced the outflow of glutamate, aspartate, GABA and adenosine. At 24 h after MCAo, the rats showed severe sensorimotor deficit and damage in both the striatum and cortex. SCH 58261 significantly reduced cortical damage but did not protect against the sensorimotor deficit. The protective effect of SCH 58261 against turning behavior and increased outflow of excitatory amino acids in the first hours after MCAo suggests the potential utility of selective adenosine A(2A) antagonists when administered in the first hours after ischemia. Furthermore, this study, for the first time, proposes that turning behavior after permanent intraluminal MCAo, be used as a precocious index of neurological deficit and neuronal damage.

Involvement of mitogen protein kinase cascade in agonist-mediated human A(3) adenosine receptor regulation

It has been suggested that A(3) adenosine receptors (ARs) play a role in the pathophysiology of cerebral ischemia with dual and opposite neuroprotective and neurodegenerative effects. This could be due to a receptor regulation mediated by rapid phosphorylation and desensitization carried out by intracellular kinases. In this study, we investigated the involvement of extracellular regulated kinase (ERK 1 and 2), members of the mitogen-activated protein kinase (MAPK) family, in A(3) AR phosphorylation. A(3) AR mediated the activation of ERK 1/2 with a typical transient monophasic kinetics (5 min). The activation was not affected by hypertonic sucrose cell pre-treatment, suggesting that this effect occurred independently of receptor internalization. The involvement of MAPK cascade in the A(3) AR regulation process was evaluated using two well-characterized MAPK kinase inhibitors, PD98059 (2-(2'-amino-3'-methoxyphenyl)oxanaphthalen-4-one) and U0126 (1,4-diamino-2,3-dicyano-1,4-bis (aminophenylthio) butadiene). The exposure of cells to PD98059 prevented MAPK activation and inhibited homologous A(3) AR desensitization and internalization, impairing agonist-mediated receptor phosphorylation. PD98059 inhibited the membrane translocation of G protein-coupled receptor kinase (GRK(2)), which is involved in A(3) AR homologous phosphorylation, suggesting this kinase as a target for the MAPK cascade. On the contrary, the chemically unrelated inhibitor of the MAPK cascade, U0126, did not significantly affect GRK(2) membrane translocation or receptor internalization. Nevertheless, the inhibitor induced a significant impairment of receptor phosphorylation and desensitization. These results suggested that the MAPK cascade is involved in A(3) AR regulation by a feedback mechanism which controls GRK(2) activity and probably involves a direct receptor phosphorylation.

Malignant infarction in cats after prolonged middle cerebral artery occlusion: glutamate elevation related to decrease of cerebral perfusion pressure

BACKGROUND AND PURPOSE

To study the putative role and predictive significance of glutamate elevation in space-occupying ischemic stroke, we investigated the correlation between perfusional disturbances and glutamate alterations in a transient ischemia model in cats that is susceptible to secondary deterioration after reperfusion.

METHODS

In 10 halothane-anesthetized cats, the left middle cerebral artery was occluded for 3 hours, followed by 6 hours of reperfusion. Laser-Doppler flowmetry (LDF) probes, microdialysis/high-performance liquid chromatography, and pressure sensors measured simultaneously regional cerebral blood flow (CBF), extracellular amino acids, mean arterial blood pressure, and intracranial pressure, respectively. Cerebral perfusion pressure (CPP) was calculated. In complementary experiments (n=2), regional CBF was assessed by sequential positron emission tomography.

RESULTS

Middle cerebral artery occlusion reduced LDF-measured CBF in all animals to <25% of control. In 5 of 10 cats, glutamate rose approximately 30-fold during ischemia. LDF-measured CBF and glutamate primarily recovered after reperfusion. Glutamate rose again in the late reperfusion phase, when CPP decreased to <60 mm Hg, and symptoms of transtentorial herniation were recognized. Positron emission tomography revealed ischemic thresholds of 15 to 20 mL/100 g per minute for secondary deterioration. In the other 5 cats, ischemic elevation of glutamate was significantly smaller, and signs of secondary deterioration were not recognized.

CONCLUSIONS

Glutamate determinations during ischemia predict fatal outcome, as do intracranial pressure and CPP measurements during early reperfusion. Secondary amino acid elevation during reperfusion is presumably caused by a drastic decrease of CPP to <50 mm Hg in the final stage of space-occupying, malignant focal ischemia. At this stage, a further progression of injury due to increased glutamate may be irrelevant with respect to fatal outcome.

Elevation of extracellular glutamate in the final, ischemic stage of progressive epidural mass lesion in cats

Epidural mass lesions may cause ischemia due to progressive intracranial hypertension. In order to investigate the impact of intracranial pressure on accumulation of neuroactive substances, we gradually raised intracranial pressure in five halothane anesthetized cats by inflation of an epidural balloon. We evaluated in the parietal cortex contralateral to the site of balloon inflation, alterations of extracellular glutamate and purine catabolites and of the lactate/pyruvate ratio in relation to changes of intracranial, cerebral perfusion and mean arterial blood pressure. In a complementary experiment, regional cerebral blood flow was assessed by sequential positron emission tomography. In this simplified mass lesion model, extracellular glutamate increased in all cats at a late, critical stage after tentorial herniation, when intracranial pressure had increased to more than 90 mm Hg, cerebral perfusion pressure had decreased below 40-50 mm Hg. Positron emission tomography assessments revealed that the ischemic threshold for glutamate accumulation was in the range of 15-20 mL/100 g/min. Purine catabolites and the lactate/pyruvate ratio increased somewhat earlier than glutamate, but also after reaching the critical, terminal stage. We conclude that in this model of progressive epidural compression, glutamate-mediated excitotoxic processes at sites remote from the initial focal lesion depend on processes such as delayed ischemia in combination with tentorial herniation and systemic hypotension. These processes seem to be initiated by a decrease of cerebral perfusion pressure below a threshold of 40-50 mm Hg.

Increased adenosine in cerebrospinal fluid after severe traumatic brain injury in infants and children: association with severity of injury and excitotoxicity

OBJECTIVES

To measure adenosine concentration in the cerebrospinal fluid of infants and children after severe traumatic brain injury and to evaluate the contribution of patient age, Glasgow Coma Scale score, mechanism of injury, Glasgow Outcome Score, and time after injury to cerebrospinal fluid adenosine concentrations. To evaluate the relationship between cerebrospinal fluid adenosine and glutamate concentrations in this population.

DESIGN

Prospective survey.

SETTING

Pediatric intensive care unit in a university-based children's hospital.

PATIENTS

Twenty-seven critically ill infants and children who had severe traumatic brain injury (Glasgow Coma Scale < 8), who required placement of an intraventricular catheter and drainage of cerebrospinal fluid as part of their neurointensive care.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Patients ranged in age from 2 months to 14 yrs. Cerebrospinal fluid samples (n = 304) were collected from 27 patients during the first 7 days after traumatic brain injury. Control cerebrospinal fluid samples were obtained from lumbar puncture on 21 infants and children without traumatic brain injury or meningitis. Adenosine concentration was measured by using high-pressure liquid chromatography. Adenosine concentration was increased markedly in cerebrospinal fluid of children after traumatic brain injury vs. controls (p < .001). The increase in cerebrospinal fluid adenosine was independently associated with Glasgow Coma Scale < or = 4 vs. > 4 and time after injury (both p < .005). Cerebrospinal fluid adenosine concentration was not independently associated with either age (< or = 4 vs. > 4 yrs), mechanism of injury (abuse vs. other), or Glasgow Outcome Score (good/moderately disabled vs. severely disabled, vegetative, or dead). Of the 27 patients studied, 18 had cerebrospinal fluid glutamate concentration previously quantified by high-pressure liquid chromatography. There was a strong association between increases in cerebrospinal fluid adenosine and glutamate concentrations (p < .005) after injury.

CONCLUSIONS

Cerebrospinal fluid adenosine concentration is increased in a time- and severity-dependent manner in infants and children after severe head injury. The association between cerebrospinal fluid adenosine and glutamate concentrations may reflect an endogenous attempt at neuroprotection against excitotoxicity after severe traumatic brain injury.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

Production of leukotrienes in a model of focal cerebral ischaemia in the rat

1. The aim of this work was to evaluate the role of leukotrienes in brain damage in vivo in a model of focal cerebral ischaemia in the rat, obtained by permanent occlusion of middle cerebral artery. 2. A significant (P < 0.01) elevation of LTC(4), LTD(4) and LTE(4) (cysteinyl-leukotrienes) levels occurred 4 h after ischaemia induction in the ipsilateral cortices of ischaemic compared to sham-operated animals (3998 +/- 475 and 897 +/- 170 fmol g(-1) tissue, respectively, P < 0.01). 3. The NMDA receptor antagonist MK-801 and the adenosine A(2A) receptor antagonist SCH 58261 were administered in vivo at doses known to reduce infarct size and compared with the leukotriene biosynthesis inhibitor MK-886. 4. MK-886 (0.3 and 2 mg kg(-1) i.v.) and MK-801 (3 mg kg(-1) i.p.) decreased cysteinyl-leukotriene levels (-78%, P < 0.05; -100%, P < 0.01; -92%, P < 0.01, respectively) 4 h after permanent occlusion of the middle cerebral artery, whereas SCH 58261 (0.01 mg kg(-1) i.v.) had no significant effects. 5. MK-886 (2 mg kg(-1) i.v.) was also able to significantly reduce the cortical infarct size by 30% (P < 0.05). 6. We conclude that cysteinyl-leukotriene formation is associated with NMDA receptor activation, and that it represents a neurotoxic event, the inhibition of which is able to reduce brain infarct area in a focal ischaemic event.

Adenosine, oxidative stress and cytoprotection

Adenosine, a metabolite of ATP, serves a number of important physiological roles in the body. These actions contribute to sedation, bradycardia, vasorelaxation, inhibition of lipolysis and regulation of the immune system and are mediated, in part, through activation of three distinct adenosine receptor (AR) subtypes. To date, four receptor types have been cloned: A1, A2A, A2B and A3. It is becoming increasing clear that adenosine contributes significantly to cytoprotection, a function mediated principally by the A1AR and A3AR. In this review, we survey the literature on the role of adenosine and the mechanisms underlying cytoprotection and ischemic preconditioning, a process characterized by cytoprotection derived from repeated brief ischemic challenges. An important recent observation is that the expression of several AR subtypes could be regulated by oxidative stress to provide a greater cytoprotective role. Thus, like other proteins known to be regulated during ischemia, the A1AR and A3AR can be considered as being inducible receptors.

Adenosine in relation to calcium homeostasis: comparison between gray and white matter ischemia

In vitro studies suggest that adenosine may attenuate anoxic white matter damage as an intrinsic protective substance. The authors investigated ischemic alterations of purines in relation to tissue depolarization and extracellular calcium and amino acid concentrations in vivo using microdialysis and ion-selective electrodes in cortical gray and subcortical white matter of 10 cats during 120 minutes of global brain ischemia. Immediately on induction of ischemia, regional cerebral blood flow ceased in all cats in both gray and white matter. The direct current potential rapidly decreased, the decline being slower and shallower in white matter. Extracellular calcium levels decreased in gray matter. In contrast, they first increased in white matter and started to decrease below control levels only after approximately 30 minutes. Adenosine levels transiently increased in both tissue compartments; the peak was delayed by 30 minutes in white matter. Thereafter, levels declined faster in gray than in white matter and remained elevated in the latter tissue compartment. Inosine and hypoxanthine elevations were progressive in both regions but smaller in white matter. Levels of gamma-aminobutyric acid, another putatively protective agent, steadily increased, starting immediately in gray matter and delayed by almost 1 hour in white matter. The delayed and prolonged accumulation of adenosine correlates with a slower adenosine triphosphate breakdown in white matter ischemia and may result in protection of white matter by suspending cellular calcium influx.

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.

Adenosine induces initial hypoxic-ischemic depression of synaptic transmission in the rat hippocampus in vivo

The present study was designed to investigate the role of adenosine in the hypoxic depression of synaptic transmission in rat hippocampus. An in vivo model of hypoxic synaptic depression was developed in which the common carotid artery was occluded on one side in the urethane-anesthetized rat. Inspired oxygen levels were controlled through a tracheal cannula. Rats were placed in a stereotaxic apparatus for stimulation and recording of bilateral hippocampal field excitatory postsynaptic potentials. The percent inspired oxygen could be reduced to levels that produced a reversible and repeatable depression of evoked synaptic transmission restricted to the hippocampus ipsilateral to the occlusion. Further reduction in the level of inspired oxygen depressed synaptic transmission recorded from both hippocampi. The adenosine nonselective antagonist caffeine and the A(1) selective antagonist 8-cyclopentyltheophylline prevented the initial depression in synaptic transmission. We conclude that the initial depression of synaptic transmission observed in the rat hippocampus in vivo is due to endogenous adenosine acting at neuronal adenosine A(1) receptors.