Adenosine and neurotrauma: therapeutic perspectives

Serum adenosine deaminase activity and acute cerebral infarction: a retrospective case-control study based on 7913 participants

BACKGROUND

Adenosine deaminase (ADA) is a key enzyme that catalyzes the deamination of adenosine into inosine, which eventually decomposes into uric acid (UA). A body of papers have reported that adenosine and UA are closely related to cerebrovascular events. However, the association between serum ADA activity and acute cerebral infarction (ACI) remains unclear.

METHODS

7913 subjects were enrolled, including 3968 ACI patients and 3945 controls, in this study. An automatic biochemistry analyzer was used to determine serum activity.

RESULTS

Serum ADA activity was found that was significantly decreased in patients with ACI (10.10 ± 3.72 U/L) compared to those without ACI (11.07 ± 2.85 U/L, p < 0.001). After Logistic regression analysis, ADA concentrations were negatively correlated with ACI (OR = 1.161, 95% CI: 1.140-1.183, p < 0.001). Smoking and alcohol consumption decreased serum ADA concentrations in patients with ACI, whereas diabetes and hypertension had the opposite effect.

CONCLUSIONS

Serum ADA concentrations in patients with ACI are markedly decreased, suggesting that the decreased ADA concentrations may be involved in the pathogenesis of ACI. We hypothesized that decreased ADA activity may be an adaptive mechanism to maintain adenosine levels and protect against ischemic brain injury.

Adenosine as a Key Mediator of Neuronal Survival in Cerebral Ischemic Injury

Several experimental studies have linked adenosine's neuroprotective role in cerebral ischemia. During ischemia, adenosine is formed due to intracellular ATP breakdown into ADP, further when phosphate is released from ADP, the adenosine monophosphate is formed. It acts via A1, A2, and A3 receptors found on neurons, blood vessels, glial cells, platelets, and leukocytes. It is related to various effector systems such as adenyl cyclase and membrane ion channels via G-proteins. Pharmacological manipulation of adenosine receptors by agonists (CCPA, ADAC, IB-MECA) increases ischemic brain damage in various in vivo and in vitro models of cerebral ischemia whereas, agonist can also be neuroprotective. Mainly, receptor antagonists (CGS15943, MRS1706) indicated neuroprotection. Later, various studies also revealed that the downregulation or upregulation of specific adenosine receptors is necessary during the recovery of cerebral ischemia by activating several downstream signaling pathways. In the current review, we elaborate on the dual roles of adenosine and its receptor subtypes A1, A2, and A3 and their involvement in the pathobiology of cerebral ischemic injury. Adenosine-based therapies have the potential to improve the outcomes of cerebral injury patients, thereby providing them with a more optimistic future.

CD73 alleviates GSDMD-mediated microglia pyroptosis in spinal cord injury through PI3K/AKT/Foxo1 signaling

BACKGROUND

Neuroinflammation-induced secondary injury is an important cause of sustained progression of spinal cord injury. Inflammatory programmed cell death pyroptosis executed by the pore-forming protein gasdermin D (GSDMD) is an essential step of neuroinflammation. However, it is unclear whether CD73, a widely accepted immunosuppressive molecule, can inhibit pyroptosis via mediating GSDMD.

METHODS

C57BL/6J CD73 deficient mice and wild-type mice, Lipopolysaccharide (LPS)-induced primary microglia and BV2 cells were respectively used to illustrate the effect of CD73 on microglia pyroptosis in vivo and in vitro. A combination of molecular and histological methods was performed to assess pyroptosis and explore the mechanism both in vivo and in vitro.

RESULTS

We have shown molecular evidence for CD73 suppresses the activation of NLRP3 inflammasome complexes to reduce the maturation of GSDMD, leading to decreased pyroptosis in microglia. Further analysis reveals that adenosine-A2B adenosine receptor-PI3K-AKT-Foxo1 cascade is a possible mechanism of CD73 regulation. Importantly, we determine that CD73 inhibits the expression of GSDMD at the transcriptional level through Foxo1. What's more, we confirm the accumulation of HIF-1α promotes the overexpression of CD73 after spinal cord injury (SCI), and the increased CD73 in turn upregulates the expression of HIF-1α, eventually forming a positive feedback regulatory loop.

CONCLUSION

Our data reveal a novel function of CD73 on microglia pyroptosis, suggesting a unique therapeutic opportunity for mitigating the disease process in SCI.

Ecto-5'-nucleotidase (CD73) attenuates inflammation after spinal cord injury by promoting macrophages/microglia M2 polarization in mice

Background

Immune activation, specifically activation of macrophages and resident microglia, leading to inflammation is a key component in the progression of spinal cord injury (SCI). Macrophages/microglia exist in two states—the classically activated M1 phenotype that confers pro-inflammatory effects or the alternatively activated M2 phenotype that confers anti-inflammatory effects. Ecto-5′-nucleotidase (CD73) is an immunosuppressive molecule intricately involved in adaptive and innate immune responses and is able to dephosphorylate AMP to adenosine. However, it is not known if CD73 is able to modulate the macrophages/microglia transformation between the M1 and M2 phenotypes.

Methods

We used gene-deficient mice to determine the role of CD73 in macrophages/microglia polarization post-SCI in vivo. We used small interference RNA (siRNA) or pcDNA3.1 to inhibit or overexpress CD73 in BV2 cells to verify anterior discovery in vitro. A combination of molecular and histological methods was used to detect the macrophages/microglia polarization and explore the mechanism both in vivo and in vitro.

Results

We found that SCI induced the upregulation of CD73 expression. CD73 deficient mice were noted to demonstrate overwhelming immune responses, few anti-inflammatory phenotype macrophages/microglia, and had a poorer locomotor recovery in comparison to wild-type mice that were also inflicted with SCI. In vitro studies found that CD73 suppression inhibited the expression of characteristic microglial anti-inflammatory polarization markers in BV2 cells, while the converse was noted in CD73 overexpression. Subsequent experiments confirmed that CD73 promoted microglia alternative activation by stimulating p38 MAPK.

Conclusion

We were able to conclude that CD73 imparts neuroprotective effects by mediating macrophages/microglia polarization. These findings allow for better understanding of the modulatory factors involved in triggering the change in macrophages/microglia phenotypes, therefore uncovering additional molecules and pathways that may be targeted in the innovation of novel SCI therapies.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1183-8) contains supplementary material, which is available to authorized users.

Discovery and Roles of 2',3'-cAMP in Biological Systems

In 2009, investigators using ultra-performance liquid chromatography-tandem mass spectrometry to measure, by selected reaction monitoring, 3',5'-cAMP in the renal venous perfusate from isolated, perfused kidneys detected a large signal at the same m/z transition (330 → 136) as 3',5'-cAMP but at a different retention time. Follow-up experiments demonstrated that this signal was due to a positional isomer of 3',5'-cAMP, namely, 2',3'-cAMP. Soon thereafter, investigative teams reported the detection of 2',3'-cAMP and other 2',3'-cNMPs (2',3'-cGMP, 2',3'-cCMP, and 2',3'-cUMP) in biological systems ranging from bacteria to plants to animals to humans. Injury appears to be the major stimulus for the release of these unique noncanonical cNMPs, which likely are formed by the breakdown of RNA. In mammalian cells in culture, in intact rat and mouse kidneys, and in mouse brains in vivo, 2',3'-cAMP is metabolized to 2'-AMP and 3'-AMP; and these AMPs are subsequently converted to adenosine. In rat and mouse kidneys and mouse brains, injury releases 2',3'-cAMP, 2'-AMP, and 3'-AMP into the extracellular compartment; and in humans, traumatic brain injury is associated with large increases in 2',3'-cAMP, 2'-AMP, 3'-AMP, and adenosine in the cerebrospinal fluid. These findings motivate the extracellular 2',3'-cAMP-adenosine pathway hypothesis: intracellular production of 2',3'-cAMP → export of 2',3'-cAMP → extracellular metabolism of 2',3'-cAMP to 2'-AMP and 3'-AMP → extracellular metabolism of 2'-AMP and 3'-AMP to adenosine. Since 2',3'-cAMP has been shown to activate mitochondrial permeability transition pores (mPTPs) leading to apoptosis and necrosis and since adenosine is generally tissue protective, the extracellular 2',3'-cAMP-adenosine pathway may be a protective mechanism [i.e., removes 2',3'-cAMP (an intracellular toxin) and forms adenosine (a tissue protectant)]. This appears to be the case in the brain where deficiency in CNPase (the enzyme that metabolizes 2',3'-cAMP to 2-AMP) leads to increased susceptibility to brain injury and neurological diseases. Surprisingly, CNPase deficiency in the kidney actually protects against acute kidney injury, perhaps by preventing the formation of 2'-AMP (which turns out to be a renal vasoconstrictor) and by augmenting the mitophagy of damaged mitochondria. With regard to 2',3'-cNMPs and their downstream metabolites, there is no doubt much more to be discovered.

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'.

A systematic review of neuroprotective strategies after cardiac arrest: from bench to bedside (part II-comprehensive protection)

Neurocognitive deficits remain a significant source of morbidity in survivors of cardiac arrest. We conducted a literature review of treatment protocols designed to evaluate neurologic outcome and survival following global cerebral ischemia associated with cardiac arrest. The search was limited to investigational therapies that were implemented either during cardiopulmonary resuscitation or after return of spontaneous circulation in studies that included assessment of impact on neurologic outcome. Given that complex pathophysiology underlies global brain hypoxic ischemia following cardiac arrest, neuroprotective strategies targeting multiple stages of neuropathologic cascades should promise to improve survival and neurologic outcomes in cardiac arrest victims. In Part II of this review, we discuss several approaches that can provide comprehensive protection against global brain injury associated with cardiac arrest, by modulating multiple targets of neuropathologic cascades. Pharmaceutical approaches include adenosine and growth factors/hormones including brain-derived neurotrophic factor, insulin-like growth factor-1 and glycine-proline-glutamate, granulocyte colony stimulating factor and estrogen. Preclinical studies of these showed some benefit but were inconclusive in models of global brain injury involving systemic ischemia. Several medical gases that can mediate neuroprotection have been evaluated in experimental settings. These include hydrogen sulfide, hyperbaric oxygen and molecular hydrogen. Hyperbaric oxygen and molecular hydrogen showed promising results; however, further investigation is required prior to clinical application of these agents in cardiac arrest patients.

Adenosine neuromodulation and traumatic brain injury

Adenosine is a ubiquitous signaling molecule, with widespread activity across all organ systems. There is evidence that adenosine regulation is a significant factor in traumatic brain injury (TBI) onset, recovery, and outcome, and a growing body of experimental work examining the therapeutic potential of adenosine neuromodulation in the treatment of TBI. In the central nervous system (CNS), adenosine (dys)regulation has been demonstrated following TBI, and correlated to several TBI pathologies, including impaired cerebral hemodynamics, anaerobic metabolism, and inflammation. In addition to acute pathologies, adenosine function has been implicated in TBI comorbidities, such as cognitive deficits, psychiatric function, and post-traumatic epilepsy. This review presents studies in TBI as well as adenosine-related mechanisms in co-morbidities of and unfavorable outcomes resulting from TBI. While the exact role of the adenosine system following TBI remains unclear, there is increasing evidence that a thorough understanding of adenosine signaling will be critical to the development of diagnostic and therapeutic tools for the treatment of TBI.

Increases in cerebrospinal fluid caffeine concentration are associated with favorable outcome after severe traumatic brain injury in humans

Caffeine, the most widely consumed psychoactive drug and a weak adenosine receptor antagonist, can be neuroprotective or neurotoxic depending on the experimental model or neurologic disorder. However, its contribution to pathophysiology and outcome in traumatic brain injury (TBI) in humans is undefined. We assessed serial cerebrospinal fluid (CSF) concentrations of caffeine and its metabolites (theobromine, paraxanthine, and theophylline) by high-pressure liquid chromatography/ultraviolet in 97 ventricular CSF samples from an established bank, from 30 adults with severe TBI. We prospectively selected a threshold caffeine level of > or = 1 micromol/L (194 ng/mL) as clinically significant. Demographics, Glasgow Coma Scale (GCS) score, admission blood alcohol level, and 6-month dichotomized Glasgow Outcome Scale (GOS) score were assessed. Mean time from injury to initial CSF sampling was 10.77+/-3.13 h. On initial sampling, caffeine was detected in 24 of 30 patients, and the threshold was achieved in 9 patients. Favorable GOS was seen more often in patients with CSF caffeine concentration > or = versus < the threshold (55.6 versus 11.8%, P=0.028). Gender, age, admission CGS score, admission blood alcohol level, and admission systolic arterial blood pressure did not differ between patients with CSF caffeine concentration > or = versus < the threshold. Increases in CSF concentrations of the caffeine metabolites theobromine and paraxanthine were also associated with favorable outcome (P=0.018 and 0.056, respectively). Caffeine and its metabolites are commonly detected in CSF in patients with severe TBI and in an exploratory assessment are associated with favorable outcome. We speculate that caffeine may be neuroprotective by long-term upregulation of adenosine A1 receptors or acute inhibition of A2a receptors.

The role of ATP and adenosine in the brain under normoxic and ischemic conditions

By taking advantage of some recently synthesized compounds that are able to block ecto-ATPase activity, we demonstrated that adenosine triphosphate (ATP) in the hippocampus exerts an inhibitory action independent of its degradation to adenosine. In addition, tonic activation of P2 receptors contributes to the normally recorded excitatory neurotransmission. The role of P2 receptors becomes critical during ischemia when extracellular ATP concentrations increase. Under such conditions, P2 antagonism is protective. Although ATP exerts a detrimental role under ischemia, it also exerts a trophic role in terms of cell division and differentiation. We recently reported that ATP is spontaneously released from human mesenchymal stem cells (hMSCs) in culture. Moreover, it decreases hMSC proliferation rate at early stages of culture. Increased hMSC differentiation could account for an ATP-induced decrease in cell proliferation. ATP as a homeostatic regulator might exert a different effect on cell trophism according to the rate of its efflux and receptor expression during the cell life cycle. During ischemia, adenosine formed by intracellular ATP escapes from cells through the equilibrative transporter. The protective role of adenosine A(1) receptors during ischemia is well accepted. However, the use of selective A(1) agonists is hampered by unwanted peripheral effects, thus attention has been focused on A(2A) and A(3) receptors. The protective effects of A(2A) antagonists in brain ischemia may be largely due to reduced glutamate outflow from neurones and glial cells. Reduced activation of p38 mitogen-activated protein kinases that are involved in neuronal death through transcriptional mechanisms may also contribute to protection by A(2A) antagonism. Evidence that A(3) receptor antagonism may be protective after ischemia is also reported.

A(2b) receptor mediates adenosine inhibition of taurine efflux from pituicytes

BACKGROUND INFORMATION

Recent work suggests that part of the control of vasopressin output is mediated by taurine released from pituicytes, the astroglial cells of the neurohypophysis. Taurine release, in turn, is stimulated by hypotonic conditions and by vasopressin itself. As adenosine is generated from ATP co-released with vasopressin, it appeared important to study its effects on taurine efflux from pituicytes.

RESULTS

We measured radioactive efflux from cultured pituicytes and whole neurohypophyses pre-loaded with [(3)H]taurine. Cultured pituicytes were also used to study adenosine-receptor mRNA expression. Taurine efflux elicited by hypotonic shocks is approximately 30-50% smaller in the presence of 10 microM adenosine or 1 microM NECA (5'-N-ethylcarboxamidoadenosine). Both compounds also inhibited basal efflux in a manner that was not immediately reversible. Agonists of the adenosine A1-, A2a- or A3-receptor subtypes have no relevant effect on basal taurine release, and the A1-receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) has no effect on the inhibition of release by NECA. In turn, the A2b-receptor antagonists MRS 1706 {N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide} or alloxazine partially reverse the inhibition of basal or hypotonicity-evoked efflux by NECA. Both A1- and A2b-receptor mRNAs are expressed in pituicytes, which is consistent with an A1-receptor-mediated effect on cell morphology and an A2b-receptor-mediated effect on taurine release. Forskolin and dibutyryl cAMP mimic the inhibitory effects of purinergics on basal taurine efflux, and the adenylate cyclase inhibitor DDA (2',5'-dideoxyadenosine) partially reverses the inhibition of the hypotonic response by NECA.Conclusions. Our results suggest that purinergic inhibition of taurine efflux from pituicytes operates through A2b receptors coupled to intracellular cAMP increase. They point to a possible modulation of neurohypophysial hormone output by endogenous adenosine released in either physiological or pathological situations.

Cell death in rat cerebellar granule neurons induced by hydrogen peroxide in vitro: mechanisms and protection by adenosine receptor ligands

Oxidative stress, resulting from excessive production of reactive oxygen species (ROS), is a pathological state that causes profound cellular damage and eventual death resulting from the overactivation of glutamate receptors, and the generation of nitric oxide, superoxide and hydrogen peroxide (H(2)O(2)). As such, H(2)O(2) represents an important model for studying the neuropathology of oxidative stress in a variety of CNS disorders. The effects of H(2)O(2) on the viability of post-natal cerebellar granule neurons (CGNs), the nature of the cell death involved and the potential protection by adenosine receptors against the damage were examined in the current study. Hydrogen peroxide (10-400 microM) reduced CGN viability in a concentration- and time-dependent manner. The addition of catalase (100 U/ml) prevented this effect, and the non-specific COX inhibitor aspirin (1 mM) also alleviated the damage. A combination of H(2)O(2) (5 microM) and Cu(2+) (0.5 mM) resulted in a significant damage that was not prevented by the hydroxyl radical scavenger mannitol (50 mM). The permeability transition pore blocker cyclosporin A, the caspase-3 inhibitor Z-DEVD-fmk (40 microM) and the PARP-1 inhibitor DPQ (10 microM) each significantly protected against peroxide damage. While the A(1) adenosine receptor agonist CPA and the A(2A) receptor antagonist ZM241385 (each at 100 nM) elicited protection, the A(1) adenosine receptor blocker DPCPX and the A(2A) receptor agonist CGS21680 (each at 100 nM) showed no effect. The data demonstrate that H(2)O(2) induced oxidative stress in CGNs, involving both apoptotic and necrotic death, and this can be ameliorated by A(1) receptor activation or A(2A) receptor blockade.

Hydrogen peroxide-induced oxidative stress in MC3T3-E1 cells: The effects of glutamate and protection by purines

Glutamate has toxic effects on a number of tissues, partly by inducing toxic (e.g., oxidative) stress, whereas adenosine can be protective. Since there is evidence that glutamate and adenosine receptors are present in bone, we set out to study whether oxidative stress, induced by hydrogen peroxide (H2O2), affected viability in the MC3T3-E1 osteoblast-like cell line and whether treatment with adenosine receptor ligands attenuated this. Hydrogen peroxide (100 microM to 5 mM) reduced the viability of the MC3T3-E1 cells, while catalase reversed this cell loss and itself had some mitogenic effect. Superoxide dismutase (SOD) increased the number of viable cells alone but failed to modify significantly the effect of H2O2 treatments. Glutamate (100 microM, 1 mM) and NMDA (10 microM), applied alone for up to 1 h, had a mitogenic effect (P < 0.05). Adenosine A1 and A2A receptor agonists and antagonists at low and high concentrations showed some mitogenic effects when added singly, but only high concentrations of the agonists showed significant protection against cell death resulting from H2O2 treatments. Contributions from both apoptotic and necrotic pathways were implicated in the H2O2-induced cell loss as was demonstrated by the use of the caspase-3 inhibitor (Z-DEVD-fmk) and the PARP-1 inhibitor (DPQ). The results demonstrate that hydrogen peroxide was toxic to MC3T3-E1 cells, whereas glutamate was not and may even have a trophic influence. Adenosine and its receptors afforded some protection to osteoblasts against cellular death mediated partly by apoptosis and partly by necrosis.

Isoflurane tolerance against focal cerebral ischemia is attenuated by adenosine A1 receptor antagonists

PURPOSE

To investigate the role of the adenosine A1 receptor in the rapid tolerance to cerebral ischemia induced by isoflurane preconditioning.

METHODS

Seventy-five rats were randomly assigned into five groups (n = 15 each): Control, 8-cyclopentyl-1,3-dipropulxanthine (DPCPX), Isoflurane, DPCPX+Isoflurane and Vehicle+Isoflurane groups. All animals underwent right middle cerebral artery occlusion (MCAO) for two hours. Isoflurane preconditioning was conducted one hour before MCAO in Isoflurane, DPCPX+Isoflurane and Vehicle+Isoflurane groups by exposing the animals to 1.5% isoflurane in 98% oxygen for one hour. In the Control and DPCPX groups, animals were exposed to 98% oxygen one hour before MCAO for one hour. A selective adenosine A1 receptor antagonist, DPCPX, was administered (0.1 mg x kg(-1)) 15 min before isoflurane/oxygen exposure in the DPCPX and DPCPX+Isoflurane groups to evaluate the effect of adenosine A1 receptor antagonist on isoflurane preconditioning. Dimethyl sulfoxide, the solvent of DPCPX, was administered (1 mL x kg(-1)) 15 min before isoflurane exposure in the Vehicle+Isoflurane group. Neurological deficit scores and brain infarct volumes were evaluated 24 hr after reperfusion.

RESULTS

Animals in the Isoflurane and Vehicle+Isoflurane groups developed lower neurological deficit scores and smaller brain infarct volumes than the Control group (P < 0.01). Animals in the DPCPX+Isoflurane group developed higher neurological deficit scores and larger brain infarct volumes than the Isoflurane and Vehicle+Isoflurane groups (P < 0.01).

CONCLUSION

The present study demonstrates that preconditioning with isoflurane reduces focal cerebral ischemic injury in rats, and the adenosine A1 receptor antagonist (DPCPX) attenuates the neuroprotection induced by isoflurane preconditioning.

A3 adenosine receptor antagonists delay irreversible synaptic failure caused by oxygen and glucose deprivation in the rat CA1 hippocampus in vitro

The role of adenosine A3 receptor activation during ischaemia-like conditions produced by oxygen and glucose deprivation (OGD) was evaluated with extracellular recordings from the CA1 region of rat hippocampal slices. In all, 7 min of OGD evoked tissue anoxic depolarisation (AD, peak at approximately 7 min from OGD start, n=20) and were invariably followed by irreversible loss of electrically evoked field epsps (fepsps, n=42).The selective adenosine A3 antagonists 3-propyl-6-ethyl-5[(ethylthio)carbonyl]-2-phenyl-4-propyl-3-pyridinecarboxylate (MRS 1523, 1-100 nM, n=31), N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-yl]benzeneacetamide (MRS 1220, 100 nM, n=7), N-(2-methoxyphenyl)-N'-[2-(3-pyrindinyl)-4-quinazolinyl]-urea, (VUF 5574, 100 nM, n=3) and 5-[[(4-pyridyl)amino]carbonyl]amino-8-methyl-2-(2-furyl)-pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine hydrochloride (1 nM, n=4), prevented the irreversible failure of neurotransmission induced by 7 min OGD (n=45) and the development of AD in 20 out of 22 monitored slices. When tested on OGD episodes of longer duration (8-10 min, n=18), 100 nM MRS 1523 prevented or delayed the appearance of AD and exerted a protective effect on neurotransmission for episodes of up to 9 min duration. In the absence of AD, the fepsp recovery was almost total, regardless of OGD episode duration. These findings support the notion that A3 receptor stimulation is deleterious during ischaemia and suggest that selective A3 receptor block may substantially increase the resistance of the CA1 hippocampal region to ischaemic damage.

Adenosine A1 receptor knockout mice develop lethal status epilepticus after experimental traumatic brain injury

Adenosine, acting at A1 receptors, exhibits anticonvulsant effects in experimental epilepsy--and inhibits progression to status epilepticus (SE). Seizures after traumatic brain injury (TBI) may contribute to pathophysiology. Thus, we hypothesized that endogenous adenosine, acting via A1 receptors, mediates antiepileptic benefit after experimental TBI. We subjected A1-receptor knockout (ko) mice, heterozygotes, and wild-type (wt) littermates (n=115) to controlled cortical impact (CCI). We used four outcome protocols in male mice: (1) observation for seizures, SE, and mortality in the initial 2 h, (2) assessment of seizure score (electroencephalogram (EEG)) in the initial 2 h, (3) assessment of mortality at 24 h across injury levels, and (4) serial assessment of arterial blood pressure, heart rate, blood gases, and hematocrit. Lastly, to assess the influence of gender on this observation, we observed female mice for seizures, SE, and mortality in the initial 2 h. Seizure activity was noted in 83% of male ko mice in the initial 2 h, but was seen in no heterozygotes and only 33% of wt (P<0.05). Seizures in wt were brief (1 to 2 secs). In contrast, SE involving lethal sustained (>1 h) tonic clonic activity was uniquely seen in ko mice after CCI (50% incidence in males), (P<0.05). Seizure score was twofold higher in ko mice after CCI versus either heterozygote or wt (P<0.05). An injury-intensity dose-response for 24 h mortality was seen in ko mice (P<0.05). Physiologic parameters were similar between genotypes. Seizures were seen in 100% of female ko mice after CCI versus 14% of heterozygotes and 25% wt (P<0.05) and SE was restricted to the ko mice (83% incidence). Our data suggest a critical endogenous anticonvulsant action of adenosine at A1 receptors early after experimental TBI.

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.

Adenosine uptake inhibitors

Adenosine is a purine nucleoside and modulates a variety of physiological functions by interacting with cell-surface adenosine receptors. Under several adverse conditions, including ischemia, trauma, stress, seizures and inflammation, extracellular levels of adenosine are increased due to increased energy demands and ATP metabolism. Increased adenosine could protect against excessive cellular damage and organ dysfunction. Indeed, several protective effects of adenosine have been widely reported (e.g., amelioration of ischemic heart and brain injury, seizures and inflammation). However, the effects of adenosine itself are insufficient because extracellular adenosine is rapidly taken up into adjacent cells and subsequently metabolized. Adenosine uptake inhibitors (nucleoside transport inhibitors) could retard the disappearance of adenosine from the extracellular space by blocking adenosine uptake into cells. Therefore, it is expected that adenosine uptake inhibitors will have protective effects in various diseases, by elevating extracellular adenosine levels. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain, and inflammatory diseases have been reported. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors.

Antagonism of adenosine receptors by caffeine and caffeine metabolites in equine forebrain tissues

OBJECTIVE

To determine the presence of adenosine receptor subtypes A1 and A2a in equine forebrain tissues and to characterize the interactions of caffeine and its metabolites with adenosine receptors in the CNS of horses.

SAMPLE POPULATION

Brain tissue specimens obtained during necropsy from 5 adult male research horses.

PROCEDURE

Membrane-enriched homogenates from cerebral cortex and striatum were evaluated by radioligand binding assays with the A1-selective ligand [3H]DPCPX and the A2a-selective ligand [3H]ZM241385. Functional responses to adenosine receptor agonists and antagonists were determined by a nucleotide exchange assay using [35S]-guanosine 5'-(gamma-thio) triphosphate ([35S]GTPgammaS).

RESULTS

Saturable high affinity [3H]DPCPX binding (A1) sites were detected in cerebral cortex and striatum, whereas high-affinity [3H]ZM241385 binding (A2a) sites were detected only in striatum. Caffeine and related methylxanthines had similar binding affinities at A1 and A2a sites with rank orders of drug binding affinities (theophylline > paraxanthine > or = caffeine >> theobromine) similar to other species. [35S]GTPgammaS exchange revealed that caffeine and its metabolites act as pure adenosine receptor antagonists at concentrations that correspond to A1 and A2a receptor binding affinities.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of our study affirm the presence of guanine nucleotide binding protein linked adenosine receptors (ie, high-affinity A1 and A2a adenosine receptors) in equine forebrain tissues and reveal the antagonistic actions by caffeine and several biologically active caffeine metabolites. Antagonism of adenosine actions in the equine CNS by these stimulants may be responsible for some central actions of methylxanthine drugs, including motor stimulation and enhanced racing performance.

Recent advances in pathophysiology and treatment of spinal cord injury

Thirty years ago, patients with spinal cord injury (SCI) and their families were told "nothing can be done" to improve function. Since the SCI patient population is reaching normal life expectancy through better health care, it has become an obviously worthwhile enterprise to devote considerable research effort to SCI. Targets for intervention in SCI toward improved function have been identified using basic research approaches and can be simplified into a list: (1) reduction of edema and free-radical production, (2) rescue of neural tissue at risk of dying in secondary processes such as abnormally high extracellular glutamate concentrations, (3) control of inflammation, (4) rescue of neuronal/glial populations at risk of continued apoptosis, (5) repair of demyelination and conduction deficits, (6) promotion of neurite growth through improved extracellular environment, (7) cell replacement therapies, (8) efforts to bridge the gap with transplantation approaches, (9) efforts to retrain and relearn motor tasks, (10) restoration of lost function by electrical stimulation, and (11) relief of chronic pain syndromes. Currently, over 70 clinical trials are in progress worldwide. Consequently, in this millennium, unlike in the last, no SCI patient will have to hear "nothing can be done."

Reduction in intracellular calcium levels induces injury in developing neurons

The neurotransmitter glutamate influences intracellular Ca(2+) levels and plays an essential role in maintaining neuronal viability during early development. Blockade of NMDA receptors induces cell death in the neonatal forebrain via mechanisms that are not understood. Other neuromodulators that can influence intracellular Ca(2+) levels include the nucleoside adenosine, which acts via A(1) adenosine receptors subtypes (A(1)ARs). Because A(1)AR activation inhibits glutamate release and action, A(1)AR activation may also contribute to neonatal brain injury. To examine this possibility, we treated primary neuronal cultures with the A(1)AR agonist CPA, the NMDAR antagonist MK801, or CPA + MK801. Combined MK801 + CPA treatment resulted in profound cellular injury, exceeding that seen in other groups. In keeping with the hypothesis that altered Ca(2+) signaling mediates CPA + MK801 injury, reduction of Ca(2+) levels with EGTA, thapsigargin, or BAPTA-AM enhanced CPA + MK801-induced neuronal damage. In contrast, increasing intracellular Ca(2+) using ionomycin reversed CPA + MK801 toxicity. Direct visualization of intracellular Ca(2+) by confocal microscopy revealed that CPA + MK801 inhibited KCl-evoked increases in intracellular Ca(2+). Supporting the concept that A(1)AR activation and NMDAR blockade results in brain injury, neonatal rats injected with A(1)AR agonists + MK801 showed widespread apoptosis in many brain regions. These observations show that A(1)AR activation and NMDAR blockade lead to early postnatal cell injury by mechanisms that involve inhibition of intracellular Ca(2+) signaling.

Mechanisms of apoptosis induced by purine nucleosides in astrocytes

Astrocytes release adenine-based and guanine-based purines under physiological and, particularly, pathological conditions. Thus, the aim of this study was to determine if adenosine induced apoptosis in cultured rat astrocytes. Further, if guanosine, which increases the extracellular concentration of adenosine, also induced apoptosis determined using the TUNEL and Annexin V assays. Adenosine induced apoptosis in a concentration-dependent manner up to 100 microM. Inosine, hypoxanthine, guanine, and guanosine did not. Guanosine or adenosine (100 microM) added to the culture medium was metabolized, with 35% or 15%, respectively, remaining after 2-3 h. Guanosine evoked the extracellular accumulation of adenosine, and particularly of adenine-based nucleotides. Cotreatment with EHNA and guanosine increased the extracellular accumulation of adenosine and induced apoptosis. Inhibition of the nucleoside transporters using NBTI (100 microM) or propentophylline (100 microM) significantly decreased but did not abolish the apoptosis induced by guanosine + EHNA or adenosine + EHNA, respectively. Apoptosis produced by either guanosine + EHNA or adenosine + EHNA was unaffected by A(1) or A(2) adenosine receptor antagonists, but was significantly reduced by MRS 1523, a selective A(3) adenosine receptor antagonist. Adenosine + EHNA, not guanosine + EHNA, significantly increased the intracellular concentration of S-adenosyl-L-homocysteine (SAH) and greatly reduced the ratio of S-adenosyl-L-methioine to SAH, which is associated with apoptosis. These data demonstrate that adenosine mediates apoptosis of astrocytes both, via activation of A(3) adenosine receptors and by modulating SAH hydrolase activity. Guanosine induces apoptosis by accumulating extracellular adenosine, which then acts solely via A(3) adenosine receptors.