Effects Of A2A Receptor Activation Research Articles

The Good, the Bad, and the Deadly: Adenosinergic Mechanisms Underlying Sudden Unexpected Death in Epilepsy.

Adenosine is an inhibitory modulator of neuronal excitability. Neuronal activity results in increased adenosine release, thereby constraining excessive excitation. The exceptionally high neuronal activity of a seizure results in a surge in extracellular adenosine to concentrations many-fold higher than would be observed under normal conditions. In this review, we discuss the multifarious effects of adenosine signaling in the context of epilepsy, with emphasis on sudden unexpected death in epilepsy (SUDEP). We describe and categorize the beneficial, detrimental, and potentially deadly aspects of adenosine signaling. The good or beneficial characteristics of adenosine signaling in the context of seizures include: (1) its direct effect on seizure termination and the prevention of status epilepticus; (2) the vasodilatory effect of adenosine, potentially counteracting postictal vasoconstriction; (3) its neuroprotective effects under hypoxic conditions; and (4) its disease modifying antiepileptogenic effect. The bad or detrimental effects of adenosine signaling include: (1) its capacity to suppress breathing and contribute to peri-ictal respiratory dysfunction; (2) its contribution to postictal generalized EEG suppression (PGES); (3) the prolonged increase in extracellular adenosine following spreading depolarization waves may contribute to postictal neuronal dysfunction; (4) the excitatory effects of A2A receptor activation is thought to exacerbate seizures in some instances; and (5) its potential contributions to sleep alterations in epilepsy. Finally, the adverse effects of adenosine signaling may potentiate a deadly outcome in the form of SUDEP by suppressing breathing and arousal in the postictal period. Evidence from animal models suggests that excessive postictal adenosine signaling contributes to the pathophysiology of SUDEP. The goal of this review is to discuss the beneficial, harmful, and potentially deadly roles that adenosine plays in the context of epilepsy and to identify crucial gaps in knowledge where further investigation is necessary. By better understanding adenosine dynamics, we may gain insights into the treatment of epilepsy and the prevention of SUDEP.

Adenosine and Forskolin Inhibit Platelet Aggregation by Collagen but not the Proximal Signalling Events.

The G protein-coupled receptor, adenosine A2A, signals through the stimulatory G protein, Gs, in platelets leading to activation of adenylyl cyclase and elevation of cyclic adenosine monophosphate (cAMP) and inhibition of platelet activation. This article investigates the effect of A2A receptor activation on signalling by the collagen receptor glycoprotein (GP) VI in platelets. Washed human platelets were stimulated by collagen or the GPVI-specific agonist collagen-related peptide (CRP) in the presence of the adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) or the adenylyl cyclase activator, forskolin and analysed for aggregation, adenosine triphosphate secretion, protein phosphorylation, spreading, Ca2+ mobilisation, GPVI receptor clustering, cAMP, thromboxane B2 (TxB2) and P-selectin exposure. NECA, a bioactive adenosine analogue, partially inhibits aggregation and secretion to collagen or CRP in the absence or presence of the P2Y12 receptor antagonist, cangrelor and the cyclooxygenase inhibitor, indomethacin. The inhibitory effect in the presence of the three inhibitors is largely overcome at higher concentrations of collagen but not CRP. Neither NECA nor forskolin altered clustering of GPVI, elevation of Ca2+ or spreading of platelets on a collagen surface. Further, neither NECA nor forskolin, altered collagen-induced tyrosine phosphorylation of Syk, LAT nor PLCγ2. However, NECA and forskolin inhibited platelet activation by the TxA2 mimetic, U46619, but not the combination of adenosine diphosphate and collagen. NECA and forskolin have no effect on the proximal signalling events by collagen. They inhibit platelet activation in a response-specific manner in part through inhibition of the feedback action of TxA2.

Adenosine A2A receptor agonist ameliorates EAE and correlates with Th1 cytokine-induced blood brain barrier dysfunction via suppression of MLCK signaling pathway.

IntroductionMultiple sclerosis (MS) disease activity is associated with blood‐brain barrier (BBB) disruption, which is mediated by inflammatory cytokines released by CD4+ lymphocytes. To assess the effects of adenosine A2A receptors on BBB permeability in vitro and in vivo.MethodsA2A receptor expression was detected by immunostaining in experimental autoimmune encephalomyelitis (EAE) C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein (MOG)35–55, and human MS brain. F‐actin and the tight junction protein Claudin‐5 were assessed in endothelial cells treated with an A2A receptor specific agonist (CGS‐21680) after Th1 cytokine stimulation. EAE mice were divided into control and CGS‐21680 (50 µg/kg, i.p., daily) groups. Disease scores were recorded daily to evaluate neurological impairment. The effects of A2A receptor on inflammation and demyelination were assessed after euthanasia by immunostaining or histology; BBB permeability was measured by sodium fluoride (Na‐F) and FITC‐dextran amounts.ResultsEndothelial A2A receptor was detected in demyelination areas of MS brain samples. In EAE lesions, A2A receptor was expressed in the endothelium in association with immune cell infiltration. Treatment with CGS‐21680 counteracted the effects of Th1 cytokines on endothelial cells in vitro, preventing the reduction of tight junction protein expression and stress fiber formation. The effects of A2A receptor activation were correlated with MLCK phosphorylation signaling repression. In EAE, A2A receptor agonist decreased BBB permeability and inhibited EAE neurologic deficiency in mice.ConclusionsA2A receptor activation at EAE onset helps reduce the effects of Th1 stimulation on BBB permeability, indicating that A2A receptor mediates BBB function in CNS demyelinated disease.

Age dependent changes in A 2A receptor expression in PreBotzinger Complex

Adenosine is a modulator of respiratory output, and can both increase and decrease respiratory frequency in an age and receptor dependent manner. We have shown an in vivo, age dependent, effect of A2A receptor activation on apneas in rats which is attenuated from postnatal day (P)14 to P21 through adulthood. We hypothesized that the attenuated response to A2A activation was due to decreased A2A receptor expression in the PreBötzinger complex (pBc). To test this hypothesis, we used unbiased stereology to quantify changes in A2A expression in pBc over the first three weeks of life. Sprague Dawley rat pups were perfused at P0, 4, 7, 14, and 21. The brains were cryosectioned and 4 identical sets of slides made from each brain. These slides were immunostained for NK-1R and somatostatin to deliniate the pBc. The remaining slides were stained with A2A, NeuN or S100β antibodies. The number of immunopositive cells in the pBc and the pBc volume were quantified using unbiased stereology (n=5 for each antibody at each age). The number of cells immunopositive for A2A per 1X105μm3 of pBc decreased significantly between P0 and P7 (p < 0.006), and rose again by P21 (P < 0.2). The number of NeuN and S100β immunopositive cells per 1X105μm3 decreased significantly between P0 and P21 (p < 0.001 for both). We conclude that A2A expression changes with age, but these changes are not correlated with the pattern of A2A dependent apneas seen in vivo.

Activation of adenosine A2A receptor on GABAergic inputs to 2nd‐order neurons of peripheral chemoreceptors in the NTS

Hypoxia elevates extracellular adenosine level in the NTS. Both adenosine A1 and A2A receptors exist in the NTS. Unlike inhibitory A1 receptor, activation of adenosine A2A receptor usually has a facilitation effect on neural function. In this project, we studied the effect of A2A receptor activation on GABAergic inputs to second‐order NTS neurons. Whole‐cell recordings were performed in NTS neurons in brainstem slices. Second‐order neurons were identified following DiA labeling of carotid bodies. Recordings were performed with CNQX but no TTX to study action potential‐dependent spontaneous pre‐synaptic GABA release (sIPSCs). In 3/7 NTS neurons, application of A2A receptor agonist CGS 21680 at 100 nM induced a significant increase in the frequency of sIPSCs (155±12% of control, p&lt;0.05) without a significant change of the amplitude (95±3% of control, p&gt;0.05). In the other 4/7 NTS neurons, CGS 21680 did not induce a significant change in sIPSCs frequency (93±4% of control, p&gt;0.05). In 3 of these 4 neurons, CGS 21680 induced a significant decrease in sIPSCs amplitude (86±2% of control, p&lt;0.05). The results indicate a heterogeneous and complex function of adenosine A2A receptor in the NTS. The lack of response in some neurons may be due to a lack of receptor on certain GABAergic neurons or indirect mechanisms involving other neurochemicals and ion channels. This work is supported by AHA BGIA 0865269F and NIH HL88052.

A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine regulates the function of the innate and adaptive immune systems through targeting virtually every cell type that is involved in orchestrating an immune/inflammatory response. Of the four adenosine receptors (A(1), A(2A), A(2B), A(3)), A(2A) receptors have taken center stage as the primary anti-inflammatory effectors of extracellular adenosine. This broad, anti-inflammatory effect of A(2A) receptor activation is a result of the predominant expression of A(2A) receptors on monocytes/macrophages, dendritic cells, mast cells, neutrophils, endothelial cells, eosinophils, epithelial cells, as well as lymphocytes, NK cells, and NKT cells. A(2A) receptor activation inhibits early and late events occurring during an immune response, which include antigen presentation, costimulation, immune cell trafficking, immune cell proliferation, proinflammatory cytokine production, and cytotoxicity. In addition to limiting inflammation, A(2A) receptors participate in tissue remodeling and reparation. Consistent with their multifaceted, immunoregulatory action on immune cells, A(2A) receptors have been shown to impact the course of a wide spectrum of ischemic, autoimmune, infectious, and allergic diseases. Here, we review the regulatory roles of A(2A) receptors in immune/inflammatory diseases of various organs, including heart, lung, gut, liver, kidney, joints, and brain, as well as the role of A(2A) receptors in regulating multiple organ failure and sepsis.

Inhibitory action of CGS 21680 on cerebral cortical neurons is antagonized by bicuculline and picrotoxin—is GABA involved?

The possibility of an involvement of endogenously released GABA in the inhibitory actions of A 1 and A 2a adenosine receptor agonists on rat cerebral cortical neurons discharges was examined using the GABA A antagonists bicuculline and picrotoxin. The A 1 agonist N 6-cyclopentyladenosine (CPA), the A 2a agonist CGS 21680 and the non-selective receptor agonist, adenosine, depressed neuronal firing. Applications of bicuculline or picrotoxin enhanced the spontaneous firing rate of cortical neurons, indicating the presence of ongoing GABA-ergic inhibition. Antagonism of GABA A receptors blocked the depressant effects of CGS 21680 on neuronal firing; was without effect on CPA-evoked inhibitions and tended to reduce the duration of adenosine-evoked inhibitions. These results suggest that the depressant effects of A 2a receptor activation are due to an increase in GABA-ergic inhibition, likely as a consequence of increased GABA release. GABA does not appear to be involved in adenosine A 1 receptor-mediated inhibition of neuronal firing.

Stimulation of high-affinity adenosine A2 receptors decreases the affinity of dopamine D2 receptors in rat striatal membranes.

Since high-affinity adenosine A2 receptors (A2a) are localized exclusively in dopamine-rich regions in the central nervous system and mediate inhibition of locomotor activity, we have examined the effect of A2a receptor activation on D1 and D2 receptor binding in membrane preparations of the rat striatum. The A2a agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'- N-ethylcarboxamidoadenosine (CGS 21680) increased the Kd of the dopamine D2 agonist L-(-)-N-[3H]propylnorapomorphine without affecting the Bmax. The increase in Kd was maximal (40%) at 30 nM CGS 21680. CGS 21680 (30 nM) decreased the dopamine-induced inhibition of [3H]raclopride (a D2 antagonist) binding due to an increase (about 3-fold) in KH and KL, the dissociation constants of high- and low-affinity binding sites. The effects of CGS 21680 were antagonized by the adenosine antagonist 8-phenyltheophylline (10 microM). (-)-N6-(2-Phenylisopropyl)adenosine produced an effect similar to that of CGS 21680, provided the concentration used was high enough to stimulate A2a receptors (300 nM). GTP (50 microM) also decreased the dopamine-induced inhibition of [3H]raclopride binding but, in contrast to CGS 21680, GTP decreased the proportion of D2 receptors in the high-affinity state. CGS 21680 (30 nM) did not affect the Kd or Bmax of [3H]raclopride and failed to affect ligand binding to D1 receptors. Thus, stimulation of A2a receptors potently reduces the affinity of D2 agonist binding sites within the plasma membrane of striatal neurons. This A2a-D2 interaction may underlie the neuroleptic-like actions of adenosine agonists and the enhancing effects of adenosine antagonists, such as caffeine, on locomotor activity.