Effect of adenosine on adenosine triphosphate-sensitive potassium channel during hypoxia in rat hippocampal neurons

Adenosine Receptors in Modulation of Central Nervous System Disorders

The ubiquitous signaling nucleoside molecule, adenosine is found in different cells of the human body to provide its numerous pharmacological role. The associated actions of endogenous adenosine are largely dependent on conformational change of the widely expressed heterodimeric G-protein-coupled A1, A2A, A2B, and A3 adenosine receptors (ARs). These receptors are well conserved on the surface of specific cells, where potent neuromodulatory properties of this bioactive molecule reflected by its easy passage through the rigid blood-brainbarrier, to simultaneously act on the central nervous system (CNS). The minimal concentration of adenosine in body fluids (30-300 nM) is adequate to exert its neuromodulatory action in the CNS, whereas the modulatory effect of adenosine on ARs is the consequence of several neurodegenerative diseases. Modulatory action concerning the activation of such receptors in the CNS could be facilitated towards neuroprotective action against such CNS disorders. Our aim herein is to discuss briefly pathophysiological roles of adenosine on ARs in the modulation of different CNS disorders, which could be focused towards the identification of potential drug targets in recovering accompanying CNS disorders. Researches with active components with AR modulatory action have been extended and already reached to the bedside of the patients through clinical research in the improvement of CNS disorders. Therefore, this review consist of recent findings in literatures concerning the impact of ARs on diverse CNS disease pathways with the possible relevance to neurodegeneration.

Preconditioning of mesenchymal stem cells with 2,4-dinitrophenol improves cardiac function in infarcted rats

AIMS

The aim of this study is to determine if preconditioning of bone marrow derived mesenchymal stem cells (MSCs) with 2,4-dinitrophenol (DNP) improves survival of transplanted stem cells in a rat model of myocardial infarction (MI), and to asses if this strategy has measurable impact on cardiac function.

MAIN METHODS

MSCs were preconditioned with DNP. In vitro cell adhesion assay and qRT-PCR were performed to analyze the expression of genes involved in cardiomyogenesis, cell adhesion and angiogenesis. MI was produced by occlusion of left anterior descending coronary artery. One million cells were transplanted by intramyocardial injection into the infarcted myocardium. Echocardiography was performed after two and four weeks of cellular transplantation. Hearts were harvested after four weeks and processed for histological analysis.

KEY FINDINGS

DNP treated MSCs adhered to the surface more (p<0.001) as compared to the normal MSCs. Gene expression levels were significantly upregulated in case of DNP treatment. The number of viable MSCs was more (p<0.001) in animals that received DNP treated MSCs, leading to significant improvement in cardiac function. Histological analysis revealed significant reduction in scar formation (p<0.001), maintenance of left ventricular wall thickness (p<0.001), and increased angiogenesis (p<0.01).

SIGNIFICANCE

The study evidenced for the first time that MSCs preconditioned with DNP improved cardiac function after transplantation. This can be attributed to improved survival, homing, adhesion, and cardiomyogenic and angiogenic differentiation of DNP treated MSCs in vivo.

Adenosine inhibits paraventricular pre-sympathetic neurons through ATP-dependent potassium channels

Adenosine produces cardiovascular depressor effects in various brain regions. However, the cellular mechanisms underlying these effects remain unclear. The pre-sympathetic neurons in the hypothalamic paraventricular nucleus (PVN) play an important role in regulating arterial blood pressure and sympathetic outflow through projections to the spinal cord and brainstem. In this study, we performed whole-cell patch-clamp recordings on retrogradely labeled PVN neurons projecting to the intermediolateral cell column of the spinal cord in rats. Adenosine (10-100 microM) decreased the firing activity in a concentration-dependent manner, with a marked hyperpolarization in 12 of 26 neurons tested. Blockade of A(1) receptors with the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine or intracellular dialysis of guanosine 5'-O-(2-thodiphosphate) eliminated the inhibitory effect of adenosine on labeled PVN neurons. Immunocytochemical labeling revealed that A(1) receptors were expressed on spinally projecting PVN neurons. Also, blocking ATP-dependent K(+) (K(ATP)) channels with 100 microM glibenclamide or 200 microM tolbutamide, but not the G protein-coupled inwardly rectifying K(+) channels blocker tertiapin-Q, abolished the inhibitory effect of adenosine on the firing activity of PVN neurons. Furthermore, glibenclamide or tolbutamide significantly decreased the adenosine-induced outward currents in labeled neurons. The reversal potential of adenosine-induced currents was close to the K(+) equilibrium potential. In addition, adenosine decreased the frequency of both spontaneous and miniature glutamatergic excitatory post-synaptic currents and GABAergic inhibitory post-synaptic currents in labeled neurons, and these effects were also blocked by 8-cyclopentyl-1,3-dipropylxanthine. Collectively, our findings suggest that adenosine inhibits the excitability of PVN pre-sympathetic neurons through A(1) receptor-mediated opening of K(ATP) channels.

Adenosine A(1) receptor: Functional receptor-receptor interactions in the brain

Over the past decade, many lines of investigation have shown that receptor-mediated signaling exhibits greater diversity than previously appreciated. Signal diversity arises from numerous factors, which include the formation of receptor dimers and interplay between different receptors. Using adenosine A₁ receptors as a paradigm of G protein-coupled receptors, this review focuses on how receptor-receptor interactions may contribute to regulation of the synaptic transmission within the central nervous system. The interactions with metabotropic dopamine, adenosine A_2A, A₃, neuropeptide Y, and purinergic P2Y₁ receptors will be described in the first part. The second part deals with interactions between A₁Rs and ionotropic receptors, especially GABA_A, NMDA, and P2X receptors as well as ATP-sensitive K⁺ channels. Finally, the review will discuss new approaches towards treating neurological disorders.

A1 adenosine receptor-mediated modulation of neuronal ATP-sensitive K channels in rat substantia nigra

ATP-sensitive K (K(ATP)) channels, widely expressed in cytoplasmic membranes of neurons, couple cell metabolism to excitability. They are considered to play important roles in controlling seizure activity during hypoxia and in neuroprotection against cell damage during hypoxia, ischemia and excitotoxicity. It is known that adenosine augments the opening of cardiac surface K(ATP) channels by reducing the sensitivity of these channels to ATP blockade. We investigated whether a similar modulation occurs in neuronal channels. Whole cell voltage-clamp recordings were made using rat midbrain slices to record the membrane current and conductance in principal neurons of the substantia nigra pars compacta (SNc). When the pipette solution contained 1 mM ATP, the membrane current at -60 mV and cellular conductance remained stable for at least 15 min. When slices were treated with (-)-N(6)-2-phenylisopropyl adenosine (R-PIA), a selective agonist for A(1) adenosine receptors, in the same condition, the outward current developed slowly to the amplitude of 109.9+/-26.6 pA, and conductance increased to 229+/-50% of the baseline. These changes were strongly inhibited by 200 microM tolbutamide, a K(ATP) channel blocker, suggesting that opening of K(ATP) channels mediated these changes. Pretreatment with 8-cyclopentyltheophylline (CPT), a selective A(1) adenosine receptor antagonist, abolished the outward current and conductance increases. Treatment of adenosine resulted in the similar changes sensitive to tolbutamide. These changes were abolished by CPT. These results suggest that activation of A(1) adenosine receptors promotes the opening of K(ATP) channels in principal neurons of the SNc by removing the blockade by ATP.

Pharmacological characterization of FR194921, a new potent, selective, and orally active antagonist for central adenosine A1 receptors

Adenosine A1 receptors in the brain are believed to play an important role in brain functioning. We have discovered a novel adenosine A1 receptor antagonist, FR194921 (2-(1-methyl-4-piperidinyl)-6-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-3(2H)-pyridazinone), and characterized the pharmacological activity in the present study. FR194921 showed potent and selective affinity for the adenosine A1 receptor without affinity for A2A and A3 receptors and did not show any species differences in binding affinity profile among human, rat, and mouse. Pharmacokinetic study in rats revealed that FR194921 was orally active and highly brain penetrable. Oral administration of FR194921 dose-dependently ameliorated the hypolocomotion induced by the A1 receptor agonist N6-cyclopentyladenosine in rats, indicating this compound exerts A1-antagonistic action in vivo. In the passive avoidance test, scopolamine (1 mg/kg)-induced memory deficits were significantly ameliorated by FR194921 (0.32, 1 mg/kg). In two animal models of anxiety, the social interaction test and elevated plus maze, FR194921 showed specific anxiolytic activity without significantly influencing general behavior. In contrast, FR194921 did not show antidepressant activity even at a dose of 32 mg/kg in the rat forced swimming test. These results indicate that the novel, potent, and selective adenosine A1 receptor antagonist FR194921 exerts both cognitive-enhancing and anxiolytic activity, suggesting the therapeutic potential of this compound for dementia and anxiety disorders.

Protection against hypoxic–ischemic injury in transgenic mice overexpressing Kir6.2 channel pore in forebrain

The role of the K-ATP channel pore-forming subunit Kir6.2 on protection from cerebral hypoxic-ischemic injury was assessed in transgenic mice overexpressing normal Kir6.2 or a dominant negative form (AFA) of this subunit in the forebrain. The resulting mice overexpress either the Kir6.2 or the AFA transgene mainly in the cerebral cortex and hippocampus. The Kir6.2 transgenic mice are resistant to hypoxic-ischemic injury showing a decreased region of cortical damage as compared to the dominant negative AFA and the wild-type mice. Moreover, the overexpression of Kir6.2 allowed an important silencing of the neurons present in forebrain regions thus protecting them from ischemic injury. Interestingly, the phenotype observed in Kir6.2 transgenic mice was observed without increased sulfonylurea binding. Taken together, these results indicate that the transgenic overexpression of Kir6.2 in forebrain significantly protects mice from hypoxic-ischemic injury and neuronal damage seen in stroke.

Mechanisms for maintaining extracellular glutamate levels in the anoxic turtle striatum

The turtle Trachemys scripta is one of a limited group of vertebrates that can withstand hours to days without oxygen. One facet of anoxic survival is the turtle's ability to maintain basal extracellular glutamate levels, whereas in most vertebrates, anoxia triggers massive excitotoxic glutamate release. We investigated glutamate release and reuptake in the anoxic turtle and the effects of adenosine and ATP-sensitive potassium (K(ATP)) channels on glutamate homeostasis. Striatal extracellular glutamate was measured in anesthetized T. scripta by microdialysis in normoxia and over 2-h anoxia. Glutamate release is decreased by 44% in the early anoxic turtle; this anoxia-induced decrease in glutamate release was prevented when K(ATP) channels and adenosine receptors were blocked simultaneously but not when either mechanism was blocked individually. We hypothesize that the continued release and reuptake of glutamate during anoxia help maintain neuronal tone and aid in the recovery of a functional neuronal network after long periods of anoxia, whereas activation of adenosine and/or K(ATP) conserves energy by reducing glutamate release and lowering transport costs.