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

Astrocytic P2X7 receptor in retrosplenial cortex drives electroacupuncture analgesia

P2X7 receptor (P2X7R) has been found to contribute to the peripheral mechanism of acupuncture analgesia (AA). However, whether it plays an important role in central mechanism remains unknown. In this study, we aimed to reveal the role of astrocytic P2X7R in retrosplenial cortex (RSC) in AA and provide new evidence for underlying the central mechanism of AA. We applied the chemogenetic receptors hM3Dq to stimulate or hM4Di to inhibit astrocytes ligand clozapine-N-oxide (CNO) following injection of adeno-associated virus (AAV) into the bilateral RSC, or pharmacologically intervened in the activity of the purinergic receptor P2X7R. Current data indicated that chemogenetic inhibition of astrocytes or injection of P2X7R agonist Bz-ATP in the bilateral RSC significantly reverses the analgesic effect of electroacupuncture (EA) in formalin tests while the bilateral injection of the P2X7R antagonist A438079 alleviated formalin-induced nociceptive behavior. Additionally, chemogenetic suppression of astrocytic P2X7R by injection of AAV in the bilateral RSC decreased hind paw flinches induced by formalin in the mice. These findings indicate the participation of both astrocytes and P2X7R in the RSC in EA analgesic. Moreover, P2X7R on astrocytes in the RSC appears to play a critical role in the ability of EA to attenuate formalin-induced pain responses in mice.

Neuroprotection by trans-resveratrol in rats with N-methyl-D-aspartate (NMDA)-induced retinal injury: Insights into the role of adenosine A1 receptors

Adenosine A1 receptors (AA1R) have been shown to counteract N-methyl-D-aspartate (NMDA)-mediated glutamatergic excitotoxicity. In the present study, we investigated the role of AA1R in neuroprotection by trans-resveratrol (TR) against NMDA-induced retinal injury. In total, 48 rats were divided into the following four groups: normal rats pretreated with vehicle; rats that received NMDA (NMDA group); rats that received NMDA after pretreatment with TR; and rats that received NMDA after pretreatment with TR and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an AA1R antagonist. Assessment of general and visual behaviour was performed using the open field test and two-chamber mirror test, respectively, on Days 5 and 6 post NMDA injection. Seven days after NMDA injection, animals were euthanized, and eyeballs and optic nerves were harvested for histological parameters, whereas retinae were isolated to determine the redox status and expression of pro- and anti-apoptotic proteins. In the present study, the retinal and optic nerve morphology in the TR group was protected from NMDA-induced excitotoxic damage. These effects were correlated with the lower retinal expression of proapoptotic markers, lipid peroxidation, and markers of nitrosative/oxidative stress. The general and visual behavioural parameters in the TR group showed less anxiety-related behaviour and better visual function than those in the NMDA group. All the findings observed in the TR group were abolished by administration of DPCPX.

G-Protein-Coupled Receptors and Ischemic Stroke: a Focus on Molecular Function and Therapeutic Potential

In ischemic stroke, there is only one approved drug, tissue plasminogen activator, to be used in clinical conditions for thrombolysis. New neuroprotective therapies for ischemic stroke are desperately needed. Several targets and pathways have been shown to confer neuroprotective effects in ischemic stroke. G-protein-coupled receptors (GPCRs) are one of the most frequently targeted receptors for developing novel therapeutics for central nervous system disorders. GPCRs are a large family of cell surface receptors that response to a wide variety of extracellular stimuli. GPCRs are involved in a wide range of physiological and pathological processes. More than 90% of the identified non-sensory GPCRs are expressed in the brain, where they play important roles in regulating mood, pain, vision, immune responses, cognition, and synaptic transmission. There is also good evidence that GPCRs are implicated in the pathogenesis of stroke. This review narrates the pathophysiological role and possible targeted therapy of GPCRs in ischemic stroke.

Sobriety and Satiety: Is NAD+ the Answer?

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that has garnered considerable interest in the last century due to its critical role in cellular processes associated with energy production, cellular protection against stress and longevity. Research in NAD+ has been reinvigorated by recent findings that components of NAD+ metabolism and NAD-dependent enzymes can influence major signalling processes associated with the neurobiology of addiction. These studies implicate raising intracellular NAD+ levels as a potential target for managing and treating addictive behaviour and reducing cravings and withdrawal symptoms in patients with food addiction and/or substance abuse. Since clinical studies showing the use of NAD+ for the treatment of addiction are limited, this review provides literature evidence that NAD+ can influence the neurobiology of addiction and may have benefits as an anti-addiction intervention.

Characteristics and the role of purinergic receptors in pathophysiology with focus on immune response

The nucleotide adenosine-5'-triphosphate (ATP) is mostly thought to be energy carrier, but evidence presented in multiple studies proves ATP involvement into variety of processes, due to its neuromodulatory capabilities. ATP and its metabolite-adenosine, bind to the purinergic receptors, which are divided into two types: adenosine binding P1 receptor and ADP/ATP binding P2 receptor. These receptors are expressed in different tissues and organs. Recent studies report their immunomodulatory characteristics, connected with varying immunological processes, such as immunological response or antigen presentation. Besides, they seem to play an important role in medical conditions such as bronchial asthma or variety of cancers. In this article, we would like to review recent discoveries on the field of purinergic receptors research focusing on their role in immunological system, and shed a new light upon the importance of these receptors in modern medicine development.

Neuroprotection by trans-resveratrol against collagenase-induced neurological and neurobehavioural deficits in rats involves adenosine A1 receptors

Objective: Trans-resveratrol has been shown to have neuroprotective effects and could be a promising therapeutic agent in the treatment of intracerebral haemorrhage (ICH). This study aimed to investigate the involvement of the adenosine A1 receptor (A1R) in trans-resveratrol-induced neuroprotection in rats with collagenase-induced ICH.Methods: Sixty male Sprague-Dawley rats weighing 330-380 g were randomly divided into five groups (n = 12): (i) control, (ii) sham-operated rats, (iii) ICH rats pretreated with vehicle (0.1% DMSO saline, i.c.v.), (iv) ICH rats pretreated with trans-resveratrol (0.9 µg, i.c.v.) and (v) ICH rats pretreated with trans-resveratrol (0.9 µg) and the A1R antagonist, DPCPX (2.5 µg, i.c.v.). Thirty minutes after pretreatment, ICH was induced by intrastriatal injection of collagenase (0.04 U). Forty-eight hours after ICH, the rats were assessed using a variety of neurobehavioural tests. Subsequently, rats were sacrificed and brains were subjected to gross morphological examination of the haematoma area and histological examination of the damaged area.Results: Severe neurobehavioural deficits and haematoma with diffuse oedema were observed after intrastriatal collagenase injection. Pretreatment with trans-resveratrol partially restored general locomotor activity, muscle strength and coordination, which was accompanied with reduction of haematoma volume by 73.22% (P < 0.05) and damaged area by 60.77% (P < 0.05) in comparison to the vehicle-pretreated ICH group. The trans-resveratrol-induced improvement in neurobehavioural outcomes and morphological features of brain tissues was inhibited by DPCPX pretreatment.Conclusion: This study demonstrates that the A1R activation is possibly the mechanism underlying the trans-resveratrol-induced neurological and neurobehavioural protection in rats with ICH.

Inhibition of the Adenosine A2A Receptor Mitigates Excitotoxic Injury in Organotypic Tissue Cultures of the Rat Cochlea

The primary loss of cochlear glutamatergic afferent nerve synapses due to noise or ageing (cochlear neuropathy) often presents as difficulties in speech discrimination in noisy conditions (hidden hearing loss (HHL)). Currently, there is no treatment for this condition. Our previous studies in mice with genetic deletion of the adenosine A_2A receptor (A_2AR) have demonstrated better preservation of cochlear afferent synapses and spiral ganglion neurons after noise exposure compared to wildtype mice. This has informed our current targeted approach to cochlear neuroprotection based on pharmacological inhibition of the A_2AR. Here, we have used organotypic tissue culture of the Wistar rat cochlea at postnatal day 6 (P6) to model excitotoxic injury induced by N-methyl-d-aspartate (NMDA)/kainic acid (NK) treatment for 2 h. The excitotoxic injury was characterised by a reduction in the density of neural processes immediately after NK treatment and loss of afferent synapses in the presence of intact sensory hair cells. The administration of istradefylline (a clinically approved A_2AR antagonist) reduced deafferentation of inner hair cells and improved the survival of afferent synapses after excitotoxic injury. This study thus provides evidence that A_2AR inhibition promotes cochlear recovery from excitotoxic injury, and may have implications for the treatment of cochlear neuropathy and prevention of HHL.

Adenosine Signaling through A1 Receptors Inhibits Chemosensitive Neurons in the Retrotrapezoid Nucleus

A subset of neurons in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors by regulating depth and frequency of breathing in response to changes in tissue CO₂/H⁺. The activity of chemosensitive RTN neurons is also subject to modulation by CO₂/H⁺-dependent purinergic signaling. However, mechanisms contributing to purinergic regulation of RTN chemoreceptors are not entirely clear. Recent evidence suggests adenosine inhibits RTN chemoreception in vivo by activation of A1 receptors. The goal of this study was to characterize effects of adenosine on chemosensitive RTN neurons and identify intrinsic and synaptic mechanisms underlying this response. Cell-attached recordings from RTN chemoreceptors in slices from rat or wild-type mouse pups (mixed sex) show that exposure to adenosine (1 µM) inhibits chemoreceptor activity by an A1 receptor-dependent mechanism. However, exposure to a selective A1 receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, DPCPX; 30 nM) alone did not potentiate CO₂/H⁺-stimulated activity, suggesting activation of A1 receptors does not limit chemoreceptor activity under these reduced conditions. Whole-cell voltage-clamp from chemosensitive RTN neurons shows that exposure to adenosine activated an inward rectifying K⁺ conductance, and at the network level, adenosine preferentially decreased frequency of EPSCs but not IPSCs. These results show that adenosine activation of A1 receptors inhibits chemosensitive RTN neurons by direct activation of a G-protein-regulated inward-rectifier K⁺ (GIRK)-like conductance, and presynaptically, by suppression of excitatory synaptic input to chemoreceptors.

Altered Purinergic Receptor Sensitivity in Type 2 Diabetes-Associated Endothelial Dysfunction and Up₄A-Mediated Vascular Contraction

Purinergic signaling may be altered in diabetes accounting for endothelial dysfunction. Uridine adenosine tetraphosphate (Up₄A), a novel dinucleotide substance, regulates vascular function via both purinergic P1 and P2 receptors (PR). Up₄A enhances vascular contraction in isolated arteries of diabetic rats likely through P2R. However, the precise involvement of PRs in endothelial dysfunction and the vasoconstrictor response to Up₄A in diabetes has not been fully elucidated. We tested whether inhibition of PRs improved endothelial function and attenuated Up₄A-mediated vascular contraction using both aortas and mesenteric arteries of type 2 diabetic (T2D) Goto Kakizaki (GK) rats vs. control Wistar (WT) rats. Endothelium-dependent (EDR) but not endothelium-independent relaxation was significantly impaired in both aortas and mesenteric arteries from GK vs. WT rats. Non-selective inhibition of P1R or P2R significantly improved EDR in aortas but not mesenteric arteries from GK rats. Inhibition of A1R, P2X₇R, or P2Y₆R significantly improved EDR in aortas. Vasoconstrictor response to Up₄A was enhanced in aortas but not mesenteric arteries of GK vs. WT rats via involvement of A1R and P2X₇R but not P2Y₆R. Depletion of major endothelial component nitric oxide enhanced Up₄A-induced aortic contraction to a similar extent between WT and GK rats. No significant differences in protein levels of A1R, P2X₇R, and P2Y₆R in aortas from GK and WT rats were observed. These data suggest that altered PR sensitivity accounts for endothelial dysfunction in aortas in diabetes. Modulating PRs may represent a potential therapy for improving endothelial function.

The P2X4 receptor is required for neuroprotection via ischemic preconditioning

Ischemic preconditioning (IPC), a procedure consisting of transient ischemia and subsequent reperfusion, provides ischemic tolerance against prolonged ischemia in the brain. Although the blood flow changes mediated by IPC are primarily perceived by vascular endothelial cells, the role of these cells in ischemic tolerance has not been fully clarified. In this study, we found that the P2X4 receptor, which is abundantly expressed in vascular endothelial cells, is required for ischemic tolerance following middle artery occlusion (MCAO) in mice. Mechanistically, the P2X4 receptor was stimulated by fluid shear stress, which mimics reperfusion, thus promoting the increased expression of osteopontin, a neuroprotective molecule. Furthermore, we found that the intracerebroventricular administration of osteopontin was sufficient to exert a neuroprotective effect mediated by preconditioning-stimulated P2X4 receptor activation. These results demonstrate a novel mechanism whereby vascular endothelial cells are involved in ischemic tolerance.

Protein phosphatase role in adenosine A1 receptor-induced AMPA receptor trafficking and rat hippocampal neuronal damage in hypoxia/reperfusion injury

Adenosine signaling via A1 receptor (A1R) and A2A receptor (A2AR) has shown promise in revealing potential targets for neuroprotection in cerebral ischemia. We recently showed a novel mechanism by which A1R activation with N(6)-cyclopentyl adenosine (CPA) induced GluA1 and GluA2 AMPA receptor (AMPAR) endocytosis and adenosine-induced persistent synaptic depression (APSD) in rat hippocampus. This study further investigates the mechanism of A1R-mediated AMPAR internalization and hippocampal slice neuronal damage through activation of protein phosphatase 1 (PP1), 2A (PP2A), and 2B (PP2B) using electrophysiological, biochemical and imaging techniques. Following prolonged A1R activation, GluA2 internalization was selectively blocked by PP2A inhibitors (okadaic acid and fostriecin), whereas inhibitors of PP2A, PP1 (tautomycetin), and PP2B (FK506) all prevented GluA1 internalization. Additionally, GluA1 phosphorylation at Ser831 and Ser845 was reduced after prolonged A1R activation in hippocampal slices. PP2A inhibitors nullified A1R-mediated downregulation of pSer845-GluA1, while PP1 and PP2B inhibitors prevented pSer831-GluA1 downregulation. Each protein phosphatase inhibitor also blunted CPA-induced synaptic depression and APSD. We then tested whether A1R-mediated changes in AMPAR trafficking and APSD contribute to hypoxia-induced neuronal injury. Hypoxia (20 min) induced A1R-mediated internalization of both AMPAR subunits, and subsequent normoxic reperfusion (45 min) increased GluA1 but persistently reduced GluA2 surface expression. Neuronal damage after hypoxia-reperfusion injury was significantly blunted by pre-incubation with the above protein phosphatase inhibitors. Together, these data suggest that A1R-mediated protein phosphatase activation causes persistent synaptic depression by downregulating GluA2-containing AMPARs; this previously undefined role of A1R stimulation in hippocampal neuronal damage represents a novel therapeutic target in cerebral ischemic damage.

Duration of activity and mode of action of modafinil: Studies on sleep and wakefulness in humans

The duration of activity of modafinil was investigated in healthy male volunteers in two double-blind crossover studies. Mode of action was explored using a statistical model concerned with the relationship between total sleep duration and that of rapid eye movement (REM) sleep. Nocturnal sleep (23:00-07:00) followed by next-day performance (09:00-17:00) was studied in 12 subjects administered 100, 200, 300 mg modafinil and placebo, 0.5 h before bedtime. Performance overnight (19:00-08:45) followed by sleep (09:15-15:15) was studied in nine subjects administered 100, 200, 300, 400 mg modafinil, 300 mg caffeine and placebo at 22:15. Modafinil dose-dependently reduced sleep duration (nocturnal: 200 mg, p<0.05; 300 mg, p<0.001; morning: 300 and 400 mg, p<0.05) and REM sleep (nocturnal: 300 mg; morning: 400 mg; p<0.05). The statistical model revealed that reduced REM sleep was due to alerting activity, with no evidence of direct suppression of REM sleep, suggesting dopaminergic activity. Enhanced performance with modafinil during overnight work varied with dose (200 mg>100 mg; 300, 400 mg>200, 100 mg, caffeine). However, in the study of next-day performance, the enhancement was attenuated at the highest dose (300 mg) by the greater disturbance of prior sleep. These findings indicate that modafinil has a long duration of action, with alerting properties arising predominantly from dopaminergic activity.

3-Mercaptopropionic acid-induced repetitive seizures increase GluN2A expression in rat hippocampus: a potential neuroprotective role of cyclopentyladenosine

The N-methyl-D-aspartate receptor (NMDAR) is involved in synaptic plasticity, learning, memory, and neurological diseases like epilepsy and it is the major mediator of excitotoxicity. Functional NMDARs in the mature brain are heteromeric complexes composed of different subunits: GluN1 and GluN2. There are four different GluN2 subunits (A-D) and each of them critically determines the pharmacological and electrophysiological properties of NMDARs. GluN1 is ubiquitously expressed in the central nervous system while the highest GluN2A expression is in the hippocampus. Adenosine, an endogenous anticonvulsant, is a neuromodulator with a critical role in the regulation of neuronal activity, mediating its effect on specific receptors, among which adenosine A1 receptor is highly expressed in the hippocampus. In the present work hippocampal GluN2A expression after the convulsant drug 3-mercaptopropionic acid (MP) induced seizures and the effect of cyclopentyladenosine (CPA) given alone or prior to MP (CPA + MP) in an acute or repetitive experimental model was studied. CPA administered to rats for one or 4 days increases seizure threshold induced by MP. After one administration of MP, no significant difference in GluN2A expression was observed in CPA and CPA + MP by Western blot, although immunohistochemistry revealed an increase in CA2/3 area. However, repetitive MP administration during 4 days showed a significant increase of GluN2A expression, and the repetitive administration of CPA 30 min prior to MP caused a significant decrease of GluN2A expression with respect to MP treatment, returning to control levels. These results show that GluN2A subunit is involved in repetitive MP-induced seizures, while CPA administration displays a protective effect against it.

Purines, purinergic receptors, and cancer

Purines were long thought to be restricted to the intracellular compartment, where they are used for energy transactions, nucleic acid synthesis, and a multiplicity of biochemical reactions. However, it is now clear that both adenosine and adenosine triphosphate are (i) abundant biochemical components of the tumor microenvironment, (ii) potent modulators of immune cell responses and cytokine release, and (iii) key players in host-tumor interaction. Moreover, both ATP and adenosine directly affect tumor cell growth. Adenosine is a powerful immunosuppressant (mainly acting at A2A receptors) and a modulator of cell growth (mainly acting at A3 receptors). ATP is a proinflammatory (acting at P2Y1, P2Y2, P2Y4, P2Y6, and P2Y12, and at P2X4 and P2X7 receptors), an immunosuppressant (acting at P2Y11), and a growth-promoting agent (acting at P2Y1, P2Y2, and P2X7 receptors). This complex signaling network generates an array of inhibitory and stimulatory responses that affect immune cell function, tumor growth, and metastatic dissemination. Investigation of purinergic signaling has increased our understanding of the tumor microenvironment and opened new and exciting avenues for the development of novel therapeutics.

Interaction of purinergic receptors with GPCRs, ion channels, tyrosine kinase and steroid hormone receptors orchestrates cell function

Extracellular purines and pyrimidines have emerged as key regulators of a wide range of physiological and pathophysiological cellular processes acting through P1 and P2 cell surface receptors. Increasing evidence suggests that purinergic receptors can interact with and/or modulate the activity of other classes of receptors and ion channels. This review will focus on the interactions of purinergic receptors with other GPCRs, ion channels, receptor tyrosine kinases, and steroid hormone receptors. Also, the signal transduction pathways regulated by these complexes and their new functional properties are discussed.

An essential role for adenosine signaling in alcohol abuse

In the central nervous system (CNS), adenosine plays an important role in regulating neuronal activity and modulates signaling by other neurotransmitters, including GABA, glutamate, and dopamine. Adenosine suppresses neurotransmitter release, reduces neuronal excitability, and regulates ion channel function through activation of four classes of G protein-coupled receptors, A(1), A(2A), A(2B), and A(3). Central adenosine are largely controlled by nucleoside transporters, which transport adenosine levels across the plasma membrane. Adenosine has been shown to modulate cortical glutamate signaling and ventral-tegmental dopaminergic signaling, which are involved in several aspects of alcohol use disorders. Acute ethanol elevates extracellular adenosine levels by selectively inhibiting the type 1 equilibrative nucleoside transporter, ENT1. Raised adenosine levels mediate the ataxic and sedative/hypnotic effects of ethanol through activation of A(1) receptors in the cerebellum, striatum, and cerebral cortex. Recently, we have shown that pharmacological inhibition or genetic deletion of ENT1 reduces the expression of excitatory amino acid transporter 2 (EAAT2), the primary regulator of extracellular glutamate, in astrocytes. These lines of evidence support a central role for adenosine-mediated glutamate signaling and the involvement of astrocytes in regulating ethanol intoxication and preference. In this paper, we discuss recent findings on the implication of adenosine signaling in alcohol use disorders.

Adenosine A1 receptors mediate local anti-nociceptive effects of acupuncture

Acupuncture is an invasive procedure commonly used to relieve pain. Acupuncture is practiced worldwide, despite difficulties in reconciling its principles with evidence-based medicine. We found that adenosine, a neuromodulator with anti-nociceptive properties, was released during acupuncture in mice and that its anti-nociceptive actions required adenosine A1 receptor expression. Direct injection of an adenosine A1 receptor agonist replicated the analgesic effect of acupuncture. Inhibition of enzymes involved in adenosine degradation potentiated the acupuncture-elicited increase in adenosine, as well as its anti-nociceptive effect. These observations indicate that adenosine mediates the effects of acupuncture and that interfering with adenosine metabolism may prolong the clinical benefit of acupuncture.

Metabolic autocrine regulation of neurons involves cooperation among pannexin hemichannels, adenosine receptors, and KATP channels

Metabolic perturbations that decrease or limit blood glucose-such as fasting or adhering to a ketogenic diet-reduce epileptic seizures significantly. To date, the critical links between altered metabolism and decreased neuronal activity remain unknown. More generally, metabolic changes accompany numerous CNS disorders, and the purines ATP and its core molecule adenosine are poised to translate cell energy into altered neuronal activity. Here we show that nonpathological changes in metabolism induce a purinergic autoregulation of hippocampal CA3 pyramidal neuron excitability. During conditions of sufficient intracellular ATP, reducing extracellular glucose induces pannexin-1 hemichannel-mediated ATP release directly from CA3 neurons. This extracellular ATP is dephosphorylated to adenosine, activates neuronal adenosine A(1) receptors, and, unexpectedly, hyperpolarizes neuronal membrane potential via ATP-sensitive K(+) channels. Together, these data delineate an autocrine regulation of neuronal excitability via ATP and adenosine in a seizure-prone subregion of the hippocampus and offer new mechanistic insight into the relationship between decreased glucose and increased seizure threshold. By establishing neuronal ATP release via pannexin hemichannels, and hippocampal adenosine A(1) receptors coupled to ATP-sensitive K(+) channels, we reveal detailed information regarding the relationship between metabolism and neuronal activity and new strategies for adenosine-based therapies in the CNS.

Adenosine augmentation therapies (AATs) for epilepsy: prospect of cell and gene therapies

Deficiencies in the brain's own adenosine-based seizure control system contribute to seizure generation. Consequently, reconstitution of adenosinergic neuromodulation constitutes a rational approach for seizure control. This review will critically discuss focal adenosine augmentation strategies and their potential for antiepileptic and disease modifying therapy. Due to systemic side effects of adenosine focal adenosine augmentation--ideally targeted to an epileptic focus--becomes a therapeutic necessity. This has experimentally been achieved in kindled seizure models as well as in post-status epilepticus models of spontaneous recurrent seizures using three different therapeutic strategies that will be discussed here: (i) polymer-based brain implants that were loaded with adenosine; (ii) brain implants comprised of cells engineered to release adenosine and embedded in a cell-encapsulation device; (iii) direct transplantation of stem cells engineered to release adenosine. To meet the therapeutic goal of focal adenosine augmentation, genetic disruption of the adenosine metabolizing enzyme adenosine kinase (ADK) in rodent and human cells was used as a molecular strategy to induce adenosine release from cellular brain implants, which demonstrated antiepileptic and neuroprotective properties. New developments and therapeutic challenges in using AATs for epilepsy therapy will critically be evaluated.