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

Microglial polarization in Alzheimer's disease: Mechanisms, implications, and therapeutic opportunities

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the accumulation of amyloid-β plaques, neurofibrillary tangles, and chronic neuroinflammation. Microglial cells, the resident immune cells in the central nervous system, play a crucial role in the pathogenesis of AD. Microglia can undergo polarization, shifting between pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes in response to different stimuli. Dysregulation of microglial polarization towards the pro-inflammatory phenotype leads to the release of inflammatory cytokines, oxidative stress, and synaptic dysfunction. These processes contribute to neuronal damage and cognitive decline in AD. However, several challenges remain in this field. The complex molecular mechanisms governing microglial polarization in AD need to be further elucidated. In this review, we discuss the mechanisms underlying microglial polarization in AD and its implications in disease progression.

Targeting the adenosine A2A receptor for neuroprotection and cognitive improvement in traumatic brain injury and Parkinson's disease

Adenosine exerts its dual functions of homeostasis and neuromodulation in the brain by acting at mainly 2 G-protein coupled receptors, called A1 and A2A receptors. The adenosine A2A receptor (A2AR) antagonists have been clinically pursued for the last 2 decades, leading to final approval of the istradefylline, an A2AR antagonist, for the treatment of OFF-Parkinson's disease (PD) patients. The approval paves the way to develop novel therapeutic methods for A2AR antagonists to address 2 major unmet medical needs in PD and traumatic brain injury (TBI), namely neuroprotection or improving cognition. In this review, we first consider the evidence for aberrantly increased adenosine signaling in PD and TBI and the sufficiency of the increased A2AR signaling to trigger neurotoxicity and cognitive impairment. We further discuss the increasing preclinical data on the reversal of cognitive deficits in PD and TBI by A2AR antagonists through control of degenerative proteins and synaptotoxicity, and on protection against TBI and PD pathologies by A2AR antagonists through control of neuroinflammation. Moreover, we provide the supporting evidence from multiple human prospective epidemiological studies which revealed an inverse relation between the consumption of caffeine and the risk of developing PD and cognitive decline in aging population and Alzheimer's disease patients. Collectively, the convergence of clinical, epidemiological and experimental evidence supports the validity of A2AR as a new therapeutic target and facilitates the design of A2AR antagonists in clinical trials for disease-modifying and cognitive benefit in PD and TBI patients.

Inhibition of the interaction between microglial adenosine 2A receptor and NLRP3 inflammasome attenuates neuroinflammation posttraumatic brain injury

AIMS

Adenosine 2A receptor (A2A R) is widely expressed in the brain and plays important roles in neuroinflammation, and the nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing protein 3 (NLRP3) inflammasome is a crucial component of the innate immune system while the regulation of A2A R on it in the central nervous system (CNS) has not been clarified.

METHODS

The effects of microglial A2A R on NLRP3 inflammasome assembly and activation were investigated in wild-type, A2A R- or NLRP3-knockout primary microglia with pharmacological treatment. Microglial A2A R or NLRP3 conditional knockout mice were used to interrogate the effects of this regulation on neuroinflammation posttraumatic brain injury (TBI).

RESULTS

We found that A2A R directly interacted with NLRP3 and facilitated NLRP3 inflammasome assembly and activation in primary microglia while having no effects on mRNA levels of inflammasome components. Inhibition of the interaction via A2A R agonist or knockout attenuated inflammasome assembly and activation in vitro. In the TBI model, microglial A2A R and NLRP3 were co-expressed at high levels in microglia next to the peri-injured cortex, and abrogating of this interaction by microglial NLRP3 or A2A R conditional knockout attenuated the neurological deficits and neuropathology post-TBI via reducing the NLRP3 inflammasome activation.

CONCLUSION

Our results demonstrated that inhibition of the interaction between A2A R and NLRP3 in microglia could mitigate the NLRP3 inflammasome assembly and activation and ameliorate the neuroinflammation post-TBI. It provides new insights into the effects of A2A R on neuroinflammation regulation post-TBI and offers a potential target for the treatment of NLRP3 inflammasome-related CNS diseases.

Prenatal exposition to haloperidol: A preclinical narrative review

Pre-existing maternal mental disorders may affect the early interactions between mother and baby, impacting the child's psychoemotional development. The typical antipsychotic haloperidol can be used during pregnancy, even with some restrictions. Its prescription is not limited to psychotic disorders, but also to other psychiatric conditions of high incidence and prevalence in the woman's fertile period. The present review focused on the preclinical available data regarding the biological and behavioral implications of embryonic exposure to haloperidol. The understanding of the effects of psychotropic drugs during neurodevelopment is important for its clinical aspect since there is limited evidence regarding the risks of antipsychotic drug treatment in pregnant women and their children. Moreover, a better comprehension of the mechanistic events that can be affected by antipsychotic treatment during the critical period of neurodevelopment may offer insights into the pathophysiology of neurodevelopmental disorders. The findings presented in this review converge to the existence of several risks associated with prenatal exposure to such medication and emphasize the need for further studies regarding its dimensions.

Molecular and Structural Insight into Adenosine A2A Receptor in Neurodegenerative Disorders: A Significant Target for Efficient Treatment Approach

All biological tissues and bodily fluids include the autacoid adenosine. The P1 class of purinergic receptors includes adenosine receptors. Four distinct G-protein-coupled receptors on the cellular membrane mediate the effects of adenosine, whose cytoplasmic content is regulated by producing/degrading enzymes and nucleoside transporters. A2A receptor has received a great deal of attention in recent years because it has a wide range of potential therapeutic uses. A2B and, more significantly, A2A receptors regulate numerous physiological mechanisms in the central nervous system (CNS). The inferior targetability of A2B receptors towards adenosine points that they might portray a promising medicinal target since they are triggered only under pharmacological circumstances (when adenosine levels rise up to micromolar concentrations). The accessibility of specific ligands for A2B receptors would permit the exploration of such a theory. A2A receptors mediate both potentially neurotoxic and neuroprotective actions. Hence, it is debatable to what extent they play a role in neurodegenerative illnesses. However, A2A receptor blockers have demonstrated clear antiparkinsonian consequences, and a significant attraction exists in the role of A2A receptors in other neurodegenerative disorders. Amyloid peptide extracellular accumulation and tau hyperphosphorylation are the pathogenic components of AD that lead to neuronal cell death, cognitive impairment, and memory loss. Interestingly, in vitro and in vivo research has shown that A2A adenosine receptor antagonists may block each of these clinical symptoms, offering a crucial new approach to combat a condition for which, regrettably, only symptomatic medications are currently available. At least two requirements must be met to determine whether such receptors are a target for diseases of the CNS: a complete understanding of the mechanisms governing A2A-dependent processes and the availability of ligands that can distinguish between the various receptor populations. This review concisely summarises the biological effects mediated by A2A adenosine receptors in neurodegenerative disorders and discusses the chemical characteristics of A2A adenosine receptor antagonists undergoing clinical trials. Selective A2A receptor blocker against neurodegenerative disorders.

A2AR and traumatic brain injury

Accumulating evidence has revealed the adenosine 2A receptor is a key tuner for neuropathological and neurobehavioral changes following traumatic brain injury by experimental animal models and a few clinical trials. Here, we highlight recent data involving acute/sub-acute and chronic alterations of adenosine and adenosine 2A receptor-associated signaling in pathological conditions after trauma, with an emphasis of traumatic brain injury, including neuroinflammation, cognitive and psychiatric disorders, and other severe consequences. We expect this would lead to the development of therapeutic strategies for trauma-related disorders with novel mechanisms of action.

Changes in adenosine receptors and neurotrophic factors in the SOD1G93A mouse model of amyotrophic lateral sclerosis: Modulation by chronic caffeine

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of corticospinal tract motor neurons. Previous studies showed that adenosine-mediated neuromodulation is disturbed in ALS and that vascular endothelial growth factor (VEGF) has a neuroprotective function in ALS mouse models. We evaluated how adenosine (A1R and A2AR) and VEGF (VEGFA, VEGFB, VEGFR-1 and VEGFR-2) system markers are altered in the cortex and spinal cord of pre-symptomatic and symptomatic SOD1G93A mice. We then assessed if/how chronic treatment of SOD1G93A mice with a widely consumed adenosine receptor antagonist, caffeine, modulates VEGF system and/or the levels of Brain-derived Neurotrophic Factor (BDNF), known to be under control of A2AR. We found out decreases in A1R and increases in A2AR levels even before disease onset. Concerning the VEGF system, we detected increases of VEGFB and VEGFR-2 levels in the spinal cord at pre-symptomatic stage, which reverses at the symptomatic stage, and decreases of VEGFA levels in the cortex, in very late disease states. Chronic treatment with caffeine rescued cortical A1R levels in SOD1G93A mice, bringing them to control levels, while rendering VEGF signaling nearly unaffected. In contrast, BDNF levels were significantly affected in SOD1G93A mice treated with caffeine, being decreased in the cortex and increased in spinal the cord. Altogether, these findings suggest an early dysfunction of the adenosinergic system in ALS and highlights the possibility that the negative influence of caffeine previously reported in ALS animal models results from interference with BDNF rather than with the VEGF signaling molecules.

Sirolimus relieves seizures and neuropsychiatric symptoms via changes of microglial polarity in tuberous sclerosis complex model mice

Tuberous sclerosis complex (TSC) is a genetic disorder involving a variety of physical manifestations, and is associated with epilepsy and multiple serious neuropsychiatric symptoms. These symptoms are collectively known as TSC-associated neuropsychiatric disorders (TAND), which is a severe burden for patients and their families. Overactivation of the mechanistic target of rapamycin complex 1 (mTORC1) by mutations in TSC1 or TSC2 is thought to cause TSC, and mTORC1 inhibitors such as sirolimus and everolimus are reported to be effective against various tumor types of TSC. However, there are various reports on the effect of mTORC1 inhibitor therapy on TAND in patients with TSC, which may or may not be effective. In our previous investigations, we generated TSC2 conditional knockout mice (Mitf-Cre, Tsc2 KO; Tsc2 cKO). These mice developed spontaneous epileptic activity. In the current study, we further analyzed the detailed behaviors of Tsc2 cKO mice and confirmed that they exhibited phenotypes of TAND as well as epileptic seizures, indicating that Tsc2 cKO mice are a useful model for TAND. Furthermore, the olfactory bulb and piriform cortex caused epilepsy and TAND in Tsc2 cKO mice, and neurodegeneration was observed. Immunohistology and immunophenotypic analysis of cells, and quantitative RT-PCR suggested that changes in microglial polarity were involved in the onset of TSC epilepsy and neuropsychiatric symptoms. Although the effect of mTORC1 inhibitors on TAND has not been established, the results of this study might help elucidate the mechanism of TAND pathogenesis and suggest that sirolimus may be a valuable therapeutic tool for TAND.

Inosine attenuates 3-nitropropionic acid-induced Huntington's disease-like symptoms in rats via the activation of the A2AR/BDNF/TrKB/ERK/CREB signaling pathway

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disease characterized by involuntary bizarre movements, psychiatric symptoms, dementia, and early death. Several studies suggested neuroprotective activities of inosine; however its role in HD is yet to be elucidated. The current study aimed to demonstrate the neuroprotective effect of inosine in 3-nitropropionic acid (3-NP)-induced neurotoxicity in rats while investigating possible underlying mechanisms. Rats were randomly divided into five groups; group 1 received i.p. injections of 1% DMSO, whereas groups 2, 3, 4, and 5 received 3-NP (10 mg/kg, i.p.) for 14 days, concomitantly with inosine (200 mg/kg., i.p.) in groups 3, 4, and 5, SCH58261, a selective adenosine 2A receptor (A2AR) antagonist, (0.05 mg/kg, i.p.) in group 4, and PD98059, an extracellular signal-regulated kinase (ERK) inhibitor, (0.3 mg/kg, i.p.) in group 5. Treatment with inosine mitigated 3-NP-induced motor abnormalities and body weight loss. Moreover, inosine boosted the striatal brain-derived neurotrophic factor (BDNF) level, p-tropomyosin receptor kinase B (TrKB), p-ERK, and p-cAMP response element-binding protein (CREB) expression, which subsequently suppressed oxidative stress biomarkers (malondialdehyde and nitric oxide) and pro-inflammatory cytokines (tumor necrosis factor alpha and interleukin-1β) and replenished the glutathione content. Similarly, histopathological analyses revealed decreased striatal injury score, the expression of the glial fibrillary acidic protein, and neuronal loss after inosine treatment. These effects were attenuated by the pre-administration of SCH58261 or PD98059. In conclusion, inosine attenuated 3-NP-induced HD-like symptoms in rats, at least in part, via the activation of the A2AR/BDNF/TrKB/ERK/CREB signaling pathway.

Caffeine Protects Against Retinal Inflammation

Caffeine, one of the most consumed central nervous system (CNS) stimulants, is an antagonist of A₁ and A_2A adenosine receptors. In this study, we investigated the potential protective effects of this methylxanthine in the retinal tissue. We tested caffeine by using in vitro and in vivo paradigms of retinal inflammation. Human retinal pigment epithelial cells (ARPE-19) were exposed to lipopolysaccharide (LPS) with or without caffeine. This latter was able to reduce the inflammatory response in ARPE-19 cells exposed to LPS, attenuating the release of IL-1β, IL-6, and TNF-α and the nuclear translocation of p-NFκB. Additionally, caffeine treatment restored the integrity of the ARPE-19 monolayer assessed by transepithelial electrical resistance (TEER) and the sodium fluorescein permeability test. Finally, the ischemia reperfusion (I/R) injury model was used in C57BL/6J mice to induce retinal inflammation and investigate the effects of caffeine treatment. Mouse eyes were treated topically with caffeine, and a pattern electroretinogram (PERG) was used to assess the retinal ganglion cell (RGC) function; furthermore, we evaluated the levels of IL-6 and BDNF in the retina. Retinal BDNF dropped significantly (p < 0.05) in the I/R group compared to the control group (normal mice); on the contrary, caffeine treatment maintained physiological levels of BDNF in the retina of I/R eyes. Caffeine was also able to reduce IL-6 mRNA levels in the retina of I/R eyes. In conclusion, these findings suggest that caffeine is a good candidate to counteract inflammation in retinal diseases.

High glutamate concentration reverses the inhibitory effect of microglial adenosine 2A receptor on NLRP3 inflammasome assembly and activation

NLRP3 inflammasome plays a crucial role in the innate immune system. Our group previously reported that the microglial adenosine 2A receptor (A_2AR) regulates canonical neuroinflammation, which is affected by the glutamate concentration. However, the regulatory effect of A_2AR on NLRP3 inflammasome and the effects of glutamate concentration remain unknown. Therefore, we aimed to investigate the regulatory effect of microglial A_2AR on NLRP3 inflammasome assembly and activation as well as the effects of glutamate concentration on the inflammasome assembly and activation. Experiments were conducted on magnetically sorted primary microglia from P14 mice. The results showed that pharmacological A_2AR activation ameliorated NLRP3 activation under no or low glutamate concentrations, but this effect was reversed by high glutamate concentrations. Moreover, the mRNA levels of NLRP3 inflammasome-related genes were not affected by A_2AR activation or the glutamate concentration. We further demonstrated that A_2AR activation inhibited the interaction between NLRP3 and caspase 1 under no or low glutamate concentrations while promoting their interaction under high glutamate concentrations. The oligomerization of ASC also showed a similar trend. In conclusion, our findings proved that the high glutamate concentration could reverse the inhibition of A_2AR on NLRP3 inflammasome activation by modulating its assembly, which provides new insights into the regulatory effect of A_2AR on neuroinflammation under different pathological conditions.

ATP and Adenosine in the Retina and Retinal Diseases

Extracellular ATP and its ultimate degradation product adenosine are potent extracellular signaling molecules that elicit a variety of pathophysiological pathways in retina through the activation of P2 and P1 purinoceptors, respectively. Excessive build-up of extracellular ATP accelerates pathologic responses in retinal diseases, whereas accumulation of adenosine protects retinal cells against degeneration or inflammation. This mini-review focuses on the roles of ATP and adenosine in three types of blinding diseases including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR). Several agonists and antagonists of ATP receptors and adenosine receptors (ARs) have been developed for the potential treatment of glaucoma, DR and AMD: antagonists of P2X7 receptor (P2X7R) (BBG, MRS2540) prevent ATP-induced neuronal apoptosis in glaucoma, DR, and AMD; A1 receptor (A1R) agonists (INO-8875) lower intraocular pressure in glaucoma; A2A receptor (A2AR) agonists (CGS21680) or antagonists (SCH58261, ZM241385) reduce neuroinflammation in glaucoma, DR, and AMD; A3 receptor (A3R) agonists (2-Cl-lB-MECA, MRS3558) protect retinal ganglion cells (RGCs) from apoptosis in glaucoma.

Microglial Adenosine Receptors: From Preconditioning to Modulating the M1/M2 Balance in Activated Cells

Neuronal survival depends on the glia, that is, on the astroglial and microglial support. Neurons die and microglia are activated not only in neurodegenerative diseases but also in physiological aging. Activated microglia, once considered harmful, express two main phenotypes: the pro-inflammatory or M1, and the neuroprotective or M2. When neuroinflammation, i.e., microglial activation occurs, it is important to achieve a good M1/M2 balance, i.e., at some point M1 microglia must be skewed into M2 cells to impede chronic inflammation and to afford neuronal survival. G protein-coupled receptors in general and adenosine receptors in particular are potential targets for increasing the number of M2 cells. This article describes the mechanisms underlying microglial activation and analyzes whether these cells exposed to a first damaging event may be ready to be preconditioned to better react to exposure to more damaging events. Adenosine receptors are relevant due to their participation in preconditioning. They can also be overexpressed in activated microglial cells. The potential of adenosine receptors and complexes formed by adenosine receptors and cannabinoids as therapeutic targets to provide microglia-mediated neuroprotection is here discussed.

Role of Cardiac A2A Receptors Under Normal and Pathophysiological Conditions

This review presents an overview of cardiac A2A-adenosine receptors The localization of A2A-AR in the various cell types that encompass the heart and the role they play in force regulation in various mammalian species are depicted. The putative signal transduction systems of A2A-AR in cells in the living heart, as well as the known interactions of A2A-AR with membrane-bound receptors, will be addressed. The possible role that the receptors play in some relevant cardiac pathologies, such as persistent or transient ischemia, hypoxia, sepsis, hypertension, cardiac hypertrophy, and arrhythmias, will be reviewed. Moreover, the cardiac utility of A2A-AR as therapeutic targets for agonistic and antagonistic drugs will be discussed. Gaps in our knowledge about the cardiac function of A2A-AR and future research needs will be identified and formulated.

Role and Function of Adenosine and its Receptors in Inflammation, Neuroinflammation, IBS, Autoimmune Inflammatory Disorders, Rheumatoid Arthritis and Psoriasis

The physiological effects of endogenous adenosine on various organ systems are very complex and numerous which are elicited upon activation of any of the four G-protein-coupled receptors (GPCRs) denoted as A1, A2A, A2B and A3 adenosine receptors (ARs). Several fused heterocyclic and non-xanthine derivatives are reported as a possible target for these receptors due to physiological problems and lack of selectivity of xanthine derivatives. In the present review, we have discussed the development of various new chemical entities as a target for these receptors. In addition, compounds acting on adenosine receptors can be utilized in treating diseases like inflammation, neuroinflammation, autoimmune and related diseases.

Regulation of Microglial Functions by Purinergic Mechanisms in the Healthy and Diseased CNS

Microglial cells, the resident macrophages of the central nervous system (CNS), exist in a process-bearing, ramified/surveying phenotype under resting conditions. Upon activation by cell-damaging factors, they get transformed into an amoeboid phenotype releasing various cell products including pro-inflammatory cytokines, chemokines, proteases, reactive oxygen/nitrogen species, and the excytotoxic ATP and glutamate. In addition, they engulf pathogenic bacteria or cell debris and phagocytose them. However, already resting/surveying microglia have a number of important physiological functions in the CNS; for example, they shield small disruptions of the blood–brain barrier by their processes, dynamically interact with synaptic structures, and clear surplus synapses during development. In neurodegenerative illnesses, they aggravate the original disease by a microglia-based compulsory neuroinflammatory reaction. Therefore, the blockade of this reaction improves the outcome of Alzheimer’s Disease, Parkinson’s Disease, multiple sclerosis, amyotrophic lateral sclerosis, etc. The function of microglia is regulated by a whole array of purinergic receptors classified as P2Y12, P2Y6, P2Y4, P2X4, P2X7, A2A, and A3, as targets of endogenous ATP, ADP, or adenosine. ATP is sequentially degraded by the ecto-nucleotidases and 5′-nucleotidase enzymes to the almost inactive inosine as an end product. The appropriate selective agonists/antagonists for purinergic receptors as well as the respective enzyme inhibitors may profoundly interfere with microglial functions and reconstitute the homeostasis of the CNS disturbed by neuroinflammation.

Keep an eye on adenosine: Its role in retinal inflammation

Adenosine is an endogenous purine nucleoside ubiquitously distributed throughout the body that interacts with G protein-coupled receptors, classified in four subtypes: A₁R, A_2AR, A_2BR and A₃R. Among the plethora of functions of adenosine, it has been increasingly recognized as a key mediator of the immune response. Neuroinflammation is a feature of chronic neurodegenerative diseases and contributes to the pathophysiology of several retinal degenerative diseases. Animal models of retinal diseases are helping to elucidate the regulatory roles of adenosine receptors in the development and progression of those diseases. Mounting evidence demonstrates that the adenosinergic system is altered in the retina during pathological conditions, compromising retinal physiology. This review focuses on the roles played by adenosine and the elements of the adenosinergic system (receptors, enzymes, transporters) in the neuroinflammatory processes occurring in the retina. An improved understanding of the molecular and cellular mechanisms of the signalling pathways mediated by adenosine underlying the onset and progression of retinal diseases will pave the way towards the identification of new therapeutic approaches.

Intravitreal injection of adenosine A2A receptor antagonist reduces neuroinflammation, vascular leakage and cell death in the retina of diabetic mice

Diabetic retinopathy is a major complication of diabetes mellitus and a leading cause of blindness. The pathogenesis of diabetic retinopathy is accompanied by chronic low-grade inflammation. Evidence shows that the blockade of adenosine A_2A receptors (A_2AR) affords protection to the retina through the control of microglia-mediated neuroinflammation. Herein, we investigated the therapeutic potential of an antagonist of A_2AR in a model of diabetic retinopathy. Type 1 diabetes was induced in 4–5 months old C57BL/6 J mice with a single intraperitoneal injection streptozotocin. Animals were treated one month after the onset of diabetes. The A_2AR antagonist was delivered by intravitreal injection once a week for 4 weeks. Microglia reactivity and inflammatory mediators were increased in the retinas of diabetic animals. The treatment with the A_2AR antagonist was able to control microglial reactivity and halt neuroinflammation. Furthermore, the A_2AR antagonist rescued retinal vascular leakage, attenuated alterations in retinal thickness, decreased retinal cell death and the loss of retinal ganglion cells induced by diabetes. These results demonstrate that intravitreal injection of the A_2AR antagonist controls inflammation, affords protection against cell loss and reduces vascular leakage associated with diabetes, which could be envisaged as a therapeutic approach for the early complications of diabetes in the retina.

Purinergic Signaling and Cochlear Injury-Targeting the Immune System?

Hearing impairment is the most common sensory deficit, affecting more than 400 million people worldwide. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy largely due to the insufficient knowledge of the pathomechanism. Purinergic signaling plays a substantial role in cochlear (patho)physiology. P2 (ionotropic P2X and the metabotropic P2Y) as well as adenosine receptors expressed on cochlear sensory and non-sensory cells are involved mostly in protective mechanisms of the cochlea. They are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics. Cochlear blood flow is also regulated by purines. Here, we propose to comprehend this field with the purine-immune interactions in the cochlea. The role of harmful immune mechanisms in sensorineural hearing losses has been emerging in the horizon of cochlear pathologies. In addition to decreasing hearing sensitivity and increasing cochlear blood supply, influencing the immune system can be the additional avenue for pharmacological targeting of purinergic signaling in the cochlea. Elucidating this complexity of purinergic effects on cochlear functions is necessary and it can result in development of new therapeutic approaches in hearing disabilities, especially in the noise-induced ones.

Pro-inflammatory Effect of Downregulated CD73 Expression in EAE Astrocytes

CD73, an ectonucleotidase, participates in the regulation of immune responses by controlling the conversion of extracellular AMP to adenosine. In this study, we investigated whether any type of brain cells, especially neuroglia cells, exhibit altered CD73 expression, localization or activity upon experimental autoimmune uveitis (EAU) induction and whether altered CD73 manipulates the activation of effector T cells that interact with such cell types. First, the amount of cell membrane-exposed CD73 was detected by flow cytometry in various types of brain cells collected from either naïve or EAE mice. Compared to that in astrocytes from naïve control mice, the amount of membrane-bound CD73 was significantly decreased in astrocytes from EAE mice, while no significant differences were detected in other cell types. Thereafter, wild-type and CD73-/- astrocytes were used to study whether CD73 influences the function of inflammatory astrocytes, such as the production of cytokines/chemokines and the activation of effector T cells that interact with astrocytes. The results indicated that the addition of exogenous AMP significantly inhibited cytokine/chemokine production by wild type astrocytes but had no effect on CD73-/- astrocytes and that the effect of AMP was almost completely blocked by the addition of either a CD73 inhibitor (APCP) or an adenosine receptor A1 subtype (ARA1) antagonist (DPCPX). Although the addition of AMP did not affect CD73-/- astrocytes, the addition of adenosine successfully inhibited their cytokine/chemokine production. The antigen-specific interaction of astrocytes with invading CD4 cells caused CD73 downregulation in astrocytes from mice that underwent EAE induction. Collectively, our findings support the conclusion that, upon EAE induction, likely due to an interaction with invading CD4+ cells, astrocytes lose most of their membrane-localized CD73; this inhibits the generation of adenosine in the local microenvironment. As adenosine has anti-inflammatory effects on astrocytes and CNS-infiltrating effector T cells in EAE, the downregulation of CD73 in astrocytes may be considered a pro-inflammatory process for facilitating the pathogenesis of EAE.

Time-Dependent Bidirectional Neuroprotection by Adenosine 2A Receptor in Experimental Traumatic Brain Injury

BACKGROUND

Traumatic brain injury (TBI) results in both focal and diffuse brain pathological features that become severely exacerbated after the initial injury. Owing to this disease complexity, no effective therapeutic measure has yet been devised aimed directly at these pathological processes. We developed a clinically relevant model of TBI and tested the bidirectional neuroprotective role of adenosine 2A receptors (A2ARs) at different times.

METHODS

Wistar rats were divided into 4 treatment groups (sham, TBI, A2AR agonist [CGS-21680], and A2AR antagonist [SCH-58261]) and 4 post-TBI intervals (15 minutes and 1, 12, and 24 hours). A2AR agonist and antagonist effects were tested by the neurological functional score (NFS) and levels of cyclic adenosine monophosphate, interleukin-1β, oxidative stress antioxidant markers, and caspase-3.

RESULTS

The A2AR agonist-treated group showed significant NFS improvement at 15 minutes and 1 hour after TBI compared with the TBI group. However, no improvement was observed at 12 and 24 hours. The A2AR antagonists resulted in no NFS improvement at 15 minutes and 1 hour, and significant improvement observed at 12 and 24 hours. Significant neuroprotective effect with an A2AR agonist were observed with cyclic adenosine monophosphate, interleukin-1β, oxidative stress markers, catalase, and caspase-3 levels at 15 minutes and 1 hour after TBI. The A2AR antagonist showed no effect at these intervals but showed a protective effect at 12 and 24 hours after TBI.

CONCLUSIONS

The A2AR agonist showed a beneficial neuroprotective effect at the early stages after TBI, and the A2AR antagonist showed a benefit at the later stages after TBI. These findings suggest that A2AR agonists and antagonists should be used in accordance with the point at which the TBI occurred.

RNA-Sequencing Analysis Revealed a Distinct Motor Cortex Transcriptome in Spontaneously Recovered Mice After Stroke

Background and Purpose- Many restorative therapies have been used to study brain repair after stroke. These therapeutic-induced changes have revealed important insights on brain repair and recovery mechanisms; however, the intrinsic changes that occur in spontaneously recovery after stroke is less clear. The goal of this study is to elucidate the intrinsic changes in spontaneous recovery after stroke, by directly investigating the transcriptome of primary motor cortex in mice that naturally recovered after stroke. Methods- Male C57BL/6J mice were subjected to transient middle cerebral artery occlusion. Functional recovery was evaluated using the horizontal rotating beam test. A novel in-depth lesion mapping analysis was used to evaluate infarct size and locations. Ipsilesional and contralesional primary motor cortices (iM1 and cM1) were processed for RNA-sequencing transcriptome analysis. Results- Cluster analysis of the stroke mice behavior performance revealed 2 distinct recovery groups: a spontaneously recovered and a nonrecovered group. Both groups showed similar lesion profile, despite their differential recovery outcome. RNA-sequencing transcriptome analysis revealed distinct biological pathways in the spontaneously recovered stroke mice, in both iM1 and cM1. Correlation analysis revealed that 38 genes in the iM1 were significantly correlated with improved recovery, whereas 74 genes were correlated in the cM1. In particular, ingenuity pathway analysis highlighted the involvement of cAMP signaling in the cM1, with selective reduction of Adora2a (adenosine receptor A2A), Drd2 (dopamine receptor D2), and Pde10a (phosphodiesterase 10A) expression in recovered mice. Interestingly, the expressions of these genes in cM1 were negatively correlated with behavioral recovery. Conclusions- Our RNA-sequencing data revealed a panel of recovery-related genes in the motor cortex of spontaneously recovered stroke mice and highlighted the involvement of contralesional cortex in spontaneous recovery, particularly Adora2a, Drd2, and Pde10a-mediated cAMP signaling pathway. Developing drugs targeting these candidates after stroke may provide beneficial recovery outcome.

Inflammation in stroke: the role of cholinergic, purinergic and glutamatergic signaling

The inflammatory response is a major factor in stroke pathophysiology and contributes to secondary neuronal damage in both acute and chronic stages of the ischemic injury. Recent work in experimental cerebral ischemia has demonstrated the involvement of neurotransmitter signaling in the modulation of neuroinflammation. The present review discusses recent findings on the therapeutic potential and diagnostic perspectives of cholinergic, purinergic and glutamatergic receptors and transporters in experimental stroke. It provides evidence of the role of neurotransmission signaling as a promising inflammatory biomarker in stroke. Finally, recent molecular imaging studies using positron emission tomography of cholinergic receptors and glutamatergic transporters are outlined along with their potential as novel anti-inflammatory therapy to reduce the outcome of cerebral ischemia.

Amyotrophic Lateral Sclerosis (ALS) and Adenosine Receptors

In the present review we discuss the potential involvement of adenosinergic signaling, in particular the role of adenosine receptors, in amyotrophic lateral sclerosis (ALS). Though the literature on this topic is not abundant, the information so far available on adenosine receptors in animal models of ALS highlights the interest to continue to explore the role of these receptors in this neurodegenerative disease. Indeed, all motor neurons affected in ALS are responsive to adenosine receptor ligands but interestingly, there are alterations in pre-symptomatic or early symptomatic stages that mirror those in advanced disease stages. Information starts to emerge pointing toward a beneficial role of A_2A receptors (A_2AR), most probably at early disease states, and a detrimental role of caffeine, in clear contrast with what occurs in other neurodegenerative diseases. However, some evidence also exists on a beneficial action of A_2AR antagonists. It may happen that there are time windows where A_2AR prove beneficial and others where their blockade is required. Furthermore, the same changes may not occur simultaneously at the different synapses. In line with this, it is not fully understood if ALS is a dying back disease or if it propagates in a centrifugal way. It thus seems crucial to understand how motor neuron dysfunction occurs, how adenosine receptors are involved in those dysfunctions and whether the early changes in purinergic signaling are compensatory or triggers for the disease. Getting this information is crucial before starting the design of purinergic based strategies to halt or delay disease progression.

Adenosine A2A receptor involves in neuroinflammation-mediated cognitive decline through activating microglia under acute hypobaric hypoxia

Hypobaric hypoxia (HH) at high altitudes leads to a wide range of cognitive impairments which can handicap human normal activities and performances. However, the underlying mechanism is still unclear. Adenosine A_2A receptors (A_2ARs) of the brain are pivotal to synaptic plasticity and cognition. Besides, insult-induced up-regulation of A_2AR regulates neuroinflammation and therefore induces brain damages in various neuropathological processes. The present study was designed to determine whether A_2AR-mediate neuroinflammation involves in cognitive impairments under acute HH. A_2AR knock-out and wild-type male mice were exposed to a simulated altitude of 8000 m for 7 consecutive days in a hypobaric chamber and simultaneously received behavioral tests including Morris water maze test and open filed test. A_2AR expression, the activation of microglia and the production of TNF-α were evaluated in the hippocampus by immunohistochemistry and ELISA, respectively. Behavioral tests showed that acute HH exposure caused the dysfunction of spatial memory and mood, while genetic inactivation of A_2AR attenuated the impairment of spatial memory but not that of mood. Double-labeled immunofluorescence showed that A_2ARs were mainly expressed on microglia and up-regulated in the hippocampus of acute HH model mice. Acute HH also induced the accumulation of microglia and increased production of TNF-α in the hippocampus, which could be markedly inhibited by A_2AR inactivation. These findings indicate that microglia-mediated neuroinflammation triggered by A_2AR activation involves in acute HH-induced spatial memory impairment and that A_2AR could be a new target for the pharmacotherapy of cognitive dysfunction at high altitudes.

Sustained reversal of central neuropathic pain induced by a single intrathecal injection of adenosine A2A receptor agonists

Central neuropathic pain is a debilitating outcome of spinal cord injury (SCI) and current treatments to alleviate this pain condition are ineffective. A growing body of literature suggests that activating adenosine A_2A receptors (A_2ARs) decreases the production of proinflammatory cytokines and increases the production of anti-inflammatory cytokines. Here, the effect of administering intrathecal A_2AR agonists on central neuropathic pain was measured using hindpaw mechanical allodynia in a rat model of SCI termed spinal neuropathic avulsion pain (SNAP). Other models of SCI cause extensive damage to the spinal cord, resulting in paralysis and health problems. SNAP rats with unilateral low thoracic (T13)/high lumbar (L1) dorsal root avulsion develop below-level bilateral allodynia, without concomitant motor or health problems. A single intrathecal injection of the A_2AR agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine HCl (CGS21680) reversed SCI-induced allodynia for at least 6 weeks. The reversal is likely in part mediated by interleukin (IL)-10, as intrathecally administering neutralizing IL-10 antibodies 1 week after CGS21680 abolished the anti-allodynic effect of CGS21680. Dorsal spinal cord tissue from the ipsilateral site of SCI (T13/L1) was assayed 1 and 6 weeks after CGS21680 for IL-10, CD11b, and tumor necrosis factor (TNF) gene expression. CGS21680 treatment did not change IL-10 gene expression but did significantly decrease CD11b and TNF gene expression at both timepoints. A second A_2AR agonist, 4-(3-(6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9H-purin-2-yl)prop-2-ynyl)piperidine-1-carboxylic acid methyl ester (ATL313), was also able to significantly prevent and reverse SCI-induced allodynia for several weeks after a single intrathecal injection, providing converging lines of evidence of A_2AR involvement. The enduring pain reversal after a single intrathecal injection of A_2AR agonists suggests that A_2AR agonists could be exciting new candidates for treating SCI-induced central neuropathic pain.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Inflammatory aspects of epileptogenesis: contribution of molecular inflammatory mechanisms

OBJECTIVE

Epilepsy is a chronic neurological disease characterised with seizures. The aetiology of the most generalised epilepsies cannot be explicitly determined and the seizures are pronounced to be genetically determined by disturbances of receptors in central nervous system. Besides, neurotransmitter distributions or other metabolic problems are supposed to involve in epileptogenesis. Lack of adequate data about pharmacological agents that have antiepileptogenic effects point to need of research on this field. Thus, in this review, inflammatory aspects of epileptogenesis has been focussed via considering several concepts like role of immune system, blood-brain barrier and antibody involvement in epileptogenesis.

METHODS

We conducted an evidence-based review of the literatures in order to evaluate the possible participation of inflammatory processes to epileptogenesis and also, promising agents which are effective to these processes. We searched PubMed database up to November 2015 with no date restrictions.

RESULTS

In the present review, 163 appropriate articles were included. Obtained data suggests that inflammatory processes participate to epileptogenesis in several ways like affecting fibroblast growth factor-2 and tropomyosin receptor kinase B signalling pathways, detrimental proinflammatory pathways [such as the interleukin-1 beta (IL-1β)-interleukin-1 receptor type 1 (IL-1R1) system], mammalian target of rapamycin pathway, microglial activities, release of glial inflammatory proteins (such as macrophage inflammatory protein, interleukin 6, C-C motif ligand 2 and IL-1β), adhesion molecules that are suggested to function in signalling pathways between neurons and microglia and also linkage between these molecules and proinflammatory cytokines.

CONCLUSION

The literature research indicated that inflammation is a part of epileptogenesis. For this reason, further studies are necessary for assessing agents that will be effective in clinical use for therapeutic treatment of epileptogenesis.

Having a Coffee Break: The Impact of Caffeine Consumption on Microglia-Mediated Inflammation in Neurodegenerative Diseases

Caffeine is the major component of coffee and the most consumed psychostimulant in the world and at nontoxic doses acts as a nonselective adenosine receptor antagonist. Epidemiological evidence suggests that caffeine consumption reduces the risk of several neurological and neurodegenerative diseases. However, despite the beneficial effects of caffeine consumption in human health and behaviour, the mechanisms by which it impacts the pathophysiology of neurodegenerative diseases still remain to be clarified. A promising hypothesis is that caffeine controls microglia-mediated neuroinflammatory response associated with the majority of neurodegenerative conditions. Accordingly, it has been already described that the modulation of adenosine receptors, namely, the A2A receptor, affords neuroprotection through the control of microglia reactivity and neuroinflammation. In this review, we will summarize the main effects of caffeine in the modulation of neuroinflammation in neurodegenerative diseases.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Pathophysiological links between traumatic brain injury and post-traumatic headaches

This article reviews possible ways that traumatic brain injury (TBI) can induce migraine-type post-traumatic headaches (PTHs) in children, adults, civilians, and military personnel. Several cerebral alterations resulting from TBI can foster the development of PTH, including neuroinflammation that can activate neural systems associated with migraine. TBI can also compromise the intrinsic pain modulation system and this would increase the level of perceived pain associated with PTH. Depression and anxiety disorders, especially post-traumatic stress disorder (PTSD), are associated with TBI and these psychological conditions can directly intensify PTH. Additionally, depression and PTSD alter sleep and this will increase headache severity and foster the genesis of PTH. This article also reviews the anatomic loci of injury associated with TBI and notes the overlap between areas of injury associated with TBI and PTSD.

The role of adenosine and adenosine receptors in the immunopathogenesis of multiple sclerosis

INTRODUCTION

Multiple sclerosis (MS) is a heterogeneous neurological disorder with multifactorial etiologies characterized by demyelination, axonal degeneration, and oligodendroglial death. It is believed that both genetics and environmental risk factors such as infection are involved in disease etiology. Accumulating evidence indicates that alteration in purinergic system signaling is involved in immunity and inflammation. Adenosine, a key purine nucleoside, has been shown to be produced during metabolic stress, including ischemia, inflammatory condition, and tissue injury.

METHODS

Extracellular adenosine directly affects various physiological and pathological processes of MS by stimulating G protein-coupled adenosine receptors A1, A2A, A2B, and A3 on the surface of immune cells. It has been suggested that promotion of adenosinergic system may be an important factor in MS pathophysiology and considered as promising therapeutic target for this disease.

CONCLUSION

In this review, we will discuss about the immunopathologic effects of adenosine on MS and its animal model, experimental autoimmune encephalomyelitis.

Selective A2A receptor antagonist prevents microglia-mediated neuroinflammation and protects retinal ganglion cells from high intraocular pressure-induced transient ischemic injury

Glaucoma is a leading cause of vision loss and blindness worldwide, characterized by chronic and progressive neuronal loss. Reactive microglial cells have been recognized as a neuropathologic feature, contributing to local inflammation and retinal neurodegeneration. In a recent in vitro work (organotypic cultures), we demonstrated that blockade of adenosine A2A receptor (A2AR) prevents the neuroinflammatory response and affords protection to retinal ganglion cells (RGCs) against exposure to elevated hydrostatic pressure (EHP), to mimic elevated intraocular pressure (IOP), the main risk factor for glaucoma development. Herein, we investigated whether a selective A2AR antagonist (SCH 58261) could modulate retinal microglia reactivity and their inflammatory response. Furthermore, we took advantage of the high IOP-induced transient ischemia (ischemia-reperfusion, I-R) animal model to evaluate the protective role of A2AR blockade in the control of retinal neuroinflammation and neurodegeneration. Primary microglial cell cultures were challenged either with lipopolysaccharide or with EHP, in the presence or absence of A2AR antagonist SCH 58261 (50 nM). In addition, I-R injury was induced in adult Wistar rats after intravitreal administration of SCH 58261 (100 nM, 5 μL). Our results showed that SCH 58261 attenuated microglia reactivity and the increased expression and release of proinflammatory cytokines. Moreover, intravitreal administration of SCH 58261 prevented I-R-induced cell death and RGC loss, by controlling microglial-mediated neuroinflammatory response. These results prompt the proposal that A2AR blockade may have great potential in the management of retinal neurodegenerative diseases characterized by microglia reactivity and RGC death, such as glaucoma and ischemic diseases.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Phosphoinositide 3-Kinase γ Restrains Neurotoxic Effects of Microglia After Focal Brain Ischemia

Phosphoinositide 3-kinase γ (PI3Kγ) is linked to neuroinflammation and phagocytosis. This study was conducted to elucidate conjectural differences of lipid kinase-dependent and kinase-independent functions of PI3Kγ in the evolvement of brain damage induced by focal cerebral ischemia/reperfusion. Therefore, PI3Kγ wild-type, knockout, and kinase-dead mice were subjected to middle cerebral artery occlusion followed by reperfusion. Tissue damage and cellular composition were assessed by immunohistochemical stainings. In addition, microglial cells derived from respective mouse genotypes were used for analysis of PI3Kγ effects on phagocytic activity, matrix metalloproteinase-9 release, and cAMP content under conditions of oxygen/glucose deprivation and recovery. Brain infarction was more pronounced in PI3Kγ-knockout mice compared to wild-type and kinase-dead mice 48 h after reperfusion. Immunohistochemical analyses revealed a reduced amount of galectin-3/MAC-2-positive microglial cells indicating that activated phagocytosis was reduced in ischemic brains of knockout mice. Cell culture studies disclosed enhanced metalloproteinase-9 secretion in supernatants derived from microglia of PI3Kγ-deficient mice after 2-h oxygen/glucose deprivation and 48-h recovery. Furthermore, PI3Kγ-deficient microglial cells showed a failed phagocytic activation throughout the observed recovery period. Lastly, PI3Kγ-deficient microglia exhibited strongly increased cAMP levels in comparison with wild-type microglia or cells expressing kinase-dead PI3Kγ after oxygen/glucose deprivation and recovery. Our data suggest PI3Kγ kinase activity-independent control of cAMP phosphodiesterase as a crucial mediator of microglial cAMP regulation, MMP-9 expression, and phagocytic activity following focal brain ischemia/recirculation. The suppressive effect of PI3Kγ on cAMP levels appears critical for the restriction of ischemia-induced immune cell functions and in turn tissue damage.

Adenosine 2A receptor agonism: A single intrathecal administration attenuates motor paralysis in experimental autoimmune encephalopathy in rats

A single intrathecal dose of adenosine 2A receptor (A2AR) agonist was previously reported to produce a multi-week reversal of allodynia in two different models of neuropathic pain in addition to downregulating glial activation markers in the spinal cord. We aimed to determine whether a single intrathecal administration of an A2AR agonist was able to attenuate motor symptoms induced by experimental autoimmune encephalopathy. Two A2AR agonists (CGS21680 and ATL313) significantly attenuated progression of motor symptoms following a single intrathecal administration at the onset of motor symptoms. OX-42, a marker of microglial activation, was significantly attenuated in the lumbar spinal cord following A2AR administration compared to vehicle. Therefore, A2AR agonists attenuate motor symptoms of EAE by acting on A2AR in the spinal cord.

Time-dependent bidirectional effects of chronic caffeine on functional recovery of the dorsal light reflex after hemilabyrinthectomy in the goldfish Carassius auratus

Caffeine works through a variety of complex mechanisms to exert an often bidirectional set of functional and structural neurological changes in vertebrates. We investigated the effects of chronic caffeine exposure on functional recovery of the dorsal light reflex (DLR) in hemilabyrinthectomized common goldfish, Carassius auratus. In this lesion model, the unilateral removal of the vestibular organs results in a temporary loss of gravitationally modulated postural control which is quantifiable via the DLR. We compared the functional recovery over 24 days of post-surgery goldfish continuously held in a caffeine solution of 2.5mg/L (n=10), 5.0mg/L (n=10), 10.0mg/L (n=11), or 0.0mg/L control (n=9). Comparison to a sham surgery group (n=11) indicated statistically significant changes in the DLR of all hemilabyrinthectomized fish on day 1. The control group recovered over the study period and approached, but did not reach sham surgery DLR. Although the caffeine-treated fishes appeared to initiate some postural recovery within the first 2 weeks, beginning on day 10, all caffeine groups diverged from the control group with a deterioration of postural control. All three caffeine groups were significantly deficient in comparison with the control on days 10-24. These results suggest that caffeine exposure can at first be benign, but that high dosage or prolonged exposure hinders functional recovery.

2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine inhibit TNF-α and CXCL10 production from activated primary murine microglia via A2A receptors

BACKGROUND

Some cells, tissues and organs release 2',3'-cAMP (a positional isomer of 3',5'-cAMP) and convert extracellular 2',3'-cAMP to 2'-AMP plus 3'-AMP and convert these AMPs to adenosine (called the extracellular 2',3'-cAMP-adenosine pathway). Recent studies show that microglia have an extracellular 2',3'-cAMP-adenosine pathway. The goal of the present study was to investigate whether the extracellular 2',3'-cAMP-adenosine pathway could have functional consequences on the production of cytokines/chemokines by activated microglia.

METHODS

Experiments were conducted in cultures of primary murine microglia. In the first experiment, the effect of 2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine on LPS-induced TNF-α and CXCL10 production was determined. In the next experiment, the first protocol was replicated but with the addition of 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX) (0.1 μM; antagonist of adenosine receptors). The last experiment compared the ability of 2-chloro-N(6)-cyclopentyladenosine (CCPA) (10 μM; selective A1 agonist), 5'-N-ethylcarboxamide adenosine (NECA) (10 μM; agonist for all adenosine receptor subtypes) and CGS21680 (10 μM; selective A2A agonist) to inhibit LPS-induced TNF-α and CXCL10 production.

RESULTS

(1) 2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine similarly inhibited LPS-induced TNF-α and CXCL10 production; (2) DPSPX nearly eliminated the inhibitory effects of 2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine on LPS-induced TNF-α and CXCL10 production; (3) CCPA did not affect LPS-induced TNF-α and CXCL10; (4) NECA and CGS21680 similarly inhibited LPS-induced TNF-α and CXCL10 production.

CONCLUSIONS

2',3'-cAMP and its metabolites (3'-AMP, 2'-AMP and adenosine) inhibit LPS-induced TNF-α and CXCL10 production via A2A-receptor activation. Adenosine and its precursors, via A2A receptors, likely suppress TNF-α and CXCL10 production by activated microglia in brain diseases.

Neuroprotective potential of adenosine A2A and cannabinoid CB1 receptor antagonists in an animal model of Parkinson disease

The development of nondopaminergic therapeutic strategies that may improve motor and nonmotor deficits, while possibly slowing down the neurodegenerative process and associated neuroinflammation,is a primary goal of Parkinson disease (PD) research. We investigated the neuroprotective and anti-inflammatory potential of combined and single treatment with adenosine A2A and cannabinoid CB1 receptor antagonists MSX-3 and rimonabant, respectively, in a rodent model of PD. Rats bearing a unilateral intrastriatal 6-hydroxydopamine lesion were treated chronically with MSX-3 (0.5or 1 mg/kg/d) and rimonabant (0.1 mg/kg/d) given as monotherapy or combined. The effects of the treatments to counteract dopaminergic cell death and neuroinflammation were assessed by immunohistochemistry for tyrosine hydroxylase and glial cell markers, respectively. Both rimonabant and MSX-3 (1 mg/kg/d) promoted dopaminergic neuron survival in the substantia nigra pars compacta (SNc) when given alone; this effect was weakened when the compounds were combined. Glial activation was not significantly affected by MSX-3 (1 mg/kg/d), whereas rimonabant seemed to increase astrocyte cell density in the SNc. Our findings demonstrate the neuroprotective potential of single treatments and suggest that glial cells might be involved in this protective effect. The results also indicate that the neuroprotective potential of combined therapy may not necessarily reflect or promote single-drug effects and point out that special care should be taken when considering multidrug therapies in PD.

Acetate supplementation modulates brain adenosine metabolizing enzymes and adenosine A₂A receptor levels in rats subjected to neuroinflammation

Background

Acetate supplementation reduces neuroglia activation and pro-inflammatory cytokine expression in rat models of neuroinflammation and Lyme neuroborreliosis. Because single-dose glyceryl triacetate (GTA) treatment increases brain phosphocreatine and reduces brain AMP levels, we postulate that GTA modulates adenosine metabolizing enzymes and receptors, which may be a possible mechanism to reduce neuroinflammation.

Methods

To test this hypothesis, we quantified the ability of GTA to alter brain levels of ecto-5’-nucleotidase (CD73), adenosine kinase (AK), and adenosine A_2A receptor using western blot analysis and CD73 activity by measuring the rate of AMP hydrolysis. Neuroinflammation was induced by continuous bacterial lipopolysaccharide (LPS) infusion in the fourth ventricle of the brain for 14 and 28 days. Three treatment strategies were employed, one and two where rats received prophylactic GTA through oral gavage with LPS infusion for 14 or 28 days. In the third treatment regimen, an interventional strategy was used where rats were subjected to 28 days of neuroinflammation, and GTA treatment was started on day 14 following the start of the LPS infusion.

Results

We found that rats subjected to neuroinflammation for 28 days had a 28% reduction in CD73 levels and a 43% increase in AK levels that was reversed with prophylactic acetate supplementation. CD73 activity in these rats was increased by 46% with the 28-day GTA treatment compared to the water-treated rats. Rats subjected to neuroinflammation for 14 days showed a 50% increase in levels of the adenosine A_2A receptor, which was prevented with prophylactic acetate supplementation. Interventional GTA therapy, beginning on day 14 following the induction of neuroinflammation, resulted in a 67% increase in CD73 levels and a 155% increase in adenosine A_2A receptor levels.

Conclusion

These results support the hypothesis that acetate supplementation can modulate brain CD73, AK and adenosine A_2A receptor levels, and possibly influence purinergic signaling.

Using caffeine and other adenosine receptor antagonists and agonists as therapeutic tools against neurodegenerative diseases: a review

Caffeine is the most consumed pychostimulant in the world, and it is known to affect basic and fundamental human processes such as sleep, arousal, cognition and learning and memory. It works as a nonselective blocker of adenosine receptors (A1, A2a, A2b and A3) and has been related to the regulation of heart rate, the contraction/relaxation of cardiac and smooth muscles, and the neural signaling in the central nervous system (CNS). Since the late 1990s, studies using adenosine receptor antagonists, such as Caffeine, to block the A1 and A2a adenosine receptor subtypes have shown to reduce the physical, cellular and molecular damages caused by a spinal cord injury (SCI) or a stroke (cerebral infarction) and by other neurodegenerative diseases such as Parkinson's and Alzheimer's diseases. Interestingly, other studies using adenosine receptor agonists have also shown to provide a neuroprotective effect on various models of neurodegenerative diseases through the reduction of excitatory neurotransmitter release, apoptosis and inflammatory responses, among others. The seemingly paradoxical use of both adenosine receptor agonists and antagonists as neuroprotective agents has been attributed to differences in dosage levels, drug delivery method, extracellular concentration of excitatory neurotransmitters and stage of disease progression. We discuss and compare recent findings using both antagonists and agonists of adenosine receptors in animal models and patients that have suffered spinal cord injuries, brain strokes, and Parkinson's and Alzheimer's diseases. Additionally, we propose alternative interpretations on the seemingly paradoxical use of these drugs as potential pharmacological tools to treat these various types of neurodegenerative diseases.

Intrathecal injection of adenosine 2A receptor agonists reversed neuropathic allodynia through protein kinase (PK)A/PKC signaling

A single intrathecal dose of adenosine 2A receptor (A2AR) agonist was previously reported to produce a multi-week reversal of allodynia in a chronic constriction injury (CCI) model of neuropathic pain. We aimed to determine if this long-term reversal was induced by A2AR agonism versus more generalized across adenosine receptor subtypes, and begin to explore the intracellular signaling cascades involved. In addition, we sought to identify whether the enduring effect could be extended to other models of neuropathic pain. We tested an A1R and A2BR agonist in CCI and found the same long duration effect with A2BR but not A1R agonism. An A2AR agonist (ATL313) produced a significant long-duration reversal of mechanical allodynia induced by long established CCI (administered 6 weeks after surgery), spinal nerve ligation and sciatic inflammatory neuropathy. To determine if ATL313 had a direct effect on glia, ATL313 was coadministered with lipopolysaccharide to neonatal microglia and astrocytes in vitro. ATL313 significantly attenuated TNFα production in both microglia and astrocytes but had no effect on LPS induced IL-10. Protein kinase C significantly reversed the ATL313 effects on TNFα in vitro in microglia and astrocytes, while a protein kinase A inhibitor only effected microglia. Both intrathecal PKA and PKC inhibitors significantly reversed the effect of the A2AR agonist on neuropathic allodynia. Therefore, A2AR agonists administered IT remain an exciting novel target for the treatment of neuropathic pain.

Both MC5r and A2Ar are required for protective regulatory immunity in the spleen of post-experimental autoimmune uveitis in mice

The ocular microenvironment uses a poorly defined mela5 receptor (MC5r)-dependent pathway to recover immune tolerance following intraocular inflammation. This dependency is seen in experimental autoimmune uveoretinitis (EAU), a mouse model of endogenous human autoimmune uveitis, with the emergence of autoantigen-specific regulatory immunity in the spleen that protects the mice from recurrence of EAU. In this study, we found that the MC5r-dependent regulatory immunity increased CD11b(+)F4/80(+)Ly-6C(low)Ly-6G(+)CD39(+)CD73(+) APCs in the spleen of post-EAU mice. These MC5r-dependent APCs require adenosine 2A receptor expression on T cells to activate EAU-suppressing CD25(+)CD4(+)Foxp3(+) regulatory T cells. Therefore, in the recovery from autoimmune disease, the ocular microenvironment induces tolerance through a melanocortin-mediated expansion of Ly-6G(+) regulatory APCs in the spleen that use the adenosinergic pathway to promote activation of autoantigen-specific regulatory T cells.

Role of CNPase in the oligodendrocytic extracellular 2',3'-cAMP-adenosine pathway

Extracellular adenosine 3',5'-cyclic monophosphate (3',5'-cAMP) is an endogenous source of localized adenosine production in many organs. Recent studies suggest that extracellular 2',3'-cAMP (positional isomer of 3',5'-cAMP) is also a source of adenosine, particularly in the brain in vivo post-injury. Moreover, in vitro studies show that both microglia and astrocytes can convert extracellular 2',3'-cAMP to adenosine. Here, we examined the ability of primary mouse oligodendrocytes and neurons to metabolize extracellular 2',3'-cAMP and their respective adenosine monophosphates (2'-AMP and 3'-AMP). Cells were also isolated from mice deficient in 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase). Oligodendrocytes metabolized 2',3'-cAMP to 2'-AMP with 10-fold greater efficiency than did neurons (and also more than previously examined microglia and astrocytes); whereas, the production of 3'-AMP was minimal in both oligodendrocytes and neurons. The production of 2'-AMP from 2',3'-cAMP was reduced by 65% in CNPase -/- versus CNPase +/+ oligodendrocytes. Oligodendrocytes also converted 2'-AMP to adenosine, and this was also attenuated in CNPase -/- oligodendrocytes. Inhibition of classic 3',5'-cAMP-3'-phosphodiesterases with 3-isobutyl-1-methylxanthine did not block metabolism of 2',3'-cAMP to 2'-AMP and inhibition of classic ecto-5'-nucleotidase (CD73) with α,β-methylene-adenosine-5'-diphosphate did not attenuate the conversion of 2'-AMP to adenosine. These studies demonstrate that oligodendrocytes express the extracellular 2',3'-cAMP-adenosine pathway (2',3'-cAMP → 2'-AMP → adenosine). This pathway is more robustly expressed in oligodendrocytes than in all other CNS cell types because CNPase is the predominant enzyme that metabolizes 2',3'-cAMP to 2-AMP in CNS cells. By reducing levels of 2',3'-cAMP (a mitochondrial toxin) and increasing levels of adenosine (a neuroprotectant), oligodendrocytes may protect axons from injury.