Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosinergic system and nucleoside transporters in attention deficit hyperactivity disorder: Current findings

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder with high heterogeneity that can affect individuals of any age. It is characterized by three main symptoms: inattention, hyperactivity, and impulsivity. These neurobehavioral alterations and neurochemical and pharmacological findings are mainly attributed to unbalanced catecholaminergic signaling, especially involving dopaminergic pathways within prefrontal and striatal areas. Dopamine receptors and transporters are not solely implicated in this imbalance, as evidence indicates that the dopaminergic signaling is modulated by adenosine activity. To this extent, alterations in adenosinergic signaling are probably involved in ADHD. Here, we review the current knowledge about adenosine's role in the modulation of chemical, behavioral and cognitive parameters of ADHD, especially regarding dopaminergic signaling. Current literature usually links adenosine receptors signaling to the dopaminergic imbalance found in ADHD, but there is evidence that equilibrative nucleoside transporters (ENTs) could also be implicated as players in dopaminergic signaling alterations seen in ADHD, since their involvement in other neurobehavioral impairments.

Muscarinic Modulation of Synaptic Transmission and Short-Term Plasticity in the Dorsal and Ventral Hippocampus

Muscarinic neurotransmission is fundamentally involved in supporting several brain functions by modulating flow of information in brain neural circuits including the hippocampus which displays a remarkable functional segregation along its longitudinal axis. However, how muscarinic neuromodulation contributes to the functional segregation along the hippocampus remains unclear. In this study we show that the nonselective muscarinic receptor agonist carbachol similarly suppresses basal synaptic transmission in the dorsal and ventral CA1 hippocampal field, in a concentration-depended manner. Furthermore, using a ten-pulse stimulation train of varying frequency we found that carbachol changes the frequency filtering properties more in ventral than dorsal hippocampus by facilitating synaptic inputs at a wide range of input frequencies in the ventral compared with dorsal hippocampus. Using the M2 receptor antagonist gallamine and the M4 receptor antagonist tropicamide, we found that M2 receptors are involved in controlling basal synaptic transmission and short-term synaptic plasticity (STSP) in the ventral but not the dorsal hippocampus, while M4 receptors participate in modulating basal synaptic transmission and STSP in both segments of the hippocampus. These results were corroborated by the higher protein expression levels of M2 receptors in the ventral compared with dorsal hippocampus. We conclude that muscarinic transmission modulates excitatory synaptic transmission and short-term synaptic plasticity along the entire rat hippocampus by acting through M4 receptors and recruiting M2 receptors only in the ventral hippocampus. Furthermore, M4 receptors appear to exert a permissive role on the actions of M2 receptors on STSP in the ventral hippocampus. This dorsoventral differentiation of muscarinic modulation is expected to have important implications in information processing along the endogenous hippocampal circuitry.

A review of the direct targets of the cannabinoids cannabidiol, Δ9-tetrahydrocannabinol, N-arachidonoylethanolamine and 2-arachidonoylglycerol

Marijuana has been used by humans for thousands of years for both medicinal and recreational purposes. This included the treatment of pain, inflammation, seizures, and nausea. In the 1960s, the structure of the principal psychoactive ingredient Δ9-tetrahydrocannabinol was determined, and over the next few decades, two cannabinoid receptors were characterized along with the human endocannabinoid system and what it affects. This includes metabolism, the cardiovascular and reproductive systems, and it is involved in such conditions as inflammation, cancer, glaucoma, and liver and musculoskeletal disorders. In the central nervous system, the endocannabinoid system has been linked to appetite, learning, memory, and conditions such as depression, anxiety, schizophrenia, stroke, multiple sclerosis, neurodegeneration, addiction, and epilepsy. It was the profound effectiveness of cannabidiol, a non-psychoactive ingredient of marijuana, to relieve the symptoms of Dravet syndrome, a severe form of childhood epilepsy, that recently helped spur marijuana research. This has helped substantially to change society's attitude towards this potential source of useful drugs. However, research has also revealed that the actions of endocannabinoids, such as anandamide and 2-arachidonoylglycerol, and the phytocannabinoids, tetrahydrocannabinol and cannabidiol, were not just due to interactions with the two cannabinoid receptors but by acting directly on many other targets including various G-protein receptors and cation channels, such as the transient receptor potential channels for example. This mini-review attempts to survey the effects of these 4 important cannabinoids on these currently identified targets.

Epigenetics and suicide: investigating altered H3K14ac unveiled differential expression in ADORA2A, B4GALT2 and MMP14

Background: Environmental factors make an important contribution to suicide. Histone tails are prone to different modifications, leading to changes of chromatin (de)condensation and consequently gene expression. Materials & methods: Level of H3K14ac was studied with chromatin immunoprecipitation followed by high-throughput DNA sequencing. Genes were further validated with RT-qPCR; using hippocampal tissue. Results: We showed lowered H3K14ac levels in individuals who died by suicide. The genes ADORA2A, B4GALT2 and MMP14 showed differential expression in individuals who died by suicide. Identified genetic and protein interactions among genes show interactions with suicide-related genes. Conclusion: Further investigations of histone modifications in association with DNA methylation and miRNA are needed to expand our knowledge of the genes that could significantly contribute to suicide.

Extracellular Purine Metabolism-Potential Target in Multiple Sclerosis

The purinergic signaling system comprises a complex network of extracellular purines and purine-metabolizing ectoenzymes, nucleotide and nucleoside receptors, ATP release channels, and nucleoside transporters. Because of its immunomodulatory function, this system is critically involved in the pathogenesis of multiple sclerosis (MS) and its best-characterized animal model, experimental autoimmune encephalomyelitis (EAE). MS is a chronic neuroinflammatory demyelinating and neurodegenerative disease with autoimmune etiology and great heterogeneity, mostly affecting young adults and leading to permanent disability. In MS/EAE, alterations were detected in almost all components of the purinergic signaling system in both peripheral immune cells and central nervous system (CNS) glial cells, which play an important role in the pathogenesis of the disease. A decrease in extracellular ATP levels and an increase in its downstream metabolites, particularly adenosine and inosine, were frequently observed at MS, indicating a shift in metabolism toward an anti-inflammatory environment. Accordingly, upregulation of the major ectonucleotidase tandem CD39/CD73 was detected in the blood cells and CNS of relapsing-remitting MS patients. Based on the postulated role of A2A receptors in the transition from acute to chronic neuroinflammation, the association of variants of the adenosine deaminase gene with the severity of MS, and the beneficial effects of inosine treatment in EAE, the adenosinergic system emerged as a promising target in neuroinflammation. More recently, several publications have identified ADP-dependent P2Y12 receptors and the major extracellular ADP producing enzyme nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) as novel potential targets in MS.

JM-20 potently prevents the onset of caffeine-induced anxiogenic phenotypes in zebrafish (Danio rerio)

Anxiety is among the most prevalent mental disorders present in the general population. Benzodiazepines are the most commonly prescribed drugs for the treatment of anxiety. Using zebrafish as a model organism, we investigated the anxiolytic activity of JM-20, a novel hybrid molecule with a 1,5-benzodiazepine ring fused to a dihydropyridine moiety. Firstly, we carried out some assays to analyze the possible toxicity mediated by JM-20. For this, zebrafish were exposed to different JM-20 concentrations (0-5 μM) for 96 h. Then, using the novel tank test, we evaluated both locomotor and anxiety-like behavior of the animals. Furthermore, brain, liver and plasma were removed to assess toxicity parameters. JM-20 exposure did not cause changes on novel tank, and also did not alter brain viability, hepatic LDH and plasma ALT levels. Afterward, we investigated whether a pre-exposure to JM-20 would prevent the anxiogenic effect evoked by caffeine. In the novel tank test, caffeine significantly decreased the time spent at the top, as well as the number of transitions to the top area. Moreover, caffeine decreased both the total and average time spent in the lit area, as well as increased the number of risk episodes evaluated by the light-dark test. Whole-body cortisol levels were also increased by caffeine exposure. Interestingly, pre-treatment with JM-20 abolished all alterations induced by caffeine. The anxiolytic effect profile of JM-20 was similar to those found for diazepam (positive control). Our findings show, for the first time, the anxiolytic effect of JM-20 in zebrafish, and its relationship with cortisol regulation.

Sound-evoked adenosine release in cooperation with neuromodulatory circuits permits auditory cortical plasticity and perceptual learning

Meaningful auditory memories are formed in adults when acoustic information is delivered to the auditory cortex during heightened states of attention, vigilance, or alertness, as mediated by neuromodulatory circuits. Here, we identify that, in awake mice, acoustic stimulation triggers auditory thalamocortical projections to release adenosine, which prevents cortical plasticity (i.e., selective expansion of neural representation of behaviorally relevant acoustic stimuli) and perceptual learning (i.e., experience-dependent improvement in frequency discrimination ability). This sound-evoked adenosine release (SEAR) becomes reduced within seconds when acoustic stimuli are tightly paired with the activation of neuromodulatory (cholinergic or dopaminergic) circuits or periods of attentive wakefulness. If thalamic adenosine production is enhanced, then SEAR elevates further, the neuromodulatory circuits are unable to sufficiently reduce SEAR, and associative cortical plasticity and perceptual learning are blocked. This suggests that transient low-adenosine periods triggered by neuromodulatory circuits permit associative cortical plasticity and auditory perceptual learning in adults to occur.

Transient Adenosine Modulates Serotonin Release Indirectly in the Dorsal Raphe Nuclei

Rapid adenosine transiently regulates dopamine and glutamate via A1 receptors, but other neurotransmitters, such as serotonin, have not been studied. In this study, we examined the rapid modulatory effect of adenosine on serotonin release in the dorsal raphe nuclei (DRN) of mouse brain slices by using fast-scan cyclic voltammetry. To mimic adenosine release during damage, a rapid microinjection of adenosine at 50 pmol was applied before electrical stimulation of serotonin release. Transient adenosine significantly reduced electrically evoked serotonin release in the first 20 s after application, but serotonin release recovered to baseline as adenosine was cleared from the slice. The continuous perfusion of adenosine did not change the evoked serotonin release. Surprisingly, the modulatory effects of adenosine were not regulated by A1 receptors as adenosine still inhibited serotonin release in A1KO mice and also after perfusion of an A1 antagonist (8-cyclopentyl-1,3-dipropyl xanthine). The inhibition was also not regulated by A3 receptors as perfusion of the A3 antagonist (MRS 1220) in A1KO brain slices did not eliminate the inhibitory effects of transient adenosine. In addition, adenosine also inhibited serotonin release in A2AKO mice, showing that A2A did not modulate serotonin. However, perfusion of a selective 5HT1A autoreceptor antagonist drug [(S)-WAY 100135 dihydrochloride] abolished the inhibitory effect of transient adenosine on serotonin release. Thus, the transient neuromodulatory effect of adenosine on DRN serotonin release is regulated by serotonin autoreceptors and not by adenosine receptors. Rapid, transient adenosine modulation of neurotransmitters such as serotonin may have important implications for diseases such as depression and brain injury.

Essential Role of Astrocytes in Learning and Memory

One of the most biologically relevant functions of astrocytes within the CNS is the regulation of synaptic transmission, i.e., the physiological basis for information transmission between neurons. Changes in the strength of synaptic connections are indeed thought to be the cellular basis of learning and memory. Importantly, astrocytes have been demonstrated to tightly regulate these processes via the release of several gliotransmitters linked to astrocytic calcium activity as well as astrocyte-neuron metabolic coupling. Therefore, astrocytes seem to be integrators of and actors upon learning- and memory-relevant information. In this review, we focus on the role of astrocytes in learning and memory processes. We delineate the recognized inputs and outputs of astrocytes and explore the influence of manipulating astrocytes on behaviour across diverse learning paradigms. We conclude that astrocytes influence learning and memory in various manners. Appropriate astrocytic Ca2+ dynamics are being increasingly identified as central contributors to memory formation and retrieval. In addition, astrocytes regulate brain rhythms essential for cognition, and astrocyte-neuron metabolic cooperation is required for memory consolidation.

Enhancement of the Evoked Excitatory Transmission in the Nucleus Tractus Solitarius Neurons after Sustained Hypoxia in Mice Depends on A2A Receptors

The first synapses of the afferents of peripheral chemoreceptors are located in the Nucleus Tractus Solitarius (NTS) and there is evidence that short-term sustained hypoxia (SH - 24 h, FiO2 0.1) facilitates glutamatergic transmission in NTS neurons of rats. Adenosine is an important neuromodulator of synaptic transmission and hypoxia contributes to increase its extracellular concentration. The A2A receptors mediate the excitatory actions of adenosine and are active players in the modulation of neuronal networks in the NTS. Herein, we used knockout mice for A2A receptors (A2AKO) and electrophysiological recordings of NTS neurons were performed to evaluate the contribution of these receptors in the changes in synaptic transmission in NTS neurons of mice submitted to SH. The membrane passive properties and excitability of NTS neurons were not affected by SH and were similar between A2AKO and wild-type mice. The overall amplitude of spontaneous glutamatergic currents in NTS neurons of A2AKO mice was lower than in Balb/c WT mice. SH increased the amplitude of evoked glutamatergic currents of NTS neurons from WT mice by a non-presynaptic mechanism, but this enhancement was not observed in NTS neurons of A2AKO mice. Under normoxia, the amplitude of evoked glutamatergic currents was similar between WT and A2AKO mice. The data indicate that A2A receptors (a) modulate spontaneous glutamatergic currents, (b) do not modulate the evoked glutamatergic transmission in the NTS neurons under control conditions, and (c) are required for the enhancement of glutamatergic transmission observed in the NTS neurons of mice submitted to SH.

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Cardiovascular and respiratory evaluation in adenosine A2A receptor knockout mice submitted to short-term sustained hypoxia

Sustained hypoxia (SH) in mice induces changes in the respiratory pattern and increase in the parasympathetic tone to the heart. Among adenosine G-protein-coupled receptors (GPCRs), the A2A receptors are especially important in mediating adenosine actions during hypoxia due to their expression in neurons involved with the generation and modulation of the autonomic and respiratory functions. Herein, we performed an in vivo evaluation of the baseline cardiovascular and respiratory parameters and their changes in response to SH in knockout mice for A2A receptors (A2A KO). SH produced similar and significant reductions in mean arterial pressure and heart rate in both wild-type (WT) and A2A KO mice when compared to their respective normoxic controls. Mice from WT and A2A KO groups submitted to normoxia or SH presented similar cardiovascular responses to peripheral chemoreflex activation (KCN). Under normoxic conditions A2A KO mice presented a respiratory frequency (fR ) significantly higher in relation to the WT group, which was reduced in response to SH. These data show that the lack of adenosine A2A receptors in mice does not affect the cardiovascular parameters and the autonomic responses to chemoreflex activation in control (normoxia) and SH mice. We conclude that the A2A receptors play a major role in the control of respiratory frequency and in the tachypnoeic response to SH in mice. NEW FINDINGS: What is the central question of this study? Are cardiovascular and respiratory parameters and their changes in response to sustained hypoxia (SH) altered in adenosine A2A receptor knockout mice? What is the main finding and its importance? Cardiovascular parameters and their changes in response to SH were not altered in A2A KO mice. The respiratory frequency in A2A KO was higher than in WT mice. In response to SH the respiratory frequency increased in WT, while it was reduced in A2A KO mice. A2A receptors play a major role in the modulation of respiratory frequency and in the tachypnoeic response to SH in mice.

Functioning and Gene Expression of Adenosine A1 Receptor During Zebrafish (Danio rerio) Development

The A1 adenosine receptor is the most widely expressed P1 receptor in vertebrates, performing inhibitory tone of the nervous system. Increased levels of adenosine are crucial to promote tissue protection in threatening situations, such as convulsion and hypoxia. Zebrafish is an established model organism for studies on health and disease. In this study, we evaluated the functionality of A1 adenosine receptor through development of zebrafish (6-7-day-, 3-, 8-, and 24-month-old), assessing: (I) the effects of the agonist N6-cyclopenthyladenosine (CPA) over locomotor parameters, (II) the anticonvulsant properties of CPA and adenosine per se in the pentylenetetrazol-induced seizure, and (III) the gene expression of adora1b through development. CPA promoted decreased distance traveled in the highest concentrations/doses tested (larvae: 75 to 500 μM; adults: 20 mg.kg-1), altered mean velocity (larvae: 50-500 μM; adults: 20 mg.kg-1) and time in the bottom zone of apparatus (adults: decrease in 20 mg.kg-1). Adenosine increased the latency of the larvae to reach stage II at 5 and 10 μM. CPA anticonvulsant effect against convulsive stage II was reached at 75 μM, although it decreased basal locomotor activity in larvae. For adults, CPA 10 mg.kg-1 was effective as anticonvulsant without locomotory effects. Adenosine had minor anticonvulsant effects in the concentration tested (larvae: 5 and 10 μM). The level of gene expression of adora1b was stable in brain from adult animals (8- and 24-month-old animals). These results suggest that zebrafish has similar responses to CPA as mammals. To avoid confounding factors, such as locomotor effects, during any brain function investigation using A1 adenosine receptor as a target, the concentration below 75 μM or below the dose of 20 mg.kg-1 of CPA is ideal for zebrafish at larval and adult stages, respectively.

Carbon nanospikes have improved sensitivity and antifouling properties for adenosine, hydrogen peroxide, and histamine

Carbon nanospikes (CNSs) are a new nanomaterial that has enhanced surface roughness and surface oxide concentration, increasing the sensitivity for dopamine detection. However, CNS-modified electrodes (CNSMEs) have not been characterized for other neurochemicals, particularly those with higher oxidation potentials. The purpose of this study was to evaluate CNSMEs for the detection of adenosine, hydrogen peroxide (H2O2), and histamine. The sensitivity increased with CNSs, and signals at CNSMEs were about 3.3 times higher than CFMEs. Normalizing for surface area differences using background currents, CNSMEs show an increased signal of 4.8 times for adenosine, 1.5 times for H2O2, and 2 times for histamine. CNSMEs promoted the formation of secondary products for adenosine and histamine, which enables differentiation from other analytes with similar oxidation potentials. CNSs also selectively enhance the sensitivity for adenosine and histamine compared to H2O2. A scan rate test reveals that adenosine is more adsorption-controlled at CNS electrodes than CFMEs. CNSMEs are antifouling for histamine, with less fouling because the polymers formed after histamine electrooxidation do not adsorb due to an elevated number of edge planes. CNSMEs were useful for detecting each analyte applied in brain slices. Because of the hydrophilic surface compared to CFMEs, CNSMEs also have reduced biofouling when used in tissue. Therefore, CNSMEs are useful for tissue measurements of adenosine, hydrogen peroxide, and histamine with high selectivity and low fouling.

Purinergic receptors in cognitive disturbances

Purinergic receptors (Rs) of the ATP/ADP, UTP/UDP (P2X, P2Y) and adenosine (A1, A2A)-sensitive classes broadly interfere with cognitive processes both under quasi normal and disease conditions. During neurodegenerative illnesses, high concentrations of ATP are released from the damaged neuronal and non-neuronal cells of the brain; then, this ATP is enzymatically degraded to adenosine. Thus, the primary injury in neurodegenerative diseases appears to be caused by various protein aggregates on which a superimposed damage mediated by especially P2X7 and A2AR activation develops; this can be efficiently prevented by small molecular antagonists in animal models of the above diseases, or are mitigated in the respective knockout mice. Dementia is a leading symptom in Alzheimer's disease (AD), and accompanies Parkinson's disease (PD) and Huntington's disease (HD), especially in the advanced states of these illnesses. Animal experimentation suggests that P2X7 and A2ARs are also involved in a number of psychiatric diseases, such as major depressive disorder (MDD), obsessive compulsive behavior, and attention deficit hyperactivity disorder. In conclusion, small molecular antagonists of purinergic receptors are expected to supply us in the future with pharmaceuticals which are able to combat in a range of neurological/psychiatric diseases the accompanying cognitive deterioration.

Effects of electroconvulsive therapy on cerebral A1 adenosine receptor availability: a PET study in patients suffering from treatment-resistant major depressive disorder

Introduction

Sleep deprivation and electroconvulsive therapy (ECT) effectively ameliorate symptoms in major depressive disorder (MDD). In rodents, both are associated with an enhancement of cerebral adenosine levels, which in turn likely influence adenosinergic receptor expression. The aim of the current study was to investigate cerebral A1 adenosine receptor (A1AR) availability in patients with MDD as a potential mediating factor of antidepressant effects of ECT using [18F]CPFPX and positron emission tomography (PET).

Methods

Regional A1AR availability was determined before and after a series of ECT applications (mean number ± SD 10.4 ± 1.2) in 14 subjects (4 males, mean age 49.5 ± 11.8 years). Clinical outcome, measured by neuropsychological testing, and ECT parameters were correlated with changes in A1AR availability.

Results

ECT had a strong antidepressive effect (p < 0.01) while on average cerebral A1AR availability remained unaltered between pre-and post-ECT conditions (F = 0.65, p = 0.42, mean difference ± SD 3.93% ± 22.7%). There was no correlation between changes in clinical outcome parameters and regional A1AR availability, although individual patients showed striking bidirectional alterations of up to 30-40% in A1AR availability after ECT. Solely, for the mean seizure quality index of the applied ECTs a significant association with changes in A1AR availability was found (rs = -0.6, p = 0.02).

Discussion

In the present study, therapeutically effective ECT treatment did not result in coherent changes of A1AR availability after a series of ECT treatments. These findings do not exclude a potential role for cerebral A1ARs in ECT, but shift attention to rather short-termed and adaptive mechanisms during ECT-related convulsive effects.

Altered Purinergic Signaling in Neurodevelopmental Disorders: Focus on P2 Receptors

With the umbrella term 'neurodevelopmental disorders' (NDDs) we refer to a plethora of congenital pathological conditions generally connected with cognitive, social behavior, and sensory/motor alterations. Among the possible causes, gestational and perinatal insults have been demonstrated to interfere with the physiological processes necessary for the proper development of fetal brain cytoarchitecture and functionality. In recent years, several genetic disorders caused by mutations in key enzymes involved in purine metabolism have been associated with autism-like behavioral outcomes. Further analysis revealed dysregulated purine and pyrimidine levels in the biofluids of subjects with other NDDs. Moreover, the pharmacological blockade of specific purinergic pathways reversed the cognitive and behavioral defects caused by maternal immune activation, a validated and now extensively used rodent model for NDDs. Furthermore, Fragile X and Rett syndrome transgenic animal models as well as models of premature birth, have been successfully utilized to investigate purinergic signaling as a potential pharmacological target for these diseases. In this review, we examine results on the role of the P2 receptor signaling in the etiopathogenesis of NDDs. On this basis, we discuss how this evidence could be exploited to develop more receptor-specific ligands for future therapeutic interventions and novel prognostic markers for the early detection of these conditions.

Adenosine A2A receptors and sleep

Adenosine, a known endogenous somnogen, induces sleep via A1 and A2A receptors. In this chapter, we review the current knowledge regarding the role of the adenosine A2A receptor and its agonists, antagonists, and allosteric modulators in sleep-wake regulation. Although many adenosine A2A receptor agonists, antagonists, and allosteric modulators have been identified, only a few have been tested to see if they can promote sleep or wakefulness. In addition, the growing popularity of natural sleep aids has led to an investigation of natural compounds that may improve sleep by activating the adenosine A2A receptor. Finally, we discuss the potential therapeutic advantage of allosteric modulators of adenosine A2A receptors over classic agonists and antagonists for treating sleep and neurologic disorders.

Purinergic Tuning of the Tripartite Neuromuscular Synapse

The vertebrate neuromuscular junction (NMJ) is a specialised chemical synapse involved in the transmission of bioelectric signals between a motor neuron and a skeletal muscle fiber, leading to muscle contraction. Typically, the NMJ is a tripartite synapse comprising (a) a presynaptic region represented by the motor nerve ending, (b) a postsynaptic skeletal motor endplate area, and (c) perisynaptic Schwann cells (PSCs) that shield the motor nerve terminal. Increasing evidence points towards the role of PSCs in the maintenance and control of neuromuscular integrity, transmission, and plasticity. Acetylcholine (ACh) is the main neurotransmitter at the vertebrate skeletal NMJ, and its role is fine-tuned by co-released purinergic neuromodulators, like adenosine 5'-triphosphate (ATP) and its metabolite adenosine (ADO). Adenine nucleotides modulate transmitter release and expression of postsynaptic ACh receptors at motor synapses via the activation of P2Y and P2X receptors. Endogenously generated ADO modulates ACh release by acting via co-localised inhibitory A₁ and facilitatory A_2A receptors on motor nerve terminals, whose tonic activation depends on the neuronal firing pattern and their interplay with cholinergic receptors and neuropeptides. Thus, the concerted action of adenine nucleotides, ADO, and ACh/neuropeptide co-transmitters is paramount to adapting the neuromuscular transmission to the working load under pathological conditions, like Myasthenia gravis. Unravelling these functional complexities prompted us to review our knowledge about the way purines orchestrate neuromuscular transmission and plasticity in light of the tripartite synapse concept, emphasising the often-forgotten role of PSCs in this context.

Neuronal activity-induced, equilibrative nucleoside transporter-dependent, somatodendritic adenosine release revealed by a GRAB sensor

The purinergic signaling molecule adenosine (Ado) modulates many physiological and pathological functions in the brain. However, the exact source of extracellular Ado remains controversial. Here, utilizing a newly optimized genetically encoded GPCR-Activation-Based Ado fluorescent sensor (GRABAdo), we discovered that the neuronal activity-induced extracellular Ado elevation is due to direct Ado release from somatodendritic compartments of neurons, rather than from the axonal terminals, in the hippocampus. Pharmacological and genetic manipulations reveal that the Ado release depends on equilibrative nucleoside transporters but not the conventional vesicular release mechanisms. Compared with the fast-vesicular glutamate release, the Ado release is slow (~40 s) and requires calcium influx through L-type calcium channels. Thus, this study reveals an activity-dependent second-to-minute local Ado release from the somatodendritic compartments of neurons, potentially serving modulatory functions as a retrograde signal.

Striatopallidal adenosine A2A receptor modulation of goal-directed behavior: Homeostatic control with cognitive flexibility

Dysfunction of goal-directed behaviors under stressful or pathological conditions results in impaired decision-making and loss of flexibility of thoughts and behaviors, which underlie behavioral deficits ranging from depression, obsessive-compulsive disorders and drug addiction. Tackling the neuromodulators fine-tuning this core behavioral element may facilitate the development of effective strategies to control these deficits present in multiple psychiatric disorders. The current investigation of goal-directed behaviors has concentrated on dopamine and glutamate signaling in the corticostriatal pathway. In accordance with the beneficial effects of caffeine intake on mood and cognitive dysfunction, we now propose that caffeine's main site of action - adenosine A2A receptors (A2AR) - represent a novel target to homeostatically control goal-directed behavior and cognitive flexibility. A2AR are abundantly expressed in striatopallidal neurons and colocalize and interact with dopamine D2, NMDA and metabotropic glutamate 5 receptors to integrate dopamine and glutamate signaling. Specifically, striatopallidal A2AR (i) exert an overall "break" control of a variety of cognitive processes, making A2AR antagonists a novel strategy for improving goal-directed behavior; (ii) confer homeostatic control of goal-directed behavior by acting at multiple sites with often opposite effects, to enhance cognitive flexibility; (iii) integrate dopamine and adenosine signaling through multimeric A2AR-D2R heterocomplexes allowing a temporally precise fine-tuning in response to local signaling changes. As the U.S. Food and Drug Administration recently approved the A2AR antagonist Nourianz® (istradefylline) to treat Parkinson's disease, striatal A2AR-mediated control of goal-directed behavior may offer a new and real opportunity for improving deficits of goal-directed behavior and enhance cognitive flexibility under various neuropsychiatric conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Neuromodulatory effect of the combination of metformin and vitamin D3 triggered by purinergic signaling in type 1 diabetes induced-rats

Several studies have indicated the vitamin D deficiency in the development of macro- and microvascular complications of diabetes mellitus (DM) including DM-related cognitive dysfunction. The purinergic system plays an important role in the modulation of a variety of mechanisms, including neuroinflammation, plasticity, and cell-cell communication. In addition, purines, their receptors, and enzymes can regulate the purinergic axis at different levels in type 1 DM (T1DM). This study evaluated the effects of vitamin D₃ alone or in combination with metformin in the behavioral performance of streptozotocin-induced T1DM rats. The effects of this combination on the metabolism of ATP and ADP were also studied by NTPDase (CD39), AMP by 5'-nucleotidase (CD73), and adenosine by adenosine deaminase (E-ADA) in the brain and peripheral lymphocytes of type 1 diabetic STZ-induced rats. The results showed that anxiety and memory loss from the DM condition reverted after 30 days of vitamin D₃ treatment. Furthermore, the DM state affected systemic enzymes, with no effect on the central enzymes hydrolyzing extracellular nucleotides and nucleosides. Vitamin D₃ treatment positively regulated ectonucleotidase (NTPDase and 5'-nucleotidase) activity, E-ADA, and the purinergic receptors as a mechanism to prevent oxidative damage in the cerebral cortex of T1DM rats. A neuroprotector effect of vitamin D₃ through adenosine signaling was also observed, by regulating A1 and A_2A receptors proteins levels. The present findings suggest that purinergic signaling through vitamin D₃ modulation may be a novel alternative strategy for T1DM treatment, and may compensate for the negative changes in the central nervous system.

The Memory Orchestra: Contribution of Astrocytes

For decades, memory research has centered on the role of neurons, which do not function in isolation. However, astrocytes play important roles in regulating neuronal recruitment and function at the local and network levels, forming the basis for information processing as well as memory formation and storage. In this review, we discuss the role of astrocytes in memory functions and their cellular underpinnings at multiple time points. We summarize important breakthroughs and controversies in the field as well as potential avenues to further illuminate the role of astrocytes in memory processes.

Extracellular ATP: A powerful inflammatory mediator in the central nervous system

Nucleotides play a crucial role in extracellular signaling across species boundaries. All the three kingdoms of life (Bacteria, Archea and Eukariota) are responsive to extracellular ATP (eATP) and many release this and other nucleotides. Thus, eATP fulfills different functions, many related to danger-sensing or avoidance reactions. Basically all living organisms have evolved sensors for eATP and other nucleotides with very different affinity and selectivity, thus conferring a remarkable plasticity to this signaling system. Likewise, different intracellular transduction systems were associated during evolution to different receptors for eATP. In mammalian evolution, control of intracellular ATP (iATP) and eATP homeostasis has been closely intertwined with that of Ca²⁺, whether in the extracellular milieu or in the cytoplasm, establishing an inverse reciprocal relationship, i.e. high extracellular Ca²⁺ levels are associated to negligible eATP, while low intracellular Ca²⁺ levels are associated to high eATP concentrations. This inverse relationship is crucial for the messenger functions of both molecules. Extracellular ATP is sensed by specific plasma membrane receptors of widely different affinity named P2 receptors (P2Rs) of which 17 subtypes are known. This confers a remarkable plasticity to P2R signaling. The central nervous system (CNS) is a privileged site for purinergic signaling as all brain cell types express P2Rs. Accruing evidence suggests that eATP, in addition to participating in synaptic transmission, also plays a crucial homeostatic role by fine tuning microglia, astroglia and oligodendroglia responses. Drugs modulating the eATP concentration in the CNS are likely to be the new frontier in the therapy of neuroinflammation. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

UHPLC-ESI-QE-Orbitrap-MS based metabolomics reveals the antioxidant mechanism of icaritin on mice with cerebral ischemic reperfusion

Background

Icaritin (ICT) has been previously demonstrated to display protective effects against cerebral ischemic reperfusion (I/R) by inhibiting oxidative stress, but the mechanism remains unclear. This study aimed to explore the mechanism from the perspective of metabolomics.

Methods

A mice cerebral artery occlusion/reperfusion (MCAO/R) model was explored to mimic cerebral ischemic reperfusion and protective effect of ICT was assessed by neurologic deficit scoring, infarct volume and brain water content. Ultra-high-performance liquid chromatography electrospray ionization orbitrap tandem mass spectrometry (UHPLC-ESI-QE-Orbitrap-MS) based metabolomic was performed to explore potential biomarkers. Brain tissue metabolic profiles were analyzed and metabolic biomarkers were identified through multivariate data analysis. The protein levels of Nrf2, HO-1 and HQO1 were assayed by western blot. The release of malondialdehyde (MDA) and the activity of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) were detected using corresponding assay kits.

Results

The results showed that after ICT treatment, the neurological deficit, cerebral infarction area, brain edema and the level of MDA in brain tissue of MCAO/R mice were significantly reduced. Meanwhile, ICT enhanced the activity of SOD, CAT and GSH-Px. Western blot results confirmed that ICT up-regulated the protein levels of antioxidant-related protein including Nrf2, HO-1 and NQO1. According to the metabolomic profiling of brain tissues, clear separations were observed among the Sham, Model and ICT groups. A total of 44 biomarkers were identified, and the identified biomarkers were mainly related to linoleic acid metabolism, arachidonic acid metabolism, alanine, aspartate and glutamate metabolism, arginine biosynthesis, arginine and proline metabolism, D-glutamine and D-glutamate metabolism, taurine and hypotaurine metabolism and purine metabolism, respectively. At the same time, the inhibitory effect of ICT on arachidonic acid and linoleic acid in brain tissue, as well as the promoting effect on taurine, GABA, NAAG, may be the key factors for the anti-neurooxidative function of mice after MCAO/R injury.

Conclusion

Our results demonstrate that ICT has benefits for MCAO/R injury, which are partially related to the suppression of oxidative stress via stimulating the Nrf2 signaling and regulating the production of arachidonic acid, linoleic acid, taurine, GABA, NAAG in brain tissue.

Adenosine A1 receptor agonist, N6-cyclohexyladenosine, attenuates Huntington's disease via stimulation of TrKB/PI3K/Akt/CREB/BDNF pathway in 3-nitropropionic acid rat model

Huntington's disease (HD) is an inherited neurodegenerative disease characterized by progressive motor, behavioral, and cognitive impairments. Intrastriatal injection of 3- nitropropionic acid (3NP) was used to induce HD-like symptoms by inhibiting succinate dehydrogenase enzyme (SDH) in the mitochondrial complex II. The adenosine A₁ receptor has long been known to have a crucial role in neuroprotection, mainly by blocking Ca²⁺ influx, which causes inhibition of glutamate (Glu) and a decline in its excitatory effects at the postsynaptic level. To this end, this study investigated the possible involvement of TrKB/PI3K/Akt/CREB/BDNF pathway in mediating protection afforded by the central N6-cyclohexyladenosine (CHA), an adenosine A₁ receptor agonist. A single intrastriatal CHA injection (6.25 nM/1 μL); 45min after 3-NP injection, attenuated neuronal death, and improved cognitive and motor deficits caused by 3-NP neurotoxin. This effect was shown to parallel an enhanced activation of PI3K/Akt/CREB/BDNF axis as well as boosting pERK_1/2 levels. Moreover, CHA attenuated neuroinflammatory and oxidative stress status via reducing NFκB p65, TNFα and iNOS contents and increasing SOD. Furthermore, immunohistochemical data showed a reduction in the glial fibrillary acidic protein (GFAP) immunoreactivity to a marker for astrocyte and microglia activation following CHA treatment. The results of this study suggest that CHA may have protective effect against HD via modulating oxidative stress, excitotoxic and inflammatory pathways.

Rapid astrocyte-dependent facilitation amplifies multi-vesicular release in hippocampal synapses

Synaptic facilitation is a major form of short-term plasticity typically driven by an increase in residual presynaptic calcium. Using near-total internal reflection fluorescence (near-TIRF) imaging of single vesicle release in cultured hippocampal synapses, we demonstrate a distinctive, release-dependent form of facilitation in which probability of vesicle release is higher following a successful glutamate release event than following a failure. This phenomenon has an onset of ≤500 ms and lasts several seconds, resulting in clusters of successful release events. The release-dependent facilitation requires neuronal contact with astrocytes and astrocytic glutamate uptake by EAAT1. It is not observed in neurons grown alone or in the presence of astrocyte-conditioned media. This form of facilitation dynamically amplifies multi-vesicular release. Facilitation-evoked release events exhibit spatial clustering and have a preferential localization toward the active zone center. These results uncover a rapid astrocyte-dependent form of facilitation acting via modulation of multi-vesicular release and displaying distinctive spatiotemporal properties.

Adenosine A1R/A3R agonist AST-004 reduces brain infarction in mouse and rat models of acute ischemic stroke

Acute ischemic stroke (AIS) is the second leading cause of death globally. No Food and Drug Administration (FDA) approved therapies exist that target cerebroprotection following stroke. Our group recently reported significant cerebroprotection with the adenosine A1/A3 receptor agonist, AST-004, in a transient stroke model in non-human primates (NHP) and in a preclinical mouse model of traumatic brain injury (TBI). However, the specific receptor pathway activated was only inferred based on in vitro binding studies. The current study investigated the underlying mechanism of AST-004 cerebroprotection in two independent models of AIS: permanent photothrombotic stroke in mice and transient middle cerebral artery occlusion (MCAO) in rats. AST-004 treatments across a range of doses were cerebroprotective and efficacy could be blocked by A3R antagonism, indicating a mechanism of action that does not require A1R agonism. The high affinity A3R agonist MRS5698 was also cerebroprotective following stroke, but not the A3R agonist Cl-IB-MECA under our experimental conditions. AST-004 efficacy was blocked by the astrocyte specific mitochondrial toxin fluoroacetate, confirming an underlying mechanism of cerebroprotection that was dependent on astrocyte mitochondrial metabolism. An increase in A3R mRNA levels following stroke suggested an intrinsic cerebroprotective response that was mediated by A3R signaling. Together, these studies confirm that certain A3R agonists, such as AST-004, may be exciting new therapeutic avenues to develop for AIS.

Effect of methylmercury on fetal neurobehavioral development: an overview of the possible mechanisms of toxicity and the neuroprotective effect of phytochemicals

Methylmercury (MeHg) is a global environmental pollutant with neurotoxic effects. Exposure to MeHg via consumption of seafood and fish can severely impact fetal neurobehavioral development even when MeHg levels in maternal blood are as low as about 5 μg/L, which the mother tolerates well. Persistent motor dysfunctions and cognitive deficits may result from trans-placental exposure. The present review summarizes current knowledge on the mechanisms of MeHg toxicity during the period of nervous system development. Although cerebellar Purkinje cells are MeHg targets, the actions of MeHg on thiol components in the neuronal cytoskeleton as well as on mitochondrial enzymes and induction of disturbances of glutamate signaling can impair extra-cerebellar functions, also at levels well tolerated by adult individuals. Numerous herbal substances possess neuroprotective effects, predominantly represented by natural polyphenolic molecules that might be utilized to develop natural drugs to alleviate neurotoxicity symptoms caused by MeHg or other Hg compounds.

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

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

Regulation of blood vessels by ATP in the ventral medullary surface in a rat model of Parkinson's disease

Parkinson's disease (PD) patients often experience impairment of autonomic and respiratory functions. These include conditions such as orthostatic hypotension and sleep apnea, which are highly correlated with dysfunctional central chemoreception. Blood flow is a fundamental determinant of tissue CO₂/H⁺, yet the extent to which blood flow regulation within chemoreceptor regions contributes to respiratory behavior during neurological disease remains unknown. Here, we tested the hypothesis that 6-hydroxydopamine injection to inducing a known model of PD results in dysfunctional vascular homeostasis, biochemical dysregulation, and glial morphology of the ventral medullary surface (VMS). We show that hypercapnia (FiCO₂ = 10%) induced elevated VMS pial vessel constriction in PD animals through a P2-receptor dependent mechanism. Similarly, we found a greater CO₂-induced vascular constriction after ARL67156 (an ectonucleotidase inhibitor) in control and PD-induced animals. In addition, we also report that weighted gene correlational network analysis of the proteomic data showed a protein expression module differentially represented between both groups. This module showed that gene ontology enrichment for components of the ATP machinery were reduced in our PD-model compared to control animals. Altogether, our data indicate that dysfunction in purinergic signaling, potentially through altered ATP bioavailability in the VMS region, may compromise the RTN neuroglial vascular unit in a PD animal model.

Septotemporal variation in modulation of synaptic transmission, paired-pulse ratio and frequency facilitation/depression by adenosine and GABAB receptors in the rat hippocampus

Short-term synaptic plasticity represents a fundamental mechanism in neural information processing and is regulated by neuromodulators. Here, using field recordings from the CA1 region of adult rat hippocampal slices, we show that excitatory synaptic transmission is suppressed by strong but not moderate activation of adenosine A₁ receptors by 2-Chloro-N⁶-cyclopentyladenosine (CCPA) more in the dorsal than the ventral hippocampus; in contrast, both mild and strong activation of GABA_B receptors by baclofen (1 μM, 10 μM) suppress synaptic transmission more in the ventral than the dorsal hippocampus. Using a 10-pulse stimulation train of variable frequency, we found that CCPA modulates short-term synaptic plasticity independently of the suppression of synaptic transmission in both segments of the hippocampus and at stimulation frequencies greater than 10 Hz. However, specifically regarding the paired-pulse ratio (PPR) and frequency facilitation/depression (FF/D) we found significant drug action before but not after adjusting conditioning responses to control levels. Activation of GABA_BRs by baclofen suppressed synaptic transmission more in the ventral than the dorsal hippocampus. Furthermore, relatively high (10 μM) but not low (1 μM) baclofen concentration enhanced both PPR and FF in both hippocampal segments at stimulation frequencies greater than 1 Hz, independently of the suppression of synaptic transmission by baclofen. These results show that A₁Rs and GABA_BRs control synaptic transmission more effectively in the dorsal and the ventral hippocampus, respectively, and suggest that these receptors modulate PPR and FF/D at different frequency bands of afferent input, in both segments of the hippocampus.

A peptide blocking the ADORA1-neurabin interaction is anticonvulsant and inhibits epilepsy in an Alzheimer's model

Epileptic seizures are common sequelae of stroke, acute brain injury, and chronic neurodegenerative diseases, including Alzheimer's disease (AD), and cannot be effectively controlled in approximately 40% of patients, necessitating the development of novel therapeutic agents. Activation of the A1 receptor (A1R) by endogenous adenosine is an intrinsic mechanism to self-terminate seizures and protect neurons from excitotoxicity. However, targeting A1R for neurological disorders has been hindered by side effects associated with its broad expression outside the nervous system. Here we aim to target the neural-specific A1R/neurabin/regulator of G protein signaling 4 (A1R/neurabin/RGS4) complex that dictates A1R signaling strength and response outcome in the brain. We developed a peptide that blocks the A1R-neurabin interaction to enhance A1R activity. Intracerebroventricular or i.n. administration of this peptide shows marked protection against kainate-induced seizures and neuronal death. Furthermore, in an AD mouse model with spontaneous seizures, nasal delivery of this blocking peptide reduces epileptic spike frequency. Significantly, the anticonvulsant and neuroprotective effects of this peptide are achieved through enhanced A1R function in response to endogenous adenosine in the brain, thus, avoiding side effects associated with A1R activation in peripheral tissues and organs. Our study informs potentially new anti-seizure therapy applicable to epilepsy and other neurological illness with comorbid seizures.

Neurovascular coupling mechanisms in health and neurovascular uncoupling in Alzheimer's disease

To match the metabolic demands of the brain, mechanisms have evolved to couple neuronal activity to vasodilation, thus increasing local cerebral blood flow and delivery of oxygen and glucose to active neurons. Rather than relying on metabolic feedback signals such as the consumption of oxygen or glucose, the main signalling pathways rely on the release of vasoactive molecules by neurons and astrocytes, which act on contractile cells. Vascular smooth muscle cells and pericytes are the contractile cells associated with arterioles and capillaries, respectively, which relax and induce vasodilation.

Much progress has been made in understanding the complex signalling pathways of neurovascular coupling, but issues such as the contributions of capillary pericytes and astrocyte calcium signal remain contentious. Study of neurovascular coupling mechanisms is especially important as cerebral blood flow dysregulation is a prominent feature of Alzheimer’s disease. In this article we will discuss developments and controversies in the understanding of neurovascular coupling and finish by discussing current knowledge concerning neurovascular uncoupling in Alzheimer’s disease.

4-Heteroaryl Substituted Amino-3,5-Dicyanopyridines as New Adenosine Receptor Ligands: Novel Insights on Structure-Activity Relationships and Perspectives

A new set of amino-3,5-dicyanopyridines was synthesized and biologically evaluated at the adenosine receptors (ARs). This chemical class is particularly versatile, as small structural modifications can influence not only affinity and selectivity, but also the pharmacological profile. Thus, in order to deepen the structure–activity relationships (SARs) of this series, different substituents were evaluated at the diverse positions on the dicyanopyridine scaffold. In general, the herein reported compounds show nanomolar binding affinity and interact better with both the human (h) A₁ and A_2A ARs than with the other subtypes. Docking studies at hAR structure were performed to rationalize the observed affinity data. Of interest are compounds 1 and 5, which can be considered as pan ligands as binding all the ARs with comparable nanomolar binding affinity (A₁AR: 1, K_i = 9.63 nM; 5, K_i = 2.50 nM; A_2AAR: 1, K_i = 21 nM; 5, Ki = 24 nM; A₃AR: 1, Ki = 52 nM; 5, Ki = 25 nM; A_2BAR: 1, EC₅₀ = 1.4 nM; 5, EC₅₀ = 1.12 nM). Moreover, these compounds showed a partial agonist profile at all the ARs. This combined AR partial agonist activity could lead us to hypothesize a potential effect in the repair process of damaged tissue that would be beneficial in both wound healing and remodeling.

International Union of Basic and Clinical Pharmacology. CXII: Adenosine Receptors: A Further Update

Our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors (2011) contained a number of emerging developments with respect to this G protein-coupled receptor subfamily, including protein structure, protein oligomerization, protein diversity, and allosteric modulation by small molecules. Since then, a wealth of new data and results has been added, allowing us to explore novel concepts such as target binding kinetics and biased signaling of adenosine receptors, to examine a multitude of receptor structures and novel ligands, to gauge new pharmacology, and to evaluate clinical trials with adenosine receptor ligands. This review should therefore be considered a further update of our previous reports from 2001 and 2011. SIGNIFICANCE STATEMENT: Adenosine receptors (ARs) are of continuing interest for future treatment of chronic and acute disease conditions, including inflammatory diseases, neurodegenerative afflictions, and cancer. The design of AR agonists ("biased" or not) and antagonists is largely structure based now, thanks to the tremendous progress in AR structural biology. The A2A- and A2BAR appear to modulate the immune response in tumor biology. Many clinical trials for this indication are ongoing, whereas an A2AAR antagonist (istradefylline) has been approved as an anti-Parkinson agent.

Molecular Simulations and Drug Discovery of Adenosine Receptors

G protein-coupled receptors (GPCRs) represent the largest family of human membrane proteins. Four subtypes of adenosine receptors (ARs), the A1AR, A2AAR, A2BAR and A3AR, each with a unique pharmacological profile and distribution within the tissues in the human body, mediate many physiological functions and serve as critical drug targets for treating numerous human diseases including cancer, neuropathic pain, cardiac ischemia, stroke and diabetes. The A1AR and A3AR preferentially couple to the Gi/o proteins, while the A2AAR and A2BAR prefer coupling to the Gs proteins. Adenosine receptors were the first subclass of GPCRs that had experimental structures determined in complex with distinct G proteins. Here, we will review recent studies in molecular simulations and computer-aided drug discovery of the adenosine receptors and also highlight their future research opportunities.

Dual-Channel Electrochemical Measurements Reveal Rapid Adenosine is Localized in Brain Slices

Rapid adenosine signaling has been detected spontaneously or after mechanical stimulation in the brain, providing rapid neuromodulation in a local area. To measure rapid adenosine signaling, a single carbon-fiber microelectrode has traditionally been used, which limits spatial resolution and an understanding of regional coordination. In this study, we utilized dual-channel fast-scan cyclic voltammetry to measure the spontaneous or mechanically stimulated adenosine release at two electrodes placed at different spacings in hippocampal CA1 mouse brain slices. For mechanically stimulated adenosine release, adenosine can be detected up to 150 μm away from where it was stimulated, although the signal is smaller and delayed. While spontaneous adenosine transients were detected at both electrodes, only 10 percent of the events were detected concurrently, and that number was similar at 50 and 200 μm electrode spacings. Thus, most adenosine transients were not caused by the widespread coordination of release. There was no evidence of diffusion of spontaneous transients to a second electrode 50-200 μm away. This study shows that spontaneous adenosine events are very localized and thus provide only local neuromodulation. Injury, such as mechanical stimulation, allows adenosine to diffuse farther, but the neuroprotective effects are still regional. These results provide a better understanding of the spatial and temporal profiles of adenosine available to act at receptors, which is crucial for future studies that design neuroprotective treatments based on rapid adenosine signaling.

Metabolomic Alterations in the Tear Fluids of Patients With Superior Limbic Keratoconjunctivitis

Purpose

Superior limbic keratoconjunctivitis (SLK) is a bilateral, chronic inflammatory disease that recurs for up to several years; however, the fundamental processes involved in its pathogenic mechanisms remain unknown. We aimed to investigate the metabolomic alterations in the tear fluids of patients with superior limbic keratoconjunctivitis (SLK) compared with those of healthy volunteers (Ctrl group).

Methods

We performed a cross-sectional study involving 42 subjects. Tear fluid was taken from one eye of 24 SLK patients (40.13 ± 14.55 years, 83.33% female) and 18 healthy volunteers (Ctrl, 39.89 ± 9.2 years, 72.22% female) using Schirmer strips. After the liquid extraction of tear metabolites, samples were infused into the QE HFX Orbitrap mass spectrometer in both positive and negative ion mode. Metabolites were quantitatively analyzed and matched with entries in the HMDB database. Metabolic differences between the SLK group and the control group were identified based on multivariate statistical analysis. Open database sources, including SMPDB and MetaboAnalyst, were used to identify metabolic pathways.

Results

Among 179 metabolites retained for annotation, 133 metabolites were finally identified, among which 50 were found to be significantly changed in SLK patients. Of these 50 metabolites, 31 metabolites significantly increased and 19 metabolites decreased in SLK patients. The altered metabolites are mainly involved in α linolenic acid and linoleic acid metabolism, ketone body metabolism, butyrate metabolism, mitochondrial electron transport chain, carnitine synthesis, and so on. The most significantly changed pathway was linoleic acid metabolism. To explore the utility of tear biomarkers, a model combining 9 metabolites (phenol, ethyl glucuronide, eicosapentaenoic acid, 12-keto-leukotriene B4, linoleic acid, hypoxanthine, triethanolamine, 1-nitrohexane, and terephthalic acid) was selected as a candidate biomarker.

Conclusion

The results reveal that SLK has a specific metabolomic profile, of which some key elements can serve as potential biomarkers of SLK for diagnostic and prognostic purposes. The findings of this study are novel and provide a basis for further investigations of the mechanism of SLK.

Effects of caffeine on anxiety and panic attacks in patients with panic disorder: A systematic review and meta-analysis

BACKGROUND

Caffeine has been purported to have anxiogenic and panicogenic properties, specifically salient in patients with panic disorder (PD). However, compilations of the magnitude of the effect of caffeine on anxiety and panic attacks are lacking and potential dose-response relationships have not been examined.

OBJECTIVES

In the present systematic review and meta-analysis, we aimed to examine the acute effects of placebo-controlled caffeine challenge on occurrence of panic attacks and subjective anxiety in patients with PD and healthy controls (HC), including dose-response relationships.

METHODS

Systematic searches were performed in six databases. We included blinded placebo-controlled studies of acute caffeine challenge on panic attacks and/or subjective anxiety in adult patients with PD.

RESULTS

Of the 1893 identified articles, ten met our inclusion criteria. The 9 studies investigating panic attacks included 237 patients, of which 51.1% had a panic attack following caffeine, but none after placebo. Six of these studies compared 128 patients with 115 healthy controls (HC), finding that patients (53.9%) were more vulnerable than HC (1.7%) for panic attacks following caffeine (log RR: 3.47; 95% CI 2.06-4.87). Six studies investigated subjective anxiety in 121 patients and 111 HC following caffeine, with an overall effect indicating increased sensitivity to the anxiogenic effects of caffeine in the patient group (Hedges' g = 1.02 [95% CI: 0.09-1.96]). The restricted range of caffeine employed [400-750 mg] and few studies (3) not using 480 mg prevented any meaningful analysis of a dose-response relationship.

LIMITATIONS

Of the ten studies included, only 2 reported anxiety data for the placebo condition, precluding a proper meta-analysis comparing anxiogenic effects of caffeine and placebo. The restricted dose range used prevented assessment of dose-response relationships.

CONCLUSIONS

The results confirm that caffeine at doses roughly equivalent to 5 cups of coffee induces panic attacks in a large proportion of PD patients and highly discriminates this population from healthy adults. Caffeine also increases anxiety in PD patients as well as among healthy adults at these doses although the exact relationship between caffeine-induced anxiety and panic attacks remains uncertain. The results suggest that caffeine targets important mechanisms related to the pathophysiology of PD.

IMPLICATIONS

Future studies should employ a wider range of caffeine doses and investigate contributions of biological and psychological mechanisms underlying the anxiogenic and panicogenic effects of caffeine. In the clinic, patients with PD should be informed about the panicogenic and anxiogenic effects of caffeine, with the caveat that little is known regarding smaller doses than 480 mg. Registration. PROSPERO (www.crd.york.ac.uk/prospero) registration number CRD42019120220.

Ectonucleotidases in the hippocampus: Spatial distribution and expression after ovariectomy and estradiol replacement

Extracellular purine nucleotides, such as adenosine 5'-triphosphate (ATP), are important modulators of hippocampal function and plasticity. In the extracellular space, ATP is inherently short-lived molecule, which undergoes rapid enzymatic degradation to adenosine by ectonucleotidases. Given that ectonucleotidases have distinct and overlapping distribution in the hippocampus, and as ovarian hormones participate in a formation, maturation, and a refinement of synaptic contacts, both during development and in adulthood, the present chapter summarizes known data about spatial distribution of selected ecto-enzymes and estradiol-induced effects on ectonucleotidases in the rat hippocampus.

The Pathophysiology and Treatment of Essential Tremor: The Role of Adenosine and Dopamine Receptors in Animal Models

Essential tremor (ET) is one of the most common neurological disorders that often affects people in the prime of their lives, leading to a significant reduction in their quality of life, gradually making them unable to independently perform the simplest activities. Here we show that current ET pharmacotherapy often does not sufficiently alleviate disease symptoms and is completely ineffective in more than 30% of patients. At present, deep brain stimulation of the motor thalamus is the most effective ET treatment. However, like any brain surgery, it can cause many undesirable side effects; thus, it is only performed in patients with an advanced disease who are not responsive to drugs. Therefore, it seems extremely important to look for new strategies for treating ET. The purpose of this review is to summarize the current knowledge on the pathomechanism of ET based on studies in animal models of the disease, as well as to present and discuss the results of research available to date on various substances affecting dopamine (mainly D3) or adenosine A1 receptors, which, due to their ability to modulate harmaline-induced tremor, may provide the basis for the development of new potential therapies for ET in the future.

Research progress in pharmacological activities and structure-activity relationships of tetralone scaffolds as pharmacophore and fluorescent skeleton

The tetralone and tetralone derivatives, as crucial structural scaffolds of potential novel drugs targeted at multiple biological end-points, are normally found in several natural compounds and also, it can be used as parental scaffold and/or intermediate for the synthesis of a series of pharmacologically active compounds with a broad-spectrum of bioactivities including antibacterial, antitumor, CNS effect and so on. Meanwhile, SAR information of its analogues has drawn attentions among medicinal chemists, which could contribute to the further research related to tetralone derivatives aimed at multiple targets. This review encompasses pharmacological activities, SAR analysis and docking study of tetralone and its derivatives, expecting to provide a general retrospect and prospect on tetralone derivatives.

Targeting purinergic receptors to suppress the cytokine storm induced by SARS-CoV-2 infection in pulmonary tissue

The etiological agent of coronavirus disease (COVID-19) is the new member of the Coronaviridae family, a severe acute respiratory syndrome coronavirus 2 virus (SARS-CoV-2), responsible for the pandemic that is plaguing the world. The single-stranded RNA virus is capable of infecting the respiratory tract, by binding the spike (S) protein on its viral surface to receptors for the angiotensin II-converting enzyme (ACE2), highly expressed in the pulmonary tissue, enabling the interaction of the virus with alveolar epithelial cells promoting endocytosis and replication of viral material. The infection triggers the activation of the immune system, increased purinergic signaling, and the release of cytokines as a defense mechanism, but the response can become exaggerated and prompt the so-called “cytokine storm”, developing cases such as severe acute respiratory syndrome (SARS). This is characterized by fever, cough, and difficulty breathing, which can progress to pneumonia, failure of different organs and death. Thus, the present review aims to compile and correlate the mechanisms involved between the immune and purinergic systems with COVID-19, since the modulation of purinergic receptors, such as A2A, A2B, and P2X7 expressed by immune cells, seems to be effective as a promising therapy, to reduce the severity of the disease, as well as aid in the treatment of acute lung diseases and other cases of generalized inflammation.

Caffeine as a Factor Influencing the Functioning of the Human Body-Friend or Foe?

Nowadays, caffeine is one of the most commonly consumed substances, which presents in many plants and products. It has both positive and negative effects on the human body, and its activity concerns a variety of systems including the central nervous system, immune system, digestive system, respiratory system, urinary tract, etc. These effects are dependent on quantity, the type of product in which caffeine is contained, and also on the individual differences among people (sex, age, diet etc.). The main aim of this review was to collect, present, and analyze the available information including the latest discoveries on the impact of caffeine on human health and the functioning of human body systems, taking into account the role of caffeine in individual disease entities. We present both the positive and negative sides of caffeine consumption and the healing properties of this purine alkaloid in diseases such as asthma, Parkinson's disease, and others, not forgetting about the negative effects of excess caffeine (e.g., in people with hypertension, children, adolescents, and the elderly). In summary, we can conclude, however, that caffeine has a multi-directional influence on various organs of the human body, and because of its anti-oxidative properties, it was, and still is, an interesting topic for research studies including those aimed at developing new therapeutic strategies.

Emerging Nondopaminergic Medications for Parkinson's Disease: Focusing on A2A Receptor Antagonists and GLP1 Receptor Agonists

Parkinson's disease (PD) is a severe neurodegenerative disease characterized by classic motor features associated with the loss of dopaminergic neurons and appearance of Lewy bodies in the substantia nigra. Due to the complexity of PD, a definitive diagnosis in the early stages and effective management of symptoms in later stages are difficult to achieve in clinical practice. Previous research has shown that colocalization of A2A receptors (A2AR) and dopamine D2 receptors (D2R) may induce an antagonistic interaction between adenosine and dopamine. Clinical trials have found that the A2AR antagonist istradefylline decreases dyskinesia in PD and could be used as an adjuvant to levodopa treatment. Meanwhile, the incretin hormone glucagon-like peptide 1 (GLP1) mainly facilitates glucose homeostasis and insulin signaling. Preclinical experiments and clinical trials of GLP1 receptor (GLP1R) agonists show that they may be effective in alleviating neuroinflammation and sustaining cellular functions in the central nervous system of patients with PD. In this review, we summarize up-to-date findings on the usefulness of A2AR antagonists and GLP1R agonists in PD management. We explain the molecular mechanisms of these medications and their interactions with other neurotransmitter receptors. Furthermore, we discuss the efficacy and limitations of A2AR antagonists and GLP1R agonists in clinical practice.

Electroacupuncture Pretreatment Prevents Cognitive Impairment Induced by Cerebral Ischemia-Reperfusion via Adenosine A1 Receptors in Rats

A previous study has demonstrated that pretreatment with electroacupuncture (EA) induces rapid tolerance to focal cerebral ischemia. In the present study, we investigated whether adenosine receptor 1 (A1 R) is involved in EA pretreatment-induced cognitive impairment after focal cerebral ischemia in rats. Two hours after EA pretreatment, focal cerebral ischemia was induced by middle cerebral artery occlusion for 120 min in male Sprague-Dawley rats. The neurobehavioral score, cognitive function [as determined by the Morris water maze (MWM) test], neuronal number, and the Bax/Bcl-2 ratio was evaluated at 24 h after reperfusion in the presence or absence of CCPA (a selective A1 receptor agonist), DPCPX (a selective A1 receptor antagonist) into left lateral ventricle, or A1 short interfering RNA into the hippocampus area. The expression of the A1 receptor in the hippocampus was also investigated. The result showed that EA pretreatment upregulated the neuronal expression of the A1 receptor in the rat hippocampus at 90 min. And EA pretreatment reversed cognitive impairment, improved neurological outcome, and inhibited apoptosis at 24 h after reperfusion. Pretreatment with CCPA could imitate the beneficial effects of EA pretreatment. But the EA pretreatment effects were abolished by DPCPX. Furthermore, A1 receptor protein was reduced by A1 short interfering RNA which attenuated EA pretreatment-induced cognitive impairment.

Quercetin enhances sexual behavior and improves ectonucleotidases activity in the hypothalamus of rats treated with cyclosporine

In order to confirm the erectile potential of Quercetin (Q) in cyclosporine-induced hypertensive rats, this research assesses the influence of Q on the ectonucleotidases and adenosine deaminase (ADA) in the brains of rats. Male Wistar rats (200 g-250 g) were divided into five groups at random: Normal control (CTRL), cyclosporine-induced hypertensive rats (25 mg kg day-1 ) (HT) group, positive control (Sildenafil [SIL], 5 mg kg day-1 ), Quercetin 25 mg kg day-1 (25 Q), and Quercetin 50 mg kg day-1 (50 Q). Aside from standard diet-fed male rats; cyclosporine was given i.p for the period of 30 days as well as Q orally while the female rats were only given a standard diet. The animals were subjected to sexual activity (copulation) after which the male rat hypothalamus was dissected for biochemical examination (E-NTPDase activities, ecto-5'-nucleotidase as well as ADA and also levels of nitric oxide [NO]). We observed that Q enhanced copulatory behavior as evident in mounting, intromission, ejaculation numbers, and latencies. A substantial (p < .05) increase in the activity of E-NTPDase (ATP and ADP as substrate) without any notable difference in the action of ecto-5' nucleotidase was facilitated by cyclosporine-induction when compared to the CTRL. The 50 mg/kg, however, had the highest reversal effect in accordance with dose manner. Also, cyclosporine increased ADA activity with a concomitant reduction of NO level while both doses of Q down-regulated ADA activity and, increased NO levels. Enhanced sexual behavior, modulation of ectonucleotidases as well as ADA activity and increased NO levels suggest that Q-rich plant foods may be promising sources of dietary phytonutrients for erectile dysfunction (ED) management. PRACTICAL APPLICATIONS: Behavioral and biochemical assays evaluated showed that Q significantly enhanced sexual behavior as well as improved ATP bioavailability in cyclosporine-induced erectile dysfunctional rats. The modulatory effects of Q on ectonucleotidases, along with its ability to minimize adenosine deaminase activity and increase nitric oxide levels, indicate that Q-rich plants and/or plant foods may be promising sources of dietary phytonutrients for erectile dysfunction management.

Assessment of sexual behavior and neuromodulation of Cyperus esculentus L. and Tetracarpidium conophorum Müll. Arg dietary supplementation regulating the purinergic system in the cerebral cortex of L-NAME-challenged rats

This study examined the behavioral responses, purinergic receptor densities, ectonucleotidases (E-NTPDase), adenosine deaminase (ADA) activity, and nitric oxide (NO) levels along with the parameters of oxidative stress-related to erectile function in the cerebral cortex (CC) of L-NAME-challenged rats pretreated with tigernut (TN) and walnut (WN) dietary supplementation. Wistar rats (male) of 70 total animals (250-300 g) were used in this research and hence separated into seven groups (n = 10): Group I: normal control-fed basal diet; Group II: positive control-fed basal diet/L-NAME/Sildenafil citrate (5 mg kg-1  day-1 ); Group III: ED-induced (placed on a basal diet/L-NAME); Group IV: diet supplemented with processed TN (20%)/L-NAME; Group V: diet supplemented with raw TN (20%)/L-NAME; Group VI: diet supplemented with processed WN (20%)/L-NAME; and Group VII placed on a diet supplemented with raw WN (20%)/L-NAME. The rats were pretreated for 2 weeks before the L-NAME (40 mg kg-1  day-1 ) challenge on their respective diet. L-NAME brought about a decrease in the sexual behaviors evaluated while the effect was significantly reversed by supplemented diets containing TN and WN. L-NAME increased the levels of reactive oxygen species and malondialdehyde, E-NTPDase as well as ADA activities, and caused the level of NO in the CC as well as the purinoreceptor densities to be downregulated. Treatments with enriched diets, however, greatly reverse these effects. The behavioral responses and neuromodulatory capacity of the nuts displayed on the CC can, therefore, further support their aphrodisiac property. PRACTICAL APPLICATIONS: The results revealed the ability of tigernut (TN; Cyperus esculentus L.) and walnut (WN; Tetracarpidium conophorum Müll. Arg.) to enhance behavioral responses; modulate purinergic receptor densities, E-NTPDase, and ADA activities; increase NO levels; and prevent oxidative stress related to erectile function in the CC of L-NAME-challenged Wistar rats. The results show that these nuts are useful feeds for both animal and human nutrition.