An essential role for adenosine signaling in alcohol abuse

Detection of interactions between genetic marker sets and environment in a genome-wide study of hypertension

As human complex diseases are influenced by the interplay of genes and environment, detecting gene-environment interactions ( G × E ) can shed light on biological mechanisms of diseases and play an important role in disease risk prediction. Development of powerful quantitative tools to incorporate G × E in complex diseases has potential to facilitate the accurate curation and analysis of large genetic epidemiological studies. However, most of existing methods that interrogate G × E focus on the interaction effects of an environmental factor and genetic variants, exclusively for common or rare variants. In this study, we proposed two tests, MAGEIT_RAN and MAGEIT_FIX, to detect interaction effects of an environmental factor and a set of genetic markers containing both rare and common variants, based on the MinQue for Summary statistics. The genetic main effects in MAGEIT_RAN and MAGEIT_FIX are modeled as random or fixed, respectively. Through simulation studies, we illustrated that both tests had type I error under control and MAGEIT_RAN was overall the most powerful test. We applied MAGEIT to a genome-wide analysis of gene-alcohol interactions on hypertension in the Multi-Ethnic Study of Atherosclerosis. We detected two genes, CCNDBP1 and EPB42, that interact with alcohol usage to influence blood pressure. Pathway analysis identified sixteen significant pathways related to signal transduction and development that were associated with hypertension, and several of them were reported to have an interactive effect with alcohol intake. Our results demonstrated that MAGEIT can detect biologically relevant genes that interact with environmental factors to influence complex traits.

Caffeine Delays Ethanol-Induced Sedation in Drosophila

Caffeine and ethanol are among the most widely available and commonly consumed psychoactive substances. Both interact with adenosine receptor-mediated signaling which regulates numerous neurological processes including sleep and waking behaviors. In mammals, caffeine is an adenosine receptor antagonist and thus acts as a stimulant. Conversely, ethanol is a sedative because it promotes GABAergic neurotransmission, inhibits glutamatergic neurotransmission, and increases the amount of adenosine in the brain. Despite seemingly overlapping interactions, not much is known about the effect of caffeine on ethanol-induced sedation in Drosophila. In this study, using Drosophila melanogaster as a model, we show that caffeine supplementation in food delays the onset of ethanol-induced sedation in males and females of different strains. The resistance to sedation reverses upon caffeine withdrawal. Heterozygous adenosine receptor mutant flies are resistant to sedation. These findings suggest that caffeine and adenosine receptors modulate the sedative effects of ethanol in Drosophila.

The Role of the Adenosine System on Emotional and Cognitive Disturbances Induced by Ethanol Binge Drinking in the Immature Brain and the Beneficial Effects of Caffeine

Binge drinking intake is the most common pattern of ethanol consumption by adolescents, which elicits emotional disturbances, mainly anxiety and depressive symptoms, as well as cognitive alterations. Ethanol exposure may act on the adenosine neuromodulation system by increasing adenosine levels, consequently increasing the activation of adenosine receptors in the brain. The adenosine modulation system is involved in the control of mood and memory behavior. However, there is a gap in the knowledge about the exact mechanisms related to ethanol exposure’s hazardous effects on the immature brain (i.e., during adolescence) and the role of the adenosine system thereupon. The present review attempts to provide a comprehensive picture of the role of the adenosinergic system on emotional and cognitive disturbances induced by ethanol during adolescence, exploring the potential benefits of caffeine administration in view of its action as a non-selective antagonist of adenosine receptors.

Insight into the Nucleoside Transport and Inhibition of Human ENT1

The human equilibrative nucleoside transporter 1 (hENT1) is an effective controller of adenosine signaling by regulating its extracellular and intracellular concentration, and has become a solid drug target of clinical used adenosine reuptake inhibitors (AdoRIs). Currently, the mechanisms of adenosine transport and inhibition for hENT1 remain unclear, which greatly limits the in-depth understanding of its inner workings as well as the development of novel inhibitors. In this work, the dynamic details of hENT1 underlie adenosine transport and the inhibition mechanism of the non-nucleoside AdoRIs dilazep both were investigated by comparative long-time unbiased molecular dynamics simulations. The calculation results show that the conformational transitions of hENT1 from the outward open to metastable occluded state are mainly driven by TM1, TM2, TM7 and TM9. One of the trimethoxyphenyl rings in dilazep serves as the adenosyl moiety of the endogenous adenosine substrate to competitively occupy the orthosteric site of hENT1. Due to extensive and various VDW interactions with N30, M33, M84, P308 and F334, the other trimethoxyphenyl ring is stuck in the opportunistic site near the extracellular side preventing the complete occlusion of thin gate simultaneously. Obviously, dilazep shows significant inhibitory activity by disrupting the local induce-fit action in substrate binding cavity and blocking the transport cycle of whole protein. This study not only reveals the nucleoside transport mechanism by hENT1 at atomic level, but also provides structural guidance for the subsequent design of novel non-nucleoside AdoRIs with enhanced pharmacologic properties.

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The transitions of hENT1 from the outward open to metastable occluded state are mainly driven by TM1, TM2, TM7 and TM9.

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The induce-fit action by adenosine recognition precedes. inward contraction of the extracellular side.

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Dilazep exerts its special hENT1 inhibitory function through competitive binding and allosteric regulation.

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A gating strategy of extracellular loop is revealed to ensure adenosine is firmly located in the transport cavity.

Recent advances on the inhibition of human solute carriers: Therapeutic implications and mechanistic insights

Solute carriers (SLCs) are membrane transport proteins tasked with mediating passage of hydrophilic molecules across lipid bilayers. Despite the extensive roles played in all aspects of human biology, SLCs remain vastly under-explored as therapeutic targets. In this brief review, we first discuss a few successful cases of drugs that exert their mechanisms of action through inhibition of human SLCs, and introduce select examples of human SLCs that have untapped therapeutic potential. We then highlight two recent structural studies which uncovered detailed structural mechanisms of inhibition exhibited against two different human major facilitator superfamily (MFS) transporters of clinical relevance.

Stimulants and Depressor Drugs in the Sleep-Wake Cycle Modulation: The case of alcohol and cannabinoids

A complex neurobiological network drives the sleep-wake cycle. In addition, external stimuli, including stimulants or depressor drugs, also influence the control of sleep. Here we review the recent advances that contribute to the comprehensive understanding of the actions of stimulants and depressor compounds, such as alcohol and cannabis, in sleep regulation. The objective of this review is to highlight the neurobiological mechanism engaged by alcohol and cannabis in sleep control.

Neurochemical and Behavioral Consequences of Ethanol and/or Caffeine Exposure: Effects in Zebrafish and Rodents

Zebrafish are increasingly being utilized to model the behavioral and neurochemical effects of pharmaceuticals and, more recently, pharmaceutical interactions. Zebrafish models of stress establish that both caffeine and ethanol influence anxiety, though few studies have implemented co-administration to assess the interaction of anxiety and reward-seeking. Caffeine exposure in zebrafish is teratogenic, causing developmental abnormalities in the cardiovascular, neuromuscular, and nervous systems of embryos and larvae. Ethanol is also a teratogen and, as an anxiolytic substance, may be able to offset the anxiogenic effects of caffeine. Co-exposure to caffeine and alcohol impacts neuroanatomy and behavior in adolescent animal models, suggesting stimulant substances may moderate the impact of alcohol on neural circuit development. Here, we review the literature describing neuropharmacological and behavioral consequences of caffeine and/or alcohol exposure in the zebrafish model, focusing on neurochemistry, locomotor effects, and behavioral assessments of stress/anxiety as reported in adolescent/juvenile and adult animals. The purpose of this review is twofold: (1) describe the work in zebrafish documenting the effects of ethanol and/or caffeine exposure and (2) compare these zebrafish studies with comparable experiments in rodents. We focus on specific neurochemical pathways (dopamine, serotonin, adenosine, GABA, adenosine), anxiety-type behaviors (assessed with novel tank, thigmotaxis, shoaling), and locomotor changes resulting from both individual and co-exposure. We compare findings in zebrafish with those in rodent models, revealing similarities across species and identifying conservation of mechanisms that potentially reinforce co-addiction.

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.

Toward a Molecular Basis of Cellular Nucleoside Transport in Humans

Nucleosides play central roles in all facets of life, from metabolism to cellular signaling. Because of their physiochemical properties, nucleosides are lipid bilayer impermeable and thus rely on dedicated transport systems to cross biological membranes. In humans, two unrelated protein families mediate nucleoside membrane transport: the concentrative and equilibrative nucleoside transporter families. The objective of this review is to provide a broad outlook on the current status of nucleoside transport research. We will discuss the role played by nucleoside transporters in human health and disease, with emphasis placed on recent structural advancements that have revealed detailed molecular principles of these important cellular transport systems and exploitable pharmacological features.

Interactions of neuroimmune signaling and glutamate plasticity in addiction

Chronic use of drugs of abuse affects neuroimmune signaling; however, there are still many open questions regarding the interactions between neuroimmune mechanisms and substance use disorders (SUDs). Further, chronic use of drugs of abuse can induce glutamatergic changes in the brain, but the relationship between the glutamate system and neuroimmune signaling in addiction is not well understood. Therefore, the purpose of this review is to bring into focus the role of neuroimmune signaling and its interactions with the glutamate system following chronic drug use, and how this may guide pharmacotherapeutic treatment strategies for SUDs. In this review, we first describe neuroimmune mechanisms that may be linked to aberrant glutamate signaling in addiction. We focus specifically on the nuclear factor-kappa B (NF-κB) pathway, a potentially important neuroimmune mechanism that may be a key player in driving drug-seeking behavior. We highlight the importance of astroglial-microglial crosstalk, and how this interacts with known glutamatergic dysregulations in addiction. Then, we describe the importance of studying non-neuronal cells with unprecedented precision because understanding structure-function relationships in these cells is critical in understanding their role in addiction neurobiology. Here we propose a working model of neuroimmune-glutamate interactions that underlie drug use motivation, which we argue may aid strategies for small molecule drug development to treat substance use disorders. Together, the synthesis of this review shows that interactions between glutamate and neuroimmune signaling may play an important and understudied role in addiction processes and may be critical in developing more efficacious pharmacotherapies to treat SUDs.

Use of knockout mice to explore CNS effects of adenosine

The initial exploration using pharmacological tools of the role of adenosine receptors in the brain, concluded that adenosine released as such acted on A₁R to inhibit excitability and glutamate release from principal neurons throughout the brain and that adenosine A_2A receptors (A_2AR) were striatal-'specific' receptors controlling dopamine D₂R. This indicted A₁R as potential controllers of neurodegeneration and A_2AR of psychiatric conditions. Global knockout of these two receptors questioned the key role of A₁R and instead identified extra-striatal A_2AR as robust controllers of neurodegeneration. Furthermore, transgenic lines with altered metabolic sources of adenosine revealed a coupling of ATP-derived adenosine to activate A_2AR and a role of A₁R as a hurdle to initiate neurodegeneration. Additionally, cell-selective knockout of A_2AR unveiled the different roles of A_2AR in different cell types (neurons/astrocytes) in different portions of the striatal circuits (dorsal versus lateral) and in different brain areas (hippocampus/striatum). Finally, a new transgenic mouse line with deletion of all adenosine receptors seems to indicate a major allostatic rather than homeostatic role of adenosine and may allow isolating P2R-mediated responses to unravel their role in the brain, a goal close to heart of Geoffrey Burnstock, to whom we affectionately dedicate this review.

Dynamic-related protein 1 inhibitor eases epileptic seizures and can regulate equilibrative nucleoside transporter 1 expression

Background

Dynamic-related protein 1 (Drp1) is a key protein involved in the regulation of mitochondrial fission, and it could affect the dynamic balance of mitochondria and appears to be protective against neuronal injury in epileptic seizures. Equilibrative nucleoside transporter 1 (ENT1) is expressed and functional in the mitochondrial membrane that equilibrates adenosine concentration across membranes. Whether Drp1 participates in the pathogenesis of epileptic seizures via regulating function of ENT1 remains unclear.

Methods

In the present study, we used pilocarpine to induce status epilepticus (SE) in rats, and we used mitochondrial division inhibitor 1 (Mdivi-1), a selective inhibitor to Drp1, to suppress mitochondrial fission in pilocarpine-induced SE model. Mdivi-1administered by intraperitoneal injection before SE induction, and the latency to firstepileptic seizure and the number of epileptic seizures was thereafter observed. The distribution of Drp1 was detected by immunofluorescence, and the expression patterns of Drp1 and ENT1 were detected by Western blot. Furthermore, the mitochondrial ultrastructure of neurons in the hippocampal CA1 region was observed by transmission electron microscopy.

Results

We found that Drp1 was expressed mainly in neurons and Drp1 expression was significantly upregulated in the hippocampal and temporal neocortex tissues at 6 h and 24 h after induction of SE. Mitochondrial fission inhibitor 1 attenuated epileptic seizures after induction of SE, reduced mitochondrial damage and ENT1 expression.

Conclusions

These data indicate that Drp1 is upregulated in hippocampus and temporal neocortex after pilocarpine-induced SE and the inhibition of Drp1 may lead to potential therapeutic target for SE by regulating ENT1 after pilocarpine-induced SE.

Type 1 equilibrative nucleoside transporter (ENT1) regulates sex-specific ethanol drinking during disruption of circadian rhythms

Disruptions in circadian rhythms are risk factors for excessive alcohol drinking. The ethanol-sensitive adenosine equilibrative nucleoside transporter type 1 (ENT1, slc29a1) regulates ethanol-related behaviors, sleep, and entrainment of circadian rhythms. However, the mechanism underlying the increased ethanol consumption in ENT1 knockout (KO) mice in constant light (LL) and whether there are sex differences in ethanol consumption in ENT1 mice are less studied. Here, we investigated the effects of loss of ENT1, LL, and sex on ethanol drinking using two-bottle choice. In addition, we monitored the locomotor activity rhythms. We found that LL increased ethanol drinking and reduced accumbal ENT1 expression and adenosine levels in male but not female mice, compared with control mice. Interestingly, only LL-exposed male, not female, ENT1 KO mice exhibited higher ethanol drinking and a longer circadian period with a higher amplitude compared with wild-type (WT) mice. Furthermore, viral-mediated rescue of ENT1 expression in the NAc of ENT1 KO mice reduced ethanol drinking, demonstrating a possible causal link between ENT1 expression and ethanol drinking in males. Together, our findings indicate that deficiency of ENT1 expression contributes to excessive ethanol drinking in a sex-dependent manner.

Neuronal modulation of hepatic lipid accumulation induced by bingelike drinking

Excessive alcohol consumption, including binge drinking, is a common cause of fatty liver disease. Binge drinking rapidly induces hepatic steatosis, an early step in the pathogenesis of chronic liver injury. Despite its prevalence, the process by which excessive alcohol consumption promotes hepatic lipid accumulation remains unclear. Alcohol exerts potent effects on the brain, including hypothalamic neurons crucial for metabolic regulation. However, whether or not the brain plays a role in alcohol-induced hepatic steatosis is unknown. In the brain, alcohol increases extracellular levels of adenosine, a potent neuromodulator, and previous work implicates adenosine signaling as being important for the development of alcoholic fatty liver disease. Acute alcohol exposure also increases both the activity of agouti-related protein (AgRP)-expressing neurons and AgRP immunoreactivity. Here, we show that adenosine receptor A2B signaling in the brain modulates the extent of alcohol-induced fatty liver in mice and that both the AgRP neuropeptide and the sympathetic nervous system are indispensable for hepatic steatosis induced by bingelike alcohol consumption. Together, these results indicate that the brain plays an integral role in alcohol-induced hepatic lipid accumulation and that central adenosine signaling, hypothalamic AgRP, and the sympathetic nervous system are crucial mediators of this process.

Adenosine-Related Mechanisms in Non-Adenosine Receptor Drugs

Many ligands directly target adenosine receptors (ARs). Here we review the effects of noncanonical AR drugs on adenosinergic signaling. Non-AR mechanisms include raising adenosine levels by inhibiting adenosine transport (e.g., ticagrelor, ethanol, and cannabidiol), affecting intracellular metabolic pathways (e.g., methotrexate, nicotinamide riboside, salicylate, and 5-aminoimidazole-4-carboxamide riboside), or undetermined means (e.g., acupuncture). However, other compounds bind ARs in addition to their canonical ‘on-target’ activity (e.g., mefloquine). The strength of experimental support for an adenosine-related role in a drug’s effects varies widely. AR knockout mice are the ‘gold standard’ method for investigating an AR role, but few drugs have been tested on these mice. Given the interest in AR modulation for treatment of cancer, CNS, immune, metabolic, cardiovascular, and musculoskeletal conditions, it is informative to consider AR and non-AR adenosinergic effects of approved drugs and conventional treatments.

Probenecid Reduces Alcohol Drinking in Rodents. Is Pannexin1 a Novel Therapeutic Target for Alcohol Use Disorder?

AIMS

The development of novel and more effective medications for alcohol use disorder (AUD) is an important unmet medical need. Drug repositioning or repurposing is an appealing strategy to bring new therapies to the clinic because it greatly reduces the overall costs of drug development and expedites the availability of treatments to those who need them. Probenecid, p-(di-n-propylsulfamyl)-benzoic acid, is a drug used clinically to treat hyperuricemia and gout due to its activity as an inhibitor of the kidneys' organic anion transporter that reclaims uric acid from urine. Probenecid also inhibits pannexin1 channels that are involved in purinergic neurotransmission and inflammation, which have been implicated in alcohol's effects and motivation for alcohol. Therefore, we tested the effects of probenecid on alcohol intake in rodents.

METHODS

We tested the effects of probenecid on operant oral alcohol self-administration in alcohol-dependent rats during acute withdrawal as well as in nondependent rats and in the drinking-in-the-dark (DID) paradigm of binge-like drinking in mice.

RESULTS

Probenecid reduced alcohol intake in both dependent and nondependent rats and in the DID paradigm in mice without affecting water or saccharin intake, indicating that probenecid's effect was selective for alcohol and not the result of a general reduction in reward.

CONCLUSIONS

These results raise the possibility that pannexin1 is a novel therapeutic target for the treatment of AUD. The clinical use of probenecid has been found to be generally safe, suggesting that it can be a candidate for drug repositioning for the treatment of AUD.

Neurochemical Evidence of Preclinical and Clinical Reports on Target-Based Therapy in Alcohol Used Disorder

Alcohol use disorder (AUD) is a chronic relapsing disorder, which enforces a person to compulsively seek alcohol, restricting control over alcohol intake leads to emergence of an undesired emotional state during abstinence. There are recent advances for better understanding of neurocircuitry involved in the pathophysiology of AUD. Alcohol interaction with neuronal membrane proteins results in changes in neuronal circuits. It is also linked with the potential medication and their clinical validation concerning their pharmacological targets for alcoholic abstinence. This review covers research work from the past few decades on the therapeutic advances on treatment of alcohol dependence; further detailing the fundamental neurochemical mechanisms after alcohol administration. It also covers interaction of alcohol with GABAergic, glutaminergic, dopaminergic, serotonergic and opioid systems. This review further elaborated the neurobiology of noradrenergic, cholinergic and cannabinoid systems and their interaction with AUD. Elaborative information of potential drug targets under current exploration for AUD treatment with their mechanisms are reported here along with clinical outcomes and the associated side effects.

Epigenetic aging is accelerated in alcohol use disorder and regulated by genetic variation in APOL2

To investigate the potential role of alcohol use disorder (AUD) in aging processes, we employed Levine's epigenetic clock (DNAm PhenoAge) to estimate DNA methylation age in 331 individuals with AUD and 201 healthy controls (HC). We evaluated the effects of heavy, chronic alcohol consumption on epigenetic age acceleration (EAA) using clinical biomarkers, including liver function test enzymes (LFTs) and clinical measures. To characterize potential underlying genetic variation contributing to EAA in AUD, we performed genome-wide association studies (GWAS) on EAA, including pathway analyses. We followed up on relevant top findings with in silico expression quantitative trait loci (eQTL) analyses for biological function using the BRAINEAC database. There was a 2.22-year age acceleration in AUD compared to controls after adjusting for gender and blood cell composition (p = 1.85 × 10^-5). This association remained significant after adjusting for race, body mass index, and smoking status (1.38 years, p = 0.02). Secondary analyses showed more pronounced EAA in individuals with more severe AUD-associated phenotypes, including elevated gamma-glutamyl transferase (GGT) and alanine aminotransferase (ALT), and higher number of heavy drinking days (all ps < 0.05). The genome-wide meta-analysis of EAA in AUD revealed a significant single nucleotide polymorphism (SNP), rs916264 (p = 5.43 × 10^-8), in apolipoprotein L2 (APOL2) at the genome-wide level. The minor allele A of rs916264 was associated with EAA and with increased mRNA expression in hippocampus (p = 0.0015). Our data demonstrate EAA in AUD and suggest that disease severity further accelerates epigenetic aging. EAA was associated with genetic variation in APOL2, suggesting potential novel biological mechanisms for age acceleration in AUD.

Increased Ethanol Consumption and Locomotion Develop upon Ethanol Deprivation in Rats Overexpressing the Adenosine (A)2A Receptor

Preclinical data indicate that ethanol produces behavioral effects that can be regulated by many neurotransmitters and neuromodulators like adenosine (A). The most important receptors with respect to the rewarding effects of ethanol seem to be the A_2A receptors. This study used a transgenic strategy, specifically rats overexpressing the A_2A receptor, to characterize the neurobiological mechanisms of ethanol consumption as measured by intermittent access to 20% ethanol in a two-bottle choice paradigm. In this model, no change in ethanol consumption was observed in transgenic animals compared to wild type controls during the acquisition/maintenance phase. Following alcohol deprivation, only transgenic rats overexpressing the A_2A receptor exhibited escalation of ethanol consumption and drank more (by ca. 90%), but not significantly, ethanol than did the wild type rats. During ethanol withdrawal, the immobility time of rats overexpressing the A_2A receptor in the forced swim test was lower than that of wild type rats. Moreover, transgenic rats withdrawn from ethanol, compared to the drug-naive transgenic animals, exhibited an increase above 70% in locomotion. The results indicated that the overexpression of A_2A receptors may be a risk factor for the escalation of ethanol consumption despite the reduction in depression-like signs of ethanol withdrawal.

Novel Adenosine Analog, N6-(4-Hydroxybenzyl)-Adenosine, Dampens Alcohol Drinking and Seeking Behaviors

Adenosine signaling is associated with ethanol-related behaviors. We previously found that adenosine A_2A receptor (A_2AR) activation dampens ethanol drinking behaviors in equilibrative nucleoside transporter 1 (ENT1) knockout mice, and A_2AR inhibition augments reward-seeking behavior in wild-type mice. The novel adenosine analog N6-(4-hydroxybenzyl)-adenosine (NHBA), which is isolated from the rhizomes of Gastrodia elata, activates A_2AR and inhibits ENT1. Here, we examined the effects of NHBA on ethanol drinking in the two-bottle choice test and operant ethanol seeking behaviors. We selected mice exhibiting high ethanol drinking behavior in the two-bottle choice test. NHBA (0.1 mg/kg, i.p.) reduced ethanol drinking behavior in a limited-access 3-hour drinking session in high-consumption ethanol drinking mice, and NHBA (0.1 mg/kg, i.p.) did not alter locomotor activity in the open-field test. Operant conditioning with 10% ethanol and 10% sucrose (10E10S) reward increased zone entries and time spent in the ethanol zone, while NHBA (0.1 mg/kg, i.p.) dampened ethanol zone preference in the Y-maze. Furthermore, NHBA (0.1 mg/kg, i.p.) devalued 10E10S and 10% ethanol (10E) reward after operant conditioning with 10E10S and 10E. Taken together, NHBA through A_2AR activation and ENT1 modulation may dampen ethanol drinking and seeking behaviors, suggesting that NHBA is a potential therapeutic agent for treating alcohol use disorder. SIGNIFICANCE STATEMENT: Our work highlights that A_2AR activation and ENT1 inhibition by a novel adenosine analog isolated from Gastrodia elata, N6-(4-hydroxybenzyl)-adenosine, decreases ethanol drinking and seeking behaviors. We suggest that NHBA is a potential therapeutic agent to treat alcohol use disorder.

Early exposure to ethanol is able to affect the memory of adult zebrafish: Possible role of adenosine

Ethanol is one of the most widely consumed drugs in the world, and the effects of ethanol during early development include morphological and cognitive problems. The regulation of adenosine levels is essential for the proper function of major neurotransmitter systems in the brain, particularly glutamate and dopamine; thus, the investigation of the relation of adenosine and memory after early ethanol exposure becomes relevant. Embryos of zebrafish were exposed to 1% ethanol during two distinct developmental stages: gastrula/segmentation or pharyngula. The evaluation of memory, morphology, and locomotor parameters was performed when fish were 3 months old. The effect of ecto-5'-nucleotidase and adenosine deaminase inhibition on the consequences of ethanol exposure with regard to memory formation was observed. Morphological evaluation showed decreases in body length and the relative telencephalic and cerebellar areas in ethanol exposed animals. The locomotor parameters evaluated were not affected by ethanol. In the inhibitory avoidance paradigm, ethanol exposure during the gastrula/segmentation and pharyngula stages decreased zebrafish memory retention. When ethanol was given in the pharyngula stage, the inhibition of ecto-5'-nucleotidase in the acquisition phase of memory tests was able to revert the effects of ethanol on the memory of adults. These findings suggest that the increased adenosine levels caused by ethanol could alter the neuromodulation of important components of memory formation, such as neurotransmitters. The adjustment of adenosine levels through ecto-5'-nucleotidase inhibition appears to be effective at restoring normal adenosine levels and the acquisition of memory in animals exposed to ethanol during the pharyngula stage.

DPCPX, a selective adenosine A1 receptor antagonist, enhances the antidepressant-like effects of imipramine, escitalopram, and reboxetine in mice behavioral tests

The main goal of the present study was to evaluate the influence of the adenosine A1 receptor (A1R) antagonist — DPCPX — on depressive-like behavior in mice, as well as the effect of DPCPX on the activity of imipramine, escitalopram, and reboxetine, each at non-effective doses. The influence of DPCPX on behavior and its influence on the activity of selected antidepressants was evaluated in the forced swim test (FST) and the tail suspension test (TST) in mice. Locomotor activity was measured to verify and exclude false-positive data obtained in the FST and TST. Moreover, serum and brain concentrations of tested antidepressants were determined using HPLC. DPCPX, at doses of 2 and 4 mg/kg, exhibited antidepressant activity in the FST and TST, which was not related to changes in the spontaneous locomotor activity. Co-administration of DPCPX with imipramine, escitalopram, or reboxetine, each at non-active doses, significantly reduced the immobilization period in the FST and TST in mice, which was not due to the increase in locomotor activity. Both antagonists of 5-HT receptors (WAY 100635 and ritanserin) completely antagonized the effect elicited by DPCPX in the behavioral tests. Results of assessment of the nature of the interaction between DPCPX and test drugs show that in the case of DPCPX and imipramine or reboxetine, there were pharmacodynamic interactions, whereas the DPCPX-escitalopram interaction is at least partially pharmacokinetic in nature. Presented outcomes indicate that an inhibition of A1Rs and an increase of monoaminergic transduction in the CNS may offer a novel strategy for the development of antidepressant drugs.

Central adenosine A1 receptors inhibit cough via suppression of excitatory glutamatergic and tachykininergic neurotransmission

BACKGROUND AND PURPOSE

The adenosine A1 receptor is reported to mediate several excitatory effects in the airways and has inhibitory effects in the CNS. In this study, we investigated the role of peripheral and central A1 receptors in regulating cough and airway obstruction.

EXPERIMENTAL APPROACH

Drugs were administered to guinea pigs via inhalation or i.c.v. infusion. Following the administration of different drugs, cough was induced by exposing guinea pigs to aerosolized 0.4 M citric acid. An automated analyser recorded both cough and airway obstruction simultaneously using whole-body plethysmography.

KEY RESULTS

The A1 receptor agonist, cyclopentyladenosine (CPA, administered by inhalation), dose-dependently inhibited cough and also inhibited airway obstruction. Similarly, CPA, administered i.c.v., inhibited both the citric acid-induced cough and airway obstruction; this was prevented by pretreatment with the A1 receptor antagonist DPCPX (i.c.v.). Treatment with DPCPX alone dose-dependently enhanced the citric acid-induced cough and airway obstruction. This effect was reversed following treatment with either the glutamate GluN1 receptor antagonist D-AP5 or the neurokinin NK1 receptor antagonist FK-888.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that activation of either peripheral or central adenosine A1 receptors inhibits citric acid-induced cough and airway obstruction. The data also suggest that tonic activation of central adenosine A1 receptors serves as a negative regulator of cough and airway obstruction, secondary to inhibition of excitatory glutamatergic and tachykininergic neurotransmission.

Acute ethanol exposure has bidirectional actions on the endogenous neuromodulator adenosine in rat hippocampus

Background and Purpose

Ethanol is a widely used recreational drug with complex effects on physiological and pathological brain function. In epileptic patients, the use of ethanol can modify seizure initiation and subsequent seizure activity with reports of ethanol being both pro‐ and anticonvulsant. One proposed target of ethanol's actions is the neuromodulator adenosine, which is released during epileptic seizures to feedback and inhibit the occurrence of subsequent seizures. Here, we investigated the actions of acute ethanol exposure on adenosine signalling in rat hippocampus.

Experimental Approach

We have combined electrophysiology with direct measurements of extracellular adenosine using microelectrode biosensors in rat hippocampal slices.

Key Results

We found that ethanol has bidirectional actions on adenosine signalling: depressant concentrations of ethanol (50 mM) increased the basal extracellular concentration of adenosine under baseline conditions, leading to the inhibition of synaptic transmission, but it inhibited adenosine release during evoked seizure activity in brain slices. The reduction in activity‐dependent adenosine release was in part produced by effects on NMDA receptors, although other mechanisms also appeared to be involved. Low concentrations of ethanol (10–15 mM) enhanced pathological network activity by selectively blocking activity‐dependent adenosine release.

Conclusions and Implications

The complex dose‐dependent actions of ethanol on adenosine signalling could in part explain the mixture of pro‐convulsant and anticonvulsant actions of ethanol that have previously been reported.

Adenosine role in brain functions: Pathophysiological influence on Parkinson's disease and other brain disorders

Although adenosine plays a key role in multiple motor, affective, and cognitive processes, it has received less attention in the neuroscience field compared to other neurotransmitters (e.g., dopamine). In this review, we highlight the role of adenosine in behavior as well as its interaction with other neurotransmitters, such as dopamine. We also discuss brain disorders impacted by alterations to adenosine, and how targeting adenosine can ameliorate Parkinson's disease motor symptoms. We also discuss the role of caffeine (as an adenosine antagonist) on cognition as well as a neuroprotective agent against Parkinson's disease (PD).

Differential SLC1A2 Promoter Methylation in Bipolar Disorder With or Without Addiction

While downregulation of excitatory amino acid transporter 2 (EAAT2), the main transporter removing glutamate from the synapse, has been recognized in bipolar disorder (BD), the underlying mechanisms of downregulation have not been elucidated. BD is influenced by environmental factors, which may, via epigenetic modulation of gene expression, differentially affect illness presentation. This study thus focused on epigenetic DNA methylation regulation of SLC1A2, encoding for EAAT2, in BD with variable environmental influences of addiction. High resolution melting PCR (HRM-PCR) and thymine–adenine (TA) cloning with sequence analysis were conducted to examine methylation of the promoter region of the SLC1A2. DNA was isolated from blood samples drawn from BD patients (N = 150) with or without addiction to alcohol, nicotine, or food, defined as binge eating, and matched controls (N = 32). In comparison to controls, the SLC1A2 promoter region was hypermethylated in BD without addiction but was hypomethylated in BD with addiction. After adjusting for age and sex, the association of methylation levels with nicotine addiction (p = 0.0009) and binge eating (p = 0.0002) remained significant. Consistent with HRM-PCR, direct sequencing revealed increased methylation in CpG site 6 in BD, but decreased methylation in three CpG sites (6, 48, 156) in BD with alcohol and nicotine addictions. These results suggest that individual point methylation within the SLC1A2 promoter region may be modified by exogenous addiction and may have a potential for developing clinically valuable epigenetic biomarkers for BD diagnosis and monitoring.

Effects of adenosine receptor antagonists in MPTP mouse model of Parkinson's disease: mitochondrial DNA integrity

INTRODUCTION

In Parkinson's disease (PD), compelling data indicate a functional link between adenosine/dopamine receptors and the progression of the neurodegenerative process. The present study was carried out to evaluate the effect of the non-selective adenosine receptor (ADR) antagonist caffeine, as well as the selective antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an ADRsA₁ antagonist, and ((E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methyl-3,7-dihydro-1H-purine-2,6-dione) (KW-6002), an ADRsA_2A antagonist, on the prevention of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism in mice.

MATERIAL AND METHODS

Mice were allocated to five groups: group I - control group; group II: MPTP group, received four injections of MPTP (20 mg/kg, i.p.) at 2 h intervals; groups III, IV, V: received MPTP and i.p. caffeine (20 mg/kg/day) or DPCPX (5 mg/kg/day) or KW-6002 (10 mg/kg/day) starting one week before MPTP injection and continuing for 2 weeks.

RESULTS

Therapy with caffeine or KW-6002 not only led to the reversibility of movement dysfunction and increased the concentrations of dopamine and ATP levels (p < 0.05), but also, ameliorates the dopaminergic neuron loss and restored the mtDNA and nDNA integrity (p < 0.05). Furthermore, in passive avoidance test, caffeine and DPCPX significantly (p < 0.05) reversed the MPTP-induced memory deficits, whereas the specific ADRsA2A antagonist did not.

CONCLUSIONS

The current results provide evidence that blockade of both ADRsA₁ and ADRsA_2A has therapeutic implications in alleviating MPTP-induced motor and cognitive dysfunction and might be a promising candidate for treatment of PD.

Alcohol and basal ganglia circuitry: Animal models

Brain circuits that include the cortex and basal ganglia make up the bulk of the forebrain, and influence behaviors related to almost all aspects of affective, cognitive and sensorimotor functions. The learning of new actions as well as association of existing action repertoires with environmental events are key functions of this circuitry. Unfortunately, the cortico-basal ganglia circuitry is also the target for all drugs of abuse, including alcohol. This makes the circuitry susceptible to the actions of chronic alcohol exposure that impairs circuit function in ways that contribute to cognitive dysfunction and drug use disorders. In the present review, we describe the connectivity and functions of the associative, limbic and sensorimotor cortico-basal ganglia circuits. We then review the effects of acute and chronic alcohol exposure on circuit function. Finally, we review studies examining the roles of the different circuits and circuit elements in alcohol use and abuse. We attempt to synthesize information from a variety of studies in laboratory animals and humans to generate hypotheses about how the three circuits interact with each other and with the other brain circuits during exposure to alcohol and during the development of alcohol use disorders. This article is part of the Special Issue entitled "Alcoholism".

Equilibrative nucleoside transporters-A review

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that mediate the transport of nucleosides, nucleobases, and therapeutic analogs. The best-characterized ENTs are the human transporters hENT1 and hENT2. However, non-mammalian eukaryotic ENTs have also been studied (e.g., yeast, parasitic protozoa). ENTs are major pharmaceutical targets responsible for modulating the efficacy of more than 30 approved drugs. However, the molecular mechanisms and chemical determinants of ENT-mediated substrate recognition, binding, inhibition, and transport are poorly understood. This review highlights findings on the characterization of ENTs by surveying studies on genetics, permeant and inhibitor interactions, mutagenesis, and structural models of ENT function.

Common genes regulate food and ethanol intake in Drosophila

The abuse liability of alcohol (ethanol) is believed to result in part from its actions on neurobiological substrates that underlie the motivation toward food and other natural reinforcers, and a growing body of evidence indicates that these substrates are broadly conserved among animal phyla. Understanding the extent to which the substrates regulating ethanol and food intake overlap is an important step toward developing therapeutics that selectively reduce ethanol intake. In the current experiments, we measured food and ethanol intake in Recombinant Inbred (RI) lines of Drosophila melanogaster using several assays, and then calculated genetic correlations to estimate the degree to which common genes might underlie behavior in these assays. We found that food intake and ethanol intake as measured in the capillary assay are genetically correlated traits in D. melanogaster, as well as in a panel of 11 Drosophila species that we tested subsequently. RI line differences in food intake in a dyed food assay were genetically unrelated to ethanol intake in the capillary assay or to ethanol preference measured using an olfactory trap apparatus. Using publicly available gene expression data, we found that expression profiles across the RI lines of a number of genes (including the D2-like dopamine receptor, DOPA decarboxylase, and fruitless) correlated with the RI line differences in food and ethanol intake we measured, while the expression profiles of other genes, including NPF, and the NPF and 5-HT2 receptors, correlated only with ethanol intake or preference. Our results suggest that food and ethanol intake are regulated by some common genes in Drosophila, but that other genes regulate ethanol intake independently of food intake. These results have implications toward the development of therapeutics that preferentially reduce ethanol intake.

Adenosinergic regulation of striatal clock gene expression and ethanol intake during constant light

Circadian rhythm and sleep disruptions occur frequently in individuals with alcohol use disorders (AUD) and present significant barriers to treatment. Recently, a variant of adenosine transporter, equilibrative nucleoside transporter 1 (ENT1), was associated with the co-occurrence of sleep problems and AUD. We have previously shown that mice lacking ENT1 (ENT1 KO) have reduced adenosine levels in the striatum and drink more alcohol compared with wild types (WT). However, it is unknown whether ENT1 deletion disrupts circadian rhythms, which may contribute to alcohol preference in ENT1 KO mice. Here we used these mice to determine whether endogenous adenosine regulates circadian genetic and behavioral rhythms and influences alcohol intake during chronodisruption. We examined circadian locomotor activity in ENT1 KO vs WT littermates and found that ENT1 KO mice were both active earlier and hyperactive compared with WT mice at night. We used real-time PCR and immunohistochemistry to estimate striatal clock gene levels and found that PER2 expression in the striatum was blunted by ENT1 deletion or A2A receptor (A2AR) antagonism. Next, we exposed ENT1 KO and WT mice to constant light (LL) and found further elevation in ethanol intake in ENT1 KO, but not in WT mice, supporting the notion that circadian dysfunction may contribute to increased alcohol intake in ENT1 KO mice. Finally, we showed that A2AR agonist administration normalized PER1 and PER2 expression and circadian locomotor activity in ENT1 KO mice. Together, our results demonstrate that adenosine signaling regulates cellular and behavioral circadian timing and influences alcohol intake during chronodisruption.

Aberrant bone density in aging mice lacking the adenosine transporter ENT1

Adenosine is known to regulate bone production and resorption in humans and mice. Type 1 equilibrative nucleoside transporter (ENT1) is responsible for the majority of adenosine transport across the plasma membrane and is ubiquitously expressed in both humans and mice. However, the contribution of ENT1-mediated adenosine levels has not been studied in bone remodeling. With the recent identification of the importance of adenosine signaling in bone homeostasis, it is essential to understand the role of ENT1 to develop novel therapeutic compounds for bone disorders. Here we examined the effect of ENT1 deletion on bone density using X-ray, dual energy X-ray absorptiometry and micro-computerized tomography analysis. Our results show that bone density and bone mineral density is reduced in the lower thoracic and lumbar spine as well as the femur of old ENT1 null mice (>7 months) compared to wild-type littermates. Furthermore, we found increased mRNA expression of tartrate-resistant acid phosphatase (TRAP), an osteoclast marker, in isolated long bones from 10 month old ENT1 null mice compared to wild-type mice. In addition, aged ENT1 null mice displayed severe deficit in motor coordination and locomotor activity, which might be attributed to dysregulated bone density. Overall, our study suggests that ENT1-regulated adenosine signaling plays an essential role in lumbar spine and femur bone density.

Adenosine and glutamate in neuroglial interaction: implications for circadian disorders and alcoholism

Recent studies have demonstrated that the function of glia is not restricted to the support of neuronal function. In fact, astrocytes are essential for neuronal activity in the brain and play an important role in the regulation of complex behavior. Astrocytes actively participate in synapse formation and brain information processing by releasing and uptaking glutamate, D-serine, adenosine 5'-triphosphate (ATP), and adenosine. In the central nervous system, adenosine-mediated neuronal activity modulates the actions of other neurotransmitter systems. Adenosinergic fine-tuning of the glutamate system in particular has been shown to regulate circadian rhythmicity and sleep, as well as alcohol-related behavior and drinking. Adenosine gates both photic (light-induced) glutamatergic and nonphotic (alerting) input to the circadian clock located in the suprachiasmatic nucleus of the hypothalamus. Astrocytic, SNARE-mediated ATP release provides the extracellular adenosine that drives homeostatic sleep. Acute ethanol increases extracellular adenosine, which mediates the ataxic and hypnotic/sedative effects of alcohol, while chronic ethanol leads to downregulated adenosine signaling that underlies insomnia, a major predictor of relapse. Studies using mice lacking the equilibrative nucleoside transporter 1 have illuminated how adenosine functions through neuroglial interactions involving glutamate uptake transporter GLT-1 [referred to as excitatory amino acid transporter 2 (EAAT2) in human] and possibly water channel aquaporin 4 to regulate ethanol sensitivity, reward-related motivational processes, and alcohol intake.

Cell type-specific effects of adenosine on cortical neurons

The neuromodulator adenosine is widely considered to be a key regulator of sleep homeostasis and an indicator of sleep need. Although the effect of adenosine on subcortical areas has been previously described, the effects on cortical neurons have not been addressed systematically to date. To that purpose, we performed in vitro whole-cell patch-clamp recordings and biocytin staining of pyramidal neurons and interneurons throughout all layers of rat prefrontal and somatosensory cortex, followed by morphological analysis. We found that adenosine, via the A1 receptor, exerts differential effects depending on neuronal cell type and laminar location. Interneurons and pyramidal neurons in layer 2 and a subpopulation of layer 3 pyramidal neurons that displayed regular spiking were insensitive to adenosine application, whereas other pyramidal cells in layers 3-6 were hyperpolarized (range 1.2-10.8 mV). Broad tufted pyramidal neurons with little spike adaptation showed a small adenosine response, whereas slender tufted pyramidal neurons with substantial adaptation showed a bigger response. These studies of the action of adenosine at the postsynaptic level may contribute to the understanding of the changes in cortical circuit functioning that take place between sleep and awakening.

Adenosine signaling in striatal circuits and alcohol use disorders

Adenosine signaling has been implicated in the pathophysiology of alcohol use disorders and other psychiatric disorders such as anxiety and depression. Numerous studies have indicated a role for A1 receptors (A1R) in acute ethanol-induced motor incoordination, while A2A receptors (A2AR) mainly regulate the rewarding effect of ethanol in mice. Recent findings have demonstrated that dampened A2AR-mediated signaling in the dorsomedial striatum (DMS) promotes ethanol-seeking behaviors. Moreover, decreased A2AR function is associated with decreased CREB activity in the DMS, which enhances goal-oriented behaviors and contributes to excessive ethanol drinking in mice. Interestingly, caffeine, the most commonly used psychoactive substance, is known to inhibit both the A1R and A2AR. This dampened adenosine receptor function may mask some of the acute intoxicating effects of ethanol. Furthermore, based on the fact that A2AR activity plays a role in goal-directed behavior, caffeine may also promote ethanol-seeking behavior. The A2AR is enriched in the striatum and exclusively expressed in striatopallidal neurons, which may be responsible for the regulation of inhibitory behavioral control over drug rewarding processes through the indirect pathway of the basal ganglia circuit. Furthermore, the antagonistic interactions between adenosine and dopamine receptors in the striatum also play an integral role in alcoholism and addiction-related disorders. This review focuses on regulation of adenosine signaling in striatal circuits and the possible implication of caffeine in goal-directed behaviors and addiction.

Effects of caffeine on alcohol consumption and nicotine self-administration in rats

BACKGROUND

Caffeine, alcohol, and nicotine are 3 of the most widespread self-administered psychoactive substances, which are known to be extensively co-administered. However, little is known about the degree to which they may mutually potentiate each other's consumption.

METHODS

In the current set of studies, we examined in rats the effect of caffeine administration on alcohol drinking and intravenous (i.v.) self-administration of nicotine. In male alcohol-preferring (P) rats, caffeine (5, 10, and 20 mg/kg) or the saline vehicle was administered acutely either by subcutaneous (S.C.) injection or orally (PO) by gavage. In a chronic study, the effect of PO caffeine (5 and 20 mg/kg) on alcohol intake over a 10-day period was tested. In another experiment, the effect of acute PO administration of caffeine (20 mg/kg) or saline on saccharin intake (0.2% solution) was determined in P rats. Effects of 20 mg/kg caffeine on motor activity were also determined in P rats. Finally, the effects of acute PO caffeine administration on nicotine self-administration in Sprague-Dawley rats were also determined.

RESULTS

Both routes of administration of caffeine, S.C. and PO, caused a significant dose-related decrease in alcohol intake and preference during free access to alcohol and after 4-day deprivation of alcohol. However, the low dose of 5 mg/kg caffeine increased alcohol intake. Acute PO caffeine also reduced saccharin intake. Acute systemic administration of 20 mg/kg caffeine did not exert a significant effect on motor activity. In Sprague-Dawley rats trained to self-administer i.v. nicotine, acute PO administration of caffeine significantly increased self-administration of nicotine in a dose-related manner.

CONCLUSIONS

These results suggest that adenosine receptor systems may play a role in both alcohol and nicotine intake and deserve further study regarding these addictions.

The Impact of Caffeine on the Behavioral Effects of Ethanol Related to Abuse and Addiction: A Review of Animal Studies

The impact of caffeine on the behavioral effects of ethanol, including ethanol consumption and abuse, has become a topic of great interest due to the rise in popularity of the so-called energy drinks. Energy drinks high in caffeine are frequently taken in combination with ethanol under the popular belief that caffeine can offset some of the intoxicating effects of ethanol. However, scientific research has not universally supported the idea that caffeine can reduce the effects of ethanol in humans or in rodents, and the mechanisms mediating the caffeine-ethanol interactions are not well understood. Caffeine and ethanol have a common biological substrate; both act on neurochemical processes related to the neuromodulator adenosine. Caffeine acts as a nonselective adenosine A1 and A2A receptor antagonist, while ethanol has been demonstrated to increase the basal adenosinergic tone via multiple mechanisms. Since adenosine transmission modulates multiple behavioral processes, the interaction of both drugs can regulate a wide range of effects related to alcohol consumption and the development of ethanol addiction. In the present review, we discuss the relatively small number of animal studies that have assessed the interactions between caffeine and ethanol, as well as the interactions between ethanol and subtype-selective adenosine receptor antagonists, to understand the basic findings and determine the possible mechanisms of action underlying the caffeine-ethanol interactions.

Striatal adenosine signaling regulates EAAT2 and astrocytic AQP4 expression and alcohol drinking in mice

Adenosine signaling is implicated in several neuropsychiatric disorders, including alcoholism. Among its diverse functions in the brain, adenosine regulates glutamate release and has an essential role in ethanol sensitivity and preference. However, the molecular mechanisms underlying adenosine-mediated glutamate signaling in neuroglial interaction remain elusive. We have previously shown that mice lacking the ethanol-sensitive adenosine transporter, type 1 equilibrative nucleoside transporter (ENT1), drink more ethanol compared with wild-type mice and have elevated striatal glutamate levels. In addition, ENT1 inhibition or knockdown reduces glutamate transporter expression in cultured astrocytes. Here, we examined how adenosine signaling in astrocytes contributes to ethanol drinking. Inhibition or deletion of ENT1 reduced the expression of type 2 excitatory amino-acid transporter (EAAT2) and the astrocyte-specific water channel, aquaporin 4 (AQP4). EAAT2 and AQP4 colocalization was also reduced in the striatum of ENT1 null mice. Ceftriaxone, an antibiotic compound known to increase EAAT2 expression and function, elevated not only EAAT2 but also AQP4 expression in the striatum. Furthermore, ceftriaxone reduced ethanol drinking, suggesting that ENT1-mediated downregulation of EAAT2 and AQP4 expression contributes to excessive ethanol consumption in our mouse model. Overall, our findings indicate that adenosine signaling regulates EAAT2 and astrocytic AQP4 expressions, which control ethanol drinking in mice.

Tonic modulatory role of mouse cerebellar α- and β-adrenergic receptors in the expression of ethanol-induced ataxia: role of AC-cAMP

To further study neurochemical basis of ethanol-induced ataxia (EIA), we investigated role of cerebellar α and β-adrenergic receptors. Male CD-1 mice received intracerebellar microinfusion of adrenergic drugs to evaluate their effect on EIA (2g/kg; ip) by Rotorod. Isoproterenol, phenylephrine (4, 8, 16 ng each), methoxamine (8 ng), and atenolol (2, 4, 8 ng), propranolol (4, 8, 16 ng), markedly attenuated and accentuated, respectively, EIA indicating the tonic nature of modulation. The attenuation of EIA by isoproterenol is β(1)-receptor mediated because it is blocked by atenolol. Tonic β(1) modulation is functionally correlated with EIA potentiation by atenolol and propranolol. The prazosin-induced attenuation of EIA, initially thought of α(1)-receptor mediated, appeared instead β(1)-receptor modulated because of: (i) blockade by atenolol; and (ii) phosphodiesterase inhibition by prazosin. The phenylephrine/methoxamine-induced attenuation of EIA seems paradoxical as the response is similar to antagonist prazosin. However, functionally the attenuation seems β(1) receptor-mediated since atenolol blocked it but prazosin did not. Also norepinephrine (NE) attenuated EIA that was inhibited by atenolol suggesting role of β(1) receptors. Similarly yohimbine and rauwolscine attenuated EIA that indicates α(2)-receptor modulation associated with stimulation of AC-cAMP pathway. The results of study support the hypothesis that attenuation and potentiation of EIA is mediated by activation and inhibition of AC-cAMP pathway, respectively, in agreement with our previous reports, via direct and/or indirect activation of β-receptor.

Neurochemical and neurostructural plasticity in alcoholism

The behavioral manifestations of alcoholism are primarily attributable to the numerous and lasting adaptations that occur in the brain as a result of chronic heavy alcohol consumption. As will be reviewed here, these adaptations include alcohol-induced plasticity in chemical neurotransmission, density and morphology of dendritic spines, as well as neurodegeneration and cerebral atrophy. Within the context of these neuroadaptations that have been observed in both human and animal studies, we will discuss how these changes potentially contribute to the cognitive and behavioral dysfunctions that are hallmark features of alcoholism, as well as how they reveal novel potential pharmacological targets for the treatment of this disorder.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Adenosine and glutamate signaling in neuron-glial interactions: implications in alcoholism and sleep disorders

Recent studies have demonstrated that the function of glia is not restricted to the support of neuronal function. Especially, astrocytes are essential for neuronal activity in the brain. Astrocytes actively participate in synapse formation and brain information processing by releasing or uptaking gliotransmitters such as glutamate, d-serine, adenosine 5'-triphosphate (ATP), and adenosine. In the central nervous system, adenosine plays an important role in regulating neuronal activity as well as in controlling other neurotransmitter systems such as GABA, glutamate, and dopamine. Ethanol (EtOH) increases extracellular adenosine levels, which regulates the ataxic and hypnotic/sedative (somnogenic) effects of EtOH. Adenosine signaling is also involved in the homeostasis of major inhibitory/excitatory neurotransmission (i.e., GABA or glutamate) through neuron-glial interactions, which regulates the effect of EtOH and sleep. Adenosine transporters or astrocytic SNARE-mediated transmitter release regulates extracellular or synaptic adenosine levels. Adenosine then exerts its function through several adenosine receptors and regulates glutamate levels in the brain. This review presents novel findings on how neuron-glial interactions, particularly adenosinergic signaling and glutamate uptake activity involving glutamate transporter 1 (GLT1), are implicated in alcoholism and sleep disorders.

Sex-Specific Regulation of Depression, Anxiety-Like Behaviors and Alcohol Drinking in Mice Lacking ENT1

OBJECTIVES: Adenosine signaling has been implicated in the pathophysiology of several psychiatric disorders including alcoholism, depression, and anxiety. Adenosine levels are controlled in part by transport across the cell membrane by equilibrative nucleoside transporters (ENTs). Recent evidence showed that a polymorphism in the gene encoding ENT1 is associated with comorbid depression and alcoholism in women. We have previously shown that deletion of ENT1 reduces ethanol intoxication and elevates alcohol intake in mice. Interestingly, ENT1 null mice display decreased anxiety-like behavior compared to wild-type littermates. However, our behavioral studies were performed only in male mice. Here, we extend our research to include female mice, and test the effect of ENT1 knockout on other behavioral correlates of alcohol drinking, including depressive and compulsive behavior, in mice. METHODS: To assess depression-like behavior, we used a forced swim test modified for mice. We examined anxiety-like behavior and locomotor activity in open field chambers, and perseverant behavior using the marble-burying test. Finally, we investigated alcohol consumption and preference in female mice using a two-bottle choice paradigm. RESULTS: ENT1 null mice of both sexes showed reduced immobility time in the forced swim test and increased time in the center of the open field compared to wild-type littermates. ENT1 null mice of both sexes showed similar locomotor activity levels and habituation to the open field chambers. Female ENT1 null mice displayed increased marble-burying compared to female wild-types, but no genotype difference was evident in males. Female ENT1 null mice showed increased ethanol consumption and preference compared to female wild-types. CONCLUSIONS: Our findings suggest that ENT1 contributes to several important behaviors involved in psychiatric disorders. Inhibition of ENT1 may be beneficial in treating depression and anxiety, while enhancement of ENT1 function may reduce compulsive behavior and drinking, particularly in females.

Alcohol and Caffeine: The Perfect Storm

Although it is widely believed that caffeine antagonizes the intoxicating effects of alcohol, the molecular mechanisms underlying their interaction are incompletely understood. It is known that both caffeine and alcohol alter adenosine neurotransmission, but the relationship is complex, and may be dose dependent. In this article, we review the available literature on combining caffeine and alcohol. Ethical constraints prohibit laboratory studies that would mimic the high levels of alcohol intoxication achieved by many young people in real-world settings, with or without the addition of caffeine. We propose a possible neurochemical mechanism for the increase in alcohol consumption and alcohol-related consequences that have been observed in persons who simultaneously consume caffeine. Caffeine is a nonselective adenosine receptor antagonist. During acute alcohol intake, caffeine antagonizes the "unwanted" effects of alcohol by blocking the adenosine A₁ receptors that mediate alcohol's somnogenic and ataxic effects. The A₁ receptor-mediated "unwanted" anxiogenic effects of caffeine may be ameliorated by alcohol-induced increase in the extracellular concentration of adenosine. Moreover, by means of interactions between adenosine A_2A and dopamine D₂ receptors, caffeine-mediated blockade of adenosine A_2A receptors can potentiate the effects of alcohol-induced dopamine release. Chronic alcohol intake decreases adenosine tone. Caffeine may provide a "treatment" for the withdrawal effects of alcohol by blocking the effects of upregulated A₁ receptors. Finally, blockade of A_2A receptors by caffeine may contribute to the reinforcing effects of alcohol.

Neuroadaptations in adenosine receptor signaling following long-term ethanol exposure and withdrawal

Ethanol affects the function of neurotransmitter systems, resulting in neuroadaptations that alter neural excitability. Adenosine is one such receptor system that is changed by ethanol exposure. The current review is focused on the A(1) and the A(2A) receptor subtypes in the context of ethanol-related neuroadaptations and ethanol withdrawal because these subtypes (i) are activated by basal levels of adenosine, (ii) have been most well-studied for their role in neuroprotection and ethanol-related phenomena, and (iii) are the primary site of action for caffeine in the brain, a substance commonly ingested with ethanol. It is clear that alterations in adenosinergic signaling mediate many of the effects of acute ethanol administration, particularly with regard to motor function and sedation. Further, prolonged ethanol exposure has been shown to produce adaptations in the cell surface expression or function of both A(1) and the A(2A) receptor subtypes, effects that likely promote neuronal excitability during ethanol withdrawal. As a whole, these findings demonstrate a significant role for ethanol-induced adaptations in adenosine receptor signaling that likely influence neuronal function, viability, and relapse to ethanol intake following abstinence.

Type 1 equilibrative nucleoside transporter regulates ethanol drinking through accumbal N-methyl-D-aspartate receptor signaling

BACKGROUND

Mice lacking type 1 equilibrative nucleoside transporter (ENT1(-/-)) exhibit increased ethanol-preferring behavior compared with wild-type littermates. This phenotype of ENT1(-/-) mice appears to be correlated with increased glutamate levels in the nucleus accumbens (NAc). However, little is known about the downstream consequences of increased glutamate signaling in the NAc.

METHODS

To investigate the significance of the deletion of ENT1 and its effect on glutamate signaling in the NAc, we employed microdialysis and iTRAQ proteomics. We validated altered proteins using Western blot analysis. We then examined the pharmacological effects of the inhibition of the N-methyl-D-aspartate (NMDA) glutamate receptor and protein kinase Cγ (PKCγ) on alcohol drinking in wild-type mice. In addition, we investigated in vivo cyclic adenosine monophosphate response element binding activity using cyclic adenosine monophosphate response element-β-galactosidase mice in an ENT1(-/-) background.

RESULTS

We identified that NMDA glutamate receptor-mediated downregulation of intracellular PKCγ-neurogranin-calcium-calmodulin dependent protein kinase type II signaling is correlated with reduced cyclic adenosine monophosphate response element binding activity in ENT1(-/-) mice. Inhibition of PKCγ promotes ethanol drinking in wild-type mice to levels similar to those of ENT1(-/-) mice. In contrast, an NMDA glutamate receptor antagonist reduces ethanol drinking of ENT1(-/-) mice.

CONCLUSIONS

These findings demonstrate that the genetic deletion or pharmacological inhibition of ENT1 regulates NMDA glutamate receptor-mediated signaling in the NAc, which provides a molecular basis that underlies the ethanol-preferring behavior of ENT1(-/-) mice.