Combined antagonism of glutamate mGlu5 and adenosine A2A receptors interact to regulate alcohol-seeking in rats

Ethanol-induced AMPA alterations are mediated by mGLU5 receptors through miRNA upregulation in hippocampal slices

Prenatal alcohol exposure (PAE) affects neuronal networks and brain development causing a range of physical, cognitive and behavioural disorders in newborns that persist into adulthood. The array of consequences associated with PAE can be grouped under the umbrella-term 'fetal alcohol spectrum disorders' (FASD). Unfortunately, there is no cure for FASD as the molecular mechanisms underlying this pathology are still unknown. We have recently demonstrated that chronic EtOH exposure, followed by withdrawal, induces a significant decrease in AMPA receptor (AMPAR) expression and function in developing hippocampus in vitro. Here, we explored the EtOH-dependent pathways leading to hippocampal AMPAR suppression. Organotypic hippocampal slices (2 days in cultures) were exposed to EtOH (150 mM) for 7 days followed by 24 h EtOH withdrawal. Then, the slices were analysed by means of RT-PCR for miRNA content, western blotting for AMPA and NMDA related-synaptic proteins expression in postsynaptic compartment and electrophysiology to record electrical properties from CA1 pyramidal neurons. We observed that EtOH induces a significant downregulation of postsynaptic AMPA and NMDA subunits and relative scaffolding protein expression and, accordingly, a decrease of AMPA-mediated neurotransmission. Simultaneously, we found that chronic EtOH induced-upregulation of miRNA 137 and 501-3p and decreased AMPA-mediated neurotransmission are prevented by application of the selective mGlu5 antagonist MPEP during EtOH withdrawal. Our data indicate mGlu5 via miRNA137 and 501-3p expression as key factors in the regulation of AMPAergic neurotransmission that may contribute, at least in part, to the pathogenesis of FASD.

Adenosine receptor stimulation inhibits methamphetamine-associated cue seeking

BACKGROUND

Methamphetamine (METH) is a psychostimulant drug that remains a popular and threatening drug of abuse with high abuse liability. There is no established pharmacotherapy to treat METH dependence, but evidence suggests that stimulation of adenosine receptors reduces the reinforcing properties of METH and could be a potential pharmacological target. This study examines the effects of adenosine receptor subtype stimulation on METH seeking using both a cue-induced reinstatement and cue-craving model of relapse.

METHODS

Male and female rats were trained to self-administer METH during daily 2-h sessions. Cue-induced reinstatement of METH seeking was evaluated after extinction training. A systemic pretreatment of an adenosine A1 receptor (A1R) or A2A receptor (A2AR) agonist was administered prior to an extinction or cue session to evaluate the effects of adenosine receptor subtype stimulation on METH seeking. The effects of a systemic pretreatment of A1R or A2AR agonists were also evaluated in a cue-craving model where the cued-seeking test was conducted after 21 days of forced home-cage abstinence without extinction training.

RESULTS

Cue-induced reinstatement was reduced in both male and female rats that received A1R or A2AR agonist pretreatments. Similarly, an A1R or A2AR agonist pretreatment also inhibited cue craving in both male and female rats.

CONCLUSION

Stimulation of either adenosine A1R or A2AR subtypes inhibits METH-seeking behavior elicited by METH-associated cues. These effects may be attributed to the ability of A1R and A2AR stimulation to disrupt cue-induced dopamine and glutamate signaling throughout the brain.

The role of metabotropic glutamate receptors in neurobehavioral effects associated with methamphetamine use

Metabotropic glutamate (mGlu) receptors are expressed throughout the central nervous system and act as important regulators of drug-induced neuroplasticity and behavior. Preclinical research suggests that mGlu receptors play a critical role in a spectrum of neural and behavioral consequences arising from methamphetamine (meth) exposure. However, an overview of mGlu-dependent mechanisms linked to neurochemical, synaptic, and behavioral changes produced by meth has been lacking. This chapter provides a comprehensive review of the role of mGlu receptor subtypes (mGlu1-8) in meth-induced neural effects, such as neurotoxicity, as well as meth-associated behaviors, such as psychomotor activation, reward, reinforcement, and meth-seeking. Additionally, evidence linking altered mGlu receptor function to post-meth learning and cognitive deficits is critically evaluated. The chapter also considers the role of receptor-receptor interactions involving mGlu receptors and other neurotransmitter receptors in meth-induced neural and behavioral changes. Taken together, the literature indicates that mGlu5 regulates the neurotoxic effects of meth by attenuating hyperthermia and possibly through altering meth-induced phosphorylation of the dopamine transporter. A cohesive body of work also shows that mGlu5 antagonism (and mGlu2/3 agonism) reduce meth-seeking, though some mGlu5-blocking drugs also attenuate food-seeking. Further, evidence suggests that mGlu5 plays an important role in extinction of meth-seeking behavior. In the context of a history of meth intake, mGlu5 also co-regulates aspects of episodic memory, with mGlu5 stimulation restoring impaired memory. Based on these findings, we propose several avenues for the development of novel pharmacotherapies for Methamphetamine Use Disorder based on the selective modulation mGlu receptor subtype activity.

Adenosine receptor signalling in Alzheimer’s disease

Alzheimer’s disease (AD) is the most common dementia in the elderly and its increasing prevalence presents treatment challenges. Despite a better understanding of the disease, the current mainstay of treatment cannot modify pathogenesis or effectively address the associated cognitive and memory deficits. Emerging evidence suggests adenosine G protein-coupled receptors (GPCRs) are promising therapeutic targets for Alzheimer’s disease. The adenosine A₁ and A_2A receptors are expressed in the human brain and have a proposed involvement in the pathogenesis of dementia. Targeting these receptors preclinically can mitigate pathogenic β-amyloid and tau neurotoxicity whilst improving cognition and memory. In this review, we provide an accessible summary of the literature on Alzheimer’s disease and the therapeutic potential of A₁ and A_2A receptors. Although there are no available medicines targeting these receptors approved for treating dementia, we provide insights into some novel strategies, including allosterism and the targeting of oligomers, which may increase drug discovery success and enhance the therapeutic response.

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.

An investigation of glutamate quantification with PRESS and MEGA-PRESS

Glutamate is an important neurotransmitter. Although many studies have measured glutamate concentration in vivo using magnetic resonance spectroscopy (MRS), researchers have not reached a consensus on the accuracy of glutamate quantification at the field strength of 3 T. Besides, there is not an optimal MRS protocol for glutamate measurement. In this work, both simulation and phantom scans indicate that glutamate can be estimated with reasonable accuracy (<10% error on average) using the standard Point-RESolved Spectroscopy (PRESS) technique with TE 30 ms; glutamine, however, is likely underestimated, which is also suggested by results from human scans using the same protocol. The phantom results show an underestimation of glutamate and glutamine for PRESS with long TE and MEGA-PRESS off-resonance spectra. Despite the underestimation, there is a high correlation between the measured values and the true values (r > 0.8). Our results suggest that the quantification of glutamate and glutamine is reliable but can be off by a scaling factor, depending on the imaging technique. The outputs from all three PRESS sequences (TE = 30, 68 and 80 ms) are also highly correlated with each other (r > 0.7) and moderately correlated (r > 0.5) with the results from the MEGA-PRESS difference spectra with moderate to good shimming (linewidth < 16 Hz).

The role of glutamate mGlu5 and adenosine A2a receptor interactions in regulating working memory performance and persistent cocaine seeking in rats

Cocaine use disorder (CUD) is associated with neurobehavioral deficits that are resistant to current treatments. While craving and high rates of relapse are prominent features of CUD, persistent cognitive impairments are common and linked to poorer treatment outcomes. Here we sought to develop an animal model to study post-cocaine changes in drug seeking and working memory, and to evaluate 'therapeutic' effects of combined glutamate mGlu5 and adenosine A2a receptor blockade. As mGlu5 antagonists reduce drug seeking, and A2a blockade ameliorates working memory impairment, we hypothesized that mGlu5 + A2a antagonist cocktail would reduce both cocaine relapse and post-cocaine working memory deficits. Adult male Sprague-Dawley rats were first trained and tested in an operant delayed match-to-sample (DMS) task to establish the working memory baseline, followed by 6 days of limited and 12 days of extended access cocaine self-administration. Chronic cocaine reduced working memory performance (abstinence day 30-40) and produced robust time-dependent cocaine seeking at 45-, but not 120-days of abstinence. Systemic administration of A2a antagonist KW-6002 (0.125 and 1 mg/kg) failed to rescue post-cocaine working memory deficit. It also failed to reverse working memory impairment produced by mGlu5 NAM MTEP (1 mg/kg). Finally, KW-6002 prevented the ability of MTEP to reduce cocaine seeking and increased locomotor behavior. Thus, despite mGlu5 and A2a being exclusively co-localized in the striatum and showing behavioral synergism towards reducing cocaine effects in some studies, our findings advocate against the use of mGlu5 + A2a antagonist cocktail as it may further compromise cognitive deficits and augment drug craving in CUD.

Adenosine A2AReceptors in Substance Use Disorders: A Focus on Cocaine

Several psychoactive drugs can evoke substance use disorders (SUD) in humans and animals, and these include psychostimulants, opioids, cannabinoids (CB), nicotine, and alcohol. The etiology, mechanistic processes, and the therapeutic options to deal with SUD are not well understood. The common feature of all abused drugs is that they increase dopamine (DA) neurotransmission within the mesocorticolimbic circuitry of the brain followed by the activation of DA receptors. D₂ receptors were proposed as important molecular targets for SUD. The findings showed that D₂ receptors formed heteromeric complexes with other GPCRs, which forced the addiction research area in new directions. In this review, we updated the view on the brain D₂ receptor complexes with adenosine (A)2A receptors (A_2AR) and discussed the role of A_2AR in different aspects of addiction phenotypes in laboratory animal procedures that permit the highly complex syndrome of human drug addiction. We presented the current knowledge on the neurochemical in vivo and ex vivo mechanisms related to cocaine use disorder (CUD) and discussed future research directions for A_2AR heteromeric complexes in SUD.

Allosteric modulation of metabotropic glutamate receptors in alcohol use disorder: Insights from preclinical investigations

Metabotropic glutamate (mGlu) receptors are family C G protein-coupled receptors (GPCRs) that modulate neuronal excitability and synaptic transmission throughout the nervous system. Owing to recent advances in development of subtype-selective allosteric modulators of mGlu receptors, individual members of the mGlu receptor family have been proposed as targets for treating a variety of neurological and psychiatric disorders, including substance use disorders. In this chapter, we highlight preclinical evidence that allosteric modulators of mGlu receptors could be useful for reducing alcohol consumption and preventing relapse in alcohol use disorder (AUD). We begin with an overview of the preclinical models that are used to study mGlu receptor involvement in alcohol-related behaviors. Alcohol exposure causes adaptations in both expression and function of various mGlu receptor subtypes, and pharmacotherapies aimed at reversing these adaptations have the potential to reduce alcohol consumption and seeking. Positive allosteric modulators (PAMs) of mGlu₂ and negative allosteric modulators of mGlu₅ show particular promise for reducing alcohol intake and/or preventing relapse. Finally, this chapter discusses important considerations for translating preclinical findings toward the development of clinically useful drugs, including the potential for PAMs to avoid tolerance issues that are frequently observed with repeated administration of GPCR agonists.

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.

Functional Cross-Talk between Adenosine and Metabotropic Glutamate Receptors

Abstract: G-protein coupled receptors are transmembrane proteins widely expressed in cells and their transduction pathways are mediated by controlling second messenger levels through different G-protein interactions. Many of these receptors have been described as involved in the physiopathology of neurodegenerative diseases and even considered as potential targets for the design of novel therapeutic strategies. Endogenous and synthetic allosteric and orthosteric selective ligands are able to modulate GPCRs at both gene and protein expression levels and can also modify their physiological function. GPCRs that coexist in the same cells can homo- and heteromerize, therefore, modulating their function. Adenosine receptors are GPCRs which stimulate or inhibit adenylyl cyclase activity through Gi/Gs protein and are involved in the control of neurotransmitter release as glutamate. In turn, metabotropic glutamate receptors are also GPCRs which inhibit adenylyl cyclase or stimulate phospholipase C activities through Gi or Gq proteins, respectively. In recent years, evidence of crosstalk mechanisms be-tween different GPCRs have been described. The aim of the present review was to summarize the described mechanisms of interaction and crosstalking between adenosine and metabotropic glutamate receptors, mainly of group I, in both in vitro and in vivo systems, and their possible use for the design of novel ligands for the treatment of neurodegenerative diseases.

Metabotropic Glutamate Receptor Subtype 5 in Alcohol-Induced Negative Affect

Allosteric modulators of metabotropic glutamate 5 receptors (mGlu5 receptors) have been identified as a promising treatment to independently alleviate both negative affective states and ethanol-seeking and intake. However, these conditions are often comorbid and might precipitate one another. Acute and protracted ethanol withdrawal can lead to negative affective states. In turn, these states are primary drivers of alcohol relapse, particularly among women. The current review synthesizes preclinical studies that have observed the role of mGlu5 receptor modulation in negative affective states following ethanol exposure. The primary behavioral assays discussed are ethanol-seeking and intake, development and extinction of ethanol-associated cues and contexts, behavioral despair, and anxiety-like activity. The work done to-date supports mGlu5 receptor modulation as a promising target for mediating negative affective states to reduce ethanol intake or prevent relapse. Limitations in interpreting these data include the lack of models that use alcohol-dependent animals, limited use of adolescent and female subjects, and a lack of comprehensive evaluations of negative affective-like behavior.

Glutamate Receptor-Mediated Neurotoxicity in a Model of Ethanol Dependence and Withdrawal in Rat Organotypic Hippocampal Slice Cultures

Long-term alcohol use can lead to alterations in brain structure and functions and, in some cases, to neurodegeneration. Several mechanisms have been proposed to explain ethanol (EtOH)-related brain injury. One of the most relevant mechanisms of alcohol-induced neurodegeneration involves glutamatergic transmission, but their exact role is not yet fully understood. We investigated the neurochemical mechanisms underlying the toxicity induced by EtOH dependence and/or withdrawal by exposing rat organotypic hippocampal slices to EtOH (100–300 mM) for 7 days and then incubating the slices in EtOH-free medium for the subsequent 24 h. EtOH withdrawal led to a dose-dependent CA1 pyramidal cell injury, as detected with propidium iodide fluorescence. Electron microscopy of hippocampal slices revealed that not only EtOH withdrawal but also 7 days chronic EtOH exposure elicited signs of apoptotic cell death in CA1 pyramidal cells. These data were supported by electrophysiological recordings of spontaneus Excitatory Post Synaptic Currents (sEPSCs) from CA1 pyramidal cells. The average amplitude of sEPSCs in slices treated with EtOH for 7 days was significantly increased, and even more so during the first 30 min of EtOH withdrawal, suggesting that the initial phase of the neurodegenerative process could be due to an excitotoxic mechanism. We then analyzed the expression levels of presynaptic (vGlut1, vGlut2, CB1 receptor, synaptophysin) and postsynaptic (PSD95, GluN1, GluN2A, GluN2B, GluA1, GluA2, mGluR1 and mGluR5) proteins after 7 days EtOH incubation or after EtOH withdrawal. We found that only GluA1 and mGluR5 expression levels were significantly increased after EtOH withdrawal and, in neuroprotection experiments, we observed that AMPA and mGluR5 antagonists attenuated EtOH withdrawal-induced toxicity. These data suggest that chronic EtOH treatment promotes abnormal synaptic transmission that may lead to CA1 pyramidal cell death after EtOH withdrawal through glutamate receptors and increased excitotoxicity.

Role of glutamatergic system and mesocorticolimbic circuits in alcohol dependence

Emerging evidence demonstrates that alcohol dependence is associated with dysregulation of several neurotransmitters. Alterations in dopamine, glutamate and gamma-aminobutyric acid release are linked to chronic alcohol exposure. The effects of alcohol on the glutamatergic system in the mesocorticolimbic areas have been investigated extensively. Several studies have demonstrated dysregulation in the glutamatergic systems in animal models exposed to alcohol. Alcohol exposure can lead to an increase in extracellular glutamate concentrations in mesocorticolimbic brain regions. In addition, alcohol exposure affects the expression and functions of several glutamate receptors and glutamate transporters in these brain regions. In this review, we discussed the effects of alcohol exposure on glutamate receptors, glutamate transporters and glutamate homeostasis in each area of the mesocorticolimbic system. In addition, we discussed the genetic aspect of alcohol associated with glutamate and reward circuitry. We also discussed the potential therapeutic role of glutamate receptors and glutamate transporters in each brain region for the treatment of alcohol dependence. Finally, we provided some limitations on targeting the glutamatergic system for potential therapeutic options for the treatment alcohol use disorders.

Effects of Alcohol Cues on MRS Glutamate Levels in the Anterior Cingulate

Growing evidence suggests that glutamate neurotransmission plays a critical role in alcohol addiction. Cue-induced change of glutamate has been observed in animal studies but never been investigated in humans. This work investigates cue-induced change in forebrain glutamate in individuals with alcohol use disorder (AUD). A total of 35 subjects (17 individuals with AUD and 18 healthy controls) participated in this study. The glutamate concentration was measured with single-voxel 1H-MR spectroscopy at the dorsal anterior cingulate. Two MRS sessions were performed in succession, the first to establish basal glutamate levels and the second to measure the change in response to alcohol cues. The changes in glutamate were quantified for both AUD subjects and controls. A mixed model ANOVA and t-tests were performed for statistical analysis. ANOVA revealed a main effect of cue-induced decrease of glutamate level in the anterior cingulate cortex (ACC). A significant interaction revealed that only AUD subjects showed significant decrease of glutamate in the ACC. There were no significant group differences in the level of basal glutamate. However, a negative correlation was found between the basal glutamate level and the number of drinking days in the past 2 weeks for the AUD subjects. Collectively, our results indicate that glutamate in key areas of the forebrain reward circuit is modulated by alcohol cues in early alcohol dependence.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Hurdles in Basic Science Translation

In the past century there have been incredible advances in the field of medical research, but what hinders translation of this knowledge into effective treatment for human disease? There is an increasing focus on the failure of many research breakthroughs to be translated through the clinical trial process and into medical practice. In this mini review, we will consider some of the reasons that findings in basic medical research fail to become translated through clinical trials and into basic medical practices. We focus in particular on the way that human disease is modeled, the understanding we have of how our targets behave in vivo, and also some of the issues surrounding reproducibility of basic research findings. We will also look at some of the ways that have been proposed for overcoming these issues. It appears that there needs to be a cultural shift in the way we fund, publish and recognize quality control in scientific research. Although this is a daunting proposition, we hope that with increasing awareness and focus on research translation and the hurdles that impede it, the field of medical research will continue to inform and improve medical practice across the world.

Glutamate plasticity woven through the progression to alcohol use disorder: a multi-circuit perspective

Glutamate signaling in the brain is one of the most studied targets in the alcohol research field. Here, we report the current understanding of how the excitatory neurotransmitter glutamate, its receptors, and its transporters are involved in low, episodic, and heavy alcohol use. Specific animal behavior protocols can be used to assess these different drinking levels, including two-bottle choice, operant self-administration, drinking in the dark, the alcohol deprivation effect, intermittent access to alcohol, and chronic intermittent ethanol vapor inhalation. Importantly, these methods are not limited to a specific category, since they can be interchanged to assess different states in the development from low to heavy drinking. We encourage a circuit-based perspective beyond the classic mesolimbic-centric view, as multiple structures are dynamically engaged during the transition from positive- to negative-related reinforcement to drive alcohol drinking. During this shift from lower-level alcohol drinking to heavy alcohol use, there appears to be a shift from metabotropic glutamate receptor-dependent behaviors to N-methyl-D-aspartate receptor-related processes. Despite high efficacy of the glutamate-related pharmaceutical acamprosate in animal models of drinking, it is ineffective as treatment in the clinic. Therefore, research needs to focus on other promising glutamatergic compounds to reduce heavy drinking or mediate withdrawal symptoms or both.

Metabotropic and ionotropic glutamate receptors as potential targets for the treatment of alcohol use disorder

Emerging evidence indicates that dysfunctional glutamate neurotransmission is critical in the initiation and development of alcohol and drug dependence. Alcohol consumption induced downregulation of glutamate transporter 1 (GLT-1) as reported in previous studies from our laboratory. Glutamate is the major excitatory neurotransmitter in the brain, which acts via interactions with several glutamate receptors. Alcohol consumption interferes with the glutamatergic signal transmission by altering the functions of these receptors. Among the glutamate receptors involved in alcohol-drinking behavior are the metabotropic receptors such as mGluR1/5, mGluR2/3, and mGluR7, as well as the ionotropic receptors, NMDA and AMPA. Preclinical studies using agonists and antagonists implicate these glutamatergic receptors in the development of alcohol use disorder (AUD). Therefore, the purpose of this review is to discuss the neurocircuitry involving glutamate transmission in animals exposed to alcohol and further outline the role of metabotropic and ionotropic receptors in the regulation of alcohol-drinking behavior. This review provides ample information about the potential therapeutic role of glutamatergic receptors for the treatment of AUD.

Nucleus incertus Orexin2 receptors mediate alcohol seeking in rats

Alcoholism is a chronic relapsing disorder and a major global health problem. Stress is a key precipitant of relapse in human alcoholics and in animal models of alcohol seeking. The brainstem nucleus incertus (NI) contains a population of relaxin-3 neurons that are highly responsive to psychological stressors; and the ascending NI relaxin-3/RXFP3 signalling system is implicated in stress-induced reinstatement of alcohol seeking. The NI receives orexinergic innervation and expresses orexin1 (OX1) and orexin2 (OX2) receptor mRNA. In alcohol-preferring (iP) rats, we examined the impact of yohimbine-induced reinstatement of alcohol seeking on orexin neuronal activation, and the effect of bilateral injections into NI of the OX1 receptor antagonist, SB-334867 (n = 16) or the OX2 receptor antagonist, TCS-OX2-29 (n = 8) on stress-induced reinstatement of alcohol seeking. We also assessed the effects of orexin-A on NI neuronal activity and the involvement of OX1 and OX2 receptors using whole cell patch-clamp recordings in rat brain slices. Yohimbine-induced reinstatement of alcohol seeking activated orexin neurons. Bilateral NI injections of TCS-OX2-29 attenuated yohimbine-induced reinstatement of alcohol seeking. In contrast, intra-NI injection of SB-334867 had no significant effect. In line with these data, orexin-A (600 nM) depolarized a majority of NI neurons recorded in coronal brain slices (18/28 cells), effects prevented by bath application of TCS-OX2-29 (10 μM), but not SB-334867 (10 μM). These data suggest an excitatory orexinergic input to NI contributes to yohimbine-induced reinstatement of alcohol seeking, predominantly via OX2 receptor signalling.

A critical role of striatal A2A R-mGlu5 R interactions in modulating the psychomotor and drug-seeking effects of methamphetamine

Addiction to psychostimulants is a major public health problem with no available treatment. Adenosine A2A receptors (A2A R) co-localize with metabotropic glutamate 5 receptors (mGlu5 R) in the striatum and functionally interact to modulate behaviours induced by addictive substances, such as alcohol. Using genetic and pharmacological antagonism of A2A R in mice, we investigated whether A2A R-mGlu5 R interaction can regulate the locomotor, stereotypic and drug-seeking effect of methamphetamine and cocaine, two drugs that exhibit distinct mechanism of action. Genetic deletion of A2A R, as well as combined administration of sub-threshold doses of the selective A2A R antagonist (SCH 58261, 0.01 mg/kg, i.p.) with the mGlu5 R antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (0.01 mg/kg, i.p.), prevented methamphetamine- but not cocaine-induced hyperactivity and stereotypic rearing behaviour. This drug combination also prevented methamphetamine-rewarding effects in a conditioned-place preference paradigm. Moreover, mGlu5 R binding was reduced in the nucleus accumbens core of A2A R knockout (KO) mice supporting an interaction between these receptors in a brain region crucial in mediating addiction processes. Chronic methamphetamine, but not cocaine administration, resulted in a significant increase in striatal mGlu5 R binding in wild-type mice, which was absent in the A2A R KO mice. These data are in support of a critical role of striatal A2A R-mGlu5 R functional interaction in mediating the ambulatory, stereotypic and reinforcing effects of methamphetamine but not cocaine-induced hyperlocomotion or stereotypy. The present study highlights a distinct and selective mechanistic role for this receptor interaction in regulating methamphetamine-induced behaviours and suggests that combined antagonism of A2A R and mGlu5 R may represent a novel therapy for methamphetamine addiction.

Adenosine 2A receptors modulate reward behaviours for methamphetamine

Addiction to methamphetamine (METH) is a global health problem for which there are no approved pharmacotherapies. The adenosine 2A (A2 A ) receptor presents a potential therapeutic target for METH abuse due to its modulatory effects on striatal dopamine and glutamate transmission. Notably, A2 A receptor signalling has been implicated in the rewarding effects of alcohol, cocaine and opiates; yet, the role of this receptor in METH consumption and seeking is essentially unknown. Therefore, the current study used A2 A knockout (KO) mice to assess the role of A2 A in behaviours relevant to METH addiction. METH conditioned place preference was absent in A2 A KO mice compared with wild-type (WT) littermates. Repeated METH treatment produced locomotor sensitization in both genotypes; however, sensitization was attenuated in A2 A KO mice in a dose-related manner. METH intravenous self-administration was intact in A2 A KO mice over a range of doses and schedules of reinforcement. However, the motivation to self-administer was reduced in A2 A KO mice. Regression analysis further supported the observation that the motivation to self-administer METH was reduced in A2 A KO mice even when self-administration was similar to WT mice. Sucrose self-administration was also reduced in A2 A KO mice but only at higher schedules of reinforcement. Collectively, these data suggest that A2 A signalling is critically required to integrate rewarding and motivational properties of both METH and natural rewards.

The metabotropic glutamate 5 receptor is necessary for extinction of cocaine-associated cues

BACKGROUND AND PURPOSE

There is currently no medication approved specifically to treat cocaine addiction. Behavioural interventions such as cue exposure therapy (CET) rely heavily on new learning. Antagonism of the metabotropic glutamate 5 (mGlu5 ) receptor has emerged as a potential treatment, by reducing the reinforcing properties of cocaine. However, mGlu5 receptor activity is necessary for learning; therefore, such agents could interfere with behavioural treatments. We used a novel rodent model of CET to test the effects of mGlu5 negative and positive allosteric modulators (NAM and PAM) on behavioural therapy.

EXPERIMENTAL APPROACH

Rats were trained to press a lever for cocaine in the presence of a discrete cue [conditioned stimulus (CS)] and then extinguished in the absence of the CS. Following lever extinction, half the rats received CS extinction in the same chambers but with the levers withdrawn; the remaining rats received no CS extinction. Before this session, rats received a systemic administration of either vehicle or a mGlu5 NAM (MTEP, experiment 1) or PAM (CDPPB, experiment 2). Cue-induced reinstatement was tested in a drug-free session the following day.

KEY RESULTS

At reinstatement, rats that had received CS extinction showed reduced responding. This effect was attenuated by MTEP treatment before CS extinction. In contrast, administration of CDPPB (PAM) led to decreased reinstatement the following day, regardless of extinction condition.

CONCLUSION AND IMPLICATIONS

These results suggest that mGlu5 receptor activity is both necessary and sufficient for efficient extinction of a cocaine-associated CS. Therefore, mGlu5 PAMs could enhance the efficacy of CET.

Rodent models and mechanisms of voluntary binge-like ethanol consumption: Examples, opportunities, and strategies for preclinical research

Binge ethanol consumption has widespread negative consequences for global public health. Rodent models offer exceptional power to explore the neurobiology underlying and affected by binge-like drinking as well as target potential prevention, intervention, and treatment strategies. An important characteristic of these models is their ability to consistently produce pharmacologically-relevant blood ethanol concentration. This review examines the current available rodent models of voluntary, pre-dependent binge-like ethanol consumption and their utility in various research strategies. Studies have demonstrated that a diverse array of neurotransmitters regulate binge-like drinking, resembling some findings from other drinking models. Furthermore, repeated binge-like drinking recruits neuroadaptive mechanisms in mesolimbocortical reward circuitry. New opportunities that these models offer in the current context of mechanistic research are also discussed.

Negative Allosteric Modulators of Metabotropic Glutamate Receptors Subtype 5 in Addiction: a Therapeutic Window

BACKGROUND

Abundant evidence at the anatomical, electrophysiological, and molecular levels implicates metabotropic glutamate receptor subtype 5 (mGluR5) in addiction. Consistently, the effects of a wide range of doses of different mGluR5 negative allosteric modulators (NAMs) have been tested in various animal models of addiction. Here, these studies were subjected to a systematic review to find out if mGluR5 NAMs have a therapeutic potential that can be translated to the clinic.

METHODS

Literature on consumption/self-administration and reinstatement of drug seeking as outcomes of interest published up to April 2015 was retrieved via PubMed. The review focused on the effects of systemic (i.p., i.v., s.c.) administration of the mGluR5 NAMs 3-((2-Methyl-4-thiazolyl)ethynyl)pyridine (MTEP) and 2-Methyl-6-(phenylethynyl)pyridine (MPEP) on paradigms with cocaine, ethanol, nicotine, and food in rats.

RESULTS

MTEP and MPEP were found to reduce self-administration of cocaine, ethanol, and nicotine at doses ≥1mg/kg and 2.5mg/kg, respectively. Dose-response relationship resembled a sigmoidal curve, with low doses not reaching statistical significance and high doses reliably inhibiting self-administration of drugs of abuse. Importantly, self-administration of cocaine, ethanol, and nicotine, but not food, was reduced by MTEP and MPEP in the dose range of 1 to 2mg/kg and 2.5 to 3.2mg/kg, respectively. This dose range corresponds to approximately 50% to 80% mGluR5 occupancy. Interestingly, the limited data found in mice and monkeys showed a similar therapeutic window.

CONCLUSION

Altogether, this review suggests a therapeutic window for mGluR5 NAMs that can be translated to the treatment of substance-related and addictive disorders.

Adenosinergic regulation of binge-like ethanol drinking and associated locomotor effects in male C57BL/6J mice

We recently observed that the addition of caffeine (a nonselective adenosine receptor antagonist) to a 20% ethanol solution significantly altered the intoxication profile of male C57BL/6J (B6) mice induced by voluntary binge-like consumption in the 'Drinking-in-the-Dark' (DID) paradigm. In the current study, the roles of A1 and A2A adenosine receptor subtypes, specifically, in binge-like ethanol consumption and associated locomotor effects were explored. Adult male B6 mice (PND 60-70) were allowed to consume 20% ethanol (v/v) or 2% sucrose (w/v) for 6days via DID. On day 7, mice received a systemic administration (i.p.) of the A1 antagonist DPCPX (1, 3, 6mg/kg), the A2A antagonist MSX-3 (1, 2, 4mg/kg), or vehicle immediately prior to fluid access in DID. Antagonism of the A1 receptor via DPCPX was found to dose-dependently decrease binge-like ethanol intake and associated blood ethanol concentrations (p's<0.05), although no effect was observed on sucrose intake. Antagonism of A2A had no effect on ethanol or sucrose consumption, however, MSX-3 elicited robust locomotor stimulation in mice consuming either solution (p's<0.05). Together, these findings suggest unique roles for the A1 and A2A adenosine receptor subtypes in binge-like ethanol intake and its associated locomotor effects.

Habitual alcohol seeking: modeling the transition from casual drinking to addiction

The transition from goal-directed actions to habitual ethanol seeking models the development of addictive behavior that characterizes alcohol use disorders. The progression to habitual ethanol-seeking behavior occurs more rapidly than for natural rewards, suggesting that ethanol may act on habit circuit to drive the loss of behavioral flexibility. This review will highlight recent research that has focused on the formation and expression of habitual ethanol seeking, and the commonalities and distinctions between ethanol and natural reward-seeking habits, with the goal of highlighting important, understudied research areas that we believe will lead toward the development of novel treatment and prevention strategies for uncontrolled drinking.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.

Functional characterization of G-protein-coupled receptors: a bioinformatics approach

Complex molecular and cellular mechanisms regulate G protein-coupled receptors (GPCRs). It is suggested that proteins intrinsically disordered regions (IDRs) are to play a role in GPCR's intra and extracellular regions plasticity, due to their potential for post-translational modification and interaction with other proteins. These regions are defined as lacking a stable three-dimensional (3D) structure. They are rich in hydrophilic and charged, amino acids and are capable to assume different conformations which allow them to interact with multiple partners. In this study we analyzed 75 GPCR involved in synaptic transmission using computational tools for sequence-based prediction of IDRs within a protein. We also evaluated putative ligand-binding motifs using receptor sequences. The disorder analysis indicated that neurotransmitter GPCRs have a significant amount of disorder in their N-terminus, third intracellular loop (3IL) and C-terminus. About 31%, 39% and 53% of human GPCR involved in synaptic transmission are disordered in these regions. Thirty-three percent of receptors show at least one predicted PEST motif, this being statistically greater than the estimate for the rest of human GPCRs. About 90% of the receptors had at least one putative site for dimerization in their 3IL or C-terminus. ELM instances sampled in these domains were 14-3-3, SH3, SH2 and PDZ motifs. In conclusion, the increased flexibility observed in GPCRs, added to the enrichment of linear motifs, PEST and heteromerization sites, may be critical for the nervous system's functional plasticity.

Effectiveness of Ceftriaxone Treatment in Preventing Relapse-like Drinking Behavior Following Long-term Ethanol Dependence in P Rats

Objective

To evaluate the effectiveness of ceftriaxone treatment in attenuating relapse-like ethanol drinking behavior in male P rats following 14-weeks of continuous ethanol consumption.

Methods

After 14-weeks of continuous access to free choice of 15% and 30% ethanol, male P rats were deprived of ethanol for two weeks. On the last five days of abstinence period, P rats were treated, once a day, with either saline or ceftriaxone (50 or 200 mg/kg; i.p.). This was followed by re-exposure to ethanol for the next 10 days to simulate the relapse-like ethanol drinking behavior.

Results

Ceftriaxone treatment (during abstinence) reduced ethanol intake upon re-exposure to ethanol, compared to the saline treated P rats. This statistically significant reduction in ethanol consumption in P rats following treatment with ceftriaxone (200 mg/kg/day) was observed from Day 2 to Day 9. Similarly, water consumption in P rats treated with ceftriaxone was significantly higher than the saline treated group between Day 2 and Day 7. Importantly, ceftriaxone treatment at both doses did not cause any significant changes in body weight compared to saline treated group.

Conclusions

We report here that ceftriaxone at higher dose has been found to be effective in the attenuation of relapse-like ethanol-drinking behavior in chronic ethanol intake model. This is in accordance with previous data from our lab in cocaine animal model demonstrating that only higher dose of ceftriaxone has been effective in attenuating cocaine relapse.

Interaction of adenosine receptors with other receptors from therapeutic perspective in Parkinson's disease

Altered dopaminergic neurotransmission in the basal ganglia is observed in Parkinson's disease (PD) and L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias (LID). An attractive alternative for treating LID is to use adjunct drugs to modulate nondopaminergic neurotransmitter systems in the basal ganglia. For example, adenosine receptors have received attention over the past years for the treatment of PD and LID. Adenosine interacts closely with dopamine and plays an important role in the function of striatal GABAergic efferent neurons. Excitatory glutamatergic neurotransmission is also modulated by adenosine in the striatum. Hence, based on the unique cellular and regional distribution of this system, adenosine neurotransmission could have an important implication for the development of new therapeutic strategies targeting the basal ganglia disorders. Indeed, A2A adenosine receptor antagonists were shown to improve motor deficits in PD and to reduce the severity of LID. A2A receptor subtypes are selectively found on striatopallidal neurons and can couple with receptors of interest in PD, such as D2 dopamine and metabotropic glutamate receptor type 5 (mGlu5) receptors, and form functional heteromeric complexes. This chapter will review relevant studies investigating the role and contribution of adenosine receptor subtypes in pathophysiology of PD and LID. The interactions of adenosine receptors, especially A1 and A2A receptor subtypes, with other receptors implicated in the pathophysiology of PD and LID such as dopaminergic and glutamatergic receptors will be reviewed. The implication of these interactions in the development and expression of PD symptoms and LID needs further investigation to find novel drug targets.

Glutamatergic targets for new alcohol medications

RATIONALE

An increasingly compelling literature points to a major role for the glutamate system in mediating the effects of alcohol on behavior and the pathophysiology of alcoholism. Preclinical studies indicate that glutamate signaling mediates certain aspects of ethanol's intoxicating and rewarding effects, and undergoes adaptations following chronic alcohol exposure that may contribute to the withdrawal, craving and compulsive drug-seeking that drive alcohol abuse and alcoholism.

OBJECTIVES

We discuss the potential for targeting the glutamate system as a novel pharmacotherapeutic approach to treating alcohol use disorders, focusing on five major components of the glutamate system: the N-methyl-D-aspartate (NMDA) receptor and specific NMDA subunits, the glycineB site on the NMDA receptors (NMDAR), L-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid ionotropic (AMPA) and kainate (KAR) receptors, metabotropic receptors (mGluR), and glutamate transporters.

RESULTS

Chronic alcohol abuse produces a hyperglutamatergic state, characterized by elevated extracellular glutamate and altered glutamate receptors and transporters. Pharmacologically manipulating glutamatergic neurotransmission alters alcohol-related behaviors including intoxication, withdrawal, and alcohol-seeking, in rodents and human subjects. Blocking NMDA and AMPA receptors reduces alcohol consumption in rodents, but side-effects may limit this as a therapeutic approach. Selectively targeting NMDA and AMPA receptor subunits (e.g., GluN2B, GluA3), or the NMDAR glycineB site offers an alternative approach. Blocking mGluR5 potently affects various alcohol-related behaviors in rodents, and mGluR2/3 agonism also suppresses alcohol consumption. Finally, glutamate transporter upregulation may mitigate behavioral and neurotoxic sequelae of excess glutamate caused by alcohol.

CONCLUSIONS

Despite the many challenges that remain, targeting the glutamate system offers genuine promise for developing new treatments for alcoholism.

The adenosine A2A receptor agonist CGS 21680 decreases ethanol self-administration in both non-dependent and dependent animals

There is emerging evidence that the adenosinergic system might be involved in drug addiction and alcohol dependence. We have already demonstrated the involvement of A2A receptors (A2AR) in ethanol-related behaviours in mice. Here, we investigated whether the A2AR agonist CGS 21680 can reduce ethanol operant self-administration in both non-dependent and ethanol-dependent Wistar rats. To rule out a potential involvement of the A1R in the effects of CGS 21680, we also tested its effectiveness to reduce ethanol operant self-administration in both heterozygous and homozygous A1R knockout mice. Our results demonstrated that CGS 21680 (0.065, 0.095 and 0.125 mg/kg, i.p.) had a bimodal effect on 10% ethanol operant self-administration in non-dependent rats. The intermediate dose was also effective in reducing 2% sucrose self-administration. Interestingly, the intermediate dose reduced 10% ethanol self-administration in dependent animals more effectively (75% decrease) when compared with non-dependent animals (57% decrease). These results suggest that the A2AR are involved in CGS 21680 effects since the reduction of ethanol self-administration was not dependent upon the presence of A1R in mice. In conclusion, our findings demonstrated the effectiveness of the A2AR agonist CGS 21680 in a preclinical model of alcohol addiction and suggested that the adenosinergic pathway is a promising target to treat alcohol 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.

Current perspectives on the neurobiology of drug addiction: a focus on genetics and factors regulating gene expression

Drug addiction is a chronic, relapsing disorder defined by cyclic patterns of compulsive drug seeking and taking interspersed with episodes of abstinence. While genetic variability may increase the risk of addictive behaviours in an individual, exposure to a drug results in neuroadaptations in interconnected brain circuits which, in susceptible individuals, are believed to underlie the transition to, and maintenance of, an addicted state. These adaptations can occur at the cellular, molecular, or (epi)genetic level and are associated with synaptic plasticity and altered gene expression, the latter being mediated via both factors affecting translation (epigenetics) and transcription (non coding microRNAs) of the DNA or RNA itself. New advances using techniques such as optogenetics have the potential to increase our understanding of the microcircuitry mediating addictive behaviours. However, the processes leading to addiction are complex and multifactorial and thus we face a major contemporary challenge to elucidate the factors implicated in the development and maintenance of an addicted state.

Adenosine A2A receptor in the monkey basal ganglia: ultrastructural localization and colocalization with the metabotropic glutamate receptor 5 in the striatum

The adenosine A(2A) receptor (A(2A) R) is a potential drug target for the treatment of Parkinson's disease and other neurological disorders. In rodents, the therapeutic efficacy of A(2A) R modulation is improved by concomitant modulation of the metabotropic glutamate receptor 5 (mGluR5). To elucidate the anatomical substrate(s) through which these therapeutic benefits could be mediated, pre-embedding electron microscopy immunohistochemistry was used to conduct a detailed, quantitative ultrastructural analysis of A(2A) R localization in the primate basal ganglia and to assess the degree of A(2A) R/mGluR5 colocalization in the striatum. A(2A) R immunoreactivity was found at the highest levels in the striatum and external globus pallidus (GPe). However, the monkey, but not the rat, substantia nigra pars reticulata (SNr) also harbored a significant level of neuropil A(2A) R immunoreactivity. At the electron microscopic level, striatal A(2A) R labeling was most commonly localized in postsynaptic elements (58% ± 3% of labeled elements), whereas, in the GPe and SNr, the labeling was mainly presynaptic (71% ± 5%) or glial (27% ± 6%). In both striatal and pallidal structures, putative inhibitory and excitatory terminals displayed A(2A) R immunoreactivity. Striatal A(2A) R/mGluR5 colocalization was commonly found; 60-70% of A(2A) R-immunoreactive dendrites or spines in the monkey striatum coexpress mGluR5. These findings provide the first detailed account of the ultrastructural localization of A(2A) R in the primate basal ganglia and demonstrate that A(2A) R and mGluR5 are located to interact functionally in dendrites and spines of striatal neurons. Together, these data foster a deeper understanding of the substrates through which A(2A) R could regulate primate basal ganglia function and potentially mediate its therapeutic effects in parkinsonism.

mGlu5 and adenosine A2A receptor interactions regulate the conditioned effects of cocaine

Adenosine A2A receptors and metabotropic glutamate type 5 (mGlu5) receptors are co-localized in the striatum and can functionally interact to regulate drug-seeking. We further explored this interaction using antagonism of mGlu5 receptors with 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) in combination with genetic deletion of A2A receptors. The conditioned rewarding and locomotor-activating properties of cocaine were evaluated via conditioned place preference (CPP). Vehicle-treated mice of both genotypes expressed a CPP to cocaine while MTEP abolished cocaine CPP in wild-type, but not A2A knockout, mice. These results were mirrored when conditioned hyperactivity was assessed. In contrast, MTEP attenuated the acute locomotor-activating properties of cocaine similarly in both genotypes. These data provide evidence for a functional interaction between adenosine A2A and mGlu5 receptors in mediating the conditioned effects of cocaine but not direct cocaine-induced hyperactivity. This functional interaction is supported by modulation of 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolol[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol ([125I]ZM241385) binding to the A2A receptor by MTEP.

mGlu5 Receptor Functional Interactions and Addiction

The idea of “receptor mosaics” is that proteins may form complex and dynamic networks with respect to time and composition. These have the potential to markedly expand the diversity and specificity of G protein-coupled receptors (GPCR) signaling, particularly in neural cells, where a few key receptors have been implicated in many neurological and psychiatric disorders, including addiction. Metabotropic glutamate type 5 receptors (mGlu5) can form complexes with other GPCRs, including adenosine A_2A and dopamine D₂ receptors. mGlu5-containing complexes have been reported in the striatum, a brain region critical for mediating the rewarding and incentive motivational properties of drugs of abuse. mGlu5-containing complexes and/or downstream interactions between divergent receptors may play roles in addiction–relevant behaviors. Interactions between mGlu5 receptors and other GPCRs can regulate the rewarding and conditioned effects of drugs as well as drug-seeking behaviors. mGlu5 complexes may influence striatal function, including GABAergic output of striatopallidal neurons and glutamatergic input from corticostriatal afferents. Given their discrete localization, mGlu5-[non-mGlu5] receptor interactions and/or mGlu5-containing complexes may minimize off-target effects and thus provide a novel avenue for drug discovery. The therapeutic targeting of receptor–receptor functional interactions and/or receptor mosaics in a tissue specific or temporal manner (for example, a sub-population of receptors in a “pathological state”) might reduce detrimental side effects that may otherwise impair vital brain functions.

Adenosinergic system: an assorted approach to therapeutics for drug addiction

Adenosine is an endogenous purine nucleoside and it is extensively present in the brain. It exerts several metabolic and neuromodulatory roles in the body. Adenosine also acts as an important messenger molecule for extracellular signaling and shows a homeostatic neuromodulatory function at the synaptic level. Extracellular adenosine exerts a wide variety of biological actions through four cell surface G-protein-coupled receptor subtypes, namely A1, A2A, A2B and A3 adenosine receptors. The extracellular levels of adenosine have been found to be enhanced in several neuropathological conditions, including drug addiction, and thus a neuroprotective role of adenosine was perceived by various experimental studies. The aversive withdrawal symptoms emanating from drug discontinuation provokes rebound drug intake patterns. In addition, alteration of neurotransmitter(s) release and changes in receptor expression contribute to the behavioral changes of drug withdrawal. Furthermore, the abuse of major drugs such as alcohol and opioids are reported to modulate extracellular adenosine levels. In this context, the neuromodulatory functions of adenosine would be valuable if projected to the clinical applications and thus, an increasing attention is currently given to the functional role of adenosine in human addictive disorders. This review will focus on recent clinical and experimental studies that reveal the actions of adenosine and related ligands in drug addiction and various drug-withdrawal syndromes. The evidence and reports provided in this review highlight the looming therapeutic potential of purinergic drugs, with a hope that new therapeutic interventions based on the adenosinergic concept will emerge in the coming years for the management of drug withdrawal syndrome.

Effects of A₂A adenosine receptor blockade or stimulation on alcohol intake in alcohol-preferring rats

RATIONALE

A(2A) adenosine receptors (A(2A)ARs) have been proposed to be involved in drug addiction; however, preclinical studies about the effects of A(2A)AR ligands on alcohol consumption have provided inconsistent results.

OBJECTIVES

The present study evaluated the effect of intraperitoneal injections of the A(2A)AR antagonist ANR 94, and the A(2A)AR agonists CGS 21680 and VT 7 on voluntary drinking and operant self-administration of 10% ethanol in Marchigian Sardinian alcohol-preferring (msP) rats.

RESULTS

Voluntary ethanol drinking was increased by ANR 94 in acute and subchronic experiments, while it was reduced by A(2A)AR agonists. The effect of CGS 21680 was abolished by a low dose of ANR 94, confirming its mediation by A(2A)ARs. Ethanol self-administration was reduced by CGS 21680 and VT 7, while ANR 94 slightly but significantly increased it. Blood alcohol levels were not modified by A(2A)AR agonists, indicating that their effect is not related to ethanol pharmacokinetics. The effect of VT 7 on ethanol drinking was behaviourally selective; ethanol and food intake were reduced, but water intake was increased, and total fluid intake was not different from that of controls. Moreover, VT 7 did not affect locomotor activity. CGS 21680 (0.1 mg/kg) did not modify total fluid intake, but 0.2 and 0.3 mg/kg reduced total fluid intake and locomotor activity.

CONCLUSION

These results provide evidence that A(2A)AR agonists reduce ethanol consumption in msP rats, which represent an animal model of alcohol abuse related to stress, anxiety and depression. A(2A)ARs may represent a potential target for treatment of alcohol abuse.

mGluR5 receptors in the basolateral amygdala and nucleus accumbens regulate cue-induced reinstatement of ethanol-seeking behavior

Pharmacological blockade of the type 5 metabotropic glutamate receptor (mGluR5) attenuates cue-induced reinstatement of ethanol-seeking behavior, yet the brain regions involved in these effects are not yet known. The purpose of the present study was to determine if local blockade of mGluR5 receptors in the basolateral amygdala (BLA) and/or the nucleus accumbens (NAc), two brain regions known to be involved in stimulus-reward associations, attenuate the reinstatement of ethanol-seeking behavior induced by ethanol-paired cues. As a control for possible non-specific effects, the effects of mGluR5 blockade in these regions on cue-induced reinstatement of sucrose-seeking were also assessed. Male Wistar rats were implanted with bilateral microinjection cannulae aimed at the BLA or NAc. Following recovery, animals were trained to self-administer ethanol (10% w/v) or 45 mg sucrose pellets on an FR1 schedule of reinforcement in 30 min daily sessions using a sucrose fading procedure. Following stabilization of responding, animals underwent extinction training. Next, animals received infusions of vehicle or the selective mGluR5 antagonist MTEP (3 μg/μl) into the BLA or NAc prior to cue-induced reinstatement testing sessions. mGluR5 blockade eliminated cue-induced reinstatement of alcohol - but not sucrose-seeking behavior. Results from this study indicate that mGluR5 receptors in the BLA and NAc mediate cue-induced reinstatement of ethanol-seeking behavior, and provide two potential neuroanatomical sites of action where systemically administered mGluR5 antagonists attenuate cue-induced reinstatement. These data are consistent with previous findings that cue-induced reinstatement of ethanol-seeking increases neuronal activity and glutamatergic transmission in these two regions.

Differential Modulation of Thresholds for Intracranial Self-Stimulation by mGlu5 Positive and Negative Allosteric Modulators: Implications for Effects on Drug Self-Administration

Pharmacological manipulation of the type 5 metabotropic glutamate (mGlu5) receptor alters various addiction related behaviors such as drug self-administration and the extinction and reinstatement of drug-seeking behavior. However, the effects of pharmacological modulation of mGlu5 receptors on brain reward function have not been widely investigated. We examined the effects of acute administration of positive and negative allosteric modulators (PAMs and NAMs, respectively) on brain reward function by assessing thresholds for intracranial self-stimulation (ICSS). In addition, when acute effects were observed, we examined changes in ICSS thresholds following repeated administration. Male Sprague-Dawley rats were implanted with bipolar electrodes into the medial forebrain bundle and trained to respond for ICSS, followed by assessment of effects of mGlu5 ligands on ICSS thresholds using a discrete trials current–intensity threshold determination procedure. Acute administration of the selective mGlu5 NAMs MTEP (0, 0.3, 1, or 3 mg/kg) and fenobam (0, 3, 10, or 30 mg/kg) dose-dependently increased ICSS thresholds (∼70% at the highest dose tested), suggesting a deficit in brain reward function. Acute administration of the mGlu5 PAMs CDPPB (0, 10, 30, and 60 mg/kg) or ADX47273 (0, 10, 30, and 60 mg/kg) was without effect at any dose tested. When administered once daily for five consecutive days, the development of tolerance to the ability of threshold-elevating doses of MTEP and fenobam to increase ICSS thresholds was observed. We conclude that mGlu5 PAMs and NAMs differentially affect brain reward function, and that tolerance to the ability of mGlu5 NAMs to reduce brain reward function develops with repeated administration. These brain reward deficits should be taken into consideration when interpreting acute effects of mGlu5 NAMs on drug self-administration, and repeated administration of these ligands may be an effective method to reduce these deficits.