Selective attenuation of psychostimulant-induced behavioral responses in mice lacking A(2A) adenosine receptors

Adenosine A2A and dopamine D2 receptor interaction controls fatigue resistance

Introduction: Caffeine and the selective A2A receptor antagonist SCH58261 both have ergogenic properties, effectively reducing fatigue and enhancing exercise capacity. This study investigates in male Swiss mice the interaction between adenosine A2A receptors and dopamine D2 receptors controlling central fatigue, with a focus on the striatum where these receptors are most abundant. Methods: We employed DPCPX and SCH58261 to antagonize A1 and A2A receptors, caffeine as a non-competitive antagonist for both receptors, and haloperidol as a D2 receptor antagonist; all compounds were tested upon systemic application and caffeine and SCH58261 were also directly applied in the striatum. Behavioral assessments using the open field, grip strength, and treadmill tests allowed estimating the effect of treatments on fatigue. Results and discussion: The results suggested a complex interplay between the dopamine and adenosine systems. While systemic DPCPX had little effect on motor performance or fatigue, the application of either caffeine or SCH58261 was ergogenic, and these effects were attenuated by haloperidol. The intra-striatal administration of caffeine or SCH58261 was also ergogenic, but these effects were unaffected by haloperidol. These findings confirm a role of striatal A2A receptors in the control of central fatigue but suggest that the D2 receptor-mediated control of the ergogenic effects of caffeine and of A2A receptor antagonists might occur outside the striatum. This prompts the need of additional efforts to unveil the role of different brain regions in the control of fatigue.

Ketogenic Diet Decreases Alcohol Intake in Adult Male Mice

The classic ketogenic diet is a diet high in fat, low in carbohydrates, and well-adjusted proteins. The reduction in glucose levels induces changes in the body’s metabolism, since the main energy source happens to be ketone bodies. Recent studies have suggested that nutritional interventions may modulate drug addiction. The present work aimed to study the potential effects of a classic ketogenic diet in modulating alcohol consumption and its rewarding effects. Two groups of adult male mice were employed in this study, one exposed to a standard diet (SD, n = 15) and the other to a ketogenic diet (KD, n = 16). When a ketotic state was stable for 7 days, animals were exposed to the oral self-administration paradigm to evaluate the reinforcing and motivating effects of ethanol. Rt-PCR analyses were performed evaluating dopamine, adenosine, CB1, and Oprm gene expression. Our results showed that animals in a ketotic state displayed an overall decrease in ethanol consumption without changes in their motivation to drink. Gene expression analyses point to several alterations in the dopamine, adenosine, and cannabinoid systems. Our results suggest that nutritional interventions may be a useful complementary tool in treating alcohol-use disorders.

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.

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.

Adenosine A2A receptors modulate the dopamine D2 receptor-mediated inhibition of synaptic transmission in the mouse prefrontal cortex

Prefrontal cortex (PFC) circuits are modulated by dopamine acting on D₁ - and D₂ -like receptors, which are pharmacologically exploited to manage neuropsychiatric conditions. Adenosine A_2A receptors (A_2A R) also control PFC-related responses and A_2A R antagonists are potential anti-psychotic drugs. As tight antagonistic A_2A R-D₂ R and synergistic A_2A R-D₁ R interactions occur in other brain regions, we now investigated the crosstalk between A_2A R and D₁ /D₂ R controlling synaptic transmission between layers II/III and V in mouse PFC coronal slices. Dopamine decreased synaptic transmission, a presynaptic effect based on the parallel increase in paired-pulse responses. Dopamine inhibition was prevented by the D₂ R-like antagonist sulpiride but not by the D₁ R antagonist SCH23390 and was mimicked by the D₂ R agonist sumanirole, but not by the agonists of either D₄ R (A-412997) or D₃ R (PD128907). Dopamine inhibition was prevented by the A_2A R antagonist, SCH58261, and attenuated in A_2A R knockout mice. Accordingly, triple-labelling immunocytochemistry experiments revealed the co-localization of A_2A R and D₂ R immunoreactivity in glutamatergic (vGluT1-positive) nerve terminals of the PFC. This reported positive A_2A R-D₂ R interaction controlling PFC synaptic transmission provides a mechanistic justification for the anti-psychotic potential of A_2A R antagonists.

The Role of Adenosine Receptors in Psychostimulant Addiction

Adenosine receptors (AR) are a family of G-protein coupled receptors, comprised of four members, named A1, A2A, A2B, and A3 receptors, found widely distributed in almost all human body tissues and organs. To date, they are known to participate in a large variety of physiopathological responses, which include vasodilation, pain, and inflammation. In particular, in the central nervous system (CNS), adenosine acts as a neuromodulator, exerting different functions depending on the type of AR and consequent cellular signaling involved. In terms of molecular pathways and second messengers involved, A1 and A3 receptors inhibit adenylyl cyclase (AC), through Gi/o proteins, while A2A and A2B receptors stimulate it through Gs proteins. In the CNS, A1 receptors are widely distributed in the cortex, hippocampus, and cerebellum, A2A receptors are localized mainly in the striatum and olfactory bulb, while A2B and A3 receptors are found at low levels of expression. In addition, AR are able to form heteromers, both among themselves (e.g., A1/A2A), as well as with other subtypes (e.g., A2A/D2), opening a whole range of possibilities in the field of the pharmacology of AR. Nowadays, we know that adenosine, by acting on adenosine A1 and A2A receptors, is known to antagonistically modulate dopaminergic neurotransmission and therefore reward systems, being A1 receptors colocalized in heteromeric complexes with D1 receptors, and A2A receptors with D2 receptors. This review documents the present state of knowledge of the contribution of AR, particularly A1 and A2A, to psychostimulants-mediated effects, including locomotor activity, discrimination, seeking and reward, and discuss their therapeutic relevance to psychostimulant addiction. Studies presented in this review reinforce the potential of A1 agonists as an effective strategy to counteract psychostimulant-induced effects. Furthermore, different experimental data support the hypothesis that A2A/D2 heterodimers are partly responsible for the psychomotor and reinforcing effects of psychostimulant drugs, such as cocaine and amphetamine, and the stimulation of A2A receptor is proposed as a potential therapeutic target for the treatment of drug addiction. The overall analysis of presented data provide evidence that excitatory modulation of A1 and A2A receptors constitute promising tools to counteract psychostimulants addiction.

Effects of intra-accumbal or intra-prefrontal cortex microinjections of adenosine 2A receptor ligands on responses to cocaine reward and seeking in rats

RATIONALE AND OBJECTIVES

Many studies indicated that adenosine via its A_2A receptors influences the behavioral effects of cocaine by modulating dopamine neurotransmission. The hypothesis was tested that A_2A receptors in the nucleus accumbens (NAc) or the prefrontral cortex (PFc) may modulate cocaine reward and/or cocaine seeking behavior in rats.

METHODS

The effects of local bilateral microinjections of the selective A_2A receptor agonist CGS 21680 or the A_2A receptor antagonists KW 6002 and SCH 58261 were investigated on cocaine self-administration on reinstatement of cocaine seeking.

RESULTS

The intra-NAc shell, but not intra-infralimbic PFc, administration of CGS 21680 significantly reduced the number of active lever presses and the number of cocaine (0.25 mg/kg) infusions. However, tonic activation of A_2A receptors located in the NAc or PFc did not play a role in modulating the rewarding actions of cocaine since neither KW 6002 nor SCH 58261 microinjections altered the cocaine (0.5 mg/kg) infusions. The intra-NAc but not intra-PFc microinjections of CGS 21680 dose- dependently attenuated the reinstatement of active lever presses induced by cocaine (10 mg/kg, i.p.) and the drug-associated combined conditioned stimuli using the subthreshold dose of cocaine (2.5 mg/kg, i.p.). On the other hand, the intra-NAc pretreatment with SCH 58261, but not with KW 6002, given alone evoked reinstatement of cocaine seeking behavior.

CONCLUSION

The results strongly support the involvement of accumbal shell A_2A receptors as a target, the activation of which exerts an inhibitory control over cocaine reward and cocaine seeking.

The melanoma-linked "redhead" MC1R influences dopaminergic neuron survival

OBJECTIVE

Individuals with Parkinson disease are more likely to develop melanoma, and melanoma patients are reciprocally at higher risk of developing Parkinson disease. Melanoma is strongly tied to red hair/fair skin, a phenotype of loss-of-function polymorphisms in the MC1R (melanocortin 1 receptor) gene. Loss-of-function variants of MC1R have also been linked to increased risk of Parkinson disease. The present study is to investigate the role of MC1R in dopaminergic neurons in vivo.

METHODS

Genetic and pharmacological approaches were employed to manipulate MC1R, and nigrostriatal dopaminergic integrity was determined by comprehensive behavioral, neurochemical, and neuropathological measures.

RESULTS

MC1R^e/e mice, which carry an inactivating mutation of MC1R and mimic the human redhead phenotype, have compromised nigrostriatal dopaminergic neuronal integrity, and they are more susceptible to dopaminergic neuron toxins 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Furthermore, a selective MC1R agonist protects against MPTP-induced dopaminergic neurotoxicity.

INTERPRETATION

Our findings reveal a protective role of MC1R in the nigrostriatal dopaminergic system, and they provide a rationale for MC1R as a potential therapeutic target for Parkinson disease. Together with its established role in melanoma, MC1R may represent a common pathogenic pathway for melanoma and Parkinson disease. Ann Neurol 2017;81:395-406.

Sigma-1 Receptor and Pain

There is a critical need for new analgesics acting through new mechanisms of action, which could increase the efficacy respect to existing therapies and/or reduce their unwanted effects. Current preclinical evidence supports the modulatory role of the sigma-1 receptor (σ1R) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists on pain of different etiology, very consistently in neuropathic pain, but also in nociceptive, inflammatory, and visceral pain. σ1R is highly expressed in different pain areas of the CNS and the periphery, particularly dorsal root ganglia (DRG), and interacts and modulates the functionality of different receptors and ion channels. Accordingly, antinociceptive effects of σ1R antagonists both acting alone and in combination with other analgesics have been reported at both central and peripheral sites. At the central level, behavioral, electrophysiological, neurochemical, and molecular findings support a role for σ1R antagonists in inhibiting augmented excitability secondary to sustained afferent input. Moreover, the involvement of σ1R in mechanisms regulating pain at the periphery has been recently confirmed. Unlike opioids, σ1R antagonists do not modify normal sensory mechanical and thermal sensitivity thresholds but they exert antihypersensitivity effects (antihyperalgesic and antiallodynic) in sensitizing conditions, enabling the reversal of nociceptive thresholds back to normal values. These are distinctive features allowing σ1R antagonists to exert a modulatory effect specifically in pathophysiological conditions such as chronic pain.

RasGRP1 promotes amphetamine-induced motor behavior through a Rhes interaction network ("Rhesactome") in the striatum

The striatum of the brain coordinates motor function. Dopamine-related drugs may be therapeutic to patients with striatal neurodegeneration, such as Huntington's disease (HD) and Parkinson's disease (PD), but these drugs have unwanted side effects. In addition to stimulating the release of norepinephrine, amphetamines, which are used for narcolepsy and attention-deficit/hyperactivity disorder (ADHD), trigger dopamine release in the striatum. The guanosine triphosphatase Ras homolog enriched in the striatum (Rhes) inhibits dopaminergic signaling in the striatum, is implicated in HD and L-dopa-induced dyskinesia, and has a role in striatal motor control. We found that the guanine nucleotide exchange factor RasGRP1 inhibited Rhes-mediated control of striatal motor activity in mice. RasGRP1 stabilized Rhes, increasing its synaptic accumulation in the striatum. Whereas partially Rhes-deficient (Rhes+/-) mice had an enhanced locomotor response to amphetamine, this phenotype was attenuated by coincident depletion of RasGRP1. By proteomic analysis of striatal lysates from Rhes-heterozygous mice with wild-type or partial or complete knockout of Rasgrp1, we identified a diverse set of Rhes-interacting proteins, the "Rhesactome," and determined that RasGRP1 affected the composition of the amphetamine-induced Rhesactome, which included PDE2A (phosphodiesterase 2A; a protein associated with major depressive disorder), LRRC7 (leucine-rich repeat-containing 7; a protein associated with bipolar disorder and ADHD), and DLG2 (discs large homolog 2; a protein associated with chronic pain). Thus, this Rhes network provides insight into striatal effects of amphetamine and may aid the development of strategies to treat various neurological and psychological disorders associated with the striatal dysfunction.

Genetic blockade of adenosine A2A receptors induces cognitive impairments and anatomical changes related to psychotic symptoms in mice

Schizophrenia is a chronic severe mental disorder with a presumed neurodevelopmental origin, and no effective treatment. Schizophrenia is a multifactorial disease with genetic, environmental and neurochemical etiology. The main theories on the pathophysiology of this disorder include alterations in dopaminergic and glutamatergic neurotransmission in limbic and cortical areas of the brain. Early hypotheses also suggested that nucleoside adenosine is a putative affected neurotransmitter system, and clinical evidence suggests that adenosine adjuvants improve treatment outcomes, especially in poorly responsive patients. Hence, it is important to elucidate the role of the neuromodulator adenosine in the pathophysiology of schizophrenia. A2A adenosine receptor (A2AR) subtypes are expressed in brain areas controlling motivational responses and cognition, including striatum, and in lower levels in hippocampus and cerebral cortex. The aim of this study was to characterize A2AR knockout (KO) mice with complete and specific inactivation of A2AR, as an animal model for schizophrenia. We performed behavioral, anatomical and neurochemical studies to assess psychotic-like symptoms in adult male and female KO and wild-type (WT) littermates. Our results show impairments in inhibitory responses and sensory gating in A2AR KO animals. Hyperlocomotion induced by d-amphetamine and MK-801 was reduced in KO animals when compared to WT littermates. Moreover, A2AR KO animals show motor disturbances, social and cognitive alterations. Finally, behavioral impairments were associated with enlargement of brain lateral ventricles and decreased BDNF levels in the hippocampus. These data highlight the role of adenosine in the pathophysiology of schizophrenia and provide new possibilities for the therapeutic management of schizophrenia.

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.

Deletion of adenosine A2A receptors from astrocytes disrupts glutamate homeostasis leading to psychomotor and cognitive impairment: relevance to schizophrenia

BACKGROUND

Adenosine A2A receptors (A2AR) modulate dopamine and glutamate signaling and thereby may influence some of the psychomotor and cognitive processes associated with schizophrenia. Because astroglial A2AR regulate the availability of glutamate, we hypothesized that they might play an unprecedented role in some of the processes leading to the development of schizophrenia, which we investigated using a mouse line with a selective deletion of A2AR in astrocytes (Gfa2-A2AR knockout [KO] mice].

METHODS

We examined Gfa2-A2AR KO mice for behaviors thought to recapitulate some features of schizophrenia, namely enhanced MK-801 psychomotor response (positive symptoms) and decreased working memory (cognitive symptoms). In addition, we probed for neurochemical alterations in the glutamatergic circuitry, evaluating glutamate uptake and release and the levels of key proteins defining glutamatergic signaling (glutamate transporter-I [GLT-I], N-methyl-D-aspartate receptors [NMDA-R] and α-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors [AMPA-R]) to provide a mechanistic understanding of the phenotype encountered.

RESULTS

We show that Gfa2-A2AR KO mice exhibited enhanced MK-801 psychomotor response and decreased working memory; this was accompanied by a disruption of glutamate homeostasis characterized by aberrant GLT-I activity, increased presynaptic glutamate release, NMDA-R 2B subunit upregulation, and increased internalization of AMPA-R. Accordingly, selective GLT-I inhibition or blockade of GluR1/2 endocytosis prevented the psychomotor and cognitive phenotypes in Gfa2-A2AR KO mice, namely in the nucleus accumbens.

CONCLUSIONS

These results show that the dysfunction of astrocytic A2AR, by controlling GLT-I activity, triggers an astrocyte-to-neuron wave of communication resulting in disrupted glutamate homeostasis, thought to underlie several endophenotypes relevant to schizophrenia.

Purinergic system dysfunction in mood disorders: a key target for developing improved therapeutics

Uric acid and purines (such as adenosine) regulate mood, sleep, activity, appetite, cognition, memory, convulsive threshold, social interaction, drive, and impulsivity. A link between purinergic dysfunction and mood disorders was first proposed a century ago. Interestingly, a recent nationwide population-based study showed elevated risk of gout in subjects with bipolar disorder (BD), and a recent meta-analysis and systematic review of placebo-controlled trials of adjuvant purinergic modulators confirmed their benefits in bipolar mania. Uric acid may modulate energy and activity levels, with higher levels associated with higher energy and BD spectrum. Several recent genetic studies suggest that the purinergic system - particularly the modulation of P1 and P2 receptor subtypes - plays a role in mood disorders, lending credence to this model. Nucleotide concentrations can be measured using brain spectroscopy, and ligands for in vivo positron emission tomography (PET) imaging of adenosine (P1) receptors have been developed, thus allowing potential target engagement studies. This review discusses the key role of the purinergic system in the pathophysiology of mood disorders. Focusing on this promising therapeutic target may lead to the development of therapies with antidepressant, mood stabilization, and cognitive effects.

Mice lacking the circadian modulators SHARP1 and SHARP2 display altered sleep and mixed state endophenotypes of psychiatric disorders

Increasing evidence suggests that clock genes may be implicated in a spectrum of psychiatric diseases, including sleep and mood related disorders as well as schizophrenia. The bHLH transcription factors SHARP1/DEC2/BHLHE41 and SHARP2/DEC1/BHLHE40 are modulators of the circadian system and SHARP1/DEC2/BHLHE40 has been shown to regulate homeostatic sleep drive in humans. In this study, we characterized Sharp1 and Sharp2 double mutant mice (S1/2-/-) using online EEG recordings in living animals, behavioral assays and global gene expression profiling. EEG recordings revealed attenuated sleep/wake amplitudes and alterations of theta oscillations. Increased sleep in the dark phase is paralleled by reduced voluntary activity and cortical gene expression signatures reveal associations with psychiatric diseases. S1/2-/- mice display alterations in novelty induced activity, anxiety and curiosity. Moreover, mutant mice exhibit impaired working memory and deficits in prepulse inhibition resembling symptoms of psychiatric diseases. Network modeling indicates a connection between neural plasticity and clock genes, particularly for SHARP1 and PER1. Our findings support the hypothesis that abnormal sleep and certain (endo)phenotypes of psychiatric diseases may be caused by common mechanisms involving components of the molecular clock including SHARP1 and SHARP2.

Cocaine-induced changes of synaptic transmission in the striatum are modulated by adenosine A2A receptors and involve the tyrosine phosphatase STEP

The striatum is a brain area implicated in the pharmacological action of drugs of abuse. Adenosine A2A receptors (A2ARs) are highly expressed in the striatum and mediate, at least in part, cocaine-induced psychomotor effects in vivo. Here we studied the synaptic mechanisms implicated in the pharmacological action of cocaine in the striatum and investigated the influence of A2ARs. We found that synaptic transmission was depressed in corticostriatal slices after perfusion with cocaine (10 μM). This effect was reduced by the A2AR antagonist ZM241385 and almost abolished in striatal A2AR-knockout mice (mice lacking A2ARs in striatal neurons, stA2ARKO). The effect of cocaine on synaptic transmission was also prevented by the protein tyrosine phosphatases (PTPs) inhibitor sodium orthovanadate (Na3VO4). In synaptosomes prepared from striatal slices, we found that the activity of striatal-enriched protein tyrosine phosphatase (STEP) was upregulated by cocaine, prevented by ZM241385, and absent in synaptosomes from stA2ARKO. The role played by STEP in cocaine modulation of synaptic transmission was investigated in whole-cell voltage clamp recordings from medium spiny neurons of the striatum. We found that TAT-STEP, a peptide that renders STEP enzymatically inactive, prevented cocaine-induced reduction in AMPA- and NMDA-mediated excitatory post-synaptic currents, whereas the control peptide, TAT-myc, had no effect. These results demonstrate that striatal A2ARs modulate cocaine-induced synaptic depression in the striatum and highlight the potential role of PTPs and specifically STEP in the effects of cocaine.

The role of adenosine receptor agonist and antagonist on Hippocampal MDMA detrimental effects; a structural and behavioral study

There is abundant evidence showing that repeated use of MDMA (3, 4-Methylenedioxymethamphetamine, ecstasy) has been associated with depression, anxiety and deficits in learning and memory, suggesting detrimental effects on hippocampus. Adenosine is an endogenous purine nucleoside that has a neuromodulatory role in the central nervous system. In the present study, we investigated the role of A2a adenosine receptors agonist (CGS) and antagonist (SCH) on the body temperature, learning deficits, and hippocampal cell death induced by MDMA administration. In this study, 63 adult, male, Sprague - Dawley rats were subjected to MDMA (10 and 20 mg/kg) followed by intraperitoneal CGS (0.03 mg/kg) or SCH (0.03 mg/kg) injection. The animals were tested for spatial learning in the Morris water maze (MWM) task performance, accompanied by a recording of body temperature, electron microscopy and stereological study. Our results showed that MDMA treatment increased body temperature significantly, and impaired the ability of rats to locate the hidden platform(P < 0.05). The number of hippocampal dark neurons also increased especially in CA1. These impairments were aggravated by co-administration of A2a antagonist (SCH) with MDMA. Furthermore, the administration of the A2a receptor agonist (CGS) provided partial protection against MWM deficits and hippocampal cell death(P < 0.05). This study provides for the first time evidence that, in contrast to A2a antagonist (SCH) effects, co-administration of A2a agonist (CGS) with MDMA can protect against MDMA hippocampal neurotoxic effects; providing a potential value in the prevention of learning deficits observed in MDMA users. However, the exact mechanism of these interactions requires further studies.

Caffeine increases psychomotor performance on the effort expenditure for rewards task

Preclinical studies suggest that cost/benefit decision-making involves interactions between adenosine and dopamine (DA). In rats, DA depletion decreases willingness to incur effort costs, while adenosine antagonism reverses these effects, likely by increasing DA transmission. Caffeine is a non-selective adenosine antagonist commonly used to facilitate effortful tasks, and thus may affect decisions involving effort costs in humans. The current study examined acute effects of 200 mg of caffeine on willingness to exert effort for monetary rewards at varying levels of reward value and reward probability, in young adult light caffeine users. Based on previous findings with amphetamine, we predicted that caffeine would increase willingness to exert effort. At separate sessions, 23 healthy normal adults received placebo or 200 mg caffeine under counterbalanced double-blind conditions, then completed the effort expenditure for rewards task (EEfRT). Measures of subjective and cardiovascular effects were obtained at regular intervals. Caffeine produced small but significant subjective and cardiovascular effects, and sped psychomotor performance on the EEfRT. Caffeine did not alter willingness to exert effort, except in high cardiovascular responders to caffeine, in whom it decreased willingness to exert effort. These results were contrary to our predictions, but consistent with rodent studies suggesting that moderate doses of caffeine alone do not affect effort, but rather only influence effort in the context of DA antagonism. Our results demonstrate that psychomotor speeding and decisional effects on the EEfRT are dissociable, providing additional evidence for the EEfRT as a specific measure of effort-based decision-making. This study provides a starting point for exploring contributions of the adenosine system to motivation in humans.

Adenosine augmentation ameliorates psychotic and cognitive endophenotypes of schizophrenia

An emerging theory of schizophrenia postulates that hypofunction of adenosine signaling may contribute to its pathophysiology. This study was designed to test the "adenosine hypothesis" of schizophrenia and to evaluate focal adenosine-based strategies for therapy. We found that augmentation of adenosine by pharmacologic inhibition of adenosine kinase (ADK), the key enzyme of adenosine clearance, exerted antipsychotic-like activity in mice. Further, overexpression of ADK in transgenic mice was associated with attentional impairments linked to schizophrenia. We observed that the striatal adenosine A2A receptor links adenosine tone and psychomotor response to amphetamine, an indicator of dopaminergic signaling. Finally, intrastriatal implants of engineered adenosine-releasing cells restored the locomotor response to amphetamine in mice overexpressing ADK, whereas the same grafts placed proximal to the hippocampus of transgenic mice reversed their working memory deficit. This functional double dissociation between striatal and hippocampal adenosine demonstrated in Adk transgenic mice highlights the independent contributions of these two interconnected brain regions in the pathophysiology of schizophrenia and thus provides the rationale for developing local adenosine augmentation therapies for the treatment of schizophrenia.

Time and sex-dependent effects of an adenosine A2A/A1 receptor antagonist on motivation to self-administer cocaine in rats

Adenosine is an important neuromodulator, known to interact with both dopaminergic and glutamatergic systems to influence psychostimulant action. In the present study, we examined the effects of ATL444, a novel adenosine receptor antagonist, on motivation for cocaine in male and female rats. Adult male and female Sprague-Dawley rats were trained to self-administer cocaine (1.5mg/kg/infusion) on a fixed-ratio 1 schedule with a daily maximum of 20 infusions. Following 5 consecutive sessions during which all 20 available infusions were obtained, motivation for cocaine (0.5 mg/kg/infusion) was assessed under a progressive ratio (PR) schedule, and once responding stabilized, the effect of treatment with ATL444 (0, 15, and 30 mg/kg, i.p.) was examined. As a control, we also assessed its effects on PR responding for sucrose. Binding studies revealed that ATL 444 was 3-fold, 25-fold, and 400-fold more selective for the A2A receptor as compared to A1, A2B, and A3 receptors, respectively. ATL444 produced a significant increase in motivation for cocaine on the day of treatment in females with a trend for an increase in males. In addition, over the two PR sessions following ATL444 treatment a significant decrease in responding was observed in males but not females. Responding for sucrose was unaffected by ATL444 treatment. Our results reveal that adenosine receptor blockade may mediate both acute increases in the reinforcing effects of cocaine, and longer term inhibitory effects on cocaine reinforcement that differ according to sex.

Adenosine A2A receptors in the nucleus accumbens bi-directionally alter cocaine seeking in rats

Repeated cocaine administration enhances dopamine D(2) receptor sensitivity in the mesolimbic dopamine system, which contributes to drug relapse. Adenosine A(2A) receptors are colocalized with D(2) receptors on nucleus accumbens (NAc) medium spiny neurons where they antagonize D(2) receptor activity. Thus, A(2A) receptors represent a target for reducing enhanced D(2) receptor sensitivity that contributes to cocaine relapse. The aim of these studies were to determine the effects of adenosine A(2A) receptor modulation in the NAc on cocaine seeking in rats that were trained to lever press for cocaine. Following at least 15 daily self-administration sessions and 1 week of abstinence, lever pressing was extinguished in daily extinction sessions. We subsequently assessed the effects of intra-NAc core microinjections of the A(2A) receptor agonist, CGS 21680 (4-[2-[[6-amino-9-(N-ethyl-b-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride), and the A(2A) receptor antagonist, MSX-3 (3,7-dihydro-8-[(1E)-2-(3-methoxyphenyl)ethenyl]-7-methyl-3-[3-(phosphonooxy)propyl-1-(2-propynyl)-1H-purine-2,6-dione disodium salt hydrate), in modulating cocaine- and quinpirole-induced reinstatement to cocaine seeking. Intra-NAc pretreatment of CGS 21680 reduced both cocaine- and quinpirole-induced reinstatement. These effects were specific to cocaine reinstatement as intra-NAc CGS 21680 had no effect on sucrose seeking in rats trained to self-administer sucrose pellets. Intra-NAc treatment with MSX-3 modestly reinstated cocaine seeking when given alone, and exacerbated both cocaine- and quinpirole-induced reinstatement. Interestingly, the exacerbation of cocaine seeking produced by MSX-3 was only observed at sub-threshold doses of cocaine and quinpirole, suggesting that removing tonic A(2A) receptor activity enables behaviors mediated by dopamine receptors. Taken together, these findings suggest that A(2A) receptor stimulation reduces, while A(2A) blockade amplifies, D(2) receptor signaling in the NAc that mediates cocaine relapse.

In vivo dopaminergic and behavioral responses to acute cocaine are altered in adenosine A(2A) receptor knockout mice

Adenosine, acting on adenosine A(2A) receptors (A2ARs), regulates addictive processes induced by drugs of abuse. This study investigates the role of A(2A) adenosine receptors in neurochemical and behavioral responses to an acute cocaine challenge. Changes in the extracellular levels of dopamine (DA) in the nucleus accumbens (NAc) of mice lacking A(2A) adenosine receptors and wild type (WT) littermates after an acute cocaine (20 mg/kg) administration were evaluated by in vivo microdialysis studies. Locomotor effects induced by cocaine were measured during the microdialysis procedure. Cocaine-evoked increases in extracellular DA were not sustained in mice lacking A(2A) Rs in comparison with wild-type mice (P < 0.05). Cocaine administration significantly increased ambulatory activity in both genotypes. However, overall locomotor activity was further increased, whereas rest and small local movement measures were significantly attenuated in the A(2A) R knockout mice compared with WT littermates (P < 0.05). Our findings support an important role for adenosine A(2A) R in modulating the acute effects of cocaine, as demonstrated by the decrease in cocaine-evoked dopaminergic transmission in the NAc. Furthermore, the results support an important antagonistic role of A(2A) R in vivo in regulating psychostimulant-induced hyperlocomotion.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Unraveling the differential functions and regulation of striatal neuron sub-populations in motor control, reward, and motivational processes

The striatum, the major input structure of the basal ganglia, is critically involved in motor control and learning of habits and skills, and is also involved in motivational and reward processes. The dorsal striatum, caudate–putamen, is primarily implicated in motor functions whereas the ventral striatum, the nucleus accumbens, is essential for motivation and drug reinforcement. Severe basal ganglia dysfunction occurs in movement disorders as Parkinson's and Huntington's disease, and in psychiatric disorders such as schizophrenia and drug addiction. The striatum is essentially composed of GABAergic medium-sized spiny neurons (MSNs) that are output neurons giving rise to the so-called direct and indirect pathways and are targets of the cerebral cortex and mesencephalic dopaminergic neurons. Although the involvement of striatal sub-areas in motor control and motivation has been thoroughly characterized, major issues remained concerning the specific and respective functions of the two MSNs sub-populations, D₂R-striatopallidal (dopamine D₂ receptor-positive) and D₁R-striatonigral (dopamine D₁ receptor-positive) neurons, as well as their specific regulation. Here, we review recent advances that gave new insight in the understanding of the differential roles of striatopallidal and striatonigral neurons in the basal ganglia circuit. We discuss innovative techniques developed in the last decade which allowed a much precise evaluation of molecular pathways implicated in motivational processes and functional roles of striatopallidal and striatonigral neurons in motor control and in the establishment of reward-associated behavior.

Long-term (trophic) purinergic signalling: purinoceptors control cell proliferation, differentiation and death

The purinergic signalling system, which uses purines and pyrimidines as chemical transmitters, and purinoceptors as effectors, is deeply rooted in evolution and development and is a pivotal factor in cell communication. The ATP and its derivatives function as a 'danger signal' in the most primitive forms of life. Purinoceptors are extraordinarily widely distributed in all cell types and tissues and they are involved in the regulation of an even more extraordinary number of biological processes. In addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion, there is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body. In this article, we focus on the latter in the immune/defence system, in stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption, as well as in cancer.

A(2A) adenosine receptor binding parameters in platelets from patients affected by pathological gambling

BACKGROUND/AIMS

A structural and functional interaction between A(2A) adenosine receptors and D(2) dopamine receptors has been implicated in the pathophysiology of impulse control disorders. The aim of this study was to use platelet membranes to assess A(2A) adenosine receptor affinity and density in patients affected by pathological gambling (PG; which is classified as a specific impulse control disorder) with respect to those of control subjects.

METHODS

Twelve drug-free PG patients and 12 age- and sex-matched healthy controls were enrolled in the study. PG was diagnosed according to the Structured Clinical Interview for DSM-IV - Patient Version 2.0 and the South Oaks Gambling Screen. A(2A) adenosine receptor binding parameters were evaluated using a [(3)H]ZM(241385) binding assay; affinity and density (B(max)) were determined by means of saturation binding studies with platelet membranes.

RESULTS

The A(2A) adenosine receptor binding affinity was found to be significantly higher in patients affected by PG than in healthy subjects; in contrast, no significant differences in B(max) were observed between the 2 groups.

CONCLUSIONS

The elevated A(2A) adenosine receptor binding affinity in platelets from PG patients with respect to control subjects demonstrates for the first time a change in adenosine receptor parameters, and it suggests the involvement of the adenosine system in this pathology. The previously demonstrated hyperactivity of the dopamine system in PG may modulate the A(2A) adenosine receptor, supporting a role for this receptor as a peripheral marker of dopamine dysfunction. Because it is not possible to directly measure the D(2) dopamine receptor in human platelets, these data are particularly relevant to the detection of dopamine dysfunction.

Electroconvulsive therapy: a novel hypothesis for the involvement of purinergic signalling

It is proposed that ATP is released from both neurons and glia during electroconvulsive therapy (ECT) and that this leads to reduction of depressive behaviour via complex stimulation of neurons and glia directly via P2X and P2Y receptors and also via P1 receptors after extracellular breakdown of ATP to adenosine. In particular, A(1) adenosine receptors inhibit release of excitatory transmitters, and A(2A) and P2Y receptors may modulate the release of dopamine. Sequential ECT may lead to changes in purinoceptor expression in mesolimbic and mesocortical regions of the brain implicated in depression and other mood disorders. In particular, increased expression of P2X7 receptors on glial cells would lead to increased release of cytokines, chemokines and neurotrophins. In summary, we suggest that ATP release following ECT involves neurons, glial cells and neuron-glial interactions acting via both P2 and after breakdown to adenosine via P1 receptors. We suggest that ecto-nucleotidase inhibitors (increasing available amounts of ATP) and purinoceptor agonists may enhance the anti-depressive effect of ECT.

Recurrent perseveration correlates with abnormal repetitive locomotion in adult mink but is not reduced by environmental enrichment

We analysed the relationship between abnormal repetitive behaviour (ARB), the presence/absence of environmental enrichment, and two types of behavioural disinhibition in farmed American mink, Neovison vison. The first type, recurrent perseveration, the inappropriate repetition of already completed responses, was assessed using three indices of excessive response repetition and patterning in a bias-corrected serial two-choice guessing task. The second type, disinhibition of prepotent responses to reward cues, a form of impulsivity, was tested in a locomotive detour task adapted from primate reaching tasks: subjects were required to walk around, rather than directly into, a transparent barrier behind which food was visible. In older adult females, recurrent perseveration positively predicted pre-feeding abnormal repetitive locomotion (ARL) in Non-enriched housing. High-ARL subjects also performed repeated (same-choice) responses more rapidly than low-ARL animals, even when statistically controlling for alternated (different-choice) response latency. Mink performed much less ARL following transfer to Enriched housing, but there was no corresponding change in recurrent perseveration. Thus, elevated recurrent perseveration is not sufficient for frequent ARL; and while captive environments do determine ARL frequency, in mink, they do not necessarily do so by modifying levels of perseveration. Disinhibition of prepotent responses to reward cues, meanwhile, did not predict ARL. In a separate sample of differentially housed young adults, neither type of behavioural disinhibition predicted ARL, and again, whether or not housing was enriched did not affect behavioural disinhibition despite affecting ARL. Thus, the relationship between recurrent perseveration and ARB may only develop with age; longitudinal studies are now required for confirmation.

The A2a adenosine receptor modulates the reinforcement efficacy and neurotoxicity of MDMA

Adenosine is an endogenous purine nucleoside that plays a neuromodulatory role in the central nervous system. A2a adenosine receptors have been involved in reward-related processes, inflammatory phenomena and neurotoxicity reactions. In the present study, we investigated the role of A2a adenosine receptors on the acute pharmacological effects, reinforcement and neuroinflammation induced by MDMA administration. First, the acute effects of MDMA on body temperature, locomotor activity and anxiety-like responses were measured in A2a knockout mice and wild-type littermates. Second, MDMA reinforcing properties were evaluated using the intravenous self-administration paradigm. Finally, we assessed striatal astrogliosis and microgliosis as markers of MDMA neurotoxicity. Our results showed that acute MDMA produced a biphasic effect on body temperature and increased locomotor activity and anxiogenic-like responses in both genotypes. However, MDMA reinforcing properties were dramatically affected by the lack of A2a adenosine receptors. Thus, wild-type mice maintained MDMA self-administration under a fixed ratio 1 reinforcement schedule, whereas the operant response appeared completely abolished in A2a knockout mice. In addition, the MDMA neurotoxic regime produced an enhanced inflammatory response in striatum of wild-type mice, revealed by a significant increase in glial expression, whereas such activation was attenuated in mutant mice. This is the first report indicating that A2a adenosine receptors play a key role in reinforcement and neuroinflammation induced by the widely used psychostimulant.

Increased non-rapid eye movement sleep by cocaine withdrawal: possible involvement of A2A receptors

This study attempted to clarify whether cocaine withdrawal altered sleep architecture and the role of adenosine receptors in this process. Cocaine (20 mg/kg) was administered subcutaneously once per day for 7 days to rat implanted with sleep/wake recording electrode. Polygraphic signs of undisturbed sleep/wake activities were recorded for 24 h before cocaine administration (basal recording as control); withdrawal-day 1 (after 1 day of repeated cocaine administration), withdrawal-day 8 (after 8 days of repeated cocaine administration), and withdrawal-day 14 (after 14 days of repeated cocaine administration), respectively. On cocaine withdrawal-day 1, wakefulness was significantly increased, total sleep was decreased, non-rapid eye movement sleep was markedly reduced, and rapid eye movement sleep was enhanced. Sleep/wake cycles were also increased on cocaine withdrawal day 1. However, non-rapid eye movement sleep was increased on withdrawal-day 8 and 14, whereas rapid eye movement sleep was decreased and no significant changes were observed in the total sleep and sleep/wake cycles during these periods. Adenosine A(2A) receptors expression was increased on withdrawal-day 8 and 14, whereas A(1) receptors levels were reduced after 14 days of withdrawal and the A(2B) receptors remained unchanged. Our findings suggest that alterations of sleep and sleep architecture during cocaine subacute and subchronic withdrawals after repeated cocaine administration may be partially involved in A(2A) receptors over-expression in the rat hypothalamus.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

Adenosine hypothesis of schizophrenia--opportunities for pharmacotherapy

Pharmacotherapy of schizophrenia based on the dopamine hypothesis remains unsatisfactory for the negative and cognitive symptoms of the disease. Enhancing N-methyl-D-aspartate receptors (NMDAR) function is expected to alleviate such persistent symptoms, but successful development of novel clinically effective compounds remains challenging. Adenosine is a homeostatic bioenergetic network modulator that is able to affect complex networks synergistically at different levels (receptor-dependent pathways, biochemistry, bioenergetics, and epigenetics). By affecting brain dopamine and glutamate activities, it represents a promising candidate for reversing the functional imbalance in these neurotransmitter systems believed to underlie the genesis of schizophrenia symptoms, as well as restoring homeostasis of bioenergetics. Suggestion of an adenosine hypothesis of schizophrenia further posits that adenosinergic dysfunction might contribute to the emergence of multiple neurotransmitter dysfunctions characteristic of schizophrenia via diverse mechanisms. Given the importance of adenosine in early brain development and regulation of brain immune response, it also bears direct relevance to the aetiology of schizophrenia. Here, we provide an overview of the rationale and evidence in support of the therapeutic potential of multiple adenosinergic targets, including the high-affinity adenosine receptors (A(1)R and A(2A)R), and the regulatory enzyme adenosine kinase (ADK). Key preliminary clinical data and preclinical findings are reviewed.

Normal and abnormal functions of adenosine receptors in the central nervous system revealed by genetic knockout studies

Endogenous adenosine is a widely distributed upstream regulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways that converge to contribute to the expression of an array of important brain functions. Over the past decade, the generation and characterization of genetic knockout models for all four G-protein coupled adenosine receptors, the A1 and A2A receptors in particular, has confirmed and extended the neuromodulatory and integrated role of adenosine receptors in the control of a broad spectrum of normal and abnormal brain functions. After a brief introduction of the available adenosine receptor knockout models, this review focuses on findings from the genetic knockout approach, placing particular emphasis on the most recent findings. This review is organized into two sections to separately address (i) the role of adenosine receptors in normal brain processes including neuroplasticity, sleep-wake cycle, motor function, cognition, and emotion-related behaviors; and (ii) their role in the response to various pathologic insults to brain such as ischemic stroke, neurodegeneration, or brain dysfunction/disorders. We largely limit our overview to the prominent adenosine receptor subtypes in brain-the A1 and A2A receptors-for which numerous genetic knockout studies on brain function are available. A1 and A2A receptor knockouts have provided significant new insights into adenosine's control of complex physiologic (e.g., cognition) and pathologic (e.g., neuroinflammation) phenomena. These findings extend and strengthen the support for A1 and A2A receptors in brain as therapeutic targets in several neurologic and psychiatric diseases. However, they also emphasize the importance of considering the disease context-dependent effect when developing adenosine receptor-based therapeutic strategies.

The adenosine A2A receptor is associated with methamphetamine dependence/psychosis in the Japanese population

BACKGROUND

Several lines of evidence suggest that the dopaminergic nervous system contributes to methamphetamine (METH) dependence, and there is increasing evidence of antagonistic interactions between dopamine and adenosine receptors. We therefore hypothesized that variations in the A2A adenosine receptor (ADORA2A) gene modify genetic susceptibility to METH dependence/psychosis.

METHODS

We first analyzed variations in the exons and exon-intron boundaries of the ADORA2A gene in METH dependent/psychotic patients. Then an association analysis between these single nucleotide polymorphisms and METH dependence/psychosis was performed using a total of 171 METH dependent/psychotic patients and 229 controls.

RESULTS

We found 6 variations, of which one single nucleotide polymorphism (SNP) was novel. Significant associations were observed between the allelic and genotypic frequencies of the Exon2+751 (rs5751876) SNP and METH dependence/psychosis. These associations were observed especially in females. In the clinical feature analyses, significant associations were observed between the SNP and the patient subgroup using METH alone (i.e., without concomitant use of other substances of abuse).

CONCLUSIONS

These results suggest that the ADORA2A gene could be a vulnerability factor for METH dependence/psychosis, especially in females and/or in patients using only METH.

Adenosine A1 and A2A receptors are not upstream of caffeine's dopamine D2 receptor-dependent aversive effects and dopamine-independent rewarding effects

Caffeine is widely consumed throughout the world, but little is known about the mechanisms underlying its rewarding and aversive properties. We show that pharmacological antagonism of dopamine not only blocks conditioned place aversion to caffeine, but also reveals dopamine blockade-induced conditioned place preferences. These aversive effects are mediated by the dopamine D(2) receptor, as knockout mice showed conditioned place preferences in response to doses of caffeine that C57Bl/6 mice found aversive. Furthermore, these aversive responses appear to be centrally mediated, as a quaternary analog of caffeine failed to produce conditioned place aversion. Although the adenosine A(2A) receptor is important for caffeine's physiological effects, this receptor seems only to modulate the appetitive and aversive effects of caffeine. A(2A) receptor knockout mice showed stronger dopamine-dependent aversive responses to caffeine than did C57Bl/6 mice, which partially obscured the dopamine-independent and A(2A) receptor-independent preferences. Additionally, the A(1) receptor, alone or in combination with the A(2A) receptor, does not seem to be important for caffeine's rewarding or aversive effects. Finally, excitotoxic lesions of the tegmental pedunculopontine nucleus revealed that this brain region is not involved in dopamine blockade-induced caffeine reward. These data provide surprising new information on the mechanism of action of caffeine, indicating that adenosine receptors do not mediate caffeine's appetitive and aversive effects. We show that caffeine has an atypical reward mechanism, independent of the dopaminergic system and the tegmental pedunculopontine nucleus, and provide additional evidence in support of a role for the dopaminergic system in aversive learning.

Lower frequency of the low activity adenosine deaminase allelic variant (ADA1*2) in schizophrenic patients

OBJECTIVE: Adenosine may play a role in the pathophysiology of schizophrenia, since it modulates the release of several neurotransmitters such as glutamate, dopamine, serotonin and acetylcholine, decreases neuronal activity by pos-synaptic hyperpolarization and inhibits dopaminergic activity. Adenosine deaminase participates in purine metabolism by converting adenosine into inosine. The most frequent functional polymorphism of adenosine deaminase (22G→A) (ADA1*2) exhibits 20-30% lower enzymatic activity in individuals with the G/A genotype than individuals with the G/G genotype. The aim of this study was to evaluate the ADA polymorphism 22G→A (ADA1*2) in schizophrenic patients and healthy controls. METHOD: The genotypes of the ADA 22G→A were identified with allele-specific PCR strategy in 152 schizophrenic patients and 111 healthy individuals. RESULTS: A significant decrease in the frequency of the G/A genotype was seen in schizophrenic patients (7/152 - 4.6%) relative to controls (13/111 - 11.7%, p = 0.032, OR = 2.6). CONCLUSION: These results suggest that the G/A genotype associated with low adenosine deaminase activity and, supposingly, with higher adenosine levels is less frequent among schizophrenic patients.

Adenosine A2A receptors and their role in drug addiction

The specific events between initial presumably manageable drug intake and the development of a drug- addicted state are not yet known. Drugs of abuse have varying mechanisms of action that create a complex pattern of behaviour related to drug consumption, drug-seeking, withdrawal and relapse. The neuromodulator adenosine has been shown to play a role in reward-related behaviour, both as an independent mediator and via interactions of adenosine receptors with other receptors. Adenosine levels are elevated upon exposure to drugs of abuse and adenosine A2A receptors are enriched in brain nuclei known for their involvement in the processing of drug-related reinforcement processing. A2A receptors are found in receptor clusters with dopamine and glutamate receptors. A2A receptors are thus ideally situated to influence the signalling of neurotransmitters relevant in the neuronal responses and plasticity that underlie the development of drug taking and drug-seeking behaviour. In this review, we present evidence for the role of adenosine and A2A receptors in drug addiction, thereby providing support for current efforts aimed at developing drug therapies to combat substance abuse that target adenosine signalling via A2A receptors.

Alterations in tryptophan and purine metabolism in cocaine addiction: a metabolomic study

BACKGROUND

Mapping metabolic "signatures" can provide new insights into addictive mechanisms and potentially identify biomarkers and therapeutic targets.

OBJECTIVE

We examined the differences in metabolites related to the tyrosine, tryptophan, purine, and oxidative stress pathways between cocaine-dependent subjects and healthy controls. Several of these metabolites serve as biological indices underlying the mechanisms of reinforcement, toxicity, and oxidative stress.

METHODS

Metabolomic analysis was performed in 18 DSM-IV-diagnosed cocaine-dependent individuals with at least 2 weeks of abstinence and ten drug-free controls. Plasma concentrations of 37 known metabolites were analyzed and compared using a liquid chromatography electrochemical array platform. Multivariate analyses were used to study the relationship between severity of drug use [Addiction Severity Index (ASI) scores] and biological measures.

RESULTS

Cocaine subjects showed significantly higher levels of n-methylserotonin (p < 0.0017) and guanine (p < 0.0031) and lower concentrations of hypoxanthine (p < 0.0002), anthranilate (p < 0.0024), and xanthine (p < 0.012), compared to controls. Multivariate analyses showed that a combination of n-methylserotonin and xanthine contributed to 73% of the variance in predicting the ASI scores (p < 0.0001). Logistic regression showed that a model combining n-methylserotonin, xanthine, xanthosine, and guanine differentiated cocaine and control groups with no overlap.

CONCLUSIONS

Alterations in the methylation processes in the serotonin pathways and purine metabolism seem to be associated with chronic exposure to cocaine. Given the preliminary nature and cross-sectional design of the study, the findings need to be confirmed in larger samples of cocaine-dependent subjects, preferably in a longitudinal design.

Effects of adenosine A2A receptor stimulation on cocaine-seeking behavior in rats

RATIONALE

Dopamine (DA) receptor stimulation in the nucleus accumbens (NAc) plays an important role in regulating cocaine-seeking behavior. Adenosine receptors antagonize the effects of DA receptor stimulation on intracellular signaling, neuronal output, and behavior.

OBJECTIVES

The goal of the present study is to determine the effects of adenosine A(2A) receptor stimulation on reinstatement of cocaine-seeking behavior in rats.

METHODS

Rats were trained to lever press for cocaine in daily self-administration sessions on a fixed-ratio 1 schedule for 3 weeks. After 1 week of abstinence, lever pressing was extinguished in six daily extinction sessions. We subsequently assessed the effects of the adenosine A(2A) receptor agonist, CGS 21680, on cocaine-, quinpirole (D(2) agonist)-, and cue-induced reinstatement to cocaine seeking. We also assessed the effects of CGS 21680 on sucrose seeking in rats extinguished from sucrose self-administration.

RESULTS

Pretreatment of CGS 21680 dose-dependently blunted cocaine-induced reinstatement (15 mg/kg, i.p.). Pretreatment with CGS 21680 (0.03 mg/kg, i.p.) also attenuated quinpirole- and cue-induced reinstatement. A minimally effective dose of CGS 21680 failed to alter cocaine-induced locomotor activity or sucrose seeking.

CONCLUSIONS

Stimulation of adenosine A(2A) receptors antagonizes reinstatement of cocaine seeking elicited by cocaine, DA D(2)-receptor stimulation, and cocaine-conditioned cues. These findings suggest that adenosine A(2A) receptor stimulation may oppose DA D(2) receptor signaling in the NAc that mediates cocaine relapse.

Cocaine exposure modulates dopamine and adenosine signaling in the fetal brain

Exposure to cocaine during the fetal period can produce significant lasting changes in the structure and function of the brain. Cocaine exerts its effects on the developing brain by blocking monoamine transporters and impairing monoamine receptor signaling. Dopamine is a major central target of cocaine. In a mouse model, we show that cocaine exposure from embryonic day 8 (E8) to E14 produces significant reduction in dopamine transporter activity, attenuation of dopamine D1-receptor function and upregulation of dopamine D2-receptor function. Cocaine's effects on the D1-receptor are at the level of protein expression as well as activity. The cocaine exposure also produces significant increases in basal cAMP levels in the striatum and cerebral cortex. The increase in the basal cAMP levels was independent of dopamine receptor activity. In contrast, blocking the adenosine A2a receptor downregulated the basal cAMP levels in the cocaine-exposed brain to physiological levels, suggesting the involvement of adenosine receptors in mediating cocaine's effects on the embryonic brain. In support of this suggestion, we found that the cocaine exposure downregulated adenosine transporter function. We also found that dopamine D2- and adenosine A2a-receptors antagonize each other's function in the embryonic brain in a manner consistent with their interactions in the mature brain. Thus, our data show that prenatal cocaine exposure produces direct effects on both the dopamine and adenosine systems. Furthermore, the dopamine D2 and adenosine A2a receptor interactions in the embryonic brain discovered in this study unveil a novel substrate for cocaine's effects on the developing brain.

A differential role for the adenosine A2A receptor in opiate reinforcement vs opiate-seeking behavior

The adenosine A(2A) receptor is specifically enriched in the medium spiny neurons that make up the 'indirect' output pathway from the ventral striatum, a structure known to have a crucial, integrative role in processes such as reward, motivation, and drug-seeking behavior. In the present study we investigated the impact of adenosine A(2A) receptor deletion on behavioral responses to morphine in a number of reward-related paradigms. The acute, rewarding effects of morphine were evaluated using the conditioned place preference paradigm. Operant self-administration of morphine on both fixed and progressive ratio schedules as well as cue-induced drug-seeking was assessed. In addition, the acute locomotor response to morphine as well as sensitization to morphine was evaluated. Decreased morphine self-administration and breakpoint in A(2A) knockout mice was observed. These data support a decrease in motivation to consume the drug, perhaps reflecting diminished rewarding effects of morphine in A(2A) knockout mice. In support of this finding, a place preference to morphine was not observed in A(2A) knockout mice but was present in wild-type mice. In contrast, robust cue-induced morphine-seeking behavior was exhibited by both A(2A) knockout and wild-type mice after a period of withdrawal. The acute locomotor response to morphine in the A(2A) knockout was similar to wild-type mice, yet A(2A) knockout mice did not display tolerance to chronic morphine under the present paradigm. Both genotypes display locomotor sensitization to morphine, implying a lack of a role for the A(2A) receptor in the drug-induced plasticity necessary for the development or expression of sensitization. Collectively, these data suggest a differential role for adenosine A(2A) receptors in opiate reinforcement compared to opiate-seeking.

Adenosine antagonists reverse the cataleptic effects of haloperidol: implications for the treatment of Parkinson's disease

The effects of adenosine antagonists were compared in two rodent models of Parkinsonian symptoms. In the first experiment the dopamine D2 antagonist, haloperidol, was used to induce catalepsy. It was found that treatment with the non-selective adenosine antagonist caffeine significantly reduced catalepsy at each dose. Treatment with the selective A1 antagonist CPT also produced a significant reduction in catalepsy, as did treatment with the selective A2A antagonist SCH58261. In the second experiment haloperidol was used to suppress locomotor activity in an open field test. Treatment with caffeine significantly increased locomotion reduced by haloperidol, but not at all doses tested. Treatment with CPT also increased haloperidol-suppressed locomotor activity in dose-dependent manner. Surprisingly, treatment with SCH58261 did not significantly increase locomotor activity in animals treated with haloperidol at any dose tested. While some of these results were unexpected, the overall pattern suggests that adenosine antagonists would be useful as therapies for Parkinsonian patients as they appear to increase movement. The results also suggest that in acute timelines A1 antagonists may be more beneficial than previously supposed.

Perinatal caffeine, acting on maternal adenosine A(1) receptors, causes long-lasting behavioral changes in mouse offspring

BACKGROUND

There are lingering concerns about caffeine consumption during pregnancy or the early postnatal period, partly because there may be long-lasting behavioral changes after caffeine exposure early in life.

METHODOLOGY/PRINCIPAL FINDINGS

We show that pregnant wild type (WT) mice given modest doses of caffeine (0.3 g/l in drinking water) gave birth to offspring that as adults exhibited increased locomotor activity in an open field. The offspring also responded to cocaine challenge with greater locomotor activity than mice not perinatally exposed to caffeine. We performed the same behavioral experiments on mice heterozygous for adenosine A(1) receptor gene (A(1)RHz). In these mice signaling via adenosine A(1) receptors is reduced to about the same degree as after modest consumption of caffeine. A(1)RHz mice had a behavioral profile similar to WT mice perinatally exposed to caffeine. Furthermore, it appeared that the mother's genotype, not offspring's, was critical for behavioral changes in adult offspring. Thus, if the mother partially lacked A(1) receptors the offspring displayed more hyperactivity and responded more strongly to cocaine stimulation as adults than did mice of a WT mother, regardless of their genotype. This indicates that long-term behavioral alterations in the offspring result from the maternal effect of caffeine, and not a direct effect on fetus. WT offspring from WT mother but having a A(1)R Hz grandmother preserved higher locomotor response to cocaine.

CONCLUSIONS/SIGNIFICANCE

We suggest that perinatal caffeine, by acting on adenosine A(1) receptors in the mother, causes long-lasting behavioral changes in the offspring that even manifest themselves in the second generation.

Purinergic signalling and disorders of the central nervous system

Purines have key roles in neurotransmission and neuromodulation, with their effects being mediated by the purine and pyrimidine receptor subfamilies, P1, P2X and P2Y. Recently, purinergic mechanisms and specific receptor subtypes have been shown to be involved in various pathological conditions including brain trauma and ischaemia, neurodegenerative diseases involving neuroimmune and neuroinflammatory reactions, as well as in neuropsychiatric diseases, including depression and schizophrenia. This article reviews the role of purinergic signalling in CNS disorders, highlighting specific purinergic receptor subtypes, most notably A(2A), P2X(4) and P2X(7), that might be therapeutically targeted for the treatment of these conditions.