Adenosine A(2A) Receptors and A(2A) Receptor Heteromers as Key Players in Striatal Function

Acute caffeine differently affects risk-taking and the expression of BDNF and of adenosine and opioid receptors in rats with high or low anxiety-like behavior

Anxiety disorders are common psychiatric conditions with a partially elucidated neurobiology. Caffeine, an unspecific adenosine receptor antagonist, is a common psychostimulant with anxiogenic effects in sensitive individuals. High doses of caffeine produce anxiety-like behavior in rats but it is not known if this is specific for rats with high baseline anxiety-like behavior. Thus, the aim of this study was to investigate general behavior, risk-taking, and anxiety-like behavior, as well as mRNA expression (adenosine A2A and A1, dopamine D2, and, μ, κ, δ opioid, receptors, BDNF, c-fos, IGF-1) in amygdala, caudate putamen, frontal cortex, hippocampus, hypothalamus, after an acute dose of caffeine. Untreated rats were screened using the elevated plus maze (EPM), giving each rat a score on anxiety-like behavior based on their time spent in the open arms, and categorized into a high or low anxiety-like behavior group accordingly. Three weeks after categorization, the rats were treated with 50 mg/kg caffeine and their behavior profile was studied in the multivariate concentric square field (MCSF) test, and one week later in the EPM. qPCR was performed on selected genes and corticosterone plasma levels were measured using ELISA. The results demonstrated that the high anxiety-like behavior rats treated with caffeine spent less time in risk areas of the MCSF and resituated towards the sheltered areas, a behavior accompanied by lower mRNA expression of adenosine A2A receptors in caudate putamen and increased BDNF expression in hippocampus. These results support the hypothesis that caffeine affects individuals differently depending on their baseline anxiety-like behavior, possibly involving adenosine receptors. This highlights the importance of adenosine receptors as a possible drug target for anxiety disorders, although further research is needed to fully elucidate the neurobiological mechanisms of caffeine on anxiety disorders.

The adenosine A2A receptor in the basal ganglia: Expression, heteromerization, functional selectivity and signalling

Adenosine is a neuroregulatory nucleoside that acts through four G protein-coupled receptors (GPCRs), A1, A2A, A2B and A3, which are widely expressed in cells of the nervous system. The A2A receptor (A2AR), the GPCR with the highest expression in the striatum, has a similar role to that of receptors for dopamine, one of the main neurotransmitters. Neuronal and glial A2ARs participate in the modulation of dopaminergic transmission and act in almost any action in which the basal ganglia is involved. This chapter revisits the expression of the A2AR in the basal ganglia in health and disease, and describes the diversity of signalling depending on whether the receptors are expressed as monomer or as heteromer. The A2AR can interact with other receptors as adenosine A1, dopamine D2, or cannabinoid CB1 to form heteromers with relevant functions in the basal ganglia. Heteromerization, with these and other GPCRs, provides diversity to A2AR-mediated signalling and to the modulation of neurotransmission. Thus, selective A2AR antagonists have neuroprotective potential acting directly on neurons, but also through modulation of glial cell activation, for example, by decreasing neuroinflammatory events that accompany neurodegenerative diseases. In fact, A2AR antagonists are safe and their potential in the therapy of Parkinson's disease has already led to the approval of one of them, istradefylline, in Japan and United States. The receptor also has a key role in reward circuits and, again, heteromers with dopamine receptors, but also with cannabinoid CB1 receptors, participate in the events triggered by drugs of abuse.

Role of adenosine A2A receptors in hot and cold cognition: Effects of single-dose istradefylline in healthy volunteers

The role of the adenosine neurochemical system in human cognition is under-studied, despite such receptors being distributed throughout the brain. The aim of this study was to shed light on the role of the adenosine A2A receptors in human cognition using single-dose istradefylline. Twenty healthy male participants, aged 19-49, received 20 mg istradefylline and placebo, in a randomized, double-blind, placebo-controlled cross-over design. Cognition was assessed using computerized cognitive tests, covering both cold (non-emotional) and hot (emotion-laden) domains. Cardiovascular data were recorded serially. Cognitive effects of istradefylline were explored using repeated measures analysis of variance and paired t-tests as appropriate. On the EMOTICOM battery, there was a significant effect of istradefylline versus placebo on the Social Information Preference task (t = 2.50, p = 0.02, d=-0.59), indicating that subjects on istradefylline interpreted social situations more positively. No other significant effects were observed on other cognitive tasks, nor in terms of cardiovascular measures (pulse and blood pressure). De-briefing indicated that blinding was successful, both for participants and the research team. Further exploration of the role of adenosine A2A receptors in emotional processing may be valuable, given that abnormalities in related cognitive functions are implicated in neuropsychiatric disorders. The role of adenosine systems in human cognition requires further clarification, including with different doses of istradefylline and over different schedules of administration.

Disrupting D1-NMDA or D2-NMDA receptor heteromerization prevents cocaine's rewarding effects but preserves natural reward processing

Addictive drugs increase dopamine in the nucleus accumbens (NAc), where it persistently shapes excitatory glutamate transmission and hijacks natural reward processing. Here, we provide evidence, from mice to humans, that an underlying mechanism relies on drug-evoked heteromerization of glutamate N-methyl-d-aspartate receptors (NMDAR) with dopamine receptor 1 (D1R) or 2 (D2R). Using temporally controlled inhibition of D1R-NMDAR heteromerization, we unraveled their selective implication in early phases of cocaine-mediated synaptic, morphological, and behavioral responses. In contrast, preventing D2R-NMDAR heteromerization blocked the persistence of these adaptations. Interfering with these heteromers spared natural reward processing. Notably, we established that D2R-NMDAR complexes exist in human samples and showed that, despite a decreased D2R protein expression in the NAc, individuals with psychostimulant use disorder display a higher proportion of D2R forming heteromers with NMDAR. These findings contribute to a better understanding of molecular mechanisms underlying addiction and uncover D2R-NMDAR heteromers as targets with potential therapeutic value.

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.

Functional roles of ST8SIA3-mediated sialylation of striatal dopamine D2 and adenosine A2A receptors

Sialic acids are typically added to the end of glycoconjugates by sialyltransferases. Among the six ST8 α-N-acetyl-neuraminide α-2,8-sialyltransferases (ST8SIA) existing in adult brains, ST8SIA2 is a schizophrenia-associated gene. However, the in vivo substrates and physiological functions of most sialyltransferases are currently unknown. The ST8SIA3 is enriched in the striatum. Here, we showed that ablation of St8sia3 in mice (St8sia3-KO) led to fewer disialylated and trisialylated terminal glycotopes in the striatum of St8sia3-KO mice. Moreover, the apparent sizes of several striatum-enriched G-protein-coupled receptors (GPCRs) (including the adenosine A_2A receptor (A_2AR) and dopamine D₁/D₂ receptors (D₁R and D₂R)) were smaller in St8sia3-KO mice than in WT mice. A sialidase treatment removed the differences in the sizes of these molecules between St8sia3-KO and WT mice, confirming the involvement of sialylation. Expression of ST8SIA3 in the striatum of St8sia3-KO mice using adeno-associated viruses normalized the sizes of these proteins, demonstrating a direct role of ST8SIA3. The lack of ST8SIA3-mediated sialylation altered the distribution of these proteins in lipid rafts and the interaction between D₂R and A_2AR. Locomotor activity assays revealed altered pharmacological responses of St8sia3-KO mice to drugs targeting these receptors and verified that a greater population of D₂R formed heteromers with A_2AR in the striatum of St8sia3-KO mice. Since the A_2AR-D₂R heteromer is an important drug target for several basal ganglia diseases (such as schizophrenia and Parkinson's disease), the present study not only reveals a crucial role for ST8SIA3 in striatal functions but also provides a new drug target for basal ganglia-related diseases.

Indirect Medium Spiny Neurons in the Dorsomedial Striatum Regulate Ethanol-Containing Conditioned Reward Seeking

Adenosine 2A receptor (A_2AR)-containing indirect medium spiny neurons (iMSNs) in the dorsomedial striatum (DMS) contribute to reward-seeking behaviors. However, those roles for ethanol-seeking behaviors remain unknown. To investigate ethanol-seeking behaviors, we used an ethanol-containing reward (10% ethanol and 10% sucrose solution; 10E10S). Upon conditioning with 10E10S, mice that initially only preferred 10% sucrose, not 10E10S, showed a stronger preference for 10E10S. Then, we investigated whether the manipulation of the DMS-external globus pallidus (GPe) iMSNs circuit alters the ethanol-containing reward (10E10S) seeking behaviors using the combination of pharmacologic and optogenetic approaches. DMS A_2AR activation dampened operant conditioning-induced ethanol-containing reward, whereas A_2AR antagonist abolished the effects of the A_2AR agonist and restored ethanol-containing reward-seeking. Moreover, pre-ethanol exposure potentiated the A_2AR-dependent reward-seeking. Interestingly, mice exhibiting ethanol-containing reward-seeking showed the reduction of the DMS iMSNs activity, suggesting that disinhibiting iMSNs decreases reward-seeking behaviors. In addition, we found that A_2AR activation reversed iMSNs neural activity in the DMS. Similarly, optogenetic stimulation of the DMS-GPe iMSNs reduced ethanol-containing reward-seeking, whereas optogenetic inhibition of the DMS-GPe iMSNs reversed this change. Together, our study demonstrates that DMS A_2AR and iMSNs regulate ethanol-containing reward-seeking behaviors.SIGNIFICANCE STATEMENT Our findings highlight the mechanisms of how operant conditioning develops the preference of ethanol-containing conditioned reward. Mice exhibiting ethanol-containing reward-seeking showed a reduction of the indirect medium spiny neuronal activity in the dorsomedial striatum. Pharmacological activation of adenosine A_2A receptor (A_2AR) or optogenetic activation of indirect medium spiny neurons dampened operant conditioned ethanol-containing reward-seeking, whereas inhibiting this neuronal activity restored ethanol-containing reward-seeking. Furthermore, repeated intermittent ethanol exposure potentiated A_2AR-dependent reward-seeking. Therefore, our finding suggests that A_2AR-containing indirect medium spiny neuronal activation reduces ethanol-containing reward-seeking, which may provide a potential therapeutic target for alcohol use disorder.

Essential Control of the Function of the Striatopallidal Neuron by Pre-coupled Complexes of Adenosine A2A-Dopamine D2 Receptor Heterotetramers and Adenylyl Cyclase

The central adenosine system and adenosine receptors play a fundamental role in the modulation of dopaminergic neurotransmission. This is mostly achieved by the strategic co-localization of different adenosine and dopamine receptor subtypes in the two populations of striatal efferent neurons, striatonigral and striatopallidal, that give rise to the direct and indirect striatal efferent pathways, respectively. With optogenetic techniques it has been possible to dissect a differential role of the direct and indirect pathways in mediating "Go" responses upon exposure to reward-related stimuli and "NoGo" responses upon exposure to non-rewarded or aversive-related stimuli, respectively, which depends on their different connecting output structures and their differential expression of dopamine and adenosine receptor subtypes. The striatopallidal neuron selectively expresses dopamine D₂ receptors (D2R) and adenosine A_2A receptors (A2AR), and numerous experiments using multiple genetic and pharmacological in vitro, in situ and in vivo approaches, demonstrate they can form A2AR-D2R heteromers. It was initially assumed that different pharmacological interactions between dopamine and adenosine receptor ligands indicated the existence of different subpopulations of A2AR and D2R in the striatopallidal neuron. However, as elaborated in the present essay, most evidence now indicates that all interactions can be explained with a predominant population of striatal A2AR-D2R heteromers forming complexes with adenylyl cyclase subtype 5 (AC5). The A2AR-D2R heteromer has a tetrameric structure, with two homodimers, which allows not only multiple allosteric interactions between different orthosteric ligands, agonists, and antagonists, but also the canonical Gs-Gi antagonistic interaction at the level of AC5. We present a model of the function of the A2AR-D2R heterotetramer-AC5 complex, which acts as an integrative device of adenosine and dopamine signals that determine the excitability and gene expression of the striatopallidal neurons. The model can explain most behavioral effects of A2AR and D2R ligands, including the psychostimulant effects of caffeine. The model is also discussed in the context of different functional striatal compartments, mainly the dorsal and the ventral striatum. The current accumulated knowledge of the biochemical properties of the A2AR-D2R heterotetramer-AC5 complex offers new therapeutic possibilities for Parkinson's disease, schizophrenia, SUD and other neuropsychiatric disorders with dysfunction of dorsal or ventral striatopallidal neurons.

Adenosine A2A Receptor Signaling in the Immunopathogenesis of Experimental Autoimmune Encephalomyelitis

Our increasing appreciation of adenosine as an endogenous signaling molecule that terminates inflammation has generated excitement regarding the potential to target adenosine receptors (ARs) in the treatment of multiple sclerosis (MS), a disease of chronic neuroinflammation. Of the four G protein-coupled ARs, A2ARs are the principal mediator of adenosine’s anti-inflammatory effects and accordingly, there is a growing body of evidence surrounding the role of A2ARs in experimental autoimmune encephalomyelitis (EAE), the dominant animal model of MS. Such evidence points to a complex, often paradoxical role for A2ARs in the immunopathogenesis of EAE, where they have the ability to both exacerbate and alleviate disease severity. This review seeks to interpret these paradoxical findings and evaluate the therapeutic promise of A2ARs. In essence, the complexities of A2AR signaling arise from two properties. Firstly, A2AR signaling downregulates the inflammatory potential of TH lymphocytes whilst simultaneously facilitating the recruitment of these cells into the CNS. Secondly, A2AR expression by myeloid cells – infiltrating macrophages and CNS-resident microglia – has the capacity to promote both tissue injury and repair in chronic neuroinflammation. Consequently, the therapeutic potential of targeting A2ARs is greatly undermined by the risk of collateral tissue damage in the periphery and/or CNS.

Inhibition of phosphodiesterases as a strategy to achieve neuroprotection in Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative condition, due to a mutation in the IT15 gene encoding for huntingtin. Currently, disease-modifying therapy is not available for HD, and only symptomatic drugs are administered for the management of symptoms. In the last few years, preclinical and clinical studies have indicated that pharmacological strategies aimed at inhibiting cyclic nucleotide phosphodiesterase (PDEs) may develop into a novel therapeutic approach in neurodegenerative disorders. PDEs are a family of enzymes that hydrolyze cyclic nucleotides into monophosphate isoforms. Cyclic nucleotides are second messengers that transduce the signal of hormones and neurotransmitters in many physiological processes, such as protein kinase cascades and synaptic transmission. An alteration in their balance results in the dysregulation of different biological mechanisms (transcriptional dysregulation, immune cell activation, inflammatory mechanisms, and regeneration) that are involved in neurological diseases. In this review, we discuss the action of phosphodiesterase inhibitors and their role as therapeutic agents in HD.

Angiotensin II type 1/adenosine A 2A receptor oligomers: a novel target for tardive dyskinesia

Tardive dyskinesia (TD) is a serious motor side effect that may appear after long-term treatment with neuroleptics and mostly mediated by dopamine D₂ receptors (D₂Rs). Striatal D₂R functioning may be finely regulated by either adenosine A_2A receptor (A_2AR) or angiotensin receptor type 1 (AT₁R) through putative receptor heteromers. Here, we examined whether A_2AR and AT₁R may oligomerize in the striatum to synergistically modulate dopaminergic transmission. First, by using bioluminescence resonance energy transfer, we demonstrated a physical AT₁R-A_2AR interaction in cultured cells. Interestingly, by protein-protein docking and molecular dynamics simulations, we described that a stable heterotetrameric interaction may exist between AT₁R and A_2AR bound to antagonists (i.e. losartan and istradefylline, respectively). Accordingly, we subsequently ascertained the existence of AT₁R/A_2AR heteromers in the striatum by proximity ligation in situ assay. Finally, we took advantage of a TD animal model, namely the reserpine-induced vacuous chewing movement (VCM), to evaluate a novel multimodal pharmacological TD treatment approach based on targeting the AT₁R/A_2AR complex. Thus, reserpinized mice were co-treated with sub-effective losartan and istradefylline doses, which prompted a synergistic reduction in VCM. Overall, our results demonstrated the existence of striatal AT₁R/A_2AR oligomers with potential usefulness for the therapeutic management of TD.

In vivo evaluation of [11C]preladenant positron emission tomography for quantification of adenosine A2A receptors in the rat brain

[¹¹C]Preladenant was developed as a novel adenosine A_2A receptor positron emission tomography radioligand. The present study aims to evaluate the suitability of [¹¹C]preladenant positron emission tomography for the quantification of striatal A_2A receptor density and the assessment of striatal A_2A receptor occupancy by KW-6002. Sixty- or ninety-minute dynamic positron emission tomography imaging was performed on rats. Tracer kinetics was quantified by the two-tissue compartment model, Logan graphical analysis and several reference tissue-based models. Test-retest reproducibility was assessed by repeated imaging on two consecutive days. Two-tissue compartment model and Logan plot estimated comparable distribution volume ( V_T) values of ∼10 in the A_2A receptor-rich striatum and substantially lower values in all extra-striatal regions (∼1.5-2.5). The simplified reference tissue model with midbrain or occipital cortex as the reference region proved to be the best non-invasive model for quantification of A_2A receptor, showing a striatal binding potential ( BP_ND) value of ∼5.5, and a test-retest variability of ∼5.5%. The brain metabolite analysis showed that at 60-min post injection, 17% of the radioactivity in the brain was due to radioactive metabolites. The ED₅₀ of KW-6002 in rat striatum for i.p. injection was 0.044-0.062 mg/kg. The study demonstrates that [¹¹C]preladenant is a suitable tracer to quantify striatal A_2A receptor density and assess A_2A receptor occupancy by A_2A receptor-targeting molecules.

Adult neurogenesis and neurodegenerative diseases: A systems biology perspective

New neurons are generated throughout adulthood in two regions of the brain, the olfactory bulb and dentate gyrus of the hippocampus, and are incorporated into the hippocampal network circuitry; disruption of this process has been postulated to contribute to neurodegenerative diseases including Alzheimer's disease and Parkinson's disease. Known modulators of adult neurogenesis include signal transduction pathways, the vascular and immune systems, metabolic factors, and epigenetic regulation. Multiple intrinsic and extrinsic factors such as neurotrophic factors, transcription factors, and cell cycle regulators control neural stem cell proliferation, maintenance in the adult neurogenic niche, and differentiation into mature neurons; these factors act in networks of signaling molecules that influence each other during construction and maintenance of neural circuits, and in turn contribute to learning and memory. The immune system and vascular system are necessary for neuronal formation and neural stem cell fate determination. Inflammatory cytokines regulate adult neurogenesis in response to immune system activation, whereas the vasculature regulates the neural stem cell niche. Vasculature, immune/support cell populations (microglia/astrocytes), adhesion molecules, growth factors, and the extracellular matrix also provide a homing environment for neural stem cells. Epigenetic changes during hippocampal neurogenesis also impact memory and learning. Some genetic variations in neurogenesis related genes may play important roles in the alteration of neural stem cells differentiation into new born neurons during adult neurogenesis, with important therapeutic implications. In this review, we discuss mechanisms of and interactions between these modulators of adult neurogenesis, as well as implications for neurodegenerative disease and current therapeutic research. © 2016 Wiley Periodicals, Inc.

Preclinical Evaluation and Quantification of 18F-Fluoroethyl and 18F-Fluoropropyl Analogs of SCH442416 as Radioligands for PET Imaging of the Adenosine A2A Receptor in Rat Brain

The cerebral adenosine A_2A receptor is an attractive therapeutic target for neuropsychiatric disorders. ¹⁸F-fluoroethyl and ¹⁸F-fluoropropyl analogs of ¹⁸F-labeled pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH442416) (¹⁸F-FESCH and ¹⁸F-FPSCH, respectively) were developed as A_2A receptor-specific PET ligands. Our aim was to determine an appropriate compartmental model for tracer kinetics, evaluate a reference tissue approach, and select the most suitable PET ligand. Methods: A 90-min dynamic PET scan with arterial blood sampling and metabolite analysis was acquired for 22 healthy male Wistar rats starting at the time of ¹⁸F-FESCH (n = 12) and ¹⁸F-FPSCH (n = 10) injection. For each tracer, half the animals were vehicle-treated whereas the other half were pretreated with the A_2A receptor-selective antagonist KW-6002, inducing full blocking. Regional tissue total volume of distribution (V_T) was estimated by 1- and 2-tissue-compartment modeling (1TCM and 2TCM, respectively) and Logan graphical analysis. Midbrain, cerebellum, and hippocampus were evaluated as the reference region by comparing baseline V_T with V_T under full blocking conditions and comparing striatal nondisplaceable binding potential (BP_ND) using a simplified reference tissue model (SRTM) with distribution volume ratio minus 1 (DVR - 1) for 60- and 90-min scans. Results: On the basis of the Akaike information criterion, 1TCM and 2TCM were the most appropriate models for ¹⁸F-FPSCH (baseline striatal V_T, 3.7 ± 1.1) and ¹⁸F-FESCH (baseline striatal V_T, 5.0 ± 2.0), respectively. Baseline striatal V_T did not significantly differ between tracers. After pretreatment, striatal V_T was reduced significantly, with no significant decrease in hippocampus, midbrain, or cerebellum V_T Baseline striatal SRTM BP_ND did not differ significantly from DVR - 1 except for ¹⁸F-FPSCH when using a 60-min scan and midbrain as the reference region, whereas Bland-Altman analysis found a smaller bias for ¹⁸F-FESCH and a 60-min scan. After pretreatment, striatal SRTM BP_ND did not significantly differ from zero except for ¹⁸F-FPSCH when using hippocampus as the reference region. Striatal SRTM BP_ND using midbrain or cerebellum as the reference region was significantly lower for ¹⁸F-FPSCH (range, 1.41-2.62) than for ¹⁸F-FESCH (range, 1.64-3.36). Conclusion: Dynamic PET imaging under baseline and blocking conditions determined ¹⁸F-FESCH to be the most suitable PET ligand for quantifying A_2A receptor expression in the rat brain. Accurate quantification is achieved by a 60-min dynamic PET scan and the use of either cerebellum or midbrain as the reference region.

2-Amino[1,2,4]triazolo[1,5-c]quinazolines and Derived Novel Heterocycles: Syntheses and Structure-Activity Relationships of Potent Adenosine Receptor Antagonists

2-Amino[1,2,4]triazolo[1,5-c]quinazolines were identified as potent adenosine receptor (AR) antagonists. Synthetic strategies were devised to gain access to a broad range of derivatives including novel polyheterocyclic compounds. Potent and selective A₃ AR antagonists were discovered, including 3,5-diphenyl[1,2,4]triazolo[4,3-c]quinazoline (17, K_i human A₃ AR 1.16 nm) and 5'-phenyl-1,2-dihydro-3'H-spiro[indole-3,2'-[1,2,4]triazolo[1,5-c]quinazolin]-2-one (20, K_i human A₃ AR 6.94 nm). In addition, multitarget antagonists were obtained, such as the dual A₁ /A₃ antagonist 2,5-diphenyl[1,2,4]triazolo[1,5-c]quinazoline (13 b, K_i human A₁ AR 51.6 nm, human A₃ AR 11.1 nm), and the balanced pan-AR antagonists 5-(2-thienyl)[1,2,4]triazolo[1,5-c]quinazolin-2-amine (11 c, K_i human A₁ AR 131 nm, A_2A AR 32.7 nm, A_2B AR 150 nm, A₃ AR 47.5 nm) and 9-bromo-5-phenyl[1,2,4]triazolo[1,5-c]quinazolin-2-amine (11 q, K_i human A₁ AR 67.7 nm, A_2A AR 13.6 nm, A_2B AR 75.0 nm, A₃ AR 703 nm). In many cases, significantly different affinities for human and rat receptors were observed, which emphasizes the need for caution in extrapolating conclusions between different species.

Phosphodiesterase Inhibition and Regulation of Dopaminergic Frontal and Striatal Functioning: Clinical Implications

BACKGROUND

The fronto-striatal circuits are the common neurobiological basis for neuropsychiatric disorders, including schizophrenia, Parkinson's disease, Huntington's disease, attention deficit hyperactivity disorder, obsessive-compulsive disorder, and Tourette's syndrome. Fronto-striatal circuits consist of motor circuits, associative circuits, and limbic circuits. All circuits share 2 common features. First, all fronto-striatal circuits consist of hyper direct, direct, and indirect pathways. Second, all fronto-striatal circuits are modulated by dopamine. Intracellularly, the effect of dopamine is largely mediated through the cyclic adenosine monophosphate/protein kinase A signaling cascade with an additional role for the cyclic guanosine monophosphate/protein kinase G pathway, both of which can be regulated by phosphodiesterases. Phosphodiesterases are thus a potential target for pharmacological intervention in neuropsychiatric disorders related to dopaminergic regulation of fronto-striatal circuits.

METHODS

Clinical studies of the effects of different phosphodiesterase inhibitors on cognition, affect, and motor function in relation to the fronto-striatal circuits are reviewed.

RESULTS

Several selective phosphodiesterase inhibitors have positive effects on cognition, affect, and motor function in relation to the fronto-striatal circuits.

CONCLUSION

Increased understanding of the subcellular localization and unraveling of the signalosome concept of phosphodiesterases including its function and dysfunction in the fronto-striatal circuits will contribute to the design of new specific inhibitors and enhance the potential of phosphodiesterase inhibitors as therapeutics in fronto-striatal circuits.

Cross-signaling in metabotropic glutamate 2 and serotonin 2A receptor heteromers in mammalian cells

We previously reported that co-expression of the Gi-coupled metabotropic glutamate receptor 2 (mGlu2R) and the Gq-coupled serotonin (5-HT) 2A receptor (2AR) in Xenopus oocytes (Fribourg et al. Cell 147:1011-1023, 2011) results in inverse cross-signaling, where for either receptor, strong agonists suppress and inverse agonists potentiate the signaling of the partner receptor. Importantly, through this cross-signaling, the mGlu2R/2AR heteromer integrates the actions of psychedelic and antipsychotic drugs. To investigate whether mGlu2R and 2AR can cross-signal in mammalian cells, we stably co-expressed them in HEK293 cells along with the GIRK1/GIRK4 channel, a reporter of Gi and Gq signaling activity. Crosstalk-positive clones were identified by Fura-2 calcium imaging, based on potentiation of 5-HT-induced Ca(2+) responses by the inverse mGlu2/3R agonist LY341495. Cross-signaling from both sides of the complex was confirmed in representative clones by using the GIRK channel reporter, both in whole-cell patch-clamp and in fluorescence assays using potentiometric dyes, and further established by competition binding assays. Notably, only 25-30 % of the clones were crosstalk-positive. The crosstalk-positive phenotype correlated with (a) increased colocalization of the two receptors at the cell surface, (b) lower density of mGlu2R binding sites and higher density of 2AR binding sites in total membrane preparations, and (c) higher ratios of mGlu2R/2AR normalized surface protein expression. Consistent with our results in Xenopus oocytes, a combination of ligands targeting both receptors could elicit functional crosstalk in a crosstalk-negative clone. Crosstalk-positive clones can be used in high-throughput assays for identification of antipsychotic drugs targeting this receptor heterocomplex.

The GPCR heterotetramer: challenging classical pharmacology

Two concepts are gaining increasing acceptance in G protein-coupled receptor (GPCR) pharmacology: (i) pre-coupling of GPCRs with their preferred signaling molecules, and (ii) GPCR oligomerization. This is begging for the introduction of new models such as GPCR oligomer-containing signaling complexes with GPCR homodimers as functional building blocks. This model favors the formation of GPCR heterotetramers - heteromers of homodimers coupled to their cognate G protein. The GPCR heterotetramer offers an optimal framework for a canonical antagonistic interaction between activated Gs and Gi proteins, which can simultaneously bind to their respective preferred receptors and to adenylyl cyclase (AC) catalytic units. This review addresses the current evidence for pre-coupling of the various specific components that provide the very elaborate signaling machinery exemplified by the Gs-Gi-AC-coupled GPCR heterotetramer.

Presynaptic adenosine A2A receptors dampen cannabinoid CB1 receptor-mediated inhibition of corticostriatal glutamatergic transmission

BACKGROUND AND PURPOSE

Both cannabinoid CB1 and adenosine A2A receptors (CB1 receptors and A2A receptors) control synaptic transmission at corticostriatal synapses, with great therapeutic importance for neurological and psychiatric disorders. A postsynaptic CB1 -A2A receptor interaction has already been elucidated, but the presynaptic A2A receptor-mediated control of presynaptic neuromodulation by CB1 receptors remains to be defined. Because the corticostriatal terminals provide the major input to the basal ganglia, understanding the interactive nature of converging neuromodulation on them will provide us with novel powerful tools to understand the physiology of corticostriatal synaptic transmission and interpret changes associated with pathological conditions.

EXPERIMENTAL APPROACH

Pharmacological manipulation of CB1 and A2A receptors was carried out in brain nerve terminals isolated from rats and mice, using flow synaptometry, immunoprecipitation, radioligand binding, ATP and glutamate release measurement. Whole-cell patch-clamp recordings were made in horizontal corticostriatal slices.

KEY RESULTS

Flow synaptometry showed that A2A receptors were extensively co-localized with CB1 receptor-immunopositive corticostriatal terminals and A2A receptors co-immunoprecipitated CB1 receptors in these purified terminals. A2A receptor activation decreased CB1 receptor radioligand binding and decreased the CB1 receptor-mediated inhibition of high-K(+) -evoked glutamate release in corticostriatal terminals. Accordingly, A2A receptor activation prevented CB1 receptor-mediated paired-pulse facilitation and attenuated the CB1 receptor-mediated inhibition of synaptic transmission in glutamatergic synapses of corticostriatal slices.

CONCLUSIONS AND IMPLICATIONS

Activation of presynaptic A2A receptors dampened CB1 receptor-mediated inhibition of corticostriatal terminals. This constitutes a thus far unrecognized mechanism to modulate the potent CB1 receptor-mediated presynaptic inhibition, allowing frequency-dependent enhancement of synaptic efficacy at corticostriatal synapses.

Intracellular calcium levels determine differential modulation of allosteric interactions within G protein-coupled receptor heteromers

The pharmacological significance of the adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) heteromer is well established and it is being considered as an important target for the treatment of Parkinson’s disease and other neuropsychiatric disorders. However, the physiological factors that control its distinctive biochemical properties are still unknown. We demonstrate that different intracellular Ca2+ levels exert a differential modulation of A2AR-D2R heteromer-mediated adenylyl-cyclase and MAPK signaling in striatal cells. This depends on the ability of low and high Ca2+ levels to promote a selective interaction of the heteromer with the neuronal Ca2+-binding proteins NCS-1 and calneuron-1, respectively. These Ca2+-binding proteins differentially modulate allosteric interactions within the A2AR-D2R heteromer, which constitutes a unique cellular device that integrates extracellular (adenosine and dopamine) and intracellular (Ca+2) signals to produce a specific functional response.

Synthesis and preclinical evaluation of 2-(2-furanyl)-7-[2-[4-[4-(2-[11C]methoxyethoxy)phenyl]-1-piperazinyl]ethyl]7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine-5-amine ([11C]Preladenant) as a PET tracer for the imaging of cerebral adenosine A2A receptors

2-(2-Furanyl)-7-[2-[4-[4-(2-[(11)C]methoxyethoxy)phenyl]-1-piperazinyl]ethyl]7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine-5-amine [(11)C]-3 ([(11)C]Preladenant) was developed for mapping cerebral adenosine A2A receptors (A2ARs) with PET. The tracer was synthesized in high specific activity and purity. Tissue distribution was studied by PET imaging, ex vivo biodistribution (BD), and in vitro autoradiography (ARG) experiments. Regional brain uptake of [(11)C]-3 was consistent with known A2ARs distribution, with highest uptake in striatum. The results indicate that [(11)C]-3 has favorable brain kinetics and exhibits suitable characteristics as an A2AR PET tracer.

The effects of nucleus accumbens μ-opioid and adenosine 2A receptor stimulation and blockade on instrumental learning

Prior research has shown that glutamate and dopamine receptors in the nucleus accumbens (NAcc) core are critical for the learning of an instrumental response for food reinforcement. It has also been demonstrated that μ-opioid and adenosine A2A receptors within the NAcc impact feeding and motivational processes. In these experiments, we examined the potential roles of NAcc μ-opioid and A2A receptors on instrumental learning and performance. Sprague-Dawley rats were food restricted and trained to lever press following daily intra-accumbens injections of the A2A receptor agonist CGS 21680 (at 0.0, 6.0, or 24.0ng/side), the A2A antagonist pro-drug MSX-3 (at 0.0, 1.0, or 3.0μg/side), the μ-opioid agonist DAMGO (at 0.0, 0.025, or 0.025μg/side), or the opioid receptor antagonist naltrexone (at 0.0, 2.0 or 20.0μg/side). After five days, rats continued training without drug injections until lever pressing rates stabilized, and were then tested with a final drug test to assess potential performance effects. Stimulation, but not inhibition, of NAcc adenosine A2A receptors depressed lever pressing during learning and performance tests, but did not impact lever pressing on non-drug days. Both μ-opioid receptor stimulation and blockade inhibited learning of the lever-press response, though only naltrexone treatment caused impairments in lever-pressing after the task had been learned. The effect of A2A receptor stimulation on learning and performance were consistent with known effects of adenosine on effort-related processes, whereas the pattern of lever presses, magazine approaches, and pellet consumption following opioid receptor manipulations suggested that their effects may have been driven by drug-induced shifts in the incentive value of the sugar reinforcer.

Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A 2A receptors during extinction training in rats

RATIONALE

Adenosine receptor stimulation and blockade have been shown to modulate a variety of cocaine-related behaviors.

OBJECTIVES

These studies identify the direct effects of adenosine receptor stimulation on cocaine seeking during extinction training and the persistent effects on subsequent reinstatement to cocaine seeking.

METHODS

Rats self-administered cocaine on a fixed ratio one schedule in daily sessions over 3 weeks. Following a 1-week withdrawal, the direct effects of adenosine receptor modulation were tested by administering the adenosine A1 receptor agonist, N(6)-cyclopentyladenosine (CPA, 0.03 and 0.1 mg/kg), the adenosine A2A agonist, CGS 21680 (0.03 and 0.1 mg/kg), the presynaptic adenosine A2A receptor antagonist, SCH 442416 (0.3, 1, and 3 mg/kg), or vehicle prior to each of six daily extinction sessions. The persistent effects of adenosine receptor modulation during extinction training were subsequently tested on reinstatement to cocaine seeking induced by cues, cocaine, and the dopamine D2 receptor agonist, quinpirole.

RESULTS

All doses of CPA and CGS 21680 impaired initial extinction responding; however, only CPA treatment during extinction produced persistent impairment in subsequent cocaine- and quinpirole-induced seeking. Dissociating CPA treatment from extinction did not alter extinction responding or subsequent reinstatement. Administration of SCH 442416 had no direct effects on extinction responding but produced dose-dependent persistent impairment of cocaine- and quinpirole-induced seeking.

CONCLUSIONS

These findings demonstrate that adenosine A1 or A2A receptor stimulation directly impair extinction responding. Interestingly, adenosine A1 receptor stimulation or presynaptic adenosine A2A receptor blockade during extinction produces lasting changes in relapse susceptibility.

Adenosine receptors: expression, function and regulation

Adenosine receptors (ARs) comprise a group of G protein-coupled receptors (GPCR) which mediate the physiological actions of adenosine. To date, four AR subtypes have been cloned and identified in different tissues. These receptors have distinct localization, signal transduction pathways and different means of regulation upon exposure to agonists. This review will describe the biochemical characteristics and signaling cascade associated with each receptor and provide insight into how these receptors are regulated in response to agonists. A key property of some of these receptors is their ability to serve as sensors of cellular oxidative stress, which is transmitted by transcription factors, such as nuclear factor (NF)-κB, to regulate the expression of ARs. Recent observations of oligomerization of these receptors into homo- and heterodimers will be discussed. In addition, the importance of these receptors in the regulation of normal and pathological processes such as sleep, the development of cancers and in protection against hearing loss will be examined.

Comparison of Caffeine and d-amphetamine in Cocaine-Dependent Subjects: Differential Outcomes on Subjective and Cardiovascular Effects, Reward Learning, and Salivary Paraxanthine

Due to indirect modulation of dopamine transmission, adenosine receptor antagonists may be useful in either treating cocaine use or improving disrupted cognitive-behavioral functions associated with chronic cocaine use. To compare and contrast the stimulant effects of adenosine antagonism to direct dopamine stimulation, we administered 150 mg and 300 mg caffeine, 20 mg amphetamine, and placebo to cocaine-dependent vs. healthy control subjects, matched on moderate caffeine use. Data were obtained on measures of cardiovascular effects, subjective drug effects (ARCI, VAS, DEQ), and a probabilistic reward-learning task sensitive to dopamine modulation. Levels of salivary caffeine and the primary caffeine metabolite paraxanthine were obtained on placebo and caffeine dosing days. Cardiovascular results revealed main effects of dose for diastolic blood pressure and heart rate; follow up tests showed that controls were most sensitive to 300 mg caffeine and 20 mg amphetamine; cocaine-dependent subjects were sensitive only to 300 mg caffeine. Subjective effects results revealed dose × time and dose × group interactions on the ARCI A, ARCI LSD, and VAS 'elated' scales; follow up tests did not show systematic differences between groups with regard to caffeine or d-amphetamine. Large between-group differences in salivary paraxanthine (but not salivary caffeine) levels were obtained under both caffeine doses. The cocaine-dependent group expressed significantly higher paraxanthine levels than controls under 150 mg and 3-4 fold greater levels under 300 mg at 90 min and 150 min post caffeine dose. However, these differences also covaried with cigarette smoking status (not balanced between groups), and nicotine smoking is known to alter caffeine/paraxanthine metabolism via cytochrome P450 enzymes. These preliminary data raise the possibility that adenosine antagonists may affect cocaine-dependent and non-dependent subjects differently. In conjunction with previous preclinical and human studies, the data suggest that adenosine modulating drugs may have value in the treatment of stimulant use disorders.

L-DOPA-treatment in primates disrupts the expression of A(2A) adenosine-CB(1) cannabinoid-D(2) dopamine receptor heteromers in the caudate nucleus

The molecular basis of priming for L-DOPA-induced dyskinesias in Parkinson's disease (PD), which depends on the indirect pathway of motor control, is not known. In rodents, the indirect pathway contains striatopallidal GABAergic neurons that express heterotrimers composed of A(2A) adenosine, CB(1) cannabinoid and D(2) dopamine receptors that regulate dopaminergic neurotransmission. The present study was designed to investigate the expression of these heteromers in the striatum of a primate model of Parkinson's disease and to determine whether their expression and pharmacological properties are altered upon L-DOPA treatment. By using the recently developed in situ proximity ligation assay and by identification of a biochemical fingerprint, we discovered a regional distribution of A(2A)/CB(1) /D(2) receptor heteromers that predicts differential D(2)-mediated neurotransmission in the caudate-putamen of Macaca fascicularis. Whereas heteromers were abundant in the caudate nucleus of both naïve and MPTP-treated monkeys, L-DOPA treatment blunted the biochemical fingerprint and led to weak heteromer expression. These findings constitute the first evidence of altered receptor heteromer expression in pathological conditions and suggest that drugs targeting A(2A)-CB(1) -D(2) receptor heteromers may be successful to either normalize basal ganglia output or prevent L-DOPA-induced side effects.

1,3-Dialkyl-substituted tetrahydropyrimido[1,2-f]purine-2,4-diones as multiple target drugs for the potential treatment of neurodegenerative diseases

Adenosine receptors and monoamine oxidases are drug targets for neurodegenerative diseases such as Parkinson's and Alzheimer's disease. In the present study we prepared a library of 55 mostly novel tetrahydropyrimido[2,1-f]purinediones with various substituents in the 1- and 3-position (1,3-dimethyl, 1,3-diethyl, 1,3-dipropyl, 1-methyl-3-propargyl) and broad variation in the 9-position. A synthetic strategy to obtain 3-propargyl-substituted tetrahydropyrimido[2,1-f]purinedione derivatives was developed. The new compounds were evaluated for their interaction with all four adenosine receptor subtypes and for their ability to inhibit monoamine oxidases (MAO). Introduction of mono- or di-chloro-substituted phenyl, benzyl or phenethyl residues at N9 of the 1,3-dimethyl series led to the discovery of a novel class of potent MAO-B inhibitors, the most potent compound being 9-(3,4-dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)-dione (21g, IC(50) human MAO-B: 0.0629 μM), which displayed high selectivity versus the other investigated targets. Potent dually active A1/A2A adenosine receptor antagonists were identified, for example, 9-benzyl-1-methyl-3-propargyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)dione (19f, Ki, human receptors, A1: 0.249 μM, A2A: 0.253 μM). Several compounds showed triple-target inhibition, the best compound being 9-(2-methoxybenzyl)-1-methyl-3-(prop-2-ynyl)-6,7,8,9-tetrahydro pyrimido [1,2-f]purine-2,4(1H,3H)-dione (19g, Ki A1: 0.605 μM, Ki A2A: 0.417 μM, IC(50) MAO-B: 1.80 μM). Compounds inhibiting several different targets involved in neurodegeneration may exhibit additive or even synergistic effects in vivo.

Pre- and postsynaptic dopamine SPECT in idiopathic Parkinsonian diseases: a follow-up study

We prospectively evaluated the diagnostic contribution of (123)I-FP-Cit (DAT) and (123)I-IBZM (IBZM) SPECT in 29 patients with Parkinson's disease (PD) (74.4 ± 4.2 years) and 28 patients with atypical parkinsonian diseases (APD) (74.3 ± 9.2 years). Twelve had multiple system atrophy (MSA) and 16 progressive supranuclear palsy (PSP). Sixteen age-matched healthy controls (HC) were included. DAT and IBZM SPECTs were made at baseline and after 1 year in all PD patients and in 20 (DAT) and 18 (IBZM) of the APD patients, and after 3 years in 22 (DAT) and 17 (IBZM) of the PD patients and in 10 (DAT) and 10 (IBZM) of the APD patients. The relative DAT uptake decrease was faster in PD and PSP than in HC and MSA. In PSP the DAT uptake was lower than in MSA after 1 year but not after 3 years. Baseline IBZM uptake was not significantly different between patients and HC or between PD and APD. One year after initiated dopaminergic treatment the mean IBZM uptake in the MSA patients remained high compared to PSP and after 3 years compared to PD, PSP, and HC. Thus, the pattern of uptake of these ligands over time may be of value in discriminating between these diagnoses.

Exercise modifies α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor expression in striatopallidal neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mouse

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid-type glutamate receptor (AMPAR) plays a critical role in modulating experience-dependent neuroplasticity, and alterations in AMPAR expression may underlie synaptic dysfunction and disease pathophysiology. Using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of dopamine (DA) depletion, our previous work showed exercise increases total GluA2 subunit expression and the contribution of GluA2-containing channels in MPTP mice. The purpose of this study was to determine whether exercise-dependent changes in AMPAR expression after MPTP are specific to the striatopallidal (D2 R) or striatonigral (D1 R) medium spiny neuron (MSN) striatal projection pathways. Drd2 -eGFP-BAC transgenic mice were used to delineate differences in AMPAR expression between striatal D2 R-MSNs and D1 R-MSNs. Striatal AMPAR expression was assessed by immunohistochemical (IHC) staining, Western immunoblotting (WB) of preparations enriched for postsynaptic density (PSD), and alterations in the current-voltage relationship of MSNs. We found DA depletion results in the emergence of GluA2-lacking AMPARs selectively in striatopallidal D2 R-MSNs and that exercise reverses this effect in MPTP mice. Exercise-induced changes in AMPAR channels observed after DA depletion were associated with alterations in GluA1 and GluA2 subunit expression in postsynaptic protein, D2 R-MSN cell surface expression, and restoration of corticostriatal plasticity. Mechanisms regulating experience-dependent changes in AMPAR expression may provide innovative therapeutic targets to increase the efficacy of treatments for basal ganglia disorders, including Parkinson's disease.

Distribution and compartmental organization of GABAergic medium-sized spiny neurons in the mouse nucleus accumbens

The nucleus accumbens (NAc) is a critical brain region involved in many reward-related behaviors. The NAc comprises major compartments the core and the shell, which encompass several subterritories. GABAergic medium-sized spiny neurons (MSNs) constitute the output neurons of the NAc core and shell. While the functional organization of the NAc core outputs resembles the one described for the dorsal striatum, a simple classification of the NAc shell neurons has been difficult to define due to the complexity of the compartmental segregation of cells. We used a variety of BAC transgenic mice expressing enhanced green fluorescence (EGFP) or the Cre-recombinase (Cre) under the control of the promoter of dopamine D1, D2, and D3 receptors and of adenosine A2a receptor to dissect the microanatomy of the NAc. Moreover, using various immunological markers we characterized in detail the distribution of MSNs in the mouse NAc. In addition, cell-type specific extracellular signal-regulated kinase (ERK) phosphorylation in the NAc subterritories was analyzed following acute administration of SKF81297 (a D1R-like agonist), quinpirole (a D2 receptors (D2R)-like agonist), apomorphine (a non-selective DA receptor agonist), raclopride (a D2R-like antagonist), and psychostimulant drugs, including cocaine and d-amphetamine. Each drug generated a unique topography and cell-type specific activation of ERK in the NAc. Our results show the existence of marked differences in the receptor expression pattern and functional activation of MSNs within the shell subterritories. This study emphasizes the anatomical and functional heterogeneity of the NAc, which will have to be considered in its further study.

Adenosine A(2A)-cannabinoid CB(1) receptor interaction: an integrative mechanism in striatal glutamatergic neurotransmission

The striatum is a subcortical area involved in sensorimotor, cognitive and emotional processes. Adenosine A(2A) receptors (A(2A)Rs) are highly expressed in the striatum, and their ability to establish functional and molecular interactions with many other receptors attributes to a pivotal role in the modulation and integration of striatal neurotransmission. This review will focus on the interaction between A(2A)Rs and cannabinoid CB(1) receptors (CB(1)Rs), taking it as a paradigmatic example of synaptic integration. Indeed, A(2A)Rs can exert an opposite (permissive vs. inhibitory) influence on CB1-dependent synaptic effect. These apparently irreconcilable functions could depend on a different role of pre- vs. postsynaptic A(2A)Rs, on their interaction with other receptors (namely adenosine A(1), metabotropic glutamate 5 and dopamine D2 receptors), and on whether A(2A)Rs form or not heteromers with CB(1)Rs. Besides providing a good example of the intricate pattern of events taking place in striatal synapses, the A(2A)/CB(1)R interaction proves very informative to understand the physiology of the basal ganglia and the mechanisms of related diseases. This article is part of a Special Issue entitled: Brain Integration.

Cardiovascular and Subjective Effects of the Novel Adenosine A(2A) Receptor Antagonist SYN115 in Cocaine Dependent Individuals

A(2A) receptor antagonists have been proposed as therapeutic tools for dopaminergically-relevant diseases, including Parkinson's disease and substance dependence. The acute subjective and cardiovascular effects of a novel, selective adenosine A(2A) receptor antagonist (SYN115) were examined. Across an 8-hour experimental testing day, 22 non-treatment seeking cocaine-dependent subjects received either placebo capsules (PO) at both the AM and PM dosing times (Plc/Plc, N = 9), or placebo in the AM and 100 mg SYN115 in the PM (Plc/SYN115, N =13). Cardiovascular measures (HR, BP) were obtained across the test day, and subjective effects (ARCI, VAS) were obtained once before and once after the AM and PM doses (four time points total). There were no between-group effects on cardiovascular function, however subjective effects consistent with stimulation were observed on the VAS scales in the SYN115 group. In cocaine-dependent subjects, SYN115 may produce stimulant-like effects through a unique mechanism of action. Due to known monoamine dysfunction related to chronic cocaine use, these effects may be specific to this population relative to healthy control or other patient populations.

Adenosine A2A receptor contributes to the anti-inflammatory effect of the fixed herbal combination STW 5 (Iberogast®) in rat small intestinal preparations

STW 5 (Iberogast®), an established herbal combination, was effective in randomized, double blind clinical studies in functional dyspepsia and irritable bowel syndrome. Since STW 5 was found to influence intestinal motility and has anti-inflammatory properties, this study investigated the expression of adenosine receptors and characterized their role in the control of the anti-inflammatory action of STW 5 and its fresh plant component STW 6 in inflammation-disturbed rat small intestinal preparations. The inflammation was induced by intraluminal instillation of 2,4,6-trinitrobenzene sulfonic acid (TNBS, 0.01 M). The effects of coincubation with selective receptor agonists and antagonists, STW 5, STW 6, or combinations of these compounds on acetylcholine (ACh)-evoked contraction of ileum/jejunum preparations were tested. Adenosine receptor mRNA expression was examined by reverse transcription-polymerase chain reaction (RT-PCR). In untreated preparations, RT-PCR revealed the presence of all adenosine receptor subtypes. Suppressed expression was detected for all subtypes in inflamed tissues, except for A(2B)R mRNA, which was unaffected. STW 5 reversed these effects and enhanced A(2A)R expression above control levels. Radioligand binding assays confirm the affinity of STW 5 to the A(2A)R, and the A(2A)R antagonist was able to prevent the effect of STW 5 on TNBS-induced attenuation of the ACh contraction. Our findings provide evidence that STW 5, but not STW 6 interacts with A(2A)R, which is involved in the anti-inflammatory action of STW 5. STW 6 did not contribute to adenosine A(2A)R-mediated anti-inflammatory effect of STW 5. Other signaling pathways could be involved in the mechanism of action of STW 6.

Functional changes in postsynaptic adenosine A(2A) receptors during early stages of a rat model of Huntington disease

Huntington disease (HD) is a neurodegenerative disorder involving preferential loss of striatal GABAergic medium spiny neurons. Adenosine A(2A) receptors (A(2A)Rs) are present in the striatum at both presynaptic and post-synaptic levels. Blocking pre-synaptic A(2A)Rs, localized in glutamatergic terminals that contact striatal GABAergic dynorphinergic neurons, reduces glutamate release, which could be beneficial in HD. On the other hand, blockade of post-synaptic A(2A)Rs, localized in striatal GABAergic enkephalinergic neurons, could exacerbate the motor dysfunction. To evaluate the function of pre- or post-synaptic A(2A)Rs in HD we used selective antagonists for these receptors in a transgenic rat model of HD. Locomotor activity after systemic administration of the postsynaptic A(2A)R antagonist KW-6002 was used to investigate the function of post-synaptic A(2A)Rs. The role of pre-synaptic A(2A)Rs was instead evaluated by measuring the reduction of the electromyographic response of mastication muscles during electrical stimulation of the orofacial motor cortex after the systemic administration of the presynaptic A(2A)R antagonist SCH-442416. The ability of KW-6002 to produce locomotor activation was lost at 6 and 12 month-old of age in heterozygous and homozygous transgenic rats, but not in wild-type littermates. Nevertheless, no significant changes were observed up to 12 months of age in the potency of SCH-442416 to decrease the electromyographic response after cortical electrical stimulation. These results agree with a selective impairment of the striatal GABAergic enkephalinergic neuronal function during pre-symptomatic stages in HD. Since presynaptic A(2A)R function is not impaired, this receptor could probably be used as a target for the symptomatic treatment of the disease.