Adenosine A2a receptor/dopamine D2 receptor hetero-oligomerization: a hypothesis that may explain behavioral sensitization to psychostimulants and schizophrenia

Novel approaches to drug design for the treatment of schizophrenia

INTRODUCTION

Schizophrenia is an important health issue affecting almost 1% of the population with significant unmet medical needs. The classical drug targets for the treatment of schizophrenia are dopamine D2 receptors. Second-generation ('atypical') drugs block more receptors of the G-protein-coupled receptor (GPCR) class 1 (e.g., clozapine is a D(2)-5HT(2) antagonist).

AREAS COVERED

In this article, the author presents the new targets for GPCR as well as ligand-gated ion. Furthermore, the author reviews the opportunities for drug design offered by the structures solved recently.

EXPERT OPINION

For drug design, the availability of these protein structures, or the possibility to build high quality models, allows to shift the paradigm from ligand-based to target-based drug design. The analysis of the drugs, both on the market and in development, shows that numerous targets are being considered which may reveal an ambiguity on the ideal drug target. This situation may be simplified, in the future, owing to recent integrative projects: the 'Human Brain Project' and the 'Brain Activity Map' that aim at modeling the brain as well as the Allen Atlas.

Receptor crosstalk: haloperidol treatment enhances A(2A) adenosine receptor functioning in a transfected cell model

A(2A) adenosine receptors are considered an excellent target for drug development in several neurological and psychiatric disorders. It is noteworthy that the responses evoked by A(2A) adenosine receptors are regulated by D(2) dopamine receptor ligands. These two receptors are co-expressed at the level of the basal ganglia and interact to form functional heterodimers. In this context, possible changes in A(2A) adenosine receptor functional responses caused by the chronic blockade/activation of D(2) dopamine receptors should be considered to optimise the therapeutic effectiveness of dopaminergic agents and to reduce any possible side effects. In the present paper, we investigated the regulation of A(2A) adenosine receptors induced by antipsychotic drugs, commonly acting as D(2) dopamine receptor antagonists, in a cellular model co-expressing both A(2A) and D(2) receptors. Our data suggest that the treatment of cells with the classical antipsychotic haloperidol increased both the affinity and responsiveness of the A(2A) receptor and also affected the degree of A(2A)-D(2) receptor heterodimerisation. In contrast, an atypical antipsychotic, clozapine, had no effect on A(2A) adenosine receptor parameters, suggesting that the two classes of drugs have different effects on adenosine-dopamine receptor interaction. Modifications to A(2A) adenosine receptors may play a significant role in determining cerebral adenosine effects during the chronic administration of antipsychotics in psychiatric diseases and may account for the efficacy of A(2A) adenosine receptor ligands in pathologies associated with dopaminergic system dysfunction.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9201-z) contains supplementary material, which is available to authorized users.

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.

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.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

G protein-coupled receptor dimerisation: Molecular basis and relevance to function

The belief that G protein-coupled receptors exist and function as monomeric, non-interacting species has been largely supplanted in recent years by evidence, derived from a range of approaches, that indicate they can form dimers and/or higher-order oligomeric complexes. Key roles for receptor homo-dimerisation include effective quality control of protein folding prior to plasma membrane delivery and interactions with hetero-trimeric G proteins. Growing evidence has also indicated the potential for many co-expressed G protein-coupled receptors to form hetero-dimers/oligomers. The relevance of this to physiology and function is only beginning to be unravelled but may offer great potential for more selective therapeutic intervention.

Neonatal Handling, Sweet Food Ingestion and Ectonucleotidase Activities in Nucleus Accumbens at Different Ages

Neonatal handled rats ingest more sweet food than non-handled ones, but it was documented only after puberty. Here, we studied the purinergic system in the nucleus accumbens, a possible target for the alteration in the preference for palatable food. We measured the ATP, ADP and AMP hydrolysis mediated by ectonucleotidases in synaptosomes of the nucleus accumbens in periadolescent and adult rats from different neonatal environments: non-handled and handled (10 min/day, first 10 days of life). Before adolescence, we found a decreased ingestion of sweet food in the neonatally handled group, with no effect on ATP, ADP or AMP hydrolysis. In adults, we found a greater ingestion of sweet food in the neonatally handled group, with no effect on ATPase or ADPase activities, but a decreased AMP hydrolysis. The nucleus accumbens is a site of intensive interaction between the adenosinergic and dopaminergic systems. Therefore, adenosine may modulate accumbens' dopamine neurotransmission differently in neonatally handled rats.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Association studies of the adenosine A2a receptor (1976T > C) genetic polymorphism in Parkinson’s disease and schizophrenia

Given the implications with respect to the pathogenesis of dopaminergic dysfunction in schizophrenia and Parkinson's disease (PD), as well as the reciprocal antagonistic interactions between adenosine A2a receptor (A2aAR) and the dopamine D2 receptors, A2aAR may be a candidate gene conferring susceptibility to PD or schizophrenia. In this study, we tested the hypothesis that the A2aAR 1976T > C genetic variant confers susceptibility to or is related to the onset age of schizophrenia or PD using a sample population consisting of 94 PD and 227 schizophrenic patients. We also tested whether the A2aAR 1976T > C relates to antipsychotic-induced tardive dyskinesia in the schizophrenic population. The results demonstrated that in comparing PD patients and controls the distribution of the A2aAR 1976T > C genotypes (P=0.788) and alleles (P=0.702) did not vary significantly. Furthermore, the PD onset age was not significantly different amongst the three A2aAR 1976T > C genotypic groups. In comparing schizophrenic patients and controls, the distribution of the A2aAR genotypes (P=0.330) and alleles (P=0.632) also did not differ significantly. The onset age of schizophrenia and tardive dyskinesia (evaluated with Abnormal Involuntary Movements Scale) were similar within the three A2aAR genotypic groups. Our findings suggest that it is unlikely that the A2aAR 1976T > C polymorphism plays a major role in the pathogenesis of PD, schizophrenia, or antipsychotic-induced tardive dyskinesia in the Chinese population.