Adenosine as a volume transmission signal. A feedback detector of neuronal activation

Effects of caffeine on temporal perception in Rattus norvegicus

We report two studies that tested the effects of caffeine, the world's most widely used psychoactive drug, on temporal perception. We trained Wistar rats using the Bisection Procedure (Experiment 1) or the Stubbs' Procedure (Experiment 2) to discriminate between short and long light stimuli. Once training finished, we administered caffeine orally (0, 9.6, and 96.0 mg/kg for Experiment 1 and 0, 9.6, 19.2, and 38.4 mg/kg for Experiment 2) 15 minutes prior to testing. Relative to the control condition, the 9.6 mg/kg condition (Experiments 1 and 2) and the 19.2 mg/kg condition (Experiment 2) resulted in an increase in proportion of choosing the long response. Meanwhile, overall accuracy was not affected by any condition in both experiments. Taken together, these results are consistent with the notion that caffeine, at some doses, speeds up temporal perception. However, it is not clear why the effect disappears at higher doses.

Multiple Adenosine-Dopamine (A2A-D2 Like) Heteroreceptor Complexes in the Brain and Their Role in Schizophrenia

In the 1980s and 1990s, the concept was introduced that molecular integration in the Central Nervous System could develop through allosteric receptor–receptor interactions in heteroreceptor complexes presents in neurons. A number of adenosine–dopamine heteroreceptor complexes were identified that lead to the A_2A-D₂ heteromer hypothesis of schizophrenia. The hypothesis is based on strong antagonistic A_2A-D₂ receptor–receptor interactions and their presence in the ventral striato-pallidal GABA anti-reward neurons leading to reduction of positive symptoms. Other types of adenosine A_2A heteroreceptor complexes are also discussed in relation to this disease, such as A_2A-D₃ and A_2A-D₄ heteroreceptor complexes as well as higher order A_2A-D₂-mGluR5 and A_2A-D₂-Sigma1R heteroreceptor complexes. The A_2A receptor protomer can likely modulate the function of the D₄ receptors of relevance for understanding cognitive dysfunction in schizophrenia. A_2A-D₂-mGluR5 complex is of interest since upon A_2A/mGluR5 coactivation they appear to synergize in producing strong inhibition of the D2 receptor protomer. For understanding the future of the schizophrenia treatment, the vulnerability of the current A_2A-D₂like receptor complexes will be tested in animal models of schizophrenia. A_2A-D₂-Simag1R complexes hold the highest promise through Sigma1R enhancement of inhibition of D2R function. In line with this work, Lara proposed a highly relevant role of adenosine for neurobiology of schizophrenia.

Multiple D2 heteroreceptor complexes: new targets for treatment of schizophrenia

The dopamine (DA) neuron system most relevant for schizophrenia is the meso-limbic-cortical DA system inter alia densely innervating subcortical limbic regions. The field of dopamine D2 receptors and schizophrenia changed markedly with the discovery of many types of D2 heteroreceptor complexes in subcortical limbic areas as well as the dorsal striatum. The results indicate that the D2 is a hub receptor which interacts not only with many other G protein-coupled receptors (GPCRs) including DA isoreceptors but also with ion-channel receptors, receptor tyrosine kinases, scaffolding proteins and DA transporters. Disturbances in several of these D2 heteroreceptor complexes may contribute to the development of schizophrenia through changes in the balance of diverse D2 homo- and heteroreceptor complexes mediating the DA signal, especially to the ventral striato-pallidal γ-aminobutyric acid (GABA) pathway. This will have consequences for the control of this pathway of the glutamate drive to the prefrontal cortex via the mediodorsal thalamic nucleus which can contribute to psychotic processes. Agonist activation of the A2A protomer in the A2A-D2 heteroreceptor complex inhibits D2 Gi/o mediated signaling but increases the D2 β-arrestin2 mediated signaling. Through this allosteric receptor-receptor interaction, the A2A agonist becomes a biased inhibitory modulator of the Gi/o mediated D2 signaling, which may the main mechanism for its atypical antipsychotic properties especially linked to the limbic A2A-D2 heterocomplexes. The DA and glutamate hypotheses of schizophrenia come together in the signal integration in D2-N-methyl-d-aspartate (NMDA) and A2A-D2-metabotropic glutamate receptor 5 (mGlu5) heteroreceptor complexes, especially in the ventral striatum. 5-Hydroxytryptamine 2A (5-HT2A)-D2 heteroreceptor complexes are special targets for atypical antipsychotics with high potency to block their 5-HT2A protomer signaling in view of the potential development of pathological allosteric facilitatory 5-HT2A-D2 interaction increasing D2 protomer signaling. Neurotensin (NTS1)-D2 heterocomplexes also exist in the ventral and dorsal striatum, and likely also in midbrain DA nerve cells as NTS1-D2 autoreceptor complexes where neurotensin produces antipsychotic and propsychotic actions, respectively.

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

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

Extrasynaptic neurotransmission in the modulation of brain function. Focus on the striatal neuronal-glial networks

Extrasynaptic neurotransmission is an important short distance form of volume transmission (VT) and describes the extracellular diffusion of transmitters and modulators after synaptic spillover or extrasynaptic release in the local circuit regions binding to and activating mainly extrasynaptic neuronal and glial receptors in the neuroglial networks of the brain. Receptor-receptor interactions in G protein-coupled receptor (GPCR) heteromers play a major role, on dendritic spines and nerve terminals including glutamate synapses, in the integrative processes of the extrasynaptic signaling. Heteromeric complexes between GPCR and ion-channel receptors play a special role in the integration of the synaptic and extrasynaptic signals. Changes in extracellular concentrations of the classical synaptic neurotransmitters glutamate and GABA found with microdialysis is likely an expression of the activity of the neuron-astrocyte unit of the brain and can be used as an index of VT-mediated actions of these two neurotransmitters in the brain. Thus, the activity of neurons may be functionally linked to the activity of astrocytes, which may release glutamate and GABA to the extracellular space where extrasynaptic glutamate and GABA receptors do exist. Wiring transmission (WT) and VT are fundamental properties of all neurons of the CNS but the balance between WT and VT varies from one nerve cell population to the other. The focus is on the striatal cellular networks, and the WT and VT and their integration via receptor heteromers are described in the GABA projection neurons, the glutamate, dopamine, 5-hydroxytryptamine (5-HT) and histamine striatal afferents, the cholinergic interneurons, and different types of GABA interneurons. In addition, the role in these networks of VT signaling of the energy-dependent modulator adenosine and of endocannabinoids mainly formed in the striatal projection neurons will be underlined to understand the communication in the striatal cellular networks.

G protein-coupled receptor oligomerization and brain integration: focus on adenosinergic transmission

The control of glutamatergic corticostriatal transmission is essential for the induction and expression of plasticity mechanisms in the striatum, a phenomenon thickly regulated by G protein-coupled receptors (GPCRs). Interestingly, in addition to dopamine receptors, adenosine and metabotropic glutamate receptors also play a key role in striatal functioning. The existence of a supramolecular organization (i.e. oligomer) containing dopamine, adenosine and metabotropic glutamate receptors in the striatal neurons is now being widely accepted by the scientific community. Indeed, these oligomers may enhance the diversity and performance by which extracellular striatal signals are transferred to the G-proteins in the process of receptor transduction, and also may allow unpredictable receptor-receptor allosteric regulations. Overall, here we want to review how formations of adenosine, dopamine and metabotropic glutamate receptors-containing oligomers impinge into striatal functioning in both normal and pathological conditions. This article is part of a Special Issue entitled: Brain Integration.

Adenosine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Adenosine-dopamine interactions in the central nervous system (CNS) have been studied for many years in view of their relevance for disorders of the CNS and their treatments. The discovery of adenosine and dopamine receptor containing receptor mosaics (RM, higher-order receptor heteromers) in the striatum opened up a new understanding of these interactions. Initial findings indicated the existence of A(2A)R-D(2)R heterodimers and A(1)R-D(1)R heterodimers in the striatum that were followed by indications for the existence of striatal A(2A)R-D(3)R and A(2A)R-D(4)R heterodimers. Of particular interest was the demonstration that antagonistic allosteric A(2A)-D(2) and A(1)-D(1) receptor-receptor interactions take place in striatal A(2A)R-D(2)R and A(1)R-D(1)R heteromers. As a consequence, additional characterization of these heterodimers led to new aspects on the pathophysiology of Parkinson's disease (PD), schizophrenia, drug addiction, and l-DOPA-induced dyskinesias relevant for their treatments. In fact, A(2A)R antagonists were introduced in the symptomatic treatment of PD in view of the discovery of the antagonistic A(2A)R-D(2)R interaction in the dorsal striatum that leads to reduced D(2)R recognition and G(i/o) coupling in striato-pallidal GABAergic neurons. In recent years, indications have been obtained that A(2A)R-D(2)R and A(1)R-D(1)R heteromers do not exist as heterodimers, rather as RM. In fact, A(2A)-CB(1)-D(2) RM and A(2A)-D(2)-mGlu(5) RM have been discovered using a sequential BRET-FRET technique and by using the BRET technique in combination with bimolecular fluorescence complementation. Thus, other pathogenic mechanisms beside the well-known alterations in the release and/or decoding of dopamine in the basal ganglia and limbic system are involved in PD, schizophrenia and drug addiction. In fact, alterations in the stoichiometry and/or topology of A(2A)-CB(1)-D(2) and A(2A)-D(2)-mGlu5 RM may play a role. Thus, the integrative receptor-receptor interactions in these RM give novel aspects on the pathophysiology and treatment strategies, based on combined treatments, for PD, schizophrenia, and drug addiction.

Adenosine receptors interacting proteins (ARIPs): Behind the biology of adenosine signaling

Adenosine is a well known neuromodulator in the central nervous system. As a consequence, adenosine can be beneficial in certain disorders and adenosine receptors will be potential targets for therapy in a variety of diseases. Adenosine receptors are G protein-coupled receptors, and are also expressed in a large variety of cells and tissues. Using these receptors as a paradigm of G protein-coupled receptors, the present review focus on how protein-protein interactions might contribute to neurotransmitter/neuromodulator regulation, based on the fact that accessory proteins impinge on the receptor/G protein interaction and therefore modulate receptor functioning. Besides affecting receptor signaling, these accessory components also play a key role in receptor trafficking, internalization and desensitization, as it will be reviewed here. In conclusion, the finding of an increasing number of adenosine receptors interacting proteins, and specially the molecular and functional integration of these accessory proteins into receptorsomes, will open new perspectives in the understanding of particular disorders where these receptors have been proved to be involved.

Adenosine A(2A) receptors, dopamine D(2) receptors and their interactions in Parkinson's disease

Future therapies in Parkinson's disease may substantially build on the existence of intra-membrane receptor-receptor interactions in DA receptor containing heteromeric receptor complexes. The A(2A)/D(2) heteromer is of substantial interest in view of its specific location in cortico-striatal glutamate terminals and in striato-pallidal GABA neurons. Antagonistic A(2A)/D(2) receptor interactions in this heteromer demonstrated at the cellular level, and at the level of the striato-pallidal GABA neuron and at the network level made it possible to suggest A(2A) antagonists as anti-parkinsonian drugs. The major mechanism is an enhancement of D(2) signaling leading to attenuation of hypokinesia, tremor, and rigidity in models of Parkinson's disease with inspiring results in two clinical trials. Other interactions are antagonism at the level of the adenylyl cyclase; heterologous sensitization at the A(2A) activated adenylyl cyclase by persistent D(2) activation and a compensatory up-regulation of A(2A) receptors in response to intermittent Levodopa treatment. An increased dominance of A(2A) homomers over D(2) homomers and A(2A)/D(2) heteromers after intermittent Levodopa treatment may therefore contribute to development of Levodopa induced dyskinesias and to the wearing off of the therapeutic actions of Levodopa giving additional therapeutic roles of A(2A) antagonists. Their neuroprotective actions may involve an increase in the retrograde trophic signaling in the nigro-striatal DA system.

Purinergic-receptor oligomerization: implications for neural functions in the central nervous system

It is becoming clear that the functions of G protein-coupled receptors (GPCRs), the largest family of plasma membrane-localized receptors, are regulated by direct oligomeric formation between GPCRs, as either homo- or hetero-oligomers. This review article explores the mechanistic implications of GPCR dimerization, especially among purinergic receptors, adenosine receptors and P2 receptors, which play critical roles in the regulation of neurotransmission in the central nervous system. Briefly, adenosine receptors are able to form a heteromeric complex with P2 receptors that generates an adenosine receptor with P2 receptor-like agonistic pharmacology. This mechanism may be used to fine-tune purinergic inhibition locally at sites where there is a particular oligomerization structure between purinergic receptors, and to explain the undefined adenosine-like purinergic functions of adenine nucleotides. Purinergic receptors also form oligomers with GPCRs of other families present in the brain, such as dopamine receptors and metabotropic glutamate receptors, to alter the functional properties. The effect of GPCR oligomerization on receptor functions is thus considered as an important system in the central nervous system.

G-protein coupled receptor oligomerization in neuroendocrine pathways

Protein-protein interactions are fundamental processes for many biological systems including those involving the superfamily of G-protein coupled receptors (GPCRs). A growing body of biochemical and functional evidence supports the existence of GPCR-GPCR homo- and hetero-oligomers. In particular, hetero-oligomers can display pharmacological and functional properties distinct from those of the homodimer or oligomer thus adding another level of complexity to how GPCRs are activated, signal and traffick in the cell. Dimerization may also play a role in influencing the activity of agonists and antagonists. We are only beginning to unravel how and why such complexes are formed, the functional implications of which will have an enormous impact on GPCR biology. Future research that studies GPCRs as dimeric or oligomeric complexes will enhance not only our understanding of GPCRs in cellular function but will also be critical for novel drug design and improved treatment of the vast array of GPCR-related conditions.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Synergistic interaction between adenosine A2A and glutamate mGlu5 receptors: implications for striatal neuronal function

The physiological meaning of the coexpression of adenosine A2A receptors and group I metabotropic glutamate receptors in gamma- aminobutyric acid (GABA)ergic striatal neurons is intriguing. Here we provide in vitro and in vivo evidence for a synergism between adenosine and glutamate based on subtype 5 metabotropic glutamate (mGluR5) and adenosine A2A (A2AR) receptor/receptor interactions. Colocalization of A2AR and mGluR5 at the membrane level was demonstrated in nonpermeabilized human embryonic kidney (HEK)-293 cells transiently cotransfected with both receptors by confocal laser microscopy. Complexes containing A2AR and mGluR5 were demonstrated by Western blotting of immunoprecipitates of either Flag-A2AR or hemagglutinin-mGluR5 in membrane preparations from cotransfected HEK-293 cells and of native A2AR and mGluR5 in rat striatal membrane preparations. In cotransfected HEK-293 cells a synergistic effect on extracellular signal-regulated kinase 1/2 phosphorylation and c-fos expression was demonstrated upon A2AR/mGluR5 costimulation. No synergistic effect was observed at the second messenger level (cAMP accumulation and intracellular calcium mobilization). Accordingly, a synergistic effect on c-fos expression in striatal sections and on counteracting phencyclidine-induced motor activation was also demonstrated after the central coadministration of A2AR and mGluR5 agonists to rats with intact dopaminergic innervation. The results suggest that a functional mGluR5/A2AR interaction is required to overcome the well-known strong tonic inhibitory effect of dopamine on striatal adenosine A2AR function.

Coaggregation, cointernalization, and codesensitization of adenosine A2A receptors and dopamine D2 receptors

Antagonistic and reciprocal interactions are known to exist between adenosine and dopamine receptors in the striatum. In the present study, double immunofluorescence experiments with confocal laser microscopy showed a high degree of colocalization of adenosine A(2A) receptors (A(2A)R) and dopamine D(2) receptors (D(2)R) in cell membranes of SH-SY5Y human neuroblastoma cells stably transfected with human D(2)R and in cultured striatal cells. A(2A)R/D(2)R heteromeric complexes were demonstrated in coimmunoprecipitation experiments in membrane preparations from D(2)R-transfected SH-SY5Y cells and from mouse fibroblast Ltk(-) cells stably transfected with human D(2)R (long form) and transiently cotransfected with the A(2A)R double-tagged with hemagglutinin. Long term exposure to A(2A)R and D(2)R agonists in D(2)R-cotransfected SH-SY5Y cells resulted in coaggregation, cointernalization and codesensitization of A(2A)R and D(2)R. These results give a molecular basis for adenosine-dopamine antagonism at the membrane level and have implications for treatment of Parkinson's disease and schizophrenia, in which D(2)R are involved.