Adenosine A 2A receptor (A 2A R)-dopamine D 2 receptor (D 2 R) heteromers are key modulators of striatal neuronal function. It has been suggested that the psychostimulant effects of caffeine depend on its ability to block an allosteric modulation within the A 2A R-D 2 R heteromer, by which adenosine decreases the affinity and intrinsic efficacy of dopamine at the D 2 R. We describe novel unsuspected allosteric mechanisms within the heteromer by which not only A 2A R agonists, but also A 2A R antagonists, decrease the affinity and intrinsic efficacy of D 2 R agonists and the affinity of D 2 R antagonists. Strikingly, these allosteric modulations disappear on agonist and antagonist coadministration. This can be explained by a model that considers A 2A R-D 2 R heteromers as heterotetramers, constituted by A 2A R and D 2 R homodimers, as demonstrated by experiments with bioluminescence resonance energy transfer and bimolecular fluorescence and bioluminescence com- Most evidence indicates that G protein-coupled receptors (GPCRs) form homodimers and heteromers. Homodimers seem to be a predominant species, and oligomeric entities can be viewed as multiples of dimers (1). It has been proposed that GPCR heteromers are constituted mainly by heteromers of homodimers (1, 2). Allosteric mechanisms determine a multiplicity of unique pharmacologic properties of GPCR homodimers and heteromers (1, 3). First, binding of a ligand to one of the receptors in the heteromer can modify the affinity of ligands for the other receptor (1, 3, 4). The most widely reproduced allosteric modulation of ligand-binding properties in a GPCR heteromer is the ability of adenosine A 2A receptor (A 2A R) agonists to decrease the affinity of dopamine D 2 receptor (D 2 R) agonists in the A 2A R-D 2 R heteromer (5). A 2A R-D 2 R heteromers have been revealed both in transfected cells (6, 7), striatal neurons in culture (6,8) and in situ, in mammalian striatum (9, 10), where they play an important role in the modulation of GABAergic striatopallidal neuronal function (9, 11).In addition to ligand-binding properties, unique properties for each GPCR oligomer emerge in relation to the varying intrinsic efficacy of ligands for different signaling pathways (1-3). Intrinsic efficacy refers to the power of the agonist to induce a functional response, independent of its affinity for the receptor. Thus, allosteric modulation of an agonist can potentially involve changes in affinity and/or intrinsic efficacy (1, 3). This principle can be observed in the A 2A R-D 2 R heteromer, where a decrease in D 2 R agonist affinity cannot alone explain the ability of an A 2A R agonist to abolish the decreased excitability of GABAergic striatopallidal neurons induced by high concentration of a D 2 R agonist (9), which should overcome the decrease in affinity. Furthermore, a differential effect of allosteric modulations of different agonist-mediated signaling responses (i.e., functional selectivity) can occur within GPCR heteromers (1, 2, 8 It has been hypothesized that the allos...
Adenosine A2A-dopamine D2 receptor interactions play a very important role in striatal function. A2A-D2 receptor interactions provide an example of the capabilities of information processing by just two different G protein-coupled receptors. Thus, there is evidence for the coexistence of two reciprocal antagonistic interactions between A2A and D2 receptors in the same neurons, the GABAergic enkephalinergic neurons. An antagonistic A2A-D2 intramembrane receptor interaction, which depends on A2A-D2 receptor heteromerization and Gq/11-PLC signaling, modulates neuronal excitability and neurotransmitter release. On the other hand, an antagonistic A2A-D2 receptor interaction at the adenylyl-cyclase level, which depends on Gs/olf-and Gi/o-type V adenylyl-cyclase signaling, modulates protein phosphorylation and gene expression. Finally, under conditions of upregulation of an activator of G protein signaling (AGS3), such as during chronic treatment with addictive drugs, a synergistic A2A-D2 receptor interaction can also be demonstrated. AGS3 facilitates a synergistic interaction between Gs/olf -and Gi/o-coupled receptors on the activation of types II/IV adenylyl cyclase, leading to a paradoxical increase in protein phosphorylation and gene expression upon co-activation of A2A and D2 receptors. The analysis of A2-D2 receptor interactions will have implications for the pathophysiology and treatment of basal ganglia disorders and drug addiction.Key Words: Adenosine A 2A Receptor, Dopamine D 2 Receptor, G Protein-Coupled Receptors, Receptor Heteromers, Striatum, Basal Ganglia Disorders, Drug Addiction. LOCALIZATION OF THE A 2A -D 2 RECEPTOR HETERO-MERApplying a broad definition of "neurotransmitter" [1], adenosine can be considered as an important neurotransmitter in the CNS, which acts through different subtypes of G protein-coupled receptors (GPCRs). From the four cloned adenosine receptors (adenosine A 1 , A 2A , A 2B and A 3 receptors ) , A 1 and A 2A receptors are the main targets for the physiological effects of adenosine in the brain [2]. A 1 receptor is widely distributed in the brain, including the striatum, while A 2A receptor is mostly concentrated in the striatum [2,3]. It is becoming increasingly obvious that the modulatory role of adenosine in the striatum is related to the ability of A 1 and A 2A receptors to heteromerize with themselves and with other GPCRs, such as dopamine, glutamate, cannabinoid and ATP receptors [4][5][6][7][8][9][10][11][12][13][14]. The present review focuses on the role of one particular adenosine receptor heteromer, the one constituted by the A 2A and the dopamine D 2 receptor, which is already having important implications for the treatment of neuropathologies involving the striatum (see below).Striatal medium spiny neurons are GABAergic efferent neurons which constitute more that 95% of the striatal neuronal population. They receive two main afferents, cortical-limbic-thalamic glutamatergic inputs and dopaminergic mesencephalic inputs, from the substantia nigra pars compac...
Bursting activity of striatal medium spiny neurons results from membrane potential oscillations between a down- and an upstate that could be regulated by G-protein-coupled receptors. Among these, dopamine D2 and adenosine A2A receptors are highly enriched in striatal neurons and exhibit strong interactions whose physiological significance and molecular mechanisms remain partially unclear. More particularly, respective involvements of common intracellular signaling cascades and A2A–D2 receptor heteromerization remain unknown. Here we show, by performing perforated-patch-clamp recordings on brain slices and loading competitive peptides, that D2 and A2A receptors regulate the induction by N-methyl-D-aspartate of a depolarized membrane potential plateau through mechanisms relying upon specific protein–protein interactions. Indeed, D2 receptor activation abolished transitions between a hyperpolarized resting potential and a depolarized plateau potential by regulating the CaV1.3a calcium channel activity through interactions with scaffold proteins Shankl/3. Noticeably, A2A receptor activation had no effect per se but fully reversed the effects of D2 receptor activation through a mechanism in which A2A–D2 receptors heteromerization is strictly mandatory, demonstrating therefore a first direct physiological relevance of these heteromers. Our results show that membrane potential transitions and firing patterns in striatal neurons are tightly controlled by D2 and A2A receptors through specific protein–protein interactions including A2A–D2 receptors heteromerization.
The striatum is the major input structure of basal ganglia and is involved in adaptive control of behaviour through the selection of relevant informations. Dopaminergic neurons that innervate striatum die in Parkinson disease, leading to inefficient adaptive behaviour. Neuronal activity of striatal medium spiny neurons (MSN) is modulated by dopamine receptors. Although dopamine signalling had received substantial attention, consequences of dopamine depletion on MSN intrinsic excitability remain unclear. Here we show, by performing perforated patch clamp recordings on brain slices, that dopamine depletion leads to an increase in MSN intrinsic excitability through the decrease of an inactivating A-type potassium current, I A. Despite the large decrease in their excitatory synaptic inputs determined by the decreased dendritic spines density and the increase in minimal current to evoke the first EPSP, this increase in intrinsic excitability resulted in an enhanced responsiveness to their remaining synapses, allowing them to fire similarly or more efficiently following input stimulation than in control condition. Therefore, this increase in intrinsic excitability through the regulation of I A represents a form of homeostatic plasticity allowing neurons to compensate for perturbations in synaptic transmission and to promote stability in firing. The present observations show that this homeostatic ability to maintain firing rates within functional range also occurs in pathological conditions, allowing stabilizing neural computation within affected neuronal networks.
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