Most evidence indicates that, as for family C G protein-coupled receptors (GPCRs), family A GPCRs can form homo and heteromers. Homodimers seem to be a predominant species with potential dynamic formation of higher-order oligomers (1). The pentameric structure constituted by one GPCR homodimer and one heterotrimeric G protein may provide a main functional unit at the plasma membrane and oligomeric entities can be viewed as multiples of dimers (Ferr et al., 2014). It still needs to be resolved if GPCR heteromers are preferentially heterodimers or if they are mostly constituted by heteromers of homodimers. Allosteric mechanisms determine a multiplicity of possible unique pharmacological properties of GPCR homomers and heteromers. Some general mechanisms seem to apply, particularly at the level of ligand-binding properties (1). Furthermore, in addition to ligand-binding properties, unique properties for each GPCR oligomer emerge in relation to different intrinsic efficacy of ligands for different signaling pathways (functional selectivity) (1). The striatal dopamine D1 receptor-D3 receptor (D1R-D3R) heteromer is being considered as a potential therapeutic target for neuropsychiatric disorders. Previous studies suggested that this heteromer could be involved in the ability of D3R agonists to potentiate locomotor activation induced by D1R agonists. It was also postulated that its striatal overexpression plays a role in L-dopa-induced dyskinesia and in drug addiction. By combining bioluminescence resonance energy transfer, bimolecular complementation techniques and cell signaling experiments in transfected cells, we have obtained evidence for a tetrameric stoichiometry of the D1R-D3R heteromer, constituted by two interacting D1R and D3R homodimers coupled to Gs and Gi proteins, respectively (2). Co-activation of both receptors led to the canonical negative interaction at the level of adenylyl cyclase signaling, to a strong recruitment of β-arrestin-1 and to a positive crosstalk of D1R and D3R agonists at the level of mitogen-activated protein kinase (MAPK) signaling. D1R or D3R antagonists counteracted β-arrestin-1 recruitment and MAPK activation induced by D3R and D1R agonists, respectively (cross-antagonism) (2). Positive crosstalk and cross-antagonism at the MAPK level were counteracted by specific synthetic peptides with amino acid sequences corresponding to D1R transmembrane (TM) domains TM5 and TM6, which also selectively modified the quaternary structure of the D1R-D3R heteromer, as demonstrated by bimolecular complementation (2). These results demonstrate functional selectivity of allosteric modulations within the D1R-D3R heteromer, which can be involved with the reported behavioral synergism of D1R and D3R agonists. We have previously found that striatal D1R also form heteromers with histamine H3 receptors (H3R) and σ1 receptors (σ1R) and hypothesized that allosteric interactions within the σ1R-D1R heteromer can explain the predominant D1R mediated acute effects of cocaine, a ligand for σ1R, including cocaine toxicity. We now found that σ1R-D1-H3 oligomeric complexes are in fact relevant targets for the striatal neuronal effects of cocaine, which disrupts H3R-mediated inhibitory allosteric modulation of D1R signaling within the complex (3). Cocaine-mediated disruption leaves an uninhibited D1R that activates Gs, freely recruits β-arrestin, activates MAPK and induces cell death when over-activated (3). With in vitro experiments in transfected cells and ex vivo experiments using both rats acutely treated or self-administered with cocaine along with mice depleted of σ1R, we found that blockade of σ1R by an antagonist restores the protective H3R receptor-mediated brake on D1R receptor signaling and prevents the cell death from elevated D1R signaling. These findings suggest that a combination therapy of σ1R antagonists with H3R receptor agonists could serve to reduce some effects of cocaine. Although it is quite accepted that psychostimulants produce reinforcing effects by directly increasing extracellular levels of dopamine in the striatum and opioids produce reinforcing effects mostly by acting on mesencephalic dopaminergic cells in the VTA, the mechanism behind the reinforcing effects of cannabinoids is still a matter of debate. Our studies suggest another mechanism: an increase in cortico-striatal neurotransmission, probably mediated by a CB1 receptor (CB1R)-mediated decrease in cortical GABAergic neurotransmission. The concomitant striatal glutamate release would then locally produce a glutamate-dependent striatal dopamine release. This is based on the ability of drugs that decrease cortico-striatal glutamatergic neurotransmission to decrease reinforcing effects of cannabinoids, but not cocaine, in experimental animals. Those include acetylcholine nicotinic α7 receptor (α7nAChR) and adenosine A2A receptor (A2AR) antagonists (4,5). Kynurenic acid (KYNA) is an endogenous negative allosteric modulator of α7nAChR. The kynurenine 3-monooxygenase (KMO) inhibitor Ro 61-8048 increases brain KYNA levels and attenuates cannabinoid-induced self-administration in squirrel monkeys and rats, and it also prevented relapse to drug-seeking induced by re-exposure to cannabinoids or cannabinoid-associated cues (4). Systemic administration of a preferentially presynaptic A2AR antagonist (SCH-442416) reduced reinforcing effects of THC in squirrel monkeys, as demonstrated by a rightward shift of THC dose-response curves. In contrast, treatment with a preferentially postsynaptic A2A receptor antagonist (KW-6002) shifted dose-response curves for THC to the left, consistent with potentiation of THCs reinforcing effects (5). We previously showed that pre- and postsynaptic effects of A2AR antagonists depend on the differential affinity of ligands for different A2AR heteromers, which are differentially localized in different striatal neuronal elements. The study provides a proof of concept for receptor heteromers as a new kind of pharmacological target, based on the ability of one of the receptor units to act as an allosteric modulator of specific ligands binding to the second receptor unit in the receptor heteromer, which gives the possibility of finding selective ligands for this second receptor unit. In a previous study we found that dopamine D2 receptor (D2R) heteromerizes with dopamine D4 receptor (D4R) and that D2R-D4R heteromers potently modulate striatal glutamatergic neurotransmission. Furthermore, we found that the polymorphic variant D4.7R, which has been suggested to be associated with ADHD and substance use disorders (SUD), does not form functional heteromers with D2R. In our search for the functional significance of these findings we reviewed the endophenotype concept and applied it to personality traits (6). We identified three high-order personality traits (positive emotionality/extroversion, negative emotionality/neuroticism and constrain) that are tied to specific brain systems and genes, and that offer a tractable approach to studying SUD. These brain systems, and the genes that moderate them, interact dynamically with the environment and with the drugs themselves to ultimately determine an individuals vulnerability or resilience to developing SUD. The trait constrain implies intentional, volitional motor control, which can be operationally measured by laboratory tests of response inhibition. Neuropsychological studies after brain lesions and imaging studies are providing a clear picture of the prefrontal-basal ganglia network involved in volitional inhibitory control of action. We are now intensively studying this circuitry with multiple approaches to demonstrate its modulation by D2R-D4R heteromers and the differential effects of D4R variants.
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