Evidence against dopamine D1/D2 receptor heteromers

Abstract

Hetero-oligomers of G-protein-coupled receptors have become the subject of intense investigation because their purported potential to manifest signaling and pharmacological properties that differ from the component receptors makes them highly attractive for the development of more selective pharmacological treatments. In particular, dopamine D1 and D2 receptors have been proposed to form hetero-oligomers that couple to Gαq proteins, and SKF83959 has been proposed to act as a biased agonist that selectively engages these receptor complexes to activate Gαq and thus phospholipase C. D1/D2 heteromers have been proposed as relevant to the pathophysiology and treatment of depression and schizophrenia. We used in vitro bioluminescence resonance energy transfer (BRET), ex vivo analyses of receptor localization and proximity in brain slices, and behavioral assays in mice to characterize signaling from these putative dimers/oligomers. We were unable to detect Gαq or Gα11 protein coupling to homomers or heteromers of D1 or D2 receptors using a variety of biosensors. SKF83959-induced locomotor and grooming behaviors were eliminated in D1 receptor knockout mice, verifying a key role for D1-like receptor activation. In contrast, SKF83959-induced motor responses were intact in D2 receptor and Gαq knockout mice, as well as in knock-in mice expressing a mutant Ala286-CaMKIIα, that cannot autophosphorylate to become active. Moreover, we found that in the shell of the nucleus accumbens, even in neurons in which D1 and D2 receptor promoters are both active, the receptor proteins are segregated and do not form complexes. These data are not compatible with SKF83959 signaling through Gαq or through a D1–D2 heteromer and challenge the existence of such a signaling complex in the adult animals that we used for our studies.

Oligomerization of GPCRs involved in endocrine regulation

“…with the D1R, D4R, D5R, adenosine A2 receptor (A2R), serotonin 2A receptor (5HT-2AR), OTR, and the histamine H3 receptor (H3R). A functional relevance for the D1R/D2R heterodimer has not yet been confirmed (Frederick et al 2015). Recently, in vivo relevance for GHSR/D1R interrelation was found to be involved in hippocampal behavior and memory (Kern et al 2015).…”

In Vivo Significance of In Vitro Studies on G-Protein-Coupled Receptor Heteromers

“…For example, physical formation of GPCR heteromers could result in a novel signaling mechanism that is distinct from those of individual GPCR monomers or their homomer. However, significance of various studies on dopamine (DA) D 1 /D 2 receptor heteromers was challenged by a recent report [1]. Javitch et al [1] provided compelling evidence against D 1 /D 2 heteromers, one of the most prominent subjects of interest in neuropsychiatry field.…”

“…In sumary, the failure to demonstrate in vivo the presence of DA D 1 /D 2 receptor heteromers [1] undoubtly suggests reconsideration of the use of the "in vitro preclinical" models using receptor couplings. There is a clear need to reassess the validity of this approach for the development of medications discovery regarding various neuropsychiatric diseases and symptoms.…”

“…Surprisingly, despite the successful creation of DA D 1 /D 2 receptor heteromers "in vitro", their immunohistochemical studies discovered the virtual absence of physical DA D 1 /D 2 receptor heteromers in the shell region of the nucleus accumbens (NAS) from C57Bl/6J mouse brain [1]. In the NAS, DA D 1 and D 2 receptors seem to co-express in the same neuron for some neurons but did not localize [1]. The virtual absence of physical DA D 1 /D 2 receptor heteromers "in vivo" was further substantiated by the discovery of lack of localization across species.…”

“…In the study Javitch et al performed an "in vivo" assessment of the first demonstration for the presence of DA D 1 /D 2 receptor heteromers in animal tissues [1]. Surprisingly, despite the successful creation of DA D 1 /D 2 receptor heteromers "in vitro", their immunohistochemical studies discovered the virtual absence of physical DA D 1 /D 2 receptor heteromers in the shell region of the nucleus accumbens (NAS) from C57Bl/6J mouse brain [1].…”