Complexes of ghrelin GHS-R1a, GHS-R1b and dopamine D1 receptors localized in the ventral tegmental area as main mediators of the dopaminergic effects of ghrelin.

Ghrelin receptor, also known as growth hormone secretagogue receptor or GHS-R1a, is co-expressed with its truncated isoform GHS-R1b, which does not bind ghrelin or signal, but oligomerizes with GHS-R1a, exerting a complex modulatory role that depends on its relative expression. D1 and D5 dopamine receptors (D1R and D5R) constitute the two D1-like receptor subtypes. Previous studies showed GHS-R1b also facilitates oligomerization of GHS-R1a with D1R, conferring GHS-R1a distinctive pharmacological properties. Those include a switch in the preferred coupling of GHS-R1a from Gq to Gs and the ability of D1R/D5R agonists and antagonists to counteract GHS-R1a signaling. Activation of ghrelin receptors localized in the ventral tegmental area (VTA) seems to play a significant role in ghrelin's contribution to motivated behavior. In view of the evidence indicating that dopaminergic cells of the VTA express ghrelin receptors and D5R but not D1R, we investigated the possible existence of functional GHS-R1a:GHS-R1b:D5R oligomeric complexes in the VTA. GHS-R1a:GHS-R1b:D5R oligomers were first demonstrated in mammalian transfected cells and their pharmacological properties were found to be different from those of GHS-R1a:GHS-R1b:D1R oligomers, including weak Gs coupling and the ability of D1R/D5R antagonists, but not agonists, to counteract the effects of ghrelin. However, analyzing the effect of ghrelin in the rodent VTA on MAPK activation with ex vivo experiments, on somato-dendritic dopamine release with in vivo microdialysis and on activation of dopaminergic cells with patch-clamp electrophysiology, provided evidence for a predominant role of GHS-R1a:GHS-R1b:D1R oligomers in the rodent VTA as main mediators of the dopaminergic effects of ghrelin.SIGNIFICANCE STATEMENT:The activation of ghrelin receptors localized in the ventral tegmental area (VTA) plays a significant role in ghrelin's contribution to motivated behavior. We present evidence that indicates these receptors form part of oligomeric complexes that include the functional ghrelin receptor GHS-R1a, its truncated non-signaling isoform GHS-R1b and the dopamine D1 receptor (D1R). Binding of ghrelin to these complexes promotes activation of the dopaminergic neurons of the VTA by activation of AC-PKA signaling, which can be counteracted by both GHS-R1a and D1R antagonists. Our study provides evidence for a predominant role of GHS-R1a:GHS-R1b:D1R oligomers in rodent VTA as main mediators of the dopaminergic effects of ghrelin.