Summary We identified subsets of neurons in the brain that co-express the dopamine receptor subtype-2 (DRD2) and the ghrelin receptor (GHSR1a). Combination of FRET confocal microscopy and Tr-FRET established the presence of GHSR1a:DRD2 heteromers in hypothalamic neurons. To interrogate function, mice were treated with the selective DRD2 agonist cabergoline which produced anorexia in wild-type and ghrelin−/− mice; intriguingly, ghsr−/− mice were refractory illustrating dependence on GHSR1a, but not ghrelin. Elucidation of mechanism showed that formation of GHSR1a:DRD2 heteromers allosterically modifies canonical DRD2 dopamine signaling resulting in Gβγ subunit-dependent mobilization of [Ca2+]i independent of GHSR1a basal activity. By targeting the interaction between GHSR1a and DRD2 in wild-type mice with a highly selective GHSR1a antagonist (JMV2959) cabergoline–induced anorexia was blocked. Inhibiting dopamine signaling in subsets of neurons with a GHSR1a antagonist has profound therapeutic implications by providing enhanced selectivity because neurons expressing DRD2 alone would be unaffected.

The ghrelin receptor (GHSR1a) and dopamine receptor-1 (DRD1) are coexpressed in hippocampal neurons, yet ghrelin is undetectable in the hippocampus; therefore, we sought a function for apo-GHSR1a. Real-time single-molecule analysis on hippocampal neurons revealed dimerization between apo-GHSR1a and DRD1 that is enhanced by DRD1 agonism. In addition, proximity measurements support formation of preassembled apo-GHSR1a:DRD1:Gαq heteromeric complexes in hippocampal neurons. Activation by a DRD1 agonist produced non-canonical signal transduction via Gαq-PLC-IP3-Ca2+ at the expense of canonical DRD1 Gαs cAMP signaling to result in CaMKII activation, glutamate receptor exocytosis, synaptic reorganization, and expression of early markers of hippocampal synaptic plasticity. Remarkably, this pathway is blocked by genetic or pharmacological inactivation of GHSR1a. In mice, GHSR1a inactivation inhibits DRD1-mediated hippocampal behavior and memory. Our findings identify a previously unrecognized mechanism essential for DRD1 initiation of hippocampal synaptic plasticity that is dependent on GHSR1a, and independent of cAMP signaling.

The relaxin receptor (LGR7, relaxin family peptide receptor 1) is a member of the leucine-rich repeat containing G protein-coupled receptors subgroup C. This and the LGR8 (relaxin family peptide receptor 2) receptor are unique in having a low-density lipoprotein class A (LDL-A) module at their N termini. This study was designed to show the role of the LDL-A in LGR7 expression and function. Point mutants for the conserved cysteines (Cys(47) and Cys(53)) and for calcium binding asparagine (Asp(58)), a mutant with deleted LDL-A domain and chimeric LGR7 receptor with LGR8 LDL-A all showed no cAMP response to human relaxins H1 or H2. We have shown that their cell surface delivery was uncompromised. The mutation of the putative N-linked glycosylation site (Asn(36)) decreased cAMP production and reduced cell surface expression to 37% of the wild-type LGR7. All point mutant, chimeric, and wild-type receptor proteins were expressed as the two forms. The immature or precursor form of the receptor was 80 kDa, whereas the mature receptor, delivered to the cell surface was 95 kDa. The glycosylation mutant was also expressed as two forms with appropriately smaller molecular masses. Deletion of the LDL-A module resulted in expression of the mature receptor only. These data suggest that the LDL-A module of LGR7 influences receptor maturation, cell surface expression, and relaxin-activated signal transduction.