Novel inverse agonists for the orphan G protein-coupled receptor 6

Abstract

The orphan G protein-coupled receptor 6 (GPR6) displays unique promise as a therapeutic target for the treatment of neuropsychiatric disorders due to its high expression in the striatopallidal neurons of the basal ganglia. GPR6, along with closely related orphan receptors GPR3 and GPR12, are phylogenetically related to CB1 and CB2 cannabinoid receptors. In the current study, we performed concentration-response studies on the effects of three different classes of cannabinoids: endogenous, phyto-, and synthetic, on both GPR6-mediated cAMP accumulation and β-arrestin2 recruitment. In addition, structure-activity relationship studies were conducted on cannabidiol (CBD), a recently discovered inverse agonist for GPR6. We have identified four additional cannabinoids, cannabidavarin (CBDV), WIN55212-2, SR141716A and SR144528, that exert inverse agonism on GPR6. Furthermore, we have discovered that these cannabinoids exhibit functional selectivity toward the β-arrestin2 recruitment pathway. These novel, functionally selective inverse agonists for GPR6 can be used as research tools and potentially developed into therapeutic agents.

Discovery of endogenous inverse agonists for G protein-coupled receptor 6

“…While a variety of exogenous compounds, including the phytocannabinoids CBD and CBDV, have been shown to act on orphan receptor GPR6 [14,15], an endogenous ligand has yet to be confirmed for this receptor. The present study was designed to test endocannabinoid-like compounds (N-acylamides) on GPR6-mediated cell signaling with the goal of deorphanizing the receptor.…”

“…A number of arachidonic acid derivatives have been reported as endocannabinoids, including anandamide (AEA), 2-arachidonoylglycerol (2-AG), noladin ether, virodhamine and NADA [21,22]. Our previous study tested the first four of these compounds and demonstrated that none of these endocannabinoids (AEA, 2-AG, noladin ether, and virodhamine) were able to alter GPR6-mediated either β-arrestin2 recruitment or cAMP accumulation [15]. However, NADA was not investigated in the previous study, and acted as the starting point for the present study.…”

“…Our recent reports have revealed that GPR6 is a novel molecular target for the phytocannabinoid cannabidiol (CBD), a major nonpsychoactive component of Cannabis sativa, along with its analog cannabidivarin (CBDV) and the synthetic cannabinoids WIN55,212-2, SR141716A and SR144528 [14,15]. Homology clustering analysis has revealed GPR6's phylogenetic relationship to the cannabinoid receptors CB1 and CB2, sharing 35% amino acid sequence identity in the transmembrane regions [16].…”

GPR6 Structural Insights: Homology Model Construction and Docking Studies

“…Recently, GPR6 has been shown to also signal through a G-protein independent, β-arrestin signaling pathway. GPR6 displays high constitutive activity in β-arrestin2 recruitment assays in CHO cells co-expressing GPR6-PK1 and EA-β-arrestin2 [11,12]. The IC conformational changes due to the β-arrestin activation mechanism are not fully elucidated and studied as G-protein activation among GPCRs.…”

“…Very recently endocannabinoid-like N-acylamides such as N-arachidonoyl dopamine, N-oleoyl dopamine and N-palmitoyl dopamine have been shown to exert inverse agonism at GPR6 in the micromolar range [10]. Some phytocannabinoids such as cannabidiol [11], and cannabinoid antagonists including SR144528 [12], and aminoalkylindole cannabinoid agonist WIN55212-2 [12,13] have shown activity on GPR6.…”