BACKGROUND AND PURPOSE:

GPR18 is a candidate cannabinoid receptor, but its classification as such is controversial. The rationale of the study presented herein was to consider the effects of N- arachidonoyl glycine (NAGly) and cannabinoids via differential G-protein coupled pathways, in addition to β-arrestin signaling. Cellular localization of GPR18 receptors was also examined.

EXPERIMENTAL APPROACH:

Calcium mobilization and extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation were quantitated in a cell line stably expressing GPR18 (HEK293/GPR18 cells). In addition, using the DiscoveRx PathHunter® CHO-K1 GPR18 β-arrestin cell line, recruitment of β-arrestin was quantified.

KEY RESULTS:

Concentration dependent increases in intracellular calcium and ERK1/2 phosphorylation were observed in the presence of NAGly, abnormal cannabidiol (AbnCBD), O-1602, O-1918 and Δ9 -tetrahydrocannabinol (Δ9 -THC) in HEK293/GPR18 cells. The initial rise in intracellular calcium in the presence of NAGly, O1918 and THC was blocked by either Gαq or Gαi/oinhibition. The ERK1/2 phosphorylation was inhibited by pertussis toxin and N-Arachidonoyl-l-serine (NARAS). Recruitment of β-arrestin in the PathHunter® CHO-K1 GPR18 cell line revealed a differential pattern of GPR18 activation; of all the ligands tested, only Δ9 -THC produced a concentration-dependent response. The localization of GPR18 receptors within the HEK293/GPR18 cells is both intracellular, and on the plasma membrane.

CONCLUSIONS AND IMPLICATIONS:

These findings suggest that GPR18 activation involves several signal transduction pathways indicative of biased agonism, thereby providing a plausible explanation for the apparent discrepancies in GPR18 activation found in the literature. Additionally, the results presented herein provide further evidence for GPR18 as a candidate cannabinoidreceptor.
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PMID:

 

24762058

 

[PubMed – as supplied by publisher]