N-Arachidonyl Glycine Does Not Activate G Protein-Coupled Receptor 18 Signaling via Canonical Pathways

Recent studies propose that N-arachidonyl glycine (NAGly), a carboxylic analogue of anandamide, is an endogenous ligand of the G alpha(i/o) protein-coupled receptor 18 (GPR18). However, a high-throughput beta-arrestin-based screen failed to detect activation of GPR18 by NAGly (Yin et al., 2009; JBC, 18: 12328). To address this inconsistency, this study investigated GPR18 coupling in a native neuronal system with endogenous signaling pathways and effectors. GPR18 was heterologously expressed in rat sympathetic neurons, and the modulation of N-type (Ca(v)2.2) calcium channels was examined. Proper expression and trafficking of receptor were confirmed by the rim-like fluorescence of fluorescently tagged receptor and the positive staining of external hemagglutinin-tagged GPR18-expressing cells. Application of NAGly on GPR18-expressing neurons did not inhibit calcium currents but instead potentiated currents in a voltage-dependent manner, similar to what has previously been reported (Guo et al., 2008; J Neurophysiol, 100: 1147). Other proposed agonists of GPR18, including anandamide and abnormal cannabidiol, also failed to induce inhibition of calcium currents. Mutants of GPR18, designed to constitutively activate receptors, did not tonically inhibit calcium currents, indicating a lack of GPR18 activation or coupling to endogenous G proteins. Other downstream effectors of G alpha(i/o)-coupled receptors, G protein-coupled inwardly rectifying potassium channels and adenylate cyclase, were not modulated by GPR18 signaling. Furthermore, GPR18 did not couple to other G proteins tested: G alpha(s), G alpha(z), and G alpha(15). These results suggest NAGly is not an agonist for GPR18 or that GPR18 signaling involves noncanonical pathways not examined in these studies.