Bioluminescence resonance energy transfer (BRET) is a well-established method for investigating protein-protein interactions. Here we present a novel BRET approach to monitor ligand binding to G protein-coupled receptors (GPCRs) on the surface of living cells made possible by the use of fluorescent ligands in combination with a novel bioluminescent protein (NanoLuc) that can be readily expressed on the N-terminus of GPCRs.

Graphical abstract

Cannabidiol (CBD) decreases insulitis, inflammation, neuropathic pain, and myocardial dysfunction in preclinical models of diabetes. We recently showed that CBD also improves vasorelaxation in the Zucker diabetic fatty (ZDF) rat, and the objective of the present study was to establish the mechanisms underlying this effect. Femoral arteries from ZDF rats and ZDF lean controls were isolated, mounted on a myograph, and incubated with CBD (10 mM) or vehicle for 2 hours. Subsequent vasorelaxant responses were measured in combination with various interventions. Prostaglandin metabolites were detected using enzyme immunoassay. Direct effects of CBD on cyclooxygenase (COX) enzyme activity were measured by oxygraph assay. CBD enhanced the maximum vasorelaxation to acetylcholine (ACh) in femoral arteries from ZDF lean rats (P , 0.01) and especially ZDF rats (P , 0.0001). In ZDF arteries, this enhancement persisted after cannabinoid receptor (CB) type 1, endothelial CB, or peroxisome proliferator-activated receptor-g antagonism but was inhibited by CB 2 receptor antagonism. CBD also uncovered a vasorelaxant response to a CB 2 agonist not previously observed. The CBD-enhanced ACh response was endothelium-, nitric oxide-, and hydrogen peroxide-independent. It was, however, COX-1/2-and superoxide dismutase-dependent, and CBD enhanced the activity of both purified COX-1 and COX-2. The CBD-enhanced ACh response in the arteries was inhibited by a prostanoid EP 4 receptor antagonist. Prostaglandin E 2 metabolite levels were below the limits of detection, but 6-keto prostaglandin F 1a was decreased after CBD incubation. These data show that CBD exposure enhances the ability of arteries to relax via enhanced production of vasodilator COX-1/2-derived products acting at EP 4 receptors.