Pharmacological characterization of receptor redistribution and beta-arrestin recruitment assays for the cannabinoid receptor 1.

Receptor redistribution and beta-arrestin recruitment assays provide a G-protein-subtype-independent method to measure ligand-stimulated activation of G-protein-coupled receptors. In particular beta-arrestin assays are becoming an increasingly popular tool for drug discovery. The authors have compared a high-content-imaging-based Redistribution assay and 2 nonimaging-based beta-arrestin recruitment assays, Tango and PathHunter, for the cannabinoid receptor 1. Inasmuch as all 3 assays use receptors that are modified at the C-terminus, the authors verified their pharmacology via detection of Galpha(i) coupling of the receptor in cAMP assays using reference ligands. The potencies and efficacies of the cannabinoid receptor agonists CP55,940 and WIN55,212-2 correlated well between the 3 assays, and are comparable with the measured ligand binding affinities. The inverse agonist SR141716 decreased basal signal in all 3 assays, but only in the Tango bla assay a reliable EC50 could be determined for this compound, suggesting that Tango is the most suitable assay for the identification of new inverse agonists. Both the Redistribution and the PathHunter assay could discriminate partial agonists from full agonists, whereas in the Tango assay partial agonists behaved as full agonists. Only the PathHunter cells allowed detection of cannabinoid receptor activation via beta-arrestin recruitment and Galpha(i)-protein-mediated inhibition of cAMP, thus enabling the identification of biased ligands that differ in these cellular effects. The characteristics and limitations of the different assays are discussed.