BACKGROUND AND PURPOSE: G protein-biased μ opioid receptor (MOPr) agonists have the potential to induce less receptor desensitization and tolerance than balanced opioids. Here we demonstrate that the cyclic endomorphin analogue Tyr-c[D-Lys-Phe-Tyr-Gly] (Compound 1) is a G protein-biased MOPr agonist and characterise its ability to induce rapid receptor desensitization in mammalian neurones.
EXPERIMENTAL APPROACH: The signalling and trafficking properties of opioids were characterised using BRET assays, ELISA and phosphosite-specific immunoblotting in HEK 293 cells. Desensitization of opioid-induced currents was studied in rat locus coeruleus neurones using whole-cell patch clamp electrophysiology. The mechanism of Compound 1-induced MOPr desensitization was probed using kinase inhibitors.
KEY RESULTS: Compound 1 is a G protein-biased MOPr agonist with a similar intrinsic activity for G protein signalling as morphine. As predicted for a G protein-biased MOPr agonist, Compound 1 induced minimal agonist-induced internalization and minimal phosphorylation at intracellular MOPr serine/threonine residues known to be involved in GRK-mediated desensitization. However, Compound 1 induced robust rapid MOPr desensitization in locus coeruleus neurons, to a greater degree than morphine. The extent of Compound 1-induced desensitization was unaffected by activation or inhibition of PKC but was significantly reduced by inhibition of G protein-coupled receptor kinase (GRK).
CONCLUSION AND IMPLICATIONS: Compound 1 is a novel G protein-biased MOPr agonist which induces substantial rapid receptor desensitization in mammalian neurons. Surprisingly, Compound 1-induced desensitization was demonstrated to be GRK-dependent despite its G protein-bias. Our findings refute the assumption that G protein-biased agonists will evade receptor desensitization and tolerance.