Morphine-induced internalization of the L83I mutant of the rat μ-opioid receptor

BACKGROUND AND PURPOSE: Naturally occurring single-nucleotide polymorphisms (SNPs) within G protein-coupled receptors (GPCRs) can result in alterations in various pharmacological parameters. Understanding the regulation and function of endocytic trafficking of the μ-opioid receptor (MOPr) is of great importance given its implication in the development of opioid tolerance. This study has compared the agonist-dependent trafficking and signalling of L83I, the rat orthologue of a naturally occurring variant of the MOPr.

EXPERIMENTAL APPROACH: Cell surface ELISA, confocal microscopy and immunoprecipitation assays were used to characterize the trafficking properties of the MOPr-L83I variant in comparison to the wild-type receptor in HEK 293 cells. Functional assays were used to compare the ability of the L83I variant to signal to several downstream pathways.

KEY RESULTS: Morphine-induced internalization of L83I was markedly increased in comparison to the wild-type receptor. The altered trafficking of this variant was found to be specific to morphine and was both GRK- and dynamin-dependent. The enhanced internalization of L83I in response to morphine was not due to increased phosphorylation of Serine 375, arrestin association or an increased ability to signal.

CONCLUSIONS AND IMPLICATIONS: These results suggest that morphine promotes a specific conformation of the L83I variant that makes it more liable to internalize to morphine unlike the wild-type receptor which undergoes significantly less morphine-stimulated internalization, providing an example of a ligand-selective biased receptor. The presence of this SNP within an individual may consequently affect the development of tolerance and analgesic responses.