| Summary: | The existence of functional selectivity at the mu-opioid receptor was examined by determining the efficacy of a range of opioid agonists for promoting G protein activation and arrestin-3 translocation. In general, there is a good correlation between the efficacy of an opioid agonist at promoting G protein signaling, and the efficacy at recruiting arrestin-3 to the receptor. Endomorphin 2 appears to be an example of a biased ligand with significantly higher efficacy for arrestin-3 translocation, while morphine does not appear to be biased. The kinetics of binding for DAMGO, morphine and endomorphin 2 were determined by competition kinetic assay as a potential explanation for the apparent bias of endomorphin 2. Mean occupancy times of DAMGO, endomorphin 2 and morphine at MOPr are similar to the time required for GRK2-mediated phosphorylation, indicating that kinetics of binding may be a determinant of their ability to promote arrestin-3 signaling at MOPr. The abilities of DAMGO, morphine and endomorphin 2 to induce desensitization of GIRK currents in AtT-20 cells expressing wild type mu-opioid receptor, mu-opioid receptor containing S261/363A substitutions, and mu-opioid receptor with a C terminal truncation from amino acid 354-398 were examined. From the results, it appears that agonist-induced acute desensitization in AtT-20 cells has multiple components. C terminal truncation of MOPr resulted in a slight inhibition of endomorphin 2-induced desensitization. Alanine substitution at serine 261 and 363 inhibited desensitization induced by endomorphin 2. Treatment with the GRK2 inhibitor 5-[2-(5-nitro-2- furyl)vinyl]-2-furoate slightly but significantly inhibited desensitization induced by DAMGO, and to a very small extent morphine, but not endomorph in 2, which may indicate that biased ligands trigger receptor regulation in a different manner to unbiased ligands.
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