RT Journal Article SR Electronic T1 Separation of acute desensitization and long-term tolerance of mu-opioid receptors is determined by the degree of C-terminal phosphorylation JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP mol.119.117358 DO 10.1124/mol.119.117358 A1 John T. Williams A1 Seksiri Arttamangkul A1 Emily R Leff A1 Omar Koita A1 William T Birdsong YR 2019 UL http://molpharm.aspetjournals.org/content/early/2019/08/05/mol.119.117358.abstract AB Phosphorylation of sites on the C-terminus of the mu opioid receptor (MOR) result in the induction of acute desensitization that is thought to be a precursor for the development of long-term tolerance. Alanine mutations of all 11 phosphorylation sites on the C-terminus of MORs almost completely abolished desensitization and one measure of tolerance in locus coeruleus neurons when phosphorylation deficient MORs were virally expressed in MOR knockout rats. The present work identifies specific residues that underlie acute desensitization, receptor internalization and tolerance. Four MORs variants with different alanine or glutamate mutations in the C-terminus were examined. Alanine mutations in the sequence between amino acids 375 and 379 (STANT-3A) and the sequence between amino acids 363 and 394 with 4 additional alanine substitutions (STANT+7A), reduced desensitization, blocked internalization and two measures of long term tolerance. Following chronic morphine treatment, alanine mutations in the sequence between 354 and 357 (TSST-4A) blocked one measure of long-term tolerance (increased acute desensitization and slowed recovery from desensitization) but did not change a second (decreased sensitivity to morphine). With the expression of receptors having glutamate substitutions in the TSST sequence (TSST-4E), an increase in acute desensitization was present after chronic morphine treatment but the sensitivity to morphine was not changed. The results show that all 11 phosphorylation sites contribute, in varying degrees, to the induction of acute desensitization and long-term tolerance and that acute desensitization and tolerance are not necessarily linked. This study illustrates the complexity of events that are triggered by chronic treatment with morphine.SIGNIFICANCE STATEMENT This work shows that the degree of the phosphorylation sites on the C-terminus of the mu opioid receptor alters acute desensitization, internalization and measures of long term tolerance to morphine. The primary conclusion is that the degree of phosphorylation on the 11 possible cites of the C-terminus have different roles for expression of the multiple adaptive mechanisms that follow acute and long term agonist activation. Although the idea that acute desensitization and tolerance are intimately linked is generally supported, these results indicate that disruption of one phosphorylation cassette of the C-terminus, TSST distinguishes the two processes.