Phosphorylation-deficient G-protein-biased μ-opioid receptors improve analgesia and diminish tolerance but worsen opioid side effects

Opioid analgesics are powerful pain relievers; however, over time, pain control diminishes as analgesic tolerance develops. The molecular mechanisms initiating tolerance have remained unresolved to date. We have previously shown that desensitization of the mu-opioid receptor and interaction with beta-arrestins is controlled by carboxyl-terminal phosphorylation. Here we created knockin mice with a series of serine- and threonine-to-alanine mutations that render the receptor increasingly unable to recruit beta-arrestins. Desensitization is inhibited in locus coeruleus neurons of mutant mice. Opioid-induced analgesia is strongly enhanced and analgesic tolerance is greatly diminished. Surprisingly, respiratory depression, constipation, and opioid withdrawal signs are unchanged or exacerbated, indicating that beta-arrestin recruitment does not contribute to the severity of opioid side effects and, hence, predicting that G-protein-biased mu-agonists are still likely to elicit severe adverse effects. In conclusion, our findings identify carboxyl-terminal multisite phosphorylation as key step that drives acute mu-opioid receptor desensitization and long-term tolerance.