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

Abstract

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 μ-opioid receptor and interaction with β-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 β-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 β-arrestin recruitment does not contribute to the severity of opioid side effects and, hence, predicting that G-protein-biased µ-agonists are still likely to elicit severe adverse effects. In conclusion, our findings identify carboxyl-terminal multisite phosphorylation as key step that drives acute μ-opioid receptor desensitization and long-term tolerance.

Synthesis and pharmacological characterization of ethylenediamine synthetic opioids in human μ‐opiate receptor 1 (OPRM1) expressing cells

“…It has been proposed that hMOR endocytosis is closely associated with the development of drug tolerance in the opiate user and in mouse models . Our data reveal that A01 and A02 demonstrated low levels (~15%) of hMOR internalization with similar drug potency for Gα i coupling.…”

“…Ser 375 , that are phosphorylated by GRKs reveal that respiratory depression and constipation are significantly exacerbated when MOR receptors fail to internalize after agonist binding 14 while analgesic tolerance is reduced. Thus, perhaps the pursuit of new opioids that better mimic the endogenous system might result in novel more "balanced" agents with improved clinical utility over morphine-like derivatives.…”

“…However, recent events, such as the failure of FDA approval for oliceridine in 2018 and new studies using mouse models with targeted mutations in the OPRM1 gene that prevent MOR internalization, suggest that this may not be the best approach for developing safer opiates with fewer side effects. Mutations of carboxyl tail serine and threonine residues, including Ser 375 , that are phosphorylated by GRKs reveal that respiratory depression and constipation are significantly exacerbated when MOR receptors fail to internalize after agonist binding while analgesic tolerance is reduced. Thus, perhaps the pursuit of new opioids that better mimic the endogenous system might result in novel more “balanced” agents with improved clinical utility over morphine‐like derivatives.…”

Fentanyl depression of respiration: Comparison with heroin and morphine

“…Conversely, the hyper locomotor activity induced by heroin in mice may stimulate respiration and obscure any decrease in tidal volume. This would not occur with the high dose of fentanyl we tested as it reduced locomotor activity but may occur at lower doses which do enhance locomotor activity (Kliewer et al, ; Varshneya et al, ).…”

“…We have previously reported that fentanyl does not exhibit arrestin bias in arrestin translocation and GTPγS binding assays (McPherson et al, ; Rivero et al, ). Furthermore, in transgenic mice in which all the phosphorylation sites on the C tail of the μ‐opioid receptor had been mutated to alanine and which are therefore not phosphorylated by G protein receptor kinases (GRKs) and do not recruit and bind arrestins, fentanyl still depressed respiration (Kliewer et al, ). This observation brings into question the concept that opioid depression of respiration is a function of arrestin signalling (Montandon & Slutsky, ).…”