Morphine-induced respiratory depression is independent of β-arrestin2 signalling

Background and Purpose GPCRs can signal through both G proteins and beta-arrestin2. For the mu-opioid receptor, early experimental evidence from a single study suggested that G protein signalling mediates analgesia, whereas beta-arrestin2 signalling mediates respiratory depression and constipation. Consequently, for more than a decade, much research effort has been focused on developing biased mu-opioid agonists that preferentially target G protein signalling over beta-arrestin signalling, as it was believed that such drugs would be analgesics devoid of respiratory depressant activity. However, the prototypical compounds that have been developed based on this concept have so far failed in clinical and preclinical development. Experimental Approach The present study was set up to re-examine opioid-induced respiratory depression in beta-arrestin2 knockout mice. To this end, a consortium was formed consisting of three different laboratories located in different countries to evaluate independently opioid-induced respiratory depression. Key Results Our consensus results unequivocally demonstrate that the prototypical mu-opioid agonist morphine (3.75-100 mg center dot kg(-1) s.c. or 3-30 mg center dot kg(-1) i.p.) as well as the potent opioid fentanyl (0.05-0.35 mg center dot kg(-1) s.c.) do indeed induce respiratory depression and constipation in beta-arrestin2 knockout mice in a dose-dependent manner indistinguishable from that observed in wild-type mice. Conclusion and Implications Our findings do not support the original suggestion that beta-arrestin2 signalling plays a key role in opioid-induced respiratory depression and call into question the concept of developing G protein-biased mu-opioid receptor agonists as a strategy for the development of safer opioid analgesic drugs.