Hyperactivity in mice induced by opioid agonists with partial intrinsic efficacy and biased agonism; alone and in combination with morphine
Opioid analgesics like morphine and fentanyl induce mu-opioid receptor (MOR)-mediated hyperactivity in mice. Here we show that morphine, fentanyl, SR-17018, and oliceridine have submaximal intrinsic efficacy in the mouse striatum using 35S-GTP{gamma}S binding assays. While all of the agonists act as partial agonists for stimulating G protein coupling in striatum, morphine, fentanyl and oliceridine are fully efficacious in stimulating locomotor activity; meanwhile, the noncompetitive biased agonists, SR-17018 and SR-15099 produce submaximal hyperactivity. Moreover, the combination of SR-17018 and morphine attenuates hyperactivity while antinociceptive efficacy is increased. The combination of oliceridine with morphine increases hyperactivity which is maintained over time. These findings provide evidence that noncompetitive agonists at MOR can be used to suppress morphine-induced hyperactivity while enhancing antinociceptive efficacy; moreover, they demonstrate that intrinsic efficacy measured at the receptor level is not directly proportional to drug efficacy in the locomotor activity assay.