The μ opioid receptor gene (MOR) was mutated in mice by a gene targeting procedure. In
these MOR-knockout mice, the analgesic effects of morphine, its major metabolites,
morphine-6-glucuronide (M-6-G) and morphine-6-ethereal sulfate (M-6-S), and
endomorphin-2, as well as morphine-induced lethality, were drastically reduced, whereas
the effects of DPDPE and U50488 remained unchanged. It is concluded that analgesic
effects of μ-specific opioid ligands and acute morphine lethality are mediated by the μ …