Behavior tests

All the experiments complied with the policy about animal use. Opioid-induced analgesia was assessed by tail immersion test. One-third of mouse tail was immersed in 52°C water, and the time of tail flicking out from water was recorded. The basal response of mice was first examined, and then opioid drugs were administrated subcutaneously (s.c.). The post-drug latency of mice was recorded 30 min after the drug treatment. The cutoff time was set at 10 s to avoid scald. Antinociception was evaluated by the change of tail response after the drug treatment: % maximum possible effect (M.P.E) = 100 × (post latency − basal latency)/(10 − basal latency).²¹