Fig. 5. Effect of intravenous nalmefene in attenuating intrathecal morphine-induced scratching responses in four monkeys. Each value represents the mean ± SEM (two to three replications). Abscissae (all panels): different doses (micrograms per kilogram) of intravenous nalmefene. Ordinates (all panels): total scratches accumulated from the second to sixth session after intravenous injection. Asterisks represent a significant difference (*P < 0.05; **P < 0.01) from the vehicle (VEH) injection. Other details are as in figures 1 and 2.