Fig. 2. Dose–response of the fentanyl hyperalgesic effects on the nociceptive threshold as measured by the paw-pressure vocalization test (days 1 through 5 after the treatment [D+1–D+5]). Fentanyl was injected four times (20, 40, 60, 80, or 100 μg/kg per injection; subcutaneously) at 15-min intervals, resulting in overall doses of 80, 160, 240, 320, and 400 μg/kg, respectively. The histogram represents the algesic index determined by the mean of the surfaces (± SEM). *P < 0.5, **P < 0.01 with the Dunnett test compared with the saline group.