Fig. 3. Long-lasting effect of fentanyl in an experimental group that had not received nociceptive inputs associated with the measurement procedure on the day of the treatment (D⁰). The fentanyl injections were performed on D⁰. The basal nociceptive threshold was evaluated before any treatment on D⁻¹and D⁰and daily after treatment for 5 days (D⁺¹–D⁺⁵). The evaluation of the analgesic effect on day D⁰was not performed. Mean nociceptive thresholds (± SEM) were expressed in grams. Fentanyl was injected four times (60 μg/kg fentanyl per injection, subcutaneously; arrows) at 15-min intervals, resulting in overall doses of 240 μg/kg. The gray shaded surfaces represent the overall long-lasting effect of fentanyl. **P < 0.01 with the Dunnett test compared with the basal nociceptive threshold value.