Fig. 2. Effects of fentanyl mechanical hyperalgesia ( A ), tactile allodynia ( B ), and weight-bearing changes ( C ) induced by left hind paw plantar incision. A hind paw plantar incision was realized on rats during halothane anesthesia on D0. One fentanyl (100 μg/kg) or saline injection was performed four consecutive times every 15 min, resulting in a total dose of 400 μg/kg subcutaneously administered (n = 12). Surgery was performed just after the second fentanyl injection. The three pain parameters were evaluated before surgery on D−2, D−1, and D0; 2, 4, and 6h after surgery on D0; and subsequently once daily for 8 days. At the end of the experiment (D8), all rats were injected with naloxone (1 mg/kg subcutaneous), and the three pain parameters were measured 5 min later. ( Inset ) Algesic index showing the variations of mechanical hyperalgesia, tactile allodynia, and weight bearing on the days after the incision. Pain parameter values and algesic index are expressed as mean ± SD. # Dunnett test, P < 0.05 compared with the D0basal value. * Dunnett test, P < 0.05 for comparison between groups. $ P < 0.05, Mann–Whitney test for algesic index comparison. Open circles = saline-treated rats; filled circles = fentanyl-treated rats.