Fig. 2. Dose-response of the effects of gabapentin on fentanyl-induced hyperalgesia as measured from D+1to D+5. Four fentanyl (60 μg/kg) injections performed every 15 min on the day of the test (D0) resulted in a decrease in the nociceptive threshold below the basal value as measured by the paw-pressure test. Intraperitoneal gabapentin or saline were administered 30 min before the first subcutaneous fentanyl injection. The algesic index was determined by the mean of the surfaces (±SD) calculated by summing the nociceptive threshold values measured every day from D+1to D+5. The histogram represents the algesic index after various intraperitoneal doses (30, 75, 150, or 300 mg/kg) of gabapentin (n = 8, 7, 10, and 7, respectively). * P < 0.05 with the Dunnett test when compared with the saline group (n = 6).