Fig. 2. Effect of different concentrations of premixed nitrous oxide–oxygen–nitrogen (N2O–O2–N2) treatment on fentanyl-induced hyperalgesia. The fentanyl injections were performed on D0(total dose of 4 × 100 μg/kg). Gas mixtures were administered to rats starting 15 min before the fentanyl injection and for 4 h. ( A ) N2O–O2–N2= 10/50/40%; ( B ) N2O–O2–N2= 20/50/30%; ( C ) N2O–O2–N2= 30/50/20%; ( D ) N2O–O2–N2= 40/50/10%. Nociceptive threshold at the hind paw level was assessed on D−2,D−1, and D0; then every 30 min after the first fentanyl injection until 420 min on D0; and subsequently once daily for 7 days. Once the rats had returned to the basal nociceptive threshold (D7), naloxone was injected (1 mg/kg subcutaneously), and the nociceptive threshold was evaluated 5 min later. Nociceptive threshold is expressed as mean ± SD. * Dunnett test, P < 0.05 compared with the D0basal value. # Dunnett test, P < 0.05 for comparison between groups. Open circles = N2O–O2–N2-treated rats (n = 10); filled circles = air-treated rats (n = 10).