Fig. 1. Spinal cord transection blocked the analgesic action of N2O. The spinal cords of rats were sectioned at the T3–T4 level before exposure to 70% N2O. The analgesia was measured after 30 min of N2O exposure. ( A ) Tail flick latencies before and after N2O in sham and transected animals. Spinal transection did not affect baseline tail flick latencies but did abolish N2O analgesia. Data are expressed as mean ± SEM. *** P < 0.001 Bonferroni multiple comparison test (n = 9). ( B ) The percent maximal possible effect of N2O was reduced in transected animals. Data are expressed as mean ± SEM. *** P < 0.001 unpaired t test (n = 9).