To study strain differences in antinociceptive effects of nitrous oxide (N2O), we examined various outbred and inbred stains of rats by using tail flick latency response. All outbred strains, i.e., Sprague-Dawley from two different breeders, Wistar, and Long-Evans, showed a similar antinociceptive response. Namely, the peak response occurred after 30 min of exposure, and tolerance to N2O developed within 60 to 90 min. Each of the four inbred stains examined, i.e., Wistar-Kyoto, Brown-Norway, Fischer, and Lewis, displayed a unique pattern of antinociceptive response to N2O. Wistar-Kyoto and Brown-Norway strains showed somewhat similar patterns as those observed in outbred strains, apart from the fact that the Wistar-Kyoto displayed a more distinct development of tolerance, whereas, the Brown-Norway strain had a lower peak effect. The Fischer strain displayed the greatest antinociceptive response to N2O, and did not develop tolerance. The Lewis strain showed no antinociceptive response to N2O. These results indicate differences in the durability and the magnitude of the antinociceptive response to N2O among various strains of rats.
Implications: Because of the variability that already exists, we recommend that animal studies examining the antinociceptive effects of nitrous oxide should be performed on inbred rat strains.