Nitrous oxide lacks the antinociceptive effect on the tail flick test in newborn rats.

Nitrous oxide (N 2 O) is commonly used for pediatric anesthesia under the assumption that it produces a similar analgesic response to that seen in adults. We examined the antinociceptive effect of 75% N 2 O on tail flick latency response in newborn rats at postnatal Day 1 (PD 1), PD 8, PD 15, PD 22, and PD 29. Up to PD 15, rats showed no analgesic effect to N 2 O. By PD 29, rats exhibited a comparable analgesic effect to that seen in adult animals. These data are consistent with the fact that the descending noradrenergic neurons, which are required for the analgesic action of N 2 O, are not anatomically or functionally developed at birth and take more than three weeks to fully develop in rats.