Acute Tolerance to the Analgesic Action of Nitrous Oxide Does Not Develop in Rats

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Abstract
The time course of N2O analgesia was studied in rats with a behavioral criterion, the tail-flick test to radiant heat. All rats were placed individually in a Plexiglas tube and exposed to either nitrous oxide, 75% in O2 or room air (control) for 1 h. Analgesic potency was evaluated by prolongation of the time required to induce tail-flick. Although individual animals showed variability in the tail-flick time during exposure to nitrous oxide, no animal showed a tendency toward the development of tolerance and a statistically significant sustained prolongation of tail-flick time was produced.