Comparison of nitrous oxide, morphine and diazepam effects in the mouse staircase test.

Mice were exposed for 10-20 min to room air, 100% oxygen (O2) or increasing concentrations of nitrous oxide (N2O) in O2, then tested for 3 min in a staircase inside a glovebag. N2O produced a concentration-dependent increase in the number of steps ascended (NSA) but no change in the number of rears (NR). Pretreatment with naloxone reversed the increase in NSA and also unmasked N2O reduction in NR. By comparison, increasing doses of the narcotic standard morphine reduced NSA and NR; these changes in NSA and NR were sensitive to antagonism by naloxone. The benzodiazepine standard diazepam produced a dose-related reduction in NR while reducing NSAd only at higher doses. These data indicate that N2O influences on NSA and NR resemble neither morphine nor diazepam. In addition, it appears that the opioid activity of N2O might mask its antianxiety activity in this particular paradigm.