Mice tolerant to nitrous oxide are not tolerant to barbiturates.

Mice continuously exposed to 50% nitrous oxide for 2 weeks or more become tolerant to nitrous oxide, as measured by an 8% to 19% increase in their nitrous oxide ED50 (the dose required to abolish the righting reflex in 50% of the animals). Control mice tolerant to nitrous oxide were given intraperitoneal injections of secobarbital (50 mg/kg), phenobarbital (120 mg/kg), or pentobarbital (55 or 65 mg/kg). Sleep onset times, sleep times, and serum levels of barbiturates upon awakening did not differ between the two groups. These results suggest that nitrous oxide and barbiturates do not share a common mechanism of action.