Discriminative stimulus effects of nitrous oxide in mice: comparison with volatile hydrocarbons and vapor anesthetics

Abstract

The abuse-related behavioral effects produced by nitrous oxide (N₂O) gas have been suggested as being unique compared with other abused inhalants. The drug discrimination paradigm in animals can be used to study subjective effects of drugs in humans and to test this hypothesis. The goals of the present experiment were to establish N₂O discrimination in mice and to compare its discriminative stimulus effects with those of abused volatile vapors and vapor anesthetics. Sixteen B6SJLF1/J mice were trained to discriminate between 10 min of exposure to 60% N₂O+40% oxygen (O₂) and 10 min of exposure to 100% O₂. The time course of N₂O discrimination was examined, followed by cross-substitution testing with abused vapors, volatile anesthetics, ethanol, D-amphetamine, and 2-butanol. Mice acquired the ability to discriminate between N₂O and O₂ in 40 days. N₂O fully substituted for 10 min of exposure to 60% N₂O in a concentration-dependent manner. Full substitution required 7 min of 60% N₂O exposure, but the offset of stimulus effects following the cessation of exposure was more rapid. The aromatic hydrocarbon toluene almost fully substituted for N₂O. 1,1,1-Trichloroethane, methoxyflurane, isoflurane, and ethanol showed lesser degrees of substitution. D-amphetamine and the odorant 2-butanol did not substitute for N₂O. Given the varying degrees of incomplete substitution by test compounds, the discriminative stimulus properties of N₂O and, perhaps, its subjective effects in humans are probably not unique. As none of the inhalants tested fully mimicked N₂O, its overall effects may include one or more novel stimulus components.

Fig 1

Percentage of mice meeting both the correct first fixed ratio and greater than 80% total training-appropriate responding across consecutive training sessions (n=16). Each point represents two-day means for O₂ (open circles) and N₂O (closed circles).

Fig 2

Concentration-effect curve of N₂O in mice trained to discriminate 60% N₂O+40% O₂ from 100% O₂ (n=16). Points above O₂ and N₂O represent 100% O₂ (open circle) and 60% N₂O+40% O₂ (closed circle) control test sessions. Upper panel: mean (±SEM) percentage N₂O-lever responding; lower panel: mean (±SEM) response rates.

Fig 3

Upper panel: mean (±SEM) percentage N₂O-lever responding produced by increasing durations of exposure to 60% N₂O+40% O₂ (n=9). Lower panel: mean (±SEM) percentage N₂Olever responding following increasing durations of delay after cessation of 10 min of exposure to 60% N₂O+40% O₂ (lower panel, n=5). Points above O₂ are the results of 100% O₂ control sessions.

Fig 4

Concentration-effect curves of TCE (n=8) and toluene (n=8) vapor in mice trained to discriminate 10 min exposure to 60% N₂O+40% O₂ from 100% O₂. Points above O₂ represent control sessions. Points above N₂O represent 60% N₂O control test sessions. Upper panel: mean (±SEM) percentage N₂O-lever responding for TCE (closed circles) and toluene (closed squares). Lower panel: mean (±SEM) response rates (responses/s). Numbers in parentheses indicate the number of subjects within the group which emitted at least one complete FR at that test concentration. * indicate statistically significant (p <0.05) differences from O₂ control session.

Fig 5

Concentration-effect curves of isoflurane (n=8) and methoxyflurane (n=9) vapor and doseeffect curve of ethanol (n=8) in mice trained to discriminate 10 min exposure to 60% N₂O+40% O₂ from 100% O_2. Points above O₂ represent O₂ control sessions. Points above N₂O represent 60% N₂O+40% O₂ control test sessions. Upper panel: mean (±SEM) percentage N₂O-lever responding for isoflurane (closed circles), methoxyflurane (closed squares) and ethanol (closed triangles). Lower panel: mean (±SEM) response rates (responses/s). Numbers in parentheses indicate the number of subjects within the group which emitted at least one complete FR at that test concentration. * indicate statistically significant (p <0.05) differences from O₂ control session.