Figure 1.

Operant sufentanil vapor self-administration. (a) Concentration–response function for sufentanil vapor self-administration. Rats (n=8) self-administered sufentanil vapor (10 s delivery) in 2 h sessions according to the following concentration schedule (in order of presentation): 3.33 mg/ml for six consecutive sessions, 1.65 mg/ml for eight consecutive sessions, and 10 mg/ml for four consecutive sessions. Bars represent the mean number of reinforcers (+SEM) earned across sessions for each of the three different sufentanil concentrations. **p<0.01, different from 3.33 mg/ml. (b) Effect of the opioid receptor inverse agonist naloxone on sufentanil vapor self-administration. The same rats (n=8) that were tested in panel (a) self-administered sufentanil vapor (10 mg/ml) in 2 h sessions. They were subcutaneously injected with 0.12, 0.25, 0.5, and 1 mg/kg naloxone immediately prior to sufentanil self-administration (one self-administration session per concentration). The baseline data are expressed as the average of five baseline sessions that were conducted intercalated with the naloxone tests. *p<0.05, **p<0.01, ***p<0.0001, different from baseline at the same time of the session.