Concentration–Response Function for Sufentanil Vapor Self-Administration

All behavioral tests were conducted during the dark phase of the light/dark cycle, 3–5 days per week. No food or water restriction was used to establish operant responding at any point of training. The rats did not have access to food or water while they were tested in ShA sessions (1–2 h). The rats (n=8) were trained to nosepoke on a fixed-ratio 1 (FR1) schedule of reinforcement (ie, each operant response in the active hole resulted in drug delivery) to obtain 10 s of sufentanil vapor (3.33 mg/ml) delivery and 60 s vapor evacuation shut off in 2 h sessions for six sessions. This initial training dose was determined in pilot studies to achieve approximately five drug deliveries per hour, which is comparable to the number of infusions per hour that was obtained in our previous studies using intravenous heroin (60 μg/kg/infusion) self-administration (Vendruscolo et al, 2011). The rats were then given the opportunity to self-administer 1.65 mg/ml sufentanil for eight sessions and 10 mg/ml for four subsequent sessions. The data are expressed as the number of reinforcers (ie, drug vapor deliveries) that were obtained in 2 h.