Fig. 1.

Self-administration of fentanyl and alprazolam by rhesus monkeys (N = 6, 3 males, 3 females) trained under a progressive-ratio schedule of i.v. remifentanil (R, 0.3 μg/kg/injection) injection. Self-administration maintenance sessions alternating remifentanil and its vehicle (saline) were conducted until a consistent difference between drug and saline self-administration was obtained. Once self-administration was stable, test sessions with fentanyl or alprazolam and their vehicle (V) were added to an alternating sequence of remifentanil and saline sessions. Grouped data are expressed as the unit dose that engendered highest number of injections/session (EDMax) and one-half log-step unit dose below (−0.5 EDMax) and above (+0.5 EDMax) the EDMax dose. Data are represented as mean number of injections/session ± S.E.M., out of a total of 20 injections available in a daily session. Note that symbols obscure error bars in some instances. *P < 0.05 versus vehicle (V) (Bonferroni t tests).