Fig. 2.

Individual subject data (top panel: males; bottom panel: females) showing self-administration of fentanyl, alprazolam and 3 different ratios of fentanyl-alprazolam combinations (1:1, 1:3 and 3:1 fentanyl:alprazolam, with 1:1 being based on the ED₅₀s of drugs alone). Monkeys were trained under a progressive-ratio schedule of i.v. remifentanil (R, 0.3 μg/kg/injection) injection, and self-administration 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, alprazolam or fentanyl-alprazolam combinations and their vehicle (V) were added to an alternating sequence of remifentanil and saline sessions. For drug combination tests, data are plotted as a function of the fentanyl dose included in each combination. 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.