FIGURE 4.

Effects of previous morphine experience and withdrawal on oxycodone self-administration (SA; 0.06 mg/kg/infusion, FR1 schedule of reinforcement). (A–C) Prior morphine experience and withdrawal potentiate acquisition of oxycodone SA in male, but not female, rats. Total number of oxycodone infusions (A), active lever presses (B), and inactive lever presses (C) in 1-h daily sessions. (D–F) Prior morphine experience and withdrawal reduce the amount of oxycodone SA in female, but not male, rats during the last 4 days of the 2-h/days maintenance phase. Total number of oxycodone infusions (D), active lever presses (E), and inactive lever presses (F) in 2-h daily sessions. *p < 0.05 comparing female morphine to female vehicle; ^###p < 0.001 main effect of time; ^{^}p < 0.05 main effect of treatment. Insets: For each graph (A–F), data are also expressed as the total of infusions or lever presses over either the acquisition or maintenance phases of SA. *p < 0.05. **p < 0.01 comparing groups indicated with brackets; ^##p < 0.01 main effect of sex. Data are expressed as mean ± SEM; N = 20–26/group