Figure 2.
Mice that intravenously self-administered morphine exhibited general drug-seeking behavior.
Either food pellets or intravenous morphine infusions was earned as rewards during the consecutive self-administration sessions. (A) Over five consecutive food-training sessions, the number of active lever presses to acquire food pellets gradually increased during the 1-hour sessions. Additionally, the mice underwent two additional food-training sessions following the catheter implantation surgery to reinstate their memory of the procedure. After food training, we conducted 6–9 consecutive self-administration. (B) Number of morphine infusions earned and lever presses performed to receive a morphine infusion (0.3 mg/kg/infusion) during consecutive self-administration sessions. (C) Number of morphine infusions earned and lever presses performed to receive a morphine infusion (0.03 mg/kg/infusion) during consecutive self-administration sessions. (D) Number of morphine infusions earned and lever presses performed to receive a morphine infusion (1 mg/kg/infusion) during consecutive self-administration sessions. (E) Number of morphine infusions earned and lever presses performed to receive a morphine infusion (0.1 mg/kg/infusion) during consecutive self-administration sessions. (F) Number of morphine infusions earned and lever presses performed to receive a morphine infusion (3 mg/kg/infusion) during consecutive self-administration sessions. All data are reported as the mean ± SE (n = 8–9 mice per dose of morphine). FT: Food training; Re-FT: re-food training.