Instrumental requirement for PR9-4 and PR3-4 schedules.

Lever response requirement values indicate the number of lever presses necessary for the acquisition of each subsequent infusion (infusion step number). Progressive increases vary between the two schedules such that for the 10th infusion, 4 lever presses will result in an infusion using the PR9-4 schedule [42], whereas 32 lever presses are required for the 10th infusion using PR3-4 schedule [43]. Accordingly, the PR3-4 schedule is deemed more difficult as it progressively requires a greater lever response for an infusion than the PR9-4 schedule. Rat responding for morphine was assessed in the current study using both schedules.