Figure 4.

Animals that have self-administered rapid cocaine injections (delivered over 5 vs 90 s) are more motivated to obtain the drug. In panels a and b, different doses of cocaine were delivered over 5 s for the 5-s groups and over 90 s for the 90-s groups, under a progressive ratio schedule. In panels c and d, cocaine was delivered over 5 and 90 s during progressive ratio testing in rats that had previously self-administered 0.25 mg/kg/infusion or 0.5 mg/kg/infusion cocaine delivered over 5 or 90 s. Values are mean±SEM of 2–3 sessions/dose or injection speed. n=11–16/group. PR 5 s, cocaine was delivered over 5 s during progressive ratio testing. PR 90 s, cocaine was delivered over 90 s during progressive ratio testing. *P<0.05 compared with the 90-s group. ^#P<0.0001 compared with the 90-s group.