The effects of exogenous CCK-8 on the acquisition and expression of morphine-induced CPP

► CCK-8 (0.01-1 μg) blocked the acquisition of CPP when co-administered with morphine. ► CCK-8 (1 μg) increased CPP expression and, along with lower doses (0.1 μg), reduced its extinction. ► CCK-8 (1 μg) suppressed locomotor activity.