Post-training and post-reactivation administration of amphetamine enhances morphine conditioned place preference

Amphetamine has been shown to enhance consolidation in a variety of memory paradigms. However, it is not known if amphetamine can modulate the consolidation of the types of context-reward associations involved in drug addiction, such as those formed in the conditioned place preference (CPP) task. Also, some types of memory exhibit a second period of lability following memory reactivation, and it is not known whether amphetamine administered during this period can modulate CPP. Our study investigated whether amphetamine can enhance morphine CPP when administered during the consolidation period or the post-reactivation period. Subjects were trained in the CPP task and injected with amphetamine or vehicle immediately or 6 h after each training session. The day after the completion of training, they were tested. Amphetamine injected immediately but not 6 h after training enhanced morphine CPP. In separate experiments, subjects were first trained in the CPP task. The day following the completion of training, subjects were given a memory reactivation session and injected with amphetamine or vehicle immediately or 6 h after reactivation. Subjects were tested the next day. Amphetamine injected immediately but not 6 h after memory reactivation enhanced morphine CPP. However, amphetamine injected without memory reactivation had no effect on the expression of morphine CPP. Our results suggest that amphetamine enhances the consolidation of morphine CPP and that morphine CPP exhibits a temporally limited period of post-reactivation lability during which the memory can be modulated.