Amphetamine-induced enhancement of responding for conditioned reward in rats: interactions with repeated testing

The mesolimbic dopamine system underlies the ability of reward-related stimuli to control operant behavior. Previous work has shown that amphetamine potentiates operant responding for conditioned rewards (CRs). Here, we asked whether the profile of this amphetamine-produced potentiation changes with repeated CR presentation, i.e., as the CR is being extinguished. Amphetamine (0-1.0 mg/kg, i.p.), administered over four daily sessions using a Latin square design, dose-dependently increased lever pressing for a 'lights-off' stimulus previously paired with food in rats. The amphetamine-produced enhancement of responding for CR was significantly modulated with repeated CR exposure: it was strongest on day 1 and became less pronounced in subsequent sessions whereas the CR effect persisted. In further experiments, rats receiving LiCl devaluation of the primary reward failed to show a significant reduction in the amphetamine-produced enhancement of responding for CR. The nature of the dissociable effects of amphetamine on responding for CR versus the CR effect itself remains to be elucidated.