Fig 1.

Schematic of behavioral protocol by postnatal day (PND). Self-administration began with a 5-day acquisition period in which rats were limited to 20 infusions/day under a fixed ratio 1 (FR1) schedule. Subsequently, rats were permitted an unlimited number of infusions for 23-hours/day during the 10-day extended access period. Next, infusions were again limited to 20 infusions/day for 2 days prior to forced abstinence. During the 10-day abstinence period rats had 2-hour access to either a locked or unlocked running wheel each day. After the last exercise session rats were returned to self-administration boxes and the following day began a within-session extinction/cue-induced reinstatement paradigm