Morphine Self-Administration

Rats recovered from surgery for at least 7 days prior to morphine self-administration. Sires were placed in operant chambers and allowed to lever press on a fixed ratio 1 (FR1) schedule for morphine infusions (0.75mg/kg/infusion over 5s). Infusions of morphine were accompanied by a 5-second light cue and followed by a 20-second timeout period during which the house light turned off and lever presses were recorded but were not reinforced by drug infusions. Animals had daily 3-hour access to morphine self-administration. Control animals underwent the same catheterization surgeries and self-administration protocol but only had access to saline and were never exposed to morphine.