Prior to surgery, all animals were trained to lever press for 45mg Noyes food pellets (BioServe; Port Washington, NY), initially on a fixed ratio (FR-1) reinforcement schedule in 3-hr daily sessions. Illumination of the house light and the extension of the active lever into the chamber indicated the start of the session. Active lever presses resulted in the following events: a food pellet was dispensed onto the receptacle tray, and retraction of the active lever, and illumination of the stimulus light above the active lever for five seconds, signaled a time out period (TO), during which reward was unavailable. When an animal earned more than 10 pellets on the FR-1 reinforcement schedule (usually in 1-3 days), the schedule was increased to an FR-3 in the next session, followed by a PR reinforcement schedule in the following session. Maximum session lengths were four hours. Food training for the PR schedule was essential for increasing the number of animals that learned to lever press for cocaine and to decrease the time needed to train the animals after surgery. Complications associated with intravenous
