2.5. Cocaine Self-administration.

Mice were given 3–4 d to recover from surgery before the initiation of behavioral testing. The cocaine self-administration behavior was tested in 2-hour sessions (6 d per week) in the same chamber used for sucrose pellet self-administration. However, wheel responding now delivered an intravenous cocaine infusion (0.6 mg/kg/infusion), paired with the same compound cue, under the same fixed ratio (FR1) schedule as food training. After 10 days of a fixed ratio reward schedule, the mice were tested for 1 day on a progressive ratio (PR) schedule, where the response requirement for each infusion increased until the subject did not fulfill the requirement. The response requirement was defined as R(i)=[5e^0.2i-5] and the session ended if the animal took longer than 30 minutes to meet the requirement. The breakpoint is defined as the total number of responses or the final ratio completed. Following PR, cocaine-seeking behavior was extinguished by replacing cocaine with 0.9% saline and removing the light and tone cues, previously paired with cocaine delivery. Daily 2 h extinction sessions occurred until animals performed <25% of their self-administration responding (average of last 3 days). Twenty-four hours after meeting this extinction criterion, animals underwent a cue-induced reinstatement session. The light and tone cues were presented non-contingently for 20 seconds, every 2 minutes during the first 10 minutes of the session. For the remainder of the session, the cues were presented contingent with operant responding, as they were during the cocaine self-administration phase. During the reinstatement session however, animals received saline infusions following responses on the active wheel. Catheter patency was tested once a week and animals were removed from the study if found to have lost patency.