Cocaine self-administration

Rats were anesthetized and implanted with intravenous catheters with back mounts according to previously established methods [19–23]. Cocaine self-administration training consisted of daily 180-min sessions, during which rats were trained to lever press for intravenous cocaine infusions (0.75 mg/kg/0.1 mL infusion) starting on a fixed ratio (FR) of one press per infusion (FR1) and finally stabilizing on an FR5 schedule. Stability criteria were at least seven infusions each hour of session and less than 10% variability each day for three days [19, 20]. Following FR training, rats were advanced onto a PR schedule of reinforcement where exponentially more lever presses are required for each successive cocaine reinforcement [4, 5]. Rats responding for cocaine on a PR schedule maintain stable number of infusions over time [24], and stability was defined as less than 10% variability in the total daily infusions for three days.

Two cohorts of rats were employed in this behavior procedure. Rats (n = 62) were included in the pharmacological tests with NMU. Rats were randomized for between-subjects pharmacological tests before FR testing and then equally distributed into to new groups before PR testing. A second cohort (n = 20) received viral knockdown with a short hairpin against NMUR2 (shNMUR2) or non-silencing control (shCTRL) in the NAcSh immediately following catheter implantation. The non-silencing control expresses a short hairpin that was designed to lack efficacy to alter any target [25]. Rats were removed from the study if the catheter patency was lost or the viral injection as mistargeted (total n = 18 for both cohorts).