Male, Sprague–Dawley rats, approximately 90–120 days old (275–350 g) were used as subjects (n = 31 total; 8 for Experiment 1, 23 for Experiment 2). All animals were surgically prepared for self-administration via implantation of a catheter into the jugular vein under ketamine hydrochloride (100 mg/kg) and xylazine hydrochloride (20 mg/kg) anesthesia using established procedures (Carelli, 2000). All surgical procedures were approved by the UNC Institutional Animal Care and Use Committee (IACUC) and in concordance with the NIH Guide for the Care and Use of Animals.
