Subjects

A total of 227 male rats were used. Of these, 177 were Long Evans (LE) and 50 were Fos-lacZ transgenic (on Sprague Dawley background)⁵⁴. Long Evans rats were purchased from Charles River, Inc. (Wilmington, MA). Fos-lacZ rats were bred at The Scripps Research Institute, and genotyped by Laragen, Inc. (Culver City, CA). Rats weighing 250–300 g at the start of experiments were housed in a temperature and humidity-controlled room, maintained on a 12 h/12 h reverse light/dark cycle. The rats were always trained and tested during the dark (active) phase in dedicated operant conditioning chambers (“chamber”). Each chamber was equipped with two retractable levers (one “active lever” and one “inactive lever”), a “light-cue,” a pump, and either a liquid swivel system for cocaine or a drinking well for alcohol. At all times, water and food were available ad libitum.