Male Sprague-Dawley rats (Charles River Laboratories, Wilmington, MA; Harlan-Nossan, Milan, Italy) weighing 300-350g were housed in temperature- and humidity-controlled rooms on a 12-hr light/dark cycle. Experiments were conducted during the light phase. For self-administration experiments, food intake was limited to 20 g/day. For drug discrimination experiments, food was restricted to maintain weight at ≥85% of the subject's highest recorded weight.
