Experimentally naive adult male C57BL/6 mice (Jackson Laboratories, Bar Harbor, ME) were used for both mouse drug discrimination experiments. Adult male ICR mice (Harlan, Dublin, VA) were used for the in vitro experiments. Adult male Long-Evans rats (Harlan Sprague Dawley, Inc., Indianapolis, IN) were used for the rat drug discrimination studies. All rodents were housed individually in clear plastic cages with steel wire fitted tops and wood-chip bedding. They were kept in a light- (12-h light:dark cycle; lights on at 0600) and temperature- (20–22°C) controlled vivarium, except during experimental sessions which occurred during the light component. Mice in the discrimination experiments were maintained at 85–90% of free-feeding body weight. Food was not restricted for mice in the in vitro experiments. Body weights for the rats were determined at approximately 3 months of age and then the rats were gradually reduced to 85% of their free-feeding weights and maintained there by supplemental post-session feedings for the remainder of the study. Water was available ad libitum in the home cage for all rodents. Animals used in this study were cared for in
