Animals

Male, Sprague Dawley rats (Harlan Laboratories Inc., Indianapolis, IN) weighing 250 to 350 g were housed in climate-controlled rooms on a 12-hour light–dark cycle. All animal procedures were approved by the Institutional Animal Care and Use Committee at Eli Lilly, and these procedures were in accordance with guidelines set by the National Institutes of Health and the International Association for the Study of Pain. Animals were monitored throughout the duration of the study to reduce any unnecessary stress and/or pain. Subjects were divided experimentally into SNL (N = 16), sham (N = 15), and naïve (N = 12) populations. Animals were housed individually for the duration of the experiment (~11 weeks) to control feeding conditions during training periods involving food restriction. During food restriction (~4 weeks), animals were monitored, handled, and tested in the operant chambers by experienced personnel. Data on motivational responses were collected from the same cohort of rats and previously reported [51].