Rats were trained to lever press for sucrose after 6 weeks of home cage access to two bottles, one filled with tap water and the other 2% sucrose (w/v). Bottles were placed on subjects’ cages every other day, with water alone available on the intervening days. After these 6 weeks, operant training for sucrose was conducted, using the same chambers as used for operant training for alcohol. Rats were initially trained in two overnight sessions under an FR1 schedule with 0.1 ml of a 2% sucrose solution as the reinforcer. Subsequently, rats were trained 5 days a week (before virus infusions), with the FR schedule progressively increased to FR2 over the course of 2 weeks. After 2 months of training, surgery to infuse viruses occurred. After a 5-day recovery period, 30-minute operant sessions resumed 7 days a week for 30 days.
