Between 13:00 and 17:00, rats were placed in standard rat operant chambers (Med Associates, Georgia, VT). Responding on the lever associated with food delivery resulted in delivery of a high-fat food pellet (45% energy from fat, 45mg, Bioserv F06162, Frenchtown, NJ). Rats were trained in half-hour sessions on a fixed ratio (FR) 1 schedule, where a single response on the active lever is needed to receive a pellet.
Once the percentage of responses on the active lever exceeded 85% for three consecutive days (30 (link)), animals were advanced to an FR3 and then an FR5 schedule, which require 3 and 5 correct responses for pellet delivery, respectively. Once animals have reached this criterion on the FR5, they move to a progressive ratio (PR) schedule, where earning each successive high-fat food pellet within the session requires a greater number of responses (1,2,3,6,9,12,15,20,25,32,40,50,62,77,95). Responses on the active, reinforced lever within a 60-minute test session were quantified. On test day, animals (n=11/group) received an IP injection of 0.3mg/kg NMU in DMSO (6 (link)), or vehicle alone fifteen minutes prior to testing (total volume 3mL).
Once the percentage of responses on the active lever exceeded 85% for three consecutive days (30 (link)), animals were advanced to an FR3 and then an FR5 schedule, which require 3 and 5 correct responses for pellet delivery, respectively. Once animals have reached this criterion on the FR5, they move to a progressive ratio (PR) schedule, where earning each successive high-fat food pellet within the session requires a greater number of responses (1,2,3,6,9,12,15,20,25,32,40,50,62,77,95). Responses on the active, reinforced lever within a 60-minute test session were quantified. On test day, animals (n=11/group) received an IP injection of 0.3mg/kg NMU in DMSO (6 (link)), or vehicle alone fifteen minutes prior to testing (total volume 3mL).