Progressive ratio performance varies with body weight in rats

The progressive ratio (PR) performance of male rats was assessed under varying states of food restriction (75-100% of free-feeding body weight). Because performance of this task requires subjects to increase, within each session, the amount of effort (lever presses) expended for subsequent food reinforcers, it is thought that measures of performance in this task provide an index of `motivation' to work for food. In earlier studies, subjects were `prefed' their daily food allotments at a variety of intervals (0.25-6 h) prior to PR testing. In those studies, it was determined that such feeding episodes did not significantly affect PR performance and thus, motivation to work for food. The present studies demonstrate that PR behavior varies significantly as a function of body weight. Thus, in the rat, body weight per se has a greater influence over food-related satiety than does the recent ingestion of a meal.