The effects of wheel running, a light/dark cycle, and the instrumental cost of food on the intake of food in a closed economy

Abstract

The effects of wheel running on the food intake of rats, and on the extent to which rats defend their daily food intake against increases in the instrumental cost of food, were studied in a closed economy. Rats lived in cages that were attached to running wheels. Within each cage, water was freely available and lever pressing was required for access to food; a fixed number of presses was required for the delivery of a single food pellet. All cages were located in an environmental chamber where a 12-h light/dark cycle was continuously maintained. During stage I, the entrance to each running wheel was blocked and two series (series 1 and 2) of progressively increasing fixed ratios (FRs) of presses per pellet were imposed. Each FR was used for a single day. During stage II, the entrance to each wheel was unblocked and wheel running and food intake were allowed to stabilize. During stage III, the running wheels remained available and the FR was again increased (series 3). In stage I, increases in the FR reduced food intake proportionally more rapidly during the light than during the dark, and this reduction in the light was greater during series 1 than series 2. During stage II, food intake was transiently reduced during the first week of access to running wheels, but recovered by the end of the second week. During stage III, increases in the FR again reduced food intake proportionally more rapidly during the light than during the dark. Wheel running did not affect the extent to which food intake was reduced within light and dark periods. However, as food intake was reduced by increasingly larger FRs, wheel running increased during the light but was unaffected during the dark. This selective increase in wheel running was attributed to the proportionally greater reduction of food intake and, by implication, body weight in the light.