Effects of chronic stress and time of day on preference for sucrose.

Rats were allowed to lever press for two types of food pellets of equivalent caloric value and total carbohydrate content. One type of food pellet provided more of the calories as sucrose. During a 16-day prestress period, lever presses for 12 rats were recorded hourly. Following the baseline period, four rats (stressed group) were shaped to pull a ceiling chain to avoid or escape signalled foot shock presented intermittently around-the-clock. Four additional rats (yoked group) were each paired to one of the chain-pulling rats such that the rat trained to pull the ceiling chain controlled stressor termination for both rats. A third group of four rats served as the control group and received no shock. We have previously reported that rats in this model of chronic stress tolerate the paradigm well, continuing to gain weight, eat, drink water, and groom and escape more than 99% of the trials presented. During the baseline period, the sweeter pellet was preferred by most rats, but differences in preference among rats and in preference at different times of day were observed. The preference for the high-sucrose pellet was most marked in the hours preceding lights off. Overall, no changes in food preference were seen as a function of stress condition during the 14-day stress period, although one rat in the yoked group increased preference for the sweeter pellet during stress and returned to prestress food preferences when stress was terminated.