The effect of chronic food and water restriction on open-field behaviour and serum corticosterone levels in rats

In operant conditioning experiments, two methods are commonly used to motiv ate laboratory rats to perform designated tasks. The first is restricting f ood so that rats are forced to lose 20% of body weight within one week, fol lowed by maintenance at 80% of the baseline weight for the remainder of the experiment. The second is restricting access to water to 15 min in each 24 h period. These methods are effective in motivating the animals. There is, however, little information available on the effects on performance in tes ts of behaviour that are not related to operant conditioning. In addition, it is not clear if these commonly used methods of food and water restrictio n will lead to physiological stress as indicated by an elevation of serum c orticosterone. Male rats were either food-restricted to reduce and maintain their weight at 80% of baseline weight, or were restricted to 15 min acces s to water every 24 h. Activity in the open field was significantly greater in food-restricted rats than in water-restricted or control rats, but free zing behaviour was similar in all experimental groups. Food-restricted rats had a higher mean serum corticosterone level than water-restricted and con trol rats 37 days after the start of the experimental period. These data su ggested that chronically restricting food and maintenance of body weight at 80% of baseline body weight led to significant behavioural changes and phy siological stress. In contrast, water restriction did not lead to changes i n behaviour or corticosterone levels. A second experiment was conducted to compare the effects of food restriction to 80% of baseline body weight, as described above, with a less stringent protocol in which test rats were ini tially reduced to 80% of baseline weight, but were then maintained at 80% o f an ad libitum fed control rat's weight. Serum corticosterone levels and a drenal gland weights were measured after the initial week of forced weight loss and after maintenance for 21 days. Forced loss of 20% of body weight i n the first week led to significantly increased serum corticosterone levels and adrenal gland weights compared to ad libitum fed controls. Serum corti costerone levels and adrenal gland weights in rats maintained at 80% of the ir initial body weight for 21 days remained higher than ad Libitum fed cont rol rats. However, rats maintained at 80% of an ad libitum fed control rat' s weight did not differ from control rats in serum corticosterone levels or adrenal gland weights at the end of the 21-day study period. Adjustment of the feeding regimen in this manner eliminated physiological evidence of ch ronic stress.