Effects of mild food deprivation on the estrous cycle of rats.

It has long been known that severe food deprivation disrupts the estrous cycle. One of the main problems with behavioral tasks that use food for reinforcement is the requirement that the animal be food deprived. This manipulation could be problematic in studies using female animals, since it may interfere with the estrous cycle of the animals. The purpose of the present study was to investigate: (1) the effect of mild food deprivation on four different strains of rats, (2) factors in the food deprivation procedure that could affect the estrous cycle, and (3) the possible effect of enriched diets during food deprivation on the estrous cycle. A comparison of the estrous cycle in four different rat strains revealed differences in the reliability of the estrous cycle even before the onset of food deprivation. Fischer, Long-Evans, and Sprague-Dawley rats all showed reliable cycle patterns. This was not the case for Brown Norway rats. During food deprivation, the cycle of the Fischer rats was disrupted, whereas the Long-Evans and Sprague-Dawley animals continued to cycle. Both the rate of weight loss and the percent of ad libitum body weight were related to cessation of the estrous cycle. However, enriching an animal's diet with sugar or oil additives delayed the disruption of the estrous cycle. Additionally, animals resumed cycling when returned to ad libitum weight levels. The present findings suggest that when animals need to be food deprived, preference should be given to using Long-Evans or Sprague-Dawley rats. If Fischer rats must be used, they should not be deprived below 90-95% of their ad libitum body weight. Strategies for future food deprivation studies are discussed, as well as a comparison of the effects of mild and severe food deprivation.