CONSTANT BRIGHT LIGHT (LL) DURING LACTATION IN RATS PREVENTS ARHYTHMICITY DUE TO LL

Light has a strong effect on the circadian system. Light-dark (LD) cyc les are the main zeitgebers for practically all organisms, and the exp osure of animals to constant bright light (LL) alters the manifestatio n of circadian rhythms. In rats, exposure to LL in adulthood produces an arhythmic pattern in their motor activity, with a large number of u ltradian components. In previous experiments, we found that rats born and kept under LL during lactation develop, after weaning, a circadian rhythm which is maintained for at least a couple of months. Here, we examined motor activity rhythms under LL of two groups of rats which d iffered in the lighting conditions under which they were kept during l actation: 1) rats kept under LL during lactation (LL-rats), which mani fested a circadian rhythm after weaning, and 2) rats kept under consta nt darkness (DD-rats), which were arhythmic after weaning. We investig ated whether the presence of rhythmicity under LL in LL-rats is a tran sitory effect or whether it persists throughout most of the life of th e rat. Moreover, we examined motor activity rhythms of both groups of rats under different lighting conditions to find out other possible di fferences in the manifestation of their circadian rhythms. Results sho wed that there are no differences in the capacity of entrainment of bo th groups of rats to LD cycles or in the rhythm that rats show under D D. Most of the LL-rats maintained their circadian rhythms for the dura tion of the experiment (1 year), although we found differences in the rhythms manifested between males and females. We found that most of th e LL-males became arhythmic; consequently, at the end of the experimen t, there were no differences in the number of males showing circadian rhythm in the LL-and DD-groups. Most of the females in the LL-group sh owed a clear circadian rhythm under LL during the entire experiment. T hus, LL during lactation has a protective effect against the disruptiv e effect of LL on the circadian rhythm, although it is only clearly ma nifested in females. (C) 1998 Elsevier Science Inc.