Constant bright light (LL) during lactation in rats prevents arrhythmicity due to LL.

Light has a strong effect on the circadian system. Light-dark (LD) cycles are the main zeitgebers for practically all organisms, and the exposure of animals to constant bright light (LL) alters the manifestation of circadian rhythms. In rats, exposure to LL in adulthood produces an arrhythmic pattern in their motor activity, with a large number of ultradian 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 differed in the lighting conditions under which they were kept during lactation: 1) rats kept under LL during lactation (LL-rats), which manifested a circadian rhythm after weaning, and 2) rats kept under constant darkness (DD-rats), which were arrhythmic after weaning. We investigated whether the presence of rhythmicity under LL in LL-rats is a transitory effect or whether it persists throughout most of the life of the rat. Moreover, we examined motor activity rhythms of both groups of rats under different lighting conditions to find out other possible differences in the manifestation of their circadian rhythms. Results showed that there are no differences in the capacity of entrainment of both groups of rats to LD cycles or in the rhythm that rats show under DD. Most of the LL-rats maintained their circadian rhythms for the duration of the experiment (1 year), although we found differences in the rhythms manifested between males and females. We found that most of the LL-males became arrhythmic; consequently, at the end of the experiment, 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 showed a clear circadian rhythm under LL during the entire experiment. Thus, LL during lactation has a protective effect against the disruptive effect of LL on the circadian rhythm, although it is only clearly manifested in females.