MANIFESTATION OF CIRCADIAN-RHYTHM UNDER CONSTANT LIGHT DEPENDS ON LIGHTING CONDITIONS DURING LACTATION

Adult rats transferred to continuous illumination (LL) show a disrupti on of circadian rhythms, although the mechanisms underlying this effec t are not yet well known. In previous experiments, we found that when rats were born and raised under LL they showed an ultradian pattern du ring the first 10 days after weaning, but afterward they generated a c ircadian rhythm that was maintained until adulthood. It was not clear whether this evolution was attributable to the influence of the rhythm of the mother or to the effect of constant light. Here, we have studi ed the motor activity rhythm of young rats maintained under LL after w eaning, taking into account the conditions to which they were exposed during lactation [LL or continuous darkness (DD)]. To check the possib le effect of the rhythm of the dam, on the day of delivery some of the dams were blinded, others were subjected to a restricted feeding sche dule of 3 h/day, and the others were used as controls. For each rat, t he period of the circadian rhythm and the percentage of variance expla ined by this rhythm were calculated. Results show that all rats mainta ined under LL during lactation expressed a circadian rhythm in their m otor activity. However, rats maintained under DD during lactation did not. This effect did not seem to be dependent on the type of dam. Thes e results suggest that the rhythm of the dams does not affect the mani festation of the rhythm of the pups and that the expression of circadi an rhythmicity under constant bright light depends on the lighting con ditions under which the animals were maintained during lactation, whic h could affect the development of the circadian pacemaker or the retin a.