Developmental appearance of light-dark entrainment in the rat.

Previous studies show that the developing circadian system is entrained by a maternal signal during the prenatal and early postnatal period. The present study investigated the developmental onset of retina-mediated, light-dark entrainment. Juvenile rats were exposed to phase-shifts in the light-dark cycle at different ages and the pineal N-acetyltransferase rhythm was monitored on postnatal day 10. The results show that retina-mediated, light-dark entrainment begins by postnatal day 6 and overrides maternal entrainment by postnatal day 8.