Split circadian rhythms of female Syrian hamsters and their offspring

In several mammalian species, circadian pacemakers of breeding females synchronize the developing clocks of offspring by as of yet unspecified mechanisms. The present study assessed whether maternal communication of circadian rhythms extends beyond setting pacemaker phase to include transfer of a fundamental reorganization of component circadian oscillators from dams to pups. In Experiment 1, a regimen of daily novel wheelrunning previously demonstrated to split activity rhythms of adult male hamsters into two discrete components was shown to similarly reorganize female hamster rhythms. In Experiment 2, females split by this method and unsplit controls exposed to similar light environments were mated with males. Split and unsplit females were equally fecund, but the former weaned pups of lower body weight. After weaning into running wheel cages, offspring of split dams were more likely to exhibit split activity rhythms than were offspring of unsplit females. Among pups not categorized as split, moreover, maternal entrainment nonetheless influenced distribution of pup activity across the 24-h cycle. Entrainment patterns of split and unsplit pups resembled those of adults. Thus, split and unsplit hamster dams provide different entraining signals to their developing offspring. The influence of maternal rhythms extends beyond entraining phase to alter interactions between component circadian oscillators that underlie split activity bouts. Maternal effects did not persist beyond the second week postweaning in split or unsplit hamsters, however, and rhythms of many split pups later joined. Thus, the maternal influence on the pup's circadian pacemaker may be transient.