MATERNAL ENTRAINMENT OF THE DEVELOPING CIRCADIAN SYSTEM IN THE SIBERIAN HAMSTER (PHODOPUS-SUNGORUS)

The aim of these studies was to investigate maternal entrainment of de veloping circadian locomotor activity rhythms in the Siberian hamster. In Experiment 1, mothers were transferred from a 16:8 LD cycle into c onstant dim red light (DD) from the day of parturition, and wheel-runn ing activity of the mother and pups was individually monitored from th e time of weaning. The phases of the individual pups' rhythms were fou nd to be synchronized both to the phase of the mother and to the phase of lights off (ZT 12) of the photo cycle that the mother was exposed to until the day of parturition. To investigate whether this synchrony might reflect direct effects of light acting upon the fetal circadian system in late gestation, the experiment was repeated but with mother s placed into DD early in pregnancy (less than or equal to day 7 of ge station). The results were similar to the first study, suggesting that the mother rather than the photo cycle during the latter part of gest ation entrains the developing circadian system. The third experiment i nvestigated whether this entrainment occurred during the postnatal per iod. Breeding pairs were maintained on alternative light-dark cycles, LD and DL, that were 12 h out of phase. Litters born to mothers on one light-dark cycle were exchanged on the day of birth with foster mothe rs from the reversed light-dark cycle, then raised in DD. Control Litt ers exchanged between mothers from the same light-dark cycle had simil ar Litter synchrony as shown by nonfostered Litters of Experiment 1. H owever, pups cross-fostered with mothers on reversed LD cycles showed a very different distribution of pup phases. Pups were not synchronize d to their natural mother but to their foster mother. Moreover, pups w ere more scattered over the 24-h period and were found to be significa ntly synchronized to the phase of the reversed LD cycle. These results demonstrate the occurrence of postnatal entrainment in the Siberian h amster. The increased scatter produced by the cross-fostering paradigm results from some Litters being completely entertained to the phase o f the foster mother, some with an intermediate distribution between th e phases of the natural and foster mothers, and a minority being assoc iated with the phase of the natural mother. These results suggest that Siberian hamster pups are initially synchronized either prenatally or at birth but that the mother continues to provide entrainment signals during the postnatal period.