Abstract
Maternal entrainment of the circadian wheel-running activity rhythm was examined in Syrian hamsters heterozygous for a single gene mutation ( tau) that affects the free-running period of circadian rhythms. Heterozygous tau pups were bom to and raised by wild-type mothers under constant dim light. The pups' wheel-running activity was recorded after weaning on postnatal day 18 or 24. Pups weaned on day 18 had an average free-running period of 21.70 hr, demonstrating that the tau phenotype was fully expressed at this age. Using the activity onset of the postnatal free-running rhythms as a phase reference, we estimated the phase relationships between the pups and their mothers on days 18 and 24. In contrast to results with wild-type pups, the activity rhythms of tau pups were not in phase with the rhythms of their wild-type mothers; that is, activity onsets of mothers and pups did not coincide. The pups did, however, show synchrony among themselves, indicating that they had been exposed to a synchronizing signal sometime during development. It is likely that this synchronizing signal was provided by the mothers, since pups from different litters showed phase relationships similar to those of their mothers. Thus the mothers provided a signal that was sufficient to cause entrainment, despite the 2-hr difference in free-running period between the mothers and pups. Although the pups' activity rhythms appeared to have been entrained by the mothers, they were clearly free-running by postnatal day 18. The mechanism for entrainment is lost during the course of development, despite continued interaction between the mothers and pups.