Manipulation of potential perinatal zeitgebers for the juvenile circadian temperature rhythm in rats.

We investigated the importance of pre- and postnatal maternal rhythmicity for the development and synchronization of the juvenile circadian core temperature (Tc) rhythm in rats by evaluating the Tc of artificially reared pups in six litters derived from mothers maintained in continuous bright light (LL) and impregnated after drinking behavior stopped showing circadian periodicity. Pups removed from their aperiodic mothers on postnatal day 9 and artificially reared for 3 days showed a free-running Tc rhythm whose amplitude (3.1 +/- 0.1 degrees C, n = 47) was only slightly smaller than that of control pups born to mothers maintained in a 12:12-h light-dark cycle. In four litters the acrophases were not much less synchronized than in the four control litters (mean vector lengths 0.79 +/- 0.05 vs. 0.94 +/- 0.04). In two litters synchronization among littermates was not significant. Additional experiments with cross-fostered pups showed that synchronization is not caused by the time of birth. We conclude that synchronization among littermates can develop even when maternal rhythmicity has been suppressed even before conception, possibly because the Tc decrease of several littermates happening to have similar phases at birth acts as zeitgeber.