The fetal circadian pacemaker is not involved in the timing of birth in hamsters

The role of the fetal circadian pacemaker in the timing of birth was examined in Syrian hamsters (Mesocricetus auratus). Two groups of pregnant hamsters with lesions of the suprachiasmatic nucleus received daily intraperitoneal injections of melatonin (25 micrograms) on Days 9-15 of gestation. One group was injected with melatonin in the evening and the other in the morning. After the last set of injections, the animals were observed every hour until they gave birth. The timing of birth was not significantly different in the two groups, indicating that it was not affected by the melatonin injections. In contrast, the average phases of the pups' activity rhythms at weaning were significantly different in the two groups; this difference, approximately 10 h, indicated that the prenatal melatonin injections set the phases of the pups' circadian pacemakers. These results suggest that the fetal pacemaker is not involved in the timing of birth, since the pacemaker could be set to different phases without affecting this timing. In a separate study, time of birth and length of gestation were measured for heterozygous tau mutant pups, which express a circadian rhythm with a period of approximately 22 h. The results were compared with similar measurements obtained from a previous study of wild-type pups that express a circadian rhythm with a period of approximately 24 h. In both cases, the pups had been born to wild-type mothers. The timing of birth was similar in the two groups, indicating that the tau mutation does not influence the length of gestation or the time of day when birth occurs.