THE CIRCADIAN-GATED TIMING OF BIRTH IN RATS: DISRUPTION BY MATERNAL SCN LESIONS OR BY REMOVAL OF THE FETAL BRAIN

Abstract

In rats, the hour of birth is gated over a 36-hr temporal window by the phase of the daily light-dark cycle during pregnancy. We have previously shown that the suprachiasmatic nuclei (SCN), the site of a known circadian pacemaker, are oscillating in phase with the prevailing light-dark cycle in the fetus. Since the onset of parturition is governed by the fetal brain in some species, we have speculated that a possible role of a functioning and entrainable circadian clock during fetal life is that it might be involved in the circadian-gated initiation of parturition. First, we showed the circadian gating of birth in our animals by exposing different groups of dams to lighting cycles of opposite phase during pregnancy. Regardless of the phase of the prenatal lighting cycle, the time of birth was gated over a 36-hr temporal window so that most births occurred during the daytime hours. Next, we found that destruction of the maternal SCN (on day 7 of gestation) eliminated the circadian gating; births occurred in a single distribution that peaked in the middle of the 36-hr window. Finally, removal of all the fetal brains from each litter also disrupted the circadian gating of birth; dams of brain-aspirated fetuses no longer exhibited a daytime preference for births. These results show that the maternal SCN are necessary for the normal circadian gating of birth and are also consistent with a role for the fetal brain (and possibly the fetal SCN) in this process. Support by HD14427.