Photoperiodic regulation of the time of birth in rats: involvement of circadian endogenous mechanisms.

A resonance experiment was undertaken to demonstrate that photoperiod regulates birth time by endogenous circadian mechanisms. Pregnant rats were maintained on a standard light-dark (LD) cycle (14L-10D; lights on from 0600 to 2000 hr) or on fixed LD cycles with periods of 12, 24, 36 and 48 hours after day 8 of gestation. In these groups, the light phase (2 hr) started between 0600 and 0800 hr or between 1800 and 2000 hr illuminating exclusively (for periods of 24 and 48 hr) or alternatively (for periods of 12 and 36 hr) the hours corresponding to morning (M) or evening (E) of the standard light regimen. At the end of gestation, the general activity was manifested mainly at moments corresponding to the night of the standard regimen in most groups; it was delayed in the two groups lit up exclusively at E hours. In groups receiving light exclusively at M hours, birth times were delayed compared to the deliveries in groups receiving light at E hours only. An intermediate distribution of birth times was observed when M and E hours were stimulated every 12 hr but not every 36 hr. The apparent stability of the diurnal rhythm of activity and the difference in birth time distributions due to the period of light phase indicate that the regulation of birth time by photoperiod is due to a circadian mechanism in rats. This mechanism implicates at least two endogenous systems which are apparently antagonists with regard to birth.