Time of parturition in rats after melatonin administration or change of photoperiod

Two experiments were carried out with rats isolated at mating (Day 1 of gestation) and kept in a standard light regimen of 14 h light (14L:10D). All treatments started on Day 8 of gestation; periodicities of 23:45 h, 24:00 h and 24:15 h were applied to the light phase (14L) in Exp. I and to daily treatment with melatonin (0.3 mg/rat) or its vehicle in Exp. II. In Exp. II, the animals were placed in a continuous dim light regimen and injections were given at a time corresponding to lights off. In all groups, rats delivered on the afternoon of Day 22 and on the morning of Day 23 after a cessation of parturitions. The rates of births during these two times depended on the periodicities of the light phase or those of melatonin administration. With a periodicity of 24:15 h, 85.7% of rats in Exp. I and 85.7% of rats in Exp. II delivered on Day 22. With a periodicity of 23:45 h, 83.0% of births occurred on Day 23 in Exp. I and 57.7% in Exp. II with melatonin instead of 25.9% in the corresponding vehicle controls. These results suggest that melatonin secretion may be a mechanism whereby photoperiod regulates the time of parturition in the rat.