Definition of a prenatal sensitive period for maternal-fetal communication of day length.

The duration of the nocturnal elevation of melatonin in the dam is a key feature in providing the Djungarian hamster fetus with a prenatal photoperiodic history. The developing animal compares the prenatal photoperiod with the photoperiod experienced during the postnatal period to properly time puberty. In the present report, we define the period during gestation when melatonin administered to the dam is effective in providing the fetus with a prenatal photoperiodic history. The administration of melatonin (50 ng delivered over 10 h at night) by timed infusions to pregnant, pinealectomized Djungarian hamsters for the last 4-7 days of gestation stimulated testicular weights of male pups reared in 14 h of light/day. Single 10-h melatonin infusions were ineffective in stimulating reproductive development (testes weights on day 34) of the male pups, irrespective of the gestational age of the fetus at the time of treatment. In contrast, two consecutive nightly 10-h infusions during gestation clearly stimulated testicular development. The gestational age of the fetus at the time of infusion strongly influenced the response, however. Treatments beginning between 6 and 3 days before birth were effective in stimulating postnatal reproductive development of the offspring. These results indicate that there is a well-delineated, sensitive period during prenatal development when melatonin can provide the fetus with a prenatal photoperiodic history.