Prenatal photoperiod influences neonatal prolactin secretion in the sheep

This study tested the hypothesis that the fetal sheep can respond to photoperiod cues. Pregnant Suffolk ewes were maintained in artificial photoperiod of either long days [16 h of light, 8 h of dark (16L:8D)] or short days (8L:16D) from approximately 100 days of gestation until term at approximately 147 days. On the day of birth, all lambs and their mothers were transferred to an intermediate photoperiod of 12L:12D; both groups were housed together. To provide an index of response to photoperiod, serum PRL concentrations were measured in blood samples collected daily 3-4 h after lights on. In lambs (n = 8 male; n = 7 female) born to mothers on long days, serum PRL concentrations were high (greater than 200 ng/ml) for the first few days after birth, but fell rapidly to low levels (less than 50 ng/ml) within 14 days postnatally in 12L:12D. Conversely, lambs (n = 8 male; n = 7 female) born to mothers on short days initially had low PRL concentrations, but these gradually increased in the postnatal 12L:12D photoperiod to 150 ng/ml by 32 days of age. Thus, serum PRL concentrations in lambs at birth reflect the photoperiodic treatment of their mother, and the subsequent PRL response to an intermediate photoperiod of 12L:12D depends on the photoperiodic history received in utero. We infer from these findings that the fetal lamb receives and responds to information about day length in utero and begins developing a seasonal photoperiod history before birth.