MELATONIN AND THE DEVELOPMENT OF CIRCADIAN AND SEASONAL RHYTHMICITY

We have investigated whether the maternal melatonin rhythm provides th e fetus with either a circadian or seasonal 'signal' during developmen t. Our findings provide evidence that melatonin can generate and entra in the early evening peak in the daily rhythm of fetal breathing movem ents. In contrast, daily variations in maternal and fetal prolactin co ncentrations are present in pinealectomized ewes, are altered by chang es in the time of onset of darkness, but are unaltered by changes in t he phase of the daily melatonin rhythm. The mechanisms that generate a nd control the daily prolactin rhythm before and after birth are there fore unknown. It is clear from a number of studies that the duration o f the nocturnal melatonin signal provides the adult and fetal sheep wi th photoperiodic information. We investigated whether there are differ ences in the fetal plasma concentrations of prolactin in ewes held in long and short photoperiods after surgical disconnection of the fetal hypothalamus and pituitary and demonstrated that there was a fetal pro lactin response to the external photoperiod in sheep fetuses in which the hypothalamo-pituitary axis was either intact or surgically disconn ected. We have suggested that one potential extrahypothalamic site of action of maternal melatonin is at the pars tuberalis of the fetal pit uitary. It appears therefore that there are a number of different mech anisms for the neuroendocrine transmission of information about the ti me of day and duration of the external photoperiod to the sheep fetus throughout late gestation.