Melatonin and the development of circadian and seasonal rhythmicity.

We have investigated whether the maternal melatonin rhythm provides the fetus with either a circadian or seasonal 'signal' during development. Our findings provide evidence that melatonin can generate and entrain the early evening peak in the daily rhythm of fetal breathing movements. In contrast, daily variations in maternal and fetal prolactin concentrations are present in pinealectomized ewes, are altered by changes in the time of onset of darkness, but are unaltered by changes in the phase of the daily melatonin rhythm. The mechanisms that generate and control the daily prolactin rhythm before and after birth are therefore unknown. It is clear from a number of studies that the duration of the nocturnal melatonin signal provides the adult and fetal sheep with photoperiodic information. We investigated whether there are differences 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 prolactin response to the external photoperiod in sheep fetuses in which the hypothalamo-pituitary axis was either intact or surgically disconnected. We have suggested that one potential extrahypothalamic site of action of maternal melatonin is at the pars tuberalis of the fetal pituitary. It appears therefore that there are a number of different mechanisms for the neuroendocrine transmission of information about the time of day and duration of the external photoperiod to the sheep fetus throughout late gestation.