We have investigated the effect of pinealectomy of ewes in pregnancy on the presence of the diurnal rhythm in fetal and maternal plasma concentrations of melatonin. Six ewes were pinealectomized between 104 and 118 days of gestation. Fetal and maternal blood samples were collected during 24-h periods between 125 and 140 days of gestation in the pinealectomized ewes and in an intact control (n = 4). There was a significant diurnal rhythm in both fetal and maternal plasma concentrations of melatonin in the control group. In this group, the fetal and maternal plasma melatonin concentrations were significantly higher in the dark (128·4±6·2 and 192·2± 10·7 pmol/l respectively) than in the light (46·2 ± 4·2 and 25·8 ± 2·1 pmol/l respectively). However there was no diurnal rhythm in either the fetal or maternal plasma melatonin concentrations in the pinealectomized group between 125 and 140 days of gestation. In contrast to the control animals, there was also no light–dark difference in the fetal or maternal plasma melatonin concentrations in four pinealectomized animals sampled frequently in the 3–7 days preceding delivery (mean length of gestation 146·5 days). However, in the pinealectomized sheep there was a gradual increase in the combined light–dark fetal plasma melatonin concentrations during late gestation from 27·9 ± 2·8 pmol/l (at 15–20 days before delivery) to 95·2± 14·1 pmol/l on the day of delivery.
We have therefore demonstrated that the maternal pineal is the major source of the diurnal rhythm in maternal and fetal plasma melatonin concentrations. However maternal pinealectomy does not appear to remove all the melatonin immunoreactivity from the maternal and fetal plasma in late gestation.
Journal of Endocrinology (1989) 120, 459–464