Diurnal Rhythms in Plasma Melatonin Concentrations in the Fetal Sheep and Pregnant Ewe during Late Gestation*

Abstract

We have measured plasma melatonin (MT) concentrations in the pregnant ewe and fetal sheep during 24-h periods between 114 and 142 days gestation. There was a clear diurnal rhythm in the plasma MT concentrations in both the ewe and fetus from 114 days gestation. Blood samples were also collected from the pregnant ewe and fetus during the day every 2-3 days from 112 days gestation to term. There was no gestational age trend in maternal or fetal day time plasma MT concentrations during late pregnancy. To establish whether there was transplacental transfer of MT, pregnant ewes were injected with [3H]MT, and total radioactivity (disintegrations per min) was measured in maternal and fetal arterial plasma and in amniotic fluid collected before and for 1 h after the [3H]MT injection. Two minutes after [3H]MT injection, radioactivity was detected in both maternal and fetal sheep plasma. Extraction of fetal plasma with chloroform indicated that [3H]MT accounted for 48.0 +/- 7.2 (SE) % of total radioactivity at 2 min after the injection. In one pregnant ewe infused with unlabeled MT (0.3 microgram/ml saline.min for 20 min) maternal and fetal plasma MT concentrations increased within 6 min after the start of the MT infusion. We conclude that there is a diurnal rhythm in the plasma concentrations of MT in the fetal lamb and pregnant ewe between 114 and 142 days gestation, and that MT crosses the ovine placenta from the maternal to the fetal circulation. Therefore, the MT present in the fetal sheep circulation may be solely of maternal origin or it may be derived from both fetal and maternal sources.

Third Trimester Fetal Heart Rate and Doppler Middle Cerebral Artery Blood Flow Velocity Characteristics During Prenatal Selective Serotonin Reuptake Inhibitor Exposure

“…The mechanisms involved in this circadian rhythm have not been elucidated in the human, but in pregnant sheep, fetal circadian rhythms in activity, behavior, and cardiovascular function are linked to a fetal plasma melatonin rhythm, which is derived at least in part from the maternal melatonin rhythm via placental transfer (26,27). Why the fHR variables in the EXP group did not show the AM to PM changes remains unclear.…”

Genetic variability in melatonin secretion originates in the number of pinealocytes in sheep

“…Regarding the onset of melatonin secretion in life, although a night-time increase in plasma melatonin concentrations has been reported to occur in sheep fetuses (Yellon & Longo 1987, Zemdegs et al 1988) as a direct consequence of melatonin secretion from the maternal pineal gland (Yellon & Longo 1988, McMillen & Nowak 1989 and in the newborn lambs by 1-6 weeks of age (Rodway et al 1985, Claypool et al 1999, it is not known whether the genetic difference in plasma melatonin concentrations previously described in adult ewes is already present soon after birth, or whether it is progressively acquired with age.…”

The characteristics of the melatonin secretory rhythm are not modified by the stage of pregnancy in ewes

“…This information may be used jointly with the postnatal photoperiod to induce seasonally appropriate reproductive and behavioral responses in the young lamb (Zemdegs et al, 1987;Ebling et al, 1989;Foster et al, 1989 (Birau et al, 1984;Kiveld, 1991 (Yellon and Longo, 1987;Zemdegs et al, 1987).…”