A Daily Prolactin Rhythm Persists in the Ewe, Foetus and Newborn Lamb After Maternal Pinealectomy in Late Gestation

We have investigated the effect of maternal pinealectomy between 104 and 120 days gestation on the daily rhythm in plasma prolactin concentrations in the ewe, foetus and newborn lamb. There was a 24 h prolactin rhythm present in both the intact (n = 4) and pinealectomized (n = 4) ewes. Plasma prolactin concentrations were highest at 2100 h (i.e. 2 h after lights off) and a daily prolactin rhythm was present until at least 24 h before delivery. The mean 24 h foetal plasma concentration of prolactin was significantly higher (P<0.001) in the pinealectomized (35.3 + 1.9 mug/1) than in the intact group (6.9 +/- 0.6 mug/1). There was a significant 24 h rhythm in foetal prolactin concentrations and the timing of the daily peak (2100 h) was the same in the foetuses of intact and pinealectomized ewes. During the first 10 weeks after birth there was a significant effect of time of day on prolactin concentrations but the 24 h prolactin profile was significantly different in the lambs delivered to pinealectomized ewes than in the lambs delivered to intact ewes. In the lambs delivered after maternal pinealectomy, plasma prolactin concentrations were higher between 1900 and 2300 h than between 0700 and 1100 h, whereas in lambs delivered to intact ewes, prolactin concentrations were higher at 0100 h than between 1500 and 1700 h. We conclude that the ontogenesis of the daily rhythm in foetal and newborn lamb plasma prolactin concentrations is not dependent on a daily melatonin signal from the ewe.