EFFECT OF CONSTANT LIGHT ON FETAL AND MATERNAL PROLACTIN RHYTHMS IN SHEEP

A 24-h rhythm of plasma PRL is present in fetal sheep. This rhythm is synchronized to an environmental clue (zeitgeber). We determined wheth er the light-dark cycle (L:D) is a zeitgeber for the fetal PRL rhythm and, if so, whether the mother might convey this zeitgeber to the fetu s. We kept nine ewes (twin pregnancies) in constant light (L:L) and fi ve ewes (singleton) in 14:10 L:D from 110 days gestation. Fetuses and mothers were catheterized at 119 days gestation. Blood samples were ta ken hourly for 24 h after 16 days under L:L or L:D. A mean 24-h rhythm of PRL was found (by RIA) in fetuses under L:D, but not in those unde r L:L. However, fetuses under L:L showed individual 24-h PRL rhythms ( cosinor analysis) whose acrophases were distributed around the clock. Nonsynchronized rhythms persisted after 23 and 30 days of L:L. Acropha ses of PRL rhythms within a set of twins were closer than those betwee n sets, suggesting that twins were responding to a common signal. Thes e findings indicate that the L:D cycle is a zeitgeber for the PRL rhyt hm in fetal sheep and suggest that the mother might convey the zeitgeb er.