Effects of altered photoperiod or maternal melatonin administration on plasma prolactin concentrations in fetal lambs

Abstract

During winter (December to March), when late-pregnant ewes were maintained under an artificial long-day photoperiod (16 h light) for 3 weeks or more before insertion of fetal vascular cannulae between 118 and 120 days of gestation (full term, 147 days), plasma prolactin concentrations in their fetal lambs were significantly increased throughout the last 3 weeks of gestation in comparison with values in similar aged fetuses from ewes experiencing only the natural short-day (less than 9 h light) winter photoperiod. When additional lighting was given only after vascular cannulation, fetal plasma prolactin increased steadily from low values, characteristic of winter pregnancies, to high values, characteristic of long-day (16 h light:8 h darkness) pregnancies. Maternal plasma prolactin concentrations changed in a similar way. During summer pregnancies (greater than 16 h light), plasma prolactin in fetal lambs was significantly reduced within 48 h when ewes were given melatonin by i.v. infusion for 14 h each night to simulate the winter duration of the nocturnal increase in plasma melatonin. Maternal plasma prolactin concentrations also decreased significantly when melatonin was given for 3 weeks, but not in a shorter experiment. Increases in fetal plasma prolactin were proportional to the basal prolactin concentration in fetuses injected i.v. with TRH or a dopaminergic antagonist, metoclopramide, to assess how photoperiod influenced the responsiveness of prolactin secretion to acute stimulation. The results confirm that photoperiod, rather than developmental maturity, is the principal determinant of plasma prolactin in the fetal lamb during the last third of gestation, and provide evidence that photoperiodic information is transmitted to the fetus through the diurnal rhythm of melatonin in the ewe.

Seasonal effects on birth weight of lambs born to prolific ewes maintained under intensive management

“…Seasonal variation in melatonin secretion may affect fetal growth also indirectly, through regulating prolactin secretion by the maternal and the fetal pituitaries (Bassett et al, 1989;Houghton et al, 1997). As melatonin negatively regulate prolactin secretion, relatively high prolactin levels during the long day periods may interfere with the Placental Lactogen promoting activity on uterine gland development and on nutrient delivery and utilization by the fetus.…”

Maternal photoperiod programs hypothalamic thyroid status via the fetal pituitary gland

“…This phenomenon is best described in rodents, where it plays a major role in ensuring that offspring reproductive development proceeds rapidly in spring-born animals but is arrested to the following year in autumnborn animals (4). MPP has also been described in larger mammals (5) and is probably of broad adaptive significance for the setting of growth and maturation trajectories. This programming, which depends on the length of daily light exposure (photoperiod) experienced by the mother being relayed to the fetus, sets initial postnatal developmental trajectories as well as postnatal sensitivity to subsequent photoperiodic experience.…”

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

“…There is a daily rhythm in plasma melatonin concentrations in both the ewe and foetus which is dependent on the presence of the maternal pineal gland (3-6) and it has been proposed that the maternal melatonin signal provides the foetus with photopc: iodic information (7). Bassett and coworkers found that during sui I liner pregnancies, foetal prolactin concentrations decreased within 48 h when ewes were infused with melatonin for 14 h each nigiit to simulate the winter nocturnal increase in melatonin (7). In !he present study, whilst plasma concentrations of prolactin appcared higher in the pregnant ewe and her foetus after maternal piriulcctomy in late pregnancy, the change in prolactin concentratioli.…”

“…It is possible that photoperiodic information may be ti-ansmitted to the foetus by the diurnal rhythm in maternal nidatonin as melatonin crosses the ovine placenta and it has been shown that the pineal gland of the pregnant ewe is the source of thc plasma melatonin rhythm in both ewe and foetus (3)(4)(5)(6). h r i n g summer pregnancies when melatonin is infused into ewes to simulate the winter duration of the nocturnal melatonin inmase, there is a reduction in the foetal and maternal plasma cmcentrations of prolactin (7). Whilst it appears that the changes in foetal prolactin associated with changes in photoperiod are dcimdent on a maternal melatonin signal, the role of melatonin in the control of the daily variation in foetal and maternal Prolactin concentrations is unknown.…”