MELATONIN AND THE LIGHT-DARK CYCLE SEPARATELY INFLUENCE DAILY BEHAVIORAL AND HORMONAL RHYTHMS IN THE PREGNANT EWE AND SHEEP FETUS

We have studied the effects of independently altering the time of dark ness and the phase of the daily melatonin rhythm during a 12-h photope riod on the diurnal rhythms of fetal breathing movements (FBMs), low v oltage electrocortical activity, and maternal and fetal plasma concent rations of PRL. Thirteen pregnant ewes were pinealectomized at 99-100 days gestation and held under a normal lighting regime (12-h photoperi od, lights off at 1900 h) until 135 days gestation. All ewes were then exposed to an altered lighting regime (12-h photoperiod, lights off a t 1100 h) until 145 days gestation. Six of these ewes were infused wit h melatonin (12-15 mug/h iv) from 1100 h for 12 h each day between 125 and 135 days gestation while in the normal lighting regime, and from 1900 h while in the altered lighting regime. The remaining seven ewes were infused with saline at corresponding times. Infusion of melatonin produced a significant daily rhythm in maternal and fetal melatonin c oncentrations; concentrations were highest throughout the 12-h infusio n period in each lighting regime. In the normal lighting regime (i.e. lights off at 1900 h) the peak incidence of FBM occurred at between 10 00 and 1300 h in the ewes infused with saline or melatonin. Under the altered lighting conditions there was no 24-h variation in the inciden ce of FBMs in the saline-infused group, but a significant daily variat ion was present in the melatonin-infused group with peak incidence occ urring at 1900-2200 h. The daily variation in maternal PRL concentrati ons was not affected by changes in the time of onset of melatonin infu sion but was different in each of the two lighting regimes. In the nor mal lighting regime, PRL concentrations were highest at 2100 h (56.2 /- 21.6 ng/ml) whereas in the altered lighting conditions the peak in maternal PRL occurred at 1700 h (90.4 +/- 25.1 ng/ml). The daily varia tion in fetal PRL concentrations was also not affected by changes in t he time of onset of melatonin infusion but was different in the two li ghting regimes; in the normal lighting regime, fetal PRL reached a pea k (13.0 +/- 3.7 ng/ml) at 0100-0300 h whereas in the altered lighting regime the peak (29.2 +/- 9.8 ng/ml) occurred at 1700-2100 h. We have presented evidence therefore that the daily melatonin rhythm may entra in the daily rhythm in fetal breathing activity. In contrast, diurnal variations in maternal and fetal PRL concentrations are present in pin ealectomized ewes, are unaltered by changes in the phase of the daily melatonin rhythm and appear to be under the control of the external li ght-dark cycle.