DAYTIME DARK EXPOSURE INCREASES PINEAL MELATONIN IN RAT PUPS

Photic regulation of the pineal melatonin synthesis was studied in 3- to 21-day-old rat pups by exposing the animals to light at night (30-4 0 min) or to darkness during the day (30-240 min). The pineal melatoni n contents were measured by radioimmunoassay, A significant day/night difference in the melatonin content and the nocturnal light-induced de crease were not found until second postnatal week. A novel finding was that at the age of 13-17 days a daytime dark exposure elevated the pi neal melatonin content; it was twofold as compared with the normal day time level and about half of the nocturnal peak level. In 21-day-old r ats the response had disappeared, while the nocturnal suppression by l ight persisted. The dark-induced increase of the melatonin synthesis w as independent of the opening of the eyelids which occurs in pups at t he age of two weeks, but it was greater in maternally isolated than no n-isolated pups. The results suggest that one component of the circadi an regulatory system matures at the end of the third postnatal week. T his mechanism inhibits the elevation of the melatonin synthesis by dar kness during the daytime.