Patterns of melatonin secretion during sexual maturation in female ferrets.

These studies were an examination of the changes in function of the pineal gland and its secretory product, melatonin, during sexual maturation induced by stimulatory photoperiod in ferrets. Plasma melatonin patterns were determined for 48-h intervals in ferrets undergoing photoperiod-stimulated sexual maturation. Sampling regimens were conducted when ferrets were 10, 15, 16, and 23 wk of age. Clear daily rhythms in melatonin concentrations were apparent in ferrets as young as 10 wk of age. Melatonin secretion increased with the onset of darkness each day and fell with lights-on the next morning. When the photoperiod was changed from a short day (8L:16D) to a long day (16L:8D) at 15 wk of age, the melatonin rise did not contract immediately to the shorter dark phase for at least the first two nights. By 1 wk after imposition of the new, long-day photoperiod, however, the pattern of melatonin concentrations in plasma was synchronized to the new photoperiod. These studies show that ferrets can detect photoperiod at a much younger age than that at which they can respond to long days with accelerated maturation and further, that while the pineal gland mediates the photoperiod-induced maturation of ferrets, other factors are involved in the 5-6-wk interval between the onset of long days and the maturational response of the hypothalamo-hypophyseal-gonadal axis in this species.