Biochemical and ultrastructural approaches to the onset of the pineal melatonin rhythm in the rat.

To determine the pineal gland content of melatonin during rat development and the onset of the circadian rhythm, a biochemical and ultrastructural experimental model was designed. Body and pineal gland weights and DNA and protein contents of the pineal glands increased rapidly during ontogenic development. No significant differences in the values of these parameters were observed when the data were obtained during the day or at night. Pineal melatonin content as expressed by pineal gland or by micrograms of DNA, was very small in 21-day-old fetuses and 5- and 10-day-old suckling rats and the values determined were similar for both day and night. However, in 20-day-old suckling and adult rats higher melatonin concentrations and significant differences between the values obtained during the day and at night were observed. The onset of the circadian melatonin rhythm coincided with the time at which the components involved in melatonin synthesis reached adult levels and pinealocytes adopted an adult morphological appearance as observed with the electron microscope. These results indicate that the appearance of a circadian melatonin rhythm in the rat pineal gland is delayed until the end of the suckling period, although the transfer of melatonin from the mother to the pups may be important for postnatal reproductive and somatic development.