Scheduled Feeding and 24-Hour Rhythms of N-Acetylserotonin and Melatonin in Rats*

Male rats, kept under a lighting condition of 14-h light, 10-h dark, were subjected to scheduled feeding regimens. Food was available either in the early light phase or the early dark phase. The 24-h rhythms of serum corticosterone and of N-acetylserotonin (NAS) and melatonin (MT) in the pineal and serum were determined. It was found that whereas serum corticosterone and NAS rhythms responded to the feeding schedules, the rhythms of pineal NAS and of serum and pineal MT remained synchronized with the light-dark cycle. These findings indicated that the pineal was not the major source of circulating NAS. Whereas environmental lighting was the dominant "Zeitgeber" for the NAS rhythms in the pineal and the MT rhythms in the pineal and serum, for serum NAS rhythm, food presentation was the stronger Zeitgeber.