Pineal and associated neuroendocrine rhythms

(1) The mammalian pineal gland exhibits a number of biosynthetic rhythms. (2) Pineal serotonin levels are highest during the day and lowest at night. (3) Conversely, the activity of serotonin N-acetyltransferase, which converts serotonin to N-acetylserotonin, is greatly increased at night over daytime values. (4) As a consequence, N-acetylserotonin content increases in the pineal gland at night as does the product of its metabolism, melatonin. (5) Either indoleamines, such as melatonin, or polypeptides, such as arginine vasotocin, may be the pineal substances which influence the secretion of hormones from other endocrine glands. (6) Besides the obvious circadian rhythms within the pineal gland there are indications that the activity of this organ also changes on a seasonal basis, being more active during the winter than during the summer. (7) One physiological niche of the pineal gland may be to act as an intermediary between seasonal photoperiodic changes and the neuroendocrin reproductive axis. (8) In long day breeders, the pineal is believed tobe most active during the winter and, as a result, its hormonal products suppress sexual physiology. (9) This ensures that animals can only breed during restricted periods of the year.