Normal patterns of melatonin levels in the pineal gland and body fluids of humans and experimental animals.

The normal 24 hours patterns of melatonin were described in experimental animals and man. The rhythms are generated by neuronal activity originating in the suprachiasmatic nuclei of the hypothalamus; the activity of these nuclei are synchronized by the prevailing light:dark environment as perceived by the retinas. As a consequence of this synchronization, pineal melatonin levels are always high at night and low during the day, irrespective of whether the animal being studied is nocturnally or diurnally active. The rhythmic production of melatonin in the pineal gland is interrupted by the exposure of animals, or man, to light during the normal dark period. Several factors may change the 24 hours pattern of melatonin production. Because daylength changes throughout the year there are seasonal effects on the melatonin rhythm. Also, age is a major factor in determining the ability of the pineal gland to metabolize serotonin to melatonin. In advanced age pineal melatonin production is severely limited in both the human and in experimental animals. A variety of endocrine manipulations have been tested in terms of their impact on the ability of the pineal to produce melatonin. The majority of these have been rather innocuous although hypophysectomy does significantly curtail the synthesis of melatonin. To date, melatonin cycles have been usually described in rodents maintained under controlled environmental conditions in the laboratory; it can only be assumed that these rhythms are similar in animals in their natural habitat.