Developmental changes in hypothalamic melatonin levels of male rats

Hypothalamic melatonin levels of Long-Evans male rats were studied at three ages (25, 55-60 and 90 days), at four times of the day in the autumn (6:00, 12:00, 18:00 and 24:00), and at two times (12:00 and 24:00) in the spring using radiommunoassay. Melatonin levels increased markedly at noon at 55-60 days of age, compared with the levels at the same time of the day at 25 and 90 days. This increase persisted in autumn and spring. The 24-hr pattern in hypothalamic melatonin was the inverse of that in the pineal, with the levels at noon higher than those at midnight. This pattern was detectable at 25 days of age although the difference in melatonin between 12:00 and 24:00 hr was not great. The day/night difference was prominent by 55-60 days of age and disappeared by adulthood (90 days). This 24-hr pattern was similar in spring and autumn in the three ages studied. Although in the 55-60-day-old group the melatonin ratio (noon/midnight) was the same in autumn and spring, the absolute levels of melatonin in spring were significantly lower. The findings are consistent with the general concept of a modulatory role of melatonin in control of hypothalamo-hypophyseal GnRH and gonadotropin function, and the timing of the developmental maturation of this neuroendocrine axis. Demonstration of the mechanism of melatonin's action at the hypothalamic level will be facilitated by further definition of quantitative developmental changes.