Changes in the hypothalamic serotonergic function may mediate the endocrine effects of melatonin.

The effects of a single injection of melatonin on serum thyroxine, corticosterone and prolactin levels, and the associated changes in the metabolism of serotonin (5-HT) in several hypothalamic regions of male rats kept under a 14-h light 10-h dark cycle (lights on at 08:00 h), are investigated. Melatonin (500 micrograms/kg, s.c.) or replacing saline was injected at 18:00 h, and 8 animals from treated and control groups were killed 1 h (19:00 h), 12 h (06:00 h) and 18 h (12:00 h) later. Melatonin significantly reduced serum thyroxine, corticosterone and prolactin levels within 1 h of treatment, indicating the existence of an acute inhibitory effect on hormonal secretion. Serum prolactin levels were increased 18 h after treatment, suggesting the implication of a different long-term regulatory mechanism. Injected melatonin induced several acute changes in the metabolism of 5-HT in the hypothalamus. Thus, a significant increase of 5-HT content in the preoptic area-anterior hypothalamic (POA-AH) and medial hypothalamic regions of melatonin-treated rats was observed within 1 h of treatment. The content of 5-hydroxyindoleacetic acid (5-HIAA) increased in medial hypothalamus, and the ratio 5-HIAA/5-HT decreased in POA-AH. The results show that melatonin acutely modifies the serotonergic activity in POA-AH and medial hypothalamus, and simultaneously inhibits thyroid, adrenal and pituitary hormonal secretion. The results are discussed suggesting that the hypothalamic serotonergic system could be an ideal mechanism underlying short-term melatonin effects on endocrine function.