CHANGES IN THE HYPOTHALAMIC SEROTONERGIC FUNCTION MAY MEDIATE THE ENDOCRINE EFFECTS OF MELATONIN

The effects of a single injection of melatonin on serum thyroxine, cor ticosterone and prolactin levels, and the associated changes in the me tabolism of serotonin (5-HT) in several hypothalamic regions of male r ats kept under a 14-h light 10-h dark cycle (lights on at 08:00 h), ar e investigated. Melatonin (500 mu g/kg, s.c.) or replacing saline was injected at 18:00 h, and 8 animals from treated and control groups wer e killed 1 h (19:00 h), 12 h (06:00 h) and 18 h (12:00 h) later. Melat onin significantly reduced serum thyroxine, corticosterone and prolact in levels within 1 h of treatment, indicating the existence of an acut e inhibitory effect on hormonal secretion. Serum prolactin levels were increased 18 h after treatment, suggesting the implication of a diffe rent long-term regulatory mechanism. Injected melatonin induced severa l acute changes in the metabolism of 5-HT in the hypothalamus. Thus, a significant increase of 5-HT content in the preoptic area-anterior hy pothalamic (POA-AH) and medial hypothalamic regions of melatonin-treat ed rats was observed within 1 h of treatment. The content of 5-hydroxy indoleacetic acid (5-HIAA) increased in medial hypothalamus, and the r atio 5-HIAA/5-HT decreased in POA-AH. The results show that melatonin acutely modifies the serotonergic activity in POA-AH and medial hypoth alamus, and simultaneously inhibits thyroid, adrenal and pituitary hor monal secretion. The results are discussed suggesting that the hypotha lamic serotonergic system could be an ideal mechanism underlying short -term melatonin effects on endocrine function.