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.
Míguez
J M
JM
Laboratorio de Fisioloxía Animal, Facultade de Bioloxía, Universidade de Santiago de Compostela, Spain. fsmiguez@uscmail.usc.es
Aldegunde
M
M
D000818:Animals; D003345:Corticosterone / Q000097:blood; D004334:Drug Administration Schedule; D006897:Hydroxyindoleacetic Acid / Q000737:chemistry; D007031:Hypothalamus / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology*; D007279:Injections, Subcutaneous; D008297:Male; D008550:Melatonin / Q000008:administration & dosage / Q000494:pharmacology*; D011388:Prolactin / Q000097:blood; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000737:chemistry; D013974:Thyroxine / Q000097:blood; D013997:Time Factors; D014364:Tryptophan / Q000737:chemistry
D012701:Serotonin; D006897:Hydroxyindoleacetic Acid; D014364:Tryptophan; D011388:Prolactin; D008550:Melatonin; D013974:Thyroxine; D003345:Corticosterone