MELATONIN ADMINISTRATION AND PITUITARY-HORMONE SECRETION

OBJECTIVE The relationship between the pineal gland and pituitary func tion remains controversial, while the role of melatonin in the adaptat ion of the organism to the light-dark cycle of the environment is beco ming increasingly recognized. The aim of this study was to investigate the effect of a manipulation of the melatonin rhythm on pituitary hor mone secretion in man. DESIGN Double-blind controlled clinical study. SUBJECTS Ten adult healthy male volunteers, aged 21-33 years, were stu died on two occasions: once after the administration of melatonin 5 mg orally for 4 days at 1700 hours and once after the administration of placebo, at similar times. On the day of each study the subjects under took their normal duties but refrained from taking heavy exercise, fro m smoking and drinking alcohol. MEASUREMENTS Serum cortisol, growth ho rmone, prolactin and plasma vasopressin, oxytocin, melatonin, sodium, potassium, osmolality and packed cell volume were measured over the fo llowing 24 hours. RESULTS The cortisol peak was advanced and prolactin release increased after melatonin administration, while growth hormon e was not affected. Vasopressin and oxytocin levels were found to incr ease during the night in the control study, but the period of the noct urnal increase in vasopressin concentrations was reduced after the adm inistration of melatonin and the nocturnal increase of oxytocin was ab sent. CONCLUSION Altering the melatonin rhythm may affect neuroendocri ne function, influencing the nocturnal pattern of neurohypophysial hor mone secretion, augmenting prolactin release and advancing the peak of cortisol release.