The effect of melatonin administration on pituitary hormone secretion in man

OBJECTIVE Evidence is accumulating that the nocturnal increase in melatonin may influence pituitary hormone secretion. The aim of this study was to de termine the effect of exogenous melatonin, in concencetrations spanning the physiological range, on the release of pituitary hormones in man during da ylight hours DESIGN A double blind, randomized, crossover study, SUBJECTS Eight healthy male volunteers with a mean age of 21 +/- 0.5 years were studied on four occasions, observations being made after the adminstra tion of melatonin in doses of 0.05, 0.5 or 5.0 mg or placebo. They refraine d from taking heavy exercise, alcohol and from smoking for 24 h prior to th e study, MEASUREMENTS Serum cortisol, growth hormone, prolactin and plasma oxytocin, vasopressin, sodium, osmolality and packed cell volume were measured in sa mples taken at 30 minutes intervals for 150 minutes after the administratio n of melatonin. RESULTS Melatonin produced dose-dependent changes in circulating concentrat ions of oxytocin and vasopressin, the 0.5 mg dose being stimulatory, while 5.0 mg was inhibitory. These two doses stimulated growth hormone release, w hile there was no significant effect on prolactin or cortisol release. CONCLUSIONS These results confirm that the nocturnal increase in melatonin could contribute to the patterns of oxytocin, vasopressin and growth hormon e release seen over 24 h.