The effect of exogenous melatonin on stimulated neurohypophysial hormone release in man

OBJECTIVE Melatonin is known to have a dose-dependent effect on basal neurohypophysial hormone release in man, low physiological doses being stimulatory and high doses inhibitory. We have performed a study to determine if this is true also for stimulated release. DESIGN This was a double-blind randomized crossover study with subjects being given oral doses of melatonin (0.5 mg or 5.0 mg) or placebo, before infusion of hypertonic saline or exercise. SUBJECTS Studies were performed on a total of 24 male subjects aged 19-23 years, who were entrained to a normal light-dark cycle and who refrained from taking heavy exercise, alcohol and smoking for 24 h prior to study. MEASUREMENTS Plasma vasopressin, oxytocin, sodium and osmolality and packed cell volume were measured in control samples and at regular intervals after the administration of melatonin and the challenge tests. Blood pressure and urine flow, sodium potassium and osmolality were determined during hypertonic saline infusion and blood pressure and pulse rate during exercise. RESULTS Plasma vasopressin increased during hypertonic saline infusion and exercise, the responses being attenuated by 5.0 mg melatonin and augmented with 0.5 mg melatonin. Although exercise had no effect on oxytocin release, there was a small but statistically significant ( P < 0.05) increase in plasma oxytocin after hypertonic saline administration, which was inhibited by 5.0 mg and augmented by 0.5 mg melatonin. CONCLUSIONS Melatonin modulates the neurohypophysial response to different stimuli, low doses enhancing the response. This effect could contribute to the night-time increase in circulating concentrations of the hormones.