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.


METHODS
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.


METHODS
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.


METHODS
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.
Forsling
Mary L
ML
Neuroendocrine Laboratories, Guy's, King's and St Thomas' School of Medicine, Guy's Campus, London, UK. mary.forsling@kcl.ac.uk
Williams
A J
AJ
D000328:Adult; D019540:Area Under Curve; D018592:Cross-Over Studies; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D015444:Exercise; D006400:Hematocrit; D006801:Humans; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D009994:Osmolar Concentration; D010121:Oxytocin / Q000097:blood; D012462:Saline Solution, Hypertonic; D012964:Sodium / Q000097:blood; D013268:Stimulation, Chemical; D014667:Vasopressins / Q000097:blood*
D012462:Saline Solution, Hypertonic; D014667:Vasopressins; D010121:Oxytocin; D012964:Sodium; D008550:Melatonin