Melatonin modulates neurohypophysial hormone release in response to elevated plasma osmolality in both man and rat (Forsling, 2000). A study has been performed on the effect of melatonin on pituitary hormone release in man in response to an effective reduction in central venous volume induced by lower body negative pressure (LBNP). A randomised double-blind study was performed on eight male subjects with approval of the local ethics committee. The subjects, aged 20-21 years, gave informed consent and refrained from taking heavy exercise, alcohol and from smoking prior to the study. Each subject participated on two occasions, receiving either placebo or 5.0 mg melatonin. After lying supine for 30 min, they were subjected to LBNP of -50 mmHg for 30 min followed by a recovery period of 30 min. Blood pressure and heart rate were monitored throughout and blood samples taken at intervals for determination of packed cell volume (PCV) and plasma osmolality, vasopressin, oxytocin and cortisol.

After 30 min LBNP mean heart rate increased from 68 ± 3.4 to 89 ± 5.4 beats min^-1 (S.E.M., ANOVA, P < 0.01) following placebo treatment, with a similar increase following melatonin. Blood pressure, which was initially 97 ± 5.6 and 95 ± 7.7 mmHg, respectively, in the placebo and melatonin-treated groups, did not change significantly. Plasma osmolality also remained constant throughout, while PCV showed a transient increase on LBNP (P < 0.05). Vasopressin concentrations were increased from 1.2 ± 0.43 pmol l^-1 to a peak of 2.2 ± 0.38 pmol l^-1 (P < 0.05) at the end of 30 min of LBNP. Following melatonin treatment vasopressin concentrations showed a greater increase to 6.2 ± 12 pmol l^-1 (P < 0.02). Oxytocin increased from 3.7 ± 0.4 to 6.3 ± 0.4 pmol l^-1 at the start of LBNP. A peak of 11.0 ± 1.0 pmol l^-1 was seen during LNBP following melatonin. The area under the curve for cortisol release was also significantly greater after melatonin ingestion. Thus the pituitary response to LBNP is enhanced by melatonin in doses similar to those recommended for the prevention of jet lag.