Effect of melatonin on L-tryptophan- and apomorphine-stimulated growth hormone secretion in man.

The effect of subacute melatonin (250 mg every 8 h over a period of 40 h) and placebo treatment on L-tryptophan- and apomorphine-stimulated GH secretion was tested in 13 volunteers. Melatonin premedication significantly reduced the GH response to peroral L-tryptophan loading, suggesting that melatonin affects GH secretion in man by interfering with serotonergic transmission. In the apomorphine test, no significant difference in GH response was observed after melatonin when compared to placebo treatment. This suggests that dopaminergic mechanisms do not have a central role in mediating actions of melatonin on the hypothalamic-pituitary axis. It is further proposed that melatonin influences GH secretion at a suprahypophyseal level.