Acute administration of melatonin at two opposite circadian stages does not change responses to gonadotropin releasing hormone, thyrotropin releasing hormone and ACTH in healthy adult males.

We evaluated the effect of a single oral administration of 100 mg melatonin (MT) vs placebo (PL) on the pituitary release of LH, FSH, TSH and prolactin (PRL) after GnRH + TRH and on the adrenocortical release of cortisol, aldosterone and progesterone after ACTH in healthy adult males. We carried out a double blind study in 6 volunteers in winter, at two opposite stages of the circadian cycle: 08:00 and 20:00 h. Injection of GnRH (100 micrograms), TRH (200 micrograms) and ACTH (10 micrograms of the synthetic ACTH 1-17 analogue, Alsactide) was performed one h after MT or PL ingestion. Plasma MT levels were 200-4,000-fold higher after MT than PL thus confirming the effective gastrointestinal absorption of the pineal hormone. The hormonal patterns were superimposable after MT and PL. A higher response of PRL, FSH and cortisol was observed in the evening vs morning protocols independently of previous MT or PL. Our data demonstrate that the acute oral administration of a pharmacological dose of MT at two opposite circadian stages is ineffective to change a variety of pituitary and adrenocortical responses in human male subjects. The circadian chronosusceptibility of pituitary and adrenocortical cells to specific stimuli deserves interest to future investigation.