Ralf C. Zimmermann

Melatonin and the ovulatory luteinizing hormone surge

A newly developed 125I-radioimmunoassay allows for the accurate determination of physiological concentrations of plasma melatonin. Melatonin secretion does not change significantly on the day before and the day of the luteinizing hormone (LH) surge when compared with the early follicular phase. In addition, it was confirmed that the beginning of the LH surge frequently occurs in the morning and is associated with low melatonin values (six of nine women). Supraphysiological melatonin concentrations did not decrease the midcycle LH secretion in four women studied.

Urinary 6-hydroxymelatonin sulfate as a measure of melatonin secretion during acute tryptophan depletion

The essential amino acid tryptophan (TRP) is the precursor for serotonin (5-HT), which is in turn an intermediary product in the synthesis of melatonin. Acute TRP depletion has recently been shown to decrease nocturnal plasma levels of melatonin in humans. Melatonin is metabolized to 6-hydroxymelatonin sulfate (6-SM), a highly stable end-product which is excreted into urine. To determine the effects of TRP bioavailability on 6-SM, 11 healthy volunteers underwent active and sham TRP depletion in a randomized, double-blind fashion. Samples of plasma free and total TRP, plasma melatonin, and urinary 6-SM were obtained before and after the depletion. Acute TRP depletion decreased free and total plasma tryptophan levels by more than 80% from baseline levels. Nocturnal 6-SM excretion was significantly decreased and highly correlated with decreases in plasma melatonin. These results suggest that nocturnal urinary 6-SM excretion is a valid measure of melatonin secretion under conditions of decreased 5-HT function. Collection of urine for 6-SM is considerably easier than nocturnal plasma sampling for melatonin. Further studies are needed to clarify the relationship between 6-SM excretion and other measures of 5-HT function in neuropsychiatric disorders.