5-Methoxypsoralen increases the plasma melatonin levels in humans.

The aim of this study was to evaluate the acute effects of 5-methoxypsoralen (5-MOP) on the melatonin secretion in humans. Eleven normal volunteers were investigated before (drug-free) and after 6-day periods of treatment with oral 5-MOP, first administered daily at 9 A.M. and after a 1-week free interval administered daily at 9 P.M. Under nyctohemeral conditions, the plasma melatonin levels were evaluated over a 24-h period in each session by hourly blood samples and radioimmunoassay. The sensitivity of the retina to light was also evaluated by means of electroretinography performed at 11 A.M. before and after a morning administration of 5-MOP. Plasma levels of melatonin were significantly increased from the second hour after 5-MOP administration. The hourly mean levels were significantly higher after 5-MOP administration compared to baseline values. This increased secretion was more pronounced after evening than after morning administration. Also, 5-MOP increases the sensitivity of the retina to light under photopic conditions and in the early stages of the dark adaptation period, as observed under scotopic conditions.