Light exposure reduces and pinealectomy virtually stops urinary excretion of 6-hydroxymelatonin by rhesus monkeys

The major metabolite of the pineal hormone melatonin, conjugated 6-hydroxymelatonin, was hydrolyzed, separated from the urine of rhesus monkeys, and assayed mass spectrometrically. The daily excretion pattern reflected pineal melatonin synthetic activity, being 5- to 16-fold higher at night than during the day. Progressive lengthening of the daily photoperiod beyond 12 h decreased the daily excretion of 6-hydroxymelatonin proportional to the increase duration of light exposure. Constant light reduced daily excretion by 90%, and pinealectomy reduced daily excretion by 96%. These results demonstrate the absence of significant extrapineal contributions to the urinary melatonin metabolite and confirm the use of 6-hydroxymelatonin excretion rates as a valid index of pineal gland melatonin synthesis.