EFFECT OF PHOTOPERIOD ON THE DIURNAL MELATONIN AND 5-METHOXYTRYPTOPHOL RHYTHMS IN THE HUMAN PINEAL-GLAND

The mammalian pineal organ is a major endocrine component of the circa dian timing system, primarily concerned with the regulation of photope riodic responses. The effect of different photoperiods on the diurnal variation in the concentration of melatonin and 5-methoxytryptophol (5 -ML) was studied in the pineal glands, obtained at autopsy, of 38 huma n subjects, and measured by radioimmunoassay. A diurnal rhythm in pine al melatonin was evident only in the long photoperiod (April-September ) with melatonin concentrations being 4.2 times higher at night (22.00 -10.00 h) than during the day (10.00-22.00 h). In contrast, diurnal va riations in the pineal 5-ML content were only observed in the short ph otoperiod (October-March) with high concentrations during day-time and low concentrations during night-time. In general, night-time concentr ations of both melatonin and 5-ML were higher in summer than in winter . These data suggest that the synthesis of indoleamines in the human p ineal exhibits a diurnal rhythm which is affected by seasonal changes in day length.