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 circadian timing system, primarily concerned with the regulation of photoperiodic 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 human subjects, and measured by radioimmunoassay. A diurnal rhythm in pineal 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 variations in the pineal 5-ML content were only observed in the short photoperiod (October-March) with high concentrations during day-time and low concentrations during night-time. In general, night-time concentrations of both melatonin and 5-ML were higher in summer than in winter. These data suggest that the synthesis of indoleamines in the human pineal exhibits a diurnal rhythm which is affected by seasonal changes in day length.