Regulation of the circadian rhythm in pineal melatonin production.

The pineal gland exhibits a circadian rhythm in the activity of N-acetyltransferase, the enzyme responsible for the rhythmic production of melatonin. Our data are consistent with the hypothesis that the N-acetyltransferase rhythm is regulated by a two-oscillator pacemaker. The evening N-acetyltransferase rise is controlled by an evening oscillator coupled to dusk, the morning decline by a morning oscillator coupled to dawn. The phase-relationship between both oscillators may determine the period of high night N-acetyltransferase activity and hence the period of elevated melatonin level. This period is different during long and short days and it may provide animals with information about environmental day duration.