Entrainment of the rat pineal rhythm in melatonin production by light.

Environmental light entrains the rat pineal N-acetyltransferase rhythm which controls melatonin production. One day after 1 min light pulses applied before midnight, or after delays in the evening switch off of light, or after a delay of the light-dark cycle, the evening N-acetyltransferase rise and the morning decline are phase delayed almost to the same extent. Consequently, the pattern of the rhythm does not change. One day after 1 min light pulses applied past midnight, or after bringing forward the morning light onset, or after an advance of the light dark-cycle, the morning N-acetyltransferase decline is phase advanced, but the evening rise is either not phase shifted or it may be even phase delayed. Consequently, the pattern of the rhythm may be changed considerably or the rhythm may be abolished. The data are consistent with an hypothesis of a two-component pacemaker controlling the N-acetyltransferase rhythm. Under all photoperiods which we encounter in nature, the pattern of the N-acetyltransferase rhythm is determined by the entraining effect of light on the pacemaker, but not by the suppressant effect of light.