Effect of Photoperiod and of One Minute Light at Night-time on the Pineal Rhythm on N-Acetyltransferase Activity in the Djungarian Hamster Phodopus sungorus

Abstract

The proposition that the temporal pattern of pineal melatonin formation and release may serve as a transducer of photic into humoral stimuli was tested. Adult male Djungarian hamsters were maintained for 3–4 weeks under 16L:8D, 8L:16D, and 8L:16D with additional 1 min light in the middle of the dark period. Pineal N-acetyltransferase activity was estimated at different daytimes as an indicator of melatonin synthesis. The pattern of the rhythms in N-acetyltransferase was similar under 16L:8D and 8L:16D + 1 min light and dissimilar to the pattern under 8L:16D. The high nocturnal N-acetyltransferase activity lasted for ∿5 h under 16L:8D, 6 h under 8L:16D + 1 min light, and 12 h under 8L:16D. The demonstration that the regimens 16L:8D and 8L:16D + 1 min light lead to similar rhythms in N-acetyltransferase and hence in melatonin formation, together with our previous finding that the same regimen (8L:16D + 1 min light in the middle of the dark period) mimics the effect of long photoperiods on gonadal development and body weight, further supports the hypothesis that the changing rhythmic pattern of melatonin formation may be involved in conveying the photoperiodic effects.