SEASONAL-VARIATIONS IN THE DAILY RHYTHM OF MELATONIN AND NAT ACTIVITY IN THE HARDERIAN-GLAND, RETINA, PINEAL-GLAND, AND SERUM OF THE GREEN FROG, RANA-ESCULENTA

Day-night variations of melatonin content and N-acetyltransferase (NAT) activity were studied in the Harderian gland (HG), retina, pineal gland, and serum of the green frog Rana esculenta. Throughout the year the retinal melatonin content was correlated with retinal NAT activity and was always higher than those in the pineal gland and HG. On the other hand, in these structures diurnal fluctuations in NAT activity were observed. There were clear seasonal differences in the magnitude of the nocturnal increase of retinal melatonin levels as well as in the nocturnal pattern of retinal NAT activity. In summer day-night variations of melatonin and NAT are absent. The prevailing photoperiod seems to affect melatonin and NAT circadian rhythms in R. esculenta. (C) 1994 Academic Press, Inc.