SEASONAL-VARIATIONS IN THE DAILY RHYTHM OF MELATONIN AND NAT ACTIVITYIN 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 gl and, and serum of the green frog Rana esculenta. Throughout the year t he retinal melatonin content was correlated with retinal NAT activity and was always higher than those in the pineal gland and HG. On the ot her hand, in these structures diurnal fluctuations in NAT activity wer e observed. There were clear seasonal differences in the magnitude of the nocturnal increase of retinal melatonin levels as well as in the n octurnal pattern of retinal NAT activity. In summer day-night variatio ns of melatonin and NAT are absent. The prevailing photoperiod seems t o affect melatonin and NAT circadian rhythms in R. esculenta. (C) 1994 Academic Press, Inc.