Effect of environmental temperature and photoperiod on the melatonin levels in the pineal, lateral eye, and plasma of the frog, Rana perezi: importance of ocular melatonin.

Day-night melatonin concentrations were studied in the pineal body, lateral eye, and plasma of the frog Rana perezi in animals maintained in February and July under long (18L:6D) or short (6L:18D) photoperiod and high (25 +/- 1 degree) or low (6 +/- 1 degree) temperature in order to evaluate the influence of these environmental factors. When frogs were kept under short photoperiod and low temperature in February, no melatonin rhythm was observed in the pineal, ocular tissue, and plasma. High temperature at this period of the year induced a day-night rhythm of melatonin levels in the lateral eye and plasma. In July, under long photoperiod and high temperature, animals showed pronounced rhythms of melatonin in the pineal, eye, and plasma. A decrease of environmental temperature in this season abolished the melatonin rhythm. When animals were maintained in August under high (25 +/- 1 degree) temperature and long (18L:6D) or short (6L:18D) photoperiod, the duration of high night time ocular melatonin levels was correlated to the length of the dark phase. In all experiments the high ocular melatonin concentrations and the close parallelism observed between ocular and circulating melatonin profiles suggest that in this species melatonin could be released from the eyes in the general circulation.