Opposite effects of photoperiod and temperature on circadian rhythms of pineal arylalkylamine N-acetyltransferas (AANAT) activity and circulating levels of thyroid hormones in airbreathing catfish, Clarias gariepinus.

Effects of different photoperiods (9L-15D, 12L-12D and 15L-9D at a constant temperature 25 ± 1°C) and simulated temperatures (15°C, 25°C and 35°C under a common photoperiod 12L-12D) were investigated on the circadian rhythms of plasma levels of thyroid hormones [thyroxine (T4) and triiodothyronine (T3)] and pineal arylalkylamine N-acetyltransferase (AANAT) activity in the fish, Clarias gariepinus during winter and summer seasons. Irrespective of the seasons, plasma levels of thyroid hormones (THs) and pineal AANAT activity exhibited circadian rhythm under all the photoperiodic regimes and simulated temperatures. The levels of THs were found to be directly related to the length of photoperiod as well as temperature. In contrast, pineal AANAT activity was inversely related to the length of photoperiod and temperature. On the basis of these findings, we suggest that photoperiod and temperature jointly regulate the inverse temporal relationship between the circadian rhythms of plasma levels of THs and pineal AANAT activity in C. gariepinus. [ABSTRACT FROM AUTHOR]