Melatonin rhythms in European sea bass plasma and eye: influence of seasonal photoperiod and water temperature

The transduction of seasonal information from the environment (i.e., photoperiod and water temperature) into melatonin rhythms was studied in sea bass. Plasma and ocular melatonin (N-acetyl-5-methoxytryptamine) was determined in autumn, winter, spring and summer (experiment 1) under natural culture conditions, and in the summer and winter solstices under both natural and "6-month out-of-phase" photoperiods (experiment 2). At each sampling, 48 sea bass were sacrificed at a rate of 6 fish every 3 hr and the level of melatonin was determined in plasma and eye cup samples by ELISA. In experiment 1, significant diel changes were observed in plasma melatonin, with nocturnal melatonin varying from 144 pg/mL (summer) to 23 pg/mL (autumn), while diurnal melatonin remained low, around 8 pg/mL throughout the year. In experiment 2, the photoperiod length was shown to control the duration of the nocturnal melatonin rise, while the water temperature determined the amplitude of the melatonin rhythm. Ocular melatonin peaked during daytime in autumn and winter, but no significant changes were detected in summer and spring. In conclusion, plasma melatonin rhythms in sea bass reflect the pineal capacity to integrate seasonal information and supply precise calendar information, which may synchronize different physiological processes such as annual reproduction and feeding rhythms.