The melatonin hormone is produced and secreted by the pineal gland during the dark phase of the photoperiod. It hence provides information such as time of the day and season for cells and organs. As in mammals, melatonin in fish is known to act on important physiological functions, including thermoregulation, reproduction and development. However, while well described in mammals, few studies have investigated its potential role on immune functions in teleost. In pikeperch, we defined in previous experiments a potential dual action of cortisol and melatonin hormones on immune defenses. In addition, we characterized daily cyclic activities of humoral innate immune markers that are correlated with the cyclic release of melatonin by the pineal gland.

Nocturnal peak of melatonin production and release is directly proportional to the length of the night and hence provides a direct transduction of night length. In order to deepen our knowledge on the immune modulation by the melatonin hormone, we hypothesized that changing photoperiod influences the fish immune functions through the modulation of melatonin synthesis. The study thus investigated the effects of two natural photoperiod regimes simulating the fall and the spring in western Europe on melatonin secretion, stress and immune markers.

Daily cyclic activities were observed for plasma melatonin and cortisol, but also for several innate immune markers, including lysozyme, peroxidase and complement activities in plasma and phagocytic activity in spleen. Nocturnal plasma melatonin values were influenced by the seasonal simulated photoperiods with progressive increase or decrease for the photoperiods simulating the fall and the spring respectively. No photoperiod effect was detected on cortisol release. Moreover, the exposure to the fall-simulated photoperiod induced several effects on immune markers, including increases in lysozyme, peroxidase and complement activities. Analyses of immune-relevant gene expression are ongoing. Our results bring an additional evidence supporting the potential immunomodulatory action of the melatonin hormone in teleosts with a stimulation of the innate immunity following the increase in melatonin production in response to the fall-simulated photoperiod