Abstract
Melatonin is considered as the time-keeping hormone acting on important physiological functions of teleosts. While the influence of melatonin on reproduction and development is well described, its potential role on immune functions has little been considered. In order to better define an immune modulation by the melatonin hormone, we hypothesized that natural variations of photoperiod and subsequent changes in melatonin release profile may act on immune status of pikeperch. Therefore, we investigated during 70 days the effects of two photoperiod regimes simulating the fall and spring in western Europe, on pikeperch physiological and immune responses. Samples were collected at 04:00 and 15:00 at days 1, 37 and 70. Growth, plasma melatonin levels, innate immune markers and expression of immune-relevant genes in head kidney tissue were assessed. While growth and stress level were not affected by the seasonal simulated photoperiods, nocturnal levels of plasma melatonin were photoperiod-dependent. Innate immune markers, including lysozyme, complement, peroxidase and phagocytic activities, were stimulated by the fall-simulated photoperiod and a significant correlation was made with plasma melatonin. In addition to bring the first evidence of changes in fish immunocompetence related to photoperiod, our results provide an additional indication supporting the immunomodulatory action of melatonin in teleosts.
Highlights
As photoperiod transducer, the melatonin hormone is mainly produced and secreted by the pineal gland during the night[1,2,3,4,5]
In order to better define in fish the effects of photoperiodic changes on the immunocompetence and the potential key role of the melatonin hormone in this regulation, this study investigated in pike perch the effects of two photoperiod regimes simulating the fall and the spring in western Europe
At D70, while the tested photoperiods did not influence the gonadosomatic index for females (0.30 ± 0.04%), a significant difference for males was detected with increased gonadal development with the fall-simulated photoperiod (FSP) (0.41 ± 0.17%) compared to the spring-simulated photoperiod (SSP) (0.22 ± 0.09%)
Summary
The melatonin hormone is mainly produced and secreted by the pineal gland during the night[1,2,3,4,5]. A potential dual action of cortisol and melatonin hormones on immune defenses was described[23,24]. These experiments defined a correlation between daily cyclic activities of humoral innate immune markers and the nocturnal peak of plasma melatonin. Considering the potential immunomodulatory action of the melatonin hormone and the annual rhythmicity of melatonin secretion by the pineal gland, it is feasible that changing photoperiod co-ordinates fish immunity through the modulation of melatonin secretion. The eyes of this species possess a tapetum lucidum that is a specific tissue of the retina which greatly amplify the eye sensitivity to light[24,39]. As the third light characteristic, photoperiod and its potential effects on fish immunity have still not been considered
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