CHANGES OF MELATONIN LEVELS IN ZEBRAFISH WITH ALTERED PHOTOPERIOD

Introduction In zebrafish, the pineal gland is a photosensitive organ that plays an important endocrine and neuronal role consisting in the regulation of the circadian clock through the melatonin secretion. Melatonin is released following a rhythmic pattern characterized by low concentrations during the day and significantly higher levels at night. This trend reflects the activity of AANAT2, protein which plays a key role in the melatonin biosynthesis. The synthesis of melatonin start with the environmental light capture by the rhodopsin photopigment (Falcon et al., 2003; Laurà et al., 2011). In fish the pineal gland functions and melatonin secretion are involved in the control of the locomotors activity, sleep, skin pigmentation, feeding, growth and reproduction (Falcon et al., 2010). Therefore, in this study we analyzed the effects of melatonin in the biology of adult zebrafish pineal gland Material and Method We used adult zebrafish (n = 90) divided in 5 experimental groups. From which the first group (n=30) was maintained under natural photoperiod 14 h light and 10 h dark and used as control (ctrl), 2 groups were submitted to 7 and 30 days of continuous darkness (DD) and 2 groups to 7 and 30 days of continuous light (LL) respectively. The animals were anaesthetized and the blood was collected and immediately processed for melatonin High performance liquid chromatography (HPLC) assay. Moreover, the pineal glands were removed and analyzed for rhodopsin and AANAT2 mRNA expression. Results The Rhodopsin and AANAT2 showed a trend of mRNA expression with evident changes between the different animal groups analyzed. The results of the melatonin dosage on blood of zebrafish maintained under normal and altered photoperiod show changes during the Light/Dark cycle. In conclusion, we demonstrated that the key components of the biosynthetic pathway of melatonin follow a trend directly proportional between them, confirming the close relationship between the factors that compose the melatonin pathway.