The seasonal light periodicity is a major regulator of numerous important biological processes and life history transition stages in Atlantic salmon such as growth, smoltification and reproduction . The light period is reflected internally by the time keeping hormone melatonin which signals “darkness” with high blood plasma levels during the night and low blood plasma levels during the day . Artificial light conditions are often used in the Atlantic salmon aquaculture to promote growth. However, very little is known about the effects of such light regimes, like for example keeping the fish in constant light conditions . In teleosts , the pineal organ is directly photoreceptive and responsible for both light detection and melatonin synthesis. This organ is located dorsally in the brain where the skin is less pigmented and the skull is quite transparent, providing a window for light to enter. The photoreceptive cells of the pineal organ express o psins that are G-protein coupled receptor proteins capable of receiving light information which in turn translates to melatonin blood plasma levels by regulation of melatonin synthesis of the cell. However, which photoreceptive input is responsible for the circadian melatonin rhythmicity is still unknown.
In this study, we have mapped several opsins and melatonin synthesizing factors of the pineal organ by using RNA sequencing, in situ hybridization and immunohistochemistry both in first feeding larva and at the parr stage of Atlantic salmon . We are also in the process of analyzing CRISPR/Cas9 opsin mutants .
The results revealed that seven opsins are expressed in the pineal organ, including exorhodopsin , parapinopsin , peropsin and teleost multiple tissue opsin. Many of the opsins are expressed in cells towards the lumen of the pineal organ, but we also find one opsin located in the dorsal sac (Figure 1). Many of the opsins are found both in the first feeding larvae and at the parr stage. We also find important factors of the melatonin synthesis in the photoreceptive cells of the pineal such as serotonin, aryl-alkylamine N-acetyltransferase 2 (aanat2) and acetylserotonin O-methyltransferase (asmt).
The location of the studied opsins and the melatonin synthesizing factors suggest a link between specific opsin activation and melatonin synthesis. Importantly, an in-dept an alysis of the Atlantic salmon pineal organ provides the basis for understanding the impact of light environment on life history transitions such as smoltification and hold the potential to increase animal welfare and yield of the aquaculture industry.