EFFECT OF CONTINUOUS LIGHT ON DAILY LEVELS OF PLASMA MELATONIN AND CORTISOL AND EXPRESSION OF CLOCK GENES IN PINEAL GLAND, BRAIN, AND LIVER IN ATLANTIC SALMON POSTSMOLTS

Continuous light is a common practice in salmon farming, where it is used to enhance growth, induce smoltification, and regulate puberty. However, knowledge about how different tissues receive information about daylength is limited. The aim of the present study was to evaluate the daily expression of clock (Per1-like, Cry2, and Clock), the nuclear transcription factor (peroxisome proliferator-activated receptor, PPAR; CCAAT/enhancer binding protein, C/EBP), and the endoplasmic reticulum (ER) stress (protein disulfide isomerase associated 3, PDIA3) genes in the pineal gland, brain, and liver of Atlantic salmon postsmolts reared under 12-h light:12-h dark (LD) regimes or under continuous light (LL) for 6 wks following transfer to seawater. All measured clock mRNAs displayed daily variations in one or more organs under LD, as well as plasma levels of melatonin. Similar variations were noted in the liver c/ebp alpha, pineal c/ebp delta, and pdia3 mRNAs. Under LL, the clock and nuclear transcription factor mRNAs did not show any daily variation in the studied organs, with the exception of pineal pdia3. Furthermore, LL had the opposite effect on the levels of melatonin and cortisol, as observed by the increase in pineal Clock, Per2, ppar alpha, and c/ebp alpha and c/ebp delta mRNAs and decrease in liver Clock, Per2, and ppar alpha mRNAs compared to those under LD. The present findings show that the expression of clock genes is affected by the light across organs and that there is a relation between PPAR, C/EBP, and clock mRNAs; however, the functional role of the individual nuclear transcription factors related to this observation remains to be established in the pineal gland and liver. (Author correspondence: Tihu@nifes.no).