ere stained with Oil Red O to visualize lipid droplet accumulation. Cells were fixed overnight in 4% neutral buffered formalin, then washed with 60% isopropanol and air-dried. A fresh 60% Oil Red O working solution was prepared from a stock solution and filtered through a 45 mM syringe filter. Cells were stained with the working solution for 45 min, washed five times with distilled water, and imaged at room temperature with an inverted microscope equipped with Zeiss A-Plan 106 and LD A-Plan 326 objectives. Images captured by a Sony Exwave HAD CCD camera were acquired using ImageJ software. Photoshop software was used to adjust levels and color balance. To measure the amount of Oil Red O in the stained samples, Oil Red O was eluted from the cells by adding 100% isopropanol and incubating for 1 hr, and then transferred to a 96well plate and measured spectrophotometrically at 500 nm. Statistics Statistically significant differences between groups were determined by performing a two-tailed Student’s t-test. Differences were considered significant if p,0.05, unless otherwise noted. Acknowledgments We thank Monica Mugnier for help with early depolarization studies, Barry A. Trimmer for help with intracellular recordings, and Dany S. Adams for MedChemExpress 1260907-17-2 advice with use of fluorescent reporter dyes. Skeletal muscle is an important tissue for whole body metabolic homeostasis and for locomotion. In fish, skeletal muscle may represent approximately half of their body mass and provides the engine for swimming, an intrinsic and 20171952 characteristic behaviour of this group of vertebrates. From a functional point of view, two types of skeletal muscle can be identified in fish: white skeletal muscle, which is anaerobic and fuels burst swimming, and red skeletal muscle, which is aerobic and fuels sustained swimming. For many fish species, their life history is intimately linked to their ability to perform under swimming-induced exercise conditions that, in turn, is dependent on the functionality of skeletal muscle. Among migrant fish species, the most extreme exercise conditions are experienced during the anorexic reproductive migration, as performed by salmonid species. Fish that migrate long distances to reach their spawning grounds like salmonids face two major challenges before they can successfully reproduce: to swim and to sexually mature. Recently, we applied exercise experimentally to investigate its effects on sexual maturation in female rainbow trout. The main conclusion of that study was that swimming suppresses ovarian development at the start of vitellogenesis. Swimming requires streamlining of the body and muscle building for optimal performance. However, the progression of oocyte growth may cause a change in body shape that, in turn, could increase drag resistance, and may also lead to muscle atrophy, leading to decreased swimming efficiency. Therefore, long distance migrants need to up-regulate the energetic processes in the muscle that provide fuel for contraction and for muscle growth, and to 
suppress vitellogenesis: the migration phenotype. When there is a need to start vitellogenesis, the situation in the muscle and the ovary is reversed: the sexual maturation phenotype. Despite 15771452 the important role of skeletal muscle for swimming in fish, relatively little is known regarding the molecular events that take place in red and white skeletal muscle in response to swimming-induced activity. In this study, we have used deep RNA Deep RNA Sequencing of Trout
