Changes in Transcript Levels of Gill Cortisol Receptor during Smoltification in Wild Masu Salmon, Oncorhynchus masou

Abstract

We developed a quantitative PCR assay to investigate transcript levels of gill cortisol receptor (CR) in masu salmon (Oncorhynchus masou). Using this system, we examined changes in transcript levels of gill CR during smoltification of wild masu salmon while tracking serum cortisol and growth hormone (GH) concentrations patterns. The masu salmon were parr in January and February, and thereafter smoltified, migrating from the river to the sea in May. Gill CR transcript levels were very low in January and February, but thereafter increased and reached a maximum in April (5 fold increase over levels observed in January). In May, when smolt enter the sea, the gill CR transcript levels decreased. Serum cortisol concentrations were also low from January to March and increased to the peak in April. These changes correlated well with changes in CR transcript levels from March to April. In contrast, serum GH concentration began to increase in January, peaked in March and decreased from March to May. These results elucidated patterns in transcript levels of gill CR during smoltification of wild masu salmon and suggested that gill CR transcription was positively regulated by cortisol until serum cortisol level reached a peak, and negatively done after the peak in masu salmon.