Pulsed expression of growth hormone mRNA in the pituitary of juvenile Pacific bluefin tuna under aquacultured conditions

The daily expression profile of growth hormone (GH) mRNA in juvenile Pacific bluefin tuna (Thunnus orientalis) was investigated under aquacultured conditions. Total RNA from pituitaries (n = 5) was sampled from 10 am to 12 pm the next day at 1 h intervals. The expression levels of GH mRNA were evaluated using real-time PCR normalized against the β-actin gene. The expression level of GH transcripts reached a peak at 3–4 am at which time it was about 10 times higher than at any other time period. Considering their high growth rate compared to red seabream (Pagrus major) and other aquacultured species which shows continuous GH mRNA expression patterns, it can be concluded that the pulsed expression of GH mRNA just before daybreak is a key for the extremely high somatic growth rate of Pacific bluefin tuna.