Changes in the levels of mRNAs for GH/prolactin/somatolactin family and Pit-1/GHF-1 in the pituitaries of pre-spawning chum salmon

Changes in the levels of pituitary mRNAs encoding GH, prolactin (PRL) and s omatolactin (SL) were determined in pre-spawning chum salmon (Oncorhynchus Keta) caught at a few key points along their homing pathway in 1994 and 199 5. Furthermore, we analyzed relationships between expression of pituitary-s pecific POU homeodomain transcription factor (Pit-1/GHF-1) and GH/PRL/SL fa mily genes. In 1994, seawater (SW) fish and matured fresh-water (FW) fish w ere sequentially captured at two points along their homing pathway, the coa st and the hatchery. In addition to these two points, maturing FW fish were captured at the intermediate of the two points in 1995. The levels of horm onal mRNAs were determined by a quantitative dot blot analysis using single -stranded sense DNA as the standard. Relative levels of Pit-1/GHF-1 mRNAs w ere estimated by Northern blot analysis. In 1994, the levels of GH/PRL/SL f amily mRNAs except for PRL mRNA in the male FW fish were 1.8-4 times higher than those in the SW fish. In 1995, the level of PRL mRNA was somewhat sha rply elevated in the maturing FW fish soon after entry into the FW environm ent, while that of SL mRNA was gradually increased during upstream migratio n from the coast to the hatchery. The levels of 3 kb Pit-1/GHF-1 mRNA in th e FW fish were higher than those in the SW fish in both 1994 and 1995. The present results indicate that expression of genes for the GH/PRL/SL family and Pit-1/GHF-1 is coincidentally enhanced in homing chum salmon. Moreover, the present study suggests that expression of the SL gene is elevated with sexual maturation, whereas that of PRL gene is elevated with osmotic chang e during the final stages of spawning migration.