The pituitary levels of mRNAs encoding gonadotropin (GTH) subunits (GTH α2 and IIβ), prolactin (PRL), and somatolactin (SL) increased in chum salmon during the last stages of spawning migration. In the present study, changes in pituitary levels of mRNAs encoding GTH α2, Iβ, and IIβ; growth hormone (GH); PRL; and SL were examined in homing chum salmon of Sanriku stock to clarify whether the changes are associated with final maturation or freshwater (FW) adaptation. In 1993, fish were caught at four areas: off the coast of Sanriku (off-coast), the mouth of Otsuchi Bay (ocean), inside of Otsuchi Bay (bay), and the Otsuchi River (river). In addition, effects of hypoosmotic stimulation by transition from seawater (SW) to FW were examined in 1994 and 1995. The amounts of mRNAs were determined by dot-blot analyses or real-time polymerase chain reactions. The levels of GTH α2 and IIβ mRNAs in the ocean, bay, and river fish were two to five times those in the off-coast fish, and the levels of SL mRNAs in the bay fish were two to four times those in the off-coast fish. The levels of GH and PRL mRNAs in the ocean and bay fish were significantly lower than those in the off-coast fish, and those in the river fish were three to five times those in the ocean and bay fish. In the SW-to-FW transition experiment in 1994, the levels of GTH α2, Iβ, and IIβ mRNAs transiently increased, whereas changes were insignificant in 1995. The levels of GH, PRL, and SL mRNAs increased in both SW and FW environments, and no apparent effects of SW-to-FW transition were observed. The present study suggests that in prespawning chum salmon, expression of genes encoding GTH α2, IIβ, and SL elevates with final maturation regardless of osmotic environment. Hypoosmotic stimulation by transition from the SW-to-FW environment is not critical to modulate expression of genes for PRL. PRL gene expression can be elevated in SW fish that were sexually almost matured.
