Changes in prolactin mRNA levels during downstream migration of the amphidromous teleost, ayu Plecoglossus altivelis

Changes in mRNA levels of prolactin (PRL) during seaward migration and after experimental transfer from fresh water (FW) to seawater (SW) were examined in larvae of the amphidromous fish, ayu Plecoglossus altivelis. In the field study, ayu larvae caught in the surf zone showed lower levels of PRL mRNA than those in the river, while growth hormone (GH) levels showed no significant change. Decrease in PRL gene transcription was also observed 24 h after direct transfer from FW to SW, whereas there was no significant influence of water temperature. On the other hand, there was no significant change in GH mRNA levels in relation to SW transfer or environmental temperature. In a raceway with a vertical salinity gradient, PRL mRNA levels of ayu larvae showed a significant reduction during spontaneous migration from FW to SW, which mimicked the results from the field observation and the transfer experiment, and then a gradual increase during the course of development. Whole body water and sodium contents of larvae in a salinity gradient were stable during migration to SW. Results in this study indicated the importance of regulation of PRL gene expression in the downstream migration and acclimation to SW during the early development of ayu.