Dynamic responses of prolactin, growth hormone and their receptors to hyposmotic acclimation in the olive flounder Paralichthys olivaceus.

Prolactin (PRL) and growth hormone (GH) play important roles in regulating salt and water balance through osmoregulatory organs in vertebrates. The aim of this study was to investigate the dynamic changes of GH/PRL hormone gene expressions in the pituitary gland and their receptors in gill and kidney, as well as the plasma osmolality when the olive flounder fish Paralichthys olivaceus were acclimated in freshwater (FW) conditions. After transfer from seawater (SW) to freshwater (FW), the osmolality of FW-adaption fish reached the lowest level at 1d which rose slightly afterwards. However, the hormone gene expression of PRL increased from 2d, reaching its peak at 5d, and then decreased at 14d. At this time, the value was still significantly higher than the control, showing a similar trend to the plasma hormone PRL. In contrast, the pituitary mRNA level of GH significantly decreased at 1d and then returned to normal levels. The mRNA levels of PRL receptor (PRLR) in both gill and kidney displayed a similar trend to the pituitary PRL. We also observed the synchronous expression trend of the renal PRLR with pituitary PRL (5d) and the asynchronous expression peaks between branchial (8d) and renal PRLR (5d). Significant responses of GH and its receptor (GHR) in both gill and kidney during the FW-acclimation were not observed. Nevertheless, the gene expression of GH receptor variant (GHR-V) in both gill and kidney declined at 2d, indicating unknown osmoregulatory functions of GHR-V. Collectively, our results provided more insights of the PRL, GH and their corresponding receptors in modulating osmoregulatory responses, representing an important aspect of FW-acclimation in flounder fish.