Prolactin177, prolactin188 and prolactin receptor 2 in the pituitary of the euryhaline tilapia, Oreochromis mossambicus, are differentially osmosensitive

Abstract

Two forms of prolactin (Prl), prolactin 177 (Prl177) and prolactin 188 (Prl188), are produced in therostral pars distalis(RPD) of the pituitary gland of euryhaline Mozambique tilapia,Oreochromis mossambicus. Consistent with their roles in fresh water (FW) osmoregulation, release of both Prls is rapidly stimulated by hyposmotic stimuli, bothin vivoandin vitro. We examined the concurrent dynamics of Prl177and Prl188hormone release and mRNA expression from Prl cells in response to changes in environmental salinityin vivoand to changes in extracellular osmolalityin vitro. In addition, mRNA levels of Prl receptors 1 and 2 (prlr1andprlr2) and osmotic stress transcription factor 1 (ostf1) were measured. Following transfer from seawater (SW) to FW, plasma osmolality decreased, while plasma levels of Prl177and Prl188and RPD mRNA levels ofprl177andprl188increased. The opposite pattern was observed when fish were transferred from FW to SW. Moreover, hyposmotically induced release of Prl188was greater in Prl cells isolated from FW-acclimated fish after 6 h of incubation, while the hyposmotically induced increase inprl188mRNA levels was only observed in SW-acclimated fish. In addition,prlr2andostf1mRNA levels in Prl cells from both FW- and SW-acclimated fish increased in direct proportion to increases in extracellular osmolality bothin vivoandin vitro. Taken together, these results indicate that the osmosensitivity of the tilapia RPD is modulated by environmental salinity with respect to hormone release and gene expression.