Two forms of prolactin (Prl), prolactin 177 (Prl 177 ) and prolactin 188 (Prl 188 ), are produced in the rostral 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, both in vivo and in vitro. We examined the concurrent dynamics of Prl 177 and Prl 188 hormone release and mRNA expression from Prl cells in response to changes in environmental salinity in vivo and to changes in extracellular osmolality in vitro. In addition, mRNA levels of Prl receptors 1 and 2 (prlr1 and prlr2) and osmotic stress transcription factor 1 (ostf1) were measured. Following transfer from seawater (SW) to FW, plasma osmolality decreased, while plasma levels of Prl 177 and Prl 188 and RPD mRNA levels of prl 177 and prl 188 increased. The opposite pattern was observed when fish were transferred from FW to SW. Moreover, hyposmotically induced release of Prl 188 was greater in Prl cells isolated from FW-acclimated fish after 6 h of incubation, while the hyposmotically induced increase in prl 188 mRNA levels was only observed in SW-acclimated fish. In addition, prlr2 and ostf1 mRNA levels in Prl cells from both FW-and SW-acclimated fish increased in direct proportion to increases in extracellular osmolality both in vivo and in 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. Journal of Endocrinology (2012) 213, 89-98Introduction A necessary component of the systems that direct osmoregulatory processes are osmoreceptors that detect deviations from salt and water balance and initiate adaptive responses to restore equilibrium. The prolactin (Prl)-secreting cells of the euryhaline Mozambique tilapia (Oreochromis mossambicus) comprise a nearly homogeneous portion of the rostral pars distalis (RPD) of the pituitary (conservatively O95%) and are directly stimulated by physiologically relevant decreases in extracellular osmolality in vitro , Seale et al. 2006b. Consistent with its role as an essential hormone for fresh water (FW) acclimation (reviewed by Hirano (1986), Manzon (2002) and Sakamoto & McCormick (2006)) plasma Prl levels increase in vivo when tilapia are transferred from seawater (SW) to FW (Yada et al. 1994, Seale et al. 2002. The operation of tilapia Prl cells as true osmoreceptors and the anatomical organization of these cells in tilapia RPD provide an accessible model to integrate osmosensing mechanisms with gene transcription, hormone synthesis and hormone secretion (Seale et al. 2005).Hyposmotically induced Prl release from tilapia lactotrophs is dependent on the entry of extracellular Ca 2C through stretch-activated ion channels upon cell swelling (Seale et al. 2003). There is evidence that the influx of extracellular Ca 2C may have distinct intracellular effects depending on the acclimation salinity of the animal fro...

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