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 (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...

The effects of acute salinity challenges on osmoregulation in Mozambique tilapia reared in a tidally changing salinity

“…Similarly, the extent of the osmolality increase and survival rate is dependent on the rate at which the fish are transferred. Tilapia are able to survive when transferred to an intermediate salinity before being transferred to SW, but are unable to survive a direct transfer (Yada et al, 1994;Seale et al, 2002Seale et al, , 2012bBreves et al, 2010d). It has been shown through many studies that the survival threshold is between 450 and 550 mOsmolal (Yada et al, 1994;Seale et al, 2002Seale et al, , 2012bKajimura et al, 2004;Wang et al, 2009;Breves et al, 2010c,d).…”

“…Tilapia are able to survive when transferred to an intermediate salinity before being transferred to SW, but are unable to survive a direct transfer (Yada et al, 1994;Seale et al, 2002Seale et al, , 2012bBreves et al, 2010d). It has been shown through many studies that the survival threshold is between 450 and 550 mOsmolal (Yada et al, 1994;Seale et al, 2002Seale et al, , 2012bKajimura et al, 2004;Wang et al, 2009;Breves et al, 2010c,d). Tilapia reared in a tidally changing salinity and then exposed to SW for 7 days showed no mortality, suggesting that they were completely acclimated to SW. We have observed previously that the plasma osmolality of tilapia reared in a tidally changing salinity fluctuated between 320 and 345 mOsmolal in the FW and SW phases of the cycle, respectively (Moorman et al, 2014).…”

“…Consistent with its activity in FW, PRL release in Mozambique tilapia increases in response to physiologically relevant reductions in extracellular osmolality both in vivo and in vitro (Grau et al, 1981;Yada et al, 1994;Seale et al, 2002Seale et al, , 2006bSeale et al, , 2012c. Conversely, Mozambique tilapia transferred from FW to brackish water (BW; 22‰) or SW, showed a reduction in plasma levels of PRL within 6 h (Yada et al, 1994;Seale et al, 2012b). Prolactin is thought to act principally by stimulating ion uptake and reducing water intake across osmoregulatory epithelia, following binding to its receptors (PRLRs).…”

“…Prolactin is thought to act principally by stimulating ion uptake and reducing water intake across osmoregulatory epithelia, following binding to its receptors (PRLRs). The receptors PRLR1 and PRLR2 are disparately regulated by extracellular osmolality Fiol et al, 2009;Seale et al, 2012b). Branchial PRLR1 expression is induced when tilapia are transferred from SW to FW, in concert with the rise in PRL release from the pituitary.…”

“…Extensive work has provided an understanding of how tilapia in FW and SW adapt to acute osmotic challenges (Dharmamba et al, 1967(Dharmamba et al, , 1973Ayson et al, 1993;Yada et al, 1994;Heijden et al, 1997;Sakamoto et al, 1997;Shepherd et al, 1999;Seale et al, 2002Seale et al, , 2006bSeale et al, , 2012bInokuchi et al, 2008Inokuchi et al, , 2009Ouattara et al, 2009;Breves et al, 2010bBreves et al, ,c,d, 2011Velan et al, 2011). While the effects of a salinity change on PRL cells and branchial mediators of ion transport are well documented in tilapia subjected to one-way transfers between two salinities, less is known about fish subjected to cyclic variations in salinity.…”