Is the primitive regulation of pituitary prolactin (tPRL177 and tPRL188) secretion and gene expression in the euryhaline tilapia (Oreochromis mossambicus) hypothalamic or environmental?

Abstract

We examined the effects of environmental salinity on circulating levels of the two prolactins (tPRL 177 and tPRL 188 ) and levels of pituitary tPRL 177 and tPRL 188 mRNA in the euryhaline tilapia, Oreochromis mossambicus. Fish were sham-operated or hypophysectomized and the rostral pars distalis (RPD) autotransplanted onto the optic nerve. Following post-operative recovery in ¼ seawater, tilapia were transferred to fresh water (FW), ¼ seawater (SW) or SW. Serum tPRL 177 and tPRL 188 levels in sham-operated and RPD-autotransplanted fish were highest in FW and decreased as salinity was increased. tPRL 177 and tPRL 188 mRNA levels in RPD implants as well as in pituitaries from the sham-operated fish were also highest in FW and decreased with increasing salinity. Serum osmolality increased with salinity, with the highest levels occurring in the seawater groups. We conclude that some plasma factor (probably plasma osmolality), in the absence of hypothalamic innervation, exerts a direct regulatory action on prolactin release and gene expression in the pituitary of O. mossambicus. This regulation is in accord with the actions of the two prolactins in the freshwater osmoregulation of the tilapia.

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

“…Likewise, mRNA levels for both prl genes were higher in pituitaries of tilapia acclimated to FW than in pituitaries of fish acclimated to 1 ⁄ 4 SW or SW (Shepherd et al 1999). More recently, we showed with dispersed Prl cells that Prl 188 release in response to in vitro reductions in osmolality is more sensitive in Prl cells of FW fish than in Prl cells of fish acclimated to SW (Seale et al 2006a).…”

“…This issue has been recently revisited by Fiol et al (2009) Early studies have shown greater RPD and Prl cell size (Dharmamba & Nishioka 1968), greater Prl cell activity (Nagahama et al 1975), and greater hypophyseal Prl content (Nicoll et al 1981) in tilapia maintained in FW compared to tilapia maintained in SW. Later, Yoshikawa-Ebesu et al (1995) reported that hyposmotically induced Prl 177 and Prl 188 synthesis is more robust in RPDs of fish acclimated to FW than in RPDs of fish acclimated to SW. Likewise, mRNA levels for both prl genes were higher in pituitaries of tilapia acclimated to FW than in pituitaries of fish acclimated to 1 ⁄ 4 SW or SW (Shepherd et al 1999). More recently, we showed with dispersed Prl cells that Prl 188 release in response to in vitro reductions in osmolality is more sensitive in Prl cells of FW fish than in Prl cells of fish acclimated to SW (Seale et al 2006a).…”

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

“…Prolactin acts by reducing water permeability and increasing ion uptake, at least in part by upregulating NCC mRNA expression in the gills (Breves et al, 2010b). Pituitary PRL release is regulated directly by decreases in extracellular osmolality that accompany FW acclimation (Nagahama et al, 1975;Wigham et al, 1977;Grau et al, 1981;Helms et al, 1991;Borski et al, 1992;Shepherd et al, 1999;Seale et al, 2002Seale et al, , 2006bSeale et al, , 2012a. Likewise, circulating plasma PRL levels in FW tilapia are higher than in SW tilapia (Yada et al, 1994;Seale et al, 2002Seale et al, , 2006b).…”

“…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.…”

The role of prolactin in fish reproduction

“…Circulating PRL levels are in most cases correlated with the amount of PRL mRNA present in the pituitary [99] The fact that each form of PRL is encoded by an independent gene makes it a particularly suitable candidate for this method. In addition, microarray technologies provide a tool for system-wide investigations of gene expression and have been used extensively to investigate various aspects of fish biology [reviewed in 28, eg.…”