Endocrine regulation of prolactin cell function and modulation of osmoreception in the Mozambique tilapia.

Prolactin (PRL) cells of the Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors by virtue of their intrinsic osmosensitivity coupled with their ability to directly regulate hydromineral homeostasis through the actions of PRL. Layered upon this fundamental osmotic reflex is an array of endocrine control of PRL synthesis and secretion. Consistent with its role in fresh water (FW) osmoregulation, PRL release in tilapia increases as extracellular osmolality decreases. The hyposmotically-induced release of PRL can be enhanced or attenuated by a variety of hormones. Prolactin release has been shown to be stimulated by gonadotropin-releasing hormone (GnRH), 17-ß-estradiol (E2), testosterone (T), thyrotropin-releasing hormone (TRH), atrial natriuretic peptide (ANP), brain-natriuretic peptide (BNP), C-type natriuretic peptide (CNP), ventricular natriuretic peptide (VNP), PRL-releasing peptide (PrRP), angiotensin II (ANG II), leptin, insulin-like growth factors (IGFs), ghrelin, and inhibited by somatostatin (SS), urotensin-II (U-II), dopamine, cortisol, ouabain and vasoactive intestinal peptide (VIP). This review is aimed at providing an overview of the hypothalamic and extra-hypothalamic hormones that regulate PRL release in euryhaline Mozambique tilapia, particularly in the context on how they may modulate osmoreception, and mediate the multifunctional actions of PRL. Also considered are the signal transduction pathways through which these secretagogues regulate PRL cell function.

Osmoreception: perspectives on signal transduction and environmental modulation.

Osmoregulation is essential to life in vertebrates and osmoreception is a fundamental element in osmoregulation. Progress in characterizing the mechanisms that mediate osmoreception has been made possible by using a uniquely accessible cell model, the prolactin (PRL) cell of the euryhaline tilapia, Oreochromis mossambicus. In addition to a brief historical overview, we offer a summary of our recent progress on signal transduction and osmosensitivity in the tilapia PRL cell model. Prolactin is a central regulator of hydromineral balance in teleosts in freshwater (FW). Consistent with its essential role in FW osmoregulation, PRL release in tilapia is inversely related to extracellular osmolality, both in vivo and in vitro. Osmotically-driven changes in PRL cell volume control PRL release. A decrease in extracellular osmolality increases cell volume, leading to a rapid influx of Ca(2+) through stretch-activated channels followed by a sharp rise in PRL release. Our recent studies also suggest that cAMP is involved in the osmotic signal transduction, and that acclimation salinity can modulate PRL cell osmosensitivity. Prolactin cells from FW tilapia show a larger rise in PRL release after a reduction in medium osmolality than those from SW fish. Paradoxically, hyposmotically-induced increase in PRL mRNA was observed only in cells from SW fish. Our studies have revealed differences in the abundance of the water channel, aquaporin 3 (AQP3), and the stretch activated Ca(2+) channel, transient receptor potential vanilloid 4 (TRPV4) in PRL cells of FW and SW fish that may explain their differing osmosensitivity and osmoreceptive output in differing acclimation salinities.

Switching of Na+, K+-ATPase isoforms by salinity and prolactin in the gill of a cichlid fish.

We identified and investigated the changes in expression of two gill Na(+), K(+)-ATPase α-subunit isoforms (α-1a and α-1b) in relationship with salinity acclimation in a cichlid fish, Mozambique tilapia. Transfer of freshwater (FW)-acclimated fish to seawater (SW) resulted in a marked reduction in α-1a expression within 24 h and a significant increase in α-1b expression with maximum levels attained 7 days after the transfer. In contrast, transfer of SW-acclimated fish to FW induced a marked increase in α-1a expression within 2 days, while α-1b expression decreased significantly after 14 days. Hypophysectomy resulted in a virtual shutdown of α-1a mRNA expression in both FW- and SW-acclimated fish, whereas no significant effect was observed in α-1b expression. Replacement therapy by ovine prolactin (oPrl) fully restored α-1a expression in FW-acclimated fish, while cortisol had a modest, but significant, stimulatory effect on α-1a expression. In hypophysectomized fish in SW, replacement therapy with oPrl alone or in combination with cortisol resulted in a marked increase in α-1a mRNA to levels far exceeding those observed in sham-operated fish. Expression of α-1b mRNA was unaffected by hormone treatment either in FW-acclimated fish or in SW-acclimated fish. The mRNA expression of fxyd-11, a regulatory Na(+), K(+)-ATPase subunit, was transiently enhanced during both FW and SW acclimation. In hypophysectomized fish in FW, oPrl and cortisol stimulated fxyd-11 expression in a synergistic manner. The clear Prl dependence of gill α-1a expression may partially explain the importance of this hormone to hyperosmoregulation in this species.

Disparate release of prolactin and growth hormone from the tilapia pituitary in response to osmotic stimulation.

In most teleost fishes, prolactin (PRL) plays a key role in freshwater (FW) adaptation, whereas growth hormone (GH) is involved in seawater (SW) adaptation in salmonids and certain euryhaline species including the tilapia, Oreochromis mossambicus. Consistent with its osmoregulatory activity, PRL release increases in response to physiologically relevant reductions in extracellular osmolality. When dispersed PRL and GH cells from FW-acclimatized fish were incubated in media of varying osmolalities, PRL release increased significantly in response to a 12% reduction in medium osmolality during 1 and 4h of exposure. By contrast, cells from SW-acclimatized fish responded only to a 24% reduction in osmolality. Growth hormone release on the other hand increased whether medium osmolality was reduced or raised. Cell volume increased together with PRL release during the perifusion of dispersed PRL cells in direct proportion to the reduction in medium osmolality. Growth hormone release increased whether GH cell volume increased or decreased. In in vivo studies, circulating PRL levels increased as early as 1h after the transfer of fish from SW to FW, whereas GH levels remained unchanged during 24h of acclimatization. These results indicate that while PRL and GH cells are osmosensitive, the PRL cells respond to reductions in extracellular osmolality in a manner that is consistent with PRL's physiological role in the tilapia. While the rise in GH release following the reduction in osmolality is of uncertain physiological significance, the rise in GH release with the elevation of medium osmolality may be connected to its role in SW adaptation.

Cell volume increase and extracellular Ca2+ are needed for hyposmotically induced prolactin release in tilapia.

In the tilapia (Oreochromis mossambicus), as in many euryhaline teleost fish, prolactin (PRL) plays a central role in freshwater adaptation, acting on osmoregulatory surfaces to reduce ion and water permeability and increase solute retention. Consistent with these actions, PRL release is stimulated as extracellular osmolality is reduced both in vivo and in vitro. In the current experiments, a perfusion system utilizing dispersed PRL cells was developed for permitting the simultaneous measurement of cell volume and PRL release. Intracellular Ca(2+) was monitored using fura 2-loaded cells under the same conditions. When PRL cells were exposed to hyposmotic medium, an increase in PRL cell volume preceded the increase in PRL release. Cell volume increased in proportion to decreases of 15 and 30% in osmolality. However, regulatory volume decrease was clearly seen only after a 30% reduction. The hyposmotically induced PRL release was sharply reduced in Ca(2+)-deleted hyposmotic medium, although cell volume changes were identical to those observed in normal hyposmotic medium. In most cells, a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) during hyposmotic stimulation was dependent on the availability of extracellular Ca(2+), although small transient increases in [Ca(2+)](i) were sometimes observed upon introduction of Ca(2+)-deleted media of the same or reduced osmolality. These results indicate that an increase in cell size is a critical step in the transduction of an osmotic signal into PRL release and that the hyposmotically induced increase in PRL release is greatly dependent on extracellular Ca(2+).

Evidence that signal transduction for osmoreception is mediated by stretch-activated ion channels in tilapia.

Prolactin (PRL) plays a central role in the freshwater osmoregulation of teleost fish, including the tilapia (Oreochromis mossambicus). Consistent with this action, PRL release from the tilapia pituitary increases as extracellular osmolality is reduced both in vitro and in vivo. Dispersed tilapia PRL cells were incubated in a perfusion chamber that allowed simultaneous measurements of cell volume and PRL release. Intracellular Ca(2+) concentrations were measured from fura 2-loaded PRL cells treated in a similar way. Gadolinium (Gd(3+)), known to block stretch-activated cation channels, inhibited hyposmotically induced PRL release in a dose-related manner without preventing cell swelling. Nifedipine, an L-type Ca(2+) channel blocker, did not prevent the increase in PRL release during hyposmotic stimulation. A high, depolarizing concentration of KCl induced a transient and marked increase of intracellular Ca(2+) and release of PRL but did not prevent the rise in intracellular Ca(2+) and PRL release evoked by exposure to hyposmotic medium. These findings suggest that a decrease in extracellular osmolality stimulates PRL release through the opening of stretch-activated ion channels, which allow extracellular Ca(2+) to enter the cell when it swells.

Isolation and characterization of a homologue of mammalian prolactin-releasing peptide from the tilapia brain and its effect on prolactin release from the tilapia pituitary.

In the tilapia (Oreochromis mossambicus), as in many teleosts, prolactin (PRL) plays a major role in osmoregulation in freshwater. Recently, PRL-releasing peptides (PrRPs) have been characterized in mammals. Independently, a novel C-terminal RF (arginine-phenylalanine) amide peptide (Carrasius RF amide; C-RFa), which is structurally related to mammalian PrRPs, has been isolated from the brain of the Japanese crucian carp. The putative PrRP was purified from an acid extract of tilapia brain by affinity chromatography with antibody against synthetic C-RFa and HPLC on a reverse-phase ODS-120 column. The tilapia PrRP cDNA was subsequently cloned by polymerase chain reaction. The cDNA consists of 619 bp encoding a preprohormone of 117 amino acids. Sequence comparison of the isolated peptide and the preprohormone revealed that tilapia PrRP contains 20 amino acids and is identical to C-RFa. Incubation of the tilapia pituitary with synthetic C-RFa (100 nM) significantly stimulated the release of two forms of tilapia PRL (PRL188 and PRL177). However, the effect of C-RFa was less pronounced than the marked increase in PRL release in response to hyposmotic medium. The ability of C-RFa to stimulate PRL release appears to be specific, since C-RFa failed to stimulate growth hormone release from the pituitary in organ culture. In contrast, rat and human PrRPs had no effect on PRL release. C-RFa was equipotent with chicken GnRH in stimulating PRL release in the pituitary preincubated with estradiol 17beta. Circulating levels of PRL were significantly increased 1 h after intraperitoneal injection of 0.1 microg/g of C-RFa in female tilapia in freshwater but not in males. These results suggest that C-RFa is physiologically involved in the control of PRL secretion in tilapia.