Regulation of gill prolactin receptors in tilapia (Oreochromis niloticus) after a change in salinity or hypophysectomy

Effects of Osmotic Stress on the mRNA Expression of prl, prlr, gr, gh, and ghr in the Pituitary and Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii

In euryhaline teleost black porgy, Acanthopagrus schlegelii, the glucocorticoid receptor (gr), growth hormone receptor (ghr), prolactin (prl)-receptor (prlr), and sodium–potassium ATPase alpha subunit (α-nka) play essential physiological roles in the osmoregulatory organs, including the gill, kidney, and intestine, during osmotic stress. The present study aimed to investigate the impact of pituitary hormones and hormone receptors in the osmoregulatory organs during the transfer from freshwater (FW) to 4 ppt and seawater (SW) and vice versa in black porgy. Quantitative real-time PCR (Q-PCR) was carried out to analyze the transcript levels during salinity and osmoregulatory stress. Increased salinity resulted in decreased transcripts of prl in the pituitary, α-nka and prlr in the gill, and α-nka and prlr in the kidney. Increased salinity caused the increased transcripts of gr in the gill and α-nka in the intestine. Decreased salinity resulted in increased pituitary prl, and increases in α-nka and prlr in the gill, and α-nka, prlr, and ghr in the kidney. Taken together, the present results highlight the involvement of prl, prlr, gh, and ghr in the osmoregulation and osmotic stress in the osmoregulatory organs (gill, intestine, and kidney). Pituitary prl, and gill and intestine prlr are consistently downregulated during the increased salinity stress and vice versa. It is suggested that prl plays a more significant role in osmoregulation than gh in the euryhaline black porgy. Furthermore, the present results highlighted that the gill gr transcript’s role was solely to balance the homeostasis in the black porgy during salinity stress.

Secretion and Function of Pituitary Prolactin in Evolutionary Perspective

The hypothalamo-pituitary system developed in early vertebrates. Prolactin is an ancient vertebrate hormone released from the pituitary that exerts particularly diverse functions. The purpose of the review is to take a comparative approach in the description of prolactin, its secretion from pituitary lactotrophs, and hormonal functions. Since the reproductive and osmoregulatory roles of prolactin are best established in a variety of species, these functions are the primary subjects of discussion. Different types of prolactin and prolactin receptors developed during vertebrate evolution, which will be described in this review. The signal transduction of prolactin receptors is well conserved among vertebrates enabling us to describe the whole subphylum. Then, the review focuses on the regulation of prolactin release in mammals as we have the most knowledge on this class of vertebrates. Prolactin secretion in response to different reproductive stimuli, such as estrogen-induced release, mating, pregnancy and suckling is detailed. Reproduction in birds is different from that in mammals in several aspects. Prolactin is released during incubation in avian species whose regulation and functional significance are discussed. Little information is available on prolactin in reptiles and amphibians; therefore, they are mentioned only in specific cases to explain certain evolutionary aspects. In turn, the osmoregulatory function of prolactin is well established in fish. The different types of pituitary prolactin in fish play particularly important roles in the adaptation of eutherian species to fresh water environments. To achieve this function, prolactin is released from lactotrophs in hyposmolarity, as they are directly osmosensitive in fish. In turn, the released prolactin acts on branchial epithelia, especially ionocytes of the gill to retain salt and excrete water. This review will highlight the points where comparative data give new ideas or suggest new approaches for investigation in other taxa.

Dynamic responses of prolactin, growth hormone and their receptors to hyposmotic acclimation in the olive flounder Paralichthys olivaceus

Prolactin (PRL) and growth hormone (GH) play important roles in regulating salt and water balance through osmoregulatory organs in vertebrates. The aim of this study was to investigate the dynamic changes of GH/PRL hormone gene expressions in the pituitary gland and their receptors in gill and kidney, as well as the plasma osmolality when the olive flounder fish Paralichthys olivaceus were acclimated in freshwater (FW) conditions. After transfer from seawater (SW) to freshwater (FW), the osmolality of FW-adaption fish reached the lowest level at 1d which rose slightly afterwards. However, the hormone gene expression of PRL increased from 2d, reaching its peak at 5d, and then decreased at 14d. At this time, the value was still significantly higher than the control, showing a similar trend to the plasma hormone PRL. In contrast, the pituitary mRNA level of GH significantly decreased at 1d and then returned to normal levels. The mRNA levels of PRL receptor (PRLR) in both gill and kidney displayed a similar trend to the pituitary PRL. We also observed the synchronous expression trend of the renal PRLR with pituitary PRL (5d) and the asynchronous expression peaks between branchial (8d) and renal PRLR (5d). Significant responses of GH and its receptor (GHR) in both gill and kidney during the FW-acclimation were not observed. Nevertheless, the gene expression of GH receptor variant (GHR-V) in both gill and kidney declined at 2d, indicating unknown osmoregulatory functions of GHR-V. Collectively, our results provided more insights of the PRL, GH and their corresponding receptors in modulating osmoregulatory responses, representing an important aspect of FW-acclimation in flounder fish.

The effects of ammonia and water hardness on the hormonal, osmoregulatory and metabolic responses of the freshwater silver catfish Rhamdia quelen

The aim of the present study was to assess the effects of ammonia and water hardness on endocrine, osmoregulatory and metabolic parameters in silver catfish (Rhamdia quelen). The specimens (60-120g) were subjected to six treatments in triplicate, combining three levels of un-ionized ammonia (NH3) (0.020±0.008mg/L [1.17±0.47μM], 0.180±0.020mg/L [10.57±1.17μM] and 0.500±0.007mg/L [29.36±0.41μM]) and two levels of water hardness (normal: 25mgCaCO3/L and high: 120mgCaCO3/L), and sampled after two exposure times (1 and 5 days post-transfer). Plasma cortisol, metabolites, osmolality and ionic values were determined concomitantly with the mRNA expression levels of different adenohypophyseal hormones (growth hormone, GH; prolactin, PRL; and somatolactin, SL). Previously, full-length PRL and SL as well as β-actin cDNAs from R. quelen were cloned. Exposure to high NH3 levels enhanced plasma cortisol levels in fish held under normal water hardness conditions but not in those kept at the high hardness value. The increase in water hardness did not alter plasma metabolites, whereas it modulated the osmolality and ion changes induced by high NH3 levels. However, this hardness increase did not lead to the decreased GH expression that was observed 5 days after exposure to 0.18mg/L NH3 in fish held at the normal water hardness level, whereas PRL expression was enhanced after one day of exposure under the increased hardness conditions. Additionally, SL expression decreased in specimens exposed for 5 days to 0.18mg/L NH3 and maintained at the high water hardness level. The results showed that increasing water hardness attenuated the hormonal parameters evaluated in R. quelen specimens exposed to high NH3 levels, although plasma metabolism do not appear to suffer major changes.

The antidepressant drug carbamazepine induces differential transcriptome expression in the brain of Atlantic salmon, Salmo salar

Concerns are being expressed recently over possible environmental effects of human pharmaceuticals. Although the likelihood of acute toxicity is low, the continuous discharge of pharmaceuticals into the aquatic environment means that sublethal effects on non-target organisms need to be seriously considered. One-year-old Atlantic salmon parr were exposed to 7.85±0.13μgL(-1) of the antidepressant drug Carbamazepine (CBZ) for five days to investigate changes of mRNA expression in the brain by means of a custom 17k Atlantic salmon cDNA microarray. The selected concentration is similar to upper levels that can be found in hospital and sewage treatment plant effluents. After treatment, 373 features were differently expressed with 26 showing up- or down-regulation of ≥2-fold (p≤0.05). Among the mRNAs showing the highest change were the pituitary hormones encoding features somatolactin, prolactin and somatotropin, or growth hormone. Functional enrichment and network analyses of up- and down-regulated genes showed that CBZ induced a highly different gene expression profile in comparison to untreated organisms. CBZ induced expression of essential genes of the focal adhesion and extracellular matrix - receptor interaction pathways most likely through integrin alpha-6 (itga6) activation. Negative regulation of apoptotic process, extracellular matrix organization and heme biosynthesis were the most enriched biological process related GO-terms, with the simultaneous enrichment of collagen and extracellular region related cellular component GO-terms, and extracellular matrix structural constituent, hormone activity and chromatin binding molecular function related GO-terms. These results show that relatively low doses of CBZ may affect brain physiology in exposed salmon parr, targeting similar processes as in human, indicating a high degree of conservation of targets of CBZ action. However, and since the mRNAs showing most changes in expression are critical for adaptation to different stressors and life history transitions in Atlantic salmon, more research should be undertaken to assess CBZ effects to avoid impairment of normal development and maintenance of natural populations.

Dynamic gene expression of GH/PRL-family hormone receptors in gill and kidney during freshwater-acclimation of Mozambique tilapia

In teleosts, prolactin (PRL) and growth hormone (GH) act at key osmoregulatory tissues to regulate hydromineral balance. This study was aimed at characterizing patterns of expression for genes encoding receptors for the GH/PRL-family of hormones in the gill and kidney of Mozambique tilapia (Oreochromis mossambicus) during freshwater (FW)-acclimation. Transfer of seawater (SW)-acclimated tilapia to FW elicited rapid and sustained increases in plasma levels and pituitary gene expression of PRL177 and PRL188; plasma hormone and pituitary mRNA levels of GH were unchanged. In the gill, PRL receptor 1 (PRLR1) mRNA increased markedly after transfer to FW by 6h, while increases in GH receptor (GHR) mRNA were observed 48 h and 14 d after the transfer. By contrast, neither PRLR2 nor the somatolactin receptor (SLR) was responsive to FW transfer. Paralleling these endocrine responses were marked increases in branchial gene expression of a Na+/Cl- cotransporter and a Na+/H+ exchanger, indicators of FW-type mitochondrion-rich cells (MRCs), at 24 and 48 h after FW transfer, respectively. Expression of Na+/K+/2Cl- cotransporter, an indicator of SW-type MRCs, was sharply down-regulated by 6h after transfer to FW. In kidney, PRLR1, PRLR2 and SLR mRNA levels were unchanged, while GHR mRNA was up-regulated from 6h after FW transfer to all points thereafter. Collectively, these results suggest that the modulation of the gene expression for PRL and GH receptors in osmoregulatory tissues represents an important aspect of FW-acclimation of tilapia.

A novel tilapia prolactin receptor is functionally distinct from its paralog

A novel tilapia prolactin (PRL) receptor (OmPRLR2) was identified based on its induction during hyperosmotic stress. OmPRLR2 protein shows 28% identity to tilapia OmPRLR1 and 26% identity to human PRLR. Comparison of OmPRLR1 and OmPRLR2 revealed conserved features of cytokine class I receptors (CKR1): a WS domain and transmembrane domain, two pairs of cysteines and N-glycosylation motifs in the extracellular region, CKR1 boxes I and II, and three tyrosines in the intracellular region. However, OmPRLR2 lacked the ubiquitin ligase and 14-3-3 binding motifs. OmPRLR2 mRNA was present in all tissues analyzed, with highest expression in gills, intestine, kidney and muscle, similar to OmPRLR1. Transfer of fish from fresh water to sea water transiently increased gill OmPRLR2 mRNA levels within 4 h but decreased its protein abundance in the long term. OmPRLR2 is expressed in part as a truncated splice variant of 35 kDa in addition to the 55 kDa full-length protein. Cloning of the mRNA encoding the 35 kDa variant revealed that it lacks the extracellular region. It is expressed at significantly higher levels in males than in females. In stably transfected HEK293 cells over-expressing tetracycline-inducible OmPRLR1 and OmPRLR2, activation of these receptors by tilapia PRL177 and PRL188 triggered different downstream signaling pathways. Moreover, OmPRLR2 significantly increased HEK293 salinity tolerance. Our data reveal that tilapia has two PRLR genes whose protein products respond uniquely to PRL and activate different downstream pathways. Expression of a short PRLR2 variant may serve to inhibit PRL binding during osmotic stress and in male tissues.

Pituitary gene and protein expression under experimental variation on salinity and temperature in gilthead sea bream Sparus aurata

Temperature and salinity are important factors that affect several physiological processes in aquatic organisms, which could be produced by variation of certain hormones. In this study, the expression of pituitary hormones involved in the acclimation to different temperatures and salinities was examined in Sparus aurata, a euryhaline and eurytherm species, by Q-Real Time RT-PCR and Western blot analyses for mRNA and protein expression, respectively. Three different experimental conditions were designed with specimens (10 per treatment) acclimated to: a) low salinity water; b) sea water; and c) high salinity water. Additionally, fish under different salinities were acclimated to three different temperatures: 12, 19 and 26 degrees C. Animals were maintained seven weeks before sampling pituitary glands. Our results provided enough evidence for a differential expression of PRL, GH and SL in the pituitary of gilthead sea bream, under different temperature and salinity regimes.

Expression of pituitary prolactin, growth hormone and somatolactin is modified in response to different stressors (salinity, crowding and food-deprivation) in gilthead sea bream Sparus auratus

Prolactin (PRL), growth hormone (GH) and somatolactin (SL) expression was studied in gilthead sea bream (Sparus auratus) in response to several different stressors (salinity, food deprivation or stocking density). In the first experiment, specimens were acclimated during 100 days at three different environmental salinities: low salinity water (LSW, 6 ppt), brackish water (BW, 12 ppt) and seawater (SW, 38 ppt). Osmoregulatory parameters corresponded to those previously reported for this species under similar osmotic conditions. Pituitary PRL expression increased with decreasing environmental salinity, and was significantly different between SW- and LSW-acclimated fish. Pituitary GH expression was similar between SW- and BW-acclimated fish but decreased in LSW-acclimated specimens. Pituitary SL expression had a "U-shaped" relationship to environmental salinity with the lowest expression in BW-acclimated fish. In a second experiment SW-acclimated specimens were randomly assigned to one of four treatments and maintained for 14 days: (1) fed fish under low density (LD, 4 kg m(-3)); (2) fed fish under high density (HD, 70 kg m(-3)); (3) food deprived fish under LD; and (4) food deprived fish under HD. Plasma glucose and cortisol levels corresponded to those previously reported in S. auratus under similar experimental conditions. Pituitary PRL and SL expression increased in fish maintained under HD and decreased in food deprived fish. In conclusion, an effect of environmental salinity on pituitary PRL and GH expression has been demonstrated. In addition, crowding stress seems to interact with food deprivation in S. auratus and this is reflected by changes in pituitary PRL, GH and SL expression levels.

Prolactin receptor, growth hormone receptor, and putative somatolactin receptor in Mozambique tilapia: tissue specific expression and differential regulation by salinity and fasting

In fish, pituitary growth hormone family peptide hormones (growth hormone, GH; prolactin, PRL; somatolactin, SL) regulate essential physiological functions including osmoregulation, growth, and metabolism. Teleost GH family hormones have both differential and overlapping effects, which are mediated by plasma membrane receptors. A PRL receptor (PRLR) and two putative GH receptors (GHR1 and GHR2) have been identified in several teleost species. Recent phylogenetic analyses and binding studies suggest that GHR1 is a receptor for SL. However, no studies have compared the tissue distribution and physiological regulation of all three receptors. We sequenced GHR2 from the liver of the Mozambique tilapia (Oreochromis mossambicus), developed quantitative real-time PCR assays for the three receptors, and assessed their tissue distribution and regulation by salinity and fasting. PRLR was highly expressed in the gill, kidney, and intestine, consistent with the osmoregulatory functions of PRL. PRLR expression was very low in the liver. GHR2 was most highly expressed in the muscle, followed by heart, testis, and liver, consistent with this being a GH receptor with functions in growth and metabolism. GHR1 was most highly expressed in fat, liver, and muscle, suggesting a metabolic function. GHR1 expression was also high in skin, consistent with a function of SL in chromatophore regulation. These findings support the hypothesis that GHR1 is a receptor for SL. In a comparison of freshwater (FW)- and seawater (SW)-adapted tilapia, plasma PRL was strongly elevated in FW, whereas plasma GH was slightly elevated in SW. PRLR expression was reduced in the gill in SW, consistent with PRL's function in freshwater adaptation. GHR2 was elevated in the kidney in FW, and correlated negatively with plasma GH, whereas GHR1 was elevated in the gill in SW. Plasma IGF-I, but not GH, was reduced by 4 weeks of fasting. Transcript levels of GHR1 and GHR2 were elevated by fasting in the muscle. However, liver levels of GHR1 and GHR2 transcripts, and liver and muscle levels of IGF-I transcripts were unaffected by fasting. These results clearly indicate tissue specific expression and differential physiological regulation of GH family receptors in the tilapia.

Hormone receptors in gills of smolting Atlantic salmon, Salmo salar: expression of growth hormone, prolactin, mineralocorticoid and glucocorticoid receptors and 11beta-hydroxysteroid dehydrogenase type 2

This is the first study to report concurrent dynamics in mRNA expression of growth hormone receptor (GHR), prolactin receptor (PRLR), gluco- and mineralocorticoid receptor (GR and MR) and the 11beta-hydroxysteroid dehydrogenase type-2 enzyme (11beta-HSD2) in Atlantic salmon (Salmo salar) gill during smoltification. Transcript levels were analysed by quantitative PCR in fresh water (FW) fish and after a 24-h salt water (SW) challenge. GHR transcript levels increased concurrent with gill Na(+),K(+)-ATPase activity in FW fish consistent with the SW-adaptive role of GH. SW-transfer induced an increased GHR expression levels in the early stages of smoltification but a decrease in expression at the peak of smoltification. PRLR transcript levels decreased steadily during smoltification in agreement with the recognized hyper-osmoregulatory role of PRL. Surprisingly, PRLR levels increased after SW transfer during the course of smoltification. GR mRNA levels were low early on during smoltification but increased at the peak of smoltification and remained high during de-smoltification, indicative of increased cortisol signalling at this point. Coherently, SW transfer increased GR levels to smolt levels prior to the smoltification peak. 11beta-HSD2 levels increased at the smoltification peak and MR levels increased during de-smoltification, suggesting a need for protection of MR from cortisol signalling during smoltification. This is supported by the fact that SW-transfer results in a profound up-regulation of 11beta-HSD2, whereas SW transfer down-regulates MR levels. The study concludes that GR and MR may have distinctive roles in developing hypo- and hyper-osmoregulatory mechanisms during smoltification and de-smoltification, respectively.

Gene structure and seasonal expression of carp fish prolactin short receptor isoforms

The complex adaptive mechanisms that eurythermal fish have evolved in response to the seasonal changes of the environment include the transduction of the physical parameter variations into neuroendocrine signals. Studies in carp (Cyprinus carpio) have indicated that prolactin (PRL) and growth hormone (GH) expression is associated with acclimatization, suggesting that the pituitary gland is a relevant physiological node in this adaptive process. Also, the distinctive pattern of expression that carp prolactin receptor (PRLr) protein depicts upon seasonal acclimatization supports the hypothesis that PRL and its receptor clearly are involved in the new homeostatic stage that the eurythermal fish needs to survive during the cyclical changes of its habitat. Here, we characterize the first prolactin receptor gene in a teleost and show that its expression is not associated with alternative promoters, unlike in humans and rodents. Using the regulatory region to direct the transcription of green fluorescent protein (GFP) in zebrafish embryos, we mapped the appearance of this hormone receptor during fish development. This is the first report identifying a fish prolactin receptor gene expressing transcript isoforms encoding for short forms of the protein (45 kDa). These have been found in osmoregulatory tissues of the carp and are regulated in connection with the seasonal acclimatization of the fish.

Prolactin and prolactin receptor expressions in a marine teleost, pufferfish Takifugu rubripes

To investigate the physiological significance of prolactin (PRL) in a marine teleost, pufferfish (or fugu), Takifugu rubripes, we cloned and characterized cDNAs encoding its PRL and PRL receptor (PRLR) from the pituitary and gills, respectively. The fugu PRL cDNA consisted of 995 bp and encoded a protein of 213 amino acids. The PRLR, consisting of 510 amino acids, contained a putative signal peptide, an extracellular domain with two pairs of cysteines, a WSXWS motif, a single transmembrane domain, and a cytoplasmic (intracellular) domain with box 1 and box 2 regions, all of which are characteristic of the cytokine receptor superfamily. Reverse transcription-PCR showed the expression of PRLR mRNA in osmoregulatory organs, such as gills, kidney, and intestine, whereas pufferfish PRL mRNA was detected only in the pituitary. Furthermore, in situ hybridization revealed the expression of pufferfish PRLR in branchial chloride cells, kidney tubule cells, and intestinal epithelia. The PRL-gene expression levels in the pituitary were about five times higher in 25%-diluted seawater than in full-strength seawater. These results suggest that fugu PRL regulates water and electrolyte balances through PRLR expressed in the osmoregulatory organs, as is the case with freshwater-adapted euryhaline species.

Growth hormone and prolactin actions on osmoregulation and energy metabolism of gilthead sea bream (Sparus auratus)

The gilthead sea bream (Sparus auratus) is an euryhaline fish where prolactin (PRL) and growth hormone (GH) play a role in the adaptation to different environmental salinities. To find out the role of these pituitary hormones in osmoregulation and energy metabolism, fish were implanted with slow release implants of ovine GH (oGH, 5 microg g(-1) body mass) or ovine prolactin (oPRL, 5 microg g(-1) body mass), and sampled 7 days after the start of the treatment. GH increased branchial Na(+),K(+)-ATPase activity and decreased sodium levels in line with its predicted hypoosmoregulatory action. GH had metabolic effects as indicated by lowered plasma protein and lactate levels, while glucose, triglycerides and plasma cortisol levels were not affected. Also, GH changed liver glucose and lipid metabolism, stimulated branchial and renal glucose metabolism and glycolytic activity, and enhanced glycogenolysis in brain. PRL induced hypernatremia. Furthermore, this hormone decreased liver lipid oxidation potential, and increased glucose availability in kidney and brain. Both hormones have opposite osmoregulatory effects and different metabolic effects. These metabolic changes may support a role for both hormones in the control of energy metabolism in fish that could be related to the metabolic changes occurring during osmotic acclimation.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Seasonal environmental changes modulate the prolactin receptor expression in an eurythermal fish

Eurythermal fish have evolved compensatory responses to the cyclical seasonal changes of the environment. The complex adaptive mechanisms include the transduction of the physical parameters variations into molecular signals. Studies in carp have indicated that prolactin and growth hormone expression is associated with acclimatization, suggesting that the pituitary gland is a relevant physiological node in the generation of the homeostatic rearrangement that occurs in this adaptive process. Here, we report the cloning and characterization of a full-length carp prolactin receptor cDNA, which codes for the long form of the protein resembling that found in mammalian prolactin receptors. We identified up to three receptor transcript isoforms in different tissues of the teleost and assessed cell- and temporal-specific transcription and protein expression in carp undergoing seasonal acclimatization. The distinctive pattern of expression that carp prolactin receptor (cPRLr) depicts upon seasonal acclimatization supports the hypothesis that prolactin and its receptor are clearly involved in the new homeostatic stage that the eurythermal fish needs to survive during the cyclical changes of its habitat.

Osmoregulatory action of PRL, GH, and cortisol in the gilthead seabream (Sparus aurata L)

The osmoregulatory actions of ovine prolactin (oPRL), ovine growth hormone (oGH), and cortisol were tested in the euryhaline gilthead seabream Sparus aurata. Acclimated to sea water (SW, 40 ppt salinity, 1000 mOsm/kg H(2)O) or brackish water (BW, 5 ppt, salinity, 130 mOsm/kg H(2)O), injected every other day for one week (number of injections, 4) with saline (0.9% NaCl), oPRL (4 microg/g body weight), oGH (4 microg/g body weight) or cortisol (5 microg/g body weight), and transferred from SW to BW or from BW to SW 24h after the last injection. Fish were sampled before and 24h after transfer. Gill Na(+), K(+)-ATPase activity, plasma osmolality, plasma ions (sodium and chloride), plasma glucose, and muscle water moisture were examined. SW-adapted fish showed higher gill Na(+), K(+)-ATPase activity, plasma osmolality, and plasma ions levels than BW-adapted fish. Transfer from SW to BW decreased plasma osmolality and ions levels after 24h, while transfer from BW to SW increased these parameters, whereas gill Na(+),K(+)-ATPase activity was unaffected. oPRL treatment significantly decreased gill Na(+),K(+)-ATPase activity and increased plasma osmolality and ions in SW- and BW-adapted fish. This treatment minimizes loss of osmolality and ions in plasma after transfer to BW and increased these values after transfer to SW. No significant changes were observed in gill Na(+),K(+)-ATPase activity, plasma osmolality, and plasma ions in oGH-treated group with respect to saline group before or after transfer from SW to BW or from BW to SW. Treatment with cortisol induced, in SW-adapted fish, a significant increase of gill Na(+),K(+)-ATPase activity and decrease of plasma osmolality and plasma ions. In BW-adapted fish this treatment induced a significant increases in gill Na(+),K(+)-ATPase activity, plasma osmolality, and plasma ions. After transfer to SW cortisol-treated fish had higher plasma osmolality than the saline group. Our results support the osmoregulatory role of PRL in the adaptation to hypoosmotic environment in the gilthead seabream S. aurata. Further studies will be necessary to elucidate the osmoregulatory role of GH in this species. Cortisol results suggest a "dual osmoregulatory role" of this hormone in S. aurata.

Cell signaling and ion transport across the fish gill epithelium

A large array of circulating and local signaling agents modulate transport of ions across the gill epithelium of fishes by either affecting transport directly or by altering the size and distribution of transporting cells in the epithelium. In some cases, these transport effects are in addition to cardiovascular effects of the same agents, which may affect the perfusion pathways in the gill vasculature and, in turn, affect epithelial transport indirectly. Prolactin is generally considered to function in freshwater, because it is the only agent that allows survival of some hypophysectomized fish species in freshwater. It appears to function by either reducing branchial permeability, Na,K-activated ATPase activity, or reducing the density of chloride cells. Cortisol was initially considered to produce virtually opposite effects (e.g., stimulation of Na,K-activated ATPase and of chloride cell size and density), but more recent studies have found that this steroid stimulates ionic uptake in freshwater fishes, as well as the activity of H-ATPase, an enzyme thought to be central to ionic uptake. Thus, cortisol may function in both high and low salinities. Growth hormone and insulin-like growth factor appear to act synergistically to affect ion regulation in seawater fishes, stimulating both Na,K-activated ATPase and Na-K-2Cl co-transporter activity, and chloride cell size, independent of their effects on growth. Some of the effects of the GH-IGF axis may be via stimulation of the number of cortisol receptors. Thyroid hormones appear to affect seawater ion regulation indirectly, by stimulating the GH-IGF axis. Natriuretic peptides were initially thought to stimulate gill ionic extrusion, but recent studies have not corroborated this finding, so it appears that the major mode of action of these peptides may be reduction of salt loading by inhibition of oral ingestion and intestinal ionic uptake. Receptors for both arginine vasotocin and angiotensin have been described in the gill epithelium, but their respective roles and importance in fish ion regulation remains unknown. The gill epithelium may be affected by both circulating and local adrenergic agents, and a variety of studies have demonstrated that stimulation of alpha-adrenergic versus beta-adrenergic receptors produces inhibition or stimulation of active salt extrusion, respectively. Local effectors, such as prostaglandins, nitric oxide, and endothelin, may affect active salt extrusion as well as gill perfusion. Recent studies have suggested that the endothelin inhibition of salt extrusion is actually mediated by the release of both NO and prostaglandins. It is hoped that modern molecular techniques, combined with physiological measurements, will allow the dissection of the relative roles in ion transport across the fish gill epithelium of this surprisingly large array of putative signaling agents.

Does prolactin affect steroid secretion by isolated rainbow trout ovarian cells?

The in vitro secretion of progesterone (P(4)), androgen (A) and estradiol (E(2)) by follicular cells, isolated monthly from the rainbow trout ovaries during the whole annual cycle, was studied. Cells were cultured as monolayers in control and prolactin (PRL) supplemented media. E(2) secretion showed two distinct maxima in September and January: 4959+/-220 pg/ml and 3166+/-121 pg/ml, respectively, i.e. during vitellogenesis and before the spawning time. PRL had a significant (16%) suppressive effect on E(2) secretion when the level of secreted steroid was at its highest (4167+/-193 pg/ml) at the end of vitellogenesis and by 32% (2157+/-124 pg/ml), before ovulation. Increased P(4) levels observed in February (988+/-69 pg/ml) and March (2008+/-74 pg/ml) may be connected with the need for a substrate for the synthesis of 17alpha20betaOH-P (MIS). At this time, the secretion of P(4) was also suppressed by PRL and was reduced to 1395+/-78 pg/ml. Our results indicate that PRL may play a role in fish reproduction.

The role of prolactin in fish osmoregulation: a review

The protein hormone prolactin (PRL) was first discovered as an anterior pituitary factor capable of stimulating milk production in mammals. We now know that PRL has over 300 different functions in vertebrates. In fish, PRL plays an important role in freshwater osmoregulation by preventing both the loss of ions and the uptake of water. This paper will review what is currently known about the structure and evolution of fish PRL and its mechanisms of action in relation to the maintenance of hydromineral balance. Historically, functional studies of fish PRL were carried out using heterologous PRLs and the results varied greatly between experiments and species. In some cases this variability was due to the ability of these PRLs to bind to both growth hormone and PRL receptors. In fact, a recurring theme in the literature is that the actions of PRL cannot be generalized to all fish due to marked differences between species. Many of the effects of PRL on hydromineral balance are specific to euryhaline fish, which is appropriate given that they frequently experience sudden changes in environmental salinity. Much of the recent work has focused on the isolation and characterization of fish PRLs and their receptors. These studies have provided the necessary tools to obtain a better understanding of the evolution of PRL and its role in osmoregulation.

Transfer of tilapia (Oreochromis niloticus) to a hyperosmotic environment is associated with sustained expression of prolactin receptor in intestine, gill, and kidney

Expression of the tilapia prolactin receptor (tiPRL-R) has been characterized in the intestine of Oreochromis niloticus and the levels of both tiPRL-R transcripts and tiPRL binding sites have been further analyzed in this organ, as well as in gill and kidney, during adaptation of tilapia to a hyperosmotic environment. A single high-affinity binding site for tilapia PRL-I (tiPRL-I) was determined in full-length intestine by Scatchard analysis. A heterogeneous distribution of tiPRL-R was detected in this organ, with the posterior part always displaying a higher expression of both tiPRL-R transcript and tiPRL binding sites than the anterior and medial parts. Transfer of tilapia to brackish water (BW) led to an apparent increase in the specific binding of tiPRLs in intestine and gill even for long-term-adapted fish, whereas the high level of kidney tiPRL binding sites measured in control fish reared in fresh water was still detected in BW-adapted tilapia. There was no overall significant modification of tiPRL-R transcript levels in any organ during short-term or long-term adaptation, although a limited decrease occurred in the gill of BW-adapted fish, as shown earlier. Therefore, in O. niloticus adapted to BW, high and sustained levels of tiPRL-R were observed in the three major osmoregulatory organs, gill, kidney, and intestine.

Structure and tissue distribution of prolactin receptor mRNA in Japanese flounder (Paralichtys olivaceus): conserved and preferential expression in osmoregulatory organs

In diadromous and euryhaline teleosts, it has been established that prolactin (PRL) is a major hormone regulating the maintenance of water and electrolyte homeostasis by acting on its receptor (PRLR) expressed in the osmoregulatory organs. To investigate the major physiological role of PRL in a marine teleost, cDNA for the Japanese flounder (Paralichtys olivaceus) prolactin receptor (fPRLR) has been cloned and characterized. The predicted fPRLR is composed of 636 amino acids conserving common structural features, such as the WSXWS motif and box 1, that are observed in the members of the cytokine receptor superfamily. By Northern blot analysis, 3.5-kb transcripts for fPRLR were clearly detected in the gill, kidney, and intestine. By RNase protection assay, similarly high levels of mRNA expression were detected in these osmoregulatory organs and lower expression levels were seen in the brain for both males and females. Interestingly, a distinct expression level of fPRLR mRNA was observed in the testis, but not in the ovary. The present results suggest that PRL may play an important role in the control of water and electrolyte balance through PRLR expressed in the osmoregulatory organs in the marine teleost the Japanese flounder as well as in other teleosts. Furthermore, PRL may differentially regulate gonadal functions in males and females of Japanese flounder.

Cloning, characterization, and tissue distribution of prolactin receptor in the sea bream (Sparus aurata)

The prolactin receptor (PRLR) was cloned and its tissue distribution characterized in adults of the protandrous hermaphrodite marine teleost, the sea bream (Sparus aurata). An homologous cDNA probe for sea bream PRLR (sbPRLR) was obtained by RT-PCR using gill mRNA. This probe was used to screen intestine and kidney cDNA libraries from which two overlapping clones (1100 and 2425 bp, respectively) were obtained. These clones had 100% sequence identity in the overlapping region (893 bp) and were used to deduce the complete amino acid sequence of sbPRLR. The receptor spans 2640 bp and encodes a protein of 537 amino acids. Features characteristic of PRLR, two pairs of cysteines, WS box, hydrophobic transmembrane domain, box 1, and box 2, were identified and showed a high degree of sequence identity to PRLRs from other vertebrate species. SbPRLR is 29 and 32% identical to tilapia (Oreochromis niloticus) and goldfish (Carassius auratus) PRLRs, respectively. In the sea bream two PRLR transcripts of 2.8 and 3.2 kb were detected in the intestine, kidney, and gills and a single transcript of 2.8 kb was detected in skin and pituitary by Northern blot. Spermiating gonads (more than 95% male tissue; gonado-somatic index of 0.6) contained, in addition to the 2.8-kb transcript, three more transcripts of 1.9, 1.3, and 1.1 kb. RT-PCR, which is a far more sensitive method than Northern blot, detected PRLR mRNA in gills, intestine, brain, pituitary, kidney, liver, gonads, spleen, head-kidney, heart, muscle, and bone. Immunohistochemistry using specific polyclonal antibodies raised against an oligopeptide from the extracellular domain of sbPRLR detected PRLR in several epithelial tissues of juvenile sea bream, including the anterior gut, renal tubule, choroid membrane of the third ventricle, saccus vasculosus, branchial chloride cells, and branchial cartilage.

Molecular characterization of the prolactin receptor in two fish species, tilapia Oreochromis niloticus and rainbow trout, Oncorhynchus mykiss: A comparative approach

We present recent information on the molecular characterization of the prolactin receptor (PRL-R) in two teleost species, tilapia (Oreochromis niloticus) and rainbow trout (Oncorhynchus mykiss), in the perspective of improved understanding of the physiological differences in the control of osmoregulatory function between these two fish species. Although our interest will mainly focus on osmoregulatory organs, we will also discuss evidence of the presence of PRL-R in other tissues such as gonads and hematopoietic organs. The first fish PRL-R was characterized in tilapia. This receptor is similar to that of the long form of mammalian PRL-R, but the most conserved region (extracellular domain) has only 53% identity with mammalian PRL-R. A rainbow trout PRL-R cDNA has been also isolated and appeared very similar in structure to tilapia PRL-R. Expression of the PRL-R gene was studied by Northern blotting for various tissues from tilapia and trout, and a unique transcript size of 3.2-3.4 kb was observed in all tissues studied (including male and female gonads, skin, brain, spleen, head, kidney, and circulating lymphocytes). Osmoregulatory organs (gills, kidney, intestine) were the richest tissues. Using in situ hybridization, PRL-R transcripts were localized in gill chloride cells, both in trout and tilapia. Analysis of PRL-R transcript levels in gills, kidney, and intestine indicated the maintenance of a high level of expression during adaptation to a hyperosmotic environment. These results support PRL being a pleiotropic hormone in fish and suggest the presence of a unique PRL-R form in tilapia and in trout. Finally, characterization of hormone receptor binding has been carried out in both species using a radioreceptor assay (in tilapia) or surface plasmon resonance (SPR) technology (in trout). These studies indicated the presence of a stable hormone-receptor complex in tilapia, while PRL binds to its receptor through an unstable homodimeric complex in trout. Thus, the characteristics of PRL binding on its receptor appear to be significantly different in tilapia and trout. Whether such differences may lead to different signal transduction mechanisms and osmoregulatory actions of PRL in these two euryhaline species merits further investigation.Key words: prolactin receptor, genetic expression, hormone-receptor interaction, surface plasmon resonance, fish osmoregulation.

Effects of prolactin and growth hormone on strategies of hypoosmotic adaptation in a marine teleost, Sparus sarba

Silver seabream (Sparus sarba) held in seawater (33 per thousand) or acclimated to a hypoosmotic environment of 6 per thousand were given intraperitoneal injections of saline (0.8% NaCl), recombinant bream growth hormone (rbGH, 1 microg/g), or ovine prolactin (oPRL, 6microg/g) for 7 consecutive days. Serum Na+ levels were unaffected by hypoosmotic acclimation and rbGH and oPRL treatment. Treatment of seawater fish with oPRL resulted in hyperchloremia. In 6 per thousand, saline-treated fish exhibited elevated branchial chloride cell (CC) numbers and exposure indices, all of which were markedly reduced by oPRL. CC numbers and morphometrics were unaffected by oPRL in seawater fish. In contrast, rbGH treatment of seawater fish resulted in elevated CC numbers, apical area, and fractional area and, in 6 per thousand fish, elevated CC fractional area and exposure numbers. Branchial Na+-K+-ATPase activity reduced in saline-treated fish adapted to 6% but was unaffected by rbGH regardless of salinity. oPRL reduced activity in both seawater and 6 per thousand-adapted fish. Neither hypoosmotic adaptation nor oPRL had any effect on renal Na+-K+-ATPase activity whereas rbGH reduced activity in both 33 and 6 per thousand. Saline-treated fish adapted to 6 per thousand exhibited reduced Na+-K+-ATPase activity in most regions of the intestine. Treatment with rbGH did not change intestinal Na+-K+-ATPase activity of seawater fish but elevated activity in the anterior regions (esophagus and stomach) of 6 per thousand-adapted fish. Treatment with oPRL elevated Na+-K+-ATPase activity throughout the gastrointestinal tract of seawater fish and in the anterior reaches of 6 per thousand-adapted fish. The data indicated that the as yet uncharacterized osmoregulatory roles of PRL and GH in seabream may warrant further attention as the present study connoted differing responses to that of other teleosts studied.

Activation of gene transcription by tilapia prolactin variants tiPRL188 and tiPRL177

In the tilapia species Oreochromis niloticus, the pituitary releases two forms of prolactins (tiPRL188 and tiPRL177). The binding parameters and the activation of tiPRL-induced JAK2/Stat5 signalling pathway were analysed using a mammalian cell line transiently transfected with the tiPRL receptor (tiPRLR). Our data indicate that the tiPRLR is able to mediate transcriptional activation of the PRL responsive element. At nanomolar concentrations, tiPRL188 activates gene transcription whereas at micromolar concentrations it inhibits luciferase transcription from the lactogenic responsive element. This is consistent with a model of receptor dimerisation. In contrast, the activation by tiPRL177 was only reached at high (microM) concentrations. The transcriptional activities induced by tiPRL177 and tiPRL188 are discussed in the context of the physiology of these hormones.

Somatotropic actions of the homologous growth hormone and prolactins in the euryhaline teleost, the tilapia, Oreochromis mossambicus

It is increasingly clear that growth hormone (GH) has growth-promoting effects in fishes, which are mediated in part by the insulin-like growth factor (IGF)-I. Growth-promoting actions of prolactin (PRL) have been reported in higher vertebrates, but are less well established in teleosts. We examined the effects of injecting homologous GH or the two homologous tilapia PRLs (tPRL177 and tPRL188) on the in vitro incorporation of [35S] sulfate (extracellular matrix synthesis) and [3H]thymidine (DNA synthesis) by ceratobranchial cartilage explants and on IGF-I mRNA levels in tilapia liver. Tilapia GH (tGH) and tPRL177 stimulated sulfate uptake at the highest doses examined. Thymidine incorporation was stimulated by tPRL177. tPRL188 was without these effects. Consistent with its somatotropic actions, tGH elevated IGF-I mRNA levels in the liver. tPRL177 also elevated liver IGF-I levels. Consistent with the previously described osmoregulatory actions of GH and PRL in teleosts, we observed that tGH elevated and tPRL177 and tPRL188 lowered levels of gill Na+,K+-ATPase activity. High-affinity, low-capacity binding sites for tGH in the tilapia liver were identified. tPRL177 binds with lower affinity than tGH to these sites but can displace 125I-labeled tGH from its receptor. The ability of tPRL177 to displace tGH was similar to that of ovine GH. tPRL188 did not displace 125I-labeled tGH binding. Collectively, this work suggests that tPRL177 may possess somatotropic actions similar to tGH, but only in freshwater tilapia where tPRL177 levels are sufficiently high for it to act as a competitive ligand for GH receptors.