Prolactin receptors in tilapia (Sarotherodon mossambicus) tissues: binding studies using 125I-labeled ovine prolactin

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.

Transcriptomes of testis and pituitary from male Nile tilapia (O. niloticus L.) in the context of social status

African cichlids are well established models for studying social hierarchies in teleosts and elucidating the effects social dominance has on gene expression. Ascension in the social hierarchy has been found to increase plasma levels of steroid hormones, follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) as well as gonadosomatic index (GSI). Furthermore, the expression of genes related to gonadotropins and steroidogenesis and signaling along the brain-pituitary-gonad axis (BPG-axis) is affected by changes of an animal’s social status. In this study, we use RNA-sequencing to obtain an in-depth look at the transcriptomes of testes and pituitaries from dominant and subordinate male Nile tilapia living in long-term stable social hierarchies. This allows us to draw conclusions about factors along the brain-pituitary-gonad axis that are involved in maintaining dominance over weeks or even months. We identify a number of genes that are differentially regulated between dominant and subordinate males and show that in high-ranking fish this subset of genes is generally upregulated. Genes differentially expressed between the two social groups comprise growth factors, related binding proteins and receptors, components of Wnt-, Tgfβ- and retinoic acid-signaling pathway, gonadotropin signaling and steroidogenesis pathways. The latter is backed up by elevated levels of 11-ketotestosterone, testosterone and estradiol in dominant males. Luteinizing hormone (Lh) is found in higher concentration in the plasma of long-term dominant males than in subordinate animals. Our results both strengthen the existing models and propose new candidates for functional studies to expand our understanding of social phenomena in teleost fish.

Tilapia prolactin cells are thermosensitive osmoreceptors

Prolactin (PRL) cells within the rostral pars distalis (RPD) of euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, rapidly respond to a hyposmotic stimulus by releasing two distinct PRL isoforms, PRL₁₈₈ and PRL₁₇₇. Here, we describe how environmentally relevant temperature changes affected mRNA levels of PRL₁₈₈ and PRL₁₇₇ and the release of immunoreactive prolactins from RPDs and dispersed PRL cells. When applied under isosmotic conditions (330 mOsm/kg), a 6 °C rise in temperature stimulated the release of PRL₁₈₈ and PRL₁₇₇ from both RPDs and dispersed PRL cells under perifusion. When exposed to this same change in temperature, ~50% of dispersed PRL cells gradually increased in volume by ~8%, a response partially inhibited by the water channel blocker, mercuric chloride. Following their response to increased temperature, PRL cells remained responsive to a hyposmotic stimulus (280 mOsm/kg). The mRNA expression of transient potential vanilloid 4, a Ca²⁺-channel involved in hyposomotically-induced PRL release, was elevated in response to a rise in temperature in dispersed PRL cells and RPDs at 6 and 24 h, respectively; prl₁₈₈ and prl₁₇₇ mRNAs were unaffected. Our findings indicate that thermosensitive PRL release is mediated, at least partially, through a cell-volume dependent pathway similar to how osmoreceptive PRL release is achieved.

Hormonal and neuroendocrine control of reproductive function in teleost fish

Reproduction is one of the important physiological events for the maintenance of the species. Hormonal and neuroendocrine regulation of teleost requires multiple and complex interactions along the hypothalamic-pituitary-gonad (HPG) axis. Within this axis, gonadotropin-releasing hormone (GnRH) regulates the synthesis and release of gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Steroidogenesis drives reproduction function in which the development and differentiation of gonads. In recent years, new neuropeptides have become the focus of reproductive physiology research as they are involved in the different regulatory mechanisms of these species' growth, metabolism, and reproduction. However, especially in fish, the role of these neuropeptides in the control of reproductive function is not well studied. The study of hormonal and neuroendocrine events that regulate reproduction is crucial for the development and success of aquaculture.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

In fishes, prolactin (Prl) signaling underlies the homeostatic regulation of hydromineral balance by controlling essential solute and water transporting functions performed by the gill, gastrointestinal tract, kidney, urinary bladder, and integument. Comparative studies spanning over 60 years have firmly established that Prl promotes physiological activities that enable euryhaline and stenohaline teleosts to reside in freshwater environments; nonetheless, the specific molecular and cellular targets of Prl in ion- and water-transporting tissues are still being resolved. In this short review, we discuss how particular targets of Prl (e.g., ion cotransporters, tight-junction proteins, and ion pumps) confer adaptive functions to the esophagus and intestine. Additionally, in some instances, Prl promotes histological and functional transformations within esophageal and intestinal epithelia by regulating cell proliferation. Collectively, the demonstrated actions of Prl in the gastrointestinal tract of teleosts indicate that Prl operates to promote phenotypes supportive of freshwater acclimation and to inhibit phenotypes associated with seawater acclimation. We conclude our review by underscoring that future investigations are warranted to determine how growth hormone/Prl-family signaling evolved in basal fishes to support the gastrointestinal processes underlying hydromineral balance.

Morphological and immunohistochemical comparison of the pituitary gland between a tropical Paracheirodon axelrodi and a subtropical Aphyocharax anisitsi characids (Characiformes: Characidae)

ABSTRACT Cardinal tetra Paracheirodon axelrodi and bloodfin tetra Aphyocharax anisitsi are two species of characids with high trade value as ornamental fish in South America. Although both species inhabit middle water layers, cardinal neon exhibits a tropical distribution and bloodfin tetra a subtropical one. Generally, these species are difficult to grow, so it becomes essential to know some key components of the neuroendocrine system to achieve their reproduction in captivity. Considering the importance of deepening the knowledge of the reproductive physiology through functional morphology, for the first time in this work we performed an anatomical, morphological and immunohistochemical analysis of the pituitary gland of these two species. In both species, a leptobasic type pituitary is found in the ventral zone of the hypothalamus and it is characterized by a neurohypophysis which has a well-developed pituitary stalk and a globular adenohypophysis. The pituitary components, characterized by histochemistry and immunohistochemistry, shows a distribution pattern of cells types similar to other teleost species, with only slight differences in the distribution of βFSH and βLH for P. axelrodi.

Depletion of Tissue-Specific Ion Transporters Causes Differential Expression of PRL Targets in Response to Increased Levels of Endogenous PRL

Prolactin (PRL) has been considered a key regulator of ion uptake in zebrafish. The genes slc12a10.2 and slc12a3, which are Na⁺ and chloride Cl^- co-transporters, have been reported to be regulated by PRL in freshwater fish. The integrative network of PRL signaling dissected from the knockout of tissue-specific downstream PRL ion transporters remains poor. In the present study, zebrafish models with increased endogenous levels of PRL were generated through the knockout of slc12a10.2 or slc12a3, and the developmental consequences were analyzed. The increased levels of pituitary PRL were observed in both slc12a10.2- and slc12a3-deficient fish. Unlike the slc12a3-deficient fish, which could survive to adulthood, the slc12a10.2-deficient fish began to die at 9 days post-fertilization (dpf) and did not survive beyond 17 dpf. This survival defect is a result of defective Cl^- uptake in this mutant, indicating that Slc12a10.2 plays an essential role in Cl^- uptake. Intriguingly, compared to the levels in control fish, no significant differences in the levels of Na⁺ in the body were observed in slc12a10.2- or slc12a3-deficient zebrafish. The upregulations of the PRL downstream transporters, slc9a3.2, slc12a10.2, and atp1a1a.5 were observed in slc12a3-deficient fish in both the gills/skin and the pronephric duct. However, this type of response was not observed in the pronephric duct of slc12a10.2-deficient fish, except under Na⁺-deprived conditions. Our results show that PRL is susceptible to deficiencies in downstream ion transporters. Moreover, both the gills/skin and pronephric duct show differential expression of downstream PRL targets in response to increased levels of pituitary PRL caused by the depletion of tissue-specific ion transporters.

Gonadal Transcriptome Analysis in Sterile Double Haploid Japanese Flounder

Sterility is a serious problem that can affect all bionts. In teleosts, double haploids (DHs) induced by mitogynogenesis are often sterile. This sterility severely restricts the further application of DHs for production of clones, genetic analysis, and breeding. However, sterile DH individuals are good source materials for investigation of the molecular mechanisms of gonad development, especially for studies into the role of genes that are indispensable for fish reproduction. Here, we used the Illumina sequencing platform to analyze the transcriptome of sterile female DH Japanese flounder in order to identify major genes that cause sterility and to provide a molecular basis for an intensive study of gonadal development in teleosts. Through sequencing, assembly, and annotation, we obtained 52,474 contigs and found that 60.7% of these shared homologies with existing sequences. A total of 1225 differentially expressed unigenes were found, including 492 upregulated and 733 downregulated genes. Gene Ontology and KEGG analyses showed that genes showing significant upregulation, such as CYP11A1, CYP11B2, CYP17, CYP21, HSD3β, bcl2l1, and PRLR, principally correlated with sterol metabolic process, steroid biosynthetic process, and the Jak-stat signaling pathway. The significantly downregulated genes were primarily associated with immune response, antigen processing and presentation, cytokine–cytokine receptor interaction, and protein digestion and absorption. Using a co-expression network analysis, we conducted a comprehensive comparison of gene expression in the gonads of fertile and sterile female DH Japanese flounder. Identification of genes showing significantly different expression will provide further insights into DH reproductive dysfunction and oocyte maturation processes in teleosts.

Prolactin 177, prolactin 188, and extracellular osmolality independently regulate the gene expression of ion transport effectors in gill of Mozambique tilapia

This study characterized the local effects of extracellular osmolality and prolactin (PRL) on branchial ionoregulatory function of a euryhaline teleost, Mozambique tilapia (Oreochromis mossambicus). First, gill filaments were dissected from freshwater (FW)-acclimated tilapia and incubated in four different osmolalities, 280, 330, 380, and 450 mosmol/kg H2O. The mRNA expression of Na(+)/K(+)-ATPase α1a (NKA α1a) and Na(+)/Cl(-) cotransporter (NCC) showed higher expression with decreasing media osmolalities, while Na(+)/K(+)/2Cl(-) cotransporter 1a (NKCC1a) and PRL receptor 2 (PRLR2) mRNA levels were upregulated by increases in media osmolality. We then incubated gill filaments in media containing ovine PRL (oPRL) and native tilapia PRLs (tPRL177 and tPRL188). oPRL and the two native tPRLs showed concentration-dependent effects on NCC, NKAα1a, and PRLR1 expression; Na(+)/H(+) exchanger 3 (NHE3) expression was increased by 24 h of incubation with tPRLs. Immunohistochemical observation showed that oPRL and both tPRLs maintained a high density of NCC- and NKA-immunoreactive ionocytes in cultured filaments. Furthermore, we found that tPRL177 and tPRL188 differentially induce expression of these ion transporters, according to incubation time. Together, these results provide evidence that ionocytes of Mozambique tilapia may function as osmoreceptors, as well as directly respond to PRL to modulate branchial ionoregulatory functions.

Prolactin and teleost ionocytes: new insights into cellular and molecular targets of prolactin in vertebrate epithelia

The peptide hormone prolactin is a functionally versatile hormone produced by the vertebrate pituitary. Comparative studies over the last six decades have revealed that a conserved function for prolactin across vertebrates is the regulation of ion and water transport in a variety of tissues including those responsible for whole-organism ion homeostasis. In teleost fishes, prolactin was identified as the "freshwater-adapting hormone", promoting ion-conserving and water-secreting processes by acting on the gill, kidney, gut and urinary bladder. In mammals, prolactin is known to regulate renal, intestinal, mammary and amniotic epithelia, with dysfunction linked to hypogonadism, infertility, and metabolic disorders. Until recently, our understanding of the cellular mechanisms of prolactin action in fishes has been hampered by a paucity of molecular tools to define and study ionocytes, specialized cells that control active ion transport across branchial and epidermal epithelia. Here we review work in teleost models indicating that prolactin regulates ion balance through action on ion transporters, tight-junction proteins, and water channels in ionocytes, and discuss recent advances in our understanding of ionocyte function in the genetically and embryonically accessible zebrafish (Danio rerio). Given the high degree of evolutionary conservation in endocrine and osmoregulatory systems, these studies in teleost models are contributing novel mechanistic insight into how prolactin participates in the development, function, and dysfunction of osmoregulatory systems across the vertebrate lineage.

The role of prolactin in fish reproduction

Prolactin (PRL) has one of the broadest ranges of functions of any vertebrate hormone, and plays a critical role in regulating aspects of reproduction in widely divergent lineages. However, while PRL structure, mode of action and functions have been well-characterised in mammals, studies of other vertebrate lineages remain incomplete. As the most diverse group of vertebrates, fish offer a particularly valuable model system for the study of the evolution of reproductive endocrine function. Here, we review the current state of knowledge on the role of prolactin in fish reproduction, which extends to migration, reproductive development and cycling, brood care behaviour, pregnancy, and nutrient provisioning to young. We also highlight significant gaps in knowledge and advocate a specific bidirectional research methodology including both observational and manipulative experiments. Focusing research efforts towards the thorough characterisation of a restricted number of reproductively diverse fish models will help to provide the foundation necessary for a more explicitly evolutionary analysis of PRL function.

Prolactin regulates transcription of the ion uptake Na+/Cl- cotransporter (ncc) gene in zebrafish gill

Prolactin (PRL) is a well-known regulator of ion and water transport within osmoregulatory tissues across vertebrate species, yet how PRL acts on some of its target tissues remains poorly understood. Using zebrafish as a model, we show that ionocytes in the gill directly respond to systemic PRL to regulate mechanisms of ion uptake. Ion-poor conditions led to increases in the expression of PRL receptor (prlra), Na(+)/Cl(-) cotransporter (ncc; slc12a10.2), Na(+)/H(+) exchanger (nhe3b; slc9a3.2), and epithelial Ca(2+) channel (ecac; trpv6) transcripts within the gill. Intraperitoneal injection of ovine PRL (oPRL) increased ncc and prlra transcripts, but did not affect nhe3b or ecac. Consistent with direct PRL action in the gill, addition of oPRL to cultured gill filaments stimulated ncc in a concentration-dependent manner, an effect blocked by a pure human PRL receptor antagonist (Δ1-9-G129R-hPRL). These results suggest that PRL signaling through PRL receptors in the gill regulates the expression of ncc, thereby linking this pituitary hormone with an effector of Cl(-) uptake in zebrafish for the first time.

Regulation of epithelial calcium transport by prolactin: from fish to mammals

Among the reported ∼300 biological actions, the established role of prolactin (PRL) is to act as a vertebrate hypercalcemic hormone that regulates epithelial calcium transport in several organs, such as the gills, intestine, and kidney. In fish, PRL stimulates the branchial calcium transport by increasing the activity of Ca(2+)-ATPase. Although this calciotropic hormone also induces hypercalcemia in amphibians, reptiles and birds, little has been known regarding the underlying mechanism. In contrast, the effects of PRL on the epithelial calcium transport in mammals are well documented. In rodents, PRL has been shown to stimulate the renal tubular calcium reabsorption and intestinal calcium absorption, the latter of which is mediated by the PRL-induced upregulation of calcium transporter gene expression and activities. Recently, we demonstrated that the duodenal calcium absorption in lactating rats was markedly enhanced by the suckling-induced PRL surge, presumably to provide calcium for milk production. The cellular and molecular mechanisms of the PRL-stimulated calcium transport in mammals have been elaborated in this review.

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.

Prolactin receptor mRNA is upregulated in discus fish (Symphysodon aequifasciata) skin during parental phase

Prolactin (PRL) has been shown to directly influence parental-care associated behavior in many vertebrate species. The discus fish (Symphysodon aequifasciata) displays extensive parental care behavior through utilization of epidermal mucosal secretion to raise free-swimming fry. Here, we cloned the full-length cDNA sequence of the S. aequifasciata prolactin receptor (dfPRLR) and investigated the mRNA expression pattern in several adult tissues. Bioinformatic analysis showed the dfPRLR shared rather high identity (79 and 67%) with the Nile tilapia PRLR 1 and black seabream PRLR 1, respectively. The presence of dfPRLR in several osmoregulatory tissues including kidney, gill and intestine is consistent with the known role of PRL in mediating hydromineral balance in teleosts. In addition, upregulated expression of PRLR mRNA was observed in skin of parental fish compared to non-parental fish, indicating possibility of a role of the PRL hormonal signaling in regulation of mucus production in relation to parental care behaviour.

Developmental ontogeny of prolactin and its receptor in fish

Prolactin (PRL) is a member of a family of structurally similar proteins which includes growth hormone (GH) and somatolactin (SL) in teleost fish. The genes encoding these proteins are expressed principally in the pituitary gland and sequence analysis reveals they share considerable similarity. GH, PRL, and SL bring about their physiological action by binding to specific receptors localised in the membrane of cells in target tissue. The PRL receptor (PRLR) and GH receptor (GHR) have been identified in a number of teleosts but the SL receptor remains to be characterised. On hormone binding, receptors dimerise, and signal transduction occurs via the JAK/STAT signalling pathway. The principal action of PRL in fish is freshwater osmoregulation, although it has also been implicated in reproduction, behaviour, growth, and immunoregulation. The role of PRL in early development and metamorphosis is well established, respectively, in mammals and amphibians, although its role in fish is not so well known. Studies have shown that PRL mRNA and protein are restricted to the developing pituitary gland in fish embryos and larvae. PRLR mRNA and protein is also present in fish embryos and has a widespread tissue distribution in larvae. The levels of PRLR and PRL mRNA vary throughout embryonic and early larval development. The potential role of PRL in fish embryos and larvae is considered in relation to their physiological status.

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.

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.

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.

Purification, characterization, and bioassay of prolactin and growth hormone from temperate basses, genus Morone

Prolactin (PRL) and two variants of growth hormone (GH), purified from pituitaries of striped bass (Morone saxatilis) and its hybrid with white bass (M. saxatilis x M. chrysops) by gel filtration chromatography under alkaline conditions followed by reversed-phase high pressure liquid chromatography, appear similar between species. Both the minor (GH I) and the major (GH II) forms of purified GH appeared as single bands (M(r) approximately 23,000) after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, as did the purified PRL (M(r) approximately 24,000). The molecular weights of GH II and PRL determined by MALDI TOF mass spectroscopy were 21.2 and 21.3 kDa, respectively. In Western blotting experiments, an antiserum against tilapia (Oreochromis mossambicus) 24K PRL specifically recognized Morone PRL, while an antiserum against tilapia GH specifically recognized Morone GH I and II. Chemical identities of the putative PRL and GH I were further confirmed by N-terminal peptide sequencing, while internal sequence analysis was performed on GH II because it was blocked at its N-terminus. Over a stretch of 29 amino acids, Morone PRL was found to be 76% identical to tilapia 24K PRL, 72% identical to tilapia 20K PRL, 72% identical to chum salmon (Oncorhynchus keta) PRL I, and 69% identical to eel (Anguilla japonica) PRL I. Alignment of the hybrid striped bass GH sequences with those of several other advanced marine teleosts indicated 75-85% sequence identity for GH I (40 amino acids) and 95-98% identity for GH II (45 amino acids). Biological activity of striped bass GH II was confirmed using a heterologous in vitro assay of insulin-like growth factor I mRNA production by coho salmon (On. kisutch) hepatocytes. An in vivo bioassay, involving hypophysectomy of hybrid striped bass and treatment of the fish maintained in fresh water with homologous PRL, confirmed that the purified striped bass PRL was also bioactive.

Effect of egg deprivation on sex steroids, gonadotropin, prolactin, and growth hormone profiles during the reproductive cycle of the mouthbrooding cichlid fish Oreochromis niloticus

Various hormones were analyzed during the course of a reproductive cycle in the cichlid fish Oreochromis niloticus: plasma levels of the gonadal steroids 17beta-estradiol (E2), testosterone (T), 17, 20beta-OH progesterone (17,20beta-P), gonadotropin (taGtH), and plasma and pituitary concentrations of prolactin (tiPRL(I) and tiPRL(II)) and growth hormone (tiGH). Two categories of fish were sampled and sacrificed on days 1 and 3 postspawning and at 3-day intervals thereafter: typical incubating females (INC), and nonincubating females (NI), deprived of their eggs just after spawning. Such deprivation is known to suppress maternal behavior and to accelerate ovarian development and especially vitellogenesis, thus shortening the mean interspawning interval. In both groups, variations of the plasma concentrations of E2 and T appeared to depend on ovarian stages, and differences between groups appeared to reflect underlying differences in the kinetics of ovarian development. The observation of noticeable levels of 17,20beta-P in plasma before spawning, when high values of taGtH could also be detected in NI females, suggests the implication of this progestin in the control of final maturation events, as in some other teleosts. Moreover, 17,20beta-P, which was still detected a few days after spawning, but at low concentrations and only in the plasma of INC females, might play a role at the beginning of the reproductive cycle in incubating females (maternal behavior and/or slowing down of ovarian growth). The pituitary and plasma profiles of both tiPRLs isoforms appeared to depend mainly on the kinetics of ovarian development in each group of fish, suggesting a role during the beginning of vitellogenesis. However, the variance of plasma tiPRL(II), which was significantly enhanced during maternal behavior in INC females, also suggests an implication of this hormone in the control of that behavior. Concerning tiGH, comparison of the plasma profiles in INC and NI fish also suggest an influence on the control of maternal behavior, but a main effect of starvation of INC during mouthbrooding cannot be excluded.

Expression cloning of a cDNA encoding a fish prolactin receptor

By using an expression cloning strategy, we isolated a single positive clone encoding a tilapia prolactin (PRL) receptor. Tilapia PRL188 was used to screen a freshwater tilapia kidney expression library transfected in COS cells. The tilapia PRL receptor is a mature protein of 606 amino acids. The extracellular domain is devoid of the tandem repeat units present in birds and has two pairs of cysteine residues, a Trp-Ser-Xaa-Trp-Ser motif, and two potential N-glycosylation sites. The cytoplasmic domain contains 372 amino acids, including box 1, a sequence previously shown to be important for signal transduction in mammalian species. Thus, the general structure is similar to the long form of mammalian PRL receptors; however, amino acid comparisons reveal a rather low identity (approximately 37%). Northern blot analysis shows the existence of a single transcript in osmoregulatory tissues and reproductive organs. This localization is in agreement with known functions of PRL in teleosts.

In vitro effects of homologous prolactins on testosterone production by testes of tilapia (Oreochromis mossambicus)

The tilapia, Oreochromis mossambicus, produces two prolactins designated tPRL177 and tPRL188 to indicate the number of amino acid residues in each. The direct in vitro actions of tPRL177 and tPRL188 on basal and ovine luteinizing hormone (LH)-induced testosterone production in minced testes of courting and noncourting (bachelor) tilapia were examined. Courting males were housed with females and were used as soon as they built a spawning pit; noncourting males were housed with other males and were used when they appeared female-like in coloration. The in vitro culture system consisted of two phases. In phase 1 (0-10 hr), testicular tissue was incubated without (control) or with the tPRLs--both alone and together and in doses ranging from 0.5 to 32 microM. In phase 2 (10-16 hr), the culture medium was replaced with medium containing 6.25 microM (0.25 micrograms/ml) ovine LH. Testosterone production during both phases was determined by radioimmunoassay of the medium. The tPRLs stimulated testosterone production during phase 1 in courting males (by 25%) but not in noncourting males. Exposure to either or both tPRLs in phase 1 enhanced the response of testicular tissue from courting males to ovine LH (by 10%) and inhibited the response of minced testes taken from noncourting males (by 12%). This is the first evidence to demonstrate direct gonadal activity of homologous PRL in a teleost.

Growth hormone binding sites in tilapia (Oreochromis mossambicus) liver

125I-labeled bovine and tilapia growth hormones were used to assess the presence of growth hormone receptors in membranes prepared from tissues of the tilapia Oreochromis mossambicus. The highest level of specific binding was detected in liver membranes from animals of both sexes and the binding was protein-dependent. Tilapia growth hormone, bovine growth hormone, and ovine prolactin, but not tilapia prolactin, potently inhibited the hepatic binding of 125I-labeled bovine growth hormone. Scatchard analysis of the 125I-labeled bovine growth hormone binding data revealed a Bmax (maximum binding) value of 180 fmol/mg protein and a Kd (dissociation constant) value of 13 nM. Tilapia growth hormone potently inhibited hepatic binding of 125I-labeled tilapia growth hormone. Scatchard analysis revealed a single class of binding sites with Bmax and Kd values of 390 fmol/mg protein and 2.5 nM, respectively. Bovine growth hormone and ovine prolactin were less potent while tilapia prolactin was inactive in inhibiting hepatic 125I-labeled tilapia growth hormone binding.

Immunocytochemical identification of growth hormone, prolactin, and gonadotropin cells in the pituitary of male plaice (Pleuronectes platessa) during gonadal maturation

Somatotrophs, gonadotrophs, and prolactin (PRL) cells have been demonstrated in male plaice (Pleuronectes platessa) pituitary by immunocytochemistry. All cell types exhibited patterns of activity correlated with gonadal maturity. Immature and maturing male plaice (gonads very small or filling with sperm, stages I and III) had numerous densely staining somatotrophs and PRL cells but only a few weakly stained gonadotrophs. Mature plaice (fish in which sperm could be extruded under light pressure, stage VI) contained two populations of gonadotrophs. The PRL cells of these fish were densely stained and the somatotrophs more lightly stained. Spent plaice (thin, flabby gonads, stage VII) had little or no staining in PRL cells. Somatotrophs and gonadotrophs from these fish stained weakly with the antisera used in the study and, in some fish, vacuoles were observed in areas of the pituitary normally occupied by gonadotrophs.

Presence of prolactin receptors in eel liver and carp kidney and growth hormone receptor in eel liver

1. 125I-labelled ovine prolactin and bovine growth hormone were used to test for the presence of prolactin and growth hormone receptors in membrane prepared from tissues of the white eel Anguilla japonica, the carp Ctenopharynogodon idellus and the ricefield eel Monopterus albus. 2. High levels of specific 125I-labelled ovine prolactin binding were found in white eel liver membranes and carp kidney membranes. 3. High levels of specific 125I-labelled bovine growth hormone binding were detected in white eel liver membranes. 4. Tissues of the ricefield eel did not bind 125I-labelled ovine prolactin or bovine growth hormone. 5. The results suggest the presence of prolactin receptors in white eel liver and carp kidney membranes and growth hormone receptors in white eel liver membranes.

Presence of specific growth hormone binding sites in rainbow trout (Oncorhynchus mykiss) tissues: characterization of the hepatic receptor

The present work outlines the presence of specific binding for chinook salmon growth hormone (sGH) in different tissue preparations of rainbow trout. Optimal incubation conditions (pH, Tris, MgCl2) were determined. Specific binding was very sensitive to salt concentration during incubation. The specific binding reached a plateau after 15 and 25 hr of incubation at 12 and 4 degrees. At 20 degrees, specific and nonspecific binding were not stable. Specific binding dissociation was slower than association and was only partial. The binding was saturable (Bmax = 187 +/- 167 pmol), of high affinity (Ka = 2.4 +/- 0.8 10(9) M-1), and very specific for GH, properties which are in agreement with the characteristics of hormonal receptors. Sea bream and mammalian GH appeared 2- and 30-fold, respectively, less potent than cold sGH2 for displacing 125I-sGH2. Tissue preparations from ovary, testis, fat, skin, cartilage, gill, blood pellet, brain, spleen, kidney, and muscle showed significant saturable binding.

Presence of prolactin receptors in kidney and large intestine of the snake Ptyas mucosa

125I-labeled ovine prolactin was used to test for the presence of prolactin receptors in membranes prepared from tissues of the common rat snake Ptyas mucosa. High levels of specific binding were found in kidney and large intestine membranes prepared from snakes of both sexes. The binding was time, temperature, and protein dependent. The presence of Ca2+ in the incubation medium enhanced the binding, with optimal concentrations at 10-20 mM. Specificity of binding was established by employing different hormones as the displacing species in the radioreceptor assay. The results suggested the presence of lactogenic receptors in snake kidney and large intestine membranes. Scatchard analysis of the binding data indicated the presence of a single class of binding sites in snake kidney membranes with a dissociation constant of 0.83 nM. The present study is the first report of the presence of prolactin receptors in snake kidney and large intestine membranes.

Effect of hypophysectomy, replacement therapy with ovine prolactin, and cortisol and triiodothyronine treatment on prolactin receptors of the tilapia (Oreochromis mossambicus)

The effects of hypophysectomy and subsequent replacement therapy with ovine prolactin (oPRL) on specific binding of 125I-oPRL to gill, kidney, and liver membranes of male tilapia were examined. The possible control of prolactin receptors by cortisol (F) and triiodothyronine (T3) was also studied using intact animals. In gill and kidney, hypophysectomy resulted in a significant decrease in specific binding that was partially restored (threefold increase) by three injections of oPRL, suggesting a role of the pituitary in the control of prolactin receptors. However, removal of the pituitary and replacement therapy with oPRL had no effect on binding by liver membranes. Cortisol and T3 treatment, alone or in combination, did not significantly affect prolactin binding by any of the tissues tested.

Prolactin receptors in liver, kidney, and gill of the tilapia (Oreochromis mossambicus): characterization and effect of salinity on specific binding of iodinated ovine prolactin

Specific binding of 125I-ovine prolactin (oPRL) to microsomal fractions from gill, kidney, and liver of adult tilapia was determined. Specific binding varied among tissues, the highest values being displayed by kidney membranes. In the liver, the binding of oPRL was not strongly displaced by tilapia prolactins (tPRL177 and tPRL188), although tPRL177 was six times more potent than tPRL188. On the other hand, in kidney and gill membranes, the two tPRLs were equipotent. Tilapia PRLs showed low potency in competing for oPRL-binding sites when pregnant rat liver membranes were utilized. Tilapia growth hormone (tGH) and human growth hormone (hGH) displaced 125I-oPRL from liver as well as did tPRL177 but were not recognized well by renal or branchial receptors. Two 125I-oPRL-binding sites were detected in every tissue tested. These binding sites are subject to physiological regulation since adaptation to seawater resulted in a significant decrease in specific binding.

Further study on the changes in the concentration of prolactin-binding sites in different organs of Xenopus laevis male and female, kept under dry conditions and then returned to water (their natural habitat)

The binding of 125I-labeled ovine prolactin (125I-oPRL) to membranes from the kidney and liver of Xenopus laevis male and female specimens (Experiment I) and from the kidney, epidermis, and liver of female specimens (Experiment II) (1) kept in an aquatic environment, (2) exposed for 2 weeks to dehydrating conditions, and (3) then placed back into their aquaria after exposure to dehydrating conditions (Experiment II) was studied. No significant sex differences in PRL binding to kidney, epidermis, and liver were found. A highly significant drop in PRL specific binding to the membranes from the kidney and epidermis is brought about in the specimens from both sexes exposed to dehydrating conditions. The results obtained by MgCl2 treatment in vitro of the membranes under study for an estimation of total PRL receptor concentrations seem to point to an actual decrease in PRL specific binding sites. The values of PRL specific binding to the membranes from the liver are not affected by dehydration of the animals (Experiment I and II) or their subsequent rehydration (Experiment II). In rehydrated females (Experiment II), PRL binding values were closely related to those recorded in females permanently maintained in water (controls).

Purification of PRL receptors from toad kidney: comparisons with rabbit mammary PRL receptors

The binding characteristics of the prolactin (PRL) receptors present in toad (Bufo marinus) kidneys were investigated and compared to those of PRL receptors present in rabbit mammary glands. The molecular characteristics of the Triton X-100 solubilized renal and mammary PRL receptors were assessed by gel filtration and by migration analysis on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) after affinity labeling of the binding sites with 125I-human growth hormone. Similar results were obtained for both receptors. Partial purification of the toad PRL receptor could be achieved by affinity chromatography. The molecular weight of this purified receptor could be determined by analysis on SDS-PAGE. With the use of a polyclonal antiserum raised against a purified preparation of rabbit mammary PRL receptor, one or several antigenic epitope(s) could be identified on the core of the toad renal PRL receptor. In conclusion, although the structure and the biological role(s) of PRL have substantially changed during evolution, the receptor for this hormone has retained many of its structural features as could be assessed between an amphibian and a mammalian species on functionally different target tissues.

The localization of labelled fish prolactin in various tissues of rainbow trout (Salmo gairdneri)

Rostral lobes of the pars distalis from rainbow trout, Salmo gairdneri, were incubated in vitro in a medium containing 14C-labelled lysine. The labelled proteins in these lobes and medium were separated by polyacrylamide gel electrophoresis, the prolactin eluted from the appropriate band, and injected into intact trout. Following the injections, various tissues were dissected out and observed autoradiographically. There was no binding of labelled prolactin to tissues of pituitary gland, thyroid, pyloric caecum, stomach, pancreas or muscle. There was, however, significant labelling in liver, intestine, kidney, bladder, skin and gill. The binding of labelled chum salmon prolactin to these latter tissues in vitro was significantly reduced when unlabelled hormone was also added.

Characterization of renal prolactin-binding sites of two amphibians (Ambystoma tigrinum and Rana catesbeiana) and a reptile (Pseudemys scripta elegans)

The specific binding of 125I-ovine prolactin (oPRL) to renal membrane preparations from Pseudemys scripta elegans, Ambystoma tigrinum, and Rana catesbeiana was characterized. All three membranes showed specific oPRL binding that was dependent upon time, temperature, pH, and membrane concentration. Scatchard analysis of the binding of 125I-oPRL revealed high-affinity sites with KA values of 2.1 X 10(10), 3.6 X 10(10), and 1.25 X 10(10) M-1 for kidney membranes of Pseudemys, Ambystoma, and Rana, respectively. In addition, there was a low-affinity site on the Ambystoma membranes. The binding capacities ranged from 31 to 70 fmol/mg of membrane protein. The hormonal specificity of these membranes was studied by competing increasing amounts of oPRL, human growth hormone (hGH) derived from recombinant DNA techniques, bovine (b) GH, and human placental lactogen (hPL) with 125I-oPRL. The oPRL standard and hGH were the most potent competitors in all three assays, although hGH was not as potent as the oPRL. Human PL was moderately active in the turtle kidney assay, weakly active in the Ambystoma radioreceptor assay, and inactive in the bullfrog assay. Bovine GH had low potency on the turtle membranes and was inactive in the amphibian assays. The results of these studies indicate that the characteristics of renal PRL receptors of Pseudemys, Ambystoma, and Rana are similar to those of lactogenic receptors throughout the vertebrates. In addition, these data provide evidence for the first time of renal PRL receptors in a reptile.

Prolactin binding sites in Xenopus laevis tissues: comparison between normal and dehydrated animals

The binding of 125I-labeled ovine prolactin (125I-oPRL) to membranes from the kidney, epidermis, liver, and testis of Xenopus laevis adult specimens either kept in an aquatic environment or exposed for 2 weeks to dehydrating conditions was studied. Prolactin binding specificity was assayed through competition with several unlabeled hormones (oPRL, hGH, rGH, rLH, and porcine insulin). In the animal exposed to dehydrating conditions a statistically highly significant reduction in prolactin binding to the membranes from the kidney and epidermis was recorded. No significant variations were revealed by the membranes from the liver and testis. The reduction detected in the binding of 125I-oPRL is not related to the dissociation constant, but to the number of PRL binding sites. Since PRL ranks among the few peptide hormones whose rise in the bloodstream promotes an increase in the number of their own receptors, the reduction of its binding sites in Xenopus specimens exposed to dehydration might lend some support to our earlier hypothesis that transfer to a dehydrating environment may bring about, in this totally aquatic species, some decrease in the blood PRL levels.

In vitro effects of somatostatin and urotensin II on prolactin and growth hormone secretion in tilapia, Oreochromis mossambicus

The control of release of two recently characterized forms of prolactin (PRL) of molecular mass 24 and 20 kDa was investigated. The rostral pars distalis of male tilapia was incubated singly in a hypotonic modified Krebs-Ringer bicarbonate medium in order to stimulate PRL release; for comparison, the proximal pars distalis containing growth hormone (GH) cells was incubated in isotonic medium with or without 1 microgram/ml cortisol to stimulate GH release. The release of both PRLs and GH into the medium was measured by sodium dodecyl sulfate (SDS)--polyacrylamide gel electrophoresis followed by densitometry. Both somatostatin and synthetic (Gillichthys) urotensin II, a partial somatostatin homolog and analog from the teleost caudal neurosecretory system, significantly inhibited the release of both PRLs. Somatostatin significantly inhibited GH release, but urotensin II had no significant effect.

Plasma and pituitary prolactin levels in rainbow trout during adaptation to different salinities

The development of a highly specific radioimmunoassay for salmonid prolactin (PRL) using chinook salmon PRL allowed us to study plasma and pituitary PRL profiles in large sedentary rainbow trout (Salmo gairdneri) transferred from fresh water to seawater and vice versa. Plasma osmotic pressure and chloride levels were also measured for 3 weeks following change of salinity. Within 1 day after transfer to full seawater we observed a plasma PRL decrease, which stayed significantly lower (3-5 ng/ml) than the fresh water control group (10-15 ng/ml) during the entire experiment. Pituitary PRL content showed an initial abrupt increase, but after 3 weeks in seawater pituitary PRL content had decreased to the same level as in the fresh water control group. On the contrary, transfer from seawater to fresh water was followed within 1 day by a rise in plasma PRL levels, which stayed high (10-15 ng/ml) after 3 weeks in fresh water. Simultaneously, pituitary PRL content decreased significantly. These results may indicate an important role of PRL in fresh water adaptation of sedentary rainbow trout.

Development and validation of a salmon prolactin radioimmunoassay

A highly specific radioimmunoassay (RIA) for the measurement of prolactin (PRL) in the plasma and pituitary of salmonid fishes was developed using a rabbit antiserum to chinook salmon (Oncorhynchus tschawytscha) PRL. The PRLs purified from chinook salmon and chum salmon (O. keta) pituitaries showed exactly the same competitive inhibition curves in the RIA, regardless of iodination of either hormone. The displacement curves for pituitary extracts and plasma from several salmonids, including chum, coho, and amago salmon, rainbow trout, and Japanese charr, were parallel to the salmon PRL standard, whereas those from the eel, goldfish, carp, and tilapia showed negligible cross-reactivity. Negligible cross-reactivity was also seen with plasma from hypophysectomized rainbow trout or coho salmon. None of the mammalian PRL or growth hormone (GH) preparations, bullfrog PRL, or presumptive chum salmon "gonadotropin" and eel "PRL" cross-reacted in the PRL RIA. Presumptive chum salmon GH showed less than 0.05% cross-reactivity. The RIA sensitivity was less than 0.1 ng of the salmon PRL standard per milliliter. The immunoreactive plasma PRL levels in mature chum salmon were below 1 ng/ml in seawater. The plasma PRL in females increased to about 8 ng/ml 1 day after transfer to fresh water, and high levels (2-4 ng/ml) were maintained during 3-7 days after the transfer. In contrast, when males were transferred to fresh water, an increase in plasma PRL was seen only 1 day after the transfer. A significant decrease in plasma osmolality was observed in both males and females after transfer to fresh water. No change was observed either in plasma PRL or osmolality, when fish were transferred from seawater to seawater.