Hormonal modulation of branchial Na+-K+-ATPase subunit mRNA in a marine teleost Sparus sarba

Extra-pituitary prolactin (PRL) and prolactin-like protein (PRL-L) in chickens and zebrafish

It is generally believed that in vertebrates, prolactin (PRL) is predominantly synthesized and released by pituitary lactotrophs and plays important roles in many physiological processes via activation of PRL receptor (PRLR), including water and electrolyte balance, reproduction, growth and development, metabolism, immuno-modulation, and behavior. However, there is increasing evidence showing that PRL and the newly identified 'prolactin-like protein (PRL-L)', a novel ligand of PRL receptor, are also expressed in a variety of extra-pituitary tissues, such as the brain, skin, ovary, and testes in non-mammalian vertebrates. In this brief review, we summarize the recent research progress on the structure, biological activities, and extra-pituitary expression of PRL and PRL-L in chickens (Gallus gallus) and zebrafish (Danio rerio) from our and other laboratories and briefly discuss their potential paracrine/autocrine roles in non-mammalian vertebrates, which may promote us to rethink the broad spectrum of PRL actions previously attributed to pituitary PRL only.

The relationship between adrenocortical function and Hsp70 expression in socially isolated Japanese quail

Physiological responses to social isolation stress were compared in 56-day-old male Japanese quail. Birds were fed pretreated diets for 3 days as follows: (i) Basal diet (control); (ii) Basal diet+1500 mg/kg metyrapone (BM); (iii) Basal diet+30 mg/kg corticosterone (BCO); (iv) Basal diet+250 mg/kg ascorbic acid (BC); (v) Basal diet+250 mg/kg α-tocopherol (BE); (vi) Basal diet+250 mg/kg ascorbic acid and 250 mg/kg α-tocopherol (BCE). The birds were subsequently socially isolated in individual opaque brown paper box for 2 hours. Plasma corticosterone (CORT) concentration and heart and brain heat shock protein 70 (Hsp 70) expressions were determined before stress and immediately after stress. Two hours of isolation stress elevated CORT concentration significantly in the control and BE birds but not in the BC, BCE and BM birds. There was a significant reduction in CORT concentration after isolation stress in the BCO group. Isolation stress increased Hsp 70 expression in the brain and heart of control and BM birds. However, brain and heart Hsp 70 expressions were not significantly altered in the isolated BC, BCE and BE birds. Although, the CORT concentration of BM birds was not affected by isolation stress, Hsp70 expression in both brain and heart were significantly increased. Moreover, exogenous corticosterone supplementation did not result in elevation of Hsp 70 expression. It can be concluded that, although Hsp 70 induction had not been directly affected by CORT concentration, it may be modulated by the HPA axis function via activation of ACTH.

Aquaporin 1a expression in gill, intestine, and kidney of the euryhaline silver sea bream

This study aimed to investigate the effects of chronic salinity acclimation, abrupt salinity transfer, and cortisol administration on aquaporin 1 (AQP1) expression in gill, intestine, and kidney of silver sea bream (Sparus sarba). An AQP1a cDNA was cloned and found to share 83-96% amino acid sequence identity with AQP1 genes from several fish species. Tissue distribution studies of AQP1a mRNA demonstrated that it was expressed in gill, liver, intestine, rectum, kidney, heart, urinary bladder, and whole blood. Semi-quantitative RT-PCR analysis was used to measure AQP1a transcript abundance in sea bream that were acclimated to salinity conditions of 0, 6, 12, 33, 50, and 70 ppt for 1 month. The abundance of gill AQP1a transcript was highest in sea bream acclimated to 0 ppt whereas no differences were found among 0-50 ppt groups. For intestine, the highest AQP1a transcript amounts were found in sea bream acclimated to 12 and 70 ppt whereas the transcript abundance of kidney AQP1a was found to be unchanged amongst the different salinity groups. To investigate the effects of acute salinity alterations on AQP1a expression, sea bream were abruptly transferred from 33 to 6 ppt. For intestine AQP1a levels were altered at different times, post transfer, but remained unchanged in gill and kidney. To study the effects of cortisol on AQP1a expression, sea bream were administered a single dose of cortisol followed by a 3-day acclimation to either 33 or 6 ppt. The findings from this experiment demonstrated that cortisol administration resulted in alterations of AQP1a transcript in gill and intestine but not in kidney.

Salinity adaptation and gene profiling analysis in the European eel (Anguilla anguilla) using microarray technology

The life cycle of the European eel (Anguilla anguilla) includes two long migratory periods, when the newly hatched leptocephali larvae drift on ocean currents from the Sargasso Sea to the shores of Western Europe and then again up to 30 years later when adult eels swim back to their place of birth for reproductive purposes. Prior to the migration from fresh water (FW) to sea water (SW) adult yellow eels undergo various anatomical and physiological adaptations (silvering) which promote sexual development and aid the transition to increased environmental salinities. The aim of this study was to identify and characterise changes in gene expression within the major osmoregulatory tissues of the eel which enable these fish to make the physiological adaptations required for transfer to SW environments. In particular, changes in the expression of the FW-adapting hormone prolactin were correlated with differential expression of known osmoregulatory important genes within the gill, intestine and kidney following the acclimation of eels to SW. Various tissues were sampled from individual fish at selected intervals over a 5-month period following FW/SW transfer and RNA was isolated. Suppressive subtractive hybridization (SSH) was used for enrichment of differentially expressed genes. Microarrays comprising 6144 cDNAs spotted in triplicate, from brain, gill, intestine and kidney libraries (1536 randomly selected clones per tissue library), were hybridized with appropriate targets and analysed. Microarray results were validated using known genes implicated in osmoregulation, such as prolactin, growth hormone, Na, K-ATPase and some unknown genes, the role of which in osmoregulation needs to be elucidated.

Prolactin directly enhanced Na+/K+- and Ca2+-ATPase activities in the duodenum of female rats

Prolactin has recently been shown to directly stimulate 2 components of the active duodenal calcium transport in female rats, i.e., solvent drag-induced and transcellular-active calcium transport. Since the basolateral Na+/K+- and Ca2+-ATPases, respectively, play important roles in these 2 transport mechanisms, the present study aimed to examine the direct actions of prolactin on the activities of both transporters in sexually mature female Wistar rats. The results showed that 200, 400, and 800 ng/mL prolactin produced a significant increase in the total ATPase activity of duodenal crude homogenate in a dose-dependent manner within 60 min (i.e., from a control value of 1.53 ± 0.13 to 2.29 ± 0.21 (p < 0.05), 2.68 ± 0.19 (p < 0.01), and 3.92 ± 0.33 (p < 0.001) µmol Pi·(mg protein)–1·min–1, respectively). Activity of Na+/K+-ATPase was increased by 800 ng/mL prolactin from 0.17 ± 0.03 to 1.18 ± 0.29 µmol Pi·(mg protein)–1·min–1 (p < 0.01). Prolactin at doses of 400 and 600 ng/mL also significantly increased the activities of Ca2+-ATPase in crude homogenate from a control value of 0.84 ± 0.03 to 1.75 ± 0.29 (p < 0.05), and 2.30 ± 0.37 (p < 0.001) µmol Pi·(mg protein)–1·min–1. When the crude homogenate was purified for the basolateral membrane, the Na+/K+-ATPase activities were elevated 10-fold. In the purified homogenate, 800 ng/mL prolactin increased Na+/K+-ATPase activity from 1.79 ± 0.38 to 2.63 ± 0.44 µmol Pi·(mg protein)–1·min–1 (p < 0.05), and Ca2+-ATPase activity from 0.08 ± 0.14 to 2.03 ± 0.23 µmol Pi·(mg protein)–1·min–1 (p < 0.001). Because the apical calcium entry was the first important step for the transcellular active calcium transport, the brush border calcium uptake was also investigated in this study. We found that, 8 min after being directly exposed to 800 ng/mL prolactin, the brush border calcium uptake into the duodenal epithelial cells was increased from 0.31 ± 0.02 to 0.80 ± 0.28 nmol·(mg protein)–1 (p < 0.05). It was concluded that prolactin directly and rapidly enhanced the brush border calcium uptake as well as the activities of the basolateral Na+/K+- and Ca2+-ATPases in the duodenal epithelium of female rats. These findings explained the mechanisms by which prolactin stimulated duodenal active calcium absorption.

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.

Differential effects of thyroxine on metabolic enzymes and other macromolecules in a freshwater teleost

The effects of thyroxine (T(4)) on citrate synthase (CS), glucose 6-phosphate dehydrogenase (G6-PDH), lactate dehydrogenase (LDH), DNA, RNA, and protein of various tissues were studied to elucidate the hormonal control of metabolism in a freshwater catfish, Clarias batrachus. T(4) did not produce any significant effect on DNA content of the fish. The CS, RNA, and protein contents of brain, liver, and skeletal muscle of the fish exposed to thiourea for 28 days decreased approximately 50-58% as compared to their levels in control individuals. Injection of T(4) to thiourea-exposed fish produced about three-fold increases in CS, RNA, and protein. These macromolecular inductions by T(4) were blocked by actinomycin D or cycloheximide. This suggests T(4)-induced de novo synthesis of macromolecules and stimulation of aerobic capacity. However, the activities of G6-PDH and LDH of brain, liver, and skeletal muscle of the fish exposed to thiourea increased two times that of the activities in control individuals. Administration of T(4) to thiourea-exposed fish reduced LDH and G6-PDH activities by about 64-74%, which reflects T(4)-dependent inhibition in anaerobic power and selective anabolic activities of the HMP pathway. These differential effects of T(4) on some metabolic enzymes and other important macromolecules may be to meet the other T(4)-induced responses in the freshwater catfish.

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.

Production of recombinant goldfish prolactin and its applications in radioreceptor binding assay and radioimmunoassay

Goldfish prolactin cDNA was subcloned into a pRSET A vector and expressed in Escherichia coli. Recombinant goldfish prolactin was expressed mainly as insoluble inclusion bodies in the form of N-terminal 6x His-tagged fusion protein. This fusion protein was purified, refolded, and (125)I-labeled to generate a radioligand for receptor binding and validation of a radioimmunoassay for goldfish prolactin. Using goldfish gill membrane as the substrate for prolactin receptor binding, both recombinant and native forms of goldfish prolactin were effective in displacing the specific binding of the radioligand in a similar dose range, suggesting that the fusion protein was refolded properly and could be recognized by goldfish prolactin receptors. To quantify prolactin contents in biological samples from the goldfish, a radioimmunoassay using the (125)I-labeled recombinant prolactin as a tracer was established. This assay was shown to be selective for goldfish prolactin without cross-reactivity with mammalian prolactin and pituitary hormones from other fish species (e.g., growth hormone and gonadotropin II). This newly validated assay system was used to investigate neuroendocrine and signal transduction mechanisms regulating prolactin release in the goldfish. In this case, the Ca(2+) ionophore A23187 and protein kinase C activator TPA were effective in elevating basal levels of prolactin secretion in perifused goldfish pituitary cells. In parallel studies using a static incubation approach, somatostatin and dopamine, but not vasoactive intestinal polypeptide, were inhibitory to basal prolactin release in goldfish pituitary cells. These results suggest that somatostatin and dopamine may serve as negative regulators of basal prolactin secretion and that extracellular Ca(2+) influx and protein kinase C activation may be important signaling events mediating prolactin release in the goldfish.

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.

Hormonal status and phagocytic activity in sea bream infected with vibriosis

Serum taken from female, sexually mature, silver sea bream (Sparus sarba) displaying either no symptoms of vibriosis, mild infection, severe infection or moribundity were assayed for a number of key hormones. Serum cortisol levels were not significantly different among symptomless, mildly- and severely-infected groups, whereas moribund fish displayed hypercortisolemia with a 14-fold increase in serum cortisol in comparison to symptomless fish. Serum estradiol levels were significantly reduced 19-fold in mildly-infected fish and remained at a low level as infection progressed, whereas serum testosterone increased gradually during vibriosis with a 1.8-fold increase in moribund groups in comparison to symptomless groups. Both serum thyroxine (T(4)) and triiodothyronine (T(3)) gradually decreased during vibriosis being 26- and 2.8-fold lower, respectively, in moribund fish in comparison to symptomless fish. The non-specific immune response, as determined by phagocytic activity, was also assessed using macrophages isolated from the pronephros and spleen of infected fish. Phagocytic indices significantly increased in mildly- and severely-infected fish and then decreased from these stimulated levels in moribund fish.

Hypercortisolemia does not affect the branchial osmoregulatory responses of the marine teleost Sparus sarba

The effect of cortisol treatment on branchial Na(+)-K(+)-ATPase subunit mRNA abundance, enzyme activity, chloride cell number/morphometrics and serum electrolyte levels were investigated for the marine teleost Sparus sarba. Groups of fish received intraperitoneal injections of cortisol at a concentration of 4 micrograms/g body weight, daily, over a seven-day period. This dose of cortisol was sufficiently high enough to maintain a condition of hypercortisolemia as serum cortisol levels in treated fish were eleven fold higher than controls at time of sacrifice. By using branchial Na(+)-K(+)-ATPase alpha- and beta-subunit cDNA clones we were able to demonstrate that cortisol administration to S. sarba caused a significant elevation in the abundance of alpha-mRNA whereas the levels of beta-mRNA were unchanged. In addition Na(+)-K(+)-ATPase activity remained unaltered by cortisol administration. Branchial chloride cell number, exposure, apical area as well as serum Na+ and Cl- levels remained unchanged after cortisol administration. The results of this study suggest that elevated cortisol level may not necessarily translate into modulated branchial Na(+)-K(+)-ATPase activity and chloride cell function in hypo-osmoregulating marine fish.