Endocrine mediators of seasonal growth in gilthead sea bream (Sparus aurata): the growth hormone and somatolactin paradigm

Genotype-temperature interaction in the regulation of development, growth, and morphometrics in wild-type, and growth-hormone transgenic coho salmon

Background

The neuroendocrine system is an important modulator of phenotype, directing cellular genetic responses to external cues such as temperature. Behavioural and physiological processes in poikilothermic organisms (e.g. most fishes), are particularly influenced by surrounding temperatures.

Methodology/Principal Findings

By comparing the development and growth of two genotypes of coho salmon (wild-type and transgenic with greatly enhanced growth hormone production) at six different temperatures, ranging between 8° and 18°C, we observed a genotype-temperature interaction and possible trend in directed neuroendocrine selection. Differences in growth patterns of the two genotypes were compared by using mathematical models, and morphometric analyses of juvenile salmon were performed to detect differences in body shape. The maximum hatching and alevin survival rates of both genotypes occurred at 12°C. At lower temperatures, eggs containing embryos with enhanced GH production hatched after a shorter incubation period than wild-type eggs, but this difference was not apparent at and above 16°C. GH transgenesis led to lower body weights at the time when the yolk sack was completely absorbed compared to the wild genotype. The growth of juvenile GH-enhanced salmon was to a greater extent stimulated by higher temperatures than the growth of the wild-type. Increased GH production significantly influenced the shape of the salmon growth curves.

Conclusions

Growth hormone overexpression by transgenesis is able to stimulate the growth of coho salmon over a wide range of temperatures. Temperature was found to affect growth rate, survival, and body morphology between GH transgenic and wild genotype coho salmon, and differential responses to temperature observed between the genotypes suggests they would experience different selective forces should they ever enter natural ecosystems. Thus, GH transgenic fish would be expected to differentially respond and adapt to shifts in environmental conditions compared with wild type, influencing their ability to survive and interact in ecosystems. Understanding these relationships would assist environmental risk assessments evaluating potential ecological effects.

Influences of the environment on the endocrine and paracrine fish growth hormone-insulin-like growth factor-I system

Insulin-like growth factor-I (IGF-I) is a key component of the complex system that regulates differentiation, development, growth and reproduction of fishes. The IGF-I gene is mainly expressed in the liver that represents the principal source of endocrine IGF-I but also in numerous other organs where the hormone most probably acts in an autocrine-paracrine manner. The primary stimulus for synthesis and release of IGF-I is growth hormone (GH) from the anterior pituitary. Thus, in analogy to mammals, it is usual to speak of a fish 'GH-IGF-I axis'. The GH-IGF-I system is affected by changes in the environment and probably represents a target of endocrine disrupting compounds (EDC) that impair many physiological processes in fishes. Thus, the review deals with the influences of changes in different environmental factors, such as food availability, temperature, photoperiod, season, salinity and EDCs, on GH gene expression in pituitary, IGF-I gene expression in liver and extrahepatic sites and the physiological effects resulting from the evoked alterations in endocrine and local IGF-I. Environmental influences certainly interact with each other but for convenience of the reader they will be dealt with in separate sections. Current trends in GH-IGF-I research are analysed and future focuses are suggested at the end of the sections.

Growth hormone receptors in zebrafish (Danio rerio): adult and embryonic expression patterns

Growth hormone receptor (GHR) is a critical regulator of growth and metabolism. Although two GHRs have been characterized in many fish species, their functional characteristics, mechanisms of regulation and roles in embryonic development remain unclear. The zebrafish (Danio rerio) is an excellent model organism to study both developmental and physiological processes. In the present work, we characterized the complete cDNA sequences of zebrafish GHRs, ghra and ghrb, and their gene structures. We studied the expression of both receptors in adult tissues, and during embryonic development and larval stages by means of RT-PCR and whole-mount in situ hybridization. We determined that both transcripts are maternal ones, with specific expression patterns during development. Both GHR transcripts are mainly expressed in the notochord, myotomes, anterior structures and in the yolk cell. Interestingly, their expression became undetectable at 96h post-fertilization. Unlike other reports in fish, ghrs expression could not be detected in brain when adult tissues were used, and we detected ghrb but not ghra transcripts in muscle. In addition, we determined alternative transcript sequences for ghra with specific domain deletions, and alternative transcripts for ghrb that generate a premature stop codon and codify for truncated isoforms. These isoforms lack intracellular regions necessary for the activation of signal transducers and activators of transcription (STAT) family transcription factors 5.

Gene expression survey of mitochondrial uncoupling proteins (UCP1/UCP3) in gilthead sea bream (Sparus aurata L.)

The aim of this work is to underline the biological significance of mitochondrial uncoupling proteins (UCPs) in ectothermic fish using the gilthead sea bream (Sparus aurata L.) as an experimental model. A contig of 1,990 bp in length was recognized as a UCP1 ortholog after initial searches in the gilthead sea bream AQUAFIRST database ( http://www.sigenae.org/aquafirst ). Additional searches were performed in skeletal muscle by RT-PCR, and the amplified PCR product was recognized as UCP3 after sequence completion by 5'- and 3'RACE. UCP1 expression was mostly detected in liver, whereas UCP3 transcripts were only found in skeletal and cardiac muscle fibres (white skeletal muscle > red skeletal muscle > heart). Specific gene regulation of UCP1 (liver) and UCP3 (white skeletal muscle) was addressed in physiological models of age, seasonal growth and energy-metabolic unbalances. Both the increase in energy demand (stress confinement) and the reduction in energy supply during adaptive cold response in winter down-regulated UCP1 expression. Conversely, transcript levels of UCP3 were higher with age, seasonal fattening and dietary deficiencies in essential fatty acids leading to the increase in fatty acid flux towards the muscle. This close association between UCP1 and UCP3 with the oxidative and metabolic tissue status is perhaps directly related to the ancestral protein UCP function, and allows the use of UCPs as lipotoxicity markers in ectothermic fish.

Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.

Effects of insulin-like growth factor-I, insulin, and leucine on protein turnover and ubiquitin ligase expression in rainbow trout primary myocytes

The effects of insulin-like growth factor-I (IGF-I), insulin, and leucine on protein turnover and pathways that regulate proteolytic gene expression and protein polyubiquitination were investigated in primary cultures of 4-day-old rainbow trout myocytes. Supplementing media with 100 nM IGF-I increased protein synthesis by 13% (P < 0.05) and decreased protein degradation by 14% (P < 0.05). Treatment with 1 microM insulin increased protein synthesis by 13% (P < 0.05) and decreased protein degradation by 17% (P < 0.05). Supplementing media containing 0.6 mM leucine with an additional 2.5 mM leucine did not increase protein synthesis rates but reduced rates of protein degradation by 8% (P < 0.05). IGF-I (1 nM-100 nM) and insulin (1 nM-1 microM) independently reduced the abundance of ubiquitin ligase mRNA in a dose-dependent manner, with maximal reductions of approximately 70% for muscle atrophy F-box (Fbx) 32, 40% for Fbx25, and 25% for muscle RING finger-1 (MuRF1, P < 0.05). IGF-I and insulin stimulated phosphorylation of FOXO1 and FOXO4 (P < 0.05), which was inhibited by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin, and decreased the abundance of polyubiquitinated proteins by 10-20% (P < 0.05). Supplementing media with leucine reduced Fbx32 expression by 25% (P < 0.05) but did not affect Fbx25 nor MuRF1 transcript abundance. Serum deprivation decreased rates of protein synthesis by 60% (P < 0.05), increased protein degradation by 40% (P < 0.05), and increased expression of all ubiquitin ligases. These data suggest that, similar to mammals, the inhibitory effects of IGF-I and insulin on proteolysis occur via P I3-kinase/protein kinase B signaling and are partially responsible for the ability of these compounds to promote protein accretion.

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.

Dynamics of liver GH/IGF axis and selected stress markers in juvenile gilthead sea bream (Sparus aurata) exposed to acute confinement: differential stress response of growth hormone receptors

The time courses of liver GH/IGF axis and selected stress markers were analyzed in juvenile gilthead sea bream (Sparus aurata) sampled at zero time and at fixed intervals (1.5, 3, 6, 24, 72 and 120 h) after acute confinement (120 kg/m(3)). Fish remained unfed throughout the course of the confinement study, and the fasting-induced increases in plasma growth hormone (GH) levels were partially masked by the GH-stress inhibitory tone. Hepatic mRNA levels of growth hormone receptor-I (GHR-I) were not significantly altered by confinement, but a persistent 2-fold decrease in GHR-II transcripts was found at 24 and 120 h. A consistent decrease in circulating levels of insulin-like growth factor-I (IGF-I) was also found through most of the experimental period, and the down-regulated expression of GHR-II was positively correlated with changes in hepatic IGF-I and IGF-II transcripts. This stress-specific response was concurrent with plasma increases in cortisol and glucose levels, reflecting the cortisol peak (60-70 ng/mL), the intensity and duration of the stressor when data found in the literature were compared. Adaptive responses against oxidative damage were also found, and a rapid enhanced expression was reported in the liver tissue for mitochondrial heat-shock proteins (glucose regulated protein 75). At the same time, the down-regulated expression of proinflammatory cytokines (tumour necrosis factor-alpha) and detoxifying enzymes (cytochrome P450 1A1) might dictate the hepatic depletion of potential sources of reactive oxygen species. These results provide suitable evidence for a functional partitioning of hepatic GHRs under states of reduced IGF production and changing cellular environment resulting from acute confinement.

Seasonal and sex-specific mRNA levels of key endocrine genes in adult yellow perch (Perca flavescens) from Lake Erie

To better understand the endocrine mechanisms that underlie sexually dimorphic growth (females grow faster) in yellow perch (Perca flavescens), real-time quantitative polymerase chain reaction (qPCR) was used to measure pituitary, liver, and ovary mRNA levels of genes related to growth and reproduction-sex in this species. Adult perch were collected from Lake Erie and body mass, age, gonadosomatic index (I (G)), hepatosomatic index (I (H)), and gene expression for growth hormone (GH), prolactin, somatolactin, insulin-like growth factor Ib (IGF-Ib), estrogen receptor alpha (esr1), estrogen receptor betaa (esr2a), and aromatase (cyp19a1a) were measured. Females had higher body mass, I (H), and liver esr1 mRNA level than males, while males had higher liver IGF-Ib, liver esr2a, and liver cyp19a1a mRNA levels. In both sexes, season had a significant effect on GH and liver IGF-Ib mRNAs with higher levels occurring in spring, which also corresponded with higher liver cyp19a1a mRNA levels. For females, I (G), liver esr1, and ovary cyp19a1a mRNA levels were higher in autumn than the spring, and ovary cyp19a1a mRNA levels showed a significant negative correlation with pituitary GH and liver IGF-Ib mRNA levels. The most significant (p </= 0.001) relationships across the parameters measured were positive correlations between liver IGF-Ib and esr2a mRNA levels and liver IGF-Ib and cyp19a1a mRNA levels. This study shows significant effects of season and sex on adult yellow perch endocrine physiology.

Gene and protein expression for prolactin, growth hormone and somatolactin in Sparus aurata: seasonal variations

The seasonal variation of PRL, GH and SL gene and protein expression has been analyzed in gilthead sea bream (Sparus aurata) pituitaries using Real-Time Q-PCR and Western Blots, respectively. Animals were cultured in earthen ponds under natural photoperiod, temperature and salinity conditions. Samples were taken during winter 2005 (January), spring 2005 (April), summer 2005 (July) and autumn 2005 (October). Beta-actin, used as the housekeeping gene both for Q-RT-PCR and Western analysis, did not present significant differences among seasons. Higher expression was observed during spring and autumn for PRL, summer and winter for GH, and spring for SL. Expression of PRI, GH and SL, presented seasonal variation, suggesting that these hormones could play a role in the molecular signal transduction of environmental factors (especially of photoperiod and temperature) in eurythermal fish.

Cloning of somatolactin alpha, beta forms and the somatolactin receptor in Atlantic salmon: seasonal expression profile in pituitary and ovary of maturing female broodstock

BACKGROUND

Somatolactin (Sl) is a fish specific adenohypophyseal peptide hormone related to growth hormone (Gh). Some species, including salmonids, possess two forms: Sl alpha and Sl beta. The somatolactin receptor (slr) is closely related to the growth hormone receptor (ghr). Sl has been ascribed many physiological functions, including a role in sexual maturation. In order to clarify the role of Sl in the sexual maturation of female Atlantic salmon (Salmo salar), the full length cDNAs of slr, Sl alpha and Sl beta were cloned and their expression was studied throughout a seasonal reproductive cycle using real-time quantitative PCR (RTqPCR).

METHODS

Atlantic salmon Sl alpha, Sl beta and slr cDNAs were cloned using a PCR approach. Gene expression of Sl alpha, SL beta and slr was studied using RTqPCR over a 17 month period encompassing pre-vitellogenesis, vitellogenesis, ovulation and post ovulation in salmon females. Histological examination of ovarian samples allowed for the classification according to the degree of follicle maturation into oil drop, primary, secondary or tertiary yolk stage.

RESULTS

The mature peptide sequences of Sl alpha, Sl beta and slr are highly similar to previously cloned salmonid forms and contained the typical motifs. Phylogenetic analysis of Atlantic salmon Sl alpha and Sl beta shows that these peptides group into the two Sl clades present in some fish species. The Atlantic salmon slr grouped with salmonid slr amongst so-called type I ghr. An increase in pituitary Sl alpha and Sl beta transcripts before and during spawning, with a decrease post-ovulation, and a constant expression level of ovarian slr were observed. There was also a transient increase in Sl alpha and Sl beta in May prior to transfer from seawater to fresh water and ensuing fasting.

CONCLUSION

The up-regulation of Sl alpha and Sl beta during vitellogenesis and spawning, with a subsequent decrease post-ovulation, supports a role for Sl during gonadal growth and spawning. Sl could also be involved in calcium/phosphate mobilization associated with vitellogenesis or have a role in energy homeostasis associated with lipolysis during fasting. The up-regulation of both Sl alpha and Sl beta prior to fasting and freshwater transfer, suggests a role for Sl linked to reproduction that may be independent of the maturation induced fasting.

Direct actions of cortisol, thyroxine and growth hormone on IGF-I mRNA expression in sea bream hepatocytes

The present study aims to investigate potential regulatory effect of different growth-related hormones including growth hormone (GH), human insulin-like growth factor-I (hIGF-I), thyroxine (T(4)), triiodothyronine (T(3)) and cortisol, on insulin-like growth factor-I (IGF-I) mRNA expression of hepatocytes isolated from silver sea bream. By using real-time PCR, IGF-I mRNA expression profiles of hepatocytes in response to individual hormones were determined in vitro. Hepatocytes incubated with GH at concentrations of 10-1000 ng/mL showed significantly higher IGF-I expression, but the elevation was attenuated at high concentration of GH (1000 ng/mL). IGF-I expression remained unchanged in hepatocytes after incubation with hIGF-I. Hepatocytes incubated with T(4) at concentration of 1000 ng/mL exhibited a significant elevation in IGF-I expression, whereas no difference in IGF-I expression was demonstrated in hepatocytes after incubation with T(3). Upon incubation with cortisol (1-1000 ng/mL), IGF-I expression was significantly decreased in hepatocytes in a dose-dependent manner. Our study demonstrated that GH, T(4), and cortisol had direct modulatory effects on IGF-I expression in fish hepatocytes in vitro.

High levels of vegetable oils in plant protein-rich diets fed to gilthead sea bream ( Sparus aurata L.): growth performance, muscle fatty acid profiles and histological alterations of target tissues

The feasibility of fish oil (FO) replacement by vegetable oils (VO) was investigated in gilthead sea bream (Sparus aurata L.) in a growth trial conducted for the duration of 8 months. Four isolipidic and isoproteic diets rich in plant proteins were supplemented with L-lysine (0.55 %) and soya lecithin (1 %). Added oil was either FO (control) or a blend of VO, replacing 33 % (33VO diet), 66 % (66VO diet) and 100 % (VO diet) of FO. No detrimental effects on growth performance were found with the partial FO replacement, but feed intake and growth rates were reduced by about 10 % in fish fed the VO diet. The replacement strategy did not damage the intestinal epithelium, and massive accumulation of lipid droplets was not found within enterocytes. All fish showed fatty livers, but signs of lipoid liver disease were only found in fish fed the VO diet. Muscle fatty acid profiles of total lipids reflected the diet composition with a selective incorporation of unsaturated fatty acids in polar lipids. The robustness of the phospholipid fatty acid profile when essential fatty acid requirements were theoretically covered by the diet was evidenced by multivariate principal components analysis in fish fed control, 33VO and 66VO diets.

Relationships between environmental changes, maturity, growth rate and plasma insulin-like growth factor-I (IGF-I) in female rainbow trout

Size reflecting growth rate, energy balance or nutritional status is regarded as an important determinant of the ability of trout to undergo puberty. The relationship of a change in photoperiod, either natural (SNP) or advancing (ADV), with growth, IGF-I and reproduction was investigated in virgin female rainbow trout. Under SNP 63% of the population attained maturity while only 29% spawned 6 months in advance in the ADV regime. Under SNP both size and growth rate in late spring-early summer appeared to determine whether an individual may initiate reproduction while condition factor appeared to be a better predictor in the ADV regime. A complete seasonal relationship between plasma IGF-I, daylength and temperature was demonstrated under natural conditions, and provides direct evidence for the relationship between reproduction and IGF-I. Conversely, trout maintained under ADV exhibited a significantly different plasma IGF-I profile relative to those under a natural photoperiod. Furthermore, IGF-I levels accurately reflected growth rate prior to elevations in sex steroids, suggesting that IGF-I may provide an endocrine signal between the somatotropic and reproductive axes that growth rate and/or size is sufficient to initiate gonad development. In addition, maturing individuals under SNP typically expressed higher circulating IGF-I levels than those that remained immature and may reflect a greater opportunity for IGF-I to act on the pituitary to stimulate gonadotropin production. We observed elevated levels in maturing fish for 3 months under SNP compared to only 1 month under ADV were observed. This may reflect a reduction in the window of opportunity to initiate reproduction under advancing photoperiods and hence explain the reduction in fish successfully recruited.

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.

Conjugated linoleic acid affects lipid composition, metabolism, and gene expression in gilthead sea bream (Sparus aurata L)

To maximize growth, farmed fish are fed high-fat diets, which can lead to high tissue lipid concentrations that have an impact on quality. The intake of conjugated linoleic acid (CLA) reduces body fat in mammals and this study was undertaken to determine the effects of dietary CLA on growth, composition, and postprandial metabolic variables in sea bream. Fish were fed 3 diets containing 48 g/100 g protein and 24 g/100 g fat, including fish oil supplemented with 0 (control), 2, or 4% CLA for 12 wk. Feed intake, specific growth rate, total body fat, and circulating somatolactin concentration were lower in fish fed CLA than in controls. Feed efficiency was greater in fish fed 2% CLA than in controls. Liver triglyceride concentrations were higher in fish fed 4% CLA and muscle triglyceride concentrations were lower in fish fed both CLA diets than in controls. Hepatic fatty acyl desaturase and elongase mRNA levels in fish fed CLA were lower than in controls. Metabolic differences between controls and CLA-fed fish were observed at 6 h but not at 24 h after the last meal, including lower postprandial circulating triglyceride concentrations, higher hepatic acyl-CoA-oxidase, and lower L-3-hydroxyacyl-CoA dehydrogenase activities in CLA-fed fish than in controls. Dietary CLA did not affect enzymes involved in lipogenesis including hepatic fatty acid synthase and malic enzyme, but it decreased glucose 6-phosphate dehydrogenase activity at 24 h, but not at 6 h after feeding. The data suggest that CLA intake in sea bream has little effect on hepatic lipogenesis, channels dietary lipid from adipose tissue to the liver, and switches hepatic mitochondrial to peroxisomal beta-oxidation.

Insulin-like growth factor I (IGF-I) in a growth-enhanced transgenic (GH-overexpressing) bony fish, the tilapia (Oreochromis niloticus): indication for a higher impact of autocrine/paracrine than of endocrine IGF-I

Several lines of growth hormone (GH)-overexpressing fish have been produced and analysed for growth and fertility parameters. However, only few data are available on the growth-promoting hormone insulin-like growth factor I (IGF-I) that mediates most effects of GH, and these are contradictory. Using quantitative real-time RT-PCR, radioimmunoassay, in situ hybridization, immunohistochemistry, and radiochromatography we investigated IGF-I and IGF binding proteins (IGFBPs) in an adult (17 months old) transgenic (GH-overexpressing) tilapia (Oreochromis niloticus). The transgenics showed an around 1.5-fold increase in length and an approximately 2.3-fold higher weight than the non-transgenics. Using radioimmunoassay, the serum IGF-I levels were lower (6.22 +/- 0.75 ng/ml) in transgenic than in wild-type (15.01 +/- 1.49 ng/ml) individuals (P = 0.0012). Radioimmunoassayable IGF-I in transgenic liver was 4.2-times higher than in wild-type (16.0 +/- 2.21 vs. 3.83 +/- 0.71 ng/g, P = 0.0017). No hepatocytes in wild-type but numerous hepatocytes in transgenic liver contained IGF-I-immunoreactivity. RT-PCR revealed a 1.4-times higher IGF-I mRNA expression in the liver of the transgenics (10.51 +/- 0.82 vs. 7.3 +/- 0.49 pg/microg total RNA, P = 0.0032). In correspondence, in situ hybridization showed more IGF-I mRNA containing hepatocytes in the transgenics. A twofold elevated IGF-I mRNA expression was determined in the skeletal muscle of transgenics (0.33 +/- 0.02 vs. 0.16 +/- 0.01 pg/microg total RNA, P < 0.0001). Both liver and serum of transgenics showed increased IGF-I binding. The increased IGFBP content in the liver may lead to retention of IGF-I, and/or the release of IGF-I into the circulation may be slower resulting in accumulation of IGF-I in the hepatocytes. Our results indicate that the enhanced growth of the transgenics likely is due to enhanced autocrine/paracrine action of IGF-I in extrahepatic sites, as shown here for skeletal muscle.

Neuroendocrine control of growth hormone in fish

The biological actions of growth hormone (GH) are pleiotropic, including growth promotion, energy mobilization, gonadal development, appetite, and social behavior. Accordingly, the regulatory network for GH is complex and includes many endocrine and environmental factors. In fish, the neuroendocrine control of GH is multifactorial with multiple inhibitors and stimulators of pituitary GH secretion. In fish, GH release is under a tonic negative control exerted mainly by somatostatin. Sex steroid hormones and nutritional status influence the level of brain expression and effectiveness of some of these GH neuroendocrine regulatory factors, suggesting that their relative importance differs under different physiological conditions. At the pituitary level, some, if not all, somatotropes can respond to multiple regulators. Therefore, ligand- and function-specificity, as well as the integrative responses to multiple signals must be achieved at the level of signal transduction mechanisms. Results from investigations on a limited number of stimulatory and inhibitory GH-release regulators indicate that activation of different but convergent intracellular pathways and the utilization of specific intracellular Ca(2+) stores are some of the strategies utilized. However, more work remains to be done in order to better understand the integrative mechanisms of signal transduction at the somatotrope level and the relevance of various GH regulators in different physiological circumstances.

Tumour necrosis factor (TNF)α as a regulator of fat tissue mass in the Mediterranean gilthead sea bream (Sparus aurata L.)

The study was undertaken to analyze the lipolytic effect and transcriptional regulation of tumour necrosis factor (TNF)alpha in gilthead sea bream (Sparus aurata L.). The study was also focused on the transcriptional regulation and analysis of the 5-flanking region of lipoprotein lipase (LPL) in an attempt to identify cis-regulatory elements that would support the TNFalpha-mediated effects. The lipolytic effect of TNFalpha was evidenced by the increased release of glycerol in the culture medium of freshly isolated adipocytes. This observation, in addition to the summer up-regulation of TNFalpha transcripts in liver and mesenteric adipose tissue, supported a key role of TNFalpha as a fish limiting factor of tissue fat mass. Accordingly, TNFalpha expression in liver and mesenteric adipose tissue was reduced by fasting. Furthermore, the up-regulated expression of TNFalpha in the skeletal muscle of older fish can represent an adaptive response to limit the enhanced lipid influx towards muscle. A close positive association between LPL and TNFalpha transcripts supported the contribution of TNFalpha as a part of a regulatory network that exerts an inhibitory tonus upon the expression of LPL, which in turns limits the tissue uptake of fatty acids and the ultimate increase of tissue lipid reservoirs. The precise mechanism for the inhibition of LPL gene expression by TNFalpha remains to be established in fish, but analysis of the 5'-flanking region evidenced the conservation through vertebrate evolution of a functional OCT-1/NF-Y site that would mediate the TNFalpha effects on LPL expression.

Two growth hormone receptors in Nile tilapia (Oreochromis niloticus): molecular characterization, tissue distribution and expression profiles in the gonad during the reproductive cycle

Growth hormone (GH) has many important physiological roles in the control of growth, metabolism and reproduction, which is mediated by growth hormone receptor (GHR). In this study, two cDNAs encoding GHR were isolated from the liver of Nile tilapia (Oreochromis niloticus). The two cDNAs, one consisting of 1908 bp and the other of 1728 bp, encoding for putative 635- and 575-amino acid GHR (designated ntGHR1 and ntGHR2, respectively), shared 34.4% identity in nucleotide sequence and 29.6% in deduced amino acid sequence. Northern blot analysis indicated a single 6.0-kb transcript of ntGHR1 and a single 4.0-kb transcript of ntGHR2 in the liver. Real-time RT-PCR analysis showed that both ntGHR1 and ntGHR2 mRNAs were presented in all tissues tested and expressed extremely highly in the liver. In most tissues, ntGHR2 expressed significantly higher than ntGHR1. Analysis of the ntGHRs expression profiles in the gonad during reproductive cycle indicated that the mRNA levels of ntGHRs in ovary were significantly higher at sexual matured stage while those in testis were significantly higher at sexual recrudescent stage, suggesting that GH/IGF-I axis might be involved in reproduction under a regulatory mechanism of GHR gene expression.

The dawn and evolution of hormones in the adenohypophysis

The adenohypophysial hormones have been believed to have evolved from several ancestral genes by duplication followed by evolutionary divergence. To understand the origin and evolution of the endocrine systems in vertebrates, we have characterized adenohypophysial hormones in an agnathan, the sea lamprey Petromyzon marinus. In gnathostomes, adrenocorticotropin (ACTH) and melanotropin (MSH) together with beta-endorphins (beta-END) are encoded in a single gene, designated as proopiomelanocortin (POMC), however in sea lamprey, ACTH and MSH are encoded in two distinct genes, proopoicortin (POC) gene and proopiomelanotropin (POM) gene, respectively. The POC and POM genes are expressed specifically in the rostral pars distalis (RPD) and the pars intermedia (PI), respectively. Consequently, the final products from both tissues are the same in all vertebrates, i.e., ACTH from the PD and MSH from the PI. The POMC gene might have been established in the early stages of invertebrate evolution by internal gene duplication of the MSH domains. The ancestral gene might be then inherited in lobe-finned fish and tetrapods, while internal duplication and deletion of MSH domains as well as duplication of whole POMC gene took place in lamprey and gnathostome fish. Sea lamprey growth hormone (GH) is expressed in the cells of the dorsal half of the proximal pars distalis (PPD) and stimulates the expression of an insulin-like growth factor (IGF) gene in the liver as in other vertebrates. Its gene consists of 5 exons and 4 introns spanning 13.6 kb, which is the largest gene among known GH genes. GH appears to be the only member of the GH family in the sea lamprey, which suggests that GH is the ancestral hormone of the GH family that originated first in the molecular evolution of the GH family in vertebrates and later, probably during the early evolution of gnathostomes. The other member of the gene family, PRL and SL, appeared by gene duplication. A beta-chain cDNA belonging to the gonadotropin (GTH) and thyrotropin (TSH) family was cloned. It is expressed in cells of the ventral half of PPD. Since the expression of this gene is stimulated by lamprey gonadotropin-releasing hormone, it was assigned to be a GTHbeta. This GTHbeta is far removed from beta-subunits of LH, FSH, and TSH in an unrooted tree derived from phylogenetic analysis, and takes a position as an out group, suggesting that lampreys have a single GTH gene, which duplicated after the agnathans and prior to the evolution of gnathostomes to give rise to LH and FSH.

Effects of aquaculture related stressors and nutritional restriction on circulating growth factors (GH, IGF-I and IGF-II) in Atlantic salmon and rainbow trout

The effects of aquaculture related stressors on circulating levels of GH, IGF-I and for the first time, IGF-II in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were investigated. Specifically, circulating growth factor levels were measured in four different experiments. Two 24 h confinement stressor procedures, (one with Atlantic salmon, the other with rainbow trout); following a hypo-osmotic stressor (freshwater bath) in salt water acclimated, adult, Atlantic salmon; and during a 22 day starvation and re-feeding protocol with juvenile Atlantic salmon. Handling and confinement resulted in significant decreases in circulating levels of all three growth factors in Atlantic salmon, and IGF-I and IGF-II (but not GH) in rainbow trout. A 2-3 h freshwater bath to remove gill parasites on a commercial Atlantic salmon aquaculture operation caused a significant decrease in circulating GH and IGF-I concentrations, but no significant change in IGF-II concentration, 2 days post bathing. Starvation for a period of 15 days in Atlantic salmon resulted in a significant increase in circulating GH levels and a significant decrease in circulating IGF-I and IGF-II. Re-feeding of starved fish for 7 days resulted in a significant decrease in GH to the concentration measured in continually fed fish, however re-feeding did not change plasma levels of IGF-I and IGF-II relative to continually starved fish. The results presented here confirm previously observed handling and confinement stressor induced effects on GH and IGF-I and, for the first time, on IGF-II in salmonids. Furthermore this study confirms the nutritional regulation of GH, IGF-I and IGF-II in juvenile Atlantic salmon.

Production and purification of recombinant somatolactin beta and its effects on melanosome aggregation in zebrafish

A second form of somatolactin, somatolactin beta (SLbeta), was recently discovered in zebrafish (Danio rerio). This novel subtype of somatolactin is distantly related to somatolactin alpha (SLalpha) found in teleost species and is produced in a different region of the pituitary. To date, no physiological study of SLbeta has been reported. In order to study the physiological functions of SLbeta, recombinant SLbeta protein has been produced and purified. The cDNA of zebrafish SLbeta was cloned into a pET100 bacteria expression vector and His-tagged fusion proteins were produced in BL21 (DE3) Escherichia coli cells. The majority of recombinant somatolactins produced by E. coli were isolated in inclusion bodies although a small percentage of recombinant proteins (<1%) were also found in soluble form. Fusion proteins were solubilized from inclusion bodies using 6M guanidine hydrochloride. Pure recombinant somatolactins were obtained by affinity purification. The estimated molecular weight of 28 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis correlates with the molecular mass calculated from the deduced amino acid sequence of SLbeta. Thereafter, specific polyclonal antibodies against the recombinant SLbeta were developed. These antibodies recognized specifically a group of cells located in the anterior pars intermedia of the pituitary. The antibodies did not react with SLalpha, growth hormone or prolactin cells in the zebrafish pituitary glands. Furthermore, recombinant SLbeta induced melanosome aggregation in a concentration-dependent manner in skin of zebrafish scales. Significant melanosome aggregation was observed in zebrafish melanophores at a concentration of 1 microg/ml. These results, combined with previous reports demonstrate that the recombinant SLbeta proteins produced here are bioactive. The function of inducing melanosome aggregation is conserved among the somatolactin functions.

Molecular characterization of gilthead sea bream (Sparus aurata) lipoprotein lipase. Transcriptional regulation by season and nutritional condition in skeletal muscle and fat storage tissues

Lipoprotein lipase (LPL) of gilthead sea bream (Sparus aurata) was cloned and sequenced using a RT-PCR approach completed by 3' and 5'RACE assays. The nucleotide sequence covered 1669 bp with an open reading frame of 525 amino acids, including a putative signal peptide of 23 amino acids long. Sequence alignment and phylogenetic analysis revealed a high degree of conservation among most fish and higher vertebrates, retaining the consensus sequence the polypeptide "lid", the catalytic triad and eight cysteine residues at the N-terminal region. A tissue-specific regulation of LPL was also found on the basis of changes in season and nutritional condition as a result of different dietary protein sources. First, the expression of LPL in mesenteric adipose tissue was several times higher than in liver and skeletal muscle. Secondly, the spring up-regulation of LPL expression in the mesenteric adipose tissue was coincident with a pronounced increase of whole body fat content. Thirdly, the highest expression of LPL in the skeletal muscle was found in summer, which may serve to cover the increased energy demands for muscle growth and protein accretion. Further, in fish fed plant-protein-based diets, hepatic LPL expression was up-regulated whereas an opposite trend was found in the mesenteric adipose tissue, which may contribute to drive dietary lipids towards liver fat storage. Finally, it is of interest that changes in circulating triglyceride (TG) levels support the key role of LPL in the clearance of TG-rich lipoproteins. This study is the first report in fish of a co-regulated expression of LPL in oxidative and fat storage tissues under different physiological conditions.

Nutritional assessment of somatolactin function in gilthead sea bream (Sparus aurata): concurrent changes in somatotropic axis and pancreatic hormones

The role of somatolactin (SL) in the regulation of energy homeostasis in gilthead sea bream (Sparus aurata) has been analysed. First, a down-regulation of plasma SL levels in response to gross shifts in dietary amino acid profile and the graded replacement of fish meal by plant protein sources (50%, 75% and 100%) has been observed. Thus, the impaired growth performance with changes in dietary amino acid profile and dietary protein source was accompanied by a decrease in plasma SL levels, which also decreased over the course of the post-prandial period irrespective of dietary nitrogen source. Secondly, we examined the effect of SL and growth hormone (GH) administration on voluntary feed intake. A single intraperitoneal injection of recombinant gilthead sea bream SL (0.1 microg/g fish) evoked a short-term inhibition of feed intake, whereas the same dose of GH exerted a marked enhancement of feed intake that still persisted 1 week later. Further, we addressed the effect of arginine (Arg) injection upon SL and related metabolic hormones (GH, insulin-like growth factor-I (IGF-I), insulin and glucagon) in fish fed diets with different nitrogen sources. A consistent effect of Arg injection (6.6 micromol/g fish) on plasma GH and IGF-I levels was not found regardless of dietary treatment. In contrast, the insulinotropic effect of Arg was found irrespective of dietary treatment, although the up-regulation of plasma glucagon and glucose levels was more persistent in fish fed a fish meal based diet (diet FM) than in those fed a plant protein diet with a 75% replacement (diet PP75). In the same way, a persistent and two-fold increase in plasma SL levels was observed in fish fed diet FM, whereas no effect was found in fish fed diet PP75. Taken together, these findings provide additional evidence for a role of SL as a marker of energy status, which may be perceived by fish as a daily and seasonal signal of abundant energy at a precise calendar time.

Growth hormone and insulin-like growth factors in fish: where we are and where to go

This communication summarizes viewpoints, discussion, perspectives, and questions, put forward at a workshop on "Growth hormone and insulin-like growth factors in fish" held on September 7th, 2004, at the 5th International Symposium on Fish Endocrinology in Castellon, Spain.

A peak in gh-receptor expression is associated with growth activation in Atlantic salmon vertebrae, while upregulation of igf-I receptor expression is related to increased bone density

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) play major roles in the endocrine regulation of fish growth, but their interdependency and mode of action has not been well elucidated. The GH-IGF-I system is essential for normal vertebral growth in mouse, but this has not been studied in fish. To study the interplay between GH, IGF-I, and their receptors, postsmolt Atlantic salmon were studied during spring growth (January-June 2003). From January to June, fish were sampled regularly for plasma and vertebral bone. The vertebra was collected from the same anterior-posterior position. The growth hormone receptor (ghr) (There is no determined nomenclature of salmon genes but we stick to the nomenclature which is consequent for zebrafish, where all gene names are named with small letters and in italic.) expression in the vertebrae peaked in the end of February coinciding with high levels of plasma GH and IGF-I, and an increase of vertebral growth rate. From April to June, plasma IGF-I levels decreased together with ghr expression in the vertebrae, while plasma GH did not decrease. In May and June, expression of the igf-I receptor (igf-Ir) increased 4- to 5-fold, which coincided with an increase in bone density. The changes seen in gene expression of the IGF-I and GH receptors suggest that these hormones are involved in vertebral growth and bone density.

Photoperiod influences growth rate and plasma insulin-like growth factor-I levels in juvenile rainbow trout, Oncorhynchus mykiss

The effect of different photoperiod regimes and the subsequent influence of melatonin on growth and insulin-like growth factor-I (IGF-I) were assessed in juvenile rainbow trout. In Experiment 1, triplicate groups of all female underyearling rainbow trout were exposed to one of three photoperiods; simulated natural photoperiod (SNP), constant short-days (LD 8:16), or constant long-days (LD 18:6) from June to December 2000 under ambient water temperatures. Fish exposed to LD 18:6 grew to a significantly heavier mean weight than the other treatments. Regression analysis showed a strong correlation between circulating plasma IGF-I, growth rate and temperature. Furthermore, it was apparent that fish exposed to LD 18:6 expressed significantly higher circulating levels of IGF-I. In a second experiment, duplicate groups of all female yearling trout were exposed to one of three photoperiods; SNP, LD 8:16, or constant light (LL), with sub groups receiving either a slow-release melatonin implant (18 mg), sham implant or left intact (control). LL increased growth rate in controls, reaching a significantly greater weight than SNP or LD 8:16 photoperiods but did not affect circulating IGF-I levels. Melatonin implants reduced growth rate in all photoperiod treatments below that of their respective controls but again did not affect circulating IGF-I levels. No differences in growth rate were found in implanted fish between photoperiods suggesting that a diel cycle of melatonin is necessary for the perception of daylength. These results would indicate that extended photoperiods (LD 18:6) may cause direct photostimulation of growth through up-regulation of IGF-I production. In contrast, in the absence of a changing diel melatonin signal, growth appeared to be maintained by a possible underlying endogenous rhythm, which was phase advanced under LL, as such plasma IGF-I levels simply reflected growth rate rather than photostimulation of the somatotropic axis. Overall, these findings indicate that measuring plasma IGF-I may be a useful tool for studying environmental influences on growth in rainbow trout.

Identification of the salmon somatolactin receptor, a new member of the cytokine receptor family

Somatolactin (SL) is a pituitary hormone of the GH/prolactin (PRL) family that so far has been found only in fish. Compared with GH and PRL, the primary structure of SL is highly conserved among divergent fish species, suggesting it has an important function and a discriminating receptor that constrains structural change. However, SL functions are poorly understood, and receptors for SL have not yet been identified. During cloning of GH receptor cDNA from salmon, we found a variant with relatively high (38-58%) sequence identity to vertebrate GH receptors and low (28-33%) identity to PRL receptors; however, the recombinant protein encoding the extracellular domain showed only weak binding of GH. Ligand binding of the recombinant extracellular domain for this receptor confirmed that the cDNA encoded a specific receptor for SL. The SL receptor (SLR) has common features of a GH receptor including FGEFS motif, six cysteine residues in the extracellular domain, a single transmembrane region, and Box 1 and 2 regions in the intracellular domain. These structural characteristics place the SLR in the cytokine receptor type I homodimeric group, which includes receptors for GH, PRL, erythropoietin, thrombopoietin, granulocyte-colony stimulating factor, and leptin. Transcripts for SLR were found in 11 tissues with highest levels in liver and fat, supporting the notion that a major function of SL is regulation of lipid metabolism. Cloning SLR cDNA opens the way for discovery of new SL functions and target tissues in fish, and perhaps novel members of this receptor family in other vertebrates.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

Nutritional and hormonal control of lipolysis in isolated gilthead seabream (Sparus aurata) adipocytes

We examined the effects of diet composition and fasting on lipolysis of freshly isolated adipocytes from gilthead seabream (Sparus aurata). We also analyzed the effects of insulin, glucagon, and growth hormone (GH) in adipocytes isolated from fish fed with different diets. Basal lipolysis, measured as glycerol release, increased proportionally with cell concentration and time of incubation, which validates the suitability of these cell preparations for the study of hormonal regulation of this metabolic process. Gilthead seabream were fed two different diets, FM (100% of fish meal) and PP (100% of plant protein supplied by plant sources) for 6 wk. After this period, each diet group was divided into two groups: fed and fasted (for 11 days). Lipolysis was significantly higher in adipocytes from PP-fed fish than in adipocytes from FM-fed fish. Fasting provoked a significant increase in the lipolytic rate, about threefold in isolated adipocytes regardless of nutritional history. Hormone effects were similar in the different groups: glucagon increased the lipolytic rate, whereas insulin had almost no effect. GH was clearly lipolytic, although the relative increase in glycerol over control was lower in isolated adipocytes from fasted fish compared with fed fish. Together, we demonstrate for the first time that lipolysis, measured in isolated seabream adipocytes, is affected by the nutritional state of the fish. Furthermore, our data suggest that glucagon and especially GH play a major role in the control of adipocyte lipolysis.

Production and characterization of recombinantly derived peptides and antibodies for accurate determinations of somatolactin, growth hormone and insulin-like growth factor-I in European sea bass (Dicentrarchus labrax)

A specific radioimmunoassay (RIA) for European sea bass (Dicentrarchus labrax) growth hormone (GH) was developed and validated. For this purpose, a stable source of GH was produced by means of recombinant DNA technology in a bacteria system. The identity of the purified protein (ion exchange chromatography) was demonstrated by Western blot and a specific GH antiserum was raised in rabbit. In Western blot and RIA system, this antiserum recognized specifically native and recombinant GH, and it did not cross-react with fish prolactin (PRL) and somatolactin (SL). In a similar way, a specific polyclonal antiserum against the now available recombinant European sea bass SL was raised and used in the RIA system to a sensitivity of 0.3 ng/ml (90% of binding of tracer). Further, European sea bass insulin-like growth factor-I (IGF-I) was cloned and sequenced, and its high degree of identity with IGF-I peptides of barramundi, tuna, and sparid fish allowed the use of a commercial IGF-I RIA based on barramundi IGF-I antiserum. These assay tools assisted for the first time accurate determinations of SL and GH-IGF-I axis activity in a fish species of the Moronidae family. Data values were compared to those found with gilthead sea bream (Sparus aurata), which is currently used as a Mediterranean fish model for growth endocrinology studies. As a characteristic feature, the average concentration year round of circulating GH in growing mature males of European sea bass was higher than in gilthead sea bream. By contrast, the average concentration of circulating SL was lower. Concerning to circulating concentration of IGF-I, the measured plasma values for a given growth rate were also lower in European sea bass. These findings are discussed on the basis of a different energy status that might allowed a reduced but more continuous growth in European sea bass.

Overwinter fasting and re-feeding in rainbow trout: plasma growth hormone and cortisol levels in relation to energy mobilisation

This study investigated the roles of cortisol and growth hormone (GH) during a period of fasting in overwintering salmonid fish. Indices of carbohydrate (plasma glucose, liver glycogen), lipid (plasma free fatty acids (FFAs)) and protein metabolism (plasma protein, total plasma amino acids) were determined, together with plasma GH, cortisol and somatolactin (SL) levels at intervals in three groups of rainbow trout (continuously fed; fasted for 9 weeks then fed; fasted for 17 weeks). In fasted fish, a decline in body weight and condition factor was accompanied by reduced plasma glucose and hepatic glycogen and increased plasma FFA. No consistent elevation of plasma GH occurred until after 8 weeks of fasting when plasma GH levels increased ninefold. No changes were observed in plasma total protein and AA until between weeks 13 and 17 when both were reduced significantly. When previously fasted fish resumed feeding, plasma glucose and FFA, and hepatic glycogen levels rapidly returned to control values and weight gain resumed. No significant changes in plasma cortisol levels, related to feeding regime, were evident at any point during the study and there was no evidence that SL played an active role in the response to fasting. The results suggest that overwinter fasting may not represent a significant nutritional stressor to rainbow trout and that energy mobilisation during fasting may be achieved without the involvement of GH, cortisol or SL.

Response of the somatotropic axis of juvenile coho salmon to alterations in plane of nutrition with an analysis of the relationships among growth rate and circulating IGF-I and 41 kDa IGFBP

The effect of different feeding levels on plasma levels of insulin-like growth factor-I (IGF-I), 41 kDa insulin-like growth factor binding protein (41 kDa IGFBP), and growth hormone (GH) were assessed in post-smolt coho salmon. Fish were fed at either stable (1 and 2% body weight/day) or varying (1-0.5-1%, 2-0.5-2% body weight/day) feeding rates and plasma was sampled from 10 fish/treatment at 2-3 week intervals over five dates from June to September, resulting in a total of 200 samples. Fish fed at higher rates grew faster and had higher plasma IGF-I and 41 kDa IGFBP levels. Plasma GH levels were variable but generally showed an inverse relationship to feeding rate. Both plasma IGF-I and 41 kDa IGFBP increased seasonally, average IGF-I levels doubled from June to September, regardless of feeding rate. On any one date both IGF-I and 41 kDa IGFBP were highly related to growth rate with regression coefficients ranging from 0.36 to 0.68 (IGF-I) and from 0.33 to 0.70 (41 kDa IGFBP). No relationship was found between IGF-I:41 kDa IGFBP ratio and individual growth rate. Overall, both feeding rate and date were important in explaining variation in IGF-I and 41 kDa IGFBP levels.

Development of an enzyme-linked immunosorbent assay for the measurement of plasma growth hormone (GH) levels in channel catfish (Ictalurus punctatus): assessment of environmental salinity and GH secretogogues on plasma GH levels

We report the development of a sensitive, and specific, competitive, antigen-capture enzyme-linked immunosorbent assay for the measurement of channel catfish (Ictalurus punctatus) growth hormone (cfGH). The detection limit of the assay (90% binding) was 2.0ng/ml and the ED(50) value (standard curve range 150-0.59 ng/ml) was 67.3 ng/ml. Recovery of cfGH-spiked plasma samples was determined to be 102%. Dose-response inhibition curves using serially diluted pituitary homogenates and plasma samples consistently showed parallelism with the standard curves using purified cfGH. The GH antibody (rabbit anti-catfish GH) specificity was demonstrated in competitive binding curves employing heterologous hormones and purified channel catfish prolactin (cfPRL). These studies show that there was no significant (0.006%) binding of cfPRL (competitive inhibition of cfGH binding), or heterologous hormones, within the working range of the assay. To physiologically validate the assay, catfish were injected (100 microg/g body weight, 3 injections every 5 days) with either bovine GHRH(1-29)-amide or the synthetic hexapeptide GHRP-2 (KP-102: D-Ala-D-beta-Nal-Ala-Trp-D-Phe-Lys-NH(2)) suspended in corn oil. Following the last injection, half of the animals were sampled for plasma and the remaining transferred from fresh water (FW) to 12 ppt seawater (BW: brackish water). Twenty-four hours after transfer to BW, animals were again sampled for plasma. Plasma GH levels were significantly (p<0.001) elevated in all the BW groups (control, KP-102, and bGHRH), compared with the FW (fresh water) groups. In addition, plasma GH levels were significantly (p<0.001) elevated by treatment with either of the GH secretogogues, KP-102 or bGHRH. Our findings demonstrate that two regulatory mechanisms of GH elevation, one which is seen in euryhaline teleosts (salinity-induced GH levels) and another, which has been recently described in teleosts (GHRP-induced GH levels), are present in the stenohaline channel catfish.

Molecular cloning and characterization of gilthead sea bream (Sparus aurata) growth hormone receptor (GHR). Assessment of alternative splicing

The full-length growth hormone receptor (GHR) of gilthead sea bream (Sparus aurata) was cloned and sequenced by RT-PCR and rapid amplification of 5'and 3'ends. The open reading frame codes for a mature 609 amino acid protein with a hydrophobic transmembrane region and all the characteristic motifs of GHRs. Sequence analysis revealed a 96 and 76% of amino acid identity with black sea bream (Acanthopagrus schlegeli) and turbot (Scophthalmus maximus) GHRs, respectively, but this amino acid identity decreases up to 52% for goldfish (Carassius auratus) GHR. By means of real-time PCR assays, concurrent changes in the hepatic expression of GHRs and insulin-like growth factor-I (IGF-I) was evidenced. Moreover, their regulation occurred in conjunction with the summer spurt of growth rates and circulating levels of GH and IGF-I. Search of alternative splicing was carried out exhaustively for gilthead sea bream GHR, but Northern blot and 3' RACE failed to demonstrate the occurrence of short alternative messengers. Besides, RT-PCR screening did not reveal deletions or insertions that could lead to alternative reading frames. In agreement with this, cross-linking assays only evidenced two protein bands that match well with the size of glycosylated and non-glycosylated forms of the full-length GHR. If so, it appears that alternative splicing at the 3'end does not occur in gilthead sea bream, although different messengers for truncated or longer GHR variants already exist in turbot and black sea bream, respectively. The physiological relevance of this finding remains unclear, but perhaps it points out large inter-species differences in the heterogeneity of the GHR population.

Environmental temperature increases plasma GH levels independently of nutritional status in rainbow trout (Oncorhynchus mykiss)

Like many poecilotherms, salmonids exhibit seasonal variations of growth rate in relation with seasonal temperatures and plasma GH level. However, temperature alters other parameters like food intake, which may directly modify the level of plasma GH. In order to determine whether temperature regulates plasma GH levels independently of nutritional status, fish were reared at 8, 12, or 16 degrees C and either fed ad libitum (fish with different food intake) to determine the global effect of temperature, or with the same ration (1.2%/body weight) to observe the temperature effect in fish with the same growth rate. Plasma insulin level was inversely proportional to the temperature (8, 12, and 16 degrees C) in fish fed ad libitum (12.1+/-0.3 ng/ml, 10.9+/-0.3 ng/ml, 9.5+/-0.4 ng/ml; P<0.001) and in restricted fish (14.0+/-0.3 ng/ml, 11.3+/-0.3 ng/ml, 10.0+/-0.2 ng/ml; P<0.0001), probably due to a prolonged nutrient absorption, and delayed recovery of basal insulin level at low temperature. Conversely, temperature did not affect plasma T3 level of fish fed ad libitum (2.5+/-0.2 ng/ml, 2.4+/-0.1 ng/ml, 2.5+/-0.1 ng/ml at 8, 12, and 16 degrees C) while fish fed with the same ration present less T3 at 16 degrees C than at 8 degrees C (1.83+/-0.1 ng/ml versus 1.2+/-0.1 ng/ml; P<0.001) throughout the experiment; these observations indicate that different plasma T3 levels reflect the different nutritional status of the fish. The levels of GH1 and GH2 mRNA, and GH1/GH2 ratio were not different for whatever the temperature or the nutritional status. Pituitary GH content, of fish fed ad libitum did not exhibit obvious differences at 8, 12, or 16 degrees C (254+/-9 ng/g bw, 237+/-18 ng/g bw, 236+/-18 ng/g bw), while fish fed with the same ration have higher pituitary GH contents at 16 degrees C than at 8 degrees C (401+/-30 ng/g bw versus 285+/-25 ng/g bw; P<0.0001). Interestingly, high temperature strongly increases plasma GH levels (2.5+/-0.3 ng/ml at 8 degrees C versus 4.8+/-0.6 ng/ml at 16 degrees C; P<0.0001) to the same extent in both experiments, since at a given temperature average plasma GH was similar between fish fed ad libitum or a restricted diet. Our results, demonstrate that temperature regulates plasma GH levels specifically but not pituitary GH content, nor the levels of GH1 and GH2 mRNA. In addition no differential regulation of both GH genes was evidenced whatever the temperature.