The interrelation between photoperiod, growth hormone, and sexual maturation of adult Atlantic salmon (Salmo salar)

Seasonal Changes in Photoperiod: Effects on Growth and Redox Signaling Patterns in Atlantic Salmon Postsmolts

Farmed Atlantic salmon reared under natural seasonal changes in sea-cages had an elevated consumption of antioxidants during spring. It is, however, unclear if this response was caused by the increase in day length, temperature, or both. The present study examined redox processes in Atlantic salmon that were reared in indoor tanks at constant temperature (9 °C) under a simulated natural photoperiod. The experiment lasted for 6 months, from vernal to autumnal equinoxes, with the associated increase and subsequent decrease in day length. We found that intracellular antioxidants were depleted, and there was an increase in malondialdehyde (MDA) levels in the liver and muscle of Atlantic salmon with increasing day length. Antioxidant enzyme activity in liver and muscle and their related gene profiles was also affected, with a distinct upregulation of genes involved in maintaining redox homeostasis, such as peroxiredoxins in the brain in April. This study also revealed a nuclear factor-erythroid 2-related factor 2 (Nrf2)-mediated oxidative stress response in muscle and liver, suggesting that fish integrate environmental signals through redox signaling pathways. Furthermore, growth and expression profiles implicated in growth hormone (GH) signaling and cell cycle regulation coincided with stress patterns. The results demonstrate that a change in photoperiod without the concomitant increase in temperature is sufficient to stimulate growth and change the tissue oxidative state in Atlantic salmon during spring and early summer. These findings provide new insights into redox regulation mechanisms underlying the response to the changing photoperiod, and highlight a link between oxidative status and physiological function.

The effect of environmental stressors on growth in fish and its endocrine control

Fish body growth is a trait of major importance for individual survival and reproduction. It has implications in population, ecology, and evolution. Somatic growth is controlled by the GH/IGF endocrine axis and is influenced by nutrition, feeding, and reproductive-regulating hormones as well as abiotic factors such as temperature, oxygen levels, and salinity. Global climate change and anthropogenic pollutants will modify environmental conditions affecting directly or indirectly fish growth performance. In the present review, we offer an overview of somatic growth and its interplay with the feeding regulatory axis and summarize the effects of global warming and the main anthropogenic pollutants on these endocrine axes.

Roles of leptin in initiation of acquired growth hormone resistance and control of metabolism in rainbow trout

Catabolic conditions often induce concomitant changes in plasma leptin (Lep), growth hormone (GH) and insulin growth factor I (IGF-I) levels in teleost fish, but it is unclear whether these parts of the endocrine system are responding independently or functionally linked. In this study, fasted rainbow trout was used to study the effects of Lep on the GH-IGF-I system and metabolism. Fish were implanted intraperitoneally with recombinant rainbow trout Lep pellets and remained unfed. After 4 days, plasma GH levels were elevated in the Lep-treated fish in a dose-dependent manner; the expression of hepatic igf1 and plasma IGF-I levels were suppressed accordingly. In vitro Lep treatment reversed ovine GH (oGH)-stimulated expression of igf1 and igf2 in hepatocytes isolated from fasted fish, similar to the inhibitory effects of the MEK1/2 inhibitor U0126 treatment. However, Lep treatment alone had no effect on the expression of igfs or oGH-stimulated ghr2a expression in the hepatocytes. These results demonstrate an additive effect of Lep on suppression of IGF-I under catabolic conditions, indicating that Lep is likely involved in initiation of acquired GH resistance. Although the Lep-implant treatment had no effect on standard metabolic rate, it significantly suppressed gene expression of hepatic hydroxyacyl-CoA dehydrogenase, phosphoenolpyruvate carboxykinase and glucose 6-phosphatase, which are key enzymes in lipid utilization and gluconeogenesis, in different patterns. Overall, this study indicates that the Lep increase in fasting salmonids is an important regulatory component for physiological adaptation during periods of food deprivation, involved in suppressing growth and hepatic metabolism to spare energy expenditure.

Sexual dimorphism in ultradian and 24h rhythms in plasma levels of growth hormone in Indian walking catfish, Clarias batrachus

Growth hormone (GH), a key regulator of somatic and reproductive growth in vertebrates, has been extensively studied, although primarily in female fish. Despite numerous reports about sex- and species-specific growth patterns in fish, to our knowledge, there is no report about the 24 h rhythm of plasma GH in male fish. Thus, we aimed to investigate temporal variations in plasma GH levels and the existence of any rhythms therein during the reproductively active months of March to August in the male walking catfish, Clarias batrachus. We also aimed to compare the secretory temporal patterns of GH in male-female specimens of C. batrachus to decipher sexual dimorphism in GH secretions in fish. After 14 days of acclimation to the natural environment, male catfish (N = 240 in total) were sorted and randomly divided into eight groups for study at ZT0 (sunrise ~06:00 h), 3, 6, 9, 12, 15, 18, and 21. During each month, physical parameters like duration of photoperiod and water temperature were measured. Male catfish (n = 40/month) in all eight groups were sampled (n = 5/group) at each time point under the natural time-of-year 24 h light-dark (LD) cycle. Male catfish were anesthetized and blood was collected through a caudal puncture, centrifuged, and plasma isolated. Plasma GH was measured using a competitive homologous enzyme-linked immunosorbent assay. Further, testes were removed, weighed, and the gonadosomatic index (GSI) was calculated. A significant effect of time and season (p ˂ 0.05, two-way ANOVA) on plasma GH level was detected. Cosinor analyses verified the existence of statistically significant (p ˂ 0.05) ultradian (12 h) and 24 h rhythms of plasma GH in male C. batrachus, with the higher values of Mesor (time series mean) and amplitude (one-half peak-to-trough difference) of the periodicities from March to July. Mapping of the acrophases (peak times) showed two ultradian and one 24 h acrophase of GH during the early photophase and early scotophase from March to August. Distinct sexual-dimorphism in plasma GH Mesors and acrophases was noticed between male and female catfish. GSI values of male catfish indicate males mature a little earlier than females in terms of size and reproductive activity. The findings that plasma GH show 24 h and seasonal fluctuations in a sex-specific manner collectively demonstrate the importance of considering the effect of biological 24 h and seasonal time and sex on the GH level in regulating the physiology of somatic growth and reproduction in catfish.

Photoperiodic regulation of pituitary thyroid-stimulating hormone and brain deiodinase in Atlantic salmon

Seasonal timing is important for many critical life history events of vertebrates, and photoperiod is often used as a reliable seasonal cue. In mammals and birds, it has been established that a photoperiod-driven seasonal clock resides in the brain and pituitary, and is driven by increased levels of pituitary thyroid stimulating hormone (TSH) and brain type 2 iodothyronine deiodinase (DIO2), which leads to local increases in triiodothyronine (T₃). In order to determine if a similar mechanism occurs in fish, we conducted photoperiod manipulations in anadromous (migratory) Atlantic salmon (Salmo salar) that use photoperiod to time the preparatory development of salinity tolerance which accompanies downstream migration in spring. Changing daylength from short days (light:dark (LD) 10:14) to long days (LD 16:8) for 20 days increased gill Na⁺/K⁺-ATPase (NKA) activity, gill NKAα1b abundance and plasma growth hormone (GH) levels that normally accompany increased salinity tolerance of salmon in spring. Long-day exposure resulted in five-fold increases in pituitary tshβb mRNA levels after 10 days and were sustained for at least 20 days. tshβb mRNA levels in the saccus vasculosus were low and not influenced by photoperiod. Increased daylength resulted in significant increases in dio2b mRNA levels in the hypothalamus and midbrain/optic tectum regions of the brain. The results are consistent with the presence of a photoperiod-driven seasonal clock in fish which involves pituitary TSH, brain DIO2 and the subsequent production of T₃, supporting the hypothesis that this is a common feature of photoperiodic regulation of seasonality in vertebrates.

Cortisol regulates insulin-like growth-factor binding protein (igfbp) gene expression in Atlantic salmon parr

The growth hormone (Gh)/insulin-like growth-factor (Igf)/Igf binding protein (Igfbp) system regulates growth and osmoregulation in salmonid fishes, but how this system interacts with other endocrine systems is largely unknown. Given the well-documented consequences of mounting a glucocorticoid stress response on growth, we hypothesized that cortisol inhibits anabolic processes by modulating the expression of hepatic igfbp mRNAs. Atlantic salmon (Salmo salar) parr were implanted intraperitoneally with cortisol implants (0, 10, and 40 μg g^-1 body weight) and sampled after 3 or 14 days. Cortisol elicited a dose-dependent reduction in specific growth rate (SGR) after 14 days. While plasma Gh and Igf1 levels were unchanged, hepatic igf1 mRNA was diminished and hepatic igfbp1b1 and -1b2 were stimulated by the high cortisol dose. Plasma Igf1 was positively correlated with SGR at 14 days. Hepatic gh receptor (ghr), igfbp1a, -2a, -2b1, and -2b2 levels were not impacted by cortisol. Muscle igf2, but not igf1 or ghr, levels were stimulated at 3 days by the high cortisol dose. As both cortisol and the Gh/Igf axis promote seawater (SW) tolerance, and particular igfbps respond to SW exposure, we also assessed whether cortisol coordinates the expression of branchial igfbps and genes associated with ion transport. Cortisol stimulated branchial igfbp5b2 levels in parallel with Na⁺/K⁺-ATPase (NKA) activity and nka-α1b, Na⁺/K⁺/2Cl^--cotransporter 1 (nkcc1), and cystic fibrosis transmembrane regulator 1 (cftr1) mRNA levels. The collective results indicate that cortisol modulates the growth of juvenile salmon via the regulation of hepatic igfbp1s whereas no clear links between cortisol and branchial igfbps previously shown to be salinity-responsive could be established.

Establishment of time-resolved fluoroimmunoassays for detection of growth hormone and insulin-like growth factor I in rainbow trout plasma

The GH/IGF-I axis influences many aspects of salmonid life history and is involved in a variety of physiological processes that are related to somatic growth (e.g., reproduction, smoltification, and the response to fasting and stress). As such, fisheries studies utilize GH/IGF-I axis components as indicators of growth and metabolic status. This study established time-resolved fluoroimmunoassays (TR-FIAs) for rainbow trout plasma GH and IGF-I using commercially available reagents. For the GH TR-FIA, the ED₈₀ and ED₂₀ were 0.6 and 28.1 ng/mL, the minimum detection limit was 0.2 ng/mL, and the intra- and inter-assay coefficients of variation (%CV) were 4.1% and 13.4%, respectively. Ethanol remaining from acid-ethanol cryoprecipitation (AEC) of plasma samples to remove IGF binding proteins reduced binding and increased variability in the IGF-I TR-FIA. Drying down and reconstituting extracted samples restored binding and reduced variability. The extraction efficiency of IGF-I standards through AEC, drying down, and reconstitution did not vary over the working range of the assay. For the IGF-I TR-FIA, the ED₈₀ and ED₂₀ were 0.2 and 6.5 ng/mL, the minimum detection limit was 0.03 ng/mL, and the intra- and inter-assay %CV were 3.0% and 6.5%, respectively. Biological validation was provided by GH injection and fasting studies in rainbow trout. Intraperitoneal injection with bovine GH increased plasma IGF-I levels. Four weeks of fasting decreased body weight, increased plasma GH levels, and decreased plasma IGF-I levels. The GH and IGF-I TR-FIAs established herein provide a cost-comparable, non-radioisotopic method for quantifying salmonid plasma GH and IGF-I using commercially available reagents.

Tracking long-distance migration of marine fishes using compound-specific stable isotope analysis of amino acids

The long-distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (δ¹⁵ N_Base ), which can be estimated by using compound-specific isotope analysis. δ¹⁵ N_Base exclusively reflects the δ¹⁵ N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. In other words, δ¹⁵ N_Base allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ¹⁵ N_Base isoscape in the northern North Pacific by bulk and compound-specific isotope analyses of copepods (n = 360 and 24, respectively), and then we determined retrospective δ¹⁵ N_Base values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon). We then estimated the migration routes of chum salmon during their skeletal growth by using a state-space model. Our isotope tracking method successfully reproduced a known chum salmon migration route between the Okhotsk and Bering seas, and our findings suggest the presence of a new migration route to the Bering Sea Shelf during a later growth stage.

Development of a time-resolved fluoroimmunoassay for measuring plasma growth hormone in Nile tilapia (Oreochromis niloticus)

Growth hormone is a hormone secreted from the pituitary and is involved in the regulation of most major physiological processes such as growth, development and metabolism. Therefore, an accurate and sensitive detection method is needed for the detection of tilapia serum Gh level. Phage display technology is widely used in the expression of antibody fragments, in which fragments of antibodies are expressed as a fusion with phage proteins and are displayed on the phage surface for easy screening. Time-resolved fluorescence immunoassay (TRFIA) is a microanalysis method developed nearly two decades ago and is one of the most sensitive analytical techniques. With the use of a special lanthanide, the detection background can be distinguished, which can greatly improve the sensitivity of detection. In this report, we cloned the V_H and V_L DNA fragments from the lymphocytes of rabbits immunized with recombinant Gh and assembled them with a linker to form a single-chain variable fragment (scFv) gene pool. Using phage display technology, we isolated scFv DNA fragments from the pool, which encode a protein that specifically binds to tilapia Gh. We then established Eu-DTTA-based TRFIA for measuring plasma Gh in tilapia. The sensitivity of double antibody sandwich Gh-TRFIA was 0.225 ng/ml, and the linear range of the standard curve was 0.225-250 ng/ml. The intra- and interassay coefficients of variation (CVs) were <9.1 and <4.5%, respectively. The cross-reactivities (CRs) of 1 μg/ml recombinant tilapia somatolactin (rtSl), prolactin (rtPrl) and thyroid-stimulating hormone beta subunit (rtTshb) were 0.042%, 0.472% and 0.036%, respectively. The sensitivity of direct competitive Gh-TRFIA was 0.208 ng/ml, and the linear range of the standard curve was 0.208-500 ng/ml. The intra- and interassay CVs were <4.8 and <7.1%, respectively. The CRs of 1 μg/ml rtSl, rtPrl and rtTshb were 0.041%, 0.079% and 0.073%, respectively. In conclusion, Gh-TRFIA is a safe (no concerns about radioactive isotopes), economical, and efficient detection method for the quantification of plasma Gh. Thus, the application of phage display technology for antibody screening and the use of TRFIA for tilapia Gh detection are conducive to research in the field of fish endocrinology.

Secretory Proteins in the Skin Mucus of Nile Tilapia (Oreochromis niloticus) are Modulated Temporally by Photoperiod and Bacterial Endotoxin Cues

Although it is well known that the biological and physical characteristics of skin mucus in fishes are strongly affected by changes in environmental conditions, the influence of photoperiod and time-dependent bacterial endotoxin stimulation is not well documented. In the present study, we determined the diel variations in the basal activities of secretory proteins with known defense functions in the skin mucus of Nile tilapia (Oreochromis niloticus) maintained under two photic environments: equal length of day and night (12L:12D, LD) or total darkness (0L:24D, DD). A second experiment was conducted to determine how time-dependent (i.e., day versus night) lipopolysaccharide (LPS) challenge could influence these skin mucosal defenses. The results revealed that LD signal differentially modulated the activities of mucosal immune molecules. Fish subjected to LD regime showed significantly higher levels of skin mucus lysozyme and protease at nighttime than at daytime. This distinct feature was not observed in fish under DD. There was no general mucosal response patterns to time-dependent LPS challenge. Nonetheless, protease and lysozyme, which were identified to be at elevated levels at night, were significantly modulated when the endotoxin was administered at nighttime. Ceruloplasmin was the only molecule that responded to LPS challenge at daytime, where its activity significantly increased at 8 h post-stimulation. Collectively, the results revealed that photoperiod cues influenced the activities of mucosal defenses and this may play, at least in part, in the temporal sensitivity to bacterial endotoxin.

Loss of Growth Hormone Gene (gh1) in Zebrafish Arrests Folliculogenesis in Females and Delays Spermatogenesis in Males

As a master hormone controlling growth and metabolism, GH is also known to regulate reproduction. Studies in mammals have shown that mutations in GH or its receptor (GHR) not only result in retardation in body growth but also reproductive dysfunctions in both sexes. However, the roles of GH in reproduction of other vertebrates are poorly defined. In this study, we created two zebrafish GH (gh1) mutant lines using CRISPR/Cas9. The mutant developed normally up to 14 days postfertilization (dpf); however, a high rate of mortality was observed afterward in both lines, and only a small number of mutant fish could survive to adult stage. The body growth of the mutants was significantly retarded in both sexes in a gene dose-dependent manner compared with their wild-type siblings. A severe dysfunction of gonadal development was observed in survived mutant females, with ovarian folliculogenesis being arrested completely at primary growth stage until 100 dpf. Interestingly, the folliculogenesis in the mutant resumed after months of delay with a certain number of follicles entering vitellogenic growth. As for male reproduction, although the spermatogenesis in mutant males seemed normal in adults, the GH-insufficient heterozygote showed an obvious delay of spermatogenesis (puberty onset) at early developmental stages. The adult mutant males could not breed with wild-type females through natural spawning; however, the sperm isolated from the mutant testes could fertilize eggs through artificial fertilization. This study provides further genetic evidence for the dependence of puberty onset on somatic growth, but not age, in fish.

The evolutionary consequences for seawater performance and its hormonal control when anadromous Atlantic salmon become landlocked

Populations of anadromous fish have become landlocked in relatively recent geological history (<14,000 years), but the evolutionary impacts of this altered lifecycle on traits underlying seawater performance have not been established. In order to examine the effects of relaxed selection on seawater traits, anadromous and landlocked Atlantic salmon were reared under identical conditions and examined for differences in seawater performance and its underlying physiological and endocrine control during the time of spring downstream migration. Salinity tolerance, survival and initial growth in seawater were greater in anadromous than in landlocked salmon. Abundance of the seawater isoform of gill Na⁺/K⁺-ATPase increased in spring in both strains but was greater in anadromous salmon. Hormones associated with seawater acclimation (adrenocorticotropic hormone, cortisol and growth hormone) increased in spring in both strains but were higher in anadromous salmon, whereas plasma thyroid hormones did not differ. Hypothalamic urotensin I mRNA levels also increased in spring and were higher in the anadromous strain. The results provide evidence that salinity tolerance and associated physiological traits are regulated by seasonal stimulation of the hypothalamic-pituitary-interrenal axis, and that relaxed selection on seawater entry traits has decreased this stimulation in landlocked salmon.

The effect of salinity and photoperiod on thermal tolerance of Atlantic and coho salmon reared from smolt to adult in recirculating aquaculture systems

Land-based, closed containment salmon aquaculture involves rearing salmon from smolt to adult in recirculating aquaculture systems (RAS). Unlike in open-net pen aquaculture, rearing conditions can be specified in RAS in order to optimize growth and physiological stress tolerance. The environmental conditions that yield optimal stress tolerance in salmon are, however, unknown. To address this knowledge gap, we reared Atlantic (Salmo salar) and coho (Oncorhynchus kisutch) salmon in 7 separate RASs for 400 days post-smoltification under 2 photoperiods (24:0 or 12:12, light:dark) and 4 salinities (2.5, 5, 10 or 30 ppt.) and assessed the effects of these conditions on thermal tolerance. We found that over the first 120 days post-smoltification, rearing coho under a 24:0 photoperiod resulted in a ~2 °C lower critical thermal maxima (CT_max) than in coho reared under a 12:12 photoperiod. This photoperiod effect did not persist at 200 and 400 days, which was coincident with an overall decrease in CTmax in coho. Finally, Atlantic salmon had a higher CT_max (~28 °C) compared to coho (~26 °C) at 400 days post-smoltification. Overall, these findings are important for the future implications of RAS and for the aquaculture industry to help identify physiologically sensitive time stages.

In vivo effects of 17α-ethinylestradiol, 17β-estradiol and 4-nonylphenol on insulin-like growth-factor binding proteins (igfbps) in Atlantic salmon

Feminizing endocrine disrupting compounds (EDCs) affect the growth and development of teleost fishes. The major regulator of growth performance, the growth hormone (Gh)/insulin-like growth-factor (Igf) system, is sensitive to estrogenic compounds and mediates certain physiological and potentially behavioral consequences of EDC exposure. Igf binding proteins (Igfbps) are key modulators of Igf activity, but their alteration by EDCs has not been examined. We investigated two life-stages (fry and smolts) of Atlantic salmon (Salmo salar), and characterized how the Gh/Igf/Igfbp system responded to waterborne 17α-ethinylestradiol (EE₂), 17β-estradiol (E₂) and 4-nonylphenol (NP). Fry exposed to EE₂ and NP for 21 days had increased hepatic vitellogenin (vtg) mRNA levels while hepatic estrogen receptor α (erα), gh receptor (ghr), igf1 and igf2 mRNA levels were decreased. NP-exposed fry had reduced body mass and total length compared to controls. EE₂ and NP reduced hepatic igfbp1b1, -2a, -2b1, -4, -5b2 and -6b1, and stimulated igfbp5a. In smolts, hepatic vtg mRNA levels were induced following 4-day exposures to all three EDCs, while erα only responded to EE₂ and E₂. EDC exposures did not affect body mass or fork length; however, EE₂ diminished plasma Gh and Igf1 levels in parallel with reductions in hepatic ghr and igf1. In smolts, EE₂ and E₂ diminished hepatic igfbp1b1, -4 and -6b1, and stimulated igfbp5a. There were no signs of compromised ionoregulation in smolts, as indicated by unchanged branchial ion pump/transporter mRNA levels. We conclude that hepatic igfbps respond (directly and/or indirectly) to environmental estrogens during two key life-stages of Atlantic salmon, and thus may modulate the growth and development of exposed individuals.

The Impact of Initial Energy Reserves on Growth Hormone Resistance and Plasma Growth Hormone-Binding Protein Levels in Rainbow Trout Under Feeding and Fasting Conditions

The growth hormone (GH)-insulin-like growth factor I (IGF-I) system regulates important physiological functions in salmonid fish, including hydromineral balance, growth, and metabolism. While major research efforts have been directed toward this complex endocrine system, understanding of some key aspects is lacking. The aim was to provide new insights into GH resistance and growth hormone-binding proteins (GHBPs). Fish frequently respond to catabolic conditions with elevated GH and depressed IGF-I plasma levels, a condition of acquired GH resistance. The underlying mechanisms or the functional significance of GH resistance are, however, not well understood. Although data suggest that a significant proportion of plasma GH is bound to specific GHBPs, the regulation of plasma GHBP levels as well as their role in modulating the GH-IGF-I system in fish is virtually unknown. Two in vivo studies were conducted on rainbow trout. In experiment I, fish were fasted for 4 weeks and then refed and sampled over 72 h. In experiment II, two lines of fish with different muscle adiposity were sampled after 1, 2, and 4 weeks of fasting. In both studies, plasma GH, IGF-I, and GHBP levels were assessed as well as the hepatic gene expression of the growth hormone receptor 2a (ghr2a) isoform. While most rainbow trout acquired GH resistance within 4 weeks of fasting, fish selected for high muscle adiposity did not. This suggests that GH resistance does not set in while fat reserves as still available for energy metabolism, and that GH resistance is permissive for protein catabolism. Plasma GHBP levels varied between 5 and 25 ng ml-1, with large fluctuations during both long-term (4 weeks) fasting and short-term (72 h) refeeding, indicating differentiated responses depending on prior energy status of the fish. The two opposing functions of GHBPs of prolonging the biological half-life of GH while decreasing GH availability to target tissues makes the data interpretation difficult, but nutritional regulatory mechanisms are suggested. The lack of correlation between hepatic ghr2a expression and plasma GHBP levels indicate that ghr2a assessment cannot be used as a proxy measure for GHBP levels, even if circulating GHBPs are derived from the GH receptor molecule.

Variation in branchial expression among insulin-like growth-factor binding proteins (igfbps) during Atlantic salmon smoltification and seawater exposure

Background

In preparation for migration from freshwater to marine habitats, Atlantic salmon (Salmo salar L.) undergo smoltification, a transformation that includes the acquisition of hyposmoregulatory capacity. The growth hormone (Gh)/insulin-like growth-factor (Igf) axis promotes the development of branchial ionoregulatory functions that underlie ion secretion. Igfs interact with a suite of Igf binding proteins (Igfbps) that modulate hormone activity. In Atlantic salmon smolts, igfbp4,−5a,−5b1,−5b2,−6b1 and−6b2 transcripts are highly expressed in gill. We measured mRNA levels of branchial and hepatic igfbps during smoltification (March, April, and May), desmoltification (July) and following seawater (SW) exposure in March and May. We also characterized parallel changes in a broad suite of osmoregulatory (branchial Na⁺/K⁺-ATPase (Nka) activity, Na⁺/K⁺/2Cl⁻cotransporter 1 (nkcc1) and cystic fibrosis transmembrane regulator 1 (cftr1) transcription) and endocrine (plasma Gh and Igf1) parameters.

Results

Indicative of smoltification, we observed increased branchial Nka activity, nkcc1 and cftr1 transcription in May. Branchial igfbp6b1 and -6b2 expression increased coincidentally with smoltification. Following a SW challenge in March, igfbp6b1 showed increased expression while igfbp6b2 exhibited diminished expression. igfbp5a,−5b1 and−5b2 mRNA levels did not change during smolting, but each had lower levels following a SW exposure in March.

Conclusions

Salmonids express an especially large suite of igfbps. Our data suggest that dynamic expression of particular igfbps accompanies smoltification and SW challenges; thus, transcriptional control of igfbps may provide a mechanism for the local modulation of Igf activity in salmon gill.

Hepatic insulin-like growth-factor binding protein (igfbp) responses to food restriction in Atlantic salmon smolts

The growth hormone (Gh)/insulin-like growth-factor (Igf) system plays a central role in the regulation of growth in fishes. However, the roles of Igf binding proteins (Igfbps) in coordinating responses to food availability are unresolved, especially in anadromous fishes preparing for seaward migration. We assayed plasma Gh, Igf1, thyroid hormones and cortisol along with igfbp mRNA levels in fasted and fed Atlantic salmon (Salmo salar). Fish were fasted for 3 or 10days near the peak of smoltification (late April to early May). Fasting reduced plasma glucose by 3days and condition factor by 10days. Plasma Gh, cortisol, and thyroxine (T4) were not altered in response to fasting, whereas Igf1 and 3,5,3'-triiodo-l-thyronine (T3) were slightly higher and lower than controls, respectively. Hepatic igfbp1b1, -1b2, -2a, -2b1 and -2b2 mRNA levels were not responsive to fasting, but there were marked increases in igfbp1a1 following 3 and 10days of fasting. Fasting did not alter hepatic igf1 or igf2; however, muscle igf1 was diminished by 10days of fasting. There were no signs that fasting compromised branchial ionoregulatory functions, as indicated by unchanged Na(+)/K(+)-ATPase activity and ion pump/transporter mRNA levels. We conclude that dynamic hepatic igfbp1a1 and muscle igf1 expression participate in the modulation of Gh/Igf signaling in smolts undergoing catabolism.

Cadmium effects on the retina of adult Danio rerio

The aim of this work is to describe the effects of cadmium pollution on the vision of adult zebrafish, Danio rerio. Retinal morpho-cytological alterations were investigated by light and electron microscopy, while the functionality of cadmium-exposed retinae was assessed by re-illumination behavioral tests with white or colored light. Our results demonstrate that cadmium toxicity causes significant degeneration and loss of organization at both macro and microscopic levels. These alterations impair functional responses particularly through an increase in light sensitivity. Metallothioneins were not seen to be up-regulated, while the recovery of visual acuity is due to a regenerative process by Müller cells.

Plasma growth hormone-binding protein levels in Atlantic salmon Salmo salar during smoltification and seawater transfer

Specific growth hormone (GH)-binding protein (Ghbp) was purified from Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss plasma with immunoprecipitation and characterized in cross-linking studies using autoradiography and western blots. The size of the Ghbp was estimated to be c. 53 kDa. A radioimmunoassay was established to measure Ghbp in salmonids, using antibodies specific against the extracellular segment of the S. salar growth hormone receptor 1 (grh1; GenBank AY462105). Plasma Ghbp levels were measured in S. salar smolts in fresh water and after transfer to seawater (SW; experiments 1 and 2), and in post-smolts kept at different salinities (0, 12, 22 and 34) for 3 months (experiment 3). A transient increase in plasma Ghbp, which lasted for 1 month or less, was noted in smolts after transfer to SW. Concomitantly, plasma GH and gill Na(+) -K(+) -ATPase activity increased during smoltification (in experiment 2). No difference in plasma Ghbp was evident between post-smolts kept at different salinities, although the fish kept at salinity 34 had higher plasma GH than the group kept at salinity 22 and higher hepatic ghr1 expression than post-smolts kept at salinity 12. This suggests that plasma Ghbp regulation may respond to salinity changes in the short term. The lack of correlation between Ghbp, plasma GH and hepatic ghr1 expression in the long-term post-smolt experiment indicates that Ghbp levels may be regulated independently of other components of the endocrine GH system in salmonids.

Long-term effects of photoperiod, temperature and their interaction on growth, gill Na⁺, K⁺-ATPase activity, seawater tolerance and plasma growth-hormone levels in Atlantic salmon Salmo salar

This study was undertaken to examine the long-term effects of photoperiod, temperature and their interaction on growth, gill Na⁺,K⁺-ATPase (NKA) activity, seawater tolerance and plasma growth-hormone levels in Atlantic salmon Salmo salar pre-smolts and smolts. The fish (mean ± s.e. initial body mass = 15·9 ± 0·4 g) were reared on two photoperiods (continuous light, LL, and simulated natural photoperiod, LDN, 60° 25' N) and two temperatures (8·3 and 12·7° C) from June to May of the following year. Mean body mass was affected by photoperiod, temperature and their interactions. Both temperature groups on LL developed peak levels in gill NKA activity from October to November, 4-5 months prior to the natural season for the parr-smolt transformation. Fish at 12° C showed peak levels in NKA activity 4-6 weeks before the fish at 8° C. Fish in all four experimental groups showed maximum NKA activity within a similar size range (113-162 g). The present findings further indicate that smoltification in S. salar is to some extent driven by size, and that S. salar will develop smolt characteristics, e.g. a marked increase in NKA activity, within a similar size range. Faster-growing S. salar will, thus, reach this size threshold at a relatively younger age.

Physiology during smoltification in Atlantic salmon: effect of melatonin implants

Melatonin implants were used to override natural melatonin rhythm in groups of juvenile Atlantic salmon, Salmo salar, raised at simulated natural photoperiod (SNP) and constant light (LL) from mid-March until end of August. The experiment contained also both sham control (with non-melatonin implants) and control (no implants). No differences were found in the experimental variables between these two control groups. Growth and food intake were negatively affected by melatonin implantation. Overall, higher GH levels were observed in the SNP melatonin-implanted group, whereas no differences in GH levels were seen between the SNP control, LL control, or the LL melatonin-implanted groups. Highest food intake was seen in the LL control group. No differences in food intake were recorded between the LL melatonin-implanted and SNP control groups. Gill Na(+), K(+), ATPase (NKA) activity was influenced by time as well as the interaction between photoperiod and time. No differences in gill NKA activity or plasma chloride levels following transfer to seawater were seen between the groups with melatonin implants and their controls. Based on the present results, it seems apparent that melatonin does play a role in regulating food intake and growth in Atlantic salmon smolts.

Differential regulation of sodium–potassium pump isoforms during smolt development and seawater exposure of Atlantic salmon

Freshwater and seawater isoforms of the alpha subunit of Na+/K+-ATPase (NKA) have previously been identified in gill ionocytes of Atlantic salmon (Salmo salar). In the present study we examine the abundance and cellular localization of these isoforms during the parr–smolt transformation, a developmental process that is preparatory for seawater entry. The abundance of NKAα1a was lower in smolts than in parr, remained relatively constant during spring and decreased in summer. NKAα1b increased tenfold in smolts during spring, peaking in late April, coincident with downstream migration and increased salinity tolerance. NKAα1b increased a further twofold after seawater exposure of smolts, whereas NKAα1a decreased by 98%. The abundance of NKAα1b-positive, and NKAα1b and NKAα1a co-labeled ionocytes increased during smolt development, whereas the number of NKAα1a cells decreased. After seawater exposure of smolts, NKAα1b-positive ionocytes increased, NKAα1a-positive cells decreased, and co-labeled cells disappeared. Plasma growth hormone and cortisol increased during spring in smolts, but not in parr, peaking just prior to the highest levels of NKAα1b. The results indicate that the increase in the abundance of NKAα1b during smolt development is directly linked to the increase in salinity tolerance that occurs at this stage, but that significant changes also occur after seawater exposure. Spring increases in circulating levels of growth hormone and cortisol indicate that these hormones may be instrumental in upregulating NKAα1b during smolt development.

Estrogenic compounds decrease growth hormone receptor abundance and alter osmoregulation in Atlantic salmon

Exposure of Atlantic salmon smolts to estrogenic compounds is shown to compromise several aspects of smolt development. We sought to determine the underlying endocrine mechanisms of estrogen impacts on the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis. Smolts in freshwater (FW) were either injected 3 times over 10 days with 2 μgg(-1) 17β-estradiol (E2) or 150μgg(-1) 4-nonylphenol (NP). Seawater (SW)-acclimated fish received intraperitoneal implants of 30 μgg(-1) E2 over two weeks. Treatment with these estrogenic compounds increased hepatosomatic index and total plasma calcium. E2 and NP reduced maximum growth hormone binding by 30-60% in hepatic and branchial membranes in FW and SW, but did not alter the dissociation constant. E2 and NP treatment decreased plasma levels of IGF-I levels in both FW and SW. In FW E2 and NP decreased plasma GH whereas in SW plasma GH increased after E2 treatment. Compared to controls, plasma chloride concentrations of E2-treated fish were decreased 5.5mM in FW and increased 10.5mM in SW. There was no effect of NP or E2 on gill sodium-potassium adenosine triphosphatase (Na(+)/K(+)-ATPase) activity in FW smolts, whereas E2 treatment in SW reduced gill Na(+)/K(+)-ATPase activity and altered the number and size of ionocytes. Our data indicate that E2 downregulates the GH/IGF-I-axis and SW tolerance which may be part of its normal function for reproduction and movement into FW. We conclude that the mechanism of endocrine disruption of smolt development by NP is in part through alteration of the GH/IGF-I axis via reduced GH receptor abundance.

The role of growth hormone in growth, lipid homeostasis, energy utilization and partitioning in rainbow trout: interactions with leptin, ghrelin and insulin-like growth factor I

The growth-promoting effects of in vivo growth hormone (GH) treatment were studied in relation to size and lipid content of energy stores including liver, mesentery, white muscle and belly flap in rainbow trout. In order to elucidate endocrine interactions and links to regulation of growth, adiposity and energy metabolism, plasma levels of GH, insulin-like growth factor I (IGF-I), leptin (Lep) and ghrelin, were assessed and correlated to growth and energy status. In addition tissue-specific expression of lepa1 mRNA was examined. Juvenile rainbow trout were implanted with sustained-release bovine GH implants and terminally sub-sampled at 1, 3 and 6 weeks. GH increased specific growth rate, reduced condition factor (CF) and increased feed conversion efficiency resulting in a redistribution of energy stores. Thus, GH decreased mesenteric (MSI) and liver somatic index (LSI). Lipid content of the belly flap increased following GH-treatment while liver and muscle lipid content decreased. Independent of GH substantial growth was accompanied by an increase in muscle lipids and a decrease in belly flap lipids. The data suggest that the belly flap may function as an energy buffering tissue during episodes of feeding and lean growth. Liver and muscle lipids were positively correlated to body weight, indicating a size-dependent change in adiposity. Hepatic lepa1 mRNA positively correlated to MSI and CF and its expression decreased following GH treatment, coinciding with decreased hepatic lipid content. Plasma Lep was positively correlated to MSI and belly flap lipid content, suggesting that Lep may communicate energy status. In summary, the observed GH tissue-specific effects on lipid metabolism in rainbow trout highlight the complex physiology of the energy reserves and their endocrine control.

Physiology and mRNA expression in rainbow trout (Oncorhynchus mykiss) after long-term exposure to the new antifoulant medetomidine

Medetomidine is under evaluation for use as an antifouling agent, and its effects on non-target aquatic organisms are therefore of interest. In this study, rainbow trout was exposed to low (0.5 and 5.0nM) concentrations of medetomidine for up to 54 days. Recently we have reported on effects on paleness and melanophore aggregation of medetomidine in these fish. Here, specific growth rates were investigated together with a broad set of physiological parameters including plasma levels of growth hormone (GH), insulin-like growth factor-I (IGF-I) and leptin, glucose and haemoglobin (Hb), hematocrit (Ht), condition factor, liver and heart somatic indexes (LSI, HSI). Hepatic enzyme activities of CYP1A (EROD activity), glutathione S-transferases (GST) and glutathione reductase (GR) were also measured. Additionally, hepatic mRNA expression was analysed through microarray and quantitative PCR in fish sampled after 31 days of exposure. Medetomidine at both concentrations significantly lowered blood glucose levels and the higher concentration significantly reduced the LSI. The mRNA expression analysis revealed few differentially expressed genes in the liver and the false discovery rate was high. Taken together, the results suggest that medetomidine at investigated concentrations could interfere with carbohydrate metabolism of exposed fish but without any clear consequences for growth.

An in vitro model for evaluating peripheral regulation of growth in fish: effects of 17β-estradiol and testosterone on the expression of growth hormone receptors, insulin-like growth factors, and insulin-like growth factor type 1 receptors in rainbow trout (Oncorhynchus mykiss)

A central component of growth coordination in vertebrates is the growth hormone (GH)-insulin-like growth factor-1 (IGF-1) system. To date, most studies on the control of vertebrate growth have focused on regulation of pituitary GH production and release. In this study, we used liver, muscle, and gill tissue from sexually immature rainbow trout incubated in vitro to evaluate the extrapituitary effects of 17β-estradiol (E2) and testosterone (T) on mRNA and functional expression of growth hormone receptors (GHR), insulin-like growth factors 1 and 2 (IGF-1, IGF-2), and type 1 IGF receptors (IGFR1). E2 significantly decreased steady-state levels of GHR1, GHR2, and IGF-1 mRNAs in liver as well as of GHR1 and GHR2 mRNAs in muscle and of IGF-1 and IGF-2 mRNAs in gill in a time- and concentration-dependent manner. E2 had no effect on levels of IGFR1 mRNAs in muscle or on GHR and IGFR1 mRNAs in gill. Functional expression of GHRs as assessed by (125)I-GH binding capacity was reduced by E2 in liver and muscle; however, E2 did not affect (125)I-IGF-1 binding capacity in muscle or (125)I-GH and (125)I-IGF-1 binding capacity in gill. By contrast, T increased steady-state levels of GHR1, GHR2, IGF-1, and IGF-2 mRNAs in liver, of GHR1, GHR2, IGFR1A, and IGFR1B in muscle, and of GHR1, GHR2, IGF-1, IGF-2, IGFR1A, and IGFR1B mRNAs in gill in a time- and concentration-dependent manner. Binding capacity of (125)I-GH in liver and of (125)I-GH and (125)I-IGF-1 in both muscle and gill also was increased by T. These data indicate that E2 and T directly affect peripheral aspects of the GH-IGF system, and suggest, at least in immature rainbow trout, that E2 reduces hepatic sensitivity to GH as well as reduces peripheral production of IGFs and that T increases peripheral sensitivity to GH and IGF as well as increases peripheral production of IGFs.

Pituitary and plasma growth hormone dynamics during sexual maturation of female Atlantic salmon

Growth hormone in fish regulates many important physiological processes including growth, metabolism and potentially reproduction. In salmonid fish, GH secretion is episodic with irregularly spaced GH peaks. Plasma GH reflects secretion episodes as well as the clearance rate of the hormone, and plasma levels may thus not always reflect the level of activation of the GH axis. This study measured the production dynamics of GH over a 17-month period in sexually maturing female Atlantic salmon which included final maturation and spawning. For the first time, the level of pituitary GH mRNA, pituitary GH protein and plasma GH protein were analyzed concurrently in the same individuals. mRNA and protein were extracted in parallel from the same samples with subsequent real time quantitative PCR to measure mRNA transcripts and radioimmunoassay to measure pituitary and plasma GH protein. Further, the effects of photoperiod manipulation on these parameters were studied. The results show no correlation between mRNA and protein levels except at some time points, and indicate that it is inappropriate to correlate pooled temporal data and averages in time series unless the relationship among the variables is stable over time. The results indicate complex and shifting relationships between pituitary GH mRNA expression, pituitary GH content and plasma GH levels, which could result from changes in clearance rather than secretion rate at different times and its episodic secretion. The study also suggests that there is a functionally important activation of the GH system during spring leading up to maturation and spawning.

Light, time, and the physiology of biotic response to rapid climate change in animals

Examination of temperate and polar regions of Earth shows that the nonbiological world is exquisitely sensitive to the direct effects of temperature, whereas the biological world is largely organized by light. Herein, we discuss the use of day length by animals at physiological and genetic levels, beginning with a comparative experimental study that shows the preeminent role of light in determining fitness in seasonal environments. Typically, at seasonally appropriate times, light initiates a cascade of physiological events mediating the input and interpretation of day length to the output of specific hormones that ultimately determine whether animals prepare to develop, reproduce, hibernate, enter dormancy, or migrate. The mechanisms that form the basis of seasonal time keeping and their adjustment during climate change are reviewed at the physiological and genetic levels. Future avenues for research are proposed that span basic questions from how animals transition from dependency on tropical cues to temperate cues during range expansions, to more applied questions of species survival and conservation biology during periods of climatic stress.

Ghrelin decreases food intake in juvenile rainbow trout (Oncorhynchus mykiss) through the central anorexigenic corticotropin-releasing factor system

Ghrelin stimulates pituitary growth hormone (GH) release, and has a key role in the regulation of food intake and adiposity in vertebrates. To investigate the central effect of native rainbow trout ghrelin (rtghrelin) on food intake in rainbow trout, as well as its possible mode of action, four groups of fish received a single injection into the third brain ventricle (i.c.v. injection): (1) control group (physiological saline) (2) ghrelin-treated group (2.0 ng rtghrelin g bwt(-1)), (3) group given the corticotropin-releasing hormone receptor antagonist alpha-helical CRF 9-41 (ahCRF) (4.0 ng g bwt(-1)) and (4) group receiving the same dose of both ghrelin and ahCRF. Food intake was assessed 1h after treatment. In addition, the presence of the GHS-R (the ghrelin receptor) in the rainbow trout CNS was examined with Western blot. To investigate peripheral effects of ghrelin, rainbow trout received an intraperitoneal cholesterol-based implant with or without rtghrelin, and daily food intake was measured during 14 days. Weight and length were measured at the start and termination of the experiment and specific growth rates were calculated. Mesenteric fat stores, muscle and liver lipid content were analysed after the treatment period. Central ghrelin injections decreased food intake compared with controls, and treatment with ahCRF abolished the ghrelin-effect. Western blot analysis of the GHS-R revealed a single band at around 60 kDa in pituitary, hypothalamus, brain and stomach. Long-term peripheral ghrelin treatment decreased daily food intake compared with controls. This was reflected in a ghrelin-induced decrease in weight growth rate (p<0.06). There was no effect of ghrelin on plasma GH levels or tissue fat stores. The conclusion from this study is that the GHS-R is indicated in the CNS in rainbow trout and that ghrelin may act there as an anorexigenic hormone, through a CRF-mediated pathway. Elevated peripheral ghrelin levels also seem to lead to decreased feed intake in the longer term.

Control of puberty in farmed fish

Puberty comprises the transition from an immature juvenile to a mature adult state of the reproductive system, i.e. the individual becomes capable of reproducing sexually for the first time, which implies functional competence of the brain-pituitary-gonad (BPG) axis. Early puberty is a major problem in many farmed fish species due to negative effects on growth performance, flesh composition, external appearance, behaviour, health, welfare and survival, as well as possible genetic impact on wild populations. Late puberty can also be a problem for broodstock management in some species, while some species completely fail to enter puberty under farming conditions. Age and size at puberty varies between and within species and strains, and are modulated by genetic and environmental factors. Puberty onset is controlled by activation of the BPG axis, and a range of internal and external factors are hypothesised to stimulate and/or modulate this activation such as growth, adiposity, feed intake, photoperiod, temperature and social factors. For example, there is a positive correlation between rapid growth and early puberty in fish. Age at puberty can be controlled by selective breeding or control of photoperiod, feeding or temperature. Monosex stocks can exploit sex dimorphic growth patterns and sterility can be achieved by triploidisation. However, all these techniques have limitations under commercial farming conditions. Further knowledge is needed on both basic and applied aspects of puberty control to refine existing methods and to develop new methods that are efficient in terms of production and acceptable in terms of fish welfare and sustainability.

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.

A homologous salmonid leptin radioimmunoassay indicates elevated plasma leptin levels during fasting of rainbow trout

The present study was conducted to establish a homologous radioimmunoassay (RIA) for quantifying plasma leptin (Lep) levels in salmonid species, and to study Lep levels in relation to nutritional status. A part of the Lep peptide, a 14 amino acid long sequence, identical between a Salmo and an Oncorhynchus species was synthesised. Polyclonal antibodies were raised in rabbit against this antigen and both were subsequently used in the development of a RIA protocol for assessing plasma Lep levels. The limit of detection of the assay was 0.3 nM, and intra- and interassay coefficient of variation (CV) were 8.4% and 13%, respectively. Apart from Atlantic salmon and rainbow trout, the assay exhibits measuring parallelism for a range of fish species, including arctic char, Atlantic cod and turbot, suggesting that the established RIA is useful for quantifying Lep levels in several fish species. The RIA indicates that Lep is found in salmonid plasma at levels of 0.5-5 nM, which is comparable with other peptide hormones, and well within the measuring range of the RIA. A study of fed and fasted rainbow trout showed elevated plasma Lep levels during fasting. In addition there was no correlation between Lep levels and condition factor. These data suggest that the relation between circulating Lep levels and energy status differs from that in mammals. While Lep is linked to energy balance, it may not act as an adiposity signal in salmonids, possibly pointing to functional divergence among ectothermic and endothermic vertebrates.

Seasonal and circadian time-dependent dual action of GH on somatic growth and ovarian development in the Asian catfish, Clarias batrachus (Linn.): role of temperature

Effect of growth hormone (GH), injected either at the morning or evening, on the somatic and ovarian development of the Asian catfish, Clarias batrachus was studied during the late quiescence and early recrudescence phase of the reproductive cycle. GH treatment during the late quiescence phase (December-January; 11L: 13D, average water temperature 16.2+/-1 degrees C) failed to exert any impact on the somatic or ovarian development, except that it increased plasma testosterone marginally, and decreased hepatic level of triglycerides (TG); while during the early recrudescence phase (March-April; 12.4L: 11.20D; average water temperature-28.4+/-2 degrees C), GH had significant circadian time-dependent effect on somatic and ovarian development. GH injection at morning stimulated ovarian growth and development, as was evident from the dose-dependent significant increase in ovarian weight, gonadosomatic index, plasma levels of testosterone and estradiol-17beta following GH treatment. The ovarian morphology also revealed the advancement in follicular development in GH treated fish. GH also induced remarkable increase in ovarian levels of TG, free cholesterol (CF) and phospholipids (PL) with concomitant declines in their hepatic levels. However, evening hour GH injection provoked appreciable increase in body weight without having any significant impact on ovarian weight and GSI. Few oocytes in the ovary of evening hour GH treated fish exhibited advanced stages of development, but number of such advanced stage oocytes was significantly much less than the morning hour GH injected fish. Ovarian TG and PL remained unchanged, while their hepatic levels were increased. The temporal synergism between GH and other intrinsic factors regulating body growth and gonadal development may be suggested herewith to be considered prior to administration of GH in fishes. This study has immense practical use in aquaculture.

Integrative and genomics approaches to uncover the mechanistic bases of fish behavior and its diversity

Understanding the molecular mechanisms underlying fish behavior is of fundamental importance to further our understanding of the proximate and ultimate causes of variation in this trait and informs us on issues of animal husbandry, conservation, and welfare. One way to approach this question is to study variation in gene expression in individuals exhibiting different behaviors and relating it to variations at other phenotypic levels in an organismic, ecological and evolutionary context. Here we review studies that have shown that the use of such an integrative and genomics approach is greatly useful for shedding new light on the mechanisms of behaviors as diverse as social dominance, mate choice, reproduction and migration. We present studies that use functional genomics tools and integrate several biological levels of organization, including transcription variation, which are important in the context of integrative biology and genomics of fish behavior. We review studies of phenotype-level variation in transcription but also studies that focus on variation at the individual-level. Dissecting the molecular bases of among-individual variation in behavior, including the study of variation in temperament (behavioral syndrome/coping style) within and among populations, will gain importance in the field in the years to come.

Effects of short-term acid and aluminum exposure on the parr-smolt transformation in Atlantic salmon (Salmo salar): disruption of seawater tolerance and endocrine status

Episodic acidification resulting in increased acidity and inorganic aluminum (Al(i)) is known to interfere with the parr-smolt transformation of Atlantic salmon (Salmo salar), and has been implicated as a possible cause of population decline. To determine the extent and mechanism(s) by which short-term acid/Al exposure compromises smolt development, Atlantic salmon smolts were exposed to either control (pH 6.7-6.9) or acid/Al (pH 5.4-6.3, 28-64 microgl(-1) Al(i)) conditions for 2 and 5 days, and impacts on freshwater (FW) ion regulation, seawater (SW) tolerance, plasma hormone levels and stress response were examined. Gill Al concentrations were elevated in all smolts exposed to acid/Al relative to controls confirming exposure to increased Al(i). There was no effect of acid/Al on plasma ion concentrations in FW however, smolts exposed to acid/Al followed by a 24h SW challenge exhibited greater plasma Cl(-) levels than controls, indicating reduced SW tolerance. Loss of SW tolerance was accompanied by reductions in gill Na(+),K(+)-ATPase (NKA) activity and Na(+),K(+),2Cl(-) (NKCC) cotransporter protein abundance. Acid/Al exposure resulted in decreased plasma insulin-like growth factor (IGF-I) and 3,3',5'-triiodo-l-thyronine (T(3)) levels, whereas no effect of treatment was seen on plasma cortisol, growth hormone (GH), or thyroxine (T(4)) levels. Acid/Al exposure resulted in increased hematocrit and plasma glucose levels in FW, but both returned to control levels after 24h in SW. The results indicate that smolt development and SW tolerance are compromised by short-term exposure to acid/Al in the absence of detectable impacts on FW ion regulation. Loss of SW tolerance during short-term acid/Al exposure likely results from reductions in gill NKA and NKCC, possibly mediated by decreases in plasma IGF-I and T(3).

Endocrine systems in juvenile anadromous and landlocked Atlantic salmon (Salmo salar): seasonal development and seawater acclimation

The present study compares developmental changes in plasma levels of growth hormone (GH), insulin-like growth factor I (IGF-I) and cortisol, and mRNA levels of their receptors and the prolactin receptor (PRLR) in the gill of anadromous and landlocked Atlantic salmon during the spring parr-smolt transformation (smoltification) period and following four days and one month seawater (SW) acclimation. Plasma GH and gill GH receptor (GHR) mRNA levels increased continuously during the spring smoltification period in the anadromous, but not in landlocked salmon. There were no differences in plasma IGF-I levels between strains, or any increase during smoltification. Gill IGF-I and IGF-I receptor (IGF-IR) mRNA levels increased in anadromous salmon during smoltification, with no changes observed in landlocked fish. Gill PRLR mRNA levels remained stable in both strains during spring. Plasma cortisol levels in anadromous salmon increased 5-fold in May and June, but not in landlocked salmon. Gill glucocorticoid receptor (GR) mRNA levels were elevated in both strains at the time of peak smoltification in anadromous salmon, while mineralocorticoid receptor (MR) mRNA levels remained stable. Only anadromous salmon showed an increase of gill 11beta-hydroxysteroid dehydrogenase type-2 (11beta-HSD2) mRNA levels in May. GH and gill GHR mRNA levels increased in both strains following four days of SW exposure in mid-May, whereas only the anadromous salmon displayed elevated plasma GH and GHR mRNA after one month in SW. Plasma IGF-I increased after four days in SW in both strains, decreasing in both strains after one month in SW. Gill IGF-I mRNA levels were only increased in landlocked salmon after 4days in SW. Gill IGF-IR mRNA levels in SW did not differ from FW levels in either strain. Gill PRLR mRNA did not change after four days of SW exposure, and decreased in both strains after one month in SW. Plasma cortisol levels did not change following SW exposure in either strain. Gill GR, 11beta-HSD2 and MR mRNA levels increased after four days in SW in both strains, whereas only the anadromous strain maintained elevated gill GR and 11beta-HSD2 mRNA levels after one month in SW. The results indicate that hormones and receptors of the GH and cortisol axes are present at significantly lower levels during spring development and SW acclimation in landlocked relative to anadromous salmon. These findings suggest that attenuation of GH and cortisol axes may, at least partially, result in reduced preparatory upregulation of key gill ion-secretory proteins, possibly a result of reduced selection pressure for marine adaptations in landlocked salmon.

Effects of hexazinone and atrazine on the physiology and endocrinology of smolt development in Atlantic salmon

Exposure to hexazinone (HEX) and atrazine (ATZ), highly mobile and widely used herbicides along rivers in the United States, is potentially harmful to Atlantic salmon, which have been listed as an endangered species. To determine the effects of these contaminants on smolt development, juvenile Atlantic salmon were exposed under flow-through conditions to 100 microgl(-1) HEX, 10 and 100 microgl(-1) ATZ in fresh water (FW) for 21 days at 10 degrees C beginning in mid-April. Twelve fish per treatment were sampled in FW, following a 24h seawater (SW) challenge and after growth for 3 months in SW. Exposure to 100 microgl(-1) HEX or 10microgl(-1) ATZ caused no mortalities of smolts in FW or after SW challenge, while 9% of the fish exposed to 100 microgl(-1) ATZ died during exposure. Fish exposed to 100 microgl(-1) ATZ reduced feeding after 10 days of exposure and had an impaired growth rate in FW and during the first month in SW; compensatory growth occurred in the second and third month in SW. HEX and ATZ at 10 microgl(-1) exposure had no effect on plasma levels of cortisol, growth hormone (GH), insulin growth factor I (IGF-I), thyroxine (T(4)) and plasma 3,5,3'-triiodo-l-thyronine (T(3)), Cl(-), Mg(2+), Na(+), Ca(2+) in FW or after SW challenge. FW smolts exposed to 100 microgl(-1) ATZ had decreased plasma Cl(-), Mg(2+), Na(+) and Ca(2+) ions and increased cortisol. No effect on plasma levels of GH, IGF-I, T(4) or T(3) was found in FW smolts exposed to 100 microgl(-1) ATZ. Following SW challenge, fish previously exposed to 100 microgl(-1) ATZ had significant increases in hematocrit, plasma cortisol, Cl(-), Mg(2+), Na(+), Ca(2+) and a decrease in T(4) and T(3). It is concluded that under the conditions imposed in this study, HEX does not affect salinity tolerance of Atlantic salmon smolts, while ATZ causes ionoregulatory, growth and endocrine disturbance.

Plasma ghrelin levels in rainbow trout in response to fasting, feeding and food composition, and effects of ghrelin on voluntary food intake

Ghrelin, a peptide hormone which stimulates growth hormone (GH) release, appetite and adiposity in mammals, was recently identified in fish. In this study, the roles of ghrelin in regulating food intake and the growth hormone (GH)-insulin-like growth factor I (IGF-I) system of rainbow trout (Oncorhynchus mykiss) were investigated in three experiments: 1) Pre- and postprandial plasma levels of ghrelin were measured in relation to dietary composition and food intake through dietary inclusion of radio-dense lead-glass beads, 2) the effect of a single intraperitoneal (i.p.) injection with rainbow trout ghrelin on short-term voluntary food intake was examined and 3) the effect of one to three weeks fasting on circulating ghrelin levels and the correlation with plasma GH and IGF-I levels, growth and lipid content in the liver and muscle was studied. There was no postprandial change in plasma ghrelin levels. Fish fed a normal-protein/high-lipid (31.4%) diet tended to have higher plasma ghrelin levels than those fed a high-protein/low-lipid (14.1%) diet. Plasma ghrelin levels decreased during fasting and correlated positively with specific growth rates, condition factor, liver and muscle lipid content, and negatively with plasma GH and IGF-I levels. An i.p. ghrelin injection did not affect food intake during 12-hours post-injection. It is concluded that ghrelin release in rainbow trout may be influenced by long-term energy status, and possibly by diet composition. Further, in rainbow trout, ghrelin seems to be linked to growth and metabolism, but does not seem to stimulate short-term appetite through a peripheral action.

Larval exposure to 4-nonylphenol and 17beta-estradiol affects physiological and behavioral development of seawater adaptation in Atlantic salmon smolts

Population declines of anadromous salmonids are attributed to anthropogenic disturbances including dams, commercial and recreational fisheries, and pollutants, such as estrogenic compounds. Nonylphenol (NP), a xenoestrogen, is widespread in the aquatic environment due to its use in agricultural, industrial, and household products. We exposed Atlantic salmon yolk-sac larvae to waterborne 10 or 100 microg L(-1) NP (NP-L or NP-H, respectively), 2 microg L(-1) 17beta-estradiol (E2), or vehicle, for 21 days to investigate their effects on smolt physiology and behavior 1 year later. NP-H caused approximately 50% mortality during exposure, 30 days after exposure, and 60 days after exposure. Mortality rates of NP-L and E2 fish were not affected until 60 days after treatment, when they were 4-fold greater than those of controls. Treatment with NP-L or E2 as yolk-sac larvae decreased gill sodium-potassium-activated adenosine triphosphatase (Na+,K(+)-ATPase) activity and seawater (SW) tolerance during smolt development, 1 year after exposure. Exposure to NP-L and E2 resulted in a latency to enter SW and reduced preference for SW approximately 2- and 5-fold, respectively. NP-L-exposed fish had 20% lower plasma insulin-like growth factor I (IGF-I) levels and 35% lower plasma triiodothyronine (T3). Plasma growth hormone and thyroxine (T4) were unaffected. Exposure to E2 did not affect plasma levels of IGF-I, GH, T3, or T4. Both treatment groups exhibited increased plasma cortisol and decreased osmoregulatory capacity in response to a handling stressor. These results suggest that early exposure to environmentally relevant concentrations of NP, and other estrogenic compounds, can cause direct and delayed mortalities and that this exposure can have long-term, "organizational" effects on life-history events in salmonids.

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.

Growth hormone and insulin-like growth factor-I act together and independently when regulating growth in vertebral and muscle tissue of atlantic salmon postsmolts

Aiming to elucidate the role of GH and IGF-I with regard to vertebral and white muscle growth, gene expression of the GH and IGF-I receptors (ghr and igf-Ir, respectively) and local IGF-I (igf-I) were analyzed during spring growth (January-June) in Atlantic salmon postsmolts. One group of fish was reared under natural light (NL), while one group was reared under continuous light (LL). Growth rate of fork length was higher in the LL group for a short period after onset of continuous light (LL: 0.50+/-0.02 mm day(-1), NL: 0.43+/-0.01 mm day(-1)) and for a longer period at the end of the experiment in June (LL: 1.18+/-0.06 mm day(-1), NL: 0.75+/-0.02 mm day(-1)). Likewise, growth rate in length of vertebra No. 40 in the LL group was higher than in the NL group the first period after onset of light (LL: 0.015+/-0.002 mm day(-1), NL: 0.008+/-0.001 mm day(-1)). Plasma GH levels peaked in late February and were higher in the LL group than in the NL group (LL: 7.27+/-0.61 ng ml(-1), NL: 2.60+/-0.50 ng ml(-1)), whereas plasma IGF-I levels peaked in early February and were unaffected by photoperiod. ghr expression was upregulated in late February in liver (12-fold), white muscle (6-fold) and vertebral tissue (3-fold) and higher in the LL group than in the NL group (2-fold) in vertebral tissue in late March. White muscle expression of igf-I and igf-Ir decreased from initial levels throughout the experiment. Hepatic gene expression of igf-I doubled in both groups in late February, followed by a 4-fold upregulation in June in the LL group only. Vertebral tissue expression of igf-I (4-fold) and igf-Ir (6-fold) increased in May and were unaffected by photoperiod. One exception was a smaller upregulation of igf-I (2-fold) in the LL group in early February. In conclusion, GH appears to have an initial role in stimulating vertebral growth, while IGF-I seems to stimulate growth during late spring. It is suggested that local IGF-I acts as a paracrine agent, evaluated from the concurrent upregulation of igf-I and igf-Ir. The upregulation of ghr in white muscle tissue, concurrent with a downregulation of muscle igf-I and igf-Ir, indicate that GH stimulated growth or metabolism independent of IGF-I.