Immunoreactive somatolactin cells in the pituitary of young, migrating, spawning and spent chinook salmon, Oncorhynchus tshawytscha

Effect of stress during handling, seawater acclimation, confinement, and induced spawning on plasma ion levels and somatolactin-expressing cells in mature female Liza ramada

The present experiments were designed to determine the effect of different stress factors; handling, seawater acclimation, confinement, and induced spawning on plasma cortisol, hydro mineral balance as well as changes in size, number and integrated intensity of somatolactin (SL)-expressing cells in Liza ramada mature females confined to fresh water ponds. The plasma levels of cortisol, PO(4)(3-), Na(+), and K(+) were higher, while Ca(2+) and Mg(2+) were lower than controls during transportation without anesthesia. By using clove oil (5 mg L(-1)) as an anesthetic during transportation, the plasma cortisol, PO(4) (3-), Na(+), and K(+) were similar to controls, while Ca(2+) and Mg(2+) were higher. During seawater acclimation, the plasma cortisol and minerals were significantly higher except Na(+) which was lower than controls. In addition, during induction of spawning, the plasma levels of cortisol, PO(4)(3-), Na(+), K(+), and Mg(2+) were significantly higher than controls. The SL-producing cells are located in the pars intermedia (PI) bordering the neurohypophysis. The stress affected the number, size, and immunostaining of SL-expressing cells. During seawater acclimation, the size and the integrated intensity of SL immunoreactivity were lower, but the number of these cells was higher than controls. Furthermore, the number, size, and the integrated intensity of SL immunoreactivity were significantly lower than controls during handling and after spawning, which was opposite to confinement. The response of SL-expressing cells in PI in parallel with changes in cortisol and hydro mineral balance induced by stress support the possible role of SL in the adaptive response of fish to stress.

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.

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.

In vitro effect of leptin on somatolactin release in the European sea bass (Dicentrarchus labrax): dependence on the reproductive status and interaction with NPY and GnRH

The aim of the present work was to investigate the neuroendocrine control of pituitary somatolactin (SL) release using dispersed pituitary cell culture obtained from male European sea bass (Dicentrarchus labrax) at different stages of sexual development. The effect of mouse recombinant leptin, sea bream gonadotropin releasing-hormone (sbGnRH) and porcine neuropeptide Y (pNPY) and their potential interaction on the SL release were investigated. High doses of leptin (10(-8)-10(-6)M) were differentially effective in inducing SL release depending on the sexual developmental stage. Porcine NPY alone was not effective on basal SL release, but it dose-dependently (0.1 and 1 nM) enhanced SL release induced by leptin (10(-6) and 10(-8)M) in late pre-pubertal but not in post-pubertal stages. No effect of sbGnRH in association or not with leptin was observed on SL release. These findings are the first evidences that leptin and pNPY can play an important role in the neuroendocrine control of pars intermedia function and SL release in fish. In addition, the sensitivity of SL producing cells to leptin and NPY only in prepubertal and pubertal stages, provides the potential role of SL in the nutritional control of the onset of puberty.

Relation between the reproductive status and somatolactin cell activity in the pituitary of pejerrey, Odontesthes bonariensis (Atheriniformes)

The aim of this study was to analyze the relationship between somatolactin (SL) expressing cells and the reproductive status in a multiple spawning fish, the pejerrey Odontesthes bonariensis. Somatolactin cells were identified in adults of both sexes by immunocytochemistry using a heterologous piscine antiserum. The area of the cells that showed immunoreactivity to SL (ir-SL) was compared in specimens with different degrees of reproductive activity as inferred from histological examination of the gonads and calculation of the gonadosomatic index (GSI %). The results showed a significant difference between the area of ir-SL cells of resting/regressing (62.9 +/- 2.1 micron 2) and sexually active/vitellogenic (76.8 +/- 2.3 micron 2) females and a significant positive correlation between the ir-SL cellular area and the GSI % (P < 0.01 in both cases). In males, the correlation between the area of ir-SL cells and the GSI % was not statistically significant. However, in those animals with the highest GSI % values, the ir-SL cells appeared more numerous and showed an increase in the immunostained area when compared to individuals with lower GSI % values. The present in morphological observations are in accordance with biochemical data obtained from other species and support the assumption that SL might be involved in the regulation of reproduction in fish.

Somatotropic regulation of fish growth and adiposity: growth hormone (GH) and somatolactin (SL) relationship

Growth hormone (GH) and insulin-like growth factors (IGFs) play a major role in fish development and metabolism, and several studies have allowed discernment of a complex and tissue-specific collection of salmonid IGF-I transcripts (Ea-4, Ea-3, Ea-2, Ea-1), which are the result of the alternative splicing of the E-domain region. However, the pattern of IGF-I expression is different in non-salmonid fish, and only one or two transcripts (Ea-4, Ea-2) have been detected in hepatic and extrahepatic tissues of common carp, barramundi, black sea bream and gilthead sea bream. Despite this, when comparisons are made within Mediterranean fish species (European sea bass, common dentex and gilthead sea bream), plasma IGF-I levels are consistent with fish species differences in growth rates. Changes of growth rates, and plasma IGF-I and GH levels are also found in response to changes in diet composition and ration size, which may serve to assess the suitability of feeding regimes in aquaculture practice. Regulation of plasma somatolactin (SL) levels is also examined in gilthead sea bream, and the resulting plasma SL profile differs from that of GH. Thus, in contrast to GH, plasma SL levels augment with the increase of ration size and fish size (advancement of age). A transient increase in plasma SL levels is also found in short-term fasted fish, and this fish peptide may act as an anti-obesity hormone helping to expedite growth-reproductive processes following replenishment of fat stores, and/or mediate the adaptation to fasting until the lipolytic action of GH and/or other endocrine factors is fully accomplished. This agrees with the known increase of plasma SL levels during acute stress and exhaustive exercise. However, a causal link between SL and energy mobilisation (lipid metabolism) remains to be established, and further research is needed to determine the extent to which SL and GH act in a complementary manner to make available metabolic fuels and to regulate body fat mass and feeding behaviour.

Implication of somatolactin in the regulation of sexual maturation and spawning of Mugil cephalus

Specific antibody for chum salmon somatolactin (SL) was used for immunocytochemical investigation of SL cell activity of Mugil cephalus during the gonadal cycle in both natural habitat and captivity. The SL-immunoreactive cells showed strong and specific immunoreactivity to antichum salmon SL. The number of SL-immunoreactive cells increased, as did the secretory and synthetic activity during sexual maturation and spawning in the natural habitat. The SL cells were rather small and moderately immunoreactive in immature fish; they were enlarged, their numbers increased, and they frequently showed more SL immunoreactivity during gonadal development. In addition, during late stages of maturation, small cell size with more or less SL immunoreactive cells were noted, indicating an active release of SL granules. Prespawning females tended to have more enlarged SL cells with stronger immunoreactivity than equivalent males. The SL cells showed an increase in the secretory activity during spawning as indicated by small size and weak immunoreactivity. The SL cells of M. cephalus reared in captivity showed high activity. This may be due to the low concentration of calcium in fresh water. The gradual stimulation of SL synthesis and release during sexual maturation and spawning of M. cephalus suggest that SL may be involved in the control of some steps of reproductive processes, such as steroidogenesis, calcium metabolism, and energy mobilization.

Isolation and cDNA cloning of somatolactin in rabbitfish (Siganus guttatus)

We report the isolation and cDNA cloning of somatolactin (SL) from rabbitfish, Siganus guttatus. Rabbitfish SL was isolated from an alkaline extract of the pituitary glands by gel filtration chromatography on Sephadex G-100 and reversed-phase high-performance liquid chromatography. SL was monitored by immunoblotting with flounder SL antiserum. The preparation (yield: 0.86 mg/g wet tissues) contained two immunoreactive bands of 24 and 28 kDa on SDS-PAGE. Overlapping partial cDNA clones corresponding to teleost SLs were amplified by PCR from single-strand cDNA from pituitary glands. Excluding the poly(A) tail, rabbitfish SL cDNA is 1605 bp long. It contains a 693-bp open reading frame encoding a signal peptide of 24 amino acids (aa) and a mature protein of 207 aa. Rabbitfish SL has two possible N-glycosylation sites at positions 11 and 121 and seven half Cys residues. The deduced amino acid sequence shows over 80% identity with those of advanced teleosts like sea bream, red drum, and flounder, 76% with the salmonids, 57% with the eel, and 46% with the goldfish SL.

Somatolactin in the white sturgeon and African lungfish and its evolutionary significance

Somatolactin (SL) is a newly characterized pituitary hormone belonging to the growth hormone-prolactin family. Until now SL has been identified only in teleosts, the most highly derived ray-finned fishes. We report here the cloning of SL cDNAs from two species of bony fish, the white sturgeon (Acipenser transmontanus) and the African lungfish (Protopterus annectens). Overlapping partial cDNA clones corresponding to teleost SLs were amplified by polymerase chain reaction (PCR) from either single-strand or double-strand cDNA from pituitary glands. Excluding the poly(A) tail, the sturgeon SL cDNA is 881 base pairs (bp). This is comparable to 1.0 kb estimated by Northern blot analysis. It contains a 696-bp open reading frame encoding a prehormone of 232 amino acids (aa) with a signal peptide of 24 aa and a mature protein of 208 aa. Excluding the poly(A) tail, the lungfish SL cDNA is 938 bp. This is comparable to 1.1 kb estimated by Northern blot analysis. It contains a 696-bp open reading frame encoding a prehormone of 232 aa with a signal peptide of 26 aa and a mature protein of 206 aa. The deduced aa sequences of sturgeon and lungfish SLs show 76-60% and 65-54% identity with teleost SLs, respectively. These values are significantly higher than the 30% identity with nonteleostean growth hormones and prolactins. Immunostaining of sturgeon pituitary with anti-salmon SL serum demonstrated that the SL cells were localized in the pars intermedia, as in teleosts. The present results demonstrate that the SL gene is present in two divergent lineages, the Actinopterygii (Chondrostei: white sturgeon) and the Sarcopterygii (Dipnoi: African lungfish).

Effects of gonadectomy and steroids on plasma and pituitary levels of somatolactin in Atlantic salmon, Salmo salar

Somatolactin (SL) is a hormone recently isolated and characterized from fish pituitaries. Although the functions of SL are still largely unknown, it has been implicated in reproduction. In this study, the effects of gonadal steroids on SL secretion were investigated in Atlantic salmon male parr in two experiments. In both, mature males were gonadectomized in the autumn and implanted with Silastic capsules containing testosterone (T), 11-ketoandrostenedione (11kA), or 17 alpha,20 beta-dihydroxy-4-pregnene-3-one (20-P), gonadectomized alone, or sham-operated. In addition, immature males were implanted with T or 11kA in experiment 1. After 4-5 weeks pituitaries and plasma were sampled and SL levels measured by radioimmunoassay (RIA). Plasma levels of T, 11-ketotestosterone, and 20-P were also measured by RIA. In experiment 1, initial immature males had lower (0.7 +/- 0.4 ng/ml) plasma SL levels than initial mature males (3.3 +/- 0.4 ng/ml), whereas pituitary content was not influenced. Gonadectomy significantly reduced both plasma SL levels (experiment 1, sham controls 5.6 +/- 0.5 ng/ml, castrated 1.6 +/- 0.5 ng/ml; experiment 2, sham controls 6.5 +/- 1.1 ng/ml, castrated 3.3 +/- 0.4 ng/ml) and the pituitary content of SL (experiment 1, sham controls 1206 +/- 187 ng/pituitary, castrated 663 +/- 104 ng/pituitary; experiment 2, sham controls 1043 +/- 199 ng/pituitary, castrated 629 +/- 70 ng/pituitary), suggesting that the testes stimulated the synthesis and release of pituitary SL. Overall, the effects of steroid replacement were inconsistent between the experiments, although in experiment 2 castrated males receiving the highest dose of T had significantly higher plasma SL levels (8.2 +/- 1.2 ng/ml) than all other castrated groups (1.8-4.3 ng/ml).

Immunocytochemical and ultrastructural characterization of prolactin, growth hormone, and somatolactin cells from the Mediterranean yellowtail (Seriola dumerilii, Risso 1810)

BACKGROUND

Prolactin (PRL), growth hormone (GH), and somatolactin (SL) are structurally related pituitary hormones that belong to a peptide family. Whereas growth hormone and prolactin are present in the hypophysis of all vertebrates, somatolactin, a recently discovered hormone, has been found only in fish. It has been demonstrated immunocytochemically in a few teleost species; ultrastructurally, cells producing this hormone have been characterized only in one species of salmon. In this paper, we identify and characterize ultrastructurally the cells producing these three hormones in Mediterranean yellowtail (Seriola dumerilii).

METHODS

Pituitaries from adult specimens were dissected out and processed for electron microscopy. The immunogold technique was performed in some ultrathin sections using fish primary antibodies.

RESULTS

PRL cells had round, peripherally distributed, very electron-dense, homogeneous secretory granules of variable size. GH cells had dense, round secretory granules with a conspicuous scalloped membrane, which were grouped in the cell area near the neurohypophysis. SL cells had round, polymorphic, or very irregularly shaped secretory granules, the last seeming to arise from the fusion of various secretory granules. The population of secretory granules varied greatly from one cell to another. In all cases, immunogold labeling was seen exclusively in the secretory granules. Exocytosis was observed in all cell types. Some of the PRL, GH, and SL cells showed involutive features.

CONCLUSIONS

PRL, GH, and SL, although structurally and functionally related, are secreted by ultrastructurally different cells in the pituitary of M. yellowtail.

The adenohypophysis of Mediterranean yellowtail, Seriola dumerilii (Risso, 1810): an immunocytochemical study

The adenohypophysis (ADH) of the Mediterranean yellowtail was studied using the peroxidase-antiperoxidase technique. Human corticotropin (ACTH) (1-24)-immunoreactive (ir) cells were found bordering the neurohypophysis (NH) and salmon prolactin (PRL)-ir cells were arranged in thick cords, both in the rostral pars distalis (RPD). Gonadotropin (GTH)-, thyrotropin (TSH)- and growth hormone (GH)-ir cells were observed in the proximal pars distalis (PPD). Anti-chum salmon GTH I and anti-chum salmon GTH II immunostained the same cells in the outermost part of the ADH at the level of the PPD and the PI. In addition to these cells, some cells grouped in the inner areas of the posterior PPD were revealed by catfish alpha, beta-GTH antiserum. Human beta-TSH-ir cells formed small groups and discontinuous strands in the PPD often in contact with the NH. Tilapia GH-ir cells formed cords mainly surrounding the NH in the central PPD, while cod somatolactin- and alpha MSH-ir cells mainly surrounded the NH branches in the PI.

Cell biology of somatolactin

Somatolactin (SL) is a novel pituitary protein, isolated for the first time from the Atlantic cod. The corresponding proteins have been identified in several toleost species, but not in other classes of vertebrates. Comparison of amino acid sequence has revealed SL molecules to be related to growth hormone (GH) and prolactin (PRL) in teleosts and other vertebrates, suggesting that SL is a new member of the GH/PRL family. Unlike GH and PRL, SL can exist in either glycosylated or nonglycosylated form, depending on the species; most teleosts possess glycosylated SLs, except for salmonids whose SLs are simple proteins. The SL-producing cells are located in the pars intermedia bordering the neurohypophysis and are distinct from melanocyte-stimulating hormone (MSH)-producing cells. The SL cells are PAS positive in most teleosts but chromophobic in salmonids, which may reflect the glycosylation status of SL. Its biochemical and molecular features have become increasingly clear, whereas its physiological significance is still poorly understood. Several possible roles for SL have been suggested, including roles in maturation, calcium regulation, stress response, acid-base regulation, fat metabolism, and background adaptation. Although direct evidence is lacking for any of the proposed functions, this involvement in acid-base regulation appears most probable, since other proposed biological events linked to SL should more or less affect the acid-base status in fish. More detailed studies are needed to define the function of SL.