Effects of aromatase inhibitors and different doses of testosterone on gonadotropins in one year old male Atlantic salmon (Salmo salar)

Effect of prolonged feeding of broodstock diet with increased inclusion of essential n-3 fatty acids on maturing and spawning performance in 3-year-old Atlantic salmon (Salmo salar)

Atlantic salmon (Salmo salar) broodstock recruits are normally fed a specialized diet with a higher content of essential nutrients for a limited time period prior to fasting and transfer to freshwater. Typically, this period lasts for about six months, but may vary among producers. Reduced use of marine ingredients in commercial salmon diets during the last decades has affected the content of essential nutrients, such as n-3 long chained polyunsaturated fatty acids (LC-PUFA), minerals and vitamins. Furthermore, to minimize the risk of losses and implement new breeding achievements faster, breeding companies have shortened the production cycle of broodstock from 4 to 3 years, which may affect the number of fish that are large enough to mature. In the present study, we have extended the broodstock feeding period from 6 to 15 months prior to the freshwater transfer giving a higher content of n-3 LC-PUFA (higher inclusion of marine oils) from February to December (Phase 1), and thereafter a diet with a higher energy content to ensure growth towards the spring and maturation (Phase 2). Four sea cages with approximately 80.000 salmon postsmolt, two sea cages with males and two with females, were given a control diet and an experimental diet. Samples were taken in Phase 1 at start (1.7 kg), mid (3.4 kg) and end Phase 1/start of Phase 2 (8.3 kg), and end of Phase 2 (13.4 kg). The fish were thereafter fasted, and selected fish transferred to landbased freshwater tanks where light and temperature were used to manipulate the spawning time of the fish in two groups (early or late). Due to disease in the facility, measures of egg quality and hatching were only obtained from the early group. During the trial and spawning period, biometrical measurements were recorded, and samples of liver, gonad, fillet and red blood cells (RBC) were collected for fatty acid composition and blood plasma for analysis of lipid and health-related parameters. Samples were also collected for gonadal transcriptomic analysis by microarray and qPCR (end Phase 2) and plasma steroids (end Phase 2, mid maturation and spawning). Males fed the test diet had a larger body size compared to the control group at the end of Phase 2, while no differences were observed between dietary groups for the females. Total mortality in the trial was lower in the test group compared to the control, losses were caused mainly by sea lice treatments, loser fish or cardiomyopathy syndrome (CMS). The dietary LC-PUFA levels in the test diet were reflected in the tissues particularly during Phase 1, but only different in the fillet samples and eggs at the end of Phase 2 and at spawning. Plasma sex steroids content increased at mid maturation and showed lower levels of androgens and estrogens in females fed the test diet compared to the control. At the end of Phase 2, transcriptional analysis showed upregulation of steroidogenic enzymes, although not reflected in changes in plasma steroids in Phase 2, indicating changes to come during maturation. The differences in LC-PUFA content in tissues and plasma steroids did not appear to affect fecundity, sperm quality, egg survival or hatching rate, but the test group had larger eggs compared to the control in the early spawner-group. Prolonged feeding of n-3 LC-PUFA to pre-puberty Atlantic salmon broodstock appears to be important for higher survival in challenging sea cage environments and has an effect on sex steroid production that, together with high energy diet during early maturation, cause the test group to produce larger eggs.

Pituitary Gonadotropin Gene Expression During Induced Onset of Postsmolt Maturation in Male Atlantic Salmon: In Vivo and Tissue Culture Studies

Precocious male maturation causes reduced welfare and increased production costs in Atlantic salmon (Salmo salar) aquaculture. The pituitary produces and releases follicle-stimulating hormone (Fsh), the gonadotropin triggering puberty in male salmonids. However, little is known about how Fsh production is regulated in Atlantic salmon. We examined, in vivo and ex vivo, transcriptional changes of gonadotropin-related genes accompanying the initial steps of testis maturation, in pituitaries of males exposed to photoperiod and temperature conditions promoting maturation (constant light and 16°C). Pituitary fshb, lhb and gnrhr2bba transcripts increased in vivo in maturing males (gonado-somatic index > 0.1%). RNA sequencing (RNAseq) analysis using pituitaries from genetically similar males carrying the same genetic predisposition to mature, but differing by responding or not responding to stimulatory environmental conditions, revealed 144 differentially expressed genes, ~2/3rds being up-regulated in responders, including fshb and other pituitary hormones, steroid-related and other puberty-associated transcripts. Functional enrichment analyses confirmed gene involvement in hormone/steroid production and gonad development. In ex vivo studies, whole pituitaries were exposed to a selection of hormones and growth factors. Gonadotropin-releasing hormone (Gnrh), 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) up-regulated gnrhr2bba and lhb, while fshb was up-regulated by Gnrh but down-regulated by 11-KT in pituitaries from immature males. Also pituitaries from maturing males responded to Gnrh and sex steroids by increased gnrhr2bba and lhb transcript levels, but fshb expression remained unchanged. Growth factors (inhibin A, activin A and insulin-like growth factor 1) did not change gnrhr2bba, lhb or fshb transcript levels in pituitaries either from immature or maturing males. Additional pituitary ex vivo studies on candidates identified by RNAseq showed that these transcripts were preferentially regulated by Gnrh and sex steroids, but not by growth factors, and that Gnrh/sex steroids were less effective when incubating pituitaries from maturing males. Our results suggest that a yet to be characterized mechanism up-regulating fshb expression in the salmon pituitary is activated in response to stimulatory environmental conditions prior to morphological signs of testis maturation, and that the transcriptional program associated with this mechanism becomes unresponsive or less responsive to most stimulators ex vivo once males had entered pubertal developmental in vivo.

Phospholipid and LC-PUFA metabolism in Atlantic salmon (Salmo salar) testes during sexual maturation

The importance of dietary lipids in male reproduction are not as well understood as in females, in which dietary lipids, such as phospholipids (PL) and associated fatty acids (FA), are important structural components of the eggs and provide energy for their offspring. In mammals, lipids are suggested to be important for spermatogenesis and to structural components of the spermatozoa that could improve fertilization rates. New knowledge of how lipids affect sexual maturation in male Atlantic salmon (Salmo salar), an important global aquaculture species, could provide tools to delay maturation and/or improve reproductive success. Therefore, changes in testicular composition of lipids and gene transcripts associated with spermatogenesis and lipid metabolism were studied in sexually maturing male salmon compared to immature males and females. An increase in total testis content of FA and PL, and a shift to higher PL composition was observed in maturing males, concomitant with increases in mRNA levels for genes involved in spermatogenesis, FA uptake and synthesis, and production of long chain-polyunsaturated fatty acids (LC-PUFA) and PL. A particularly interesting finding was elevated testis expression of acyl-CoA synthetase 4 (acsl4), and acyl-CoA thioesterase 2 (acot2), critical enzymes that regulate intra-mitochondrial levels of 20:4n-6 FA (arachidonic acid), which have been associated with improved cholesterol transport during steroidogenesis. This suggested that FA may have direct effects on sex steroid production in salmon. Furthermore, we observed increased testis expression of genes for endogenous synthesis of 16:0 and elongation/desaturation to 22:6n-3 (docosahexaenoic acid) in sexually maturing males relative to immature fish. Both of these FA are important structural components of the PL, phosphatidylcholine (PC), and were elevated concomitant with increases in the content of phosphatidic acid, an important precursor for PC, in maturing males compared to immature fish. Overall, this study suggests that, similar to mammals, lipids are important to spermatogenesis and serve as structural components during testicular growth and maturation in Atlantic salmon.

Effects of androgens on the leptin system in immature male Atlantic salmon parr

Leptin modulates all levels of the reproductive endocrine axis in mammals, and in turn, both leptin and the leptin receptor are regulated by sex steroids. The aim of this study was to investigate if sex steroids regulate the leptin system also in fish. Immature one-year old male Atlantic salmon parr were implanted with Silclear capsules that were either empty or filled with 11-ketoandrostenedione (11KA) or testosterone (T) and the effects of 35-days treatment were investigated on measures of maturation, gene expression of leptin (lepa1, lepa2), leptin receptor (lepra1) and circulating plasma leptin. Both 11-KA and T stimulated the reproductive axis by increasing testes weight and up-regulated pituitary lh-β mRNA levels and for T also fsh-β. T up-regulated transcription levels of lepa1 and lepra1 in the pituitary, while 11-KA had no effect. Leptin receptor expression in the testis was unaltered by either androgen. T up-regulated lepa1 mRNA levels significantly also in the liver, but had no effect on lepa2, and 11KA did not affect hepatic gene expression of either lepa1 or lepa2. Plasma leptin levels did not differ significantly between treatments. The results indicate that androgens regulate gene expression of leptin and the leptin receptor in different tissues in fish and that the effects of leptin might be tissue specific considering plasma levels remained unaltered. Overall, the results suggest a role for leptin in fish reproduction, where sex steroids are able to regulate components of the leptin system differentially in liver and important tissues of the reproductive axis.

Development of approaches to induce puberty in cultured female sablefish (Anoplopoma fimbria)

Efforts to establish sustainable and efficient aquaculture production of sablefish (Anoplopoma fimbria) have been constrained by delayed puberty in cultured females. This study integrates a series of experiments aimed at gaining an understanding of the reproductive physiology of puberty in female sablefish. We detected transcripts for the dopamine D2 receptor (drd2) in brain, pituitary and ovary of sablefish, and prepubertal females exhibited significantly elevated brain and pituitary drd2 expression relative to wild maturing females. Treatments with sustained-release cholesterol pellets containing testosterone (T) and the dopamine D2 receptor antagonist, metoclopramide (Met), stimulated expression of pituitary luteinizing hormone beta subunit (lhb) and follicle-stimulating hormone beta subunit (fshb), respectively, in prepubertal females, whereas a combination of T and gonadotropin-releasing hormone agonist (GnRHa) had a strong synergistic effect on lhb expression (2000-fold higher than control). Although T induced a significant increase in the maximum ovarian follicle volume, none of the treatments tested stimulated onset of vitellogenesis. Using liquid chromatography/tandem mass spectrometry, we demonstrated that Met stimulated production of T by previtellogenic ovarian follicles in vitro, whereas gonadotropin preparations enhanced 17α-hydroxyprogesterone, androstenedione (A4), T and 17β-estradiol (E2) production. Treatment with T increased production of A4, 11β-hydroxyandrostenedione, 11β-hydroxytestosterone, E2, 11-ketotestosterone, and 5α-dihydrotestosterone (DHT). Interestingly, in the presence of high doses of T the previtellogenic ovary preferentially produced A4 and DHT over any other metabolite. Our data suggest the existence of dopamine inhibition of the reproductive axis in female sablefish. Treatments with Met and T elevated gonadotropin mRNAs in prepubertal females but failed to stimulate the transition into vitellogenic growth, suggesting a possible failure in pituitary gonadotropin protein synthesis/release. Previtellogenic ovarian follicles of sablefish are equipped to synthesize steroids, including those required for vitellogenic growth, and DHT, a steroid hormone whose role in reproduction of fishes remains unknown.

Seasonal changes and endocrine regulation of pejerrey (Odontesthes bonariensis) spermatogenesis in the wild

The endocrine mechanisms that regulate spermatogenesis and their interaction with environmental cues have been poorly studied compared with oogenesis in fish. The aim of this work was to study the spermatogenesis in pejerrey under the influence of photoperiod and water temperature fluctuation in the wild, evaluating the transcript levels of brain Gnrh variants and cyp19a1b, pituitary Gth subunits, gonadal Gth receptors, 11β-hsd, and 11-KT plasma levels. Males at spermiogenic stage were observed during spring and autumn, under a photoperiod above 11h of light and a water temperature below 23 °C. Most arrested males were observed in summer when water temperatures increased above 23 °C. Males at spermatogonial stage were mainly observed in autumn, while most males at spermatocytary stage were caught in winter. An increase of gnrh-I, cyp19a1b, fshb, gpha and 11β-hsd transcripts and 11-KT plasma levels was observed during spermatogonial and/or spermatocytary stage (early spermatogenesis). The spermiogenic stage was associated to the maximum gnrh-I gene expression level and a significant increase of Gth receptors transcripts, being this fact more evident for lhcgr. During this last gonadal stage, cyp19a1b transcript level remained high, while fshb mRNA and 11-KT plasma levels showed a significant decreased compared to that occurred at the spermatocytary stage. Also, gphα and 11β-hsd gene expression levels fell during spermiation up to similar values to those observed in arrested males. A significant correlation between 11-KT and gnrh-I, cyp19a1b, gphα, fshb, 11β-hsd transcripts, and the number of spermatocytes was observed during spermatogenesis. All these findings suggested that in pejerrey, the spermatocyte proliferation occurs mainly during winter under the stimulation of 11-KT induced by FSH through the stimulation of specific enzymes, including the 11β-hsd while spermiation occurs after photoperiod increase and with temperatures of the water below 23 °C, through the stimulation of gnrh-I, cyp19a1b and lhcgr.

Androgens directly stimulate spermatogonial differentiation in juvenile Atlantic salmon (Salmo salar)

We studied the effects of androgens on early stages of spermatogenesis along with androgen receptor binding characteristics and the expression of selected testicular and pituitary genes. To this end, immature Atlantic salmon postsmolts received testosterone (T), adrenosterone (OA, which is converted in vivo into 11-ketotestosterone, 11-KT) or a combination of the two androgens (T+OA). Treatment with OA and T elevated the plasma levels of 11-KT and T, respectively, and co-injection of OA with T lead to high 11-KT levels but prevented plasma T levels to reach the levels observed after injecting T alone. Clear stimulatory effects were recorded as regards pituitary lhb and gnrhr4 transcript levels in fish receiving T, and to a lesser extent in fish receiving OA (but for the lhb transcript only). The two androgen receptors (Ara1 and Ara2) we cloned bound T and 11-KT and responded to these androgens in a similar way. Both androgens down-regulated testicular amh and increased igf3 transcript levels after 1 week of treatment, but effects on growth factor gene expression required sustained androgen stimulation and faded out in the groups with the decreasing T plasma levels. In fish exhibiting a sustained elevation of 11-KT plasma levels (OA and T+OA groups) for 2 weeks, the number of differentiating spermatogonia had increased while the number of undifferentiated spermatogonia decreased. Previous work showed that circulating gonadotropin levels did not increase following androgen treatments of gonad-intact immature male salmonids. Taken together, androgen treatment of immature males modulated testicular growth factor expression that, when sustained for 2 weeks, stimulated differentiation, but not self-renewal, of undifferentiated type A spermatogonia.

The progestin levonorgestrel disrupts gonadotropin expression and sex steroid levels in pubertal roach (Rutilus rutilus)

The aim of the present study was to investigate the effects of the synthetic progestin levonorgestrel (LNG) on the reproductive endocrine system of a teleost fish, the roach (Rutilus rutilus). Pubertal roach were exposed for 28 days in a flow-through system to four concentrations of LNG (3, 31, 312, and 3124 ng/l). Both males and females treated with 3124 ng/l LNG exhibited the upregulated levels of vitellogenin and oestrogen receptor 1 mRNA in the liver. At the same concentration, LNG caused a significant upregulation of the mRNA expression of the gene encoding luteinising hormone β-subunit (lhβ) and the suppression of the mRNA expression of the gene encoding follicle-stimulating hormone β-subunit (fshβ) in the pituitary of both male and female roach. A lower LNG concentration (312 ng/l) suppressed mRNA expression of fshβ in males only. Females treated with 3124 ng/l LNG exhibited significantly lower plasma 11-ketotestosterone (11-KT) and oestradiol (E2) concentrations, whereas their testosterone (T) level was higher compared with the control. Females exposed to 312 ng/l LNG presented significantly lower plasma E2 concentrations. Males exposed to ≥31 ng/l LNG exhibited significantly reduced 11-KT levels. As determined through a histological analysis, the ovaries of females were not affected by LNG exposure, whereas the testes of males exposed to 31 and 312 ng/l LNG exhibited a significantly higher percentage of spermatogonia B compared with the control. The results of the present study demonstrate that LNG disrupts the reproductive system of pubertal roach by affecting the pituitary gonadotropin expression and the sex steroid levels. This disruption was determined to occur in males after exposure to an environmentally relevant concentration (31 ng/l). Moreover, the highest tested concentration of LNG (3124 ng/l) exerted an oestrogenic effect on fish of both sexes.

Salinity and photoperiod modulate pubertal development in Atlantic salmon (Salmo salar)

The Atlantic salmon shows substantial life cycle plasticity, which also applies to the timing of puberty. While it is characterized by the activation of the brain-pituitary-gonad axis, many morphophysiological aspects of puberty and the influence of environmental conditions, such as water salinity, are not well understood in fish. Here, 12-month-old Atlantic salmon coming from an out-of-season smoltification regime in December were exposed to freshwater (FW) or seawater (SW) at 16 °C to stimulate puberty under a 24-h constant light (LL) or 12 h light:12 h darkness (LD) photoperiod. These four treatment groups (FWLL, SWLL, FWLD, and SWLD) were studied from January to March. Next to 11-ketotestosterone (11-KT) plasma levels, the expression of pituitary genes (gnrhr4, fshb, and lhb) and spermatogenesis was quantified. When spermatogonial proliferation started, fshb mRNA levels increased steeply and began to decrease when spermatogonial mitosis approached completion and most germ cells had reached meiotic or post-meiotic stages. Conversely, lhb mRNA levels increased progressively during spermatogenesis. Most males in all treatment groups matured, but exposure to SW resulted in the strongest stimulation of the onset of spermatogenesis and elevation of pituitary gnrhr4 and fshb mRNA levels. Later on, the LD photoperiod accelerated, irrespective of the salinity, the completion of spermatogenesis, associated with higher lhb mRNA and 11-KT plasma levels than in the LL groups. We find that both salinity and photoperiod modulated different aspects of spermatogenesis, and resulted in a differential activation of pituitary and testis functions; SW stimulating the onset and the shorter photoperiod the completion of spermatogenesis.

Teleost maturation-inducing hormone, 17,20β-dihydroxypregn-4-en-3-one, peaks after spawning in Tinca tinca

During an eight month study of the reproductive cycle in two age groups, and in both sexes, of tench (Tinca tinca L.), it was found that plasma concentrations of the presumptive 'maturation inducing hormone (MIH)' 17,20β-dihydroxypregn-4-en-3-one (17,20β-P) did not reach a peak during the spawning season, but as much as two months after spawning had ceased. The cessation of the spawning season was confirmed by histological examination of the gonads and by measurement of 11-ketotestosterone and 17β-estradiol in the plasma of males and females, respectively. Measurements were also made of the 'alternative MIH' 17,20β,21-trihydroxypregn-4-en-3-one in the older fish. However, this steroid did not show the same pattern as 17,20β-P. An assessment was made of the prevalence of primary spermatocytes in the testes of post-spawned fish - to test an alternative hypothesis that 17,20β-P might be involved in the stimulation of meiosis. However, there was no evidence for any increase in testis differentiation post-spawning. In fact the testes became increasingly undifferentiated as the autumn progressed. The role, if any, of this 'unseasonal' peak of 17,20β-P production remains to be determined.