Onset of gonadotropic hormone accumulation in the immature trout pituitary gland in response to estrogen or aromatizable androgen steroid hormones

Advances in Reproductive Endocrinology and Neuroendocrine Research Using Catfish Models

Catfishes, belonging to the order siluriformes, represent one of the largest groups of freshwater fishes with more than 4000 species and almost 12% of teleostean population. Due to their worldwide distribution and diversity, catfishes are interesting models for ecologists and evolutionary biologists. Incidentally, catfish emerged as an excellent animal model for aquaculture research because of economic importance, availability, disease resistance, adaptability to artificial spawning, handling, culture, high fecundity, hatchability, hypoxia tolerance and their ability to acclimate to laboratory conditions. Reproductive system in catfish is orchestrated by complex network of nervous, endocrine system and environmental factors during gonadal growth as well as recrudescence. Lot of new information on the molecular mechanism of gonadal development have been obtained over several decades which are evident from significant number of scientific publications pertaining to reproductive biology and neuroendocrine research in catfish. This review aims to synthesize key findings and compile highly relevant aspects on how catfish can offer insight into fundamental mechanisms of all the areas of reproduction and its neuroendocrine regulation, from gametogenesis to spawning including seasonal reproductive cycle. In addition, the state-of-knowledge surrounding gonadal development and neuroendocrine control of gonadal sex differentiation in catfish are comprehensively summarized in comparison with other fish models.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.

Plasticity in medaka gonadotropes via cell proliferation and phenotypic conversion

Follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) produced by the gonadotropes play a major role in control of reproduction. Contrary to mammals and birds, Lh and Fsh are mostly produced by two separate cell types in teleost. Here, we investigated gonadotrope plasticity, using transgenic lines of medaka (Oryzias latipes) where DsRed2 and hrGfpII are under the control of the fshb and lhb promotors respectively. We found that Fsh cells appear in the pituitary at 8 dpf, while Lh cells were previously shown to appear at 14 dpf. Similar to Lh cells, Fsh cells show hyperplasia from juvenile to adult stages. Hyperplasia is stimulated by estradiol. Both Fsh and Lh cells show hypertrophy during puberty with similar morphology. They also share similar behavior, using their cellular extensions to make networks. We observed bi-hormonal gonadotropes in juveniles and adults but not in larvae where only mono-hormonal cells are observed, suggesting the existence of phenotypic conversion between Fsh and Lh in later stages. This is demonstrated in cell culture, where some Fsh cells start to produce Lhβ, a phenomenon enhanced by gonadotropin-releasing hormone (Gnrh) stimulation. We have previously shown that medaka Fsh cells lack Gnrh receptors, but here we show that with time in culture, some Fsh cells start responding to Gnrh, while fshb mRNA levels are significantly reduced, both suggestive of phenotypic change. All together, these results reveal high plasticity of gonadotropes due to both estradiol-sensitive proliferation and Gnrh promoted phenotypic conversion, and moreover, show that gonadotropes lose part of their identity when kept in cell culture.

Wastewater treatment plant effluent alters pituitary gland gonadotropin mRNA levels in juvenile coho salmon (Oncorhynchus kisutch)

It is well known that endocrine disrupting compounds (EDCs) present in wastewater treatment plant (WWTP) effluents interfere with reproduction in fish, including altered gonad development and induction of vitellogenin (Vtg), a female-specific egg yolk protein precursor produced in the liver. As a result, studies have focused on the effects of EDC exposure on the gonad and liver. However, impacts of environmental EDC exposure at higher levels of the hypothalamic-pituitary-gonad axis are less well understood. The pituitary gonadotropins, follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) are involved in all aspects of gonad development and are subject to feedback from gonadal steroids making them a likely target of endocrine disruption. In this study, the effects of WWTP effluent exposure on pituitary gonadotropin mRNA expression were investigated to assess the utility of Lh beta-subunit (lhb) as a biomarker of estrogen exposure in juvenile coho salmon (Oncorhynchus kisutch). First, a controlled 72-h exposure to 17α-ethynylestradiol (EE2) and 17β-trenbolone (TREN) was performed to evaluate the response of juvenile coho salmon to EDC exposure. Second, juvenile coho salmon were exposed to 0, 20 or 100% effluent from eight WWTPs from the Puget Sound, WA region for 72h. Juvenile coho salmon exposed to 2 and 10ng EE2L(-1) had 17-fold and 215-fold higher lhb mRNA levels relative to control fish. Hepatic vtg mRNA levels were dramatically increased 6670-fold, but only in response to 10ng EE2L(-1) and Fsh beta-subunit (fshb) mRNA levels were not altered by any of the treatments. In the WWTP effluent exposures, lhb mRNA levels were significantly elevated in fish exposed to five of the WWTP effluents. In contrast, transcript levels of vtg were not affected by any of the WWTP effluent exposures. Mean levels of natural and synthetic estrogens in fish bile were consistent with pituitary lhb expression, suggesting that the observed lhb induction may be due to estrogenic activity of the WWTP effluents. These results suggest that lhb gene expression may be a sensitive index of acute exposure to estrogenic chemicals in juvenile coho salmon. Further work is needed to determine the kinetics and specificity of lhb induction to evaluate its utility as a potential indicator of estrogen exposure in immature fish.

Triacylglyceride physiology in the short-finned eel, Anguilla australis--the effects of androgen

The importance of androgens (especially 11-ketotestosterone) during previtellogenesis in eels is well established. In wild pubertal migrants, circulating 11-ketotestosterone levels correlate with a number of morphological and molecular changes. Here, we test the prediction that this correlation represents a causal relationship by artificially raising the levels of circulating 11-ketotestosterone in prepubertal nonmigratory female and pubertal, migratory male short-finned eels (Anguilla australis) using sustained-release hormone implants. In females, increases in hepatosomatic index and transcript copy numbers of hepatic apolipoprotein B and microsomal triacylglyceride transfer protein indicated increased repackaging of endogenously sourced triacylglycerides. These changes in liver measures were reflected in increased concentrations of serum triacylglycerides. However, despite a small increase in gonadosomatic index, ovarian lipoprotein receptor transcript abundances were not affected by 11-ketotestosterone. Interestingly, no such changes in hepatic gene expression were detected in a dose-response experiment using males. We propose that the androgens are inducing the observed changes in previtellogenic females, although it remains unclear to what extent these effects are direct or indirect.

Environmental concentrations of an androgenic progestin disrupts the seasonal breeding cycle in male three-spined stickleback (Gasterosteus aculeatus)

Synthetic steroid hormones from contraceptive pharmaceuticals have become global aquatic contaminants. Progestins, the synthetic analogs to progesterone, are receiving increasing attention as contaminants and have been shown to impair reproduction in fish and amphibians at low ng L(-1) concentrations. Certain progestins, such as levonorgestrel have androgenic properties and seem to be several orders of magnitude more potent in terms of reproductive impairment in fish than non-androgenic progestins and progestagens. We recently reported that levonorgestrel has strong androgenic effects in female three-spined sticklebacks (Gasterosteus aculeatus), including induction of the normally male-specific glue protein spiggin and suppression of vitellogenesis. In light of this we investigated if exposure to levonorgestrel could disrupt the highly androgen-dependent seasonal reproductive cycle in male sticklebacks. Male sticklebacks that were in the final stage of a breeding period were exposed to various concentrations of levonorgestrel for six weeks in winter conditions in terms of light and temperature, after which reproductive status was evaluated from gross morphology, histology and key gene transcript levels. During the experimental period the controls had transitioned from full breeding condition into the non-breeding state, including regression of secondary sex characteristics, cessation of spiggin production in the kidney, and resumption of spermatogenesis in the testes. This is ascribed to the natural drop in plasma androgen levels after breeding. However, in the groups concurrently exposed to levonorgestrel, transition to the non-breeding condition was dose-dependently inhibited. Our results show that levonorgestrel can disrupt the seasonal breeding cycle in male sticklebacks. The fitness costs of such an effect could be detrimental to natural stickleback populations. Some effects occurred at a levonorgestrel concentration of 6.5 ng L(-1), well within the range of levonorgestrel levels in surface waters and may therefore occur in progestin-contaminated waters. Furthermore, the effects by levonorgestrel in the present study were likely mediated mainly by its androgenic activity, and the low concentration at which they occurred makes levonorgestrel one of the most potent androgenic contaminants known.

Androgen feedback effects on LH and FSH, and photoperiodic control of reproduction in male three-spined sticklebacks, Gasterosteus aculeatus

Sexual maturation in the stickleback is controlled by photoperiod. The aim of this study was to find out whether changes in feedback effects exerted by sex steroids could mediate the photoperiodic effect, which is regarded to be of an all-or-nothing character. To that end, males were castrated and treated with different doses of testosterone (T) and in one experiment also with the aromatase inhibitor fadrozole (AI) and kept under different photoperiods. In control fish, long day (LD 16:8) stimulated maturation, associated with more hypertrophied kidneys (a secondary sexual character) and higher levels of pituitary lhb and fshb mRNA than under short day conditions (LD 8:16). Under LD 8:16, low doses of T suppressed both lhb and fshb mRNA levels. However, with the use of high doses of T and/or longer photoperiods the inhibitory effects on lhb and fshb mRNA levels became less clear or instead positive effects were observed. Under intermediate photoperiod conditions, the negative feedback effect of a low dose of T on fshb was more prominent with shorter photoperiods, whereas no such shift was observed for lhb mRNA. The inhibitory effect of the low dose of T on lhb mRNA levels under LD 8:16 was abolished by AI, whereas the stimulatory effect of the high dose of T was not. The negative feedback effects were more marked under short days than under long days, whereas positive feedback effects were more marked under long days. The suppression of both fshb and lhb mRNA levels by low androgen levels, especially under short days, may inhibit maturation completely unless a rise of androgens above threshold levels would allow complete maturation.

Androgen-specific regulation of FSH signalling in the previtellogenic ovary and pituitary of the New Zealand shortfinned eel, Anguilla australis

The evidence for androgens having a pivotal role in the functioning of the female reproductive axis--such as initiating puberty or vitellogenesis--is mounting. However, the use of aromatizable androgens and the tissue-specific focus of most studies often make it unclear if androgenic effects throughout the axis proceed via androgen or estrogen signalling mechanisms. In this study, we assessed the effects of 11-ketotestosterone (11KT, a non-aromatizable androgen) on the pituitary and ovary of previtellogenic (PV) freshwater eels Anguilla australis, comparing them with eels naturally undergoing early vitellogenesis (EV). We found that 11KT treatment produces molecular and morpho-physiological phenotypes that were generally intermediate between PV and EV. Most notably, we demonstrated that 11KT induces effects on follicle-stimulating hormone (FSH) signalling in the pituitary and ovaries that are in opposition to each other. Thus, 11KT significantly reduced fshβ subunit expression in the pituitary. At the same time, 11KT dramatically increased mRNA levels of ovarian FSH receptor and plasma levels of estradiol-17β, very likely sensitizing the previtellogenic follicle to the FSH signal. Androgens therefore may be important in facilitating puberty in the eel.

Testing the synergistic effects of GnRH and testosterone on the reproductive physiology of pre-adult pink salmon Oncorhynchus gorbuscha

To test the hypothesis that the hypothalmic gonadotropin-releasing hormone (GnRH) and testosterone (T) co-treatment stimulates both the hypothalmo-pituitary-gonadal (HPG) and hypothalmo-pituitary-interrenal axes, the reproductive and osmoregulatory responses of pre-adult pink salmon Oncorhynchus gorbuscha were compared after GnRH and T administration either alone or in combination. Relative to controls, neither GnRH nor T treatment resulted in significantly greater ovarian or testicular growth, but co-treatment significantly increased ovarian growth after 5 months. Interestingly, the stimulation was undetectable after 3 months. However, once daily photoperiod began shortening after the summer solstice, c. 2 months before the natural spawning date, GnRH+T-treated females were stimulated to produce larger ovaries. Final fish body length and the size of individual eggs did not differ among treatment groups. GnRH+T eggs, however, showed signs of advanced vitellogenesis relative to GnRH-treated and control eggs, whereas T-treated eggs became atretic. Testis size increased significantly from initial values and most males were spermiating, but this growth and development were independent of hormone treatments. Final plasma ion, metabolite and cortisol concentrations did not differ among treatment groups. It is concluded that GnRH+T co-treatment was effective in stimulating female but not male maturation. GnRH and T treatment, however, presumably had little effect on the hypothalmo-pituitary-interrenal axis as observed by ionoregulatory status.

Effects of castration and androgen-treatment on the expression of FSH-beta and LH-beta in the three-spine stickleback, gasterosteus aculeatus--feedback differences mediating the photoperiodic maturation response?

In many animals, including the three-spine stickleback (Gasterosteus aculeatus), photoperiod strongly influences reproduction. The aim of this study was to investigate if feedback mechanisms on the brain-pituitary-gonadal axis play a role in mediating the photoperiodic response in the stickleback. To that end, stickleback males, exposed to either non-stimulatory short photoperiod (light/dark 8:16) or under stimulatory long photoperiod (LD 16:8), were subjected to either sham-operation, castration, castration combined with treatment with the androgens 11-ketoandrostenedione (11KA) and testosterone (T), and the effects on levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)-beta mRNA were analyzed. During breeding season the kidney of the stickleback male hypertrophies and produces a glue used for building nests. Kidney weight and expression of both LH-beta and FSH-beta were higher in sham-operated fish kept under long than under short photoperiod. Under both photoperiods, LH-beta mRNA levels were lower in castrated males compared to sham-operated males and treatment with 11KA and T increased expression, indicating a positive feedback. A positive feedback was also found on FSH-beta expression under long photoperiod, where castration decreased, and androgen replacement restored FSH-beta mRNA expression. On the contrary, castration under short photoperiod instead increased FSH-beta levels whereas treatment with 11KA and T decreased FSH-beta expression, indicating a negative feedback on FSH-beta under these conditions. The positive feedback on FSH-beta expression under stimulatory photoperiod may accelerate maturation, whereas the negative feedback under inhibitory photoperiod may suppress maturation. This could be part of the mechanisms by which photoperiod controls maturation.

Temporal dynamics of oocyte development, plasma sex steroids and somatic energy reserves during seasonal ovarian maturation in captive Murray cod Maccullochella peelii peelii

The temporal dynamics of oocyte growth, plasma sex steroids and somatic energy stores were examined during a 12 month ovarian maturation cycle in captive Murray cod Maccullochella peelii peelii under simulated natural photothermal conditions. Ovarian function was found to be relatively uninhibited in captivity, with the exception that post-vitellogenic follicles failed to undergo final maturation, resulting in widespread pre-ovulatory atresia. Seasonal patterns of oocyte growth were characterised by cortical alveoli accumulation in March, deposition of lipids in April, and vitellogenesis between May and September. Two distinct batches of vitellogenic oocytes were found in Murray cod ovaries, indicating a capacity for multiple spawns. Plasma profiles of 17beta-oestradiol and testosterone were both highly variable during the maturation period suggesting that multiple roles exist for these steroids during different stages of oocyte growth. Condition factor, liver size and visceral fat stores were all found to increase prior to, or during the peak phase of vitellogenic growth. Murray cod appear to strategically utilise episodes of high feeding activity to accrue energy reserves early in the reproductive cycle prior to its deployment during periods of rapid ovarian growth.

Immunohistochemical detection of gonadotropin-like material in the pituitary of brown hagfish (Paramyxine atami) correlated with their gonadal functions and effect of estrogen treatment

Since hagfish are members of the most primitive group of living vertebrates, studies on their reproduction are indispensable for understanding phylogenetic aspects of vertebrate reproductive system. Nevertheless, our knowledge of the reproductive physiology of the hagfish, especially of the pituitary-gonadal axis, is almost completely lacking. In the present study, the relationship between the amount of immunoreactive gonadotropin (GTH)-like material in the pituitary gland and gonadal conditions was examined in the brown hagfish, Paramyxine atami. First, pituitary sections were stained immunohistochemically with anti-ovine LHbeta, and the degrees of the accumulation of GTH-like material were compared among three different groups of gonadal conditions; juveniles and adults with and without developing gonads. Immunoreactive GTH-like material was heavily accumulated in adults with developing gonads, whereas it was not or only weakly accumulated in juveniles or adults without developing gonads. Thus, there was a strong positive correlation between the amount of GTH-like material and gonadal conditions. Second, effect of estradiol benzoate on GTH-like material was examined using three groups of juvenile hagfish: initial control, sham control, and experimental animals. Experimental animals received estradiol benzoate resolved in sesame oil intraperitoneally every third day for 1 month, whereas sham control animals received the same doses of sesame oil. GTH-like material was heavily or moderately accumulated in most estrogen-treated animals, whereas it was not or weakly accumulated in initial or sham control animals. Thus, estrogen treatment in juvenile hagfish resulted in the large increase in the amount of GTH-like material. From these results, it is suggested the presence of not only GTH but also the hypophysial-gonadal feedback system in the hagfish.

Molecular regulation of gonadotropin secretion by gonadotropin-releasing hormone in salmonid fishes

Gonadotropin-releasing hormone (GnRH) plays a central role in the control of reproductive function in vertebrates. In salmonids, salmon GnRH (sGnRH) secreted by preoptic GnRH neurons regulates gonadal maturation through stimulation of synthesis and release of pituitary gonadotropins (GTHs). In addition, several lines of our evidence indicate that sGnRH is involved in spawning behavior, and serves to integrate the gonadal maturation with the reproductive behavior. A growing number of studies show that the effects of GnRH are mediated by multiple subtypes of GnRH receptors, successive multiple signaling pathways, and finally multiple transcription factors which act cooperatively to stimulate transcription of GTH subunit genes. This complex regulatory system of the action of GnRH may serve as a molecular basis of divergent physiological strategies of reproductive success in various vertebrate species. In this article, recent data on the molecular mechanisms of action of GnRH are reviewed with special reference to the regulation of synthesis and release of GTHs in the pituitary of salmonids to elucidate the multifunctional action of GnRH.

Estradiol-17β Induced a Reversible Sex Change in the Fingerlings of Protandrous Black Porgy, Acanthopagrus schlegeli Bleeker: The Possible Roles of Luteinizing Hormone in Sex Change1

The objectives of the present study were to investigate the effects of oral administration of estradiol-17beta (E(2)) on luteinizing hormone (LH) in plasma, aromatase activity in gonad, and sex change in the fingerlings of protandrous black porgy, Acanthopagus schlegeli Bleeker. The expression of estrogen receptor (ER) and androgen receptor (AR) transcripts in gonad was also analyzed. Undifferentiated (2-mo-old) black porgy were divided into two groups, one fed a control diet and the other a diet mixed with E(2) (6.0 mg/kg feed) for 5 mo. Fish treated with E(2) for 3 mo showed complete suppression of spermatogenesis and spermiation and induced sex change with primary oocytes. Aromatase activity in forebrain and midbrain was increased in the control in December-March (during the spawning season). E(2) stimulated aromatase activity in the brain. Higher gonadal aromatase activity in concordance with elevated levels of plasma LH was observed in the E(2) group compared with the control. After 2-mo of E(2) termination, regressed testicular tissue recovered and controlled females gradually reversed back to functional males in January and March. Plasma LH levels were higher in the E(2)-terminated group during the period of reversible sex change (from a controlled female to male) compared with the control. The expression of ER and AR transcripts was closely related to the development of testis and ovary. The data showed that E(2) induced a reversible sex change with high plasma LH. Increase of gonadal aromatase and decrease of ER/AR were associated with controlled sex change. Plasma LH levels were correlated with the conversion from a controlled female to male in black porgy.

Synergistic effects of salmon gonadotropin-releasing hormone and estradiol-17beta on gonadotropin subunit gene expression and release in masu salmon pituitary cells in vitro

Effects of salmon gonadotropin-releasing hormone (sGnRH) and estradiol-17beta (E2) on gene expression and release of gonadotropins (GTHs) were examined in masu salmon (Oncorhynchus masou) using primary pituitary cell cultures at three reproductive stages, initiation of sexual maturation in May, pre-spawning in July, and spawning in September. Amounts of GTH subunit mRNAs were determined by real-time polymerase chain reaction, and levels of GTH released in the medium were determined by RIA. In control cells, the amounts of three GTH subunit mRNAs (alpha2, FSHbeta, and LHbeta) peaked in July prior to spawning. FSH release spontaneously increased with gonadal maturation and peaked in September, whereas LH release remained low until July and extensively increased in September. Addition of E2 to the culture extensively increased the amounts of LHbeta mRNA in May and July in both sexes. It also increased the alpha2 mRNA in July in the females. In contrast, sGnRH alone did not have any significant effects on the amounts of three GTH subunit mRNAs at all stages, except for the elevation of alpha2 and FSHbeta mRNAs in July in the females. Nevertheless, synergistic effects by sGnRH and E2 were evident for all three GTH subunit mRNAs. In May, sGnRH in combination with E2 synergistically increased the amounts of LHbeta mRNA in the males and alpha2 mRNA in the females. However, in July the combination suppressed the amounts of alpha2 and FSHbeta mRNAs in the females. sGnRH alone stimulated LH release at all stages in both sexes, and the release was synergistically enhanced by E2. Synergistic stimulation of FSH release was also observed in May and July in both sexes. These results indicate that a functional interaction of sGnRH with E2 is differently involved in synthesis and release of GTH. The synergistic interaction modulates GTH synthesis differentially, depending on subunit, stage, and gender, whereas it potentiates the activity of GnRH to release GTH in any situation.

In vitro steroid production by triploid ovarian follicles

Triploid female fish are unlike normal diploids in that they show impaired oocyte growth and are unable to produce viable offspring. Compared to diploid siblings, triploid female brook trout (Salvelinus fontinalis) have significantly lower levels of plasma vitellogenin, testosterone (T), and estradiol-17beta (E(2)), and show significantly lower gonadosomatic indices. The purpose of this study was to compare in vitro steroid production by diploid and triploid ovarian follicles in order to determine whether delayed oocyte growth in triploids is the result of a decreased capacity for E(2) synthesis by triploid follicles. Intact oocytes (i.e., including follicular tissue) from several age 3(+) triploids were pooled and sorted according to small (<2mm), medium (2.2-3.0mm), and large (4.0-5.0mm) diameter size, and compared to same-sized vitellogenic oocytes from 3(+) diploids. Oocytes were incubated for 8h at 15 degrees C in the presence or absence of gonadotropin stimulus (SG-G100) and the precursors E(2), T, and cholesterol. Estradiol-17beta output following SG-G100 treatment was significantly higher than in controls for both diploid and triploid follicles. Production of E(2) was dependent on stage of oocyte and follicular development in triploids, with triploid and diploid follicles of similar diameter producing similar amounts of E(2) with SG-G100 stimulus. This suggests that ovarian steroidogenesis is similar in diploids and triploids.

Effects of progesterone and estradiol on the reproductive axis in immature diploid and triploid rainbow trout

In fish species, many studies demonstrated the crucial role of estradiol (E2) in the development of the reproductive axis, but progesterone (P) has been described mainly as a precursor steroid and no clear role by itself has been reported. Moreover, a cooperative effect of P (or another progestin) and E2 in fish has never been reported to our knowledge. In the present work, we investigated the effects of P, alone or in combination with E2, on the reproductive-axis of immature rainbow trout (Oncorhynchus mykiss). Liver vitellogenin and estradiol receptor (rtER) mRNA levels increased after E2 treatment, but were unchanged by P treatments as a reflection of peripheral action of steroids. In contrast, at the pituitary level, LH contents increased after E2 and/or P treatments. Focusing on the brain level, we confirmed a clear up regulation of rtER expression by E2 in sterile triploid females, and we also demonstrated a similar stimulating effect of P alone but no cooperative effect together with E2. In conclusion, our data demonstrate that in immature trout, prior to the beginning of the first reproductive cycle, unlike E2, P is able to stimulate the reproductive brain-pituitary axis without affecting vitellogenin synthesis in the liver.

Effects of 4-nonylphenol on gene expression of pituitary hormones in juvenile Atlantic salmon (Salmo salar)

Alkylphenols such as 4-nonylphenol (NP) are one of the wide variety of environmental chemicals reported to have estrogenic effects in both in vitro and in vivo studies. Induction of eggshell zona radiata proteins (Zrp) and vitellogenin (Vtg) mRNA and protein synthesis in the liver are widely used biomarkers for xenoestrogen exposure in fish. However, little work has been done to characterize the molecular effects of xenoestrogens on other potential target organs such as the pituitary. To evaluate pituitary effects and develop new potential biomarkers for xenoestrogens, the influences of NP and 17beta-estradiol (E2) on the mRNA levels of pituitary gonadotropic hormone (GTH) beta subunits [leutinizing hormone beta (LH beta or GTH II beta) and follicle stimulating hormone beta (FSH beta or GTH I beta)], prolactin (PRL), growth hormone (GH) and the pituitary specific transcription factor (Pit-1) were investigated in individual male and female juvenile Atlantic salmon (Salmo salar), 3 days after a single intraperitoneal (i.p.) injection. In one experiment, fish were injected with NP (125 mg/kg body weight (BW)) or E2 (5 mg/kg BW) and a semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) method was used to analyze LH beta and FSH beta mRNA levels. In the second experiment, fish were injected with three doses of NP (10, 50, 125 mg/kg BW) or a single dose of E2 (5 mg/kg BW) and Northern blot analysis was used to quantify LH beta, FSH beta, PRL, GH and Pit-1 mRNAs. Both NP (50 and 125 mg/kg BW) and E2 significantly induced LH beta mRNA levels (P<0.01), but only in females. The highest dose of NP (125 mg/kg BW) significantly induced Pit-1 mRNA in males (P<0.01). NP did not have significant effects on any of the other pituitary transcripts. NP induced LH beta mRNA synthesis in females by up to 6-fold and the changes appeared to correlate with the increases in hepatic Vtg and Zrp mRNA levels. The results show that LH beta mRNA assay in female juvenile salmonids may be used as a marker for pituitary effects of xenoestrogens. The data also suggest that NP may have the potential to perturb the regulation of LH beta gene expression by mimicking E2.

Testosterone triggers the brain-pituitary-gonad axis of juvenile female catfish (Heteropneustes fossilis Bloch) for precocious ovarian maturation

The brain-pituitary-gonad axis of precociously matured females (PMFs) of Indian catfish (Heteropneustes fossilis), produced by testosterone treatment during juvenile stages, was analyzed by studies on immunoreactive gonadotropin-releasing hormone (ir-GnRH) secreting cells of the preoptic area of brain, plasma levels of gonadotropin (GtH-II), testosterone (T), and estradiol-17 beta (E(2)). GnRH cells of PMFs were large and strongly immunoreactive in comparison to control females. PMFs showed higher plasma levels of GtH-II, T, and E(2) than did control females. The ovaries of PMFs contained ripe ova, whereas control females had ova at maturing stages. This study suggests testosterone-mediated activation of the brain-pituitary-ovarian axis for precocious maturation in juvenile catfish.

Molecular characterization of LH-beta and FSH-beta subunits and their regulation by estrogen in the goldfish pituitary

The gonadal steroids, along with gonadotropin-releasing hormone (GnRH) are involved in the regulation of gonadotropin (GtH) production in vertebrates. Goldfish have an annual reproductive cycle, characterized by seasonal fluctuations in the circulating levels of the reproductive hormones, including 17beta-estradiol (E2). The purpose of the present study was to investigate the effect of E2 on basal and GnRH-induced GtH subunit (alpha, FSH-beta and LH-beta) gene expression in the goldfish pituitary. Northern analyses were performed to determine changes in steady state mRNA levels. Both in vivo and in vitro treatment with E2 resulted in a stimulation of all three GtH subunit mRNA levels, although a higher concentration was required for the stimulation of the FSH-beta subunit mRNA levels. The effect of E2 on GnRH-induced GtH mRNA level was also investigated and demonstrated that E2 influences the GnRH-induced GtH subunit mRNA levels in a seasonally dependent manner. Overall, the present results indicate that E2 stimulates GtH subunit mRNA levels directly at the level of the pituitary in a seasonally dependent manner in goldfish.

Effects of photoperiod on pituitary gonadotropin levels in masu salmon

We examined the effects of photoperiod on pituitary levels of two types of gonadotropin (GTH), GTH I and GTH II, in masu salmon Oncorhynchus masou to study their mechanism of synthesis. In Experiment 1, the effects of long or short photoperiod combined with castration were examined using 8-month-old precocious males. Castration was carried out in early August and then the fish were reared under a short (8L16D) or long (16L8D) photoperiod for 60 days. In Experiment 2, the effects of photoperiod combined with testosterone treatment were examined using 12-month-old immature females. Silastic tubes containing testosterone (500 microg /fish) or vehicle were implanted intra-peritoneally in early October. Fish were reared under 16L8D for 60 days, and then half of the fish were transferred to 8L16D, while the remaining fish were kept under 16L8D until Day 90. In Experiment 1, GTH I contents were higher under 16L8D than under 8L16D in the castrated group on Day 30. Moreover, GTH I contents were higher in the castrated group than the control group under 16L8D on Day 30. GTH II contents increased with testicular maturation in the control groups, whereas they remained at low levels in the castrated groups regardless of photoperiodic treatment. In Experiment 2, GTH I contents did not change remarkably in all the groups, while GTH II contents were remarkably increased by testosterone treatment regardless of photoperiodic treatment. These results indicate that the synthesis of GTH I and GTH II are differently regulated by photoperiod and testosterone in masu salmon.

Regulation of plasma gonadotropin II secretion by sex steroids, aromatase inhibitors, and antiestrogens in the protandrous black porgy, Acanthopagrus schlegeli Bleeker

Plasma gonadotropin II (GTH II) concentrations were significantly higher (approx. 15-20-fold) in estradiol-17beta (E(2)) treated (1.0 microg or 2.5 microg g(-1) body weight) female black porgy from days 4 to 12 compared with the control. E(2) (1 microg g(-1) wt.) had a stronger stimulation on plasma GTH II in early recrudescent phase (low GSI) males (11-fold) than in high GSI and late spermiating males (2.6-fold, P< 0.05). No effect of androgens (testosterone, T; 5alpha-dihydrotestosterone, DHT) on plasma GTH II levels was observed either sex. The levels of plasma GTH II were stimulated in 1,4,6-androstatriene-3,17-dione (ATD, 1 microg g(-1), 2 microg g(-1) body wt.) and fadrozole-treated (1 microg g(-1), 3 microg g(-1) body wt.) groups compared to control. Tamoxifen (1 microg g(-1), 3 microg g(-1) body wt.) but not enclomiphene could stimulate high GTH II levels in plasma. In another experiment of ATD in combination with T, T treatment further attenuated the ATD stimulation of plasma GTH II levels. We concluded that GTH II secretion is positively regulated by an estrogen-specific effect in female and male black porgy. Gonadal stage had significant effects on the responsiveness of GTH II to E(2) stimulation in males. A negative aromatase-dependent feedback control of plasma GTH II levels was also suggested in the protandrous black porgy, Acanthopagrus schlegeli.

cDNA cloning of two gonadotropin beta subunits (GTH-Ibeta and -IIbeta) and their expression profiles during gametogenesis in the Japanese flounder (Paralichthys olivaceus)

To clarify the profiles of two distinct gonadotropin (GTH-I and -II) mRNA levels during gametogenesis in a multiple spawner, the Japanese flounder (Paralichthys olivaceus), the cDNAs encoding GTH-Ibeta and -IIbeta from the pituitary gland have been cloned and sequenced. The nucleotide sequence of GTH-Ibeta was 542 bp long, encoding 120 amino acids, and that of GTH-IIbeta was 554 bp long, encoding 145 amino acids. In females, Northern blot analysis has revealed that relative mRNA levels of GTH-Ibeta and -IIbeta were low in immature fish, showed a gradual increase with ovarian development, and reached the highest level at the maturation stage. Both GTH-Ibeta and -IIbeta mRNA levels were highly correlated with gonadosomatic index (GSI) values and with circulating estradiol-17beta and testosterone (T) levels. In males, the mRNA levels of GTH-Ibeta increased with the increase in GSI values and in circulating 11-ketotestosterone and T levels, whereas the mRNA levels of GTH-IIbeta did not show any correlation with GSI values and with circulating steroid levels, suggesting a difference in regulatory mechanisms of GTH-I and -II synthesis in males. The similar changes in GTH-Ibeta and -IIbeta mRNA levels during oogenesis are considered to be characteristic of GTH synthesis in multiple spawners, differing from the differential changes reported in annual spawners such as salmonids.

Estradiol-17beta stimulates gonadotropin II expression and release in the protandrous male black porgy Acanthopagrus schlegeli Bleeker: a possible role in sex change

The objective of the present study was to investigate the in vivo effects of sex steroids (estradiol-17beta, E(2); testosterone, T) and the nonaromatizable androgen 5alpha-dihydrotestosterone (DHT) on the levels of gonadotropin II (GTH II) in plasma and pituitary and on aromatase activity in 2-year-old male black porgy, Acanthopagrus schlegeli, during the prereproductive season. Black porgy GTH II and GTH II beta subunits were purified and anti-GTH II beta serum was induced. A specific radioimmunoassay for black porgy GTH II was developed. cDNA GTH II beta was also cloned from a black porgy pituitary cDNA library for use as a probe for Northern analysis. Male fish were divided into eight groups (n = 64): control; E(2) (3 doses, 2.4 ng, 72 ng, and 2.2 microg/g body weight); T (2 doses, 72 ng and 2.2 microg/g body weight); and DHT (2 doses, 72 ng and 2.2 microg/g body weight). Fish were injected with the respective vehicle or different doses of material on days 0, 8, and 16. Plasma was collected at 4-day intervals from days 4 to 20. Plasma GTH II concentrations were significantly increased (up to 45-fold) in the E(2) group from days 4 to 20 in a dose-dependent manner. In a further experiment during the late reproductive season, plasma GTH II levels increased at 4 h and on days 1 and 2 following a single injection of 1.0 microg E(2)/g body weight (on day 0). Androgens (T or DHT) had little or no effect on plasma GTH II. Pituitary GTH II contents on day 20 were significantly lower in the 72-ng E(2) and 2.2-microg E(2) groups but not in the 2.4-ng E(2) group compared with the control group. Pituitary GTH II beta mRNA levels were significantly stimulated in the 72-ng and 2.2-microg E(2) groups on day 20. Gonadal aromatase activity was not significantly changed in any of the treated or control groups. It is concluded that GTH II secretion in black porgy is regulated by an estrogen-specific effect. Increased plasma GTH II levels or other factors in addition to E(2) might be involved in the regulation of gonadal aromatase activity and sex change in protandrous black porgy.

Pituitary levels of three forms of GnRH in the male European sea bass (Dicentrarchus labrax, L.) during sex differentiation and first spawning season

In the present study, levels of three GnRH forms [seabream GnRH (sbGnRH), chicken GnRH-II (cGnRH-II), and salmon GnRH (sGnRH)] were analyzed in the pituitary of male sea bass during sex differentiation and the first spawning season. Plasma levels of gonadotropin (GTH-2), testosterone (T), and 11-ketotestosterone (11-KT) were determined during the same periods. All GnRH forms were present in the pituitary. sbGnRH levels were 9-fold higher than cGnRH-II and 17-fold higher than sGnRH levels. The highest GnRHs levels were detected in November 1995, when fish were 9 months old and when the gonads started to differentiate. Levels of the three forms decreased and remained low during the first spawning season, with the exception of sbGnRH, which showed a significant increase in November 1996. Plasma GTH-2 levels were lowest in November 1995, later increasing 2.5 times during the next months. During the first spawning season, plasma GTH-2 levels peaked in December 1996, 1 month after the peak of sbGnRH. During sex differentiation, plasma T levels were high in November 1995 but decreased over the next months, while levels of 11-KT remained low and unchanged. During the first spawning season, both steroids peaked in January 1997. These results suggest a possible role for all three GnRH forms in achieving gonadal differentiation, while sbGnRH may be the most relevant form in the regulation of the first spawning season in male sea bass. Moreover, GTH-2 and 11-KT may play important roles in gonadal maturation, since plasma GTH-2 and 11-KT levels were high throughout the period of spermiation.

Profiles of gonadal development, sex steroids, aromatase activity, and gonadotropin II in the controlled sex change of protandrous black porgy, Acanthopagrus schlegeli Bleeker

The objectives of the present study were to investigate the effects of oral administration of estradiol-17 beta (E(2)) on the sex steroids and gonadotropin II (GTH II) in plasma, aromatase activity in gonad and brain, and sex change in protandrous black porgy, Acanthopagus schlegeli Bleeker. Two-year-old black porgy were divided into two groups, one fed a control diet and the other a diet mixed with E(2) (4.0 mg/kg feed) for 7 months. Significantly higher GSI was observed in the E(2) group. Fish treated with E(2) showed complete suppression of spermatogenesis and spermiation and induced sex change with vitellogenic oocytes and large primary oocytes. Lower levels of plasma 11-ketotestosterone and higher levels of plasma vitellogenin were shown in the E(2) group. Higher gonadal aromatase activity in concordance with elevated plasma levels of GTH II was observed in the E(2) group. The highest aromatase activity in the forebrain in the control group was observed in January. Higher aromatase activity in forebrain, midbrain, and hindbrain was also detected in the E(2) group than the control group. The change of aromatase activity in brain was more susceptible than that in gonad. The data showed that the increase of plasma GTH II levels and gonadal aromatase activity may be important to the controlled sex change by administration in black porgy.

Feedback control of gonadotropins in Atlantic salmon, Salmo salar, male parr.I. Castration effects in rematuring and nonrematuring fish

Sexual maturation of Atlantic salmon (Salmo salar) male parr is a seasonally recurrent "all or none" response; either a fish matures fully or it does not mature. To study whether gonadal feedback on gonadotropic hormones, GTH I and GTH II, is involved in the control of maturation, previously mature Atlantic salmon male parr were either sham-operated or castrated in spring. They were then sampled during the onset of gonadal growth (late June-early July) or shortly before the breeding season (late September). In autumn, sham-operated males separated into two groups: nonrematuring males with low pituitary and plasma levels of both GTH I and GTH II, and those rematuring with high levels of gonadotropins. Castrated males had low GTH I and GTH II plasma and pituitary levels, similar to those of the nonrematuring fish, suggesting positive feedback mechanisms, separating the sham-operated fish into low and high GTH level groups. In the summer, plasma GTH II was nondetectable in all fish. Pituitary GTH II content was lower in nonrematuring, than in rematuring males and was even lower in castrated fish. In contrast, castration increased pituitary and plasma levels of GTH I in the summer, suggesting a negative feedback at this reproductive stage. There were no significant differences in immunoassayable levels of GTH I in plasma in rematuring and nonrematuring sham-operated males at this time. The control of rematuration is complex and may involve factors other than circulating GTH I levels, possibly with differences in gonadal sensitivity to GTH I.

Feedback control of gonadotropins in Atlantic salmon, Salmo salar, male parr.II. Aromatase inhibitor and androgen effects

Both positive and negative feedback on the (hypothalmus)- pituitary-gonad axis occur in salmonids. The aim of the present study was to investigate the role of different androgens, and in particular the involvement of aromatization of androgens to estrogens in feedback mechanisms. Previously mature Atlantic salmon, Salmo salar, male parr were studied in two experiments. In the first experiment, intact fish were implanted with Silastic capsules filled with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) in spring and sampled in the autumn when the rematuring males were starting to display running milt. In the second experiment, castrated males were implanted with capsules containing ATD, the androgens testosterone (T) and 11-ketoandrostenedione (11KA), or ATD and T combined in spring. These fish were sampled in the summer when rematuring fish were starting to show signs of gonadal growth. Pituitary and plasma gonadotropins (GTH I and GTH II) were studied using radioimmunoassay. In autumn, ATD treatment reduced pituitary and plasma GTH II levels. In summer, GTH II was consistently nondetectable in plasma. Castration diminished pituitary GTH II content. Treatment with T increased pituitary GTH II content, an effect that was attenuated when T treatment was combined with ATD. All these results are consistent with the presence of an aromatase-dependent positive feedback of T on GTH II. 11KA also had a stimulatory effect on GTH II, although weaker than that of T. Testicular size and spermiation was reduced by ATD in autumn; the latter of these results is likely to be due to the inhibitory effect of ATD on GTH II. Positive effects of ATD on plasma GTH I were found in autumn, indicative of an aromatase-dependent negative feedback in this phase. On the other hand, castration increased plasma and pituitary GTH I levels in summer, indicating that the gonads in this phase exert a predominantely negative control of GTH I. In summer, negative effects of T on GTH I pituitary levels were not suppressed, but were rather enhanced, by the combined treatment with ATD. Furthermore, plasma GTH I levels were lower after treatment with T in combination with ATD than with T or ATD separately. These negative effects of T were not diminished by ATD, so that they are nonaromatase-dependent. Furthermore, since they were actually more pronounced in the presence of ATD it is suggested that there is also a positive aromatase-dependent feedback component in this phase. In addition, 11KA had a negative effect on plasma and pituitary GTH I in castrated previously mature males. Thus, GTH I secretion is controlled by both aromatase-dependent and nonaromatase-dependent feedback effects, of which at least the former may be positive or negative depending on season. In summary, the feedback control of GTH I appears to be more complex than that of GTH II.

Cortisol selectively stimulates pituitary gonadotropin beta-subunit in a primitive teleost, Anguilla anguilla

It has been suggested that in mammals, glucocorticoids, beside their stress-related inhibitory effects on reproductive function, may also play a stimulatory role at the onset of puberty. Using the juvenile female eel as a model, we investigated the potential stimulatory role of cortisol (F) on pituitary gonadotropin (GtH-II). GtH-II levels were measured by RIA, and messenger RNA (mRNA) levels for alpha- and GtH-II beta-subunits were determined by dot blot using homologous probes. F treatment increased eel pituitary GtH-II content in vivo and in vitro. Using a long term, serum-free primary culture of pituitary cells, we studied the direct effect of F on GtH-II production. F increased the GtH-II cellular content in vitro in a dose- and time-dependent manner. The relative potencies of various corticosteroids on GtH-II were: triamcinolone acetonide > dexamethasone > F >> cortisone and aldosterone, indicating a glucocorticoid-specific receptor (GR). F stimulated GtH-II production through a selective increase in mRNA levels for GtH-II beta-subunit; no significant effect was observed on alpha-subunit mRNA levels. This stimulatory effect of F on GtH-II beta, played out directly at the pituitary cell level, recalls that of F on FSHbeta in the rat. The present study, performed in a primitive teleost at the juvenile stage, suggests that the role of F in the positive regulation of gonadotropins at puberty may have arisen early in vertebrate evolution.

Release of pituitary gonadotrophins GtH I and GtH II in the rainbow trout (Oncorhynchus mykiss): modulation by estradiol and catecholamines

The present study focused on the role of catecholaminergic neurons and estrogens on the release of gonadotropins I and II in immature and early vitellogenic female rainbow trout. The ovariectomy-induced increase of GtH I blood levels (from about 10 to 15 ng/ml) was prevented in vitellogenic fish by E2 supplementation. E2 implantation of immature fish decreased blood GtH I levels (from about 6 to 1 ng/ml). Blood levels of GtH II were low (about 0.5 ng/ml) and not altered by ovariectomy and E2 treatment. These data demonstrate that estrogens exert a negative feedback on the release of GtH I in trout. A treatment with alpha-methyl-p-tyrosine (MPT), an inhibitor of catecholamine synthesis, increased blood GtH II levels of sham-operated vitellogenic fish and ovariectomized fish implanted with E2, but had no effects in ovariectomized fish. MPT did not modify blood GtH I levels in any experimental group. A treatment of E2-implanted immature or vitellogenic fish with the dopamine antagonist pimozide also increased blood GtH II levels, but did not significantly change blood GtH I levels. These data demonstrate that release of GtH II, but not of GtH I, depends on an E2-activated DA inhibitory tone.

Effects of steroids on GTH I and GTH II secretion and pituitary concentration in the immature rainbow trout Oncorhynchus mykiss

Using specific radio-immunoassays for rainbow trout GTH I and GTH II, the effects of testosterone and estradiol 17 beta have been studied or reinvestigated on the regulation of the secretion and the synthesis of the these two pituitary gonadotropins in the immature rainbow trout. After steroid implantation, the GTH II pituitary concentration is stimulated by testosterone and estradiol 17 beta for the entire period during which the plasma levels of these hormones are maintained to values comparable to those measured in the adult vitellogenic female rainbow trout. On the other hand, only testosterone induced a transient increase in the GTH I pituitary content 15 days after implantation, and estradiol provoked a decrease at day 30. The secretion of both GTH I and GTH II is stimulated by testosterone but not by estradiol 17 beta. Altogether, these results show that in the immature rainbow trout, testosterone preferentially modifies GTH I secretion, but not that of GTH II. They confirm that the stimulation of GTH II accumulation after testosterone or estradiol treatment would correspond to a stimulation of hormone synthesis. They evidence a differential action of both steroids on the synthesis of the two gonadotropins, especially a possible inhibition of GTH I synthesis by estradiol. They let suppose that the regulation of GTH I synthesis would involve factors other than steroids.

Effects of gonadectomy on plasma gonadotropins I and II in coho salmon, Oncorhynchus kisutch

Studies of both salmon and trout have indicated that the levels of gonadotropins, GTH I and GTH II, in the pituitary and peripheral circulation vary during the reproductive cycle. To evaluate the possible feedback relationship between the gonads and pituitary GTH secretion, we studied the effects of bilateral gonadectomy on plasma levels of GTH I and GTH II in coho salmon, Oncorhynchus kisutch. During late spermatogenesis in males and late vitellogenesis in females, plasma GTH I levels increased significantly after gonadectomy, approximately 6- and 5-fold over presurgery levels at 3 and 14 days after surgery, respectively, and then declined to near presurgery levels by Day 17. No change in GTH I levels occurred in sham-operated fish. In all groups, GTH II levels were nondetectable and did not change significantly up to 17 days postsurgery. In males gonadectomized during spermiation, plasma GTH I levels increased significantly, approximately 10-fold over presurgery levels by 7 days postsurgery, and remained elevated thereafter. In contrast to the males in late spermatogenesis, the spermiating fish had detectable levels of GTH II (2-3 ng/ml), and significant elevations in plasma GTH II levels (approximately 60-fold) were observed 7 days after gonadectomy. These experiments demonstrate that the gonads exert negative feedback effects on secretion of both GTH I and GTH II, but the effect varies seasonally and the nature of the specific factor(s) from the gonads that inhibits and/or stimulates GTH production and secretion remains to be clarified.

Synthesis and regulation of 17α-hydroxy-20β-dihydroprogesterone in immature males of Oncorhynchus mykiss

Three experimental approaches were chosen to study the question if the progestin 17α-hydroxy-20β-dihydroprogesterone (17α20βOHP) is synthesised in testes of young Oncorhynchus mykiss, in which the absence of spermatozoa was verified histologically: first, in order to detect 20β-hydroxysteroid dehydrogenase activity (20βHSD), testes homogenates were incubated with (3)H-labeled 17αOHP.Metabolites were analysed by TLC, HPLC, and repeated crystallization to constant isotope ratios. One of the metabolites was identified as 17α20βOHP-(3)H, indicating that already immature testes contain 20βHSD activity and are able to produce 20β-reduced steroids. Second, 17α20βOHP was quantified by radioimmunoassay in incubates of testes fragments. The sensitivity of the gonads to gonadotropin II (GtH II) became evident when comparing incubations in the absence and presence of GtH II. Third, plasma levels of 17α20βOHP were significantly higher in animals injected with partially purified salmon gonadotropin, compared to controls. Thus, for the first time, it could be shown that 20βHSD is present in testicular cells other than spermatozoa. Furthermore, 17α20βOHP is indeed secreted at a very early stage of testicular development; 17α20βOHP secretion is also responsive to GtH II. Future studies will have to show if the functions of this progestin include the stimulation of spermatogenesis.

Effects of aromatase inhibitors on sexual maturation in Atlantic salmon, Salmo salar, male parr

Atlantic salmon, Salmo salar, male parr were implanted with Silastic capsules filled with different aromatase inhibitors: 1,4,6-androstatriene-3,17-dione (ATD), 4-hydroxy-4-androstene-3,17-dione (4OH), and the non-steroidal CGS16949 A, 4-benzonitrile monohydrochloride (CGS). Aromatization in brain homogenates were lower in salmon implanted with CGS and ATD than in controls. This was not the case for 4OH, but administration of 4OH to brain homogenates reduced the aromatase activity. All three aromatase inhibitors had effected gonadal weights in fish sampled in the summer, but the effects were markedly different among inhibitors. Plasma levels of the androgen 11-ketotestosterone (11KT) and the progestin 17α, 20β-dihydroxy-4-pregnen-3-one (17,20P) were measured by means of radioimmunoassay. CGS and ATD, but not 4OH, significantly decreased the plasma 17,20P levels in the autumn. Plasma levels of 11 KT were not influenced by ATD or CGS treatment, but 4OH had a lowering effect in one autumn sampling. ATD and 4OH (CGS not tested) increased the proportion of maturing males.These findings suggest that aromatization is of physiological importance in different mechanisms controlling reproduction in salmon.

Endocrine and gonadal changes during the annual reproductive cycle of the freshwater teleost,Stizostedion vitreum

The annual reproductive cycle of walleye (Stizostedion vitreum) was characterized by documenting changes in gonadal development and serum levels of estradiol-17β (E2), testosterone (T), 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P), and 11-ketotestosterone (11-KT) in wild fish captured from upper midwestern lakes and rivers throughout the year. Fish from the populations used in this study spawn annually in early- to mid-April. Walleye showed group synchronous ovarian development with exogenous vitellogenesis beginning in autumn. Oocyte diameters increased rapidly from ∼ 200 μm in October to ∼ 1,000 μm in November, and reached a maximum of 1,500 μm just prior to spawning. Changes in gonadosomatic indices (GSIs) paralleled changes in oocyte diameters. Serum E2 levels in females increased rapidly from low values in October (< 0.1 ng ml(-1)) to peak levels of 3.7 ng ml(-1) in November, coinciding with the period of the most rapid ovarian growth. Subsequently, E2 levels decreased from December through spawning. Serum T levels exhibited a bimodal pattern, increasing to 1.6 ng ml(-1) in November, and peaking again at 3.3 ng ml(-1) just prior to spawning. We detected 11-KT in the serum of some females at concentrations up to 5.6 ng ml(-1), but no seasonal pattern was apparent. In this study (unlike our results in a related study) 17,20-P was not detected. In males, differentiation of spermatogonia began in late August, and by January the testes were filled (> 95% of germ cells) with spermatozoa. Mature spermatozoa could be expressed from males from January through April. GSIs ranged from 0.2% (post-spawn) to 3.2% (pre-spawn). Serum T levels rose from undetectable levels in post-spawn males to 1.6 ng ml(-1) by November, remained elevated throughout the winter, and peaked at 2.8 ng ml(-1) I prior to spawning. Levels of 11-KT in males remained low (< 10 ng ml(-1), from post-spawning through January, then increased significantly by March and peaked just prior to spawning at 39.7 ng ml(-1). Our results indicate that vitellogenesis and spermatogenesis are complete or nearly so, in walleye by early winter, and suggest that it may be possible to induce spawning in this species several months prior to the normal spawning season by subjecting fish to relatively simple environmental and hormonal treatments.

Serum thyroxine, estradiol-17β, and testosterone profiles during the parr-smolt transformation of masu salmon, Oncorhynchus masou

Changes in serum thyroxine (T4), estradiol-17β (E2) and testosterone (T) levels during the parr-smolt transformation (smoltification) were investigated in the masu salmon (Oncorhynchus masou) in 1985 and 1987. T4 showed a peak in levels at the early stage of smoltification and E2 and T levels peaked almost at the same time. There were no significant differences between the concentrations of serum hormones in female and males. During smoltification, germ cells in the peri-nucleolus and spermatogonia stage were present in the ovary and testis, respectively. These results suggest that E2 and T may be involved in smoltification in the masu salmon.

Absence of direct regulation of prolactin cells by estradiol-17 beta in rainbow trout (Oncorhynchus mykiss)

The effects of estradiol-17 beta (E2) implants on plasma prolactin (PRL) concentrations, pituitary PRL content and pituitary PRL mRNA levels were examined in rainbow trout (Oncorhynchus mykiss). Intact immature fish treated with 1 mg estradiol-17 beta did not show significant changes in both PRL mRNA levels and pituitary PRL content after 3 days of treatment. In a similar experiment, no changes were observed in plasma PRL levels followed during 7 days. Similarly, lack of estradiol-17 beta effect on plasma PRL levels and on final PRL pituitary content was observed in ovariectomized female rainbow trout treated during 48 days with 25 mg estradiol-17 beta and in mature male fish over a 3-week treatment period. Localization of estradiol receptor (ER) mRNAs in the pituitary was carried out by Northern blot analysis using a full-length rainbow trout estrogen receptor (rtER) cDNA as a probe. The rostral pars distalis of the pituitary which contained mostly PRL cells showed the lower amount of rtER mRNA when compared to other parts of the pituitary. Moreover, two mRNAs of different size (3.5 and 1.4 kb) were detected in different parts of the pituitary. Further hybridization experiments using probes containing part of the rtER cDNA (E domain or C and D domains) indicated that the small-sized mRNA (1.4 kb) probably encodes a truncated ER protein lacking hormone binding domain or an ER-related protein. Thus, only the 3.56 kb mRNA appeared to be involved in the regulation of pituitary function by estradiol. In situ hybridization analysis allowed a more precise localization of this rtER mRNA in the pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal changes accompanying sexual maturation in captive milkfish (Chanos chanos Forsskal)

Steroid hormone profiles accompanying sexual maturation in captive milkfish are described. There were no significant differences in levels of serum estradiol 17-β (E2) and testosterone (T) between immature male and female fish. Mean E2 levels rose from 0.54±0.11 ng/ml in immature females (Stage 1) to 4.53±1.16 ng/ml in vitellogenic females (Stage 5), while T levels increased from 2.06±0.28 ng/ml to 38.4±9.26 ng/ml. E2 and T levels were positively correlated to GSI and oocyte diameter. In males, serum T levels increased from 2.5±0.40 ng/ml in immature males to 27.73±5.02 ng/ml in spermiating males. A significantly higher T level was found in males with thick and scantly milt (spermiation index, SPI, 2) compared to males with scanty milt (SPI, 1) or males with copious, fluid milt (SPI, 3).Serum levels of E2 and T, and the GSI in females rose significantly during the breeding season (April-June 1983). The levels of both steroids dropped below 1 ng/ml in spent females sampled in succeeding months. In immature males, T levels ranged from 1.11 ng/ml to 2.78 ng/ml and rose significantly to 21.52±8.38 ng/ml during the breeding season when GSI peaked. Serum T levels dropped to around 10 ng/ml in the succeeding months when only spent or regressed males were sampled.

Effects of estradiol on brain aminergic turnover of the female rainbow trout (Oncorhynchus mykiss) at the beginning of vitellogenesis

Brain serotonin and dopamine (DA) turnovers in the female rainbow trout were studied at the beginning of the vitellogenesis and related to blood estradiol (E2) levels; pituitary and plasma gonadotropin (GtH) were also assayed. Ovariectomy did not modify brain aminergic turnover. E2 replacement on ovariectomized fish increased hypothalamic DA turnover (increased DA and increased DA metabolites). E2 stimulated GtH synthesis (positive feedback) but did not enhance GtH release; hypothalamic E2-mediated aminergic inhibition upon release was suspected. Individual relations between blood E2 levels and catecholaminergic neurotransmitters were determined. A linear positive correlation (r = 0.82) was found for the hypothalamus, but not for the pituitary, the preoptic area, or the telencephalon. These data suggest that an activation of hypothalamic tyrosine hydroxylase (the limiting step of catecholamines synthesis) by E2 could develop as vitellogenesis proceeds.

Influence of dihydrotestosterone on sex determination in channel catfish and blue catfish: period of developmental sensitivity

Treatment of channel catfish with 0.2, 20, or 200 mg/liter of dihydrotestosterone (DHT) in the water during the egg stage or during egg and sac-fry stages did not alter the expected 1:1 sex ratio of the progeny. Feeding DHT at 200 mg/kg of feed for the first 21 days after yolk sac absorption resulted in 80% females; this proportion was increased by combining feeding with treatment of 200 mg DHT/liter in the sac-fry stage (90%) or in the egg and sac-fry stage (97%). In contrast, treatment of blue catfish sac-fry with 200 mg DHT/liter, with or without the combination of feeding DHT at 200 mg/kg food, resulted in 100% female populations. Neither clomiphene citrate, an estrogen-receptor blocking agent, nor clofibrate, an inhibitor of hepatic synthesis of cholesterol, affected the sex ratio of channel catfish, and neither of these compounds altered the feminizing effect of 200 mg DHT/kg when fed in combination with DHT. The nonaromatizable androgen DHT is not as effective as many other androgens in producing paradoxical female populations of channel catfish. However, feminization of blue catfish by treatment of sac-fry indicates that this species is more susceptible to hormonal manipulation and that the period of sex determination may occur earlier in development than in channel catfish.