Seasonal variations in plasma and pituitary levels of gonadotrophin in males and females of two strains of rainbow trout (Salmo gairdneri)

In silico predicted transcriptional regulatory control of steroidogenesis in spawning female fathead minnows (Pimephales promelas)

Oocyte development and maturation (or oogenesis) in spawning female fish is mediated by interrelated transcriptional regulatory and steroidogenesis networks. This study integrates a transcriptional regulatory network (TRN) model of steroidogenic enzyme gene expressions with a flux balance analysis (FBA) model of steroidogenesis. The two models were functionally related. Output from the TRN model (as magnitude gene expression simulated using extreme pathway (ExPa) analysis) was used to re-constrain linear inequality bounds for reactions in the FBA model. This allowed TRN model predictions to impact the steroidogenesis FBA model. These two interrelated models were tested as follows: First, in silico targeted steroidogenic enzyme gene activations in the TRN model showed high co-regulation (67-83%) for genes involved with oocyte growth and development (cyp11a1, cyp17-17,20-lyase, 3β-HSD and cyp19a1a). Whereas, no or low co-regulation corresponded with genes concertedly involved with oocyte final maturation prior to spawning (cyp17-17α-hydroxylase (0%) and 20β-HSD (33%)). Analysis (using FBA) of accompanying steroidogenesis fluxes showed high overlap for enzymes involved with oocyte growth and development versus those involved with final maturation and spawning. Second, the TRN model was parameterized with in vivo changes in the presence/absence of transcription factors (TFs) during oogenesis in female fathead minnows (Pimephales promelas). Oogenesis stages studied included: PreVitellogenic-Vitellogenic, Vitellogenic-Mature, Mature-Ovulated and Ovulated-Atretic stages. Predictions of TRN genes active during oogenesis showed overall elevated expressions for most genes during early oocyte development (PreVitellogenic-Vitellogenic, Vitellogenic-Mature) and post-ovulation (Ovulated-Atretic). Whereas ovulation (Mature-Ovulated) showed highest expression for cyp17-17α-hydroxylase only. FBA showed steroid hormone productions to also follow trends concomitant with steroidogenic enzyme gene expressions. General trends predicted by in silico modeling were similar to those observed in vivo. The integrated computational framework presented was capable of mechanistically representing aspects of reproductive function in fish. This approach can be extended to study reproductive effects under exposure to adverse environmental or anthropogenic stressors.

Photoperiod modulates the reproductive axis of European sea bass through regulation of kiss1 and gnrh2 neuronal expression

The onset of puberty is characterized by activation of the brain-pituitary-gonad axis. However, the molecular and endocrine mechanism involved in the process of puberty and the influence of environmental conditions, such as photoperiod signalling, are not well understood in fish. In this study, 1-year-old male European sea bass (Dicentrarchus labrax) were exposed to photoperiod manipulation in combination with size-sorting. Two treatment groups, a puberty accelerating photoperiod (AP) group and a continuous light (LL) group, were studied from August to February. Our results indicate that AP and LL are able to entrain the rhythms of both kiss1 and gnrh2 mRNA levels in the brain, while kiss2 and gnrh1 mRNA expression does not seem to be directly affected by the photoperiod, at least during testicular growth. It is likely that AP and LL photoperiod regimes affected both plasma Fsh and 11-KT profiles, which might explain, respectively, the phase shift and reduction of testes maturation seen under these conditions. We therefore hypothesize that the unbalanced production of this androgen regulated by circulating Fsh might be limiting the stimulation of germ cell proliferation in European sea bass males. In summary, our study establishes that photoperiod modulates the expression of kiss1 and gnrh2 in the forebrain-midbrain, which may be involved in the translation of the light stimulus to activate the reproductive axis.

New insights into the factors mediating the onset of puberty in sea bass

In populations of 1-year-old male European sea bass (Dicentrarchus labrax), only large males are able to acquire for the first time a functional competence of their reproductive axis; in other words, to attain puberty. To examine the causes and mechanisms involved in the onset of puberty in this species, a size sorting sampling was carried out to obtain two experimental groups of small and large male fish exhibiting different growth rates. As expected, only large fish reached full spermiogenesis (stage V of testicular development) by the end of the experiment. Our study suggests that fish size is a permissive condition to ensure full effectiveness of the hormonal (Gnrh1, gonadotropins and sexual steroids) actions. Thus, though small fish had endocrine profiles similar to those of large fish, their amplitude was much lower, and was most likely the reason why functional competence of the reproductive axis was not achieved. Moreover, this work provides evidence of the involvement of kisspeptin and Gnrh1 systems in the onset of puberty in a marine teleost fish. It also indicates that very likely kisspeptin and Gnrh1 may regulate gonadotropins and sex steroids at specific stages of testicular development.

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.

The molecular regulation of Chinook salmon gonadotropin beta-subunit gene transcription

Expression of the gonadotropin beta-subunit genes is tightly regulated both cell-specifically and by the regulatory hormones to achieve the appropriate gonadotropic hormone levels required for reproductive development and function. Although the cDNA sequences of these genes are highly conserved across species, their promoter sequences are not and few functional studies have been carried out to understand the molecular mechanisms through which their expression is regulated. We and others have carried out several studies on the LHbeta gene promoter of Chinook salmon (Oncorhynchus tschawytscha), and also isolated the FSHbeta gene from the same species. We present here a review of these studies and also novel data pertaining to both genes, in an attempt to collate the current understanding of the molecular regulation of the gonadotropin beta-subunit genes in these fish.

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.

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.

The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

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.

Molecular biology of channel catfish gonadotropin receptors: 1. Cloning of a functional luteinizing hormone receptor and preovulatory induction of gene expression

There is little known about the molecular biology of piscine gonadotropin receptors, and information about gene expression during reproductive development is particularly lacking. We have cloned the LH receptor (LHR) in the channel catfish (cc), and examined its gene expression throughout a reproductive cycle. A cDNA encoding the receptor was isolated from the testis using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends procedures. It encoded a 696-amino acid protein that showed the greatest homology (46-50% identity) with the known LHRs and lesser similarity with FSH receptors and thyroid-stimulating hormone receptors (44-47% and 42-44% identity, respectively). In addition, two characteristics unique to the LHRs were conserved in the cloned receptor and the encoding gene: presence of an intron corresponding to intron 10 in mammals and turkey and occurrence of a double cysteine residue in the cytoplasmic tail for potential palmitoylation. The ccLHR gene was well expressed in the gonads and kidney and merely detectable in the gills, muscle, and spleen. The isolated cDNA encoded an active ccLHR protein, as the recombinant receptor expressed in COS7 cells activated a cAMP response element-driven reporter gene (luciferase) upon exposure to hCG in a dose-dependent manner. Seasonal changes in the ovarian expression of the ccLHR gene, as examined by measuring the transcript abundance by quantitative real-time RT-PCR, remained rather low during most of the reproductive cycle but was acutely induced around the time of spawning. This pattern of expression correlates well with the reported expression of its ligand (LH) in fishes and concurs with the notion that LH is a key regulator of the periovulatory maturational events.

The brain-pituitary-gonadal axis of female rainbow trout Oncorhynchus mykiss: effects of photoperiod manipulation1

Two groups of post-spawned female rainbow trout were exposed to two different photoperiods, an ambient photoperiod (56 degrees N) and a combination of long and short photoperiods (a constant 18L:6D from February 1 until May 10, then a constant 6L:18D), which acted to advance maturation and spawning. The stimulatory long-short photoperiod advanced spawning by 3-4 months and correspondingly advanced peaks in serum levels of 17beta-estradiol, testosterone, calcium (an index of vitellogenin), and GTH II. Earlier events in gonadal recrudescence appeared to be less affected by the photoperiod. The initiation of exogenous vitellogenesis coincided with high levels of both pituitary salmon gonadotropin-releasing hormone (sGnRH) content and serum follicle-stimulating hormone (FSH, GTH I) levels. High levels of serum FSH were associated with rapid gonadal growth in the fish exposed to the stimulatory long-short photoperiod. In contrast, the fish exposed to the ambient photoperiod showed gonadal steroid production, formation of vitellogenin, and secondary oocyte growth without any detectable increase in serum FSH levels. The possible roles and interactions of sGnRH, gonadotropins, and steroids with respect to normal and artificially stimulated ovarian maturation are discussed.

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.

Gonadotropins I and II do not stimulate thein vitro secretion of 17α,20β-dihydroxy-4-pregnen-3-one 20-sulphate by rainbow trout gonads during final sexual maturation

Thein vitro secretion of 17α,20β-dihydroxy-4-pregnen-3-one 20-sulphate (17α,20β-P-sulphate) and the free steroid 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-P), by rainbow trout (Oncorhynchus mykiss) gonads, in response to gonadotropin (GTH) I and GTH II, were studied during the final stages of sexual maturation. Substantial amounts of 17α,20β-P-sulphate were produced, by both mature ovaries and testes, indicating considerable 20β-hydroxysteroid sulphotransferase (20β-HST) activity within these tissues. In the post-ovulatory ovary the level of 17α,20β-P-sulphate (36.6 ng ml(-1)) greatly exceeded that of 17α,20β-P (8.59 ng ml(-1)). The amount of 17α,20β-P-sulphate produced in incubations of both mature ovary and testes was unaffected by either GTH I or GTH II treatment at physiological concentrations up to 100 ng ml(-1). Similarly, incubations of maturing ovary and testes, treated with GTH I or GTH II, in the presence of added 17α,20β-P at 100 ng ml(-1) of medium, produced levels of 17α,20β-P-sulphate that were similar to those of the controls. In incubations of mature ovarian follicles at the stages of germinal vesicle breakdown and preovulation, both GTHs significantly stimulated secretion of 17α,20β-P, although GTH II was always more potent than GTH I. GTH II significantly elevated the levels of 17α,20β-P in testicular incubations from mature males more than 4-fold relative to GTH I and controls, which did not differ from one another.In conclusion, 20β-HST, the enzyme responsible for the sulphate conjugation of 17α,20β-P, was found to be active in the ovaries and testes of rainbow troutin vitro. However, the levels of this enzyme do not appear to be regulated by either GTH I or GTH II.

Male dominance, plasma hormone concentrations, and availability of milt in male rainbow trout (Oncorhynchus mykiss)

The relationships among social status, endocrine profile, and amounts of milt available by stripping were examined in pairs of male rainbow trout (Oncorhynchus mykiss) competing for access to sexually active females in laboratory spawning channels. Gonadotropic hormone (GtH II), the gonadal steroids 11-ketotestosterone, testosterone, and 17α,20β-dihydroxy-4-pregnen-3-one, and milt volumes increased in dominant males in response to stimuli provided by a nesting female. Subordinate males in the same location did not show endocrine or milt responses. Following the removal of the dominant male of each pair, plasma hormone and milt levels increased in the previously subordinate males. This result indicates that endocrine and spermiation responses depend upon close-range sensory and behavioural interaction with the sexually active female. Treatment with testosterone did not cause a change in behaviour or social status of smaller males predicted to be subordinate.

Differential regulation of the estrogen receptor mRNA by estradiol in the trout hypothalamus and pituitary

In an attempt to understand the molecular mechanisms by which steroids can modulate brain functions in fish, we first localized the cells which produce estrogen receptor mRNA in the rainbow trout forebrain (Salbert et al., 1991). We now report how estradiol itself can alter the estrogen receptor mRNA content of these cells in a sterile strain of female rainbow trout. We also examined liver and pituitary levels of the estrogen receptor mRNA under the same estrogenic treatment. As revealed by slot blot and in situ hybridisations, a single injection (1.5 mg/kg) of estradiol can induce a strong increase (about five-fold) in the estrogen receptor mRNA levels in the liver, as well as a moderate increase (about two-fold) in two nuclei of the hypothalamus/preoptic area: the nucleus lateralis tuberis and the nucleus preopticus periventricularis. Conversely, no modifications of these levels were observed in the pars intermedia and the proximal pars distalis of the pituitary. Moreover, a comparison between estrogen receptor mRNA levels in the brain of sexually active female trout and in the brain of these sterile animals revealed that, in these latter, estrogen receptor mRNA levels are lower but can be increased by a single estradiol injection and reached the levels observed in mature females.

Purification of gonadotropins (PmGTH I and II) from red seabream (Pagrus major) and development of a homologous radioimmunoassay for PmGTH II

Two gonadotropic glycoproteins (PmGTH I and II) were purified by ion-exchange chromatography, gel filtration and preparative SDS-PAGE, from pituitaries of red seabream, a marine teleost which has an asynchronous-type ovary and spawns almost daily during the spawning season. The glycoproteins were composed of distinct subunits and the molecular weights were estimated to be 32 and 38 kDa for PmGTH I and PmGTH II, respectively. Both PmGTH I and II were active in two homologous bioassays: in vitro oocyte maturation and/or in vitro estradiol-17β production assays. These two GTHs were distinct in electrostatic properties, molecular weight, stability and yields from pituitaries during the spawning season. These properties suggest that PmGTH I and II correspond to salmon GTH I and II, respectively.A homologous radioimmunoassay with which to measure PmGTH II was developed using a rabbit antiserum against the β subunit of PmGTH II and intact PmGTH II as standards and radioactive competitors. Competition curves for red seabream plasma and pituitary extract were parallel to the standard curve, while PmGTH I had low cross-reactivity (3.1 %) with the antibody. This specific RIA system showed an in vivo LHRHa induced GTH surge in the plasma of female red seabream.

Pituitary as a target organ for toxic effects of P4501A1 inducing chemicals

We report here that cytochrome P4501A1 in the male rainbow trout pituitary is highly inducible by beta-naphthoflavone. Pituitary cells containing inducible P4501A1 were identified by double immunostaining as gonadotrophs containing gonadotropin II. Thus, the pituitary gonadotrophs may be target cells for polyaromatic hydrocarbons. Elevated plasma levels of gonadotropin II (GTH II) and testosterone in the induced fish indicated that the functioning of the pituitary was disturbed. Because GTH II regulate the final stage of sexual maturation the results implies that exposure to P4501A1 inducing compounds may disturb this development stage.

Isolation of maturational gonadotropin subunits from spotted seatrout (Cynoscion nebulosus) and development of a beta-subunit-directed radioimmunoassay for gonadotropin measurement in sciaenid fishes

Maturational gonadotropin (GTH) subunits were isolated from pituitaries of the spotted seatrout (Cynoscion nebulosus), a marine perciform teleost, by ethanolic extraction and ion-exchange, gel-filtration, and reverse-phase chromatography. Partial amino acid sequencing of the N-terminal regions of the alpha and beta subunits indicated 60-80% identities with various carp and salmon GTH subunits. The spotted seatrout GTH beta-subunit was used as radioligand in a radioimmunoassay (RIA) with Atlantic croaker (Micropogonias undulatus) GTH antiserum. Pituitary extracts and plasmas from a variety of sciaenid fishes diluted parallel to the croaker GTH standard in the RIA. These data suggest that there is a high degree of immunological similarity among the GTH beta subunits of sciaenid fishes. The RIA measured increased levels of GTH in the plasmas of three species of sciaenid fishes, spotted seatrout, orangemouth corvina (Cynoscion xanthulus), and red drum (Sciaenops ocellatus), following injections of a luteinizing hormone-releasing hormone analog. The beta-subunit-directed GTH RIA increases considerably the number of species in which studies of GTH physiology can now be conducted.

Immunocytochemistry of somatotrophs, gonadotrophs, prolactin and adrenocorticotropin cells in larval sea bream (Sparus auratus) pituitaries

The chronological appearance of endocrine cells in the pituitary of sea-bream (Sparus auratus) larvae was studied using antisera against salmon prolactin, trout growth hormone, salmon gonadotropin and N-terminal human adrenocorticotropin. The larval pituitary (1-12 days after hatching) was oval in shape and was composed of a dense mass of cells with few neurohypophysial fibres. By 60 days after hatching it began to resemble the adult and was divisible into a distinct rostral pars distalis containing prolactin and adrenocorticotropin cells; a proximal pars distalis containing somatotrophs and gonadotrophs and a pars intermedia. Cells immunoreactive with antisera against growth hormone were observed immediately after hatching (2 days post-fertilization). Weakly staining prolactin cells were observed 2 days later in the region corresponding to the rostral pars distalis. Cells immunoreactive with antigonadotropin and anti-adrenocorticotropin sera were observed in the pituitary 6 and 8 days after hatching, respectively. All the cell-types studied were immunoreactive from the time they were first identified until the final samples 90 days after hatching.

Effect of salmon gonadotropic hormone on sex steroids in male rainbow trout: plasma levels and testicular secretion in vitro

Male rainbow trout were treated with salmon gonadotropic hormone (GTH) at different stages of the circannual reproductive cycle; spawning fish were also treated with an antiserum against salmon GTH. Injection of GTH led to a several-fold increase of plasma sex steroid levels during spermatogenesis and in the spawning season but was without effect at early stages of testicular development. GTH neutralization during the spawning season was followed by a several-fold decrease of plasma sex steroid levels. During spermatogenesis and in the spawning season, both treatment regimes resulted in an increased sensitivity of testicular explants in response to a subsequent stimulation of steroid secretion in vitro. This up-regulatory response may facilitate and maintain the high sex steroid plasma levels observed during the spawning season. It may also be necessary to allow for concomitant peak values of plasma GTH and sex steroids in the spawning season, a situation difficult to understand within the negative feedback concept. The adaptive capacities of the testicular steroidogenic system indicate that it is not only an effector site for GTH but also an active part of the endocrine system controling reproduction.

Involvement of gonadotropin in the uptake of vitellogenin into vitellogenic oocytes of the rainbow trout, Oncorhynchus mykiss

The effects of two fully characterized, structurally distinct gonadotropins, GtH I and GtH II, on the uptake of vitellogenin (VTG) into oocytes of the rainbow trout, Oncorhynchus mykiss, were investigated both in vivo and in vitro. GtH I, injected into maturing vitellogenic females at a dose of 10 micrograms.kg body wt-1, increased the rate of [3H]VTG uptake into oocytes by more than two-fold, effectively doubling their rate of growth. Ovaries from females similarly treated with GtH II sequestered VTG at rates similar to controls. In vitro, GtH I stimulated VTG uptake in a dose-dependent manner. At a GtH I concentration of 100 ng.ml-1 and above, the rate of VTG uptake was significantly greater than that of the controls and at 1000 ng.ml-1 the rate of uptake was more than doubled. GtH II did not significantly increase VTG sequestration into isolated oocytes at concentrations up to, and including, 1000 ng.ml-1. These data provide the first evidence that GtH I has a primary function in stimulating VTG uptake and strongly support the contention that at least two functionally distinct GtHs occur in fish.

Stimulation of in vitro ovarian estradiol-17 beta synthesis in the rainbow trout by the carbohydrate-poor protein fraction from sockeye salmon pituitary glands

The role of carbohydrate-poor (Con A I) and carbohydrate-rich (Con A II) pituitary protein fractions, isolated from sockeye salmon (Oncorhynchus nerka), were investigated pertaining to in vitro estradiol-17 beta (E2) production by rainbow trout (Oncorhynchus mykiss) ovarian follicles. During the early vitellogenic phase of the reproductive cycle, using defolliculated ovarian follicle preparations (outer epithelium-thecal layer absent), it was demonstrated that the Con A I fraction was capable of increasing E2 production, in the presence of exogenous testosterone (T) as the substrate. Under similar conditions the Con A II fraction (containing the maturational gonadotropin) was inactive. However the Con A II fraction or T, separately, increased E2 production by intact ovarian follicles, whereas the Con A I fraction did not. A mechanism proposed to explain the regulation of ovarian E2 synthesis involves the Con A I fraction enhancing aromatase activity in granulosa cells permitting an increased conversion of T to E2.

In vitro effects of arginine vasotocin on testosterone production by testes of rainbow trout (Oncorhynchus mykiss)

The objective of this study was to investigate the effect of arginine vasotocin (AVT) on testosterone (T2) production by the rainbow trout testis in culture. AVT increased T2 production in a dose-dependent manner in immature testes. The maximum response (10 nM AVT) increased T2 production 6-fold over basal (3-6 pg/mg tissue protein). Mature testes did not respond to AVT. Salmon gonadotropin (2.5 nM sGtH, SGA-GtH) stimulated T2 production by both juvenile (5-fold) and mature (11-fold) testes. When AVT (100 nM) was added in addition to salmon gonadotropin (sGtH; from 0.1 to 100 nM), AVT had a stimulatory effect at the submaximal doses of sGtH, but T2 production did not exceed the maximum level obtained with sGtH (100 nM) alone. Isotocin also increased T2 production although to a lesser degree than AVT. The maximum response at 10 nM increased T2 production 4-fold over basal. beta-Endorphin had no effect on T2 production.

Plasma growth hormone levels during sexual maturation in diploid and triploid rainbow trout (Oncorhynchus mykiss)

Plasma growth hormone concentrations were determined in diploid and triploid rainbow trout of both sexes during sexual maturation. Diploid females grow large ovaries, whereas triploid female trout show no ovarian development. The plasma growth hormone concentration in triploid female trout remained low and unchanged throughout the study, whereas it rose slightly, but significantly, in the diploid females that matured, but not in those that remained immature. On the other hand, triploid males do develop tests like their diploid counterparts. In both groups spermiation was accompanied by a steady rise in the plasma growth hormone concentration. The results suggest that the elevated growth hormone concentration in mature male trout was a consequence not of reproduction per se, but of the loss of condition that accompanied spawning. This hypothesis was supported by the results from the females. Apart from a temporary loss in the mature diploids caused by stripping of the eggs, female trout did not lose condition, neither did they show any change in the plasma growth hormone concentration during the period when they ovulated. These results suggest that it was the nutritional insufficiency accompanying reproduction in male fish that caused the elevation in growth hormone concentration. In fact, a strong negative correlation between the plasma growth hormone concentration and the condition factor of the fish was observed.

Steroid secretion of rainbow trout testis in vitro: variation during the reproductive cycle

Testicular tissue collected at different stages of gonadal development was incubated with a pituitary extract (PE) from mature salmon. Three androgens (11-ketotestosterone, OT; 11 beta-hydroxytestosterone, OHT; and testosterone, T) and 17 alpha,20 beta-dihydroxyprogesterone (17-20 beta P) were quantified by radioimmunoassay in incubation media. OHT and OT were secreted in larger quantities than T and 17-20 beta P. The PE dose that evoked a half-maximal response (ED50), the ratios of maximum stimulated vs baseline secretion, and total testicular steroid output all changed during the reproductive cycle. Androgen secretion in response to PE was low in immature and spent fish, both in terms of ED50 and the ratio of maximum stimulated vs baseline secretion. This ratio increased in testes showing the first signs of maturation and remained elevated during rapid testicular growth, before reaching maximum values at full maturity. The lowest ED50 values were found at the end of spermatogenesis and during the peak spawning period. 17-20 beta P secretion could not be stimulated noticeably until the fish had entered the spawning period and, as opposed to androgens, remained stimulable in spent fish. ED50 values for 17-20 beta P ranged, without showing clear-cut variations, above those calculated for androgens. The changes in PE reactivity and steroid secretion capacity during the reproductive cycle are likely to contribute to the changes in circulating steroid concentrations and may allow modulations of testicular steroid production without large changes in circulating GTH levels.

The dynamics of vitellogenin sequestration into vitellogenic ovarian follicles of the rainbow trout, Salmo gairdneri

This study provides quantitative data on the dynamics of protein sequestration into vitellogenic follicles of the rainbow trout, Salmo gairdneri. The ovarian uptake of both radiolabelled vitellogenin (VTG) and bovine serum albumin (BSA) were investigated in a homogenous population of maturing vitellogenic females.Ten fish were injected with (3)H. VTG directly into the bloodstream. Concomitantly, five of these fish received an equal amount of (14)C.BSA. Twenty two hours after injection, of the tissues sampled, the greatest proportions of (3)H. VTG were present in the blood (into which the radio labels were administered) and in the ovary (up to 28% and 46% of that originally present in the blood, respectively). VTG uptake was both selective, rates of uptake far exceeding that of the (14)C.BSA, and rapid. (3)H. VTG was sequestered at rates of between 35 to 390 ng.mm(2) follicle surface(-1).h(-1) in the different fish. The rates of VTG uptake into similarly sized follicles varied both between different sites within the ovary (by up to 30%) and also between the ovaries (by up to 38%) of an individual fish.

The deleterious effects of cortisol implantation on reproductive function in two species of trout, Salmo trutta L. and Salmo gairdneri Richardson

Implantation of a cortisol-releasing pellet (60 mg kg-1 fish) into the peritoneal cavity of brown trout, Salmo trutta L. (sexually maturing males and females), and rainbow trout, Salmo gairdneri Richardson (maturing males and immature fish of both sexes), significantly elevated their plasma cortisol level. At 18 days postimplantation, cortisol-implanted sexually maturing male brown trout had smaller gonads, a lower plasma testosterone level, and less gonadotropin in their pituitary gland than control fish. Plasma levels of 11-ketotestosterone and gonadotropin were not significantly affected. Cortisol-implanted sexually maturing female brown trout had smaller gonads, reduced plasma levels of 17 beta-oestradiol, testosterone, and vitellogenin, and a lower pituitary gland gonadotropin content than control fish. The plasma gonadotropin level was unaffected. At 36 days post-implantation, cortisol treatment of maturing male rainbow trout significantly suppressed plasma gonadotropin levels. Plasma levels of testosterone, 11-ketotestosterone, and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one, pituitary gonadotropin content, and gonad size were not significantly affected. In sexually immature female rainbow trout, cortisol administration suppressed the level of vitellogenin in the plasma, compared to control-implanted fish. The 17 beta-oestradiol level was not affected. Cortisol implantation did not affect the plasma testosterone level in sexually immature male trout. These results suggest that prolonged elevation of plasma cortisol, to levels well within physiological range, can affect a wide range of reproductive parameters in both brown and rainbow trout. Further, some effects are manifest in immature as well as in mature fish. These findings are discussed in relation to the effects of cortisol treatment on the state of health of the treated fish.