Gonadotropins beta-GtHI and beta-GtHII from the gilthead seabream, Sparus aurata

Pituitary multi-hormone cells in mammals and fish: history, origin, and roles

The vertebrate pituitary is a dynamic organ, capable of adapting its hormone secretion to different physiological demands. In this context, endocrinologists have debated for the past 40 years if endocrine cells are mono- or multi-hormonal. Since its establishment, the dominant "one cell, one hormone" model has been continuously challenged. In mammals, the use of advanced multi-staining approaches, sensitive gene expression techniques, and the analysis of tumor tissues have helped to quickly demonstrate the existence of pituitary multi-hormone cells. In fishes however, only recent advances in imaging and transcriptomics have enabled the identification of such cells. In this review, we first describe the history of the discovery of cells producing multiple hormones in mammals and fishes. We discuss the technical limitations that have led to uncertainties and debates. Then, we present the current knowledge and hypotheses regarding their origin and biological role, which provides a comprehensive review of pituitary plasticity.

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

Development of a giant grouper Luteinizing Hormone (LH) Enzyme-Linked Immunosorbent Assay (ELISA) and its use towards understanding sexual development in grouper

A recombinant giant grouper Luteinizing Hormone (LH) consisting of tethered beta and alpha subunits was produced in a yeast expression system. The giant grouper LH β-subunit was also produced and administered to rabbits for antibody development. The recombinant LH and its antibody were used to develop an Enzyme Linked Immunosorbent Assay (ELISA). This ELISA enabled detection of plasma LH levels in groupers at a sensitivity between 391 pg/ml and 200 ng/ml. Different species of grouper were assayed with this ELISA in conjunction with gonadal histology and body condition data to identify links between circulating LH levels and sexual development. We found that circulating levels of LH decreased when oocytes began to degenerate, and sex-transition gonadal characteristics were apparent when LH levels decreased further. When circulating LH levels were related to body condition (body weight/ body length), transitioning-stage fish had relatively high body condition but low plasma LH levels. This observation was similar across multiple grouper species and indicates that plasma LH levels combined with body condition may be a marker for early male identification in the protogynous hermaphrodite groupers.

The involvement of gonadotropins and gonadal steroids in the ovulatory dysfunction of the potamodromous Salminus hilarii (Teleostei: Characidae) in captivity

Potamodromous teleosts that require migration to reproduce show dysfunctions that block ovulation and spawning while in captivity. To understand the physiological basis of these reproductive dysfunctions, follicle-stimulating hormone b subunit (fshb) and luteinizing hormone b subunit (lhb) gene expression analyses by real-time quantitative PCR, together with measurements of estradiol (E 2), 17α-hydroxyprogesterone (17α-OHP) and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DHP) levels, were carried out throughout the reproductive cycle of the potamodromous Salminus hilarii. The following reproductive stages were evaluated in captive and wild females: previtellogenic (PV), advanced maturation/mature (AM) and regression/spent (REG/SPENT). In the wild females, fshb expression decreased from the PV to the AM stage, and the opposite pattern was detected for E 2, which increased from the PV to the AM stage. fshb was expressed at lower levels in captive than in wild females, and this difference did not change during the reproductive cycle. lhb expression also increased from the PV to the AM stage in both groups, but the wild females at the AM and REG/SPENT stages showed higher lhb expression levels than the captive females. The concentrations of 17α-OHP did not change during the reproductive cycle, and the levels were higher in the captive than in the wild females at all reproductive stages. 17α,20β-DHP levels did not change between wild and captive females. However, in captive females, the transition from PV to AM stage was followed by an increase in 17α,20β-DHP levels. These data indicate that dysfunctions in the gonadotropins and steroids synthesis pathways cause the ovulation failure in captive S. hilarii.

Intra-pituitary relationship of follicle stimulating hormone and luteinizing hormone during pubertal development in Atlantic bluefin tuna (Thunnus thynnus)

As part of the endeavor aiming at the domestication of Atlantic bluefin tuna (BFT; Thunnus thynnus), first sexual maturity in captivity was studied by documenting its occurrence and by characterizing the key hormones of the reproductive axis: follicle stimulating hormone (FSH) and luteinizing hormone (LH). The full length sequence encoding for the related hormone β-subunits, bftFSHβ and bftLHβ, were determined, revealing two bftFSHβ mRNA variants, differing in their 5' untranslated region. A quantitative immuno-dot-blot assay to measure pituitary FSH content in BFT was developed and validated enabling, for the first time in this species, data sets for both LH and FSH to be compared. The expression and accumulation patterns of LH in the pituitary showed a steady increase of this hormone, concomitant with fish age, reaching higher levels in adult females compared to males of the same age class. Conversely, the pituitary FSH levels were elevated only in 2Y and adult fish. The pituitary FSH to LH ratio was consistently higher (>1) in immature than in maturing or pubertal fish, resembling the situation in mammals. Nevertheless, the results suggest that a rise in the LH storage level above a minimum threshold may be an indicator of the onset of puberty in BFT females. The higher pituitary LH levels in adult females over males may further support this notion. In contrast three year-old (3Y) males were pubertal while cognate females were still immature. However, it is not yet clear whether the advanced puberty in the 3Y males was a general feature typifying wild BFT populations or was induced by the culture conditions. Future studies testing the effects of captivity and hormonal treatments on precocious maturity may allow for improved handling of this species in a controlled environment which would lead to more cost-efficient farming.

GnRH isoforms expression in relation to the gonadal cycle and to dominance rank in the gilthead seabream, Sparus aurata

The manner in which behavior influences the gonadotropin-releasing hormone (GnRH) axis in hermaphroditic fishes is not understood. The Gilthead seabream, Sparus aurata, is a protandrous hermaphrodite with a complex gonadal cycle consisting of a quiescent, pre-spawning, spawning, and post-spawning stage. On two separate experiments, I used real-time quantitative PCR to measure the mRNA expression of three GnRH isoforms in homogenized seabream whole-brain extracts. In the first experiment, I measured the levels of GnRH-1, GnRH-2, and GnRH-3 mRNA throughout the gonad cycle. All three GnRH mRNAs increase around the peak of the spawning season (December). GnRH-3 mRNA expression is also elevated in August, which coincides with the beginning of gonad differentiation. All three GnRH mRNAs have the lowest expression levels in the month of September. There was no difference between males and females in the expression levels of any of the three GnRH mRNA. In the second experiment, I measured individual dominance ranks in six groups of fish, three during quiescence and three during spawning. GnRH-1 mRNA expression was positively correlated with dominance rank only during the quiescent period. The more dominant fish tended to have higher GnRH-1 mRNA expression. The existence of a quiescent-only correlation between GnRH-1 mRNA and dominance rank suggests a mechanism by which activation of gonad maturation could occur first in the most dominant ambisexual fish.

Molecular cloning and localization of the luteinizing hormone beta subunit and glycoprotein hormone alpha subunit from Japanese anchovy Engraulis japonicus

Although Clupeiformes contain many economically important species, there is limited information on their reproductive physiology. To obtain more insight into reproductive mechanisms in clupeiform fishes, molecular cloning of the Japanese anchovy Engraulis japonicus luteinizing hormone beta (LHbeta) and glycoprotein hormone alpha (GPHalpha) subunits, and immunocytochemistry of gonadotrophs in the pituitary using antisera raised against the synthetic peptides for both subunits were carried out. The cDNAs for LHbeta and GPHalpha subunits consisted of 963 and 535 nucleotides encoding 141 and 122 amino acids, respectively. The deduced amino acid sequences of the E. japonicus LHbeta subunit showed a 60% similarity to the Pacific herring Clupea pallasii LHbeta subunit and 24-31% similarities to FSHbeta subunits of other fish species. The E. japonicus GPHalpha subunit showed 52-57% similarities to anguilliform and cypriniform GPHalpha subunits. Both the subunits have typical structural characteristics of each subunit such as N-linked glycosylation sites, conserved cysteine residues and highly conserved short amino acid sequences. These results indicate that cDNAs cloned in this study encode the E. japonicus LHbeta and GPHalpha subunits. Reverse-transcription polymerase chain reaction (RT-PCR) revealed that both the LHbeta and GPHalpha subunit genes were abundantly expressed in the pituitary, and the GPHalpha subunit was observed to be weakly expressed in the extrapituitary tissues. Immunocytochemistry of the E. japonicus pituitary showed that cells that immunoreacted with antiserum against the LHbeta subunit were distributed in the peripheral regions of proximal pars distalis, and these cells were also immunoreactive to antiserum against the GPHalpha subunit. An abundant number of both LHbeta and GPHalpha cells in the pituitary of matured fish were observed, in comparison with immature fish. These results indicate that the E. japonicus LH is involved in the final reproductive maturation as well as those of other teleosts.

Expression of recombinant zebrafish follicle-stimulating hormone (FSH) in methylotropic yeast Pichia pastoris

Pituitary gonadotropin follicle-stimulating hormone (FSH) was identified in fish two decades ago, but its functional importance in fish reproduction remains poorly defined, especially in non-salmonid species. This gap in our knowledge is partially due to the lack of the hormone in pure form in most of the species studied. We describe here the production of two different forms of biologically active recombinant zebrafish FSH (zfFSH and zfFSH(HIS)) using methylotrophic yeast, Pichia pastoris, as the bioreactor. One form (zfFSH) was produced as the molecule closer to the native form, with the two subunits (Cga and Fshb) expressed separately under different promoters. The other form (zfFSH(HIS)) was produced as a single polypeptide, with the cDNAs for the two subunits joined to form a fusion gene that contained a 6X His tag as part of the linker between the two subunits. The culture conditions were optimized for pH and incubation time for maximal production of the proteins. Using a zebrafish FSH receptor (Fshr)-based reporter gene assay, we tested and compared the biological activities of the two forms of recombinant zebrafish FSH. Our results provide useful information for the future production of recombinant gonadotropins in other fish species.

Molecular cloning, characterization and expression of FSH and LH beta subunits from grass carp (Ctenopharyngodon idella)

Follicle-stimulating hormone beta subunit (FSHbeta) and luteinizing hormone beta subunit (LHbeta) have been cloned and characterized from the pituitary of grass carp (Ctenopharyngodon idella). The full length of FSHbeta and LHbeta cDNA was 393 bp and 441 bp, with open reading frame encoding proteins of 130 and 146 amino acids, respectively. Phylogenetic analysis of the proteins of FSHbeta and LHbeta showed a high homology with other fishes. Homology analysis also indicated that LHbeta has higher conservation than FSHbeta. The expression analysis of grass carp FSHbeta and LHbeta by RT-PCR suggested that they were only expressed in the pituitary. Real-time quantitative PCR protocols were developed and validated to measure FSHbeta and LHbeta mRNAs during ovarian development. The FSHbeta and LHbeta mRNA level was very low in the pituitaries of early-pubertal fish and significantly increased during the ovulation period. These results suggested that in grass carp the gonadotropins synthesized synchronously in order for asynchronous oogenesis to take place.

Molecular characterization of Chinese sturgeon gonadotropins and cellular distribution in pituitaries of mature and immature individuals

Chinese sturgeon (Acipenser sinensis) is a rare and endangered species, and also an important resource for the sturgeon aquaculture industry. To understand molecular characterization of Chinese sturgeon gonadotropins (GTHs), we cloned the full-length cDNAs of gonadotropin subunits common alpha (GTH-alpha), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from a pituitary cDNA library of mature female. Two subtypes of GTH-alpha were identified. The nucleotide sequences of A. sinensis common alpha I (AsGTH-alpha I), common alpha II (AsGTH-alpha II), FSHbeta (AsFSHbeta) and LHbeta (AsLHbeta) subunit cDNAs are 345, 363, 387 and 414bp in length, and encode mature peptides of 115, 121, 129 and 138aa, respectively. Then, three polyclonal antibodies were prepared from the in vitro expressed AsGTH-alpha I, AsFSHbeta and AsLHbeta mature proteins, respectively. Significant expression differences were revealed between immature and mature sturgeon pituitaries. Western blot detection and immunofluoresence localization revealed the existence of three-gonadotropin subunits (AsGTH-alpha, AsFSHbeta and AsLHbeta) in mature sturgeon pituitaries, but only AsFSHbeta was detected in immature individual pituitaries during early stages in the sturgeon life, and obvious difference was observed between males and females. In males, AsFSHbeta was expressed in 4-year-old individuals, whereas in females, AsFSHbeta was just expressed in 5-year-old individuals.

Quantification of gonadotropin subunits GPalpha, FSHbeta, and LHbeta mRNA expression from Atlantic cod (Gadus morhua) throughout a reproductive cycle

To elucidate the role of the gonadotropins in Atlantic cod (Gadus morhua), complete coding sequences with partially or fully un-translated regions for the three subunits GPalpha, FSHbeta, and LHbeta were determined. The sequences of the corresponding genomic loci were also determined, allowing the design of mRNA-targeting quantitative PCR assays. Relative expression was analyzed during a complete seasonal sexual maturation cycle in Atlantic cod females. Increasing levels of lhbeta mRNA were observed during gonadal growth, peaking at spawning in February-March which corresponds to maximum gonadosomatic index. In contrast, both gpalpha and fshbeta gradually increased to a peak in December, two months before spawning started, and decreased in January just prior to spawning. Both mRNAs increased again and remained high during the spawning season, with a decline at the end of the spawning period, a further decrease in spent females, followed by a new gradual increase concurrent with the start of the next reproductive cycle. In addition to its role in vitellogenesis prior to spawning, FSH seems to have additional functions during the spawning period, possibly related to vitellogenesis that runs in parallel with final oocyte maturation and ovulation of the multiple batch spawner Atlantic cod.

Comparative gene expression of gonadotropins (FSH and LH) and peptide levels of gonadotropin-releasing hormones (GnRHs) in the pituitary of wild and cultured Senegalese sole (Solea senegalensis) broodstocks

The Senegalese sole (Solea senegalensis) is a valuable flatfish for aquaculture, but it presents important reproductive problems in captivity. Spawning is achieved by wild-caught breeders but cultured broodstocks fail to spawn spontaneously and, when they do, eggs are unfertilized. To gain knowledge on the physiological basis underlying this reproductive dysfunction, this study aimed at analyzing comparative hormone levels between wild and cultured broodstocks at the spawning season. The Senegalese sole gonadotropin (GTH) subunits, FSHbeta, LHbeta and GPalpha, were cloned and qualitative (in situ hybridization) and quantitative (real-time PCR) assays developed to analyze pituitary GTH gene expression. In females, FSHbeta and GPalpha mRNA levels were higher in wild than in cultured broodstocks, whereas in males all three subunits were highest in cultured. By ELISA, three GnRH forms were detected in the pituitary, displaying a relative abundance of GnRH2>GnRH1>GnRH3. All GnRHs were slightly more abundant in wild than cultured females, whereas no differences were observed in males. Plasma levels of vitellogenin and sex steroids were also analyzed. Results showed endocrine differences between wild and cultured broodstocks at the spawning period, which could be related to the endocrine failure of the reproductive axis in cultured breeders.

Molecular characterization and quantification of the gonadotropin receptors FSH-R and LH-R from Atlantic cod (Gadus morhua)

In order to elucidate regulatory mechanisms during puberty final oocyte maturation and spawning, full-length sequences coding for the receptors for follicle-stimulating hormone (FSH-R) and luteinizing hormone (LH-R) were isolated from female Atlantic cod (Gadus morhua) by a RACE-PCR based strategy. The nucleotide and amino acid sequences showed high homologies with the corresponding sequences of other fish species but contained some distinct differences. Conserved features important for functionality, such as a long N-terminal extracellular domain (ECD), seven transmembrane domains and a short C-terminal intracellular domain, were identified in both predicted proteins. Partial genomic sequences for these genes were also determined, allowing the design of mRNA-specific quantitative PCR assays. Due to suspected alternative splicing during expression of these genes, additional real-time PCR assays detecting variants containing the membrane-anchoring domain were established. Besides the expected expression of FSH-R and LH-R mRNA in the gonads similarly strong signals for LH-R were also obtained in male gill, and in female and male brain. When relative expression was analysed at different stages of sexual maturation, levels for FSH-R increased moderately during gonadal growth whereas those of LH-R showed a high peak at spawning.

The gonadotropin receptors FSH-R and LH-R of Atlantic halibut (Hippoglossus hippoglossus)--2. Differential follicle expression and asynchronous oogenesis

The biological activity and spatio-temporal expression patterns of the gonadotropin receptors FSH-R and LH-R were examined in the repetitive spawner Atlantic halibut to elucidate the gonadotropic regulation of the asynchronous follicle development. The cloned receptors were expressed in mammalian COS-7 cells, and stimulation with sea bass FSH and LH increased the cAMP production. The halibut FSH-R and LH-R genes were shown to be highly expressed in the gonads of sexually mature fish, but the transcripts were also found in extra-gonadal tissues such as pituitary and brain. Different expression patterns of FSH-R and LH-R in the developing follicles were documented by semi-quantitative RT-PCR. Abundant FSH-R mRNA was found in the small follicles during primary growth and vitellogenesis, and the signals were localized to the granulosa cells by in situ hybridization. In contrast, follicular LH-R mRNA was hardly detectable during the early stages. Conversely, in follicles during final maturation FSH-R mRNA levels tended to decrease, while the expression of LH-R was highly upregulated. Whereas the pituitary FSH and LH are asynchronously expressed in annual spawners, both gonadotropins were expressed in the female halibut pituitary throughout the reproductive cycle, except in the prespawning females. Hence, the sequential gonadotropic activation of ovarian follicle growth and maturation in repetitive spawners is probably regulated by modulating the temporal expression of FSH-R and LH-R in the follicle membrane.

Gonadotropin hormone and receptor sequences from model teleost species

Fish offer some advantages for the study of vertebrate reproductive physiology. Only a few of the genes encoding the components of the hypothalamic-pituitary-gonadal axis have been identified from model teleosts. This study describes a combination of database searching and molecular approaches to identify the FSH and LH gonadotropin beta-subunits (fshb and lhb, respectively), and the LH receptor (lhr) from two model teleost species: zebrafish (Danio rerio) and Fugu (Takifugu rubripes). Sequence and phylogenetic analyses were used to examine the relationships that exist between gonadotropins and their receptors from species representing several piscine orders. The gonadotropin alpha-subunit (Cga) is highly conserved among teleosts and tetrapods. The presence of a genomic pseudogene (cgap) was also noted in zebrafish. Generally, teleostean FSHbeta protein sequences share less identity with each other than do LHbeta protein sequences, supporting the hypothesis that FSHbeta diverged more rapidly during teleost evolution. Interestingly, and uniquely, zebrafish Fshb lacked two highly conserved cysteine residues in the "determinant loop" which is thought to contribute towards receptor binding and specificity. Teleost gonadotropin receptor sequences clearly diverged into two distinct groups, FSHR and LHR. As has been seen with mammalian gonadotropin receptor transcripts, splice variants of zebrafish lhr were also observed.

Molecular cloning of cDNAs and structural model analysis of two gonadotropin beta-subunits of snakehead fish (Channa maculata)

The cDNAs encoding beta-subunits of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) have been cloned from the pituitary of snakehead fish, Channa maculata, and the three-dimensional structural models of the encoded FSH and LH were investigated. The cloned cDNAs, including 5'-untranslated region (UTR), open-reading frame, and 3'-UTR followed by a poly(A) tail, were obtained by reverse transcription-polymerase chain reaction and rapid amplification of cDNA end methods. The open-reading frames of FSH-beta cDNA encodes a 120-amino acid protein with a signal peptide of 18 amino acids and a mature protein of 102 amino acids; while LH-beta cDNA encodes a 140-amino acid protein with a signal peptide of 33 amino acids and a mature protein of 115 amino acids. The amino acid sequence identities of snakehead fish FSH-beta and LH-beta in comparison with other fish are 27.8-81.9% and 45.2-88.8%, respectively; while in comparison with tetrapods are 26.2-28.9% and 37.5-51.2%, respectively. Both FSH-beta and LH-beta of snakehead fish resemble most to those of Perciformes, implying their closer phylogenetic relationship. All 12 cysteine residues are conserved in snakehead fish LH-beta; while 11 cysteine residues are conserved in its FSH-beta. The third cysteine is absent in snakehead fish FSH-beta; instead, a positionally shifted cysteine residue is present at the N-terminus, as found in some phylogenetic related fish. The structure models of snakehead fish FSH and LH, constructed by using the crystal structures of human FSH and human chorionic gonadotropin as respective template, showed that the positionally shifted N-terminal cysteine residue of snakehead fish FSH-beta likely can substitute the third cysteine to form a disulfide bond with the 12th cysteine.

Hormonal regulation of hepatic IGF-I and IGF-II gene expression in the marine teleost Sparus aurata

The present work aimed to determine whether GnRH potentiates the effect of growth hormone (GH) on insulin-like growth factors (IGF-I and IGF-II) hepatic gene expression in Sparus aurata liver. Since several hepatic genes were shown to underlie direct regulation via the hepatic estrogen receptor, another aim was to extend our understanding of direct estrogen effects on liver IGFs gene expression. Pre-reproductive sea bream females were treated with GH, GnRH, estradiol-17beta, GH plus GnRH, and estradiol-17beta plus GH. After 72 hr, all treatment induced an increase of plasma estradiol well correlated with the increase of plasma vitellogenin (VTG) levels. IGF-I and IGF-II expression in the liver of treated females was determined by semi-quantitative RT-PCR, using beta-actin as internal standard. The results reported here show that GH significantly stimulates hepatic transcription of IGF-I and IGF-II genes. Surprisingly, E2 and GnRH treatments decreased both IGF-I and IGF-II mRNA levels. In fishes treated with GH plus GnRH, the GnRH contrasted the GH effect: the IGF-I mRNA levels were still significantly higher than in controls, while the effect of GH on IGF-II gene expression was totally abolished. At the same time, in the combined treatment with GH plus E2, the E2 counteracted the stimulatory effect of GH on both IGF-I and IGF-II genes expression.

Zebrafish gonadotropins and their receptors: II. Cloning and characterization of zebrafish follicle-stimulating hormone and luteinizing hormone subunits--their spatial-temporal expression patterns and receptor specificity

Gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) play critical roles in vertebrate reproduction. In the present study, we cloned and characterized zebrafish FSHbeta (fshb), LHbeta (lhb), and GTHalpha (cga) subunits. Compared with the molecules of other teleosts, the cysteine residues and potential glycosylation sites are fully conserved in zebrafish Lhb and Cga but not in Fshb, whose cysteines exhibit unique distribution. Interestingly, in addition to the pituitary, fshbeta, lhbeta, and cga were also expressed in some extrapituitary tissues, particularly the gonads and brain. In situ hybridization showed that zebrafish fshbeta and lhbeta were expressed in two distinct populations of gonadotrophs in the pituitary. Real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that all the three subunits increased expression before ovulation (0100-0400) when the germinal vesicles in the full-grown follicles were migrating toward the periphery, but the levels dropped at 0700, when ovulation occurred. Recombinant zebrafish FSH (zfFSH) and LH (zfLH) were produced in the Chinese hamster ovary (CHO) cells and their effects on the cognate receptors (zebrafish Fshr and Lhr) tested. Interestingly, zfFSH specifically activated zebrafish Fshr expressed together with a cAMP-responsive reporter gene in the CHO cells, whereas zfLH could stimulate both Fshr and Lhr. In conclusion, the present study systematically investigated gonadotropins in the zebrafish in terms of their structure, spatial-temporal expression patterns, and receptor specificity. These results, together with the availability of recombinant zfFSH and zfLH, provide a solid foundation for further studies on the physiological relevance of FSH and LH in the zebrafish, one of the top biological models in vertebrates.

Cloning of FSHbeta, LHbeta, and glycoprotein alpha subunits from the Russian sturgeon (Acipenser gueldenstaedtii), beta-subunit mRNA expression, gonad development, and steroid levels in immature fish

The Russian sturgeon, Acipenser gueldenstaedtii, is a late-maturing Acipenseriformes. To elucidate the role of FSH and LH in its reproduction, we cloned its glycoprotein alpha-subunit (GPalpha) and gonadotropin beta-subunits (FSHbeta and LHbeta) using 5' and 3' RACE-PCR. The nucleotide sequences of the Russian sturgeon (st) GPalpha, FSHbeta, and LHbeta are 345, 384, and 411 bp long, encoding peptides of 91, 115, and 114 amino acids, respectively. The deduced amino acid sequence of each mature subunit showed high similarity with those of other teleosts. Sequence analysis showed that stFSHbeta is more similar to higher vertebrate FSHbetas (35-37%) than to higher vertebrate LHbetas (26-30%). The next objective of this work was to compare the development of sturgeon gonads at the very first stages of their growth with the expression of their gonadotropins. Sturgeons at ages 1, 2, 3 or 4 years were sacrificed. The expression of their gonadotropin beta-subunits was determined using quantitative real-time PCR, and their gonads were examined histologically, followed by a determination of the plasma levels of estradiol in females and 11-ketotestosterone (11-KT) in males. The expression levels of stFSHbeta subunit was found to be higher in fish at 3 and 4 years of age than in 1-year olds. mRNA levels of stLHbeta were higher than those of stFSHbeta in both genders. Moreover mRNA levels of stFSHbeta detected in females were significantly higher than those found in males. Even at age 4 years, all female Russian sturgeons tested contained gonads at the pre-vitellogenic stage, with small oocytes and very low levels of estradiol in the plasma. However, among the males, at ages 3 and 4 years, we found testes that contained spermatids and spermatozoa. Those males were found to have significantly high GSI (gonadosomatic index; gonadal weight as a percentage of BW) levels, stLHbeta expression and 11-KT levels.

Novel expression of gonadotropin subunit genes in oocytes of the gilthead seabream (Sparus aurata)

It is widely believed that FSH and LH, which are known to play key roles in controlling the production of functional oocytes in vertebrates, are synthesized and secreted exclusively by the anterior pituitary. Here we present evidence for the novel expression of FSHbeta, LHbeta, and the common glycoprotein-alpha (Cgalpha) in the gilthead seabream ovary. Using in situ hybridization and immunocytochemistry, FSHbeta was detected in primary-growth and secondary-growth-I oocytes, LHbeta was found in secondary-growth oocytes, and Cgalpha was observed in both primary and secondary-growth oocytes. Northern blot analyses demonstrated that Fshbeta transcript is 0.6 kb in both pituitary and ovary, whereas the ovarian Lhbeta transcript (1.1 kb), unexpectedly, is longer than the known pituitary Lhbeta transcript (0.6 kb). Sequence analyses revealed that ovarian Lhbeta is driven by a different promoter than pituitary Lhbeta, which generates an additional 459 bases at the distal portion of the 5'-untranslated region of the ovarian Lhbeta. Furthermore, using in vitro ovarian fragment incubation, we demonstrated that mammalian GnRH analog agonist enhanced the expression of ovarian Fshbeta (up to 2.7-fold), Lhbeta (up to 1.4-fold), Cgalpha (up to 1.8-fold), and the secretion of ovarian LH (up to 2.2-fold). In contrast, GnRH antagonist, analog E, suppressed the secretion of ovarian LH. Our findings suggest that a GnRH-gonadotropin axis is present in the gilthead seabream ovary and that FSH and LH, the well-characterized pituitary hormones, may have prominent novel roles in teleost intraovarian communication between oocytes and ovarian follicle cells.

Developmental Expression of Three Forms of Gonadotropin-Releasing Hormone and Ontogeny of the Hypothalamic-Pituitary-Gonadal Axis in Gilthead Seabream (Sparus aurata)1

To address the complexity of the origin of the GnRH system in perciforms, we investigated the ontogenic expression of three GnRHs in gilthead seabream. Using in situ hybridization, chicken (c) GnRH-II mRNA-expressing cells were detected in the hindbrain at 1.5 days postfertilization (DPF) and in the midbrain at 2 DPF and thereafter; the hindbrain signals became undetectable after 10 DPF. Salmon (s) GnRH mRNA-expressing cells were first seen in the olfactory placode at 3 DPF, started caudal migration at 14 DPF, and reached the preoptic areas at 59 DPF. Seabream (sb) GnRH mRNA-expressing cells were first detected in the terminal nerve ganglion cells (TNgc), ventral part of the ventral telencephalon, nucleus preopticus parvocellularis, and thalamus at 39 DPF, and extended to the nucleus preopticus magnocellularis at 43 DPF, ventrolateral hypothalamus at 51 DPF, and nucleus lateralis tuberis and posterior tuberculum at 59 DPF. Coexpression of sbGnRH and sGnRH transcripts was found in the TNgc. Using real-time fluorescence-based quantitative polymerase chain reaction, transcript levels of cGnRH-II and sGnRH were first detected at 1 and 1.5 DPF, respectively, and increased and remained high thereafter. Transcript levels of sbGnRH remained low after first detection at 1 DPF. Furthermore, these GnRH expression profiles were correlated with the expression profiles of reproduction-related genes in which at least four concomitant increases of GnRH, GnRH receptor, gonadotropin, gonadotropin receptor, and Vasa transcripts were found at 5, 8, 14, and 28 DPF. Our data provide an expanded view of the ontogeny of the GnRH system and reproductive axis in perciforms.

Temporal profile of beta follicle-stimulating hormone, beta luteinizing hormone, and growth hormone gene expression in the protandrous hermaphrodite, gilthead seabream, Sparus aurata

The temporal profile of betaFSH, betaLH, and growth hormone (GH) gene expression was measured throughout the periods of gonadal development, spawning, and post-spawning in the protandrous hermaphrodite gilthead seabream (sb), Sparus aurata (L.). Sampling was carried out monthly, covering a 8-31 month fish age. Pituitary RNA was extracted individually. The levels of betaFSH, betaLH, and GH mRNA were measured by dot blot hybridization using sb betaFSH, betaLH, and GH cDNA as probes and analyzed by computing densitometer (values standardized using individuals' beta-actin pituitary mRNA levels). All three genes, betaFSH, betaLH, and GH were expressed throughout the year, with seasonal variations. However, transcript levels of betaLH were consistently higher than those of betaFSH. During the spawning season (which lasts for about 4 months), the mRNA levels of both betaFSH and betaLH subunits increased dramatically. betaFSH peaked at the start of the spawning season for both males and females, and was significantly higher in males. As for betaLH transcripts, a statistical interaction between sex and date was observed. No significant differences between males and females were found for GH. The pattern of GH expression levels was found to be correlated to that of betaLH.

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.

Cloning and sequencing of the FSH-beta and LH beta-subunit in the three-spined stickleback, Gasterosteus aculeatus, and effects of photoperiod and temperature on LH-beta and FSH-beta mRNA expression

The aim of this study was to characterize the FSH-beta and LH-beta subunit in the three-spined stickleback, a fish used extensively in experimental studies, and to use the cloned cDNAs as probes for measuring FSH-beta and LH-beta mRNA expression in sticklebacks treated with different photoperiods and temperatures. A first strand cDNA was prepared from 10 pituitaries from male sticklebacks, and cDNA fragments were amplified by PCR using degenerated primers based on highly conserved regions of known teleost FSH-beta and LH-beta cDNA sequences. To obtain full-length cDNAs of FSH-beta and LH-beta, RACE amplifications were performed. The cDNA of FSH-beta was 540 bp long, encoding a protein of 122 amino acids and LH-beta cDNA was 568 bp long, encoding a protein of 148 amino acids. Of gonadotropin (GTH) beta-subunits published so far, those most similar to stickleback GTHs are found among other percomorph fishes, with amino acid similarities of 46-55% for FSH-beta and 75-77% for LH-beta subunits.The cloned cDNAs were used as probes to analyze LH and FSH mRNA expression in pituitaries from sticklebacks exposed to different photoperiods and temperatures. Two hundred males were divided into four groups and kept under different photoperiods and temperatures; light:dark (LD) 16:8, 20 degrees C; LD 16:8, 7 degrees C; LD 8:16, 20 degrees C, and LD 8:16, 7 degrees C. Red breeding colors and a marked kidney hypertrophy, androgen-dependent male secondary sexual characters in the stickleback, appeared in the group kept under LD 16:8 at 20 degrees C, but not in the other groups. The kidney epithelium height (KEH) was significantly lower in the LD 8:16, 20 degrees C group than in all other groups, and this was also the only group with active spermatogenesis. The LD 8:16 20 degrees C had significantly lower expression of both FSH-beta and LH-beta than all other groups. Thus, both GTHs displayed their lowest expression when spermatogenesis was active and when low KEH indicated that androgens levels were at their lowest.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

Cloning of European eel (Anguilla anguilla) FSH-β subunit, and expression of FSH-β and LH-β in males and females after sex determination

The European eel (Anguilla anguilla) is a catadromic teleost species with a complex life cycle, both in sea and freshwater environments. The sex determination phase of gonadal development occurs in a freshwater environment. Polymorphism occurs in increasing rates with respect to gender. While males stop growing at approximately 150 g, females continue to grow to being much larger. In this study, we cloned the cDNA FSH-beta subunit of the European eel (A. anguilla), and measured the mRNA levels of FSH-beta and LH-beta in males and females after sex determination. The FSH-beta subunit cDNA consisted of 1068 bp, encoding a 127 amino acid peptide. A comparison between European and Japanese eels of the FSH-beta amino acid sequence showed 98% similarity.

Molecular characterization of sea bass gonadotropin subunits (alpha, FSHbeta, and LHbeta) and their expression during the reproductive cycle

Reproduction is controlled by two pituitary gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This study reports the cloning, sequence analysis, and gene expression of gonadotropin (GTH) subunits from the European sea bass (Dicentrarchus labrax). The GTH subunits were cloned from a sea bass brain-pituitary cDNA library. The nucleotide sequences of the common alpha, the FSHbeta, and the LHbeta subunit cDNAs were 625, 521, and 591 base pair (bp) long, respectively, encoding for mature peptides of 94, 105, and 115 amino acids (aa), respectively. Sequence analysis showed that sea bass FSHbeta is more similar to higher vertebrate FSHbeta's (35-37%) than to LHbeta's (26-30%), whereas sea bass LHbeta is more similar to LHbeta's (40-53%) than to FSHbeta's (26-41%). Phylogenetic analysis of fish GTH sequences grouped the beta subunits into two groups, FSH and LH, distributed into four classes, corresponding to the accepted divisions of Elopomorphs, Ostariophysis, Salmonids, and Percomorphs. A dot-blot technique was developed to analyze GTH pituitary mRNA levels during the reproductive cycle of male sea bass. From October (initiation of gametogenesis) to February (spermiation), the expression of all three subunits in the pituitary increased in parallel, concomitantly with the gonadosomatic index (GSI) and the accumulation of LH protein in the pituitary, all values declining sharply at post-spermiation. This study demonstrates that the pituitary of sea bass contains two gonadotropin hormones and that both gonadotropins are probably involved in the control of gametogenesis, gamete maturation, and spermiation.

Immunocytochemical characterization of adenohypophyseal cells in the greater weever fish (Trachinus draco)

The adenohypophysis of the greater weever fish (Trachinus draco) was studied using histochemical and immunocytochemical methods. The adenohypophysis comprised the rostral pars distalis (RPD), the proximal pars distalis (PPD), and the pars intermedia (PI). Neurohypophysis showed a patent hypophyseal stalk which was divided into several branches intermingled with the adenohypophysis. Salmon prolactin (PRL)-immunoreactive (ir) cells, arranged in follicles, resided in the RPD and the most rostral part of the ventral PPD. Human adrenocorticotropin (ACTH)-ir cells were located in the RPD between PRL-ir cells and the neurohypophyseal processes. Salmon and seabream somatotropin (GH)-ir cells were located in both the dorsal and the ventral PPD. Some GH-ir cells were seen in surrounding and in contact with neurohypophyseal branches, whereas other isolated or clustered GH-ir cells were embedded in adenohypophyseal cells of the PPD. In addition, isolated or clustered GH-ir cells were also detected in the tissue of the PPD covering the most rostral part of PI. Only one class of salmon and carp gonadotropin (GTH)-ir cells was detected. Isolated or clustered GTH-ir cells resided in both the dorsal and the ventral PPD and were seen surrounding the PI and in the tissue of the PPD covering the most rostral part of PI. In addition, a few scattered GTH-ir cells were observed in the ventral RPD. Scattered groups of thyrotropin (TSH)-ir cells were present in the anteroventral PPD. Salmon and seabream somatolactin (SL)-ir and bovine melanotropin (MSH)-ir cells were intermingled surrounding the neurohypophyseal tissue. SL-ir cells were negative to periodic acid-Schiff technique. MSH-ir cells showed a very weak immunoreactivity to anti-human ACTH((1-24)) serum. In addition to the PI location, few isolated or clustered SL- and MSH-ir cells were observed in the dorsal PPD.

Immunocytochemical applications of specific antisera raised against synthetic fragment peptides of mummichog GtH subunits: examining seasonal variations of gonadotrophs (FSH cells and LH cells) in the mummichog and applications to other acanthopterygian fishes

Two distinct types of gonadotrophs, FSH (GtH I) cells and LH (GtH II) cells, were immunocytochemically identified from mummichog (Fundulus heteroclitus; Cyprinodontiformes, Acanthopterygii) pituitary using antisera raised against synthetic fragment peptides of FSHbeta and LHbeta. Both cell types were abundant during the spawning period (spring and early summer) and decreased in number during the post-spawning immature period. The number of FSH cells increased again during the early phases of gonadal development (cortical alveoli accumulation in the oocytes and basal spermatogenesis) in early winter, whereas the number of LH cells did not. Only FSH cells were abundant during the latter phases of gonadal development (vitellogenesis and active spermatogenesis) in early spring. These observations suggest that both GtHs have important yet different roles for reproduction in this species. Antisera against the conservative region of the FSHbeta and the LHbeta subunits immunostained FSH cells and LH cells, respectively, also in red seabream (Pagrus major; Perciformes, Acanthopterygii) and small mouth bass (Micropterus dolomieu; Perciformes, Acanthopterygii), suggesting the possibility of their use for other acanthopterygian fishes.

Differential regulation of tyrosine hydroxylase and estradiol receptor expression in the rainbow trout brain

In numerous fish species, dopamine has been found to strongly inhibit gonadotropin release. Among the enzymes that regulate dopamine turnover, tyrosine hydroxylase (TH), the rate-limiting anabolic enzyme, could be a target for endocrine feedback regulation. Since dopamine turnover is stimulated by estradiol in rainbow trout, we have investigated the effect of estradiol on TH and estradiol receptor expression. In situ hybridization was used to quantify mRNA levels in the brain of ovariectomized female rainbow trout implanted or not with estradiol pellets. We demonstrated that preoptic TH and estradiol receptor mRNA levels are greatly decreased by gonadectomy during vitellogenesis. For TH expression, this effect was reversed in part by estradiol supplementation. We have also confirmed the existence of an inhibitory gonadal feedback on FSH secretion, mediated by estradiol. The stimulating effect of estradiol on TH expression found in this study could be a pathway involved in gonadal feedback on gonadotropin release.

Investigation into the duality of gonadotropic cells of Mediterranean yellowtail (Seriola dumerilii, Risso 1810): immunocytochemical and ultrastructural studies

Two antisera against the follicle-stimulating hormone-like gonadotropin (FSH) of Mediterranean (M.) yellowtail, anti-My FSHa and anti-My FSHb, were obtained. Anti-My FSHa serum specifically recognized FSH cells and did not react with any other pituitary cell type, while anti-My FSHb serum recognized the alpha-subunit of the pituitary glycoprotein hormones and immunostained FSH, luteinizing hormone-like gonadotropin (LH), and thyrotropin (TSH) cells. Anti-My FSHa serum, together with a previously obtained anti-My LHbeta serum, were used to further investigate the duality of gonadotropic cells in M. yellowtail by light and electron microscopic immunocytochemistry; three immunologically different gonadotropic cell populations expressing FSH, LH, or both hormones, were revealed. The three cell populations had the same regional distribution in the pituitary gland: the proximal pars distalis, including the thin ring surrounding the pars intermedia. However, while FSH cells were found isolated or forming small clusters, LH cells formed strands or compact groups, and were more numerous than FSH cells. FSH/LH cells were scarce. At the ultrastructural level, vesicular, granular, and intermediate FSH, LH, and FSH/LH cells were found; secretory granules and globules, on the one hand, or conspicuous dilated cisternae of rough endoplasmic reticulum (or both) predominated, respectively, in these cell types. The production of either FSH or LH, or both hormones, was not reflected in the ultrastructural features of gonadotropic cells. Thus, a single morphological cell type of varying ultrastructure depending on the functional stage seemed to encompass all gonadotropic cells in M. yellowtail. All forms of FSH, LH, and FSH/LH cells were found in involution.

Dopamine D2 receptors and secretion of FSH and LH: role of sexual steroids on the pituitary of the female rainbow trout

The role of sexual steroids in the modulation of a dopaminergic inhibitory tone on FSH and LH release was studied in the rainbow trout. The experiments were performed on previtellogenic trout, implanted or not with estradiol (E(2)), and vitellogenic trout. E(2) implant increased the circulating levels of LH and decreased the circulating levels of FSH in previtellogenic fish. The catecholamine inhibitor alphaMPT increased the circulating levels of LH, implanted or not with E(2). AlphaMPT increased circulating levels of LH in vitellogenic fish. This increase could be prevented by the dopaminergic agonist bromocryptine. The dopaminergic drugs had no effect on the circulating levels of FSH in all groups. E(2) decreased mRNA levels of sGnRH1 and sGnRH2 in the telencephalon of previtellogenic fish. The dopaminergic treatments had no effect on mRNA levels of both forms of sGnRH in previtellogenic and vitellogenic fish. Primary cultures of pituitary cells were primed for three days with steroids (E(2) or 17alpha-hydroxy, 20beta-dihydroprogesterone (17alpha20betaP)) before treatment with increasing doses of bromocryptine, associated or not with sGnRH. E(2), but not 17alpha20betaP, potentiated the sGnRH-induced release of LH. Bromocryptine induced a slight dose-dependent decrease of sGnRH-induced release of LH. This decrease was potentiated by 17alpha20betaP. E(2) and 17alpha20betaP had no effect on the release of FSH, but bromocryptine decreased the 10(-8)M sGnRH-induced release of FSH. In conclusion, the development of the dopaminergic inhibitory tone on gonadotropin release, at the onset of vitellogenesis, requires factors other than estradiol. E(2) should contribute in part to decrease the release of FSH. At the end of the reproductive cycle, 17alpha20betaP should reinforce the dopaminergic inhibitory tone.

Regulation of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) gene expression by gonadotropin-releasing hormone (GnRH) and sexual steroids in the Mediterranean Sea bass

The secretion of gonadotropins, the key reproductive hormones in vertebrates, is controlled from the brain by the gonadotropin-releasing hormone (GnRH), but also by complex steroid feedback mechanisms. In this study, after the recent cloning of the three gonadotropin subunits of sea bass (Dicentrarchus labrax), we aimed at investigating the effects of GnRH and sexual steroids on pituitary gonadotropin mRNA levels, in this valuable aquaculture fish species. Implantation of sea bass, in the period of sexual resting, for 12 days with estradiol (E2), testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT), almost suppressed basal expression of FSHbeta (four to 15-fold inhibition from control levels), while slightly increasing that of alpha (1.5-fold) and LHbeta (approx. twofold) subunits. Further injection with a GnRH analogue (15 microg/kg BW; [D-Ala6, Pro9-Net]-mGnRH), had no effect on FSHbeta mRNA levels, but stimulated (twofold) pituitary alpha and LHbeta mRNA levels in sham- and T-implanted fish, and slightly in E2- and DHT-implanted fish (approx. 1.5-fold). The GnRHa injection, as expected, elevated plasma LH levels with a parallel decrease on LH pituitary content, with no differences between implanted fish. In conclusion, high circulating steroid levels seems to exert different action on gonadotropin secretion, inhibiting FSH while stimulating LH synthesis. In these experimental conditions, the GnRHa stimulate LH synthesis and release, but have no effect on FSH synthesis.

Physiological changes in the spawning gilthead seabream, Sparus aurata, succeeding the removal of males

The physiological effects triggered in females by the removal of males from a group of spawning fish were examined in the multiple batch spawner, the gilthead seabream, Sparus aurata. One week after the removal of males, a large portion of the oocytes underwent atresia, and sporadic release of low quality eggs continued at low frequency over a period of seven weeks. The transcript levels of the three native gonadotropin releasing hormone (GnRH) forms, salmon (s)GnRH, seabream (sb)GnRH, and chicken (c)GnRH-II, and the two beta GtH subunits were measured. Brain mRNA levels for all three GnRHs and pituitary beta LH mRNA levels significantly declined in the females as a result of removing the males compared to females that were maintained with males. Pituitary beta FSH mRNA levels showed the opposite trend and were significantly higher in females that were separated from males. Circulating levels of LH, testosterone, estradiol, 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one, and 17 alpha, 20 beta,21-trihydroxy-4-pregnen-3-one all declined in the group of females without males. These results imply the existence of an endocrine response to socio-sexual stimuli during the reproductive process in the gilthead seabream.

Purification of mummichog (Fundulus heteroclitus) gonadotropins and their subunits, using an immunochemical assay with antisera raised against synthetic peptides

To detect mummichog gonadotropins (GtHs) and their subunits immunochemically, fragment peptides with amino acid sequences corresponding to cDNA data were synthesized, and antisera were raised against them. In the case of GtH-IIbeta, large loops such as the second loop and the "seat belt" structure (deduced from the hCG 3D structural data) were considered to be favorable regions for antigen, although further examination is needed to determine if this is the case of GtH-Ibeta and GtH-alpha. In the purification process, glycoprotein was extracted from acetone-dried mummichog pituitary and separated by various liquid chromatography procedures. Each fraction was assayed by immunoblotting with the appropriate antisera against synthetic peptides. Subunits (GtH-alpha, GtH-Ibeta, and GtH-IIbeta) were obtained through gel filtration, anion-exchange chromatography, and reverse-phase HPLC. Intact bioactive GtH-I and GtH-II were obtained through gel filtration, anion-exchange chromatography, and hydrophobic chromatography. Both GtH-I and GtH-II dissociated into subunits under acidic conditions. Nominal MW of each subunit was estimated from SDS-PAGE as 23,000 for GtH-alpha from GtH-I, 22,000 for GtH-alpha from GtH-II, 18,000 for GtH-Ibeta, and 21,000 for GtH-IIbeta.

Follicle-stimulating hormone and its alpha and beta subunits in rainbow trout (Oncorhynchus mykiss): purification, characterization, development of specific radioimmunoassays, and their seasonal plasma and pituitary concentrations in females

Gonad development in fish, as in mammals, is regulated by two gonadotropins (GTHs), FSH and LH. The function of LH in fish has been clearly established; however, the function(s) of FSH is less certain. The lack of specific and sensitive assays to quantify FSH and its alpha and beta subunits has hindered studies to assess physiological function. In this study, gel filtration chromatography, ion exchange chromatography, and HPLC were employed to purify FSH and its subunits from pituitary glands of rainbow trout (Oncorhynchus mykiss), and the identities of the isolates were confirmed by amino acid analysis. Polyclonal antibodies were raised against the free GTHalpha2 and free FSHbeta subunits to develop specific RIAs. The sensitivities of the intact FSH, GTHalpha2, and FSHbeta assays were 1 ng/ml, 0.2 ng/ml, and 0.1 ng/ml, respectively, and the cross-reaction of these molecules with each other and with intact LH in the heterologous assays was <10.4% throughout. Pituitary and plasma samples diluted in parallel with the standards in all three assays and spiked sample recoveries were >90% throughout. Measurement of plasma and pituitary concentrations of intact FSH in female rainbow trout confirmed the established seasonal profiles. Concentrations of free GTHalpha2 subunit were elevated both in the plasma and in the pituitary in females at ovulation (maximum concentrations: 34.93 +/- 6.3 ng/ml in plasma; 37.63 +/- 5.79 microg/pituitary). In both the plasma and the pituitary, free FSHbeta subunit was present throughout the reproductive cycle but at very low concentrations when compared with both free GTHalpha2 and intact FSH. The presence of free GTHalpha2 subunit in the plasma similarly occurs in mammals, but its functional significance in fish has yet to be established.

Characterization of tilapia FSHbeta gene and analysis of its 5' flanking region

The objective of the current study was to unveil molecular mechanisms underlying transcriptional regulation of the FSHbeta gene expression in the pituitary of tilapia (Oreochromis mossambicus). The full-length sequence of tilapia FSHbeta (tFSHbeta) gene was determined. Its transcriptional unit (2.7 kb) exhibits the conserved genomic organization, i.e. three exons and two introns. Primer extension and RT-PCR analysis revealed heterogeneity of the tFSHbeta transcripts, due to alternate mRNA splicing and multiple initiation sites for transcription. Examination of the 5' flanking region (5'FR) of the tFSHbeta gene identified potential CAAT and TATA promoter proximal elements as well as several sequences of cis-acting motifs known to dictate inducible and tissue-specific transcriptional regulation in other gonadotropin genes. Chimeric constructs containing 1.7 kb of the tFSHbeta 5'FR fused to a luciferase (LUC) reporter gene were transiently transfected into primary culture of tilapia pituitary cells. The tFSHbeta-LUC construct was efficiently expressed under basal conditions and was rapidly induced by GnRH stimulation. Our data indicate that the 5'FR contains a functional promoter, which is responsive to GnRH treatment. In addition, 5' deletion analysis showed that the 1.7 kb, DNA sequence of the FSHbeta 5'FR encompasses both positive and negative regulatory elements.

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.

Differential expression of three different prepro-GnRH (gonadotrophin-releasing hormone) messengers in the brain of the european sea bass (Dicentrarchus labrax)

The expression sites of three prepro-gonadotrophin-releasing hormones (GnRHs), corresponding to seabream GnRH (sbGnRH: Ser(8)-mGnRH, mammalian GnRH), salmon GnRH (sGnRH: Trp(7)Leu(8)-mGnRH), and chicken GnRH-II (cGnRH-II: His(5)Trp(7)Tyr(8)-mGnRH) forms were studied in the brain of a perciform fish, the European sea bass (Dicentrarchus labrax) by means of in situ hybridization. The riboprobes used in this study correspond to the three GnRH-associated peptide (GAP)-coding regions of the prepro-GnRH cDNAs cloned from the same species (salmon GAP: sGAP; seabream GAP: sbGAP; chicken GAP-II: cIIGAP), which show little oligonucleotide sequence identity (sGAP versus sbGAP: 42%; cIIGAP versus sbGAP: 36%; sGAP versus cIIGAP: 41%). Adjacent paraffin sections (6 mm) throughout the entire brain were treated in parallel with each of the three anti-sense probes and the corresponding sense probes, demonstrating the high specificity of the hybridization signal. The results showed that both sGAP and sbGAP mRNAs had a broader expression in the olfactory bulbs, ventral telencephalon, and preoptic region, whereas cIIGAP mRNA expression was confined to large cells of the nucleus of the medial longitudinal fascicle. In the olfactory bulbs, both the signal intensity and the number of positive cells were higher with the sGAP probe, whereas sbGAP mRNA-expressing cells were more numerous and intensely stained in the preoptic region. Additional isolated sbGAP-positive cells were detected in the ventrolateral hypothalamus. These results demonstrate a clear overlapping of sGAP- and sbGAP-expressing cells in the forebrain of the European sea bass, in contrast to previous reports in other perciforms showing a clear segregation of these two cell populations.

An immunocytochemical study of the pituitary gland of the white seabream (Diplodus sargus)

The adenohypophysis of the white seabream (Diplodus sargus) was studied using histochemical and immunocytochemical techniques. The adenohypophysis was composed of rostral pars distalis, proximal pars distalis and pars intermedia. Prolactin (anti-chum salmon prolactin positive) and adrenocorticotropic (anti-human ACTH positive) cells were found in the rostral pars distalis. Prolactin cells were organized into follicles, while ACTH cells were arranged in cords around neurohypophyseal tissue branches that penetrated the rostral pars distalis. In the proximal pars distalis, somatotropic (anti-chum salmon and anti-gilthead seabream growth hormone positive), gonadotropic (anti-chum salmon beta-gonadotrophin II and anti-carp beta-gonadotrophin II positive, but anti-chum salmon beta-gonadotrophin I negative) and thyrotropic (anti-human beta-thyrotropin positive) cells were observed. Growth hormone cells were restricted to the dorsal and ventral part of the proximal pars distalis. They were clustered or surrounded the neurohypophyseal branches. Only one type of gonadotrophin cell was identified and they were clustered or isolated in the proximal pars distalis. Scattered groups of thyrotropin cells were located throughout the proximal pars distalis. In the pars intermedia somatolactin (anti-chum salmon and anti-gilthead seabream somatolactin positive) and melanotropic (anti-alpha-melanotropic hormone positive) cells were localized. In addition, gonadotrophin cells surrounded the pars intermedia or distributed evenly between somatolactin and melanotropic hormone cells. Somatolactin cells were periodic acid-Schiff negative and surrounded the neurohypophyseal branches intermingled with melanotropic cells. These cells were also immunoreactive to anti-human ACTH antiserum.

Roles of the activin regulatory system in fish reproduction

Activin (βAβA, βAβB, and βBβb) is a dimeric growth factor with diverse biological activities in vertebrate reproduction. Activin exerts its actions by binding to its specific type II and type I receptors. The activity of activin is regulated by follistatin, its binding protein, and the antagonists inhibin and antivin. All major components of the activin-inhibin-follistatin system have been identified in fish except the α subunit of inhibin. Using goldfish as a model, we have demonstrated that activin is expressed in the pituitary and the recombinant goldfish activin B has novel inverse effects on the expression of GTH β subunits. Activin increases the mRNA level of GTH-Iβ while significantly suppressing the expression of GTH-IIβ. We have also demonstrated the expression of activin and its receptors in the goldfish and zebrafish ovary. Using an in vitro ovarian follicle incubation as the system, we have investigated the involvement of the activin system in the process of final oocyte maturation. Our evidence clearly indicates that activin has potent effect of promoting final oocyte maturation, and that it may play a role in mediating the stimulatory effect of pituitary gonadotropin in the event of oocyte maturation. Key words: activin, inhibin, follistatin, fish, reproduction.

Modulation of pituitary dopamine D1 or D2 receptors and secretion of follicle stimulating hormone and luteinizing hormone during the annual reproductive cycle of female rainbow trout

The two gonadotrophins follicle stimulating hormone (FSH) and luteinizing hormone (LH) have distinct temporal expression and release profiles in fish, but little is known regarding their neuroendocrine control, especially for FSH. The present experiments were performed on previtellogenic, mature and preovulatory female trout. The catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, increased plasma LH and FSH concentrations of mature fish. The dopamine agonist apomorphine decreased and the dopamine antagonist domperidone increased plasma LH concentration of preovulatory fish and delayed ovulation, but did not modify plasma FSH concentration. The dopamine D2 agonist bromocryptine inhibited LH release in cultured gonadotrophs from mature and preovulatory fish, but not from previtellogenic fish. Bromocryptine also significantly inhibited basal and salmon gonadotrophin releasing-hormone (sGnRH)-induced FSH release from cultured gonadotrophs of mature fish, but not of preovulatory fish, and increased FSH release from gonadotrophs of previtellogenic fish. The dopamine D1 agonist SKF 38393 had no observed effect on the release of FSH and LH, at any reproductive stage studied. The D1 agonist SKF 38393, the D2 agonist bromocriptine and sGnRH had no observed effects on cell contents of FSH and LH. Taken together, these data suggest that, at the level of the pituitary, dopamine inhibits LH release as vitellogenesis proceeds, via activation of dopamine D2 receptors. We demonstrate for the first time in fish a control of FSH release (a dopamine control), especially in mature fish which have low circulating concentrations of FSH.

Unique expression of gonadotropin-I and -II subunit genes in male and female red seabream (Pagrus major) during sexual maturation

Two distinct gonadotropins (GTHs) have been demonstrated in a number of teleost fishes. Although the physiological roles of GTHs have been extensively studied in salmonids, little is known about their biological functions in nonsalmonid fishes. In this study, to elucidate the role of GTH-I and GTH-II in reproduction, we cloned the alpha-glycoprotein subunit (alphaGSU) and gonadotropin beta subunits (Ibeta and IIbeta) of red seabream using the 5'- and 3'-RACE methods and used these cDNA probes to reveal changes in mRNA levels of each subunit during sexual maturation of both male and female red seabream. The nucleotide sequences of alphaGSU, Ibeta, and IIbeta are 629, 531, and 557 base pairs long, encoding peptides of 117, 120, and 146 amino acids, respectively. The deduced amino acid sequence of each mature subunit showed high homology with those of other teleosts. Northern blot analysis showed that Ibeta mRNA levels of males increase in association with gonadal development, whereas those of females remain low throughout sexual maturation, indicating sexual dimorphism in the expression pattern of Ibeta. In contrast, IIbeta mRNA levels of both sexes are maintained at high levels from the beginning of gametogenesis to spawning season. These results are different than those of salmonids and suggest that GTH-I may have important roles in male, but not female, gametogenesis. GTH-II may be involved in regulation of early and late gametogenesis in both male and female red seabream.

Recombinant goldfish activin B stimulates gonadotropin-Ibeta but inhibits gonadotropin-IIbeta expression in the goldfish, Carassius auratus

It is well documented that the pituitary in teleosts produces two gonadotropins, namely gonadotropin-I (GTH-I) and gonadotropin-II (GTH-II), which may regulate different phases of the reproductive cycle. However, unlike in mammals, very little is known about the differential regulation of the two GTHs in fish. Using goldfish as a model, the present study demonstrates, for the first time, that activin, a protein factor that plays a critical role in the differential regulation of mammalian FSH and LH, has opposite effects on GTH-Ibeta and GTH-IIbeta mRNA expression. Recombinant goldfish activin B stimulates GTH-Ibeta but significantly suppresses GTH-IIbeta mRNA levels in a dose-dependent manner in cultured goldfish pituitary cells. Administration of recombinant human follistatin completely abolished the effects of activin, thus demonstrating the specificity of the activin activities. The novel opposite effects of activin on the two goldfish GTHs make goldfish a very unique vertebrate model for activin studies. The present study not only contributes to our understanding of the mechanisms that control the temporal expression patterns of the two GTHs during the fish reproductive cycle, but also provides important information on the evolution of gonadotropin regulation in vertebrates.

Levels of the native forms of GnRH in the pituitary of the gilthead seabream, Sparus aurata, at several characteristic stages of the gonadal cycle

Brains of the gilthead seabream, Sparus aurata, contain three different forms of gonadotropin-releasing hormone (GnRH): seabream (sb) GnRH, chicken (c) GnRH-II, and salmon (s) GnRH. In the present study, we developed three specific enzyme-linked-immunosorbent assays (ELISA) for sbGnRH, cGnRH-II, and sGnRH and used them to measure the levels of each GnRH form in the pituitary of male and female seabream at different stages of gametogenesis. The sensitivity was 6 pg/well for the sbGnRH assay, 7 pg/well for the cGnRH-II assay, and 2 pg/well for the sGnRH assay. Levels of each of the three GnRH forms were measured in pituitaries from fish sampled at the beginning of gonadal recrudescence and during the spawning season. Of the three forms, only sbGnRH and cGnRH-II were detected in the pituitary, irrespective of reproductive state or sex. Recrudescent fish had similar levels of sbGnRH and cGnRH-II in the pituitary. In sexually mature fish, the levels of sbGnRH were higher than those in recrudescent fish while pituitary cGnRH-II content remained unchanged. Consequently, sbGnRH levels were 3- to 17-fold higher than cGnRH-II levels in mature fish. Positive correlations also existed between pituitary sbGnRH content and pituitary and plasma gonadotropin (GtH) II levels. Surprisingly, mature 1-year-old males had significantly higher levels of sbGnRH in the pituitary than mature 3-year-old males, while pituitary and plasma GtH II levels were similar between these two groups. Although the reason for this difference in sbGnRH levels is unclear, a possible role of sbGnRH in the processes of puberty or sex-inversion is implied. Based on the present results, it can be suggested that in the gilthead seabream, sbGnRH is the most relevant form of GnRH in the control of reproduction.

GTH II but not GTH I induces final maturation and the development of maturational competence of oocytes of red seabream in vitro

The effects of red seabream gonadotropins (PmGTH I and PmGTH II) on the induction in vitro of germinal vesicle breakdown (GVBD) and the development of maturational competence (responsiveness to maturation-inducing steroid) were examined in the oocytes of red seabream. PmGTH II was highly effective in inducing GVBD in both maturationally incompetent (45.6 +/- 3.2% GVBD at a concentration of 300 ng/ml) and competent oocytes (42.3 +/- 0.4% GVBD at a concentration of 300 ng/ml). 17,20beta-Dihydroxy-4-pregnen-3-one (DHP, 10 ng/ml) increased the frequency of GVBD induced by PmGTH II. PmGTH I (33, 100, 300, and 900 ng/ml) was unable to induce GVBD at any tested concentration in the presence or absence of DHP. Actinomycin D (1 microgram/ml) and cycloheximide (1 microgram/ml) totally inhibited the PmGTH II-induced GVBD in the presence and in the absence of DHP. Both PmGTH I and PmGTH II stimulated in vitro production of 11-ketotestosterone in sliced testes of red seabream in a similar potency. These results indicate that PmGTH II, but not PmGTH I, induces the final maturation of oocytes, as well as the development of the maturational competence of oocytes, in red seabream.

Endocrine regulation of gonadotropin and growth hormone gene transcription in fish

The pituitary of a number of teleosts contains two gonadotropins (GtHs) which are produced in distinct populations of cells; the beta subunit of the GtH I being found in close proximity to the somatotrophs, while the II beta cells are more peripheral. In several species the GtH beta subunits are expressed at varying levels throughout the reproductive cycle, the I beta dominating in early maturing fish, after which the II beta becomes predominant. This suggests differential control of the beta subunit synthesis which may be regulated by both hypothalamic hormones and gonadal steroids. At ovulation and spawning, changes also occur in the somatotrophs, which become markedly more active, while plasma growth hormone (GH) levels increase. In a number of species, GnRH elevates either the I beta or the II beta mRNA levels, depending on the reproductive state of the fish. In tilapia, the GnRH effect on the II beta appears to be mediated through both cAMP-PKA and PKC pathways. GnRH also stimulates GH release in both goldfish and tilapia, but it increases the GH transcript levels only in goldfish; both GnRH and direct activation of PKC are ineffective in altering GH mRNA in tilapia pituitary cells. Dopamine (DA) does not alter II beta transcript levels in cultured tilapia pituitary cells, but increases GH mRNA levels in both rainbow trout and tilapia, in a PKA-dependent manner. This effect appears to be through interactions with Pit-1 and also by stabilizing the mRNA. Somatostatin (SRIF) does not alter GH transcript levels in either tilapia or rainbow trout, although it may alter GH synthesis by modulation of translation. Gonadal steroids appear to have differential effects on the transcription of the beta subunits. In tilapia, testosterone (T) elevates I beta mRNA levels in cells from immature or early maturing fish (in low doses), but depresses them in cells from late maturing fish and is ineffective in cells from regressed fish. Similar results were seen in early recrudescing male coho salmon injected with T or E2. T or E2 administered in vivo has dramatic stimulatory effects on the II beta transcript levels in immature fish of a number of species, while less powerful effects are seen in vitro. A response is also seen in cells from early maturing rainbow trout or tilapia, or regressed tilapia, but not in cells from late maturing or spawning fish. These results are substantiated by the finding that the promoter of the salmon II beta gene contains several estrogen responsive elements (EREs) which react and interact differently when exposed to varying levels of E2. In addition, activator protein-1 (AP-1) and steroidogenic factor-1 (SF-1) response elements are also found in the salmon II beta promoter; the AP-1 site is located close to a half ERE, while the SF-1 acts synergistically with the E2 receptor. The mRNA levels of both AP-1 and SP-1 are elevated, at least in mammals, by GnRH, suggesting possible sites for cross-talk between GnRH and steroid activated pathways. Reports of the effects of T or E2 on GH transcription differ. No effect is seen in vitro in pituitaries of tilapia, juvenile rainbow trout or common carp, but T does increase the transcript levels in pituitaries of both immature and mature goldfish. Reasons for these discrepancies are unclear, but other systemic hormones may be more instrumental than the gonadal steroids in regulating GH transcription. These include T3 which increases both GH mRNA levels and de novo synthesis (in tilapia and common carp) and insulin-like growth factor-I (IGF-I) which reduces GH transcript levels as well as inhibiting GH release.