Activation of salmon gonadotropin-releasing hormone synthesis by 17 alpha-methyltestosterone administration in yearling masu salmon, Oncorhynchus masou

The effects of 17α-methyltestosterone on gonadal histology and gene expression along hypothalamic-pituitary-gonadal axis, germ cells, sex determination, and hypothalamus-pituitary-thyroid axis in zebrafish (Danio rerio)

As a synthetic androgen, 17α-methyltestosterone (MT) is widely used in aquaculture to induce sex reversal and may pose a potential risk to aquatic organisms. This ecological risk has attracted the attention of many scholars, but it is not comprehensive enough. Thus, the adverse effects of MT on zebrafish (Danio rerio) were comprehensively evaluated from gonadal histology, as well as the mRNA expression levels of 47 genes related to hypothalamic-pituitary-gonadal (HPG) axis, germ cell differentiation, sex determination, and hypothalamus-pituitary-thyroid (HPT) axis. Adult zebrafish with a female/male ratio of 5:7 were exposed to a solvent control (0.001% dimethyl sulfoxide) and three measured concentrations of MT (5, 51 and 583 ng/L) for 50 days. The results showed that MT had no significant histological effects on the ovaries of females, but the frequency of late-mature oocytes (LMO) showed a downward trend, indicating that MT could induce ovarian suppression to a certain extent. The transcriptional expression of activating transcription factor 4b1 (atf4b1), activating transcription factor 4b2 (atf4b2), calcium/calmodulin-dependent protein kinase II delta 1 (camk2d1), calcium/calmodulin-dependent protein kinase II delta 2 (camk2d2) and calcium/calmodulin-dependent protein kinase II inhibitor 2 (camk2n2) genes in the brain of females increased significantly at all treatment groups of MT, and the mRNA expression of forkhead box L2a (foxl2) and ovarian cytochrome P450 aromatase (cyp19a1a) genes in the ovaries were down-regulated by 5 and 583 ng/L group, which would translate into inhibition of oocyte development. As compared to females, MT had relatively little effects on the reproductive system of males, and only the transcriptional alterations of synaptonemal complex protein 3 (sycp3) and 17-alpha-hydroxylase/17,20-lyase (cyp17) genes were observed in the testes, not enough to affect testicular histology. In addition, MT at all treatments strongly increased corticotropin-releasing hormone (crh) transcript in the brain of females, as well as deiodinase 2 (dio2) transcript in the brain of males. The paired box protein 8 (pax8) gene was significantly decreased at 51 or 583 ng/L of MT in both female and male brains. The above results suggest that MT can pose potential adverse effects on the reproductive and thyroid endocrine system of fish.

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

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

Effects of 17 α-methyltestosterone on the transcriptome, gonadal histology and sex steroid hormones in Pseudorasbora parva

In recent years, the increasing use of environmental endocrine disruptors has caused serious environmental pollution and hurt aquatic organisms. It is still risky for aquatic species and humans exposed to 17α-methyltestosterone (MT), however, the harmful effect of MT on fish is still poorly understood. The main purpose of this study is to evaluate the influence of MT on Pseudorasbora parva at multi-levels. We analyzed gonadal histology, the sex steroid hormones, steroidogenic genes expression, and transcriptome profiling of gonads in response to MT in adult P. parva. Through this study, we found MT could inhibit the gonadal development of P. parva, and the growth and development of fish could be delayed by exposure to MT at 200 ng/L. MT could produce disruption effects on fish from multiple pathways, while its interference to the HPGL axis happens primarily through the steroidogenic pathway, e.g., disturbing the expression of crucial genes and sex steroids synthesis. Besides, we constructed 4 RNAseq libraries and obtained 7758 and 11,543 DEGs in females and males, respectively. Interestingly, we found MT had more obvious disruption effects on males than the females, mainly reflected in the immune system. Interestingly, we found three common pathways in both sexes after MT exposure, i.e., cell adhesion molecules, cytokine-cytokine receptor interaction, and neuroactive ligand-receptor interaction. These results confirm the suitability of P. parva as a model fish for aquatic toxicological study and provide us a multidimensional sight for the disruption effects of MT on fish.

Ameliorative effect of chitosan-conjugated 17α-methyltestosterone on testicular development in Clarias batrachus

Chitosan nanoparticles conjugated with 17α-methyltestosterone (CS + MT) were used for studying their effect on the testicular development of Clarias batrachus during different reproductive phases. The size of chitosan nanoparticles was 127.2 nm and the nano-conjugated 17α methyltestosterone (17α-MT) was 196.1 nm (20 mg/100 ml of chitosan). Single injections of CS + MT at different doses such as 0.01, 0.1 and 0.5 μg/g body weight were administered to adults during the pre-spawning, spawning and post-spawning phase. Nano-conjugated steroid was effective at the lower dose; showing an increase in the Gonadosomatic Index (GSI) and 11-ketotestosterone level compared to the control group. Histological observations confirmed the dose-dependent advancement in spermatogenesis. These findings indicate the possibility of using CS + MT for enhancing gonadal maturity of C. batrachus.

GnRHa-mediated stimulation of the reproductive endocrine axis in captive Atlantic bluefin tuna, Thunnus thynnus

A controlled-release implant loaded with GnRH agonist (GnRHa) was used to induce spawning in Atlantic bluefin tuna (Thunnus thynnus) during two consecutive reproductive seasons. The fish were implanted underwater and sampled between days 2 and 8 after treatment. At the time of GnRHa treatment, females were in full vitellogenesis and males in spermiation. There was a rapid burst of pituitary luteinizing hormone (LH) release at day 2 after treatment in GnRHa-treated fish, and circulating LH remained elevated up to day 8 after treatment. In contrast, control fish had significantly lower levels in the plasma, but higher LH content in the pituitary, as observed in many other cultured fishes that fail to undergo oocyte maturation, ovulation and spawning unless induced by an exogenous GnRHa. Plasma testosterone (T) and 17β-estradiol (E(2)) were elevated in response to the GnRHa treatment in females, while 11-ketotestosterone (11-KT) but not T was elevated in males. Even though oocyte maturation and ovulation did occur in GnRHa-induced fish, no significant elevations in 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) or 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S), in either the free, conjugated or 5β-reduced,3α-hydroxylated forms was observed in fish sampled within 6 days after treatment. Interestingly, a significant peak in plasma free 17,20β-P levels occurred in both males and females at day 8 after treatment. Histological sections of the ovaries in these females contained oocytes at the migrating germinal vesicle stage, suggesting the role of this hormone as a maturation-inducing steroid in Atlantic bluefin tuna. In conclusion, the GnRHa implants activated effectively the reproductive endocrine axis in captive Atlantic bluefin tuna broodstocks, through stimulation of sustained elevations in plasma LH, which in turn evoked the synthesis and secretion of the relevant sex steroids leading to gamete maturation and release.

Bisphenol A modulates expression of gonadotropin subunit genes in the hermaphroditic fish, Kryptolebias marmoratus

Pituitary gonadotropins (GTHs), follicle stimulating hormone beta (FSH-beta), and luteinizing hormone beta (LH-beta) are the key hormones in the hypothalamus-pituitary-gonad (HPG) axis, and form the heterodimers between a common alpha subunit (gonadotropin-alpha) and FSH-beta and/or LH-beta. To obtain a better understanding on the modulation of gonadotropin subunit genes expression upon bisphenol A (BPA) exposure in hermaphroditic fish, we studied differential regulation of gonadotropin subunit genes from Kryptolebias marmoratus after the exposure of several EDCs. Expression profiles of these three genes when using quantitative real-time RT-PCR revealed that brain/pituitary tissues were highly expressed in these genes compared to other tissues. At different developmental stages, expression of those genes dramatically increased over the course of development but showed a decrease in expression at the secondary male (showing atresia) stage. When adult fish were exposed to BPA (600 microg/L for 96 h), a significant upregulation of these three genes was observed in the brain/pituitary. A time course study also revealed the increased expression of gonadotropin subunit genes over 12 h with a more pronounced effect on the expression of FSH-beta and LH-beta genes, indicating that both genes were associated with the BPA exposure on the transcriptional regulation. This is the first report of gonadotropin subunit genes from K. marmoratus, with particular emphasis on the modulation of their expressions by EDCs. In addition, these findings suggest that EDCs modulate the expression of gonadotropin subunit genes and would act as potential biomarkers upon EDCs exposure.

Expression of GnRH genes is elevated in discrete brain loci of chum salmon before initiation of homing behavior and during spawning migration

Our previous studies suggested the importance of gonadotropin-releasing hormones (GnRHs) for initiation of spawning migration of chum salmon, although supporting evidence had been not available from oceanic fish. In farmed masu salmon, the amounts of salmon GnRH (sGnRH) mRNAs in the forebrain increased in the pre-pubertal stage from winter through spring, followed by a decrease toward summer. We thus hypothesized that gene expression for GnRHs in oceanic chum salmon changes similarly, and examined this hypothesis using brain samples from winter chum salmon in the Gulf of Alaska and summer fish in the Bering Sea. They were classified into sexually immature and maturing adults, which had maturing gonads and left the Bering Sea for the natal river by the end of summer. The absolute amounts of GnRH mRNAs were determined by real-time PCRs. The amounts of sGnRH mRNA in the maturing winter adults were significantly larger than those in the maturing summer adults. The amounts of sGnRH and chicken GnRH mRNAs then peaked during upstream migration from the coast to the natal hatchery. Such changes were observed in various brain loci including the olfactory bulb, terminal nerve, ventral telencephalon, nucleus preopticus parvocellularis anterioris, nucleus preopticus magnocellularis and midbrain tegmentum. These results suggest that sGnRH neurons change their activity for gonadal maturation prior to initiation of homing behavior from the Bering Sea. The present study provides the first evidence to support a possible involvement of neuropeptides in the onset of spawning migration.

Androgen receptors in a cichlid fish, Astatotilapia burtoni: structure, localization, and expression levels

Androgens are an important output of the hypothalamic-pituitary-gonadal (HPG) axis that controls reproduction in all vertebrates. In male teleosts two androgens, testosterone and 11-ketotestosterone, control sexual differentiation and development in juveniles and reproductive behavior in adults. Androgenic signals provide feedback at many levels of the HPG axis, including the hypothalamic neurons that synthesize and release gonadotropin-releasing hormone 1 (GnRH1), but the precise cellular site of androgen action in the brain is not known. Here we describe two androgen receptor subtypes, ARalpha and ARbeta, in the cichlid Astatotilapia burtoni and show that these subtypes are differentially located throughout the adult brain in nuclei known to function in the control of reproduction. ARalpha was expressed in the ventral part of the ventral telencephalon, the preoptic area (POA) of the hypothalamus and the ventral hypothalamus, whereas ARbeta was more widely expressed in the dorsal and ventral telencephalon, the POA, and the ventral and dorsal hypothalamus. We provide the first evidence in any vertebrate that the GnRH1-releasing neurons, which serve as the central control point of the HPG axis, express both subtypes of AR. Using quantitative real-time PCR, we show that A. burtoni AR subtypes have different expression levels in adult tissue, with ARalpha showing significantly higher expression than ARbeta in the pituitary, and ARbeta expressed at a higher level than ARalpha in the anterior and middle brain. These data provide important insight into the role of androgens in regulating the vertebrate reproductive axis.

Reproduction phase-related variations in neuropeptide Y immunoreactivity in the olfactory system, forebrain, and pituitary of the female catfish,Clarias batrachus (Linn.)

The aim of this study was to determine whether neuropeptide Y (NPY) immunoreactivity in the cells and fibers in the forebrain and pituitary of Clarias batrachus is linked to the annual reproductive cycle. A steady rise in luteinizing hormone (LH) immunoreactivity was seen in the pituitary through preparatory (February-April) and prespawning (May-June) phases; it was greatly reduced during spawning (July-August; P < 0.001) and partially replenished during postspawning (September-November; P < 0.01) through resting (December-January) phases. Although NPY immunoreactivity in olfactory receptor neurons and olfactory nerve layer in olfactory bulb was gradually augmented during resting through prespawning phases (P < 0.001), attaining a peak in spawning phase (P < 0.001), a dramatic decline was encountered during postspawning phase (P < 0.001). A similar pattern was also observed in NPY-containing fibers of the medial olfactory tract (MOT) and pituitary. However, a different pattern of NPY immunoreactivity was observed in the neurons of nucleus entopeduncularis (NE) and nucleus preopticus periventricularis (NPP). Whereas these neurons and fibers in the forebrain showed significant augmentation during the resting through prespawning phases (P < 0.001), the immunoreactivity dramatically declined during spawning (P < 0.001) and was partially replenished in the postspawning phase. Testosterone injection of juveniles significantly augmented (P < 0.001) NPY immunoreactivity in NE neurons. We suggest that NPY cells of NE and NPP, and related fiber systems, might be involved in processing of sex steroid-borne information and regulation of the gonadotropin-releasing hormone-LH axis.

Effects of a gonadotropin-releasing hormone antagonist on gonadotropin levels in masu salmon and sockeye salmon

The salmon gonadotropin-releasing hormone (sGnRH) is considered to be involved in gonadal maturation via gonadotropin (GTH) secretion in salmonid fishes. However, there is no direct evidence for endogenous sGnRH-stimulated GTH secretion in salmonids. In this study, to clarify whether endogenous sGnRH stimulates GTH secretion, we examined the effects of the mammalian GnRH (mGnRH) antagonist [Ac-Delta(3)-Pro(1), 4FD-Phe(2), D-Trp(3,6)]-mGnRH on luteinizing hormone (LH) levels in 0-year-old masu salmon Oncorhynchus masou and sockeye salmon Oncorhynchus nerka. First, the effects of the GnRH antagonist on LH release were examined in 0-year-old precocious male masu salmon. GnRH antagonist treatment for 3 hr significantly inhibited an increase in plasma LH levels that was artificially induced by exogenous sGnRH administration, indicating that the GnRH antagonist is effective in inhibiting LH release from the pituitary. Subsequently, we examined the effect of the GnRH antagonist on LH synthesis in 0-year-old immature sockeye salmon that were pretreated with exogenous testosterone for 42 days to increase the pituitary LH contents; the testosterone treatment did not affect the plasma LH levels. GnRH antagonist treatment slightly but significantly inhibited an increase in the testosterone-stimulated pituitary LH content levels. However, no significant differences in the plasma LH levels were observed between the GnRH antagonist-treated and control groups. These results suggest that endogenous sGnRH is involved in LH secretion in salmonid fishes.

The mRNA expression of P450 aromatase, gonadotropin β-subunits and FTZ-F1 in the orange-spotted grouper (Epinephelus Coioides) during 17α-methyltestosterone-induced precocious sex change

The orange-spotted grouper Epinephelus coioides is a protogynous hermaphroditic fish, but the physiological basis of its sex change remains largely unknown. In the present study, the 2-year-old orange-spotted grouper was induced to change sex precociously by oral administration of 17alpha-methyltestosterone (MT, 50 mg/Kg diet, twice a day at daily ration of 5% bodyweight) for 60 days. The serum testosterone levels were significantly elevated after MT treatment for 20 and 40 days as compared to control, but the levels of serum estradiol (E(2)) remained unchanged. The expression of P450aromA in the gonad significantly decreased after MT treatment for 20, 40, and 60 days. Accordingly, the enzyme activity of gonadal aromatase was also lower. The expression of FSHbeta subunit in the pituitary was significantly decreased after MT treatment for 20 days, but returned to the control levels after 40 and 60 days; however, the expression of LHbeta subunit was not altered significantly by MT treatment. The expression of FTZ-F1 in the gonad also decreased significantly in response to MT treatment for 40 and 60 days, but its expression in the pituitary was not altered significantly. Interestingly, when tested in vitro on ovarian fragments, MT had no direct effect on the expression of P450aromA and FTZ-F1 as well as the activity of gonadal aromatase, suggesting that the inhibition of gonadal P450aromatase and FTZ-F1 by MT may be mediated at upper levels of the brain-pituitary-gonadal axis. Taken together, these results indicated that FSH, P450aromA, FTZ-F1, and serum testosterone are associated with the MT-induced sex change of the orange-spotted grouper, but the cause-effect relationship between these factors and sex change in this species remains to be characterized.

Regulation of salmon gonadotrophin-releasing hormone gene expression by sex steroids in rainbow trout brain

Salmon gonadotrophin-releasing hormone (sGnRH) is the major form of gonadotrophin-releasing hormone in the brain of Salmonids and is encoded by two different genes: sGnRH1 and sGnRH2. In the present study, we examined the expression patterns of these two genes during development and throughout the reproductive cycle of the female rainbow trout (Oncorhynchus mykiss), and also investigated the feedback action of sex steroids on brain mRNA levels. Both genes are expressed as early as 14 days postfertilisation and show a similar expression pattern during early life stages. In the adult female, sGnRH1 and sGnRH2 mRNAs are both present in neurones located in the ventral forebrain. This gene expression in the brain appears to be low during early vitellogenesis, and increases during oocyte maturation to reach a maximum after ovulation. The expression of sGnRH1 was not modified by in vivo steroid treatments in any experiment; however, testosterone and 5alpha-dihydrotestosterone down-regulate brain sGnRH2 gene in immature and adult ovariectomised females. Oestradiol treatment decreases sGnRH2 mRNA levels in the brain of adult ovariectomised females only. In the triploid fish brain, none of the steroids affect brain sGnRH mRNA levels. Our results suggest that, unlike sGnRH1, the sGnRH2 gene is under a strongly androgenic inhibitory control in the immature and adult female rainbow trout.

Morphological changes of Sertoli cells during the male reproductive cycle of the teleost Piaractus mesopotamicus (Holmberg, 1887)

An investigation of the histological and ultrastructural changes of Sertoli cells during the male reproductive cycle in Piaractus mesopotamicus was made. The results showed that the Sertoli cell development is closely related with germ cell maturation. Therefore, these cells may have some role in germ cell maturation during the reproductive cycle of this species, whether in forming a tissue framework for the developing spermatogenic cysts, aiding in testes reorganization for a new reproductive cycle, in addition to other possible functions discussed in the text.

Elevation of gene expression for salmon gonadotropin-releasing hormone in discrete brain loci of prespawning chum salmon during upstream migration

Our previous studies suggested that salmon gonadotropin-releasing hormone (sGnRH) neurons regulate both final maturation and migratory behavior in homing salmonids. Activation of sGnRH neurons can occur during upstream migration. We therefore examined expression of genes encoding the precursors of sGnRH, sGnRH-I, and sGnRH-II, in discrete forebrain loci of prespawning chum salmon, Oncorhynchus keta. Fish were captured from 1997 through 1999 along their homing pathway: coastal areas, a midway of the river, 4 km downstream of the natal hatchery, and the hatchery. Amounts of sGnRH mRNAs in fresh frozen sections including the olfactory bulb (OB), terminal nerve (TN), ventral telencephalon (VT), nucleus preopticus parvocellularis anterioris (PPa), and nucleus preopticus magnocellularis (PM) were determined by quantitative real-time polymerase chain reactions. The amounts of sGnRH-II mRNA were higher than those of sGnRH-I mRNA, while they showed similar changes during upstream migration. In the OB and TN, the amounts of sGnRH mRNAs elevated from the coast to the natal hatchery. In the VT and PPa, they elevated along with the progress of final maturation. Such elevation was also observed in the rostroventral, middle, and dorsocaudal parts of the PM. The amounts of gonadotropin IIbeta and somatolactin mRNAs in the pituitary also increased consistently with the elevation of gene expression for sGnRH. These results, in combination with lines of previous evidence, indicate that sGnRH neurons are activated in almost all the forebrain loci during the last phases of spawning migration, resulting in coordination of final gonadal maturation and migratory behavior to the spawning ground.

In vivo and in vitro sex steroids stimulate seabream gonadotropin-releasing hormone content and release in the protandrous black porgy, Acanthopagrus schlegeli

The objective of the present study was to investigate the regulation of seabream gonadotropin-releasing hormone (sbGnRH) release using in vivo and in vitro approaches in the protandrous black porgy, Acanthopagrus schlegeli. Estradiol-17beta (E2), testosterone (T), and 11-ketotestosterone (11-KT) were found to significantly stimulate the increase of sbGnRH levels in pituitary of black porgy after 5-96 h of injection. An in vitro culture system using dispersed brain neurons was also developed to investigate the effects of various steroids on sbGnRH release. Different doses (10(-6) - 10(-12) M) of E2, T, 11-KT, and cortisol were applied during 6 h experiment. KCl stimulated sbGnRH release at a dose- and time-dependent manner. The concentration of sbGnRH increased 2-fold in the highest dose of KCl treatment compared to the control. Treatments with E2, T, 11-KT and cortisol significantly stimulated the release of sbGnRH from the cultured brain neurons. The concentration of sbGnRH in medium was increased by 2-, 1.9-, 2.1-, and 4.9-fold when treated with E2, T, 11-KT, and cortisol, respectively, as compared to the respective control. Cholesterol did not have any stimulatory effects in the release of sbGnRH. The results showed that sex steroids and cortisol had direct effect on brain neuronal cells stimulating the release of sbGnRH.

Different modes of gonadotropin-releasing hormone (GnRH) release from multiple GnRH systems as revealed by radioimmunoassay using brain slices of a teleost, the dwarf gourami (Colisa lalia)

It has become a general notion that there are multiple GnRH systems in the vertebrate brains. To measure GnRH release activities from different GnRH systems, we conducted a static incubation of brain-pituitary slices under various conditions, and GnRH released into the incubation medium was measured by RIA. The slices were divided into two parts, one containing GnRH neurons in the preoptic area and axon terminals in the pituitary (POA-GnRH slices), and the other containing the cell bodies and fibers of terminal nerve-GnRH neurons and midbrain tegmentum-GnRH neurons (TN-TEG-GnRH slices). We demonstrated that GnRH release was evoked by high [K(+)](o) depolarizing stimuli (in both POA-GnRH and TN-TEG-GnRH slices) via Ca(2+) influx through voltage-gated Ca(2+) channels. The most prominent result was the presence of conspicuous sexual difference in the amount of GnRH release in the POA-GnRH slices. The GnRH release from TN-TEG-GnRH slices also showed a small sexual difference, which was by far more inconspicuous than that of POA-GnRH slices. Immunohistochemical analysis using an antiserum specific to the seabream GnRH (sbGnRH; suggested to be specific to POA-GnRH neurons) revealed the presence of a much larger number of POA-GnRH neurons in males than in females. This clear morphological sexual difference is suggested to underlie that of GnRH release in the POA-GnRH slices.

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.

Year-to-year differences in plasma levels of steroid hormones in pre-spawning chum salmon

Changes in plasma levels of steroid hormones in pre-spawning chum salmon (Oncorhynchus keta) were examined for 6 years in association with sexual maturation. Fish were sampled along their homing pathway from the coastal sea to the spawning ground from 1995 to 2000. Plasma levels of testosterone (T), 11-ketotestosterone (11KT), estradiol-17beta (E2), 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP), and cortisol were determined by enzyme immunoassays. Sexual maturity was comprehensively estimated by gonadosomatic indices, histology of gonads, nuptial color, spermiation or ovulation ratio. Since the plasma levels of steroid hormones and sexual maturation differed from year to year, they were compared with year-to-year variation of sea surface temperature (SST) of coastal sea to study influence of oceanographic environment on these physiological data. The SST of the migratory route varied among the years, so that we classified the 6 years into cool, intermediate, and warm years. Concerning maturity, the males that returned to the natal hatchery in the warm years were sexually more advanced than those in the cool years. Furthermore, histological data suggested that final oocyte maturation occurred before arrival at the hatchery in one of the warm years, i.e., 1999, while it occurred at the hatchery in one of the intermediate years, i.e., 2000. In the males, T and 11KT levels increased significantly on midway of the homing route in the warm years, whereas they did not show any noticeable changes in the cool years. Furthermore, the levels of T and 11KT on midway of the homing route in the warm years, i.e., 1998 and 1999, were significantly higher than those in one of the cool years, i.e., 1995, in both sexes. In the females, the levels of E2 decreased during upstream migration. Conversely, those of DHP considerably elevated at spawning ground in all years examined. The levels of cortisol were different from year to year regardless of the SST. The present results showed that there were year-to-year differences in plasma levels of steroid hormones and maturity, and some of them may be influenced by the year-to-year variation of SST.

Seasonal variation of the three native gonadotropin-releasing hormone messenger ribonucleic acids levels in the brain of female red seabream

We studied the seasonal variation of the expression of genes encoding the three native gonadotropin-releasing hormones (GnRHs), namely salmon(s) GnRH, chicken(c) GnRH-II, and seabream(sb) GnRH in red seabream, Pagrus (Chrysophrys) major, in order to better understand the regulatory mechanisms of GnRH gene expression by environmental and endocrine factors. Female red seabream, reared under natural conditions, were collected monthly or bimonthly from October to June, and the levels of the three distinct GnRH messenger ribonucleic acids (mRNAs) in the brains of those fish (n = 4-6) were determined by ribonuclease (RNase) protection analysis. The levels of sbGnRH mRNA correlated well with the observed ovarian histology; the levels of sbGnRH mRNA of immature fish in October and December were low, and increased in February and March in conjunction with active vitellogenesis. The sbGnRH mRNA levels reached a maximum level in April (spawning season), after which they rapidly decreased together with the observed ovarian regression in June. In contrast, the levels of sGnRH mRNA showed no variation, while those of cGnRH-II mRNA were elevated only slightly in March and April. The increase in sbGnRH mRNA levels correlates with the increase in day length, water temperature and serum steroids levels, suggesting that these factors are candidates for regulators of sbGnRH synthesis.

Spermatogenesis and related plasma androgen levels in Atlantic halibut (Hippoglossus hippoglossus L.)

Spermatogenesis in male Atlantic halibut (Hippoglossus hippoglossus L.) was investigated by sampling blood plasma and testicular tissue from 15-39-month-old fish. The experiment covered a period in which all fish reached puberty and completed sexual maturation at least once. The germinal compartment in Atlantic halibut testis appears to be organized in branching lobules of the unrestricted spermatogonial type, because spermatocysts with spermatogonia were found throughout the testis. Spermatogenesis was characterized histologically, and staged according to the most advanced type of germ cell present: spermatogonia (Stage I), spermatogonia and spermatocytes (Stage II), spermatogonia, spermatocytes and spermatids (Stage III), spermatogonia, spermatocytes, spermatids and spermatozoa (Stage IV), and regressing testis (Stage V). Three phases could be distinguished: first, an initial phase with low levels of circulating testosterone (T; quantified by RIA) and 11-ketotestosterone (11-KT; quantified by ELISA), spermatogonial proliferation, and subsequently the initiation of meiosis marked by the formation of spermatocytes (Stage I and II). Secondly, a phase with increasing T and 11-KT levels and with haploid germ cells including spermatozoa present in the testis (Stage III and IV). Thirdly, a phase with low T and 11-KT levels and a regressing testis with Sertoli cells displaying signs of phagocytotic activity (Stage V). Circulating levels of 11-KT were at least four-fold higher than those of T during all stages of spermatogenesis. Increasing plasma levels of T and 11-KT were associated with increasing testicular mass throughout the reproductive cycle. The absolute level of, or the relation between, testis growth and circulating androgens were not significantly different in first time spawners compared to fish that underwent their second spawning season. These results provide reference levels for Atlantic halibut spermatogenesis.

Ontogenic origin of salmon GnRH neurons in the ventral telencephalon and the preoptic area in masu salmon

During the ontogeny of masu salmon Oncorhynchus masou, neurons producing the salmon type of gonadotropin-releasing hormone (sGnRH) were first detected in the olfactory epithelium of the eyed egg and, subsequently, in the brain, suggesting a migration of these cells. Among sGnRH neurons distributed from the olfactory nerve (ON) through the preoptic area (POA), those in the ventral telencephalon (VT) and the POA are indicated to regulate gonadotropin secretion. Thus, it is of interest to know whether all the sGnRH neurons originate from the olfactory epithelium. In the present study, we examined by in situ hybridization whether sGnRH neurons are present in the VT-POA of fish, whose olfactory epithelia including sGnRH clusters were cauterized just after hatching (44 days after fertilization). Fish were sampled in June (212 days after the operation). Neurons expressing sGnRH mRNA were detected in the VT-POA as well as in the ON, ventral olfactory bulb, and transitional area between the olfactory bulb and telencephalon (which is considered to correspond to the terminal nerve ganglion) in the control group. In contrast, neurons expressing sGnRH mRNA were not detected in the VT-POA in the olfactory epithelium lesioned (OEL) group. Furthermore, pituitary sGnRH content in the OEL group was just above the detectable limit and was significantly lower than that in the corresponding control group in both sexes. These results indicate that sGnRH neurons in the VT-POA are derived from the olfactory epithelium in masu salmon, although the possibility cannot be ruled out that sGnRH neurons in the VT-POA arise from the VT-POA, but were delayed in expressing sGnRH because of the trauma of cauterization.

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

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

Three mRNA species for mammalian-type gonadotropin-releasing hormone in the brain of the eel Anguilla japonica

A variant mRNA species for mammalian-type gonadotropin-releasing hormone (mGnRH) that retains first intron has recently been found in the brain of a primitive teleost, the eel Anguilla japonica. Here we have found that this mRNA species is generated from an alternative upstream transcription start site of the mGnRH gene. In addition, the upstream start site produces another mRNA species that lacks first intron. Both of these two variant mRNA species, however, have the same open reading frame as the downstream mGnRH mRNA, whose introns are all spliced out, and therefore, all three mGnRH mRNA species expressed in the eel brain would translate a common prepro-mGnRH polypeptide. Quantitative analysis revealed that the upstream mRNAs are less abundant accounting for not more than 1% of the total mGnRH mRNAs. When testosterone was administered (1 mg per fish per day) continuously for 5 days to juvenile male eels, both upstream and downstream mGnRH mRNA species in the brain were up-regulated. However, the level of the upstream mRNAs increased by 3-5-fold after 12 days of the treatment, whereas that of the downstream mRNA showed a more modest increase (about 2-fold) although this increase was more rapid. These results demonstrate that both upstream and downstream mGnRH mRNAs in the eel brain are under the hormonal control, but they are regulated in different and independent manners.

Molecular cloning of three cDNAs encoding different GnRHs in the brain of barfin flounder

To examine the reproductive endocrinology of a large pleuronectiform fish, barfin flounder, Verasper moseri, a promising candidate for aquaculture and resource enhancement in northern Japan due to its high commercial value, three gonadotropin-releasing hormones (GnRHs) in the brain was identified by isolation of their cDNAs. This species had three molecular forms of GnRH; salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and seabream GnRH (sbGnRH). Each GnRH cDNA encoded a signal peptide (SP), GnRH, and a GnRH-associated peptide (GAP), which was connected to GnRH by a Gly-Lys-Arg sequence. The sGnRH cDNA encoded an SP composed of 23 amino acids and a GAP composed of 54 amino acids. The cGnRH-II cDNA encoded an SP of 23 amino acids and a GAP of 49 amino acids. The sbGnRH cDNA encoded an SP of 26 amino acids and a GAP of 57 amino acids. In situ hybridization showed that the genes for sGnRH, cGnRH-II, and sbGnRH are expressed in the ventromedial olfactory bulbs and the terminal nerve ganglion, the midbrain tegmentum, and the preoptic area, respectively. These results indicate that sbGnRH neurons in the preoptic area are involved in gonadotropin secretion in barfin flounder.

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

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

Two different messenger RNAs for salmon gonadotropin-releasing hormone are expressed in rainbow trout (Oncorhynchus mykiss) brain

Two different precursor genes encoding the decapeptide salmon GnRH (sGnRH) are present in most salmonid species. In rainbow trout, a precedent Southern blot study revealed the existence of two different sGnRH genes and, recently, two different genes and their complementary DNAs that encode the identical peptide sGnRH were isolated from ovary and testis. Our study confirms the existence of two different mRNAs encoding sGnRH (sGnRH mRNA-I and sGnRH mRNA-II) in the brain of rainbow trout and, for the first time, full-length complementary DNA sequences are given. Central and peripheral distributions of the two messengers are described and seem to indicate different regulation of their expression. sGnRH mRNA-I is found essentially in the olfactory bulbs and telencephalon, whereas sGnRH mRNA-II is more widely expressed in the brain. Our observations allow speculation on the respective roles of two genes encoding the same decapeptide.

Sex differences in the brain of goldfish: gonadotropin-releasing hormone and vasotocinergic neurons

The differences between male and female behaviors are reflected in sexual dimorphism of brain structures and are found throughout the nervous system in a variety of vertebrates. The present study examined neurons immunolabeled for gonadotropin-releasing hormone and arginine vasotocin in the brain of the goldfish Carassius auratus to determine if these neurons are sexually dimorphic. There was no sex difference or influence of sex steroids on the neuronal volume and optical density of staining of arginine vasotocin neurons. Similarly, gonadotropin-releasing hormone neurons of the terminal nerve and midbrain tegmentum did not differ between sexually mature males, females and maturing females replaced with sex steroids with respect to distribution, numbers, optical density of staining, or gross morphology. In maturing females, testosterone specifically recruited additional preoptic gonadotropin-releasing hormone neurons to equal those in sexually mature individuals. Since estrogen had no effect, the influence of testosterone on gonadotropin-releasing hormone neuronal numbers appears to be independent of aromatization. Specifically, the preoptic gonadotropin-releasing hormone neuronal size was significantly larger in sexually mature males than females. 11-Ketotestosterone-replacement to ovariectomized maturing females induced male-typical secondary characters and male-type courtship behavior but did not masculinize the preoptic gonadotropin-releasing hormone neuronal size. Our results show that the sexually dimorphic preoptic gonadotropin-releasing hormone neuronal size is determined by factors (genetic) other than gonadal steroids. Further, we propose the hypothesis that phenotypic and behavioral sex differences need not be accompanied by structural differences in gonadotropin-releasing hormone and arginine vasotocin in the brain.

Effects of photoperiod on pituitary gonadotropin levels in masu salmon

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

The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou

Effects of testosterone (T) on upstream migratory behavior in masu salmon, Oncorhynchus masou, were studied by use of artificial raceways. In Experiment 1, yearling precocious males castrated in August were implanted with a capsule of medical silicone tube containing 500 microg of T in September. Their upstream migration was recorded in an artificial raceway with castrated and sham-operated precocious males implanted with a capsule containing vehicle for 2 months. In Experiment 2, upstream migratory behavior of yearling immature parr implanted with a capsule containing T 500 microg or vehicle was observed from September through November. In Experiment 3, upstream migratory behavior of castrated, castrated + T 50 microg, castrated + T 500 microg, and sham-operated precocious males was observed from September through October. In Experiment 4, upstream migratory behavior of the control, T 50 microg-, T 500 microg-, and T 1000 microg-treated immature parr was observed from September through October. In each experiment, plasma and pituitary samples were taken from the fish that moved upstream and remained to measure levels of T, gonadotropin (GTH) II, thyroxine (T(4)), and triiodothyronine (T(3)) by radioimmunoassay. Administration of T caused increases in plasma T levels within the physiological range. In Experiment 1, the frequency of migration upstream was 0, 19.2, and 35.7% in the castrated, castrated + T 500 microg, and sham-operated groups, respectively. The frequency was higher in the castrated + T 500 microg and sham-operated groups than in the castrated group. In Experiment 2, the frequency was higher in T 500 microg-treated parr (22%) than in the control (2.6%). In Experiments 1 and 2, pituitary contents of GTH II in the T-treated groups and precocious males were higher than those in the castrated precocious males and immature parr. In Experiment 3, castrated + T 50 microg, castrated + T 500 microg, and sham-operated fish showed upstream migratory behavior, whereas castrated fish without T did not. In Experiment 4, the frequency was 5.7, 22.9, 17.1, and 28.6% in the control, T 50 microg-, T 500 microg-, and T 1000 microg-treated groups, respectively. In each experiment, plasma levels of T(4) in migrants were lower than those in nonmigrants, whereas plasma levels of T(3) did not show such changes. From these results, it is inferred that T is a factor influencing upstream migration in masu salmon.

Gonadal steroids and the maturation of the species-specific gonadotropin-releasing hormone system in brain and pituitary of the male African catfish (Clarias gariepinus)

The effect of testosterone (T), 11-ketotestosterone (KT) and estradiol (E(2)) on the development of the catfish gonadotropin-releasing hormone system (cfGnRH) of male African catfish (Clarias gariepinus), at the onset of puberty [between 10 and 12 weeks post hatching (ph)] was investigated. The cfGnRH neurons, located in the ventral forebrain, were visualized by immunofluorescence and their numbers were determined and the amounts of cfGnRH-associated peptide (cfGAP) in the pituitary were measured by RIA. Steroid treatments did not significantly alter the numbers of immunoreactive GnRH neurons. However, T and E(2) caused an increase in the amount of GnRH, demonstrated by the intensity of the immunostaining of GnRH neurons and fibers in the brain and the amount of cfGAP in the pituitary. Treatment with KT, the main circulating androgen in adult male catfish, neither changed the number of cfGnRH neurons, nor elevated the cfGnRH content in the pituitary. In previous experiments with younger, prepubertal fish (2-6 weeks ph), T caused an elevation of the number of cfGnRH neurons to the same level as present in pubertal fish of 12-14 weeks. We conclude that the onset of puberty in the male African catfish coincides with the completion of the steroid-dependent structural maturation of the cfGnRH system in the brain. T and/or E(2), however, are still able to exert a positive influence on the amounts of cfGnRH during the later stages of pubertal development, thus still playing a role in the control of the cfGnRH system.

Steroid hormones stimulate gonadotrophs in juvenile male African catfish (Clarias gariepinus)

In juvenile African catfish (Clarias gariepinus), the pituitary LH content strongly increased after the beginning of spermatogonial proliferation. We hypothesized that a signal of testicular origin is involved in stimulating the gonadotrophs. We investigated the effects of castration and sex steroid treatment on gonadotrophs in juvenile males by quantifying LH production and release and LH subunit transcript levels and by examining gonadotroph morphology and proliferation. Castration reduced but did not abolish the maturation-associated elevation in pituitary LH content. Treatment with testosterone but not with 11-ketotestosterone, an otherwise potent androgen in fish, reversed the castration-induced decrease of pituitary LH levels. An increased pituitary LH content was accompanied by an increased number of cytologically mature gonadotrophs. However, no evidence was found for gonadotroph proliferation, so that quiescent gonadotrophs may have become activated. Although 11-ketotestosterone treatments had no effect in castrated males, this androgen attenuated gonadotroph activation in intact males. Because androgen production in juvenile catfish is downregulated by treatment with 11-ketotestosterone, its inhibitory effects on gonadotrophs in gonad-intact males may be due to suppression of Leydig cell testosterone production, which appears to be a limiting factor for the activation of catfish gonadotrophs. Aromatizable androgens may have opposite effects on fish (stimulatory) and mammalian (inhibitory) gonadotrophs.

Gonadotropin-releasing hormone fibers innervate the pituitary of the male African catfish (Clarias gariepinus) during puberty

The development of the catfish gonadotropin-releasing hormone (cfGnRH) fiber network in the pituitary of male African catfish (Clarias gariepinus) was investigated in relation to puberty. Double immunolabeling studied by confocal laser scanning microscopy revealed a concomitant development of gonadotropes and of pituitary cfGnRH innervation during the first wave of spermatogenesis. Catfish GnRH-immunoreactive fibers in the proximal pars distalis (PPD) of the pituitary were initially observed at the age of 10 weeks (onset of spermatogonial proliferation) and gradually reached the adult pattern at the age of 20 weeks (spermatozoa present in the testis). The content of cfGnRH-associated peptide (cfGAP, part of the prohormone) in the pituitary similarly increased during puberty. At the electron microscopical level, fibers containing cfGAP-ir granules came into close proximity of the gonadotropes at 18 weeks of age. In vitro studies indicated a progressively increasing basal and cfGnRH-stimulated luteinizing hormone (LH) secretion during pubertal development. The LH secretion patterns were similar in response to exogenous cfGnRH (0.1 microM) or to endogenous cfGnRH, the release of which was induced by forskolin (1 microM). Castration experiments demonstrated that the innervation of the pituitary with cfGnRH fibers continued after surgery, accompanied by an increase in the cfGAP levels. However, gonadotrope development was retarded, suggesting a differential regulation of the two maturational processes. Since testosterone stimulates both processes, other testicular factors may also be involved. Puberty-associated changes in LH release patterns appear to reflect changes in the GnRH sensitivity and in the pool of releasable LH, while availability of cfGnRH does not appear to be a limiting factor.

A novel form of gonadotropin-releasing hormone in the medaka, Oryzias latipes

The present study has identified three molecular forms of gonadotropin-releasing hormone (GnRH) in the brain of a teleost, the medaka, by isolation of their cDNAs. This species has a novel GnRH, which is here named medaka-type GnRH (mdGnRH), in addition to two characterized forms, chicken-II-type GnRH (cGnRH-II) and salmon-type GnRH (sGnRH). Phylogenetic analysis showed that mdGnRH is a medaka homolog of and seabream-type GnRH (sbGnRH) and mammalian-type GnRH (mGnRH) in other species, and suggested that all vertebrates have three distinct GnRHs. Furthermore, in situ hybridization revealed that the mdGnRH gene is expressed only in neurons clustered within the preoptic area as sbGnRH and mGnRH genes in other species are, while the genes for cGnRH-II and sGnRH are only in the midbrain tegmentum and nucleus olfactoretinalis, respectively. This result suggested that mdGnRH is a hypophysiotropic factor and the other two forms are involved in other physiological events as neuromodulators or neurotransmitters.

The effects of oral administration with 17 alpha-methyltestosterone on chromosomal synapsis in Oreochromis niloticus (Pisces, Cichlidae)

The pattern of chromosomal synapsis after treatment with 17 alpha-methyltestosterone (MT), a testosterone analogue routinely used for the reversal of phenotypic sex in aquaculture, was investigated using the Nile tilapia (Oreochromis niloticus) as a model teleost species. Progeny-tested, monosex diploid (2n = 44) individuals were orally administered with diets containing 50 mg/kg MT for 30 days after first feeding (XX(MT) neomales and XY(MT) males) and compared to controls (XY males). The formation and structure of the synaptonemal complex (SC) and the nature of chromosomal synapsis were investigated in control and treated groups by computer-assisted image analysis of transmission electron microscope (TEM) microphotographs taken from SC spreads. Nuclei at the pachytene stage were first observed in XX(MT) neomales, indicating an earlier commitment of genetically female spermatocytes to enter the first meiotic prophase. Administration of MT did not result in obvious SC lesions, breakage, asynapsis or formation of multivalents in genotypic females (XX(MT) neomales). Administration of MT resulted in a significant increase in the SC lengths in XY(MT) males, although it did not significantly alter the pattern of synapsis (SC structure and number and morphology of bivalents) in comparison to XY controls. The significance of the effects and the putative mode(s) of action of MT on chromosomal synapsis in teleosts is discussed.

Effects of photoperiod on salmon GnRH mRNA levels in brain of castrated underyearling precocious male masu salmon

Previous studies have suggested that activation of salmon gonadotropin-releasing hormone (sGnRH)-producing neurons is induced by the combined effects of photoperiod and steroid hormones in underyearling males of the masu salmon, Oncorhynchus masou. The present study further assesses the effects of photoperiod and steroid hormones on sGnRH synthetic activity and examines the changes in sGnRH mRNA levels in the brains of castrated underyearling precocious male masu salmon by manipulating the photoperiod for 60 days from August through October. In castrated males in which plasma testosterone levels decreased to low levels, sGnRH mRNA levels in the preoptic area (POA) increased under a short photoperiod (8L-16D), whereas they remained at low levels under a long photoperiod (16L-8D) for a 2-month duration. In sham-operated males, sGnRH mRNA levels in the ventral telencephalon and those in the POA increased in October with testicular maturation even under a long photoperiod with a delay of 1 month compared with the short photoperiod group. These results suggest that preoptic sGnRH-producing neurons receive short photoperiodic signals and that either short photoperiod or steroid hormone secretion is required for the activation of sGnRH synthesis in underyearling precocious male masu salmon.

Testosterone accelerates the development of the catfish GnRH system in the brain of immature African catfish (Clarias gariepinus)

The effects of two endogenous steroids on the maturation of the catfish GnRH and the chicken GnRH-II system in the African catfish were investigated. Immature fish (2 weeks of age, which is before sexual differentiation; thus male and female genotypes present) were fed with food pellets containing either testosterone (T), 11beta-hydroxyandrostenedione (OHA) or no steroid (control). After 2 and 4 weeks of treatment, the effects on the two GnRH systems were investigated immunocytochemically, using specific antibodies against the respective GnRH-associated peptides. By means of fluorescence microscopy the number of GnRH perikarya and the cell surfaces were determined. Confocal laser scanning microscopy was applied to verify spatial distribution and staining intensity. After 2 weeks of treatment no difference in any of the parameters between the groups was observed. However, 4 weeks T treatment resulted in significantly more cfGnRH-ir perikarya in the brain compared to the OHA and control groups. In addition, in the T group the number of immunoreactive fibers was markedly higher and the staining of the perikarya and axons was more intense. The distribution of cfGnRH-ir neurons over the ventral forebrain differed between the two age groups: in 4-week-old fish, the largest concentration of neurons was localized in the ventral telencephalon, while 2 weeks later the number of neurons in the supraoptic area had markedly increased, suggesting that the cfGnRH system is still undergoing developmental changes during this period. In 6-week-old fish the average volume of the cfGnRH perikarya (expressed as surface size in the microscopical sections) in both the OHA and the T group was significantly bigger than that in the control group. The cGnRH-II-ir neurons in the midbrain tegmentum showed strong immunoreactivity in all groups, both treated and nontreated. In contrast to the cfGnRH neurons, the staining intensity and the number of cGnRH-II neurons did not change after steroid treatment. The results of this study show that T is able to accelerate the development of the cfGnRH system, whereas OHA has only minimal effects; the cGnRH-II system develops independent from these steroids.

Testosterone differentially regulates expression of GnRH messenger RNAs in the terminal nerve, preoptic and midbrain of male tilapia

The purpose of the present study was to examine the regulation of three molecular variants of gonadotropin-releasing hormone (GnRH)-encoding mRNAs by testosterone in the male tilapia Oreochromis niloticus. Tilapias castrated for two weeks were injected intraperitoneally with sesame oil or 5 microgram/g testosterone for 7 days. In situ hybridization histochemistry was performed using 35S-labelled 30-mer antisense oligonucleotide probes complementary to exon two (bases 1-30) of salmon-, seabream-, and chicken II-GnRH. Computerized image analysis was performed to quantify GnRH mRNA expression in the terminal nerve ganglia (nucleus olfactoretinalis) and in individual cells of the preoptic area and the midbrain tegmentum. Testosterone treatment significantly elevated terminal nerve salmon-GnRH mRNA, reduced preoptic seabream-GnRH mRNA but had no effect on midbrain chicken II-GnRH mRNA levels. The total number and size of preoptic and midbrain GnRH mRNA-containing neurons or the total volume of the terminal nerve ganglia in testosterone-treated animals did not differ significantly from oil-treated animals. The midbrain chicken II-GnRH neurons are not targets of testosterone. These results demonstrate for the first time differential regulation of subpopulations of GnRH neurons with molecular diversity and different topography.

Fish as models for the neuroendocrine regulation of reproduction and growth

Models are essential for the full understanding of neuroendocrine control processes. In this regard fish offer a rich source of biological material. They have diverse growth and reproductive strategies, inhabiting most of the Earth's aquatic ecological niches. Fish possess many of the common vertebrate features but also offer several unique aspects to allow the biologist easy access to the study of hypothalamic and pituitary function. Several key examples of how teleosts, or the bony fish, can offer insight into fundamental mechanisms of vertebrate sex differentiation, growth and reproduction are reviewed.

Activation of salmon GnRH mRNA expression prior to differentiation of precocious males in Masu salmon

Changes in salmon gonadotropin-releasing hormone (sGnRH) mRNA levels in the brain of underyearling male masu salmon, Oncorhynchus masou, were investigated to clarify sGnRH participation in differentiation of precocious males. Fish were immature with low gonadosomatic index (GSI < 0.100%) on April 16 and May 7. On June 11, fish were differentiated into two groups: future precocious males and immature males. Fish in the former group had high GSI (0.203%) with proliferation of primary spermatocytes in the testis, while the latter had low GSI (0.055%) with primary spermatogonia. sGnRH mRNA levels (the number of neurons expressing sGnRH mRNA and the total number of silver grains per unit area) in the ventral telencephalon (VT) and in the VT plus preoptic area (POA) already increased in May before the differentiation of precocious males. GSI was positively correlated with sGnRH mRNA levels in the VT and in the VT plus POA in April, and with mRNA levels in the POA in May. Activated sGnRH synthesis is concluded to be related to the appearance of precocious males in masu salmon.

Testicular responsiveness to gonadotropic hormonein vitro and Leydig and Sertoli cell ultrastructure during pubertal development of male African catfish (Clarias gariepinus)

The gonadotropin (GTH)-stimulated testicular androgen secretionin vitro and the ultrastructure of Leydig and Sertoli cells was studied during the pubertal development in male African catfish. Testicular weight increased from less than 1 mg in the ninth week of age to nearly 600 mg in the 28th week. Immature testes (stage I: spermatogonia) were highly sensitive to GTH and secreted very high amounts of androgens per mg of tissue. The secretion per mg tissue decreased gradually in stages II (spermatogonia and spermatocytes) and III (spermatogonia, spermatocytes, and spermatids), but precipitously in stage IV (all germ cell stages, including spermatozoa). However, due to the testicular weight gain, the total androgen output per pair of testes increased slightly in stage III and strongly in stage IV. The sensitivity to GTH decreased with the appearance of haploid germ cells in stage III. Leydig cells but not Sertoli cells showed the ultrastructural characteristics of steroid producing cells. Leydig cell morphology did not change in stages I-III, while in stage IV, more smooth endoplasmic reticulum was present. The ultrastructural characteristics of Sertoli cells did not change prominently. Thus, spermatogonial multiplication and spermatocyte formation takes place when the testicular steroidogenic system is highly active and responsive to GTH; whereas the differentiation of haploid germ cells is accompanied by a reduced responsiveness to GTH and by the secretion of several-fold lower androgen amounts per mg of tissue.

Do gonadotrophin-releasing hormone neurons express estrogen receptors in the rainbow trout? A double immunohistochemical study

A double immunocytochemical procedure, with two different chromogens, was used to compare the respective distributions of estrogen receptor-immunoreactive cells and gonadotrophin-releasing hormone-immunoreactive neurons on the same sections of the brains of adult male and female rainbow trout (Oncorhynchus mykiss). Estrogen receptor-immunoreactive cells were observed in the ventral and lateral telencephalon, the preoptic region, the mediobasal hypothalamus, and the ventromedial thalamic nucleus. Gonadotrophin-releasing hormone-immunoreactive perikarya were detected in the olfactory bulbs, the ventral telencephalon, the preoptic area, and the mediobasal hypothalamus. Double-staining studies showed that, although some estrogen receptor-positive cells were in close proximity to gonadotrophin-releasing hormone-immunoreactive perikarya, careful examination of 550 gonadotrophin-releasing hormone-positive cells from five adult females and two adult males failed to demonstrate any evidence that gonadotrophin-releasing hormone neurons coexpress estrogen receptor in the brain of the rainbow trout. The present study provides, for the first time in teleosts, morphological evidence that gonadotrophin-releasing hormone neurons do not represent major direct targets for estradiol, suggesting that the positive feedback effects of estradiol onto the gonadotrophin-releasing hormone system are likely to be conveyed via other cell populations.