Induction of gonadotropin surge by steroid hormone implantation in ovariectomized and sexually regressed female goldfish

Larval exposure to environmentally relevant concentrations of triclosan impairs metamorphosis and reproductive fitness in zebrafish

Developmental exposure to endocrine disruptors can cause organizational changes resulting in latent and transgenerational disease. We exposed zebrafish to environmentally relevant concentrations of triclosan during the critical period of metamorphosis and somatic sex differentiation to determine effects on metamorphosis and reproduction. We use biological and morphological biomarkers to predict potential modes of action. Larval exposure to environmentally relevant concentrations of triclosan was sufficient to cause adverse effects in adults and their offspring. TCS exposure delays metamorphosis and impairs fecundity and fertility. Offspring from TCS-exposed fish show decreased survival and delayed maturation, but their reproductive capacity is not altered. Delays in metamorphosis in conjunction with morphological indicators suggest that toxicity may result from lowered thyroid hormones in parental fish. This work illustrates the importance of evaluating the latent effects of early exposure to environmental contaminants, and that further studies to evaluate the effects of triclosan on the thyroid axis are warranted.

Sex steroids differentially regulate fshb, lhb and gnrhr expression in Atlantic cod (Gadus morhua)

Depending on the stage of gonad maturation, as well as other factors, gonadal steroids can exert either a positive or negative feedback at the brain and pituitary level. While this has been demonstrated in many teleost species, little is known about the nature of steroid feedback in Gadiform fish. Using an optimized in vitro model system of the Atlantic cod pituitary, the present study investigated the potential effects of two physiologically relevant doses of estradiol, testosterone (TS) or dihydrotestosterone (DHTS) on cell viability and gene expression of gonadotropin subunits (fshb/lhb) and two suggested reproduction-relevant gonadotropin-releasing hormone receptors (gnrhr1b/gnrhr2a) during three stages of sexual maturity. In general, all steroids stimulated cell viability in terms of metabolic activity and membrane integrity. Furthermore, all steroids affected fshb expression, with the effect depending on both the specific steroid, dose and maturity status. Conversely, only DHTS exposure affected lhb levels, and this occurred only during the spawning season. Using single-cell qPCR, co-transcription of gnrhr1b and gnrhr2a was confirmed to both fshb- and lhb- expressing gonadotropes, with gnrhr2a being the most prominently expressed isoform. While steroid exposure had no effect on gnrhr1b expression, all steroids affected gnrhr2a transcript levels in at least one maturity stage. These and previous results from our group point to Gnrhr2a as the main modulator of gonadotropin regulation in cod and that regulation of its gene expression level might function as a direct mechanism for steroid feedback at the pituitary level.

Gonadotropin-Inhibitory Hormone, the Piscine Ortholog of LPXRFa, Participates in 17β-Estradiol Feedback in Female Goldfish Reproduction

Gonadotropin-inhibitory hormone (GnIH) plays a critical role in regulating gonadotropin-releasing hormone, gonadotropin hormone, and steroidogenesis in teleosts. In the present study, we sought to determine whether 17β-estradiol (E2) acts directly on GnIH neurons to regulate reproduction in goldfish, a seasonal breeder, and we investigated the role of estrogen receptors (ERs) in mediating this process. We found that GnIH neurons coexpress three types of ERs. Ovariectomy and letrozole implantation into female goldfish at the vitellogenic stage elicited a substantial decrease in the expression of GnIH messenger RNA (mRNA), and E2 supplementation abolished this effect. In primary cultured hypothalamus cells, E2 increased GnIH mRNA levels; surprisingly, selective ERα and ERβ agonists showed opposite effects in regulating GnIH mRNA levels. Using genome walking, we isolated a 2329-bp section of the GnIH promoter sequence, and 7 half-estrogen response elements (EREs) were found in the promoter region. Luciferase assays and electrophoretic mobility shift assay results show that the half-ERE element at -2203 is the key site for competitive binding between ERα and ERβ. Ovariectomy and letrozole implantation into female goldfish in the maturating stage did not change the GnIH mRNA expression levels. Taken together, these findings suggest that E2 binds to multiple types of ERs, which competitively bind to the same half-ERE binding site of the GnIH promoter to achieve both positive and negative feedback in response to estrogen to regulate goldfish reproduction at different stages of ovarian development.

Steroid sensitive kiss2 neurones in the goldfish: evolutionary insights into the duplicate kisspeptin gene-expressing neurones

The KISS1/Kiss1/kiss1 gene product kisspeptin is suggested to be involved in the steroid feedback system in vertebrates. In addition to kiss1, kiss2 has been identified in many vertebrates, including some mammals, suggesting that the both genes were originally expressed in the common ancestor of teleosts and tetrapods. Moreover, peptides from both genes have been shown to activate the kisspeptin receptors. To investigate the involvement of kiss1 or kiss2 neurones in steroid feedback, we used a seasonal breeder, the goldfish (Carassius auratus). We found that kiss2 is expressed in the preoptic area (POA), nucleus lateralis tuberis and nucleus recessus lateralis, and that kiss1 is expressed in the habenula. Greater mRNA expression in breeding than in nonbreeding condition animals and conspicuous up-regulation of gene expression by gonadal steroids was seen only in the kiss2 neurones of the POA. Furthermore, double in situ hybridisation suggested that these neurones express oestrogen receptors. Given that amphibians express kiss2 in POA and mammalian anteroventral periventricular nucleus/POA Kiss1 neurones show similar expression dynamics as goldfish POA Kiss2 neurones, we hypothesise that kiss1 and kiss2 share the same evolutionary origin; and, after the loss of kiss2, kiss1 became active for steroid feedback in mammals.

Endocrine control of sexual behavior in teleost fish

Sexual behavior is one of the most profound events during the life cycle of animals that reproduce sexually. After completion of gonadal development that is mediated by various hormones, oviparous teleosts perform a suite of behaviors, often termed as spawning behavior. This is particularly important for teleosts that have their gametes fertilized externally as the behavior patterns ensures the close proximity of both sexes for gamete release, fusion and ultimately the production of offspring. As in other vertebrates, sexual behavior of fish is also under the control of hormones. Testicular androgen is a requirement for male sexual behavior to occur in most fish species that have been studied. Unlike tetrapods, however, ovarian estrogen does not appear to be essential for the occurrence of female sexual behavior for fish that have their gametes fertilized externally. Prostaglandins produced in the ovary after ovulation act as a trigger in some teleosts to induce female sexual behavior. Potentiating effects of gonadotropin-releasing hormone in the brain on sexual behavior are reported in some species. Under endocrine regulation, male and female fish exhibit gender-typical behavior during spawning, but in some fish species there is also some plasticity in their sexual behavior. Sex changing fish can perform both male-typical and female-typical sexual behaviors during their lifetime and this sexual plasticity can also be observed in non-sex changing fish when undergoing hormonal treatment. Although the neuroanatomical basis is not clear in fish, results of field and laboratory observations suggest that some teleosts possess a sexually bipotential brain which can regulate two types of behaviors unlike most other vertebrates which have a discrete sex differentiation of their brain and can only perform gender-typical sexual behavior.

Neurogenic and neuroendocrine effects of goldfish pheromones

Goldfish (Carassius auratus) use reproductive hormones as endocrine signals to synchronize sexual behavior with gamete maturation and as exogenous signals (pheromones) to mediate spawning interactions between conspecifics. We examined the differential effects of two hormonal pheromones, prostaglandin F(2alpha) (PGF(2alpha)) and 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) on neurogenesis, neurotransmission, and neuronal activities, and on plasma androstenedione (AD) levels. Exposure to waterborne PGF(2alpha) induced a multitude of changes in male goldfish brain. Histological examination indicated an increase in the number of dividing cells in male diencephalon (p < 0.05, Kruskal-Wallis test). Real-time quantitative PCR tests showed elevated levels of transcripts for the salmon gonadotropin-releasing hormone (GnRH) in the male telencephalon and cerebellum (p < 0.005, one-way ANOVA) and for ChAT (choline acetyltransferase) in the male vagal lobe and the brainstem underneath the vagal lobe (p < 0.05, one-way ANOVA). Therefore, PGF(2alpha) seemed to modulate male brain plasticity that coincided with behavioral changes during spawning season. Exposure to waterborne 17,20beta-P, however, increased circulatory levels of immunoreactive AD in males and the transcripts of androgen receptor and cGnRH-II (chicken-II GnRH) in the female cerebellum (p < 0.05, one-way ANOVA). PGF(2alpha) and 17,20beta-P thereby seemed to act through distinct pathways to elicit different responses in the neuroendocrine system. This is the first finding that links a specific pheromone molecule (PGF(2alpha)) to neurogenesis in a vertebrate animal.

Inhibition of follicular development, vitellogenesis, and serum 17beta-estradiol concentrations in zebrafish following chronic, sublethal dietary exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent endocrine disruptor with the ability to affect several biologic processes, including reproduction. In fish, sublethal exposure to TCDD is known to modulate overall reproductive capacity, but impacts on follicular development and vitellogenesis are unknown. Here we show that chronic, dietary exposure to 0.08, 0.32, or 0.80 ng TCDD female(-1) day(-1) decreased egg production by more than 50% and that spawning success was reduced by as much as 96%. Serum estradiol concentrations were decreased more than twofold, accounting, in part, for observed decreases in serum vitellogenin concentrations by as much as 29%. Our data suggest that decreased egg production is likely the result of TCDD-mediated inhibition of the transition from pre-vitellogenic stage follicles to vitellogenic stage follicles, as well as the induction of follicular atresia. The majority of reproductive toxicity of TCDD is likely due to direct impacts on the ovary, yet histopathologic observations suggest liver toxicity could also contribute to observed impacts on vitellogenesis. Importantly, even when overall egg production is not significantly affected, our data show that subtle physiologic changes induced by TCDD can lead to altered gonadogenesis. This suggests that long-term exposure to very low concentrations of TCDD could greatly affect fecundity and reproductive success in fishes.

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.

Estradiol-17beta triggers luteinizing hormone release in the protandrous black porgy (Acanthopagrus schlegeli Bleeker) through multiple interactions with gonadotropin-releasing hormone control

We investigated the mechanism of estradiol-17beta (E2) action on stimulation of LH (=gonadotropin II) release in the black porgy fish (Acanthopagrus schlegeli Bleeker) using an in vivo approach and primary cultures of dispersed pituitary cells in vitro. In vivo, E2 but not androgens (testosterone [T] and 11-ketotestosterone [11-KT]) significantly stimulated plasma LH in a dose-dependent manner. Estradiol-17beta also increased brain content of seabream GnRH. GnRH antagonist prevented E2 stimulation of LH release in vivo, indicating that the effect of E2 on LH was mediated by GnRH. In vitro, sex steroids (E2, T, 11-KT) alone had no effect on basal LH release in the cultured pituitary cells, but GnRH significantly stimulated LH release. Estradiol-17beta potentiated GnRH stimulation of LH release, an effect that was inhibited by GnRH antagonist, and 11-KT, but not T, also potentiated GnRH stimulation of LH release. The potentiating effect of 11-KT on GnRH-induced LH release in vitro was stronger than that of E2. These data suggest that E2 triggers LH release in vivo by acting both on GnRH production at the hypothalamus and on GnRH action at the pituitary. In contrast, 11-KT may only stimulate GnRH action at the pituitary. The E2) induction of LH release, through multiple interactions with GnRH control, supports a possible central role of E2in the sex change observed in the protandrous black porgy.

In vitro application of testosterone potentiates gonadotropin-releasing hormone-stimulated gonadotropin-II secretion from cultured goldfish pituitary cells

The in vitro effects of overnight treatment with testosterone (T) on gonadotropin (GTH)-II secretion from primary cultures of dispersed female goldfish pituitary cells were examined. T (100 nM) did not affect basal GTH-II release, but increased GTH-II responses to initial applications of 0.5-h pulses of sGnRH or cGnRH-II in cells from females at sexually regressed, recrudescing, or mature (prespawning) stages. Pretreatment with 10 nM T was also effective, except in experiments with cells from sexually regressed females. Analysis of GTH-II response profiles to the first GnRH pulse revealed that T increased the size of the initial (first 15 min) and sustained (rest of response) release phases, and the duration of the total response to both GnRHs. These results indicate that direct positive influence of T on GnRH-stimulated GTH-II release is demonstrable in cells from female goldfish at all ovarian maturational stages; in addition, T affects both the initial and the sustained response phases. However, compared to the initial GnRH challenge, responses to a second 0.5-h GnRH pulse were decreased in T-treated but not in control cells, suggesting that T also enhanced desensitization. Ovarian maturational conditions modulated the effects of T on the GTH-II response kinetics. In cells prepared from sexually regressed females, T treatment changed the "monophasic" (initial phase only) GTH-II response to sGnRH to a "biphasic" one characteristic of cells prepared from fish at later stages of gonadal recrudescence. Advancing gonadal maturity increased the magnitude of both initial and sustained phases of the T-enhanced GTH-II response to sGnRH, but only elevated the initial phase of T-potentiated cGnRH-II-induced release. Direct actions of T on pituitary cells may play a role in ovarian steroid feedback regulation of GTH-II secretion during the seasonal reproductive cycle in goldfish.

In vitro action of testosterone in potentiating gonadotropin-releasing hormone-stimulated gonadotropin-II secretion in goldfish pituitary cells: involvement of protein kinase C, calcium, and testosterone metabolites

Overnight preincubation of goldfish pituitary cell culture with testosterone (T) enhanced the gonadotropin (GTH)-II responses to GTH-releasing hormone (GnRH). In this study, the involvement of GnRH signal transduction components and the requirement for T metabolism in mediating this direct, pituitary cell action of T were examined using cultured pituitary cells from both male and female goldfish. Each sets of related experiments were done in at least two different stages of the gonadal reproductive cycle and similar effects were observed. Overnight treatment with 10 nM T increased GTH-II responses to maximal stimulatory doses (100 nM) of either salmon (s)GnRH or chicken (c)GnRH-II, but not the total cellular GTH-II contents measured prior to and after a 2-h GnRH challenge. T increased the efficacy and sensitivity of the GTH-II response to stimulation by a protein kinase C (PKC) activator, tetradecanoyl phorbol acetate (TPA) without altering the ED50 of the dose-response curve. In T-treated cells, addition of a PKC inhibitor attenuated GTH-II responses to 100 nM doses of sGnRH, cGnRH-II, or TPA. T did not affect the GTH-II release stimulated by high concentrations of the Ca2+ ionophore ionomycin (100 microM) and the voltage-sensitive Ca2+ channel (VSCC) agonist Bay K 8644 (10 microM); similarly, the sensitivity of the GTH-II response to ionomycin and Bay K 8644 was also unaltered. Taken together, these data suggest that T potentiates GnRH-stimulated GTH-II release by enhancing the effectiveness of PKC-dependent pathways, but not by increasing the total Ca2+-sensitive GTH-II pool, the sensitivity of the release response to increases in intracellular Ca2+, or the amount of available GTH-II. However, the VSCC agonist nifedipine reduced sGnRH- and cGnRH-II-elicited GTH-II release in T-treated as well as in non-T-treated cells, suggesting that VSCC dependence is still present in the GnRH-induced response following exposure to T. Since total cGnRH-II binding to pituitary cells was not increased by T, increases in GnRH receptor capacity are unlikely following T treatment. The ability of T to increase GnRH-stimulated GTH-II secretion was not mimicked by 11-ketotestosterone or dihydrotestosterone, but was abolished by coincubation with an aromatase inhibitor. When viewed together, these observations suggest that aromatization of T may be required for the pituitary action of T on GnRH-induced GTH-II release.

Induction of male-type gonadotropin secretion by implantation of 11-ketotestosterone in female goldfish

In goldfish, plasma gonadotropin levels increase during spawning in both males and females (GTH surge). A female-typical GTH surge induces ovulation (ovulatory surge), and a male-typical surge triggers milt production in response to sex pheromones released from ovulatory females. This study examined whether the male-typical GTH surge occurs in adult females that are implanted with 11-ketotestosterone (KT), which induces male-typical sexual behavior in adult female goldfish. When KT-implanted females were exposed to ovulatory females, a GTH surge occurred without ovulation. No GTH surge was observed when KT-females were exposed to nonovulatory females. The GTH secretion in KT-females was further characterized by exposure to 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20-P), a female sex pheromone that induces the GTH surge in males. Exposure to waterborne 17,20-P caused an elevation of GTH levels in KT-females as well as in males. The elevation of GTH levels induced by 17,20-P exposure was abolished when the KT-females were rendered anosmic. Unlike the female-typical ovulatory GTH surge that occurs in synchrony with photoperiod and peaks in the dark phase of the day, the 17,20-P-induced surge did not show a peak in the dark phase. These results indicate that the GTH surge in KT-females is a male-typical surge. Together with a previous study showing KT-induced behavioral masculinization (N. E. Stacey and M. Kobayashi, 1996, Horm. Behav. 30, 434-445), this adult gonochoristic species was shown to possess sexual plasticity of brain function in behavior and GTH secretion in response to sex steroid.

Molecular cloning of cDNA encoding two types of pituitary gonadotropin alpha subunit from the goldfish, Carassius auratus

Two types of cDNA encoding the alpha subunit of pituitary gonadotropin (GTH) were cloned by polymerase chain reaction (PCR) for goldfish pituitary cDNA library. The goldfish GTH-alpha subunit cDNAs, designated as alpha 1 and alpha 2, encode 117 and 118 amino acids, respectively, including a 23-amino-acid signal peptide. These two types of cDNAs showed high homology in nucleotide and amino acid sequence, but deletion of a triplet nucleotides was present in alpha 1. Deduced amino acid sequences represented differences in four residues between alpha 1 and alpha 2 subunits. Analyses of goldfish genomic DNA revealed that each individual goldfish possesses two distinct genes relevant to GTH-alpha 1 and -alpha 2 subunits. Results of reverse transcriptase-PCR analysis suggest that these two GTH-alpha subunit genes are expressed at different levels without relation to sex and gonadal maturity.

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

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

Estradiol increases susceptibility of goldfish to Trypanosoma danilewskyi

We examined the in vivo and in vitro effects of estradiol on the host defense of goldfish against a hemoflagellate Trypanosoma danilewskyi. Estradiol (10(-6) M), significantly suppressed the mitogen-induced proliferation of peripheral blood lymphocytes in vitro. The parasitemia and mortality of goldfish implanted with tubing containing estradiol were significantly higher than that of sham operated control fish. In addition, the mitogen-induced proliferation of peripheral blood lymphocytes obtained from estradiol-implanted fish was significantly impaired when compared to that of lymphocytes obtained from untreated sham operated control fish. Estradiol implantation did not influence the ability of goldfish to resist a challenge infection with T. danilewskyi. These results indicate that estradiol can modulate the susceptibility to hemoflagellates by suppressing the host immune response.

Seasonal variation of neuropeptide Y actions on growth hormone and gonadotropin-II secretion in the goldfish: effects of sex steroids

The effects of neuropeptide Y (NPY) on growth hormone (GH) and gonadotropin-II (GtH-II) release in different reproductive stages were studied using perifused pituitary fragments of female goldfish. The GH and GtH-II release responses to 5-min pulses of NPY were relatively small in sexually regressed fish (July), intermediate in recrudescent fish (December), and maximal in sexually mature (= prespawning) fish (May). To test if sex steroids can modulate NPY action, the effects of in vivo implantation of 17 beta-estradiol (E2) and testosterone (T) (both at 100 micrograms/g dosage) on NPY-induced GH and GtH-II secretion were examined. In sexually regressed goldfish, implantation of T significantly enhanced NPY-induced GH and GtH-II release from perifused pituitary fragments; implantation of E2 potentiated the NPY-induced GtH-II, but not GH release. However, steroid implantation did not affect responses to NPY when this experiment was repeated using pituitaries from sexually mature fish. To test the hypothesis that steroids may act directly at the level of the pituitary to potentiate NPY action, pituitary fragments taken from sexually regressed goldfish were incubated with 100 nM T for 24 h, and the GH and GtH-II responses to 5-min challenges of NPY assessed in the presence of T. Both GH and GtH-II responses to NPY were not affected by treatment with T in vitro, suggesting that T does not act directly at the level of the pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of common carp gonadotropin I and gonadotropin II

Two gonadotropins, GtH I and GtH II, were extracted with 35% ethanol-10% ammonium acetate, pH 6.1, from female common carp pituitary glands and purified by ion-exchange chromatography on a DE-52 column followed by gel filtration on a Sephadex G-75 column. Molecular weights of GtH I and GtH II as determined by SDS-PAGE were 45,000 and 35,000, respectively. Both GtHs dissociate into two subunits following reduction with beta-mercaptoethanol. These subunits contain different N-terminal amino acids (Tyr and Gly for GtH I; Tyr and Ser for GtH II). GtH I was acid stable and did not dissociate into subunits following treatment with 0.1% trifluoroacetic acid; GtH II readily dissociated into subunits by this treatment. GtH I and GtH II have distinct elution profiles on reverse-phase HPLC. The N-terminal amino acid sequence of the beta-subunit of GtH II was identical to that of common carp maturational GtH described by other workers suggesting that GtH I is a newly identified molecule. This was supported by radioimmunoassay analysis. GtH II and a common carp maturational GtH preparation (F11 cGtH; Peter et al., 1982, J. Interdiscipl. Cycle Res. 13, 229-239) had similar immunological activity in tests with antisera to the beta-subunit of maturational GtH whereas GtH I had low (less than 6%) cross-reactivity. GtH I, GtH II, and F11 cGtH were equipotent in tests with antisera to the alpha-subunit of maturational GtH suggesting these molecules contain a similar alpha-subunit. In vitro bioassays using goldfish revealed that GtH I and GtH II share the same spectrum of biological activities causing stimulation of ovarian and testicular steroidogenesis and induction of oocyte final maturation. The demonstration of two chemically distinct GtHs in common carp is similar to what has been described for chum and coho salmon.

Progestins and cortisol delay while estradiol-17 beta induces early parturition in the guppy, Poecilia reticulata

Fertilization and gestation are intrafollicular in the guppy (Poecilia reticulata), and ovulation occurs at the end of gestation prior to parturition. In this study, the effects in vivo of the ovarian steroids, progesterone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-P), cortisol and estradiol-17 beta, the antiprogestin RU 486, and aromatase inhibitor, 4-hydroxyandrost-4-ene-3,17-dione (4-HAD), on gestation and parturition were studied in the guppy. Progesterone (0.05 and 0.10 micrograms/ml of water), 17 alpha,20 beta-P (0.01 micrograms/ml and greater), cortisol (0.10 micrograms/ml) and 4-HAD (0.10 micrograms/ml) all prolonged gestation presumably by inhibiting ovulation. 17 alpha,20 beta-P was most effective in inhibiting ovulation and parturition for up to 36 days postpartum. This inhibition was reversed when fish were transferred to steroid-free water. Besides extending gestation, 17 alpha,20 beta-P and 4-HAD also inhibited development of vitellogenic oocytes. Estradiol-17 beta (0.05 and 0.10 micrograms/ml) and RU 486 (10 micrograms/g body weight) both induced premature parturition presumably by accelerating onset of ovulation. These results, together with our previous observations on the steroid profile in the guppy, strongly suggest roles for estradiol-17 beta and cortisol in regulating ovulation and parturition.

Nonmammalian vertebrate models in studies of brain-steroid interactions

Estrogen formation in brain and pituitary mediates certain androgen actions in central targets. Goldfish (Carassius auratus) and quail (Coturnix coturnix japonica) have been advantageous for studying the role of locally formed estrogen in autoregulating aromatization and in controlling estrogen receptor occupancy, androgen receptor levels, and behavioral expression. Data from these two experimental models reveal a molecular basis for androgen-estrogen synergism in neuroendocrine tissues and for alterations in androgen sensitivity/responsiveness. These mechanisms are essential components of seasonal reproduction in the test species and may have wider relevance for cyclicity in other vertebrates, including mammals.

Involvement of steroid hormones in the preovulatory gonadotropin surge in female goldfish

Goldfish,Carassius auratus, spawn several times within a spawning season. A gonadotropin (GtH) surge occurs at the time of ovulation in this cyprinid species. This ovulatory GtH surge mediates the processes of final oocyte maturation and ovulation, and occurs at the end of each spawning cycle. Within a cycle, there is a shift in the predominant plasma steroid from estradiol to testosterone, and finally 17α, 20β-dihydroxy-4-pregnen-3-one at the time of the ovulatory GtH surge. High levels of testosterone were always observed before ovulation. When ovariectomized or sexually regressed female goldfish were implanted with testosterone, they exhibited a GtH surge which was similar to those normally observed at ovulation. These results strongly suggest that elevated plasma testosterone is an important physiological requirement for the occurrence of the GtH surge.