The progestin levonorgestrel disrupts gonadotropin expression and sex steroid levels in pubertal roach (Rutilus rutilus)

The aim of the present study was to investigate the effects of the synthetic progestin levonorgestrel (LNG) on the reproductive endocrine system of a teleost fish, the roach (Rutilus rutilus). Pubertal roach were exposed for 28 days in a flow-through system to four concentrations of LNG (3, 31, 312, and 3124 ng/l). Both males and females treated with 3124 ng/l LNG exhibited the upregulated levels of vitellogenin and oestrogen receptor 1 mRNA in the liver. At the same concentration, LNG caused a significant upregulation of the mRNA expression of the gene encoding luteinising hormone β-subunit (lhβ) and the suppression of the mRNA expression of the gene encoding follicle-stimulating hormone β-subunit (fshβ) in the pituitary of both male and female roach. A lower LNG concentration (312 ng/l) suppressed mRNA expression of fshβ in males only. Females treated with 3124 ng/l LNG exhibited significantly lower plasma 11-ketotestosterone (11-KT) and oestradiol (E2) concentrations, whereas their testosterone (T) level was higher compared with the control. Females exposed to 312 ng/l LNG presented significantly lower plasma E2 concentrations. Males exposed to ≥31 ng/l LNG exhibited significantly reduced 11-KT levels. As determined through a histological analysis, the ovaries of females were not affected by LNG exposure, whereas the testes of males exposed to 31 and 312 ng/l LNG exhibited a significantly higher percentage of spermatogonia B compared with the control. The results of the present study demonstrate that LNG disrupts the reproductive system of pubertal roach by affecting the pituitary gonadotropin expression and the sex steroid levels. This disruption was determined to occur in males after exposure to an environmentally relevant concentration (31 ng/l). Moreover, the highest tested concentration of LNG (3124 ng/l) exerted an oestrogenic effect on fish of both sexes.

Expression profiles of LHbeta, FSHbeta and their gonadal receptor mRNAs during sexual differentiation of Xenopus laevis tadpoles

The gonadotropins, luteinising hormone (LH) and follicle stimulating hormone (FSH), are important hormones regulating reproductive biology in vertebrates, especially the processes of steroidogenesis and gamete maturation. Despite the role of gonadotropins during the reproductive cycle in amphibians is well established, much less is known about the functional maturation of the hypothalamus-pituitary-gonad axis during larval development. Therefore, the present study aimed to analyze the expression profiles of hypophyseal LHbeta and FSHbeta mRNA and of their corresponding gonadal receptors (LH-R, FSH-R) in Xenopus laevis tadpoles during their ontogeny and sexual differentiation. The first significant elevation of LHbeta and FSHbeta mRNA was observed at late premetamorphosis. A clear raise of LHbeta mRNA was present during prometamorphic stages especially in males, while the LH-R only slowly increased during ontogeny with highest levels during metamorphic climax. In contrast, FSHbeta mRNA expression only slightly increased during ontogeny, however in both sexes the FSH-R mRNA was considerably elevated at prometamorphosis and further at metamorphic climax. Our results suggest that LHbeta and LH-R mRNA expression might be involved in initial maturation events of gametes, at least in males, while the gradually increase of FSH-R mRNA coincided with the advancing process of gamete maturation in both sexes. The present study provides for the first time evidence based on expression of gonadotropins and their corresponding gonadal receptors that the hypothalamus-pituitary-gonad axis evolves already at early stages of ontogeny and sexual differentiation in amphibians.

Exposure of Xenopus laevis tadpoles to finasteride, an inhibitor of 5-alpha reductase activity, impairs spermatogenesis and alters hypophyseal feedback mechanisms

Sexual steroids have major regulatory functions in gonadal development, maturation of gametes and sexual differentiation in vertebrates. Previous studies in amphibians provided evidence that dihydrotestosterone and activity of 5-alpha reductases might play a significant role in androgen-mediated reproductive biology. To test the involvement of 5-alpha reductases in maturation of gametes in amphibians, Xenopus laevis was exposed to finasteride (FIN), a known inhibitor of 5-alpha reductase enzyme activity. In a long-term exposure from stage 46 to 66, severe disruption of spermatogenesis was observed in histological analysis of testes as detected by occurrence of empty spermatocysts, while ovaries remained unaffected. Real-time PCR analyses of male and female brain revealed an increase of LHbeta mRNA and a decrease of FSHbeta mRNA in males, suggesting a signalling on testes that could result in increased steroidogenesis and reduced Sertoli cell proliferation. Accordingly, the mRNA expression of P450 side chain cleavage enzyme and 5-alpha reductase type 2 was increased in testes, while no effects could be observed on steroidogenic genes in ovaries. A short-term exposure to testosterone, FIN and testosterone+FIN showed that transient effects of FIN targeted males selectively and, in particular, interfered with the hypothalamus-pituitary-gonad axis. Furthermore, a negative feedback of testosterone on LHbeta was observed on males and females. This study provides evidence that exposure of X. laevis to FIN, an inhibitor of 5-alpha reductases, impaired spermatogenesis and involved sex-specific hypophyseal feedback mechanisms.

Aromatase, steroid-5-alpha-reductase type 1 and type 2 mRNA expression in gonads and in brain of Xenopus laevis during ontogeny

The key enzymes involved in the production of endogenous sex steroids are steroid-5-alpha-reductase and aromatase converting testosterone (T) into dihydrotestosterone (DHT) and into estradiol (E2), respectively. To gain more insights into the molecular mechanisms of sexual differentiation of amphibians, we determined the mRNA expression of steroid-5-alpha-reductase type1 (Srd5a1), type2 (Srd5a2) and aromatase (Aro) during ontogeny starting from the egg and ending after completion of metamorphosis in Xenopus laevis. Expression of all three enzymes was measured by means of semi-quantitative RT-PCR, determining for the first time Srd5a1 and Srd5a2 mRNA expression in amphibians. mRNA was analyzed in whole body homogenates from stage 12 to 48, while brain and gonads with kidney were studied separately from stage 48 to 66. Different ontogenetic mRNA expression patterns were observed for all genes analyzed, revealing early mRNA expression of Srd5a1 already in the egg at stage 12 whereas Srd5a2 and Aro was detected at stage 39. Sex-specific mRNA expressions of Srd5a2 and of Aro were determined in the gonads with kidney but not in brain. Srd5a2 was two-fold higher expressed in testes than in ovaries while Aro mRNA was ten-fold higher in ovaries. No gender-specific mRNA expression was observed for Srd5a1 in gonads and in brain. The ontogenetic patterns of Aro, Srd5a1 and Srd5a2 suggest that these genes are involved in sexual differentiation of gonads and brain already in early developmental stages. Especially in gonads Srd5a2 seems to be important for physiological regulation of testis development while Aro is associated with the development of ovaries.

Luteinizing hormone, follicle stimulating hormone, and gonadotropin releasing hormone mRNA expression of Xenopus laevis in response to endocrine disrupting compounds affecting reproductive biology

Environmental pollutants can interfere with the endocrine system of a variety of animals and are suggested to contribute to the worldwide decline of amphibians. In this study, the effects of endocrine disrupting compounds (EDC) on the hypothalamus-pituitary-gonad axis, regulating reproduction, were investigated in Xenopus laevis by determining their potential impact on gene expression of gonadotropin releasing hormone (GnRH), luteinizing hormone beta-subunit (LHbeta) and follicle-stimulating hormone beta-subunit (FSHbeta) in brain and pituitary using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). One environmental sample and four model compounds, ethinylestradiol (EE2), tamoxifen (TAM), methyldihydrotestosterone (MDHT), and flutamide (FLU), corresponding to (anti)estrogenic and (anti)androgenic modes of action were used at 10(-8)M during a four weeks exposure of adults of both sexes. In general, males had a higher LHbeta mRNA level compared to females, while the mRNA expression of FSHbeta and GnRH did not differ between both sexes. EE2 and MDHT treatment decreased LHbeta mRNA expression in the brain of male X. laevis, while only EE2 but not MDHT reduced LHbeta mRNA in females indicating classical negative feed-back mechanisms on hypophyseal gonadotropin expression. TAM increased LHbeta mRNA and FSHbeta mRNA expression in female X. laevis while none of the other treatments showed an effect on FSHbeta mRNA expression. GnRH expression was not changed by any treatment and exposure of X. laevis to Lambro river water had no significant effect on any of the genes examined. It is reported for the first time in amphibians that gonadotropin mRNA expression is differentially regulated by (anti)estrogenic and (anti)androgenic EDC and that gender-specific patterns of gene expression exist.

Effects of infection with Anguillicola crassus and simultaneous exposure with Cd and 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126) on the levels of cortisol and glucose in European eel ( Anguilla anguilla )

To investigate whether the stress response of European eels infected with Anguillicola crassus is influenced by environmental pollutants, experimentally infected eels were exposed to Cd and/or to 3,3', 4,4', 5-pentachlorobiphenyl (PCB 126). Serum cortisol and glucose concentrations of these eels were monitored over a period of 103 days and were compared with data from infected, unexposed eels as well as with data from uninfected eels. Additionally, the levels of cortisol were correlated with concentrations of Anguillicola-specific antibodies. All eels showed an initial increase of the cortisol levels until day 63. This general elevation of plasma cortisol is most likely due to handling stress, as all eels were repeatedly netted and afterwards inoculated with a feeding tube. At the end of the exposure period eels which were infected and those which were infected and simultaneously exposed to Cd and PCB showed significantly higher levels than the controls. The general course of serum glucose levels in eels resembled that of cortisol. Accordingly, Spearman correlation analysis revealed that an increase in serum cortisol concentrations is correlated with rising levels of glucose. With respect to immune-endocrine interactions a significant negative correlation between cortisol and anti-A. crassus antibodies was found. Our data show that A. crassus is the most potent stressor for European eels among the treatments tested within this study. This is important in terms of ecotoxicological studies as the main effects are caused by parasites rather than chemicals. Accordingly, effects of parasites on the physiological homeostasis of organisms must be considered in ecotoxicology. From the parasitological point of view our results suggest that probably as part of an unbalanced host-parasite interaction A. crassus evokes a strong cortisol response in A. anguilla, thereby suppressing the immune response which in turn enables the parasite to establish. The parasite-induced stress response in the newly adopted European eel might be one of the factors which contributes to the extremely effective colonizing strategy of A. crassus.

Endocrine effects of environmental pollution on Xenopus laevis and Rana temporaria

To determine the capacity of sewage treatment work effluents to disrupt the endocrine system under semifield conditions, two amphibian species, Xenopus laevis and Rana temporaria, were exposed to the effluent of a regional sewage treatment plant in South Bavaria during larval development until completion of metamorphosis. Exposure was carried out in river water (Würm) as a reference, and a 1:12-mixture sewage effluent representing the real situation on the spot, and in a higher concentration of sewage using a 1:2 mixture. An accidental impact of industrial wastewater into the reference and dilution medium, Würm, which was caused by a spate in the respective area during the sensitive period of sex differentiation of amphibian larvae, is assumed to be responsible for the relatively high percentage of females observed by histological analysis in all treatment groups. All of these values were higher than those determined in controls exposed to artificial tap water in laboratory experiments conducted in a comparable study design. Sex ratios between species, revealed by the semifield study with decreasing portions of females from control to 1:12 to 1:2, were strongly correlated. Determination of biomarker-mRNA-levels in Xenopus liver using semiquantitative RT-PCR at the end of the experimental phase, when exposure regime has turned into the initially expected situation with the highest load of potential estrogens in the effluent, followed by 1:2 and 1:12 mixture, resulted in a significant increase of Vitellogenin-mRNA in female juveniles exposed to the highest portion of sewage, whereas expression of both androgen and estrogen receptor-mRNA showed no clear differences. The results concerning the induction of estrogenic biomarkers are in accordance with our findings for estrogen receptor binding of sample extracts from the Würm and sewage taken in parallel at the end of the experiment, when sewage extracts possessed a much higher ability to displace [3H]estradiol from the estrogen receptor than the ones extracted from the mixtures.

Environmental estrogens and reproductive biology in amphibians

In this paper, the effects of an estrogenic compound, 4-nonyl-phenol (NP), on the amphibians Rana esculenta and Triturus carnifex are described together with those on sexual differentiation in Xenopus laevis. NP increased plasma vitellogenin in male frogs and newts in a dose-related manner; moreover, inhibitory effects on gonadotropin and prolactin (PRL) secretion by pituitary were found together with an elevation of plasma androgens. NP treatment also caused a remarkable increase in number of prolactin-immunolabeled cells, suggesting that xenoestrogen might induce, at least in the newt pituitary, a PRL accumulation possibly due to a reduction of the hormone release. In addition, both NP and bisphenol A caused feminization by increasing the percentage of female phenotypes in X. laevis, and the in vivo effects were more pronounced than those of estradiol-17beta.

Primary reactions of sensory rhodopsins

The first steps in the photocycles of the archaeal photoreceptor proteins sensory rhodopsin (SR) I and II from Halobacterium salinarum and SRII from Natronobacterium pharaonis have been studied by ultrafast pump/probe spectroscopy and steady-state fluorescence spectroscopy. The data for both species of the blue-light receptor SRII suggests that their primary reactions are nearly analogous with a fast decay of the excited electronic state in 300-400 fs and a transition between two red-shifted product states in 4-5 ps. Thus SRII behaves similarly to bacteriorhodopsin. In contrast for SRI at pH 6.0, which absorbs in the orange part of the spectrum, a strongly increased fluorescence quantum yield and a drastically slower and biexponential decay of the excited electronic state occurring on the picosecond time scale (5 ps and 33 ps) is observed. The results suggest that the primary reactions are controlled by the charge distribution in the vicinity of the Schiff base and demonstrate that there is no direct connection between absorption properties and reaction dynamics for the retinal protein family.

Amphibians as a model to study endocrine disruptors: II. Estrogenic activity of environmental chemicals in vitro and in vivo

Several environmental chemicals are known to have estrogenic activity by interacting with development and functions of endocrine systems in nearly all classes of vertebrates. In order to get a better insight of potential estrogenic effects on amphibians caused by environmental pollution this study aims to develop a model for investigating endocrine disruptors using the amphibian Xenopus laevis. In that model the potential estrogenic activity of endocrine disruptors is determined at several levels of investigation: (I) binding to liver estrogen receptor; (II) estrogenicity in vitro by inducing vitellogenin synthesis in primary cultured hepatocytes; and (III) in vivo effects on sexual development. Here we deal with establishing methods to assay estrogenic activity of environmental chemicals in vitro and in vivo. In vitro we used a semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) technique to determine mRNA-induction of the estrogenic biomarker vitellogenin in primary cultured hepatocytes of male Xenopus laevis. Time courses of vitellogenin-mRNA in the presence and absence of 10(-6) M 17 beta-estradiol (E2) resulted in a marked loss of mRNA from controls after 2 days while E2 treatment kept vitellogenin-mRNA at a relatively stable level. After 36 h of incubation estrogenic activities of E2, 4-nonylphenol (NP), and 2,2-bis-(4-hydroxyphenyl)-propan (bisphenol A) at concentrations ranging from 10(-10) to 10(-5) M were assayed by RT-PCR of vitellogenin-mRNA and showed the following ranking of dose-dependent potency: E2 > NP > bisphenol A. These in vitro results were confirmed further by in vivo experiments determining sexual differentiation of Xenopus laevis after exposure to E2 and environmental chemicals during larval development. Concentrations of 10(-7) and 10(-8) M E2 as well as 10(-7) M of NP or bisphenol A caused a significant higher number of female phenotypes compared to controls indicating a similar ranking of estrogenic potencies in vivo as in vitro. In addition, butylhydroxyanisol and octylphenol, both showed feminization at 10(-7) M while octylphenol was also effective at 10(-8) M. In summary these results demonstrate for the first time the use of a semiquantitative RT-PCR technique for screening estrogenicity by assaying mRNA induction of the estrogenic biomarker vitellogenin in vitro. The combination of this newly developed method with classical exposure experiments is necessary for determination of the biological significance of estrogenic chemicals.

Amphibians as a model to study endocrine disruptors: I. Environmental pollution and estrogen receptor binding

Many chemicals released into the environment without toxicological risks have the capacities to disrupt the function of endocrine systems. These endocrine disruptors disturb normal endocrine mechanisms and have been observed in nearly all classes of vertebrates. The aim of this research is to develop a comprehensive model to study endocrine disruption using the amphibian Xenopus laevis. The assessment of estrogenic potencies of endocrine disruptors includes several levels of investigation: (I) binding to liver estrogen receptor, (II) estrogenic activity in vitro by inducing vitellogenin synthesis in primary cultured hepatocytes, and (III) in vivo effects on sexual development caused by exposure of larvae. The present paper is focused on the first part by establishing a radioreceptorassay for [3H]17 beta-estradiol ([3H]E2) binding using liver cytosol fraction. In order to get optimum binding conditions we performed kinetic, saturation, and competitive displacement experiments. Association of [3H]E2 to estrogen receptor revealed that maximum specific binding is achieved between 18 and 48 h of incubation. Scatchard analyses of saturation experiments resulted in a homogenous saturable population of estrogen receptors having no significant differences of binding parameters between both sexes. The values of Kd (dissociation constant) in males and females were 22.4 +/- 6.0 and 15.0 +/- 2.8 nM (mean +/- S.E.M.; n = 5), respectively, while corresponding Bmax (maximum binding capacity) revealed 89 +/- 46 and 136 +/- 46 fmol [3H]E2/mg protein. The specificity of estrogen receptors as shown by competitive displacement experiments demonstrated receptors being highly specific just for estrogens, but not for other endogenous steroids having the following ranking of binding affinities: E2 > estrone > dehydroepiandrosterone > aldosterone > or = testosterone > or = corticosterone > or = progesterone. The affinity ranking of environmental chemicals compared to E2 was: E2 > tetrachlorbiphenyl > diethylphthalate > 2,2-bis-(4-hydroxyphenyl)-propan (bisphenol A) > or = 4-nonylphenol > or = 3-t-butyl-4-hydroxyanisole > or = 4-octylphenol > dichlor-diphenyl-trichlor-ethan (4,4'-DDT). Analyses of five sewage effluents for displacement of [3H]E2 binding resulted in three samples displacing more than 50% of specific binding at their original concentration. Taken together the established radioreceptorassay for [3H]E2 binding in Xenopus laevis liver cytosol is useful to screen estrogen receptor binding of pure compounds or complex mixtures of them, which is the prerequisite for causing either estrogenic or antiestrogenic effects.