Alterations in the GnRH-LH system in relation to gonadal stage and Aroclor 1254 exposure in Atlantic croaker

Sex Dependent Action of Aroclor 1254 on Basal and sGnRHa-Stimulated Secretion of LH from the Pituitary Cells of Common Carp, Cyprinus carpio L

Polychlorinated biphenyls (PCBs) affect the hypothalamic-pituitary-gonadal axis in many vertebrates, changing the hormonal regulation of reproduction. To identify one of the possible sites of action of PCBs on gonadotropin release in common carp, the direct effects of Aroclor 1254 on luteinizing hormone (LH) secretion from dispersed pituitary cells were investigated. Pituitary cells were obtained from sexually mature male and female common carp (Cyprinus carpio L.) at the time of natural spawning. The cells were incubated with different concentrations of Aroclor 1254 (5, 10, 50 and 100 ng mL–1 medium) and/or salmon gonadotropin-releasing hormone analogue (sGnRHa) at a concentration of 10−8 M. LH levels were measured in the cultured medium by the ELISA method after 10 hours of cell incubation. Incubation of male pituitary cells in the presence of tested concentrations of Aroclor did not change the basal LH secretion to the media. In the female pituitary cell incubations Aroclor (5, 10 and 100 ng mL–1 medium) caused a significant increase in LH concentrations in comparison to control incubations. In the case of sGnRHastimulated LH secretion in incubations of cells of both sexes, all the concentrations of Aroclor significantly stimulated LH release and potentiated stimulatory effects of sGnRH analogue. These results indicate that endocrine disrupters, such as Aroclor 1254, may affect reproduction in fish, acting also directly on gonadotrophs at the level of the pituitary gland, changing LH secretion.

Endocrine disrupting chemicals (EDCs) and the neuroendocrine system: Beyond estrogen, androgen, and thyroid

Hundreds of anthropogenic chemicals occupy our bodies, a situation that threatens the health of present and future generations. This chapter focuses on endocrine disrupting compounds (EDCs), both naturally occurring and man-made, that affect the neuroendocrine system to adversely impact health, with an emphasis on reproductive and metabolic pathways. The neuroendocrine system is highly sexually dimorphic and essential for maintaining homeostasis and appropriately responding to the environment. Comprising both neural and endocrine components, the neuroendocrine system is hormone sensitive throughout life and touches every organ system in the body. The integrative nature of the neuroendocrine system means that EDCs can have multi-system effects. Additionally, because gonadal hormones are essential for the sex-specific organization of numerous neuroendocrine pathways, endocrine disruption of this programming can lead to permanent deficits. Included in this review is a brief history of the neuroendocrine disruption field and a thorough discussion of the most common and less well understood neuroendocrine disruption modes of action. Also provided are extensive examples of how EDCs are likely contributing to neuroendocrine disorders such as obesity, and evidence that they have the potential for multi-generational effects.

Polychlorinated biphenyls and its potential role in endometriosis

With the progress of global industrialization and environmental deterioration, the relationship between human health and the living environment has become an increasing focus of attention. Polychlorinated biphenyls (PCBs, including dioxin-like polychlorinated biphenyls and non-dioxin-like polychlorinated biphenyls), as part of the organic chlorine contaminants, have been suspected as playing a role in the etiopathogenesis of endometriosis. Several population-based studies have proposed that exposure to PCBs may increase the risk of developing endometriosis, while some epidemiological studies have failed to find any association between PCBs and endometriosis. The purpose of this review is to discuss the potential pathophysiological relationship between endometriosis and PCBs with a focus on both dioxin-like polychlorinated biphenyls and non-dioxin-like polychlorinated biphenyls.

Comparative study of 17 β-estradiol on endocrine disruption and biotransformation in fingerlings and juveniles of Japanese sea bass Lateolabrax japonicus

Estrogenic contaminants in the aquatic environment are associated with endocrine disruption and feminization in fish. The effects of endocrine disrupting chemicals (EDCs) on fish have been well documented. However, very few studies have focused on 17 β-estradiol (E2) and its effects on endocrine system and biotransformation in a single prolonged exposure. This study investigated changes in the levels of serotonin (5-hydroxytryptamine) and acetyl choline esterase (AchE) in brain, cortisol in plasma and Ethoxyresorufin-O-deethylase (EROD) activity in gill of two different size groups (fingerlings and juveniles) of Japanese sea bass (Lateolabrax japonicus) upon exposure to two sub-lethal concentrations (200 and 2000 ng L(-1)) of E2 for 30 d. The results indicate that cortisol level and EROD activity significantly increased in both groups, whereas serotonin level increased in juveniles and decreased in fingerlings due to E2 exposure. The correlation analysis revealed that E2 significantly affected the endocrine and biotransformation systems in both age groups.

Patterns in stream fish assemblage structure and function associated with a PCB gradient

Stream fish assemblage structure and function were examined for significant response along a polychlorinated biphenyl (PCB) gradient from two PCB-contaminated streams (Clear Creek and Richland Creek watershed) at three locations and a control stream (Little Indian Creek), Indiana, USA. Fish were sampled in the summer months of 1995 and from 1999 to 2002. 51 fish assemblage attributes-including structure (i.e., fish composition) and function (i.e., trophic, reproductive, condition guilds), biomass, and index of biotic integrity (IBI) metric scores-were evaluated for significance according to an increasing PCB gradient. Eight biomass attributes of fish assemblages decreased with increasing PCB concentration: number of species biomass, number of sunfish biomass, percent sunfish biomass, number of sucker biomass, percent sucker biomass, biomass of sensitive species, percent sensitive species biomass, and percent carnivore biomass. Three biomass attributes increased with PCB concentration: percent minnow biomass, percent pioneer species biomass, and percent tolerant species biomass. Seven species composition and relative abundance characters decreased with increasing PCB concentration: number of species; number of darter, madtom, and sculpin; number of darter; number of sunfish; number of sucker; number of sensitive species; and percent individuals as carnivores. Percent individuals as pioneer species increased with increasing PCB concentration. Two IBI metrics, percent individuals as headwater species and number of minnow species, increased as PCB concentrations increased, whereas number of sucker species and percent individuals as pioneer species decreased with increasing PCB concentration class. We observed a direct response between decreased relative abundance and biomass of carnivores and increased relative abundance minnows as the PCB gradient increased. Total IBI score did not detect subtle changes to the fish community that were observed along a PCB gradient, whereas diagnostic analysis of the individual metrics did.

Neuroendocrine effects of endocrine disruptors in teleost fish

Because a large proportion of potential endocrine disruptors (EDC) end up in surface waters, aquatic species are particularly vulnerable to their potential adverse effects. Recent studies identified a number of brain targets for EDC commonly present in environmentally relevant concentrations in surface waters. Among those neuronal systems disrupted by EDC are the gonadotropin-releasing hormone (GnRH) neurons, the dopaminergic and serotoninergic circuits, and more recently the Kiss/GPR54 system, which regulates gonadotropin release. However, one of the most striking effects of EDC, notably estrogen mimics, is their impact on the cyp19a1b gene that encodes the brain aromatase isoform in fish. Moreover, this is the only example in which the molecular basis of endocrine disruption is fully understood. The aims of this review were to (1) synthesize the most recent discoveries concerning the EDC effects upon neuroendocrine systems of fish and (2) provide, when possible, the underlying molecular basis of disruption for each system concerned. The potential adverse effects of EDC on neurogenesis, puberty, and brain sexualization are also described. It is important to point out the future environmental, social, and economical issues arising from endocrine disruption studies in the context of risk assessment.

Tryptophan hydroxylase: a target for neuroendocrine disruption

Tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin (5-HT) synthesis, performs an essential role in the maintenance of serotonergic functions in the central nervous system (CNS), including regulation of the neuroendocrine system controlling reproduction. The results of recent studies in a teleost model of neuroendocrine disruption, Atlantic croaker, indicated that hypothalamic TPH is a major site of interference of hypothalamic-pituitary-gonadal function by environmental stressors. The effects of exposure to two different types of environmental stressors, low dissolved oxygen (hypoxia) and a polychlorinated biphenyl mixture (Aroclor 1254), on the stimulatory brain serotonergic system controlling reproductive neuroendocrine function in Atlantic croaker are reviewed. Exposure to both stressors produced decreases in TPH activity, which were accompanied by a fall in hypothalamic 5-HT and gonadotropin-releasing hormone (GnRH I) content in the preoptic-anterior hypothalamic area and were associated with reduction in luteinizing hormone (LH) secretion and gonadal development. Pharmacological restoration of hypothalamic 5-HT levels after exposure to both stressors also restored neuroendocrine and reproductive functions, indicating that the serotonergic system is an important site for hypoxia- and Aroclor 1254-induced inhibition of reproductive neuroendocrine functions. The mechanisms underlying downregulation of TPH activity by these stressors remain unclear but may involve alterations in hypothalamic antioxidant status. In support of this hypothesis, treatment with an antioxidant, vitamin E, was found to reverse the inhibitory effects of Aroclor 1254 on TPH activity. The results suggest that TPH is a major target for neuroendocrine disruption by diverse environmental stressors.

Effects of monocrotophos on the reproductive axis in the male goldfish (Carassius auratus): potential mechanisms underlying vitellogenin induction

Monocrotophos (MCP) is a highly toxic organophosphorus pesticide that has been banned in many countries. Both vitellogenin mRNA expression and secretion were significantly induced in male goldfish by exposure to an MCP-based pesticide, suggesting that MCP has significant estrogenic properties. To elucidate the mechanisms of action of MCP on vitellogenin induction, we used radioimmunoassay to examine the effect of MCP treatment on plasma 17beta-estradiol and testosterone levels in male goldfish (Carassius auratus). We also investigated the potential impacts of MCP treatment on aromatase expression, on the synthesis and secretion of pituitary gonadotropins and on the regulation of hypothalamic gonadotropin-releasing hormones by real-time PCR and radioimmunoassay. Experiments were carried out during the period of gonadal late recrudescence following a 21-day exposure to 0.01, 0.10 and 1.00 mg L(-1) of a pesticide containing 40% MCP in a semi-static exposure system. The results indicated that males in each MCP treatment group had much higher plasma levels of 17beta-estradiol, suggesting that the induction of VTG production by MCP was indirectly caused by elevated levels of endogenous 17beta-estradiol. MCP-induced plasma 17beta-estradiol levels via interference with the reproductive axis at multiple potential sites in male goldfish: (a) MCP exposure enhanced the mRNA expression of gonadal aromatase, the enzyme that converts androgens into estrogens, consequently reducing plasma levels of testosterone and increasing plasma concentrations of 17beta-estradiol; (b) MCP treatment increased follicle-stimulating hormone beta subunit mRNA expression and protein secretion and decreased luteinizing hormone beta subunit mRNA expression and protein secretion, thus interfering with gonadotropin synthesis and secretion at the pituitary level and leading to the disruption of reproductive endocrine control and androgen and estrogen balance.

Effects of monocrotophos on the reproductive axis in the female goldfish (Carassius auratus)

Monocrotophos (MCP) is a highly toxic organophosphorus pesticide. To elucidate the influence of MCP on female fish reproduction, plasma 17beta-estradiol, testosterone and gonadotropin levels and aromatase and gonadotropin beta subunit gene expression levels were examined in female goldfish (Carassius auratus) following a 21-day exposure to 0.01, 0.10 and 1.00 mg L-(1) 40% MCP-based pesticide in a semi-static exposure system. The results indicated that MCP induced increases in plasma 17beta-estradiol levels and the 17beta-estradiol/testosterone ratio via interference with the reproductive axis at multiple potential sites through two mechanisms: (a) MCP exposure enhanced the mRNA expression of gonadal aromatase, the enzyme that converts androgens into estrogens, consequently reducing plasma testosterone levels and increasing plasma concentrations of 17beta-estradiol; and (b) MCP treatment increased follicle-stimulating hormone beta subunit mRNA expression and secretion and decreased luteinizing hormone beta subunit mRNA expression and secretion, leading to the disruption of reproductive endocrine control and androgen and estrogen balance. This study provided convincing evidence for reproductive toxicology of MCP by disrupting of the HPG axis at multiple sites in female goldfish.

Changes in gene expression profile of medaka with acute toxicity of Arochlor 1260, a polychlorinated biphenyl mixture

Differential gene expression profiling was performed with a cDNA microarray in the liver tissue of the medaka fish, Oryzias latipes, after exposure to Arochlor 1260, a polychlorinated biphenyl (PCB) mixture, which is used as a coolant and insulating fluid for transformers and capacitors and is classified as a persistent organic pollutant. Twenty-six differentially expressed candidate genes were identified. The expression of 12 genes was up-regulated and that of 14 genes was down-regulated. These genes are associated with the cytoskeleton, development, endocrine/reproduction, immunity, metabolism, nucleic acid/protein binding, and signal transduction, or are uncategorized. The transcription of molecular biomarkers known to be involved in endocrine disruption (e.g., vitellogenins, choriogenins, and estrogen receptor alpha) was highly up-regulated. The same tendencies in gene expression changes were observed with real-time quantitative PCR (qRT-PCR) analysis, which was conducted to examine 12 selected candidate genes. These genes could be used as molecular biomarkers for biological responses to toxic chemicals, especially endocrine disrupting and carcinogenic chemical contamination in aquatic environments.

Alterations in the brain monoaminergic neurotransmitters of rainbow trout related to naphthalene exposure at the beginning of vitellogenesis

The contents of dopamine (DA), noradrenaline (NA), serotonin (5HT), and some related metabolites were studied in different brain regions of rainbow trout at two different stages of sexual maturation (at the beginning of vitellogenesis), after naphthalene (NAP) administration. The effects of NAP varied according to duration of exposure, brain region and vitellogenesis stage of the trout, and were more significant during previtellogenesis. The changes observed in DA metabolism were generally stimulatory after exposure for 3 h, and either stimulatory or inhibitory (depending on the brain regions) after exposure for 3 days to NAP. NA levels were altered by NAP in various brain regions, but only during previtellogenesis. With respect to 5HT, treatment with NAP reduced levels of the amine and/or its main metabolite in most of the brain regions studied, particularly 3 h after treatment. The results suggest that NAP might interfere with the processes regulating brain monoamine metabolism, either locally or indirectly by altering steroid feedback to brain centres, and thus disrupt endocrine control of reproductive development through the brain-pituitary axis.

Estrogenic environmental endocrine-disrupting chemical effects on reproductive neuroendocrine function and dysfunction across the life cycle

Endocrine disrupting chemicals (EDCs) are natural or synthetic compounds that interfere with the normal function of an organism's endocrine system. Many EDCs are resistant to biodegradation, due to their structural stability, and persist in the environment. The focus of this review is on natural and artificial EDCs that act through estrogenic mechanisms to affect reproductive neuroendocrine systems. This endocrine axis comprises the hypothalamic gonadotropin-releasing hormone (GnRH), pituitary gonadotropins, and gonadal steroid hormones, including estrogens. Although it is not surprising that EDCs that mimic or antagonize estrogen receptors may exert actions upon reproductive targets, the mechanisms for these effects are complex and involve all three levels of the hypothalamic-pituitary-gonadal (HPG) system. Nevertheless, considerable evidence links exposure to estrogenic environmental EDCs with neuroendocrine reproductive deficits in wildlife and in humans. The effects of an EDC are variable across the life cycle of an animal, and are particularly potent when exposure occurs during fetal and early postnatal development. As a consequence, abnormal sexual differentiation, disrupted reproductive function, or inappropriate sexual behavior may be detected later in life. This review will cover the effects of two representative classes of estrogenic EDCs, phytoestrogens and polychlorinated biphenyls (PCBs), on neuroendocrine reproductive function, from molecules to behavior, across the vertebrate life cycle. Finally, we identify the gaps of knowledge in this field and suggest future directions for study.

Development of quantitative real-time PCR assays for fathead minnow (Pimephales promelas) gonadotropin beta subunit mRNAs to support endocrine disruptor research

Fathead minnows (Pimephales promelas) are a widely-used small fish model for regulatory ecotoxicology testing and research related to endocrine disrupting chemicals (EDCs). Quantitative real-time PCR assays for measuring fathead minnow gonadotropin (GtH) beta subunit transcripts were developed and "baseline" transcript levels in pituitary tissue were examined over a range of age classes and spawning states. Among females, GtHbeta transcripts did not vary significantly with gonadal-somatic index or gonad stage. However, in males, follicle-stimulating hormone beta subunit transcripts decreased significantly with increasing gonad stage, while mean luteinizing hormone beta subunit expression trended in the opposite direction. GtHbeta transcript levels measured in pituitaries from fish that had spawned within the preceding 24 h were not significantly different from those from fish that were 2-3 days post-spawn. Exposure to the fungicide ketoconazole, a known steroidogenesis inhibitor, for 21 days significantly affected the abundance of GtHbeta transcripts in pituitary tissue in males, but not females. This study provides critical data needed to design and interpret effective experiments for studying direct and indirect effects of EDCs on GtH subunit mRNA expression. Results of such experiments should facilitate a greater understanding of integrated system-wide responses of the fathead minnow brain-pituitary-gonadal axis to stressors including EDCs.

Effects of acute and prolonged naphthalene exposure on brain monoaminergic neurotransmitters in rainbow trout (Oncorhynchus mykiss)

We have shown previously that acute (1 to 6 h) and prolonged (1 to 5 days) exposure of rainbow trout to naphthalene resulted in decreased plasmatic cortisol and 17-beta-estradiol levels. In order to elucidate the mechanisms through which naphthalene might disrupt endocrine regulation, the present study investigated whether brain monoaminergic neurotransmitters are altered by the action of this polycyclic aromatic hydrocarbon. In a first experiment, immature rainbow trout were injected with vegetable oil alone or containing naphthalene (10 and 50 mg/kg, i.p.), and sacrificed 1, 3 and 6 h after treatment. In a second experiment, slow-coconut oil implants alone or containing naphthalene (doses of 10 and 50 mg/kg) were i.p. located and fish sacrificed 1, 3 and 5 days after treatment. Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and noradrenaline (NA) were measured in several brain regions by HPLC. The results show that short-term naphthalene increases DA and 5-HT contents in hypothalamus and telencephalon, but differentially alter contents of the acid metabolites. Implants with naphthalene reduced DA content in hypothalamus and preoptic region but increased in telencephalon. 5-HT metabolism was decreased in hypothalamus, preoptic region, pituitary and brain stem after 3 to 6 days of treatment. In addition, the levels of NA were increased in hypothalamus and telencephalon after acute treatment and in hypothalamus and preoptic area after several days of exposure to naphthalene. These data suggest that brain neurotransmitter systems are sensitive to polycyclic aromatic hydrocarbons and could represent a target of the naphthalene-induced neuroendocrine disruption.

Contaminant-induced feminization and demasculinization of nonmammalian vertebrate males in aquatic environments

Many chemicals introduced into the environment by humans adversely affect embryonic development and the functioning of the male reproductive system. It has been hypothesized that these developmental alterations are due to the endocrine-disruptive effects of various environmental contaminants. The endocrine system exhibits an organizational effect on the developing embryo. Thus, a disruption of the normal hormonal signals can permanently modify the organization and future function of the male reproductive system. A wide range of studies examining wildlife either in laboratories or in natural settings have documented alterations in the development of males. These studies have begun to provide the causal relationships between embryonic contaminant exposure and reproductive abnormalities that have been lacking in pure field studies of wild populations. An understanding of the developmental consequences of endocrine disruption in wildlife can lead to new indicators of exposure and a better understanding of the most sensitive life stages as well as the consequences of exposure during these periods.

Expression of gonadotropin subunit genes following 4-nonylphenol exposure in masu salmon: Effects on transcript levels and promoter activities via estrogen receptor alpha

The 4-nonylphenol (NP) group is classified as some of the most potent endocrine disrupting chemicals (EDCs) reported to have estrogenic effects on reproductive endocrine system in vertebrates. In the present study, we investigated the effect of NP on expression of gonadotropin (GTH) subunit genes in masu salmon (Oncorhynchus masou) to clarify pituitary-based reproductive impact by EDCs. Female juvenile fish were injected with NP (a mixture of ring and chain isomers; 10 or 50 mg kg(-1) body weight) and maintained for 3 days post-injection. A semi-quantitative reverse transcription-polymerase chain reaction was used to analyze the pituitary GTHalpha, follicle-stimulating hormone beta (FSHbeta), and luteinizing hormone beta (LHbeta), and hepatic vitellogenin (VTG) mRNA levels. A low dose of NP induced the GTHalpha and LHbeta mRNA levels. High dose of NP slightly reduced FSHbeta mRNA levels in contrast to increased VTG mRNA levels. In a promoter study, NP (1-10 nM) increased the activities of luciferase reporter gene located downstream of masu salmon GTHalpha or LHbeta 5'-flanking region depending on the estrogen receptor alpha (ERalpha) in transiently transfected mammalian cells. In contrast, the luciferase activity of FSHbeta was elevated by NP in an ERalpha-independent manner. These results suggest that GTH subunit gene expression of masu salmon may be affected by EDCs at the transcription level and that the genes are useful markers for pituitary effects of xenoestrogens.

Effects of the maturation-inducing steroid on LH secretion and the GnRH system at different stages of the gonadal cycle in Atlantic croaker

The effects of treatment with the maturation-inducing steroid 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S) on luteinizing hormone-releasing hormone analog (LHRHa)-induced LH secretion were examined during several phases of the gonadal cycle in Atlantic croaker, Micropogonias undulatus. 20beta-S (1 and 5 microg/g of body wt) was administered by intraperitoneal (ip) injection, 24 h prior to injection with LHRHa (10-50 ng/g of body wt) and fish were bled 1 h after LHRHa injection. Treatment with both doses of 20beta-S resulted in plasma concentrations of the steroid within the normal physiological range for this species during final oocyte maturation and ovulation. The 20beta-S treatments altered the LH response to LHRHa throughout the reproductive cycle in both sexes, but the direction and magnitude of the response varied. 20beta-S treatment decreased the LH response to LHRHa in fish with recrudescing and fully recrudesced gonads and in females with regressed gonads. On the other hand, 20beta-S treatment significantly increased the LH response to LHRHa in males with regressing or regressed gonads. 20beta-S treatment also altered preoptic anterior hypothalamic (POAH) and pituitary seabream gonadotropin-releasing hormone (sbGnRH) contents, and the patterns of these changes were similar to those observed in LH secretion. The finding that moderate increases in plasma 20beta-S concentrations, similar to those occurring during final oocyte maturation, significantly inhibit the LH response to LHRHa at the end of the reproductive cycle suggests that this action of 20beta-S is of physiological importance during the periovulatory period. Moreover, the fact that concurrent changes occur in POAH and pituitary sbGnRH contents suggests that the actions of 20beta-S on LH secretion are at least partly mediated via the GnRH system.