| The thyroid hormones (THs)3,3’,5-triiodo-L-thyronine (T3) andL-thyroxine (T4)exert a wide range of biological effects on physiological processes of fish. Toelucidate the thyroid disrupting effects of monocrotophos (MCP), anorganophosphate pesticide, on adult goldfish (Carassius auratus), plasma total T3(TT3), total T4(TT4), free T3(FT3), and free T4(FT4) levels, and the mRNAexpression of indices involved in the hypothalamic-pituitary-thyroid axis (HPT axis)were examined, by which effects of MCP pesticide on the HPT axis and theirunderlying mechanisms were evaluated in both hormone levels and genetranscription levels. Furthermore, it is inferred that there might be compositemechanisms of action of MCP on the endocrine disruption in fish. Thus, in order toevaluate the cross-talk between disruption of these two axes, we also investigated theeffect of MCP on plasma levels of THs and sex hormones, and elucidated thepotential underlying mechanisms by assessing the changes in endocrine-mediatedresponses along the HPT axis, including the regulation of thyrotropin-releasinghormone (TRH) and corticotropin-releasing hormone (CRH), synthesis and secretionof thyroid-stimulating hormone (TSH), and expression of deiodinases (d1, d2, andd3), as well as endocrine-mediated responses along the hypothalamic-pituitary-gonad axis (HPG axis), including the regulation of gonadotropic releasing hormone(GnRHs), synthesis and secretion of gonadotropins (GtHs), and expression of gonadaromatase. These experimental data would be helpful in providing a furthertheoretical basis for ecological risk assessment, especially regarding certainchemicals, such as MCP, which could simultaneously cause a variety of toxic effectsin different endocrine systems of fish. The results revealed that:(1) Following21-day exposure to0.01,0.10, and1.00mg/L of a40%MCP-based pesticide, significant hyperplasia with increased number of epithelial cells,mild colloid depletion, and moderate hypertrophy (follicle with low columnarepithelium) were observed in thyroid follicles of male goldfish. Plasma THs levelswere examined by the radioimmunoassay (RIA). In males,0.01,0.10, and1.00mg/Lof MCP pesticide caused significant decreases in plasma TT3levels but not in TT4levels, which also resulted in decreased plasma TT3-to-TT4ratios; plasma FT3levelswere decreased at0.01and1.00mg/L, while plasma FT4levels were decreased at0.10mg/L. In females, MCP pesticide also caused reduced plasma TT3levels and adose-related decrease in the ratio of TT3to TT4; plasma FT3levels were decreased inall MCP treatments, while unlikely as in males, plasma FT4levels were increased at0.01mg/L but decreased at0.10mg/L. This provided convincing evidence fordisruption of MCP on thyroid, by which TH homeostasis was disturbed in both malesand females.(2) Quantitative real-time PCR was employed to measure the expression of genesinvolved in the HPT axis of goldfish following a21-day exposure to0.01,0.10and1.00mg/L40%MCP-based pesticide. Profiles of the changes in the relativeabundance of deiodinase (type I deiodinases, D1; type II deiodinases, D2; and type IIIdeiodinases, D3) transcripts were observed in the liver, brain, and kidneys, duringMCP exposure. An increase in the metabolism of T3, expressed as highly elevatedhepatic d1and d3mRNA levels in both male and female goldfish, might beassociated with the reduction in plasma TT3levels in both the0.01and0.10mg/Lgroups, while in the1.00mg/L MCP group, inhibited hepatic d2transcripts mighthave also resulted in decreased TT3levels in males by preventing the activation of T4to T3, and stimulation of hepatic d3transcripts also resulted in declined TT3levels infemales due to the increased metabolism of T3. As a compensatory response todecreased T3levels, pituitary thyroid-stimulating hormone β subunit mRNAtranscription was up-regulated by the MCP pesticide. Decreases in plasma FT3levelswere also correlated with the modulation of hepatic transthyretin mRNA expression.The responses of hypothalamic trh and crh mRNA to MCP exposure are very complicated, and hence, elucidating the thyroid-disrupting properties of this pollutantmight not be possible.(3) Disturbed plasma TT3, TT4,17β-estradiol (E2), and testosterone (T) levelswere observed by RIA in goldfish exposed to0.004,0.040, and0.400mg/L MCP(purity=99.3%) for2,4,8, and12days, and we also found that treatment with MCPresulted in altered transcriptional profiles of the thyroidal axis and reproductive axisby real-time PCR. Although results of this short-time test were not consistent withthose of the21-d exposure test, one could deduce that there were interactionsbetween the the thyroidal axis and reproductive axis in goldfish after MCP exposure.It was hypothesized that the inhibited hepatic d2and d3mRNA expression levelsmight be, at least partly attributed to the increased plasma E2levels following2-and4-d MCP exposure. Besides, stimulated transcriptions of gonad aromtase weresupposed to be associated with the decreased plasma T3levels induced by MCP,revealed by the decreased mRNA expression levels of thyroid hormone receptor(TRα-1and TRβ).In summary, this study indicated that MCP pesticide at thyroid-disrupting levelscould perturb circulating TH homeostasis in male and female goldfish, and thispesticide can most likely impair certain TH-dependent physiological functions. ThemRNA expressions of genes regulating TH synthesis, transport, and metabolism atmultiple sites of the HPT axis might serve as sensitive biomarkers underlying MCPpesticide exposure. Especially, there were changes in tissue-specific profiles of therelative abundance of deiodinase transcripts in the liver, brain, and kidney. Besides,MCP, the effective component of this pesticide, caused disruption of plasma THs andsex hormone levels and modulation of transcriptional profiles of the thyroidal axis andreproductive axis in a short time. Our findings would be helpful to predict interactionsbetween disturbance of HPT axis and HPG axis in goldfish after the MCP exposure,although in-vitro study should be exercised to identify this cross-talk underlying theMCP-induced effect. |
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