Impact of co-exposure with butachlor and triadimefon on thyroid endocrine system in larval zebrafish

IntroductionButachlor (BTL) and triadimefon (TDF), the widely used herbicide and fungicide, are unavoidable enter into the aquatic environment. However, there were limited study regarding to the joint toxicity of these two pesticides on fish at present.AimTo evaluate the potential thyroid-disrupting toxicity and exposed to different concentrations of BTL mixed with TDF.Materials and methodsZebrafish embryo (n = 3) were exposed to 0.01 and 0.05 fold of LC50 from the acute joint toxicity test, of which 0.32 mg/L (BTL) and 9.41 mg/L (TDF) for single or mixture agents (BTL: 0.0064 mg/L, 0.032 mg/L; TDF: 0.1882 mg/L, 0.9410 mg/L; co-exposure: 0.0032 mg/L BTL + 0.0941 mg/L TDF, 0.016 mg/l BTL + 0.4705 mg/L TDF) after 10-day post-fertilization. Hatching, malformation, survival rates and thyroid hormones (THs), genes expression involved in HPT-axis of embryos were measured and detected in control and separately/co-exposure treatments. THs contents were evaluated by ELISA kit and the expression levels of genes were determined by RT-PCR.ResultsHatching, malformation and survival rates of embryos exposed to single BTL exhibited no statistically significant difference from the control besides decreased of high concentration in survival rates. Exposure to TDF reduced hatching, survival rate and increased malformation. The combined exposure to BTL and TDF resulted in greater adverse effects on embryonic development. BTL exposure significantly increased free T3 and T4 contents. Elevated free T3 content was also observed in the larvae exposed with single BTL. Co-exposure of the two pesticides caused greater enhanced of T3 and T4 levels. Furthermore, gene data showed BTL up-regulated the mRNA expression of tpo, tshβ, tg, ttr, dio2, TDF up-regulated the mRNA expression of tpo, trα, ttr, dio2 and down-regulated trβ gene. The mixture of the two pesticides caused up-regulation mRNA expression of trα, trβ, tg, ttr, dio2.ConclusionBTL and TDF resulted in adverse effects on zebrafish embryonic development and caused thyroid endocrine disruption, BTL and TDF have a synergistic effect on development and thyroid endocrine by enhanced level of thyroid hormone.