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Effects of Bifenthrin on the Estrogenic and Dopaminergic Signaling Pathways in Fish


Bifenthrin (BF) is a pyrethroid insecticide used in urban and agricultural applications. Previous studies have shown that environmentally relevant (ng/L) concentrations of BF increased plasma concentrations of 17β-estradiol (E2) and altered the expression of dopaminergic (DA) pathway components. DA neurons can indirectly regulate E2 biosynthesis, suggesting that BF may disrupt the hypothalamic-pituitary-gonadal (HPG) axis. This study sought to answer the hypothesis that BF affects dopaminergic and E2 signaling pathways through a unique target, causing in vivo estrogenic activity in fish. Since embryos do not have a complete HPG axis, the hypothesis that BF impairs DA regulation was tested in embryonic and one-month-old juvenile zebrafish (Danio rerio) with exposure to measured concentrations of 0.34 and 3.1 μg/L BF for 96 hours. Significant decreases of TH and DR1 transcripts, and HVA levels, as well as ratios of HVA/DA and HVA+DOPAC/DA in zebrafish embryos, were observed after BF treatment. In juveniles, a significant increase in the expression of ERβ1 and the DOPAC/DA ratio was noted. These results show a possible anti-estrogenic effect of BF in embryos, and estrogenicity in juveniles. To examine the role of the estrogen receptor (ER) in BF’s toxicity, embryos were exposed for 96 hours to a mixture of BF and an ER agonist (17α-ethynylestradiol – EE2). Results showed that only exposure to EE2 had a significant effect on gene expression and DA, HVA, and DOPAC levels after co-exposure with BF. Additionally, ERα knockdown did not rescue BF effects in embryos of zebrafish. Results demonstrated that ER might not be the target of BF’s toxicity at the zebrafish embryonic stage. Furthermore, the hypothesis that BF impairs sex differentiation was tested in larval and embryonic Japanese Medaka. Fish were exposed to BF, an ER antagonist (ICI 182,780), and an ER agonist (E2). Results showed a trend towards masculinization of Japanese Medaka exposed at the larval stage for 30 days to the lowest concentration of BF. However, genotypic gender ratios were not altered. Exposures at embryonic stages of development also did not alter phenotypic gender. These results show sex differentiation was not significantly altered by embryonic and larval exposure to BF. Overall, these results will be very useful in risk assessment strategies and inform studies of developmental mechanisms of toxicity of bifenthrin.

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