Bifenthrin (BF) is a pyrethroid insecticide widely used in urban and agricultural
applications. Previous studies in embryos of zebrafish have shown that BF can affect
estradiol biosynthesis and the dopaminergic system. To examine the role of the estrogen
receptor (ER) in the endocrine effects of BF, embryos were exposed for 96 h to a mixture
of 0.15 and 1.5 μg/L BF and an ER agonist (17α-ethynylestradiol – EE2) at 0.09 μg/L.
Transcripts related to estrogenic (vitellogenin VTG) and dopaminergic (tyrosine hydroxylase
(TH), dopamine receptor 1 (DR1), monoamine oxidase (MAO), and catechol-O-methyltransferase
b (COMTb)) signaling pathways were investigated by qRT-PCR. Dopamine (DA) and its
metabolites (homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC)) were
also measured. There was a significant increase in VTG, DR1, MAO and COMTb mRNA levels
and HVA-DA ratios within all zebrafish embryos exposed to EE2, including EE2 alone,
0.15 μg/L BF + EE2 and 1.5 μg/L BF + EE2. A significant decrease in homogenate concentrations
of DA was observed within all zebrafish embryos exposed to EE2, which included EE2
alone, 0.15 μg/L BF + EE2 and 1.5 μg/L BF + EE2. Co-exposure of BF with EE2 failed
to diminish estrogenic or dopaminergic signaling in embryos. Additionally, embryos
with diminished ERα expression by morpholino injection were exposed to 0.15 μg/L BF,
1.5 μg/L BF and 0.09 μg/L EE2, with subsequent gene expression measurements. ERα knockdown
did not prevent the effects of BF, indicating ERα may have a limited role in the estrogenic
and dopaminergic effects caused by BF in zebrafish embryos.