Thiram-induced notochord malformation in zebrafish embryos and its association with thyroxine

Tetramethyl thiram disulfide (thiram) is a dimethyldithiocarbamate (DTC) pesticide and is widely used in China, but its high residue has caused widespread concern. In this study, we used zebrafish (Danio rerio) embryos as animal models to explore the relationship between notochord curvature and thyroxine caused by thiram in zebrafish embryos. Zebrafish embryos at 3 hours post-fertilization (3 hpf) were exposed to low concentrations of thiram (0, 0.0024, 0.024, and 0.24 mg/L) for short-term (10, 30 min) or continuous exposure (1-4 d), and the relevant indicators were observed at 24, 48, 72, and 96 hpf. HPLC analysis results showed that the detectable concentration of thiram in zebrafish was 0.13 mg/kg (exposure from 3 hpf to 72 hpf). Morphological results showed that even short-term (30 min) thiram exposure increased mortality, delayed hatching, and caused notochord curvature in zebrafish embryos. Thiram exposure resulted in a decrease in gene expression of the thyroxine-related gene thyroid peroxidase (TPO). The detection of hormone levels indicated that thiram caused a decrease in the levels of tetraiodothyronine (T4) and triiodothyronine (T3). The results of this study indicate that one-time, short-term exposure to thiram still causes toxic effects on zebrafish embryos, and this effect may be related to thyroid hormones. This study also provides ideas for biological monitoring of dithiocarbamate pesticides.