Morphological and Transcriptomic Analyses Reveal the Toxicological Mechanism and Risk of Nitrate Exposure in Bufo gargarizans Embryos.

Simple Summary: The ingestion of excessive nitrate can affect the thyroid gland and cause thyroid dysfunction in humans. In the present study, amphibian embryos were exposed to nitrate, thyroxine and methimazole (a thyroid peroxidase inhibitor) during embryonic development to further explore the effects of nitrate on the thyroid. The results showed that nitrate, thyroxine and methimazole inhibited embryo growth and development. Additionally, methimazole and high concentrations of nitrate downregulated the genes related to thyroid morphogenesis and cholesterol metabolism, while upregulating the genes related to inflammation and apoptosis. These suggested that nitrate not only damaged the thyroid gland, but also affected the formation of the thyroid, thus affecting embryonic development. In recent years, nitrate (NO3-N) pollution in water bodies has been increasing due to the excessive use of nitrogen-based fertilizers. Exposure to NO3-N during the development of amphibian embryos may have lasting effects on the growth and development of individuals and even threaten their survival, but the toxicity mechanism of NO3-N in amphibian embryos prior to thyroid morphogenesis remains unclear. In the present study, Bufo gargarizans was selected as the model organism to investigate the toxic effects of 10 mg/L and 100 mg/L NO3-N exposure (N10 and N100) on amphibian embryos using methimazole (MMI) and exogenous thyroxine (T4) as the reference groups. We found that T4, MMI, N10 and N100 inhibited B. gargarizans embryo growth and development, with MMI and N100 showing the earliest and strongest effects. Transcriptome analysis revealed that MMI and NO3-N (especially N100) significantly downregulated genes related to thyroid morphogenesis and cholesterol metabolism, while upregulating genes related to inflammation and apoptosis. Together, these results contribute to a deeper understanding of the complex mechanisms by which NO3-N disrupts B. gargarizans embryonic development, reveal the potential risks of NO3-N pollution to other aquatic organisms, and provide insights into the conservation of a broader ecosystem. [ABSTRACT FROM AUTHOR]