Abstract
This study aimed to evaluate the response mechanisms of zebrafish larvae to Norfloxacin nicotinate (NOR-N) exposure. Embryos were exposed to NOR-N from 4 h post-fertilization (hpf) until 96 hpf. The exposure concentrations included 0.002, 0.2, 1, and 5 mg/L (simulating both normal and exceptionally high environmentally relevant levels of NOR), as well as a high dose of 25 mg/L. Subsequent analyses focused on apoptosis, neurodevelopment, and DNA methylation in the resulting zebrafish larvae. The results showed that high-dose NOR-N (≥5 mg/L) induced obvious apoptotic cell death in zebrafish larvae, accompanied by increased activities of Cas3 and Cas9, up-regulated P53, Bax, Puma, Apaf1, Cas3 and Cas9 genes expression, and reduced Mdm2 levels and Bcl2/Bax ratio. Moreover, exposure to 5 and/or 25 mg/L NOR-N resulted in a significant up-regulation of neurodevelopment-related genes (Sox2, Sox3 and Sox19a), concomitantly with a marked decline in the transcription of DNA methylation genes, including Dnmt1, Dnmt3a1, Dnmt3b1, Dnmt3b2 and Dnmt3b4. Overall, our findings demonstrated that NOR-N exposure could induce apoptosis, developmental neurotoxicity and aberrant DNA methylation in zebrafish larvae. These findings provide insights to guide the safe application of NOR-N in aquaculture and support the assessment of its potential ecological risks to aquatic ecosystems.