[Comparison on neurotoxicity and immunotoxicity between 6PPD and 6PPD-Q in zebrafish under equimolar exposure...

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its derivative N-(1,3-dimethylbutyl)-N'-phenyl-p-benzoquinone (6PPD-Q) have been widely detected in the environment and pose potential threats to ecosystems and human health. Given that 6PPD can be equimolarly converted to 6PPD-Q in the environment and organisms, the toxicity differences and molecular mechanisms of action between the two under equimolar exposure remain unclear. This study systematically compared the neurotoxicity and immunotoxicity of the two substances under equimolar conditions. The results from the zebrafish model showed that both 6PPD and 6PPD-Q induced excessive production of reactive oxygen species (ROS) and inhibited the expression of antioxidant enzymes, leading to oxidative damage and immune dyshomeostasis. Meanwhile, they activated the innate immune system and caused an increase in immune cells. In terms of neural development, both disturbed the expression of neurofunctional genes and induced malformations such as pericardial edema, delayed swim bladder closure, and spinal curvature. Notably, at environmentally relevant concentrations, the two showed similar acute toxicity. However, at sublethal levels, 6PPD-Q exhibited stronger toxicity, with oxidative damage, immunotoxicity, teratogenicity, and neurotoxicity being 1-3 times higher than those of 6PPD, and it significantly impaired the sensory and motor abilities of larval fish. The results of real-time quantitative PCR (RT-qPCR) indicated that both substances regulated the expression of neuro- and inflammation-related genes in a dose-dependent pattern. The gene ontology (GO), Kyoto encyclopedia of genes and genomes (KEGG), disease ontology (DO), and hub gene analyses further revealed differences in the molecular-level action focuses between 6PPD and 6PPD-Q. This study provides new evidence for the identification and early warning of the potential risks of 6PPD and 6PPD-Q at environmental and sublethal levels.