Co-exposure with titanium dioxide nanoparticles exacerbates MCLR-induced brain injury in zebrafish

Owing to the eutrophication in freshwater and industrial emissions, the detected concentrations of MCLR and nano-TiO₂ in nature water increase year by year. The purpose of this study was to evaluate the joint effect of microcystin-LR (MCLR) and titanium dioxide nanoparticles (nano-TiO₂) on the zebrafish brain and to investigate the underlying mechanisms. In this study, four-month old zebrafish were exposed to 0, 0.5, 4, and 32 μg/L MCLR and MCLR-co-nano-TiO₂ (100 μg/L) for 45 days. Obvious brain injury characterized by formation of glial scars and ventriculomegaly was observed in both MCLR groups and MCLR-co-nano-TiO₂ groups. In addition, our results showed the existence of nano-TiO₂ aggravated MCLR-induced abnormity of swimming behavior and social behavior of zebrafish. To clarify the mechanisms of nano-TiO₂ aggravated MCLR-induced brain injury, we firstly examined the reactive oxygen species (ROS) generation in the zebrafish brain. The results showed that co-exposure with nano-TiO₂ could further increase ROS content compared with MCLR only groups. We also detected a significant change of lipid peroxidation products (MDA, malondialdehyde) content, antioxidant enzyme (SOD, superoxide dismutase) activity, and non-enzymatic antioxidant (GSH, glutathione) content in MCLR-co-nano-TiO₂ groups. Transcriptional analysis indicated the expression of genes related to the antioxidant system was significantly altered in the zebrafish brain. Collectively, the observations in this study showed that the existence of nano-TiO₂ could exacerbate the damage of the zebrafish brain through the aggravation of MCLR-induced oxidative stress, ultimately leading to the abnormity of swimming behavior and social behavior.