Combined neurotoxicity of aged microplastics and thiamethoxam in the early developmental stages of zebrafish (Danio rerio)

Microplastics (MPs) pollution, together with its consequential effects on aquatic biota, represents a burgeoning environmental concern that has garnered significant scholarly attention. Thiamethoxam (TMX), a prevalently utilized neonicotinoid insecticide, is renowned for its neurotoxic impact and selective action against targeted pests. The aquatic environment serves as a receptacle for numerous pollutants, such as MPs and neonicotinoid insecticides. However, there is currently a paucity of comprehensive understanding regarding the toxic effects of co-exposure to aged MPs and neonicotinoid insecticides in aquatic organisms. Therefore, we endeavor to elucidate the deleterious impacts of aged PS and TMX on zebrafish (Danio rerio) larvae when present at concentrations typical of natural environments, and to unravel the underlying molecular mechanisms driving these effects. Our study revealed that exposure to aged PS, TMX, or their combination notably impeded the heart rate and locomotion of zebrafish larvae, with a pronounced effect observed under combined exposure. Isolated instances of aged PS or TMX contamination were found to diminish the activity of antioxidative enzymes, including SOD, CAT, and GST, elevate MDA levels, and disrupt neurotransmitter homeostasis (5-HT, GABA and Ach). Notably, the mixtures exhibited a synergistic effect. Moreover, gene expression pertinent to oxidative stress (e.g., gstr1, gpx1a, sod1, cat1, p38a, ho-1, and nrf2b) and neurotransmission (e.g., ache, ChAT, gat1, gabra1, 5ht1b, and 5ht1aa) was significantly altered upon co-exposure to aged PS and TMX in larval zebrafish. In summary, our findings supporting the harmful effects of aged MPs and the neonicotinoid insecticides they loaded on aquatic organisms. Results from this study enhance our understanding of the biological risks of MPs and insecticides, as well as help fill existing knowledge gaps on neonicotinoid insecticides and MPs coexistence toxicity in aquatic environment.