Combined exposure to microplastics and cetylpyridinium chloride disturbs the sociability and brain monoamine neurotransmitters in juvenile zebrafish.

Although the co-occurrence of microplastics (MPS) and cetylpyridinium chloride (CPC) is common in natural aquatic ecosystems, our understanding of their combined neurotoxic effects on fish remains limited. In this study, we exposed juvenile zebrafish (Danio rerio) to MPS (2 μm, 40 μg/L), CPC (30 μg/L), and their mixture (MIX) for 21 days to investigate responses in behavioral traits and central monoaminergic systems. Furthermore, the interaction pattern between MPS and CPC was cross-validated using Generalized Linear Models (GLM) and Independent Action (IA) models. The results showed that exposure to MPS and/or CPC profoundly impaired zebrafish sociability, with the co-exposure inducing a more pronounced detrimental effect through an additive joint action. Although single-substance exposure to MPS or CPC did not significantly affect brain levels of serotonin (5-HT) or 5-hydroxyindoleacetic acid (5-HIAA), combined exposure significantly reduced the 5-HIAA/5-HT ratio through a synergistic effect. Regarding the dopaminergic system, the most severe inhibition of the dopamine turnover ratio was observed in the co-exposure scenario, driven by the additive interaction of MPS and CPC. At the transcriptional level, combined exposure to MPS and CPC induced a distinct synergistic downregulation of the mao gene, alongside antagonistic suppression of dbh and group-specific reductions in drd3 and htr1b expression. Spearman correlation analysis revealed that social behavior deficits were associated with suppressed dopaminergic metabolic activity and altered transcriptional profiles. These findings highlight the neurotoxic potential of these co-occurring pollutants in aquatic ecosystems.