Micro- and nanoplastic exposure as an emerging risk factor for depressive-like phenotypes across species: a systematic review.

The pervasive environmental presence of micro- and nanoplastics (MNPs), combined with their ability to cross biological barriers and accumulate in the brain, has raised significant concerns regarding their neurotoxic potential. While neuroinflammatory and neurodegenerative effects are increasingly documented, a focused synthesis of their impact on depression-relevant behaviors is lacking. This systematic review evaluates the evidence linking MNPs exposure to depressive-like phenotypes across species. Following PRISMA guidelines, nine animal studies and one human cross-sectional study met the inclusion criteria. Rodent models exposed to MNPs, primarily polystyrene, showed consistent induction of depressive-like behaviors, whereas aquatic models exhibited neurobehavioral alterations compatible with depression-relevant phenotypes. Effects were dose-dependent and influenced by particle characteristics, exposure window, and sex. Preliminary human data suggest a positive association between estimated environmental MNPs exposure and depressive symptoms. Although evidence remains limited and methodologically heterogeneous, converging findings across experimental models indicate that MNPs exposure may represent a potential emerging risk factor for depression-related outcomes, possibly mediated by neuroinflammatory processes and neuronal damage. Future research requires standardized behavioral assessments, environmentally relevant exposure models, and longitudinal human studies with direct exposure biomonitoring to establish causality and public health relevance.