Selective serotonin reuptake inhibitor (SSRI) antidepressants consumption has increased significantly worldwide leading to its environmental dissemination. Their frequent and increasing detection in different environmental matrices is thus an emerging environmental concern as these drugs are biologically active substances that, although designed to modulate human behaviour, have the ability to alter fish behaviour, physiology, and gene expression, even at low concentrations. The available studies with paroxetine are scarce and there is little information on how chronic exposure to SSRIs may influence the response to other environmental contaminants like nanoplastics (NPLs), which is an environmentally relevant condition as SSRIs do not exist alone in the environment. Accordingly, in this study, the effects of zebrafish 21 days exposure to 0, 40, and 400 µg/L of the antidepressant paroxetine (PAR) was assessed by studying fish behaviour. Moreover, this study also assessed how chronic exposure to PAR would affect the response to acute exposure to 1 and 10 µg/L of 50 nm polymethylmethacrylate NPLs (PMMA-NPLs).

Overall, chronic exposure to PAR decreased fish basal locomotor activity during both light and dark periods and interfered with stress response. When considering fish that were pre-exposed to PAR and then subjected to PMMA acute exposure, a significant interaction between PAR and PMMA-NPLs was found when considering fish swimming behaviour under light conditions. These results highlight the ability of SSRIs to modulate effects of other contaminants and the need to consider organisms’ previous contamination history.