Abstract

Paroxetine, a potent selective serotonin reuptake inhibitor (SSRI) frequently detected in aquatic environments, has raised concerns about its neurobehavioral impacts on non-target organisms. However, the neurobehavioral effects and underlying mechanisms at environmentally relevant concentrations are often overlooked. In this study, we investigated the chronic (28-day) effects of environmentally relevant PRX concentrations (100 and 500 ng/L) on the Western mosquitofish ( Gambusia affinis ) using an integrated multi-level approach. Exposure to PRX resulted in a non-monotonic restructuring of behavioral syndromes. Notably, 100 ng/L PRX produced the most significant alterations, concurrently reducing anxiety-like behavior and neophobia, yet impairing social preference, indicating a dissociation of typically correlated behavioral traits. This behavioral disruption was underpinned by a complex dysregulation of monoaminergic neurotransmission. At the neurochemical level, 100 ng/L PRX significantly elevated brain serotonin (5-HT) and dopamine (DA) levels, which normalized at 500 ng/L. Molecular analysis revealed coordinated gene expression changes, including downregulation of serotonin and dopamine transporters ( slc6a4a , slc6a3 ) and widespread downregulation of receptor genes ( htr1b , htr2c , drd1b , drd2a , drd3 ), suggesting a compensatory neuroadaptive response. Molecular docking confirmed PRX's high binding affinity for these key targets, providing a structural basis for the initiation of this adverse outcome pathway. Strong correlations were established between specific behavioral syndromes and the altered neurochemical and molecular profiles. Our findings underscore the critical need to incorporate behavioral syndromes and non-monotonic response models into the ecological risk assessment of psychoactive pharmaceuticals.