An unwanted prescription: Trace fluoxetine exposure disrupts the enteric nervous system, gut-liver axis and somatic growth in zebrafish (Danio rerio).

Fluoxetine (FLX), a widely prescribed selective serotonin reuptake inhibitor, frequently enters aquatic ecosystems through human excretion and improper disposal, reaching organisms at trace but biologically active concentrations. As FLX disrupts multiple physiological pathways in aquatic species, we aimed to describe its chronic effects on the enteric nervous system (ENS), serotonergic enteric neurons (SENs), gut-liver axis, and somatic growth at trace concentrations (0.5, 5, and 50 ng L-1) on adult zebrafish (Danio rerio). Fish were exposed for 21 days, and gut mucosa, liver, ENS, and growth parameters were assessed through histopathology, immunohistochemistry, and morphometric analyses. All FLX concentrations induced pronounced disruption of the ENS, reducing HuC/D+ cells density by more than 30%. SEN modulation was concentration-dependent: 5 ng L-1 increased SEN prevalence via selective loss of other neuronal phenotypes, while 50 ng L-1 increased SEN density itself. Gut mucosa exhibited inflammatory remodeling at 0.5 and 5 ng L-1, with significant goblet cell and intraepithelial lymphocyte hyperplasia and epithelial thickening, indicating chronic mucosal activation and potential barrier dysfunction. Hepatic alterations occurred in a concentration-dependent manner, including hepatocyte hypertrophy, glycogen dysregulation, vascular injury, and reduced hepatosomatic index, revealing disruption of the gut-liver axis. At the organismal level, all concentrations reduced body length, while only 50 ng L-1 increased body mass, resulting in a concentration-dependent increase in Fulton's condition largely driven by impaired linear growth. In conclusion, FLX caused profound neurogenic, immunological, metabolic, and organismal disturbances, with the lowest and intermediate concentrations eliciting the strongest effects. This non-monotonic toxicity shows that trace FLX acts as a potent disruptor of ENS integrity, serotonergic signaling, intestinal homeostasis, hepatic metabolism, and somatic growth, ultimately compromising gut-brain and gut-liver axes communication. These findings highlight the ENS as a highly sensitive biomarker of pharmaceutical pollution and reinforce the need to reassess environmental safety thresholds for antidepressants.