Persistent and Dose-Dependent Neural and Metabolic Gene Expression Changes Induced by Transient Citalopram Exposure in Zebrafish Embryos

Citalopram, a common selective serotonin reuptake inhibitor (SSRI), has been increasingly detected in aquatic environments due to ineffective removal and improper disposal. Although developmental exposure to SSRIs is linked to neurotoxicity, little is known about the persistence of gene expression alterations following limited exposure periods. Zebrafish embryos were exposed from 2 to 24 h post-fertilization (hpf) at concentrations of citalopram hydrobromide spanning surface water to therapeutic serum levels (0.03, 0.9, 50, and 250 μg/L), followed by removal of the citalopram and development until 48 hpf. Whole-embryo RNA sequencing at 48 hpf revealed a non-linear dose–response wherein the lowest dose resulted in the induction of the highest number of differentially expressed genes (DEGs). Gene set enrichment analyses (GSEA) and overrepresentation analyses (ORAs) showed that 0.03 μg/L citalopram caused upregulation of metabolic and developmental pathway genes, but suppressed synaptic membrane genes, whereas 0.9 μg/L resulted in strong downregulation of key neurotransmitter receptors. At 50 μg/L, genes linked to oxidative stress (glutathione metabolism and ferroptosis) were upregulated, and at 250 μg/L, stress and apoptotic processes were increased, while glutamate receptor genes were repressed. All four citalopram doses suggested synaptic and neurotransmitter alterations, implying that persistent neurodevelopmental impacts resulted from a limited early window of exposure. These data highlight that transient, low-level SSRI exposures shape long-term embryonic gene expression.