Immersion exposure to okadaic acid triggers neurotoxic and hepatotoxic responses in zebrafish larvae (Danio rerio).

Okadaic acid (OA) has the potential to induce sublethal effects during the early life stages of fish, which may negatively affect natural populations. The aim of this study was to evaluate the hepatotoxic and neurotoxic effects of OA in 5-day post-fertilization zebrafish (Danio rerio) larvae exposed to concentrations spanning environmentally relevant levels and mechanistically informative doses. Hepatotoxicity was assessed by quantifying lipid droplet size in the liver, while neurotoxicity was evaluated through locomotor activity assays under light-dark cycle conditions and through molecular analysis of acetylcholinesterase (ache) and myelin basic protein (mbp) gene expression. The results demonstrate that OA induces both hepatotoxic and neurotoxic effects following direct immersion exposure; however, the exposure times required to elicit significant responses differ between endpoints. Hepatotoxicity, characterized by macrovesicular steatosis, was evident after 24 h of exposure, whereas neurotoxicity was observed after 80 min. Specifically, larvae exposed to OA exhibited reduced locomotor activity (hypoactivity) compared to unexposed controls, a pattern consistent with previous reports of impaired swimming performance in adult fish following OA ingestion. Together, these findings suggest that chronic exposure to sublethal concentrations of OA during the larval stages may induce progressive multi-system damage, potentially compromising not only individual survival but also behavioral performance, with implications for population dynamics in natural environments.