Microcystin-LR (MC-LR) is the most prevalent and toxic microcystin congeners, posing a significant threat to aquatic organisms as well as humans; however, its underlying toxic mechanisms remain incompletely elucidated. In this study, the negative impacts of MC-LR and the underlying mechanisms in zebrafish larvae were investigated. The results demonstrated that MC-LR could penetrate zebrafish larvae and induce developmental toxicity, characterized by reduced heart rate, decreased body length, and smaller eye area. H&E staining revealed that MC-LR exposure significantly reduced the thickness of retinal layers. qPCR analysis showed altered expression levels of phototransduction and retinoic acid metabolism related genes (rho, gnat1, gnat2, opn1sw1, opn1lw1, opn1mw1, rdh1, rbp4, cyp26a1, and aldh1a2). These findings suggest that MC-LR may disrupt retinal structure and impair normal visual function in larvae. Behavioral analyses indicated that MC-LR exposure weakened spontaneous movements in embryos and impaired swimming ability in larvae, potentially due to significant alterations in the levels of glutamate, γ-aminobutyric acid, and brain-derived neurotrophic factor. Additionally, MC-LR exposure reduced visuomotor responses, delayed reactions to external stimuli, and disrupted circadian rhythms, which may be attributed to altered expression levels of circadian rhythm-related genes (clock1a, bmal1a, per1b, cry1a, and per2), as well as changes in melatonin and arylalkylamine N-acetyltransferase 2 levels. Overall, these findings indicate that MC-LR exposure induce developmental neurotoxicity in zebrafish, and that impaired visual function and disrupted circadian rhythm could serve as key contributing factors to MC-LR-induced behavioral abnormalities, which warrant further emphasis in future ecological and health risk assessments.
