Microcystin-leucine arginine causes brain injury and functional disorder in Lithobates catesbeianus tadpoles by oxidative stress and inflammation

Microcystin-leucine arginine (MC-LR) is a toxin commonly found in eutrophic waters worldwide, but its potential effects on amphibian brain toxicity and exposure mechanisms are unclear. In this study, Lithobates catesbeianus tadpoles were exposed to MC-LR for 30 days at realistic ambient concentrations (0, 0.5, and 2 µg/L) to reveal its effects on brain health. The MC-LR bioaccumulation in the brain increased in dependence on the concentration of MC-LR exposure. Exposure to 0.5 and 2 µg/L MC-LR resulted in a significant down-regulation of the expression of structural components of the blood-brain barrier (CLDN1), while the expression of genes associated with inflammation (NLRP3, TNF, IL-1β, and CXCL12) was significantly up-regulated with increased number of eosinophils. In the hippocampal and hypothalamic regions, the number of vacuolated neuropils increased with increasing MC-LR exposure concentration, while the expression of genes associated with neuronal development (LGALS1, CACNA2D2, and NLGN4X) and neurotransmitter transmission (SLC6A13 and AChE) was significantly down-regulated. Moreover, the levels of neurotransmitters (5-HT, glutamate, GABA, and ACh) were significantly reduced. These results provide strong evidence that MC-LR exposure at realistic ambient concentrations of 0.5 and 2 µg/L can break the blood-brain barrier and raise the accumulation of MC-LR in the brain tissue, causing structural damage and functional disorder to brain neurons. Further, based on transcriptomic and biochemical analysis, it was revealed that MC-LR exposure induces DNA damage through oxidative stress and may be an important pathway causing brain structural damage and functional disorder. Overall, this study demonstrates the significant effects of MC-LR on the brain tissue of amphibians, highlighting the sensitivity of amphibians to MC-LR.