Endocrine-disrupting effects and reproductive toxicity of low dose MCLR on male frogs (Rana nigromaculata) in vivo

• Low-dose MCLR (1 μg/L) elicits a potential ecological effect on amphibian populations.
• MCLR can induce abnormal sperm morphologies and activities on male frogs.
• MCLR can induce a decrease in serum testosterone and an increase in serum estradiol of male frogs.
• MCLR can increase SF-1 protein levels and decrease P450 aromatase levels in the gonads of frogs.

Toxic cyanobacterial blooms are potential global threats to aquatic ecosystems and human health. The World Health Organization has set a provisional guideline limit of 1 μg/L microcystin-LR (MCLR) in freshwater. However, MCLR concentrations in several water bodies have exceeded this level. Despite this recommended human safety standard, MCLR-induced endocrine-disrupting effects and reproductive toxicity on male frog (Rana nigromaculata) were demonstrated in this study. Results showed that sperm motility and sperm count were significantly and negatively correlated with exposure time and concentration. By contrast, abnormal sperm rate was positively correlated with both parameters. Ultrastructural observation results revealed abnormal sperm morphologies, vacuoles in spermatogenic cells, cell dispersion, incomplete cell structures, and deformed nucleoli. These results indicated that MCLR could induce toxic effects on the reproductive system of frogs, significantly decrease testosterone content, and rapidly increase estradiol content. Prolonged exposure and increased concentration enhanced the relative expression levels of P450 aromatase and steroidogenic factor 1; thus, endocrine function in frogs was disrupted. This study is the first to demonstrate in vivo MCLR toxicity in the reproductive system of male R. nigromaculata. This study provided a scientific basis of the global decline in amphibian populations.