Waterborne exposure to microcystin-LR causes thyroid hormone metabolism disturbances in juvenile Chinese rare minnow (Gobiocypris rarus).

Microcystin-LR (MC-LR) has the potential to disturb thyroid hormone homeostasis, but little is known about the underlying mechanisms of MC-LR in fish. In the present study, juvenile Chinese rare minnows (Gobiocypris rarus) were exposed to various concentrations of MC-LR (0 µg/L, 50 µg/L, 100 µg/L, and 500 µg/L) for 7 d. The whole-body thyroid hormone content, the histology of thyroid follicle epithelial cells, the activities of hepatic iodothyronine deiodinases, and the transcription of selected genes associated with thyroid hormone synthesis, transport, and metabolism were analyzed. Following exposure to MC-LR, whole-body concentrations of both thyroxine (T4 ) and triiodothyronine (T3 ) were significantly decreased. The levels of messenger RNA for sodium/iodide symporter, transthyretin, thyroid hormone receptor-α, iodothyronine deiodinase2, and iodothyronine deiodinase3 were significantly down-regulated after exposure to 500 µg/L MC-LR. A significant decrease in ID2 activity was also observed in the 500-µg/L MC-LR exposure group. Moreover, hypertrophy of thyroid follicle epithelial cells was observed after exposure to MC-LR. The results indicate that acute MC-LR exposure has the potential to disturb the homeostasis of thyroid hormone metabolism, leading to a hypothyroidism state in the juvenile Chinese rare minnow.