Inhibited Metamorphosis and Disruption of Antioxidant Defenses and Thyroid Hormone Systems in Bufo gargarizans Tadpoles Exposed to Copper

Copper is a common aqueous pollutant that is known to cause oxidative stress and disrupt the thyroid axis in amphibians. In the present study, tadpoles of the Chinese toad (Bufo gargarizans) were exposed to 1, 6.4, 32, and 64 μg L of copper from Gosner stages 26 to 42. We aimed to examine the influence of copper on thyroid hormone-responsive and stress-associated gene expression in the hind-limb, tail, and liver of B. gargarizans tadpoles. Exposure to 64 μg L copper decreased percent metamorphosis and increased length of both hind-limb and tail of B. gargarizans tadpoles at Gs 42. In addition, according to real-time PCR results, exposure to 64 μg L copper induced downregulation of Dio2, Dio3, TRα, and TRβ mRNA levels in all tissues examined. We inferred that copper might induce a considerable reduction of TH levels through downregulation of Dio2 and Dio3 mRNA levels in peripheral tissues. Decreased TH levels may then decrease the expressions of TRα and TRβ. Also, HSP, SOD, and PHGPx transcript levels were measured to assess cellular stress which might affect TH signaling and metamorphosis. We found that copper significantly downregulated the level of HSP, SOD, and PHGPx transcripts in the hind-limb and tail. This demonstrates that high concentrations of copper could disrupt the antioxidant system of B. gargarizans tadpoles and increase oxidative damage. Therefore, we conclude that copper could disrupt the antioxidant system and cause thyroid hormone disruption in B. gargarizans tadpoles.