Monocrotophos Pesticide Decreases the Plasma Levels of Total 3,3′,5-Triiodo-l-Thyronine and Alters the Expression of Genes Associated with the Thyroidal Axis in Female Goldfish (Carassius auratus)

Our recent study showed that monocrotophos (MCP) pesticide disrupted the hypothalamic-pituitary-thyroid (HPT) axis in male goldfish ( Carassius auratus ); however, the effects of MCP on the thyroid system in female goldfish are remain unclear. In the present study, plasma thyroid hormone (TH) and thyroid-stimulating hormone (TSH) levels were evaluated in female goldfish exposed to 0.01, 0.10, and 1.00 mg/L of 40% MCP-based pesticide for 21 days in a semi-static exposure system. Expression profiles of HPT axis-responsive genes, including transthyretin ( ttr ), deiodinases ( d1 , d2 , and d3 ), tshβ , thyrotropin-releasing hormone ( trh ), and corticotrophin-releasing hormone ( crh ), were determined. The results indicated that MCP decreased the plasma levels of total 3,3′,5-triiodo- l -thyronine (TT 3 ) and the ratio of TT 3 to total 3,3′,5,5′- l -thyroxine (TT 4 ), and induced alternative expression of TH-related genes. Exposure to 0.01 and 0.10 mg/L MCP pesticide resulted in the up-regulation of ttr mRNA. The reduction of plasma TT 3 levels was partly attributed to an increase in the metabolism of T 3 in the liver, as revealed by the highly elevated hepatic d1 and d3 mRNA levels in the MCP treatment groups, and the expression of hepatic d3 showed a negative correlation with the plasma TT 3 /TT 4 levels in females. Moreover, the plasma TSH levels were lower in females exposed to 0.01 and 0.10 mg/L MCP pesticide, whereas the up-regulation of tshβ mRNA levels was compensated by the decreased plasma TT 3 levels. These results indicated that MCP had the potential to influence several pathways of HPT axis homeostasis in female goldfish.