Abstract
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 (TT3) and the ratio of TT3 to total 3,3′,5,5′-l-thyroxine (TT4), 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 TT3 levels was partly attributed to an increase in the metabolism of T3 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 TT3/TT4 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 TT3 levels. These results indicated that MCP had the potential to influence several pathways of HPT axis homeostasis in female goldfish.
Highlights
In fish, growth and reproduction are, at least partly, under the control of thyroid hormones (THs), 3,39,5,59-L-thyroxine (T4) and 3,39,5-triiodo-L-thyronine (T3) [1,2]
The basal free T3 (FT3) levels in the plasma of the control female goldfish were 2.3960.39 pg/mL; they were significantly reduced by 32%, 32%, and 50% in the 0.01, 0.10, and 1.00 mg/L MCP pesticidetreated groups, respectively (P,0.01; Fig. 2A)
The mRNA expression of hepatic ttr was significantly higher after treatment with 0.01 and 0.10 mg/L of MCP pesticide (P,0.01), whereas there was no significant difference in the hepatic ttr mRNA expression between the group treated with the highest dose and the control group (Fig. 2C)
Summary
Growth and reproduction are, at least partly, under the control of thyroid hormones (THs), 3,39,5,59-L-thyroxine (T4) and 3,39,5-triiodo-L-thyronine (T3) [1,2]. Since THs have profound effects on energy metabolism, for example, they are the major regulators of oxidative energy metabolism at the level of the mitochondria in teleost fish [3], they should invariably be involved in the multifactorial regulation of metabolism associated with reproduction. They might act as direct modulators of the reproductive cycle. The expression of steroidogenic enzymes and steroid receptors in goldfish could be modulated by THs [5]
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