Chronic exposures to the herbicide atrazine and the pharmaceutical finasteride disrupt sex steroid and thyroid hormone signalling and gonadal development in frogs

Endocrine disrupting chemicals (EDCs) such as pesticides and pharmaceuticals can upset amphibian development and contribute to worldwide amphibian population declines. The first objective of this doctoral research was to investigate if the widely used herbicide atrazine (ATZ; used on corn and soy crops) alters Lithobates (Rana) pipiens (Northern leopard frog) development using concentrations of ATZ reported in Canadian ecosystems. Chronic exposures to 0.1 and 1.8 mug/L ATZ on L. pipiens tadpoles were performed in semi-controlled mesocosms. Atrazine reduced metamorphosic success, induced female-biased sex ratio, altered the hepatic activity of steroid 5beta-reductase (srd5beta; an enzyme involves in the conversion of testosterone into 5beta-dihydrotestosterone) and affected the expression of estrogen receptor alpha in brain and deiodinase type 3 in tail. The second objective was to characterize the functions of the steroid 5alpha-reductase (srd5alpha; type 1, 2 and 3) and srd5beta in frogs. These enzymes represent a convergence in evolution: they share similar biological functions (e.g., testosterone reduction, bile acid biosynthesis and erythropoesis), but srd5alpha and srd5beta do not have a common ancestor. Using real-time RT-PCR analysis, these enzymes were detected throughout Silurana (Xenopus) tropicalis (Western clawed frog) early development. The prostate drug finasteride (a 5alpha-reductase type 2 and srd5beta inhibitor in humans) was used in short-term (25, 50 and 100 muM) and chronic (25 muM) exposures of S. tropicalis under laboratory conditions. Finasteride inhibited srd5beta and aromatase (cyp19) activities in whole embryos after short-term exposures. However, chronically exposed S. tropicalis until metamorphosis, revealed no effects of finasteride on cyp19 activity and an increase in male hepatic srd5beta activity. Furthermore, chronic treatment with finasteride induced testicular oocytes in developing males (also referred to as the intersex condition). In addition, real-time RT-PCR analysis showed that finasteride treatments altered sex steroid- and thyroid hormone-related gene expression. Alterations of thyroid hormone gene expression following the inhibition of srd5alpha and srd5beta suggest a complex relationship between the thyroid hormone-responsive genes and the androgen status in early frog development and at metamorphosis. In conclusion, real-time RT-PCR, enzymatic activity and histology analyses demonstrated that srd5alpha and srd5beta are important enzymes during frog development and are 'new' targets to EDCs