The effects of methimazole on development of the fathead minnow, Pimephales promelas, from embryo to adult.

The importance of thyroid hormones in regulating early developmental processes of many amphibian and fish species is well known, but the impacts of exposure to disrupters of thyroid homeostasis during the embryo-larval-juvenile transitions are unclear. To investigate these impacts, fathead minnows, Pimephales promelas, were exposed to a model thyroid axis disrupter, methimazole, an inhibitor of thyroid hormone synthesis, at control (0), 32, 100, and 320 mug/l, starting at <24-h postfertilization, for 28, 56, and 83/84 days postfertilization (dpf). Thyroid disruption was evident at 28 dpf, when survival was significantly reduced by 32 or 100 mug/l methimazole concomitant with a reduced thyroxine (T(4)) content. However, the T(3) content of these fish was similar to that of control fish, and body mass was unaffected (as in all groups), suggesting compensatory mechanisms overcame reduced T(4) synthesis. At the highest concentration of methimazole (320 mug/l), activation of feedback mechanisms on the hypothalamic-pituitary-thyroid axis was suggested by the normal T(4) content after 28 dpf exposure to methimazole, although triiodothyronine (T(3)) content of these fish was significantly reduced. The generally less pronounced disruption of thyroid hormone homeostasis after 56 days exposure to methimazole also suggests compensatory mechanisms in juvenile/adult fish that may regulate T(4) content, despite exposure to methimazole at 32 or 100 mug/l (in fish held in 320 mug/l methimazole, the T(4) content was significantly higher than in controls). Whole body T(3) content at 56 dpf was significantly depressed only in fish held in 100 mug/l methimazole. By 83/84 dpf, length, body mass, and thyroid hormone concentrations were similar in all experimental groups and controls, indicating that adult fish may achieve regulation of their thyroid axis despite prolonged exposures to thyroid disruptors throughout early development.

Effects of ammonium perchlorate on thyroid function in developing fathead minnows, Pimephales promelas.

Perchlorate is a known environmental contaminant, largely due to widespread military use as a propellant. Perchlorate acts pharmacologically as a competitive inhibitor of thyroidal iodide uptake in mammals, but the impacts of perchlorate contamination in aquatic ecosystems and, in particular, the effects on fish are unclear. Our studies aimed to investigate the effects of concentrations of ammonium perchlorate that can occur in the environment (1, 10, and 100 mg/L) on the development of fathead minnows, Pimephales promelas. For these studies, exposures started with embryos of < 24-hr postfertilization and were terminated after 28 days. Serial sectioning of thyroid follicles showed thyroid hyperplasia with increased follicular epithelial cell height and reduced colloid in all groups of fish that had been exposed to perchlorate for 28 days, compared with control fish. Whole-body thyroxine (T4) content (a measure of total circulating T4 in fish exposed to 100 mg/L perchlorate was elevated compared with the T4 content of control fish, but 3,5,3-triiodothyronine (T3) content was not significantly affected in any exposure group. Despite the apparent regulation of T3, after 28 days of exposure to ammonium perchlorate, fish exposed to the two higher levels (10 and 100 mg/L) were developmentally retarded, with a lack of scales and poor pigmentation, and significantly lower wet weight and standard length than were control fish. Our study indicates that environmental levels of ammonium perchlorate affect thyroid function in fish and that in the early life stages these effects may be associated with developmental retardation.

Developmental changes of thyroid hormones in the fathead minnow, Pimephales promelas.

The fathead minnow is widely used in ecotoxicological studies and such investigations have begun to focus on potential disruption of the thyroid axis. However, normal levels of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) and their developmental patterns are unknown. To provide these baseline data, radioimmunoassays were developed and validated for analysis of T4 and T3 after extraction from plasma or whole fish. Female fish showed consistently higher plasma levels of T4 than male fish. Analysis of thyroid hormones during development showed a significant rise in both T4 and T3 during the pre-hatch period, indicating embryonic production of both thyroid hormones. After hatching, whole-body content of both T4 and T3 significantly increased in early development, peaking at 16 days post-hatch (dph) and 9 dph, respectively, when T4 reached 32.88 +/- 3.30 ng g(-1) body weight and T3 reached 24.17 +/- 3.15 ng g(-1) body weight. Thyroid hormones subsequently declined to a low plateau in later development with approximately 5 ng g(-1) body weight T4 and 2 ng g(-1) body weight T3. These data suggest a prominent role for thyroid hormones in early developmental processes when we predict that the ecotoxicological effects of thyroid disrupters will be most significant.