Tamoxifen protects against 17α-ethynylestradiol-induced liver damage and the development of urogenital papillae in the rainbow darter (Etheostoma caeruleum)

Juvenile rainbow darters (Etheostoma caeruleum) were exposed to nominal concentrations of 20 to 1,000 ng/L of 17α‐ethynylestradiol (EE2) at 120 d posthatch and in a subsequent experiment to 200 ng/L of EE2 with 2.0 to 20,000 ng/L of tamoxifen (TMX) at 150 d posthatch to determine the threshold of estrogen‐induced morphological and histological changes in a sexually dimorphic benthic fish species ecologically relevant to southern Minnesota (USA). 17α‐Ethynylestradiol induced female‐associated urogenital papillae in males at 200 ng/L, enlargement and development of fibrosis in male testes, enlargement of ovary and oocyte size in females, and large fatty inclusions in the liver of both sexes. Exposure to 1,000 ng/L of EE2 caused gross hypertrophy of the liver and kidneys and high mortalities, predominantly in male fish. A low incidence of ovotestes found in all treatment groups was unaffected by EE2, which may be unusual to this species or a response to unknown water contaminants present during the hatching or early development of the darters. Gonadosomatic index was not altered for either sex by any treatment. A TMX level equal to or less than that of EE2 decreased fat accumulation in the liver in both sexes, and a TMX level greater than that of EE2 appeared to prevent urogenital papilla in males. Tamoxifen did not significantly alter fibrosis caused by EE2 in testes. It appears that the presence of TMX in the environment can mask many signs of estrogen exposure, including secondary sexual characteristics, hypertrophy of ovaries and testes, and fatty infiltration of organs. Ovotestes did not prove to be a good indicator of estrogen exposure at this late stage of juvenile darter development.