Thyroid hormone regulation of mRNAs encoding thyrotropin beta-subunit, glycoprotein alpha-subunit, and thyroid hormone receptors alpha and beta in brain, pituitary gland, liver, and gonads of an adult teleost, Pimephales promelas.

Thyroid hormones (THs) regulate growth, morphological development, and migratory behaviors in teleost fish, yet little is known about the transcriptional dynamics of gene targets for THs in these taxa. Here, we characterized TH regulation of mRNAs encoding thyrotropin subunits and thyroid hormone receptors (TRs) in an adult teleost fish model, the fathead minnow (Pimephales promelas). Breeding pairs of adult minnows were fed diets containing 3,5,3'-triiodo-L-thyronine (T(3)) or the goitrogen methimazole for 10 days. In males and females, dietary intake of exogenous T(3) elevated circulating total T(3), while methimazole depressed plasma levels of total thyroxine (T(4)). In both sexes, this methimazole-induced reduction in T(4) led to elevated mRNA abundance for thyrotropin beta-subunit (tshbeta) in the pituitary gland. Fish treated with T(3) had elevated transcript levels for TR isoforms alpha and beta (tralpha and trbeta) in the liver and brain, but reduced levels of brain mRNA for the immediate-early gene basic transcription factor-binding protein (bteb). In the ovary and testis, exogenous T(3) elevated gene transcripts for tshbeta, glycoprotein hormone alpha-subunit (gphalpha), and trbeta, while not affecting tralpha levels. Taken together, these results demonstrate negative feedback of T(4) on pituitary tshbeta, identify tralpha and trbeta as T(3)-autoinduced genes in the brain and liver, and provide new evidence that tshbeta, gphalpha, and trbeta are THs regulated in the gonad of teleosts. Adult teleost models are increasingly used to evaluate the endocrine-disrupting effects of chemical contaminants, and our results provide a systemic assessment of TH-responsive genes during that life stage.