Down-Regulation of TSH Subunit mRNA Levels by Thyroid Hormones in the European Eel

Abstract

The effects of 3,5,3′-triiodothyronine (T3) and thyroxine (T4) on α and thyroid-stimulating hormone (TSH) β subunit mRNA pituitary levels were examined in a teleost, the European silver eel. Northern blot analysis showed that the number and length of mRNAs encoding TSH β varied among individuals, a variability apparently not related to thyroidal status. When several bands were present, their intensities were summed for quantitative analysis. Increasing circulating thyroid hormones (THs) by implantation of T3or T4significantly decreased TSH β mRNA levels. Depression of circulating THs by thiourea treatment increased α and TSH β mRNA levels.In vitrostudies showed that T3and T4decrease TSH β mRNA levels in primary cultures of eel pituitary cells. In conclusion,in vivoandin vitroexperiments indicate that T3and T4exert a negative feedback action on pituitary TSH β mRNA level in the European eel and that this effect might be exerted, at least partly, through a direct action on the pituitary.