Fish Physiology and Biochemistry 20: 193–199, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 193 Thyroid hormones down-regulate thyrotropin β mRNA level in vivo in the turbot (Psetta maxima) B. Pradet-Balade 1 , C. Burel 2 , S. Dufour 3 , T. Boujard 2 , S. J. Kaushik 2 , B. Qu´ erat 3,∗ and G. Boeuf 4 1 CSIC-Centro Nacional de Biotecnologia, Campus de Canto Bianco-Ctra de Colmenar Viejo, Km 15,500, 28049 Madrid Spain; 2 Laboratoire de Nutrition des Poissons, Unit´ e mixte INRA-IFREMER, Station d’Hydrobiologie, BP 3, 64310 Saint P´ ee-sur-Nivelle, France; 3 Laboratoire de Physiologie G´ en´ erale et Compar´ ee, Mus´ eum National d’Histoire Naturelle, 7, Rue Cuvier, 75231 Paris C´ edex 05; 4 Laboratoire de Physiologie des Poissons, IFREMER, Station de Brest, BP 70, 29280 Plouzan´ e, France; ∗ Author for correspondence (Phone: 33-140793620; Fax: 33-140793618; E-mail: querat@mnhn.fr) Accepted: July 20, 1998 Key words: TSH regulation, TSH β subunit, T3 feed-back, teleost Abstract Thyroid stimulating hormone (TSH) is a vertebrate pituitary heterodimeric hormone that stimulates the thyroid gland to produce the thyroid hormones, T 3 and T 4 . We report here the cloning, by PCR on reverse-transcribed pituitary RNAs, of a 180 bp fragment of the cDNA encoding TSH β subunit in the turbot (Psetta maxima). The deduced amino acid sequence displayed 66 and 75% identity with the corresponding sequence from the European eel (Anguilla anguilla) and the rainbow trout (Oncorchyncus mykiss), respectively. This cDNA was then used as a probe for densitometric analysis of individual pituitary Northern blots. TSH β mRNA levels were quantified in turbot where circulating thyroid hormones were modified by dietary treatments or hormone supplementation. Recombinant rainbow trout growth hormone had no effect on circulating thyroid hormone levels or on pituitary TSH β mRNA level. In turbot fed heat-treated rapeseed meal, plasma T 4 levels were lowered and TSH β mRNA increased more than two fold. In contrast, when turbot were fed a standard fish-meal supplemented with T 3 , circulating T 3 levels were elevated and there was a dramatic decrease in TSH β mRNA level. It is concluded that both thyroid hormones are able to down-regulate TSH β mRNA level in vivo in the turbot. These results are discussed in the context of the evolution of the TH feed-back on TSH production. Introduction Thyrotropin – or thyroid stimulating hormone (TSH) – is a member of the vertebrate glycoprotein hormone family, which also comprises the pituitary and chori- onic gonadotropins. All are composed of two different, non-covalently bound subunits, α and β , encoded by separate genes. In a given species, the α subunit is common to all glycoprotein hormones whereas the β subunit is specific to each hormone (Pierce and Par- sons 1981; Pierce 1988; Quérat 1994). Hence, in TSH, the α subunit is common to the gonadotropin hormones while the β is TSH-specific and considered to be the rate-limiting subunit for the dimer formation (Chin et al. 1993). In vertebrates, TSH stimulates the thyroid gland to produce the thyroid hormones (TH) thyroxine (T 4 ) and 3,5,3 ′ -triiodo-L-thyronine (T 3 ). In mammals, TSH β cDNAs have been cloned in several species and the negative feedback exerted by T 3 and T 4 on TSH has been extensively described (for review Chin et al. 1993). Briefly, both THs down-regulate the synthesis of TSH and, in particular, of its mRNAs, and TSH re- lease. In contrast, much less is known about TSH reg- ulation in non-mammalian species. cDNAs encoding the TSH β subunit were cloned in two teleost fishes,