Thyroid hormones down-regulate thyrotropin beta mRNA level in vivo in the turbot (Psetta maxima)

Thyroid stimulating hormone (TSH) is a vertebrate pituitary heterodimeric h ormone 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 pitui tary RNAs, of a 180 bp fragment of the cDNA encoding TSH beta subunit in th e turbot (Psetta maxima). The deduced amino acid sequence displayed 66 and 75% identity with the corresponding sequence from the European eel (Anguill a anguilla) and the rainbow trout (Oncorchyncus mykiss), respectively. This cDNA was then used as a probe for densitometric analysis of individual pit uitary Northern blots. TSH beta mRNA levels were quantified in turbot where circulating thyroid hormones were modified by dietary treatments or hormon e supplementation. Recombinant rainbow trout growth hormone had no effect o n circulating thyroid hormone levels or on pituitary TSH beta mRNA level. I n turbot fed heat-treated rapeseed meal, plasma T-4 levels were lowered and TSH beta 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 beta mRNA level. It is c oncluded that both thyroid hormones are able to down-regulate TSH beta mRNA level in viva in the turbot. These results are discussed in the context of the evolution of the TH feed-back on TSH production.