Thyroid hormone receptor β2 is strongly up-regulated at all levels of the hypothitlamo-pituitary-thyroidal axis during late embryogenesis in chicken

In this study, we tried to elucidate the changes in thyroid hormone (TH) receptor beta 2 (TR beta 2) expression at the different levels of the hypothalamo-pituitary-thyroidal (HPT) axis during the last week of chicken embryonic development and hatching, a period characterized by an augmented activity of the HPT axis. We quantified TR beta 2 mRNA in retina, pineal gland, and the major control levels of the HPT axis - brain, pituitary, and thyroid gland - at day 18 of incubation, and found the most abundant mRNA content in retina and pituitary. Thyroidal TR beta 2 mRNA content increased dramatically between embryonic day 14 and I day post-hatch. In pituitary and hypothalamus, TR beta 2 mRNA expression rose gradually, in parallel with increases in plasma thyroxine concentrations. Using in situ hybridization, we have demonstrated the presence of TR beta 2 mRNA throughout the diencephalon and confirmed the elevation in TR beta 2 mRNA expression in the hypophyseal thyrotropes. In vitro incubation with THs caused a down-regulation of TR beta 2 mRNA levels in embryonic but not in post-hatch pituitaries. The observed expression patterns in pituitary and diencephalon may point to substantial changes in TR beta 2-mediated TH feedback active during the perinatal period. The strong rise in thyroidal TR beta 2 mRNA content could be indicative of an augmented modulation of thyroid development and/or function by THs toward and after hatching. Finally, THs proved to exert an age-dependent effect on pituitary TK beta 2 mRNA expression.