Precocious auto-induction of thyroid hormone receptors in embryonic chick hypothalamic neurons.

Qualitative immunocytochemical and quantitative autoradiographic approaches were used to examine the presence of thyroid hormone receptor (TR) proteins in embryonic chick hypothalamic cultures and the effect of T3 on their expression. We used antibodies raised against rat TRs having first verified that they recognised nuclear T3 binding proteins in extracts from chick brains. TRs were expressed in embryonic hypothalamic neurons from 6 day-old chick embryos maintained 4 days in vitro, and their expression was up-regulated by T3. This demonstration of TRs in embryonic hypothalamic neurons suggests that a physiological mechanism for early regulation of the hypothalamic neuropeptide, thyrotropin releasing hormone, is installed prior to the onset of thyroid function.