Regulation of the hypothalamic release of thyrotropin-releasing hormone (TRH)

The tripeptide TRH has diverse endocrine and non-endocrine functions and is ubiquitously located in the body. Its endocrine functions relate to its role as a hypothalamic factor which stimulates the release of TSH and PRL from the adenohypophysial thyrotroph and lactotroph, respectively. The role of TRH in the regulation of thyroid function and lactation is exerted by the binding of the hormone to its pituitary receptors. Regulation of the hypothalamic release of TRH may be an important factor in the control of its bioavailability. Indeed, indirect studies suggest that TRH plays an important role in TSH secretion during altered thyroid status, the low T3 syndrome, and acute cold exposure. Moreover, PRL secretion seems partly dependent on hypothalamic TRH and, therefore, TRH is also considered a PRL-releasing factor. In the litterature, however, only sparse data are available on in vivo and in vitro release of hypothalamic TRH in conditions associated with altered plasma levels of TSH and/or PRL. Moreover, these few studies are not reconciable and the exact role of TRH in the regulation of thyroid function and lactation is far from clear. Therefore, the scope of this thesis was to hypothalamic secretion of TRH is regulated with regard to investigate how thyroid function and lactation. Firstly, different neuroendocrine techniques were validated to measure hypothalamic release of TRH (Chapter II). Secondly, the regulation of the secretion of TRH was investigated in conditions associated with altered plasma levels of TSH (Chapter III) and PRL (Chapter IV).