Alteration by thyroid hormones of TRH in the rat median eminence: role of the hypothalamic paraventricular nucleus.

We have shown that hypothyroidism produced a progressive decrease in the TRH content of the hypothalamic median eminence. In the present study, we explored whether thyroid hormones might alter TRH content in the median eminence by acting on the hypothalamic paraventricular nucleus (PVN) in the rat. After thyroidectomy, adult male rats received a hypothalamic deafferentation or electrolytic lesions in the bilateral PVNs. These rats were i.p. administered daily with T4 in a dose of 1.5 micrograms per 100 g body weight. Two weeks later, these rats were decapitated, and TRH in the median eminence and TSH in the peripheral blood were determined by RIAs. Thyroidectomy caused a significant reduction in TRH content of the median eminence. The hypothalamic deafferentation and PVN-lesions further reduced the TRH content. T4-replacement restored the decreased content of median eminence TRH to the normal level. The hypothalamic deafferentation and PVN-lesions prevented T4-induced restoration of TRH content. The present data imply that thyroid hormones regulate the amount of THR in the median eminence through acting on the hypothalamic PVN.