A time course of altered thyroid states on the noradrenergic system in rat brain by quantitative autoradiography

The present study was designed to investigate the time course of alterations in beta 1- and beta 2-adrenoceptor subtypes, alpha 2-adrenoceptors and uptake sites for norepinephrine in the rat brain following thyroidectomy (TXT) and thyroxine replacement. The results indicate that alterations in the thyroid state lead to changes in the pre- and postsynaptic noradrenergic system that are both region- and receptor-specific. TXT caused the binding of 125-iodopindolol to beta 1-adrenoceptors to decrease in the cortex and hippocampus and caused its binding to beta 2-adrenoceptors to decrease in the hypothalamus. TXT caused 3H-idazoxan binding to alpha 2-adrenoceptors to be reduced specifically in the amygdala. Following TXT, the binding of 3H-nisoxetine to norepinephrine uptake sites was found to be increased in the hippocampus and decreased in the hypothalamus. In most brain regions, thyroxine replacement for 7 or 28 days caused radioligand binding to recover to control levels. Thus it appears that a neuromodulatory link between thyroid hormone and the noradrenergic system exists in many regions of the rat brain.