Regulation of thyrotropin secretion by the central epinephrine system. Studies in the chronically cannulated rat

In the present investigation CNS epinephrine (EPI) biosynthesis was selectively interrupted with specific norepinephrine N-methyltransferase (NMT) inhibitors, SK&F 64139 (Smith, Kline & French Laboratories) and LY 78335 (Eli Lilly & Co. Research Laboratories), to determine the effects of central EPI depletion on basal and cold, thyrotropin-releasing hormone, and hypothalamic somatostatin antiserum induced thyrotropin (TSH) secretion in chronically cannulated rats. Because these NMT inhibitors also are alpha 2-adrenergic receptor blockers, the effects of alpha 2- and alpha 1-adrenergic blockade and alpha 2-activation on plasma TSH were assessed with rauwolscine and corynanthine and B-HT 933, respectively. Serum T4 and plasma corticosterone were also measured. Blockade of CNS EPI synthesis resulted in inhibition of basal and cold and thyrotropin-releasing hormone induced TSH release, suppression of serum T4, and increased corticosterone release. The stimulatory effect of SRIF antiserum on plasma TSH was not altered by SK&F 64139. alpha 2-adrenergic blockade suppressed plasma TSH levels, but not to the same degree as the NMT inhibitors; activation of alpha 2-receptors enhanced TSH secretion. Thus, it is possible that part of the effect of the NMT inhibitors on TSH was due to alpha 2-blockade. alpha 1-adrenergic blockade also lowered plasma TSH. These results indicate that central EPI systems have a stimulatory role in TSH regulation, possibly mediated by alpha 2-adrenergic receptors, cold-induced TSH release is mediated, in part, by EPI, and central EPI systems exert an inhibitory effect on the hypothalamic-pituitary-adrenal axis.