Selective suppression of pressor and sympathetic responses to centrally infused TRH in hypothyroid rats.

To determine whether hypothyroidism alters responsiveness to centrally infused thyrotropin-releasing hormone (TRH), blood pressure and sympathetic nerve responses to various stimuli were recorded in urethane-anesthetized rats. Four weeks after thyroidectomy or treatment with propylthiouracil, pressor responses elicited by intracerebroventricular- (ICV)-infused TRH or by electrical stimulation of the posterior or ventromedial hypothalamus were always accompanied by increased sympathetic nerve activity in all rats. When serum thyroxine levels were lowered in hypothyroid rats, pressor responses to most stimuli, including intravenous injections of norepinephrine, angiotensin, and vasopressin, were inhibited, indicating that peripheral pressor responsiveness had been reduced. For centrally acting stimuli, pressor inhibition was also considered partly caused by a selective decrease in sympathetic vasomotor tone because sympathetic excitation was reduced only during ICV infusions of TRH, but not during posterior hypothalamic stimulation. On the other hand, there was no obvious explanation why both pressor and sympathetic responses to ventromedial hypothalamic stimulation were spared from inhibition. Because sympathetic nerve responses to ICV-infused TRH were reduced by thyroid suppression, these results imply that sympathetic activation by TRH in intact rats is mediated, at least in part, indirectly by way of TRH stimulation of thyroid activity.