Neuroendocrine regulation of adrenal gland and hypothalamus 5'deiodinase activity. II. Effects of splanchnicotomy and hypophysectomy.

This study analyzes the role of the autonomic nervous system, the pituitary gland, ACTH, dexamethasone (DEX), and thyroid hormones in the regulation of 5'deiodinase (5'D) in the hypothalamus (HP) and adrenal gland (AG) of the rat. 5'D activity was analyzed in rats under basal conditions (22 C) and during cold exposure (4 C, during 15, 30, 60, and 120 min). Several experimental groups were formed: intact animals (INT), unilateral (left) splanchnicotomized, sham splanchnicotomized, hypophysectomized (HPX), and sham hypophysectomized. Results in the hypothalamus were: 1) independent of the experimental group, the HP 5'D activity increased during the first 15-30 min of cold exposure; however, this increase was greater in operated animals than in INT rats; and 2) basal 5'D activity was increased in HPX rats and was also regulated by thyroid hormones. Results in the adrenal gland were: 1) INT rats exhibited a biphasic pattern of 5'D activation during cold stress (30 and 60 min of exposure); 2) the splanchnic nerve exerted a tonic-stimulatory effect on basal AG 5'D activity; 3) the denervated gland preserved its ability to respond to cold; 4) in INT animals DEX but not ACTH had a stimulatory effect on basal activity; 5) the high 5'D activity post-HPX was reverted to basal values by T4 and DEX administration; 6) SHAM-HPX also was followed by a large increase in basal 5'D activity, and 7) this hyperresponse was abolished by acute ACTH and DEX administration. In conclusion, our results demonstrate that the mechanisms that participate in the regulation and activation of 5'D in the adrenal gland and the hypothalamus are of a neuroendocrine nature. Also, in both organs, but mainly in the HP, 5'D activity is T4-dependent. In addition to the tonic-stimulatory influence conveyed by the splanchnic nerve, AG 5'D activity is influenced by thyroid hormones, glucocorticoids, and probably extrapituitary factors whose nature is unknown yet.

Neuroendocrine regulation of adrenal 5'-monodeiodination during acute cold exposure in the rat. I. Effects of hypophysectomy.

Circulating levels of T4, T3, corticosterone, noradrenaline, and adrenaline, as well as 5'-monodeiodinase activity (5'-MA) were measured in control and hypophysectomized rats acutely exposed to cold environment (15-120 min, 4 C). In addition to the well known activation of the sympathoadrenomedullary system and the hypothalamic-pituitary-adrenal and-thyroid axes, cold exposure was followed by a rapid and sustained increase of 5'-MA in the hypothalamus, and a byphasic course of activation in the adrenal gland in control rats. The adrenal rapid activation (30 min) corresponded to the medulla and the slower activation (120 min) to the cortex. Both, the basal adrenal 5'-MA and the response to cold in adrenal and hypothalamus were 2-fold higher in hypophysectomized rats compared to control. The time course of enzyme activation in these structures suggests that: 1) organ-specific increases in 5'-MA may be associated to a simultaneous rise in their metabolic and/or functional activity, 2) the triggering mechanisms involves an immediate sympathetic signal activating the hypothalamic-adrenal medulla response and a pituitary signal eliciting a slower adrenocortical response, and 3) the compensatory sympathetic hyperactivity after panhypopituitarism contribute to enhance both the adrenal enzyme basal activity and the hypothalamic and adrenal hyperresponse to cold stress.