Effects of peripheral versus central administration of the endogenous glucocorticoid, corticosterone, and the glucocorticoid receptor agonist, RU 28362, on LH release in male rats

Effects of hydrocortisone on the formation of gap junctions and the abnormal growth of cilia within the rat anterior pituitary gland: possible role of gap junctions on the regulation of cell development

We investigated the effects of hydrocortisone on the formation of gap junctions in and the growth of cilia on folliculo-stellate cells. The male rats of experimental groups were given daily intraperitoneal injections of 5 mg/kg of hydrocortisone from Day 20 to 60. Five rats were killed at ages 10, 20, 30 and 40 days after initiation of injections, and the pituitary gland was removed from each rat. Then, the specimens were prepared for observation by transmission electron microscopy. A delay in the formation of gap junctions between folliculo-stellate cells was observed in hydrocortisone treated rats compared with control rats on Day 30, 40 and 50. Another finding in the present study was the increase of ciliated follicles on Day 40 and 50 in the hydrocortisone treated groups, simultaneous with the delay in gap junction formation. The results suggest that hydrocortisone has a suppressive effect on the gap junction formation between folliculo-stellate cells, and loss of intercellular communication by way of gap junctions may lead to alteration of morphological development of the cell.

Induction of immediate-early gene expression in preoptic and hypothalamic neurons by the glucocorticoid receptor agonist, dexamethasone

Glucocorticoid receptors (GR) exist in several preoptic and hypothalamic nuclei that participate in neuroendocrine control of anterior pituitary function. GR may mediate effects of endogenous steroids on hormone secretion, since intracerebral administration of exogenous ligands alters plasma levels of several pituitary hormones. The following studies utilized selective antisera for the transcriptional proteins, Fos and Jun, to examine whether these immediate-early gene products are upregulated in response to the GR agonist, dexamethasone (DEX). DEX was administered to groups of male rats by either a subcutaneous (s.c., 5.0 mg/kg) or intracerebroventricular route (i.c.v., 10.0 microg/rat); matched controls received vehicle only. Two hours later, the rats were sacrificed by transcardial perfusion, and serial 25 microm sections through the preoptic area and hypothalamus were processed by avidin-biotin immunocytochemistry for Fos- and Jun-like proteins. Animals treated with DEX i.c.v. exhibited Fos-like immunoreactivity (-li) in several sites in close proximity to the third ventricle, including the preoptic and anterior hypothalamic nuclei, and the periventricular zone of the paraventricular nucleus. In the same group, Jun-li was detected only in the arcuate and suprachiasmatic nuclei. Subcutaneous injection of DEX resulted in more widespread immunostaining for Fos, which occurred in lateral, as well as medial, loci in the preoptic area and hypothalamus, whereas Jun-li was restricted to only medial sites. These data show that discrete populations of preoptic/hypothalamic neurons express c fos and/or jun in response to GR activation. The differential distribution of Fos-li following s.c. vs. i.c.v. administration of DEX suggests that steroid induction and/or amplification of this cellular signaling cascade may depend upon resultant hormone concentrations in neural tissue. In addition, the wide pattern of immunolabeling for Fos in the systematically treated group may reflect both central and peripheral (indirect) steroid effects. Additional studies are in progress to characterize those neurons within sites of neuroendocrine significance that exhibit possible upregulation of these regulatory gene products in response to GR stimulation.

Stimulatory vs. inhibitory effects of acute stress on plasma LH: differential effects of pretreatment with dexamethasone or the steroid receptor antagonist, RU 486

Acute stress elicits variable patterns of pituitary LH release in intact rats. While the pituitary-adrenal axis is capable of discrimination between stressors of graded intensity, the effects of variable glucocorticoid output on the direction and magnitude of LH release during stress remain unclear. The present studies compared the effects of a psychological stress and two different physical stressors on peripheral corticosterone (CORT) and LH concentrations. Plasma CORT levels were elevated during each stress, but this increase in hormone release was significantly greater in response to physical stress. This differential CORT sensitivity to psychological vs. physical stress was correlated with divergent patterns of pituitary LH release; novel environment (NE) stress resulted in a transient increase in plasma LH, whereas both physical stressors ultimately caused a reduction in circulating hormone levels. Pretreatment with the glucocorticoid receptor (GR) antagonist, RU 486, reversed physical stress-induced decreases in LH release, but did not further facilitate circulating LH during NE stress. Other studies showed that stimulation of GRs prior to stress with the potent ligand, dexamethasone (DEX), blunted the stimulatory effects of NE stress on circulating LH. Additional experiments investigated whether prolonged exposure to elevated glucocorticoid levels elicits adaptive responses from the hypothalamic-pituitary LH axis to acute stress. Chronic DEX administration resulted in a significant attenuation of the inhibitory LH response to acute immobilization, but had no impact upon the facilatory effects of NE stress on LH release. The current studies confirm previous reports of variation in the magnitude of CORT secretion elicited by stressors of different intensity, and provide new evidence that inhibitory patterns of pituitary LH release may be correlated with a high degree of activation of the pituitary-adrenal axis. Attenuation of the facilatory effects of novel environment stress on LH release by pretreatment with the GR agonist, DEX, suggests that GR-induced inhibition of LH requires occupation of GRs beyond that which occurs during this mild stressor. The present findings that stress-induced decreases in plasma LH are blunted by chronic glucocorticoid exposure support a role for glucocorticoid-dependent mechanisms in adaptation of GR-mediated inhibitory responses to stress.

The antiglucocorticoid action of mifepristone

Glucocorticoid hormones influence the physiological activity of almost all cell types in the mammal. This is accomplished via a soluble receptor that, in the presence of an appropriate steroid, modifies the activity of RNA polymerase by binding to the site where different factors assemble for the initiation of cell transcription. The development of antiglucocorticoids has permitted the molecular elucidation of a number of underlying events. Contrary to the classical view, it is now clear that the affinity, stability and activability of the glucocorticoid receptor in the presence of a steroid are cell- and/or tissue-dependent events. The antiglucocorticoid RU 38486 can even activate transcription by binding to sites distinct from those that process transactivation by the agonist. Furthermore, glucocorticoids can sometimes activate the mineralocorticoid receptor, whereas mineralocorticoids can bind the glucocorticoid receptor. Since mifepristone is devoid of adverse toxicity, it has been used for the paraclinical diagnosis of the hypothalamus-pituitary-adrenal axis in normal volunteers, subjects with disorders of the behaviour, and the treatment of Cushing's disease. However, the whole spectrum of cell-specific processes that are antagonized by RU 38486 suggests wide ranging possibilities in the eventual application of antigluco-corticoids.