Inhibition of aldosterone turn-off phenomenon following chronic adrenocorticotropin treatment with in vivo administration of antiglucocorticoid and antioxidants in rats

Altered Expression of Phox2 Transcription Factors in the Locus Coeruleus in Major Depressive Disorder Mimicked by Chronic Stress and Corticosterone Treatment In Vivo and In Vitro

Phox2a and Phox2b are two homeodomain transcription factors playing a pivotal role in the development of noradrenergic neurons during the embryonic period. However, their expression and function in adulthood remain to be elucidated. Using human postmortem brain tissues, rat stress models and cultured cells, this study aimed to examine the alteration of Phox2a and Phox2b expression. The results show that Phox2a and Phox2b are normally expressed in the human locus coeruleus (LC) in adulthood. Furthermore, the levels of Phox2a protein and mRNA and protein levels of Phox2b were significantly elevated in the LC of brain donors that suffered from the major depressive disorder, as compared to age-matched and psychiatrically normal control donors. Fischer 344 rats subjected to chronic social defeat showed higher mRNA and protein levels of Phox2a and Phox2b in the LC, as compared to non-stressed control rats. In rats chronically administered oral corticosterone, mRNA and protein levels of Phox2b, but not Phox2a, in the LC were significantly increased. In addition, the corticosterone-induced increase in Phox2b protein was reversed by simultaneous treatment with either mifepristone or spironolactone. Exposing SH-SY5Y cells to corticosterone significantly increased expression of Phox2a and Phox2b, which was blocked by corticosteroid receptor antagonists. Taken together, these experiments reveal that Phox2 genes are expressed throughout the lifetime in the LC of humans and Fischer 344 rats. Alterations in their expression may play a role in major depressive disorder and possibly other stress-related disorders through their modulatory effects on the noradrenergic phenotype.

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

Corticosterone administration up-regulated expression of norepinephrine transporter and dopamine β-hydroxylase in rat locus coeruleus and its terminal regions

Stress has been reported to activate the locus coeruleus (LC)-noradrenergic system. In this study, corticosterone (CORT) was orally administrated to rats for 21 days to mimic stress status. In situ hybridization measurements showed that CORT ingestion significantly increased mRNA levels of norepinephrine transporter (NET) and dopamine β-hydroxylase (DBH) in the LC region. Immunofluorescence staining and western blotting revealed that CORT treatment also increased protein levels of NET and DBH in the LC, as well as NET protein levels in the hippocampus, the frontal cortex and the amygdala. However, CORT-induced increase in DBH protein levels only appeared in the hippocampus and the amygdala. Elevated NET and DBH expression in most of these areas (except for NET protein levels in the LC) was abolished by simultaneous treatment with combination of corticosteroid receptor antagonist mifepristone and spironolactone (s.c. for 21 days). Also, treatment with mifepristone alone prevented CORT-induced increases of NET expression and DBH protein levels in the LC. In addition, behavioral tasks showed that CORT ingestion facilitated escape in avoidance trials using an elevated T-maze, but interestingly, there was no significant effect on the escape trial. Corticosteroid receptor antagonists failed to counteract this response in CORT-treated rats. In the open-field task, CORT treatment resulted in less activity in a defined central zone compared to controls and corticosteroid receptor antagonist treatment alleviated this increase. In conclusion, this study demonstrates that chronic exposure to CORT results in a phenotype that mimics stress-induced alteration of noradrenergic phenotypes, but the effects on behavior are task dependent. As the sucrose consumption test strongly suggests CORT ingestion-induced depression-like behavior, further elucidation of underlying mechanisms may improve our understanding of the correlation between stress and the development of depression.

Effects of chronic social defeat on expression of dopamine β-hydroxylase in rat brains

It is documented that stress activates the locus coeruleus-norepinephrine system. However, there are far few reports regarding effects of stress on the expression of dopamine β-hydroxylase, a hallmark enzyme of the noradrenergic neuron. In the present study, adult Fischer 344 rats were subjected to chronic social defeat for 4 weeks. Dopamine β-hydroxylase expressional levels in the locus coeruleus and its terminal regions were measured by in situ hybridization and western blotting. The results showed that immediately following chronic social defeat there are significantly increased mRNA and protein levels of dopamine β-hydroxylase in the locus coeruleus, and dopamine β-hydroxylase protein levels in the hippocampus, frontal cortex and amygdala, compared with those in the control. This chronic social defeat-induced upregulation of dopamine β-hydroxylase was completely abolished by adrenalectomy, and/or by treatment with corticosteroid receptor antagonists, mifepristone and spironolactone, either alone or in combination. Furthermore, treatment with desipramine, an antidepressant with specific inhibitory effects on norepinephrine transport, prevented an increased dopamine β-hydroxylase expression by chronic social defeat in the locus coeruleus and its main terminal regions such as the hippocampus, frontal cortex and amygdala. However, treatment with fluoxetine, an antidepressant with specific inhibition for serotonin transport, only selectively blocked increased dopamine β-hydroxylase protein levels in the hippocampus caused by CSD. The present findings indicate that chronic social defeat activates the locus coeruleus-norepinephrine system by upregulating the expression of dopamine β-hydroxylase, which may increase norepinephrine synthesis. This chronic social defeat induced upregulation of DBH expression was mediated through corticosterone and corticosteroid receptors, with possible interference from antidepressants.

Chronic social defeat up-regulates expression of norepinephrine transporter in rat brains

Stress has been reported to activate the locus coeruleus (LC)-noradrenergic system. However, the molecular link between chronic stress and noradrenergic neurons remains to be elucidated. In the present study adult Fischer 344 rats were subjected to a regimen of chronic social defeat (CSD) for 4weeks. Measurements by in situ hybridization and Western blotting showed that CSD significantly increased mRNA and protein levels of the norepinephrine transporter (NET) in the LC region and NET protein levels in the hippocampus, frontal cortex and amygdala. CSD-induced increases in NET expression were abolished by adrenalectomy or treatment with corticosteroid receptor antagonists, suggesting the involvement of corticosterone and corticosteroid receptors in this upregulation. Furthermore, protein levels of protein kinase A (PKA), protein kinase C (PKC), and phosphorylated cAMP-response element binding (pCREB) protein were significantly reduced in the LC and its terminal regions by the CSD paradigm. Similarly, these reduced protein levels caused by CSD were prevented by adrenalectomy. However, effects of corticosteroid receptor antagonists on CSD-induced down-regulation of PKA, PKC, and pCREB proteins were not consistent. While mifeprestone and spironolactone, either alone or in combination, totally abrogate CSD effects on these protein levels of PKA, PKC and pCREB in the LC and those in the hippocampus, frontal cortex and amygdala, their effects on PKA and PKC in the hippocampus, frontal cortex and amygdala were region-dependent. The present findings indicate a correlation between chronic stress and activation of the noradrenergic system. This correlation and CSD-induced alteration in signal transduction molecules may account for their critical effects on the development of symptoms of major depression.

Altered corticosteroid metabolism differentially affects pituitary corticotropin response

To evaluate the effects of altered corticosteroid metabolism on the hypothalamic-pituitary-adrenal axis, we examined rats treated with glycyrrhizic acid (G rats) or rifampicin (R rats) for 7 days. The half-life of exogenously administered hydrocortisone as a substitute for corticosterone was longer in G rats and shorter in R rats, with no differences in basal plasma levels of ACTH or corticosterone. The ACTH responses to human corticotropin-releasing factor (CRF) or insulin-induced hypoglycemia were greater in G rats and tended to be smaller in R rats compared with those in the control rats, whereas the corticosterone response was similar. No difference was observed in the content and mRNA level of hypothalamic CRF among the groups. The number and mRNA level of CRF receptor and type 1 11 beta-hydroxysteroid dehydrogenase (11-HSD1) mRNA level in the pituitary were increased in G rats but not changed in R rats, suggesting that chronically increased intrapituitary corticosterone upregulates pituitary CRF receptor expression. In contrast, CRF mRNA levels in the pituitary were increased in R rats. Our data indicate novel mechanisms of corticosteroid metabolic modulation and the involvement of pituitary 11-HSD1 and CRF in glucocorticoid feedback physiology.

Role of rat adrenal antioxidant defense systems in the aldosterone turn-off phenomenon

The mechanism(s) of the "aldosterone turn-off phenomenon", hypoaldosteronemia following chronic ACTH administration, remains unclear. Our previous observation that antioxidants prevented turn-off prompted us to evaluate the chronic effect of ACTH on the enzymatic antioxidant system as well as P450aldo activity and expression of CYP11B2 in adrenal zona glomerulosa. Male Wistar rats were administered ACTH-Z for 5 days with or without antioxidants, vitamin E or DMSO. Adrenal capsules were prepared for P450aldo activity measurement and mRNA content determination by competitive RT-PCR, and immunoreactivity of Mn-SOD in whole adrenals was evaluated. ACTH decreased the P450aldo activity and mRNA level of CYP11B2 in adrenal capsules, while co-administration of vitamin E or DMSO partially blocked this inhibition. ACTH increased Mn-SOD mRNA and immunoreactivity but decreased GPx mRNA. These results suggest that prolonged ACTH treatment increases oxidative stress in the zona glomerulosa and an imbalance in the ratio of Mn-SOD to GPx, possibly via corticosterone overproduction in the zona fasciculata, resulting in the downregulation of CYP11B2. Vitamin E and DMSO might thus protect CYP11B2 expression through their antioxidant actions.

Effects of ACTH on adrenal angiotensin II receptor subtype expression in vivo

Control of adrenal aldosterone secretion is an important endocrine mechanism mediated by angiotensin II (Ang. II). Recently three subtypes of angiotensin receptors have been demonstrated in the rat adrenal. Our aim was to examine if these receptors are affected by hormone treatment in vivo. Treatment of rats with ACTH resulted in a decrease in AT2 receptor binding from 766 +/- 95 to 310 +/- 51 fmol/mg protein (P < 0.05), without significant changes in AT1 receptors. AT2 receptor mRNA was also decreased (18 +/- 2%, of control, P < 0.05; 25+/-7% of control, P < 0.05) after both 2 and 7 day treatment with ACTH. Changes in AT1a receptor mRNA or total AT1 mRNA did not reach statistical significance. Moreover, treatment with dexamethasone or aldosterone did not affect AT1a, AT1b, or AT2 mRNA. These results demonstrate that ACTH treatment results in subtype-specific changes in adrenal AT receptor gene expression in vivo.