Mineralocorticoid receptor blockade by canrenoate increases both spontaneous and stimulated adrenal function in humans

Glucocorticoid Negative Feedback in Regulation of the Hypothalamic-Pituitary-Adrenal Axis in Rhesus Monkeys With Various Types of Adaptive Behavior: Individual and Age-Related Differences

The study of the mechanisms underlying the increased vulnerability of the individual to stressful environmental factors in different age periods is of great relevance for prevention and effective treatment of stress-dependent diseases that are widespread in the population of aging individuals. The purpose of our study was to investigate the individual and age-related features of the glucocorticoid negative feedback in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the key adaptive neuroendocrine system, in experiments with physically healthy young and old female rhesus monkeys with administration of mineracorticoid receptor (fludrocortisone) and glucocorticoid receptor (dexamethasone) agonists. We studied the monkeys with increased trait anxiety and depression-like behavior (DAB) characterized, as previously was shown, by the increased vulnerability to acute stress and the animals with normal standard behavior (SB) as the control. The pronounced individual differences in the reaction of HPA axis to fludrocortisone and dexamethasone in young animals were found. Young animals with DAB showed a lower sensitivity of HPA axis to the inhibitory effect of both fludrocortisone and dexamethasone compared with young animals with SB. At the same time, there were no significant intergroup differences in the concentration of ACTH and cortisol in response to placebo injection, i.e., in basal conditions. The old individuals with DAB demonstrated the essential relative resistance of HPA axis to fludrocortisone test and higher basal plasma levels of cortisol and ACTH in the evening (the period of HPA axis low circadian activity) compared to old SB animals. In the same time, the intergroup differences in the response of HPA axis to dexamethasone administration were leveled due to age-related increase in sensitivity of HPA axis to dexamethasone in animals with DAB. These data testify the pronounced intergroup and age differences in the feedback regulation of HPA axis, presumably resulting from unequal individual, and age-related changes in the activity of mineralcorticoid and glucocorticoid receptors in the brain structures supporting the functions of HPA axis. The maximum age disorders in functioning of the negative feedback mechanism in the regulation of HPA axis are characteristic of animals with DAB, which, apparently, underlie the increased vulnerability of these animals to stress exposure.

Regulation of aldosterone secretion by mineralocorticoid receptor-mediated signaling

We posit the existence of a paracrine/autocrine negative feedback loop, mediated by the mineralocorticoid receptor (MR), regulating aldosterone secretion. To assess this hypothesis, we asked whether altering MR activity in zona glomerulosa (ZG) cells affects aldosterone production. To this end, we studied ex vivo ZG cells isolated from male Wistar rats fed chow containing either high (1.6% Na⁺ (HS)) or low (0.03% Na⁺ (LS)) amount of sodium. Western blot analyses demonstrated that MR was present in both the ZG and zona fasciculata/zona reticularis (ZF/ZR/ZR). In ZG cells isolated from rats on LS chow, MR activation by fludrocortisone produced a 20% and 60% reduction in aldosterone secretion basally and in response to angiotensin II (ANGII) stimulation, respectively. Corticosterone secretion was increased in these cells suggesting that aldosterone synthase activity was being reduced by fludrocortisone. In contrast, canrenoic acid, an MR antagonist, enhanced aldosterone production by up to 30% both basally and in response to ANGII. Similar responses were observed in ZG cells from rats fed HS. Modulating glucocorticoid receptor (GR) activity did not alter aldosterone production by ZG cells; however, altering GR activity did modify corticosterone production from ZF/ZR/ZR cells both basally and in response to adrenocorticotropic hormone (ACTH). Additionally, activating the MR in ZF/ZR/ZR cells strikingly reduced corticosterone secretion. In summary, these data support the hypothesis that negative ultra-short feedback loops regulate adrenal steroidogenesis. In the ZG, aldosterone secretion is regulated by the MR, but not the GR, an effect that appears to be secondary to a change in aldosterone synthase activity.

A role for mineralocorticoid receptors in the physiology of the ovine fetus: effects on ACTH and lung liquid composition

In the human and ovine fetus, the presence of 11β-hydroxysteroid dehydrogenase 1 allows cortisol and other corticosteroids to act at mineralocorticoid receptors (MRs) in lung and brain. To test the physiologic role of MRs in the late gestation fetus, fetal lambs were infused with a specific MR antagonist for 12 h. Infusion of the MR antagonist significantly increased plasma ACTH and cortisol concentrations. Infusion of the MR antagonist also significantly increased fetal Pco2 and hematocrit, and decreased fetal pH, but did not alter fetal heart rate or blood pressure. Infusion of the MR antagonist altered the ratio of Na⁺ to K⁺ in lung fluid but did not alter the rate of production of lung liquid or the expression of the epithelial sodium channel α or of the Na,K ATPaseα1 in lung. These results suggest that corticosteroids act at MR to regulate ACTH and blood volume and modulate lung fluid composition in the fetus, but basal levels of corticosteroids do not alter lung liquid production rate through effects on MR.

Differential effects of mineralocorticoid blockade on the hypothalamo-pituitary-adrenal axis in pregnant and nonpregnant ewes

During pregnancy, plasma ACTH and cortisol are chronically increased; this appears to occur through a reset of hypothalamo-pituitary-adrenal (HPA) activity. We have hypothesized that differences in mineralocorticoid receptor activity in pregnancy may alter feedback inhibition of the HPA axis. We tested the effect of MR antagonism in pregnant and nonpregnant ewes infused for 4 h with saline or the MR antagonist canrenoate. Pregnancy significantly increased plasma ACTH, cortisol, angiotensin II, and aldosterone. Infusion of canrenoate increased plasma ACTH, cortisol, and aldosterone in both pregnant and nonpregnant ewes; however, the temporal pattern of these responses differed between these two reproductive states. In nonpregnant ewes, plasma ACTH and cortisol transiently increased at 1 h of infusion, whereas in pregnant ewes the levels gradually increased and were significantly elevated from 2 to 4 h of infusion. MR blockade increased plasma aldosterone from 2 to 4 h in the pregnant ewes but only at 4 h in the nonpregnant ewes. In both pregnant and nonpregnant ewes, the increase in plasma aldosterone was significantly related to the timing and magnitude of the increase in plasma potassium. The results indicate a differential effect of MR activity in pregnant and nonpregnant ewes and suggest that the slow changes in ACTH, cortisol, and aldosterone are likely to be related to blockade of MR effects in the kidney rather than to effects of MR blockade in hippocampus or hypothalamus.

Arginine vasopressin, fluid balance and exercise: is exercise-associated hyponatraemia a disorder of arginine vasopressin secretion?

The ability of the human body to regulate plasma osmolality (POsm) within a very narrow and well defined physiological range underscores the vital importance of preserving water and sodium balance at rest and during exercise. The principle endocrine regulator of whole body fluid homeostasis is the posterior pituitary hormone, arginine vasopressin (AVP). Inappropriate AVP secretion may perpetuate either slow or rapid violation of these biological boundaries, thereby promoting pathophysiology, morbidity and occasional mortality. In the resting state, AVP secretion is primarily regulated by changes in POsm (osmotic regulation). The osmotic regulation of AVP secretion during exercise, however, may possibly be enhanced or overridden by many potential non-osmotic factors concurrently stimulated during physical activity, particularly during competition. The prevalence of these highly volatile non-osmotic AVP stimuli during strenuous or prolonged physical activity may reflect a teleological mechanism to promote water conservation during exercise. However, non-osmotic AVP secretion, combined with high fluid availability plus sustained fluid intake (exceeding fluid output), has been hypothesized to lead to an increase in both the incidence and related deaths from exercise-associated hyponatraemia (EAH) in lay and military populations. Inappropriately, high plasma AVP concentrations ([AVP](p)) associated with low blood sodium concentrations facilitate fluid retention and sodium loss, thereby possibly reconciling both the water intoxication and sodium loss theories of hyponatraemia that are currently under debate. Therefore, given the potential for a variety of exercise-induced non-osmotic stimuli for AVP secretion, hydration strategies must be flexible, individualized and open to change during competitive events to prevent the occurrence of rare, but life-threatening, EAH. This review focuses on the potential osmotic and non-osmotic stimuli to AVP secretion that may affect fluid homeostasis during physical activity. Recent laboratory and field data support: (i) stimulatory effects of exercise intensity and duration on [AVP](p); (ii) possible relationships between changes in POsm with changes in both sweat and urinary osmolality; (iii) alterations in the AVP osmoregulatory set-point by sex steroid hormones; (iv) differences in [AVP](p) in trained versus untrained athletes; and (v) potential inter-relationships between AVP and classical (aldosterone, atrial natriuretic peptide) and non-classical (oxytocin, interleukin-6) endocrine mediators. The review concludes with a hypothesis on how sustained fluid intakes beyond the capacity for fluid loss might possibly facilitate the development of hyponatraemia if exercise-induced non-osmotic stimuli override 'normal' osmotic suppression of AVP when hypo-osmolality exists.

The impact of mineralocorticoid receptor ISO/VAL genotype (rs5522) and stress on reward learning

Research suggests that stress disrupts reinforcement learning and induces anhedonia. The mineralocorticoid receptor (MR) determines the sensitivity of the stress response, and the missense iso/val polymorphism (Ile180Val, rs5522) of the MR gene (NR3C2) has been associated with enhanced physiological stress responses, elevated depressive symptoms and reduced cortisol-induced MR gene expression. The goal of these studies was to evaluate whether rs5522 genotype and stress independently and interactively influence reward learning. In study 1, participants (n = 174) completed a probabilistic reward task under baseline (i.e. no-stress) conditions. In study 2, participants (n = 53) completed the task during a stress (threat-of-shock) and no-stress condition. Reward learning, i.e. the ability to modulate behavior as a function of reinforcement history, was the main variable of interest. In study 1, in which participants were evaluated under no-stress conditions, reward learning was enhanced in val carriers. In study 2, participants developed a weaker response bias toward a more frequently rewarded stimulus under the stress relative to no-stress condition. Critically, stress-induced reward learning deficits were largest in val carriers. Although preliminary and in need of replication due to small sample size, findings indicate that psychiatrically healthy individuals carrying the MR val allele, gene, which has been recently linked to depression, showed a reduced ability to modulate behavior as a function of reward when facing an acute, uncontrollable stressor. Future studies are warranted to evaluate whether rs5522 genotype interacts with naturalistic stressors to increase the risk of depression and whether stress-induced anhedonia might moderate such risk.

Mineralocorticoid receptor overexpression in forebrain decreases anxiety-like behavior and alters the stress response in mice

Although numerous stress-related molecules have been implicated in vulnerability to psychiatric illness, especially major depression and anxiety disorders, the role of the brain mineralocorticoid receptor (MR) in stress, depression, and affective function is not well defined. MR is a steroid hormone receptor that detects circulating glucocorticoids with high affinity and has been primarily implicated in controlling their basal level and circadian rhythm. To specifically address the role of MR in hypothalamic-pituitary-adrenal axis activity and anxiety-related behaviors, we generated transgenic mice with increased levels of MR in the forebrain (MRov mice) by using the forebrain-specific calcium/calmodulin-dependent protein kinase II alpha promoter to direct expression of MR cDNA. A mild but chronic elevation in forebrain MR results in decreased anxiety-like behavior in both male and female transgenic mice. Female MRov mice also exhibit a moderate suppression of the corticosterone response to restraint stress. Increased forebrain MR expression alters the expression of two genes associated with stress and anxiety, leading to a decrease in the hippocampal glucocorticoid receptor (GR) and an increase in serotonin receptor 5HT-1a, consistent with the decreased anxiety phenotype. These data suggest that the functions of forebrain MR may overlap with GR in hypothalamic-pituitary-adrenal axis regulation, but they dissociate significantly from GR in the modulation of affective responses, with GR overexpression increasing anxiety-like behavior and MR overexpression dampening it. These findings point to the importance of the MR:GR ratio in the control of emotional reactivity.