11beta-hydroxysteroid dehydrogenase type 2 activity in hypothalamic paraventricular nucleus modulates sympathetic excitation

Central infusion of aldosterone synthase inhibitor attenuates left ventricular dysfunction and remodelling in rats after myocardial infarction

AIMS

Blockade of mineralocorticoid receptors in the central nervous system (CNS) prevents sympathetic hyperactivity and improves left ventricle (LV) function in rats post-myocardial infarction (MI). We examined whether aldosterone produced locally in the brain may contribute to the activation of mineralocorticoid receptors in the CNS.

METHODS AND RESULTS

Two days after coronary artery ligation, Wistar rats received an intra-cerebroventricular (icv) infusion via osmotic mini-pumps of the aldosterone synthase inhibitor FAD286 at 100 microg/kg/day or vehicle for 4 weeks. LV function was assessed by echocardiography at 2 and 4 weeks, and by Millar catheter at 4 weeks. At 4 weeks post-MI, aldosterone in the hippocampus was increased by 70% and tended to increase in the hypothalamus by 20%. These increases were prevented by FAD286. Across groups, aldosterone in the hippocampus and hypothalamus showed a high correlation. There were no differences in brain corticosterone levels. Compared to sham rats, at both 2 and 4 weeks post-MI rats treated with vehicle showed increased LV dimensions and decreased LV ejection fraction. Icv infusion of FAD286 attenuated these changes in LV dimensions and ejection fraction by approximately 30%. At 4 weeks post-MI, LV peak systolic pressure (LVPSP) and dP/dt(max/min) were decreased and LV end-diastolic pressure (LVEDP) was increased. In rats treated with icv FAD286, LVPSP and dP/dt(min) remained normal and LVEDP and dP/dt(max) were markedly improved. Post-MI increases in cardiac fibrosis and cardiomyocyte diameter were substantially attenuated by icv FAD286.

CONCLUSION

These data suggest that aldosterone produced locally in the brain acts as the main agonist of mineralocorticoid receptors in the CNS and contributes substantially to the progressive heart failure post MI.

Chronic treatment with the mineralocorticoid hormone aldosterone results in increased anxiety-like behavior

Aldosterone is the last component of the renin-angiotensin-aldosterone system inducing its peripheral effects via mineralocorticoid receptors (MR). Brain MR bind preferentially glucocorticoids. So far, the role of MR in behavioral functions has been investigated almost exclusively in relation to glucocorticoids. Recently, aldosterone itself has been linked to affective disorders. The aim of this study was to test the hypothesis that chronic elevation of circulating levels of aldosterone leads to increased anxiety. We have investigated the effects of chronic aldosterone treatment on (1) anxiety-like behavior, and (2) basal and stress-induced levels of selected hormones. Forty male Wistar rats were subcutaneously implanted with osmotic minipumps and treated with aldosterone (2 microg/100 g/day) or vehicle for two weeks. Aldosterone concentrations in plasma showed a mild (approximately four-fold) increase at the end of two-week aldosterone treatment. This mild hyperaldosteronism resulted in a significant enhancement of anxiety as demonstrated by alterations in all indicators of anxiety-like behavior measured in the open field and elevated plus-maze tests, without significant changes in measures of general locomotor activity. Aldosterone treatment affected not only the spatiotemporal measures of anxiety, but also the ethological parameters related to exploration and risk assessment. Chronic treatment with aldosterone was associated with increased water intake and decreased plasma renin activity, but failed to modify basal or stress-induced activity of the hypothalamic-pituitary-adrenocortical axis. The results provide evidence on anxiogenic action of prolonged increase in circulating aldosterone concentrations. Thus, aldosterone may represent an important target for future antidepressant and anxiolytic drug development.

Central infusion of aldosterone synthase inhibitor prevents sympathetic hyperactivity and hypertension by central Na+ in Wistar rats

In Wistar rats, increasing cerebrospinal fluid (CSF) Na+ concentration ([Na+]) by intracerebroventricular (ICV) infusion of hypertonic saline causes sympathetic hyperactivity and hypertension that can be prevented by blockade of brain mineralocorticoid receptors (MR). To assess the role of aldosterone produced locally in the brain in the activation of MR in the central nervous system (CNS), Wistar rats were infused ICV with artificial CSF (aCSF), Na+ -rich (800 mmol/l) aCSF, aCSF plus the aldosterone synthase inhibitor FAD286 (100 microg x kg(-1) x day(-1)), or Na+ -rich aCSF plus FAD286. After 2 wk of infusion, rats treated with Na+ -rich aCSF exhibited significant increases in aldosterone and corticosterone content in the hypothalamus but not in the hippocampus, as well as increases in resting blood pressure (BP) and sympathoexcitatory responses to air stress, and impairment of arterial baroreflex function. Concomitant ICV infusion of FAD286 prevented the Na+ -induced increase in hypothalamic aldosterone but not corticosterone and prevented most of the increases in resting BP and sympathoexcitatory and pressor responses to air stress and the baroreflex impairment. FAD286 had no effects in rats infused with ICV aCSF. In another set of rats, 24-h BP and heart rate were recorded via telemetry before and during a 14-day ICV infusion of Na+ -rich aCSF with or without FAD286. Na+ -rich aCSF without FAD286 caused sustained increases ( approximately 10 mmHg) in resting mean arterial pressure that were absent in the rats treated with FAD286. These data suggest that in Wistar rats, an increase in CSF [Na+] may increase the biosynthesis of corticosterone and aldosterone in the hypothalamus, and mainly aldosterone activates MR in the CNS leading to sympathetic hyperactivity and hypertension.

Aldosterone acts centrally to increase brain renin-angiotensin system activity and oxidative stress in normal rats

Aldosterone acts upon mineralocorticoid receptors in the brain to increase blood pressure and sympathetic nerve activity, but the mechanisms are still poorly understood. We hypothesized that aldosterone increases sympathetic nerve activity by upregulating the renin-angiotensin system (RAS) and oxidative stress in the brain, as it does in peripheral tissues. In Sprague-Dawley rats, aldosterone (Aldo) or vehicle (Veh) was infused for 1 wk via an intracerebroventricular (ICV) cannula, while RU-28318 (selective mineralocorticoid receptor antagonist), Tempol (superoxide dismutase mimetic), losartan [angiotensin II type 1 receptor (AT1R) antagonist], or Veh was infused simultaneously via a second ICV cannula. After 1 wk of ICV Aldo, plasma norepinephrine was increased and mean arterial pressure was slightly elevated, but heart rate was unchanged. These effects were ameliorated by ICV infusion of RU-28318, Tempol or losartan. Aldo increased expression of AT1R and angiotensin-converting enzyme (ACE) mRNA in hypothalamic tissue. RU-28318 minimized and Tempol prevented the increase in AT1R mRNA; RU-28318 prevented the increase in ACE mRNA. Losartan had no effect on AT1R or ACE mRNA. Immunohistochemistry revealed Aldo-induced increases in dihydroethidium staining (indicating oxidative stress) and Fra-like activity (indicating neuronal excitation) in neurons of the hypothalamic paraventricular nucleus (PVN). RU-28318 prevented the increases in superoxide and Fra-like activity in PVN; Tempol and losartan minimized these effects. Acute ICV infusions of sarthran (AT1R antagonist) or Tempol produced greater sympathoinhibition in Aldo-treated than in Veh-treated rats. Thus aldosterone upregulates key elements of brain RAS and induces oxidative stress in the hypothalamus. Aldosterone may increase sympathetic nerve activity by these mechanisms.

Does aldosterone upregulate the brain renin-angiotensin system in rats with heart failure?

The brain renin-angiotensin system (RAS) contributes to increased sympathetic drive in heart failure (HF). The factors upregulating the brain RAS in HF remain unknown. We hypothesized that aldosterone (ALDO), a downstream product of the systemic RAS that crosses the blood-brain barrier, signals the brain to increase RAS activity in HF. We examined the relationship between circulating and brain ALDO in normal intact rats, in adrenalectomized rats receiving subcutaneous infusions of ALDO, and in rats with ischemia-induced HF and sham-operated controls. Brain ALDO levels were proportional to plasma ALDO levels across the spectrum of rats studied. Compared with sham-operated controls rats, HF rats had higher plasma and hypothalamic tissue levels of ALDO. HF rats also had higher expression of mRNA and protein for angiotensin-converting enzyme and angiotensin type 1 receptors in the hypothalamus, increased reduced nicotinamide-adenine dinucleotide phosphate oxidase activity and superoxide generation in the paraventricular nucleus of the hypothalamus, increased excitation of paraventricular nucleus neurons, and increased plasma norepinephrine. HF rats treated for 4 weeks with intracerebroventricular RU28318 (1 microg/h), a selective mineralocorticoid receptor antagonist, had less hypothalamic angiotensin-converting enzyme and angiotensin type 1 receptor mRNA and protein, less reduced nicotinamide-adenine dinucleotide phosphate-induced superoxide in the paraventricular nucleus, fewer excited paraventricular nucleus neurons, and lower plasma norepinephrine. RU28318 had no effect on plasma ALDO or on angiotensin-converting enzyme or angiotensin type 1 receptor expression in brain cortex. The data demonstrate that ALDO of adrenal origin enters the hypothalamus in direct proportion to plasma levels and suggest that ALDO contributes to the upregulation of hypothalamic RAS activity and sympathetic drive in heart failure.

Adrenalectomy potentiates noradrenergic suppression of GABAergic transmission in parvocellular neurosecretory neurons of hypothalamic paraventricular nucleus

Glucocorticoids are known to regulate both the noradrenergic and GABAergic inputs to the paraventricular nucleus (PVN). However, little is known about the effects of glucocorticoids on the interaction of these two input systems. Here we examined the effects of bilateral adrenalectomy (ADX) on the noradrenergic modulation of GABAergic transmission in the type II PVN neurons labeled with a retrograde dye injected into the pituitary stalk. Noradrenaline either reduced or augmented the frequency of spontaneous inhibitory postsynaptic current (sIPSC) without changing the amplitude and decay time constant. These effects were blocked by alpha2A- and alpha(1A/1L)-adrenoceptor antagonists, respectively. ADX increased the proportion of the neurons showing the noradrenergic reduction and the extent of reduction in the IPSC frequency. The ADX-induced changes were reversed by supplementation of ADX rats with corticosterone (10-mg pellet). ADX also potentiated the noradrenergic reduction in the frequency of miniature IPSC and paired-pulse facilitation of evoked IPSC. BRL 44408 (3 microM), a alpha2A-adrenoceptor antagonist, blocked the noradrenergic reduction in ADX rats. Corticotropin-releasing hormone and/or vasopressin transcripts were detected in neurons displaying noradrenergic augmentation or reduction of IPSC frequency. ADX enhanced the proportion of neurons expressing corticotropin-releasing hormone. Collectively, the results suggest that depletion of corticosterone by ADX markedly potentiates the noradrenergic suppression of GABAergic transmission mediated by the alpha2A-adrenoceptors on the GABAergic terminals in the parvocellular neurosecretory PVN neurons. These results may provide a novel synaptic mechanism for the glucocorticoid-induced plasticity in the noradrenergic modulation of neuroendocrine function of the PVN.

Sympathoexcitation by oxidative stress in the brain mediates arterial pressure elevation in salt-sensitive hypertension

Central sympathoexcitation is involved in the pathogenesis of salt-sensitive hypertension. We have suggested that oxidative stress in the brain modulates the sympathetic regulation of arterial pressure. Thus, we investigated whether oxidative stress could mediate central sympathoexcitation in salt-sensitive hypertension. Five- to 6-week-old male Dahl salt-sensitive rats and salt-resistant rats were fed with a normal (0.3%) or high- (8%) salt diet for 4 weeks. In urethane-anesthetized and artificially ventilated rats, arterial pressure, renal sympathetic nerve activity, and heart rate decreased in a dose-dependent fashion, when 20 or 40 micromol of tempol, a membrane-permeable superoxide dismutase mimetic, was infused into the lateral cerebral ventricle. The same degree of reduction was noted in salt-sensitive and salt-resistant rats without salt loading. Salt loading significantly increased central tempol-induced reductions in arterial pressure (-29.1+/-4.8% versus -10.6+/-3.3% at 40 micromol; P<0.01), sympathetic nerve activity (-18.7+/-2.0% versus -7.1+/-1.8%; P<0.01), and heart rate (-10.7+/-2.8% versus -2.0+/-0.7%; P<0.05) in salt-sensitive rats but not in salt-resistant rats. Intracerebroventricular diphenyleneiodonium, a reduced nicotinamide-adenine dinucleotide phosphate oxidase inhibitor, also elicited significantly greater reduction in each parameter in salt-loaded salt-sensitive rats. Moreover, salt loading increased reduced nicotinamide-adenine dinucleotide phosphate-dependent superoxide production in the hypothalamus in salt-sensitive rats but not in salt-resistant rats. In addition, reduced nicotinamide-adenine dinucleotide phosphate oxidase subunits p22(phox), p47(phox), and gp91(phox) mRNA expression significantly increased in the hypothalamus of salt-loaded salt-sensitive rats. In conclusion, in salt-sensitive hypertension, increased oxidative stress in the brain, possibly via activation of reduced nicotinamide-adenine dinucleotide phosphate oxidase, may elevate arterial pressure through central sympathoexcitation.

Cortisol metabolism in hypertension

Corticosteroids are critically involved in blood pressure regulation. Lack of adrenal steroids in Addison's disease causes life-threatening hypotension, whereas glucocorticoid excess in Cushing's syndrome invariably results in high blood pressure. At a pre-receptor level, glucocorticoid action is modulated by 11beta-hydroxysteroid dehydrogenases (11beta-HSDs). 11Beta-HSD1 activates cortisone to cortisol to facilitate glucocorticoid receptor (GR)-mediated action. By contrast, 11beta-HSD2 plays a pivotal role in aldosterone target tissues where it catalyses the opposite reaction (i.e. inactivation of cortisol to cortisone) to prevent activation of the mineralocorticoid receptor (MR) by cortisol. Mutations in the 11beta-HSD2 gene cause a rare form of inherited hypertension, the syndrome of apparent mineralocorticoid excess (AME), in which cortisol activates the MR resulting in severe hypertension and hypokalemia. Ingestion of competitive inhibitors of 11beta-HSD2 such as liquorice and carbenoxolone result in a similar but milder clinical phenotype. Epidemiological data suggests that polymorphic variability in the HSD11B2 gene determines salt sensitivity in the general population, which is a key predisposing factor to adult onset hypertension in some patients. Extrarenal sites of glucocorticoid action and metabolism that might impact on blood pressure include the vasculature and the central nervous system. Intriguingly, increased exposure to glucocorticoids during fetal life promotes high blood pressure in adulthood suggesting an early programming effect. Thus, metabolism and action in many peripheral tissues might contribute to the pathophysiology of human hypertension.