Chronic AT1 receptor blockade normalizes NMDA-mediated changes in renal sympathetic nerve activity and NR1 expression within the PVN in rats with heart failure

Role of the central renin‑angiotensin system in hypertension (Review)

Present in more than one billion adults, hypertension is the most significant modifiable risk factor for mortality resulting from cardiovascular disease. Although its pathogenesis is not yet fully understood, the disruption of the renin-angiotensin system (RAS), consisting of the systemic and brain RAS, has been recognized as one of the primary reasons for several types of hypertension. Therefore, acquiring sound knowledge of the basic science of RAS and the under- lying mechanisms of the signaling pathways associated with RAS may facilitate the discovery of novel therapeutic targets with which to promote the management of patients with cardiovascular and kidney disease. In total, 4 types of angiotensin II receptors have been identified (AT1R-AT4R), of which AT1R plays the most important role in vasoconstriction and has been most extensively studied. It has been found in several regions of the brain, and its distribution is highly associated with that of angiotensin-like immunoreactivity in nerve terminals. The effect of AT1R involves the activation of multiple media and signaling pathways, among which the most important signaling pathways are considered to be AT1R/JAK/STAT and Ras/Raf/MAPK pathways. In addition, the regulation of the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and cyclic AMP response element-binding (CREB) pathways is also closely related to the effect of ATR1. Their mechanisms of action are related to pro-inflammatory and sympathetic excitatory effects. Central AT1R is involved in almost all types of hypertension, including spontaneous hypertension, salt-sensitive hypertension, obesity-induced hypertension, renovascular hypertension, diabetic hypertension, L-NAME-induced hypertension, stress-induced hypertension, angiotensin II-induced hyper- tension and aldosterone-induced hypertension. There are 2 types of central AT1R blockade, acute blockade and chronic blockade. The latter can be achieved by chemical blockade or genetic engineering. The present review article aimed to high- light the prevalence, functions, interactions and modulation means of central AT-1R in an effort to assist in the treatment of several pathological conditions. The identification of angiotensin-derived peptides and the development of AT-2R agonists may provide a wider perspective on RAS, as well as novel therapeutic strategies.

Sympathetic neural responses in heart failure during exercise and after exercise training

The sympathetic nervous system coordinates the cardiovascular response to exercise. This regulation is impaired in both experimental and human heart failure with reduced ejection fraction (HFrEF), resulting in a state of sympathoexcitation which limits exercise capacity and contributes to adverse outcome. Exercise training can moderate sympathetic excess at rest. Recording sympathetic nerve firing during exercise is more challenging. Hence, data acquired during exercise are scant and results vary according to exercise modality. In this review we will: (1) describe sympathetic activity during various exercise modes in both experimental and human HFrEF and consider factors which influence these responses; and (2) summarise the effect of exercise training on sympathetic outflow both at rest and during exercise in both animal models and human HFrEF. We will particularly highlight studies in humans which report direct measurements of efferent sympathetic nerve traffic using intraneural recordings. Future research is required to clarify the neural afferent mechanisms which contribute to efferent sympathetic activation during exercise in HFrEF, how this may be altered by exercise training, and the impact of such attenuation on cardiac and renal function.

Central Ang II (Angiotensin II)-Mediated Sympathoexcitation: Role for HIF-1α (Hypoxia-Inducible Factor-1α) Facilitated Glutamatergic Tone in the Paraventricular Nucleus of the Hypothalamus

Central infusion of Ang II (angiotensin II) has been associated with increased sympathetic outflow resulting in neurogenic hypertension. In the present study, we appraised whether the chronic increase in central Ang II activates the paraventricular nucleus of the hypothalamus (PVN) resulting in elevated sympathetic tone and altered baro- and chemoreflexes. Further, we evaluated the contribution of HIF-1α (hypoxia-inducible factor-1α), a transcription factor involved in enhancing the expression of N-methyl-D-aspartate receptors and thus glutamatergic-mediated sympathetic tone from the PVN. Ang II infusion (20 ng/minute, intracerebroventricular, 14 days) increased mean arterial pressure (126±9 versus 84±4 mm Hg), cardiac sympathetic tone (96±7 versus 75±6 bpm), and decreased cardiac parasympathetic tone (16±2 versus 36±3 versus bpm) compared with saline-infused controls in conscious rats. The Ang II-infused group also showed an impaired baroreflex control of heart rate (-1.50±0.1 versus -2.50±0.3 bpm/mm Hg), potentiation of the chemoreflex pressor response (53±7 versus 30±7 mm Hg) and increased number of FosB-labeled cells (53±3 versus 19±4) in the PVN. Concomitant with the activation of the PVN, there was an increased expression of HIF-1α and N-Methyl-D-Aspartate-type1 receptors in the PVN. Further, Ang II-infusion showed increased renal sympathetic nerve activity (20.5±2.3% versus 6.4±1.9% of Max) and 3-fold enhanced renal sympathetic nerve activity responses to microinjection of N-methyl-D-aspartate (200 pmol) into the PVN of anesthetized rats. Further, silencing of HIF-1α in NG108 cells abrogated the expression of N-methyl-D-aspartate-N-methyl-D-aspartate-type1 induced by Ang II. Taken together, our studies suggest a novel Ang II-HIF-1α-N-methyl-D-aspartate receptor-mediated activation of preautonomic neurons in the PVN, resulting in increased sympathetic outflow and alterations in baro- and chemoreflexes.

Angiotensin-converting enzyme 2 activator, DIZE in the basolateral amygdala attenuates the tachycardic response to acute stress by modulating glutamatergic tone

The basolateral amygdala (BLA) is critical in the control of the sympathetic output during stress. Studies demonstrated the involvement of the renin-angiotensin system components in the BLA. Angiotensin-(1-7) [Ang-(1-7)], acting through Mas receptors, reduces stress effects. Considering that angiotensin-converting enzyme 2 (ACE2) is the principal enzyme for the production of Ang-(1-7), here we evaluate the cardiovascular reactivity to acute stress after administration of the ACE2 activator, diminazene aceturate (DIZE) into the BLA. We also tested whether systemic treatment with DIZE could modify synaptic activity in the BLA and its effect directly on the expression of the N-methyl-d-aspartate receptors (NMDARs) in NG108 neurons in-vitro. Administration of DIZE into the BLA (200 pmol/100 nL) attenuated the tachycardia to stress (ΔHR, bpm: vehicle = 103 ± 17 vs DIZE = 49 ± 7 p = 0.018); this effect was inhibited by Ang-(1-7) antagonist, A-779 (ΔHR, bpm: DIZE = 49 ± 7 vs A-779 + DIZE = 100 ± 15 p = 0.04). Systemic treatment with DIZE attenuated the excitatory synaptic activity in the BLA (Frequency (Hz): vehicle = 2.9 ± 0.4 vs. DIZE =1.8 ± 0.3 p < 0.04). NG108 cells treated with DIZE demonstrated decreased expression of l subunit NMDAR-NR1 (NR1 expression (a.u): control = 0.534 ± 0.0593 vs. DIZE = 0.254 ± 0.0260) of NMDAR and increases of Mas receptors expression. These data demonstrate that DIZE attenuates the tachycardia evoked by acute stress. This effect results from a central action in the BLA involving activation of Mas receptors. The ACE2 activation via DIZE treatment attenuated the frequency of excitatory synaptic activity in the basolateral amygdala and this effect can be related with the decreases of the NMDAR-NR1 receptor expression.

Exercise training augments neuronal nitric oxide synthase dimerization in the paraventricular nucleus of rats with chronic heart failure

Exercise training (ExT) is an established non-pharmacological therapy that improves the health and quality of life in patients with chronic heart failure (CHF). Exaggerated sympathetic drive characterizes CHF due to an imbalance of the autonomic nervous system. Neuronal nitric oxide synthase (nNOS) in the paraventricular nucleus (PVN) produce nitric oxide (NO•), which is known to regulate the sympathetic tone. Previously we have shown that during CHF, the catalytically active dimeric form of nNOS is significantly decreased with a concurrent increase in protein inhibitor of nNOS (PIN) expression, a protein that dissociates dimeric nNOS to monomers and facilitates its degradation. Dimerization of nNOS also requires (6R)-5,6,7,8-tetrahydrobiopterin (BH4) for stability and activity. Previously, we have shown that ExT improves NO-mediated sympathetic inhibition in the PVN; however, the molecular mechanism remains elusive. We hypothesized; ExT restores the sympathetic drive by increasing the levels and catalytically active form of nNOS by abrogating changes in the PIN in the PVN of CHF rats. CHF was induced in adult male Sprague-Dawley rats by coronary artery ligation, which reliably mimics CHF in patients with myocardial infarction. After 4 weeks of surgery, Sham and CHF rats were subjected to 3 weeks of progressive treadmill exercise. ExT significantly (p < 0.05) decreased PIN expression and increased dimer/monomer ratio of nNOS in the PVN of rats with CHF. Moreover, we found decreased GTP cyclohydrolase 1(GCH1) expression: a rate-limiting enzyme for BH4 biosynthesis in the PVN of CHF rats suggesting that perhaps reduced BH4 availability may also contribute to decreased nNOS dimers. Interestingly, CHF induced decrease in GCH1 expression was increased with ExT. Our findings revealed that ExT rectified decreased PIN and GCH1 expression and increased dimer/monomer ratio of nNOS in the PVN, which may lead to increase NO• bioavailability resulting in amelioration of activated sympathetic drive during CHF.

Prenatal cold exposure causes hypertension in offspring by hyperactivity of the sympathetic nervous system

Environmental temperature plays a role in the variation of blood pressure. Maternal cold stress could affect the physiological phenotype of the offspring, including blood pressure elevation. In the present study, we found that adult offspring of dams exposed to cold have increased systolic and diastolic blood pressure, and decreased urine volume and sodium excretion, accompanied by increased heart rate and heart rate variability, secondary to increased activity of the sympathetic nervous system. Renal denervation or adrenergic receptor blockade decreased blood pressure and increased sodium excretion. The increase in peripheral sympathetic nerve activity can be ascribed to the central nervous system because administration of clonidine, a centrally acting α₂ adrenergic receptor agonist, lowered blood pressure to a greater degree in the prenatal cold-exposed than control offspring. Moreover, these prenatal cold-exposed offspring had hypothalamic paraventricular nucleus (PVN) disorder because magnetic resonance spectroscopy showed decreased N-acetylaspartate and increased choline and creatine ratios in the PVN. Additional studies found that prenatal cold exposure impaired the balance between inhibitory and excitatory neurons. This led to PVN overactivation that was related to enhanced PVN-angiotensin II type 1 (AT₁) receptor expression and function. Microinjection of the AT₁ receptor antagonist losartan in the PVN lowered blood pressure to a greater extent in prenatal cold-exposed that control offspring. The present study provides evidence for overactive peripheral and central sympathetic nervous systems in the pathogenesis of prenatal cold-induced hypertension. Central AT₁ receptor blockade in the PVN may be a key step for treatment of this type hypertension.

Central blockade of the AT1 receptor attenuates pressor effects via reduction of glutamate release and downregulation of NMDA/AMPA receptors in the rostral ventrolateral medulla of rats with stress-induced hypertension

Glutamatergic activity in the rostral ventrolateral medulla (RVLM), which is an important brain area where angiotensin II (Ang II) elicits its pressor effects, contributes to the onset of hypertension. The present study aimed to explore the effect of central Ang II type 1 receptor (AT₁R) blockade on glutamatergic actions in the RVLM of stress-induced hypertensive rats (SIHR). The stress-induced hypertension (SIH) model was established by electric foot shocks combined with noises. Normotensive Sprague-Dawley rats (control) and SIHR were intracerebroventricularly infused with the AT₁R antagonist candesartan or artificial cerebrospinal fluid for 14 days. Mean arterial pressure (MAP), heart rate (HR), plasma norepinephrine (NE), glutamate, and the expression of N-methyl-D-aspartic acid (NMDA) receptor subunit NR1, and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in the RVLM increased in the SIH group. These increases were blunted by candesartan. Bilateral microinjection of the ionotropic glutamate receptor antagonist kynurenic acid, the NMDA receptor antagonist D-2-amino-5-phosphonopentanoate, or the AMPA/kainate receptors antagonist 6-cyano-7-nitroquinoxaline-2,3-dione into the RVLM caused a depressor response in the SIH group, but not in other groups. NR1 and AMPA receptors expressed in the glutamatergic neurons of the RVLM, and glutamate levels, increased in the intermediolateral column of the spinal cord of SIHR. Central Ang II elicits release of glutamate, which binds to the enhanced ionotropic NMDA and AMPA receptors via AT₁R, resulting in activation of glutamatergic neurons in the RVLM, increasing sympathetic excitation in SIHR.

Regulation of sympathetic vasomotor activity by the hypothalamic paraventricular nucleus in normotensive and hypertensive states

The hypothalamic paraventricular nucleus (PVN) is a unique and important brain region involved in the control of cardiovascular, neuroendocrine, and other physiological functions pertinent to homeostasis. The PVN is a major source of excitatory drive to the spinal sympathetic outflow via both direct and indirect projections. In this review, we discuss the role of the PVN in the regulation of sympathetic output in normal physiological conditions and in hypertension. In normal healthy animals, the PVN presympathetic neurons do not appear to have a major role in sustaining resting sympathetic vasomotor activity or in regulating sympathetic responses to short-term homeostatic challenges such as acute hypotension or hypoxia. Their role is, however, much more significant during longer-term challenges, such as sustained water deprivation, chronic intermittent hypoxia, and pregnancy. The PVN also appears to have a major role in generating the increased sympathetic vasomotor activity that is characteristic of multiple forms of hypertension. Recent studies in the spontaneously hypertensive rat model have shown that impaired inhibitory and enhanced excitatory synaptic inputs to PVN presympathetic neurons are the basis for the heightened sympathetic outflow in hypertension. We discuss the molecular mechanisms underlying the presynaptic and postsynaptic alterations in GABAergic and glutamatergic inputs to PVN presympathetic neurons in hypertension. In addition, we discuss the ability of exercise training to correct sympathetic hyperactivity by restoring blood-brain barrier integrity, reducing angiotensin II availability, and decreasing oxidative stress and inflammation in the PVN.

Evidence-based nutritional and pharmacological interventions targeting chronic low-grade inflammation in middle-age and older adults: A systematic review and meta-analysis

Growing evidence suggests chronic low-grade inflammation (LGI) as a possible mechanism underlying the aging process. Some biological and pharmaceutical compounds may reduce systemic inflammation and potentially avert functional decline occurring with aging. The aim of the present meta-analysis was to examine the association of pre-selected interventions on two established biomarkers of inflammation, interleukin-6 (IL-6), and C-reactive protein (CRP) in middle-age and older adults with chronic LGI. We reviewed the literature on potential anti-inflammatory compounds, selecting them based on safety, tolerability, acceptability, innovation, affordability, and evidence from randomized controlled trials. Six compounds met all five inclusion criteria for our systematic review and meta-analysis: angiotensin II receptor blockers (ARBs), metformin, omega-3, probiotics, resveratrol and vitamin D. We searched in MEDLINE, PubMed and EMBASE database until January 2017. A total of 49 articles fulfilled the selection criteria. Effect size of each study and pooled effect size for each compound were measured by the standardized mean difference. I² was computed to measure heterogeneity of effects across studies. The following compounds showed a significant small to large effect in reducing IL-6 levels: probiotics (-0.68 pg/ml), ARBs (-0.37 pg/ml) and omega-3 (-0.19 pg/ml). For CRP, a significant small to medium effect was observed with probiotics (-0.43 mg/L), ARBs (-0.2 mg/L), omega-3 (-0.17 mg/L) and metformin (-0.16 mg/L). Resveratrol and vitamin D were not associated with any significant reductions in either biomarker. These results suggest that nutritional and pharmaceutical compounds can significantly reduce established biomarkers of systemic inflammation in middle-age and older adults. The findings should be interpreted with caution, however, due to the evidence of heterogeneity across the studies.

Effect of the changes of NMDA receptor in hypothalamic paraventricular nucleus on cardiac function and sympathetic nervous activity in rats with heart failure

OBJECTIVE

To observe the effect of the changes of n-methyl-d-aspartate receptor 1 (NMDAR1), tyrosine hydroxylase (TH), and glutamic acid decarboxylase 67 (GAD67) in the hypothalamic paraventricular nucleus (PVN) on cardiac function and sympathetic nervous activity in rats with heart failure (HF).

METHODS

Thirty-six adult male SD rats were randomly divided into the heart failure group (HF), the heart failure + NMDA receptors agonist AP5 intervention group (HF-AP5), and the Sham-operation group (SO) (n = 12). HF model in SD rats was induced by ligation of left coronary artery. AP5 (0.02 μg/h) was administrated by the paraventricular nucleus subsequently for 4 weeks. The cardiac function, renal sympathetic nerve activity (RSNA), lung/body weight ratio (L/BW), and right ventricle/body weight ratio (RV/BW), as well as the plasma noradrenaline (NE) and Angiotensin II (Ang II) level and the expressions of NMDAR1, GAD67, and TH in PVN, in different groups were recorded 4 weeks after the establishment of HF model.

RESULTS

After the coronary artery was ligated, LVEDP was increased, ±dp/dt max and LVEF were decreased, lung/BW and RV/BW were raised. RSNA, Ang II and NE were raised. Expression of NMDAR1 and TH were increased, but GAD67 was decreased. The levels of LVEDP, lung/BW, and RV/BW in group HF-AP5 were reduced while ± dp/dtmax was increased after the treatment. The blood Ang II and NE content was decreased, RSNA was reduced, expression of NMDAR1 and TH were downregulated, but GAD67 was upregulated.

CONCLUSIONS

NMDAR1 is significantly activated in PVN of HF rats, the activity of TH is increased, GAD67 is downregulated, RSNA is increased, and the heart function is decreased. NMDA receptor blockers can alleviate HF.

TNF-α receptor 1 knockdown in the subfornical organ ameliorates sympathetic excitation and cardiac hemodynamics in heart failure rats

In systolic heart failure (HF), circulating proinflammatory cytokines upregulate inflammation and renin-angiotensin system (RAS) activity in cardiovascular regions of the brain, contributing to sympathetic excitation and cardiac dysfunction. Important among these is the subfornical organ (SFO), a forebrain circumventricular organ that lacks an effective blood-brain barrier and senses circulating humors. We hypothesized that the tumor necrosis factor-α (TNF-α) receptor 1 (TNFR1) in the SFO contributes to sympathetic excitation and cardiac dysfunction in HF rats. Rats received SFO microinjections of a TNFR1 shRNA or a scrambled shRNA lentiviral vector carrying green fluorescent protein, or vehicle. One week later, some rats were euthanized to confirm the accuracy of the SFO microinjections and the transfection potential of the lentiviral vector. Other rats underwent coronary artery ligation (CL) to induce HF or a sham operation. Four weeks after CL, vehicle- and scrambled shRNA-treated HF rats had significant increases in TNFR1 mRNA and protein, NF-κB activity, and mRNA for inflammatory mediators, RAS components and c-Fos protein in the SFO and downstream in the hypothalamic paraventricular nucleus, along with increased plasma norepinephrine levels and impaired cardiac function, compared with vehicle-treated sham-operated rats. In HF rats treated with TNFR1 shRNA, TNFR1 was reduced in the SFO but not paraventricular nucleus, and the central and peripheral manifestations of HF were ameliorated. In sham-operated rats treated with TNFR1 shRNA, TNFR1 expression was also reduced in the SFO but there were no other effects. These results suggest a key role for TNFR1 in the SFO in the pathophysiology of systolic HF.NEW & NOTEWORTHY Activation of TNF-α receptor 1 in the subfornical organ (SFO) contributes to sympathetic excitation in heart failure rats by increasing inflammation and renin-angiotensin system activity in the SFO and downstream in the hypothalamic paraventricular nucleus. Cytokine receptors in the SFO may be a target for central intervention in cardiovascular conditions characterized by peripheral inflammation.

Hypoxia-Inducible Factor-1α Mediates Increased Sympathoexcitation via Glutamatergic N-Methyl-d-Aspartate Receptors in the Paraventricular Nucleus of Rats With Chronic Heart Failure

BACKGROUND

Increased sympathetic outflow is a major contributor to the progression of chronic heart failure (CHF). Potentiation of glutamatergic tone has been causally related to the sympathoexcitation in CHF. Specifically, an increase in the N-methyl-d-aspartate-type 1 receptor (NMDA-NR₁) expression within the paraventricular nucleus (PVN) is critically linked to the increased sympathoexcitation during CHF. However, the molecular mechanism(s) for the upregulation of NMDA-NR₁ remains unexplored. We hypothesized that hypoxia via hypoxia-inducible factor 1α (HIF-1α) might contribute to the augmentation of the NMDA-NR₁-mediated sympathoexcitatory responses from the PVN in CHF.

METHODS AND RESULTS

Immunohistochemistry staining, mRNA, and protein for hypoxia-inducible factor 1α were upregulated within the PVN of left coronary artery-ligated CHF rats. In neuronal cell line (NG108-15) in vitro, hypoxia caused a significant increase in mRNA and protein for HIF-1α (2-fold) with the concomitant increase in NMDA-NR₁ mRNA, protein levels, and glutamate-induced Ca⁺ influx. Chromatin immunoprecipitation assay identified HIF-1α binding to NMDA-NR₁ promoter during hypoxia. Silencing of HIF-1α in NG108 cells leads to a significant decrease in expression of NMDA-NR₁, suggesting that expression of HIF-1α is necessary for the upregulation of NMDA-NR₁. Consistent with these observations, HIF-1α silencing within the PVN abrogated the increased basal sympathetic tone and sympathoexcitatory responses to microinjection of NMDA in the PVN of rats with CHF.

CONCLUSIONS

These results uncover a critical role for HIF-1 in the upregulation of NMDA-NR₁ to mediate sympathoexcitation in CHF. We conclude that subtle hypoxia within the PVN may act as a metabolic cue to modulate sympathoexcitation during CHF.

A novel role for miR-133a in centrally mediated activation of the renin-angiotensin system in congestive heart failure

An activated renin-angiotensin system (RAS) within the central nervous system has been implicated in sympathoexcitation during various disease conditions including congestive heart failure (CHF). In particular, activation of the RAS in the paraventricular nucleus (PVN) of the hypothalamus has been recognized to augment sympathoexcitation in CHF. We observed a 2.6-fold increase in angiotensinogen (AGT) in the PVN of CHF. To elucidate the molecular mechanism for increased expression of AGT, we performed in silico analysis of the 3'-untranslated region (3'-UTR) of AGT and found a potential binding site for microRNA (miR)-133a. We hypothesized that decreased miR-133a might contribute to increased AGT in the PVN of CHF rats. Overexpression of miR-133a in NG108 cells resulted in 1.4- and 1.5-fold decreases in AGT and angiotensin type II (ANG II) type 1 receptor (AT₁R) mRNA levels, respectively. A luciferase reporter assay performed on NG108 cells confirmed miR-133a binding to the 3'-UTR of AGT. Consistent with these in vitro data, we observed a 1.9-fold decrease in miR-133a expression with a concomitant increase in AGT and AT₁R expression within the PVN of CHF rats. Furthermore, restoring the levels of miR-133a within the PVN of CHF rats with viral transduction resulted in a significant reduction of AGT (1.4-fold) and AT₁R (1.5-fold) levels with a concomitant decrease in basal renal sympathetic nerve activity (RSNA). Restoration of miR-133a also abrogated the enhanced RSNA responses to microinjected ANG II within the PVN of CHF rats. These results reveal a novel and potentially unique role for miR-133a in the regulation of ANG II within the PVN of CHF rats, which may potentially contribute to the commonly observed sympathoexcitation in CHF.NEW & NOTEWORTHY Angiotensinogen (AGT) expression is upregulated in the paraventricular nucleus of the hypothalamus through posttranscriptional mechanism interceded by microRNA-133a in heart failure. Understanding the mechanism of increased expression of AGT in pathological conditions leading to increased sympathoexcitation may provide the basis for the possible development of new therapeutic agents with enhanced specificity.

Early Training-Induced Reduction of Angiotensinogen in Autonomic Areas-The Main Effect of Exercise on Brain Renin-Angiotensin System in Hypertensive Rats

Background

Exercise training (T) blunts functional deficits and renin-angiotensin system (RAS) hyperactivity in hypertensive individuals. There is no information on T-induced temporal changes of brain RAS. We evaluate now the simultaneous effects of T on functional responses and time course changes in the expression/activity of brain RAS components in autonomic cardiovascular-controlling areas.

Methods and Results

Spontaneously hypertensive rats (SHR) and age-matched normotensive controls (WKY) were trained for 0, 1, 2, 4, 8 and 12 weeks. Sedentary (S) groups served as time-controls. After arterial pressure (AP) and heart rate (HR) recordings at rest, fresh and fixed brains were harvested for qPCR and immunofluorescence assays. SHR-S vs. WKY-S exhibited higher mean AP (MAP) and HR, increased pressure variability and sympathetic activity, elevated AT₁ receptor (AT₁) expression in nucleus tractus solitarii (NTS) and higher Mas receptor expression in the rostroventrolateral medulla (RVLM). In SHR, T promptly (T₂ on) reduced sympathetic variability to heart/vessels and largely decreased angiotensinogen expression in the paraventricular hypothalamic nucleus (PVN) and NTS, with a late RVLM reduction (T₄). AT₁ expression was only reduced at T₁₂ (PVN and NTS) with transient, not maintained Mas receptor changes in PVN and RVLM. These responses were accompanied by baseline MAP and HR reduction in the SHR-T (from T₄ on). In the SHR group, PVN angiotensinogen expression correlated positively with sympathetic activity, resting MAP and HR. In WKY-T, a precocious (T₂-T₁₂) RVLM AT₁ decrease preceded the appearance of resting bradycardia (from T₈ on).

Conclusions

Early and maintained reduction of angiotensinogen content in autonomic areas of the SHR is the most prominent effect of training on brain RAS. Down-regulation of PVN RAS expression is an essential factor to drive cardiovascular benefits in SHR-T, while resting bradycardia in WKY-T is correlated to RVLM AT₁ reduction.

Circulating angiotensin II deteriorates left ventricular function with sympathoexcitation via brain angiotensin II receptor

Sympathoexcitation contributes to the progression of heart failure. Activation of brain angiotensin II type 1 receptors (AT₁R) causes central sympathoexcitation. Thus, we assessed the hypothesis that the increase in circulating angiotensin II comparable to that reported in heart failure model affects cardiac function through the central sympathoexcitation via activating AT₁R in the brain. In Sprague-Dawley rats, the subcutaneous infusion of angiotensin II for 14 days increased the circulating angiotensin II level comparable to that reported in heart failure model rats after myocardial infarction. In comparison with the control, angiotensin II infusion increased 24 hours urinary norepinephrine excretion, and systolic blood pressure. Angiotensin II infusion hypertrophied left ventricular (LV) without changing chamber dimensions while increased end-diastolic pressure. The LV pressure–volume relationship indicated that angiotensin II did not impact on the end-systolic elastance, whereas significantly increased end-diastolic elastance. Chronic intracerebroventricular infusion of AT₁R blocker, losartan, attenuated these angiotensin II-induced changes. In conclusion, circulating angiotensin II in heart failure is capable of inducing sympathoexcitation via in part AT₁R in the brain, subsequently leading to LV diastolic dysfunction.

Effect of Prolonged Moderate Exercise on the Changes of Nonneuronal Cells in Early Myocardial Infarction

Myocardial infarction (MI) is one of the leading causes of death in developed countries and it is characterized by several associated symptomatologies and poor quality of life. Recent data showed a possible interaction between infarction and brain inflammation and activity. Previous studies have demonstrated the beneficial effect of exercise training on deterioration in cardiac function after MI. In this study we analyzed in sedentary and trained rats the microglia and astrocytes 48 hours after MI in PVN, thalamus, prefrontal cortex, and hippocampus through immunofluorescence approach. We found significant changes in specific microglia phenotypes in the brain areas analyzed together with astrocytes activation. Prolonged exercise normalized these morphological changes of microglia and astrocytes in the prefrontal cortex, hippocampus, and thalamus but not in the PVN. Our data suggest that there is an early brain reaction to myocardial infarction induction, involving nonneuronal cells, that is attenuated by the prolonged exercise.

Modulation of angiotensin II signaling following exercise training in heart failure

Sympathetic activation is a consistent finding in the chronic heart failure (CHF) state. Current therapy for CHF targets the renin-angiotensin II (ANG II) and adrenergic systems. Angiotensin converting enzyme (ACE) inhibitors and ANG II receptor blockers are standard treatments along with β-adrenergic blockade. However, the mortality and morbidity of this disease is still extremely high, even with good medical management. Exercise training (ExT) is currently being used in many centers as an adjunctive therapy for CHF. Clinical studies have shown that ExT is a safe, effective, and inexpensive way to improve quality of life, work capacity, and longevity in patients with CHF. This review discusses the potential neural interactions between ANG II and sympatho-excitation in CHF and the modulation of this interaction by ExT. We briefly review the current understanding of the modulation of the angiotensin type 1 receptor in sympatho-excitatory areas of the brain and in the periphery (i.e., in the carotid body and skeletal muscle). We discuss possible cellular mechanisms by which ExT may impact the sympatho-excitatory process by reducing oxidative stress, increasing nitric oxide. and reducing ANG II. We also discuss the potential role of ACE2 and Ang 1-7 in the sympathetic response to ExT. Fruitful areas of further investigation are the role and mechanisms by which pre-sympathetic neuronal metabolic activity in response to individual bouts of exercise regulate redox mechanisms and discharge at rest in CHF and other sympatho-excitatory states.

Neuroprotective effects of angiotensin receptor blockers

Angiotensin II receptor blockers (ARBs, collectively called sartans) are widely used compounds therapeutically effective in cardiovascular disorders, renal disease, the metabolic syndrome, and diabetes. It has been more recently recognized that ARBs are neuroprotective and have potential therapeutic use in many brain disorders. ARBs ameliorate inflammatory and apoptotic responses to glutamate, interleukin 1β and bacterial endotoxin in cultured neurons, astrocytes, microglial, and endothelial cerebrovascular cells. When administered systemically, ARBs enter the brain, protecting cerebral blood flow, maintaining blood brain barrier function and decreasing cerebral hemorrhage, excessive brain inflammation and neuronal injury in animal models of stroke, traumatic brain injury, Alzheimer's and Parkinson's disease and other brain conditions. Epidemiological analyses reported that ARBs reduced the progression of Alzheimer's disease, and clinical studies suggested amelioration of cognitive loss following stroke and aging. ARBs are pharmacologically heterogeneous; their effects are not only the result of Ang II type 1(AT1) receptor blockade but also of additional mechanisms selective for only some compounds of the class. These include peroxisome proliferator-activated receptor gamma activation and other still poorly defined mechanisms. However, the complete pharmacological spectrum and therapeutic efficacy of individual ARBs have never been systematically compared, and the neuroprotective efficacy of these compounds has not been rigorously determined in controlled clinical studies. The accumulation of pre-clinical evidence should promote further epidemiological and controlled clinical studies. Repurposing ARBs for the treatment of brain disorders, currently without effective therapy, may be of immediate and major translational value.

Angiotensin II type 1 receptor expression in astrocytes is upregulated leading to increased mortality in mice with myocardial infarction-induced heart failure

Enhanced central sympathetic outflow worsens left ventricular (LV) remodeling and prognosis in heart failure after myocardial infarction (MI). Previous studies suggested that activation of brain angiotensin II type 1 receptors (AT1R) in the brain stem leads to sympathoexcitation due to neuronal AT1R upregulation. Recent studies, however, revealed the importance of astrocytes for modulating neuronal activity, but whether changes in astrocytes influence central sympathetic outflow in heart failure is unknown. In the normal state, AT1R are only weakly expressed in astrocytes. We hypothesized that AT1R in astrocytes are upregulated in heart failure and modulate the activity of adjacent neurons, leading to enhanced sympathetic outflow. In the present study, by targeting deletion of astrocyte-specific AT1R, we investigated whether AT1R in astrocytes have a key role in enhancing central sympathetic outflow, and thereby influencing LV remodeling process and the prognosis of MI-induced heart failure. Using the Cre-LoxP system, we generated glial fibrillary acidic protein (GFAP)-specific AT1R knockout (GFAP/AT1RKO) mice. Urinary norepinephrine excretion for 24 h, as an indicator of sympathoexcitation, was significantly lower in GFAP/AT1RKO-MI mice than in control-MI mice. LV size and heart weight after MI were significantly smaller in GFAP/AT1RKO mice than in control mice. Prognosis was significantly improved in GFAP/AT1RKO-MI mice compared with control-MI mice. Our findings indicated that AT1R expression was upregulated in brain stem astrocytes in MI-induced heart failure, which worsened LV remodeling and prognosis via sympathoexcitation. Thus, in addition to neuronal AT1R, AT1R in astrocytes appear to have a key role in enhancing central sympathetic outflow in heart failure.

Disruption of cardiovascular circadian rhythms in mice post myocardial infarction: relationship with central angiotensin II receptor expression

Angiotensin II (Ang II) is well known to participate in the abnormal autonomic cardiovascular control that occurs during the development of chronic heart failure (CHF). Disrupted cardiovascular circadian rhythm in CHF is also well accepted; however, the mechanisms underlying and the role of central Ang II type 1 receptors (AT1R) and oxidative stress in mediating such changes are not clear. In a post myocardial infarction (MI) CHF mouse model we investigated the circadian rhythm for mean arterial pressure (MAP), heart rate (HR), and baroreflex sensitivity (BRS) following MI. The cardiovascular parameters represent the middle 6‐h averages during daytime (6:00–18:00) and nighttime (18:00–6:00). HR increased with the severity of CHF reaching its maximum by 12 weeks post‐MI; loss of circadian HR and BRS rhythms were observed as early as 4 weeks post‐MI in conjunction with a significant blunting of the BRS and an upregulation in the AT1R and gp91^phox proteins in the brainstem. Loss of MAP circadian rhythm was observed 8 weeks post‐MI. Circadian AT1R expression was demonstrated in sham animals but was lost 8 weeks following MI. Losartan reduced AT1R expression in daytime (1.18 ± 0.1 vs. 0.85 ± 0.1; P < 0.05) with a trend toward a reduction in the AT1R mRNA expression in the nighttime (1.2 ± 0.1 vs. 1.0 ± 0.1; P > 0.05) but failed to restore circadian variability. The disruption of circadian rhythm for HR, MAP and BRS along with the upregulation of AT1 and gp91^phox suggests a possible role for central oxidative stress as a mediator of circadian cardiovascular parameters in the post‐MI state.

Increases in central angiotenisn II signaling provide a driving force for sympatho‐excitation in heart failure. In this study, we show a loss of circadian variability in angiotensin type 1 receptor expression in the brainstem of mice post myocardial infarction. These changes correlate with a loss of cardiovascular circadian variability. These data suggest that sympatho‐ excitation may be increased in the post‐MI state at times when sympathetic outflow is normally reduced.

Angiotensin II induces membrane trafficking of natively expressed transient receptor potential vanilloid type 4 channels in hypothalamic 4B cells

Transient receptor potential vanilloid family type 4 (TRPV4) channels are expressed in central neuroendocrine neurons and have been shown to be polymodal in other systems. We previously reported that in the rodent, a model of dilutional hyponatremia associated with hepatic cirrhosis, TRPV4 expression is increased in lipid rafts from the hypothalamus and that this effect may be angiotensin dependent. In this study, we utilized the immortalized neuroendocrine rat hypothalamic 4B cell line to more directly test the effects of angiotensin II (ANG II) on TRPV4 expression and function. Our results demonstrate the expression of corticotropin-releasing factor (CRF) transcripts, for sex-determining region Y (SRY) (male genotype), arginine vasopressin (AVP), TRPV4, and ANG II type 1a and 1b receptor in 4B cells. After a 1-h incubation in ANG II (100 nM), 4B cells showed increased TRPV4 abundance in the plasma membrane fraction, and this effect was prevented by the ANG II type 1 receptor antagonist losartan (1 μM) and by a Src kinase inhibitor PP2 (10 μM). Ratiometric calcium imaging experiments demonstrated that ANG II incubation potentiated TRPV4 agonist (GSK 1016790A, 20 nM)-induced calcium influx (control 18.4 ± 2.8% n = 5 and ANG II 80.5 ± 2.4% n = 5). This ANG II-induced increase in calcium influx was also blocked by 1 μM losartan and 10 μM PP2 (losartan 26.4 ± 3.8% n = 5 and PP2 19.7 ± 3.9% n = 5). Our data suggests that ANG II can increase TRPV4 channel membrane expression in 4B cells through its action on AT1R involving a Src kinase pathway.

Overexpression of angiotensin-converting enzyme 2 attenuates tonically active glutamatergic input to the rostral ventrolateral medulla in hypertensive rats

The rostral ventrolateral medulla (RVLM) plays a key role in cardiovascular regulation. It has been reported that tonically active glutamatergic input to the RVLM is increased in hypertensive rats, whereas angiotensin-converting enzyme 2 (ACE2) in the brain has been suggested to be beneficial to hypertension. This study was designed to determine the effect of ACE2 gene transfer into the RVLM on tonically active glutamatergic input in spontaneously hypertensive rats (SHRs). Lentiviral particles containing enhanced green fluorescent protein (lenti-GFP) or ACE2 (lenti-ACE2) were injected bilaterally into the RVLM. Both protein expression and activity of ACE2 in the RVLM were increased in SHRs after overexpression of ACE2. A significant reduction in blood pressure and heart rate in SHRs was observed 6 wk after lenti-ACE2 injected into the RVLM. The concentration of glutamate in microdialysis fluid from the RVLM was significantly reduced by an average of 61% in SHRs with lenti-ACE2 compared with lenti-GFP. ACE2 overexpression significantly attenuated the decrease in blood pressure and renal sympathetic nerve activity evoked by bilateral injection of the glutamate receptor antagonist kynurenic acid (2.7 nmol in 100 nl) into the RVLM in SHRs. Therefore, we suggest that ACE2 overexpression in the RVLM attenuates the enhanced tonically active glutamatergic input in SHRs, which may be an important mechanism underlying the beneficial effect of central ACE2 to hypertension.

The central renin-angiotensin system and sympathetic nerve activity in chronic heart failure

CHF (chronic heart failure) is a multifactorial disease process that is characterized by overactivation of the RAAS (renin-angiotensin-aldosterone system) and the sympathetic nervous system. Both of these systems are chronically activated in CHF. The RAAS consists of an excitatory arm involving AngII (angiotensin II), ACE (angiotensin-converting enzyme) and the AT1R (AngII type 1 receptor). The RAAS also consists of a protective arm consisting of Ang-(1-7) [angiotensin-(1-7)], the AT2R (AngII type 2 receptor), ACE2 and the Mas receptor. Sympatho-excitation in CHF is driven, in large part, by an imbalance of these two arms, with an increase in the AngII/AT1R/ACE arm and a decrease in the AT2R/ACE2 arm. This imbalance is manifested in cardiovascular-control regions of the brain such as the rostral ventrolateral medulla and paraventricular nucleus in the hypothalamus. The present review focuses on the current literature that describes the components of these two arms of the RAAS and their imbalance in the CHF state. Moreover, the present review provides additional evidence for the relevance of ACE2 and Ang-(1-7) as key players in the regulation of central sympathetic outflow in CHF. Finally, we also examine the effects of exercise training as a therapeutic strategy and the molecular mechanisms at play in CHF, in part, because of the ability of exercise training to restore the balance of the RAAS axis and sympathetic outflow.

Possible synergistic effect of direct angiotensin II type 2 receptor stimulation by compound 21 with memantine on prevention of cognitive decline in type 2 diabetic mice

Type 2 diabetes mellitus (T2DM) is known to be associated with increased risk of cognitive impairment including Alzheimer disease. Recent studies have suggested an interaction between angiotensin II and N-methyl-d-aspartic acid (NMDA) glutamate receptors. We previously reported that stimulation of the angiotensin II type 2 (AT2) receptor exerts brain protective effects. A newly developed AT2 receptor agonist, compound 21 (C21), has enabled examination of the direct effect of AT2 receptor stimulation in vivo. Accordingly, we examined the possible synergistic effect of C21 and memantine on cognitive impairment in T2DM mice, KKAy. KKAy were divided into four groups; (1) control, (2) treatment with C21 (10 μg/kg/day), (3) treatment with memantine (20mg/kg/day), and (4) treatment with both for 4 weeks, and subjected to Morris water maze tasks. Treatment with C21 or memantine alone at these doses tended to shorten escape latency compared to that in the control group. C21 treatment increased cerebral blood flow (CBF), but memantine did not influence CBF. Treatment with C21 or C21 plus memantine increased hippocampal field-excitatory postsynaptic potential (f-EPSP). Moreover, treatment with memantine or C21 increased acetylcholine level, which was lower in KKAy than in wild-type mice, and C21 plus memantine treatment enhanced memantine or C21-induced acetylcholine secretion. This study provides an insight into new approaches to understand the interaction of angiotensin II and neurotransmitters. We can anticipate a new therapeutic approach against cognitive decline using C21 and memantine.

Ang II-salt hypertension depends on neuronal activity in the hypothalamic paraventricular nucleus but not on local actions of tumor necrosis factor-α

Development of angiotensin II (Ang II)-dependent hypertension involves microglial activation and proinflammatory cytokine actions in the hypothalamic paraventricular nucleus (PVN). Cytokines activate receptor signaling pathways that can both acutely grade neuronal discharge and trigger long-term adaptive changes that modulate neuronal excitability through gene transcription. Here, we investigated contributions of PVN cytokines to maintenance of hypertension induced by subcutaneous infusion of Ang II (150 ng/kg per min) for 14 days in rats consuming a 2% NaCl diet. Results indicate that bilateral PVN inhibition with the GABA-A receptor agonist muscimol (100 pmol/50 nL) caused significantly greater reductions of renal and splanchnic sympathetic nerve activity (SNA) and mean arterial pressure in hypertensive than in normotensive rats (P<0.01). Thus, ongoing PVN neuronal activity seems required for support of hypertension. Next, the role of the prototypical cytokine tumor necrosis factor-α was investigated. Whereas PVN injection of tumor necrosis factor-α (0.3 pmol/50 nL) acutely increased lumbar and splanchnic SNA and mean arterial pressure, interfering with endogenous tumor necrosis factor-α by injection of etanercept (10 μg/50 nL) was without effect in hypertensive and normotensive rats. Next, we determined that although microglial activation in PVN was increased in hypertensive rats, bilateral injections of minocycline (0.5 μg/50 nL), an inhibitor of microglial activation, failed to reduce lumbar or splanchnic SNA or mean arterial pressure in hypertensive or in normotensive rats. Collectively, these findings indicate that established Ang II-salt hypertension is supported by PVN neuronal activity, but short term maintenance of SNA and arterial blood pressure does not depend on ongoing local actions of tumor necrosis factor-α.

Centrally mediated erectile dysfunction in rats with type 1 diabetes: role of angiotensin II and superoxide

INTRODUCTION

Erectile dysfunction is a serious complication of diabetes mellitus. Apart from the peripheral actions, central mechanisms are also responsible for penile erection.

AIM

This study aims to determine the contribution of angiotensin (ANG) II in the dysfunction of central N-methyl-D-aspartic acid (NMDA)- and nitric oxide (NO)-induced erectile responses in streptozotocin-induced type 1 diabetic (T1D) rats.

METHODS

Three weeks after streptozotocin injections, rats were randomly treated with the angiotensin-converting enzyme inhibitor-enalapril, or the ANG II type 1 receptor blocker, losartan, or the superoxide dismutase mimetic, tempol, or vehicle via chronic intracerebroventricular infusion by osmotic mini-pump for 2 weeks.

MAIN OUTCOME MEASURE

Central NMDA receptor stimulation or the administration of the NO donor, sodium nitroprusside (SNP)-induced penile erectile responses and concurrent behavioral responses were monitored in conscious rats.

RESULTS

Two weeks of enalapril, losartan, or tempol treatment significantly improved the erectile responses to central microinjection of both NMDA and SNP in the paraventricular nucleus (PVN) of conscious T1D rats (NMDA responses-T1D+enalapril: 1.7 ± 0.6, T1D+losartan: 2.0 ± 0.3, T1D+tempol: 2.0 ± 0.6 vs. T1D+vehicle: 0.6 ± 0.3 penile erections/rat in the first 20 minutes, P < 0.05; SNP responses-T1D+enalapril: 0.9 ± 0.3, T1D+losartan: 1.3 ± 0.3, T1D+tempol: 1.4 ± 0.4 vs. T1D+vehicle: 0.4 ± 0.2 penile erections/rat in the first 20 minutes, P < 0.05). Concurrent behavioral responses including yawning and stretching, induced by central NMDA and SNP microinjections, were also significantly increased in T1D rats after enalapril, losartan, or tempol treatments. Neuronal NO synthase expression within the PVN was also significantly increased, and superoxide production was reduced in T1D rats after these treatments.

CONCLUSIONS

These data strongly support the contention that enhanced ANG II mechanism/s within the PVN of T1D rats contributes to the dysfunction of central NMDA-induced erectile responses in T1D rats via stimulation of superoxide.

Angiotensin II-mediated posttranslational modification of nNOS in the PVN of rats with CHF: role for PIN

An increased sympathetic drive is an adverse characteristic in chronic heart failure (CHF). The protein expression of neuronal nitric oxide synthase (nNOS)- and hence nitric oxide (NO)-mediated sympathoinhibition is reduced in the paraventricular nucleus (PVN) of rats with CHF. However, the molecular mechanism(s) of nNOS downregulation remain(s) unclear. The aim of the study was to reveal the underlying molecular mechanism for the downregulation of nNOS in the PVN of CHF rats. Sprague-Dawley rats with CHF (6-8 wk after coronary artery ligation) demonstrated decreased nNOS dimer/monomer ratio (42%), with a concomitant increase in the expression of PIN (a protein inhibitor of nNOS known to dissociate nNOS dimers into monomers) by 47% in the PVN. Similarly, PIN expression is increased in a neuronal cell line (NG108) treated with angiotensin II (ANG II). Furthermore, there is an increased accumulation of high-molecular-weight nNOS-ubiquitin (nNOS-Ub) conjugates in the PVN of CHF rats (29%). ANG II treatment in NG108 cells in the presence of a proteasome inhibitor, lactacystin, also leads to a 69% increase in accumulation of nNOS-Ub conjugates immunoprecipitated by an antiubiquitin antibody. There is an ANG II-driven, PIN-mediated decrease in the dimeric catalytically active nNOS in the PVN, due to ubiquitin-dependent proteolytic degradation in CHF. Our results show a novel intermediary mechanism that leads to decreased levels of active nNOS in the PVN, involved in subsequent reduction in sympathoinhibition during CHF, offering a new target for the treatment of CHF and other cardiovascular diseases.

Neuroimmune communication in hypertension and obesity: a new therapeutic angle?

Hypertension is an epidemic health concern and a major risk factor for the development of cardiovascular disease. Although there are available treatment strategies for hypertension, numerous hypertensive patients do not have their clinical symptoms under control and it is imperative that new avenues to treat or prevent high blood pressure in these patients are developed. It is well established that increases in sympathetic nervous system (SNS) outflow and enhanced renin-angiotensin system (RAS) activity are common features of hypertension and various pathological conditions that predispose individuals to hypertension. More recently, hypertension has also become recognized as an immune condition and accumulating evidence suggests that interactions between the RAS, SNS and immune systems play a role in blood pressure regulation. This review summarizes what is known about the interconnections between the RAS, SNS and immune systems in the neural regulation of blood pressure. Based on the reviewed studies, a model for RAS/neuroimmune interactions during hypertension is proposed and the therapeutic potential of targeting RAS/neuroimmune interactions in hypertensive patients is discussed. Special emphasis is placed on the applicability of the proposed model to obesity-related hypertension.

Construction and validation of lentiviral vector carrying rat neuronal nitric oxide synthase in vitro and in vivo

In the present study, we developed a lentiviral vector with human cytomegalovirus promoter permitting high-level of nNOS expression. Neuronal cell line NG108 was used as an in vitro model to check the validity of gene transfer. The cells were infected with lenti-EGFP or lenti-nNOS particles for 24h. Lenti-nNOS infection in the NG108 cells induced dose dependent increase in mRNA and protein for nNOS; with a dose of 2.5 × 10⁴ pfu/ml, nNOS mRNA expression increased by 40-fold while protein expression was increased by 2.5-fold compared to lenti-EGFP. Moreover, lenti-nNOS infection caused a greater increase in nNOS immunoreactivity in NG108 cells compared to lenti-EGFP as shown by immonocytochemistry. nNOS expression showed time dependent increases with lenti-nNOS infection with maximum up-regulation observed after two weeks of infection. Moreover, in vivo, unilateral injection of lenti-nNOS into the paraventricular nucleus (PVN) of rats induced a 27-fold increase of nNOS protein level in the injected side compared to non-injected side and this escalation was sustained up to three weeks. Overall, lenti-EGFP injection in the PVN did not show any significant change in nNOS expression. Furthermore, NADPH-diaphorase staining of nNOS in the PVN infected with lenti-nNOS induced a visible increase in nNOS expression compared with contralateral non-injected side up to three weeks. These results indicate that this approach of lentiviral mediated gene transfer of nNOS may provide a new means to up-regulate the nNOS expression for longer periods of time compared to adenoviral transfection and can be used as a research tool and potentially a therapy for chronic diseases involving impaired nNOS expression.

Parallel changes in neuronal AT1R and GRK5 expression following exercise training in heart failure

Although exercise training (ExT) is an important therapeutic strategy for improving quality of life in patients with chronic heart failure (CHF), the central mechanisms by which ExT is beneficial are not well understood. The angiotensin II type 1 receptor (AT1R) plays a pivotal role in the development of CHF and is upregulated in a number of tissues owing, in part, to transcription factor nuclear factor kappa B (NF-κB). In addition, AT1R is marked for internalization and recycling via G-protein-coupled receptor kinase (GRK) phosphorylation. Because previous studies have shown that the beneficial effects of ExT in CHF rely on a reduction in angiotensin II, we hypothesized ExT would decrease AT1R, GRK5, and NF-κB protein expression in the paraventricular nucleus and rostral ventrolateral medulla of CHF rats. Following infarction by coronary artery ligation, animals were exercised 4 weeks postsurgery on a treadmill at a final speed of 25 miles per minute for 60 minutes, 5 days per week for 6 weeks. Western blot analysis of paraventricular nucleus and rostral ventrolateral medulla micropunches revealed an upregulation of AT1R, GRK5, and NF-κB in the infarcted group that was reversed by ExT. Furthermore, the relative expression of phosphorylated AT1R and AT1R/GRK5 physical association was increased in the CHF sedentary group and reversed by ExT. Overexpression of GRK5 in cultured CATH.a neurons blunted angiotensin II-mediated upregulation of AT1R and NF-κB; conversely, silencing of GRK5 exacerbated angiotensin II-mediated AT1R and NF-κB upregulation. Taken together, increased GRK5 may regulate AT1R expression in CHF, and ExT mitigates AT1R and its pathway components.

Sustained activation of microglia in the hypothalamic PVN following myocardial infarction

Microglia are the immune cells in the central nervous system and can produce cytokines when they are activated by an insult or injury. In the present study, we investigated in detail the time frame of the activation of microglia in the hypothalamic paraventricular nucleus (PVN) following myocardial infarction in rats. Morphological changes and immunohistochemistry to detect CD11b (clone OX-42) were used to identify activated microglia. Compared to rats that had undergone sham surgical procedures, there was a significant increase of between 40 and 50% in the proportion of activated microglia in the PVN 4-16 weeks following myocardial infarction (P<0.001, One way ANOVA). At 24h or 1 week post myocardial infarction, however, there was no significant increase in the proportion of activated microglia. Echocardiography and haemodynamic parameters after myocardial infarction indicated significantly reduced left ventricular function. In conclusion, following myocardial infarction, activation of microglia in the PVN does not occur immediately but once manifested, activation is sustained. Thus, activated microglia may contribute to the chronic elevation in cytokine levels observed following myocardial infarction. Since cytokines elicit sympatho-excitatory effects when locally microinjected into the PVN, activated microglia may contribute to the mechanisms mediating the chronic increase in sympathetic nerve activity in animals with reduced left ventricular function induced following myocardial infarction.

Enhanced activation of RVLM-projecting PVN neurons in rats with chronic heart failure

Previous studies have indicated that there is increased activation of the paraventricular nucleus (PVN) in rats with chronic heart failure (CHF); however, it is not clear if the preautonomic neurons within the PVN are specifically overactive. Also, it is not known if these neurons have altered responses to baroreceptor or osmotic challenges. Experiments were conducted in rats with CHF (6-8 wk after coronary artery ligation). Spontaneously active neurons were recorded in the PVN, of which 36% were antidromically activated from the rostral ventrolateral medulla (RVLM). The baseline discharge rate in RVLM-projecting PVN (PVN-RVLM) neurons from CHF rats was significantly greater than in sham-operated (sham) rats (6.0 ± 0.6 vs. 2.6 ± 0.3 spikes/s, P < 0.05). Picoinjection of the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid significantly decreased the basal discharge of PVN-RVLM neurons by 80% in CHF rats compared with 37% in sham rats. Fifty-two percent of spontaneously active PVN-RVLM neurons responded to changes in the mean arterial pressure (MAP). The changes in discharge rate in PVN-RVLM neurons after a reduction in MAP (+52 ± 7% vs. +184 ± 61%) or an increase in MAP (-42 ± 8% vs. -71 ± 6%) were significantly attenuated in rats with CHF compared with sham rats. Most PVN-RVLM neurons (63%), including all barosensitive PVN-RVLM neurons, were excited by an internal carotid artery injection of hypertonic NaCl (2.1 osmol/l), whereas a smaller number (7%) were inhibited. The increase in discharge rate in PVN-RVLM neurons to hypertonic stimulation was significantly enhanced in rats with CHF compared with sham rats (134 ± 15% vs. 92 ± 13%). Taken together, these data suggest that PVN-RVLM neurons are more active under basal conditions and this overactivation is mediated by an enhanced glutamatergic tone in rats with CHF. Furthermore, this enhanced activation of PVN-RVLM neurons may contribute to the altered responses to baroreceptor and osmotic challenges observed during CHF.

Nitric oxide inhibits the expression of AT1 receptors in neurons

We have previously observed an increased of angiotensin II (ANG II) type 1 receptor (AT(1)R) with enhanced AT(1)R-mediated sympathetic outflow and concomitant downregulation of neuronal nitric oxide (NO) synthase (nNOS) with reduced NO-mediated inhibition from the paraventricular nucleus (PVN) in rats with heart failure. To test the hypothesis that NO exerts an inhibitory effect on AT(1)R expression in the PVN, we used primary cultured hypothalamic cells of neonatal rats and neuronal cell line NG108-15 as in vitro models. In hypothalamic primary culture, NO donor sodium nitroprusside (SNP) induced dose-dependent decreases in mRNA and protein of AT(1)R (10(-5) M SNP, AT(1)R protein was 10 ± 2% of control level) while NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) induced dose-dependent increases in mRNA and protein levels of AT(1)R (10(-5) M l-NMMA, AT(1)R protein was 148 ± 8% of control level). Similar effects of SNP and l-NMMA on AT(1)R expression were also observed in NG108-15 cell line (10(-6) M SNP, AT(1)R protein was 30 ± 4% of control level while at the dose of 10(-6) M l-NMMA, AT(1)R protein was 171 ± 15% of the control level). Specific inhibition of nNOS, using antisense, caused an increase in AT(1)R expression while overexpression of nNOS, using adenoviral gene transfer (Ad.nNOS), caused an inhibition of AT(1)R expression in NG108 cells. Antisense nNOS transfection augmented the increase while Ad.nNOS infection blunted the increase in intracellular calcium concentration in response to ANG II treatment in NG108 cells. In addition, downregulation of AT(1)R mRNA as well as protein level in neuronal cell line in response to S-nitroso-N-acetyl pencillamine (SNAP) treatment was blocked by protein kinase G (PKG) inhibitor, while the peroxynitrite scavenger deforxamine had no effect. These results suggest that NO acts as an inhibitory regulator of AT(1)R expression and the activation of PKG is the required step in the regulation of AT(1)R gene expression via cGMP-dependent signaling pathway.

Peroxisome proliferator-activated receptor-γ regulates inflammation and renin-angiotensin system activity in the hypothalamic paraventricular nucleus and ameliorates peripheral manifestations of heart failure

Activation of peroxisome proliferator-activated receptor (PPAR)-γ, a nuclear transcription factor, has been shown to inhibit the production of proinflammatory cytokines and, in peripheral tissues, to downregulate the renin-angiotensin system. PPAR-γ is expressed in key brain areas involved in cardiovascular and autonomic regulation. We hypothesized that activation of central PPAR-γ would reduce sympathetic excitation and ameliorate peripheral manifestations of heart failure (HF) by inhibiting central inflammation and brain renin-angiotensin system activity. Two weeks after coronary artery ligation, HF rats received an intracerebroventricular infusion of the PPAR-γ agonist pioglitazone or vehicle for another 2 weeks. PPAR-γ expression in the paraventricular nucleus of hypothalamus, an important cardiovascular region, was unchanged in HF compared with sham-operated rats. However, PPAR-γ DNA binding activity was reduced, nuclear factor-κB activity was increased, and expression of proinflammatory cytokines and angiotensin II type-1 receptor was augmented in the HF rats. Mean blood pressure response to ganglionic blockade was greater; plasma norepinephrine levels, lung/body weight, right ventricle/body weight, and left ventricular end-diastolic pressure were increased; and maximal left ventricular dP/dt was decreased. All of these findings were ameliorated in HF rats treated with intracerebroventricular pioglitazone, which increased PPAR-γ expression and DNA binding activity in the paraventricular nucleus of hypothalamus. The results demonstrate that cardiovascular and autonomic mechanisms leading to heart failure after myocardial infarction can be modulated by activation of PPAR-γ in the brain. Central PPAR-γ may be a novel target for treatment of sympathetic excitation in myocardial infarction-induced HF.

Decreased nNOS in the PVN leads to increased sympathoexcitation in chronic heart failure: role for CAPON and Ang II

AIMS

Previously, we showed an enhanced excitatory (N-methyl d-aspartate receptor-NR(1)) and decreased inhibitory neuronal nitric oxide (NO) synthase (nNOS) influence within the paraventricular nucleus (PVN) of rats with chronic heart failure (CHF). Although NR(1) and nNOS are normally linked, they can be disconnected by nNOS sequestering with nNOS-associated protein (CAPON). The aim of this study was to elucidate the underlying mechanism for the disconnection between increased expression of NR(1) and decreased nNOS in the PVN of rats with CHF which leads to enhanced sympathoexcitation.

METHODS AND RESULTS

CAPON expression was augmented while nNOS expression was decreased in the PVN of rats with CHF (6-8 weeks after left coronary artery ligation). Angiotensin II (Ang II) type I receptor (AT(1)) antagonist losartan (Los) treatment in rats with CHF reduced renal sympathetic nerve activity with concomitant normalization of protein expression of CAPON and nNOS in the PVN. Los treatment also reversed the blunting of endogenous NO-mediated sympatho-inhibition in rats with CHF. Moreover, Ang II-induced increase in CAPON expression in NG108 neuronal cells was also ameliorated by Los.

CONCLUSION

Blocking AT(1) receptors prevents the overexpression of CAPON and concomitant decrease in nNOS in the PVN, resulting in attenuation of sympathoexcitation commonly observed in CHF. Taken together, our data highlight the importance of altered expression and subsequent interaction of nNOS and CAPON within the PVN, leading to increased sympathoexcitation in CHF. Identifying this crucial nNOS/CAPON interaction regulated by AT(1) receptors may provide an important potential therapeutic target in CHF.

Brain-selective overexpression of angiotensin-converting enzyme 2 attenuates sympathetic nerve activity and enhances baroreflex function in chronic heart failure

Angiotensin-converting enzyme 2 (ACE2) has been suggested to be involved in the central regulation of autonomic function. During chronic heart failure (CHF), elevated central angiotensin II signaling contributes to the sustained increase of sympathetic outflow. This is accompanied by a downregulation of ACE2 in the brain. We hypothesized that central overexpression of ACE2 decreases sympathetic outflow and enhances baroreflex function in CHF. Transgenic mice overexpressing human ACE2 selectively in the brain (SYN-hACE2 [SA]) and wild-type littermates (WT) were used. CHF was induced by permanent coronary artery ligation. Four weeks after coronary artery ligation, both WT and SA mice exhibited a significant decrease in left ventricular ejection fraction (<40%). A slight decrease in mean arterial pressure was found only in SA mice. Compared with WT mice with CHF, brain-selective ACE2 overexpression attenuated left ventricular end-diastolic pressure; decreased urinary norepinephrine excretion; baseline renal sympathetic nerve activity (WT CHF: 71.6±7.6% max versus SA CHF: 49.3±6.1% max); and enhanced baroreflex sensitivity (maximum slope: WT sham: 1.61±0.16%/mm Hg versus SA CHF: 1.51±0.17%/mm Hg). Chronic subcutaneous blockade of mas receptor increased renal sympathetic nerve activity in SA mice with CHF (A779: 67.3±5.8% versus vehicle: 46.4±3.6% of max). An upregulation in angiotensin II type 1 receptor expression was detected in medullary nuclei in WT CHF mice, which was significantly attenuated in SA mice with CHF. These data suggest that central ACE2 overexpression exerts a potential protective effect in CHF through attenuating sympathetic outflow. The mechanism for this effect involves angiotensin (1-7) mas signaling, as well as a decrease in angiotensin II type 1 receptor signaling in the medulla.

Inhibitory-excitatory synaptic balance is shifted toward increased excitation in magnocellular neurosecretory cells of heart failure rats

Despite the well-established contribution of neurohumoral activation to morbidity and mortality in heart failure (HF) patients, relatively little is known about the underlying central nervous system mechanisms. In this study, we aimed to determine whether changes in GABAergic inhibitory and glutamatergic excitatory synaptic function contribute to altered hypothalamic magnocellular neurosecretory cell (MNC) activity in HF rats. Patch-clamp recordings were obtained from MNCs in brain slices from sham and HF rats. Glutamate excitatory (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs) were simultaneously recorded, and changes in their strengths, as well as their interactions, were evaluated. We found a diminished GABAergic synaptic strength in MNCs of HF rats, reflected as faster decaying IPSCs and diminished mean IPSC charge transfer. Opposite changes were observed in glutamate EPSC synaptic strength, resulting in a shift in the GABA-glutamate balance toward a relatively stronger glutamate influence in HF rats. The prolongation of glutamate EPSCs during HF was mediated, at least in part, by an enhanced contribution of AMPA receptor desensitization to the EPSC decay time course. EPSC prolongation, and consequently increased unitary strength, resulted in a stronger AMPA receptor-mediated excitatory drive to firing discharge in MNCs of HF rats. Blockade of GABA(A) synaptic activity diminished the EPSC waveform variability observed among events in sham rats, an effect that was blunted in HF rats. Together, our results suggest that opposing changes in postsynaptic properties of GABAergic and glutamatergic synaptic function contribute to enhanced magnocellular neurosecretory activity in HF rats.

Chronic AT1 receptor blockade normalizes NMDA-mediated changes in renal sympathetic nerve activity and NR1 expression within the PVN in rats with heart failure

Exercise training normalizes enhanced glutamatergic mechanisms within the paraventricular nucleus (PVN) concomitant with the normalization of increased plasma ANG II levels in rats with heart failure (HF). We tested whether ANG II type 1 (AT(1)) receptors are involved in the normalization of PVN glutamatergic mechanisms using chronic AT(1) receptor blockade with losartan (Los; 50 mg.kg(-1).day(-1) in drinking water for 3 wk). Left ventricular end-diastolic pressure was increased in both HF + vehicle (Veh) and HF + Los groups compared with sham-operated animals (Sham group), although it was significantly attenuated in the HF + Los group compared with the HF + Veh group. The effect of Los on cardiac function was similar to exercise training. At the highest dose of N-methyl-d-aspartate (NMDA; 200 pmol) injected into the PVN, the increase in renal sympathetic nerve activity was 93 +/- 13% in the HF + Veh group, which was significantly higher (P < 0.05) than the increase in the Sham + Veh (45 +/- 2%) and HF + Los (47 +/- 2%) groups. Relative NMDA receptor subunit NR(1) mRNA expression within the PVN was increased 120% in the HF + Veh group compared with the Sham + Veh group (P < 0.05) but was significantly attenuated in the HF + Los group compared with the HF + Veh group (P < 0.05). NR(1) protein expression increased 87% in the HF + Veh group compared with the Sham + Veh group but was significantly attenuated in the HF + Los group compared with the HF + Veh group (P < 0.05). Furthermore, in in vitro experiments using neuronal NG-108 cells, we found that ANG II treatment stimulated NR(1) protein expression and that Los significantly ameliorated the NR(1) expression induced by ANG II. These data are consistent with our hypothesis that chronic AT(1) receptor blockade normalizes glutamatergic mechanisms within the PVN in rats with HF.