Tempol Attenuates Excitatory Actions of Angiotensin II in the Rostral Ventrolateral Medulla During Emotional Stress

Augmented fear bradycardia in rats with heart failure

In congestive heart failure (CHF), while resting parasympathetic activity becomes reduced, parasympathetically-mediated responses to stressors have not been described. This study aimed to (1) elucidate the effect of CHF on fear bradycardia, a parasympathetically-mediated response, and (2) examine if brain oxidative stress of CHF mediates fear bradycardia. White noise sound (WNS) exposure to conscious rats induced freezing behavior and elicited bradycardia. WNS exposure-elicited bradycardia was greater in rats with CHF than in controls. Superoxide dismutase mimetics administered in the lateral/ventrolateral midbrain periaqueductal gray (l/vlPAG), a region that contributes to the generation of fear bradycardia, had no effect on the bradycardia response in control and CHF rats. Dihydroethidium staining in situ showed that superoxide generation in the l/vlPAG of CHF rats was increased as compared to controls. These results demonstrate that CHF leads to the augmentation of fear bradycardia. Moreover, oxidative stress in the l/vlPAG of CHF unlikely mediates the augmented fear bradycardia.

Circadian Differences in the Contribution of the Brain Renin-Angiotensin System in Genetically Hypertensive Mice

Objective: Genetically hypertensive BPH/2J mice are recognized as a neurogenic model of hypertension, primarily based on sympathetic overactivity and greater neuronal activity in cardiovascular regulatory brain regions. Greater activity of the central renin angiotensin system (RAS) and reactive oxygen species (ROS) reportedly contribute to other models of hypertension. Importantly the peripheral RAS contributes to the hypertension in BPH/2J mice, predominantly during the dark period of the 24 h light cycle. The aim of the present study was to determine whether central AT₁ receptor stimulation and the associated ROS signaling contribute to hypertension in BPH/2J mice in a circadian dependent manner.

Methods: Blood pressure (BP) was measured in BPH/2J and normotensive BPN/3J mice (n = 7–8) via pre-implanted telemetry devices. Acute intracerebroventricular (ICV) microinjections of AT₁ receptor antagonist, candesartan, and the superoxide dismutase (SOD) mimetic, tempol, were administered during the dark and light period of the 24 h light cycle via a pre-implanted ICV guide cannula. In separate mice, the BP effect of ICV infusion of the AT₁ receptor antagonist losartan for 7 days was compared with subcutaneous infusion to determine the contribution of the central RAS to hypertension in BPH/2J mice.

Results: Candesartan administered ICV during the dark period induced depressor responses which were 40% smaller in BPH/2J than BPN/3J mice (P_strain < 0.05), suggesting AT₁ receptor stimulation may contribute less to BP maintenance in BPH/2J mice. During the light period candesartan had minimal effect on BP in either strain. ICV tempol had comparable effects on BP between strains during the light and dark period (P_strain > 0.08), suggesting ROS signaling is also not contributing to the hypertension in BPH/2J mice. Chronic ICV administration of losartan (22 nmol/h) had minimal effect on BPN/3J mice. By contrast in BPH/2J mice, both ICV and subcutaneously administered losartan induced similar hypotensive responses (−12.1 ± 1.8 vs. −14.7 ± 1.8 mmHg, P_route = 0.31).

Conclusion: While central effects of peripheral losartan cannot be excluded, we suggest the hypotensive effect of chronic ICV losartan was likely peripherally mediated. Thus, based on both acute and chronic AT₁ receptor inhibition and acute ROS inhibition, our findings suggest that greater activation of central AT₁ receptors or ROS are unlikely to be mediating the hypertension in BPH/2J mice.

Overexpression of AT2R in the solitary-vagal complex improves baroreflex in the spontaneously hypertensive rat

The aim of this study was to investigate the physiological effects of increased angiotensin II type 2 receptor (AT2R) expression in the solitary-vagal complex (nucleus of the solitary tract/dorsal motor nucleus of the vagus; NTS/DVM) on baroreflex function in non-anaesthetised normotensive (NT) and spontaneously hypertensive rats (SHR). Ten week old NT Holtzman and SHR were microinjected with either an adeno-associated virus expressing AT2R (AAV2-CBA-AT2R) or enhanced green fluorescent protein (control; AAV2-CBA-eGFP) into the NTS/DVM. Baroreflex and telemetry recordings were performed on four experimental groups: 1) NTeGFP, 2) NTAT2R, 3) SHReGFP and 4) SHRAT2R (n=4-7/group). Following in-vivo experimental procedures, brains were harvested for gene expression analysis. Impaired bradycardia in SHReGFP was restored in SHR rats overexpressing AT2R in the NTS/DMV. mRNA levels of angiotensin converting enzyme decreased and angiotensin converting enzyme 2 increased in the NTS/DMV of SHRAT2R compared to SHReGFP. Increased levels of pro-inflammatory cytokine mRNA levels in the SHReGFP group also decreased in the SHRAT2R group. AT2R overexpression did not elicit any significant change in mean arterial pressure (MAP) in all groups from baseline to 4weeks post viral transfection. Both SHReGFP and SHRAT2R showed a significant elevation in MAP compared to the NTeGFP and NTAT2R groups. Increased AT2R expression within the NTS/DMV of SHR was effective at improving baroreflex function but not MAP. We propose possible mediators involved in improving baroreflex are in the ANG II/ACE2 axis, suggesting a potential beneficial modulatory effect of AT2R overexpression in the NTS/DMV of neurogenic hypertensive rats.

Effect of p22phox depletion on sympathetic regulation of blood pressure in SHRSP: evaluation in a new congenic strain

Oxidative stress in the rostral ventrolateral medulla (RVLM), a sympathetic center in the brainstem, was implicated in the regulation of sympathetic activity in various hypertensive models including stroke-prone spontaneously hypertensive rats (SHRSP). In this study, we evaluated the role of the NADPH oxidases (NOX) in the blood pressure (BP) regulation in RVLM in SHRSP. The P22PHOX-depleted congenic SHRSP (called SP.MES) was constructed by introducing the mutated p22phox gene of Matsumoto Eosinophilic Shinshu rat. BP response to glutamate (Glu) microinjection into RVLM was compared among SHRSP, SP.MES, SHR and Wistar Kyoto (WKY); the response to Glu microinjection was significantly greater in SHRSP than in SP.MES, SHR and WKY. In addition, tempol, losartan and apocynin microinjection reduced the response to Glu significantly only in SHRSP. The level of oxidative stress, measured in the brainstem using lucigenin and dihydroethidium, was reduced in SP.MES than in SHRSP. BP response to cold stress measured by telemetry system was also blunted in SP.MES when compared with SHRSP. The results suggested that oxidative stress due to the NOX activation in RVLM potentiated BP response to Glu in SHRSP, which might contribute to the exaggerated response to stress in this strain.

Lack of Reactive Oxygen Species Deteriorates Blood Pressure Regulation in Acute Stress

This study investigated the contribution of reactive oxygen species (ROS) to blood pressure regulation in conscious adult male Wistar rats exposed to acute stress. Role of ROS was investigated in rats with temporally impaired principal blood pressure regulation systems using ganglionic blocker pentolinium (P, 5 mg/kg), angiotensin converting enzyme inhibitor captopril (C, 10 mg/kg), nitric oxide synthase inhibitor L-NAME (L, 30 mg/kg) and superoxide dismutase mimeticum tempol (T, 25 mg/kg). Mean arterial pressure (MAP) was measured by the carotid artery catheter and inhibitors were administered intravenously. MAP was disturbed by a 3-s air jet, which increased MAP by 35.2±3.0 % vs. basal MAP after the first exposure. Air jet increased MAP in captopril- and tempol-treated rats similarly as observed in saline-treated rats. In pentolinium-treated rats stress significantly decreased MAP vs. pre-stress value. In L-NAME-treated rats stress failed to affect MAP significantly. Treatment of rats with P+L+C resulted in stress-induced MAP decrease by 17.3±1.3 % vs. pre-stress value and settling time (20.1±4.2 s). In P+L+C+T-treated rats stress led to maximal MAP decrease by 26.4±2.2 % (p<0.005 vs. P+L+C) and prolongation of settling time to 32.6±3.3 s (p<0.05 vs. P+L+C). Area under the MAP curve was significantly smaller in P+L+C-treated rats compared to P+L+C+T-treated ones (167±43 vs. 433±69 a.u., p<0.008). In conclusion, in rats with temporally impaired blood pressure regulation, the lack of ROS resulted in greater stress-induced MAP alterations and prolongation of time required to reach new post-stress steady state.

Central mechanisms regulating coordinated cardiovascular and respiratory function during stress and arousal

Actual or potentially threatening stimuli in the external environment (i.e., psychological stressors) trigger highly coordinated defensive behavioral responses that are accompanied by appropriate autonomic and respiratory changes. As discussed in this review, several brain regions and pathways have major roles in subserving the cardiovascular and respiratory responses to threatening stimuli, which may vary from relatively mild acute arousing stimuli to more prolonged life-threatening stimuli. One key region is the dorsomedial hypothalamus, which receives inputs from the cortex, amygdala, and other forebrain regions and which is critical for generating autonomic, respiratory, and neuroendocrine responses to psychological stressors. Recent studies suggest that the dorsomedial hypothalamus also receives an input from the dorsolateral column in the midbrain periaqueductal gray, which is another key region involved in the integration of stress-evoked cardiorespiratory responses. In addition, it has recently been shown that neurons in the midbrain colliculi can generate highly synchronized autonomic, respiratory, and somatomotor responses to visual, auditory, and somatosensory inputs. These collicular neurons may be part of a subcortical defense system that also includes the basal ganglia and which is well adapted to responding to threats that require an immediate stereotyped response that does not involve the cortex. The basal ganglia/colliculi system is phylogenetically ancient. In contrast, the defense system that includes the dorsomedial hypothalamus and cortex evolved at a later time, and appears to be better adapted to generating appropriate responses to more sustained threatening stimuli that involve cognitive appraisal.

Effects of tempol on baroreflex neural arc versus peripheral arc in normotensive and spontaneously hypertensive rats

Although oxidative redox signaling affects arterial pressure (AP) regulation via modulation of vascular tone and sympathetic nerve activity (SNA), it remains unknown which effect plays a dominant role in the determination of AP in vivo. Open-loop systems analysis of the carotid sinus baroreflex was conducted to separately quantify characteristics of the neural arc from baroreceptor pressure input to SNA and the peripheral arc from SNA to AP in normotensive Wistar-Kyoto (WKY; n = 8) and spontaneously hypertensive rats (SHR; n = 8). Responses in SNA and AP to a staircase-wise increase in carotid sinus pressure were examined before and during intravenous administration of the membrane-permeable superoxide dismutase mimetic tempol (30 mg/kg bolus followed by 30 mg·kg−1·h−1). Two-way ANOVA indicated that tempol significantly decreased the response range of SNA (from 89.1 ± 2.4% to 60.7 ± 2.5% in WKY and from 77.5 ± 3.2% to 56.9 ± 7.3% in SHR, P < 0.001) without affecting the lower plateau of SNA (from 12.5 ± 2.4% to 9.5 ± 2.5% in WKY, and from 28.8 ± 2.8% to 30.4 ± 5.7% in SHR, P = 0.800) in the neural arc. While tempol did not affect the peripheral arc characteristics in WKY, it yielded a downward change in the regression line of AP vs. SNA in SHR. In conclusion, oxidative redox signaling plays an important role, not only in the pathological AP elevation, but also in the baroreflex-mediated physiological AP regulation. The effect of modulating oxidative redox signaling on the peripheral arc contributed to the determination of AP in SHR but not in WKY.

Central command dysfunction in rats with heart failure is mediated by brain oxidative stress and normalized by exercise training

Sympathoexcitation elicited by central command, a parallel activation of the motor and autonomic neural circuits in the brain, has been shown to become exaggerated in chronic heart failure (CHF). The present study tested the hypotheses that oxidative stress in the medulla in CHF plays a role in exaggerating central command-elicited sympathoexcitation, and that exercise training in CHF suppresses central command-elicited sympathoexcitation through its antioxidant effects in the medulla. In decerebrate rats, central command was activated by electrically stimulating the mesencephalic locomotor region (MLR) after neuromuscular blockade. The MLR stimulation at a current intensity greater than locomotion threshold in rats with CHF after myocardial infarction (MI) evoked larger (P < 0.05) increases in renal sympathetic nerve activity and arterial pressure than in sham-operated healthy rats (Sham) and rats with CHF that had completed longterm (8–12 weeks) exercise training (MI + TR). In the Sham and MI + TR rats, bilateral microinjection of a superoxide dismutase (SOD) mimetic Tempol into the rostral ventrolateral medulla (RVLM) had no effects on MLR stimulation-elicited responses. By contrast, in MI rats, Tempol treatment significantly reduced MLR stimulation-elicited responses. In a subset of MI rats, treatment with Tiron, another SOD mimetic, within the RVLM also reduced responses. Superoxide generation in the RVLM, as evaluated by dihydroethidium staining, was enhanced in MI rats compared with that in Sham and MI + TR rats. Collectively, these results support the study hypotheses. We suggest that oxidative stress in the medulla in CHF mediates central command dysfunction, and that exercise training in CHF is capable of normalizing central command dysfunction through its antioxidant effects in the medulla.

Brain stem NOS and ROS in neural mechanisms of hypertension

SIGNIFICANCE

There is now compelling evidence to substantiate the notion that by depressing baroreflex regulation of blood pressure and augmenting central sympathetic outflow through their actions on the nucleus tractus solitarii (NTS) and rostral ventrolateral medulla (RVLM), brain stem nitric oxide synthase (NOS) and reactive oxygen species (ROS) are important contributing factors to neural mechanisms of hypertension. This review summarizes our contemporary views on the impact of NOS and ROS in the NTS and RVLM on neurogenic hypertension, and presents potential antihypertensive strategies that target brain stem NOS/ROS signaling.

RECENT ADVANCES

NO signaling in the brain stem may be pro- or antihypertensive depending on the NOS isoform that generates this gaseous moiety and the site of action. Elevation of the ROS level when its production overbalances its degradation in the NTS and RVLM underlies neurogenic hypertension. Interventional strategies with emphases on alleviating the adverse actions of these molecules on blood pressure regulation have been investigated.

CRITICAL ISSUES

The pathological roles of NOS in the RVLM and NTS in neural mechanisms of hypertension are highly complex. Likewise, multiple signaling pathways underlie the deleterious roles of brain-stem ROS in neurogenic hypertension. There are recent indications that interactions between brain stem ROS and NOS may play a contributory role.

FUTURE DIRECTIONS

Given the complicity of action mechanisms of brain-stem NOS and ROS in neural mechanisms of hypertension, additional studies are needed to identify the most crucial therapeutic target that is applicable not only in animal models but also in patients suffering from neurogenic hypertension.

Angiotensin-generated reactive oxygen species in brain and pathogenesis of cardiovascular diseases

SIGNIFICANCE

Overproduction of angiotensin II (Ang II) in brain contributes to the pathogenesis of cardiovascular diseases. One of the most promising theses that emerged during the last decade is that production of reactive oxygen species (ROS) and activation of redox-dependent signaling cascades underlie those Ang II actions. This review summarizes our status of understanding on the roles of ROS and redox-sensitive signaling in brain Ang II-dependent cardiovascular diseases, using hypertension and heart failure as illustrative examples.

RECENT ADVANCES

ROS generated by NADPH oxidase, mitochondrial electron transport chain, and proinflammatory cytokines activates mitogen-activated protein kinases and transcription factors, which in turn modulate ion channel functions and ultimately increase neuronal activity and sympathetic outflow in brain Ang II-dependent cardiovascular diseases. Antioxidants targeting ROS have been demonstrated to be beneficial to Ang II-induced hypertension and heart failure via protection from oxidative stress in brain regions that subserve cardiovascular regulation.

CRITICAL ISSUES

Intra-neuronal signaling and the downstream redox-sensitive proteins involved in controlling the neuronal discharge rate, the sympathetic outflow, and the pathogenesis of cardiovascular diseases need to be identified. The cross talk between Ang II-induced oxidative stress and neuroinflammation in neural mechanisms of cardiovascular diseases also warrants further elucidation.

FUTURE DIRECTIONS

Future studies are needed to identify new redox-based therapeutics that work not only in animal models, but also in patients suffering from the prevalent diseases. Upregulation of endogenous antioxidants in the regulation of ROS homeostasis is a potential therapeutic target, as are small molecule- or nanoformulated conjugate-based antioxidant therapy.

Losartan reduces oxidative stress within the rostral ventrolateral medulla of rats with renovascular hypertension

BACKGROUND

Previous studies showed that the microinjection of antioxidants or the overexpression of superoxide dismutase within the rostral ventrolateral medulla (RVLM) reduces hypertension and sympathoexcitation in the 2-kidney, 1-clip (2K-1C) model. In this study, we hypothesized that angiotensin II (ANG II) type 1 receptor (AT1R) is involved in the oxidative stress within the RVLM and contributes to cardiovascular dysfunction in renovascular hypertension.

METHODS

Losartan (30mg/kg/day, oral gavage) was administered for 7 consecutive days by week 5 after implantation of the clip (gap width = 0.2mm). Mean arterial pressure, baroreflex, and renal sympathetic nerve activity (rSNA) were evaluated. Superoxide production was evaluated by dihydroethidium (DHE) staining within the RVLM and within a control area. Systemic oxidative stress was characterized by measurement of thiobarbituric acid reactive substances (TBARS) and total glutathione (tGSH) in the blood.

RESULTS

AT1R blockade significantly (P < 0.05) reduced hypertension by approximately 20% (n = 11) and sympathoexcitation to the kidneys by approximately 41% (n = 6) in the 2K-1C rats. Losartan treatment increased the baroreflex sensitivity of rSNA to pressor (67%) and depressor (140%) stimuli in the 2K-1C rats. AT1R blockade caused a significant (66%) reduction in DHE staining within the RVLM but not within the control area, reduced plasma TBARS (from 1.6±0.1 to 1.0±0.1 nmol/ml), and increased tGSH (from 3.4±0.4 to 5.2±0.3 μmol/g Hb) in the 2K-1C group only.

CONCLUSIONS

Our findings suggest that the beneficial effects of ANG II blockade in renovascular hypertension are partly due to preferential reduction of oxidative stress in the RVLM.

Detraining differentially preserved beneficial effects of exercise on hypertension: effects on blood pressure, cardiac function, brain inflammatory cytokines and oxidative stress

Aims

This study sought to investigate the effects of physical detraining on blood pressure (BP) and cardiac morphology and function in hypertension, and on pro- and anti-inflammatory cytokines (PICs and AIC) and oxidative stress within the brain of hypertensive rats.

Methods and Results

Hypertension was induced in male Sprague-Dawley rats by delivering AngiotensinII for 42 days using implanted osmotic minipumps. Rats were randomized into sedentary, trained, and detrained groups. Trained rats underwent moderate-intensity exercise (ExT) for 42 days, whereas, detrained groups underwent 28 days of exercise followed by 14 days of detraining. BP and cardiac function were evaluated by radio-telemetry and echocardiography, respectively. At the end, the paraventricular nucleus (PVN) was analyzed by Real-time RT-PCR and Western blot. ExT in AngII-infused rats caused delayed progression of hypertension, reduced cardiac hypertrophy, and improved diastolic function. These results were associated with significantly reduced PICs, increased AIC (interleukin (IL)-10), and attenuated oxidative stress in the PVN. Detraining did not abolish the exercise-induced attenuation in MAP in hypertensive rats; however, detraining failed to completely preserve exercise-mediated improvement in cardiac hypertrophy and function. Additionally, detraining did not reverse exercise-induced improvement in PICs in the PVN of hypertensive rats; however, the improvements in IL-10 were abolished.

Conclusion

These results indicate that although 2 weeks of detraining is not long enough to completely abolish the beneficial effects of regular exercise, continuing cessation of exercise may lead to detrimental effects.

Excessive leukotriene B4 in nucleus tractus solitarii is prohypertensive in spontaneously hypertensive rats

Inflammation within the brain stem microvasculature has been associated with chronic cardiovascular diseases. We found that the expression of several enzymes involved in arachidonic acid-leukotriene B4 (LTB4) production was altered in nucleus tractus solitarii (NTS) of spontaneously hypertensive rat (SHR). LTB4 produced from arachidonic acid by 5-lipoxygenase is a potent chemoattractant of leukocytes. Leukotriene B4-12-hydroxydehydrogenase (LTB4-12-HD), which degrades LTB4, was downregulated in SHR rats compared with that in Wistar-Kyoto rats. Quantitative real-time PCR revealed that LTB4-12-HD was reduced by 63% and 58% in the NTS of adult SHR and prehypertensive SHR, respectively, compared with that in age-matched Wistar-Kyoto rats (n=6). 5-lipoxygenase gene expression was upregulated in the NTS of SHR (≈50%; n=6). LTB4 levels were increased in the NTS of the SHR, (17%; n=10, P<0.05). LTB4 receptors BLT1 (but not BLT2) were expressed on astroglia in the NTS but not neurons or vessels. Microinjection of LTB4 into the NTS of Wistar-Kyoto rats increased both leukocyte adherence and arterial pressure for over 4 days (peak: +15 mm Hg; P<0.01). In contrast, blockade of NTS BLT1 receptors lowered blood pressure in the SHR (peak: -13 mm Hg; P<0.05) but not in Wistar-Kyoto rats. Thus, excessive amounts of LTB4 in NTS of SHR, possibly as a result of upregulation of 5-lipoxygenase and downregulation of LTB4-12-HD, can induce inflammation. Because blockade of NTS BLT1 receptors lowered arterial pressure in the SHR, their endogenous activity may contribute to the hypertensive state of this rodent model. Thus, inflammatory reactions in the brain stem are causally associated with neurogenic hypertension.

Abnormal baroreflex function is dissociated from central angiotensin II receptor expression in chronic heart failure

Neurohumoral disturbances characterize chronic heart failure (CHF) and are reflected, in part, as impairment of baroreflex sensitivity (BRS) and sympathetic function. However, the mechanisms that trigger these neurohumoral abnormalities in CHF are not clear. We hypothesized that the BRS is blunted early in CHF and that the humoral effects occur later and contribute to progressive loss of cardiovascular control in CHF. We assessed the BRS (beats/min per mmHg) and recorded renal sympathetic nerve activity (RSNA) in four groups of conscious rabbits at varying time intervals: control, 1-week CHF, 2-week CHF, and 3-week CHF. Chronic heart failure was induced by ventricular pacing at 360 beats/min and was assessed by echocardiography. Arterial blood pressure and heart rate were recorded by an implanted telemetric device and RSNA through an implanted electrode. A significant fall in the ejection fraction, fractional shortening, and an increase in left ventricular end-systolic diameter and left ventricular end-diastolic diameter were observed in all CHF groups. The BRS was significantly reduced in all the CHF groups with no significant change in the basal RSNA (% of maximum) after 1 week of pacing; a small but insignificant rise in RSNA was seen at 2 weeks, and a significant rise in RSNA was observed at 3 weeks. Angiotensin II type 1 (AT-1) receptor protein (Western Blot) and mRNA (reverse transcriptase-polymerase chain reaction) expression in the rostral ventrolateral medulla exhibited a progressive increase with the duration of CHF, reaching significance after 3 weeks, the same time point in which RSNA was significantly elevated. These data are the first to examine early changes in central AT-1 receptors in CHF and suggest that the fall in BRS and hemodynamic changes occur early in the development of CHF followed by sympathoexcitation and overexpression of AT-1 receptors with the progression of CHF, causing further impairment of cardiovascular control.

Oxidative stress in the rostral ventrolateral medulla modulates excitatory and inhibitory inputs in spontaneously hypertensive rats

OBJECTIVES

The rostral ventrolateral medulla (RVLM) of the brainstem and the paraventricular nucleus (PVN) of the hypothalamus play crucial roles in central cardiovascular regulation. In hypertensive rats, an imbalance of excitatory and inhibitory inputs to the RVLM enhances central sympathetic outflow. Increased reactive oxygen species (ROS) in the RVLM also contribute to sympathoexcitation, leading to hypertension. The aim of the present study was to elucidate whether ROS in the RVLM modulate synaptic transmission via excitatory and inhibitory amino acids and influence the excitatory inputs to the RVLM from the PVN in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

We transfected adenovirus vectors encoding the manganese superoxide dismutase (AdMnSOD) gene to scavenge ROS in the RVLM both in Wistar-Kyoto rats and SHRs. The decreases in blood pressure and renal sympathetic nerve activity (RSNA) evoked by injecting kynurenic acid, a glutamate receptor blocker, into the RVLM were attenuated, and the increases in blood pressure and RSNA evoked by injecting bicuculline, a γ-amino butyric acid (GABA) receptor blocker, into the RVLM were enhanced in AdMnSOD-transfected SHRs compared with adenovirus vectors encoding the β-galactosidase (AdLacZ) gene-transfected SHRs. Furthermore, the increases in blood pressure and RSNA evoked by injecting bicuculline into the PVN were attenuated in AdMnSOD-transfected SHRs compared with AdLacZ-transfected SHRs.

CONCLUSION

These findings suggest that ROS in the RVLM enhance glutamatergic excitatory inputs and attenuate GABAergic inhibitory inputs to the RVLM, thereby increasing sympathoexcitatory input to the RVLM from the PVN in SHRs.

Chronic exercise modulates RAS components and improves balance between pro- and anti-inflammatory cytokines in the brain of SHR

Recently, exercise has been recommended as a part of lifestyle modification for all hypertensive patients; however, the precise mechanisms of its effects on hypertension are largely unknown. Therefore, this study aimed to investigate the mechanisms within the brain that can influence exercise-induced effects in an animal model of human essential hypertension. Young normotensive WKY rats and SHR were given moderate-intensity exercise for 16 weeks. Blood pressure was measured bi-weekly by tail-cuff method. Animals were then euthanized; paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM), important cardiovascular regulatory centers in the brain, were collected and analyzed by real-time RT-PCR, Western blot, EIA, and fluorescent microscopy. Exercise of 16-week duration attenuated systolic, diastolic, and mean arterial pressure in SHR. Sedentary SHR exhibited increased pro-inflammatory cytokines (PICs) and decreased anti-inflammatory IL-10 levels in the PVN and RVLM. Furthermore, SHR(sed) rats exhibited elevated levels of ACE, AT1R, and decreased levels of ACE2 and receptor Mas in the PVN and RVLM. Chronic exercise not only prevented the increase in PICs (TNF-α, IL-1β), ACE, and AT1R protein expression in the brain of SHR, but also dramatically upregulated IL-10, ACE2, and Mas receptor expression in SHR. In addition, these changes were associated with reduced plasma AngII levels, reduced neuronal activity, reduced NADPH-oxidase subunit gp91(phox) and inducible NO synthase in trained SHRs indicating reduced oxidative stress. These results suggest that chronic exercise not only attenuates PICs and the vasoconstrictor axis of the RAS but also improves the anti-inflammatory defense mechanisms and vasoprotective axis of the RAS in the brain, which, at least in part, explains the blood pressure-lowering effects of exercise in hypertension.

Sympathoexcitation by brain oxidative stress mediates arterial pressure elevation in salt-induced chronic kidney disease

Hypertension is very prevalent in chronic kidney disease and critical for its prognosis. Sympathoexcitation and oxidative stress have been demonstrated to be involved in chronic kidney disease. We have shown previously that sympathoexcitation by brain oxidative stress mediates arterial pressure elevation in the salt-sensitive hypertension model, Dahl salt-sensitive rats. Thus, we investigated whether sympathoexcitation by excessive brain oxidative stress could contribute to arterial pressure elevation in salt-induced chronic kidney disease model rats. Young (3-week-old) male Sprague-Dawley rats were randomly assigned to a uninephrectomy or sham operation and then subjected to either a normal salt (0.5%) or high-salt (8.0%) diet for 4 weeks. The young salt-loaded uninephrectomized rats exhibited sympathoexcitation, hypertension, and renal injury, proteinuria and global glomerulosclerosis together with tubulointerstitial damage. Under urethane anesthesia and artificial ventilation, renal sympathetic nerve activity, arterial pressure, and heart rate decreased to a greater degree in the salt-loaded uninephrectomized rats than in the nonsalt-loaded uninephrectomized rats and the salt-loaded or nonsalt-loaded sham-operated rats, when Tempol, a membrane-permeable superoxide dismutase mimetic, was infused acutely into the lateral cerebral ventricle. Oxidative stress in the hypothalamus, measured by lucigenin chemiluminescence, was also significantly greater. Furthermore, in the salt-loaded uninephrectomized rats, antioxidant treatment with chronic intracerebroventricular Tempol decreased sympathetic nerve activity and arterial pressure, which, in turn, led to a decrease in renal damage. Similar effects were elicited by treatment with oral moxonidine, the central sympatholytic agent. In conclusion, sympathoexcitation by brain oxidative stress may mediate arterial pressure elevation in salt-induced chronic kidney disease.

Effect of age on high-fat diet-induced hypertension

Aging and obesity both have a significant impact on central blood pressure (BP) regulation, and previous studies indicated that changes in central redox signaling with age may affect high-fat (HF) diet-induced cardiovascular responses. Therefore, we investigated the effects of 60% HF feeding on BP regulation in young adult (5 mo) and old (26 mo) Fischer-344 × Brown-Norway rats. Radiotelemetric transmitters were implanted to measure BP, heart rate (HR), locomotor activity, and spontaneous baroreflex sensitivity. Expression and activity of NADPH oxidase and ANG II type 1 receptor were assessed in the hypothalamus and in the nucleus tractus solitarii. Old animals gained more weight on HF diet compared with young, whereas central NADPH oxidase expression and activity elevated similarly in the two age groups. After an initial hypotensive and tachycardic response during the first week of HF feeding, BP in young animals increased and became significantly elevated after 6 wk of HF feeding. In contrast, BP in old animals remained depressed. Nighttime HR and locomotor activity decreased in both young and old rats fed with HF diet, but these changes were more significant in young rats. As a result, amplitudes of circadian variation of BP, HR, and activity that were originally higher in young rats declined significantly and became similar in the two age groups. In conclusion, our experiments led to the surprising finding that HF diet has a more serious impact on cardiovascular regulation in young animals compared with old.

Role of angiotensin in the rostral ventrolateral medulla in the development and maintenance of hypertension

Whilst crucial for behavioural and homeostatic responses to environmental challenges, chronic elevation of sympathetic nervous system activity to specific vascular beds is associated with hypertension. Indeed such elevated activity may drive the increase in blood pressure seen in some people and in some experimental models of hypertension. This review discusses the neural circuitry involved in generating and modulating sympathetic efferent nerve activity, focusing on the premotor neurons of the rostral ventrolateral medulla. Neurons in the rostral ventrolateral medulla show altered responses to angiotensin II in experimental models of hypertension, suggesting that this might be an important node for interaction between these two systems that are crucial for regulation of blood pressure.

Central TNF inhibition results in attenuated neurohumoral excitation in heart failure: a role for superoxide and nitric oxide

This study examined the effect of central tumor necrosis factor-alpha (TNF) blockade on the imbalance between nitric oxide and superoxide production in the paraventricular nucleus (PVN) and ventrolateral medulla (VLM), key autonomic regulators, and their contribution to enhanced sympathetic drive in mice with congestive heart failure (CHF). We also used a TNF gene knockout (KO) mouse model to study the involvement of TNF in body fluid homeostasis and sympathoexcitation in CHF. After implantation of intracerebroventricular (ICV) cannulae, myocardial infarction (MI) was induced in wild-type (WT) and KO mice by coronary artery ligation. Osmotic mini-pumps were implanted into one set of WT + MI/Sham mice for continuous ICV infusion of Etanercept (ETN), a TNF receptor fusion protein, or vehicle (VEH). Gene expressions of neuronal nitric oxide synthase (NOS) and angiotensin receptor-type 2 were reduced, while those of inducible NOS, Nox2 homologs, superoxide, peroxynitrite and angiotensin receptor-type 1 were elevated in the brainstem and hypothalamus of MI + VEH. Plasma norepinephrine levels and the number of Fos-positive neurons were also increased in the PVN and VLM in MI + VEH. MI + ETN and KO + MI mice exhibited reduced oxidative stress, reduced sympathoexcitation and an improved cardiac function. These changes in WT + MI were associated with increased sodium and fluid retention. These results indicate that elevated TNF in these autonomic regulatory regions of the brain alter the production of superoxide and nitric oxide, contributing to fluid imbalance and sympathoexcitation in CHF.

AT₁ receptor and NAD(P)H oxidase mediate angiotensin II-stimulated antioxidant enzymes and mitogen-activated protein kinase activity in the rat hypothalamus

INTRODUCTION

Angiotensin II (AngII) regulates blood pressure and water and electrolyte metabolism through the stimulation of NAD(P)H oxidase and production of reactive oxygen species (ROS) such as O₂⁻, which is metabolised by superoxide dismutase, catalase and glutathione peroxidase. We assessed the role of AT₁ and AT₂ receptors, NAD(P)H oxidase and protein kinase C (PKC) in Ang II-induced sodium and water excretion and their capacity to stimulate antioxidant enzymes in the rat hypothalamus, a brain structure known to express a high density of AngII receptors.

MATERIALS AND METHODS

Male Sprague-Dawley rats were intracerebroventricularly (ICV) injected with AngII and urinary sodium and water excretion was assessed. Urine sodium concentration was determined using flame photometry. After decapitation the hypothalamus was microdissected under stereomicroscopic control. Superoxide dismutase, catalase and glutathione peroxidase activity were determined spectrophotometrically and extracellular signal-regulated kinase (ERK1/2) activation was analysed by Western blot.

RESULTS

AngII-ICV resulted in antidiuresis and natriuresis. ICV administration of losartan, PD123319, apocynin and chelerythrine blunted natriuresis. In hypothalamus, AngII increased catalase, superoxide dismutase and glutation peroxidase activity and ERK1/2 phosphorylation. These actions were prevented by losartan, apocynin and chelerythrine, and increased by PD123319.

CONCLUSIONS

AT₁ and AT₂ receptors, NAD(P)H oxidase and PKC pathway are involved in the regulation of hydromineral metabolism and antioxidant enzyme activity induced by AngII.

Animal aging and regulation of sympathetic nerve discharge

Studies completed in human subjects have made seminal contributions to understanding the effects of age on sympathetic nervous system (SNS) regulation. Numerous experimental constraints limit the design of studies involving human subjects; therefore, completion of studies in animal models of aging would be expected to provide additional insight regarding mechanisms mediating age-related changes in sympathetic nerve discharge (SND) regulation. The present review assesses the current state of the literature regarding contributions from animal studies on the effects of advancing age on SND regulation, focusing primarily on studies that have used direct recordings of sympathetic nerve outflow. Few studies using direct SND recordings have been completed in animal models of aging, regardless of the fundamental component of SND regulation reviewed (basal levels, acute responsiveness, relationships between the discharges in sympathetic nerves, central neural regulation). SNS responsiveness to various acute stressors is altered in aged compared with young animals; however, mechanisms remain virtually unexplored. There is a marked dearth of studies that have used central neural microinjection techniques in conjunction with SND recordings in aged animals, making it difficult to develop an evidence-based framework regarding potential age-associated effects on central regulation of SND. Determination of age-related changes in mechanisms regulating SND is important for understanding relationships between chronic disease development and changes in SNS function; however, this can only be achieved by substantially extending the current knowledge base regarding the effects of age on SND regulation in animal studies.

Endothelin activation of reactive oxygen species mediates stress-induced pressor response in Dahl salt-sensitive prehypertensive rats

Experiments were designed to test the hypothesis that endothelin (ET) and/or reactive oxygen species contribute to the pressor response induced by acute air jet stress in normotensive Dahl salt-sensitive rats maintained on a normal salt diet (prehypertensive). Mean arterial pressure was chronically monitored by telemetry before and after 3-day treatment with the free radical scavenger 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (Tempol) or ET receptor antagonists ABT-627 (ET A antagonist) or A-182086 (ET A/B antagonist) supplied in the drinking water. Rats were restrained and subjected to pulsatile air jet stress (3 minutes). Plasma samples at baseline and during acute stress were analyzed for 8-isoprostane (measure of reactive oxygen species production) and ET. Neither Tempol nor ET receptor antagonist treatment had an effect on baseline mean arterial pressure or plasma 8-isoprostane. The pressor response to acute stress was accompanied by significant increases in plasma 8-isoprostane and ET. Tempol significantly reduced both the total pressor response (area under the curve) and the stress-mediated increase in plasma 8-isoprostane; conversely, Tempol had no effect on the stress-induced increase in plasma ET. Combined ET(A/B) antagonism, but not selective ET(A) receptor blockade, similarly suppressed the pressor response to stress and stress-mediated rise in 8-isoprostane. Together these results indicate that reactive oxygen species contribute to the pressor response to acute air jet stress. Furthermore, the increase in reactive oxygen species occurs downstream of ET(B) receptor activation.

Reactive oxygen species in rostral ventrolateral medulla modulate cardiac sympathetic afferent reflex in rats

AIM

The aim of the present study was to investigate whether reactive oxygen species (ROS) in rostral ventrolateral medulla (RVLM) modulate cardiac sympathetic afferent reflex (CSAR) and the enhanced CSAR response caused by microinjection of angiotensin II (Ang II) into the paraventricular nucleus (PVN).

METHODS

Under urethane and alpha-chloralose anaesthesia, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded in sinoaortic-denervated and cervical-vagotomized rats. The CSAR was evaluated by the RSNA response to epicardial application of capsaicin (1.0 nmol).

RESULTS

Bilateral RVLM microinjection of tempol (a superoxide anion scavenger) or polyethylene glycol-superoxide dismutase (PEG-SOD, an analogue of endogenous superoxide dismutase) attenuated the CSAR, but did not cause significant change in baseline RSNA and MAP. NAD(P)H oxidase inhibitors apocynin or phenylarsine oxide (PAO) also showed similar effects, but SOD inhibitor diethyldithio-carbamic acid (DETC) enhanced the CSAR and baseline RSNA, and increased the baseline MAP. Bilateral PVN microinjection of Ang II (0.3 nmol) enhanced the CSAR and increased RSNA and MAP, which was inhibited by the pre-treatment with RVLM administration of tempol, PEG-SOD, apocynin or PAO. The pre-treatment with DETC in the RVLM only showed a tendency in potentiating the CSAR response of Ang II in the PVN, but significantly potentiated the RSNA and MAP responses of Ang II.

CONCLUSION

These results suggest that the NAD(P)H oxidase-derived ROS in the RVLM modulate the CSAR. The ROS in the RVLM is necessary for the enhanced CSAR response caused by Ang II in the PVN.

Central oxytocin modulation of acute stress-induced cardiovascular responses after myocardial infarction in the rat

The present study was aimed at determining the role of centrally released oxytocin in regulation of blood pressure and heart rate (HR) under resting conditions and during an acute air-jet stress in rats with a myocardial infarction and controls infarcted. Four weeks after ligation of a coronary artery or sham surgery, conscious Sprague Dawley rats were subjected to one of the following intracerebroventricular (ICV) infusions: (1) 0.9% NaCl (control), (2) oxytocin, (3) oxytocin receptor antagonist {desGly-NH(2)-d(CH(2))(5)[D-Tyr(2)Thr(4)]OVT}(OXYANT). Resting arterial blood pressure and HR were not affected by any of the ICV infusions either in the infarcted or sham-operated rats. In the control experiments, the pressor and tachycardic responses to the air jet of infarcted rats were significantly greater than in the sham-operated rats. OXYANT significantly enhanced the cardiovascular responses to stress only in the sham-operated rats whereas oxytocin significantly attenuated both responses in the infarcted but not in the sham-operated rats. The results suggest that centrally released endogenous oxytocin significantly reduces the cardiovascular responses to the acute stressor in control rats. This buffering function of the brain-oxytocin system is not efficient during the post-myocardial infarction state, however it may be restored by central administration of exogenous oxytocin.

Effect of high-fat diet feeding on hypothalamic redox signaling and central blood pressure regulation

We examined the effect of high-fat (HF) feeding on blood pressure (BP) regulation, including hypothalamic redox signaling, as well as the changes in diurnal patterns and responses to restraint stress. Furthermore, we investigated whether HF feeding affects catecholamine and neuropeptide Y (NPY) biosynthesis in the adrenal medulla. Male obesity-prone Sprague-Dawley rats were fed with standard rat chow or 60% HF diet for 6 months. BP and heart rate (HR) were measured by telemetry, and circadian changes as well as responses to 20 min restraint stress were analyzed. Mean arterial BP was significantly elevated in HF rats both during daytime and nighttime compared with controls, whereas HR was elevated only during the day. BP and HR increased similarly in response to stress in both experimental groups; however, post-stress recovery of BP and HR were significantly delayed in HF animals. Protein levels of angiotensin II type 1 receptor (AT(1)) and NOX2, p67(phox) and p47(phox) subunits of NADPH oxidase, as well as NADPH oxidase activity increased significantly in the hypothalamus with HF feeding, whereas levels of antioxidant enzymes and nitric oxide synthases remained unchanged. In addition, HF diet also elevated the adrenomedullary protein levels of tyrosine hydroxylase and NPY. This study shows that feeding obesity-prone Sprague-Dawley rats with a HF diet results in elevated BP and HR and delayed cardiovascular post-stress recovery, and that these changes are paralleled by increases in the expression and activity of NADPH oxidase in the hypothalamus without a compensatory increase in the antioxidant enzyme levels, possibly leading to superoxide-mediated sympathoexcitation and hypertension.

Role of oxidative stress in the natriuresis induced by central administration of angiotensin II

Introduction. Central administration of angiotensin II (Ang II) is known to reduce urinary volume and to increase sodium and potassium excretion. Recently, a novel signalling mechanism for Ang II in the periphery has been shown to involve reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase-derived reactive oxygen species (ROS).Although ROS are now known to be involved in numerous Ang II-regulated processes in peripheral tissues, and are increasingly implicated in CNS neurodegenerative diseases, the role of ROS in central regulation of Ang II-induced hydromineral metabolism remains unexplored.The hypothesis that ROS are involved in central Ang II signalling and in Ang II-dependent antidiuresis, natriuresis and kaliuresis was tested by the use of selective antagonists of the NAD(P)H oxidase cascade. Materials and methods. In intracerebroventricular (ICV)-cannulated rats,Ang II was injected ICV and urinary sodium and potassium excretion was assessed at 1-, 3-, and 6-hour periods of urine collection. Urine sample was analysed for sodium and potassium concentration using a flame photometer. The role of NAD(P)H oxidase-dependent signalling cascade was evaluated using the selective NAD(P)H oxidase inhibitor, apocynin; the superoxide dismutase mimetic, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol); and the protein kinase C inhibitor, chelerythrine. Results. ICV administration of Ang II to conscious hydrated rats resulted in a significant decrease in urinary volume in the first hour, and an increased sodium and potassium excretion during the 6-hour period of urine collection, which was most effective during the 3 and 6 h. Interference with the NAD(P)H oxidase signalling by central administration of apocynin, tempol or chelerythrine, blunted the natriuretic and kaliuretic effect induced by central administration of Ang II, without affecting its antidiuretic action.

Conclusion.This study demonstrates that increases of urinary sodium and potassium excretion elicited by central administration of Ang II are mediated by NAD(P)H oxidase- dependent production of superoxide and protein kinase C activation in conscious hydrated rats.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Sympathoexcitation by oxidative stress in the brain mediates arterial pressure elevation in obesity-induced hypertension

BACKGROUND

Obesity is one of the major risk factors for cardiovascular disease and is often associated with increased oxidative stress and sympathoexcitation. We have already suggested that increased oxidative stress in the brain modulates the sympathetic regulation of arterial pressure in salt-sensitive hypertension, which is often associated with obesity. The present study was performed to determine whether oxidative stress could mediate central sympathoexcitation in the initial stage of obesity-induced hypertension.

METHODS AND RESULTS

Four-week-old male Sprague-Dawley rats were fed a high-fat (45% kcal as fat) or low-fat (10% kcal as fat) diet for 6 weeks. Fat loading elicited hypertension and sympathoexcitation, along with visceral obesity. In urethane-anesthetized and artificially ventilated rats, arterial pressure and renal sympathetic nerve activity decreased in a dose-dependent fashion when 53 or 105 mumol/kg tempol, a membrane-permeable superoxide dismutase mimetic, was infused into the lateral cerebral ventricle. Central tempol reduced arterial pressure and renal sympathetic nerve activity to a significantly greater extent in high-fat diet-fed hypertensive rats than in low-fat diet-fed normotensive rats. Intracerebroventricular apocynin or diphenyleneiodonium, a reduced NADPH oxidase inhibitor, also elicited markedly greater reductions in arterial pressure and renal sympathetic nerve activity in the high-fat diet-fed rats. In addition, fat loading increased NADPH oxidase activity and NADPH oxidase subunit p22(phox), p47(phox), and gp91(phox) mRNA expression in the hypothalamus.

CONCLUSIONS

In obesity-induced hypertension, increased oxidative stress in the brain, possibly via activation of NADPH oxidase, may contribute to the progression of hypertension through central sympathoexcitation.

Oxidative Impairment of Mitochondrial Electron Transport Chain Complexes in Rostral Ventrolateral Medulla Contributes to Neurogenic Hypertension

The role for mitochondrial electron transport chain (ETC) in neurogenic hypertension is unidentified. We evaluated the hypothesis that feedforward depression of mitochondrial ETC functions by superoxide anion (O 2 ·− ) and hydrogen peroxide (H 2 O 2 ) in rostral ventrolateral medulla (RVLM), a brain stem site that maintains sympathetic vasomotor tone and contributes to oxidative stress and neural mechanism of hypertension. Compared with normotensive Wistar-Kyoto rats, spontaneously hypertensive rats exhibited mitochondrial ETC dysfunctions in RVLM in the forms of depressed complex I or III activity and reduced electron coupling capacity between complexes I and III or II and III. Microinjection of coenzyme Q 10 into RVLM of spontaneously hypertensive rats reversed the depressed ETC activity and augmented O 2 ·− production and hypertensive phenotypes. This mobile electron carrier also antagonized the elevated H 2 O 2 in RVLM and vasopressor responses to complex I (rotenone) or III (antimycin A) inhibitor in Wistar-Kyoto or prehypertensive rats. Intracerebroventricular infusion of angiotensin II promoted mitochondrial ETC dysfunctions in Wistar-Kyoto rats, and coenzyme Q 10 or gene knockdown of the p22 phox subunit of NADPH oxidase antagonized the resultant elevation of H 2 O 2 in RVLM. Overexpression of superoxide dismutase or catalase in RVLM of spontaneously hypertensive rats by gene transfer reversed mitochondrial dysfunctions and blunted the augmented O 2 ·− and H 2 O 2 in RVLM. We conclude that O 2 ·− - and H 2 O 2 -dependent feedforward impairment of mitochondrial ETC complexes because of predisposed downregulation of superoxide dismutase or catalase and a cross-talk between NADPH oxidase-derived O 2 ·− and ETC enzymes contribute to chronic oxidative stress in the RVLM of spontaneously hypertensive rats, leading to augmented sympathetic vasomotor tone and hypertension.

Regional expression of NAD(P)H oxidase and superoxide dismutase in the brain of rats with neurogenic hypertension

BACKGROUND

Single injection of small quantities of phenol into the kidney cortex causes hypertension which is mediated by renal afferent sympathetic pathway activation. This phenomenon can be prevented by superoxide dismutase (SOD) infusion in the lateral ventricle, suggesting the role of superoxide (O(2)(-).) in noradrenergic control of arterial pressure. Since NAD(P)H oxidase is a major source of O(2)(-)., we tested the hypothesis that hypertension in this model may be associated with upregulation of NAD(P)H oxidase in relevant regions of brain.

METHODS

NAD(P)H oxidase subunits, mitochondrial (MnSOD) and cytoplasmic (CuZnSOD) SOD were measured in rats 4 weeks after injection of phenol or saline in the left kidney cortex.

RESULTS

Phenol-injected rats exhibited hypertension, upregulation of gp91(phox), p22(phox), p47(phox) and p67(phox) in the medulla, gp91(phox) and p22(phox) in pons and gp91(phox) in hypothalamus. This was associated with upregulation of MnSOD with little change in CuZnSOD.

CONCLUSIONS

Chronic hypertension in phenol-injected rats is associated with upregulation of NAD(P)H oxidase and hence increased O(2)(-). production capacity in the key regions of the brain involved in regulation of blood pressure. Since reactive oxygen species can intensify central noradrenergic activity, the observed maladaptive changes may contribute to the genesis and maintenance of the associated hypertension.

Differential sensitisation to central cardiovascular effects of angiotensin II in rats with a myocardial infarct: relevance to stress and interaction with vasopressin

The purpose of the present study was to elucidate if rats with myocardial infarction manifest altered responsiveness to central cardiovascular effects of low doses of angiotensin II (ANG II), and if ANG II and vasopressin (VP) cooperate in the central regulation of cardiovascular functions at rest and during stress. Conscious Sprague-Dawley rats with myocardial infarction induced by left coronary artery ligation, or sham-ligated (SL) controls were infused intracerebroventricularly with artificial cerebrospinal fluid (aCSF), ANG II, ANG II + VP or ANG II + V1a receptor antagonist (V1ANT) 4 weeks after cardiac surgery. In the infarcted but not in the SL rats, the resting mean arterial blood pressure (MABP) was significantly elevated by infusions of ANG II and ANG II + VP, while infusion of ANG II + V1ANT was not effective. During administration of aCSF, the pressor, and tachycardic responses to an air-jet stressor were significantly greater in the infarcted than in the SL rats. In the SL rats, the pressor responses to the stressor were significantly greater during infusions of ANG II, ANG II + VP and ANG II + V1ANT than during infusion of aCSF. The tachycardic response in the SL rats was enhanced only by the combined infusion of ANG II + VP. In the infarcted rats, the pressor and the tachycardic responses to the stressor were similar in all groups. It is concluded that: (1) under resting conditions the infarcted rats manifest sensitisation to the central pressor effect of ANG II and that this effect depends on concomitant stimulation of V1a VP receptors, (2) central ANG II may enhance the pressor response to an alarming stressor in the SL rats through an action which does not depend on the concomitant stimulation of V1a receptors, (3) the cooperative action of ANG II and VP is required for intensification of the tachycardic response to the alarming stressor in the SL rats and (4) exaggeration of the cardiovascular responses to the alarming stressor in the infarcted rats cannot be further augmented by an additional stimulation of central ANG II receptors or combined stimulation of ANG II and VP receptors.

Ultrastructural changes in the lateral reticular nucleus of the medulla oblongata in prepubertal rats under the effect of long-term stress exposure

Ultrastructural changes in the lateral reticular nuclei of the medulla oblongata of rat pups developing under the effect of chronic mental and pain stress indicate impaired histogenesis of structures of the medulla oblongata reticular formation and appearance of pronounced morphofunctional differences between the neurons.

Nitric oxide synthase inhibition in rostral ventrolateral medulla attenuates pressor response to psychological stress in rabbits

Nitric oxide (NO) has been critically implicated in the central regulation of autonomic function. We recently found, however, that acute (up to 30min) blockade of NO synthase (NOS) in the rostral ventrolateral medulla (RVLM) inhibited sympathetic baroreflex transmission, without altering the cardiovascular response to psychological (air-jet) stress in rabbits. In the present study, we examined the effect of the later phase (1-3h) of NOS inhibition in the RVLM on the pressor and sympathetic responses to air-jet stress in conscious rabbits. Air-jet evoked a sustained increase in blood pressure (+14+/-2mmHg), heart rate (+37+/-9beats/min) and renal sympathetic nerve activity (+52+/-8%). Bilateral microinjection of a NOS inhibitor l-NAME (10nmol) into RVLM did not affect resting parameters or stress responses during the first 30min after injection. Conversely, in the later phase of NOS inhibition, the pressor, tachycardic and renal sympathetic responses to air-jet stress were reversibly attenuated by 48-72%. Microinjection of l-NAME outside the RVLM did not change stress responses. Microinjection of glutamate (3nmol) into the RVLM induced similar pressor effects before and after l-NAME (+30+/-6mmHg and +26+/-6mmHg, respectively). Microinjection of d-NAME altered neither stress responses nor pressor response to glutamate. These results suggest that NOS inhibition in the RVLM has a dual effect on the autonomic response to psychological stress. In the early phase, NOS inhibition has little impact on this response. However, in the later phase, NOS inhibition attenuates the stress response, perhaps via indirect mechanisms such as altering the local redox state.

Changes in the subcellular distribution of NADPH oxidase subunit p47phox in dendrites of rat dorsomedial nucleus tractus solitarius neurons in response to chronic administration of hypertensive agents

NADPH oxidase-generated superoxide can modulate crucial intracellular signaling cascades in neurons of the nucleus tractus solitarius (NTS), a brain region that plays an important role in cardiovascular processes. Modulation of NTS signaling by superoxide may be linked to the subcellular location of the mobile NADPH oxidase p47(phox) subunit, which is known to be present in dendrites of NTS neurons. It is not known, however, if hypertension can produce changes in the trafficking of p47(phox) in defined NTS subregions, particularly the preferentially barosensitive dorsomedial NTS (dmNTS), or preferentially gastrointestinal medial NTS (mNTS). We used immunogold electron microscopy to determine if p47(phox) localization was differentially affected in dendritic profiles of neurons from these NTS subregions of the rat in response to distinct models of hypertension, namely chronic 7-day subcutaneous administration of angiotensin II (AngII), or phenylephrine. In small (<1 microm) dendritic processes, both AngII and phenylephrine produced a decrease in intracellular p47(phox) labeling selectively in dmNTS neurons. In intermediate-size (1-2 microm) dendritic profiles in the dmNTS region only, there was an increase in p47(phox) labeling in response to each hypertensive agent, although these changes occurred in different subcellular compartments. There was an increase in non-vesicular labeling in response to AngII, but an increase in surface labeling with phenylephrine. Moreover, each of the changes in p47(phox) targeting mentioned above occurred in dendritic profiles with, or without immunoperoxidase labeling for the AngII AT-1A receptor subtype (AT-1A). These results indicate that chronic administration of agents that induce hypertension can also produce changes in the subcellular localization in p47(phox) in dmNTS neurons. Thus, systemic hypertension may produce alterations in the trafficking of proteins associated with superoxide production in central autonomic neurons, thus revealing a potentially important neurogenic component of free radical production and systemic blood pressure elevation.

Junctional Adhesion Molecule-1 Is Upregulated in Spontaneously Hypertensive Rats

Junctional adhesion molecule-1 (JAM-1) forms part of the tight junction between adjacent endothelial cells. Using microarray technology, we showed previously that JAM-1 was differentially expressed in the brain stem of spontaneously hypertensive rats compared with normotensive Wistar-Kyoto (WKY) rats. In this study, we quantified the expression of JAM-1 in the brain stem of spontaneously hypertensive rats and WKY rats and established whether any differential expression was confined to this region of the brain or was ubiquitous throughout the central nervous system and, indeed, the whole body. Because the nucleus tractus solitarii plays a pivotal role in arterial pressure regulation, we assessed whether JAM-1 in this region affects the chronic regulation of arterial pressure. Real time RT-PCR revealed that JAM-1 mRNA was upregulated in multiple regions of the brain and all of the peripheral vascular beds studied. In the nucleus tractus solitarii, the level of JAM-1 mRNA was significantly higher in both young (3-week-old, prehypertensive) and adult male spontaneously hypertensive rats (15 to 18 weeks old) than that of age-matched WKY rats (fold differences; prehypertensives: 1.01+/-0.06 versus 1.59+/-0.13; n=10; P<0.01; adult: 1.08+/-0.14 versus 2.86+/-0.57; n=10; P<0.01). After adenoviral-mediated expression of JAM-1 in the nucleus tractus solitarii of adult WKY rats (15 weeks old; n=6), systolic pressure was increased from 120+/-4 to 132+/-4 mm Hg (P<0.01). Our data suggest that JAM-1 expression in the spontaneously hypertensive rat is upregulated throughout the body compared with the WKY rat and that this is not secondary to the hypertension. When JAM-1 is expressed in the nucleus tractus solitarii, it raises arterial pressure, suggesting a novel prohypertensive role for this protein within the brain stem.

Brain superoxide as a key regulator of the cardiovascular response to emotional stress in rabbits

Cardiovascular reactivity, an abrupt increase in blood pressure and heart rate in response to emotional stress, is a risk factor for hypertension and heart disease. Brain angiotensin II (Ang II) type 1 (AT(1)) receptor is increasingly recognized as an important regulator of cardiovascular reactivity. Given that a wide variety of AT(1) receptor signalling pathways exists in neurones, the precise molecular mechanisms that underlie central cardiovascular actions of Ang II during emotional stress are yet to be determined. Growing evidence, however, indicates that reactive oxygen species, and in particular superoxide (.O(2)(-)), are important intracellular messengers of many actions of brain Ang II. In particular, studies employing microinjection of .O(2)(-) scavengers directly into the rostral ventrolateral medulla (RVLM) and dorsomedial hypothalamus of rabbits have shown that the activation of AT(1) receptor-.O(2)(-) signalling is required for full manifestation of the cardiovascular response to emotional stress. This role of .O(2)(-) appears to be highly specific, because .O(2)(-) scavengers in the RVLM do not alter the sympathoexcitatory response to baroreceptor unloading or sciatic nerve stimulation. The subcellular mechanisms for the stress-induced .O(2)(-) production are likely to include the activation of NADPH oxidase and are essentially independent of nitric oxide. This review summarizes current knowledge of redox-sensitive signalling mechanisms in the brain that regulate cardiovascular effects of stress. Additionally, it presents initial evidence that .O(2)(-) may be less important in the activation of central pressor pathways mediating cardiovascular arousal associated with appetitive events, such as food anticipation and feeding.

Interaction of AT1 receptors and V1a receptors-mediated effects in the central cardiovascular control during the post-infarct state

Experimental objectives. Because myocardial infarct is associated with overactivation of brain angiotensin II (ANG II) and vasopressin (AVP) V1a receptors we decided to determine whether AT1 and V1a receptors-mediated effects of ANG II and AVP interact in central cardiovascular control during the post-infarct state. Four groups of infarcted and four groups of sham-operated conscious rats entered the study. Results. In the infarcted rats cerebroventricular infusion of AT1 (AT1ANT, losartan) and V1a antagonist {V1aANT,d(CH(2))(5)[Tyr(Me)(2)Ala-NH(2)(9)]VP} and combined infusion of both these compounds performed 4 weeks after induction of the infarct significantly and comparably reduced mean arterial blood pressure (MABP) in comparison to control experiments (artificial cerebrospinal fluid infusion). In the sham rats MABP was not affected by any of the infusions. In control experiments MABP and HR responses to an alarming air jet stress were significantly higher in the infarcted than in the sham rats. Both responses were normalized with the same effectiveness by administration of AT1ANT, V1aANT and AT1ANT+V1aANT. In the sham rats administration of these compounds did not affect MABP and HR responses to stress.

CONCLUSION

The results provide evidence for interaction of AT1 and V1a receptors-mediated effects of ANG II and AVP in the central cardiovascular control during the post-infarct state.

Azelnidipine Attenuates Cardiovascular and Sympathetic Responses to Air-Jet Stress in Genetically Hypertensive Rats

Azelnidipine is a new dihydropyridine calcium channel blocker that causes minimal stimulation of the sympathetic nervous system despite its significant depressor effect. In the present study, we examined the effects of oral or intravenous administration of azelnidipine on cardiovascular and renal sympathetic nerve activity (RSNA) responses to air-jet stress in conscious, unrestrained stroke-prone spontaneously hypertensive rats. Oral administration of high-dose azelnidipine (10 mg/kg per day) or nicardipine (150 mg/kg per day) for 10 days caused a significant and comparable decrease in blood pressure, but low-dose azelnidipine (3 mg/kg per day) did not. Air-jet stress increased mean arterial pressure (MAP), heart rate (HR) and RSNA. High-dose azelnidipine significantly attenuated the increases in MAP, HR and RSNA in response to air-jet stress while nicardipine did not. Low-dose azelnidipine significantly attenuated the pressor response with a trend of decrease in RSNA. Intravenous injection of azelnidipine induced a slowly developing depressor effect. To obtain a similar time course of decrease in MAP by azelnidipine, nicardipine was continuously infused at adjusted doses. Both drugs increased HR and RSNA significantly, while the change in RSNA was smaller in the azelnidipine group. In addition, intravenous administration of azelnidipine attenuated the responses of MAP, HR, and RSNA to air-jet stress; by comparison, the inhibitory actions of nicardipine were weak. In conclusion, oral or intravenous administration of azelnidipine inhibited cardiovascular and sympathetic responses to air-jet stress. This action of azelnidipine may be mediated at least in part by the inhibition of the sympathetic nervous system.

Subcellular localization of nicotinamide adenine dinucleotide phosphate oxidase subunits in neurons and astroglia of the rat medial nucleus tractus solitarius: relationship with tyrosine hydroxylase immunoreactive neurons

Superoxide produced by the enzyme nicotinamide adenine dinucleotide phosphate (NADPH) oxidase mediates crucial intracellular signaling cascades in the medial nucleus of the solitary tract (mNTS), a brain region populated by catecholaminergic neurons, as well as astroglia that play an important role in autonomic function. The mechanisms mediating NADPH oxidase (phagocyte oxidase) activity in the neural regulation of cardiovascular processes are incompletely understood, however the subcellular localization of superoxide produced by the enzyme is likely to be an important regulatory factor. We used immunogold electron microscopy to determine the phenotypic and subcellular localization of the NADPH oxidase subunits p47(phox), gp91(phox,) and p22(phox) in the mNTS in rats. The mNTS contains a large population of neurons that synthesize catecholamines. Significantly, catecholaminergic signaling can be modulated by redox reactions. Therefore, the relationship of NADPH oxidase subunit labeled neurons or glia with respect to catecholaminergic neurons was also determined by dual labeling for the superoxide producing enzyme and tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. In the mNTS, NADPH oxidase subunits were present primarily in somatodendritic processes and astrocytes, some of which also contained TH, or were contacted by TH-labeled axons, respectively. Immunogold quantification of NADPH oxidase subunit localization showed that p47(phox) and gp91(phox) were present on the surface membrane, as well as vesicular organelles characteristic of calcium storing smooth endoplasmic reticula in dendritic and astroglial processes. These results indicate that NADPH oxidase assembly and consequent superoxide formation are likely to occur near the plasmalemma, as well as on vesicular organelles associated with intracellular calcium storage within mNTS neurons and glia. Thus, NADPH oxidase-derived superoxide may participate in intracellular signaling pathways linked to calcium regulation in diverse mNTS cell types. Moreover, NADPH oxidase-derived superoxide in neurons and glia may directly or indirectly modulate catecholaminergic neuron activity in the mNTS.

NADPH oxidases of the brain: distribution, regulation, and function

The NADPH oxidase is a multi-subunit enzyme that catalyzes the reduction of molecular oxygen to form superoxide (O(2)(-)). While classically linked to the respiratory burst in neutrophils, recent evidence now shows that O(2)(-) (and associated reactive oxygen species, ROS) generated by NADPH oxidase in nonphagocytic cells serves myriad functions in health and disease. An entire new family of NADPH Oxidase (Nox) homologues has emerged, which vary widely in cell and tissue distribution, as well as in function and regulation. A major concept in redox signaling is that while NADPH oxidase-derived ROS are necessary for normal cellular function, excessive oxidative stress can contribute to pathological disease. This certainly is true in the central nervous system (CNS), where normal NADPH oxidase function appears to be required for processes such as neuronal signaling, memory, and central cardiovascular homeostasis, but overproduction of ROS contributes to neurotoxicity, neurodegeneration, and cardiovascular diseases. Despite implications of NADPH oxidase in normal and pathological CNS processes, still relatively little is known about the mechanisms involved. This paper summarizes the evidence for NADPH oxidase distribution, regulation, and function in the CNS, emphasizing the diversity of Nox isoforms and their new and emerging role in neuro-cardiovascular function. In addition, perspectives for future research and novel therapeutic targets are offered.

Reduction in molecular synthesis or enzyme activity of superoxide dismutases and catalase contributes to oxidative stress and neurogenic hypertension in spontaneously hypertensive rats

A balance between production and elimination of reactive oxygen species such as superoxide anion (O2*-) and hydrogen peroxide (H2O2) tightly regulates the homeostasis of cellular oxidative stress, which contributes to a variety of cardiovascular diseases, including hypertension. The present study assessed the hypothesis that O2*- or H2O2 levels augmented by the reduced molecular synthesis or enzyme activity of superoxide dismutase (SOD), catalase (CAT), or glutathione peroxidase (GPx) in the rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons that generate tonic vasomotor tone are located, contribute to the pathogenesis of hypertension. We found that copper/zinc SOD (SOD1), manganese SOD (SOD2), or CAT, but not GPx, mRNA or protein expression and enzyme activity in the RVLM of spontaneously hypertensive rats (SHR) were significantly lower than those in normotensive Wistar-Kyoto (WKY) rats, along with a significantly higher level of O2*- or H2O2. A causative relationship between these biochemical correlates of oxidative stress and neurogenic hypertension was established when gene transfer by microinjection of adenovirus encoding SOD1, SOD2, or CAT into the bilateral RVLM promoted a long-lasting reduction in arterial pressure in SHR, but not WKY rats, accompanied by an enhanced SOD1, SOD2, or CAT protein expression or enzyme activity and reduced O2*- or H2O2 level in the RVLM. These results together suggest that downregulation of gene expression and enzyme activity of the antioxidant SOD1, SOD2, or CAT may underlie the augmented levels of O2*- and H2O2 in the RVLM, leading to oxidative stress and hypertension in SHR.

Acute pressor effect of central angiotensin II is mediated by NAD(P)H-oxidase-dependent production of superoxide in the hypothalamic cardiovascular regulatory nuclei

BACKGROUND

Centrally applied angiotensin II (Ang II) increases sympathetic nervous activity and mean arterial blood pressure (MAP), but the mediation of these effects is not fully understood.

OBJECTIVE

To test the hypothesis that central effects of Ang II are mediated by reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H]-oxidase-dependent production of superoxide in the hypothalamus.

METHODS

Under isoflurane anesthesia, male Sprague-Dawley rats were given an intracerebroventricular infusion of either artificial cerebrospinal fluid or apocynin (4 microg/kg per min), a selective inhibitor for NAD(P)H oxidase, for 30 min, followed by Ang II (20 ng) or carbachol (200 ng), while MAP and heart rate were measured at the femoral artery. At the end of the experiments, hydroethidine, a superoxide-sensitive fluorescent dye, was infused intravenously for 10 min, and superoxide production was assessed in the vasoregulatory hypothalamic nuclei using confocal microscopy.

RESULTS

Ang II elicited a rapid 11 +/- 2-mmHg increase in MAP and a 16 +/- 2-beats/min decrease in heart rate. Apocynin abolished these effects of Ang II in a specific manner, as carbachol-induced increases in MAP were unaffected by the inhibition of NAD(P)H oxidase (MAP increased by 9 +/- 2 and 8 +/- 1 mmHg in the absence and presence of apocynin, respectively). In response to Ang II, apocynin-sensitive production of superoxide increased significantly in the nuclei of the anterior hypothalamus, in the subfornical organ, and in the paraventricular nucleus of the hypothalamus.

CONCLUSION

These findings demonstrate that acute pressor responses of central Ang II are mediated by NAD(P)H-oxidase-dependent production of superoxide in the hypothalamus.

Angiotensin II in dorsomedial hypothalamus modulates cardiovascular arousal caused by stress but not feeding in rabbits

The dorsomedial hypothalamus (DMH) is critically implicated in the cardiovascular response to emotional stress. This study aimed to determine whether the DMH is also important in cardiovascular arousal associated with appetitive feeding behavior and, if so, whether locally released angiotensin II and glutamate are important in this arousal. Emotional (air-jet) stress and feeding elicited similar tachycardic (+51 and +45 beats/min, respectively) and pressor (+16 and +9 mmHg, respectively) responses in conscious rabbits. Bilateral microinjection of GABAA agonist muscimol (500 pmol) into the DMH, but not nearby hypothalamic regions, attenuated pressor and tachycardic responses to air-jet by 56–63% and evoked anorexia. Conversely, stimulation of the DMH with the glutamate analog kainic acid (250 pmol) elicited hypertension (+25 mmHg) and tachycardia (+114 beats/min) and activated feeding behavior. Local microinjection of a glutamate receptor antagonist, kynurenic acid (10 nmol), decreased pressor responses to stress and eating by 46 and 72%, respectively, without affecting feeding behavior. Bilateral microinjection of a selective AT1-receptor antagonist, candesartan (500 pmol), into the DMH, but not nearby sites, attenuated pressor and tachycardic stress responses by 31 and 33%, respectively. Candesartan did not alter feeding behavior or circulatory response to feeding. These results indicate that, in addition to its role in mediating stress responses, the DMH may be important in regulating cardiovascular arousal associated with feeding. Local glutamatergic inputs appear to regulate cardiovascular response to both stress and feeding. Conversely, angiotensin II, acting via AT1 receptors, may selectively modulate, in the DMH, cardiovascular response to stress, but not feeding.

Central autonomic integration of psychological stressors: Focus on cardiovascular modulation

During stress the sympathoadrenal system and the hypothalamo-pituitary-adrenal axis act in a coordinated manner to force changes within an animal's current physiological and behavioral state. Such changes have been described as 'fight flight' or stress responses. The central nervous system may generate a stress response by different neural circuits, this being dependent upon the type of stressor presented. For instance, the central control of the autonomic function during physical stress would seem to be based on existing homeostatic mechanisms. In contrast, with exposure to psychological stress the means by which autonomic outflow is regulated has not been fully established. This review discusses recent observations of autonomic flow, cardiovascular components in particular, during psychological stress and the possible implications these may have for our understanding of the central nervous system. In addition, an update of recent findings concerning several regions thought to be important to the regulation of autonomic function during psychological stress exposure is provided.

NADPH oxidase-derived superoxide anion mediates angiotensin II-induced pressor effect via activation of p38 mitogen-activated protein kinase in the rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons are located, is a central site via which angiotensin II (Ang II) elicits its pressor effect. We tested the hypothesis that NADPH oxidase-derived superoxide anion (O2*-) in the RVLM mediates Ang II-induced pressor response via activation of mitogen-activated protein kinase (MAPK) signaling pathways. Bilateral microinjection of Ang II into the RVLM resulted in an angiotensin subtype 1 (AT1) receptor-dependent phosphorylation of p38 MAPK and extracellular signal-regulated protein kinase (ERK)1/2, but not stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK), in the ventrolateral medulla. The Ang II-induced p38 MAPK or ERK1/2 phosphorylation was attenuated by application into the RVLM of a NADPH oxidase inhibitor, diphenyleneiodonium chloride (DPI), an antisense oligonucleotide that targets against p22phox or p47phox subunit of NADPH oxidase mRNA, or the superoxide dismutase mimetic tempol. DPI or antisense p22phox or p47phox oligonucleotide treatment also attenuated the AT1 receptor-dependent increase in O2*- production in the ventrolateral medulla elicited by Ang II at the RVLM. Functionally, Ang II-elicited pressor response in the RVLM was attenuated by DPI, tempol, or a p38 MAPK inhibitor, SB203580. The AT1 receptor-mediated enhancement of the frequency of glutamate-sensitive spontaneous excitatory postsynaptic currents induced by Ang II in RVLM neurons was also abolished by SB203580. These results suggest that NADPH oxidase-derived O2*- underlies the activation of p38 MAPK or ERK1/2 by Ang II in the ventrolateral medulla. Furthermore, the p38 MAPK signaling pathway may mediate Ang II-induced pressor response via enhancement of presynaptic release of glutamate to RVLM neurons.

Selective Sensitization by Nitric Oxide of Sympathetic Baroreflex in Rostral Ventrolateral Medulla of Conscious Rabbits

Nitric oxide (NO) deficiency in the rostral ventrolateral medulla (RVLM) has been implicated in impaired baroreflex control in hypertensive and heart failure animals. However, the role of local NO in normal baroreflex regulation remains unclear. This study aimed to examine the role of NO in tonic and baroreflex control of blood pressure (BP) in the RVLM of conscious rabbits. Microinjections of NO donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside (5 to 20 nmol), or NO itself (20 to 200 pmol) into the RVLM dose-dependently increased BP. Bilateral microinjections of an NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 nmol), its inactive enantiomer D-NAME, or soluble guanylate cyclase (sGC) inhibitors, 1-H-[1,2,4]oxadiaolo[4,3-a]quinoxalin-1-one (ODQ, 250 pmol) and methylene blue (10 nmol), into the RVLM did not affect resting BP, heart rate, or renal sympathetic nerve activity (RSNA). However, L-NAME, methylene blue, and ODQ decreased RSNA baroreflex gain by 42% to 55%, whereas D-NAME did not affect this reflex. Co-microinjections of L-NAME and superoxide scavenger tempol (20 nmol) decreased RSNA baroreflex gain by 37+/-8%. Microinjections of a neuronal NOS (nNOS) inhibitor, 7-nitroindazole (500 pmol), into the RVLM decreased RSNA baroreflex gain by 42+/-12%, without altering resting BP, heart rate, or RSNA. Local administration of inducible NOS (iNOS) inhibitors, S-methylisothiourea (0.25 nmol) and aminoguanidine (0.25 and 2.5 nmol), affected neither resting nor baroreflex parameters. These results suggest that nNOS-derived NO facilitates sympathetic baroreflex transmission in the RVLM at least in part via a sGC-dependent, superoxide-independent mechanism. However, local nNOS and iNOS play little role in the tonic support of BP in conscious rabbits.

NAD(P)H oxidase inhibition attenuates neuronal chronotropic actions of angiotensin II

It is well established that the central cardiovascular effects of angiotensin II (Ang II) involve superoxide production. However, the intracellular mechanism by which reactive oxygen species (ROS) signaling regulates neuronal Ang II actions remains to be elucidated. In the present study, we have used neuronal cells in primary cultures from the hypothalamus and brain stem areas to study the role of ROS on the cellular actions of Ang II. Ang II increases neuronal firing rate, an effect mediated by the AT(1) receptor subtype and involving inhibition of the delayed rectifier potassium current (I(Kv)). This increase in neuronal activity was associated with increases in NADPH oxidase activity and ROS levels within neurons, the latter evidenced by an increase in ethidium fluorescence. The increases in NADPH oxidase activity and ethidium fluorescence were blocked by either the AT(1) receptor antagonist losartan or by the selective NAD(P)H oxidase inhibitor gp91ds-tat. Extracellular application of the ROS scavenger, Tempol, attenuated the Ang II-induced increase in neuronal firing rate by 70%. In addition, gp91ds-tat treatment resulted in a 50% inhibition of Ang II-induced increase in firing rate. In contrast, the ROS generator Xanthine-Xanthine oxidase significantly increased neuronal firing rate. Finally, Ang II inhibited neuronal I(Kv,) and this inhibition was abolished by gp91ds-tat treatment. These observations demonstrate, for the first time, that Ang II regulates neuronal activity via a series of events that includes ROS generation and inhibition of I(Kv). This signaling seems to be a critical cellular event in central Ang II regulation of cardiovascular function.

Sympathoexcitation by central ANG II: roles for AT1 receptor upregulation and NAD(P)H oxidase in RVLM

Chronic heart failure is often associated with sympathoexcitation and blunted arterial baroreflex function. These phenomena have been causally linked to elevated central ANG II mechanisms. Recent studies have shown that NAD(P)H oxidase-derived reactive oxygen species (ROS) are important mediators of ANG II signaling and therefore might play an essential role in these interactions. The aims of this study were to determine whether central subchronic infusion of ANG II in normal animals has effects on O2- production and expression of NAD(P)H oxidase subunits as well as ANG II type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM). Twenty-four male New Zealand White rabbits were divided into four groups and separately received a subchronic intracerebroventricular infusion of saline alone, ANG II alone, ANG II with losartan, and losartan alone for 1 wk. On day 7 of intracerebroventricular infusion, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) values were recorded, and arterial baroreflex sensitivity was evaluated while animals were in the conscious state. We found that ANG II significantly increased baseline RSNA (161.9%; P < 0.05), mRNA and protein expression of AT1 receptors (mRNA, 66.7%; P < 0.05; protein, 85.1%; P < 0.05), NAD(P)H oxidase subunits (mRNA, 120.0-200.0%; P < 0.05; protein, 90.9-197.0%; P < 0.05), and O2- production (83.2%; P < 0.05) in the RVLM. In addition, impaired baroreflex control of HR (Gain(max) reduced by 48.2%; P < 0.05) and RSNA (Gain(max) reduced by 53.6%; P < 0.05) by ANG II was completely abolished by losartan. Losartan significantly decreased baseline RSNA (-49.5%; P < 0.05) and increased baroreflex control of HR (Gain(max) increased by 64.8%; P < 0.05) and RSNA (Gain(max) increased by 67.9%; P < 0.05), but had no significant effects on mRNA and protein expression of AT1 receptor and NAD(P)H oxidase subunits and O2- production in the RVLM. These data suggest that in normal rabbits, NAD(P)H oxidase-derived ROS play an important role in the modulation of sympathetic activity and arterial baroreflex function by subchronic central treatment of exogenous ANG II via AT1 receptors.