Participation of AT1 and AT2 receptor subtypes in the tonic inhibitory modulation of baroreceptor reflex response by endogenous angiotensins at the nucleus tractus solitarii in the rat

Sleep deprivation reduces the baroreflex sensitivity through elevated angiotensin (Ang) II subtype 1 receptor expression in the nucleus tractus solitarii

Introduction

Sleep insufficiency has been linked to an increased risk of high blood pressure and cardiovascular diseases. Emerging studies have demonstrated that impaired baroreflex sensitivity (BRS) is involved in the adverse cardiovascular effects caused by sleep deprivation, however, the underlying mechanisms remain unknown. Therefore, the present study aims to clarify the role of abnormal renin-angiotensin system in the nucleus tractus solitarii (NTS) in impaired BRS induced by sleep deprivation.

Methods

Rats were randomly divided into two groups: normal sleep (Ctrl) and chronic sleep deprivation (CSD) group. Rats were sleep deprived by an automated sleep deprivation system. The blood pressure, heart rate, BRS, the number of c-Fos positive cells and the expression of angiotensin (Ang) II subtype 1 receptors (AT1R) in the NTS of rats were assessed.

Results

Compared to Ctrl group, CSD group exhibited a higher blood pressure, heart rate, and reduced BRS. Moreover, the number of c-Fos positive cells and local field potential in the NTS in CSD group were increased compared with the Ctrl group. It was shown that the expression of the AT1R and the content of Ang II and the ratio of Ang II to Ang-(1-7) were increased in the NTS of rats in CSD group compared to Ctrl group. In addition, microinjection of losartan into the NTS significantly improved the impaired BRS caused by sleep deprivation.

Discussion

In conclusion, these data suggest that the elevated AT1R expression in the NTS mediates the reduced BRS induced by chronic sleep deprivation.

Investigation of the Role of AT2 Receptors in the Nucleus Tractus Solitarii of Normotensive Rats in Blood Pressure Control

Aim

The nucleus tractus solitarii (NTS) densely expresses angiotensin II type 2 receptors (AT2R), which are mainly located on inhibitory gamma-aminobutyric acid (GABA) neurons. Central AT2R stimulation reduces blood pressure, and AT2R stimulation in the rostral ventrolateral medulla (RVLM), mediates a hypotensive response through a GABAergic mechanism. We aimed to test the hypothesis that an AT2R mediated inhibition of the GABA release within the NTS might be involved in this hypotensive response, by assessing possible alterations in blood pressure and heart rate, as well as in GABA levels in normotensive Wistar rats.

Methods

In vivo microdialysis was used for measurement of extracellular GABA levels and for perfusion of the selective AT2R agonist, Compound 21, within the NTS. Our set-up allowed to determine simultaneously the excitatory glutamate dialysate levels. The mean arterial pressure and heart rate responses were monitored with a pressure transducer.

Results

Local perfusion of Compound 21 into the NTS did not modify blood pressure and heart rate, nor glutamate and GABA levels compared to baseline concentrations. A putative effect was also not unmasked by concomitant angiotensin II type 1 receptor blockade with candesartan. Positive control experiments confirmed that the experimental set up had enough sensitivity to detect a reduction in GABA dialysate levels and blood pressure.

Conclusion

The results did not provide evidence for a role of the AT2R within the NTS in the control of blood pressure, nor for an interaction with local GABAergic signaling in normotensive rats.

Exploring the complexity: the interplay between the angiotensin-converting enzyme insertion/deletion polymorphism and the sympathetic response to hemodialysis

Patients on hemodialysis (HD) are at increased risk for arrhythmias and sudden cardiac death. Autonomic nervous system (ANS) dysfunction seems to participate in the arrhythmogenic process. Genetic factors have an impact on ANS modulation, but the specific role of the insertion/deletion (I/D) polymorphism in the gene for angiotensin-converting enzyme (ACE) has not been investigated. Since the D allele increases gene expression, it is a candidate polymorphism to interact with the ANS. The aim of the present study was to compare the behavior of heart rate variability (HRV) during HD, as a surrogate for ANS response to stressors, between the ACE genotypes. In a sample of patients with chronic kidney disease I/D ACE genotypes were assessed with PCR and HRV was measured before, in the second hour, and after a HD session. HRV parameters in the time and frequency domains were analyzed by repeated-measures mixed models according to the time of measurement and ACE polymorphism. HRV parameters in the frequency domain presented significantly different variations during the HD session between patients with or without the D allele. Only patients with the II genotype presented an increase in low-frequency normalized units and in the low frequency-to-high frequency ratio throughout HD. Patients with the II genotype seemed to have a more physiological response to the volemic and electrolytic changes that occur during HD, with greater sympathetic activation than patients with ID and DD genotypes. NEW & NOTEWORTHY Adding to the effort to understand the complexity of cardiovascular system regulation, we have found that the autonomic nervous system response to the acute volume removal during hemodialysis may be different between angiotensin-converting enzyme insertion/deletion polymorphisms. To our knowledge, this is the first time that this specific interaction was analyzed during a volume removal intervention.

The role of brain angiotensin II (type 2) receptors and nitric oxide in the renal sympathoinhibitory response to acute volume expansion in conscious rats

OBJECTIVE

The study was performed to investigate the role of angiotensin II type 2 (AT2) receptors and nitric oxide in the renal sympathoinhibitory response to volume expansion (VEP).

METHOD

Conscious rats were subjected to volume expansion (VEP) [0.25% body weight/min saline for 10 min intravenously (i.v.)] following intracerebroventricular (i.c.v.) infusion of either saline or angiotensin II (Ang II), or a combination of Ang II with either losartan, PD123319, or N-nitro-L-arginine methyl ester (L-NAME).

RESULTS

Intracerebroventricular losartan, PD123319, or L-NAME did not change baseline mean arterial pressure, heart rate, or renal sympathetic nerve activity (RSNA). However, i.c.v. Ang II increased mean arterial pressure and decreased heart rate and RSNA baselines (113 ± 2 vs. 107 ± 2 mmHg, 365 ± 7 vs. 379 ± 5 beats/min, 1.03 ± 0.13 vs. 1.29 ± 0.15 μV.s, respectively, all P < 0.05). During i.c.v. saline infusion, VEP decreased RSNA by 27 ± 2% (P < 0.05) after 10 min and the magnitude of this response was unchanged during i.c.v. infusion of Ang II, losartan, or PD123319 but was decreased by L-NAME compared with that obtained with i.c.v. saline (14 ± 3 vs. 30 ± 5%, P < 0.05). i.c.v. Ang II in combination with losartan enhanced (41 ± 3 vs. 29 ± 5%) but with PD123319 decreased (15 ± 2 vs. 28 ± 4%, P < 0.05) the renal sympathoinhibition compared with Ang II alone. The renal sympathoinhibitory response was enhanced (43 ± 5 vs. 29 ± 1%, P < 0.05) by i.c.v. infusion of an AT2 agonist, CGP42112 the magnitude of which was unchanged when combined with L-NAME. The sympathoinhibitory response to VEP following Ang II plus L-NAME was similar to Ang II alone.

CONCLUSION

These findings suggest that activation of central AT2 receptors enhances the renal sympathoinhibitory response to VEP but this effect is not dependent on nitric oxide.

Angiotensin III increases monocyte chemoattractant protein-1 expression in cultured human proximal tubular epithelial cells

BACKGROUND/AIMS

We investigated whether angiotensin III (Ang III) is involved in monocyte recruitment through regulation of the chemokine monocyte chemoattractant protein-1 (MCP-1) in cultured human proximal tubular epithelial cells (HK-2 cells).

METHODS

We measured MCP-1 levels in HK-2 cells that had been treated with various concentrations of Ang III and Ang II type-1 (AT1) receptor antagonists at various time points. The phosphorylation states of p38, c-Jun N-terminal kinases (JNK), and extracellular-signal-regulated kinases were measured in Ang III-treated cells to explore the mitogen-activated protein kinase (MAPK) pathway. MCP-1 levels in HK-2 cell-conditioned media were measured after pre-treatment with the transcription factor inhibitors curcumin or pyrrolidine dithiocarbamate.

RESULTS

Ang III increased MCP-1 protein production in dose- and time-dependent manners in HK-2 cells, which was inhibited by the AT1 receptor blocker losartan. p38 MAPK activity increased significantly in HK-2 cells exposed to Ang III for 30 minutes, and was sustained at higher levels after 60 minutes (p < 0.05). Total phosphorylated JNK protein levels tended to increase 20 minutes after stimulation with Ang III. Pre-treatment with a p38 inhibitor, a JNK inhibitor, or curcumin significantly inhibited Ang III-induced MCP-1 production.

CONCLUSIONS

Ang III increases MCP-1 synthesis via stimulation of intracellular p38 and JNK MAPK signaling activity and subsequent activated protein-1 transcriptional activity in HK-2 cells.

The cough reflex is upregulated by lisinopril microinjected into the caudal nucleus tractus solitarii of the rabbit

We have previously shown that cough potentiation induced by intravenous administration of the AT1 receptor antagonist losartan is lower than that induced by the ACE inhibitor lisinopril in anesthetized and awake rabbits. Since losartan and lisinopril cross the blood-brain barrier, their central action on the cough reflex can be hypothesized. Mechanical stimulation of the tracheobronchial tree and citric acid inhalation were used to induce cough reflex responses in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections (30-50 nl) of losartan (5mM), lisinopril (1mM), bradykinin (0.05 mM), HOE-140 (0.2mM, a bradykinin B2 receptor antagonist) and CP-99,994 (1mM, an NK1 receptor antagonist) were performed into the caudal nucleus tractus solitarii, the predominant site of termination of cough-related afferents. Lisinopril, but not losartan increased the cough number. This effect was reverted by HOE-140 or CP-99,994. Cough potentiation was also induced by bradykinin. The results support for the first time a central protussive action of lisinopril mediated by an accumulation of bradykinin and substance P.

Intranasal Angiotensin II in Humans Reduces Blood Pressure When Angiotensin II Type 1 Receptors Are Blocked

Intranasal administration of angiotensin II (ANGII) affects blood pressure in a mode different from intravenously administered ANGII via a direct access to the brain bypassing the blood–brain barrier. This clinical study investigated blood pressure regulation after intranasal ANGII administration in healthy humans, whereas systemic, blood-mediated effects of ANGII were specifically blocked. In a balanced crossover design, men (n=8) and women (n=8) were intranasally administered ANGII (400 μg) or placebo after ANGII type 1 receptors had been blocked by pretreatment with valsartan (80 mg; 12 and 6 hours before intranasal administration). Plasma levels of ANGII, aldosterone, renin, vasopressin, and norepinephrine were measured; blood pressure and heart rate were recorded continuously. Intranasal ANGII acutely decreased blood pressure without altering the heart rate. Plasma levels of vasopressin and norepinephrine remained unaffected. Plasma ANGII levels were increased throughout the recording period. Aldosterone levels increased despite the peripheral ANGII type 1 receptor blockade, indicating an aldosterone escape phenomenon. In conclusion, intranasal ANGII reduces blood pressure in the presence of selective ANGII type 1 receptor blockade. Intranasal ANGII administration represents a useful approach for unraveling the role of this peptide in blood pressure regulation in humans.

Angiotensin II-induced activation of central AT1 receptors exerts endocannabinoid-mediated gastroprotective effect in rats

The aim of the present study was to analyze whether angiotensin II via the endocannabinoid system can induce gastric mucosal protection, since transactivation of cannabinoid CB1 receptors by angiotensin AT1 receptor in CHO cells was described. Experimental ulcer was induced by acidified ethanol given orally in male Wistar rats, CB1(+/+) wild type and CB1(-/-) knockout mice. The compounds were administered intracerebroventricularly. It was found, that 1. Angiotensin II inhibited the ethanol-induced gastric lesions (11.9-191pmol); the effect of angiotensin II (191pmol) was inhibited by the CB1 receptor inverse agonist AM 251 (1.8nmol) and the inhibitor of diacylglycerol lipase (DAGL), tetrahydrolipstatin (0.2nmol). 2. Angiotensin II exerted gastroprotection in wild type, but not in CB1(-/-) mice. 3. The gastroprotective effect of angiotensin II (191pmol) was reduced by atropine (1mg/kg i.v.) and bilateral cervical vagotomy. In conclusion, stimulation of central angiotensin AT1 receptors via activation of cannabinoid CB1 receptors induces gastroprotection in a DAGL-dependent and vagus-mediated mechanism.

Brain stem oxidative stress and its associated signaling in the regulation of sympathetic vasomotor tone

There is now compelling evidence from studies in humans and animals that overexcitation of the sympathetic nervous system plays an important role in the pathogenesis of cardiovascular diseases. An excellent example is neurogenic hypertension, in which central sympathetic overactivation is involved in the development, staging, and progression of the disease, and one of the underlying mechanisms involves oxidative stress in key brain stem sites that are engaged in the regulation of sympathetic vasomotor tone. Using the rostral ventrolateral medulla (RVLM) and nucleus tractus solitarii (NTS) as two illustrative brain stem neural substrates, this article provides an overview of the impact of reactive oxygen species and antioxidants on RVLM and NTS in the pathogenesis of neurogenic hypertension. This is followed by a discussion of the redox-sensitive signaling pathways, including several kinases, ion channels, and transcription factors that underpin the augmentation in sympathetic vasomotor tone. In addition, the emerging view that brain stem oxidative stress is also causally related to a reduction in sympathetic vasomotor tone and hypotension during brain stem death, methamphetamine intoxication, and temporal lobe status epilepticus will be presented, along with the causal contribution of the oxidant peroxynitrite formed by a reaction between nitric oxide synthase II (NOS II)-derived nitric oxide and superoxide. Also discussed as a reasonable future research direction is dissection of the cellular mechanisms and signaling cascades that may underlie the contributory role of nitric oxide generated by different NOS isoforms in the differential effects of oxidative stress in the RVLM or NTS on sympathetic vasomotor tone.

Angiotensin II type 2 receptor-coupled nitric oxide production modulates free radical availability and voltage-gated Ca2+ currents in NTS neurons

The medial region of the nucleus tractus solitarius (mNTS) is a key brain stem site controlling cardiovascular function, wherein ANG II modulates neuronal L-type Ca(2+) currents via activation of ANG II type 1 receptors (AT(1)R) and production of reactive oxygen species (ROS). ANG II type 2 receptors (AT(2)R) induce production of nitric oxide (NO), which may interact with ROS and modulate AT(1)R signaling. We sought to determine whether AT(2)R-mediated NO production occurs in mNTS neurons and, if so, to elucidate the NO source and the functional interaction with AT(1)R-induced ROS or Ca(2+) influx. Electron microscopic (EM) immunolabeling showed that AT(2)R and neuronal NO synthase (nNOS) are coexpressed in neuronal somata and dendrites receiving synapses in the mNTS. In the presence of the AT(1)R antagonist losartan, ANG II increased NO production in isolated mNTS neurons, an effect blocked by the AT(2)R antagonist PD123319, but not the angiotensin (1-7) antagonist D-Ala. Studies in mNTS neurons of nNOS-null or endothelial NOS (eNOS)-null mice established nNOS as the source of NO. ANG II-induced ROS production was enhanced by PD123319, the NOS inhibitor N(G)-nitro-l-arginine (LNNA), or in nNOS-null mice. Moreover, in the presence of losartan, ANG II reduced voltage-gated L-type Ca(2+) current, an effect blocked by PD123319 or LNNA. We conclude that AT(2)R are closely associated and functionally coupled with nNOS in mNTS neurons. The resulting NO production antagonizes AT(1)R-mediated ROS and dampens L-type Ca(2+) currents. The ensuing signaling changes in the NTS may counteract the deleterious effects of AT(1)R on cardiovascular function.

Robust dynamic balance of AP-1 transcription factors in a neuronal gene regulatory network

BACKGROUND

The octapeptide Angiotensin II is a key hormone that acts via its receptor AT1R in the brainstem to modulate the blood pressure control circuits and thus plays a central role in the cardiac and respiratory homeostasis. This modulation occurs via activation of a complex network of signaling proteins and transcription factors, leading to changes in levels of key genes and proteins. AT1R initiated activity in the nucleus tractus solitarius (NTS), which regulates blood pressure, has been the subject of extensive molecular analysis. But the adaptive network interactions in the NTS response to AT1R, plausibly related to the development of hypertension, are not understood.

RESULTS

We developed and analyzed a mathematical model of AT1R-activated signaling kinases and a downstream gene regulatory network, with structural basis in our transcriptomic data analysis and literature. To our knowledge, our report presents the first computational model of this key regulatory network. Our simulations and analysis reveal a dynamic balance among distinct dimers of the AP-1 family of transcription factors. We investigated the robustness of this behavior to simultaneous perturbations in the network parameters using a novel multivariate approach that integrates global sensitivity analysis with decision-tree methods. Our analysis implicates a subset of Fos and Jun dependent mechanisms, with dynamic sensitivities shifting from Fos-regulating kinase (FRK)-mediated processes to those downstream of c-Jun N-terminal kinase (JNK). Decision-tree analysis indicated that while there may be a large combinatorial functional space feasible for neuronal states and parameters, the network behavior is constrained to a small set of AP-1 response profiles. Many of the paths through the combinatorial parameter space lead to a dynamic balance of AP-1 dimer forms, yielding a robust AP-1 response counteracting the biological variability.

CONCLUSIONS

Based on the simulation and analysis results, we demonstrate that a dynamic balance among distinct dimers of the AP-1 family of transcription factors underlies the robust activation of neuronal gene expression in the NTS response to AT1R activation. Such a differential sensitivity to limited set of mechanisms is likely to underlie the stable homeostatic physiological response.

Neurochemical modulation of central cardiovascular control: the integrative role of galanin

Galanin (GAL) is a peptide involved in multiple functions, including central cardiovascular control. In this review, the role of GAL and its fragments in the modulation of cardiovascular neuronal networks in the nucleus of the solitary tract is presented, including its interaction with the classical neurotransmitters and other neuropeptides involved in cardiovascular responses in this nucleus. First, we describe the cardiovascular responses of GAL and the pathway involved in these responses. Then we summarize findings obtained in our laboratory on how GAL, through its receptors, interacts with two other neuropeptides--Neuropeptide Y and Angiotensin II and their receptors--as they have particularly conspicuous cardiovascular effects. All these results strengthen the role of GAL in central cardiovascular control and indicate the existence of interactions among GAL receptor subtypes and alpha2-adrenergic receptors, AT1, and Y1 receptor subtypes. These interactions are crucial for understanding the integrative mechanisms responsible for the organization of the cardiovascular responses from the NTS.

Angiotensin II type 1 receptor-mediated reduction of angiotensin-converting enzyme 2 activity in the brain impairs baroreflex function in hypertensive mice

Angiotensin-converting enzyme 2 (ACE2), a new component of the brain renin-angiotensin system, has been suggested to participate in the central regulation of blood pressure (BP). To clarify the relationship between ACE2 and other brain renin-angiotensin system components, we hypothesized that central angiotensin II type 1 receptors reduce ACE2 expression/activity in hypertensive mice, thereby impairing baroreflex function and promoting hypertension. To test this hypothesis, chronically hypertensive mice (RA) with elevated angiotensin II levels were treated with losartan (angiotensin II type 1 receptor blocker) or PD123319 (angiotensin II type 2 antagonist; 10 mg/kg per day, SC) for 2 weeks. Baseline spontaneous baroreflex sensitivity and brain ACE2 activity were dramatically decreased in RA compared with nontransgenic mice, whereas peripheral ACE2 activity/expression remained unaffected. Losartan, but not PD123319, increased central ACE2 activity, spontaneous baroreflex sensitivity, and normalized BP in RA mice. To confirm the critical role of central ACE2 in BP regulation, we generated a triple-transgenic model with brain ACE2 overexpression on a hypertensive RA background. Triple-transgenic-model mice exhibit lower BP and blunted water intake versus RA, suggesting lower brain angiotensin II levels. Moreover, the impaired spontaneous baroreflex sensitivity, parasympathetic tone, and increased sympathetic drive, observed in RA, were normalized in triple-transgenic-model mice. These data suggest that angiotensin II type 1 receptors inhibit ACE2 activity in RA mice brain, thus contributing to the maintenance of hypertension. In addition, overexpression of ACE2 in the brain reduces hypertension by improving arterial baroreflex and autonomic function. Together, our data suggest that angiotensin II type 1 receptor-mediated ACE2 inhibition impairs baroreflex function and support a critical role for ACE2 in the central regulation of BP and the development of hypertension.

Interaction between cardiac sympathetic afferent reflex and chemoreflex is mediated by the NTS AT1 receptors in heart failure

Several sympathoexcitatory reflexes, such as the cardiac sympathetic afferent reflex (CSAR) and arterial chemoreflex, are significantly augmented and contribute to elevated sympathetic outflow in chronic heart failure (CHF). This study was undertaken to investigate the interaction between the CSAR and the chemoreflex in CHF and to further identify the involvement of angiotensin II type 1 receptors (AT1Rs) in the nucleus of the tractus solitarius (NTS) in this interaction. CHF was induced in rats by coronary ligation. Acute experiments were performed in anesthetized rats. The chemoreflex-induced increase in cardiovascular responses was significantly greater in CHF than in sham-operated rats after either chemical or electrical activation of the CSAR. The inhibition of the CSAR by epicardial lidocaine reduced the chemoreflex-induced effects in CHF rats but not in sham-operated rats. Bilateral NTS injection of the AT1R antagonist losartan (10 and 100 pmol) dose-dependently decreased basal sympathetic nerve activity in CHF but not in sham-operated rats. This procedure also abolished the CSAR-induced enhancement of the chemoreflex. The discharge and chemosensitivity of NTS chemosensitive neurons were significantly increased following the stimulation of the CSAR in sham-operated and CHF rats, whereas CSAR inhibition by epicardial lidocaine significantly attenuated chemosensitivity of NTS neurons in CHF but not in sham-operated rats. Finally, the protein expression of AT1R in the NTS was significantly higher in CHF than in sham-operated rats. These results demonstrate that the enhanced cardiac sympathetic afferent input contributes to an excitatory effect of chemoreflex function in CHF, which is mediated by an NTS-AT1R-dependent mechanism.

Differential baroreflex control of sympathetic drive by angiotensin II in the nucleus tractus solitarii

Microinjection of angiotensin II into the nucleus tractus solitarii attenuates the baroreceptor reflex-mediated bradycardia by inhibiting both vagal and cardiac sympathetic components. However, it is not known whether the baroreflex modulation of other sympathetic outputs (i.e., noncardiac) also are inhibited by exogenous angiotensin II (ANG II) in nucleus tractus solitarii (NTS). In this study, we determined whether there was a difference in the baroreflex sensitivity of sympathetic outflows at the thoracic and lumbar levels of the sympathetic chain following exogenous delivery of ANG II into the NTS. Experiments were performed in two types of in situ arterially perfused decerebrate rat preparations. Sympathetic nerve activity was recorded from the inferior cardiac nerve, the midthoracic sympathetic chain, or the lower thoracic-lumbar sympathetic chain. Increases in perfusion pressure produced a reflex bradycardia and sympathoinhibition. Microinjection of ANG II (500 fmol) into the NTS attenuated the reflex bradycardia (57% attenuation, P < 0.01) and sympathoinhibition of both the inferior cardiac nerve (26% attenuation, P < 0.05) and midthoracic sympathetic chain (37% attenuation, P < 0.05) but not the lower thoracic-lumbar chain (P = 0.56). We conclude that ANG II in the nucleus tractus solitarii selectively inhibits baroreflex responses in specific sympathetic outflows, possibly dependent on the target organ innervated.

Evidence for a role of AT(2) receptors at the CVLM in the cardiovascular changes induced by low-intensity physical activity in renovascular hypertensive rats

In the present study, we evaluated the involvement of the rennin-angiotensin system (RAS) in the control of the blood pressure (BP), baroreceptor-mediated bradycardia and the reactivity of caudal ventrolateral medulla (CVLM) neurons to Ang II and to AT(2) receptor antagonist in sedentary or trained renovascular hypertensive rats. Physical activity did not significantly change the baseline mean arterial pressure (MAP), heart rate (HR) or the sensitivity of the baroreflex bradycardia in normotensive Sham rats. However, in 2K1C hypertensive rats, physical activity induced a significant fall in baseline MAP and HR and produced an improvement of the baroreflex function (bradycardic component). The microinjections of Ang II into the CVLM produced similar decreases in MAP in all groups, Sham and 2K1C, sedentary and trained rats. The hypotensive effect of Ang II at the CVLM was blocked by previous microinjection of the AT(2) receptors antagonist, PD123319, in all groups of rats. Unexpectedly, microinjection of PD123319 at the CVLM produced a depressor effect in 2K1C sedentary that was attenuated in 2K1C trained rats. No significant changes in MAP were observed after PD123319 in Sham rats, sedentary or trained. These data showed that low-intensity physical activity is effective in lowering blood pressure and restoring the sensitivity of the baroreflex bradycardia, however these cardiovascular effects are not accompanied by changes in the responsiveness to Ang II at CVLM in normotensive or hypertensive, 2K1C rats. In addition, the blood pressure changes observed after AT(2) blockade in 2K1C rats suggest that hypertension may trigger an imbalance of AT(1)/AT(2) receptors at the CVLM that may be restored, at least in part, by low-intensity physical activity.

Cardiac sympathetic afferent stimulation augments the arterial chemoreceptor reflex in anesthetized rats

Chronic heart failure (CHF) is well known to be associated with both an enhanced chemoreceptor reflex and an augmented cardiac "sympathetic afferent reflex" (CSAR). The augmentation of the CSAR may play an important role in the enhanced chemoreceptor reflex in the CHF state because the same central areas are involved in the sympathetic outputs of both reflexes. We determined whether chemical and electrical stimulation of the CSAR augments chemoreceptor reflex function in normal rats. Under anesthesia, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded. The chemoreceptor reflex was tested by unilateral intra-carotid artery bolus injection of potassium cyanide (KCN) and nicotine. We found that 1) left ventricular epicardial application of capsaicin increased the pressor responses and the RSNA responses to chemoreflex activation induced by both KCN and nicotine; 2) when the central end of the left cardiac sympathetic nerve was electrically stimulated, both the pressor and the RSNA responses to chemoreflex activation induced by KCN were increased; 3) pretreatment with intracerebroventricular injection of losartan (500 nmol) completely prevented the enhanced chemoreceptor reflex induced by electrical stimulation of the cardiac sympathetic nerve; and 4) bilateral microinjection of losartan (250 pmol) into the nucleus tractus solitarii (NTS) completely abolished the enhanced chemoreceptor reflex by epicardial application of capsaicin. These results suggest that both the chemical and electrical stimulation of the CSAR augments chemoreceptor reflex and that central ANG II, specially located in the NTS, plays a major role in these reflex interactions.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Intracisternal galanin/angiotensin II interactions in central cardiovascular control

The aim of this work was to investigate the interactions between angiotensin II (Ang II) and galanin(1-29) [GAL(1-29)] or its N-terminal fragment galanin(1-15) [GAL(1-15)] on central cardiovascular control. The involvement of angiotensin type1 (AT1) receptor subtype was analyzed by the AT1 antagonist, DuP 753. Anesthesized male Sprague-Dawley rats received intracisternal microinjections of Ang II (3 nmol) with GAL(1-29) (3 nmol) or GAL(1-15) (0.1 nmol) alone or in combination. The changes in mean arterial pressure (MAP) and heart rate (HR) recorded from the femoral artery were analyzed. The injection of Ang II and GAL(1-15) alone did not produce any change in MAP. However, coinjections of both Ang II and GAL(1-15) elicited a significant vasopressor response. This response was blocked by DuP 753. Ang II and GAL(1-15) alone produced an increase in HR. The coinjections of Ang II with GAL(1-15) induced an increase in HR not significantly different from the tachycardia produced by each peptide. The presence of DuP 753 counteracted this response. GAL(1-29) alone elicited a transient vasopressor response that disappeared in the presence of Ang II. The coinjections of Ang II with GAL(1-29) and with DuP 753 restored the transient vasopressor effect produced by GAL(1-29). GAL(1-29) produced a slight but significant tachycardic effect that was not modified in the presence of Ang II. The presence of DuP 753 did not modify the tachycardic response produced by Ang II and GAL(1-29). These results give indications for the existence of a differential modulatory effect of Ang II with GAL(1-15) and GAL(1-29) on central blood pressure response that might be dependent on the activity of the angiotensin AT1 receptor subtype.

Angiotensin II modulates the cardiovascular responses to microinjection of NPY Y1 and NPY Y2 receptor agonists into the nucleus tractus solitarii of the rat

The aim of this work was to investigate the modulation of the cardiovascular effects of neuropeptide Y (NPY) in the nucleus tractus solitarii (NTS) by angiotensin II (Ang II) and to determine the NPY receptor subtype involved in this modulation. Anesthesized Sprague-Dawley rats received microinjections in the NTS of Ang II (threshold and ED(50) doses) with NPY Y(1) agonist Leu(31)Pro(34)NPY and NPY Y(2) agonist NPY(13-36) (threshold and ED(50) doses). The changes in mean arterial pressure (MAP) and heart rate (HR) recorded in the femoral artery were analyzed during 60 min after the microinjections. The injection of threshold doses of Ang II, Y(1) agonist or Y(2) agonist alone did not produce any change in cardiovascular parameters. However, the co-injections into the NTS of threshold doses of both Ang II and the Y(1) agonist elicited significant increases of MAP and HR of about 12 and 10%, respectively. The co-administration of threshold doses of Ang II with the Y(2) agonist also induced a significant vasopressor response. The vasodepressor and bradycardiac effect of an ED(50) dose of the Y(1) agonist was significantly counteracted (P<0.01) by a threshold dose of Ang II. The vasopressor effect elicited by an ED(50) dose of the Y(2) agonist was significantly enhanced by a threshold dose of Ang II (P<0.01). No significant change of cardiovascular responses elicited by an ED(50) dose of Ang II was observed in the presence of threshold doses of the Y(1) agonist or of the Y(2) agonist. The present study gives functional evidences for a differential modulatory activity of Ang II on the cardiovascular responses mediated by Y(1) and Y(2) receptor subtypes, which may be of relevance for central cardiovascular regulation in the NTS.

Angiotensin potentiates excitatory sensory synaptic transmission to medial solitary tract nucleus neurons

Femtomole doses of angiotensin (ANG) II microinjected into nucleus tractus solitarii (nTS) decrease blood pressure and heart rate, mimicking activation of the baroreflex, whereas higher doses depress this reflex. ANG II might generate cardioinhibitory responses by augmenting cardiovascular afferent synaptic transmission onto nTS neurons. Intracellular recordings were obtained from 99 dorsal medial nTS region neurons in rat medulla horizontal slices to investigate whether ANG II modulated short-latency excitatory postsynaptic potentials (EPSPs) evoked by solitary tract (TS) stimulation. ANG II (200 fmol) increased TS-evoked EPSP amplitudes 20-200% with minimal membrane depolarization in 12 neurons excited by ANG II and glutamate, but not substance P (group A). Blockade of non-N-methyl-d-aspartate receptors eliminated TS-evoked EPSPs and responses to ANG II. ANG II did not alter TS-evoked EPSPs in 14 other neurons depolarized substantially by ANG II and substance P (group B). ANG II appeared to selectively augment presynaptic sensory transmission in one class of nTS neurons but had only postsynaptic effects on another group of cells. Thus ANG II is likely to modulate cardiovascular function by more than one nTS neuronal pathway.

Brain renin-angiotensin system dysfunction in hypertension: recent advances and perspectives

This review focuses on the dysfunction of the intrinsic brain renin-angiotensin system (RAS) in the pathogenesis of hypertension. Hyperactivity of the brain RAS plays a critical role in mediating hypertension in both humans and animal models of hypertension, including the spontaneously hypertensive rat (SHR). The specific mechanisms by which increased brain RAS activity results in hypertension are not well understood but include increases in sympathetic vasomotor tone and impaired arterial baroreflex function. We discuss the contribution of endogenous angiotensin (Ang) II actions on presympathetic vasomotor rostral ventrolateral medulla neurons to enhance sympathetic activity and maintain hypertension. In addition, we discuss Ang II-induced attenuation of afferent baroreceptor feedback within the nucleus tractus solitarius and its relevance to the development of hypertension. We also outline the cellular and molecular mechanisms of Ang II signal transduction that may be critical for the initiation and establishment of hypertension. In particular, we present evidence for a phosphoinositide-3-kinase-dependent signaling pathway that appears to contribute to hypertension in the SHR, possibly via augmented Ang II-induced increases in neuronal firing rate and enhanced transcriptional noradrenaline neuromodulation. Finally, we outline future directions in utilizing our understanding of the brain RAS dysfunction in hypertension for the development of improved therapeutic intervention in hypertension.

Localization of AT(2) receptors in the nucleus of the solitary tract of spontaneously hypertensive and Wistar Kyoto rats using [125I] CGP42112: upregulation of a non-angiotensin II binding site following unilateral nodose ganglionectomy

We have examined the binding distribution of a selective AT(2) receptor ligand [125I] CGP42112 in the brain of adult Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). AT(2) receptor localization was also examined in the rat brainstem following unilateral nodose ganglionectomy. Specific [125I] CGP42112 binding was observed in discrete brain regions from both rat strains, including the nucleus of the solitary tract (NTS), and did not differ between WKY and SHR. [125I] CGP42112 binding in the NTS revealed an AT(2) receptor component that was displaceable by PD 123319 and Ang II (50-58%), as well as a non-angiotensin II receptor component (42-49%). Following unilateral nodose ganglionectomy, [125I] CGP42112 binding density on the denervated side of the NTS was increased approximately two-fold in both WKY and SHR. This increased [125I] CGP42112 binding density in the ipsilateral NTS was comprised of a greater non-angiotensin II component than that observed in the sham groups, since only approximately 30% was displaced by PD123319 and angiotensin II. Furthermore, [125I] CGP42112 also revealed high binding density on the denervated side in the dorsal motor nucleus and the nucleus ambiguus in both WKY and SHR. AT(2) receptor immunoreactivity was also visualised in the NTS of sham operated rats, but was not observed in the dorsal motor nucleus or the nucleus ambiguus, nor was it up-regulated following nodose ganglionectomy. These results demonstrate, for the first time, an AT(2) receptor binding site in the NTS, as well as a non-angiotensin II [125I] CGP42112 binding site. These studies also demonstrate that nodose ganglionectomy represents a useful model in which to study a non-angiotensin II [125I] CGP42112 binding site that is up-regulated following degeneration of afferent vagal nerves.

Chronic inhibition of endothelial nitric oxide synthase activity in nucleus tractus solitarii enhances baroreceptor reflex in conscious rats

In acute experiments, we demonstrated previously that nitric oxide (NO) donors exogenously applied to the nucleus tractus solitarii (NTS) depressed the baroreceptor cardiac reflex. In this study, we determined a role for endogenous endothelial nitric oxide synthase (eNOS) activity in the NTS for chronically regulating baroreceptor reflex function in conscious rats. A recombinant adenoviral vector directing expression of a truncated form of eNOS was microinjected bilaterally into the NTS to inhibit endogenous eNOS activity. Arterial pressure was monitored continuously using radio-telemetry in freely moving animals and spontaneous baroreceptor reflex gain (sBRG) determined by a time-series method. sBRG showed a gradual increase from day 7 to 21 after gene transfer and the value at day 21 (1.68 +/- 0.20 ms mmHg(-1), n = 6) was significantly higher than that before gene transfer (1.13 +/- 0.09 ms mmHg(-1), P < 0.001). This value was also significantly higher than that in rats in which enhanced green fluorescent protein (eGFP) was expressed in the NTS (1.04 +/- 0.21 ms mmHg(-1); n = 6, P < 0.01) and saline-treated groups (1.12 +/- 0.15 ms mmHg(-1); n = 4, P < 0.05), which did not change from control levels. In addition, heart rate decreased from 336 +/- 6 to 318 +/- 8 b.p.m. (P < 0.05) 21 days after gene transfer. This value was also significantly lower than that in control groups (eGFP: 348 +/- 9 b.p.m., n = 6, P < 0.01; saline: 347 +/- 5 b.p.m., n = 4, P < 0.05). Gene transfer did not affect arterial pressure. These findings suggest that in the conscious rat eNOS is constitutively active within the NTS and is a factor regulating baroreceptor reflex gain and heart rate.

Genetic and pharmacological dissection of pathways involved in the angiotensin II-mediated depression of baroreflex function

Heart failure and hypertension are associated with increases in angiotensin II (ANG II) activity. One brain area where ANG II effects may be particularly important in these situations is the nucleus of the solitary tract (NTS). Located in the dorsomedial medulla, the NTS is the termination site of baroreceptor afferents and is essential for mediating the baroreflex. In hypertensive animals the baroreflex is impaired; this may be reversed by antagonizing ANG II AT1 receptors in the NTS. Recently, we showed that the baroreflex depressant action of ANG II in the NTS is mediated by activation of endothelial nitric oxide synthase (eNOS) and enhanced release of GABA. Using conventional pharmacological tools and a range of adenoviral-mediated expression of dominant negative proteins, we have determined the intracellular pathway(s) in the NTS by which ANG II activates eNOS. Our data indicate that ANG II acting in the NTS depresses the baroreflex via a Gq protein-mediated activation of phospholipase C, which through 1,4,5-inositol triphosphate causes release of calcium from the IP3-sensitive intracellular stores and calcium-calmodulin formation. In contrast, multiple site disruption of a pathway leading to eNOS activation via the serine/threonine kinase Akt was ineffective

Augmented upregulation by c-fos of angiotensin subtype 1 receptor in nucleus tractus solitarii of spontaneously hypertensive rats

Our laboratory demonstrated previously that spontaneously hypertensive rats (SHR) exhibited an elevated basal Fos expression in the nucleus tractus solitarii (NTS), the terminal site for primary baroreceptor afferents, and that Fos protein is required for the re-expression of angiotensin subtype 1 receptor (AT1R) mRNA in the NTS after baroreceptor activation. The present study evaluated the hypothesis that this re-expression of AT1R is augmented in SHR and is promoted by the heightened Fos expression. Reverse transcription-polymerase chain reaction analysis revealed that baroreceptor activation via sustained increase in systemic arterial pressure resulted in a discernible reduction in the expression of AT1R mRNA at the dorsomedial medulla of SHR and normotensive Wistar-Kyoto rats. However, SHR manifested an appreciably larger magnitude of decline, followed by a faster time course of re-expression in AT1R mRNA. Parallel findings were obtained from the pressor response induced by microinjection unilaterally of angiotensin II (40 pmol) into the NTS. Whereas the re-expression of AT1R at both transcriptional and functional expression levels after baroreceptor activation was discernibly blunted by prior bilateral application into the NTS of an antisense c-fos oligonucleotide (50 pmol), the suppression in SHR was again significantly more intense. Control pretreatment with the corresponding sense or scrambled c-fos oligonucleotide was ineffective. We conclude that the heightened Fos expression in SHR is causatively related to the augmented re-expression of AT1R in the NTS at both transcriptional and functional levels.

Heart rate variability and baroreflex function in AT2 receptor-disrupted mice

We adapted telemetry and sequence analysis employed in humans to mice and measured heart rate variability and the spontaneous baroreflex sensitivity in angiotensin II type 2 (AT2) receptor-deleted (AT2 -/-) and wild-type (AT2 +/+) mice with either deoxycorticosterone acetate (DOCA)-salt hypertension or N(omega)-nitro-L-arginine methylester hydrochloride (L-NAME) hypertension. Mean arterial pressure leveled during the day at 101+/-1 mm Hg and during the night at 109+/-1 mm Hg in AT2 receptor-deleted mice, compared with 98+/-2 mm Hg (day) and 104+/-2 mm Hg (night) in wild-type mice. Mean arterial pressure increased in AT2 receptor-deleted mice with L-NAME to 114+/-1 mm Hg (day) and 121+/-1 mm Hg (night), compared with 105+/-2 mm Hg (day) and 111+/-2 mm Hg (night), respectively. DOCA-salt also increased day and night blood pressures in AT2 receptor-deleted mice to a greater degree than in wild-type mice. Heart rate variability in the time and frequency domain was not different between AT2 receptor-deleted mice and AT2 receptor-deleted mice at baseline. Systolic blood pressure variability in the low frequency band was lower in AT2 receptor-deleted mice (0.6+/-0.1 ms2 versus 3.9+/-1.3 ms2) than in wild-type mice. Baroreceptor-heart rate reflex sensitivity was significantly increased in AT2 receptor-deleted mice compared with wild-type mice (3.4+/-0.6 versus 2.1+/-0.5 ms/mm Hg). These differences remained after DOCA-salt and L-NAME treatments. We conclude that activation of the AT2 receptor impairs arterial baroreceptor reflex function, probably by a central action. These data support the existence of an inhibitory central effect of the AT2 receptor on baroreflex function.

Angiotensin peptides as neurotransmitters/neuromodulators in the dorsomedial medulla

1. The present review provides an update on evidence of the neurotransmitter pathways and location of receptors within the nucleus tractus solitarii (NTS) mediating the baroreflex and other haemodynamic actions of angiotensin (Ang) II. 2. A series of studies suggests a significant role for substance P in the acute cardiovascular and carotid sinus chemoreceptor facilitatory actions of AngII in the NTS. The use of antisense oligonucleotides to AT1 receptors indicates both pre- and post-synaptic AngII receptors are likely to be involved in these actions. 3. With respect to baroreceptor reflex actions, it is clear that endogenous AngII impairs the gain for operation of the baroreceptor reflex, because AT1 receptor antagonists facilitate reflex function. This effect is either independent of substance P or involves inhibition of release. Moreover, initial data obtained using antisense oligonucleotides to AT1 receptors suggest that, in the NTS, the effect of endogenous AngII on the baroreceptor reflex is mainly due to presynaptic actions on vagal or carotid sinus afferent fibres. In contrast, the level of endogenous AngII within the NTS appears to have variable effects on activation of cardiopulmonary vagal afferent fibres by phenylbiguanide. These results indicate a divergence of effects of AngII on reflexes evoked by these two different types of sensory input. 4. Use of transgenic rats with alterations in brain angiotensin peptides allowed us to assess the effect of long-term alterations in brain Ang peptides on reflex function. We studied (mRen2)27 transgenic rats (TGR(mRen2)) with high brain medulla AngII levels and transgenic rats with angiotensinogen (Aogen) antisense linked to glial fibrillary acidic protein promoter (TGR(ASrAogen)) with greatly reduced brain Aogen. The reflex evoked by activation of cardiac vagal chemosensitive afferent fibres was enhanced in TGR(ASrAogen), whereas the baroreceptor reflex control of heart rate was attenuated in TGR(mRen2), further confirming a divergence of effects of AngII on these two sensory modalities. 5. The overall results are consistent with a sustained inhibitory effect of AngII on the baroreceptor reflexes, with dose-dependent or activation-dependent effects on cardiac vagal afferent fibre activation. Moreover, alterations in substance P pathways may contribute to the actions of AngII on reflex function.

Angiotensin III activates nuclear transcription factor-kappaB in cultured mesangial cells mainly via AT(2) receptors: studies with AT(1) receptor-knockout mice

Nuclear factor-kappaB (NF-kappaB) regulates many genes involved in renal pathophysiologic processes. It was previously demonstrated that angiotensin II (AngII) and its amino-terminal degradation product AngIII activate NF-kappaB in mesangial cells. However, which are the Ang receptor subtypes involved in the NF-kappaB pathway and whether these Ang peptides act through the same or different receptors in mesangial cells have not been evaluated. Under the culture conditions used, quiescent rat mesangial cells expressed both AT(1) and AT(2) receptors. To investigate the receptors involved in the NF-kappaB pathway, two different approaches were used, i.e., pharmacologic studies, using specific AT(1) and AT(2) receptor antagonists and agonists, and studies in AT(1) receptor-knockout mice. In cultured rat mesangial cells, both AT(1) and AT(2) receptor antagonists inhibited AngII-induced NF-kappaB DNA binding activity, whereas NF-kappaB activation elicited by AngIII was mainly blocked by the AT(2) receptor antagonist. Similar results were observed for cytosolic IkappaBalpha degradation. An AT(2) receptor agonist also activated NF-kappaB. In AT(1) receptor-knockout murine mesangial cells, AngIII and AngII increased NF-kappaB activity and degraded cytosolic IkappaBalpha; both processes were blocked by the AT(2) receptor antagonist. These data demonstrate that, in mesangial cells, NF-kappaB activation is mediated by AT(1) and AT(2) receptors, suggesting a novel intracellular signaling mechanism for AT(2) receptors in the kidney. Some differences in Ang peptide receptor-mediated responses were also observed. AngII activates NF-kappaB via AT(1) and AT(2) receptors, whereas AngIII acts mainly via AT(2) receptors. These results suggest the potential involvement of the AngIII/AT(2) receptor/NF-kappaB pathway in pathophysiologic processes in the kidney and provide a better understanding of the renin-angiotensin system.

Unravelling mechanisms of action of angiotensin II on cardiorespiratory function using in vivo gene transfer

We review recent and ongoing work from our laboratory that has shed novel insights into the effects of angiotensin II (ANGII) on the baroreflex at the level of the nucleus of the solitary tract (NTS). The NTS is the site of termination for baroreceptor afferents and is a potentially powerful region for neuronal modulation. ANGII applied to this nucleus attenuated the cardiac vagal and cardiac sympathetic components of the baroreceptor reflex. This effect was antagonized by blockade of either gamma-amino butyric acid receptors or nitric oxide synthase within the NTS. Interestingly, nitric oxide donors microinjected into the NTS mimicked the effect of ANGII. Using an adenovirus we showed that ANGII activated the endothelial isoform of nitric oxide synthase. The NTS was transfected to express a dominant negative truncated mutant form of endothelial nitric oxide synthase that prevented the depressant effect of ANGII on the baroreflex. Endothelial nitric oxide synthase was present in both neurones and endothelium in the NTS. A possibility is that ANGII activation of endothelial nitric oxide synthase is calcium dependent. However, in most NTS neurones tested, ANGII failed to elevate intracellular calcium concentration. We conclude that ANGII activates endothelial nitric oxide synthase to release nitric oxide which enhances gamma-amino butyric acid transmission destined for circuitry mediating the baroreflex. We discuss the contribution of endothelial cells within the nucleus of the solitary tract as a potential target for both circulating and/or centrally produced ANGII. These data have relevance to patients with essential hypertension and left heart failure, conditions in which ANGII activity is elevated and the baroreceptor reflex is depressed.

Inhibition of baroreflex by angiotensin II via Fos expression in nucleus tractus solitarii of the rat

We evaluated the modulatory action of angiotensin II at the nucleus tractus solitarii on spontaneous baroreceptor reflex response, the angiotensin subtype receptors involved, and the role of Fos protein in this process, using Sprague-Dawley rats anesthetized with pentobarbital sodium. Microinjection bilaterally of angiotensin (Ang ) II (5, 10, 20, or 40 pmol) into the nucleus tractus solitarii significantly suppressed the spontaneous baroreceptor reflex, as represented by the magnitude of transfer function between systemic arterial pressure and heart rate signals. There also was a concomitant increase in Fos-like immunoreactivity in the nucleus tractus solitarii. Both the suppression of spontaneous baroreceptor reflex and Fos expression in nucleus tractus solitarii neurons elicited by Ang II were discernibly attenuated by pretreatment with or comicroinjection into the bilateral nucleus tractus solitarii of a 15-mer antisense c-fos oligonucleotide that targets against the initiation codon of c-fos mRNA. In addition, those 2 actions of Ang II were reversed by the coadministration of the nonpeptide Ang II type 1 (AT(1)) receptor antagonist losartan (1.6 nmol) but not by the nonpeptide AT(2) receptor antagonist PD 123,319 (1.6 nmol). Control treatments with artificial cerebrospinal fluid, sense cDNA, or antisense oligonucleotide with a scrambled sequence were ineffective. We conclude that under minimal cardiovascular perturbation, Fos expression mediated via activation of AT(1) subtype receptors may underlie the inhibitory modulation of beat-to-beat baroreflex control of blood pressure by Ang II at the nucleus tractus solitarii.

Central angiotensin and baroreceptor control of circulation

Angiotensin (Ang) receptors are located in many important central nuclei involved in the regulation of the cardiovascular system. While most interest has focused on forebrain circumventricular actions, areas of the brainstem such as the nucleus of the solitary tract and the ventrolateral medulla contain high concentrations of AT1 receptors. The present review encompasses the physiological role of Ang II in the hindbrain, particularly in relation to its influence on baroreflex control mechanisms. In rabbits there are sympatho-excitatory AT1 receptors in the rostral ventrolateral medulla (RVLM), accessible to Ang II from the cerebrospinal fluid. Activation of these receptors acutely increases renal sympathetic nerve activity (RSNA) and RSNA baroreflex responses. However, blockade of endogenous Ang receptors in the brainstem also shows sympathoexcitation, suggesting there is greater endogenous activity of a sympathoinhibitory Ang II action. Microinjections of angiotensin antagonists into the RVLM showed relatively little tonic activity of endogenous Ang II influencing sympathetic activity in conscious rabbits. However, Ang II receptors in the RVLM mediate sympathetic responses to airjet stress in conscious rabbits. Similarly with respect to heart rate baroreflexes, there appears to be little tonic effect of angiotensin in the brainstem in normal conscious animals. Chronic infusion of Ang II for two weeks into the fourth ventricle of conscious rabbits inhibits the cardiac baroreflex while infusion of losartan increases the gain of the reflex. These actions suggest that Ang II in the brainstem modulates sympathetic responses depending on specific afferent and synaptic inputs in both the short term but importantly also in the long term, thus forming an important mechanism for increasing the range of adaptive response patterns.

Baroreflex inhibition of cardiac sympathetic outflow is attenuated by angiotensin II in the nucleus of the solitary tract

Homeostatic regulation of arterial pressure is maintained by arterial baroreceptors. Activation of these receptors results in an inhibition of sympathetic activity to the heart. It is known that angiotensin II in the nucleus tractus solitarii attenuates the baroreceptor reflex-evoked vagal bradycardia. Here, we determined whether the cardiac sympathetic component of the baroreceptor reflex could be modulated by angiotensin II in the nucleus of the solitary tract. An in situ, arterially perfused working heart--brainstem preparation of rat was employed and the sympathetic inferior cardiac nerve recorded. Increases in perfusion pressure caused a reflex bradycardia and inhibition of inferior cardiac nerve activity. Microinjection of angiotensin II (500 fmol) in the nucleus of the solitary tract attenuated significantly both the reflex bradycardia and inhibition of inferior cardiac nerve activity (P<0.01). The latter was reversible and sensitive to losartan, an angiotensin II type 1 receptor antagonist. In contrast, the peripheral chemoreceptor reflex evoked an increase in inferior cardiac nerve activity that was not affected by angiotensin II applied exogenously in the nucleus of the solitary tract. We conclude that within the nucleus of the solitary tract angiotensin II exerts a powerful and specific inhibitory modulation of the baroreceptor reflex control of sympathetic nerve activity destined for the heart. We suggest that our data may have clinical implications relating to hypertension, a condition when angiotensin II activity is heightened in the brain and the efficacy of the baroreflex is reduced.

Fos protein is required for the re-expression of angiotensin II type 1 receptors in the nucleus tractus solitarii after baroreceptor activation in the rat

We evaluated in Sprague--Dawley rats the hypothesis that Fos protein induced by baroreceptor activation in the nucleus tractus solitarii participates in transcriptional regulation of the expression of angiotensin receptor genes. Reverse transcription-polymerase chain reaction revealed that baroreceptor activation elicited by sustained hypertension resulted in a transient decrease in angiotensin II subtype 1, but not subtype 2, receptor messenger RNA, in the dorsomedial medulla, including the nucleus tractus solitarii. There was subsequently a transitory reduction in the pressor response elicited by microinjection bilaterally of angiotensin II (40 pmol) into the nucleus tractus solitarii, followed by an increase in c-fos messenger RNA and Fos immunoreactivity at the same nucleus. Both the re-expression of angiotensin II subtype 1 receptor messenger RNA and restoration of pressor response to angiotensin II after baroreceptor activation were significantly blunted by bilateral application into the nucleus tractus solitarii of an antisense oligonucleotide (50 pmol) that targets against the initiation codon of c-fos messenger RNA. Control pretreatment with the corresponding sense oligonucleotide (50 pmol), or an antisense c-fos oligonucleotide that targets against a different portion of the coding sequence of the c-fos messenger RNA (50 pmol), was ineffective. At the receptor level, the angiotensin II-induced pressor response was antagonized by the subtype 1 receptor antagonist losartan (1.6 nmol), but not by the subtype 2 receptor antagonist PD-123319 (1.6 nmol). These findings suggest that sustained hypertension down-regulates angiotensin II subtype 1 receptors at both messenger RNA and functional expression levels in the nucleus tractus solitarii. Furthermore, Fos protein induced in the nucleus tractus solitarii by baroreceptor activation may play a permissive role in the transcriptional regulation of the re-expression of this subtype of angiotensin receptors.

Tonic suppression of spontaneous baroreceptor reflex by endogenous angiotensins via AT(2) subtype receptors at nucleus reticularis ventrolateralis in the rat

We evaluated the role of endogenous angiotensins at the rostral nucleus reticularis ventrolateralis (NRVL) in the modulation of spontaneous baroreceptor reflex (BRR) response and the subtype of angiotensin receptors involved using rats anesthetized and maintained with pentobarbital sodium. Bilateral microinjection of angiotensin II (ANG II) or its active metabolite angiotensin III (ANG III) (5, 10, or 20 pmol) into the NRVL significantly suppressed the spontaneous BRR response, as represented by the magnitude of transfer function between systemic arterial pressure and heart rate signals. The inhibitory effect of ANG III (20 pmol) was discernibly reversed by coadministration with its peptide antagonist, [Ile(7)]ANG III (1.6 nmol), or the nonpeptide AT(2) receptor antagonist, PD-123319 (1.6 nmol), but not by the nonpeptide AT(1) receptor antagonist, losartan (1.6 nmol). On the other hand, the peptide antagonist, [Sar(1), Ile(8)]ANG II (1.6 nmol) or both non-peptide antagonists appreciably reversed the suppressive action of ANG II (20 pmol). Whereas losartan produced minimal effect, blocking the endogenous activity of the angiotensins by microinjection into the bilateral NRVL of PD-123319, [Sar(1), Ile(8)]ANG II or [Ile(7)]ANG III elicited significant enhancement of the spontaneous BRR response. We conclude that under physiologic conditions both endogenous ANG II and ANG III may exert a tonic inhibitory modulation on the spontaneous BRR response by acting selectively on the AT(2) subtype receptors at the NRVL.

Brain angiotensin receptors and sympathoadrenal regulation during insulin-induced hypoglycemia

Simultaneous blockade of systemic AT1 and AT2 receptors or converting enzyme inhibition (CEI) attenuates the hypoglycemia-induced reflex increase of epinephrine (Epi). To examine the role of brain AT1 and AT2 receptors in the reflex regulation of Epi release, we measured catecholamines, hemodynamics, and renin during insulin-induced hypoglycemia in conscious rats pretreated intracerebroventricularly with losartan, PD-123319, losartan and PD-123319, or vehicle. Epi and norepinephrine (NE) increased 60-and 3-fold, respectively. However, the gain of the reflex increase in plasma Epi (Deltaplasma Epi/Deltaplasma glucose) and the overall Epi and NE responses were similar in all groups. The ensuing blood pressure response was similar between groups, but the corresponding bradycardia was augmented after PD-123319 (P < 0.05 vs. vehicle) or combined losartan and PD-123319 (P < 0.01 vs. vehicle). The findings indicate 1) brain angiotensin receptors are not essential for the reflex regulation of Epi release during hypoglycemia and 2) the gain of baroreceptor-mediated bradycardia is increased by blockade of brain AT2 receptors in this model.

Adenoviral vector demonstrates that angiotensin II-induced depression of the cardiac baroreflex is mediated by endothelial nitric oxide synthase in the nucleus tractus solitarii of the rat

Angiotensin II (ANGII) acting on ANGII type 1 (AT1) receptors in the solitary tract nucleus (NTS) depresses the baroreflex. Since ANGII stimulates the release of nitric oxide (NO), we tested whether the ANGII-mediated depression of the baroreflex in the NTS depended on NO release. In a working heart-brainstem preparation (WHBP) of rat NTS microinjection of either ANGII (500 fmol) or a NO donor (diethylamine nonoate, 500 pmol) both depressed baroreflex gain by -56 and -67 %, respectively (P < 0.01). In contrast, whilst ANGII potentiated the peripheral chemoreflex, the NO donor was without effect. NTS microinjection of non-selective NO synthase (NOS) inhibitors (L-NAME; 50 pmol) or (L-NMMA; 200 pmol) prevented the ANGII-induced baroreflex attenuation (P > 0.1). In contrast, a neurone-specific NOS inhibitor, TRIM (50 pmol), was without effect. Using an adenoviral vector, a dominant negative mutant of endothelial NOS (TeNOS) was expressed bilaterally in the NTS. Expression of TeNOS affected neither baseline cardiovascular parameters nor baroreflex sensitivity. However, ANGII microinjected into the transfected region failed to affect the baroreflex.Immunostaining revealed that eNOS-positive neurones were more numerous than those labelled for AT1 receptors. Neurones double labelled for both AT1 receptors and eNOS comprised 23 +/- 5.4 % of the eNOS-positive cells and 57 +/- 9.2 % of the AT1 receptor-positive cells. Endothelial cells were also double labelled for eNOS and AT1 receptors. We suggest that ANGII activates eNOS located in either neurones and/or endothelial cells to release NO, which acts selectively to depress the baroreflex.

Pharmacological profile of depressor response elicited by sarthran in rat ventrolateral medulla

Injection of sarthran, an angiotensin receptor antagonist, bilaterally into the rostral ventrolateral medulla (RVLM) of alpha-chloralose-anesthetized rats decreases arterial pressure (AP) to the same extent as total autonomic blockade. This response is not reproduced by selective AT(1) antagonists. To examine the pharmacological profile of the response elicited by [Sar(1), Thr(8)]ANG II (sarthran), the ability of angiotensin analogs to inhibit the effect of sarthran injected into the RVLM was tested. Coinjection of angiotensin II (ANG II) prevented the sarthran-evoked decrease in AP, but this action of ANG II was markedly attenuated by pretreatment of the RVLM with the aminopeptidase inhibitor amastatin. Coinjection of ANG(3-8) or a selective agonist of AT(4) receptors prevented the effect of sarthran injected into the RVLM. ANG(1-7) was also able to prevent the effect of sarthran. None of the angiotensin fragments tested substantially altered blood pressure when injected alone into the RVLM. These results suggest that the depressor action of sarthran injected into the RVLM is not dependent on ANG II receptors, though the nature of the site or sites of action of sarthran within the RVLM remains uncertain.

Differential effects of angiotensin II on cardiorespiratory reflexes mediated by nucleus tractus solitarii - a microinjection study in the rat

1. The effect of microinjecting angiotensin II (ANGII) into the nucleus of the solitary tract (NTS) on both baroreceptor and peripheral chemoreceptor reflexes was compared. 2. Experiments were performed in a working heart-brainstem preparation of rat. Baroreceptors were stimulated by raising perfusion pressure and chemoreceptors were activated with aortic injections of sodium cyanide (0.025 %, 25-75 microl). Reflex changes in phrenic nerve activity and heart rate were measured after bilateral NTS microinjection (50 nl) of ANGII (0.5-5000 fmol). 3. NTS microinjection of 5 fmol ANGII elicited a transient (28.2 +/- 6 s; mean +/- s.e.m.) bradycardia (-18 +/- 3 beats min-1), and decreased phrenic nerve activity cycle length and amplitude (P < 0.05). At higher doses of ANGII a similar respiratory response was seen but heart rate changes were inconsistent. 4. The baroreceptor reflex bradycardia was depressed significantly by NTS microinjections of ANGII (5-5000 fmol) in a dose-dependent manner with the reflex gain decreasing from 1.7 +/- 0.16 to 0.66 +/- 0.1 beats min-1 mmHg-1 (P < 0.01) at 5000 fmol. Although the chemoreceptor reflex bradycardia was depressed at a low dose of ANGII (5 fmol), all higher doses (50-5000 fmol) produced a dose-dependent potentiation of the reflex bradycardia (maximally +64 +/- 8 %). The respiratory component was unaffected. The effects of ANGII on both reflexes were blocked by an ANGII type 1 (AT1) receptor antagonist, losartan (20 microM). 5. The potentiating action of ANGII on the chemoreceptor reflex cardiac response was abolished by a neurokinin type 1 (NK1) receptor blocker (CP-99,994, 5 microM) but this had no effect on the baroreceptor reflex. 6. AT1 receptors in the NTS can depress the baroreceptor reflex bradycardia which is independent of NK1 receptors. The ANGII effect on the cardiac component of the chemoreceptor reflex is bi-directional being inhibited at low concentrations and potentiated at higher concentrations; the latter involves NK1 receptors and presumably results from release of substance P.

Differential effects of angiotensin II in the nucleus tractus solitarii of the rat--plausible neuronal mechanism

1. Cellular mechanisms of the actions of angiotensin II (ANGII) within the nucleus of the solitary tract (NTS) were studied using rat brain slices in 78 neurones recorded in the whole-cell configuration. Twenty-nine per cent of cells had an on-going activity and with only one exception these cells responded to tractus solitarii (TS) stimulation with a monophasic excitatory postsynaptic potential (EPSP). In approximately half of the silent cells, TS stimulation evoked an EPSP-inhibitory postsynaptic potential (IPSP) complex. 2. The ANGII (200 or 1000 nM) effect on TS-evoked EPSPs depended on the cell subpopulation. In cells with on-going activity, ANGII (1000 nM) increased evoked EPSP amplitude by +70 +/- 13 % (means +/- s.e.m., n = 5) but reduced it (200 and 1000 nM) in silent cells where both evoked EPSPs and IPSPs were present. ANGII either increased TS-evoked IPSP conductances in cells where they were detectable or revealed an evoked IPSP (200 nM ANGII: IPSP conductance increased from 70 +/- 29 to 241 +/- 34 pS; n = 11). All ANGII effects were prevented by the ANGII type 1 (AT1) receptor blocker losartan. Since 200 nM ANGII did not increase responses to iontophoretically applied GABA, the effect of ANGII on TS-evoked IPSPs may occur presynaptically. 3. The neurokinin type 1 (NK1) receptor antagonist CP-99,994 (5 microM) blocked the ANGII-induced increase in EPSPs but had no effect on TS-evoked IPSP potentiation by ANGII. 4. Thus, ANGII can potentiate both inhibitory and excitatory synaptic transmission within different subpopulations of NTS neurones. Potentiation of evoked EPSPs, but not of IPSPs, involves activation of NK1 receptors. The balance of these actions of ANGII could be reflex specific: for the baroreflex circuitry the inhibitory action might predominate while the peripheral chemoreceptor reflex may be facilitated due to enhanced excitatory transmission.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.