Immunocytochemical localization of angiotensin-(1-7) in the rat forebrain

Targeting the Renin-Angiotensin System (RAS) for Neuropsychiatric Disorders

BACKGROUND

Neuropsychiatric disorders, such as mood disorders, schizophrenia, and Alzheimer's disease (AD) and related dementias, are associated to significant morbidity and mortality worldwide. The pathophysiological mechanisms of neuropsychiatric disorders remain to be fully elucidated, which has hampered the development of effective therapies. The Renin Angiotensin System (RAS) is classically viewed as a key regulator of cardiovascular and renal homeostasis. The discovery that RAS components are expressed in the brain pointed out a potential role for this system in central nervous system (CNS) pathologies. The understanding of RAS involvement in the pathogenesis of neuropsychiatric disorders may contribute to identifying novel therapeutic targets.

AIMS

We aim to report current experimental and clinical evidence on the role of RAS in physiology and pathophysiology of mood disorders, schizophrenia, AD and related dementias. We also aim to discuss bottlenecks and future perspectives that can foster the development of new related therapeutic strategies.

CONCLUSION

The available evidence supports positive therapeutic effects for neuropsychiatric disorders with the inhibition/antagonism of the ACE/Ang II/AT1 receptor axis or the activation of the ACE2/Ang-(1-7)/Mas receptor axis. Most of this evidence comes from pre-clinical studies and clinical studies lag much behind, hampering a potential translation into clinical practice.

The hypothalamus as a key regulator of glucose homeostasis: emerging roles of the brain renin-angiotensin system

The regulation of plasma glucose levels is a complex and multifactorial process involving a network of receptors and signaling pathways across numerous organs that act in concert to ensure homeostasis. However, much about the mechanisms and pathways by which the brain regulates glycemic homeostasis remains poorly understood. Understanding the precise mechanisms and circuits employed by the central nervous system to control glucose is critical to resolving the diabetes epidemic. The hypothalamus, a key integrative center within the central nervous system, has recently emerged as a critical site in the regulation of glucose homeostasis. Here, we review the current understanding of the role of the hypothalamus in regulating glucose homeostasis, with an emphasis on the paraventricular nucleus, the arcuate nucleus, the ventromedial hypothalamus, and lateral hypothalamus. In particular, we highlight the emerging role of the brain renin-angiotensin system in the hypothalamus in regulating energy expenditure and metabolic rate, as well as its potential importance in the regulation of glucose homeostasis.

Hemodynamic phenotyping of transgenic rats with ubiquitous expression of an angiotensin-(1-7)-producing fusion protein

Activation of the angiotensin (Ang)-converting enzyme (ACE) 2/Ang-(1-7)/MAS receptor pathway of the renin-angiotensin system (RAS) induces protective mechanisms in different diseases. Herein, we describe the cardiovascular phenotype of a new transgenic rat line (TG7371) that expresses an Ang-(1-7)-producing fusion protein. The transgene-specific mRNA and the corresponding protein were shown to be present in all evaluated tissues of TG7371 with the highest expression in aorta and brain. Plasma Ang-(1-7) levels, measured by radioimmunoassay (RIA) were similar to control Sprague-Dawley (SD) rats, however high Ang-(1-7) levels were found in the hypothalamus. TG7371 showed lower baseline mean arterial pressure (MAP), assessed in conscious or anesthetized rats by telemetry or short-term recordings, associated with increased plasma atrial natriuretic peptide (ANP) and higher urinary sodium concentration. Moreover, evaluation of regional blood flow and hemodynamic parameters with fluorescent microspheres showed a significant increase in blood flow in different tissues (kidneys, mesentery, muscle, spleen, brown fat, heart and skin), with a resulting decrease in total peripheral resistance (TPR). TG7371 rats, on the other hand, also presented increased cardiac and global sympathetic tone, increased plasma vasopressin (AVP) levels and decreased free water clearance. Altogether, our data show that expression of an Ang-(1-7)-producing fusion protein induced a hypotensive phenotype due to widespread vasodilation and consequent fall in peripheral resistance. This phenotype was associated with an increase in ANP together with an increase in AVP and sympathetic drive, which did not fully compensate the lower blood pressure (BP). Here we present the hemodynamic impact of long-term increase in tissue expression of an Ang-(1-7)-fusion protein and provide a new tool to investigate this peptide in different pathophysiological conditions.

Angiotensin-(1-7) and Mas receptor in the brain

The renin-angiotensin system (RAS) is a key regulator of blood pressure and electrolyte homeostasis. Besides its importance as regulator of the cardiovascular function, the RAS has also been associated to the modulation of higher brain functions, including cognition, memory, depression and anxiety. For many years, angiotensin II (Ang II) has been considered the major bioactive component of the RAS. However, the existence of many other biologically active RAS components has currently been recognized, with similar, opposite, or distinct effects to those exerted by Ang II. Today, it is considered that the RAS is primarily constituted by two opposite arms. The pressor arm is composed by Ang II and the Ang II type 1 (AT1) receptor (AT1R), which mediates the vasoconstrictor, proliferative, hypertensive, oxidative and pro-inflammatory effects of the RAS. The depressor arm is mainly composed by Ang-(1-7), its Mas receptor (MasR) which mediates the depressor, vasodilatory, antiproliferative, antioxidant and anti-inflammatory effects of Ang-(1-7) and the AT2 receptor (AT2R), which opposes to the effects mediated by AT1R activation. Central Ang-(1-7) is implicated in the control of the cardiovascular function, thus participating in the regulation of blood pressure. Ang-(1-7) also exerts neuroprotective actions through MasR activation by opposing to the harmful effects of the Ang II/AT1R axis. This review is focused on the expression and regulation of the Ang-(1-7)/MasR axis in the brain, its main neuroprotective effects and the evidence regarding its involvement in the pathophysiology of several diseases at cardiovascular and neurological level.

The Renin Angiotensin System and Bipolar Disorder: A Systematic Review

Bipolar Disorder (BD) is a chronic a multifactorial psychiatric illness that affects mood, cognition, and functioning. BD is associated with several psychiatric conditions as well clinical comorbidities, particularly cardiovascular diseases. The neurobiology of BD is complex and multifactorial and several systems have been implicated. Considering that the Renin Angiotensin System (RAS) plays an important role in cardiovascular diseases and that recently evidence has suggested its role in psychiatric disorders, the aim of the present study is to summarize and to discuss recent findings related to the modulation of RAS components in BD. A systematic search of the literature using the electronic databases MEDLINE and LILACS was conducted through March 2019. The search terms were: "Bipolar Disorder"; "Renin Angiotensin System"; "Angiotensin 2"; "Angiotensin receptors"; "Angiotensin 1-7"; "ACE"; "ACE2"; "Mas Receptor". We included original studies assessing RAS in BD patients. Two hundred twenty-two citations were initially retrieved. Eleven studies were included in our systematic review. In the majority of studies (6 of 8), the ACE insertion/deletion (I/D) polymorphism did not differ between BD patients and controls. BD patients presented higher plasma renin activity in comparison with controls. The studies evaluating the RAS molecules in BD are very scarce and heterogeneous. The literature suggests a potential role of RAS in BD. Further studies are necessary to investigate this relationship.

Pathobiological Interactions of Local Bone Marrow Renin-Angiotensin System and Central Nervous System in Systemic Arterial Hypertension

Circulating renin-angiotensin system (RAS) and local paracrin-autocrin-intracrin tissue-based RAS participate in numerous pathobiological events. Pro-inflammatory, pro-fibrotic, and pro-thrombotic consequences associated with local RAS activation have been detected at cellular and molecular level. Regenerative progenitor cell therapy in response to RAS modulating pharmacotherapy has emerged as an adjunct in the context of endothelial cell injury and regeneration to improve regeneration of the vascular endothelium. Local hematopoietic bone marrow (BM) RAS symbolizes the place of cross-interaction between vascular biology and cellular events from embryogenesis to definitive hematopoiesis underlying vascular atherosclerosis. The BM microenvironment also contains Mas receptors, which control the proliferative role of Ang 1-7 on hematopoietic stem cells. Ang 1-7 is produced from Ang-II or Ang-I with the help of ACE2. Various tissues and organs also have an effect on the RAS system. The leukocytes contain and synthesize immunoreactive angiotensinogen species capable of producing angiotensin in the basal state or after incubation with renin. The significance of RAS employment in atherosclerosis and hypertension was indicated by novel bidirectional Central Nervous System (CNS) RAS-BM RAS communications. Myeloid cells generated within the context of hematopoietic BM RAS are considered as the initiators and decision shapers in atherosclerosis. Macrophages in the atherosclerotic lesions contain angiotensin peptides by which RAS blockers inhibit monocyte activation and adherence. Furthermore, vascular biology in relation to inflammation and neoplasia is also affected by local tissue RAS. The purpose of this article is to outline interactions of circulating and local angiotensin systems, especially local bone marrow RAS, in the vascular pathobiological microenvironment of CNS.

Blockade of Endogenous Angiotensin-(1-7) in Hypothalamic Paraventricular Nucleus Attenuates High Salt-Induced Sympathoexcitation and Hypertension

Angiotensin (Ang)-(1-7) is an important biologically-active peptide of the renin-angiotensin system. This study was designed to determine whether inhibition of Ang-(1-7) in the hypothalamic paraventricular nucleus (PVN) attenuates sympathetic activity and elevates blood pressure by modulating pro-inflammatory cytokines (PICs) and oxidative stress in the PVN in salt-induced hypertension. Rats were fed either a high-salt (8% NaCl) or a normal salt diet (0.3% NaCl) for 10 weeks, followed by bilateral microinjections of the Ang-(1-7) antagonist A-779 or vehicle into the PVN. We found that the mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and plasma norepinephrine (NE) were significantly increased in salt-induced hypertensive rats. The high-salt diet also resulted in higher levels of the PICs interleukin-6, interleukin-1beta, tumor necrosis factor alpha, and monocyte chemotactic protein-1, as well as higher gp91^phox expression and superoxide production in the PVN. Microinjection of A-779 (3 nmol/50 nL) into the bilateral PVN of hypertensive rats not only attenuated MAP, RSNA, and NE, but also decreased the PICs and oxidative stress in the PVN. These results suggest that the increased MAP and sympathetic activity in salt-induced hypertension can be suppressed by blockade of endogenous Ang-(1-7) in the PVN, through modulation of PICs and oxidative stress.

The depressor axis of the renin–angiotensin system and brain disorders: a translational approach

All the components of the classic renin–angiotensin system (RAS) have been identified in the brain. Today, the RAS is considered to be composed mainly of two axes: the pressor axis, represented by angiotensin (Ang) II/angiotensin-converting enzyme/AT1 receptors, and the depressor and protective one, represented by Ang-(1–7)/ angiotensin-converting enzyme 2/Mas receptors. Although the RAS exerts a pivotal role on electrolyte homeostasis and blood pressure regulation, their components are also implicated in higher brain functions, including cognition, memory, anxiety and depression, and several neurological disorders. Overactivity of the pressor axis of the RAS has been implicated in stroke and several brain disorders, such as cognitive impairment, dementia, and Alzheimer or Parkinson’s disease. The present review is focused on the role of the protective axis of the RAS in brain disorders beyond its effects on blood pressure regulation. Furthermore, the use of drugs targeting centrally RAS and its beneficial effects on brain disorders are also discussed.

The ACE2/Angiotensin-(1-7)/MAS Axis of the Renin-Angiotensin System: Focus on Angiotensin-(1-7)

The renin-angiotensin system (RAS) is a key player in the control of the cardiovascular system and hydroelectrolyte balance, with an influence on organs and functions throughout the body. The classical view of this system saw it as a sequence of many enzymatic steps that culminate in the production of a single biologically active metabolite, the octapeptide angiotensin (ANG) II, by the angiotensin converting enzyme (ACE). The past two decades have revealed new functions for some of the intermediate products, beyond their roles as substrates along the classical route. They may be processed in alternative ways by enzymes such as the ACE homolog ACE2. One effect is to establish a second axis through ACE2/ANG-(1-7)/MAS, whose end point is the metabolite ANG-(1-7). ACE2 and other enzymes can form ANG-(1-7) directly or indirectly from either the decapeptide ANG I or from ANG II. In many cases, this second axis appears to counteract or modulate the effects of the classical axis. ANG-(1-7) itself acts on the receptor MAS to influence a range of mechanisms in the heart, kidney, brain, and other tissues. This review highlights the current knowledge about the roles of ANG-(1-7) in physiology and disease, with particular emphasis on the brain.

The Suprachiasmatic Nucleus of the Dromedary Camel (Camelus dromedarius): Cytoarchitecture and Neurochemical Anatomy

In mammals, biological rhythms are driven by a master circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Recently, we have demonstrated that in the camel, the daily cycle of environmental temperature is able to entrain the master clock. This raises several questions about the structure and function of the SCN in this species. The current work is the first neuroanatomical investigation of the camel SCN. We carried out a cartography and cytoarchitectural study of the nucleus and then studied its cell types and chemical neuroanatomy. Relevant neuropeptides involved in the circadian system were investigated, including arginine-vasopressin (AVP), vasoactive intestinal polypeptide (VIP), met-enkephalin (Met-Enk), neuropeptide Y (NPY), as well as oxytocin (OT). The neurotransmitter serotonin (5-HT) and the enzymes tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) were also studied. The camel SCN is a large and elongated nucleus, extending rostrocaudally for 9.55 ± 0.10 mm. Based on histological and immunofluorescence findings, we subdivided the camel SCN into rostral/preoptic (rSCN), middle/main body (mSCN) and caudal/retrochiasmatic (cSCN) divisions. Among mammals, the rSCN is unusual and appears as an assembly of neurons that protrudes from the main mass of the hypothalamus. The mSCN exhibits the triangular shape described in rodents, while the cSCN is located in the retrochiasmatic area. As expected, VIP-immunoreactive (ir) neurons were observed in the ventral part of mSCN. AVP-ir neurons were located in the rSCN and mSCN. Results also showed the presence of OT-ir and TH-ir neurons which seem to be a peculiarity of the camel SCN. OT-ir neurons were either scattered or gathered in one isolated cluster, while TH-ir neurons constituted two defined populations, dorsal parvicellular and ventral magnocellular neurons, respectively. TH colocalized with VIP in some rSCN neurons. Moreover, a high density of Met-Enk-ir, 5-HT-ir and NPY-ir fibers were observed within the SCN. Both the cytoarchitecture and the distribution of neuropeptides are unusual in the camel SCN as compared to other mammals. The presence of OT and TH in the camel SCN suggests their role in the modulation of circadian rhythms and the adaptation to photic and non-photic cues under desert conditions.

Central angiotensin-(1-7) increases osmotic thirst

NEW FINDINGS

What is the central question of this study? The central goal of this study was to understand the effects of central angiotensin-(1-7) on basal and osmotically stimulated water intake in rats. What is the main finding and its importance? This study demonstrated that central administration of angiotensin-(1-7) did not induce thirst in basal conditions but increased water intake after osmotic stimulation, such as water deprivation and salt loading. These results indicate a new function for this peptide, which, in turn, allows for future research on the mechanisms through which angiotensin-(1-7) influences osmotic thirst. Angiotensin-(1-7) [Ang-(1-7)] is generated by type 2 angiotensin-converting enzyme (ACE2) and binds to the MAS receptor. Although it is well known that Ang-(1-7) functionally antagonizes the effects of the classical renin-angiotensin system in several situations, the role of Ang-(1-7) in hydromineral homeostasis is not clear. The aim of this study was to assess the role of Ang-(1-7) on neuroendocrine responses to hyperosmolality in rats. Male Wistar rats were divided into the following three groups: control; 24 h of water deprivation (WD); and 24 h of salt loading (SL; 1.8% NaCl). Intracerebroventricular (i.c.v.) injections of Ang-(1-7) or vehicle were given to assess water intake and plasma concentration of vasopressin. Additionally, the brains from control and WD groups were collected to evaluate gene expression in the subfornical organ (SFO), paraventricular nucleus (PVN) and supraoptic nucleus (SON). It was found that i.c.v. Ang-(1-7) did not change water and salt intake in control rats; however, Ang-(1-7) increased water intake after WD and SL, with no change in salt intake. Plasma vasopressin was not changed by i.c.v. Ang-(1-7) in control or WD rats. Moreover, WD increased Mas gene expression in the SON and PVN, with no changes in Ace2 mRNA levels. In conclusion, Ang-(1-7) increases thirst after osmotic stimuli, indicating that a previous sensitization to its action is necessary. This finding is consistent with the increased Mas gene expression in the PVN and SON after water deprivation.

Brain angiotensin-(1-7)/Mas axis: A new target to reduce the cardiovascular risk to emotional stress

Emotional stress is now considered a risk factor for several diseases including cardiac arrhythmias and hypertension. It is well known that the activation of neuroendocrine and autonomic mechanisms features the response to emotional stress. However, its link to cardiovascular diseases and the regulatory mechanisms involved remain to be further comprehended. The renin-angiotensin system (RAS) plays an important role in homeostasis on all body systems. Specifically in the brain, the RAS regulates a number of physiological aspects. Recent data indicate that the activation of angiotensin-converting enzyme/angiotensin II/AT1 receptor axis facilitates the emotional stress responses. On the other hand, growing evidence indicates that its counterregulatory axis, the angiotensin-converting enzyme 2 (ACE2)/(Ang)iotensin-(1-7)/Mas axis, reduces anxiety and attenuates the physiological responses to emotional stress. The present review focuses on angiotensin-(1-7)/Mas axis as a promising target to attenuate the physiological response to emotional stress reducing the risk of cardiovascular diseases.

Protective axis of the renin-angiotensin system in the brain

The RAS (renin-angiotensin system) is composed of two arms: the pressor arm containing AngII (angiotensin II)/ACE (angiotensin-converting enzyme)/AT1Rs (AngII type 1 receptors), and the depressor arm represented by Ang-(1-7) [angiotensin-(1-7)]/ACE2/Mas receptors. All of the components of the RAS are present in the brain. Within the brain, Ang-(1-7) contributes to the regulation of BP (blood pressure) by acting at regions that control cardiovascular function such that, when Ang-(1-7) is injected into the nucleus of the solitary tract, caudal ventrolateral medulla, paraventricular nucleus or anterior hypothalamic area, a reduction in BP occurs; however, when injected into the rostral ventrolateral medulla, Ang-(1-7) stimulates an increase in BP. In contrast with AngII, Ang-(1-7) improves baroreflex sensitivity and has an inhibitory neuromodulatory role in hypothalamic noradrenergic neurotransmission. Ang-(1-7) not only exerts effects related to BP regulation, but also acts as a cerebroprotective component of the RAS by reducing cerebral infarct size and neuronal apoptosis. In the present review, we provide an overview of effects elicited by Ang-(1-7) in the brain, which suggest a potential role for Ang-(1-7) in controlling the central development of hypertension.

Activation of angiotensin-converting enzyme 2/angiotensin-(1-7)/Mas axis attenuates the cardiac reactivity to acute emotional stress

Recent data indicate the brain angiotensin-converting enzyme/ANG II/AT1 receptor axis enhances emotional stress responses. In this study, we investigated whether its counterregulatory axis, the angiotensin-converting enzyme 2 (ACE2)/ANG-(1-7)/Mas axis, attenuate the cardiovascular responses to acute emotional stress. In conscious male Wistar rats, the tachycardia induced by acute stress (air jet 10 l/min) was attenuated by intravenous injection of ANG-(1-7) [Δ heart rate (HR): saline 136 ± 22 vs. ANG-(1-7) 61 ± 25 beats/min; P < 0.05]. Peripheral injection of the ACE2 activator compound, XNT, abolished the tachycardia induced by acute stress. We found a similar effect after intracerebroventricular injections of either ANG-(1-7) or XNT. Under urethane anesthesia, the tachycardia evoked by the beta-adrenergic agonist was markedly reduced by ANG-(1-7) [ΔHR: saline 100 ± 16 vs. ANG-(1-7) 18 ± 15 beats/min; P < 0.05]. The increase in renal sympathetic nerve activity (RSNA) evoked by isoproterenol was also abolished after the treatment with ANG-(1-7) [ΔRSNA: saline 39% vs. ANG-(1-7) -23%; P < 0.05]. The tachycardia evoked by disinhibition of dorsomedial hypothalamus neurons, a key nucleus for the cardiovascular response to emotional stress, was reduced by ∼45% after intravenous injection of ANG-(1-7). In cardiomyocyte, the incubation with ANG-(1-7) (1 μM) markedly attenuated the increases in beating rate induced by isoproterenol. Our data show that activation of the ACE2/ANG-(1-7)/Mas axis attenuates stress-induced tachycardia. This effect might be either via the central nervous system reducing anxiety level and/or interfering with the positive chronotropy mediated by activation of cardiac β adrenergic receptors. Therefore, ANG-(1-7) might contribute to reduce the sympathetic load to the heart during situations of emotional stress, reducing the cardiovascular risk.

Angiotensin II and angiotensin-(1-7) in paraventricular nucleus modulate cardiac sympathetic afferent reflex in renovascular hypertensive rats

BACKGROUND

The enhanced cardiac sympathetic afferent reflex (CSAR) is involved in the sympathetic activation that contributes to the pathogenesis and progression of hypertension. Activation of AT(1) receptors by angiotension (Ang) II in the paraventricular nucleus (PVN) augments the enhanced CSAR and sympathetic outflow in hypertension. The present study is designed to determine whether Ang-(1-7) in PVN plays the similar roles as Ang II and the interaction between Ang-(1-7) and Ang II on CSAR in renovascular hypertension.

METHODOLOGY/PRINCIPAL FINDINGS

The two-kidney, one-clip (2K1C) method was used to induce renovascular hypertension. The CSAR was evaluated by the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to epicardial application of capsaicin in sinoaortic-denervated and cervical-vagotomized rats with urethane and α-chloralose anesthesia. Either Ang II or Ang-(1-7) in PVN caused greater increases in RSNA and MAP, and enhancement in CSAR in 2K1C rats than in sham-operated (Sham) rats. Mas receptor antagonist A-779 and AT(1) receptor antagonist losartan induced opposite effects to Ang-(1-7) or Ang II respectively in 2K1C rats, but losartan had no effects in Sham rats. Losartan but not the A-779 abolished the effects of Ang II, while A-779 but not the losartan blocked the effects of Ang-(1-7). PVN pretreatment with Ang-(1-7) dose-dependently augmented the RSNA, MAP, and CSAR responses to the Ang II in 2K1C rats. Ang II level, AT(1) receptor and Mas receptor protein expression in PVN increased in 2K1C rats compared with Sham rats but Ang-(1-7) level did not.

CONCLUSIONS

Ang-(1-7) in PVN is as effective as Ang II in enhancing the CSAR and increasing sympathetic outflow and both endogenous Ang-(1-7) and Ang II in PVN contribute to the enhanced CSAR and sympathetic outflow in renovascular hypertension. Ang-(1-7) in PVN potentiates the effects of Ang II in renovascular hypertension.

Angiotensin-(1-7) in paraventricular nucleus modulates sympathetic activity and cardiac sympathetic afferent reflex in renovascular hypertensive rats

Background

Excessive sympathetic activity contributes to the pathogenesis and progression of hypertension. Enhanced cardiac sympathetic afferent reflex (CSAR) is involved in sympathetic activation. This study was designed to determine the roles of angiotensin (Ang)-(1–7) in paraventricular nucleus (PVN) in modulating sympathetic activity and CSAR and its signal pathway in renovascular hypertension.

Methodology/Principal Findings

Renovascular hypertension was induced with two-kidney, one-clip method. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded in sinoaortic-denervated and cervical-vagotomized rats with anesthesia. CSAR was evaluated with the RSNA and MAP responses to epicardial application of capsaicin. PVN microinjection of Ang-(1–7) and cAMP analogue db-cAMP caused greater increases in RSNA and MAP, and enhancement in CSAR in hypertensive rats than in sham-operated rats, while Mas receptor antagonist A-779 produced opposite effects. There was no significant difference in the angiotensin-converting enzyme 2 (ACE2) activity and Ang-(1–7) level in the PVN between sham-operated rats and hypertensive rats, but the Mas receptor protein expression in the PVN was increased in hypertensive rats. The effects of Ang-(1–7) were abolished by A-779, adenylyl cyclase inhibitor SQ22536 or protein kinase A (PKA) inhibitor Rp-cAMP. SQ22536 or Rp-cAMP reduced RSNA and MAP in hypertensive rats, and attenuated the CSAR in both sham-operated and hypertensive rats.

Conclusions

Ang-(1–7) in the PVN increases RSNA and MAP and enhances the CSAR, which is mediated by Mas receptors. Endogenous Ang-(1–7) and Mas receptors contribute to the enhanced sympathetic outflow and CSAR in renovascular hypertension. A cAMP-PKA pathway is involved in the effects of Ang-(1–7) in the PVN.

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.

ACE2/ANG-(1-7)/Mas pathway in the brain: the axis of good

The last decade has seen the discovery of several new components of the renin-angiotensin system (RAS). Among them, angiotensin converting enzyme-2 (ACE2) and the Mas receptor have forced a reevaluation of the original cascade and led to the emergence of a new arm of the RAS: the ACE2/ANG-(1-7)/Mas axis. Accordingly, the new system is now seen as a balance between a provasoconstrictor, profibrotic, progrowth axis (ACE/ANG-II/AT(1) receptor) and a provasodilatory, antifibrotic, antigrowth arm (ACE2/ANG-(1-7)/Mas receptor). Already, this simplistic vision is evolving and new components are branching out upstream [ANG-(1-12) and (pro)renin receptor] and downstream (angiotensin-IV and other angiotensin peptides) of the classical cascade. In this review, we will summarize the role of the ACE2/ANG-(1-7)/Mas receptor, focusing on the central nervous system with respect to cardiovascular diseases such as hypertension, chronic heart failure, and stroke, as well as neurological diseases. In addition, we will discuss the new pharmacological (antagonists, agonists, activators) and genomic (knockout and transgenic animals) tools that are currently available. Finally, we will review the latest data regarding the various signaling pathways downstream of the Mas receptor.

Chronic infusion of angiotensin receptor antagonists in the hypothalamic paraventricular nucleus prevents hypertension in a rat model of sleep apnea

Sleep apnea is characterized by increased sympathetic activity and is associated with systemic hypertension. Angiotensin (Ang) peptides have previously been shown to participate in the regulation of sympathetic tone and arterial pressure in the hypothalamic paraventricular nucleus (PVN) neurons. We investigated the role of endogenous Ang peptides within the PVN to control blood pressure in a rat model of sleep apnea-induced hypertension. Male Sprague-Dawley rats (250 g), instrumented with bilateral guide cannulae targeting the PVN, received chronic infusion of Ang antagonists (A-779, Ang-(1-7) antagonist; losartan and ZD7155, AT(1) antagonists; PD123319, AT(2) receptor antagonist, or saline vehicle). A separate group received an infusion of the GABA(A) receptor agonist (muscimol) to inhibit PVN neuronal activity independent of angiotensin receptors. After cannula placement, rats were exposed during their sleep period to eucapnic intermittent hypoxia (IH; nadir 5% O(2); 5% CO(2) to peak 21% O(2); 0% CO(2)) 20 cycles/h, 7 h/day, for 14 days while mean arterial pressure (MAP) was measured by telemetry. In rats receiving saline, IH exposure significantly increased MAP (+12±2 mm Hg vs. Sham -2±1 mm Hg P<0.01). Inhibition of PVN neurons with muscimol reversed the increase in MAP in IH rats (MUS: -9±4 mm Hg vs. vehicle +12±2 mm Hg; P<0.01). Infusion of any of the Ang antagonists also prevented the rise in MAP induced by IH (A-779: -5±1 mm Hg, losartan: -9±4 mm Hg, ZD7155: -11±4 mm Hg and PD123319: -4±3 mm Hg; P<0.01). Our results suggest that endogenous Ang peptides acting in the PVN contribute to IH-induced increases in MAP observed in this rat model of sleep apnea-induced hypertension.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Advances in the renin angiotensin system focus on angiotensin-converting enzyme 2 and angiotensin-(1-7)

The contribution of the renin angiotensin system to physiology and pathology is undergoing a rapid reconsideration of its mechanisms from emerging new concepts implicating angiotensin-converting enzyme 2 and angiotensin-(1-7) as new elements negatively influencing the vasoconstrictor, trophic, and pro-inflammatory actions of angiotensin II. This component of the system acts to oppose the vasoconstrictor and proliferative effects on angiotensin II through signaling mechanisms mediated by the mas receptor. In addition, a reduced expression of the vasodepressor axis composed by angiotensin-converting enzyme 2 and angiotensin-(1-7) may contribute to the expression of essential hypertension, the remodeling of heart and renal function associated with this disease, and even the physiology of pregnancy and the development of eclampsia.

RNA interference shows interactions between mouse brainstem angiotensin AT1 receptors and angiotensin-converting enzyme 2

Angiotensin (Ang) AT1 receptors and Ang-converting enzymes (ACE and ACE2) are expressed in the dorsal vagal complex (DVC) of the brainstem. The aim of this study was to examine in vivo interactions between brainstem Ang AT1 receptors, ACE and ACE2 using small, hairpin RNA (shRNA) gene-silencing methods. The study takes advantage of the bilateral brainstem expression of renin-angiotensin system (RAS) markers. Adenovirus vectors (Ad, 2.0 x 10(9) c.f.u. ml(-1), 200 nl) carrying interference small hairpin RNA (shRNA) for either AngAT1a (Ad-AT1a-shRNA) or AngAT1b (Ad-AT1b-shRNA) were microinjected into the right side of the brainstem DVC. The Ad-LacZ control was injected into the left side. Brainstems were processed with in situ hybridization and immunochemistry. Results showed that: (1) Ad-AT1a-shRNA downregulated Ang AT1a mRNA by 61.2 +/- 6.8% (P < 0.01) and Ad-AT1b-shRNA downregulated Ang AT1b mRNA by 51.6 +/- 5.2% (P < 0.01); (2) downregulation of Ang AT1a mRNA was associated with decreased ACE2 mRNA expression (decrease of 29.0 +/- 14.5%, P < 0.01), while reduction in Ang Ad-AT1b mRNA had no effect; (3) ACE mRNA expression was not altered by either RNA interference (RNAi) treatment; and (4) immunochemical staining for Ang AT1 receptors, ACE and ACE2 were in agreement with the mRNA changes observed. These results demonstrate the utility of in vivo gene silencing to examine functional specificity. Both Ad-AT1a-shRNA and Ad-AT1b-shRNA induced site- and subtype-specific downregulation of receptor expression. Gene silencing showed that there were interactions between brainstem Ang AT1a receptors and the RAS regulatory enzyme, ACE2.

Brain angiotensin-converting enzymes: role of angiotensin-converting enzyme 2 in processing angiotensin II in mice

Angiotensin (Ang)‐converting enzyme 2 (ACE2) metabolizes Ang II to the vasodilatory peptide Ang(1–7), while neprilysin (NEP) generates Ang(1–7) from Ang I. Experiments used novel Surface Enhanced Laser Desorption Ionization‐Time of Flight (SELDI‐TOF) mass spectroscopic (MS) assays to study Ang processing. Mass spectroscopy was used to measure proteolytic conversion of Ang peptide substrates to their specific peptide products. We compared ACE/ACE2 activity in plasma, brain and kidney from C57BL/6 and NEP^−/− mice. Plasma or tissue extracts were incubated with Ang I or Ang II (1296 or 1045, m/z, respectively), and generated peptides were monitored with MS. Angiotensin‐converting enzyme 2 activity was detected in kidney and brain, but not in plasma. Brain ACE2 activity was highest in hypothalamus. Angiotensin‐converting enzyme 2 activity was inhibited by the specific ACE2 inhibitor, DX600 (10 μm, 99% inhibition), but not by the ACE inhibitor, captopril (10 μm). Both MS and colorimetric assays showed high ACE activity in plasma and kidney with low levels in brain. To extend these findings, ACE measurements were made in ACE overexpressing mice. Angiotensin‐converting enzyme four‐copy mice showed higher ACE activity in kidney and plasma with low levels in hypothalamus. In hypothalamus from NEP^−/− mice, generation of Ang(1–7) from Ang I was decreased, suggesting a role for NEP in Ang metabolism. With Ang II as substrate, there was no difference between NEP^−/− and wild‐type control mice, indicating that other enzymes may contribute to generation of Ang(1–7). The data suggest a predominant role of hypothalamic ACE2 in the processing of Ang II, in contrast to ACE, which is most active in plasma.

Immunofluorescence localization of the receptor Mas in cardiovascular-related areas of the rat brain

The G protein-coupled receptor Mas was recently described as an angiotensin-(1–7) [ANG-(1–7)] receptor. In the present study we evaluated the anatomical localization of Mas using immunofluorescence in the central nervous system of adult male Wistar rats. An abundant labeling was found in the hippocampus, amigdala, anterodorsal thalamic nucleus, cortex, and hypoglossal nucleus. More importantly, a dense ANG-(1–7) receptor Mas immunoreactivity was observed in cardiovascular-related areas of the medulla and forebrain, shown in several previous studies as sites for the action of ANG-(1–7) in the brain. A strong staining was found in the nucleus of the solitary tract, caudal and rostral ventrolateral medulla, inferior olive, parvo and magnocellular portions of the paraventricular hypothalamic nucleus, supraoptic nucleus, and lateral preoptic area. Furthermore, Mas staining was predominantly present in neurons. At the medullary sites, a specific and high-intensity binding for rhodamine-ANG-(1–7) was also shown. The specific ANG-(1–7) binding was completely displaced by the anti-Mas antibody or by the ANG-(1–7) antagonist, A-779. The data presented provide the first anatomical basis for the physiological role of ANG-(1–7)/Mas axis in the modulation of different cardiovascular functions and give new insights for clarifying the role of ANG-(1–7) in the central nervous system.

Regulation of Cardiovascular Control Mechanisms by Angiotensin-(1–7) and Angiotensin-Converting Enzyme 2

Among the molecular forms of angiotensin peptides generated by the action of renin on angiotensinogen (Aogen), both angiotensin II (Ang II) and the amino terminal heptapeptide angiotensin-(1–7) [Ang-(1–7)] are critically involved in the long-term control of tissue perfusion, cell-cell communication, development, and growth. Whereas an impressive body of literature continues to uncover pleiotropic effects of Ang II in the regulation of cell function, research on Ang-(1–7) has a shorter history as it was only 16 yr ago that a biological function for this heptapeptide was first demonstrated in the isolated rat neuro-hypophysial explant preparation (1). On the contrary, the synthesis of angiotonin/ hypertensin (now Ang II) was first obtained in 1957 (2), three decades ahead of the discovery of Ang-(1–7) biological properties.

Isolation and identification of proangiotensin-12, a possible component of the renin–angiotensin system

The renin-angiotensin (RA) system plays an important role in regulating blood pressure and fluid balance. In the search for bioactive peptides with an antibody binding to the N-terminal portion of angiotensin II (Ang II), we isolated a new angiotensinogen-derived peptide from the rat small intestine. Consisting of 12 amino acids, this peptide was termed proangiotensin-12 based on its possible role of an Ang II precursor. Proangiotensin-12 constricted aortic strips and, when infused intravenously, raised blood pressure in rats, while both the vasoconstrictor and pressor response to proangiotensin-12 were abolished by captopril and by CV-11974, an Ang II type I receptor blocker. Proangiotensin-12 is abundant in a wide range of organs and tissues including the small intestine, spleen, kidneys, and liver of rats. The identification of proangiotensin-12 suggests a processing cascade of the RA system, different from the cleavage of angiotensinogen to Ang I by renin.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Blockade of endogenous angiotensin-(1-7) in the hypothalamic paraventricular nucleus reduces renal sympathetic tone

In this study, we tested the hypothesis that angiotensin-(1-7) [Ang-(1-7)] acting in the neurons of paraventricular hypothalamic nucleus (PVN) contributes to the maintenance of sympathetic activity and blood pressure. For this purpose, the effects of microinjection of the A-779, the receptor Mas antagonist, into the PVN on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were evaluated. In rats anesthetized with urethane (1.2 to 1.4 g/kg IP), bilateral microinjections of A-779 (0.1 nmol) into the PVN resulted in a selective and significant decrease in RSNA (-26+/-6% versus -2+/-3% vehicle; saline 0.9%). The magnitude of the decrease in RSNA produced by A-779 was comparable to that observed after microinjection of muscimol (1 nmol; -26+/-4%), a powerful neuronal inhibitor. A higher dose of A-779 (1 nmol) caused a reduction in RSNA (-21+/-4%) that was comparable in magnitude to the reduction observed with the lower dose. When compared with vehicle solution, no significant changes in MAP or HR were observed with both doses of A-779 tested. A decrease in RSNA was also observed after microinjections into the PVN of the angiotensin II type 2 (AT2) receptor antagonist PD123319 (1 nmol; -18+/-4%). Microinjections of the AT1 antagonist losartan but not CV 11974 reduced MAP without changing RSNA. These results suggest that Ang-(1-7) Mas receptors and AT2 receptors in the PVN neurons play a role in mediating the tonic maintenance of RSNA.

Organ-specific distribution of ACE2 mRNA and correlating peptidase activity in rodents

Biochemical analysis revealed that angiotensin-converting enzyme related carboxy-peptidase (ACE2) cleaves angiotensin (Ang) II to Ang-(1-7), a heptapeptide identified as an endogenous ligand for the G protein-coupled receptor Mas. No data are currently available that systematically describe ACE2 distribution and activity in rodents. Therefore, we analyzed the ACE2 expression in different tissues of mice and rats on mRNA (RNase protection assay) and protein levels (immunohistochemistry, ACE2 activity, western blot). Although ACE2 mRNA in both investigated species showed the highest expression in the ileum, the mouse organ exceeded rat ACE2, as also demonstrated in the kidney and colon. Corresponding to mRNA, ACE2 activity was highest in the ileum and mouse kidney but weak in the rat kidney, which was also confirmed by immunohistochemistry. Contrary to mRNA, we found weak activity in the lung of both species. Our data demonstrate a tissue- and species-specific pattern for ACE2 under physiological conditions.

Vasopressinergic hypothalamic neurons are recruited during the audiogenic seizure of WARs

The Wistar Audiogenic Rat (WAR) is a genetic model of reflex epilepsy with seizures induced by high-intensity sound stimulation (120 dB SPL). In spite of the known neural substrates involved in WAR seizure phenotype, neuroendocrine hypothalamic neurons were never investigated. In this work, AVP immunohistochemistry in the hypothalamus and radioimmunoassay (RIA) in plasma and in hypothalamic and hypophysial tissues were performed on both controls and WARs in order to evaluate the dynamics of AVP release due to seizure induction. Susceptible animals (WARs) displayed at least tonic-clonic convulsions followed by clonic spasms, while resistant Wistar rats (R) had no convulsive behavior. Animals were sacrificed at 3 instances: basal condition (without stimulus) and at 3 and 10 min after sound stimulation. For the immunohistochemistry AVP study, brains were harvested and processed by the avidin-biotin-peroxidase detection method. Optic densitometry was used for quantifying AVP labeling in supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. SON presented higher densitometry levels (%D--relative to background) for both WARs and R when compared to PVN. Nevertheless, both nuclei presented a marked decrease, referenced to basal levels, in %D for WARs at 3 min (approximately 35%) against a discrete change for R (approximately 90%). RIA results were significantly higher in the hypophysis of WARs when compared to R rats, at 3 min. Also, at 3 min, plasma AVP in WARs (89.32 +/- 24.81 pg/mL) were higher than in R (12.01 +/- 2.39 pg/mL). We conclude, based on the AVP releasing profiles, that vasopressinergic hypothalamic neurons are recruited during the audiogenic seizure of WARs.

Angiotensin-(1-7) stimulates water transport in rat inner medullary collecting duct: evidence for involvement of vasopressin V2 receptors

The peptide angiotensin-(1-7) [Ang-(1-7)] is known to enhance water transport in rat inner medullary collecting duct (IMCD). The aim of this study was to determine the mechanism of the Ang-(1-7) effect on osmotic water permeability (Pf). Pf was measured in the normal rat IMCD perfused in vitro in presence of agonists [Ang-(1-7), arginine vasopressin (AVP) and Ang-(3-8)], and antagonists of the angiotensin and the vasopressin cascade. Ang-(1-7), but not Ang-(3-8), increased Pf significantly. The effect of Ang-(1-7) on Pf was abolished by its selective antagonist, A-779, added before or after Ang-(1-7). Prostaglandin E2 and the protein kinase A inhibitor H8 also blocked the Ang-(1-7) effect. Blockade of vasopressin V1 receptors by antagonists did not change the Ang-(1-7) effect, but pre-treatment with a V2 antagonist abolished the effect of Ang-(1-7) on Pf. Similarly, pre-treatment with A-779 inhibited AVP's effect on Pf. Forskolin-stimulated Pf was blocked both by A-779 and by the V2 antagonist. Finally, Ang-(1-7) increased cAMP levels in fresh IMCD cell suspensions whilst the forskolin-stimulated cAMP synthesis was decreased by A-779 and the V2 antagonist. These data provide evidence that Ang-(1-7) interacts via its receptor with the AVP V2 system through a mechanism involving adenylate-cyclase activation.

Characterization of a new selective antagonist for angiotensin-(1-7), D-pro7-angiotensin-(1-7)

Angiotensin-(1-7) [Ang-(1-7)] has biological actions that can often be distinguished from those of angiotensin II (Ang II). Recent studies indicate that the effects of Ang-(1-7) are mediated by specific receptor(s). We now report the partial characterization of a new antagonist selective for Ang-(1-7), D-Pro7-Ang-(1-7). D-Pro7-Ang-(1-7) (50 pmol) inhibited the hypertensive effect induced by microinjection of Ang-(1-7) [4+/-1 vs 21+/-2 mm Hg, 25 pmol Ang-(1-7) alone] into the rostral ventrolateral medulla without changing the effect of Ang II (16+/-2.5 vs 19+/-2.5 mm Hg after 25 pmol Ang II alone). At 10(-7) mol/L concentration, it completely blocked the endothelium-dependent vasorelaxation produced by Ang-(1-7) (10(-10) to 10(-6) mol/L) in the mouse aorta. The antidiuresis produced by Ang-(1-7) (40 pmol/100 g body weight) in water-loaded rats was also blocked by its analog [1 microg/100 g body weight; 3.08+/-0.8 vs 1.27+/-0.33 mL in Ang-(1-7)-treated rats]. D-Pro7-Ang-(1-7) at a molar ratio of 40:1 did not change the hypotensive effect of bradykinin. Moreover, D-Pro7-Ang-(1-7) did not affect the dipsogenic effect produced by intracerebroventricular administration of Ang II (11.4+/-1.15 vs 8.8+/-1.2 mL/h after Ang II) and did not show any demonstrable angiotensin-converting enzyme inhibitory activity in assays with the synthetic substrate Hip-His-Leu and rat plasma as a source of enzyme. Autoradiography studies with 125I-Ang-(1-7) in mouse kidney slices showed that D-Pro7-Ang-(1-7) competed for the binding of Ang-(1-7) to the cortical supramedullary region. In Chinese hamster ovary cells stably transfected with the AT1 receptor subtype, D-Pro7-Ang-(1-7) did not compete for the specific binding of 125I-Ang-II in concentrations up to 10(-6) mol/L. There was also no significant displacement of Ang II binding to angiotensin type 2 receptors in membrane preparations of adrenal medulla. These data indicate that D-Pro7-Ang-(1-7) is a selective antagonist for Ang-(1-7), which can be useful to clarify the functional role of this heptapeptide.

Angiotensin-(1-7) stimulates oxidative stress in rat kidney

The effect of two different doses of angiotensin-(1-7) and angiotensin II on the oxidative stress generation was analyzed in rat kidney. Animals were injected intraperitoneally with a single dose of angiotensin-(1-7) or angiotensin II (20 or 50 nmol/kg body weight) and killed 3 h after injection. Production of thiobarbituric acid reactive substances (TBARS), measured as indicator of oxidative stress induction, was significantly increased in rat kidney after Ang-(1-7) administration up to 30% and 50% over controls, at 20 and 50 nmol/kg, respectively. Reduced glutathione (GSH), the most important soluble antioxidant defense in mammalian cells, showed a significant decrease of 13% and 20% at 20 and 50 nmol/kg of angiotensin-(1-7), respectively. When the antioxidant enzyme activities were determined, it was found that catalase activity was not altered by the assayed angiotensin-(1-7) doses while superoxide dismutase and glutathione peroxidase activities were significantly reduced by injection of 20 nmol/kg (34% and 13%, with respect to controls) and 50 nmol/kg of angiotensin-(1-7) (54% and 22%, respectively). In contrast, angiotensin II injections did not produce significant changes neither in TBARS levels nor in soluble and enzymatic defense parameters at the two doses used in this work. These results suggest that angiotensin-(1-7) is undoubtedly related to oxidative stress induction.

Modulation of the baroreflex control of heart rate by angiotensin-(1-7) at the nucleus tractus solitarii of normotensive and spontaneously hypertensive rats

OBJECTIVES

In the present study, we evaluated the effect of angiotensin-(1-7) [Ang-(1-7)] and its selective antagonist, D-Ala7-Ang-(1-7) (A-779), at the nucleus tractus solitarii (nTS), in the modulation of the bradycardic component of the baroreceptor reflex.

METHODS

Mean arterial pressure (MAP) and heart rate were continuously recorded. Reflex changes in heart rate elicited by bolus injection of graded doses of phenylephrine were evaluated before and after bilateral microinjection (glass micropipette) of Ang-(1-7) (10 pmol or 25 pmol), A-779 (50 pmol) or saline (vehicle) into the nTS of urethane anesthetized male Wistar rats or spontaneously hypertensive rats (SHR). The averaged ratio between reflex changes in heart rate and changes in MAP was used as index of baroreflex sensitivity.

RESULTS

Microinjection of Ang-(1-7) into the nTS elicited significant decreases in MAP and heart rate in both Wistar and SHR. While the decrease in MAP was similar in both strains, the changes in heart rate were smaller in SHR. A-779 produced small changes in MAP and heart rate that were no different from those induced by saline. After microinjection of 10 pmol of Ang-(1-7) into the nTS of normotensive rats, there was a significant increase in baroreflex sensitivity. In SHR, only the microinjection of a higher dose (25 pmol) of Ang-(1-7) produced a significant increase in baroreflex sensitivity. A significant reduction inbaroreflex sensitivity was observed after microinjection of A-779 (50 pmol) in both strains.

CONCLUSIONS

These results indicate that Ang-(1-7) exerts a tonic modulatory effect on the baroreflex control of heart rate at the nTS, probably through a non-AT1 non-AT2 receptor subtype. In addition, our data showed a reduced sensitivity to Ang-(1-7) at the nTS of SHR, that could be accounting, at least in part, for the decreased baroreflex sensitivity present in this model of hypertension.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Angiotensin-(1-7): an update

The renin-angiotensin system is a major physiological regulator of arterial pressure and hydro-electrolyte balance. Evidence has now been accumulated that in addition to angiotensin (Ang) II other Ang peptides [Ang III, Ang IV and Ang-(1-7)], formed in the limited proteolysis processing of angiotensinogen, are importantly involved in mediating several actions of the RAS. In this article we will review our knowledge of the biological actions of Ang-(1-7) with focus on the puzzling aspects of the mediation of its effects and the interaction Ang-(1-7)-kinins. In addition, we will attempt to summarize the evidence that Ang-(1-7) takes an important part of the mechanisms aimed to counteract the vasoconstrictor and proliferative effects of Ang II.

Angiotensin peptides acting at rostral ventrolateral medulla contribute to hypertension of TGR(mREN2)27 rats

We have previously demonstrated that microinjections of the selective angiotensin-(1-7) [ANG-(1-7)] antagonist, A-779, into the rostral ventrolateral medulla (RVLM) produces a significant fall in mean arterial pressure (MAP) and heart rate (HR) in both anesthetized and conscious rats. In contrast, microinjection of angiotensin II (ANG II) AT(1) receptor antagonists did not change MAP in anesthetized rats and produced dose-dependent increases in MAP when microinjected into the RVLM of conscious rats. In the present study, we evaluated whether endogenous ANG-(1-7) and ANG II acting at the RVLM contribute to the hypertension of transgenic rats harboring the mouse renin Ren-2 gene, TGR(mREN2)27. Unilateral microinjection of A-779 (0.1 nmol) produced a significant fall in MAP (-25 +/- 5 mmHg) and HR (-57 +/- 20 beats/min) of awake TGR rats. The hypotensive effect was greater than that observed in Sprague-Dawley (SD) rats (-9 +/- 2 mmHg). Microinjection of the AT(1) antagonist CV-11974 (0.2 nmol) produced a fall in MAP in TGR rats (-14 +/- 4 mmHg), contrasting with the pressor effect observed in SD rats (33 +/- 9 mmHg). These results indicate that endogenous ANG-(1-7) exerts a significant pressor action in the RVLM, contributing to the hypertension of TGR(mREN2)27 transgenic rats. The role of ANG II at the RVLM seems to be dependent on its endogenous level in this area.

Effect of angiotensin-(1-7) on ATPase activities in several tissues

The present investigation was undertaken to determine whether Ang-(1-7) is able to modify ATPase activities in membrane fractions prepared from several tissues. In the presence of 10(-6) M Ang-(1-7), total (Na , K+, Mg2+)-ATPase activity decreased 31% in rat atrium and 13% in sheep atrium but was unmodified in sheep liver, rat ventricle or crude brain membranes. In rat brain synaptosomal membranes, Ang-(1-7) at 10(-8) and 10(-7) M concentrations activated Na+, K+-ATPase 20 and 24%, respectively. Rat kidney Na+, K+-ATPase activity decreased roughly 40-70% with 10(-10)-10(-6) M Ang-(1-7)), but increased 22% with 10(-12) M peptide concentration, thus indicating a biphasic effect. Our findings showing that ATPase from several tissues responds differently to Ang-(1-7) are attributable to enzyme tissue specificity.

Angiotensin receptors in the nervous system

In addition to its traditional role as a circulating hormone, angiotensin is also involved in local functions through the activity of tissue renin-angiotensin systems that occur in many organs, including the brain. In the brain, both systemic and presumptive neurally derived angiotensin and angiotensin metabolites act through specific receptors to modulate many functions. This review examines the distribution of these specific angiotensin receptors and discusses evidence regarding the function of angiotensin peptides in various brain regions. Angiotensin AT1 and AT2 receptors occur in characteristic distributions that are highly correlated with the distribution of angiotensin-like immunoreactivity in nerve terminals. Acting through the AT1 receptor in the brain, angiotensin has effects on fluid and electrolyte homeostasis, neuroendocrine systems, autonomic pathways regulating cardiovascular function and behavior. Angiotensin AT1 receptors are also found in many afferent and efferent components of the peripheral autonomic nervous system. The role of the AT2 receptor in the brain is less well understood, although recent knockout studies point to an involvement with behavioral and cardiovascular functions. In addition to the AT1 and AT2 receptors, receptors for other fragments of angiotensin have been proposed. The AT4 binding site, which binds angiotensin, has a widespread distribution in the brain quite distinct from that of the AT1 and AT2 receptors. It is associated with many cholinergic neuronal groups and also several sensory nuclei, but its function remains to be determined. Our discovery that another brain-derived peptide binds to the AT4 binding site in the brain and may represent the native ligand is discussed. Overall, the distribution of angiotensin receptors in the brain indicate that they play diverse and important physiological roles in the nervous system.

Effect of selective angiotensin antagonists on the antidiuresis produced by angiotensin-(1-7) in water-loaded rats

In the present study we evaluated the nature of angiotensin receptors involved in the antidiuretic effect of angiotensin-(1-7) (Ang-(1-7)) in water-loaded rats. Water diuresis was induced in male Wistar rats weighing 280 to 320 g by water load (5 ml/100 g body weight by gavage). Immediately after water load the rats were treated subcutaneously with (doses are per 100 g body weight): 1) vehicle (0.05 ml 0.9% NaCl); 2) graded doses of 20, 40 or 80 pmol Ang-(1-7); 3) 200 nmol Losartan; 4) 200 nmol Losartan combined with 40 pmol Ang-(1-7); 5) 1.1 or 4.4 nmol A-779; 6) 1.1 nmol A-779 combined with graded doses of 20, 40 or 80 pmol Ang-(1-7); 7) 4.4 nmol A-779 combined with graded doses of 20, 40 or 80 pmol Ang-(1-7); 8) 95 nmol CGP 42112A, or 9) 95 nmol CGP 42112A combined with 40 pmol Ang-(1-7). The antidiuretic effect of Ang-(1-7) was associated with an increase in urinary Na+ concentration, an increase in urinary osmolality and a reduction in creatinine clearance (CCr: 0.65 +/- 0.04 ml/min vs 1.45 +/- 0.18 ml/min in vehicle-treated rats, P < 0.05). A-779 and Losartan completely blocked the effect of Ang-(1-7) on water diuresis (2.93 +/- 0.34 ml/60 min and 3.39 +/- 0.58 ml/60 min, respectively). CGP 42112A, at the dose used, did not modify the antidiuretic effect of Ang-(1-7). The blockade produced by Losartan was associated with an increase in CCr and with an increase in sodium and water excretion as compared with Ang-(1-7)-treated rats. When Ang-(1-7) was combined with A-779 there was an increase in CCr and natriuresis and a reduction in urine osmolality compared with rats treated with Ang-(1-7) alone. The observation that both A-779, which does not bind to AT1 receptors, and Losartan blocked the effect of Ang-(1-7) suggests that the kidney effects of Ang-(1-7) are mediated by a non-AT1 angiotensin receptor that is recognized by Losartan.

Angiotensin-(1-7) immunoreactivity in the hypothalamus of the (mRen-2d)27 transgenic rat

The distribution of angiotensin-(1-7) immunoreactive neurons was compared to those of vasopressin-(VP) and oxytocin-(OT) immunoreactive (IR) neurons in the hypothalamus of adult (mRen-2d)27 transgenic hypertensive and Sprague-Dawley rats. In both strains, angiotensin (Ang)-(1-7)-IR cells were found in the supraoptic nucleus (SON), and in the anterior (ap-), medial (mp-), and lateral (lp-) parvocellular, and posterior magnocellular (pm-) subdivisions of the paraventricular (PVN) nucleus. Three-dimensional reconstructions showed that cells immunoreactive to Ang-(1-7) and VP were specifically co-distributed in the SON and in the pmPVN. Double-labeling neurons for both peptides revealed that both Ang-(1-7) and VP were colocalized in a subpopulation of neurons in the pmPVN and SON. In combination with previous studies, our results suggest that Ang-(1-7) and VP are colocalized, co-released and may have a combined action at a common target. In addition, the introduction of the mouse submandibular renin (mRen-2d) transgene into Sprague-Dawley rats does not appear to have altered the fundamental organization of hypothalamic peptide systems involved in fluid homeostasis.

Diuresis and natriuresis produced by long term administration of a selective Angiotensin-(1-7) antagonist in normotensive and hypertensive rats

In this study we evaluated the renal effects of chronic administration of the selective Angiotensin-(1-7)[Ang-(1-7)] antagonist, A-779, in normotensive and spontaneously hypertensive rats (SHR). Male adult SHR and Wistar rats were housed in metabolic cages with tap water and standard chow, for three-five days before starting infusion (Alzet osmotic mini-pumps) of A-779 (Wistar: 1 microg/h, n = 9; 2.5 microg/h, n = 6; SHR:2.5 microg/h, n = 6) or vehicle (0.9% NaCl - 1 microl/h, n = 7 and n = 10 for SHR and Wistar rats, respectively). Urine volume, water and food intake and urinary Na+ were measured daily. On the last day of infusions mean arterial pressure (MAP) was recorded and urine and blood samples were collected to determine renal function parameters. Chronic infusion of A-779 produced a sustained increase in diuresis in normotensive rats [seventh day values: 0.75+/-0.08 ml/h (1 microg/h) and 0.94+/-0.13 ml/h (2.5 microg/h) vs. 0.42 + 0.03 ml/h for the control group, P<0.05] associated to a dose-dependent increase in the creatinine clearance. In SHR, diuresis increased significantly after chronic infusion of A-779 (fifth day values: 0.44 + 0.06 ml/h vs. 0.25+/-0.04 ml/h for the control group, P<0.05), without changes in creatinine clearance. Infusion of A-779 in normotensive rats produced a decrease in water reabsorption. A-779 infusion also produced a dose-dependent increase in urinary Na+ excretion (1.49 + 0.14 mEq, 1 microg/h vs. 2.37+/-0.22 mEq, 2.5 microg/h, P<0.05), in Wistar rats, without modifying the fractional excretion of Na+. In SHR, urinary Na+ excretion was also increased by A-779 (2.21+/-0.46 mEq vs. 0.94+/-0.22 mEq for the control group, P<0.05). No significant changes in blood pressure were observed. These findings suggest that endogenous Ang-(l-7) participates in the control of hydroelectrolyte balance by modulating water excretion, acting at tubular and glomerular sites.

Renal actions of angiotensin-(1-7)

The heptapeptide angiotensin-(1-7) is considered to be a biologically active endproduct of the renin-angiotensin system. This angiotensin, which is devoid of the most known actions of angiotensin II such as induction of drinking behavior and vasoconstriction, has several selective effects in the brain and periphery. In the present article we briefly review recent evidence for a physiological role of angiotensin-(1-7) in the control of hydroelectrolyte balance.

Lateral hypothalamic stimulation inhibits dentate granule cell LTP: direct connections

We discovered that angiotensin II (Ang II) applied directly to the dentate gyrus inhibited LTP induction in medial perforant path-dentate granule cell synapses and that the inhibition can be blocked by losartan, an Ang II AT1 receptor specific antagonist. In the first part of this study we found that electrical stimulation of the lateral hypothalamus (LH) inhibits LTP in these synapses and the inhibition can be blocked by pretreating the animals with losartan, indicating that LH angiotensin-containing neurons project to the dentate gyrus. Results of the second part of the study demonstrate clearly that some angiotensin-containing LH neurons project directly to dentate granule cells. LH neurons were identified by retrograde tracers applied to the granule cell layer. Double-labeled neurons containing angiotensin and HRP were sparsely distributed and both fusiform and multipolar LH neurons appeared in a small cluster lateral and ventral to the fornix at the level of the paraventricular nucleus. Large numbers of angiotensin staining neurons were observed in the hypothalamus. Results support our hypothesis that some angiotensin containing LH neurons project directly to the dentate gyrus.

Changes in the baroreflex control of heart rate produced by central infusion of selective angiotensin antagonists in hypertensive rats

We have recently shown that an angiotensin-(1-7) [Ang-(I-7)] analogue, D-Ala7-Ang-(1-7) (A-779), is a selective Ang-(1-7) antagonist with no significant action on angiotensin type 1 or type 2 receptors. The availability of selective angiotensin antagonists prompted us to evaluate the role of Ang-(1-7) and Ang II on central modulation of the baroreflex control of heart rate in normotensive Wistar rats and spontaneously hypertensive rats (SHR). Blood pressure recording and reflex changes in heart rate elicited by intravenous bolus injections of phenylephrine were made before and within 1 and 3 hours of intracerebroventricular (ICV, lateral ventricle) infusion of saline (8 microL/h), A-779 (4 microg/h), DuP 753 (100 microg/h), or CGP 42112A (50 mu g/h) in conscious rats. The slope of the relationship between changes in pulse interval versus changes in mean arterial pressure was used as an index of the baroreflex control of heart rate. ICV infusion of saline or any of the antagonists did not significantly change basal levels of mean arterial pressure and heart rate in SHR (170 +/- 6 mm Hg nd 360 +/- 9 beats per minute, respectively; n = 29) or Wistar rats (108 +/- 2 mm Hg and 377 +/- 6 beats per minute, respectively; n=29). Three hours of ICV infusion of A-779 markedly decreased baroreflex sensitivity in Wistar rats (from a basal slope of 1.09 +/- O.3). In contrast, A-779 did not significantly alter the depressed baroreflex sensitivity of SHR (0.61 +/- O.l). ICV infusion of DuP 753 produced a significant increase (60 percent) in baroreflex control of heart rate in both Wistar rats and SHR. Saline or CGP 42112A infusions did not significantly alter baroreflex control of heart rate. These results suggest that endogenous Ang II and Ang-(1-7) are differentially affecting central baroreflex modulation, acting probably through distinct receptor subtypes. Although the central Ang II inhibitory effect is mediated by the type 1 receptor subtype, the facilitatory effect of Ang-(1-7) might be mediated by a different, unidentified receptor.

Effect of acute volume expansion associated with salt load on the profile of plasma angiotensins in rats

We measured the changes produced in renin and the peptide components of the circulating renin-angiotensin system by acute volume expansion alone or associated with salt load in rats. With both maneuvers, there was a decrease of plasma renin activity (PRA) and plasma angiotensin (Ang) concentrations, but the changes were more pronounced in the salt-load group. In contrasting to Ang I and Ang II, plasma Ang-(1-7) concentration decreased significantly only within 60 min of 5.0% NaCl infusion (19.5 +/- 2.9 vs 30.5 +/- 1.9 pg/ml in the control group). As expected, plasma vasopressin increased markedly in the animals submitted to acute salt load (144.0 +/- 42.0 vs 5.0 +/- 0.2 pg/ml in the control group). The dissociation between the changes in plasma Ang-(1-7) and that of other plasma angiotensins is consistent with our previous studies using chronic salt load, and suggests that during acute volume expansion associated with salt load Ang I is preferentially converted to Ang-(1-7). In contrast to vasopressin, however, plasma Ang-(1-7) concentration increased only after chronic salt load, suggesting that this angiotensin may be more involved with long-term mechanisms of control of hydromineral balance.