Protective Angiotensin Type 2 Receptors in the Brain and Hypertension

Sex-Dependent Effects of Angiotensin Type 2 Receptor-Expressing Medial Prefrontal Cortex Interneurons in Fear Extinction Learning

Background

The renin-angiotensin system has been identified as a potential therapeutic target for posttraumatic stress disorder, although its mechanisms are not well understood. Brain angiotensin type 2 receptors (AT2Rs) are a subtype of angiotensin II receptors located in stress and anxiety-related regions, including the medial prefrontal cortex (mPFC), but their function and mechanism in the mPFC remain unexplored. Therefore, we used a combination of imaging, cre/lox, and behavioral methods to investigate mPFC-AT2R-expressing neurons in fear and stess related behavior.

Methods

To characterize mPFC-AT2R-expressing neurons in the mPFC, AT2R-Cre/tdTomato male and female mice were used for immunohistochemistry. mPFC brain sections were stained with glutamatergic or interneuron markers, and density of AT2R+ cells and colocalization with each marker were quantified. To assess fear-related behaviors in AT2R-flox mice, we selectively deleted AT2R from mPFC neurons using a Cre-expressing adeno-associated virus. Mice then underwent Pavlovian auditory fear conditioning, elevated plus maze, and open field testing.

Results

Immunohistochemistry results revealed that AT2R was densely expressed throughout the mPFC and primarily expressed in somatostatin interneurons in a sex-dependent manner. Following fear conditioning, mPFC-AT2R Cre-lox deletion impaired extinction and increased exploratory behavior in female but not male mice, while locomotion was unaltered by mPFC-AT2R deletion in both sexes.

Conclusions

These results identify mPFC-AT2R+ neurons as a novel subgroup of somatostatin interneurons and reveal their role in regulating fear learning in a sex-dependent manner, potentially offering insights into novel therapeutic targets for posttraumatic stress disorder.

Impacts of SARS-CoV-2 on brain renin angiotensin system related signaling and its subsequent complications on brain: A theoretical perspective

Cellular ACE2 (cACE2), a vital component of the renin-angiotensin system (RAS), possesses catalytic activity to maintain AngII and Ang 1-7 balance, which is necessary to prevent harmful effects of AngII/AT2R and promote protective pathways of Ang (1-7)/MasR and Ang (1-7)/AT2R. Hemostasis of the brain-RAS is essential for maintaining normal central nervous system (CNS) function. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a viral disease that causes multi-organ dysfunction. SARS-CoV-2 mainly uses cACE2 to enter the cells and cause its downregulation. This, in turn, prevents the conversion of Ang II to Ang (1-7) and disrupts the normal balance of brain-RAS. Brain-RAS disturbances give rise to one of the pathological pathways in which SARS-CoV-2 suppresses neuroprotective pathways and induces inflammatory cytokines and reactive oxygen species. Finally, these impairments lead to neuroinflammation, neuronal injury, and neurological complications. In conclusion, the influence of RAS on various processes within the brain has significant implications for the neurological manifestations associated with COVID-19. These effects include sensory disturbances, such as olfactory and gustatory dysfunctions, as well as cerebrovascular and brain stem-related disorders, all of which are intertwined with disruptions in the RAS homeostasis of the brain.

Sex-dependent effects of angiotensin type 2 receptor expressing medial prefrontal cortex (mPFC) interneurons in fear extinction learning

Background

The renin-angiotensin system (RAS) has been identified as a potential therapeutic target for PTSD, though its mechanisms are not well understood. Brain angiotensin type 2 receptors (AT2Rs) are a subtype of angiotensin II receptors located in stress and anxiety-related regions, including the medial prefrontal cortex (mPFC), but their function and mechanism in the mPFC remain unexplored. We therefore used a combination of imaging, cre/lox, and behavioral methods to investigate mPFC-AT2R-expressing neuron involvement in fear learning.

Methods

To characterize mPFC-AT2R-expressing neurons in the mPFC, AT2R-Cre/td-Tomato male and female mice were used for immunohistochemistry (IHC). mPFC brain sections were stained with glutamatergic or interneuron markers, and density of AT2R+ cells and colocalization with each marker was quantified. To assess fear-related behaviors in AT2R-flox mice, we selectively deleted AT2R from mPFC neurons using an AAV-Cre virus. Mice then underwent Pavlovian auditory fear conditioning, approach/avoidance, and locomotion testing.

Results

IHC results revealed that AT2R is densely expressed in the mPFC. Furthermore, AT2R is primarily expressed in somatostatin interneurons in females but not males. Following fear conditioning, mPFC-AT2R deletion impaired extinction in female but not male mice. Locomotion was unaltered by mPFC-AT2R deletion in males or females, while AT2R-deleted females had increased exploratory behavior.

Conclusion

These results lend support for mPFC-AT2R+ neurons as a novel subgroup of somatostatin interneurons that influence fear extinction in a sex-dependent manner. This furthers underscores the role of mPFC in top-down regulation and a unique role for peptidergic (ie., angiotensin) mPFC regulation of fear and sex differences.

The Renin Angiotensin System as a Therapeutic Target in Traumatic Brain Injury

Traumatic brain injury (TBI) is a major public health problem, with limited pharmacological options available beyond symptomatic relief. The renin angiotensin system (RAS) is primarily known as a systemic endocrine regulatory system, with major roles controlling blood pressure and fluid homeostasis. Drugs that target the RAS are used to treat hypertension, heart failure and kidney disorders. They have now been used chronically by millions of people and have a favorable safety profile. In addition to the systemic RAS, it is now appreciated that many different organ systems, including the brain, have their own local RAS. The major ligand of the classic RAS, Angiotensin II (Ang II) acts predominantly through the Ang II Type 1 receptor (AT1R), leading to vasoconstriction, inflammation, and heightened oxidative stress. These processes can exacerbate brain injuries. Ang II receptor blockers (ARBs) are AT1R antagonists. They have been shown in several preclinical studies to enhance recovery from TBI in rodents through improvements in molecular, cellular and behavioral correlates of injury. ARBs are now under consideration for clinical trials in TBI. Several different RAS peptides that signal through receptors distinct from the AT1R, are also potential therapeutic targets for TBI. The counter regulatory RAS pathway has actions that oppose those stimulated by AT1R signaling. This alternative pathway has many beneficial effects on cells in the central nervous system, bringing about vasodilation, and having anti-inflammatory and anti-oxidative stress actions. Stimulation of this pathway also has potential therapeutic value for the treatment of TBI. This comprehensive review will provide an overview of the various components of the RAS, with a focus on their direct relevance to TBI pathology. It will explore different therapeutic agents that modulate this system and assess their potential efficacy in treating TBI patients.

Recent Advances in the Endogenous Brain Renin-Angiotensin System and Drugs Acting on It

The RAS (renin-angiotensin system) is the part of the endocrine system that plays a prime role in the control of essential hypertension. Since the discovery of brain RAS in the seventies, continuous efforts have been put by the scientific committee to explore it more. The brain has shown the presence of various components of brain RAS such as angiotensinogen (AGT), converting enzymes, angiotensin (Ang), and specific receptors (ATR). AGT acts as the precursor molecule for Ang peptides—I, II, III, and IV—while the enzymes such as prorenin, ACE, and aminopeptidases A and N synthesize it. AT1, AT2, AT4, and mitochondrial assembly receptor (MasR) are found to be plentiful in the brain. The brain RAS system exhibits pleiotropic properties such as neuroprotection and cognition along with regulation of blood pressure, CVS homeostasis, thirst and salt appetite, stress, depression, alcohol addiction, and pain modulation. The molecules acting through RAS predominantly ARBs and ACEI are found to be effective in various ongoing and completed clinical trials related to cognition, memory, Alzheimer's disease (AD), and pain. The review summarizes the recent advances in the brain RAS system highlighting its significance in pathophysiology and treatment of the central nervous system-related disorders.

Brain Areas Involved in Modulating the Immune Response Participating in Hypertension and Its Target Organ Damage

Significance: Hypertension is a multifactorial disease ensuing from the continuous challenge imposed by several risk factors on the cardiovascular system. Classically known pathophysiological alterations associated with hypertension comprise neurogenic mechanisms dysregulating the autonomic nervous system (ANS), vascular dysfunction, and excessive activation of the renin angiotensin system. During the past few years, a considerable number of studies indicated that immune activation and inflammation also have an important role in the onset and maintenance of hypertension. Critical Issues: On these premises, it has been necessary to reconsider the pathophysiological mechanisms underlying hypertension development, taking into account the potential interactions established between classically known determinants of high blood pressure and the immune system. Recent Advances: Interestingly, central nervous system areas controlling cardiovascular functions are enriched with Angiotensin II receptors. Observations showing that these brain areas are crucial for mediating peripheral ANS and immune responses were suggestive of a critical role of neuroimmune interactions in hypertension. In fact, the ANS, characterized by an intricate network of afferent and efferent fibers, represents an intermediate between the brain and peripheral responses that are essential for blood pressure regulation. Future Directions: In this review, we will summarize studies showing how specific brain areas can modulate immune responses that are involved in hypertension. Antioxid. Redox Signal. 35, 1515-1530.

A comprehensive guide to the pharmacologic regulation of angiotensin converting enzyme 2 (ACE2), the SARS-CoV-2 entry receptor

The recent emergence of coronavirus disease-2019 (COVID-19) as a global pandemic has prompted scientists to address an urgent need for defining mechanisms of disease pathology and treatment. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent for COVID-19, employs angiotensin converting enzyme 2 (ACE2) as its primary target for cell surface attachment and likely entry into the host cell. Thus, understanding factors that may regulate the expression and function of ACE2 in the healthy and diseased body is critical for clinical intervention. Over 66% of all adults in the United States are currently using a prescription drug and while earlier findings have focused on possible upregulation of ACE2 expression through the use of renin angiotensin system (RAS) inhibitors, mounting evidence suggests that various other widely administered drugs used in the treatment of hypertension, heart failure, diabetes mellitus, hyperlipidemias, coagulation disorders, and pulmonary disease may also present a varied risk for COVID-19. Specifically, we summarize mechanisms on how heparin, statins, steroids and phytochemicals, besides their established therapeutic effects, may also interfere with SARS-CoV-2 viral entry into cells. We also describe evidence on the effect of several vitamins, phytochemicals, and naturally occurring compounds on ACE2 expression and activity in various tissues and disease models. This comprehensive review aims to provide a timely compendium on the potential impact of commonly prescribed drugs and pharmacologically active compounds on COVID-19 pathology and risk through regulation of ACE2 and RAS signaling.

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

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

Angiotensin receptors and neuropathic pain

Growing evidence implicates the renin-angiotensin system (RAS) in multiple facets of neuropathic pain (NP). This narrative review focuses primarily on the major bioactive RAS peptide, Angiotensin II (Ang II), and its receptors, namely type 1 (AT1R) and type 2 (AT2R). Both receptors are involved in the development of NP and represent potential therapeutic targets. We first discuss the potential role of Ang II receptors in modulation of NP in the central nervous system. Ang II receptor expression is widespread in circuits associated with the perception and modulation of pain, but more studies are required to fully characterize receptor distribution, downstream signaling, and therapeutic potential of targeting the central nervous system RAS in NP. We then describe the peripheral neuronal and nonneuronal distribution of the RAS, and its contribution to NP. Other RAS modulators (such as Ang (1-7)) are briefly reviewed as well. AT1R antagonists are analgesic across different pain models, including NP. Several studies show neuronal protection and outgrowth downstream of AT2R activation, which may lead to the use of AT2R agonists in NP. However, blockade of AT2R results in analgesia. Furthermore, expression of the RAS in the immune system and a growing appreciation of neuroimmune crosstalk in NP add another layer of complexity and therapeutic potential of targeting this pathway. A growing number of human studies also hint at the analgesic potential of targeting Ang II signaling. Altogether, Ang II receptor signaling represents a promising, far-reaching, and novel strategy to treat NP.

Renin-Angiotensin System and Alzheimer's Disease Pathophysiology: From the Potential Interactions to Therapeutic Perspectives

New roles of the Renin-Angiotensin System (RAS), apart from fluid homeostasis and Blood Pressure (BP) regulation, are being progressively unveiled, since the discoveries of RAS alternative axes and local RAS in different tissues, including the brain. Brain RAS is reported to interact with pathophysiological mechanisms of many neurological and psychiatric diseases, including Alzheimer's Disease (AD). Even though AD is the most common cause of dementia worldwide, its pathophysiology is far from elucidated. Currently, no treatment can halt the disease course. Successive failures of amyloid-targeting drugs have challenged the amyloid hypothesis and increased the interest in the inflammatory and vascular aspects of AD. RAS compounds, both centrally and peripherally, potentially interact with neuroinflammation and cerebrovascular regulation. This narrative review discusses the AD pathophysiology and its possible interaction with RAS, looking forward to potential therapeutic approaches. RAS molecules affect BP, cerebral blood flow, neuroinflammation, and oxidative stress. Angiotensin (Ang) II, via angiotensin type 1 receptors may promote brain tissue damage, while Ang-(1-7) seems to elicit neuroprotection. Several studies dosed RAS molecules in AD patients' biological material, with heterogeneous results. The link between AD and clinical conditions related to classical RAS axis overactivation (hypertension, heart failure, and chronic kidney disease) supports the hypothesized role of this system in AD. Additionally, RAStargeting drugs as Angiotensin Converting Enzyme inhibitors (ACEis) and Angiotensin Receptor Blockers (ARBs) seem to exert beneficial effects on AD. Results of randomized controlled trials testing ACEi or ARBs in AD are awaited to elucidate whether AD-RAS interaction has implications on AD therapeutics.

Anti-Hypertensive Potential and Epigenetics of Angiotensin II type 2 Receptor (AT2R)

BACKGROUND

Renin angiotensin system (RAS) is a critical pathway involved in blood pressure regulation. Octapeptide, angiotensin II (Ang II), is a biologically active compound of RAS pathway which mediates its action by binding to either angiotensin II type 1 receptor (AT1R) or angiotensin II type 2 receptor (AT2R). Binding of Ang II to AT1R facilitates blood pressure regulation, whereas AT2R is primarily involved in wound healing and tissue remodeling.

OBJECTIVES

Recent studies have highlighted the additional role of AT2R to counterbalance the detrimental effects of AT1R. Activation of angiotensin II type 2 receptor using AT2R agonist has shown the effect on natriuresis and release of nitric oxide. Additionally, AT2R activation has been found to inhibit angiotensin converting enzyme (ACE) and enhance angiotensin receptor blocker (ARB) activity. These findings highlight the potential of AT2R as a novel therapeutic target against hypertension.

CONCLUSION

The potential role of AT2R highlights the importance of exploring additional mechanisms that might be crucial for AT2R expression. Epigenetic mechanisms, including DNA methylation and histone modification, have been explored vastly with relation to cancer, but the role of such mechanisms in the expression of AT2R has recently gained interest.

Brain angiotensin converting enzyme-2 in central cardiovascular regulation

The brain renin-angiotensin system (RAS) plays an important role in the regulation of autonomic and neuroendocrine functions, and maintains cardiovascular homeostasis. Ang-II is the major effector molecule of RAS and exerts most of its physiological functions, including blood pressure (BP) regulation, via activation of AT1 receptors. Dysregulation of brain RAS in the central nervous system results in increased Ang-II synthesis that leads to sympathetic outflow and hypertension. Brain angiotensin (Ang) converting enzyme-2 (ACE2) was discovered two decades ago as an RAS component, exhibiting a counter-regulatory role and opposing the adverse cardiovascular effects produced by Ang-II. Studies using synthetic compounds that can sustain the elevation of ACE2 activity or genetically overexpressed ACE2 in specific brain regions found various beneficial effects on cardiovascular function. More recently, ACE2 has been shown to play critical roles in neuro-inflammation, gut dysbiosis and the regulation of stress and anxiety-like behaviors. In the present review, we aim to highlight the anatomical locations and functional implication of brain ACE2 related to its BP regulation via modulation of the sympathetic nervous system and discuss the recent developments and future directions in the ACE2-mediated central cardiovascular regulation.

Angiotensinergic receptors in the medial amygdaloid nucleus differently modulate behavioral responses in the elevated plus-maze and forced swimming test in rats

The brain renin-angiotensin system (RAS) has been implicated in anxiety and depression disorders, but the specific brain sites involved are poorly understood. The medial amygdaloid nucleus (MeA) is involved in expression of behavioral responses. However, despite evidence of the presence of all angiotensinergic receptors in this amygdaloid nucleus, regulation of anxiety- and depressive-like behaviors by angiotensinergic neurotransmissions within the MeA has never been reported. Thus, the present study aimed to investigate the role angiotensin II (AT₁ and AT₂ receptors) and angiotensin-(1-7) (Mas receptor) receptors present within the MeA in behavioral responses in the elevated plus-maze (EPM) and forced swimming test (FST). For this, male Wistar rats had cannula-guide bilaterally implanted into the MeA, and independent sets of animals received bilateral microinjections of either the selective AT₁ receptor antagonist losartan, the selective AT₂ receptor antagonist PD123319, the selective Mas receptor antagonist A-779 or vehicle into the MeA before the EPM and FST. Treatment of the MeA with either PD123319 or A-779 decreased the EPM open arms exploration, while losartan did not affect behavioral responses in this apparatus. However, intra-MeA microinjection of losartan decreased immobility in the FST. Administration of either PD123319 or A-779 into the MeA did not affect the immobility during the FST, but changed the pattern of the active behaviors swimming and climbing. Altogether, these results indicate the presence of different angiotensinergic mechanisms within the MeA controlling behavioral responses in the FST and EPM.

Maternal high-fat diet increases vascular contractility in adult offspring in a sex-dependent manner

A maternal high-fat diet (HFD) is a risk factor for cardiovascular diseases in offspring. The aim of the study was to determine whether maternal HFD causes the epigenetic programming of vascular angiotensin II receptors (ATRs) and leads to heightened vascular contraction in adult male offspring in a sex-dependent manner. Pregnant rats were treated with HFD (60% kcal fat). Aortas were isolated from adult male and female offspring. Maternal HFD increased phenylephrine (PE)-and angiotensin II (Ang II)-induced contractions of the aorta in male but not female offspring. N^G-nitro-L-arginine (ʟ-NNA; 100 μM) abrogated the maternal HFD-induced increase in PE-mediated contraction. HFD caused a decrease in endothelium-dependent relaxations induced by acetylcholine in male but not female offspring. However, it had no effect on sodium nitroprusside-induced endothelium-independent relaxations of aortas regardless of sex. The AT₁ receptor (AT₁R) antagonist losartan (10 μM), but not the AT₂ receptor (AT₂R) antagonist PD123319 (10 μM), blocked Ang II-induced contractions in both control and HFD offspring in both sexes. Maternal HFD increased AT₁R but decreased AT₂R, leading to an increased ratio of AT₁R/AT₂R in HFD male offspring, which was associated with selective decreases in DNA methylation at the AT_1aR promoter and increases in DNA methylation at the AT₂R promoter. The vascular ratio of AT₁R/AT₂R was not significantly different in HFD female offspring compared with the control group. Our results indicated that maternal HFD caused a differential regulation of vascular AT₁R and AT₂R gene expression through a DNA methylation mechanism, which may be involved in HFD-induced vascular dysfunction and the development of a hypertensive phenotype in adulthood in a sex-dependent manner.

Brain Angiotensin Type-1 and Type-2 Receptors in Physiological and Hypertensive Conditions: Focus on Neuroinflammation

PURPOSE OF REVIEW

To review recent data that suggest opposing effects of brain angiotensin type-1 (AT1R) and type-2 (AT2R) receptors on blood pressure (BP). Here, we discuss recent studies that suggest pro-hypertensive and pro-inflammatory actions of AT1R and anti-hypertensive and anti-inflammatory actions of AT2R. Further, we propose mechanisms for the interplay between brain angiotensin receptors and neuroinflammation in hypertension.

RECENT FINDINGS

The renin-angiotensin system (RAS) plays an important role in regulating cardiovascular physiology. This includes brain AT1R and AT2R, both of which are expressed in or adjacent to brain regions that control BP. Activation of AT1R within those brain regions mediate increases in BP and cause neuroinflammation, which augments the BP increase in hypertension. The fact that AT1R and AT2R have opposing actions on BP suggests that AT1R and AT2R may have similar opposing actions on neuroinflammation. However, the mechanisms by which brain AT1R and AT2R mediate neuroinflammatory responses remain unclear. The interplay between brain angiotensin receptor subtypes and neuroinflammation exacerbates or protects against hypertension.

GABA is a mediator of brain AT1 and AT2 receptor-mediated blood pressure responses

The nucleus tractus solitarius (NTS), paraventricular nucleus (PVN), and rostral ventrolateral medulla (RVLM) are the most targeted regions of central blood pressure control studies. Glutamate and gamma-aminobutyric acid (GABA) interact within these brain regions to modulate blood pressure. The brain renin-angiotensin system also participates in central blood pressure control. Angiotensin II increases blood pressure through the stimulation of angiotensin II type 1 (AT₁) receptors within the PVN and RVLM and attenuates baroreceptor sensitivity, resulting in elevated blood pressure within the NTS. Angiotensin II type 2 (AT₂) receptors in cardiovascular control centers in the brain also appear to be involved in blood pressure control and counteract AT₁ receptor-mediated effects. The current review is focused on the interaction of GABA with AT₁ and AT₂ receptors in the control of blood pressure within the RVLM, PVN and NTS. Within the NTS, GABA is released from local GABAergic interneurons that are stimulated by local AT₁ receptors and mediates a hypertensive response. In contrast, the local increase in GABA levels observed after AT₂ receptor stimulation within the RVLM, likely from GABAergic nerve endings originating in the caudal ventrolateral medulla, is important in the mediation of the hypotensive response. Preliminary results suggest that the hypertensive response to AT₁ receptor stimulation within the RVLM is associated with a reduction in GABA release. The current experimental evidence therefore indicates that GABA is an important mediator of brainstem responses to AT₁ and AT₂ receptor stimulation and that increased GABA release may play a role in hypertensive and hypotensive responses, depending on the site of action.

The Role of Angiotensin-(1-7)/Mas Axis and Angiotensin Type 2 Receptors in the Central Nervous System in Cardiovascular Disease and Therapeutics: A Riddle to be Solved

In recent years, the Angiotensin-(1-7)/Mas receptor [Ang-(1-7)/Mas] sub-branch of the Renin-Angiotensin System (RAS) in the brain, and Angiotensin Type 2 Receptors (AT2R), have attracted scientific interest, as there is evidence that they constitute an essential pathway in cardiovascular regulation, in health and in disease. By acting centrally, the Ang-(1-7)/Mas axis - that has been termed 'the axis of good'- can exert blood pressure-lowering effects, while also favourably altering baroreflex sensitivity and noradrenergic neurotransmission. Thus, research has focused on the possible neuro- and cardioprotective effects of this pathway in the setting of cardiovascular disease, ultimately aiming to evaluate the potential for development of novel therapeutic strategies based on its modulation. We summarize the available evidence from experimental studies in this context, aiming to assess current limits of scientific knowledge relevant to this newly-described 'player' in haemodynamic regulation, that may become a potential therapeutic target.

Brain angiotensin type-1 and type-2 receptors: cellular locations under normal and hypertensive conditions

Brain angiotensin-II (Ang-II) type-1 receptors (AT1Rs), which exert profound effects on normal cardiovascular, fluid, and metabolic homeostasis, are overactivated in and contribute to chronic sympathoexcitation and hypertension. Accumulating evidence indicates that the activation of Ang-II type-2 receptors (AT2Rs) in the brain exerts effects that are opposite to those of AT1Rs, lowering blood pressure, and reducing hypertension. Thus, it would be interesting to understand the relative cellular localization of AT1R and AT2R in the brain under normal conditions and whether this localization changes during hypertension. Here, we developed a novel AT1aR-tdTomato reporter mouse strain in which the location of brain AT1aR was largely consistent with that determined in the previous studies. This AT1aR-tdTomato reporter mouse strain was crossed with our previously described AT2R-eGFP reporter mouse strain to yield a novel dual AT1aR/AT2R reporter mouse strain, which allowed us to determine that AT1aR and AT2R are primarily localized to different populations of neurons in brain regions controlling cardiovascular, fluid, and metabolic homeostasis. Using the individual AT1aR-tdTomato reporter mice, we also demonstrated that during hypertension induced by the administration of deoxycorticosterone acetate-salt, there was no shift in the expression of AT1aR from neurons to microglia or astrocytes in the paraventricular nucleus, a brain area important for sympathetic regulation. Using AT2R-eGFP reporter mice under similar hypertensive conditions, we demonstrated that the same was true of AT2R expression in the nucleus of the solitary tract (NTS), an area critical for baroreflex control. Collectively, these findings provided a novel means to assess the colocalization of AT1R and AT2R in the brain and a novel view of their cellular localization in hypertension.

Angiotensin II Type 2 Receptor-Expressing Neurons in the Central Amygdala Influence Fear-Related Behavior

BACKGROUND

The renin-angiotensin system has been implicated in posttraumatic stress disorder; however, the mechanisms responsible for this connection and the therapeutic potential of targeting the renin-angiotensin system in posttraumatic stress disorder remain unknown. Using an angiotensin receptor bacterial artificial chromosome (BAC) and enhanced green fluorescent protein (eGFP) reporter mouse, combined with neuroanatomical, pharmacological, and behavioral approaches, we examined the role of angiotensin II type 2 receptor (AT₂R) in fear-related behavior.

METHODS

Dual immunohistochemistry with retrograde labeling was used to characterize AT₂R-eGFP⁺ cells in the amygdala of the AT₂R-eGFP-BAC reporter mouse. Pavlovian fear conditioning and behavioral pharmacological analyses were used to demonstrate the effects of AT₂R activation on fear memory in male C57BL/6 mice.

RESULTS

AT₂R-eGFP⁺ neurons in the amygdala were predominantly expressed in the medial amygdala and the medial division of the central amygdala (CeM), with little AT₂R-eGFP expression in the basolateral amygdala or lateral division of the central amygdala. Characterization of AT₂R-eGFP⁺ neurons in the CeM demonstrated distinct localization to gamma-aminobutyric acidergic projection neurons. Mice receiving acute intra-central amygdala injections of the selective AT₂R agonist compound 21 prior to tests for cued or contextual fear expression displayed less freezing. Retrograde labeling of AT₂R-eGFP⁺ neurons projecting to the periaqueductal gray revealed AT₂R-eGFP⁺ neuronal projections from the CeM to the periaqueductal gray, a key brain structure mediating fear-related freezing.

CONCLUSIONS

These findings suggest that CeM AT₂R-expressing neurons can modulate central amygdala outputs that play a role in fear expression, providing new evidence for a novel angiotensinergic circuit in the regulation of fear.

Distinct effects of antihypertensives on depression in the real-world setting: A retrospective cohort study

BACKGROUND

Evidence is increasing that pathways of antihypertensives may have a role in the pathogenesis of depression. However, how the class of antihypertensives affects depression risk remains unclear.

METHODS

The effects of different classes of antihypertensives on depression were explored using an insurance database in Beijing, China. Antihypertensives in our study included calcium channel blockers (CCBs), angiotensin-converting enzyme inhibitors (ACEIs), angiotensin II receptor blockers (ARBs), beta-blockers (BBs), and diuretics (DIUs). Those initially treated with only one class of antihypertensives were included. Stratified analysis was conducted for demographic characteristics, comorbidities, and statin prescriptions.

RESULTS

In total, 181,709 newly detected hypertension patients were included. The median follow-up period was 4.33 years and 19,030 participants were with depression by the end. After adjusting for covariates, the incidence density (95% confidence interval, CI) of depression in the BB, ACEI, DIU, CCB, and ARB groups was 3.16 (2.98-3.33), 3.10 (2.91-3.29), 2.70 (2.45-2.94), 2.67 (2.53-2.81), and 2.30 (2.16-2.43) per 100 person-years, respectively. Compared with ARB group, the hazard ratio (95% CI) of depression for BB, ACEI, DIU, and CCB group was 1.37 (1.32-1.43), 1.35 (1.28-1.42), 1.17 (1.08-1.27), and 1.16 (1.12-1.21), respectively. Stratified analysis suggested the highest depression ID remained within the BB or ACEI group.

LIMITATIONS

Detailed clinical information was unavailable, which may introduce bias. Patients on monotherapy as initial treatment were included and caution is needed for extrapolation.

CONCLUSIONS

Compared with ARBs, there may be a class effect of other antihypertensives on the risk of depression.

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.

(Pro)renin receptor knockdown in the paraventricular nucleus of the hypothalamus attenuates hypertension development and AT1 receptor-mediated calcium events

Activation of the brain renin-angiotensin system (RAS) is a pivotal step in the pathogenesis of hypertension. The paraventricular nucleus (PVN) of the hypothalamus is a critical part of the angiotensinergic sympatho-excitatory neuronal network involved in neural control of blood pressure and hypertension. However, the importance of the PVN (pro)renin receptor (PVN-PRR)-a key component of the brain RAS-in hypertension development has not been examined. In this study, we investigated the involvement and mechanisms of the PVN-PRR in DOCA-salt-induced hypertension, a mouse model of hypertension. Using nanoinjection of adeno-associated virus-mediated Cre recombinase expression to knock down the PRR specifically in the PVN, we report here that PVN-PRR knockdown attenuated the enhanced blood pressure and sympathetic tone associated with hypertension. Mechanistically, we found that PVN-PRR knockdown was associated with reduced activation of ERK (extracellular signal-regulated kinase)-1/2 in the PVN and rostral ventrolateral medulla during hypertension. In addition, using the genetically encoded Ca²⁺ biosensor GCaMP6 to monitor Ca²⁺-signaling events in the neurons of PVN brain slices, we identified a reduction in angiotensin II type 1 receptor-mediated Ca²⁺ activity as part of the mechanism by which PVN-PRR knockdown attenuates hypertension. Our study demonstrates an essential role of the PRR in PVN neurons in hypertension through regulation of ERK1/2 activation and angiotensin II type 1 receptor-mediated Ca²⁺ activity. NEW & NOTEWORTHY PRR knockdown in PVN neurons attenuates the development of DOCA-salt hypertension and autonomic dysfunction through a decrease in ERK1/2 activation in the PVN and RVLM during hypertension. In addition, PRR knockdown reduced AT_1aR expression and AT₁R-mediated calcium activity during hypertension. Furthermore, we characterized the neuronal targeting specificity of AAV serotype 2 in the mouse PVN and validated the advantages of the genetically encoded calcium biosensor GCaMP6 in visualizing neuronal calcium activity in the PVN.

The renin-angiotensin system: going beyond the classical paradigms

Thirty years ago, a novel axis of the renin-angiotensin system (RAS) was unveiled by the discovery of angiotensin-(1−7) [ANG-(1−7)] generation in vivo. Later, angiotensin-converting enzyme 2 (ACE2) was shown to be the main mediator of this reaction, and Mas was found to be the receptor for the heptapeptide. The functional analysis of this novel axis of the RAS that followed its discovery revealed numerous protective actions in particular for cardiovascular diseases. In parallel, similar protective actions were also described for one of the two receptors of ANG II, the ANG II type 2 receptor (AT₂R), in contrast to the other, the ANG II type 1 receptor (AT₁R), which mediates deleterious actions of this peptide, e.g., in the setting of cardiovascular disease. Very recently, another branch of the RAS was discovered, based on angiotensin peptides in which the amino-terminal aspartate was replaced by alanine, the alatensins. Ala-ANG-(1−7) or alamandine was shown to interact with Mas-related G protein-coupled receptor D, and the first functional data indicated that this peptide also exerts protective effects in the cardiovascular system. This review summarizes the presentations given at the International Union of Physiological Sciences Congress in Rio de Janeiro, Brazil, in 2017, during the symposium entitled “The Renin-Angiotensin System: Going Beyond the Classical Paradigms,” in which the signaling and physiological actions of ANG-(1−7), ACE2, AT₂R, and alatensins were reported (with a focus on noncentral nervous system-related tissues) and the therapeutic opportunities based on these findings were discussed.

Renin-Angiotensin System Responds to Prolonged Hypotensive Effect Induced by Mandibular Extension in Spontaneously Hypertensive Rats

There is an ongoing interest in the renin-angiotensin system (RAS) contribution either to pathological mechanisms leading to hypertension (mainly regarding the ACE/AngII/AT1R axis), or, to RAS protective and pro-regenerative actions, primarily ascribed to the mediation of the AT2R and the MAS1 receptor. In the present study, we evaluated the modulation of gene expression and protein levels of “deleterious” (ACE/AngII/AT1R) and “protective” [ACE/AngII/AT2R and ACE2/Ang(1-7)/MAS1 arms] RAS components in parietal and frontal areas of cerebral cortex of spontaneously hypertensive rats (SHRs), after two periods of mandibular extensions (MEs). Blood pressure, BP and heart rate, HR were also measured. While no significant changes in BP and HR were present in the sham operated (SO) group, in rats after two MEs (2-ME rats), BP displayed a marked decrease (p < 0.001) at ME2, and remained then stably low for the subsequent observation period. In gene expression analysis, in SHRs undergoing two MEs, either in parietal or frontal cortex, we did not observe any significant variation of AT2R and ACE2 with respect to SO rats. In contrast, we observed a decrease in Mas1 gene expression in parietal area (p < 0.01) and an increase in frontal region (p < 0.01). AT1R and ACE gene expression was significantly higher in 2-ME rats than SO in parietal cortex (p < 0.05) but no difference was observed in the frontal area. Concerning protein levels, in parietal area, AT1R and AT2R did not change whereas MAS1 significantly decreased in 2-ME rats (p < 0.05). In frontal area, both AT1R and AT2R significantly decreased in 2-ME rats (p < 0.05), whereas MAS1 did not significantly change. Gene expression analysis in normotensive (NT) rats revealed the non-detectability of AT1R in both parietal and frontal zone. In parietal area, AT2R (p < 0.0001) and Mas1 (p < 0.01) were significantly decreased in 2-ME NT rats, when compared to SO, and ACE and ACE2 resulted not detectable whereas there was some expression of these genes after 2-ME procedure. In conclusion, our data in rat models indicated that a 2-ME procedure induced a hypotensive response and that a modulation of gene expression and protein levels of RAS components occurred in different cerebral cortex areas.

Molecular Basis of the Brain Renin Angiotensin System in Cardiovascular and Neurologic Disorders: Uncovering a Key Role for the Astroglial Angiotensin Type 1 Receptor AT1R

The central renin angiotensin system (RAS) is one of the most widely investigated cardiovascular systems in the brain. It is implicated in a myriad of cardiovascular diseases. However, studies from the last decade have identified its involvement in several neurologic abnormalities. Understanding the molecular functionality of the various RAS components can thus provide considerable insight into the phenotypic differences and mechanistic drivers of not just cardiovascular but also neurologic disorders. Since activation of one of its primary receptors, the angiotensin type 1 receptor (AT1R), results in an augmentation of oxidative stress and inflammatory cytokines, it becomes essential to investigate not just neuronal RAS but glial RAS as well. Glial cells are key homeostatic regulators in the brain and are critical players in the resolution of overt oxidative stress and neuroinflammation. Designing better and effective therapeutic strategies that target the brain RAS could well hinge on understanding the molecular basis of both neuronal and glial RAS. This review provides a comprehensive overview of the major studies that have investigated the mechanisms and regulation of the brain RAS, and it also provides insight into the potential role of glial AT1Rs in the pathophysiology of cardiovascular and neurologic disorders.

Integrative Physiological Aspects of Brain RAS in Hypertension

PURPOSE OF REVIEW

The renin-angiotensin system (RAS) plays an important role in modulating cardiovascular function and fluid homeostasis. While the systemic actions of the RAS are widely accepted, the role of the RAS in the brain, its regulation of cardiovascular function, and sympathetic outflow remain controversial. In this report, we discuss the current understanding of central RAS on blood pressure (BP) regulation, in light of recent literature and new experimental techniques.

RECENT FINDINGS

Studies using neuronal or glial-specifc mouse models have allowed for greater understanding into the site-specific expression and role centrally expressed RAS proteins have on BP regulation. While all components of the RAS have been identified in cardiovascular regulatory regions of the brain, their actions may be site specific. In a number of animal models of hypertension, reduction in Ang II-mediated signaling, or upregulation of the central ACE2/Ang 1-7 pathway, has been shown to reduce BP, via a reduction in sympathetic signaling and increase parasympathetic tone, respectively. Emerging evidence also suggests that, in part, the female protective phenotype against hypertension may be due to inceased ACE2 activity within cardiovascular regulatory regions of the brain, potentially mediated by estrogen. Increasing evidence suggests the importance of a central renin-angiotensin pathway, although its localization and the mechanisms involved in its expression and regulation still need to be clarified and more precisely defined. All reported studies/experiments with human or animal subjects performed by the authors have been previously published and complied with all applicable ethical standards (including the Helsinki declaration and its amendments, institutional/national research committee standards, and international/national/institutional guidelines).