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How Is the Brain Renin–Angiotensin System Regulated?

2017; Lippincott Williams & Wilkins; Volume: 70; Issue: 1 Linguagem: Inglês

10.1161/hypertensionaha.117.08550

1524-4563

Pablo Nakagawa, Curt D. Sigmund,

Cardiovascular, Neuropeptides, and Oxidative Stress Research

HomeHypertensionVol. 70, No. 1How Is the Brain Renin–Angiotensin System Regulated? Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBHow Is the Brain Renin–Angiotensin System Regulated? Pablo Nakagawa and Curt D. Sigmund Pablo NakagawaPablo Nakagawa From the Department of Pharmacology, UIHC Center for Hypertension Research, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City. and Curt D. SigmundCurt D. Sigmund From the Department of Pharmacology, UIHC Center for Hypertension Research, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City. Originally published30 May 2017https://doi.org/10.1161/HYPERTENSIONAHA.117.08550Hypertension. 2017;70:10–18Other version(s) of this articleYou are viewing the most recent version of this article. Previous versions: January 1, 2017: Previous Version 1 The renin–angiotensin system (RAS) is one of the most studied physiological pathways controlling arterial blood pressure (BP). The longevity of RAS blockers as effective antihypertensive therapies evidences the continued clinical importance of the RAS. New components of the RAS and novel mechanisms continue to be discovered with regularity. For example, the RAS has recently been implicated in metabolic control and its association with metabolic pathways.1–3 Moreover, the RAS is one of the most complex systems because of the interaction between circulating and tissue RAS, among RAS in different tissues, and the activity of counter-regulatory peptides and receptors, vis-a-vis Ang-II/angiotensin type 1 (AT1) receptor versus Ang-(1–7)/Mas. The intricate neural circuitry between cardiovascular, fluid homeostasis, and metabolic control regions, some of which have the capacity for the independent generation and action of angiotensin peptides further complicates our understanding of the brain RAS. Instead of an all-encompassing review of the brain RAS, which can be found elsewhere, this article will focus primarily on investigations into how RAS activity in the brain is regulated.4AngiotensinogenThere have been numerous studies examining the transcriptional mechanisms regulating expression of the angiotensinogen (AGT) gene. A vast majority of these studies have focused on hepatocytes and renal proximal tubule. In the brain, AGT is expressed in astrocytes and in some neurons, particularly in regions of the brain controlling cardiovascular and metabolic function.5–7 Expression of AGT in glial cells is of clear importance as overexpression of the human RAS in glial cells results in hypertension, and its removal from glia significantly blunts hypertension in mice.8,9 Similarly, transgenic rats with decreased glial AGT exhibit severe defects in water homeostasis and a blunting of hypertension when bred with a rat model of increased brain Ang-II.10It has been widely presumed that AGT is constitutively released from astrocytes into the extracellular space where it can be cleaved by neuron-derived renin. Interestingly, AGT protein in astrocytes may also be localized in the nucleus.11 Although the function of nuclear AGT remains unclear, activation of nuclear Ang-II receptors has been attributed to alter production of reactive oxygen species and nitric oxide (NO) in renal cells.12There is experimental evidence that AGT expression in brain may be regulated independently of the circulating (hepatic-derived) AGT. For example, chronic angiotensin-converting enzyme (ACE) blockade decreased AGT levels in plasma, but not in the cerebrospinal fluid, concomitant with increased AGT expression in the anterior hypothalamus of stroke-prone spontaneously hypertensive and Wistar Kyoto rats.13 AGT expression in cultured primary astrocytes from mouse and rat is downregulated by Ang-II.14,15 Genetic variation in the AGT promoter region also differentially influences AGT promoter activity in cultured astrocytes, proximal tubules, and hepatocytes.16 That AGT is required for the initiation of the RAS cascade provides an impetus for a better understanding of its regulation in the brain.ReninRenin is the rate-limiting step of the RAS cascade and consequently the most highly regulated RAS component. Although the active form of renin is thought to be exclusively generated in the kidney,17 it now recognized that prorenin is expressed in many extrarenal tissues including the brain. The first reported evidence for the presence of renin-like activity in the central nervous system was in 1971, but the presence of renin mRNA in brain tissue was not confirmed until 1986.18–20 For years, the lack of tools to detect the low levels of renin in the brain was a barrier toward determining the location of renin in the brain. To address the cellular location of renin in the brain, mice that express green fluorescent protein driven by the renin promoter, in combination with well-characterized markers for astrocytes, neurons, and oligodendrocytes were used to determine that renin is expressed mainly in neurons and in close proximity to AGT-expressing cells.7,21 Importantly, enhanced green fluorescent protein distribution matched closely the distribution that has been reported for both AGT and Ang-II in regions controlling cardiovascular function. Transgenic models expressing RAS components in glia or neurons have provided important tools evidencing the importance of the brain RAS.22 Nevertheless, the evidence that naturally occurring brain renin mediates RAS activation in the brain remains surprisingly controversial.23,24It is well recognized that there is a species specificity in the enzymatic reaction between renin and AGT.25 That transgenic mice substantially overexpressing human AGT do not exhibit elevated arterial BP has been used to argue that the processing of AGT in the brain is likely to be renin dependent.26 On the contrary, the low level of renin in the brain has led to speculation that AGT is cleaved directly into Ang-II by other processing proteases including tonin and cathepsin G with a broad spectrum of substrate affinities.27 Ang-(1–12), a peptide fragment of AGT has also been reported to be expressed in the brain; and chymase or ACE can generate Ang-II from Ang-(1–12).28,29 It was argued that Ang-(1–12) may be a bona fide precursor to Ang-II evidencing a nonrenin Ang-II generation pathway because immunoneutralization of Ang-(1–12) reduces BP in (mRen2)27 rats.30 However, does the antisera have the selectivity to specifically target Ang-(1–12) and not full-length AGT? Thus, whereas the evidence favors a renin-dependent model of Ang-II generation, and although there is no direct evidence supporting renin-independent Ang-II generation, we cannot formally rule out the possibility that cathepsin G or an Ang-(1–12)–hydrolyzing enzymes is involved in the nonrenin-dependent generation of Ang-II in the brain. Indeed, if these enzymes, like renin, act species specifically, it could explain why mice expressing vast quantities of human AGT are not hypertensive. It is interesting to note that human cathepsin G has a broadened specificity than the mouse isoform and Ang-(1–12) processing mechanisms differ between species.31,32There is more known about the mechanisms governing expression of the renin gene than any other RAS gene. The isolation of the As4.1 cell line, a renin-expressing cell line derived from renin-expressing kidney tumor, became a turning point leading us and others to identify critical DNA sequences in the proximal promoter and distal enhancer that were necessary for the maintenance of high-level renin expression (Figure 1).33–36 Deletion of the human or mouse renin distal enhancer preserves tissue specificity in mice but results in a decrease in the overall level of renin expression in kidney.37,38 Interestingly, deletion of the distal enhancer also blunts the renin protein response to physiological cues in several tissues including the brain.39 Whether there are specific sequences in the renin enhancer or promoter that direct expression of renin to specific populations of neurons remains unknown, as are the control elements that regulate the production of renin in response to physiological or pathophysiological cues. Moreover, a further complication in understanding the regulation of renin in the brain is that transcription of renin occurs from an alternative promoter encoding an mRNA starting with a new first exon (exon 1b).40,41 In fact, there are no studies assessing whether the proximal and distal (enhancer) transcriptional regulatory sequences controlling the brain-specific promoter are the same as the classical promoter active in kidney. Similarly, there have been no studies to define the mechanisms for selecting which promoter is activated in the brain (Figure 2). That there is a switch in which isoform of renin is expressed in the brain suggests that their regulation may be independent and distinct.42 If this is indeed the case, additional studies must be performed to identify those regulatory sequences.Download figureDownload PowerPointFigure 1. Transcriptional regulation of renin. Schematic representation of the regulatory region of the renin gene is shown. In kidney, transcription is initiated at exon-1a, whereas in brain, transcription is initiated at exon 1b. Both exon-1a and exon-1b splice to exon 2. Sequences controlling expression of renin in As4.1 cells, a juxtaglomerular cell line, have been identified in 2 regions upstream of the transcription start site, the proximal promoter region and the distal enhancer. Numerous transcription factors have been identified to bind to those sites, and in some cases, there are multiple binding sites for some transcription factors. Experimental data strongly supports requirement for the distal enhancer to mediate high-level renin expression in As4.1 cells. However, whether these transcription factors operate in vivo and how they are regulated under different physiological conditions is unknown. Moreover, it is unclear which transcription factor binding sites and transcription factors are required to mediate expression of renin in the brain.Download figureDownload PowerPointFigure 2. Hypothetical models for expression of renin in the brain. Under baseline conditions, renin expression in the brain is initiated from the exon-1b promoter that excludes expression of exon-1a. Under certain conditions, for example, deletion of exon-1b and surrounding sequences (renin-bNull mice) or deoxycorticosterone acetate (DOCA)–salt hypertension, expression reverts to exon-1a suggesting some dynamic mechanism regulating differential promoter activity or promoter selection in the brain. The mechanisms responsible for this have yet to be examined experimentally. These could include (A) some mechanism controlling the selection for RNA polymerase (RNAP) binding to promoter for exon-1a vs exon-1b, (B) the exclusion of RNAP binding to the exon-1a promoter or inhibition of RNAP procession along the DNA template when RNAP is bound to the promoter for exon-1b, or (C) the presence of some negative regulatory element (NRE) in the DNA sequence surrounding exon-1b, which inhibits the binding or procession of RNAP at the promoter for exon-1a. Models B and C are consistent with the activation of exon-1a expression when exon-1b and its surrounding sequences (≈500 bp upstream and downstream) are deleted.Renin-b lacks both the signal peptide and the first third of the prosegment. Consequently, renin-b likely remains intracellular (Figure 3). Based on these observations, we hypothesized that renin-b could be the missing link defining Ang-II as a neurotransmitter.43 To test this hypothesis, we generated mice that selectively lack renin-b while preserving renin-a, the form expressed in renal JG cells. The deletion of renin-b resulted in a paradoxical increase in BP and renal sympathetic nerve activity. It is paradoxical because both of these phenotypes are more consistent with brain RAS activation than repression. Moreover, both effects were blunted when the brain RAS was blocked by intracerebroventricular infusion of captopril or losartan, indicating that renin-b may be acting, by an unknown molecular mechanism, as a negative regulator of central RAS activation.44 Whereas our data failed to provide evidence supporting a specific role for intracellular renin, it lead us to the concept that expression of renin-b and renin-a occurs in opposition, with expression of renin-b precluding expression of renin-a (Figure 2). Perhaps, this is another example of a counter-regulatory system built into the RAS. Indeed, this is potentially attractive because presumably, AGT is constitutively released from glial cells and bathes the extracellular space. The natural extension of this concept dictates that the failure of the tonic inhibition of brain RAS by renin-b could be a pathophysiological mechanism that contributes to the development of neurogenic hypertension. Future studies will be necessary to address the mechanisms that regulate the switch between the 2 renin isoforms under pathophysiological conditions.Download figureDownload PowerPointFigure 3. Hypothetical model for renin synthesis in the brain. We hypothesize that expression of renin-a and renin-b occurs in opposition, and this dynamic control of renin-a vs renin-b expression might represent a counter-regulatory system designed to control the activity of the brain renin–angiotensin system. Renin-b is transcribed at baseline. If translated, renin-b would lack the signal peptide and the first third of the prosegment, which is normally encoded in preprorenin (renin-a). Although the translation product of renin-b has been shown to be enzymatically active, there is no direct experimental evidence that an intracellular renin is synthesized and is active under normal conditions. Conversely, during fetal development, and under some pathophysiological conditions (eg, deoxycorticosterone acetate-salt hypertension), renin-a is the predominant isoform of brain renin. Renin-a encodes preprorenin. Prorenin is predicted to be secreted from neurons into the extracellular space. It remains unclear whether prorenin is enzymatically processed to renin in the brain, a proportion becomes active through a conformational change that removes the prosegment from the active site or becomes active through an interaction with (pro)renin receptor (PRR). PRR is abundantly expressed in the brain. Active renin or activated prorenin generates angiotensin II (Ang-II) by the sequential cleavage of angiotensinogen (AGT), released in abundance by glial cells, and then by angiotensin-converting enzyme (ACE), which is also expressed throughout the brain.(Pro)renin ReceptorIn the kidney, renin is generated by the enzymatic removal of the prosegment from prorenin. Surprisingly, little is known about the true identity of the prorenin processing enzyme.45 Because prorenin is inactive, it cannot explain why the brain, which ostensibly only produces prorenin, has a significant amount of Ang-II.46,47 The discovery of the (Pro)renin receptor (PRR), which has the ability to activate prorenin independently of conventional enzymatic hydrolysis of the prosegment, revealed a potential new mechanism of RAS regulation.48 The PRR is a 350 amino acid membrane protein highly expressed in the brain, heart, and placenta, and at lower levels in liver and kidney. In the brain, PRR mRNA and protein are expressed primarily in neurons and in some astrocytes. Like renin, there is more known about the regulation of PRR expression and its signaling pathways in kidney than in brain.49–53 In brain, PRR expression was reported to be increased by deoxycorticosterone acetate-salt through an Ang-II/AT1 receptor–dependent mechanism involving cAMP response element binding protein and activator protein-1.54By binding to renin and prorenin, PRR might play a key role in the regulation of local RAS activation.55 Mice specifically lacking PRR in neurons provided experimental evidence that PRR mediates Ang-II formation in the brain and BP elevation when prorenin is infused intracerebroventricular.56 Subfornical organ-selective deletion of the PRR by intracerebroventricular adeno-associated virus-PRR-short-hairpin RNA delivery in double-transgenic mice expressing human renin and AGT significantly decreased BP.57 Similarly, PRR deletion in the SON reduced BP in SHR rats.58 Neuronal deletion of PRR or infusion of the inhibitory PRO20 peptide prevented hypertension induced by deoxycorticosterone acetate-salt.56,59 The main question that remains to be answered is whether the effect of PRR on BP is mediated entirely by an Ang-II–dependent or Ang-II–independent mechanism involving direct signaling through the PRR or some combination of both (Figure 4). It remains possible that each mechanism may be operant in the brain, perhaps in different neuronal connections, or mediating different neuronal outputs.Download figureDownload PowerPointFigure 4. Model for (Pro)renin receptor (PRR) activity in the brain. Prorenin binding to PRR results in the generation of angiotensin (Ang) peptides and the subsequent activation of angiotensin type 1 receptors (AT1R). Ang-II binding to AT1R initiates GPCR (G-protein–coupled receptor)–dependent and GPCR-independent (β-arrestin) signaling. The binding of renin or prorenin to PRR can also induce Ang-II–independent signaling pathways such as PI3K (phosphoinositide 3-kinase)/Akt, ERK1/2 (extracellular signal-regulated kinase 1/2), and MAPK (mitogen-activated protein kinase), which subsequently activate several proinflammatory and profibrotic genes. PRR was also reported to be involved in the activation of canonical wnt/β-catenin signaling pathway because the acidification of the signaling endosome subsequent to the internalization of Fz (Frizzled) and LRP5/5 (low-density lipoprotein receptor–related protein) requires both PRR and V-ATPases.Angiotensin-Converting EnzymeACE is expressed in many regions of the brain including areas that regulate BP.60,61 ACE is present in the endothelium of the cerebral vasculature, choroid plexus, and astrocytes in circumventricular organs and coincides with Ang-II immunoreactivity and angiotensin receptors. ACE inhibition has been shown to attenuate drinking because of water deprivation, hypovolemia, or central administration of plasma-renin substrates or intracranial Ang-I.62–64 Intracerebroventricular transfection of human ACE into the brain of Sprague–Dawley rats increased BP which was associated with an increased cerebrospinal fluid Ang-II and activation of the sympathetic nervous system.65Although it is well accepted that ACE is a key component of the brain RAS, little is known about its regulation. Hypertensive animals exhibit larger antihypertensive effects of ACE inhibition in the brain than normotensive animals, suggesting upregulation of brain ACE expression or activity in hypertension, but the mechanism remains unknown. High-brain ACE activity is also observed in heart disease. For example, increased ACE and decreased ACE2 expression in the rostral ventrolateral medulla, nucleus tractus solitarii, and paraventricular nucleus (PVN) of rabbits was reported in chronic heart failure.66 Evidence that brain ACE is regulated by dietary sodium intake comes from studies showing upregulation of hypothalamic and pons ACE activity and ACE mRNA in Dahl S rats (but not in Dahl R rats) after 5 weeks of high-salt diet.67 Leptin was also reported to increase ACE in the subfornical organ.68 This is interesting because leptin increases sensitivity to Ang-II hypertension and mediates increased ACE expression in the lamina terminalis and PVN in response to high-fat diet.69 Future studies will be necessary to address the mechanisms that induce ACE activation in the brain. This is important as ACE not only mediates brain RAS activation but may also be involved in the generation of several other peptides with biological activity.ACE2/Ang-(1–7)/Mas AxisACE2/Ang-(1–7)/Mas is considered a counter-regulatory arm of the RAS because Ang-(1–7) is widely considered to exert physiological effects that are in opposition of Ang-II. For example, Ang-(1–7) stimulates NO release, improves baroreceptor reflex sensitivity, and has an inhibitory neuromodulatory role in hypothalamic noradrenergic neurotransmission.70 ACE2 cleaves Ang-I to generate Ang-(1–9), which can be further processed to Ang-(1–7) by ACE or other peptidases. ACE2 can also proteolytically cleave Ang-II to generate Ang-(1–7). Ang-(1–7) is the ligand for the Mas receptor.71Neuronal overexpression of ACE2 reduces BP in Ang II–induced hypertension, deoxycorticosterone acetate-salt hypertension, and SHR.72–74 Numerous studies indicate that ACE2 can downregulate other components of the brain RAS, suggesting some coordinate regulation. ACE2 overexpression in the PVN decreases AT1 receptor and ACE mRNA expression concomitant with Mas receptor and AT2 receptor overexpression resulting in a decreased pressor and drinking response to Ang-II.75 Similarly, brain-specific Ang-(1–7) overexpression by Ang-(1–7) fusion protein infusion in the cisterna magna enhanced baroreflex sensitivity concomitant with reduced renin and AT1 receptor expression in transgenic (mRen2) 27 hypertensive rats.76 AT1 receptor activation decreases ACE2 activity, whereas long-term blockade of AT1 receptors increases ACE2 and Mas receptor expression in the dorsomedial medulla of fisher 344 rats, suggesting that AT1 receptors and ACE2/Ang-(1–7)/Mas axis may be coordinately regulated.77 ACE2 expression is decreased in pathological conditions such as in patients with multiple sclerosis and rabbits with heart failure.66,78 Comparably, fetal exposure to glucocorticoids induces elevated BP with impaired baroreflex sensitivity, which is associated with decreased Mas receptor expression in the dorsal medulla.79 The ACE2/Ang-(1–7)/Mas axis is upregulated in ischemic stroke, and evidence indicates that it also participates in the pathogenesis of cerebrovascular disease.80,81 Interestingly, brain expression of Ang-(1–7), Mas receptor and neuronal NO synthase is controlled by female sex hormones, which may explain the reduced risk to cardiovascular disease and stroke in females.82 The elucidation of the exact mechanisms regulating expression of ACE2 and Mas receptors in the brain will be important as it is suggested that insufficient endogenous Ang-(1–7) rather than an excess of Ang-II in the brain elicits impaired tonic baroreceptor reflex in hypertensive rats.83,84 Exercise training upregulates this protective axis of the RAS in the brain and this might be because of the inhibition of cytokines and signaling cascades related to inflammation and possibly changes in micro-RNAs in the brain.85Angiotensin ReceptorsIn the brain, Ang-II binds to 2 main G-protein–coupled receptors, AT1 and type 2 (AT2) receptors. There is a consensus that AT1 mediates most of the detrimental effects of Ang-II, whereas AT2 receptors counteract the action of the AT1 receptor, promoting vasodilatation, apoptosis, and antiproliferative effects. Numerous studies showed that the blockade of AT1 receptors in different regions of the brain induces large BP-lowering effects in both genetic and experimental models of hypertension.86,87It is not uncommon for the expression of a receptor to decrease in response to its agonist, a phenomenon known as receptor desensitization. Paradoxically, the repetitive or chronic intracerebroventricular infusion of Ang-II increases AT1 receptor expression in many regions of the brain, suggesting the existence of a positive feed-forward mechanism regulating the brain RAS activity.88,89 Water deprivation increases systemic Ang-II levels within the physiological range with concomitant AT1 receptor upregulation in the PVN and in the medial and caudal regions of the subfornical organ, but not in other brain regions.62,90 Nuclear factor-κB and cAMP response element binding protein are required for the agonist-induced AT1 receptor gene transcription in CATH.a neuron cell line.91 Dietary salt negatively regulates the systemic RAS, but its effects on the brain RAS remain controversial. One study showed that both high-salt and low-salt diet decreased AT1 receptor expression in the hypothalamus, whereas another showed that elevated dietary salt intake does not affect AT1 receptor expression levels.92,93 In Dahl salt-sensitive rats, high-salt diet for 1 week increased AT1 receptor binding in the subfornical organ, median preoptic nucleus, PVN, and suprachiasmatic nucleus, whereas in Dahl salt-resistant rats, no changes were observed. However, after 4 weeks, the high-salt diet induced AT1 overexpression in Dahl salt-resistant rats as well.94Hypothalamic AT1 receptors are upregulated in pathological states, namely heart failure and chronic renal failure.95,96 In heart failure rats, the upregulation of AT1 receptor in the hypothalamus is mediated by mitogen-activated protein kinases p44/42, p38, and c-Jun N-terminal kinase activation.97 NO donor S-nitroso-N-acetyl pencillamine decreases AT1 receptor mRNA and protein levels via cGMP-dependent signaling pathway in primary cultured hypothalamic cells of neonatal rats and neuronal cell line NG108-15.98 Although the AT1 receptor is expressed abundantly in several regions, the expression of AT2 receptor exhibits a more restricted and lower level of expression in the brain.99 Whereas most studies showed that systemic AT2 receptor stimulation does not decrease BP, overexpression of AT2 receptor in the solitary–vagal complex prevents renovascular hypertension and reverses the impairment of the baroreflex in rats.100,101 Studies conducted in models of heart failure have shown the central AT2 receptor overexpression or pharmacological activation decrease SNA, which is accompanied by downregulation of the AT1 receptors in regions of the brain controlling BP and SNA, such as the PVN.102,103 In contrast, intracerebroventricular infusion of an AT2 receptor antagonist significantly augmented the pressor effect of deoxycorticosterone acetate-salt in females, but not in males, indicating that central AT2 receptors play a protective role in women.104 It is not surprising that the brain RAS is differentially regulated depending on sex differences because steroid hormones regulate AT1 receptor binding and mRNA in the hypothalamus of female rats.105 AT2 receptor agonist, compound 21, improves neurological outcomes and reduces mortality in mice subjected to middle cerebral artery occlusion, suggesting a neuroprotective role of AT2 receptors.106 Although evidence suggests a functional role for AT2 receptor in the brain, our understanding of the regulation of AT2 receptors expression and localization within the brain was previously limited. Recently, studies of a novel bacterial artificial chromosome transgenic AT2R–enhanced green fluorescent protein reporter mouse evidenced that AT2R is expressed in neurons localized within cardiovascular control nuclei such as NTS and the median preoptic nucleus, but surprisingly not the PVN.107 The recent development of new genetic and molecular tools such as bacterial artificial chromosome transgenic AT2R–enhanced green fluorescent protein mice or new methods designed to detect mRNAs such as RNAscope might be useful to decipher how these receptors are regulated in different physiological and pathological conditions.Coordinate Regulation: Role of the Brain in BP Sensitivity to Ang-IISeveral studies using an experimental design of induction-delay-expression demonstrated that preconditioning to several distinct prohypertensive stressors can sensitize rats to a subsequent Ang-II–elicited hypertension (Figure 5). Animals exposed to a nonpressor dose of Ang-II for a week subsequently develop an exaggerated response to Ang-II–induced BP elevation.108 Sensitization to Ang-II hypertension was also observed when the animals were preconditioned with aldosterone, leptin, high-fat diet, and gestational hypertension.69,109–111 Each study provided experimental evidence that activation of the brain RAS was involved in sensitization. First, essentially all of the stressors increased expression of ≥1 RAS genes in the PVN and lamina terminalis. Second, central blockade of AT1 receptors prevented the sensitization to both Ang-II and high-fat diet.108,110 The above-mentioned agonist-induced AT1 receptor upregulation mechanism, which occurs specifically in the brain, could possibly explain the autopotentiation capacity of Ang-II. This is important because it is likely that repetitive activation of the brain RAS, especially during early life, predisposes and sensitizes individuals to hypertension. Perhaps, novel AT1 receptor desensitizing drugs such as biased AT1 receptor ligands that operate through selectively activating certain protective signal transduction pathways such as β-arrestin could be promising strategies to counter-regulate the positive feed-forward activation of the RAS in the brain.Download figureDownload PowerPointFigure 5. The brain renin–angiotensin system (RAS) mediates blood pressure sensitivity. Preconditioning to several hypertensive stressors such as a nonpressor dose of Ang-II, aldosterone, high- or low-salt diet, high-fat diet, inflammation, and maternal hypertension induces an Ang-II–sensitization state with increased brain RAS activity, but with normal blood pressure (BP). The sensitization to Ang-II–elicited hypertension may be mediated by the activation of the RAS in specific regions of the brain including the lamina terminalis (LT) and the paraventricular nucleus (PVN) as the hypertension can be prevented by intracerebroventricular infusion of angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. Under normal conditions, RAS gene expression