Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

Function and pharmacology of spinally-projecting sympathetic pre-autonomic neurones in the paraventricular nucleus of the hypothalamus

The paraventricular nucleus (PVN) of the hypothalamus has been described as the "autonomic master controller". It co-ordinates critical physiological responses through control of the hypothalamic-pituitary-adrenal (HPA)-axis, and by modulation of the sympathetic and parasympathetic branches of the central nervous system. The PVN comprises several anatomical subdivisions, including the parvocellular/ mediocellular subdivision, which contains neurones projecting to the medulla and spinal cord. Consensus indicates that output from spinally-projecting sympathetic pre-autonomic neurones (SPANs) increases blood pressure and heart rate, and dysfunction of these neurones has been directly linked to elevated sympathetic activity during heart failure. The influence of spinally-projecting SPANs on cardiovascular function high-lights their potential as targets for future therapeutic drug development. Recent studies have demonstrated pharmacological control of these spinally-projecting SPANs with glutamate, GABA, nitric oxide, neuroactive steroids and a number of neuropeptides (including angiotensin, substance P, and corticotrophin-releasing factor). The underlying mechanism of control appears to be a state of tonic inhibition by GABA, which is then strengthened or relieved by the action of other modulators. The physiological function of spinally-projecting SPANs has been subject to some debate, and they may be involved in physiological stress responses, blood volume regulation, glucose regulation, thermoregulation and/or circadian rhythms. This review describes the pharmacology of PVN spinally-projecting SPANs and discusses their likely roles in cardiovascular control.

Angiotensin II type 1 receptor signaling regulates feeding behavior through anorexigenic corticotropin-releasing hormone in hypothalamus

The activation of renin-angiotensin system contributes to the development of metabolic syndrome and diabetes as well as hypertension. However, it remains undetermined how renin-angiotensin system is implicated in feeding behavior. Here, we show that angiotensin II type 1 (AT(1)) receptor signaling regulates the hypothalamic neurocircuit that is involved in the control of food intake. Compared with wild-type Agtr1a(+/+) mice, AT(1) receptor knock-out (Agtr1a(-/-)) mice were hyperphagic and obese with increased adiposity on an ad libitum diet, whereas Agtr1a(-/-) mice were lean with decreased adiposity on a pair-fed diet. In the hypothalamus, mRNA levels of anorexigenic neuropeptide corticotropin-releasing hormone (Crh) were lower in Agtr1a(-/-) mice than in Agtr1a(+/+) mice both on an ad libitum and pair-fed diet. Furthermore, intracerebroventricular administration of CRH suppressed food intake both in Agtr1a(+/+) and Agtr1a(-/-) mice. In addition, the Crh gene promoter was significantly transactivated via the cAMP-responsive element by angiotensin II stimulation. These results thus demonstrate that central AT(1) receptor signaling plays a homeostatic role in the regulation of food intake by maintaining gene expression of Crh in hypothalamus and suggest a therapeutic potential of central AT(1) receptor blockade in feeding disorders.

Weight loss and hypophagia after high-dose AT1-blockade is only observed after high dosing and depends on regular leptin signalling but not blood pressure

AT(1)-blockade has been shown to induce weight loss in animals or patients. The aim of this study was to investigate whether weight reduction after AT(1)-blockade is dependent on dose, blood pressure reduction and leptin signalling. Spontaneously hypertensive rats (SHR) and lean and obese Zucker rats were treated for 4 weeks with candesartan (0, 2, 6 or 16 mg/kg/day). Body weight, food intake and hypothalamic mRNA levels of (an)orexigenic peptides were determined. Obese Zucker rats served as a model of primary leptin resistance. In SHR, body mass index and food intake were decreased selectively by 16 mg/kg/day candesartan but not after using normal (2 mg/kg/day) or supranormal (6 mg/kg/day) doses. Correlation analysis between blood pressure and body weight indicated no relationship of hypotensive potency on weight loss. The hypothalamic mRNA levels of the orexigenic peptide MCH (melanin-concentrating hormone) were diminished in parallel. Consistent to the results in SHRs, 16 mg/kg/day candesartan revealed a decrease of body weight, food intake and hypothalamic MCH mRNA levels in lean Zucker rats. In obese Zucker rats, none of these parameters were reduced by candesartan. Loss of body weight and hypophagia are not general features of AT(1)-blockers, since neither was seen after normal or moderately supranormal doses, but they were, after the highest doses. These actions of AT(1)-blockers occur independently of their ability to lower blood pressure. They do depend on an intact leptin signalling, since they were absent in obese Zucker rats that feature a genetic mutation of the leptin receptor.

Angiotensin II, a Neuropeptide at the Frontier between Endocrinology and Neuroscience: Is There a Link between the Angiotensin II Type 2 Receptor and Alzheimer's Disease?

Amyloid-β peptide deposition, abnormal hyperphosphorylation of tau, as well as inflammation and vascular damage, are associated with the development of Alzheimer's disease (AD). Angiotensin II (Ang II) is a peripheral hormone, as well as a neuropeptide, which binds two major receptors, namely the Ang II type 1 receptor (AT1R) and the type 2 receptor (AT2R). Activation of the AT2R counteracts most of the AT1R-mediated actions, promoting vasodilation, decreasing the expression of pro-inflammatory cytokines, both in the brain and in the cardiovascular system. There is evidence that treatment with AT1R blockers (ARBs) attenuates learning and memory deficits. Studies suggest that the therapeutic effects of ARBs may reflect this unopposed activation of the AT2R in addition to the inhibition of the AT1R. Within the context of AD, modulation of AT2R signaling could improve cognitive performance not only through its action on blood flow/brain microcirculation but also through more specific effects on neurons. This review summarizes the current state of knowledge and potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT2R activation by non-peptide and highly selective agonists, acting on neuronal plasticity, could represent new pharmacological tools that may help improve impaired cognitive performance in AD and other neurological cognitive disorders.

Altered dipsogenic responses and expression of angiotensin receptors in the offspring exposed to prenatal high sucrose

The present study determined water and salt intake as well as expression of AT(1) and AT(2) receptors in the brain and kidney in the adult offspring rats prenatally exposed to high sucrose. Following the exposure during pregnancy, water intake and salt intake at baseline levels were not changed in the adult offspring. However, after 24h water deprivation, consumption of water and salt was significantly increased compared to that of the control. Plasma sodium and osmolality levels remained the same between the offspring in the control and the exposed groups, while hematocrit was higher in the offspring exposed to prenatal high sucrose immediately following water deprivation. Density of renal AT(1) receptor protein was the same between the control and the exposed group, while AT(2) receptor protein in the kidney was significantly increased in the offspring exposed to prenatal high sucrose in association of thicker basal membrane of glomerular. In the forebrain, both AT(1) and AT(2) receptor levels were significantly increased in the offspring with history of prenatal high sucrose. In addition, water deprivation induced more c-fos expression in the central dipsogenic areas, including the paraventricular and supraoptic nuclei in the offspring exposed to prenatal high sucrose. The results suggested that prenatal high intake of sucrose may affect development of pathways in regulation of dipsogenic behavior in face of dehydration, which was associated with altered expression of AT(1) or/and AT(2) receptors in the kidney and brain.

Adenoviral inhibition of AT1a receptors in the paraventricular nucleus inhibits acute increases in mean arterial blood pressure in the rat

Brain and peripheral renin-angiotensin systems are important in blood pressure maintenance. Circulating ANG II stimulates brain RAS to contribute to the increase mean arterial pressure (MAP). This mechanism has not been fully clarified, so it was hypothesized that reducing angiotensin type 1a (AT1a) receptors (AT1aRs) in the paraventricular nucleus (PVN) would diminish intravenous ANG II-induced increases in MAP. Adenoviruses (Ad) encoding AT1a small hairpin RNA (shRNA) or Ad-LacZ (marker gene) were injected into the PVN [1 × 109 plaque-forming units/ml, bilateral (200 nl/site)] of male Sprague-Dawley rats instrumented with radiotelemetry transmitters for MAP and heart rate measurements and with venous catheters for drug administration. No differences in weight gain or basal MAP were observed. ANG II (30 ng·kg−1·min−1 iv, 15 μl/min for 60 min) was administered 3, 7, 10, and 14 days after PVN Ad injection to increase blood pressure. ANG II-induced elevations in MAP were significantly reduced in PVN Ad-AT1a shRNA rats compared with Ad-LacZ rats (32 ± 6 vs. 8 ± 9 mmHg at 7 days, 35 ± 6 vs. 10 ± 6 mmHg at 10 days, and 32 ± 2 vs. 1 ± 5 mmHg at 14 days; P < 0.05). These observations were confirmed by acute administration of losartan (20 nmol/l, 100 nl/site) in the PVN prior to short-term infusion of ANG II; the ANG II-pressor response was attenuated by 69%. In contrast, PVN Ad-AT1a shRNA treatment did not influence phenylephrine-induced increases in blood pressure (30 μg·kg−1·min−1 iv, 15 μl/min for 30 min). Importantly, PVN Ad-AT1a shRNA did not alter superior mesenteric arterial contractility to ANG II or norepinephrine; ACh-induced arterial relaxation was also unaltered. β-Galactosidase staining revealed PVN Ad transduction, and Western blot analyses revealed significant reductions of PVN AT1 protein. In conclusion, PVN-localized AT1Rs are critical for short-term circulating ANG II-mediated elevations of blood pressure. A sustained suppression of AT1aR expression by single administration of shRNA can interfere with short-term actions of ANG II.

Renin angiotensin system gene polymorphisms modify angiotensin-converting enzyme inhibitors' effect on cognitive function: the health, aging and body composition study

OBJECTIVES

To investigate the effect of polymorphisms in renin angiotensin system genes on the association between angiotensin-converting enzyme inhibitor (ACEI) exposure and global and executive cognitive function in the Health, Aging and Body Composition study.

DESIGN

Cohort study.

SETTING

Community.

PARTICIPANTS

Three thousand seventy-five participants: mean age 73.6, 58% Caucasian, 52% female, 15% taking ACE-Is, 8 years of follow-up.

MEASUREMENTS

The outcomes were longitudinal change in Executive Clock Drawing Test-1 (CLOX1), the Digit Symbol Substitution test, and the Modified Mini-Mental State Examination. The genetic polymorphisms included angiotensin-converting enzyme insertion deletion (ACEID) in the ACE gene and the M235T and 6AG polymorphisms in the angiotensinogen (AGT) gene.

RESULTS

For the CLOX1 outcome, there was significant interaction between 6AG and M235T polymorphisms in the AGT gene and angiotensin-converting enzyme inhibitors (ACE-Is) in Caucasian participants (P=.01 for both polymorphisms) independent of blood pressure levels. Specifically, ACE-I exposure was protective against CLOX1 score decline in carriers of the AA genotype of the 6AG and the CC genotype of the M235T (for the ACE-I vs non-ACE-I groups, P=.01 for 6AG and P=.005 for M235T) but not the other genotypes. These associations were not significant with other cognitive tests, with ACEID, or in African Americans.

CONCLUSION

ACE-Is may provide a protective effect on executive function in Caucasians with AGT gene polymorphisms known to be associated with greater renin angiotensin system activity. If confirmed in a pharmacogenetic trial, ACE-Is may be found to have additional cognitive protection in a select group of elderly individuals.

Angiotensin II-induced upregulation of AT(1) receptor expression: sequential activation of NF-kappaB and Elk-1 in neurons

It has been clearly established that increased circulating angiotensin II (ANG II) with concurrent upregulation of brain and peripheral ANG II type 1 receptors (AT(1)R) are important mediators in the pathophysiology of several diseases characterized by sympatho-excitation. In an effort to further understand the regulation of AT(1)R expression in neurons, we determined the role of sequential activation of the transcription factors nuclear factor-kappaB (NF-kappaB) and Ets-like protein 1 (Elk-1) in AT(1)R upregulation. We used CATH.a neurons as our neuronal cell model. Cells were treated with ANG II (100 nM) over a preset time course. Following ANG II activation, there was a temporal increase in the p65 subunit of NF-kappaB that was observed at 30 min, peaked at 1 h, and was sustained up to 24 h. There was a concomitant decrease of IkappaB and increased IkappaK expression. We also observed an increase in AT(1)R expression which followed the temporal increase of NF-kappaB. The activation of NF-kappaB was blocked by using the inhibitors parthenolide or p65 small interfering RNA (siRNA) which both led to a decrease in AT(1)R expression. The expression of Elk-1 was upregulated over a time period following ANG II activation and was decreased following NF-kappaB inhibition. p65-DNA binding was assessed using electrophoretic mobility shift assay, and it was shown that there was a time-dependent increased binding that was inhibited by means of parthenolide pretreatment or siRNA-mediated p65 gene silencing. Therefore, our results suggest a combined role for the transcription factors NF-kappaB and Elk-1 in the upregulation of AT(1)R in the CATH.a cell neuronal model. These data imply a positive feedback mechanism that may impact neuronal discharge sensitivity in response to ANG II.

Central angiotensin II stimulates cutaneous water intake behavior via an angiotensin II type-1 receptor pathway in the Japanese tree frog Hyla japonica

Angiotensin II (Ang II) stimulates oral water intake by causing thirst in all terrestrial vertebrates except anurans. Anuran amphibians do not drink orally but absorb water osmotically through ventral skin. In this study, we examined the role of Ang II on the regulation of water-absorption behavior in the Japanese tree frog (Hyla japonica). In fully hydrated frogs, intracerebroventricular (ICV) and intralymphatic sac (ILS) injection of Ang II significantly extended the residence time of water in a dose-dependent manner. Ang II-dependent water uptake was inhibited by ICV pretreatment with an angiotensin II type-1 (AT(1)) receptor antagonist but not a type-2 (AT(2)) receptor antagonist. These results suggest that Ang II stimulates water-absorption behavior in the tree frog via an AT(1)-like but not AT(2)-like receptor. We then cloned and characterized cDNA of the tree frog AT(1) receptor from the brain. The tree frog AT(1) receptor cDNA encodes a 361 amino acid residue protein, which is 87% identical to the toad (Bufo marinus) AT(1) receptor and exhibits the functional characteristics of an Ang II receptor. AT(1) receptor mRNAs were found to be present in a number of tissues including brain (especially in the diencephalon), lung, large intestine, kidney and ventral pelvic skin. When tree frogs were exposed to dehydrating conditions, AT(1) receptor mRNA significantly increased in the diencephalon and the rhombencephalon. These data suggest that central Ang II may control water intake behavior via an AT(1) receptor on the diencephalon and rhombencephalon in anuran amphibians and may have implications for water consumption in vertebrates.

Fyn is involved in angiotensin II type 2 receptor-induced neurite outgrowth, but not in p42/p44mapk in NG108-15 cells

In NG108-15 cells, activation of p42/p44(mapk) is essential for induction of neurite outgrowth by angiotensin II (Ang II) type 2 receptor (AT(2)). The aim was to verify whether Fyn, a member of the Src family kinases (SFK), is involved in neurite outgrowth induced by AT(2) activation. Preincubation of cells with PP1, a general inhibitor of the SKF, decreased activation of Rap1 and p42/p44(mapk) and abolished TrkA activation by Ang II or by the AT(2) agonist, CGP42112A. NG108-15 cells were transfected with a Fyn-WT and a Fyn-DN expressing vector. Fyn-WT was sufficient to induce neurite outgrowth, although transfection with Fyn-DN abolished neurite elongation. However, the Fyn-DN form failed to affect activation of TrkA, Rap1 or p42/p44(mapk) by Ang II. Thus, although SKF activity is required to achieve AT(2)-induced activation of TrkA, Rap1 and p42/p44(mapk), Fyn is essential for AT(2) receptor-induced neurite outgrowth, but not in AT(2) signaling leading to p42/p44(mapk) activation.

Changed salt appetite and central angiotensin II-induced cellular activation in rat offspring following hypoxia during fetal stages

Hypoxia in pregnancy may induce fetal growth restriction and cause functional abnormalities during development. The present study determined the long-term influence of hypoxia in fetal life on dipsogenic behavior linked to central angiotensin (Ang) network in the offspring rats. Fetal blood pO(2) and body weight were decreased by hypoxia during pregnancy, followed by a postnatal "catch-up" growth. Subcutaneous hypertonic saline or intracerebroventricular Ang II significantly increased salt intake in the offspring prenatally exposed to hypoxia, while water intake was the same between the two groups. Ang II-induced c-fos expression was detected in the paraventricular nuclei, median preoptic nuclei, supraoptic nuclei, and subfornical organ in the brain, in association with reduced forebrain AT(2) receptor protein abundance in the offspring prenatally exposed to hypoxia. Levels of central AT(1) receptor protein were not changed between the two groups. Hypoxia during pregnancy could be linked to developmental problems related to behavioral dysfunctions in body fluid regulations in later life, in association with the change in central angiotensin II-mediated neural activation and expression of the Ang II receptor in the brain.

The renin angiotensin system and the metabolic syndrome

The renin angiotensin system (RAS; most well-known for its critical roles in the regulation of cardiovascular function and hydromineral balance) has regained the spotlight for its potential roles in various aspects of the metabolic syndrome. It may serve as a causal link among obesity and several co-morbidities. Drugs that reduce the synthesis or action of angiotensin-II (A-II; the primary effector peptide of the RAS) have been used to treat hypertension for decades and, more recently, clinical trials have determined the utility of these pharmacological agents to prevent insulin resistance. Moreover, there is evidence that the RAS contributes to body weight regulation by acting in various tissues. This review summarizes what is known of the actions of the RAS in the brain and throughout the body to influence various metabolic disorders. Special emphasis is given to the role of the RAS in body weight regulation. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

The role of the central renin-angiotensin system in Parkinson's disease

Since the discovery of a renin-angiotensin system (RAS) in the brain, several studies have linked this central RAS to neurological disorders such as ischaemia, Alzheimer's disease and depression. In the last decade, evidence has accumulated that the central RAS might also play a role in Parkinson's disease. Although the exact cause of this progressive neurodegenerative disorder of the basal ganglia remains unidentified, inflammation and oxidative stress have been suggested to be key factors in the pathogenesis and the progression of the disease. Since angiotensin II is a pro-inflammatory compound that can induce the production of reactive oxygen species due to activation of the NADPH-dependent oxidase complex, this peptide might contribute to dopaminergic cell death. In this review, three different strategies to interfere with the pathogenesis or the progression of Parkinson's disease are discussed. They include inhibition of the angiotensin-converting enzyme, blockade of the angiotensin II type 1 receptor and stimulation of the angiotensin II type 2 receptor.

Dietary sodium deprivation evokes activation of brain regional neurons and down-regulation of angiotensin II type 1 receptor and angiotensin-convertion enzyme mRNA expression

Previous studies have indicated that the renin-angiotensin-aldosterone system (RAAS) is implicated in the induction of sodium appetite in rats and that different dietary sodium intakes influence the mRNA expression of central and peripheral RAAS components. To determine whether dietary sodium deprivation activates regional brain neurons related to sodium appetite, and changes their gene expression of RAAS components of rats, the present study examined the c-Fos expression after chronic exposure to low sodium diet, and determined the relationship between plasma and brain angiotensin I (ANG I), angiotensin II (ANG II) and aldosterone (ALD) levels and the sodium ingestive behavior variations, as well as the effects of prolonged dietary sodium deprivation on ANG II type 1 (AT1) and ANG II type 2 (AT2) receptors and angiotensin-convertion enzyme (ACE) mRNA levels in the involved brain regions using the method of real-time polymerase chain reaction (PCR). Results showed that the Fos immunoreactivity (Fos-ir) expression in forebrain areas such as subfornical organ (SFO), paraventricular hypothalamic nuclei (PVN), supraoptic nucleus (SON) and organum vasculosum laminae terminalis (OVLT) all increased significantly and that the levels of ANG I, ANG II and ALD also increased in plasma and forebrain in rats fed with low sodium diet. In contrast, AT1, ACE mRNA in PVN, SON and OVLT decreased significantly in dietary sodium depleted rats, while AT2 mRNA expression did not change in the examined areas. These results suggest that many brain areas are activated by increased levels of plasma and/or brain ANG II and ALD, which underlies the elevated preference for hypertonic salt solution after prolonged exposure to low sodium diet, and that the regional AT1 and ACE mRNA are down-regulated after dietary sodium deprivation, which may be mediated by increased ANG II in plasma and/or brain tissue.

The effect of angiotensin-converting enzyme inhibition using captopril on energy balance and glucose homeostasis

Increasing evidence suggests that the renin-angiotensin-system contributes to the etiology of obesity. To evaluate the role of the renin-angiotensin-system in energy and glucose homeostasis, we examined body weight and composition, food intake, and glucose tolerance in rats given the angiotensin-converting enzyme inhibitor, captopril ( approximately 40 mg/kg . d). Rats given captopril weighed less than controls when fed a high-fat diet (369.3 +/- 8.0 vs. 441.7 +/- 8.5 g after 35 d; P < 0.001) or low-fat chow (320.1 +/- 4.9 vs. 339.8 +/- 5.1 g after 21 d; P < 0.0001). This difference was attributable to reductions in adipose mass gained on high-fat (23.8 +/- 2.0 vs. 65.12 +/- 8.4 g after 35 d; P < 0.0001) and low-fat diets (12.2 +/- 0.7 vs. 17.3 +/- 1.3 g after 21 d; P < 0.001). Rats given captopril ate significantly less [3110.3 +/- 57.8 vs. 3592.4 +/- 88.8 kcal (cumulative 35 d high fat diet intake); P < 0.001] despite increased in neuropeptide-Y mRNA expression in the arcuate nucleus of the hypothalamus and had improved glucose tolerance compared with free-fed controls. Comparisons with pair-fed controls indicated that decreases in diet-induced weight gain and adiposity and improved glucose tolerance were due, primarily, to decreased food intake. To determine whether captopril caused animals to defend a lower body weight, animals in both groups were fasted for 24 h and subsequently restricted to 20% of their intake for 2 d. When free food was returned, captopril and control rats returned to their respective body weights and elicited comparable hyperphagic responses. These results suggest that angiotensin-converting enzyme inhibition protects against the development of diet-induced obesity and glucose intolerance.

Prenatal blockade of Ang II receptors affects neonatal rat hindbrain structure and receptor localization

The development of knock-out mice for Angiotensin II (Ang II) AT(2) receptors, which exhibited altered exploratory behavior, prompted us to investigate the cerebellum and brainstem. We evaluated the effect of stimulation/inhibition of Ang II receptors on hindbrain development, in offspring (postnatal days P0, P8) of pregnant rats treated during late pregnancy (Ang II, Losartan or PD123319, 1 mg/kg/day). Receptor localization by autoradiography showed in P0 and P8 hindbrains, that most structures expressed AT(2) subtype: cerebellar cortex, cerebellar nuclei, genu facial nucleus, inferior colicullus, inferior olive. In the cerebellar cortex, [(125)I]Ang II AT(2) binding was predominant, while low AT(1) binding was observed in adjacent layers of the cerebellar cortex. Blockade of AT(2) receptors with PD123319 increased binding in cerebellar nuclei (p<0.05) and brainstem nuclei at P0, P8, in correlation with increased AT(2) receptor expression by RT-PCR. The enlarged external granular layer (EGL) in PD123319-treated P0 pups contrast with the significant decrease in Ang II binding (p<0.001) in the cerebellar cortex. Blockade of AT(2) receptors during late pregnancy seems to arrest cerebellar cortex development in P0 animals. On the contrary, increased AT(2) binding was observed in cerebellar cortex and DTg nucleus in PD123319-treated P8 animals (p<0.001). Ang II treatment leads to increased binding in the brainstem. In spite of the low doses of Ang II antagonists used, treatments were performed during a time-frame critical for hindbrain development, leading to remarkable effects. The present study makes a contribution to understand the role of Ang II receptors during hindbrain development.

Angiotensin II type-2 (AT2) receptor antagonism alters cardiovascular responses to static exercise and simultaneously changes glutamate/GABA levels within the ventrolateral medulla

Angiotensin II receptors (Ang II), classified into AT1 and AT2 subtypes, are located in different regions of the central nervous system, including the cardiovascular control centers in the medulla oblongata. We previously reported the role of Ang II AT1 receptors within the medulla on cardiovascular responses and glutamate/GABA neurotransmission during the exercise pressor reflex [Patel, D., Böhlke, M., Phattanarudee, S., Kabadi, S., Maher, T.J., Ally, A., 2008. Cardiovascular responses and neurotransmitter changes during blockade of angiotensin II receptors within the ventrolateral medulla. Neurosci. Res. 60 (3), 340-348]. In this study, we investigated the role of the AT2 receptor subtype within the ventrolateral medullary region (VLM) in modulating increases in mean arterial pressure (MAP) and heart rate (HR) in response to static skeletal muscle contraction.

METHODS

Using microdialysis methods in anesthetized rats, we administered AR-AT2 antagonists into the rostral (RVLM) and caudal (CVLM) VLM and determined its effects on cardiovascular responses and glutamate/GABA neurotransmission following muscle contraction. Bilateral microdialysis of a selective AT2 antagonist, PD 123319 (10 microM), for 30 min into the RVLM augmented MAP and HR responses during a static muscle contraction. Simultaneously, the drug increased glutamate and decreased GABA levels within the RVLM. After 60 min of discontinuation of the drug, only MAP and HR values but not the neurotransmitter levels in response to a muscle contraction returned to baseline. In contrast, bilateral microdialysis of the drug into the CVLM attenuated cardiovascular responses during a static muscle contraction, decreased glutamate and increased GABA. However, only the cardiovascular responses recovered after 60 min of discontinuation of the drug. These results demonstrate that AT2 within both RVLM and CVLM plays important differential roles in modulating neurotransmission and cardiovascular function during the exercise pressor reflex.

Type-1 angiotensin receptors are expressed and transported in motor and sensory axons of rat sciatic nerves

Angiotensin II (Ang II) and its type-1 receptor (AT(1)) occur in neurons at multiple locations within the organism, but the basic biology of the receptor in the nervous system remains incompletely understood. We previously observed abundant AT(1)-like binding sites and intense expression of AT(1) immunoreactivity in perikarya of the dorsal root ganglion and ventral horn of the rat spinal cord. We have now examined the receptor in rat sciatic nerve, including the dynamics of its axonal transport. Ligand-binding autoradiography of resting nerve showed "hot spots" of (125)I-Ang II binding that could be specifically blocked by the AT(1) antagonist, losartan. Immunohistochemistry with an AT(1)-antibody validated by Western blots also showed patches of AT(1)-reactivity in nerve. These patches were localized around large myelinated axons with faint immunoreactivity in their lumens. Sixteen hours after nerve ligation there was no change in the patches or hot spots, but luminal AT(1)-reactivity increased dramatically in a narrow zone immediately above the ligature. With double ligation there was a pronounced accumulation of AT(1) immunoreactivity proximal to the upstream ligature and a very slight accumulation distal to the second ligature. This asymmetric pattern of accumulation, confirmed by quantitative receptor binding autoradiography, probably reflected axonal transport rather than local production of receptor. Retrograde tracing and stereological analysis to determine the source of transported AT(1) indicated that many AT(1)-positive fibers arise in the ventral horn, and a larger number arise in dorsal root ganglia. A corresponding result was obtained with double-label immunohistochemistry of ligated nerve, which showed AT(1) accumulations in both motor and sensory fibers. We conclude that somatic sensory and motor neurons of the rat export substantial quantities of AT(1) into axons, which transport them to the periphery. The physiologic implications of this finding require further investigation.

Angiotensin II inhibits GABAergic synaptic transmission in dorsolateral periaqueductal gray neurons

The purpose of this study was to determine the role of angiotensin II (Ang II) in modulating inhibitory and excitatory synaptic inputs to the dorsolateral periaqueductal gray (dl-PAG). The whole cell voltage-clamp recording was performed to examine inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs) of the dl-PAG neurons. Ang II, at the concentration of 2microM, decreased the frequency of miniature IPSCs from 0.83+/-0.02 to 0.45+/-0.03Hz (P<0.05) in 10 tested neurons. This did not significantly affect the amplitude and decay time constant. The effect of Ang II on miniature IPSCs was blocked by the prior application of Ang II AT1 receptor antagonist losartan, but not by AT2 receptor antagonist PD123319. Additionally, Ang II decreased the amplitude of evoked IPSCs from 148+/-15 to 89+/-7pA (P<0.05), and increased the paired-pulse ratio from 96+/-5% to 125+/-7% (P<0.05) in eight tested neurons. In contrast, Ang II had no distinct effects on the EPSCs. Our data suggest that Ang II inhibits GABAergic synaptic inputs to the dl-PAG through activation of presynaptic AT1 receptors.

Angiotensin II upregulates hypothalamic AT1 receptor expression in rats via the mitogen-activated protein kinase pathway

ANG II type 1 receptors (AT(1)R) mediate most of the central effects of ANG II on cardiovascular function, fluid homeostasis, and sympathetic drive. The mechanisms regulating AT(1)R expression in the brain are unknown. In some tissues, the AT(1)R can be upregulated by prolonged exposure to ANG II. We examined the hypothesis that ANG II upregulates the AT(1)R in the brain by stimulating the intracellular mitogen-activated protein kinase (MAPK) signaling pathway. Using molecular and immunochemical approaches, we examined expression of the AT(1)R and phosphorylated MAPK in the paraventricular nucleus of the hypothalamus (PVN) and the subfornical organ (SFO) of rats receiving a chronic (4-wk) subcutaneous infusion of ANG II (0.6 microg/h) or saline (vehicle control), with or without concomitant (4-wk) intracerebroventricular (ICV) infusions of MAPK inhibitors or the AT(1)R blocker losartan. Subcutaneous infusion of ANG II markedly increased phosphorylation of MAPK and expression of AT(1)R mRNA and protein and AT(1)R-like immunoreactivity in the PVN and SFO. ANG II-induced AT(1)R expression was blocked by ICV infusion of the p44/42 MAPK inhibitor PD-98059 (0.025 microg/h) and the JNK inhibitor SP-600125 (0.125 microg/h), but not by the p38 MAPK inhibitor SB-203580 (0.125 microg/h). Upregulation of the AT(1)R in the PVN and SFO by peripheral ANG II was abolished by ICV losartan (10 microg/h). The data indicate that blood-borne ANG II upregulates brain AT(1)R by activating intracellular p44/42 MAPK and JNK signaling pathways.

The role of nuclear factor-kappaB in the effect of angiotensin II in the paraventricular nucleus in protecting the gastric mucosa from ischemia-reperfusion injury in rats

BACKGROUND

The purpose of this study was to elucidate the role of nuclear factor kappaB (NF-kappaB) in the development of gastric ischemia-reperfusion (GI-R) injury and in mediating the effects of angiotensin II (Ang II) in the paraventricular nucleus (PVN) on GI-R injury.

METHODS

GI-R injury was induced in rats by clamping the celiac artery for 30 min and then reperfusing for 1 h. A cannula was inserted into the unilateral PVN for microinjection of Ang II. The expressions and levels of NF-kappaB (p65), IkappaB-alpha, and phosphorylated IkappaB-alpha in rat gastric mucosa were examined by Western blotting and immunohistochemistry. A laser Doppler flowmeter was used to assess gastric blood flow (GBF). Malondialdehyde (MDA) was determined using the thiobarbituric acid (TBA) method, and superoxide dismutase (SOD) activity was determined by the xanthine/xanthine oxidase method.

RESULTS

Microinjection of Ang II (3, 30, and 300 ng) into the PVN dose-dependently inhibited GI-R injury. The levels and expressions of NF-kappaB (p65) and phosphospecific IkappaB-alpha protein increased 1 h after GI-R and were markedly reduced by microinjection of Ang II into the PVN. In contrast, the level and expression of IkappaB-alpha protein decreased 1 h after ischemia-reperfusion and recovered to the normal level by microinjection of Ang II into the PVN. The effects of Ang II were prevented by pretreatment with the Ang II AT1 receptor antagonist losartan (5 microg) microinjected into the lateral cerebral ventricle. Inhibition of NF-kappaB activity by pyrrolidine dithiocarbamate (PDTC, 200 mg/kg) produced similar effects in rats subjected to ischemia-reperfusion with or without microinjection of Ang II into the PVN. Administration of PDTC attenuated gastric mucosal injury and suppressed the activation of NF-kappaB (p65). Ang II microinjection into the PVN increased GBF and decreased the MDA content but did not alter SOD activity in the gastric mucosa following ischemia-reperfusion.

CONCLUSIONS

NF-kappaB plays a role in PVN Ang II-mediated protection against GI-R injury. These central effects of Ang II are mediated by AT1 receptors.

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

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

Distribution of cells expressing human renin-promoter activity in the brain of a transgenic mouse

Renin plays a critical role in fluid and electrolyte homeostasis by cleaving angiotensinogen to produce Ang peptides. Whilst it has been demonstrated that renin mRNA is expressed in the brain, the distribution of cells responsible for this expression remains uncertain. We have used a transgenic mouse approach in an attempt to address this question. A transgenic mouse, in which a 12.2 kb fragment of the human renin promoter was used to drive expression of Cre-recombinase, was crossed with the ROSA26-lac Z reporter mouse strain. Cre-recombinase mediated excision of the floxed stop cassette resulted in expression of the reporter protein, beta-galactosidase. This study describes the distribution of beta-galactosidase in the brain of the crossed transgenic mouse. In all cases where it was examined the reporter protein was co-localized with the neuronal marker NeuN. An extensive distribution was observed with numerous cells labeled in the somatosensory, insular, piriform and retrosplenial cortices. The motor cortex was devoid of labeled cells. Several other regions were labeled including the parts of the amygdala, periaqueductal gray, lateral parabrachial nucleus and deep cerebellar nuclei. Overall the distribution shows little overlap with those regions that are known to express receptors for the renin-angiotensin system in the adult brain. This transgenic approach, which demonstrates the distribution of cells which have activated the human renin promoter at any time throughout development, yields a unique and extensive distribution of putative renin-expressing neurons. Our observations suggest that renin may have broader actions in the brain and may indicate a potential for interaction with the (pro)renin receptor or production of a ligand for non-AT(1)/AT(2) receptors.

Imbalance of angiotensin type 1 receptor and angiotensin II type 2 receptor in the rostral ventrolateral medulla: potential mechanism for sympathetic overactivity in heart failure

Upregulation of angiotensin II type 1 receptors (AT(1)R) in the rostral ventrolateral medulla (RVLM) contributes to the sympathoexcitation in the chronic heart failure (CHF). However, the role of angiotensin II type 2 receptor (AT(2)R) is not clear. In this study, we measured AT(1)R and AT(2)R protein expression in the RVLM and determined their effects on renal sympathetic nerve activity, blood pressure, and heart rate in anesthetized sham and CHF rats. We found that (1) although AT(1)R expression in the RVLM was upregulated, the AT(2)R was significantly downregulated (CHF: 0.06+/-0.02 versus sham: 0.15+/-0.02, P<0.05); (2) simultaneously stimulating RVLM AT(1)R and AT(2)R by angiotensin II evoked sympathoexcitation, hypertension, and tachycardia in both sham and CHF rats with greater responses in CHF; (3) stimulating RVLM AT1R with angiotensin II plus the specific AT(2)R antagonist PD123319 induced a larger sympathoexcitatory response than simultaneously stimulating AT(1)R and AT(2)R in sham rats, but not in CHF; (4) activating RVLM AT(2)R with CGP42112 induced a sympathoinhibition, hypotension, and bradycardia only in sham rats (renal sympathetic nerve activity: 36.4+/-5.1% of baseline versus 102+/-3.9% of baseline in artificial cerebrospinal fluid, P<0.05); (5) pretreatment with 5,8,11,14-eicosatetraynoic acid, a general inhibitor of arachidonic acid metabolism, into the RVLM attenuates the CGP42112-induced sympathoinhibition. These results suggest that AT(2)R in the RVLM exhibits an inhibitory effect on sympathetic outflow, which is, at least partially, mediated by an arachidonic acid metabolic pathway. These data implicate a downregulation in the AT(2)R as a contributory factor in the sympathoexcitation in CHF.

Mitogen-activated protein kinases mediate upregulation of hypothalamic angiotensin II type 1 receptors in heart failure rats

In heart failure (HF), angiotensin II type 1 receptor (AT(1)-R) expression is upregulated in brain regions regulating sympathetic drive, blood pressure, and body fluid homeostasis. However, the mechanism by which brain AT(1)-R are upregulated in HF remains unknown. The present study examined the hypothesis that the angiotensin II (Ang II)-triggered mitogen-activated protein kinases (MAPKs) p44/42, p38, and c-Jun N-terminal kinase contribute to upregulation of the AT(1)-R in the hypothalamus of rats with HF. AT(1)-R protein, AT(1)-R mRNA, and AT(1)-R immunoreactivity increased in the paraventricular nucleus of hypothalamus and the subfornical organ of rats with ischemia-induced HF compared with sham-operated controls. Phosphorylated p44/42 MAPK, c-Jun N-terminal kinase, and p38 MAPK also increased in paraventricular nucleus and subfornical organ. A 4-week ICV infusion of the AT(1)-R antagonist losartan decreased AT(1)-R protein and phosphorylation of p44/42 MAPK, c-Jun N-terminal kinase, and p38 MAPK in the HF rats. A 4-week ICV infusion of the p44/42 MAPK inhibitor PD98059 or the c-Jun N-terminal kinase inhibitor SP600125 significantly decreased AT(1)-R protein and AT(1)-R immunoreactivity in the paraventricular nucleus and subfornical organ, but the p38 MAPK inhibitor SB203580 did not. Treatment with ICV losartan, PD98059, and SP600125 had no effect on AT(1)-R expression by Western blot in sham-operated rats. In untreated HF rats 4 weeks after coronary ligation, a 3-hour ICV infusion of PD98059, SP600125, or losartan reduced AT(1)-R mRNA in paraventricular nucleus and subfornical organ. These data indicate that MAPK plays an important role in the upregulation of AT(1)-R in the rat forebrain in HF and suggest that Ang II upregulates its own receptor by this mechanism.

Vasopressin and angiotensin receptors of the medial septal area in the control of mean arterial pressure induced by vasopressin

Introduction. Brain arginine 8-vasopressin (AVP), through the V1a- and V2-receptors, is essential for the maintenance of mean arterial pressure (MAP). Central AVP interacts with the components of the renin-angiotensin system, which participate in MAP regulation. This study aimed to determine the effects of V1a-, V2- and V1a/V2-AVP selective antagonists and AT1- and AT2-angiotensin II (Ang II) selective antagonists on the MAP induced by AVP injected into the medial septal area (MSA) of the brain.

Materials and methods. Male Holtzman rats with stainless steel cannulae implanted into the MSA were used in experiments. Direct MAP was recorded in conscious rats.

Results. AVP administration into the MSA caused a prompt and potent pressor response in a dose-dependent fashion. Pretreatment with the V1a- and V2-antagonists reduced, whereas prior injection of the V1a/V2-antagonist induced a decrease in the MAP that remained below the baseline. Both AT 1- and AT2-antagonists elicited a decrease, while simultaneous injections of two antagonists were more effective in decreasing the MAP induced by AVP.

Conclusion. These results indicate there is a synergism between the V1a- and V2-AVP and AT1and AT2-Ang II receptors in the MSA in the regulation of MAP.

Angiotensin II stimulates water and NaCl intake through separate cell signalling pathways in rats

Angiotensin II (AngII) stimulation of water and NaCl intake is a classic model of the behavioural effects of hormones. In vitro studies indicate that the AngII type 1 (AT(1)) receptor stimulates intracellular pathways that include protein kinase C (PKC) and mitogen-activated protein (MAP) kinase activation. Previous studies support the hypotheses that PKC is involved in AngII-induced water, but not NaCl intake and that MAP kinase plays a role in NaCl consumption, but not water intake, after injection of AngII. The present experiments test these hypotheses in rats using central injections of AngII in the presence or absence of a PKC inhibitor or a MAP kinase inhibitor. Pretreatment with the PKC inhibitor chelerythrine attenuated AngII-induced water intake, but NaCl intake was unaffected. In contrast, pretreatment with U0126, a MAP kinase inhibitor, had no effect on AngII-induced water intake, but attenuated NaCl intake. These data support the working hypotheses and significantly extend our earlier findings and those of others. Perhaps more importantly, these experiments demonstrate the remarkable diversity of peptide receptor systems and add support for the surprising finding that intracellular signalling pathways can have divergent behavioural relevance.

Subfornical organ differentially modulates baroreflex function in normotensive and two-kidney, one-clip hypertensive rats

During activation of the renin-angiotensin system, hindbrain circumventricular organs such as the area postrema have been implicated in modulating the arterial baroreflex. This study was undertaken to test the hypothesis that the subfornical organ (SFO), a forebrain circumventricular structure, may also modulate the baroreflex. Studies were performed in rats with two-kidney, one-clip (2K,1C) hypertension as a model of endogenously activated renin-angiotensin system. Baroreflex function was ascertained during ramp infusions of phenylephrine and nitroprusside in conscious sham-clipped and 5-wk 2K,1C rats with either a sham or electrolytically lesioned SFO. Lesioning significantly decreased mean arterial pressure in 2K,1C rats from 158 +/- 7 to 131 +/- 4 mmHg but not in sham-clipped rats. SFO-lesioned, sham-clipped rats had a significantly higher upper plateau and range of the renal sympathetic nerve activity-mean arterial pressure relationship compared with sham-clipped rats with SFO ablation. In contrast, lesioning the SFO in 2K,1C rats significantly decreased both the upper plateau and range of the baroreflex control of renal sympathetic nerve activity, but only the range of the baroreflex response of heart rate decreased. Thus, during unloading of the baroreceptors, the SFO differentially modulates the baroreflex responses in sham-clipped vs. 2K,1C rats. Since lesioning the SFO did not influence plasma angiotensin II (ANG II), the effects of the SFO lesion are not caused by changes in circulating levels of ANG II. These findings support a pivotal role for the SFO in the sympathoexcitation observed in renovascular hypertension and in baroreflex regulation of sympathetic activity in both normal and hypertensive states.

Osmoregulatory fluid intake but not hypovolemic thirst is intact in mice lacking angiotensin

Water intakes in response to hypertonic, hypovolemic, and dehydrational stimuli were investigated in mice lacking angiotensin II as a result of deletion of the angiotensinogen gene (Agt−/− mice), and in C57BL6 wild-type (WT) mice. Baseline daily water intake in Agt−/− mice was approximately threefold that of WT mice because of a renal developmental disorder of the urinary concentrating mechanisms in Agt−/− mice. Intraperitoneal injection of hypertonic saline (0.4 and 0.8 mol/l NaCl) caused a similar dose-dependent increase in water intake in both Agt−/− and WT mice during the hour following injection. As well, Agt−/− mice drank appropriate volumes of water following water deprivation for 7 h. However, Agt−/− mice did not increase water or 0.3 mol/l NaCl intake in the 8 h following administration of a hypovolemic stimulus (30% polyethylene glycol sc), whereas WT mice increased intakes of both solutions during this time. Osmoregulatory regions of the brain [hypothalamic paraventricular and supraoptic nuclei, median preoptic nucleus, organum vasculosum of the lamina terminalis (OVLT), and subfornical organ] showed an increased number of neurons exhibiting Fos-immunoreactivity in response to intraperitoneal hypertonic NaCl in both Agt−/− mice and WT mice. Polyethylene glycol treatment increased Fos-immunoreactivity in the subfornical organ, OVLT, and supraoptic nuclei in WT mice but only increased Fos-immunoreactivity in the supraoptic nucleus in Agt−/− mice. These data show that brain angiotensin is not essential for the adequate functioning of neural pathways mediating osmoregulatory thirst. However, angiotensin II of either peripheral or central origin is probably necessary for thirst and salt appetite that results from hypovolemia.

Baroreflex control of heart rate and renal sympathetic nerve activity in rats with low brain angiotensinogen

The main objective of the present study was to evaluate baroreceptor control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in transgenic rats (TG) with low angiotensinogen production in glial cells, TGR(ASrAogen)-680. In addition, the sympathetic and vagal autonomic tonus to the heart was investigated. As previously shown, TG rats presented a lower arterial pressure (AP) and HR. However, TG rats had decreased AP variability during the night (8.9+/-0.4 mmHg vs 9.8+/-0.3 mmHg, in SD) accompanied by an increase in HR variability (39+/-1 beats/min vs 35+/-1 beats/min, in SD) and augmented locomotor activity during the night (3.5+/-0.3 counts/min vs 2.5+/-0.2 counts/min, in SD). In addition, TG rats presented increased baroreflex sensitivity for the RSNA (slope of line that correlates decreases in RSNA and increases in AP=1.36+/-0.18 vs 0.77+/-0.1, in SD) and an increased sensitivity for both the baroreflex bradycardia (0.79+/-0.04 ms/mmHg vs 0.52+/-0.04 ms/mmHg, in SD) and tachycardia (1.46+/-0.1 ms/mmHg vs 0.93+/-0.01 ms/mmHg, in SD). Further, TG rats had increased vagal tonus (25+/-3 beats/min vs 11+/-4 beats/min in SD) without significant change in the sympathetic tonus to the heart. These results confirm and extend previous observations showing that glial angiotensinogen, the main source of brain RAS peptides, importantly modulates sympathetic tonus, at least to the renal nerve, and vagal tonus to the heart.

Human brain aminopeptidase A: biochemical properties and distribution in brain nuclei

Aminopeptidase A (APA) generated brain angiotensin III, one of the main effector peptides of the brain renin angiotensin system, exerting a tonic stimulatory effect on the control of blood pressure in hypertensive rats. The distribution of APA in human brain has not been yet studied. We first biochemically characterized human brain APA (apparent molecular mass of 165 and 130 kDa) and we showed that the human enzyme exhibited similar enzymatic characteristics to recombinant mouse APA. Both enzymes had similar sensitivity to Ca(2+). Kinetic studies showed that the K(m) (190 mumol/L) of the human enzyme for the synthetic substrate-l-glutamyl-beta-naphthylamide was close from that of the mouse enzyme (256 mumol/L). Moreover, various classes of inhibitors including the specific and selective APA inhibitor, (S)-3-amino-4-mercapto-butyl sulfonic acid, had similar inhibitory potencies toward both enzymes. Using (S)-3-amino-4-mercapto-butyl sulfonic acid, we then specifically measured the activity of APA in 40 microdissected areas of the adult human brain. Significant heterogeneity was found in the activity of APA in the various analyzed regions. The highest activity was measured in the choroids plexus and the pineal gland. High activity was also detected in the dorsomedial medulla oblongata, in the septum, the prefrontal cortex, the olfactory bulb, the nucleus accumbens, and the hypothalamus, especially in the paraventricular and supraoptic nuclei. Immunostaining of human brain sections at the level of the medulla oblongata strengthened these data, showing for the first time a high density of immunoreactive neuronal cell bodies and fibers in the motor hypoglossal nucleus, the dorsal motor nucleus of the vagus, the nucleus of the solitary tract, the Roller nucleus, the ambiguus nucleus, the inferior olivary complex, and in the external cuneate nucleus. APA immunoreactivity was also visualized in vessels and capillaries in the dorsal motor nucleus of the vagus and the inferior olivary complex. The presence of APA in several human brain nuclei sensitive to angiotensins and involved in blood pressure regulation suggests that APA in humans is an integral component of the brain renin angiotensin system and strengthens the idea that APA inhibitors could be clinically tested as an additional therapy for the treatment of certain forms of hypertension.

Administration of angiotensin II in the paraventricular nucleus protects gastric mucosa from ischemia-reperfusion injury

Our previous study demonstrated that electrical stimulation of the hypothalamic paraventricular nucleus (PVN) protects against gastric ischemia-reperfusion (GI-R) injury, but it is still unknown whether angiotensin II (Ang II) in the PVN plays a role in the development of GI-R. The purpose of this study was to investigate the effect of Ang II in the PVN on GI-R injury. GI-R injury was induced in rats by clamping the celiac artery for 30 min, and then reperfusing for 30 min, 1 h, 3 h, 6 h or 24 h, respectively. A cannula was inserted into the unilateral PVN for microinjection of Ang II. The extent of gastric mucosal damage was determined by gross and histological methods. We found that microinjection of pharmacological doses of Ang II (3, 30, and 300 ng) into the PVN dose-dependently inhibited GI-R injury, and that Ang II (30 ng) markedly attenuated GI-R injury at 1 h and 3 h after reperfusion. The effect of Ang II was prevented by pretreatment with the Ang II AT1 receptor antagonist losartan (5 microg) into the lateral cerebral ventricle. Furthermore, the protective effect of Ang II on GI-R injury was abolished by propranolol (1 mg/kg, i.v.) or disconnection of the nerves innervating the adrenal glands, was augmented by sympathectomy or phentolamine (1 mg/kg, i.v.), and was not affected by subdiaphragmatic vagotomy or atropine (1 mg/kg, i.v.). These results indicate that the PVN is a responsive site for central Ang II-induced protection against GI-R injury. The central effects of Ang II are mediated by AT1 receptors in the PVN, and the peripheral effects by a sympathetic-adrenal gland/beta-adrenoceptor pathway.

Jacques Benoit lecture: the neuroendocrine view of the angiotensin and apelin systems

The hyperactivity of the brain renin-angiotensin system (RAS) has been implicated in the development and maintenance of hypertension in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III display the same affinity for type 1 and type 2 Ang II receptors. Both peptides, injected intracerebroventricularly, similarly increase arginine vasopressin (AVP) release and blood pressure (BP); however, because Ang II is converted in vivo to Ang III, the identity of the true effector is unknown. We review new insights into the predominant role of brain Ang III in the control of BP, underlining the fact that brain aminopeptidase A (APA), the enzyme generating brain Ang III, may therefore be an interesting candidate target for the treatment of hypertension. This justifies the development of potent systemically active APA inhibitors, such as RB150, as prototypes of a new class of antihypertensive agents for the treatment of certain forms of hypertension. We also searched for a putative angiotensin receptor subtype specific for Ang III and isolated a seven transmembrane-domain G protein-coupled receptor corresponding to the receptor for apelin, a newly-discovered peptide isolated from bovine stomach. Apelin and its receptor are expressed in magnocellular vasopressinergic neurones in the hypothalamus. The central injection of apelin in lactating rats decreases the phasic electrical activity of vasopressinergic neurones and the systemic secretion of AVP, inducing water diuresis. Apelin is therefore a natural inhibitor of the antidiuretic effect of AVP. In addition, systemic administration of apelin decreases BP, improves cardiac contractility and reduces cardiac loading. The development of nonpeptide agonists of the apelin receptor may provide new therapeutic tools for treating water retention, hyponatraemia and cardiovascular diseases. Angiotensins and apelin thus exert opposing but complementary effects, and are thereby determinant for the maintenance of body fluid homeostasis and cardiovascular functions.

Hypertension, RAS, and gender: what is the role of aminopeptidases?

Hypertension is the major risk factor for coronary heart disease, stroke, and renal disease. Also, it is probably the most important risk factor for peripheral vascular disease and vascular dementia. Although hypertension occurs in both men and women, gender differences have been observed. However, whether sex hormones are responsible for the observed gender-associated differences in arterial blood pressure, and which is their mechanism of action, remains unclear. Local and circulating renin-angiotensin systems (RAS) are examples of systems that may be involved in the pathogenesis of hypertension. Classically, angiotensin II (Ang II) has been considered as the effector peptide of the RAS, but Ang II is not the only active peptide. Several of its degradation products, including angiotensin III (Ang III) and angiotensin IV (Ang IV) also possess biological functions. These peptides are formed via the activity of several aminopeptidases. This review will briefly summarize what is known about gender differences in RAS-regulating aminopeptidase activities, their relationship with sex hormones, and their potential role in controlling blood pressure acting through local and circulating RAS.

Aminopeptidase A inhibitors as centrally acting antihypertensive agents

Among the main bioactive peptides of the brain renin-angiotensin system, angiotensin (Ang) II and AngIII exhibit the same affinity for the type 1 and type 2 Ang receptors. Both peptides, injected intracerebroventricularly, cause similar increase in blood pressure (BP). Because AngII is converted in vivo to AngIII, the identity of the true effector is unknown. This review summarized recent insights into the predominant role of brain AngIII in the central control of BP underlining the fact that brain aminopeptidase A (APA), the enzyme forming central AngIII, could constitute a putative central therapeutic target for the treatment of hypertension. This led to the development of potent, systematically active APA inhibitors, such as RB150, as a prototype of a new class of centrally acting antihypertensive agents for the treatment of certain forms of hypertension.

The area postrema: a brain monitor and integrator of systemic autonomic state

The area postrema is a medullary structure lying at the base of the fourth ventricle. The area postrema's privileged location outside of the blood-brain barrier make this sensory circumventricular organ a vital player in the control of autonomic functions by the central nervous system. By virtue of its lack of tight junctions between endothelial cells in this densely vascularized structure and the presence of fenestrated capillaries, peptide and other physiological signals borne in the blood have direct access to neurons that project to brain areas with important roles in the autonomic control of many physiological systems, including the cardiovascular system and systems controlling feeding and metabolism. However, the area postrema is not simply a conduit through which signals flow into the brain, but it is now being recognized as the initial site of integration for these signals as they enter the circuitry of the central nervous system.

Selective angiotensin II AT2 receptor agonists devoid of the imidazole ring system

A versatile parallel synthetic method to obtain three series of non-cyclic analogues of the first drug-like selective angiotensin II AT(2) receptor agonist (1) has been developed. In analogy with the transformation of losartan to valsartan it was demonstrated that a non-cyclic moiety could be employed as an imidazole replacement to obtain AT(2) selective compounds. In all the three series, AT(2) receptor ligands with affinities in the lower nanomolar range were found. None of the analogues exhibited any affinity for the AT(1) receptor. Four compounds, 17, 22, 39 and 51, were examined in a neurite outgrowth cell assay. All four compounds were found to exert a high agonistic effect as deduced from their capacity to induce neurite elongation in neuronal cells, as does angiotensin II.

Central nitric oxide blocks vasopressin, oxytocin and atrial natriuretic peptide release and antidiuretic and natriuretic responses induced by central angiotensin II in conscious rats

The presence of nitric oxide synthase (NOS), the enzyme that catalyses the formation of nitric oxide (NO), in the circumventricular organs and magnocellular neurones suggests an important role of NO in the modulation of vasopressin (AVP) and oxytocin (OT) release. Intracerebroventricular (I.C.V.) injection of angiotensin II (Ang II) stimulates the release of AVP, OT and atrial natriuretic peptide (ANP), with the resultant antidiuretic and natriuretic effects. This study investigated the interaction between nitrergic and angiotensinergic pathways on the release of AVP, OT and ANP and on urinary volume and sodium excretion in water-loaded rats. Unanaesthetized, freely moving, male Wistar rats received two water loads followed by an injection into the lateral ventricle of an inhibitor of NOS (L-NAME), a NO donor [3-morpholinylsydnoneimine chloride (SIN-1) or S-nitroso-N-acetyl penicillamine (SNAP)] or vehicle (isotonic saline) and, 20 min after, they received a second I.C.V. injection of Ang II or vehicle. Injections of L-NAME or Ang II produced an increase in plasma levels of AVP, OT and ANP, a reduction in urinary volume and an increase in sodium excretion. Pretreatment with L-NAME enhanced the Ang II-induced increase in AVP, OT and ANP release, as well as the antidiuresis and natriuresis. Injection of SIN-1 or SNAP did not modify hormonal plasma levels and urinary parameters. In contrast SNAP blocked the AVP, OT and ANP release, as well as antidiuretic and natriuretic responses induced by ANG-II. Thus, the central nitrergic system can act to inhibit AVP, OT and ANP secretion and the antidiuretic and natriuretic effects in response to Ang II.

Involvement of angiotensin-(1-7) in the hypothalamic hypotensive effect of captopril in sinoaortic denervated rats

The role of anterior hypothalamic angiotensin-(1-7) (Ang-(1-7)) on blood pressure regulation was studied in sinoaortic denervated (SAD) rats. Since angiotensin-converting enzyme inhibitors increase endogenous levels of Ang-(1-7), we addressed the involvement of Ang-(1-7) in the hypotensive effect induced by captopril in SAD rats. Wistar rats 7 days after SAD or sham operation (SO) were anaesthetized and the carotid artery was cannulated for monitoring mean arterial pressure (MAP). A needle was inserted into the anterior hypothalamus for drug administration. Intrahypothalamic administration of Ang-(1-7) (5 pmol) was without effect in SO rats but reduced MAP in SAD rats by 15.5+/-3.2 mm Hg and this effect was blocked by 250 pmol [D-Ala(7)]-Ang-(1-7), a Mas receptor antagonist. Angiotensin II (Ang II) induced an increase in MAP in both groups being the effect greater in SAD rats (DeltaMAP=15.8+/-1.4 mm Hg) than in SO rats (DeltaMAP=9.6+/-1.0 mm Hg). Ang-(1-7) partially abolished the pressor response caused by Ang II in SAD rats. Whilst the captopril intrahypothalamic injection did not affect MAP in SO animals, it significantly reduced MAP in SAD rats (DeltaMAP=-13.3+/-1.9 mm Hg). Either [D-Ala(7)]-Ang-(1-7) or an anti-Ang-(1-7) polyclonal antibody partially blocked the MAP reduction caused by captopril. In conclusion, whilst Ang-(1-7) does not contribute to hypothalamic blood pressure regulation in SO normotensive animals, in SAD rats the heptapeptide induces a reduction of blood pressure mediated by Mas receptor activation. Although Ang-(1-7) is not formed in enough amount in the AHA of SAD animals to exert cardiovascular effects in normal conditions, our results suggest that enhancement of hypothalamic Ang-(1-7) levels by administration of captopril is partially involved in the hypotensive effect of the ACE inhibitor.

Upregulation of angiotensin AT1 receptor and intracellular kinase gene expression in hypertensive rats

1. Activation of angiotensin II AT1 receptors (AT1R) stimulates catecholamine systems within both central and peripheral tissues that are associated with blood pressure control. In the present study, we sought to determine whether the hypertensive phenotype of the spontaneously hypertensive rat (SHR) is associated with changes in AT1R gene expression and whether gene expression of downstream signalling molecules was coupled to catecholamine gene expression, both in key brainstem nuclei and in peripheral sites implicated in cardiovascular control. 2. Gene expression levels of AT1R, extracellular signal-regulated kinase (ERK) 1 and 2 and phosphatidylinositol 3-kinase (PI3-K) were quantified in Wistar-Kyoto (WKY) rats and SHR. Messenger RNA expression levels were quantified using real time reverse transcription-polymerase chain reaction. In addition, we investigated whether there was a relationship between gene expression and systolic blood pressure. 3. The gene expression levels of AT1R, ERK2 and PI3-K were significantly higher in the paraventricular nucleus of the hypothalamus (4.12-, 1.40- and 1.38-fold, respectively), rostral ventrolateral medulla (2.71-, 1.33- and 2.73-fold, respectively), spinal cord (30.5-, 2.72- and 1.53-fold, respectively), adrenal medulla (1.68-, 1.55- and 1.76-fold, respectively) and coeliac ganglion (1.39-, 1.35- and 1.12-fold, respectively) in SHR compared with WKY rats. There was no significant difference in the level of ERK1 gene expression between the two strains. The gene expression levels of AT1R and ERK2 were positively correlated with blood pressure in all central nervous tissues investigated in the SHR, but not in WKY rats. Gene expression levels of the AT1R in the coeliac ganglion and adrenal medulla were also positively correlated with increased systolic blood pressure. 4. The present data suggest that a defect in AT1R expression (that may further alter downstream signalling pathways) in the SHR may be responsible, at least in part, for the hypertensive phenotype.

Training-induced pressure fall in spontaneously hypertensive rats is associated with reduced angiotensinogen mRNA expression within the nucleus tractus solitarii

Knowing that exercise training reduces arterial pressure in hypertensive individuals and that pressure fall is accompanied by blockade of brain renin-angiotensin system, we sought to investigate whether training (T) affects central renin-angiotensin system. Spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto controls (WKY) were submitted to training or kept sedentary (S) for 3 months. After functional recordings, brain was removed and processed for autoradiography (brain stem sequential slices hybridized with (35)S-oligodeoxynucleotide probes for angiotensinogen [Aogen] and angiotensin II type 1 [AT(1A)] receptors). Resting arterial pressure and heart rate were higher in SHR(S) (177+/-2 mm Hg, 357+/-12 bpm versus 121+/-1 mm Hg, 320+/-9 bpm in WKY(S); P<0.05). Training was equally effective to enhance treadmill performance and to cause resting bradycardia (-10%) in both groups. Training-induced blood pressure fall (-6.3%) was observed only in SHR(T). In SHR(S) (versus WKY(S)) AT(1A) and Aogen mRNA expression were significantly increased within the NTS and area postrema (average of +67% and +41% for AT(1A) and Aogen, respectively; P<0.05) but unchanged in the gracilis nucleus. Training did not change AT(1A) expression but reduced NTS and area postrema Aogen mRNA densities specifically in SHR(T) (P<0.05 versus SHR(S), with values within the range of WKY groups). In SHRs, NTS Aogen mRNA expression was correlated with resting pressure (y=5.95x +41; r=0.55; P<0.05), with no significant correlation in the WKY group. Concurrent training-induced reductions of both Aogen mRNA expression in brain stem cardiovascular-controlling areas and mean arterial pressure only in SHRs suggest that training is as efficient as the renin-angiotensin blockers to reduce brain renin-angiotensin system overactivity and to decrease arterial pressure.

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

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

Angiotensin Type 2 Receptor in Resistance Arteries of Type 2 Diabetic Hypertensive Patients

The role of angiotensin type 2 receptor (AT 2 R) on vascular responses to angiotensin II in humans remains unclear. In this study we explored whether AT 2 R is expressed and functionally active on peripheral resistance arteries of hypertensive diabetic patients treated for 1 year with either the angiotensin receptor blocker valsartan or the β-blocker atenolol. Twenty-six hypertensive type 2 diabetic patients treated with oral hypoglycemic and antihypertensive agents (not receiving angiotensin receptor blockers or β-blockers) were randomly assigned to double-blind treatment for 1 year with valsartan or atenolol once daily added to their previous therapy in a clinical trial that we reported recently and compared with 10 normal subjects. Resistance arteries dissected from gluteal subcutaneous tissues were assessed on a pressurized myograph. Vasomotor response curves to angiotensin II (1 nmol/L to 1 μmol/L) were performed on norepinephrine precontracted vessels in the presence of valsartan (10 μmol/L) with or without the AT 2 R inhibitor PD123319 (1 μmol/L). AT 2 R expression was evaluated by confocal microscopy. After 1 year of treatment, systolic and diastolic blood pressure was controlled and comparable in the valsartan and atenolol groups. Angiotensin II evoked a significant vasodilatory response only on resistance arteries from patients treated with valsartan, effect blocked by PD123319. AT 2 R expression was 4-fold higher in small arteries of valsartan-treated patients. In conclusion, AT 2 Rs are upregulated and contribute to angiotensin II–induced vasodilation in resistance arteries of hypertensive diabetic patients treated with angiotensin type 1 receptor blockers and may mediate, in part, vascular actions of these drugs in high cardiovascular risk patients.

Heat acclimation and cross-tolerance against novel stressors: genomic–physiological linkage

Heat acclimation (AC) is a "within lifetime" reversible phenotypic adaptation, enhancing thermotolerance and heat endurance via a transition to "efficient" cellular performance when acclimatory homeostasis is reached. An inseparable outcome of AC is the development of cross-tolerance (C-T) against novel stressors. This chapter focuses on central plasticity and the molecular-physiological linkage of acclimatory and C-T responses. A drop in temperature thresholds (T-Tsh) for activation of heat-dissipation mechanisms and an elevated T-Tsh for thermal injury development imply autonomic nervous system (ANS) and cytoprotective network involvement in these processes. During acclimation, the changes in T-Tsh for heat dissipation are biphasic. Initially T-Tsh drops, signifying the early autonomic response, and is associated with perturbed peripheral effector cellular performance. Pre-acclimation values return when acclimatory homeostasis is achieved. The changes in the ANS suggest that acclimatory plasticity involves molecular and cellular changes. These changes are manifested by the activation of central peripheral molecular networks and post-translational modifications. Sympathetic induction of elevated HSP 72 reservoirs, with faster heat shock response, is only one example of this. The global genomic response, detected using gene-chips and cluster analyses imply upregulation of genes encoding ion channels, pumps, and transporters (markers for neuronal excitability) in the hypothalamus at the onset of AC and down regulation of metabotrophic genes upon long term AC. Peripherally, the transcriptional program indicates a two-tier defense strategy. The immediate transient response is associated with the maintenance of DNA and cellular integrity. The sustained response correlates with long-lasting cytoprotective-signaling networks. C-T is recorded against cerebral hypoxia, hyperoxia, and traumatic brain injury. Using the highly developed ischemic/reperfused heart model as a baseline, it is evident that C-T stems via protective shared pathways developed with AC. These comprise constitutive elevation of HIF 1alpha and associated target pathways, HSPs, anti-apoptosis, and antioxidative pathways. Collectively the master regulators of AC and C-T are still enigmatic; however, cutting-edge investigative techniques, using a broad molecular approach, challenge current ideas, and the data accumulated will pinpoint novel pathways and provide new perspectives.

Selective Angiotensin II AT2 Receptor Agonists: Arylbenzylimidazole Structure−Activity Relationships

Structural alterations in the 2- and 5-positions of the first drug-like selective angiotensin II AT2 receptor agonist (1) have been performed. The imidazole ring system was proven to be a strong determinant for the AT2 selectivity, and with few exceptions all variations gave good AT2 receptor affinities and with retained high AT2/AT1 selectivities. On the contrary to the findings with AT1 receptor agonists, the impact of structural modifications in the 5-position of the AT2 selective compounds were less pronounced regarding activation of the AT2 receptor. The butyloxyphenyl (56) and the propylthienyl (50) derivatives were found to exert a high agonistic effect as deduced from their capacity to induce neurite elongation in neuronal cells, as does angiotensin II.