Angiotensin receptors in the brain

Neurogenic Hypertension, the Blood-Brain Barrier, and the Potential Role of Targeted Nanotherapeutics

Hypertension is a major health concern globally. Elevated blood pressure, initiated and maintained by the brain, is defined as neurogenic hypertension (NH), which accounts for nearly half of all hypertension cases. A significant increase in angiotensin II-mediated sympathetic nervous system activity within the brain is known to be the key driving force behind NH. Blood pressure control in NH has been demonstrated through intracerebrovascular injection of agents that reduce the sympathetic influence on cardiac functions. However, traditional antihypertensive agents lack effective brain permeation, making NH management extremely challenging. Therefore, developing strategies that allow brain-targeted delivery of antihypertensives at the therapeutic level is crucial. Targeting nanotherapeutics have become popular in delivering therapeutics to hard-to-reach regions of the body, including the brain. Despite the frequent use of nanotherapeutics in other pathological conditions such as cancer, their use in hypertension has received very little attention. This review discusses the underlying pathophysiology and current management strategies for NH, as well as the potential role of targeted therapeutics in improving current treatment strategies.

The AT1 Receptor Blocker Telmisartan Reduces Intestinal Mucus Thickness in Obese Mice

The angiotensin II (type 1) (AT₁) receptor blocker telmisartan (TEL) is beneficial for the treatment of individuals suffering from metabolic syndrome. As we have shown that TEL has an impact on gut microbiota, we investigated here whether TEL influences gut barrier function. C57BL/6N mice were fed with chow or high-fat diet (HFD) and treated with vehicle or TEL (8 mg/kg/day). Mucus thickness was determined by immunohistochemistry. Periodic Acid-Schiff staining allowed the number of goblet cells to be counted. Using western blots, qPCR, and immunohistochemistry, factors related to mucus biosynthesis (Muc2, St6galnac), proliferation (Ki-67), or necroptosis (Rip3) were measured. The influence on cell viability was determined in vitro by using losartan, as the water solubility of TEL was too low for in vitro experiments. Upon HFD, mice developed obesity as well as leptin and insulin resistance, which were prevented by TEL. Mucus thickness upon HFD-feeding was diminished. Independent of feeding, TEL additionally reduced mucus thickness. Numbers of goblet cells were not affected by HFD-feeding and TEL. St6galnac expression was increased by TEL. Rip3 was increased in TEL-treated and HFD-fed mice, while Ki-67 decreased. Cell viability was diminished by using >1 mM losartan. The anti-obese effect of TEL was associated with a decrease in mucus thickness, which was likely not related to a lower expression of Muc2 and goblet cells. A decrease in Ki-67 and increase in Rip3 indicates lower cell proliferation and increased necroptosis upon TEL. However, direct cell toxic effects are ruled out, as in vivo concentrations are lower than 1 mM.

Renin-Angiotensin System Alterations in the Human Alzheimer's Disease Brain

BACKGROUND

Understanding Alzheimer's disease (AD) in terms of its various pathophysiological pathways is essential to unravel the complex nature of the disease process and identify potential therapeutic targets. The renin-angiotensin system (RAS) has been implicated in several brain diseases, including traumatic brain injury, ischemic stroke, and AD.

OBJECTIVE

This study was designed to evaluate the protein expression levels of RAS components in postmortem cortical and hippocampal brain samples obtained from AD versus non-AD individuals.

METHODS

We analyzed RAS components in the cortex and hippocampus of postmortem human brain samples by western blotting and immunohistochemical techniques in comparison with age-matched non-demented controls.

RESULTS

The expression of AT1R increased in the hippocampus, whereas AT2R expression remained almost unchanged in the cortical and hippocampal regions of AD compared to non-AD brains. The Mas receptor was downregulated in the hippocampus. We also detected slight reductions in ACE-1 protein levels in both the cortex and hippocampus of AD brains, with minor elevations in ACE-2 in the cortex. We did not find remarkable differences in the protein levels of angiotensinogen and Ang II in either the cortex or hippocampus of AD brains, whereas we observed a considerable increase in the expression of brain-derived neurotrophic factor in the hippocampus.

CONCLUSION

The current findings support the significant contribution of RAS components in AD pathogenesis, further suggesting that strategies focusing on the AT1R and AT2R pathways may lead to novel therapies for the management of AD.

Effects of Hyperosmolality on Hypothalamic Astrocytic Area, mRNA Expression and Glutamate Balance In Vitro

During prolonged dehydration, body fluid homeostasis is challenged by extracellular fluid (ECF) hyperosmolality, which induce important functional changes in the hypothalamus, in parallel with other effector responses, such as the activation of the local renin-angiotensin system (RAS). Therefore, in the present study we investigated the role of sodium-driven ECF hyperosmolality on glial fibrillary acid protein (GFAP) immunoreactivity and protein expression, membrane capacitance, mRNA expression of RAS components and glutamate balance in cultured hypothalamic astrocytes. Our data show that hypothalamic astrocytes respond to increased hyperosmolality with a similar decrease in GFAP expression and membrane capacitance, indicative of reduced cellular area. Hyperosmolality also downregulates the transcript levels of angiotensinogen and both angiotensin-converting enzymes, whereas upregulates type 1a angiotensin II receptor mRNA. Incubation with hypertonic solution also decreases the immunoreactivity to the membrane glutamate/aspartate transporter (GLAST) as well as tritiated-aspartate uptake by astrocytes. This latter effect is completely restored to basal levels when astrocytes previously exposed to hypertonicity are incubated under isotonic conditions. Together with a direct effect on two important local signaling systems (glutamate and RAS), these synaptic rearrangements driven by astrocytes may accomplish for a coordinated increase in the excitatory drive onto the hypothalamic neurosecretory system, ultimately culminating with increased AVP release in response to hyperosmolality.

Telmisartan prevents diet-induced obesity and preserves leptin transport across the blood-brain barrier in high-fat diet-fed mice

Obesity is a global health problem and treatment options are still insufficient. When chronically treated with the angiotensin II receptor blocker telmisartan (TEL), rodents do not develop diet-induced obesity (DIO). However, the underlying mechanism for this is still unclear. Here we investigated whether TEL prevents leptin resistance by enhancing leptin uptake across the blood-brain barrier (BBB). To address this question, we fed C57BL/6 mice a high-fat diet (HFD) and treated them daily with TEL by oral gavage. In addition to broadly characterizing the metabolism of leptin, we determined leptin uptake into the brain by measuring BBB transport of radioactively labeled leptin after long-term and short-term TEL treatment. Additionally, we assessed BBB integrity in response to angiotensin II in vitro and in vivo. We found that HFD markedly increased body weight, energy intake, and leptin concentration but that this effect was abolished under TEL treatment. Furthermore, glucose control and, most importantly, leptin uptake across the BBB were impaired in mice on HFD, but, again, both were preserved under TEL treatment. BBB integrity was not impaired due to angiotensin II or blocking of angiotensin II receptors. However, TEL did not exhibit an acute effect on leptin uptake across the BBB. Our results confirm that TEL prevents DIO and show that TEL preserves leptin transport and thereby prevents leptin resistance. We conclude that the preservation of leptin sensitivity is, however, more a consequence than the cause of TEL preventing body weight gain.

Targeting Renin-Angiotensin System Against Alzheimer's Disease

Renin Angiotensin System (RAS) is a hormonal system that regulates blood pressure and fluid balance through a coordinated action of renal, cardiovascular, and central nervous systems. In addition to its hemodynamic regulatory role, RAS involves in many brain activities, including memory acquisition and consolidation. This review has summarized the involvement of RAS in the pathology of Alzheimer’s disease (AD), and the outcomes of treatment with RAS inhibitors. We have discussed the effect of brain RAS in the amyloid plaque (Aβ) deposition, oxidative stress, neuroinflammation, and vascular pathology which are directly and indirectly associated with AD. Angiotensin II (AngII) via AT1 receptor is reported to increase brain Aβ level via different mechanisms including increasing amyloid precursor protein (APP) mRNA, β-secretase activity, and presenilin expression. Similarly, it was associated with tau phosphorylation, and reactive oxygen species generation. However, these effects are counterbalanced by Ang II mediated AT2 signaling. The protective effect observed with angiotensin receptor blockers (ARBs) and angiotensin converting enzyme inhibitors (ACEIs) could be as the result of inhibition of Ang II signaling. ARBs also offer additional benefit by shifting the effect of Ang II toward AT2 receptor. To conclude, targeting RAS in the brain may benefit patients with AD though it still requires further in depth understanding.

The brain renin-angiotensin system plays a crucial role in regulating body weight in diet-induced obesity in rats

BACKGROUND AND PURPOSE

Reduced weight gain after treatment with AT1 receptor antagonists may involve a brain-related mechanism. Here, we investigated the role of the brain renin-angiotensin system on weight regulation and food behaviour, with or without additional treatment with telmisartan.

METHODS

Transgenic rats with a brain-specific deficiency in angiotensinogen (TGR(ASrAOGEN)) and the corresponding wild-type, Sprague Dawley (SD) rats were fed (3 months) with a high-calorie cafeteria diet (CD) or standard chow. SD and TGR(ASrAOGEN) rats on the CD diet were also treated with telmisartan (8 mg·kg(-1) ·d(-1) , 3 months).

RESULTS

Compared with SD rats, TGR(ASrAOGEN) rats (i) had lower weights during chow feeding, (ii) did not become obese during CD feeding, (iii) had normal baseline leptin plasma concentrations independent of the feeding regimen, whereas plasma leptin of SD rats was increased due to CD, (iv) showed a reduced energy intake, (v) had a higher, strain-dependent energy expenditure, which is additionally enhanced during CD feeding, (vi) had enhanced mRNA levels of pro-opiomelanocortin and (vii) showed improved glucose control. Weight gain and energy intake in rats fed the CD diet were markedly reduced by telmisartan in SD rats but only to a minor extent in TGR(ASrAOGEN) rats.

CONCLUSIONS

The brain renin-angiotensin system affects body weight regulation, feeding behaviour and metabolic disorders. When angiotensin II levels are low in brain, rats are protected from developing diet-induced obesity and obesity-related metabolic impairments. We further suggest that telmisartan at least partly lowers body weight via a CNS-driven mechanism.

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

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

Impact of taurine supplementation on blood pressure in gestational protein-restricted offspring: Effect on the medial solitary tract nucleus cell numbers, angiotensin receptors, and renal sodium handling

OBJECTIVE

The current study considers changes of the postnatal brainstem cell number and angiotensin receptors by maternal protein restriction (LP) and LP taurine supplementation (LPT), and its impact on arterial hypertension development in adult life.

METHODS AND RESULTS

The brain tissue studies were performed by immunoblotting, immunohistochemistry, and isotropic fractionator analysis. The current study shows that elevated blood pressure associated with decreased fractional urinary sodium excretion (FENa) in adult LP offspring was reverted by diet taurine supplementation. Also, that 12-day-old LP pups present a reduction of 21% of brainstem neuron counts, and, immunohistochemistry demonstrates a decreased expression of type 1 angiotensin II receptors (AT1R) in the entire medial solitary tract nuclei (nTS) of 16-week-old LP rats compared to age-matched NP and LPT offspring. Conversely, the immunostained type 2 AngII (AT2R) receptors in 16-week-old LP nTS were unchanged.

CONCLUSION

The present investigation shows a decreased FENa that occurs despite unchanged creatinine clearance. It is plausible to hypothesize an association of decreased postnatal nTS cell number, AT1R/AT2R ratio and FENa with the higher blood pressure levels found in taurine-deficient progeny (LP) compared with age-matched NP and LPT offspring.

Neuroprotective role of angiotensin II type 2 receptor after transient focal ischemia in mice brain

This study assessed the time course of angiotensin (Ang) II type 1 and type 2 receptor expression after 60 min of ischemia/reperfusion in mice treated with a nonhypotensive dose of valsartan, an angiotensin II type 1 receptor antagonist. We also examined the potential neuroprotective mechanisms mediated by angiotensin II type 2 receptor. Mice were divided into two groups (n=64, each): valsartan-treated and control, vehicle groups. Infarct volume and neurological deficit scores were evaluated at several time points after ischemia, while immunohistochemical analyses were performed at serial time points after reperfusion. Valsartan significantly reduced the infarct volume and improved the neurological deficit scores (P<0.05). Both angiotensin II type 1 and type 2 receptors were upregulated at 24h and peaked at 72 h with type I receptors dominating in the ischemic penumbra of the vehicle group. Interestingly, angiotensin II type 2 receptor expression levels were significantly higher in the valsartan group than vehicle controls (P<0.001). Moreover, angiotensin II type 2 receptor upregulated phosphosignal transducer and activator of transcription-3, and B-cell lymphoma protein-2 (P<0.05). Our results indicated that angiotensin II type 2 receptor has antiapoptotic activity by activating the B-cell lymphoma protein-2 via the janus kinase/signal transducer and activator of transcription signaling pathway.

Angiotensin type 1 receptor antagonist losartan, reduces MPTP-induced degeneration of dopaminergic neurons in substantia nigra

Background

Recent attention has focused on understanding the role of the brain-renin-angiotensin-system (RAS) in stroke and neurodegenerative diseases. Direct evidence of a role for the brain-RAS in Parkinson's disease (PD) comes from studies demonstrating the neuroprotective effect of RAS inhibitors in several neurotoxin based PD models. In this study, we show that an antagonist of the angiotensin II (Ang II) type 1 (AT₁) receptor, losartan, protects dopaminergic (DA) neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity both in primary ventral mesencephalic (VM) cultures as well as in the substantia nigra pars compacta (SNpc) of C57BL/6 mice (Fig. 1).

Results

In the presence of exogenous Ang II, losartan reduced MPP⁺ (5 μM) induced DA neuronal loss by 72% in vitro. Mice challenged with MPTP showed a 62% reduction in the number of DA neurons in the SNpc and a 71% decrease in tyrosine hydroxylase (TH) immunostaining of the striatum, whereas daily treatment with losartan lessened MPTP-induced loss of DA neurons to 25% and reduced the decrease in striatal TH⁺ immunostaining to 34% of control.

Conclusion

Our study demonstrates that the brain-RAS plays an important neuroprotective role in the MPTP model of PD and points to AT₁ receptor as a potential novel target for neuroprotection.

Angiotensin II inhibition of Ca2+ currents is independent of ATR1 angiotensin II receptor activation in rat adult vagal afferent neurons

Angiotensin II (ANG II) has the ability to modulate the activity of neurons involved in the cardiovascular regulation. One effective way of doing that is by changing calcium currents. In the present study, we investigated the effects of ANG II on high-voltage-activated (HVA) Ca2+ currents measured in adult vagal afferent neurons using the whole-cell patch-clamp technique. In addition, we demonstrated the presence of ATR1 and ATR2 receptors mRNA at nodose neurons using conventional reverse transcriptase-polymerase chain reaction (RT-PCR). ANG II (100 nM) decreased the HVA Ca2+ current (peak current recorded at 0 mV: -60.9+/-8.7 pA/pF in control conditions versus -31.9+/-5.7 pA/pF in the presence of ANG II) and shifted the Ca2+ current activation to a more negative membrane potential (control V0.5=-12.5+/-1.5 mV versus -18.4+/-2.8 mV during perfusion with ANG II). Losartan (500 nM) was not able to prevent the ANG II effect on the HVA Ca2+ current making unlikely the involvement of the ATR1 receptor. When ANG II was perfused in the continuous presence of saralasin, a non-selective ANG II receptor antagonist, we observed a faster but transient inhibition of HVA Ca2+ current. The inhibition was not sustained as observed when we applied ANG II alone and the HVA Ca2+ current recovered with time reaching levels close to the control. Unexpectedly, treatment with the ATR2 blocker PD 123,319 (500 nM) caused a significant inhibition on the HVA Ca2+ current making rather difficult any further conclusions. The above results allow us to conclude that ANG II induced inhibition on the HVA Ca2+ current is probably not via ATR1 receptor activation.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

Drinking behavior elicited by central injection of angiotensin II: roles for protein kinase C and Ca2+/calmodulin-dependent protein kinase II

Prior studies utilizing neurons cultured from the hypothalamus and brain stem of newborn rats have demonstrated that ANG II-induced modulation of neuronal firing involves activation of both protein kinase C (PKC) and Ca2+/calmodulin-dependent protein kinase II (CaMKII). The present studies were performed to determine whether these signaling molecules are also involved in physiological responses elicited by ANG II in the brain in vivo. Central injection of ANG II (10 ng/2 microl) into the lateral cerebroventricle (icv) of Sprague-Dawley rats increased water intake in a time-dependent manner. This ANG II-mediated dipsogenic response was attenuated by central injection of the PKC inhibitors chelerythrine chloride (0.5-50 microM, 2 microl) and Go-6976 (2.3 nM, 2 microl) and by the CaMKII inhibitor KN-93 (10 microM, 2 microl). Conversely, icv injection of chelerythrine chloride (50 microM, 2 microl) and KN-93 (10 microM, 2 microl) had no effect on the dipsogenic response elicited by central injection of carbachol (200 ng/2 microl). Furthermore, injection of ANG II (10 ng/2 microl) icv increases the activity of both PKC-alpha and CaMKII in rat septum and hypothalamus. These data suggest that signaling molecules involved in ANG II-induced responses in vitro are also relevant in physiological responses elicited by ANG II in the whole animal model.

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

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

Angiotensin AT1 receptor signalling pathways in neurons

1. The aim of the present article is to review the intracellular signal transduction pathways that are influenced by the peptide angiotensin (Ang) II, acting via its type 1 (AT1) receptor, in neurons. 2. The AT1 receptors couple to a wide variety of signalling pathways in peripheral tissues, such as kidney, heart and vascular smooth muscle. A similar diversity of signalling mechanisms exists for AT1 receptors in neurons. 3. We outline the known neuronal AT1 receptor signalling pathways as they relate to function. Pathways that couple activation of AT1 receptors to short-term changes in neuronal membrane ionic currents and firing rate will be reviewed. These are different from the pathways that elicit longer-term changes in enzyme activity and gene expression and, ultimately, increases in noradrenaline synthesis. 4. Novel AT1 receptor signalling pathways discovered through gene expression profiling and their potential functional significance have been discussed.

Brain angiotensinergic mediation of enhanced water consumption in lactating rats

The mechanism by which lactating rats increase fluid consumption to meet the demands of milk production is unknown. Because ANG II is the most potent dipsogenic stimulus known, this study examined whether angiotensinergic signaling plays a role in enhanced drinking in lactating rats. ANG II administered intracerebroventricularly caused a significantly greater dipsogenic response in lactating rats than in control rats, suggesting that dipsogenic responsivity to ANG II is enhanced in the brains of lactating rats. The angiotensin type 1 (AT1) ANG II receptor subtype antagonist SKF-108566, also given intracerebroventricularly, caused a significant reduction in water consumption in lactating rats, whereas it did not significantly affect water intake in control rats. In contrast, stimulation of drinking by the muscarinic agonist carbachol, also administered intracerebroventricularly, did not differ between lactating and control rats. Inhibition of drinking by the muscarinic antagonist atropine also did not differ significantly between lactating and control rats. These results suggest that the increased drinking in lactating rats involves an increased responsivity to ANG II in neurons that mediate dipsogenesis, as well as an enhancement in the amount of angiotensinergic input to these ANG II-responsive neurons.

AT(1)-receptor blockade in the hypothalamic PVN reduces central hyperosmolality-induced renal sympathoexcitation

Autonomic neurons in the hypothalamic paraventricular nucleus (PVN) are innervated by osmotic-sensitive regions of the lamina terminalis, receive input from ANG II-containing cells, and express AT(1) ANG II receptors. Therefore, we hypothesized that ANG II actions within the PVN could underlie hyperosmolality-induced increases in renal sympathetic nerve activity (RSNA). In anesthetized baroreceptor-denervated rats, graded concentrations of NaCl (0.30, 0.9, 1.5, and 2.1 osmol/l) were injected (300 microl) centrally via the internal carotid artery (ICA) and produced corresponding increases in mean arterial pressure (MAP) and RSNA. In addition, equivalent hyperosmotic loads (1.5 osmol/l) of NaCl, glucose, and mannitol each significantly (P < 0.05) increased MAP and RSNA. The same stimuli had no effect when administered intravenously. Bilateral PVN microinjections (100 nl) of the AT(1)-receptor antagonist losartan (80 nmol) before osmotic challenge had no effect on resting RSNA but significantly (P < 0.05) reduced RSNA responses to hyperosmotic NaCl (n = 7), glucose (n = 6), and mannitol (n = 6). Increases in RSNA evoked by hyperosmotic NaCl were significantly (P < 0.05) attenuated approximately 20 min after losartan injection and recovered within 60-120 min. In contrast, losartan outside the PVN as well as vehicle (saline) within the PVN failed to alter RSNA responses to ICA hyperosmotic NaCl. Results suggest that elevated RSNA after central sodium/osmotic activation is mediated, at least in part, by a synaptic mechanism involving AT(1)-receptor activation within the PVN.

A role for the angiotensin IV/AT4 system in mediating natriuresis in the rat

Angiotensin II (AngII) or Angiotensin IV (AngIV) was infused into the renal artery of anesthetized rats while renal cortical blood flow was measured via laser Doppler flowmetry. The infusion of AngII produced a significant elevation in mean arterial pressure (MAP) with an accompanying decrease in cortical blood flow, glomerular filtration rate (GFR), urine volume, and urine sodium excretion. The infusion of AngIV induced significant increases in renal cortical blood flow and urine sodium excretion, without altering MAP, GFR, and urine volume. Pretreatment infusion with a specific AT1 receptor antagonist, DuP 753, blocked or attenuated the subsequent AngII effects, while pretreatment infusion with the specific AT4 receptor antagonist, Divalinal-AngIV, blocked the AngIV effects. These results support distinct and opposite roles for AngII and AngIV, i.e. AngII acts as an anti-natriuretic agent, while AngIV acts as a natriuretic agent.

Strain differences in the anxiolytic effects of losartan in the mouse

Anxiolytic effects of the angiotensin AT(1) receptor antagonist losartan were studied in the elevated plus maze (EPM) and the light/dark test (LDT) in different mouse strains as were responses to angiotensin II and acetylcholine in isolated ascending colon. There were no significant strain differences in behaviour on the EPM, and diazepam was anxiolytic in C57BL/6, DBA/2 and BKW mice. Losartan was anxiolytic in BKW only. In the LDT, there were significant strain differences, with BKW mice exhibiting greatest anxiety-like behaviour; losartan was ineffective in this test. In vitro responses to angiotensin II and acetylcholine were significantly smaller in BKW than in C57BL/6 and DBA/2. These results indicate that the mouse strain exhibiting least angiotensin receptor function is the most responsive to the anxiolytic effects, suggesting a possible relationship between angiotensin receptor function and anxiolytic response to losartan.

Effect of repetitive icv injections of ANG II on c-Fos and AT(1)-receptor expression in the rat brain

ANG II has been implicated in neuroplastic processes via stimulation of inducible transcription factors (ITF) in the brain. In the present study, we investigated the effects of acute vs. repetitive once daily intracerebroventricular injections of ANG II for 7 days on the expression of ITF and constitutive transcription factor (CTF) and the AT1 receptor in the median preoptic area (MnPO), the subfornical organ (SFO), and the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON). After repetitive injections, the expression of c-Fos declined by approximately 50% in MnPO, SFO, PVN, and SON compared with controls injected once. The desensitization of c-Fos occurred on the transcriptional level as shown in the SON by RT-PCR. Apart from a novel expression of c-Jun in the SON, the ITF c-Jun, JunB, JunD, and Krox-24 did not change after repetitive stimulation. Neither were the CTF, calcium response element binding protein, activating transcription factor 2, and serum response factor altered after repetitive vs. single injections of ANG II. The AT1 receptor was coexpressed with c-Fos/c-Jun. Immunohistochemical stainings suggest an increase in AT1-receptor number in MnPO, SFO, PVN, and SON on chronic stimulation compared with once-injected controls. These findings demonstrate that repetitive periventricular stimulation with ANG II essentially alters the expression of transcription factors compared with acute stimulation and suggest c-Fos and c-Jun as major intermediates of the AT1-receptor transcription.

Novel nuclear signaling pathway mediates activation of fibroblast growth factor-2 gene by type 1 and type 2 angiotensin II receptors

In bovine adrenal medullary cells synergistically acting type 1 and type 2 angiotensin II (AII) receptors activate the fibroblast growth factor-2 (FGF-2) gene through a unique AII-responsive promoter element. Both the type 1 and type 2 AII receptors and the downstream cyclic adenosine 1',3'-monophosphate- and protein kinase C-dependent signaling pathways activate the FGF-2 promoter through a novel signal-transducing mechanism. This mechanism, which we have named integrative nuclear FGF receptor-1 signaling, involves the nuclear translocation of FGF receptor-1 and its subsequent transactivation of the AII-responsive element in the FGF-2 promoter.

Emerging features of brain angiotensin receptors

In mammalian brain, angiotensin II AT1 and AT2 receptor subtypes are apparently expressed only in neurons and not in glia. AT1 and AT2 receptor subtypes are sometimes closely associated, but apparently expressed in different neurons. Brain AT1/AT2 interactions may occur in selective cases as inter-neuron cross talk. There are two AT1 isoforms in rodents. AT1A, which predominates, and AT1B. There are also important inter-species differences in receptor expression. Relative lack of amino acid conservation in the gerbil gAT1A receptor substantially decreases affinity for the AT1 antagonists. AT1 receptors are expressed in brain areas regulating autonomic and hormonal responses. AT1A receptors are heterogeneously regulated in a number of experimental conditions. In specific areas, AT1A receptors are not normally expressed, but are induced under influence of reproductive hormones in dopaminergic neurons. There are AT1 and AT2 receptors also in areas related to limbic, sensory and motor functions and their expression is developmentally regulated. A picture is emerging of widespread, neuronally localized, heterogeneously regulated, closely associated brain angiotensin receptor subtypes, modulating multiple functions including neuroendocrine and autonomic responses, stress, cerebrovascular flow, and perhaps brain maturation, neuronal plasticity, memory and behavior.

Blockade of central angiotensin AT(1) receptors improves neurological outcome and reduces expression of AP-1 transcription factors after focal brain ischemia in rats

BACKGROUND AND PURPOSE

Angiotensin-converting enzyme inhibitors have been shown to protect against stroke in hypertensive rats and to improve neurological outcome after cerebral ischemia in normotensive rats. The present study was designated to test the hypothesis that blockade of brain AT(1) receptors improves the recovery from focal cerebral ischemia and reduces expression of AP-1 transcription factors c-Fos and c-Jun, which have been associated with programmed cell death and neurodegeneration.

METHODS

Experiments were carried out in normotensive male Wistar rats. Focal cerebral ischemia was induced by middle cerebral artery occlusion lasting for 90 minutes and followed by reperfusion. The selective AT(1) receptor antagonist irbesartan was infused intracerebroventricularly over a 5-day period before the induction of ischemia at a dose that inhibited brain but not vascular AT(1) receptors. Twenty-four hours after ischemia, neurological outcome was evaluated and expression of c-Fos and c-Jun proteins in the brain was studied immunocytochemically.

RESULTS

Focal brain ischemia resulted in a strong induction of c-Fos and c-Jun proteins in the cortex, which positively correlated with the degree of neurological deficits. Treatment of rats with irbesartan significantly improved neurological outcome of focal cerebral ischemia when compared with the vehicle-treated group and markedly reduced the expression of c-Fos and c-Jun proteins in the cortex on the ligated side of the brain. Irbesartan pretreatment completely abolished the ischemia-induced c-Fos expression in the hippocampus.

CONCLUSIONS

The present study shows a relationship between c-Fos and c-Jun expression and neurological outcome after focal brain ischemia. Our data indicate that long-term blockade of central AT(1) receptors improves the recovery from brain ischemia and reduces the expression of c-Fos and c-Jun proteins in the brain. Pretreatment with an AT(1) receptor antagonist has beneficial effects after cerebral ischemia.

Practical considerations of the pharmacology of angiotensin receptor blockers

A review of the drug class of angiotensin receptor blockers (ARBs) as well as the ARBs currently available by prescription in the United States is presented. The importance of angiotensin II production by non-angiotensin-converting enzyme (non-ACE) pathways, particularly human chymase, is discussed. Emphasis is placed on the mechanism of action of ARBs and the different binding kinetics of these agents. Although all ARBs, as a group, block the AT1 receptor, they may differ in the pharmacological characteristics of their binding and be classified as either surmountable or insurmountable antagonists. Mechanisms of surmountable and insurmountable antagonism as well as possible benefits of these blocking characteristics are discussed in relation to the various ARBs. The cardiovascular effects of activation of the two main subtypes of angiotensin receptors (AT1 and AT2) are presented. In addition to their treatment of hypertension, ACE inhibitors are recognized as being effective in the management of heart failure, left ventricular hypertrophy, recurrent myocardial infarctions, and renal disease. ARBs are currently indicated only for the treatment of hypertension; however, in vitro and in vivo pharmacological studies as well as preliminary clinical data suggest that ARBs, like ACE inhibitors, may also provide effective protection against end-organ damage in these conditions.

Renal and hemodynamic effects of losartan in conscious dogs during controlled mechanical ventilation

In 12 conscious dogs, we investigated whether the angiotensin II-receptor antagonist losartan increases renal sodium excretion and urine volume during controlled mechanical ventilation (CMV) with positive end-expiratory pressure. In four experimental protocols, the dogs were extracellular volume (ECV) expanded (electrolyte solution, 0.5 ml. kg-1. min-1 iv) or not and received losartan (100 micrograms. kg-1. min-1 iv) or not. They breathed spontaneously during the 1st and 4th hour and received CMV with positive end-expiratory pressure (mean airway pressure 20 cmH2O) during the 2nd and 3rd hours. In the expansion group, dogs with losartan excreted approximately 18% more sodium (69 +/- 7 vs. 38 +/- 5 micromol. min-1. kg-1) and 15% more urine during the 2 h of CMV because of a higher glomerular filtration rate (5.3 +/- 0.3 vs. 4.5 +/- 0.2 ml. min-1. kg-1) and the tubular effects of losartan. In the group without expansion, sodium excretion (2.0 +/- 0.6 vs. 2.6 +/- 1.0 micromol. min-1. kg-1) and glomerular filtration rate (3.8 +/- 0.3 vs. 3.8 +/- 0.4 ml. min-1. kg-1) did not change, and urine volume decreased similarly in both groups during CMV. Plasma vasopressin and aldosterone increased in both groups, and plasma renin activity increased from 4.9 +/- 0.7 to 7.8 +/- 1.3 ng ANG I. ml-1. h-1 during CMV in nonexpanded dogs without losartan. Mean arterial pressure decreased by 10 mmHg in nonexpanded dogs with losartan. In conclusion, losartan increases sodium excretion and urine volume during CMV if the ECV is expanded. If the ECV is not expanded, a decrease in mean arterial blood pressure and/or an increase in aldosterone and vasopressin during CMV attenuates the renal effects of losartan.

The renin-angiotensin system and its receptors

The renin-angiotensin system (RAS) plays an important role in blood pressure control and in water and salt homeostasis. It is involved in the pathophysiology of hypertension and structural alterations of the vasculature, kidney, and heart, including neointima formation, nephrosclerosis, postinfarction remodeling, and cardiac left ventricular hypertrophy (LVH). Recently, an increased knowledge of the effector peptides of the RAS, their receptors, and their respective functions has led to a new principle of treatment for hypertension: the inhibition of angiotensin (Ang) II via angiotensin-converting enzyme inhibitors or Ang II-receptor antagonists. In this review, the Ang receptors AT1 and AT2 and the potential roles of shorter angiotensin fragments, including Ang III(2-8), Ang IV(3-8), and Ang(1-7), are discussed.

Effects of systemic treatment with irbesartan and losartan on central responses to angiotensin II in conscious, normotensive rats

Angiotensin AT1 receptor antagonists represent a novel class of cardiovascular drugs. In conscious, normotensive rats, irbesartan ((2-n-butyl-3-[(2'-(1H-tetrazol-5-yl)-biphenyl-4-yl) methyl]-1,3-diaza-spiro[4,4]non) and losartan ((2 n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphenyl -4-yl) methyl] imidazol), two specific, high- affinity angiotensin AT1 receptor antagonists administered intravenously (i.v.) at doses of 0.3, 1, 3 and 10 mg/kg body weight, or orally (p.o.) at doses of 1, 3, 10 and 30 mg/kg body weight, antagonized the pressor responses to i.v. angiotensin II (50 ng/kg body weight) in a dose-related manner and with similar potency. In the following sets of experiments, we tested the hypothesis that these angiotensin AT1 receptor antagonists, when applied systemically, can inhibit the effects of angiotensin AT1 receptor stimulation in the brain. Irbesartan and losartan were administered i.v. or p.o. at doses of 3, 10, 30 and 100 mg/kg body weight. The responses to 100 ng angiotensin II injected into the lateral brain ventricle (i.c.v.), namely blood pressure increase, vasopressin release into the circulation and drinking, were recorded for up to 3 h. While both angiotensin AT1 receptor antagonists dose-dependently attenuated the pressor responses to central angiotensin AT1 receptor stimulation to a similar degree (maximal inhibition, irbesartan: 62% i.v., 39% p.o.; losartan: 62% i.v., 46% p.o.; respectively), irbesartan was more effective with respect to the inhibition of vasopressin release (76% i.v., 65% p.o.) and drinking (63% i.v., 79% p.o.) than losartan (58% i.v., 33% p.o and 22% i.v., 56% p.o., respectively). We conclude that systemically administered angiotensin AT1 receptor antagonists have access to central angiotensin receptors. The degree of central angiotensin AT1 receptor blockade following peripheral application may vary between different representatives of this class of drugs.

Mapping of angiotensin AT1 receptors in the rat limbic system

The AT1 receptor is one of the two receptor subtypes able to bind angiotensin II. In the present study, immunohistochemical examination of the distribution of the AT1 receptor in several limbic structures of female rats has been done, revealing new aspects of the distribution of AT1-positive cells. The presence of AT1 receptor expressing cells in the hippocampus and the amygdala is described, but their distribution in these regions has not been examined in a detailed way. We found some notable differences in the distribution of these cells: in female rats, we detected high amounts of labeled cells in the hippocampus, the entorhinal cortex and piriform cortex. In somewhat lower amounts, stained cells could be found in several nuclei of the amygdala (in the basomedial, basolateral, lateral, central and medial nucleus of the amygdala, in the amygdalopiriform transition area and in the amygdalohippocampal transition area as well as in the bed nucleus of the stria terminalis).

Immunohistochemical localization of the angiotensin II type 1 receptor in human hypothalamus and brainstem

We studied the distribution of the angiotensin II type 1 (AT-1) receptor using a polyclonal rabbit antibody in two human brains. AT-1 receptor immunoreactivity was detected in several hypothalamic nuclei, substantia nigra, locus coeruleus, nucleus tractus solitarius, ventrolateral medulla, pontine nuclei, and inferior olivary nucleus. This provides direct evidence of neuronal localization of the AT-1 receptor in autonomic and motor areas in human brain.

In vitro effect of angiotensin II on GABA release in rat hippocampus

In rat hippocampal slices [3H]GABA release evoked by 25 mM KCI consisted of Ca2+-dependent and Ca2+-independent fractions. Angiotensin II (AngII) at a concentration of 1 microM inhibited K+-stimulated [3H]GABA release. The effect of AngII (20% inhibition) on [3H]GABA release was decreased by the addition of 0.01 mM nipecotic acid to the superfusion medium. AngII also decreased the Ca2+-independent carrier-mediated [3H]GABA release (25% inhibition at a concentration of 1 microM). Different mechanisms of the neuromodulatory action of AngII on GABA release are discussed.

Dopaminergic control of angiotensin II-induced vasopressin secretion in vitro

Because dopamine influences arginine vasopressin (AVP) release, the present studies were designed to ascertain the dopamine receptor subtype that potentiates angiotensin II-induced AVP secretion in cultured hypothalamo-neurohypophysial explants. Dopamine (a nonselective D1/D2 agonist), apomorphine (a D2 >> D1 agonist), and SKF-38393 (a selective D1 agonist) dose dependently increased AVP secretion. Maximal AVP release was observed with 5 microM dopamine, 307 +/- 66% . explant-1 . h-1, 1 microM SKF-38393, 369 +/- 41% . explant-1 . h-1, and 0.1 microM apomorphine, 374 +/- 67% . explant-1 . h-1. Selective D1 antagonism with 1 microM SCH-23390 blocked AVP secretion to values no different from basal. Domperidone (D2 antagonist), phenoxybenzamine (nonselective adrenergic antagonist), and prazosin (alpha1-antagonist) failed to prevent release. D1 antagonism also prevented AVP secretion to 1 microM angiotensin II [angiotensin II, 422 +/- 87% . explant-1 . h-1 vs. angiotensin II plus SCH-23390, 169 +/- 28% . explant-1 . h-1 (P < 0.05)], but D2 and alpha1-adrenergic blockade did not. In contrast, AT1 receptor inhibition with 0.5 microM losartan blocked angiotensin II- but not dopamine-induced AVP release. AT2 antagonism had no effect. Although subthreshold doses of the agonists did not increase AVP secretion (0. 05 microM dopamine, 133 +/- 44% . explant-1 . h-1; 0.01 microM SKF-38393, 116 +/- 26% . explant-1 . h-1;and 0.001 microM angiotensin II, 104 +/- 29% . explant-1 . h-1 ), the combination of dopamine and angiotensin II provoked a significant rise in AVP [420 +/- 83% . explant-1 . h-1 (P < 0.01)]. Similar results were observed with SKF-38393 and angiotensin II, and the AVP response was blocked to basal levels by either D1 or AT1 antagonism. These findings support a role for D1 receptor activation to increase AVP release and mediate angiotensin II-induced AVP release within the hypothalamo-neurohypophysial system. The data also suggest that the combined subthreshold stimulation of receptors that use distinct intracellular pathways can prompt substantial AVP release.

Central angiotensin AT1 and muscarinic receptors in ITF expression on intracerebroventricular NaCl

In the present study, we investigated the expression pattern of the inducible transcription factors (ITF) c-Fos, c-Jun, JunB, JunD, and Krox-24 following intracerebroventricular injections of hyperosmolar saline (0.2, 0.3, and 0.6 M NaCl) and its mediation via angiotensin and/or muscarinic receptors. c-Fos, c-Jun, and Krox-24 were differentially expressed in organum vasculosum laminae terminalis, median preoptic area, subfornical organ (SFO), and paraventricular and supraoptic nuclei. Expression of c-Fos and c-Jun was inhibited by pretreatment with the angiotensin AT1 receptor antagonist losartan (10 and 20 nmol icv) following 0.20 and 0.30 M saline. Pretreatment with atropine (15 nmol icv) inhibited the 0.30 and 0.60 M NaCl-induced expression of c-Fos, c-Jun, and Krox-24 in all areas except the SFO. Coexpression of the ITF with vasopressin and oxytocin, the major effector peptides in osmoregulation, was demonstrated, implying the corresponding genes as putative target genes of the ITF. The results show a highly differentiated ITF expression pattern in the brain mediated by angiotensinergic and muscarinergic pathways, suggesting a finely tuned regulation of target genes.

Differential behavioral effects of angiotensin II microinjected unilaterally into the CA1 hippocampal area

The behavioral responses of rats to unilateral microinjections of angiotensin II (ATII) into the left or right CA1 hippocampal area were studied. Unilateral (left or right) injections of ATII at a dose of 0.5 microg decreased locomotor activity but, at a dose of 1.0 microg, ATII increased it compared to the respective controls. The effect was more pronounced when ATII was microinjected into the left CAI hippocampal area. The elevated plus-maze experiments showed that ATII microinjections into the right CA1 hippocampal area at a dose of 0.5 microg decreased the ratio of the number of entries into the open arms to the total number of entries (into the open and closed arms). These findings suggest some asymmetric effects of ATII, depending on the dose, the behavioral test and the microinjected hemisphere.

Central amino acids mediate cardiovascular response to angiotensin II in the rat

To elucidate the role of the rostral ventrolateral medulla (RVLM) in cardiovascular control through the release of central amino acid neurotransmitters, experiments were performed in Sprague-Dawley (normotensive) rats and spontaneously hypertensive rats (SHR) anesthetized with urethane by using microdialysis sampling from the RVLM for determination of amino acid neurotransmitters. The baseline release of the excitatory amino acid neurotransmitter, glutamate (GLU) from the RVLM in SHR was higher and those of the inhibitory amino acid neurotransmitters, glycine (GLY), taurine (TAU), and gamma-aminobutyric acid (GABA), were lower than in normotensive rats. Microinjection of angiotensin II (ANG II) into the RVLM caused a dose-dependent increase in mean arterial pressure (MAP) and heart rate (HR), accompanied by increased release of GLU in the RVLM. In contrast, microinjection of the ANG II type 1 receptor (AT1) antagonist CV 11974 into the RVLM reduced MAP and HR, accompanied by increased release of GLY, TAU and GABA. These changes in MAP and HR after administration of ANG II or AT1 antagonist were partially blocked by the use of the corresponding antagonist of each amino acid neurotransmitter. Furthermore, these effects were more prominently seen in SHR than in normotensive rats. These results suggest that the release of amino acid neurotransmitters mediate the cardiovascular effects of the angiotensin system in the RVLM, which may be involved in the generation of hypertension in SHR.

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

We evaluated the endogenous action of angiotensin II (AII) and its active metabolite, angiotensin III (AIII), at the nucleus tractus solitarii (NTS) in the modulation of baroreceptor reflex (BRR) response, and the subtype(s) of angiotensin receptors involved in this process. Adult, male Sprague-Dawley rats that were anesthetized and maintained with pentobarbital sodium were used. Bilateral microinjection of AII or AIII (10, 20 or 40 pmol) into the NTS significantly and dose-dependently suppressed the BRR response, which was evoked by transient hypertension induced by phenylephrine (5 micrograms/kg, i.v.). The suppressive effect of AII (40 pmol) was reversed by co-administration of the non-peptide AT1 receptor antagonist, losartan (1.6 nmol), but only partially by the non-peptide AT2 receptor antagonist, PD-123319. On the other hand, both angiotensin receptor antagonists appreciably reversed the depressive action of AIII (40 pmol). Blocking the endogenous activity of the angiotensins by microinjection into the bilateral NTS of losartan (1.6 nmol) or PD-123319 (1.6 nmol) elicited a significant enhancement of the BRR response. An interruption of the conversion of AII to AIII with the aminopeptidase A inhibitor, amastatin (3.3 nmol), attenuated, but did not eliminate, the AII-induced inhibition of the BRR response. We conclude that whereas the endogenous AIII may exert a tonic inhibitory modulation on the BRR response by acting on both the AT1 and AT2 receptor subtypes, the same action of the endogenous AII engaged only the AT1 receptor subtype at the NTS. Furthermore, at least part of the suppressive action of AII may result from its metabolic conversion to AIII.

Dose-dependent inhibitory effects of angiotensin II on visual responses of the rat superior colliculus: AT1 and AT2 receptor contributions

Angiotensin II (Ang II) has traditionally been regarded as a peripherally circulating and acting hormone involved in fluid homeostasis and blood pressure regulation. With the rather recent localization of Ang II receptors within the mammalian brain, renewed interest has emerged in the hope of elucidating the central impact and function of this hormone. One region that has been clearly demonstrated to express Ang II receptors is the superior colliculus (SC). This mesencephalic structure plays an important role in sensory visuomotor integration. Receptors for Ang II (of both the AT1 and AT2 subtypes) have been localized within the superficial layers of this structure, i.e. the areas that are visually responsive. In the hopes of characterizing the role of Ang II in the SC, we have attempted to physiologically activate these receptors in vivo and observe the effects of Ang II on visually evoked responses. In the attempt to identify the receptor subtype(s) responsible in mediating these effects, Ang II was injected concomitantly with selective receptor ligands. Experiments were performed on adult rats prepared in classical fashion for electrophysiological studies. Through microinjection of Ang II, and the simultaneous recording of visually evoked potentials to flash stimulation, we have observed that this peptide yields a strong suppressive effect on visual neuronal activity. By injecting Ang II at various concentrations (10(-3)-10(-10) M), we have further observed that the effects of this peptide express a dose-related dependency. Injection of Ang II in progressively more ventral layers yielded less pronounced effects, demonstrating physiologically the discrete localization of these receptors in the stratum griseum superficiale. Coinjection of Ang II with Losartan yielded a near complete blockade of Ang II suppressive effects, suggesting that AT1 receptors play a prominent role in mediating these responses. However, coinjection of Ang II with PD 123,319 yielded a slight, yet significant partial blockade. Coinjection of Ang II with both the AT1 and AT2 receptor antagonists yielded a complete blockade of the Ang II effect. Finally, some of the results suggest that the AT2 receptor ligand CGP 42,112 may possess agonist properties. Taken together, these findings suggest that the AT1 receptor is predominantly involved in mediating Ang II responses in the SC and there also appears to be some indication of AT2 receptor involvement. However, the underlying mechanisms (such as receptor interactions), the exact specificity of the ligands used, and the possibility of other receptor subtype implication have yet to be explored fully.

6-OHDA lesions to the central amygdala abolish angiotensins facilitation of object recognition in rats

1. We have previously reported that the dopaminergic projection from A10 ventral tegmental neurons and A9 neurons of substantia nigra to the central amygdala (CA) is, in part, responsible for the facilitatory effect of angiotensin II (AII) and its 3-7 fragment [AII(3-7)] on the retrieval of information in memory that is motivated affectively. 2. In this study, the influence of both angiotensins, given intracerebroventricularly at the dose of 1 nmol each in rats lesioned with 6-OHDA to CA, on recognition memory, was tested. 3. AII and its 3-7 fragment significantly improved object recognition in sham-operated groups of rats. Bilateral 6-OHDA lesions to CA totally abolished the facilitatory effect of both angiotensins on object recognition. As insignificant increase of spontaneous locomotor activity in rats lesioned to CA did not interfere with the cognitive effect of AII and AII(3-7). 4. These results suggest that the dopaminergic projection at the CA takes part in the facilitatory effect of angiotensins on recognition memory.

The role of angiotensin receptors in cardiovascular diseases

As an antihypertensive regimen, angiotensin I-converting enzyme (ACE) inhibition appears to have an antiproliferative cardiovascular effect that is not caused by blood pressure reduction alone. On the other hand, ACE inhibition has been shown to induce neocapillarization in hypertrophied myocardium. The possible mechanisms behind these beneficial cardiovascular effects of ACE inhibition are the suppression of angiotensin II formation and the potentiation of bradykinin. Angiotensin II receptor antagonism appears to have a similar antiproliferative effect on myocardium and vascular smooth muscle as ACE inhibition. This suggests that the antiproliferative action of both regimens is due only to the reduction of the pressor and growth effects of angiotensin II, or that both regimens have an additional, similarly effective antiproliferative action. Recently, knowledge about angiotensin II receptors has almost exponentially expanded. The two main classes of angiotensin II receptors, type 1 and 2 (AT1 and AT2), have been shown to belong to the same receptor family. However, their signal transduction and function seem to differ totally. The function and signal transduction of AT1 are to a large extent known. All the well-known physiological and pathophysiological effects of angiotensin II have been attributed to AT1. On the other hand, AT2 has quite recently been shown to mediate antiproliferation and differentiation at least in some tissues and cells, e.g. in vascular endothelial cells and some cells of neuronal origin. This review highlights the recent findings on angiotensin II receptors, and discusses the mechanisms behind the beneficial cardiovascular effects of interfering with the renin-angiotensin system.

Behavioral effects of angiotensin II microinjected into CA1 hippocampal area

The behavioral responses of rats to bilateral microinjections of angiotensin II (ATII) at doses of 0.1, 0.5 and 1.0 microg into the hippocampal CA1 area of male Sprague-Dawley rats were studied. ATII affected locomotor activity (the number of horizontal and vertical movements) in a dose-related U-shaped manner. In the elevated plus-maze experiments, ATII at a dose of 0.1 microg microinjected bilaterally into the CA1 hippocampal area increased the number of entries into the open arms, suggesting some anxiolytic effect.

Complex activation of inducible transcription factors in the brain of normotensive and spontaneously hypertensive rats following central angiotensin II administration

The effects of intracerebroventricular (i.c.v.) injections of angiotensin II (Ang II) on the expression of inducible transcription factors (ITF) (c-Fos, FosB, c-Jun, JunB, JunD, Krox-20 and Krox-24) in the brain of conscious rats were assessed immunohistochemically using polyclonal antisera. Ang II (1, 10, 100 ng) induced after 90 min a dose-dependent expression of c-Fos, FosB, c-Jun, JunB and Krox-24, which was confined to four specific brain areas, namely the subfornical organ (SFO), median preoptic area (MnPO), paraventricular nucleus (PVN) and supraoptic nucleus (SON). In the above-mentioned regions, JunD exhibited a high basal staining which was not visibly altered by Ang II. Krox 20 was not induced by AnG II. FosB was only induced 4 h after i.c.v. injection of 100 ng Ang II in the MnPO and PVN. The Ang II-AT1 receptor antagonist, losartan, applied i.c.v. 5 min prior to Ang II (100 ng, i.c.v.) prevented the Ang II-induced ITF expression. In spontaneously hypertensive rats (SHR) but not in Wistar rats with nephrogenic hypertension due to aortic banding (WIab), the Ang II-induced expression of c-Fos, and c-Jun was enhanced in all four areas when compared to normotensive Wistar Kyoto (WKY)- and Wistar (WI) rats. The Ang II-induced expression of Krox-24 in the SFO, MnPO and PVN in SHR was also significantly increased when compared to WKY, WI and WIab rats. Our data demonstrate that a stimulation of periventricular Ang II-AT1 receptors induces a temporally and spatially highly differentiated expression pattern of ITFs restricted to four distinct regions of the forebrain involved in blood pressure regulation and body fluid homeostasis. The points to a strictly regulated expression of target genes in the respective regions. The enhanced Ang II-induced expression of ITFs in SHR compared to normotensive controls is not due to elevated blood pressure itself, since it was not observed in secondary hypertensive rats WIab. Thus, the increased sensitivity to Ang II in SHR appears to be genetically determined. The target genes regulated by Ang II-induced ITFs will have to be identified.