An angiotensin system in the anterior hypothalamic area anterior is involved in the maintenance of hypertension in spontaneously hypertensive rats

Protective effects of losartan on some type 2 diabetes mellitus-induced complications in Wistar and spontaneously hypertensive rats

Diabetes mellitus type 2 (T2DM) is characterized by resistance of insulin receptors and/or inadequate insulin secretion resulting in metabolic and structural complications including vascular diseases, arterial hypertension and different behavioral alterations. We aimed to study the effects of the antihypertensive angiotensin AT1 receptor antagonist losartan on the T2DM-induced changes of exploratory behavior, anxiety, nociception and short term memory in normotensive Wistar and spontaneously hypertensive rats (SHRs). The experimental model of T2DM induced by a combination of high fat diet and streptozotocin, decreased exploratory activity and increased the level of carbonylated proteins in selected brain structures in both strains; as well it increased corticosterone level, pain threshold, anxiety-like behavior, and decline short term memory only in SHRs. Losartan treatment alleviated some of the T2DM- induced metabolic complications, abolished the T2DM-induced hypo activity, and normalized the corticosterone level, carbonylated proteins in brain, nociception and memory. Losartan did not exert effect on the anxiety behavior in both strains. We showed that T2DM exerted more pronounced negative effects on the rats with comorbid hypertension as compared to normotensive rats. Overall effects on the studied behavioral parameters are related to decreased exploration of the new environment, increased anxiety-like behavior, and decline in short-term memory. The systemic sub-chronic treatment with an angiotensin AT1 receptor antagonist losartan ameliorated most of these complications.

The Renin-Angiotensin System in the Central Nervous System and Its Role in Blood Pressure Regulation

Purpose of the Review

The main goal of this article is to discuss how the development of state-of-the-art technology has made it possible to address fundamental questions related to how the renin-angiotensin system (RAS) operates within the brain from the neurophysiological and molecular perspective.

Recent Findings

The existence of the brain RAS remains surprisingly controversial. New sensitive in situ hybridization techniques and novel transgenic animals expressing reporter genes have provided pivotal information of the expression of RAS genes within the brain. We discuss studies using genetically engineered animals combined with targeted viral microinjections to study molecular mechanisms implicated in the regulation of the brain RAS. We also discuss novel drugs targeting the brain RAS that have shown promising results in clinical studies and trials.

Summary

Over the last 50 years, several new physiological roles of the brain RAS have been identified. In the coming years, efforts to incorporate cutting-edge technologies such as optogenetics, chemogenetics, and single-cell RNA sequencing will lead to dramatic advances in our full understanding of how the brain RAS operates at molecular and neurophysiological levels.

Hypertension in mice with transgenic activation of the brain renin-angiotensin system is vasopressin dependent

An indispensable role for the brain renin-angiotensin system (RAS) has been documented in most experimental animal models of hypertension. To identify the specific efferent pathway activated by the brain RAS that mediates hypertension, we examined the hypothesis that elevated arginine vasopressin (AVP) release is necessary for hypertension in a double-transgenic model of brain-specific RAS hyperactivity (the "sRA" mouse model). sRA mice experience elevated brain RAS activity due to human angiotensinogen expression plus neuron-specific human renin expression. Total daily loss of the 4-kDa AVP prosegment (copeptin) into urine was grossly elevated (≥8-fold). Immunohistochemical staining for AVP was increased in the supraoptic nucleus of sRA mice (~2-fold), but no quantitative difference in the paraventricular nucleus was observed. Chronic subcutaneous infusion of a nonselective AVP receptor antagonist conivaptan (YM-087, Vaprisol, 22 ng/h) or the V(2)-selective antagonist tolvaptan (OPC-41061, 22 ng/h) resulted in normalization of the baseline (~15 mmHg) hypertension in sRA mice. Abdominal aortas and second-order mesenteric arteries displayed AVP-specific desensitization, with minor or no changes in responses to phenylephrine and endothelin-1. Mesenteric arteries exhibited substantial reductions in V(1A) receptor mRNA, but no significant changes in V(2) receptor expression in kidney were observed. Chronic tolvaptan infusion also normalized the (5 mmol/l) hyponatremia of sRA mice. Together, these data support a major role for vasopressin in the hypertension of mice with brain-specific hyperactivity of the RAS and suggest a primary role of V(2) receptors.

Central angiotensinergic mechanisms associated with hypertension

Following its generation by both systemic and tissue-based renin-angiotensin systems, angiotensin II interacts with specific, G-protein coupled receptors to modulate multiple physiological systems, including the cardiovascular system. Genetic models in which the different components of the renin-angiotensin system have been deleted show large changes in resting blood pressure. Interruption of the generation of angiotensin II, or its interaction with these receptors, decreases blood pressure in hypertensive humans and experimental animal models of hypertension. Whilst the interaction of angiotensin II with the kidney is pivotal in this modulation of blood pressure, an involvement of the system in other tissues is important. Both systemic angiotensins, acting via the blood-brain barrier deficient circumventricular organs, and centrally-generated angiotensin modulate cardiovascular control by regulating fluid and electrolyte ingestion, autonomic activity and neuroendocrine function. This review discusses the pathways in the brain that are involved in this regulation of blood pressure as well as examining the sites in which altered angiotensin function might contribute to the development and maintenance of high blood pressure.

Direct Corticosteroid Modulation of GABAergic Neurons in the Anterior Hypothalamic Area of GAD65-eGFP Mice

Corticosterone is known to modulate GABAergic synaptic transmission in the hypothalamic paraventricular nucleus. However, the underlying receptor mechanisms are largely unknown. In the anterior hypothalamic area (AHA), the sympathoinhibitory center that project GABAergic neurons onto the PVN, we examined the expression of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) of GABAergic neurons using intact GAD65-eGFP transgenic mice, and the effects of corticosterone on the burst firing using adrenalectomized transgenic mice. GR or MR immunoreactivity was detected from the subpopulations of GABAergic neurons in the AHA. The AHA GABAergic neurons expressed mRNA of GR (42%), MR (38%) or both (8%). In addition, in brain slices incubated with corticosterone together with RU486 (MR-dominant group), the proportion of neurons showing a burst firing pattern was significantly higher than those in the slices incubated with vehicle, corticosterone, or corticosterone with spironolactone (GR-dominant group; 64 vs. 11~14%, p< 0.01 by χ(2)-test). Taken together, the results show that the corticosteroid receptors are expressed on the GABAergic neurons in the AHA, and can mediate the corticosteroid-induced plasticity in the firing pattern of these neurons. This study newly provides the experimental evidence for the direct glucocorticoid modulation of GABAergic neurons in the AHA in the vicinity of the PVN.

The hypothalamic arcuate nucleus: a new site of cardiovascular action of angiotensin-(1-12) and angiotensin II

The hypothalamic arcuate nucleus (ARCN) has been reported to play a significant role in cardiovascular regulation. It has been hypothesized that the ARCN may be one of the sites of cardiovascular actions of angiotensins (ANGs). Experiments were carried out in urethane-anesthetized, artificially ventilated, adult male Wistar rats. The ARCN was identified by microinjections of N-methyl-d-aspartic acid (NMDA; 10 mM). Microinjections (50 nl) of ANG-(1-12) (1 mM) into the ARCN elicited increases in mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA). The tachycardic responses to ANG-(1-12) were attenuated by bilateral vagotomy. The cardiovascular responses elicited by ANG-(1-12) were attenuated by microinjections of ANG II type 1 receptor (AT(1)R) antagonists but not ANG type 2 receptor (AT(2)R) antagonist. Combined inhibition of ANG-converting enzyme (ACE) and chymase in the ARCN abolished ANG-(1-12)-induced responses. Microinjections of ANG II (1 mM) into the ARCN also increased MAP and HR. Inhibition of ARCN by microinjections of muscimol (1 mM) attenuated the pressor and tachycardic responses to intravenously administered ANG-(1-12) and ANG II (300 pmol/kg each). These results indicated that 1) microinjections of ANG-(1-12) into the ARCN elicited increases in MAP, HR, and GSNA; 2) HR responses were mediated via both sympathetic and vagus nerves; 3) AT(1)Rs, but not AT(2)Rs, in the ARCN mediated ANG-(1-12)-induced responses; 4) both ACE and chymase were needed to convert ANG-(1-12) to ANG II in the ARCN; and 5) ARCN plays a role in mediating the cardiovascular responses to circulating ANGs.

The Renin-Angiotensin System in the Development of Salt-Sensitive Hypertension in Animal Models and Humans

Hypertension is still one of the major causes of death from cardiovascular failure. Increased salt intake may aggravate the rise in blood pressure and the development of consequential damage of the heart, the vessels and other organs. The general necessity of restricted salt intake regardless of blood pressure or salt sensitivity has been a matter of debate over the past decades. This review summarizes the main pathogenic mechanisms of hypertension and salt sensitivity in rat models, particularly in the spontaneously hypertensive rat (SHR), and in patients with essential hypertension (EH). Although SHRs are commonly considered to be salt-resistant, there is much evidence that salt loading may deteriorate blood pressure and cardiovascular function even in these animals. Similarly, EH is not a homogenous disorder - some patients, but not all, exhibit pronounced salt sensitivity. The renin-angiotensin system (RAS) plays a key role in the regulation of blood pressure and salt and fluid homeostasis and thus is one of the main targets of antihypertensive therapy. This review focuses on the contribution of the RAS to the pathogenesis of salt-sensitive hypertension in SHRs and patients with EH.

Differential sensitisation to central cardiovascular effects of angiotensin II in rats with a myocardial infarct: relevance to stress and interaction with vasopressin

The purpose of the present study was to elucidate if rats with myocardial infarction manifest altered responsiveness to central cardiovascular effects of low doses of angiotensin II (ANG II), and if ANG II and vasopressin (VP) cooperate in the central regulation of cardiovascular functions at rest and during stress. Conscious Sprague-Dawley rats with myocardial infarction induced by left coronary artery ligation, or sham-ligated (SL) controls were infused intracerebroventricularly with artificial cerebrospinal fluid (aCSF), ANG II, ANG II + VP or ANG II + V1a receptor antagonist (V1ANT) 4 weeks after cardiac surgery. In the infarcted but not in the SL rats, the resting mean arterial blood pressure (MABP) was significantly elevated by infusions of ANG II and ANG II + VP, while infusion of ANG II + V1ANT was not effective. During administration of aCSF, the pressor, and tachycardic responses to an air-jet stressor were significantly greater in the infarcted than in the SL rats. In the SL rats, the pressor responses to the stressor were significantly greater during infusions of ANG II, ANG II + VP and ANG II + V1ANT than during infusion of aCSF. The tachycardic response in the SL rats was enhanced only by the combined infusion of ANG II + VP. In the infarcted rats, the pressor and the tachycardic responses to the stressor were similar in all groups. It is concluded that: (1) under resting conditions the infarcted rats manifest sensitisation to the central pressor effect of ANG II and that this effect depends on concomitant stimulation of V1a VP receptors, (2) central ANG II may enhance the pressor response to an alarming stressor in the SL rats through an action which does not depend on the concomitant stimulation of V1a receptors, (3) the cooperative action of ANG II and VP is required for intensification of the tachycardic response to the alarming stressor in the SL rats and (4) exaggeration of the cardiovascular responses to the alarming stressor in the infarcted rats cannot be further augmented by an additional stimulation of central ANG II receptors or combined stimulation of ANG II and VP receptors.

Hypothalamic angiotensinergic–noradrenergic systems interaction in fructose induced hypertension

OBJECTIVE

Several studies suggest the importance of the interaction between the renin angiotensin and sympathetic nervous systems in blood pressure control, especially in clinical situations such as the metabolic syndrome. Previously, we have demonstrated changes in noradrenergic hypothalamic control of blood pressure in an animal model of insulin resistance and hypertension. The aim of the present study was to evaluate the effects of the interaction between the noradrenergic and angiotensinergic systems on hypothalamic blood pressure regulation in fructose hypertensive rats.

METHODS

In control (C) and fructose-fed hypertensive (F) rats, we studied: 1) the effects of hypothalamic perfusion of irbesartan (AT(1) angiotensin receptor antagonist, 50 and 500 microg ml(-1)) and metoprolol (beta(1) adrenergic receptor antagonist, 10 and 100 microg ml(-1)) on blood pressure, heart rate and noradrenaline intrahypothalamic levels, by means of the microdialysis technique; and 2) the effects of intrahypothalamic microinjection of angiotensin II alone or after metoprolol pre-administration, on blood pressure and heart rate.

RESULTS

Meanwhile irbesartan perfusion did not modify neither mean arterial pressure (MAP) nor heart rate or noradrenaline hypothalamic levels in the C group, its highest dose diminished MAP (DeltaMAP: F: - 16.3+/-1 mm Hg, p<0.05) and noradrenaline levels (% of basal levels: 58+/-7%, p<0.05) in the F group, without affecting heart rate. Intrahypothalamic perfusion of metoprolol diminished MAP only in the F group (DeltaMAP: F: -12.1+/-1.1 mm Hg, p<0.05), but did not modify heart rate in both groups. On the other hand, it diminished noradrenaline hypothalamic levels in C (% of basal levels: 53+/-6%, p<0.05) but not in the F group. The pressor response to angiotensin II microinjection was increased in F rats (DeltaMAP: F: 13.3+/-1.5 mm Hg vs. C: 6.9+/-1.8 mm Hg; p<0.05). Previous administration of metoprolol markedly abolished this increment.

CONCLUSIONS

Our results suggest the existence of an increase in AT(1) and beta(1) adrenergic receptors tone in the hypothalamus of F rats, which could be related to the increase in blood pressure present in this experimental model. On the other hand, considering that the enhanced pressor response to angiotensin II intrahypothalamic injection in F rats was abolished by previous administration of a beta(1) adrenergic receptor antagonist, these results would indicate that beta(1) adrenergic receptors activation participates in the pressor response to angiotensin II in this experimental model of insulin resistance and hypertension.

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.

γ-Aminobutyric acid in the lateral septal area is involved in mediation of the inhibition of hypothalamic angiotensin II-sensitive neurons induced by blood pressure increases in rats

Previously, we have demonstrated that intravenous phenylephrine-induced increases in blood pressure inhibit angiotensin II-sensitive neurons via gamma-aminobutyric acid (GABA) inputs in the anterior hypothalamic area (AHA). The lateral septal area (LSV) is also demonstrated to be involved in mediation of the baroreceptor reflex. To investigate central mechanisms involved in mediating the baroreceptor reflex, we examined whether GABA in the LSV is involved in mediation of the phenylephrine-induced inhibition of AHA angiotensin II-sensitive neurons. Microinjection of GABA into the LSV inhibited angiotensin II-sensitive neurons in the AHA of rats. The LSV GABA-induced inhibition of AHA neurons was abolished by pressure application of bicuculline onto the same AHA neurons. Intravenous injection of phenylephrine also inhibited AHA angiotensin II-sensitive neurons and the phenylephrine-induced inhibition of AHA neurons was abolished by microinjection of the GABAA receptor antagonist bicuculline into the LSV. In contrast, the LSV microinjection of bicuculline did not affect the inhibition of firing of AHA neurons induced by GABA pressure-applied in the AHA. These findings suggest that intravenous phenylephrine inhibits AHA angiotensin II-sensitive neurons via release of GABA in the LSV.

Intracerebroventricular injection of losartan inhibits angiotensin II-sensitive neurons via GABA inputs in the anterior hypothalamic area of rats

Previously, we have demonstrated that pressure-ejected application of angiotensin II and losartan, an angiotensin AT1 receptor antagonist, onto some neurons in the anterior hypothalamic area (AHA) of the rat increases and decreases, respectively, the basal firing rate of the neurons. To investigate possible participation of these AHA neurons in the brain angiotensin system, we examined whether intracerebroventricular injection of the angiotensin AT1 receptor antagonist losartan inhibits the neuronal activity of angiotensin II-sensitive neurons via GABA inputs in the AHA of rats. Intracerebroventricular injection of losartan decreased the firing rate of AHA angiotensin II-sensitive neurons. However, the intracerebroventricular injection of losartan did not affect the increase in firing rate of AHA angiotensin II-sensitive neurons induced by pressure application of angiotensin II onto the same neurons, although losartan similarly injected abolished the increase in firing rate of AHA angiotensin II-sensitive neurons induced by intracerebroventricular injection of angiotensin II. The losartan-induced decrease of unit firing in AHA neurons was abolished by pressure application of the GABAA receptor antagonist bicuculline onto the same neurons. Bicuculline itself did not affect the basal firing rate of AHA neurons. These findings suggest that intracerebroventricular injection of losartan inhibits AHA angiotensin II-sensitive neurons via GABA inputs to the neurons.

[Mechanisms of hypertension in the central nervous system]

This article reviews studies by the author on central mechanisms of hypertension. Spontaneously hypertensive rats (SHR) have been developed as a rat model of genetic hypertension, and central acetylcholine has been implicated in hypertension in SHR. The rostral ventrolateral medulla (RVL), a major source of efferent sympathetic activity, has cholinergic pressor systems. The release of acetylcholine is enhanced in the RVL of SHR, leading to hypertension. The alteration of the RVL cholinergic system in SHR results from enhanced angiotensin systems in the anterior hypothalamic area (AHA). Angiotensin II-sensitive neurons are present in the AHA and they are tonically activated by endogenous angiotensins. The basal activity of AHA angiotensin II-sensitive neurons is enhanced in SHR, mainly due to enhanced sensitivity of AHA neurons to angiotensin II. The AHA angiotensin system is also responsible for hypertension induced by emotional stress and central Na(+) increases. These findings suggest that the AHA angiotensin system may play a critical role in the development of hypertension.

Hypothalamic cardiovascular effects of angiotensin-(1–7) in spontaneously hypertensive rats

The objective of the present work was to study the cardiovascular actions of the intrahypothalamic injection of Ang-(1-7) and its effects on the pressor response to Ang II in spontaneously hypertensive (SH) rats and Wistar Kyoto (WKY) animals. In anaesthetized SH and WKY rats, a carotid artery was cannulated for mean arterial pressure (MAP) measurement and a stainless-steel needle was inserted into the anterior hypothalamus for drug administration. The cardiovascular effects of the intrahypothalamic administration of Ang-(1-7) were determined in SH and WKY rats. In SH rats, the effect of irbesartan and D-Ala-Ang-(1-7) on Ang-(1-7) cardiovascular effect was also evaluated. Ang II was administered in the hypothalamus of SH and WKY rats and changes in blood pressure and heart rate were measured followed by the administration of Ang II, Ang II+Ang-(1-7) or Ang II+D-Ala-Ang-(1-7). Ang-(1-7) did not the change basal MAP in WKY rats, but induced a pressor response in SH animals. Whilst the co-administration of D-Ala-Ang-(1-7) did not affect the response to Ang-(1-7), the previous administration of irbesartan prevented the effect of the peptide. The intrahypothalamic injection of Ang II induced a significantly greater pressor response in SH animals compared to normotensive rats. The co-administration of Ang-(1-7) with Ang II did not affect the pressor response to Ang II in the WKY group. In SH rats, whilst the co-administration of Ang-(1-7) with Ang II reduced the pressor response to Ang II, the concomitant application of D-Ala-Ang-(1-7) with Ang II increased the pressor response to the octapeptide after 5 and 10 min of intrahypothalamic administration. In conclusion, our result demonstrated that the biologically active peptide Ang-(1-7) did not participate in the hypothalamic blood pressure regulation of WKY animals. In SH rats, Ang-(1-7) exerted pleiotropic effects on blood pressure regulation. High dose of the heptapeptide produced a pressor response because of an unspecific action by activation of AT1 receptors. The concomitant administration of lower doses of Ang-(1-7) with Ang II reduced the pressor response to the octapeptide. Finally, the effect of AT(1-7) antagonist on Ang II pressor response suggested that hypothalamic formed Ang-(1-7) are implicated in the regulation of the cardiovascular effects of Ang II.

Evidence suggesting that angiotensins released not via synaptic inputs are involved in the basal activity of anterior hypothalamic neurons in rats

Previously, we have demonstrated that angiotensin II-sensitive neurons exist in the anterior hypothalamic area (AHA) and that these neurons are tonically activated by endogenous angiotensins in rats. Chemical stimulation of the lateral septal area (LSV) and medial amygdaloid nucleus (MeA), and intracerebroventricular injection of hypertonic saline, activated AHA angiotensin II-sensitive neurons. To investigate mechanisms of the basal activity of AHA angiotensin II-sensitive neurons, we examined the effect of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), a calmodulin inhibitor, applied onto AHA neurons on the basal activity and the stimulus-evoked activation of these neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjections of carbachol into the LSV and corticotropin-releasing factor into the MeA, and intracerebroventricular injection of hypertonic saline, activated AHA angiotensin II-sensitive neurons. These three kinds of injection-induced activations of AHA neurons were abolished by pressure application of W7 onto the same neurons, while the calmodulin inhibitor did not affect the increase in firing of AHA neurons induced by pressure application of angiotensin II onto the same neurons. The pressure application of W7 did not affect the basal activity of AHA angiotensin II-sensitive neurons, whereas the angiotensin AT1 receptor antagonist losartan similarly applied inhibited it. These findings suggest that the basal activity of AHA angiotensin II-sensitive neurons is mediated by angiotensins released not via synaptic inputs.

Angiotensin II sensitivity of anterior hypothalamic area neurons is enhanced in both spontaneously hypertensive rats and Dahl salt-sensitive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins. Furthermore, we have demonstrated that intracerebroventricular injection of hypertonic saline increases the firing rate of AHA angiotensin II-sensitive neurons via angiotensins and that the central sodium-induced activation of AHA neurons is enhanced in spontaneously hypertensive rats (SHR) and Dahl salt-sensitive (Dahl S) rats. In this study, we examined whether sensitivities of AHA angiotensin II-sensitive neurons to angiotensin II are enhanced in SHR and Dahl S rats as compared with their respective controls. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY), and male 15- to 16-week-old Dahl S rats and Dahl R rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. In SHR, pressure application of angiotensin II (3 x 10(-9) to 3 x 10(-8) M) onto AHA angiotensin II-sensitive neurons increased their firing rate in a concentration-dependent manner. In WKY, only the highest concentration of angiotensin II increased the firing rate, while the lower concentrations of angiotensin II did not affect it. In Dahl S rats, pressure application of angiotensin II (10(-8) and 3 x 10(-8) M) onto AHA neurons increased their firing rate, while angiotensin II (3 x 10(-9) M) did not affect it. In Dahl R rats, the highest concentration of angiotensin II increased the firing rate, while the lower concentrations of angiotensin II did not affect it. These findings indicate that the sensitivity of AHA neurons to angiotenisn II is enhanced in SHR and Dahl S rats as compared with their respective controls.

Enhanced central hypertonic saline-induced activation of angiotensin II-sensitive neurons in the anterior hypothalamic area of spontaneously hypertensive and Dahl S rats

High dietary salt intake activates the brain renin-angiotensin system in spontaneously hypertensive rats (SHR) and Dahl S rats, resulting in sympathetic hyperactivity and hypertension. Increases of sodium concentration in cerebrospinal fluid (CSF) and/or enhanced responses to CSF sodium are considered to be involved in the high dietary salt-induced activation of central nervous system pathways in those rats. Previously we have demonstrated that intracerebroventricular injection of hypertonic saline increases the neural activity of angiotensin II-sensitive neurons trans-synaptically via endogenous angiotensins in the anterior hypothalamic area (AHA) of rats. In the present study, we examined whether the AHA angiotensin II-sensitive neuron response to hypertonic saline would differ in SHR and Dahl S rats from those of their controls. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY), Dahl S rats and Dahl R rats and Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Intracerebroventricular injection of hypertonic saline increased the firing rate of AHA angiotensin II-sensitive neurons. The threshold sodium concentration for the central sodium-induced increase of neural firing was lower in SHR than those of WKY, Dahl S rats, Dahl R rats and Wistar rats. The increase in neural firing induced by hypertonic saline (250 mM) was greater in SHR than those of other four kinds of rats. Similarly, the threshold sodium concentration was lower in Dahl S rats than those of WKY, Dahl R rats and Wistar rats and the increase in neural firing induced by hypertonic saline (250 mM) was greater in Dahl S rats than those of WKY, Dahl R rats and Wistar rats. In SHR, intracerebroventricular injection of the amiloride-sensitive sodium channel blocker benzamil abolished the hypertonic saline (250 mM)-induced increase in neural firing, but the sodium channel blocker itself did not affect the basal firing of these neurons. These findings indicate that central sodium-induced activation of AHA angiotensin II-sensitive neurons is enhanced in SHR and Dahl S rats.

Activities of hypothalamic angiotensin II-sensitive neurons are greately enhanced even in prehypertensive spontaneously hypertensive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) of rats are tonically activated by endogenous angiotensins and that reactivities of these neurons to angiotensin II are enhanced in 15- to 16-week-old spontaneously hypertensive rats (SHR). To investigate whether the enhanced reactivity of SHR AHA neurons to angiotensin II is secondary to raised blood pressure, we examined whether the enhanced reactivity to angiotensin II also occurs in prehypertensive SHR. We also examined whether reactivities of AHA angiotensin II-sensitive neurons to intracerebroventricular hypertonic saline are enhanced in prehypertensive SHR, since intracerebroventricular injection of hypertonic saline increases the firing rate of AHA neurons via release of angiotensins at AHA neuron levels. Male 4-week-old SHR and age-matched Wistar Kyoto rats (WKY) were used in this study. There was no difference in systolic blood pressure between both rats. They were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of angiotensin II onto some AHA neurons increased their firing rate. The basal firing rate of angiotensin II-sensitive neurons was increased in SHR as compared with WKY. The increase of unit firing by angiotenisn II was enhanced in SHR as compared with WKY. Intracerebroventricular injection of hypertonic saline increased the firing rate of AHA angiotensin II-sensitive neurons. The average threshold sodium concentration for the saline-induced increase of neural firing was lower in SHR than in WKY. These findings demonstrate that basal activities and responsiveness to angiotensin II in AHA angiotensin II-sensitive neurons are enhanced in prehypertensive SHR as compared with age-matched WKY. In addition, these findings indicate that central saline-induced activation of AHA angiotensin II-sensitive neurons is also enhanced in SHR. It appears that the enhanced reactivity of SHR AHA neurons to angiotensin II occurs primarily in nature but not secondarily to raised blood pressure in SHR.

Posterior hypothalamus cholinergic stimulation-induced activation of anterior hypothalamic area neurons is enhanced in spontaneously hypertensive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA neurons are enhanced in spontaneously hypertensive rats (SHR). In addition, we have demonstrated that cholinergic mechanisms in the posterior hypothalamic nucleus (PHN) are involved in the activation of AHA angiotensin-II-sensitive neurons. It has been suggested that cholinergic function in the posterior hypothalamus is enhanced in SHR and that this hyperactivity plays a role in hypertension in SHR. In the present study, we examined whether the PHN cholinergic stimulation-induced activation of AHA angiotensin-II-sensitive neurons is altered in SHR. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY) were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of the cholinoceptor agonist carbachol, the cholinesterase inhibitor physostigmine and the excitatory amino acid glutamate into the PHN caused increases in firing rate of AHA angiotensin-II-sensitive neurons in anesthetized WKY and SHR. The increase in firing rate of AHA neurons induced by these drugs was enhanced in SHR as compared to WKY. The enhancement of the physostigmine-induced activation of AHA neurons in SHR was similar to that of the carbachol-induced activation of AHA neurons in SHR. The enhancement of the glutamate-induced activation of AHA neurons in SHR was similar to that of the carbachol-induced activation of AHA neurons in SHR. Microinjection of scopolamine, a cholinoceptor antagonist, into the PHN caused a small but significant decrease of firing rate of AHA angiotensin-II-sensitive neurons in SHR but not in WKY. These findings indicate that the PHN cholinergic stimulation-induced activation of AHA angiotensin-II-sensitive neurons is enhanced in SHR and that PHN cholinergic mechanisms are involved in tonic activation of angiotensin-II-sensitive neurons in the AHA of SHR. It appears that the enhancement of the PHN cholinergic stimulation-induced activation of AHA neurons in SHR results mainly from the enhanced neural reactivity to angiotensins in AHA neurons of SHR.

Cholinergic stimulation in the posterior hypothalamic nucleus activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats (SHR). Acetylcholine in the posterior hypothalamic nucleus (PHN) has been implicated in hypertension in SHR. It is suggested that there exist neuronal projections from the PHN to the AHA in rats. In this study, we examined whether cholinergic stimulation in the PHN activates AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of carbachol, physostigmine and glutamate into the PHN caused an increase in firing rate of AHA angiotensin II-sensitive neurons in anesthetized rats. The carbachol-induced increase of firing rate was inhibited by pressure application of the AT1 receptor antagonist losartan onto AHA angiotensin II-sensitive neurons. The glutamate-induced increase of firing rate was also inhibited by the pressure application of losartan. PHN microinjections of carbachol and glutamate did not affect blood pressure in these anesthetized rats. In conscious rats, PHN microinjection of carbachol produced an increase of blood pressure and the carbachol-induced pressor response was inhibited by bilateral microinjections of losartan into the AHA. These findings indicate that cholinergic stimulation in the PHN activates AHA angiotensin II-sensitive neurons. It seems likely that the activation of AHA angiotensin II-sensitive neurons induced by PHN cholinergic stimulation is partly mediated via release of angiotensins at AHA angiotensin II-sensitive neuron levels.

Anterior hypothalamic neurons respond to blood pressure changes via γ-aminobutyric acid and angiotensins in rats

It has been suggested that neurons in the hypothalamus respond to baroreflex activation and deactivation. In this study, we examined whether angiotensin II-sensitive neurons in the anterior hypothalamic area (AHA) respond to baroreflex activation and deactivation, and which neurotransmitters are involved in mediating the baroreflex responses. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Increases in blood pressure induced by intravenous phenylephrine completely inhibited the firing of AHA angiotensin II-sensitive neurons. The phenylephrine-induced inhibition of neuronal firing was blocked and enhanced by the pressure application of bicuculline and nipecotic acid, respectively, onto the same neurons. In contrast, decreases in blood pressure induced by intravenous nitroprusside increased the firing of angiotensin II-sensitive neurons. The nitroprusside-induced increase of neuronal firing was blocked by the pressure application of losartan onto the same neurons. These findings suggest that angiotensin II-sensitive neurons in the AHA respond to blood pressure changes via gamma-aminobutyric acid and angiotensins in rats.

Cholinergic stimulation in the lateral septal area activates anterior hypothalamic area neurons via excitatory amino acid receptors in rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats. It is suggested that there exist neuronal projections from the lateral septal area (LSV) to the AHA in rats. In this study, we examined whether neurons in the LSV are involved in activation of AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of carbachol into the LSV caused an increase in firing rate of AHA angiotensin II-sensitive neurons. The carbachol-induced increase of firing rate of AHA angiotensin II-sensitive neurons was inhibited by pressure application of the excitatory amino acid receptor antagonist kynurenate but not by the AT1 receptor antagonist losartan onto the same neurons. Microinjection of carbachol into the LSV also increased the firing rate of AHA ACh-sensitive neurons, and the carbachol-induced increase of firing rate of ACh-sensitive neurons was again abolished by pressure application of kynurenate but not by the muscarinic receptor antagonist scopolamine onto the same neurons. Microinjection of the muscarinic receptor antagonist 4-DAMP into the LSV did not affect the firing rate of AHA angiotensin II-sensitive neurons. These findings indicate that neurons in the LSV are involved in activation of AHA angiotensin II-sensitive neurons. It seems likely that the carbachol-induced activation of AHA angiotensin II-sensitive neurons is mainly mediated via excitatory amino acid receptors at AHA neurons.

Central injection of hypertonic saline activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

We have previously reported that microinjection of angiotensin II into the anterior hypothalamic area (AHA) produces pressor responses and that angiotensin II-sensitive neurons in the AHA are tonically activated by endogenous angiotensins in rats. Central injection of hypertonic saline causes pressor responses via release of angiotensins in brain. In this study, we examined whether angiotensin II-sensitive neurons in the AHA are responsive to intracerebroventricular injection of hypertonic saline and whether endogenous angiotensins in the AHA are involved in the central hypertonic saline-induced pressor response. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Intraventricular injection of hypertonic saline increased the neural activity of angiotensin II-sensitive neurons, whereas pressure application of hypertonic saline onto angiotensin II-sensitive neurons themselves did not affect their neural activities. The intraventricular hypertonic saline-induced increase of unit activity of AHA neurons was inhibited by pressure application of the angiotensin AT1 receptor antagonist losartan onto the same neurons. The hypertonic saline-induced increase of unit firing was also blocked by intraventricular injection of the amiloride-sensitive sodium channel blocker benzamil. In conscious rats, intraventricular injection of hypertonic saline produced pressor responses, and the hypertonic saline-induced pressor response was inhibited by bilateral microinjection of losartan into the AHA. Repeated intraventricular injection of hypertonic saline caused an increase in the release of angiotensins in the AHA of anesthetized rats. These findings indicate that intracerebroventricular injection of hypertonic saline increases neural activity of angiotensin II-sensitive neurons trans-synaptically via endogenous angiotensins in the AHA. In addition, these findings also indicate that the intracerebroventricular injection of hypertonic saline produces a pressor response at least partly via release of angiotensins in the AHA.

Role of protein kinase Cβ in phorbol ester-induced c-fos gene expression in neurons of normotensive and spontaneously hypertensive rat brains

We have previously demonstrated that pressure application of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) onto some neurons in the anterior hypothalamic area of rats increases neural activity in vivo and that this PKC activation-induced increase of neural activity is enhanced in spontaneously hypertensive rats (SHR), an animal model for genetic hypertension. Activation of PKC increases expression of the c-fos gene, an important transcription factor and proto-oncogene thought to be a marker of neural activity. To evaluate PKC isoforms responsible for neural activation, we examined which isoforms of PKC are involved in the PKC activation-induced c-fos gene expression in neuronal cultures of Wistar rat and spontaneously hypertensive rat (SHR) brains. PMA increased c-fos gene expression in neuronal cultures of Wistar rat brain and the PMA-induced c-fos gene expression was inhibited by the PKC inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7). The PKCalpha,beta,gamma activator thymeleatoxin also increased c-fos gene expression, while the PKCdelta,epsilon activator ingenol did not affect it. In addition, the PMA-induced c-fos gene expression was inhibited by PKCbetaantisense oligonucleotides (AON) but not by PKCalpha and PKCgammaAONs. In SHR brain neuronal cultures, the PMA-induced c-fos gene expression was enhanced as compared with that of Wistar Kyoto rats (WKY), while basal c-fos gene expression was almost the same in both neuronal cultures. The enhancement of PMA-induced c-fos gene expression in SHR brain cultures was abolished by PKCbetaAON. These findings suggest that in rat brain neuronal cultures, PMA increases c-fos gene expression via activation of PKC and that PKCbetaisoforms are partly involved in the PMA-induced c-fos gene expression. In neuronal cultures of SHR brain, it appears that the PMA-induced c-fos gene expression is also enhanced via PKCbeta.

The medial amygdaloid area is involved in activation of angiotensin II-sensitive neurons in the anterior hypothalamic area

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that the activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats. It is suggested that there exist neural projections from the medial amygdala to the AHA in rats. In this study, we examined whether neurons in the medial amygdaloid area (MeA) are involved in the activation of AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of glutamate into the MeA caused an increase in the firing rate of AHA angiotensin II-sensitive neurons. The glutamate-induced increase of firing rate was inhibited by pressure application of the AT1 receptor antagonist losartan onto AHA angiotensin II-sensitive neurons. The microinjection of glutamate into the central amygdaloid area also increased the firing rate of AHA angiotensin II-sensitive neurons, but the glutamate-induced increase of firing rate was not affected by pressure application of losartan onto AHA angiotensin II-sensitive neurons. The microinjection of corticotropin-releasing factor (CRF) into the MeA also increased the firing rate of AHA angiotensin II-sensitive neurons, but the CRF-induced increase of firing rate was not inhibited by pressure application of losartan onto AHA angiotensin II-sensitive neurons. Repeated microinjection of glutamate into the MeA caused an increase in the release of angiotensins in the AHA. These findings indicate that neurons in the MeA are involved in the activation of AHA angiotensin II-sensitive neurons. It seems likely that the activation of AHA angiotensin II-sensitive neurons induced by glutamate but not CRF is partly mediated via the release of angiotensins at AHA angiotensin II-sensitive neuron levels.

Protein kinase C activation-induced increases of neural activity are enhanced in the hypothalamus of spontaneously hypertensive rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these angiotensin II-sensitive neurons in the AHA are enhanced in spontaneously hypertensive rats (SHR). In addition, neural activations induced by both angiotensin II and glutamate were enhanced in the AHA of SHR. In this study, we examined whether intracellular neural activation mechanisms via protein kinase C (PKC) and a potassium channel are altered in angiotensin II-sensitive neurons in the AHA of SHR. Male 15- to 16-week-old SHR and age-matched Wistar-Kyoto rats (WKY) and Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of the PKC activator phorbol 12-myristate 13-acetate (PMA) onto angiotensin II-sensitive neurons in the AHA of Wistar rats increased their firing rate. The increase of unit activity by PMA was inhibited by the potent inhibitor of PKC, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), but not by the weak PKC inhibitor, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (HA1004). The increase of unit firing by PMA was enhanced in SHR as compared with WKY. Pressure application of H-7 alone decreased the basal firing activity of angiotensin II-sensitive neurons in SHR but not in WKY. HA1004 did not affect the basal firing activity of angiotensin II-sensitive neurons in SHR. Angiotensin II-induced increases of firing rate in AHA neurons were inhibited by H-7 and the inhibition by H-7 was enhanced in SHR as compared with WKY. Pressure application of 4-aminopyridine, a blocker of the transient potassium current, onto angiotensin II-sensitive neurons increased their firing rate and the increase of unit firing rate was almost the same in WKY and SHR. These findings indicate that activation of PKC increases neural activity in angiotensin II-sensitive neurons in the AHA and that this PKC activation-induced increase of neural activity is enhanced in the AHA of SHR. It seems likely that the enhanced PKC activation effect is responsible for the enhanced basal neural activity seen in the AHA of SHR.

Anterior hypothalamic beta-adrenoceptors in chronic aortic-coarctated hypertensive rats: An interaction with central angiotensin II receptors

1. The aim of the present study was to investigate the activity of anterior hypothalamic beta-adrenoceptors and angiotensin (Ang) II receptors on blood pressure in normotensive rats and aortic-coarctated (ACo) animals at a chronic stage of hypertension. A possible interaction between beta-adrenoceptors and AngII pressor activity was also investigated. 2. Injection of isoproterenol (0.1-10 nmol) in the anterior hypothalamic area induced a dose-dependent decrease in mean arterial pressure (MAP) in sham-operated (SO), but not in ACo, animals. Isoproterenol (1 nmol) reduced blood pressure in SO rats (DeltaMAP -10.1+/-1.4 mmHg; n=10) but not in ACo animals (DeltaMAP -0.9+/-1.6 mmHg; n=10; P<0.05 vs SO rats). Whereas previous administration of atenolol (40 nmol) enhanced the cardiovascular effect of isoproterenol (1 nmol) in ACo rats but not in SO animals, propranolol (40 nmol) prevented the hypotensive action of isoproterenol in both experimental groups. Intrahypothalamic administration of clenbuterol decreased MAP in a dose-dependent manner; however, the depressor response to clenbuterol (10 nmol) was greater in ACo rats than in SO rats (DeltaMAP -26.8+/-3.2 vs -14.4+/-2.4 mmHg, respectively; n=5 for both; P<0.05). When AngII (50 ng) was injected into the anterior hypothalamic area, a greater pressor response was observed in ACo rats than in SO rats (DeltaMAP 19.6+/-1.1 vs 11.3+/-0.6 mmHg, respectively; n=5 for both; P<0.05). Atenolol (40 nmol) pretreatment partially and significantly prevented the pressor response to AngII in ACo rats, but not in SO rats. 3. In conclusion, these results provide pharmacological evidence for the existence of a beta1-adrenoceptor-mediated pressor mechanism in the anterior hypothalamic area of ACo rats that is absent in SO rats. The enhanced depressor beta2-adrenoceptor activity observed in chronic ACo rats could be a compensatory adjustment to pressor beta1-adrenoceptor activity. Conversely, pressor overactivity of AngII was observed in the anterior hypothalamic area of ACo rats at a chronic hypertensive stage; this enhancement could be explained, at least in part, by the pressor beta1-adrenoceptor activity.

Basal transcriptional regulation of rat AT1 angiotensin II receptor gene expression

1. Angiotensin II AT1A receptors are thought to play an important role in the development of hypertension. The transcriptional factor Sp1 is a ubiquitous transcriptional factor associated with GC-rich promoters and involved in basal promoter activity. 2. To examine basal transcriptional levels regulation of the rat AT1A receptor gene, we determined whether two GC-box-related regions within 100 bp of the rat AT1A receptor gene promoter are involved in the basal expression of the gene in A10 cells, a vascular smooth muscle cell line. 3. The electrophoretic mobility shift assay demonstrated that incubation of the -98/-79 region and -58/-34 region sequence oligonucleotides with nuclear extracts of rat hypothalamus, liver and adrenal formed DNA-protein complexes and that the addition of unlabelled oligonucleotides containing the Sp1 consensus sequence blocked the formation of the DNA-protein complex. The addition of antibody against Sp1 also blocked the formation of the DNA-protein complex. 4. The promoter/luciferase reporter assay demonstrated that the reporter activity of AT1A receptor promoters mutated either within the -98/-79 or the -58/-34 region was lower than that of intact AT1A receptor promoters. 5. The promoter activity of AT1A receptor promoters mutated within those two regions was lower than that of promoters mutated within either the -98/-79 or the -58/-34 region. 6. These findings suggest that GC-box-regulated sequences within the -98/-79 region and the -58/-34 region are additively involved in basal expression level of the AT1A receptor gene in A10 cells.

Enhanced activity of angiotensin II-sensitive neurons in the anterior hypothalamic area of spontaneously hypertensive rats

We have previously reported that an angiotensin system in the anterior hypothalamic area (AHA) is enhanced in spontaneously hypertensive rats (SHRs) and that this enhancement is involved in hypertension in this strain. In addition, we have reported that some neurons in the AHA are tonically activated by endogenous angiotensins in rats. In this study, we examined whether activities of neurons receiving tonic angiotensinergic inputs in the AHA are enhanced in SHR as compared with those of Wistar Kyoto rats (WKY). Male 15- to 16- or 6-week-old SHR and age-matched WKY were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of angiotensin II onto some neurons in the AHA increased their firing rate. The basal firing rate of angiotensin II-sensitive neurons was increased in both 15- to 16- and 6-week-old SHR than in age-matched WKY. The increase of unit firing by angiotenisn II was enhanced in both 15- to 16- and 6-week-old SHR as compared with age-matched WKY. Pressure application of losartan, an angiotensin type 1 (AT1) receptor antagonist, alone decreased the basal firing rate of angiotensin II-sensitive neurons in 15- to 16-week-old SHR and WKY. The decrease of unit firing by losartan was also enhanced in SHR as compared with WKY. Pressure application of glutamate onto angiotensin II-sensitive neurons increased their firing rate and the increase of unit firing by glutamate was enhanced in 15- to 16-week-old SHR as compared with age-matched WKY. These findings suggest that activities of angiotensin II-sensitive neurons in the AHA are enhanced in SHR as compared with WKY. It is possible that the enhanced activity of angiotensin II-sensitive neurons in the AHA of SHR is partly due to enhanced neuronal reactivity to angiotensin II.

Tonic angiotensinergic inputs to neurons in the anterior hypothalamic area of rats

We have previously reported that microinjection of angiotensin II into the anterior hypothalamic area (AHA) produces a pressor response in rats and that the angiotensin AT1 receptor antagonist, losartan, similarly injected causes a depressor response in hypertensive rats. In this study, we examined whether endogenous angiotensins are involved in activation of neurons in the AHA. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure-ejected application of angiotensin II and glutamate onto some neurons in the AHA increased their firing rate. The increase of unit firing induced by angiotensin II but not by glutamate was inhibited by losartan. Application of losartan alone inhibited the basal firing rate of angiotensin II-sensitive neurons in a concentration-dependent manner. Application of the angiotensin AT2 receptor antagonist, PD123319, did not affect the increase of unit firing induced by angiotensin II and the basal firing rate of angiotensin II-sensitive neurons. Pressure application of angiotensin I onto angiotensin II-sensitive neurons also increased firing rate and the increase of unit firing by angiotensin I was inhibited by the angiotensin converting enzyme inhibitor, captopril. Captopril alone inhibited the basal firing rate of angitensin II-sensitive neurons. Acetylcholine did not affect unit firing of angiotensin II-sensitive neurons, whereas it increased the firing rate of some angiotensin II-insensitive neurons in the AHA. Increases of blood pressure by intravenous phenylephrine completely inhibited the basal firing rate of angiotensin II-sensitive neurons. These findings suggest that some neurons in the AHA are tonically activated by endogenous angiotensins. It seems likely that newly synthesized angiotensins are used for the angiotensinergic transmission in the AHA.

Chronic production of angiotensin IV in the brain leads to hypertension that is reversible with an angiotensin II AT1 receptor antagonist

Angiotensin IV (Ang IV) is a metabolite of the potent vasoconstrictor angiotensin II (Ang II). Because specific binding sites for this peptide have been reported in numerous tissues including the brain, it has been suggested that a specific Ang IV receptor (AT4) might exist. Bolus injection of Ang IV in brain ventricles has been implicated in learning, memory, and localized vasodilatation. However, the functions of Ang IV in a physiological context are still unknown. In this study, we generated a transgenic (TG) mouse model that chronically releases Ang IV peptide specifically in the brain. TG mice were found to be hypertensive by the tail-cuff method as compared with control littermates. Treatment with the angiotensin-converting enzyme inhibitor captopril had no effect on blood pressure, but surprisingly treatment with the Ang II AT1 receptor antagonist candesartan normalized the blood pressure despite the fact that the levels of Ang IV in the brains of TG mice were only 4-fold elevated over the normal endogenous level of Ang peptides. Calcium mobilization assays performed on cultured CHO cells chronically transfected with the AT1 receptor confirm that low-dose Ang IV can mobilize calcium via the AT1 receptor only in the presence of Ang II, consistent with an allosteric mechanism. These results suggest that chronic elevation of Ang IV in the brain can induce hypertension that can be treated with angiotensin II AT1 receptor antagonists.

Sp1 decoy oligodeoxynucleotide decreases angiotensin receptor expression and blood pressure in spontaneously hypertensive rats

The transcriptional factor Sp1 is associated with GC-rich promoters and involved in basal promoter activity. A GC-box-related sequence is located within the -58 to -34 base pair region of the angiotensin type 1 receptor gene promoter. We examined whether Sp1 in the hypothalamus was increased in spontaneously hypertensive rats (SHR) and whether inhibition of Sp1 binding sites suppressed angiotensin type 1 receptor expression and thus decreased blood pressure in SHR. Western blot analysis showed that Sp1 protein levels were increased in nuclear extracts of hypothalamus from SHR. Electrophoretic mobility shift assay (EMSA) using oligonucleotides containing Sp1 consensus sequence and -58 to -34 region sequence oligonucleotides showed that DNA-protein complexes were greater in nuclear extracts of hypothalamus from SHR than those of Wistar Kyoto rats (WKY). Sp1 decoy phosphorothioate oligodeoxynucleotides injected into the lateral ventricle produced a decrease in blood pressure in SHR, and decreased angiotensin type 1 receptor mRNA levels and number of angiotensin receptors in the hypothalamus of SHR. Pressor responses to angiotensin II but not to carbachol injected into the lateral ventricle were decreased in the Sp1 decoy-treated SHR. The results of the present study suggest that Sp1 levels in the hypothalamus of SHR are increased, and that inhibition of the binding of Sp1 to its binding sites decreases angiotensin type 1 receptor expression and blood pressure in SHR. The possibility cannot be ruled out that the Sp1 decoy oligodeoxynucleotides (ODN) also suppressed transcriptions of genes other than the angiotensin type 1 receptor gene.

Existence of a mutation of angiotensin AT1 receptor gene promoter region involved in inhibition of AT1 receptor gene transcription in spontaneously hypertensive rats

The angiotensin type 1a (AT1a) receptor gene is overexpressed in the brain and peripheral tissues of spontaneously hypertensive rats (SHR). We examined whether there are mutations responsible for overexpression of the AT1a receptor gene in the SHR AT1a receptor promoter region. Genomic DNA was extracted from the livers of SHR and Wistar Kyoto rats (WKY) of Izumo strain (SHR/Izm and WKY/Izm, respectively). Fragments of the AT1a receptor gene promoter region were amplified by polymerase chain reaction (PCR). Amplified fragments were purified by agarose gel electrophoresis, and the purified fragments were cloned using pTBlue T-Vector. Sequence analysis identified one single base mutation unique to the SHR AT1a receptor gene promoter region when compared to that of WKY. The sequence of the mutation site in SHR was the same as that of Sprague Dawley rats. Using an electrophoretic mobility shift assay, we compared gel patterns formed by DNA-protein complexes using ds-oligonucleotides representing region-1624 to-1595 of the SHR and WKY AT1a receptor promoters. There were 3 major similar DNA-protein complexes against WKY and SHR oligonucleotides. In addition, the oligonucleotide bearing the SHR sequence produced an extra band. Promoter/luciferase reporter assay demonstrated that the promoter activity of SHR AT1a receptor promoters (-2050 to +57) was lower than that of WKY. These results suggest that there is one single mutation unique to the SHR AT1a receptor gene promoter region, but that the mutation is not responsible for overexpression of the AT1 a receptor gene in SHR.

Cholinergic mechanism in the lateral septal area is involved in the stress-induced blood pressure increase in rats

Previously, we demonstrated that the rostral part of the ventral zone of the lateral septal area (LSV) was involved in the restraint stress-induced pressor response. It is suggested that there exist acetylcholine receptors responsible for blood pressure increase in the caudal part of the lateral septal area. In this study, we examined whether acetylcholine receptors responsible for pressor responses also exist in the rostral part of the LSV and whether these acetylcholine receptors are involved in the stress-induced pressor response in rats. Microinjection of either carbachol (10-100pmol) or physostigmine (0.46 and 1.5nmol) into the LSV caused a dose-dependent increase in blood pressure. The pressor response to carbachol (30pmol) was inhibited by the M1 antagonist pirenzepine and the M3 antagonist 4-DAMP mustard but not by the M2 antagonist methoctramine injected into the LSV. Bilateral microinjections of the M1/M3 antagonist 4-DAMP (1nmol) inhibited the restraint stress-induced pressor response. These findings suggest that M1/M3 muscarinic receptors responsible for blood pressure increase exist in the rostral part of the LSV and they are partly involved in the stress-induced pressor response.

Activation of hypothalamic angiotensin receptors produces pressor responses via cholinergic inputs to the rostral ventrolateral medulla in normotensive and hypertensive rats

We have previously reported that the angiotensin system in the anterior hypothalamic area (AHA) is enhanced in spontaneously hypertensive rats (SHR) and that this enhancement is involved in hypertension in SHR. In addition, acetylcholine (ACh) release is increased in the rostral ventrolateral medulla (RVLM) of SHR, which has also been shown to be involved in hypertension in SHR. In this study, we examined whether the enhanced angiotensin system in the AHA of SHR is related to the increase in cholinergic inputs to the RVLM. Electrical stimulation in the AHA produced a pressor response and an increase in firing rate of RVLM barosensitive neurons. These responses were inhibited and enhanced by RVLM application of the muscarinic receptor antagonist scopolamine and the cholinesterase inhibitor physostigmine, respectively. AHA stimulation also produced release of ACh in the RVLM. Microinjections of angiotensin II and carbachol into the AHA produced pressor responses. The pressor response to angiotensin II was inhibited by scopolamine microinjected into the RVLM, although this produced no effect on the response to carbachol. In SHR, although not in Wistar-Kyoto rats, microinjection of losartan into the AHA inhibited pressor responses to physostigmine. However inhibition was not observed in response to the directly acting muscarinic receptor agonist carbachol, injected into the RVLM. These findings demonstrate that angiotensin receptor activation or electrical stimulation in the AHA produce a pressor response via an increase in ACh release in the RVLM. In addition, the present study suggests that the enhanced angiotensin system in the AHA of SHR increases cholinergic inputs to the RVLM, which leads to increases in blood pressure.

Angiotensin receptor blockade in the anterior hypothalamic area inhibits stress-induced pressor responses in rats

Central angiotensin systems are involved in expression of pressor responses induced by immobilization stress. In this study, we examined whether angiotensin receptors in the anterior hypothalamic area are involved in the pressor response during stress exposure in rats. Intracerebroventricular injections of the angiotensin AT1-receptor antagonist losartan (6.5 and 22 nmol) attenuated pressor responses to immobilization stress dose-dependently. Injections of losartan (0.065 and 0.22 nmol) into the anterior hypothalamic area also suppressed the stress-induced pressor response dose-dependently, whereas intraventricular injection of losartan (2.2 nmol) did not affect it. Immobilization stress caused increases in plasma catecholamine levels. The stress-induced increase of plasma catecholamine levels was also inhibited by angiotensin receptor blockade in the anterior hypothalamic area. The present results suggest that angiotensin receptors in the anterior hypothalamic area are involved in expression of the pressor response and sympathetic activation induced by immobilization stress.

Renin antisense injected intraventricularly decreases blood pressure in spontaneously hypertensive rats

Brain renin-angiotensin system plays an important role in blood pressure regulation and is suggested to play a role in the development and maintenance of hypertension. To test the hypothesis that brain renin may play a significant role in hypertension in spontaneously hypertensive rats (SHR), phosphorothioated antisense oligodeoxynucleotides targeted to renin mRNA were administered intracerebroventricularly in SHR. Administration of an antisense but not its sense oligodeoxynucleotide produced a prolonged duration of decrease in blood pressure. Intra-arterial administration of the antisense oligodeoxynucleotide at the same dose that decreased blood pressure when administered intraventricularly did not affect blood pressure. Furthermore, renin mRNA but not angiotensin AT1 receptor mRNA levels were decreased in the hypothalamus of the antisense oligodeoxynucleotide-treated rats. These results suggest that brain renin may play a significant role in hypertension in SHR.