Angiotensin-like immunoreactivity in the brain of the spontaneously hypertensive rat

Chronic Intermittent Hypobaric Hypoxia Reduces Hypothalamic N-Methyl-d-Aspartate Receptor Activity and Sympathetic Outflow in Spontaneously Hypertensive Rats

Guo, Xinqi, Hongyu Ma, Ziye Cui, Qiyue Zhao, Ying Zhang, Lu Jia, Liping Zhang, Hui Guo, Xiangjian Zhang, Yi Zhang, Yue Guan, and Huijie Ma. Chronic intermittent hypobaric hypoxia reduces hypothalamic N-Methyl-d-Aspartate Receptor activity and sympathetic outflow in spontaneously hypertensive rats. High Alt Med Biol. 25:77-88, 2024. Objective: This study aims to determine the role of hypothalamic renin-angiotensin system (RAS) in the antihypertensive effect of chronic intermittent hypobaric hypoxia (CIHH). Methods: Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) received 35 days of hypobaric hypoxia simulating an altitude of 4,000 m, 5 h/day. The levels of RAS, blood pressure, and N-methyl-d-aspartate receptor (NMDAR) activities of hypothalamic paraventricular nucleus (PVN) presympathetic neurons from each group of rats were determined. Results: The systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure (MAP) of SHRs significantly decreased from the third week of CIHH treatment. This blood pressure reduction effect could be maintained for at least 2 weeks after stopping the CIHH treatment. CIHH treatment also attenuated the decrease in MAP and renal sympathetic nerve activity induced by hexamethonium administration in SHRs, but not in WKY rats. Furthermore, CIHH reversed the increase in serum angiotensin (Ang)II concentration and the expression of PVN angiotensin-converting enzyme (ACE) and AngII type 1 (AT1) receptors, as well as the decrease in serum Ang1-7 concentration and the expression of PVN ACE2 and Mas receptors in SHRs. In addition, the administration of CIHH resulted in a reduction in the frequency of miniature excitatory postsynaptic currents and amplitude of NMDAR current in PVN presympathetic neurons of SHRs, which means that CIHH decreased the pre- and postsynaptic NMDAR activity of PVN presympathetic neurons in SHRs. However, pretreatment with A779 (a Mas receptor blocker) or AngII abrogated the above effects. Meanwhile, Ang1-7 pretreatment mimicked the CIHH effect on pre- and postsynaptic NMDAR activity of presympathetic neurons in SHRs. Conclusions: Our data indicate that CIHH reduces pre- and postsynaptic NMDAR activity of PVN presympathetic neurons, sympathetic outflow, and blood pressure by decreasing the activity of the ACE/AngII/AT1 axis and increasing the activity of ACE2/Ang1-7/Mas axis in the hypothalamus in hypertension.

Conventional cardiovascular risk factors in Transient Global Amnesia: Systematic review and proposition of a novel hypothesis

Transient Global Amnesia (TGA) is an enigmatic amnestic syndrome. We conducted a systematic review to investigate the relationship between the conventional cardiovascular risk factors and TGA. MEDLINE, CENTRAL, EMBASE and PsycINFO were comprehensively searched and 23 controlled observational studies were retrieved. The prevalence of hypertension, diabetes mellitus, dyslipidemia and smoking was lower among patients with TGA compared to Transient Ischemic Attack. Regarding the comparison of TGA with healthy individuals, there was strong evidence suggesting a protective effect of diabetes mellitus on TGA and weaker evidence for a protective effect of smoking. Hypertension was associated with TGA only in more severe stages, while dyslipidemia was not related. In view of these findings, a novel pathophysiological hypothesis is proposed, in which the functional interactions of Angiotensin-II type-1 and N-methyl-D-aspartate receptors are of pivotal importance. The whole body of clinical evidence (nature of precipitating events, associations with migraine, gender-based association patterns) was integrated.

Parental Renovascular Hypertension-Induced Autonomic Dysfunction in Male Offspring Is Improved by Prenatal or Postnatal Treatment With Hydrogen Sulfide

Increasing evidence indicates there is a strong association between parental health during pregnancy and incidence of cardiovascular disease in adult offspring. Recently, hydrogen sulfide (H₂S) has been demonstrated to be a powerful vasodilator of the placental vasculature, improving intrauterine growth restriction. In this study, we investigated whether parental hypertension induces autonomic dysfunction in male adult offspring, and the H₂S mechanism underlying this autonomic dysfunction. 2-kidney-1-clip method was employed to induce parental hypertension during pregnancy and lactation in rats. Basal blood pressure (BP) and autonomic function of male offspring in adulthood was evaluated. Additionally, either maternal hypertensive dams or their male offspring after weaning were treated with H₂S to determine improving effects of H₂S on autonomic dysfunction. The BP was significantly increased in male offspring of renovascular hypertensive dams when compared to that in offspring of normotensive dams. The offspring of renovascular hypertensive dams also exhibited blunted baroreflex sensitivity, increased sympathetic effect and sympathetic tonus. Western blotting analysis revealed downregulation of endogenous H₂S catalyzed enzyme and upregulation of angiotensin Ang II type 1 receptor (AT1R) pathway in the nucleus tractus solitarius and rostral ventrolateral medulla, two hindbrain nuclei involved in BP and autonomic regulation, in these offspring. Either prenatal or postnatal treatment with H₂S improved the adverse effects. The results suggest that parental hypertension results in elevated BP and autonomic dysfunction in adult male offspring through activation of AT1R pathway and inhibition of endogenous H₂S production in the brain.

Endogenous AT1 receptor-protein kinase C activity in the hypothalamus augments glutamatergic input and sympathetic outflow in hypertension

KEY POINTS

The angiotensin AT1 receptor expression and protein kinase C (PKC)-mediated NMDA receptor phosphorylation levels in the hypothalamus are increased in a rat genetic model of hypertension. Blocking AT1 receptors or PKC activity normalizes the increased pre- and postsynaptic NMDA receptor activity of hypothalamic presympathetic neurons in hypertensive animals. Inhibition of AT1 receptor-PKC activity in the hypothalamus reduces arterial blood pressure and sympathetic nerve discharges in hypertensive animals. AT1 receptors in the hypothalamus are endogenously activated to sustain NMDA receptor hyperactivity and elevated sympathetic outflow via PKC in hypertension.

ABSTRACT

Increased synaptic N-methyl-d-aspartate receptor (NMDAR) activity in the hypothalamic paraventricular nucleus (PVN) plays a major role in elevated sympathetic output in hypertension. Although exogenous angiotensin II (AngII) can increase NMDAR activity in the PVN, whether endogenous AT1 receptor-protein kinase C (PKC) activity mediates the augmented NMDAR activity of PVN presympathetic neurons in hypertension is unclear. Here we show that blocking AT1 receptors with losartan or inhibiting PKC with chelerythrine significantly decreased the frequency of NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) and the amplitude of puff NMDA currents of retrogradely labelled spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs). Also, treatment with chelerythrine abrogated the potentiating effect of AngII on mEPSCs and puff NMDA currents of labelled PVN neurons in SHRs. In contrast, neither losartan nor chelerythrine had any effect on mEPSCs or puff NMDA currents in labelled PVN neurons in Wistar-Kyoto (WKY) rats. Furthermore, levels of AT1 receptor mRNA and PKC-mediated NMDAR phosphorylation in the PVN were significantly higher in SHRs than in WKY rats. In addition, microinjection of losartan or chelerythrine into the PVN substantially reduced blood pressure and renal sympathetic nerve discharges in SHRs but not in WKY rats. Chelerythrine blocked sympathoexcitatory responses to AngII microinjected into the PVN. Our findings suggest that endogenous AT1 receptor-PKC activity is essential for presynaptic and postsynaptic NMDAR hyperactivity of PVN presympathetic neurons and for the augmented sympathetic outflow in hypertension. This information advances our mechanistic understanding of the interplay between angiotensinergic and glutamatergic excitatory inputs in hypertension.

Differential control of vasomotion by angiotensins in the rostral ventrolateral medulla of hypertensive rats

The central and peripheral renin-angiotensin systems are known for playing a key role in cardiovascular control. In the present study, we evaluated the hemodynamic effects produced by nanoinjections of angiotensin II (Ang II) or angiotensin-(1-7) [Ang-(1-7)] into the rostral ventrolateral medulla (RVLM) of adult male normotensive (Wistar-WT) and spontaneously hypertensive rats (SHR). Animals were anesthetized (urethane 1.2g/kg) and instrumented for recording blood pressure (BP), heart rate (HR) and blood flow (BF) in the femoral, renal or mesenteric arteries. Afterwards, rats were positioned in a stereotaxic and prepared for nanoinjections (100 nl) of saline (NaCl 0.9%), Ang-(1-7) (40 ng) or Ang II (40 ng) into the RVLM. The vascular resistance (VR) was calculated by ΔMAP/ΔBF ratio. In WT, Ang-(1-7) or Ang II caused equipotent pressor effects that were not accompanied by changes in vascular resistance. However, MAP changes were greater in SHR. This strain also showed a concomitant increase in relative vascular resistance (ΔVR/VRbaseline) of renal (0.31 ± 0.07 and 0.3 ± 0.07 vs. 0.02 ± 0.01; Ang-(1-7), Ang II and Saline, respectively) and mesenteric beds (0.3 ± 0.06 and 0.33 ± 0.04 vs. 0.05 ± 0.02; Ang-(1-7), Ang II and saline, respectively). We conclude that Ang II and Ang-(1-7) at the RVLM control the vascular resistance of renal and mesenteric beds during hypertension.

A current view of brain renin-angiotensin system: Is the (pro)renin receptor the missing link?

The renin-angiotensin system (RAS) plays a central role in the brain to regulate blood pressure (BP). This role includes the modulation of sympathetic nerve activity (SNA) that regulates vascular tone; the regulation of secretion of neurohormones that have a critical role in electrolyte as well as fluid homeostasis; and by influencing behavioral processes to increase salt and water intake. Based on decades of research it is clear that angiotensin II (Ang II), the major bioactive product of the RAS, mediates these actions largely via its Ang II type 1 receptor (AT1R), located within hypothalamic and brainstem control centers. However, the mechanisms of brain RAS function have been questioned, due in large part to low expression levels of the rate limiting enzyme renin within the central nervous system. Tissue localized RAS has been observed in heart, kidney tubules and vascular cells. Studies have also given rise to the hypothesis for localized RAS function within the brain, so that Ang II can act in a paracrine manner to influence neuronal activity. The recently discovered (pro)renin receptor (PRR) may be key in this mechanism as it serves to sequester renin and prorenin for localized RAS activity. Thus, the PRR can potentially mitigate the low levels of renin expression in the brain to propagate Ang II action. In this review we examine the regulation, expression and functional properties of the various RAS components in the brain with particular focus on the different roles that PRR may have in BP regulation and hypertension.

Brain renin angiotensin in disease

A brain renin angiotensin system (RAS) and its role in cardiovascular control and fluid homeostasis was at first controversial. This was because a circulating kidney-derived renin angiotensin system was so similar and well established. But, the pursuit of brain RAS has proven to be correct. In the course of accepting brain RAS, high standards of proof attracted state of the art techniques in all the new developments of biolo1gy. Consequently, brain RAS is a robust concept that has enlightened neuroscience as well as cardiovascular physiology and is a model neuropeptide system. Molecular biology confirmed the components of brain RAS and their location in the brain. Transgenic mice and rats bearing renin and extra copies of angiotensinogen genes revealed the importance of brain RAS. Cre-lox delivery in vectors has enabled pinpoint gene deletion of brain RAS in discrete brain nuclei. The new concept of brain RAS includes ACE-2, Ang1–7, and prorenin and Mas receptors. Angiotensin II (ANG II) generated in the brain by brain renin has many neural effects. It activates behavioral effects by selective activation of ANG II receptor subtypes in different locations. It regulates sympathetic activity and baroreflexes and contributes to neurogenic hypertension. New findings implicate brain RAS in a much wider range of neural effects. We review brain RAS involvement in Alzheimer’s disease, stroke memory, and learning alcoholism stress depression. There is growing evidence to consider developing treatment strategies for a variety of neurological disease states based on brain RAS.

Differential regulation of the renin-angiotensin system by nicotine in WKY and SHR glia

Given that (1) the renin-angiotensin system (RAS) is compartmentalized within the central nervous system in neurons and glia (2) the major source of brain angiotensinogen is the glial cells, (3) the importance of RAS in the central control of blood pressure, and (4) nicotine increases the probability of development of hypertension associated to genetic predisposition; the objective of the present study was to evaluate the effects of nicotine on the RAS in cultured glial cells from the brainstem and hypothalamus of Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Ligand binding, real-time PCR and western blotting assays were used to compare the expression of angiotensinogen, angiotensin converting enzyme, angiotensin converting enzyme 2 and angiotensin II type1 receptors. We demonstrate, for the first time, that there are significant differences in the basal levels of RAS components between WKY and SHR rats in glia from 1-day-old rats. We also observed that nicotine is able to modulate the renin-angiotensin system in glial cells from the brainstem and hypothalamus and that the SHR responses were more pronounced than WKY ones. The present data suggest that nicotine effects on the RAS might collaborate to the development of neurogenic hypertension in SHR through modulation of glial cells.

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.

Centrally injected angiotensin II trans-synaptically activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

Previously, we have demonstrated that pressure-ejected application of angiotensin II onto some neurons in the anterior hypothalamic area (AHA) of rats increases their firing rate. In contrast, pressure application of the angiotensin AT1 receptor antagonist losartan onto AHA neurons blocked the basal firing of the neurons. To investigate possible participation of these AHA neurons in the brain angiotensin system, we examined whether intracerebroventricular injection of angiotensin II results in an activation of angiotensin II-sensitive neurons in the AHA of rats. Intracerebroventricular injection of angiotensin II increased the firing rate of AHA angiotensin II-sensitive neurons. The angiotensin II-induced increase of unit firing in AHA neurons was abolished by pressure application of losartan onto the same neurons. In addition, the angiotensin II-induced increase of firing in AHA neurons was abolished by pressure application of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), a calmodulin inhibitor, onto the same neurons. Pressure application of W7 onto AHA neurons affected neither the basal firing rate nor the increase in unit firing induced by pressure application of angiotensin II onto the same neurons. Intracerebroventricular injection of the cholinergic agonist carbachol did not affect the firing rate of angiotensin II-sensitive neurons in the AHA. These findings suggest that intracerebroventricular injection of angiotensin II activates AHA angiotensin II-sensitive neurons via angiotensinergic inputs to the neurons.

The design and properties of N-carboxyalkyldipeptide inhibitors of angiotensin-converting enzyme

Angiotensin-converting enzyme inhibitors promise to make important therapeutic contributions to the control of hypertension and congestive heart failure. The nonapeptide teprotide was the first of these inhibitors to be tested clinically. It was followed by orally active inhibitors, captopril in 1977 and enalapril in 1980. The latter is representative of a new design for the inhibition of metallopeptidases and is the subject of this review. The best of the N-carboxyalkyldipeptide inhibitors inhibits angiotensin-converting enzyme with a Ki of 7.6 X 10(-11) M. This compound is the most potent competitive inhibitor of a metallopeptidase yet to have been reported. The basis of this high potency is beginning to be understood and in part is considered to involve precisely arranged multiple interactions within the enzyme active site. X-ray crystallography of a thermolysin-inhibitor complex has been achieved. Assuming that similar interactions within the active site of angiotensin-converting enzyme are mechanistically probable, the authors hypothesize the binding of enalaprilat to converting enzyme as shown in Figure 24. Such interactions are consistent with kinetic studies (Section V) with the understanding that binding to the enzyme is not sensitive to the inhibitor's state of NH protonation. The reason for this surprising conclusion has not been established. Perhaps counterbalancing factors are involved in the energetics of binding or there may be compensating adjustments made in the enzyme which permit NH protonated and nonprotonated inhibitor to bind equally well. Figure 24 also summarizes present understanding of the conformation of enalaprilat when bound to angiotensin-converting enzyme. From studies on conformationally defined analogs of enalaprilat, it seems likely that the Ala-Pro segment of enalaprilat binds in a conformation that is close to a minimum energy conformer. This situation no doubt contributes to the potency of enalaprilat, since little binding energy would be needed to induce conformational changes in this part-structure of enalaprilat when it is bound to the enzyme. The phenethyl group of enalaprilat is believed to be near the alpha-hydrogen of the L-Ala residue in the enzyme-inhibitor complex. However, the synthesis of conformationally restricted analogs to establish this point has not yet been reached. The N-carboxyalkylpeptide design was developed from Wolfenden's collected product inhibitors of carboxypeptidase-A. Whether or not N-carboxyalkyldipeptides should be classified as collected product or transition state inhibitors is unclear.(ABSTRACT TRUNCATED AT 400 WORDS)

[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.

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.

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.

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.

Plasma sodium and hypertension

Dietary salt is the major cause of the rise in the blood pressure with age and the development of high blood pressure in populations. However, the mechanisms whereby salt intake raises the blood pressure are not clear. Existing concepts focus on the tendency for an increase in extracellular fluid volume (ECV), but an increased salt intake also induces a small rise in plasma sodium, which increases a transfer of fluid from the intracellular to the extracellular space, and stimulates the thirst center. Accordingly, the rise in plasma sodium is responsible for the tendency for an increase in ECV. Although the change in ECV may have a pressor effect, the associated rise in plasma sodium itself may also cause the blood pressure to rise. There is some evidence in patients with essential hypertension and the spontaneously hypertensive rat (SHR) that plasma sodium may be raised by 1 to 3 mmol/L. An experimental rise in sodium concentration greater than 5 mmol/L induces pressor effects on the brain and on the renin-angiotensin system. Such a rise can also induce changes in cultured vascular tissue similar to those that occur in the vessels of humans and animals on a high sodium diet, independent of the blood pressure. We suggest that a small increase in plasma sodium may be part of the mechanisms whereby dietary salt increases the blood pressure.

Immunohistochemical localization of angiotensin-converting enzyme, angiotensin II and AT1 receptor in human ocular tissues

We investigated the immunohistochemical distribution of 3 components of the renin-angiotensin system (RAS), angiotensin-converting enzyme (ACE), angiotensin II (AngII) and AT1 receptor (AT1), in the human eye. ACE and AngII were localized to nonpigmented epithelial cells of the ciliary body, to endothelial and epithelial cells of the cornea, to epithelial cells of the conjunctiva and to trabecular meshwork cells in the anterior part of the eye. In the posterior part of the eye, ACE and AngII were localized to ganglion cells, some cells in the inner nuclear layer, photoreceptor cells and to endothelial cells of the retinal and choroidal vessels. The overall intensity of AT1 immunoreactivity was weak in all ocular tissues, but the main localization was in ganglion cells. As a preliminary investigation, we were able to include 2 Alzheimer's disease (AD) cases. In AD, no differences from controls were found in the cellular distribution and staining intensity of all 3 antigens. The manifold localization sites of the observed antigens point to rather generalized functions of the RAS in human ocular tissues, such as regulatory effects on neuronal cells, vessels and vitreous humor homeostasis.

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.

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.

Physiological genomic analysis of the brain renin-angiotensin system

The brain renin-angiotensin system (RAS) has long been considered pivotal in cardiovascular regulation and important in the pathogenesis of hypertension and heart failure. However, despite more than 30 years of study, the brain RAS continues to defy explanation. Our lack of understanding of how the brain RAS is organized at the cellular and regional levels has made it difficult to resolve long-sought questions of how ANG II is produced in the brain and the precise mechanisms by which it exerts its actions. A major reason for this is the difficulty in experimentally dissecting the brain RAS at the regional, cellular, and whole organism levels. Recently, we and others developed a series of molecular tools for selective manipulation of the murine brain RAS, in parallel with technologies for integrative analysis of cardiovascular and volume homeostasis in the conscious mouse. This review, based in part on a lecture given in conjunction with the American Physiological Society Young Investigator Award in Regulatory and Integrative Physiology (Water and Electrolyte Homeostasis Section), outlines the physiological genomics strategy that we have taken in an effort to unravel some of the complexities of this system. It also summarizes the principles, progress, and prospects for a better understanding of the brain RAS in health and disease.

Brain renin-angiotensin system dysfunction in hypertension: recent advances and perspectives

This review focuses on the dysfunction of the intrinsic brain renin-angiotensin system (RAS) in the pathogenesis of hypertension. Hyperactivity of the brain RAS plays a critical role in mediating hypertension in both humans and animal models of hypertension, including the spontaneously hypertensive rat (SHR). The specific mechanisms by which increased brain RAS activity results in hypertension are not well understood but include increases in sympathetic vasomotor tone and impaired arterial baroreflex function. We discuss the contribution of endogenous angiotensin (Ang) II actions on presympathetic vasomotor rostral ventrolateral medulla neurons to enhance sympathetic activity and maintain hypertension. In addition, we discuss Ang II-induced attenuation of afferent baroreceptor feedback within the nucleus tractus solitarius and its relevance to the development of hypertension. We also outline the cellular and molecular mechanisms of Ang II signal transduction that may be critical for the initiation and establishment of hypertension. In particular, we present evidence for a phosphoinositide-3-kinase-dependent signaling pathway that appears to contribute to hypertension in the SHR, possibly via augmented Ang II-induced increases in neuronal firing rate and enhanced transcriptional noradrenaline neuromodulation. Finally, we outline future directions in utilizing our understanding of the brain RAS dysfunction in hypertension for the development of improved therapeutic intervention in hypertension.

The predominant role of brain angiotensinogen and angiotensin in environmentally induced hypertension

Rats exposed chronically to a cold environment (5 degrees C/4 degrees F) develop hypertension. This cold-induced hypertension (CIH) is a non-genetic, non-pharmacological, non-surgical model of environmentally induced hypertension in rats. The renin-angiotensin system (RAS) appears to play a role in both initiating and/or maintaining the high blood pressure in CIH. The goal of the present study was to evaluate the role of central and peripheral circulating RAS components, angiotensinogen (AGT), angiotensin-converting enzyme (ACE) and angiotensin (Ang) II, in CIH. Seventy-two Sprague-Dawley adult male rats were used. Thirty-six rats were kept in cold room at 5 degrees C while the other 36 were at 24 degrees C as controls for 5 weeks. Systolic blood pressure (SBP) was recorded by tail cuff. The SBP was increased in rats exposed to cold within 1 week, and this increase was significant for the next 2-5 weeks of the cold exposure (p<0.01). Three subgroups of the cold-treated and control rats (n=12) were sacrificed at 1, 3 and 5 weeks. The brain and liver were removed and plasma was saved. The AGT mRNA significantly increased in the hypothalamus and liver in cold-treated rats from the first week of exposure to cold, and was maintained throughout the time of exposure to cold (n=4, p<0.01). The AGT protein levels in the brain, liver and plasma did not differ significantly between cold-treated and control rats (p>0.05, n=4). The hypothalamic Ang II levels were significantly increased, whereas plasma Ang II levels significantly decreased, in the rats of 5 weeks of cold exposure (n=8, p<0.05). Plasma ACE significantly increased in the rats of 1 week of cold exposure (p<0.05, n=12). The results show differential regulation of RAS components, AGT, ACE and Ang II, between brain and periphery in cold-exposed rats. We conclude that the exposure to low temperature initially increases plasma RAS but with continuous exposure to cold, the brain RAS maintains the hypertension, probably by sustained sympathetic activation, which would provide increased metabolism but also vasoconstriction leading to hypertension.

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.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

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.

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

An overactive brain renin-angiotensin system is one of the factors contributing to the pathogenesis of hypertension in spontaneously hypertensive rats (SHR). We examined brain sites where enhanced activity of an angiotensin system is responsible for the pathogenesis of hypertension in SHR. The angiotensin receptor antagonist, losartan was injected into tissues around rostral parts of the third ventricle in conscious rats. Losartan (0.22 nmol) injected into the anterior hypothalamic area, anterior (AHA) produced a depressor response in SHR but not in Wistar Kyoto rats (WKY). Angiotensin II (0.091-0.91 pmol) injected into the AHA produced a pressor response in both WKY and SHR, and the pressor response to angiotensin II was greater in SHR than that of WKY. Carbachol (3 pmol) injected into the AHA also produced a pressor response in WKY and SHR, and the pressor response to carbachol was almost the same in both strains of rats. Release of angiotensin peptides in the AHA was greater in SHR than that of WKY. These findings suggest that an angiotensin system in the AHA is enhanced and this enhancement of angiotensin system is involved in the maintenance of hypertension in SHR. Both increased pressor reactivity to angiotensin II and increased release of angiotensin peptides in the AHA appear to be related to this enhancement of angiotensin system in SHR.

Cosegregation of spontaneously hypertensive rat renin gene with elevated blood pressure in an F2 generation

OBJECTIVE

To investigate the role of the renin gene in the hypertension of the spontaneously hypertensive rat (SHR) of the Okamoto strain.

METHODS

We determined whether the SHR renin allele was cosegregated with high blood pressure in 137 F2 rats derived from inbred SHR and Wistar-Kyoto rats. Systolic blood pressure in conscious rats was measured by the tail-cuff method, whereas mean arterial pressure was determined from an indwelling catheter in the left carotid artery. Renin genotypes of F2 rats were determined using a SHR-specific Bg/II restriction fragment length polymorphism that we have previously described.

RESULTS

The SHR renin allele was cosegregated significantly with higher systolic blood pressure in male F2 rats aged 8-24 weeks and in females aged 12-24 weeks. The greatest differences in blood pressure between SHR renin allele homozygotes and Wistar-Kyoto rat renin allele homozygotes were 35 mmHg for males and 17 mmHg for females aged 24 weeks. The SHR renin allele was also associated with a higher mean arterial pressure in rats aged 24 weeks and cosegregated with higher body weight of male F2 rats aged 12-24 weeks but not with that of the females. In contrast to the relationship with blood pressure, the SHR renin allele was segregated with lower plasma renin concentrations in rats aged 24 weeks.

CONCLUSION

These results are consistent with the SHR renin gene being one of the loci determining high blood pressure in rats of this strain, possibly through action at some extra-renal site subserving control of blood pressure.

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.

Effect of angiotensin II on striatal dopamine release in the spontaneous hypertensive rat

We have previously demonstrated that angiotensin II stimulates the release of dopamine from the normotensive rat striatum via the AT1 receptor. In this study, the effect of angiotensin II-stimulated striatal dopamine release in the spontaneous hypertensive rat was compared to normotensive controls. In the spontaneous hypertensive rat, angiotensin II stimulated dopamine release to 169 +/- 13% (P < 0.05) in the experimental period, with levels remaining high in the recovery phase, 158 +/- 16% (P < 0.05). This effect was not significantly different from the response in normotensive controls, in which angiotensin II stimulated dopamine release to 149 +/- 18% (P < 0.05) in the experimental period, with the effect also persisting through the recovery period, 244 +/- 62% (P < 0.05). Thus, despite reports of increased activity of the brain angiotensin II and dopamine systems in the spontaneous hypertensive rat, there is no evidence of abnormal regulation of the striatonigral dopamine system.

Aminopeptidase A: distribution in rat brain nuclei and increased activity in spontaneously hypertensive rats

Aminopeptidase A is a membrane-bound zinc metalloprotease which cleaves angiotensin II into angiotensin III. Using a new specific aminopeptidase A inhibitor, EC33, we evaluated its enzymatic activity in several microdissected brain nuclei involved in the control of cardiovascular functions and in the pituitary. We compared this distribution with that of the angiotensin I-converting enzyme which converts angiotensin I to angiotensin II. Aminopeptidase A activity was heterogenously distributed with a 150-fold difference between the lowest and the highest levels. The pituitary and the circumventricular organs were the richest source of enzyme, followed by the median eminence, the arcuate nucleus, the area postrema, the choroid plexus and the supraotic and paraventricular nuclei. We did not find any close parallel between aminopeptidase A and angiotensin I-converting enzyme distributions. We examined both enzymatic activities in brain nuclei of spontaneously hypertensive rats. Aminopeptidase A activity was higher in the spontaneously hypertensive rats than in age-matched Wistar Kyoto control rats. The difference was up to 2.5-fold in several brain nuclei involved in the blood pressure regulation; in contrast, no differences in angiotensin I-converting enzyme activity were found in the same regions. The close correspondence between the distribution of aminopeptidase A activity and angiotensin receptors and nerve terminals in the brain associated with the observation that aminopeptidase A activity was overactivated in the spontaneously hypertensive rats suggest that this enzyme may contribute, at least in part, to the regulation of cardiovascular functions by its ability to convert angiotensin II to angiotensin III.

Changes in the baroreflex control of heart rate produced by central infusion of selective angiotensin antagonists in hypertensive rats

We have recently shown that an angiotensin-(1-7) [Ang-(I-7)] analogue, D-Ala7-Ang-(1-7) (A-779), is a selective Ang-(1-7) antagonist with no significant action on angiotensin type 1 or type 2 receptors. The availability of selective angiotensin antagonists prompted us to evaluate the role of Ang-(1-7) and Ang II on central modulation of the baroreflex control of heart rate in normotensive Wistar rats and spontaneously hypertensive rats (SHR). Blood pressure recording and reflex changes in heart rate elicited by intravenous bolus injections of phenylephrine were made before and within 1 and 3 hours of intracerebroventricular (ICV, lateral ventricle) infusion of saline (8 microL/h), A-779 (4 microg/h), DuP 753 (100 microg/h), or CGP 42112A (50 mu g/h) in conscious rats. The slope of the relationship between changes in pulse interval versus changes in mean arterial pressure was used as an index of the baroreflex control of heart rate. ICV infusion of saline or any of the antagonists did not significantly change basal levels of mean arterial pressure and heart rate in SHR (170 +/- 6 mm Hg nd 360 +/- 9 beats per minute, respectively; n = 29) or Wistar rats (108 +/- 2 mm Hg and 377 +/- 6 beats per minute, respectively; n=29). Three hours of ICV infusion of A-779 markedly decreased baroreflex sensitivity in Wistar rats (from a basal slope of 1.09 +/- O.3). In contrast, A-779 did not significantly alter the depressed baroreflex sensitivity of SHR (0.61 +/- O.l). ICV infusion of DuP 753 produced a significant increase (60 percent) in baroreflex control of heart rate in both Wistar rats and SHR. Saline or CGP 42112A infusions did not significantly alter baroreflex control of heart rate. These results suggest that endogenous Ang II and Ang-(1-7) are differentially affecting central baroreflex modulation, acting probably through distinct receptor subtypes. Although the central Ang II inhibitory effect is mediated by the type 1 receptor subtype, the facilitatory effect of Ang-(1-7) might be mediated by a different, unidentified receptor.

The developmental increase of the AT1A, but not the AT1B, receptor mRNA level at the preoptic area in spontaneously hypertensive rats

To determine the possible involvement of the central angiotensin system in hypertension, the angiotensin II type-1 receptor subtype mRNA levels in the preoptic area (POA) and hypothalamus were measured by means of a reverse-transcriptase/polymerase chain reaction in spontaneously hypertensive rats (SHR), and the results were then compared with the findings in age-matched normotensive Wistar-Kyoto rats (WKY). In 4-week-old (prehypertensive stage) and 7-week-old (evolving stage) SHR, the AT1A and AT1B receptor subtype mRNA levels in the POA and hypothalamus did not show any significant difference between the SHR and WKY. However, in the 16-week-old SHR (hypertensive stage), AT1A receptor subtype mRNA at POA was approximately 2-fold higher than in the WKY, while the AT1B receptor subtype mRNA showed no difference. On the other hand, neither the AT1A nor the AT1B mRNA level at the hypothalamus were different between the 16-week-old SHR and WKY. These results suggest that the AT1A mRNA level, but not the AT1B mRNA level, increases in the POA in hypertensive stage of SHR and the increase may therefore be related in some way to the state of hypertension.

Brain angiotensin and the female reproductive cycle

The results consistently show from experiment to experiment that there is a surge of brain Ang II prior to the well known preovulatory LH surge. It should be pointed out that these experiments have been carried out by two different laboratories and with the help of different experimenters and some of the experiments have been repeated. Therefore, the consistency of the results is reassuring. It does appear that Ang II increases in the brain, specifically in the hypothalamus, probably in cells of the paraventricular nucleus about 1 hour before the LH levels in plasma rise to a peak. Since LH release from the anterior pituitary gland is stimulated by the release of LHRH from the arcuate nucleus into the median eminence, the results would suggest that Ang II stimulates the release of LHRH. The peak in the OVX of Ang II treated rats is sharp and short-lasting with a second, later peak. The LH surge follows the first peak and a second rise in LH follows the second Ang II peak. These data suggest that brain Ang II synthesized and stored in the brain plays a critical role in the female reproductive cycle by initiating the LH surge. The regulation of Ang II may be by estrogen and progesterone, but as the increase in angiotensinogen mRNA was not marked, the surge of Ang II appears to result more from the sudden release of stored Ang II than its synthesis. Thus, the question is what releases Ang II. Earlier studies showed that catecholamines release Ang II from neurons and not from glia involving alpha 2 receptor blockade to increase norepinephrine by inhibiting reuptake (7). An interaction between catecholamines, Ang II and LH had also been suggested earlier (18, 19). Therefore, a series of events triggered by steroids in proestrus may begin with increases in norepinephrine activating neuronal alpha 2 receptors and precipitating release of brain Ang II. This is represented diagrammatically in Figure 15. The Ang II surge stimulates the cells containing GnRH (gonadotropin releasing hormone) in the arcuate nucleus. The effect of Ang II on multiple GnRH cells amplifies the effect and GnRH is released into the portal vessels of the pituitary to stimulate the large LH release, from gonadotrope cells in the anterior pituitary, into the plasma that produces the LH surge. The effect of the LH surge is ovulation which ends the estrogen build up.(ABSTRACT TRUNCATED AT 400 WORDS)

Anomalous effects of losartan on aminopeptidase-induced reductions of blood pressure in SHR

The present investigation initially determined that a commercially available aminopeptidase M (AmM, Sigma Chemical) can lower blood pressure when intracerebroventricularly (ICV) infused in spontaneously hypertensive rats (SHR). Pretreatment with the angiotensin II (AngII) receptor subtype 1 (AT1) antagonist, DuP 753 (losartan) significantly attenuated this hypotensive effect, in a dose-dependent manner, while pretreatment with the AngII receptor subtype 2 (AT2) antagonist, PD123177, did not influence AmM-induced hypotension. These results suggest that AT1 receptors may be involved in the hypotension accompanying the ICV infusion of AmM; however, the relationship among available AT1 sites, angiotensin ligands, and peptidase activity appears to be complicated with the likely involvement of additional, as yet unspecified, brain peptide systems possessing cardiovascular action.

Angiotensin II depresses glutamate depolarizations and excitatory postsynaptic potentials in locus coeruleus through angiotensin II subtype 2 receptors

A previously reported depression of glutamate responses by angiotensin II was investigated to define the nature of this neuromodulatory effect. Studies were carried out in an vitro brain slice preparation containing the locus coeruleus, using intracellular recordings, and iontophoretic, micropressure and bath perfusion methods for application of drugs. The angiotensin action was found to be blocked by a non-peptide antagonist specific for the angiotensin type 2 receptor, and not by an antagonist selective for the type 1 receptor. Excitatory postsynaptic potentials mediated primarily by excitatory amino acids were also depressed by angiotensin II. The angiotensin II depressions of glutamate were shown to be strong and highly specific. The low effectiveness of bath-applied compared with iontophoretically or micropressure-applied angiotensin II was found to be at least partly explained by a rapid degradation by peptidases. Ammonium ions and hydrogen ions were also able to depress glutamate responses, but these effects were not specific for locus coeruleus neurons and were mediated independently of the angiotensin actions. Strong depression by angiotensin II of excitatory postsynaptic potentials as well as exogenously applied glutamate strengthens the strong possibility of a physiological role for this neuromodulatory mechanism. The identification of the type 2 angiotensin receptor subtype as the mediator of this effect indicates a novel functional role for this receptor, since previously recognized functions of angiotensin II in the brain, such as vascular and body fluid regulation, have been associated with the type 1 receptor.

Salt-induced hypertension in normotensive spontaneously hypertensive rats

Lifetime treatment with oral captopril prevents the development of hypertension in spontaneously hypertensive rats (SHR). We tested the hypothesis that this treatment also prevents the hypertensive response that occurs when untreated NaCl-sensitive SHR are placed on a high NaCl diet. Female SHR were continuously treated with oral captopril before conception and throughout lactation, and the offspring were similarly treated with oral captopril throughout life. At 6 weeks of age, treated male SHR were placed on an 8% (or remained on a 1%) NaCl diet, and systolic arterial pressure, heart rate, and body weight were monitored for 2 weeks. The 8% NaCl diet caused a rapid increase in arterial pressure in the lifetime captopril-treated rats, and 18 days after the initiation of the diet, the mean arterial pressure of this group was 136 +/- 7 mm Hg compared with 100 +/- 2 mm Hg in the 1% NaCl diet rats. The results of a second experiment confirmed the hypertensive effect of the high NaCl diet in lifetime captopril-treated SHR and demonstrated that after 18 days on the diet the dietary NaCl-induced hypertensive response was greater in magnitude in lifetime captopril-treated compared with untreated SHR. The results also demonstrated that lifetime captopril-treated Wistar-Kyoto rats, which are normotensive irrespective of captopril treatment, display no significant increase in arterial pressure when given a high NaCl diet. A third experiment demonstrated that rapidly progressing NaCl sensitivity is also present in female lifetime captopril-treated SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental differences of angiotensinogen mRNA in the preoptic area between spontaneously hypertensive and age-matched Wistar-Kyoto rats

In order to know the possible involvement of the central angiotensin system in hypertension, angiotensinogen mRNA (AomRNA) levels of eight discrete brain areas were measured by Northern blot hybridization analysis in the spontaneously hypertensive rats (SHR), compared with those in age-matched normotensive Wistar-Kyoto rats (WKY). In 16-week-old SHR (hypertensive stage), AomRNA levels in the preoptic area (POA), but not in the ventromedial hypothalamus, lateral hypothalamus and mammillary body, among the hypothalamic nuclei, were higher (approximate 50%) than in WKY. There were no differences in other brain areas, such as the striatum, septum, amygdala and cerebellum between both the strains. The AomRNA levels in POA were already higher (38%) in 4-week-old SHR (prehypertensive stage) without significance, and the difference was augmented (82%) in 7-week-old SHR (evolving stage). These results suggest that the developmental changes of AomRNA levels at POA may be related in some aspect to hypertension process.

Differences between perinatal angiotensin binding in the brains of SHR and WKY rats

A growing body of evidence suggests that angiotensin may have a functional role in growth and development, in addition to its classical role in the maintenance of body water homeostasis. Components of the renin-angiotensin system have been identified in the rat fetus. Because of the association between the renin-angiotensin system and hypertension, we quantified angiotensin receptor binding sites in the brains of spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) rats during perinatal development. Using in vitro receptor autoradiography we identified specific 125I-Sar1,Ile8 AII binding in several areas of the brains of perinatal rats of both strains and observed significant differences in the concentration of binding sites, at different ages in several brain nuclei. With the knowledge that components of the renin-angiotensin system appear early in development and are known to have an association with cellular growth, it is possible that an irregularity in this system occurring during neurogenesis could contribute to developmental abnormalities, as well as subsequent hypertension.

Blocking hypothalamic AT1 receptors lowers blood pressure in salt-sensitive rats

Previous studies from our laboratory have shown that microinjection of DuP 753 (2-n-butyl-4-chloro-5-(hydroxymethyl)-1-[[2'-(1H-tetrazol-5-yl) biphenyl-4-yl]methyl]imidazole, potassium salt), a highly selective nonpeptide antagonist of type 1 angiotensin II receptors, into the anterior hypothalamic area produces a dose-related depressor response in salt-sensitive spontaneously hypertensive rats fed a basal (1%) salt diet. The current study tested the hypothesis that the depressor response to anterior hypothalamic type 1 angiotensin II receptor blockade with DuP 753 or its metabolite EXP 3174 is enhanced by high (8%) salt feeding in this model. DuP 753 or EXP 3174 (40 micrograms in 100 nl artificial cerebrospinal fluid vehicle) or vehicle alone was microinjected into the anterior hypothalamic area of conscious salt-sensitive spontaneously hypertensive and Wistar-Kyoto rats that had been fed 1% or 8% salt diets for 3 weeks. Both DuP 753 and EXP 3174 caused significant decreases in mean arterial pressure in spontaneously hypertensive but not in Wistar-Kyoto rats fed either diet. The magnitude and duration of the depressor responses to DuP 753 and EXP 3174 were significantly greater in the 8% salt-fed spontaneously hypertensive rats than in 1% salt-fed rats. Vehicle injections had no effect on blood pressure in either strain-diet group. Microinjection of angiotensin II (2 micrograms in 100 nl artificial cerebrospinal fluid vehicle) into the anterior hypothalamic area caused significant pressor and bradycardiac responses in all strain-diet groups; dietary salt supplementation enhanced these effects in salt-sensitive spontaneously hypertensive rats but not in Wistar-Kyoto rats. These responses were blocked by pretreatment with EXP 3174.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II binding sites in the hamster brain: localization and subtype distribution

This study was designed to characterize the distribution of angiotensin II (AII) binding sites in the hamster brain. Brain sections were incubated with [125I][sar1,ile8]-angiotensin II in the absence and presence of angiotensin II receptor subtype selective compounds, losartan (AT1 subtype) and PD123177 (AT2 subtype). Binding was quantified by densitometric analysis of autoradiograms and localized by comparison with adjacent thionein stained sections. The distribution of AII binding sites was similar to that found in the rat, with some exceptions. [125I][sar1,ile8]-angiotensin II binding was not evident in the subthalamic nucleus and thalamic regions, inferior olive, suprachiasmatic nucleus, and piriform cortex of the hamster, regions of prominent binding in the rat brain. However, intense binding was observed in the interpeduncular nucleus and the medial habenula of the hamster, nuclei void of binding in the rat brain. Competition with receptor subtype selective compounds revealed a similar AII receptor subtype profile in brain regions where binding is evident in both species. One notable exception is the medial geniculate nucleus, predominately AT1 binding sites in the hamster but AT2 in the rat. Generally, the AII binding site distribution in the hamster brain parallels that of the other species studied, particularly in brain regions associated with cardiovascular and dipsogenic functions. Functional correlates for AII binding sites have not been elucidated in the majority of brain regions and species mismatches might provide clues in this regard.

Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.

Depressor effect of blocking angiotensin subtype 1 receptors in anterior hypothalamus

Previous studies have shown that the anterior hypothalamic area participates in the centrally mediated pressor response to exogenous angiotensin II. The current study was designed to test the hypothesis that endogenous anterior hypothalamic angiotensin II plays a significant role in blood pressure control. Type 1 angiotensin II receptors in the anterior hypothalamic area were blocked by local microinjection of DuP 753 (2-n-butyl-4-chloro-5-(hydroxymethyl)-1-[(2'-(1H-tetrazol-5-yl) biphenyl-4-yl)methyl]imidazole, potassium salt), a highly selective nonpeptide antagonist. DuP 753 (20 or 40 micrograms; in 100 nl artificial cerebrospinal fluid) or vehicle alone was microinjected into the anterior hypothalamic area of conscious NaCl-sensitive spontaneously hypertensive rats and Wistar-Kyoto controls. DuP 753 caused significant dose-related decreases in mean arterial pressure (maximal decrease, 22.5 +/- 1.8 mm Hg) with unchanged heart rate in NaCl-sensitive spontaneously hypertensive rats but effected no change in Wistar-Kyoto rats. Injections of equal volumes of artificial cerebrospinal fluid into the anterior hypothalamic area had no effect in either strain. Further, microinjection of DuP 753 into the posterior hypothalamic area produced no significant effect on blood pressure or heart rate in NaCl-sensitive spontaneously hypertensive rats. Microinjection into the anterior hypothalamic area of the selective type 2 angiotensin II receptor antagonist PD 123319 did not affect blood pressure or heart rate in NaCl-sensitive spontaneously hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Central administration of angiotensin II receptor antagonists and arterial pressure regulation: a note of caution

The blunting of arterial pressure increases to a variety of pressor agents or the lowering of arterial pressure in some models of hypertension following intracerebroventricular administration of an angiotensin II (AII) antagonist, has been interpreted as prima facie evidence for the involvement of the central AII system in these situations. Central administration of vasopressin or carbachol (a cholinergic agonist) produces pressor effects which have been reported to be due to an increase in the activity of the sympathetic nervous system. We now report that central administration of AII antagonists [either (Sar-1, Ile-8) AII or (Sar-1, Ala-8) AII] in rats prevents the majority (greater than 70%) of the pressor effects of intraventricular vasopressin or carbachol. These results can be interpreted in two ways. The first is that all of these pressor agents use a central angiotensinergic mechanism(s) to increase sympathetic nervous system activity. An alternative hypothesis is that centrally administered AII antagonists non-specifically inhibit sympathetic nervous system function.

Use of aminopeptidase M as a hypotensive agent in spontaneously hypertensive rats

The present investigation determined that a commercially available aminopeptidase M (AmM, Sigma Chemical) can be utilized to lower blood pressure in normotensive and hypertensive rats. In vitro analyses indicated that the predominant peptidase present in this preparation was AmM; however, it also contained some aminopeptidase A (AmA) and less DAP IV. Although no DAP IV-mediated metabolism of angiotensin II (AII) or angiotensin III (AIII) was measured, both AmM and AmA metabolized AII and AIII. Upon further examination, it appeared that AII could be converted to AIII by either AmM or AmA; however, Arg was cleaved from the N-Terminal of AIII predominantly by AmM. The aminopeptidase inhibitors actinonin (AC), amastatin (AM), and bestatin (BE) effectively blocked the AmM-induced hydrolysis of the Asp-Arg bond of AII, and the Arg-Val bond of AIII. The activity of AmA was inhibited by AM but was relatively resistant to inhibition by AC and BE. Next, exogenous aminopeptidase replacement was employed in the anesthetized spontaneously hypertensive rat (SHR) in an attempt to temporarily correct a hypothesized brain deficiency of receptor-associated peptidases and lower blood pressure. Third-ventricle infusion of AmM produced significant drops in blood pressure and heart rate in both SHRs and Wistar-Kyoto normotensive controls. Pretreatment with AC or BE was particularly effective at interfering with the subsequent AmM-induced hypotensive effect, while AM was less effective. The central mechanisms underlying these effects are in need of further investigation; however, they are at least partially dependent upon the brain angiotensin system.

Alterations in vasopressin mechanisms in captopril-treated spontaneously hypertensive rats

The effects of lifetime captopril treatment on vasopressin (VP) were assessed in spontaneously hypertensive rats (SHR). Pregnant and nursing dams were treated with oral Captopril (100 mg/kg/day). After weaning, the pups were maintained on Captopril (50/kg/day) for 19-20 wks. Blood pressures of Captopril-treated SHR were in the normotensive range and significantly lower (p less than .001) than SHR control rats. Control and Captopril-treated SHR were perfused and brains were sectioned for immunohistochemical staining with a polyclonal antibody directed against vasopressin (VP). Compared to control SHR, Captopril-treated rats displayed decreased VP-like immunoreactivity in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Captopril treatment also selectively decreased the number of brightly labeled cell bodies in the SON and PVN and reduced VP-like labeling in the axons of the neurons in these nuclei. Concurrent with a decrease in VP-like immunoreactivity, Captopril treatment reduced plasma VP levels (RIA) (p less than 0.01, Captopril, 5.6 +/- 0.5 pg/ml; control, 11.8 +/- 2.2 pg/ml). Scatchard analysis of 3H-VP binding indicated that Captopril treatment increased the number but not the affinity of VP receptors in the hypothalamus and brain stem of SHR. These results suggest that in SHR oral Captopril treatment attenuates the synthesis and release of VP, an effect that may contribute to the blood pressure lowering effect of converting enzyme inhibitors.