Contribution of the subfornical organ to angiotensin II-induced hypertension

Previous studies clearly demonstrated acute actions of angiotensin II (ANG II) at one of the central circumventricular organs, the subfornical organ (SFO), but studies demonstrating a role for the SFO in the chronic actions of ANG II remain uncertain. The purpose of this study was to examine the role of the SFO in the chronic hypertensive phase of ANG II-induced hypertension. We hypothesized that the SFO is necessary for the full hypertensive response observed during the chronic phase of ANG II-induced hypertension. To test this hypothesis, male Sprague-Dawley rats were subjected to sham operation (sham rats) or electrolytic lesion of the SFO (SFOx rats). After 1 wk, the rats were instrumented with venous catheters and radiotelemetric transducers for intravenous administration of ANG II and measurement of blood pressure and heart rate, respectively. Rats were then allowed 1 wk for recovery. After 3 days of saline control infusion (7 ml of 0.9% NaCl/day), sham and SFOx rats were infused with ANG II at 10 ng.kg(-1).min(-1) i.v. for 10 consecutive days and then allowed to recover for 3 days. A 0.4% NaCl diet and distilled water were provided ad libitum. At day 5 of ANG II infusion, mean arterial pressure increased 11.7 +/- 3.0 mmHg in sham rats (n = 9) but increased only 3.7 +/- 1.4 mmHg in SFOx rats (n = 9). This trend continued through day 10 of ANG II treatment. These results support the hypothesis that the SFO is necessary for the full hypertensive response to chronic ANG II administration.     

Acute sympathoexcitatory action of angiotensin II in conscious baroreceptor-denervated rats

Angiotensin II (ANG II) has complex actions on the cardiovascular system. ANG II may act to increase sympathetic vasomotor outflow, but acutely the sympathoexcitatory actions of exogenous ANG II may be opposed by ANG II-induced increases in arterial pressure (AP), evoking baroreceptor-mediated decreases in sympathetic nerve activity (SNA). To examine this hypothesis, the effect of ANG II infusion on lumbar SNA was measured in unanesthetized chronic sinoaortic-denervated rats. Chronic sinoaortic-denervated rats had no reflex heart rate (HR) responses to pharmacologically evoked increases or decreases in AP. Similarly, in these denervated rats, nitroprusside-induced hypotension had no effect on lumbar SNA; however, phenylephrine-induced increases in AP were still associated with transient decreases in SNA. In control rats, infusion of ANG II (100 ng x kg(-1) x min(-1) iv) increased AP and decreased HR and SNA. In contrast, ANG II infusion increased lumbar SNA and HR in sinoaortic-denervated rats. In rats that underwent sinoaortic denervation surgery but still had residual baroreceptor reflex-evoked changes in HR, the effect of ANG II on HR and SNA was variable and correlated to the extent of baroreceptor reflex impairment. The present data suggest that pressor concentrations of ANG II in rats act rapidly to increase lumbar SNA and HR, although baroreceptor reflexes normally mask these effects of ANG II. Furthermore, these studies highlight the importance of fully characterizing sinoaortic-denervated rats used in experiments examining the role of baroreceptor reflexes.     

Glucocorticoids potentiate central actions of angiotensin to increase arterial pressure

Experiments were performed to determine if glucocorticoids potentiate central hypertensive actions of ANG II. Male Sprague-Dawley rats were treated for 3 days to 3 wk with corticosterone (Cort). Experiments were performed in conscious rats that had previously been instrumented with arterial and venous catheters and an intracerebroventricular guide cannula in a lateral ventricle. Baseline arterial pressure (AP) was greater in Cort-treated rats than in control rats (119 +/- 2 vs. 107 +/- 1 mmHg, P < 0.01). Microinjection of ANG II intracerebroventricularly produced a significantly larger increase in AP in Cort-treated rats than in control rats. For example, at 30 ng ANG II, AP increased by 23 +/- 1 and 16 +/- 2 mmHg in Cort-treated and control rats, respectively (P < 0.01). Microinjection of an angiotensin type 1 receptor antagonist significantly decreased AP (-6 +/- 2 mmHg) and heart rate (-26 +/- 7 beats/min) in Cort-treated but not control rats. Increases in AP produced by intravenous administration of ANG II were not different between control and Cort-treated rats. Intravenous injections of ANG II antagonist had no significant effects on mean AP or heart rate in control or Cort-treated rats. Therefore, a sustained increase in plasma Cort augments the central pressor effects of ANG II without altering the pressor response to peripheral administration of the hormone.     

Central mechanisms of acute ANG II modulation of arterial baroreflex control of renal sympathetic nerve activity

Short-term intravenous infusion of angiotensin II (ANG II) into conscious rabbits reduces the range of renal sympathetic nerve activity (RSNA) by attenuating reflex disinhibition of RSNA. This action of ANG II to attenuate the arterial baroreflex range is exaggerated when ANG II is directed into the vertebral circulation, which suggests a mechanism involving the central nervous system. Because an intact area postrema (AP) is required for ANG II to attenuate arterial baroreflex-mediated bradycardia and is also required for maintenance of ANG II-dependent hypertension, we hypothesized that attenuation of maximum RSNA during infusion of ANG II involves the AP. In conscious AP-lesioned (APX) and AP-intact rabbits, we compared the effect of a 5-min intravenous infusion of ANG II (10 and 20 ng x kg(-1) x min(-1)) on the relationship between mean arterial blood pressure (MAP) and RSNA. Intravenous infusion of ANG II into AP-intact rabbits resulted in a dose-related attenuation of maximum RSNA observed at low MAP. In contrast, ANG II had no effect on maximum RSNA in APX rabbits. To further localize the central site of ANG II action, its effect on the arterial baroreflex was assessed after a midcollicular decerebration. Decerebration did not alter arterial baroreflex control of RSNA compared with the control state, but as in APX, ANG II did not attenuate the maximum RSNA observed at low MAP. The results of this study indicate that central actions of peripheral ANG II to attenuate reflex disinhibition of RSNA not only involve the AP, but may also involve a neural interaction rostral to the level of decerebration.     

Effect of NMDA-Induced Lesion of the Subfornical Organ on the Angiotensin II Binding Sites Density and Acetylcholinesterase or NADPH-Diphorase Activities in the Lamina Terminalis of the Rat Brain

1. Neural angiotensinergic circuitry located in the lamina terminalis has been proposed to be involved in blood pressure regulation and fluid homeostasis.2. ANG II binding sites have been described to be localized throughout the lamina terminalis including the subfornical organ (SFO), the median preoptic nucleus (MnPO), and the organum vasculosum lamina terminalis (OVLT).3. The present experiment was designed to investigate the ANG II binding sites localization in the lamina terminalis. For this purpose, we have compared the ANG II binding sites, acetylcholinesterase, and NADPH-diaphorase distributions throughout the lamina terminalis. Additionally, we have studied the effect ofthe preferential lesion of SFO neuronal cell bodies by local injection of NMDA on the ANG II binding sites density in different areas of the lamina terminalis.4. Male Wistar rats were anesthetized, immobilized in a stereotaxic apparatus, and 500 nl of saline or 250 nmol NMDA was injected into the SFO.5. Animals were sacrificed 1 week later, the brain was removed, frozen, and sagittal 16 m slices were cut in a cryostat. Alternate brain slices were incubated with [¹²⁵I]-Sar¹-ANG II for receptor autoradiography or histochemically stained for visualization of acetylcholinesterase and NADPH-diaphorase activities. Binding capacity was determined by computerized quantitative densitometry of autoradiograms. The intensity of histochemical reactions was measured as relative units obtained by computerized densitometry processing of the brain slices stained for either activity.6. Acetylcholinesterase staining was mainly located in the SFO, with faint staining reaction in other areas of the lamina terminalis. NADPH-diaphorase staining was homogeneously distributed throughout the lamina terminalis. A significant positive correlation was observed between acetylcholinesterase and NADPH-diaphorase stainings in the SFO of control and NMDA-lesioned rats.ANG II binding sites were localized throughout the lamina terminalis. A significant positive correlation was observed between the density of ANG II binding sites and the intensity of acetylcholinesterase or NADPH-diaphorase staining in the SFO of control and NMDA-lesioned rats.8. The distribution of the NADPH-diaphorase staining was found to closely match the distribution of the ANG II binding sites in the lamina terminalis.9. Neuronal lesion of the SFO caused significant reductions in the density of ANG II biding sites in the SFO (–68%) and the MnPO (–48%). No changes were observed either in the OVLT or outside the lamina terminalis in the superior colliculus.10. The present results indicate the following: first, the presence ofhigh levels of acetylcholinesterase staining in the SFO and of NADPH-diaphorasethroughout the lamina terminalis; second, that ANG II binding sites in the SFO and possibly in the MnPO are localized in neuronal cell bodies; third, that SFOlesion did not affect the expression of ANG II binding sites in the OVLT, thus suggesting that these binding sites correspond to different angiotensinergic system; and finally, the existence of a striking correlation between the distribution of the ANG II binding sites and NADPH-diaphorase throughout the lamina terminalis, thus suggesting a interrelation between angiotensinergic and nitrergicsystems in the lamina terminalis.     

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.     

Angiotensin II increases GABAB receptor expression in nucleus tractus solitarii of rats

Increasing evidence indicates that both the angiotensin II (ANG II) and gamma-aminobutyric acid (GABA) systems play a very important role in the regulation of blood pressure (BP). However, there is little information concerning the interactions between these two systems in the nucleus tractus solitarii (NTS). In the present study, we examined the effects of ANG II on GABAA and GABAB receptor (GAR and GBR) expression in the NTS of Sprague-Dawley rats. The direct effect of ANG II on GBR expression was determined in neurons cultured from NTS. Treatment of neuronal cultures with ANG II (100 nM, 5 h) induced a twofold increase in GBR1 expression, as detected with real-time RT-PCR and Western blots, but had no effect on GBR2 or GAR expression. In electrophysiological experiments, perfusion of neuronal cultures with the GBR agonist baclofen decreased neuronal firing rate by 39% and 63% in neurons treated with either PBS (control) or ANG II, respectively, indicating that chronic ANG II treatment significantly enhanced the neuronal response to GBR activation. In contrast, ANG II had no significant effect on the inhibitory action of the GAR agonist muscimol. In whole animal studies, intracerebroventricular infusion of ANG II induced a sustained increase in mean BP and an elevation of GBR1 mRNA and protein levels in the NTS. These results indicate that ANG II stimulates GBR expression in NTS neurons, and this could contribute to the central nervous system actions of ANG II that result in dampening of baroreflexes and elevated BP in the central actions of ANG II.     

Angiotensin II reduces serotonin release in the rat subfornical organ area

In the present study we used intracerebral microdialysis techniques to examine whether angiotensin II (ANG II) modulates the release of serotonin (5-hydroxytryptamine, 5-HT) in the subfornical organ (SFO) in freely moving rats. Extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the region of the SFO were significantly decreased by microinjection of ANG II (10 pmol, 50 nl), but not by vehicle, into the dialysis site. No significant changes in the 5-HT and 5-HIAA levels caused by ANG II were observed in the sites away from the SFO. Water ingestion significantly enhanced the amount of the decrease in the 5-HT and 5-HIAA concentrations in the SFO area elicited by the ANG II injection. These results show that ANG II may reduce the release of 5-HT in the SFO area, and imply that the 5-HT receptor mechanism in the SFO area may participate in the elicitation of the drinking behavior to ANG II.     

Evidence for a functional intracellular angiotensin system in the proximal tubule of the kidney

ANG II is the most potent and important member of the classical renin-angiotensin system (RAS). ANG II, once considered to be an endocrine hormone, is now increasingly recognized to also play novel and important paracrine (cell-to-cell) and intracrine (intracellular) roles in cardiovascular and renal physiology and blood pressure regulation. Although an intracrine role of ANG II remains an issue of continuous debates and requires further confirmation, a great deal of research has recently been devoted to uncover the novel actions and elucidate underlying signaling mechanisms of the so-called intracellular ANG II in cardiovascular, neural, and renal systems. The purpose of this article is to provide a comprehensive review of the intracellular actions of ANG II, either administered directly into the cells or expressed as an intracellularly functional fusion protein, and its effects throughout a variety of target tissues susceptible to the impacts of an overactive ANG II, with a particular focus on the proximal tubules of the kidney. While continuously reaffirming the roles of extracellular or circulating ANG II in the proximal tubules, our review will focus on recent evidence obtained for the novel biological roles of intracellular ANG II in cultured proximal tubule cells in vitro and the potential physiological roles of intracellular ANG II in the regulation of proximal tubular reabsorption and blood pressure in rats and mice. It is our hope that the new knowledge on the roles of intracellular ANG II in proximal tubules will serve as a catalyst to stimulate further studies and debates in the field and to help us better understand how extracellular and intracellular ANG II acts independently or interacts with each other, to regulate proximal tubular transport and blood pressure in both physiological and diseased states.     

Central angiotensin II-induced pressor responses and neural activity in utero and hypothalamic angiotensin receptors in preterm ovine fetus

The central renin-angiotensin system is important in the control of blood pressure in the adult. However, few data exist about the in utero development of central angiotensin-mediated pressor responses. Our recent studies have shown that the application of ANG II into the fetal brain can increase blood pressure at near term. The present study determined fetal blood pressure and heart rate in response to a central application of ANG II in the chronically prepared preterm ovine fetus, determined the action sites marked by c-Fos expression in the fetal central pathways after intracerebroventricular injection of ANG II in utero, and determined angiotensin subtype 1 receptors in the fetal hypothalamus. Central injection of ANG II significantly increased fetal mean arterial pressure (MAP). Adjusted fetal MAP against amniotic pressure was also increased by ANG II. Fetal heart rate was subsequently decreased after the central administration of ANG II and/or the increase of blood pressure. ANG II induced c-Fos expression in the central putative cardiovascular area, the paraventricular nuclei in the brain sympathetic pathway. Application of ANG II also caused intense Fos immunoreactivity in the tractus solitarius nuclei in the hindbrain. In addition, intense angiotensin subtype 1 receptors were expressed in the hypothalamus at preterm. These data demonstrate that central ANG II-related pressor centers start to function as early as at preterm and suggest that the central angiotensin-related sympathetic pathway is likely intact in the control of blood pressure in utero.     

Circumventricular organs and ANG II-induced salt appetite: blood pressure and connectivity

A lesion of the subfornical organ (SFO) may reduce sodium depletion-induced salt appetite, which is largely dependent on ANG II, and yet ANG II infusions directly into SFO do not provoke salt appetite. Two experiments were designed to address this apparent contradiction. In experiment 1 sustained infusions of ANG II into SFO did not produce a sustained elevation of blood pressure, and neither a reduction of blood pressure alone with minoxidil and captopril nor a reduction of both blood pressure and volume with furosemide and captopril enhanced salt appetite. Infusions of ANG II in the organum vasculosum laminae terminalis (OVLT) did evoke salt appetite without raising blood pressure. In experiment 2 knife cuts of the afferent and efferent fibers of the rostroventral pole of the SFO abolished water intake during an infusion of ANG II into the femoral vein but failed to reduce salt appetite during an infusion of ANG II into the OVLT. We conclude that 1) hypertension does not account for the failure of infusions of ANG II in the SFO to generate salt appetite and 2) the OVLT does not depend on its connectivity with the SFO to generate salt appetite during ANG II infusions.     

Agonist activation of cytosolic Ca2+ in subfornical organ cells projecting to the supraoptic nucleus

The subfornical organ (SFO) is sensitive to both ANG II and ACh, and local application of these agents produces dipsogenic responses and vasopressin release. The present study examined the effects of cholinergic drugs, ANG II, and increased extracellular osmolarity on dissociated, cultured cells of the SFO that were retrogradely labeled from the supraoptic nucleus. The effects were measured as changes in cytosolic calcium in fura 2-loaded cells by using a calcium imaging system. Both ACh and carbachol increased intracellular ionic calcium concentration ([Ca2+]i). However, in contrast to the effects of muscarinic receptor agonists on SFO neurons, manipulation of the extracellular osmolality produced no effects, and application of ANG II produced only moderate effects on [Ca2+]i in a few retrogradely labeled cells. The cholinergic effects on [Ca2+]i could be blocked with the muscarinic receptor antagonist atropine and with the more selective muscarinic receptor antagonists pirenzepine and 4-diphenylacetoxy-N-methylpiperdine methiodide (4-DAMP). In addition, the calcium in the extracellular fluid was required for the cholinergic-induced increase in [Ca2+]i. These findings indicate that ACh acts to induce a functional cellular response in SFO neurons through action on a muscarinic receptor, probably of the M1 subtype and that the increase of [Ca2+]i, at least initially, requires the entry of extracellular Ca2+. Also, consistent with a functional role of M1 receptors in the SFO are the results of immunohistochemical preparations demonstrating M1 muscarinic receptor-like protein present within this forebrain circumventricular organ.     

Aortic stiffness affects the coronary blood flow response to percutaneous coronary intervention

Chronic subcutaneous infusion of ouabain causes hypertension via central pathways involving angiotensin type 1 (AT(1)) receptor stimulation. The present study assessed plasma and tissue ANG I and II levels as well as AT1 receptor and angiotensin-converting enzyme (ACE) mRNA levels and binding densities by real-time PCR and in vitro autoradiography in relevant brain nuclei and peripheral tissues (heart and kidney) in rats at 1 and/or 2 wk after start of ouabain infusion at 50 microg/day. After 2 wk (but not after 1 wk), blood pressures significantly increased (+15 mmHg). At 2 wk, plasma ANG I and II levels were markedly suppressed by ouabain. In contrast, in the heart and kidneys, ANG I levels were not affected, and ANG II levels tended to decrease, whereas in the hypothalamus ANG II content clearly increased. At 1 wk, no changes in ACE and AT1 receptor densities were seen. After 2 wk, there were significant decreases in AT(1) receptor mRNA and densities in the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and paraventricular nucleus (PVN). ACE densities decreased only in the OVLT and SFO, but ACE mRNA showed more variable responses (decrease in OVLT vs. increase in PVN). In the kidneys, at 2 wk both AT1 receptor and ACE densities were decreased, but mRNA abundance did not change. The heart showed no significant changes. The increase in hypothalamic ANG II content and associated decreases in central AT1 receptor and ACE densities support the involvement of the brain renin-angiotensin system in the central hypertensive mechanism of action of ouabain.     

Angiotensin-(1-7) attenuates the chronotropic response to angiotensin II via stimulation of PTEN in the spontaneously hypertensive rat neurons

Several studies have focused on the beneficial effects of peripheral angiotensin-(1-7) [Ang-(1-7)] in the regulation of cardiovascular function, showing its counterregulatory effect against the actions of angiotensin II (ANG II). However, its actions in the central nervous system are not completely understood. In the present study, we investigated the intracellular mechanisms underlying the action of ANG-(1-7) using the patch-clamp technique in neurons cultured from the hypothalamus of neonatal spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Superfusion of neurons with ANG II (100 nM) significantly increased neuronal firing in both strains of rats, and this chronotropic effect of ANG II was significantly enhanced in prehypertensive SHR neurons compared with WKY rat neurons. The enhanced chronotropic effect of ANG II was attenuated by a phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, LY 294002 (10 μM). Superfusion of neurons with ANG-(1-7) (100 nM) did not alter the neuronal firing rate in either SHR or WKY neurons; however, it significantly attenuated the chronotropic action of ANG II exclusively in prehypertensive SHR neurons. This counterregulatory effect of ANG-(1-7) on ANG II action in prehypertensive SHR neurons was attenuated by cotreatment with either A-779, a Mas receptor antagonist, or bisperoxovanadium, a phosphatase and tensin homologue deleted on chromosome ten (PTEN) inhibitor. In addition, incubation of WKY and prehypertensive SHR neurons with ANG-(1-7) significantly increased PTEN activity. The data demonstrate that ANG-(1-7) counterregulates the chronotropic action of ANG II via a PTEN-dependent signaling pathway in prehypertensive SHR neurons.     

Central ventilatory and cardiovascular actions of angiotensin peptides in trout

In the brains of teleosts, angiotensin II (ANG II), one of the main effector peptides of the renin-angiotensin system, is implicated in various physiological functions notably body fluid and electrolyte homeostasis and cardiovascular regulation, but nothing is known regarding the potential action of ANG II and other angiotensin derivatives on ventilation. Consequently, the goal of the present study was to determine possible ventilatory and cardiovascular effects of intracerebroventricular injection of picomole doses (5-100 pmol) of trout [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, ANG IV, and ANG 1-7 into the third ventricle of unanesthetized trout. The central actions of these peptides were also compared with their ventilatory and cardiovascular actions when injected peripherally. Finally, we examined the presence of [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, and ANG IV in the brain and plasma using radioimmunoassay coupled with high-performance liquid chromatography. After intracerebroventricular injection, [Asn(1)]-ANG II and [Asp(1)]-ANG II two ANG IIs, elevated the total ventilation through a selective stimulatory action on the ventilation amplitude. However, the hyperventilatory effect of [Asn(1)]-ANG II was threefold higher than the effect of [Asp(1)]-ANG II at the 50-pmol dose. ANG III, ANG IV, and ANG 1-7 were without effect. In addition, ANG IIs and ANG III increased dorsal aortic blood pressure (P(DA)) and heart rate (HR). After intra-arterial injections, none of the ANG II peptides affected the ventilation but [Asn(1)]-ANG II, [Asp(1)]-ANG II, and ANG III elevated P(DA) (50 pmol: +80%, +58% and +48%, respectively) without significant decrease in HR. In brain tissue, comparable amounts of [Asn(1)]-ANG II and [Asp(1)]-ANG II were detected (ca. 40 fmol/mg brain tissue), but ANG III was not detected, and the amount of ANG IV was about eightfold lower than the content of the ANG IIs. In plasma, ANG IIs were also the major angiotensins (ca. 110 fmol/ml plasma), while significant but lower amounts of ANG III and ANG IV were present in plasma. In conclusion, our study suggests that the two ANG II isoforms produced within the brain may act as a neurotransmitter and/or neuromodulator to regulate the cardioventilatory functions in trout. In the periphery, two ANG IIs and their COOH-terminal peptides may act as a circulating hormone preferentially involved in cardiovascular regulations.     

Imidazoline receptors of the paraventricular nucleus on the pressor response induced by stimulation of the subfornical organ

In the present experiments we investigated a possible involvement of imidazoline receptors of the paraventricular nucleus (PVN) of the hypothalamus on the pressor effects of the angiotensin II (ANG II) injected into the subfornical organ (SFO), in male Holtzman rats (250–300 g) with a cannula implanted into the third ventricle (3rdV), PVN and SFO. At first we tested the participation of α₂ and imidazoline agonist and antagonist compounds on the pressor effect of ANG II injected into the 3rdV. Based on the results we may conclude that clonidine associated with rilmenidine was able to block the hypertensive response to ANG II. The ANG II (20 pmol) injected into SFO induced a robust increase in blood pressure (37 ± 2 mmHg). Isotonic saline (0.15 M) NaCl did not produce any change in blood pressure (5 ± 2 mmHg). The injection of rilmenidine (30 μg/kg/1 μL), an imidazoline agonist agent injected into PVN before ANG II injection into SFO, blocked the pressor effect of ANG II (5 ± 2 mmHg). Also, the injection of idazoxan (60 μg/kg/μL) before rilmenidine blocked the inhibitory effect of rilmenidine on blood pressure (39 ± 4 mmHg). The injection of clonidine (20 nmol/μL) prior to ANG II into the 3rdV produced a decreased in arterial blood pressure (37 ± 2 mmHg) to (15 ± 4 mmHg). The injection of yohimbine (80 nmol/μL) prior to clonidine blocked the effect of clonidine on the effect of ANG II (27 ± 2 mmHg). The injection of rilmenidine prior to ANG II also induced a decrease in arterial blood pressure (10 ± 3 mmHg). The injection of idazoxan prior to rilmenidine also blocked the inhibitory effect of rilmenidine (24 ± 3 mmHg). In summary, the present study demonstrated that rilmenidine decreases the hypertensive effect of ANG II, with more potency than clonidine, even when injected into 3rdV or PVN. This study established that the PVN interacts with SFO by imidazoline receptors in order to control the arterial blood pressure.     

Paraventricular noradrenergic systems participate in angiotensin II-induced drinking

The present study was designed to examine the role of noradrenergic systems in the hypothalamic paraventricular nucleus (PVN) in the drinking response induced by microinjection of angiotensin II (ANG II) into the subfornical organ (SFO) in the awake rat. Intracerebral microdialysis techniques were utilized to quantify the extracellular concentration of noradrenaline (NA) in the region of the PVN. Injections of ANG II (10⁻⁶ M, 0.2 μl) into the SFO significantly increased NA release in the PVN area. The increase in the NA concentration caused by the ANG II injection was significantly attenuated by water ingestion. In urethane-anesthetized rats, injections of ANG II into the SFO elicited an elevation in mean arterial pressure (MAP). On the other hand, intravenous injections of the -agonist metaraminol (5 μg) slightly decreased the release of NA in the PVN area that accompanied an elevation in MAP. These results show that the noradrenergic system in the PVN area may be involved in the dipsogenic response induced by ANG II acting at the SFO.     

Nucleus of the solitary tract lesions enhance drinking, but not vasopressin release, induced by angiotensin

Rats with chronic nucleus of the solitary tract lesions (NTS-X) drink water and release vasopressin (VP) in response to reduced blood volume despite an absence of neural signals from cardiac and arterial baroreceptors. The present study determined whether rats with NTS-X have a greater sensitivity to circulating ANG II, which may contribute to the drinking and VP responses to hypovolemia. In conscious control rats and rats with NTS-X, ANG II was infused intravenously for 1 h at 10, 100, or 250 ng. kg(-1). min(-1). At the two higher doses, ANG II stimulated more water intake with a shorter latency to drink in rats with NTS-X than in control rats. In contrast, infusion of ANG II produced comparable increases in plasma VP in the two groups. At the two higher doses, ANG II produced an enhanced increase in arterial pressure (AP) in rats with NTS-X, and the bradycardia seen in control rats was reversed to a tachycardia. Infusion of hypertonic saline, which did not alter AP or heart rate, produced comparable drinking and VP release in the two groups. These results demonstrate that chronic NTS-X increases the dipsogenic response of rats to systemic ANG II but has no effect on ANG II-induced VP release or the osmotic stimulation of these responses.     

Role of angiotensin in body fluid homeostasis of mice: effect of losartan on water and NaCl intakes

It is known that mice injected peripherally with ANG II do not show a drinking response but that cFos immunoreactivity (ir) is induced in brain regions similar to those in rats. We now show in Crl:CD1(ICR) mice that peripheral injection of the ANG II type 1 receptor antagonist losartan was sufficient to prevent this induction of Fos-ir in the subfornical organ (SFO). Injection of ANG II into the lateral cerebral ventricle produced a robust water intake in mice and induced Fos-ir in SFO, as well as in median preoptic (MnPO) and paraventricular (PVN) nuclei. Peripheral injection of losartan blocked this drinking response and prevented the induction of Fos-ir in each of these brain regions. Hypovolemia produced by polyethylene glycol (PEG) produced a robust water intake but no evidence of sodium appetite, and it induced Fos-ir in SFO, MnPO, and PVN. Peripheral injection of losartan did not affect this drinking response. Fos-ir induced by PEG in SFO and MnPO was reduced by treatment with losartan, while that induced in the PVN was further increased by losartan. Sodium depletion with furosemide and low-sodium diet produced a strong sodium appetite and induced Fos-ir in SFO and MnPO. Treatment with losartan completely blocked the sodium appetite, as well as the induction of Fos-ir in these brain regions. These data indicate that endogenous production of ANG II and action at forebrain receptors is critically involved in depletion-related sodium appetite in mice. The absence of an effect of losartan on PEG-induced drinking suggests the critical involvement of other factor(s) such as arterial or venous baroreceptor input, and we discuss how this factor could also explain why peripheral ANG II is not dipsogenic in mice.     

ANG II-induced hypertension and the role of the area postrema during normal and increased dietary salt

It has been shown that the area postrema (AP) plays a role in the development of certain types of chronic angiotensin II (ANG II)-induced hypertension in the rat but is not of great importance in the salt sensitivity of arterial pressure. It has recently been proposed, however, that elevated sodium levels may exacerbate the hypertensive effects of ANG II, which by itself dramatically affects salt sensitivity, by acting at sodium-sensing neurons in certain circumventricular organs of the brain. Thus the interactions of ANG II, sodium, and the central nervous system remain to be fully understood. The purpose of this study was to examine the role of the AP in ANG II-induced hypertension during periods of normal and elevated dietary salt. We hypothesized that an intact AP was necessary for the full development of hypertension under chronic ANG II infusion and that its role would be pronounced during periods of increased dietary sodium. To test this, male Sprague-Dawley rats underwent ablation of the area postrema (APx, n = 6) or sham operation (sham, n = 6). After 3 wk of recovery, rats were instrumented with radiotelemetry transducers for constant blood pressure and heart rate monitoring and venous catheters for vehicle infusion. After a 3-day control period of 0.9% saline infusion (7 ml/day) and 0.4% dietary sodium, a 10-day period of ANG II infusion (10 ng.kg(-1).min(-1)) was begun, immediately followed by a second 10-day period during which rats were fed a 4.0% sodium diet. By day 6 of ANG II infusion, mean arterial pressure (MAP) in APx rats had increased to 139 +/- 4 mmHg, whereas MAP in sham rats had increased to 126 +/- 3 mmHg. This difference was found to be significant and continued through day 1 of the high-salt period, after which MAP of the two groups had risen to similar levels. On day 9 of high salt, MAP was again observed to be significantly higher (162 +/- 1 mmHg) in APx rats when compared with sham rats (147 +/- 4 mmHg.) These results do not support the hypothesis that the AP is necessary for the full development of ANG II-induced hypertension at normal or elevated levels of dietary sodium.