Forebrain circumventricular organs mediate salt appetite induced by intravenous angiotensin II in rats

Brain Angiotensinergic Regulation of the Immune System: Implications for Cardiovascular and Neuroendocrine Responses

OBJECTIVE

The Renin-Angiotensin-Aldosterone System (RAAS) plays a major role in the regulation of cardiovascular functions, water and electrolytic balance, and hormonal responses. We perform a review of the literature, aiming at providing the current concepts regarding the angiotensin interaction with the immune system in the brain and the related implications for cardiovascular and neuroendocrine responses.

METHODS

Appropriate keywords and MeSH terms were identified and searched in Pubmed. Finally, references of original articles and reviews were examined.

RESULTS

Angiotensin II (ANG II), beside stimulating aldosterone, vasopressin and CRH-ACTH release, sodium and water retention, thirst, and sympathetic nerve activity, exerts its effects on the immune system via the Angiotensin Type 1 Receptor (AT 1R) that is located in the brain, pituitary, adrenal gland, and kidney. Several actions are triggered by the binding of circulating ANG II to AT 1R into the circumventricular organs that lack the Blood-Brain-Barrier (BBB). Furthermore, the BBB becomes permeable during chronic hypertension thereby ANG II may also access brain nuclei controlling cardiovascular functions. Subfornical organ, organum vasculosum lamina terminalis, area postrema, paraventricular nucleus, septal nuclei, amygdala, nucleus of the solitary tract and retroventral lateral medulla oblongata are the brain structures that mediate the actions of ANG II since they are provided with a high concentration of AT 1R. ANG II induces also T-lymphocyte activation and vascular infiltration of leukocytes and, moreover, oxidative stress stimulating inflammatory responses via inhibition of endothelial progenitor cells and stimulation of inflammatory and microglial cells facilitating the development of hypertension.

CONCLUSION

Besides the well-known mechanisms by which RAAS activation can lead to the development of hypertension, the interactions between ANG II and the immune system at the brain level can play a significant role.

Molecular Basis of the Brain Renin Angiotensin System in Cardiovascular and Neurologic Disorders: Uncovering a Key Role for the Astroglial Angiotensin Type 1 Receptor AT1R

The central renin angiotensin system (RAS) is one of the most widely investigated cardiovascular systems in the brain. It is implicated in a myriad of cardiovascular diseases. However, studies from the last decade have identified its involvement in several neurologic abnormalities. Understanding the molecular functionality of the various RAS components can thus provide considerable insight into the phenotypic differences and mechanistic drivers of not just cardiovascular but also neurologic disorders. Since activation of one of its primary receptors, the angiotensin type 1 receptor (AT1R), results in an augmentation of oxidative stress and inflammatory cytokines, it becomes essential to investigate not just neuronal RAS but glial RAS as well. Glial cells are key homeostatic regulators in the brain and are critical players in the resolution of overt oxidative stress and neuroinflammation. Designing better and effective therapeutic strategies that target the brain RAS could well hinge on understanding the molecular basis of both neuronal and glial RAS. This review provides a comprehensive overview of the major studies that have investigated the mechanisms and regulation of the brain RAS, and it also provides insight into the potential role of glial AT1Rs in the pathophysiology of cardiovascular and neurologic disorders.

Blood pressure regulation by the angiotensin type 1 receptor in the proximal tubule

PURPOSE OF REVIEW

The renin-angiotensin system (RAS) plays a critical role in the pathogenesis of hypertension. Homeostatic actions of the RAS, such as increasing blood pressure (BP) and vasoconstriction, are mediated via type 1 (AT1) receptors for angiotensin II. All components of the RAS are present in the renal proximal tubule, which reabsorbs the bulk of the glomerular filtrate, making this segment of the nephron a location of great interest for solute handling under RAS influence. This review highlights recent studies that illustrate the key role of renal proximal tubule AT1 receptors in BP regulation.

RECENT FINDINGS

A variety of investigative approaches have demonstrated that angiotensin II signaling via AT1a receptors, specifically in the renal proximal tubule, is a major regulator of BP and sodium homeostasis. Reduction of proximal tubule AT1a receptors led to lower BPs, whereas overexpression generally caused increased BPs.

SUMMARY

AT1a receptors in the proximal tubule are critical to the regulation of BP by the kidney and the RAS. The pattern of BP modulation is associated with alterations in sodium transporters. As a key site for sodium homeostasis, the renal proximal tubule could hence be a potential target in the treatment of hypertension.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

Behavioral responses and fluid regulation in male rats after combined dietary sodium deficiency and water deprivation

Most investigators use a single treatment such as water deprivation or dietary sodium deficiency to evaluate thirst or sodium appetite, which underlie behavioral responses to body fluid challenges. The goal of the present experiments was to assess the effects of combined treatments in driving behaviors. Therefore, we evaluated the effect of combined overnight water deprivation and dietary sodium deficiency on water intake and salt intake by adult male rats in 2-bottle (0.5M NaCl and water) tests. Overnight water deprivation alone increased water intake, and 10days of dietary sodium deficiency increased 0.5M NaCl intake, with a secondary increase in water intake. During combined water deprivation and dietary sodium deficiency, water intake was enhanced and 0.5M NaCl was reduced, but not eliminated, suggesting that physiologically relevant behavioral responses persist. Nonetheless, the pattern of fluid intake was altered by the combined treatments. We also assessed the effect of these behaviors on induced deficits in body sodium and fluid volume during combined treatments and found that, regardless of treatment, fluid ingestion partially repleted the induced deficits. Finally, we examined urine volume and sodium excretion during dietary sodium deficiency with or without overnight water deprivation and found that, whether or not rats were water deprived, and regardless of water consumption, sodium excretion was minimal. Thus, the combination of water deprivation and dietary sodium deficiency appears to arouse drives that stimulate compensatory behavioral responses. These behaviors, in conjunction with physiological adaptations to the treatments, underlie body sodium and volume repletion in the face of combined water deprivation and dietary sodium deficiency.

Evidence for intraventricular secretion of angiotensinogen and angiotensin by the subfornical organ using transgenic mice

Direct intracerebroventricular injection of angiotensin II (ANG II) causes increases in blood pressure and salt and water intake, presumably mimicking an effect mediated by an endogenous mechanism. The subfornical organ (SFO) is a potential source of cerebrospinal fluid (CSF), ANG I, and ANG II, and thus we hypothesized that the SFO has a secretory function. Endogenous levels of angiotensinogen (AGT) and renin are very low in the brain. We therefore examined the immunohistochemical localization of angiotensin peptides and AGT in the SFO, and AGT in the CSF in two transgenic models that overexpress either human AGT (A⁺ mice), or both human AGT (hAGT) and human renin (SRA mice) in the brain. Measurements were made at baseline and following volumetric depletion of CSF. Ultrastructural analysis with immunoelectron microscopy revealed that superficially located ANG I/ANG II and AGT immunoreactive cells in the SFO were vacuolated and opened directly into the ventricle. Withdrawal of CSF produced an increase in AGT in the CSF that was accompanied by a large decline in AGT immunoreactivity within SFO cells. Our data provide support for the hypothesis that the SFO is a secretory organ that releases AGT and possibly ANG I/ANG II into the ventricle at least under conditions when genes that control the renin-angiotensin system are overexpressed in mice.

Serotonin release in the subfornical organ area induced by sodium and water intake in the rat

Previous studies have shown that the subfornical organ (SFO) is involved in the modulation of sodium intake in rats. To clarify whether serotonergic mechanisms in the SFO participate in the modulatory system, the present study was carried out to examine the effects of sodium and water intake on serotonin (5-hydroxytryptamine, 5-HT) release in the subfornical organ (SFO) in freely moving rats. The ingestion of 0.3M NaCl and water was induced by subcutaneous injections of the diuretic furosemide (FURO, 10mg/kg) and the angiotensin converting enzyme inhibitor captopril (CAP, 5mg/kg), and extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the region of the SFO were measured using intracerebral microdialysis techniques. The combined treatment with FURO and CAP elicited significant decreases in the 5-HT and 5-HIAA levels in the SFO area. The 0.3M NaCl and water intake caused by the treatment significantly enhanced the 5-HT and 5-HIAA concentrations in the SFO area compared with the basal levels. No significant changes in the 5-HT and 5-HIAA levels caused by either the FURO and CAP treatment or water and NaCl intake were observed in the sites away from the SFO. These results suggest that the serotonergic mechanism in the SFO may be important for the control of sodium appetite and thirst.

Angiotensin II (de)sensitization: Fluid intake studies with implications for cardiovascular control

Cardiovascular disease is the leading cause of death worldwide and hypertension is the most common risk factor for death. Although many anti-hypertensive pharmacotherapies are approved for use in the United States, rates of hypertension have increased over the past decade. This review article summarizes a presentation given at the 2015 meeting of the Society for the Study of Ingestive Behavior. The presentation described work performed in our laboratory that uses angiotensin II-induced drinking as a model system to study behavioral and cardiovascular effects of the renin-angiotensin system, a key component of blood pressure regulation, and a common target of anti-hypertensives. Angiotensin II (AngII) is a potent dipsogen, but the drinking response shows a rapid desensitization after repeated injections of AngII. This desensitization appears to be dependent upon the timing of the injections, requires activation of the AngII type 1 (AT1) receptor, requires activation of mitogen-activated protein (MAP) kinase family members, and involves the anteroventral third ventricle (AV3V) region as a critical site of action. Moreover, the response does not appear to be the result of a more general suppression of behavior, a sensitized pressor response to AngII, or an aversive state generated by the treatment. More recent studies suggest that the treatment regimen used to produce desensitization in our laboratory also prevents the sensitization that occurs after daily bolus injections of AngII. Our hope is that these findings can be used to support future basic research on the topic that could lead to new developments in treatments for hypertension.

Mechanisms of brain renin angiotensin system-induced drinking and blood pressure: importance of the subfornical organ

It is critical for cells to maintain a homeostatic balance of water and electrolytes because disturbances can disrupt cellular function, which can lead to profound effects on the physiology of an organism. Dehydration can be classified as either intra- or extracellular, and different mechanisms have developed to restore homeostasis in response to each. Whereas the renin-angiotensin system (RAS) is important for restoring homeostasis after dehydration, the pathways mediating the responses to intra- and extracellular dehydration may differ. Thirst responses mediated through the angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptors (AT2R) respond to extracellular dehydration and intracellular dehydration, respectively. Intracellular signaling factors, such as protein kinase C (PKC), reactive oxygen species (ROS), and the mitogen-activated protein (MAP) kinase pathway, mediate the effects of central angiotensin II (ANG II). Experimental evidence also demonstrates the importance of the subfornical organ (SFO) in mediating some of the fluid intake effects of central ANG II. The purpose of this review is to highlight the importance of the SFO in mediating fluid intake responses to dehydration and ANG II.

Salt appetite is reduced by a single experience of drinking hypertonic saline in the adult rat

Salt appetite, the primordial instinct to favorably ingest salty substances, represents a vital evolutionary important drive to successfully maintain body fluid and electrolyte homeostasis. This innate instinct was shown here in Sprague-Dawley rats by increased ingestion of isotonic saline (IS) over water in fluid intake tests. However, this appetitive stimulus was fundamentally transformed into a powerfully aversive one by increasing the salt content of drinking fluid from IS to hypertonic saline (2% w/v NaCl, HS) in intake tests. Rats ingested HS similar to IS when given no choice in one-bottle tests and previous studies have indicated that this may modify salt appetite. We thus investigated if a single 24 h experience of ingesting IS or HS, dehydration (DH) or 4% high salt food (HSD) altered salt preference. Here we show that 24 h of ingesting IS and HS solutions, but not DH or HSD, robustly transformed salt appetite in rats when tested 7 days and 35 days later. Using two-bottle tests rats previously exposed to IS preferred neither IS or water, whereas rats exposed to HS showed aversion to IS. Responses to sweet solutions (1% sucrose) were not different in two-bottle tests with water, suggesting that salt was the primary aversive taste pathway recruited in this model. Inducing thirst by subcutaneous administration of angiotensin II did not overcome this salt aversion. We hypothesised that this behavior results from altered gene expression in brain structures important in thirst and salt appetite. Thus we also report here lasting changes in mRNAs for markers of neuronal activity, peptide hormones and neuronal plasticity in supraoptic and paraventricular nuclei of the hypothalamus following rehydration after both DH and HS. These results indicate that a single experience of drinking HS is a memorable one, with long-term changes in gene expression accompanying this aversion to salty solutions.

Activation of the renin-angiotensin system, specifically in the subfornical organ is sufficient to induce fluid intake

Increased activity of the renin-angiotensin system within the brain elevates fluid intake, blood pressure, and resting metabolic rate. Renin and angiotensinogen are coexpressed within the same cells of the subfornical organ, and the production and action of ANG II through the ANG II type 1 receptor in the subfornical organ (SFO) are necessary for fluid intake due to increased activity of the brain renin-angiotensin system. We generated an inducible model of ANG II production by breeding transgenic mice expressing human renin in neurons controlled by the synapsin promoter with transgenic mice containing a Cre-recombinase-inducible human angiotensinogen construct. Adenoviral delivery of Cre-recombinase causes SFO-selective induction of human angiotensinogen expression. Selective production of ANG II in the SFO results in increased water intake but did not change blood pressure or resting metabolic rate. The increase in water intake was ANG II type 1 receptor-dependent. When given a choice between water and 0.15 M NaCl, these mice increased total fluid and sodium, but not water, because of an increased preference for NaCl. When provided a choice between water and 0.3 M NaCl, the mice exhibited increased fluid, water, and sodium intake, but no change in preference for NaCl. The increase in fluid intake was blocked by an inhibitor of PKC, but not ERK, and was correlated with increased phosphorylated cyclic AMP response element binding protein in the subfornical organ. Thus, increased production and action of ANG II specifically in the subfornical organ are sufficient on their own to mediate an increase in drinking through PKC.

Activity of protein kinase C-α within the subfornical organ is necessary for fluid intake in response to brain angiotensin

Angiotensin-II production in the subfornical organ acting through angiotensin-II type-1 receptors is necessary for polydipsia, resulting from elevated renin-angiotensin system activity. Protein kinase C and mitogen-activated protein kinase pathways have been shown to mediate effects of angiotensin-II in the brain. We investigated mechanisms that mediate brain angiotensin-II-induced polydipsia. We used double-transgenic sRA mice, consisting of human renin controlled by the neuron-specific synapsin promoter crossed with human angiotensinogen controlled by its endogenous promoter, which results in brain-specific overexpression of angiotensin-II, particularly in the subfornical organ. We also used the deoxycorticosterone acetate-salt model of hypertension, which exhibits polydipsia. Inhibition of protein kinase C, but not extracellular signal-regulated kinases, protein kinase A, or vasopressin V₁A and V₂ receptors, corrected the elevated water intake of sRA mice. Using an isoform selective inhibitor and an adenovirus expressing dominant negative protein kinase C-α revealed that protein kinase C-α in the subfornical organ was necessary to mediate elevated fluid and sodium intake in sRA mice. Inhibition of protein kinase C activity also attenuated polydipsia in the deoxycorticosterone acetate-salt model. We provide evidence that inducing protein kinase C activity centrally is sufficient to induce water intake in water-replete wild-type mice, and that cell surface localization of protein kinase C-α can be induced in cultured cells from the subfornical organ. These experimental findings demonstrate a role for central protein kinase C activity in fluid balance, and further mechanistically demonstrate the importance of protein kinase C-α signaling in the subfornical organ in fluid intake stimulated by angiotensin-II in the brain.

Natriorexigenic effect of DAMGO is decreased by blocking AT1 receptors in the central nucleus of the amygdala

μ-Opioid receptor (μ-OR) activation with agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) in the central nucleus of the amygdala (CeA) induces sodium (0.3M NaCl) intake in rats. The purpose of this study was to examine the effects of pre-injections of losartan (AT1 angiotensin receptor antagonist) into the CeA on 0.3 M NaCl and water intake induced by DAMGO injected bilaterally in the same area in rats submitted to water deprivation-partial rehydration (WD-PR) and in rats treated with the diuretic furosemide (FURO) combined with a low dose of the angiotensin-converting enzyme inhibitor captopril (CAP) injected subcutaneously (FURO/CAP). Male Sprague-Dawley rats with stainless steel cannulas implanted bilaterally into the CeA were used. In WD-PR rats, bilateral injections of DAMGO (2 nmol in 0.5 μL) into the CeA induced 0.3 M NaCl and water intake, and pre-treatment with losartan (108 nmol in 0.5 μL) injected into the CeA reduced 0.3 M NaCl and water intake induced by DAMGO. In FURO/CAP rats, pre-treatment with losartan (108 nmol in 0.5 μL) injected into the CeA attenuated the increase in 0.3M NaCl and water intake induced by DAMGO (2 nmol in 0.5 μL) injected into the same site. The results suggest that the natriorexigenic effect of DAMGO injected into the CeA is facilitated by endogenous angiotensin II acting on AT1 receptors in the CeA, which drives rats to ingest large amounts of hypertonic NaCl.

Involvement of brain ANG II in acute sodium depletion induced salty taste changes

Many investigations have been devoted to determining the role of angiotensin II (ANG II) and aldosterone (ALD) in sodium-depletion-induced sodium appetite, but few were focused on the mechanisms mediating the salty taste changes accompanied with sodium depletion. To further elucidate the mechanism of renin-angiotensin-aldosterone system (RAAS) action in mediating sodium intake behavior and accompanied salty taste changes, the present study examined the salty taste function changes accompanied with sodium depletion induced by furosemide (Furo) combined with different doses of angiotensin converting enzyme (ACE) inhibitor, captopril (Cap). Both the peripheral and central RAAS activity and the nuclei Fos immunoreactivity (Fos-ir) expression in the forebrain area were investigated. Results showed that sodium depletion induced by Furo+low-Cap increased taste preference for hypertonic NaCl solution with amplified brain action of ANG II but without peripheral action, while Furosemide combined with a high dose of captopril can partially inhibit the formation of brain ANG II, with parallel decreased effects on salty taste changes. And the resulting elevating forebrain ANG II may activate a variety of brain areas including SFO, PVN, SON and OVLT in sodium depleted rats injected with Furo+low-Cap, which underlines salty taste function and sodium intake behavioral changes. Neurons in SFO and OVLT may be activated mainly by brain ANG II, while PVN and SON activation may not be completely ANG II dependent. These findings suggested that forebrain derived ANG II may play a critical role in the salty taste function changes accompanied with acute sodium depletion.

Water deprivation increases angiotensin-converting enzyme but not AT(1) receptor expression in brainstem and paraventricular nucleus of the hypothalamus of the rat

The rostral ventrolateral medulla (RVLM) is critical to the maintenance of blood pressure. It has been proposed that blood-borne Ang II can influence the RVLM via a neural connection between the circumventricular organs and paraventricular nucleus of the hypothalamus (PVH) and that a component of this pathway is angiotensinergic. A period of water deprivation leads to increased ability of angiotensin type 1 (AT(1)) receptor antagonists to reduce blood pressure when administered into the RVLM and PVH. We studied the differences in AT(1) receptor and angiotensin-converting enzyme (ACE) expression in these and other brain regions involved in blood pressure regulation and water intake following dehydration. AT(1) receptor and ACE expression in brains of rats deprived of water for 48 h were compared to that of water-replete rats by quantitative receptor autoradiography. AT(1) receptor expression was increased in the subfornical organ and periventricular nucleus of the hypothalamus, but not in other brain regions measured. ACE expression was increased in the RVLM, PVH, choroid plexus, median preoptic nucleus, and organosum vasculosum of the lamina terminalis. These findings suggest that increased Ang II production but not increased receptor expression in the PVH and RVLM is the mechanism by which Ang II in the brain helps to sustain systemic blood pressure during periods of water deprivation.

Angiotensin modulation of rostral ventrolateral medulla (RVLM) in cardiovascular regulation

The rostral ventrolateral medulla (RVLM) and the presympathetic bulbospinal neurons in this region play a critical role in cardiovascular regulation. However, there is ambiguity regarding the precise anatomical coordinates of the RVLM and much still needs to be learned regarding the regulation and neurochemistry of this region. This brief review discusses some of these issues and focuses on the role of angiotensin-mediated signaling in the RVLM in blood pressure regulation.

In hypertension, the kidney breaks your heart

The renin-angiotensin system (RAS) is a master regulator of blood pressure and fluid homeostasis. Because RAS components are expressed in several tissues that may influence blood pressure, studies using conventional gene targeting to globally interrupt the RAS have not determined the contributions of angiotensin II receptor type 1 (AT(1)) receptors in specific tissue pools to blood pressure regulation and tissue injury. Recent experiments using kidney cross-transplantation and mice lacking the dominant murine AT(1) receptor isoform, AT(1A), have demonstrated that 1) AT(1) receptors inside and outside the kidney make equivalent contributions to normal blood pressure homeostasis, 2) activation of renal AT(1) receptors is required for the development of angiotensin II-dependent hypertension, and 3) this blood pressure elevation rather than activation of AT(1) receptors in the heart drives angiotensin II-induced cardiac hypertrophy. These findings, together with previous experiments, confirm the kidney's critical role in the pathogenesis of hypertension and its complications.

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

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

Angiotensin and NMDA receptors in the median preoptic nucleus mediate hemodynamic response patterns to stress

The brain renin-angiotensin system plays an important role in the regulation of arterial pressure in response to stress, in part due to activation of AT1 receptors in the hypothalamic median preoptic nucleus (MnPO) by endogenous angiotensin II (ANG II). N-methyl-d-aspartate (NMDA) receptors are also involved in the angiotensinergic signaling pathway through the MnPO. We investigated whether AT1 and NMDA receptors in the MnPO are responsible for variable hemodynamic response patterns to stress. Cocaine or startle with cold water evoked a pressor response in Sprague-Dawley rats due, in some rats [vascular responders (VR)], to a large increase in systemic vascular resistance (SVR) and, in other rats [mixed responders (MR)], to small increases in SVR and cardiac output (CO). Microinjection of the GABAA agonist muscimol into the MnPO to block synaptic transmission attenuated the cocaine- or stress-induced increase in SVR and the decrease in CO seen in VR without altering either response in MR. Likewise, administration of either an AT1 receptor antagonist, losartan, or an NMDA receptor antagonist, MK-801, attenuated the increase in SVR and the decrease in CO in VR in response to either cocaine (losartan and MK-801) or startle with cold water (losartan) without altering either response in MR. We propose that the MnPO is responsible for greater SVR responses in VR and that AT1 and NMDA receptors play an important role in greater SVR responses in VR. These data provide additional support for the critical role of the MnPO in cardiovascular responses to stress.

Involvement of tumor necrosis factor-alpha in angiotensin II-mediated effects on salt appetite, hypertension, and cardiac hypertrophy

Hypertension is considered a low-grade inflammatory condition induced by various proinflammatory cytokines, including tumor necrosis factor (TNF)-alpha. Recent studies have implicated an involvement of TNF-alpha in the development of salt-sensitive hypertension induced by angiotensin II (Ang II). To understand further the relationship between TNF-alpha and Ang II, we examined the responses to Ang II in TNF-alpha knockout (TNF-alpha(-/-)) mice in the present study. A continuous infusion of Ang II (1 microg/kg per minute) for 2 weeks was given to both TNF-alpha(-/-) and wild-type (WT) mice with implanted osmotic minipumps. Daily measurement of water intake, salt intake, and urine output were performed using metabolic cages. Blood pressure was monitored continuously with implanted radiotelemetry. Ang II administration for 2 weeks caused increases in salt (0.2+/-0.07 to 5.6+/-0.95 mL/d) and water (5.4+/-0.34 to 11.5+/-1.2 mL/d) intake and in mean arterial pressure (115+/-1 to 151+/-3 mm Hg) in wild-type mice, but these responses were absent in TNF-alpha(-/-) mice (0.2+/-0.04 to 0.3+/-0.09 mL/d, 5.5+/-0.2 to 6.1+/-0.07 mL/d, and 113+/-2 to 123+/-3 mm Hg, respectively). Cardiac hypertrophy induced by Ang II was significantly attenuated in TNF-alpha(-/-) mice compared with wild-type mice. In a group of TNF-alpha(-/-) mice, when replacement therapy was made with recombinant TNF-alpha, Ang II induced similar responses in salt appetite, mean arterial pressure, and cardiac hypertrophy, as observed in wild-type mice. These results suggest that TNF-alpha plays a mechanistic role in mediating chronic Ang II-induced effects on salt appetite and blood pressure, as well as on cardiac hypertrophy.

The neural substrates of enhanced salt appetite after repeated sodium depletions

Sodium appetite is associated with a form of behavioral plasticity in which animals experimentally depleted of sodium progressively increase their intake of hypertonic NaCl over several successive (on 2 to 4 occasions) depletion. The present experiment explored the nature of this plasticity by quantifying Fos immunoreactivity (Fos-ir) in structures implicated in the mediation of sodium appetite and in the signaling of reward. Rats were depleted of sodium with the diuretic furosemide three times (3F), one time (2V1F) or sham depleted (i.e., vehicle treated; 3V). Rats were given sodium appetite tests for the first two treatments. The sodium appetite test was omitted after the third treatment. Fos-ir activity was quantified in the paraventricular nucleus (PVN), subfornical organ (SFO), supraoptic nucleus (SON), nucleus accumbens (NAc) shell and core, basolateral (BLA) and central amygdala (CeA), and medial prefrontal cortex (mPFC). Animals receiving repeated sodium depletions increased sodium ingestion across initial depletions. Fos-ir activity was markedly enhanced in the SFO, BLA, and shell of the NAc of 3F rats relative to 2V1F and 3V animals. These results indicate that repeated experience with sodium depletion and ingestion affects both behavioral and neural responses to sodium. Experience with sodium depletion enhances its ingestion and may have a direct impact on central structures implicated in sodium appetite and reward signaling.

In hypertension, the kidney rules

The renin-angiotensin system (RAS) is a critical regulator of blood pressure and fluid homeostasis. Components of the RAS, including renin, angiotensin-converting enzyme (ACE), and angiotensin type 1 (AT1) receptors, are expressed throughout the body in tissues that may impact blood pressure control. Blocking actions of individual components of the RAS lowers blood pressure. Although it has been suggested that control of sodium excretion by the kidney is the dominant mechanism for blood pressure regulation by the RAS, pharmacologic antagonists or conventional gene targeting experiments globally interrupt the RAS and cannot discriminate its actions in the kidney from other tissue compartments. Recent experiments using kidney cross-transplantation and genetically engineered mice have confirmed a major role for angiotensin II acting via AT1 receptors in the kidney in hypertension. These actions of renal AT1 receptors are required for the development of angiotensin II-dependent hypertension and cardiac hypertrophy. These findings, with previous experiments, clearly establish the critical role of the kidney in the pathogenesis of hypertension and its cardiovascular complications.

Intravenous angiotensin and salt appetite in rats

Circulating angiotensin II is crucial for the activation of salt appetite after sodium depletion. We tested if angiotensin (ANG) II infused intravenously at 50 ng/kg/min overnight (chronic) can mimic the rapid salt appetite similar to furosemide and overnight sodium depletion. In experiment 1, rats received chronic ANG II or vehicle infusions all night with access to water and chow but no saline solution. In the morning, the infusions continued, but half of the vehicle-infused group was switched to ANG II (acute). Thirty minutes after the switch, all rats received 10 mg/kg furosemide SC. One hour later they were provided water and 0.3 M NaCl to drink. Rats infused with vehicle or acute ANG drank little, but the chronic ANG group drank 11+/-1 ml of saline in 90 min. In experiment 2, the furosemide was omitted, and a group receiving a chronic infusion of phenylephrine at 6.25 microg/kg/min was included. The chronic ANG group drank 10+/-1 ml saline in 90 min, but the phenylephrine group, which also incurred a significant negative sodium balance overnight, drank little. Thus, an overnight infusion of ANG II is sufficient to mimic the robust expression of salt appetite as observed after furosemide and overnight sodium depletion.

AT1 Receptors and Control of Blood Pressure: The Kidney and More…

The renin-angiotensin system (RAS) is a critical regulator of blood pressure and fluid homeostasis. The components of the RAS including renin, angiotensin-converting enzyme, and angiotensin receptors are expressed throughout the body in tissues that may impact blood pressure control. Blocking actions of individual components of the RAS including renin, angiotensin-converting enzyme, or the type 1 (AT(1)) receptor lowers blood pressure. Although it has been suggested that control of sodium excretion by the kidney is the dominant mechanism for blood pressure regulation by the RAS, pharmacologic antagonists or conventional gene-targeting experiments globally interrupt the RAS and cannot discriminate its actions in the kidney from other tissue compartments. Recent experiments with the use of kidney cross-transplantation and genetically engineered mice suggest independent and equivalent effects of angiotensin II acting via AT(1) receptors in the kidney and in extrarenal tissues to maintain the normal level of blood pressure. However, the nature and relative contributions of these actions may differ in hypertension.

Induction and expression of salt appetite: effects on Fos expression in nucleus accumbens

Sodium depletion is a strong natural motivator that creates a pronounced sodium appetite and has been shown to activate neural regions associated with fluid and sodium balance. However, it is not known whether sodium appetite affects the mesolimbic circuitry associated with reward motivation. The present studies examined expression of the immediate early gene Fos in the nucleus accumbens (NAc) as a marker of neuronal activation following the induction and expression of furosemide-induced sodium appetite. During sodium appetite expression, sham-drinking and normal drinking were used to dissociate effects of NaCl taste stimulation from the repletion that follows absorption of sodium. These studies revealed that the combination of NaCl taste stimulation and persistent sodium depletion experienced by sham-drinking animals dramatically activates the NAc, while neither induction nor expression of sodium appetite alone is sufficient to increase Fos expression in this region. Results are discussed in terms of current theories of reward motivation.

Divergent behavioral roles of angiotensin receptor intracellular signaling cascades

Central injections of angiotensin II (AngII) increase both water and NaCl intake. These effects of AngII occur largely through stimulation of the AngII type 1 (AT(1)) receptor. Stimulation of the AT(1) receptor leads to a number of intracellular events, including phospholipase C (PLC) activation and the subsequent formation of diacylglycerol and inositol trisphosphate (IP(3)), which then activate protein kinase C (PKC) and increase intracellular calcium, respectively. In addition, AT(1) receptor stimulation leads to the activation of MAPK family members. Recent experiments using mutated AT(1) receptor constructs or the AngII analog Sar(1),Ile(4),Ile(8)-AngII (SII) revealed that MAPK activation can occur independent of PLC/PKC/IP(3) activation. The present experiments used in vitro and in vivo approaches to clarify the cellular and behavioral responses to SII. Specifically, SII mimicked AngII stimulation of MAPK in AT(1) receptor-transfected COS-1 cells and rat brain but blocked the effects of AngII in two distinct settings: in vitro stimulation of IP(3) and in vivo increases in water intake. Moreover, SII increased intake of 1.5% NaCl, despite the SII blockade of IP(3) formation and water intake. Examination of brain tissue showed increases in Fos expression in several AngII-sensitive brain areas after injection of AngII, but not SII. The lack of SII-induced IP(3) production, water intake, and Fos expression strongly suggest that the PLC/PKC/IP(3) pathway is required for water intake, but not NaCl consumption stimulated by AngII. Collectively, these results support the hypothesis that divergent intracellular signals from a single receptor type can give rise to separable behavioral phenomena.

Roles of brain angiotensins II and III in thirst and sodium appetite

The current study examined the effects of intracerebroventricular (icv) infused aminopeptidase-resistant analogs of angiotensin II (AngII) and angiotensin III (AngIII) on thirst and sodium appetite. The analogs, [D-Asp1D-Arg2]AngII and [D-Arg1]AngIII, were further protected from degradation by pretreatment with the aminopeptidase A inhibitor, EC33, or the aminopeptidase N inhibitor, PC18. Prior to icv infusions, rats were sodium depleted with furosemide, followed by the angiotensin-converting enzyme inhibitor captopril, to block endogenous angiotensin formation. Both angiotensin analogs, at either of the two doses, were capable of eliciting fluid intakes of water and 0.3 M NaCl. Water and saline intakes were increased to a similar extent by 125 and 1250 pmol of [D-Asp1D-Arg2]AngII. [D-Arg1]AngIII produced a dose-dependent increase in water intake, whereas saline intake was equivalently increased by the 125 and 1250 pmol infusions. Pretreatment with EC33 or PC18 decreased water and saline intakes in response to [D-Asp1D-Arg2]AngII, while pretreatment with PC18 altered the time course of the [D-Arg1]AngIII-induced water and saline intakes. The ability of both inhibitors to decrease, but not completely block, AngII analog-induced intakes, coupled with the altered time course of the responses induced by the AngIII analog in the presence of PC18, supports the hypothesis that both AngII and AngIII are active ligands in brain angiotensin-mediated thirst and sodium appetite. However, these results do not resolve the primary question of whether conversion of AngII to AngIII is a prerequisite to dipsogenic and salt appetite responses in the brain.

Effects of forebrain circumventricular organ ablation on drinking or salt appetite after sodium depletion or hypernatremia

In many previous studies, one or the other forebrain circumventricular organ, the subfornical organ (SFO) or organum vasculosum laminae terminalis (OVLT), was lesioned to test whether it was critical for the behavioral or physiological responses to sodium depletion and hypernatremia. These studies conflict in their conclusions. The present study was designed to create discrete lesions of both the SFO and OVLT in the same animals and to compare these with rats having a lesion of only the SFO or OVLT. Both the OVLT-lesioned group and the combined SFO + OVLT-lesioned group drank significantly more water and saline on a daily basis than Controls or SFO-lesioned rats. In both sodium depletion and hypertonic saline testing, rats with SFO lesions displayed transient deficits in salt appetite or thirst responses, whereas the rats with single OVLT lesions did not. In the sodium depletion test, but not in the hypernatremia test, rats with lesions of both the SFO and OVLT exhibited the largest deficit. The data support the hypothesis that a combined lesion eliminates redundancy and is more effective than a single lesion in sodium depletion tests. The interpretation of the OVLT lesion-only data may have been complicated by a tendency to drink more fluid on a daily basis, because some of those animals drank copious water in addition to saline even very early during the salt appetite test.

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.

Neuroendocrine control of body fluid metabolism

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

Effects of central oxytocin receptor blockade on water and saline intake, mean arterial pressure, and c-Fos expression in rats

Central injection of ANG II has been proposed to have dual effects on salt appetite including a direct stimulatory effect and an indirect inhibitory effect through an activation of central oxytocinergic neurons. The inhibition was demonstrated by pretreating rats with central ornithine vasotocin (OVT; oxytocin antagonist) 30 min before a central ANG II injection. The OVT pretreatment produced a large increase in ANG II-induced saline intake. The present paper reports a failure to replicate that influential experiment. However, we also report for the first time that OVT by itself: 1) provokes drinking of both water and saline solution with a latency almost as short as that produced by ANG II; 2) produces a mild pressor response; and 3) increases c-Fos expression in the organum vasculosum laminae terminalis (OVLT) and the median preoptic nucleus (MnPO). Oxytocin activity may provide an inhibitory control of drinking responses as has been suggested, but the inhibition is tonic and includes both water and saline drinking. Inhibition of this tonic activity may stimulate drinking by increasing neural activity in the OVLT and MnPO.

Salt appetite of adrenalectomized rats after a lesion of the SFO

Circumventricular organs such as the subfornical organ (SFO) may mediate the effects of circulating angiotensin (ANG) II on salt appetite under conditions of sodium depletion in the rat. We studied the effects of an electrolytic lesion of SFO on salt appetite after adrenalectomy (ADX) in Long-Evans rats. The SFO lesion had no effect on saline intake, but it did abolish water intake after acute peripheral treatments with 2 mg/kg of captopril or a 10 mg/kg of furosemide. These findings contrast with other recent data from this laboratory demonstrating large reductions in salt appetite in adrenal-intact rats with lesions of either SFO or the organum vasculosum laminae terminalis during acute iv infusions of ANG II. Thus, the SFO may contribute to the salt appetite response to circulating ANG II, but it is not essential for the response to adrenalectomy.