Role of brain angiotensin in thirst and sodium appetite of rats

Sodium Intake and Disease: Another Relationship to Consider

Sodium (Na+) is crucial for numerous homeostatic processes in the body and, consequentially, its levels are tightly regulated by multiple organ systems. Sodium is acquired from the diet, commonly in the form of NaCl (table salt), and substances that contain sodium taste salty and are innately palatable at concentrations that are advantageous to physiological homeostasis. The importance of sodium homeostasis is reflected by sodium appetite, an "all-hands-on-deck" response involving the brain, multiple peripheral organ systems, and endocrine factors, to increase sodium intake and replenish sodium levels in times of depletion. Visceral sensory information and endocrine signals are integrated by the brain to regulate sodium intake. Dysregulation of the systems involved can lead to sodium overconsumption, which numerous studies have considered causal for the development of diseases, such as hypertension. The purpose here is to consider the inverse-how disease impacts sodium intake, with a focus on stress-related and cardiometabolic diseases. Our proposition is that such diseases contribute to an increase in sodium intake, potentially eliciting a vicious cycle toward disease exacerbation. First, we describe the mechanism(s) that regulate each of these processes independently. Then, we highlight the points of overlap and integration of these processes. We propose that the analogous neural circuitry involved in regulating sodium intake and blood pressure, at least in part, underlies the reciprocal relationship between neural control of these functions. Finally, we conclude with a discussion on how stress-related and cardiometabolic diseases influence these circuitries to alter the consumption of sodium.

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.

Interaction between angiotensin II and glucose sensing at the subfornical organ

The subfornical organ (SFO) lacks the normal blood-brain barrier and senses the concentrations of many different circulating signals, including glucose and angiotensin II (ANG II). ANG II has recently been implicated in the control of food intake and body weight gain. The present study assessed whether single SFO neurones sense changes in glucose and ANG II, and also whether changes in glucose concentration alter the responsiveness of these neurones to ANG II. SFO neurones dissociated from male Sprague-Dawley rats (100-175 g) were used. We first examined whether glucose concentration modulates AT₁ receptor expression. Similar AT_1a mRNA expression levels were found at glucose concentrations of 1, 5 and 10 mmol L^-1 in dissociated SFO neurones. Glucose responsiveness of SFO neurones was assessed using perforated current-clamp recordings and switching between 5 and 10 mmol L^-1 glucose artificial cerebrospinal fluid to classify single neurones as nonresponsive (nGS), glucose-excited (GE) or glucose-inhibited (GI). In total, 26.7% of the SFO neurones were GI (n = 24 of 90), 21.1% were GE (n = 19 of 90) and 52.2% were nGS (n = 47 of 90). Once classified, the effects of 10 nmol L^-1 ANG II on the excitability of these neurones were tested, with 52% of GE (n = 10 of 19), 71% of GI (n = 17 of 24) and 43% of nGS (n = 20 of 47) neurones being ANG II sensitive. Finally, we tested whether acute changes in glucose concentration modified the response to ANG II and showed that some neurones (4/17) only respond to ANG II at 10 mmol L^-1 glucose. Our data demonstrate that the same SFO neurone can sense glucose and ANG II and that acute changes in glucose concentration may change ANG II responsiveness.

Neuronal (pro)renin receptor regulates deoxycorticosterone-induced sodium intake

Increased sodium appetite is a physiological response to sodium deficiency; however, it has also been implicated in disease conditions such as congestive heart failure, kidney failure, and salt-sensitive hypertension. The central nervous system is the major regulator of sodium appetite and intake behavior; however, the neural mechanisms underlying this behavior remain incompletely understood. Here, we investigated the involvement of the (pro)renin receptor (PRR), a component of the brain renin-angiotensin system, in the regulation of sodium intake in a neuron-specific PRR knockout (PRRKO) mouse model generated previously in our laboratory. Sodium intake following deoxycorticosterone (DOCA) stimulation was tested by assessing the preference of mice for 0.9% saline or regular water in single-animal metabolic cages. Blood pressure was monitored in conscious, freely moving mice by a telemetry system. We found that saline intake and total fluid intake were significantly reduced in PRRKO mice following DOCA treatment compared with that in wild-type (WT) mice, whereas regular water intake was similar between the genotypes. Sodium preference and total sodium intake were significantly reduced in PRRKO mice compared with WT mice. PRRKO mice also excreted less urine and urinary sodium compared with WT mice following DOCA treatment, whereas potassium excretion was similar between the two groups. Finally, we found that the sodium balance, calculated by subtracting urinary sodium excretion from sodium intake, was greater in WT mice than in PRRKO mice. Collectively, these findings suggest that the neuronal PRR plays a regulatory role in DOCA-induced sodium intake.

In-Depth Duodenal Transcriptome Survey in Chickens with Divergent Feed Efficiency Using RNA-Seq

Since the feed cost is a major determinant of profitability in poultry industry, how to improve feed efficiency through genetic selection is an intriguing subject for breeders and producers. As a more suitable indicator assessing feed efficiency, residual feed intake (RFI) is defined as the difference between observed and expected feed intake based on maintenance and growth. However, the genetic mechanisms responsible for RFI in chickens are still less well appreciated. In this study, we investigated the duodenal transcriptome architecture of extreme RFI phenotypes in the six brown-egg dwarf hens (three per group) using RNA sequencing technology. Among all mapped reads, an average of 75.62% fell into annotated exons, 5.50% were located in introns, and the remaining 18.88% were assigned to intergenic regions. In total, we identified 41 promising candidate genes by differential expression analysis between the low and high RFI groups. Furthermore, qRT-PCR assays were designed for 10 randomly chosen genes, and nine (90.00%) were successfully validated. Functional annotation analyses revealed that these significant genes belong to several specific biological functions related to digestibility, metabolism and biosynthesis processes as well as energy homeostasis. We also predicted 253 intergenic coding transcripts, and these transcripts were mainly involved in fundamental biological regulation and metabolism processes. Our findings provided a pioneering exploration of biological basis underlying divergent RFI using RNA-Seq, which pinpoints promising candidate genes of functional relevance, is helpful to guide future breeding strategies to optimize feed efficiency and assists in improving the current gene annotation in chickens.

Osmotic homeostasis

Alterations in water homeostasis can disturb cell size and function. Although most cells can internally regulate cell volume in response to osmolar stress, neurons are particularly at risk given a combination of complex cell function and space restriction within the calvarium. Thus, regulating water balance is fundamental to survival. Through specialized neuronal "osmoreceptors" that sense changes in plasma osmolality, vasopressin release and thirst are titrated in order to achieve water balance. Fine-tuning of water absorption occurs along the collecting duct, and depends on unique structural modifications of renal tubular epithelium that confer a wide range of water permeability. In this article, we review the mechanisms that ensure water homeostasis as well as the fundamentals of disorders of water balance.

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Mitogen-activated protein kinase is required for the behavioural desensitization that occurs after repeated injections of angiotensin II

Angiotensin II (Ang II) acts on central angiotensin type 1 (AT(1)) receptors to increase water and saline intake. Prolonged exposure to Ang II in cell culture models results in a desensitization of the AT(1) receptor that is thought to involve receptor internalization, and a behavioural correlate of this desensitization has been shown in rats after repeated central injections of Ang II. Specifically, rats given repeated injections of Ang II drink less water than control animals after a subsequent test injection of Ang II. In the same conditions, however, repeated injections of Ang II have no effect on Ang II-induced saline intake. Given earlier studies indicating that separate intracellular signalling pathways mediate Ang II-induced water and saline intake, we hypothesized that the desensitization observed in rats may be incomplete, leaving the receptor able to activate mitogen-activated protein (MAP) kinases (ERK1/2), which play a role in Ang II-induced saline intake without affecting water intake. In support of this hypothesis, we found no difference in MAP kinase phosphorylation after an Ang II test injection in rats given prior treatment with repeated injections of vehicle, Ang II or Sar(1),Ile(4),Ile(8)-Ang II (SII), an Ang II analogue that activates MAP kinase without G protein coupling. In addition, we found that pretreatment with the MAP kinase inhibitor U0126 completely blocked the desensitizing effect of repeated Ang II injections on water intake. Furthermore, Ang II-induced water intake was reduced to a similar extent by repeated injections of Ang II or SII. The results suggest that G protein-independent signalling is sufficient to produce behavioural desensitization of the angiotensin system and that the desensitization requires MAP kinase activation.

Angiotensin receptor type 1 antagonists protect against neuronal injury induced by oxygen-glucose depletion

BACKGROUND AND PURPOSE

Several clinical trials and in vivo animal experiments have suggested that blockade of angiotensin receptor type 1 (AT(1)) improves ischaemic outcomes. However, the mechanism(s) underlying these effects has not been elucidated. Here, we have investigated the protective effects of pretreatment with AT(1) receptor antagonists, losartan or telmisartan, against ischaemic insult to neurons in vitro.

EXPERIMENTAL APPROACH

Primary rat neuron-astrocyte co-cultures and astrocyte-defined medium (ADM)-cultured pure astrocyte cultures were prepared. Ischaemic injury was modelled by oxygen-glucose depletion (OGD) and lactate dehydrogenase release after OGD was measured with or without AT(1) receptor antagonists or agonists (L162313), AT(2) receptor antagonist (PD123319) or agonist (CGP-42112A) pretreatment, for 48 h. Activity of glutamate transporter 1 (GLT-1) was evaluated by [(3)H]-glutamate uptake assays, after AT(1) receptor agonists or antagonists. Immunoblot and real-time PCR were used for analysis of protein and mRNA levels of GLT-1.

KEY RESULTS

AT(1) receptor agonists augmented OGD-induced cellular damage, which was attenuated by AT(1) receptor antagonists. AT(1) receptor antagonists also suppressed OGD-induced extracellular glutamate release, reactive oxygen species production and nitric oxide generation. GLT-1 expression and glutamate uptake activity were significantly enhanced by AT(1) receptor antagonists and impaired by AT(1) receptor agonists. AT(1) receptor stimulation suppressed both ADM-induced GLT-1 protein expression and mRNA levels. AT(1)b receptor knock-down with siRNA enhanced GLT-1 expression. In postnatal (P1-P21) rat brains, protein levels of GLT-1 and AT(1) receptors were inversely correlated.

CONCLUSIONS AND IMPLICATIONS

Suppression of AT(1) receptor stimulation induced GLT-1 up-regulation, which ameliorated effects of ischaemic injury.

AT₁ receptor and NAD(P)H oxidase mediate angiotensin II-stimulated antioxidant enzymes and mitogen-activated protein kinase activity in the rat hypothalamus

INTRODUCTION

Angiotensin II (AngII) regulates blood pressure and water and electrolyte metabolism through the stimulation of NAD(P)H oxidase and production of reactive oxygen species (ROS) such as O₂⁻, which is metabolised by superoxide dismutase, catalase and glutathione peroxidase. We assessed the role of AT₁ and AT₂ receptors, NAD(P)H oxidase and protein kinase C (PKC) in Ang II-induced sodium and water excretion and their capacity to stimulate antioxidant enzymes in the rat hypothalamus, a brain structure known to express a high density of AngII receptors.

MATERIALS AND METHODS

Male Sprague-Dawley rats were intracerebroventricularly (ICV) injected with AngII and urinary sodium and water excretion was assessed. Urine sodium concentration was determined using flame photometry. After decapitation the hypothalamus was microdissected under stereomicroscopic control. Superoxide dismutase, catalase and glutathione peroxidase activity were determined spectrophotometrically and extracellular signal-regulated kinase (ERK1/2) activation was analysed by Western blot.

RESULTS

AngII-ICV resulted in antidiuresis and natriuresis. ICV administration of losartan, PD123319, apocynin and chelerythrine blunted natriuresis. In hypothalamus, AngII increased catalase, superoxide dismutase and glutation peroxidase activity and ERK1/2 phosphorylation. These actions were prevented by losartan, apocynin and chelerythrine, and increased by PD123319.

CONCLUSIONS

AT₁ and AT₂ receptors, NAD(P)H oxidase and PKC pathway are involved in the regulation of hydromineral metabolism and antioxidant enzyme activity induced by AngII.

Changed salt appetite and central angiotensin II-induced cellular activation in rat offspring following hypoxia during fetal stages

Hypoxia in pregnancy may induce fetal growth restriction and cause functional abnormalities during development. The present study determined the long-term influence of hypoxia in fetal life on dipsogenic behavior linked to central angiotensin (Ang) network in the offspring rats. Fetal blood pO(2) and body weight were decreased by hypoxia during pregnancy, followed by a postnatal "catch-up" growth. Subcutaneous hypertonic saline or intracerebroventricular Ang II significantly increased salt intake in the offspring prenatally exposed to hypoxia, while water intake was the same between the two groups. Ang II-induced c-fos expression was detected in the paraventricular nuclei, median preoptic nuclei, supraoptic nuclei, and subfornical organ in the brain, in association with reduced forebrain AT(2) receptor protein abundance in the offspring prenatally exposed to hypoxia. Levels of central AT(1) receptor protein were not changed between the two groups. Hypoxia during pregnancy could be linked to developmental problems related to behavioral dysfunctions in body fluid regulations in later life, in association with the change in central angiotensin II-mediated neural activation and expression of the Ang II receptor in the brain.

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 II receptor signalling

Angiotensin II plays a key role in the regulation of body fluid homeostasis. To correct body fluid deficits that occur during hypovolaemia, an animal needs to ingest both water and electrolytes. Thus, it is not surprising that angiotensin II, which is synthesized in response to hypovolaemia, acts centrally to increase both water and NaCl intake. Here, we review findings relating to the properties of angiotensin II receptors that give rise to changes in behaviour. Data are described to suggest that divergent signal transduction pathways are responsible for separable behavioural responses to angiotensin II, and a hypothesis is proposed to explain how this divergence may map onto neural circuits in the brain.

Richter and sodium appetite: from adrenalectomy to molecular biology

Nearly three-quarters of a century ago, Curt Richter removed the adrenal glands from rats and noted that the animal's vitality was dependent on its increased consumption of sodium chloride. In doing so, Richter revealed an innate behavioral mechanism that serves to maintain the hydromineral balance of an animal faced with sodium deficit. This experiment and others like it, led to the development of a field of research devoted to the investigation of salt appetite. The following is a discussion of how Richter's initial observations gave birth to an evolving field that incorporates multiple approaches to examine the drive to consume sodium.

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.

Effects of i.c.v. losartan on the angiotensin II-mediated vasopressin release and hypothalamic fos expression in near-term ovine fetuses

Our previous studies have shown that central administration of angiotensin (ANG II) causes arginine vasopressin (AVP) release in the fetus at 70-90% gestation. This is evidence that the hypothalamic-neurohypophysial system is relatively mature before birth. However, few data exist regarding central ANG receptor mechanisms-mediated AVP response during fetal life. To determine roles of brain ANG receptor subtypes in this response, AT1 and AT2 receptor antagonists, losartan and PD123319, were investigated in the brain in chronically prepared ovine fetuses at the last third of gestation. Application of losartan intracerebroventricularly (i.c.v.) at 0.5 mg/kg suppressed central ANG II-stimulated plasma AVP release. Losartan at 5 mg/kg (i.c.v.) demonstrated a significant enhancement of AVP increase to i.c.v. ANG II. Associated with the increase of plasma vasopressin levels, c-fos expression in the hypothalamic neurons was significantly different between the low and high doses of losartan. The low dose losartan markedly reduced the dual immunoreactivity for FOS and AVP in the supraoptic nuclei and paraventricular nuclei after i.c.v. ANG II, whereas the high dose losartan together with ANG II, significantly increased the co-localization of positive FOS in the AVP-containing neurons than that induced by i.c.v. ANG II alone. Central ANG II induced fetal plasma vasopressin increase was not altered by PD123319. The data suggest that losartan in the fetal brain has remarkably different effects based on the doses administrated on central ANG II-related neuroendocrine effects at the late gestation, and that the AT1 mechanism is critical in the regulation of fetal body fluid homeostasis related to plasma AVP levels.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Effects of intracerebroventricular losartan on angiotensin II-mediated pressor responses and c-fos expression in near-term ovine fetus

The renin-angiotensin system plays an important role in cardiovascular control. Intracerebroventricular (i.c.v.) angiotensin (ANG) II causes a reliable pressor response in the fetus at 90% gestation. To determine the roles of brain AT1 and AT2 receptors in this response, the effects of the central AT1 and AT2 receptor antagonists losartan and PD123319 were investigated in chronically prepared near-term ovine fetuses. Losartan at 0.5 mg/kg (i.c.v.) abolished central ANG II-induced pressor responses. High-dose losartan (5 mg/kg, i.c.v.) showed a potentiation of the pressor response to i.c.v. ANG II, accompanied by bradycardia. Associated with the pressor responses, c-fos expression in the cardiovascular controlling areas was significantly different between the low and high doses of losartan. These areas included the subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis, and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, lateral parabrachial nuclei in the hindbrain. Low-dose losartan markedly reduced c-fos in these areas after i.c.v. ANG II, while the high-dose losartan together with ANG II elicited a much stronger FOS-immunoreactivity in these areas than that induced by i.c.v. ANG II alone. This is a novel finding, that c-fos expression in the brain can be both activated and inhibited under the same condition. Central ANG II-induced fetal pressor responses were not altered by PD123319 (0.8 mg/kg). These results indicate that i.c.v. losartan at a high and a low dose has strikingly different effects on central ANG II-induced pressor responses in fetuses at late gestation, and that the AT1 mechanism plays an important role in fetal cardiovascular regulation.

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.

N-methyl-D-aspartate glutamate receptor mediates spontaneous and angiotensin II-stimulated ovine fetal swallowing

BACKGROUND

In adult rats, N-methyl-D-aspartate (NMDA) receptors have been implicated in the central control of body fluid homeostasis, as intracerebroventricular (ICV) injection of NMDA receptor antagonists suppresses stimulated drinking behavior. Fetal swallowing occurs at a significantly higher rate as compared to adult drinking, contributing to amniotic fluid volume regulation and fetal gastrointestinal development. The aim of present study was to determine the role of central NMDA receptors in the modulation of fetal swallowing activity.

METHODS

Eight time-dated pregnant ewes and fetuses were chronically prepared with fetal vascular and ICV catheters, electrocorticogram (ECoG), and esophageal electromyogram electrodes and studied at 130 +/- 1 days' gestation. Following an initial 2-hour baseline period (time 2 h), the NMDA receptor antagonist, dizocipline (1 mg), was injected ICV. At time 4 h, the dose of dizocipline was repeated, together with angiotensin II (AngII, 6.4 microg). Fetal swallowing was monitored for 2 hours after each injection. Four of these fetuses also received an identical control study (on an alternate day) in which dizocipline was replaced with artificial cerebrospinal fluid (aCSF).

RESULTS

ICV dizocipline injection nearly abolished spontaneous fetal swallowing activities (0.6 +/- 0.1 to 0.2 +/- 0.1 swallows/min; P < .001). ICV AngII in the presence of dizocipline did not demonstrate a dipsogenic effect on fetal swallowing (0.1 +/- 0.1; P < .001). In the control study, ICV injection of aCSF did not change fetal swallowing activity (1.0 +/- 0.1 swallows/min), while ICV AngII resulted in a significant increase in fetal swallowing (2.0 +/- 0.1 swallows/min; P < .001).

CONCLUSIONS

This study demonstrates that central NMDA-glutamate receptor-mediated activity contributes to the high rate of spontaneous and AngII-stimulated fetal swallowing. We speculate that reduced NMDA receptor expression within the forebrain dipsogenic neurons may account for observed differences in drinking activities between the fetus/neonate and the adult.

Induced fetal depressor or pressor responses associated with c-fos by intravenous or intracerebroventricular losartan

Previous fetal studies have indicated depressor responses of intravenous (i.v.) administration of angiotensin antagonists. However, little is known of central effects of angiotensin blockers on fetal cardiovascular controlling. The cardiovascular effects of central administration of the angiotensin-1 (AT(1)) and angiotensin-2 (AT(2)) receptor antagonists, losartan and PD123319, were investigated in the chronically catheterized near-term ovine fetuses. Intravenous losartan produced within 1.5 min a significant and persistent depressor response [maximum Delta mean arterial pressure (MAP)=9 mm Hg] without altering fetal heart rate. Intracerebroventricular (i.c.v.) administration of losartan (1-5 mg/kg) increased fetal arterial pressures (Delta MAP=9-14 mm Hg). Central application of losartan (1 mg/kg) also increased fetal heart rate (maximum Delta heart rate=33 beat per minute). Losartan increased c-fos expression in the median preoptic nucleus and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, the lateral parabrachial nuclei, and the ventrolateral medullabrain. These brain sectors are with abundant AT(1) receptors and have been demonstrated in the involvement in cardiovascular regulation. In contrast, intracerebroventricular injection of the AT(2) receptor antagonist PD123319 had no effect on fetal arterial pressure and heart rate. The results demonstrate strikingly functional differences of losartan on the fetal cardiovascular regulation in central and peripheral sides.

Effect of intracerebroventricular angiotensin II on body weight and food intake in adult rats

We recently reported that intracerebroventricular infusions of ANG II decreased food intake and increased energy expenditure in young rats. The aim of the present study was to determine if intracerebroventricular ANG II has similar effects in adult rats. The time course of the effect was also investigated with the idea that at earlier time points, a potential role for increased hypothalamic expression of corticotropin-releasing hormone (CRH) in the anorexia could be established. Finally, the contribution of ANG II-induced water drinking to the decrease in food intake was directly investigated. Rats received intracerebroventricular saline or ANG II using osmotic minipumps. Food intake, water intake, and body weight were measured daily. Experiments were terminated 2, 5, or 11 days after the beginning of the infusions. ANG II (approximately 32 ng.kg(-1).min(-1)) produced a transient decrease in food intake that lasted for 4-5 days although body weight continued to be decreased for the entire experiment most likely due to increased energy expenditure as evidenced by increased uncoupling protein-1 mRNA expression in brown adipose tissue. At 11 and 5 days, the expression of CRH mRNA was decreased. At 2 days, CRH expression was not suppressed even though body weight was decreased. The decrease in food intake and body weight was identical whether or not rats were allowed to increase water consumption. These data suggest that in adult rats ANG II acts within the brain to affect food intake and energy expenditure in a manner that is not related to water intake.

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

Two circumventricular organs, the subfornical organ (SFO) and organum vasculosum laminae terminalis (OVLT), may mediate salt appetite in response to acute intravenous infusions of angiotensin (ANG) II. Fluid intakes and mean arterial pressures were measured in rats with sham lesions or electrolytic lesions of the SFO or OVLT during an intravenous infusion of 30 ng/min ANG II. Beginning 21 h before the 90-min infusion, the rats were depleted of sodium with furosemide and given a total of 300 mg/kg captopril in 75 ml/kg water in three spaced gavages to block the usual salt appetite and to hydrate the rats. No other food or fluids were available for ingestion. Sham-lesioned rats drank 9.3+/-1.2 ml if 0.3 M NaCl alone was available and drank 8.9+/-1.6 ml of saline and 3.7+/-1.6 ml of water if both were available. Either SFO or OVLT lesions reduced the intakes of saline to <5 ml in both conditions and of water to <1 ml. Mean arterial pressure did not differ among the groups and was maintained above 100 mmHg after the depletion and captopril treatments because of the large doses of water. Thus, a full expression of salt appetite in response to an acute intravenous infusion of ANG II requires the integrity of both the SFO and OVLT.

Relaxin and drinking in pregnant rats

Work reported in this chapter describes the potential role of relaxin in resetting cardiovascular thresholds in pregnant rats. Relaxin, a polypeptide produced primarily by the ovary in pregnant animals in many species, is also produced in the brain. Exogenous administration of relaxin into the brain causes a profound drinking response which is negated by pretreatment with a specific monoclonal antibody to rat relaxin when the antibody is injected into the brain. Neutralizing the action of endogenous brain relaxin in pregnant rats also blocks the normal increase in drinking that is observed in rats at night during the second half of pregnancy. Relaxin acts through the forebrain angiotensin system at the level of the subfornical organ (an important interface between the blood, the brain and the cerebrospinal fluid) as blockade of the angiotensin II receptor action negates several central actions of relaxin. Expression of angiotensin II AT1 receptors in the subfornical organ increases in parallel with the increase in circulating relaxin seen in the second half of pregnancy. Neutralizing the effects of endogenous brain relaxin, using central injections of the monoclonal antibody, blocks this increase in the expression of angiotensin II AT1 receptors in subfornical organ. These data imply that relaxin in the brain may act to affect central cardiovascular thresholds in rats and this may be important for the normal physiology of pregnancy.

Effects of SFO lesions on salt appetite during multiple sodium depletions

A depletion of sodium may increase sodium intake by increasing the synthesis of angiotensin (ANG) II in the blood and by stimulating ANG II receptors in the subfornical organ (SFO) of the rat. Lesions of SFO reportedly reduce these intakes. The present experiments tested the effects of SFO lesions on salt appetite after three successive depletions. After a furosemide-induced natriuresis, Long-Evans rats had free access to water- and sodium-deficient diet for 22 h. Water and 0.3 M NaCl were given for 2 h, and then the rats received regular chow, water, and 0.3 M NaCl until the next injection 5 or 7 days later. SFO lesions reduced water intake 1-2 h after each furosemide injection but not during the overnight periods. The lesions did not affect salt appetite the next day, 24-26 h after furosemide, but they did prevent the expected increase in the chronic daily 0.3 M NaCl intake after repeated depletions. The second experiment was similar to the first except that three subcutaneous injections of 100 mg/kg captopril were given at 1, 18, and 20 h after furosemide for the second depletion only. After the first depletion, the results were similar to those of the same condition of the first experiment. After the second depletion, captopril greatly reduced water intake and salt appetite in all rats including those with SFO lesions. Thus, we found that the lesion reduced chronic intake, but we did not replicate results showing large effects of SFO lesions on acute salt appetite. This residual acute appetite after SFO lesion remains dependent on the synthesis of ANG II.

Time of day and access to food alter water intake in rats after water deprivation

1. Drinking behaviour after water deprivation is one of the standard tests used to study thirst in humans and animals. Diurnal cycle and food availability are known to influence water intake, but have not been considered in previous studies of thirst after water deprivation. In the present study, we examined the effects of diurnal variation and food availability on water intake after 24 h water deprivation in rats. 2. All rats cycled through four treatments in varying order. These treatments were: (i) 24 h water deprivation with free access to food from 1900 h one day to 1900 h the next day, followed by free access to both food and water (Night-with-Food); (ii) 24 h water deprivation with free access to food from from 1900 h one day to 1900 h the next day, followed by free access to water but not food (Night-without-Food); (iii) 24 h water deprivation with free access to food from 0700 h one day to 0700 h the next day, followed by free access to both food and water (Day-with-Food); or (iv) 24 h water deprivation with free access to food from 0700 h one day to 0700 h the next day, followed by free access to water but not food (Day-without-Food). The amount of water consumed during the first 6 h, post-24 h water deprivation, was examined under each condition. 3. There was a significant diurnal effect (P < 0.001) and a significant food availability effect (P = 0.007) on the water consumed in the 6 h period after water deprivation. Most water was consumed by the Night-with-Food group and the least amount of water was consumed by the Day-without-Food group. These effects persisted after correction for water intake during 6 h periods from 0700 and 1900 h with and without food but without previous water deprivation. The diurnal and food availability effects on water consumption were independent (P = 0.5). 4. The coefficient of variability for each group suggests that the most sensitive measurements of water intake are obtained during the day in the absence of food. 5. We conclude that both the time of day and access to food independently alter water intake in rats subjected to a previous 24 h water deprivation. Our study also supports the validity of performing water intake measurements in thirst studies in rats during the day.

Atypical angiotensin receptors may mediate water intake induced by central injections of angiotensin II and of serotonin in pigeons

Intracerebroventricular (i.c.v.) injection of serotonin (5-HT) in pigeons dose-dependently evokes a prompt and intense drinking behavior, which resembles that evoked by i.c.v. injections of angiotensin II (ANGII) in the same species. In the present study, we have examined the possible participation of central ANGII receptors in both ANGII- and 5-HT-evoked drinking behavior. The effects of i.c.v. injections of 5-HT (155 nmol), avian ANGII ([Asp(1),Val(5)]-ANGII, 0.1 nmol) or vehicle were studied in pigeons pretreated 20 min before with i.c.v. injections of the nonspecific ANGII receptor antagonist [Sar(1),Ile(8)]-ANGII (SAR; 1, 0.1 or 0.01 nmol), the AT(1) receptor antagonist losartan (2 or 4 nmol), the AT(2) receptor antagonist PD 123,319 (2 or 4 nmol) or vehicle (NaCl 0.15 M, 1 microl, n = 8/group). Immediately after treatment, they were given free access to water and drinking behavior was recorded during the next 60 min. At the doses presently used both 5-HT and ANGII treatments evoked comparable water intake amounts with similar behavioral profiles. While pretreatment with SAR dose-dependently reduced the water intake evoked by both 5-HT and ANGII, neither losartan nor PD 123,319 pretreatment affected the drinking induced by these treatments. The present results indicate that ANGII- and 5-HT-induced drinking in pigeons may be mediated by AT receptors possibly different from mammalian AT(1) and AT(2) receptors and suggest that activation of ANGII central circuits is a necessary step for the intense drinking induced by i.c.v. injections of 5-HT in this species.

Angiotensin II stimulates intake of ethanol in C57BL/6J mice

The influence of intracerebroventricular (i.c.v.) infusion of angiotensin II on intake of water and ethanol solutions was determined in C57BL/6J mice. Compared to other mice, C57 mice do not show an aversion to ethanol solutions. With both water and ethanol solutions available, the C57 mice consumed 40-60% of their total daily fluid intake as ethanol solution when the concentration of ethanol solution offered was 4-14%. When given a choice between 0.3 M KCl and either 4 or 10% ethanol solution, the mice clearly preferred the ethanol solution. With water only available, i.c.v. infusion of angiotensin II increased intake from 3-5 mL/day (baseline) to 11-12 mL/ day (Day 4 of infusion). A similar increase in intake occurred in mice with access to a nonpreferred solution of 0.3 M KCl. In comparison, when only 4% ethanol solution was available, angiotensin II increased intake to 7-8 mL/day, and when only 10% ethanol solution was available, intake was transiently increased. The results demonstrated that thirst for water caused by i.c.v. infusion of angiotensin II in C57 mice is similar to that observed in BALB/C mice. Unlike BALB/C mice, however, i.c.v. infusion of angiotensin II stimulated intake of ethanol solution. The failure of angiotensin II to cause a large increase in 4% ethanol solution or a sustained increase in 10% ethanol solution intake does not seem to be caused by an aversion to the taste of ethanol solution, but most likely due to postingestional factors.

Intracerebroventricular infusion of angiotensin II increases water and ethanol intake in rats

The influence of prolonged ingestion of ethanol on stimulation of water or ethanol intake by intracerebroventricular infusion of ANG II was evaluated in rats. Animals were maintained for 5-6 mo with either 10% ethanol solution or water as their only source of fluid. In both groups of rats, infusion of ANG II caused a large increase in water intake (7-fold) and a lesser increase in 10% ethanol intake (2-fold). The effect of ANG II on the volume of ethanol solution ingested, however, was inversely related to the concentration of the ethanol solution. As the concentration of ethanol solution was decreased, frequency and duration of drinking bouts increased. The intake of sweetened 10% ethanol solution or commercially produced wine during infusion of ANG II was similar to the intake of 10% ethanol and not related to taste preference. In conclusion, chronic consumption of ethanol solution did not appear to adversely effect ANG II stimulation of water intake. The intake of ethanol solution during infusion of ANG II was inhibited by a direct effect of ingested ethanol and/or by indirect effect from metabolized ethanol.

Effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides from 1994 through 1998

Effects of peptides on the central nervous system are presented in two ways so as to provide a cross-listing. In the first table, the peptides are listed alphabetically. In the second table, the central nervous system effects are arranged alphabetically. No longer can there be any doubt that peptides affect the central nervous system, sometimes in several ways.