Angiotensin in the brain

Recent Advances in the Endogenous Brain Renin-Angiotensin System and Drugs Acting on It

The RAS (renin-angiotensin system) is the part of the endocrine system that plays a prime role in the control of essential hypertension. Since the discovery of brain RAS in the seventies, continuous efforts have been put by the scientific committee to explore it more. The brain has shown the presence of various components of brain RAS such as angiotensinogen (AGT), converting enzymes, angiotensin (Ang), and specific receptors (ATR). AGT acts as the precursor molecule for Ang peptides—I, II, III, and IV—while the enzymes such as prorenin, ACE, and aminopeptidases A and N synthesize it. AT1, AT2, AT4, and mitochondrial assembly receptor (MasR) are found to be plentiful in the brain. The brain RAS system exhibits pleiotropic properties such as neuroprotection and cognition along with regulation of blood pressure, CVS homeostasis, thirst and salt appetite, stress, depression, alcohol addiction, and pain modulation. The molecules acting through RAS predominantly ARBs and ACEI are found to be effective in various ongoing and completed clinical trials related to cognition, memory, Alzheimer's disease (AD), and pain. The review summarizes the recent advances in the brain RAS system highlighting its significance in pathophysiology and treatment of the central nervous system-related disorders.

The role of the brain renin-angiotensin system (RAS) in mild traumatic brain injury (TBI)

There is considerable interest in traumatic brain injury (TBI) induced by repeated concussions suffered by athletes in sports, military personnel from combat-and non-combat related activities, and civilian populations who suffer head injuries from accidents and domestic violence. Although the renin-angiotensin system (RAS) is primarily a systemic cardiovascular regulatory system that, when dysregulated, causes hypertension and cardiovascular pathology, the brain contains a local RAS that plays a critical role in the pathophysiology of several neurodegenerative diseases. This local RAS includes receptors for angiotensin (Ang) II within the brain parenchyma, as well as on circumventricular organs outside the blood-brain-barrier. The brain RAS acts primarily via the type 1 Ang II receptor (AT₁R), exacerbating insults and pathology. With TBI, the brain RAS may contribute to permanent brain damage, especially when a second TBI occurs before the brain recovers from an initial injury. Agents are needed that minimize the extent of injury from an acute TBI, reducing TBI-mediated permanent brain damage. This review discusses how activation of the brain RAS following TBI contributes to this damage, and how drugs that counteract activation of the AT₁R including AT₁R blockers (ARBs), renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, and agonists at type 2 Ang II receptors (AT₂) and at Ang (1-7) receptors (Mas) can potentially ameliorate TBI-induced brain damage.

A microanalytical capillary electrophoresis mass spectrometry assay for quantifying angiotensin peptides in the brain

The renin-angiotensin system (RAS) of the brain produces a series of biologically active angiotensinogen-derived peptides involved in physiological homeostasis and pathophysiology of disease. Despite significant research efforts to date, a comprehensive understanding of brain RAS physiology is lacking. A significant challenge has been the limited set of bioanalytical assays capable of detecting angiotensin (Ang) peptides at physiologically low concentrations (2-15 fmol/g of wet tissue) and sufficient chemical specificity for unambiguous molecular identifications. Additionally, a complex brain anatomy calls for microanalysis of specific tissue regions, thus further taxing sensitivity requirements for identification and quantification in studies of the RAS. To fill this technology gap, we here developed a microanalytical assay by coupling a laboratory-built capillary electrophoresis (CE) nano-electrospray ionization (nano-ESI) platform to a high-resolution mass spectrometer (HRMS). Using parallel reaction monitoring, we demonstrated that this technology achieved confident identification and quantification of the Ang peptides at approx. 5 amol to 300 zmol sensitivity. This microanalytical assay revealed differential Ang peptide profiles between tissues that were micro-sampled from the subfornical organ and the paraventricular nucleus of the hypothalamus, important brain regions involved in thirst and water homeostasis and neuroendocrine regulation to stress. Microanalytical CE-nano-ESI-HRMS extends the analytical toolbox of neuroscience to help better understand the RAS.

Hypothalamic and inflammatory basis of hypertension

Hypertension is a major health problem with great consequences for public health. Despite its role as the primary cause of significant morbidity and mortality associated with cardiovascular disease, the pathogenesis of essential hypertension remains largely unknown. The central nervous system (CNS) in general, and the hypothalamus in particular, are intricately involved in the development and maintenance of hypertension. Over the last several decades, the understanding of the brain's role in the development of hypertension has dramatically increased. This brief review is to summarize the neural mechanisms of hypertension with a focus on neuroendocrine and neurotransmitter involvement, highlighting recent findings that suggest that hypothalamic inflammation disrupts key signalling pathways to affect the central control of blood pressure, and therefore suggesting future development of interventional strategies that exploit recent findings pertaining to the hypothalamic control of blood pressure as well as the inflammatory-sympathetic mechanisms involved in hypertension.

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

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

The effects of para-chloromercuribenzoic acid and different oxidative and sulfhydryl agents on a novel, non-AT1, non-AT2 angiotensin binding site identified as neurolysin

A novel, non-AT1, non-AT2 brain binding site for angiotensin peptides that is unmasked by p-chloromercuribenzoate (PCMB) has been identified as a membrane associated variant of neurolysin. The ability of different organic and inorganic oxidative and sulfhydryl reactive agents to unmask or inhibit 125I-Sar1Ile8 angiotensin II (SI-Ang II) binding to this site was presently examined. In tissue membranes from homogenates of rat brain and testis incubated in assay buffer containing losartan (10 μM) and PD123319 (10 μM) plus 100 μM PCMB, 5 of the 39 compounds tested inhibited 125I-SI Ang II binding in brain and testis. Mersalyl acid, mercuric chloride (HgCl2) and silver nitrate (AgNO3) most potently inhibited 125I-SI Ang II binding with IC50s ~1-20 μM. This HgCl2 inhibition was independent of any interaction of HgCl2 with angiotensin II (Ang II) based on the lack of effect of HgCl2 on the dipsogenic effects of intracerebroventricularly administered Ang II and 125I-SI Ang II binding to AT1 receptors in the liver. Among sulfhydryl reagents, cysteamine and reduced glutathione (GSH), but not oxidized glutathione (GSSG) up to 1mM, inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis. Thimerosal and 4-hydroxymercuribenzoate moderately inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis at 100 μM; however, they also unmasked non-AT1, non-AT2 binding independent of PCMB. 4-Hydroxybenzoic acid did not promote 125 I-SI Ang II binding to this binding site indicating that only specific organomercurial compounds can unmask the binding site. The common denominator for all of these interacting substances is the ability to bind to protein cysteine sulfur. Comparison of cysteines between neurolysin and the closely related enzyme thimet oligopeptidase revealed an unconserved cysteine (cys650, based on the full length variant) in the proposed ligand binding channel (Brown et al., 2001) [45] near the active site of neurolysin. It is proposed that the mercuric ion in PCMB and closely related organomercurial compounds binds to cys650, while the acidic anion forms an ionic bond with a nearby arginine or lysine along the channel to effect a conformational change in neurolysin that promotes Ang II binding.

Angiotensin II-stimulated secretion of arginine vasopressin is inhibited by atrial natriuretic peptide in humans

We investigated the effect of the intravenous infusion of atrial natriuretic peptide (ANP) on the response of plasma arginine vasopressin (AVP) levels to intravenous infusion of angiotensin II (ANG II) in healthy individuals. Intravenous infusion of ANP (10 ng·kg−1·min−1) slightly but significantly decreased plasma AVP levels, while intravenous infusion of ANG II (10 ng·kg−1·min−1) resulted in slightly increased plasma AVP levels. ANG II infused significant elevations in arterial blood pressure and central venous pressure (CVP). Because the elevation in blood pressure could have potentially inhibited AVP secretion via baroreceptor reflexes, the effect of ANG II on blood pressure was attenuated by the simultaneous infusion of nitroprusside. ANG II alone produced a remarkable increase in plasma AVP levels when infused with nitroprusside, whereas the simultaneous ANP intravenous infusion (10 ng·kg−1·min−1) abolished the increase in plasma AVP levels induced by ANG II when blood pressure elevation was attenuated by nitroprusside. Thus, ANG II increased AVP secretion and ANP inhibited not only basal AVP secretion but also ANG II-stimulated AVP secretion in humans. These findings support the hypothesis that circulating ANP modulates AVP secretion, in part, by antagonizing the action of circulating ANG II.

Blockade of brain angiotensin II AT1 receptors ameliorates stress, anxiety, brain inflammation and ischemia: Therapeutic implications

Poor adaptation to stress, alterations in cerebrovascular function and excessive brain inflammation play critical roles in the pathophysiology of many psychiatric and neurological disorders such as major depression, schizophrenia, post traumatic stress disorder, Parkinson's and Alzheimer's diseases and traumatic brain injury. Treatment for these highly prevalent and devastating conditions is at present very limited and many times inefficient, and the search for novel therapeutic options is of major importance. Recently, attention has been focused on the role of a brain regulatory peptide, Angiotensin II, and in the translational value of the blockade of its physiological AT(1) receptors. In addition to its well-known cardiovascular effects, Angiotensin II, through AT(1) receptor stimulation, is a pleiotropic brain modulatory factor involved in the control of the reaction to stress, in the regulation of cerebrovascular flow and the response to inflammation. Excessive brain AT(1) receptor activity is associated with exaggerated sympathetic and hormonal response to stress, vulnerability to cerebrovascular ischemia and brain inflammation, processes leading to neuronal injury. In animal models, inhibition of brain AT(1) receptor activity with systemically administered Angiotensin II receptor blockers is neuroprotective; it reduces exaggerated stress responses and anxiety, prevents stress-induced gastric ulcerations, decreases vulnerability to ischemia and stroke, reverses chronic cerebrovascular inflammation, and reduces acute inflammatory responses produced by bacterial endotoxin. These effects protect neurons from injury and contribute to increase the lifespan. Angiotensin II receptor blockers are compounds with a good margin of safety widely used in the treatment of hypertension and their anti-inflammatory and vascular protective effects contribute to reduce renal and cardiovascular failure. Inhibition of brain AT(1) receptors in humans is also neuroprotective, reducing the incidence of stroke, improving cognition and decreasing the progression of Alzheimer's disease. Blockade of AT(1) receptors offers a novel and safe therapeutic approach for the treatment of illnesses of increasing prevalence and socioeconomic impact, such as mood disorders and neurodegenerative diseases of the brain.

Preliminary biochemical characterization of the novel, non-AT1, non-AT2 angiotensin binding site from the rat brain

A novel binding site for angiotensins II and III was recently discovered in brain membranes in the presence of the sulfhydryl reactive angiotensinase inhibitor parachloromercuribenzoate. This binding site is distinctly different from the other known receptors for angiotensins: AT₁, AT₂, AT₄, and mas oncogene protein (Ang 1-7 receptor). Preliminary biochemical characterization studies have been done on this protein by crosslinking it with (125)I-labeled photoaffinity probes and solubilizing the radiolabeled binding site. Polyacrylamide gel electrophoresis studies and isoelectric focusing indicate that this membrane bound binding site is a protein with a molecular weight of 70-85 kDa and an isoelectric point of ~7. Cyanogen bromide hydrolysis of the protein yielded two radiolabeled fragments of 12.5 and 25 kDa. The protein does not appear to be N-glycosylated based upon the failure of PNGaseF to alter its migration rate on a 7.5% polyacrylamide gel. The binding of angiotensin II to this protein is not affected by GTPγS or Gpp(NH)p, suggesting that it is not a G protein-coupled receptor. Further characterization studies are directed to identify this protein either as a novel angiotensin receptor, an angiotensin scavenger (clearance receptor) or an angiotensinase.

Antisense oligodeoxynucleotides for estrogen receptor-beta and alpha attenuate estradiol's modulation of affective and sexual behavior, respectively

Estradiol (E(2)) modulates affective and socio-sexual behavior of female rodents. E(2)'s functional effects may involve actions through alpha and beta isoforms of estrogen receptor (ERs). The importance of E(2)'s actions at these isoforms for anxiety (open field, elevated plus maze), depression (forced swim test), and sexual behavior (lordosis) was investigated using an antisense oligonucleotide (AS-ODN) strategy. If ERbeta is required for anti-anxiety and antidepressant-like effects, and ERalpha is required for sexual receptivity, of E(2), then intracerebroventricular administration of AS-ODNs against these ERs should attenuate these effects and reduce immunoreactivity of ERs in brain regions that mediate these behaviors, such as the hippocampus and ventral medial hypothalamus (VMH). Ovariectomized rats were primed with 17beta-E(2) (10 microg) 48 h before testing (hour 0). At hours 0, 24, and 47.5, rats were infused with saline vehicle, scrambled control AS-ODNs, or AS-ODNs targeted against ERalpha and/or ERbeta, and were tested at hour 48. Rats infused with ERbeta AS-ODNs, alone, or with ERalpha AS-ODNs had significantly decreased open field central entries, decreased plus maze open arm time and entries, increased time spent immobile, and decreased time spent swimming in the forced swim test, and decreased ERbeta immunoreactivity in the brain than did rats administered ERalpha AS-ODNs, vehicle, or scrambled AS-ODNs. Rats that were administered ERalpha AS-ODNs, alone, or with ERbeta AS-ODNs had significantly decreased lordosis and decreased ERalpha immunoreactivity in the brain compared to rats administered ERbeta AS-ODNs, vehicle, or scrambled AS-ODNs. Thus, ERbeta and ERalpha may be required for E(2)'s modulation of affective and sexual behavior, respectively.

Enzymatic pathways of the brain renin-angiotensin system: unsolved problems and continuing challenges

The brain renin-angiotensin system continues to be enigmatic more than 40 years after the brain was first recognized to be a site of action of angiotensin II. This review focuses on the enzymatic pathways for the formation and degradation of the growing number of active angiotensins in the brain. A brief description and nomenclature of the peptidases involved in the processing of angiotensin peptides in the brain is given. Of primary interest is the array of enzymes that degrade radiolabeled angiotensins in receptor binding assays. This poses major challenges to studies of brain angiotensin receptors and it is debatable whether an accurate determination of brain angiotensin receptor binding kinetics has yet been made. The quandary facing the investigator of brain angiotensin receptors is the need to protect the radioligand from metabolic alteration while maintaining the characteristics of the receptors in situ. It is the tenet of this review that we have yet to fully understand the binding characteristics of brain angiotensin receptors and the extent of their distribution in the brain because of our inability to fully protect the angiotensins from metabolic alteration until equilibrium binding conditions can be attained.

Plasma sodium and hypertension

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

Vasopressin mechanism-mediated pressor responses caused by central angiotensin II in the ovine fetus

AVP not only influences renal water excretion but also has profound cardiovascular effects in adults. Our recent studies have demonstrated that central angiotensin induced fetal pressor responses accompanied with AVP release. However, little is known of hormonal mechanisms in angiotensin-mediated fetal blood pressure (BP) changes. The present study determined AVP mechanisms in central angiotensin-mediated fetal pressor responses. The V1-receptor antagonist or V2-receptor antagonist was infused intravenously into the ovine fetus at 90% gestation. Angiotensin II (Ang II; 1.5 microg/kg) was then injected intracerebroventricularly into the chronically instrumented fetus. Ang II produced a significant increase in fetal systolic, diastolic, and mean arterial pressure adjusted to amniotic pressure (A-MAP). The enhanced fetal A-MAP was associated with intense c-fos expression in the central putative cardiovascular area: the paraventricular nuclei (PVN). Double labeling demonstrated that a number of the AVP-containing neurons in the PVN were expressing c-fos in response to central Ang II. Consistent with the activation of AVP neurons in the PVN, fetal plasma AVP was markedly enhanced. Fetal i.v. V1-receptor antagonist or V2-receptor antagonist had no effect on either fetal or maternal baseline BP. However, intracerebroventricular Ang II-increased BP was partially inhibited, although not completely abolished, by the V1-receptor blockade. In contrast, fetal i.v. infusion of V2-receptor antagonist had no effect on the pressor responses induced by central Ang II. The results suggest that the central Ang II-mediated pressor responses at the last third of gestation is mediated partially by the AVP mechanism via V1 not V2 receptors.

Effects of angiotensin II and AIII microinjections into the zona incerta after intra- and extracellular fluid loss

Our recent results showed that angiotensin II or III (AII, AIII) microinjected into the zona incerta (ZI) significantly increased water intake. The most effective doses of AII and AIII were also defined. The two neuropeptides had their effects differently on drinking via different receptors. AII bound to AT(1) that was blocked by AT(1) receptor antagonist Losartan and the effect of AIII was eliminated by prior application of AT(2) receptor antagonist PD 123319. After different hydrational challenges, the effects of AII and AIII in the ZI have never been experimented, however. In the present experiments, the previously defined effective doses of AII (100 ng) or AIII (200 ng) were microinjected into the ZI after different types of challenges: (1). lowered thirst motivation when animals ingested approximately 40% of their daily fluid need during the consequent 60-min-daily-drinking period before the injection, (2). 48-h water deprivation, (3). intracellular dehydration and (4). extracellular dehydration. In all of the cases, incertally injected AII increased the animals' water ingestion. While Losartan could block these effects, PD 123319 was ineffective. Experiments were repeated by AIII, but in none of the cases differences were experienced between the groups. The finding that following hydrational challenges water intake increased only after AII injections and it could be blocked only by Losartan suggests that AII and AT(1) receptor play a pivotal role in the ZI in maintaining the body water balance.

Different brain structures mediate drinking and sleep suppression elicited by the somatostatin analog, octreotide, in rats

When injected into the cerebral ventricles, the somatostatin analog, octreotide (OCT) elicits prompt drinking, vasopressin secretion and increases in blood pressure that are attributed to the activation of the intracerebral angiotensinergic system. In addition, OCT induces sleep responses that might be mediated by an inhibition of hypothalamic neurons producing growth hormone-releasing hormone (GHRH). OCT (0.02 microg in 0.2 microl) was microinjected into various brain sites to determine the structures inducing drinking and/or sleep suppression in response to OCT in rats. Drinking (>1 ml water in 10 min) was elicited in 17 rats out of 86 tested. The positive drinking sites resided in or around the subfornical organ (SFO) and the paraventricular nucleus. Both structures are part of the reported angiotensinergic dipsogenic circuit of the brain. These microinjections failed to elicit consistent sleep effects. Sleep suppression (>10% recording time in hour 1) was observed after injection of OCT either into the arcuate nucleus (n=7), where the majority of GHRHergic neurons reside, or into the medial preoptic area/anterior hypothalamus (n=8), where GHRH acts to promote sleep. Administration of OCT into far lateral sites of the lateral preoptic area and lateral hypothalamus stimulated sleep in hour 1 (n=10), perhaps via inhibiting cholinergic neurons previously implicated in arousal. The results are consistent with the hypothesis that somatostatin is involved in the regulation of both water intake and sleep, and suggest that different structures, and therefore different somatostatinergic neuronal pools, mediate these actions.

Central depletion of angiotensinogen is associated with elevated AT1 receptors in the SFO and PVN

The brain renin-angiotensin system (RAS) is important in fluid balance and blood pressure regulation. In this study, we compared angiotensin (Ang) receptor density in the subfornical organ (SFO) and paraventricular nucleus (PVN) of a) brain angiotensinogen deficient rats (ASrAogen); b) those with high levels of brain Ang II [(mRen2)27]; c) Hannover Sprague Dawley (SD) rats at 48 and 68 wks of age. Since there was no difference between the two ages in any of the three strains, the data from the 48 and 68 wk time points were combined. There was a significantly higher level of AT1 receptors in the SFO and PVN of ASrAogen animals compared to both the SD and (mRen2)27 rats. This suggests that the brain RAS is important in regulating receptor density and that the differences may be explained by lower levels of the peptide locally. These higher levels of receptors suggest that the ASrAogen animals in adulthood and early aging would be more sensitive to either circulating or endogenous brain Ang II than the SD animals of similar age. In contrast, the similar receptor density in the (mRen2)27 and SD rats suggest that previous reports of reduced responses in the (mRen2)27 rats may result from differences in post receptor mechanisms such as intracellular signaling. Moreover, our data reveal that functional assessments are necessary in addition to receptor density levels to understand the consequences of long-term alterations in brain tissue peptides.

Angiotensin II and III microinjections into the zona incerta influence drinking behavior

Different doses of angiotensin II (AII) or angiotensin III (AIII) microinjections into the zona incerta have been studied on drinking of rats in separate experiments during the consequent 60-min-daily-drinking period. Also, the dipsogen power of only the effective dose of AII and AIII was compared to vehicle treated rats. After, angiotensin receptor (AT(1), AT(2)) antagonists on AII or AIII induced drinking were tested. In the first and second experiments only the 100 ng AII and the 200 ng AIII increased water intake significantly. In the third experiment the AII started its dipsogen effect earlier, at the 5 min measuring time, compared to the AIII. Both effects kept on lasting parallel from the 10 min on. Considering the antagonist pre-treatments in the fourth experiment, animals were injected with 90 ng losartan, an AT(1) antagonist, 180 ng PD 123319 or 200 ng CGP 42112, both AT(2) antagonists, respectively. Both AII and AIII increased water consumption. The effect of AII could be blocked by losartan, but not by PD 123319 or CGP 42112. On the other hand, the effect of AIII could not be blocked by losartan, but by both the PD 123319 and CGP 42112. Since, the effects of AII, AIII and angiotensin antagonists have not been tested in the zona incerta, the finding that water intake increased after AII or AIII injections and it could be blocked only by either of the antagonists suggests that AT(1) and AT(2) receptors play partially different roles in the regulation of water intake.

The brain renin-angiotensin system: location and physiological roles

Angiotensinogen, the precursor molecule for angiotensins I, II and III, and the enzymes renin, angiotensin-converting enzyme (ACE), and aminopeptidases A and N may all be synthesised within the brain. Angiotensin (Ang) AT(1), AT(2) and AT(4) receptors are also plentiful in the brain. AT(1) receptors are found in several brain regions, such as the hypothalamic paraventricular and supraoptic nuclei, the lamina terminalis, lateral parabrachial nucleus, ventrolateral medulla and nucleus of the solitary tract (NTS), which are known to have roles in the regulation of the cardiovascular system and/or body fluid and electrolyte balance. Immunohistochemical and neuropharmacological studies suggest that angiotensinergic neural pathways utilise Ang II and/or Ang III as a neurotransmitter or neuromodulator in the aforementioned brain regions. Angiotensinogen is synthesised predominantly in astrocytes, but the processes by which Ang II is generated or incorporated in neurons for utilisation as a neurotransmitter is unknown. Centrally administered AT(1) receptor antagonists or angiotensinogen antisense oligonucleotides inhibit sympathetic activity and reduce arterial blood pressure in certain physiological or pathophysiological conditions, as well as disrupting water drinking and sodium appetite, vasopressin secretion, sodium excretion, renin release and thermoregulation. The AT(4) receptor is identical to insulin-regulated aminopeptidase (IRAP) and plays a role in memory mechanisms. In conclusion, angiotensinergic neural pathways and angiotensin peptides are important in neural function and may have important homeostatic roles, particularly related to cardiovascular function, osmoregulation and thermoregulation.

Systemic site of action for pressor effect of angiotensin II injected into the fourth cerebral ventricle of rats?

Angiotensin II (ANG II) causes a systemic pressor effect when injected into the cerebral ventricles. In the rat fourth ventricle, the effective doses for the ANG II pressor effect are over 100 times larger than in the systemic circulation. Considering the discrepancy of doses, the possibility that ANG II may reach the systemic circulation and promote pressor effects, following injection into the fourth ventricle, was investigated. The effects on blood pressure of different vasoactive peptides that produce pressor responses when injected into the central nervous system were compared. Dose-response curves were obtained for intravenous or fourth cerebroventricular injections of ANG II, lysyl-vasopressin (LVP), bradykinin (BK), or endothelin-1 (ET-1). The ED50 ratios for intracerebroventricular/intraveneous injections were 110 for ANG II, 109 for LVP, 0.01 for BK, and approximately 0.4 for ET-1. In cross-circulation preparations, pressor responses occurred in the donor rat following injection into the fourth cerebral ventricle of the recipient animal, showing that effective doses of ANG II, administered to the fourth cerebral, reach the systemic circulation. The same results were obtained for the microinjection of 4 nmol of LVP into the fourth cerebral ventricle of recipient animals. High-performance reverse-phase liquid chromatography analyses of arterial blood showed that approximately 1% of the [125I]ANG II injected into the fourth cerebral ventricle may be recovered from the systemic circulation a few seconds after the microinjection. The systemic administration of the ANG II receptor antagonist losartan blocked the response to ANG II injected into the fourth ventricle whereas antagonist administration in the same ventricle did not. Angiotensin injections into the lateral ventricle produced pressor responses that were reduced by antagonist administration to the same ventricle but not by systemic administration of the antagonist. The data suggest that the pressor effect resulting from ANG II or LVP injections into the fourth cerebral ventricle may be due to the action of this peptide in the systemic circulation. On the other hand, the pressor effect due to ANG II microinjection into the lateral ventricle apparently results from the direct stimulation of central periventricular structures.

The hypothalamus and hypertension

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

Nitric oxide modulates angiotensin II-induced drinking behavior in the near-term ovine fetus

OBJECTIVE

Human and ovine fetuses demonstrate an enhanced rate of spontaneous and angiotensin II-stimulated swallowing. Angiotensin II and nitric oxide synthase have been localized to thirst centers in the brain. This study was performed to determine whether central nitric oxide contributes to the regulation of angiotensin II-induced fetal swallowing.

STUDY DESIGN

Six pregnant ewes with near-term singleton fetuses were chronically prepared with fetal vascular and lateral ventricle catheters and electrocorticogram and esophageal electromyogram electrodes. After a 2-hour control period, fetuses were administered serial lateral ventricle injections (1 mL) of angiotensin II (3.2 microg; time, 2 hours) and N omega-nitro-L -arginine methyl ester (3 mg; time, 3 hours) and a repeat angiotensin II injection (3.2 microg; time, 5 hours). All fetuses received an additional control study of lateral ventricle injections of artificial cerebrospinal fluid on a previous day.

RESULTS

Angiotensin II injection significantly increased mean +/- SEM fetal swallowing (0.9 +/- 0.1 to 2.7 +/- 0.4 swallows/min). N omega-nitro-L -arginine methyl ester significantly decreased fetal swallowing to below the basal rate (0.4 +/- 0.1 swallows/min), and swallowing did not increase with the second angiotensin II dose (in the presence of nitric oxide blockade).

CONCLUSIONS

These results demonstrate that inhibition of central nitric oxide suppresses fetal swallowing behavior in response to central angiotensin II. We speculate that tonic nitric oxide facilitates angiotensin II swallowing stimulation by maintenance of glutamate activation of hypothalamic N -methyl-D -aspartate receptors.

The brain renin-angiotensin system contributes to the hypertension in mice containing both the human renin and human angiotensinogen transgenes

We have previously shown that mice transgenic for both the human renin and human angiotensinogen genes (RA+) exhibit appropriate tissue- and cell-specific expression of both transgenes, have 4-fold higher plasma angiotensin II (AII) levels, and are chronically hypertensive. However, the relative contribution of circulating and tissue-derived AII in causing hypertension in these animals is not known. We hypothesized that the brain renin-angiotensin system contributes to the elevated blood pressure in this model. To address this hypothesis, mean arterial pressure (MAP) and heart rate were measured in conscious, unrestrained mice after they were instrumented with intracerebroventricular cannulae and carotid arterial and jugular vein catheters. Intracerebroventricular administration of the selective AII type 1 (AT-1) receptor antagonist losartan (10 microgram, 1 microL) caused a significantly greater peak fall in MAP in RA+ mice than in nontransgenic RA- controls (-29+/-4 versus -4+/-2 mm Hg, P<0.01). To explore the mechanism of a central renin-angiotensin system-dependent hypertension in RA+ mice, we determined the relative depressor responses to intravenous administration of the ganglionic blocking agent hexamethonium (5 mg/kg) or an arginine vasopressin (AVP) V1 receptor antagonist (AVPX, 10 microgram/kg). Hexamethonium caused equal lowering of MAP in RA+ mice and controls (-46+/-3 versus -52+/-3, P>0.05), whereas AVPX caused a significantly greater fall in MAP in RA+ compared with RA- mice (-24+/-2 versus -6+/-1, P<0.01). Consistent with this was the observation that circulating AVP was 3-fold higher in RA+ mice than in control mice. These results suggest that increased activation of central AT-1 receptors, perhaps those located at sites involved in AVP release from the posterior pituitary gland, plays a role in the hypertension in RA+ mice. Furthermore, our finding that both human transgenes are expressed in brain regions of RA+ mice known to be involved in cardiovascular regulation raises the possibility that augmented local production of AII and increased activation of AT-1 receptors at these sites is involved.

Enhanced angiotensin II stimulates renal disorders in transgenic Tsukuba hypertensive mice

Tsukuba hypertensive mice (THMs) are transgenic mice carrying human renin-angiotensin system (RAS) genes. The aim of this study is to evaluate whether renal disorders are present in THMs. Twenty-week-old THMs and C57BL/6 mice (C57s) were used for this study. Each group consisted of 8 mice. Systolic blood pressure, urinary volume, water intake and urinary albumin excretion were measured in each mouse. Each mouse was then euthanized, and the renal glomerulosclerosis index and glomerular size were measured. Systolic blood pressure of THMs was about 40 mmHg higher than that of C57s. Urinary volume, water intake and urinary albumin excretion were significantly higher in THMs than in C57s. The renal glomerulosclerosis index and glomerular size were also significantly higher in THMs than in C57s. These results suggested that an enhanced renin-angiotensin system, including its hypertensive effects, stimulates albuminuria, renal glomerulosclerosis and glomerular hypertrophy in THMs.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Pressor effect mediated by bradykinin in the paratrigeminal nucleus of the rat

1. The participation of the paratrigeminal nucleus (Pa5) in the pressor response produced by bradykinin in the dorsolateral medulla of rats was investigated. The microinjection of 6 pmol of bradykinin directly over the paratrigeminal nucleus of unanaesthetized rats produced a significant increase in arterial pressure and a moderate increase in heart rate. 2. Bradykinin microinjections in different sites surrounding the Pa5 compromising the external cuneate nucleus, the trigeminal nucleus, the lateral and ventral spinal trigeminal tract and the dorsal trigeminal tract rostral and caudal to the Pa5 did not elicit significant pressor responses. In contrast, microinjections in the paratrigeminal nucleus produced pressor effects. Injections in the dorsolateral medulla directly over the paratrigeminal nucleus produced larger responses than when injections were made in the nucleus. Saline injections in the different nuclei did not produce pressor effects. 3. Neurochemical lesioning of the Pa5, with microinjections of ibotenic acid in the Pa5, abolished the pressor response to bradykinin injected over the lesioned nucleus. The effect was present, however, when bradykinin was injected on the contralateral side to the lesion, over the intact nucleus of the same animal. Pretreatment with capsaicin (injected in the lateral cerebral ventricle), which causes selective degeneration of afferent sensory fibres, did not alter the pressor effect of bradykinin injected over the paratrigeminal nucleus. 4. Dose-related responses were produced by different concentrations of bradykinin (0.6-1.8 pmol) microinjected over the nucleus. The bradykinin receptor antagonist HOE 140, injected over the paratrigeminal nucleus 30 min earlier, abolished the pressor response caused by bradykinin. 5. Low doses of bradykinin injected in or directly over the paratrigeminal nucleus increased arterial pressure and caused a small increase in heart rate by stimulating kinin receptors of the paratrigeminal nucleus in the dorsolateral medulla of awake and unrestrained rats. The pattern of the response was consistent with that of sympathetic stimulation. The paratrigeminal nucleus, which receives primary afferents and projects to the nucleus tractus solitarii and the rostral ventral lateral medulla, may be positioned as relay nucleus possibly connecting sensory input to structures that regulate blood pressure.

Angiotensin II interacts with nitric oxide-cyclic GMP pathway in the central control of drinking behaviour: mapping with c-fos and NADPH-diaphorase

Recognition of the role of nitric oxide in cell-to-cell communication has changed the concept of traditional neurotransmission. We have shown previously that N-methyl-D-aspartate receptors mediate dipsogenic responses and c-Fos expression induced by intracerebroventricular infusion of angiotensin II. Since these receptors are known to be linked to the nitric oxide-cyclic GMP pathway, the present study explores the contribution of this path to the behavioural and cellular effects of intracerebroventricular angiotensin II by using behavioural testing, NADPH-diaphorase histochemistry and immunocytochemical staining for the immediate-early gene, c-fos. N(G)-nitro-L-arginine methyl ester (125 and 250 microg, intracerebroventricular), an inhibitor of nitric oxide synthase, and Methylene Blue (100 microg), an inhibitor of guanylate cyclase activation, antagonized water intake induced by intracerebroventricular injection of 25 pmol angiotensin II. The effects of N(G)-nitro-L-arginine methyl ester were reversed by co-injection of L-arginine, the substrate for nitric oxide synthase. However, N(G)-nitro-L-arginine methyl ester did not alter the pattern of angiotensin II-induced c-fos expression in the organum vasculosum of the lamina terminalis, median preoptic nucleus, hypothalamic paraventricular nucleus and supraoptic nucleus. Double staining with NADPH-diaphorase histochemistry and c-Fos immunocytochemistry showed that neurons staining for both were localized to the anterior third ventricle. However, only 19-25% of the c-Fos-positive neurons expressed NADPH. There were also substantial numbers of neurons in which angiotensin II induced c-Fos that were NADPH-negative. Extensive co-distribution of NADPH-diaphorase-stained cells and those expressing c-fos in response to intracerebroventricular injection of angiotensin II, especially in the median preoptic nucleus, imply that nitric oxide might participate in the mechanism of angiotensin II-induced drinking behaviour. However, a low rate of co-localization of the two markers to individual cells suggests that angiotensin II stimulated the production of nitric oxide and c-Fos in different populations of neurons. Since our previous results showed that glutamate blockade, but not nitric oxide synthase inhibition, suppressed angiotensin II-induced c-Fos, the experiments reported here further suggest that nitric oxide release is not an essential requirement for the expression of c-fos elicited by angiotensin II. They also provide evidence that the dipsogenic and c-Fos responses to angiotensin II are dissociated at a cellular level.

C-type natriuretic peptide suppresses arginine-vasopressin secretion from dissociated magnocellular neurons in newborn rat supraoptic nucleus

Central administration of C-type natriuretic peptide (CNP) affects various neuroendocrine systems. In the present study, we examined whether CNP acts directly on arginine-vasopressin (AVP) secretion from rat supraoptic nucleus (SON) neurons, using acute dissociated cell preparations. CNP inhibited the basal secretion of AVP in a dose-dependent manner (10(-11)-10(-6) M). A- type natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) also suppressed the basal secretion of AVP, however, the effects were two-orders of magnitude less potent than CNP. CNP also suppressed All-induced AVP secretion, however, the inhibitory effect of CNP was less than that of ANP or BNP. These findings suggest that CNP inhibits the basal secretion of AVP through natriuretic peptide receptor (NPR)-B and has a role in the body water and electrolyte homeostasis in the central nervous system.

Aminopeptidase A antiserum inhibits intracerebroventricular angiotensin II-induced dipsogenic and pressor responses

Angiotensin II increases drinking and blood pressure when administered intracerebroventricularly. Intracerebroventricular injections of antiserum with anticatalytic activity against aminopeptidase A, the principal enzyme that metabolizes angiotensin II to angiotensin III, reduced the drinking and blood pressure responses to 10 pmol angiotensin II by 73% and 59%, respectively. APA antiserum had no effect on responses to angiotensin III administered intracerebroventricularly. A Glu-thiol inhibitor of aminopeptidase A also reduced angiotensin II-induced drinking. These results suggest that metabolism of angiotensin II to angiotensin III is an obligatory activation step for the brain angiotensin system.

The neuronal role of angiotensin II in thirst, sodium appetite, cognition and memory

Within the past two decades, a great deal has been learnt about the renin-angiotensin system in the brain. The renin-angiotensin system is one of the best-studied enzyme-neuropeptide systems in the brain. The diversity of localization of this peptide throughout the brain has implied a variety of potential functions. Besides its classical role in the regulation of blood pressure and body-fluid homeostasis, it has more subtle functions involving complex mechanisms such as learning and memory. The profound effects on behaviour produced by angiotensin are of broad interest to neuroscientists. The mechanisms of action differ depending on whether angiotensin is locally synthesized and whether regulation is governed by neural or metabolic inputs impinging on the neurones. Its central action is mediated through peptidergic receptors present on neurones. The description of the receptor subtypes AT1 and AT2 for angiotensin II and the development of non-peptidic specific angiotensin receptor subtype antagonists have opened a new area in this field of research. The AT1 site, which preferentially binds to angiotensin II and angiotensin III, appears to mediate the classical angiotensin functions concerned with maintenance of blood pressure and body-fluid control. In addition, most of the behavioural effects described so far are linked with AT1, although so-called psychotropic effects are presumed to be mediated by receptor systems other than the known specific angiotensin receptors. In fact, evidence for the existence of such receptors with high-affinity binding has been reported. The central action of angiotensin II mediated by AT2 is as yet unclear. Most reports concerning this receptor subtype suggest a role in differentiation and development, since the number of binding sites is higher in fetal and young rats than in adults. Furthermore, the neuronal effect of angiotensin II in the inferior olivary nucleus which is blocked specifically by AT2 antagonists suggests an involvement in motor control. Over the next few years we should find answers to many of the questions currently unanswered about angiotensin function and, given the rapid progress in research on this neuropeptide, it may serve as a model for the action of peptides on neuronal function in general.

The role of brain angiotensin in thirst and AVP release induced by hemorrhage

This study investigated the role of brain angiotensin (Ang II) in thirst induced by hemorrhage. Hemorrhage by blood withdrawal from the femoral artery to 33% and 44% blood volume loss produces a dose response increase in plasma Ang II. In the brainstem there was no Ang II response to hemorrhage. In the hypothalamus, Brain Ang II was maximally elevated to 33% hemorrhage. Thus, plasma Ang II and brain Ang II had an independent response to hemorrhage. To further test the role of central versus peripheral Ang II, we tested the effect of central (50 mg) and peripheral (50 mg/kg) administration of captopril or central injection of 1 mg losartan or 3 mg CGP 42112A prior to a 33% hemorrhage in unanesthetized male Sprague-Dawley rats (250 g). Drinking was measured and AVP blood samples were taken before and after hemorrhage. The results show that central (i.v.t.) administration of captopril and losartan inhibited drinking compared to controls (0.33 +/- 0.3 ml vs. 2.3 +/- 0.8 ml: P < 0.05 and 0.20 +/- 0.09 ml vs.3.05 +/- 0.81 ml; P < 0.01, respectively) while peripheral (i.p.) captopril alone increased drinking in response to hemorrhage (5.81 +/- 0.81 ml vs. 2.3 +/- 0.8 ml; P < 0.05). AVP levels were elevated at 5 and 15 min, but neither injections of losartan or CGP 42112A i.v.t. affected this response to hemorrhage. We conclude that increased hypothalamic brain Ang II after hypovolemic hemorrhage stimulates thirst and blood pressure restoration and acts through AT1 receptors. The release of AVP in hemorrhage, however, does not rely exclusively on the angiotensinergic pathway in the brain.

Brain angiotensin and circulatory control

1. Components of the renin-angiotensin system (RAS) are found in the brain; both outside and inside the blood-brain barrier. 2. Almost all of the classical actions of the brain RAS are attributable to angiotensin (Ang) II and mediated by AT1 receptors. 3. Circumventricular organs (CVO), which lack the blood-brain barrier, are rich in AngII receptors and monitor circulating AngII levels. In vivo binding studies suggest that the CVO are also accessible to cerebrospinal fluid-derived AngII. 4. The median preoptic nucleus, paraventricular hypothalamic nucleus, supraoptic nucleus, nucleus tractus solitarius and ventrolateral medulla are inside the blood-brain barrier and are sites of action of brain AngII. In these nuclei, AngII seems to act as an excitatory neurotransmitter or neuromodulator. 5. Actions of AngII in the brain, both inside and outside the blood-brain barrier, are implicated in the central regulation of blood pressure and sympathetic outflow, release of hypothalamic and pituitary hormones and renal sodium handling. 6. Alterations in the activity of brain AngII may be involved in the mechanisms of some types of hypertension.

Regulation of angiotensin II type 2 receptor gene by the protein kinase C-calcium pathway

In the present study, rat angiotensin II type 2 (AT2) receptor expression was upregulated in confluence-arrested PC12 cells compared with expression in proliferating cells. Treatment with cycloheximide inhibited the increase in mRNA levels in confluent cells. The state of growth arrest by serum deprivation was associated with increased expression of the AT2 receptor, which was markedly suppressed by exposure to the active phorbol ester 12-O-tetradecanoylphorbol 13-acetate and the calcium ionophore A23187. Similar inhibitions were also observed in myocytes isolated from neonatal rat heart. The change in AT2 mRNA levels by serum deprivation was due to the increase in the gene transcription rate. The effect of 12-O-tetradecanoylphorbol 13-acetate was mediated through decreases in gene transcription and mRNA stability, whereas A23187 affected mRNA stability. Vasoactive substances with the protein kinase C-calcium pathway, such as norepinephrine and angiotensin II, also downregulated the AT2 mRNA level in myocytes. These findings indicate that the expression of AT2 receptor in PC12 cells is regulated in a growth state-dependent manner, which is involved in confluence-induced new protein synthesis, thus providing a means by which cells can modulate their responsiveness to external angiotensin II stimulus. The activation of protein kinase C or calcium mobilization modifies this regulatory mechanism, suggesting that neurotransmitters or vasoactive substances with the protein kinase C-calcium pathway at least in part affect neuronal activity or blood pressure control by downregulating AT2 receptor expression.

GABAergic inhibitory inputs to subfornical organ neurons in rat slice preparations

To investigate GABAergic inhibitory inputs to neurons of the subfornical organ (SFO), intracellular recordings were made in rat brain slice preparations. Inhibitory postsynaptic potentials, which occurred spontaneously or were evoked by focal electric stimulation, had reversal potentials of approximately -60 mV, and were almost totally abolished by the GABAA antagonists bicuculline at 3-100 microM or picrotoxin at 50 microM. Following the application of bicuculline or picrotoxin, the resting membrane potentials were decreased by 4-8 mV. GABA at 10-100 microM and the GABAA agonist muscimol at 1-100 microM decreased the membrane resistance and the firing rate in all neurons tested. The reversal potential of the response to muscimol was similar to that for inhibitory postsynaptic potentials. The actions of muscimol persisted in the presence of 1 microM tetrodotoxin, implying that muscimol must act directly on the recorded neurons. These results suggest that there is a tonic inhibitory GABAergic input to SFO neurons which are mainly mediated through GABAA receptors.

Intracerebroventricular administration of angiotensin type 1 (AT1) receptor antisense oligonucleotides attenuate thirst in the rat

The central actions of the peptide hormone angiotensin II (AngII) are importantly involved in body fluid homeostasis. Included amongst these actions is a potent dipsogenic response that has been implicated in the thirst that develops during many forms of extracellular dehydration. The use of highly selective receptor antagonists has revealed that the Type 1 (AT1), and not the Type 2 (AT2), AngII receptor subtype mediates this drinking response. More recently, antisense oligonucleotides specific for the AT1 receptor have been developed and after intracerebroventricular (i.c.v.) administration, they significantly reduce the dipsogenic response elicited by a similar injection of AngII. In the present study AT1 antisense oligonucleotides were used to further investigate their effect on experimentally induced thirst in the rat. In addition, immunohistochemical analysis of biotin-labeled oligonucleotides was performed in order to correlate the behavioral effects of the oligonucleotides with their distribution in the brain. The results demonstrated that the antidipsogenic effects of the oligonucleotides were dose and time-dependent and were limited to those thirst challenges that involve activation of the renin-angiotensin system. Collectively, these results demonstrate the efficacy and behavioral specificity of these oligonucleotides, as well as their utility in investigating the physiological role of cerebral AngII receptor subpopulations in various models of thirst.

The Neuro-2a neuroblastoma cell line expresses [Met]-enkephalin and vasopressin mRNA and peptide

Mouse neuroblastoma Neuro-2a cells were examined for the expression of pro-enkephalin mRNA, protein, and Met-enkephalin ([Met]-Enk) peptide. Reverse transcriptase/polymerase chain reaction (RT/PCR) and in situ hybridization demonstrated the presence of pro-enkephalin mRNA in these cells. Immunocytochemistry using an antibody which recognizes pro-enkephalin and high pressure liquid chromatography (HPLC) followed by radioimmunoassay indicated that pro-enkephalin was synthesized in these cells and processed to yield the bioactive pentapeptide, [Met]-Enk. Furthermore, release studies showed that the [Met]-Enk was secreted from these cells with high K+ stimulation. Using double labeling, in situ hybridization combined with immunocytochemistry, we demonstrated that prohormone convertase 2 (PC2) mRNA is colocalized with pro-enkephalin in the same Neuro-2a cells, suggesting that this enzyme may be responsible for processing this precursor. we also showed the presence of vasopressin mRNA and arginine-vasopressin peptide in these cells using in situ hybridization and immunocytochemistry, respectively. Thus, the Neuro-2a cells are a multiple neuropeptide-producing cell line and an excellent model for studying the mechanisms involved in the synthesis, intracellular targeting and processing of endogenous pro-enkephalin and pro-vasopressin, as well as other transfected neuropeptide precursors.

The angiotensin type 1 and type 2 receptor families. Siblings or cousins?

The diverse actions of angiotensin II (AngII) are mediated by cell surface receptors. Molecular cloning techniques have identified two distinct subtypes of AngII receptors referred to as AT1 and AT2. It is now well accepted that multiple forms of the AT1 receptor exist, but similar diversity of the AT2 subtype has not been conclusively demonstrated. Nonetheless, several converging lines of evidence do suggest that multiple AT2 receptors may be present in brain and cultured neuron-like cells lines. For instance, some AT2 receptors are regulated by guanine nucleotides and sulfhydryl-reducing agents, whereas others are insensitive. AT2 receptor populations also exhibit differing pharmacological profiles particularly with respect to their affinity for peptidic and non-peptidic ligands. Moreover, a recently developed anti-AT2 polyclonal antisera reveals a unique pattern of immunohistochemical staining in brain and it does not immunoreact with the recently cloned AT2 receptor. Collectively, these results support the hypothesis of multiple AT2 receptors at least within the CNS. Future studies should reveal whether these putative AT2 receptor subtypes result from unique genes or cell-specific post-translational modifications of a single gene product.

Neuronal expression of Fos protein in the rat brain after baroreceptor stimulation

The purpose of this study was to identify the CNS neurons that express Fos protein after repeated activation of the baroreceptor reflex. This was done in Wistar rats anesthetized with urethane and alpha-chloralose with careful physiological controls. The intact control rat showed few Fos-immunoreactive (ir) neurons, whereas the anesthetized control rat showed many Fos-ir neurons in the CNS from the medulla oblongata to the forebrain. After repeated stimulation of baroreceptors by pressor responses to phenylephrine (dose), we counted the amounts of Fos-ir neurons (response). The correlation coefficient of the dose-response relationship was high, and significant only in the medial part of the nucleus tractus solitarii (NTS) in the medulla and periaqueductal gray (PAG) in the midbrain, whereas it was comparatively high but insignificant in the commissure and lateral parts of the NTS, caudal and rostral ventrolateral medulla, periambiguus nucleus, dorsal and ventral medullary reticular nuclei, lateral parabrachial nucleus, paraventricular nucleus thalamus, and dorsomedial nucleus hypothalamus. No significant correlation was found in the humoral control nuclei in the preoptico-hypothalamic structure. Fos expression was never detected in the sensory neurons in the ganglia petrosum and nodosum, and in the sympathetic preganglionic neurons in the intermediolateral nucleus of the thoracic spinal cord. This study shows that Fos expression in the CNS neurons is induced not only by baroreceptor stimulation but also by anesthesia and/or sham-operation, and that Fos expression in the NTSm and PAG neurons faithfully responds to baroreceptor stimulation.

The effects of central administration of angiotensin II type-1 receptor antagonist, CV-11974, in nephrectomized spontaneously hypertensive rats

1. The role of the brain renin-angiotensin system in the pathogenesis of genetic hypertension was evaluated using a specific non-peptide angiotensin II type-1 receptor antagonist, TCV-116. 2. CV-11974 (active metabolite of TCV-116) was acutely injected either intravenously (i.v.) or intracerebroventricularly (i.c.v) in male spontaneously hypertensive rats (SHR; 12 week old). In separate groups of nephrectomized and sham-operated SHR, graded doses of CV-11974 were administered either i.v. or i.c.v. for 2 days using an osmotic minipump. In another group, the effects of nephrectomy on the depressor effect of chronic treatment with CV-11974 were investigated. Haemodynamics at three points: before infusion, before nephrectomy and 48 h after nephrectomy, were monitored. 3. Acute i.c.v. injection of CV-11974 decreased blood pressure in the presence of the kidney. Prolonged i.c.v. administration of the drug for 2 days decreased blood pressure even at the lowest dosage, which had no hypotensive effects when given i.v. The hypotensive effect of centrally administered CV-11974 was noted even 48 h after bilateral nephrectomy. 4. These results suggest that the brain renin-angiotensin system has a primary role in the maintenance of hypertension after eliminating the circulating renin-angiotensin system in SHR.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Involvement of angiotensin receptor subtypes in osmotically induced release of vasopressin

We have previously shown that AT1 and AT2 angiotensin II (Ang II) receptors mediate the release of arginine vasopressin (AVP) to central injections of Ang II. In this study we have tested the hypothesis that Ang II, acting at AT1 and AT2 receptors in the brain, is involved in mediating osmotically stimulated AVP release. Adult Sprague-Dawley rats were fitted with intraventricular (i.v.t.) cannulas and catheters in the carotid artery and the femoral vein. Intraventricular injections of Ang II receptor antagonists specific to different subtypes of the receptor (AT1 and AT2) were given before a 30 min infusion of hypertonic (2.5 M) saline. Arterial blood samples were collected 5 min before and at two time points after (+15 min and +30 min) beginning the saline infusion. We found that both losartan (AT1 specific) and CGP42112A (AT2 specific) significantly reduced osmotically induced release of AVP. PD123319 (AT2 specific) had no effect of osmotically stimulated AVP release. A combined treatment of losartan + PD123319 was no more effective than losartan in blocking the AVP response. Since losartan was the most rapid and effective antagonist of osmotically stimulated AVP release, we conclude that AT1 receptors are directly involved in the response. However, but since CGP42112A was also an effective antagonist and since, AT2 receptors are located at sites distant from the hypothalamus, such as the locus ceruleus, they may also contribute to this response. We conclude that brain Ang II receptors are involved in osmotically stimulated AVP release.

Forebrain lesions that disrupt water homeostasis do not eliminate the sodium appetite of sodium deficiency in sheep

Brain structures located within the anterior wall of the third brain ventricle (subfornical organ, median preoptic nucleus and organum vasculosum of the lamina terminalis) are known to be involved in thirst as well as other aspects of body fluid and electrolyte balance. The present studies evaluated the role of these structures in the Na appetite of mildly or moderately Na-depleted sheep (sheep with a parotid fistula deprived of Na solution for 22 or 46 h). In addition, the role of these structures was tested in mildly Na-depleted sheep in which the Na appetite was enhanced by decreasing cerebrospinal fluid and brain extracellular fluid Na concentration (i.e., i.c.v. infusion of hypertonic saccharide solution) or was decreased by systemic infusion of hypertonic saline. The results indicated that sheep with lesions which reduced or eliminated daily water intake or water intake in response to hypertonicity of body fluids had, in all situations tested, appropriate changes in Na appetite (i.e., similar to their prelesion changes). Thus, the present experiments demonstrated that the brain areas involved in thirst as well as other aspects of body fluid and electrolyte balance are anatomically different from those involved in regulating Na appetite.

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

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

Losartan potassium, a nonpeptide antagonist of angiotensin II, chronically administered p.o. does not readily cross the blood-brain barrier

Recently several novel nonpeptide antagonists of angiotensin II (Ang II) have been identified. One of these, losartan potassium (formerly DuP 753) was developed as an orally active and highly selective antagonist for Ang II. As it is inhibited by sulfhydryl agents, it is specific for the AT1 receptor subtype. Since Ang II has both central and peripheral effects, we investigated whether losartan, given p.o. chronically, crosses the blood-brain barrier. The effects of chronic administration of losartan orally (p.o.) at 3 mg/kg per day for three days on the dipsogenic and pressor responses to a pre-established dose of Ang II i.v.t. (50 ng) were studied. Three series of experiments were carried out using conscious normotensive Sprague-Dawley rats. The rats were injected with Ang II intraventricularly (i.v.t.) before and after treatment of losartan p.o. and blood pressure and drinking responses measured. The experiments established that 3 mg/kg losartan p.o. for 3 days antagonized pressor effects of Ang II intravenously (i.v.), but did not antagonize the pressor or drinking effects of Ang II i.v.t. Daily water intake significantly increased with chronic losartan p.o.. Since chronic administration of losartan p.o. was able to block the effects of Ang II i.v. but had no effect on Ang II i.v.t. we conclude that losartan potassium does not readily cross the blood-brain barrier using this dose regimen.

Three-dimensional structure of an angiotensin II-Fab complex at 3 A: hormone recognition by an anti-idiotypic antibody

The elucidation of bioactive conformations of small peptide hormones remains an elusive goal to structural chemists because of the inherent flexibility of these molecules. Angiotensin II (AII), the major effector of the renin-angiotensin system, is an octapeptide hormone for which no clear structural models exist. Peptide hormones such as AII share the property that they bind to their receptors with high affinities, in spite of the fact that they must overcome an extremely large conformational entropy barrier to bind in one conformation. A "surrogate system" that consists of a high-affinity monoclonal antibody (MAb) and AII has been used to study a bound conformation of AII. The crystallographic structure of the complex reveals a structure of AII that is compatible with predicted bioactive conformations of AII derived from structure-activity studies and theoretical calculations. In the complex, the deeply bound hormone is folded into a compact structure in which two turns bring the amino and carboxyl termini close together. The antibody of this complex (MAb 131) has the unusual property that it was not generated against AII, but rather against an anti-idiotypic antibody reactive with a MAb to AII, which renders this antibody an anti-anti-idiotypic antibody. The high affinity for AII of the original MAb to AII was passed on to MAb 131 through a structural determinant on the anti-idiotypic antibody. Strikingly, the conformation of AII in this complex is highly similar to complementarity determining region loops of antibodies, possibly indicating that a true molecular mimic of bound AII was present on the anti-idiotypic antibody against which MAb 131 was elicited.