Angiotensin-converting enzyme is present in the subfornical organ and other circumventricular organs of the rat

Understanding the heart-brain axis response in COVID-19 patients: A suggestive perspective for therapeutic development

In-depth characterization of heart-brain communication in critically ill patients with severe acute respiratory failure is attracting significant interest in the COronaVIrus Disease 19 (COVID-19) pandemic era during intensive care unit (ICU) stay and after ICU or hospital discharge. Emerging research has provided new insights into pathogenic role of the deregulation of the heart-brain axis (HBA), a bidirectional flow of information, in leading to severe multiorgan disease syndrome (MODS) in patients with confirmed infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Noteworthy, HBA dysfunction may worsen the outcome of the COVID-19 patients. In this review, we discuss the critical role HBA plays in both promoting and limiting MODS in COVID-19. We also highlight the role of HBA as new target for novel therapeutic strategies in COVID-19 in order to open new translational frontiers of care. This is a translational perspective from the Italian Society of Cardiovascular Researches.

Central nervous system circuits modified in heart failure: pathophysiology and therapeutic implications

The pathophysiology of heart failure (HF) is characterized by an abnormal activation of neurohumoral systems, including the sympathetic nervous and the renin-angiotensin-aldosterone systems, which have long-term deleterious effects on the disease progression. Perpetuation of this neurohumoral activation is partially dependent of central nervous system (CNS) pathways, mainly involving the paraventricular nucleus of the hypothalamus and some regions of the brainstem. Modifications in these integrative CNS circuits result in the attenuation of sympathoinhibitory and exacerbation of sympathoexcitatory pathways. In addition to the regulation of sympathetic outflow, these central pathways coordinate a complex network of agents with an established pathophysiological relevance in HF such as angiotensin, aldosterone, and proinflammatory cytokines. Central pathways could be potential targets in HF therapy since the current mainstay of HF pharmacotherapy aims primarily at antagonizing the peripheral mechanisms. Thus, in the present review, we describe the role of CNS pathways in HF pathophysiology and as potential novel therapeutic targets.

Role of neurons and glia in the CNS actions of the renin-angiotensin system in cardiovascular control

Despite tremendous research efforts, hypertension remains an epidemic health concern, leading often to the development of cardiovascular disease. It is well established that in many instances, the brain plays an important role in the onset and progression of hypertension via activation of the sympathetic nervous system. Further, the activity of the renin-angiotensin system (RAS) and of glial cell-mediated proinflammatory processes have independently been linked to this neural control and are, as a consequence, both attractive targets for the development of antihypertensive therapeutics. Although it is clear that the predominant effector peptide of the RAS, ANG II, activates its type-1 receptor on neurons to mediate some of its hypertensive actions, additional nuances of this brain RAS control of blood pressure are constantly being uncovered. One of these complexities is that the RAS is now thought to impact cardiovascular control, in part, via facilitating a glial cell-dependent proinflammatory milieu within cardiovascular control centers. Another complexity is that the newly characterized antihypertensive limbs of the RAS are now recognized to, in many cases, antagonize the prohypertensive ANG II type 1 receptor (AT1R)-mediated effects. That being said, the mechanism by which the RAS, glia, and neurons interact to regulate blood pressure is an active area of ongoing research. Here, we review the current understanding of these interactions and present a hypothetical model of how these exchanges may ultimately regulate cardiovascular function.

Proinflammatory cytokines upregulate sympathoexcitatory mechanisms in the subfornical organ of the rat

Our previous work indicated that the subfornical organ (SFO) is an important brain sensor of blood-borne proinflammatory cytokines, mediating their central effects on autonomic and cardiovascular function. However, the mechanisms by which SFO mediates the central effects of circulating proinflammatory cytokines remain unclear. We hypothesized that proinflammatory cytokines act within the SFO to upregulate the expression of excitatory and inflammatory mediators that drive sympathetic nerve activity. In urethane-anesthetized Sprague-Dawley rats, direct microinjection of tumor necrosis factor (TNF)-α (25 ng) or interleukin (IL)-1β (25 ng) into SFO increased mean blood pressure, heart rate, and renal sympathetic nerve activity within 15 to 20 minutes, mimicking the response to systemically administered proinflammatory cytokines. Pretreatment of SFO with microinjections of the angiotensin II type-1 receptor blocker losartan (1 μg), angiotensin-converting enzyme inhibitor captopril (1 μg) or cyclooxygenase-2 inhibitor NS-398 (2 μg) attenuated those responses. Four hours after the SFO microinjection of TNF-α (25 ng) or IL-1β (25 ng), mRNA for angiotensin-converting enzyme, angiotensin II type-1 receptor, TNF-α and the p55 TNF-α receptor, IL-1β and the IL-1R receptor, and cyclooxygenase-2 had increased in SFO, and mRNA for angiotensin-converting enzyme, angiotensin II type-1 receptor, and cyclooxygenase-2 had increased downstream in the hypothalamic paraventricular nucleus. Confocal immunofluorescent images revealed that immunoreactivity for the p55 TNF-α receptor and the IL-1 receptor accessory protein, a subunit of the IL-1 receptor, colocalized with angiotensin-converting enzyme, angiotensin II type-1 receptor-like, cyclooxygenase-2, and prostaglandin E2 EP3 receptor immunoreactivity in SFO neurons. These data suggest that proinflammatory cytokines act within the SFO to upregulate the expression of inflammatory and excitatory mediators that drive sympathetic excitation.

Angiotensin II's role in sodium lactate-induced panic-like responses in rats with repeated urocortin 1 injections into the basolateral amygdala: amygdalar angiotensin receptors and panic

Rats treated with three daily urocortin 1 (UCN) injections into the basolateral amygdala (BLA; i.e., UCN/BLA-primed rats) develop prolonged anxiety-associated behavior and vulnerability to panic-like physiological responses (i.e., tachycardia, hypertension and tachypnea) following intravenous infusions of 0.5 M sodium lactate (NaLac, an ordinarily mild interoceptive stressor). In these UCN-primed rats, the osmosensitive subfornical organ (SFO) may be a potential site that detects increases in plasma NaLac and mobilizes panic pathways since inhibiting the SFO blocks panic following NaLac in this model. Furthermore, since SFO neurons synthesize angiotensin II (A-II), we hypothesized that the SFO projects to the BLA and releases A-II to mobilizing panic responses in UCN/BLA-primed rats following NaLac infusions. To test this hypothesis, rats received daily bilateral injections of UCN or vehicle into the BLA daily for 3 days. Five to seven days following the intra-BLA injections, we microinjected either the nonspecific A-II type 1 (AT1r) and 2 (AT2r) receptor antagonist saralasin, or the AT2r-selective antagonist PD123319 into the BLA prior to the NaLac challenge. The UCN/BLA-primed rats pre-injected with saralasin, but not PD123319 or vehicle, had reduced NaLac-induced anxiety-associated behavior and panic-associated tachycardia and tachypnea responses. We then confirmed the presence of AT1rs in the BLA using immunohistochemistry which, combined with the previous data, suggest that A-II's panicogenic effects in the BLA is AT1r dependent. Surprisingly, the SFO had almost no neurons that directly innervate the BLA, which suggests an indirect pathway for relaying the NaLac signal. Overall these results are the first to implicate A-II and AT1rs as putative neurotransmitter-receptors in NaLac induced panic-like responses in UCN/BLA-primed rats.

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

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

beta-Amyloid degradation and Alzheimer's disease

Extensive β-amyloid (Aβ) deposits in brain parenchyma in the form of senile plaques and in blood vessels in the form of amyloid angiopathy are pathological hallmarks of Alzheimer's disease (AD). The mechanisms underlying Aβ deposition remain unclear. Major efforts have focused on Aβ production, but there is little to suggest that increased production of Aβ plays a role in Aβ deposition, except for rare familial forms of AD. Thus, other mechanisms must be involved in the accumulation of Aβ in AD. Recent data shows that impaired clearance may play an important role in Aβ accumulation in the pathogenesis of AD. This review focuses on our current knowledge of Aβ-degrading enzymes, including neprilysin (NEP), endothelin-converting enzyme (ECE), insulin-degrading enzyme (IDE), angiotensin-converting enzyme (ACE), and the plasmin/uPA/tPA system as they relate to amyloid deposition in AD.

Circulating signals as critical regulators of autonomic state--central roles for the subfornical organ

To maintain homeostasis autonomic control centers in the hypothalamus and medulla must respond appropriately to both external and internal stimuli. Although protected behind the blood-brain barrier, neurons in these autonomic control centers are known to be influenced by changing levels of important signaling molecules in the systemic circulation (e.g., osmolarity, glucose concentrations, and regulatory peptides). The subfornical organ belongs to a group of specialized central nervous system structures, the circumventricular organs, which are characterized by the lack of the normal blood-brain barrier, such that circulating lipophobic substances may act on neurons within this region and via well-documented efferent neural projections to hypothalamic autonomic control centers, influence autonomic function. This review focuses on the role of the subfornical organ in sensing peripheral signals and transmitting this information to autonomic control centers in the hypothalamus.

The renin angiotensin system and the metabolic syndrome

The renin angiotensin system (RAS; most well-known for its critical roles in the regulation of cardiovascular function and hydromineral balance) has regained the spotlight for its potential roles in various aspects of the metabolic syndrome. It may serve as a causal link among obesity and several co-morbidities. Drugs that reduce the synthesis or action of angiotensin-II (A-II; the primary effector peptide of the RAS) have been used to treat hypertension for decades and, more recently, clinical trials have determined the utility of these pharmacological agents to prevent insulin resistance. Moreover, there is evidence that the RAS contributes to body weight regulation by acting in various tissues. This review summarizes what is known of the actions of the RAS in the brain and throughout the body to influence various metabolic disorders. Special emphasis is given to the role of the RAS in body weight regulation. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Vascular expression of germinal ACE fails to maintain normal blood pressure in ACE-/- mice

Maintenance of normal blood pressure is critical for preserving the integrity of the cardiovascular system. Angiotensin 1-converting enzyme (ACE) regulates normal blood pressure and fluid homeostasis through its action in the renin-angiotensin-aldosterone system (RAAS) and the renal tubuloglomerular feedback response. Although the two structurally related isozymic forms of ACE both generate the vasoactive octapeptide angiotensin II (Ang II) with equal efficiency, both are expressed in a nonoverlapping tissue-restricted fashion. To discriminate the precise physiological role of each ACE in its requisite tissue in vivo, we expressed one ACE isoform exclusively in a single cell type of an Ace null mouse. Previously, we demonstrated that vascular endothelial cell-specific expression of transgenic somatic ACE (sACE) could restore normal blood pressure of Ace-null mice. In this current study, we expressed germinal ACE (gACE) in the vascular endothelial cells of the Ace null mouse. These mice exhibited correct renal structure, renal function, and normal growth rates. Although the mice had elevated levels of gACE bound to vascular endothelial cells and high levels of gACE and Ang II in the circulating serum, blood pressure was restored only partially. This study demonstrated that gACE, even when expressed in the vasculature, could not functionally substitute for sACE.

Angiotensin-II is a putative neurotransmitter in lactate-induced panic-like responses in rats with disruption of GABAergic inhibition in the dorsomedial hypothalamus

Intravenous sodium lactate infusions or the noradrenergic agent yohimbine reliably induce panic attacks in humans with panic disorder but not in healthy controls. However, the exact mechanism of lactate eliciting a panic attack is still unknown. In rats with chronic disruption of GABA-mediated inhibition in the dorsomedial hypothalamus (DMH), achieved by chronic microinfusion of the glutamic acid decarboxylase inhibitor L-allylglycine, sodium lactate infusions or yohimbine elicits panic-like responses (i.e., anxiety, tachycardia, hypertension, and tachypnea). In the present study, previous injections of the angiotensin-II (A-II) type 1 receptor antagonist losartan and the nonspecific A-II receptor antagonist saralasin into the DMH of "panic-prone" rats blocked the anxiety-like and physiological components of lactate-induced panic-like responses. In addition, direct injections of A-II into the DMH of these panic-prone rats also elicited panic-like responses that were blocked by pretreatment with saralasin. Microinjections of saralasin into the DMH did not block the panic-like responses elicited by intravenous infusions of the noradrenergic agent yohimbine or by direct injections of NMDA into the DMH. The presence of the A-II type 1 receptors in the region of the DMH was demonstrated using immunohistochemistry. Thus, these results implicate A-II pathways and the A-II receptors in the hypothalamus as putative substrates for sodium lactate-induced panic-like responses in vulnerable subjects.

Brain angiotensin-converting enzyme activity and autonomic regulation in heart failure

Several recent studies suggest an important role for the brain renin-angiotensin system in the pathogenesis of heart failure. Angiotensin-converting enzyme (ACE) activity and binding of angiotensin type 1 (AT1) receptors, which mediate the central effects of ANG II, are increased in heart failure. The present study examined the relationship between brain ACE activity and the autonomic dysregulation characteristic of rats with congestive heart failure. Rats with heart failure (HF) induced by coronary artery ligation and sham-operated control (SHAM) rats were treated with chronic (28 days) third cerebral ventricle [intracerebroventricular (ICV)] or intraperitoneal (IP) infusion of a low dose of the ACE inhibitor enalaprilat (ENL) or vehicle (VEH). VEH-treated HF rats had increased sodium consumption, reduced urine sodium and urine volume, and increased sympathetic nerve activity with impaired baroreflex regulation. These responses were minimized or prevented by ICV ENL started 24 h after coronary ligation. IP ENL at the low dose used in these studies had no beneficial effects on HF rats. Neither IP nor ICV ENL had any substantial effect on the SHAM rats. The findings confirm a critically important contribution of the brain renin-angiotensin system to the pathophysiology of congestive heart failure.

Angiotensin II receptors in the arcuate nucleus mediate stress-induced reduction of prolactin secretion in steroid-primed ovariectomized and lactating rats

Angiotensin II (Ang II) is a peptide that exerts an inhibitory effect upon pituitary prolactin (PRL) release through the hypothalamic arcuate nucleus (ARC). Since both PRL and Ang II are known to be affected by stress, the experiments reported here were conducted to investigate the possible participation of Ang II in the stress-induced response of PRL in situations in which pre-stress PRL levels are high, as during the PRL surge induced by estradiol (E(2)) and progesterone (P) in ovariectomized rats (OVXE(2)P) and lactating females on day 7 post-partum. Adult female rats were stereotactically implanted with bilateral guide-cannulae in the ARC; 3 days later, they were microinjected with saline or losartan and, after a 15-min interval, they were submitted to stress by ether inhalation during 1 min. Five minutes after stress, trunk blood samples were collected. Plasma PRL was measured by radioimmunoassay (RIA). In OVXE(2)P and lactating rats, a significant reduction in PRL levels was detected after stress compared to non-stressed animals. The microinjection of losartan in the ARC before stress blocked the reduction of PRL in both OVXE(2)P and lactating females. In conclusion, the stress-induced reduction of plasma PRL in OVXE(2)P and lactating rats is mediated by Ang II through AT(1) receptors in the ARC.

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.

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.

Heart failure and the brain: new perspectives

Despite recent therapeutic advances, the prognosis for patients with heart failure remains dismal. Unchecked neurohumoral excitation is a critical element in the progressive clinical deterioration associated with the heart failure syndrome, and its peripheral manifestations have become the principal targets for intervention. The link between peripheral systems activated in heart failure and the central nervous system as a source of neurohumoral drive has therefore come under close scrutiny. In this context, the forebrain and particularly the paraventricular nucleus of the hypothalamus have emerged as sites that sense humoral signals generated peripherally in response to the stresses of heart failure and contribute to the altered volume regulation and augmented sympathetic drive that characterize the heart failure syndrome. This brief review summarizes recent studies from our laboratory supporting the concept that the forebrain plays a critical role in the pathogenesis of ischemia-induced heart failure and suggesting that the forebrain contribution must be considered in designing therapeutic strategies. Forebrain signaling by neuroactive products of the renin-angiotensin system and the immune system are emphasized.

The renin-angiotensin-aldosterone system excites hypothalamic paraventricular nucleus neurons in heart failure

The paraventricular nucleus (PVN) of the hypothalamus has critical homeostatic functions, including the regulation of fluid balance and sympathetic drive. It has been suggested that altered activity of this nucleus contributes to the progression of congestive heart failure (HF). We hypothesized that forebrain influences of the renin-angiotensin-aldosterone system augment the activity of PVN neurons in HF. The rate of PVN neurons (n = 68) from rats with ischemia-induced HF was higher than that of PVN neurons (n = 42) from sham-operated controls (8.7 +/- 0.8 vs. 2.7 +/- 0.3 spikes/s, P < 0.001, HF vs. SHAM). Forebrain-directed intracarotid artery injections of the angiotensin type 1 receptor antagonist losartan, the angiotensin-converting enzyme inhibitor captopril, and the mineralocorticoid receptor antagonist spironolactone all significantly (P < 0.05) reduced PVN neuronal activity in HF rats. These findings demonstrate that the renin-angiotensin-aldosterone system drives PVN neuronal activity in HF, likely resulting in increased sympathetic drive and volume accumulation. This mechanism of neurohumoral excitation in HF is accessible to manipulation by blood-borne therapeutic agents.

Forebrain renin-angiotensin system has a tonic excitatory influence on renal sympathetic nerve activity

All elements of the renin-angiotensin system (RAS) are present in the forebrain, particularly in circumventricular organs surrounding the third cerebral ventricle. We tested the hypothesis that forebrain angiotensin-converting enzyme (ACE) has a tonic excitatory influence on sympathetic drive. Neurally intact and sinoaortic-denervated pentobarbital-anesthetized rats were treated with forebrain-directed intracarotid artery (ICA) versus intravenous injections of angiotensin I (ANG I) and of the ACE inhibitor captopril. In intact rats, ICA ANG I elicited a rise in arterial pressure and a concomitant reduction in renal sympathetic nerve activity (RSNA; ICA captopril elicited the opposite responses). In barodenervated rats, ICA ANG I increased and ICA captopril decreased arterial pressure and RSNA in parallel; intravenous ANG I had no effect on RSNA. The findings suggest that the intrinsic forebrain RAS has a tonic excitatory influence on sympathetic drive that is overshadowed in normal rats by baroreflex mechanisms, but may assume a more prominent role in pathophysiological states (e.g., heart failure) in which baroreflex mechanisms are impaired and RAS activity is augmented.

Functional evidence for subfornical organ-intrinsic conversion of angiotensin I to angiotensin II

Using extracellular electrophysiological recording in an in vitro slice preparation, we investigated whether ANG I can be locally converted to the functionally active ANG II within the rat subfornical organ (SFO). ANG I and ANG II (10(-8)-10(-7) M) excited approximately 75% of all neurons tested with both peptides (n = 25); the remainder were insensitive. The increase in firing rate and the duration and the latency of the responses of identical neurons, superfused with equimolar concentrations of ANG I and ANG II, were not different. The threshold concentrations of the ANG I- and ANG II-induced excitations were both 10(-9) M. Inhibition of the angiotensin-converting enzyme by captopril (10(-4) M; n = 8) completely blocked the ANG I-induced excitation, a 10-fold lower dose was only effective in two of four neurons. The AT1-receptor antagonist losartan (10(-5) M; n = 6) abolished the excitation caused by ANG I and ANG II. Subcutaneous injections of equimolar doses of ANG I and ANG II (200 microliters; 2 x 10(-4) M) in water-sated rats similarly increased water intake by 2.4 +/- 0.5 (n = 16) and 2. 7 +/- 0.4 ml (n = 20) after 1 h, respectively. Control rats receiving saline drank 0.07 +/- 0.06 ml under these conditions. Pretreatment with a low dose of captopril (2.3 x 10(-3) M) 10 min before the injection of ANG I caused a water intake of 2.8 +/- 0.5 ml (n = 10), whereas a high dose of captopril (4.6 x 10(-1) M) suppressed the dipsogenic response of ANG I entirely (n = 11). These data provide direct functional evidence for an SFO-intrinsic renin-angiotensin system (RAS) and underline the importance of the SFO as a central nervous interface connecting the peripheral with the central RAS.

Aminopeptidases in the circumventricular organs of the mouse brain: a histochemical study

The localization of four membrane-bound aminopeptidases--aminopeptidase A, aminopeptidase M, dipeptidylpeptidase IV, and gamma-glutamyl transpeptidase--known as characteristic enzymes of the blood-brain barrier was studied in the microvasculature of some circumventricular organs of the mouse brain (subfornical organ, area postrema, choroid plexus, and neurohypophysis). Enzyme activities were demonstrated histochemically in chloroform-acetone-pretreated cryostat sections applying an azo-coupling method. Reactions were evaluated using light microscopy and end-point microdensitometry. The results revealed differences in microvascular enzyme pattern between circumventricular organs and regions having a blood-brain barrier. Moreover, the cytochemical picture of the circumventricular organs themselves was not uniform. Dipeptidylpeptidase IV reaction showed a strongly reduced activity in the microvessels of all studied circumventricular organs. On the other hand, aminopeptidase M seemed to be present in both the leaky and the tight capillaries. Only a low activity of aminopeptidase A was found in parts of the choroid endothelium and the subfornical organ microvasculature. gamma-Glutamyl transpeptidase could neither be detected in the capillary part of the choroid plexus nor in the neurohypophysis. We are led to conclude that at least dipeptidylpeptidase IV might be involved in special mechanisms of the blood-brain barrier.

The kallikrein-kinin system in the rat hypothalamus. Immunohistochemical localization of high molecular weight kininogen and T kininogen in different neuronal systems

High molecular weight kininogen (HKg) and T kininogen (TKg) were detected and localized by immunocytochemistry in adult rat hypothalamus. In addition, kininogens were measured by their direct radioimmunoassay (RIA) or by indirect estimation of kinins released after trypsin hydrolysis and high pressure liquid chromatography (HPLC) separation of bradykinin (BK) and T kinin. A specific HKg immunoreactivity demonstrated with antibodies directed against the light chain (LC) of HKg was colocated with SRIF in neurons of hypothalamic periventricular area (PVA) projecting to external zone (ZE) of median eminence (ME). Heavy chain (HC) immunoreactivity which could be related to HKg or to low molecular weight kininogen (LKg) was detected in some other systems: i) parvocellular neurons of suprachiasmatic (SCN) and arcuate nuclei containing SRIF, ii) magnocellular neurons (mostly oxytocinergic) of paraventricular (PVN) and supraoptic (SON) nuclei, iii) neurons of dorsomedian and lateral hypothalamic areas. TKg immunostaining was restricted to magnocellular neurons of PVN, SON, accessory nuclei (mostly vasopressinergic) and to parvocellular neurons of SCN (vasopressinergic). TKg projections are directed towards the internal zone (ZI) of ME, but very few immunoreactive terminals are detectable in neurohypophysis. TKg staining parallels with vasopressin during water deprivation, and is undetectable in homozygous Brattleboro rats. In some magnocellular neurons, TKg and HC (related to HKg or LKg) are coexpressed. TKg, was also detected in hypothalamus and cerebellum extracts by direct RIA, and BK and T kinin were identified after trypsin hydrolysis. HKg and LKg can act as precursor of BK which can play a physiological role as releasing factor, neuromodulator--neurotransmitter,--or modulator of local microcirculation in hypothalamus. The three kininogens are also potent thiolprotease inhibitors which could modulate both the maturation processes of peptidic hormones and their inactivation and catabolism.

Comparative quantification of rat brain and pituitary angiotensin-converting enzyme with autoradiographic and enzymatic methods

Angiotensin-converting enzyme (ACE, kininase II, EC 3.4.15.1) was quantified in selected areas of the rat brain by 3 different methods. The enzyme activity was quantified by an enzymatic assay in homogenates of discrete brain areas with the use of the artificial substrate hippuryl-His-Leu-[14C]glycine. Binding of the specific ACE inhibitor [125I]351A was quantified after incubation of thin brain sections followed by quantitative autoradiography with comparison to [125I]standards. The antigenic sites of the enzyme were quantified by immunoautoradiography, using a polyclonal antibody to ACE in combination with [125I]protein A followed by quantitative autoradiography. There was a good correlation between the ACE activity, as determined by the quantitative enzymatic method, and both the enzyme inhibitor binding and the [125I]protein A binding, as determined by quantitative autoradiography, in brain areas and in anterior and posterior lobes of the pituitary gland. The present data demonstrate that ACE activity and ACE binding can be quantified in discrete tissue areas with different methods in a variety of conditions and with a high degree of accuracy.

Effect of chronic administration of the converting enzyme inhibitor enalapril (MK 421) on brain atrial natriuretic peptide receptors in Wistar-Kyoto and spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) showed lower brain ANP binding density when compared with normotensive Wistar-Kyoto (WKY) rats. In the WKY, angiotensin converting enzyme inhibitor, enalapril (25 mg/kg, p.o. for 14 days), decreased the number of ANP binding sites selectively in the subfornical organ and area postrema. Conversely, enalapril increased ANP binding density in the SHR, but only in the area postrema. Enalapril has central effects on ANP binding sites, specific to the circumventricular organs.

Gamma-aminobutyric acid and taurine antagonize the central effects of angiotensin II and renin on the intake of water and salt, and on blood pressure in rats

Antagonism by neuro-amino acids of the central effects of angiotensin II and renin in rats was investigated. Angiotensin II (100 ng), injected into the preoptic area, stimulated the intake of water but not salt, to a lesser extent and with a shorter duration as compared with that induced by renin (5 mU), injected into the preoptic area. This angiotensin II-induced intake of water was markedly inhibited by [Sar1, Ile8]-angiotensin II, an angiotensin II receptor antagonist, but not by captopril, an angiotensin-converting enzyme inhibitor, previously administered through the same cannula. The angiotensin II-induced intake of water was also inhibited in a dose-dependent manner by gamma-aminobutyric acid (GABA) (50-100 micrograms), muscimol, a GABA agonist, (100-200 ng), taurine (100-200 micrograms) and hypotaurine (100-200 micrograms), administered into the cerebroventricle and by GABA (5-10 micrograms), muscimol (10-20 ng) and taurine (10-20 micrograms) injected into the preoptic area in smaller doses. Renin (5 mU), injected into the preoptic area, elicited a marked increase in the intake of water and salt, which lasted for about 3 days. The effect of renin was inhibited by [Sar1, Ile8]-angiotensin II (10 micrograms) and was eliminated by captopril (25 micrograms) injected into the preoptic area. This effect of renin was not influenced by the peripheral administration of captopril. The effect of renin was also inhibited by GABA, muscimol or taurine injected into the cerebroventricle, in larger doses, or into the preoptic area in smaller doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of angiotensin II receptors along the anteroventral third ventricle area of the rat brain

Autoradiographic techniques were utilized to localize and to quantify angiotensin II (ANG) binding sites in rat forebrain. Specific, localized ANG binding sites were demonstrated in midline sagittal sections, corresponding to the entire anteroventral third ventricle (AV3V) area, including the nucleus preopticus medianus and the subependymal area of the anterior third ventricle from the nucleus preopticus medianus to the organon vasculosum laminae terminalis. A continuous band of ANG receptors extended dorsally from the nucleus preopticus medianus along the subependymal area of the third ventricle to the organon subfornicalis. Scatchard analysis performed with consecutive sections from single animals revealed a single class of high-affinity ANG receptors in both the organon subfornicalis and the organon vasculosum laminae terminalis. In addition, ANG receptors were localized in areas anatomically and physiologically related to the AV3V area, including the nuclei paraventricularis and periventricularis and the eminentia mediana. These results support the idea that ANG may act as both a hormone and a neurotransmitter in the central regulation of fluid balance and cardiovascular function, and suggest that the circumventricular organs are the most likely sites for an interaction between the peripheral and central ANG systems.

Angiotensin converting enzyme in rat brain visualized by quantitative in vitro autoradiography

Angiotensin converting enzyme was localized in rat brain by quantitative in vitro autoradiography using an [125I]labelled converting enzyme inhibitor called "351A". This radioligand was found to bind with high affinity and specificity to angiotensin converting enzyme. Very high levels of converting enzyme were observed in the ventricular choroid plexus, ependyma of all ventricles and large and medium blood vessels, subfornical organ, and organum vasculosum of the lamina terminalis. High levels of converting enzyme were found in the basal ganglia including caudate putamen, nucleus accumbens, globus pallidus, entopenduncular nucleus and substantia nigra pars reticulata. The neurosecretory nuclei, paraventricular nucleus and supraoptic nucleus, as well as the median eminence and posterior pituitary displayed high levels of the enzyme. In the amygdala, basolateral, lateral, basomedial, medial and anterior cortical nuclei showed moderate converting enzyme activity. The medial habenula and molecular layer of the dentate gyrus showed high levels of activity. In the cerebellum, dense labelling was observed in the Purkinje cell layer. Moderate levels of converting enzyme occurred in the gelatinosus subnucleus of the caudal part of the nucleus of the spinal tract of the trigeminal. There was a close correspondence between the distribution of angiotensin converting enzyme and angiotensin II in the neurosecretory nuclei (paraventricular and supraoptic nuclei) and median eminence and this suggests a role of angiotensin converting enzyme in the production of angiotensin II in this system. There was also a good correspondence between the distribution of angiotensin converting enzyme and angiotensin II in the subfornical organ, median preoptic nucleus, and organum vasculosum of the lamina terminalis, structures abutting the anterior wall of the third ventricle which are implicated in fluid and electrolyte homeostasis. A striking discrepancy occurs in the basal ganglia which is reported to contain very little angiotensin II or angiotensin II receptors but is very rich in angiotensin converting enzyme. It is concluded that the enzyme may act to convert circulating angiotensin I to angiotensin II in circumventricular organs; generate intraneuronal angiotensin II in pathways such as the hypothalamic-hypophyseal tract; and process neuropeptides other than angiotensin II in regions such as basal ganglia.

Autoradiographic localization and characterization of circumventricular angiotensin II receptors in duck brain

Binding of [125I-5Val]angiotensin II and [125I-1Sar, 8Ile]angiotensin II to target sites in the hypothalamus of the Pekin duck was determined by quantitative receptor autoradiography and conventional membrane binding techniques. Circumventricular areas involved in body fluid homeostasis like the subfornical organ (SFO), median eminence and the anterior-ventral region of the third ventricle showed highest labeling density. Binding sites for angiotensin II were also found in the paraventricular and supraoptic nuclei. The physiological relevance of the labeled SFO angiotensin II receptor is indicated by similarities between rank orders of potency of angiotensin II analogues in displacing radiolabeled ligands and their physiological osmoregulatory potencies. Receptor density in the SFO of saltwater-acclimated ducks was increased 3-fold compared to non-acclimated freshwater ducks, indicating an up-regulation of the angiotensinergic system in ducks under conditions of dehydration or high salt intake.

Quantitative distribution of angiotensin II binding sites in rat brain by autoradiography

Angiotensin II binding sites were localized and quantified in individual brain nuclei from single rats by incubation of tissue sections with 1 nM 125I-[Sar1]-angiotensin II, [3H]-Ultrofilm autoradiography, computerized microdensitometry and comparison with 125I-standards. High angiotensin II binding was present in the circumventricular organs (organon vasculosum laminae terminalis, organon subfornicalis and area postrema), in selected hypothalamic nuclei (nuclei suprachiasmatis, periventricularis and paraventricularis) and in the nucleus tractus olfactorii lateralis, the nucleus preopticus medianus, the dorsal motor nucleus of the vagus and the nucleus tractus solitarii. High affinity (KA from 0.3 to 1.5 X 10(9) M-1) angiotensin II binding sites were demonstrated in the organon subfornicalis, the nucleus tractus solitarii and the area postrema after incubation of consecutive sections from single rat brains with 125I-[Sar1]-angiotensin II in concentrations from 100 pM to 5 nM. These results demonstrate and characterize brain binding sites for angiotensin II of variable high affinity binding both inside and outside the blood-brain barrier.

Quantitative distribution of angiotensin-converting enzyme (kininase II) in discrete areas of the rat brain by autoradiography with computerized microdensitometry

We report the localization of angiotensin-converting enzyme (kininase II, EC 3.4.15.1) in discrete nuclei and areas of the rat brain by a quantitative autoradiographic technique using image processing coupled to computerized microdensitometry, after incubation of brain sections with the specific converting enzyme inhibitor [125I]351A. High angiotensin-converting enzyme levels are present in circumventricular organs (organon subfornicalis and area postrema), the choroid plexus, and extrapyramidal areas (nucleus caudatus, globus pallidus and substantia nigra) with intermediate levels in selected hypothalamic, septal, habenular and brainstem nuclei. Our results support the idea that angiotensin II could be formed in specific brain areas, both outside and inside the blood-brain barrier. In other brain structures, such as the extrapyramidal areas, kininase II could be involved in the processing or metabolism of other brain peptides.

Angiotensin II binding sites in the anteroventral-third ventricle (AV3V) area and related structures of the rat brain

Angiotensin II binding sites were localized in the rat brain by incubation of sagittal sections with 125I-[Sar1]-angiotensin II, followed by [3H]Ultrofilm autoradiography. Binding sites were localized in the subfornical organ and the entire anteroventral-third ventricle (AV3V) area, including the subependymal area of the anterior third ventricle, the median preoptic nucleus and the organon vasculosum laminae terminalis. Binding sites were also present in the paraventricular and periventricular nuclei and in the median eminence. These findings support the hypothesis of an extensive involvement of angiotensin in the neuronal circuits connecting the subfornical organ, the AV3V area, the paraventricular nucleus and the median eminence, and in the regulation of fluid balance, blood pressure and pituitary secretion.

Cerebral renin-angiotensin mediation of isoproterenol-induced thirst in the dog

Pretreatment of dogs with s.c. isoproterenol (10 micrograms/kg) caused a significant increase in drinking when 100 ng renin substrate was administered 3 min later to the lateral cerebral ventricles or subfornical organ. Isoproterenol itself was a potent peripheral (10 micrograms/kg), but unreliable central (0.01-1 microgram) dipsogen. The increased drinking after combined s.c. isoproterenol and intracerebroventricular (i.c.v.) renin substrate injections was significantly attenuated by i.c.v. captopril (20 micrograms), but was not influenced by s.c. captopril (500 micrograms/kg). However, combined i.c.v./s.c. pretreatment with captopril nearly abolished drinking to peripheral isoproterenol, or the combination of s.c. isoproterenol and i.c.v. renin substrate. Finally, single intracranial injections of the components of the renin-angiotensin system elicited dose-dependent and site-specific drinking. Renin substrate, angiotensin I and angiotensin III produced greater intakes at forebrain tissue sites than after i.c.v. or subfornical organ injections. Renin, on the other hand, was more potent i.c.v. than at forebrain loci. These results suggest that the cerebral renin-angiotensin system may participate in beta-adrenergic thirst mechanisms by increasing local angiotensin II biosynthesis in specific areas of the brain.

Binding of angiotensin and atrial natriuretic peptide in brain of hypertensive rats

Atrial natriuretic peptides, produced in the mammalian cardiac atrium, are released into the general circulation and may be actively involved in the control of blood pressure and in fluid homeostasis as antagonists of the peripheral angiotensin system. Certain cardiovascular effects of atrial natriuretic peptides may be centrally mediated, as binding sites for atrial natriuretic factor (8-33) (ANF) have been localized to the subfornical organ. This circumventricular structure lacks a blood-brain barrier and is therefore accessible to circulating peptides. It contains large numbers of angiotensin II (AII) binding sites, and has been suggested as the main central site of action for circulating AII in the regulation of blood pressure and fluid metabolism. Here we have studied binding sites for rat atrial natriuretic peptide(6-33) (rANP) and AII in the brains of spontaneously (genetic) hypertensive rats (SHR) and their normotensive controls, Wistar Kyoto (WKY) rats, by quantitative autoradiography. Binding sites for both peptides were highly localized in the subfornical organ. The number of rANP binding sites was decreased in the subfornical organ of both young (4 weeks old) and adult (14 weeks old) SHR compared with age-matched normotensive controls. Conversely, the number of AII binding sites was higher in both young and adult SHR compared with WKY rats. Our results suggest a central role for rANP and AII in genetic hypertension; they may act as mutual antagonists in brain areas involved in control of blood pressure and fluid regulation.

Quantitative autoradiography of 125I-[Sar1, Ile8]-angiotensin II binding in the brain of spontaneously hypertensive rats

The brain contains its own angiotensin II (AII) system. To better understand the role of central AII in cardiovascular regulation, we used 125I-[Sar1, Ile8]-AII (125I-SI-AII), radioactive AII antagonist, to autoradiographically localize putative AII receptor binding in many parts of the central nervous system of the spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. With 125I-SI-AII binding on brain membrane preparations. Scatchard analysis indicated that Kd values were from 0.10 +/- 0.04 nM to 0.13 +/- 0.05 nM, whereas Bmax values (femtomol/mg protein) were found to be from 6.95 +/- 1.60 to 15.52 +/- 4.99 among brain regions studied. Various SI-AII receptor binding activities among brain regions revealed in this study were therefore most likely due to differences in AII receptor density with high affinity binding of 125I-AII. Using 125I-SI-AII, specific binding for SI-AII was found in the nucleus tractus solitarius (NTS), paraventricular hypothalamic nucleus (PVN), subfornical organ (SFO), suprachiasmatic nucleus (SCN), area postrema, the dorsal motor nucleus of the vagus (DMX), and the nucleus of spinal tract of the trigeminal system (NSV). With quantitative receptor autoradiography in conjunction with radioactive standards, we have observed that the NTS possesses the highest SI-AII binding, followed by the PVN, SFO, NTS, DMX, and NSV. No significant differences were observed between the SHR and WKY rats in the SI-AII binding within the SFO, PVN and NTS. However, SHR at early hypertensive (7 weeks) and established hypertensive (16 weeks) stages contained significantly higher SI-AII bindings in the NSV, as compared to age-matched WKY rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic determination of angiotensin-converting enzyme (kininase II) binding in individual rat brain nuclei with 125I-351A, a specific enzyme inhibitor

We report the localization and characterization of angiotensin-converting enzyme (kininase II) in discrete nuclei from individual rat brains by a quantitative autoradiographic technique coupled to computerized microdensitometry. The enzyme was quantitated by incubation of 16-micron-thick brain sections with 0.07-2 nM of the converting enzyme inhibitor 125I-351A and comparison to 125I-standards. This technique can be applied to the study of other enzymes in single rat brain nuclei.

Angiotensin-converting enzyme in discrete forebrain areas of spontaneously hypertensive rats

Angiotensin-converting enzyme (ACE, kininase II, EC 3.4.15.1) activity was measured in forebrain nuclei of 4- and 16-week-old spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) normotensive controls. Young SHR showed lower enzyme activity in the nucleus paraventricularis, and higher activity in the nuclei septalis medialis and habenularis medialis. Adult SHR exhibited lower ACE activity in the nucleus septalis medialis, the organon subfornicalis, the organon vasculosum laminae terminalis, and the nuclei periventricularis and suprachiasmatis.

Localization of angiotensin converting enzyme in rat forebrain and other tissues by in vitro autoradiography using 125I-labelled MK351A

The potent angiotensin converting enzyme inhibitor, MK351A, was radioiodinated and found to show saturable, high affinity, reversible binding to membrane fractions of rat lung. At 20 degrees C the labelled inhibitor bound to lung membranes with a T1/2 of congruent to 2-3 min and reached a plateau at 15 min; the complex dissociated with a T1/2 congruent to 65 min on addition of excess unlabelled MK521. The potency of a series of converting enzyme inhibitors in displacing the radioactive ligand closely paralleled their anticatalytic potency, strongly suggesting that the ligand labels the active site of converting enzyme. This ligand has the desired properties as a probe for autoradiographic localization of converting enzyme since it exhibits high affinity, stable binding to the enzyme with very low non-specific binding (less than 2% of total). In vitro autoradiographic analysis using 125I-MK351A revealed a very high density of converting enzyme in lung, small bowel mucosa, adrenal zona glomerulosa and medulla and renal cortex. In rat forebrain a very high density of binding was found in the choroid plexus, subfornical organ and caudate-putamen. This technique provides a promising method for the quantitative localization of angiotensin converting enzyme in tissues.

A new feature of angiotensin-converting enzyme in the brain: hydrolysis of substance P

Highly purified rat brain angiotensin-converting enzyme hydrolyzes substance P which contains a C-terminal amino acid with an amidated carboxyl group. The hydrolysis of substance P verified by amino-group fluorometry and by high-performance liquid chromatography is inhibited by captopril, but not by phosphoramidon. The presence of sodium chloride is essential for the hydrolysis. The analyses of cleavage products indicate that the enzyme hydrolyzes substance P between Phe7-Phe8 and Phe8-Gly9 by an endopeptidase action, followed by successive release of dipeptides by a dipeptidyl carboxypeptidase action.

Selective increase of angiotensin-converting enzyme activity in discrete extrahypothalamic areas of Brattleboro rats

We studied the activity of angiotensin I converting enzyme (ACE, kininase II, E.C. 3.4.15.1) in discrete areas of the brainstem and limbic system, and in circumventricular organs, pineal gland and choroid plexus of homozygous Brattleboro rats (DI) which are characterized by vasopressin deficiency and diabetes insipidus, with or without vasopressin replacement. We also determined ACE activity in heterozygous Brattleboro (HZ) and Long-Evans (LE) control rats. We found changes in ACE activity in several brain areas and the pineal gland of Brattleboro rats. ACE activity was increased in DI rats with respect to HZ and LE controls in the A1 area of the brainstem, locus coeruleus, and triangular nucleus of the septum. ACE activity in the A2 area of the brainstem, the nucleus tractus spinalis nervi trigeminii and the pineal gland was enhanced in both HZ and DI rats with respect to that of LE controls, but was not different between HZ and DI rats. ACE activity did not change in the other extrahypothalamic areas studied. The elevated ACE activity in extrahypothalamic areas of DI rats was not reversed by vasopressin replacement. These results suggest that a relationship between central vasopressin and angiotensin or bradykinin systems may exist in selective extrahypothalamic areas of the rat brain, and that peripherally administered vasopressin cannot influence this relationship.

Purification and inhibition by neuropeptides of angiotensin-converting enzyme from rat brain

Angiotensin-converting enzyme was solubilized with papain from a particulate fraction of rat brain and purified to apparent homogeneity by a procedure including DEAE-cellulose, hydroxylapatite, Sephadex G-200, Cys(Bzl)-Pro-Sepharose, and ricin-Sepharose chromatography. Bradykinin potentiators, SQ 14,225, and Arg-Pro-Pro strongly inhibited the activity of the purified enzyme, whereas Phe-Ala, phosphoramidon, and pentobarbital exerted little inhibitory effect on the activity. Among neuropeptides investigated, substance P, bradykinin, and Leu-enkephalin (Arg6) exerted strong inhibitory actions on the enzyme. Furthermore, the latter two peptides were shown to be good substrates for the enzyme. Thus, angiotensin-converting enzyme of rat brain is distinct from endogenous enkephalinase and may interact with various neuropeptides located in the brain.