Localization and characterization of angiotensin II receptor binding and angiotensin converting enzyme in the human medulla oblongata

Role of the angiotensin type 1 receptor in modulating the carotid chemoreflex in an ovine model of renovascular hypertension

OBJECTIVE

The carotid body has been implicated as an important mediator and putative target for hypertension. Previous studies have indicated an important role for angiotensin II in mediating carotid body function via angiotensin type-1 receptors (AT1R); however, their role in modulating carotid body function during hypertension is unclear.

METHODS

Using a large preclinical ovine model of renovascular hypertension, we hypothesized that acute AT1R blockade would lower blood pressure and decrease carotid body-mediated increases in arterial pressure. Adult ewes underwent either unilateral renal artery clipping or sham surgery. Two weeks later, flow probes were placed around the contralateral renal and common carotid arteries.

RESULTS

In both hypertensive and sham animals, carotid body stimulation using potassium cyanide caused dose-dependent increases in mean arterial pressure but a reduction in renal vascular conductance. These responses were not different between groups. Infusion of angiotensin II led to an increase in arterial pressure and reduction in renal blood flow. The sensitivity of the renal vasculature to angiotensin II was significantly attenuated in hypertension compared with the sham animals. Systemic inhibition of the AT1R did not alter blood pressure in either group. Interestingly carotid body-evoked arterial pressure responses were attenuated by AT1R blockade in renovascular hypertension but not in shams.

CONCLUSION

Taken together, our findings indicate a decrease in vascular reactivity of the non-clipped kidney to angiotensin II in hypertension. The CB-evoked increase in blood pressure in hypertension is mediated in part, by the AT1R. These findings indicate a differential role of the AT1R in the carotid body versus the renal vasculature.

Acute and chronic neurological disorders in COVID-19: potential mechanisms of disease

Coronavirus disease 2019 (COVID-19) is a global pandemic caused by SARS-CoV-2 infection and is associated with both acute and chronic disorders affecting the nervous system. Acute neurological disorders affecting patients with COVID-19 range widely from anosmia, stroke, encephalopathy/encephalitis, and seizures to Guillain-Barré syndrome. Chronic neurological sequelae are less well defined although exercise intolerance, dysautonomia, pain, as well as neurocognitive and psychiatric dysfunctions are commonly reported. Molecular analyses of CSF and neuropathological studies highlight both vascular and immunologic perturbations. Low levels of viral RNA have been detected in the brains of few acutely ill individuals. Potential pathogenic mechanisms in the acute phase include coagulopathies with associated cerebral hypoxic-ischaemic injury, blood-brain barrier abnormalities with endotheliopathy and possibly viral neuroinvasion accompanied by neuro-immune responses. Established diagnostic tools are limited by a lack of clearly defined COVID-19 specific neurological syndromes. Future interventions will require delineation of specific neurological syndromes, diagnostic algorithm development and uncovering the underlying disease mechanisms that will guide effective therapies.

Neurology and the COVID-19 Pandemic: Gathering Data for an Informed Response

PURPOSE OF REVIEW

The current coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is one of the greatest medical crises faced by our current generation of health care providers. Although much remains to be learned about the pathophysiology of SARS-CoV-2, there is both historical precedent from other coronaviruses and a growing number of case reports and series that point to neurologic consequences of COVID-19.

RECENT FINDINGS

Olfactory/taste disturbances and increased risk of strokes and encephalopathies have emerged as potential consequences of COVID-19 infection. Evidence regarding whether these sequelae result indirectly from systemic infection or directly from neuroinvasion by SARS-CoV-2 is emerging.

SUMMARY

This review summarizes the current understanding of SARS-CoV-2 placed in context with our knowledge of other human coronaviruses. Evidence and data regarding neurologic sequelae of COVID-19 and the neuroinvasive potential of human coronaviruses are provided along with a summary of patient registries of interest to the Neurology community.

Resting regional brain activity and connectivity vary with resting blood pressure but not muscle sympathetic nerve activity in normotensive humans: An exploratory study

Blood pressure is tightly controlled by the central nervous system, particularly the brainstem. The aim of this study was to investigate the relationship between mean blood pressure (MBP), muscle sympathetic nerve activity (MSNA) and resting regional brain activity in healthy human subjects. Pseudocontinuous arterial spin labeling and functional magnetic resonance imaging of the brain were performed immediately following a laboratory microneurography recording of MSNA and BP measurement in 31 young, healthy normotensive subjects. Regional cerebral blood flow (CBF) correlated significantly with resting MBP levels in the region encompassing the rostroventrolateral medulla (RVLM), dorsolateral pons, and insular, prefrontal and cingulate cortices. Functional connectivity analysis revealed that the ventrolateral prefrontal cortex displayed greater resting connectivity strength within the RVLM in the lower compared with the higher MBP group. No significant differences in CBF were found when subjects were divided based on their MSNA levels. These results suggest that even subtle differences in resting MBP are associated with significant differences in resting activity in brain regions, which are well known to play a role in cardiovascular function. These data raise the question of the potential long-term consequences of differences in regional brain activity levels and their relationship with systemic blood pressure.

The angiotensin receptor blocker, Losartan, inhibits mammary tumor development and progression to invasive carcinoma

Drugs that target the Renin-Angiotensin System (RAS) have recently come into focus for their potential utility as cancer treatments. The use of Angiotensin Receptor Blockers (ARBs) and Angiotensin-Converting Enzyme (ACE) Inhibitors (ACEIs) to manage hypertension in cancer patients is correlated with improved survival outcomes for renal, prostate, breast and small cell lung cancer. Previous studies demonstrate that the Angiotensin Receptor Type I (AT₁R) is linked to breast cancer pathogenesis, with unbiased analysis of gene-expression studies identifying significant up-regulation of AGTR1, the gene encoding AT₁R in ER^+ve/HER2^−ve tumors correlating with poor prognosis. However, there is no evidence, so far, of the functional contribution of AT₁R to breast tumorigenesis. We explored the potential therapeutic benefit of ARB in a carcinogen-induced mouse model of breast cancer and clarified the mechanisms associated with its success.

Mammary tumors were induced with 7,12-dimethylbenz[α]antracene (DMBA) and medroxyprogesterone acetate (MPA) in female wild type mice and the effects of the ARB, Losartan treatment assessed in a preventative setting (n = 15 per group). Tumor histopathology was characterised by immunohistochemistry, real-time qPCR to detect gene expression signatures, and tumor cytokine levels measured with quantitative bioplex assays. AT₁R was detected with radiolabelled ligand binding assays in fresh frozen tumor samples.

We showed that therapeutic inhibition of AT₁R, with Losartan, resulted in a significant reduction in tumor burden; and no mammary tumor incidence in 20% of animals. We observed a significant reduction in tumor progression from DCIS to invasive cancer with Losartan treatment. This was associated with reduced tumor cell proliferation and a significant reduction in IL-6, pSTAT3 and TNFα levels. Analysis of tumor immune cell infiltrates, however, demonstrated no significant differences in the recruitment of lymphocytes or tumour-associated macrophages in Losartan or vehicle-treated mammary tumors.

Analysis of AT₁R expression with radiolabelled ligand binding assays in human breast cancer biopsies showed high AT₁R levels in 30% of invasive ductal carcinomas analysed. Furthermore, analysis of the TCGA database identified that high AT₁R expression to be associated with luminal breast cancer subtype.

Our in vivo data and analysis of human invasive ductal carcinoma samples identify the AT₁R is a potential therapeutic target in breast cancer, with the availability of a range of well-tolerated inhibitors currently used in clinics. We describe a novel signalling pathway critical in breast tumorigenesis, that may provide new therapeutic avenues to complement current treatments.

Renal sympathetic denervation lowers arterial pressure in canines with obesity-induced hypertension by regulating GAD65 and AT1R expression in rostral ventrolateral medulla

To explore the roles of glutamate acid decarboxylase 65 (GAD65) and angiotensin II type 1 receptor (AT₁R) in the action of renal sympathetic denervation (RSD) on obesity-induced hypertension in canines. Thirty-two beagles were randomly divided into a hypertensive model (n = 22) and control (n = 10) groups. A hypertensive canine model was established by feeding a high-fat diet. Twenty hypertensive beagles were randomized equally to a sham-surgery and RSD-treated group receiving catheter-based radiofrequency RSD. Compared with the control group, the sham-surgery group exhibited significant increases in blood pressure, serum angiotensin II level, rostral ventrolateral medulla (RVLM) glutamate level, and AT₁R mRNA and protein expression and decreases in γ-amino acid butyric acid (γ-GABA) level and GAD65 mRNA and protein expression in the RVLM (all P < 0.05). Treatment with RSD significantly attenuated the above abnormal alterations (all P < 0.05). Linear correlation analysis revealed that angiotensin II level was positively correlated with glutamate level (r = 0.804) and inversely correlated with γ-GABA level (r = -0.765). GAD65 protein expression was positively correlated with γ-GABA level (r = 0.782). Catheter-based radiofrequency RSD can decrease blood pressure in obesity-induced hypertensive canines. The antihypertensive mechanism might be linked to upregulation of GAD65 and downregulation of AT₁R in the RVLM.

Viscerosensory input drives angiotensin II type 1A receptor-expressing neurons in the solitary tract nucleus

Homeostatic regulation of visceral organ function requires integrated processing of neural and neurohormonal sensory signals. The nucleus of the solitary tract (NTS) is the primary sensory nucleus for cranial visceral sensory afferents. Angiotensin II (ANG II) is known to modulate peripheral visceral reflexes, in part, by activating ANG II type 1A receptors (AT_1AR) in the NTS. AT_1AR-expressing NTS neurons occur throughout the NTS with a defined subnuclear distribution, and most of these neurons are depolarized by ANG II. In this study we determined whether AT_1AR-expressing NTS neurons receive direct visceral sensory input, and whether this input is modulated by ANG II. Using AT_1AR-GFP mice to make targeted whole cell recordings from AT_1AR-expressing NTS neurons, we demonstrate that two-thirds (37 of 56) of AT_1AR-expressing neurons receive direct excitatory, visceral sensory input. In half of the neurons tested (4 of 8) the excitatory visceral sensory input was significantly reduced by application of the transient receptor potential vallinoid type 1 receptor agonist, capsaicin, indicating AT_1AR-expressing neurons can receive either C- or A-fiber-mediated input. Application of ANG II to a subset of second-order AT_1AR-expressing neurons did not affect spontaneous, evoked, or asynchronous glutamate release from visceral sensory afferents. Thus it is unlikely that AT_1AR-expressing viscerosensory neurons terminate on AT_1AR-expressing NTS neurons. Our data suggest that ANG II is likely to modulate multiple visceral sensory modalities by altering the excitability of second-order AT_1AR-expressing NTS neurons.

Functional and neurochemical characterization of angiotensin type 1A receptor-expressing neurons in the nucleus of the solitary tract of the mouse

Angiotensin II acts via two main receptors within the central nervous system, with the type 1A receptor (AT_1AR) most widely expressed in adult neurons. Activation of the AT₁R in the nucleus of the solitary tract (NTS), the principal nucleus receiving central synapses of viscerosensory afferents, modulates cardiovascular reflexes. Expression of the AT₁R occurs in high density within the NTS of most mammals, including humans, but the fundamental electrophysiological and neurochemical characteristics of the AT_1AR-expressing NTS neurons are not known. To address this, we have used a transgenic mouse, in which the AT_1AR promoter drives expression of green fluorescent protein (GFP). Approximately one-third of AT_1AR-expressing neurons express the catecholamine-synthetic enzyme tyrosine hydroxylase (TH), and a subpopulation of these stained for the transcription factor paired-like homeobox 2b (Phox2b). A third group, comprising approximately two-thirds of the AT_1AR-expressing NTS neurons, showed Phox2b immunoreactivity alone. A fourth group in the ventral subnucleus expressed neither TH nor Phox2b. In whole cell recordings from slices in vitro, AT_1AR-GFP neurons exhibited voltage-activated potassium currents, including the transient outward current and the M-type potassium current. In two different mouse strains, both AT_1AR-GFP neurons and TH-GFP neurons showed similar AT_1AR-mediated depolarizing responses to superfusion with angiotensin II. These data provide a comprehensive description of AT_1AR-expressing neurons in the NTS and increase our understanding of the complex actions of this neuropeptide in the modulation of viscerosensory processing.

Blockade of angiotensin AT1-receptors in the rostral ventrolateral medulla of spontaneously hypertensive rats reduces blood pressure and sympathetic nerve discharge

Microinjections of angiotensin II (Ang II) into the rostral ventrolateral medulla (RVLM) induce a sympathetically-mediated increase in blood pressure (BP), through an interaction with AT1-receptors. Under basal conditions in anaesthetised animals, microinjections of AT 1-receptor antagonists into the RVLM have little, or no effect on BP, suggesting that the angiotensin input to this nucleus is not tonically active. In contrast, microinjections of AT1-receptor antagonists into the RVLM of sodium-deplete rats and TGR(mRen2)27 rats, induce a depressor response through sympatho-inhibition. This indicates that when the renin-angiotensin system is activated, angiotensin can act in the RVLM to support sympathetic nerve discharge and BP. This study examined whether angiotensin inputs to the RVLM are activated in the spontaneously hypertensive rat — a pathophysiological model which displays increases in both brain angiotensin levels and sympathetic nerve activity. Bilateral microinjections of the AT 1-receptor antagonist candesartan cilexetil, (1 nmol in 100 nl), into the RVLM of the spontaneously hypertensive rat induced a significant decrease in lumbar sympathetic nerve discharge (-18±2%) and BP (140±6 to 115±6 mmHg). In contrast, similar microinjections in the Wistar-Kyoto (WKY) rat had no effect on BP or sympathetic nerve discharge. These results are interpreted to suggest that Ang II inputs to the RVLM are activated in the spontaneously hypertensive rat to maintain an elevated level of sympathetic nerve discharge, even in the face of increased BP.

Association between resting-state brain functional connectivity and muscle sympathetic burst incidence

The insula (IC) and cingulate are key components of the central autonomic network and central nodes of the salience network (SN), a set of spatially distinct but temporally correlated brain regions identified with resting-state (task free) functional MRI (rsMRI). To examine the SN's involvement in sympathetic outflow, we tested the hypothesis that individual differences in intrinsic connectivity of the SN correlate positively with resting postganglionic muscle sympathetic nerve activity (MSNA) burst incidence (BI) in subjects without and with obstructive sleep apnea (OSA). Overnight polysomnography, 5-min rsMRI, and fibular MSNA recording were performed in 36 subjects (mean age 57 yr; 10 women, 26 men). Independent component analysis (ICA) of the entire cohort identified the SN as including bilateral IC, pregenual anterior cingulate cortex (pgACC), midcingulate cortex (MCC), and the temporoparietal junction (TPJ). There was a positive correlation between BI and the apnea-hypopnea index (AHI) (P < 0.001), but dual-regression analysis identified no differences in SN functional connectivity between subjects with no or mild OSA (n = 17) and moderate or severe (n = 19) OSA. Correlation analysis relating BI to the strength of connectivity within the SN revealed large (i.e., spatial extent) and strong correlations for the left IC (P < 0.001), right pgACC/MCC (P < 0.006), left TPJ (P < 0.004), thalamus (P < 0.035), and cerebellum (P < 0.013). Indexes of sleep apnea were unrelated to BI and the strength of SN connectivity. There were no relationships between BI and default or sensorimotor network connectivity. This study links connectivity within the SN to MSNA, demonstrating several of its nodes to be key sympathoexcitatory regions.

Selective C1 Lesioning Slightly Decreases Angiotensin II Type I Receptor Expression in the Rat Rostral Ventrolateral Medulla (RVLM)

Cardiovascular homeostasis is regulated in large part by the rostral ventrolateral medulla (RVLM) in mammals. Projections from the RVLM to the intermediolateral column of the thoracolumbar spinal cord innervate preganglionic neurons of the sympathetic nervous system causing elevation of blood pressure and heart rate. A large proportion, but not all, of the neurons in the RVLM contain the enzymes necessary for the production of epinephrine and are identified as the C1 cell group. Angiotensin II (Ang II) activates the RVLM acting upon AT1 receptors. To assess the proportion of AT1 receptors that are located on C1 neurons in the rat RVLM this study employed an antibody to dopamine-beta-hydroxylase conjugated to saporin, to selectively destroy C1 neurons in the RVLM. Expression of tyrosine hydroxylase immunoreactive neurons in the RVLM was reduced by 57 % in the toxin injected RVLM compared to the contralateral RVLM. In contrast, densitometric analysis of autoradiographic images of (125)I-sarcosine(1), isoleucine(8) Ang II binding to AT1 receptors of the injected side RVLM revealed a small (10 %) reduction in AT1-receptor expression compared to the contralateral RVLM. These results suggest that the majority of AT1 receptors in the rat RVLM are located on non-C1 neurons or glia.

Canadian Association of Neuroscience Review: Respiratory Control and Behavior in Humans: Lessons from Imaging and Experiments of Nature

The purpose of this review is to demonstrate that respiration is a complex behavior comprising both brainstem autonomic control and supramedullary influences, including volition. Whereas some fundamental mechanisms had to be established using animal models, this review focuses on clinical cases and physiological studies in humans to illustrate normal and abnormal respiratory behavior. To summarize, central respiratory drive is generated in the rostroventrolateral medulla, and transmitted to both the upper airway and to the main and accessory respiratory muscles. Afferent feedback is provided from lung and muscle mechnoreceptors, peripheral carotid and aortic chemoreceptors, and multiple central chemoreceptors. Supramedullary regions, including cortex and subcortex, modulate or initiate breathing with volition, emotion and at the onset of exercise. Autonomic breathing control can be perturbed by brainstem pathology including space occupying lesions, compression, congenital central hypoventilation syndrome and sudden infant death syndrome. Sleep-wake states are important in regulating breathing. Thus, respiratory control abnormalities are most often evident during sleep, or during transition from sleep to wakefulness. Previously undiagnosed structural brainstem pathology may be revealed by abnormal breathing during sleep. Ondine's curse and 'the locked-in syndrome' serve to distinguish brainstem from supramedullary regulatory mechanisms in humans: The former comprises loss of autonomic respiratory control and requires volitional breathing for survival, and the latter entails loss of corticospinal or corticobulbar tracts required for volitional breathing, but preserves autonomic respiratory control.

Stimulation of angiotensin type 1A receptors on catecholaminergic cells contributes to angiotensin-dependent hypertension

Hypertension contributes to multiple forms of cardiovascular disease and thus morbidity and mortality. The mechanisms inducing hypertension remain unclear although the involvement of homeostatic systems, such as the renin-angiotensin and sympathetic nervous systems, is established. A pivotal role of the angiotensin type 1 receptor in the proximal tubule of the kidney for the development of experimental hypertension is established. Yet, other systems are involved. This study tests whether the expression of angiotensin type 1A receptors in catecholaminergic cells contributes to hypertension development. Using a Cre-lox approach, we deleted the angiotensin type 1A receptor from all catecholaminergic cells. This deletion did not alter basal metabolism or blood pressure but delayed the onset of angiotensin-dependent hypertension and reduced the maximal response. Cardiac hypertrophy was also reduced. The knockout mice showed attenuated activation of the sympathetic nervous system during angiotensin II infusion as measured by spectral analysis of the blood pressure. Increased reactive oxygen species production was observed in forebrain regions, including the subfornical organ, of the knockout mouse but was markedly reduced in the rostral ventrolateral medulla. These studies demonstrate that stimulation of the angiotensin type 1A receptor on catecholaminergic cells is required for the full development of angiotensin-dependent hypertension and support an important role for the sympathetic nervous system in this model.

Central angiotensinergic mechanisms associated with hypertension

Following its generation by both systemic and tissue-based renin-angiotensin systems, angiotensin II interacts with specific, G-protein coupled receptors to modulate multiple physiological systems, including the cardiovascular system. Genetic models in which the different components of the renin-angiotensin system have been deleted show large changes in resting blood pressure. Interruption of the generation of angiotensin II, or its interaction with these receptors, decreases blood pressure in hypertensive humans and experimental animal models of hypertension. Whilst the interaction of angiotensin II with the kidney is pivotal in this modulation of blood pressure, an involvement of the system in other tissues is important. Both systemic angiotensins, acting via the blood-brain barrier deficient circumventricular organs, and centrally-generated angiotensin modulate cardiovascular control by regulating fluid and electrolyte ingestion, autonomic activity and neuroendocrine function. This review discusses the pathways in the brain that are involved in this regulation of blood pressure as well as examining the sites in which altered angiotensin function might contribute to the development and maintenance of high blood pressure.

Angiotensin type 1A receptors in C1 neurons of the rostral ventrolateral medulla modulate the pressor response to aversive stress

The rise in blood pressure during an acute aversive stress has been suggested to involve activation of angiotensin type 1A receptors (AT(1A)Rs) at various sites within the brain, including the rostral ventrolateral medulla. In this study we examine the involvement of AT(1A)Rs associated with a subclass of sympathetic premotor neurons of the rostral ventrolateral medulla, the C1 neurons. The distribution of putative AT(1A)R-expressing cells was mapped throughout the brains of three transgenic mice with a bacterial artificial chromosome-expressing green fluorescent protein under the control of the AT(1A)R promoter. The overall distribution correlated with that of the AT(1A)Rs mapped by other methods and demonstrated that the majority of C1 neurons express the AT(1A)R. Cre-recombinase expression in C1 neurons of AT(1A)R-floxed mice enabled demonstration that the pressor response to microinjection of angiotensin II into the rostral ventrolateral medulla is dependent upon expression of the AT(1A)R in these neurons. Lentiviral-induced expression of wild-type AT(1A)Rs in C1 neurons of global AT(1A)R knock-out mice, implanted with radiotelemeter devices for recording blood pressure, modulated the pressor response to aversive stress. During prolonged cage-switch stress, expression of AT(1A)Rs in C1 neurons induced a greater sustained pressor response when compared to the control viral-injected group (22 ± 4 mmHg for AT(1A)R vs 10 ± 1 mmHg for GFP; p < 0.001), which was restored toward that of the wild-type group (28 ± 2 mmHg). This study demonstrates that AT(1A)R expression by C1 neurons is essential for the pressor response to angiotensin II and that this pathway plays an important role in the pressor response to aversive stress.

Distribution of cells expressing human renin-promoter activity in the brain of a transgenic mouse

Renin plays a critical role in fluid and electrolyte homeostasis by cleaving angiotensinogen to produce Ang peptides. Whilst it has been demonstrated that renin mRNA is expressed in the brain, the distribution of cells responsible for this expression remains uncertain. We have used a transgenic mouse approach in an attempt to address this question. A transgenic mouse, in which a 12.2 kb fragment of the human renin promoter was used to drive expression of Cre-recombinase, was crossed with the ROSA26-lac Z reporter mouse strain. Cre-recombinase mediated excision of the floxed stop cassette resulted in expression of the reporter protein, beta-galactosidase. This study describes the distribution of beta-galactosidase in the brain of the crossed transgenic mouse. In all cases where it was examined the reporter protein was co-localized with the neuronal marker NeuN. An extensive distribution was observed with numerous cells labeled in the somatosensory, insular, piriform and retrosplenial cortices. The motor cortex was devoid of labeled cells. Several other regions were labeled including the parts of the amygdala, periaqueductal gray, lateral parabrachial nucleus and deep cerebellar nuclei. Overall the distribution shows little overlap with those regions that are known to express receptors for the renin-angiotensin system in the adult brain. This transgenic approach, which demonstrates the distribution of cells which have activated the human renin promoter at any time throughout development, yields a unique and extensive distribution of putative renin-expressing neurons. Our observations suggest that renin may have broader actions in the brain and may indicate a potential for interaction with the (pro)renin receptor or production of a ligand for non-AT(1)/AT(2) receptors.

Human brain aminopeptidase A: biochemical properties and distribution in brain nuclei

Aminopeptidase A (APA) generated brain angiotensin III, one of the main effector peptides of the brain renin angiotensin system, exerting a tonic stimulatory effect on the control of blood pressure in hypertensive rats. The distribution of APA in human brain has not been yet studied. We first biochemically characterized human brain APA (apparent molecular mass of 165 and 130 kDa) and we showed that the human enzyme exhibited similar enzymatic characteristics to recombinant mouse APA. Both enzymes had similar sensitivity to Ca(2+). Kinetic studies showed that the K(m) (190 mumol/L) of the human enzyme for the synthetic substrate-l-glutamyl-beta-naphthylamide was close from that of the mouse enzyme (256 mumol/L). Moreover, various classes of inhibitors including the specific and selective APA inhibitor, (S)-3-amino-4-mercapto-butyl sulfonic acid, had similar inhibitory potencies toward both enzymes. Using (S)-3-amino-4-mercapto-butyl sulfonic acid, we then specifically measured the activity of APA in 40 microdissected areas of the adult human brain. Significant heterogeneity was found in the activity of APA in the various analyzed regions. The highest activity was measured in the choroids plexus and the pineal gland. High activity was also detected in the dorsomedial medulla oblongata, in the septum, the prefrontal cortex, the olfactory bulb, the nucleus accumbens, and the hypothalamus, especially in the paraventricular and supraoptic nuclei. Immunostaining of human brain sections at the level of the medulla oblongata strengthened these data, showing for the first time a high density of immunoreactive neuronal cell bodies and fibers in the motor hypoglossal nucleus, the dorsal motor nucleus of the vagus, the nucleus of the solitary tract, the Roller nucleus, the ambiguus nucleus, the inferior olivary complex, and in the external cuneate nucleus. APA immunoreactivity was also visualized in vessels and capillaries in the dorsal motor nucleus of the vagus and the inferior olivary complex. The presence of APA in several human brain nuclei sensitive to angiotensins and involved in blood pressure regulation suggests that APA in humans is an integral component of the brain renin angiotensin system and strengthens the idea that APA inhibitors could be clinically tested as an additional therapy for the treatment of certain forms of hypertension.

Paraventricular nucleus, stress response, and cardiovascular disease

The paraventricular nucleus of the hypothalamus (PVN) is a complex effector structure that initiates endocrine and autonomic responses to stress. It receives inputs from visceral receptors, circulating hormones such as angiotensin II, and limbic circuits and contains neurons that release vasopressin, activate the adrenocortical axis, and activate preganglionic sympathetic or parasympathetic outflows. The neurochemical control of the different subgroups of PVN neurons is complex. The PVN has been implicated in the pathophysiology of congestive heart failure and the metabolic syndrome.

NITRIC OXIDE and SYMPATHETIC NERVE ACTIVITY IN THE CONTROL OF BLOOD PRESSURE

1. Endothelial dysfunction marked by impairment in the release of nitric oxide (NO) is seen very early in the development of hypertension and is considered important in mediating the impaired vascular tone evident in essential hypertensive patients. 2. Recently, a hypothesis has emerged that NO acting as a neurotransmitter in the brain can modulate levels of sympathetic nerve activity and thereby blood pressure. The NO inhibition model of hypertension has been used to explore the possibility that a decrease in levels of NO can cause an increase in levels of sympathetic nerve activity that can mediate the hypertension. 3. In the present review, we examine the literature regarding the role of NO in setting the mean level of sympathetic nerve activity and blood pressure. Although the acute effects of NO inhibition are well understood, the chronic interaction between the sympathetic nervous system and NO has only been investigated using indirect measures of sympathetic nerve activity, such as ganglionic blockade. This has led to inconsistent results regarding the role of NO in modulating sympathetic nerve activity chronically. 4. Some of the conflicting results may be explained by differences in the 'background' levels of angiotensin (Ang) II. Evidence suggests that NO may interact with AngII and baroreceptor afferent inputs in the central nervous system to set the mean level of sympathetic nerve activity. 5. We suggest chronic NO inhibition can increase sympathetic nerve activity if baroreceptor input is intact and AngII levels are elevated. Although studies exploring the actions of NO or AngII in isolation are useful for gathering initial information, future studies should focus on their interactions and their role in setting the long-term levels of sympathetic activity and blood pressure.

Intracisternal galanin/angiotensin II interactions in central cardiovascular control

The aim of this work was to investigate the interactions between angiotensin II (Ang II) and galanin(1-29) [GAL(1-29)] or its N-terminal fragment galanin(1-15) [GAL(1-15)] on central cardiovascular control. The involvement of angiotensin type1 (AT1) receptor subtype was analyzed by the AT1 antagonist, DuP 753. Anesthesized male Sprague-Dawley rats received intracisternal microinjections of Ang II (3 nmol) with GAL(1-29) (3 nmol) or GAL(1-15) (0.1 nmol) alone or in combination. The changes in mean arterial pressure (MAP) and heart rate (HR) recorded from the femoral artery were analyzed. The injection of Ang II and GAL(1-15) alone did not produce any change in MAP. However, coinjections of both Ang II and GAL(1-15) elicited a significant vasopressor response. This response was blocked by DuP 753. Ang II and GAL(1-15) alone produced an increase in HR. The coinjections of Ang II with GAL(1-15) induced an increase in HR not significantly different from the tachycardia produced by each peptide. The presence of DuP 753 counteracted this response. GAL(1-29) alone elicited a transient vasopressor response that disappeared in the presence of Ang II. The coinjections of Ang II with GAL(1-29) and with DuP 753 restored the transient vasopressor effect produced by GAL(1-29). GAL(1-29) produced a slight but significant tachycardic effect that was not modified in the presence of Ang II. The presence of DuP 753 did not modify the tachycardic response produced by Ang II and GAL(1-29). These results give indications for the existence of a differential modulatory effect of Ang II with GAL(1-15) and GAL(1-29) on central blood pressure response that might be dependent on the activity of the angiotensin AT1 receptor subtype.

Central angiotensin II AT1 receptors mediate fetal swallowing and pressor responses in the near-term ovine fetus

Swallowed volumes in the fetus are greater than adult values (per body weight) and serve to regulate amniotic fluid volume. Central ANG II stimulates swallowing, and nonspecific ANG II receptor antagonists inhibit both spontaneous and ANG II-stimulated swallowing. In the adult rat, AT1 receptors mediate both stimulated drinking and pressor activities, while the role of AT2 receptors is controversial. As fetal brain contains increased ANG II receptors compared with the adult brain, we sought to investigate the role of both AT1 and AT2 receptors in mediating fetal swallowing and pressor activities. Five pregnant ewes with singleton fetuses (130 +/- 1 days) were prepared with fetal vascular and lateral ventricle (LV) catheters and electrocorticogram and esophageal electromyogram electrodes and received three studies over 5 days. On day 1 (ANG II), following a 2-h basal period, 1 ml artificial cerebrospinal fluid (aCSF) was injected in the LV. At time 4 h, ANG II (6.4 microg) was injected in the LV, and the fetus was monitored for a final 2 h. On day 3, AT1 receptor blocker (losartan 0.5 mg) was administered at 2 h, and ANG II plus losartan was administered at 4 h. On day 5, AT2 receptor blocker (PD-123319; 0.8 mg was administered at 2 h and ANG II plus PD-123319 at 4 h. In the ANG II study, LV injection of ANG II significantly increased fetal swallowing (0.9 +/- 0.1 to 1.4 +/- 0.1 swallows/min; P < 0.05). In the losartan study, basal fetal swallowing significantly decreased in response to blockade of AT1 receptors (0.9 +/- 0.1 to 0.4 +/- 0.1 swallows/min; P < 0.05), while central injection of ANG II in the presence of AT1 receptor antagonism did not increase fetal swallowing (0.6 +/- 0.1 swallows/min). In the PD-123319 study, basal fetal swallowing did not change in response to blockade of AT2 receptor (0.9 +/- 0.1 swallows/min), while central injection of ANG II in the presence of AT2 blockade significantly increased fetal swallowing (1.5 +/- 0.1 swallows/min; P < 0.05). ANG II caused significant pressor responses in the control and PD-123319 studies but no pressor response in the presence of AT1 blockade. These data demonstrate that in the near-term ovine fetus, AT1 receptor but not AT2 receptors accessible via CSF contribute to dipsogenic and pressor responses.

Angiotensin II modulates the cardiovascular responses to microinjection of NPY Y1 and NPY Y2 receptor agonists into the nucleus tractus solitarii of the rat

The aim of this work was to investigate the modulation of the cardiovascular effects of neuropeptide Y (NPY) in the nucleus tractus solitarii (NTS) by angiotensin II (Ang II) and to determine the NPY receptor subtype involved in this modulation. Anesthesized Sprague-Dawley rats received microinjections in the NTS of Ang II (threshold and ED(50) doses) with NPY Y(1) agonist Leu(31)Pro(34)NPY and NPY Y(2) agonist NPY(13-36) (threshold and ED(50) doses). The changes in mean arterial pressure (MAP) and heart rate (HR) recorded in the femoral artery were analyzed during 60 min after the microinjections. The injection of threshold doses of Ang II, Y(1) agonist or Y(2) agonist alone did not produce any change in cardiovascular parameters. However, the co-injections into the NTS of threshold doses of both Ang II and the Y(1) agonist elicited significant increases of MAP and HR of about 12 and 10%, respectively. The co-administration of threshold doses of Ang II with the Y(2) agonist also induced a significant vasopressor response. The vasodepressor and bradycardiac effect of an ED(50) dose of the Y(1) agonist was significantly counteracted (P<0.01) by a threshold dose of Ang II. The vasopressor effect elicited by an ED(50) dose of the Y(2) agonist was significantly enhanced by a threshold dose of Ang II (P<0.01). No significant change of cardiovascular responses elicited by an ED(50) dose of Ang II was observed in the presence of threshold doses of the Y(1) agonist or of the Y(2) agonist. The present study gives functional evidences for a differential modulatory activity of Ang II on the cardiovascular responses mediated by Y(1) and Y(2) receptor subtypes, which may be of relevance for central cardiovascular regulation in the NTS.

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.

Angiotensin and baroreflex control of the circulation

There is a close association between the location of angiotensin (Ang) receptors and many important brain nuclei involved in the regulation of the cardiovascular system. The present review encompasses the physiological role of Ang II in the brainstem, particularly in relation to its influence on baroreflex control of the heart and kidney. Activation of AT1 receptors in the brainstem by fourth ventricle (4V) administration to conscious rabbits or local administration of Ang II into the rostral ventrolateral medulla (RVLM) of anesthetized rabbits acutely increases renal sympathetic nerve activity (RSNA) and RSNA baroreflex responses. Administration of the Ang antagonist Sarile into the RVLM of anesthetized rabbits blocked the effects of Ang II on the RSNA baroreflex, indicating that the RVLM is the major site of sympathoexcitatory action of Ang II given into the cerebrospinal fluid surrounding the brainstem. However, in conscious animals, blockade of endogenous Ang receptors in the brainstem by the 4V AT1 receptor antagonist losartan resulted in sympathoexcitation, suggesting an overall greater activity of endogenous Ang II within the sympathoinhibitory pathways. However, the RSNA response to airjet stress in conscious rabbits was markedly attenuated. While we found no effect of acute central Ang on heart rate baroreflexes, chronic 4V infusion inhibited the baroreflex and chronic losartan increased baroreflex gain. Thus, brainstem Ang II acutely alters sympathetic responses to specific afferent inputs thus forming part of a potentially important mechanism for the integration of autonomic response patterns. The sympathoexcitatory AT1 receptors appear to be activated during stress, surgery and anesthesia.

Role of angiotensin II receptors in the regulation of vasomotor neurons in the ventrolateral medulla

1. There is a high density of angiotensin type 1 (AT1) receptors in various brain regions involved in cardiovascular regulation. The present review will focus on the role of AT1 receptors in regulating the activity of sympathetic premotor neurons in the rostral part of the ventrolateral medulla (VLM), which are known to play a pivotal role in the tonic and phasic regulation of sympathetic vasomotor activity and arterial pressure. 2. Microinjection of angiotensin (Ang) II into the rostral VLM (RVLM) results in an increase in arterial pressure and sympathetic vasomotor activity. These effects are blocked by prior application of losartan, a selective AT1 receptor antagonist, indicating that they are mediated by AT1 receptors. However, microinjection of AngII into the RVLM has no detectable effect on respiratory activity, indicating that AT1 receptors are selectively or even exclusively associated with vasomotor neurons in this region. 3. Under normal conditions in anaesthetized animals, AT1 receptors do not appear to contribute significantly to the generation of resting tonic activity in RVLM sympathoexcitatory neurons. However, recent studies suggest that they contribute significantly to the tonic activity of these neurons under certain conditions, such as salt deprivation or heart failure, or in spontaneously hypertensive or genetically modified rats in which the endogenous levels of AngII are increased or in which AT1 receptors are upregulated. 4. Recent evidence also indicates that AT1 receptors play an important role in mediating phasic excitatory inputs to RVLM sympathoexcitatory neurons in response to activation of some neurons within the hypothalamic paraventricular nucleus. The physiological conditions that lead to activation of these AT1 receptor-mediated inputs are unknown. Further studies are also required to determine the cellular mechanisms of action of AngII in the RVLM and its interactions with other neurotransmitters in that region.

Central chemosensitivity and breathing asleep in unilateral medullary lesion patients: comparisons to animal data

The rostro-ventrolateral medulla (RVLM) is a site of chemosensitivity in animals; such site(s) have not been defined in humans. We studied the effect of unilateral focal lesions in the rostrolateral medulla (RLM) of man, on the ventilatory CO(2) sensitivity and during awake and sleep breathing. Nine patients with RLM lesions (RLM group), and six with lesions elsewhere (non-RLM group) were studied. The ventilatory CO(2) sensitivity was lower in the RLM compared with the non-RLM group (mean (S.D.), RLM, 1.4 (0.9), non-RLM 3.0 (0.6) L min(-1) mmHg(-1)). In both groups resting breathing was normal. During sleep all RLM patients had frequent arousals, four had significant sleep disordered breathing (SDB), only one non-RLM patient had SDB. Our findings in humans resemble those in animals with focal RVLM lesions. This review provides evidence that in humans there is an area of chemosensitivity in the RLM. We propose that in humans, dorsal displacement of the RVLM area of chemosensitivity in animals, arises from development of the olive plus the consequences of the evolution of the cerebellum/inferior peduncle.

Central angiotensin and baroreceptor control of circulation

Angiotensin (Ang) receptors are located in many important central nuclei involved in the regulation of the cardiovascular system. While most interest has focused on forebrain circumventricular actions, areas of the brainstem such as the nucleus of the solitary tract and the ventrolateral medulla contain high concentrations of AT1 receptors. The present review encompasses the physiological role of Ang II in the hindbrain, particularly in relation to its influence on baroreflex control mechanisms. In rabbits there are sympatho-excitatory AT1 receptors in the rostral ventrolateral medulla (RVLM), accessible to Ang II from the cerebrospinal fluid. Activation of these receptors acutely increases renal sympathetic nerve activity (RSNA) and RSNA baroreflex responses. However, blockade of endogenous Ang receptors in the brainstem also shows sympathoexcitation, suggesting there is greater endogenous activity of a sympathoinhibitory Ang II action. Microinjections of angiotensin antagonists into the RVLM showed relatively little tonic activity of endogenous Ang II influencing sympathetic activity in conscious rabbits. However, Ang II receptors in the RVLM mediate sympathetic responses to airjet stress in conscious rabbits. Similarly with respect to heart rate baroreflexes, there appears to be little tonic effect of angiotensin in the brainstem in normal conscious animals. Chronic infusion of Ang II for two weeks into the fourth ventricle of conscious rabbits inhibits the cardiac baroreflex while infusion of losartan increases the gain of the reflex. These actions suggest that Ang II in the brainstem modulates sympathetic responses depending on specific afferent and synaptic inputs in both the short term but importantly also in the long term, thus forming an important mechanism for increasing the range of adaptive response patterns.

Central inhibition of AT1receptors by eprosartan--in vitro autoradiography in the brain

All components of the renin-angiotensin system have been demonstrated in the brain and AT1 receptors have been localized in brain areas involved in central cardiovascular regulation. It is currently unclear whether AT1 receptor antagonists, which are increasingly used in the treatment of arterial hypertension and chronic heart failure, have the potential to mediate action via the central renin-angiotensin system. Therefore, we tested the in vivo access of the non-peptide AT1 receptor antagonist, eprosartan (30 and 60 mg per kg of body weight (BW) for 4 weeks, i.p. administered by osmotic minipumps), to angiotensin II receptors in the rat brain by in vitro autoradiography with 125I- (Sar1- Ile8) angiotensin II as a ligand. Eprosartan significantly increased plasma renin activity by four-fold and six-fold at doses of 30 and 60 mg x kg(-1), respectively (P< 0.05 vs CTRL). In the brain, eprosartan produced a dose-dependent inhibition of AT receptor binding in the median cerebral artery ( 850 +/- 249 and 650 +/- 106 vs 1072 +/- 116 dpm x mm(-2) of CTRL; P< 0.05). Furthermore, eprosartan inhibited angiotensin II receptor binding in discrete brain areas, which express exclusively, or predominantly, AT1 receptors both outside and within the blood-brain barrier, such as the paraventricular nucleus ( 180 +/- 47 and 130 +/- 18 vs 545 +/- 99 dpm x mm(-2)of CTRL; P< 0.05), the subfornical organ ( 106 +/- 26 and 112 +/- 17 vs 619 +/- 256 dpm x mm(-2)of CTRL; P< 0.05), and the organum vasculosum laminae terminalis ( 461 +/- 110 and 763 +/- 136 vs 1033 +/- 123 dpmx mm(-2)of CTRL; P< 0.05). These results emphasize that eprosartan readily crosses the blood-brain barrier in vivo and selectively inhibits binding to AT1 receptors in specific brain nuclei. The modulation of central regulatory mechanisms might contribute to AT1 receptor antagonists overall therapeutic efficacy in cardiovascular disease.

Does angiotensin II have a significant tonic action on cardiovascular neurons in the rostral and caudal VLM?

The peptidic ANG II receptor antagonists [Sar(1),Ile(8)]ANG II (sarile) or [Sar(1),Thr(8)]ANG II (sarthran) are known to decrease arterial pressure and sympathetic activity when injected into the rostral part of the ventrolateral medulla (VLM). In anesthetized rabbits and rats, the profound depressor and sympathoinhibitory response after bilateral microinjections of sarile or sarthran into the rostral VLM was unchanged after prior selective blockade of angiotensin type 1 (AT(1)) and ANG-(1---7) receptors, although this abolished the effects of exogenous ANG II. Unlike the neuroinhibitory compounds muscimol or lignocaine, microinjections of sarile in the rostral VLM did not affect respiratory activity. Sarile or sarthran in the caudal VLM resulted in a large pressor and sympathoexcitatory response, which was also unaffected by prior blockade of AT(1) and ANG-(1---7) receptors. The results indicate that the peptidic ANG receptor antagonists profoundly inhibit the tonic activity of cardiovascular but not respiratory neurons in the VLM and that these effects are independent of ANG II or ANG-(1---7) receptors.

The physiological role of AT1 receptors in the ventrolateral medulla

Neurons in the rostral and caudal parts of the ventrolateral medulla (VLM) play a pivotal role in the regulation of sympathetic vasomotor activity and blood pressure. Studies in several species, including humans, have shown that these regions contain a high density of AT1 receptors specifically associated with neurons that regulate the sympathetic vasomotor outflow, or the secretion of vasopressin from the hypothalamus. It is well established that specific activation of AT1 receptors by application of exogenous angiotensin II in the rostral and caudal VLM excites sympathoexcitatory and sympathoinhibitory neurons, respectively, but the physiological role of these receptors in the normal synaptic regulation of VLM neurons is not known. In this paper we review studies which have defined the effects of specific activation or blockade of these receptors on cardiovascular function, and discuss what these findings tell us with regard to the physiological role of AT1 receptors in the VLM in the tonic and phasic regulation of sympathetic vasomotor activity and blood pressure.

Angiotensin peptides and baroreflex control of sympathetic outflow: pathways and mechanisms of the medulla oblongata

The baroreceptor reflex is a relatively high gain control system that maintains arterial pressure within normal limits. To a large extent, this is accomplished through central neural pathways responsible for autonomic outflow residing in the medulla oblongata. The circulating renin-angiotensin system also contributes to the regulation of blood pressure, predominantly through its effects on the control of hydromineral balance and fluid volume. All the components of the renin-angiotensin system are also found in the brain. One of the principal products of the renin-angiotensin system cascade (brain or blood), angiotensin II, modulates the baroreceptor reflex by diminishing the sensitivity of the reflex and shifting the operating point for regulation of sympathetic outflow to higher blood pressures. This paper reviews our current knowledge about the neuronal pathways in the medulla oblongata through which angiotensin peptides alter the baroreceptor reflex control of sympathetic nerve activity. Emphasis is placed on the probable components and neural mechanisms of the medullary baroreflex arc that account for the ability of angiotensin peptides to change the sensitivity of the baroreceptor reflex and to shift the baroreceptor reflex control of sympathetic outflow to higher blood pressures in a pressure-independent manner.

Activation of brain neurons by circulating angiotensin II: direct effects and baroreceptor-mediated secondary effects

Circulating angiotensin II acts on neurons in circumventricular organs, leading to activation of central pathways involved in blood pressure regulation and body fluid homeostasis. Apart from this primary effect, an increase in the level of circulating angiotensin II may also activate brain neurons as a secondary consequence of the associated increase in blood pressure, which will stimulate arterial baroreceptors and thus activate central neurons that are part of the central baroreceptor reflex pathway. The aim of this study was to identify the population of neurons that are activated as a consequence of the direct actions of circulating angiotensin II on the brain, independent of secondary baroreceptor-mediated effects. For this purpose, we have mapped the distribution of neurons in the brainstem and forebrain that are immunoreactive for Fos (a marker of neuronal activation) following intravenous infusion of angiotensin II in conscious rabbits with chronically denervated carotid sinus and aortic baroreceptors. The distribution was compared with that evoked by the same procedure in two separate groups of barointact rabbits, in which angiotensin II was infused either at a rate similar to that in the barodenervated group, or at a rate approximately five times greater. In barodenervated rabbits, angiotensin II infusion evoked a significant increase in Fos expression, compared to control animals infused with the vehicle solution alone, in several forebrain nuclei (organum vasculosum of the lamina terminalis, subfornical organ, median preoptic nucleus, supraoptic nucleus, paraventricular nucleus, bed nucleus of the stria terminalis and suprachiasmatic nucleus), but little or no increase in Fos expression in any lower brainstem region. In barointact rabbits infused with angiotensin II at a similar rate to that in barodenervated rabbits, a similar degree of Fos expression was evoked in all of the above forebrain regions, but in addition a significantly greater degree of Fos expression was evoked in several medullary regions (nucleus tractus solitarius, area postrema, and ventrolateral medulla), even though the angiotensin II-evoked increase in mean arterial pressure (17 +/- 3 mmHg) was less than that evoked in the barodenervated rabbits (26 +/- 2 mmHg). In barointact rabbits infused with angiotensin II at the higher rate, the increase in mean arterial pressure was 29 +/- 3 mmHg. In these animals, the pattern of Fos expression was similar to that evoked in barointact rabbits infused at the lower rate, but the degree of Fos expression in all medullary regions and in some forebrain regions was significantly greater. The results of the present study, together with those of previous studies from our laboratory in which we determined the effects of phenylephrine-induced hypertension on brain Fos expression [Li and Dampney (1994) Neuroscience 61, 613-634; Potts et al. (1997) Neuroscience 77, 503-520], indicate that in conscious rabbits circulating angiotensin II activates primarily circumventricular neurons within the organum vasculosum of the lamina terminalis and subfornical organ, but not the area postrema, and this in turn leads to activation of neurons in other forebrain regions, including the median preoptic, supraoptic, paraventricular and suprachiasmatic nucleus as well as the bed nucleus of the stria terminalis. In contrast, the activation of neurons in medullary regions evoked by an increase in the level of circulating angiotensin II is primarily a secondary effect resulting from stimulation of arterial baroreceptors.

Angiotensin II receptors in the human brain

The distribution of angiotensin AT1 and AT2 receptors in the human central nervous system has been mapped and is reviewed here. The results discussed provide the anatomical basis for inferences regarding the physiological role of angiotensin in the human brain. The distribution of the AT2 receptor is very restricted in the human brain and shows a high degree of variability across species. The physiological role of this receptor in the adult central nervous system is not clear. In contrast, a high correlation exists between the distributions of AT1 receptors in the human and other mammalian brains studied. This pattern of distribution suggests that angiotensin, acting through the AT1 receptor, would act as a neuromodulator or neurotransmitter in the human central nervous system to influence fluid and electrolyte homeostasis, pituitary hormone release and autonomic control of cardiovascular function.

Mapping tissue angiotensin-converting enzyme and angiotensin AT1, AT2 and AT4 receptors

BACKGROUND

The renin-angiotensin system (RAS) functions as both a circulating endocrine system and a tissue paracrine/autocrine system. As a circulating peptide, angiotensin II (Ang II) plays a prominent role in blood-pressure control and body fluid and electrolyte balance by acting on the AT1 receptor in the brain and peripheral tissues. As a paracrine/autocrine peptide, locally formed Ang II also plays additional roles in tissues involving the regulation of regional haemodynamics, cell growth and remodelling, and neurotransmitter release. Evidence is emerging that Ang II is not the only active peptide of the RAS, and other Ang II fragments may also have important biological activities.

OBJECTIVES

To provide a morphological basis for understanding novel actions of angiotensin-converting enzyme (ACE), Ang II and related peptides in tissues, this article will review the localization of ACE and AT1, AT2 and AT4 receptors in the central nervous system, blood vessels and kidney.

RESULTS AND CONCLUSION

Autoradiographic mapping of the major components of the RAS has proved a valuable strategy to reveal, or suggest, cellular sites of novel actions for Ang II and related peptides in tissues. First, colocalization of ACE and AT1 receptors in the substantia nigra, the caudate nucleus and putamen of human and rat brain, which contain the dopamine-synthesizing neurons, suggests that the central RAS may be important in modulating central dopamine release. Secondly, the distribution of AT4 receptors with a striking association with cholinergic neurons, motor and sensory nuclei in the brain reveals that Ang IV may modulate central motor and sensory activities and memory. Thirdly, the occurrence of high levels of ACE and AT1 and/or AT2 receptors in the adventitia of blood vessels suggests important paracrine roles of the vascular RAS. Finally, the identification of abundant AT1 receptor and elucidation of its roles in the renomedullary interstitial cells of the kidney may provide a new impetus to study further the role of Ang II in the regulation of renal medullary function and blood pressure. Overall, circulating and locally produced Ang II and related peptides may exert a remarkable range of actions in the brain, kidney and cardiovascular system through multiple angiotensin receptors.

Angiotensin receptors in the nervous system

In addition to its traditional role as a circulating hormone, angiotensin is also involved in local functions through the activity of tissue renin-angiotensin systems that occur in many organs, including the brain. In the brain, both systemic and presumptive neurally derived angiotensin and angiotensin metabolites act through specific receptors to modulate many functions. This review examines the distribution of these specific angiotensin receptors and discusses evidence regarding the function of angiotensin peptides in various brain regions. Angiotensin AT1 and AT2 receptors occur in characteristic distributions that are highly correlated with the distribution of angiotensin-like immunoreactivity in nerve terminals. Acting through the AT1 receptor in the brain, angiotensin has effects on fluid and electrolyte homeostasis, neuroendocrine systems, autonomic pathways regulating cardiovascular function and behavior. Angiotensin AT1 receptors are also found in many afferent and efferent components of the peripheral autonomic nervous system. The role of the AT2 receptor in the brain is less well understood, although recent knockout studies point to an involvement with behavioral and cardiovascular functions. In addition to the AT1 and AT2 receptors, receptors for other fragments of angiotensin have been proposed. The AT4 binding site, which binds angiotensin, has a widespread distribution in the brain quite distinct from that of the AT1 and AT2 receptors. It is associated with many cholinergic neuronal groups and also several sensory nuclei, but its function remains to be determined. Our discovery that another brain-derived peptide binds to the AT4 binding site in the brain and may represent the native ligand is discussed. Overall, the distribution of angiotensin receptors in the brain indicate that they play diverse and important physiological roles in the nervous system.

Angiotensin AT1 receptor-mediated excitation of rat carotid body chemoreceptor afferent activity

1. A high density of angiotensin II receptors was observed in the rat carotid body by in vitro autoradiography employing 125I-[Sar1, Ile8]-angiotensin II as radioligand. Displacement studies demonstrated that the receptors were of the AT1 subtype. 2. The binding pattern indicated that the AT1 receptors occurred over clumps of glomus cells, the principal chemoreceptor cell of the carotid body. Selective lesions of the sympathetic or afferent innervation of the carotid body had little effect on the density of receptor binding, demonstrating that the majority of AT1 receptors were intrinsic to the glomus cells. 3. To determine the direct effect of angiotensin II on chemoreceptor function, without the confounding effects of the vasoconstrictor action of angiotensin II, carotid sinus nerve activity was recorded from the isolated carotid body in vitro. The carotid body was superfused with Tyrode solution saturated with carbogen (95 % O2, 5 % CO2), maintained at 36 C, and multi-unit nerve activity recorded with a suction electrode. 4. Angiotensin II elicited a dose-dependent excitation of carotid sinus nerve activity (maximum increase of 36 +/- 11 % with 10 nM angiotensin II) with a threshold concentration of 1 nM. The response was blocked by the addition of an AT1 receptor antagonist, losartan (1 microM), but not by the addition of an AT2 receptor antagonist, PD123319 (1 microM). 5. In approximately 50 % of experiments the excitation was preceded by an inhibition of activity (maximum decrease of 24 +/- 8 % with 10 nM angiotensin II). This inhibitory response was markedly attenuated by losartan but not affected by PD123319. 6. These observations demonstrate that angiotensin II, acting through AT1 receptors located on glomus cells in the carotid body, can directly alter carotid chemoreceptor afferent activity. This provides a means whereby humoral information about fluid and electrolyte homeostasis might influence control of cardiorespiratory function.

Separation of peripheral and central cardiovascular actions of angiotensin II

The pressor and vasoconstrictor action of angiotensin II (ANG II) is considered to be caused by a combination of its direct and indirect vascular effects, the latter mediated by the sympathetic nervous system. The purpose of this study was to determine the extent to which the direct and indirect actions of ANG II contribute to its pressor and vascular effects. Blood pressure, cutaneous vascular, and plasma norepinephrine responses to intravenous ANG II were measured in conscious rabbits before and after inhibition of central sympathetic outflow with intravenous and intracisternal clonidine and after ganglionic blockade with intravenous pentolinium. Intravenous ANG II caused a similar dose-related rise in blood pressure before and after sympathetic blockade with intravenous clonidine, intracisternal clonidine, and intravenous pentolinium. In contrast, the dose-related fall in cutaneous ear blood flow and cutaneous ear temperature and rise in cutaneous ear vascular resistance induced by intravenous ANG II were abolished after intravenous clonidine, intracisternal clonidine, and intravenous pentolinium. Heart rate was unchanged after ANG II. There were no changes in back skin or rectal temperature. There was a nonsignificant fall in plasma norepinephrine and no change in epinephrine after ANG II. These results demonstrate that the acute pressor response to intravenous ANG II is mediated by its direct vascular effects and is not dependent on central or ganglionic stimulation of the sympathetic nervous system, in contrast to the effect of ANG II on cutaneous ear vasoconstriction, which is predominantly caused by a centrally mediated increase in sympathetic nervous activity. Our results separate, in conscious rabbits, the direct vascular effects of ANG II from its indirect vascular actions, which are mediated by central sympathetic stimulation in the brain.

Central origin of decreased heart rate variability in patients with cardiovascular diseases

Sympathetic activity influences the total power of the heart rate variability spectrum. Decreased heart rate variability and increased sympathetic tone have been related to poor prognosis and an increased risk of sudden cardiac death in patients with myocardial infarction, congestive heart failure or hypertension. On the basis of recent experimental data, I propose that these disease states are associated with altered spectral characteristics of the central sympathetic activity generated within rostral ventrolateral medulla neurones, which subsequently leads to decreased heart rate variability and increased systemic sympathetic tone.

Medullary neurons activated by angiotensin II in the conscious rabbit

Previous studies have shown that angiotensin II (Ang II) can activate cardiovascular neurons within the medulla oblongata via an action on specific receptors. The purpose of this study was to determine the distribution of neurons within the medulla activated by infusion of Ang II into the fourth ventricle of conscious rabbits, using the expression of Fos, the protein product of the immediate early gene c-fos as a marker of neuronal activation. Experiments were done in both intact and barodenervated animals. In comparison with a control group infused with Ringer's solution alone, in both intact and barodenervated animals, fourth ventricular infusion of Ang II (4 to 8 pmol/min) induced a significant increase in the number of Fos-positive neurons in the nucleus of the solitary tract and in the rostral, intermediate, and caudal parts of the ventrolateral medulla. Double-labeling for Fos and tyrosine hydroxylase immunoreactivity showed that 50% to 75% of Fos-positive cells in the rostral, intermediate, and caudal ventrolateral medulla and 30% to 40% of Fos-positive cells in the nucleus of the solitary tract were also positive for tyrosine hydroxylase in both intact and barodenervated animals. The distribution of Fos-positive neurons corresponded very closely to the location of Ang II receptor binding sites as previously determined in the rabbit. The results indicate that medullary neurons activated by Ang II are located in discrete regions within the nucleus of the solitary tract and ventrolateral medulla and include, in all of these regions, both catecholamine and noncatecholamine neurons.

Angiotensin II decreases a resting K+ conductance in rat bulbospinal neurons of the C1 area

In the rostral ventrolateral medulla (RVLM), angiotensin II (Ang II) receptors are concentrated in the region that contains neurons innervating sympathetic preganglionic neurons. We sought to determine whether these bulbospinal cells are sensitive to Ang II. Retrogradely labeled bulbospinal RVLM neurons (N = 125) were recorded in thin slices from neonatal rats. Most (33 of 46) histologically recovered bulbospinal neurons were C1 cells (immunoreactive for tyrosine hydroxylase [TH-ir] or phenylethanolamine N-methyltransferase [PNMT-ir]). Bulbospinal RVLM neurons were spontaneously active (2.7 +/- 0.2 spikes per second, n = 69) with 'resting' potential of -54 +/- 0.4 mV (n = 77) and input resistance of 879 +/- 53 M omega (n = 47). Ang II (0.3 to 1 mumol/L) increased the spontaneous firing rate of most bulbospinal neurons (+250%, 28 of 39). In current-clamp mode, Ang II (1 mumol/L) produced depolarization (+6.8 +/- 0.6 mV, n = 59 neurons) and increased input resistance (+21 +/- 2%, n = 36 neurons). In voltage-clamp mode, Ang II elicited an inward current (9.7 +/- 0.9 pA; holding potential, -40 to -55 mV; n = 25 neurons) that reversed polarity at the K+ equilibrium potential (n = 8 neurons) and was barium sensitive (n = 4 neurons). Ang II-evoked conductance change was voltage independent (-40 to -140 mV, n = 8 neurons). The effects of Ang II were blocked by losartan (9 of 9 neurons) but persisted in low Ca2+/high Mg2+ (7 of 7 neurons). Ang II-sensitive cells were inhibited by alpha 2-adrenergic receptor agonists (12 of 15 neurons). Ang II excited 91% (30 of 33) of TH-ir or PNMT-ir cells but 23% (3 of 13) of non-TH-ir neurons. In conclusion, RVLM bulbospinal cells express Ang II type-1 receptors whose activation leads to a reduction in resting K+ conductance.

Interactions of angiotensin II with central dopamine

There is a large body of evidence to support the concept of a relationship between brain Ang II and catecholamine systems. This interaction may participate in some central actions of Ang II such as cardiovascular control, dipsogenesis, and complex behaviours. It also extends to the nigrostriatal dopaminergic system which bear AT1 receptors, both on their cell bodies in the substantia nigra presynaptically, and on their terminals in the striatum, where Ang II can markedly potentiate DA release. This observation suggests that drugs which modulate central Ang II may be useful in regulating central dopaminergic activity.

Heterogeneity of angiotensin type 2 (AT2) receptors

Evidence continues to accumulate that strengthens the proposal of heterogeneity within both the AT1 and the AT2 receptor subtypes. Pharmacologic, biochemical and immunological studies of AT2 receptors expressed in N1E-115 cells strengthen the hypothesis of AT2 receptor heterogeneity. However, it is important to reassess these studies, especially in terms of how these results correlate with other reports of AT2 receptor heterogeneity. For example, AT2 receptor immunoreactivity was absent in some neuronal regions which have previously been proposed to express the AT2 receptor subtype. In particular, AT2 receptor staining was not seen in the inferior olive, a region which is known to express a high density of AT2 receptors. Upon first examination, these results were somewhat troubling. However, when compared with earlier reports, these results should not have been unexpected. For instance, Tsutsumi and Saaverdra previously have shown that AT2 receptors in the locus coeruleus are sensitive to the actions of guanine nucleotides, while AT2 receptors in the inferior olive are insensitive (21). These antisera were raised against a population of AT2 receptors which are sensitive to GTP gamma S and therefore, the lack of AT2 receptor staining in the inferior olive, as well as the presence of AT2 receptor immunoreactivity in the locus coeruleus, confirms and extends these earlier reports. In addition the AT2 receptors expressed in the locus coeruleus have been shown to be functionally distinct from AT2 receptors in the inferior olive. In this regard, Ang II has been shown to depress glutamate-induced EPSPs in the locus coeruleus, an effect which is mediated through the AT2 receptor (19). Conversely, AT2 receptors have been shown to increase the firing rate of neurons in the inferior olive (20). Collectively, these results would predict that staining should be absent in the inferior olive using these AT2-directed antisera. Indeed, in view of these earlier physiological and pharmacological studies, the presence of AT2 receptor immunoreactivity in the inferior olive would have been surprising. The most convincing example of AT2 receptor heterogeneity is the characterization of AT2 receptors present in N1E-115 cells. Separation of solubilized N1E-115 membranes by heparin-Sepharose chromatography generates two populations of AT2 receptors which are pharmacologically and biochemically distinct. In particular, CGP42112A was approximately 2 orders of magnitude more selective for Peak III AT2 receptors than was PD123319. Binding activity of Peak I and Peak III AT2 receptor populations also differed in their responses to GTP gamma S and DTT treatment. Lastly, the AT2-directed antisera, raised against the Peak I population of AT2 receptors, were not able to immunodetect the Peak III population of AT2 receptors in immunoblot analysis, or immunoprecipiatate AT2 binding activity from Peak III material. Pharmacological, biochemical and immunological analysis of the AT2 receptor clone isolated from N1E-115 cells revealed that it has the identical characteristics or properties of the Peak III receptor. The AT2 receptor isolated from N1E-115 cells exhibited a similar pharmacology as the Peak III AT2 receptor, in that CGP42112A was more effective at displacing 125I-Ang II binding activity than was PD123319. The AT2 receptor clone was also shown to be insensitive to the actions of GTP gamma S, as well as demonstrated increased binding activity in the presence of DTT, identical to the Peak III AT2 receptor. Lastly, immunoblot analysis of membranes prepared from COS-1 cells transfected with the AT2 receptor cDNA from N1E-115 cells did not demonstrate any immune-specific bands with the AT2-directed antisera. Characterization of an AT2 receptor cDNA isolated from N1E-115 cells reveals that this clone is identical to the Peak III type of AT2 receptor.

Human intermediate reticular zone: a cyto- and chemoarchitectonic study

The primary aim of this study was to provide a comprehensive account of the morphology, topography, and frequency of tyrosine hydroxylase- and substance P-like (TH-LI, SP-LI) immunoreactive neurons of the human intermediate reticular zone (IRt), the putative autonomic zone of the medullary reticular formation. A further aim is to examine the IRt from a three-dimensional perspective using computer reconstruction techniques and compare its relationship with other structures in the rest of the medullary reticular formation. Six adult human brains were obtained from individuals with no sign of cerebral disease and were perfusion fixed. Free-floating transverse sections were immunostained with monoclonal antibodies against tyrosine hydroxylase and substance P by the avidin-biotin-peroxidase technique. The entire IRt displays TH-LI cell bodies and fibers, and thus it is readily distinguishable from the neighbouring gigantocellular and parvicellular reticular nuclei. In contrast, SP-LI cells are restricted to the external part of the IRt that is found in the open medulla, while SP-LI fibers are more widely distributed. The IRt displays TH-LI neurons which are fusiform, oval, and round in shape. The SP-LI neurons of the IRt are primarily oval and fusiform. In preparations stained for Nissl substance, IRt cells were classified as pigmented and nonpigmented. A characteristic feature of the IRt is that its cells are larger (20 +/- 4 micrograms) than those of the laterally adjoining parvicellular (12 +/- 2 micrograms) and clearly smaller than those of the medially adjoining gigantocellular nuclei (33 +/- 6 micrograms). The shape of the IRt is in keeping with the radial organization of the medulla with zones emanating from the fourth ventricle. Three-dimensional computer reconstructions of the cell plots show that 1) TH-LI neurons extend through the entire IRt and densely packed in the rostral part of the ventrolateral IRt and 2) SP-LI neurons are found only in the rostral half of the medulla oblongata.

Angiotensin II receptor subtypes in the human central nervous system

The distribution of the AT1 and AT2 subtypes of angiotensin II receptor was mapped in the adult human central nervous system using quantitative in vitro autoradiography. Binding in all forebrain, midbrain, pontine, medullary and spinal cord sites where angiotensin II receptors have previously been described is of the AT1 subtype, as is binding in the small and large arteries in the adjacent meninges and in choroid plexus. By contrast, both AT1 and AT2 receptors occur in the molecular layer of the cerebellum. Angiotensin II AT1 receptors in the brain show a moderate degree of conservation across mammalian species studied so far, whereas expression of AT2 receptors is more variable, and is more restricted in the human CNS than in many other mammals. These differences between the subtype distributions in humans and other animals indicate the need for care when extrapolating the results of animal studies involving the brain angiotensin system.

Neurons in the barosensory area of the caudal ventrolateral medulla project monosynaptically on to sympathoexcitatory bulbospinal neurons in the rostral ventrolateral medulla

Neurons in the caudal ventrolateral medulla may function as interneurons in the baroreceptor reflex are by inhibiting sympathoexcitatory bulbospinal neurons in rostral ventrolateral medulla. While some caudal ventrolateral medullary neurons are excited orthodromically by baroreceptors and antidromically from the rostral ventrolateral medulla, there is no anatomical evidence to prove that these barosensory neurons of the caudal ventrolateral medulla monosynaptically innervate the bulbospinal neurons in the rostral ventrolateral medulla. To establish the presence of such a direct projection, barosensory neurons were identified in the rostral caudal ventrolateral medulla of anesthetized rats by criteria that they spontaneously discharged with a cardiac rhythm and were excited by baroreceptor stimulation. The anterograde tracer biocytin was iontophoresed onto these neurons and, in the same animal, the retrograde tracer wheatgerm-agglutinated apo-horseradish peroxidase conjugated to gold particles was injected by micropressure into the ipsilateral spinal (thoracic level 3) intermediolateral cell column to label bulbospinal neurons. After 18-24 h, rats were killed and sections through the rostral ventrolateral medulla were processed for both markers. By light microscopy, numerous biocytin-labeled varicose processes overlapped rostral ventrolateral medullary neurons containing wheatgerm-agglutinated apo-horseradish peroxidase conjugated to gold particles. By electron microscopy, biocytin was found in axons and terminals. The terminals (n = 76) were large (0.6-1.2 microns in diameter), contained numerous small, clear vesicles and formed primarily symmetric synapses on perikarya and large (1.5-4.5 microns) dendrites within the rostral ventrolateral medulla. Some of these target neurons contained wheatgerm-agglutinated apo-horseradish peroxidase conjugated to gold particles associated with lysosomes and multivesicular bodies in the cytoplasm. The results indicate that: (i) neurons in the barosensory sympathoinhibitory region of the caudal ventrolateral medulla directly synapse on bulbospinal neurons in the rostral ventrolateral medulla; and (ii) the synaptic profile (symmetric synapse) and location (perikarya and large dendrites) is consistent with the conclusion that baroreceptor neurons of the caudal ventrolateral medulla potently and monosynaptically inhibit sympathoexcitatory neurons of the rostral ventrolateral medulla. The findings support the hypothesis that the barosensory region of the rostral caudal ventrolateral medulla is an intermediate relay in the baroreceptor reflex are.