Vascular-brain signaling in hypertension: role of angiotensin II and nitric oxide

In search of a marker of altered cerebrovascular function in hypertension: Analysis of the fractional amplitude of low-frequency fluctuations in UK Biobank resting state fMRI data

The selfish brain mechanism proposes that in some patients with impaired cerebral blood flow (CBF) or cerebrovascular function, hypertension may develop as a compensatory mechanism that aims to maintain CBF by increasing systemic blood pressure through an increase in cardiovascular sympathetic tone. The amplitude of low frequency fluctuations (ALFF) in the resting state blood oxygenation level dependent (BOLD) functional MRI signal has been previously posited as an index of cerebrovascular reactivity. We investigated whether regional fractional ALFF (fALFF) differs between 2054 hypertensives and 1724 normotensives using data from the UK Biobank dataset. Our primary hypothesis was that cerebrovascular function in the medulla and other regions involved in sympathetic regulation differs between hypertensives and normotensives, and that this is reflected by regional variations in fALFF. There is a significant regional variation in fALFF (F(14) =1126.17, p < 2 × 10-16, partial η2 = 0.22), but this regional variation does not differ between hypertensives and normotensives (F(14) = 0.23, p = 0.99, partial η2 = 8 × 10-5). Prospective longitudinal studies of cerebral haemodynamics in hypertensives and normotensives are required to further investigate the selfish brain mechanism.

Role of neuronal nitric oxide synthase on cardiovascular functions in physiological and pathophysiological states

This review describes and summarizes the role of neuronal nitric oxide synthase (nNOS) on the central nervous system, particularly on brain regions such as the ventrolateral medulla (VLM) and the periaqueductal gray matter (PAG), and on blood vessels and the heart that are involved in the regulation and control of the cardiovascular system (CVS). Furthermore, we shall also review the functional aspects of nNOS during several physiological, pathophysiological, and clinical conditions such as exercise, pain, cerebral vascular accidents or stroke and hypertension. For example, during stroke, a cascade of molecular, neurochemical, and cellular changes occur that affect the nervous system as elicited by generation of free radicals and nitric oxide (NO) from vulnerable neurons, peroxide formation, superoxides, apoptosis, and the differential activation of three isoforms of nitric oxide synthases (NOSs), and can exert profound effects on the CVS. Neuronal NOS is one of the three isoforms of NOSs, the others being endothelial (eNOS) and inducible (iNOS) enzymes. Neuronal NOS is a critical homeostatic component of the CVS and plays an important role in regulation of different systems and disease process including nociception. The functional and physiological roles of NO and nNOS are described at the beginning of this review. We also elaborate the structure, gene, domain, and regulation of the nNOS protein. Both inhibitory and excitatory role of nNOS on the sympathetic autonomic nervous system (SANS) and parasympathetic autonomic nervous system (PANS) as mediated via different neurotransmitters/signal transduction processes will be explored, particularly its effects on the CVS. Because the VLM plays a crucial function in cardiovascular homeostatic mechanisms, the neuroanatomy and cardiovascular regulation of the VLM will be discussed in conjunction with the actions of nNOS. Thereafter, we shall discuss the up-to-date developments that are related to the interaction between nNOS and cardiovascular diseases such as hypertension and stroke. Finally, we shall focus on the role of nNOS, particularly within the PAG in cardiovascular regulation and neurotransmission during different types of pain stimulus. Overall, this review focuses on our current understanding of the nNOS protein, and provides further insights on how nNOS modulates, regulates, and controls cardiovascular function during both physiological activity such as exercise, and pathophysiological conditions such as stroke and hypertension.

The effect of stenting on blood pressure in hypertensive patients with symptomatic proximal subclavian or vertebral artery stenosis

OBJECTIVE

This study aimed to evaluate the effect of stenting on blood pressure in hypertensive patients with symptomatic proximal subclavian or vertebral artery stenosis.

BACKGROUND

Whether posterior circulation revascularization could reduce blood pressure in hypertensive patients with posterior circulation hypoperfusion has not been investigated in humans.

METHODS

A total of 48 patients with essential hypertension (33 males; mean age 63.0 ± 8.7 years) with symptomatic proximal subclavian or vertebral artery stenosis who underwent stenting successfully at the Fuwai Hospital were prospectively enrolled between January 2014 and December 2015. All 48 patients were followed up at 1, 3, and 6 months after the procedure. Blood pressure, use of antihypertensive agents, and complications were investigated.

RESULTS

Baseline values included office blood pressure of 132/77 ± 10/8 mmHg, mean 24-hr blood pressure of 127/75 ± 12/9 mmHg, and mean antihypertensive agents used of 1.6 ± 0.8. Stenosis of the subclavian and vertebral arteries decreased from 88.9 ± 9.5% and 85.8 ± 7.4% to 5.5 ± 3.5% and 4.6 ± 3.7%, respectively, immediately after the procedure. Reductions in office blood pressure were - 7/-3 (SD 3/2), -9/-4 (5/3), and - 10/-5 (7/5) mmHg at 1, 3, and 6 months, respectively. While 24-hr blood pressures after the procedure reduced by -5/-3 mmHg at 6 months, the total number of antihypertensive agents used at the aforementioned time points was unchanged.

CONCLUSION

This first prospective cohort study in humans showed that posterior circulation stenting is apparently effective in reducing blood pressure in hypertensive patients with symptomatic proximal subclavian or vertebral artery stenosis.

Brain renin-angiotensin system in broiler chickens with cold-induced pulmonary hypertension

1. The relative expression of angiotensinogen (AGT), renin, angiotensin-converting enzyme (ACE) and angiotensin II type 1 receptor (AT₁R) was determined using quantitative real-time PCR on tissue from the brain (forebrain, midbrain and hindbrain) to investigate the effect of cold-induced pulmonary hypertension syndrome (PHS) in broilers aged 42 days. Brain angiotensin II (Ang II) and AT₁R levels were measured using enzyme immunoassay. 2. The right ventricle/total ventricles (RV/TV) ratio of the heart was increased in broilers exposed to cold stress (PHS group) at the end of the experiment. 3. ACE and renin transcripts in three parts of the brain were significantly increased in the PHS group at 42 d of age compared to controls while AGT transcript was significantly increased only in the hindbrain of PHS birds. The amount of AT1R transcript did not differ between control and PHS groups. 4. The amount of Ang II significantly decreased only in the midbrain of PHS birds compared with controls while the amounts of AT₁R were not different between treatments in the three segments of the brain. 5. It was concluded that brain gene expression of AGT (in the hindbrain), renin, and ACE was upregulated in broilers with PHS whereas Ang II and AT1R levels were not changed. These results provided evidence of diminished involvement of the renin-angiotensin system in the pathogenesis of chicken pulmonary hypertension.

Autonomic regulation of T-lymphocytes: Implications in cardiovascular disease

The nervous and immune systems both serve as essential assessors and regulators of physiological function. Recently, there has been a great interest in how the nervous and immune systems interact to modulate both physiological and pathological states. In particular, the autonomic nervous system has a direct line of communication with immune cells anatomically, and moreover, immune cells possess receptors for autonomic neurotransmitters. This circumstantial evidence is suggestive of a functional interplay between the two systems, and extensive research over the past few decades has demonstrated neurotransmitters such as the catecholamines (i.e. dopamine, norepinephrine, and epinephrine) and acetylcholine have potent immunomodulating properties. Furthermore, immune cells, particularly T-lymphocytes, have now been found to express the cellular machinery for both the synthesis and degradation of neurotransmitters, which suggests the ability for both autocrine and paracrine signaling from these cells independent of the nervous system. The details underlying the functional interplay of this complex network of neuroimmune communication are still unclear, but this crosstalk is suggestive of significant implications on the pathogenesis of a number of autonomic-dysregulated and inflammation-mediated diseases. In particular, it is widely accepted that numerous forms of cardiovascular diseases possess imbalanced autonomic tone as well as altered T-lymphocyte function, but a paucity of literature exists discussing the direct role of neurotransmitters in shaping the inflammatory microenvironment during the progression or therapeutic management of these diseases. This review seeks to provide a fundamental framework for this autonomic neuroimmune interaction within T-lymphocytes, as well as the implications this may have in cardiovascular diseases.

Effects of Mandibular Extension on Pial Arteriolar Diameter Changes in Glucocorticoid-Induced Hypertensive Rats

Previously, in normotensive rats, it has been observed that a repetitive sub-maximal mouth opening (mandibular extension, ME) obtained by placing a home-made U-shaped dilator between the superior and inferior dental arches of the rat caused modulation of pial arteriolar tone. The present study was aimed to characterize pial microcirculation in two different cortical brain regions and to assess the hemodynamic effects of a single or double ME on pial arteriolar rhythmic diameter changes in rats rendered hypertensive by dexamethasone administrations. Cranial windows were prepared on parietal and frontal region. Pial arterioles were classified by Strahler method in five orders by in vivo fluorescence microscopy technique associated with a computerized system that permits off-line measurements of arteriolar diameter changes. Two 10 min ME at 10 min interval were applied; then the animals were monitored for further 240 min. Dexamethasone-treated rats exhibited a marked arterial rarefaction and asymmetry of bifurcation in the pial microvascular networks more evident in the frontal region. Starting from ME1, in both cortical areas, the arterioles dilated, and the vasodilation became significant compared to baseline after ME2 for the entire observation period. The spectral analysis carried out on order 2 arteriolar diameter change tracings, showed that double ME increased the spectral density of the frequency components related to endothelial, neuronal and myogenic activities in both the cortical regions studied. In conclusion, double ME has a generalized effect in the cortical areas by restoring the physiological vasomotion of the pial arterioles that was severely impaired by the experimentally hypertension.

Differential roles of hippocampal nNOS and iNOS in the control of baroreflex function in conscious rats

The baroreflex is a prominent moment-to-moment mechanism regulating the blood pressure. The hippocampus is a limbic structure in which has been pointed out as part of central network regulating baroreflex. However, the local neurochemical mechanisms involved in control of baroreflex function are not completely understood. Thus, this study aimed to investigate the involvement of nitrergic neurotransmission present in the dorsal hippocampus in baroreflex control of heart rate in conscious rats. For this, we evaluated the effect of bilateral microinjection into the dorsal hippocampus of either the nitric oxide (NO) scavenger carboxy-PTIO, the selective neuronal nitric oxide synthase (nNOS) inhibitor Nω-Propyl-l-arginine (NPLA) or the selective inducible nitric oxide synthase (iNOS) inhibitor 1400 W in bradycardia evoked by blood pressure increases in response to intravenous infusion of phenylephrine, and tachycardia caused by blood pressure decreases evoked by intravenous infusion of sodium nitroprusside. Bilateral microinjection of carboxy-PTIO into the dorsal hippocampus decreased the baroreflex tachycardic response without affecting the reflex bradycardia. Hippocampus treatment with NPLA increased the baroreflex bradycardia gain without affecting the reflex tachycardia. Bilateral hippocampal treatment with 1400 W decreased the reflex tachycardia and increased the baroreflex bradycardic response. Overall, these findings provide evidence that hippocampal nitrergic mechanisms acting in a NOS isoform-specific manner plays a prominent role in control of baroreflex function. Indeed, the results indicate that nNOS and iNOS exerts an inhibitory influence on reflex bradycardia, whereas iNOS mediates the reflex tachycardia.

Exaggerated blood pressure response to fasudil or nifedipine in hypertensive Ren-2 transgenic rats: role of altered baroreflex

Basal calcium sensitization is decreased in spontaneously hypertensive rats, although their blood pressure (BP) response to acute Rho-kinase inhibition is enhanced. Using fasudil (Rho-kinase inhibitor) or nifedipine (L-VDCC blocker), we evaluated the contribution of calcium sensitization and calcium entry to BP maintenance in hypertensive transgenic Ren-2 rats (TGR) focusing on the influence of major vasoactive systems and/or baroreflex efficiency on BP responses to these two drugs. Homozygous TGR and normotensive Hannover Sprague-Dawley (HanSD) control rats aged 5, 11, or 22 weeks were used. The acute BP-lowering effects of fasudil or nifedipine were studied in intact rats, nitric oxide-deficient L-NAME-pretreated rats and rats subjected to combined blockade of the renin-angiotensin system (RAS), sympathetic nervous system (SNS) and nitric oxide synthase (NOS). Fasudil- or nifedipine-induced BP reduction increased during hypertension development in TGR. By contrast, the nifedipine-induced BP response decreased, whereas the fasudil-induced BP response increased with age in HanSD controls. Our data indicated a major contribution of nifedipine-sensitive calcium entry and relative attenuation of calcium sensitization in hypertensive rats compared with normotensive controls. The BP responses to fasudil or nifedipine were enhanced by NOS inhibition and combined blockade in normotensive HanSD rats but not in hypertensive TGR. In conclusion, calcium  sensitization is attenuated by endogenous nitric oxide in normotensive HanSD rats but not in hypertensive TGR. Moreover, BP reduction elicited by acute Rho-kinase inhibition is partially compensated by enhanced sympathetic vasoconstriction. The decreased compensation in hypertensive rats with impaired baroreflex efficiency explains their greater BP response to fasudil than in normotensive animals.

Basal and Activated Calcium Sensitization Mediated by RhoA/Rho Kinase Pathway in Rats with Genetic and Salt Hypertension

Calcium sensitization mediated by RhoA/Rho kinase pathway can be evaluated either in the absence (basal calcium sensitization) or in the presence of endogenous vasoconstrictor systems (activated calcium sensitization). Our aim was to compare basal and activated calcium sensitization in three forms of experimental hypertension with increased sympathetic tone and enhanced calcium entry—spontaneously hypertensive rats (SHR), heterozygous Ren-2 transgenic rats (TGR), and salt hypertensive Dahl rats. Activated calcium sensitization was determined as blood pressure reduction induced by acute administration of Rho kinase inhibitor fasudil in conscious rats with intact sympathetic nervous system (SNS) and renin-angiotensin system (RAS). Basal calcium sensitization was studied as fasudil-dependent difference in blood pressure response to calcium channel opener BAY K8644 in rats subjected to RAS and SNS blockade. Calcium sensitization was also estimated from reduced development of isolated artery contraction by Rho kinase inhibitor Y-27632. Activated calcium sensitization was enhanced in all three hypertensive models (due to the hyperactivity of vasoconstrictor systems). In contrast, basal calcium sensitization was reduced in SHR and TGR relative to their controls, whereas it was augmented in salt-sensitive Dahl rats relative to their salt-resistant controls. Similar differences in calcium sensitization were seen in femoral arteries of SHR and Dahl rats.

Vasculo-Neuronal Coupling: Retrograde Vascular Communication to Brain Neurons

Continuous cerebral blood flow is essential for neuronal survival, but whether vascular tone influences resting neuronal function is not known. Using a multidisciplinary approach in both rat and mice brain slices, we determined whether flow/pressure-evoked increases or decreases in parenchymal arteriole vascular tone, which result in arteriole constriction and dilation, respectively, altered resting cortical pyramidal neuron activity. We present evidence for intercellular communication in the brain involving a flow of information from vessel to astrocyte to neuron, a direction opposite to that of classic neurovascular coupling and referred to here as vasculo-neuronal coupling (VNC). Flow/pressure increases within parenchymal arterioles increased vascular tone and simultaneously decreased resting pyramidal neuron firing activity. On the other hand, flow/pressure decreases evoke parenchymal arteriole dilation and increased resting pyramidal neuron firing activity. In GLAST-CreERT2; R26-lsl-GCaMP3 mice, we demonstrate that increased parenchymal arteriole tone significantly increased intracellular calcium in perivascular astrocyte processes, the onset of astrocyte calcium changes preceded the inhibition of cortical pyramidal neuronal firing activity. During increases in parenchymal arteriole tone, the pyramidal neuron response was unaffected by blockers of nitric oxide, GABA_A, glutamate, or ecto-ATPase. However, VNC was abrogated by TRPV4 channel, GABA_B, as well as an adenosine A₁ receptor blocker. Differently to pyramidal neuron responses, increases in flow/pressure within parenchymal arterioles increased the firing activity of a subtype of interneuron. Together, these data suggest that VNC is a complex constitutive active process that enables neurons to efficiently adjust their resting activity according to brain perfusion levels, thus safeguarding cellular homeostasis by preventing mismatches between energy supply and demand.

SIGNIFICANCE STATEMENT

We present evidence for vessel-to-neuron communication in the brain slice defined here as vasculo-neuronal coupling. We showed that, in response to increases in parenchymal arteriole tone, astrocyte intracellular Ca²⁺ increased and cortical neuronal activity decreased. On the other hand, decreasing parenchymal arteriole tone increased resting cortical pyramidal neuron activity. Vasculo-neuronal coupling was partly mediated by TRPV4 channels as genetic ablation, or pharmacological blockade impaired increased flow/pressure-evoked neuronal inhibition. Increased flow/pressure-evoked neuronal inhibition was blocked in the presence of adenosine A1 receptor and GABA_B receptor blockade. Results provide evidence for the concept of vasculo-neuronal coupling and highlight the importance of understanding the interplay between basal CBF and resting neuronal activity.

Centrally administered TNF increases arterial blood pressure independently of nitric oxide synthase

INTRODUCTION

Emerging evidence indicates that increased levels of TNF in the brain are associated with hypertension. Nitric oxide synthase (NOS) is involved in the central control of the cardiovascular system, exerting both pro- and antihypertensive effects. TNF induces hypothalamic synthesis of nitric oxide.

AIM

We checked if acutely administered TNF into the cerebral ventricles affects arterial blood pressure, heart rate and baroreflex sensitivity, and whether TNF actions are dependent on NOS in normotensive rats.

METHODS

We carried out hemodynamic measurements in 6 groups of freely moving, adult Sprague-Dawley male rats, intracerebroventricularly (ICV) infused with either: 1) saline (5μl/h); 2) TNF (200ng/5μl/h); 3) non-selective NO synthase inhibitor - l-NG-Nitroarginine Methyl Ester (l-NAME) (1mg/5μl/h); 4) TNF together with l-NAME (200ng and 1mg/5μl/h, respectively); 5) neuronal NO synthase inhibitor - 7-nitroindazole sodium salt (7-NI) (20μg/10μl/h); 6) or TNF together with 7-NI (200ng and 20μg/10μl/h, respectively). Mean arterial blood pressure (MABP), heart rate (HR) and spontaneous baroreflex sensitivity (sBRS) evaluated by the sequence method were analysed.

RESULTS

ICV infusion of TNF caused a significant increase in MABP accompanied by a transient increase in HR, and a decrease in sBRS. ICV infusion of l-NAME increased MABP, but it did not change HR, nor sBRS. ICV infusion of 7-NI did not affect MABP, nor HR, nor sBRS. TNF administered together with l-NAME increased MABP with a transient increase in HR without changes of sBRS. Similarly, ICV infusion of TNF with 7-NI increased MABP without changes in HR and sBRS.

CONCLUSIONS

Centrally administered TNF increases MABP and HR and blunts sBRS. The pressor effect of TNF appears to be independent of NOS activity in the brain. Inhibition of nNOS restores sBRS in TNF treated rats.

Complementing the Latest APA Definition of Hypnosis: Sensory-Motor and Vascular Peculiarities Involved in Hypnotizability

The aim of this article is to complement the recently revised American Psychological Association (APA) definition of hypnotizability. It (a) lists a few differences in sensorimotor integration between subjects with high (highs) and low (lows) hypnotizability scores in the ordinary state of consciousness and in the absence of suggestions, (b) proposes that hypnotizability-related cerebellar peculiarities may account for them,

Leptin influences the excitability of area postrema neurons

The area postrema (AP) is a circumventricular organ with important roles in central autonomic regulation. This medullary structure has been shown to express the leptin receptor and has been suggested to have a role in modulating peripheral signals, indicating energy status. Using RT-PCR, we have confirmed the presence of mRNA for the leptin receptor, ObRb, in AP, and whole cell current-clamp recordings from dissociated AP neurons demonstrated that leptin influenced the excitability of 51% (42/82) of AP neurons. The majority of responsive neurons (62%) exhibited a depolarization (5.3 ± 0.7 mV), while the remaining affected cells (16/42) demonstrated hyperpolarizing effects (-5.96 ± 0.95 mV). Amylin was found to influence the same population of AP neurons. To elucidate the mechanism(s) of leptin and amylin actions in the AP, we used fluorescence resonance energy transfer (FRET) to determine the effect of these peptides on cAMP levels in single AP neurons. Leptin and amylin were found to elevate cAMP levels in the same dissociated AP neurons (leptin: % total FRET response 25.3 ± 4.9, n = 14; amylin: % total FRET response 21.7 ± 3.1, n = 13). When leptin and amylin were coapplied, % total FRET response rose to 53.0 ± 8.3 (n = 6). The demonstration that leptin and amylin influence a subpopulation of AP neurons and that these two signaling molecules have additive effects on single AP neurons to increase cAMP, supports a role for the AP as a central nervous system location at which these circulating signals may act through common intracellular signaling pathways to influence central control of energy balance.

Intracellular Information Processing through Encoding and Decoding of Dynamic Signaling Features

Cell signaling dynamics and transcriptional regulatory activities are variable within specific cell types responding to an identical stimulus. In addition to studying the network interactions, there is much interest in utilizing single cell scale data to elucidate the non-random aspects of the variability involved in cellular decision making. Previous studies have considered the information transfer between the signaling and transcriptional domains based on an instantaneous relationship between the molecular activities. These studies predict a limited binary on/off encoding mechanism which underestimates the complexity of biological information processing, and hence the utility of single cell resolution data. Here we pursue a novel strategy that reformulates the information transfer problem as involving dynamic features of signaling rather than molecular abundances. We pursue a computational approach to test if and how the transcriptional regulatory activity patterns can be informative of the temporal history of signaling. Our analysis reveals (1) the dynamic features of signaling that significantly alter transcriptional regulatory patterns (encoding), and (2) the temporal history of signaling that can be inferred from single cell scale snapshots of transcriptional activity (decoding). Immediate early gene expression patterns were informative of signaling peak retention kinetics, whereas transcription factor activity patterns were informative of activation and deactivation kinetics of signaling. Moreover, the information processing aspects varied across the network, with each component encoding a selective subset of the dynamic signaling features. We developed novel sensitivity and information transfer maps to unravel the dynamic multiplexing of signaling features at each of these network components. Unsupervised clustering of the maps revealed two groups that aligned with network motifs distinguished by transcriptional feedforward vs feedback interactions. Our new computational methodology impacts the single cell scale experiments by identifying downstream snapshot measures required for inferring specific dynamical features of upstream signals involved in the regulation of cellular responses.

Single cell studies have shown that differential patterns in the dynamics of signaling proteins, transcription factor activity, gene expression, etc. produce distinct downstream outcomes. The opposite also holds true where particular cellular outcomes have been found to be associated with the dynamical pattern of one or more signaling molecules. Signaling pathways, therefore, serve as signal processing units to inform specific downstream regulation. However, the functional capabilities of the dynamic aspects of signaling are not well understood. To address this issue, we developed a new approach that evaluates information processing between dynamic features in signaling patterns and transcriptional regulatory activity. Our work demonstrates that the information transfer occur through decoding of temporal history of signals rather than only through instantaneous correlations. Moreover, our results identify regulatory network motifs as the critical components in the information processing and filtering of variability in signaling dynamics to produce distinct patterns of downstream transcriptional responses. Our methodology can be broadly applied to single cell scale data on experimentally accessible downstream measures to infer dynamic aspects of upstream signaling.

Revealing the role of the autonomic nervous system in the development and maintenance of Goldblatt hypertension in rats

Despite extensive use of the renovascular/Goldblatt model of hypertension—2K-1C, and the use of renal denervation to treat drug resistant hypertensive patients, autonomic mechanisms that underpin the maintenance of this hypertension are important yet remain unclear. Our aim was to analyse cardiovascular autonomic function by power spectral density analysis of both arterial pressure and pulse interval measured continuously by radio telemetry for 6 weeks after renal artery clipping. Mean arterial pressure increased from 106 ± 5 to 185 ± 2 mm Hg during 5 weeks post clipping when it stabilized. A tachycardia developed during the 4th week, which plateaued between weeks 5 and 6. The gain of the cardiac vagal baroreflex decreased immediately after clipping and continued to do so until the 5th week when it plateaued (from − 2.4 ± 0.09 to − 0.8 ± 0.04 bpm/mm Hg; P < 0.05). A similar time course of changes in the high frequency power spectral density of the pulse interval was observed (decrease from 13.4 ± 0.6 to 8.3 ± 0.01 ms²; P < 0.05). There was an increase in both the very low frequency and low frequency components of systolic blood pressure that occurred 3 and 4 weeks after clipping, respectively. Thus, we show for the first time the temporal profile of autonomic mechanisms underpinning the initiation, development and maintenance of renovascular hypertension including: an immediate depression of cardiac baroreflex gain followed by a delayed cardiac sympathetic predominance; elevated sympathetic vasomotor drive occurring after the initiation of the hypertension but coinciding during its mid-development and maintenance.

Brain stem NOS and ROS in neural mechanisms of hypertension

SIGNIFICANCE

There is now compelling evidence to substantiate the notion that by depressing baroreflex regulation of blood pressure and augmenting central sympathetic outflow through their actions on the nucleus tractus solitarii (NTS) and rostral ventrolateral medulla (RVLM), brain stem nitric oxide synthase (NOS) and reactive oxygen species (ROS) are important contributing factors to neural mechanisms of hypertension. This review summarizes our contemporary views on the impact of NOS and ROS in the NTS and RVLM on neurogenic hypertension, and presents potential antihypertensive strategies that target brain stem NOS/ROS signaling.

RECENT ADVANCES

NO signaling in the brain stem may be pro- or antihypertensive depending on the NOS isoform that generates this gaseous moiety and the site of action. Elevation of the ROS level when its production overbalances its degradation in the NTS and RVLM underlies neurogenic hypertension. Interventional strategies with emphases on alleviating the adverse actions of these molecules on blood pressure regulation have been investigated.

CRITICAL ISSUES

The pathological roles of NOS in the RVLM and NTS in neural mechanisms of hypertension are highly complex. Likewise, multiple signaling pathways underlie the deleterious roles of brain-stem ROS in neurogenic hypertension. There are recent indications that interactions between brain stem ROS and NOS may play a contributory role.

FUTURE DIRECTIONS

Given the complicity of action mechanisms of brain-stem NOS and ROS in neural mechanisms of hypertension, additional studies are needed to identify the most crucial therapeutic target that is applicable not only in animal models but also in patients suffering from neurogenic hypertension.

Chronic knockdown of the nucleus of the solitary tract AT1 receptors increases blood inflammatory-endothelial progenitor cell ratio and exacerbates hypertension in the spontaneously hypertensive rat

AT1 receptor subtype a (AT1Ra) expression is increased in the nucleus of the solitary tract (NTS) in spontaneously hypertensive rat (SHR) compared with Wistar Kyoto controls. However, the chronic role of AT1Ra in the NTS for cardiovascular control is not well understood. In this study, we investigated the hypothesis that the NTS AT1Ra is involved in the neural regulation of the peripheral inflammatory status and linked with hypertension. Transduction of brain neuronal cultures with recombinant adeno-associated virus type 2 (AAV2)-AT1R-small hairpin RNA (shRNA) resulted in a 72% decrease in AT1Ra mRNA and attenuated angiotensin II-induced increase in extracellular signal-regulated kinase 1/2 phosphorylation and neuronal firing. Specific NTS microinjection of AAV2-AT1R-shRNA vector in the SHR resulted in a ≈30 mm Hg increase in the mean arterial pressure compared with control vector-injected animals (Sc-shRNA: 154±4 mm Hg; AT1R-shRNA: 183±10 mm Hg) and induced a resetting of the baroreflex control of heart rate to higher mean arterial pressure. In addition, AAV2-AT1R-shRNA-treated SHRs exhibited a 74% decrease in circulating endothelial progenitor cells (CD90+, CD4- / CD5- / CD8-) and a 300% increase in the circulating inflammatory cells, including CD4+ + CD8+, CD45+ / 3+ T lymphocytes, and macrophages (CD68+). As a result, the endothelial progenitor cell/inflammatory cells ratio was decreased by 8- to 15-fold in the AT1R-shRNA-treated SHR. However, identical injection of AAV2-AT1R-shRNA into the NTS of Wistar Kyoto rats had no effect on mean arterial pressure and inflammatory cells. These observations suggest that increased expression of the AT1Ra in SHR NTS may present a counterhypertensive mechanism involving inflammatory/angiogenic cells.

Neurogenic hypertension and elevated vertebrobasilar arterial resistance: is there a causative link?

There is evidence of sympathetic overdrive in a significant proportion of patients with essential hypertension and an animal model of the condition, the spontaneously hypertensive rat (SHR). The reasons for this remain elusive. However, there is also evidence of narrowing of the arteries supplying the brainstem in the SHR and hypertensive humans. In this review, we discuss the possible role of brainstem hypoperfusion in driving increased sympathetic activity and hypertension.

The sympathetic nervous system and blood pressure in humans: implications for hypertension

A neurogenic component to primary hypertension (hypertension) is now well established. Along with raised vasomotor tone and increased cardiac output, the chronic activation of the sympathetic nervous system in hypertension has a diverse range of pathophysiological consequences independent of any increase in blood pressure. This review provides a perspective on the actions and interactions of angiotensin II, inflammation and vascular dysfunction/brain hypoperfusion in the pathogenesis and progression of neurogenic hypertension. The optimisation of current treatment strategies and the exciting recent developments in the therapeutic targeting of the sympathetic nervous system to control hypertension (for example, catheter-based renal denervation and carotid baroreceptor stimulation) will be outlined.

Decreased expression of neuronal nitric oxide synthase in the nucleus tractus solitarii inhibits sympathetically mediated baroreflex responses in rat

Despite numerous studies it remains controversial whether nitric oxide (NO·) synthesized by neuronal NOS (nNOS) plays an excitatory or inhibitory role in transmission of baroreflex signals in the nucleus tractus solitarii (NTS). In the current studies we sought to test the hypothesis that nNOS is involved in excitation of baroreflex pathways in NTS while excluding pharmacological interventions in assessing the influence of nNOS. We therefore developed, validated and utilized a short hairpin RNA (shRNA) to reduce expression of nNOS in the NTS of rats whose baroreflex activity was then studied. We demonstrate downregulation of nNOS through transduction with adeno-associated virus type 2 (AAV2) carrying shRNA for nNOS. When injected bilaterally into NTS AAV2nNOSshRNA significantly reduced reflex tachycardic responses to acute hypotension while not affecting reflex bradycardic responses to acute increases of arterial pressure. Control animals treated with intravenous propranolol to block sympathetically mediated chronotropic responses manifested the same baroreflex responses as animals that had been treated with AAV2nNOSshRNA. Neither AAV2 eGFP nor AAV2nNOScDNA affected baroreflex responses. Blocking cardiac vagal influences with atropine similarly reduced baroreflex-mediated bradycardic responses to increases in arterial pressure both in control animals and in those treated with AAV2nNOSshRNA. We conclude that NO· synthesized by nNOS in the NTS is integral to excitation of baroreflex pathways involved in reflex tachycardia, a largely sympathetically mediated response, but not reflex bradycardia, a largely parasympathetically mediated response. We suggest that, at the basal state, nNOS is maximally engaged. Thus, its upregulation does not augment the baroreflex.

Cardiovascular actions of leptin in the subfornical organ are abolished by diet-induced obesity

The subfornical organ (SFO), a sensory circumventricular organ lacking the normal blood-brain barrier with well documented roles in cardiovascular regulation, has recently been identified as a potential site at which the adipokine, leptin, may act to influence central autonomic pathways. Systemic and central leptin administration has been shown to increase blood pressure and it has been suggested that selective leptin resistance contributes to obesity-related hypertension. Given the relationship between obesity and hypertension, the present study aimed to investigate the cardiovascular consequences of the direct administration of leptin into the SFO of young lean rats and in the diet-induced obesity (DIO) rat model, which has been shown to be leptin-resistant. Leptin administration (500 fmol) directly into the SFO of young rats resulted in rapid decreases in blood pressure (BP) [mean area under the curve (AUC) = -677.8 ± 167.1 mmHg*s; n = 9], without an effect on heart rate (mean AUC = -21.2 ± 13.4 beats; n = 9), and these effects were found to be dose-related as microinjection of 5 pmol of leptin into the SFO had a larger effect on BP (mean AUC = -972.3 ± 280.1 mmHg*s; n = 4). These BP effects were also shown to be site-specific as microinjection of leptin into non-SFO regions or into the ventricle was without effect on BP (non-SFO: mean AUC = -22.4 ± 55.3 mmHg*s; n = 4; ventricle: mean AUC = 194.0 ± 173.0 mmHg*s; n = 6). By contrast, microinjection of leptin into leptin-resistant DIO rats was without effect on BP (mean AUC = 205.2 ± 75.1 mmHg*s; n = 4). These observations suggest that the SFO may be an important relay centre through which leptin, in normal weight, leptin responsive rats, acts to maintain BP within normal physiological limits through descending autonomic pathways involved in cardiovascular control and that, in obese, leptin-resistant, rats leptin no longer influences SFO neurones, resulting in an elevated BP, thus contributing to obesity-related hypertension.

Contributions of vascular inflammation in the brainstem for neurogenic hypertension

Essential hypertension is idiopathic although it is accepted as a complex polygenic trait with underlying genetic components, which remain unknown. Our supposition is that primary hypertension involves activation of the sympathetic nervous system. One pivotal region controlling arterial pressure set point is nucleus tractus solitarii (NTS). We recently identified that pro-inflammatory molecules, such as junctional adhesion molecule-1, were over expressed in endothelial cells of the microvasculature supplying the NTS in an animal model of human hypertension (the spontaneously hypertensive rat: SHR) compared to normotensive Wistar Kyoto (WKY) rats. We have also shown endogenous leukocyte accumulation inside capillaries within the NTS of SHR but not WKY rats. Despite the inflammatory state in the NTS of SHR, transcripts of some inflammatory molecules such as chemokine (C-C motif) ligand 5 (Ccl5), and its receptors, chemokine (C-C motif) receptor 1 and 3 were down-regulated in the NTS of SHR compared to WKY rats. This may be compensatory to avoid further strong inflammatory activity. More importantly, we found that down-regulation of Ccl5 in the NTS of SHR may be pro-hypertensive since microinjection of Ccl5 into the NTS of SHR decreased arterial pressure but was less effective in WKY rats. Leukocyte accumulation of the NTS microvasculature may also induce an increase in vascular resistance and hypoperfusion within the NTS; the latter may trigger release of pro-inflammatory molecules which via paracrine signaling may affect central neural cardiovascular activity conducive to neurogenic hypertension. All told, we suggest that vascular inflammation within the brainstem contributes to neurogenic hypertension by multiple pathways.

Acute reductions in blood flow restricted to the dorsomedial medulla induce a pressor response in rats

OBJECTIVES

The brainstem nucleus of the solitary tract (nucleus tractus solitarii, NTS) is a pivotal region for regulating the set-point of arterial pressure, the mechanisms of which are not fully understood. Based on evidence that the NTS exhibits O2-sensing mechanisms, we examined whether a localized disturbance of blood supply, resulting in hypoxia in the NTS, would lead to an acute increase in arterial pressure.

METHODS

Male Wistar rats were used. Cardiovascular parameters were measured before and after specific branches of superficial dorsal medullary veins were occluded; we assumed these were drainage vessels from the NTS and would produce stagnant hypoxia. Hypoxyprobe-1, a marker for detecting cellular hypoxia in the post-mortem tissue, was used to reveal whether vessel occlusion induced hypoxia within the NTS.

RESULTS

Following vessel occlusion, blood flow in the dorsal surface of the medulla oblongata including the NTS region showed an approximately 60% decrease and was associated with hypoxia in neurons located predominantly in the caudal part of the NTS as revealed using hypoxyprobe-1. Arterial pressure increased and this response was pronounced significantly in both magnitude and duration when baroreceptor reflex afferents were sectioned.

CONCLUSION

These results suggest that localized hypoxia in the NTS increases arterial pressure. We suggest this represents a protective mechanism whereby the elevated systemic pressure is a compensatory mechanism to enhance cerebral perfusion. Whether this physiological mechanism has any relevance to neurogenic hypertension is discussed.

Elevated vertebrobasilar artery resistance in neonatal spontaneously hypertensive rats

There is a strong correlation between increased vertebral artery resistance and arterial blood pressure in humans. The reasons for this increased resistance at high systemic pressure remain unknown, but may include raised sympathetic activity. With the recent finding that prehypertensive spontaneously hypertensive (PHSH) rats, which have raised sympathetic nerve activity, but a blood pressure comparable to normotensive rat strains, we hypothesized that its vertebrobasilar vascular resistance would already be raised and, as a consequence, would exhibit a more responsive Cushing response (e.g., brain ischemia evoked sympathoexcitation and a pressor response). We report that PHSH rats exhibited a remodeling of the basilar artery (i.e., increased wall thickness and lower lumen-to-wall thickness ratio) that occurred before the onset of hypertension. In a novel in vitro vascularly isolated, arterially perfused brain stem preparation of PHSH rats of 4-5 wk of age, brain stem vascular resistance was raised by ∼35% relative to age- and sex-matched normotensive rats (P < 0.05). In the in situ arterial perfused working heart-brain stem preparation, occlusion of both vertebral arteries in the PHSH rat resulted in a significantly greater increase in sympathetic activity (57 vs. 20%, PHSH vs. control; P < 0.01) that triggered a greater increase in arterial perfusion pressure (8 vs. 3 mmHg, PHSH vs. control; P < 0.01) compared with normotensive rats. These data indicate raised vertebrobasilar artery resistance before the onset of hypertension in the PHSH rat. With the raised responsiveness of the Cushing response in the PHSH rat, we discuss the possibility of brain stem perfusion as a central nervous system determinant of the set point of vasomotor sympathetic tone in the hypertensive condition.

The central nervous system and inflammation in hypertension

In recent years a major research effort has focused on the role of inflammation, and in particular adaptive immunity, in the genesis of hypertension. Hypertension stimulates the accumulation of inflammatory cells including macrophages and T lymphocytes in peripheral tissues important in blood pressure control, such as the kidney and vasculature. Angiotensin II modulates blood pressure via actions on the central nervous system (CNS) and the adaptive immune system. Recent work suggests that the central actions of angiotensin II via the circumventricular organs lead to activation of circulating T-cells and vascular inflammation. The neuro-immune system plays an essential role in the pathogenesis of hypertension and further understanding of this relationship could lead to the development of new treatment strategies.

Imbalance of central nitric oxide and reactive oxygen species in the regulation of sympathetic activity and neural mechanisms of hypertension

Nitric oxide (NO) and reactive oxygen species (ROS) play important roles in blood pressure regulation via the modulation of the autonomic nervous system, particularly in the central nervous system (CNS). In general, accumulating evidence suggests that NO inhibits, but ROS activates, the sympathetic nervous system. NO and ROS, however, interact with each other. Our consecutive studies and those of others strongly indicate that an imbalance between NO bioavailability and ROS generation in the CNS, including the brain stem, activates the sympathetic nervous system, and this mechanism is involved in the pathogenesis of neurogenic aspects of hypertension. In this review, we focus on the role of NO and ROS in the regulation of the sympathetic nervous system within the brain stem and subsequent cardiovascular control. Multiple mechanisms are proposed, including modulation of neurotransmitter release, inhibition of receptors, and alterations of intracellular signaling pathways. Together, the evidence indicates that an imbalance of NO and ROS in the CNS plays a pivotal role in the pathogenesis of hypertension.

NOS antagonism using viral vectors as an experimental strategy: implications for in vivo studies of cardiovascular control and peripheral neuropathies

Nitric oxide, a free gaseous signalling molecule, has attracted the attention of numerous biologists and has been implicated in the regulation of the cardiovascular, nervous and immune system. However, the cellular mechanisms mediating nitric oxide modulation remain unclear. Upregulation by gene over-expression or down-regulation by gene inactivation of nitric oxide synthase has generated quantitative changes in abundance thereby permitting functional insights. We have tested and proved that genetic nitric oxide synthase antagonism using viral vectors, particularly with dominant negative mutants and microRNA 30-based short hairpin RNA, is an efficient and effective experimental approach to manipulate nitric oxide synthase expression both in vitro and in vivo.

Endothelin receptor antagonists: potential in Alzheimer's disease

Alzheimer's disease (AD) is believed to be initiated by the accumulation of neurotoxic forms of Aβ peptide within the brain. AD patients show reduction of cerebral blood flow (CBF), the extent of the reduction correlating with the impairment of cognition. There is evidence that cerebral hypoperfusion precedes and may even trigger the onset of dementia in AD. Cerebral hypoperfusion impairs neuronal function, reduces the clearance of Aβ peptide and other toxic metabolites from the brain, and upregulates Aβ production. Studies in animal models of AD have shown the reduction in CBF to be more than would be expected for the reduction in neuronal metabolic activity. Aβ may contribute to the reduction in CBF in AD, as both Aβ₁₋₄₀ and Aβ₁₋₄₂ induce cerebrovascular dysfunction. Aβ₁₋₄₀ acts directly on cerebral arteries to cause cerebral smooth muscle cell contraction. Aβ₁₋₄₂ causes increased neuronal production and release of endothelin-1 (ET-1), a potent vasoconstrictor, and upregulation of endothelin-converting enzyme-2 (ECE-2), the enzyme which cleaves ET-1 from its inactive precursor. ET-1 and ECE-2 are also elevated in AD, making it likely that upregulation of the ECE-2-ET-1 axis by Aβ₁₋₄₂ contributes to the chronic reduction of CBF in AD. At present, only a few symptomatic treatment options exist for AD. The involvement of ET-1 in the pathogenesis of endothelial dysfunction associated with elevated Aβ indicates the potential for endothelin receptor antagonists in the treatment of AD. It has already been demonstrated that the endothelin receptor antagonist bosentan, preserves aortic and carotid endothelial function in Tg2576 mice, and our findings suggest that endothelin receptor antagonists may be beneficial in maintaining CBF in AD.

Robust dynamic balance of AP-1 transcription factors in a neuronal gene regulatory network

BACKGROUND

The octapeptide Angiotensin II is a key hormone that acts via its receptor AT1R in the brainstem to modulate the blood pressure control circuits and thus plays a central role in the cardiac and respiratory homeostasis. This modulation occurs via activation of a complex network of signaling proteins and transcription factors, leading to changes in levels of key genes and proteins. AT1R initiated activity in the nucleus tractus solitarius (NTS), which regulates blood pressure, has been the subject of extensive molecular analysis. But the adaptive network interactions in the NTS response to AT1R, plausibly related to the development of hypertension, are not understood.

RESULTS

We developed and analyzed a mathematical model of AT1R-activated signaling kinases and a downstream gene regulatory network, with structural basis in our transcriptomic data analysis and literature. To our knowledge, our report presents the first computational model of this key regulatory network. Our simulations and analysis reveal a dynamic balance among distinct dimers of the AP-1 family of transcription factors. We investigated the robustness of this behavior to simultaneous perturbations in the network parameters using a novel multivariate approach that integrates global sensitivity analysis with decision-tree methods. Our analysis implicates a subset of Fos and Jun dependent mechanisms, with dynamic sensitivities shifting from Fos-regulating kinase (FRK)-mediated processes to those downstream of c-Jun N-terminal kinase (JNK). Decision-tree analysis indicated that while there may be a large combinatorial functional space feasible for neuronal states and parameters, the network behavior is constrained to a small set of AP-1 response profiles. Many of the paths through the combinatorial parameter space lead to a dynamic balance of AP-1 dimer forms, yielding a robust AP-1 response counteracting the biological variability.

CONCLUSIONS

Based on the simulation and analysis results, we demonstrate that a dynamic balance among distinct dimers of the AP-1 family of transcription factors underlies the robust activation of neuronal gene expression in the NTS response to AT1R activation. Such a differential sensitivity to limited set of mechanisms is likely to underlie the stable homeostatic physiological response.

Vascular tone and neurovascular coupling: considerations toward an improved in vitro model

Neurovascular research has made significant strides toward understanding how the brain neurovascular unit accomplishes rapid and spatial increases in blood flow following neuronal activation. Among the experimental models used, the in vitro brain slice preparation provides unique information revealing the potential signals and cellular mechanisms involved in functional hyperemia. The most crucial limitation of this model, however, is the lack of intraluminal pressure and flow in the vessels being studied. Moreover, differences in basal vascular tone have led to varied interpretations regarding the polarity of vascular responses following neuron-to-glial stimulation. Given the complexity of astrocyte-induced neurovascular responses, we propose the use of a modified in vitro brain slice preparation, where intraluminal arteriolar pressure and flow are retained. Throughout this review, we discuss the advantages and disadvantages to be considered when using brain slices for neurovascular studies. Potential ways to overcome the current limitations are proposed.

Central and peripheral mechanisms of T-lymphocyte activation and vascular inflammation produced by angiotensin II-induced hypertension

RATIONALE

We have previously found that T lymphocytes are essential for development of angiotensin II-induced hypertension; however, the mechanisms responsible for T-cell activation in hypertension remain undefined.

OBJECTIVE

We sought to study the roles of the CNS and pressure elevation in T-cell activation and vascular inflammation caused by angiotensin II.

METHODS AND RESULTS

To prevent the central actions of angiotensin II, we created anteroventral third cerebral ventricle (AV3V) lesions in mice. The elevation in blood pressure in response to angiotensin II was virtually eliminated by AV3V lesions, as was activation of circulating T cells and the vascular infiltration of leukocytes. In contrast, AV3V lesioning did not prevent the hypertension and T-cell activation caused by the peripheral acting agonist norepinephrine. To determine whether T-cell activation and vascular inflammation are attributable to central influences or are mediated by blood pressure elevation, we administered hydralazine (250 mg/L) in the drinking water. Hydralazine prevented the hypertension and abrogated the increase in circulating activated T cells and vascular infiltration of leukocytes caused by angiotensin II.

CONCLUSIONS

We conclude that the central and pressor effects of angiotensin II are critical for T-cell activation and development of vascular inflammation. These findings also support a feed-forward mechanism in which modest degrees of blood pressure elevation lead to T-cell activation, which in turn promotes inflammation and further raises blood pressure, leading to severe hypertension.

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.

Antihypertensive effects of central ablations in spontaneously hypertensive rats

Commissural nucleus of the solitary tract (commNTS) lesions transitorily (first 5 days) reduce mean arterial pressure (MAP) in spontaneously hypertensive rats (SHR), and lesions of the tissue surrounding the anteroventral third ventricle (AV3V region) chronically reduce MAP in other models of hypertension. In the present study, we investigated the effects of combined AV3V+commNTS electrolytic lesions on MAP and heart rate (HR) in conscious SHR. Baseline MAP and HR were recorded in male SHR before and for the next 40 days after sham or AV3V lesions combined with sham or commNTS lesions. The AV3V lesions produced no change in MAP in SHR, while commNTS lesions reduced MAP acutely (121 ± 2 to 127 ± 3 mmHg in the 1st and 5th days, respectively, vs. prelesion: 192 ± 4 mmHg) but not chronically (from 10 to 40 days). However, combined AV3V+commNTS lesions reduced MAP of SHR chronically (119 ± 2 to 161 ± 4 mmHg, in the 1st and 40th day, respectively, vs. prelesion levels: 186 ± 4 mmHg) or sham-lesioned SHR (187 ± 4 to 191 ± 6 mmHg). Sympathetic and angiotensinergic blockade produced less reduction in MAP in SHR with AV3V+commNTS-lesions, and there was no relationship between changes on water and food intake, body weight, or urinary excretion produced by AV3V+commNTS lesions with the changes in MAP. The present findings suggest that in the absence of the commNTS, the AV3V region contributes to the hypertension observed in SHR by mechanisms that appear to involve enhanced angiotensinergic and sympathetic activity.

Is neurogenic hypertension related to vascular inflammation of the brainstem?

Essential hypertension is idiopathic although it is accepted as a complex polygenic trait with underlying genetic components, which remain unknown. Our supposition is that hypertension involves activation of the sympathetic nervous system. One pivotal region controlling arterial pressure set point is nucleus tractus solitarii (NTS). We recently identified that pro-inflammatory molecules, such as junctional adhesion molecule-1 (JAM-1), were over expressed in endothelial cells of the microvasculature supplying the NTS in an animal model of human hypertension (the spontaneously hypertensive rat) compared to normotensive Wistar-Kyoto rats (WKY). Over expression of JAM-1 in NTS of WKY rats was pro-hypertensive and induced leukocyte adherence to the microvasculature. Since leukocyte adhesion causes cytokine release, we found expression of monocyte chemoattractant protein-1 (MCP-1) was higher in the NTS of SHR while inter-leukin-6 (IL-6) was lower compared to the WKY rat. Inflammation of the brainstem microvasculature may increase vascular resistance within the brainstem. High brainstem vascular resistance and its inflammation may release pathological paracrine signaling molecules affecting central neural cardiovascular activity conducive to neurogenic hypertension.

Harvey Cushing and the regulation of blood pressure in giraffe, rat and man: introducing 'Cushing's mechanism'

The fundamental mechanism that underlies essential hypertension is a high total peripheral resistance. We review here possible origins of high total peripheral resistance in physiologically hypertensive giraffes, spontaneously hypertensive rats and humans with essential hypertension. We propose that a common link could be reduced brainstem perfusion, as first suggested by Cushing in 1901. Any tendency towards reduction of cerebral blood flow to the cardiovascular control centres in rest and sleep will be prevented by activation of a response arising in the brainstem. The response will proportionately increase systemic blood pressure and return cerebral blood flow to a new homeostatic level. New evidence we review here supports this idea and leads us to suggest that central regulation of blood pressure has two components: the classic Cushing's response, which is a terminal event, and a Cushing's mechanism, which is a physiological mechanism for long-term control of mean arterial pressure. In giraffes, Cushing's mechanism is activated by increasing neck length during growth and subsequent gravitational hypotension that stimulates a rise in basal arterial blood pressure. In man and rats, the mechanism is activated by narrowing of the arteries supplying the brainstem. If we are correct, future successful treatment of essential hypertension in man will include methods of reducing cerebral arterial resistance.

The subfornical organ: a central nervous system site for actions of circulating leptin

Adipose tissue plays a critical role in energy homeostasis, secreting adipokines that control feeding, thermogenesis, and neuroendocrine function. Leptin is the prototypic adipokine that acts centrally to signal long-term energy balance. While hypothalamic and brain stem nuclei are well-established sites of action of leptin, we tested the hypothesis that leptin signaling occurs in the subfornical organ (SFO). The SFO is a circumventricular organ (CVO) that lacks the normal blood-brain barrier, is an important site in central autonomic regulation, and has been suggested to have a role in modulating peripheral signals indicating energy status. We report here the presence of mRNA for the signaling form of the leptin receptor in SFO and leptin receptor localization by immunohistochemistry within this CVO. Central administration of leptin resulted in phosphorylation of STAT3 in neurons of SFO. Whole cell current-clamp recordings from dissociated SFO neurons demonstrated that leptin (10 nM) influenced the excitability of 64% (46/72) of SFO neurons. Leptin was found to depolarize the majority of responsive neurons with a mean change in membrane potential of 7.3 +/- 0.6 mV (39% of all SFO neurons), while the remaining cells that responded to leptin hyperpolarized (-6.9 +/- 0.7 mV, 25% of all SFO neurons). Similar depolarizing and hyperpolarizing effects of leptin were observed in recordings from acutely prepared SFO slice preparations. Leptin was found to influence the same population of SFO neurons influenced by amylin as three of four cells tested for the effects of bath application of both amylin and leptin depolarized to both peptides. These observations identify the SFO as a possible central nervous system location, with direct access to the peripheral circulation, at which leptin may act to influence hypothalamic control of energy homeostasis.

Concepts of neural nitric oxide-mediated transmission

As a chemical transmitter in the mammalian central nervous system, nitric oxide (NO) is still thought a bit of an oddity, yet this role extends back to the beginnings of the evolution of the nervous system, predating many of the more familiar neurotransmitters. During the 20 years since it became known, evidence has accumulated for NO subserving an increasing number of functions in the mammalian central nervous system, as anticipated from the wide distribution of its synthetic and signal transduction machinery within it. This review attempts to probe beneath those functions and consider the cellular and molecular mechanisms through which NO evokes short- and long-term modifications in neural performance. With any transmitter, understanding its receptors is vital for decoding the language of communication. The receptor proteins specialised to detect NO are coupled to cGMP formation and provide an astonishing degree of amplification of even brief, low amplitude NO signals. Emphasis is given to the diverse ways in which NO receptor activation initiates changes in neuronal excitability and synaptic strength by acting at pre- and/or postsynaptic locations. Signalling to non-neuronal cells and an unexpected line of communication between endothelial cells and brain cells are also covered. Viewed from a mechanistic perspective, NO conforms to many of the rules governing more conventional neurotransmission, particularly of the metabotropic type, but stands out as being more economical and versatile, attributes that presumably account for its spectacular evolutionary success.

Neurohormonal regulation of the sympathetic nervous system: new insights into central mechanisms of action

To regulate blood pressure, the brain controls circulating hormones, which influence the brain by binding to brain neurons that lie outside the blood-brain barrier. Recent work has demonstrated that "cardiovascular" hormones are synthesized and released in the brain as neurotransmitters/neuromodulators and can, in some cases, signal through the blood-brain barrier. The renin-angiotensin system is a prototype for these newly appreciated mechanisms. The brain's intrinsic renin-angiotensin system plays an important role in blood pressure control. Angiotensin II in brain neurons affects other neurons both through activation of angiotensin receptors and via generation of nitric oxide and reactive oxygen molecules. Similarly, angiotensin in blood vessels activates endothelial nitric oxide, which can diffuse across the blood-brain barrier and thereby alter neuronal activity in cardiovascular control nuclei. The relative importance of these mechanisms to blood pressure control remains to be fully elucidated.

Signalling across the blood brain barrier by angiotensin II: novel implications for neurogenic hypertension

Angiotensin II (AngII) is a major culprit in essential hypertension. Based on a genetic rodent model of hypertension, we review here evidence that AngII may signal across the blood brain barrier to affect neuronal circuits within the nucleus tractus solitarii (NTS) of the brainstem, a pivotal region regulating both the baroreceptor reflex and set point control of arterial pressure. We have termed this form of signalling as vascular-neuronal signalling. We describe that the depressant action of AngII in NTS on the baroreceptor reflex is mediated via activation of endothelial nitric oxide synthase (eNOS) releasing NO that promotes release of the inhibitory transmitter-GABA. This could shunt the incoming excitatory baroreceptor afferent traffic impinging on NTS neurones. Chronic studies recording arterial pressure in conscious unrestrained rats using radio-telemetry have revealed that eNOS in NTS plays an endogenous physiological role in the homeostatic regulation of the gain of the cardiac baroreceptor reflex. However, in the spontaneously hypertensive rat, eNOS mRNA was higher (compared to normotensive rats), and its chronic blockade in NTS restored the abnormally depressed cardiac baroreceptor reflex to levels akin to normotensive rats, improved heart rate variability and lowered arterial pressure. Hence, it seems that excessive eNOS activity in NTS of the SHR contributes to the pathological state of this animal model's cardiovascular autonomic nervous system. We speculate on why eNOS activity may be up regulated in the NTS of the SHR and propose that it is a consequence of high cerebral vascular resistance and inadequate blood perfusion of the brainstem.

The area postrema: a brain monitor and integrator of systemic autonomic state

The area postrema is a medullary structure lying at the base of the fourth ventricle. The area postrema's privileged location outside of the blood-brain barrier make this sensory circumventricular organ a vital player in the control of autonomic functions by the central nervous system. By virtue of its lack of tight junctions between endothelial cells in this densely vascularized structure and the presence of fenestrated capillaries, peptide and other physiological signals borne in the blood have direct access to neurons that project to brain areas with important roles in the autonomic control of many physiological systems, including the cardiovascular system and systems controlling feeding and metabolism. However, the area postrema is not simply a conduit through which signals flow into the brain, but it is now being recognized as the initial site of integration for these signals as they enter the circuitry of the central nervous system.