PVN lesions prevent the endothelin 1-induced increase in arterial pressure and vasopressin

The Causal Relationship between Endothelin-1 and Hypertension: Focusing on Endothelial Dysfunction, Arterial Stiffness, Vascular Remodeling, and Blood Pressure Regulation

Hypertension (HTN) is one of the most prevalent diseases worldwide and is among the most important risk factors for cardiovascular and cerebrovascular complications. It is currently thought to be the result of disturbances in a number of neural, renal, hormonal, and vascular mechanisms regulating blood pressure (BP), so crucial importance is given to the imbalance of a number of vasoactive factors produced by the endothelium. Decreased nitric oxide production and increased production of endothelin-1 (ET-1) in the vascular wall may promote oxidative stress and low-grade inflammation, with the development of endothelial dysfunction (ED) and increased vasoconstrictor activity. Increased ET-1 production can contribute to arterial aging and the development of atherosclerotic changes, which are associated with increased arterial stiffness and manifestation of isolated systolic HTN. In addition, ET-1 is involved in the complex regulation of BP through synergistic interactions with angiotensin II, regulates the production of catecholamines and sympathetic activity, affects renal hemodynamics and water–salt balance, and regulates baroreceptor activity and myocardial contractility. This review focuses on the relationship between ET-1 and HTN and in particular on the key role of ET-1 in the pathogenesis of ED, arterial structural changes, and impaired vascular regulation of BP. The information presented includes basic concepts on the role of ET-1 in the pathogenesis of HTN without going into detailed analyses, which allows it to be used by a wide range of specialists. Also, the main pathological processes and mechanisms are richly illustrated for better understanding.

Chronic exposure to low doses of estradiol-17ß increases blood pressure in young female rats: A possible role for central Endothelin-1

Previously, we demonstrated that chronic exposure to low levels of estradiol-17β (E2) increases mean arterial pressure (MAP) in young female Sprague-Dawley (SD) rats, however, the underlying mechanisms are unclear. Since endothelin-1 (ET-1) is implicated in blood pressure (BP) regulation, we hypothesized that E2’s effects on MAP are mediated through central ET-1. To test this, young female SD rats were either sham implanted or implanted s.c. with slow-release E2 pellets (20 ng/day for 90 days). BP was monitored by telemetry. After 75 days of E2 exposure, ET_A antagonist or vehicle was administered i.c.v. After 90 days of E2 exposure, rats were sacrificed, and the paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM) were microdissected for gene expression and protein analysis of ET-1 and its receptors. E2 exposure increased MAP after pellet implantation. Gene expression of ET-1 and ET_A but not ET_B receptors were upregulated in the PVN and RVLM of E2 treated animals. Further, the protein levels of ET_A receptor were also increased in the PVN of E2 treated animals. However, i.c.v. infusion of the ET_A antagonist did not completely block the increase in blood pressure. Our results suggest that increases in central ET-1 activity could possibly play a role in chronic E2-induced increase in BP but further studies are needed to completely understand the contribution of ET-1 in this phenomenon.

Bidirectional neuro-glial signaling modalities in the hypothalamus: role in neurohumoral regulation

Maintenance of bodily homeostasis requires concerted interactions between the neuroendocrine and the autonomic nervous systems, which generate adaptive neurohumoral outflows in response to a variety of sensory inputs. Moreover, an exacerbated neurohumoral activation is recognized to be a critical component in numerous disease conditions, including hypertension, heart failure, stress, and the metabolic syndrome. Thus, the study of neurohumoral regulation in the brain is of critical physiological and pathological relevance. Most of the work in the field over the last decades has been centered on elucidating neuronal mechanisms and pathways involved in neurohumoral control. More recently however, it has become increasingly clear that non-neuronal cell types, particularly astrocytes and microglial cells, actively participate in information processing in areas of the brain involved in neuroendocrine and autonomic control. Thus, in this work, we review recent advances in our understanding of neuro-glial interactions within the hypothalamic supraoptic and paraventricular nuclei, and their impact on neurohumoral integration in these nuclei. Major topics reviewed include anatomical and functional properties of the neuro-glial microenvironment, neuron-to-astrocyte signaling, gliotransmitters, and astrocyte regulation of signaling molecules in the extracellular space. We aimed in this review to highlight the importance of neuro-glial bidirectional interactions in information processing within major hypothalamic networks involved in neurohumoral integration.

Endothelin-1 in paraventricular nucleus modulates cardiac sympathetic afferent reflex and sympathetic activity in rats

Background

Cardiac sympathetic afferent reflex (CSAR) is a positive-feedback, sympathoexcitatory reflex. Paraventricular nucleus (PVN) is an important component of the central neurocircuitry of the CSAR. The present study is designed to determine whether endothelin-1 (ET-1) in the PVN modulates the CSAR and sympathetic activity, and whether superoxide anions are involved in modulating the effects of ET-1 in the PVN in rats.

Methodology/Principal Findings

In anaesthetized Sprague–Dawley rats with cervical vagotomy and sinoaortic denervation, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded. The CSAR was evaluated by the responses of the RSNA and MAP to epicardial application of capsaicin. Microinjection of ET-1 into the bilateral PVN dose-dependently enhanced the CSAR, increased the baseline RSNA and MAP. The effects of ET-1 were blocked by PVN pretreatment with the ET_A receptor antagonist BQ-123. However, BQ-123 alone had no significant effects on the CSAR, the baseline RSNA and MAP. Bilateral PVN pretreatment with either superoxide anion scavenger tempol or polyethylene glycol-superoxide dismutase (PEG-SOD) inhibited the effects of ET-1 on the CSAR, RSNA and MAP. Microinjection of ET-1 into the PVN increased the superoxide anion level in the PVN, which was abolished by PVN pretreatment with BQ-123. Epicardial application of capsaicin increased superoxide anion level in PVN which was further enhanced by PVN pretreatment with ET-1.

Conclusions

Exogenous activation of ET_A receptors with ET-1 in the PVN enhances the CSAR, increases RSNA and MAP. Superoxide anions in PVN are involved in the effects of ET-1 in the PVN.

Endothelin-mediated calcium responses in supraoptic nucleus astrocytes influence magnocellular neurosecretory firing activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca(2+) imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca(2+) ](i) levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET(B) receptors and mobilisation of thapsigargin-sensitive Ca(2+) stores. The presence of ET(B) receptors in SON astrocytes was also verified immunohistochemically. ET(B) receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET(B) -mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca(2+) ](i) were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca(2+) chelation in the recorded SON neurones failed to block ET(B) -mediated effects. In summary, our results indicate that ET(B) receptor activation in SON astrocytes induces the mobilisation of [Ca(2+) ](i) , likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.

The paraventricular nucleus modulates thyroidal motilin release and rat gastric motility

Motilin, an important endocrine regulator of gastrointestinal motility, was once considered to be produced in the gastrointestinal tract and brain. In recent years, however, motilin has been found in the human thyroid, as well as in that of the guinea pig. The physiological function and central modulation of thyroidal motilin remain poorly understood. To determine the functional role of thyroidal motilin, we observed the concentration of motilin in the plasma and also gastric motility before and after thyroidectomy. Our studies show that both the concentration of plasma motilin and gastric motility were decreased after thyroidectomy. To explore modulation-related nuclei, a c-Fos immune response experiment was carried out. The PVN of the hypothalamus was the main area of reactivity after thyroidectomy. Subsequently, we studied the effects of electrical excitation and PVN lesions on gastric motility and the expression of motilin in the thyroid and plasma. Excitation of the PVN was shown to prompt gastric motility that was partly prevented by the motilin receptor antagonist, GM-109. The effects of PVN excitation on gastric contraction were significantly reduced in thyroidectomised rats. In addition, the expression of motilin in the thyroid was significantly increased after PVN excitation and decreased after PVN lesions. The changes in the concentration of motilin in plasma induced by PVN stimulation were positively correlated with changes of gastric motility. In our in vitro study, the motilin secreted from TT cells (a parafollicular cell line originating from human thyroid medullary carcinoma) gradually increased on day 6 of culture, and motilin and calcitonin (CT) were co-expressed in TT cells. These results demonstrate that motilin from the thyroid could be secreted into the peripheral plasma and affect gastric motility and that PVN was a central nucleus for modulating gastric motility and motilin expression in the thyroid.

Effect of chronic central endothelin-1 on hemodynamics and plasma vasopressin in conscious rats

OBJECTIVES

These studies were designed to test whether chronic central administration of endothelin-1 induces changes in systemic hemodynamics and plasma vasopressin similar to those observed with acute microinjections of endothelin-1.

METHODS

Sprague Dawley rats underwent sham denervation or sinoaortic denervation. Three days later, baseline mean arterial blood pressure, heart rate, and vasopressin were assessed in conscious rats. Then, a cannula was stereotaxically inserted into the lateral ventricle and attached to an osmotic minipump that delivered one of the following: (i) artificial cerebrospinal fluid; (ii) endothelin-1, 10 pmol/hour; (iii) BQ-123, 400 pmol/hour; or (iv) endothelin-1+BQ-123. Mean arterial blood pressure and heart rate were monitored daily and blood was obtained for plasma vasopressin on days 3 and 9. On day 10, the rats were euthanized, the hypothalami were removed, and vasopressin messenger ribonucleic acid content was assessed.

RESULTS

The pressor effect of intracerebroventricular endothelin-1 was similar in intact and sinoaortic denervation rats and was prevented by endothelin receptor A antagonism with BQ-123. Administration of BQ-123 alone resulted in a depressor and bradycardia in sinoaortically denervated rats. Chronic endothelin-1 administration did not change plasma vasopressin but resulted in a significant decrease in hypothalamic vasopressin messenger ribonucleic acid levels, which was reversed by endothelin receptor A inhibition.

DISCUSSION

Although the pressor effect of chronic central endothelin-1 is similar to that reported with acute endothelin-1, plasma vasopressin levels do not increase, at least in part, due to downregulation of hypothalamic vasopressin gene expression. Sinoaortic denervation increases endogenous central endothelin receptor A tone. Furthermore, these observations confirm that the pressor effect of central endothelin-1 is not mediated by plasma vasopressin.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Cyclic intermittent hypoxia enhances renal sympathetic response to ICV ET-1 in conscious rats

To test the hypothesis that central changes in sympathoregulation might contribute to sympathoexcitation after cyclic intermittent hypoxia (CIH) we exposed male Sprague-Dawley rats to CIH or to room air sham (Sham) for 8h/d for 3 weeks. After completion of the exposure we assessed heart rate, mean arterial pressure and renal sympathetic nerve activity in conscious animals before and after intracerebroventricular (i.c.v.) administration of endothelin-1 (ET-1, 3 pmol). CIH-exposed animals had a significantly greater sympathetic response to ET-1 than did Sham-exposed animals (CIH 137.8+/-15.6% of baseline; Sham 112.2+/-10.0% of baseline; CIH vs. Sham, P=0.0373). This enhanced sympathetic response to i.c.v. ET-1 was associated with greater expression of endothelin receptor A (ETA) protein in the subfornical organs of CIH-exposed relative to Sham-exposed rats. We conclude that 3-week CIH exposure enhances central ET-1 receptor expression and the sympathetic response to i.c.v. ET-1 suggesting central endothelin may contribute to the sympathetic and hemodynamic response to cyclic intermittent hypoxia.

Long-term modulation of tyrosine hydroxylase activity and expression by endothelin-1 and -3 in the rat anterior and posterior hypothalamus

Brain catecholamines are involved in the regulation of biological functions, including cardiovascular activity. The hypothalamus presents areas with high density of catecholaminergic neurons and the endothelin system. Two hypothalamic regions intimately related with the cardiovascular control are distinguished: the anterior (AHR) and posterior (PHR) hypothalamus, considered to be sympathoinhibitory and sympathoexcitatory regions, respectively. We previously reported that endothelins (ETs) are involved in the short-term tyrosine hydroxylase (TH) regulation in both the AHR and PHR. TH is crucial for catecholaminergic transmission and is tightly regulated by well-characterized mechanisms. In the present study, we sought to establish the effects and underlying mechanisms of ET-1 and ET-3 on TH long-term modulation. Results showed that in the AHR, ETs decreased TH activity through ETBreceptor activation coupled to the nitric oxide, phosphoinositide, and CaMK-II pathways. They also reduced total TH level and TH phosphorylated forms (Ser 19 and 40). Conversely, in the PHR, ETs increased TH activity through a G protein-coupled receptor, likely an atypical ET receptor or the ETCreceptor, which stimulated the phosphoinositide and adenylyl cyclase pathways, as well as CaMK-II. ETs also increased total TH level and the Ser 19, 31, and 40 phosphorylated sites of the enzyme. These findings support that ETs are involved in the long-term regulation of TH activity, leading to reduced sympathoinhibition in the AHR and increased sympathoexcitation in the PHR. Present and previous studies may partially explain the cardiovascular effects produced by ETs when applied to the brain.

Paraventricular nucleus is involved in the central pathway of cardiac sympathetic afferent reflex in rats

Our previous studies have shown that angiotensin II and reactive oxygen species in the paraventricular nucleus (PVN) modulate the cardiac sympathetic afferent reflex (CSAR). The present study was designed to demonstrate more conclusively that the PVN is an important component of the central neurocircuitry of the CSAR. In anaesthetized Sprague-Dawley rats with sinoaortic denervation and cervical vagotomy, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were continuously recorded. The CSAR was evaluated by the response of the RSNA to epicardial application of bradykinin or capsaicin. Bilateral microinjection of the anaesthetic, lignocaine, into the PVN abolished the CSAR without significant effects on the baseline RSNA and MAP, while l-glutamate, which excites the neurons in the PVN, enhanced the CSAR and increased the baseline RSNA and MAP. Bilateral electrolytic lesions of the PVN irreversibly abolished the CSAR without significant effects on the baseline RSNA and MAP. Bilateral selective lesions of the neurons in the PVN with kainic acid induced rapid and great increases in both RSNA and MAP which returned to nearly normal levels in 60 min. At the 90th minute after kainic acid, epicardial application of bradykinin or capsaicin failed to induce the CSAR. These results indicate that inhibition or lesion of the PVN abolishes the CSAR, but excitation of the neurons in the PVN enhances the CSAR, suggesting that the PVN is an important component of the central neurocircuitry of the CSAR.

Physiological basis for a causal relationship of obstructive sleep apnoea to hypertension

Obstructive sleep apnoea (OSA) is causally related to systemic hypertension through sustained sympathoexcitation. The causes of this sympathoexcitation remain uncertain; however, substantial animal and human data suggest that cyclic intermittent hypoxia (CIH), as is experienced at night by patients with OSA, provides the causal link between upper airway obstruction during sleep and sympathetic activation during waking. Direct and indirect evidence indicates that CIH leads to sympathoexcitation by two mechanisms: (1) augmentation of peripheral chemoreflex sensitivity (hypoxic acclimatization); and (2) direct effects on sites of central sympathetic regulation, such as the subfornical organ and the paraventricular nucleus of the hypothalamus. Initial reports suggest that the molecular mechanisms influencing peripheral chemoreflex sensitivity and central sympathetic activity may be the same, involving such neuromodulators as angiotensin II, endothelin and nitric oxide.

Involvement of nitric oxide pathways in short term modulation of tyrosine hydroxylase activity by endothelins 1 and 3 in the rat anterior hypothalamus

The ability of endothelins 1 and 3 (ET-1 and ET-3) to reduce neuronal norepinephrine release through ETB receptor activation involving nitric oxide (NO) pathways in the rat anterior hypothalamus region (AHR) was previously reported. In the present work, we studied the effects of ET-1 and -3 on tyrosine hydroxylase (TH) activity and the possible involvement of NO pathways. Results showed that ET-1 and -3 (10 nM) diminished TH activity in AHR and this effect was blocked by a selective ETB receptor antagonist (100 nM BQ-788), but not by a ET(A) receptor antagonist (BQ-610). To confirm these results, 1 microM IRL-1620 (ET(B) agonist) reduced TH activity whereas 300 nM sarafotoxin S6b falled to modify it. N(omega)-Nitro-L-arginine methyl ester (10 microM), 7-nitroindazole (10 microM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-ona (10 microM), KT5823 (2 microM), inhibitors of nitric oxide synthase, neuronal nitric oxide synthase, NO-sensitive-guanylyl cyclase, and protein kinase G, respectively, did not modify the reduction of TH activity produced by ETs. In addition, both 100 microM sodium nitroprusside and 50 microM 8-bromoguanosine-3',5'-cyclic monophosphate (NO donor and guanosine-3',5'-cyclic monophosphate analog, respectively) diminished TH activity. Present results showed that ET-1 and ET-3 diminished TH activity through the activation of ET(B) receptors involving the NO/guanosine-3',5'-cyclic monophosphate/protein kinase G pathway. Taken jointly present and previous results it can be concluded that both ETs play an important role as modulators of norepinephrine neurotransmission in the rat AHR.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

The influence of dorsomedial hypothalamic nucleus on contralateral paraventricular nucleus in NMDA-mediated cardiovascular responses

Dorsomedial (DMH) and paraventricular nuclei (PVN) are two important hypothalamic structures involved in the central regulation of cardiovascular regulation. L-Glutamic acid and gamma-aminobutyric acid (GABA) were demonstrated to elicit cardiovascular responses when administered via intracerebroventricular injection or parenchymal microinjections into the hypothalamic nuclei, participating in central cardiovascular regulation. In this study the interaction between the DMH and the PVN were investigated by means of microinjection and microdialysis techniques in Sprague-Dawley rats. Stereotaxic surgery was performed for the insertion of intracerebral parenchymal microinjection cannula into the right DMH and microdialysis probe into the left PVN. After a recovery period of 3 days, the iliac artery was cannulated for monitoring pulsatile blood pressure and heart rate by means of pressure transducer connected to a polygraph. Microinjection of 50 pmol NMDA into the DMH was performed and microdialysis perfusates were collected simultaneously from the PVN in the conscious rat model. L-Glutamic acid and GABA levels were analyzed by an isocratic HPLC method with the aid of a fluorescent detector. Microinjection of 50 pmol NMDA into the DMH produced significant increases in mean arterial pressure and heart rate. NMDA microinjection into the DMH produced a significant increase in L-glutamic acid release in the PVN, but no significant change in GABA release was observed. These results may indicate that stimulation of the DMH by NMDA results in subsequent stimulation of the PVN.

Inhibition of sympathetic responses at birth in sheep by lesion of the paraventricular nucleus

Birth is characterized by a surge in sympathetic outflow, heart rate (HR), mean arterial blood pressure (MABP) and circulating catecholamines. The paraventricular nucleus (PVN) of the hypothalamus is an important central regulatory site of sympathetic activity, but its role in the regulation of sympathoexcitation at birth is unknown. To test the hypothesis that the PVN regulates sympathetic activity at birth, experiments were performed in chronically instrumented near-term (137- to 142-day gestation, term 145 days) sheep before and after delivery by cesarean section. Stereotaxic guided electrolytic lesioning of the PVN (n = 6) or sham lesioning (n = 6) was performed 48 h before study. At 30 min after birth, renal sympathetic nerve activity (RSNA) increased 128 +/- 26% above fetal values in the sham-lesioned animals (P < 0.05). In contrast, at a similar time point, RSNA decreased to 52 +/- 12% of the fetal value in the PVN-lesioned animals. Lesioning of the PVN did not affect the usual postnatal increases in MABP and epinephrine levels although HR failed to rise above fetal values. ANG II but not arginine vasopressin or norepinephrine levels increased in PVN-lesioned animals after birth, whereas all three hormones increased (P < 0.05) in sham-lesioned animals. Fetal and newborn HR baroreflex responses were similar in both groups. However, the usual postnatal attenuation of baroreflex-mediated inhibition of RSNA was blunted in the PVN-lesioned group. The results of this study demonstrate that ablation of the PVN abolishes sympathoexcitation with birth at near-term gestation. The PVN may play a critical role in physiological adaptation at birth.

Co-localisation of nitric oxide synthase and endothelin in the rat supraoptic nucleus

The co-localisation of neuronal nitric oxide synthase and endothelin-1 was studied in the rat supraoptic nucleus at the electron microscopy level. Double pre-embedding immunocytochemistry was performed using ExtrAvidin-horseradish peroxidase and immunogold-silver techniques. Immunoreactivities to neuronal nitric oxide synthase and endothelin-1 were co-localised in sub-populations of endocrine neurones (cell bodies) and dendrites. Double-labelled axon terminals making asymmetrical synapses on unlabelled dendrites were also observed. The findings are discussed in terms of the possible role and significance of nitric oxide and endothlin-1 in the hypothalamo-neurohypophysial system.