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

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

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

Renal Sensory Activity Regulates the γ-Aminobutyric Acidergic Inputs to the Paraventricular Nucleus of the Hypothalamus in Goldblatt Hypertension

Renal sensory activity is centrally integrated within brain nuclei involved in the control of cardiovascular function, suggesting that renal afferents regulate basal and reflex sympathetic vasomotor activity. Evidence has shown that renal deafferentation (DAx) evokes a hypotensive and sympathoinhibitory effect in experimental models of cardiovascular diseases; however, the underlying mechanisms involved in this phenomenon need to be clarified, especially those related to central aspects. We aimed to investigate the role of renal afferents in the control of γ-aminobutyric acid (GABA)ergic inputs to the paraventricular nucleus (PVN) of the hypothalamus in renovascular hypertensive (2K1C) rats and their influence in the regulation of cardiovascular function. Hypertension was induced by clipping the left renal artery. After 4 weeks, renal DAx was performed by exposing the left renal nerve to a 33 mM capsaicin solution for 15 min. After 2 weeks of DAx, microinjection of muscimol into the PVN was performed in order to evaluate the influence of GABAergic activity in the PVN and its contribution to the control of renal sympathetic nerve activity (rSNA) and blood pressure (BP). Muscimol microinjected into the PVN triggered a higher drop in BP and rSNA in the 2K1C rats and renal DAx mitigated these responses. These results suggest that renal afferents are involved in the GABAergic changes found in the PVN of 2K1C rats. Although the functional significance of this phenomenon needs to be clarified, it is reasonable to speculate that GABAergic alterations occur to mitigate microglia activation-induced sympathoexcitation in the PVN of 2K1C rats.

Afferent innervation of the ischemic kidney contributes to renal dysfunction in renovascular hypertensive rats

The ablation of renal nerves, by destroying both the sympathetic and afferent fibers, has been shown to be effective in lowering blood pressure in resistant hypertensive patients. However, experimental studies have reported that the removal of sympathetic fibers may lead to side effects, such as the impairment of compensatory cardiorenal responses during a hemodynamic challenge. In the present study, we evaluated the effects of the selective removal of renal afferent fibers on arterial hypertension, renal sympathetic nerve activity, and renal changes in a model of renovascular hypertension. After 4 weeks of clipping the left renal artery, afferent renal denervation (ARD) was performed by exposing the left renal nerve to a 33 mM capsaicin solution for 15 min. After 2 weeks of ARD, we found reduced MAP (~ 18%) and sympathoexcitation to both the ischemic and contralateral kidneys in the hypertensive group. Moreover, a reduction in reactive oxygen species was observed in the ischemic (76%) and contralateral (27%) kidneys in the 2K1C group. In addition, ARD normalized renal function markers and proteinuria and podocin in the contralateral kidney. Taken altogether, we show that the selective removal of afferent fibers is an effective method to reduce MAP and improve renal changes without compromising the function of renal sympathetic fibers in the 2K1C model. Renal afferent nerves may be a new target in neurogenic hypertension and renal dysfunction.

Renal denervation reduces sympathetic overactivation, brain oxidative stress, and renal injury in rats with renovascular hypertension independent of its effects on reducing blood pressure

The underlying mechanisms by which renal denervation (RD) decreases blood pressure (BP) remain incompletely understood. In this study, we investigated the effects of ischemic kidney denervation on different sympathetic outflows, brain and renal expression of angiotensin-II receptors, oxidative stress and renal function markers in the 2-kidney, 1-clip (2K-1C) rat model. Surgical RD was performed in Wistar male rats 4-5 weeks after clip implantation. After 10 days of RD, BP, and the activity of sympathetic nerves projecting to the contralateral kidney (rSNA) and splanchnic region were partially reduced in 2K-1C rats, with no change in systemic renin-angiotensin system (RAS). To distinguish the effects of RD from the reduction in BP, 2K-1C rats were treated with hydralazine by oral gavage (25 mg/kg/day for 1 week). RD, but not hydralazine, normalized oxidative stress in the sympathetic premotor brain regions and improved intrarenal RAS, renal injury, and proteinuria. Furthermore, different mechanisms led to renal injury and oxidative stress in the ischemic and contralateral kidneys of 2K-1C rats. Injury and oxidative stress in the ischemic kidney were driven by the renal nerves. Although RD attenuated rSNA, injury and oxidative stress persisted in the contralateral kidney, probably due to increased BP. Therefore, nerves from the ischemic kidney at least partially contribute to the increase in BP, sympathetic outflows, brain oxidative stress, and renal alterations in rats with renovascular hypertension. Based on these findings, the reduction in oxidative stress in the brain is a central mechanism that contributes to the effects of RD on Goldblatt hypertension.

Blood-borne interleukin-1β acts on the subfornical organ to upregulate the sympathoexcitatory milieu of the hypothalamic paraventricular nucleus

We previously reported that microinjection of the proinflammatory cytokine interleukin-1β (IL-1β) into the subfornical organ (SFO) elicits a pressor response accompanied by increases in inflammation and renin-angiotensin system (RAS) activity in the SFO and hypothalamic paraventricular nucleus (PVN). The present study sought to determine whether blood-borne IL-1β induces similar neurochemical changes in the SFO and PVN and, if so, whether increased inflammation and RAS activity at the SFO level orchestrate the sympathoexcitatory response to circulating IL-1β. In urethane-anesthetized male Sprague-Dawley rats, intravenous injection of IL-1β (500 ng) increased blood pressure, heart rate, renal sympathetic nerve activity, and mRNA for angiotensin-converting enzyme, angiotensin II type 1a receptor, cyclooxygenase-2, tumor necrosis factor-α, and IL-1β, as well as the tumor necrosis factor-α p55 receptor and the IL-1 receptor, in the SFO and PVN. Pretreatment with SFO microinjections of the angiotensin II type 1a receptor blocker losartan (1 µg), the angiotensin-converting enzyme inhibitor captopril (1 µg), or the cyclooxygenase-2 inhibitor NS-398 (2 µg) attenuated expression of these excitatory mediators in the SFO and downstream in the PVN and the IL-1β-induced pressor responses. An SFO lesion minimized the IL-1β-induced expression of inflammatory and RAS components as well as c-Fos, an indicator of neuronal excitation, in the PVN. These studies demonstrate that circulating IL-1β, which increases in cardiovascular disorders such as hypertension and heart failure, acts on the SFO to increase inflammation and RAS activity in the SFO and PVN and that intervening in these neurochemical processes in the SFO can significantly reduce the sympathetic response.

Atrial Fibrillation in Acute Obstructive Sleep Apnea: Autonomic Nervous Mechanism and Modulation

Background

The mechanisms of atrial fibrillation (AF) induced by obstructive sleep apnea (OSA) are not completely understood. This study investigated the roles of the intrinsic and extrinsic cardiac autonomic nervous system in OSA‐induced AF and provided noninvasive autonomic nervous modulation for the suppression of OSA‐induced AF by using low‐level transcutaneous electrical stimulation (LL‐TS) of the auricular branch of the vagus nerve at the tragus.

Methods and Results

Eighteen dogs received tracheostomy under general anesthesia and were randomly divided into 3 groups: the OSA group (OSA was simulated via clamping of the endotracheal tube at end expiration for 1.5 minutes every 10 minutes, n=6), the LL‐TS + OSA group (simulated OSA plus LL‐TS, at 80% of the slowing sinus rate, n=6), and the control group (sham surgery without stimulation, n=6). The effective refractory period was significantly shortened after 1 hour of simulated OSA, and the window of vulnerability and plasma norepinephrine levels were both markedly increased in the OSA group. OSA dramatically increased the neural function and activity of the intrinsic and extrinsic cardiac autonomic nervous system, including the superior left ganglionated plexus, the left stellate ganglion, and the left renal sympathetic nerve. OSA also significantly upregulated the expression levels of c‐fos and nerve growth factor in the superior left ganglionated plexus and the left stellate ganglion. However, LL‐TS markedly improved these parameters.

Conclusions

These findings suggest that the intrinsic and extrinsic cardiac autonomic nervous system plays crucial roles in the acute stage of OSA‐induced AF. Noninvasive LL‐TS suppressed shortening of atrial refractoriness and autonomic remodeling, which prevented OSA‐induced AF.

Administration of exogenous 1,25(OH)2D3 normalizes overactivation of the central renin-angiotensin system in 1α(OH)ase knockout mice

Previously, we reported that active vitamin D deficiency in mice causes secondary hypertension and cardiac dysfunction, but the underlying mechanism remains largely unknown. To clarify whether exogenous active vitamin D rescues hypertension by normalizing the altered central renin-angiotensin system (RAS) via an antioxidative stress mechanism, 1-alpha-hydroxylase [1α(OH)ase] knockout mice [1α(OH)ase(-/-)] and their wild-type littermates were fed a normal diet alone or with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or a high-calcium, high-phosphorus "rescue" diet with or without antioxidant N-acetyl-l-cysteine (NAC) supplementation for 4 weeks. Compared with their wild-type littermates, 1α(OH)ase(-/-)mice had high mean arterial pressure, increased levels of renin, angiotensin II (Ang II), and Ang II type 1 receptor, and increased malondialdehyde levels, but decreased anti-peroxiredoxin I and IV proteins and the antioxidative genes glutathione reductase (Gsr) and glutathione peroxidase 4 (Gpx4) in the brain samples. Except Ang II type 1 receptor, these pathophysiological changes were rescued by exogenous 1,25(OH)2D3 or NAC plus rescue diet, but not by rescue diet alone. We conclude that 1,25(OH)2D3 normalizes the altered central RAS in 1α(OH)ase(-/-)mice, at least partially, through a central antioxidative mechanism.

Central SDF-1/CXCL12 expression and its cardiovascular and sympathetic effects: the role of angiotensin II, TNF-α, and MAP kinase signaling

The chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) and its receptors are expressed by neurons and glial cells in cardiovascular autonomic regions of the brain, including the hypothalamic paraventricular nucleus (PVN), and contribute to neurohumoral excitation in rats with ischemia-induced heart failure. The present study examined factors regulating the expression of SDF-1 in the PVN and mechanisms mediating its sympatho-excitatory effects. In urethane anesthetized rats, a 4-h intracerebroventricular (ICV) infusion of angiotensin II (ANG II) or tumor necrosis factor-α (TNF-α) in doses that increase mean blood pressure (MBP) and sympathetic drive increased the expression of SDF-1 in PVN. ICV administration of SDF-1 increased the phosphorylation of p44/42 mitogen-activated protein kinase (MAPK), JNK, and p38 MAPK in PVN, along with MBP, heart rate (HR), and renal sympathetic nerve activity (RSNA), but did not affect total p44/42 MAPK, JNK, and p38 MAPK levels. ICV pretreatment with the selective p44/42 MAPK inhibitor PD98059 prevented the SDF-1-induced increases in MBP, HR, and RSNA; ICV pretreatment with the selective JNK and p38 MAPK inhibitors attenuated but did not block these SDF-1-induced excitatory responses. ICV PD98059 also prevented the sympatho-excitatory response to bilateral PVN microinjections of SDF-1. ICV pretreatment with SDF-1 short-hairpin RNA significantly reduced ANG II- and TNF-α-induced phosphorylation of p44/42 MAPK in PVN. These findings identify TNF-α and ANG II as drivers of SDF-1 expression in PVN and suggest that the full expression of their cardiovascular and sympathetic effects depends upon SDF-1-mediated activation of p44/42 MAPK signaling.

Aldosterone-induced brain MAPK signaling and sympathetic excitation are angiotensin II type-1 receptor dependent

Angiotensin II (ANG II)-induced mitogen-activated protein kinase (MAPK) signaling upregulates angiotensin II type-1 receptors (AT(1)R) in hypothalamic paraventricular nucleus (PVN) and contributes to AT(1)R-mediated sympathetic excitation in heart failure. Aldosterone has similar effects to increase AT(1)R expression in the PVN and sympathetic drive. The present study was undertaken to determine whether aldosterone also activates the sympathetic nervous system via MAPK signaling and, if so, whether its effect is independent of ANG II and AT(1)R. In anesthetized rats, a 4-h intravenous infusion of aldosterone induced increases (P < 0.05) in phosphorylated (p-) p44/42 MAPK in PVN, PVN neuronal excitation, renal sympathetic nerve activity (RSNA), mean blood pressure (MBP), and heart rate (HR). Intracerebroventricular or bilateral PVN microinjection of the p44/42 MAPK inhibitor PD-98059 reduced the aldosterone-induced RSNA, HR, and MBP responses. Intracerebroventricular pretreatment (5 days earlier) with pooled small interfering RNAs targeting p44/42 MAPK reduced total and p-p44/42 MAPK, aldosterone-induced c-Fos expression in the PVN, and the aldosterone-induced increases in RSNA, HR, and MBP. Intracerebroventricular infusion of either the mineralocorticoid receptor antagonist RU-28318 or the AT(1)R antagonist losartan blocked aldosterone-induced phosphorylation of p44/42 MAPK and prevented the increases in RSNA, HR, and MBP. These data suggest that aldosterone-induced sympathetic excitation depends upon that AT(1)R-induced MAPK signaling in the brain. The short time course of this interaction suggests a nongenomic mechanism, perhaps via an aldosterone-induced transactivation of the AT(1)R as described in peripheral tissues.

EP₃ receptors mediate PGE₂-induced hypothalamic paraventricular nucleus excitation and sympathetic activation

Prostaglandin E(2) (PGE(2)), an important mediator of the inflammatory response, acts centrally to elicit sympathetic excitation. PGE(2) acts on at least four E-class prostanoid (EP) receptors known as EP(1), EP(2), EP(3), and EP(4). Since PGE(2) production within the brain is ubiquitous, the different functions of PGE(2) depend on the expression of these prostanoid receptors in specific brain areas. The type(s) and location(s) of the EP receptors that mediate sympathetic responses to central PGE(2) remain unknown. We examined this question using PGE(2), the relatively selective EP receptor agonists misoprostol and sulprostone, and the available selective antagonists for EP(1), EP(3), and EP(4). In urethane-anesthetized rats, intracerebroventricular (ICV) administration of PGE(2), sulprostone or misoprostol increased renal sympathetic nerve activity, blood pressure, and heart rate. These responses were significantly reduced by ICV pretreatment with the EP(3) receptor antagonist; the EP(1) and EP(4) receptor antagonists had little or no effect. ICV PGE(2) or misoprostol increased the discharge of neurons in the hypothalamic paraventricular nucleus (PVN). ICV misoprostol increased the c-Fos immunoreactivity of PVN neurons, an effect that was substantially reduced by the EP(3) receptor antagonist. Real-time PCR detected EP(3) receptor mRNA in PVN, and immunohistochemical studies revealed sparsely distributed EP(3) receptors localized in GABAergic terminals and on a few PVN neurons. Direct bilateral PVN microinjections of PGE(2) or sulprostone elicited sympathoexcitatory responses that were significantly reduced by the EP(3) receptor antagonist. These data suggest that EP(3) receptors mediate the central excitatory effects of PGE(2) on PVN neurons and sympathetic discharge.

The role of the renin—angiotensin system in the pathogenesis of intracranial aneurysms

Introduction: Recent work has begun to elucidate the pathogenesis of intracranial aneurysms (IA) and has shown that many genes are involved in the risk for this condition. There has also been increasing research interest in the renin—angiotensin system (RAS) in the brain and its involvement in a range of cardiovascular and neurological disorders. The possibility that the RAS is implicated in the pathogenesis of IA merits further investigation. The aim of this article is to review the literature on the pathogenesis of IA and the pathophysiological significance of the brain RAS, and to identify directions for research into their association.

Methods and results : A survey of the literature in these fields shows that although factors contributing to systemic hypertension predispose to IA, a large number of genes involved in endothelial cell adhesion, smooth muscle activity, extracellular matrix dynamics and the inflammatory and immune responses are also implicated. The brain RAS has a significant role in regulating blood pressure and in maintaining cerebrovascular autoregulation, but angiotensin II receptors are also involved in the maintenance of endothelial cell and vascular smooth muscle function and in the inflammatory response in the brain.

Conclusions: There is strong, albeit largely circumstantial, evidence in the literature for a relationship between the brain RAS and the formation of IA. Research on the association between polymorphisms in RAS-related genes and the incidence of unruptured and ruptured IA is indicated.

Centrally administered lipopolysaccharide elicits sympathetic excitation via NAD(P)H oxidase-dependent mitogen-activated protein kinase signaling

OBJECTIVE

The mechanisms by which inflammation activates sympathetic drive in heart failure and hypertension remain ill-defined. In this study, an intracerebroventricular injection of lipopolysaccharide (LPS) was used to induce the expression of cytokines and other inflammatory mediators in the brain, in the absence of other excitatory mediators, and the downstream signaling pathways leading to sympathetic activation were examined using intracerebroventricular injections of blocking or inhibiting agents.

METHODS AND RESULTS

In anesthetized rats, intracerebroventricular injection of LPS (5 microg) increased (P < 0.05) renal sympathetic nerve activity, blood pressure and heart rate. LPS increased (P < 0.05) hypothalamic mRNA for NAD(P)H oxidase subunits p47 and gp91, NAD(P)H oxidase-dependent superoxide generation, hypothalamic mRNA for tumor necrosis factor-alpha, cyclooxygenase-2 and cerebrospinal fluid levels of tumor necrosis factor-alpha and prostaglandin E2. In the paraventricular nucleus of hypothalamus, dihydroethidium staining for superoxide expression and c-Fos activity (indicating neuronal excitation) increased. The superoxide scavenger tempol significantly (P < 0.05) diminished the expression of inflammatory mediators, as well as superoxide expression and neuronal excitation in paraventricular nucleus. SB203580 (p38 mitogen-activated protein kinase inhibitor) also reduced the expression of inflammatory mediators in hypothalamus and cerebrospinal fluid. Tempol, apocynin [NAD(P)H oxidase inhibitor], SB203580 and NS398 (cyclooxygenase-2 inhibitor) all reduced cerebrospinal fluid prostaglandin E2 and the sympathoexcitatory response to LPS. LPS also increased angiotensin II type 1 receptor mRNA, a response blocked by apocynin and tempol but not by SB203580.

CONCLUSION

These findings suggest that central inflammation in pathophysiological conditions activates the sympathetic nervous system via NAD(P)H oxidase and p38 mitogen-activated protein kinase-dependent synthesis of prostaglandin E2.

A current view of brain renin-angiotensin system: Is the (pro)renin receptor the missing link?

The renin-angiotensin system (RAS) plays a central role in the brain to regulate blood pressure (BP). This role includes the modulation of sympathetic nerve activity (SNA) that regulates vascular tone; the regulation of secretion of neurohormones that have a critical role in electrolyte as well as fluid homeostasis; and by influencing behavioral processes to increase salt and water intake. Based on decades of research it is clear that angiotensin II (Ang II), the major bioactive product of the RAS, mediates these actions largely via its Ang II type 1 receptor (AT1R), located within hypothalamic and brainstem control centers. However, the mechanisms of brain RAS function have been questioned, due in large part to low expression levels of the rate limiting enzyme renin within the central nervous system. Tissue localized RAS has been observed in heart, kidney tubules and vascular cells. Studies have also given rise to the hypothesis for localized RAS function within the brain, so that Ang II can act in a paracrine manner to influence neuronal activity. The recently discovered (pro)renin receptor (PRR) may be key in this mechanism as it serves to sequester renin and prorenin for localized RAS activity. Thus, the PRR can potentially mitigate the low levels of renin expression in the brain to propagate Ang II action. In this review we examine the regulation, expression and functional properties of the various RAS components in the brain with particular focus on the different roles that PRR may have in BP regulation and hypertension.

Systemically administered tempol reduces neuronal activity in paraventricular nucleus of hypothalamus and rostral ventrolateral medulla in rats

OBJECTIVE

Systemic administration of the superoxide scavenger tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) reduces blood pressure (BP), heart rate (HR) and sympathetic nerve activity in normotensive and hypertensive animals. The global nature of the depressor response to tempol suggests an inhibitory influence on cardiovascular presympathetic regions of the brain. This study examined several possible mechanisms for such an effect.

METHODS AND RESULTS

In urethane anesthetized rats, as expected, intravenous tempol (120 microg mol/kg) reduced mean arterial pressure, HR and renal sympathetic nerve activity (RSNA). Concomitant central neuronal recordings revealed reduced spontaneous discharge (spikes/s) of neurons in the paraventricular nucleus of hypothalamus (from 2.9 +/- 0.4 to 0.8+/- 0.2) and the rostral ventrolateral medulla (RVLM; from 9.8 +/- 0.5 to 7.2 +/-0.4), two cardiovascular and autonomic regions of the brain. Baroreceptor-denervated rats had exaggerated sympathetic and cardiovascular responses. Pretreatment with the hydroxyl radical scavenger dimethyl sulfoxide (intravenous) attenuated the tempol-induced decreases in BP, HR and RSNA, but the nitric oxide synthesis inhibitor NG-nitro-L-arginine methyl ester (intravenous or intracerebroventricular) had no effect.

CONCLUSION

These findings suggest that systemically administered tempol acts upon neurons in paraventricular nucleus and RVLM to reduce BP, HR and RSNA, perhaps by reducing the influence of reactive oxygen species in those regions. The arterial baroreflex modulates the depressor responses to tempol. These central mechanisms must be considered in interpreting data from studies using systemically administered tempol to assess the role of reactive oxygen species in cardiovascular regulation.

Hypothalamic corticotrophin-releasing factor and norepinephrine mediate sympathetic and cardiovascular responses to acute intracarotid injection of tumour necrosis factor-alpha in the rat

Systemic administration of tumour necrosis factor (TNF)-alpha induces the release of norepinephrine in the paraventricular nucleus (PVN) of hypothalamus and an increase in expression of corticotrophin-releasing factor (CRF) and CRF type 1 receptors. We explored the hypothesis that CRF and norepinephrine in PVN mediate the cardiovascular and sympathetic responses to acute systemic administration of TNF-alpha. In anaesthetised rats, the increases in arterial pressure and heart rate induced by intracarotid artery injection of TNF-alpha were attenuated by intracerebroventricular (ICV) injection of either the alpha 1-adrenergic antagonist prazosin or the CRF antagonist alpha-helical CRF. Prazosin blocked the TNF-alpha-induced increase in renal sympathetic nerve activity (RSNA), whereas alpha-helical CRF substantially reduced the RSNA response. Conversely, CRF and the alpha 1-adrenergic agonist phenylephrine, administered ICV, both elicited increases in PVN neuronal activity, RSNA, arterial pressure and heart rate. Microinjection of CRF and phenylephrine directly into PVN evoked smaller responses. These results are consistent with the hypothesis that norepinephrine and CRF in the PVN mediate the cardiovascular and sympathetic responses to acute systemic administration of TNF-alpha.

Angiotensin II-triggered p44/42 mitogen-activated protein kinase mediates sympathetic excitation in heart failure rats

Angiotensin II (Ang II), acting via angiotensin type 1 receptors in the brain, activates the sympathetic nervous system in heart failure (HF). We reported recently that Ang II stimulates mitogen-activated protein kinase (MAPK) to upregulate brain angiotensin type 1 receptors in HF rats. In this study we tested the hypothesis that Ang II-activated MAPK signaling pathways contribute to sympathetic excitation in HF. Intracerebroventricular administration of PD98059 and UO126, 2 selective p44/42 MAPK inhibitors, induced significant decreases in mean arterial pressure, heart rate, and renal sympathetic nerve activity in HF rats, but had no effect on these variables in sham-operated rats. Pretreatment with losartan attenuated the effects of PD98059. Intracerebroventricular administration of the p38 MAPK inhibitor SB203580 and the c-Jun N-terminal kinase inhibitor SP600125 had no effect on mean arterial pressure, heart rate, or renal sympathetic nerve activity in HF. The phosphatidylinositol 3-kinase inhibitor LY294002 induced a small decrease in mean arterial pressure and heart rate but no change in renal sympathetic nerve activity. Immunofluorescent staining demonstrated increased p44/42 MAPK activity in neurons of the paraventricular nucleus of the hypothalamus of HF rats, colocalized with Fra-like activity (indicating chronic neuronal excitation). Intracerebroventricular PD98059 and UO126 reduced Fra-like activity in the paraventricular nucleus of the hypothalamus neurons in HF rats. In confirmatory acute studies, intracerebroventricular Ang II increased mean arterial pressure, heart rate, and renal sympathetic nerve activity in baroreceptor-denervated rats and Fra-like immunoreactivity in the paraventricular nucleus of the hypothalamus of neurally intact rats. Central administration of PD98059 markedly reduced these responses. These data demonstrate that intracellular p44/42 MAPK activity contributes to Ang II-induced neuronal excitation in the paraventricular nucleus of the hypothalamus and augmented sympathetic nerve activity in rats with HF.

Cardiovascular and renal sympathetic activation by blood-borne TNF-alpha in rat: the role of central prostaglandins

In pathophysiological conditions, increased blood-borne TNF-alpha induces a broad range of biological effects, including activation of the hypothalamic-pituitary-adrenal axis and sympathetic drive. In urethane-anesthetized adult Sprague-Dawley rats, we examined the mechanisms by which blood-borne TNF-alpha activates neurons in paraventricular nucleus (PVN) of hypothalamus and rostral ventrolateral medulla (RVLM), two critical brain regions regulating sympathetic drive in normal and pathophysiological conditions. TNF-alpha (0.5 microg/kg), administered intravenously or into ipsilateral carotid artery (ICA), activated PVN and RLVM neurons and increased sympathetic nerve activity, arterial pressure, and heart rate. Responses to intravenous TNF-alpha were not affected by vagotomy but were reduced by mid-collicular decerebration. Responses to ICA TNF-alpha were substantially reduced by injection of the cyclooxygenase inhibitor ketorolac (150 microg) into lateral ventricle. Injection of PGE(2) (50 ng) into lateral ventricle or directly into PVN increased PVN or RVLM activity, respectively, and sympathetic drive, with shorter onset latency than blood-borne TNF-alpha. These findings suggest that blood-borne cytokines stimulate cardiovascular and renal sympathetic responses via a prostaglandin-dependent mechanism operating at the hypothalamic level.

Programming the cardiovascular system, kidney and the brain--a review

The concept that 'life before birth' or the 'first environment' is important in determining subsequent risk for the development of cardiovascular/metabolic disease is now gaining acceptance. There are substantial data from animal experiments that complement and enhance the epidemiological data from human studies. We argue that any factor which disrupts nephrogenesis, and lowers nephron number, during the period of active nephrogenesis, will induce malapadaptive changes in the future functioning of that kidney and predispose to the onset of adult hypertension. Such factors include exposure of the mother, to a particular low-protein diet, excess synthetic or natural glucocorticoid at certain critical periods, mild vitamin A deficiency, elevated blood glucose, unilateral nephrectomy during the period of nephrogenesis, as well as the deletion of one allele of a gene (GDNF) involved in normal metanephric development. All of these stresses are associated with a reduction (20-40 per cent) in total nephron number in the adult, and the development of hypertension. In some hypertensive models, (rats) there is evidence of alterations in the components of the hippocampal/hypothalamic/pituitary/adrenal axis, whereas in others (sheep) there are alterations in the expression of angiotensinogen (hypothalamus) and angiotensin II receptor type I (AT(1)) in the medulla oblongata. The surprising finding is that the period when the kidney and brain are most vulnerable is very early in development, when both organs are in an extremely primitive state of development.

Heart failure and the brain: new perspectives

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

Biological mechanisms in the relationship between depression and heart disease

Psychological depression is shown to be associated with several aspects of coronary artery disease (CAD), including arrhythmias, myocardial infarction, heart failure and sudden death. The physiological mechanisms accounting for this association are unclear. Hypothalamic-pituitary-adrenal dysregulation, diminished heart rate variability, altered blood platelet function and noncompliance with medial treatments have been proposed as mechanisms underlying depression and cardiovascular disease. Recent evidence also suggests that reduced baroreflex sensitivity, impaired immune function, chronic fatigue and the co-morbidity of depression and anxiety may be involved in the relationship between depression and cardiovascular dysregulation. An experimental strategy using animal models for investigating underlying physiological abnormalities in depression is presented. A key to understanding the bidirectional association between depression and heart disease is to determine whether there are common changes in brain systems that are associated with these conditions. Such approaches may hold promise for advancing our understanding of the interaction between this mood disorder and CAD.

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

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

Forebrain-mediated adaptations to myocardial infarction in the rat

Recent studies suggest that the forebrain contributes to the circulatory derangements leading to heart failure after myocardial injury. We tested that hypothesis by examining the effect of myocardial infarction (MI) or sham MI (MI-s) on neurohumoral regulation in rats with prior anteroventral (AV) third ventricle lesion (AV3V-x) or sham lesion (AV3V-s). AV3V-s/MI rats had higher sodium intake, lower urine volume, and lower urinary sodium excretion than AV3V-s/MI-s rats. AV3V-x/MI rats had lower sodium intake and higher urine volume than AV3V-s/MI or AV3V-s/MI-s rats and urinary sodium excretion comparable to AV3V-s/MI-s rats. AV3V-x had no effect on baseline plasma renin activity (PRA). One week after MI, PRA had increased in AV3V-s but decreased in AV3V-x rats. AV3V-x reduced renal sympathetic nerve activity in MI and MI-s rats. AV3V-x improved baroreflex function in MI rats but diminished it in MI-s rats. Survival beyond 2 wk was lower in the AV3V-x/MI rats than in all other groups. These results confirm a critical role for the forebrain in the neurohumoral adjustments to MI.