AT(1) and glutamatergic receptors in paraventricular nucleus support blood pressure during water deprivation

A Critical Role for the Paraventricular Nucleus of the Hypothalamus in the Regulation of the Volume Reflex in Normal and Various Cardiovascular Disease States

PURPOSE OF REVIEW

This review focuses on studies implicating forebrain neural pathways and neuromodulator systems, particularly, the nitric oxide system within the paraventricular nucleus of the hypothalamus in regulating neurohumoral drive, autonomic pathways, and fluid balance.

RECENT FINDINGS

Accumulating evidence from animals with experimental models of hypertension and heart failure as well as humans with hypertension suggests that alterations in central neural pathways, particularly, within the PVN neuromodulated by neuronal nitric oxide, are involved in regulating sympathetic outflow particularly to the kidney resulting in alterations in fluid balance commonly observed in hypertension and heart failure states. The characteristics of the hypertensive and heart failure states include alterations in neuronal nitric oxide within the PVN to cause an increase in renal sympathetic nerve activity to result in sodium and fluid retention in these diseases. A comprehensive understanding of these mechanisms will enhance our ability to treat hypertensive and heart failure conditions and their cardiovascular complications more efficiently.

BDNF shifts excitatory-inhibitory balance in the paraventricular nucleus of the hypothalamus to elevate blood pressure

Presympathetic neurons in the paraventricular nucleus of the hypothalamus (PVN) play a key role in cardiovascular regulation. We have previously shown that brain-derived neurotrophic factor (BDNF), acting in the PVN, increases sympathetic activity and blood pressure and serves as a key regulator of stress-induced hypertensive responses. BDNF is known to alter glutamatergic and GABA-ergic signaling broadly in the central nervous system, but whether BDNF has similar actions in the PVN remains to be investigated. Here, we tested the hypothesis that increased BDNF expression in the PVN elevates blood pressure by enhancing N-methyl-d-aspartate (NMDA) receptor (NMDAR)- and inhibiting GABA_A receptor (GABA_AR)-mediated signaling. Sprague-Dawley rats received bilateral PVN injections of AAV2 viral vectors expressing green fluorescent protein (GFP) or BDNF. Three weeks later, cardiovascular responses to PVN injections of NMDAR and GABA_AR agonists and antagonists were recorded under α-chloralose-urethane anesthesia. In addition, expressions of excitatory and inhibitory signaling components in the PVN were assessed using immunofluorescence. Our results showed that NMDAR inhibition led to a greater decrease in blood pressure in the BDNF vs. GFP group, while GABA_AR inhibition led to greater increases in blood pressure in the GFP group compared to BDNF. Conversely, GABA_AR activation decreased blood pressure significantly more in GFP vs. BDNF rats. In addition, immunoreactivity of NMDAR1 was upregulated, while GABA_AR-α1 and K⁺/Cl^- cotransporter 2 were downregulated by BDNF overexpression in the PVN. In summary, our findings indicate that hypertensive actions of BDNF within the PVN are mediated, at least in part, by augmented NMDAR and reduced GABA_AR signaling.NEW & NOTEWORTHY We have shown that BDNF, acting in the PVN, elevates blood pressure in part by augmenting NMDA receptor-mediated excitatory input and by diminishing GABA_A receptor-mediated inhibitory input to PVN neurons. In addition, we demonstrate that elevated BDNF expression in the PVN upregulates NMDA receptor immunoreactivity and downregulates GABA_A receptor as well as KCC2 transporter immunoreactivity.

Role of Mineralocorticoid and Angiotensin Type 1 Receptors in the Paraventricular Nucleus in Angiotensin-Induced Hypertension

The hypothalamic paraventricular nucleus (PVN) is an important site where an interaction between circulating angiotensin (Ang) and mineralocorticoid receptor (MR) activity may modify sympathetic nerve activity (SNA) to influence long-term elevation of blood pressure. We examined in conscious Ang II-treated rabbits, the effects on blood pressure and tonic and reflex renal SNA (RSNA) of microinjecting into the PVN either RU28318 to block MR, losartan to block Ang (AT1) receptors or muscimol to inhibit GABA A receptor agonist actions. Male rabbits received a moderate dose of Ang II (24 ng/kg/min subcutaneously) for 3 months (n = 13) or sham treatment (n = 13). At 3 months, blood pressure increased by +19% in the Ang II group compared to 10% in the sham (P = 0.022) but RSNA was similar. RU28318 lowered blood pressure in both Ang II and shams but had a greater effect on RSNA and heart rate in the Ang II-treated group (P < 0.05). Losartan also lowered RSNA, while muscimol produced sympatho-excitation in both groups. In Ang II-treated rabbits, RU28318 attenuated the blood pressure increase following chemoreceptor stimulation but did not affect responses to air jet stress. In contrast losartan and muscimol reduced blood pressure and RSNA responses to both hypoxia and air jet. While neither RU28318 nor losartan changed the RSNA baroreflex, RU28318 augmented the range of the heart rate baroreflex by 10% in Ang II-treated rabbits. Muscimol, however, augmented the RSNA baroreflex by 11% in sham animals and none of the treatments altered baroreflex sensitivity. In conclusion, 3 months of moderate Ang II treatment promotes activation of reflex RSNA principally via MR activation in the PVN, rather than via activation of AT1 receptors. However, the onset of hypertension is independent of both. Interestingly, the sympatho-excitatory effects of muscimol in both groups suggest that overall, the PVN regulates a tonic sympatho-inhibitory influence on blood pressure control.

Severe food restriction activates the central renin angiotensin system

We previously showed that 2 weeks of a severe food restricted (sFR) diet (40% of the caloric intake of the control (CT) diet) up‐regulated the circulating renin angiotensin (Ang) system (RAS) in female Fischer rats, most likely as a result of the fall in plasma volume. In this study, we investigated the role of the central RAS in the mean arterial pressure (MAP) and heart rate (HR) dysregulation associated with sFR. Although sFR reduced basal mean MAP and HR, the magnitude of the pressor response to intracerebroventricular (icv) microinjection of Ang‐[1‐8] was not affected; however, HR was 57 ± 13 bpm lower 26 min after Ang‐[1‐8] microinjection in the sFR rats and a similar response was observed after losartan was microinjected. The major catabolic pathway of Ang‐[1‐8] in the hypothalamus was via Ang‐[1‐7]; however, no differences were detected in the rate of Ang‐[1‐8] synthesis or degradation between CT and sFR animals. While sFR had no effect on the AT₁R binding in the subfornical organ (SFO), the organum vasculosum laminae terminalis (OVLT) and median preoptic nucleus (MnPO) of the paraventricular anteroventral third ventricle, ligand binding increased 1.4‐fold in the paraventricular nucleus (PVN) of the hypothalamus. These findings suggest that sFR stimulates the central RAS by increasing AT₁R expression in the PVN as a compensatory response to the reduction in basal MAP and HR. These findings have implications for people experiencing a period of sFR since an activated central RAS could increase their risk of disorders involving over activation of the RAS including renal and cardiovascular diseases.

Activation of ADAM17 (A Disintegrin and Metalloprotease 17) on Glutamatergic Neurons Selectively Promotes Sympathoexcitation

Chronic activation of the brain renin-angiotensin system contributes to the development of hypertension by altering autonomic balance. Beyond the essential role of Ang II (angiotensin II) type 1 receptors, ADAM17 (A disintegrin and metalloprotease 17) is also found to promote brain renin-angiotensin system overactivation. ADAM17 is robustly expressed in various cell types within the central nervous system. The aim of this study was to determine whether ADAM17 modulates presympathetic neuronal activity to promote autonomic dysregulation in salt-sensitive hypertension. To test our hypothesis, ADAM17 was selectively knocked down in glutamatergic neurons using Cre-loxP technology. In mice lacking ADAM17 in glutamatergic neurons, the blood pressure increase induced by deoxycorticosterone acetate-salt treatment was blunted. Deoxycorticosterone acetate-salt significantly elevated cardiac and vascular sympathetic drive in control mice, while such effects were reduced in mice with ADAM17 knockdown. This blunted sympathoexcitation was extended to the spleen, with a lesser activation of the peripheral immune system, translating into a sequestration of circulating T cells within this organ, compared with controls. Within the paraventricular nucleus, Ang II-induced activation of kidney-related presympathetic glutamatergic neurons was reduced in ADAM17 knockdown mice, with the majority of cells no longer responding to Ang II stimulation, confirming the supportive role of ADAM17 in increasing presympathetic neuronal activity. Overall, our data highlight the pivotal role of neuronal ADAM17 in regulating sympathetic activity and demonstrate that activation of ADAM17 in glutamatergic neurons leads to a selective increase of sympathetic output, but not vagal tone, to specific organs, ultimately contributing to dysautonomia and salt-sensitive hypertension.

Hydration Status and Cardiovascular Function

Hypohydration, defined as a state of low body water, increases thirst sensations, arginine vasopressin release, and elicits renin–angiotensin–aldosterone system activation to replenish intra- and extra-cellular fluid stores. Hypohydration impairs mental and physical performance, but new evidence suggests hypohydration may also have deleterious effects on cardiovascular health. This is alarming because cardiovascular disease is the leading cause of death in the United States. Observational studies have linked habitual low water intake with increased future risk for adverse cardiovascular events. While it is currently unclear how chronic reductions in water intake may predispose individuals to greater future risk for adverse cardiovascular events, there is evidence that acute hypohydration impairs vascular function and blood pressure (BP) regulation. Specifically, acute hypohydration may reduce endothelial function, increase sympathetic nervous system activity, and worsen orthostatic tolerance. Therefore, the purpose of this review is to present the currently available evidence linking acute hypohydration with altered vascular function and BP regulation.

(Pro)renin receptor knockdown in the paraventricular nucleus of the hypothalamus attenuates hypertension development and AT1 receptor-mediated calcium events

Activation of the brain renin-angiotensin system (RAS) is a pivotal step in the pathogenesis of hypertension. The paraventricular nucleus (PVN) of the hypothalamus is a critical part of the angiotensinergic sympatho-excitatory neuronal network involved in neural control of blood pressure and hypertension. However, the importance of the PVN (pro)renin receptor (PVN-PRR)-a key component of the brain RAS-in hypertension development has not been examined. In this study, we investigated the involvement and mechanisms of the PVN-PRR in DOCA-salt-induced hypertension, a mouse model of hypertension. Using nanoinjection of adeno-associated virus-mediated Cre recombinase expression to knock down the PRR specifically in the PVN, we report here that PVN-PRR knockdown attenuated the enhanced blood pressure and sympathetic tone associated with hypertension. Mechanistically, we found that PVN-PRR knockdown was associated with reduced activation of ERK (extracellular signal-regulated kinase)-1/2 in the PVN and rostral ventrolateral medulla during hypertension. In addition, using the genetically encoded Ca²⁺ biosensor GCaMP6 to monitor Ca²⁺-signaling events in the neurons of PVN brain slices, we identified a reduction in angiotensin II type 1 receptor-mediated Ca²⁺ activity as part of the mechanism by which PVN-PRR knockdown attenuates hypertension. Our study demonstrates an essential role of the PRR in PVN neurons in hypertension through regulation of ERK1/2 activation and angiotensin II type 1 receptor-mediated Ca²⁺ activity. NEW & NOTEWORTHY PRR knockdown in PVN neurons attenuates the development of DOCA-salt hypertension and autonomic dysfunction through a decrease in ERK1/2 activation in the PVN and RVLM during hypertension. In addition, PRR knockdown reduced AT_1aR expression and AT₁R-mediated calcium activity during hypertension. Furthermore, we characterized the neuronal targeting specificity of AAV serotype 2 in the mouse PVN and validated the advantages of the genetically encoded calcium biosensor GCaMP6 in visualizing neuronal calcium activity in the PVN.

Regulation of sympathetic vasomotor activity by the hypothalamic paraventricular nucleus in normotensive and hypertensive states

The hypothalamic paraventricular nucleus (PVN) is a unique and important brain region involved in the control of cardiovascular, neuroendocrine, and other physiological functions pertinent to homeostasis. The PVN is a major source of excitatory drive to the spinal sympathetic outflow via both direct and indirect projections. In this review, we discuss the role of the PVN in the regulation of sympathetic output in normal physiological conditions and in hypertension. In normal healthy animals, the PVN presympathetic neurons do not appear to have a major role in sustaining resting sympathetic vasomotor activity or in regulating sympathetic responses to short-term homeostatic challenges such as acute hypotension or hypoxia. Their role is, however, much more significant during longer-term challenges, such as sustained water deprivation, chronic intermittent hypoxia, and pregnancy. The PVN also appears to have a major role in generating the increased sympathetic vasomotor activity that is characteristic of multiple forms of hypertension. Recent studies in the spontaneously hypertensive rat model have shown that impaired inhibitory and enhanced excitatory synaptic inputs to PVN presympathetic neurons are the basis for the heightened sympathetic outflow in hypertension. We discuss the molecular mechanisms underlying the presynaptic and postsynaptic alterations in GABAergic and glutamatergic inputs to PVN presympathetic neurons in hypertension. In addition, we discuss the ability of exercise training to correct sympathetic hyperactivity by restoring blood-brain barrier integrity, reducing angiotensin II availability, and decreasing oxidative stress and inflammation in the PVN.

Effect of Chronic Intermittent Hypoxia on Angiotensin II Receptors in the Central Nervous System

Chronic intermittent hypoxia (CIH) increases basal sympathetic nervous system activity, augments chemoreflex-induced sympathoexcitation, and raises blood pressure. All effects are attenuated by systemic or intracerebroventricular administration of angiotensin II type 1 receptor (AT1R) antagonists. This study aimed to quantify the effects of CIH on AT1R- and AT2R-like immunoreactivity in the rostroventrolateral medulla (RVLM) and paraventricular nucleus of the hypothalamus (PVN), central regions that are important components of the extended chemoreflex pathway. Eighteen Sprague-Dawley rats were exposed to intermittent hypoxia (FIO2 = 0.10, 1 min at 4-min intervals) for 10 hr/day for 1, 5, 10, or 21 days. After exposure, rats were deeply anesthetized and transcardially perfused with phosphate buffered saline (PBS) followed by 4% paraformaldehyde in PBS. Brains were removed and sectioned coronally into 50 µm slices. Immunohistochemistry was used to quantify AT1R and AT2R in the RVLM and the PVN. In the RVLM, CIH significantly increased the AT1R-like immunoreactivity, but did not alter AT2R immunoreactivity, thereby augmenting the AT1R:AT2R ratio in this nucleus. In the PVN, CIH had no effect on immunoreactivity of either receptor subtype. The current findings provide mechanistic insight into increased basal sympathetic outflow, enhanced chemoreflex sensitivity, and blood pressure elevation observed in rodents exposed to CIH.

Delineation of the central melanocortin circuitry controlling the kidneys by a virally mediated transsynaptic tracing study in transgenic mouse model

To examine if brain neurons involved in the efferent control of the kidneys possess melanocortin-4 receptor (MC4-R) and/or tryptophan hydroxylase (TPH). Retrograde tracing pseudorabies virus (PRV)-614 was injected into the kidneys in adult male MC4R-green fluorescent protein (GFP) transgenic mice. After a survival time of 3-7 days, spinal cord and brain were removed and sectioned, and processed for PRV-614 visualization. The neurochemical phenotype of PRV-614-positive neurons was identified using double or triple immunocytochemical labeling against PRV-614, MC4R, or TPH. Double and triple labeling was quantified using microscopy. The majority of PRV-614 immunopositive neurons which also expressed immunoreactivity for MC4R were located in the ipsilateral intermediolateral cell column (IML) of the thoracic spinal cord, the paraventricular nucleus (PVN) of the hypothalamus, and raphe pallidus (RPa), nucleus raphe magnus (NRM) and ventromedial medulla (VMM) of the brainstem. Triple-labeled MC4R/PRV-614/TPH neurons were concentrated in the PVN, RPa, NRM and VMM. These data strongly suggest that central MC4R and TPH are involved in the efferent neuronal control of the kidneys.

Cardiovascular responses to l-glutamate microinjection into the NTS are abrogated by reduced glutathione

Redox imbalance in regions of the CNS controlling blood pressure is increasingly recognized as a leading factor for hypertension. Nucleus tractus solitarius (NTS) of the dorsomedial medulla is the main region receiving excitatory visceral sensory inputs that modulate autonomic efferent drive to the cardiovascular system. This study sought to determine the capacity of reduced glutathione, a major bioactive antioxidant, to modulate NTS-mediated control of cardiovascular function in unanaesthetized rats. Male Fischer 344 rats were used for microinjection experiments. Cardiovascular responses to l-glutamate were first used to verify accurate placement of injections into the dorsomedial region comprising the NTS. Next, responses to GSH or vehicle were recorded followed by responses to l-glutamate again at the same site. GSH microinjection increased mean arterial pressure (MAP) compared to vehicle and abrogated responses to subsequent injection of l-glutamate. These data indicate that GSH microinjection into the NTS affects blood pressure regulation by dorsomedial neuronal circuits and blunts l-glutamate driven excitation in this region. These findings raise the possibility that increased antioxidant actions of GSH in NTS could contribute to autonomic control dysfunctions of the cardiovascular system.

Astrocytes Contribute to Angiotensin II Stimulation of Hypothalamic Neuronal Activity and Sympathetic Outflow

Angiotensin II (AngII) is a key neuropeptide that acting within the brain hypothalamic paraventricular nucleus regulates neurohumoral outflow to the circulation. Moreover, an exacerbated AngII action within the paraventricular nucleus contributes to neurohumoral activation in hypertension. Although AngII effects involve changes in paraventricular nucleus neuronal activity, the precise underlying mechanisms, cellular targets, and distribution of AngII receptors within the paraventricular nucleus remain largely unknown. Thus, whether AngII effects involve direct actions on paraventricular neurons, or whether it acts via intermediary cells, such as astrocytes, is still controversial. To address this important gap in our knowledge, we used a multidisciplinary approach combining patch-clamp electrophysiology in presympathetic paraventricular neurons and astrocytes, along with in vivo sympathetic nerve recordings and astrocyte-targeted gene manipulations. We present evidence for a novel mechanism underlying central AngII actions, which involves astrocytes as major intermediary cellular targets. We found that AngII type 1 receptor mRNA is expressed in paraventricular astrocytes. Moreover, we report that AngII inhibited glutamate transporter function, increasing in turn extracellular glutamate levels. This resulted in the activation of neuronal extrasynaptic NMDA (N-methyl-d-aspartate) receptors, increased presympathetic neuronal activity, enhanced sympathoexcitatory outflow, and increased blood pressure. Together, our studies support astrocytes as critical intermediary cell types mediating brain AngII regulation of the circulation and indicate that AngII-mediated neuronal and sympathoexcitatory effects are dependent on a unique neuroglial signaling modality involving nonsynaptic glutamate transmission.

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

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

Female protection from slow-pressor effects of angiotensin II involves prevention of ROS production independent of NMDA receptor trafficking in hypothalamic neurons expressing angiotensin 1A receptors

Renin–angiotensin system overactivity, upregulation of postsynaptic NMDA receptor function, and increased reactive oxygen species (ROS) production in the hypothalamic paraventricular nucleus (PVN) are hallmarks of angiotensin II (AngII)-induced hypertension, which is far more common in young males than in young females. We hypothesize that the sex differences in hypertension are related to differential AngII-induced changes in postsynaptic trafficking of the essential NMDA receptor GluN1 subunit and ROS production in PVN cells expressing angiotensin Type 1a receptor (AT1aR). We tested this hypothesis using slow-pressor (14-day) infusion of AngII (600 ng/kg/min) in mice, which elicits hypertension in males but not in young females. Two-month-old male and female transgenic mice expressing enhanced green fluorescent protein (EGFP) in AT1aR-containing cells were used. In males, but not in females, AngII increased blood pressure and ROS production in AT1aR–EGFP PVN cells at baseline and following NMDA treatment. Electron microscopy showed that AngII increased cytoplasmic and total GluN1–silver-intensified immunogold (SIG) densities and induced a trend toward an increase in near plasmalemmal GluN1–SIG density in AT1aR–EGFP dendrites of males and females. Moreover, AngII decreased dendritic area and diameter in males, but increased dendritic area of small (<1 µm) dendrites and decreased diameter of large (>1 µm) dendrites in females. Fluorescence microscopy revealed that AT1aR and estrogen receptor β do not colocalize, suggesting that if estrogen is involved, its effect is indirect. These data suggest that the sexual dimorphism in AngII-induced hypertension is associated with sex differences in ROS production in AT1aR-containing PVN cells but not with postsynaptic NMDA receptor trafficking.

Activation of the hypothalamic paraventricular nucleus by forebrain hypertonicity selectively increases tonic vasomotor sympathetic nerve activity

We recently reported that mean arterial pressure (MAP) is maintained in water-deprived rats by an irregular tonic component of vasomotor sympathetic nerve activity (SNA) that is driven by neuronal activity in the hypothalamic paraventricular nucleus (PVN). To establish whether generation of tonic SNA requires time-dependent (i.e., hours or days of dehydration) neuroadaptive responses or can be abruptly generated by even acute circuit activation, forebrain sympathoexcitatory osmosensory inputs to PVN were stimulated by infusion (0.1 ml/min, 10 min) of hypertonic saline (HTS; 1.5 M NaCl) through an internal carotid artery (ICA). Whereas isotonic saline (ITS; 0.15 M NaCl) had no effect (n = 5), HTS increased (P < 0.001; n = 6) splanchnic SNA (sSNA), phrenic nerve activity (PNA), and MAP. Bilateral PVN injections of muscimol (n = 6) prevented HTS-evoked increases of integrated sSNA and PNA (P < 0.001) and attenuated the accompanying pressor response (P < 0.01). Blockade of PVN NMDA receptors with d-(2R)-amino-5-phosphonovaleric acid (AP5; n = 6) had similar effects. Analysis of respiratory rhythmic bursting of sSNA revealed that ICA HTS increased mean voltage (P < 0.001) without affecting the amplitude of inspiratory or expiratory bursts. Analysis of cardiac rhythmic sSNA likewise revealed that ICA HTS increased mean voltage. Cardiac rhythmic sSNA oscillation amplitude was also increased, which is consistent with activation of arterial baroreceptor during the accompanying pressor response. Increased mean sSNA voltage by HTS was blocked by prior PVN inhibition (muscimol) and blockade of PVN NMDA receptors (AP5). We conclude that even acute glutamatergic activation of PVN (i.e., by hypertonicity) is sufficient to selectively increase a tonic component of vasomotor SNA.

Activation of corticotropin-releasing factor receptors in the rostral ventrolateral medulla is required for glucose-induced sympathoexcitation

Energy expenditure is determined by metabolic rate and diet-induced thermogenesis. Normally, energy expenditure increases due to neural mechanisms that sense plasma levels of ingested nutrients/hormones and reflexively increase sympathetic nerve activity (SNA). Here, we investigated neural mechanisms of glucose-driven sympathetic activation by determining contributions of neuronal activity in the hypothalamic paraventricular nucleus (PVN) and activation of corticotropin-releasing factor (CRF) receptors in the rostral ventrolateral medulla (RVLM). Glucose was infused intravenously (150 mg/kg, 10 min) in male rats to raise plasma glucose concentration to a physiological postprandial level. In conscious rats, glucose infusion activated CRF-containing PVN neurons and TH-containing RVLM neurons, as indexed by c-Fos immunofluorescence. In α-chloralose/urethane-anesthetized rats, glucose infusion increased lumbar and splanchnic SNA, which was nearly prevented by prior RVLM injection of the CRF receptor antagonist astressin (10 pmol/50 nl). This cannot be attributed to a nonspecific effect, as sciatic afferent stimulation increased SNA and ABP equivalently in astressin- and aCSF-injected rats. Glucose-stimulated sympathoexcitation was largely reversed during inhibition of PVN neuronal activity with the GABA-A receptor agonist muscimol (100 pmol/50 nl). The effects of astressin to prevent glucose-stimulated sympathetic activation appear to be specific to interruption of PVN drive to RVLM because RVLM injection of astressin prior to glucose infusion effectively prevented SNA from rising and prevented any fall of SNA in response to acute PVN inhibition with muscimol. These findings suggest that activation of SNA, and thus energy expenditure, by glucose is initiated by activation of CRF receptors in RVLM by descending inputs from PVN.

Coping with dehydration: sympathetic activation and regulation of glutamatergic transmission in the hypothalamic PVN

Autonomic and endocrine profiles of chronic hypertension and heart failure resemble those of acute dehydration. Importantly, all of these conditions are associated with exaggerated sympathetic nerve activity (SNA) driven by glutamatergic activation of the hypothalamic paraventricular nucleus (PVN). Here, studies sought to gain insight into mechanisms of disease by determining the role of PVN ionotropic glutamate receptors in supporting SNA and mean arterial pressure (MAP) during dehydration and by elucidating mechanisms regulating receptor activity. Blockade of PVN N-methyl-D-aspartate (NMDA) receptors reduced (P < 0.01) renal SNA and MAP in urethane-chloralose-anesthetized dehydrated (DH) (48 h water deprivation) rats, but had no effect in euhydrated (EH) controls. Blockade of PVN α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors had no effect in either group. NMDA in PVN caused dose-dependent increases of renal SNA and MAP in both groups, but the maximum agonist evoked response (Emax) of the renal SNA response was greater (P < 0.05) in DH rats. The latter was not explained by increased PVN expression of NMDA receptor NR1 subunit protein, increased PVN neuronal excitability, or decreased brain water content. Interestingly, PVN injection of the pan-specific excitatory amino acid transporter (EAAT) inhibitor DL-threo-β-benzyloxyaspartic acid produced smaller sympathoexcitatory and pressor responses in DH rats, which was associated with reduced glial expression of EAAT2 in PVN. Like chronic hypertension and heart failure, dehydration increases excitatory NMDA receptor tone in PVN. Reduced glial-mediated glutamate uptake was identified as a key contributing factor. Defective glutamate uptake in PVN could therefore be an important, but as yet unexplored, mechanism driving sympathetic hyperactivity in chronic cardiovascular diseases.

Roles of the subfornical organ and area postrema in arterial pressure increases induced by 48-h water deprivation in normal rats

In rats, water deprivation (WD) increases arterial blood pressure (BP) in part due to actions of elevated osmolality in the brain to increase vasopressin levels and sympathetic activity. However, the osmoreceptors that mediate this response have not been identified. To test the hypothesis that osmoregulatory circumventricular organs are involved, BP and heart rate (HR) were continuously recorded telemetrically during 48 h of WD in normal rats with lesions (x) or sham lesions (sham) of the subfornical organ (SFO) or area postrema (AP). Although WD increased BP in SFOx and SFOsham rats, no significant difference in the hypertensive response was observed between groups. HR decreased transiently but similarly in SFOx and SFOsham rats during the first 24 h of WD. When water was reintroduced, BP and HR decreased rapidly and similarly in both groups. BP (during lights off) and HR were both lower in APx rats before WD compared to APsham. WD increased BP less in APx rats, and the transient bradycardia was eliminated. Upon reintroduction of drinking water, smaller falls in both BP and HR were observed in APx rats compared to APsham rats. WD increased plasma osmolality and vasopressin levels similarly in APx and APsham rats, and acute blockade of systemic V1 vasopressin receptors elicited similar depressor responses, suggesting that the attenuated BP response is not due to smaller increases in vasopressin or osmolality. In conclusion, the AP, but not the SFO, is required for the maximal hypertensive effect induced by WD in rats.

Sympathetic network drive during water deprivation does not increase respiratory or cardiac rhythmic sympathetic nerve activity

Effects of water deprivation on rhythmic bursting of sympathetic nerve activity (SNA) were investigated in anesthetized, bilaterally vagotomized, euhydrated (control) and 48-h water-deprived (WD) rats (n = 8/group). Control and WD rats had similar baseline values of mean arterial pressure, heart rate, end-tidal CO2, and central respiratory drive. Although integrated splanchnic SNA (sSNA) was greater in WD rats than controls (P < 0.01), analysis of respiratory rhythmic bursting of sSNA revealed that inspiratory rhythmic burst amplitude was actually smaller (P < 0.005) in WD rats (+68 ± 6%) than controls (+208 ± 20%), and amplitudes of the early expiratory (postinspiratory) trough and late expiratory burst of sSNA were not different between groups. Further analysis revealed that water deprivation had no effect on either the amplitude or periodicity of the cardiac rhythmic oscillation of sSNA. Collectively, these data indicate that the increase of sSNA produced by water deprivation is not attributable to either increased respiratory or cardiac rhythmic burst discharge. Thus the sympathetic network response to acute water deprivation appears to differ from that of chronic sympathoexcitation in neurogenic forms of arterial hypertension, where increased respiratory rhythmic bursting of SNA and baroreflex adaptations have been reported.

Central mineralocorticoid receptors and the role of angiotensin II and glutamate in the paraventricular nucleus of rats with angiotensin II-induced hypertension

A chronic increase in circulating angiotensin II (Ang II) activates an aldosterone-mineralocorticoid receptor-ouabain neuromodulatory pathway in the brain that increases neuronal activation in hypothalamic nuclei, such as the paraventricular nucleus (PVN) and causes progressive hypertension. Several models of chronic sympathetic hyperactivity are associated with an increase in AT1 and glutamate receptor activation in the PVN. The current study evaluated whether increased angiotensin type 1 (AT1) and glutamate receptor-dependent signaling in the PVN contributes to the maintenance of blood pressure (BP) in Ang II-hypertensive Wistar rats, and the role of aldosterone-mineralocorticoid receptor pathway in this enhanced signaling. After subcutaneous infusion of Ang II for 2 weeks, in conscious rats BP and heart rate were recorded after (1) 10-minute bilateral infusions of candesartan and kynurenate in the PVN; (2) 1 hour intracerebroventricular infusion of eplerenone, and (3) candesartan and kynurenate after eplerenone. Candesartan or kynurenate in the PVN fully reversed the increase in BP from circulating Ang II. Kynurenate after candesartan or candesartan after kynurenate did not further lower BP. Intracerebroventricular infusion of eplerenone at 16 hours after its infusion fully reversed the increase in BP from circulating Ang II. After eplerenone, candesartan and kynurenate in the PVN did not further decrease BP. These findings suggest that increased mineralocorticoid receptor activation in the brain activates a slow neuromodulatory pathway that maintains enhanced AT1 and glutamate receptor-dependent signaling in the PVN, and thereby the hypertension from a chronic increase in circulating Ang II.

Regulation of arterial pressure by the paraventricular nucleus in conscious rats: interactions among glutamate, GABA, and nitric oxide

The paraventricular nucleus (PVN) of the hypothalamus is an important site for autonomic and neuroendocrine regulation. Experiments in anesthetized animals and in vitro indicate an interaction among gamma-aminobutyric acid (GABA), nitric oxide (NO), and glutamate in the PVN. The cardiovascular role of the PVN and interactions of these neurotransmitters in conscious animals have not been evaluated fully. In chronically instrumented conscious rats, mean arterial pressure (MAP) and heart rate (HR) responses to microinjections (100 nl) in the region of the PVN were tested. Bilateral blockade of ionotropic excitatory amino acid (EAA) receptors (kynurenic acid, Kyn) in the PVN produced small but significant decreases in MAP and HR. GABA_A receptor blockade (bicuculline, Bic), and inhibition of NO synthase [(NOS), N-(G)-monomethyl-L-arginine, L-NMMA] each increased MAP and HR. The NO donor sodium nitroprusside (SNP) produced depressor responses that were attenuated by Bic. NOS inhibition potentiated both pressor responses to the selective EAA agonist, N-methyl-D-aspartic acid (NMDA), and depressor responses to Kyn. Increases in MAP and HR due to Bic were blunted by prior blockade of EAA receptors. Thus, pressor responses to GABA blockade require EAA receptors and GABA neurotransmission contributes to NO inhibition. Tonic excitatory effects of glutamate in the PVN are tonically attenuated by NO. These data demonstrate that, in the PVN of conscious rats, GABA, glutamate, and NO interact in a complex fashion to regulate arterial pressure and HR under normal conditions.

Role of the Median Preoptic Nucleus in Arterial Pressure Regulation and Sodium and Water Homeostasis during High Dietary Salt Intake

Changes in the osmolality and level of angiotensin II (ANG II) are important peripheral signals modulating appropriate central sympathetic output and maintaining a normal arterial pressure during high salt intake. The median preoptic nucleus (MnPO) receives reciprocal inputs from the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT), the circumventricular organs that have been shown to be necessary in multiple central effects of changes in the osmolality and circulating ANG II directed toward the maintenance of sodium and water homeostasis. We, therefore, hypothesized that the MnPO is a crucial part of the central neuronal mechanisms mediating the blood pressure control by altered osmolality and/or ANG II signaling during chronic high dietary salt intake. Male Sprague-Dawley rats were randomly assigned to either sham (operation), or electrolytic lesion of the MnPO. After a 7-day recovery, rats were instrumented with radiotelemetric transducers and aortic flow probes for the measurement of the mean arterial pressure + heart rate (HR) and cardiac output (CO), respectively. Femoral venous catheters were also implanted to collect blood for the measurements of plasma osmolality and sodium concentration, as well as plasma renin activity. Rats were given another 10 days to recover and then were subjected to a 28-day-long study protocol that included a 7-day control period (1.0% NaCl diet), followed by 14 days of high salt (4.0% NaCl), and a 7-day recovery period (1.0% NaCl). The data showed, that despite a slight increase in the MAP observed in both MnPO- (n = 12) and sham-lesioned (n = 8) rats during the high-salt period, there were no significant differences between the MAP, HR, and CO in the two groups throughout the study protocol. These findings do not support the hypothesis that the MnPO is necessary to maintain normal blood pressure during high dietary salt intake. However, MnPO-lesioned rats showed less sodium balance than sham-lesioned rats during the first 4 days of high salt intake. Although, these results may be explained partly by the plasma hyperosmolarity and hypernatremia observed in MnPO-lesioned rats; they also shed light on the role of the MnPO in central neuronal control of renal sodium handling during chronic high dietary salt intake.

Endogenous hydrogen peroxide in the hypothalamic paraventricular nucleus regulates sympathetic nerve activity responses to L-glutamate

The hypothalamic paraventricular nucleus (PVN) is important for maintenance of sympathetic nerve activity (SNA) and cardiovascular function. PVN-mediated increases of SNA often involve the excitatory amino acid L-glutamate (L-glu), whose actions can be positively and negatively modulated by a variety of factors, including reactive oxygen species. Here, we determined modulatory effects of the highly diffusible reactive oxygen species hydrogen peroxide (H(2)O(2)) on responses to PVN L-glu. Renal SNA (RSNA), arterial blood pressure, and heart rate were recorded in anesthetized rats. L-Glu (0.2 nmol in 100 nl) microinjected unilaterally into PVN increased RSNA (P < 0.05), without affecting mean arterial blood pressure or heart rate. Effects of endogenously generated H(2)O(2) were determined by comparing responses to PVN L-glu before and after PVN injection of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ; 100 nmol/200 nl, n = 5). ATZ alone was without effect on recorded variables, but attenuated the increase of RSNA elicited by PVN L-glu (P < 0.05). PVN injection of exogenous H(2)O(2) (5 nmol in 100 nl, n = 4) and vehicle (artificial cerebrospinal fluid) were without affect, but H(2)O(2), like ATZ, attenuated the increase of RSNA to PVN L-glu (P < 0.05). Tonic effects of endogenous H(2)O(2) were determined by PVN injection of polyethylene glycol-catalase (1.0 IU in 200 nl, n = 5). Whereas polyethylene glycol-catalase alone was without effect, increases of RSNA to subsequent PVN injection of L-glu were increased (P < 0.05). From these data, we conclude that PVN H(2)O(2) tonically, but submaximally, suppresses RSNA responses to L-glu, supporting the idea that a change of H(2)O(2) availability within PVN could influence SNA regulation under physiological and/or disease conditions.

Effect of global and regional sympathetic blockade on arterial pressure during water deprivation in conscious rats

Forty-eight hours of water deprivation (WD) in conscious rats results in a paradoxical increase in mean arterial pressure (MAP). Previous studies suggest this may be due to increased sympathetic nerve activity (SNA). However, this remains to be investigated in conscious, freely behaving animals. The purpose of this study was to determine, in conscious rats, the role of the sympathetic nervous system (SNS) in mediating WD-induced increases in MAP and to identify which vascular beds are targeted by increased SNA. Each rat was chronically instrumented with a radiotelemetry transmitter to measure MAP and heart rate (HR) and an indwelling venous catheter for plasma sampling and/or drug delivery. MAP and HR were continuously measured during a 2-day baseline period followed by 48 h of WD and then a recovery period. By the end of the WD period, MAP increased by ∼15 mmHg in control groups, whereas HR did not change significantly. Chronic blockade of α(1)/β(1)-adrenergic receptors significantly attenuated the WD-induced increase in MAP, suggesting a role for global activation of the SNS. However, the MAP response to WD was unaffected by selective denervations of the hindlimb, renal, or splanchnic vascular beds, or by adrenal demedullation. In contrast, complete adrenalectomy (with corticosterone and aldosterone replaced) significantly attenuated the MAP response to WD in the same time frame as α(1)/β(1)-adrenergic receptor blockade. These results suggest that, in conscious water-deprived rats, the SNS contributes to the MAP response and may be linked to release of adrenocortical hormones. Finally, this sympathetically mediated response is not dependent on increased SNA to one specific vascular bed.

Cardiovascular effects of angiotensin II and glutamate in the PVN of Dahl salt-sensitive rats

Several models of chronic sympathetic hyperactivity are associated with an increase in excitatory angiotensinergic and glutamatergic activity, and a decrease in GABAergic activity in the PVN. The present study evaluated whether activation of glutamate and AT1 receptors in the PVN contributes to the maintenance of resting BP in Dahl salt sensitive (S) rats on regular or high salt diet for 4-6 weeks. Candesartan and kynurenate were infused bilaterally into the PVN and BP and heart rate (HR) were recorded. Both candesartan and kynurenate in the PVN did not change MAP and HR in normotensive Dahl salt resistant (R) and S rats on regular salt diet or in R rats on high salt diet. In hypertensive Dahl S rats on high salt diet, candesartan decreased MAP (-14±2 mm Hg), and tended to increase HR (22±5 bpm). Kynurenate decreased both MAP (-22±3 mm Hg) and HR (-42±7 bpm) in these rats. At the peak BP decrease by candesartan, kynurenate in the PVN further decreased BP by ~50% (-14±2 mm Hg), whereas candesartan did not further decrease BP at the peak BP response to kynurenate (-4±2 mm Hg). These results indicate that activation of glutamate and AT1-receptors in the PVN contributes to the maintenance of BP in hypertensive Dahl S rats, but not normotensive Dahl S and R rats. The increased BP response to AT1-receptor activation in the PVN of hypertensive Dahl S appears to be mediated by enhanced local glutamate receptor activation, but another mechanism(s) appears to further enhance glutamate responses.

Mechanisms mediating sodium-induced pressor responses in the PVN of Dahl rats

Intracerebroventricular infusion of Na(+)-rich artificial cerebrospinal fluid (aCSF) causes larger sympathetic and pressor responses in Dahl salt-sensitive (S) than -resistant (R) or Wistar rats. Enhanced activity of the aldosterone-"ouabain" pathway or decreased nitric oxide (NO) release may contribute to this enhanced responsiveness. Where in the brain these mechanisms interact is largely unknown. The present study evaluated whether Na(+) in the paraventricular nucleus (PVN) causes larger pressor responses in Dahl S (SS/Mcw) than R (Dahl SS.BN13) rats and whether mineralocorticoid receptors, benzamil-blockable Na(+) channels, "ouabain," angiotensin type 1 receptors, or NO mediates these enhanced responses. Na(+)-rich aCSF in the PVN caused 30-40% larger increases in blood pressure and heart rate in Dahl S than R or Wistar rats, whereas responses to ouabain, ANG II, or N(ω)-nitro-l-arginine methyl ester hydrochloride (l-NAME) in the PVN were the same. These responses to Na(+) were not affected by eplerenone, benzamil, or Fab fragments, whereas they were fully blocked by losartan, in Dahl S and R rats. l-NAME enhanced them more in Dahl R than S rats, thereby equalizing the responses in the two strains. Pressor responses to l-NAME in the PVN were attenuated by a high-salt diet in Dahl S, but not R, rats. The results indicate that acute and chronic increases in Na(+) concentration in the PVN inhibit NO release in the PVN of Dahl S, but not R, rats, thereby contributing to the enhanced pressor responses to Na(+) in Dahl S rats.

Switching control of sympathetic activity from forebrain to hindbrain in chronic dehydration

We investigated the mechanisms responsible for increased blood pressure and sympathetic nerve activity (SNA) caused by 2-3 days dehydration (DH) both in vivo and in situ preparations. In euhydrated (EH) rats, systemic application of the AT(1) receptor antagonist Losartan and subsequent pre-collicular transection (to remove the hypothalamus) significantly reduced thoracic (t)SNA. In contrast, in DH rats, Losartan, followed by pre-collicular and pontine transections, failed to reduce tSNA, whereas transection at the medulla-spinal cord junction massively reduced tSNA. In DH but not EH rats, selective inhibition of the commissural nucleus tractus solitarii (cNTS) significantly reduced tSNA. Comparable data were obtained in both in situ and in vivo (anaesthetized/conscious) rats and suggest that following chronic dehydration, the control of tSNA transfers from supra-brainstem structures (e.g. hypothalamus) to the medulla oblongata, particularly the cNTS. As microarray analysis revealed up-regulation of AP1 transcription factor JunD in the dehydrated cNTS, we tested the hypothesis that AP1 transcription factor activity is responsible for dehydration-induced functional plasticity. When AP1 activity was blocked in the cNTS using a viral vector expressing a dominant negative FosB, cNTS inactivation was ineffective. However, tSNA was decreased after pre-collicular transection, a response similar to that seen in EH rats. Thus, the dehydration-induced switch in control of tSNA from hypothalamus to cNTS seems to be mediated via activation of AP1 transcription factors in the cNTS. If AP1 activity is blocked in the cNTS during dehydration, sympathetic activity control reverts back to forebrain regions. This unique reciprocating neural structure-switching plasticity between brain centres emphasizes the multiple mechanisms available for the adaptive response to dehydration.

Forebrain origins of glutamatergic innervation to the rat paraventricular nucleus of the hypothalamus: differential inputs to the anterior versus posterior subregions

The hypothalamic paraventricular nucleus (PVN) regulates numerous homeostatic systems and functions largely under the influence of forebrain inputs. Glutamate is a major neurotransmitter in forebrain, and glutamate neurosignaling in the PVN is known to mediate many of its functions. Previous work showed that vesicular glutamate transporters (VGluTs; specific markers for glutamatergic neurons) are expressed in forebrain sites that project to the PVN; however, the extent of this presumed glutamatergic innervation to the PVN is not clear. In the present study retrograde FluoroGold (FG) labeling of PVN-projecting neurons was combined with in situ hybridization for VGluT1 and VGluT2 mRNAs to identify forebrain regions that provide glutamatergic innervation to the PVN and its immediate surround in rats, with special consideration for the sources to the anterior versus posterior PVN. VGluT1 mRNA colocalization with retrogradely labeled FG neurons was sparse. VGluT2 mRNA colocalization with FG neurons was most abundant in the ventromedial hypothalamus after anterior PVN FG injections, and in the lateral, posterior, dorsomedial, and ventromedial hypothalamic nuclei after posterior PVN injections. Anterograde tract tracing combined with VGluT2 immunolabeling showed that 1) ventromedial nucleus-derived glutamatergic inputs occur in both the anterior and posterior PVN; 2) posterior nucleus-derived glutamatergic inputs occur predominantly in the posterior PVN; and 3) medial preoptic nucleus-derived inputs to the PVN are not glutamatergic, thereby corroborating the innervation pattern seen with retrograde tracing. The results suggest that PVN subregions are influenced by varying amounts and sources of forebrain glutamatergic regulation, consistent with functional differentiation of glutamate projections.

Role of angiotensin in the rostral ventrolateral medulla in the development and maintenance of hypertension

Whilst crucial for behavioural and homeostatic responses to environmental challenges, chronic elevation of sympathetic nervous system activity to specific vascular beds is associated with hypertension. Indeed such elevated activity may drive the increase in blood pressure seen in some people and in some experimental models of hypertension. This review discusses the neural circuitry involved in generating and modulating sympathetic efferent nerve activity, focusing on the premotor neurons of the rostral ventrolateral medulla. Neurons in the rostral ventrolateral medulla show altered responses to angiotensin II in experimental models of hypertension, suggesting that this might be an important node for interaction between these two systems that are crucial for regulation of blood pressure.

Macrophage migration inhibitory factor in the paraventricular nucleus plays a major role in the sympathoexcitatory response to salt

Central hyperosmotic stimulation (HS) evokes increases in sympathetic nerve activity mediated by activation of angiotensin type 1 receptors in the hypothalamic paraventricular nucleus (PVN). Macrophage inhibitory migration factor (MIF) is an intracellular inhibitory regulator of angiotensin type 1 receptor-mediated actions of angiotensin II within neurons of the PVN. MIF mediates its actions via its intrinsic thiol-protein oxidoreductase activity. We demonstrate that intracerebroventricular injection of hypertonic saline into Sprague-Dawley rats elicits a significant (≈112%) increase in MIF mRNA expression in the PVN. Next, we evaluated the effect of viral-mediated expression of either MIF or [C60S]-MIF (which lacks thiol-protein oxidoreductase activity) in the PVN on the sympathoexcitation evoked by HS. We used a decorticate, arterially perfused in situ preparation of male Wistar rats (60 to 80 g). HS was induced by raising perfusate osmolality from 290 to 380 milliosmoles for 40 seconds. Seven to 10 days before experiments, rats were injected bilaterally (500 nL per side) with 0.9% saline (control) or with adenoassociated virus to express MIF, [C60S]-MIF, or enhanced green fluorescent protein in the PVN. HS produced sympathoexcitation in both the 0.9% saline and enhanced green fluorescent protein groups (sympathetic nerve activity increase of +27±4% and +25±4%, respectively; P<0.05), an effect that was not observed in the MIF group (+4±5%). Conversely, the HS-induced increase in sympathetic nerve activity was potentiated in the [C60S]-MIF group (+45±6%; P<0.05). We propose that MIF acting within the PVN is a major counterregulator of HS-induced sympathoexcitation, an effect that depends on thiol-protein oxidoreductase activity.

Pregnancy and the endocrine regulation of the baroreceptor reflex

The purpose of this review is to delineate the general features of endocrine regulation of the baroreceptor reflex, as well as specific contributions during pregnancy. In contrast to the programmed changes in baroreflex function that occur in situations initiated by central command (e.g., exercise or stress), the complex endocrine milieu often associated with physiological and pathophysiological states can influence the central baroreflex neuronal circuitry via multiple sites and mechanisms, thereby producing varied changes in baroreflex function. During pregnancy, baroreflex gain is markedly attenuated, and at least two hormonal mechanisms contribute, each at different brain sites: increased levels of the neurosteroid 3alpha-hydroxy-dihydroprogesterone (3alpha-OH-DHP), acting in the rostral ventrolateral medulla (RVLM), and reduced actions of insulin in the forebrain. 3alpha-OH-DHP appears to potentiate baroreflex-independent GABAergic inhibition of premotor neurons in the RVLM, which decreases the range of sympathetic nerve activity that can be elicited by changes in arterial pressure. In contrast, reductions in the levels or actions of insulin in the brain blunt baroreflex efferent responses to increments or decrements in arterial pressure. Although plasma levels of angiotensin II are increased in pregnancy, this is not responsible for the reduction in baroreflex gain, although it may contribute to the increased level of sympathetic nerve activity in this condition. How these different hormonal effects are integrated within the brain, as well as possible interactions with additional potential neuromodulators that influence baroreflex function during pregnancy and other physiological and pathophysiological states, remains to be clearly delineated.

Hyperosmotic activation of CNS sympathetic drive: implications for cardiovascular disease

Evidence now indicates that exaggerated sympathetic nerve activity (SNA) significantly contributes to salt-sensitive cardiovascular diseases. Although CNS mechanisms that support the elevation of SNA in various cardiovascular disease models have been intensively studied, many mechanistic details remain unknown. In recent years, studies have shown that SNA can rise as a result of both acute and chronic increases of body fluid osmolality. These findings have raised the possibility that salt-sensitive cardiovascular diseases could result, at least in part, from direct osmosensory activation of CNS sympathetic drive. In this brief review we emphasize recent findings from several laboratories, including our own, which demonstrate that neurons of the forebrain organum vasculosum laminae terminalis (OVLT) play a pivotal role in triggering hyperosmotic activation of SNA by recruiting neurons in specific regions of the hypothalamus, brainstem and spinal cord. Although OVLT neurons are intrinsically osmosensitive and shrink when exposed to extracellular hypertonicity, it is not yet clear if these processes are functionally linked. Whereas acute hypertonic activation of OVLT neurons critically depends on TRPV1 channels, studies in TRPV1(-/-) mice suggest that acute and long-term osmoregulatory responses remain largely intact. Therefore, acute and chronic osmosensory transduction by OVLT neurons may be mediated by distinct mechanisms. We speculate that organic osmolytes such as taurine and possibly novel processes such as extracellular acidification could contribute to long-term osmosensory transduction by OVLT neurons and might therefore participate in the elevation of SNA in salt-sensitive cardiovascular diseases.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Water deprivation increases angiotensin-converting enzyme but not AT(1) receptor expression in brainstem and paraventricular nucleus of the hypothalamus of the rat

The rostral ventrolateral medulla (RVLM) is critical to the maintenance of blood pressure. It has been proposed that blood-borne Ang II can influence the RVLM via a neural connection between the circumventricular organs and paraventricular nucleus of the hypothalamus (PVH) and that a component of this pathway is angiotensinergic. A period of water deprivation leads to increased ability of angiotensin type 1 (AT(1)) receptor antagonists to reduce blood pressure when administered into the RVLM and PVH. We studied the differences in AT(1) receptor and angiotensin-converting enzyme (ACE) expression in these and other brain regions involved in blood pressure regulation and water intake following dehydration. AT(1) receptor and ACE expression in brains of rats deprived of water for 48 h were compared to that of water-replete rats by quantitative receptor autoradiography. AT(1) receptor expression was increased in the subfornical organ and periventricular nucleus of the hypothalamus, but not in other brain regions measured. ACE expression was increased in the RVLM, PVH, choroid plexus, median preoptic nucleus, and organosum vasculosum of the lamina terminalis. These findings suggest that increased Ang II production but not increased receptor expression in the PVH and RVLM is the mechanism by which Ang II in the brain helps to sustain systemic blood pressure during periods of water deprivation.

In vivo discharge properties of hypothalamic paraventricular nucleus neurons with axonal projections to the rostral ventrolateral medulla

The hypothalamic paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM) are key components of a neural network that generates and regulates sympathetic nerve activity (SNA). Although each region has been extensively studied, little is presently known about the in vivo discharge properties of individual PVN neurons that directly innervate the RVLM. Here extracellular recording was performed in anesthetized rats, and antidromic stimulation was used to identify single PVN neurons with axonal projections to the RVLM (n = 94). Neurons were divided into two groups that had either unbranched axons terminating in the RVLM (i.e., PVN-RVLM neurons, n = 65) or collateralized axons targeting both the RVLM and spinal cord [i.e., PVN-RVLM/intermediolateral cell column (IML) neurons, n = 29]. Many PVN-RVLM (32/65, 49%) and PVN-RVLM/IML (17/29, 59%) neurons were spontaneously active. The average firing frequency was not different across groups. Spike-triggered averaging revealed that spontaneous discharge of most neurons was temporally correlated with renal SNA (PVN-RVLM: 12/21, 57%; PVN-RVLM/IML: 6/9, 67%). Time histograms triggered by the electrocardiogram (ECG) R-wave indicated that discharge of most cells was also cardiac rhythmic (PVN-RVLM: 25/32, 78%; PVN-RVLM/IML: 10/17, 59%). Raising and lowering arterial blood pressure to increase and decrease arterial baroreceptor input caused a corresponding decrease and increase in firing frequency among cells of both groups (PVN-RVLM: 9/13, 69%; PVN-RVLM/IML: 4/4, 100%). These results indicate that PVN-RVLM and PVN-RVLM/IML neurons are both capable of contributing to basal sympathetic activity and its baroreflex modulation.

Anteroposterior distribution of AT(1) angiotensin receptors in caudal brainstem cardiovascular regulatory centers of the rat

Angiotensin II acts on Ang II type 1 (AT(1)) receptors in areas of the caudal brainstem involved in cardiovascular regulation. In particular, activation of AT(1) receptors in the rostral ventrolateral medulla (RVLM) has been suggested to contribute to hypertension. However, the characteristics of AT(1) receptors in the RVLM of rat, the species in which the most experimental work has been done, are not well documented. This study evaluated AT(1) receptor binding along a 2.7-mm length of rat medulla, which included the full extent of the RVLM and the caudal ventrolateral medulla (CVLM). Sections of medulla from female rats cut on a cryostat were incubated with five concentrations of (125)I-sarcosine(1), isoleucine(8) angiotensin II to assess the density (B(max)) and dissociation constant (K(D)) of the receptors for the radioligand. The dorsomedial medulla (DMM) displayed a high density of AT(1) binding (1207+/-100 fmol/g), which peaked at 0.4 mm rostral to the calamus scriptorius (approximately 14 mm caudal to Bregma). The RVLM and CVLM displayed significantly lower (p<0.01) densities of AT(1) binding, 278+/-38 and 379+/-64 fmol/g, respectively. However, the dissociation constants were significantly lower (i.e., higher affinity) in RVLM and CVLM (164+/-38 and 178+/-27 pM, respectively,) than in DMM (328+/-12 pM, p<0.01 and p<0.05, respectively). These results provide an anatomical and pharmacological framework for future studies on the role in cardiovascular regulation of AT(1) receptors in the caudal brainstem.

Hypothalamic paraventricular nucleus mediates sodium-induced changes in cardiovascular and renal function in conscious sheep

The contribution of the paraventricular nucleus of the hypothalamus (PVN) in mediating cardiovascular, renal, hormonal, and sympathetic nerve responses to increased cerebrospinal fluid (CSF) [Na+] was investigated in conscious sheep. Intracerebroventricular hypertonic NaCl (0.5 mol/l, 20 μl/min for 60 min) increased arterial blood pressure [AP; +13.4 (sd 2.0) mmHg, P < 0.001] and central venous pressure [CVP; +2.8 (sd 1.3) mmHg, P < 0.001], but did not significantly change heart rate or cardiac output ( n = 6). Elevated CSF [Na+] also lowered plasma ANG II levels [−3.3 (sd 1.6) pmol/l, P = 0.004] and increased creatinine clearance [+31.5 (sd 32.7) ml/min, P = 0.03] and renal sodium excretion [+9.2 (sd 9.2) mmol/h, P = 0.003]. Lidocaine injection (1 μl, 2%) into the PVN prior to the ICV infusion had no apparent effect per se, but it abolished the AP, CVP, creatinine clearance, and ANG II responses to hypertonic NaCl, as well as reducing the increase in renal sodium excretion ( n = 6). Subsequent studies were performed in conscious sheep with chronically implanted electrodes for measurement of renal sympathetic nerve activity (RSNA). The effects of ICV hypertonic NaCl on AP and RSNA were measured before and after PVN-injection of glycine (250 nmol in 500 nl artificial CSF). ICV NaCl increased AP and decreased RSNA ( P < 0.001). These effects were significantly reduced by glycine ( P = 0.02–0.001, n = 5). Saline injected into the PVN ( n = 5) or lidocaine injected outside the PVN ( n = 6) had no effect on the response to ICV hypertonic NaCl. These results indicate that the PVN is an important mediator of cerebrally induced homeostatic responses to elevated sodium concentration/hyperosmolality.

Pregnancy decreases GABAergic inhibition of the hypothalamic paraventricular nucleus

Depressor responses to peripheral or central infusion of Angiotensin II type 1 (AT(1)) receptor antagonists (AT(1)X) are greater in pregnant (P) compared to nonpregnant (NP) animals. AT(1) and ionotropic excitatory amino acid (EAA) receptors contribute to pressor responses to GABA(A) receptor blockade with bicuculline (Bic) in the paraventricular nucleus (PVN) of male rats. Therefore, we hypothesized that GABAergic inhibition is decreased and AT(1) receptors play a greater excitatory role in the PVN of P versus NP rats. Unilateral microinjection of Bic was performed before (Bic(1)), after AT(1)X (Bic(2)), and after AT(1)X + EAA blockade (kynurenate, Kyn) (Bic(3)) in the PVN. Increases in mean arterial pressure (MAP: NP=20+/-2; P=12+/-2 mmHg), heart rate (HR: NP=57+/-6; P=19+/-6 beats/min) and renal sympathetic nerve activity (RSNA: NP=70+/-9; P=33+/-7%) due to Bic (Bic(1)) were attenuated in P rats. Responses to AT(1)X and Kyn alone were insignificant in both groups. In NP rats, AT(1)X attenuated (+12+/-4 mmHg), and AT(1)X + Kyn further decreased the pressor response to Bic in the PVN (+6+/-2 mmHg). In P rats AT(1)X reduced the pressor response to Bic (+5+/-1 mm Hg), and Kyn had no additional effect (+3+/-1 mmHg). Effects of PVN Bic to alter the autospectra of RSNA were suppressed by prior AT(1)X and Kyn in both groups. Thus, tonic GABAergic inhibition is decreased and the contribution of AT(1) receptors in the PVN may be greater in P rats.

Excess dietary salt alters angiotensinergic regulation of neurons in the rostral ventrolateral medulla

Excess dietary salt intake contributes to or exacerbates some forms of hypertension by increasing sympathetic nerve activity (SNA) and arterial blood pressure (ABP) through angiotensin II (Ang II) type 1 receptor activation in the rostral ventrolateral medulla (RVLM). Despite this interaction among dietary salt, Ang II, and the RVLM, no studies have directly examined whether dietary salt by itself alters Ang II-dependent responses and regulation of RVLM neurons, SNA, and ABP. Therefore, the present study directly tested this hypothesis. Male Sprague-Dawley rats were fed normal chow and given access to water or 0.9% NaCl solution for 14 days. Unilateral injection of Ang II (0.6, 6, and 60 pmol) into the RVLM produced a significantly greater increase in renal SNA and mean ABP of rats drinking 0.9% NaCl versus water. However, dietary salt did not alter mRNA levels of RVLM Ang II type 1a receptors or the SNA and ABP responses to stimulation of the dorsolateral funinculus. Additional experiments demonstrate that blockade of RVLM Ang II type 1 receptors significantly reduced renal SNA, splanchnic SNA, and mean ABP of rats drinking 0.9% NaCl but not water. Blockade of iontotropic glutamate receptors had no effect. Altogether, these findings suggest that elevated dietary salt enhances the sympathoexcitatory actions of Ang II in the RVLM via changes in the intrinsic properties of RVLM neurons. Moreover, elevated dietary salt intake differentially affects the tonic activity of the peripheral versus brain RVLM Ang II type 1 receptors to regulate baseline SNA and ABP.

Role of the hypothalamic PVN in the reflex reduction in mesenteric blood flow elicited by hyperthermia

The hypothalamic paraventricular nucleus (PVN) is an important integrative center in the brain. In the present study, we investigated whether the PVN is a key region in the mesenteric vasoconstriction that normally accompanies an increase in core body temperature. Anesthetized rats were monitored for blood pressure, heart rate, mesenteric blood flow, and vascular conductance. In control rats, elevation of core body temperature to 41 degrees C had no significant effect on blood pressure, increased heart rate, and reduced mesenteric blood flow by 21%. In a separate group of rats, muscimol was microinjected bilaterally (1 nmol/side) into the PVN. Compared with the control group, there was no significant difference in the blood pressure and heart rate responses elicited by the increase in core body temperature. In contrast to control animals, however, mesenteric blood flow did not fall in the muscimol-treated rats in response to the elevation in core body temperature. In a separate group, in which muscimol was microinjected into regions outside the PVN, elevating core body temperature elicited the normal reduction in mesenteric blood flow. The results suggest that the PVN may play a key role in the reflex decrease in mesenteric blood flow elicited by hyperthermia.

Forebrain osmotic regulation of the sympathetic nervous system

1. Accumulating evidence in both humans and animals indicates that acute increases in plasma osmolality elevate sympathetic nerve activity (SNA). In addition, plasma hyperosmolality (or hypernatraemia) can produce sustained increases in SNA and arterial blood pressure (ABP) through stimulation of forebrain osmoreceptors. 2. Although an abundance of information exists regarding the osmoregulatory circuits for thirst and secretion of antidiuretic hormone, much less is known about those pathways and synaptic mechanisms linking osmotic perturbations and SNA. To date, the available evidence suggests that osmosensitive sites within the forebrain lamina terminalis, such as the organum vasculosum of the lamina terminalis, are key elements that link plasma hypertonicity to elevated SNA. 3. The major efferent target of osmosensitive regions in the forebrain lamina terminalis is the hypothalamic paraventricular nucleus (PVH). Evidence from a number of studies indicates that the PVH contributes to both acute and chronic osmotically driven increases in SNA. In turn, PVH neurons increase SNA through a direct vasopressinergic spinal pathway and/or a glutamatergic pathway to bulbospinal sympathetic neurons of the rostral ventrolateral medulla. 4. Future studies are needed to: (i) define the contribution of various osmosensitive regions of the forebrain lamina terminalis to acute and chronic osmotically driven increases in SNA; (ii) identify the cellular mechanisms and neural circuitry linking plasma osmolality and SNA; and (iii) define whether such mechanisms contribute to elevated SNA in salt-sensitive hypertension.

Organum vasculosum laminae terminalis contributes to increased sympathetic nerve activity induced by central hyperosmolality

The contribution of the organum vasculosum laminae terminalis (OVLT) in mediating central hyperosmolality-induced increases of sympathetic nerve activity (SNA) and arterial blood pressure (ABP) was assessed in anesthetized rats. Solutions of graded NaCl concentration (150, 375, and 750 mM) were injected (150 mul) into the forebrain vascular supply via an internal carotid artery (ICA). Time-control experiments (n = 6) established that ICA NaCl injections produced short-latency, transient increases of renal SNA (RSNA) and mean ABP (MAP) (P < 0.05-0.001). Responses were graded, highly reproducible, and unaltered by systemic blockade of vasopressin V1 receptors (n = 4). In subsequent studies, stimulus-triggered averaging of RSNA was used to accurately locate the OVLT. Involvement of OVLT in responses to ICA NaCl was assessed by recording RSNA and MAP responses before and 15 min after electrolytic lesion of the OVLT (n = 6). Before lesion, NaCl injections increased RSNA and MAP (P < 0.05-0.001), similar to time control experiments. After lesion, RSNA responses were significantly reduced (P < 0.05-0.001), but MAP responses were unaltered. To exclude a role for fibers of passage, the inhibitory GABA-A receptor agonist muscimol was microinjected into the OVLT (50 pmol in 50 nl) (n = 6). Before muscimol, hypertonic NaCl increased RSNA, lumbar SNA (LSNA), and MAP (P < 0.05-0.001). After muscimol, both RSNA and LSNA were significantly reduced in response to 375 and 750 mM NaCl (P < 0.05). MAP responses were again unaffected. Injections of vehicle (saline) into OVLT (n = 6) and muscimol lateral to OVLT (n = 5) each failed to alter responses to ICA NaCl. We conclude that OVLT neurons contribute to sympathoexcitation by central hyperosmolality.