Effect of dietary sodium intake on the responses to bicuculline in the paraventricular nucleus of rats

Involvement of GABAergic and Adrenergic Neurotransmissions on Paraventricular Nucleus of Hypothalamus in the Control of Cardiac Function

Sympathetic premotor neurons of the paraventricular hypothalamus (PVN) play a role in hemodynamics adjustments during changes in body fluid homeostasis. However, PVN contribution to the tonic control of cardiac function remains to be systematically studied. In this study, we assessed whether GABAergic and adrenergic synapses, known for being active in the PVN, are involved in the control of cardiac function. Adult male Wistar rats (250–350 g; n = 27) were anesthetized with urethane (1.2–1.4 g/kg i.p.) and underwent catheterization of femoral artery to record blood pressure and heart rate. The femoral vein was used to inject the vasoactive agents phenylephrine (10 μg/kg) and sodium nitroprusside (10 μg/kg) and to supplement anesthesia. The cardiac left ventricle was catheterized to record left ventricular pressure and its derivative. Craniotomy allowed for injections (100 nL) into the PVN of: muscimol (20 mM), bicuculline methiodide (0.4 mM), propranolol (10 mM), isoproterenol (100 μM), phentolamine (13 mM), phenylephrine (30 nM). We found that: (i) inhibition of PVN by muscimol, reduced arterial pressure, cardiac chronotropy and inotropy; (ii) disinhibition of PVN neurons by bicuculline evoked positive chronotropy and inotropy, and increase blood pressure; (iii) PVN alpha adrenergic receptors control cardiac chronotropy and inotropy; (iv) beta adrenergic receptors of the PVN do not influence cardiac function; (v) afterload does not contribute to the PVN-evoked inotropy. Our results indicate that the modulation of the activity of PVN neurons exerted by GABAergic and adrenergic mechanisms contribute to the control of cardiac function.

Understanding the Two Faces of Low-Salt Intake

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

Do the carotid body chemoreceptors mediate cardiovascular and sympathetic adjustments induced by sodium overload in rats?

Acute plasma hypernatremia induces several cardiovascular and sympathetic responses. It is conceivable that these responses contribute to rapid sodium excretion and restoration of normal conditions. Afferent pathways mediating these responses are not entirely understood. The present study analyses the effects of acute carotid chemoreceptor inactivation on cardiovascular and sympathetic responses induced by infusion of hypertonic saline (HS). All experiments were performed on anesthetized male Wistar rats instrumented for recording of arterial blood pressure (ABP), renal blood flow (RBF) and renal sympathetic nerve activity (RSNA). Animals were subjected to sham surgery or carotid chemoreceptor inactivation by bilateral ligation of the carotid body artery (CBA). In sham rats (n=8), intravenous infusion of HS (3 M NaCl, 1.8 ml/kg b.wt.) elicited a transient increase (9±2mmHg) in ABP, and long lasting (30 min) increases in RBF (138±5%) and renal vascular conductance (RVC) (128±5%) with concurrent decrease in RSNA (-19±4%). In rats submitted to CBA ligation (n=8), the pressor response to HS was higher (24±2mmHg; p<0.05). However, RBF and RVC responses to HS infusion were significantly reduced (113±5% and 93±4%, respectively) while RSNA was increased (13±2%). When HS (3M NaCl, 200μl) was administrated into internal carotid artery (ICA), distinct sympathetic and cardiovascular responses were observed. In sham-group, HS infusion (3M NaCl, 200μl) into ICA promoted an increase in ABP (26±8mmHg) and RSNA (29±13%). In CBA rats, ABP (-3±5.6mmHg) remained unaltered despite sympathoinhibition (-37.6±5.4%). These results demonstrate that carotid body chemoreceptors play a role in the development of hemodynamic and sympathetic responses to acute HS infusion.

Excess dietary salt intake alters the excitability of central sympathetic networks

The ingestion of excess dietary salt (defined as NaCl) is strongly correlated with cardiovascular disease, morbidity, mortality, and is regarded as a major contributing factor to the pathogenesis of hypertension. Although several mechanisms contribute to the adverse consequences of dietary salt intake, accumulating evidence suggests that dietary salt loading produces neurogenically-mediated increases in total peripheral resistance to raise arterial blood pressure (ABP). Evidence from clinical studies and experimental models clearly establishes a hypertensive effect of dietary salt loading in a subset of individuals who are deemed "salt-sensitive". However, we will discuss and present evidence to develop a novel hypothesis to suggest that while chronic increases in dietary salt intake do not elevate mean ABP in "non-salt-sensitive" animals, dietary salt intake does enhance several sympathetic reflexes thereby predisposing these animals and/or individuals to the development of salt-sensitive hypertension. Additional evidence raises an intriguing hypothesis that these enhanced sympathetic reflexes are largely attributed to the ability of excess dietary salt intake to selectively enhance the excitability of sympathetic-regulatory neurons in the rostral ventrolateral medulla. Insight into the cellular mechanisms by which dietary salt intake alters the responsiveness of RVLM circuits will likely provide a foundation for developing new therapeutic approaches to treat salt-sensitive hypertension. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

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.

Dietary salt loading exacerbates the increase in sympathetic nerve activity caused by intravenous insulin infusion in rats

Obesity and type 2 diabetes mellitus frequently produce chronic elevations in blood insulin levels. Importantly, hyperinsulinemia stimulates increases in sympathetic nerve activity that may predispose to hypertension, atherosclerosis, and end-organ damage. Because depletion of dietary salt (NaCl) increases angiotensin II levels, which has been shown to enhance sympathetic responses to excitatory stimuli such as thermal stimulation and bicuculline in the hypothalamus, we predicted that insulin-induced elevations in lumbar sympathetic activity would be augmented by low NaCl and suppressed by high dietary NaCl. Adult male Sprague-Dawley rats were randomized into groups receiving low (0.0 mEq/d, n = 10), normal (2.0 mEq/d, n = 10), and high (5.7 mEq/d, n = 10) NaCl for a period of 8 days. After this, the animals were anesthetized for measurement of heart rate, mean arterial pressure, and lumbar sympathetic nerve activity during 110 minutes of intravenous insulin infusion (15 mU/kg per minute) with euglycemic clamp. Insulin administration caused modest blood pressure decreases accompanied by heart rate increases that were similar across the 3 dietary groups. Unexpectedly, sympathetic increases to insulin were lowest in the low-NaCl group (100%-135% +/- 24%), moderate in the normal-NaCl group (100%-170% +/- 23%), and greatest in the high-NaCl group (100%-252% +/- 39%). Dietary NaCl level did not affect baseline blood glucose or insulin sensitivity as assessed by euglycemic clamp. These findings indicate that dietary salt loading exacerbates the lumbar sympathoexcitatory response to intravenous insulin infusion in rats.

A spinal vasopressinergic mechanism mediates hyperosmolality-induced sympathoexcitation

An elevation in plasma osmolality elicits a complex neurohumoral response, including an activation of the sympathetic nervous system and an increase in arterial pressure. Using a combination of in vivo and in situ rat preparations, we sought to investigate whether hypothalamic vasopressinergic spinally projecting neurones are activated during increases in plasma osmolality to elicit sympathoexcitation. Hypertonic saline (HS, i.v. bolus), which produced a physiological increase in plasma osmolality to 299 +/- 1 mosmol (kg water)(-1), elicited an immediate increase in mean arterial pressure (MAP) (from 101 +/- 1 to 121 +/- 3 mmHg) in vivo. Pre-treatment with prazosin reversed the HS-induced pressor response to a hypotensive response (from 121 +/- 3 to 68 +/- 2 mmHg), indicating significant activation of the sympathetic nervous system. In an in situ arterially perfused decorticate rat preparation, hyperosmotic perfusate consisted of either 135 mm NaCl, or a non-NaCl osmolyte, mannitol (0.5%); both increased lumbar sympathetic nerve activity (LSNA) by 32 +/- 5% (NaCl) and 21 +/- 1% (mannitol), which was attenuated after precollicular transection (7 +/- 3% and 1 +/- 1%, respectively). Remaining experiments used the NaCl hyperosmotic stimulus. In separate preparations the hyperosmotic-induced sympathoexcitation (21 +/- 2%) was also significantly attenuated after transection of the circumventricular organs (2 +/- 1%). Either isoguvacine (a GABA(A) receptor agonist) or kynurenic acid (a non-selective ionotropic glutamate receptor antagonist) microinjected bilaterally into the paraventricular nucleus (PVN) attenuated the increase in LSNA induced by the hyperosmotic stimulus (control: 25 +/- 2%; after isoguvacine: 7 +/- 2%; after kynurenic: 8 +/- 3%). Intrathecal injection of a V(1a) receptor antagonist also reduced the increase in LSNA elicited by the hyperosmotic stimulus (control: 29 +/- 6%; after blocker: 4 +/- 1%). These results suggest that a physiological hyperosmotic stimulus produces sympathetically mediated hypertension in conscious rats. These data are substantiated by the in situ decorticate preparation in which sympathoexcitation was also evoked by comparable hyperosmotic stimulation. Our findings demonstrate the importance of vasopressin acting on spinal V(1a) receptors for mediating sympathoexcitatory response to acute salt loading.

The contribution of brain angiotensin II to the baroreflex regulation of renal sympathetic nerve activity in conscious normotensive and hypertensive rats

Angiotensin II receptor density in the brain is elevated when dietary salt intake is raised or in the state of hypertension. The aim of this study was to evaluate whether the angiotensin II modulation of the baroreceptor control of renal sympathetic nerve activity was altered under these conditions. Wistar rats, fed either a regular (0.25% w/w sodium) or high-salt diet (3.1% w/w sodium), or stroke-prone spontaneously hypertensive rats (SHRSPs) were implanted with cannulae in the carotid artery, jugular vein and the cerebroventricle and with recording electrodes on the renal sympathetic nerves. Three days later, baroreceptor gain curves were generated for renal sympathetic nerve activity and heart rate before and following intracerebroventricular (i.c.v.) administration of losartan (15 mug) to block angiotensin AT1 receptors. The rats fed a regular diet had a mean blood pressure of 116 +/- 3 mmHg and heart rate of 467 +/- 25 beats min(-1), which remained unchanged after the i.c.v. administration of losartan. The sensitivity or curvature coefficient of the baroreceptor curve for renal sympathetic nerve activity was increased by 36% (P < 0.05) following losartan. In the rats fed a high-salt diet, all cardiovascular variables and the losartan-induced increase in the baroreceptor curvature coefficient for renal sympathetic nerve activity (29%) were similar to values in rats on the regular sodium diet. The heart rate baroreceptor curvature coefficient was not altered in either the rats fed a regular or a high-salt diet. The slope of the renal sympathetic nerve activity baroreflex gain curve in the SHRSPs was less and the increase following administration of losartan (54%) was greater than in the Wistar rats. These data indicate that in the conscious state, the tonic inhibitory action of brain angiotensin II on the baroreflex regulation of renal sympathetic nerve activity was unaffected by raised dietary sodium, but its role was enhanced in the SHRSPs.

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

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

Effect of intravenous angiotensin II infusion on responses to hypothalamic PVN injection of bicuculline

The hypothalamic paraventricular nucleus (PVN) plays an important role in the sympathoexcitatory response to elevated plasma angiotensin II (Ang II). However, the mechanism by which Ang II influences sympathetic activity is not fully understood. In this study, we tested the hypothesis that GABA(gamma-aminobutyric acid)-ergic function in the PVN is reduced by peripheral infusion of Ang II. To accomplish this, rats received either intravenous Ang II (12 ng/kg per minute) or vehicle (D5W) for 7 days, and renal sympathetic nerve activity (SNA), mean arterial pressure (MAP), and heart rate (HR) responses were recorded after unilateral PVN microinjection of the GABA-A receptor antagonist bicuculline methiodide (BMI, 0.1 nmol). Results indicate that in contrast to a significant increase in renal SNA, MAP, and HR observed in vehicle-infused rats (P<0.05), BMI injection into the PVN of Ang II-infused animals was without effect on all recorded variables. In a separate groups of animals, ganglionic blockade produced a significantly greater fall in MAP (P<0.01) in Ang II-infused rats than in vehicle-infused control rats, indicating that the contribution of SNA to the maintenance of blood pressure was elevated in the Ang II-infused group. Overall, these data indicate that cardiovascular and sympathoexcitatory responses to acute GABA-A receptor antagonism in the PVN are significantly blunted in rats after 7 days of intravenous infusion of Ang II. We conclude that an Ang II-induced reduction in GABAergic inhibition within the PVN may contribute to elevated SNA observed in this study.

Responses to GABA-A receptor blockade in the hypothalamic PVN are attenuated by local AT1 receptor antagonism

Blockade of GABA-A receptors in the hypothalamic paraventricular nucleus (PVN) has been repeatedly shown to increase arterial blood pressure (ABP), heart rate (HR), and sympathetic nerve activity (SNA), but the mechanism(s) that underlies this response has not been determined. Here, we tested whether full expression of the response requires activation of local ANG II AT1 receptors. ABP, HR, and renal SNA responses to PVN microinjection of bicuculline methobromide (BIC; 0.1 nmol) were recorded before and after microinjection of vehicle (saline); losartan (or L-158809), to block local AT1 receptors; or PD123319, to block AT2 receptors. After PVN microinjection of vehicle or PD123319 (10 nmol), BIC significantly (P < 0.05) increased mean arterial pressure (MAP), HR, and renal SNA. However, PVN microinjection of 2 and 20 nmol of losartan dose dependently reduced responses to PVN-injected BIC, with the 20-nmol dose nearly abolishing MAP (P < 0.005), HR (P < 0.05), and renal SNA (P < 0.005) responses. Another AT1 receptor antagonist, L-158809 (10 nmol), produced similar effects. Neither losartan nor L-158809 altered baseline parameters. Responses to PVN injection of BIC were unchanged by losartan (20 nmol) given intravenously or into the PVN on the opposite side. MAP, HR, and renal SNA responses to PVN microinjection of l-glutamate (10 nmol) were unaffected by PVN injection of losartan (20 nmol), indicating that effects of losartan were not due to nonspecific depression of neuronal excitability. We conclude that pressor, tachycardic, and renal sympathoexcitatory responses to acute blockade of GABA-A receptors in the PVN depend on activation of local AT1 receptors.

Physiological genomic analysis of the brain renin-angiotensin system

The brain renin-angiotensin system (RAS) has long been considered pivotal in cardiovascular regulation and important in the pathogenesis of hypertension and heart failure. However, despite more than 30 years of study, the brain RAS continues to defy explanation. Our lack of understanding of how the brain RAS is organized at the cellular and regional levels has made it difficult to resolve long-sought questions of how ANG II is produced in the brain and the precise mechanisms by which it exerts its actions. A major reason for this is the difficulty in experimentally dissecting the brain RAS at the regional, cellular, and whole organism levels. Recently, we and others developed a series of molecular tools for selective manipulation of the murine brain RAS, in parallel with technologies for integrative analysis of cardiovascular and volume homeostasis in the conscious mouse. This review, based in part on a lecture given in conjunction with the American Physiological Society Young Investigator Award in Regulatory and Integrative Physiology (Water and Electrolyte Homeostasis Section), outlines the physiological genomics strategy that we have taken in an effort to unravel some of the complexities of this system. It also summarizes the principles, progress, and prospects for a better understanding of the brain RAS in health and disease.

Endogenous angiotensin affects responses to stimulation of baroreceptor afferent nerves

OBJECTIVE

To study effects of endogenous angiotensin II on responses to standardized stimulation of afferent neural input into the central portion of the arterial and cardiac baroreflexes.

DESIGN

Different dietary sodium intakes were used to physiologically alter endogenous angiotensin II activity. Candesartan, an angiotensin II type 1 receptor antagonist, was used to assess dependency of observed effects on angiotensin II stimulation of angiotensin II type 1 receptors. Electrical stimulation of arterial and cardiac baroreflex afferent nerves was used to provide a standardized input to the central portion of the arterial and cardiac baroreflexes.

METHODS

In anesthetized rats in balance on low, normal and high dietary sodium intake, arterial pressure, heart rate and renal sympathetic nerve activity responses to electrical stimulation of vagus and aortic depressor nerves were determined. Compared with plasma renin activity values in normal dietary sodium intake rats, those from low dietary sodium intake rats were higher and those from high dietary sodium intake rats were lower. During vagus nerve stimulation, the heart rate, arterial pressure and renal sympathetic nerve activity responses were similar in all three dietary sodium intake groups. During aortic depressor nerve stimulation, the heart rate and arterial pressure responses were similar in all three dietary sodium intake groups. However, the renal sympathetic nerve activity response was significantly greater in the low sodium group than in the normal and high sodium group at 4, 8 and 16 Hz. Candesartan administered to low dietary sodium intake rats had no effect on the heart rate and arterial pressure responses to either vagus or aortic depressor nerve stimulation but increased the magnitude of the renal sympathoinhibitory responses.

CONCLUSIONS

Increased endogenous angiotensin II in rats on a low dietary sodium intake attenuates the renal sympathoinhibitory response to activation of the cardiac and sinoaortic baroreflexes by standardized vagus and aortic depressor nerve stimulation, respectively.

Central angiotensin modulation of baroreflex control of renal sympathetic nerve activity in the rat: influence of dietary sodium

AIM

Administration of angiotensin II (angII) into the cerebral ventricles or specific brain sites impairs arterial baroreflex regulation of renal sympathetic nerve activity (SNA). Further insight into this effect was derived from: (a) using specific non-peptide angII receptor antagonists to assess the role of endogenous angII acting on angII receptor subtypes, (b) microinjection of angII receptor antagonists into brain sites behind an intact blood-brain barrier to assess the role of endogenous angII of brain origin and (c) alterations in dietary sodium intake, a known physiological regulator of activity of the renin-angiotensin system (RAS), to assess the ability to physiologically regulate the activity of the brain RAS.

METHODS

In rats in balance on low, normal or dietary sodium intake, losartan or candesartan was injected into the lateral cerebral ventricle or the rostral ventrolateral medulla (RVLM) and the effects on basal renal SNA and the arterial baroreflex sigmoidal relationship between renal SNA and arterial pressure were determined.

RESULTS

With both routes of administration, the effects were proportional to the activity of the RAS as indexed by plasma renin activity (PRA). The magnitude of both the decrease in basal renal SNA and the parallel resetting of arterial baroreflex regulation of renal SNA to a lower arterial pressure was greatest in low-sodium rats with highest PRA and least in high-sodium rats with lowest PRA. Disinhibition of the paraventricular nucleus (PVN) by injection of bicuculline causes pressor, tachycardic and renal sympathoexcitatory responses mediated via an angiotensinergic projection from PVN to RVLM. In comparison with responses in normal sodium rats, these responses were greatly diminished in high-sodium rats and greatly enhanced in low-sodium rats.

CONCLUSION

Physiological changes in the activity of the RAS produced by alterations in dietary sodium intake regulate the contribution of endogenous angII of brain origin in the modulation of arterial baroreflex regulation of renal SNA.

The paraventricular nucleus: an important component of the central neurocircuitry regulating sympathetic nerve outflow

AIM

The sympathetic nervous system plays an important role in the regulation of physiological homeostasis under basal conditions and in response to acute and chronic stressors. It is known that multiple levels of the neuroaxis, including the paraventricular nucleus (PVN) of the hypothalamus, are involved in regulation of efferent sympathetic nerve discharge (SND). This review focuses on the role of the PVN in regulation of functional characteristics of efferent SND.

RESULTS

The available experimental evidence indicates that the level of efferent sympathetic nerve activity is altered after microinjection of numerous substances into the PVN, including excitatory amino acids, gamma-aminobutyric acid (GABAA) receptor agonists and antagonists, and PVN nitric oxide synthase inhibitors. In addition, antagonism of PVN GABAA receptors changes the pattern of synchronized discharge bursts in efferent sympathetic nerves and enhances the frequency-domain coupling between low-frequency bursts in sympathetic nerve pairs. Finally, PVN microinjections of excitatory amino acids (L-glutamate, D,L-homocysteic acid) have been shown to produce non-uniform changes in the level of efferent sympathetic nerve activity.

CONCLUSION

These findings support the concept that the PVN is an important component of the central neurocircuitry regulating functional characteristics (basal level of activity, bursting pattern, and relationships between discharges in nerves innervating different targets) of efferent sympathetic nerve outflow.

Modifications to central neural circuitry during heart failure

AIM

During heart failure (HF), excess sodium retention is triggered by increased plasma renin-angiotensin-aldosterone activity and increased basal sympathetic nerve discharge (SND). Enhanced basal SND in the renal nerves plays a role in sodium retention. Therefore, as a hypothetical model for the central sympathetic control pathways that are dysregulated as a consequence of HF, the central neural pathways regulating the sympathetic motor output to the kidney are reviewed in the context of their role during HF.

CONCLUSION

From these findings, a model of the neuroanatomical circuitry that may be affected during HF is constructed.

Effect of dietary sodium intake on central angiotensinergic pathways

The role of central angiotensinergic pathways in the cardiovascular regulation has been examined using the microinjection of angiotensin peptides and angiotensin receptor antagonists. However, in such studies, neither the overall nor the local level of activity of the renin-angiotensin system is generally known. Herein, physiological changes in the endogenous level of activity of the renin-angiotensin system were produced by alterations in the dietary sodium intake. Microinjection of the angiotensin II AT1 receptor antagonists losartan or candesartan into the rostral ventrolateral medulla produced the bradycardic, depressor and renal sympathoinhibitory responses which were greater in low sodium diet rats with stimulated activity of the renin-angiotensin system than in high sodium diet rats with suppressed activity of the renin-angiotensin system activity. The renal sympathoexcitatory responses to activation of the paraventricular nucleus by microinjection of bicuculline, known to be dependent on the excitatory synaptic inputs to the rostral ventrolateral medulla mediated by AT1 receptors, were greater in low sodium diet rats than in high sodium rats. These observations support the view that physiologically regulated angiotensin peptides of the brain origin exert a local paracrine or autocrine action on sites that influence the renal sympathetic nerve activity.