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

Frequency-coded patterns of sympathetic vasomotor activity are differentially evoked by the paraventricular nucleus of the hypothalamus in the Goldblatt hypertension model

Introduction

The paraventricular nucleus of the hypothalamus (PVN) contains premotor neurons involved in the control of sympathetic vasomotor activity. It is known that the stimulation of specific areas of the PVN can lead to distinct response patterns at different target territories. The underlying mechanisms, however, are still unclear. Recent evidence from sympathetic nerve recording suggests that relevant information is coded in the power distribution of the signal along the frequency range. In the present study, we addressed the hypothesis that the PVN is capable of organizing specific spectral patterns of sympathetic vasomotor activation to distinct territories in both normal and hypertensive animals.

Methods

To test it, we investigated the territorially differential changes in the frequency parameters of the renal and splanchnic sympathetic nerve activity (rSNA and sSNA, respectively), before and after disinhibition of the PVN by bicuculline microinjection. Subjects were control and Goldblatt rats, a sympathetic overactivity-characterized model of neurogenic hypertension (2K1C). Additionally, considering the importance of angiotensin II type 1 receptors (AT1) in the sympathetic responses triggered by bicuculline in the PVN, we also investigated the impact of angiotensin AT1 receptors blockade in the spectral features of the rSNA and sSNA activity.

Results

The results revealed that each nerve activity (renal and splanchnic) presents its own electrophysiological pattern of frequency-coded rhythm in each group (control, 2K1C, and 2K1C treated with AT1 antagonist losartan) in basal condition and after bicuculline microinjection, but with no significant differences regarding total power comparison among groups. Additionally, the losartan 2K1C treated group showed no decrease in the hypertensive response triggered by bicuculline when compared to the non-treated 2K1C group. However, their spectral patterns of sympathetic nerve activity were different from the other two groups (control and 2K1C), suggesting that the blockade of AT1 receptors does not totally recover the basal levels of neither the autonomic responses nor the electrophysiological patterns in Goldblatt rats, but act on their spectral frequency distribution.

Discussion

The results suggest that the differential responses evoked by the PVN were preferentially coded in frequency, but not in the global power of the vasomotor sympathetic responses, indicating that the PVN is able to independently control the frequency and the power of sympathetic discharges to different territories.

Melatonin suppresses sympathetic vasomotor tone through enhancing GABAA receptor activity in the hypothalamus

Introduction: Melatonin (5-methoxy-N-acetyl-tryptamine) is a circadian hormone synthesized and secreted by the pineal gland. In addition to regulating circadian rhythms of many physiological functions, melatonin is involved in regulating autonomic nervous function and blood pressure. Hypothalamus paraventricular nucleus (PVN), receiving melatonin projections from the superchiasmatic nucleus, is a critical brain region to regulate neuroendocrine and cardiovascular function. Here, we determined the synaptic mechanisms involved in the effect of melatonin on the sympathetic outflow and blood pressure.

Methods and Results: Microinjection of melatonin into the PVN produced a depressor effect and decreased renal sympathetic nerve activity (RSNA). While microinjection of luzindole, a non-selective melatonin receptor antagonist, into the PVN did not change melatonin-induced sympathoinhibition, GABAA receptor antagonist bicuculline eliminated melatonin-induced sympathoinhibition. Furthermore, melatonin decreased firing rate of retrogradely labeled PVN neurons which project to the rostral ventrolateral medulla (RVLM), an effect was not altered by luzindole but eliminated by bicuculline. Melatonin significantly increased the amplitude of spontaneous and evoked GABAergic inhibitory synaptic currents, as well as GABA-induced currents.

Conclusion: These data suggest that melatonin in the PVN suppresses sympathetic vasomotor tone through enhancing GABAA receptor activity. This study provides novel information for understanding the cellular mechanisms involved in the effect of melatonin on regulating blood pressure and sympathetic output.

Effects of chronic mild stress on GABAergic system in the paraventricular nucleus of hypothalamus associated with cardiac autonomic activity

Stress is associated with cardiovascular diseases. One possible mechanism is the reduction in gamma-aminobutyric acid (GABA)ergic transmission at the paraventricular nucleus (PVN), which contributes to the disinhibition of sympathoexcitatory circuits and activates sympathetic outflow. At present, the mechanism of chronic mild stress (CMS) on GABAergic transmission at the PVN and cardiac autonomic activity is not yet fully clarified. Therefore, this study was designed to investigate the effects of CMS on the GABAergic system at the PVN and on the cardiac autonomic activity. Adult male Sprague-Dawley rats were randomly assigned to control (left undisturbed in their home cage) or CMS (subjected to various mild stressors for 4 weeks). Cardiac autonomic activities were determined by heart rate variability (HRV) analysis, and GABAergic alterations at the PVN were determined from GABA levels and mRNA expression of GABA-related activities. Results showed that the CMS group had decreased HRV as determined by the standard deviation of all R-R intervals (SDNN). The low frequency (LF) and high frequency (HF) powers of the CMS group were higher than those of the control. Hence, the LF/HF ratio was consequently unaffected. These findings indicated that despite the increase in sympathetic and parasympathetic activities, the autonomic balance was preserved at 4 weeks post CMS. For the GABAergic-related parameters, the CMS group had decreased mRNA expression of glutamic acid decarboxylase-65 (GAD-65), the GABA-synthesizing enzyme, and increased mRNA expression of gamma-aminobutyric acid transporter-1 (GAT-1). Moreover, the GAD-65 mRNA expression was negatively correlated with LF. In conclusion, 4-week CMS exposure in male rats could attenuate GABAergic transmission at the PVN and alter cardiac autonomic activities.

Cross-Talk between CB1, AT1, AT2 and Mas Receptors Responsible for Blood Pressure Control in the Paraventricular Nucleus of Hypothalamus in Conscious Spontaneously Hypertensive Rats and Their Normotensive Controls

We have previously shown that in urethane-anaesthetized rats, intravenous injection of the angiotensin II (Ang II) AT1 receptor antagonist losartan reversed the pressor effect of the cannabinoid CB1 receptor agonist CP55940 given in the paraventricular nucleus of hypothalamus (PVN). The aim of our study was to determine the potential interactions in the PVN between CB1 receptors and AT1 and AT2 receptors for Ang II and Mas receptors for Ang 1–7 in blood pressure regulation in conscious spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. The pressor effects of Ang II, Ang 1–7 and CP55940 microinjected into the PVN were stronger in SHRs than in WKYs. Increases in blood pressure in response to Ang II were strongly inhibited by antagonists of AT1 (losartan), AT2 (PD123319) and CB1 (AM251) receptors, to Ang 1–7 by a Mas antagonist (A-779) and AM251 and to CP55940 by losartan, PD123319 and A-779. Higher (AT1 and CB1) and lower (AT2 and Mas) receptor expression in the PVN of SHR compared to WKY may partially explain the above differences. In conclusion, blood pressure control in the PVN depends on the mutual interaction of CB1, AT1, AT2 and Mas receptors in conscious spontaneously hypertensive rats and their normotensive controls.

Salusin-β in Intermediate Dorsal Motor Nucleus of the Vagus Regulates Sympathetic-Parasympathetic Balance and Blood Pressure

The dorsal motor nucleus of the vagus (DMV) is known to control vagal activity. It is unknown whether the DMV regulates sympathetic activity and whether salusin-β in the DMV contributes to autonomic nervous activity. We investigated the roles of salusin-β in DMV in regulating sympathetic-parasympathetic balance and its underline mechanisms. Microinjections were carried out in the DMV and hypothalamic paraventricular nucleus (PVN) in male adult anesthetized rats. Renal sympathetic nerve activity (RSNA), blood pressure and heart rate were recorded. Immunohistochemistry for salusin-β and reactive oxidative species (ROS) production in the DMV were examined. Salusin-β was expressed in the intermediate DMV (iDMV). Salusin-β in the iDMV not only inhibited RSNA but also enhanced vagal activity and thereby reduced blood pressure and heart rate. The roles of salusin-β in causing vagal activation were mediated by NAD(P)H oxidase-dependent superoxide anion production in the iDMV. The roles of salusin-β in inhibiting RSNA were mediated by not only the NAD(P)H oxidase-originated superoxide anion production in the iDMV but also the γ-aminobutyric acid (GABA)_A receptor activation in PVN. Moreover, endogenous salusin-β and ROS production in the iDMV play a tonic role in inhibiting RSNA. These results indicate that salusin-β in the iDMV inhibits sympathetic activity and enhances vagal activity, and thereby reduces blood pressure and heart rate, which are mediated by NAD(P)H oxidase-dependent ROS production in the iDMV. Moreover, GABA_A receptor in the PVN mediates the effect of salusin-β on sympathetic inhibition. Endogenous salusin-β and ROS production in the iDMV play a tonic role in inhibiting sympathetic activity.

Increased (Pro)renin Receptor Expression in the Hypertensive Human Brain

Overactivation of the renin-angiotensin system (RAS) – a central physiological pathway involved in controlling blood pressure (BP) – leads to hypertension. It is now well-recognized that the central nervous system (CNS) has its own local RAS, and the majority of its components are known to be expressed in the brain. In physiological and pathological states, the (pro)renin receptor (PRR), a novel component of the brain RAS, plays a key role in the formation of angiotensin II (Ang II) and also mediates Ang II-independent PRR signaling. A recent study reported that neuronal PRR activation is a novel mechanism for cardiovascular and metabolic regulation in obesity and diabetes. Expression of the PRR is increased in cardiovascular regulatory nuclei in hypertensive (HTN) animal models and plays an important role in BP regulation in the CNS. To determine the clinical significance of the brain PRR in human hypertension, we investigated whether the PRR is expressed and regulated in the paraventricular nucleus of the hypothalamus (PVN) and rostral ventrolateral medulla (RVLM) – two key cardiovascular regulatory nuclei – in postmortem brain samples of normotensive (NTN) and HTN humans. Here, we report that the PRR is expressed in neurons, but not astrocytes, of the human PVN and RVLM. Notably, PRR immunoreactivity was significantly increased in both the PVN and RVLM of HTN subjects. In addition, PVN-PRR immunoreactivity was positively correlated with systolic BP (sBP) and showed a tendency toward correlation with age but not body mass index (BMI). Collectively, our data provide clinical evidence that the PRR in the PVN and RVLM may be a key molecular player in the neural regulation of BP and cardiovascular and metabolic function in humans.

Exercise training increases GAD65 expression, restores the depressed GABAA receptor function within the PVN and reduces sympathetic modulation in hypertension

GABAergic inhibitory input within the paraventricular hypothalamic nucleus (PVN) plays a key role in restraining sympathetic outflow. Although experimental evidence has shown depressed GABA_A receptor function plus sympathoexcitation in hypertension and augmented GABA levels with reduced sympathetic activity after exercise training (T), the mechanisms underlying T‐induced effects remain unclear. Here we investigated in T and sedentary (S) SHR and WKY: (1) time‐course changes of hemodynamic parameters and PVN glutamic acid decarboxylase (GAD) isoforms’ expression; (2) arterial pressure (AP) and heart rate (HR) responses, sympathetic/parasympathetic modulation of heart and vessels and baroreflex sensitivity to GABA_A receptor blockade within the PVN. SHR‐S versus WKY‐S exhibited higher AP and HR, increased sympathetic reduced parasympathetic modulation, smaller baroreflex sensitivity, and reduced PVN GAD65 immunoreactivity. SHR‐T and WKY‐T showed prompt maintained increase (2–8 weeks) in GAD65 expression (responsible for GABA vesicular pool synthesis), which occurred simultaneously with HR reduction in SHR‐T and preceded MAP fall in SHR‐T and resting bradycardia in WKY‐T. There was no change in GAD67 expression (mainly involved with GABA metabolic pool). Resting HR in both groups and basal MAP in SHR were negatively correlated with PVN GAD65 expression. Normalized baroreflex sensitivity and autonomic control observed only in SHR‐T were due to recovery of GABA_A receptor function into the PVN since bicuculline administration abolished these effects. Data indicated that training augments in both groups the expression/activity of GABAergic neurotransmission within presympathetic PVN neurons and restores GABA_A receptors′ function specifically in the SHR, therefore strengthening GABAergic modulation of sympathetic outflow in hypertension.

Interaction between angiotensin II and GABA in the spinal cord regulates sympathetic vasomotor activity in Goldblatt hypertension

Previous studies have been described changes in brain regions contributing to the sympathetic vasomotor overactivity in Goldblatt hypertension (2K1C). Furthermore, changes in the spinal cord are also involved in the cardiovascular and autonomic dysfunction in renovascular hypertension, as intrathecal (i.t.) administration of Losartan (Los) causes a robust hypotensive/sympathoinhibitory response in 2K1C but not in control rats. The present study evaluated the role of spinal γ-aminobutyric acid (GABA)-ergic inputs in the control of sympathetic vasomotor activity in the 2K1C rats. Hypertension was induced by clipping the renal artery. After six weeks, a catheter (PE-10) was inserted into the subarachnoid space and advanced to the T10-11 vertebral level in urethane-anaesthetized rats. The effects of i.t. injection of bicuculline (Bic) on blood pressure (BP), renal and splanchnic sympathetic nerve activity (rSNA and sSNA, respectively) were evaluated over 40 consecutive minutes in the presence or absence of spinal AT1 antagonism. I.t. Bic triggered a more intense pressor and sympathoexcitatory response in 2K1C rats, however, these responses were attenuated by previous i.t. Los. No differences in the gene expression of GAD 65 and GABA-A receptors subunits in the spinal cord segments were found. Thus, the sympathoexcitation induced by spinal GABA-A blockade is dependent of local AT1 receptor in 2K1C but not in control rats. Excitatory angiotensinergic inputs to sympathetic preganglionic neurons are tonic controlled by spinal GABAergic actions in Goldblatt hypertension.

Gut microbiota and neuroinflammation in pathogenesis of hypertension: A potential role for hydrogen sulfide

Inflammation and gut dysbiosis are hallmarks of hypertension (HTN). Hydrogen sulfide (H2S) is an important freely diffusing molecule that modulates the function of neural, cardiovascular and immune systems, and circulating levels of H2S are reduced in animals and humans with HTN. While most research to date has focused on H₂S produced endogenously by the host, H2S is also produced by the gut bacteria and may affect the host homeostasis. Here, we review an association between neuroinflammation and gut dysbiosis in HTN, with special emphasis on a potential role of H2S in this interplay.

The Potential Therapeutic Capacity of Inhibiting the Brain Renin-Angiotensin System in the Treatment of Co-Morbid Conditions in Epilepsy

Epilepsy is one of the most prevalent neurological diseases and although numerous novel anticonvulsants have been approved, the proportion of patients who are refractory to medical treatment of seizures and have progressive co-morbidities such as cognitive impairment and depression remains at about 20-30%. In the last decade, extensive research has identified a therapeutic capacity of the components of the brain renin-angiotensin system (RAS) in seizure- and epilepsy-related phenomena. Alleviating the activity of RAS in the central nervous system is considered to be a potential adjuvant strategy for the treatment of numerous detrimental consequences of epileptogenesis. One of the main advantages of RAS is associated with its modulatory influence on different neurotransmitter systems, thereby exerting a fine-tuning control mechanism for brain excitability. The most recent scientific findings regarding the involvement of the components of brain RAS show that angiotensin II (Ang II), angiotensin-converting enzyme (ACE), Ang II type 1 (AT₁) and type 2 (AT₂) receptors are involved in the control of epilepsy and its accompanying complications, and therefore they are currently of therapeutic interest in the treatment of this disease. However, data on the role of different components of brain RAS on co-morbid conditions in epilepsy, including hypertension, are insufficient. Experimental and clinical findings related to the involvement of Ang II, ACE, AT₁, and AT₂ receptors in the control of epilepsy and accompanying complications may point to new therapeutic opportunities and adjuvants for the treatment of common co-morbid conditions of epilepsy.

ACE2 and ADAM17 Interaction Regulates the Activity of Presympathetic Neurons

Brain renin angiotensin system within the paraventricular nucleus plays a critical role in balancing excitatory and inhibitory inputs to modulate sympathetic output and blood pressure regulation. We previously identified ACE2 and ADAM17 as a compensatory enzyme and a sheddase, respectively, involved in brain renin angiotensin system regulation. Here, we investigated the opposing contribution of ACE2 and ADAM17 to hypothalamic presympathetic activity and ultimately neurogenic hypertension. New mouse models were generated where ACE2 and ADAM17 were selectively knocked down from all neurons (AC-N) or Sim1 neurons (SAT), respectively. Neuronal ACE2 deletion revealed a reduction of inhibitory inputs to AC-N presympathetic neurons relevant to blood pressure regulation. Primary neuron cultures confirmed ACE2 expression on GABAergic neurons synapsing onto excitatory neurons within the hypothalamus but not on glutamatergic neurons. ADAM17 expression was shown to colocalize with angiotensin-II type 1 receptors on Sim1 neurons, and the pressor relevance of this neuronal population was demonstrated by photoactivation. Selective knockdown of ADAM17 was associated with a reduction of FosB gene expression, increased vagal tone, and prevented the acute pressor response to centrally administered angiotensin-II. Chronically, SAT mice exhibited a blunted blood pressure elevation and preserved ACE2 activity during development of salt-sensitive hypertension. Bicuculline injection in those models confirmed the supporting role of ACE2 on GABAergic tone to the paraventricular nucleus. Together, our study demonstrates the contrasting impact of ACE2 and ADAM17 on neuronal excitability of presympathetic neurons within the paraventricular nucleus and the consequences of this mutual regulation in the context of neurogenic hypertension.

The Possible Role of the Angiotensin System in the Pathophysiology of Schizophrenia: Implications for Pharmacotherapy

A growing body of literature has elucidated the involvement of the central renin-angiotensin system (RAS) in various neuropsychiatric diseases. While consensus on the exact mechanism of the central RAS in schizophrenia pathophysiology does not currently exist, increasing evidence reveals promise in harnessing the therapeutic potential of RAS modulation in the treatment of schizophrenia. In this review, we examine how the central RAS affects inflammation, glutamate, dopamine, gamma-aminobutyric acid (GABA), and peroxisome proliferator-activated receptor (PPAR)-γ, all of which are associated with schizophrenia etiology. In addition, a recent study has demonstrated the therapeutic potential of RAS modulators, especially angiotensin II type 1 receptor blockers (ARBs), as adjunctive therapy to the currently available antipsychotic medications for schizophrenia treatment. With a greater understanding of how RAS inhibition directly modulates neurotransmitter balance in the brain, it is possible that compounds with RAS-inhibiting properties could be used to optimize physiological levels of glutamate, dopamine, and GABA, and the balance among the three neurotransmitters, analogously to how antipsychotic medications mediate the dopaminergic pathways. It can be hoped that a novel approach based on this concept, such as adjunctive telmisartan therapy, may offer practical interventional strategies to address currently unmet therapeutic needs in patients with schizophrenia, especially those with treatment-resistant schizophrenia.

(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.

Long-Term High Salt Intake Involves Reduced SK Currents and Increased Excitability of PVN Neurons with Projections to the Rostral Ventrolateral Medulla in Rats

Evidence indicates that high salt (HS) intake activates presympathetic paraventricular nucleus (PVN) neurons, which contributes to sympathoexcitation of salt-sensitive hypertension. The present study determined whether 5 weeks of HS (2% NaCl) intake alters the small conductance Ca²⁺-activated potassium channel (SK) current in presympathetic PVN neurons and whether this change affects the neuronal excitability. In whole-cell voltage-clamp recordings, HS-treated rats had significantly decreased SK currents compared to rats with normal salt (NS, 0.4% NaCl) intake in PVN neurons. The sensitivity of PVN neuronal excitability in response to current injections was greater in HS group compared to NS controls. The SK channel blocker apamin augmented the neuronal excitability in both groups but had less effect on the sensitivity of the neuronal excitability in HS group compared to NS controls. In the HS group, the interspike interval (ISI) was significantly shorter than that in NS controls. Apamin significantly shortened the ISI in NS controls but had less effect in the HS group. This data suggests that HS intake reduces SK currents, which contributes to increased PVN neuronal excitability at least in part through a decrease in spike frequency adaptation and may be a precursor to the development of salt-sensitive hypertension.

Sympathoexcitation in Rats With Chronic Heart Failure Depends on Homeobox D10 and MicroRNA-7b Inhibiting GABBR1 Translation in Paraventricular Nucleus

BACKGROUND

Chronic heart failure (CHF) increases sympathoexcitation through angiotensin II (ANG II) receptors (AT1R) in the paraventricular nucleus (PVN). Recent publications indicate both γ-aminobutyric acid B-type receptor 1 (GABBR1) and microRNA-7b (miR-7b) are expressed in the PVN. We hypothesized that ANG II regulates sympathoexcitation through homeobox D10 (HoxD10), which regulates miR-7b in other tissues.

METHODS AND RESULTS

Ligation of the left anterior descendent coronary artery in rats caused CHF and sympathoexcitation. PVN expression of AT1R, HoxD10, and miR-7b was increased, whereas GABBR1 was lower in CHF. Infusion of miR-7b in the PVN caused sympathoexcitation in control animals and enhanced the changes in CHF. Antisense miR-7b infused in PVN normalized GABBR1 expression while attenuating CHF symptoms, including sympathoexcitation. A luciferase reporter assay detected miR-7b binding to the 3' untranslated region of GABBR1 that was absent after targeted mutagenesis. ANG II induced HoxD10 and miR-7b in NG108 cells, effects blocked by AT1R blocker losartan and by HoxD10 silencing. miR-7b transfection into NG108 cells decreased GABBR1 expression, which was inhibited by miR-7b antisense. In vivo PVN knockdown of AT1R attenuated the symptoms of CHF, whereas HoxD10 overexpression exaggerated them. Finally, in vivo PVN ANG II infusion caused dose-dependent sympathoexcitation that was abrogated by miR-7b antisense and exaggerated by GABBR1 silencing.

CONCLUSIONS

There is an ANG II/AT1R/HoxD10/miR-7b/GABBR1 pathway in the PVN that contributes to sympathoexcitation and deterioration of cardiac function in CHF.

Early Training-Induced Reduction of Angiotensinogen in Autonomic Areas-The Main Effect of Exercise on Brain Renin-Angiotensin System in Hypertensive Rats

Background

Exercise training (T) blunts functional deficits and renin-angiotensin system (RAS) hyperactivity in hypertensive individuals. There is no information on T-induced temporal changes of brain RAS. We evaluate now the simultaneous effects of T on functional responses and time course changes in the expression/activity of brain RAS components in autonomic cardiovascular-controlling areas.

Methods and Results

Spontaneously hypertensive rats (SHR) and age-matched normotensive controls (WKY) were trained for 0, 1, 2, 4, 8 and 12 weeks. Sedentary (S) groups served as time-controls. After arterial pressure (AP) and heart rate (HR) recordings at rest, fresh and fixed brains were harvested for qPCR and immunofluorescence assays. SHR-S vs. WKY-S exhibited higher mean AP (MAP) and HR, increased pressure variability and sympathetic activity, elevated AT₁ receptor (AT₁) expression in nucleus tractus solitarii (NTS) and higher Mas receptor expression in the rostroventrolateral medulla (RVLM). In SHR, T promptly (T₂ on) reduced sympathetic variability to heart/vessels and largely decreased angiotensinogen expression in the paraventricular hypothalamic nucleus (PVN) and NTS, with a late RVLM reduction (T₄). AT₁ expression was only reduced at T₁₂ (PVN and NTS) with transient, not maintained Mas receptor changes in PVN and RVLM. These responses were accompanied by baseline MAP and HR reduction in the SHR-T (from T₄ on). In the SHR group, PVN angiotensinogen expression correlated positively with sympathetic activity, resting MAP and HR. In WKY-T, a precocious (T₂-T₁₂) RVLM AT₁ decrease preceded the appearance of resting bradycardia (from T₈ on).

Conclusions

Early and maintained reduction of angiotensinogen content in autonomic areas of the SHR is the most prominent effect of training on brain RAS. Down-regulation of PVN RAS expression is an essential factor to drive cardiovascular benefits in SHR-T, while resting bradycardia in WKY-T is correlated to RVLM AT₁ reduction.

Sympathoexcitation in ANG II-salt hypertension involves reduced SK channel function in the hypothalamic paraventricular nucleus

Hypertension (HTN) resulting from subcutaneous infusion of ANG II and dietary high salt (HS) intake involves sympathoexcitation. Recently, we reported reduced small-conductance Ca(2+)-activated K(+) (SK) current and increased excitability of presympathetic neurons in the paraventricular nucleus (PVN) in ANG II-salt HTN. Here, we hypothesized that ANG II-salt HTN would be accompanied by altered PVN SK channel activity, which may contribute to sympathoexcitation in vivo. In anesthetized rats with normal salt (NS) intake, bilateral PVN microinjection of apamin (12.5 pmol/50 nl each), the SK channel blocker, remarkably elevated splanchnic sympathetic nerve activity (SSNA), renal sympathetic nerve activity (RSNA), and mean arterial pressure (MAP). In contrast, rats with ANG II-salt HTN demonstrated significantly attenuated SSNA, RSNA, and MAP (P < 0.05) responses to PVN-injected apamin compared with NS control rats. Next, we sought to examine the individual contributions of HS and subcutaneous infusion of ANG II on PVN SK channel function. SSNA, RSNA, and MAP responses to PVN-injected apamin in rats with HS alone were significantly attenuated compared with NS-fed rats. In contrast, sympathetic nerve activity responses to PVN-injected apamin in ANG II-treated rats were slightly attenuated with SSNA, demonstrating no statistical difference compared with NS-fed rats, whereas MAP responses to PVN-injected apamin were similar to NS-fed rats. Finally, Western blot analysis showed no statistical difference in SK1-SK3 expression in the PVN between NS and ANG II-salt HTN. We conclude that reduced SK channel function in the PVN is involved in the sympathoexcitation associated with ANG II-salt HTN. Dietary HS may play a dominant role in reducing SK channel function, thus contributing to sympathoexcitation in ANG II-salt HTN.

Cardiovascular and behavioral effects produced by administration of liposome-entrapped GABA into the rat central nervous system

Liposomes are nanosystems that allow a sustained release of entrapped substances. Gamma-aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter of the central nervous system (CNS). We developed a liposomal formulation of GABA for application in long-term CNS functional studies. Two days after liposome-entrapped GABA was injected intracerebroventricularly (ICV), Wistar rats were submitted to the following evaluations: (1) changes in mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) to ICV injection of bicuculline methiodide (BMI) in anesthetized rats; (2) changes in cardiovascular reactivity to air jet stress in conscious rats; and (3) anxiety-like behavior in conscious rats. GABA and saline-containing pegylated liposomes were prepared with a mean diameter of 200 nm. Rats with implanted cannulas targeted to lateral cerebral ventricle (n = 5-8/group) received either GABA solution (GS), empty liposomes (EL) or GABA-containing liposomes (GL). Following (48 h) central microinjection (2 μL, 0.09 M and 99 g/L) of liposomes, animals were submitted to the different protocols. Animals that received GL demonstrated attenuated response of RSNA to BMI microinjection (GS 48 ± 9, EL 43 ± 9, GL 11 ± 8%; P < 0.05), blunted tachycardia in the stress trial (ΔHR: GS 115 ± 14, EL 117 ± 10, GL 74 ± 9 bpm; P<0.05) and spent more time in the open arms of elevated plus maze (EL 6 ± 2 vs. GL 18 ± 5%; P = 0.028) compared with GS and EL groups. These results indicate that liposome-entrapped GABA can be a potential tool for exploring the chronic effects of GABA in specific regions and pathways of the central nervous system.

Nitric oxide impacts on angiotensin AT2 receptor modulation of high-pressure baroreflex control of renal sympathetic nerve activity in anaesthetized rats

Aim

Nitric oxide (NO) interacts with the local brain renin-angiotensin system to modulate sympathetic outflow and cardiovascular homoeostasis. This study investigated whether NO influenced the ability of angiotensin AT2 receptor activation to modify the high-pressure baroreceptor regulation of renal sympathetic nerve activity (RSNA) and heart rate (HR).

Methods

Anaesthetized (chloralose/urethane) rats were prepared to allow generation of baroreflex gain curves for RSNA or HR following intracerebroventricular (I.C.V.) CGP42112 (AT2 receptor agonist), PD123319 (AT2 receptor antagonist) or losartan (AT1 receptor antagonist), and then in combination with L-NAME (NO synthase inhibitor).

Results

I.C.V. PD123319, CGP42112, and Losartan did not change baseline mean arterial pressure, HR or RSNA. Baroreflex sensitivities for RSNA and HR were increased following AT2 receptor activation with CGP42112 by 112 and 157%, respectively, but were reduced following PD123319 by 20% (all P < 0.05). L-NAME alone increased baroreflex sensitivity for both RSNA and HR, by 62 and 158%, respectively, but when co-infused with either CGP42112 or PD123319, the baroreflex sensitivity fell to values comparable to those obtained during I.C.V. saline infusion. The baroreflex sensitivities for RSNA and HR were increased by losartan by 92% and 192%, respectively, but in the presence of L-NAME were no different from those obtained during I.C.V. saline infusion.

Conclusion

There is an important facilitatory role for AT2 receptors in the high-pressure baroreflex regulation of RSNA and HR which is dependent on a functional NO/NOS system. Conversely, AT1 receptors have an inhibitory effect on the baroreflex, an action that relies on a tonic inhibition of NO.

Paraventricular nucleus control of blood pressure in two-kidney, one-clip rats: effects of exercise training and resting blood pressure

Exercise-induced changes in γ-aminobutyric acid (GABA) or nitric oxide signaling within the paraventricular nucleus (PVN) have not been studied in renovascular hypertension. We tested whether exercise training decreases mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) in two-kidney, one-clip (2K-1C) hypertensive rats due to enhanced nitric oxide or GABA signaling within PVN. Conscious, unrestrained male Sprague-Dawley rats with either sham (Sham) or right renal artery clipping (2K-1C) were assigned to sedentary (SED) or voluntary wheel running (ExT) for 6 or 12 wk. MAP and angiotensin II (ANG II) were elevated in 2K-1C SED rats. The 2K-1C ExT rats displayed lower MAP at 6 wk that did not decline further by 12 wk. Plasma ANG II was lower in 2K-1C ExT rats. Increases in MAP, heart rate, and RSNA to blockade of PVN nitric oxide in 2K-1C SED rats were attenuated compared with either Sham group. Exercise training restored the responses in 2K-1C ExT rats. The increase in MAP in response to bicuculline was inversely correlated with baseline MAP. The rise in MAP was lower in 2K-1C SED vs. either Sham group and was normalized in the 2K-1C ExT rats. Paradoxically, heart rate and RSNA responses were not diminished in 2K-1C SED rats but were significantly lower in the 2K-1C ExT rats. Thus the decrease in arterial pressure in 2K-1C hypertension associated with exercise training is likely due to diminished excitatory inputs to PVN because of lower ANG II and higher nitritergic tone rather than enhanced GABA inhibition of sympathetic output.

Purinergic and glutamatergic interactions in the hypothalamic paraventricular nucleus modulate sympathetic outflow

P2X receptors are expressed on ventrolateral medulla projecting paraventricular nucleus (PVN) neurons. Here, we investigate the role of adenosine 5'-triphosphate (ATP) in modulating sympathetic nerve activity (SNA) at the level of the PVN. We used an in situ arterially perfused rat preparation to determine the effect of P2 receptor activation and the putative interaction between purinergic and glutamatergic neurotransmitter systems within the PVN on lumbar SNA (LSNA). Unilateral microinjection of ATP into the PVN induced a dose-related increase in the LSNA (1 nmol: 38 ± 6 %, 2.5 nmol: 72 ± 7 %, 5 nmol: 96 ±13 %). This increase was significantly attenuated by blockade of P2 receptors (pyridoxalphosphate-6-azophenyl-20,40-disulphonic acid, PPADS) and glutamate receptors (kynurenic acid, KYN) or a combination of both. The increase in LSNA elicited by L-glutamate microinjection into the PVN was not affected by a previous injection of PPADS. Selective blockade of non-N-methyl-D-aspartate receptors (6-cyano-7-nitroquinoxaline-2,3-dione disodium salt, CNQX), but not N-methyl-D-aspartate receptors (NMDA) receptors (DL-2-amino-5-phosphonopentanoic acid, AP5), attenuated the ATP-induced sympathoexcitatory effects at the PVN level. Taken together, our data show that purinergic neurotransmission within the PVN is involved in the control of SNA via P2 receptor activation. Moreover, we show an interaction between P2 receptors and non-NMDA glutamate receptors in the PVN suggesting that these functional interactions might be important in the regulation of sympathetic outflow.

Blunted endogenous GABA-mediated inhibition in the hypothalamic paraventricular nucleus of rats with streptozotocin-induced diabetes

The hypothalamic paraventricular nucleus (PVN) is involved in the regulation of sympathetic outflow and particularly affects the heart. This study sets out to determine the role of GABA of the paraventricular nucleus (PVN) in cardiovascular regulation in streptozotocin-induced diabetic rats. Pharmacological stimulation of glutamatergic receptors with DL-Homocysteic acid (200 mM in 100 nL) in the PVN region showed a significant depression in both mean arterial pressure (MAP) and heart rate (HR) of diabetic rats (Diabetic vs. non-diabetic: MAP 15.0 ± 1.5 vs. 35.8 ± 2.8 mmHg; HR 3.0 ± 2.0 vs. 30.0 ± 6.0 bpm, P < 0.05). Microinjection of bicuculline methiodide (1 mM in 100 nL), a GABAA receptor antagonist, produced an increase in baseline MAP and HR in both non-diabetic and diabetic rats. In the diabetic rats, bicuculline injection into the PVN reduced the pressor and HR responses (Diabetic vs. non-diabetic: MAP 6.2 ± 0.8 vs. 25.1 ± 2.2 mmHg; HR 1.8 ± 1.1 vs. 25.4 ± 6.2 bpm, P < 0.05). A microinjection of muscimol (2 mM in 100 nL), which is a GABAA receptor agonist, in the PVN elicited decreases in MAP and HR in both groups. The diabetic group showed a significantly blunted reduction in HR, but not MAP (Diabetic vs. non-diabetic: MAP -15.7 ± 4.0 vs. -25.0 ± 3.8 mmHg; HR -5.2 ± 2.1 vs. -39.1 ± 7.9 bpm). The blunted vasopressor and tachycardic responses to bicuculline microinjection in the diabetic rats are likely to result from decreased GABAergic inputs, attenuated release of endogenous GABA or alterations in GABAA receptors within the PVN.

Reduced GABAergic inhibition of kidney-related PVN neurons in streptozotocin-treated type 1 diabetic mouse

Activity of presympathetic neurons in the paraventricular nucleus (PVN) of the hypothalamus is known to play an important role in the regulation of sympathetic outflow. Sympathetic overactivity is associated with many pathophysiological conditions such as diabetes mellitus and hypertension; however, the underlying synaptic mechanisms are poorly understood. In this study, we examined the GABAergic inhibitory synaptic control of kidney-related presympathetic PVN neurons in the streptozotocin-treated type 1 diabetic mouse model, using patch-clamp slice electrophysiology in combination with retrograde labeling. Type 1 diabetes resulted in decreased frequency of miniature inhibitory postsynaptic currents (mIPSCs). Our data also demonstrated a reduction of mIPSC amplitude and mean inhibitory current without alteration of input resistance. Furthermore, our data revealed decreased tonic GABAergic inhibition of kidney-related PVN neurons in diabetic conditions, which was consistent with the observed increased excitability of the presympathetic PVN neurons. In summary, our data demonstrated decreased phasic and tonic inhibitory control of kidney-related presympathetic PVN neurons that suggest altered sympathetic circuitry in type 1 diabetes.

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.

Exercise training normalizes enhanced sympathetic activation from the paraventricular nucleus in chronic heart failure: role of angiotensin II

Exercise training (ExT) normalizes the increased sympathetic outflow in heart failure (HF), but the underlying mechanisms are not known. We hypothesized ExT would normalize the augmented activation of the paraventricular nucleus (PVN) via an angiotensinergic mechanism during HF. Four groups of rats used were the following: 1) sham-sedentary (Sed); 2) sham-ExT; 3) HF-Sed, and 4) HF-ExT. HF was induced by left coronary artery ligation. Four weeks after surgery, 3 wk of treadmill running was performed in ExT groups. The number of FosB-positive cells in the PVN was significantly increased in HF-Sed group compared with the sham-Sed group. ExT normalized (negated) this increase in the rats with HF. In anesthetized condition, the increases in renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) in response to microinjection of angiotensin (ANG) II (50∼200 pmol) in the PVN of HF-Sed group were significantly greater than of the sham-Sed group. In the HF-ExT group the responses to microinjection of ANG II were not different from sham-Sed or sham-ExT groups. Blockade of ANG II type 1 (AT(1)) receptors with losartan in the PVN produced a significantly greater decrease in RSNA, MAP, and HR in HF-Sed group compared with sham-Sed group. ExT prevented the difference between HF and sham groups. AT(1) receptor protein expression was increased 50% in HF-Sed group compared with sham-Sed group. In the HF-ExT group, AT(1) receptor protein expression was not significantly different from sham-Sed or sham-ExT groups. In conclusion, one mechanism by which ExT alleviates elevated sympathetic outflow in HF may be through normalization of angiotensinergic mechanisms within the PVN.

Role of small conductance calcium-activated potassium channels expressed in PVN in regulating sympathetic nerve activity and arterial blood pressure in rats

Small conductance Ca(2+)-activated K(+) (SK) channels regulate membrane properties of rostral ventrolateral medulla (RVLM) projecting hypothalamic paraventricular nucleus (PVN) neurons and inhibition of SK channels increases in vitro excitability. Here, we determined in vivo the role of PVN SK channels in regulating sympathetic nerve activity (SNA) and mean arterial pressure (MAP). In anesthetized rats, bilateral PVN microinjection of SK channel blocker with peptide apamin (0, 0.125, 1.25, 3.75, 12.5, and 25 pmol) increased splanchnic SNA (SSNA), renal SNA (RSNA), MAP, and heart rate (HR) in a dose-dependent manner. Maximum increases in SSNA, RSNA, MAP, and HR elicited by apamin (12.5 pmol, n = 7) were 330 ± 40% (P < 0.01), 271 ± 40% (P < 0.01), 29 ± 4 mmHg (P < 0.01), and 34 ± 9 beats/min (P < 0.01), respectively. PVN injection of the nonpeptide SK channel blocker UCL1684 (250 pmol, n = 7) significantly increased SSNA (P < 0.05), RSNA (P < 0.05), MAP (P < 0.05), and HR (P < 0.05). Neither apamin injected outside the PVN (12.5 pmol, n = 6) nor peripheral administration of the same dose of apamin (12.5 pmol, n = 5) evoked any significant changes in the recorded variables. PVN-injected SK channel enhancer 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (DCEBIO, 5 nmol, n = 4) or N-cyclohexyl-N-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-4-pyrimidin]amine (CyPPA, 5 nmol, n = 6) did not significantly alter the SSNA, RSNA, MAP, and HR. Western blot and RT-PCR analysis of punched PVN tissue showed abundant expression of SK1-3 channels. We conclude that SK channels expressed in the PVN play an important role in the regulation of sympathetic outflow and cardiovascular function.

Angiotensin II AT1 receptor blocker candesartan prevents the fast up-regulation of cerebrocortical benzodiazepine-1 receptors induced by acute inflammatory and restraint stress

Centrally acting Angiotensin II AT(1) receptor blockers (ARBs) protect from stress-induced disorders and decrease anxiety in a model of inflammatory stress, the systemic injection of bacterial endotoxin lipopolysaccharide (LPS). In order to better understand the anxiolytic effect of ARBs, we treated rats with LPS (50 μg/kg) with or without 3 days of pretreatment with the ARB candesartan (1mg/kg/day), and studied cortical benzodiazepine (BZ) and corticotrophin-releasing factor (CRF) receptors. We compared the cortical BZ and CRF receptors expression pattern induced by LPS with that produced in restraint stress. Inflammation stress produced a generalized increase in cortical BZ(1) receptors and reduced mRNA expression of the GABA(A) receptor γ(2) subunit in cingulate cortex; changes were prevented by candesartan pretreatment. Moreover, restraint stress produced similar increases in cortical BZ(1) receptor binding, and candesartan prevented these changes. Treatment with candesartan alone increased cortical BZ(1) binding, and decreased γ(2) subunit mRNA expression in the cingulate cortex. Conversely, we did not find changes in CRF(1) receptor expression in any of the cortical areas studied, either after inflammation or restraint stress. Cortical CRF(2) receptor binding was undetectable, but CRF(2) mRNA expression was decreased by inflammation stress, a change prevented by candesartan. We conclude that stress promotes rapid and widespread changes in cortical BZ(1) receptor expression; and that the stress-induced BZ(1) receptor expression is under the control of AT(1) receptor activity. The results suggest that the anti-anxiety effect of ARBs may be associated with their capacity to regulate stress-induced alterations in cortical BZ(1) receptors.

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.

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.

Central angiotensin II has catabolic action at white and brown adipose tissue

Considerable evidence implicates the renin-angiotensin system (RAS) in the regulation of energy balance. To evaluate the role of the RAS in the central nervous system regulation of energy balance, we used osmotic minipumps to chronically administer angiotensin II (Ang II; icv; 0.7 ng/min for 24 days) to adult male Long-Evans rats, resulting in reduced food intake, body weight gain, and adiposity. The decrease in body weight and adiposity occurred relative to both ad libitum- and pair-fed controls, implying that reduced food intake in and of itself does not underlie all of these effects. Consistent with this, rats administered Ang II had increased whole body heat production and oxygen consumption. Additionally, chronic icv Ang II increased uncoupling protein-1 and β(3)-adrenergic receptor expression in brown adipose tissue and β3-adrenergic receptor expression in white adipose tissue, which is suggestive of enhanced sympathetic activation and thermogenesis. Chronic icv Ang II also increased hypothalamic agouti-related peptide and decreased hypothalamic proopiomelanocortin expression, consistent with a state of energy deficit. Moreover, chronic icv Ang II increased the anorectic corticotrophin- and thyroid-releasing hormones within the hypothalamus. These results suggest that Ang II acts in the brain to promote negative energy balance and that contributing mechanisms include an alteration in the hypothalamic circuits regulating energy balance, a decrease in food intake, an increase in energy expenditure, and an increase in sympathetic activation of brown and white adipose tissue.

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.

GABA in the paraventricular nucleus tonically suppresses baroreflex function: alterations during pregnancy

It is well established that GABAergic inputs to the paraventricular nucleus of the hypothalamus (PVN) tonically suppress heart rate and the activity of several sympathetic nerves. However, whether GABA similarly inhibits PVN control of baroreflex function has not been previously investigated. To test this hypothesis, it was determined whether microinjection of the GABA(A) antagonist, bicuculline, into the PVN enhances the baroreflex in anesthetized female virgin rats. In addition, because GABAergic inhibition of PVN preautonomic neurons is decreased during pregnancy, it was also determined whether the effects of PVN bicuculline administration on baroreflex function were less in pregnant animals. In virgin rats, PVN microinjection of bicuculline increased (P < 0.05) baroreflex gain and maximum levels of heart rate (gain, from 1.6 ± 0.6 to 3.8 ± 1.3 bpm/mmHg; maximum, from 406 ± 18 to 475 ± 14 bpm) and of lumbar sympathetic nerve activity (gain from 2.6 ± 0.7 to 4.8 ± 1.6%/mmHg; maximum, 149 ± 32 to 273 ± 48%), indicating that PVN GABA normally suppresses baroreflex function. Pregnancy decreased heart rate baroreflex gain (pregnant, 0.9 ± 0.3 bpm/mmHg; virgin, 1.9 ± 0.2 bpm/mmHg; P < 0.05). Following PVN bicuculline administration in pregnant rats, smaller (P < 0.01) increments in baroreflex gain (pregnant, 0.6 ± 0.1 bpm/mmHg; virgin, 2.4 ± 0.9 bpm/mmHg) and maximum (pregnant, 33 ± 7 bpm; virgin, 75 ± 12 bpm; P < 0.05) were produced. Collectively, these data suggest that the PVN normally inhibits the baroreflex via tonic GABAergic inputs and that this inhibition is less during pregnancy.

Sympathetic response to insulin is mediated by melanocortin 3/4 receptors in the hypothalamic paraventricular nucleus

Hyperinsulinemia increases sympathetic nerve activity and contributes to cardiovascular dysfunction in obesity and diabetes. Neurons of the hypothalamic paraventricular nucleus (PVN) regulate sympathetic nerve activity through mono- and poly-synaptic connections to preganglionic neurons in the spinal cord. The purpose of the present study was to determine whether PVN neurons mediate the sympathetic response to insulin. Hyperinsulinemic-euglycemic clamps were performed in α-chloralose-anesthetized, male Sprague-Dawley rats (280-420 g) by an infusion of insulin (3.75 mU/kg per min) and 50% dextrose (0.75-2.0 mL/h) for 120 minutes. At 90 minutes, insulin significantly increased lumbar sympathetic nerve activity without any change in renal sympathetic nerve activity, heart rate, or blood glucose levels. Inhibition of the PVN with bilateral injection of the GABA(A) receptor agonist muscimol completely reversed the sympathoexcitatory response. However, direct injection of insulin into the PVN did not alter lumbar sympathetic nerve activity, and thereby suggests that insulin activates neurons upstream of the PVN. Interestingly, the sympathetic response to insulin was eliminated by PVN injection of the melanocortin 3/4 receptor antagonist SHU9119, but was unaffected by the angiotensin II type 1 receptor antagonist losartan. A final set of experiments suggests activation of PVN neurons during hyperinsulinemia increases glutamatergic drive to the rostral ventrolateral medulla. Collectively, these findings indicate that insulin activates a melanocortin-dependent pathway to the PVN that increases glutamatergic drive to the rostral ventrolateral medulla and alters cardiovascular function.

The hypothalamic arcuate nucleus: a new site of cardiovascular action of angiotensin-(1-12) and angiotensin II

The hypothalamic arcuate nucleus (ARCN) has been reported to play a significant role in cardiovascular regulation. It has been hypothesized that the ARCN may be one of the sites of cardiovascular actions of angiotensins (ANGs). Experiments were carried out in urethane-anesthetized, artificially ventilated, adult male Wistar rats. The ARCN was identified by microinjections of N-methyl-d-aspartic acid (NMDA; 10 mM). Microinjections (50 nl) of ANG-(1-12) (1 mM) into the ARCN elicited increases in mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA). The tachycardic responses to ANG-(1-12) were attenuated by bilateral vagotomy. The cardiovascular responses elicited by ANG-(1-12) were attenuated by microinjections of ANG II type 1 receptor (AT(1)R) antagonists but not ANG type 2 receptor (AT(2)R) antagonist. Combined inhibition of ANG-converting enzyme (ACE) and chymase in the ARCN abolished ANG-(1-12)-induced responses. Microinjections of ANG II (1 mM) into the ARCN also increased MAP and HR. Inhibition of ARCN by microinjections of muscimol (1 mM) attenuated the pressor and tachycardic responses to intravenously administered ANG-(1-12) and ANG II (300 pmol/kg each). These results indicated that 1) microinjections of ANG-(1-12) into the ARCN elicited increases in MAP, HR, and GSNA; 2) HR responses were mediated via both sympathetic and vagus nerves; 3) AT(1)Rs, but not AT(2)Rs, in the ARCN mediated ANG-(1-12)-induced responses; 4) both ACE and chymase were needed to convert ANG-(1-12) to ANG II in the ARCN; and 5) ARCN plays a role in mediating the cardiovascular responses to circulating ANGs.

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Chronic infusion of angiotensin receptor antagonists in the hypothalamic paraventricular nucleus prevents hypertension in a rat model of sleep apnea

Sleep apnea is characterized by increased sympathetic activity and is associated with systemic hypertension. Angiotensin (Ang) peptides have previously been shown to participate in the regulation of sympathetic tone and arterial pressure in the hypothalamic paraventricular nucleus (PVN) neurons. We investigated the role of endogenous Ang peptides within the PVN to control blood pressure in a rat model of sleep apnea-induced hypertension. Male Sprague-Dawley rats (250 g), instrumented with bilateral guide cannulae targeting the PVN, received chronic infusion of Ang antagonists (A-779, Ang-(1-7) antagonist; losartan and ZD7155, AT(1) antagonists; PD123319, AT(2) receptor antagonist, or saline vehicle). A separate group received an infusion of the GABA(A) receptor agonist (muscimol) to inhibit PVN neuronal activity independent of angiotensin receptors. After cannula placement, rats were exposed during their sleep period to eucapnic intermittent hypoxia (IH; nadir 5% O(2); 5% CO(2) to peak 21% O(2); 0% CO(2)) 20 cycles/h, 7 h/day, for 14 days while mean arterial pressure (MAP) was measured by telemetry. In rats receiving saline, IH exposure significantly increased MAP (+12±2 mm Hg vs. Sham -2±1 mm Hg P<0.01). Inhibition of PVN neurons with muscimol reversed the increase in MAP in IH rats (MUS: -9±4 mm Hg vs. vehicle +12±2 mm Hg; P<0.01). Infusion of any of the Ang antagonists also prevented the rise in MAP induced by IH (A-779: -5±1 mm Hg, losartan: -9±4 mm Hg, ZD7155: -11±4 mm Hg and PD123319: -4±3 mm Hg; P<0.01). Our results suggest that endogenous Ang peptides acting in the PVN contribute to IH-induced increases in MAP observed in this rat model of sleep apnea-induced hypertension.

Modulation of cardiorespiratory function mediated by the paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) coordinates autonomic and neuroendocrine systems to maintain homeostasis and to respond to stress. Neuroanatomic and neurophysiologic experiments have provided insight into the mechanisms by which the PVN acts. The PVN projects directly to the spinal cord and brainstem and, specifically, to sites that control cardio-respiratory function: the intermediolateral cell columns and phrenic motor nuclei in the spinal cord and rostral ventrolateral medulla (RVLM) and the rostral nuclei in the ventral respiratory column (rVRC) in the brainstem. Activation of the PVN increases ventilation (both tidal volume and frequency) and blood pressure (both heart rate and sympathetic nerve activity). Excitatory and inhibitory neurotransmitters including glutamate and GABA converge in the PVN to influence its neuronal activity. However, a tonic GABAergic input to the PVN directly modulates excitatory outflow from the PVN. Further, even within the PVN, microinjection of GABA(A) receptor blockers increases glutamate release suggesting an indirect mechanism by which GABA control contributes to PVN functions. PVN activity alters blood pressure and ventilation during various stresses, such as maternal separation, chronic intermittent hypoxia (CIH), dehydration and hemorrhage. Among the several PVN neurotransmitters and neurohormones, vasopressin and oxytocin modulate ventilation and blood pressure. Here, we review our data indicating that increases in vasopressin and vasopressin type 1A (V(1A)) receptor signalling in the RVLM and rVRC are mechanisms increasing blood pressure and ventilation after exposure to CIH. That blockade of V(1A) receptors in the medulla normalizes baseline blood pressure as well as blunts PVN-evoked blood pressure and ventilatory responses in CIH-conditioned animals indicate the role of vasopressin in cardiorespiratory control. In summary, morphological and functional studies have found that the PVN integrates sensory input and projects to the sympathetic and respiratory control systems with descending projections to the medulla and spinal cord.

Reduction in synaptic GABA release contributes to target-selective elevation of PVN neuronal activity in rats with myocardial infarction

Neuronal activity in the paraventricular nucleus (PVN) is known to be elevated in rats with heart failure. However, the type of neurons involved and the underlying synaptic mechanisms remain unknown. Here we examined spontaneous firing activity and synaptic currents in presympathetic PVN neurons in rats with myocardial infarction (MI), using slice patch clamp combined with the retrograde labeling technique. In PVN neurons projecting to the rostral ventrolateral medulla (PVN-RVLM), MI induced a significant increase in basal firing rate (1.79 to 3.02 Hz, P < 0.05) and a reduction in the frequency of spontaneous (P < 0.05) and miniature (P < 0.01) inhibitory postsynaptic currents (IPSCs). In addition, MI induced an increase in the paired-pulse ratio of evoked IPSCs (P < 0.05). Bicuculline, a GABA(A) receptor antagonist, increased the firing rate of PVN-RVLM neurons in sham-operated (1.21 to 2.74 Hz, P < 0.05) but not MI (P > 0.05) rats. In contrast, in PVN neurons projecting to the intermediolateral horn of the spinal cord (PVN-IML), MI did not induce any significant changes in the basal firing rate and the properties of spontaneous and miniature IPSCs. The properties of spontaneous excitatory postsynaptic currents (EPSCs) were not altered in either neuron group. In conclusion, our results indicate that MI induces an elevation of firing activity in PVN-RVLM but not in PVN-IML neurons and that the elevated firing rate is largely due to a decrease in GABA release. These results provide evidence for a novel target-selective synaptic plasticity in the PVN that is associated with the sympathetic hyperactivity commonly seen in heart failure.

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.

Increased vasopressin transmission from the paraventricular nucleus to the rostral medulla augments cardiorespiratory outflow in chronic intermittent hypoxia-conditioned rats

A co-morbidity of sleep apnoea is hypertension associated with elevated sympathetic nerve activity (SNA) which may result from conditioning to chronic intermittent hypoxia (CIH). Our hypothesis is that SNA depends on input to the rostral ventrolateral medulla (RVLM) from neurons in the paraventricular nucleus (PVN) that release arginine vasopressin (AVP) and specifically, that increased SNA evoked by CIH depends on this excitatory input. In two sets of neuroanatomical experiments, we determined if AVP neurons project from the PVN to the RVLM and if arginine vasopressin (V(1A)) receptor expression increases in the RVLM after CIH conditioning (8 h per day for 10 days). In the first set, cholera toxin beta subunit (CT-beta) was microinjected into the RVLM to retrogradely label the PVN neurons. Immunohistochemical staining demonstrated that 14.6% of CT-beta-labelled PVN neurons were double-labelled with AVP. In the second set, sections of the medulla were immunolabelled for V(1A) receptors, and the V(1A) receptor-expressing cell count was significantly greater in the RVLM (P < 0.01) and in the neighbouring rostral ventral respiratory column (rVRC) from CIH- than from room air (RA)-conditioned rats. In a series of physiological experiments, we determined if blocking V(1A) receptors in the medulla would normalize blood pressure in CIH-conditioned animals and attenuate its response to disinhibition of PVN. Blood pressure (BP), heart rate (HR), diaphragm (D(EMG)) and genioglossus muscle (GG(EMG)) activity were recorded in anaesthetized, ventilated and vagotomized rats. The PVN was disinhibited by microinjecting a GABA(A) receptor antagonist, bicuculline (BIC, 0.1 nmol), before and after blocking V(1A) receptors within the RVLM and rVRC with SR49059 (0.2 nmol). In RA-conditioned rats, disinhibition of the PVN increased BP, HR, minute D(EMG) and GG(EMG) activity and these increases were attenuated after blocking V(1A) receptors. In CIH-conditioned rats, a significantly greater dose of blocker (0.4 nmol) was required to blunt these physiological responses (P < 0.05). Further, this dose normalized the baseline BP. In summary, AVP released by a subset of PVN neurons modulates cardiorespiratory output via V(1A) receptors in the RVLM and rVRC, and increased SNA in CIH-conditioned animals depends on up-regulation of V(1A) receptors in the RVLM.

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.

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.

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

Water deprivation activates sympathoexcitatory neurons in the paraventricular nucleus (PVN); however, the neurotransmitters that mediate this activation are unknown. To test the hypothesis that ANG II and glutamate are involved, effects on blood pressure (BP) of bilateral PVN microinjections of ANG II type 1 receptor (AT1R) antagonists, candesartan and valsartan, or the ionotropic glutamate receptor antagonist, kynurenate, were determined in urethane-anesthetized water-deprived and water-replete male rats. Because PVN may activate sympathetic neurons via the rostral ventrolateral medulla (RVLM) and because PVN disinhibition increases sympathetic activity in part via increased drive of AT1R in the RVLM, candesartan was also bilaterally microinjected into the RVLM. Total blockade of the PVN with bilateral microinjections of muscimol, a GABA(A) agonist, decreased BP more (P < 0.05) in water-deprived (-29 +/- 8 mmHg) than in water-replete (-7 +/- 2 mmHg) rats, verifying that the PVN is required for BP maintenance during water deprivation. PVN candesartan slowly lowered BP by 7 +/- 1 mmHg (P < 0.05). In water-replete rats, however, candesartan did not alter BP (1 +/- 1 mmHg). Valsartan also produced a slowly developing decrease in arterial pressure (-6 +/- 1 mmHg; P < 0.05) in water-deprived but not in water-replete (-1 +/- 1 mmHg) rats. In water-deprived rats, PVN kynurenate rapidly decreased BP (-19 +/- 3 mmHg), and the response was greater (P < 0.05) than in water-replete rats (-4 +/- 1 mmHg). Finally, as in PVN, candesartan in RVLM slowly decreased BP in water-deprived (-8 +/- 1 mmHg; P < 0.05) but not in water-replete (-3 +/- 1 mmHg) rats. These data suggest that activation of AT(1) and glutamate receptors in PVN, as well as of AT1R in RVLM, contributes to BP maintenance during water deprivation.

Long-term regulation of arterial blood pressure by hypothalamic nuclei: some critical questions

1. The long-term level of arterial pressure is dependent on the relationship between arterial pressure and the urinary output of salt and water, which, in turn, is affected by a number of factors, including renal sympathetic nerve activity (RSNA). In the present brief review, we consider the mechanisms within the brain that can influence RSNA, focusing particularly on hypothalamic mechanisms. 2. The paraventricular nucleus (PVN) in the hypothalamus has major direct and indirect connections with the sympathetic outflow and there is now considerable evidence that tonic activation of the PVN sympathetic pathway contributes to the sustained increased level of RSNA that occurs in conditions such as heart failure and neurogenic hypertension. The tonic activity of PVN sympathetic neurons, in turn, depends upon the balance of excitatory and inhibitory inputs. A number of neurotransmitters and neuromodulators are involved in these tonic excitatory and inhibitory effects, including glutamate, GABA, angiotensin II and nitric oxide. 3. The dorsomedial hypothalamic nucleus (DMH) also exerts a powerful influence over sympathetic activity, including RSNA, via synapses with sympathetic nuclei in the medulla and, possibly, also other brainstem regions. The DMH sympathetic pathway is an important component of the phasic sympathoexcitatory responses associated with acute stress, but there is no evidence that it is an important component of the central pathways that produce long-term changes in arterial pressure. Nevertheless, it is possible that repeated episodic activation of this pathway could lead to vascular hypertrophy and, thus, sustained changes in vascular resistance and arterial pressure. 4. Recent studies have reactivated the old debate concerning the possible role of the baroreceptor reflex in the long-term regulation of sympathetic activity. Therefore, central resetting of the baroreceptor-sympathetic reflex may be an important component of the mechanisms causing sustained changes in RSNA. However, little is known about the cellular mechanisms that could cause such resetting.

Plasticity of GABAergic control of hypothalamic presympathetic neurons in hypertension

Increased sympathetic outflow contributes to the pathogenesis of hypertension. However, the mechanisms of increased sympathetic drive in hypertension remain unclear. We examined the tonic GABAergic inhibition in control of the excitability of paraventricular (PVN) presympathetic neurons in spontaneously hypertensive rats (SHR) and normotensive controls, including Sprague-Dawley (SD) and Wistar-Kyoto (WKY) rats. Whole cell patch-clamp recordings were performed on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla (RVLM) in brain slices. The basal firing rate of PVN neurons was significantly decreased in 13-wk-old SD and WKY rats but increased in 13-wk-old SHR, compared with their respective 6-wk-old controls. The GABA(A) antagonist bicuculline consistently increased the firing of PVN neurons in normotensive controls. Surprisingly, bicuculline either decreased the firing or had no effect in 59.3% of labeled cells in 13-wk-old SHR. In contrast, the GABA(B) antagonist CGP-55845 had no effect on the firing of PVN neurons in normotensive controls but significantly increased the firing of 75% of cells studied in 13-wk-old SHR. Furthermore, the evoked GABA(A) current decreased significantly in labeled PVN neurons of 13-wk-old SHR compared with that in normotensive controls. Both the frequency and amplitude of GABAergic spontaneously inhibitory postsynaptic currents were also reduced in 13-wk-old SHR. This study demonstrates an unexpected functional change in GABA(A) and GABA(B) receptors in regulation of the firing activity of PVN-RVLM neurons in SHR. This change in GABA(A) receptor function and GABAergic inputs to PVN output neurons may contribute to increased sympathetic outflow in hypertension.

Blockade of endogenous angiotensin-(1-7) in the hypothalamic paraventricular nucleus reduces renal sympathetic tone

In this study, we tested the hypothesis that angiotensin-(1-7) [Ang-(1-7)] acting in the neurons of paraventricular hypothalamic nucleus (PVN) contributes to the maintenance of sympathetic activity and blood pressure. For this purpose, the effects of microinjection of the A-779, the receptor Mas antagonist, into the PVN on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were evaluated. In rats anesthetized with urethane (1.2 to 1.4 g/kg IP), bilateral microinjections of A-779 (0.1 nmol) into the PVN resulted in a selective and significant decrease in RSNA (-26+/-6% versus -2+/-3% vehicle; saline 0.9%). The magnitude of the decrease in RSNA produced by A-779 was comparable to that observed after microinjection of muscimol (1 nmol; -26+/-4%), a powerful neuronal inhibitor. A higher dose of A-779 (1 nmol) caused a reduction in RSNA (-21+/-4%) that was comparable in magnitude to the reduction observed with the lower dose. When compared with vehicle solution, no significant changes in MAP or HR were observed with both doses of A-779 tested. A decrease in RSNA was also observed after microinjections into the PVN of the angiotensin II type 2 (AT2) receptor antagonist PD123319 (1 nmol; -18+/-4%). Microinjections of the AT1 antagonist losartan but not CV 11974 reduced MAP without changing RSNA. These results suggest that Ang-(1-7) Mas receptors and AT2 receptors in the PVN neurons play a role in mediating the tonic maintenance of RSNA.

Angiotensin peptides modulatory system: how is it implicated in the control of seizure susceptibility?

Accumulated studies support the concept that angiotensin peptides, ANG II, ANG III, and ANG IV act as neurotransmitters or neuromodulators in specific neuronal pathways in the brain stem, the hypothalamus, and the forebrain. They have been implicated in the regulation of several physiological processes, particularly in excitable brain structures that express high concentration of their receptors. With the help of pharmacological approaches it was shown that angiotensin peptides appear to be anticonvulsant in a variety of experimental seizure models. Thus, ANG II increases the threshold for pentylenetetrazol (PTZ)-, bicuculline-and picrotoxin-induced seizures in mice. It also attenuates the intensity of clonic seizures evoked by PTZ and 3-mercaptopropionic acid and is effective in the maximal electroshock test. Furthermore, ANG II, ANG III, and ANG IV protect against the clonic convulsions in the PTZ kindling model of epilepsy in mice. From the accumulated results it could be assumed that the angiotensin peptides appear to realize their effects acting directly on their receptors (AT(1), AT(2) and AT(4)) and through close interaction with different neurotransmitter/neuromodulator systems as dopamine (DA)-, gamma-aminobutyric acid (GABA)-and adenosine. This may contribute to a new potential use of angiotensin drugs either alone or in combination with other neuroprotective agents acting through the above mentioned systems, thus providing a more rational strategy for the treatment of neurodegenerative disorders such as epilepsy.