Angiotensin II stimulates spinally projecting paraventricular neurons through presynaptic disinhibition

Protein kinase CK2 contributes to diminished small conductance Ca2+-activated K+ channel activity of hypothalamic pre-sympathetic neurons in hypertension

Small conductance calcium-activated K(+) (SK) channels regulate neuronal excitability. However, little is known about changes in SK channel activity of pre-sympathetic neurons in the hypothalamic paraventricular nucleus (PVN) in essential hypertension. SK channels, calmodulin, and casein kinase II (CK2) form a molecular complex. Because CK2 is up-regulated in the PVN in spontaneously hypertensive rats (SHRs), we hypothesized that CK2 increases calmodulin phosphorylation and contributes to diminished SK channel activity in PVN pre-sympathetic neurons in SHRs. Perforated whole-cell recordings were performed on retrogradely labeled spinally projecting PVN neurons in Wistar-Kyoto (WKY) rats and SHRs. Blocking SK channels with apamin significantly increased the firing rate of PVN neurons in WKY rats but not in SHRs. CK2 inhibition restored the stimulatory effect of apamin on the firing activity of PVN neurons in SHRs. Furthermore, apamin-sensitive SK currents and depolarization-induced medium after-hyperpolarization potentials of PVN neurons were significantly larger in WKY rats than in SHRs. CK2 inhibition significantly increased the SK channel current and medium after-depolarization potential of PVN neurons in SHRs. In addition, CK2-mediated calmodulin phosphorylation level in the PVN was significantly higher in SHRs than in WKY rats. Although SK3 was detected in the PVN, its expression level did not differ significantly between SHRs and WKY rats. Our findings suggest that CK2-mediated calmodulin phosphorylation is increased and contributes to diminished SK channel function of PVN pre-sympathetic neurons in SHRs. This information advances our understanding of the mechanisms underlying hyperactivity of PVN pre-sympathetic neurons and increased sympathetic vasomotor tone in hypertension. Small conductance calcium-activated K(+) (SK) channels, calmodulin, and protein kinase CK2 form a molecular complex and regulate neuronal excitability. Our study suggests that augmented CK2 activity in hypertension can increase calmodulin (CaM) phosphorylation, which leads to diminished SK channel function in pre-sympathetic neurons. Diminished SK channel activity plays a role in hyperactivity of pre-sympathetic neurons in the hypothalamus in hypertension.

Overexpression of angiotensin-converting enzyme 2 attenuates tonically active glutamatergic input to the rostral ventrolateral medulla in hypertensive rats

The rostral ventrolateral medulla (RVLM) plays a key role in cardiovascular regulation. It has been reported that tonically active glutamatergic input to the RVLM is increased in hypertensive rats, whereas angiotensin-converting enzyme 2 (ACE2) in the brain has been suggested to be beneficial to hypertension. This study was designed to determine the effect of ACE2 gene transfer into the RVLM on tonically active glutamatergic input in spontaneously hypertensive rats (SHRs). Lentiviral particles containing enhanced green fluorescent protein (lenti-GFP) or ACE2 (lenti-ACE2) were injected bilaterally into the RVLM. Both protein expression and activity of ACE2 in the RVLM were increased in SHRs after overexpression of ACE2. A significant reduction in blood pressure and heart rate in SHRs was observed 6 wk after lenti-ACE2 injected into the RVLM. The concentration of glutamate in microdialysis fluid from the RVLM was significantly reduced by an average of 61% in SHRs with lenti-ACE2 compared with lenti-GFP. ACE2 overexpression significantly attenuated the decrease in blood pressure and renal sympathetic nerve activity evoked by bilateral injection of the glutamate receptor antagonist kynurenic acid (2.7 nmol in 100 nl) into the RVLM in SHRs. Therefore, we suggest that ACE2 overexpression in the RVLM attenuates the enhanced tonically active glutamatergic input in SHRs, which may be an important mechanism underlying the beneficial effect of central ACE2 to hypertension.

Direct evidence of intracrine angiotensin II signaling in neurons

The existence of a local renin-angiotensin system (RAS) in neurons was first postulated 40 years ago. Further studies indicated intraneuronal generation of ANG II. However, the function and signaling mechanisms of intraneuronal ANG II remained elusive. Since ANG II type 1 receptor (AT1R) is the major type of receptor mediating the effects of ANG II, we used intracellular microinjection and concurrent Ca(2+) and voltage imaging to examine the functionality of intracellular AT1R in neurons. We show that intracellular administration of ANG II produces a dose-dependent elevation of cytosolic Ca(2+) concentration ([Ca(2+)]i) in hypothalamic neurons that is sensitive to AT1R antagonism. Endolysosomal, but not Golgi apparatus, disruption prevents the effect of microinjected ANG II on [Ca(2+)]i. Additionally, the ANG II-induced Ca(2+) response is dependent on microautophagy and sensitive to inhibition of PLC or antagonism of inositol 1,4,5-trisphosphate receptors. Furthermore, intracellular application of ANG II produces AT1R-mediated depolarization of hypothalamic neurons, which is dependent on [Ca(2+)]i increase and on cation influx via transient receptor potential canonical channels. In summary, we provide evidence that intracellular ANG II activates endolysosomal AT1Rs in hypothalamic neurons. Our results point to the functionality of a novel intraneuronal angiotensinergic pathway, extending the current understanding of intracrine ANG II signaling.

Natriorexigenic effect of DAMGO is decreased by blocking AT1 receptors in the central nucleus of the amygdala

μ-Opioid receptor (μ-OR) activation with agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) in the central nucleus of the amygdala (CeA) induces sodium (0.3M NaCl) intake in rats. The purpose of this study was to examine the effects of pre-injections of losartan (AT1 angiotensin receptor antagonist) into the CeA on 0.3 M NaCl and water intake induced by DAMGO injected bilaterally in the same area in rats submitted to water deprivation-partial rehydration (WD-PR) and in rats treated with the diuretic furosemide (FURO) combined with a low dose of the angiotensin-converting enzyme inhibitor captopril (CAP) injected subcutaneously (FURO/CAP). Male Sprague-Dawley rats with stainless steel cannulas implanted bilaterally into the CeA were used. In WD-PR rats, bilateral injections of DAMGO (2 nmol in 0.5 μL) into the CeA induced 0.3 M NaCl and water intake, and pre-treatment with losartan (108 nmol in 0.5 μL) injected into the CeA reduced 0.3 M NaCl and water intake induced by DAMGO. In FURO/CAP rats, pre-treatment with losartan (108 nmol in 0.5 μL) injected into the CeA attenuated the increase in 0.3M NaCl and water intake induced by DAMGO (2 nmol in 0.5 μL) injected into the same site. The results suggest that the natriorexigenic effect of DAMGO injected into the CeA is facilitated by endogenous angiotensin II acting on AT1 receptors in the CeA, which drives rats to ingest large amounts of hypertonic NaCl.

Centrally mediated erectile dysfunction in rats with type 1 diabetes: role of angiotensin II and superoxide

INTRODUCTION

Erectile dysfunction is a serious complication of diabetes mellitus. Apart from the peripheral actions, central mechanisms are also responsible for penile erection.

AIM

This study aims to determine the contribution of angiotensin (ANG) II in the dysfunction of central N-methyl-D-aspartic acid (NMDA)- and nitric oxide (NO)-induced erectile responses in streptozotocin-induced type 1 diabetic (T1D) rats.

METHODS

Three weeks after streptozotocin injections, rats were randomly treated with the angiotensin-converting enzyme inhibitor-enalapril, or the ANG II type 1 receptor blocker, losartan, or the superoxide dismutase mimetic, tempol, or vehicle via chronic intracerebroventricular infusion by osmotic mini-pump for 2 weeks.

MAIN OUTCOME MEASURE

Central NMDA receptor stimulation or the administration of the NO donor, sodium nitroprusside (SNP)-induced penile erectile responses and concurrent behavioral responses were monitored in conscious rats.

RESULTS

Two weeks of enalapril, losartan, or tempol treatment significantly improved the erectile responses to central microinjection of both NMDA and SNP in the paraventricular nucleus (PVN) of conscious T1D rats (NMDA responses-T1D+enalapril: 1.7 ± 0.6, T1D+losartan: 2.0 ± 0.3, T1D+tempol: 2.0 ± 0.6 vs. T1D+vehicle: 0.6 ± 0.3 penile erections/rat in the first 20 minutes, P < 0.05; SNP responses-T1D+enalapril: 0.9 ± 0.3, T1D+losartan: 1.3 ± 0.3, T1D+tempol: 1.4 ± 0.4 vs. T1D+vehicle: 0.4 ± 0.2 penile erections/rat in the first 20 minutes, P < 0.05). Concurrent behavioral responses including yawning and stretching, induced by central NMDA and SNP microinjections, were also significantly increased in T1D rats after enalapril, losartan, or tempol treatments. Neuronal NO synthase expression within the PVN was also significantly increased, and superoxide production was reduced in T1D rats after these treatments.

CONCLUSIONS

These data strongly support the contention that enhanced ANG II mechanism/s within the PVN of T1D rats contributes to the dysfunction of central NMDA-induced erectile responses in T1D rats via stimulation of superoxide.

Regulation of Hypothalamic Presympathetic Neurons and Sympathetic Outflow by Group II Metabotropic Glutamate Receptors in Spontaneously Hypertensive Rats

Increased glutamatergic input in the hypothalamic paraventricular nucleus (PVN) plays an important role in the development of hypertension. Group II metabotropic glutamate receptors are expressed in the PVN, but their involvement in regulating synaptic transmission and sympathetic outflow in hypertension is unclear. Here, we show that the group II metabotropic glutamate receptors agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) produced a significantly greater reduction in the frequency of spontaneous and miniature excitatory postsynaptic currents and in the amplitude of electrically evoked excitatory postsynaptic currents in retrogradely labeled spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs) than in normotensive control rats. DCG-IV similarly decreased the frequency of GABAergic inhibitory postsynaptic currents of labeled PVN neurons in the 2 groups of rats. Strikingly, DCG-IV suppressed the firing of labeled PVN neurons only in SHRs. DCG-IV failed to inhibit the firing of PVN neurons of SHRs in the presence of ionotropic glutamate receptor antagonists. Lowering blood pressure with celiac ganglionectomy in SHRs normalized the DCG-IV effect on excitatory postsynaptic currents to the same level seen in control rats. Furthermore, microinjection of DCG-IV into the PVN significantly reduced blood pressure and sympathetic nerve activity in SHRs. Our findings provide new information that presynaptic group II metabotropic glutamate receptor activity at the glutamatergic terminals increases in the PVN in SHRs. Activation of group II metabotropic glutamate receptors in the PVN inhibits sympathetic vasomotor tone through attenuation of increased glutamatergic input and neuronal hyperactivity in SHRs.

Hypertonic saline resuscitation enhances blood pressure recovery and decreases organ injury following hemorrhage in acute alcohol intoxicated rodents

BACKGROUND

Acute alcohol intoxication (AAI) impairs the hemodynamic and arginine vasopressin (AVP) counter-regulation to hemorrhagic shock (HS) and lactated Ringer's solution (LR) fluid resuscitation (FR). The mechanism of AAI-induced suppression of AVP release in response to HS involves accentuated nitric oxide (NO) inhibitory tone. In contrast, AAI does not prevent AVP response to increased osmolarity produced by hypertonic saline (HTS) infusion. We hypothesized that FR with HTS during AAI would enhance AVP release by decreasing periventricular nucleus NO inhibitory tone, subsequently improving mean arterial blood pressure (MABP) and organ perfusion.

METHODS

Male Sprague-Dawley rats received a 15-hour alcohol infusion (2.5 g/kg + 0.3 g/kg/h) or dextrose (DEX) before HS (40 mm Hg × 60 minutes) and FR with HTS (7.5%, 4 ml/kg) or LR (2.4 × blood volume removed). Organ blood flow was determined, and brains were collected for NO content at 2 hours after FR.

RESULTS

HTS improved MABP recovery in AAI (109 vs. 80 mm Hg) and DEX (114 vs. 83 mm Hg) animals compared with LR. This was associated with higher (>60%) circulating AVP levels at 2 hours after FR compared with those detected in LR animals in both groups. Neither AAI alone nor HS in DEX animals resuscitated with LR altered organ blood flow. In AAI animals, HS and FR with LR reduced blood flow to the liver (72%), small intestine (65%), and large intestine (67%) compared with shams. FR with HTS improved liver (threefold) and small intestine (twofold) blood flow compared with LR in AAI-HS animals. The enhanced MABP response to HTS was prevented by pretreatment with a systemic AVP V1a receptor antagonist. HTS decreased periventricular nucleus NO content in both groups 2 hours after FR.

CONCLUSION

These results suggest that FR with HTS in AAI results in the removal of central NO inhibition of AVP, restoring AVP levels and improving MABP and organ perfusion in AAI-HS.

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

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

Neuroimmune communication in hypertension and obesity: a new therapeutic angle?

Hypertension is an epidemic health concern and a major risk factor for the development of cardiovascular disease. Although there are available treatment strategies for hypertension, numerous hypertensive patients do not have their clinical symptoms under control and it is imperative that new avenues to treat or prevent high blood pressure in these patients are developed. It is well established that increases in sympathetic nervous system (SNS) outflow and enhanced renin-angiotensin system (RAS) activity are common features of hypertension and various pathological conditions that predispose individuals to hypertension. More recently, hypertension has also become recognized as an immune condition and accumulating evidence suggests that interactions between the RAS, SNS and immune systems play a role in blood pressure regulation. This review summarizes what is known about the interconnections between the RAS, SNS and immune systems in the neural regulation of blood pressure. Based on the reviewed studies, a model for RAS/neuroimmune interactions during hypertension is proposed and the therapeutic potential of targeting RAS/neuroimmune interactions in hypertensive patients is discussed. Special emphasis is placed on the applicability of the proposed model to obesity-related hypertension.

The Prorenin and (Pro)renin Receptor: New Players in the Brain Renin-Angiotensin System?

It is well known that the brain renin-angiotensin (RAS) system plays an essential role in the development of hypertension, mainly through the modulation of autonomic activities and vasopressin release. However, how the brain synthesizes angiotensin (Ang) II has been a debate for decades, largely due to the low renin activity. This paper first describes the expression of the vasoconstrictive arm of RAS components in the brain as well as their physiological and pathophysiological significance. It then focus on the (pro)renin receptor (PRR), a newly discovered component of the RAS which has a high level in the brain. We review the role of prorenin and PRR in peripheral organs and emphasize the involvement of brain PRR in the pathogenesis of hypertension. Some future perspectives in PRR research are heighted with respect to novel therapeutic target for the treatment of hypertension and other cardiovascular diseases.

Central neuromodulatory pathways regulating sympathetic activity in hypertension

The classical neurotransmitters, glutamate and GABA, mediate fast (milliseconds) synaptic transmission and modulate its effectiveness through slow (seconds to minutes) signaling processes. Angiotensinergic pathways, from the lamina terminalis to the paraventricular nucleus (PVN)/supraoptic nucleus and rostral ventrolateral medulla (RVLM), are activated by stimuli such as circulating angiotensin type II (Ang II), cerebrospinal fluid (CSF) sodium ion concentration ([Na(+)]), and possibly plasma aldosterone, leading to sympathoexcitation, largely by decreasing GABA and increasing glutamate release. The aldosterone-endogenous ouabain (EO) pathway is a much slower neuromodulatory pathway. Aldosterone enhances EO release, and the latter increases chronic activity in angiotensinergic pathways by, e.g., increasing expression for Ang I receptor (AT(1)R) and NADPH oxidase subunits in the PVN. Blockade of this pathway does not affect the initial sympathoexcitatory and pressor responses but to a large extent, prevents chronic responses to CSF [Na(+)] or Ang II. Recruitment of these two neuromodulatory pathways allows the central nervous system (CNS) to shift gears to rapidly cause and sustain sympathetic hyperactivity in an efficient manner. Decreased GABA release, increased glutamate release, and enhanced AT(1)R activation in, e.g., the PVN and RVLM contribute to the elevated blood pressure in a number of hypertension models. In Dahl S rats and spontaneous hypertensive rats, high salt activates the CNS aldosterone-EO pathway, and the salt-induced hypertension can be prevented/reversed by specific CNS blockade of any of the steps in the cascade from aldosterone synthase to AT(1)R. Further studies are needed to advance our understanding of how and where in the brain these rapid, slow, and very slow CNS pathways are activated and interact in models of hypertension and other disease states associated with chronic sympathetic hyperactivity.

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.

Angiotensin-(1-7) inhibits neuronal activity of dorsolateral periaqueductal gray via a nitric oxide pathway

The midbrain periaqueductal gray (PAG) is a neural site for several physiological functions related to cardiovascular regulation, pain modulation and behavioral reactions. Recently, angiotensin-(1-7) [Ang-(1-7)] has been considered as an important biologically active component of the renin-angiotensin system in the CNS. The purpose of this study was to determine (1) existence of Ang-(1-7) receptor, Mas-R, within the dorsolateral PAG (dl-PAG), (2) the role for Ang-(1-7) in modulating activity of dl-PAG neurons, and (3) the mechanisms by which Ang-(1-7) plays a regulatory role. Western blot analysis showed that Mas-R appears within the dl-PAG. Whole cell patch-clamp recording demonstrated that the discharge rates of dl-PAG neurons were decreased from 4.35±0.32 Hz of control to 1.06±0.34 Hz (P<0.05, vs. control) by 100 nM of Ang-(1-7). With pretreatment of A-779, a Mas-R inhibitor, the discharge rate was 4.66±0.62 Hz (P>0.05, vs. control) during infusion of Ang-(1-7). Additionally, neuronal nitric oxide synthase (nNOS) was largely localized within the dl-PAG among the three isoforms. The effects of Ang-(1-7) on neuronal activity of the PAG were attenuated in the presence of S-methyl-L-thiocitrulline (SMTC), a nNOS inhibitor. The discharge rates were 4.21±0.39 Hz in control and 4.09±0⋅47 Hz (P>0.05, vs. control) when Ang-(1-7) was applied with pretreatment of SMTC. Those findings suggest that Ang-(1-7) plays an inhibitory role in the dl-PAG via a NO dependent signaling pathway. This offers the basis for the physiological role of Ang-(1-7) and Mas R in the regulation of various functions in the CNS.

Angiotensin-(1-12) in the rostral ventrolateral medullary pressor area of the rat elicits sympathoexcitatory responses

The rostral ventrolateral medullary pressor area (RVLM) is known to be critical in the regulation of cardiovascular function. In this study, it was hypothesized that the RVLM may be one of the sites of cardiovascular actions of a newly discovered angiotensin, angiotensin-(1-12) [Ang-(1-12)]. Experiments were carried out in urethane-anaesthetized, artificially ventilated, adult male Wistar rats. The RVLM was identified by microinjections of L-glutamate (5 mM). The volume of all microinjections into the RVLM was 100 nl. Microinjections of Ang-(1-12) (0.1-1.0 mM) into the RVLM elicited increases in mean arterial pressure and heart rate. Maximal cardiovascular responses were elicited by 0.5 mM Ang-(1-12); this concentration was used in the other experiments described. Microinjections of Ang-(1-12) increased greater splanchnic nerve activity. The tachycardic responses to Ang-(1-12) were not altered by bilateral vagotomy. The cardiovascular responses elicited by Ang-(1-12) were attenuated by microinjections of an angiotensin II type 1 receptor (AT(1)R) antagonist (losartan), but not an AT(2)R antagonist (PD123319), into the RVLM. Combined inhibition of angiotensin-converting enzyme and chymase in the RVLM abolished Ang-(1-12)-induced responses. Angiotensin-(1-12)-immunoreactive cells were present in the RVLM. Angiotensin II type 1 receptors and phenylethanolamine-N-methyl-transferase were present in the RVLM neurons retrogradely labelled by microinjections of Fluoro-Gold into the intermediolateral cell column of the thoracic spinal cord. Angiotensin-(1-12)-containing neurons in the hypothalamic paraventricular nucleus did not project to the RVLM. These results indicated that: (1) microinjections of Ang-(1-12) into the RVLM elicited increases in mean arterial pressure, heart rate and greater splanchnic nerve activity; (2) both angiotensin-converting enzyme and chymase were needed to convert Ang-(1-12) into angiotensin II; and (3) AT(1)Rs, but not AT(2)Rs, in the RVLM mediated the Ang-(1-12)-induced responses.

Casein kinase 2-mediated synaptic GluN2A up-regulation increases N-methyl-D-aspartate receptor activity and excitability of hypothalamic neurons in hypertension

Increased glutamatergic input, particularly N-methyl-D-aspartate receptor (NMDAR) activity, in the paraventricular nucleus (PVN) of the hypothalamus is closely associated with high sympathetic outflow in essential hypertension. The molecular mechanisms underlying augmented NMDAR activity in hypertension are unclear. GluN2 subunit composition at the synaptic site critically determines NMDAR functional properties. Here, we found that evoked NMDAR-excitatory postsynaptic currents (EPSCs) of retrogradely labeled spinally projecting PVN neurons displayed a larger amplitude and shorter decay time in spontaneously hypertensive rats (SHRs) than in Wistar-Kyoto (WKY) rats. Blocking GluN2B caused a smaller decrease in NMDAR-EPSCs of PVN neurons in SHRs than in WKY rats. In contrast, GluN2A blockade resulted in a larger reduction in evoked NMDAR-EPSCs and puff NMDA-elicited currents of PVN neurons in SHRs than in WKY rats. Blocking presynaptic GluN2A, but not GluN2B, significantly reduced the frequency of miniature EPSCs and the firing activity of PVN neurons in SHRs. The mRNA and total protein levels of GluN2A and GluN2B in the PVN were greater in SHRs than in WKY rats. Furthermore, the GluN2B Ser(1480) phosphorylation level and the synaptosomal GluN2A protein level in the PVN were significantly higher in SHRs than in WKY rats. Inhibition of protein kinase CK2 normalized the GluN2B Ser(1480) phosphorylation level and the contribution of GluN2A to NMDAR-EPSCs and miniature EPSCs of PVN neurons in SHRs. Collectively, our findings suggest that CK2-mediated GluN2B phosphorylation contributes to increased synaptic GluN2A, which potentiates pre- and postsynaptic NMDAR activity and the excitability of PVN presympathetic neurons in hypertension.

Renin-Angiotensin system hyperactivation can induce inflammation and retinal neural dysfunction

The renin-angiotensin system (RAS) is a hormone system that has been classically known as a blood pressure regulator but is becoming well recognized as a proinflammatory mediator. In many diverse tissues, RAS pathway elements are also produced intrinsically, making it possible for tissues to respond more dynamically to systemic or local cues. While RAS is important for controlling normal inflammatory responses, hyperactivation of the pathway can cause neural dysfunction by inducing accelerated degradation of some neuronal proteins such as synaptophysin and by activating pathological glial responses. Chronic inflammation and oxidative stress are risk factors for high incidence vision-threatening diseases such as diabetic retinopathy (DR), age-related macular degeneration (AMD), and glaucoma. In fact, increasing evidence suggests that RAS inhibition may actually prevent progression of various ocular diseases including uveitis, DR, AMD, and glaucoma. Therefore, RAS inhibition may be a promising therapeutic approach to fine-tune inflammatory responses and to prevent or treat certain ocular and neurodegenerative diseases.

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.

Nitric oxide inhibits the expression of AT1 receptors in neurons

We have previously observed an increased of angiotensin II (ANG II) type 1 receptor (AT(1)R) with enhanced AT(1)R-mediated sympathetic outflow and concomitant downregulation of neuronal nitric oxide (NO) synthase (nNOS) with reduced NO-mediated inhibition from the paraventricular nucleus (PVN) in rats with heart failure. To test the hypothesis that NO exerts an inhibitory effect on AT(1)R expression in the PVN, we used primary cultured hypothalamic cells of neonatal rats and neuronal cell line NG108-15 as in vitro models. In hypothalamic primary culture, NO donor sodium nitroprusside (SNP) induced dose-dependent decreases in mRNA and protein of AT(1)R (10(-5) M SNP, AT(1)R protein was 10 ± 2% of control level) while NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) induced dose-dependent increases in mRNA and protein levels of AT(1)R (10(-5) M l-NMMA, AT(1)R protein was 148 ± 8% of control level). Similar effects of SNP and l-NMMA on AT(1)R expression were also observed in NG108-15 cell line (10(-6) M SNP, AT(1)R protein was 30 ± 4% of control level while at the dose of 10(-6) M l-NMMA, AT(1)R protein was 171 ± 15% of the control level). Specific inhibition of nNOS, using antisense, caused an increase in AT(1)R expression while overexpression of nNOS, using adenoviral gene transfer (Ad.nNOS), caused an inhibition of AT(1)R expression in NG108 cells. Antisense nNOS transfection augmented the increase while Ad.nNOS infection blunted the increase in intracellular calcium concentration in response to ANG II treatment in NG108 cells. In addition, downregulation of AT(1)R mRNA as well as protein level in neuronal cell line in response to S-nitroso-N-acetyl pencillamine (SNAP) treatment was blocked by protein kinase G (PKG) inhibitor, while the peroxynitrite scavenger deforxamine had no effect. These results suggest that NO acts as an inhibitory regulator of AT(1)R expression and the activation of PKG is the required step in the regulation of AT(1)R gene expression via cGMP-dependent signaling pathway.

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.

Cardiovascular actions of angiotensin-(1-12) in the hypothalamic paraventricular nucleus of the rat are mediated via angiotensin II

The role of the hypothalamic paraventricular nucleus (PVN) in cardiovascular regulation is well established. In this study, it was hypothesized that the PVN may be one of the sites of cardiovascular actions of a newly discovered angiotensin, angiotensin-(1-12). Experiments were carried out in urethane-anaesthetized, artificially ventilated, adult male Wistar rats. The PVN was identified by microinjections of NMDA (10 mm). Microinjections (50 nl) of angiotensin-(1-12) (1 mm) into the PVN elicited increases in mean arterial pressure, heart rate and renal sympathetic nerve activity. The tachycardic responses to angiotensin-(1-12) were attenuated by bilateral vagotomy. The cardiovascular responses elicited by angiotensin-(1-12) were attenuated by microinjections of an angiotensin II type 1 receptor (AT(1)R) antagonist (losartan), but not an angiotensin II type 1 receptor (AT(2)R) antagonist (PD123319), into the PVN. Combined inhibition of angiotensin-converting enzyme and chymase in the PVN abolished angiotensin-(1-12)-induced responses. Angiotensin-(1-12)-immunoreactive cells and fibres were more numerous in the middle and caudal regions of the PVN. Angiotensin-(1-12) was present in many, but not all, vasopressinergic PVN cells. This peptide was also present in some non-vasopressinergic PVN cells, but not in oxytocin-containing PVN cells. These results can be summarized as follows: (1) microinjections of angiotensin-(1-12) into the PVN elicited increases in mean arterial pressure, heart rate and renal sympathetic nerve activity; (2) heart rate responses were mediated via both sympathetic and vagus nerves; (3) both angiotensin-converting enzyme and chymase were needed to convert angiotensin-(1-12) to angiotensin II in the PVN; and (4) AT(1)Rs, but not AT(2)Rs, in the PVN mediated angiotensin-(1-12)-induced responses. It was concluded that the cardiovascular actions of angiotensin-(1-12) in the PVN are mediated via its conversion to angiotensin II.

Neural degeneration in the retina of the streptozotocin-induced type 1 diabetes model

Diabetic retinopathy, a vision-threatening disease, has been regarded as a vascular disorder. However, impaired oscillatory potentials (OPs) in the electroretinogram (ERG) and visual dysfunction are recorded before severe vascular lesions appear. Here, we review the molecular mechanisms underlying the retinal neural degeneration observed in the streptozotocin-(STZ-) induced type 1 diabetes model. The renin-angiotensin system (RAS) and reactive oxygen species (ROS) both cause OP impairment and reduced levels of synaptophysin, a synaptic vesicle protein for neurotransmitter release, most likely through excessive protein degradation by the ubiquitin-proteasome system. ROS also decrease brain-derived neurotrophic factor (BDNF) and inner retinal neuronal cells. The influence of both RAS and ROS on synaptophysin suggests that RAS-ROS crosstalk occurs in the diabetic retina. Therefore, suppressors of RAS or ROS, such as angiotensin II type 1 receptor blockers or the antioxidant lutein, respectively, are potential candidates for neuroprotective and preventive therapies to improve the visual prognosis.

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.

Dehydration increases sodium-dependent glutamate uptake by hypothalamic paraventricular nucleus synaptosomes

OBJECTIVES

The purpose of this study was to identify and characterize Na+-dependent, high affinity glutamate transporter (GLUT) activity in the hypothalamic paraventricular nucleus (PVN) and to compare GLUT activity in PVN of euhydrated versus water-deprived rats.

METHODS

Sprague-Dawley rats were deprived of water for two days before sacrifice. Control rats received water ad libitum. After sacrifice, PVN and cerebrum were removed and synaptosomes were prepared using standard techniques. Glutamate uptake was measured using [3H]-glutamate as substrate, physiological buffer, approximately 100 µg of synaptosomal tissue per assay and a Brandel cell harvester.

RESULTS

Glutamate uptake was saturable in PVN synaptosomes from euhydrated, control rats with a Vmax of 541 ± 22 pmol/min-mg protein (SEM) and Km of 17.6 ± 3.8 µM (SEM). In contrast, Vmax of glutamate uptake was 808 ± 58 pmol/min-mg protein in PVN of rats deprived of water for 2 days. This was significantly higher than controls (p<0.001). Km was 21.2 ± 7.3 µM and not significantly different from controls (NS).

CONCLUSIONS

Our results suggest that water deprivation of rats results in significantly higher synaptosomal glutamate uptake in PVN. Although the exact mechanism is unknown, increased transcription of the GLUT gene and/or increased cell surface expression of GLUT may contribute to the observed increase of glutamate uptake in dehydrated rats. Increased glutamate uptake may serve to restrict dehydration-induced activation of PVN efferent pathways specifically involved in release of neurohypophysial hormones and activation of sympathetic outflow that operate to maintain body fluid balance and cardiovascular function.

Enhanced angiotensin II-mediated central sympathoexcitation in streptozotocin-induced diabetes: role of superoxide anion

Studies have shown that the superoxide mechanism is involved in angiotensin II (ANG II) signaling in the central nervous system. We hypothesized that ANG II activates sympathetic outflow by stimulation of superoxide anion in the paraventricular nucleus (PVN) of streptozotocin (STZ)-induced diabetic rats. In α-chloralose- and urethane-anesthetized rats, microinjection of ANG II into the PVN (50, 100, and 200 pmol) produced dose-dependent increases in renal sympathetic nerve activity (RSNA), arterial pressure (AP), and heart rate (HR) in control and STZ-induced diabetic rats. There was a potentiation of the increase in RSNA (35.0 ± 5.0 vs. 23.0 ± 4.3%, P < 0.05), AP, and HR due to ANG II type I (AT(1)) receptor activation in diabetic rats compared with control rats. Blocking endogenous AT(1) receptors within the PVN with AT(1) receptor antagonist losartan produced significantly greater decreases in RSNA, AP, and HR in diabetic rats compared with control rats. Concomitantly, there were significant increases in mRNA and protein expression of AT(1) receptor with increased superoxide levels and expression of NAD(P)H oxidase subunits p22(phox), p47(phox), and p67(phox) in the PVN of rats with diabetes. Pretreatment with losartan (10 mg·kg(-1)·day(-1) in drinking water for 3 wk) significantly reduced protein expression of NAD(P)H oxidase subunits (p22(phox) and p47(phox)) in the PVN of diabetic rats. Pretreatment with adenoviral vector-mediated overexpression of human cytoplasmic superoxide dismutase (AdCuZnSOD) within the PVN attenuated the increased central responses to ANG II in diabetes (RSNA: 20.4 ± 0.7 vs. 27.7 ± 2.1%, n = 6, P < 0.05). These data support the concept that superoxide anion contributes to an enhanced ANG II-mediated signaling in the PVN involved with the exaggerated sympathoexcitation in diabetes.

Regulation of posttranscriptional modification as a possible therapeutic approach for retinal neuroprotection

Understanding pathogenesis at the molecular level is the first step toward developing new therapeutic approaches. Here, we review the molecular mechanisms of visual dysfunction in two common diseases, innate chorioretinal inflammation and diabetic retinopathy, and the role of the ubiquitin-proteasome system (UPS) in both processes. In innate chorioretinal inflammation, interleukin-6 family ligands induce STAT3 activation in photoreceptors, which causes UPS-mediated excessive degradation of the visual substance, rhodopsin. In diabetic retinopathy, angiotensin II type 1 receptor (AT1R) signaling activates ERK in the inner layers of the retina, causing UPS-mediated excessive degradation of the synaptic vesicle protein, synaptophysin. This latter effect may decrease synaptic activity, in turn adversely affecting neuronal survival. Both mechanisms involve increased UPS activity and the subsequent excessive degradation of a protein required for visual function. Finally, we review the therapeutic potential of regulating the UPS to protect tissue function, citing examples from clinical applications in other medical fields.

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

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

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.

The renin angiotensin system and the metabolic syndrome

The renin angiotensin system (RAS; most well-known for its critical roles in the regulation of cardiovascular function and hydromineral balance) has regained the spotlight for its potential roles in various aspects of the metabolic syndrome. It may serve as a causal link among obesity and several co-morbidities. Drugs that reduce the synthesis or action of angiotensin-II (A-II; the primary effector peptide of the RAS) have been used to treat hypertension for decades and, more recently, clinical trials have determined the utility of these pharmacological agents to prevent insulin resistance. Moreover, there is evidence that the RAS contributes to body weight regulation by acting in various tissues. This review summarizes what is known of the actions of the RAS in the brain and throughout the body to influence various metabolic disorders. Special emphasis is given to the role of the RAS in body weight regulation. 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.

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

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

Protective actions of estrogen on angiotensin II-induced hypertension: role of central nitric oxide

The present study tested the hypotheses that 1) nitric oxide (NO) is involved in attenuated responses to ANG II in female mice, and 2) there is differential expression of neuronal NO synthase (nNOS) in the subfornical organ (SFO) and paraventricular nucleus (PVN) in response to systemic infusions of ANG II in males vs. females. Aortic blood pressure (BP) was measured in conscious mice with telemetry implants. N(G)-nitro-l-arginine methyl ester (l-NAME; 100 microg x kg(.-1)day(-1)), an inhibitor of NOS, was administrated into the lateral cerebral ventricle for 14 days before and during ANG II pump implantation. Central infusion of l-NAME augmented the pressor effects of systemic ANG II in females (Delta21.5 + or - 2.2 vs. Delta9.2 + or - 1.5 mmHg) but not in males (Delta29.4 + or - 2.5 vs. Delta30.1 + or - 2.5 mmHg). Central administration of N(5)-(1-imino-3-butenyl)-l-ornithine (l-VNIO), a selective nNOS inhibitor, also significantly potentiated the increase in BP induced by ANG II in females (Delta17.5 + or - 3.2 vs. Delta9.2 + or - 1.5 mmHg). In gonadectomized mice, central l-NAME infusion did not affect the pressor response to ANG II in either males or females. Ganglionic blockade after ANG II infusion resulted in a greater reduction in BP in central l-NAME- or l-VNIO-treated females compared with control females. Western blot analysis of nNOS protein expression indicated that levels were approximately 12-fold higher in both the SFO and PVN of intact females compared with those in intact males. Seven days of ANG II treatment resulted in a further increase in nNOS protein expression only in intact females (PVN, to approximately 51-fold). Immunohistochemical studies revealed colocalization of nNOS and estrogen receptors in the SFO and PVN. These results suggest that NO attenuates the increase in BP induced by ANG II through reduced sympathetic outflow in females and that increased nNOS protein expression associated with the presence of female sex hormones plays a protective role against ANG II-induced hypertension in female mice.

Endogenous angiotensin II facilitates GABAergic neurotransmission afferent to the Na+-responsive neurons of the rat median preoptic nucleus

The median preoptic nucleus (MnPO) is densely innervated by efferent projections from the subfornical organ (SFO) and, therefore, is an important relay for the peripheral chemosensory and humoral information (osmolality and serum levels ANG II). In this context, controlling the excitability of MnPO neuronal populations is a major determinant of body fluid homeostasis and cardiovascular regulation. Using a brain slice preparation and patch-clamp recordings, our study sought to determine whether endogenous ANG II modulates the strength of the SFO-derived GABAergic inputs to the MnPO. Our results showed that the amplitude of the inhibitory postsynaptic currents (IPSCs) were progressively reduced by 44 ± 2.3% by (Sar1, Ile8)-ANG II, a competitive ANG type 1 receptor (AT1R) antagonist. Similarly, losartan, a nonpeptidergic AT1R antagonist decreased the IPSC amplitude by 40.4 ± 5.6%. The facilitating effect of endogenous ANG II on the GABAergic input to the MnPO was not attributed to a change in GABA release probability and was mimicked by exogenous ANG II, which potentiated the amplitude of the muscimol-activated GABAA/Cl− current by 53.1 ± 11.4%. These results demonstrate a postsynaptic locus of action of ANG II. Further analysis reveals that ANG II did not affect the reversal potential of the synaptic inhibitory response, thus privileging a cross talk between postsynaptic AT1 and GABAA receptors. Interestingly, facilitation of GABAergic neurotransmission by endogenous ANG II was specific to neurons responding to changes in the ambient Na+ level. This finding, combined with the ANG II-mediated depolarization of non-Na+-responsive neurons reveals the dual actions of ANG II to modulate the excitability of MnPO neurons.

Enhanced angiotensin-mediated excitation of renal sympathetic nerve activity within the paraventricular nucleus of anesthetized rats with heart failure

Chronic heart failure (HF) is characterized by increased sympathetic drive. Enhanced angiotensin II (ANG II) activity may contribute to the increased sympathoexcitation under HF condition. The present study examined sympathoexcitation by 1) the effects of ANG II in the paraventricular nucleus (PVN) on renal sympathetic nerve activity (RSNA), and 2) the altered ANG II type 1 (AT(1)) receptor expression during HF. Left coronary artery ligation was used to induce HF. In the anesthetized Sprague-Dawley rats, microinjection of ANG II (0.05-1 nmol) into the PVN increased RSNA, mean arterial pressure (MAP), and heart rate (HR) in both sham-operated and HF rats. The responses of RSNA and HR were significantly enhanced in rats with HF compared with sham rats (RSNA: 64 +/- 8% vs. 33 +/- 4%, P < 0.05). Microinjection of AT(1) receptor antagonist losartan into the PVN produced a decrease of RSNA, MAP, and HR in both sham and HF rats. The RSNA and HR responses to losartan in HF rats were significantly greater (RSNA: -25 +/- 4% vs. -13 +/- 1%, P < 0.05). Using RT-PCR and Western blot analysis, we found that there were significant increases in the AT(1) receptor mRNA (Delta186 +/- 39%) and protein levels (Delta88 +/- 20%) in the PVN of rats with HF (P < 0.05). The immunofluorescence of AT(1) receptors was significantly higher in the PVN of rats with HF. These data support the conclusion that an increased angiotensinergic activity on sympathetic regulation, due to the upregulation of ANG II AT(1) receptors within the PVN, may contribute to the elevated sympathoexcitation that is observed during HF.

Phosphate-activated glutaminase-containing neurons in the rat paraventricular nucleus express angiotensin type 1 receptors

The centrally mediated cardiovascular regulatory actions of angiotensin II in normal and hypertensive rats include angiotensin II type 1 receptor (AT1R)-mediated actions at the paraventricular nucleus (PVN) of the hypothalamus. Because the PVN consists of multiple neuronal populations, it is important to understand which neuronal types in the PVN are influenced by angiotensin II. Here we have developed a viral vector (Adeno-associated vector 2 [AAV2]-PAG-eGFP [PAG; phosphate-activated glutaminase promoter]) to drive expression of green fluorescent protein (GFP) primarily within glutamate neurons. At 10 to 14 days after bilateral microinjection (200 nL per side; 1.2 x10(12) genome copies) of AAV2-PAG-eGFP into adult Sprague-Dawley rat PVN, animals were euthanized and brains removed and used for isolation and culture of PVN neurons. Fluorescence microscopy and immunostaining using neuron and PAG-specific antibodies revealed the presence of GFP-containing glutamatergic neurons in these PVN cultures. Whole-cell patch-clamp recordings demonstrated that angiotensin II (100 nmol/L) produced a 16% decrease in delayed rectifier potassium current in approximately 50% of the GFP-containing neurons, an effect that was abolished by the AT1R antagonist losartan (1 mumol/L). Consistently, 9 of 28 GFP/PAG-expressing neurons contained AT1R mRNA, as indicated by single-cell RT-PCR. Furthermore, specific GFP/PAG-positive neurons in the PVN that project to the rostral ventrolateral medulla of the brain stem express immunoreactive AT1R. In conclusion, we have demonstrated the presence of functional AT1R on PAG-positive (largely glutamate) neurons within rat PVN, certain of which project to the rostral ventrolateral medulla.

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.

Neurochemistry of the paraventricular nucleus of the hypothalamus: implications for cardiovascular regulation

The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic and endocrine homeostasis. The PVN integrates specific afferent stimuli to produce an appropriate differential sympathetic output. The neural circuitry and some of the neurochemical substrates within this circuitry are discussed. The PVN has at least three neural circuits to alter sympathetic activity and cardiovascular regulation. These pathways innervate the vasculature and organs such as the heart, kidney and adrenal medulla. The basal level of sympathetic tone at any given time is dependent upon excitatory and inhibitory inputs. Under normal circumstances the sympathetic nervous system is tonically inhibited. This inhibition is dependent upon GABA and nitric oxide such that nitric oxide potentiates local GABAergic synaptic inputs onto the neurones in the PVN. Excitatory neurotransmitters such as glutamate and angiotensin II modify the tonic inhibitory activity. The neurotransmitters oxytocin, vasopressin and dopamine have been shown to affect cardiovascular function. These neurotransmitters are found in neurones of the PVN and within the spinal cord. Oxytocin and vasopressin terminal fibres are closely associated with sympathetic preganglionic neurones (SPNs). Sympathetic preganglionic neurones have been shown to express receptors for oxytocin, vasopressin and dopamine. Oxytocin causes cardioacceleratory and pressor effects that are greatest in the upper thoracic cord while vasopressin cause these effects but more significant in the lower thoracic cord. Dopaminergic effects on the cardiovascular system include inhibitory or excitatory actions attributed to a direct PVN influence or via interneuronal connections to sympathetic preganglionic neurones.

Angiotensin II inhibits GABAergic synaptic transmission in dorsolateral periaqueductal gray neurons

The purpose of this study was to determine the role of angiotensin II (Ang II) in modulating inhibitory and excitatory synaptic inputs to the dorsolateral periaqueductal gray (dl-PAG). The whole cell voltage-clamp recording was performed to examine inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs) of the dl-PAG neurons. Ang II, at the concentration of 2microM, decreased the frequency of miniature IPSCs from 0.83+/-0.02 to 0.45+/-0.03Hz (P<0.05) in 10 tested neurons. This did not significantly affect the amplitude and decay time constant. The effect of Ang II on miniature IPSCs was blocked by the prior application of Ang II AT1 receptor antagonist losartan, but not by AT2 receptor antagonist PD123319. Additionally, Ang II decreased the amplitude of evoked IPSCs from 148+/-15 to 89+/-7pA (P<0.05), and increased the paired-pulse ratio from 96+/-5% to 125+/-7% (P<0.05) in eight tested neurons. In contrast, Ang II had no distinct effects on the EPSCs. Our data suggest that Ang II inhibits GABAergic synaptic inputs to the dl-PAG through activation of presynaptic AT1 receptors.

Sex differences and central protective effect of 17beta-estradiol in the development of aldosterone/NaCl-induced hypertension

The present study tested the hypotheses that male and female rats respond differently to subcutaneous infusions of aldosterone (Aldo; 1.8 microg.kg(-1).h(-1), 1% NaCl to drink; 28 days) and that central estrogen plays a protective role against the development of hypertension. In rats with blood pressure (BP) and heart rate (HR) measured by Data Sciences International telemetry, chronic Aldo/NaCl treatment induced a greater increase in BP in males (Delta25.4 +/- 2.4 mmHg) than in females (Delta7.1 +/- 2.2 mmHg). Gonadectomy augmented Aldo/NaCl-induced hypertension in females (Delta18.2 +/- 2.0 mmHg) but had no effect in males (Delta23.1 +/- 2.9 mmHg). Immunohistochemistry for Fra-like activity was higher in the paraventricular nucleus of intact males, castrated males, and ovariectomized (OVX) females compared with intact females after 28 days of Aldo/NaCl treatment. In intact males, central 17beta-estradiol (E(2)) inhibited the Aldo/NaCl increase in BP (Delta10.5 +/- 0.8) compared with that in central vehicle plus systemic Aldo/NaCl (Delta26.1 +/- 2.5 mmHg) rats. Combined administration of E(2) and estrogen receptor antagonist ICI182780 (ICI) blocked the protective effect of E(2) (Delta23.2 +/- 2.4 mmHg). In intact females central, but not peripheral, infusions of ICI augmented the Aldo/NaCl (Delta20.4 +/- 1.8 mmHg) BP increase. Finally, ganglionic blockade after Aldo infusions resulted in a smaller reduction in BP in intact females (-23.9 +/- 2.5 mmHg) and in central estrogen-treated males (-30.2 +/- 1.0 mmHg) compared with other groups (intact males, -39.3 +/- 3.4; castrated males, -41.8 +/- 1.9; intact males with central E(2) + ICI, -42.3 +/- 2.1; OVX females, -40.3 +/- 3.3; and intact females with central ICI, -39.1 +/- 1.3 mmHg). Chronic Aldo infusion produced increases in NaCl intake and decreases in HR that were both similar in all groups. Taken together, the results indicate that central estrogen plays a protective role in the development of Aldo/NaCl-induced hypertension and that this may result from reduced sympathetic outflow.

Mechanisms in the PVN mediating local and central sodium-induced hypertension in Wistar rats

Sympathoexcitatory and hypertensive responses to central infusion of Na(+)-rich artificial cerebrospinal fluid (aCSF) are enhanced by aldosterone and mediated by mineralocorticoid receptors (MRs) and benzamil-blockable Na(+) influx, leading to "ouabain" release and ANG II type 1 (AT(1)) receptor stimulation. The present study evaluated the functional role of these mechanisms in the paraventricular nucleus (PVN). In conscious Wistar rats, Na(+)-rich aCSF was infused either directly into the PVN or intracerebroventricularly preceded by aldosterone and blockers. Infusion of Na(+)-rich aCSF in the PVN caused gradual increases in blood pressure (BP) and heart rate (HR). Aldosterone and a subpressor dose of ouabain in the PVN alone did not affect BP and HR but enhanced responses to Na(+). Eplerenone, benzamil, and "ouabain"-binding Fab fragments only blocked the enhancement by aldosterone, whereas losartan blocked all responses to Na(+)-rich aCSF in the PVN. Increases in BP and HR by intracerebroventricular infusion of Na(+)-rich aCSF were enhanced by aldosterone infused intracerebroventricularly, but not in the PVN. Telmisartan in the PVN again blocked all responses. In contrast, both eplerenone and benzamil in the PVN did not change the pressor responses to intracerebroventricular infusion of aldosterone and Na(+)-rich aCSF. These findings indicate that AT(1) receptors in the PVN mediate the responses to Na(+)-rich aCSF and their enhancement by aldosterone, both locally in the PVN or in the general CSF. MRs, benzamil-blockable Na(+) channels or transporters, and "ouabain" can be functionally active in the PVN, but in Wistar rats appear not to contribute to the pressor responses to short-term increases in CSF [Na(+)].

The paraventricular nucleus of the hypothalamus - a potential target for integrative treatment of autonomic dysfunction

BACKGROUND

The paraventricular nucleus of the hypothalamus (PVN) has emerged as one of the most important autonomic control centers in the brain, with neurons playing essential roles in controlling stress, metabolism, growth, reproduction, immune and other more traditional autonomic functions (gastrointestinal, renal and cardiovascular).

OBJECTIVES

Traditionally the PVN was viewed as a nucleus in which afferent inputs from other regions were faithfully translated into changes in single specific outputs, whether neuroendocrine or autonomic. Here we present data which suggest that the PVN plays significant and essential roles in integrating multiple sources of afferent input and sculpting an integrated autonomic output by concurrently modifying the excitability of multiple output pathways. In addition, we highlight recent work that suggests that dysfunction of such intranuclear integrative circuitry contributes to the pathology of conditions such as hypertension and congestive heart failure.

CONCLUSIONS

This review highlights data showing that individual afferent inputs (subfornical organ), signaling molecules (orexins, adiponectin), and interneurons (glutamate/GABA), all have the potential to influence (and thus coordinate) multiple PVN output pathways. We also highlight recent studies showing that modifications in this integrated circuitry may play significant roles in the pathology of diseases such as congestive heart failure and hypertension.

Angiotensin II type 1 receptor signaling contributes to synaptophysin degradation and neuronal dysfunction in the diabetic retina

OBJECTIVE

Pathogenic mechanisms underlying diabetes-induced retinal dysfunction are not fully understood. The aim of the present study was to show the relationship of the renin-angiotensin system (RAS) with the synaptic vesicle protein synaptophysin and neuronal activity in the diabetic retina.

RESEARCH DESIGN AND METHODS

C57BL/6 mice with streptozotocin-induced diabetes were treated with the angiotensin II type 1 receptor (AT1R) blocker telimsartan or valsartan, and retinal function was analyzed by electroretinography. Retinal production of the RAS components and phosphorylation of ERK (extracellular-signal regulated kinase) were examined by immunoblotting. Retinal mRNA and protein levels of synaptophysin were measured by quantitative RT-PCR and immunoblot analyses, respectively. In vitro, synaptophysin levels were also evaluated using angiotensin II-stimulated PC12D neuronal cells cultured with or without the inhibition of ERK signaling or the ubiquitin-proteasome system (UPS).

RESULTS

Induction of diabetes led to a significant increase in retinal production of angiotensin II and AT1R together with ERK activation in the downstream of AT1R. AT1R blockade significantly reversed diabetes-induced electroretinography changes and reduction of synaptophysin protein, but not mRNA, levels in the diabetic retina. In agreement with the AT1R-mediated posttranscriptional downregulation of synaptophysin in vivo, in vitro application of angiotensin II to PC12D neuronal cells caused the UPS-mediated degradation of synaptophysin protein via AT1R, which proved to be induced by ERK activation.

CONCLUSIONS

These data indicate the first molecular evidence of the RAS-induced synaptophysin degradation and neuronal dysfunction in the diabetic retina, suggesting the possibility of the AT1R blockade as a novel neuroprotective treatment for diabetic retinopathy.

Angiotensin II regulates the activity of mouse suprachiasmatic nuclei neurons

Neuropeptide signaling plays key roles in coordinating cellular activity within the suprachiasmatic nuclei (SCN), site of the master circadian oscillator in mammals. The neuropeptide angiotensin II (ANGII) and its cognate receptor AT1, are both expressed by SCN cells, but unlike other SCN neurochemicals, very little is known about the cellular actions of ANGII within this circadian clock. We used multi-electrode, multiunit, extracellular electrophysiology, coupled with whole-cell voltage and current clamp techniques to investigate the actions of ANGII in mouse SCN slices. ANGII (0.001-10 microM) dose dependently stimulated and inhibited extracellularly recorded neuronal discharge in many SCN neurons ( approximately 60%). Both actions were blocked by pre-treatment with the AT1 receptor antagonist ZD7155 (0.03 microM), while suppressions but not activations were prevented by pre-treatment with the GABA A receptor antagonist bicuculline (20 microM). AT1 receptor blockade itself suppressed discharge in a subset ( approximately 30%) of SCN neurons, and this action was not blocked by bicuculline. In voltage-clamped SCN neurons (-70 mV), AT1 receptor activation dose-dependently enhanced the frequency of action potential-driven, GABA A receptor-mediated currents, but did not alter their responses to exogenously applied GABA. In current-clamped SCN neurons perfused with tetrodotoxin, ANGII induced a membrane depolarization with a concomitant decrease in input resistance. In conclusion we show that AT1 receptor activation by ANGII depolarizes SCN neurons and stimulates action potential firing, leading to increased GABA release in the mouse SCN. Additionally we provide the first evidence that endogenous AT1 receptor signaling tonically regulates the activities of some SCN neurons.

Pre- and postsynaptic plasticity underlying augmented glutamatergic inputs to hypothalamic presympathetic neurons in spontaneously hypertensive rats

Increased sympathetic outflow plays an important role in the pathogenesis of hypertension. Glutamatergic inputs in the paraventricular nucleus (PVN) of the hypothalamus maintain resting sympathetic vasomotor tone in spontaneously hypertensive rats (SHR). In this study, we determined the synaptic and cellular mechanisms of increased glutamatergic inputs to PVN presympathetic neurons in SHR. The spinally projecting PVN neurons were retrogradely labelled by fluorescent microspheres injected into the intermediolateral cell column of the spinal cord. Blockade of NMDA and non-NMDA receptors significantly decreased the firing activity of labelled PVN neurons in brain slices in SHR, but not in normotensive Wistar-Kyoto rats (WKY). The basal frequency of glutamatergic spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs, respectively) of labelled PVN neurons was significantly greater in SHR than in WKY. But the frequency of neither sEPSCs nor mEPSCs stimulated by 4-aminopyridine or capsaicin differed significantly between WKY and SHR. Furthermore, the amplitude of postsynaptic NMDA currents elicited by either electrical stimulation or puff application in labelled PVN neurons was significantly higher in SHRs than in WKY. However, the evoked AMPA current amplitude in PVN neurons was similar in WKY and SHR. This study provides new evidence of how the glutamatergic synaptic inputs to PVN presympathetic neurons are increased and how they contribute to the elevated firing activity of these neurons in SHR. The augmented glutamatergic tone in the PVN is maintained by an increase in presynaptic glutamate release and an up-regulation of postsynaptic NMDA receptor function in SHR.