Abstract 073: Age-Dependent Hypertension-Related Cognitive Impairment And Neuroinflammation In Sprague Dawley Rats Can Be Reversed By An AT 1 R Antagonism

Aim: We hypothesize paraventricular nucleus (PVN)-specific blood brain barrier (BBB) disruption and neuroinflammation increase hypertension and sympathoexcitation with age, promoting cognitive impairment, that can be attenuated by an Angiotensin II Type 1 Receptor (AT 1 R)-dependent mechanism in the aging Sprague Dawley (SD) rat.

Methods: In male and female SD rats aged 3, 8 and 16 months old (MO) (N=6/gp) and 16 MO male rats treated with losartan (21 days; sc 3 mg/kg/day), blood pressure (BP; femoral artery cannulation), sympathetic tone to the vasculature (iv hexamethonium) and plasma NE (ELISA) was measured. Memory function was assessed by the novel object recognition task. BBB disruption was assessed via FITC extravasation and IHC/IF was performed for microglia (CD11b/c), astrocytes (GFAP), IL-6 and TNF-α in the PVN.

Results: Aged male, but not female, SD rats develop HTN, sympathoexcitation, and cognitive impairment. PVN neuroinflammation, proinflammatory cytokine production and BBB disruption increased in male, but not female, rats with age. Losartan significantly lowered BP, reduced sympathoexcitation, attenuated BBB disruption and neuroinflammation, and reversed cognitive impairment in aged male rats.

Conclusions: Our data suggest there is a potential protective role of female sex steroids in preventing the development of age-dependent HTN which is associated with PVN neuroinflammation. Concurrent with recent clinical findings, we found lowering blood pressure improved cognitive function and, thus, an AT 1 R antagonism represents a new therapeutic modality to improve cognitive performance in hypertensive individuals.

Natriuresis During an Acute Intravenous Sodium Chloride Infusion in Conscious Sprague Dawley Rats Is Mediated by a Blood Pressure-Independent α1-Adrenoceptor-Mediated Mechanism

The mechanisms that sense alterations in total body sodium content to facilitate sodium homeostasis in response to an acute sodium challenge that does not increase blood pressure have not been fully elucidated. We hypothesized that the renal sympathetic nerves are critical to mediate natriuresis via α₁- or β-adrenoceptors signal transduction pathways to maintain sodium balance in the face of acute increases in total body sodium content that do not activate the pressure-natriuresis mechanism. To address this hypothesis, we used acute bilateral renal denervation (RDNX), an anteroventral third ventricle (AV3V) lesion and α₁- or β-antagonism during an acute 1M NaCl sodium challenge in conscious male Sprague Dawley rats. An acute 1M NaCl infusion did not alter blood pressure and evoked profound natriuresis and sympathoinhibition. Acute bilateral RDNX attenuated the natriuretic and sympathoinhibitory responses evoked by a 1M NaCl infusion [peak natriuresis (μeq/min) sham 14.5 ± 1.3 vs. acute RDNX: 9.2 ± 1.4, p < 0.05; plasma NE (nmol/L) sham control: 44 ± 4 vs. sham 1M NaCl infusion 11 ± 2, p < 0.05; acute RDNX control: 42 ± 6 vs. acute RDNX 1M NaCl infusion 25 ± 3, p < 0.05]. In contrast, an AV3V lesion did not impact the cardiovascular, renal excretory or sympathoinhibitory responses to an acute 1M NaCl infusion. Acute i.v. α₁-adrenoceptor antagonism with terazosin evoked a significant drop in baseline blood pressure and significantly attenuated the natriuretic response to a 1M NaCl load [peak natriuresis (μeq/min) saline 17.2 ± 1.4 vs. i.v. terazosin 7.8 ± 2.5, p < 0.05]. In contrast, acute β-adrenoceptor antagonism with i.v. propranolol infusion did not impact the cardiovascular or renal excretory responses to an acute 1M NaCl infusion. Critically, the natriuretic response to an acute 1M NaCl infusion was significantly blunted in rats receiving a s.c. infusion of the α₁-adrenoceptor antagonist terazosin at a dose that did not lower baseline blood pressure [peak natriuresis (μeq/min) sc saline: 18 ± 1 vs. sc terazosin 7 ± 2, p < 0.05]. Additionally, a s.c. infusion of the α₁-adrenoceptor antagonist terazosin further attenuated the natriuretic response to a 1M NaCl infusion in acutely RDNX animals. Collectively these data indicate a specific role of a blood pressure-independent renal sympathetic nerve-dependent α₁-adrenoceptor-mediated pathway in the natriuretic and sympathoinhibitory responses evoked by acute increases in total body sodium.

Sensory Afferent Renal Nerve Activated Gαi2 Subunit Proteins Mediate the Natriuretic, Sympathoinhibitory and Normotensive Responses to Peripheral Sodium Challenges

We have previously reported that brain Gαi2 subunit proteins are required to maintain sodium homeostasis and are endogenously upregulated in the hypothalamic paraventricular nucleus (PVN) in response to increased dietary salt intake to maintain a salt resistant phenotype in rats. However, the origin of the signal that drives the endogenous activation and up-regulation of PVN Gαi2 subunit protein signal transduction pathways is unknown. By central oligodeoxynucleotide (ODN) administration we show that the pressor responses to central acute administration and central infusion of sodium chloride occur independently of brain Gαi2 protein pathways. In response to an acute volume expansion, we demonstrate, via the use of selective afferent renal denervation (ADNX) and anteroventral third ventricle (AV3V) lesions, that the sensory afferent renal nerves, but not the sodium sensitive AV3V region, are mechanistically involved in Gαi2 protein mediated natriuresis to an acute volume expansion [peak natriuresis (μeq/min) sham AV3V: 43 ± 4 vs. AV3V 45 ± 4 vs. AV3V + Gαi2 ODN 25 ± 4, p < 0.05; sham ADNX: 43 ± 4 vs. ADNX 23 ± 6, AV3V + Gαi2 ODN 25 ± 3, p < 0.05]. Furthermore, in response to chronically elevated dietary sodium intake, endogenous up-regulation of PVN specific Gαi2 proteins does not involve the AV3V region and is mediated by the sensory afferent renal nerves to counter the development of the salt sensitivity of blood pressure (MAP [mmHg] 4% NaCl; Sham ADNX 124 ± 4 vs. ADNX 145 ± 4, p < 0.05; Sham AV3V 125 ± 4 vs. AV3V 121 ± 5). Additionally, the development of the salt sensitivity of blood pressure following central ODN-mediated Gαi2 protein down-regulation occurs independently of the actions of the brain angiotensin II type 1 receptor. Collectively, our data suggest that in response to alterations in whole body sodium the peripheral sensory afferent renal nerves, but not the central AV3V sodium sensitive region, evoke the up-regulation and activation of PVN Gαi2 protein gated pathways to maintain a salt resistant phenotype. As such, both the sensory afferent renal nerves and PVN Gαi2 protein gated pathways, represent potential targets for the treatment of the salt sensitivity of blood pressure.

Abstract P189: AT 1 R-Dependent Blood-Brain Barrier Disruption Precedes Neuroinflammation In Age-Dependent Hypertension

AIM: We hypothesized paraventricular nucleus (PVN)-specific blood-brain barrier (BBB) dysfunction and neuroinflammation contribute to hypertension and sympathoexcitation that can be attenuated by an Angiotensin II Type 1 Receptor (AT 1 R)-dependent mechanism in the aging Sprague-Dawley (SD) rat.

Methods: Naïve male SD rats aged 3-, 8- and 16-months-old (MO) (N=4-6/gp) were used in the following studies. Separate groups of 16 MO rats were administered losartan (21 days; s.c. 3 mg/kg/day) or hydrochlorothiazide (14 days; s.c. 4 mg/kg/day). Blood pressure (femoral cannulation) and plasma NE (ELISA) were assessed at end of study. In separate groups, BBB dysfunction was assessed via PVN FITC extravasation using intravascular co-infusion of FITC-Dextran (10 kDa) and rhodamine B isothiocyanate-Dextran (70 kDa). IHC/IF was performed in naive and losartan-treated rats for microglia (CD11b/c) and astrocytes (GFAP) in the PVN and subfornical organ (SFO).

Results: Male SD rats develop HTN and sympathoexcitation with age. At 8 and 16 MO, rats exhibit PVN BBB dysfunction (increased FITC extravasation). However, only 16 MO rats exhibit significant PVN neuroinflammation (increased microglial activation and astrocyte reactivity). In the SFO, there is no evidence of age-dependent neuroinflammation. Losartan and HCTZ both significantly lower blood pressure to similar levels, however, only losartan significantly attenuates PVN BBB dysfunction and neuroinflammation.

Conclusions: Within the PVN, a known neural control center, there are AT 1 R-dependent increases in PVN BBB disruption and neuroinflammation that we speculate contribute to hypertension in aging SD rats.

Sympathetic Regulation of the NCC (Sodium Chloride Cotransporter) in Dahl Salt-Sensitive Hypertension

Increased sympathoexcitation and renal sodium retention during high salt intake are hallmarks of the salt sensitivity of blood pressure. The mechanism(s) by which excessive sympathetic nervous system release of norepinephrine influences renal sodium reabsorption is unclear. However, studies demonstrate that norepinephrine can stimulate the activity of the NCC (sodium chloride cotransporter) and promote the development of SSH (salt-sensitive hypertension). The adrenergic signaling pathways governing NCC activity remain a significant source of controversy with opposing studies suggesting a central role of upstream α1- and β-adrenoceptors in the canonical regulatory pathway involving WNKs (with-no-lysine kinases), SPAK (STE20/SPS1-related proline alanine-rich kinase), and OxSR1 (oxidative stress response 1). In our previous study, α1-adrenoceptor antagonism in norepinephrine-infused male Sprague-Dawley rats prevented the development of norepinephrine-evoked SSH in part by suppressing NCC activity and expression. In these studies, we used selective adrenoceptor antagonism in male Dahl salt-sensitive rats to test the hypothesis that norepinephrine-mediated activation of the NCC in Dahl SSH occurs via an α1-adrenoceptor dependent pathway. A high-salt diet evoked significant increases in NCC activity, expression, and phosphorylation in Dahl salt-sensitive rats that developed SSH. Increases were associated with a dysfunctional WNK1/4 dynamic and a failure to suppress SPAK/OxSR1 activity. α1-adrenoceptor antagonism initiated before high-salt intake or following the establishment of SSH attenuated blood pressure in part by suppressing NCC activity, expression, and phosphorylation. Collectively, our findings support the existence of a norepinephrine-activated α1-adrenoceptor gated pathway that relies on WNK/SPAK/OxSR1 signaling to regulate NCC activity in SSH.

Abstract MP05: Pvn-specific Microgliosis And Inflammation Precedes Sympathoexcitation In gαI 2 Protein-dependent, Salt-sensitive Hypertension

Aim: Hypothalamic paraventricular nucleus (PVN) Gαi 2 proteins mediate sympathoinhibitory responses to a high salt (HS; 4% NaCl) diet. Failure to upregulate Gαi 2 proteins in response to a HS diet results in PVN inflammation and salt sensitive hypertension (SSHTN). We hypothesize that microglial-mediated PVN inflammation precedes sympathoexcitation in Gαi 2 protein-dependent SSHTN.

Methods: Three-month-old male Sprague Dawley rats implanted with ICV cannulas fitted to osmotic minipumps to centrally infuse either a control scrambled (SCR) oligodeoxynucleotide (ODN) or a Gαi 2 targeted ODN, which downregulates CNS Gαi 2 proteins by ~85%, (25μg/5μl/day/ODN) were placed on a 1-7-day normal salt (NS; 0.6% NaCl) or HS diet (n=5/group) and underwent cardiac perfusion. Brain immunohistochemistry was used to assess PVN and subfornical organ microgliosis and qualitatively assess levels of PVN pro-inflammatory cytokines (PIC) IL-1β, IL-6, and TNFα. In additional groups, MAP was assessed via radiotelemetry, and whole blood and kidneys were obtained for ELISA measurement of plasma and renal norepinephrine (NE) as estimates of sympathetic tone.

Results: By 24h in control SCR ODN infused rats a HS diet, which did not alter MAP or microglial activation, evoked sympathoinhibition. In contrast, in Gαi 2 ODN infused rats a HS diet did not result in sympathoinhibition and evoked significant increases in MAP, PVN microgliosis and PVN PIC expression within 24h, and elevated renal NE content by Day 3.

Conclusions: Our data suggest that in the male Sprague-Dawley model of PVN Gαi 2 protein-dependent SSHTN PVN inflammation (microgliosis and PIC production) precedes sympathoexcitation.

Inhibition of microglial activation in rats attenuates paraventricular nucleus inflammation in Gαi2 protein-dependent, salt-sensitive hypertension

New Findings

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What is the central question of this study?

We hypothesized that central inflammatory processes that involve activation of microglia and astrocytes contribute to the development of Gαi₂ protein‐dependent, salt‐sensitive hypertension.

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What is the main finding and its importance?

The main finding is that PVN‐specific inflammatory processes, driven by microglial activation, appear to be linked to the development of Gαi₂ protein‐dependent, salt‐sensitive hypertension in Sprague–Dawley rats. This finding might reveal new mechanistic targets in the treatment of hypertension.

Abstract

The central mechanisms underlying salt‐sensitive hypertension, a significant public health issue, remain to be established. Researchers in our laboratory have reported that hypothalamic paraventricular nucleus (PVN) Gαi₂ proteins mediate the sympathoinhibitory and normotensive responses to high sodium intake in salt‐resistant rats. Given the recent evidence of central inflammation in animal models of hypertension, we hypothesized that PVN inflammation contributes to Gαi₂ protein‐dependent, salt‐sensitive hypertension. Male Sprague–Dawley rats received chronic intracerebroventricular infusions of a targeted Gαi₂ or control scrambled oligodeoxynucleotide (ODN) and were maintained for 7 days on a normal‐salt (NS; 0.6% NaCl) or high‐salt (HS; 4% NaCl) diet; in subgroups on HS, intracerebroventricular minocycline (microglial inhibitor) was co‐infused with ODNs. Radiotelemetry was used in subgroups of rats to measure mean arterial pressure (MAP) chronically. In a separate group of rats, plasma noradrenaline, plasma renin activity, urinary angiotensinogen and mRNA levels of the PVN pro‐inflammatory cytokines TNFα, IL‐1β and IL‐6 and the anti‐inflammatory cytokine IL‐10 were assessed. In additional groups, immunohistochemistry was performed for markers of PVN and subfornical organ microglial activation and cytokine levels and PVN astrocyte activation. High salt intake evoked salt‐sensitive hypertension, increased plasma noradrenaline, PVN pro‐inflammatory cytokine mRNA upregulation, anti‐inflammatory cytokine mRNA downregulation and PVN‐specific microglial activation in rats receiving a targeted Gαi₂ but not scrambled ODN. Minocycline co‐infusion significantly attenuated the increase in MAP and abolished the increase in plasma noradrenaline and inflammation in Gαi₂ ODN‐infused animals on HS. Our data suggest that central Gαi₂ protein prevents microglial‐mediated PVN inflammation and the development of salt‐sensitive hypertension.

Abstract 086: Pvn-Specific Microgliosis Drives Inflammation in G-Alpha-i 2 Protein Dependent Salt Sensitive Hypertension and Gnai2 Snps Correlate With Essential Hypertension

Aim: Failure to upregulate hypothalamic paraventricular nucleus (PVN) Gαi 2 proteins promotes salt sensitive hypertension (ssHTN) and central inflammation plays a role in HTN. We hypothesize that 1) PVN-specific inflammation contributes to Gαi 2 protein dependent ssHTN, and 2) GNAI2 SNPs correlate with HTN.

Methods: 3-month-old male Sprague Dawley rats were implanted with I.C.V. cannulas fitted to osmotic minipumps and infused with Gαi 2 or control scrambled (SCR) oligodeoxynucleotides (ODN) (25μg/5μl/day), or ODN + minocycline (MINO) (120μg/5μl/day, HS only) and placed on a 7-day normal (NS; 0.6% NaCl) or high (HS; 4% NaCl) salt diet. Day 7 BP (femoral cannulation) and Plasma NE (ELISA) were measured. PVN and subfornical organ microglia and astrocyte activation (IHC), and PVN pro-inflammatory (PIC) and anti-inflammatory (AIC) cytokine mRNA and protein levels were analyzed. GNAI2 SNPs were examined in the UK BioBank for associations with HTN.

Results: In contrast to SCR ODN infusion, Gαi 2 ODN infusion and HS evoked HTN, elevated plasma NE, PVN-specific microgliosis, upregulation of PVN PICs and downregulation of PVN IL-10. MINO co-infusion with Gαi 2 ODN and HS diet restored PIC and AIC mRNA and protein levels. Furthermore, MINO attenuated microgliosis and reduced the magnitude of ssHTN. GNAI2 SNPs rs2298952 (p=0.0411, MAF=0.103) and rs4547694 (p=0.0117, MAF=0.381) are both associated with essential HTN in BioBank subjects.

Conclusion: PVN-specific microgliosis and inflammation may drive Gαi 2 protein dependent ssHTN and GNAI2 SNPs represent a potential biomarker of essential HTN.