Impact of anesthesia on micromagnetic stimulation (μMS) of the vagus nerve

To treat diseases associated with vagal nerve control of peripheral organs, it is necessary to selectively activate efferent and afferent fibers in the vagus. As a result of the nerve's complex anatomy, fiber-specific activation proves challenging. Spatially selective neuromodulation using micromagnetic stimulation(μMS) is showing incredible promise. This neuromodulation technique uses microcoils(μcoils) to generate magnetic fields by powering them with a time-varying current. Following the principles of Faraday's law of induction, a highly directional electric field is induced in the nerve from the magnetic field. In this study on rodent cervical vagus, a solenoidalμcoil was oriented at an angle to left and right branches of the nerve. The aim of this study was to measure changes in the mean arterial pressure (MAP) and heart rate (HR) followingμMS of the vagus. Theμcoils were powered by a single-cycle sinusoidal current varying in pulse widths(PW = 100, 500, and 1000μsec) at a frequency of 20 Hz. Under the influence of isoflurane,μMS of the left vagus at 1000μsec PW led to an average drop in MAP of 16.75 mmHg(n = 7). In contrast,μMS of the right vagus under isoflurane resulted in an average drop of 11.93 mmHg in the MAP(n = 7). Surprisingly, there were no changes in HR to either right or left vagalμMS suggesting the drop in MAP associated with vagusμMS was the result of stimulation of afferent, but not efferent fibers. In urethane anesthetized rats, no changes in either MAP or HR were observed uponμMS of the right or left vagus(n = 3). These findings suggest the choice of anesthesia plays a key role in determining the efficacy ofμMS on the vagal nerve. Absence of HR modulation uponμMS could offer alternative treatment options using VNS with fewer heart-related side-effects.

Renal and hypothalamic inflammation in renovascular hypertension: role of afferent renal nerves

Renal denervation (RDN) is a potential therapy for drug-resistant hypertension. However, whether its effects are mediated by ablation of efferent or afferent renal nerves is not clear. Previous studies have implicated that renal inflammation and the sympathetic nervous system are driven by the activation of afferent and efferent renal nerves. RDN attenuated the renal inflammation and sympathetic activity in some animal models of hypertension. In the 2 kidney,1 clip (2K1C) model of renovascular hypertension, RDN also decreased sympathetic activity; however, mechanisms underlying renal and central inflammation are still unclear. We tested the hypothesis that the mechanisms by which total RDN (TRDN; efferent + afferent) and afferent-specific RDN (ARDN) reduce arterial pressure in 2K1C rats are the same. Male Sprague-Dawley rats were instrumented with telemeters to measure mean arterial pressure (MAP), and after 7 days, a clip was placed on the left renal artery. Rats underwent TRDN, ARDN, or sham surgery of the clipped kidney and MAP was measured for 6 wk. Weekly measurements of water intake (WI), urine output (UO), and urinary copeptin were conducted, and urine was analyzed for cytokines/chemokines. Neurogenic pressor activity (NPA) was assessed at the end of the protocol calculated by the depressor response after intraperitoneal injection of hexamethonium. Rats were euthanized and the hypothalamus and kidneys removed for measurement of cytokine content. MAP, NPA, WI, and urinary copeptin were significantly increased in 2K1C-sham rats, and these responses were abolished by both TRDN and ARDN. 2K1C-sham rats presented with renal and hypothalamic inflammation and these responses were largely mitigated by TRDN and ARDN. We conclude that RDN attenuates 2K1C hypertension primarily by ablation of afferent renal nerves which disrupts bidirectional renal neural-immune pathways.NEW & NOTEWORTHY Hypertension resulting from reduced perfusion of the kidney is dependent on renal sensory nerves, which are linked to inflammation in the kidney and hypothalamus. Afferent renal nerves are required for chronic increases in both water intake and vasopressin release observed following renal artery stenosis. Findings from this study suggest an important role of renal sensory nerves that has previously been underestimated in the pathogenesis of 2K1C hypertension.

Abstract 075: The Kidney-OVLT Neuroaxis Mediates Neurogenic Hypertension And Polydipsia In 2-Kidney, 1-Clip Rats

Renal afferent nerves mediate activation of the sympathetic nervous system and hypertension in rats with unilateral renal artery stenosis (2K1C-HTN). However, the central neural structures that mediate this response are unclear. The organum vasculosum of the lamina terminalis (OVLT), has also been implicated in the regulation of mean arterial pressure (MAP) and sympathetic activity in preclinical models of hypertension. We tested the hypothesis that afferent renal nerves and the OVLT share a common neural pathway required for the development of 2K1C-HTN. Two experiments were conducted. 1: Male rats (n=5-9) were implanted with telemeters to measure MAP. 1-week later they received a clip on the left renal artery and were subjected to afferent renal denervation (ARDN) by periaxonal capsaicin (2K1C-ARDN) or sham (2K1C-sham) treatment of the clipped kidney. 2: Male rats (n=6-8) were subjected to lesioning of the OVLT (OVLTx) or sham operation. 1-week later telemeters were implanted and 7 days later the left renal artery was stenosed by application of a silver clip. In both experiments, MAP was measured continuously for 6 weeks, and water intake (WI) was measured once a week. Neurogenic pressor activity (NPA) was assessed by measuring the peak MAP response to ganglionic blockade. EXPT 1 Results: At the end of the protocol, MAP was 165±5 mmHg in 2K1C-sham compared to 131±10mmHg in 2K-1C-ARDN rats (p<0.001). WI increased in 2K1C-sham rats peaking at 110±13 ml/day at week 2. In contrast, WI was markedly reduced in 2K1C-ARDN rats at this same time point (41±3 ml/day). NPA was increased in 2K1C-sham (-50 ± 4 mmHg) but was reduced in 2K1C-ARDN (-25 ± 3 mmHg) rats (p<0.05). EXPT 2 Results: At end of the protocol MAP was 162±6 mmHg in sham-2K1C rats compared to 135±8 mmHg in OVLTx-2K1C rats (p<0.001). WI peaked at 74±9 ml/day in sham lesion-2K1C compared to 52±9 ml/day in OVLTx-2K1C rats at week 2 (p<0.05). NPA was increased in sham-2K1C rats (-51 ± 3 mmHg) but was reduced in OVLTx-2K1C rats (-31 ± 4 mmHg) (p<0.05). Since both ARDN and OVLTx attenuated MAP, WI, and NPA similarly in 2K1C rats, we hypothesize that afferent renal nerves and the OVLT are linked in a common neural pathway required for the development of hypertension following renal artery stenosis.

Chronic Low-Level Vagus Nerve Stimulation Improves Long-Term Survival in Salt-Sensitive Hypertensive Rats

Chronic hypertension (HTN) affects more than 1 billion people worldwide, and is associated with an increased risk of cardiovascular disease. Despite decades of promising research, effective treatment of HTN remains challenging. This work investigates vagus nerve stimulation (VNS) as a novel, device-based therapy for HTN treatment, and specifically evaluates its effects on long-term survival and HTN-associated adverse effects. HTN was induced in Dahl salt-sensitive rats using a high-salt diet, and the rats were randomly divided into two groups: VNS (n = 9) and Sham (n = 8), which were implanted with functional or non-functional VNS stimulators, respectively. Acute and chronic effects of VNS therapy were evaluated through continuous monitoring of blood pressure (BP) and ECG via telemetry devices. Autonomic tone was quantified using heart rate (HR), HR variability (HRV) and baroreflex sensitivity (BRS) analysis. Structural cardiac changes were quantified through gross morphology and histology studies. VNS significantly improved the long-term survival of hypertensive rats, increasing median event-free survival by 78% in comparison to Sham rats. Acutely, VNS improved autonomic balance by significantly increasing HRV during stimulation, which may lead to beneficial chronic effects of VNS therapy. Chronic VNS therapy slowed the progression of HTN through an attenuation of SBP and by preserving HRV. Finally, VNS significantly altered cardiac structure, increasing heart weight, but did not alter the amount of fibrosis in the hypertensive hearts. These results suggest that VNS has the potential to improve outcomes in subjects with severe HTN.

Targeted afferent renal denervation reduces arterial pressure but not renal inflammation in established DOCA-salt hypertension in the rat

Recent preclinical studies show renal denervation (RDNx) may be an effective treatment for hypertension; however, the mechanism remains unknown. We have recently reported total RDNx (TRDNx) and afferent-selective RDNx (ARDNx) similarly attenuated the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Whereas TRDNx abolished renal inflammation, ARDNx had a minimal effect despite an identical antihypertensive effect. Although this study established that ARDNx attenuates the development of DOCA-salt hypertension, it is unknown whether this mechanism remains operative once hypertension is established. The current study tested the hypothesis that TRDNx and ARDNx would similarly decrease mean arterial pressure (MAP) in the DOCA-salt hypertensive rat, and only TRDNx would mitigate renal inflammation. After 21 days of DOCA-salt treatment, male Sprague-Dawley rats underwent TRDNx ( n = 16), ARDNx ( n = 16), or Sham ( n = 14) treatment and were monitored for 14 days. Compared with baseline, TRDNx and ARDNx decreased MAP similarly (TRDNx -14 ± 4 and ARDNx -15 ± 6 mmHg). After analysis of diurnal rhythm, rhythm-adjusted mean and amplitude of night/day cycle were also reduced in TRDNx and ARDNx groups compared with Sham. Notably, no change in renal inflammation, injury, or function was detected with either treatment. We conclude from these findings that: 1) RDNx mitigates established DOCA-salt hypertension; 2) the MAP responses to RDNx are primarily mediated by ablation of afferent renal nerves; and 3) renal nerves do not contribute to the maintenance of renal inflammation in DOCA-salt hypertension.

Resting Afferent Renal Nerve Discharge and Renal Inflammation: Elucidating the Role of Afferent and Efferent Renal Nerves in Deoxycorticosterone Acetate Salt Hypertension

Renal sympathetic denervation (RDNx) has emerged as a novel therapy for hypertension; however, the therapeutic mechanisms remain unclear. Efferent renal sympathetic nerve activity has recently been implicated in trafficking renal inflammatory immune cells and inflammatory chemokine and cytokine release. Several of these inflammatory mediators are known to activate or sensitize afferent nerves. This study aimed to elucidate the roles of efferent and afferent renal nerves in renal inflammation and hypertension in the deoxycorticosterone acetate (DOCA) salt rat model. Uninephrectomized male Sprague-Dawley rats (275-300 g) underwent afferent-selective RDNx (n=10), total RDNx (n=10), or Sham (n=10) and were instrumented for the measurement of mean arterial pressure and heart rate by radiotelemetry. Rats received 100-mg DOCA (SC) and 0.9% saline for 21 days. Resting afferent renal nerve activity in DOCA and vehicle animals was measured after the treatment protocol. Renal tissue inflammation was assessed by renal cytokine content and T-cell infiltration and activation. Resting afferent renal nerve activity, expressed as a percent of peak afferent nerve activity, was substantially increased in DOCA than in vehicle (35.8±4.4 versus 15.3±2.8 %Amax). The DOCA-Sham hypertension (132±12 mm Hg) was attenuated by ≈50% in both total RDNx (111±8 mm Hg) and afferent-selective RDNx (117±5 mm Hg) groups. Renal inflammation induced by DOCA salt was attenuated by total RDNx and unaffected by afferent-selective RDNx. These data suggest that afferent renal nerve activity may mediate the hypertensive response to DOCA salt, but inflammation may be mediated primarily by efferent renal sympathetic nerve activity. Also, resting afferent renal nerve activity is elevated in DOCA salt rats, which may highlight a crucial neural mechanism in the development and maintenance of hypertension.

Abstract 018: Afferent-targeted Renal Denervation Attenuates Established Deoxycorticosterone-salt Hypertension: A Central Role for Renal Afferent Nerves in Hypertension?

Cardiovascular disease (CVD) remains the most pervasive cause of death worldwide. High arterial pressure, or hypertension (HTN), is the highest risk factor for CVD morbidity and mortality. Increased peripheral and renal sympathetic nerve activity (SNA) is hypothesized to be a primary contributor to HTN etiology. Moreover, recent clinical and experimental studies show total renal denervation (T-RDNx), may reverse the HTN; however, the contribution of afferent and efferent renal nerves in this effect is unknown. We have recently reported T-RDNx and afferent-specific denervation (A-RDNx) identically attenuated the development of deoxycorticostereone acetate (DOCA)-salt hypertension. However, the efficacy of T-RDNx and A-RDNx to reverse the established phase of this model of HTN is unknown. Therefore, the present study tested the hypothesis that A-RDNx and T-RDNx would similarly decrease the mean arterial pressure (MAP) in DOCA-salt rats with established HTN. Twenty-four male Sprague Dawley rats (275-300g) instrumented with radiotelemeters were administered DOCA (100mg, s.c.) and 0.9% saline to drink ad libitum for 35 days. On day 21 of DOCA-salt, rats underwent T-RDNx (n=9), A-RDNx (n=9), or sham (n=6) treatments. MAP was monitored for an additional 14 days. Neurogenic pressor activity (NPA) was assessed 14 days after treatment by measuring the MAP response to acute ganglionic blockade (hexamethonium, 30mg/kg, i.p.). Data was analyzed with a one-way ANOVA with Bonferroni post-hoc test (α=0.05). Data presented as mean ± SEM. MAP was similar across all groups prior to treatment on Day 21 of DOCA-salt (Sham: 165±6; T-RDNx: 164±3; A-RDNx: 162±7mmHg). Whereas Sham had no effect (-2±3) on MAP 14 days after treatment, both RDNx treatments decreased MAP by approximately 20 mmHg (T-RDNx -18±8; A-RDNx -22±5). NPA 14 days after treatment in Sham rats was -97±12mmHg. This response was reduced by nearly half in both T-RDNx (-50±9mmHg) and A-RDNx (-48±4mmHg) groups. We conclude from these findings that: 1) RDNx is effective in treating the established phase of DOCA-salt hypertension, 2) the MAP response to RDNx is mediated by ablation of afferent renal nerves, and 3) the antihypertensive response to RDNx is mediated by a decrease global neurogenic pressor activity.

Abstract 085: The Role of Afferent and Efferent Renal Nerves in T Lymphocyte Recruitment into the Kidneys and Development of Two Models of Salt-Dependent Hypertension

Renal denervation (RDNX) attenuates hypertension (HT) in some animal models and human patients, yet the antihypertensive mechanisms involved are not known. Because it has been suggested that efferent renal nerves may cause recruitment of T lymphocytes (T cells) into the kidneys, and T cell-secreted cytokines can activate afferent neurons, we hypothesized that RDNX attenuates HT in the angiotensin II (AngII)-salt and deoxycorticosterone acetate (DOCA)-salt models by decreasing efferent renal nerve-dependent T cell trafficking to the kidneys and T cell-mediated activation of afferent renal nerves. Male Sprague-Dawley rats underwent SHAM surgery, RDNX or selective ablation of afferent renal nerves (renal-CAP) and were subjected to either AngII-salt or DOCA-salt HT. AngII-salt rats underwent SHAM, RDNX or renal-CAP (n = 8/group) and were instrumented with an IV catheter and radio-telemeter to measure mean arterial pressure (MAP). Rats were fed a 4% NaCl diet and, after a baseline period, AngII was infused IV (10 ng/kg/min) for 16 days. DOCA-salt rats were unilaterally nephrectomized and, after 2 weeks, instrumented with a radio-telemeter, subjected to SHAM (n = 5), RDNX (n = 7) or renal-CAP (n = 6) and given 0.9% saline to drink. After a baseline period, silicone pellets containing 100mg DOCA were implanted SC and rats were sacrificed 3 weeks later. Upon sacrifice, kidneys were harvested for flow cytometry to determine the number of CD4+ and CD8+ T cells in the kidneys. In AngII-salt rats, renal CD4 and CD8 T cell counts and the change in MAP from baseline (ΔMAP) were similar between groups (SHAM = 42.3 ± 6.4 mmHg, RDNX = 32.6 ± 5.9 mmHg, renal-CAP = 36.8 ± 5.0 mmHg). In DOCA-salt rats, RDNX and renal-CAP significantly attenuated ΔMAP (SHAM = 24.2 ± 3.1 mmHg, RDNX = 12.7 ± 2.4 mmHg, renal-CAP = 13.8 ± 1.7 mmHg). CD8 T cell counts in RDNX kidneys were significantly lower than SHAM (p < 0.01) and trended lower than renal-CAP. CD4 T cell counts in RDNX kidneys were significantly lower than renal-CAP (p < 0.05) and trended lower than SHAM. These data show that renal nerves do not contribute to AngII-salt HT, but suggest that afferent renal nerves contribute to DOCA-salt HT and that this may be T cell-mediated and dependent on efferent renal nerves. Funding: R01 HL116476-01.