Changes In Renal Sympathetic Nerve Activity Research Articles

A NOVEL METHOD FOR THE ANALYSIS OF RENAL SYMPATHETIC NERVE ACTIVITY FROM MULTI-FIBER NERVE RECORDINGS

Objective: Measurement of renal sympathetic nerve activity (RSNA) via bipolar electrodes from multi-fiber preparations in various species has been used for decades in the context of autonomic regulation research. Traditionally, raw neurograms are usually integrated over short time intervals and this approach works perfectly for the analysis of dynamic RSNA changes due to acute intervention. However, comparability of baseline activities between groups is limited. We present a novel analysis method, based on the raw neurogram sampled at rate of 25 kHz. Design and method: For RSNA-burst analysis a software based programmable algorithm was used. Continuous activities longer than three single spikes (i.e., >8ms) were defined as bursts, followed by silent periods with some single spikes. Approximately 10.000 bursts/rat were analyzed and burst amplitude, burst duration, burst area (i.e., duration integral), as well as the burst frequencies were analyzed. Results: In rats with myocardial infarction (n=11) we found no difference in integrated baseline RSNA as compared to controls ((n=10). However, the new method revealed higher higher burst rate per cardiac cycle [CC] (2.65 ± 0.39 vs. 1.61 ± 0.38 bursts/CC; P<0.001) and burst frequency (15.09 ± 2.42 vs. 8.52 ±1.54 Hz; P<0.001) compared to healthy controls, indicating increased RSNA. Conclusions: Our new method of RSNA baseline analysis was able to detect subtle differences between groups of animals that could not be detected by the traditional method of integrated RSNA analysis. Furthermore, this method has the potential to give further insights into RNSA patterns and synchronization to afferent regulatory input form the kidney itself or other organs.

Abstract P410: A Novel Method For The Analysis Of Renal Sympathetic Nerve Activity From Multi-fiber Nerve Recordings

Introduction: Measurement of renal sympathetic nerve activity (RSNA) via bipolar electrodes from multi-fiber preparations in various species has been used for decades in the context of autonomic regulation research. Traditionally, raw neurograms are usually integrated over short time intervals and this approach works perfectly for the analysis of dynamic RSNA changes due to acute intervention. However, comparability of baseline activities between groups is limited. We present a novel analysis method, based on the raw neurogram sampled at rate of 25 kHz. Material and methods: For RSNA-burst analysis a software based programmable algorithm was used. Continuous activities longer than three single spikes (i.e., >8ms) were defined as bursts, followed by silent periods with some single spikes. Approximately 10.000 bursts/rat were analyzed and burst amplitude, burst duration, burst area (i.e., duration integral), as well as the burst frequencies were analyzed. Results: In rats with myocardial infarction (n=11) we found no difference in integrated baseline RSNA as compared to controls ((n=10). However, the new method revealed higher higher burst rate per cardiac cycle [CC] (2.65 ± 0.39 vs. 1.61 ± 0.38 bursts/CC; P<0.001) and burst frequency (15.09 ± 2.42 vs. 8.52 ±1.54 Hz; P<0.001) compared to healthy controls, indicating increased RSNA. Conclusion: Our new method of RSNA baseline analysis was able to detect subtle differences between groups of animals that could not be detected by the traditional method of integrated RSNA analysis. Furthermore, this method has the potential to give further insights into RNSA patterns and synchronization to afferent regulatory input form the kidney itself or other organs.

Exaggerated renal sympathetic nerve and pressor responses during spontaneously occurring motor activity in hypertensive rats.

Stimulation of the mesencephalic locomotor region elicits exaggerated sympathetic nerve and pressor responses in spontaneously hypertensive rats (SHR) as compared with normotensive Wistar-Kyoto rats (WKY). This suggests that central command or its influence on vasomotor centers is augmented in hypertension. The decerebrate animal model possesses an ability to evoke intermittent bouts of spontaneously occurring motor activity (SpMA) and generates cardiovascular responses associated with the SpMA. It remains unknown whether the changes in sympathetic nerve activity and hemodynamics during SpMA are altered by hypertension. To test the hypothesis that the responses in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) during SpMA are exaggerated with hypertension, this study aimed to compare the responses in decerebrate, paralyzed SHR, WKY, and normotensive Sprague-Dawley (SD) rats. In all strains, an abrupt increase in RSNA occurred in synchronization with tibial motor discharge (an index of motor activity) and was followed by rises in MAP and heart rate. The centrally evoked increase in RSNA and MAP during SpMA was much greater (306 ± 110%) in SHR than WKY (187 ± 146%) and SD (165 ± 44%). Although resting baroreflex-mediated changes in RSNA were not different across strains, mechanically or pharmacologically induced elevations in MAP attenuated or abolished the RSNA increase during SpMA in WKY and SD but had no effect in SHR. It is likely that the exaggerated sympathetic nerve and pressor responses during SpMA in SHR are induced along a central command pathway independent of the arterial baroreflex and/or result from central command-induced inhibition of the baroreflex.

Renal Sympathetic Nerve Activity Responses to Hypoxia and Hypercapnia in Rats Exposed to Chronic Intermittent Hypoxia

Increased responsiveness of the carotid bodies to acute hypoxia was shown in animals exposed to chronic intermittent hypoxia (CIH). The kidney is sensitive to hypoxia and decreased cortical partial pressure of oxygen (PO2) levels was revealed in rats exposed to moderate CIH. We hypothesized that the response of the renal sympathetic nerve activity (RSNA) to acute hypoxia and hypercapnia is enhanced in rats exposed to CIH. Wistar rats were exposed to 96 cycles of CIH (6% O2, 8 hr/day, n=6) or to normoxia (sham, 21% O2, n=9) for 14 days. On day 15, rats were anesthetized (induction: urethane, α‐chloralose and sodium pentobarbitone mixture i.p. (416, 27 and 33mg/kg), maintenance: urethane and α‐chloralose mixture i.v. (62 and 4mg/kg)) to record cardiovascular parameters and RSNA. Rats were provided with supplementary oxygen to maintain PO2 levels > 84 mmHg. During hypoxia, rats breathed 10% O2for two minutes. This was followed with a hypercapnic challenge during which rats breathed 10% CO2 for 5 minutes. Following a 90‐minute recovery period, capsaizepine (CPZ, 5µg/ml/hr), a TRPV1 blocker, was infused into the right kidney. Then, hypoxic and hypercapnic challenges were repeated following the intra‐renal infusion of CPZ. Data are expressed as mean±SD and were analyzed using t‐test or ANOVA, where relevant. CIH rats were hypertensive (130±9 vs. 118±9mmHg, p=0.039) with comparable basal HR (425±38 vs. 407±34bpm), and RSNA (% of max, 33±12 vs. 25±12) compared with sham rats. In response to acute hypoxia, RSNA was increased to an equivalent extent in CIH‐exposed and sham rats (Maximum change in RSNA of baseline, 128±23 vs. 125±16%; RSNA (% of max), 36±7 vs. 44±22%). Additionally, the increase in HR in response to acute hypoxia was similar between CIH‐exposed and sham rats (∆HR, +42±29 vs. +30±27bpm). Mean arterial blood pressure (MAP) was similarly decreased in CIH‐exposed and sham rats during acute hypoxia (Maximum change in MAP, ‐33±24 vs. ‐27±17mmHg). The decrease in PaO2 levels during hypoxia was comparable between exposure groups (∆PaO2, ‐48±15 vs. ‐47±10mmHg). Hypercapnia induced a comparable increase in the RSNA in CIH‐exposed and sham rats (Maximum change in RSNA of baseline, +121±15 vs. +113±8%). The analysis of absolute RSNA when normalized to maximum RSNA showed that CIH‐exposed rats had increased RSNA compared with sham rats before and after exposure to hypercapnia (RSNA (% of max), 37±8 vs. 26±10%, p=0.037). The increase in PaCO2 levels during hypercapnia was similar between CIH and sham groups (∆PaCO2, +24±8 vs. +19±6mmHg). Intra‐renal CPZ did not alter the increase in RSNA and HR in response to hypoxia and hypercapnia in CIH‐exposed and sham rats. Protein expression of TRPV1 receptors in kidney tissue was equivalent in both exposure groups (1.88±0.23 vs. 1.68±0.26ng/mg). Our results indicate that the responsiveness of RSNA to acute hypoxia and hypercapnia is not altered following exposure to moderate CIH sufficient to increase RSNA and blood pressure.

Regional Differences in Sympathetic Nerve Activity Are Generated by Multiple Arterial Baroreflex Loops Arranged in Parallel.

In this review, by evaluating the responses during freezing, rapid eye movement (REM) sleep, and treadmill exercise, we discuss how multiple baroreflex loops arranged in parallel act on different organs to modulate sympathetic nerve activity (SNA) in a region-specific and coordinated manner throughout the body. During freezing behaviors, arterial pressure (AP) remains unchanged, heart rate (HR) persistently decreases, renal SNA (RSNA) increases, and lumbar SNA (LSNA) remains unchanged. The baroreflex curve for RSNA shifts upward; that for LSNA remains unchanged; and that for HR shifts to the left. These region-specific changes in baroreflex curves are responsible for the region-specific changes in RSNA, LSNA, and HR during freezing. The decreased HR could allow the heart to conserve energy, which is offset by the increased RSNA caused by decreased vascular conductance, resulting in an unchanged AP. In contrast, the unchanged LSNA leaves the muscles in readiness for fight or flight. During REM sleep, AP increases, RSNA and HR decrease, while LSNA is elevated. The baroreflex curve for RSNA during REM sleep is vertically compressed in comparison with that during non-REM sleep. Cerebral blood flow is elevated while cardiac output is decreased during REM sleep. To address this situation, the brain activates the LSNA selectively, causing muscle vasoconstriction, which overcomes vasodilation of the kidneys as a result of the decreased RSNA and cardiac output. Accordingly, AP can be maintained during REM sleep. During treadmill exercise, AP, HR, and RSNA increase simultaneously. The baroreflex curve for RSNA shifts right-upward with the increased feedback gain, allowing maintenance of a stable AP with significant fluctuations in the vascular conductance of working muscles. Thus, the central nervous system may employ behavior-specific scenarios for modulating baroreflex loops for differential control of SNA, changing the SNA in a region-specific and coordinated manner, and then optimizing circulatory regulation corresponding to different behaviors.

Exaggerated pressor and sympathetic nerve responses during spontaneously‐occurring motor activity in decerebrate spontaneously hypertensive rats

Stimulation of the mesencephalic locomotor region (a putative component of the central command pathway) has been shown to elicit exaggerated pressor and sympathetic nerve responses in decerebrate spontaneously hypertensive rats (SHR) as compared to normotensive Wister-Kyoto rats. This finding suggests that central command activity and/or influence on caudal vasomotor centers may be augmented in hypertension. In the normotensive decerebrate rat model, mean arterial pressure (MAP) and sympathetic nerve activity are known to increase during intermittent bouts of spontaneous motor activity. Moreover, evidence suggests that simulated central command influences at the brainstem may evoke these pressor and sympathetic responses. However, it remains unknown whether these spontaneous changes in cardiovascular hemodynamics are altered with the pathogenesis of hypertension. Based on this background, we hypothesized that the pressor and sympathetic nerve responses during spontaneously-occurring motor activity are exaggerated in hypertension. MAP, renal sympathetic nerve activity (RSNA), and tibial nerve discharge were measured in decerebrate SHR (n=8) and normotensive Sprague-Dawley (SD) rats (n=6). The present study aimed to compare resting baroreflex changes in RSNA (in response to pharmacological alterations in MAP) and central command-evoked responses in MAP and RSNA during spontaneously-occurring motor activity between SHR and normotensive SD. Results demonstrated that the baroreflex-mediated RSNA response was not different between SHR and SD. In both SHR and SD, abrupt, spontaneous increases in RSNA were synchronized with tibial motor discharge and followed by a rise in MAP. These potentially central command-evoked increases in MAP and RSNA were greater in SHR than SD. Phenylephrine-induced elevations in MAP did not suppress the central command-induced RSNA increase in SHR, whereas it abolished the rise in RSNA in SD. Taken together, it is concluded that 1) the arterial baroreflex is operative during resting conditions in SHR as well as SD; 2) the pressor and sympathetic nerve responses during spontaneously-occurring motor activity are exaggerated in hypertensive rats; and 3) exaggerated RSNA responses in SHR may be induced along a central command pathway independent of the arterial baroreflex and/or as a result of central command-induced inhibition of the arterial baroreflex itself.

Intermedin in Paraventricular Nucleus Attenuates Sympathoexcitation and Decreases TLR4-Mediated Sympathetic Activation via Adrenomedullin Receptors in Rats with Obesity-Related Hypertension.

Intermedin/adrenomedullin-2 (IMD/AM2), a member of the calcitonin gene-related peptide/AM family, plays an important role in protecting the cardiovascular system. However, its role in the enhanced sympathoexcitation in obesity-related hypertension is unknown. In this study, we investigated the effects of IMD in the paraventricular nucleus (PVN) of the hypothalamus on sympathetic nerve activity (SNA), and lipopolysaccharide (LPS)-induced sympathetic activation in obesity-related hypertensive (OH) rats induced by a high-fat diet for 12 weeks. Acute experiments were performed under anesthesia. The dynamic alterations of sympathetic outflow were evaluated as changes in renal SNA and mean arterial pressure (MAP) in response to specific drugs. Male rats were fed a control diet (12% kcal as fat) or a high-fat diet (42% kcal as fat) for 12 weeks to induce OH. The results showed that IMD protein in the PVN was downregulated, but Toll-like receptor 4 (TLR4) and plasma norepinephrine (NE, indicating sympathetic hyperactivity) levels, and systolic blood pressure were increased in OH rats. LPS (0.5 µg/50 nL)-induced enhancement of renal SNA and MAP was greater in OH rats than in obese or control rats. Bilateral PVN microinjection of IMD (50 pmol) caused greater decreases in renal SNA and MAP in OH rats than in control rats, and inhibited LPS-induced sympathetic activation, and these were effectively prevented in OH rats by pretreatment with the AM receptor antagonist AM22-52. The mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) inhibitor U0126 in the PVN partially reversed the LPS-induced enhancement of SNA. However, IMD in the PVN decreased the LPS-induced ERK activation, which was also effectively prevented by AM22-52. Chronic IMD administration resulted in significant reductions in the plasma NE level and blood pressure in OH rats. Moreover, IMD lowered the TLR4 protein expression and ERK activation in the PVN, and decreased the LPS-induced sympathetic overactivity. These results indicate that IMD in the PVN attenuates SNA and hypertension, and decreases the ERK activation implicated in the LPS-induced enhancement of SNA in OH rats, and this is mediated by AM receptors.

Sympathetic nerve response to optogenetic inhibition of PVN‐RVLM neurons in anesthetized rats with heart failure

Central neuronal cell groups that underlie enhanced sympathetic nerve activity (SNA) in heart failure (HF) have not been identified. Here, we tested the hypothesis that paraventricular hypothalamic nucleus (PVN) neurons that send axonal projections to the rostral ventrolateral medulla (RVLM) mediate this abnormality by examining the effect of optogenetic inhibition of PVN‐RVLM neuron activity on renal SNA (RSNA) in anesthetized rats after myocardial infarction (MI). More than one week after left coronary artery ligation to induce MI, rats received bilateral microinjection into the PVN of an adeno‐associated virus (AAV) vector that encodes reporter protein mcherry (control‐MI rats, N=6) or the light‐activated inhibitory opsin Archaerhodopsin‐eYFP (ArchT‐MI rats, N=12). The AAV vector encoding ArchT‐eYFP was also bilaterally microinjected into the PVN of the sham‐operated rats (ArchT‐sham rats, N=6). More than three weeks later, intermittent bouts (1‐ to 4‐sec stimulation to non‐stimulation) of photostimulation (532 nm wavelength, 5 mW) for 1 min were bilaterally provided to the RVLM of rats anesthetized with a mixture of urethane and α‐chloralose. The RSNA responses to 12 optogenetic stimulations were superimposed on one another and averaged. The superimposing analysis showed that 1‐sec photostimulation significantly (P<0.05) changed RSNA from baseline in ArchT‐MI rats while this stimulation had no effect in control‐MI or ArchT‐sham rats. During the early 500‐msec period of 1‐sec photostimulation, the RSNA changes from baseline were not significant or did not differ among rat groups (+7 ± 6, +4 ± 15, and +14 ± 10% for ArchT‐MI, control‐MI, and ArchT‐sham rats, respectively). During the late 500‐msec period, on the other hand, a significant RSNA inhibition from baseline was detected in ArchT‐MI rats (−35 ± 10%), which was significantly different from the RSNA changes in control‐MI and ArchT‐sham rats (+6 ± 9 and +15 ± 11%). Left ventricular end‐diastolic pressure was increased in rats after MI [+7 ± 3 mmHg, 40 ± 5% of infarct size (IS)] as compared to sham‐operated rats (+2 ± 0 mmHg, 0 ± 0% of IS), indicating heart dysfunction in rats after MI. Taken together, we suggest that dysfunction of PVN‐RVLM neurons underlies enhanced SNA in HF.Support or Funding InformationSupported by JSPS KAKENHI 15H05367 (SK) & 16K15190 (SK), the Nakatomi Foundation grant (SK), and the Takeda Science Foundation grant (SK).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The pedunculopontine tegmentum controls renal sympathetic nerve activity and cardiorespiratory activities in nembutal-anesthetized rats.

Elevated renal sympathetic nerve activity (RSNA) accompanies a variety of complex disorders, including obstructive sleep apnea, heart failure, and chronic kidney disease. Understanding pathophysiologic renal mechanisms is important for determining why hypertension is both a common sequelae and a predisposing factor of these disorders. The role of the brainstem in regulating RSNA remains incompletely understood. The pedunculopontine tegmentum (PPT) is known for regulating behaviors including alertness, locomotion, and rapid eye movement sleep. Activation of PPT neurons in anesthetized rats was previously found to increase splanchnic sympathetic nerve activity and blood pressure, in addition to altering breathing. The present study is the first investigation of the PPT and its potential role in regulating RSNA. Microinjections of DL-homocysteic acid (DLH) were used to probe the PPT in 100-μm increments in Nembutal-anesthetized rats to identify effective sites, defined as locations where changes in RSNA could be evoked. A total of 239 DLH microinjections were made in 18 rats, which identified 20 effective sites (each confirmed by the ability to evoke a repeatable sympathoexcitatory response). Peak increases in RSNA occurred within 10–20 seconds of PPT activation, with RSNA increasing by 104.5 ± 68.4% (mean ± standard deviation) from baseline. Mean arterial pressure remained significantly elevated for 30 seconds, increasing from 101.6 ± 18.6 mmHg to 135.9 ± 36.4 mmHg. DLH microinjections also increased respiratory rate and minute ventilation. The effective sites were found throughout the rostal-caudal extent of the PPT with most located in the dorsal regions of the nucleus. The majority of PPT locations tested with DLH microinjections did not alter RSNA (179 sites), suggesting that the neurons that confer renal sympathoexcitatory functions comprise a small component of the PPT. The study also underscores the importance of further investigation to determine whether sympathoexcitatory PPT neurons contribute to adverse renal and cardiovascular consequences of diseases such as obstructive sleep apnea and heart failure.

Exaggerated sympathetic and pressor responses to exercise pressor reflex activation are attenuated by intracerebroventricular insulin administration in type 2 diabetic rats

The cardiovascular response to exercise is abnormally exaggerated in patients with type 2 diabetes mellitus (T2D). We have previously demonstrated that stimulation of the exercise pressor reflex (EPR) during muscle contraction elicits exaggerated increases in renal sympathetic nerve activity (RSNA) contributing significantly to this heightened circulatory responsiveness. However, the mechanisms underlying this EPR dysfunction remain to be elucidated. Evidence suggests that the availability of brain insulin is reduced in T2D. Given that sympathetic activity is primarily regulated within the brain, we hypothesized that a decrease in brain insulin contributes to EPR‐induced sympathetic overactivity in this disease. To test this hypothesis, we measured changes in RSNA and mean arterial pressure (MAP) during stimulation of the EPR in decerebrated Sprague‐Dawley rats given either a normal diet (control) or a high fat diet (in combination with a one‐time low dose of streptozotocin to produce T2D) for 14–16 weeks. Compared to normal diet‐fed rats, animals receiving the high fat diet had significantly increased plasma insulin (1.6±0.2 vs. 4.1±0.4 ng/mL, P<0.05) as well as fasting blood glucose (94±5 vs. 120±5 mg/dL, P<0.05). Compared to control animals, stimulation of the EPR by electrically‐induced muscle contraction evoked significantly greater increases in RSNA (Δ = 36±10 vs. 122±20 %, P<0.05) and MAP (Δ = 15±1 vs. 49±5 mmHg, P<0.05) in T2D rats. Intracerebroventricular insulin administration significantly attenuated the exaggerated sympathetic (Δ = 111±20 vs. 50±17 %, P=0.05) and pressor responses (Δ = 49±5 vs. 12±4 mmHg, P<0.05) to EPR activation in T2D rats but did not have a significant effect in control animals (RSNA: Δ = 43±2 vs. 33±13 %; MAP: Δ = 15±1 vs. 7±4 mmHg). These findings suggest that the heightened cardiovascular response to exercise in T2D may be mediated by EPR‐induced sympathetic dysfunction resulting as a consequence of decreased insulin availability within the brain.Support or Funding InformationSupported by the Lawson & Rogers Lacy Research Fund in Cardiovascular Disease.

Overactivation of cannabinoid receptor type 1 in rostral ventrolateral medulla promotes cardiovascular responses in spontaneously hypertensive rats.

Stimulation of cannabinoid type 1 (CB1) receptor in the rostral ventrolateral medulla (RVLM) increases renal sympathetic nerve activity (RSNA) and blood pressure (BP) in rats. Thus, we hypothesized that abnormal expression of CB1 receptor in the RVLM may play a critical role in the pathogenesis of essential hypertension. We evaluated the effects of intra-RVLM infusions of arachidonyl-2'-chloroethylamide (ACEA), selective CB1 receptor agonist, with or without AM251, selective CB1 receptor antagonist, on BP, heart rate (HR), and RSNA in spontaneously hypertensive rats and wild-type rats. We also assessed the protein level and surface expression of CB1 receptor in the RVLM in these rats. We found that spontaneously hypertensive rats exhibited higher basal BP, HR, and RSNA than wild-type rats. Furthermore, unilateral intra-RVLM microinjections ACEA (0, 10, or 100 nM/0.5 μl/site) increased BP, HR, and RSNA to a greater extent in spontaneously hypertensive rats than in wild-type rats. These effects were abolished by co-administrations of AM251 (500 nM/0.5 μl/site) into the RVLM. In addition, the protein level of CB1 receptor in the RVLM was robustly increased in spontaneously hypertensive rats, which is correlated with ACEA-induced maximum changes of RSNA, and this was also associated with reduced expression of β-arrestin 2 in the RVLM in spontaneously hypertensive rats. Finally, overexpression of β-arrestin 2 in the RVLM in spontaneously hypertensive rats attenuated BP, HR and RSNA. Taken together, our results suggested that alterations of CB1 receptor desensitization in the RVLM may play a role in the pathogenesis of essential hypertension.

Estradiol alters the chemosensitive cardiac afferent reflex in female rats by augmenting sympathoinhibition and attenuating sympathoexcitation.

The chemosensitive cardiac vagal and sympathetic afferent reflexes are implicated in driving pathophysiological changes in sympathetic nerve activity (SNA) in cardiovascular disease states. This study investigated the impact of sex and ovarian hormones on the chemosensitive cardiac afferent reflex. Experiments were performed in anaesthetized, sinoaortic baroreceptor denervated male, female and ovariectomized female (OVX) Wistar rats with either intact cardiac innervation or bilateral vagotomy. To investigate the chemosensitive cardiac afferent reflexes renal SNA, heart rate (HR) and arterial pressure (AP) were recorded before and following application of capsaicin onto the epicardial surface of the left ventricle. Compared to males, ovary-intact females displayed similar cardiac afferent reflex mediated changes in renal SNA albeit with a reduced maximum sympathetic reflex driven increase in renal SNA. In females, ovariectomy significantly attenuated the cardiac vagal afferent reflex mediated inhibition of renal SNA (renal SNA decreased 2 ± 17% in OVX versus -50 ± 4% in ovary-intact females, P < 0.05) and augmented cardiac sympathetic afferent reflex mediated sympathoexcitation (renal SNA increased 91 ± 11% in OVX vs 62 ± 9% in ovary-intact females, P < 0.05) so that overall increases in reflex driven sympathoexcitation were significantly enhanced. Chronic estradiol replacement, but not progesterone replacement, begun at time of ovariectomy restored cardiac afferent reflex responses to be similar as ovary-intact females. Vagal denervation eliminated all group differences. The current findings show ovariectomy in female rats, mimicking menopause in women, results in greater chemosensitive cardiac afferent reflex driven sympathoexcitation and does so, at least partly, via the loss of estradiols actions on the cardiac vagal afferent reflex pathway.

Effects of Combined Oxytocin Receptor (OTR) with Vasopressin Receptor (V1aR or V1bR) Blockade within Paraventricular Nucleus (PVN) on Neurocardiovascular Responses

Vasopressin (VP) released from dendrites of magnocellular neurons within PVN may influence systemic hemodynamics and sympathetic nerve activity. PVN neurons possess all three receptor subtypes: V1aR, V1bR and OTR. We have shown that inhibition of any one of these receptors alone dose dependently blocks the rise in MAP and reflex changes in heart rate (HR) and renal sympathetic nerve activity (RSNA) with injection of VP into PVN. Here we test the hypothesis that a subthreshold dose of OTR antagonist combined with either subthreshold V1aR or V1bR blockers within PVN will result in additive inhibition of hemodynamic and RSNA changes induced by 100 ng VP into PVN. Male Sprague Dawley rats were instrumented and studies performed in awake unrestrained rats 24 ‐ 48 hr after recovery from anesthesia. Baseline parameters did not differ. VP increased MAP by 18.2 ± 3.1 mmHg (P &lt; 0.001) while HR and RSNA decreased 33 ± 14 bpm and 13.6 ±10.6 %baseline, respectively (P &lt;0.05). The response to combined subthreshold inhibitory doses of (d(CH₂)₅¹,Tyr(Me)²,Arg⁸)‐Vasopressin (V1aR, 20 ng), nelivaptan (V1bR, 0.5 ng) and (d(CH₂)₅¹,Tyr(Me)²,Thr⁴,Orn⁸,des‐Gly‐NH₂⁹)‐Vasotocin (OTR, 1 ng) attenuated the rise in MAP by 43% as well as the reflex changes in HR and RSNA. Neither V1aR+OTR blockade nor V1bR+ OTR blockade resulted in greater blockade of the responses (45% and 49%, respectively. Vehicle did not block any responses. These findings suggest that V1aR, V1bR and OTR activated pathways share intracellular and/or neural pathways that regulate system hemodynamics and sympathoexcitatory responses to VP within the PVN.

Pyridostigmine enhances vagal influence to the heart but increases renal sympathetic nerve activity in anesthetized rats (1169.14)

We evaluated effect of acute blockade of plasma acetylcholinesterase with pyridostigmine (PYR) on cardiac sympatho‐vagal balance and on electrical activity of the renal sympathetic nerve (RSN) in anesthetized rats.Cardiac sympathetic and vagal tone were assessed in urethane anesthetized rats by heart rate (HR) responses to propranolol (2 mg/kg, N=14) or methyl‐atropine (1 mg/kg, N=18), respectively, 60 min after treatment with PYR (0.25 mg/kg iv) or saline.Other rats, also under urethane anesthesia, were instrumented for AP and RSN activity recordings. After basal recording the rats received PYR (0.25 mg/kg, iv N=6) or saline (N=4) and the experiment last for 1h.PYR elicited a prompt bradycardia (‐73±6 bpm) that partially recovered, but HR remained lower (387±7 vs 445±9 bpm), 1h after treatment. Sympathetic tone was lower (31±6 vs 69±7 bpm) and vagal tone was higher (86±7 vs 44±5 bpm) in rats treated with PYR.A progressive increase in RSN activity, with no change in BP, was observed after treatment with PYR (40±3, 81±26 and 87±6% at 20, 40 and 60 min after PYR). No change in RSN activity was observed in rats treated with saline.Acute treatment with PYR was effective in enhance parasympathetic and reduce sympathetic tonic influence to the heart. Nevertheless, RSN recording revealed an acute increase in sympathetic nerve traffic elicited by PYR.Grant Funding Source: Supported by FAPESP, CAPES, CNPq.

Involvement of Renal Sympathetic Nerve Overactivation in the Progression of Chronic Kidney Disease in Rats

Time-dependent changes in the renal sympathetic nerve activity (RSNA) in the progression of chronic kidney disease (CKD) have not been investigated, despite the fact that renal sympathetic nervous system is augmented in the condition of CKD. In the present study, we examined time-dependent changes in RSNA and renal venous norepinephrine concentrations for 12 weeks using 5 of 6 nephrectomized CKD rats. Both RSNA and norepinephrine concentrations were increased during the early phase in the progression of CKD. Urinary protein excretion and systolic blood pressure (SBP) were gradually increased during 12 weeks after 5 of 6 nephrectomy. Treatment with γ-aminobutyric acid or the combination of prazosin and propranolol in the early phase (0-4 weeks) after 5 of 6 nephrectomy significantly attenuated the increases in urinary protein excretion and SBP in 5 of 6 nephrectomized rats. On the other hand, the same treatment in the late phase (8-12 weeks) after 5 of 6 nephrectomy failed to suppress the proteinuria and increase in SBP. Treatment with hydralazine at hypotensive dose for 12 weeks also failed to affect the proteinuria in 5 of 6 nephrectomized CKD rats. In conclusion, the augmentation of renal sympathetic nervous system in early phase after 5 of 6 nephrectomy is closely related to the development of partial ablation-induced CKD in rats.

Effects of exercise training on SFO-mediated sympathoexcitation during chronic heart failure

Exercise training (ExT) has been shown to reduce sympathetic drive during heart failure (HF). The subfornical organ (SFO) is involved in the neural control of sympathetic drive. We hypothesized that an activated SFO contributes to enhanced sympathetic activity in HF. We also postulated that ExT would reduce the activation of the SFO and its contribution to the sympathetic drive during HF. Sprague-Dawley rats were subjected to coronary artery ligation to induce HF. Rats were assigned to ExT for 3-4 wk. Rats with HF had a 2.5-fold increase in FosB-positive cells in the SFO compared with sham-operated rats, and this was normalized by ExT. Microinjection of ANG II (100 pmol) into the SFO resulted in a greater increase in renal sympathetic nerve activity (RSNA), blood pressure, and heart rate in the HF group than in the sham-operated group. These responses were normalized after ExT (change in RSNA: 23 ± 3% vs. 8 ± 2%). ExT also abolished the decrease in RSNA in HF rats after the microinjection of losartan (200 pmol) into the SFO (-21 ± 4% vs. -2 ± 3%). Finally, there was elevated mRNA (5-fold) and protein expression (43%) of ANG II type 1 receptors in the SFO of rats with HF, which were reversed after ExT. These data suggest that the enhanced activity of the SFO by elevated tonic ANG II contributes to the enhanced sympathoexcitation exhibited in HF. The decrease in ANG II type 1 receptor expression in the SFO by ExT may be responsible for reversing the neuronal activation in the SFO and SFO-mediated sympathoexcitation in rats with HF.

The neurosteroid metabolite of progesterone, 3α‐OH‐dihydroprogesterone (3α‐OH‐DHP), is required for attenuated baroreflex mediated sympathoexcitation in pregnancy

Exogenous 3á‐OH‐DHP mimics effects of pregnancy on arterial baroreflex (BX) control of renal sympathetic nerve activity (RSNA). Current experiments chronically blocked endogenous 3á‐OH‐DHP formation (Finasteride administration) and evaluated BX function in NP and near‐term pregnant (P, d21) rats. Either vehicle (Veh) or Finasteride (Fin, 25mg/kg) was given daily (s.cu.) for 7 days. RSNA electrodes were implanted on d5 of treatment. On d7, reflex changes in RSNA to increases (phenylephrine, iv) and decreases (nitroprusside, iv) in mean arterial pressure (MAP) were obtained in conscious rats and data fit to a logistic curve. As seen previously, in Veh treated P rats maximum RSNA was decreased (Veh‐P, 155 ± 14%; Veh‐NP, 335 ± 44% baseline), and baroreflex curve midpoint was shifted to lower MAP (Veh‐NP, 100 ± 4; Veh‐P, 84 ± 5 mmHg). Chronic Finasteride had no effect on BX function in Veh‐NP rats, but restored maximum RSNA in P rats (Fin‐P, 309 ± 25% baseline) to NP levels, while curve midpoint remained low (Fin‐P, 74 ± 3 mmHg). Thus, 3á‐OH‐DHP, likely through positive modulation of GABAA receptors in the RVLM, contributes to attenuated BX sympathoexcitation in P rats. NIH R01 HL091164 (CMH)

Abstract 21: Leptin-Induced Arterial Pressure Increase: Key Role of the PI3K Pathway

Leptin is an adipocyte-derived hormone that acts in the brain to decrease food intake and increases energy expenditure. Leptin also raises arterial pressure by increasing sympathetic nerve activity (SNA). We previously implicated PI3K as a downstream pathway in leptin receptor (ObRb) signaling that mediates the renal SNA effects of leptin. We used genetic approaches to test the hypothesis that bidirectional changes in PI3K activity (gain vs. loss of function) will be associated with contrasting changes in renal SNA and arterial pressure responses to leptin. PI3K signaling was enhanced in leptin-sensitive neurons in Pten ΔObRb mice in which the lipid phosphatase Pten was ablated in cells that express ObRb, leading to accumulating PIP3 in these cells. For loss of function, we used p110α D933A/WT mice which carry a heterozygous mutation in the p110α subunit of PI3K specifically disrupting this pathway. Consistent with our previous findings, the increase in renal SNA caused by intracerebralventricular (ICV) injection of leptin (2 μg) was exaggerated and blunted in Pten ΔObRb mice (286±41%) and p110α D933A/WT mice (-40±6%), respectively, relative to littermate controls (70±4%). These effects were specific to leptin, as the renal SNA response to ICV MTII (melanocortin receptors agonist) was comparable (P=0.3) between controls (196±36%), Pten ΔObRb mice (159±98%) and p110α D933A/WT mice (194±36%). Interestingly, the Pten ΔObRb mice had significantly (P&lt;0.01) higher baseline radiotelemetric mean arterial pressure (MAP, 137±4 mmHg) as compared to littermate controls (107±2 mmHg). In contrast, despite being hyperleptinemic the p110α D933A/WT mice had normal (P=0.34) baseline MAP (112±1 mmHg) as compared to controls (113±2 mmHg). We further measured the arterial pressure response to leptin treatment. Intraperitoneal administration of leptin (60 μg) raised MAP in control mice by 13±5 mmHg and this response was significantly (P&lt;0.05) enhanced in the Pten ΔObRb mice (+35±14 mmHg). Conversely, the p110α D933A/WT mice had a significantly blunted (P&lt;0.05) MAP response to leptin (-3±4 mmHg). Our data demonstrate the importance of PI3K signaling in leptin receptor-containing neurons for the regulation of arterial pressure and in mediating leptin effects.

Baroreflex control of renal sympathetic nerve activity in mice with cardiac hypertrophy

Altered renal sympathetic nerve activity (RSNA) plays a major role in the progression of cardiac hypertrophy. We aimed to evaluate the baroreflex control of RSNA in mice with cardiac hypertrophy. Swiss Webster mice were treated with isoproterenol (15 μg/g/day, s.c.) or vehicle and the baroreflex evaluation was performed by measuring changes in RSNA in response to changes in arterial pressure. The maximal gain of the reflex changes in RSNA was reduced in isoproterenol-treated animals (1.39 ± 0.08%/mm Hg) in comparison with vehicle-treated animals (1.77 ± 0.10%/mm Hg). Therefore, we can conclude that cardiac hypertrophy led to a reduced sensitivity of baroreflex control of RSNA.

Effects of sex and ovarian hormones on the initial renal sympathetic nerve activity response to myocardial infarction

The physiological mechanisms contributing to sex differences following myocardial infarction (MI) are poorly understood. Given the strong relationship between sympathetic nerve activity (SNA) and outcome, we hypothesized there may be a sex difference in SNA responses to MI. In anaesthetized, open-chest male, female and ovariectomized (OVX) female Wistar rats, mean arterial pressure, heart rate and renal SNA were recorded in response to ligation of the left coronary artery. In males, renal SNA increased by 30 ± 6% in the first minute of coronary occlusion (P < 0.05) and remained elevated at 18 ± 7% above baseline (P < 0.05) at 2 h following MI. In response to MI, ovary-intact females displayed no change in renal SNA, whereas OVX females displayed a significant increase, similar to that seen in the males (increases of 43 ± 11% at 1 min and 21 ± 7% at 2 h post-MI, P < 0.05 versus intact females). Arterial baroreflex control of renal SNA had a smaller range in females (ovary intact and OVX) than males; no changes in arterial baroreflex responses were observed 1 h post-MI in males or females. Denervating the arterial baroreceptors abolished the renal SNA response to MI in the males, whereas in ovary-intact females and OVX females the response was unaltered. These findings suggest that ovarian hormones are able to blunt the initial sympathetic activation post-MI in females and that the importance of the arterial baroreflex in mediating initial sympathetic activation post-MI is different between the sexes.