Role of NaV1.7 in postganglionic sympathetic nerve function in human and guinea-pig arteries.

NaV1.7 plays a crucial role in inducing and conducting action potentials in pain-transducing sensory nociceptor fibres, suggesting that NaV1.7 blockers could be effective non-opioid analgesics. While SCN9A is expressed in both sensory and autonomic neurons, its functional role in the autonomic system remains less established. Our single neuron rt-PCR analysis revealed that 82% of sympathetic neurons isolated from guinea-pig stellate ganglia expressed NaV1.7 mRNA, with NaV1.3 being the only other tetrodotoxin-sensitive channel expressed in approximately 50% of neurons. We investigated the role of NaV1.7 in conducting action potentials in postganglionic sympathetic nerves and in the sympathetic adrenergic contractions of blood vessels using selective NaV1.7 inhibitors. Two highly selective NaV1.7 blockers, GNE8493 and PF 05089771, significantly inhibited postganglionic compound action potentials by approximately 70% (P < 0.01), with residual activity being blocked by the NaV1.3 inhibitor, ICA 121431. Electrical field stimulation (EFS) induced rapid contractions in guinea-pig isolated aorta, pulmonary arteries, and human isolated pulmonary arteries via stimulation of intrinsic nerves, which were inhibited by prazosin or the NaV1 blocker tetrodotoxin. Our results demonstrated that blocking NaV1.7 with GNE8493, PF 05089771, or ST2262 abolished or strongly inhibited sympathetic adrenergic responses in guinea-pigs and human vascular smooth muscle. These findings support the hypothesis that pharmacologically inhibiting NaV1.7 could potentially reduce sympathetic and parasympathetic function in specific vascular beds and airways. KEY POINTS: 82% of sympathetic neurons isolated from the stellate ganglion predominantly express NaV1.7 mRNA. NaV1.7 blockers inhibit action potential conduction in postganglionic sympathetic nerves. NaV1.7 blockade substantially inhibits sympathetic nerve-mediated adrenergic contractions in human and guinea-pig blood vessels. Pharmacologically blocking NaV1.7 profoundly affects sympathetic and parasympathetic responses in addition to sensory fibres, prompting exploration into the broader physiological consequences of NaV1.7 mutations on autonomic nerve activity.

Postganglionic sympathetic neurons, but not locus coeruleus optostimulation, activates neuromuscular transmission in the adult mouse in vivo.

Recent work demonstrated that sympathetic neurons innervate the skeletal muscle near the neuromuscular junction (NMJ), and muscle sympathectomy and sympathomimetic agents strongly influence motoneuron synaptic vesicle release ex vivo. Moreover, reports attest that the pontine nucleus locus coeruleus (LC) projects to preganglionic sympathetic neurons and regulates human mobility and skeletal muscle physiology. Thus, we hypothesized that peripheral and central sympathetic neurons projecting directly or indirectly to the skeletal muscle regulate NMJ transmission. The aim of this study was to define the specific neuronal groups in the peripheral and central nervous systems that account for such regulation in adult mice in vivo by using optogenetics and NMJ transmission recordings in 3-5-month-old, male and female ChR2(H134R/EYFP)/TH-Cre mice. After detecting ChR2(H134R)/EYFP fluorescence in the paravertebral ganglia and LC neurons, we tested whether optostimulating the plantar nerve near the lumbricalis muscle or LC neurons effectively modulates motor nerve terminal synaptic vesicle release in living mice. Nerve optostimulation increased motor synaptic vesicle release in vitro and in vivo, while the presynaptic adrenoceptor blockers propranolol (ß1/ß2) and atenolol (ß1) prevented this outcome. The effect is primarily presynaptic since miniature end-plate potential (MEPP) kinetics remained statistically unmodified after stimulation. In contrast, optostimulation of LC neurons did not regulate NMJ transmission. In summary, we conclude that postganglionic sympathetic neurons, but not LC neurons, increased NMJ transmission by acting on presynaptic ß1-adrenergic receptors in vivo.

Human epicardium-derived cells reinforce cardiac sympathetic innervation.

RATIONALE: After cardiac damage, excessive neurite outgrowth (sympathetic hyperinnervation) can occur, which is related to ventricular arrhythmias/sudden cardiac death. Post-damage reactivation of epicardium causes epicardium-derived cells (EPDCs) to acquire a mesenchymal character, contributing to cardiac regeneration. Whether EPDCs also contribute to cardiac re/hyperinnervation, is unknown. AIM: To investigate whether mesenchymal EPDCs influence cardiac sympathetic innervation. METHODS AND RESULTS: Sympathetic ganglia were co-cultured with mesenchymal EPDCs and/or myocardium, and neurite outgrowth and sprouting density were assessed. Results showed a significant increase in neurite density and directional (i.e. towards myocardium) outgrowth when ganglia were co-cultured with a combination of EPDCs and myocardium, as compared to cultures with EPDCs or myocardium alone. In absence of myocardium, this outgrowth was not directional. Neurite differentiation of PC12 cells in conditioned medium confirmed these results via a paracrine effect, in accordance with expression of neurotrophic factors in myocardial explants co-cultured with EPDCs. Of interest, EPDCs increased the expression of nerve growth factor (NGF) in cultured, but not in fresh myocardium, possibly due to an "ischemic state" of cultured myocardium, supported by TUNEL and Hif1α expression. Cardiac tissues after myocardial infarction showed robust NGF expression in the infarcted, but not remote area. CONCLUSION: Neurite outgrowth and density increases significantly in the presence of EPDCs by a paracrine effect, indicating a new role for EPDCs in the occurrence of sympathetic re/hyperinnervation after cardiac damage.

Sesame lignans increase sympathetic nerve activity and blood flow in rat skeletal muscles.

Beneficial effects of sesame lignans, especially antioxidative effects, have been widely reported; however, its potential effects on autonomic nerves have not yet been investigated. Therefore, the current study aimed to investigate the effect of sesame lignans on the autonomic nervous system. The sympathetic nerve activity in rat skeletal muscle was measured using electrophysiological approaches, with blood flow determined using the laser Doppler method. Sesame lignans were administered intragastrically at 2 and 20 mg/kg, and after 60 min, the sympathetic nerve activity was observed to increase by 45.2% and 66.1%, respectively. A significant increase in blood flow (39.6%) was also observed for the 20-mg/kg dose when measured at 55 min after administration. These sympathomimetic effects were completely prevented by subdiaphragmatic vagotomy, and the increase in blood flow was eliminated in the presence of the beta2-adrenergic receptor inhibitor butoxamine. Thus, it is proposed that sesame lignans can increase the blood flow of skeletal muscle, possibly by exciting sympathetic nerve activity through the afferent vagal nerve.

Elimination of spurious absent sweat response in QSWEAT recordings.

INTRODUCTION: Forearm QSWEAT recordings are occasionally absent in females, likely due to high skin resistance. METHODS: We identified consecutive subjects with no sudomotor abnormalities but absent/markedly reduced QSWEAT forearm volume, and repeated QSWEAT at the same site after gentle abrasion. RESULTS: QSWEAT volumes were absent for 4 subjects and markedly reduced for the other 4 (median 0.01, IQR 0-0.03). After gentle skin abrasion, repeat volumes were significantly higher for all subjects and became normal in 7 of 8 subjects. DISCUSSION: Skin abrasion restores QSWEAT volumes in previously absent/markedly reduced site suggesting that skin preparation using abrasion is more effective.

Dramatically Amplified Thoracic Sympathetic Postganglionic Excitability and Integrative Capacity Revealed with Whole-Cell Patch-Clamp Recordings.

Thoracic paravertebral sympathetic postganglionic neurons (tSPNs) comprise the final integrative output of the distributed sympathetic nervous system controlling vascular and thermoregulatory systems. Considered a non-integrating relay, what little is known of tSPN intrinsic excitability has been determined by sharp microelectrodes with presumed impalement injury. We thus undertook the first electrophysiological characterization of tSPN cellular properties using whole-cell recordings and coupled results with a conductance-based model to explore the principles governing their excitability in adult mice of both sexes. Recorded membrane resistance and time constant values were an order of magnitude greater than values previously obtained, leading to a demonstrable capacity for synaptic integration in driving recruitment. Variation in membrane resistivity was the primary determinant controlling cell excitability with vastly lower currents required for tSPN recruitment. Unlike previous microelectrode recordings in mouse which observed inability to sustain firing, all tSPNs were capable of repetitive firing. Computational modeling demonstrated that observed differences are explained by introduction of a microelectrode impalement injury conductance. Overall, tSPNs largely linearly encoded injected current magnitudes over a broad frequency range with distinct subpopulations differentiable based on repetitive firing signatures. Thus, whole-cell recordings reveal tSPNs have more dramatically amplified excitability than previously thought, with greater intrinsic capacity for synaptic integration and with the ability for maintained firing to support sustained actions on vasomotor tone and thermoregulatory function. Rather than acting as a relay, these studies support a more responsive role and possible intrinsic capacity for tSPNs to drive sympathetic autonomic function.

The role of the paravertebral ganglia in human sympathetic neural discharge patterns.

KEY POINTS: The mechanisms affecting recruitment patterns of postganglionic sympathetic nerves remain unclear. The divergent and convergent preganglionic innervation patterns of postganglionic neurons and the presence of differently sized postganglionic nerves suggest that the ganglia may participate in modifying the discharge patterns of single sympathetic postganglionic neurons innervating the skeletal muscle circulation. Whether the ganglia affect the ordered behaviour of varying sized postganglionic sympathetic neurons in humans has not been studied. Trimethaphan infusion produced an ordered pattern of action potential (AP) de-recruitment whereby the firing of larger, low probability APs present at baseline was abolished first, followed by progressive decreased probability of smaller APs. Although integrated sympathetic bursts were no longer detected after several minutes of trimethaphan, firing of the smallest APs was detected. These data suggest the ganglia affect the distribution of firing probabilities exhibited by differently sized sympathetic neurons. The ganglia may contribute to sympathetic neural emission patterns involved in homeostatic regulation. ABSTRACT: Do the ganglia contribute to the ordered behaviour of postganglionic neuronal discharge within the sympathetic nervous system? To further understand the functional organization of the sympathetic nervous system we employed the microneurographic approach to record muscle sympathetic nerve activity (MSNA) and a continuous wavelet transform to study postganglionic action potential (AP) behaviour during nicotinic blockade at the ganglia (trimethaphan camsylate, 1-7 mg min-1 ) in seven females (37 ± 5 years). Trimethaphan elicited a progressive reduction in sympathetic outflow characterized by fewer integrated bursts with decaying amplitude. Underlying trimethaphan-mediated attenuations in integrated MSNA were reductions in AP incidence (186 ± 101 to 29 ± 31 AP (100 beats)-1 ) and AP content per integrated burst (7 ± 2 to 3 ± 1 APs burst-1 ) (both P < 0.01) in the final minute of detectable bursting activity in the trimethaphan condition, compared to baseline. We observed an ordered de-recruitment of larger to smaller AP clusters active at baseline (14 ± 3 to 8 ± 2 active AP clusters, P < 0.01). Following cessation of integrated bursts in the trimethaphan condition, the smallest 6 ± 2 sympathetic AP clusters persisted to fire in an asynchronous pattern (49 ± 41 AP (100 beats)-1 ) in all participants. Valsalva's manoeuvre did not increase the incidence of these persistent APs (60 ± 42 AP (100 beats)-1 , P = 0.52), or recruit any larger APs in six of seven participants (6 ± 1 total AP clusters, P = 0.30). These data suggest that the ganglia participate in the ordered recruitment of differently sized postganglionic sympathetic nerves.

Physiological and pathophysiological firing properties of single postganglionic sympathetic neurons in humans.

It has long been known from microneurographic recordings in human subjects that the activity of postganglionic sympathetic axons occurs as spontaneous bursts, with muscle sympathetic nerve activity (MSNA) exhibiting strong cardiac rhythmicity via the baroreflex and skin sympathetic nerve activity showing much weaker cardiac modulation. Here we review the firing properties of single sympathetic neurons, obtained using highly selective microelectrodes. Individual vasoconstrictor neurons supplying muscle or skin, or sudomotor neurons supplying sweat glands, always discharge with a low firing probability (~30%) and at very low frequencies (~0.5 Hz). Moreover, they usually fire only once per cardiac interval but can fire greater than four times within a burst. Modeling has shown that this pattern can best be explained by individual neurons being driven by, on average, two preganglionic inputs. Unitary recordings of muscle vasoconstrictor neurons have been made in several pathophysiological states, including heart failure, hypertension, obstructive sleep apnea, bronchiectasis, chronic obstructive pulmonary disease, depression, and panic disorder. The augmented MSNA in each of these diseases features an increase in firing probability and discharge frequency of individual muscle vasoconstrictor neurons above that seen in healthy subjects, yet firing rates rarely exceed 1 Hz. However, unlike patients with heart failure, all patients with respiratory disease or panic disorder, and patients with hyperhidrosis, exhibited an increase in multiple within-burst firing, which emphasizes the different modes by which the sympathetic nervous system grades its output in pathophysiological states of high sympathetic nerve activity.

Cardiac sympathetic afferent reflex control of cardiac function in normal and chronic heart failure states.

KEY POINTS: Cardiac sympathetic afferents are considered to be essential pathways for transmission of cardiac nociception to the central nervous system during myocardial ischaemia. However, a potential contribution of the CSAR control of cardiac dysfunction in both normal and chronic heart failure (CHF) states remains unknown. We found that activation of the CSAR evokes little increase in cardiac contractility with an exaggerated peripheral vasoconstriction in the CHF state. CSAR inhibition by epicardial lidocaine decreased cardiac contractility to a greater extent in CHF rats than sham rats. Furthermore, we also found that epicardial lidocaine paradoxically decreased left ventricular end-diastolic pressure (LVEDP) and left ventricular end-diastolic volume (preload) in CHF rats, which was not observed in sham rats. Chronic ablation of the CSAR by epicardial application of the afferent neurotoxin, RTX, selectively lowered diastolic blood pressure CHF rats. The observation suggests that CSAR has a differential effect on cardiac function in normal and CHF states. CSAR activation in normal state causes significant increase in cardiac contractility and cardiac output. ABSTRACT: The enhanced 'cardiac sympathetic afferent reflex' (CSAR) critically contributes to the exaggerated global sympathetic tone in chronic heart failure (CHF). However, a potential contribution of the cardio-cardiac reflex control of cardiac function in both normal and CHF states remains unknown. In this study, we evaluated the effects of direct activation or inhibition of the CSAR on cardiac function by pressure-volume (P-V) loop analysis in ∼12-week sham-operated and myocardial infarcted (MI) rats. In sham rats, acute CSAR activation by epicardial application of bradykinin (BK) increased heart rate (HR), left ventricular systolic pressure (LVSP), the maximum first derivative of left ventricular pressure (dp/dtmax ), and the slope of the end-systolic P-V relationship (ESPVR), suggesting that acute CSAR activation in the normal state enhances myocardial contractility. CSAR activation also decreased left ventricular (LV) systolic and diastolic volumes with little effect on LV end-diastolic pressure (LVEDP) or the end-diastolic P-V relationship (EDPVR) in sham rats. Compared to sham, CHF rats exhibit a reduced increase in the slope of the ESPVR and dp/dtmax in response to BK, indicating a poor contractile response to CSAR activation. Interestingly, BK application in CHF rats increased cardiac systolic and diastolic volumes and further increased the elevated LVEDP, neither of which was seen in sham rats. Following CSAR inhibition by epicardial lidocaine, blood pressure, HR, LVSP, dp/dt, LVEDP and ESPVR decreased in CHF rats whereas lidocaine had little effect in sham rats, indicating that the CSAR is tonically active in CHF and contributes to cardiac dysfunction. Furthermore, we found that epicardial lidocaine paradoxically decreased LV end-diastolic volume (preload) in CHF rats, which was not observed in sham rats. The decreased preload by lidocaine in CHF rats may be due to a reduction in peripheral vascular resistance since epicardial lidocaine significantly lowered peripheral (renal) sympathetic nerve activity in CHF rats but not in sham rats. Furthermore, chronic ablation of CSAR by epicardial application of a selective afferent neurotoxin, resiniferatoxin, selectively lowered diastolic blood pressure both at daytime and night-time with less effect on systolic blood pressure in CHF rats. Our data suggest that there is an imbalance between cardiac and peripheral responses to CSAR in CHF animals compared to sham-operated controls.

Carotid sinus denervation ameliorates renovascular hypertension in adult Wistar rats.

KEY POINTS: Peripheral chemoreflex sensitization is a feature of renovascular hypertension. Carotid sinus nerve denervation (CSD) has recently been shown to relieve hypertension and reduce sympathetic activity in other rat models of hypertension. We show that CSD in renovascular hypertension halts further increases in blood pressure. Possible mechanisms include improvements in baroreceptor reflex sensitivity and renal function, restoration of cardiac calcium signalling towards control levels, and reduced neural inflammation. Our data suggest that the peripheral chemoreflex may be a viable therapeutic target for renovascular hypertension. ABSTRACT: The peripheral chemoreflex is known to be hyper-responsive in both spontaneously hypertensive (SHR) and Goldblatt hypertensive (two kidney one clip; 2K1C) rats. We have previously shown that carotid sinus nerve denervation (CSD) reduces arterial blood pressure (ABP) in SHR. In the present study, we show that CSD ameliorates 2K1C hypertension and reveal the potential underlying mechanisms. Adult Wistar rats were instrumented to record ABP via telemetry, and then underwent CSD (n = 9) or sham CSD (n = 9) 5 weeks after renal artery clipping, in comparison with normal Wistar rats (n = 5). After 21 days, renal function was assessed, and tissue was collected to assess sympathetic postganglionic intracellular calcium transients ([Ca2+ ]i ) and immune cell infiltrates. Hypertensive 2K1C rats showed a profound elevation in ABP (Wistar: 98 ± 4 mmHg vs. 2K1C: 147 ± 8 mmHg; P < 0.001), coupled with impairments in renal function and baroreflex sensitivity, increased neuroinflammatory markers and enhanced [Ca2+ ]I in stellate neurons (P < 0.05). CSD reduced ABP in 2K1C+CSD rats and prevented the further progressive increase in ABP seen in 2K1C+sham CSD rats, with a between-group difference of 14 ± 2 mmHg by week 3 (P < 0.01), which was accompanied by improvements in both baroreflex control and spectral indicators of cardiac sympatho-vagal balance. Furthermore, CSD improved protein and albuminuria, decreased [Ca2+ ]i evoked responses from stellate neurons, and also reduced indicators of brainstem inflammation. In summary, CSD in 2K1C rats reduces the hypertensive burden and improves renal function. This may be mediated by improvements in autonomic balance, functional remodelling of post-ganglionic neurons and reduced inflammation. Our results suggest that the peripheral chemoreflex may be considered as a potential therapeutic target for controlling renovascular hypertension.

Individual sympathetic postganglionic neurons coinnervate myenteric ganglia and smooth muscle layers in the gastrointestinal tract of the rat.

A full description of the terminal architecture of sympathetic axons innervating the gastrointestinal (GI) tract has not been available. To label sympathetic fibers projecting to the gut muscle wall, dextran biotin was injected into the celiac and superior mesenteric ganglia (CSMG) of rats. Nine days postinjection, animals were euthanized and stomachs and small intestines were processed as whole mounts (submucosa and mucosa removed) to examine CSMG efferent terminals. Myenteric neurons were counterstained with Cuprolinic Blue; catecholaminergic axons were stained immunohistochemically for tyrosine hydroxylase. Essentially all dextran-labeled axons (135 of 136 sampled) were tyrosine hydroxylase-positive. Complete postganglionic arbors (n = 154) in the muscle wall were digitized and analyzed morphometrically. Individual sympathetic axons formed complex arbors of varicose neurites within myenteric ganglia/primary plexus and, concomitantly, long rectilinear arrays of neurites within circular muscle/secondary plexus or longitudinal muscle/tertiary plexus. Very few CSMG neurons projected exclusively (i.e., ∼100% of an arbor's varicose branches) to myenteric plexus (∼2%) or smooth muscle (∼14%). With less stringent inclusion criteria (i.e., ≥85% of an axon's varicose branches), larger minorities of neurons projected predominantly to either myenteric plexus (∼13%) or smooth muscle (∼27%). The majority (i.e., ∼60%) of all individual CSMG postganglionics formed mixed, heterotypic arbors that coinnervated extensively (>15% of their varicose branches per target) both myenteric ganglia and smooth muscle. The fact that ∼87% of all sympathetics projected either extensively or even predominantly to smooth muscle, while simultaneously contacting myenteric plexus, is consistent with the view that these neurons control GI muscle directly, if not exclusively. J. Comp. Neurol. 524:2577-2603, 2016. © 2016 Wiley Periodicals, Inc.

Skin sympathetic function in complex regional pain syndrome type 1.

PURPOSE: Cutaneous sympathetic pathophysiology in complex regional pain syndrome type 1 (CRPS-1) is not yet completely understood. To evaluate cutaneous sympathetic dysfunction in CRPS-1, we evaluated sympathetic sweat response (SSwR) and skin vasomotor reflex (SkVR) in CRPS-1 patients. METHODS: We studied 10 CRPS-1 patients (age 41 ± 13 years; 5 females and 5 males; disease duration 20 ± 22 months) and 10 healthy subjects (age 44 ± 13 years; 3 females and 7 males). SkVRs and SSwRs to several sympathetic activating procedures were recorded on the palms of the CRPS-1 patients (affected side) and controls (right side). RESULTS: There were no significant differences in the baselines of sweat output and skin blood flow between the CRPS-1 and control groups. SSwR and SkVR amplitudes were significantly lower in the CRPS-1 group than in the control group. There was no significant correlation between disease duration and SSwR or SkVR amplitudes among the patients. CONCLUSIONS: The reduced SSwRs and SkVRs in the affected limb of our CRPS-1 patients may reflect underlying damage to the sympathetic postganglionic fibres.

Pulse Photoplethysmographic Analysis Estimates the Sympathetic Activity Directed to Heart and Vessels.

BACKGROUND: Novel pulse photoplethysmographic-derived indices have been proposed as tools to measure autonomic nervous system (ANS) modulation in anesthetized and awake patients, but nowadays their experimental validation is lacking. The authors aimed to investigate the ability of pulse photoplethysmographic amplitude (PPGA), ANS state (ANSS), and ANSS index (ANSSi) to measure changes of ANS modulation in response to sympathetic stimulation. METHODS: Ten awake healthy volunteers underwent two passive head-up tilts at 45° and 90°. The heart rate variability (HRV) and systolic arterial pressure variability were analyzed in the frequency domain as a measure of ANS modulation directed to the heart and the vessels. HRV, baroreflex sensitivity, and pulse photoplethysmographic indices were measured at baseline and after tilt maneuvers. The agreement between HRV-derived indices and pulse photoplethysmographic indices was assessed using Bland-Altman plots. RESULTS: PPGA, ANSS, and ANSSi changed significantly during the study protocol. Head-up tilt decreased PPGA and ANSS and increased ANNSi. There was a good agreement between ANSSi and baroreflex sensitivity explored in the high-frequency band (bias, 0.23; 95% CI, -22.7 to 23.2 normalized units) and between ANSSi and the sympathovagal modulation directed to the heart (bias, 0.96; 95% CI, -8.7 to 10.8 normalized units). CONCLUSIONS: In controlled experimental conditions, novel pulse plethysmographic indices seem to estimate the changes of the sympathetic outflow directed to the vessels and the sympathovagal balance modulating heart rate. These indices might be useful in the future to monitor the fluctuation of sympathetic activity in anesthetized patients.

The sympathetic nervous system alterations in human hypertension.

Several articles have dealt with the importance and mechanisms of the sympathetic nervous system alterations in experimental animal models of hypertension. This review addresses the role of the sympathetic nervous system in the pathophysiology and therapy of human hypertension. We first discuss the strengths and limitations of various techniques for assessing the sympathetic nervous system in humans, with a focus on heart rate, plasma norepinephrine, microneurographic recording of sympathetic nerve traffic, and measurements of radiolabeled norepinephrine spillover. We then examine the evidence supporting the importance of neuroadrenergic factors as promoters and amplifiers of human hypertension. We expand on the role of the sympathetic nervous system in 2 increasingly common forms of secondary hypertension, namely hypertension associated with obesity and with renal disease. With this background, we examine interventions of sympathetic deactivation as a mode of antihypertensive treatment. Particular emphasis is given to the background and results of recent therapeutic approaches based on carotid baroreceptor stimulation and radiofrequency ablation of the renal nerves.

Paradoxical muscle sympathetic reflex activation in human heart failure.

BACKGROUND: Muscle sympathetic activation in heart failure with reduced ejection fraction (HFrEF) has been attributed, on the basis of multiunit recordings, to attenuated inhibitory feedback from stretch-sensitive cardiopulmonary mechanoreceptors. However, such preparations integrate 2 populations of single units exhibiting directionally opposite firing when atrial pressure is perturbed. We tested the hypothesis that the proportion of single units firing paradoxically when filling pressure increases is augmented in HFrEF. METHODS AND RESULTS: Muscle sympathetic nerve activity and estimated central venous pressure were recorded during nonhypotensive lower body negative pressure (LBNP; -10 mm Hg) and nonhypertensive positive pressure (LBPP; +10 mm Hg) in 11 treated HFrEF (left ventricular ejection fraction 25 ± 6% [mean ± standard deviation]) patients and 14 similarly aged controls. Single-unit muscle sympathetic nerve activity discharge was termed either anticipated, if firing frequency exhibited classic negative-feedback responses, or paradoxical. LBNP and LBPP had no heart rate, stroke volume, or blood pressure effects (P>0.05). Estimated central venous pressure decreased with LBNP (P<0.05), increased with LBPP (P<0.05), and was consistently higher in HFrEF (P<0.05). During LBNP, the ratio of single units with anticipated and paradoxical discharge was similar in HFrEF (18:7) and controls (27:5), whereas LBPP elicited paradoxical reflex excitation in a greater proportion of HFrEF single units (7:18 versus 24:6; P=0.0001). Consequently, LBPP increased mean single-unit firing frequency (P<0.05) and did not inhibit multiunit muscle sympathetic nerve activity of HFrEF subjects (P<0.05 versus controls). Firing of 12/18 HFrEF (but no control) single units increased during both LBPP and LBNP. CONCLUSION: These findings provide the first evidence in human HFrEF for an augmented excitatory cardiopulmonary-muscle sympathetic nerve activity reflex response to increased preload, incorporating 2 distinct single-unit populations with differing firing properties.

Segmental origins of cardiac sympathetic nerve activity in rats.

The segmental origins of cardiac sympathetic nerve activity (CSNA) were investigated in 8 urethane-anesthetized, artificially ventilated rats. The left upper thoracic sympathetic chain was exposed retropleurally after removing the heads of the second to fourth ribs. The preganglionic inputs to the chain from segments T1-T3 and the trunk distal to T3 were marked for later sectioning. CSNA was recorded conventionally, amplified, rectified and smoothed. Its mean level was quantified before and after each preganglionic input was cut, usually in rostro-caudal sequence. The level after all inputs were cut (i.e. noise and residual ECG pickup) was subtracted from previous measurements. The signal decrement from cutting each preganglionic input was then calculated as a percentage. CSNA in all rats depended on preganglionic drive from two or more segments, which were not always contiguous. Over the population, most preganglionic drive came from T3 and below, while the least came from T1. But there was striking inter-individual variation, such that the strongest drive to CSNA in any one rat could come from T1, T2, T3, or below T3. These findings provide new functional data on the segmental origins of CSNA in rats.

The kidney and the sympathetic system: a short review.

Despite the well documented and very effective non pharmacologic and pharmacologic therapies, hypertension remains often poorly controlled. There is still room for improvements in blood pressure control and recent technological advances have generated a regained interest in the physiopathology of renal sympathetic innervation in hypertension. In this article we review the evidence that renal sympathetic activity is increased in essential hypertension. The postganglionic sympathetic fibers are directed to the afferent and efferent renal arterioles, the juxtaglomerular apparatus, the proximal renal tubule, the loop of Henle, as well as the distal renal tubule and are under the control of many reflex loops, which are summarized in a most comprehensive manner for an unfamiliar reader within this field of research. Studies on renal denervation have provided further insights on the role of the sympathetic system in the kidneys, however their proper interpretation requires a special attention to the experimental protocols, as is explained in the text. Last, the possibility of kidney reinnervation is discussed, as well as the emerging evidence that the kidney is also a sensory organ. In summary, this review article provides a strong scientific background to understand not only the mechanisms of the hypotensive effects, but also those of possible pitfalls, of renal denervation.

Evidence from bilateral recordings of sympathetic nerve activity for lateralisation of vestibular contributions to cardiovascular control.

Using low-frequency (0.08-0.18 Hz) sinusoidal galvanic vestibular stimulation (sGVS), we recently showed that two peaks of modulation of muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA) occurred for each cycle of stimulation: a large peak associated with the positive peak of the sinusoid (defined as the primary peak) and a smaller peak (defined as the secondary peak) related to the negative peak of the sinusoid. However, these recordings were only made from the left common peroneal nerve, so to investigate lateralisation of vestibulosympathetic reflexes, concurrent recordings were made from both sides of the body. Tungsten microelectrodes were inserted into muscle or cutaneous fascicles of the left and right common peroneal nerves in 17 healthy individuals. Bipolar binaural sinusoidal GVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08 Hz. Cross-correlation analysis revealed that vestibular modulation of MSNA (10 bilateral recordings) and SSNA (6 bilateral recordings) on the left side was expressed as a primary peak related to the positive phase of the sinusoid and a secondary peak related to the negative phase of the sinusoid. Conversely, on the right side, the primary and secondary peaks were reversed: the secondary peak on the right coincided with the primary peak on the left and vice versa. Moreover, differences in pattern of outflow were apparent across sides. We believe the results support the conclusion that the left and right vestibular nuclei send both an ipsilateral and contralateral projection to the left and right medullary output nuclei from which MSNA and SSNA originate. This causes a "flip-flop" patterning between the two sympathetic outflows: when vestibular modulation of a burst is high on the left, it is low on the right, and when modulation is low on the left, it is high on the right.

Sympathetic nerve activity in stress-induced cardiomyopathy.

PURPOSE: To evaluate directly recorded efferent sympathetic nerve traffic in patients with stress-induced cardiomyopathy (SIC). BACKGROUND: SIC is a syndrome affecting mostly postmenopausal women following severe emotional stress. Though the precise pathophysiology is not well understood, a catecholamine overstimulation of the myocardium is thought to underlie the pathogenesis. METHODS: Direct recordings of multiunit efferent postganglionic muscle sympathetic nerve activity (MSNA) were obtained from 12 female patients, 5 in the acute (24-48 h) and 7 in the recovery phase (1-6 months), with apical ballooning pattern and 12 healthy matched controls. MSNA was expressed as burst frequency (BF), burst incidence (BI) and relative median burst amplitude (RMBA %). One of the twelve patients in this study was on beta blockade treatment due to a different illness, at time of onset of SIC. All patients were investigated with ongoing medication. RESULTS: MSNA was lower in patients with SIC as compared to matched controls, but did not differ between the acute and recovery phase of SIC. RMBA %, blood pressure and heart rate did not differ between the groups. CONCLUSION: MSNA is shown to be lower in patients with SIC compared to healthy controls, suggesting that sympathetic neuronal outflow is rapidly reduced following the initial phase of SIC. A distension of the ventricular myocardium, due to excessive catecholamine release over the heart in the acute phase, may increase the firing rate of unmyelinated cardiac c-fibre afferents resulting in widespread sympathetic inhibition. Such a mechanism may underlie the lower MSNA reported in our patients.