Recording sympathetic nerve activity in conscious humans and other mammals: guidelines and the road to standardization

Inter-individual variability in muscle sympathetic nerve activity at rest and during exercise: Disconnection with blood pressure

Microneurographic recordings of muscle sympathetic nerve activity (MSNA) have provided fundamental insight into sympathetic discharge patterns at rest and during exercise in health and disease. A key feature of MSNA recordings at rest is a large inter-individual variability, even among healthy adults. The physiological consequences of inter-individual variability in MSNA are commonly discussed as being associated with the regulation of resting blood pressure. However, available evidence from large cross-sectional analyses demonstrate a near absence of an association between resting MSNA and blood pressure. Less appreciated, MSNA also exhibits inter-individual variability in response to stress, such as exercise. Again, the consequences of variability in MSNA are unclear and can be dissociated from the blood pressure response, particularly at low-to-moderate intensity muscle contractions for short durations (≤2 min). In this brief review, we summarize several examples of how inter-individual variability in MSNA is unrelated to blood pressure control at rest and during exercise and discuss potential mechanisms responsible for this observation, and key methodological considerations for future study design and interpretation. Additionally, we highlight several unanswered questions that could pave the way for future investigations in the field.

Differential effects of female aging on sympathetic blood pressure regulation at rest and during stress in humans

Older female (OF) adults exhibit blunted resting sympathetic blood pressure (BP) transduction compared with young female (YF) adults, affecting BP regulation. However, studies often lack control over health factors like body composition or habitual physical activity. Therefore, we compared resting sympathetic BP transduction and neurovascular responses during cold pressor test (CPT) and end-expiratory apnea between YF (n = 12) and OF (n = 9) matched for several health factors. We measured beat-to-beat hemodynamics and muscle sympathetic nerve activity (MSNA). OF exhibited higher resting supine BP and MSNA (ps < 0.001) than YF. OF exhibited blunted increases in mean BP and diastolic BP following spontaneous MSNA bursts at rest. During the CPT, OF exhibited a smaller percent increase in total MSNA (interaction effect: p = 0.001) but not MSNA burst frequency responses. Mean BP increases were not different between groups, but OF exhibited a higher ∆mean BP/∆MSNA burst frequency ratio (p = 0.003). During apnea, OF experienced a smaller percent MSNA increase in total MSNA (p < 0.05), larger mean BP increases (interaction effect: p = 0.031), and higher ∆mean BP/∆total MSNA ratio (p = 0.003). These findings suggest attenuated signal-averaged sympathetic BP transduction at rest but increased time-averaged sympathetic transduction to mean BP during cold and apneic laboratory stressors in OF.

The impact of oral contraceptive pill use on sympathetic transduction at rest in young females

Although previous work has demonstrated that oral contraceptive pill (OCP) use does not affect resting muscle sympathetic nerve activity (MSNA), growing evidence indicates that it attenuates neurogenic vasoconstriction. Despite these advances, it remains unknown how OCP use affects the ability of MSNA to dynamically control vascular tone and arterial blood pressure (BP) on a beat-by-beat basis. Thus, we tested the hypothesis that, compared with naturally menstruating females (MC), those using OCPs will exhibit attenuated sympathetic vascular transduction at rest. Forty-three females [MC: n = 21, 26 (4) yrs; OCP: n = 22, 24 (4) yrs; data are presented as means (SD)] completed 10 min of supine rest with continuous measurements of beat-by-beat BP, femoral artery blood flow (26 females; MC: n = 13, OCP: n = 13), and MSNA. Spike-triggered averaging was used to determine sympathetic transduction into leg vascular conductance (LVC) and BP for 12 cardiac cycles following MSNA bursts. Overall sympathetic-BP transduction (P = 0.293), as well as sympathetic-BP transduction of MSNA burst quartiles (P = 0.741) and burst firing patterns (P = 0.452) were not different between the MC and OCP groups. Conversely, sympathetic vascular transduction per unit MSNA burst amplitude (P = 0.026) and burst firing pattern (P = 0.014) were attenuated among females using OCPs. In addition, females using OCPs demonstrated progressively smaller leg vasoconstrictor responses as a function of MSNA burst firing pattern compared with MC females (P = 0.021). Collectively, these data indicate that, in premenopausal females, OCP use attenuates the leg vasoconstrictor responses to bursts of MSNA, particularly during periods of increased sympathetic neural drive, without affecting the transduction of MSNA bursts into beat-by-beat changes in BP.NEW & NOTEWORTHY This study investigated the impact of OCP use on the transduction of MSNA bursts into regional vasoconstriction and blood pressure in premenopausal females. We demonstrated that females using OCPs exhibit attenuated sympathetic transduction into LVC; however, this does not translate to reductions in sympathetic blood pressure transduction. Collectively, these data indicate that OCP use may alter the local vasoconstrictor response to bursts of MSNA; however, compensatory mechanisms may contribute to maintain sympathetic blood pressure transduction.

Acute intermittent hypoxia elicits sympathetic neuroplasticity independent of peripheral chemoreflex activation and spinal cord tissue hypoxia in a rodent model of high-thoracic spinal cord injury

The loss of medullary control of spinal circuits controlling the heart and blood vessels is a unifying mechanism linking both hemodynamic instability and the risk for cardiovascular diseases (CVD) following spinal cord injury (SCI). As such, new avenues to regulate sympathetic activity are essential to mitigate CVD in this population. Acute intermittent hypoxia (AIH) induces a type of neuroplasticity known as long-term facilitation (LTF), a persistent increase in nerve activity post-AIH in spinal motor circuits. Whether LTF occurs within the sympathetic circuit following SCI is largely unknown. We aimed to test whether AIH elicits sympathetic LTF (i.e., sLTF) and attenuates hypoactivity in sub-lesional splanchnic sympathetic circuits in a male rat model of SCI. In 3 experimental series, we tested whether 1) high-thoracic contusion SCI induces hypoactivity in splanchnic sympathetic nerve activity, 2) AIH elicits sLTF following SCI, and 3) sLTF requires carotid chemoreflex activation or spinal cord tissue hypoxia. Our results indicate that a single-session of AIH therapy (10 × 1 min of FiO2 = 0.1, interspersed with 2 min of FiO2 = 1.0) delivered at 2 weeks following SCI attenuates SCI-induced sympathetic hypoactivity by eliciting sLTF 90 min post-treatment that is independent of peripheral chemoreflex activation and/or spinal cord hypoxia. These findings advance our mechanistic understanding of AIH in the field and yield new insights into factors underpinning AIH-induced sLTF following SCI in a rat model. Our findings also set the stage for the chronic application of AIH to alleviate secondary complications resulting from sympathetic hypoactivity following SCI.

Method for measuring cervical vagal nerve activity in conscious rats

The current study aimed to propose a method to directly measure right cervical vagal nerve activity (cVNA) alongside renal sympathetic nerve activity (RSNA) in conscious rats. The right cervical vagus nerve was surgically exposed and fitted with a bipolar electrode to record cVNA. A microcatheter was used to administer levobupivacaine to selectively block afferent cVNA. Upon levobupivacaine administration, cVNA was reduced by 84%, enabling the exclusive assessment of efferent cVNA. Intravenous and intraperitoneal administration of cholecystokinin-8 (CCK-8) demonstrated that peripherally acting CCK-8 influences the central nervous system through afferent cVNA without affecting the RSNA or efferent cVNA. This technique can be highly applicable for quantifying the dynamic changes in the interaction between vagal and sympathetic nerve activities, thereby shedding light on their roles in maintaining homeostasis and developing autonomic dysfunction, as in obesity and diabetes.NEW & NOTEWORTHY This study proposed a method for directly measuring cervical vagal nerve activity and reversibly blocking afferent cVNA in conscious rats. It demonstrated that CCK-8, when administered intraperitoneally, distinctly influences peripheral afferent vagal nerve activity without affecting renal sympathetic nerve activity, arterial pressure, or heart rate.

Stress appraisal is associated with sympathetic neural reactivity to mental stress in humans

Muscle sympathetic nerve activity (MSNA) responsiveness to mental stress is highly variable between individuals. Although stress perception has been posited as a contributor to the MSNA variability during mental stress, prior studies have been inconclusive. Furthermore, the importance of stress appraisal and coping on MSNA reactivity to mental stress has not been investigated. We hypothesize that appraisal of mental stress as a threat (i.e., perceived demands of stress exceed coping resources) versus a challenge (i.e., perceived coping resources sufficient for demands of stress) would be associated with greater MSNA reactivity. Twenty healthy adults (11 males, 9 females, 21 ± 3 yr, 23 ± 3 kg/m2) participated. Beat-by-beat blood pressure (finger plethysmography), heart rate (electrocardiography), and MSNA (microneurography) were recorded during a 10-min quiet rest followed by the Trier Social Stress Test (TSST). After each phase of the TSST (i.e., speech prep, speech, and mental arithmetic), participants reported threat versus challenge appraisal. Endorsement of a threat appraisal was positively associated with changes in MSNA burst frequency (r = 0.548, P = 0.018), burst incidence (r = 0.599, P = 0.009), and total MSNA (r = 0.697, P = 0.037) during the speech stress period. Moreover, increases in threat appraisal across tasks was associated with elevated MSNA burst frequency (r = 0.531, P = 0.023), incidence (r = 0.512, P = 0.030), and total MSNA (r = 0.727, P = 0.027) responsiveness. These findings support an association between stress appraisal processes and postganglionic sympathetic neural reactivity to psychosocial stress and may partially explain interindividual variability in MSNA responses to mental stress.NEW & NOTEWORTHY The present study investigated the association between stress appraisal and muscle sympathetic nerve activity (MSNA) reactivity to the Trier Social Stress Test. Appraisal of the stress task as a threat (i.e., perceived inability to cope with the demands of the task) was associated with exaggerated MSNA reactivity to mental stress in humans. Threat appraisal is associated with exaggerated sympathetic reactivity to stress, potentially underlying the commonly observed interindividual variability in MSNA responsiveness to mental stress.

Association between skin sympathetic nerve activity and electrocardiogram alterations after subarachnoid hemorrhage

While autonomic dysregulation and repolarization abnormalities are observed in subarachnoid hemorrhage (SAH), their relationship remains unclear. We aimed to measure skin sympathetic nerve activity (SKNA), a novel method to estimate stellate ganglion nerve activity, and investigate its association with electrocardiogram (ECG) alterations after SAH. We recorded a total of 179 SKNA data from SAH patients at three distinct phases and compared them with 20 data from controls. Compared with control data, in the acute phase data (days 1-3 of SAH), T peak-to-end (Tp-e) interval was significantly prolonged (81 [interquartile range {IQR}: 71-93] vs. 58 [IQR: 54-64] ms, p < 0.001), non-burst amplitude of SKNA was significantly increased (2.4 [IQR: 1.3-4.1] vs. 0.7 [IQR: 0.5-1.7] μV, p < 0.001), and the ratio of low frequency to high frequency (HF) was significantly decreased (0.8 [IQR: 0.5-1.1] vs. 1.1 [IQR: 0.7-1.3], p = 0.028). Linear mixed model revealed a relationship between Tp-e interval and SKNA. Although these abnormalities gradually normalized, delayed cerebral ischemia episodes were associated with increased HF oscillation. Transient sympathetic dysregulation contributes to repolarization impairment after SAH. SKNA may have the potential to monitor adverse outcomes.

Renal autonomic dynamics in hypertension: how can we evaluate sympathetic activity for renal denervation?

This review explores the various pathophysiological factors influencing antihypertensive effects, involving the regulation of vascular resistance, plasma volume, cardiac function, and the autonomic nervous system, emphasizing the interconnected processes regulating blood pressure (BP). The kidney's pivotal role in BP control and its potential contribution to hypertension is complicated but important to understand the effective mechanisms of renal denervation (RDN), which may be a promising treatment for resistant hypertension. Excessive stimulation of the sympathetic nervous system or the renin-angiotensin-aldosterone system (RAAS) can elevate BP through various physiological changes, contributing to chronic hypertension. Renal sympathetic efferent nerve activation leads to elevated norepinephrine levels and subsequent cascading effects on vasoconstriction, renin release, and sodium reabsorption. RDN reduces BP in resistant hypertension by potentially disrupting sensory afferent nerves, decreasing feedback activation to the central nervous system, and reducing efferent sympathetic nerve activity in the heart and other structures. RDN may also modulate central sympathetic outflow and inhibit renal renin-angiotensin system overactivation. While evidence for RDN efficacy in hypertension is increasing, accurate patient selection becomes crucial, considering complex interactions that vary among patients. This review also discusses methods to evaluate autonomic nerve activity from the golden standard to new potential examination for finding out optimization in stimulation parameters or rigorous patient selection based on appropriate biomarkers.

Binge Alcohol Consumption Elevates Sympathetic Transduction to Blood Pressure: A Randomized Controlled Trial

BACKGROUND

Alcohol consumption is associated with cardiovascular disease, and the sympathetic nervous system is a suspected mediator. The present study investigated sympathetic transduction of muscle sympathetic nerve activity to blood pressure at rest and in response to cold pressor test following evening binge alcohol or fluid control, with the hypothesis that sympathetic transduction would be elevated the morning after binge alcohol consumption.

METHODS

Using a randomized, fluid-controlled (FC) crossover design, 26 healthy adults (12 male, 14 female, 25±6 years, 27±4 kg/m2) received an evening binge alcohol dose and a FC. All participants underwent next-morning autonomic-cardiovascular testing consisting of muscle sympathetic nerve activity, beat-to-beat blood pressure, and heart rate during a 10-minute rest period and a 2-minute cold pressor test. Sympathetic transduction was assessed at rest and during the cold pressor test in both experimental conditions.

RESULTS

Evening alcohol increased heart rate (FC: 60±9 versus alcohol: 64±9 bpm; P=0.010) but did not alter resting mean arterial pressure (FC: 80±6 versus alcohol: 80±7 mm Hg; P=0.857) or muscle sympathetic nerve activity (FC: 18±9 versus alcohol: 20±8 bursts/min; P=0.283). Sympathetic transduction to mean arterial pressure (time×condition; P=0.003), diastolic blood pressure (time×condition; P=0.010), and total vascular conductance (time×condition; P=0.004) was augmented after alcohol at rest. Sympathetic transduction during the cold pressor test was also elevated after evening binge alcohol consumption (P=0.002).

CONCLUSIONS

These findings suggest that evening binge alcohol consumption leads to augmented morning-after sympathetic transduction of muscle sympathetic nerve activity to blood pressure, highlighting a new mechanism whereby chronic or excessive alcohol consumption contributes to cardiovascular disease progression via altered end-organ responsiveness to sympathetic neural outflow.

REGISTRATION

URL: https://clinicaltrials.gov/study/NCT03567434; Unique identifier: NCT03567434.

Sympathetic toggled paroxysmal atrial fibrillation and recurrent premature atrial contractions in ambulatory patients

BACKGROUND

Autonomic nerve activity is important in the mechanisms of paroxysmal atrial fibrillation (PAF).

OBJECTIVE

The purpose of this study was to test the hypothesis that a single burst of skin sympathetic nerve activity (SKNA) can toggle on and off PAF or premature atrial contraction (PAC) clusters.

METHODS

Simultaneous recording of SKNA and electrocardiogram (neuECG) recording was performed over 7 days in patients with PAF.

RESULTS

In study 1, 8 patients (7 men and 1 woman; age 62 ± 8 years) had 124 episodes of PAF. An SKNA burst toggled both on and off PAF in 8 episodes (6.5%) (type 1), toggled on but not off in 12 episodes (9.7%) (type 2), and toggled on a PAC cluster followed by PAF in 4 episodes (3.2%) (type 3). The duration of these PAF episodes was <10 minutes. The remaining 100 episodes (80.6%) were associated with active SKNA bursts throughout PAF (type 4) and lasted longer than type 1 (P = .0185) and type 2 (P = .0027) PAF. There were 47 PAC clusters. Among them, 24 (51.1%) were toggled on and off, and 23 (48.9%) were toggled on but not off by an SKNA burst. In study 2, 17 patients (9 men and 8 women; age 58 ± 12 years) had <10 minutes of PAF (4, 8, 0, and 31 of types 1, 2, 3, and 4, respectively). There were significant circadian variations of all types of PAF.

CONCLUSION

A single SKNA burst can toggle short-duration PAF and PAC cluster episodes on and off. The absence of continued SKNA after the onset might have affected the maintenance of these arrhythmias.

Sympathetic baroreflex sensitivity is enhanced in postmenopausal women

The sympathetic nervous system is critical for regulating blood pressure (BP) via the arterial baroreflex and sympathetic transduction in the peripheral vasculature. These mechanisms interact, and both may be altered with aging and impacted by menopause. Although age-related decreases in sympathetic transduction have been demonstrated in women, it remains unclear whether sympathetic baroreflex sensitivity (BRS) is impaired in postmenopausal women (POST). We tested the hypothesis that sympathetic BRS would be enhanced in POST compared with premenopausal women (PRE). We examined beat-by-beat BP and muscle sympathetic nerve activity (MSNA) in 19 PRE (22 ± 2 yr, 22 ± 3 kg/m2) and 12 POST (57 ± 5 yr, 24 ± 2 kg/m2) during 10 min of rest. Spontaneous sympathetic BRS was quantified as the slope of a linear regression between MSNA burst incidence and diastolic BP. Sympathetic transduction to mean arterial pressure (MAP) for the 10 cardiac cycles following spontaneous MSNA bursts was assessed via signal averaging method. Resting MAP was similar (PRE: 82 ± 8 vs. POST: 85 ± 8 mmHg, P = 0.43), whereas resting MSNA was elevated in POST (PRE: 10 ± 6 vs. POST: 45 ± 16 bursts/100 heart beats, P < 0.0001). Spontaneous sympathetic BRS was enhanced in POST (PRE: -2.0 ± 1.2 vs. POST: -5.2 ± 1.9 bursts/beat/mmHg, P < 0.0005). Sympathetic transduction to MAP was attenuated in POST (time: P < 0.001, group: P < 0.001, interaction: P < 0.01). These data suggest that sympathetic BRS may be enhanced in POST. Consistent with recent hypotheses, enhanced sensitivity of the arterial baroreflex's neural arc may signify a compensatory response to reduced efficiency of the peripheral arterial baroreflex arc (i.e., sympathetic transduction) to preserve BP buffering capacity.NEW & NOTEWORTHY Studies examining sympathetic baroreflex function with aging remain equivocal, with some studies showing an increase, decrease, or no change in sympathetic baroreflex sensitivity (BRS) in older adults compared with younger adults. With aging, women experience unique physiological changes due to menopause that influence autonomic function. For the first time, we show that postmenopausal women exhibit a greater sympathetic BRS compared with young premenopausal women.

The interactive effects of posture and biological sex on the control of muscle sympathetic nerve activity during rhythmic handgrip exercise

Body posture and biological sex exhibit independent effects on the sympathetic neural responses to dynamic exercise. However, the neural mechanisms (e.g., baroreflex) by which posture impacts sympathetic outflow during rhythmic muscular contractions, and whether biological sex affects posture-mediated changes in efferent sympathetic nerve traffic during exercise, remain unknown. Thus, we tested the hypotheses that increases in muscle sympathetic nerve activity (MSNA) would be greater during upright compared with supine rhythmic handgrip (RHG) exercise, and that females would demonstrate smaller increases in MSNA during upright RHG exercise than males. Twenty young (30 [6] yr; means [SD]) individuals (9 males, 11 females) underwent 6 min of supine and upright (head-up tilt 45°) RHG exercise at 40% maximal voluntary contraction with continuous measurements of MSNA (microneurography), blood pressure (photoplethysmography), and heart rate (electrocardiogram). In the pooled group, absolute MSNA burst frequency (P < 0.001), amplitude (P = 0.009), and total MSNA (P < 0.001) were higher during upright compared with supine RHG exercise. However, body posture did not impact the peak change in MSNA during RHG exercise (range: P = 0.063-0.495). Spontaneous sympathetic baroreflex gain decreased from rest to RHG exercise (P = 0.006) and was not impacted by posture (P = 0.347). During upright RHG exercise, males demonstrated larger increases in MSNA burst amplitude (P = 0.002) and total MSNA (P = 0.001) compared with females, which coincided with greater reductions in sympathetic baroreflex gain among males (P = 0.004). Collectively, these data indicate that acute attenuation of baroreflex-mediated sympathoinhibition permits increases in MSNA during RHG exercise and that males exhibit a greater reserve for efferent sympathetic neural recruitment during orthostasis than females.NEW & NOTEWORTHY The impact of posture and sex on cardiovascular control during rhythmic handgrip (RHG) exercise is unknown. We show that increases in muscle sympathetic nerve activity (MSNA) during RHG are partly mediated by a reduction in sympathetic baroreflex gain. In addition, males demonstrate larger increases in total MSNA during upright RHG than females. These data indicate that the baroreflex partly mediates increases in MSNA during RHG and that males have a greater sympathetic vasoconstrictor reserve than females.

Nadir blood pressure responses to longer consecutive cardiac cycle sequences absent of sympathetic bursts are associated with popliteal endothelial-dependent dilation

PURPOSE

The nadir pressure responses to cardiac cycles absent of muscle sympathetic nerve activity (MSNA) bursts (or non-bursts) are typically reported in studies quantifying sympathetic transduction, but the information gained by studying non-bursts is unclear. We tested the hypothesis that longer sequences of non-bursts (≥8 cardiac cycles) would be associated with a greater nadir diastolic blood pressure (DBP) and that better popliteal artery function would be associated with an augmented reduction in DBP.

METHODS

Resting beat-by-beat DBP (via finger photoplethysmography) and common peroneal nerve MSNA (via microneurography) were recorded in 39 healthy, adults (age 23.4 ± 5.3 years; 19 females). For each cardiac cycle absent of MSNA bursts, the mean nadir DBP (ΔDBP) during the 12 cardiac cycles following were determined, and separate analyses were conducted for ≥8 or < 8 cardiac cycle sequences. Popliteal artery endothelial-dependent (via flow-mediated dilation; FMD) and endothelial-independent vasodilation (via nitroglycerin-mediated dilation; NMD) were determined.

RESULTS

The nadir DBP responses to sequences ≥8 cardiac cycles were larger (-1.40 ± 1.27 mmHg) than sequences <8 (-0.38 ± 0.46 mmHg; p < 0.001). In adjusting for sex and burst frequency (14 ± 8 bursts/min), larger absolute or relative FMD (p < 0.01), but not NMD (p > 0.53) was associated with an augmented nadir DBP. This overall DBP-FMD relationship was similar in sequences ≥8 (p = 0.04-0.05), but not <8 (p > 0.72).

CONCLUSION

The DBP responses to non-bursts, particularly longer sequences, were inversely associated with popliteal endothelial function, but not vascular smooth muscle sensitivity. This study provides insight into the information gained by quantifying the DBP responses to cardiac cycles absent of MSNA.

Actigraphy-based sleep and muscle sympathetic nerve activity in humans

Short and insufficient sleep are prevalent and associated with cardiovascular disease, with the sympathetic nervous system as a suspected mediator. The purpose of the present study was to investigate the association between objective, actigraphy-based total sleep time (TST), sleep efficiency (SE), and cardiovascular and sympathetic regulation in healthy adults. We hypothesized that short TST and low SE would be associated with elevated resting blood pressure, heart rate (HR), and muscle sympathetic nerve activity (MSNA). Participants included 94 individuals [46 males, 48 females, age: 30 ± 15 yr, body mass index (BMI): 26 ± 4 kg/m2]. All participants underwent at least 7 days of at-home, wristwatch actigraphy monitoring (avg: 10 ± 3 days). Seated blood pressures were assessed using brachial blood pressure measurements, followed by a 10-minute supine autonomic testing session consisting of continuous HR (electrocardiogram), beat-by-beat blood pressure (finger plethysmograph), and MSNA (microneurography) monitoring. Partial correlations were used to determine the relationship between sleep and cardiovascular parameters while accounting for the influence of age, sex, and BMI. TST was not associated with MAP (R = -0.105, P = 0.321), HR (R = 0.093, P = 0.383), or MSNA burst frequency (BF; R = -0.168, P = 0.112) and burst incidence (BI; R = -0.162, P = 0.124). Similarly, SE was not associated with MAP (R = -0.088, P = 0.408), HR (R = -0.118, P = 0.263), MSNA BF (R = 0.038, P = 0.723), or MSNA BI (R = 0.079, P = 0.459). In contrast to recent preliminary findings, our results do not support a significant association between actigraphy-based sleep duration or efficiency and measures of resting blood pressure, heart rate, and MSNA.NEW & NOTEWORTHY The present study investigated the independent association between actigraphy-based sleep duration, efficiency, and measures of blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) in adult males and females. Contrary to our hypothesis, the findings do not support an independent association between habitual sleep and cardiovascular or sympathetic neural activity. However, these findings do not preclude a potential association between these parameters in populations with sleep disorders and/or cardiovascular disease.

Effect of acute intranasal insulin administration on muscle sympathetic nerve activity in healthy young adults

Systemic insulin increases muscle sympathetic nerve activity (MSNA) via both central actions within the brainstem and peripheral activation of the arterial baroreflex. Augmented MSNA during hyperinsulinemia likely restrains peripheral vasodilation and contributes to the maintenance of blood pressure (BP). However, in the absence of insulin action within the peripheral vasculature, whether central insulin stimulation increases MSNA and influences peripheral hemodynamics in humans remains unknown. Herein, we hypothesized intranasal insulin administration would increase MSNA and BP in healthy young adults. Participants were assigned to time control [TC, n = 13 (5 females/8 males), 28 ± 1 yr] or 160 IU of intranasal insulin administered over 5 min [n = 15 (5 females/10 males), 26 ± 2 yr]; five (1 female/4 males) participants completed both conditions. MSNA (fibular microneurography), BP (finger photoplethysmography), and leg blood flow (LBF, femoral Doppler ultrasound) were assessed at baseline, and 15 and 30 min following insulin administration. Leg vascular conductance [LVC = (LBF ÷ mean BP) × 100] was calculated. Venous insulin and glucose concentrations remained unchanged throughout (P > 0.05). Following intranasal insulin administration, MSNA (burst frequency; baseline = 100%; minute 15, 121 ± 8%; minute 30, 118 ± 6%; P = 0.009, n = 7) and mean BP (baseline = 100%; minute 15, 103 ± 1%; minute 30, 102 ± 1%; P = 0.003) increased, whereas LVC decreased (baseline = 100%; minute 15, 93 ± 3%; minute 30, 99 ± 3%; P = 0.03). In contrast, MSNA, mean BP, and LVC were unchanged in TC participants (P > 0.05). We provide the first evidence that intranasal insulin administration in healthy young adults acutely increases MSNA and BP and decreases LVC. These results enhance mechanistic understanding of the sympathetic and peripheral hemodynamic response to insulin.NEW & NOTEWORTHY Systemic insulin increases muscle sympathetic nerve activity (MSNA) via central actions within the brainstem and peripheral activation of the arterial baroreflex. In the absence of peripheral insulin action, whether central insulin stimulation increases MSNA and influences peripheral hemodynamics in humans was unknown. We provide the first evidence that intranasal insulin administration increases MSNA and blood pressure and reduces leg vascular conductance. These results enhance mechanistic understanding of the sympathetic and hemodynamic response to insulin.

Unipolar and bipolar signal acquisition to distinguish artifacts from nervous activity

The acquisition of nervous activity is a meticulous work which requires particular attention to validate the presence of the signal of interest and to distinguish it from electrical and physiological artifacts which are constantly added to the recordings even when the experimenters have taken care to clean the measurement environment as much as possible. In this article, we propose to analyze the bipolar and unipolar recordings from the same electrode contacts, to confirm the nervous nature of the recorded signal and distinguish it from artifacts on the vagus nerve. The metrics used for the analysis of these signals confirmed the recording of nervous activity and the ability of the measuring device to distinguish and separate the respiratory muscular artifact from the respiratory nervous activity although they have very similar shapes.

Is Renal Denervation Effective in Treating Resistant Hypertension?

Resistant hypertension is diagnosed in patients whose blood pressure target is unmet despite the use of three or more antihypertensive medications. Systemic sympathetic hyperactivation is associated with the development of resistant hypertension. As the kidney is largely pervasive of the sympathetic nervous system renal denervation procedure was developed to control blood pressure by attenuating the renal and systemic sympathetic hyperactivity. Renal denervation is a minimally invasive procedure that uses radiofrequency or ultrasound energy waves to reduce the activity of the renal artery nerves. Previous clinical trials have shown conflicting results regarding the efficacy of the procedure. Symplicity HTN-1 and -2 trials showed effective blood pressure lowering results in the renal denervation group with a good safety profile. However, the Symplicity HTN-3 trial showed no difference in blood pressure lowering effect between the renal denervation and control Sham procedure groups. Notwithstanding, some recent clinical trials with Sham control and meta-analysis showed clinical benefits of renal denervation. Other clinical benefits of renal denervation include glucose control, cardiovascular protective effect, reduction of obstructive sleep apnea, and neuralgia control. A subset of patients with satisfactory blood pressure control response to the procedure may experience improved glucose control due to the overall reduced sympathetic activity and insulin resistance. Sympathetic activity control after renal denervation has cardioprotective effects, especially for those with arrhythmia and left ventricular hypertrophy. Also, renal denervation could be helpful in renalorigin pain control. Renal denervation is an effective, safe, non-invasive procedure with many clinical benefits beyond blood pressure control. Further development in the procedure technique and selection of target patients are needed for wider clinical use of renal denervation in resistant hypertension.

Influence of sex and sedentary conditions on sympathetic burst characteristics in prepubertal, postpubertal, and young adult rats

Recent evidence indicates that sex-based differences in cardiovascular disease (CVD) begin early in life, particularly when associated with risk factors such as a sedentary lifestyle. CVD is associated with elevated sympathetic nerve activity (SNA), quantified as increased SNA burst activity in humans. Whether burst characteristics are influenced by sex or sedentary conditions at younger ages is unknown. The purpose of our study is to compare SNA bursts in active and sedentary female and male rats at ages including prepuberty and young adulthood. We hypothesized that burst characteristics and blood pressure are higher under sedentary conditions and lower in female rats compared with males. We analyzed splanchnic SNA (SpSNA) recordings from Inactin-anesthetized male and female rats at 4-, 8-, and 16-wk of age. Physically active and sedentary rats were each housed in separate, environmentally controlled chambers where physically active rats had free access to an in-cage running wheel. Sympathetic bursts were obtained by rectifying and integrating the raw SpSNA signal. Burst frequency, burst height, and burst width were calculated using the Peak Parameters extension in LabChart. Our results showed that sedentary conditions produced a greater burst width in 8- and 16-wk-old rats compared with 4-wk-old rats in both males and females (P < 0.001 for both). Burst frequency and incidence were both higher in 16-wk-old males compared with 16-wk-old females (P < 0.001 for both). Our results suggest that there are sedentary lifestyle- and sex-related mechanisms that impact sympathetic regulation of blood pressure at ages that range from prepuberty into young adulthood.NEW & NOTEWORTHY The mechanisms of decreased incidence of cardiovascular disease (CVD) in reproductive-age women compared with age-matched men are unknown. The strong association between elevated sympathetic activity and CVD led us to characterize splanchnic sympathetic bursts in female and male rats. Prepubescent males and females exhibited narrower sympathetic bursts, whereas young adult males had higher resting burst frequency compared with age-matched females. Sex-based regulation of sympathetic activity suggests a need for sex-dependent therapeutic strategies to combat CVD.

Analysis of sympathetic responses to cognitive stress and pain through skin sympathetic nerve activity and electrodermal activity

We explored the non-invasive evaluation of the sympathetic nervous system (SNS) by employing two distinct physiological signals: skin sympathetic nerve activity (SKNA), extracted from electrocardiogram (ECG) signals, and electrodermal activity (EDA), a well-studied marker in the context of the SNS assessment. Our investigation focused on cognitive stress and pain; two conditions closely associated with the SNS. We sought to determine if the information and dynamics of EDA could be derived from the novel SKNA signal. To this end, ECG and EDA signals were recorded simultaneously during three experiments aimed at sympathetic stimulation, Valsalva maneuver (VM), Stroop test, and thermal-grill pain test. We calculated the integral area under the rectified SKNA signal (iSKNA) and decomposed the EDA signal to its phasic component (EDAphasic). An average delay of more than 4.6 s was observed in the onset of EDAphasic bursts compared to their corresponding iSKNA bursts. After shifting the EDAphasic segments by the extent of this delay and smoothing the corresponding iSKNA bursts, our results revealed a strong average correlation coefficient of 0.85±0.14 between the iSKNA and EDAphasic bursts, indicating a noteworthy similarity between the two signals. We also reconstructed the EDA signals with time-varying sympathetic (TVSymp) and modified TVSymp (MTVSymp) methods. Then we extracted the following features from iSKNA, EDAphasic, TVSymp, and MTVSymp signals: peak amplitude, average amplitude (aSKNA), standard deviation (vSKNA), and the cumulative duration during which the signals had higher amplitudes than a specified threshold (HaSKNA). A strong average correlation of 0.89±0.18 was found between vSKNA and subjects' self-rated pain levels during the pain test. Our statistical analysis also included applying Linear Mixed-Effects Models to check if there were significant differences in features across baseline and different levels of SNS stimulation. We then assessed the discriminating power of the features using Area Under the Receiver Operating Characteristic Curve (AUROC) and Fisher's Ratio. Finally, using all the four EDA features, a multi-layer perceptron (MLP) classifier reached the classification accuracies 95.56%, 89.29%, and 67.88% for the VM, Stroop, and thermal-grill pain control and stimulation classes. On the other hand, the highest classification accuracies based on SKNA features were achieved using K-nearest neighbors (KNN) (98.89%), KNN (89.29%), and MLP (95.11%) classifiers for the same experiments. Our comparative analysis showed the feasibility of SKNA as a novel tool for assessing the SNS with accurate classification capability, with a faster onset of amplitude increase in response to SNS activity, compared to EDA.

Guidelines on antibody use in physiology research

Antibodies are one of the most used reagents in scientific laboratories and are critical components for a multitude of experiments in physiology research. Over the past decade, concerns about many biological methods, including those that use antibodies, have arisen as several laboratories were unable to reproduce the scientific data obtained in other laboratories. The lack of reproducibility could be largely attributed to inadequate reporting of detailed methods, no or limited verification by authors, and the production and use of unvalidated antibodies. The goal of this guideline article is to review best practices concerning commonly used techniques involving antibodies, including immunoblotting, immunohistochemistry, and flow cytometry. Awareness and integration of best practices will increase the rigor and reproducibility of these techniques and elevate the quality of physiology research.

Sympathetic determinants of resting blood pressure in males and females

Discharge of postganglionic muscle sympathetic nerve activity (MSNA) is related poorly to blood pressure (BP) in adults. Whether neural measurements beyond the prevailing level of MSNA can account for interindividual differences in BP remains unclear. The current study sought to evaluate the relative contributions of sympathetic-BP transduction and sympathetic baroreflex gain on resting BP in young adults. Data were analyzed from 191 (77 females) young adults (18-39 years) who underwent continuous measurement of beat-to-beat BP (finger photoplethysmography), heart rate (electrocardiography), and fibular nerve MSNA (microneurography). Linear regression analyses were computed to determine associations between sympathetic-BP transduction (signal-averaging) or sympathetic baroreflex gain (threshold technique) and resting BP, before and after controlling for age, body mass index, and MSNA burst frequency. K-mean clustering was used to explore sympathetic phenotypes of BP control and consequential influence on resting BP. Sympathetic-BP transduction was unrelated to BP in males or females (both R2 < 0.01; P > 0.67). Sympathetic baroreflex gain was positively associated with BP in males (R2 = 0.09, P < 0.01), but not in females (R2 < 0.01; P = 0.80), before and after controlling for age, body mass index, and MSNA burst frequency. K-means clustering identified a subset of participants with average resting MSNA, yet lower sympathetic-BP transduction and lower sympathetic baroreflex gain. This distinct subgroup presented with elevated BP in males (P < 0.02), but not in females (P = 0.10). Sympathetic-BP transduction is unrelated to resting BP, while the association between sympathetic baroreflex gain and resting BP in males reveals important sex differences in the sympathetic determination of resting BP.NEW & NOTEWORTHY In a sample of 191 normotensive young adults, we confirm that resting muscle sympathetic nerve activity is a poor predictor of resting blood pressure and now demonstrate that sympathetic baroreflex gain is associated with resting blood pressure in males but not females. In contrast, signal-averaged measures of sympathetic-blood pressure transduction are unrelated to resting blood pressure. These findings highlight sex differences in the neural regulation of blood pressure.

Machine learning based hybrid anomaly detection technique for automatic diagnosis of cardiovascular diseases using cardiac sympathetic nerve activity and electrocardiogram

OBJECTIVES

Coronary artery diseases (CADs) are the leading cause of death worldwide and early diagnosis is crucial for timely treatment. To address this, our study presents a novel automated Artificial Intelligence (AI)-based Hybrid Anomaly Detection (AIHAD) technique that combines various signal processing, feature extraction, supervised, and unsupervised machine learning methods. By jointly and simultaneously analyzing 12-lead cardiac sympathetic nerve activity (CSNA) and electrocardiogram (ECG) data, the automated AIHAD technique performs fast, early, and accurate diagnosis of CADs.

METHODS

In order to develop and evaluate the proposed automated AIHAD technique, we utilized the fully labeled STAFF III and PTBD databases, which contain the 12-lead wideband raw recordings non-invasively acquired from 260 subjects. Using these wideband raw recordings, we developed a signal processing technique that simultaneously detects the 12-lead CSNA and ECG signals of all subjects. Using the pre-processed 12-lead CSNA and ECG signals, we developed a time-domain feature extraction technique that extracts the statistical CSNA and ECG features critical for the reliable diagnosis of CADs. Using the extracted discriminative features, we developed a supervised classification technique based on Artificial Neural Networks (ANNs) that simultaneously detects anomalies in the 12-lead CSNA and ECG data. Furthermore, we developed an unsupervised clustering technique based on Gaussian mixture models (GMMs) and Neyman-Pearson criterion, which robustly detects outliers corresponding to CADs.

RESULTS

Using the automated AIHAD technique, we have, for the first time, demonstrated a significant association between the increase in CSNA signals and anomalies in ECG signals during CADs. The AIHAD technique achieved highly reliable detection of CADs with a sensitivity of 98.48 %, specificity of 97.73 %, accuracy of 98.11 %, positive predictive value of 97.74 %, negative predictive value of 98.47 %, and F1-score of 98.11 %. Hence, the automated AIHAD technique demonstrates superior performance compared to the gold standard diagnostic test ECG in the diagnosis of CADs. Additionally, it outperforms other techniques developed in this study that separately utilize either only CSNA data or only ECG data. Therefore, it significantly increases the detection performance of CADs by taking advantage of the diversity in different data types and leveraging their strengths. Furthermore, its performance is comparatively better than that of most previously proposed machine and deep learning methods that exclusively used ECG data to diagnose or classify CADs. Additionally, it has a very low implementation time, which is highly desirable for real-time detection of CADs.

CONCLUSIONS

The proposed automated AIHAD technique may serve as an efficient decision-support system to increase physicians' success in fast, early, and accurate diagnosis of CADs. It may be highly beneficial and valuable, particularly for asymptomatic patients, for whom the diagnostic information provided by ECG alone is not sufficient to reliably diagnose the disease. Hence, it may significantly improve patient outcomes by enabling timely treatments and considerably reducing the mortality of cardiovascular diseases (CVDs).

Glucose metabolism and autonomic function in healthy individuals and patients with type 2 diabetes mellitus at rest and during exercise

Autonomic dysfunction is a common complication of type 2 diabetes mellitus (T2DM). However, the character of dysfunction varies in different reports. Differences in measurement methodology and complications might have influenced the inconsistent results. We sought to evaluate comprehensively the relationship between abnormal glucose metabolism and autonomic function at rest and the response to exercise in healthy individuals and T2DM patients. We hypothesized that both sympathetic and parasympathetic indices would decrease with the progression of abnormal glucose metabolism in individuals with few complications related to high sympathetic tone. Twenty healthy individuals and 11 T2DM patients without clinically evident cardiovascular disease other than controlled hypertension were examined. Resting muscle sympathetic nerve activity (MSNA), heart rate variability, spontaneous cardiovagal baroreflex sensitivity (CBRS), sympathetic baroreflex sensitivity and the MSNA response to handgrip exercise were measured. Resting MSNA was lower in patients with T2DM than in healthy control subjects (P = 0.011). Resting MSNA was negatively correlated with haemoglobin A1c in all subjects (R = -0.45, P = 0.024). The parasympathetic components of heart rate variability and CBRS were negatively correlated with glycaemic/insulin indices in all subjects and even in the control group only (all, P < 0.05). In all subjects, the MSNA response to exercise was positively correlated with fasting blood glucose (R = 0.69, P < 0.001). Resting sympathetic activity and parasympathetic modulation of heart rate were decreased in relationship to abnormal glucose metabolism. Meanwhile, the sympathetic responses to handgrip were preserved in diabetics. The responses were correlated with glucose/insulin parameters throughout diabetic and control subjects. These results suggest the importance of a comprehensive assessment of autonomic function in T2DM.

Aging and sympathetic transduction to blood pressure in humans: methodological and physiological considerations

Recent reports suggest that quantification of signal-averaged sympathetic transduction is influenced by resting muscle sympathetic nerve activity (MSNA) and burst occurrence relative to the average mean arterial pressure (MAP). Herein, we asked how these findings may influence age-related reductions in sympathetic transduction. Beat-to-beat blood pressure and MSNA were recorded during 5 min of rest in 27 younger (13 females: age, 25 ± 5 yr; BMI, 25 ± 4 kg/m2) and 26 older (15 females: age, 59 ± 5 yr; BMI, 26 ± 4 kg/m2) healthy adults. All MSNA bursts were signal averaged together. Beat-to-beat MAP values were then split into low (T1), middle (T2), and high (T3) tertiles, and signal-averaged transduction was calculated within each tertile. Resting MSNA was higher in older adults and MAP was similar between groups. Older adults exhibited blunted overall MAP transduction (younger, Δ1.5 ± 0.6 vs. older, Δ0.9 ± 0.7 mmHg; P = 0.005), which was irrespective of relation to prevailing MAP. A greater proportion of bursts occurred above the average MAP in older adults (P < 0.001), and a larger proportion of these bursts were associated with depressor responses (P = 0.005). Nonetheless, assessment of bursts above the average MAP associated with pressor responses revealed similar age-associated reductions in transduction (younger, Δ2.6 ± 1.6 vs. older, Δ1.7 ± 0.8 mmHg; P = 0.016). These findings indicate an age-related increase in burst occurrence above the average resting MAP, which alone does not explain blunted transduction, thereby supporting the physiological underpinnings of age-related decrements in sympathetic transduction to blood pressure.NEW & NOTEWORTHY The current study demonstrated that aging is associated with a greater prevalence of sympathetic bursts occurring above the average blood pressure, which offers both methodologically and physiologically relevant information regarding aging and sympathetic control of blood pressure. These data support age-related reductions in sympathetic transduction via a reduced pressor response to sympathetic bursts irrespective of the prevailing absolute blood pressure value, along with increases in sympathetic outflow necessary to maintain blood pressure.

Adjusting for muscle strength and body size attenuates sex differences in the exercise pressor reflex in young adults

Females typically exhibit lower blood pressure (BP) during exercise than males. However, recent findings indicate that adjusting for maximal strength attenuates sex differences in BP during isometric handgrip (HG) exercise and postexercise ischemia (PEI; metaboreflex isolation). In addition, body size is associated with HG strength but its contribution to sex differences in exercising BP is less appreciated. Therefore, the purpose of this study was to determine whether adjusting for strength and body size would attenuate sex differences in BP during HG and PEI. We obtained beat-to-beat BP in 110 participants (36 females, 74 males) who completed 2 min of isometric HG exercise at 40% of their maximal voluntary contraction followed by 3 min of PEI. In a subset (11 females, 17 males), we collected muscle sympathetic nerve activity (MSNA). Statistical analyses included independent t tests and mixed models (sex × time) with covariate adjustment for 40% HG force, height2, and body surface area. Females exhibited a lower absolute 40% HG force than male participants (Ps < 0.001). Females exhibited lower Δsystolic, Δdiastolic, and Δmean BPs during HG and PEI than males (e.g., PEI, Δsystolic BP, 15 ± 11 vs. 23 ± 14 mmHg; P = 0.004). After covariate adjustment, sex differences in BP responses were attenuated. There were no sex differences in MSNA. In a smaller strength-matched cohort, there was no sex × time interactions for BP responses (e.g., PEI systolic BP, P = 0.539; diastolic BP, P = 0.758). Our data indicate that sex differences in exercising BP responses are attenuated after adjusting for muscle strength and body size.NEW & NOTEWORTHY When compared with young males, females typically exhibit lower blood pressure (BP) during exercise. Adjusting for maximal strength attenuates sex differences in BP during isometric handgrip (HG) exercise and postexercise ischemia (PEI), but the contribution of body size is unknown. Novel findings include adjustments for muscle strength and body size attenuate sex differences in BP reactivity during exercise and PEI, and sex differences in body size contribute to HG strength differences.

Aging in females is associated with changes in respiratory modulation of sympathetic nerve activity and blood pressure

Sympathetic nerve activity (SNA) is tightly coupled with the respiratory cycle. In healthy human males, respiratory modulation of SNA does not change with age. However, it is unclear how this modulation is affected by age in females. We investigated whether respiratory sympathetic modulation is altered in healthy postmenopausal (PMF) versus premenopausal female (YF), and younger male (YM) adults, and determined its relationship to resting blood pressure. Muscle SNA (MSNA; microneurography), respiration (transducer belt), ECG, and continuous blood pressure were measured in 12 YF, 13 PMF, and 12 YM healthy volunteers. Respiratory modulation of MSNA was quantified during two phases of the respiratory cycle: mid-late expiration and inspiration/postinspiration. All groups showed respiratory modulation of MSNA (P < 0.0005). There was an interaction between the respiratory phase and group for MSNA [bursts/100 heartbeats (HB) (P = 0.004) and bursts/min (P = 0.029)], with smaller reductions in MSNA during inspiration observed in PMF versus the other groups. Respiratory modulation of blood pressure was also reduced in PMF versus YF (6 [2] vs. 12 [9] mmHg, P = 0.008) and YM (13 [13] mmHg, P = 0.001, median [interquartile range]). The magnitude of respiratory sympathetic modulation was related to resting blood pressure in PMF only, such that individuals with less modulation had greater resting blood pressure. The data indicate that aging in postmenopausal females is associated with less inspiratory inhibition of MSNA. This correlated with a higher resting blood pressure in PMF only. Thus, the reduced modulation of MSNA could contribute to the age-related rise in blood pressure that occurs in females.NEW & NOTEWORTHY The current study demonstrates that respiratory modulation of sympathetic nerve activity (SNA) is reduced in healthy postmenopausal (PMF) versus premenopausal females (YF). Furthermore, respiratory sympathetic modulation was negatively related to resting blood pressure in postmenopausal females, such that blood pressure was greater in individual with less modulation. Reduced respiratory sympathetic modulation may have implications for the autonomic control of blood pressure in aging postmenopausal females, by contributing to age-related sympathetic activation and reducing acute, respiratory-linked blood pressure variation.

Effects of biological sex and oral contraception on the sympathetic neurocirculatory adjustments to static handgrip exercise in humans

While biological sex affects the neurocirculatory adjustments to exercise, the effects of sex hormones on sympathetic action potential (AP) patterns and ensuing vascular transduction remain unknown. We tested the hypothesis that males, and females using oral contraceptive pills (OCPs), would demonstrate larger increases in sympathetic activation and sympathetic vascular transduction compared with naturally menstruating females during static handgrip exercise (SHG) and postexercise circulatory occlusion (PECO). Young males [n = 14, 25 (5) yr], females using OCPs [n = 16, 24 (6) yr], and naturally menstruating females [n = 18, 26 (4) yr] underwent assessments of multiunit muscle sympathetic nerve activity (MSNA)/AP discharge patterns (microneurography) and femoral artery blood flow (ultrasound) during fatiguing SHG at 40% maximum voluntary contraction and 2-min PECO. Sympathetic vascular transduction was determined as the quotient of the change in leg vascular conductance (LVC) and MSNA/AP discharge. Males demonstrated greater increases in APs/burst [males: Δ7 (6) vs. midluteal: Δ2 (3), P = 0.028] and total AP clusters [males: Δ5 (3) vs. midluteal: Δ2 (3), P = 0.008] compared with naturally menstruating females only but not those using OCPs during exercise (APs/burst: P = 0.171, total clusters: P = 0.455). Sympathetic vascular transduction of MSNA burst amplitude, APs/burst, and total AP clusters was greater in males and females using OCPs compared with naturally menstruating females (range: P = 0.004-0.044). In contrast, during PECO no group differences were observed in AP discharge (range: P = 0.510-0.872), and AP discharge was not related to LVC during PECO (range: P = 0.08-0.949). These data indicate that biological sex and OCP use impact the central generation of AP discharge, as well as the transduction of these neuronal messages into peripheral vasoconstriction during static exercise.

Telemetric long-term assessment of autonomic function in experimental heart failure

Despite medical advances in the treatment of heart failure (HF), mortality remains high. It has been shown that alterations of the autonomic-nervous-system (ANS) are associated with HF progression and increased mortality. Preclinical models are required to evaluate the effectiveness of novel treatments modulating the autonomic imbalance. However, there are neither standard models nor diagnostic methods established to measure sympathetic and parasympathetic outflow continuously. Digital technologies might be a reliable tool for continuous assessment of autonomic function within experimental HF models. Telemetry devices and pacemakers were implanted in beagle dogs (n = 6). HF was induced by ventricular pacing. Cardiac hemodynamics, plasma catecholamines and parameter describing the ANS ((heart rate variability (HRV), deceleration capacity (DC), and baroreflex sensitivity (BRS)) were continuously measured at baseline, during HF conditions and during recovery phase. The pacing regime led to the expected depression in cardiac hemodynamics. Telemetric assessment of the ANS function showed a significant decrease in Total power, DC, and Heart rate recovery, whereas BRS was not significantly affected. In contrast, plasma catecholamines, revealing sympathetic activity, showed only a significant increase in the recovery phase. A precise diagnostic of the ANS in the context of HF is becoming increasingly important in experimental models. Up to now, these models have shown many limitations. Here we present the continuous assessment of the autonomic function in the progression of HF. We could demonstrate the advantage of highly resolved ANS measurement by HR and BP derived parameters due to early detection of an autonomic imbalance in the progression of HF.

Age- and sex-related differences in sympathetic vascular transduction and neurohemodynamic balance in humans

Bursts of muscle sympathetic nerve activity (MSNA) and the ensuing vasoconstriction are pivotal determinants of beat-by-beat blood pressure regulation. Although age and sex impact blood pressure regulation, how these factors affect the central and peripheral arcs of the baroreflex remains unclear. In 27 young [25 (SD 3) yr] males (YM; n = 14) and females (YF; n = 13) and 23 older [71 (SD 5) yr] males (OM; n = 11) and females (OF; n = 12), femoral artery blood flow, blood pressure, and MSNA were recorded for 10 min of supine rest. Sympathetic baroreflex sensitivity (i.e., central arc) was quantified as the relationship between diastolic blood pressure and MSNA burst incidence. Signal averaging was used to determine sympathetic vascular transduction into leg vascular conductance (LVC) for 12 cardiac cycles following MSNA bursts (i.e., peripheral arc). Older adults demonstrated attenuated sympathetic transduction into LVC (both P < 0.001) following MSNA bursts, and smaller increases in sympathetic transduction as a function of MSNA burst size and firing pattern compared with young adults (range, P = 0.004-0.032). YM (r2 = 0.36; P = 0.032) and OM (r2 = 0.51; P = 0.014) exhibited an inverse relationship between the central and peripheral arcs of the baroreflex, whereas females did not (YF, r2 = 0.03, P = 0.621; OF, r2 = 0.06, P = 0.445). MSNA burst incidence was inversely related to sympathetic transduction in YM and OF (range, P = 0.03-0.046) but not in YF or OM (range, P = 0.360-0.603). These data indicate that age is associated with attenuated sympathetic vascular transduction, whereas age- and sex-specific changes are present in the relationship between the central and peripheral arcs of the baroreflex regulation of blood pressure.NEW & NOTEWORTHY Sympathetic vascular transduction is attenuated in older compared with young adults, regardless of biological sex. Males, but not females (regardless of age), demonstrate an inverse relationship between central (sympathetic baroreflex sensitivity) and peripheral (sympathetic vascular transduction) components of the baroreflex arc. Young males and older females exhibit an inverse relationship between resting sympathetic outflow and sympathetic vascular transduction. Our results indicate that age and sex exert independent and interactive effects on sympathetic vascular transduction and sympathetic neurohemodynamic balance in humans.

The interactive effects of age and sex on the neuro-cardiovascular responses during fatiguing rhythmic handgrip exercise

The impact of age on exercise pressor responses is equivocal, likely because of sex-specific neuro-cardiovascular changes with age. However, assessments of the interactive effects of age and sex on muscle sympathetic nerve activity (MSNA) responses to exercise are lacking. We tested the hypothesis that older females would exhibit exaggerated increases in blood pressure (BP) and MSNA discharge patterns during handgrip exercise compared with similarly aged males and young adults. Twenty-five young (25 (2) years; mean (SD)) males (YM; n = 12) and females (YF; n = 13) and 23 older (71 (5) years) males (OM; n = 11) and females (OF; n = 12) underwent assessments of BP, total peripheral resistance (TPR; Modelflow) and MSNA action potential (AP) discharge patterns (microneurography) during incremental rhythmic handgrip exercise and post-exercise circulatory occlusion (PECO). OM demonstrated larger ∆BP and ∆TPR from baseline than YM (both P < 0.001) despite smaller increases in ∆APs/burst (OM: 0.4 (3) vs. YM: 5 (3) spikes/burst, P < 0.001) and ∆AP clusters/burst (OM: 0.1 (1) vs. YM: 1.8 (1) clusters/burst, P < 0.001) during exercise. Testosterone was lower in OM than YM (P < 0.001) and was inversely related to ∆BP but positively related to ∆AP clusters/burst in males (both P = 0.03). Conversely, YF and OF demonstrated similar ∆BP and ∆AP discharge during exercise (range: P = 0.75-0.96). Age and sex did not impact haemodynamics or AP discharge during PECO (range: P = 0.08-0.94). Altogether, age-related changes in neuro-cardiovascular reactivity exist in males but not females during fatiguing exercise and seem to be related to testosterone. This sex-specific impact of age underscores the importance of considering biological sex when assessing age-related changes in neuro-cardiovascular control during exercise. KEY POINTS: Older males have the largest increase in blood pressure despite having the smallest increases in sympathetic vasomotor outflow during rhythmic handgrip exercise. Young males demonstrate greater increases in sympathetic action potential (AP) discharge compared with young females during rhythmic handgrip exercise. Older adults (regardless of sex) demonstrate smaller increases in muscle sympathetic nerve activity (MSNA) burst amplitude and total AP clusters compared with young adults during exercise, as well as smaller increases in integrated MSNA burst frequency, incidence and total MSNA activity during post-exercise circulatory occlusion (i.e. independent effect of age). Males, but not females (regardless of age), reflexively modify AP conduction velocity during exercise. Our results indicate that age and sex independently and interactively impact the neural and cardiovascular homeostatic adjustments to fatiguing small muscle mass exercise.

Blood pressure and muscle sympathetic nerve activity are associated with trait anxiety in humans

Chronic anxiety is prevalent and associated with an increased risk of cardiovascular disease. Prior studies that have reported a relationship between muscle sympathetic nerve activity (MSNA) and anxiety have focused on participants with anxiety disorders and/or metabolic syndrome. The present study leverages a large cohort of healthy adults devoid of cardiometabolic disorders to examine the hypothesis that trait anxiety severity is positively associated with resting MSNA and blood pressure. Resting blood pressure (BP) (sphygmomanometer and finger plethysmography), MSNA (microneurography), and heart rate (HR; electrocardiogram) were collected in 88 healthy participants (52 males, 36 females, 25 ± 1 yr, 25 ± 1 kg/m2). Multiple linear regression was performed to assess the independent relationship between trait anxiety, MSNA, resting BP, and HR while controlling for age and sex. Trait anxiety was significantly correlated with systolic arterial pressure (SAP; r = 0.251, P = 0.018), diastolic arterial pressure (DAP; r = 0.291, P = 0.006), mean arterial pressure (MAP; r = 0.328, P = 0.002), MSNA burst frequency (BF; r = 0.237, P = 0.026), and MSNA burst incidence (BI; r = 0.225, P = 0.035). When controlling for the effects of age and sex, trait anxiety was independently associated with SAP (β = 0.206, P = 0.028), DAP (β = 0.317, P = 0.002), MAP (β = 0.325, P = 0.001), MSNA BF (β = 0.227, P = 0.030), and MSNA BI (β = 0.214, P = 0.038). Trait anxiety is associated with increased blood pressure and MSNA, demonstrating an important relationship between anxiety and autonomic blood pressure regulation.NEW & NOTEWORTHY Anxiety is associated with development of cardiovascular disease. Although the sympathetic nervous system is a likely mediator of this relationship, populations with chronic anxiety have shown little, if any, alteration in resting levels of directly recorded muscle sympathetic nerve activity (MSNA). The present study is the first to reveal an independent relationship between trait anxiety, resting blood pressure, and MSNA in a large cohort of healthy males and females devoid of cardiometabolic comorbidities.

Differential regulation of sympathetic neural burst frequency and amplitude throughout normal pregnancy: a longitudinal study

Sympathetic activation is a hallmark of pregnancy. However, longitudinal assessments of muscle sympathetic nerve activity (MSNA) in pregnancy are scarce and have primarily focused on burst occurrence (frequency) at rest, despite burst strength (amplitude) representing distinct characteristics of sympathetic outflow. Thus, we assessed MSNA burst amplitude distributions in healthy women to determine the impact of normal pregnancy on neural discharge patterns in response to orthostatic stress. Twenty-six women were studied longitudinally during pre-, early- (4-8 wk of gestation), and late (32-36 wk) pregnancy, as well as postpartum (6-10 wk after delivery). MSNA, blood pressure (BP), and heart rate (HR) were measured in the supine posture and during graded head-up tilt (30° and 60° HUT). Mean and median MSNA burst amplitudes were used to characterize burst amplitude distribution. In late pregnancy, women demonstrated smaller increases in HR (P < 0.001) during 60° HUT and larger increases in systolic BP (P = 0.043) throughout orthostasis, compared with prepregnancy. The increase in MSNA burst frequency during late- relative to prepregnancy (Late: Δ14[10] vs. Pre: Δ21[9] bursts/min; P = 0.001) was smaller during 60° HUT, whereas increases in burst incidence were smaller in late- relative to prepregnancy throughout orthostasis (P = 0.009). Nonetheless, median burst amplitude was smaller throughout orthostasis in late compared with prepregnancy (P = 0.038). Thus, while supine MSNA burst frequency was greater in late pregnancy, increases in burst frequency and strength during orthostasis were attenuated. These smaller, orthostatically induced MSNA increases may reflect natural adaptions of pregnancy serving to prevent sympathetic hyper-reactivity that is common in pathological states.

Longitudinal interrogation of sympathetic neural circuits and hemodynamics in preclinical models

Neurological disorders, including spinal cord injury, result in hemodynamic instability due to the disruption of supraspinal projections to the sympathetic circuits located in the spinal cord. We recently developed a preclinical model that allows the identification of the topology and dynamics through which sympathetic circuits modulate hemodynamics, supporting the development of a neuroprosthetic baroreflex that precisely controls blood pressure in rats, monkeys and humans with spinal cord injuries. Here, we describe the continuous monitoring of arterial blood pressure and sympathetic nerve activity over several months in preclinical models of chronic neurological disorders using commercially available telemetry technologies, as well as optogenetic and neuronal tract-tracing procedures specifically adapted to the sympathetic circuitry. Using a blueprint to construct a negative-pressure chamber, the approach enables the reproduction, in rats, of well-controlled and reproducible episodes of hypotension-mimicking orthostatic challenges already used in humans. Blood pressure variations can thus be directly induced and linked to the molecular, functional and anatomical properties of specific neurons in the brainstem, spinal cord and ganglia. Each procedure can be completed in under 2 h, while the construction of the negative-pressure chamber requires up to 1 week. With training, individuals with a basic understanding of cardiovascular physiology, engineering or neuroscience can collect longitudinal recordings of hemodynamics and sympathetic nerve activity over several months.

Rhythmic firing of neurons in the medulla of conscious freely behaving rats: rhythmic coupling with baroreceptor input

Recent investigations emphasized the importance of neural control of cardiovascular adjustments in complex behaviors, including stress, exercise, arousal, sleep-wake states, and different tasks. Baroreceptor feedback is an essential component of this system acting on different time scales from maintaining stable levels of cardiovascular parameters on the long-term to rapid alterations according to behavior. The baroreceptor input is essentially rhythmic, reflecting periodic fluctuations in arterial blood pressure. Cardiac rhythm is a prominent feature of the autonomic control system, present on different levels, including neuron activity in central circuits. The mechanism of rhythmic entrainment of neuron firing by the baroreceptor input was studied in great detail under anesthesia, but recordings of sympathetic-related neuron firing in freely moving animals remain extremely scarce. In this study, we recorded multiple single neuron activity in the reticular formation of the medulla in freely moving rats during natural behavior. Neurons firing in synchrony with the cardiac rhythm were detected in each experiment (n = 4). In agreement with prior observations in anesthetized cats, we found that neurons in this area exhibited high neuron-to-neuron variability and temporal flexibility in their coupling to cardiac rhythm in freely moving rats, as well. This included firing in bursts at multiples of cardiac cycles, but not directly coupled to the heartbeat, supporting the concept of baroreceptor input entraining intrinsic neural oscillations rather than imposing a rhythm of solely external origin on these networks. It may also point to a mechanism of maintaining the basic characteristics of sympathetic neuron activity, i.e., burst discharge and cardiac-related rhythmicity, on the background of behavior-related adjustments in their firing rate.

Sympathetic toggled sinus rate acceleration as a mechanism of sustained sinus tachycardia in chronic orthostatic intolerance syndrome

BACKGROUND

The role of sympathetic nerve activity to maintain sinus rate acceleration remains unclear.

OBJECTIVE

The purpose of this study was to test the hypothesis that sustained (>30 seconds) sinus rate acceleration can be associated with either a sympathetic driven or a sympathetic toggled mechanism.

METHODS

We used a patch monitor to record skin sympathetic nerve activity (SKNA) and electrocardiogram over 24 hours. Study 1 included chronic orthostatic intolerance (OI) (n = 18), atrial fibrillation (n = 7), and asymptomatic normal control (n = 19) groups. Study 2 included 17 participants with chronic OI not treated with ivabradine, pyridostigmine, or β-blockers.

RESULTS

While a majority of sinus rate acceleration was driven by persistent SKNA in study 1, some episodes were toggled on and off by SKNA bursts without persistent SKNA elevation. The sympathetic toggled sinus rate acceleration episodes were found in 7 of 18 participants with chronic OI (39%), 2 of 7 participants with atrial fibrillation (29%), and 6 of 19 normal control participants (32%) (P = .847) and were faster and longer in the chronic OI group than in other groups. In study 2, there were a total of 11 episodes of sinus rate acceleration that persisted for >200 seconds. Among these episodes, 6 (35%) were toggled on and off by SKNA bursts.

CONCLUSION

Sustained sinus rate acceleration (may be toggled on or off) is associated with SKNA bursts in participants with chronic OI, participants with atrial fibrillation, and normal controls. Patients with OI had more frequent and longer episodes than did other groups.

Sympathetic neural reactivity to the Trier social stress test

Sympathetic responsiveness to laboratory mental stress is highly variable, making interpretations of its role in stress reactivity challenging. The present study assessed muscle sympathetic nerve activity (MSNA, microneurography) responsiveness to the Trier social stress test (TSST), which employs an anticipatory stress phase, followed by a public speaking and mental arithmetic task. We hypothesized that sympathetic reactivity to the anticipatory phase would offer a more uniform response between individuals due to elimination of confounds (i.e. respiratory changes, muscle movement, etc.) observed during more common stress tasks. Participants included 26 healthy adults (11 men, 15 women, age: 25 ± 6 years, body mass index: 24 ± 3 kg/m² ). Continuous heart rate (electrocardiogram) and beat-to-beat blood pressure (finger plethysmography) were recorded from all participants, while MSNA recordings were obtained in 20 participants. MSNA burst frequency was significantly reduced during anticipatory stress. During the speech, although burst frequency was unchanged, total MSNA was significantly increased. Changes in diastolic arterial pressure were predictive of changes in MSNA during anticipatory (β = -0.680, P = 0.001), but not the speech (P = 0.318) or mental maths (P = 0.051) phases. Lastly, sympathetic reactivity to anticipatory stress was predictive of subsequent reactivity to both speech (β = 0.740, P = 0.0002) and maths (β = 0.663, P = 0.001). In conclusion, anticipatory social stress may offer a more versatile means of assessing sympathetic reactivity to mental stress in the absence of confounds and appears to predict reactivity to subsequent mental stress paradigms. KEY POINTS: Cardiovascular reactivity to laboratory mental stress is predictive of future health outcomes. However, reactivity of the sympathetic nervous system to mental stress is highly variable. The current study assessed peripheral muscle sympathetic nerve activity in response to the Trier social stress test, a psychosocial stressor that includes anticipatory stress, public speaking and mental arithmetic. Our findings demonstrate that sympathetic neural activity is consistently reduced during anticipatory stress. Conversely, the classically observed inter-individual variability of sympathetic responsiveness was observed during speech and maths tasks. Additionally, sympathetic reactivity to the anticipatory period accurately predicted how an individual would respond to both speech and maths tasks, outlining the utility of anticipatory stress in future research surrounding stress reactivity. Utilization of the Trier social stress test in autonomic physiology may offer an alternative assessment of sympathetic responsiveness to stress with more consistent inter-individual responsiveness and may be a useful tool for further investigation of stress reactivity.

Aging is associated with enhanced central, but impaired peripheral arms of the sympathetic baroreflex arc

We tested the hypothesis that the baroreflex control of action potential (AP) subpopulations would be blunted in older compared to young adults. Integrated muscle sympathetic nerve activity (MSNA) and the underlying sympathetic APs were obtained using microneurography and a continuous wavelet analysis approach, respectively, during 5 minutes of supine rest in 13 older (45-75 years, 6 females) and 14 young (21-30 years, 7 females) adults. Baroreflex threshold relationships were quantified as the slope of the linear regression between MSNA burst probability (%) and diastolic blood pressure (mmHg), or AP cluster firing probability (%) and diastolic blood pressure (mmHg). Integrated MSNA baroreflex threshold gain was greater in older compared to young adults (older: -7.6±3.6 %/mmHg vs. Young: -3.5±1.5 %/mmHg, P<0.001). Similarly, the baroreflex threshold gain of AP clusters was modified by aging (group-by-cluster effect: P<0.001) such that older adults demonstrated greater baroreflex threshold gains of medium-sized AP clusters (e.g., Cluster 4, older: -8.2±3.2 %/mmHg vs. Young: -3.6±1.9 %/mmHg, P=0.003) but not for the smallest- (Cluster 1, older: -1.6±1.9 %/mmHg vs. Young: -1.0±1.7 %/mmHg, P>0.999) and largest-sized (Cluster 10, older: -0.5±0.5 %/mmHg vs. Young: -0.2±0.1 %/mmHg, P=0.819) AP clusters compared to young adults. In contrast, the peak change in mean arterial pressure (MAP) following a spontaneous MSNA burst (i.e., sympathetic transduction) was impaired with aging (older: -0.7±0.3 mmHg vs. Young: 1.8±1.2 mmHg, P<0.001). We conclude that aging is associated with elevated baroreflex control over high-probability AP content of sympathetic bursts that may compensate for impaired sympathetic neurovascular transduction.

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.

Role of the arterial baroreflex in the sympathetic response to hyperinsulinemia in adult humans

Muscle sympathetic nerve activity (MSNA) increases during hyperinsulinemia, primarily attributed to central nervous system effects. Whether peripheral vasodilation induced by insulin further contributes to increased MSNA via arterial baroreflex-mediated mechanisms requires further investigation. Accordingly, we examined baroreflex modulation of the MSNA response to hyperinsulinemia. We hypothesized that rescuing peripheral resistance with coinfusion of the vasoconstrictor phenylephrine would attenuate the MSNA response to hyperinsulinemia. We further hypothesized that the insulin-mediated increase in MSNA would be recapitulated with another vasodilator (sodium nitroprusside, SNP). In 33 young healthy adults (28 M/5F), MSNA (microneurography) and arterial blood pressure (BP, Finometer/brachial catheter) were measured, and total peripheral resistance (TPR, ModelFlow) and baroreflex sensitivity were calculated at rest and during intravenous infusion of insulin (n = 20) or SNP (n = 13). A subset of participants receiving insulin (n = 7) was coinfused with phenylephrine. Insulin infusion decreased TPR (P = 0.01) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity or BP (P > 0.05). Coinfusion with phenylephrine returned TPR and MSNA to baseline, with no effect on arterial baroreflex sensitivity (P > 0.05). Similar to insulin, SNP decreased TPR (P < 0.02) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity (P > 0.12). Acute hyperinsulinemia shifts the baroreflex stimulus-response curve to higher MSNA without changing sensitivity, likely due to insulin's peripheral vasodilatory effects. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.NEW & NOTEWORTHY We hypothesized that elevation in muscle sympathetic nervous system activity (MSNA) during hyperinsulinemia is mediated by its peripheral vasodilator effect on the arterial baroreflex. Using three separate protocols in humans, we observed increases in both MSNA and cardiac output during hyperinsulinemia, which we attributed to the baroreflex response to peripheral vasodilation induced by insulin. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.

Potentiation of GABAergic synaptic transmission by diazepam acutely increases resting beat-to-beat blood pressure variability in young adults

Resting beat-to-beat blood pressure variability is a powerful predictor of cardiovascular events and end-organ damage. However, its underlying mechanisms remain unknown. Herein, we tested the hypothesis that a potentiation of GABAergic synaptic transmission by diazepam would acutely increase resting beat-to-beat blood pressure variability. In 40 (17 females) young, normotensive subjects, resting beat-to-beat blood pressure (finger photoplethysmography) was continuously measured for 5 to 10 min, 60 min after the oral administration of either diazepam (10 mg) or placebo. The experiments were conducted in a randomized, double-blinded, and placebo-controlled design. Stroke volume was estimated from the blood pressure waveform (ModelFlow) permitting the calculation of cardiac output and total peripheral resistance. Direct recordings of muscle sympathetic nerve activity (MSNA, microneurography) were obtained in a subset of subjects (N=13) and spontaneous cardiac and sympathetic baroreflex sensitivity calculated. Compared to placebo, diazepam significantly increased the standard deviation of systolic (4.7±1.4 vs. 5.7±1.5 mmHg, P=0.001), diastolic (3.8±1.2 vs. 4.5±1.2 mmHg, P=0.007) and mean blood pressure (3.8±1.1 vs. 4.5±1.1 mmHg, P=0.002), as well as cardiac output (469±149 vs. 626±259 ml/min, P<0.001) and total peripheral resistance (1.0±0.3 vs. 1.4±0.6 mmHg/l/min, P<0.001). Similar results were found using different indices of variability. Furthermore, diazepam reduced MSNA burst frequency (placebo: 22±6 vs. diazepam: 18±8 bursts/min, P=0.025) without affecting the arterial baroreflex control of heart rate (placebo: 18.6±6.7 vs. diazepam: 18.8±7.0 ms/mmHg, P=0.87) and MSNA (placebo: -3.6±1.2 vs. diazepam: -3.4±1.5 bursts/100Hb/mmHg, P=0.55). These findings suggest that GABA_A receptors modulate resting beat-to-beat blood pressure variability in young adults.

Impaired sympathetic neural recruitment during exercise pressor reflex activation in women with post-traumatic stress disorder

Muscle sympathetic nerve activity (MSNA) increases during isometric exercise via increased firing of low-threshold action potentials (AP), recruitment of larger, higher-threshold APs, and synaptic delay modifications. Recent work found that women with post-traumatic stress disorder (PTSD) demonstrate exaggerated early-onset MSNA responses to exercise; however, it is unclear how PTSD affects AP recruitment patterns during fatiguing exercise. We hypothesized that women with PTSD (n = 11, 43 [11] [SD] years) would exhibit exaggerated sympathetic neural recruitment compared to women without PTSD (controls; n = 13, 40 [8] years). MSNA and AP discharge patterns (via microneurography and a continuous wavelet transform) were measured during 1 min of baseline, isometric handgrip exercise (IHG) to fatigue, 2 min of post-exercise circulatory occlusion (PECO), and 3 min of recovery. Women with PTSD were unable to increase AP content per burst compared to controls throughout IHG and PECO (main effect of group: P = 0.026). Furthermore, relative to controls, women with PTSD recruited fewer AP clusters per burst during the first (controls: ∆1.3 [1.2] vs. PTSD: ∆-0.2 [0.8]; P = 0.016) and second minute (controls: ∆1.2 [1.1] vs. PTSD: ∆-0.1 [0.8]; P = 0.022) of PECO, and fewer subpopulations of larger, previously silent axons during the first (controls: ∆5 [4] vs. PTSD: ∆1 [2]; P = 0.020) and second minute (controls: ∆4 [2] vs. PTSD: ∆1 [2]; P = 0.021) of PECO. Conversely, PTSD did not modify the AP cluster size-latency relationship during baseline, the end of IHG, or PECO (all P = 0.658-0.745). Collectively, these data indicate that women with PTSD demonstrate inherent impairments in the fundamental neural coding patterns elicited by the sympathetic nervous system during IHG and exercise pressor reflex activation.

Untangling Peripheral Sympathetic Neurocircuits

The sympathetic nervous system plays a critical role in regulating many autonomic functions, including cardiac rhythm. The postganglionic neurons in the sympathetic chain ganglia are essential components that relay sympathetic signals to target tissues and disruption of their activity leads to poor health outcomes. Despite this importance, the neurocircuitry within sympathetic ganglia is poorly understood. Canonically, postganglionic sympathetic neurons are thought to simply be activated by monosynaptic inputs from preganglionic cholinergic neurons of the intermediolateral cell columns of the spinal cord. Early electrophysiological studies of sympathetic ganglia where the peripheral nerve trunks were electrically stimulated identified excitatory cholinergic synaptic events in addition to retrograde action potentials, leading some to speculate that excitatory collateral projections are present. However, this seemed unlikely since sympathetic postganglionic neurons were known to synthesize and release norepinephrine and expression of dual neurochemical phenotypes had not been well recognized. In vitro studies clearly established the capacity of cultured sympathetic neurons to express and release acetylcholine and norepinephrine throughout development and even in pathophysiological conditions. Given this insight, we believe that the canonical view of ganglionic transmission needs to be reevaluated and may provide a mechanistic understanding of autonomic imbalance in disease. Further studies likely will require genetic models manipulating neurochemical phenotypes within sympathetic ganglia to resolve the function of cholinergic collateral projections between postganglionic neurons. In this perspective article, we will discuss the evidence for collateral projections in sympathetic ganglia, determine if current laboratory techniques could address these questions, and discuss potential obstacles and caveats.

Sympathetic Neural Control in Humans with Anxiety-Related Disorders

Numerous conceptual models are used to describe the dynamic responsiveness of physiological systems to environmental pressures, originating with Claude Bernard's milieu intérieur and extending to more recent models such as allostasis. The impact of stress and anxiety upon these regulatory processes has both basic science and clinical relevance, extending from the pioneering work of Hans Selye who advanced the concept that stress can significantly impact physiological health and function. Of particular interest within the current article, anxiety is independently associated with cardiovascular risk, yet mechanisms underlying these associations remain equivocal. This link between anxiety and cardiovascular risk is relevant given the high prevalence of anxiety in the general population, as well as its early age of onset. Chronically anxious populations, such as those with anxiety disorders (i.e., generalized anxiety disorder, panic disorder, specific phobias, etc.) offer a human model that interrogates the deleterious effects that chronic stress and allostatic load can have on the nervous system and cardiovascular function. Further, while many of these disorders do not appear to exhibit baseline alterations in sympathetic neural activity, reactivity to mental stress offers insights into applicable, real-world scenarios in which heightened sympathetic reactivity may predispose those individuals to elevated cardiovascular risk. This article also assesses behavioral and lifestyle modifications that have been shown to concurrently improve anxiety symptoms, as well as sympathetic control. Lastly, future directions of research will be discussed, with a focus on better integration of psychological factors within physiological studies examining anxiety and neural cardiovascular health. © 2022 American Physiological Society. Compr Physiol 12:1-33, 2022.

An open-source application for the standardized burst identification from the integrated muscle sympathetic neurogram

Muscle sympathetic nerve activity (MSNA) can be acquired from humans using the technique of microneurography. The resulting integrated neurogram displays pulse-synchronous bursts of sympathetic activity, which undergoes processing for standard MSNA metrics including burst frequency, height, area, incidence, total activity, and latency. The procedure for detecting bursts of MSNA and calculating burst metrics is tedious and differs widely among laboratories worldwide. We sought to develop an open-source, cross-platform web application that provides a standardized approach for burst identification and a tool to increase research reproducibility for those measuring MSNA. We compared the performance of this web application against a manual scoring approach under conditions of rest, chemoreflex activation (n = 9, 20-min isocapnic hypoxia), and metaboreflex activation (n = 13, 2-min isometric handgrip exercise and 4-min postexercise circulatory occlusion). The intraclass correlation coefficient (ICC) indicated good to strong agreement between scoring approaches for burst frequency (ICC = 0.92-0.99), incidence (ICC = 0.94-0.99), height (ICC = 0.76-0.88), total activity (ICC = 0.85-0.99), and latency (ICC = 0.97-0.99). Agreement with burst area was poor to moderate (ICC = 0.04-0.67) but changes in burst area were similar with chemoreflex and metaboreflex activation. Scoring using the web application was highly efficient and provided data visualization tools that expedited data processing and the analysis of MSNA. We recommend the open-source web application be adopted by the community for the analysis of MSNA.NEW & NOTEWORTHY The basic analysis of muscle sympathetic nerve activity (MSNA) requires the identification of pulse-synchronous bursts from the integrated neurogram before standard MSNA metrics can be quantified. This process is a time-consuming task requiring an experienced microneurographer to visually identify and manually label bursts. We developed an open-source, cross-platform application permitting a standardized approach for sympathetic burst identification and present the performance of this application against a manual scorer under basal conditions and during sympathoexcitatory stresses.

Blood pressure oscillations impact signal-averaged sympathetic transduction of blood pressure: implications for the association with resting sympathetic outflow

Signal-averaged sympathetic transduction of blood pressure (BP) is inversely related to resting muscle sympathetic nerve activity (MSNA) burst frequency in healthy cohorts. Whether this represents a physiological compensatory adaptation or a methodological limitation, remains unclear. The current analysis aimed to determine the contribution of methodological limitations by evaluating the dependency of MSNA transduction at different levels of absolute BP. Thirty-six healthy participants (27 ± 7 yr, 9 females) underwent resting measures of beat-to-beat heart rate, BP, and muscle sympathetic nerve activity (MSNA). Tertiles of mean arterial pressure (MAP) were computed for each participant to identify cardiac cycles occurring below, around, and above the MAP operating pressure (OP). Changes in hemodynamic variables were computed across 15 cardiac cycles within each MAP tertile to quantify sympathetic transduction. MAP increased irrespective of sympathetic activity when initiated below the OP, but with MSNA bursts provoking larger rises (3.0 ± 0.9 vs. 2.1 ± 0.7 mmHg; P < 0.01). MAP decreased irrespective of sympathetic activity when initiated above the OP, but with MSNA bursts attenuating the drop (-1.3 ± 1.1 vs. -3.1 ± 1.2 mmHg; P < 0.01). In participants with low versus high resting MSNA (12 ± 4 vs. 32 ± 10 bursts/min), sympathetic transduction of MAP was not different when initiated by bursts below (3.2 ± 1.0 vs. 2.8 ± 0.9 mmHg; P = 0.26) and above the OP (-1.0 ± 1.3 vs. -1.6 ± 0.8 mmHg; P = 0.08); however, low resting MSNA was associated with a smaller proportion of MSNA bursts firing above the OP (15 ± 5 vs. 22 ± 5%; P < 0.01). The present analyses demonstrate that the signal-averaging technique for calculating sympathetic transduction of BP is influenced by the timing of an MSNA burst relative to cyclic oscillations in BP.NEW & NOTEWORTHY The current signal-averaging technique for calculating sympathetic transduction of blood pressure does not consider the arterial pressure at which each muscle sympathetic burst occurs. A burst firing when mean arterial pressure is above the operating pressure was associated with a decrease in blood pressure. Thus, individuals with higher muscle sympathetic nerve activity demonstrate a reduced sympathetic transduction owing to the weighted contribution of more sympathetic bursts at higher levels of arterial pressure.

Inference on homeostatic belief precision

Interoception and homeostatic/allostatic control are intertwined branches of closed-loop brain-body interactions (BBI). Given their importance in mental and psychosomatic disorders, establishing computational assays of BBI represents a clinically important but methodologically challenging endeavor. This technical note presents a novel approach, derived from a generic computational model of homeostatic/allostatic control that underpins (meta)cognitive theories of affective and psychosomatic disorders. This model views homeostatic setpoints as probability distributions ("homeostatic beliefs") whose parameters determine regulatory efforts and change dynamically under allostatic predictions. In particular, changes in homeostatic belief precision, triggered by anticipated threats to homeostasis, are thought to alter cerebral regulation of bodily states. Here, we present statistical procedures for inferring homeostatic belief precision from measured bodily states and/or regulatory (action) signals. We analyze the inference problem, derive two alternative estimators of homeostatic belief precision, and apply our method to simulated data. Our proposed approach may prove useful for assessing BBI in individual subjects.

GABAA receptor activation modulates the muscle sympathetic nerve activity responses at the onset of static exercise in humans

Exercise is a well-known sympathoexcitatory stimulus. However, muscle sympathetic nerve activity (MSNA) can decrease during the onset of muscle contraction. Yet, the underlying mechanisms and neurotransmitters involved in the sympathetic responses at the onset of exercise remain unknown. Herein, we tested the hypothesis that GABA_A receptors may contribute to the MSNA responses at the onset of static handgrip in humans. Thirteen young, healthy individuals (4 females) performed 30 s of ischemic static handgrip at 30% of maximum volitional contraction before and following oral administration of either placebo or diazepam (10 mg), a benzodiazepine that enhances GABA_A activity. MSNA (microneurography), beat-to-beat blood pressure (finger photopletysmography), heart rate (electrocardiogram), and stroke volume (ModelFlow) were continuously measured. Cardiac output (CO = stroke volume × heart rate) and total vascular conductance (TVC = CO/mean blood pressure) were subsequently calculated. At rest, MSNA was reduced while hemodynamic variables were unchanged after diazepam administration. Before diazepam, static handgrip elicited a significant decrease in MSNA burst frequency (Δ-7 ± 2 bursts/min, P < 0.01 vs. baseline) and MSNA burst incidence (Δ-16 ± 2 bursts/100 heart beats, P < 0.01 vs. baseline); however, these responses were attenuated following diazepam administration (Δ-1 ± 2 bursts/min and Δ-7 ± 2 bursts/100 heart beats, respectively; P < 0.01 vs. before diazepam). Diazepam did not affect the increases in heart rate, blood pressure, CO, and TVC at the exercise onset. Importantly, the placebo had no effect on any variable at rest or exercise onset. These findings suggest that GABA_A receptor activation modulates the MSNA responses at the onset of static exercise in young, healthy humans.NEW & NOTEWORTHY In this study, we found that the reduction in muscle sympathetic nerve activity at the onset of static handgrip exercise was blunted following GABA_A receptor activation with oral administration of diazepam in young, healthy individuals. The present findings provide novel insight into neural circuitry mechanisms controlling muscle sympathetic outflow during exercise in humans.

Evidence of Reduced Efferent Renal Sympathetic Innervation After Chemical Renal Denervation in Humans

BACKGROUND

Renal denervation (RDN) is effective at lowering blood pressure. However, it is unknown if ablative procedures elicit sympathetic denervation of the kidneys in humans. The aim of this investigation was to assess sympathetic innervation of the renal cortex following perivascular chemical RDN, which may be particularly effective at ablating perivascular efferent and afferent nerves.

METHODS

Seven hypertensive patients (4F:3M; 50-65 years) completed PET-CT sympathetic neuroimaging of the renal cortex using 11C-methylreboxetine (11C-MRB, norepinephrine transporter ligand) and 6-[18F]-fluorodopamine (18F-FDA; substrate for the cell membrane norepinephrine transporter) before and 8 weeks after chemical RDN (Peregrine System Infusion Catheter, Ablative Solutions; n = 4; 2F:2M) or control renal angiography (n = 3; 2F:1M). Patients completed physiological phenotyping including 24-hour ambulatory blood pressure, hemodynamics, muscle sympathetic nerve activity, and 24-hour urine collection.

RESULTS

RDN decreased 11C-MRB-derived radioactivity by ~30% (Δ  11C-MRB/chamber: -0.95 a.u. confidence interval (CI): -1.36 to -0.54, P = 0.0002), indicative of efferent RDN. In contrast, 18F-FDA-derived radioactivity increased (Δ  18F-FDA/chamber: 2.72 a.u. CI: 0.73-4.71, P = 0.009), consistent with reduced vesicular turnover. Controls showed no change in either marker. Ambulatory systolic pressure decreased in 3 of 4 patients (-9 mm Hg CI: -27 to 9, P = 0.058), and central systolic pressure decreased in all patients (-23 mm Hg CI: -51 to 5, P = 0.095).

CONCLUSIONS

These results are the first to show efferent sympathetic denervation of the renal cortex following RDN in humans. Further studies of mechanisms underlying variable blood pressure lowering in the setting of documented RDN may provide insights into inconsistencies in clinical trial outcomes.

CLINICAL TRIALS REGISTRATION

Trial Number NCT03465917.