Increased Brain-Derived Neurotrophic Factor in Lumbar Dorsal Root Ganglia Contributes to the Enhanced Exercise Pressor Reflex in Heart Failure

Neurocardiology: translational advancements and potential

In our original white paper published in the The Journal of Physiology in 2016, we set out our knowledge of the structural and functional organization of cardiac autonomic control, how it remodels during disease, and approaches to exploit such knowledge for autonomic regulation therapy. The aim of this update is to build on this original blueprint, highlighting the significant progress which has been made in the field since and major challenges and opportunities that exist with regard to translation. Imbalances in autonomic responses, while beneficial in the short term, ultimately contribute to the evolution of cardiac pathology. As our understanding emerges of where and how to target in terms of actuators (including the heart and intracardiac nervous system (ICNS), stellate ganglia, dorsal root ganglia (DRG), vagus nerve, brainstem, and even higher centres), there is also a need to develop sensor technology to respond to appropriate biomarkers (electrophysiological, mechanical, and molecular) such that closed-loop autonomic regulation therapies can evolve. The goal is to work with endogenous control systems, rather than in opposition to them, to improve outcomes.

Diagnostic and predictive abilities of myokines in patients with heart failure

Myokines are defined as a heterogenic group of numerous cytokines, peptides and metabolic derivates, which are expressed, synthesized, produced, and released by skeletal myocytes and myocardial cells and exert either auto- and paracrine, or endocrine effects. Previous studies revealed that myokines play a pivotal role in mutual communications between skeletal muscles, myocardium and remote organs, such as brain, vasculature, bone, liver, pancreas, white adipose tissue, gut, and skin. Despite several myokines exert complete divorced biological effects mainly in regulation of skeletal muscle hypertrophy, residential cells differentiation, neovascularization/angiogenesis, vascular integrity, endothelial function, inflammation and apoptosis/necrosis, attenuating ischemia/hypoxia and tissue protection, tumor growth and malignance, for other occasions, their predominant effects affect energy homeostasis, glucose and lipid metabolism, adiposity, muscle training adaptation and food behavior. Last decade had been identified 250 more myokines, which have been investigating for many years further as either biomarkers or targets for heart failure management. However, only few myokines have been allocated to a promising tool for monitoring adverse cardiac remodeling, ischemia/hypoxia-related target-organ dysfunction, microvascular inflammation, sarcopenia/myopathy and prediction for poor clinical outcomes among patients with HF. This we concentrate on some most plausible myokines, such as myostatin, myonectin, brain-derived neurotrophic factor, muslin, fibroblast growth factor 21, irisin, leukemia inhibitory factor, developmental endothelial locus-1, interleukin-6, nerve growth factor and insulin-like growth factor-1, which are suggested to be useful biomarkers for HF development and progression.

Altered locomotor muscle metaboreflex control of ventilation in patients with COPD

We investigated the locomotor muscle metaboreflex control of ventilation, circulation, and dyspnea in patients with chronic obstructive pulmonary disease (COPD). Ten patients [forced expiratory volume in 1 second (FEV1; means ± SD) = 43 ± 17% predicted] and nine age- and sex-matched controls underwent 1) cycling exercise followed by postexercise circulatory occlusion (PECO) to activate the metaboreflex or free circulatory flow to inactivate it, 2) cold pressor test to interpret whether any altered reflex response was specific to the metaboreflex arc, and 3) muscle biopsy to explore the metaboreflex arc afferent side. We measured airflow, dyspnea, heart rate, arterial pressure, muscle blood flow, and vascular conductance during reflexes activation. In addition, we measured fiber types, glutathione redox balance, and metaboreceptor-related mRNAs in the vastus lateralis. Metaboreflex activation increased ventilation versus free flow in patients (∼15%, P < 0.020) but not in controls (P > 0.450). In contrast, metaboreflex activation did not change dyspnea in patients (P = 1.000) but increased it in controls (∼100%, P < 0.001). Other metaboreflex-induced responses were similar between groups. Cold receptor activation increased ventilation similarly in both groups (P = 0.46). Patients had greater type II skeletal myocyte percentage (14%, P = 0.010), lower glutathione ratio (-34%, P = 0.015), and lower nerve growth factor (NGF) mRNA expression (-60%, P = 0.031) than controls. Therefore, COPD altered the locomotor muscle metaboreflex control of ventilation. It increased type II myocyte percentage and elicited redox imbalance, potentially producing more muscle metaboreceptor stimuli. Moreover, it decreased NGF expression, suggesting a downregulation of metabolically sensitive muscle afferents.NEW & NOTEWORTHY This study's integrative physiology approach provides evidence for a specific alteration in locomotor muscle metaboreflex control of ventilation in patients with COPD. Furthermore, molecular analyses of a skeletal muscle biopsy suggest that the amount of muscle metaboreceptor stimuli derived from type II skeletal myocytes and redox imbalance overcame a downregulation of metabolically sensitive muscle afferents.

Antagonism of TRPV4 channels partially reduces mechanotransduction in rat skeletal muscle afferents

Mechanical distortion of working skeletal muscle induces sympathoexcitation via thin fibre afferents, a reflex response known as the skeletal muscle mechanoreflex. However, to date, the receptor ion channels responsible for mechanotransduction in skeletal muscle remain largely undetermined. Transient receptor potential vanilloid 4 (TRPV4) is known to sense mechanical stimuli such as shear stress or osmotic pressure in various organs. It is hypothesized that TRPV4 in thin-fibre primary afferents innervating skeletal muscle is involved in mechanotransduction. Fluorescence immunostaining revealed that 20.1 ± 10.1% of TRPV4 positive neurons were small dorsal root ganglion (DRG) neurons that were DiI-labelled, and among them 9.5 ± 6.1% of TRPV4 co-localized with the C-fibre marker peripherin. In vitro whole-cell patch clamp recordings from cultured rat DRG neurons demonstrated that mechanically activated current amplitude was significantly attenuated after the application of the TRPV4 antagonist HC067047 compared to control (P = 0.004). Such reductions were also observed in single-fibre recordings from a muscle-nerve ex vivo preparation where HC067047 significantly decreased afferent discharge to mechanical stimulation (P = 0.007). Likewise, in an in vivo decerebrate rat preparation, the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to passive stretch of hindlimb muscle were significantly reduced by intra-arterial injection of HC067047 (ΔRSNA: P = 0.019, ΔMAP: P = 0.002). The findings suggest that TRPV4 plays an important role in mechanotransduction contributing to the cardiovascular responses evoked by the skeletal muscle mechanoreflex during exercise. KEY POINTS: Although a mechanical stimulus to skeletal muscle reflexively activates the sympathetic nervous system, the receptors responsible for mechanotransduction in skeletal muscle thin fibre afferents have not been fully identified. Evidence suggests that TRPV4 is a mechanosensitive channel that plays an important role in mechanotransduction within various organs. Immunocytochemical staining demonstrates that TRPV4 is expressed in group IV skeletal muscle afferents. In addition, we show that the TRPV4 antagonist HC067047 decreases the responsiveness of thin fibre afferents to mechanical stimulation at the muscle tissue level as well as at the level of dorsal root ganglion neurons. Moreover, we demonstrate that intra-arterial HC067047 injection attenuates the sympathetic and pressor responses to passive muscle stretch in decerebrate rats. These data suggest that antagonism of TRPV4 attenuates mechanotransduction in skeletal muscle afferents. The present study demonstrates a probable physiological role for TRPV4 in the regulation of mechanical sensation in somatosensory thin fibre muscle afferents.

Enhanced Respiratory Frequency Response to Lower Limb Mechanoreceptors Activation in Patients with Chronic Obstructive Pulmonary Disease

PURPOSE

To investigate the mechanoreflex control of respiration and circulation in patients with chronic obstructive pulmonary disease (COPD).

METHODS

Twenty-eight patients with moderate-to-severe COPD (mean ± SD: 67.0 ± 7.9 yr, 10 women) and 14 age- and sex-matched controls (67.9 ± 2.6 yr, 7 women) participated in the study. Their dominant knee was passively moved to stimulate mechanoreceptors, whereas vastus lateralis surface electrical activity checked active contractions. A differential pressure flowmeter, an electrocardiogram, and a servo-controlled finger photoplethysmograph acquired cardiorespiratory data. To gain insight into the mechanoreflex arc, we further analyzed reduced/oxidized glutathione ratio and mechanoreceptor-related gene expression in a vastus lateralis biopsy of additional nine patients (63.9 ± 8.1 yr, 33% women) and eight controls (62.9 ± 9.1 yr, 38% women).

RESULTS

Patients with COPD had a greater peak respiratory frequency response (COPD: Δ = 3.2 ± 2.3 vs Controls: 1.8 ± 1.2 cycles per minute, P = 0.036) and a smaller peak tidal volume response to passive knee movement than controls. Ventilation, heart rate, stroke volume, and cardiac output peak responses, and total peripheral resistance nadir response, were unaltered by COPD. In addition, patients had a diminished glutathione ratio (COPD: 13.3 ± 3.8 vs controls: 20.0 ± 5.5 a.u., P = 0.015) and an augmented brain-derived neurotrophic factor expression (COPD: 2.0 ± 0.7 vs controls: 1.1 ± 0.4 a.u., P = 0.002) than controls. Prostaglandin E receptor 4, cyclooxygenase 2, and Piezo1 expression were similar between groups.

CONCLUSIONS

Respiratory frequency response to mechanoreceptors activation is increased in patients with COPD. This abnormality is possibly linked to glutathione redox imbalance and augmented brain-derived neurotrophic factor expression within locomotor muscles, which could increase mechanically sensitive afferents' stimulation and sensitivity.

Voltage-gated potassium channel dysfunction in dorsal root ganglia contributes to the exaggerated exercise pressor reflex in rats with chronic heart failure

An exaggerated exercise pressor reflex (EPR) causes excessive sympathoexcitation and exercise intolerance during physical activity in the chronic heart failure (CHF) state. Muscle afferent sensitization contributes to the genesis of the exaggerated EPR in CHF. However, the cellular mechanisms underlying muscle afferent sensitization in CHF remain unclear. Considering that voltage-gated potassium (Kv) channels critically regulate afferent neuronal excitability, we examined the potential role of Kv channels in mediating the sensitized EPR in male rats with CHF. Real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting experiments demonstrate that both mRNA and protein expressions of multiple Kv channel isoforms (Kv1.4, Kv3.4, Kv4.2, and Kv4.3) were downregulated in lumbar dorsal root ganglions (DRGs) of CHF rats compared with sham rats. Immunofluorescence data demonstrate significant decreased Kv channel staining in both NF200-positive and IB4-positive lumbar DRG neurons in CHF rats compared with sham rats. Data from patch-clamp experiments demonstrate that the total Kv current, especially I_A, was dramatically decreased in medium-sized IB4-negative muscle afferent neurons (a subpopulation containing mostly Aδ neurons) from CHF rats compared with sham rats, indicating a potential functional loss of Kv channels in muscle afferent Aδ neurons. In in vivo experiments, adenoviral overexpression of Kv4.3 in lumbar DRGs for 1 wk attenuated the exaggerated EPR induced by muscle static contraction and the mechanoreflex by passive stretch without affecting the blunted cardiovascular response to hindlimb arterial injection of capsaicin in CHF rats. These data suggest that Kv channel dysfunction in DRGs plays a critical role in mediating the exaggerated EPR and muscle afferent sensitization in CHF.NEW & NOTEWORTHY The primary finding of this manuscript is that voltage-gated potassium (Kv) channel dysfunction in DRGs plays a critical role in mediating the exaggerated EPR and muscle afferent sensitization in chronic heart failure (CHF). We propose that manipulation of Kv channels in DRG neurons could be considered as a potential new approach to reduce the exaggerated sympathoexcitation and to improve exercise intolerance in CHF, which can ultimately facilitate an improved quality of life and reduce mortality.

Macrophage activation in stellate ganglia contributes to lung injury-induced arrhythmogenesis in male rats

AIM

Patients suffering from acute lung injury (ALI) are at high risk of developing cardiac arrhythmias. We hypothesized that stellate ganglia (SG) neural inflammation contributes to ALI-induced arrhythmia.

METHODS

We created an ALI rat model using a single tracheal instillation of bleomycin (2.5 mg/kg), with saline as a sham control. We recorded ECGs by implanted radiotelemetry in male bleomycin and sham rats treated with and without oral minocycline (20 mg/kg/d), an anti-inflammatory drug that inhibits microglia/macrophage activation. The SG neuronal excitability was assessed by electrophysiology experiments.

RESULTS

ECG data showed that bleomycin-exposed rats exhibited significantly more spontaneous premature ventricular contractions (PVCs) from 1- to 3-week post-induction compared with sham rats, which was mitigated by chronic oral administration of minocycline. The bleomycin-exposed rats displayed a robust increase in both the number of Iba1-positive macrophages and protein expression of interferon regulatory factor 8 in the SG starting as early at 1-week post-exposure and lasted for at least 4 weeks, which was largely attenuated by minocycline. Heart rate variability analysis indicated autonomic imbalance during the first 2-week post-bleomycin, which was significantly attenuated by minocycline. Electrical stimulation of the decentralized SG triggered more PVCs in bleomycin-exposed rats than sham and bleomycin + minocycline rats. Patch-clamp data demonstrated enhanced SG neuronal excitability in the bleomycin-exposed rats, which was attenuated by minocycline. Co-culture of lipopolysaccharide (LPS)-pretreated macrophages with normal SG neurons enhanced SG neuronal excitability.

CONCLUSION

Macrophage activation in the SG contributes to arrhythmogenesis in bleomycin-induced ALI in male rats.

Exaggerated sympathetic and cardiovascular responses to dynamic mechanoreflex activation in rats with heart failure: Role of endoperoxide 4 and thromboxane A2 receptors

The primary purpose of this investigation was to determine the role played by endoperoxide 4 receptors (EP4-R) and thromboxane A₂ receptors (TxA₂-R) during isolated dynamic muscle mechanoreflex activation in rats with heart failure with reduced ejection fraction (HF-rEF) and sham-operated healthy controls. We found that injection of the EP4-R antagonist L-161,982 (1 μg) into the arterial supply of the hindlimb had no effect on the peak pressor response to dynamic hindlimb muscle stretch in HF-rEF (n = 6, peak ∆MAP pre: 27 ± 7; post: 27 ± 4 mm Hg; P = 0.99) or sham (n = 6, peak ∆MAP pre: 15 ± 3; post: 13 ± 3 mm Hg; P = 0.67) rats. In contrast, injection of the TxA₂-R antagonist daltroban (80 μg) into the arterial supply of the hindlimb reduced the pressor response to dynamic hindlimb muscle stretch in HF-rEF (n = 11, peak ∆MAP pre: 28 ± 4; post: 16 ± 2 mm Hg; P = 0.02) but not sham (n = 8, peak ∆MAP pre: 17 ± 3; post: 16 ± 3; P = 0.84) rats. Our data suggest that TxA₂-Rs on thin fibre muscle afferents contribute to the exaggerated mechanoreflex in HF-rEF.

Brain-Derived Neurotrophic Factor, a New Predictor of Coronary Artery Calcification

Brain-derived neurotrophic factor (BDNF) plays a functional role in vascular endothelium homeostasis and the alleviation of atherosclerosis. Matrix gla protein (MGP) and Nε-(1-carboxymethyl)-l-lysine (CML) are both confirmed to be VC predictors. This study investigated the association between BDNF, MGP, CML and coronary artery calcification (CAC). Plasma BDNF, MGP, and CML levels were measured in 274 patients who underwent computed tomography to determine the CAC score (Agatston score). It was found that patients with CAC exhibited lower BDNF and MGP and higher CML levels than those without CAC. Plasma BDNF levels in patients with diabetes or hypertension were lower compared with the control groups. In logistic regression analysis, age, hypertension, BDNF, and MGP were independent predictors of CAC. Plasma BDNF and MGP levels were both correlated with the Agatston score even after adjustment for age, total cholesterol level, triglycerides, low-density lipoprotein level, creatinine clearance rate, and the presence of hypertension and diabetes mellitus. In 167 patients with CAC, circulating BDNF level was inversely associated with CML level and positively related to MGP level. In the receiver operating characteristic analysis for CAC, the areas under the curves for BDNF, MGP, and CML were 0.757, 0.777 and 0.653, respectively. In summary, plasma BDNF levels are associated with the Agatston score, and BDNF further predicts the occurrence of CAC.

No effect of endoperoxide 4 or thromboxane A2 receptor blockade on static mechanoreflex activation in rats with heart failure

NEW FINDINGS

What is the central question of this study? Do endoperoxide 4 and thromboxane A₂ receptors, which are receptors for cyclooxygenase products of arachidonic metabolism, on thin fibre muscle afferents play a role in the chronic mechanoreflex sensitization present in rats with heart failure with reduced ejection fraction (HF-rEF)? What is the main finding and its importance? The data do not support a role for endoperoxide 4 receptors or thromboxane A₂ receptors in the chronic mechanoreflex sensitization in HF-rEF rats.

ABSTRACT

We investigated the role of cyclooxygenase metabolite-associated endoperoxide 4 receptors (EP4-R) and thromboxane A₂ receptors (TxA₂ -R) on thin fibre muscle afferents in the chronic mechanoreflex sensitization in rats with myocardial infarction-induced heart failure with reduced ejection fraction (HF-rEF). We hypothesized that injection of either the EP4-R antagonist L-161,982 (1 µg) or the TxA₂ -R antagonist daltroban (80 µg) into the arterial supply of the hindlimb would reduce the increase in blood pressure and renal sympathetic nerve activity (RSNA) evoked in response to 30 s of static hindlimb skeletal muscle stretch (a model of isolated mechanoreflex activation) in decerebrate, unanaesthetized HF-rEF rats but not sham-operated control rats (SHAM). Ejection fraction was significantly reduced in HF-rEF (45 ± 11%) compared to SHAM (83 ± 6%; P < 0.01) rats. In SHAM and HF-rEF rats, we found that the EP4-R antagonist had no effect on the peak increase in mean arterial pressure (peak ΔMAP SHAM n = 6, pre: 15 ± 7, post: 15 ± 9, P = 0.99; HF-rEF n = 9, pre: 30 ± 11, post: 32 ± 15 mmHg, P = 0.84) or peak increase in RSNA (peak ΔRSNA SHAM pre: 33 ± 14, post: 47 ± 31%, P = 0.94; HF-rEF, pre: 109 ± 47, post: 139 ± 150%, P = 0.76) response to stretch. Similarly, in SHAM and HF-rEF rats, we found that the TxA₂ -R antagonist had no effect on the peak ΔMAP (SHAM n = 7, pre: 13 ± 7, post: 19 ± 14, P = 0.15; HF-rEF n = 14, pre: 24 ± 13, post: 21 ± 13 mmHg, P = 0.47) or peak ΔRSNA (SHAM pre: 52 ± 43, post: 57 ± 67%, P = 0.94; HF-rEF, pre: 108 ± 93, post: 88 ± 72%, P = 0.30) response to stretch. The data do not support a role for EP4-Rs or TxA₂ -Rs in the chronic mechanoreflex sensitization in HF-rEF.

Insulin potentiates the response to mechanical stimuli in small dorsal root ganglion neurons and thin fibre muscle afferents in vitro

KEY POINTS

Insulin is known to activate the sympathetic nervous system centrally. A mechanical stimulus to tissues activates the sympathetic nervous system via thin fibre afferents. Evidence suggests that insulin modulates putative mechanosensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, we report the novel finding that insulin augments the mechanical responsiveness of thin fibre afferents not only at dorsal root ganglion, but also at muscle tissue levels. Our data suggest that sympathoexcitation is mediated via the insulin-induced mechanical sensitization peripherally. The present study proposes a novel physiological role of insulin in the regulation of mechanical sensitivity in somatosensory thin fibre afferents.

ABSTRACT

Insulin activates the sympathetic nervous system, although the mechanism underlying insulin-induced sympathoexcitation remains to be determined. A mechanical stimulus to tissues such as skin and/or skeletal muscle, no matter whether the stimulation is noxious or not, activates the sympathetic nervous system via thin fibre afferents. Evidence suggests that insulin modulates putative mechanosensitive channels in the dorsal root ganglion (DRG) neurons of these afferents. Accordingly, we investigated whether insulin augments whole-cell current responses to mechanical stimuli in small DRG neurons of normal healthy mice. We performed whole-cell patch clamp recordings using cultured DRG neurons and observed mechanically-activated (MA) currents induced by mechanical stimuli applied to the cell surface. Local application of vehicle solution did not change MA currents or mechanical threshold in cultured DRG neurons. Insulin (500 mU mL^-1 ) significantly augmented the amplitude of MA currents (P < 0.05) and decreased the mechanical threshold (P < 0.05). Importantly, pretreatment with the insulin receptor antagonist, GSK1838705, significantly suppressed the insulin-induced potentiation of the mechanical response. We further examined the impact of insulin on thin fibre muscle afferent activity in response to mechanical stimuli in normal healthy rats in vitro. Using a muscle-nerve preparation, we recorded single group IV fibre activity to a ramp-shaped mechanical stimulation. Insulin significantly decreased mechanical threshold (P < 0.05), although it did not significantly increase the response magnitude to the mechanical stimulus. In conclusion, these data suggest that insulin augments the mechanical responsiveness of small DRG neurons and potentially sensitizes group IV afferents to mechanical stimuli at the muscle tissue level, possibly contributing to insulin-induced sympathoexcitation.

Investigation of the mechanisms of cyclooxygenase-mediated mechanoreflex sensitization in a rat model of simulated peripheral artery disease

Mechanical and metabolic stimuli within contracting skeletal muscles reflexly increase sympathetic nervous system activity and blood pressure. That reflex, termed the exercise pressor reflex, is exaggerated in patients with peripheral artery disease (PAD) and in a rat PAD model with a chronically ligated femoral artery. The cyclooxygenase (COX) pathway contributes to the exaggerated pressor response during rhythmic skeletal muscle contractions in patients with PAD, but the specific mechanism(s) of the COX-mediated exaggeration are not known. In decerebrate, unanesthetized rats with a chronically ligated femoral artery ("ligated" rats), we hypothesized that hindlimb arterial injection of the COX inhibitor indomethacin would reduce the pressor response during 1-Hz dynamic hindlimb skeletal muscle stretch; a model of the activation of the mechanical component of the exercise pressor reflex (i.e., the mechanoreflex). In ligated rats (n = 7), indomethacin reduced the pressor response during stretch (control: 30 ± 4; indomethacin: 12 ± 3 mmHg; P < 0.01), whereas there was no effect in rats with "freely perfused" femoral arteries (n = 6, control: 18 ± 5; indomethacin: 17 ± 5 mmHg; P = 0.87). In ligated rats (n = 4), systemic indomethacin injection had no effect on the pressor response during stretch. Femoral artery ligation had no effect on skeletal muscle COX protein expression or activity or concentration of the COX metabolite prostaglandin E₂. Conversely, femoral artery ligation increased expression of the COX metabolite receptors endoperoxide 4 and thromboxane A₂-R in dorsal root ganglia tissue. We conclude that, in ligated rats, the COX pathway sensitizes the peripheral endings of mechanoreflex afferents, which occurs principally as a result of increased expression of COX metabolite receptors.NEW & NOTEWORTHY We demonstrate that the mechanoreflex is sensitized by the cyclooxygenase (COX) pathway within hindlimb skeletal muscles in the rat chronic femoral artery ligation model of simulated peripheral artery disease (PAD). The mechanism of sensitization appears attributable to increased receptors for COX metabolites on sensory neurons and not increased concentration of COX metabolites. Our data may carry important clinical implications for patients with PAD who demonstrate exaggerated increases in blood pressure during exercise compared with healthy counterparts.