Vagal Feedback Research Articles

Vagal afferent fibers contribute to the anti-inflammatory reactions by vagus nerve stimulation in concanavalin A model of hepatitis in rats

BackgroundIncreasing number of studies provide evidence that the vagus nerve stimulation (VNS) dampens inflammation in peripheral visceral organs. However, the effects of afferent fibers of the vagus nerve (AFVN) on anti-inflammation have not been clearly defined. Here, we investigate whether AFVN are involved in VNS-mediated regulation of hepatic production of proinflammatory cytokines.MethodsAn animal model of hepatitis was generated by intraperitoneal (i.p.) injection of concanavalin A (ConA) into rats, and electrical stimulation was given to the hepatic branch of the vagus nerve. AFVN activity was regulated by administration of capsaicin (CAP) or AP-5/CNQX and the vagotomy at the hepatic branch of the vagus nerve (hVNX). mRNA and protein expression in target tissues was analyzed by RT-PCR, real-time PCR, western blotting and immunofluorescence staining. Hepatic immune cells were analyzed by flow cytometry.ResultsTNF-α, IL-1β, and IL-6 mRNAs and proteins that were induced by ConA in the liver macrophages were significantly reduced by the electrical stimulation of the hepatic branch of the vagus nerve (hVNS). Alanine transaminase (ALT) and aspartate transaminase (AST) levels in serum and the number of hepatic CD4+ and CD8+ T cells were increased by ConA injection and downregulated by hVNS. CAP treatment deteriorated transient receptor potential vanilloid 1 (TRPV1)-positive neurons and increased caspase-3 signals in nodose ganglion (NG) neurons. Concomitantly, CAP suppressed choline acetyltransferase (ChAT) expression that was induced by hVNS in DMV neurons of ConA-injected animals. Furthermore, hVNS-mediated downregulation of TNF-α, IL-1β, and IL-6 expression was hampered by CAP treatment and similarly regulated by hVNX and AP-5/CNQX inhibition of vagal feedback loop pathway in the brainstem. hVNS elevated the levels of α7 nicotinic acetylcholine receptors (α7 nAChR) and phospho-STAT3 (Tyr705; pY-STAT3) in the liver, and inhibition of AFVN activity by CAP, AP-5/CNQX and hVNX or the pharmacological blockade of hepatic α7 nAChR decreased STAT3 phosphorylation.ConclusionsOur data indicate that the activity of AFVN contributes to hepatic anti-inflammatory responses mediated by hVNS in ConA model of hepatitis in rats.

Sex-specific vagal and spinal modulation of swallow and its coordination with breathing.

Swallow-breathing coordination is influenced by changes in lung volume, which is modulated by feedback from both vagal and spinal sensory afferents. The purpose of this study was to manipulate feedback from these afferents, with and without a simultaneous mechanical challenge (chest compression), in order to assess the influence of each sensory pathway on swallow in rats. We hypothesized that manipulation of afferent feedback would shift the occurrence of swallow toward the inspiratory phase of breathing. Afferent feedback was perturbed by lidocaine nebulization, extra-thoracic vagotomy, or lidocaine administration to the pleural space in sodium pentobarbital anesthetized rats (N = 43). These different afferent perturbations were performed both in control conditions (no chest compression), and with chest compression. Manipulating pulmonary stretch receptor-mediated volume feedback in male animals decreased swallow occurrence. Female rats appear to rely more on spinal afferent feedback, as swallow occurrence shifted to late expiration with chest compression and vagotomy or lidocaine injections. Results suggest that sex-specific mechanisms modulate swallow-breathing coordination, and that vagal feedback is inhibitory to swallow-related muscles, while spinal feedback from pleural afferents has excitatory effects. This study supports the theory that a balance of vagal and spinal afferent feedback is necessary to maintain an optimal swallow pattern and swallow-breathing coordination.

Stimulation of the vagus nerve reduces learning in a go/no-go reinforcement learning task

When facing decisions to approach rewards or to avoid punishments, we often figuratively go with our gut, and the impact of metabolic states such as hunger on motivation are well documented. However, whether and how vagal feedback signals from the gut influence instrumental actions is unknown. Here, we investigated the effect of non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) vs. sham (randomized cross-over design) on approach and avoidance behavior using an established go/no-go reinforcement learning paradigm in 39 healthy human participants (23 female) after an overnight fast. First, mixed-effects logistic regression analysis of choice accuracy showed that taVNS acutely impaired decision-making, p = .041. Computational reinforcement learning models identified the cause of this as a reduction in the learning rate through taVNS (∆α = −0.092, pboot = .002), particularly after punishment (∆αPun = −0.081, pboot = .012 vs. ∆αRew =−0.031, pboot = .22). However, taVNS had no effect on go biases, Pavlovian response biases or response time. Hence, taVNS appeared to influence learning rather than action execution. These results highlight a novel role of vagal afferent input in modulating reinforcement learning by tuning the learning rate according to homeostatic needs.

Vagal Feedback Obscures the Effects of Systemic Opioids on Respiratory Rate in the preBötzinger Complex in Young Rabbits in vivo

IntroductionWe have previously shown in vagus‐intact young rabbits that respiratory rate depression from clinical dose‐rates of remifentanil was partially reversed by naloxone injection into the Parabrachial Nucleus (PBN) (1). On the contrary, naloxone injection into the preBötzinger Complex (preBötC) changed the ratio between inspiratory (TI) and expiratory (TE) duration but did not reverse respiratory rate depression (2). To determine whether the entrainment of the respiratory rhythm to the ventilator contributed to the observed lack of respiratory rate reversal, we repeated the study in vagotomized animals.MethodsThe study was approved by the local Animal Care Committee and conformed to NIH standards. Young New Zealand White rabbits (16–39d, 155–547g) were anesthetized, tracheotomized, ventilated, decerebrated and vagotomized. Phrenic nerve activity was recorded from the c5 rootlet, time averaged and used to calculate TI, TE and peak phrenic activity (PPA). Gridwise localized pressure microinjections of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA, 50 μM, 35 nl) utilizing a multibarrel glass pipette with a tip diameter of 20μm into the ventral respiratory column rostral to obex identified the preBötC where AMPA injection caused significant tachypnea. We infused remifentanil until respiratory rate was depressed to 50% of control. After reaching steady‐state, we microinjected naloxone (1mM, 350nl) into the bilateral preBötC and, once the effect had reached steady‐state, we injected naloxone (100mcM) intravenously. Data are provided as mean±SD. Comparison of drug effects within one group used 1‐way repeated‐measures ANOVA. Comparison of drug effects between the current study and an earlier study in non‐vagotomized animals used 2‐way ANOVA.ResultsIn seven rabbits, remifentanil infusion reduced respiratory rate from 29±14 breaths per minute (bpm) to 15±8 bpm, which was due to an increase in TI from 0.9±0.2s to 1.4±0.4s and an increase in TE from 1.6±1.1s to 3.4±1.8s (all p<0.001). Peak phrenic activity (PPA) was not significantly decreased (78±23%). Naloxone injection into the preBötC increased respiratory rate to 21±11bpm (p=0.013) due to a decrease in TI to 1.1±0.4s (p=0.028) and TE to 2.3±1.4s (p=0.009). PPA did not change. The difference from our previous study where we did not observe a reversal of respiratory rate depression (11±6 to 15±9bpm, n=12, p=0.198) was not significant. However, there was a difference between the change in TI:TE ratio, which increased more in vagus‐intact animals (0.5 to 1.1 vs. 0.5 to 0.5, p=0.04).ConclusionSystemic opioids affect respiratory phase duration through action in the preBötC in young rabbits, however, the effect is obscured when respiratory rhythm is entrained to the ventilator in vagus‐intact animals.Support or Funding InformationR01GM112960‐01A1 (Dr. Stucke) and VA merit grant I01BX000721 (Dr. Zuperku)

Abstract 084: Gut Dysbiosis Impairs Serotonergic Gut-Brain Axis and Increases Blood Pressure

Introduction: Fecal microbiota transfer (FMT) from hypertensive to normotensive rats elevates blood pressure (BP). As gut microbiota can affect synthesis of serotonin (5-HT), a major neurotransmitter predominantly synthesized in the gut, our objective was to investigate the effect of gut dysbiosis on gut-derived 5-HT in control of BP. Methods: Gut microbiota was depleted in normotensive adult male Wistar Kyoto (WKY) rats (N=24) using a cocktail of Vancomycin/Meropenem/Omeprazole at 50mg/kg/day for 5 days, followed by FMT from either adult male spontaneously hypertensive rats (SHR) (SHR-WKY, N=12) or age-, sex-matched WKY (WKY-WKY, N=12). At 8 weeks post-FMT, the following parameters were measured: BP by tailcuff and radiotelemetry; autonomic variables by spectral analysis of waveform BP; activity of cardioregulatory brain regions by manganese-enhanced MRI; fecal and proximal colonic 5-HT levels by HPLC; and levels of 5-HT 3 receptors in the proximal colon, nodose ganglia and the nucleus of the solitary tract (NTS) by qPCR. Lastly, BP and heart rate (HR) responses to intra-mesenteric i.v. injection of 5-HT 3 receptor agonist 1-phenylbigunide (35μg) were determined in anesthetized rats. Results: Compared with WKY-WKY, SHR-WKY showed a significant increase in systolic BP (Δ=34mmHg, P<0.05). This was associated with increased low frequency (by 13%, P<0.05) and decreased high frequency of BP (by 14%, P<0.05), indicative of sympathetic and parasympathetic variables respectively. A suppression of neural activity in the NTS (by 2 fold, P=0.06), and a reduction in 5HT 3 receptors in the NTS (by 1.5 fold, P<0.01), nodose ganglia (by 2 fold, P<0.05) and proximal colon (by 1.5 fold, P=0.09) in SHR-WKY resembled those in the SHR. Lower level of 5-HT in the proximal colon (by 4 fold, P<0.01), but not in the feces of SHR-WKY were observed. Similar to SHR, WKY-SHR showed significantly blunted drop in BP (by 5 fold, P<0.05) and HR (by 2 fold, P<0.05) in response to intra-mesenteric i.v. injection of 1-phenylbigunide, compared with WKY-WKY. Conclusions: FMT from SHR to WKY is associated with imbalanced autonomic variables and perturbations in serotonergic gut-brain axis that may contribute to hypertension partly via a dampened afferent vagal feedback to the NTS.

Increased markers of cardiac vagal activity in leucine-rich repeat kinase 2-associated Parkinson's disease.

Cardiac autonomic dysfunction manifests as reduced heart rate variability (HRV) in idiopathic Parkinson's disease (PD), but no significant reduction has been found in PD patients who carry the LRRK2 mutation. Novel HRV features have not been investigated in these individuals. We aimed to assess cardiac autonomic modulation through standard and novel approaches to HRV analysis in individuals who carry the LRRK2 G2019S mutation. Short-term electrocardiograms were recorded in 14 LRRK2-associated PD patients, 25 LRRK2-non-manifesting carriers, 32 related non-carriers, 20 idiopathic PD patients, and 27 healthy controls. HRV measures were compared using regression modeling, controlling for age, sex, mean heart rate, and disease duration. Discriminant analysis highlighted the feature combination that best distinguished LRRK2-associated PD from controls. Beat-to-beat and global HRV measures were significantly increased in LRRK2-associated PD patients compared with controls (e.g., deceleration capacity of heart rate: p = 0.006) and idiopathic PD patients (e.g., 8th standardized moment of the interbeat interval distribution: p = 0.0003), respectively. LRRK2-associated PD patients also showed significantly increased irregularity of heart rate dynamics, as quantified by Rényi entropy, when compared with controls (p = 0.002) and idiopathic PD patients (p = 0.0004). Ordinal pattern statistics permitted the identification of LRRK2-associated PD individuals with 93% sensitivity and 93% specificity. Consistent results were found in a subgroup of LRRK2-non-manifesting carriers when compared with controls. Increased beat-to-beat HRV in LRRK2 G2019S mutation carriers compared with controls and idiopathic PD patients may indicate augmented cardiac autonomic cholinergic activity, suggesting early impairment of central vagal feedback loops in LRRK2-associated PD.

Central Respiration and Mechanical Ventilation in the Gating of Swallow With Breathing.

Swallow-breathing coordination safeguards the lower airways from tracheal aspiration of bolus material as it moves through the pharynx into the esophagus. Impaired movements of the shared muscles or structures of the aerodigestive tract, or disruptions in the interaction of brainstem swallow and respiratory central pattern generators (CPGs) result in dysphagia. To maximize lower airway protection these CPGs integrate respiratory rhythm generation signals and vagal afferent feedback to synchronize swallow with breathing. Despite extensive study, the roles of central respiratory activity and vagal feedback from the lungs as key elements for effective swallow-breathing coordination remain unclear. The effect of altered timing of bronchopulmonary vagal afferent input on swallows triggered during electrical stimulation of the superior laryngeal nerves or by injection of water into the pharyngeal cavity was studied in decerebrate, paralyzed, and artificially ventilated cats. We observed two types of single swallows that produced distinct effects on central respiratory-rhythm across all conditions: post-inspiratory type swallows disrupted central-inspiratory activity without affecting expiration, whereas expiratory type swallows prolonged expiration without affecting central-inspiratory activity. Repetitive swallows observed during apnea reset the E2 phase of central respiration and produced facilitation of swallow motor output nerve burst durations. Moreover, swallow initiation was negatively modulated by vagal feedback and was reset by lung inflation. Collectively, these findings support a novel model of reciprocal inhibition between the swallow CPG and inspiratory or expiratory cells of the respiratory CPG where lung distension and phases of central respiratory activity represent a dual peripheral and central gating mechanism of swallow-breathing coordination.

New insights into a decerebrate feline model of swallow‐breathing coordination

In the medulla, the swallow and respiratory central pattern generators (CPGs) interact to coordinate swallow initiation and permit swallow execution during the expiratory phase of breathing. Disruption of swallow‐breathing coordination can result in dysphagia, which is a major cause of morbidity and mortality in many neuro‐muscular diseases, especially stroke. Despite extensive study, the precise mechanisms of swallow‐breathing coordination are not known. We tested the hypothesis that both central inspiratory activity and vagal feedback from the lungs are key elements for effective swallow‐breathing coordination. Swallows were triggered by electrical stimulation of the superior laryngeal nerves (SLN) or by injection of water into the pharyngeal cavity in decerebrate, paralyzed and artificially ventilated cats while motor activities were recorded from phrenic, hypoglossal, lumbar and vagal nerves. Mechanical ventilation was either triggered by central inspiratory activity or set independently from the phrenic activity to alter the timing of bronchopulmonary vagal afferent input and allow analysis of its influence on swallow‐breathing coordination. We observed two types of swallows that produced opposite effects on central respiratory‐rhythm resetting across all conditions: post‐Inspiratory type swallows disrupted central I activity without affecting expiration, whereas expiratory type swallows prolonged expiration without affecting central inspiratory activity. Repetitive swallows observed during apnea reset the E2 phase of central respiration and produced facilitation of swallow motor output. Moreover, swallow initiation was negatively modulated by vagal feedback and was reset by lung inflation. Collectively, these findings support a reciprocal inhibition between central inspiration and the swallow CPG and they are consistent with a role for pulmonary stretch receptors in the coordination between brainstem CPGs for breathing and swallowing. They may also prove relevant for healthcare management of dysphagia.Support or Funding InformationOT2OD023854‐01, HL109025This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Abraham Guz (1929-2014).

How do you describe a giant? As a person of abnormally great stature; above the usual size; or someone with much greater powers than the average person? Abraham Guz (‘Abe’) was not a tall man, but he was a scientific giant with an absolute desire to understand and find the scientific truth. This drive led him to make many original contributions in respiratory and cardiovascular physiology. Indeed, a simple literature search produces over 250 peer reviewed papers spanning some 50 years, the top 25 of which have been quoted over 2500 times. Moreover, most of his outstanding work was published in The Journal of Physiology, and we received it with gratitude because it was always insightful and ahead of its time. Many will argue that counting citations is a poor measure of scientific endeavour and I suspect Abe himself would certainly agree. However, this approach did provide us with a place to start when faced with the task of bringing together Abe's huge and varied scientific contributions.

Hering‐Breuer evoked regulation of respiratory rhythm variability in rats (686.25)

Hering‐Breuer‐related afferent activity affects respiratory patterning in disparate ways: constant mechanical ventilation entrains the respiratory rhythm; yet removal of vagal feedback reduces respiratory rhythm variability. We hypothesize that these effects are mechanistically linked and test this hypothesis by closing the Hering Breuer Reflex feedback loop via rhythmic vagal nerve stimulation in situ before and after suppressing activity in the nucleus tractus solitarii (nTS) or Kölliker‐Fuse nuclei (KFn) by microinjection of muscimol, a GABAAR agonist. We measured feedback‐evoked entrainment of respiration and the variability of the rhythm in the presence or absence of periodic vagal feedback. Rhythmic vagal feedback evoked periods of entrainment interposed by phase slips. Analysis of respiratory rhythm variability revealed that vagal feedback amplified intrinsic respiratory rhythm variability via the transitions from a phase slip to stable entrainment. Suppressing nTS activity eliminated both entrainment and variability amplification. Suppressing KFn activity increased entrainment strength, reduced phase slip frequency and blunted variability amplification. We conclude that 1) Hering‐Breuer feedback inputs are sufficient to amplify respiratory variability via a noisy synchronization mechanism and 2) both entrainment and variability depend on KFn and nTS activity.Grant Funding Source: Supported by NS069220 and T32 HL007913

The essential role of peripheral respiratory chemoreceptor inputs in maintaining breathing revealed when CO2 stimulation of central chemoreceptors is diminished

Central sleep apnoea is a condition characterized by oscillations between apnoea and hyperpnoea during sleep. Studies in sleeping dogs suggest that withdrawal of peripheral chemoreceptor (carotid body) activation following transient ventilatory overshoots plays an essential role in causing apnoea, raising the possibility that sustaining carotid body activity during ventilatory overshoots may prevent apnoea. To test whether sustained peripheral chemoreceptor activation is sufficient to drive breathing, even in the absence of central chemoreceptor stimulation and vagal feedback, we used a vagotomized, decerebrate dual-perfused in situ rat preparation in which the central and peripheral chemoreceptors are independently and artificially perfused with gas-equilibrated medium. At varying levels of carotid body stimulation (CB PO2/PCO2: 40/60, 100/40, 200/15, 500/15 Torr), we decreased the brainstem perfusate PCO2 in 5 Torr steps while recording phrenic nerve activity to determine the central apnoeic thresholds. The central apnoeic thresholds decreased with increased carotid body stimulation. When the carotid bodies were strongly stimulated (CB 40/60), the apnoeic threshold was 3.6 ± 1.4 Torr PCO2 (mean ± SEM, n = 7). Stimulating carotid body afferent activity with either hypercapnia (60 Torr PCO2) or the neuropeptide pituitary adenylate cyclase-activating peptide restored phrenic activity during central apnoea. We conclude that peripheral stimulation shifts the central apnoeic threshold to very hypocapnic levels that would likely increase the CO2 reserve and have a protective effect on breathing. These data demonstrate that peripheral respiratory chemoreceptors are sufficient to stave off central apnoeas when the brainstem is perfused with low to no CO2.

Changes in central respiratory patterns and activity following acute lung injury in the in situ rat preparation

We have quantified changes in the variability of the respiratory patterns after acute lung injury (ALI) and reported increased respiratory rate and increased measures of linear and nonlinear variability by using Mutual Information (MI) and Nonlinear Complexity Index (NLCI). These changes are associated with lung and brainstem inflammatory responses. We hypothesize that changes in respiratory pattern after ALI are, in part, mediated centrally. To test our hypothesis we administered either bleomycin (0.5U in 50 μL of saline, n=9) or saline (50 μL, n=9) intratracheally to rats (Sprague‐Dawley, Harlan, P21–22). After 7d, central respiratory patterns were measured by recording the left phrenic, hypoglossal and vagal nerve activities using a perfused in situ preparation. Comparing in situ preparations from bleomycin to saline‐treated rats, bleomycin‐treated rats exhibited: 1) higher burst frequency (26 ± 4 vs. 20 ± 4 bpm, p=0.01), 2) lower coefficient of variation (p=0.01) integrated vagal nerve activity had 3) greater MI (p=0.01) and NLCI (p=0.01) and 4) phrenic and vagal nerve activities displayed increased early inspiratory activity. Fundamental changes in the respiratory patterns evoked by ALI persisted in the absence of vagal feedback from lung or blood chemistry indicating that central mechanisms are involved in generating the respiratory pattern after 7d of ALI. Supported by HL 087377, VA Research Service

Hypercapnia attenuates inspiratory amplitude and expiratory time responsiveness to hypoxia in vagotomized and vagal-intact rats

A negative influence of central chemosensitivity on peripheral chemoreflex response has been demonstrated recently in a decerebrate-vagotomized rat preparation in situ with separate carotid body and brainstem perfusions. Here, we report similar negative influences of hypercapnia on the hypoxic respiratory response in anesthetized, spontaneously breathing rats before and after vagotomy and anesthetized, artificially ventilated rats after vagotomy. Baseline breathing patterns and responsiveness to hypercapnia and hypoxia varied widely between the three respiratory modes. Despite this, the responses in inspiratory amplitude and expiratory duration (and hence respiratory frequency and neural ventilation) to hypoxia varied inversely with the background CO2 level in all three groups. Results demonstrate a hypoadditive hypercapnic-hypoxic interaction in vivo that resembles the hypoadditive central-peripheral chemoreceptor interaction in situ for these respiratory variables in the rat, regardless of differences in vagal feedback, body temperature and ventilation method. These observations stand in contrast to previous reports of hyperadditive peripheral-central chemoreceptor interaction.

POSTER PRESENTATIONS

POSTER PRESENTATIONS

Impact of the vagal feedback on cardiorespiratory coupling in anesthetized rats

Cardiorespiratory coupling can be significantly influenced by both pontine and vagal modulation of medullary motor and premotor areas. We investigated influences of the pontine intertrigeminal region (ITR) and peripheral vagal pathways on the coupling between systolic blood pressure (SBP) and respiration in 9 anesthetized rats. Glutamate injection into the ITR perturbed both respiration and SBP and decreased SBP-respiratory coherence (0.95±0.01 vs 0.89±0.02; (p=0.01). Intravenous infusion of serotonin (5-HT) produced apnea and hypertension and also decreased SBP-respiratory coherence (0.95±0.01 vs 0.72±0.06; p=0.04). Bilateral vagotomy eliminated the cardiorespiratory coherence perturbations induced by central (glutamate injection into the ITR: 0.89±0.03 vs 0.86±0.03; p=0.63) and peripheral (5-HT infusion: 0.89±0.03 vs 0.88±0.02; p=0.98) pharmacologic manipulations. Glutamate stimulation of the ITR postvagotomy increased the relative spectral power density of SBP in the respiratory frequency range (0.25±0.08 vs 0.55±0.06; p=0.01). The data suggest that SBP-respiratory coupling is largely mediated within the central nervous system, with vagal systems acting in a way that disrupts coherence during transient cardiorespiratory disturbances. Although decreased cardiorespiratory coherence may increase cardiac work during perturbations, this may be physiologically advantageous in restoring homeostatic equilibrium of respiration and blood pressure.

Identification of a novel form of noradrenergic-dependent respiratory motor plasticity triggered by vagal feedback.

The respiratory control system is not just reflexive, it is smart, it learns, and, in fact, it has a memory. The respiratory system listens to and carefully remembers how previous stimuli affect breathing. Respiratory memory is laid down by adjusting synaptic strength between respiratory neurons. For example, repeated hypoxic bouts trigger a form of respiratory memory that functions to strengthen the ability of respiratory motoneurons to trigger contraction of breathing muscles. This type of respiratory plasticity is known as long-term facilitation (LTF). Although chemical feedback, such as hypoxia, initiates LTF, it is unknown whether natural modulation of mechanical feedback (from vagal inputs) also causes motor plasticity. Here, we used reverse microdialysis, electrophysiology, neuropharmacology, and histology to determine whether episodic modulation of vagally mediated mechanical feedback is able to induce respiratory LTF in anesthetized adult rats. We show that repeated obstructive apneas disrupt vagal feedback and trigger LTF of hypoglossal motoneuron activity and genioglossus muscle tone. This same stimulus does not cause LTF of diaphragm activity. Hypoxic episodes do not cause apnea-induced LTF; instead, LTF is triggered by modulation of vagal feedback. Unlike hypoxia-induced respiratory plasticity, vagus-induced LTF does not require 5-HT(2) receptors but instead relies on activation of α1-adrenergic receptors on hypoglossal motoneurons. In summary, we identify a novel form of hypoxia- and 5-HT-independent respiratory motor plasticity that is triggered by physiological modulation of vagal feedback and is mediated by α1-adrenergic receptor activation on (or near) hypoglossal motoneurons.

Disrupting vagal feedback affects birdsong motor control.

Coordination of different motor systems for sound production involves the use of feedback mechanisms. Song production in oscines is a well-established animal model for studying learned vocal behavior. Whereas the online use of auditory feedback has been studied in the songbird model, very little is known about the role of other feedback mechanisms. Auditory feedback is required for the maintenance of stereotyped adult song. In addition, the use of somatosensory feedback to maintain pressure during song has been demonstrated with experimentally induced fluctuations in air sac pressure. Feedback information mediating this response is thought to be routed to the central nervous system via afferent fibers of the vagus nerve. Here, we tested the effects of unilateral vagotomy on the peripheral motor patterns of song production and the acoustic features. Unilateral vagotomy caused a variety of disruptions and alterations to the respiratory pattern of song, some of which affected the acoustic structure of vocalizations. These changes were most pronounced a few days after nerve resection and varied between individuals. In the most extreme cases, the motor gestures of respiration were so severely disrupted that individual song syllables or the song motif were atypically terminated. Acoustic changes also suggest altered use of the two sound generators and upper vocal tract filtering, indicating that the disruption of vagal feedback caused changes to the motor program of all motor systems involved in song production and modification. This evidence for the use of vagal feedback by the song system with disruption of song during the first days after nerve cut provides a contrast to the longer-term effects of auditory feedback disruption. It suggests a significant role for somatosensory feedback that differs from that of auditory feedback.

A Genetic Deficiency in Presynaptic Choline Uptake Leads to Resting Tachycardia and Diminished Vagal Reserve

The parasympathetic branch of the autonomic nervous system projects to the heart via the vagus nerve. Vagal efferents into the heart produce bradycardia through release of acetylcholine (ACh). The presynaptic synthesis of ACh is believed to be dependent on the uptake of extracellular choline via the hemicolinium‐3‐sensitive choline transporter (CHT). Attenuated parasympathetic tone has been shown to be a high risk indicator for increased mortality in patients with cardiac disease. Our studies have examined the impact of a genetic loss of CHT on cardiac function in mice. Cardiac synaptosome preparations made from CHT heterozygous (Het) mice have significantly reduced cardiac tissue levels of ACh as well as elevated choline levels, along with elevated urinary NE levels compared to wildtype mice. Conscious CHT Het mice monitored via telemetry exhibit significantly higher resting heart rates, but display similar mean blood pressure compared to wildtype mice. Functional cardiac studies using echocardiography show that CHT Het mice also exhibit reduced fractional shortening and increased left ventricular internal dimensions, but normal cardiac output. Consistent with reductions in parasympathetic tone, CHT Het mice exhibit an attenuated baroreceptor‐mediated reflex. During recovery from treadmill exercise, CHT Het mice show reduced heart rate recovery, indicative of reduced vagal feedback for HR control.

Reply from M. Dutschmann and G. M. Stettner

We appreciate the interest and comments regarding our recently published article in The Journal of Physiology (Stettner et al. 2007a). The manuscript described disturbances in vagal-pontine interactions in the control of the postinspiratory activity as a potential cause of breathing disturbances in MECP2-deficient mice, an animal model for Rett syndrome (Guy et al. 2001). The Letter by Poon & Song addresses an aspect of the publication that is illustrated in Fig. 7, which contrasts the response to repetitive electrical stimulation of the vagal nerve of wild-type (C57BL/6J) and MECP2-deficient mice. In both groups of mice, the intensity of electrical stimulation of the central end of the vagus nerve was determined by the same procedure and the stimuli elicited a postinspiratory reflex apnoea (Hering–Breuer reflex). In particular, the intensity of vagal stimulation in our study was 1–2 × above threshold (low intensity stimulation), which was similar to that used to study plasticity following repetitive vagal stimulation (Siniaia et al. 2000). The data showed that the apnoea after cessation of the electrical stimulus was significantly longer in MECP2-deficient mice than in C57BL/6J mice. Furthermore, the reflex apnoea progressively decreased with repeated vagal stimuli in the wild-type but not MECP2-deficient mice. We interpreted our data as Mecp2−/y knockout mice lacked ‘desensitization’ to vagal stimulation. This terminology was criticised in the letter to the Editor. We want to emphasize that disturbed processing of vagal afferent input remains a part of the phenotype of the MECP2-deficient mice, whether they lack desensitization or ‘classic habituation’. We agree that a full description of the underlying plasticity phenomenon would require analysis of the poststimulus rebound activity. At this point, we cannot confirm the correspondent's insights regarding Fig. 7. Differences in the stimulus intensities between our and other studies relate to different experimental tools, for instance the central vagal nerve was stimulated via suction electrodes in our experiment. This may clarify confusion regarding a potential secondary or non-specific sensitization of the vagally evoked reflex response due to activation of rapidly adapting pulmonary stretch receptor afferent fibres or cardiopulmonary C-fibres. Even though group data for the poststimulus rebound were not analysed, Fig. 7 shows that expiration lengthened after the stimulus. Poststimulus rebound activity as described for rats (Siniaia et al. 2000), however, did not occur in C57BL/6J and MECP2-deficient mice. The C57BL/6J mouse strain itself may have an imbalance in ponto-medullary circuits. We have observed spontaneous postinspiratory apnoeas in the perfused brainstem preparation in situ (Stettner et al. 2007b) and others report severe periodic breathing patterns during the recovery from a hypoxic challenge in vivo (Han et al. 2002; Gonsenhauser et al. 2004). We speculate that the pontine Kölliker–Fuse nucleus (KF) may be a part of this imbalance, not only because neurones in the KF express postinspiratory activity in many species (Dick et al. 1994; Ezure, 2006; Dutschmann & Herbert, 2006), but also because KF neuronal activity determines the timing of postinspiratory activities of laryngeal muscles in the absence of vagal afferent feedback (Dutschmann & Herbert, 2006). If pontine mechanisms in the control of breathing are similar in rats and mice, then spontaneous postinspiratory apnoeas in C57BL/6J mice indicate a genetic imbalance in pontine mechanisms controlling postinspiratory activity in C57BL/6J mice that may be accentuated under the condition of MECP2 deficiency. This may cause the unusual poststimulus rebound illustrated in Fig. 7. Interestingly, another Mecp2−/y knockout model (Chen et al. 2001) has a different genetic background. If we are correct about the intrinsic imbalance prevalent in C57BL/6J mice, then these other MECP2-deficient mice should show distinct differences in their breathing phenotype. In our opinion, further experiments are required in MECP2-deficient mice with different genetic backgrounds to fully understand disturbed synaptic plasticity associated with the processing of afferent vagal feedback.

Ventilation induced apnea and its effect on dorsal brainstem inspiratory neurones in the rat

The purpose of this study was to examine the effect of mechanical ventilation (MV) on inherent breathing and on dorsal brainstem nucleus tractus solitarius (NTS) respiratory cell function. In pentobarbitone-anaesthetised rats, application of MV at combined high frequencies and volumes (representing threshold levels) produced apnea. The apnea persisted as long as MV was maintained at or above the threshold frequency and volume. Following removal of MV, inherent breathing did not resume immediately, with the diaphragm exhibiting post-mechanical ventilation apnea. The fall in arterial P(CO2) (Pa(CO2)) levels evoked by MV-engendered hyperventilation was shown not to be the trigger for initiation of apnea. MV-induced apnea was immediately reversed by bilateral vagotomy. Further, MV-induced apnea could not be evoked in bilaterally vagotomized animals suggesting that vagal feedback is the critical pathway for its initiation. NTS inspiratory neurones were inhibited during both MV-induced apnea and post-mechanical ventilation apnea, implying the involvement of central neural mechanisms in mediating this effect.