Reflexes Evoked from Tracheobronchial Tree and Lungs

Identifying vagal bronchopulmonary afferents mediating cough response to inhaled sulfur dioxide in mice

Sulfur dioxide (SO2), a common environmental and industrial air pollutant, possesses a potent effect in eliciting cough reflex, but the primary type of airway sensory receptors involved in its tussive action has not been clearly identified. This study was carried out to determine the relative roles of three major types of vagal bronchopulmonary afferents [slowly adapting receptors (SARs), rapidly adapting receptors (RARs), and C-fibers] in regulating the cough response to inhaled SO2. Our results showed that inhalation of SO2 (300 or 600 ppm for 8 min) evoked an abrupt and intense stimulatory effect on bronchopulmonary C-fibers, which continued for the entire duration of inhalation challenge and returned toward the baseline in 1-2 min after resuming room air-breathing in anesthetized and mechanically ventilated mice. In stark contrast, the same SO2 inhalation challenge generated a distinct and consistent inhibitory effect on both SARs and phasic RARs; their phasic discharges synchronized with respiratory cycles during the baseline (breathing room air) began to decline progressively within 1-3 min after the onset of SO2 inhalation, ceased completely before termination of the 8-min inhalation challenge, and then slowly returned toward the baseline after >40 min. In a parallel study in awake mice, inhalation of SO2 at the same concentration and duration as that in the nerve recording experiments evoked cough responses in a pattern and time course similar to that observed in the C-fiber responses. Based on these results, we concluded that stimulation of vagal bronchopulmonary C-fibers is primarily responsible for triggering the cough response to inhaled SO2.NEW & NOTEWORTHY This study demonstrated that inhalation of a high concentration of sulfur dioxide, an irritant gas and common air pollutant, completely and reversibly inhibited the neural activities of both slowly adapting receptor and rapidly adapting receptor, two major types of mechanoreceptors in the lungs with their activities conducted by myelinated fibers. Furthermore, the results of this study suggested that stimulation of vagal bronchopulmonary C-fibers is primarily responsible for triggering the cough reflex responses to inhaled sulfur dioxide.

Inhibitory effect of sulfur dioxide inhalation on Hering-Breuer inflation reflex in mice: role of voltage-gated potassium channels

Slowly adapting receptors (SARs), vagal mechanosensitive receptors located in the lung, play an important role in regulating the breathing pattern and Hering-Breuer inflation reflex (HBIR). Inhalation of high concentration of sulfur dioxide (SO2), a common environmental and occupational air pollutant, has been shown to selectively block the SAR activity in rabbits, but the mechanism underlying this inhibitory effect remained a mystery. We carried out this study to determine if inhalation of SO2 can inhibit the HBIR and change the eupneic breathing pattern, and to investigate further a possible involvement of voltage-gated K+ channels in the inhibitory effect of SO2 on these vagal reflex-mediated responses. Our results showed 1) inhalation of SO2 (600 ppm; 8 min) consistently abolished both the phasic activity of SARs and their response to lung inflation in anesthetized, artificially ventilated mice, 2) inhalation of SO2 generated a distinct inhibitory effect on the HBIR and induced slow deep breathing in anesthetized, spontaneously breathing mice, and these effects were reversible and reproducible in the same animals, 3) This inhibitory effect of SO2 was blocked by pretreatment with 4-aminopyridine (4-AP), a nonselective blocker of voltage-gated K+ channel, and unaffected by pretreatment with its vehicle. In conclusion, this study suggests that this inhibitory effect on the baseline breathing pattern and the HBIR response was primarily mediated through the SO2-induced activation of voltage-gated K+ channels located in the vagal bronchopulmonary SAR neurons.NEW & NOTEWORTHY This study demonstrated that inhaled sulfur dioxide completely and reversibly abolished the activity of vagal bronchopulmonary slowly adapting receptors, significantly inhibited the apneic response to lung inflation, and induced slow deep breathing in anesthetized mice. More importantly, our results further suggested that this inhibitory effect was mediated through an action of sulfur dioxide and its derivatives on the voltage-gated potassium channels expressed in the slowly adapting receptor sensory neurons innervating the lung.

The gut-brain axis: Identifying new therapeutic approaches for type 2 diabetes, obesity, and related disorders

BACKGROUND

The gut-brain axis, which mediates bidirectional communication between the gastrointestinal system and central nervous system (CNS), plays a fundamental role in multiple areas of physiology including regulating appetite, metabolism, and gastrointestinal function. The biology of the gut-brain axis is central to the efficacy of glucagon-like peptide-1 (GLP-1)-based therapies, which are now leading treatments for type 2 diabetes (T2DM) and obesity. This success and research to suggest a much broader role of gut-brain circuits in physiology and disease has led to increasing interest in targeting such circuits to discover new therapeutics. However, our current knowledge of this biology is limited, largely because the scientific tools have not been available to enable a detailed mechanistic understanding of gut-brain communication.

SCOPE OF REVIEW

In this review, we provide an overview of the current understanding of how sensory information from the gastrointestinal system is communicated to the central nervous system, with an emphasis on circuits involved in regulating feeding and metabolism. We then describe how recent technologies are enabling a better understanding of this system at a molecular level and how this information is leading to novel insights into gut-brain communication. We also discuss current therapeutic approaches that leverage the gut-brain axis to treat diabetes, obesity, and related disorders and describe potential novel approaches that have been enabled by recent advances in the field.

MAJOR CONCLUSIONS

The gut-brain axis is intimately involved in regulating glucose homeostasis and appetite, and this system plays a key role in mediating the efficacy of therapeutics that have had a major impact on treating T2DM and obesity. Research into the gut-brain axis has historically largely focused on studying individual components in this system, but new technologies are now enabling a better understanding of how signals from these components are orchestrated to regulate metabolism. While this work reveals a complexity of signaling even greater than previously appreciated, new insights are already being leveraged to explore fundamentally new approaches to treating metabolic diseases.

Hypertonic saline stimulates vagal afferents that respond to lung deflation

In our present studies, we seek to determine whether increased osmolarity stimulates deflation-activated receptors (DARs). In anesthetized, open-chest, and mechanically ventilated rabbits, we recorded single-unit activities from typical slowly adapting receptors (SARs; responding only to lung inflation) and DAR-containing SARs (DAR-SARs; responding to both lung inflation and deflation) and identified their receptive fields in the lung. We examined responses of these two groups of pulmonary sensory units to direct injection of hypertonic saline (8.1% sodium chloride; 9-fold in tonicity) into the receptive fields. Hypertonic saline decreased the activity in most SAR units from 40.3 ± 5.4 to 34.8 ± 4.7 imp/s (P < 0.05, n = 12). In contrast, it increased the activity in DAR-SAR units quickly and significantly from 15.9 ± 2.2 to 43.4 ± 10.0 imp/s (P < 0.01, n = 10). Many units initially had increased activity, mainly in the deflation phase. DAR-SAR activities largely returned to the control level 30 s after injection. Since hypertonic saline stimulated DAR-SAR units but not SAR units, we conclude that hypertonic saline activates DARs.

Airway Mechanosensor Behavior during Application of Positive End-Expiratory Pressure

<b><i>Background:</i></b> Positive end-expiratory pressure (PEEP) is commonly used in clinical settings. It is expected to affect the input from slowly adapting pulmonary stretch receptors (SARs), leading to altered cardiopulmonary functions. However, we know little about how SARs behave during PEEP application. <b><i>Objectives:</i></b> Our study aimed to characterize the behavior of SARs during PEEP application. <b><i>Methods:</i></b> We recorded single-unit activities from 18 SARs in the cervical vagus nerve and examined their response to an increase of PEEP from 3 to 10 cm H₂O for 20 min in anesthetized, open-chest and mechanically ventilated rabbits. <b><i>Results:</i></b> The mean activity of the units increased immediately from 35.7 to 80.5 impulses per second at the fifth breath after increasing PEEP (n = 14, p < 0.001) and then gradually returned to 56.5 impulses per second at the end of 20 min of PEEP application (p < 0.001). In the meantime, peak airway pressure increased from 9.3 to 32.7 cm H₂O, and then gradually returned to 29.4 cm H₂O (n = 18; p < 0.05) after 20 min. The remaining four units ceased firing at 34.7 s (range 10-56 s) after their initial increased activity upon 10 cm H₂O PEEP application. The unit activity resumed as the PEEP was returned to 3 cm H₂O. <b><i>Conclusions:</i></b> High PEEP stimulates SARs and SAR activity gradually returns towards the baseline via multiple mechanisms including receptor deactivation, neural habituation and mechanical adaptation. Understanding of the sensory inputs during PEEP application will assist in developing better strategies of mechanical ventilation.

Effect of upper torso inclination in Fowler's position on autonomic cardiovascular regulation

The present study investigates autonomic cardiovascular regulation during postural changes while in Fowler's position. Respiratory sinus arrhythmia (RSA) and sequence baroreflex sensitivity (sBRS) were measured in 12 healthy individuals in three positions (Experiment 1). We also measured RSA, sBRS, tidal volume (TV), lung volume spectrum (LV spectrum), and transfer gain and phase between lung volume and RR interval (RSA-TF, RSATF-phase) in 11 healthy individuals in two positions (Experiment 2). All participants maintained respiratory frequency at 15 breaths/min. The three positions in Experiment 1 were 30°, 45°, and 60° of upper torso inclination with a lower torso inclination of 30° throughout all evaluations. The two positions in Experiment 2 were 30° and 60° of upper torso backrest inclination with a lower torso inclination of 30° throughout all evaluations. The results of Experiment 1 showed significantly higher RSA and sBRS at 60° and 45° than at 30°, whereas RR interval (RRI), systolic blood pressure (SBP), and diastolic blood pressure (DBP) did not differ significantly under any condition. The results of Experiment 2 showed that RSA, RSA-TF, sBRS, TV, and LV spectrum were significantly higher at 60° than at 30°, and that RRI, SBP, DBP, and the RSATF phase did not significantly differ under any condition. These findings suggested that slight flexion of the upper torso in Fowler's position activates respiratory function and increases the contribution of vagal nerve activity to the cardiovascular system in young participants under conditions of a fixed respiratory rate.

Nodose ganglia-modulatory effects on respiration

The key role of the vagus nerves in the reflex control of breathing is generally accepted. Cardiopulmonary vagal receptors and their afferent connection with the medullary respiratory centers secures the proper regulatory feedback. Section of the vagi at the midcervical level interrupts primary vagal reflexes and those due to activation of lung afferents by neuroactive substances. In this context the present review focuses on the reflex contribution of the inferior (nodose) vagal ganglia to the respiratory pattern, considering that this structure contains perikarya of vagal afferent neurons which house neurotransmitters, neuropeptides and neurochemical substances. In experimental animals with removed sensory input from the lungs (midcervical vagotomy) the following evidence was reported. Transient respiratory suppression in the form of apnoea, occurring after systemic injection of serotonin, adenosine triphosphate and anandamide (N-arachidonoyl-ethanolamine-endogenous cannabinoid neurotransmitter), which was abrogated by nodose ganglionectomy. Preserved nodose-NTS connection conditioned respiratory depression affecting the timing component of the breathing pattern evoked by N-6-cyclopentyl-adenosine (CPA) and inhibition of both respiratory constituents induced by NPY. Stimulatory effect of NPY13-36 on tidal volume required nodosal connection. The cardiovascular effects of majority of the tested substances occurred beyond the nodose ganglia (with exclusion of serotonin and anandamide).

Inhaled furosemide does not alleviate respiratory effort during flow-limited exercise in healthy subjects

Expiratory muscle loading results in increased perception of respiratory effort; this response is mediated by non-vagal reflexes originating in the chest wall. Furosemide, due to its vagal effect, might not affect the perception of respiratory effort during expiratory flow-limited incremental exercise. In this study, we compared in nine healthy subjects the following determinants of exercise performance such as respiratory effort (Borg), workload (W'), ventilation (V'E), tidal volume (VT), respiratory frequency (f), and mean inspiratory flow (VT/TI), an index of central respiratory drive, during either standard incremental cycling exercise, or expiratory flow-limited incremental exercise. In addition we examined the effect of inhaled placebo, furosemide (40 or 80 mg) on the perception of respiratory effort following standard incremental cycling exercise and expiratory flow-limited incremental exercise. Compared with standard incremental cycling exercise, expiratory flow-limited incremental exercise increased the Borg score and VT/Ti, and decreased W',V'E ,VT, and f in all subjects at iso-workload. Neither placebo nor furosemide modified peak ventilatory variables, slopes, or intercepts of the relationships of the Borg score with W', V'E, VT/TI and VT during expiratory flow-limited incremental exercise. We conclude that (a) compared with standard incremental exercise, expiratory flow limited exercise increases central respiratory drive and perception of respiratory effort, and (b) furosemide does not affect the sensation of respiratory effort under the present conditions of increased drive to the respiratory muscles.

GABA, in some cases together with glycine, is used as the inhibitory transmitter by pump cells in the Hering-Breuer reflex pathway of the rat

The Hering-Breuer reflex is one of the fundamental respiratory reflexes and is mediated by second-order relay neurons of the slowly adapting lung stretch receptors. These neurons, which are called pump cells, are located in the nucleus tractus solitarii and include a population of inhibitory neurons. We aimed to determine which transmitter, GABA or glycine, the inhibitory pump cells use. In addition, we examined whether or not second-order relay neurons of the rapidly-adapting lung stretch receptors (RAR-cells), whose excitatory or inhibitory nature is not known, use these inhibitory neurotransmitters. In Nembutal-anesthetized, neuromuscularly blocked and artificially ventilated rats, we labeled pump cells (n=33) and RAR-cells (n=26) with Neurobiotin and processed the tissues for detection of mRNA encoding either glutamic acid decarboxylase isoform 67 (GAD67) or glycine transporter 2 (GLYT2) using in situ hybridization. The pump cells were located in the interstitial nucleus and its vicinity and the RAR-cells in the commissural subnucleus. The majority (64%) of the pump cells examined for GAD67 mRNA and many (26%) of the pump cells examined for GLYT2 mRNA expressed respective mRNAs. Of the eight pump cells in which both mRNAs were double-detected, three expressed both mRNAs and one expressed GAD67 mRNA but not GLYT2 mRNA, the other four expressing neither mRNAs. On the other hand, RAR-cells expressed neither GAD67 mRNA nor GLYT2 mRNA. The results suggest that the inhibitory pump cells are basically GABAergic and some of them may corelease GABA and glycine, and that RAR-cells are neither GABAergic nor glycinergic. These findings expand our understanding of the networks of lung receptor-mediated reflexes including the Hering-Breuer reflex.

Modulation of breathing by phasic pulmonary stretch receptor feedback in an amphibian, Bufo marinus

This study examined the role of phasic pulmonary stretch receptor (PSR) feedback in ventilatory control, breath clustering and breath timing in decerebrate, paralysed and artificially-ventilated cane toads (Bufo marinus) under conditions designed to minimise tonic PSR feedback. Fictive breathing was recorded as trigeminal motor output to the buccal musculature. Artificial tidal ventilation, with hypercarbic gas mixtures, was either continuous or activated by the fictive breaths and was manipulated to provide differing amounts/patterns of phasic PSR feedback. The results demonstrate that increased amounts of phasic PSR feedback increase overall breathing frequency. Within multi-breath episodes there was an increase in the instantaneous breathing frequency during the later stages of the episode. The temporal relationship between a fictive breath and lung inflation influenced the duration of the pause between fictive breaths. The data indicate that phasic PSR feedback stimulates breathing by enhancing the occurrence of breathing episodes in this species but does not appear to modify the instantaneous breathing frequency during an episode.

Effects of gadolinium chloride on slowly adapting and rapidly adapting receptors of the rabbit lung

Effects of gadolinium chloride, an inhibitor of stretch-activated channels, on the responses of slowly adapting receptors (SARs) and rapidly adapting receptors (RARs) to hyperinflation were investigated. The increase in activity of RARs resulting from sustained elevations of left atrial pressure (LAP) was also assessed with gadolinium chloride application. Action potentials (AP) of SARs and RARs during hyperinflation were recorded from the vagus nerve of anesthetized New Zealand White rabbits before and after application of gadolinium chloride (20mM) directly on the receptor area of the nerve endings. There was a significant reduction of activity in SARs (n = 9) and RARs (n = 7) after application of gadolinium chloride. Activity of RARs (n = 6) increased when the LAP was elevated by 5 and 10 mmHg. This effect was abolished after gadolinium chloride was applied to receptor endings and the activity was restored when gadolinium chloride was removed. This suggests that stretch-activated channels play a role in SARs and RARs activity.

The effect of sulphur dioxide exposure on indices of heart rate variability in normal and asthmatic adults

Sulphur dioxide (SO2) is an important air pollutant and causes bronchoconstriction in normal and asthmatic adults. This paper has explored the autonomic consequences of SO2 exposure using the spectral analysis of heart rate variability. Electrocardiogram recordings were made in 12 normal and 12 asthmatic adults undergoing pollutant exposures. Exposures were of a 1 h duration, double blind, in random order, > or = 2 weeks apart and included air and 200 parts per billion SO2. Spectral analysis of R-R intervals was performed. SO2 exposure was associated with an increase in total power (TP) and high (HF) and low frequency (LF) power in the normal subjects, and a reduction in these indices in the subjects with asthma. The difference in TP with SO2 exposure compared to air was +1730 ms2 in the normal group and -1021 ms2 asthmatic group (p<0.003). For HF the respective values were +964 ms2 and -539 ms2 (p=0.02) and for LF, +43 7 ms2 and -57 2 ms2 (p=0.01). No change in lung function or symptoms was observed in either group. This suggests that SO2 exposure at concentrations which are frequently encountered during air pollution episodes can influence the autonomic nervous system. This may be important in understanding the mechanisms involved in SO2 induced bronchoconstriction, and of the cardiovascular effects of air pollution.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

Effect of posture change on control of ventilation

To clarify the control mechanism of ventilation during posture change, ventilatory parameters, PETCO2, and ventilatory response to CO2 were examined in 11 healthy male subjects at supine (0 degrees) and 75 degrees head-up tilt positions. Minute expiratory ventilation (V.E), tidal volume (VT), respiratory frequency (f), end-tidal and transcutaneous PCO2 and CO2 output (V.CO2), and ventilatory response to CO2 were measured during a steady state condition. V.E (V.A) and VT increased significantly at 75 degrees tilt with significant decrease in PETCO2 from 40.1 mmHg (0 degrees) to about 36.1 mmHg (75 degrees). Transcutaneous PCO2 also decreased during tilt, by 3.3 mmHg. Physiological dead space (VD/VT) and V.CO2, however, remained unchanged, and ventilatory equivalent (V.E/V.CO2, V.A/V.CO2) increased significantly. The CO2-ventilatory response curve shifted upward (or leftward) without significant change in the response slope. At 75 degrees tilt, EMG activity of gastro-cnemius muscle increased. These findings suggested that PETCO2 decreased because of increased V.E (V.A) with a leftward shift of CO2-ventilatory response curve. Various signals such as afferents from lower extremities might have net stimulatory effects on a CO2-ventilation control system to reset the controlled level of PETCO2 to a lower range, but without significant change in CO2-ventilatory response during upright position.

Mechanisms mediating the heart rate response to hypoxemia

We studied the effect of phasic pulmonary afferent information on heart rate (HR) during a progressive reduction in oxygen saturation (SaO2). The Hering-Breuer reflex was evaluated with the use of the ratio of apnea duration after lung inflation to the preceding expiratory time (dT). Phasic afferent activity was stopped in anesthetized, paralyzed dogs by constant-flow ventilation (CFV), a technique that removes cyclic changes in lung volume. During normocapnic (PaCO2 = 36.4 +/- 1.1 mm Hg) spontaneous breathing, there was a wide variability in HR response, with a mean delta HR/delta SaO2 (+/- SE) of 0.62 +/- 0.27 beats/min/% (values greater than 0 indicate a tachycardiac response). There was a good correlation between delta HR/delta SaO2 and dT (r = .79). Mean delta HR/delta SaO2 for the combined normocapnic and hypercapnic studies during CFV was lower (-1.32 +/- 0.19 bpm/%) than that during spontaneous breathing (0.23 +/- 0.19, p less than .0001). We suggest that the HR response to hypoxemia is strongly related to the strength of the Hering-Breuer reflex, which may explain the large interdog variability in HR responses.

Carotid sinus baroreceptors modulate tracheal smooth muscle tension in dogs

Arterial baroreceptors are known to influence airway smooth muscle tone. Thus, increasing carotid sinus pressure from 20 to 200 mm Hg causes reflex tracheal dilation. However, the effects of changing sinus pressure around a normal arterial pressure set-point of 100 mm Hg have not been examined. In anesthetized, artificially ventilated dogs, we distended the vascularly isolated carotid sinuses with a pulsatile pressure and recorded isometric tension in an upper tracheal segment. The aortic nerves were cut. Increasing mean carotid sinus pressure in steps between 100 and 200 mm Hg decreased tracheal tension, heart rate, and arterial blood pressure; decreasing sinus pressure between 100 and 25 mm Hg had the opposite effect. Changing carotid sinus pressure still evoked tracheal responses when systemic arterial pressure was held constant. Increasing and decreasing carotid sinus pulse pressure around a constant mean pressure evoked similar changes in tracheal tension. All reflex effects were abolished by cutting or cooling (0 degree C) the carotid sinus nerves; tracheal responses were abolished by the carotid baroreflex were of comparable magnitude to those triggered by stimulating pulmonary stretch receptors, laryngeal receptors, and pulmonary C-fibers. Our results indicate that carotid sinus baroreceptors exert a tonic influence on the upper airways by a vagal cholinergic pathway, increasing and decreasing tracheal smooth muscle tension as blood pressure varies around the normal set-point.

Stimulation of pulmonary vagal afferent C-fibers by lung edema in dogs

In anesthetized, open-chest dogs we examined the effect of pulmonary edema on the firing frequency of afferent vagal fibers arising from the lung. We recorded impulses from slips of the cervical vagus nerves and infused isotonic Krebs-Henseleit solution (20% of body weight) intravenously to increase net filtration pressure in the lung microvasculature. Measurement of extravascular lung water (6.0 +/- 0.4 g/g dry lung), and morphological examination of lung tissue (revealing various degrees of perivascular and peribronchial cuffing) confirmed that edema was present. At the end of the infusion when the lungs were congested (lung microvascular pressure, 37 cm water) and edematous, the impulse frequency of pulmonary and bronchial C-fibers and rapidly adapting receptors had increased 5-6 times. The only significant change in slowly adapting receptor activity was an increase during deflation. When lung water was still elevated but lung microvascular pressure had been restored to control by withdrawal of blood, impulse activity of rapidly and slowly adapting receptors reverted to or below control. Pulmonary C-fiber activity, although less than during congestion, remained significantly above control, several C-fibers being stimulated by interstitial edema in the absence of alveolar edema. Bronchial C-fibers were stimulated in severely edematous lung showing pronounced peribronchial cuffing and alveolar edema, but were not stimulated in milder grades of edema. Our results support the hypothesis (Paintal, 1969) that pulmonary C-fibers (J-receptors) are stimulated by an increase in interstitial pressure secondary to edema.