Vagal Bronchopulmonary C-fiber Research Articles

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.

Molecular/Ionic Basis of Vagal Bronchopulmonary C-Fiber Activation by Inflammatory Mediators.

Stimulation of bronchopulmonary vagal afferent C fibers by inflammatory mediators can lead to coughing, chest tightness, and changes in breathing pattern, as well as reflex bronchoconstriction and secretions. These responses serve a defensive function in healthy lungs but likely contribute to many of the signs and symptoms of inflammatory airway diseases. A better understanding of the mechanisms underlying the activation of bronchopulmonary C-fiber terminals may lead to novel therapeutics that would work in an additive or synergic manner with existing anti-inflammatory strategies.

Mechanisms Underlying the Stimulatory Effect of Inhaled Sulfur Dioxide on Rat Vagal Bronchopulmonary Sensory Nerves

Chronic exposure to sulfur dioxide (SO2), an air pollutant, can cause airway injury and lung diseases. Acute exposure to SO2 triggers cough and reflex bronchoconstriction, indicating a stimulatory effect of SO2 on airway sensory nerves. Indeed, a recent study in our lab has demonstrated that vagal bronchopulmonary C‐fibers are the primary target of inhaled SO2. This study was carried out to investigate the underlying mechanism of this stimulatory effect of SO2 on bronchopulmonary C‐fibers. Our results showed: 1) Inhalation of SO2 (1,000–2,000 ppm, 10 breaths) evoked a pronounced and reproducible stimulatory effect on pulmonary C‐fibers in anesthetized rats. 2) Pretreatment with an intravenous infusion of sodium bicarbonate (120 μmol/kg) raised the baseline arterial pH by 0.08 units, which profoundly diminished the stimulatory effect of SO2 on pulmonary C‐fibers by 83.9%. 3) This stimulatory effect of SO2 was also significantly attenuated by a pretreatment with amiloride (a blocker of acid‐sensing ion channels, ASICs; 11 μmol/kg) alone (Δ = 70.7%); AMG9810 (an antagonist of transient receptor potential vanilloid type‐1, TRPV1; 10 μmol/kg) alone (Δ = 76.7%); and by a combination of amiloride and AMG9810 (Δ = 91.4%). 4) To further investigate if this stimulatory effect is generated by a direct action of SO2 on sensory nerves, the change in intracellular Ca2+ concentration, [Ca2+]i, was measured in isolated rat vagal pulmonary sensory neurons. Perfusion with extracellular fluid saturated with SO2 (1,000 and 2,000 ppm) evoked a significant increase in [Ca2+]i in a concentration‐dependent manner in these neurons, and these responses to SO2 were markedly suppressed by a combined pretreatment with amiloride and AMG9810. 5) In addition, inhalation of SO2 (300 and 600 ppm for 5 min) evoked cough reflex responses consistently in awake mice in a concentration‐dependent manner; the cough responses were almost completely prevented by a pretreatment with amiloride aerosol inhalation (5 mM for 4 min) in TRPV1‐knockout mice. In conclusion, this study suggested that inhalation of SO2 lowered the pH in airway/lung tissues, which exerted a stimulatory effect on vagal bronchopulmonary C‐fibers by activating both ASICs and TRPV1 channels.Support or Funding InformationNIH grants AI123832 & UL1TR001998This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

KCNQ/M-channels regulate mouse vagal bronchopulmonary C-fiber excitability and cough sensitivity.

Increased airway vagal sensory C-fiber activity contributes to the symptoms of inflammatory airway diseases. The KCNQ/Kv7/M-channel is a well-known determinant of neuronal excitability, yet whether it regulates the activity of vagal bronchopulmonary C-fibers and airway reflex sensitivity remains unknown. Here we addressed this issue using single-cell RT-PCR, patch clamp technique, extracellular recording of single vagal nerve fibers innervating the mouse lungs, and telemetric recording of cough in free-moving mice. Single-cell mRNA analysis and biophysical properties of M-current (IM) suggest that KCNQ3/Kv7.3 is the major M-channel subunit in mouse nodose neurons. The M-channel opener retigabine negatively shifted the voltage-dependent activation of IM, leading to membrane hyperpolarization, increased rheobase, and suppression of both evoked and spontaneous action potential (AP) firing in nodose neurons in an M-channel inhibitor XE991-sensitive manner. Retigabine also markedly suppressed the α,β-methylene ATP-induced AP firing in nodose C-fiber terminals innervating the mouse lungs, and coughing evoked by irritant gases in awake mice. In conclusion, KCNQ/M-channels play a role in regulating the excitability of vagal airway C-fibers at both the cell soma and nerve terminals. Drugs that open M-channels in airway sensory afferents may relieve the sufferings associated with pulmonary inflammatory diseases such as chronic coughing.

Inhalation of electronic cigarette aerosol induces reflex bronchoconstriction by activation of vagal bronchopulmonary C-fibers.

The electronic cigarette (e-cig) has been suggested as a safer alternative to tobacco cigarettes. However, the health effects of e-cigs on the airways have not been fully investigated. Nicotine, the primary chemical constituent of the e-cig aerosol, has been shown to stimulate vagal bronchopulmonary C-fiber sensory nerves, which upon activation can elicit vigorous pulmonary defense reflexes, including airway constriction. In this study, we investigated the bronchomotor response to e-cig inhalation challenge in anesthetized guinea pigs and the mechanisms involved in regulating these responses. Our results showed that delivery of a single puff of e-cig aerosol into the lung triggered immediately a transient bronchoconstriction that sustained for >2 min. The increase in airway resistance was almost completely abolished by a pretreatment with either intravenous injection of atropine or inhalation of aerosolized lidocaine, suggesting that the bronchoconstriction was elicited by cholinergic reflex mechanism and stimulation of airway sensory nerves was probably involved. Indeed, electrophysiological recording further confirmed that inhalation of e-cig aerosol exerted a pronounced stimulatory effect on vagal bronchopulmonary C-fibers. These effects on airway resistance and bronchopulmonary C-fiber activity were absent when the e-cig aerosol containing zero nicotine was inhaled, indicating a critical role of nicotine. Furthermore, a pretreatment with nicotinic acetylcholine receptor antagonists by inhalation completely prevented the airway constriction evoked by e-cig aerosol inhalation. In conclusion, inhalation of a single puff of e-cig aerosol caused a transient bronchoconstriction that was mediated through cholinergic reflex and triggered by a stimulatory effect of nicotine on vagal bronchopulmonary C-fiber afferents.

Cough response to sulfur dioxide inhalation challenge is enhanced by tumor necrosis factor alpha: a primary role of vagal bronchopulmonary C‐fibers

Tumor necrosis factor alpha (TNFα), a pro‐inflammatory cytokine, plays a significant role in the pathogenesis of asthma. We recently reported that TNFα pretreatment elevated the sensitivities of vagal bronchopulmonary C‐fibers and silent rapidly adapting rectors (RARs) to capsaicin (Front. Physiol. 2017). In this study, we investigated the effect of TNFα on cough response to sulfur dioxide (SO2), a common irritant gas, and the relative contributions of these vagal afferents. Our results showed: 1) Inhalation of SO2 (300 and 600 ppm; 8 min each) consistently elicited coughs in a concentration‐dependent manner in awake mice moving freely in a recording chamber; coughs were recorded via telemetry sensors implanted in the intrapleural space. The cough frequencies during and immediately after the SO2 inhalation challenges increased markedly in the same animals after TNFα (10 μg/ml; 0.03 ml) was administered into the lung by intratracheal instillation, and this increase in cough sensitivity sustained for >7 days after the TNFα pretreatment. In contrast, the cough response to SO2 inhalation challenges did not change after pretreatment with vehicle (0.03 ml of PBS) in a matching group of control mice. 2) In single‐fiber recording experiments in anesthetized mice, the same SO2 inhalation challenges (for 8 min) evoked a pronounced stimulatory effect on C‐fiber afferents, reaching a peak in 1–4 min. The increased fiber activity was concentration dependent and lingered for several more minutes after the termination of SO2 challenge. This stimulatory effect of SO2 was significantly elevated in the TNFα‐treated mice. 3) The same SO2 inhalation challenges, either 300 or 600 ppm, did not activate any of the silent RARs in vehicle‐treated mice, and only 600 ppm SO2 caused a very weak stimulation in 2 out of 7 of these silent RARs in TNFα‐treated mice. 4) Inhalation of SO2 (300 or 600 ppm) consistently induced an inhibitory effect on both slowly adapting receptors and phasic RARs; these afferents ceased to discharge completely in 1–5 min after the onset of SO2 (600 ppm) inhalation challenge, and a majority of them slowly returned to controls after resuming air breathing for >1 hr. No detectable difference in this inhibitory effect was found between TNFα‐ and vehicle‐treated mice. In conclusion, the cough response to SO2 inhalation challenge is enhanced by the TNFα pretreatment in wake mice, and an increase in the bronchopulmonary C‐fiber sensitivity to SO2 is primarily responsible.Support or Funding InformationNIH grant AI123832This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Interaction between TRPA1 and TRPV1: Synergy on pulmonary sensory nerves

Transient receptor potential ankyrin type 1 (TRPA1) and vanilloid type 1 (TRPV1) receptors are co-expressed in vagal pulmonary C-fiber sensory nerves. Because both these ligand-gated non-selective cation channels are sensitive to a number of endogenous inflammatory mediators, it is highly probable that they can be activated simultaneously during airway inflammation. Studies were carried out to investigate whether there is an interaction between these two polymodal transducers upon simultaneous activation, and how it modulates the activity of vagal pulmonary C-fiber sensory nerves. Our studies showed a distinct potentiating effect induced abruptly by simultaneous activations of TRPA1 and TRPV1 by their respective selective agonists, allyl isothiocyanate (AITC) and capsaicin (Cap), at near-threshold concentrations. This synergistic effect was demonstrated in the studies of single-unit recording of vagal bronchopulmonary C-fiber afferents and the reflex responses elicited by activation of these afferents in intact animals, as well as in the isolated nodose and jugular bronchopulmonary sensory neurons. This potentiating effect was absent when either AITC or Cap was replaced by non-TRPA1 and non-TRPV1 chemical activators of these neurons, demonstrating the selectivity of the interaction between these two TRP channels. Furthermore, the synergism was dependent upon the extracellular Ca2+, and the rapid onset of the action further suggests that the interaction probably occurred locally at the sites of these channels. These findings suggest that the TRPA1–TRPV1 interaction may play an important role in regulating the function and excitability of pulmonary sensory neurons during airway inflammation, but the mechanism underlying this positive interaction is not yet fully understood.

Positive Interaction between TRPA1 and TRPV1 Channels in Rat Vagal Bronchopulmonary Sensory Neurons

Both TRPA1 and TRPV1 are selectively and abundantly expressed in vagal bronchopulmonary C-fiber sensory nerves. They can be either activated or sensitized by a number of endogenous inflammatory mediators such as H+, ROS, bradykinin, protanoids, etc. As such, it is highly probable that concurrent activation of these two receptors can occur during airway inflammatory reaction. A recent study has demonstrated a distinct synergistic effect of simultaneous activation of TRPA1 and TRPV1 in vagal pulmonary C-fiber afferents in anesthetized rats (Lin et al., JAP, 2014), but its underlying mechanism and whether this action occurred directly at the sensory neurons were not known. This study was carried out to answer these questions in isolated nodose and jugular pulmonary sensory neurons. Using the perforated patch-clamp recording technique, our results showed: 1) The current density evoked by a combination of low concentrations of allyl isothiocyanate (AITC, a TRPA1 activator; 30 µM) and capsaicin (Cap, a TRPV1 activator; 0.1-0.3 µM) was 233% of the mathematical sum of the responses to AITC and Cap when they were administered separately at the same concentration and duration (4 sec) in the same neurons. 2) The positive interaction was absent when either AITC or Cap was replaced by another chemical activator of these neurons (e.g., ATP or phenylbiguanide). 3) This synergistic effect was completely blocked when Ca2+ was removed from the extracellular solution. 4) Application of AITC alone immediately preceding Cap also significantly augmented the Cap-evoked current, whereas reversing the order did not potentiate the response to AITC. In conclusion, a positive interaction occurs directly in vagal pulmonary sensory neurons between TRPA1 and TRPV1 when both are activated, and the extracellular Ca2+ plays an important role in this action.

Bronchoconstriction induced by increasing airway temperature in ovalbumin-sensitized rats: role of tachykinins

This study was carried out to determine the effect of allergic inflammation on the airway response to increasing airway temperature. Our results showed the following: 1) In Brown-Norway rats actively sensitized by ovalbumin (Ova), isocapnic hyperventilation with humidified warm air (HWA) for 2 min raised tracheal temperature (Ttr) from 33.4 ± 0.6°C to 40.6 ± 0.1°C, which induced an immediate and sustained (>10 min) increase in total pulmonary resistance (Rl) from 0.128 ± 0.004 to 0.212 ± 0.013 cmH2O·ml(-1)·s (n = 6, P < 0.01). In sharp contrast, the HWA challenge caused the same increase in Ttr but did not generate any increase in Rl in control rats. 2) The increase in Rl in sensitized rats was reproducible when the same HWA challenge was repeated 60-90 min later. 3) This bronchoconstrictive effect was temperature dependent: a slightly smaller increase in peak Ttr (39.6 ± 0.2°C) generated a significant but smaller increase in Rl in sensitized rats. 4) The HWA-induced bronchoconstriction was not generated by the humidity delivered by the HWA challenge alone, because the same water content delivered by saline aerosol at room temperature had no effect. 5) The HWA-evoked increase in Rl in sensitized rats was not blocked by atropine but was completely prevented by pretreatment either with a combination of neurokinin (NK)-1 and NK-2 antagonists or with formoterol, a β2 agonist, before the HWA challenge. This study showed that increasing airway temperature evoked a pronounced and reversible increase in airway resistance in sensitized rats and that tachykinins released from the vagal bronchopulmonary C-fiber endings were primarily responsible.

Pulmonary chemoreflex responses are potentiated by tumor necrosis factor-alpha in mice

Inhalation of tumor necrosis factor-alpha (TNF-α), a proinflammatory cytokine, induces airway hyperresponsiveness, and the underlying mechanism is not fully understood. Hypersensitivity of vagal bronchopulmonary C-fiber afferents is known to contribute to the airway hyperresponsiveness during an airway inflammatory reaction. Because activation of these afferents can elicit pulmonary chemoreflexes, this study was designed to determine if a pretreatment with TNF-α induced airway inflammation and enhanced the pulmonary chemoreflex sensitivity in anesthetized mice; and if so, whether the effect was mediated through activation of either or both of the TNF receptors, p55 and p75. Our results showed that TNF-α instilled into the lung caused an increased sensitivity of pulmonary chemoreflex responses to various chemical stimulants of the vagal bronchopulmonary C-fiber afferents. The increased sensitivity was found 24 h later, persisted at 48 h, and then gradually declined after several days. The TNF-α-induced airway hypersensitivity was accompanied by airway inflammation as shown by a striking elevation of the levels of eosinophils and neutrophils, several potent bronchoactive inflammatory mediators, and proinflammatory cytokines in the bronchoalveolar lavage fluid. Furthermore, the increase in pulmonary chemoreflex response caused by TNF-α was partially abrogated in both p55-null and p75-null mice, but completely abolished in p55/p75-null mice. In conclusion, TNF-α pretreatment induced airway inflammation and a sustained elevation of pulmonary chemoreflex sensitivity, which was mediated through an activation of both types of TNF receptors.

Bronchoconstriction Triggered by Breathing Hot Humid Air in Patients with Asthma

Hyperventilation of hot humid air induces transient bronchoconstriction in patients with asthma; the underlying mechanism is not known. Recent studies showed that an increase in temperature activates vagal bronchopulmonary C-fiber sensory nerves, which upon activation can elicit reflex bronchoconstriction. This study was designed to test the hypothesis that the bronchoconstriction induced by increasing airway temperature in patients with asthma is mediated through cholinergic reflex resulting from activation of these airway sensory nerves. Specific airway resistance (SR(aw)) and pulmonary function were measured to determine the airway responses to isocapnic hyperventilation of humidified air at hot (49°C; HA) and room temperature (20-22°C; RA) for 4 minutes in six patients with mild asthma and six healthy subjects. A double-blind design was used to compare the effects between pretreatments with ipratropium bromide and placebo aerosols on the airway responses to HA challenge in these patients. SR(aw) increased by 112% immediately after hyperventilation of HA and by only 38% after RA in patients with asthma. Breathing HA, but not RA, triggered coughs in these patients. In contrast, hyperventilation of HA did not cause cough and increased SR(aw) by only 22% in healthy subjects; there was no difference between their SR(aw) responses to HA and RA challenges. More importantly, pretreatment with ipratropium completely prevented the HA-induced bronchoconstriction in patients with asthma. Bronchoconstriction induced by increasing airway temperature in patients with asthma is mediated through the cholinergic reflex pathway. The concomitant increase in cough response further indicates an involvement of airway sensory nerves, presumably the thermosensitive C-fiber afferents.

Airway irritation and cough evoked by acid: from human to ion channel.

Inhalation or aspiration of acid solution evokes airway defense responses such as cough and reflex bronchoconstriction, resulting from activation of vagal bronchopulmonary C-fibers and Aδ afferents. The stimulatory effect of hydrogen ion on these sensory nerves is generated by activation of two major types of ion channels expressed in these neurons: a rapidly activating and inactivating current mediated through ASICs, and a slow sustaining current via activation of TRPV1. Recent studies have shown that these acid-evoked responses are elevated during airway inflammatory reaction, revealing the potential convergence of a wide array of inflammatory signaling on these ion channels. Since pH in the airway fluid drops substantially in patients with inflammatory airway diseases, these heightened stimulatory effects of acid on airway sensory nerves may play a part in the manifestation of airway irritation and excessive cough under those pathophysiological conditions.

Vagal afferent nerves with the properties of nociceptors

Vagal afferent nerves are essential for optimal neural regulation of visceral organs, but are not often considered important for their defense. However, there are well-defined subsets of vagal afferent nerves that have activation properties indicative of specialization to detect potentially harmful stimuli (nociceptors). This is clearly exemplified by the vagal bronchopulmonary C-fibers that are quiescent in healthy lungs but are readily activated by noxious chemicals and inflammatory molecules. Vagal afferent nerves with similar activation properties have been also identified in the esophagus and probably exist in other visceral tissues. In addition, these putative vagal nociceptors often initiate defensive reflexes, can be sensitized, and have the capacity to induce central sensitization. This set of properties is a characteristic of nociceptors in somatic tissues.

Effects of airway anesthesia on dyspnea and ventilatory response to intravenous injection of adenosine in healthy human subjects

We have recently shown that intravenous injection of adenosine causes dyspnea and hyperventilation in man, and we suggested that stimulation of vagal C-fibers in the airways and lungs is involved. To test this hypothesis further, the present study was performed in healthy subjects ( n = 12 ; age 32.4±10.2 yrs, 7 females) to determine if the effect of adenosine could be attenuated by blocking the airway sensory receptors by inhalation of aerosolized lidocaine, a local anesthetic. In each subject, the effects of intravenous injection of adenosine (10 mg) on dyspneic sensation, minute ventilation, airway resistance and heart rate were measured after the subject inhaled lidocaine or placebo aerosol on two separate days. After a latency of ∼20 s, adenosine injection evoked a distinct dyspneic sensation, increase in minute ventilation ( V ˙ E ) , and transient bradycardia followed by tachycardia in all subjects. The increase in V ˙ E resulted primarily from a significant increase in tidal volume. The intensity of adenosine-induced dyspnea was markedly reduced after the lidocaine pretreatment compared to placebo. In a sharp contrast, the V ˙ E and heart rate responses to adenosine were not affected by lidocaine. These results lend further support to our previous studies indicating that the origin of the dyspnogenic action of intravenous adenosine is most likely vagal bronchopulmonary C-fiber sensory nerves.

Stimulatory effect of CO2 on vagal bronchopulmonary C-fiber afferents during airway inflammation

This study investigated 1) whether pulmonary C fibers are activated by a transient increase in the CO2 concentration of alveolar gas; and 2) if the CO2 sensitivity of these afferents is altered during airway inflammation. Single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering a CO2-enriched gas mixture (25-30% CO2, 21% O2, balance N2) for five to eight breaths, which increased alveolar CO2 concentration progressively to near or above 13% for 3-5 s and lowered the arterial pH transiently to 7.10 +/- 0.05. Our results showed the following. 1) HPC evoked only a mild stimulation in a small fraction (4/47) of pulmonary C fibers, and there was no significant change in fiber activity (change in fiber activity = 0.22 +/- 0.16 imp/s; P > 0.1, n = 47). 2) In sharp contrast, after airway exposure to poly-L-lysine, a cationic protein known to induce mucosal injury, the same challenge of transient HPC activated 87.5% of the pulmonary C fibers tested and evoked a distinct stimulatory effect on these afferents (change in fiber activity = 6.59 +/- 1.78 imp/s; P < 0. 01, n = 8). 3) Similar potentiation of the C-fiber response to HPC was also observed after acute exposure to ozone (n = 6) and during a constant infusion of inflammatory mediators such as adenosine (n = 15) or prostaglandin E2 (n = 12). 4) The enhanced C-fiber sensitivity to CO2 after poly-L-lysine was completely abrogated by infusion of NaHCO3 (1.82 mmol.kg(-1).min(-1)) that prevented the reduction in pH during HPC (n = 6). In conclusion, only a small percentage (<10%) of the bronchopulmonary C fibers exhibit CO2 sensitivity under control conditions, but alveolar HPC exerts a consistent and pronounced stimulatory effect on the C-fiber endings during airway inflammation. This effect of CO2 is probably mediated through the action of hydrogen ions.

Effect of 5-hydroxytryptamine on vagal C-fiber subtypes in guinea pig lungs

5-Hydroxytryptamine (5-HT, serotonin) evokes pulmonary reflexes partially by activating vagal bronchopulmonary C-fibers. In the guinea pig, vagal bronchopulmonary C-fibers arise from cell bodies situated in the nodose and the jugular ganglia. The nodose and the jugular C-fibers differ both pharmacologically and neurochemically, and may subserve different functions. In this study, we compared the effect of 5-HT on the nodose and jugular C-fibers with receptive fields within the guinea pig lungs. The nerve terminals of the vagal bronchopulmonary C-fibers were studied in an ex vivo isolated perfused lung nerve preparation using the extracellular recordings. All nodose C-fibers responded to transient administration of 5-HT (10 μM) and to the selective 5-HT 3 receptor agonist, 2-methyl-5-HT (10 μM), with the action potential discharge. The selective 5-HT 3 receptor antagonist ondansetron (10 μM) inhibited (by ∼90%) the activation of the nodose C-fibers evoked by 5-HT (10 μM). In contrast to the nodose C-fibers, the jugular C-fibers were unresponsive or poorly responsive to 5-HT ( n=9) and unresponsive to 2-methyl-5-HT ( n=11). A direct action of 5-HT on the C-fiber neurons was supported using the whole cell patch clamp recordings from the isolated vagal sensory neurons retrogradely labeled from the lungs. Consistently with the studies on the nerve terminals, 5-HT (10 μM) induced inward current in nodose lung-specific capsaicin-sensitive neurons. Conversely, 5-HT was inefficient to stimulate the lung-specific jugular neurons. We conclude that in the guinea pig lungs, 5-HT selectively stimulates vagal nodose, but not jugular C-fibers via the 5-HT 3 receptors in the neuronal membrane.

Pulmonary chemoreflexes elicited by intravenous injection of lactic acid in anesthetized rats.

Experiments were carried out to characterize the cardiorespiratory reflex responses to intravenous injection of lactic acid and to determine the involvement of vagal bronchopulmonary C-fiber afferents in eliciting these responses in anesthetized rats. Bolus injection of lactic acid (0.2 mmol/kg i.v.) immediately elicited apnea, bradycardia, and hypotension, which were then followed by a sustained hyperpnea. The immediate apneic and bradycardiac responses to lactic acid were completely abolished by bilateral vagotomy and were absent when the same dose of lactic acid was injected into the left ventricle. The subsequent hyperpneic response was substantially attenuated by denervation of carotid body chemoreceptors. After a perineural capsaicin treatment of both vagus nerves to block the conduction of C fibers, lactic acid no longer evoked the immediate apnea and bradycardia, whereas the hyperpneic response became more pronounced and sustained, presumably because of the removal of the inhibitory effect on breathing mediated by pulmonary C-fiber activation. Single-unit electrophysiological recording showed that intravenous injection of lactic acid consistently evoked an abrupt and intense burst of discharge from the vagal C-fiber afferent endings in the lungs. In conclusion, the cardiorespiratory depressor responses induced by lactic acid are predominantly elicited by activation of vagal pulmonary C fibers.

Vagal bronchopulmonary C-fibers and acute ventilatory response to inhaled irritants

Experiments were carried out in anesthetized Sprague-Dawley rats to determine the role of vagal bronchopulmonary C-fiber afferents in regulating the respiratory responses to inhaled irritants. Spontaneous inhalation of 2 tidal breaths of a known airway irritant (sulfur dioxide, 0.5%; ammonia, 1%; cigarette smoke, 50%) into the lower airways invariably elicited an immediate and transient inhibitory effect on breathing, characterized by apnea or bradypnea and accompanied by bradycardia, which lasted for 3–8 breaths. A delayed hyperpnea was also induced by inhalation of cigarette smoke, but not by sulfur dioxide or ammonia. After perineural capsaicin treatment of both cervical vagi to selectively block the conduction of capsaicin-sensitive C-fibers, these inhaled irritants no longer evoked any inhibitory effect on breathing; conversely, an augmented inspiration was triggered within the first 3 breaths from the onset of cigarette smoke inhalation in >85% of the rats studied, but after a delay of several breaths following inhalation of ammonia or sulfur dioxide in only 30% of the rats. The augmented breaths were completely abolished when both cervical vagi were cooled to 6–7°C. Bilateral vagotomy eliminated all the immediate responses to these irritants. These results suggest that both vagal C-fiber endings and irritant receptors in the airways are activated by these inhaled irritants, but the more dominant and consistent inhibitory effect on breathing is elicited primarily by stimulation of C-fiber afferents.

Inhibitory effect of gas phase cigarette smoke on breathing: role of hydroxyl radical

Inhaling cigarette smokes evokes immediate bradypnea in rats, resulting from stimulation of vagal bronchopulmonary C-fiber afferents by smoke constituent(s) other than nicotine. To determine the contribution of the gas phase of smoke to this irritant effect, the acute respiratory responses to both cigarette smoke and gas phase smoke were studied and compared in anesthetized Sprague-Dawley rats; smoke (6 ml, 50%) was generated by a machine from low-nicotine research cigarettes and the gas phase was obtained by passing the smoke through a glass-fiber Cambridge filter. Inhalation of gas phase smoke alone evoked a transient inhibitory effect on breathing, prolonging expiratory time (Te) to peak of 159 ± 6% of the base line; this response was very similar to that triggered by inhaling the unified smoke (Te = 177 ± 12%). The bradypnea started within 1–4 breaths after the onset of smoke inhalation, lasted for 3–5 breaths and was completely abolished by vagotomy. This inhibitory effect of gas phase smoke on breathing was also largely prevented after a pretreatment with either intravenous infusion or aerosol inhalation of hydroxyl radical scavenger, dimethylthiourea. These results suggest that the gas phase is primarily responsible for eliciting the reflexogenic bradypneic response to cigarette smoke in anesthetized rats and that hydroxyl radicals released endogenously in the lungs may be involved.

Acute effects of cigarette smoke on breathing in rats: vagal and nonvagal mechanisms

The acute ventilatory response to inhalation of cigarette smoke was studied in anesthetized Sprague-Dawley rats. Cigarette smoke (6 ml, 50%) generated by a machine was inhaled spontaneously via a tracheal cannula. Within the first two breaths of smoke inhalation, a slowing of respiration resulting from a prolonged expiratory duration (173 +/- 6% of the base line; n = 32) was elicited in 88% of the rats studied. This initial inhibitory effect on breathing was not affected either by an increase (410%) in the nicotine content of the cigarette smoke or by pretreatment with hexamethonium (33 mg/kg iv). However, bilateral vagotomy completely eliminated the initial ventilatory inhibition. Cooling both vagi to 5.1 degrees C blocked the reflex apneic response to lung inflation, but it did not abolish the inhibitory effect of smoke. After the initial response, a rapid shallow breathing pattern developed and reached its peak 5-12 breaths after inhalation of high-nicotine cigarette smoke; this delayed response could not be prevented by vagotomy and was undetectable after inhalation of low-nicotine smoke. We conclude that the initial inhibitory effect of smoke on breathing is mediated by vagal bronchopulmonary C-fiber afferents, which are stimulated by smoke constituents other than nicotine, whereas the delayed tachypneic response to smoke is caused by the absorbed nicotine.