Lu-Yuan Lee

Afferent properties and reflex functions of bronchopulmonary C-fibers.

Bronchopulmonary C-fiber afferents are characterized by their distinct sensitivity to chemical stimuli in the airways or pulmonary circulation. Responses evoked by activating these afferents are mediated by both central reflex pathways and by local or axon reflexes involving the release of tachykinins from sensory endings. Bronchopulmonary C-fiber stimulation reflexly reduces tidal volume and increases respiratory rate, constricts the airways, increases mucus secretion in the airways, and is associated with coughing. Cardiovascular effects include bradycardia, a fall in cardiac output, and bronchial vasodilation that increases airway blood flow despite systemic hypotension. In animals, C-fiber stimulation inhibits skeletal muscle activity, and in humans, is accompanied by burning and choking sensations in the throat and upper chest. Recent studies have identified additional physiologic and pharmacologic stimuli to these afferents, such as hydrogen ions, adenosine, reactive oxygen species, and hyperosmotic solutions. Furthermore, increasing evidence indicates that the excitability of these afferents is enhanced by the local release of certain autocoids (e.g. PGE2) during airway inflammation. These findings further indicate that vagal C-fiber endings in the lungs and airways play an important role in regulating the cardiopulmonary functions under both normal and abnormal physiologic conditions.

Activation of pulmonary C fibres by adenosine in anaesthetized rats: role of adenosine A1 receptors

1 Intravenous administration of adenosine (Ado) to patients can cause dyspnoea, chest discomfort and bronchoconstriction. To assess the role of vagal pulmonary C fibres in evoking these adverse reactions, the effect of Ado on single pulmonary C fibres was studied in anaesthetized and artificially ventilated rats. 2 Right‐atrial injection of Ado (320 μg kg−1) activated 68 % (73/107) of pulmonary C fibres; the total number of action potentials during a period of 15 s increased from a baseline of 0.2 ± 0.1 impulses to a peak of 16.4 ± 2.6 impulses (P < 0.01, n= 107) after Ado. Inosine, the metabolite of Ado, did not activate any of eleven C fibres tested in six rats. Furthermore, C fibres were activated only by right‐atrial and not by left‐ventricular injection of the same dose of Ado. 3 Unlike the immediate and transient stimulation of C fibres by capsaicin, the C fibre stimulation by Ado had a latency of 6.5 ± 0.3 s (range, 3‐18 s) and lasted longer. 4 The stimulation of C fibres by Ado was significantly attenuated by pretreatment with aminophylline, a non‐selective Ado receptor antagonist, was completely prevented by 1,3‐dipropyl‐8‐cyclopentylxanthine, an Ado A1 receptor antagonist, but was unaffected by 3,7‐dimethy‐1‐propargylxanthine, an A2 receptor antagonist. None of these Ado receptor antagonists prevented capsaicin‐induced C fibre stimulation. 5 In conclusion, Ado stimulates pulmonary C fibre terminals through an activation of A1 receptors. The stimulation of pulmonary C fibres may play an important role in Ado‐induced adverse respiratory effects.

Role of TRPV1 in inflammation-induced airway hypersensitivity

Airway hypersensitivity is a common pathophysiological feature in various airway inflammatory diseases. Increasing evidence suggests that activation of the transient receptor potential vanilloid type 1 receptor (TRPV1) plays an important part in the manifestation of various symptoms of airway hypersensitivity. This mini-review focuses on recent studies that have revealed several potential contributing factors to the increase in TRPV1 sensitivity in pulmonary sensory neurons during airway inflammatory reaction. In addition, chronic allergic airway inflammation induces a pronounced overexpression of TRPV1 in neurofilament-positive pulmonary sensory neurons in nodose ganglia. A better understanding of the mechanisms underlying the increase in sensitivity and/or expression of TRPV1 during acute and chronic airway inflammation should generate the necessary information for developing effective therapeutic interventions to alleviate airway hypersensitivity.

Altered expression of TRPV1 and sensitivity to capsaicin in pulmonary myelinated afferents following chronic airway inflammation in the rat

Vagal pulmonary myelinated afferents are normally not activated by capsaicin, a selective agonist of transient receptor potential vanilloid type 1 (TRPV1) receptors. This study was carried out to investigate whether the expression of TRPV1 in these afferents is altered when chronic airway inflammation is induced by ovalbumin (Ova) sensitization. Two groups of Brown–Norway rats (sensitized and control) were exposed to aerosolized Ova and vehicle, respectively, 3 days per week for 3 weeks. After the C‐fibre conduction in both vagus nerves was blocked, right‐atrial injection of capsaicin elicited augmented breaths in sensitized rats breathing spontaneously, but not in control rats, indicating a stimulation of rapidly adapting receptors (RARs) by capsaicin. Single‐unit fibre activities of RARs and slow adapting receptors (SARs), identified by their firing behaviour and adaptation indexes in response to lung inflation, were recorded in anaesthetized, vagotomized and artificially ventilated rats. Capsaicin injection evoked either negligible or no response in both RARs and SARs of control rats. However, in striking contrast, the same dose of capsaicin evoked an immediate stimulatory effect on these myelinated afferents in sensitized rats. Furthermore, the immunohistochemistry experiments showed that there was a significant increase in the proportion of TRPV1‐expressing pulmonary neurones in nodose ganglia of sensitized rats; this increase in TRPV1 expression was found mainly in neurofilament‐positive (myelinated) neurones. In conclusion, allergen‐induced airway inflammation clearly elevated capsaicin sensitivity in myelinated pulmonary afferents, which probably resulted from an increased expression of TRPV1 in these sensory nerves.

Mechanisms of Dyspnea

The mechanisms and pathways of the sensation of dyspnea are incompletely understood, but recent studies have provided some clarification. Studies of patients with cord transection or polio, induced spinal anesthesia, or induced respiratory muscle paralysis indicate that activation of the respiratory muscles is not essential for the perception of dyspnea. Similarly, reflex chemostimulation by CO₂ causes dyspnea, even in the presence of respiratory muscle paralysis or cord transection, indicating that reflex chemoreceptor stimulation per se is dyspnogenic. Sensory afferents in the vagus nerves have been considered to be closely associated with dyspnea, but the data were conflicting. However, recent studies have provided evidence of pulmonary vagal C-fiber involvement in the genesis of dyspnea, and recent animal data provide a basis to reconcile differences in responses to various C-fiber stimuli, based on the ganglionic origin of the C fibers. Brain imaging studies have provided information on central pathways subserving dyspnea: Dyspnea is associated with activation of the limbic system, especially the insular area. These findings permit a clearer understanding of the mechanisms of dyspnea: Afferent information from reflex stimulation of the peripheral sensors (chemoreceptors and/or vagal C fibers) is processed centrally in the limbic system and sensorimotor cortex and results in increased neural output to the respiratory muscles. A perturbation in the ventilatory response due to weakness, paralysis, or increased mechanical load generates afferent information from vagal receptors in the lungs (and possibly mechanoreceptors in the respiratory muscles) to the sensorimotor cortex and results in the sensation of dyspnea.

Histamine enhances vagal pulmonary C-fiber responses to capsaicin and lung inflation

Effects of histamine on the base-line activity of vagal pulmonary C-fibers and their responses to chemical and mechanical stimulations were studied in anesthetized, open-chest dogs. Histamine aerosols (5 breaths, 1% solution) increased the change in tracheal pressure per breath (delta Pt) to 53 +/- 9% (mean +/- SE) above its base line, whereas an accompanying increase in activity was found in only 27% of the C-fibers studied and hence the overall afferent activity was not significantly different from the control. After Pt returned toward the base line in several minutes, the receptor activity evoked by right atrial injection of capsaicin (0.8-4.0 micrograms/kg) increased from a base line of 0.43 +/- 0.02 imp/sec to 8.12 +/- 1.16 imp/sec (averaged over 10 sec), which was markedly greater and longer lasting than the response triggered by the same dose of capsaicin after phosphate buffer aerosols (4.72 +/- 0.71 imp/sec). Furthermore, pretreatment with histamine aerosols also enhanced the afferent responses of these receptors to static lung inflation (Pt = 20 cmH2O). We conclude that the low dose of histamine aerosols alone did not consistently stimulate the vagal pulmonary C-fibers, but it potentiated the stimulatory effects of both capsaicin and lung inflation on these receptors.

Respiratory sensations evoked by activation of bronchopulmonary C-fibers

C-fibers represent the majority of vagal afferents innervating the airways and lung, and can be activated by inhaled chemical irritants and certain endogenous substances. Stimulation of bronchopulmonary C-fibers with selective chemical activators by either inhalation or intravenous injection evokes irritation, burning and choking sensations in the throat, neck and upper chest (mid-sternum region) in healthy human subjects. These irritating sensations are often accompanied by bouts of coughs either during inhalation challenge or when a higher dose of the chemical activator is administered by intravenous injection. Dyspnea and breathless sensation are not always evoked when these afferents are activated by different types of chemical stimulants. This variability probably reflects the chemical nature of the stimulants, as well as the possibility that different subtypes of C-fibers encoded by different receptor proteins are activated. These respiratory sensations and reflex responses (e.g., cough) are believed to play an important role in protecting the lung against inhaled irritants and preventing overexertion under unusual physiological stresses (e.g., during strenuous exercise) in healthy individuals. More importantly, recent studies have revealed that the sensitivity of bronchopulmonary C-fibers can be markedly elevated in acute and chronic airway inflammatory diseases, probably caused by a sensitizing effect of certain endogenously released inflammatory mediators (e.g., prostaglandin E(2)) that act directly or indirectly on specific ion channels expressed on the sensory terminals. Normal physiological actions such as an increase in tidal volume (e.g., during mild exercise) can then activate these C-fiber afferents, and consequently may contribute, in part, to the lingering respiratory discomforts and other debilitating symptoms in patients with lung diseases.

Prostaglandin E2 potentiates a TTX‐resistant sodium current in rat capsaicin‐sensitive vagal pulmonary sensory neurones

Capsaicin‐sensitive vagal pulmonary neurones (pulmonary C neurones) play an important role in regulating airway function. During airway inflammation, the level of prostaglandin E2 (PGE2) increases in the lungs and airways. PGE2 has been shown to sensitize isolated pulmonary C neurones. The somatosensory correlate of the pulmonary C neurone, the small‐diameter nociceptive neurone of the dorsal root ganglion, contains a high percentage of tetrodotoxin‐resistant sodium currents (TTX‐R INa). Therefore, this study was carried out to determine whether these channel currents are involved in the PGE2‐induced sensitization of pulmonary C neurones. We used the perforated patch‐clamp technique to study the effects of PGE2 on the TTX‐R INa in acutely cultured capsaicin‐sensitive pulmonary neurones that were identified by retrograde labelling with a fluorescent tracer, DiI. We found that the pulmonary neurones sensitive to capsaicin had a higher percentage of TTX‐R INa than that of capsaicin‐insensitive pulmonary neurones. PGE2 exposure increased the evoked TTX‐R INa when experiments were performed at both room temperature and at 37°C. Furthermore, stimulation of the adenylyl cyclase/protein kinase A pathway with either forskolin or Sp‐5,6‐DCl‐cBiMPS potentiated the TTX‐R INa in a manner similar to that of PGE2. We conclude that these modulatory effects of PGE2 on TTX‐R INa play an important role in the sensitization of pulmonary C neurones.

Stimulation of pulmonary C fibres by lactic acid in rats: contributions of H+ and lactate ions.

1. The contributions of H+ and lactate ions to the stimulation of single pulmonary C fibres by lactic acid were examined in anaesthetized and artificially ventilated rats. 2. Lactic acid injected into the right atrium caused a transient decrease in arterial blood pH (pHa) and a short but intense burst of afferent activities in pulmonary C fibres, whereas sodium lactate had no effect. The fibre activity usually reached a peak within 1‐1.5 s, with an onset latency of < 1 s, and returned to the baseline in 5 s. 3. The injection of hydrochloric acid at the same pH as that of lactic acid did not significantly decrease pHa, nor did it stimulate any C fibres studied. 4. Formic acid has a pKa value (the negative logarithm of the dissociation constant) almost identical to that of lactic acid; thus, its injection decreased pHa to the same degree as did the injection of lactic acid. However, the response of C fibres to lactic acid was 134% stronger than that to formic acid. 5. We conclude that H+ is primarily responsible for the activation of pulmonary C fibres by lactic acid, probably through a direct effect of H+ on these afferent endings. The lactate ion, by itself, does not activate C fibres, but it seems to potentiate the stimulatory effect of H+ on these afferents.

Stimulation of pulmonary vagal C-fibres by anandamide in anaesthetized rats: role of vanilloid type 1 receptors

This study was carried out to determine the effect of intravenous injection of anandamide on pulmonary C‐fibre afferents and the cardiorespiratory reflexes. In anaesthetized, spontaneously breathing rats, intravenous bolus injection of anandamide near the right atrium immediately elicited the pulmonary chemoreflex responses, characterized by apnoea, bradycardia and hypotension. After perineural treatment of both cervical vagi with capsaicin to block the conduction of C‐fibres, anandamide no longer evoked these reflex responses. In open‐chest, and artificially ventilated rats, anandamide injection evoked an abrupt and intense discharge in vagal pulmonary C‐fibres in a dose‐dependent manner. After injection of the high dose, the fibre discharge generally started within 1 s, reached a peak in ∼2 s, and returned to baseline within 7 s. The stimulation of C‐fibres by anandamide was completely and reversibly blocked by pretreatment with capsazepine, a competitive antagonist of the vanilloid type 1 receptor. Anandamide (0.4 mg kg−1) stimulated ∼93 % of pulmonary C‐fibres that were activated by capsaicin at a much lower dose (0.6 μg kg−1); the response to anandamide showed similar intensity, but had slightly longer latency and duration than that to capsaicin. In conclusion, intravenous bolus injection of anandamide evokes a consistent and distinct stimulatory effect on pulmonary C‐fibre terminals, and this effect appears to be mediated through an activation of the vanilloid type 1 receptor.