Yu-Jung Lin

Sensitivity of vagal afferent endings to chemical irritants in the rat lung

This study was carried out to investigate the relationship between the conduction velocity of the vagal afferents arising from the rat lungs and their sensitivities to capsaicin, other chemical irritants, and lung inflation. We recorded single-unit activities of vagal pulmonary afferents (n = 205) in anesthetized, open-chest rats, and distinguished C fibers (conduction velocity < 2 m/sec) from myelinated afferents; the latter group was further classified into rapidly adapting pulmonary receptors (RARs) and slowly adapting pulmonary stretch receptors (SARs) on the basis of their adaptation indexes to lung inflation. Right-atrial injection of capsaicin (1 microg/kg) evoked an abrupt and intense stimulatory effect in 88.9% (64/72) of the pulmonary C fibers tested, but only a mild stimulation in 6.3% (3/48) of the RARs and none of the SARs. Other inhaled and injected chemical stimulants (e.g., cigarette smoke, lactic acid) activated 68.9% (42/61) of the pulmonary C fibers. The same chemical irritants exerted a mild stimulatory effect in only 14.5% (8/55) of the RARs; this subgroup of RARs exhibited a low or no baseline activity, and half of them were located near the hilum. Chemical stimulants had little or no effect on SARs. The response of pulmonary C fibers to lung inflation (tracheal pressure = 30 cm H2O) was not only extremely weak, but also showed a longer onset latency and an irregular pattern. In a sharp contrast, lung inflation evoked rapid and vigorous discharges in both RARs and SARs. In conclusion, C fibers are the primary type of chemosensitive vagal pulmonary afferents in rat lungs.

A synergistic effect of simultaneous TRPA1 and TRPV1 activations on vagal pulmonary C-fiber afferents

Transient receptor potential ankyrin type 1 (TRPA1) and vanilloid type 1 (TRPV1) receptors are coexpressed in vagal pulmonary C-fiber sensory nerves. Because both these receptors are sensitive to a number of endogenous inflammatory mediators, it is conceivable that they can be activated simultaneously during airway inflammation. This study aimed to determine 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. In anesthetized, spontaneously breathing rats, the reflex-mediated apneic response to intravenous injection of a combined dose of allyl isothiocyanate (AITC, a TRPA1 activator) and capsaicin (Cap, a TRPV1 activator) was ∼202% greater than the mathematical sum of the responses to AITC and Cap when they were administered individually. Similar results were also observed in anesthetized mice. In addition, the synergistic effect was clearly demonstrated when the afferent activity of single vagal pulmonary C-fiber afferents were recorded in anesthetized, artificially ventilated rats; C-fiber responses to AITC, Cap and AITC + Cap (in combination) were 0.6 ± 0.1, 0.8 ± 0.1, and 4.8 ± 0.6 impulses/s (n = 24), respectively. This synergism was absent when either AITC or Cap was replaced by other chemical activators of pulmonary C-fiber afferents. The pronounced potentiating effect was further demonstrated in isolated vagal pulmonary sensory neurons using the Ca(2+) imaging technique. In summary, this study showed a distinct positive interaction between TRPA1 and TRPV1 when they were activated simultaneously in pulmonary C-fiber sensory nerves.

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.

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 Ca(2+), 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.

Hemorrhagic hypotension-induced hypersensitivity of vagal pulmonary C-fibers to chemical stimulation and lung inflation in anesthetized rats

This study was carried out to investigate whether hemorrhagic hypotension (HH) altered the sensitivity of vagal pulmonary C-fibers. The fiber activity (FA) of single vagal pulmonary C-fiber was continuously recorded in anesthetized rats before, during, and after HH was induced by bleeding from the femoral arterial catheter into a blood reservoir and lowering the mean systemic arterial pressure (MSAP) to ∼40 mmHg for 20 min. Our results showed the following. First, after MSAP reached a steady state of HH, the peak FA response to intravenous injection of capsaicin was elevated by approximately fivefold. The enhanced C-fiber sensitivity continued to increase during HH and sustained even after MSAP returned to baseline during the recovery, but slowly returned to control ∼20 min later. Second, responses of FA to intravenous injections of other chemical stimulants of pulmonary C-fibers (phenylbiguanide, lactic acid, and adenosine) and a constant-pressure lung hyperinflation were all significantly potentiated by HH. Third, infusion of sodium bicarbonate alleviated the systemic acidosis during HH, and it also attenuated, but did not completely prevent, the HH-induced C-fiber hypersensitivity. In conclusion, the pulmonary C-fiber sensitivity was elevated during HH, probably caused by the endogenous release of chemical substances (e.g., lactic acid) that were produced by tissue ischemia during HH. This enhanced C-fiber sensitivity may heighten the pulmonary protective reflexes mediated through these afferents (e.g., cough, J reflex) during hemorrhage when the body is more susceptible to other hazardous insults and pathophysiological stresses.

Hypersensitivity of vagal pulmonary C-fibers induced by increasing airway temperature in ovalbumin-sensitized rats

Our recent study has shown that hyperventilation of humidified warm air (HWA) triggered cough and reflex bronchoconstriction in patients with mild asthma. We suggested that a sensitizing effect on bronchopulmonary C-fibers by increasing airway temperature was involved, but direct evidence was lacking. This study was carried out to test the hypothesis that HWA enhances the pulmonary C-fiber sensitivity in Brown-Norway rats sensitized with ovalbumin (Ova). In anesthetized rats, isocapnic hyperventilation of HWA for 3 min rapidly elevated airway temperature to a steady state of 41.7°C. Immediately after the HWA challenge, the baseline fiber activity (FA) of pulmonary C-fibers was markedly elevated in sensitized rats, but not in control rats. Furthermore, the response of pulmonary C-fibers to right atrial injection of capsaicin in sensitized rats was significantly higher than control rats before the HWA challenge, and the response to capsaicin was further amplified after HWA in sensitized rats (ΔFA = 4.51 ± 1.02 imp/s before, and 9.26 ± 1.74 imp/s after the HWA challenge). A similar pattern of the HWA-induced potentiation of the FA response to phenylbiguanide, another chemical stimulant of C-fibers, was also found in sensitized rats. These results clearly demonstrated that increasing airway temperature significantly elevated both the baseline activity and responses to chemical stimuli of pulmonary C-fibers in Ova-sensitized rats. In conclusion, this study supports the hypothesis that the increased excitability of these afferents may have contributed to the cough and reflex bronchoconstriction evoked by hyperventilation of HWA in patients with asthma.