Reflex bradypnea elicited by cigarette smoke inhaled through an isolated larynx

Menthol suppresses laryngeal C-fiber hypersensitivity to cigarette smoke in a rat model of gastroesophageal reflux disease: the role of TRPM8

Patients with gastroesophageal reflux disease (GERD) display enhanced laryngeal reflex reactivity to stimuli that may be due to sensitization of the laryngeal C-fibers by acid and pepsin. Menthol, a ligand of transient receptor potential melastatin-8 (TRPM8), relieves throat irritation. However, the possibility that GERD induces laryngeal C-fiber hypersensitivity to cigarette smoke (CS) and that menthol suppresses this event has not been investigated. We delivered CS into functionally isolated larynxes of 160 anesthetized rats. Laryngeal pH 5-pepsin treatment, but not pH 5-denatured pepsin, augmented the apneic response to CS, which was blocked by denervation or perineural capsaicin treatment (a procedure that blocks the conduction of C fibers) of the superior laryngeal nerves. This augmented apnea was partially attenuated by capsazepine [an transient receptor potential vanilloid 1 (TRPV1) antagonist], SB-366791 (a TRPV1 antagonist), and HC030031 [a transient receptor potential ankyrin 1 (TRPA1) antagonist] and was completely prevented by a combination of TRPV1 and TRPA1 antagonists. Local application of menthol significantly suppressed the augmented apnea and this effect was reversed by pretreatment with AMTB (a TRPM8 antagonist). Our electrophysiological studies consistently revealed that laryngeal pH 5-pepsin treatment increased the sensitivity of laryngeal C-fibers to CS. Likewise, menthol suppressed this laryngeal C-fiber hypersensitivity and its effect could be reversed by pretreatment with AMTB. Our results suggest that laryngeal pH 5-pepsin treatment increases sensitivity to CS of both TRPV1 and TRPA1, which are presumably located at the terminals of laryngeal C-fibers. This sensory sensitization leads to enhanced laryngeal reflex reactivity and augmentation of the laryngeal C-fiber responses to CS, which can be suppressed by menthol acting via TRPM8.

Canine models of asthma and COPD

Dogs have been extensively used to model the important components of asthma and COPD. Many of the key features of human asthma such as reversible airflow obstruction, pulmonary inflammation, airway hyperresponsiveness and cough are demonstrated in dogs after provocation with antigen, following a period of hyperventilation with dry air or after inhalation of ozone. Furthermore, standard anti-asthma drugs such as beta-adrenergic agonists, corticosteroids and leukotriene inhibitors are effective in these models. The pathology and pathophysiology of chronic bronchitis and emphysema can also be demonstrated in dogs after exposure to cigarette smoke, following inhalation of sulfur dioxide and by intra-tracheal or aerosol administration of proteolytic enzymes such as papain. These canine models of COPD have been used to evaluate a variety of new methodologies and treatments before they are tested in humans. This review highlights some of the important features of these canine models and how they have increased our understanding of the pathology, pathophysiology and control of human asthma and COPD.

Reflex apneic response evoked by laryngeal exposure to wood smoke in rats: neural and chemical mechanisms

We investigated the neural and chemical mechanisms contributing to the immediate ventilatory responses to laryngeal exposure to wood smoke in anesthetized Sprague-Dawley rats. Five milliliters of wood smoke were delivered into a functionally isolated larynx at a constant flow rate of 1.4 ml/s while the animals breathed spontaneously. Within 1 s after exposure, laryngeal wood smoke consistently triggered an apnea in each of the 42 rats tested. The apneic duration reached 1,636.4 +/- 105.4 (SE) % (n = 42) of the baseline expiratory duration. This apneic response was not affected by denervation of recurrent laryngeal nerves (n = 6) or by removal of smoke particulates (n = 14), but it was totally eliminated by topical application of an anesthetic (n = 8; lidocaine hydrochloride, 8%) to the laryngeal mucosa or by sectioning of the superior laryngeal nerves (n = 42). Furthermore, laryngeal application of a hydroxyl radical scavenger (dimethylthiourea; 500 mg/ml; n = 8) greatly diminished or abolished the smoke-induced apneic response, but it did not affect the apneic response evoked by laryngeal exposure to air saturated with 6% ammonia. These results suggest that the immediate apneic response to laryngeal wood smoke is a reflex resulting from the stimulation of the superior laryngeal afferents by the gas phase of wood smoke and that the stimulation is mediated through a hydroxyl radical mechanism.

Airway protective reflexes elicited by laryngeal ammonia: role of C-fiber afferents

Intralaryngeal ammonia induces an airway protective reflex in rats characterized by apnea, bradycardia and increased laryngeal resistance. Since ammonia is known to stimulate both the myelinated and the nonmyelinated afferents, the role of nonmyelinated afferents in eliciting the above response is unclear. The present study was designed to investigate this by utilizing two techniques that selectively block C-fiber afferents: intravenous ruthenium red and perineural application of capsaicin. Ammonia vapor was introduced into the functionally isolated larynx of anesthetized rats for 10 sec at a flow rate of 5 ml/sec. Changes in expiratory duration were expressed as the ratio between test and control expiratory time (TEmax/control). Perineural application of capsaicin (n = 8) to SLNs attenuated the responses of both intralaryngeal ammonia (TEmax/control: 12.32 +/- 1.67 to 6.89 +/- 1.62; P < 0.05) and capsaicin (8.86 +/- 2.02 to 3.35 +/- 2.17; P < 0.05). Pretreatment with ruthenium red (1 mg/kg, iv., n = 10) significantly reduced the effects of ammonia (20.43 +/- 4.38 vs. 11.66 +/- 3.21; P < 0.05) and nebulized capsaicin (16.28 +/- 4.58 to 3.90 +/- 0.83; P < 0.05). These results suggest that the C-fiber endings of the SLN play an important role in eliciting the airway protective reflexes by irritants such as ammonia.

Carbon dioxide-responsive laryngeal receptors in the dog

The purpose of this study was to relate the carbon dioxide (CO2) response of laryngeal receptors to their behavior during the breathing cycle (i.e. their response to transmural pressure changes, laryngeal movement or decreases in temperature) or during exposure to irritant stimuli (water or cigarette smoke). In 9 anesthetized mongrel dogs breathing spontaneously through a tracheostomy, unit activity from the superior laryngeal nerve was recorded while warmed and humidified gas mixtures (air or 10% CO2 in O2) were passed, for 1 min, through the functionally isolated upper airway in the expiratory direction. None of the 10 cold receptors studied were affected by CO2. Eleven of 20 laryngeal non-modulated mechano-receptors were stimulated (from 0.3 to 1.6 imp/sec) by exposure to CO2. These CO2-responsive receptors were also stimulated by known irritant stimuli (cigarette smoke, water), although not all receptors which responded to these irritants were stimulated by CO2. Twelve of 33 respiratory-modulated receptors were affected by CO2; 4 were stimulated and 8 inhibited. Receptors inhibited by CO2 were also inhibited by negative pressure while receptors stimulated by CO2 were also stimulated by negative pressure. These results show that CO2-responsive laryngeal receptors are not specialized endings. Although it is not clear to what extent each separate group of laryngeal receptors is involved, each may contribute to the reflex bradypnea which has been observed during exposure of the upper airway to elevated levels of CO2. However, the importance of CO2-responsive laryngeal receptors in physiological conditions remains unclear.