Sensory neuropeptides are not involved in acetaldehyde-induced bronchoconstriction in guinea-pigs

Perspective on the human cough reflex

This review dissects the complex human cough reflex and suggests hypotheses about the evolutionary basis for the reflex. A mechanosensory-induced cough reflex conveys through branches of myelinated Aδ nerve fibers is not chemically reactive (i.e., capsaicin, bradykinin); possibly, its evolution is to prevent the harmful effects of aspiration of gastric or particulate contents into the lungs. This became necessary as the larynx moves closer to the opening of the esophagus as human ancestors adapt phonation over olfaction beginning less than 10 million years ago. The second type of cough reflex, a chemosensory type, is carried by unmyelinated C fibers. Supposedly, its origin dates back when prehistoric humans began living in close proximity to each other and were at risk for infectious respiratory diseases or irritant-induced lung injury. The mechanism for the latter type of cough is analogous to induced pain after tissue injury; and, it is controlled by the identical transient receptor potential vanilloid cation channel (TRPV1). The airways do not normally manifest nociceptive pain from a stimulus but the only consistent response that capsaicin and lung inflammation provoke in healthy human airways is cough. TRPA1, another excitatory ion channel, has been referred to as the "irritant receptor" and its activation also induces cough. For both types of cough, the motor responses are identical and via coordinated, precisely-timed and sequential respiratory events orchestrated by complex neuromuscular networking of the diaphragm, chest and abdominal respiratory muscles, the glottis and parts of the brain.

A guinea pig model for cough variant asthma and role of tachykinins

Cough variant asthma is known as a major cause of chronic cough. Fundamental features of cough variant asthma are prolonged nonproductive cough responding to bronchodilator therapy, no history of wheezing or dyspnea attack, normal cough sensitivity, and slightly increased bronchial responsiveness. Animal model of cough variant asthma has not been reported. The aim of this study was to establish an animal model for studying detailed pathophysiology of cough variant asthma. Bronchial responsiveness to methacholine and cough reflex sensitivity to capsaicin were measured 72 hours after antigen (ovalbumin, OA) inhalation in actively sensitized guinea pigs. Next, cough number and specific airway resistance (sRaw) were measured during 20 minutes following reinhalation of OA solution, which was carried out 72 hours after the first OA inhalation, and then total cell number and cell differentials in bronchoalveolar lavage fluid (BALE) were measured. Bronchial responsiveness to methacholine, but not cough reflex sensitivity to capsaicin, was significantly increased 72 hours after the first inhalation of OA solution. Number of coughs, sRaw and total cell number in BALF increased significantly by the OA reinhalation, and the cough number and the increase in sRaw were significantly suppressed by beta2 agonist, procaterol. FK224, a specific neurokinin (NK) receptor antagonist, did not significantly influence the OA reinhalation-induced cough and increase in sRaw and total cell number in BALF in this model In conclusion, pathophysiologic feature of this animal model is similar to that of clinical cough variant asthma. Tachykinins may not play an important part in antigen-induced cough associated with bronchoconstriction and airway inflammation in cough variant asthma.