Nerve growth factor enhances cough and airway obstruction via TrkA receptor- and TRPV1-dependent mechanisms.

Background: Nerve growth factor (NGF) is an important mediator of airway hyper-responsiveness and hyperalgesia but its role in cough is unknown.
Objectives: In this study the effects of NGF on the cough reflex and airway calibre were investigated in guinea pigs. The involvement of the tropomyosin-related kinase A (TrkA) receptor and transient receptor potential vanilloid-1 (TRPV1), and the p38 mitogen-activated protein kinase (MAPK)-dependent pathway in any NGF-induced effects on cough and airway obstruction was also assessed.
Methods: Guinea pigs were placed in a transparent whole-body plethysmograph box. Cough was assessed visually, acoustically and by analysis of the airflow signal. Airway obstruction was measured using enhanced pause (Penh) as an index.
Results: Exposure of guinea pigs to NGF did not induce a cough response nor a significant airway obstruction. However, exposure of guinea pigs to NGF immediately before citric acid inhalation resulted in a significant increase in the citric acid-induced cough and airway obstruction compared with vehicle-treated animals. Pretreatment with the TrkA receptor antagonist, K252a, or the TRPV1 antagonist, iodoresiniferatoxin, significantly inhibited the NGF-enhanced cough and airway obstruction. Exposure to NGF also increased p38 MAPK phosphorylation, but pretreatment with the p38 MAPK inhibitor, SB203580, did not affect either the NGF-enhanced cough or airway obstruction despite preventing the NGF-induced elevation in p38 MAPK phosphorylation.
Conclusions: The data show that NGF can enhance both cough and airway obstruction via a mechanism that involves the activation of the TrkA receptor and TRPV1 but not the p38 MAPK-dependent pathway.