Neurotrophins in chronic cough: association with asthma but not with cough severity

Chronic cough is associated with an enhanced excitability of airway cough receptors, possibly due to action of neurotrophins. The present study aimed to compare the neurotrophin levels between healthy subjects and patients with chronic cough and to analyze the factors associated with these levels.

Serum and sputum levels of nerve growth factor (NGF), serum levels of brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were analyzed by enzyme immunoassay in 19 healthy subjects and 47 patients with chronic cough. In addition, cough sensitivity to hypertonic saline was assessed, cough diary was kept, Leicester Cough Questionnaire was filled in, peak flow was monitored and spirometry, skin prick tests, exhaled nitric oxide measurement and histamine challenge were performed.

The NGF levels did not differ between the healthy subjects and the patients with chronic cough and were not associated with any index describing cough severity. However, these levels in both serum (P = 0.01) and sputum (P = 0.025) samples were associated with asthma. There was a statistically significant association between serum and sputum NGF levels (R = 0.45, P = 0.026). The serum BDNF levels did not differ between the groups and were not associated with any of the background characteristics. The serum NT-3 levels were below the detection limit in most subjects and therefore these data were not analyzed.

Neither chronic cough nor its severity is associated with abnormal neurotrophin levels. High NGF levels among some patients with chronic cough may indicate a presence of asthma.