Identification of a new “cough receptor” that responds to irritants

Megan Grace, Mark Birrell, Sarah Maher, David Hele, Laura Sadofsky, Alyn Morice, Maria Belvisi. Imperial College, London, United Kingdom.

Cough is a common respiratory complaint, and can often become excessive when associated with inflammatory airway disease. Environmental irritants such as air pollution and cigarette smoke are thought to evoke cough by stimulating airway sensory nerve fibres, which originate in the vagal sensory ganglia and are carried to the airways via the vagus nerve. It is currently not known how airway sensory nerves are activated by many environmental irritants. We hypothesise that the Transient Receptor Potential Ankyrin 1 (TRPA1) ion channel may have a role as a novel “cough receptor” given that many potential irritants have recently been shown to activate this channel.

The effect of TRPA1-specific agonists and antagonists on guinea pig (male Dunkin Hartley, 350-500g) and mouse (male Wild Type C57bl/6j or receptor deficient Trpa1-/- mice, 18-20g) vagus nerve depolarisation was investigated using an isolated vagus nerve preparation. Guinea pig vagus nerves responded in a concentration-dependent manner to stimulation by TRPA1 agonists acrolein, cinnamaldehyde and allyl-isothiocyanate. TRPA1-specific antagonists HC-030031 (10 μM) and AP-18 (10 μM) inhibited 300 μM acrolein-induced nerve depolarisations by 90 ± 7% and 100%, respectively (n=6, p<0.05). Wild-type mouse vagus also responded in a concentration-dependent manner to acrolein stimulation. However, Trpa1-/- mouse vagus did not depolarise to 1 mM acrolein, while maintaining normal responses to 1 μM capsaicin (0.26 ± 0.06 mV, n=5), a TRPV1 agonist. Furthermore, human vagus nerve (Male donor tissue obtained from transplant surgery) responded to 1 mM allyl-isothiocyanate, cinnamaldehyde and acrolein, thus confirming a functional role for TRPA1 stimulation in both animal and human in vitro models.

In conscious guinea pigs, 10 minutes exposure to aerosolised acrolein induced a dose related increase in coughs. Pre-treatment with HC-030031 (300 mg/kg i.p.) inhibited the tussive response to acrolein exposure (1.0 ± 0.4 coughs) when compared to vehicle (4.7 ± 1.1 coughs; p<0.01).

In summary, TRPA1-specific agonists activate TRPA1 channels in murine, guinea pig and human vagal sensory nerves. The role of the TRPA1 channel was confirmed using specific inhibitors and tissue from TRPA1 gene deleted animals. Finally, a tussive effect of TRPA1 stimulation was confirmed in an in vivo guinea pig model, a response that was successfully blocked by a specific TRPA1 inhibitor. These findings could have major implications for the treatment of cough, which currently presents a significant unmet medical need.