Transient receptor potential ankyrin receptor 1 is a novel target for pro-tussive agents

Abstract

Background and purpose: The transient receptor potential ankyrin receptor 1 (TRPA1) is a cation channel, co-expressed with the pro-tussive transient receptor potential vanilloid type 1 (TRPV1) channel in primary sensory neurons. TRPA1 is activated by a series of irritant exogenous and endogenous alpha,beta-unsaturated aldehydes which seem to play a role in airway diseases. We investigated whether TRPA1 agonists provoke cough in guinea pigs and whether TRPA1 antagonists inhibit this response.

Experimental approach: Animals were placed in a Perspex box, and cough sounds were recorded and counted by observers unaware of the treatment used.

Key results: Inhalation of two selective TRPA1 agonists, allyl isothiocyanate and cinnamaldehyde, dose-dependently caused cough in control guinea pigs, but not in those with airway sensory nerves desensitized by capsaicin. Coughs elicited by TRPA1 agonists were reduced by non-selective (camphor and gentamicin) and selective (HC-030031) TRPA1 antagonists, whereas they were unaffected by the TRPV1 antagonist, capsazepine. Acrolein and crotonaldehyde, two alpha,beta-unsaturated aldehydes recently identified as TRPA1 stimulants and contained in cigarette smoke, air pollution or produced endogenously by oxidative stress, caused a remarkable tussive effect, a response that was selectively inhibited by HC-030031. Part of the cough response induced by cigarette smoke inhalation was inhibited by HC-030031, suggesting the involvement of TRPA1.

Conclusions and implications: A novel pro-tussive pathway involves the TRPA1 channel, expressed by capsaicin-sensitive airway sensory nerves and is activated by a series of exogenous (cigarette smoke) and endogenous irritants. These results suggest TRPA1 may be a novel target for anti-tussive medicines.

Figure 1

Schematic diagram of the different cough protocols. (A) Dose–response curve with allyl isothiocyanate, cinnamaldehyde or their vehicles. (B) Effect of ruthenium red, gentamicin, (+)camphor, HC-030031 and capsazepine on TRPA1/V1 agonist-induced cough. (C) Effect of ruthenium red, (+)camphor, HC-030031 and capsazepine on cigarette smoke-induced cough. (D) Effect of capsaicin desensitization on TRPA1/V1 agonist-induced cough.

Figure 4

Effect of aerosolized HC-030031 (HC, 0.3 mM) or capsazepine (CPZ, 10 µM) on cough induced by acrolein (10 mM) (A) or crotonaldehyde (10 mM) (B) in guinea pig. Open bars represent the response induced by the vehicle of the tussive stimulus (isotonic saline). VEH indicates results obtained by the stimuli after pretreatment with 6% dimethyl sulphoxide and 3.5% Tween-80. *P < 0.05, analysis of variance and Bonferroni's test versus VEH. Each column is presented as mean ± SE of at least six experiments.

Figure 2

Aerosolized allyl isothiocyanate- (1–30 mM) or cinnamaldehyde- (10–30 mM) induced cough in a concentration-dependent manner. *P < 0.05, analysis of variance and Bonferroni's test versus vehicle (VEH, 5% ethanol and 3.5% Tween-80 in isotonic saline). Each column is presented as mean ± SE of at least six experiments.

Figure 3

Effect of aerosolized ruthenium red (RR, 3 mM), gentamicin (GM, 100 µM), (+)camphor [(+)CAM, 1 mM], HC-030031 (HC, 0.3 mM), capsazepine (CPZ, 10 µM) or desensitization of guinea pig airways by repeated capsaicin administration (CPS des, 30 µM) on (A) allyl isothiocyanate- (10 mM), (B) cinnamaldehyde- (30 mM) or (C) resiniferatoxin-induced cough in guinea pig. Open bars represent the response induced by the vehicle of the tussive stimulus (5% ethanol and 3.5% Tween-80 in isotonic saline). VEH indicates results obtained by the stimuli after pretreatment with 6% dimethyl sulphoxide and 3.5% Tween-80 in isotonic saline. *P < 0.05, analysis of variance and Bonferroni's test versus vehicle (VEH). Each column is presented as mean ± SE of at least six experiments.

Figure 5

Effect of aerosolized ruthenium red (RR, 3 mM), (+)camphor [(+)CAM, 1 mM], HC-030031 (0.3 mM) or capsazepine (CPZ, 10 µM) on cigarette smoke-induced cough in guinea pig. VEH indicates results obtained by cigarette smoke after pretreatment with 6% dimethyl sulphoxide and 3.5% Tween-80. *P < 0.05, analysis of variance and Bonferroni's test versus VEH. Each column is presented as mean ± SE of at least six experiments.