Mechanism - how environmental irritants like cigarette smoke cause a person to cough..

November 26, 2009 - Scientists have revealed how environmental irritants such as air pollution and cigarette smoke cause people to cough (tussive). They have identified the reaction inside the lungs that can trigger coughing when a person is exposed to particular irritants in the air.

Coughing is the most common reason for people visiting a family doctor. This new study indicates, for the first time, how coughing can be triggered when a person is exposed to certain irritants in the air. It shows that the irritants can switch on receptor proteins called TRPA1 (TRPA1 is a member of the transient receptor potential (TRP) channel family present in sensory neurons) on the surface of nerve endings in the lungs. This switches on sensory nerves, which then trigger a cough reflex. The researchers say coughing could potentially be treated by blocking TRPA1 receptors, to stop irritants in the air from setting off this chain reaction.

Professor Maria Belvisi, corresponding author of the study from the National Heart and Lung Institute at Imperial College London, said: "Now that we think we have cracked the mechanism, we can start investigating whether we can stop people from coughing excessively by blocking the receptor protein that triggers it."

PAPER: TRPA1 Agonists Evoke Coughing in Guinea Pig and Human Volunteers
Mark A. Birrell1,2, Maria G. Belvisi1,2, Megan Grace1,2, Laura Sadofsky3, Shoaib Faruqi3, David J. Hele1,2, Sarah A. Maher1,2, Véronique Freund-Michel1 and Alyn H. Morice3 American Journal of Respiratory and Critical Care Medicine Vol 180. pp. 1042-1047, (2009) Abstract..

To reach their conclusions, the researchers first looked at sensory nerves from mice, guinea pigs and humans, and showed that the receptors on the sensory nerves were activated by a number of irritants, including a key compound in cigarette smoke (acrolein) and a chemical called cinnamaldehyde. The researchers then blocked the receptors and showed that these substances no longer activated the nerves.

To establish whether activating the receptor causes coughing, the researchers looked at the effect of acrolein on guinea pigs, as they have a coughing reflex. The researchers assessed the guinea pigs' coughing after inhaling acrolein. The compound caused coughing, and the higher the concentration, the more the guinea pigs coughed. The researchers then showed that blocking the receptor using a drug significantly reduced the guinea pigs' coughing response to the compound.

Finally, researchers led by Professor Alyn Morice at the University of Hull looked at the effect of inhaling the chemical cinnamaldehyde in humans. Ten healthy, non-smoking volunteers inhaled the chemical, as well as control substances. The researchers measured their cough response on five occasions, 2-3 days apart. All of the volunteers coughed after inhaling the compound.

The authors suggest that their findings may ultimately lead to the development of new treatments for chronic coughing.

Reference: How Coughing Is Triggered by Environmental Irritants, Science Daily, 11/25/2009.