Lining the lower airways are 'brush' cells that can sense bitter molecules and send signals to regulate breathing, perhaps providing a defence against bacteria, which produce bitter compounds.

Gabriela Krasteva at Justus Liebig University in Giessen, Germany, and her colleagues found that brush cells in the tracheae of mice express protein receptors and other key molecules involved in bitter-taste signalling. The cells make contact with vagal nerve fibres, which connect key visceral organs to the brain, and send signals to the nerves by releasing the neurotransmitter acetylcholine. When the researchers administered a bitter substance directly to the airway surfaces of mice, the animals' respiratory rate dropped suddenly.

Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1019418108 (2011)