In the 1960s, admen told Americans that Mountain Dew would "tickle your innards."
They could have been a bit more specific. What Mountain Dew actually tickles are the same pain sensors in the nose and mouth that detect spicy foods like mustard and horseradish, researchers at USC have learned.
To discover how the body experiences the burning sensation you get when you take a swig of Mountain Dew (or Coca-Cola, or Perrier), USC neurobiology professor Emily Liman and her team cultured nerve cells from the noses and mouths of mice. Pouring carbonated saline solution over the cells, they saw that only one kind of cell reacted to the carbon dioxide in the soda: the mustard-sensitive pain receptors that express the gene TRPA1.
The researchers also poured their soda over nerve cells from mice missing the TRPA1 gene. These responded less strongly to the carbon dioxide. The researchers also poured the carbonated saline over cells that originally were not sensitive to CO2 but which had the TRPA1 gene added. Those did react to the fizzy bubbles.