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Campus Currents May 2007: Volume 4, Number 2

When Fizz Fizzles

Soft drinks taste terrible once the fizz falls flat, but it’s not just the tingle on the tongue that’s missing. Neuroscientists working at UC San Diego have found bubbles of carbon dioxide contribute to flavor through the taste buds’ sense of sour.

Charles Zuker’s research group genetically engineered mice to be unable to detect one of the five fundamental tastes—sweet, sour, salty, bittter or umami. Taste nerves fired in response to club soda or ­carbon dioxide gas in all but the ‘sourless’ mice, pointing to sour-sensing cells as the source of soda’s distinctive flavor.

A search through the genes that are active in these cells turned up a likely carbon dioxide sensor: a protein tethered to the taste buds that reacts with the gas. The protein, an enzyme called carbonic anhydrase 4, splits the carbon dioxide from the bubbles into bicarbonate ions and free protons, which trigger a neural response that the team reported in a recent issue of Science.

A drug that inhibits the enzyme also dampened normal mice’s neural response to carbonation, an experience shared by some mountain climbers. Similarly acting drugs meant to counter altitude sickness sadly also dull the sparkle of a celebratory bottle of bubbly champagne at mountain summits.

—Susan Brown