Taste bud: image via Wikipedia
Each taste bud on human or animal tongues contain around 50 receptor cells. Each of the receptor cells then reacts to one of the 5 primary tastes – sour and salty are detected with ion channels while sweet, bitter and umami (savoury) are detected by G protein coupled receptors. There is some debate as to whether there is a sixth primary taste that distinguishes fat content. At one time it was thought that different parts of the tongue responded to different tastes but it is now clear that all tastes are detected by all parts of the tongue.
New research has studied the mechanism by which sour is detected.
Rui B. Chang, Hang Waters, Emily R. Liman: A proton current drives action potentials in genetically identified sour taste cells. Proceedings of the National Academy of Sciences, 2010; DOI:10.1073/pnas.1013664107
Neurobiology researchers at the University of Southern California have made a surprising discovery about how some cells respond to sour tastes.
Sour is the sensation evoked by substances that are acidic, such as lemons and pickles. The more acidic the substance, the more sour the taste. Acids release protons. How protons activate the taste system had not been understood. The USC team expected to find protons from acids binding to the outside of the cell and opening a pore in the membrane that would allow sodium to enter the cell. Sodium’s entry would send an electrical response to the brain, announcing the sensation that we perceive as sour.
Instead, the researchers found that the protons were entering the cell and causing the electrical response directly.
The finding is to be published in the Proceedings of the National Academy of Sciences (PNAS). “In order to understand how sour works, we need to understand how the cells that are responsive to sour detect the protons,” said senior author Emily Liman, associate professor of neurobiology in the USC College of Letters, Arts and Sciences.
“In the past, it’s been difficult to address this question because the taste buds on the tongue are heterogeneous. Among the 50 or so cells in each taste bud there are cells responding to each of the five tastes. But if we want to know how sour works, we need to measure activity specifically in the sour sensitive taste cells and determine what is special about them that allows them to respond to protons.”
Liman and her team bred genetically modified mice and marked their sour cells with a yellow florescent protein. Then they recorded the electrical responses from just those cells to protons. The ability to sense protons with a mechanism that does not rely on sodium has important implications for how different tastes interact, Liman speculates. “This mechanism is very appropriate for the taste system because we can eat something that has a lot of protons and not much sodium or other ions, and the taste system will still be able to detect sour,” she said. “It makes sense that nature would have built a taste cell like this, so as not to confuse salty with sour.”
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