Five taste sensations and only three types of taste cells form the world of tastes – or so we thought. A new discovery indicates that there could be more complexity to this equation than we first thought.
Researchers working with rodents have identified a new population of taste buds, and they appear to have a greater appreciation for taste than previously thought possible. Instead of being selective like other taste cells, this ‘broad responsive’ cell can respond to any known taste except salt.
Taste cells are receptors that group together to form taste buds, shaped like small garlic bulbs, on the tongue. Some taste buds can also be found on the soft palate (the roof of a person’s mouth), and on the back of the throat.
Taste cells can be messy, with good reason. If they feel one or bothersome or potentially toxic taste, our taste buds tell us to swallow and release a mouthful of food. But taste something sweet, and they will have us to ask for more.
You probably know that five flavors have been portrayed so far: sour, salty, bitter, sweet and sour. But fat can be another, and possibly stiffness. Just as people argue about whether coriander is a godsend as a taste of soap, scientists continue to debate and investigate the different types of flavors and taste cells.
Type I taste buds represent about half the number of cells in taste buds. They can react with saccharides and amino acids to produce sweet and umami-flavored sensations (umami is the pleasant hearty taste associated with the amino acids in proteins and the food additive MSG). But their main task is to act as support for other nearby nerve cells.
Type II cells can detect bitter, sweet or umami-flavored sensations, while type III cells are equally selective but make up less than one-fifth of taste cells in taste buds. They can detect sour or salty flavors.
Now, scientists at Buffalo University in New York have discovered and characterized a previously unknown subset of type III cells, isolated from transgenic mice.
These newly found cells are also sensitive to acid, but lack their usual ability to detect salt. Instead, these broad-responsive III cells use a unique signaling pathway to detect bitter, umami, and sweet tastes as well.
“Our study describes a new population of taste cells that can detect multiple types of stimuli, including chemicals from different taste qualities,” the authors explained in their paper.
(Jhanna Flora and Kathryn Medler)
The versatility of this new population of taste buds reflects nerve cells in the brain, where both generalist and more specialized neurons are used to process incoming taste information.
Although not every cell tested could be stimulated by all tastes in the experiments, 100 percent of the ‘broad responsive’ (BR) cells responded to more than one taste quality, and about 80 percent discovered three or four types of taste.
“By making broad-matched cells, the taste system could better distinguish between chemicals with similar characteristics,” said the authors of their findings, which should apply more broadly to other mammals sharing the same taste systems.
After identifying and characterizing the new taste cells, the following experiments were set up to see exactly how much taste is actually affected. It was about testing taste-evoking behaviors of mice designed to live without a significant part of the signaling pathway used by BR cells.
Both transgenic and wild-type mice were presented with drinking tubes containing fluids containing either MSG, to stimulate umami sensations, a sweeter, bitter compound called denatonium, or salt. An apt-named instrument called a lickometer measures its response to each solution, compared to water.
The transgenic mice drank the sweet, bitter, and umami liquids just like water; for them, there was no difference in taste until they reached the highest concentrations for face-crossing ranges. Without the signaling pathway of BR cells, the mice would have lost their sensation of these flavors.
“Even though the BR cells are only a subset of cells in the bud, our data suggest that these cells make a significant contribution to taste,” the authors concluded.
Scientists had suspected its presence for some time after previous research found that many receptor cells, sampled as intact taste buds of mice, responded to multiple taste qualities. But until now, no one had isolated and identified the responsible taste cells with multiple tasks.
Again, we have mice to thank for this latest discovery. Now, to design a lickometer for people …
The research is published in PLOS Genetics.
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