NEW DELHI : Most people probably think that we perceive the five basic tastes — sweet, sour, salty, bitter and umami (savory)–with our tongue, which then sends signals to our brain ‘telling’ us what we’ve tasted. However, scientists have turned this idea on its head, demonstrating in mice the ability to change the way something tastes by manipulating groups of cells in the brain. The findings were published in the online edition of Nature.
“Taste, the way you and I think of it, is ultimately in the brain,” said study leader Charles S Zuker, PhD, professor of biochemistry and molecular biophysics and of neuroscience of Columbia University Medical Center (CUMC). “Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it’s the brain that affords meaning to these chemicals.”
Over the past decade or so, Dr Zuker and his colleagues proved that there are dedicated receptors for each taste on the tongue, and that each class of receptor sends a specific signal to the brain. More recently, they demonstrated that each taste is sensed by unique sets of brain cells, located in separate locations in the brain’s cortex -generating a map of taste qualities in the brain.
The scientists used optogenetics, which allowed them to directly activate specific neurons with laser light. Yueqing Peng, a postdoctoral associate in Dr Zuker’s lab, examined whether manipulating the neurons in these brain regions could evoke the perception of sweet or bitter, without the mouse actually tasting either.
When scientists injected a substance into the mice to silence the sweet neurons, the animals could not reliably identify sweet. They could, however, still detect bitter. The animals regained their ability to taste sweet when the drug was flushed from the brain. Conversely, silencing the bitter neurons prevented the mice from recognising bitter, but they could still taste sweet.
Remarkably, the researchers were also able to make the animals think they were tasting bitter or sweet, even when the animal was only drinking water. When the researchers activated the sweet neurons during drinking, they observed behavioral responses in the mice associated with sweet, such as impressively increased licking. In contrast, stimulating bitter neurons dramatically suppressed licking, and elicited classic taste-rejection responses, including the activation of gagging behaviour.
These results showed that by manipulating the brain centres representing sweet and bitter taste they could directly control an animal’s sensory perception and behavioral actions, says Peng. (UNI)
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