New York: If you think that mouthwatering “umami” taste comes to you via your tongue which sends signals to the brain “telling” you what we have tasted, read on.
Turning this idea on its head, scientists have turned tastes on and off by activating and silencing clusters of specific brain cells in mice.
“Taste, the way you and I think of it, is ultimately in the brain,” said study leader Charles S Zuker, professor of biochemistry and molecular biophysics and of neuroscience from the Columbia University Medical Centre (CUMC).
“Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it is the brain that accords meaning to these chemicals,” he noted.
Most people think that we perceive the five basic tastes — sweet, sour, salty, bitter and umami (savoury) – with our tongue.
To look deeper, scientists used optogenetics, which allowed them to directly activate specific neurons with laser light.
Yueqing Peng, post-doctoral associate in 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.
“In this study, we wanted to know if specific regions in the brain really represent sweet and bitter. If they do, silencing these regions would prevent the animal from tasting sweet or bitter, no matter how much we gave them,” Peng said.
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 behavioural 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.
“The results showed that by manipulating the brain centres representing sweet and bitter taste they could directly control an animal’s sensory perception and behavioural actions,” Peng pointed out.
The researchers also performed optogenetic tests on animals that had never tasted sweet or bitter chemicals.
The results showed that activation of the corresponding neurons triggered the appropriate behavioural response.
“These experiments prove that the sense of taste is completely hardwired, independent of learning or experience which is different from the olfactory system,” Zuker concluded in a paper published in the prestigious journal Nature.