Washington: The ability to perceive the five basic tastes - sweet, sour, salty, bitter and umami (savoury) - is hardwired in our brains, according to scientists who were able to switch taste on and off through brain manipulation. "Taste, the way you and I think of it, is ultimately in the brain," said study leader Charles S Zuker, from the Columbia University Medical Centre (CUMC) in US.
"Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it's the brain that affords meaning to these chemicals," said Zuker. The scientists used optogenetics, which allowed them to directly activate specific neurons with laser light. Yueqing Peng, a postdoctoral 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. Sweet and bitter tastes were chosen because they are most critical and recognisable tastes for humans and other animals.
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. 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 researchers also performed optogenetic tests on animals that had never tasted sweet or bitter chemicals, and showed that activation of the corresponding neurons triggered the appropriate behavioural response.
In a final set of experiments, animals were trained to report the identity of an orally applied sweet and bitter stimulus by performing a novel behavioural task, allowing the researchers to test what the animal is tasting. In the experiments, the mice tasted real bitter, sweet and salty chemicals at times, but at other times the researchers used the laser to activate the animals' sweet or bitter cortical fields.
The behaviour of the mice did not differ between the real and virtual tastes, demonstrating that the light is mimicking the perception of bitter and sweet. The study was published in the journal Nature.
