Brain's taste secrets uncovered

By James Gallagher
Health editor, BBC News website

image source, Thinkstock

The brain has specialist neurons for each of the five taste categories - salty, bitter, sour, sweet and umami - US scientists have discovered.

The study, published in the journal Nature, should settle years of debate on how the brain perceives taste.

The Columbia University team showed the separate taste sensors on the tongue had a matching partner in the brain.

The scientists hope the findings could be used to help reverse the loss of taste sensation in the elderly.

It is a myth that you taste sweet only on the tip of the tongue.

Each of the roughly 8,000 taste buds scattered over the tongue is capable of sensing the full suite of tastes.

But specialised cells within the taste bud are tuned to either salty, bitter, sour, sweet or umami tastes.

When they detect the signal, a message is sent to the brain. Although how the brain deals with the information has been up for discussion.

Accounting for taste

A team at Columbia University engineered mice so that their taste neurons would fluoresce when they were activated.

They then trained their endoscopes on the neurons deep at their base of the brain.

The animals were fed chemicals to trigger either a salty, bitter, sour, sweet or umami response on the tongue and the researchers monitored the change in the brain.

They found a "hard wired" connection between tongue and brain.

Prof Charles Zuker told the BBC News website: "The cells were beautifully tuned to discrete individual taste qualities, so you have a very nice match between the nature of the cells in your tongue and the quality they represent [in the brain]."

It scotches the alternative idea that brain cells respond to multiple tastes.

Taste in the animal kingdom

image source, SPL

Pandas evolved from carnivorous bears to eat bamboo - they have lost the ability to taste umami, the savoury taste of meat

Cats, from big wild ones to pets, don't do sweet - they are carnivores so focus on the other tastes

Vampire bats are one of the few species to have reduced umami, sweet and bitter tastes - their diet of blood limits their exposure to other tastes

Elderly benefit

Prof Zuker said: "In the ageing population, they don't enjoy eating anymore, you cannot believe how devastating this is.

"We believe that is a reflection of the taste cells in the tongue."

Stem cells in the tongue produce new taste cells every fortnight. However, this process becomes weaker with age.

"These findings provide an interesting avenue to help deal with this problem because you have a clear understanding of how taste is functioning so you could imagine ways of enhancing that function," Prof Zuker added.

This could include ways of making the existing cells more responsive so they sent a stronger signal to brain.

However, the findings are unlikely to help devise ways to encourage children to eat their greens.

The five tastes are innate rather than learned, and bitter is the signal that something may be toxic.

Children, of course, love the taste of calories, namely sugar, and the only way to change tastes is with time.

"Give a baby sugar and it smiles, give it quinine and it frowns. But give me tonic water and I like it, I like to be on the edge," Prof Zuker said.

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