Fruit Flies Have Something To Tell You About Caffeine

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Fruit Flies Have Something To Tell You About Caffeine

http://www.medicalnewstoday.com/medicalnews.php?newsid=52224

In their hunt for genes and proteins that explain how animals discern

bitter from sweet, a team of s Hopkins researchers began by

testing whether mutant fruit flies prefer eating sugar over sugar

laced with caffeine. Using a simple behavioral test, the researchers

discovered that a single protein missing from the fly-equivalent of

our taste buds caused them to ignore caffeine's taste and consume the

caffeine as if it were not there.

" No, you won't see jittery Drosophila flitting past your bananas to

slurp your morning java anytime soon, " says Craig Montell, Ph.D., a

professor of biological chemistry in the Institute of Basic

Biomedical Sciences at Hopkins. " The bottom line is that our mutant

flies willingly drink caffeine-laced liquids and foods because they

can't taste its bitterness -- their taste receptor cells don't detect

it. "

The Hopkins flies, genetically mutated to lack a certain taste

receptor protein, have been the focus of studies to sort out how

animals taste and why we like the taste of some things but are turned

off by the taste of others.

By color-coding sweet and bitter substances eaten by fruit flies and

examining the coloring that shows up in their translucent bellies,

the Hopkins team hoped to learn whether flies missing a

specific " taste-receptor " protein changed their taste preferences.

" Normally, " Montell explains, " when given the choice between sweet

and bitter substances, flies avoid caffeine and other bitter-tasting

chemicals. But flies missing this particular taste-receptor protein,

called Gr66a, consume caffeine because their taste-receptor cells

don't fire in response to it. "

The discovery, which is the first ever example of a protein required

for both caffeine tasting and caffeine-induced behavior, was

published Sept. 19 in Current Biology.

For the study, Montell and his colleagues kept 50 fruit flies away

from food overnight and for breakfast gave the starved flies 90

minutes to eat as much as they wanted of either or both of two

concoctions: a blue-colored mixture of sugar and agar and a red-

colored mixture of caffeine, sugar and agar. The researchers then

flipped the flies onto their backs and looked at the color of their

bellies to see what they ate - blue indicating a preference for

eating sugar, red indicating a preference for bitter caffeine, and

purple indicating no preference.

Flies missing the critical taste receptor protein Gr66a consumed the

bitter caffeine solution to the same extent as the sugar-only

solution. Montell and colleagues conclude that Gr66a is crucial for

the normal caffeine avoidance behavior and without it, flies are

seemingly indifferent to the bitter taste.

The researchers went on to examine whether this indifference to

bitter was due to the taste nerves on the fly's " tongue " or some

malfunction in the fly's brain. Chemical stimulants trigger taste

receptor cells to send an electrical current to the brain where the

information is processed and often leads to a change in behavior,

such as the decision to eat or avoid.

With fine tools, the research team recorded electrical currents in

those cells known to contain the Gr66a caffeine taste receptor in the

fly's equivalent of the taste buds - dubbed the taste bristles.

Applying sugar to the taste bristles of normal flies, or to mutant

flies missing the Gr66a protein, causes the neurons to produce

electrical current " spikes " at a frequency of about 20 spikes per

second. Other bitter compounds like quinine generated electrical

current spikes at about the same frequency in the mutants.

Only flies missing the Gr66a taste receptor protein were unable to

generate any current spikes when given caffeine. " This is a clear

demonstration that Gr66a is functioning in the taste receptor cells

and is not a 'general sensor' for bitter compounds, but is required

more specifically for the caffeine response, " says Montell.

" This indicates that flies have different receptors for the response

to other types of bitter compounds, " he says.

" We also tested whether the flies avoided the related bitter

compounds found in tea and cocoa -- chocolate -- and found that Gr66a

also is required for the response to the compound in tea, but not for

the one in chocolate, " he says.

Fruit flies often are used as experimental organisms because they

grow quickly and are easy to manipulate genetically. Now that Montell

and his colleagues have a mutant fly that is unable to taste

caffeine, they hope to further examine the other genes and molecules

involved in the caffeine response and better understand the

biochemistry behind caffeine-induced behavior in other organisms,

namely humans.

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The researchers were funded by the Polycystic Kidney Disease

Foundation and the National Institute of Deafness and Communicative

Disorders of the National Institutes of Health.

Authors on this paper: Seok Jun Moon, Kottgen, Yuchen Jiao,

Hong Xu and Montell, all of Hopkins.

On the Web:

http://www.hopkinsmedicine.org/cmontell/

http://www.hopkinsmedicine.org/ibbs/index.html