Why do some people who choke down coffee dislike its taste immensely, while others don't notice the bitterness of the jolt that starts their morning? The answer may depend on the presence or lack of certain proteins in our taste buds.
A team of researchers at the Johns Hopkins University School of Medicine, using simple taste-testing methods, recently found that genetically mutated fruit flies that were missing an essential taste receptor protein were able to consume caffeine-laden sugar without noticing its taste. The study was published in Current Biology in the Sept. 19 issue.
The mutant flies lacked a receptor protein that normally allows them to discern caffeinated substances that have a bitter taste from a normal sugar mixture. In general, flies tend to avoid bitter substances. But flies missing the specific protein, known as Gr66a, lack the proper mechanisms to detect the bitterness of caffeine and respond to it.
In the study, Craig Montell, Ph.D., and his team at the Institute of Basic Biomedical Sciences at Hopkins starved 50 fruit flies overnight and then exposed them to two mixtures: a blue-colored mix of sugar and agarose and a red-colored mix of sugar, agarose and caffeine. The flies were allowed to eat as much as they liked for 90 minutes, after which they were turned upside down so their bellies could be studied.
By color-coding the mixtures and studying the color of the flies' abdomens, the researchers were able to determine whether the flies showed a preference for eating the caffeinated or decaffeinated sugar substances. A blue stomach indicated an inclination towards plain sugar, a red stomach indicated consumption of the bitter caffeine, and a purple stomach was considered neutral, with no clear preference.
The mutated flies that lacked the Gr66a receptor protein showed no overall preference for either solution, while the control group of flies with fully functional taste receptors showed a strong preference for the red solution of plain sugar. This data allowed Montell and his team to conclude that the Gr66a protein is essential in taste differentiation and avoidance of bitter foods.
Further research was conducted as to whether the receptor protein worked through the taste bristles, the fruit fly equivalent of a tongue, or through a caffeine-initiated signal that moved through the central brain processing of the fly. For most organisms, certain stimulants interact with taste receptor proteins and cause them to signal the brain.
Through the use of sensitive tips on the taste bristles known to contain the Gr66a protein, the researchers measured action potential frequency, or the rate of electrical currents being fired from the taste bristles to the brain, when the fruit flies were exposed to either sweet or bitter substances.
For both the mutant and normal flies, there was an expected increase in electrical "spikes," indicating neuronal activity, when plain sugar was applied to the taste bristles. When other bitter compounds, such as quinine, were tested, a similar response was provoked in both sets of flies.
The mutant flies were incapable of producing any such response, demonstrating that the Gr66a protein acts specifically in the pathway of caffeine response, and not just as a general promoter of the reaction to bitter complexes.
Drosophila are used in many scientific experiments due to their ability to be genetically manipulated quite easily, their short life cycles and fast growth rates, and the fact that a great deal is known about their genome, anatomy and physiology.
With this mutant fly, Montell and his colleagues will be able to better determine the molecular pathway of the caffeine response, which will eventually allow further research on the effects of caffeine on behavior in mammals. So for those of you who need a kick-start in the morning but can't stand the taste of coffee, there may be hope for you yet.
