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Researchers identify taste receptor responsible for caffeine detection

September 19, 2006

By studying how taste-receptor mutations impact fruit fly behavior, researchers have identified a taste receptor responsible for the detection of caffeine, a bitter compound known to activate certain taste-receptor neurons, as well as impact various aspects of physiology. The findings are reported by Craig Montell and colleagues at Johns Hopkins University, and appear in the September 19th issue of Current Biology, published by Cell Press.

Many proteins have been identified in mammals and in the genetic model organism Drosophila that belong to a large family of taste receptors, but relatively few of these receptors have been shown to be responsible for the detection of a specific taste compound, or "tastant." One class of tastants for which receptors have not yet been identified is methylxanthines, which include caffeine and related compounds in tea (theophylline) and cocoa (theobromine).

In the new work reported by Montell and colleagues, researchers showed that a specific taste receptor, known as Gr66a, is responsible for the perception of caffeine's bitter taste by fruit flies. The researchers showed that when the gene encoding the Gr66a receptor was disrupted, the mutant flies failed to react normally to the presentation of caffeine, which is a bitter compound that is aversive to flies. Flies with an intact Gr66a receptor gene strongly prefer to consume sucrose (sugar) when given a choice between sucrose and a mixture of sucrose and bitter compounds, but the researchers showed that flies bearing a mutation that disrupted Gr66a failed to show this strong propensity when the bitter mixture contained caffeine. The researchers also showed that activation of the gustatory-receptor neurons that express the Gr66a receptor was impaired in the mutants: Although these gustatory neurons were activated normally by a range of different tastants, they failed to be activated by caffeine, or by the related methylxanthine, theophylline. Another methylxanthine, theobromine, did elicit activation of the neurons, indicating that the Gr66a receptor exhibits some specificity in relaying the detection of different methylxanthine compounds to the nervous system. This observation also solidified the evidence that caffeine detection is functioning through a taste receptor, rather than a nonreceptor signaling route known to be affected by both caffeine and theobromine.

Though the route by which caffeine activates mammalian taste-receptor cells is not known, the new findings suggest that taste receptors may mediate caffeine detection in mammals.

Cell Press

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Common herbicides and fibrates block nutrient-sensing receptor found in gut and pancreas
According to new research from the Monell Center and the Mount Sinai School of Medicine, certain common herbicides and lipid-lowering fibrate drugs act in humans to block T1R3, a nutrient-sensing taste receptor also present in intestine and pancreas.

Variants of 'umami' taste receptor contribute to our individualized flavor worlds
Using a combination of sensory, genetic, and in vitro approaches, researchers from the Monell Center confirm that the T1R1-T1R3 taste receptor plays a role in human umami (amino acid) taste.

Red pandas reveal an unexpected (artificial) sweet tooth
Researchers from the Monell Center report that the red panda is the first non-primate mammal to display a liking for the artificial sweetener aspartame. This unexpected affinity for an artificial sweetener may reflect structural variation in the red panda's sweet taste receptor.

Brain's 'sixth sense' for calories discovered
The brain can sense the calories in food, independent of the taste mechanism, researchers have found in studies with mice.

Accessory protein determines whether pheromones are detected
Pheromones are like the molecules you taste as you chomp on a greasy french fry: big and fatty.

Discovery of 'sugar sensor' in intestine could benefit diabetes
Diabetes patients could benefit from new research at the University of Liverpool that has identified a molecule in the intestine that can 'taste' the sugar content of the diet.

Your gut has taste receptors
Researchers in the Department of Neuroscience at Mount Sinai School of Medicine have identified taste receptors in the human intestines.

Psst! Coffee drinkers: Fruit flies have something to tell you about caffeine
In their hunt for genes and proteins that explain how animals discern bitter from sweet, a team of Johns Hopkins researchers began by testing whether mutant fruit flies prefer eating sugar over sugar laced with caffeine.

Researchers identify the cells and receptor for sensing sour taste
In the last seven years, Howard Hughes Medical Institute researcher Charles S. Zuker and Nicholas J.P. Ryba at the National Institutes of Health have worked together to identify the cells, receptors and signaling mechanisms for three of the five tastes humans can sense - sweet, bitter, and umami (the taste of monosodium glutamate).

Living taste cells produced outside the body
Researchers from the Monell Chemical Senses Center have succeeded in growing mature taste receptor cells outside the body and for the first time have been able to successfully keep the cells alive for a prolonged period of time.

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