Bitter and Sweet: Fruit Flies Reveal More About the Complexity of Food

Fruit flies could tell us a little bit more about the complexity of food. These tiny insects are attracted to sugar, but are deterred when bitter compounds are added. Now, researchers have discovered how sensory input impacts fruit flies' decisions about sweet taste, revealing a little bit more about our own likes and dislikes.

When fruit flies and other animals encounter bitter food, there are two factors that cause them to stop eating. First, bitter compounds bind to proteins called bitter gustatory receptors (GRs), which then inhibit feeding. The second and more elusive factor involves inhibition of the sugar response.

It's this inhibition that really intrigued researchers. Until now, the cellular and molecular mechanisms underlying this phenomenon have been poorly understood. In order to learn a bit more about these mechanisms, the researchers employed two different types of fruit flies: wild-type and mutants missing the OBP49a protein.  This allowed them to demonstrate that bitter compounds suppress feeding behavior by binding to the OBP49a protein.

While the wild-type flies found bitter compounds aversive, the mutant flies did not. Their avoidance behavior was impaired because the bitter compounds did not inhibit the sweet response by binding to OPB49a since it simply wasn't there. However, loss of OBP49a did not affect gustatory behavior or action potentials in sugar.

In fact, the researchers found the function of an odorant-binding protein (OPB) in the gustatory system. These proteins are usually but not exclusively resident in the olfactory system. The scientists, though, found definitive evidence that an OPB not only binds bitter tastants, but also moves and binds to the surface of nearby gustatory receptor neurons (GRNs) that contain sugar-activated GRs.

"We showed that the OBP49a protein was in very close proximity or even touching the sugar GRs" said Craig Montell, one of the researchers, in a news release. "If the bitter compound weren't present, there would be normal sugar activation. We found that decreased behavioral avoidance to a sucrose/aversive mixture in the mutant flies was due to a deficit in the sugar-activated GRNS and not due to effects on GRNs activated by bitter compounds."

The findings reveal that OBP49a promotes the inhibition of sucrose-activated GRNS in fruit flies. This, in turn, could allow researchers to understand a little bit more about how pests can be controlled. In addition, it could aid in preventing insects that spread disease in the future.

The findings are published in the journal Neuron.