Instantly reacting to the sweet lure of chocolate or the bitter taste of strychnine would seem to demand that such behavioral responses be so innate as to be hard-wired into the brain. Indeed, in studies with the easily manipulable fruit fly Drosophila, Kristin Scott and colleagues reported in the January 19, 2006, issue of Neuron experiments demonstrating just such a hard-wired circuitry.

Their findings, they said, favor a model for taste encoding in the brain that holds that specific cells are dedicated to detecting specific tastes. Competing models hold that multiple neurons combine information to encode taste, or that the timing of patterns of taste information encodes taste.

In their studies, the researchers explored the behavioral effects of activating fly taste neurons that had either of two chemical taste receptors on their surface. In earlier studies, the researchers had shown that the Gr5a receptor on taste neurons was essential for response to sugar and that the Gr66a receptor was essential for response to bitter tastes. However, those studies left open the question of whether those different neurons selectively detected the different tastes and whether they generated taste behaviors.

To directly monitor taste responses of the flies, the researchers generated flies with fluorescent labels on their neurons that would signal activation of one or the other type. They used microscopic imaging through tiny windows in the fly brains to watch neuronal response when they exposed the flies to sweet or bitter chemicals

They found that a whole range of sweet substances selectively switched-on the Gr5a neurons, while a range of bitter substances switched-on the Gr66a neurons. However, the "sweet neurons" did not respond to bitter substances, and vice versa.

In behavioral studies, they found that flies preferred to spend time tasting substances that activated the Gr5a neurons and avoided substances that activated Gr66a neurons.
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Contact: Heidi Hardman
hhardman@cell.com
617-397-2879
Cell Press
18-Jan-2006

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