To tackle this question, the researchers engineered transgenic fruit flies that express in distinct nerve cells a special ion channel, "channelrhodopsin-2," whose activity is light-sensitive (the protein is normally found in green algae). As a result of expressing channelrhodopsin-2, neurons could be activated simply by illuminating fruit fly larvae with blue light. This tool allowed the researchers to test whether such an activation of certain neurons can actually substitute for external stimuli--for example, reward or punishment--in an associative-learning experiment. The researchers found that if an odor is presented while a group of dopamine-releasing neurons are experimentally light-activated, the larvae learn to avoid this odor in a later test, despite the fact that no negative stimulus was presented to the larvae along with the odor. Conversely, if an odor is presented while a different group of neurons--releasing octopamine and/or tyramine--are experimentally light-activated, the odor becomes attractive. These findings demonstrate that antagonistic subsets of neurons are responsible for assigning positive or negative values to odor stimuli. It will be of interest to see whether this principal concept of antagonistic neuronal populations mediating positive or negative values during learning holds true for much more complex mammalian brains as well.

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Contact: Heidi Hardman
hhardman@cell.com
617-397-2879
Cell Press
5-Sep-2006