Distributed Representation of Taste Quality by Second-Order Gustatory Neurons inDrosophila

SUMMARY Sweet and bitter compounds excite different sensory cells and drive opposing behaviors. It is commonly thought that the neural circuits linking taste sensation to behavior conform to a labeled-line architecture, but in Drosophila , evidence for labeled lines beyond first-order neurons is lacking. To address this, we devised trans- Tango(activity), a strategy for calcium imaging of second-order gustatory projection neurons based on trans -Tango, a genetic transsynaptic tracing technique. We found distinct projection neuron populations that respond to sweet and bitter tastants. However, the bitter-responsive population was also activated by water alone. We further discovered that bitter tastants evoke activity upon both stimulus onset and offset. Bitter offset responses are exhibited by both first- and second-order gustatory neurons, but these responses are distributed among multiple types of projection neurons in the second order. These findings suggest a more complex coding scheme for gustatory information than can be explained by a labeled line model.