Olfactory processing in the Drosophila Brain

Rachel I. Wilson (Harvard)

The chemical world has a higher dimensionality than any other class of sensory stimuli, and the olfactory system receives input from an unusually large number of unique information channels. This suggests that aspects of olfactory processing may differ fundamentally from processing in other sensory modalities. The large number of processing channels (~1000 glomeruli) in the rodent olfactory bulb has so far precluded a systematic analysis of olfactory processing in these systems. The fruit fly, by contrast, has only ~40 glomeruli, but the basic circuit architecture of the Drosophila and vertebrate olfactory systems is remarkably similar. Drosophila olfactory neurons are also electrophysiologically accessible in vivo, and can be mapped to identified glomeruli corresponding to specific olfactory receptor genes. We have exploited these advantages in performing a detailed analysis of the input/output function of six representative glomeruli. Using extracellular recording from six types of identified primary receptor neurons in the antennae, we have mapped their receptive fields using a diverse odor panel. Using whole-cell patch-clamp recording, we have also characterized the receptive fields of the six types of second-order neurons directly postsynaptic to each of these receptor neuron types. These experiments reveal a substantial transformation of olfactory representations in the Drosophila brain. Four features are prominent: the odor responses of second-order neurons are (1) amplified, (2) more tuned to stimulus change, (3) more broadly tuned, and (4) less noisy, as compared to primary receptor cells. These results are discussed in terms what we know about the connectivity of second-order neurons to their downstream targets.