Color vision, which differentiates spectral compositions independent of brightness, provides animals, from insects to primates, great power for object recognition and memory registration and retrieval. Using a combination of Drosophila genetic, histological, electrophysiological and behavioral approaches, we study how the visual system processes chromatic information to generate color percept. Previous vertebrate studies suggest that the color opponent neurons, which exhibit combination-sensitive excitatory and/or inhibitory interaction between two or more photoreceptor types, subserve color vision. Our anatomical study suggests that the Drosophila color-vision circuit in the medulla likely contains such color opponent neurons. We hypothesize that chromatic information is processed in two discrete stages in Drosophila. First, the transmedulla (Tm) neurons serve as color opponent neurons by combining multiple photoreceptor channels and relaying the information to the higher visual center, the lobula. Second, chromatic information is spatially integrated in the lobula, to subserve higher-order color-vision functions, such as color contrast and constancy. Using both light and electron microscope, we determined the synaptic circuits of the photoreceptors and their synaptic target neurons in the medulla. The chromatic photoreceptors, R7 and R8, provide inputs to a subset of first-order interneurons, which might serve as color opponent neurons. The first-order interneurons Tm5a/b/c receive direct synaptic inputs from R7 while Tm9, Tm20 and Tm5c receive inputs from R8. In addition, these Tm neurons receive indirect inputs from R1-6 via L3. These Tm neurons relay spectral information from the medulla to various lobula layers. In addition, the amacrine neuron Dm8 receives input from multiple R7s and provides input for Tm5. Functional studies further revealed that the amacrine Dm8 neurons are both required and sufficient for animals'innate spectral preference to UV light while Tm9 neurons are sufficient to drive green phototaxis. To relate neural connectivity to functions, it is critical to determine the components of neural computation machinery expressed in the neurons of interest. In particular, the usage of neurotransmitters and receptors provide crucial information about the polarity and dynamic of signal transmission. We have found that promoter constructs are not reliable reporters and instead developed a single-cell transcript profiling method, which directly probes the expression of neuronal transmitter synthesis enzymes, transporters and receptors. This RT-PCR based method is capable of assaying maximally 20 genes from one single GFP-labeled neuron and the detection sensitivity is 3-10 transcript copies. As a proof of principle, we have determined the expression of all known acetylcholine receptors in three cholinergic neurons, L2, L4, and Tm2, which interconnect with one another and mediate motion detection. We found that these neurons express type-invariant subsets of nicotinic receptors. With color-vision circuit, we have determined that L3, Tm9, Tm5c, and Dm8 neurons are glutamatergic while Tm20 is cholinergic. As a proof of principle, we have determined that the Dm8 neurons expressed the vesicular glutamate transporter (vGlut) and that knocking down vGlut in the Dm8 neurons lead to aberrant green preference, phenocopying shi(ts)-mediated inactivation of Dm8. We are now applying this approach to different Tm neurons, and determine their requirement of transmitter and receptors. This information will provide critical information for determining the logic of information transmission and color computation.
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