Melanopsin-expressing and intrinsically photosensitive retinal ganglion cells (mRGC or ipRGC) are indispensable for non-image forming (NIF) visual responses including pupillary light reflex, photoentrainment of the circadian clock, brightness perception in image-forming vision, light modulation of -activity, -sleep, and - pineal melatonin synthesis. Despite these pleiotropic roles of mRGCs, a comprehensive map of their central projections, genetically defined cell types that receive mRGC input, and synaptic ultra-structure are incomplete. This application will make innovative use of mouse genetics and viral vectors to specifically express fluorescent and electron microscopy reporter miniSOG in the mRGCs of mouse retina. Inherent fluorescence of miniSOG will guide selection of mRGC subtypes for subsequent analyses by serial blockface electron microscopy (SBEM). Automatic segmentation of miniSOG labeled neurites and soma will accelerate rapid 3D reconstruction of different subtypes of mRGCs, which will be used as scaffolds to reconstruct a comprehensive intra-retinal connectivity map of mRGCs. Using engineered rabies virus for monosynaptic retrograde transmission, mRGC cell types and their pre-synaptic retinal neurons constituting light input circuits to different target brain regions will be mapped. The mRGC connectivity in the target brain regions will be further mapped at ultrastructure level by combining electron microscopy reporter miniSOG and SBEM. All together the application will generate cellular networks of mRGC subtypes, identify cell types that receive mRGC inputs in the brain, and establish tools and techniques that can be used to unravel the connectivity of any genetically defined neuron.
Specialized groups of light sensitive neurons in the retina play an indispensable role in adapting our activity, sleep, and hormone levels to the ambient light-dark cycle. This application will elucidate the cellular circuitry by which these light sensitive neurons exert their effect on broad brain regions.
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