Neural Circuits and Synapses for Early Visual Processing

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PROJECT SUMMARY My long-term goal is to understand the biological basis of visual processing at the level of neural circuits and synapses. I am pursuing this goal in the mammalian retina, where we can efficiently study the tissue's natural function in vitro; and where we have a basic understanding of the cell types that compose the circuits: 3-4 photoreceptors (rods and cones), ~50 interneurons (horizontal, bipolar and amacrine cells), and ~20 output neurons (ganglion cells). A major obstacle to understanding retinal circuitry is the unknown function of the majority of the ~40 types of amacrine cell. These (primarily) inhibitory interneurons play a major role in ganglion cell receptive field computations, but we presently have a fairly detailed understanding of just four types, specialized for rod vision (AII, A17), direction selectivity (starburst) and neuromodulation (dopaminergic). Hypothesized functions for the remaining 30+ types include feature selectivity (e.g., orientation tuning), generation of receptive field surrounds, 'cross-over' inhibition between the ON and OFF pathways, and feed- forward inhibition within either the ON or OFF pathways. However, there are major barriers to understanding the role of amacrine cells in retinal circuitry: we lack the ability to assign specific functions to individual amacrine cell types, and in most cases we do not know a specific cell type's postsynaptic targets and its associated role in network function. To move the field forward, we propose to test between alternative hypothesized functions for novel amacrine cell circuits using an integrated, multidisciplinary approach. Aim 1 will determine receptive field properties of genetically-identified amacrine cell types by recording Ca signals in their dendrites, the sites of neurotransmitter release. Aim 2 will determine a specific amacrine cell type's postsynaptic ganglion cell targets using optogenetics. Aim 3 will determine the functional role of an amacrine cell type in the circuit by measuring how its reversible inactivation affects ganglion cell receptive field properties. The combination of these three approaches will provide major advances over existing methods for studying amacrine cells and should be broadly applicable to the study of neural circuit function throughout the mammalian central nervous system. Furthermore, studying the function of amacrine cells and their interconnections with other retinal neurons will contribute to the long-term goal of developing new diagnoses and treatments (i.e., optogenetic, prosthetic) for eye disease.