The ganglion cells of mammalian retinas exist in roughly a dozen types, each transmitting a different encoding of the visual scene to the brain. The long-term goal of this research is to learn the mechanism by which specific types of ganglion cells achieve these codlings. We have developed an interface incubation system that allows maintenance of multiple samples of adult rabbit retinas for several days in an unsupervised, culture-like system. Genes coding for RNAi or tagged synapse proteins are biolistically transfected into individual retinal ganglion cells. Two questions involve the synaptic events underlying directional selectivity in certain retinal ganglion cells. First, we propose to use RNAi to knock down GABAergic or cholinergic responsiveness in individual ganglion cells. Recording will reveal the contribution of these direct (postsynaptic) inputs to direction selectivity. Second, we will investigate the co-release of GABA and Ach by the starburst amacrine cells. Are the two neurotransmitters released from the same cellular sites or different ones? This will be studied by localizing their vesicular transport proteins. The third question is a more general one. Our recent experiments suggest that the excitatory inputs to ganglion cell dendrites are spatially distributed according to an even-spacing law: the synapses seem to repel each other. In addition, they systematically avoid branch points in the dendritic arbor. We now propose to see if these rules apply to all types of ganglion cells;to model their physiological consequences;and to see if the same rules apply to inhibitory synapses. The methods introduced here can be used in adult animals of any species. They obviate the need for transgenic animals, increase the number of samples that can be simultaneously studied, and allow labeling of proteins that have long half-lives. They can be used in normal tissue or in disease models. We hope that they will be useful to laboratories studying both basic and clinical problems.
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