Glutamate is the primary excitatory neurotransmitter of the brain. In the retina, fine differences in light contrast elicit grades of glutamate release in the photoreceptor axon terminal. Thus, visual perception is encoded in how the neurons post- synaptic to the photoreceptor respond to subtle changes in glutamate release. In the primate retina, these include 8-10 morphological types of excitatory bipolar cell that parcel bands of spatial, spectral and temporal information to different types of ganglion cell that form parallel visual pathways to the brain. Thus, functional diversity between visual pathways begins not at the ganglion cell, but with each bipolar cell's unique complement and distribution of glutamate receptors at the first retinal synapse. In primate retinal slices and whole mount preparations, iGluR and mGluR subunits will be localized to invaginating bipolar cell dendrites. In addition, the localization and distribution of GluRs across different types of bipolar cell will be determined. IG1uR and mGluR subunit mRNA expression will be localized to bipolar cell types. Bipolar-cell centers will be computed according to glutamate receptor distribution. Immunocytochemistry with both electron and light microscopy, in- situ hybridization, and intracellular-dye injections will be used. Localization of glutamate receptors to specific types of bipolar cell is essential in understanding the transmission of light information and, therefore visual processing.
