Our work focuses on specialized synapses in the inner retina. Our understanding of ribbon synaptic physiology in bipolar cells is limited to rod bipolar cells. We don't know as much about synaptic transmission from cone bipolar cells, because it is very difficult to obtain synaptically coupled cone bipolar - ganglion cell pairs. We have crossed mouse lines with genetically encoded markers identifying specific types of bipolar and ganglion cells that are very likely to be connected. In particular, we have utilized mouse lines in which type two cone bipolar cells (CBC2) can be visualized. We find that CBC2s make reciprocal synaptic connections with AII amacrine cells. We are studying bidirectional communication between these two cell types, and we are also studying how the rod bipolar cell signal is shaped by the AII amacrine cell before it is passed to the CBC2. Our preliminary findings suggest that AIIs preferentially transmit information about contrast, but not luminance, to CBCs. In addition, AIIs pass signals with different temporal dynamics to ON and OFF cone bipolar cells, likely reflecting fundamental differences between electrical and chemical synapses. We are examining various cellular and synaptic processes that underlie these transformations, and a manuscript is in preparation. To expand our understanding of ribbon synapse physiology, we are recording from genetically labeled cells that we think are likely to be synaptically connected. Recent connectomic analyses of the mouse retina predict connectivity between specific cone bipolar cell subtypes and third-order neurons (amacrine and ganglion cells). We are working to take advantage of this new information to discover reliably connected cell types that will allow us to examine the dynamics of synaptic transmission from ON and OFF cone bipolar cells. Finally, we have extended our electron microscopy studies, in collaboration with Richard Leapman (NIBIB), to explore the detailed ultrastructure of synaptic ribbons in photoreceptors and rod bipolar cells. So far, EM tomography enables us to detect protein filaments that tether synaptic vesicles to the ribbon and the presynaptic membrane. We find that vesicles adopt different tethering relationships with respect to the ribbon and the presynaptic membrane, and we believe this may reflect morphological differences between docked and primed synaptic vesicles. We have obtainedhigh-resolution three-dimensional images of rod bipolar cell ribbon synapses, using high-pressure freezing and EM tomographic techniques, under different physiological conditions that should give rise to different fractions of docked/primed vesicles. We are currently analyzing morphological differences between these different physiological states with the goal of determining the morphological substrate for vesicle priming at these synapses and to detect spatial patterns of exocytosis from the ribbon.
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