Our work focuses on several different synapses and cell types in the inner retina. We have expanded our study of inhibitory synaptic connections made by amacrine cells within the inner retina, to understand how feedforward and feedback inhibition contributes to signal processing in this network. We find that A17 amacrine cells provide rapid GABAergic feedback to rod bipolar cell terminals via a release process that is independent of membrane depolarization or voltage-gated calcium channels (Chavez, et al., 2006). This rapid feedback, driven by activation of calcium-permeable AMPA receptors in the A17 amacrine cell, may be essential to prevent the rapid depletion of readily-releasable vesicles from the rod bipolar cell synaptic terminal (Singer and Diamond, 2006). More recent work (Chavez and Diamond, 2008;Chavez, et al., 2010) indicate that two other types of feedback inhibition onto bipolar cells exhibit different characteristics and mechanisms of modulation. In addition to providing valuable information about feedback, this work is enabling us to begin to make functional sense of the vast array of amacrine cells (>two dozen cell types) in the mammalian retina. We have combined electrophysiological and imaging techniques to explore the mechanisms of signal integration in A17 amacrine cells and have found that active, calcium-dependent conductances may be essential to compartmentalize signal processing within A17 dendrites (Grimes, et al., 2010). Currently we are studying how the biophysical properties of this synapse changes with different phases of the circadian cycle. In addition, we have begin to collaborate with Dr. Kevin Briggman to mine densely reconstructed retinal tissue to determine all of the amacrine cells that contact rod bipolar cell terminals. We also have combined recordings from synaptically connected cell pairs with light-evoked responses from rod bipolar cells and AII amacrine cells to study how this important synapse in the rod pathway transmits visual information. We find that this synapse response to changes in luminance with a biphasic response. The slow, sustained component of the response encodes absolute luminance, and the transient component encodes Weber contrast (Oesch and Diamond, 2011). Recordings from synaptically coupled rod bipolar and AII amacrine cells showed that Weber contrast is computed via careful regulation of the readily releasable pool of synaptic vesicles. Future experiments are aimed at determining how synaptic inhibition influences this computation. So far, we find that feedback inhibition, by reducing the extent of presynaptic vesicle depletion, extends the range over which these synapses encode luminance and compute contrast. In addition, this reduction in depletion also enhances the amplitude of synaptic responses to the absorption of a single photon. In addition, we are combining electrophysiology, imaging approaches and cellular/network modeling to explore dendritic integration in directionally-selective ganglion cells. We are also taking advantage of numerous transgenic mouse lines to image specific cell types, which will enable us to record from synaptically coupled cone bipolar - ganglion cell pairs. In addition, we have started to investigate the physiological properties of genetically identified ON and OFF cells early in development, before their dendrites have specifically targeted the specific ON/OFF layers. Finally, we are extending our electron microscopy studies, in collaboration with Tom Reese and Richard Leapman, 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. This approach may enable us to discern morphologically, for the first time, docked and primed synaptic vesicles.
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