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David W Marshak, Andrea S Bordt, Sara Patterson, Rebecca Girresch, Christian Puller, Judith Mosinger Ogilvie, Jay Neitz; Synaptic inputs to broad thorny ganglion cells from macaque retina. Invest. Ophthalmol. Vis. Sci. 2020;61(7):5139.
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In primates, broad thorny retinal ganglion cells respond preferentially to small, moving stimuli, and they would be well suited to guide “catch-up” saccades during smooth pursuit eye movements. The neural circuit that generates these light responses has not been described, and that was the goal of this work.
Two macaque retinal connectomes were built from tissue 2 mm from the center of the fovea and analyzed as in (Patterson et al. 2019, Sci. Rep. 9, 11913). Whole-cell voltage-clamp recordings of light responses from broad thorny ganglion cells were done in control conditions and during pharmacological blockade of mGluR6, as in (Puller et al., 2015, J. Neurosci. 35, 5397).
Two broad thorny ganglion cells were partially reconstructed and identified by their characteristic morphology and stratification pattern. Only 9 of the 549 synaptic inputs analyzed to date were from bipolar cells, 6 ON and 3 OFF. The remainder of were from amacrine cells, of which approximately one-third were wide-field types. Three presynaptic cell types were identified: wiry type 2 cells, spiny cells and ON starburst cells. Inhibitory and excitatory synaptic currents occurred both at light increments and decrements in control conditions. They persisted at light decrements but were abolished at light increments during blockade of the ON pathway.
Based on the synaptic inputsof broad thorny cells, amacrine cells are likely to play a major role in generating their light responses. This may account for their response latencies being longer than other types of third-order neurons ramifying in the center of the IPL. Wide-field amacrine cells provided a major input, and this is consistent with the finding that postsynaptic inhibition of broad thorny cells increases with the stimulus size, reaching a maximum at sizes larger than the dendritic field diameter (Puller et al., 2015, J. Neurosci. 35, 5397). The electrophysiological results suggest that direct excitatory synaptic inputs via ON and OFF pathways elicit light responses in broad thorny cells and rule out cessation of crossover inhibition as the sole source of the OFF responses. Because robust excitation of broad thorny cells was observed despite the paucity of inputs from bipolar cells, there may be glutamatergic synapses from vGluT3-positive or cholinergic synapses from amacrine cells.
This is a 2020 ARVO Annual Meeting abstract.
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