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Khaldoun Hamade, William Taylor, Robert Smith; Mechanisms for enhancement of direction selectivity in the starburst amacrine cell. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2512.
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© ARVO (1962-2015); The Authors (2016-present)
The starburst amacrine cell (SBAC) is an essential component of the presynaptic circuitry of the direction-selective ganglion cell. The SBAC and its local circuit is thought to initiate direction selectivity in its dendritic arbor through several mechanisms. Its output synapses are located in the outer third of the arbor, and this region is known to generate a direction selective response, in part through partial dendritic isolation from the soma. The dendrites contain voltage-gated Na channels that can amplify EPSPs and in somatic recordings enhance the directional difference in EPSPs. We asked what mechanisms in the SBAC contribute to a direction selective response in the dendritic tip and in the soma.
Using a computational model of the SBAC and an array of presynaptic bipolar cells calibrated with real recordings, we stimulated the bipolar cell array with spots and moving bars of light. We utilized a simplified morphology comprising the soma and one dendrite, simulating a radially-symmetric stimulus that generates robust direction selective (DS) responses in real SBACs. Responses were recorded in voltage-clamp and current-clamp modes at several radii from the soma, in the presence of different density profiles of Na and K channels.
From a resting potential in the model SBAC near -70 mV, moving bars elicited EPSPs 25-30 mV in amplitude in the dendritic tips. In a model without voltage-gated channels, a bar moving from the soma to the tips (centrifugal) and from the tips to the soma (centripetal) elicited EPSPs with a peak directional difference of ~7 mV at the dendritic tip, but DS was reversed (centripetal) at the soma. When Na and K channels (~30 mS/cm2, ~5 mS/cm2) were added at the dendritic tips, the response at the tip to centripetal motion was amplified by 3-10 mV, depending on the Na channel density. The somatic DS remained reversed. However, when a gradient of Na channels was added along the dendrite, somatic DS became centrifugal. In somatic voltage-clamp, DS was reduced because regeneration in the dendrite was partially blocked by the clamp currents.
Voltage-gated Na channels can amplify the centrifugal preference in responses evoked by moving bars in SBAC dendritic tips. Somatic responses have centripetal DS in a linear model or with Na channels only at the dendritic tip, but are modified to centrifugal DS with a gradient of Na channels along the dendrites.
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