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T Euler, PB Detwiler, W Denk; Direction-Selective Calcium Signals in Starburst Cell Dendrites . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4763.
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Purpose: The involvement of starburst amacrine cells in motion processing by direction-selective ganglion cells is controversial. To further evaluate their potential contribution to directional selectivity we tested if signals evoked by moving visual stimuli in the dendrites of starburst amacrine cells depended on the direction of movement. Methods: Intracellular recordings were made from displaced (ON) starburst amacrine cells in dark-adapted, whole mount rabbit retina. After iontophoretically filling a cell with a calcium indicator (Oregon-Green-1) regions of the dendritic arbor were imaged with two-photon laser scanning microscopy. Using a laser wavelength of 930 nm caused little direct photoreceptor stimulation and allowed [Ca++] changes evoked by visual stimuli to be recorded optically. Results: Both spontaneous and light stimulus-evoked fast transient increases in [Ca++] were observed. The stimulus-evoked [Ca++] changes were more pronounced towards the dendritic tips, and were largest in the varicosities near the distal extremities. Dendritic [Ca++] increased at light-onset when the area above the imaged branch was stimulated and not when the branch on the opposite side was illuminated. Furthermore, stimulating the cell’s surround lead to a small decrease in [Ca++] (surround inhibition) lasting until light-offset when it was followed by a calcium transient, possibly reflecting the release of surround inhibition. Moving bars or gratings evoked large calcium responses only for centrifugal movement along the imaged dendrite. Movement in other directions evoked no calcium signals or smaller ones. In agreement with earlier studies (e.g. Peters & Masland, 1996), the electrical responses recorded in the soma showed no directional preference. Conclusion: The dendritic branches of starburst amacrine cells display local [Ca++] signals, which are sensitive to the direction of moving stimuli and are maximal for centrifugal motion. This indicates that starburst cells may contribute directionally tuned input to direction-selective ganglion cells.
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