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A.V. Dmitriev, K.E. Gavrikov, S.C. Mangel; COMPUTATIONAL ANALYSIS OF DIRECTION–SELECTIVE LIGHT RESPONSES OF STARBURST AMACRINE CELL DENDRITES . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4266.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Starburst amacrine cells (SACs) exhibit directionally–selective (DS) light responses such that they depolarize to stimuli that move centrifugally from their somata to the periphery, but hyperpolarize to stimuli that move centripetally from the periphery to their somata (Gavrikov et al., 2003, PNAS). Because blockade of the Na–K–Cl and K–Cl cotransporters eliminates the DS responses of SAC dendrites, it has been suggested that the differential distribution of the cotransporters generates a chloride gradient along the length of SAC dendrites and underlies their DS responses (Gavrikov et al., 2003, PNAS). We modeled the SAC dendrite to examine whether a chloride gradient along its length could produce the DS responses to centrifugal and centripetal stimulus motion that we have observed in the starburst cell body. Methods: The voltage changes at the starburst cell body and dendritic tip produced by a light stimulus moving in the centrifugal and centripetal directions were modeled with a custom–made computer program. The SAC dendrite was represented as a cable that consisted of 200 groups of transmembrane elements and was attached to a cell body of low resistance at the most proximal end. The groups were divided by radial intracellular resistances and each group consisted of resistances and batteries for Na+, K+, and Cl–, and a capacitance. The effect of a moving light was simulated as sequential decreases in the glutamate–evoked Na+ and GABA–evoked Cl– resistances. EK and ENa were the same in every cable group, but ECl sequentially became more hyperpolarized toward the dendritic tip to simulate the effect of the Cl–cotransporters. The electrical and ionic parameters that were used were selected based on our patch–clamp measurements. Results: A depolarization and a hyperpolarization to stimulus motion in the centrifugal and centripetal directions, respectively, were generated in both the starburst cell body and dendritic tip only if 1) there was a chloride gradient along the length of the dendrite, and 2) the GABA–evoked increase in the chloride conductance lasted at least 100 msec longer than the glutamate–evoked sodium conductance increase. Conclusions: Our modeling results indicate that DS responses to centrifugal and centripetal stimulus motion can be generated in both the starburst cell body and dendritic tip if there is a chloride gradient along the dendrite and the GABA–evoked increase in the chloride conductance is relatively long–lasting. Our findings therefore support the idea that the differential distribution of the Na–K–Cl and K–Cl cotransporters on SAC dendrites underlies their DS responses.
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