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S.I. Fried, T.A. Muench, F.S. Werblin; The Circuitry Underlying Directional Excitation and Inhibition to DS cells . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2266.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Our goal was to uncover the circuitry and mechanisms that mediate directional selectivity in the rabbit retina. Recent work has shown that the inputs to the directionally selective (DS) ganglion cell are themselves already directional. Here we pharmacologically identify the specific neural pathways that shape these inputs. Methods: We isolated the excitatory and inhibitory input currents to the DS cell using whole cell patch clamp in the flat mount retina. Sequences of flashes, simulating null and preferred movement, were used to determine whether directional properties arise from suppression or enhancement. Pharmacological blockers were used to dissect and identify specific circuit components. Results: Excitation and inhibition to the DS cell are both shaped by suppressive signals. The excitatory input is primarily glutamatergic for the ON system but contains an additional cholinergic component in the OFF system. Each of these excitatory components is individually suppressed. Suppression of excitation is elicited by acitivity on the null side from both within and outside the DS dendritic field and is eliminated in the presence of SR–95531, an antagonist of GABA–A receptors. Suppression of inhibition is elicited by activity from the preferred side and, in the ON system, is eliminated in the presence of curare, an antagonist of nicotinic acetylcholine receptors. Cholinergic excitatory pathways to the DS cell from outside its dendritic field were revealed in the presence of SR–95531. Conclusions: Directional selectivity is a multi–layered process, engaging a hierarchical series of spatially offset inhibitory events. This is first expressed in the asymmetric suppression of both the excitatory and inhibitory inputs to the DS cell and then again at the dendrites of the DS cell itself. Both ACh and GABA, known to be released from starburst cells, appear to be involved in suppression, implicating the starburst amacrine cell in both pre– and post–synaptic mechanisms of directional selectivity.
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