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K. Gavrikov, A. V. Dmitriev, S. C. Mangel; Voltage-Clamp Analysis of the Directionally Selective Light Responses of Starburst Amacrine Cells in the Rabbit Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3851.
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Directionally selective (DS) light responses occur first in the retina in the dendrites of starburst amacrine cells (SACs), which depolarize to stimuli that move centrifugally through the receptive field (RF) surround and hyperpolarize to stimuli that move centripetally through the surround (Gavrikov et al., 2003). We recently reported that these DS responses are highly sensitive to the activity of NKCC2 and KCC2, two subtypes of Cl cotransporter that determine whether GABA depolarizes or hyperpolarizes neurons, respectively. Specifically, our findings demonstrated that the differential distribution of NKCC2 on the proximal dendrites and KCC2 on the distal dendrites of SACs results in a GABA-evoked depolarization and hyperpolarization at the NKCC2 and KCC2 compartments, respectively, and underlies the DS light responses of SACs (Gavrikov et al., 2006). We have further examined whether the Cl cotransporters mediate SAC DS light responses by performing voltage-clamp analysis of the cells.
Whole-cell patch-clamp recordings of displaced SACs in the rabbit retina were obtained and the effects of voltage-clamping the cells on their DS light responses assessed.
When voltage-clamped at their resting potential (~ -55 mV), SACs produced a slow outward current to light stimuli that moved centripetally through the RF surround, a slow inward current to stimuli that moved centrifugally through the surround, and a fast inward current to stimulation of the RF center. The slow outward current due to centripetal stimulation of the surround was greatly reduced when the cells were voltage-clamped at ~ -80 mV, but larger in size when the cells were voltage-clamped at more positive potentials. The slow inward current due to centrifugal stimulation of the surround was greatly reduced when the cells were voltage-clamped at ~ -30 mV, but larger in size when the cells were voltage-clamped at more negative potentials. The fast inward current due to center stimulation was eliminated when the cells were voltage-clamped at ~ 0 mV, and larger in size when the cells were voltage-clamped at more negative potentials.
These findings indicate that the fast light response of SACs to RF center stimulation is mediated by glutamate, but that the slow, DS light responses to centripetal and centrifugal motion through the SAC surround are mediated by GABA and dependent on both NKCC2 and KCC2 activity.
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