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T. Ichinose, C.R. Shields, P.D. Lukasiewicz; Sodium Channels in Bipolar Cells Enhance the Light Sensitivity of Ganglion Cells . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1007.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Bipolar cells are slow potential neurons that do not fire action potentials. Recently, voltage-gated sodium channels have been identified on isolated bipolar cells. However, it is not known if these channels influence visual processing in the retina. We investigated whether sodium channels on bipolar cells affected light-evoked signaling to ganglion cells. Methods: Light-evoked responses from ganglion cells were recorded in the salamander retinal slice using whole-cell patch clamp techniques. Morphologies were determined by filling cells with Lucifer yellow. The excitatory inputs from bipolar cells were isolated by including strychnine, bicuculline, I4AA, picrotoxin, SCH23390, sulpiride, d-tubocurarine in the bath. Electroretinogram (ERG) recordings from eyecup preparations were used to assess ON bipolar cell activity. Results: The sodium channel blocker TTX reduced light-evoked EPSCs in a specific class of ganglion cells; ON-OFF ganglion cells with dendritic arbors in the mid inner plexiform layer (IPL). The intracellular sodium channel blocker QX-314 did not alter the TTX effect, indicating that TTX acted presynaptic to the ganglion cell. Amacrine cells did not mediate the EPSC suppression by TTX because it was still observed when GABAA, GABAC, glycine, dopamine, and acetylcholine receptor-specific blockers were present. To determine whether TTX affected bipolar cell light responses, we recorded photopic ERGs after ionotropic glutamate, GABA, and glycine receptors were blocked. TTX reduced the B-wave without affecting the A-wave, suggesting that sodium currents shape bipolar cell voltage responses. TTX shifted the ganglion cells intensity response curves to the right, suggesting that bipolar cell sodium channel activation enhanced ganglion cell light sensitivity. Conclusions: Sodium channels are present on a subset of bipolar cells. Activation of these channels boosts the light sensitivity of ON-OFF ganglion cells whose processes ramify in the mid IPL.
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