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T. Szikra, D. Hillier, B. Roska; Calcium Signaling in the Mouse Bipolar Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5809.
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Different bipolar cell types feed the circuitry of the inner plexiform layer (IPL) with parallel visual information, which is modulated by the inhibitory feedback from amacrine cells. Although the main components of the circuitry have been described it is not yet known to what extent the intrinsic properties of different ON bipolar cell terminals contribute to the processing in the ON sublamina of the IPL. To answer this question we analyzed the depolarization-evoked Ca transients in different ON bipolar cell populations.
Patch-clamp combined two-photon imaging was performed in mouse retinal slice and wholemount preparations. In whole-cell voltage-clamp configuration light -evoked responses were recorded in bipolar cells. Cells were loaded with calcium dye through the pipette. Steps to various depolarization levels with different lengths were applied to trigger Ca influx. Depolarization-evoked Ca transients were recorded in the bipolar cell bodies and terminals. Cells were identified by Z-stack imaging and by immunocytochemistry.
Rod bipolar cell bodies were identified from their position in the outer part of the IPL. Rod bipolar axons were descending along the IPL branching into ball-shaped processes. In contrast, the cell bodies of cone bipolars were located closer to the IPL and their axons branched forming numerous bushy terminals in layers of ON sublamina. Light-evoked currents were recorded and later fed-back into bipolar cells in the presence of GABAC and mGluRIII blockers to exclude circuitry intervention. Fed-back currents and simple depolarization steps evoked strong Ca influx in all bipolar cell terminals, but somewhat smaller Ca influx into the cell bodies. However calcium-imaging unveiled differences in the kinetics and amplitude of Ca transients between bipolar cell subpopulations.
ON signals in the retina are segregated into parallel channels provided by a group of ON bipolar cells. Our result suggests that intrinsic properties of Ca homeostasis contribute to the temporal segregation of visual information in the IPL.
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