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J.-J. Pang, J. Lem, D. E. Bramblett, D. L. Paul, S. M. Wu; Subpopulations of Rod Bipolar Cells and Cone On Bipolar Cells in the Mouse Retina Receive Direct Synaptic Inputs From Both Rods and Cones. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4562.
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© ARVO (1962-2015); The Authors (2016-present)
The objective is to determine the relative rod and cone inputs to various types of rod and cone depolarizing bipolar cell cells (DBCRs and DBCCs) in the mouse retina.
Cation currents (ΔIC) in DBCRs and DBCCs evoked by 500 nm light steps were recorded from dark-adapted mouse retinal slices by whole-cell voltage clamp techniques, and the DBC cell types were identified by their characteristic morphology revealed by Lucifer yellow fluorescent images. Photocurrents from rods and M-cones elicited by the same 500 nm lights were recorded with suction electrodes. Wildtype (C57BL/6J, WT), connexin36 knockout (Cx36-/-), Bhlhb4 knockout (Bhlhb4-/-, mouse without DBCRs) and rod transducin knockout (Tr-/-) mice were used in this study.
A pair of 500 nm (150 R*/rod-sec) light steps (0.5-sec in duration and 1-sec apart) evoked a single sustained current response in rods and two separate current responses in M-cones. DBCRs with globular axon terminals reaching the ganglion cell layer (DBCR1s) exhibit a single inward ΔIC to the light pair. DBCRs with globular axon terminals ending near 80-90% of the inner plexiform layer depth (DBCR2s) exhibit two separate inward ΔIC responses to the light pair (with the second smaller than the first). Cells with DBCR2 morphology in Tr-/- mice were much less sensitive. DBCR2s in Cx36-/- mice exhibit two separate responses. DBCCs with branching axon terminals ending near 70-80% of the inner plexiform layer depth (DBCC1s) have response threshold near that of the DBCR1s and DBCR2s (about 0.1 R*/rod-sec), whereas DBCCs with branching axon terminals ending at 65-75% of the inner plexiform layer depth (DBCC2s) have much higher response threshold (about 10 R*/rod-sec). Cells with DBCC1 morphology in either Cx36-/- or Bhlhb4-/- mice have similar response threshold as that of the WT DBCC1s.
Our results suggest that mouse DBCR1s receive inputs primarily from rods, whereas DBCR2s receive inputs from rods and cones, and the cone input is likely be mediated by direct cone→DBCR2 chemical synapses, not indirectly through rod-cone coupling. DBCC2s receive inputs primarily from cones, whereas DBCC1s receive inputs from rods and cones, and the rod input is mediated by direct rod→DBCC1 chemical synapses, not indirectly through rod-cone coupling or via the DBCR→AIIAC→DBCC pathway.
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