Abstract
Purpose: :
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.
Methods: :
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.
Results: :
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.
Conclusions: :
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.
Keywords: bipolar cells • photoreceptors • electrophysiology: non-clinical