Abstract
Purpose: :
Rods have no direct pathway to ganglion cells; instead, they control the function of cones and cone bipolar cells when cones are quiescent. Of the several pathways by which rod signals are transmitted to cone bipolar cells, the rod-AII pathway, (connecting rods to rod bipolars to AII amacrines and then onto cone bipolar cells) is widely accepted as offering the highest sensitivity. It has been proposed that GCs with decreased sensitivity1 likely receive input from ONCBs with no connectivity to AIIs 2. Here we attempted to determine which type of ONCB is not coupled to AIIs.
Methods: :
ON cone bipolars that contain glycine are coupled to AIIs. By injecting bipolar cells at random with Neurobiotin and using glycine as a marker for coupling, we attempted to find glycine negative (AII-uncoupled) ONCBs. Flat mount rabbit retinas were prepared and stained with 5mM DAPI (4, 6-diamidino-2-phenylindole) to visualize bipolar cell somata. Bipolar cells were randomly injected with Lucifer Yellow and Neurobiotin. Once impaled, cells were injected for 3 to 5 seconds. Tissues were fixed and double labeled with Rat anti-glycine (1:1000, Immunosolution #IG1002) as a marker for coupling to AIIs and Goat anti-ChAT (1:100, Chemicon #AB144P), as a marker for comparing bipolar cell axon terminal ramification depth to that of starburst amacrine cells.
Results: :
Random injections of bipolar cell somas led to non-random recovery of an over-represented cell type, which we call the ChAT-B ONCB because they ramify in the upper portion of the ChAT (starburst amacrine) plexus in sublamina b. Out of 149 bipolar cells injected only 39 were recovered, consisting of 6 RBPs (15%), 12 OFFCBs (31%), and 18 ChAT-B ONCBs (46%). None of the recovered cells contained glycine, suggesting they are not coupled to AIIs.
Conclusions: :
The skewed distribution can be attributed to the ChAT-B ONCB’s lack of coupling to AII amacrine cells, since none contained glycine. Other cell types were not recovered as readily, likely because their Neurobiotin diffused away through gap junctions into the AII network, which acts as a sink for diffusion. The ChAT-B ONCB is likely the synaptic drive for the ON starburst amacrine cells. Starburst amacrine cells are part of the circuitry that drives directionally-selective (DS) ganglion cells. The DS mechanism has been shown to rely on timing of inputs. Uncoupling of ChAT-B ONCB’s from AIIs means they will have sharper receptive fields and can operate independently of each other, providing precisely timed inputs to postsynaptic cells.1. Deans MR, Volgyi B, et al. Neuron 2002;36:703-712.2. Petrides A, Trexler EB. J Comp Neurol 2008;507:1653-1662.
Keywords: bipolar cells • gap junctions/coupling • amacrine cells