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W. Li, S. H. DeVries; Synapse Between Rods and Off Cone Bipolar Cells in a Mammalian Retina. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3229.
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© ARVO (1962-2015); The Authors (2016-present)
Recent studies suggest that there might be unorthodox connections between rods and OFF cone bipolar cells, forming alternative rod pathways. Anatomical evidence for such pathways comes from studies in a number of species (Hack, et al., 1999 & 2001; Tsukamoto, et al., 2001; Fyk-Kolodziej, et al., 2003; Li, et al., 2004). Physiological evidence, however, is indirect, mostly from ganglion cell recordings (Soucy, et al., 1998; Volgyi, et al., 2004). Our goal was to determine: 1) which type(s) of OFF cone bipolar cell contacts rods; 2) whether contacts represent functional synapses; and, 3) whether there are differences in the kinetics of synaptic transmission between a rod and a cone and a common postsynaptic cone bipolar cell.
Using dye injection, immunolabeling, and dual whole cell voltage clamp recording techniques, we have examined the synapses between both rods and cones with different types of OFF cone bipolar cells in ground squirrel retina slices.
We identified one type of OFF cone bipolar cell (the b2) that contacted rods. AMPA receptor subunits GluR2/3 was found at the tips of b2 cell dendrites, closely associated with rod invaginations. Paired recordings of rods and b2 cells revealed direct synaptic input (n = 12). A step rod depolarization produced a transient EPSC (~0.6 ms 20-80% rise and a 1.5 ms τ decay; n = 9). Transmission was effectively blocked by the AMPA receptor selective antagonist GYKI 53655 (25 µm, n = 2). Similar to cones, the ability of a rod to release transmitter recovered rapidly (~270 ms τ; n = 2) after an initial depolarization. In addition, prolonged depolarization of a rod often elicited a smaller and slower secondary peak from b2 cells, which followed a large and transient primary response. The late peaks were possibly triggered by neighboring cones that are slightly depolarized by current passing from the stimulated rod through rod-cone gap junctions.
b2 cells collect input indiscriminately from photoreceptors, and apparently subserve pathways that play a role in achromatic or luminance vision. The properties of transmission at rod and cone to b2 bipolar cell synapses were similar. We conclude that the speed of transmission at the rod to b2 synapse is probably limited by the kinetics of rod phototransduction rather than by the kinetics of transmitter release or bipolar cell receptors.
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