Abstract
Rod and cone pathways are segregated in the first stage of the retina: cones synapse with both ON- and OFF-cone bipolar cells while rods contact only rod bipolar cells. However, there is an exception to this specific wiring in that rods also contact certain OFF cone bipolar cells, providing a tertiary rod pathway. Recently, it has been proposed that there is even more crossover between rod and cone pathways. Physiological recordings suggested that rod bipolar cells receive input from cones, and ON cone bipolar cells can receive input from rods, in addition to the established pathways. To image their rod and cone contacts, we have dye-filled individual rod bipolar cells in the rabbit retina. We report that approximately half the rod bipolar cells receive one or two cone contacts. Dye-filling AII amacrine cells, combined with subtractive labeling, revealed most of the ON cone bipolar cells to which they were coupled, including the occasional blue cone bipolar cell, identified by its contacts with blue cones. Imaging the AII-coupled ON cone bipolar dendrites in this way showed that they contact cones exclusively. We conclude that there is some limited cone input to rod bipolar cells, but we could find no evidence for rod contacts with ON cone bipolar cells. The tertiary rod OFF pathway operates via direct contacts between rods and OFF cone bipolar cells. In contrast, our results do not support the presence of a tertiary rod ON pathway in the rabbit retina.
Highlights
The mammalian retina can process signals over a vast range of intensities, approximately 10 log units, from starlight to sunlight
MGluR6 labeling is located on the dendritic tips of both ON cone bipolar cell and rod bipolar cells (Nomura et al, 1994; Vardi and Morigiwa, 1997; Li et al, 2004)
Using individual dye-filled rod bipolar cells and a population of ON cone bipolar cells selectively labeled via gap junctions with AII amacrine cells, we have examined the connectivity between photoreceptors and bipolar cells in the rabbit retina
Summary
The mammalian retina can process signals over a vast range of intensities, approximately 10 log units, from starlight to sunlight. In part, this is accomplished by the use of two types of photoreceptors, rods, and cones, which operate in different intensity ranges. In the canonical primary rod pathway, rods signal to rod bipolar cells which synapse onto AII amacrine cells. The ON and OFF cone bipolar cells relay signals to their respective ON and OFF retinal ganglion cells. This pathway operates as a high gain circuit that facilitates the transmission of single-photon responses from rods. It has been reported that rod bipolar cells saturate at relatively low light levels, even before the threshold for cone vision has been reached (Field et al, 2005) In this so-called ‘‘mesopic’’ range of intensities, it is thought that additional circuits are recruited that bypass the rod bipolar
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