Rod photoreceptors in the mammalian retina allow vision under dim light conditions, when cones are not sufficiently activated. The rod light response, however, is relatively slow. Rods transmit their signals mainly to two effector cell types—to the ON bipolar cells by means of a synapse consisting of metabotropic glutamate receptors, and to adjacent cones by means of gap junctions. A third type of contact has been reported as well, between about 20% of rods in the mouse retina and a particular subset of OFF bipolar cells, the b2 cells. It was, however, not clear whether these contacts formed functional synaptic connections.
Now, Li, Chen and DeVries, on page 414 of this issue, characterize the electrophysiological properties of these contacts in slices from ground squirrel retina. The b2 bipolar cells, in contrast to the rod ON bipolar cells, express fast ionotropic AMPA-type glutamate receptors in their postsynaptic endings and could therefore mediate faster signaling.
The picture shows a b2 OFF bipolar cell (green) contacting a rod (red). The outer segment of the rod cell is stained blue for rhodopsin. Recording from such cell pairs, the authors found that kinetics of synaptic transmission between these cells is as fast and transient as transmission between cones and b2 bipolar cells and five to ten times faster than transmission between rods and rod ON bipolar cells or rods and cones. In other respects, too, such as synaptic vesicle replenishment, the rod–b2 OFF bipolar cell synapses resemble cone–b2 OFF bipolar cell synapses.
Although this study does not tease out any specific contribution of the new fast transmission circuit component to rod-mediated vision, the discovery of fast rod signaling is surprising in itself. One may speculate that at intermediate light intensities, cones and a subset of rods collaboratively activate the b2 cell–driven OFF circuitry. We look forward to future work revealing the physiological significance of this new input.
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Markus, A. Speedy rod signaling. Nat Neurosci 13, 410 (2010). https://doi.org/10.1038/nn0410-410