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Charles P. Ratliff, Steven H. DeVries; Different Bipolar Cell Types Process Information from the Cone Synapse at Different Rates. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2572.
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© ARVO (1962-2015); The Authors (2016-present)
Cone photoreceptors encode light intensity, but the fidelity with which they transmit this information to ~10 distinct types of bipolar cells is unknown. One hypothesis is that the same information is available to all bipolar cell types, and this information is relayed to the inner retina. Against this view, we observed that the 3 main types of Off bipolar cells in the ground squirrel (b2, b3, and b7) made significantly different numbers of contacts with each cone (from 1.7 to 7). The differences in contact number suggest that the reliability of transmission could differ. To resolve this, we measured synaptic transmission of information from cones to the three types of Off bipolar cells.
In retinal slices, cone membrane voltage was controlled through a perforated patch, while a synaptically connected bipolar cell was maintained at -70 mV in the whole-cell voltage clamp configuration. We assessed information transmission at the synapse by repeating white-noise stimuli (variable mean, std = 2.5 mV, cutoff frequency = 100 Hz), and sometimes pink noise, natural stimuli, or white noise with different parameters. Cells were labeled with different tracers, and synaptic contacts were imaged at high resolution using a Zeiss LSM 510 confocal microscope. Density within a class was measured by imaging retinal whole mounts labeled with bipolar cell type-specific markers.
Information rate, as measured by the white noise stimulus, differed according to bipolar cell type. b3 cells made an average of 3.8 ± 1.8 basal contacts with each of 7.7 ± 1.8 cone terminals, and had an information rate of 311 ± 94 (mean ± SD; n = 7) bits/s. b7 cells also made basal contacts (1.7 ± 0.8 per cone onto 9.3 ± 1.9 cones), and had information rates that were significantly lower (138 ± 57 bits/s; n = 8). b2 bipolar cells made invaginating contacts (7.0 ± 3.0 per cone onto 13.9 ± 2.0 cones), and had intermediate information rates (200 ± 45 bits/s; n = 6).
Different Off bipolar cell types carry information from the cone synapse at different rates. We are currently measuring cell density to verify that each class, as a whole, transmits a different amount of information. Our results suggest that the three main types of bipolar cells initiate three basic information pathways: the b3 cell initiates a high spatial resolution, low-noise "sustained" pathway; the b7 cell initiates a low-SNR, "sluggish" pathway; and, the b2 initiates a low spatial resolution, high-noise, "brisk" pathway.
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