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L. Yin, R. G. Smith, P. Sterling, D. H. Brainard; Color-Opponent Ganglion Cells in Guinea Pig Retina (S+/M- and S-/M+) Co-Stratify in the ON Stratum of the IPL. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3249.
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Most mammals have short-wavelength sensitive (S) cones and one or two types of middle/long-wavelength (M/L) sensitive cones. In primate, some ganglion cell types combine inputs from S cones antagonistically with those from M/L cones. In dichromatic mammals, no direct evidence links the morphology of a ganglion cell with physiological S/M opponency. We identified color-opponent ganglion cells in guinea pig and then studied their morphology. Because guinea pig retina exhibits a dual gradient in S/M opsin expression across the retina, we also studied whether S/M opponency varies with this gradient.
We recorded spikes from ganglion cells with extracellular loose patch electrodes from in vitro whole-mount albino guinea pig retina. Chromatic properties were characterized by responses to (a) spots produced by mixtures of two primary lights and (b) flashed monochromatic spots. After recording, we stained the cell with Lucifer yellow and quantified its dendritic morphology.
In superior retina, we found cells with strong S+/M- and S-/M+ opponency. In inferior retina, we found cells with weak S+/M- opponency. The S/M opponent cells in superior retina (n = 4) had well-balanced M and S weights, while those in inferior retina (n = 2) had unbalanced M and S weights, with the M weights being much weaker. All of the opponent cells that we identified were monostratified, with dendritic arbors in the ON sublamina of the inner plexiform layer. Two bistratified cells studied were non-opponent.
The decline in S/M opponency we found in inferior retina is consistent with the dual gradient, suggesting that retinal circuitry in inferior retina does not enhance S/M opponency by selectively amplifying the M-opsin input to S/M opponent ganglion cells.
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