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A. Arman, A. P. Sampath; OFF Bipolar Cells Do Not Carry Visual Signals Near Absolute Threshold. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3302.
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© ARVO (1962-2015); The Authors (2016-present)
In the mammalian retina several identified circuits allow signals generated by rod photoreceptors to reach ganglion cells, the retinal output. A high sensitivity pathway conserved across mammalian species carries signals near absolute visual threshold with rod signals converging downstream on a narrow field depolarizing (AII) amacrine cell. However, the path for signal flow from AII amacrine cells to OFF ganglion cells near absolute visual threshold remains unclear; namely whether signals are communicated directly to OFF ganglion cells, or are relayed to OFF ganglion cells through OFF cone bipolar cell synaptic terminals.
Rod signals were measured in whole-cell patch clamp recordings from AII amacrine, OFF bipolar, and OFF ganglion cells in dark-adapted retinas of mice lacking cone light responses (Gnat2-/-). Retinal slices were superfused with heated Ames’ media. Strychnine (20 micromolar) was used to eliminate inhibitory contributions to responses. Response threshold, or the flash strength where the signal-to-noise ratio is 1, was calculated from families of flash responses. Cell classification was accomplished using fluorescent dyes in the intracellular solution.
OFF ganglion cells demonstrated robust rod-driven signals that were attenuated by strychnine, increasing the cell’s response threshold. Mean threshold measured in OFF cone bipolar cells was ~30-fold higher than putative presynaptic AII amacrine cells. Furthermore, OFF cone bipolar cells did not demonstrate any deleterious effects of strychnine for the dimmest flashes, but rather strychnine caused their response size to increased at flash strengths ~10-fold brighter than their threshold.
OFF cone bipolar cells display a markedly lower sensitivity to strychnine compared to OFF ganglion cells for responses near their respective thresholds. These differences may be attributed to the physiological properties of the glycine receptors imparted by the lack of the anchoring protein, gephyrin, in OFF cone bipolar cells. AII amacrine cell synapses’ with OFF bipolar cells may instead modulate activity at bright light levels, however AII amacrine signals near absolute visual threshold appear to be conveyed directly to OFF ganglion cells.
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