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A. Arman, A. P. Sampath; Contribution of Rod Photoresponses to the Rod-Cone and Rod-OFF Pathways in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3044. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Three known pathways allow rod photoresponses to traverse the retinal circuitry en route to the ganglion cell layer. Of these the Rod Bipolar Pathway is believed to be the most sensitive by ~ 5- to 10-fold, relaying information near absolute visual threshold. At higher light levels the Rod-Cone and Rod-OFF pathways may participate in rod vision, however the contribution of each of these pathways is obscured by their convergence in the inner retina. The goal of these experiments is to separate the contribution of each pathway using a combination of genetics and pharmacology.
Rod contributions to the Rod-Cone and Rod-Off pathways were studied in retinal slices from mice lacking cone photoresponses (Gnat2-/-; i.e. cone transducin KO). Whole-cell voltage clamp recordings (Vm = -60 mV) were made from OFF cone bipolar cells while superfusing the slice at ~ 36oC with bicarbonate-buffered Ames media containing 100 µM strychnine to block the AII amacrine contribution to the response. Families of light responses were recorded for 10 ms flashes from an LED (max ~ 470 nm) from which the sensitivity was determined. Cellular morphology, and thus bipolar cell subtype, was determined by adding Lucifer Yellow or Alexa 488 in the intracellular solution.
Rod bipolar cells in Gnat2-/- mice showed normal sensitivity with half-maximal flash strengths of 14 ± 4.5 photons µm-2 (n = 33). Rod signals in cone bipolar cells of Gnat2-/- mice shared a similar sensitivity. OFF cone bipolar responses had half-maximal flash strengths of 17 ± 6.6 photons µm-2 (n = 26), with little variation across OFF bipolar cell subtypes.
Surprisingly we find that the sensitivity of OFF cone bipolar cells for rod signals is comparable to rod bipolar cells. This observation is consistent with previous dye-tracing studies that show coupling between rods and cones in the outer retina is substantial. Here we show functionally that this coupling allows the rod photoresponse to drive cone bipolar cells with similar sensitivity, suggesting that all the rod pathways may carry rod-driven signals at low light levels. We aim to further segregate the Rod-Cone and Rod-OFF pathways in mice lacking both cone transducin and connexin36.
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