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Sidney P. Kuo, Haruhisa Okawa, Justin Pacholec, Rachel O Wong, Fred Rieke; Divergence of visual signals in parallel ON cone bipolar cell pathways of the mouse retina. Invest. Ophthalmol. Vis. Sci. 2016;57(12):593.
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© ARVO (1962-2015); The Authors (2016-present)
Cone photoreceptor signals are conveyed to the inner retina by ~12 distinct subtypes of cone bipolar cells. This divergence of cone signals into different feed-forward bipolar pathways forms the initial basis for parallel retinal circuits dedicated to specific visual functions. We examined how distinct bipolar cell types contribute to differential temporal processing of visual signals by comparing the light response properties of two types of ON cone bipolar cell with distinct axonal morphologies and stratification patterns (type 5 and type 6), as well as excitatory synaptic currents in specific retinal ganglion cell (RGC) targets of these bipolar cells.
We used patch-clamp recordings to measure visual stimulus-evoked responses from bipolar cells and RGCs in isolated mouse retinal tissue. Recordings were restricted to ventral retina and light stimuli were delivered from a short wavelength (395 nm peak) LED at a photopic background light level. ON cone bipolar cells were targeted using 2-photon microscopy in retinal slices prepared from transgenic mice with selective fluorescent protein expression in specific bipolar cell subtypes. Bipolar cell responses were recorded using gramicidin perforated-patch recordings. Whole-cell voltage-clamp recordings were obtained from RGCs in a flat mount preparation. To identify postsynaptic targets of type 5 bipolar cells, we biolistically transfected RGCs in transgenic mouse tissue with DNA coding for a fluorescently labeled postsynaptic marker.
We found light-evoked voltage responses of type 5 and type 6 cone bipolar cells were not significantly different from one another. However, excitatory synaptic currents in RGCs measured in response to the identical stimuli as used in bipolar cell recordings exhibited substantial differences between ON alpha RGCs, a major target of type 6 bipolar cells (Schwartz et al., 2012), or a previously uncharacterized ON RGC type we identified as a postsynaptic target of type 5 bipolar cells. For example, excitatory synaptic input to this latter RGC, which we term ‘ON transient’, was less sensitive to small, rapid fluctuations in visual contrast and had distinctive kinetics compared to excitatory currents in ON alpha RGCs.
These results suggest that bipolar cell type-specific synaptic mechanisms play a critical role in divergence of visual signals across the parallel ON pathways we examined here.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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