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Reece Mazade, Erika D Eggers; Light adaptation narrows spatial inhibitory input and increases signal output of the mouse inner retina. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3223.
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© ARVO (1962-2015); The Authors (2016-present)
OFF cone bipolar cells (OFF BCs) in the inner retina bridge the rod and cone pathways by receiving excitatory input from cones and inhibitory input via amacrine cells (ACs) from both rods and cones. We have previously shown that light adaptation of OFF BC inhibition results in a switch from glycinergic to GABAergic inhibition, which changes the source of inhibitory input from morphologically narrow-field glycinergic to wide-field GABAergic ACs. However, it is unknown if this switch changes the spatial inhibitory input to OFF BCs, and as a result the input signal strength to ganglion cells (GC).
We used whole-cell voltage clamp to record light-evoked inhibitory and excitatory postsynaptic currents from dark-adapted mouse OFF BCs. A white OLED screen was used to set the background light and to generate 25 μm bars of light flashed for 1 sec to map spatial inhibition. The magnitude of light-evoked responses was measured as charge transfer and peak amplitude. The spatial distributions were averaged and compared between light conditions where significance was p<0.05.To evaluate GC input strength, we constructed a model of BC inputs to a GC using simulated BC excitation and inhibition Gaussian distributions to calculate the differences in the spatial strength to GCs under dark and light-adapted conditions.
We predicted that OFF BC spatial inhibition would widen with light adaptation as a result of the wide spatial extent of GABAergic ACs. Surprisingly, we found that the total, isolated GABAergic, and isolated glycinergic spatial inhibition to OFF BCs became narrower with light adaptation (p<0.05). There was no significant change in the spatial input of excitatory responses to the OFF BCs with light adaptation. Our model suggested that when switching from dark to light-adapted conditions, the narrower inner retinal spatial inhibition increases the excitatory strength of distinct spatial inputs to the GCs.
Here we show that light adaptation narrows OFF BC spatial inhibitory input while excitatory spatial input remains unchanged. Adjusting the inhibitory surround of BCs may be part of the mechanism for ganglion cell center-surround changes seen with light adaptation, as a simple model suggests that changes in the inner retina increase ganglion cell excitation to small spots of light. This may be one factor playing a role in increasing visual acuity during the day.
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