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Reece Mazade, Erika Eggers; Light adaptation alters the spatial distribution of inhibition to the retinal OFF pathway. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3422.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal OFF cone bipolar cells (OFF BCs) bridge the rod and cone pathways by receiving both excitatory input from cones and inhibitory input via amacrine cells (ACs) from both pathways. We previously found that OFF BC types 1,2, and 4 (OFF1,2,4) and OFF BC type 3 (OFF3) have different inhibitory inputs. OFF BC inhibitory responses were dominated by glycine in the dark but had a compensatory switch to GABA input with light adaptation. However, it is unknown how this switch affects the spatial inhibition since glycine and GABA have distinct spatial extents.
Light-evoked inhibitory postsynaptic currents (L-IPSCs) were recorded at the reversal potential for cation currents from dark-adapted mouse OFF BCs, identified via fluorescent labeling. The magnitude of L-IPSCs was measured as charge transfer. A white OLED screen was used to set the background light and to generate 25μm bars of light flashed for 500ms to map spatial inhibition. The data was averaged and the standard deviation of fitted Gaussian curves was used for comparison.
The compensatory inhibition switch with light adaptation suggested a potential widening of spatial inhibitory input due to a change from narrow-field glycinergic to wide-field GABAergic ACs. However, our initial results show that OFF3 BCs had significantly narrower spatial inhibition in the light (n = 5, p < 0.05) while OFF1,2,4 BCs spatial inhibition showed a trend toward a narrower spatial distribution (n = 4, p = 0.10). Gaussian curve fits for all OFF BCs revealed that the standard deviations were significantly smaller with light adaptation further suggesting a narrowing spatial inhibition (20% of OFF3 dark-adapted distribution, 46% of OFF1,2,4 dark-adapted distribution, p < 0.05).
We show that light adaptation changes OFF BC spatial inhibitory input. Though previous work implies a change to wider inhibitory surrounds, our initial results suggest a reduction of OFF BC total spatial inhibition. This suggests that factors in addition to the spatial extent of ACs are determining the spatial sensitivity of inhibition. Adjusting the spatial inhibitory surrounds undoubtedly affects the information that downstream ganglion cells receive likely altering their spatial acuity. Decreasing inhibitory surround size may allow for the comparison of more distinct light stimuli which may be useful under light-adapted conditions.
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