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Reece Mazade, Erika D Eggers; Spatial inhibitory input to the retinal OFF pathway narrows with light adaptation. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5007.
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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 rod and cone pathways. While OFF BC inhibition is primarily glycinergic in the dark, we previously found that there was a compensatory switch to larger GABAergic input with light adaptation. However, it is unknown how this switch will affect the spatial inhibitory input to OFF BCs as it underlies a switch from morphologically narrow-field glycinergic to wide-field GABAergic ACs.
Using whole-cell voltage clamp, light-evoked inhibitory postsynaptic currents (L-IPSCs) were recorded from dark-adapted mouse OFF BCs, identified via fluorescent labeling, while holding at the reversal potential for nonspecific cation currents. The magnitude of L-IPSCs was measured as charge transfer. A white OLED screen was used to set the background light and to generate 25 μm bars of light flashed for 500ms to map spatial inhibition. The spatial distributions were averaged and compared between light conditions where significance was p<0.05.
Due to larger GABAergic input to OFF BCs in the light, 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 spatial inhibition to OFF BCs became narrower with light adaptation (n=9, p<0.05). When specific spatial receptor inputs were isolated, we found that under both dark- and light-adapted conditions, GABAergic spatial input (dark n=6, light n=3) to OFF BCs was not different than glycinergic input (dark n=9, light n=6, p>0.05). However, unexpectedly, both GABAergic and glycinergic specific input to OFF BCs also became narrower in the light (p<0.05).
Here we show that light adaptation narrows OFF BC spatial inhibitory input. Though anatomical measurements imply a change to wider inhibitory surrounds, our initial results suggest opposite effects. Factors in addition to the spatial extent of ACs may play a role in determining the spatial sensitivity of inhibition, such as dopamine modulation of circuits and receptor properties and specific BC-AC and AC-AC interactions. Adjusting the inhibitory surround of BCs may be part of the mechanism for ganglion cell center-surround changes seen with light adaptation.
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