Light Sensitivity in Cones is Affected by the Feedback from Horizontal Cells

We examined the effect of annular illumination on resetting of the I-R relation measured intracellularly in the tiger salamander cones. Our results suggest that peripheral illumination contributes to the cellular mechanism of adaptation. This is done by a neural network involving feedback synapse from horizontal cell to cones. The effect is to unsaturate the membrane potential of a fully hyperpolarized cone, by “instantaneously” shifting cone’s I-R curves along intensity axis to be in register with ambient light level of the periphery. An equivalent electrical circuit with three different transmembrane channels, (leakage, photocurrent and feedback) was used to model static behavior of a cone. SPICE simulation showed that interactions between feedback and the light sensitive conductance can shift the I-R curves along the intensity domain, provided that phototransduction mechanism is not saturated during maximally hyperpolarized light response.