Synaptic action mediating cone responses to annular illumination in the retina of the larval tiger salamander.

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Abstract
Responses of salamander cones to steps of light on their surrounding area were intracellularly recorded through micropipettes filled with 2 M potassium acetate [KAc] or 2 M KCl. Using 2 M KAc, cone step responses showed a larger relaxation for a 1100 .mu.m diameter spot than for a 100 .mu.m spot, but dim annular illumination failed to evoke any detectable response. With 2 M KCl the larger spot or an annulus evoked responses that were mainly or purely depolarizing, respectively, while the responses to the smaller spot were still hyperpolarizing. The depolarizing response to annular illumination had an increased amplitude immediately after discontinuing the injection of inward current through a micropipette filled with 2 M KCl. Depolarization by extrinsic current reversed the polarity of the depolarizing response to annular illumination. Probably, the enhancement of the depolarizing influence of the surround is due to an increase in the intracellular concentration of Cl-, which issue from the electrodes by passive diffusion or electrophoresis. Taking into consideration the effect of depolarizing current, it would follow that surround illumination induces an increase in the Cl- conductance of the cone membrane. Preliminary observations on the properties of the enhanced surround response reveal a slow time course and a receptive field of more than 270 .mu.m radius.