Contrast encoding in retinal bipolar cells: current vs. voltage.

To investigate the influence of voltage-sensitive conductances in shaping light-evoked responses of retinal bipolar cells, whole-cell recordings were made in the slice preparation of the tiger salamander, Ambystoma tigrinum. To study contrast encoding, the retina was stimulated with 0.5-s steps of negative and positive contrasts of variable magnitude. In the main, responses recorded under voltage- and current-clamp modes were remarkably similar. In general agreement with past results in the intact retina, the contrast/response curves were relatively steep for small contrasts, thus showing high contrast gain; the dynamic range was narrow, and responses tended to saturate at relatively small contrasts. For ON and OFF cells, linear regression analysis showed that the current response accounted for 83-93% of the variance of the voltage response. Analysis of specific parameters of the contrast/response curve showed that contrast gain was marginally higher for voltage than current in three of four cases, while no significant differences were found for half-maximal contrast (C50), dynamic range, or contrast dominance. In sum, the overall similarity between current and voltage responses indicates that voltage-sensitive conductances do not play a major role in determining the shape of the bipolar cell's contrast response in the light-adapted retina. The salient characteristics of the contrast response of bipolars apparently arise between the level of the cone voltage response and the postsynaptic current of bipolar cells, via the transformation between cone voltage and transmitter release and/or via the interaction between the neurotransmitter glutamate and its postsynaptic receptors on bipolar cells.