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W.B. Thoreson, D.A. Burkhardt; Contrast Encoding in Retinal Bipolar Cells: Current vs. Voltage . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4150.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate the influence of voltage-sensitive conductances in shaping responses to contrast steps in retinal bipolar cells. Methods: 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 sec steps of negative and positive contrasts of variable magnitude. Results: 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 3 of 4 cases, while no significant differences were found for half-maximal contrast (C50), dynamic range, or contrast dominance. Waveforms of current and voltage contrast responses were quite similar to one another although inward currents exceeded corresponding depolarizing voltage responses as the membrane potential approached -30 mV. This is explained by the finding that large voltage-dependent currents begin to activate as the membrane potential approaches -30 mV and thus depolarizing voltage responses are dampened by entering a region of lower input resistance. Conclusions: 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 post-synaptic current of bipolar cells, via the transformation between cone voltage and transmitter release and/or via the interaction between the neurotransmitter glutamate and its post-synaptic receptors on bipolar cells.
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