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.
