Postsynaptic calcium feedback between rods and rod bipolar cells in the mouse retina

Light-evoked currents were recorded from rod bipolar cells in a dark-adapted mouse retinal slice preparation. Low-intensity light steps evoked a sustained inward Current. Saturating light steps evoked an inward current with an initial peak that inactivated. with a time constant of about 60-70 ms, to a steady plateau level that was maintained for the duration of the step. The inactivation was strongest at hyperpolarized potentials, and absent at positive potentials. Inactivation was mediated by an increase in the intracullular calcium concentration, as it was abolished in cells dialyzed with 10 mM BAPTA, but was present in cells dialyzed with 1 mM EGTA. Moreover, responses to brief flashed of lialit were broader in the presence of intracellular BAPTA indicating that the calcium feedback actively shapes the time Course of the light responses. Recovery from inactivation observed for paired-pulse stimuli Occurred with a time constant of about 375 ins. Calcium feedback could act to increase the dynamic range of the bipolar cells. and to reduce variability in the amplitude and duration of the sin.-le-photon signal. This may be important for nonlinear processing at downstream sites of convergence front rod bipolar cells to AII amacrine cells. A model in which intracellular calcium rapidly binds to the light-gated channel and reduces the conductance can account for the results.