Properties of depolarizing bipolar cell responses to central illumination in salamander retinal slices

The voltage and current responses of depolarizing bipolar cells to central illumination were studied by means of whole-cell recording in retinal slices of the larval tiger salamander, Ambystoma tigrinum. To stabilize the responses, it was necessary to limit exchanges between the cytosol and the solution in the patch pipette by reducing the diameter of the pipette tip opening. The current-voltage relationship of the cell membrane in darkness displayed a strong outward rectification, and the inward current evoked by light could be consistently reversed by depolarization only when tetraethylammonium was added to the pipette solution. As a result of the membrane non-linearity, increases in the intensity of bright lights caused relatively smaller amplitude increases in the voltage than in the current responses and the latter had a proportionally smaller after-effect. With larger pipette tip openings, the cytosol equilibrated with the pipette filling solution. Under these conditions the light-evoked responses gradually became slower and acquired an on-off pattern, their final amplitude and polarity being determined by the ratio of the chloride concentrations on each side of the cell membrane. This finding is interpreted as revealing the existence of two response components: a chloride-dependent on-off increase in conductance and a faster depolarizing input that was lost through diffusional exchange. Addition of GTP and ATP to the electrode filling solution had a stabilizing effect on the labile component, whether or not cyclic GMP was also included. Observations on the magnitude of the conductance changes and on diphasic reversals indicate that the labile response component, presumably representing direct input from photoreceptors, is caused by an increase in conductance. The resulting inward current was still present at a low intracellular chloride concentration and may be assumed, therefore, to be carried by a cation influx.