Response characteristics and receptive field widths of on-bipolar cells in the mouse retina

Voltage-clamp and current-clamp recordings were made from bipolar cells in dark-adapted mouse retinal slices. Light-evoked responses fell into three groups corresponding to the rod bipolar cells, on-cone bipolar cells and off-cone bipolar cells. The morphology of the recorded cells confirmed this classification. Intensity-response relations were well fitted by a Michaelis saturation function with Hill coefficients of 1.15 +/- 0.11 (n = 6) for rod bipolar cells and 2.33 +/- 0.06 (n = 4) for cone inputs onto on-cone bipolar cells. In the absence of antagonists for GABA or glycine receptors, light-evoked synaptic currents for all cells displayed linear current-voltage relations that reversed near 0 mV, indicating that very little inhibition was activated under dark-adapted recording conditions. Saturating light stimuli evoked conductances of 0.81 +/- 0.56 nS (n = 4) in rod bipolar cells and 1.1 +/- 0.8 nS (n = 4) in on-cone bipolar cells. Receptive field widths were estimated by flashing a vertical light bar at various locations along the slice. Rod and on-cone bipolar cells had receptive field widths of 67 +/- 16 micrometer (n = 6) and 43 +/- 7 microm (n = 5), respectively. The maximum spatial resolution of an array of such cone bipolar cells was estimated to be 0.3 cycles deg-1, compared with a maximum resolution of 0.5 cycles deg-1 obtained from behavioural studies in mice. Our results suggest that this limit to spatial resolution could be imposed early in the visual system by the size of the bipolar cell receptive fields.