Responses of rod bipolar cells in the dark-adapted retina of the dogfish, Scyliorhinus canicula.

1. Responses to light were recorded from bipolar cells in the retina of the dogfish, Scyliorhinus canicula, under dark-adapted conditions. The identity of the cells was confirmed by Procion Yellow staining.2. More than 95% of the bipolar cells sampled were of the type which depolarized to a spot of light. These are termed depolarizing bipolar cells. In most cells, illumination of the surround had little effect on the responses elicited from the central receptive field.3. The mean flash sensitivity of the depolarizing bipolar cells was 270 mV/Rh(**) (where Rh(**) signifies rhodopsin photoisomerization per rod for full field illumination).4. The mean flash sensitivity of horizontal cells under the same conditions was 8 mV/Rh(**). In a limited sample of hyperpolarizing bipolar cells the highest flash sensitivity was 42 mV/Rh(**).5. The high flash sensitivity of the depolarizing bipolar cells indicates a large voltage gain at its synapse with rods. On the assumption of a rod flash sensitivity of 2 mV/Rh(**) the mean gain at the synapse was 135, but for some cells the gain was in excess of 500.6. Responses of depolarizing bipolar cells to dim flashes could be approximated by the impulse response of a 12-16 stage low-pass filter, whereas horizontal cell responses could be fitted by a low-pass filter of six sections. The implied filter at the rod-bipolar cell synapse is tuned to the higher frequency components of rod signals, thereby improving temporal resolution in the rod pathway.7. Depolarizing bipolar cell responses to test flashes are reduced by weak background illumination (less than 0.1 Rh(**)/sec). This desensitization, which would not be expected to affect rod responses, could be explained by a shift in the operating point to a less sensitive region of the intensity-response curve as a result of the large depolarization elicited by the background.8. The results of current injection into the cell in darkness and during the response to light are consistent with the release by rod terminals of a transmitter which closes ionic channels in a conductance path having a reversal potential of - 8 mV, transmitter release being suppressed by light.