ON BIPOLAR CELL RESPONSES IN THE TELEOST RETINA ARE GENERATED BY 2 DISTINCT MECHANISMS

1. ON Bipolar cells were recorded in slices obtained from hybrid bass retinas. Cells were identified as bipolar cells by position in the sli ce, by characteristic voltage- and ligand-gated currents, and by filli ng with the fluorescent dye Lucifer yellow. Cells were recorded with t he use of either whole cell or perforated-patch techniques. Standard e lectrophysiological protocols were used. Drugs were applied by puffing and by local superfusion. 2. Application of exogenous glutamate to ON bipolar cells generated two characteristic responses. One effect of g lutamate was to open a conductance with a reversal potential close to the chloride equilibrium potential. The other effect of glutamate was to close a conductance with a reversal potential near 0 mV. These two effects of glutamate on ON bipolar cells match the effects of light de scribed previously with the use of intracellular recordings. Thus the effects of glutamate that we report here appear to underlie the rod an d cone inputs to these cells. 3. Many of the ON bipolar cells recorded demonstrated both classes of responses to glutamate. To isolate the t wo responses, 500 mu M glutamate was first applied, and then glutamate in the presence of 5 mu M 2-amino-4-phosphonobutyric acid (APE). APE specifically blocks the effects of glutamate on the putative rod-drive n glutamate receptor (the glutamate-elicited conductance decrease), al lowing us to study in isolation the effects of glutamate on the cone c omponent, the glutamate-activated chloride current (I-Glu). 4. By isol ating I-Glu as described above, and taking advantage of the fact that amphotericin-perforated-patch recordings limit the diffusion of chlori de ions between the patch pipette and the cell body, we found the phys iological reversal potential of I-Glu to be -58.9 +/- 7.7 (SD) mV. 5. Both the putative rod- and cone-mediated glutamatergic inputs to these bipolar cells could be activated by driving the photoreceptors with p uffs of potassium. The currents recorded with this technique were very similar to those seen with direct application of glutamate.