PHARMACOLOGY OF THE SKATE ELECTRORETINOGRAM INDICATES INDEPENDENT ON AND OFF BIPOLAR CELL PATHWAYS

Organization of afferent information into parallel ON and OFF pathways is a critical feature of the vertebrate visual system. All afferent v isual information in the vertebrate retina reaches the inner plexiform layer (IPL) via bipolar cells. It is at the bipolar cell level that s eparation of ON and OFF information first appears for afferent informa tion from cones. This may also hold true for the rod pathway of cold-b looded vertebrates, but not for mammals. The all-rod retina of the ska te presents an opportunity to examine such pathways in a retina having but a single class of photoreceptor. Immunocytochemical evidence sugg ests that both ON and OFF bipolar cells are present in the skate retin a. We examined the pharmacology of the skate electroretinogram (ERG) t o test the hypothesis that independent ON and OFF bipolar cell pathway s are functional as rod afferent pathways from outer to inner plexifor m layer in the skate. 100 mu M 2-amino-4-phosphonobutyric acid (APE) r eversibly blocked the skate ERG b-wave. A small d-wave-like OFF compon ent of the ERG revealed by DC recording of response to a prolonged (10 s) flash of light was reduced or blocked by 5 mM kynurenic acid (KYN) . We found that addition of 200 mu M picrotoxin to the Ringer's soluti on revealed prominent ON and OFF components of the skate ERG while red ucing the c-wave. These ON and OFF components were reversibly blocked by 100 mu M APE and 5 mM KYN, respectively. Reversible block of the OF F component by KYN was also accomplished in the presence of 500 mu M N -methyl-DL-aspartate. From these findings, we conclude that ON and OFF bipolar cells are likely to be functional as parallel afferent interp lexiform pathways in the all-rod retina of the skate.