LIGHT-EVOKED CHANGES IN NEAR-INFRARED TRANSMISSION BY THE ON AND OFF CHANNELS OF THE ANURAN RETINA

We have developed an optical method to monitor the activity of the ON and OFF channels in the anuran retina. The change in the fraction of near infrared that is transmitted transversely through the retina in an eyecup slice is monitored during stimulation by visible, green light. Near-infrared transmission increases both at the onset and at the termination of a step stimulus. This ''ON/OFF'' response is maximal in the neural retina. Sodium L-aspartate, which blocks the light-evoked activity of post-photoreceptor neurons, abolishes the ''ON/OFF'' response. L-AP4, used as a selective blocker of the ON channel, reduces the ''ON'' component and has little or no effect on the ''OFF'' component. The ''ON'' and ''OFF'' processes observed optically are distinct from those that generate the b- and d-waves of the electroretinogram, and the ''ON'' and ''OFF'' components may be superior to the b- and d-waves as indicators of ON and OFF channel activity. The optical method is almost as simple as electroretinography and has the advantage that responses can be spatially localized with ease.