Light-evoked Increases in Extracellular K+ in the Plexiform Layers of Amphibian Retinas

Recordings of light-evoked changes in extracellular K+ concentration (delta[K+]o) were obtained in the retinas of frog and mudpuppy. In eyecup preparations, various recording approaches were used and provided evidence for a K increase near the outer plexiform layer (distal K increase). This distal K increase could be pharmacologically dissociated from the well-known, large K increase in the proximal retina by the application of ethanol and gamma-aminobutyric acid. The distal K increase also often showed surround antagonism. A retinal slice preparation was used to permit electrode placement into the desired retinal layers under direct visual control and without the risk of electrode damage to adjacent layers. In the slice, a distinct distal K increase was found in the outer plexiform layer, in addition to the prominent K increase in the inner plexiform layer. Compared with eyecups, only weak K increases were found in the nuclear layers of the slice. This suggests that the K responses observed in the nuclear layers of eyecups may be generated by K+ diffusing along the electrode track from the plexiform layers. In the context of current models of ERG b-wave generation, the magnitude of the recorded distal K increase, compared with the proximal K increase, seems too small to give rise to the b-wave. However, the distal K increase may be differentially depressed by electrode dead space. It is also possible that if certain aspects of the models of b-wave generation were modified, then the observed distal K increase could give rise to the b-wave.