THE ORIGIN OF SLOW PIII IN FROG RETINA - CURRENT SOURCE DENSITY ANALYSIS IN THE EYECUP AND ISOLATED RETINA

The objective of this research was to determine the sources and sinks of current underlying the slow Pm component of the electroretinogram. Current source density analysis of the ERG evoked by diffuse light fla shes was performed in eyecup and isolated retinas of frog. Blockade of synaptic transmission with aminophosphonobutyric + kynurenic acids si mplified the CSD profiles through the retina. In addition to the photo receptor source/sink pair, there was evidence for a major slow Pm sour ce near the outer limiting membrane, a major sink near the inner limit ing membrane, and a small source near the inner plexiform layer. Addit ion of Ba2+ abolished the slow Pm source/sinks, and it left only the p hotoreceptor source and sink. The results support the idea that slow P III originates through K+ spatial buffering by Muller cells. Specifica lly, the light-evoked decrease in [K+](0) in the subretinal space caus es a primary K+ efflux from Muller cells (current source) and a primar y K+ influx at the Muller cell endfeet (current sink). A decrease in [ K+](0) in the proximal retina, caused by diffusion of K+ to the subret inal space, results in K+ efflux (the current source) at the inner ple xiform layer.