Origin and sensitivity of the light peak in the intact cat eye

Abstract

The light peak is a large light-induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5-13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus-response characteristics. Direct-coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular microelectrodes. Light peaks were generally evoked with 300-s periods of diffuse white illumination. Microelectrode recordings made in the subretinal space just outside the apical membrane of the retinal pigment epithelium (RPE) showed that the light peak was a change in transepithelial potential. No component was generated in the neural retina. Intracellular recordings from RPE cells showed that the change in transepithelial potential resulted from a depolarization of the basal membrane (facing the choroid). This depolarization came after the hyperpolarization of the apical membrane that gave rise to the RPE component of the c-wave of the ERG [electroretinogram]. The light peak amplitude at a constant retinal illumination was nearly linear with stimulus duration over the range 15-180 s, and saturated at .apprx. 300 s. The time-to-peak remained nearly constant at .apprx. 300 s over this range. Large light peaks could be evoked with flashes as short as 10 s if the retinal illumination was several log units above rod saturation. When stimulus duration was held constant at 300 s, light peak amplitude was graded with illumination over a wide range, from 3 log units below to 2 log units above rod saturation. The threshold of the light peak was below that of the ERG and only .apprx. 1.5-2.5 log units above the absolute threshold of the most sensitive ganglion cells. The increase of light peak amplitude above rod saturation was not due primarily to cones. The transepithelial light peak had an unusual dependence on stimulus area, being at least twice as large in response to diffuse light as it was in response to a large spot (10.degree. diameter) of the same retinal illumination. The light peak may represent a normal physiological interaction between the retina and the RPE. The interaction may involve a change in the concentration of a diffusible substance in the retina, which then either enters the RPE itself or triggers an internal messenger to cause the basal depolarization.