Origin of the light peak: in vitro study of Gekko gekko

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Abstract
The light peak is a large, light-evoked increase in standing potential recorded in mammals, birds and reptiles. The cellular origin of the light peak was studied in an in vitro preparation of neural retina-pigment epithelium (RPE)-choroid from the lizard, G. gecko. The tissue was mounted between 2 separate bathing solutions; the transtissue potential was recorded retinal-side positive; microelectrodes were introduced to measure the transepithelial potential (TEP) and to record intracellularly from the RPE. A 10-min stimulus of diffuse white light evoked an increase in transtissue potential that reached maximum amplitude, the light peak, .apprx. 15 min after stimulus onset. Since the light peak is present in vitro, it must originate in either the neural retina or the RPE. A microelectrode was positioned in the subretinal space and the transretinal potential and TEP were measured simultaneously. A 10-min stimulus produced an increase in TEP equal in magnitude and time course to the transtissue light peak; no potential was present across the retina. The light peak is thus generated solely across the RPE. Intracellular RPE recordings were made to determine whether the light peak was generated at the apical or basal membrane or across the paracellular shunt. A 10-min stimulus first caused a hyperpolarization of both membranes with a time course similar to the RPE c-wave followed by a depolarization of both membranes with the time course of the light peak. Apparently, whereas the RPE c-wave results from a hyperpolarization of the apical membrane, the light peak is generated by a depolarization of the basal membrane of the RPE. Changes in tissue resistance, Rt, and the ratio of apical to basal membrane resistances, a, were monitored during the light peak by passing current across the tissue and measuring the appropriate current-induced voltages. Rt decreased and a increased with the time course of the light peak. Assuming that the paracellular shunt resistance is constant, the light peak may be accompanied by an increase in basal membrane conductance. This is the 1st direct demonstration of an interaction between the neural retina and the basal membrane of the RPE. The light peak, initiated by absorption of light by photoreceptors, results in a depolarization and conductance increase of the basal membrane.