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

1. The light peak is a large, light-evoked increase in standing potential recorded in mammals, birds and reptiles. We have studied the cellular origin of the light peak in an in vitro preparation of neural retina-pigment epithelium (r.p.e)-choroid from the lizard, Gekko gekko. The tissue was mounted between two separate bathing solutions; the trans-tissue potential was recorded retinal-side positive; micro-electrodes were introduced to measure the trans-epithelial potential (t.e.p.) and to record intracellularly from the r.p.e.2. A 10 min stimulus of diffuse white light evoked an increase in trans-tissue potential that reached maximum amplitude, the light peak, about 15 min after stimulus onset. Since the light peak is present in vitro, it must originate in either the neural retina or the r.p.e.3. A micro-electrode was positioned in the subretinal space and the trans-retinal potential and t.e.p. were measured simultaneously. A 10 min stimulus produced an increase in t.e.p. equal in magnitude and time course to the trans-tissue light peak; no potential was present across the retina. The light peak is therefore generated solely across the r.p.e.4. Intracellular r.p.e. 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 r.p.e. c-wave followed by a depolarization of both membranes with the time course of the light peak. We conclude that whereas the r.p.e. c-wave results from a hyperpolarization of the apical membrane, the light peak is generated by a depolarization of the basal membrane of the r.p.e.5. Changes in tissue resistance, R(t), 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. R(t) decreased and a increased with the time course of the light peak. Assuming that the paracellular shunt resistance is constant, we conclude that the light peak is accompanied by an increase in basal membrane conductance.6. This and the following paper present the first direct demonstration of an interaction between the neural retina and the basal membrane of the r.p.e. The light peak, initiated by absorption of light by photoreceptors, results in a depolarization and conductance increase of the basal membrane.