Generation of the Early Receptor Potential with Long Flashes

Abstract

SINCE its discovery in 1964 by Brown and Murakami¹, the early receptor potential (ERP) has been detected in retinas isolated from a variety of vertebrates. The ERP is evoked by an intense light flash of typical duration less than a few hundred microseconds. Its latency is shorter than 0.5 µs (ref. 2), and at physiological temperatures it decays in a few milliseconds. The ERP has two components in opposite polarity, R1 and R2; R2 (vitreous negative) is dominant at physiological temperatures, while R1 can be isolated by cooling to near 0° C (ref. 3). It is almost certainly generated in the outer segments of the photoreceptors⁴, most likely by charge displacements associated with conformational changes in photopigment or of neighbouring molecules^2,5. In eyes fixed in formaldehyde the rise and decay times of the ERP are related in a simple manner to those of the stimulus flash⁶. In unfixed retinas, the ERP is more prolonged, and decays considerably later than the flash. It would be of considerable interest to know how the ERP behaves in unfixed retinas for really long flashes, particularly in view of recent results obtained from the lateral eye of Limulus by Smith and Brown⁷. They have described for the retina of this invertebrate a monophasic potential of rapid onset—the “photoelectric potential” (PEP)—with properties markedly similar to the vertebrate ERP. The stimulus was, however, a continuous arc xenon lamp focused to deliver several mwatts of radiant power to a single ommatidium, and the light pulses were essentially square waves of about 5 ms duration. The PEP recorded with a micropipette reversed in polarity as the membrane of a retinular cell was penetrated. The intracellular PEP attained a constant amplitude during the course of the light flash; it was not clear whether cooling produced a response of opposite polarity, as occurs for the vertebrate ERP.