Rod-dependent intracellular responses to light recorded from the pigment epithelium of the cat retina.

1. Intracellular recordings from pigment epithelial cells reveal rod-dependent hyperpolarizing responses to light which are identical with the c-wave of the electroretinogram. These responses were studied as a function of the duration, intensity and area of light flashes and also were compared with rod-dependent horizontal cell responses recorded under similar conditions.2. Because of the long time constant of the response (800 msec), pigment epithelial responses increased in amplitude as flash duration was increased over a wide range. The response reached a plateau at a flash duration of 2.4 sec so that longer flashes only increased the duration of the response. Responses to repetitive flashes, even at a slow rate of 2/sec were integrated to produce a sustained voltage.3. The growth of pigment epithelial response amplitude as a function of flash duration could be divided into two components. The increase in amplitude as flash duration was increased from 5-50 msec was also observed in horizontal cells and was assumed to have originated in the rods. The growth in amplitude with longer flashes was unique to the pigment epithelial response.4. The pigment epithelial response increased in amplitude as spot size was increased, reaching 90% of maximum with an 0.36 mm diameter spot. A precise measurement of the receptive field could not be obtained, because the responses were recorded in the tapetal area of dark-adapted retinas. The b-wave of the local electroretinogram which was ;picked-up' intracellularly exhibited a much greater area effect.5. Pigment epithelial responses increased in amplitude as flash intensity was increased but the amplitude-log intensity functions differed as a function of flash duration. With long flashes (> 2.4 sec) the growth in amplitude was a power function of intensity (exponent = 0.6-0.7) and the response reached an amplitude ceiling at about 3.0 log td scotopic. At higher intensities the responses increased in duration (the rod after-effect). With short flashes, e.g. 480 msec, the responses continued to increase in amplitude with flash intensity well above the intensity needed to produce amplitude-ceilings for rod-dependent responses. This additional increase in amplitude resulted from integration of the rod after-effect by the pigment epithelial response.6. Rod-dependent horizontal cell responses followed the amplitude-log intensity functions that were recorded in pigment epithelial cells with long flashes. The rod after-effect was also the same function of log intensity in both responses.7. It was concluded that the pigment epithelial response is not generated by a late photoproduct. Rather, it appears to depend upon the same mechanism of rod excitation as the horizontal cell response, and the essential characteristics of both rod-dependent responses originate in individual rods.