Noninvasive recording and response characteristics of the rat dc-electroretinogram

In response to light, the retinal pigment epithelium (RPE) generates a series of potentials that can be recorded using the dc-electroretinogram (dc-ERG). As these potentials can be related to specific cellular events, they provide information about RPE function and how that may be altered by disease or experimental manipulation. The purposes of the present study were to define a noninvasive means for recording the rat dc-ERG, to use this to define the stimulus-response properties of the major components, and to relate these results to measures of the rat electrooculogram (EOG). Parallel studies were conducted in two strains of rats (Long-Evans, LE; Sprague-Dawley, SD) that are commonly used in vision research. Rats were sedated with ketamine/xylazine and placed on a heating pad. Ag/AgCl wire electrodes were bridged with capillary tubes filled with Hanks balanced salt solution. The active electrode was placed in contact with the corneal surface and referenced to a second electrode placed within the orbit. The dc-ERG signal was amplified (dc-100 Hz), digitized, and stored offline. The duration of full-field flash stimuli was controlled using a mechanical shutter and flash luminance was controlled with neutral density filters. EOGs were recorded using subdermal platinum needle electrodes placed near the eye. In response to a 5-min light exposure, the dc-ERG of LE and SD rats included a distinct b-wave, after potential, c-wave, fast oscillation, and a slow potential of positive polarity the characteristics of which are consistent with a light peak. In LE rats, the final plateau of this slow positive potential was often lower than the prestimulus baseline; in SD rats, this potential achieved a level above the baseline. Analysis of EOGs recorded from these two strains yielded results consistent with the amplitude of the slow potential relative to the prestimulus baseline. Specifically, the amplitude of the EOG of SD rats increased when the eye was exposed to light. In LE rats, this increase did not occur, and in some cases light reduced the amplitude of the EOG. The two strains also differed with respect to c-wave implicit time, which was faster in SD rats. These results indicate that many of the major components of the dc-ERG are readily measured in the rat. Therefore, we believe that the rat may provide a useful animal model in which to conduct pharmacological analysis of nonneuronal responses and to develop animal models of human retinal disorders involving the RPE, such as Best Vitelliform Macular Dystrophy.