Electroretinogram of the parietal eye of lizards: Photoreceptor, glial, and lens cell contributions

Abstract

Local electroretinograms (ERGs) were recorded in the parietal eye of Xantusia vigilis. The responses to monochromatic light under dark- and light-adapted conditions were studied. We found that two antagonistic chromatic mechanisms dominate the overall response. With the electrode tip in the lumen of the eye, light stimulation under dark-adapted conditions evoked responses of negative polarity with maximum sensitivity to green light. Intense green background illumination saturated the green-sensitive mechanism, and superposition of a blue stimulus then elicited responses of opposite polarity, driving the potentials back toward the dark resting level. The spectral sensitivities of the two chromatic mechanisms were determined using chromatic adaptation. The lower threshold, green-sensitive mechanism has a maximum sensitivity at 495 nm while the antagonistic mechanism, with its maximal spectral sensitivity at 430 nm, is at least 2 log units less sensitive. The polarity of the ERG recording inverts as the electrode traverses the photoreceptor layer, suggesting that the photoreceptors are the major source of the ERG. This result was confirmed with intracellular recordings from photoreceptors, glial, and lens cells. The glial and lens cells of the parietal eye respond to local changes in [K⁺]_o. Intracellular recordings of the responses of these cells to light stimuli follow time courses similar to changes in extracellular potassium concentrations measured with K⁺-specific electrodes. These results suggest that the glial and lens cell membranes are highly permeable to potassium and, therefore, the electrical responses of these cells are evoked by changes in [K⁺]_o elicited by light stimulation of the photoreceptors. Nevertheless, the major component of the parietal eye ERG is the photoreceptor signal. A circuit model of the ERG sources is presented.