Characterization of zebrafish rod and cone photoresponses

Zebrafish is a popular species widely used in vision research. The zebrafish retina has one rod and four cone subtypes (UV-, blue-, green-, and red-sensitive) with 40%-rod 60%-cone ratio, making it suitable for comparable studies of rods and cones in health and disease. However, the basic photoresponse properties of the four zebrafish cone subtypes have not been described yet. Here, we established a method for collecting flash photoresponses from zebrafish rods and cones by recording membrane current with a suction electrode. Photoreceptor subtypes could be distinguished based on their characteristic morphology and spectral sensitivity. Rods showed 40-220-fold higher photosensitivity than cones. In the four cone subtypes, green-sensitive cones showed the highest sensitivity, 5.5-fold higher than that of red cones. Unexpectedly, rods produced smaller flash responses than cones despite their larger outer segments. Dim flash response analysis showed the quickest response kinetics in blue- and red-sensitive cones, with responses about 2-fold faster than the responses of UV- and green-sensitive cones, and 6.6-fold faster than the rod responses. We also obtained pharmacologically isolated photoreceptor voltage responses (a-wave) from isolated zebrafish retinas using ex vivo electroretinography (ERG). Dim flashes evoked rod-only responses, while bright flashes evoked two-component responses with a slow rod component and a fast cone component. Red- and green-sensitive cones were the dominant sources of the overall cone response. These studies provide a foundation for the use of zebrafish rods and cones to study the fundamental mechanisms that modulate the function of vertebrate photoreceptors in health and disease.