UV input to chromaticity horizontal cells in the turtle retina

Purpose. Single cones containing UV-sensitive photopigment have been presumed to exist in the turtle retina based on morphological and behavioral considerations. This study was designed to test the physiological role of these cones in the distal retina. Methods: The photoresponses of 3 Yellow/Blue and 4 Red/Green chromaticity-type horizontal cells were measured intracellularly in the turtle eyecup preparation. The action spectra were determined in the range 620-680nm. The effects of chromatic background illuminations on the action spectrum were used to reveal the contribution of UV input. Results: In the dark-adapted state the Y/B cells exhibited peak sensitivity for hyperpolarizing responses around 450nm with a secondary peak around 360nm. The action spectra of the R/G cells did not indicate an input in the UV range. Mixtures of bright red light and monochromatic lights of different wavelengths and intensities were used to reveal the action spectrum of the hyperpolarizing inputs to Y/B cels. It peaked at 450nm and at 360nm. The action spectra of the Y/B horizontal cells were also measured during continuous illumination with wavelengths longer than 520nm or with a blue band-pass filter (peak at 450nm) The difference spectra were characterized by relative augmentation of flash sensitivity in the blue and UV regions of the spectrum. Conclusions: These data are consistent with an UV input to Yellow/Blue C-type horizontal cells. The UV input contributes to the hyperpolarizing responses of these horizontal cells.