UV‐sensitive input to horizontal cells in the turtle retina

Microspectrophotometry, electroretinography and behavioural studies have indicated that ultraviolet (UV) light contributes to functional vision in various vertebrate species. Based on behavioural evidence, this was also suggested for turtle vision. In order to reveal the interactions underlying detection of UV light in the distal retina, we recorded intracellularly the photoresponses of cones and horizontal cells in retinas of Pseudemys scripta elegans and Mauremys caspica and calculated the action spectra of these cells under different conditions of adaptation. In the dark-adapted retina, all three types of horizontal cells; luminosity-type, red/green chromaticity-type and yellow/blue chromaticity-type exhibited increased sensitivity in the UV region of the spectrum. However, chromatic adaptation indicated that only the yellow/blue chromaticity-type horizontal cells received excitatory input from UV-sensitive cones with peak sensitivity approximately 360 nm. The enhanced UV sensitivity of luminosity-type horizontal cells probably reflected the beta-band of the long-wavelength sensitive visual pigment as indicated by the action spectra of dark-adapted L-cones. It is suggested that the enhanced UV sensitivity of red/green chromaticity-type horizontal cells reflects the beta-band of the medium-wavelength sensitive visual pigment. Transmission measurements of the optical media (cornea, lens and vitreous) indicated that UV vision can be functional under normal circumstances.