Chromaticity (C-type) horizontal cells in the retina of
cold-blooded vertebrates receive antagonistic inputs from cone
photoreceptors of different spectral types leading to color
opponency. The relative contribution of each spectral type of
cones can be selectively altered by chromatic background
illumination. Therefore, the spectral properties of C-type
horizontal cells are expected to change when the intensity and
color of ambient illumination are altered. In this study, we
investigated the effects of chromatic background lights upon
color opponency in Red/Green (RGH) and Yellow/Blue (YBH) C-type
horizontal cells in the everted eyecup preparation of the turtle
Mauremys caspica. Photoresponses were elicited by
long-wavelength and short-wavelength light stimuli in the
dark-adapted state and under conditions of chromatic background
illumination. We found that the total voltage range, within
which graded depolarizing and the hyperpolarizing photoresponses
could be elicited, either increased or decreased depending upon
the color of the background light. However, the maximal and
minimal potential levels determined respectively by long-wavelength
and short-wavelength light stimuli of supersaturating intensity
remained unchanged, regardless of the wavelength and intensity
of the background. These findings indicate that turtle C-type
horizontal cells operate as push–pull devices. A sufficiently
bright short-wavelength stimulus can push them all the way to
the maximal hyperpolarizing level while a very bright
long-wavelength stimulus can pull them towards the most
depolarizing potential.