Recent physiological experiments support behavioral and
morphological evidence for a fourth type of cone in the turtle
retina, maximally sensitive in the ultraviolet (UV). This cone
type has not yet been included in the models proposed for
connectivity between cones and horizontal cells. In this study,
we examined the inputs of UV, S, M, and L cones to horizontal
cells. We used the high-resolution Dynamic Constant Response
Method to measure the spectral sensitivity of horizontal cells
without background light and after adaptation to UV, blue (B),
green (G), and red (R) light. We concluded the following: (1)
Tetrachromatic input to a Y/B horizontal cell was identified.
The spectral-sensitivity curves of the cell in three of the
adaptation conditions were well represented by L-, M-, and S-cone
functions. Adaptation to blue light revealed a peak at 372 nm,
the same wavelength location as that determined behaviorally
in the turtle. A porphyropsin template could be closely fitted
to the sensitivity band in that region, strong evidence for
input from a UV cone. (2) The spectral-sensitivity functions
of R/G horizontal cells were well represented by the L- and
M-cone functions. There was no indication of UV- or S-cone inputs
into these cells. (3) The spectral sensitivities of the monophasic
horizontal cells were dominated by the L cone. However, the
shape of the spectral-sensitivity function depended on the
background wavelength, indicating secondary M-cone input.
Connectivity models of the outer retina that predict input from
all cone types are supported by the finding of tetrachromatic
input into Y/B horizontal cells. In contrast, we did not find
tetrachromatic input to R/G and monophasic horizontal cells.
Chromatic adaptation revealed the spectral-sensitivity function
of the turtle UV cone peaking at 372 nm.
GABA receptors on horizontal cells in the goldfish retina
