Spectral responses in zebrafish horizontal cells include a tetraphasic response and a novel UV-dominated triphasic response.

Zebrafish are tetrachromats with red (R, 570 nm), green (G, 480 nm), blue (B, 415 nm), and UV (U, 362 nm) cones. Although neurons in other cyprinid retinas are rich in color processing neural circuitry, spectral responses of individual neurons in zebrafish retina, a genetic model for vertebrate color vision, are yet to be studied. Using dye-filled sharp microelectrodes, horizontal cell voltage responses to light stimuli of different wavelengths and irradiances were recorded in a superfused eyecup. Spectral properties were assessed both qualitatively and quantitatively. Six spectral classes of horizontal cell were distinguished. Two monophasic response types (L1 and L2) hyperpolarized at all wavelengths. L1 sensitivities peaked at 493 nm, near the G cone absorbance maximum. Modeled spectra suggest equally weighted inputs from both R and G cones and, in addition, a "hidden opponency" from blue cones. These were classified as R-/G-/(b+). L2 sensitivities were maximal at 563 nm near the R cone absorbance peak; modeled spectra were dominated by R cones, with lesser G cone contributions. B and UV cone signals were small or absent. These are R-/g-. Four chromatic (C-type) horizontal cells were either depolarized (+) or hyperpolarized (-) depending on stimulus wavelength. These types are biphasic (R+/G-/B-) with peak excitation at 467 nm, between G and B cone absorbance peaks, UV triphasic (r-/G+/U-) with peak excitation at 362 nm similar to UV cones, and blue triphasic (r-/G+/B-/u-) and blue tetraphasic (r-/G+/B-/u+), with peak excitation at 409 and 411 nm, respectively, similar to B cones. UV triphasic and blue tetraphasic horizontal cell spectral responses are unique and were not anticipated in previous models of distal color circuitry in cyprinids.