EFFECTS OF PROLONGED DARKNESS ON LIGHT RESPONSIVENESS AND SPECTRAL SENSITIVITY OF CONE HORIZONTAL CELLS IN CARP RETINA IN-VIVO

Light responses of cone- and rod-driven horizontal cells were recorded intracellularly from opened eyes of intact, immobilized carp and the effects of prolonged darkness on these cells were examined. When the r etina was left in the dark, responses of cone horizontal cells to a mo derate test flash presented once every 10 min gradually and steadily d ecreased in size (dark suppression effect). Following a repetitive pre sentation of the test flash, the light responsiveness of the cells was enhanced. In contrast, the rod horizontal cells did not show the dark suppression effect and the light responsiveness of these cells increa sed in the dark. Effects of prolonged darkness on the spectral sensiti vity of L-type and R/G-type cone horizontal cells were also studied. T he spectral sensitivity of L-type cells, determined just after backgro und illumination was extinguished, agreed reasonably well with that of red-sensitive (R-) cones, but it matched better that of green-sensiti ve (G-) cones when determined in prolonged darkness. We further show t hat depolarizing responses of R/G-type cone horizontal cells to long-w avelength flashes, driven by R-cones, were abolished after prolonged d arkness. Taken together, these results suggest that input from R-cones converging onto the cone horizontal cells is more significantly suppr essed after prolonged dark adaptation than input from G-cones. Due to the suppression of the R-cone input, enhancement of responses of the L -type cone horizontal cells to long-wavelength stimulation in the pres ence of a green background light was no longer seen after prolonged da rkness. Following intraocular injections of B-hydroxydopamine (6-OHDA) , cone horizontal cells were still partially suppressed in the dark, b ut the suppression was much less significant than that observed in unt reated retinas, suggesting the involvement of dopamine in the dark sup pression effect. Partial dark suppression of cone horizontal cells obs erved in 6-OHDA-treated retinas also suggests that a substance(s) othe r than dopamine may contribute to the dark suppression effect.