Role of GABA neurotransmitter in chromatic processing of outer nuclear layer in turtle retina

Abstract

Information related to light stimuli is transmitted from the cones to the second (horizontal and bipolar) and to the third order (amacrine and ganglion) cells. The electrophysiological responses and the morphology of these cells are known, but little is known about the responses to ultraviolet (UV) light, despite the evidence of the ability of the turtle to discriminate color in the UV range. One objective of this study was to evaluate the cone inputs to horizontal cells. The second objective was to study the electrophysiological responses of morphologically identified amacrine and ganglion cells introducing stimulation in the UV region (300-400 nm), in addition to the spectral range previously tested by most other investigators (400¬700 nm). The activity of cells in the outer and inner retinae was recorded intracellularly in the presence of light stimuli of different wavelengths, intensity and area. The spectral sensitivity of horizontal cells was measured by the Dynamic Constant Response Method without background light and under different monochromatic background lights. Several amacrine and ganglion cells were injected with Neurobiotin and their morphology visualized with a confocal microscope. We identified inputs the of UV, B, G and R cones to one biphasic Y/B horizontal cell and it was also possible to estimate the sensitivity function of the UV cone. The input of G and R cones to biphasic R/G horizontal cells and a major input of R cones into monophasic horizontal cell was also identified. Twenty two ganglion cells were recorded from, 14 of which were labeled. Color opponency was recorded from nine of these cells. Of the labeled cells, two had ON responses, four had OFF responses and six responded with an ON-OFF. Another ganglion cell was maximally sensitive to UV light. Five amacrine cells were labled. Three among the amacrine cells were color opponent, one had ON-OFF responses and one had either ON or OFF responses, depending on the stimuli intensity used. Altogether, out of the 14 theoretical possible color opponency types, eight were recorded in the inner retina. In conclusion, in the outer retina, this work shows (1) that responses to UV light are present in all horizontal cells recorded from, (2) that there is a tetrachromatic input to one biphasic Y/B horizontal cell and (3) suggest an estimation of the spectral sensitivity of the UV cone. In the inner retina the work showed: (1) UV/blue opponency in five ganglion cells, (2) UV/red opponency in eight ganglion cells, (3) one ganglion cell whose sensitivity was specifically tuned to UV light and (4) three ganglion cells and one amacrine cells whose morphology was not seen before. The results not only support the known evidence related to turtle UV vision, but also expand and modify the previous physiological classification of turtle inner retina cells. (AU)