Light absorbed by 575-cones trigger rod disc shedding in the frog retina

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Abstract
Photoreceptors periodically shed light-sensitive membranes; rods at light onset, cones at night. The spectral characteristics of light required to initiate shedding by 502-rods were studied in the frog retina. After 24 h of white light, animals were dark primed for 1 h then presented with 1 h of nearly monochromatic light to induce the shedding response. The light delivered a total photon flux of 2.5 or 0.5 μE/m2s. Following conventional fixation and plastic embedding, 1 μm sections were examined with light microscopy. The number of photoreceptor tips (phagosomes) shed by 100 consecutive rods were counted and plotted as a function of wavelength. Bright light induced at least 15 phagosomes per 100 rods at all wavelengths tested, 420−640 nm, and this shedding was more than doubled with light from 540−600 nm. When the light was dimmed, there was no shedding response except for this 540−600 nm window. This shedding peak closely corresponds to the absorbance curve of the frog's 575-cone photopigment and implies that the 575-cone can drive rod shedding. The broad background effect further indicates that all photoreceptors have an input and suggests that a luminosity cell, such as the internal horizontal cell, may be involved.