Local stimulation induces shedding throughout the frog retina

Our previous work has demonstrated that rod shedding in the frog retina can be driven by environmental cues such as light onset. Although shedding normally occurs binocularly, we found that shedding could be initiated independently in either eye of the frog by monocular stimulation. Further, rod shedding occurs in vitro in the isolated eyecup under appropriate incubation conditions when provided with a light stimulus following a dark incubation period. Thus, the control mechanism for light induced rod shedding in the frog seems to be located within the eye, and does not seem to be systemically or centrally located. However, the exact link between light onset and shedding of the distal rod tips remains unknown. To elucidate further the control site for initiation of rod shedding, we used a variety of stimulus conditions, including front and rear screens as well as spots and slits projected directly on the retina, to stimulate a small portion of the frog retina with a range of light intensities and stimulus paradigms. In all cases where shedding occurred, it was uniform throughout the retina. Thus, it appears that the light-cued message received by a small population of photoreceptors is sufficient to initiate shedding throughout the retina. These results differ significantly from those found by Easter and Macy for light-induced photomechanical movements, which were found to be locally controlled.