Disc shedding and autophagy in the cone-dominant ground squirrel retina

The temporal pattern of cone outer-segment disc shedding was examined in the retina of the California ground squirrel, Spermophilus beecheyi, under two different lighting conditions. Squirrels were entrained to either 10-180 lx (room lighting) or 1,400 lx. Cone shedding during the dark period was biphasic in both conditions, with increases occurring at 2-3 hr and 5-6 hr after light offset. Entrainment to 1400 lx resulted in an increase in shedding at 2 hr after light offset and a slight advance in the timing of both peaks. Dense granules were often found in photoreceptors, retinal neurons, Müller cells, microglia and vascular cells. These granules, which were found primarily during the dark period and early light period, were lipofuscin-like in their lipophilia, size and autofluorescence. Many of the granules were probably autophagic in origin, but some within Müller cells may have originated via endocytosis of extracellular material which was exocytosed by photoreceptors.