Ultrastructural analysis of arrestin distribution in mouse photoreceptors during dark/light cycle

Arrestin localization was studied in BALB/c mice retinas during a 12-hr dark/light diurnal cycle and under various light/dark interruptions. Intracellular distribution of arrestin in photoreceptor cells was determined by immunocytochemistry and electron microscopy. During the light phase of the diurnal cycle, arrestin was localized mostly in the rod outer segments. During the dark phase of the cycle, arrestin was localized mostly in the inner segment, nuclei and synaptic terminals. The disc domains of the rod outer segments were labeled at a low density, but conspicuous cytoplasmic regions in the outer segment were labeled at a very high density. These cytoplasmic regions were not labeled in our illuminated retinas. Hence, intrasegmental segregation within the outer segment may be influenced by environmental lighting. During dark adaptation, increase in inner segment labeling density was observed. In previous studies, decrease in outer segment and increase in inner segment labeling density in the dark, as determined by light microscopy, was interpreted as movement of arrestin from the outer to inner segments. Our present ultrastructural analysis of arrestin distribution indicates that yet undetermined amounts of arrestin accumulate in localized regions of the outer segment in the dark. The extent of movement of arrestin to the inner segment, if it occurs, remains to be established. Localization of arrestin in phagosomes indicates that at least part of the arrestin is being degraded in the pigment epithelium.