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We studied the protective effects of midkine, a growth factor produced by a retinoic acid responsive gene, on constant light-induced retinal damage in albino BALB/C adult mice. Two days before exposure to constant light, midkine was injected into the vitreous of the left eye and a phosphate buffer saline into the right eye (control eyes). After 7 days of constant light, control eyes exhibited shortening of the photoreceptor outer segments and decreased thickness of the outer nuclear layer, whereas in midkine-treated eyes photoreceptor cells were virtually intact. Although midkine-treated eyes also showed photoreceptor damage after longer light exposure of up to 21 days, the damage was significantly less than in control eyes. Measurement of the thickness of the outer nuclear layer showed the protective effect of intravitreous midkine on the constant light-induced retinal damage. These findings suggest that midkine is a potential agent for the prevention of photoreceptor degeneration.

The distribution of alpha 2,3-sialyltransferase (alpha 2,3-ST) mRNA in the rat iris and ciliary body was investigated with in situ hybridization histochemistry. Strong expression of alpha 2,3-ST mRNA was detected in the inner epithelial layer of the ciliary body and weak expression in the iris epithelium. Since the synthesis of sialoglycoconjugates is completed by terminal sialylation by the action of sialyltransferase (ST), the ST-expressed portions are considered to produce sialoglycoconjugates. Hence, the source of the sialoglycoconjugates found in the inner epithelial layer of the ciliary body in previous histochemical studies is the same epithelial cell.

The binding of amaranthin, specific for Gal beta 1,3 GalNAc and sialic acid Gal beta 1,3 GalNAc sequences, to the human retina was investigated with avidin biotinylated peroxidase. Amaranthin bound to the cone and rod photoreceptors, inner plexiform layer, ganglion cells, and nerve fibers. Since peanut agglutinin, specific for Gal beta 1,3 GalNAc, selectively binds to cones, we conclude that O-glycoside-linked glycoconjugates are present on the surfaces of both cones and rods: Gal beta 1,3 GalNAc and sialic acid Gal beta 1,3 GalNAc are terminal sugars of the glycoconjugates around cones and rods, respectively.

We studied the distribution pattern of free ribosomes in the photoreceptor inner segments of adult mice with reference to environmental light. Eyeballs were obtained from animals at the light or dark phase of the ordinary cyclic light, and photoreceptor inner segments were examined by electron microscopy. In the light phase, free ribosomes were distributed in a random fashion, although meticulous observations revealed that they appeared to be arranged either in a spiral or linear form, or in isolation. In the dark phase, several free ribosomes were arranged in a clustered fashion. When animals were kept in continuous darkness as long as 27 hours, most of the inner segment free ribosomes showed a clustered pattern similar to that in the dark phase of the cyclic light. We discussed the light-dependent configurational changes of ribosomes with respect to their activity or inactivity in protein synthesis.