Autoradiographic and biochemical analysis of photoreceptor membrane renewal inOctopus retina

Abstract

Using autoradiographic and biochemical methods, we have demonstrated the renewal of light-sensitive membranes and photopigments in Octopus visual cells. After the injection of Octopus with [3H]leucine, electron microscope autoradiography revealed an intracellular pathway similar to that in vertebrates for the synthesis and transport of nascent protein from the inner segments to the rhabdomes. However, migration of labelled protein from synthetic sites to the light-sensitive rhabdomes took longer in Octopus than the equivalent process in vertebrates. Biochemical analysis of [3H]leucine-labelled retinas identified some of the labelled protein observed in autoradiographs of the rhabdomes as the visual pigment, rhodopsin. We have shown that retinochrome, a second photopigment in cephalopod retinas, is also renewed. Biochemical analysis 8 h after injection of [3H]leucine revealed heavy labelling of this photoprotein. Light microscope autoradiography of Octopus retina 8 h after injection of [3H]retinol showed labelling of both the rhabdomes and the myeloid bodies of the inner segments. Biochemical data gathered 8 h after injection of [3H]retinol indicated chromophore addition to both rhodopsin and retinochrome with retinochrome being more heavily labelled than rhodopsin. Thus, silver grains observed over the rhabdomes and inner segments could arise from one or both photopigments. These data suggest that retinal is stored in the myeloid bodies of the photoreceptor inner segments. Retinal could then be transferred, perhaps via retinochrome, to newly synthesized opsin before the visual pigment is assembled into new rhabdomeric membranes. Alternatively, retinochrome may serve to transport retinal from the myeloid bodies to the rhabdomes to regenerate rhodopsin as previously proposed.

Light-/dark-induced changes in rhabdom structure in the retina of Octopus bimaculoides

“…The next day the eyecups from the dark-and light-adapted octopus were cut into wedges and each piece subdivided into inner (nearest the center of the eyecup), middle and outer retinal (nearest the octopus equivalent of the ora serrata) regions. All retinal pieces were embedded in Araldite (Robles et al 1984). …”

“…The micrographs were printed to a uniform magnification. To compare ratios of the relative areas occupied by dark-or light-adapted rhabdom membranes we used the method of Weibel (1969) previously used in statistical comparisons of photoreceptor membrane compartments in the octopus (Robles et al 1984). A transparent grid constructed of 1 cm 2 squares was placed over a 12 cm 17 cm print allowing 204 points to fall within the border of the photograph.…”

“…The photoreceptors are compartmentalized into inner and outer segments connected by a region filled with pigment granules (Yamamoto et al 1965;Cohen 1973;Yamada and Usukura 1982;Robles et al 1984). The photoreceptor inner segments contain a nucleus, biosynthetic organelles and myeloid bodies, while the outer segments consist of long, cylindrical cores of cytoplasm from which project two opposing sets of rhabdomere microvilli.…”

Retinoid cycling proteins redistribute in light‐/dark‐adapted octopus retinas

“…By routine light microscopy, the myeloid bodies appear as thin, wavy lines, and by electron microscopy, the myeloid bodies are composed of closely apposed smooth membranes that are sometimes continuous with the RER (Fig. 2; Hara and Hara, 1976;Robles et al, 1984). As will be described, the myeloid bodies are difficult to distinguish when using laser scanning confocal microscopy and a more complex structure is suggested.…”

Appearance of Opsin-containing Vesicles as Rhabdomeric Precursors and Their Incorporation into the Rhabdom around Dusk in the Compound Eye of the Crab, Hemigrapsus sanguineus

“…How are they formed and incorporated into the rhabdoms? Although number of fragmentary observations in this respect are made in various species (Itaya, 1976;Stowe, 1980;Robles et al, 1984;Suzuki and Hirosawa, 1991), the picture of the process had been far from complete.…”