PubMed · 3548572
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H Wallach. 1987. Perceiving a stable environment when one moves.. https://doi.org/10.1146/annurev.ps.38.020187.000245
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In cephalopods, the complex rhodopsin-retinochrome system serves to regenerate metarhodopsin and metaretinochrome after illumination. In the dark, a soluble protein, retinal-binding protein (RALBP), shuttles 11-cis retinal released from metaretinochrome located in the photoreceptor inner segments to metarhodopsin present in the rhabdoms. While in the rhabdoms, RALBP delivers 11-cis retinal to regenerate rhodopsin and in turn binds the all-trans isomer released by metarhodopsin. RALBP then returns all-trans retinal to the inner segments to restore retinochrome. The conventional interpretation of retinoid cycling is contradicted by immunocytochemical studies showing that, in addition to rhodopsin, retinochrome is present in the rhabdomal compartment, making possible the direct exchange of chromophores between the metapigments with the potential exclusion of RALBP. By using immunofluorescence and laser scanning confocal microscopy, we have precisely located opsin, aporetinochrome, and RALBP in light-/dark-adapted octopus retinas. We found differences in the distribution of all three proteins throughout the retina. Most significantly, comparison of cross sections though light- and dark-adapted rhabdoms showed a dramatic shift in position of the proteins. In the dark, opsin and retinochrome colocalized at the base of the rhabdomal microvilli. In the light, opsin redistributed along the length of the microvillar membranes, and retinochrome retreated to a location that is perhaps extracellular. RALBP was present in the core cytoplasm of the photoreceptor outer segments in the dark, and RALBP moved to the periphery in the light. Because of the colocalization of opsin and retinochrome in the dark, we believe that the two metapigments participate directly in chromophore exchange. RALBP may serve to transport additional chromophore from the inner segments to the rhabdoms and may not be immediately involved in the exchange process.
Light-adapted and dark-adapted electroretinograms were obtained in 27 premature infants who were screened for retinopathy of prematurity shortly after birth. Thirteen showed no retinopathy and 13 had mild to moderate retinopathy, which ranged from stage I, zone III (International Classification) to stage III, zone II. Measurements were made during the first 16 months of life. The configuration of the waveforms under both photopic and scotopic conditions changed during this period showing increased amplitudes of both the a- and the b-waves. A scotopic intensity series was performed and the b-wave amplitudes and implicit times were measured. The b-wave amplitude data were averaged for three adult control subjects, for infants without retinopathy of prematurity measured at 3-4 and at 6-7 months and for infants with retinopathy of prematurity measured at 3-4 and at 6-7 months. The Naka-Rushton function was fitted to the average data. The Rmax increased from 3 to 6 months and from 6 months to adulthood, and the Isat values decreased over this age range. The b-wave implicit times were within normal range in the 6-month data. There was no difference in Rmax or Isat for infants with and without ROP.
New possible causes and unexplored aspects of the electroretinogram were evaluated in a case of vitamin A deficiency secondary to surgical therapy for morbid obesity. The Naka-Rushton equation, applied to the scotopic b-wave, demonstrated the quantal catch reduction caused by the loss of rhodopsin in the outer segment of photoreceptors. Study of the Fourier analysis of the photopic 20-Hz response suggested a primary involvement of the external retinal layers, with an indirect alteration of the inner layer. The electroretinogram oscillatory potentials showed alterations explained by the involvement of their generators related to the primary photoreceptor lesion.