Correlation of regenerable opsin with rod ERG signal in Rpe65-/- mice during development and aging

PURPOSE: RPE65 has been shown to be essential for the production of 11-cis retinal by the retinal pigment epithelium. Mutations in RPE65 are known to be associated with severe forms of early-onset retinal dystrophy. This project was designed to determine the amount of regenerable opsin in Rpe65-/- mice during development and aging, and to examine the function of this rhodopsin by electroretinography (ERG). METHODS: Young and aged Rpe65-/- and wild-type (WT) mice were dark adapted. Endogenous rhodopsin and regenerable opsin were measured using absorption-difference spectrophotometry. Photoreceptor function was assessed with scotopic single-flash ERGs and photoreceptors were counted in histologic sections. Opsin's primary structure was analyzed by mass-spectrometric mapping. RESULTS: Unlike WT mice, amounts of regenerable opsin in Rpe65-/- mice decreased significantly with age, which correlated with a decrease in the number of photoreceptors and a decline in ERG amplitudes. Opsin structure, however, did not change. No endogenous levels of rhodopsin were measurable in the Rpe65-/- mice (detection limit: 0.225 pmol). 11-cis Retinal injections resulted in the regeneration of similar amounts of rhodopsin and improved rod function in a comparable way, irrespective of age. CONCLUSIONS: In the aged Rpe65-/- mouse, opsin levels decrease because of the loss of photoreceptors. The remaining opsin is structurally intact, and the components of the phototransduction cascade and the retinal circuitry remain functional, despite the absence of normal photoreceptor activity.     

Idiopathic retinal degeneration in the dog: differential patterns of [3H]uridine incorporation and HIOMT-like immunoreactivity in surviving photoreceptors

Photoreceptor cell pathology was investigated in an 8-yr-old mixed-breed dog which had displayed visual symptoms of 1 month duration. An electroretinogram detected no light-evoked responses. Light and electron microscopic features showed marked thinning and atrophy of the outer both the tapetal and non-tapetal retina appeared to be involved. In the non-tapetal region, a majority of the rod inner segments were missing, while scattered mitochondria-filled stubby inner segments of cones were readily identified. Inner segments of both rods and cones were observed in the tapetal region. Photoreceptor outer segments were completely absent from the affected retina, and no outer segment debris was observed between the photoreceptor layer and the retinal pigmented epithelium (RPE). Autoradiographic analysis of 3-mm retinal disks from the degenerate retina following incubation with [3H]uridine indicated that only 61% +/- 13 S.D. of the remaining nuclei of rod photoreceptors were undergoing RNA synthesis, whereas more than 99% of cone nuclei incorporated the label. Normal and degenerate retina were also analysed for localization of hydroxyindole-O-methyltransferase (HIOMT)-like immunoreactivity. While the normal retina showed immunoreactivity in both rod and cone photoreceptors with more intense immunoreactivity present in cones, the degenerate retina showed HIOMT-like immunoreactivity only in the remaining cone photoreceptors. The results of this study of idiopathic photoreceptor degeneration of the canine retina suggest that although both photoreceptor types are involved, rods are more severely affected than cones.     

The retina of c-fos-/- mice: electrophysiologic, morphologic and biochemical aspects

PURPOSE: Mice without a functional c-Fos protein (c-fos-/- mice) do not exhibit light-induced apoptotic cell death of rods in contrast to their wild-type littermates (c-fos+/+ mice). To analyze the consequences of the absence of c-fos in the retina, we investigated whether the retinas of c-fos-/- mice have a reduced capacity to absorb and transduce light compared with c-fos+/+ mice. METHODS: Retinal function was evaluated in dark-adapted mice by full-field electroretinograms (ERGs) over more than 6 log units of intensity. Retinal morphology was studied by light- and electron microscopy. Arrestin and the heat shock protein 70 (Hsp70) were detected by Western blot analysis. The rhodopsin content and the kinetics of rhodopsin regeneration were determined in retinal extracts. RESULTS: Although the configuration of the ERGs was comparable in both groups of mice, c-fos-/- mice showed a marked variability in all quantitative ERG-measures with lower mean amplitudes, longer latencies, and a 0.9-log-unit lower b-wave sensitivity on average. Morphometry showed that c-fos-/- mice have 23% fewer rods on average, whereas the number of cones was comparable among c-fos+/+ and c-fos-/- mice. Arrestin levels appeared slightly reduced in c-fos-/- mice when compared with c-fos+/+ mice, whereas Hsp70 levels were comparable in both genotypes. The kinetics of rhodopsin regeneration were similar, but c-fos-/- mice had a 25% lower rhodopsin content on average. CONCLUSIONS: Compared with c-fos+/+ mice, retinal function in c-fos-/- mice is attenuated to a variable but marked degree, which may be, at least in part, related to the reduced number of rods and the reduced rhodopsin content. However, c-fos does not appear to be essential for the ability to absorb photons, nor for phototransduction or the function of second-order neurons. The resistance to light-induced apoptosis of photoreceptor cells in c-fos-/- mice may result from the acute deficit of c-fos in the apoptotic cascade rather than from developmental deficits affecting rod photoreceptor function.     

Correlation between rod photoreceptor numbers and levels of ocular pigmentation

PURPOSE: Ocular melanin synthesis modulates rod photoreceptor production, because in albino eyes, rod numbers are reduced by approximately 30%. In this study, rod numbers and ocular rhodopsin concentrations were measured in intermediate pigmentation phenotypes to determine whether proportional reductions in melanin are correlated with proportional changes in rod numbers. Further, patterns of cell production and death were examined around the time of birth, when rod production peaks, to determine whether there are abnormalities in these features associated with hypopigmentation. METHODS: Four mouse pigmentation phenotypes were used: fully pigmented, albino, Beige, and Himalayan. The latter two are intermediate-pigmentation phenotypes, with Beige having markedly more pigment than Himalayan. Ocular melanin concentrations were measured during development and at maturity. Rods were counted at maturity and measurements of ocular rhodopsin undertaken. Mitotic and pyknotic cells were also counted in neonates. RESULTS: Rods and ocular rhodopsin were reduced in both Beige and Himalayan mice below levels found in fully pigmented mice, but not to levels found in albino animals. This was more marked in Himalayan than Beige mice, reflecting the lower concentration of melanin found in the former compared with the latter, both in development and at maturity. Although patterns of cell production were elevated in the hypopigmented animals, such patterns varied. CONCLUSIONS: Rod numbers are modulated within a range between that in fully pigmented and albino phenotypes by the concentration of ocular melanin. However, in these animals, there is no obvious correlation between these events and patterns of cell production and death in neonates.     

Rod photopigment deficits in albinos are specific to mammals and arise during retinal development.

Adult albino mammals have specific retinal defects, including reduced numbers of rod photoreceptors. To examine when this rod deficit arises and whether it exists in nonmammalian albinos, we have used absorbance spectrophotometry to measure photopigment levels in dark-adapted eyes taken from three groups of pigmented and albino animals: adult rodents (rats and mice), developing rats, and mature Xenopus frogs. Rhodopsin concentrations were consistently and significantly reduced in mammalian albinos compared to their wild-type counterparts from before the time of eye opening, but photopigment levels were similar in frogs of both pigmentation phenotypes. The results strongly suggest that deficits in the rod cell population arise early in development of the mammalian albino retina, but do not generalize to nonmammalian mutants lacking retinal melanin.     

Augmented rod bipolar cell function in partial receptor loss: an ERG study in P23H rhodopsin transgenic and aging normal rats

Physiological consequences of early stages of photoreceptor degeneration were examined in heterozygous P23H rhodopsin transgenic (Tg) and in aging normal Sprague-Dawley rats. Rod photoreceptor and rod bipolar (RB) cell function were estimated with maximum value and sensitivity parameters of P3 and P2 components of the electroretinogram. In both Tg and aging normal rats, the age-related rate of decline of P3 amplitude was steeper than that of the P2 amplitude. Tg rats showed greater than normal sensitivity of the rods. A new model of distal RB pathway connectivity suggested photoreceptor loss could not be the sole cause of physiological abnormalities; there was an additional increase of post-receptoral sensitivity. We propose that changes at rod-RB synapses compensate for the partial loss of rod photoreceptors in senescence and in early stages of retinal degeneration.     

Retinal degeneration in tulp1-/- mice: vesicular accumulation in the interphotoreceptor matrix.

PURPOSE: The Tulp1 gene is a member of the tubby gene family with unknown function. Mutations in the human TULP1 gene cause autosomal recessive retinitis pigmentosa. To understand the pathogenic mechanism associated with TULP1 mutations and to explore the physiologic function of this protein, we examined tissue distribution of the Tulp1 protein in normal mice and the photoreceptor disease phenotype in Tulp1-ablated mice. METHODS: Tissue distribution of the Tulp1 protein in normal mice was examined by immunoblotting and immunocytochemistry. The disease phenotype in tulp1-/- mice was studied by light and electron microscopy, electroretinography (ERG), and immunocytochemistry. These results were compared with another mouse model of retinal degeneration carrying a rhodopsin mutation. RESULTS: Tulp1 is found exclusively in photoreceptors, localizing predominantly in the inner segments. It is a soluble protein with an apparent molecular weight of approximately 70 kDa. Photoreceptor degeneration developed in tulp1-/- mice, with early involvement of both rods and cones. At the early stage of degeneration, rod and cone opsins, but not peripherin/RDS, exhibited prominent ectopic localization. Electron microscopy revealed massive accumulation of extracellular vesicles surrounding the distal inner segments. CONCLUSIONS: The function of Tulp1 is required to maintain viability of rod and cone photoreceptors. Extracellular vesicular accumulation is not a common phenomenon associated with photoreceptor degeneration but appears to be a distinct ultrastructural feature shared by a small group of retinal disease models. The defect in tulp1-/- mice may be consistent with a loss of polarized transport of nascent opsin to the outer segments.     

The relationship between ambient lighting conditions, absolute dark-adapted thresholds, and rhodopsin in black and hypopigmented mice

Significant variation in absolute dark-adapted thresholds is observed both within and between strains of mice with differing ocular pigmentation levels. Differences in threshold within a single strain are related to the Williams' photostasis effect, that is, photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. To examine threshold differences among strains, we equalized rhodopsin levels by maintaining albino mice (c2J/c2J) at 2 x 10-4 cd/m2 (dim light) and black mice at 2 x 102 cd/m2 (bright light). This resulted in ocular rhodopsin levels for albino mice (albino--dim) of 494 +/- 11 pmoles/eye and rhodopsin levels for black mice (black--bright) of 506 +/- 25 pmoles/eye. For comparison, rhodopsin levels in black mice maintained in dim light are 586 +/- 46 pmoles/eye and 217 +/- 46 pmoles/eye in albino mice maintained in bright light. We found similar dark-adapted thresholds (6.38 log cd/m2 vs. 6.47 log cd/m2)) in albino and black mice with equivalent rhodopsin determined with a water maze test. This suggests that dark-adapted thresholds are directly related to rhodopsin levels regardless of the level of ocular melanin. The number of photoreceptors, photoreceptor layer thickness, and outer segment length did not differ significantly between albino (dark) and black mice (bright). These results demonstrate that the visual sensitivity defect found in hypopigmented animals is secondary to abnormal rhodopsin regulation and that hypopigmented animals have either an improper input to the photostasis mechanism or that the photostasis mechanism is defective.     

Ultrastructural localization of the HNK-1 carbohydrate epitope to glial and neuronal cells of the human retina

PURPOSE: To localize the cell adhesion-related HNK-1 carbohydrate epitope in the human retina at cellular and subcellular levels. METHODS: Retinas were obtained from seven normal human eyes at autopsy (age, 43-78 years). The specimens were embedded in medium-grade resin and studied by postembedding immunoelectron microscopy using the primary mouse mAb HNK-1 (Leu 7) to the HNK-1 epitope and secondary antibodies conjugated to 10-nm colloidal gold particles. RESULTS: Prominent immunolabeling with mAb HNK-1 was observed on the outer surface of the entire plasma membrane of Müller radial glial cells, including their microvilli between the inner segments of rods and cones, on the plasma membranes of astrocytes in the ganglion cell layer, in bipolar cells in the inner nuclear layer, and in photoreceptor cells in the outer nuclear layer. Fewer gold particles were present on plasma membranes of other main types of retinal neurons, including ganglion cells. Only the outer segments of rods and cones and the endothelial cells of retinal capillaries were never labeled. In the ciliary epithelium, gold particles localized to the basement membrane of the nonpigmented and pigmented layers and to the cytoplasm of the pigmented epithelium. CONCLUSIONS: Unlike in many other species, the HNK-1 epitope in the human retina is found on both glial and neuronal cells, including photoreceptors. This epitope potentially contributes to neuron-to-neuron and glia-to-neuron adhesion of human retinal cells.     

Morphological characterization of the retinal degeneration in three strains of mice carrying the rd-3 mutation

Retinal development in 3 strains of rd-3/rd-3 mutant mice, previously shown to have different rates of degeneration, was studied using light, electron, and immunofluorescence microscopy. The time course and phenotype of the degeneration as well as details on the mechanism of massive photoreceptor cell loss are compared with other known retinal degenerations in mice. Up until postnatal day (P) 10, the retinas of all three strains (RBF, 4Bnr, In-30) develop similarly to those of pigmented and nonpigmented controls. TUNEL-positive cells appear in the outer nuclear layer (ONL) by P14, and reach a maximum in all three mutant strains around P21. Scattered rods and cones form a loose, monolayered ONL by 8 weeks in the albino RBF strain, by 10 weeks in the albino 4Bnr strain, and by 16 weeks in the pigmented In-30 strain. Though the initial degeneration begins in the central retina, there is no preferred gradient of cell death between central and peripheral photoreceptors. Rods and cones are present at all ages examined. During development, stacks of outer segments (OS) form in all three strains though they never achieve full adult lengths, and often have disorganized, atypical OS. Rod opsin is expressed in the developing OS but is redistributed into plasma membrane as OS degeneration proceeds. Retinal pigment epithelial (RPE) cells of all mutant strains contain packets of phagocytosed OS, and their apical processes associate with the distal ends of the OS. At their synaptic sites, photoreceptor terminals contain ribbons apposed to apparently normal postsynaptic triads. As photoreceptors are lost, Müller cells fill in space in the ONL but they do not appear to undergo significant hypertrophy or migration, though during the degeneration, glial fibrillary acidic protein (GFAP) expression is gradually upregulated. Macrophage-like cells are found frequently in the subretinal space after the onset of photoreceptor apoptosis. As OS disappear, the RPE apical processes revert to simple microvilli. Late in the degeneration, some RPE cells die and neighboring cells appear to flatten as if to maintain confluence. In regions of RPE cell loss that happen to lie above retina where the ONL is gone, cells of the inner nuclear layer (INL), wrapped by Müller cell processes, may front directly on Bruch's membrane.     

Functional comparisons of visual arrestins in rod photoreceptors of transgenic mice

PURPOSE: To examine the biochemical characteristics of rod and cone arrestin with respect to their ability to quench the activity of light-activated rhodopsin in transgenic mice. METHODS: The mouse rod opsin promoter was used to drive expression of mouse cone arrestin in rod photoreceptor cells of rod arrestin knockout (arr1-/-) mice. Suction electrode recordings from single rods were performed to investigate cone arrestin's ability to quench the catalytic activity of light-activated rhodopsin. In addition, the ability of cone arrestin to prevent light-induced retinal damage caused by prolonged activation of the phototransduction cascade was assessed. RESULTS: Two independent lines of transgenic mice were obtained that expressed cone arrestin in rod photoreceptors, and each was bred into the arr1-/- background. Flash responses measured by suction electrode recordings showed that cone arrestin reduced signaling from photolyzed rhodopsin but was unable to quench its activity completely. Consistent with this observation, expression of mouse cone arrestin conferred dose-dependent protection against photoreceptor cell death caused by low light exposure to arr1-/- retinas, but did not appear to be as effective as rod arrestin. CONCLUSIONS: Cone arrestin can partially substitute for rod arrestin in arr1-/- rods, offering a degree of protection from light-induced damage and increasing the extent of rhodopsin deactivation in response to flashes of light. Although earlier work has shown that rod arrestin can bind and deactivate cone pigments efficiently, the results suggest that cone arrestin binds light-activated, phosphorylated rhodopsin less efficiently than does rod arrestin in vivo. These results suggest that the structural requirements for high-affinity binding are fundamentally distinct for rod and cone arrestins.     

Late stages of visual pigment photolysis in situ: cones vs. rods

Slow photolysis reactions and the regeneration of the dark pigment constitute the mechanisms of dark adaptation whereby photoreceptor cells restore their sensitivity after bright illumination. We present data on the kinetics of the late stages of the photolysis of the visual pigment in intact rods and red- and green-sensitive cones of the goldfish retina. Measurements were made on single photoreceptors by means of a fast-scanning dichroic microspectrophotometer. We show that in cones the hydrolysis of the opsin-all-trans 3-dehydroretinal linkage proceeds with a half-time of approximately 5s at 20 degrees C that is almost two orders of magnitude faster than in rods. 3-Dehydroretinol in cones is produced approximately 3-fold faster than retinol in amphibian rhodopsin rods; the rate of the reaction is limited by the speed of retinal reduction catalyzed by retinoldehydrogenase. The fast hydrolysis of the 3-dehydroretinal/opsin Schiff base and the correspondingly fast appearance of the substrates for dark visual pigment regeneration (free opsin and 3-dehydroretinol) provide essential conditions for faster dark adaptation of cone (diurnal) as compared to rod (nocturnal) vision.     

Transgenic expression of a GFP-rhodopsin COOH-terminal fusion protein in zebrafish rod photoreceptors

To facilitate the identification and characterization of mutations affecting the retina and photoreceptors in the zebrafish, a transgene expressing green fluorescent protein (GFP) fused to the C-terminal 44 amino acids of Xenopus rhodopsin (Tam et al., 2000) under the control of the 1.3-kb proximal Xenopus opsin promoter was inserted into the zebrafish genome. GFP expression was easily observed in a ventral patch of retinal cells at 4 days postfertilization (dpf). Between 45-50% of the progeny from the F1, F2, and F3 generations expressed the transgene, consistent with a single integration event following microinjection. Immunohistochemical analysis demonstrated that GFP is expressed exclusively in rod photoreceptors and not in the UV, blue, or red/green double cones. Furthermore, GFP is localized to the rod outer segments with little to no fluorescence in the rod inner segments, rod cell bodies, or rod synapse regions, indicating proper targeting and transport of the GFP fusion protein. Application of exogenous retinoic acid (RA) increased the number of GFP-expressing cells throughout the retina, and possibly the level of expressed rhodopsin. When bred to a zebrafish rod degeneration mutant, fewer GFP-expressing rods were seen in living mutants as compared to wild-type siblings. This transgenic line will facilitate the search for recessive and dominant mutations affecting rod photoreceptor development and survival as well as proper rhodopsin expression, targeting, and transport.     

AAV-mediated expression targeting of rod and cone photoreceptors with a human rhodopsin kinase promoter

PURPOSE: Gene therapy for retinal degeneration requires well-defined promoters that drive expression in rod and cone photoreceptors. This study was undertaken to develop short, active derivatives of the human rhodopsin kinase (RK) gene promoter for targeting transgene expression in rods and cones. RK, also known as G protein-coupled receptor kinase 1 (GRK1), is a component of the light adaptation pathway expressed in rods and cones. METHODS: Human RK (hRK) promoter and its concatemers or derivatives extending into the conserved 5' untranslated region (5'-UTR) were assayed for promoter activity in WERI retinoblastoma or Crx/Sp1-supplemented HEK-293 cells. The derivative displaying the highest activity was linked to a GFP reporter and packaged in a pseudotyped adenoassociated viral vector (AAV2/5). The AAV vector was tested in vivo by subretinal injections in wild-type mice, in the all-cone Nrl(-/-) mice, and in the cone-rich diurnal Nile grass rat (Arvicanthis niloticus). Control eyes received a similar AAV2/5 vector carrying a mouse rod opsin (mOps) promoter-controlled GFP reporter. RESULTS: The hRK promoter with the full 5' untranslated sequence (-112 to +180) was the most active in cell culture. Delivered by the AAV2/5 vector, RK promoter drove GFP expression specifically in photoreceptors. In rods, hRK promoter-mediated expression was as efficient as, but appeared more uniform than, mOps promoter-mediated expression. In cones, the hRK promoter drove expression, whereas the mOps promoter did not. CONCLUSIONS: The hRK promoter is active and specific for rod and cone photoreceptors. Because of its small size and proven activity in cones, it is a promoter of choice for somatic gene transfer and gene therapy targeting rods and cones.     

Non-rod,non-cone photoreception in the vertebrates

When reflected from a surface, light can provide a representation of the spatial environment, whilst gross changes in environment light can signal the time of day. The differing sensory demands of using light to detect environmental space and time appear to have provided the selection pressures for the evolution of different photoreceptor systems in the vertebrates, and probably all animals. This point has been well recognised in the non-mammals, which possess multiple opsin/vitamin A-based photoreceptor populations in a variety of sites distributed both within and outside the CNS. By contrast, eye loss in mammals abolishes all responses to light, and as a result, all photoreception was attributed to the rods and cones of the retina. However, studies over the past decade have provided overwhelming evidence that the mammalian eye contains a novel photoreceptor system that does not depend upon the input from the rods and cones. Mice with eyes but lacking rod and cone photoreceptors can still detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity, and modulate pupil size. Furthermore, action spectra for some of these responses in rodents and humans have characterised at least one novel opsin/vitamin A-based photopigment, and molecular studies have identified a number of candidate genes for this photopigment. Parallel studies in fish showing that VA opsin photopigment is expressed within sub-sets of inner retina neurones, demonstrates that mammals are not alone in having inner retinal photoreceptors. It therefore seems likely that inner retinal photoreception will be a feature of all vertebrates. Current studies are directed towards an understanding of their mechanisms, determining the extent to which they contribute to physiology and behaviour in general, and establishing how they may interact with other photoreceptors, including the rods and cones. Progress on each of these topics is moving very rapidly. As a result, we hope this review will serve as an introduction to the cascade of papers that will emerge on these topics in the next few years. We also hope to convince the more casual reader that there is much more to vertebrate photoreceptors than the study of retinal rods and cones.     

Transgenic expression of a GFP-rhodopsin COOH-terminal fusion protein in zebrafish rod photoreceptors

To facilitate the identification and characterization of mutations affecting the retina and photoreceptors in the zebrafish, a transgene expressing green fluorescent protein (GFP) fused to the C-terminal 44 amino acids of Xenopus rhodopsin (Tam et al., 2000) under the control of the 1.3-kb proximal Xenopus opsin promoter was inserted into the zebrafish genome. GFP expression was easily observed in a ventral patch of retinal cells at 4 days postfertilization (dpf). Between 45-50% of the progeny from the F1, F2, and F3 generations expressed the transgene, consistent with a single integration event following microinjection. Immunohistochemical analysis demonstrated that GFP is expressed exclusively in rod photoreceptors and not in the UV, blue, or red/green double cones. Furthermore, GFP is localized to the rod outer segments with little to no fluorescence in the rod inner segments, rod cell bodies, or rod synapse regions, indicating proper targeting and transport of the GFP fusion protein. Application of exogenous retinoic acid (RA) increased the number of GFP-expressing cells throughout the retina, and possibly the level of expressed rhodopsin. When bred to a zebrafish rod degeneration mutant, fewer GFP-expressing rods were seen in living mutants as compared to wild-type siblings. This transgenic line will facilitate the search for recessive and dominant mutations affecting rod photoreceptor development and survival as well as proper rhodopsin expression, targeting, and transport.     

Retinal damage by constant light in chimaeric mice: implications for the protective role of melanin

Adult chimaeric mice, containing varying proportions of albino and pigmented cells in their ocular tissues, were exposed to constant light for 5 weeks and the distribution of the surviving rod perikarya in the retina and of the pigmented cells in various eye tissues were compared. In chimaeras which were mostly albino, the retinal lesion was similar to that in pure strain albino mice; in chimaeras with relatively more pigmented cells in their ocular tissues, the retina was unaffected as in fully pigmented mice. In chimaeras with amounts of pigmented cells in their ocular tissues varying between these two ends, lesions of intermediate degrees could be observed. Surviving rod cells in such chimaeric retinas were always found in regions adjoining the periphery. The location of the rod perikarya in such regions did not show an exact correlation with that of the overlying pigmented cells but regions of the outer nuclear layer with surviving rod perikarya were generally located in the half or quarter of the retina in which the overlying pigment epithelium also contained more pigmented cells than in the other regions. The proportions of the surviving photoreceptor cells varied between such chimaeras. The lesion appeared to be less extensive in individuals with more pigmented cells in the epithelium but no exact correlation was recorded. The findings suggest that while pigmentation in the iris reduces the amount of light reaching the retina, melanin in the pigment epithelium, in addition to preventing light reflection, may also play an antitoxic role, possibly as an antioxidative agent.     

Postural stability changes in elderly subjects due to disruption of the somatosensory and vestibular system inputs, refractive blur and dual tasking

Purpose:  In the retina of albino mammals so far examined there is a deficit in the rod photoreceptor population. We reasoned that a consequence of the rod deficit might be a subsequent abnormality in the next level of the rod pathway, the rod bipolar cell. We therefore compared the distribution of rod bipolar cells in pigmented and albino rats.
Methods:  Rod bipolar cells were labelled in 15 µm thick sections of fixed retinas with a monoclonal antibody directed against protein kinase C, and visualized using the ABC method. Counts of the cell bodies and processes of rod bipolar cells were undertaken at various locations across the retina.
Results:  Qualitatively, the protein kinase C staining suggested differences between the albino and pigmented phenotypes both in the outer and inner plexiform layers and in the inner nuclear layer. Staining was denser in the pigmented retina and the bipolar cell bodies were arranged in multiple rows rather than a single row as found in the albino retina. The actual number of rod bipolar cells was reduced in the albino phenotype. At the neonatal age we examined (post-natal day 15), there was a 14% reduction in the total number whereas in the adult retina the reduction was around 9%.
Conclusions:  Although the reduction in the rod bipolar population is not as great as that previously found for the rod photoreceptors (∼25%), the reduction in both populations suggests a general abnormality in the rod pathway of albinos. Further, our data support the view that melanin is crucially involved in the normal development of retinal structure.