Protection by dimethylthiourea against retinal light damage in rats.

The protective effect of dimethylthiourea (DMTU) against retinal light damage was determined in albino rats reared in darkness or in weak cyclic light. Rats maintained under these conditions were treated with DMTU at different concentrations and dosing schedules and then exposed for various times to intense visible light, either intermittently (1 hr light and 2 hr dark) or continuously. The extent of retinal light damage was determined 2 weeks after light exposure by comparing rhodopsin levels in experimental rats with those in unexposed control animals. To determine the effect of DMTU on rod outer segment (ROS) membrane fatty acids, ROS were isolated immediately after intermittent light exposure, and fatty acid compositions were measured. The time course for DMTU uptake and its distribution in serum, retina, and the retinal pigment epithelium (RPE)/choroid complex was determined in other rats not exposed to intense light. After intraperitoneal injection of the drug (500 mg/kg body weight), DMTU appeared rapidly in the serum, retina, and the RPE and choroid. In the ocular tissues, it was distributed 70-80% in the retina and 20-30% in the RPE and choroid. This antioxidant appears to have a long half-life because it was present in these same tissues 72 hr after a second intraperitoneal injection. For rats reared in the weak cyclic light environment, DMTU (two injections) provided complete protection against rhodopsin loss after intense light exposures of up to 16 hr. Only 15% rhodopsin loss was found in cyclic-light DMTU-treated rats after 24 hr of intermittent or continuous light. For rats reared in darkness, DMTU treatment resulted in a rhodopsin loss of less than 20% after 8-16 hr of continuous light and approximately 40% after similar exposure to intermittent light. Irrespective of the type of light exposure, rhodopsin loss in the dark-reared DMTU-treated rats was nearly identical to that found in uninjected cyclic light-reared animals. In rats from both light-rearing environments, DMTU treatment prevented the light-induced loss of docosahexaenoic acid from ROS membranes. As measured by rhodopsin levels 2 weeks later, DMTU was most effective when given as two doses administered 24 hr before and just before intense light exposure. As a single dose given during continuous light exposure, DMTU protected cyclic light-reared rats for at least 4 hr after the start of exposure but was ineffective in dark-reared animals if injected 1 hr after the start of light. It was also ineffective in both types of rats when given after light exposure.(ABSTRACT TRUNCATED AT 400 WORDS)     

Retinal light damage in rats exposed to intermittent light. Comparison with continuous light exposure

Visible light-induced photoreceptor cell damage resulting from exposure to multiple intermittent light-dark periods was compared with damage resulting from continuous light in albino rats maintained in a weak cyclic-light environment or in darkness before light treatment. The time course of retinal damage was determined by correlative measurements of rhodopsin and visual cell DNA at various times after light exposure, and by histopathological evaluation. The effect of intense light exposures on rhodopsin regeneration and on the level of rod outer segment docosahexaenoic acid was also determined. For rats previously maintained in weak cyclic light, 50% visual cell loss was measured 2 weeks after 12 1 hr light/2 hr dark periods, or following 24 hr of continuous light. A comparable 50% loss of visual cells was found in dark-reared rats after only 5 hr of continuous illumination or 2-3 hr of intermittent light. As judged by histology, cyclic-light-reared rats incurred less retinal pigment epithelial cell damage than dark-reared animals. In both experimental rat models intermittent light exposure resulted in greater visual cell damage than continuous exposure. Visual cell damage from intermittent light was found to depend on the duration of light exposure and on the number of light doses administered. Measurements of rhodopsin and DNA 2 hr and 2 weeks after light exposure of up to 8 hr duration revealed that visual cell loss occurs largely during the 2 week dark period following light treatment. The loss of docosahexaenoic acid from rod outer segments was also greater in rats exposed to intermittent light than in animals treated with continuous light. It is concluded that intermittent light exposure exacerbates Type I light damage in rats (involving the retina and retinal pigment epithelium) and the schedule of intense light exposure is a determinant of visual cell death.     

The protective effect of ascorbic acid in retinal light damage of rats exposed to intermittent light

Retinal light damage in dark-reared rats supplemented with ascorbic acid and exposed to multiple doses of intermittent light was studied and compared with damage in unsupplemented dark-reared and cyclic-light-reared rats. The extent of photoreceptor cell loss from intense light exposure was determined by whole-eye rhodopsin levels and retinal DNA measurements two weeks after light treatment. Two weeks after 3 or 8 hr of intermittent light, ascorbate-supplemented animals had rhodopsin and retinal DNA levels that were two to three times higher than in unsupplemented dark-reared rats. In both types of rats rhodopsin levels were influenced by the number of light doses, the duration of light exposure, and to a lesser extent, by the length of the dark period between exposures. Rhodopsin levels in the dark-reared ascorbate-supplemented rats were significantly higher than in unsupplemented dark-reared rats, and were similar to the levels in unsupplemented cyclic-light-reared animals. Ascorbate treatment had no effect on the rate of rhodopsin bleaching. However, regeneration was greater in supplemented rats after multiple 1-hr light exposures. Intermittent light also resulted in lower ascorbate levels in the retinas of supplemented and unsupplemented rats, with dramatic losses from the retinal pigment epithelium (RPE)-choroid in both types of animals. We conclude that ascorbic acid protects the eye by reducing the irreversible Type I form of light damage in dark-reared rats. Ascorbate appears to shift light damage to the Type II form typical of cyclic-light-reared animals.     

Ascorbate and glutathione levels in the developing normal and dystrophic rat retina: effect of intense light exposure

Ascorbic acid and glutathione were measured in retinas excised from normal rats reared in a cyclic light or dark environment and in dystrophic rats from the dark environment. Similar measurements were made on retinas from age matched rats exposed to intense visible light for periods of up to 24 hours. In other rats, ascorbic acid was given for various periods before exposure to intense light and the degree of photoreceptor cell death determined subsequently by rhodopsin measurements. In non-intense light treated rats ascorbate and glutathione were 12.1 nmol/retina at 20 days of age and 13.3 - 15.9 nmol/retina in 60 day old animals. In dystrophic rat retinas glutathione was 4-8% higher and ascorbate 10-20% higher than in normal dark reared rats. Although the levels of ascorbate and glutathione per retina increased during development, the molar ratios of the antioxidant materials to rhodopsin decreased by 36% and 60% in normal and dystrophic rats respectively. The levels of glutathione in young cyclic light or dark reared normals were unaffected by intense light exposure of either short (2-4 hrs) or long (24 hrs) duration. However, in both 20 and 40 day old dystrophic rats, intense light exposure resulted in a significant increase in retinal glutathione. In contrast to glutathione, retinal ascorbate decreased in normal rats exposed to intense light for 24 hrs, in an age and prior light environment dependent fashion. At ages greater than 20 days, normal rats exposed to light had significantly lower retinal ascorbate levels than their non-light exposed counterparts. The levels of ascorbate in 21-40 and 41-60 day old dark reared rat retinas were also significantly lower than in comparable intense light treated-cyclic light reared rats. In the youngest dystrophic rats whole eye ascorbate (retina, RPE, choroid and sclera) was 20-30% lower than in non-light treated rats, but in older mutant rats (41-60 day) light had no effect on the level of ascorbate in the retina. As determined by the level of rhodopsin remaining in the eye two weeks after 24 hrs light exposure, cyclic light reared rats lost 50-55% of their visual cells. However, cyclic light rats supplemented with ascorbic acid before intense light exposure lost only 30-35% of their visual cells.     

Light-induced exacerbation of retinal degeneration in a rat model of Smith-Lemli-Opitz syndrome

Potentiation of retinal degeneration by intense light exposure, and its amelioration by an antioxidant, were studied in a rat model of Smith-Lemli-Opitz syndrome (SLOS), in comparison with normal (control) Sprague-Dawley rats. The SLOS model is created by treating rats with AY9944, a selective inhibitor of cholesterol synthesis at the level of 3beta-hydroxysterol-Delta7-reductase. A subset of rats was treated with dimethylthiourea (DMTU), a synthetic antioxidant, 24 and 1 hr prior to light exposure. Half of the animals (+/-DMTU) were exposed to intense, constant, green light (24hr, 1700lx, 490-580 nm), while the others were maintained in darkness. Subsequently all animals were returned to dim cyclic light (20-40 lx, 12 hr light-12 hr dark) for 2 weeks, after which electroretinograms were recorded. One eye from each rat was taken for histological and quantitative morphometric analyses; sterol analysis was performed on retinas from contralateral eyes. HPLC analysis confirmed the accumulation of 7-dehydrocholesterol (7DHC) in retinas of AY9944-treated rats; cholesterol represented >99% of the sterol in control retinas. Histology of retinas from unexposed, AY9944-treated rats (6-week-old) was normal. In contrast, age-matched, light-exposed rats exhibited massive photoreceptor cell loss in both the superior and inferior hemispheres, and concomitant rod and cone dysfunction. The severity and geographic extent of the damage was far greater than that observed in normal albino rats exposed to the same conditions. DMTU pre-treatment largely prevented these degenerative changes. These findings indicate that the AY9944-induced rat SLOS model is hypersensitive to intense light-induced retinal damage, relative to normal rats. DMTU protection against light-induced damage implicates free radical-based oxidation in the retinal degeneration process. Furthermore, the use of green light (corresponding to the absorption maxima of rhodopsin) implicates rhodopsin in the initiation of the pathobiological mechanism. We propose that generation of cytotoxic oxysterols (by-products of 7DHC oxidation) is an integral part of retinal cell death in the SLOS rat model, which is exacerbated by intense light. Furthermore, the results predict light-dependent potentiation of retinal degeneration in SLOS patients, and the possible ameliorative effects of antioxidant therapy.     

Retinal light damage reduces autofluorescent pigment deposition in the retinal pigment epithelium

Lipofuscin in the retinal pigment epithelium (RPE) is thought to be derived from phagocytosed photoreceptor outer segment disc membranes. Based on this hypothesis, one would predict that the rate of lipofuscin deposition in the RPE would be proportional to the density of photoreceptor cells in the retina. In previous studies it was demonstrated that specific loss of photoreceptor cells due to a genetic defect resulted in a substantial decrease in the rate of age-related lipofuscin accumulation in the RPE. In order to confirm that this decreased RPE lipofuscin deposition was directly related to reduced photoreceptor cell density, experiments were conducted to determine whether light-induced photoreceptor cell destruction affected RPE lipofuscin content. The effects of retinal light damage on RPE autofluorescent pigment accumulation resulting from both normal aging and vitamin E deficiency were examined. Starting immediately after weaning, albino Fisher 344 rats were fed diets either containing or lacking vitamin E. All animals were maintained on a 12 hr/12 hr light/dark cycle. During the light phases of the cycles, the cage illuminance for one-half the animals in each dietary group was 750 lux, while the remaining rats were exposed to a light level of 15 lux. Illumination was provided by 40 watt cool-white fluorescent lamps. After 17 weeks, rats in both dietary groups that were maintained under the higher light intensity had substantially reduced photoreceptor cell densities relative to animals in the same dietary group maintained under dim light conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoreceptor cell damage by light in young Royal College of Surgeons rats.

PURPOSE. To determine the effects of genetic background and light rearing conditions on intense-light-mediated retinal degeneration in young RCS rats. MATERIALS AND METHODS. Albino rats, homozygous or heterozygous for the rdy gene were bred and born in dim cyclic light. At P7 they were moved to a dark environment, and maintained there until exposure to intense visible (green) light at P18 or P25. Other rats remained in the dim cyclic light environment. At various times between P11 and P40 rats were killed for determinations of rhodopsin and photoreceptor cell DNA levels, western transblot analysis of retinal S-antigen (arrestin) and alpha-transducin, or northern slot blot analysis of their respective mRNA levels. RESULTS. At P18, unexposed dark maintained homozygous RCS rats and their phenotypically normal heterozygous counterparts have nearly equivalent rhodopsin levels and photoreceptor cell DNA. Intense light exposure at this age, to 8 hours of continuous light or 3 hours of intermittent light, did not lead to a loss of either rhodopsin or retinal DNA when compared with their respective unexposed controls. At P25 rhodopsin levels were higher than at P18, while photoreceptor cell DNA was essentially the same as in the younger rats. However, intense light exposure at P25 resulted in substantial losses of rhodopsin and photorecptor cell DNA and the losses were greater in homozygous rats than in heterozygous animals. Light damage of P25 rats maintained in dim cyclic light was essentially the same as in dark maintained homozygous rats, but no damage was found in the heterozygous animals. By western analysis, alpha-transducin levels in the retina increased with time in darkness, while retinal S-antigen levels either remained the same or decreased during the period P15-P35. For rats in the cyclic light environment S-antigen expression was greater than alpha-transducin at all ages. Slot blot analysis of mRNAs for the two proteins generally followed the patterns seen by western analysis. S-antigen mRNA was expressed at an earlier age and at higher levels than alpha-transducin in both types of rats from both light rearing conditions. Peak expression of S-antigen most often occurred at P18 in both the heterozygous and homozygous rats. CONCLUSIONS. The relative expressions of S-antigen and alpha-transducin in P18 and P25 rats correlates with their relative resistance to retinal light damage at P18 and their enhanced susceptibility at P25. Rats homozygous for the rdy gene also exhibit more damage than heterozygous animals when photoreceptor cell DNA is used to estimate the extent of retinal light damage.     

Development and degeneration of retina in rds mutant mice: altered disc shedding pattern in the albino heterozygotes and its relation to light exposure

The number of phagosomes in the retinal pigment epithelium (RPE) of normal albino mice, maintained in cyclic light, is highest at the time of onset of light. The number declines to the lowest level around the start of the dark period. If the dark period is prolonged, the pattern of shedding remains cyclic but the phagosome count is higher than normal. If the light period is prolonged the phagosome count remains low and the rise to the peak is delayed. The number of phagosomes in the RPE of albino rds/+ mice, maintained in cyclic light, peaks to higher than normal level and the peak is recorded near the end of the light period. If the dark period is prolonged the phagosome count in the RPE of rds/+ mice remains lower than in the similarly treated normal mice. The phagosome count in the RPE of rds/+ mice, exposed to a prolonged light period, remains higher than in the similarly treated normal mice. In both normal and rds/+ mice, born and reared in total darkness, the pattern of disc shedding is very different but the rds/+ mice show a relatively higher frequency of phagosomes in the RPE than the normal mice. The phagosomes in the rds/+ RPE are larger than normal. An increase in size appears to correspond with increased rate of shedding in the rds/+ mice and to a lesser extent also in normal mice. Thus, the pattern of disc shedding in the albino rds/+ mice, which is different from the normal albino mice, also shows a different reaction to changes in the environmental light.     

实验性视网膜光性损伤

我们用Wister大鼠建立了视网膜急性光性损伤实验模型．动物在全麻下照光(白光)lh，照度为20 0001x．分别于照光后24、48h，1、2、3、4、5周处死实验动物。病理组织学显示视网膜急性光性损伤始于光感受器外节，表现为碎解和消失，随之波及内节和视网膜色素上皮(RFE)，部分标本外核层亦波及。后期(照光后2-3周)有些标本显示内、外节有某种程度修复，另一些标本则无。有一个照光后5周的标本显示外核层，内、外节完全消失。所有标本未显示任何炎症反应，提示视网膜急性光性损伤是一个退行性变性过程。 （中华眼底病杂志，1994，10：84-85）     

Increased light damage susceptibility at night does not correlate with RPE65 levels and rhodopsin regeneration in rats

The susceptibility of rats to light-induced retinal degeneration is increased at night. In mice, an important determinant of light damage susceptibility is the efficacy of rhodopsin regeneration after bleaching. The rate of rhodopsin regeneration is at least partly controlled by RPE65, a protein expressed in the retinal pigment epithelium. We therefore tested a potential involvement of RPE65 and rhodopsin regeneration in the increased light damage susceptibility of rats at night. For this purpose, rats were exposed to visible light at noon or at midnight and extent of light damage was determined by retinal morphology and TUNEL staining. Rpe65 gene expression was analyzed by semiquantitative RT-PCR and levels of RPE65 protein were determined by Western blotting. Rhodopsin regeneration kinetics was determined by measuring rhodopsin content immediately after a strong bleach and after different times of recovery in darkness.Rats were more susceptible to light damage at night as described by Organisciak and collegues [Invest. Ophthalmol. Vis. Sci. 41 (2000) 3694]. Rpe65 gene expression followed a day-night rhythm with highest steady-state mRNA levels at the beginning and lowest levels at the end of the day period. However, RPE65 protein levels remained constant. Rhodopsin regeneration kinetics did not differ during day and night. We conclude that levels of RPE65 protein and rhodopsin regeneration kinetics do not correlate with the increased light damage susceptibility observed in rats at night. Additional genetic or physiologic modifiers may exist in rats that regulate the retinal responsiveness to acute light exposure.     

The diurnal susceptibility of rat retinal photoreceptors to light-induced damage

Exposure of albino rats to high intensity light results in rapid, graded loss of photoreceptors. The hormonal status and age of an animal at the time of exposure affect the severity of light-induced retinal damage. The adrenal axis and pituitary hormones (prolactin) have been demonstrated previously to affect the degree of cell death in the retina. Because circadian rhythms for adrenal and pituitary secretion have been demonstrated in the rat, a series of experiments was undertaken to determine if a diurnal pattern of retinal susceptibility to light damage exists which might be related to endogenous endocrine rhythms. Male Sprague-Dawley rats were exposed to 4 hr of high intensity fluorescent light for 8 consecutive days during different phases of the 14:10 hr light: dark animal room light cycle. Morphometric analysis performed at the light microscopic level 2 weeks after exposure demonstrated a differential susceptibility to light-induced cell death depending upon the period during the light-dark cycle when animals received their daily light exposure. Neuronal cell death was confined to the outer nuclear layer as previously described. The retinas of animals exposed during the middle of the dark period or during the first 5 hr of the light period were significantly more damaged than the retinas of animals exposed during the last 9 hr of the light period. Control groups for the relative amounts of dark-adaptation between groups suggested that the diurnal susceptibility to light damage was not solely dependent upon the degree of dark adaptation. These results demonstrate a diurnal susceptibility of photoreceptors to light-induced cell death.     

Adaptive changes in visual cell transduction protein levels: effect of light.

Long-term environmental light-mediated changes in visual cell transduction proteins were studied to assess the influence of rearing environment on their levels and their potential effects on intense light-induced retinal damage. The levels of rhodopsin, S-antigen and the alpha subunit of transducin were measured in whole eye detergent extracts, retinal homogenates or rod outer segments isolated from rats reared in weak cyclic light or darkness, and following a change in rearing environment. Rats changed from weak cyclic light to darkness had 22% more rhodopsin per eye than cyclic-light rats after 12-14 days in the new environment. Western trans-blot analysis of retinal proteins from these dark-maintained animals contained 65% higher levels of immunologically detectable alpha transducin; S-antigen levels were approximately 45% lower than in cyclic-light rats. In rats changed from the dark environment to weak cyclic light, rhodopsin levels decreased by 18% during a comparable period; retinal alpha transducin was 35% lower, S-antigen was 30% higher. At various times after the change in rearing environment, some rats were exposed to intense visible light to determine their susceptibility to retinal damage. Two weeks after an 8-hr exposure, cyclic-light reared rats had rhodopsin levels only 10% lower than control (2.1 nmol per eye). However, rhodopsin was 75% lower when cyclic-light rats were maintained in darkness for 2 weeks before intense light. For animals originally reared in darkness, rhodopsin was 78% lower following 8 hr of intense light, whereas only 30% rhodopsin loss occurred in dark-reared rats after previous maintenance for 2 weeks in weak cyclic-light.(ABSTRACT TRUNCATED AT 250 WORDS)     

Failure of ascorbate to protect against broadband blue light-induced retinal damage in rat

· Background: Excessive generation of free radicals due to light absorption is proposed as the most likely mechanism for photochemical retinal damage. The observed reduction of green light-induced retinal injury after ascorbate treatment is believed to be an antioxidative effect. The aim of the present study was to evaluate the possible protection of ascorbate against blue light-induced photoreceptor damage. · Methods: Cyclic light-reared albino rats were injected intraperitoneally with either ascorbate (1 mg/g body weight) or, as placebo, physiological saline 24 h before and just prior to exposure to blue light. After 20–22 h of dark adaptation, two groups of the rats were exposed in pairs to the blue light (400–480 nm) for 6 h at an average irradiance of 0.7 W/m² in the cage. Six days after light exposure, all rats were killed and retinal samples were analyzed. · Results: Diffuse blue light irradiation resulted in an uneven distribution of damage in the retina. As judged from the pathological changes in the retina irradiated, no microscopic difference was observed between the two groups. The preserved thickness of the outer nuclear layer was on average 61.3% in the ascorbate-treated and 66.4% in the placebo-treated group. The photoreceptor loss was not significantly different between the two groups. · Conclusion: The ascorbate did not protect the retina from blue-light induced damage. This favors the assumption that the mechanisms for blue light-induced retinal damage might differ from that for green light. Received: 13 October 1998 Revised version received: 22 January 1999 Accepted: 18 February 1999     

过度光照后诱导型一氧化氮合酶在大鼠视网膜色素上皮的表达

目的探讨过度光照诱导大鼠视网膜色素上皮（retinal pigment epithelium．RPE）细胞表达诱导型一氧化氮合酶（inducible nitric oxide synthase，iNOS）的作用及其病理学意义。方法应用免疫组织化学的方法分别检测处于自然光照环境中的正常大鼠及接受强光过度照射后大鼠的视网膜RPE细胞中iNOS的表达情况。结果正常大鼠RPE细胞中未见iNOS的表达，而过度光照后大鼠RPE细胞的胞浆中表达高水平的iNOS，同时视网膜外核层感光细胞发生结构损伤。结论过度光照可诱导RPE细胞表达iNOS．这一异常改变可能是视网膜感光细胞结构损伤的重要原因。     

Increased levels of leukotriene C4 in retinal pigment epithelium are correlated with early events in photoreceptor shedding in Xenopus laevis

The levels of 5-lipoxygenase products of arachidonic acid, leukotriene (LT) B4 and LTC4 in retinal pigment epithelia (RPE) from Xenopus laevis were measured by radioimmunoassay (RIA). RPE were isolated during various stages of photoreceptor renewal to determine possible alterations in 5-lipoxygenase activity concurrent with photoreceptor detachment and phagocytosis. Both LTC4 and LTB4 were released to RPE incubation media, although levels of LTB4 in unstimulated RPE were close to the limits of detection by RIA. Incubation of RPE with the calcium ionophore A23187 increased the levels of both LTB4 and LTC4. When animals were maintained on a cycle of 12 hr light/dark, normal photoreceptor shedding, as measured by histological quantitation of the appearance of phagosomes in the RPE, occurred 1 hr after light onset. Levels of LTC4 in RPE were lower 1 hr after light onset, as compared to 1 hr prior to light onset. Due to the low levels of LTB4, no significant differences could be detected. However, when LTB4 levels were elevated with A23187, LTB4 also declined 1 hr after light onset. When animals were maintained in constant light for 5 days, then exposed to 2 hr dark and 2 hr light, a massive shedding response occurred. Levels of LTC4 were stimulated 5 min after light onset (prior to detectable shedding) and declined below dark levels as shedding progressed. These data suggest a correlation between 5-lipoxygenase activity and the events of photoreceptor shedding and phagocytosis.     

alpha-Tocopherol in the developing rat retina: a high pressure liquid chromatographic analysis

High pressure liquid chromatography was used to measure alpha-tocopherol in the retinas of rats reared in a cyclic light or dark environment. These measurements were performed on extracts of whole retinas during the developmental period, 18-60 days, and on isolated ROS from adult animals. Similar alpha-tocopherol determinations were performed on retinas and isolated ROS following exposure of rats to intense visible light for 24 hr periods. The results show that alpha-tocopherol is chromatographically separated from the vitamin A derivatives found in the retina and is pure, as judged by mass spectrometry. In the retinas of cyclic light and dark reared rats, alpha-tocopherol accumulates in an age dependent fashion, so that at 60 days the level is nearly double that of animals at 18-20 days of age (P less than 0.001). Because the age dependent accumulation of rhodopsin is greater in dark reared rats, the average molar ratio of rhodopsin to alpha- tocopherol in the retina of dark reared animals is 25% higher than in cyclic light rats. Following exposure of rats to intense visible light for 24 hr periods, alpha-tocopherol concentrations in the retina were unchanged from the levels in control animals. In adult animals the concentration of alpha-tocopherol in ROS is 2.5-3.5 times higher than in whole retina. ROS from adult cyclic light reared rats also contain an average of 43% more alpha-tocopherol per mg protein than ROS from dark maintained animals (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of colchicine on the diurnal variation of phagocytosis in mouse retinal pigment epithelium

The effect of colchicine on the fine structure of the retinal pigment epithelium (RPE) was investigated morphometrically in ICR mice bred in a condition of 12-hr light: 12-hr dark cycle since birth. In untreated mice, the number of phagosomes in RPE, containing packets of rod outer segment (ROS) discs, increased maximally at 1.5 hr after light onset, and progressively declined to the bottom value at around the time of lights off. Intraperitoneal injection of mice with colchicine (1 mg per 100 g body weight) prior to a peak of phagocytosis (4 hr before light onset) resulted in a marked increase in the number of phagosomes and a clustering of lysosomes in the apical cytoplasm of RPE and a decrease in the number of microtubules. The colchicine treatment also induced the dispersion of mitochondria from a basal cytoplasmic portion to the whole cytoplasmic area. These changes became dominant at 8 hr after the treatment and continued thereafter for a period of 12 hr. On the other hand, when colchicine was given during the period in which phagocytosis is inactive (8 hr after light onset), it induced only a slight increase in the number of phagosomes which recovered to the normal level earlier than the above. Clustering of lysosomes and dispersion of mitochondria continued to the same extent as in mice treated with colchicine at 4 hr before light onset. It was suggested that the effect of colchicine on the phagocytosis of ROS by RPE might depend on the functional state of RPE during the lighting cycle.     

The dopaminergic amacrine system and its response to light stimulation in rats with inherited retinal dystrophy

In both the RCS mutant strain of rats with hereditary progressive retinal dystrophy and in controls, concentrations of dopamine (DA) and its metabolic DOPAC increased steeply in retina from 14 to 60 days postnatally with no further elevations in older animals indicating normal dopaminergic system evolution in the RCS rats. In 60-, 90-, and 135-day-old but not in younger (14- and 30-day-old) RCS rats killed in the light phase of the light-dark cycle, retinal DA, and more markedly DOPAC levels, were lower than those in age-matched controls. In normal rats aged 24, 35 and 70 days that were dark-adapted for 24 hr, 2-hr light exposure increased DA and mainly DOPAC levels in retina. Light stimulation after dark adaptation elevated retinal DA and DOPAC only in 24- but not in 35- or 70-day-old RCS rats. In RCS rats with advanced retinal dystrophy, decreases in retinal DA and DOPAC levels and lack of response of DA amacrine cells to light exposure are probably secondary to degeneration or impairment of photoreceptors which are no longer capable of transmitting light stimuli onto DA neurons in retina.     

Light-induced retinal damage in mice. Hydrogen peroxide production and superoxide dismutase activity in retina.

The oxidative mechanism in retinal damage due to exposure to intense light was investigated histochemically and biochemically. SMA mice (albino mice) of 2 to 3 months of age were exposed to intense light (1000-1400 lux). In the cyclic light-reared group (without dark adaptation), the outer and inner segments of the photoreceptor cells were damaged after 3 days of exposure, and severe outer nuclear layer damage was observed after 5 to 7 days of exposure. Hydrogen peroxide (H2O2) production in the outer nuclear layer increased with the progress of retinal damage. In the dark-reared group (dark adaptation of 16-18 hours), outer and inner segment damage was noted after 4 hours of light exposure, and severe outer nuclear layer damage was noted after 12 hours of light exposure. H2O2 production increased in the outer nuclear layer with retinal damage. Superoxide dismutase (SOD) activity did not change before the occurrence of retinal damage, and decreased by 25% after 3 days of exposure to light in the cyclic light-reared group. The decrease in total SOD activity corresponded to that of manganese-SOD (Mn-SOD). In the dark-reared group, SOD activity did not change, even after 1 day of exposure. There appears to be some relationship between retinal light damage and H2O2 production in the outer nuclear layer. Superoxide dismutase activity failed to provide protection against retinal oxidative damage due to intense light exposure.