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.     

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)     

The protective effect of ascorbate in retinal light damage of rats

Cyclic light and dark-reared rats were exposed to intense visible light for various periods and then rhodopsin-measured following recovery in darkness for up to 14 days. Animals were injected with ascorbic acid or ascorbate derivatives at various doses prior to light exposure in green Plexiglas chambers. The results show that ascorbic acid administration elevates retinal ascorbate and reduces the loss of rhodopsin and photoreceptor cell nuclei resulting from intense light. When given in comparable doses, L-ascorbic acid, sodium ascorbate, and dehydroascorbate were equally effective in preserving rhodopsin. The ascorbate protective effect in the retina is also dose dependent in both cyclic light and dark-reared rats and exhibits a requirement for the L-stereoisomer of the vitamin. Ascorbic acid is effective when administered before, but not after, light exposure, suggesting that protection from light damage in the retina occurs during the light period. In some experiments, rod outer segments were isolated from rats immediately after light exposure, lipids extracted, and fatty acid composition determined. As judged by the preservation of rod outer segment docosahexaenoic acid in rats given ascorbate, the vitamin may act in an antioxidative fashion by inhibiting oxidation of membrane lipids during intense light.     

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)     

Ascorbate treatment prevents accumulation of phagosomes in RPE in light damage.

In dark-reared albino rats, exposure to 2 or 3 hr of intense light interrupted by 2 hr dark periods resulted in extensive degeneration of photoreceptor cells and degeneration of the retinal pigment epithelium (RPE). Ascorbate (ie, vitamin C) administration prior to light exposure protected photoreceptors and the RPE from light damage. In the present study, ascorbate-treated and untreated rats were exposed to various cycles of intermittent light. Immediately after this light exposure, phagosome frequency in the RPE was morphologically evaluated in comparable 50 microns sections. In untreated rats, exposure to 2 or 3 hr of intermittent light resulted in a five- to sixfold increase in phagosome density compared to unexposed controls. In contrast, no increase in phagosome density was observed in ascorbate-treated rats. In these animals, under all lighting regimens, phagosome levels remained essentially identical to those in rats not exposed to light. After a single nondamaging light exposure, phagosome density remained at the level of dark controls in ascorbate-treated and untreated rats. These results indicate that phagosome frequency may serve as an index for light damage and that the protective effect of ascorbate may be linked to its capacity to prevent rod outer segment shedding and phagocytosis under intense light conditions.     

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.     

Effect of continuous versus multiple intermittent light exposures on rat retina

The damaging effects of continuous light exposure to the albino rat retina have been well documented. However, the cumulative effects of multiple light exposures are not well defined. We therefore compared the retinal injury induced by a single 24 hour light exposure with that caused by three intermittent exposures of 8 hours each. Eight dark-adapted albino Lewis rats were exposed for 24 hours to green fluorescent light (490-580 nm) at an illuminance level of 175 foot-candles. A second group of 8 rats was exposed under similar conditions in three split doses of 8 hours each at intervals of 7 days between each exposure. Recovery was allowed in total darkness, and the animals were sacrificed 2 weeks following the last exposure. Retinal damage was assessed by morphometry and light and electron microscopy. Mild cumulative retinal injury, mostly in photoreceptor cells with relative sparing of the retinal pigment epithelium, was seen in the split dose group, while extensive damage involving photoreceptor cells and retinal pigment epithelium was noted in the group exposed continuously for 24 hours.     

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.     

Susceptibility to retinal light damage in transgenic rats with rhodopsin mutations

PURPOSE: To determine relative light-induced retinal damage susceptibility in transgenic rats expressing mutations in the N- or C-terminal region of rhodopsin. METHODS: Heterozygous transgenic rats, including P23H sublines 2 and 3 and S334ter sublines 4 and 9, were reared in dim cyclic light or in darkness before visible light exposure starting at various times of the day or night. Before exposure to light, some rats were given the synthetic antioxidant dimethylthiourea (DMTU). At various times after intense light treatment, rats were killed for determinations of rhodopsin and retinal DNA recovery, DNA fragmentation patterns, and Northern blot analysis of retinal heme oxygenase (HO)-1 and interphotoreceptor retinol binding protein (IRBP). Rod outer segments (ROSs) were isolated for Western blot analysis of rhodopsin using N- and C- terminal-specific monoclonal antibodies. RESULTS: All rats incurred greater photoreceptor cell damage from exposure to light starting at 1 AM than from exposure at 5 PM. Among cyclic-light-reared rats, P23H line 3 animals were more susceptible to light-induced damage than P23H line 2 animals. S334ter rats exhibited retinal light damage profiles similar to those in normal rats. Dark-rearing potentiated retinal damage by light. However, dark-rearing alone prolonged photoreceptor cell life in P23H rats, but had no such effect in S334ter animals. DMTU pretreatment was effective in preventing or reducing light-induced retinal damage in all transgenic rats. S334ter rat ROSs contained the truncated form of rhodopsin. Intense light exposure resulted in DNA ladders typical of apoptotic cell death and the simultaneous induction of retinal HO-1 mRNA and reduced expression of IRBP. CONCLUSIONS: Light-induced retinal damage in transgenic rats depends on the time of day of exposure to light, prior light-or dark-rearing environment, and the relative level of transgene expression. Retinal light damage leads to apoptotic visual cell loss and appears to result from oxidative stress. These results suggest that reduced environmental lighting and/or antioxidant treatment may delay retinal degenerations arising from rhodopsin mutations.     

Influence of UVA light stress on photoreceptor cell metabolism: decreased rates of rhodopsin regeneration and opsin synthesis.

There is considerable evidence indicating that rhodopsin is the chromophore mediating light damage to the rat retina caused by exposure to mid-visible wavelengths. Retinal damage is, however, more effectively produced by short-wavelength light, and little is known about the initiating events for this damage class. The present study sought to determine the involvement of rhodopsin bleaching in short-wavelength damage by examining rhodopsin levels and opsin synthesis at early time points following acute ultraviolet-A (UVA) exposures of the pigmented rat eye. A gradual decline in rhodopsin to 8% of the level in non-exposed control eyes occurred over a 1 hr exposure to 1500 microW cm-2of UVA light. When animals were placed in darkness following this exposure, rhodopsin had recovered to only 27% of control levels by 2 hr post-exposure indicating a very slow rate of regeneration. For later time points, animals were returned to dim cyclic light and by 2 days following exposure, rhodopsin levels had risen to 57% of control. In contrast, opsin levels at this same time point were unaffected by UVA exposure. Other observations indicating the UVA exposure affected photoreceptor cell metabolism included a 27% decrease in the rate of opsin synthesis between 1 and 2 days following exposure, and a 69% reduction in the rate of rod outer segment disk renewal during the initial 3 days following exposure.These data show that UVA light stress in the retina causes a gradual bleaching of rhodopsin followed by a slow rate of recovery and altered photoreceptor cell metabolism. These results are consistent with the concept that rhodopsin mediates UVA-induced retinal damage and the possible mechanisms by which this might occur are discussed in relation to alternative hypotheses currently in the literature.     

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 effects of L-and D-ascorbic acid administration on retinal tissue levels and light damage in rats.

To assess the protective effect of ascorbic acid in retinal light damage of rats, we have determined the uptake and retinal tissue distributions of its L- and D- stereoisomers following interperitoneal or intraocular injections. The effects of intense-intermittent light exposure and darkness on tissue ascorbate were compared by measuring its levels in retina and retinal pigment epithelial tissues at various times after administration. The protective effects of the two forms of ascorbate against retinal light damage were also compared by measuring rhodopsin levels 2 weeks after intense light exposure. After interperitoneal injection, both forms of ascorbic acid were higher in the retinal pigment epithelial-choroid-scleral complex (eye cup) than in the retina. Over a 2 hr post-injection period, L-ascorbate in the eye cup was 2 to 4 fold higher than normal (10-11 nmol); D-ascorbate levels were between 15 and 30 nmol. During the same period retinal L-ascorbate was just above normal (12-14 nmol), whereas less than 5 nmol of D-ascorbate was present. When ascorbate was given by the intraocular route the opposite effect was found. During the 2 hr post-injection period retinal L-ascorbate levels were 2 to 5 fold higher than normal; D-ascorbate was between 25 and 50 nmol/retina. Within 1 hr post-injection, L-ascorbate in the eye cup was near normal and D-ascorbate levels were 10 nmol or less. In uninjected rats perfused with normal saline, the endogenous L-ascorbate was distributed 55% in the retina with 9% and 36%, respectively, in the RPE-choroid and sclera. Ten-thirty min after interperitoneal peritoneal injection about 40% of the L-ascorbate was present in the retina with 17% and 44% in the RPE-choroid and sclera. Total ascorbate (L + D) levels in the same tissues of D- injected rats were similar to those found for rats given L-ascorbate. Following 7 hrs of darkness, tissue ascorbate levels in the injected rats decreased to approximately the same levels present in uninjected animals. For rats exposed to intense light average retinal ascorbate levels decreased further, while RPE-choroid and scleral levels were largely unchanged from the dark control levels. About 50% of the tissue ascorbate was present in the retina 10-30 min after intraocular injection. The RPE-choroid contained between 10 and 14% of the ascorbate, with 35-40% present in the sclera. Retinal ascorbate levels remained high in the injected eyes following 2.5 hrs of darkness, but decreased as a result of intense light treatment.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effects of dim cyclic light on pigment epithelial function in the albino rat

Most pathologies of the outer retina include physiological and morphological changes in the pigment epithelium. The question of pigment epithelial involvement in retinal light damage caused by low intensities of light is still unresolved. In the present study, we investigated the effects of low intensity cyclic light on pigment epithelial function in albino rats. The functioning of the pigment epithelium was assessed electrophysiologically from d.c. recordings of ERG c-waves and sodium azide induced changes in the resting potential. Responses obtained from albino rats raised under low intensity cyclic light (0.63 ft cd. 12:12 L:D) were compared to those obtained from albino rats raised under minimal light exposure conditions (dark-reared) and pigmented rats housed under low intensity cyclic light. We report, for the first time, that albino rats raised from birth under low intensity cyclic light possess c-waves. Their responses were comparable in amplitude and latency to those recorded from pigmented rats housed under similar conditions, but were significantly smaller than those recorded from dark-reared albino rats. The reduction in the amplitudes of the c-waves recorded from cyclic light-reared albino rats was probably not due to retinal light damage. Comparisons of the amplitudes and latencies of ERG b-waves recorded from cyclic light-reared and dark-reared albino rats did not suggest that the retinas of the cyclic light-reared albino rats had been damaged by light. Light microscopic examination of these retinas also provided no evidence for light damage. The transient, positive potential changes recorded from cyclic light-reared albino rats in response to bolus injections of sodium azide were significantly smaller than those recorded from either dark-reared albino rats or pigmented rats housed under low intensity cyclic light. The results of these experiments suggest that the pigment epithelium of albino rats is functionally altered by extremely low intensities of cyclic light.     

Interaction of environmental light and eye pigmentation with inherited retinal degenerations

The effects of eye pigmentation and light deprivation on the rate of photoreceptor degeneration have been explored in rodents with inherited photoreceptor degenerations. In pink-eyed RCS rats with inherited retinal dystrophy, the rate of degeneration is slowed when the animals are dark-reared. Fully pigmented RCS rats reared in cyclic light show the same degree of slowing as seen in dark-reared, pink-eyed RCS rats. Dark-rearing in fully pigmented rats fails to result in any further slowing of the degeneration. In albino nervous mutant mice with a slow form of photoreceptor degeneration, darkrearing fails to slow the rate of degeneration. The findings are compared with those of similar experiments in mice with the retinal degeneration mutation and human retinitis pigmentosa patients. In all forms of inherited retinal degeneration tested thus far, light deprivation fails to retard the disease, at least if the eyes are pigmented and in some cases even if they are not.     

Protective effect of halothane anesthesia on retinal light damage: inhibition of metabolic rhodopsin regeneration

PURPOSE: To determine whether the volatile anesthetic halothane protects against light-induced photoreceptor degeneration in the rodent retina. METHODS: Albino mice and rats were anesthetized with halothane and exposed to high levels of white or blue light. Nonanesthetized animals served as controls. Retinal morphology was assessed by light microscopy, and apoptosis of photoreceptor cells was verified by detection of fragmented genomic DNA and in situ staining of apoptotic nuclei (TUNEL assay). Rhodopsin regeneration after bleaching was determined by measuring rhodopsin levels in retinas of mice or rats at different time points in darkness. RESULTS: Halothane anesthesia reversibly inhibited metabolic rhodopsin regeneration and thus prevented rhodopsin from absorbing high numbers of photons during light exposure. Consequently, photoreceptors of mice and rats anesthetized with halothane were completely protected against degeneration induced by white light. In remarkable contrast, however, halothane anesthesia did not protect against blue-light-induced photoreceptor cell death. CONCLUSIONS: After the initial bleach, halothane impeded photon absorption by rhodopsin by inhibiting metabolic rhodopsin regeneration. Apparently, the rhodopsin-mediated uptake of the critical number of photons to initiate white light-induced retinal degeneration was prevented. In contrast, halothane did not protect the retina against blue light. Blue light can efficiently restore functional rhodopsin from bleaching intermediates through a process termed photoreversal of bleaching. This process does not depend on the visual cycle via the pigment epithelium but nevertheless enables rhodopsin molecules to absorb the critical number of photons required to induce retinal degeneration.     

Effect of light history on retinal antioxidants and light damage susceptibility in the rat

Albino rats were born and raised to 12 weeks of age in 12L:12D light regimes of 5, 300 or 800 lx. Upon killing, the activities of the following glutathione enzymes were measured in the neuroretina: (1) glutathione peroxidase; (2) glutathione-S-transferase; and (3) glutathione reductase. Also measured were vitamin E, ascorbic acid, and the levels of oxidized and reduced glutathione. Animals raised in 800-lx cyclic light have a significant increase in the retinal activities of the three glutathione enzymes over activities measured in animals raised in the two dimmer regimes. The retinal level of vitamin E, measured per nmol of lipid phosphorus, is directly and significantly correlated with rearing illuminance (P less than 0.05). The same is true of retinal ascorbic acid, which shows a 30% increase in the 800-lx-reared rats over the level of those raised in the intermediate light regime (300 lx). Some of the animals from each group were exposed to 2000 lx for 24 hr to determine if correlations existed between the levels of retinal antioxidants listed above and susceptibility to light damage. Animals raised in 5-lx cyclic light lost almost all of their photoreceptors as a result of the exposure. Rats raised in 300-lx cyclic light lost a small but significant number (ca. 20%), while those raised in 800 lx sustained no light damage. Electroretinographic evaluation supports these morphometrical findings.     

Light exposure can reduce selectively or abolish the C-wave of the albino rat electroretinogram

The ERG (electroretinogram) of the albino rat is reported to lack a c-wave. Observations of our own suggested that the conditions of light-rearing are important. Consequently, the authors recorded c-waves in two groups of albino rats. One group was reared from birth in dim illumination (dark-reared) and the other in 12/12 cyclic light (light-reared). Rats were tested after birth from 22 days to about 1 year. All dark-reared animals had a c-wave. Rats reared in cyclic light typically had no detectable c-wave. Physiologic and anatomic evidence suggests this consistent difference, c-waves present in dark-reared animals but absent or diminished in light-reared animals, is probably not due to extensive light induced retinal damage. No consistent differences between the two groups were seen in a- or b-wave thresholds, a- or b-wave intensity-response functions, and in the time-course of b-wave dark adaptation.     

Photostasis: regulation of daily photon-catch by rat retinas in response to various cyclic illuminances

Albino rats were born and raised in 12 hr light: 12 hr dark regimes of illuminances varying from 3- to 800 lx. At 15 weeks of age, the animals were killed and determinations were made of the following: dark-adapted and steady-state rhodopsin levels; rod outer-segment length and photoreceptor-cell density; retinal topography of rhodopsin absorbance, and regeneration rate of visual pigment in vivo. It was found that there is a four-fold drop in the dark-adapted rhodopsin level of animals raised in 400-lx cyclic light compared with those raised in 3 lx. This difference can be accounted for by differences in rod outer-segment length and transverse absorbance of frozen retinal sections. Further, this change in rhodopsin content, coupled with variations in the visual pigment regeneration rate, allows the rat to control the amount of pigment in its retina at steady-state bleach. In this way, the rat can regulate the number of photons its retina catches each day. Animals raised in cyclic illuminances differing by more than two orders of magnitude catch very nearly equal number of photons (1.10 +/- 0.2 X 10(16) per eye) during the light period. A reduction in the number of photoreceptor cells also occurs with increasing illuminance, and these changes are more pronounced in the inferior region of the retina. This is not typical of the type of light-induced retinal damage caused by acute exposures.     

The role of ocular pigmentation in protecting against retinal light damage

Albino and pigmented rats exposed to similar constant light conditions were compared in terms of their susceptibility to retinal damage. Pigmented rats showed considerable resistance to light damage but, when their pupils were maximally dilated, they damaged only about twice as slowly as albinos. When compared at light intensities that caused equal steady-state rhodopsin bleaches, albino and dilated pigmented rats showed essentially the same rate of light-induced retinal degeneration. These experiments suggest that the inherent susceptibility of the retina to light damage is about the same in albino and pigmented rats and that ocular pigmentation protects against damage primarily by lowering the retinal irradiance.