Effect of light history on rod outer-segment membrane composition in the rat

Albino rats were born and raised through 12 weeks of age in 12L:12D regimes of 5, 300- or 800-lx illuminance. Upon killing, the animals' retinas were examined for the following: (1) rhodopsin of whole retina and isolated rod outer-segment membrane; (2) retinal morphology, including outer segment length and outer nuclear layer area; and (3) outer-segment membrane lipid biochemistry. The three groups of animals show significant differences with respect to one another for nearly every parameter measured. Rod outer-segment membranes of rats raised in dim cyclic light (5 lx) have high rhodopsin packing densities, high levels of polyunsaturated fatty acids, and low cholesterol levels in comparison with animals raised in brighter illuminances (300- or 800 lx). The mole ratio of phospholipid to rhodopsin in the outer-segment membrane of rats raised in 5-lx cyclic light is only 43% of that of rats raised in 800-lx cyclic light. The difference between these two groups of animals for docosahexaenoic acid is greater than three times, with dim light-reared animals showing higher levels. These rats (5 lx-reared) have less cholesterol in their photoreceptor outer segments, 6.6 mol% compared with 19.7 mol% for animals from the 800-lx regime. In all cases, rats from the intermediate rearing illuminance (300 lx) exhibit intermediate membrane composition values. It is likely that these differences in membrane composition illustrate a profound effect of light history on photoreceptor function.     

Light damage in the rat retina: the effect of dietary deprivation of N-3 fatty acids on acute structural alterations.

We investigated the effect of depleting membrane docosahexaenoic acid (DHA, 22:6n-3) content through dietary deprivation of n-3 fatty acids on the susceptibility of the photoreceptors and pigment epithelium cells to acute light-induced changes. Male Sprague-Dawley rats were raised throughout gestation, lactation and up to the age of 8 weeks on semi-purified diets containing either safflower oil (SFO, n-3 deficient diet) or soybean oil (SO) as the sole source of lipids. A third group was switched at weaning from safflower oil to soybean oil (SFO/SO). Rats were maintained on a 12 hr/12 hr light/dark cycle in which the light level at the front of the cages was 5-10 lx. Light damage was produced by exposing dark-adapted animals to diffuse white fluorescent light of 700-800 lx for 30 min followed by 90 min of darkness. In order to study recovery from light damage, additional groups of SFO and SO rats were returned to dim cyclic light for 27 hr following bright light exposure. DHA content in retinal phosphatidylethanolamine and phosphatidylcholine was 65-75% lower in rats fed SFO than in rats fed SO. The decrease was compensated for by an increase in 22:5n-6, the total content of polyunsaturated fatty acids (PUFA) being similar in both the SFO and SO groups. The SFO/SO rats had DHA levels similar to SO animals, but 22:5n-6 remained elevated resulting in a slightly higher level of total PUFA. Severe rod outer segment (ROS) membrane disruptions were seen following bright light exposure in rats on the SO and SFO/SO diets. The appearance of these disruptions did not change significantly during more than 24 hr in dim cyclic light. In contrast, there were virtually no acute ROS lesions in the SFO group. Furthermore, there was a strong light-elicited disk-shedding response in the SO rats but not in the other two groups. The pigment epithelium of the DHA deficient retinas showed a significantly greater accumulation of large lipid droplets in the dark-adapted state. Notably, whole retina rhodopsin levels were 15% higher in the SFO than in the SO group. These results indicate that depletion of retinal DHA reduces the susceptibility of the rod outer segments to acute light damage and at the same time may alter visual pigment photochemistry and other photoreceptor and pigment epithelium functions.     

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.     

Protection of photoreceptor cells in adult rats from light-induced degeneration by adaptation to bright cyclic light

Light history has been shown to affect the susceptibility of the albino rat retina to the damaging effects of constant light exposure. Retinas of animals raised in relatively bright cyclic light are protected against light-induced degeneration compared with dim-reared animals. These effects were observed in animals raised from birth in bright cyclic light and are part of an adaptive response that protects photoreceptors from stress-induced degeneration. To determine if retinas of adult animals are capable of such adaptive changes or flexibility by switching between different light environments which do not pathologically damage photoreceptor cells, albino rats were maintained in less than 250 lux cyclic light for more than 3 weeks. At 12-13 weeks of age, they were placed into 800 lux cyclic light for 1 week, after which they were exposed to constant illumination of 1500-lux for 1, 3 or 7 days. Retinal function was evaluated by electroretinography and photoreceptor cell death was quantified by measuring outer nuclear layer thickness. After 1 week in bright cyclic light, the retinas were completely protected against 1 day constant light exposure that significantly damaged retinas of animals without 800 lux cyclic light adaptation. Significant protection was also observed in 3 day constant light exposed animals; limited protection occurred after 7 days exposure. These results indicate that the retinas of adult rats adapted to bright cyclic light within certain ranges that did not significantly damage photoreceptor cells are protected from constant light challenge. This phenomenon is a post-developmental response that demonstrates a remarkable plasticity of the retina. The mechanism(s) underlying the ability of this adaptation/flexibility in protecting photoreceptors could involve endogenous molecules that encompass many aspects of retinal cell and molecular biology and physiology. Identification of these molecules may provide insight into the development of therapeutic approaches to treat retinal degeneration.     

Bright cyclic light rearing-mediated retinal protection against damaging light exposure in adrenalectomized mice

Previous studies have shown that albino rats and mice raised in bright cyclic light are protected from light-induced retinal damage. We tested if the stress response mediated by the adrenal grand is involved in the initiation of this neuroprotective phenomenon. Balb/c mice that were adrenalectomized (Adrex) or sham operated at 28 days of age were kept under dim (5 lux) or bright (400 lux) cyclic light (12h on/off) for 2 weeks. Thereafter, their electroretinogram (ERG), outer nuclear layer (ONL) thickness and area, and plasma corticosterone levels were measured in animals with (dim+light and bright+light groups) and without (dim and bright groups) damaging light exposure (3000 lux for 24 h). In the dim+light group, a- and b-wave amplitudes and the ONL thicknesses and areas were significantly higher in the Adrex animals than the sham animals, indicating that adrenalectomy itself yielded retinal protection. In the Adrex animals, the ONL areas were significantly larger in the bright+light group than the dim+light group, indicating that bright cyclic light rearing yielded further retinal tolerance, even in the absence of the adrenal gland. In sham animals, the plasma corticosterone concentration did not change between the dim and the light groups. Accordingly, glucocorticoid secreted from the adrenal gland is not likely to be required for the mechanisms of the light-adaptation neuroprotection phenomenon in mice.     

L-NAME protects against acute light damage in albino rats,but not against retinal degeneration in P23H and S334ter transgenic rats

Two previous studies have shown that N(G)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of neuronal nitric oxide synthase, protects retinas of albino rats and mice from damaging levels of light. The aims of the present study were two-fold: (1) to confirm the protective effect of L-NAME on wild type albino rats and (2) to determine if L-NAME protects the retinas of transgenic rats with P23H and S334ter rhodopsin mutations. In the first study, albino rats born and raised in 5-10 lux cyclic light were injected intraperitoneally with either L-NAME or its inactive isomer D-NAME 30 min before being placed in bright light (2700 lux) for 24hr. Electroretinograms (ERGs) were recorded before light treatment and 2 days after cessation of exposure, and eyes were enucleated for morphologic evaluation. L-NAME, but not D-NAME provided structural protection of photoreceptor cells from light damage. The functional rescue was not statistically significant between the drug treated groups. In the second study, albino WT, P23H transgenic, and S334ter transgenic rats were born and raised in 400 lux cyclic light. Three week old animals received daily intraperitoneal injections of L-NAME or D-NAME for 4 weeks, and the same drugs were added to their drinking water. At 7 weeks of age, the ERG sensitivity curves and the outer nuclear layer thickness of both transgenic groups were significantly reduced compared to WT controls. However, administration of L-NAME did not protect against retinal degeneration caused by the rhodopsin mutation in either strain of transgenic (P23H and S334ter) rats. Thus, although photoreceptor cell death in light damage and inherited retinal degenerations share a common apoptotic mechanism, there must be significant 'up-stream' differences that allow selective neuroprotection by L-NAME.     

A morphometric study of light-induced damage in transgenic rat models of retinitis pigmentosa

PURPOSE: To determine relative susceptibility to, and regional variation of, light-induced retinal damage in two rhodopsin-mutant rat models of retinitis pigmentosa, using slow- and fast-degenerating lines. METHODS: Transgenic S334ter (lines 4 and 9) and P23H (lines 2 and 3) rats were reared in dim cyclic light or darkness and then exposed to intense green light for 1 to 8 hours. Sections along the vertical meridian were collected for retinal morphology and photoreceptor morphometry 2 weeks later. Unexposed transgenic and normal Sprague-Dawley rats served as the control. Mean outer segment lengths and outer nuclear layer thicknesses were analyzed as a function of position along the vertical meridian and as averages across that vector. RESULTS: Rapidly degenerating S334ter-4 retinas, reared in dim cyclic light, exhibited no light-induced damage, whereas retinas in the other sublines sustained damage within a sensitive region in the superior hemisphere. Light-induced damage always involved loss of outer segment membrane and photoreceptors. In some cases, the retinal pigment epithelium and inner nuclear layer were also affected. Potentiation of light-induced damage by dark-rearing was increased by at least a factor of three, and in some sublines the sensitive region was enlarged to include the entire vertical meridian. CONCLUSIONS: A complex pattern of light-induced damage outcomes was identified in S334ter (sublines 4 and 9) and P23H (sublines 2 and 3) rats. The relative susceptibilities of each subline to damage by light were different, even within the same transgene, but consistent factors included a sensitive region in the superior hemisphere and potentiation by dark-rearing.     

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.     

Alleviation of constant-light-induced photoreceptor degeneration by adaptation of adult albino rat to bright cyclic light

PURPOSE: To further test the hypothesis that light-adaptation-mediated photoreceptor protection works through inhibition of apoptosis by activation and/or upregulation of neuroprotective molecules. METHODS: Albino rats were born and raised in 5-lux cyclic light (12 hours OFF and ON). At 8 weeks of age, animals were adapted to 400-lux cyclic light for different periods. Light damage was induced by exposure to constant light for 1 day at an illumination of 1700 lux. Animals were killed, and their eyes were removed for morphometric and biochemical analysis. TUNEL assay was used to evaluate photoreceptor cell apoptosis and Western blot analyses were used to determine the levels of basic fibroblast growth factor (bFGF), neuronal nitric oxide synthase (nNOS), and caspase-3. RESULTS: Exposure of dim-reared rats to constant light for 1 day dramatically increased TUNEL-positive cells in the outer nuclear layer. Adaptation to 400-lux bright cyclic light for 4 days significantly reduced TUNEL-positive cells induced by exposure to constant light, which correlated with a significant increase in bFGF expression. Compared with control retinas, caspase-3 levels were not changed by exposure to constant light or after adaptation to 400 lux. There was a significant increase in nNOS level in the constant-light-exposed group, but not in the group adapted to 400-lux bright light before exposure to constant light. CONCLUSIONS: The retina of the adult rat can rapidly upregulate neuroprotective mechanisms when switched from dim to bright cyclic light. Identification of the molecules involved in this process may allow rational development of therapeutic approaches to treat retinal degenerative diseases.     

Characterization of rhodopsin mis-sorting and constitutive activation in a transgenic rat model of retinitis pigmentosa

PURPOSE: To determine the extent to which rhodopsin mis-sorting and constitutive activation of the phototransduction cascade contribute to retinal degeneration in a transgenic rat model of retinitis pigmentosa. METHODS: Retinas from transgenic rats expressing truncated rhodopsin (Ser334ter) were examined by light and electron microscopic immunocytochemistry at several time points. Retinal degeneration in transgenic rats raised in darkness was evaluated by quantification of outer nuclear layer thickness and by electroretinography. RESULTS: Mutant rhodopsin was found at inappropriately high levels in the plasma membrane and cytoplasm of Ser334ter rat photoreceptors. When the cell death rate was high this mis-sorting was severe, but mis-sorting attenuated greatly at later stages of degeneration, as the cell death rate decreased. The distributions of two other outer segment proteins (the cGMP-gated channel and peripherin) were examined and found to be sorted normally within the photoreceptors of these rats. Raising Ser334ter transgenic rats in darkness resulted in minimal rescue from retinal degeneration. CONCLUSIONS: Because dark rearing Ser334ter rats results in little rescue, it is concluded that constitutive activation of the phototransduction cascade does not contribute significantly to photoreceptor cell death in this rat model. The nature of the rhodopsin sorting defect and the correlation between the severity of mis-sorting and rate of cell death indicate that truncated rhodopsin may cause apoptosis by interfering with normal cellular machinery in the post-Golgi transport pathway or in the plasma membrane.     

Effects of oxygen and bFGF on the vulnerability of photoreceptors to light damage

PURPOSE. To test whether tissue oxygen levels affect the vulnerability of photoreceptors to damage by bright continuous light (BCL). METHODS. Albino rats were raised in standard conditions of cyclic light (12-hour light, 12-hour darkness) with the light level at 5 to 10 lux or 40 to 65 lux. They were then exposed to BCL (1000-1400 lux), either continuously for 48 hours or for the day or night components of the 48-hour period. During BCL, some rats were kept in room air (normoxia, 21% oxygen), some in hypoxia (10%), and some in hyperoxia (70%). Their retinas were examined for cell death, for the expression of basic fibroblast growth factor (bFGF), and for response to light (electroretinogram, ERG). RESULTS. The death of retinal cells induced by BCL was confined to photoreceptors. Within the retina, the severity of death was inversely related to the level of bFGF immunolabeling in the somas of the outer nuclear layer (ONL) before exposure. The death of photoreceptors was accompanied by an upregulation of bFGF protein levels in the ONL and by a decline in the ERG. Both hypoxia and hyperoxia during BCL reduced the photoreceptor death, bFGF upregulation, and ERG decline caused by BCL. The protective effects of hyperoxia and hypoxia were evident during both the day and night halves of the daily cycle. Hypoxia or hyperoxia alone did not upregulate bFGF or ciliary neurotrophic factor (CNTF) expression in the retina. CONCLUSIONS. Photoreceptors are protected from light damage by hypoxia and hyperoxia during exposure. The protection provided by oxygen levels operates during both day and night. The protection is not mediated by an upregulation of bFGF or CNTF.     

The effect of unilateral optic nerve section on retinal light damage in rats

The effect of unilateral optic nerve section on the susceptibility of rat photoreceptors to damage by constant light was studied. Optic nerves of albino rats were cut intracranially by a ventral approach, so as not to interfere with structures in the orbit of the eye, with the brain or with the blood supply to the retina. Two separate experiments were performed on unilaterally optic nerve sectioned rats from two different sources. One group was purchased from a commercial supplier and the other group was born and raised in 3 lux cyclic light in our laboratory. For 1-4 weeks after surgery they were exposed to either 1000 lux for 24 hr or 80 lux for 48 hr. Light damage was quantified by measuring the outer nuclear layer area remaining in histological sections through the vertical meridian of the retina. It was found that retinas with optic nerves cut suffered substantially less damage from light than did those with intact optic nerves. Sham operated rats suffered the same amount of damage as did the optic nerve intact retinas of rats with one nerve cut. The extent of protection was greatest in the region previously shown to be most susceptible to damage. The protection afforded by optic nerve section could not be explained on the basis of behavior or rhodopsin photochemistry. The possible role of heat-shock proteins and a neuromodulator is discussed.     

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)     

Transcorneal electrical stimulation promotes the survival of photoreceptors and preserves retinal function in royal college of surgeons rats

PURPOSE: To determine whether transcorneal electrical stimulation (TES) has neuroprotective effects on photoreceptors and preserves retinal function in Royal College of Surgeons (RCS) rats. METHODS: Three-week-old RCS rats received TES through a contact lens electrode on the left eye weekly for 2 to 6 weeks. The right eyes received sham stimulation on the same days. Electroretinograms (ERGs) were recorded from the rats at 3 weeks (before TES), and at 5, 7, and 9 weeks of age. After the ERG recordings, the rats were killed for morphologic analyses of the retina. RESULTS: Morphologic analyses showed that the mean thickness of the outer nuclear layer (ONL) at each time point was significantly thicker in eyes treated with TES of 100 muA than in eyes with sham stimulation (TES 100 muA versus sham: 5, 7, and 9 weeks of age; P < 0.001). ERG studies showed that TES also significantly preserved retinal function up to 7 weeks of age, but did not preserve retinal function at 9 weeks of age. CONCLUSIONS: TES prolongs the survival of photoreceptors and delays the decrease of retinal function in RCS rats. Although further investigations are necessary before using TES on patients, these findings indicate that TES may be a therapeutic treatment for some patients with diseases of the photoreceptors such as retinitis pigmentosa.     

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.     

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)     

Activity of rhodopsin in vitamin A-deprived rats: light-dependent binding of G-protein

Levels of rhodopsin in the photoreceptors of Long-Evans rats were reduced by approximately 55% through dietary deprivation of vitamin A. The interaction of visual pigment with G-protein was examined in receptor outer segment (ROS) membranes obtained from these animals. A binding assay was used to quantitate affinity of the visual pigment in unbleached and bleached ROS membranes for the alpha and beta subunits of exogenous G-protein. Extents of binding were similar to those observed for ROS membranes of rats raised on a normal (vitamin A-supplemented) diet. The results are consistent with a normal capacity for G-activation by photoactivated rhodopsin (R*) in vitamin A-deprived animals. They further indicate that "free opsin" arising in vitamin A deficiency, unlike R*, has relatively low affinity for G-protein.     

Age-related changes in retinal sensitivity, rhodopsin content and rod outer segment length in hooded rats following low-level lead exposure during development

Electroretinographic, morphometric and cyclic nucleotide metabolism studies in adult hooded rats have established that low-level lead exposure during early postnatal development (postnatal days 0-21) causes long-term selective rod deficits and degeneration. To determine if this same low-level lead exposure during early postnatal development produces immediate and/or long-term alterations in retinal sensitivity we examined ERG b-wave threshold responses in dark-adapted control and lead-exposed rats at 1-, 3- and 12 months of age. In addition, to determine possible sites and mechanisms of action responsible for the observed decreases in retinal sensitivity we analyzed the rhodopsin content per eye, the lambda max of rhodopsin and rod outer segment (ROS) length in superior and inferior posterior retina at 1-, 3- and 12 months of age. Relative to adult (3-month-old) controls whose log threshold was arbitrarily set at 0 log units, the mean log relative threshold in control rats was 0.4 log units at 1 month of age and 0.2 log units at 1 yr of age. In contrast, the mean log relative threshold in lead-exposed rats was 1.3 log units at 1 month of age and 1.1-1.2 log units at 3- and 12 months of age. Thus, compared with controls, retinal sensitivity in lead-exposed rats was decreased approx. 1 log unit at all ages examined. The rhodopsin content per eye in control rats increased 13% between 1- and 3 months of age, reaching an adult value of 1.99 nmol per eye, and then decreased 8% by 1 yr of age. In contrast, the eyes from lead-exposed rats contained 30-34% less rhodopsin at all ages examined. No change in the lambda max of rhodopsin was observed in the retinas from the lead-exposed rats. In both controls and lead-exposed rats, the developmental changes in log b-wave relative threshold were paralleled by linear increases and decreases in rhodopsin content per eye such that a log-linear relation between retinal sensitivity and rhodopsin content per eye existed between 1- and 12 months of age. The developmental changes in the superior and inferior retinal ROS length were similar in control and lead-exposed rats: ROSs were at their adult length at 1 month of age and then slightly decreased by 1 yr of age.(ABSTRACT TRUNCATED AT 250 WORDS)