Experimental Retinal Reattachment: A New Perspective

In the feline model, retinal detachment initiates a cascade of changes that include photoreceptor- cell "deconstruction," apoptotic death of some photoreceptors, neurite outgrowth from second- and third-order neurons, remodeling of photoreceptor synaptic terminals, and Müller-cell gliosis. We have previously shown that reattachment within 24 h halts or reverses many of these presumed detrimental changes. However, in patients with retinal detachments, reattachment cannot always be performed within this 24-h window. Moreover, recovery of vision following successful reattachment surgery in the macula is often imperfect. Here, we examine the ability of relatively long-term reattachment (28 d) to stop or reverse several cellular events that occur at 3 d of detachment. In contrast to earlier studies of reattachment, which focused on the regeneration of outer segments, we focus our attention here on other cellular events such as neuronal remodeling and gliosis. Some of these changes are reversed by reattachment, but reattachment itself appears to stimulate other changes that are not associated with detachment. The implications of these events for the return of vision are unknown, but they do indicate that simply reattaching the retina does not return the retina to its pre-detachment state within 28 d.

Vitamin E deficiency in dogs with retinal pigment epithelial dystrophy

The role of vitamin E deficiency in the development of retinal pigment epithelial dystrophy was investigated in 11 cocker spaniels and four other dogs. The concentration of alpha-tocopherol was measured by high performance liquid chromatography in plasma samples obtained from the affected dogs and from 28 ophthalmoscopically normal, healthy control dogs. The mean (sd) plasma alpha-tocopherol concentration in the normal dogs was 20.2 (7.1) microg/ml, compared with 1.14 (0.67) microg/ml in the 11 affected cocker spaniels. The difference between the two groups remained highly significant when the alpha-tocopherol concentrations were expressed relative to the concentrations of the plasma lipids cholesterol and triglycerides. Low plasma concentrations of alpha-tocopherol were observed in the four affected dogs of other breeds, but the finding was not so consistent. The plasma lipid concentrations were normal in the affected dogs. The deficiency of alpha-tocopherol in the affected dogs appeared to be primary, because there was no clinical, biochemical or pathological evidence of underlying disease, or any indication of a dietary deficiency which might have contributed to the low concentrations of alpha-tocopherol.

Mechanism of CAR syndrome: anti-recoverin antibodies are the inducers of retinal cell apoptotic death via the caspase 9- and caspase 3-dependent pathway

Anti-recoverin autoantibodies have been associated with cancer-associated retinopathy (CAR), a paraneoplastic blinding disease. Those antibodies have been shown to induce apoptotic death of photoreceptor cells. The objective was to ascertain the mechanisms of retinal death induced by anti-recoverin antibody in vitro by examining the apoptotic pathway involved in retinal cell death. Internalization of anti-recoverin antibody or its Fab fragments by retinal cells mediated by endocytosis lead to cytotoxicity. Antibody cellular translocation induced the increase of bcl-x(s) and bax and the decrease in the bcl-x(L) protein. We detected the release of cytochrome c and down-regulation of the apaf-1 protein. This correlated with the sequential activation of caspase 9 and caspase 3, as well as the degradation of the caspase substrate PARP and the fragmentation of DNA. Our data show that anti-recoverin antibodies are inducers of apoptosis through the mitochondrial pathway involving caspases 9 and 3. We propose that a similar mechanism may be in place in patients with CAR syndrome where high levels of circulating antibodies have been associated with retinal degeneration.

Lymphoma-associated retinopathy

OBJECTIVE

To describe the clinical, electrophysiologic, and serologic findings in a patient with retinal degeneration associated with Hodgkin's lymphoma.

DESIGN

Case report with ancillary immunohistochemical studies.

METHODS

A 24-year-old woman experienced night blindness and fundus abnormalities 1 week after initiation of chemotherapy for Hodgkin's lymphoma. Visual fields and full-field electroretinograms (ERGs) were monitored over a 10-year period. Serum antibodies were studied on Western blot reactions on a solubilized extract of bovine retina. Serum antibodies were also evaluated through indirect immunohistochemistry on rhesus monkey retina.

RESULTS

Visual field and ERG amplitudes, initially abnormal, became reduced further over 10 years. Serum antibodies were identified that reacted to a retinal protein or proteins approximating 65 kd; these antibodies showed immunologic activity against photoreceptors.

CONCLUSIONS

A progressive paraneoplastic retinopathy can occur in association with Hodgkin's lymphoma. The pathogenesis of the retinal degeneration appears to be related to a serum antibody that is reacting to a retinal protein or proteins of approximately 65 kd.

Antirecoverin antibody in the aqueous humor of a patient with cancer-associated retinopathy

PURPOSE

To describe a patient with cancer-associated retinopathy (CAR) in whom antirecoverin antibody was found in the aqueous humor.

DESIGN

Interventional case report.

METHODS

A 65-year-old man underwent resection of adenocarcinoma of the lung in June 1991 and noted deterioration of vision 10 months later. Goldmann perimetry revealed a ring-like scotoma in each eye, and the electroretinogram was nonrecordable. Aqueous humor and peripheral venous blood were collected for Western blot analysis from this patient and three other patients during surgery for age-related cataract.

RESULTS

We found antirecoverin antibody within the aqueous humor and serum in the patient with CAR. In contrast, such imunoreactivities were not observed in specimens from the control patients.

CONCLUSION

These observations strongly suggest that antirecoverin antibody penetrates into the aqueous humor and vitreous cavity beyond the blood-retina barrier in CAR.

Membranes of retinal pigment epithelial cells in vitro are damaged in the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions

The ferrous ions released from haemoglobin and storage-transferrin ions cause oxidative stress in the eyes. We observed the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions in the retinal pigment epithelial (RPE) cells in vitro, and investigated how the ferrous ions influenced RPE in vitro and the photoreceptor outer segment discs. We obtained isolated photoreceptor outer segment discs using sucrose gradient of specific gravity after homogenizing porcine retinas. After bovine RPE cells were cultured with isolated photoreceptor outer segment discs containing FeCl2 for 5 and 24 h, we incubated the specimens with rhodamine phalloidin, antimouse alpha-tubulin antibody and antimouse Ig G (FITC and rhodamine labelled). We observed the specimens by a laser scanning microscopy, and made the ultrathin sections with or without 2% uranyl acetate and 2% lead acetate for examination by transmission electron microscopy. Actin filaments and microtubules of specialized cells such as RPE cells were actively involved in phagocytosis of the photoreceptor outer segment discs. Microtubules were damaged during the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions. The peroxidation increased the granular and aggregated autofluorescence of the photoreceptor outer segment discs. The membranes of the disc and the phagosomes, and lysosomes in RPE cells were damaged by ferrous ions and had fine particles with high electron density staining without uranium acetate and lead citrate. The cytoskeletons such as actin filaments and microtubules, and the membranes of the phagosomes and the lysosomes in RPE cells in vitro were damaged during the phagocytotic process of the photoreceptor outer segment discs peroxidized by ferrous ions.

Indocyanine green angiographic interpretation of reticular dystrophy of the retinal pigment epithelium complicated by choroidal neovascularization

A 37-year-old woman presented with flashes in her left eye and bilateral visual distortion. Fundal examination revealed a reticular 'fishnet' pattern of retinal pigmentation in both eyes consistent with reticular dystrophy of the retinal pigment epithelium. In the left eye there was a small haemorrhage and a shallow serous macular detachment. Fluorescein angiography demonstrated subfoveal choroidal neovascularization. Indocyanine green angiography (ICG) revealed more extensive involvement than fluorescein angiography, with small areas of intense hyperfluorescence amongst reticular areas of hypofluorescence. These changes, as interpreted in light of the known histopathological localization of ICG, are consistent with varying stages of dysfunction of the retinal pigment epithelium in this disease.

Neuronal degradation in mouse retina after a transient ischemia and protective effect of hypothermia

Temporal profile of neuronal deaths in the mouse retina evoked by a transient retinal ischemia and the protective effect of hypothermia on such deaths were evaluated. A transient ischemic insult was induced in the mouse retina by elevating the intra-ocular pressure. The retina tissue responses after reperfusion were histopathologically detected by monitoring the retinal cell death in the ganglion cell layer and inner nuclear layer, using a sequential TUNEL-staining technique, and by measuring the inner retinal thickness. Elevation of intra-ocular pressure induced a time-related appearance of TUNEL-positive cells in the mouse inner retinas. Peak TUNEL staining occurred 12 h after reperfusion. Lowering mice body temperature to 35 degrees C, 33 degrees C and 29 degrees C during the ischemia period significantly inhibited DNA fragmentation of retinal neurons in a lowering temperature dependent manner. In this experiment, the inner retinal thickness was preserved in 29 degrees C group compared with that in 37 degrees C group. From these results, the 45-min transient ischemia and histopathological examination 12 h later provided a reproducible number of retinal neuronal deaths. Furthermore, hypothermic intervention showed a protective effect to salvage retinal neuronal cells from a transient ischemic insult.

Single amino acid change in the fifth transmembrane segment of the TRP Ca2+ channel causes massive degeneration of photoreceptors

The trp gene encodes subunits of a highly Ca(2+)-permeable class of light-activated channels of Drosophila photoreceptors. The recently characterized mutation in this gene, Trp(P365), is semidominant and causes massive degeneration of photoreceptors by making the TRP channel constitutively active. We show that a single amino acid change, Phe-550 to Ile, near the beginning of the fifth transmembrane domain of TRP channel subunits is necessary to induce, and sufficient to closely mimic, the original mutant phenotypes of Trp(P365). Hypotheses are presented as to why the amino acid residues at position 550 and its immediate vicinity might be important in influencing the regulation of the TRP channel and why the substitution of Phe for Ile at this position, in particular, could result in constitutive activity of the channel.

NF-kappaB activation in light-induced retinal degeneration in a mouse model

PURPOSE

To investigate the modulation of nuclear factor (NF)-kappaB in light-induced photoreceptor degeneration in a mouse model.

METHODS

Mice were exposed to intense green light. Light-induced activation of NF-kappaB and its nuclear localization were studied by immunohistochemistry. The NF-kappaB DNA-binding activity in the retinas after exposure to light was measured by electrophoretic mobility shift assay (EMSA). Nuclear transactivation of NF-kappaB in the photoreceptor cells was determined by quantitative real-time (qRT)-PCR. The amount of NF-kappaB p65 in the photoreceptor cells after exposure to light was assessed by Western blot analysis. To obtain more photoreceptor-specific information, microdissected photoreceptor cells were used in some studies.

RESULTS

By an immunohistochemical method, the perinuclear region of the photoreceptor cells was heavily labeled with an antibody to activated NF-kappaB after a 1-hour exposure to light. Nuclear localization of NF-kappaB in the photoreceptor nucleus was seen at 12 hours. In the experiments involving 3 hours of exposure to light followed by recovery in the dark, nuclear localization of NF-kappaB was also noted after 12 hours' recovery in the dark. During continuous exposure to light, the NF-kappaB DNA-binding activity gradually increased and reached its maximum at 12 hours. There was an increase of NF-kappaB p65 protein at 3 hours. The mRNA levels of IkappaBalpha were upregulated after 6 hours' exposure to light.

CONCLUSIONS

Intense light activated NF-kappaB in the photoreceptor cells in vivo, increased the NF-kappaB DNA-binding activity, and increased the expression of mRNA of IkappaBalpha, a target gene of NF-kappaB.

Retinal sensitivity loss in third-generation n-3 PUFA-deficient rats

A previous study conducted in guinea pigs suggested that ingestion of diets high in EPA and DHA may result in suboptimal retinal function. The aim of the present study was to evaluate retinal function in pigmented (Long-Evans) rats, raised to a third generation on diets that were either deficient in n-3 PUFA or adequate (with the addition of DHA). Electroretinographic assessment employed full-field white flash stimulation. Photoreceptor responses were evaluated in terms of peak amplitudes and implicit times (a-wave, b-wave), intensity-response functions (Naka-Rushton), and the parameters of a model of transduction (P3). Retinal phospholipid FA composition was measured by capillary GLC. DHA levels were reduced by 55% in n-3-deficient animals compared with the n-3-adequate group, whereas the levels of docosapentaenoic acid n-6 were 44 times higher in n-3-deficient animals. The level of arachidonic acid was marginally higher (12.8%) in n-6-adequate animals. The n-3-deficient animals exhibited significantly reduced retinal sensitivity (sigma and S values were both affected by 0.29 log units) and increased b-wave implicit times compared with those fed the n-3-adequate diet. These data suggest that n-3 PUFA are required for development of retinal sensitivity, more so than other indices of retinal function assessed by current methods, such as maximal response amplitude. However, the benefit for retinal function of adding preformed DHA to diets already replete in n-3 PUFA remains unclear.

Molecular steps involved in light-induced oxidative damage to retinal rods

PURPOSE

To define the molecular mechanism underlying light-induced oxidative damage to retinal photoreceptors.

METHODS

Oxidative stress was induced in isolated rod photoreceptors by bright 470- to 490-nm light and monitored by measuring the conversion of dihydrorhodamine 123 to rhodamine, with fluorescence microscopy. The effect of the wavelength on oxidant generation was investigated by applying prebleaching stimuli of either 485- or 520-nm light before the bright 470- to 490-nm light. The role of internal messengers in photooxidative stress and membrane damage by bright 470- to 490-nm light was investigated by patch-clamp recording.

RESULTS

Constant illumination with bright 470- to 490-nm light caused a rapid increase in generation of oxidants, which peaked after approximately 60 seconds, and a decrease in membrane resistance, eventually producing irreversible membrane damage. The time course and extent of oxidant generation were not affected by the absence of intracellular guanosine triphosphate (GTP) or adenosine triphosphate (ATP), suggesting that oxidative stress and membrane damage induced by 470- to 490-nm light do not require coupling to a GTP-binding protein. Prebleaching exposure to 520-nm light suppressed oxidative stress and membrane damage by subsequent application of bright 470- to 490-nm light, and the extent of suppression increased with prebleaching duration.

CONCLUSIONS

Oxidative stress and damage induced in rods in response to 470- to 490-nm light require rhodopsin activation, but not visual transduction steps downstream of active rhodopsin. Prebleaching with 485- or 520-nm light has a different effect on the level of a transient rhodopsin intermediate required for lipid peroxidation by 470- to 490-nm light.

[Significance of serotonin in the pathogenesis of diabetic retinopathy and central chorioretinal dystrophy]

A total of 147 patients with preproliferative and proliferative diabetic retinopathy (DR), with the exudative hemorrhagic and nonexudative stages (93 females and 54 males), were consulted at Helmholtz Institute of Ophthalmic Diseases in 1997-2001. Control group consisted of 39 healthy subjects aged 25-76 years. The group of patients with preproliferative DR consisted of 27 patients with compensated diabetes mellitus. The course of diabetes in 39 patients with proliferative DR was evaluated as medium severe during the subcompensation stage. The clinical picture of the fundus oculi was characterized by pronounced hemorrhagic activity. Slight retinal hemorrhages were seen in the patients with preproliferative DR. Nodules, defects of pigmented epithelium, atrophic foci were seen in the central zone of the fundus oculi in patients with nonexudative stage of central chorioretinal dystrophy; edema in the central zone, polymorphic hemorrhages, solid exudate were observed in the patients with the exudative hemorrhagic stage; subretinal neovascular membrane was detected in some patients. Erythrocyte deformability coefficient, platelet aggregation coefficient to ADP, and platelet factor were evaluated by common methods. Serotonin was measured by the fluorometric method (B. M. Kogan's method) at clinical biochemical laboratory of urgent methods of examination of N. V. Sklifosovsky Institute of Emergency. The erythrocyte deformability coefficient was notably increased in the patients with proliferative DR and central chorioretinal dystrophy in comparison with the normal value. Plasma serotonin concentration was increased significantly only in the patients with proliferative DR, while in the rest groups this concentration was notably decreased. Study of platelet aggregation gave contradictory results. The values of platelet factor 4 differed from the control negligibly. Serotonin insufficiency in patients with proliferative DR was paralleled by increased plasma serotonin concentration and thrombocytopathy. In patients with preproliferative DR and central chorioretinal dystrophy serotonin insufficiency was associated with decreased concentration of serotonin and thrombocytopathy. Erythrocyte rigidity was similarly increased in patients with proliferative DR and central chorioretinal dystrophy.

Management of incontinentia pigmenti: a case of monolateral preretinal fibrovascular proliferations adjacent to snail-track degeneration areas

PURPOSE

To report a case of monolateral preretinal fibrovascularproliferations in a young adult woman, who had suffered from incontinentia pigmenti (IP) during her first month of life.

METHODS

Case report.

RESULTS

Circumscribed preretinal fibrovascular proliferations, adjacent to a mid-peripheral area of snail track degeneration, were occasionally diagnosed in the left eye of an asymptomatic 18-year-old white female. Careful ocular examination did not reveal any cause of the monolateral vascular abnormalities observed in the posterior segment. A detailed medical history brought to light that the patient has suffered infantile IP, like four other females in her family. The patient did not present any evident malformation of teeth, nails, skeleton or hair. A cytogenetic linkage study documented a chromosomal aberration in the Xq28 band, which confirmed the diagnosis of familial IP (type 2). The fluorescein angiography findings clearly illustrated the minimal retinal involvement in the course of IP.

CONCLUSIONS

This case shows that a wide range of etiologies must be considered in patients presenting monolateral preretinal fibrovascular proliferations. To correctly manage these uncommon, inherited or acquired, retinal diseases it is better to do a mid-term follow-up, rather than operate immediately, and this enabled us to observe the natural course of the lesion, while awaiting a definitive diagnosis.

Spinocerebellar ataxia type 7 (SCA7) shows a cone-rod dystrophy phenotype

Autosomal dominant spinocerebellar ataxia 7 is associated with retinal degeneration. SCA7, the causative gene, encodes ataxin-7, a ubiquitous 892 amino acid protein of variable sub-cellular localization, and the disease is due to expansion of an unstable CAG repeat in the coding region of the gene. Recent increases in understanding of the mechanisms ofSCA7 -related retinopathy from in vitro and murine model studies prompted us to perform a detailed study of the retinal phenotype of affected members of a family with SCA7 mutation (45-47 CAG repeats). There was a spectrum of severity from mild to severe dysfunction. Early functional abnormalities were at both photoreceptor and post-receptoral levels. When cone and rod photoreceptor dysfunction was present, it was approximately equal. Regional retinal dysfunction was evident: there was more dysfunction centrally than peripherally with least effect in the midperiphery. In vivo cross-sectional retinal images with optical coherence tomography showed an early disease stage of altered foveal lamination (abnormal area of low reflectivity splitting the outer retina-choroidal complex) accompanied in the parafovea by reduced retinal thickness. Later disease stages showed foveal and parafoveal retinal thinning. The phenotype in this family with SCA7 is that of a cone-rod dystrophy. These observations increase interest in a recent hypothesis that ataxin-7 may interfere with the function of CRX (cone-rod homeobox), a transcription factor regulating photoreceptor genes and a cause of a cone-rod dystrophy phenotype in man.

Autoantibodies to photoreceptor membrane proteins and outer plexiform layer in patients with cancer-associated retinopathy

Cancer-associated retinopathy (CAR) is a paraneoplastic syndrome that is characterized by degeneration of the retina as a remote effect of cancer outside the eye. The detection of autoantibodies associated with the retinopathy may precede the diagnosis of the underlying cancer. We have examined the sera of two patients with CAR by Western blot analysis. Autoantibodies to a 40kD antigen doublet and a 35 kD antigen were detected. Tissue specificity of the autoantigens was determined by testing several different tissues. The 40 kD antigen doublet was most abundant in retinal extract but was also present in lung and spleen extracts. The 35 kD antigen showed little tissue specificity and was present in all tissues tested. Fractionation of retinal proteins into water-soluble and -insoluble proteins revealed that the 40 kD antigen doublet was highly insoluble and probably represented membrane-associated proteins. Immunohistochemical analysis of the retina showed that the 40 kD antigens locate to the photoreceptors while the 35 kD antigen is located in the outer plexiform layer.

Retinal and optic nerve degeneration after chronic carotid ligation: time course and role of light exposure

BACKGROUND AND PURPOSE

Carotid artery disease can cause chronic retinal ischemia, resulting in transient or permanent blindness, pupillary reflex dysfunction, and retinal degeneration. This experiment investigated the effects of chronic retinal ischemia in an animal model involving permanent carotid occlusion. The time course of retinal pathology and the role of light in this pathology were examined.

METHODS

Sprague-Dawley rats underwent permanent bilateral occlusion of the common carotid arteries or sham surgery. Half of the animals were postsurgically housed in darkness, and half were housed in a 12-hour light/dark cycle. Animals were killed at 3, 15, and 90 days after surgery. Stereological techniques were used to count the cells of the retinal ganglion cell layer. Thy-1 immunoreactivity was assessed to specifically quantify loss of retinal ganglion cells. The thicknesses of the remaining retinal sublayers were measured. Optic nerve degeneration was quantified with the Gallyas silver staining technique.

RESULTS

Permanent bilateral occlusion of the common carotid arteries resulted in loss of the pupillary reflex to light in 58% of rats. Eyes that lost the reflex showed (1) optic nerve degeneration at 3, 15, and 90 days after surgery; (2) a reduction of retinal ganglion cell layer neurons and Thy-1 immunoreactivity by 15 and 90 days; and (3) a severe loss of photoreceptors by 90 days when postsurgically housed in the light condition only.

CONCLUSIONS

Ischemic damage to the optic nerve caused loss of pupillary reflex and death of retinal ganglion cells in a subset of rats. Subsequently, light toxicity induced death of the photoreceptors.

Long-term effects of light damage on the retina of albino and pigmented rats

PURPOSE

To observe the morphology and physiology of the retina in rats 11 weeks after a constant (24-hour) but moderate (500-lux) illumination for 1 week.

METHODS

Levels of aspartate, gamma-aminobutyric acid (GABA), glutamate, glutamine, and taurine were measured by high-pressure liquid chromatography (HPLC) in the retina and vitreous humor of albino (Wistar) and pigmented (Long-Evans) rats. Semithin sections were used to determine retinal morphology. The TUNEL method was used to detect cells degenerating by apoptosis. Because the GABAergic system has been shown to be particularly sensitive to the loss of photoreceptors, an additional immunohistochemical study using anti-GABA, anti-glutamate decarboxylase (GAD)(67) and anti-GAD(65) antibodies was performed.

RESULTS

No apparent morphologic changes were found in the retina of pigmented rats after constant illumination, whereas in albino rats disappearance of photoreceptors (except in the extreme retinal periphery) and cell bodies was observed. A significant number of TUNEL-positive nuclei also occurred in the remaining nuclear and ganglion cell layers. However, no change in the distribution of GABA, GAD(67), and GAD(65) immunoreactivities was found in either strain under constant illumination compared with control animals. Constant illumination affected the retinal levels of aspartate, glutamate, glutamine, glycine in both strains, whereas GABA contents did not change and taurine was decreased only in albino rats. A significant increase of vitreal glutamate levels was also found in both strains and of taurine levels only in albino rats.

CONCLUSIONS

Phototoxicity can provoke durable retinal alterations beyond the period of lighting, suggesting progressive and probably continuous modifications of retinal physiology, even in pigmented animals in which the retina seems morphologically normal.

[Complicated cataract. Part I: complicated cataract in uveitic patients]

The paper shows pathognomonique cases of complicated cataracts concerning the prognostic aspects of surgical treatment. The article presents methods of treatment in cases with uveitic cataract. Modern diagnostic methods are discussed, how to choose the safest surgical technique and to reach the desired postoperative refraction. More effective ways of cataract surgery in uveitis patients are proposed.

[Complicated cataract: Part II: complicated cataract in glaucoma. Part II: Complicated cataract in retinal degenerative disorders]

The paper shows pathognomonique cases of complicated cataracts concerning the prognostic aspects of surgical treatment. The article presents surgical methods of treating patients with secondary cataract in high myopia and glaucoma. Modern diagnostic methods are discussed, how to choose the safest surgical technique and to reach the desired postoperative refraction. More effective ways of cataract surgery in patients with glaucoma and in high myopia are suggested.

[Myopic peripheral retinal degenerations]

The article shows periferical retinal degenerations which appear in myopia. The places, the ophthalmoscopical aspect and histopathological aspect of each retinal change and the complications that may appear are presented. In the end some principles of treatment are presented.

Modulation of metabotropic glutamate receptors fails to prevent the loss of adult rat retinal ganglion cells following axotomy or N-methyl-D-aspartate lesion in vivo

Both optic nerve (ON) transection and intraocular injection of N-methyl-D-aspartate (NMDA) are established lesion models to cause death of retinal ganglion cells (RGCs) in the adult rat. Excitotoxic effects via glutamate receptors resulting in secondary neuronal death are discussed as possible initiators in both types of RGC damage. We examined whether modulating glutamatergic transmission through metabotropic glutamate receptors rescues RGCs from lesion-induced degeneration in vivo. Unexpectedly, repeated intraocular injection of four different agonists/antagonists on the various subtypes of mGluRs did not decrease retinal damage in both lesion paradigms as revealed by measurement of visual performance and RGC survival. We conclude that activation/inactivation of retinal mGluRs does not play an important role for the initiation and execution of secondary RGC loss after ON transection and NMDA lesion in the adult rat.

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.