Function of glial cell network as a modulator of neural cell death during retinal degeneration

PURPOSE: Recent studies have demonstrated the mechanism of neural cell death, neuroprotection, and regeneration. However, the functional importance of glial cells during retinal degeneration is not well understood. In this review, we summarize our recent progress regarding the function of glial cells in neurotrophic factor production and neural cell death during retinal degeneration. METHODS: We made a rat model of photoreceptor degeneration by continuous light exposure, and examined the distribution and expression levels of neurotrophins and their receptors. In addition, we carried out quantitative analysis of neurotrophic factor production in cultured Müller glial cells and microglia. RESULTS: In the light-degenerated retina, microglia invade the photoreceptor layer from the inner part of the retina and increase the production of nerve growth factor (NGF). NGF decreases the production of basic fibroblast growth, factor, which prevents photoreceptor cell death, in Müller glial cells through low-affinity neurotrophin receptor p 75. Blockade of p 75 decreased photoreceptor cell death during light-induced retinal degeneration. CONCLUSIONS: These results suggest that a gliaglia network plays a critical role in neural cell death during retinal degeneration. Thus, a glia-glia network as well as a glia-neuron network could be a possible therapeutic target for inhibition of retinal degeneration.     

Constitutive and AP20187-induced Ret activation in photoreceptors does not protect from light-induced damage

PURPOSE: Delivery of glial cell-derived neurotrophic factor (GDNF), either as a recombinant protein or by retinal gene transfer results in photoreceptor (PR) neuroprotection in genetic models of retinitis pigmentosa (RP). The mechanism of GDNF action and its direct targets in the retina remain unknown. The goal of the present study was to test the neuroprotective effect of GDNF from light-induced damage, a commonly used stimulus of PR degeneration, and to determine whether protection occurs directly on PRs. METHODS: Adeno-associated viral vectors (AAV) were developed that expressed either GDNF or a constitutively (RetMen2A) or pharmacologically activated chimeric GDNF receptor (Fv2Ret). Fv2Ret homodimerization and activation are induced by the administration of the small dimerizer drug AP20187. AAV2/2 vectors and the cytomegalovirus (CMV) promoter were used to transduce GDNF in the retina, whereas RetMen2A and Fv2Ret were transduced by AAV2/5 vectors and their expression restricted to PRs by the rhodopsin promoter. In vivo GDNF levels were measured by ELISA, RetMen2A and Fv2Ret expression and activation in vitro and/or in vivo were assessed by Western blot and immunofluorescence analyses. ERG measurements and histologic analyses were performed to assess morphologic and functional rescue, respectively. RESULTS: GDNF gene transfer resulted in sustained protein expression in the eye. In addition, the results confirmed in vivo that PR-restricted activation of Ret signaling occurred after either AAV-mediated expression of RetMen2A or AP20187-dependent Fv2Ret activation. However, this or AAV-mediated GDNF retinal gene transfer did not result in functional or morphologic PR protection from light-induced damage. CONCLUSIONS: The results suggest that the apoptotic pathways responsible for light-induced PR degeneration are not inhibited by GDNF. However, GDNF signaling was shown to be regulated in time and levels in the retina by the AP20187/Fv2Ret system which is therefore available to be tested as gene-based therapeutic strategy in models of PR degeneration responsive to GDNF.     

Neurotrophic signaling in normal and degenerating rodent retinas.

Several types of insult cause up-regulation of neurotrophic factors and their receptors in the retina resulting in decreased photoreceptor cell death from subsequent injury. This phenomenon is more prominent in rats than in mice and neurotrophic factors are more efficacious in rats than mice. If up-regulation of neurotrophic factor receptors on photoreceptor cells early in the course of degenerations contributes to neurotrophic factor survival-promoting activity, it may also increase the ability to detect neurotrophic factor-induced signaling in photoreceptors, particularly in rats. In this study, these hypotheses were investigated by performing immunohistochemical staining for the phosphorylated form of extracellular receptor kinase (pERK) or c-fos after intravitreous injection of neurotrophic factors in wild type rats or mice, or those with inherited retinal degenerations. In both rats and mice either early or late in the course of degeneration, or in wild type animals, intravitreous injection of brain-derived neurotrophic factor, ciliary neurotrophic factor, or fibroblast growth factor-2 caused immunostaining for pERK and c-fos in cells of the inner retina, particularly Müller cells, but not in photoreceptors. These data add to the mounting evidence suggesting that neurotrophic factors act indirectly through Müller cells to promote photoreceptor survival.     

Gene therapy for detached retina by adeno-associated virus vector expressing glial cell line-derived neurotrophic factor

PURPOSE: To examine the protective effect of glial cell line-derived neurotrophic factor (GDNF) on retinal detachment (RD)-induced photoreceptor damage by using gene delivery. METHODS: Gene delivery to photoreceptors was achieved by subretinal injection of recombinant adeno-associated virus expressing GDNF (rAAV-GDNF) in the right eyes and AAV expressing Escherichia coli LacZ (rAAV-LacZ) in the left eyes of Lewis rats. RD in bilateral eyes was induced with subretinal injection of high-density vitreous substitute in the temporal retina 3 weeks after gene delivery. The synthesis and accumulation of GDNF within the retina was monitored 3 weeks after RD by immunohistochemistry and enzyme-linked immunosorbent assay (ELISA), respectively. The rescue of photoreceptors was evaluated by monitoring the preservation of the thickness of photoreceptor outer segment (OS) and outer nuclear layer (ONL). Apoptosis in the photoreceptors was studied using the TdT-dUTP terminal nick-end labeling (TUNEL) method 2 days after RD. Müller cell activity was checked using the immunohistochemistry with glial fibrillary acidic protein (GFAP) antibody 28 days after RD. RESULTS: Gene delivery was demonstrated by immunohistochemical study. The results of ELISA confirmed that high levels of neurotrophic factors were produced in retinas. Photoreceptor OS degeneration and the gradual shortening of the ONL were noted after RD in all the eyes. However, rAAV-GDNF-treated eyes retained longer OS than rAAV-LacZ-treated eyes 7 (P = 0.012) and 28 days (P = 0.008) after RD. ONL was also longer in rAAV-GDNF-treated eyes than in rAAV-LacZ-treated eyes 7 (P = 0.012) and 28 days (P = 0.008) after RD. GDNF-treated eyes had statistically less apoptotic cells than control eyes in photoreceptor layer (P = 0.043). Subretinal proliferation of Müller cells was suppressed in the GDNF-treated group, indicating less scar formation. CONCLUSIONS: GDNF is a potential factor that can protect photoreceptors from degeneration. In addition to preserving the OS and ONL structures, GDNF may exert its protective action by preventing the apoptosis of photoreceptors after RD. GDNF gene therapy may be a valuable adjuvant to current treatments in certain complicated forms of RD.     

Cell cycle regulation in retinal progenitors by glia-derived neurotrophic factor and docosahexaenoic acid

PURPOSE: A recent study has shown that glia-derived neurotrophic factor (GDNF) and docosahexaenoic acid (DHA) promote the survival and differentiation of retina photoreceptors. The current study was undertaken to investigate whether these molecules participate in cell cycle regulation in retinal progenitors in vitro. METHODS: Developmental changes in the expression of the stem cell marker nestin and of cell cycle and differentiated neuron markers were analyzed in neuroblasts obtained from 1-day-old rat retinas. The effects of GDNF and DHA on those changes were then determined. RESULTS: Expression of nestin, found in more than one third of neuroblasts at day 1, rapidly decreased during development, with most neuroblasts acquiring the photoreceptor phenotype. GDNF increased the percentage of photoreceptor progenitors expressing nestin, whereas DHA reduced it, simultaneously enhancing photoreceptor differentiation. Several markers of cell cycle progression indicated that photoreceptor progenitors maintained an active cell cycle during the first 2 days in vitro. GDNF stimulated the cell cycle, increasing the number of dividing cells and generating more photoreceptor progenitors, whereas DHA induced cell cycle exit and photoreceptor differentiation. Analysis of the expression of the cyclin-Cdk inhibitor p27(Kip1) confirmed these results. CONCLUSIONS: GDNF and DHA acted as molecular cues, counterbalancing the decision of photoreceptors to remain in or exit the cell cycle. The results strongly suggest that both factors participate in determining the number of photoreceptors in vitro, regulating the cell cycle and survival at early and late stages of development, respectively. Hence, GDNF and DHA may coordinately control the histogenesis of photoreceptors in the retina by modulating both neurogenesis and apoptosis.     

Rod Outer Segment Maintenance Is Enhanced in the Presence of bFGF, CNTF and GDNF

We employed a morphological assay of outer segment collapse to determine if growth factors or other supplements directly affect dissociated rod photoreceptors in vitro. The morphological changes in outer segments were correlated with the light responsiveness of rods. Time-lapse video microscopy was used to observe the collapse of rod outer segments from isolated single cells and small clumps of cells. A consistent pattern of outer segment collapse into the inner segment was observed, yielding a convenient assay of the effects of neurotrophic factors on photoreceptor functional maintenance. The functional state of rods, defined as light-responsiveness, was measured with suction electrode recordings and matched with the various stages of outer segment collapse. Ciliary neurotrophic factor (CNTF) and glial cell-line-derived neurotrophic factor (GDNF) at a high concentration, yielded statistically significant improvements in rat outer segment survival times. Basic fibroblast growth factor (bFGF), which rescues photoreceptors in several rodent models of retinal degeneration, produced a significant increase in survival time in the presence of the cofactor heparin. In 4 out of 10 cases using human tisue, bFGF also yielded a significant increase in survival times. When brain-derived neurotrophic factor (BDNF) was applied to rat rods, outer segment survival times did not change. Outer segments collapsed more quickly when either pigment epithelial cell derived factor (PEDF) or sugarN-acetyld-galactosamine (NAD-gal) were present. Our results show that rod photoreceptors can respond to bFGF, GDNF and CNTF in vitro and provide evidence for a direct effect of these neurotrophic factors on rods. The rapid collapse of isolated photoreceptors in this model provides a convenient means for testing various neurotrophic agents and the induced cellular responses.     

Retinal degeneration,apoptosis and the c-fos gene

In the past few years, the interest in the research field of apoptosis in the retina has been growing rapidly. We will give a short overview of apoptosis in the context of retinal degeneration and summarize recent data obtained in our laboratory. Based on our findings, we will also discuss possible future strategies to influence apoptotic cell death in the retina and to modulate the time course of retinal dystrophies. Apoptosis is the final common pathway of photoreceptor cell death in several retinal dystrophies as well as in light-induced photoreceptor degeneration. We investigated potential signal transducers for apoptosis in our laboratory and found an essential role of the immediate-early gene product c-Fos in light-induced photoreceptor degeneration. This is of particular interest in light of the finding that c-fos is continuously upregulated concomitant with apoptotic photoreceptor death in animal models of the retinal dystrophy retinitis pigmentosa. Interference with c-fos expression or function might therefore represent a novel means to influence the time course of retinal dystrophies, which are at present incurable diseases.     

白蒺藜对光损伤小鼠光感受器细胞的保护作用

目的 探讨白蒺藜对小鼠视网膜光损伤的保护作用.方法 采用10 000 lux白炽光光照30 min建立视网膜光损伤模型,将BALB/c小鼠分为正常对照组、模型组和治疗组,每组各6只.光照前0.5h治疗组腹腔注射白蒺藜水煎液100 μL(生药100mg/20 g),正常对照组和模型组腹腔注射等量生理盐水.光照后3h、7d分别通过OCT检测小鼠视网膜形态学改变;造模后7d处死小鼠,取眼球固定、包埋、切片进行HE染色,视紫红质和短波敏感视蛋白(short wave-length sensitivity opsin,M-opsin)的免疫荧光化学染色,观察视网膜病理改变情况.结果 正常对照组内外核层结构清楚、界限明显,视网膜视杆细胞内外节排列整齐;模型组视网膜结构层次模糊且外核层变薄,与正常对照组差异显著;治疗组小鼠视网膜结构与正常对照组没有显著差异.与正常对照组比较,模型组视紫红质和M-opsin表达降低,治疗组显著提高了视紫红质和M-opsin的表达.模型组较正常对照组视网膜外核层厚度显著变薄(P＜0.05);治疗组较模型组能显著性预防小鼠视网膜外核层变薄(P＜0.05).结论 小鼠视网膜光损伤模型造模成功,白蒺藜对光损伤视网膜具有显著的保护作用.     

Paradoxical role of BDNF: BDNF+/- retinas are protected against light damage-mediated stress

PURPOSE: Photoreceptors can be prevented from undergoing apoptosis in response to constant light by the application of exogenous neuroprotective agents, including brain-derived neurotrophic factor (BDNF). BDNF, however, cannot exert its effect directly on photoreceptors because they do not express receptors for BDNF. It has been proposed that BDNF released from Müller cells provides a feed-forward loop, increasing ciliary neurotrophic factor (CNTF) and basic fibroblast growth factor (bFGF) production in Müller cells, which may enhance photoreceptor survival. The authors hypothesized that retinas with reduced BDNF levels in which the BDNF-mediated release of neuroprotective signals is dampened are more susceptible to light-induced photoreceptor degeneration. METHODS: Young adult BDNF+/+ and BDNF+/- littermates (B6.129-BDNF(tm1-LT)) were analyzed. Retinal neurotrophin and growth factor mRNA levels were determined by quantitative RT-PCR, photoreceptor function was assessed through electroretinography, and survival was documented in morphologic sections and in TUNEL assays. Oxidative stress was assayed by measuring glutathione peroxidase activity. RESULTS: At baseline, BDNF+/- animals had significantly increased levels of glial-derived neurotrophic factor (GDNF) mRNA compared with their wild-type littermates. After light damage GDNF, CNTF, and BDNF mRNA levels dropped 14- to 16-fold in the BDNF+/+ mice but remained almost unchanged compared with baseline levels in the BDNF+/- mice. Preservation of neurotrophin levels in BDNF+/- mice correlated with photoreceptor cell survival, preservation of function, and reduced oxidative stress. CONCLUSIONS: Contrary to the hypothesis, reducing BDNF levels resulted in photoreceptor protection against light damage. Survival was paralleled by a reduction in oxidative stress and the preservation of neurotrophin levels, suggesting that chronic reduction of BDNF in the retina provides a level of preconditioning against stress.     

Caspase-1 ablation protects photoreceptors in a model of autosomal dominant retinitis pigmentosa

PURPOSE: Caspase-1 gene expression has been reported to be upregulated during light-induced retinal degeneration and to be reduced after neuroprotective treatments. Thus, caspase-1 may be proapoptotic in the retina. To test directly the role of caspase-1 in photoreceptor apoptosis, three mouse models were analyzed for retinal degeneration in the presence or absence of caspase-1. METHODS: Photoreceptor apoptosis was monitored in one model of induced (exposure to light) and in two models of inherited (rd1, VPP) retinal degeneration. Retinal degeneration was assessed qualitatively by light microscopy and quantitatively by the determination of free nucleosomes with ELISA or by rhodopsin measurements. Gene expression and protein levels were assessed by real-time RT-PCR and by Western blot analysis, respectively. RESULTS: Levels of caspase-1 proenzyme increased in all models of retinal degeneration concomitantly with the onset of cell death. Maturation or classic activity of caspase-1 was not detected in the retina. Ablation of caspase-1 was protective in the model of adRP (VPP mouse), but not in the two other models. Ablation of interleukin-1 receptor type 1 was without effect. Expression of monocyte chemoattractant protein (MCP)-1 increased in the model protected by caspase-1 ablation. CONCLUSIONS: Increased retinal expression of caspase-1 proenzyme may be a common marker for photoreceptor degeneration. The differential effects of caspase-1 ablation suggests a modulatory role of caspase-1 for photoreceptor apoptosis in some but not all models. Such a modulatory activity may involve a caspase-1 function different from the classic activation of interleukin-1beta.     

Transplantation of Schwann cell line clones secreting GDNF or BDNF into the retinas of dystrophic Royal College of Surgeons rats

PURPOSE: To assess the capacity of a retrovirus-engineered Schwann cell line (SCTM41), transfected with either a glial cell line-derived neurotrophic factor (GDNF) construct or a brain-derived neurotrophic factor (BDNF) construct, to sustain visual function in the dystrophic Royal College of Surgeons (RCS) rat. METHODS: Cell suspensions were injected into the subretinal space of the right eye of 3-week-old dystrophic RCS rats through a transscleral approach. The left eye remained as an unoperated control. Sham-surgery animals received injections of carrier medium plus DNase to the right eye. All animals were placed on oral cyclosporine. At 8, 12, 16, and 20 weeks of age, animals were placed in a head-tracking apparatus and screened for their ability to track square-wave gratings at various spatial frequencies (0.125, 0.25, and 0.5 cyc/deg). At the end of the experiment, the animals were perfused and processed for histologic assessment of photoreceptor survival. RESULTS: Animals with SCTM41-GDNF-secreting cells, on average, head tracked longer than animals with SCTM41-BDNF-secreting cells, and both performed better than those injected with the parent SCTM41 line. All tracked longer than sham-surgery or nonsurgical dystrophic eyes. Each cell type demonstrated preservation of photoreceptors up to at least 4 months of age, over and above the sham-surgery control. CONCLUSIONS: Engineered Schwann cells sustain retinal structure and function in the dystrophic RCS rat. Cells overexpressing GDNF or BDNF had a greater effect on photoreceptor survival than the parent line or sham surgery. This study demonstrates that ex vivo gene therapy and subsequent cell transplantation can be effective in preserving photoreceptors from the cell death that normally accompanies retinal degeneration.     

GDNF regulates chicken rod photoreceptor development and survival in reaggregated histotypic retinal spheres

PURPOSE: To investigate the role of glial-cell-line-derived neurotrophic factor (GDNF) on proliferation, differentiation, and apoptosis of different retinal cell types--in particular, photoreceptor cells. METHODS: Reaggregated histotypic spheres, derived from retinal cells of the E6 chicken embryo were used. Under rotation, so-called rosetted spheroids arose by aggregation of dissociated retinal cells, followed by the proliferation, migration, differentiation and programmed cell death of particular cell types. Rosetted spheroids were cultured under serum-reduced conditions, either in the absence or presence of 50 ng/mL GDNF. At appropriate stages, rosetted spheroids were analyzed by using conventional staining and immunolabeling with antibodies against different retinal cell types. RESULTS: At early stages of culture, the application of GDNF to rosetted spheroids significantly increased and sustained the rate of proliferation. In particular, a de novo production of rod photoreceptors was observed, whereas cone photoreceptors and amacrine, horizontal, ganglion, and Müller cells were not affected. In addition, in GDNF-treated cultures, rod photoreceptors differentiated earlier than in nontreated cultures. In older rosetted spheroids raised in absence of GDNF, rod but not cone photoreceptors underwent apoptosis. By supplementation with GDNF, the percentage of dying rod photoreceptors was dramatically reduced (31%-6% at 8 days in culture, 71%-3% at 10 days in culture). Both the mitogenic and survival promoting effect of GDNF were dose dependent. CONCLUSIONS: The results strongly suggest that GDNF, at least in vitro, affects rod photoreceptors. Depending on the developmental stage, GDNF regulates their proliferation, differentiation, and survival.     

Neuroprotective effect of subretinal implants in the RCS rat

PURPOSE: Retinal prosthetics have been designed to interface with the neural retina by electrically stimulating the remaining retinal circuits after photoreceptor degeneration. However, the electrical stimulation provided by the subretinal implant may also stimulate neurotrophic factors that provide neuroprotection to the retina. This study was undertaken to determine whether electrical stimulation from a subretinal photodiode-based implant has a neuroprotective effect on photoreceptors in the RCS rat, a model of photoreceptor degeneration. METHODS: Eyes of RCS rats were implanted with an active or inactive device or underwent sham surgery before photoreceptor degeneration. Outer retinal function was assessed with electroretinogram (ERG) recordings weekly until 8 weeks after surgery, at which time retinal tissue was collected and processed for morphologic assessment, including photoreceptor cell counts and retinal layer thickness. RESULTS: At 4 to 6 weeks after surgery, the ERG responses in the active-implant eyes were 30% to 70% greater in b-wave amplitude than the responses from eyes implanted with inactive devices, those undergoing sham surgery, or the nonsurgical control eyes. At 8 weeks after surgery the ERG responses from active-implant eyes were not significantly different from the control groups. However, the number of photoreceptors in eyes implanted with the active or inactive device was significantly greater in the regions over and around the implant versus sham-surgical and nonsurgical control eyes. CONCLUSIONS: These results suggest that subretinal electrical stimulation provides temporary preservation of retinal function in the RCS rat. In addition, implantation of an active or inactive device into the subretinal space causes morphologic preservation of photoreceptors in the RCS rat until 8 weeks after surgery. Further studies are needed to determine whether the correlation of neuropreservation with subretinal implantation is due to electrical stimulation and/or a mechanical presence of the implant in the subretinal space.     

Expression of CNTF receptor-alpha in chick violet-sensitive cones with unique morphologic properties

PURPOSE: Application of ciliary neurotrophic factor (CNTF) can rescue mature photoreceptors from lesion-induced and hereditary degeneration. In the chick retina, expression of the CNTF receptor is present in a subpopulation of photoreceptor cells. The present study was undertaken to identify the CNTF receptor-expressing photoreceptors and to describe the subcellular localization of the receptor protein. METHODS: The localization of the CNTF receptor was analyzed by light and electron microscopic immunocytochemistry in chick retinal wholemount preparations, with an antibody for CNTF receptor alpha (CNTFRalpha). Immunoreactive cells were identified by double labeling with immunocytochemical markers for photoreceptor subpopulations. RESULTS: The CNTFRalpha antibody labeled evenly distributed outer segments (OS) of a photoreceptor subpopulation. CNTFRalpha-positive OS were associated with oil droplets of uniform size. Receptor immunoreactivity did not colocalize with markers for rods and red-green cones. Complete overlap was found after double labeling with the antibody CERN 933, which recognizes violet-sensitive cones in the chick retina. Ultrastructurally, the CNTFRalpha-immunoreactive OS showed rodlike properties: an elongated shape and stacks of membrane discs separated from the plasma membrane. Immunoreactivity was completely restricted to the plasma membrane of the OS and the inner membrane sheet of the photoreceptor calices present in avian retinas. CONCLUSIONS: CNTFRalpha expression identifies a unique type of photoreceptors in the avian retina which does not fit into the classic morphologic definition of rods and cones. The specific expression in violet-sensitive photoreceptors suggests that CNTF may have a neuroprotective role related to the specific function of these cells.     

Retinal ganglion cell numbers and delayed retinal ganglion cell death in the P23H rat retina

The P23H-1 rat strain carries a rhodopsin mutation frequently found in retinitis pigmentosa patients. We investigated the progressive degeneration of the inner retina in this strain, focussing on retinal ganglion cells (RGCs) fate. Our data show that photoreceptor death commences in the ventral retina, spreading to the whole retina as the rat ages. Quantification of the total number of RGCs identified by Fluorogold tracing and Brn3a expression, disclosed that the population of RGCs in young P23H rats is significantly smaller than in its homologous SD strain. In the mutant strain, there is also RGC loss with age: RGCs show their first symptoms of degeneration at P180, as revealed by an abnormal expression of cytoskeletal proteins which, at P365, translates into a significant loss of RGCs, that may ultimately be caused by displaced inner retinal vessels that drag and strangulate their axons. RGC axonal compression begins also in the ventral retina and spreads from there causing RGC loss through the whole retinal surface. These decaying processes are common to several models of photoreceptor loss, but show some differences between inherited and light-induced photoreceptor degeneration and should therefore be studied to a better understanding of photoreceptor degeneration and when developing therapies for these diseases.     

Light treatment enhances photoreceptor survival in dystrophic retinas of Royal College of Surgeons rats.

PURPOSE: To determine whether treatment with bright light elicits a protective response that enhances photoreceptor survival in Royal College of Surgeons (RCS) rats with inherited retinal degeneration. METHODS: RCS rats were illuminated for 10 to 12 hours with 130 foot-candles (fc) of white or green light. Untreated littermates that were kept under low cyclic light levels were used as control subjects. Photoreceptor survival was determined by quantitative analysis of photoreceptor nuclei and ultrastructural assessment of cellular organization. Basic fibroblast growth factor (bFGF) and ciliary neurotrophic factor (CNTF) gene expression were determined at the mRNA and protein levels. RESULTS: Treatments of RCS rats with a single dose of bright light on postnatal day 23 (P23) greatly enhanced photoreceptor survival. Ultrasturctural analysis revealed intact inner segments in light-treated retinas, whereas in untreated retinas only remnants of inner segments were observed. By P42, numerous viable nuclei were counted in the posterior retina of light-treated rats, whereas most of the remaining nuclei in untreated RCS rat retinas were highly pyknotic. At 2.5 days after treatment with a single dose of bright light, bFGF gene expression was significantly higher than in untreated RCS rat retinas. By P42, bFGF protein levels were still significantly higher in the treated retinas. CONCLUSIONS: Exogenous bFGF has been shown to promote photoreceptor survival in the RCS rat retina. Thus, the increased bFGF expression that was measured in the light-treated RCS rat retinas may be a protective response to light stress, which supports the observed rescue of photoreceptors in light-treated RCS rat retinas.