Electroretinogram changes associated with retinal upregulation of trophic factors: observations following optic nerve section

7,8-Dihydroxiflavone Maintains Retinal Functionality and Protects Various Types of RGCs in Adult Rats with Optic Nerve Transection

To analyze the neuroprotective effects of 7,8-Dihydroxyflavone (DHF) in vivo and ex vivo, adult albino Sprague-Dawley rats were given a left intraorbital optic nerve transection (IONT) and were divided in two groups: One was treated daily with intraperitoneal (ip) DHF (5 mg/kg) (n = 24) and the other (n = 18) received ip vehicle (1% DMSO in 0.9% NaCl) from one day before IONT until processing. At 5, 7, 10, 12, 14, and 21 days (d) after IONT, full field electroretinograms (ERG) were recorded from both experimental and one additional naïve-control group (n = 6). Treated rats were analyzed 7 (n = 14), 14 (n = 14) or 21 d (n = 14) after IONT, and the retinas immune stained against Brn3a, Osteopontin (OPN) and the T-box transcription factor T-brain 2 (Tbr2) to identify surviving retinal ganglion cells (RGCs) (Brn3a⁺), α-like (OPN⁺), α-OFF like (OPN⁺Brn3a⁺) or M4-like/α-ON sustained RGCs (OPN⁺Tbr⁺). Naïve and right treated retinas showed normal ERG recordings. Left vehicle-treated retinas showed decreased amplitudes of the scotopic threshold response (pSTR) (as early as 5 d), the rod b-wave, the mixed response and the cone response (as early as 10 d), which did not recover with time. In these retinas, by day 7 the total numbers of Brn3a⁺RGCs, OPN⁺RGCs and OPN⁺Tbr2⁺RGCs decreased to less than one half and OPN⁺Brn3a⁺RGCs decreased to approximately 0.5%, and Brn3a⁺RGCs showed a progressive loss with time, while OPN⁺RGCs and OPN⁺Tbr2⁺RGCs did not diminish after seven days. Compared to vehicle-treated, the left DHF-treated retinas showed significantly greater amplitudes of the pSTR, normal b-wave values and significantly greater numbers of OPN⁺RGCs and OPN⁺Tbr2⁺RGCs for up to 14 d and of Brn3a⁺RGCs for up to 21 days. DHF affords significant rescue of Brn3a⁺RGCs, OPN⁺RGCs and OPN⁺Tbr2⁺RGCs, but not OPN⁺Brn3a⁺RGCs, and preserves functional ERG responses after IONT.

The Timecourses of Functional, Morphological, and Molecular Changes Triggered by Light Exposure in Sprague-Dawley Rat Retinas

Retinal neurodegeneration can impair visual perception at different levels, involving not only photoreceptors, which are the most metabolically active cells, but also the inner retina. Compensatory mechanisms may hide the first signs of these impairments and reduce the likelihood of receiving timely treatments. Therefore, it is essential to characterize the early critical steps in the neurodegenerative progression to design adequate therapies. This paper describes and correlates early morphological and biochemical changes in the degenerating retina with in vivo functional analysis of retinal activity and investigates the progression of neurodegenerative stages for up to 7 months. For these purposes, Sprague-Dawley rats were exposed to 1000 lux light either for different durations (12 h to 24 h) and examined seven days afterward (7d) or for a fixed duration (24 h) and monitored at various time points following the exposure (up to 210d). Flash electroretinogram (fERG) recordings were correlated with morphological and histological analyses to evaluate outer and inner retinal disruptions, gliosis, trophic factor release, and microglial activation. Twelve hours or fifteen hours of exposure to constant light led to a severe retinal dysfunction with only minor morphological changes. Therefore, early pathological signs might be hidden by compensatory mechanisms that silence retinal dysfunction, accounting for the discrepancy between photoreceptor loss and retinal functional output. The long-term analysis showed a transient functional recovery, maximum at 45 days, despite a progressive loss of photoreceptors and coincident increases in glial fibrillary acidic protein (GFAP) and basic fibroblast growth factor-2 (bFGF-2) expression. Interestingly, the progression of the disease presented different patterns in the dorsal and ventral retina. The information acquired gives us the potential to develop a specific diagnostic tool to monitor the disease's progression and treatment efficacy.

In vivo evaluation of outer retinal function and structure after retrobulbar optic nerve crush by lateral orbitotomy in goats

Large animal model of optic nerve crush (ONC) plays an important role in translating novel therapeutic strategies developed in rodent model to clinical application. Due to the poor accessibility of the optic nerve (ON) in humans and large animals, lateral orbitotomy is needed to expose the retrobulbar ON. This study was to explore the effects of ONC and ON exposure with lateral orbitotomy (sham surgery) on the outer retinal function and structure in goats by using standard flash electroretinogram (FERG) and spectral-domain optical coherence tomography (SD-OCT). We found that ONC led to a transient reduction in FERG amplitudes at 1 week post injury (wpi), which recovered gradually over 2 months afterwards. Sham surgery alone also caused a similar pattern of amplitude reduction in FERG, although not as significantly as ONC did. Transient outer retinal thickening following ONC occurred at 4 wpi (when progressive thinning of the ganglion cell complex began), peaked at 8 wpi, then recovered gradually at 12 wpi. In contrast, outer retinal thickness remained unchanged statistically 3 months after sham surgery. Fundus fluorescein angiography showed that neither ONC nor ON exposure with lateral orbitotomy significantly caused any significant delay or absence of central retinal vascular filling. In summary, ONC with lateral orbitotomy affects outer retinal function and structure transiently.

Intravitreal injection worsens outcomes in a mouse model of indirect traumatic optic neuropathy from closed globe injury

It is well established that an intravitreal needle poke or injection of buffer is protective to the retina in models of photoreceptor degeneration due to release of endogenous neurotrophic factors. Here we assess the effect of intravitreal injection of buffer in a model of closed globe trauma that causes air-blast induced indirect traumatic optic neuropathy (bITON). We injected animals 1-day after the last bITON or sham procedure and performed assessments 1-month later. Surprisingly, we detected a lower electroretinogram (ERG), greater optic nerve damage, and increased levels of pro-inflammatory cytokines in animals given an intravitreal injection. The effect was sometimes independent of bITON and sometimes exacerbated by the injury. Retina histology appeared normal, however the total number of axons in the optic nerve was lower even in uninjured animals that were injected. The number of degenerative axons was further increased in injured animals that were injected. In contrast, we detected a decrease in the ERG a wave and b wave amplitudes, but no effect on the visual evoked potential. Levels of the pro-inflammatory cytokines, IL-1α and IL-1β were elevated in the mice that received an intravitreal injection. This increase was even greater in animals that also had a bITON. This suggests that intravitreal injections may be injurious to the optic nerve particularly during the acute stage of optic nerve injury. In addition, the data suggests a role for IL-1α and IL-1β in this response.

Biocompatibility of a Conjugated Polymer Retinal Prosthesis in the Domestic Pig

The progressive degeneration of retinal photoreceptors is one of the most significant causes of blindness in humans. Conjugated polymers represent an attractive solution to the field of retinal prostheses, and a multi-layer fully organic prosthesis implanted subretinally in dystrophic Royal College of Surgeons (RCS) rats was able to rescue visual functions. As a step toward human translation, we report here the fabrication and in vivo testing of a similar device engineered to adapt to the human-like size of the eye of the domestic pig, an excellent animal paradigm to test therapeutic strategies for photoreceptors degeneration. The active conjugated polymers were layered onto two distinct passive substrates, namely electro-spun silk fibroin (ESF) and polyethylene terephthalate (PET). Naive pigs were implanted subretinally with the active device in one eye, while the contralateral eye was sham implanted with substrate only. Retinal morphology and functionality were assessed before and after surgery by means of in vivo optical coherence tomography and full-field electroretinogram (ff-ERG) analysis. After the sacrifice, the retina morphology and inflammatory markers were analyzed by immunohistochemistry of the excised retinas. Surprisingly, ESF-based prostheses caused a proliferative vitreoretinopathy with disappearance of the ff-ERG b-wave in the implanted eyes. In contrast, PET-based active devices did not evoke significant inflammatory responses. As expected, the subretinal implantation of both PET only and the PET-based prosthesis locally decreased the thickness of the outer nuclear layer due to local photoreceptor loss. However, while the implantation of the PET only substrate decreased the ff-ERG b-wave amplitude with respect to the pre-implant ERG, the eyes implanted with the active device fully preserved the ERG responses, indicating an active compensation of the surgery-induced photoreceptor loss. Our findings highlight the possibility of developing a new generation of conjugated polymer/PET-based prosthetic devices that are highly biocompatible and potentially suitable for subretinal implantation in patients suffering from degenerative blindness.

Cord Blood Serum (CBS)-Based Eye Drops Modulate Light-Induced Neurodegeneration in Albino Rat Retinas

Age-related macular degeneration (AMD) is one of the leading causes of visual loss in western countries, it has no cure, and its incidence will grow in the future, for the overall population aging. Albino rats with retinal degeneration induced by exposure to high-intensity light (light-damage, LD) have been extensively used as a model of AMD to test neuroprotective agents. Among them, trophic factors (NGF and BDNF) have been shown to play a significant role in photoreceptors' survival. Interestingly, cord blood serum (CBS) is an extract full of chemokines and trophic factors; we, therefore, hypothesized that CBS could be an excellent candidate for neuroprotection. Here, we investigate whether CBS-based eye drops might mitigate the effects of light-induced retinal degeneration in albino rats. CBS treatment significantly preserved flash-electroretinogram (f-ERG) response after LD and reduced the "hot-spot" extension. Besides, CBS-treated animals better preserved the morphology of the outer nuclear layer, together with a reduction in microglia migration and activation. Interestingly, the treatment did not modulate reactive gliosis and activation of the self-protective mechanism (FGF2). In conclusion, our results suggest that CBS-based eye drops might be successfully used to mitigate retinal neurodegenerative processes such as AMD.

Modulation of Type-1 and Type-2 Cannabinoid Receptors by Saffron in a Rat Model of Retinal Neurodegeneration

Experimental studies demonstrated that saffron (Crocus sativus) given as a dietary supplement counteracts the effects of bright continuous light (BCL) exposure in the albino rat retina, preserving both morphology and function and probably acting as a regulator of programmed cell death [1]. The purpose of this study was to ascertain whether the neuroprotective effect of saffron on rat retina exposed to BCL is associated with a modulation of the endocannabinoid system (ECS). To this aim, we used eight experimental groups of Sprague-Dawley rats, of which six were exposed to BCL for 24 hours. Following retinal function evaluation, retinas were quickly removed for biochemical and morphological analyses. Rats were either saffron-prefed or intravitreally injected with selective type-1 (CB1) or type-2 (CB2) cannabinoid receptor antagonists before BCL. Prefeeding and intravitreally injections were combined in two experimental groups before BCL. BCL exposure led to enhanced gene and protein expression of retinal CB1 and CB2 without affecting the other ECS elements. This effect of BCL on CB1 and CB2 was reversed by saffron treatment. Selective CB1 and CB2 antagonists reduced photoreceptor death, preserved morphology and visual function of retina, and mitigated the outer nuclear layer (ONL) damage due to BCL. Of interest, CB2-dependent neuroprotection was more pronounced than that conferred by CB1. These data suggest that BCL modulates only distinct ECS elements like CB1 and CB2, and that saffron and cannabinoid receptors could share the same mechanism in order to afford retinal protection.

Cerium Oxide Nanoparticles Reduce Microglial Activation and Neurodegenerative Events in Light Damaged Retina

The first target of any therapy for retinal neurodegeneration is to slow down the progression of the disease and to maintain visual function. Cerium oxide or ceria nanoparticles reduce oxidative stress, which is known to play a pivotal role in neurodegeneration. Our aim was to investigate whether cerium oxide nanoparticles were able to mitigate neurodegeneration including microglial activation and related inflammatory processes induced by exposure to high intensity light. Cerium oxide nanoparticles were injected intravitreally or intraveinously in albino Sprague-Dawley rats three weeks before exposing them to light damage of 1000 lux for 24 h. Electroretinographic recordings were performed a week after light damage. The progression of retinal degeneration was evaluated by measuring outer nuclear layer thickness and TUNEL staining to quantify photoreceptors death. Immunohistochemical analysis was used to evaluate retinal stress, neuroinflammatory cytokines and microglial activation. Only intravitreally injected ceria nanoparticles were detected at the level of photoreceptor outer segments 3 weeks after the light damage and electoretinographic recordings showed that ceria nanoparticles maintained visual response. Moreover, this treatment reduced neuronal death and "hot spot" extension preserving the outer nuclear layer morphology. It is noteworthy that in this work we demonstrated, for the first time, the ability of ceria nanoparticles to reduce microglial activation and their migration toward outer nuclear layer. All these evidences support ceria nanoparticles as a powerful therapeutic agent in retinal neurodegenerative processes.

The reaction of cerebral cortex to a nearby lesion: damage, survival, self-protection

A needlestick injury to cerebral cortex causes immediate damage along the track of the needle (haemorrhage, cell death) and sequelae (the formation of amyloid-positive plaques, extracellular deposits of hyperphosphorylated tau protein, microglial proliferation, astrogliosis) that are long lasting, and mimic the neuropathology associated with Alzheimer's disease. We report here that the same injury also elicits a distinctive response in the cortex flanking (up to 1mm from) the direct injury. Needlestick injury was made in the neo- and hippocampal cortex of young (3 months) healthy Sprague-Dawley rats. After survival times of up to 7d, the response of the cortex was assessed with histology, immunohistochemistry and stereology. Uptake of FluoroJade B at 1d survival and labelling for 4-hydroxynonenal (4-HNE) gave measures of membrane damage; labelling for 8-hydroxy-2'-deoxyguanosine (8-OHDG) gave a measure of DNA damage, and labelling with the AT8 antibody gave a measure of the hyperphosphorylation of tau. Two probes, for neuroglobin and basic fibroblast growth factor (FGF-2), gave measures of a self-protective response in the tissue. Results indicate that neurones in the flanking region are damaged by the nearby lesion, and within 1d upregulate self-protective mechanisms. Over the next 6d survival, evidence of neuronal damage reduces markedly. In summary, cells in the region flanking a lesion are stressed by the lesion, and react to the stress with a self-protective response, which prevents their death. This response may be an important, previously unrecognised feature of brain tissue close to a focus of stress, such as a microhaemorrhage.

Slow-release drug delivery through Elvax 40W to the rat retina: implications for the treatment of chronic conditions

Diseases of the retina are difficult to treat as the retina lies deep within the eye. Invasive methods of drug delivery are often needed to treat these diseases. Chronic retinal diseases such as retinal oedema or neovascularization usually require multiple intraocular injections to effectively treat the condition. However, the risks associated with these injections increase with repeated delivery of the drug. Therefore, alternative delivery methods need to be established in order to minimize the risks of reinjection. Several other investigations have developed methods to deliver drugs over extended time, through materials capable of releasing chemicals slowly into the eye. In this investigation, we outline the use of Elvax 40W, a copolymer resin, to act as a vehicle for drug delivery to the adult rat retina. The resin is made and loaded with the drug. The drug-resin complex is then implanted into the vitreous cavity, where it will slowly release the drug over time. This method was tested using 2-amino-4-phosphonobutyrate (APB), a glutamate analogue that blocks the light response of the retina. It was demonstrated that the APB was slowly released from the resin, and was able to block the retinal response by 7 days after implantation. This indicates that slow-release drug delivery using this copolymer resin is effective for treating the retina, and could be used therapeutically with further testing.

Environmental enrichment extends photoreceptor survival and visual function in a mouse model of retinitis pigmentosa

Slow, progressive rod degeneration followed by cone death leading to blindness is the pathological signature of all forms of human retinitis pigmentosa (RP). Therapeutic schemes based on intraocular delivery of neuroprotective agents prolong the lifetime of photoreceptors and have reached the stage of clinical trial. The success of these approaches depends upon optimization of chronic supply and appropriate combination of factors. Environmental enrichment (EE), a novel neuroprotective strategy based on enhanced motor, sensory and social stimulation, has already been shown to exert beneficial effects in animal models of various disorders of the CNS, including Alzheimer and Huntington disease. Here we report the results of prolonged exposure of rd10 mice, a mutant strain undergoing progressive photoreceptor degeneration mimicking human RP, to such an enriched environment from birth. By means of microscopy of retinal tissue, electrophysiological recordings, visual behaviour assessment and molecular analysis, we show that EE considerably preserves retinal morphology and physiology as well as visual perception over time in rd10 mutant mice. We find that protective effects of EE are accompanied by increased expression of retinal mRNAs for CNTF and mTOR, both factors known as instrumental to photoreceptor survival. Compared to other rescue approaches used in similar animal models, EE is highly effective, minimally invasive and results into a long-lasting retinal protection. These results open novel perspectives of research pointing to environmental strategies as useful tools to extend photoreceptor survival.

ERG changes in albino and pigmented mice after optic nerve transection

Optic nerve transection (ONT) triggers retinal ganglion cell (RGC) death. By using this paradigm, we have analyzed for the first time in adult albino and pigmented mice, the effects of ONT in the scotopic threshold response (STR) components (negative and positive) of the full-field electroretinogram. Two weeks after ONT, when in pigmented mice approximately 18% of the RGC population survive, the STR-implicit time decreased and the p and nSTR waves diminished approximately to 40% or 55%, in albino or pigmented, respectively, with respect to the values recorded from the non-operated contralateral eyes. These changes were maintained up to 12 weeks post-ONT, demonstrating that the ERG-STR is a useful parameter to monitor RGC functionality in adult mice.

Role of the c-Jun N-terminal kinase pathway in retinal excitotoxicity, and neuroprotection by its inhibition

Retinal excitotoxicity is associated with retinal ischemia, and with glaucomatous and traumatic optic neuropathy. The present study investigates the role of c-Jun N-terminal kinase (JNK) activation in NMDA-mediated retinal excitotoxicity and determines whether neuroprotection can be obtained with the JNK pathway inhibitor, D-form of JNK-inhibitor 1 (D-JNKI-1). Young adult rats received intravitreal injections of 20 nmol NMDA, which caused extensive neuronal death in the inner nuclear and ganglion cell layers. This excitotoxicity was associated with strong activation of calpain, as revealed by fodrin cleavage, and of JNK. The cell-permeable peptide D-JNKI-1 was used to inhibit JNK. Within 40 min of its intravitreal injection, FITC-labeled D-JNKI-1 spread through the retinal ganglion cell layer into the inner nuclear layer and interfered with the NMDA-induced phosphorylation of JNK. Injections of unlabeled D-JNKI-1 gave unprecedentedly strong neuroprotection against cell death in both layers, lasting for at least 10 days. The NMDA-induced calpain-specific fodrin cleavage was likewise strongly inhibited by D-JNKI-1. Moreover the electroretinogram was partially preserved by D-JNKI-1. Thus, the JNK pathway is involved in NMDA-mediated retinal excitotoxicity and JNK inhibition by D-JNKI-1 provides strong neuroprotection as shown morphologically, biochemically and physiologically.

Prolonged illumination up-regulates arrestin and two guanylate cyclase activating proteins: a novel mechanism for light adaptation

Light adaptation in vertebrate photoreceptors is mediated by multiple mechanisms, one of which could involve nuclear feedback and changes in gene expression. Therefore, we have investigated light adaptation-associated changes in gene expression using microarrays and real-time PCR in isolated photoreceptors, in cultured isolated retinas and in acutely isolated retinas. In all three preparations after 2 h of an exposure to a bright light, we observed an up-regulation of almost 100% of three genes, Sag, Guca1a and Guca1b, coding for proteins known to play a major role in phototransduction: arrestin, GCAP1 and GCAP2. No detectable up-regulation occurred for light exposures of less than 1 h. Functional in vivo electroretinographic tests show that a partial recovery of the dark current occurred 1-2 h after prolonged illumination with a steady light that initially caused a substantial suppression of the photoresponse. These observations demonstrate that prolonged illumination results in the up-regulation of genes coding for proteins involved in the phototransduction signalling cascade, possibly underlying a novel component of light adaptation occurring 1-2 h after the onset of a steady bright light.

Retinal organization in the retinal degeneration 10 (rd10) mutant mouse: a morphological and ERG study

Retinal degeneration 10 (rd10) mice are a model of autosomal recessive retinitis pigmentosa (RP), identified by Chang et al. in 2002 (Vision Res. 42:517-525). These mice carry a spontaneous mutation of the rod-phosphodiesterase (PDE) gene, leading to a rod degeneration that starts around P18. Later, cones are also lost. Because photoreceptor degeneration does not overlap with retinal development, and light responses can be recorded for about a month after birth, rd10 mice mimic typical human RP more closely than the well-known rd1 mutants. The aim of this study is to provide a comprehensive analysis of the morphology and function of the rd10 mouse retina during the period of maximum photoreceptor degeneration, thus contributing useful data for exploiting this novel model to study RP. We analyzed the morphology and survival of retinal cells in rd10 mice of various ages with quantitative immunocytochemistry and confocal microscopy; we also studied retinal function with the electroretinogram (ERG), recorded between P18 and P30. We found that photoreceptor death (peaking around P25) is accompanied and followed by dendritic retraction in bipolar and horizontal cells, which eventually undergo secondary degeneration. ERG reveals alterations in the physiology of the inner retina as early as P18 (before any obvious morphological change of inner neurons) and yet consistently with a reduced band amplification by bipolar cells. Thus, changes in the rd10 retina are very similar to what was previously found in rd1 mutants. However, an overall slower decay of retinal structure and function predicts that rd10 mice might become excellent models for rescue approaches.