Retinal Ganglion Cell Death Induced by Retinal Ischemia

Axonal protection by combination of ripasudil and brimonidine with upregulation of p-AMPK in TNF-induced optic nerve degeneration

PURPOSE

The ROCK inhibitor ripasudil hydrochloride hydrate was shown to have axonal protective effects in TNF-induced optic nerve degeneration. The α2-adrenoreceptor agonist brimonidine was also shown to exert axonal protection. The current study aimed to elucidate whether additive axonal protection was achieved by the simultaneous injection of ripasudil and brimonidine and examine the association with AMPK activation.

METHODS

Intravitreal administration was performed in the following groups: PBS, TNF, or TNF with ripasudil, with brimonidine, or with a combination of ripasudil and brimonidine. Axon numbers were counted to evaluate the effects against axon loss. Immunoblot analysis was performed to examine phosphorylated AMPK expression in optic nerves, and immunohistochemical analysis was performed to evaluate the expression levels of p-AMPK and neurofilament in the optic nerve.

RESULTS

Both ripasudil alone or brimonidine alone resulted in significant neuroprotection against TNF-induced axon loss. The combination of ripasudil and brimonidine showed additive protective effects. Combined ripasudil and brimonidine plus TNF significantly upregulated p-AMPK levels in the optic nerve compared with the TNF groups. Immunohistochemical analysis revealed that p-AMPK is present in axons and enhanced by combination therapy.

CONCLUSION

The combination of ripasudil and brimonidine may have additive protective effects compared with single-agent treatment alone. These protective effects may be at least partially associated with AMPK activation.

Evaluation of acute effects of pulmonary involvement and hypoxia on retina and choroid in coronavirus disease 2019: An optic coherence tomography study

PURPOSE

We investigated the acute subclinical choroidal and retinal changes caused by Coronavirus Disease 2019 (COVID-19) in patients with and without pulmonary involvement, using spectral domain optic coherence tomography.

METHODS

This prospective case-control study included COVID-19 patients: 50 with pulmonary involvement and 118 with non-pulmonary involvement. All patients were examined 1 month after recovering from COVID-19. The changes were followed using optic coherence tomography parameters such as choroidal and macular thickness and retinal nerve fibre layer and ganglion cell complex measurements.

RESULTS

All choroidal thicknesses in the pulmonary involvement group were lower than in the non-pulmonary involvement group and the subfoveal choroidal thickness differed significantly (p=0.036). Although there were no significant differences between the central and average macular thicknesses in the two groups, they were slightly thicker in the pulmonary involvement group (p=0.152 and p=0.180, respectively). A significant decrease was detected in the pulmonary involvement group in all ganglion cell complex segments, except for the outer nasal inferior segment (p<0.05). In addition, a thinning tendency was observed in all retinal nerve fibre layer quadrants in the pulmonary involvement group compared to the non-pulmonary involvement group.

CONCLUSION

In COVID-19 patients with pulmonary involvement, subclinical choroidal and retinal changes may occur due to hypoxia and ischemia in the acute period. These patients may be predisposed to ischemic retinal and optic nerve diseases in the future. Therefore, COVID-19 patients with pulmonary involvement should be followed for ophthalmological diseases.

On the other end of the line: Extracellular vesicle-mediated communication in glaucoma

In the last decade, extracellular vesicles (EVs) have emerged as a promising field of research due to their ability to participate in cell-to-cell communication via the transfer of their very diverse and complex cargo. The latter reflects the nature and physiological state of the cell of origin and, as such, EVs may not only play a pivotal role in the cellular events that culminate into disease, but also hold great potential as drug delivery vehicles and biomarkers. Yet, their role in glaucoma, the leading cause of irreversible blindness worldwide, has not been fully studied. Here, we provide an overview of the different EV subtypes along with their biogenesis and content. We elaborate on how EVs released by different cell types can exert a specific function in the context of glaucoma. Finally, we discuss how these EVs provide opportunities to be used as biomarkers for diagnosis and monitoring of disease.

Retinal degeneration induced in a mouse model of ischemia-reperfusion injury and its management by pemafibrate treatment

Retinal ischemia-reperfusion (I/R) injury is a common cause of visual impairment. To date, no effective treatment is available for retinal I/R injury. In addition, the precise pathological mechanisms still need to be established. Recently, pemafibrate, a peroxisome proliferator-activated receptor α (PPARα) modulator, was shown to be a promising drug for retinal ischemia. However, the role of pemafibrate in preventing retinal I/R injury has not been documented. Here, we investigated how retinal degeneration occurs in a mouse model of retinal I/R injury by elevation of intraocular pressure and examined whether pemafibrate could be beneficial against retinal degeneration. Adult mice were orally administered pemafibrate (0.5 mg/kg/day) for 4 days, followed by retinal I/R injury. The mice were continuously administered pemafibrate once every day until the end of the experiments. Retinal functional changes were measured using electroretinography. Retina, liver, and serum samples were used for western blotting, quantitative PCR, immunohistochemistry, or enzyme linked immunosorbent assay. Retinal degeneration induced by retinal inflammation was prevented by pemafibrate administration. Pemafibrate administration increased the hepatic PPARα target gene expression and serum levels of fibroblast growth factor 21, a neuroprotective molecule in the eye. The expression of hypoxia-response and pro-and anti-apoptotic/inflammatory genes increased in the retina following retinal I/R injury; however, these changes were modulated by pemafibrate administration. In conclusion, pemafibrate is a promising preventive drug for ischemic retinopathies.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

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.

7,8-Dihydroxiflavone Protects Adult Rat Axotomized Retinal Ganglion Cells through MAPK/ERK and PI3K/AKT Activation

We analyze the 7,8-dihydroxyflavone (DHF)/TrkB signaling activation of two main intracellular pathways, mitogen-activated protein kinase (MAPK)/ERK and phosphatidylinositol 3 kinase (PI3K)/AKT, in the neuroprotection of axotomized retinal ganglion cells (RGCs). Methods: Adult albino Sprague-Dawley rats received left intraorbital optic nerve transection (IONT) and were divided in two groups. One group received daily intraperitoneal DHF (5 mg/kg) and another vehicle (1%DMSO in 0.9%NaCl) from one day before IONT until processing. Additional intact rats were employed as control (n = 4). At 1, 3 or 7 days (d) after IONT, phosphorylated (p)AKT, p-MAPK, and non-phosphorylated AKT and MAPK expression levels were analyzed in the retina by Western blotting (n = 4/group). Radial sections were also immunodetected for the above-mentioned proteins, and for Brn3a and vimentin to identify RGCs and Müller cells (MCs), respectively (n = 3/group). Results: IONT induced increased levels of p-MAPK and MAPK at 3d in DHF- or vehicle-treated retinas and at 7d in DHF-treated retinas. IONT induced a fast decrease in AKT in retinas treated with DHF or vehicle, with higher levels of phosphorylation in DHF-treated retinas at 7d. In intact retinas and vehicle-treated groups, no p-MAPK or MAPK expression in RGCs was observed. In DHF- treated retinas p-MAPK and MAPK were expressed in the ganglion cell layer and in the RGC nuclei 3 and 7d after IONT. AKT was observed in intact and axotomized RGCs, but the signal intensity of p-AKT was stronger in DHF-treated retinas. Finally, MCs expressed higher quantities of both MAPK and AKT at 3d in both DHF- and vehicle-treated retinas, and at 7d the phosphorylation of p-MAPK was higher in DHF-treated groups. Conclusions: Phosphorylation and increased levels of AKT and MAPK through MCs and RGCs in retinas after DHF-treatment may be responsible for the increased and long-lasting RGC protection afforded by DHF after IONT.

Ocular Pharmacokinetics of Brimonidine Drug Delivery System in Monkeys and Translational Modeling for Selection of Dose and Frequency in Clinical Trials

Brimonidine, a selective α ₂-adrenoceptor agonist, displays putative retinal cyto- and neuroprotective activity in vitro and in vivo. An intravitreal sustained-release brimonidine implant, Brimonidine Posterior Segment Drug Delivery System (brimonidine DDS), allowing targeted drug delivery to the retina has been developed for potential clinical application. This study evaluates the in vivo posterior segment pharmacokinetics of brimonidine DDS implant in the monkey eye and applies translational pharmacokinetic modeling to predict tissue exposure in the human eye. Anesthetized cynomolgus monkeys received a single intravitreal injection of brimonidine DDS 400 µg implant before removal of study eyes at days 7, 30, 60, 92, 120, and 150 postimplant (three to four animals per time point) for assay of brimonidine in aqueous humor, vitreous, and retina samples. Brimonidine concentrations in the human eye were modeled using a linear, three-compartment model assuming bidirectional distribution to/from the aqueous humor and retina and elimination from the aqueous humor. Monkey tissue volumes were scaled up to human values; intercompartmental and elimination rate constants were assumed to be identical in the two species. Modeling and simulations were performed using NONMEM v. 7.3, R 3.5.1. Brimonidine exposure was highest in the monkey vitreous and retina; concentrations in the central (macula) and peripheral retina were maintained at high levels (>100 ng/g) for 3 to 4 months. Simulated brimonidine concentration-time profiles in human macula indicated that brimonidine DDS 400 µg implant would deliver effective drug concentrations (20.7‒82.2 ng/g, based on animal pharmacology) for approximately 3 months. Accordingly, administration of the 400 µg implant at 3-month intervals is recommended. SIGNIFICANCE STATEMENT: Brimonidine, an α ₂-adrenoceptor agonist, is cyto- and neuroprotective in animal models of retinal/optic nerve injury. Brimonidine Posterior Segment Drug Delivery System (brimonidine DDS) is an intravitreal sustained-release implant with potential ophthalmological applications. This study explores the pharmacokinetics of brimonidine DDS 400 µg implant in the monkey eye and uses compartmental modeling to predict human ocular tissue exposure. Targeted retinal brimonidine delivery from vitreous was demonstrated in monkeys. Simulated tissue concentration-time profiles indicated persistence of pharmacologically effective brimonidine concentrations for ≈3 months in human retina.

Inhibition of the HIF-1α/BNIP3 pathway has a retinal neuroprotective effect

Retinal ischemia is a leading cause of irreversible blindness worldwide. Inner retinal dysfunction including loss of retinal ganglion cells is encountered in a number of retinal ischemic disorders. We previously reported administration of two different hypoxia-inducible factor (HIF) inhibitors exerted neuroprotective effects in a murine model of retinal ischemia/reperfusion (I/R) which mimics these disorders, as inner retinal degeneration could be involved in pathological HIF induction. However, this notion needs further investigation. Therefore, in this study, we attempted to use retina-specific Hif-1α conditional knockout (cKO) mice to uncover this notion more clearly under the same condition. Hif-1α cKO mice showed inner retinal neurodegeneration to a lesser extent than control mice. Hif-1α depletion in a murine 661W retinal cell line reduced cell death under pseudohypoxic and hypoxic conditions. Among hypoxia-related genes, the expression of BCL2 19 kDa protein-interacting protein 3 (Bnip3) was substantially upregulated in the inner retinal layer after retinal I/R. In this regard, we further examined Bnip3 depletion in retinal neurons in vitro and in vivo and found the similar neuroprotective effects. Our results support the notion that the HIF-1α/BNIP3 pathway may have a critical role in inner retinal neurodegeneration, which can be linked with the development of new promising therapeutics for inner retinal ischemic disorders.

The Role of Adrenoceptors in the Retina

The retina is a part of the central nervous system, a thin multilayer with neuronal lamination, responsible for detecting, preprocessing, and sending visual information to the brain. Many retinal diseases are characterized by hemodynamic perturbations and neurodegeneration leading to vision loss and reduced quality of life. Since catecholamines and respective bindings sites have been characterized in the retina, we systematically reviewed the literature with regard to retinal expression, distribution and function of alpha₁ (α₁)-, alpha₂ (α₂)-, and beta (β)-adrenoceptors (ARs). Moreover, we discuss the role of the individual adrenoceptors as targets for the treatment of retinal diseases.

HIF inhibitor topotecan has a neuroprotective effect in a murine retinal ischemia-reperfusion model

Purpose

The therapeutic approach for retinal ganglion cell (RGC) degeneration has not been fully established. Recently, it has been reported that hypoxia-inducible factor (HIF) may be involved with retinal neurodegeneration. In this study, we investigated neuroprotective effects of a HIF inhibitor against RGC degeneration induced in a murine model of retinal ischemia-reperfusion (I/R).

Methods

Eight-weeks-old male C57/BL6J mice were treated with intraperitoneal injection of a HIF inhibitor topotecan (1.25 mg/kg) for 14 days followed by a retinal I/R procedure. Seven days after the I/R injury, the therapeutic effect was evaluated histologically and electrophysiologically.

Results

The increase of HIF-1α expression and the decrease of retinal thickness and RGC number in I/R were significantly suppressed by administration of topotecan. Impaired visual function in I/R was improved by topotecan evaluated with electroretinogram and visual evoked potentials.

Conclusions

Topotecan administration suppressed HIF-1a expression and improved RGC survival resulting in a functional protection against retinal I/R. These data indicated that the HIF inhibitor topotecan may have therapeutic potentials for RGC degeneration induced with retinal ischemia or high intraocular pressure.

Brimonidine's Neuroprotective Effects against Transient Ischaemia-Induced Retinal Ganglion Cell Death

Purpose

Brimonidine is a lowering pressure agent currently used in glaucoma. This chronic degenerative condition is characterised by neuronal death, and an agent which offers neuroprotection may slow down or impede the progression of neuronal cell death.

Methods

The effects of brimonidine (BMD) on the short- and long-term survival of retinal ganglion cells (RGCs) after transient retinal ischaemia are reported here using a rat model. The fluorescent tracer Fluorogold (FG) was applied to both superior colliculi to retrogradely label RGCs. A ninety-minute period of ischaemia was induced and densities of surviving RGCs were estimated over time by counting FG-labelled RGCs in 12 standard regions of each retina.

Results

Seven days after inducing transient ischaemia, there was loss of approximately half of the RGC population. Topical pre-treatment with 0.1% or 0.5% BMD prevented ischaemia-induced RGC death.

Conclusions

These results indicate that optimal neuroprotective effects against the early loss of RGCs are seen with 0.1% or 0.5% BMD. Ischaemia-induced RGC loss continued between day 7 and day 21 in the vehicle treated groups and amounted to approximately 25% of the RGC population. Topical pre-treatment with 0.1% or 0.5% BMD was also effective in reducing the slow loss of RGCs.

Retinal neurodegeneration in experimental glaucoma

In rats and mice, limbar tissues of the left eye were laser-photocoagulated (LP) and ocular hypertension (OHT) effects were investigated 1 week to 6 months later. To investigate the innermost layers, retinas were examined in wholemounts using tracing from the superior colliculi to identify retinal ganglion cells (RGCs) with intact retrograde axonal transport, melanopsin immunodetection to identify intrinsically photosensitive RGCs (m(+)RGC), Brn3a immunodetection to identify most RGCs but not m(+)RGCs, RECA1 immunodetection to examine the inner retinal vessels, and DAPI staining to detect all nuclei in the GC layer. The outer retinal layers (ORLs) were examined in cross sections analyzed morphometrically or in wholemounts to study S- and L-cones. Innervation of the superior colliculi was examined 10 days to 14 weeks after LP with orthogradely transported cholera toxin subunit B. By 2 weeks, OHT resulted in pie-shaped sectors devoid of FG(+)RGCs or Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. Brn3a(+)RGCs were significantly greater than FG(+)RGCs, indicating the survival of large numbers of RGCs with their axonal transport impaired. The inner retinal vasculature showed no abnormalities that could account for the sectorial loss of RGCs. m(+)RGCs decreased to approximately 50-51% in a diffuse loss across the retina. Cross sections showed focal areas of degeneration in the ORLs. RGC loss at 1m diminished to 20-25% and did not progress further with time, whereas the S- and L-cone populations diminished progressively up to 6m. The retinotectal projection was reduced by 10 days and did not progress further. LP-induced OHT results in retrograde degeneration of RGCs and m(+)RGCs, severe damage to the ORL, and loss of retinotectal terminals.

Role of C/EBP homologous protein in retinal ganglion cell death after ischemia/reperfusion injury

PURPOSE

To investigate the role of C/EBP homologous protein (CHOP), a proapoptotic protein, and the unfolded protein response (UPR) marker that is involved in endoplasmic reticulum (ER) stress-mediated apoptosis in mouse retinal ganglion cell (RGC) death following ischemia/reperfusion (I/R) injury.

METHODS

Retinal I/R injury was induced in adult C57BL/6J wild-type (WT) and CHOP knockout (Chop(-/-)) mice by raising IOP to 120 mm Hg for 60 minutes. Expression of CHOP and other UPR markers was studied by Western blot and immunohistochemistry. Retinal ganglion cell counts were performed in retinal flat mounts stained with an RGC marker. Retinal ganglion cell function was evaluated by scotopic threshold response (STR) electroretinography.

RESULTS

In WT mice, retinal CHOP was upregulated by 30% in I/R-injured eyes compared to uninjured eyes 3 days after injury (P < 0.05). Immunohistochemistry confirmed CHOP upregulation specifically in RGCs. CHOP knockout did not affect baseline RGC density or STR amplitude. Ischemia/reperfusion injury decreased RGC densities and STR amplitudes in both WT and Chop(-/-) mice. However, survival of RGCs in I/R-injured Chop(-/-) mouse was 48% higher (P < 0.05) than that in I/R-injured WT mouse 3 days after I/R injury. Similarly, RGC density was significantly higher in Chop(-/-) eyes at 7, 14, and 28 days after I/R injury. Scotopic threshold response amplitudes of Chop(-/-) mice were significantly higher at 3 and 7 days after I/R than those of WT mice.

CONCLUSIONS

Absence of CHOP partially protects against RGC loss and reduction in retinal function after I/R injury, indicating that CHOP and, thus, ER stress play an important role in RGC apoptosis in retinal I/R injury.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Factors influencing the retrograde labeling of retinal ganglion cells with fluorogold in an animal optic nerve crush model

PURPOSE

To investigate whether different crush durations or a different fluorogold (FG) injection timing can affect the efficiency of FG retrograde labeling of retinal ganglion cells (RGCs) in the optic nerve (ON) crush model.

METHODS

We performed the ON crush in rats with a clip at different durations or a jewel forceps to compare the effects of different crush methods with FG staining. RGC density was compared between the FG injection 1 week before the sacrifice of the animals (group A) and the injection before the crush experiment (group B). Double staining with CD11b and FG in the retinal sections was conducted to investigate the relationship between the overcounting of RGCs and microglia.

RESULTS

The FG-stained particles were significantly decreased at the distal part of the crush site compared to the proximal site of the ON with a crush duration of over 30 s or when crushed with the jewel forceps. Two weeks after ON crush, the RGC count was higher both in the central and mid-peripheral retinas in group B. The percentage of CD11b-stained cells among the FG-stained cells in the RGC layer of retinas in group B was higher than that of group A (34% in group B vs. 4% in group A, p = 0.0001). Overcounting of RGC density in group B was due to additional microglia with FG engulfing.

CONCLUSIONS

Our results suggest that each laboratory should test its setting conditions to avoid factors influencing the RGC density measurement before conducting ON crush experiments.

The protective effects of dexmedetomidine against apoptosis in retinal ischemia/reperfusion injury in rats

OBJECTIVE

Dexmedetomidine is an alpha 2 adrenoceptor agonist and can be used for postoperative sedation, analgesia and anesthesia-sparing properties. Furthermore, the neuroprotective effects against ischemia/reperfusion (I/R) injury in the central nervous system have been shown in experimental studies. This study aimed to investigate the protective effects of dexmedetomidine against apoptosis in retinal I/R injury in the rat.

MATERIALS AND METHODS

Retinal I/R injury was induced by transient elevation of intraocular pressure. Eighteen animals were divided into three groups (n = 6): sham, I/R and treatment. The I/R injury and protective effects of the dexmedetomidine were evaluated by retinal thickness determined by histological sections, terminal deoxynucleotidyl transferase-mediated biotin-deoxyuridine triphosphate nick-end labeling (TUNEL) and immunohistochemistry of caspases 3.

RESULTS

A decrease in the retinal thickness and an increase in the apoptotic cells were found to be statistically significant in I/R and treatment groups when compared with the control group. However, in comparison with the I/R group we realized that the administration of dexmedetomidine reduced the thinning of retinal thickness and also decreased the number of caspases 3 and TUNEL-positive cells.

CONCLUSION

Dexmedetomidine is protective against apoptosis in retinal I/R injury in rats.

Trophic factors in the pathogenesis and therapy for retinal degenerative diseases

Trophic factors are endogenously secreted proteins that act in an autocrine and/or paracrine fashion to affect vital cellular processes such as proliferation, differentiation, and regeneration, thereby maintaining overall cell homeostasis. In the eye, the major contributors of these molecules are the retinal pigment epithelial (RPE) and Müller cells. The primary paracrine targets of these secreted proteins include the photoreceptors and choriocapillaris. Retinal degenerative diseases such as age-related macular degeneration and retinitis pigmentosa are characterized by aberrant function and/or eventual death of RPE cells, photoreceptors, choriocapillaris, and other retinal cells. We discuss results of in vitro and in vivo animal studies in which candidate trophic factors, either singly or in combination, were used in an attempt to ameliorate photoreceptor and/or retinal degeneration. We also examine current trophic factor therapies as they relate to the treatment of retinal degenerative diseases in clinical studies.

Changes in retinal morphology, electroretinogram and visual behavior after transient global ischemia in adult rats

The retina is a light-sensitive tissue of the central nervous system that is vulnerable to ischemia. The pathological mechanism underlying retinal ischemic injury is not fully understood. The purpose of this study was to investigate structural and functional changes of different types of rat retinal neurons and visual behavior following transient global ischemia. Retinal ischemia was induced using a 4-vessel occlusion model. Compared with the normal group, the number of βIII-tubulin positive retinal ganglion cells and calretinin positive amacrine cells were reduced from 6 h to 48 h following ischemia. The number of recoverin positive cone bipolar cells transiently decreased at 6 h and 12 h after ischemia. However, the fluorescence intensity of rhodopsin positive rod cells and fluorescent peanut agglutinin positive cone cells did not change after reperfusion. An electroretinogram recording showed that the a-wave, b-wave, oscillatory potentials and the photopic negative response were completely lost during ischemia. The amplitudes of the a- and b-waves were partially recovered at 1 h after ischemia, and returned to the control level at 48 h after reperfusion. However, the amplitudes of oscillatory potentials and the photopic negative response were still reduced at 48 h following reperfusion. Visual behavior detection showed there was no significant change in the time spent in the dark chamber between the control and 48 h group, but the distance moved, mean velocity in the black and white chambers and intercompartmental crosses were reduced at 48 h after ischemia. These results indicate that transient global ischemia induces dysfunction of retinal ganglion cells and amacrine cells at molecular and ERG levels. However, transient global ischemia in a 17 minute duration does not appear to affect photoreceptors.

Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Glaucoma is a common optic neuropathy with a complex etiology often linked to sensitivity to intraocular pressure. Though the precise mechanisms that mediate or transduce this sensitivity are not clear, the axon of the retinal ganglion cell appears to be vulnerable to disease-relevant stressors early in progression. One reason may be because the axon is generally thin for both its unmyelinated and myelinated segment and much longer than the thicker unmyelinated axons of other excitatory retinal neurons. This difference may predispose the axon to metabolic and oxidative injury, especially at distal sites where pre-synaptic terminals form connections in the brain. This idea is consistent with observations of early loss of anterograde transport at central targets and other signs of distal axonopathy that accompany physiological indicators of progression. Outright degeneration of the optic projection ensues after a critical period and, at least in animal models, is highly sensitive to cumulative exposure to elevated pressure in the eye. Stress emanating from the optic nerve head can induce not only distal axonopathy with aspects of dying back neuropathy, but also Wallerian degeneration of the optic nerve and tract and a proximal program involving synaptic and dendritic pruning in the retina. Balance between progressive and acute mechanisms likely varies with the level of stress placed on the unmyelinated axon as it traverses the nerve head, with more acute insult pushing the system toward quicker disassembly. A constellation of signaling factors likely contribute to the transduction of stress to the axon, so that degenerative events along the length of the optic projection progress in retinotopic fashion. This pattern leads to well-defined sectors of functional depletion, even at distal-most sites in the pathway. While ganglion cell somatic drop-out is later in progression, some evidence suggests that synaptic and dendritic pruning in the retina may be a more dynamic process. Structural persistence both in the retina and in central projection sites offers the possibility that intrinsic self-repair pathways counter pathogenic mechanisms to delay as long as possible outright loss of tissue.

Evaluation of extended release brimonidine intravitreal device in normotensive rabbit eyes

PURPOSE

To evaluate the safety profile of a brimonidine extended release intravitreal implant, in normotensive rabbit eyes.

METHODS

Devices were made from hollow poly-l-lactic acid (PLA) tubes and contained hundred micrograms of brimonidine pamoate. Device was injected intravitreally in one eye of 12 New Zealand pigmented rabbits, whereas other eye was injected with a sham implant in masked fashion. Ocular examination was conducted at baseline and months 1, 3 and 6 including dilated fundus examination and electro-retinogram (ERG). Four rabbits were sacrificed at each time-point for retinal histology. ERG data were compared between groups and time-points using anova.

RESULTS

No complications were reported from either eye of any rabbits over a 6-month period. Photopic A wave was reduced in the control eye at 1 month compared with baseline (p < 0.01). There was no significant difference in other ERG parameters between the groups at different time-points. Gross retinal histology was normal at all time-points.

CONCLUSION

Extended release intravitreal brimonidine device was found to be safe and in normotensive rabbit eyes.

Axotomy-induced retinal ganglion cell death in adult mice: quantitative and topographic time course analyses

The fate of retinal ganglion cells after optic nerve injury has been thoroughly described in rat, but not in mice, despite the fact that this species is amply used as a model to study different experimental paradigms that affect retinal ganglion cell population. Here we have analyzed, quantitatively and topographically, the course of mice retinal ganglion cells loss induced by intraorbital nerve transection. To do this, we have doubly identified retinal ganglion cells in all retinas by tracing them from their main retinorecipient area, the superior colliculi, and by their expression of BRN3A (product of Pou4f1 gene). In rat, this transcription factor is expressed by a majority of retinal ganglion cells; however in mice it is not known how many out of the whole population of these neurons express it. Thus, in this work we have assessed, as well, the total population of BRN3A positive retinal ganglion cells. These were automatically quantified in all whole-mounted retinas using a newly developed routine. In control retinas, traced-retinal ganglion cells were automatically quantified, using the previously reported method (Salinas-Navarro et al., 2009b). After optic nerve injury, though, traced-retinal ganglion cells had to be manually quantified by retinal sampling and their total population was afterwards inferred. In naïve whole-mounts, the mean (±standard deviation) total number of traced-retinal ganglion cells was 40,437(±3196) and of BRN3A positive ones was 34,697(±1821). Retinal ganglion cell loss was first significant for both markers 5 days post-axotomy and by day 21, the last time point analyzed, only 15% or 12% of traced or BRN3A positive retinal ganglion cells respectively, survived. Isodensity maps showed that, in control retinas, BRN3A and traced-retinal ganglion cells were distributed similarly, being densest in the dorsal retina along the naso-temporal axis. After axotomy the progressive loss of BRN3A positive retinal ganglion cells was diffuse and affected the entire retina. In conclusion, this is the first study assessing the values, in terms of total number and density, of the retinal ganglion cells surviving axotomy from 2 till 21 days post-lesion. Besides, we have demonstrated that BRN3A is expressed by 85.6% of the total retinal ganglion cell population, and because BRN3A positive retinal ganglion cells show the same spatial distribution and temporal course of degeneration than traced ones, BRN3A is a reliable marker to identify, quantify and assess, ex-vivo, retinal ganglion cell loss in this species.

Brain derived neurotrophic factor maintains Brn3a expression in axotomized rat retinal ganglion cells

The transcription factor Brn3a has been reported to be a good marker for adult rat retinal ganglion cells in control and injured retinas. However, it is still unclear if Brn3a expression declines progressively by the injury itself or otherwise its expression is maintained in retinal ganglion cells that, though being injured, are still alive, as might occur when assessing neuroprotective therapies. Therefore, we have automatically quantified the whole population of surviving Brn3a positive retinal ganglion cells in retinas subjected to intraorbital optic nerve transection and treated with either brain derived neurotrophic factor or vehicle. Brain derived neurotrophic factor is known to delay retinal ganglion cell death after axotomy. Thus, comparison of both groups would inform of the suitability of Brn3a as a retinal ganglion cell marker when testing neuroprotective molecules. As internal control, retinal ganglion cells were, as well, identified in all retinas by retrogradely tracing them with fluorogold. Our data show that at all the analyzed times post-lesion, the numbers of Brn3a positive retinal ganglion cells and of fluorogold positive retinal ganglion cells are significantly higher in the brain derived neurotrophic factor-treated retinas compared to the vehicle-treated ones. Moreover, detailed isodensity maps of the surviving Brn3a positive retinal ganglion cells show that a single injection of brain derived neurotrophic factor protects retinal ganglion cells throughout the entire retina. In conclusion, Brn3a is a reliable retinal ganglion cell marker that can be used to accurately measure the potential effect of a given neuroprotective therapy.

Brimonidine prevents axonal and somatic degeneration of retinal ganglion cell neurons

BACKGROUND

Brimonidine is a common drug for lowering ocular pressure and may directly protect retinal ganglion cells in glaucoma. The disease involves early loss of retinal ganglion cell transport to brain targets followed by axonal and somatic degeneration. We examined whether brimonidine preserves ganglion cell axonal transport and abates degeneration in rats with elevated ocular pressure induced by laser cauterization of the episcleral veins.

RESULTS

Ocular pressure was elevated unilaterally by 90% for a period of 8 weeks post- cauterization. During this time, brimonidine (1mg/kg/day) or vehicle (phosphate-buffered saline) was delivered systemically and continuously via subcutaneous pump. Animals received bilateral intravitreal injections of fluorescent cholera toxin subunit β (CTB) two days before sacrifice to assess anterograde transport. In retinas from the vehicle group, elevated pressure induced a 44% decrease in the fraction of ganglion cells with intact uptake of CTB and a 14-42% reduction in the number of immuno-labelled ganglion cell bodies, with the worst loss occurring nasally. Elevated pressure also caused a 33% loss of ganglion cell axons in vehicle optic nerves and a 70% decrease in CTB transport to the superior colliculus. Each of these components of ganglion cell degeneration was either prevented or significantly reduced in the brimonidine treatment group.

CONCLUSIONS

Continuous and systemic treatment with brimonidine by subcutaneous injection significantly improved retinal ganglion cell survival with exposure to elevated ocular pressure. This effect was most striking in the nasal region of the retina. Brimonidine treatment also preserved ganglion cell axon morphology, sampling density and total number in the optic nerve with elevated pressure. Consistent with improved outcome in the optic projection, brimonidine also significantly reduced the deficits in axonal transport to the superior colliculus associated with elevated ocular pressure. As transport deficits to and from retinal ganglion cell projection targets in the brain are relevant to the progression of glaucoma, the ability of brimonidine to preserve optic nerve axons and active transport suggests its neuroprotective effects are relevant not only at the cell body, but throughout the entire optic projection.

Animal models of retinal disease

Diseases of the retina are the leading causes of blindness in the industrialized world. The recognition that animals develop retinal diseases with similar traits to humans has led to not only a dramatic improvement in our understanding of the pathogenesis of retinal disease but also provided a means for testing possible treatment regimes and successful gene therapy trials. With the advent of genetic and molecular biological tools, the association between specific gene mutations and retinal signs has been made. Animals carrying natural mutations usually in one gene now provide well-established models for a host of inherited retinal diseases, including retinitis pigmentosa, Leber congenital amaurosis, inherited macular degeneration, and optic nerve diseases. In addition, the development of transgenic technologies has provided a means by which to study the effects of these and novel induced mutations on retinal structure and function. Despite these advances, there is a paucity of suitable animal models for complex diseases, including age-related macular degeneration (AMD) and diabetic retinopathy, largely because these diseases are not caused by single gene defects, but involve complex genetics and/or exacerbation through environmental factors, epigenetic, or other modes of genetic influence. In this review, we outline in detail the available animal models for inherited retinal diseases and how this information has furthered our understanding of retinal diseases. We also examine how transgenic technologies have helped to develop our understanding of the role of isolated genes or pathways in complex diseases like AMD, diabetes, and glaucoma.

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.

Staining of fluorogold-prelabeled retinal ganglion cells with calcein-AM: A new method for assessing cell vitality

PURPOSE

The number of retinal ganglion cells (RGC) is often used as an outcome measure in neuroprotection. The gold standard for staining RGC is retrograde labeling, e.g. with fluorogold (FG). However, this method alone does not permit to differentiate between viable and dead cells, because dying cells only avoid being counted once they have undergone complete microglial-phagocytosis. To differentiate between viable and dead but still existent RGC, we additionally stained FG-labeled RGC with calcein-acetoxymethylester (CAM).

METHODS

The left optic nerves of rats were crushed 6 days after stereotactical injection of FG into both superior colliculi. The right eyes served as controls. Retinal whole mounts were prepared 2, 5, 8 or 11 days after optic nerve crush (ONC), and incubated for 30min in culture media containing 0.01% CAM. RGC densities were determined in defined areas at different eccentricities under a fluorescence microscope using the appropriate filters. Twice-positive RGC were counted after merging both filters.

RESULTS

The loss of RGC induced by ONC is identified earlier when these cells are detected by FG+CAM rather than by FG-labeling alone. The percentages of FG-positive RGC stained with CAM were 83% in controls, 68% on day 2, 48% on day 5, 26% on day 8, and 9% on day 11 after ONC. The decay rate of FG-prelabeled RGC appears accelerated and becomes more linear when only viable RGC positive for CAM are counted.

CONCLUSIONS

The staining of FG-prelabeled RGC with CAM permits the discrimination between dead and viable RGC in retinal whole mounts, which enables to quantify RGC degeneration earlier after injury than by using microglial-phagocytosis-dependant retrograde labeling alone.

Changes in the inner and outer retinal layers after acute increase of the intraocular pressure in adult albino Swiss mice

In adult albino mice the effects of increased intraocular pressure on the outer retina and its circuitry was investigated at intervals ranging 3-14 weeks. Ocular hypertension (OHT) was induced by cauterizing the vessels draining the anterior part of the mice eye, as recently reported (Salinas-Navarro et al., 2009a). Electroretinographic (ERG) responses were recorded simultaneously from both eyes and compared each other prior to and at different survival intervals of 2, 8 or 12 weeks after lasering. Animals were processed at 3, 9 or 14 weeks after lasering, and radial sections were obtained in the cryostat and further processed for immunocytochemistry using antibodies against recoverin, gamma-transducin, Protein Kinase C-alpha (PKC-alpha), calbindin or synaptophysin. The synaptic ribbons were identified using an antibody against the protein bassoon, which labels photoreceptor ribbons and nuclei were identified using TO-PRO. Laser photocoagulation of the perilimbar and episcleral veins of the left eye resulted in an increase in mean intraocular pressure to approximately over twice its baseline by 24 h that was maintained for approximately five days reaching basal levels by 1 week. ERG recordings from the different groups of mice showed their a-, b-wave and scotopic threshold response (STR) amplitudes, when compared to their contralateral fellow eye, reduced to 62%, 52% and 23% at 12 weeks after lasering. Three weeks after lasering, immunostaining with recoverin and transducin antibodies could not document any changes in the outer nuclear layer (ONL) but both ON-rod bipolar and horizontal cells had lost their dendritic processes in the outer plexiform layer (OPL). Sprouting of horizontal and bipolar cell processes were observed into the ONL. Fourteen weeks after lasering, protein kinase-C antibodies showed morphologic changes of ON-rod bipolar cells and calbindin staining showed abnormal horizontal cells and a loss of their relationship with their presynaptic input. Moreover, at this time, quantitative studies indicate significant diminutions in the number of photoreceptor synaptic ribbons/100 microm, and in the thickness of the outer nuclear and plexiform layer, when compared to their fellow eyes. Increased intraocular pressure in Swiss mice results in permanent alterations of their full field ERG responses and in changes of the inner and outer retinal circuitries.

Treatment of nonarteritic anterior ischemic optic neuropathy

Nonarteritic anterior ischemic optic neuropathy (NAION) is the most common clinical presentation of acute ischemic damage to the optic nerve. Most treatments proposed for NAION are empirical and include a wide range of agents presumed to act on thrombosis, on the blood vessels, or on the disk edema itself. Others are presumed to have a neuroprotective effect. Although there have been multiple therapies attempted, most have not been adequately studied, and animal models of NAION have only recently emerged. The Ischemic Optic Neuropathy Decompression Trial, the only class I large multicenter prospective treatment trial for nonarteritic anterior ischemic optic neuropathy, found no benefit from surgical intervention. One recent large, nonrandomized controlled study suggested that oral steroids might be helpful for acute NAION. Others recently proposed interventions are intravitreal injections of steroids or anti-vascular endothelial growth factor (anti-VEGF) agents. There are no class I studies showing benefit from either medical or surgical treatments. Most of the literature on the treatment of NAION consists of retrospective or prospective case series and anecdotal case reports. Similarly, therapies aimed at secondary prevention of fellow eye involvement in NAION remain of unproven benefit.

Ocular hypertension impairs optic nerve axonal transport leading to progressive retinal ganglion cell degeneration

Ocular hypertension (OHT) is the main risk factor of glaucoma, a neuropathy leading to blindness. Here we have investigated the effects of laser photocoagulation (LP)-induced OHT, on the survival and retrograde axonal transport (RAT) of adult rat retinal ganglion cells (RGC) from 1 to 12 wks. Active RAT was examined with fluorogold (FG) applied to both superior colliculi (SCi) 1 wk before processing and passive axonal diffusion with dextran tetramethylrhodamine (DTMR) applied to the optic nerve (ON) 2 d prior to sacrifice. Surviving RGCs were identified with FG applied 1 wk pre-LP or by Brn3a immunodetection. The ON and retinal nerve fiber layer were examined by RT97-neurofibrillar staining. RGCs were counted automatically and color-coded density maps were generated. OHT retinas showed absence of FG+ or DTMR+RGCs in focal, pie-shaped and diffuse regions of the retina which, by two weeks, amounted to, approximately, an 80% of RGC loss without further increase. At this time, there was a discrepancy between the total number of surviving FG-prelabelled RGCs and of DMTR+RGCs, suggesting that a large proportion of RGCs had their RAT impaired. This was further confirmed identifying surviving RGCs by their Brn3a expression. From 3 weeks onwards, there was a close correspondence of DTMR+RGCs and FG+RGCs in the same retinal regions, suggesting axonal constriction at the ON head. Neurofibrillar staining revealed, in ONs, focal degeneration of axonal bundles and, in the retinal areas lacking backlabeled RGCs, aberrant staining of RT97 characteristic of axotomy. LP-induced OHT results in a crush-like injury to ON axons leading to the anterograde and protracted retrograde degeneration of the intraocular axons and RGCs.

Intraocular pressure lowering, change of antiapoptotic molecule expression, and neuroretinal changes by dorzolamide 2%/timolol 0.5% combination in a chronic ocular hypertension rat model

The aim of this study was to examine intraocular pressure lowering, change of antiapoptotic molecules expression, and neuroretinal changes by a commercially available dorzolamide 2%/timolol 0.5% combination in a chronic ocular hypertension rat model. Chronic ocular hypertension was induced by three episcleral vein cauterizations. The expression of antiapoptotic molecules and the effect of dorzolamide 2%/timolol 0.5% combination in chronic ocular hypertensive retina were evaluated. Retinal ganglion cell (RGC) retrograde labeling and quantification with 4-di-10-ASP (DiA) and expression of glial fibrillary acidic protein (GFAP) were detected before and after the administration of dorzolamide 2%/timolol 0.5%. Treatment of ocular hypertensive eyes with dorzolamide 2%/timolol 0.5% significantly reduced, intraocular pressure when compared to the control eyes. Labeling of RGCs with DiA showed a significant decrease in RGC loss after the administration of dorzolamide 2%/timolol 0.5%. GFAP expression revealed a significant decrease in retinal damage after dorzolamide 2%/timolol 0.5% administration. However, dorzolamide 2%/timolol 0.5% did not affect Bcl-2 and Bcl-xL mRNA expression. In conclusion, dorzolamide 2%/timolol 0.5% may have neuroprotective potential in the animal model, which is not mediated by Bcl-2 or Bcl-xL. The mechanism of neuroprotection by dorzolamide 2%/timolol 0.5% in chronic glaucoma models requires further investigation.

Retinal ganglion cell population in adult albino and pigmented mice: a computerized analysis of the entire population and its spatial distribution

In adult Swiss albino and C57 pigmented mice, RGCs were identified with a retrogradely transported neuronal tracer applied to both optic nerves (ON) or superior colliculi (SCi). After histological processing, the retinas were prepared as whole-mounts, examined and photographed under a fluorescence microscope equipped with a motorized stage controlled by a commercial computer image analysis system: Image-Pro Plus((R)) (IPP). Retinas were imaged as a stack of 24-bit color images (140 frames per retina) using IPP with the Scope-Pro plug-in 5.0 and the images montaged to create a high-resolution composite of the retinal whole-mount when required. Single images were also processed by specific macros written in IPP that apply a sequence of filters and transformations in order to separate individual cells for automatic counting. Cell counts were later transferred to a spreadsheet for statistical analysis and used to generate a RGC density map for each retina.

RESULTS

The mean total numbers of RGCs labeled from the ON, in Swiss (49,493+/-3936; n=18) or C57 mice (42,658+/-1540; n=10) were slightly higher than the mean numbers of RGCs labeled from the SCi, in Swiss (48,733+/-3954; n=43) or C57 mice (41,192+/-2821; n=42), respectively. RGCs were distributed throughout the retina and density maps revealed a horizontal region in the superior retina near the optic disk with highest RGC densities. In conclusion, the population of mice RGCs may be counted automatically with a level of confidence comparable to manual counts. The distribution of RGCs adopts a form of regional specialization that resembles a horizontal visual streak.

A computerized analysis of the entire retinal ganglion cell population and its spatial distribution in adult rats

In adult albino (SD) and pigmented (PVG) rats the entire population of retinal ganglion cells (RGCs) was quantified and their spatial distribution analyzed using a computerized technique. RGCs were back-labelled from the optic nerves (ON) or the superior colliculi (SCi) with Fluorogold (FG). Numbers of RGCs labelled from the ON [SD: 82,818+/-3,949, n=27; PVG: 89,241+/-3,576, n=6) were comparable to those labelled from the SCi [SD: 81,486+/-4,340, n=37; PVG: 87,229+/-3,199; n=59]. Detailed methodology to provide cell density information at small scales demonstrated the presence of a horizontal region in the dorsal retina with highest densities, resembling a visual streak.

Localization of alpha 2 receptors in ocular tissues

Alpha 2 adrenergic agonists are used for controlling intraocular pressure (IOP) in the treatment of glaucoma. They have also been shown to be neuroprotective to retinal cells in a variety of injury models. Despite this significance, the localization of the three known alpha 2 adrenergic receptors has not been unequivocally established. The aim of this study was to determine the location of the three alpha 2 adrenergic receptors in ocular tissues using immunohistochemical techniques. New antibodies were generated and their specificity was determined using Western blotting and preadsorption. In the anterior segment of the eye alpha 2A immunoreactivity was located in the nonpigmented ciliary epithelium, corneal, and conjunctival epithelia. Alpha 2B staining was not apparent in these tissues. Alpha 2C immunostaining was present in the membrane of pigmented ciliary epithelium and corneal and conjunctival epithelial cells. In the rat retina, all three receptor subtypes were present but were differentially localized. Alpha 2A was present in the somata of ganglion cell layer and inner nuclear layer somas, alpha 2B was located in the dendrites and axons of most of the neurons as well as glia, while alpha 2C was present in the somata and inner segment of the photoreceptors. In human and monkey retinas, similar pattern of labeling for alpha 2A and 2B receptors were observed, while alpha 2B was additionally present in the membranes of many cell somata in addition to dendrites and axons. Alpha 2C labeling was much weaker but exhibited similar pattern to that observed in the rat. These data provide additional information on the location of the alpha 2 receptors in the anterior portion of the eye and present new information on their specific location in the retina. This offers insights into possible targets for adrenergic agonists in a therapeutic context.

Neuroprotective effect of topically applied brimonidine tartrate 0.2% in endothelin-1-induced optic nerve ischaemia model

BACKGROUND

To investigate the neuroprotective effects of topically applied brimonidine tartrate 0.2% (BMD), an alpha(2)-receptor agonist, on the retinal ganglion cell (RGC) layer and inner nuclear layer (INL) of rabbit retina in endothelin-1 (ET-1)-induced optic nerve (ON) ischaemia model.

METHODS

Osmotic minipumps were surgically implanted into one eye of 16 New Zealand Albino rabbits to deliver ET-1 at the constant rate of 0.5 microL/h for 2 weeks. Eyes were divided into four groups. ET-1 was given with (Group 3) and without topical BMD therapy (Group 1). Groups 2 and 4 were taken as controls. Rabbits were sacrificed at day 14. Morphological alterations, total cell number and proportion of cells undergoing apoptosis in INL and RGC layer were assessed by histopathological analysis to determine the survival of the cells of the INL and RGC layer.

RESULTS

Endothelin-1 led to severe reduction of cells in both the RGC layer and INL in Group 1 (P < 0.05). In Group 3, the total cell number and the proportion of cells undergoing apoptosis in the RGC layer were comparable with the control group (Group 4), whereas the former was found to be higher and the latter was found to be lower than those recorded for Group 1. However, the total cell number in the INL was found to be lower in Group 3 compared with that of Group 4, despite topical BMD therapy (P < 0.05).

CONCLUSIONS

Topically applied BMD seems to be neuroprotective and antiapoptotic in the ET-1-induced ON ischaemia model, especially for RGCs. BMD might be used as an adjuvant agent for its neuroprotective effects in hypoxic-ischaemic conditions such as diabetic retinopathy, normotensive glaucoma and other retinal vascular occlusive conditions which require further investigations.

Nicotinamide attenuates retinal ischemia and light insults to neurones

The aim of the present studies was to determine whether nicotinamide is effective in blunting the negative influence of ischemia/reperfusion to the rat retina in situ and of light to transformed retinal ganglion cells (RGC-5 cells) in culture. Ischemia was delivered to the retina of one eye of rats by raising the intraocular pressure. Nicotinamide was administered intraperitoneally just before ischemia and into the vitreous immediately after the insult. Electroretinograms (ERGs) of both eyes were recorded before and 5 days after ischemia. Seven days after ischemia, retinas were analysed for the localization of various antigens. Retinal and optic nerve extracts were also prepared for analysis of specific proteins and mRNAs. Also, RGC-5 cells in culture were given a light insult (1000 lux, 48 and 96 h) and evidence for reduced viability and apoptosis determined by a variety of procedures. Nicotinamide was added to some cultures to see whether it reversed the negative effect of light. Ischemia/reperfusion to the retina affected the localization of Thy-1, neuronal nitric oxide synthase (NOS) and choline acetyltransferase (ChAT), the a- and b-wave amplitudes of the ERG, the content of various retinal and optic nerve proteins and mRNAs. Significantly, nicotinamide statistically blunted many of the effects induced by ischemia/reperfusion which included the activation of poly-ADP-ribose polymerase (PARP). Light-induced apoptosis of RGC-5 cells in culture was attenuated by nicotinamide and the PARP inhibitor NU1025. The presented data show that nicotinamide attenuates injury to the retina and RGC-5 cells in culture caused by ischemia/reperfusion and by light, respectively. Evidence is provided to suggest that nicotinamide acts as a PARP inhibitor and possibly an antioxidant.

Neuroprotective effect of the novel Na+/Ca2+ channel blocker NS-7 on rat retinal ganglion cells

PURPOSE

To investigate whether NS-7, 4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride, a novel Na(+)/Ca(2+) channel blocker, can protect the rat retina subjected to ischemia-reperfusion insult.

METHODS

To evaluate the protective effect of NS-7 against retinal damage, the drug was administered before and after ischemia-reperfusion. Damage to the retina was assessed by measuring the thickness of the inner plexiform layer (IPL) and the outer nuclear layer (ONL) of each eye. In a subsequent experiment, electroretinographic (ERG) evaluation was also used.

RESULTS

In histopathologic evaluation, ischemia-reperfusion injury caused a significant reduction of IPL thickness (measured as the IPL/ONL ratio). In the NS-7-treated group, retinal damage was partially prevented by a concentration of 0.25 mg/kg per day. In the ERG evaluation, ischemia-reperfusion injury caused a reduction of A- and B-wave amplitudes. NS-7 treatment significantly prevented the reduction of the B wave at a concentration of 0.1 or 0.3 mg/kg, while the reduction of the A wave was not significantly affected.

CONCLUSIONS

NS-7 has neuroprotective effects against retinal damage resulting from subjection to ischemia. In addition, NS-7 can be used as an agent for treating acute ischemic retinopathy, including diseases associated with very high intraocular pressure, such as acute angle-closure glaucoma.

Human ganglion cells express the alpha-2 adrenergic receptor: relevance to neuroprotection

BACKGROUND/AIM

Alpha-2alpha adrenergic receptor (alpha(2)-AR) agonists are thought to be neuroprotective, preventing retinal ganglion cell death independent of pressure reduction. Previous studies have identified alpha(2)-ARs in rat retina. The authors aimed to demonstrate the presence and localisation of alpha(2)-ARs in human and rat retina and on the rat retinal ganglion cell line, RGC-5.

METHODS

Seven postmortem human and three postmortem rat eyes were paraformaldehyde fixed and frozen. RGC-5 cells were also paraformaldehyde fixed. The expression of alpha(2A)-ARs was determined by antibody immunofluorescence.

RESULTS

alpha(2A)-AR expression was identified in the human retina, on ganglion cells, and cells in the inner and outer nuclear layers (INL, ONL). Differential alpha(2A)-AR staining patterns in the INL and ONL suggest a further restriction to as yet unidentified neuronal subclasses. The RGC-5 cell line also expressed alpha(2A)-ARs in undifferentiated cells and an increased expression upon fully differentiated cells.

CONCLUSION

alpha(2)-AR agonists in addition to their pressure lowering effects in the eye, may act directly upon retinal neurons, including retinal ganglion cells. The presence of alpha(2)-ARs on the RGC-5 cell line allows future investigation of these possible direct effects using in vitro glaucoma model systems.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Treatment of ischemic optic neuropathy

Ischemic damage of the optic nerve has no proven effective treatment. While ischemia related to vasculitis (arteritic) is treated with systemic corticosteroids, the primary goal is to prevent further damage, either in the affected or fellow eye. Thrombolytic or anticoagulation supplementive therapy may be considerations for the future. In the more common idiopathic (nonarteritic) form (NAION), multiple attempts at therapy, including systemic corticosteroids, anticoagulants and antiplatelet agents, diphenylhydantoin, hyperbaric oxygen, and optic nerve sheath decompression have been unsuccessful. The use of levodopa has been proposed but is unproven. Megadose intravenous corticosteroid therapy has not been studied in a systematic way. Neuroprotective strategies are under intense investigation for optic neuropathies including NAION, and clinical trials in humans are in progress. Optic nerve regeneration studies are ongoing in animals. Prophylaxis in NAION is unproven.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.