Vascular and glial changes in the retrolaminar optic nerve in glaucomatous monkey eyes

The role of aquaporin-4 in optic nerve head astrocytes in experimental glaucoma

PURPOSE

To study aquaporin channel expression in astrocytes of the mouse optic nerve (ON) and the response to IOP elevation in mice lacking aquaporin 4 (AQP4 null).

METHODS

C57BL/6 (B6) and AQP4 null mice were exposed to bead-induced IOP elevation for 3 days (3D-IOP), 1 and 6 weeks. Mouse ocular tissue sections were immunolabeled against aquaporins 1(AQP1), 4(AQP4), and 9(AQP9). Ocular tissue was imaged to identify normal AQP distribution, ON changes, and axon loss after IOP elevation. Ultrastructure examination, cell proliferation, gene expression, and transport block were also analyzed.

RESULTS

B6 mice had abundant AQP4 expression in Müller cells, astrocytes of retina and myelinated ON (MON), but minimal AQP4in prelaminar and unmyelinated ON (UON). MON of AQP4 nulls had smaller ON area, smaller axon diameter, higher axon density, and larger proportionate axon area than B6 (all p≤0.05). Bead-injection led to comparable 3D-IOP elevation (p = 0.42) and axonal transport blockade in both strains. In B6, AQP4 distribution was unchanged after 3D-IOP. At baseline, AQP1 and AQP9 were present in retina, but not in UON and this was unaffected after IOP elevation in both strains. In 3D-IOP mice, ON astrocytes and microglia proliferated, more in B6 than AQP4 null. After 6 week IOP elevation, axon loss occurred equally in the two mouse types (24.6%, AQP4 null vs. 23.3%, B6).

CONCLUSION

Lack of AQP4 was neither protective nor detrimental to the effects of IOP elevation. The minimal presence of AQP4 in UON may be a vital aspect of the regionally specific phenotype of astrocytes in the mouse optic nerve head.

Biomechanics of the optic nerve head and peripapillary sclera in a mouse model of glaucoma

The deformation of the mouse astrocytic lamina (AL) and adjacent peripapillary sclera (PPS) was measured in response to elevated intraocular pressure. We subjected explanted mouse eyes to inflation testing, comparing control eyes to those 3 days and 6 weeks after induction of ocular hypertension (OHT) via ocular microbead injection. Laser scanning microscopy was used with second harmonic generation to image the collagenous PPS and two-photon fluorescence to image transgenic fluorescent astrocytes in the AL. Digital volume correlation was applied to calculate strains in the PPS and AL. The specimen-averaged strains were biaxial in the AL and PPS, with greater strain overall in the x- than y-direction in the AL and greater strain in the θ- than the r-direction in the PPS. Strains increased after 3-day OHT, with greater strain overall in the 3-day AL than control AL, and greater circumferential strain in the 3-day PPS than control PPS. In the 6-week OHT eyes, AL and PPS strains were similar overall to controls. This experimental glaucoma model demonstrated a dynamic change in the mechanical behaviour of the AL and PPS over time at the site of neuronal injury and remodelling in glaucoma.

The connective tissue phenotype of glaucomatous cupping in the monkey eye - Clinical and research implications

In a series of previous publications we have proposed a framework for conceptualizing the optic nerve head (ONH) as a biomechanical structure. That framework proposes important roles for intraocular pressure (IOP), IOP-related stress and strain, cerebrospinal fluid pressure (CSFp), systemic and ocular determinants of blood flow, inflammation, auto-immunity, genetics, and other non-IOP related risk factors in the physiology of ONH aging and the pathophysiology of glaucomatous damage to the ONH. The present report summarizes 20 years of technique development and study results pertinent to the characterization of ONH connective tissue deformation and remodeling in the unilateral monkey experimental glaucoma (EG) model. In it we propose that the defining pathophysiology of a glaucomatous optic neuropathy involves deformation, remodeling, and mechanical failure of the ONH connective tissues. We view this as an active process, driven by astrocyte, microglial, fibroblast and oligodendrocyte mechanobiology. These cells, and the connective tissue phenomena they propagate, have primary and secondary effects on retinal ganglion cell (RGC) axon, laminar beam and retrolaminar capillary homeostasis that may initially be "protective" but eventually lead to RGC axonal injury, repair and/or cell death. The primary goal of this report is to summarize our 3D histomorphometric and optical coherence tomography (OCT)-based evidence for the early onset and progression of ONH connective tissue deformation and remodeling in monkey EG. A second goal is to explain the importance of including ONH connective tissue processes in characterizing the phenotype of a glaucomatous optic neuropathy in all species. A third goal is to summarize our current efforts to move from ONH morphology to the cell biology of connective tissue remodeling and axonal insult early in the disease. A final goal is to facilitate the translation of our findings and ideas into neuroprotective interventions that target these ONH phenomena for therapeutic effect.

Intravitreal injection of β-crystallin B2 improves retinal ganglion cell survival in an experimental animal model of glaucoma

Purpose of this study was to investigate firstly specific proteomic changes within the retina in the course of an animal glaucoma model and to identify secondly new approaches for neuroprotective, therapeutic options in glaucoma by addressing those specific changes. Intraocular pressure was elevated through cauterization of episcleral veins in adult Sprague Dawley rats. Molecular and morphological changes were surveyed using mass spectrometry, optical coherence tomography as well as immunohistochemical cross section- and flat mount stainings. By quantifying more than 1500 retinal proteins, it was found that the HspB5 protein and numerous beta-crystallins showed a uniform and unique shifting expression pattern as a result of different periods of elevated IOP exposure. Crystallins showed a significant downregulation (p<0.05) after 3 weeks of elevated IOP and an upregulation after 7 weeks. Counteracting those typical changes, an intravitreal injection of β-crystallin B2 at the time of IOP elevation was found to reduce retinal ganglion cell loss (p<0.05), decrease of the retinal nerve fiber layer (p<0.05) and impairment of the optic nerve. Ultimately, proteomic data revealed that β-crystallin B2 might influence calcium-depended cell signaling pathways with severe effect on apoptosis and gene regulation. In this context especially annexin A5, calcium-transporting ATPase 1 and various histone proteins seem to play a major role.

Comparative Anatomy of the Trabecular Meshwork, the Optic Nerve Head and the Inner Retina in Rodent and Primate Models Used for Glaucoma Research

Glaucoma is a heterogeneous group of ocular disorders with a multi-faceted etiology. Although numerous studies on glaucoma using different animal models have been published, it is unwise to simply generalize the results of one model to all glaucomatous situations because of the differences in the anatomy and morphology of animal eyes in comparison with humans’. In this review, we highlight the differences in the trabecular meshwork (TM) tissue, lamina cribrosa (LC) region, optic nerve head (ONH) and the inner layer of the retina in mice, rats and monkeys. In comparison with humans, non-human primates show TM, retina and ONH that are anatomically almost identical. The rat model shows many similarities in the aqueous outflow pathway compared to humans. The mouse ONH lacks collagenous LC, and this finding is observed across different mouse strains. The tissue structure of the ONH in rodents is similar to that in humans, although the blood supply shows differences. The number of cells in the ganglion layer depends on the rodent strain. Despite some differences from humans, rodents are a good choice for studying different types of glaucoma, and the modeling method should be selected based on the experimental needs and the hypothesis being tested.

The Connective Tissue Components of Optic Nerve Head Cupping in Monkey Experimental Glaucoma Part 1: Global Change

PURPOSE

To characterize optic nerve head (ONH) connective tissue change within 21 monkey experimental glaucoma (EG) eyes, so as to identify its principal components.

METHODS

Animals were imaged three to five times at baseline then every 2 weeks following chronic unilateral IOP elevation, and euthanized early through end-stage confocal scanning laser tomographic change. Optic nerve heads were serial-sectioned, three-dimensionally (3D) reconstructed, delineated, and quantified. Overall EG versus control eye differences were assessed by general estimating equations (GEE). Significant, animal-specific, EG eye change was required to exceed the maximum physiologic intereye differences in six healthy animals.

RESULTS

Overall EG eye change was significant (P < 0.0026) and animal-specific EG eye change most frequent, for five phenomena (number of EG eyes and range of animal-specific change): posterior laminar deformation (21, -29 to -437 μm), laminar thickening (11, 20-73 μm) and thinning (3, -23 to -31 μm), scleral canal expansion (17, 20-139 μm), outward anterior (16, -16 to -124 μm) and posterior (17, -22 to -279 μm) laminar insertion migration, and peripapillary scleral bowing (11, 21-77 μm). Experimental glaucoma versus control eye laminar thickness differences were bimodal in behavior, being thickened in most EG eyes demonstrating the least deformation and less thickened or thinned in most EG eyes demonstrating the greatest deformation.

CONCLUSIONS

Our postmortem studies retrospectively identify five connective tissue components of ONH "cupping" in monkey EG which serve as targets for longitudinally staging and phenotyping ONH connective tissue alteration within all forms of monkey and human optic neuropathy.

Compromised Optic Nerve Blood Flow and Autoregulation Secondary to Neural Degeneration

PURPOSE

To test the hypothesis that optic nerve head (ONH) blood flow (BF) and autoregulation compromise are consequences of optic nerve degeneration induced by surgical optic nerve transection (ONT).

METHODS

In both eyes of five nonhuman primates, peripapillary retinal nerve fiber layer thickness (RNFLT) was measured by spectral-domain optical coherence tomography. Optic nerve head BF and dynamic autoregulation responses to a rapid manometric IOP increase (from 10-40 mm Hg) were measured by Laser Speckle Flowgraphy. The measurements were conducted every 10 to 15 days before and after unilateral ONT. Post-ONT measurements were repeated until RNFLT in the ONT eye was reduced by more than 40% of baseline value.

RESULTS

After ONT, RNFLT, and ONH BF progressively declined over time (P < 0.0001 and P = 0.02, respectively). Longitudinal changes between the two were highly correlated (P < 0.0001). When data was grouped by test session, the first significant decreases for RNFLT and BF were found 13 ± 0.8 and 24 ± 3.2 days post ONT, respectively (P < 0.05, both). At the final time point (55 ± 0.5 days post ONT), RNFLT, and BF were reduced by 44% ± 2.0% and 38 ± 5.0% from baseline, respectively. Dynamic autoregulation analysis showed marginal increased response time in post-ONT eyes (P = 0.05). Control eyes showed no longitudinal changes for any parameter.

CONCLUSIONS

The close association between RNFLT loss and ONH BF decrease following optic nerve degeneration demonstrated a clear cause and effect relationship. Increased BF response time appears to be a sign of dynamic autoregulation dysfunction in this ONT model.

The non-human primate experimental glaucoma model

The purpose of this report is to summarize the current strengths and weaknesses of the non-human primate (NHP) experimental glaucoma (EG) model through sections devoted to its history, methods, important findings, alternative optic neuropathy models and future directions. NHP EG has become well established for studying human glaucoma in part because the NHP optic nerve head (ONH) shares a close anatomic association with the human ONH and because it provides the only means of systematically studying the very earliest visual system responses to chronic intraocular pressure (IOP) elevation, i.e. the conversion from ocular hypertension to glaucomatous damage. However, NHPs are impractical for studies that require large animal numbers, demonstrate spontaneous glaucoma only rarely, do not currently provide a model of the neuropathy at normal levels of IOP, and cannot easily be genetically manipulated, except through tissue-specific, viral vectors. The goal of this summary is to direct NHP EG and non-NHP EG investigators to the previous, current and future accomplishment of clinically relevant knowledge in this model.

Morphological and morphometric changes in rat optic nerve microvessels in a glaucoma experimental model

AIM

To study the morphological and morphometric changes produced in the capillaries of the optic nerve (ON) head and initial portion after the experimental increase in intraocular pressure (IOP).

MATERIAL AND METHODS

Wistar rats underwent cauterization of three episcleral veins, which produced an immediate increase in the IOP, and was maintained for 3 months. Sagittal sections of the eyeball were studied with immunohistochemical techniques, using a primary antibody to GLUT-1. The GLUT-1 positive capillaries were counted, and measurements were made of the area, perimeter and mean diameter.

RESULTS

Microscopic examination of sections of the ON of control rats revealed a lower density and larger caliber of capillaries in the prelaminar region as compared with the other regions of the ON (P<.05). Comparison between the control and the experimental groups showed a reduction in capillary density (except in the prelaminar region) and a smaller size in all the areas of the ON studied, but less evident in the initial portion (P<.05).

CONCLUSIONS

The increase in IOP was associated with significant qualitative and quantitative changes in the capillaries of the laminar and poslaminar regions of the ON head. These changes appear to return towards parameters compatible with normality in the initial portion of the ON, an area where the vascular collapse was less evident. These findings might explain the significant reduction in ocular blood flow seen in patients with primary open-angle glaucoma.

Antioxidants reduce TGF-beta2-induced gene expressions in human optic nerve head astrocytes

PURPOSE

The goal of the present study was to investigate whether the antioxidants vitamin E, vitamin C and vitamin B1 can reduce the transforming growth factor-beta2 (TGF-β2)-induced gene expressions in cultured human optic nerve head (ONH) astrocytes.

METHODS

Cultured human ONH astrocytes were pretreated with different concentrations of vitamin E, vitamin C and vitamin B1 and then exposed to 1.0 ng/ml TGF-β2 for 24 hr. Expression of the heat shock proteins Hsp27 and αB-crystallin, the extracellular matrix (ECM) component fibronectin and the ECM-modulating protein connective tissue growth factor (CTGF) was detected by immunohistochemistry or real-time PCR analysis.

RESULTS

TGF-β2 increased the expression of Hsp27, αB-crystallin, fibronectin and CTGF in human ONH astrocytes. Pretreatment with different concentrations of vitamin E, vitamin C and vitamin B1 reduced the TGF-β2-stimulated gene expressions.

CONCLUSION

In cultured human ONH astrocytes, the TGF-β2-stimulated gene expressions could be reduced by pretreatment with vitamin E, vitamin C and vitamin B1. Therefore, the use of antioxidants in glaucomatous optic neuropathy might be a promising approach to prevent TGF-β2-induced cellular changes in ONH astrocytes.

Crystallins are regulated biomarkers for monitoring topical therapy of glaucomatous optic neuropathy

Optic nerve atrophy caused by abnormal intraocular pressure (IOP) remains the most common cause of irreversible loss of vision worldwide. The aim of this study was to determine whether topically applied IOP-lowering eye drugs affect retinal ganglion cells (RGCs) and retinal metabolism in a rat model of optic neuropathy. IOP was elevated through cauterization of episcleral veins, and then lowered either by the daily topical application of timolol, timolol/travoprost, timolol/dorzolamide, or timolol/brimonidine, or surgically with sectorial iridectomy. RGCs were retrogradely labeled 4 days prior to enucleation, and counted. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), matrix-assisted laser desorption ionization mass spectrometry, Western blotting, and immunohistochemistry allowed the identification of IOP-dependent proteomic changes. Genomic changes were scrutinized using microarrays and qRT-PCR. The significant increase in IOP induced by episcleral vein cauterization that persisted until 8 weeks of follow-up in control animals (p<0.05) was effectively lowered by the eye drops (p<0.05). As anticipated, the number of RGCs decreased significantly following 8 weeks of elevated IOP (p<0.05), while treatment with combination compounds markedly improved RGC survival (p<0.05). 2D-PAGE and Western blot analyses revealed an IOP-dependent expression of crystallin cry-βb2. Microarray and qRT-PCR analyses verified the results at the mRNA level. IHC demonstrated that crystallins were expressed mainly in the ganglion cell layer. The data suggest that IOP and either topically applied antiglaucomatous drugs influence crystallin expression within the retina. Neuronal crystallins are thus suitable biomarkers for monitoring the progression of neuropathy and evaluating any neuroprotective effects.

Functional rescue of experimental ischemic optic neuropathy with αB-crystallin

PURPOSE

Anterior ischemic optic neuropathy (AION) is an important cause of acute vision loss in adults, and there is no effective treatment. We studied early changes following experimental AION and tested the benefit of a potential treatment.

MATERIALS AND METHODS

We induced experimental AION in adult mice and tested the effects of short-term (daily for 3 days) and long-term (every other day for 3 weeks) αB-crystallin (αBC) treatment using histological and serial intracranial flash visual evoked potential recordings.

RESULTS

One day after experimental AION, there was swelling at the optic nerve (ON) head and increased expression of αBC, a small heat shock protein important in ischemia and inflammation. This upregulation coincided with microglial and astrocytic activation. Our hypothesis was that αBC may be part of the endogenous protective mechanism against injury, thus we tested the effects of αBC on experimental AION. Daily intraveneous or intravitreal αBC injections did not improve visual evoked potential amplitude or latency at days 1-2. However, αBC treatment decreased swelling and dampened the microglial and astrocytic activation on day 3. Longer treatment with intravenous αBC led to acceleration of visual evoked potential latency over 3 weeks, without improving amplitude. This latency acceleration did not correlate with increased retinal ganglion cell survival but did correlate with complete rescue of the ON oligodendrocytes, which are important for myelination.

CONCLUSIONS

We identified αBC as an early marker following experimental AION. Treatment with αBC enhanced this endogenous, post-ischemic response by decreasing microglial activation and promoting ON oligodendrocyte survival.

Morphometric changes in the rat optic nerve following short-term intermittent elevations in intraocular pressure

PURPOSE

Intraocular pressure (IOP) fluctuations may occur in patients with glaucoma, but how these fluctuations affect axonal populations in the optic nerve and other structures in the eye has been difficult to assess. This study developed a rat model to evaluate the effect of intermittent controlled elevations in IOP on the morphology of the rat optic nerve.

METHODS

IOP was transiently elevated for 1 hour on each of 6 days a week over 6 weeks with an adjustable vascular loop around the right topically anesthetized eye of Sprague-Dawley rats. IOP was measured by pneumatonometer before, immediately after, and at the end of 1 hour of treatment with ligature. Globes and optic nerve segments were prepared for histology and morphometry.

RESULTS

Mean baseline IOP of 14.9 ± 1.8 mm Hg increased to 35.3 ± 2.6 mm Hg (P < 0.001) during 1-hour treatments and returned to 15.0 ± 2.2 mm Hg (P = 0.84) 1 hour after completion. The contralateral untreated eyes had a mean IOP of 14.2 ± 1.9 mm Hg at baseline and 14.6 ± 1.9 mm Hg at the end of treatment. Nerve fiber layer thinning (22%-25%) corresponded with a decrease (7%-10%) in soma number in the ganglion cell layer. Optic nerves displayed axonal degeneration with a modest axon loss of 6% and increased expression of glial acidic fibrillary protein in astrocytes.

CONCLUSIONS

Controlled daily 1-hour IOP elevations can be performed with an adjustable vascular loop in rats. After only 6 weeks, intermittent elevations in IOP produce changes in optic nerve consistent with early degeneration reported in chronic models of glaucoma.

Friend or foe? Resolving the impact of glial responses in glaucoma

Glaucomatous vision loss results from the progressive degeneration of optic nerve axons and the death of retinal ganglion cells. This process is accompanied by dramatic alterations in the functional properties and distribution of glial cells in both the retina and the optic nerve head in a reaction commonly referred to as glial activation. The recent availability of rodent and cell culture glaucoma models has substantially contributed to our knowledge of glial activation under glaucomatous conditions. Conclusions drawn from these studies have led to the refinement of existing hypotheses and the generation of new ones. Because these hypotheses encompass both protective and injurious roles for glia, the impact of specific aspects of glial activation are current topics of intensive research, speculation, and debate in the field. With these unresolved issues in mind, this review will summarize recent progress in our understanding of the process of glial activation in the glaucomatous optic nerve head and retina.

Comparative anatomy of the optic nerve head and inner retina in non-primate animal models used for glaucoma research

To judge the information of experimental settings in relation to the human situation, it is crucial to be aware of morphological differences and peculiarities in the species studied. Related to glaucoma, the most important structures of the posterior eye segment are the optic nerve head including the lamina cribrosa, and the inner retinal layers. The review highlights the differences of the lamina cribrosa and its vascular supply, the prelaminar optic nerve head, and the retinal ganglion cell layer in the most widely used animal models for glaucoma research, including mouse, rat, rabbit, pig, dog, cat, chicken, and quail. Although all species show some differences to the human situation, the rabbit seems to be the most problematic animal for glaucoma research.

Global changes in optic nerve head gene expression after exposure to elevated intraocular pressure in a rat glaucoma model

PURPOSE

In glaucoma, the optic nerve head (ONH) is the likely site of initial injury and elevated intraocular pressure (IOP) is the best-known risk factor. This study determines global gene expression changes in the pressure-injured ONH.

METHODS

Unilateral sustained IOP elevation (glaucoma, n = 46) or optic nerve transection (n = 10) was produced in rats. ONHs were removed, and the retrobulbar optic nerves were graded for degeneration. Gene expression in the glaucomatous ONH with extensive injury was compared with that in the fellow ONH (n = 6/group), by using cDNA microarrays. Data from 12 arrays were normalized, significant differences in gene expression determined, and significantly affected gene classes identified. For the remaining ONH, grouped by experimental condition and degree of injury, quantitative reverse transcriptase-PCR (qPCR) and ANOVA were used to compare selected message levels.

RESULTS

Microarray analysis identified more than 2000 significantly regulated genes. For 225 of these genes, the changes were greater than twofold. The most significantly affected gene classes were cell proliferation, immune response, lysosome, cytoskeleton, extracellular matrix, and ribosome. A 2.7-fold increase in ONH cellularity confirmed glaucoma model cell proliferation. By qPCR, increases in levels of periostin, collagen VI, and transforming growth factor beta1 were linearly correlated to the degree of IOP-induced injury. For cyclinD1, fibulin 2, tenascin C, TIMP1, and aquaporin-4, correlations were significantly nonlinear, displaying maximum change with focal injury.

CONCLUSIONS

In the ONH, pressure-induced injury results in cell proliferation and dramatically altered gene expression. For specific genes, expression levels were most altered by focal injury, suggesting that further array studies may identify initial, and potentially injurious, altered processes.

Hypoxia/reoxygenation and TGF-β increase αB-crystallin expression in human optic nerve head astrocytes

Reactive astrocytes in glaucomatous optic nerve changes are characterized by an increased expression of alphaB-crystallin and transforming growth factor-beta (TGF-beta). In the pathogenesis of glaucomatous optic nerve damage, ischemia/reperfusion injury may play an important role. The goal of the present study was to determine the influence of hypoxia/reoxygenation and TGF-beta on the expression of alphaB-crystallin in cultured human astrocytes of the optic nerve head (ONH). Cultured human astrocytes were incubated under hypoxic conditions (1% O2 for 4-12 h) with subsequent reoxygenation (12-24 h). Additionally, cells were treated with 1.0 ng/ml TGF-beta1 and TGF-beta2 for 12-48 h. Expression of alphaB-crystallin was examined by Northern- and Western-blotting. Levels of TGF-beta1 and TGF-beta2 were analyzed by RT-PCR analysis and ELISA. The effect of TGF-beta blocking on the hypoxia/reoxygenation modulated expression of alphaB-crystallin was investigated by simultaneous incubation with neutralizing antibodies against TGF-beta during the reoxygenation phase. Hypoxia/reoxygenation increased the expression of alphaB-crystallin at the mRNA (2.8- to 3.1-fold) and protein level (1.8- to 2.1-fold). Treatment with 1.0 ng/ml TGF-beta1 and TGF-beta2 for 12-48 h markedly enhanced alphaB-crystallin mRNA expression approximately three- to fourfold. Using Western blot analysis, this increase ranged from 2 to 3 times. Both cytokines showed a twofold increase after 12 and 24 h of reoxygenation at the mRNA and a two- to threefold increase at the protein level. Simultaneous treatment with neutralizing antibodies against both TGF-beta isoforms prevented the hypoxia/reoxygenation-mediated elevation of alphaB-crystallin. The process of hypoxia/reoxygenation is capable of inducing the expression of alphaB-crystallin and TGF-ss in cultured ONH astrocytes. Therefore, optimization of conditions leading to hypoxia/reoxygenation in the ONH of glaucomatous patients may help to lower the incidence of characteristic changes in the optic nerve.

Choroidal innervation and optic neuropathy in macacque monkeys with laser- or anterior chamber perfusion-induced short-term elevation of intraocular pressure

Long-term intraocular pressure (IOP) elevation leads to a significant reduction of the intrinsic choroidal innervation and axon loss in the optic nerve. In this study we investigated early changes in these tissues in order to clarify the putative sequence of events between choroidal ganglion cell loss and optic nerve axon loss in 19 monkeys with experimentally induced glaucoma. After 1 month of increased IOP, severe optic neuropathy but no reduction of choroidal ganglion cells (CGC) was detectable. Beginning at 2 months after treatment signs of degeneration in CGC morphology were observed. Laser treatment of the trabecular meshwork without IOP elevation caused no changes in choroidal innervation. Our results show that there is no apparent association between reduction of CGC and optic neuropathy in the early stages of IOP elevation.

Vascular changes in the posterior eye segment of secondary angle-closure glaucoma: cause or consequence?

BACKGROUND

To study the role of choroidal and retinal vessels in the pathology of secondary angle-closure glaucoma.

METHODS

DBA/2NNia and non-glaucomatous C57BL/6J mice over the age range 2-20 months were investigated. Corrosion cast preparations of the vasculature were studied using scanning electron microscopy. Whole mounts of the retina and choroid were stained enzyme-histochemically for NADPH diaphorase as an indicator for nitric oxide synthase activity. Semi- and ultra-thin sections of the posterior eye segment were performed and evaluated.

RESULTS

DBA/2NNia mice showed loss of choroidal pigmentation and a decrease in choriocapillary density already at 4 months of age. In animals 9 months and older, a decrease of choroidal NADPH-diaphorase positive nerve fibers was evident. The retinal vasculature showed only mild changes in NADPH-diaphorase staining, even in the oldest animals. The ultrastructural appearance of the retinal vessels was similar in both mouse strains and for all ages investigated.

CONCLUSIONS

Choroidal changes in the DBA/2NNia mouse are similar to that seen in other glaucoma models. The lack of retinal vasculature changes in adult and senescent DBA/2NNia mice suggests a normal blood supply of the retina during the progress of secondary angle-closure glaucoma in these animals.

Optic nerve degeneration in the DBA/2NNia mouse: is the lamina cribrosa important in the development of glaucomatous optic neuropathy?

To study optic nerve (ON) degeneration in the DBA/2NNia (DBA) mouse, a species lacking a lamina cribrosa and a model for secondary angle-closure glaucoma, serial semi- and ultra-thin sectioning of the myelinated ON and of the ON head was performed and sections evaluated qualitatively and quantitatively by light and electron microscopy. Immunohistochemistry was performed using antibodies against collagen type I, III, VI, laminin, and connexin43. The major finding on the myelinated ON was a significant decrease in cross section area during ON degeneration which was paralleled by a loss of axons and an increase in microglia. The number of astrocytes and blood vessels did not change. The major findings on the ON papilla were that ON heads with only mild degeneration showed a pronounced focal degeneration around the central retinal artery. In more severely degenerated ON, newly formed bundles of collagen type VI were located between astrocyte processes within the ON head. In a species that has no lamina cribrosa, DBA mice can develop typical signs of glaucomatous optic neuropathy. The entrance of the central retinal vessels into the ONH seems to be a preferentially vulnerable region for axon loss in this mouse model. In addition, astrocytes in the ON head form extracellular material similar to that found in human glaucomatous eyes.

Primate glaucoma models

The monkey model of ocular hypertension (OHT) with its resultant optic neuropathy closely reflects the optic neurodegeneration associated with human glaucoma. Utilization of the experimental glaucoma model (ExpG) in non-human primates (NHP) has led to advances in the understanding of aqueous humor dynamics, glaucomatous changes in the visual pathways from photoreceptors to the visual cortex, and anterior and posterior ocular segment pharmacological effects.

Optic nerve head neuroretinal rim blood flow differences in monkeys with laser-induced glaucoma

INTRODUCTION

The blood flow of the neuroretinal rim (NRR) of the optic nerve head (ONH) of the rhesus monkey with laser-induced glaucoma was examined.

METHODS

Argon laser photocoagulation of the trabecular meshwork to induce elevated intraocular pressure (IOP) was performed in one eye of nine normal male rhesus monkeys. The nasal and temporal NRR of the monkey ONH were examined by the Heidelberg retina tomograph/flowmeter (HRT/HRF) under neuromuscular blockade. A mixed effect analysis of variance was used to determine significant differences between eyes and between locations in the eyes.

RESULTS

The average IOP in the hypertensive glaucoma and normal eyes was 34.8 +/- 7.2 and 16.0 +/- 1.9 mmHg, respectively. The HRT determined average overall cup to disc (C/D) area ratio in the glaucoma and normal eyes, which was 0.49 +/- 0.28 and 0.22 +/- 0.16, respectively. The mean temporal NRR HRF flow in the hypertensive eyes was significantly greater than in the normotensive eyes (P < 0.0001), than in the nasal NRR of the hypertensive eyes (P < 0.0001) and than in the nasal NRR of the normotensive eyes (P < 0.01). The mean nasal NRR HRF flow in the hypertensive eyes was significantly less than in the nasal NRR of the normotensive eyes (P < 0.01). There was no statistical difference between the mean HRF flow of the temporal and nasal NRR of the normotensive eyes. The elevated IOP positively influenced the flow values in the hypertensive eye (r = 0.724).

CONCLUSIONS

The capillary microcirculation of the temporal NRR of the rhesus monkey ONH with laser-induced glaucoma has significantly increased blood flow, and the nasal NRR significantly reduced blood flow compared to blood flow in the NRR of normal normotensive monkey eyes.

Increased plasma endothelin-1 levels in patients with progressive open angle glaucoma

AIM

To compare the plasma levels of endothelin-1 (ET-1) between patients with primary open angle glaucoma with visual field progression despite normal or normalised intraocular pressure and patients with stabile visual fields in a retrospective study.

METHODS

The progressive group consisted of 16 primary open angle glaucoma patients and the group with stable visual field consisted of 15 patients. After a 30 minute rest in a supine position, venous blood was obtained for ET-1 dosing. Difference in the plasma level of ET-1 between two groups was compared by means of analysis of covariance (ANCOVA), including age, sex, and mean arterial blood pressure as covariates.

RESULTS

ET-1 plasma levels were found to be significantly increased in patients with deteriorating (3.47 (SD 0.75) pg/ml) glaucoma when compared to those with stable (2.59 (SD 0.54) pg/ml) visual fields (p = 0.0007).

CONCLUSIONS

Glaucoma patients with visual field progression in spite of normal or normalised intraocular pressure have been found to have increased plasma endothelin-1 levels. It remains to be determined if this is a secondary phenomenon or whether it may have a role in the progression of glaucomatous damage.

Endothelin: is it a contributor to glaucoma pathophysiology?

Endothelin is a vasoactive peptide that has been shown to play an important role in vascular homeostasis. Recently, endothelin and its receptors have been found in ocular tissues where it appears to have a regulatory function. Endothelin is found in both the aqueous and vitreous humors and its concentration is elevated in glaucoma patients and in animal models of glaucoma. In the current review, the authors present information about the distribution of endothelin and endothelin receptors in the eye and the ocular actions of endothelins. Specifically, endothelin/aqueous humor dynamics, endothelin/nitric oxide interactions, endothelin and ischemia, and endothelin/optic nerve head effects. Observations concerning the potential role of endothelin in glaucoma pathophysiology is presented and discussed relative to its effects on the optic nerve head and in relation to glaucoma theories.