Magnocellular and parvocellular visual pathways are both affected in a macaque monkey model of glaucoma

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

Trans-synaptic degeneration in the visual pathway: Neural connectivity, pathophysiology, and clinical implications in neurodegenerative disorders

There is a strong interrelationship between eye and brain diseases. It has been shown that neurodegenerative changes can spread bidirectionally in the visual pathway along neuronal projections. For example, damage to retinal ganglion cells in the retina leads to degeneration of the visual cortex (anterograde degeneration) and vice versa (retrograde degeneration). The underlying mechanisms of this process, known as trans-synaptic degeneration (TSD), are unknown, but TSD contributes to the progression of numerous neurodegenerative disorders, leading to clinical and functional deterioration. The hierarchical structure of the visual system comprises of a strong topographic connectivity between the retina and the visual cortex and therefore serves as an ideal model to study the cellular effect, clinical manifestations, and deterioration extent of TSD. With this review we provide comprehensive information about the neural connectivity, synapse function, molecular changes, and pathophysiology of TSD in visual pathways. We then discuss its bidirectional nature and clinical implications in neurodegenerative diseases. A thorough understanding of TSD in the visual pathway can provide insights into progression of neurodegenerative disorders and its potential as a therapeutic target.

Transneuronal Degeneration in the Brain During Glaucoma

The death of retinal ganglion cells (RGCs) is a key factor in the pathophysiology of all types of glaucoma, but the mechanism of pathogenesis of glaucoma remains unclear. RGCs are a group of central nervous system (CNS) neurons whose soma are in the inner retina. The axons of RGCs form the optic nerve and converge at the optic chiasma; from there, they project to the visual cortex via the lateral geniculate nucleus (LGN). In recent years, there has been increasing interest in the dysfunction and death of CNS and retinal neurons caused by transneuronal degeneration of RGCs, and the view that glaucoma is a widespread neurodegenerative disease involving CNS damage appears more and more frequently in the literature. In this review, we summarize the current knowledge of LGN and visual cortex neuron damage in glaucoma and possible mechanisms behind the damage. This review presents an updated and expanded view of neuronal damage in glaucoma, and reveals new and potential targets for neuroprotection and treatment.

Glaucoma: A Degenerative Optic Neuropathy Related to Neuroinflammation?

Glaucoma is one of the leading causes of irreversible blindness in the world and remains a major public health problem. To date, incomplete knowledge of this disease’s pathophysiology has resulted in current therapies (pharmaceutical or surgical) unfortunately having only a slowing effect on disease progression. Recent research suggests that glaucomatous optic neuropathy is a disease that shares common neuroinflammatory mechanisms with “classical” neurodegenerative pathologies. In addition to the death of retinal ganglion cells (RGCs), neuroinflammation appears to be a key element in the progression and spread of this disease. Indeed, early reactivity of glial cells has been observed in the retina, but also in the central visual pathways of glaucoma patients and in preclinical models of ocular hypertension. Moreover, neuronal lesions are not limited to retinal structure, but also occur in central visual pathways. This review summarizes and puts into perspective the experimental and clinical data obtained to date to highlight the need to develop neuroprotective and immunomodulatory therapies to prevent blindness in glaucoma patients.

Structural and functional analyses of the optic nerve and lateral geniculate nucleus in glaucoma

Purpose

To analyze the correlation between structural characteristics of intraorbital optic nerve (ION) and lateral geniculate nucleus (LGN) measured by 3-Tesla magnetic resonance imaging (3T MRI), and the severity of glaucomatous damage.

Methods

In this cross-sectional study, 41 glaucoma patients and 12 age- and sex-matched controls underwent standard automated perimetry (SAP) and frequency doubling technology (FDT) as functional evaluation; optic disc stereophotograph, spectral-domain optical coherence tomography (OCT) and confocal scanning laser tomography as ocular structural evaluation; and 3T MRI. Structure-structure and structure-function correlation were performed using bootstrap resampling method for clustered data.

Results

The ION mean diameter and cross-sectional area were different between glaucoma and control groups at 5mm and 10mm (all, p≤0.011) from the globe, but not at 15mm (both, p≥0.067). LGN height was significantly lower in glaucoma group (p = 0.005). OCT rim area and functional parameters (SAP and FDT) correlated significantly with all ION segments, showing stronger correlations at 10 and 15 mm. ION parameters at 10 and 15 mm presented mild-to-moderate correlation with OCT peripapillary nerve fiber layer thickness, and ION at 15mm had mild association with the neuroretinal rim area on stereophotographs. Although LGN height was significantly smaller in glaucoma group (p = 0.005), LGN parameters were not associated with any ocular structural or functional parameter.

Conclusion

Assessment of central and peripheral nervous systems using 3T MRI confirmed that glaucoma patients had smaller ION dimensions and LGN height compared to the control group. In general, ION dimensions presented mild to moderate correlations with functional and ocular structural parameters. Although ION had significant correlations at any distance from the eye, the ION distal locations correlated better with OCT results and functional parameters. However, LGN parameters were not associated with functional or ocular structural parameters.

Glaucoma and the brain: Trans-synaptic degeneration, structural change, and implications for neuroprotection

A recent hypothesis to enter the literature suggests that glaucoma is a neurodegenerative disease. The basis for this has been the finding of central nervous system changes in glaucoma patients on histology and neuroimaging. It is known that retinal ganglion cell pathology of any cause leads to anterograde and retrograde retinal ganglion cell degeneration, as well as trans-synaptic (transneuronal) anterograde degeneration. Trans-synaptic degeneration has been demonstrated in a range of optic neuropathies including optic nerve transection, optic neuritis, and hereditary optic neuropathies. More recently, similar changes have been confirmed in glaucoma patients using the neuroimaging techniques of voxel-based morphometry and diffusion tensor imaging. Some studies have reported brain changes in glaucoma outside the retino-geniculo-cortical pathway; however, these are preliminary and exploratory in nature. Further research is required to identify whether the degenerative brain changes in glaucoma are entirely secondary to the optic neuropathy or whether there is additional primary central nervous system pathology. This has critical implications for neuroprotective and regenerative treatment strategies and our basic understanding of glaucoma.

Glaucoma related Proteomic Alterations in Human Retina Samples

Glaucoma related proteomic changes have been documented in cell and animal models. However, proteomic studies investigating on human retina samples are still rare. In the present work, retina samples of glaucoma and non-glaucoma control donors have been examined by a state-of-the-art mass spectrometry (MS) workflow to uncover glaucoma related proteomic changes. More than 600 proteins could be identified with high confidence (FDR < 1%) in human retina samples. Distinct proteomic changes have been observed in 10% of proteins encircling mitochondrial and nucleus species. Numerous proteins showed a significant glaucoma related level change (p < 0.05) or distinct tendency of alteration (p < 0.1). Candidates were documented to be involved in cellular development, stress and cell death. Increase of stress related proteins and decrease of new glaucoma related candidates, ADP/ATP translocase 3 (ANT3), PC4 and SRFS1-interacting protein 1 (DFS70) and methyl-CpG-binding protein 2 (MeCp2) could be documented by MS. Moreover, candidates could be validated by Accurate Inclusion Mass Screening (AIMS) and immunostaining and supported for the retinal ganglion cell layer (GCL) by laser capture microdissection (LCM) in porcine and human eye cryosections. The workflow allowed a detailed view into the human retina proteome highlighting new molecular players ANT3, DFS70 and MeCp2 associated to glaucoma.

Reduced cortical thickness in primary open-angle glaucoma and its relationship to the retinal nerve fiber layer thickness

Objectives

To examine possible changes in cortical thickness and their relationship to retinal nerve fiber layer (RNFL) thickness in patients with primary open-angle glaucoma (POAG).

Materials and Methods

Thirty-six patients with POAG and 40 matched healthy controls were enrolled in this study. All subjects underwent a comprehensive ophthalmologic examination and a high resolution structural magnetic resonance scan. Cortical thickness analysis was used to assess the changes between patients and controls. Correlations between the thickness of the visual cortex and RNFL thickness were also analyzed. Finally, the relationship between the severity of changes in the visual cortex and RNFL thickness was evaluated by comparing patients with mild and severe groups.

Results

POAG patients showed significant bilateral cortical thinning in the anterior half of the visual cortex around the calcarine sulci (left BA 17 and BA 18, right BA17) and in some smaller regions located in the left middle temporal gyrus (BA37) and fusiform gyrus (BA19). The thickness of the visual cortex correlated positively with RNFL thickness (left, r = 0.44, p = 0.01; right, r = 0.38, p = 0.03). Significant differences between mild and severe groups were observed with regard to both RNFL thickness and the thickness of bilateral visual cortex (p < 0.05).

Conclusion

Our findings indicate that cortical thickness analysis may be sufficiently sensitive to detect cortical alterations in POAG and that the measurement has great potential for clinical application.

Anterograde degeneration along the visual pathway after optic nerve injury

PURPOSE

To investigate anterograde degenerative changes along the visual pathway in a rat model of optic nerve axotomy.

METHODS

Optic nerve transection was performed in adult Sprague-Dawley rats. Animals were sacrificed at regular time intervals and tissues harvested. Immunoblotting followed by densitometric analysis was used to determine the phosphorylation profile of Akt in the dorsal lateral geniculate nucleus (dLGN) and the primary visual cortex (V1). The neuronal cell size and cell density were measured in the dLGN and the V1 using Nissl staining. The prevalence of apoptosis was characterized by terminal deoxynucleotidyl-transferase-mediated biotin-dUTP nick end labelling (TUNEL) histochemistry. Caspase-3 antibodies were also used to identify apoptotic cells. Neurons and astrocytes were detected using NeuN and glial fibrillary acidic protein (GFAP), respectively.

RESULTS

An early and sustained loss of Akt phosphorylation was observed after optic nerve transection in both dLGN and V1. At week one, a decrease in the neuronal cell size (50.5±4.9 vs 60.3±5.0 µm(2), P = 0.042) and an increase of TUNEL positive cells (7.9±0.6 vs 1.4±0.5 ×10(2) cells/mm(2), P<0.001) were evident in the dLGN but not in V1. A significant decline in neuronal cell number (14.5±0.1 vs 17.4±1.3 ×10(2) cells/mm(2), P = 0.048), cell size (42.5±4.3 vs 62.1±4.7 µm(2), P = 0.001) and an increase in apoptotic cells (5.6±0.5 vs 2.0±0.4 ×10(2) cells/mm(2), P<0.001) appeared in V1 initially at one month post-transection. The changes in the visual pathway continued through two months. Both neuronal cells and GFAP-positive glial cells were affected in this anterograde degeneration along the visual pathway.

CONCLUSIONS

Anterograde degeneration along the visual pathway takes place in target relay (LGN) and visual cortex following the optic nerve injury. Apoptosis was observed in both neural and adjacent glial cells. Reduction of Akt phosphorylation preceded cellular and apoptotic changes.

Arterial spin labeling fMRI measurements of decreased blood flow in primary visual cortex correlates with decreased visual function in human glaucoma

PURPOSE

Altered metabolic activity has been identified as a potential contributing factor to the neurodegeneration associated with primary open angle glaucoma (POAG). Consequently, we sought to determine whether there is a relationship between the loss of visual function in human glaucoma and resting blood perfusion within primary visual cortex (V1).

METHODS

Arterial spin labeling (ASL) functional magnetic resonance imaging (fMRI) was conducted in 10 participants with POAG. Resting cerebral blood flow (CBF) was measured from dorsal and ventral V1. Behavioral measurements of visual function were obtained using standard automated perimetry (SAP), short-wavelength automated perimetry (SWAP), and frequency-doubling technology perimetry (FDT). Measurements of CBF were compared to differences in visual function for the superior and inferior hemifield.

RESULTS

Differences in CBF between ventral and dorsal V1 were correlated with differences in visual function for the superior versus inferior visual field. A statistical bootstrapping analysis indicated that the observed correlations between fMRI responses and measurements of visual function for SAP (r=0.49), SWAP (r=0.63), and FDT (r=0.43) were statistically significant (all p<0.05).

CONCLUSIONS

Resting blood perfusion in human V1 is correlated with the loss of visual function in POAG. Altered CBF may be a contributing factor to glaucomatous optic neuropathy, or it may be an indication of post-retinal glaucomatous neurodegeneration caused by damage to the retinal ganglion cells.

Glaucoma alters the expression of NGF and NGF receptors in visual cortex and geniculate nucleus of rats: effect of eye NGF application

We investigated the effect of glaucoma (GL) on nerve growth factor (NGF) presence in two brain visual areas. Rats with elevated intraocular pressure (EIOP), induced by hypertonic saline injection in the episcleral vein, were treated with eye topical application of saline or NGF. Rats were subsequently sacrificed, and brain tissues were used for immunohistochemical, biochemical, and molecular analyses. We found that GL alters the basal level of NGF and NGF receptors in brain visual centers and that NGF eye application normalized these deficits. These findings demonstrate that the reduced presence of NGF can arise due to degenerative events in retinal and brain visual areas.

Changes in visual fields and lateral geniculate nucleus in monkey laser-induced high intraocular pressure model

Monkey eyes are useful for ophthalmologic research into eye diseases because their histological and functional properties are very similar to those of humans. The monkey laser-induced high intraocular pressure (IOP) model is a common model for ophthalmologic research, especially into glaucoma. Although several studies using this model have focused on changes in visual field, retinal ganglion cells (RGC), and lateral geniculate nucleus (LGN), clear relationships among these changes in one and the same monkey have not been established. We therefore examined visual field changes, RGC and LGN numbers, and glial fibrous acidic protein (GFAP) immunohistochemistry in the LGN in each of two monkeys. Visual field sensitivity, RGC number, and neuronal density of LGN were all decreased by high IOP. The relationship between loss of RGC and decrease in visual field sensitivity depended on the eccentricity from the fovea. Moreover, LGN immunohistochemistry revealed greater increases in GFAP expression in the layers receiving a neuronal input from the high IOP eye than in those receiving a neuronal input from the contralateral untreated eye. From these results, we suggest that glaucoma may lead to changes in glial function not only in the retina, but also in the visual pathway, and that such central nervous system changes may be a hallmark of neuropathy in glaucoma, as in other neurodegenerative diseases.

What changes can we expect in the brain of glaucoma patients?

Elevated intraocular pressure in glaucoma can injure retinal ganglion cells and trigger the spread of disease to connected target vision structures of the brain. Glaucomatous degeneration has been observed in retrobulbar and intracranial optic nerve, lateral geniculate nucleus, and visual cortex of the brain. Oxidative damage and glutamate toxicity are implicated in transsynaptic central nervous system injury in glaucoma, similar to other neurodegenerative diseases. The perception of glaucoma as a disorder of "visual neurons" within the eye and brain may contribute to understanding progressive disease, and encourage comprehensive treatment strategies to prevent vision loss in glaucoma.

Glaucoma of the brain: a disease model for the study of transsynaptic neural degeneration

The identification of mechanisms precipitating neuronal death and injury is an intense area of investigation requiring reliable models to assess the effects of neuroprotective agents. Most are suboptimal since the effects of initial damage are diffuse and may not be reproducible or easily quantifiable. The ideal laboratory model should have the ability to (a) clearly detect evidence of neuronal injury and recovery, (b) accurately measure morphologically the extent of these changes, and (c) provide functional evidence for damage and recovery. Glaucoma is a disease of visual neurons in the eye and brain. In the visual system, neuroanatomical pathways and retinotopic organization are exquisitely defined, functional modalities are highly characterized and can be dissected physiologically, visual input parameters can be modified, visual functional output can be readily tested and measured, changes in the eye and the visual brain can be directly visualized and imaged, and pathological and compensatory changes in brain centers of vision can be examined and measured specifically. For these reasons, the glaucoma disease model is ideal for the study of response and recovery to injury in the central nervous system due to anterograde and retrograde degeneration from the eye to the brain and the brain to the eye, respectively. The study of this glaucoma model of transsynaptic brain injury may be relevant to understanding more complex pathways and point to new strategies to prevent disease progression in other neurodegenerative diseases.

[Consequences of glaucoma on circadian and central visual systems]

Glaucoma is a chronic optic neuropathy leading to a degeneration of retinal ganglion cells. There is accumulating evidence that glaucomatous damage extends from retinal ganglion cells to vision centers in the brain. Degenerative changes are observed in magnocellular, parvocellular, and koniocellular pathways in the lateral geniculate nucleus, and these changes are related to intraocular pressure and the severity of optic nerve damage. In addition, recent studies show that there are also changes in the visual cortex in relation to varying degrees of retinal ganglion cell loss. In a rat model of glaucoma, we have recently demonstrated a reduction of retinal projections of retinal ganglion cells, not only on the visual system but also on the suprachiasmatic nucleus. Human studies suggest that the ganglion cell degeneration caused by glaucoma could lead to a lesion of the retinohypothalamic tract, which permits the synchronization of circadian rhythms.

Glaucoma as a neurodegenerative disease

PURPOSE OF REVIEW

Glaucoma is a leading cause of irreversible world vision loss characterized by progressive retinal ganglion cell death. Elevated eye pressure is a major risk factor for glaucoma; however, despite effective medical and surgical therapies to reduce intraocular pressure, progressive vision loss among glaucoma patients is common. These observations suggest that mechanisms independent of intraocular pressure are also implicated in glaucomatous degeneration. Numerous similarities exist between glaucoma and neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Similarities include the selective loss of neuron populations, transsynaptic degeneration in which disease spreads from injured neurons to connected neurons, and common mechanisms of cell injury and death.

RECENT FINDINGS

Glaucomatous injury to retinal ganglion cells has profound effects on target vision structures within the brain, including the lateral geniculate nucleus and visual cortex in experimental primate and human glaucoma. Mechanisms involved in central visual system damage in glaucoma include oxidative injury and glutamate toxicity, as seen in neurodegenerative diseases.

SUMMARY

Glaucoma as a neurodegenerative disease is a valid working hypothesis to understand neural injury in the visual system. This paradigm may stimulate the discovery of innovative intraocular pressure-independent strategies to help prevent loss of vision in glaucoma patients.

Retinotopic organization of primary visual cortex in glaucoma: Comparing fMRI measurements of cortical function with visual field loss

Primary open angle glaucoma (POAG) is a progressive optic neuropathy characterized by retinal ganglion cell loss. Experimental primate glaucoma indicates neuronal degeneration of the lateral geniculate nucleus (LGN) and activity changes in the visual cortex (V1). Neuronal degeneration has also been shown in a post-mortem human study of the optic nerve, LGN and visual cortex. Functional magnetic resonance imaging (fMRI), a non-invasive means of inferring function-specific neuronal activity, provides an opportunity to evaluate glaucomatous changes in neuronal activity throughout the visual pathway in vivo. The purpose of this study is to demonstrate that the relationship between visual field loss in human POAG and the functional organization of V1 can be measured using novel fMRI analysis methods. Visual field defects were measured using standard automated perimetry (SAP). A retinotopic map of visual space was obtained for V1, and the retinotopy data was fit with a template. The template was used to project regions within the visual field onto a flattened representation of V1. Viewing through the glaucomatous vs. fellow eye was compared by alternately presenting each eye with a scotoma-mapping stimulus. The resulting blood oxygen level dependent (BOLD) fMRI response was compared to interocular differences in thresholds for corresponding regions of the visual field. The spatial pattern of activity observed in the flattened representation agreed with the pattern of visual field loss. Furthermore, the amplitude of the BOLD response was correlated on a pointwise basis with the difference in sensitivity thresholds between the glaucomatous and fellow eyes (r = 0.53, p < 0.0001). The BOLD signal in human V1 is altered for POAG patients in a manner consistent with the loss of visual function. FMRI of visual brain areas is a potential means for quantifying glaucomatous changes in neuronal activity. This should enhance our understanding of glaucoma, and could lead to new diagnostic techniques and therapies.

Defining the nature of motion perception deficits in glaucoma using simple and complex motion stimuli

PURPOSE

The purpose of this study is to determine the nature of motion perception deficits in primary open-angle glaucoma by measuring the sensitivity of simple (luminance-defined) and complex (texture-defined) motion, the latter requiring supplementary neural processing to be resolved. These findings will help address the possible extent of the cortical damage in glaucoma that has been recently demonstrated by anatomic and physiological studies. They also serve the purpose of establishing which motion paradigms would be most appropriate for assessing glaucoma-related functional loss.

METHODS

Direction-identification thresholds for first-order and second-order motion were measured for 26 patients with primary open-angle glaucoma (for both phakic and pseudophakic) and 18 nonglaucomatous observers.

RESULTS

The glaucomatous observers showed significantly increased motion thresholds for both first- and second-order motion conditions when compared with nonglaucomatous observers. However, the relative increase in threshold for first-order motion did not differ significantly from that of second-order motion.

CONCLUSIONS

These findings imply that there is no measurable higher-level cortical function damage caused by the glaucomatous process because no greater loss in second-order motion was observed. Based on the results, we suggest that motion paradigms used to assess functional loss in primary open-angle glaucoma should consist of simple, first-order type stimuli to minimize potential confounds such as those introduced by both the normal and pathologic aging process on complex motion processing (i.e., perimetry using complex motion stimuli).

Visual-Evoked Response, Pattern Electroretinogram, and Psychophysical Magnocellular Thresholds in Glaucoma, Optic Atrophy, and Dyslexia

PURPOSE

The purpose of this study is to compare visual-evoked response (VEP) pattern electroretinogram (PERG) and psychophysical thresholds to the same stimulus, designed to be optimal for the magnocellular system, in suspects and patients with early glaucoma, patients with optic nerve disease, dyslexic children, and age-matched controls.

METHODS

Stimuli were low spatial frequency sinusoidal luminance profile gratings abruptly phase reversing at 7.14 Hz. Electrophysiological recordings were made at 50%, 30%, 20%, 10%, and 5% contrast. Threshold was the lowest contrast evoking a clear response at the stimulus frequency. Three independent judges scored the traces. Psychophysical thresholds were obtained by ascending and descending method of limits. VEPs and PERGs to International Society for Clinical Electrophysiology of Vision (ISCEV) standards and to increasing spatial frequencies were obtained as parvocellular specific controls. Patients were diagnosed independently by the referring professionals.

RESULTS

Parvocellular-specific responses were normal, except in cases with explicable visual acuity loss. The judges scores correlated highly (> 0.9). VEPs and PERGs correlated highly and each correlated less well with psychophysics in normals, glaucoma, and dyslexia but the opposite occurred in optic nerve disease. VEPs had the lowest normal values and least variance (all adults < 5%, children < 10%, PERGs < 20%). In glaucoma, VEP magnocellular deficits occurred in 85% of recently diagnosed positive cases, 48% of high-risk suspects, 39% of low-risk suspects, and ocular hypertensives. Approximately 28% of dyslexics had VEP magnocellular deficits. PERG losses were less frequent. There was a clear dichotomy and low correlations between psychophysics and electrophysiology both within and between groups. Psychophysical threshold elevations were absent in all glaucoma groups, often large in optic atrophy and small (2.5%) but highly significant in dyslexia.

CONCLUSION

Contrast thresholds to magnocellular-specific stimuli are consistent in cortex and retina. VEPs are more reliable. Psychophysics seems to tap different mechanisms. VEPs are very sensitive to early glaucoma. The lack of VEP loss in dyslexia suggests the other losses are artifactual. Further research is needed to see if stimuli even more like the frequency-doubling technology are more useful clinically.

Repeat sample intraocular pressure variance in induced and naturally ocular hypertensive monkeys

PURPOSE

To compare repeat-sample means variance of laser induced ocular hypertension (OH) in rhesus monkeys with the repeat-sample mean variance of natural OH in age-range matched monkeys of similar and dissimilar pedigrees.

MATERIALS & METHODS

Multiple monocular, retrospective, intraocular pressure (IOP) measures were recorded repeatedly during a short sampling interval (SSI, 1-5 months) and a long sampling interval (LSI, 6-36 months). There were 5-13 eyes in each SSI and LSI subgroup. Each interval contained subgroups from the Florida with natural hypertension (NHT), induced hypertension (IHT1) Florida monkeys, unrelated (Strasbourg, France) induced hypertensives (IHT2), and Florida age-range matched controls (C). Repeat-sample individual variance means and related IOPs were analyzed by a parametric analysis of variance (ANOV) and results compared to non-parametric Kruskal-Wallis ANOV.

RESULTS

As designed, all group intraocular pressure distributions were significantly different (P < or = 0.009) except for the two (Florida/Strasbourg) induced OH groups. A parametric 2 x 4 design ANOV for mean variance showed large significant effects due to treatment group and sampling interval. Similar results were produced by the nonparametric ANOV. Induced OH sample variance (LSI) was 43x the natural OH sample variance-mean. The same relationship for the SSI was 12x.

CONCLUSION

Laser induced ocular hypertension in rhesus monkeys produces large IOP repeat-sample variance mean results compared to controls and natural OH.

Autoimmunity against neurofilament protein and its possible association with HLA-DRB1*1502 allele in glaucoma

Glaucoma is understood as a neurodegenerative disease and intraocular pressure has been regarded as the major risk factors for the optic nerve damages. However, recent studies suggested that several risk factors including autoimmunity are also shown to play important roles in glaucoma. To identify the retinal antigen in glaucoma, we used the serological analysis of recombinant cDNA expression libraries (SEREX) approach and quantified IgG antibodies directed against the identified antigens in an ELISA. We identified neurofilament protein and the prevalence of anti-bovine neurofilament light subunit (NF-L) autoantibodies in glaucomatous patients was significantly higher than in healthy controls and patients with other uveitic and optic nerve diseases (P<0.05). In addition, our immunogenetic analysis showed a possible association between HLA-DRB1*1502 allele and the patients positive for anti-NF-L autoantibodies. It suggests that the HLA class II-linked gene may be involved in development of autoimmunity in patients with glaucoma.

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.

Oxidative injury by peroxynitrite in neural and vascular tissue of the lateral geniculate nucleus in experimental glaucoma

In glaucoma, recent studies show that neural degeneration extends beyond the retinal ganglion cells to include target neurons in the lateral geniculate nucleus of the brain. The pathobiology of LGN degeneration in glaucoma is as yet unknown. We investigated whether peroxynitrite-mediated oxidative stress plays a role in glaucomatous degeneration of the LGN. Nitrotyrosine (NT), a marker for peroxynitrite-mediated oxidative injury, was studied in right LGN sections from monkeys with experimental unilateral glaucoma in the right eye and from normal controls. Immunoreactivity for NT was analyzed using bright-field microscopy. The density of NT profiles localized in neural tissue was determined for LGN layers (2,3,5) connected to the glaucoma eye and LGN layers (1,4,6) connected to the non-glaucoma eye. Density was calculated for each LGN layer by dividing the number of NT profiles by the cross-sectional area of each LGN layer. Blood vessels in each LGN were examined for NT formation. NT formation was detected in LGN layers of all monkeys with glaucoma. Quantitative analysis revealed that compared to controls, the density of NT profiles was increased in monkeys with glaucoma in LGN layers connected to glaucoma and non-glaucoma eyes. The mean density of NT profiles (+/-SEM) in neural tissue was significantly increased in glaucoma LGN layers compared to those of controls (2.30+/-0.56 vs. 0.29+/-0.12; P=0.016). Nitrotyrosine was readily apparent in LGN blood vessel endothelium in glaucoma, and not detected in blood vessels of control LGNs. The presence of NT in neural and vascular tissue of the glaucomatous LGN implicates peroxynitrite-mediated oxidative cell injury in the pathobiology of central neural degeneration in glaucoma.

Neurofilament protein expression in the geniculostriate pathway of a New World monkey ( Callithrix jacchus)

We examined the expression profile of non-phosphorylated neurofilament protein in the dorsal lateral geniculate nucleus (LGN) and striate cortex (V1) of a New World simian, the marmoset monkey, using the monoclonal antibody SMI-32. The overall distribution of neurofilament protein in the marmoset resembled that previously described in Old World monkeys. While immunostained neurones were observed throughout the LGN, there were clear laminar differences in terms of both cellular and neuropil labelling. Neurones in the magnocellular layer cells stained more densely than those in the parvocellular layers. The marmoset's well-defined koniocellular layers showed an overall light stain of both neurones and neuropil. In V1, densely stained pyramidal cells and heavy neuropil label were observed in the two sublayers that send projections to the middle temporal area (MT): a supragranular band located in layer 3C (Brodmann's layer 4B) and an infragranular band located near the top of layer 6. More lightly stained, small pyramidal cells were also found in layer 3Balpha. Accordingly, in both New World and Old World monkeys the expression of neurofilament protein is correlated with specific functional subdivisions of the geniculocortical pathway. In particular, projection neurones associated with fast-conducting pathways to the extrastriate 'dorsal stream' appear to contain higher levels of this protein.

Brain changes in glaucoma

There is evidence that glaucomatous damage extends from retinal ganglion cells to vision centers in the brain. In the lateral geniculate nucleus (LGN), the major relay center between the eye and the visual cortex, neurons should undergo degenerative and/or neurochemical changes in magno-, parvo-, and koniocellular pathways conveying motion, red-green, and blue-yellow information, respectively. Furthermore, in both the LGN and visual cortex in glaucoma, changes in metabolic activity are observed. The study of brain changes in glaucoma may provide new insights into the pathobiology of glaucomatous damage and disease progression, and may stimulate new detection and therapeutic strategies to prevent blindness.

Motion perception in glaucoma patients: a review

Most of the histopathological and psychophysical studies in glaucoma reveal a preferential damage to the magnocellular (M) pathway although a few of them support a damage to the parvocellular (P) pathway as well. In glaucoma, the visual fields are usually evaluated by conventional perimetry. However, it has been demonstrated that 20-40% of ganglion cells are lost before field defects are detected using conventional perimetry. Therefore, new psychophysical tests have recently been designed in order to specifically isolate and evaluate the visual mechanisms that are impaired at the early stages of glaucoma. In this context, several authors have addressed the issue of motion perception under the hypothesis of a predominant damage of the M pathway in glaucoma, and that motion perception is mediated mainly by M pathway. The results of these studies depict a large variation in the percentage of patients showing anomalous motion perception. Overall, motion thresholds are elevated in both glaucoma and ocular hypertensive patients as compared to control subjects, irrespective of the stimulus size and eccentricity. The test which discriminates best between patients and normal subjects is motion perimetry. The visual field defects in glaucoma patients identified by conventional perimetry and motion perimetry are similar, but the sizes of the defects are usually larger with motion perimetry. However, motion tests in central vision have no correlation with visual field defect on conventional perimetry. In glaucoma, loss of performance on motion perception tests does not necessarily support the existence of a specific deficit in the M pathway, because some behavioral studies suggest that the P pathway can also mediate motion perception. It is also difficult to conclude that motion perception is specifically affected in glaucoma because most of these studies do not yield a comparison with other visual functions. Despite these difficulties, localized motion perception tests at eccentricities of more than 15 degrees can be considered as a promising diagnostic tool.

Neurofilament triplet proteins are restricted to a subset of neurons in the rat neocortex

The cellular localisation of neurofilament triplet subunits was investigated in the rat neocortex. A subset of mainly pyramidal neurons showed colocalisation of subunit immunolabelling throughout the neocortex, including labelling with the antibody SMI32, which has been used extensively in other studies of the primate cortex as a selective cellular marker. Neurofilament-labelled neurons were principally localised to two or three cell layers in most cortical regions, but dramatically reduced labelling was present in areas such as the perirhinal cortex, anterior cingulate and a strip of cortex extending from caudal motor regions through the medial parietal region to secondary visual areas. However, quantitative analysis demonstrated a similar proportion (10-20%) of cells with neurofilament triplet labelling in regions of high or low labelling. Combining retrograde tracing with immunolabelling showed that cellular content of the neurofilament proteins was not correlated with the length of projection. Double labelling immunohistochemistry demonstrated that neurofilament content in axons was closely associated with myelination. Analysis of SMI32 labelling in development indicated that content of this epitope within cell bodies was associated with relatively late maturation, between postnatal days 14 and 21. This study is further evidence of a cell type-specific regulation of neurofilament proteins within neocortical neurons. Neurofilament triplet content may be more closely related to the degree of myelination, rather than the absolute length, of the projecting axon.

Visual field defects and neural losses from experimental glaucoma

Glaucoma is a relatively common disease in which the death of retinal ganglion cells causes a progressive loss of sight, often leading to blindness. Typically, the degree of a patient's visual dysfunction is assessed by clinical perimetry, involving subjective measurements of light-sense thresholds across the visual field, but the relationship between visual and neural losses is inexact. Therefore, to better understand of the effects of glaucoma on the visual system, a series of investigations involving psychophysics, electrophysiology, anatomy, and histochemistry were conducted on experimental glaucoma in monkeys. The principal results of the studies showed that, (1) the depth of visual defects with standard clinical perimetry are predicted by a loss of probability summation among retinal detection mechanisms, (2) glaucomatous optic atrophy causes a non-selective reduction of metabolism of neurons in the afferent visual pathway, and (3) objective electrophysiological methods can be as sensitive as standard clinical perimetry in assessing the neural losses from glaucoma. These experimental findings from glaucoma in monkeys provide fundamental data that should be applicable to improving methods for assessing glaucomatous optic neuropathy in patients.

Nitric oxide, microglial activities and neuronal cell death in the lateral geniculate nucleus of glaucomatous rats

The present study was initiated to investigate neuronal degeneration, microglial reactivity and possible roles of NO in the lateral geniculate nucleus (LGN) of glaucomatous rats. An experimental one-eye glaucoma model was created by cauterization of the limbal-derived veins. Neuronal cell viability was studied by immunostaining with antibody against neuronal nuclei. Changes of expressions of nitric oxide synthase I (NOS I), NOS II, ED 1, OX6 and OX42 in the LGN were studied by immunohistochemistry. NADPH-d histochemistry was also employed. In the experimental glaucomatous rats, the number of NeuN labelled neurons was significantly decreased in both the ipsi- and contra-lateral sides of the ventral LGN (vLGN) but not the dorsal LGN (dLGN) at 1 month post-operation and beyond. Expressions of NOS I and NADPH-d were notably increased from 1 week post-operation in the ipsilateral vLGN. In the contralateral side of the vLGN, however, this change was only observed from 1 month post-operation. No NOS II immunoreaction was observed in LGN of both the normal control and glaucomatous rats. Increased microglial reactivity as indicated by OX-42 immunoreactivity was first observed in both sides of the LGN at 1 week post-operation, and this was most significant especially at 1 and 2 months post-operation. The present results suggest that NO and microglial cells may play some important roles in the pathologic processes of neuronal degeneration in the LGN of glaucomatous rats.

A neuronal model of attentional spotlight: parietal guiding the temporal

Recent studies have reported an attentional feedback that highlights neural responses as early along the visual pathway as the primary visual cortex. Such filtering would help in reducing informational overload and in performing serial visual search by directing attention to individual locations in the visual field. The magnocellular (M) and parvocellular (P) subdivisions are two of the major parallel pathways in primate vision that originate in the retina and carry distinctly different types of information. The M pathway, characterized by its high sensitivity to movement and to low contrast stimuli, forms the predominant visual input into the dorsal, parietal stream in the neocortex. The P inputs, characterized by their colour selectivity and higher spatial resolution, are channeled mainly into the ventral, temporal stream. It is proposed that the attentional spotlight originates in the dorsal stream and helps in serially searching the field for conjunction of the relevant target features in the temporal stream, effectively performing a gating function on all visual inputs. This model predicts that a defect limited to the magnocellular or the dorsal pathway can lead to widespread deficits in cognitive abilities, including those functions that are largely based on parvocellular information. For example, the model provides a neural mechanism linking a peripheral defect in the magnocellular pathway to the reading disabilities in dyslexia. Even though there has been strong evidence for a magnocellular deficit in dyslexia, the paradox has been that the cognitive disability seems to be related to P pathway function. The scheme proposed here shows how M input may be vital for controlling sequential attention during reading.