Head-up tilt lowers IOP and improves RGC dysfunction in glaucomatous DBA/2J mice

Addressing neurodegeneration in glaucoma: Mechanisms, challenges, and treatments

Glaucoma is the leading cause of irreversible blindness globally. The disease causes vision loss due to neurodegeneration of the retinal ganglion cell (RGC) projection to the brain through the optic nerve. Glaucoma is associated with sensitivity to intraocular pressure (IOP). Thus, mainstay treatments seek to manage IOP, though many patients continue to lose vision. To address neurodegeneration directly, numerous preclinical studies seek to develop protective or reparative therapies that act independently of IOP. These include growth factors, compounds targeting metabolism, anti-inflammatory and antioxidant agents, and neuromodulators. Despite success in experimental models, many of these approaches fail to translate into clinical benefits. Several factors contribute to this challenge. Firstly, the anatomic structure of the optic nerve head differs between rodents, nonhuman primates, and humans. Additionally, animal models do not replicate the complex glaucoma pathophysiology in humans. Therefore, to enhance the success of translating these findings, we propose two approaches. First, thorough evaluation of experimental targets in multiple animal models, including nonhuman primates, should precede clinical trials. Second, we advocate for combination therapy, which involves using multiple agents simultaneously, especially in the early and potentially reversible stages of the disease. These strategies aim to increase the chances of successful neuroprotective treatment for glaucoma.

Mitochondrial Dysfunction in Primary Open-angle Glaucoma Characterized by Flavoprotein Fluorescence at the Optic Nerve Head

OBJECTIVE

To investigate the presence of flavoprotein fluorescence (FPF) at the optic nerve head (ONH) rim as a marker of mitochondrial dysfunction in primary open-angle glaucoma (POAG) and control eyes.

DESIGN

Retrospective cross-sectional study, with patients recruited from the New York Eye and Ear Infirmary of Mount Sinai.

SUBJECTS, PARTICIPANTS, AND/OR CONTROLS

A total of 86 eyes (50 eyes of 30 POAG patents and 36 eyes of 20 controls) were enrolled. The presence of POAG was defined by circumpapillary retinal nerve fiber layer thickness below the bottom fifth percentile of the normative database, glaucomatous ONH changes, and visual field defects on 24-2 tests.

METHODS, INTERVENTION, OR TESTING

POAG and control eyes were imaged using the OcuMet Beacon. A 23°x23° infrared scan was obtained, and an FPF scan was performed within a capture field spanning 13 degrees in diameter. The ONH margins on the infrared image were identified by software algorithms. FPF then was measured within an elliptical annulus around the ONH rim, with the inner and outer boundaries corresponding to 0.5 to 1.1 times the ONH rim size.

MAIN OUTCOMES MEASURES

FPF at the OHN rim in POAG and control eyes.

RESULTS

Differences in FPF between POAG and control eyes were characterized through mixed-effects logistic regression, adjusted for age and interocular pressure. FPF was significantly higher in POAG versus control eyes, with a mean±SD of 46.4±27.9 versus 28.0±11.7 (P<0.001), respectively. Evaluation of anatomical quadrants revealed greater FPF in POAG versus control eyes at the temporal (P=0.001), superior (P<0.001), nasal (P=0.002), and inferior (P=0.001) quadrants. Among POAG eyes, FPF showed correlation to visual field mean deviation (P<0.001), visual field pattern standard deviation (P=0.003), and circumpapillary retinal nerve fiber thickness (P=0.001) on linear mixed-effects models.

CONCLUSIONS

Higher FPF in POAG versus control eyes suggests the presence of mitochondrial dysfunction at the ONH rim in eyes with glaucomatous damage. The degree of FPF corresponds to disease severity, as measured by visual field and nerve fiber layer thickness metrics. FPF may thus represent a metabolic indicator of disease status that reveals the extent of injury in glaucoma.

Short-Term Changes in the Photopic Negative Response Following Intraocular Pressure Lowering in Glaucoma

Purpose

To evaluate the short-term changes in inner retinal function using the photopic negative response (PhNR) after intraocular pressure (IOP) reduction in glaucoma.

Methods

Forty-seven participants with glaucoma who were commencing a new or additional IOP-lowering therapy (treatment group) and 39 participants with stable glaucoma (control group) were recruited. IOP, visual field, retinal nerve fiber layer thickness, and electroretinograms (ERGs) were recorded at baseline and at a follow-up visit (3 ± 2 months). An optimized protocol developed for a portable ERG device was used to record the PhNR. The PhNR saturated amplitude (V_max), V_max ratio, semi-saturation constant (K), and slope of the Naka–Rushton function were analyzed.

Results

A significant percentage reduction in IOP was observed in the treatment group (28 ± 3%) compared to the control group (2 ± 3%; P < 0.0001). For PhNR V_max, there was no significant interaction (F_1,83 = 2.099, P = 0.15), but there was a significant difference between the two time points (F_1,83 = 5.689, P = 0.019). Post hoc analysis showed a significant difference between baseline and 3 months in the treatment group (mean difference, 1.23 µV; 95% confidence interval [CI], 0.24–2.22) but not in the control group (0.30 µV; 95% CI, 0.78–1.38). K and slope were not significantly different in either group. Improvement beyond test–retest variability was seen in 17% of participants in the treatment group compared to 3% in the control group (P = 0.007, χ² test).

Conclusions

The optimized protocol for measuring the PhNR detected short-term improvements in a proportion of participants following IOP reduction, although the majority showed no change.

A subacute model of glaucoma based on limbal plexus cautery in pigmented rats

Glaucoma is a neurodegenerative disorder characterized by the progressive functional impairment and degeneration of the retinal ganglion cells (RGCs) and their axons, and is the leading cause of irreversible blindness worldwide. Current management of glaucoma is based on reduction of high intraocular pressure (IOP), one of its most consistent risk factors, but the disease proceeds in almost half of the patients despite such treatments. Several experimental models of glaucoma have been developed in rodents, most of which present shortcomings such as high surgical invasiveness, slow learning curves, damage to the transparency of the optic media which prevents adequate functional assessment, and variable results. Here we describe a novel and simple method to induce ocular hypertension in pigmented rats, based on low-temperature cauterization of the whole circumference of the limbal vascular plexus, a major component of aqueous humor drainage and easily accessible for surgical procedures. This simple, low-cost and efficient method produced a reproducible subacute ocular hypertension with full clinical recovery, followed by a steady loss of retinal ganglion cells and optic axons, accompanied by functional changes detected both by electrophysiological and behavioral methods.

DBA/2J mouse model for experimental glaucoma: pitfalls and problems

BACKGROUND

The DBA/2J mouse has been described as a model for congenital experimental glaucoma. It develops anterior segment anomalies with synechiae and pigment dispersion leading to raised intraocular pressure and glaucomatous damage. However, there are serious practical considerations when using this model in longitudinal studies.

METHODS

We followed 118 mice from 12-48 weeks of age in a pharmaceutical trial. Here we report on the findings in control animals (n = 37). Intraocular pressure was measured weekly, electrophysiology and optical coherence tomography every 6 weeks. A subset also had invasive intraocular pressure measurements performed prior to euthanasia.

RESULTS

Although intraocular pressure eventually rose by 9 months in most animals, tonometry was complicated by corneal calcification in the majority of animals rendering intraocular pressure measurement unreliable. Invasive intraocular pressure did not correlate with non-invasive measures. Loss of scotopic threshold response and thinning of inner retinal layers on optical coherence tomography was observed over time, suggesting glaucomatous damage, but this occurred in some animals without raised intraocular pressure. Poor pupil dilation significantly affected electrophysiology, optical coherence tomography and fundus imaging; 22% of animals developed major systemic complications leading to high dropout rate.

CONCLUSIONS

The DBA/2J experimental glaucoma model shows variability in expression, and its pathological changes cause major difficulties in assessing disease progression. From our experience, the model presents significant challenges for drug studies in glaucoma, as there are many confounding factors: difficulty with accurate intraocular pressure measurement, in vivo imaging, and electrophysiology recording and a high dropout rate. In addition, there may be an underlying neurodegenerative process independent of intraocular pressure.

Evaluating retinal ganglion cell loss and dysfunction

Retinal ganglion cells (RGC) bear the sole responsibility of propagating visual stimuli to the brain. Their axons, which make up the optic nerve, project from the retina to the brain through the lamina cribrosa and in rodents, decussate almost entirely at the optic chiasm before synapsing at the superior colliculus. For many traumatic and degenerative ocular conditions, the dysfunction and/or loss of RGC is the primary determinant of visual loss and are the measurable endpoints in current research into experimental therapies. To actually measure these endpoints in rodent models, techniques must ascertain both the quantity of surviving RGC and their functional capacity. Quantification techniques include phenotypic markers of RGC, retrogradely transported fluorophores and morphological measurements of retinal thickness whereas functional assessments include electroretinography (flash and pattern) and visual evoked potential. The importance of the accuracy and reliability of these techniques cannot be understated, nor can the relationship between RGC death and dysfunction. The existence of up to 30 types of RGC complicates the measuring process, particularly as these may respond differently to disease and treatment. Since the above techniques may selectively identify and ignore particular subpopulations, their appropriateness as measures of RGC survival and function may be further limited. This review discusses the above techniques in the context of their subtype specificity.

Brain derived neurotrophic factor keeps pattern electroretinogram from dropping after superior colliculus lesion in mice

AIM

To determine if brain-derived neurotrophic factor (BDNF) could offer protention to retinal ganglion cells following a superior colliculus (SC) lesion in mice using pattern electroretinogram (PERG) and optical coherence tomography (OCT) as a measures of ganglion cell response and retinal health.

METHODS

Seven C57BL/6J mice with BDNF protection were tested with PERG and OCT before and after SC lesions.

RESULTS

Compared with baseline PERG, the amplitude of PERG decreased 11.7% after SC lesions, but not significantly (P>0.05). Through fast Fourier transform (FFT) analysis of the PERGs before and after SC lesions, it was found that dominant frequency of PERGs stayed unchanged, suggesting that the ganglion cells of the retina remained relatively healthy inspite of damage to the ends of the ganglion cell axons. Also, OCT showed no changes in retinal thickness after lesions.

CONCLUSION

It was concluded that BDNF is essential component of normal retinal and helps retina keeping normal function. While retina lack of BDNF, ex vivo resource of BDNF provides protection to the sick retina. It implies that BDNF is a kind therapeutic neurotrophic factor to retina neurodegeneration diseases, such as glaucoma, age related macular degeneration.

Frequency spectrum might act as communication code between retina and visual cortex I

AIM

To explore changes and possible communication relationship of local potential signals recorded simultaneously from retina and visual cortex I (V1).

METHODS

Fourteen C57BL/6J mice were measured with pattern electroretinogram (PERG) and pattern visually evoked potential (PVEP) and fast Fourier transform has been used to analyze the frequency components of those signals.

RESULTS

The amplitude of PERG and PVEP was measured at about 36.7 µV and 112.5 µV respectively and the dominant frequency of PERG and PVEP, however, stay unchanged and both signals do not have second, or otherwise, harmonic generation.

CONCLUSION

The results suggested that retina encodes visual information in the way of frequency spectrum and then transfers it to primary visual cortex. The primary visual cortex accepts and deciphers the input visual information coded from retina. Frequency spectrum may act as communication code between retina and V1.

Integrative properties of retinal ganglion cell electrical responsiveness depend on neurotrophic support and genotype in the mouse

Early stages of glaucoma and optic neuropathies are thought to show inner retina remodeling and functional changes of retinal ganglion cells (RGCs) before they die. To assess RGC functional plasticity, we investigated the contrast-gain control properties of the pattern electroretinogram (PERG), a sensitive measure of RGC function, as an index of spatio-temporal integration occurring in the inner retina circuitry subserving PERG generators. We studied the integrative properties of the PERG in mice exposed to different conditions of neurotrophic support. We also investigated the effect of genotypic differences among mouse strains with different susceptibility to glaucoma (C57BL/6J, DBA/2J, DBA/2.Gpnmb(+)). Results show that the integrative properties of the PERG recorded in the standard C57BL/6J inbred mouse strain are impaired after deficit of neurotrophic support and partially restored after exogenous neurotrophic administration. Changes in PERG amplitude, latency, and contrast-dependent responses differ between mouse strains with different susceptibility to glaucoma. Results represent a proof of concept that the PERG could be used as a tool for in-vivo monitoring of RGC functional plasticity before RGC death, the effect of neuroactive treatments, as well as for high-throughput tool for phenotypic screening of different mouse genotypes.

Electrophysiological assessment of retinal ganglion cell function

The function of retinal ganglion cells (RGCs) can be non-invasively assessed in experimental and genetic models of glaucoma by means of variants of the ERG technique that emphasize the activity of inner retina neurons. The best understood technique is the Pattern Electroretinogram (PERG) in response to contrast-reversing gratings or checkerboards, which selectively depends on the presence of functional RGCs. In glaucoma models, the PERG can be altered before histological loss of RGCs; PERG alterations may be either reversed with moderate IOP lowering or exacerbated with moderate IOP elevation. Under particular luminance-stimulus conditions, the Flash-ERG displays components that may reflect electrical activity originating in the proximal retina and be altered in some experimental glaucoma models (positive Scotopic Threshold response, pSTR; negative Scotopic Threshold Response, nSTR; Photopic Negative Response, PhNR; Oscillatory Potentials, OPs; multifocal ERG, mfERG). It is not yet known which of these components is most sensitive to glaucomatous damage. Electrophysiological assessment of RGC function appears to be a necessary outcome measure in experimental glaucoma models, which complements structural assessment and may even predict it. Neuroprotective strategies could be tested based on enhancement of baseline electrophysiological function that results in improved RGC survival. The use of electrophysiology in glaucoma models may be facilitated by specifically designed instruments that allow high throughput, robust assessment of electrophysiological function.

Non-continuous measurement of intraocular pressure in laboratory animals

Glaucoma is a leading cause of blindness, which is treatable but currently incurable. Numerous animal models therefore have both been and continue to be utilized in the study of numerous aspects of this condition. One important facet associated with the use of such models is the ability to accurately and reproducibly measure (by cannulation) or estimate (by tonometry) intraocular pressure (IOP). At this juncture there are several different approaches to IOP measurement in different experimental animal species, and the list continues to grow. We feel therefore that a review of this subject matter is timely and should prove useful to others who wish to perform similar measurements. The general principles underlying various types of tonometric and non-tonometric techniques for non-continuous determination of IOP are considered. There follows discussion of specific details as to how these techniques are applied to experimental animal species involved in the research of this disease. Specific comments regarding anesthesia, circadian rhythm, and animal handling are also included, especially in the case of rodents. Brief consideration is also given to possible future developments.

Transgenic mice expressing mutated Tyr437His human myocilin develop progressive loss of retinal ganglion cell electrical responsiveness and axonopathy with normal iop

PURPOSE

To characterize age-related changes of retinal ganglion cell (RGC) function, IOP, and anatomical markers of axon/glia integrity in a transgenic mouse expressing Tyr437His mutant of human myocilin protein.

METHODS

Retinal ganglion cell electrical responsiveness was tested with pattern electroretinogram (PERG) in 11 transgenic mice expressing mutated myocilin at different ages over 18 months under ketamine/xylazine anesthesia. Twelve age-matched C57BL/6J mice also were tested as controls. Intraocular pressure was measured with a Tonolab tonometer. Immunohistochemistry for GFAP and neurofilament was performed on dissected optic nerve heads.

RESULTS

In transgenic mice expressing mutated myocilin, the PERG amplitude progressively decreased with increasing age by approximately 50%, whereas the PERG peak latency increased by approximately 40 ms (ANOVA, P < 0.05). In contrast, PERGs of young and old control mice had similar amplitudes and peak latencies. In transgenic mice, GFAP staining was more intense and extended than in control mice, and increased with increasing age; neurofilament staining showed swollen and partially degenerated axons in old transgenic mice. The IOP of young transgenic mice was similar to that of control mice and did not significantly change with increasing age.

CONCLUSIONS

Transgenic mice expressing mutated human myocilin display progressive age-related changes in RGC electrical responsiveness that are not associated with IOP elevation but are associated with marked astrogliosis and axonopathy. Our results support the view that MYOC expression in the optic nerve may impact structural, metabolic, or neurotrophic support to RGC axons, thereby influencing their susceptibility to glaucomatous damage independently of IOP.

Robust mouse pattern electroretinograms derived simultaneously from each eye using a common snout electrode

PURPOSE

We recorded pattern electroretinograms (PERGs) simultaneously from each eye in mice using binocular stimulation and a common noncorneal electrode.

METHODS

The PERG was derived simultaneously from each eye in 71 ketamine/xylazine anesthetized mice (C57BL/6J, 4 months old) from subcutaneous needles (active, snout; reference, back of the head; ground, root of the tail) in response to contrast-reversal of gratings (0.05 cycles/deg, >95% contrast) generated on two custom-made light-emitting diode (LED) tablets alternating at slight different frequencies (OD, 0.984 Hz; OS, 0.992 Hz). Independent PERG signals from each eye were retrieved using one channel continuous acquisition and phase-locking average (OD, 369 epochs of 492 ms; OS, 372 epochs of 496 ms). The PERG was the average of three consecutive repetitions.

RESULTS

Binocular snout PERGs had high amplitude (mean, 25.3 μV, SD 6.6) and no measurable interocular cross-talk. Responses were reliable (test-retest variability within-session, 14%, SD 7; between sessions, 25%, SD 9; interocular asymmetry within-session, 9%, SD 7; between sessions, 13%, SD 5). Retinal ganglion cells (RGCs) were the main source of the binocular snout PERG, as optic nerve crush in three mice abolished the signal.

CONCLUSIONS

The PERG, a sensitive measure of RGC function, is used increasingly in mouse models of glaucoma and optic nerve disease. Compared to current methods, the binocular snout PERG represents a substantial improvement in terms of simplicity and speed. It also overcomes limitations of corneal electrodes that interfere with invasive procedures of the eye and facilitates experiments based on comparison between the responses of the two eyes.

From mechanosensitivity to inflammatory responses: new players in the pathology of glaucoma

PURPOSE OF THE STUDY

Many blinding diseases of the inner retina are associated with degeneration and loss of retinal ganglion cells (RGCs). Recent evidence implicates several new signaling mechanisms as causal agents associated with RGC injury and remodeling of the optic nerve head. Ion channels such as Transient receptor potential vanilloid isoform 4 (TRPV4), pannexin-1 (Panx1) and P2X7 receptor are localized to RGCs and act as potential sensors and effectors of mechanical strain, ischemia and inflammatory responses. Under normal conditions, TRPV4 may function as an osmosensor and a polymodal molecular integrator of diverse mechanical and chemical stimuli, whereas P2X7R and Panx1 respond to stretch- and/or swelling-induced adenosine triphosphate release from neurons and glia. Ca(2+) influx, induced by stimulation of mechanosensitive ion channels in glaucoma, is proposed to influence dendritic and axonal remodeling that may lead to RGC death while (at least initially) sparing other classes of retinal neuron. The secondary phase of the retinal glaucoma response is associated with microglial activation and an inflammatory response involving Toll-like receptors (TLRs), cluster of differentiation 3 (CD3) immune recognition molecules associated with the T-cell antigen receptor, complement molecules and cell type-specific release of neuroactive cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). The retinal response to mechanical stress thus involves a diversity of signaling pathways that sense and transduce mechanical strain and orchestrate both protective and destructive secondary responses.

CONCLUSIONS

Mechanistic understanding of the interaction between pressure-dependent and independent pathways is only beginning to emerge. This review focuses on the molecular basis of mechanical strain transduction as a primary mechanism that can damage RGCs. The damage occurs through Ca(2+)-dependent cellular remodeling and is associated with parallel activation of secondary ischemic and inflammatory signaling pathways. Molecules that mediate these mechanosensory and immune responses represent plausible targets for protecting ganglion cells in glaucoma, optic neuritis and retinal ischemia.

Head-down posture induces PERG alterations in early glaucoma

PURPOSE

To probe susceptibility of retinal ganglion cells (RGC) to physiological stressors associated with moderate head-down body tilt in patients with suspicion of glaucoma or early manifest glaucoma (EMG).

METHODS

One hundred nine subjects with best corrected visual acuity (BCVA) ≥ 20/20 and no disease other than glaucoma [glaucoma suspects (GS)=79, EMG=14, normal controls (NC)=16 and comparable age range were tested. Noncontact intraocular pressure (IOP), pattern electroretinogram (PERG), and brachial blood pressure/heart rate measurements were performed in 3 consecutive conditions (∼0038 min apart): seated (baseline), -10-degree whole body head-down tilt (HDT), and seated again (recovery). PERG amplitude and latency, IOP, and systolic/diastolic blood pressures, heart rate, calculated mean central retinal artery pressure, ocular perfusion pressure, and systolic/diastolic perfusion pressures were evaluated.

RESULTS

During HDT, IOP significantly (P<0.001) increased in all groups approximately to the same extent (approximately 20%). PERG amplitude did not change in NC but decreased significantly (P<0.001) in patients (GS, -25%, EMG -23%). PERG phase become delayed in NC (-1.6%, P=0.04) but more so in patients (GS, -2.7%, P<0.001; EMG, -6.0%, P<0.001). The proportion of patients with PERG alterations significantly (P<0.05) exceeding those occurring in age-adjusted and baseline-adjusted NC were, GS: amplitude 20%, phase 15%; EMG: amplitude 14%, phase 50%. All measures recovered baseline values after HDT.

CONCLUSIONS

Moderate HDT induces temporary worsening of RGC function in a subpopulation of GS and EMG patients. This noninvasive protocol may help disclose abnormal susceptibility of RGCs in a subset of the patients at risk of glaucoma.

Retinal ganglion cell functional plasticity and optic neuropathy: a comprehensive model

The clinical management of glaucoma and optic neuropathies has traditionally focused on stages of the diseases at which there are congruent losses of visual function and optic nerve tissue. Increasing clinical and experimental evidence suggests that the electrical activity of retinal ganglion cells, as measured by pattern electroretinogram (PERG), may be altered long before measurable changes in the thickness of the retinal nerve fiber layer. In addition, PERG alterations in early glaucoma may be either reversed by lowering the intraocular pressure or induced with head-down body posture. Here we apply the well-known concept of neural plasticity to model the reversible/inducible changes of retinal ganglion cell electrical activity during a critical period of dysfunction preceding death. Identification and characterization of this stage of modifiable retinal ganglion cell function represents both a rationale and a target for treatment to change the natural history of the disease.

A new mouse model of inducible, chronic retinal ganglion cell dysfunction not associated with cell death

PURPOSE

To develop a mouse model of inducible, chronic retinal ganglion cell (RGC) dysfunction not associated with cell death.

METHODS

Eighteen C57BL/6J mice were longitudinally tested with pattern electroretinogram (PERG) and spectral-domain optical coherence tomography (OCT) before and after aspiration of the contralateral superior colliculus (SC), which removed terminals of optic tract axons and the superficial layers of the SC. At the 4-month end points, retinas were harvested for Brn3b immunostaining and BDNF immunoblotting.

RESULTS

The PERG lost approximately 60% of its baseline amplitude (P < 0.01) within the first day after lesion, and remained at a reduced level over 4 months. At the end point, the density of Brn3b-positive RGCs was normal, but their nucleus size was reduced by approximately 24% (P < 001). OCT measurements showed thinning of the inner, but not outer, retina by approximately 9% (P < 0.01) starting 10 to 20 days after lesion. Retinal nerve fiber layer thickness was unchanged. At the end point, retinal homogenates showed a substantial overexpression of BDNF protein level.

CONCLUSIONS

Mechanical SC lesion in adult mice results in a rapid, chronic loss of RGC electrical responsiveness that is followed by cell shrinkage but not cell death. The SC-lesion mouse represents a new, inducible model that allows investigating stages and mechanisms of RGC dysfunction without the confounding effects of cell death that are common in the existing models of optic neuropathies and optic nerve lesions.

Progressive loss of retinal ganglion cell function precedes structural loss by several years in glaucoma suspects

PURPOSE

We determined the time lag between loss of retinal ganglion cell function and retinal nerve fiber layer (RNFL) thickness.

METHODS

Glaucoma suspects were followed for at least four years. Patients underwent pattern electroretinography (PERG), optical coherence tomography (OCT) of the RNFL, and standard automated perimetry testing at 6-month intervals. Comparisons were made between changes in all testing modalities. To compare PERG and OCT measurements on a normalized scale, we calculated the dynamic range of PERG amplitude and RNFL thickness. The time lag between function and structure was defined as the difference in time-to-criterion loss between PERG amplitude and RNFL thickness.

RESULTS

For PERG (P < 0.001) and RNFL (P = 0.030), there was a statistically significant difference between the slopes corresponding to the lowest baseline PERG amplitude stratum (≤50%) and the reference stratum (>90%). Post hoc comparisons demonstrated highly significant differences between RNFL thicknesses of eyes in the stratum with most severely affected PERG (≤50%) and the two strata with least affected PERG (>70%). Estimates suggested that the PERG amplitude takes 1.9 to 2.5 years to lose 10% of its initial amplitude, whereas the RNFL thickness takes 9.9 to 10.4 years to lose 10% of its initial thickness. Thus, the time lag between PERG amplitude and RNFL thickness to lose 10% of their initial values is on the order of 8 years.

CONCLUSIONS

In patients who are glaucoma suspects, PERG signal anticipates an equivalent loss of OCT signal by several years.

Retrograde signaling in the optic nerve is necessary for electrical responsiveness of retinal ganglion cells

PURPOSE

We investigated the role of retrograde signaling in the optic nerve on retinal ganglion cell (RGC) electrical responsiveness in the mouse model.

METHODS

Electrical response of RGC was measured by pattern electroretinogram (PERG) in 43 C57BL/6J mice 4 to 6 months old under ketamine/xylazine anesthesia. PERGs were recorded before and at different times after blockade of axon transport with lidocaine at either the retrobulbar level (2 μL, 40 μg/μL) or at level of the superior colliculus (SC, 1 μL, 40 μg/μL). PERGs also were recorded before and at different times after optic nerve crush 1.5 mm behind the eye, followed by TUJ1-positive RGC counts of excised retinas. As controls, PERGs also were recorded after either saline injections or sham optic nerve surgery. The photopic flash electroretinogram (FERG) and visual evoked potential (FVEP) also were recorded before lidocaine and at relevant times afterwards.

RESULTS

Lidocaine injection caused rapid (retrobulbar ~10 minutes, SC 1 hour), reversible reduction of PERG amplitude (≥50%). Optic nerve crush caused rapid (10-20 minutes), irreversible reduction of PERG amplitude (70-75%), increase of PERG latency (>25%), as well as RGC loss (88%) 1 month after crush. FVEP was unaltered by lidocaine. For all procedures, the FERG was unaltered.

CONCLUSIONS

As experimental interventions were made at postretinal level(s), PERG changes were likely associated with altered supply of retrogradely-delivered material from the SC. This implies that retrograde transport of target-derived molecules is necessary for normal RGC electrical responsiveness. The time course of early PERG changes is consistent with the speed of fast retrograde axon transport.

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.

Characterization of structure and function of the mouse retina using pattern electroretinography, pupil light reflex, and optical coherence tomography

OBJECTIVE

To perform in vivo analysis of retinal functional and structural parameters in healthy mouse eyes.

ANIMAL STUDIED

Adult C57BL/6 male mice (n = 37).

PROCEDURES

Retinal function was evaluated using pattern electroretinography (pERG) and the chromatic pupil light reflex (cPLR). Structural properties of the retina and nerve fiber layer (NFL) were evaluated using spectral-domain optical coherence tomography (SD-OCT).

RESULTS

The average pERG amplitudes were found to be 11.2 ± 0.7 μV (P50-N95, mean ± SEM), with an implicit time for P50-N95 interval of 90.4 ± 5.4 ms. Total retinal thickness was 229.5 ± 1.7 μm (mean ± SEM) in the area centralis region. The thickness of the retinal nerve fiber layer (mean ± SEM) using a circular peripapillary retinal scan centered on the optic nerve was 46.7 ± 0.9 μm (temporal), 46.1 ± 0.9 μm (superior), 45.8 ± 0.9 μm (nasal), and 48.4 ± 1 μm (inferior). The baseline pupil diameter was 2.1 ± 0.05 mm in darkness, and 1.1 ± 0.05 and 0.56 ± 0.03 mm after stimulation with red (630 nm, luminance 200 kcd/m(2)) or blue (480 nm, luminance 200 kcd/m(2)) light illumination, respectively.

CONCLUSIONS

Pattern electroretinography, cPLR and SD-OCT analysis are reproducible techniques, which can provide important information about retinal and optic nerve function and structure in mice.

Magnetic resonance imaging indicates decreased choroidal and retinal blood flow in the DBA/2J mouse model of glaucoma

PURPOSE

This study tests the hypothesis that reduced retinal and choroidal blood flow (BF) occur in the DBA/2J mouse model of glaucoma.

METHODS

Quantitative BF magnetic resonance imaging (MRI) with a resolution of 42 × 42 × 400 μm was performed on DBA/2J mice at 4, 6, and 9 months of age and C57BL/6 age-matched controls under isoflurane anesthesia. BF MRI images were acquired with echo-planar imaging using an arterial spin labeling technique and a custom-made eye coil at 7 Tesla. Automated profile analysis was performed to average layer-specific BF along the length of the retina and choroid. In separate experiments, servo-null micropressure measurements of iliac arterial pressure were performed in old mice of both strains.

RESULTS

Choroidal BF was lower in DBA/2J mice than in age-matched C57BL/6 control mice at 4, 6, and 9 months of age (P < 0.01 for all age-matched groups). Retinal BF was lower in DBA/2J mice than in C57BL/6 mice at the 9-month time point (P < 0.01). Mean arterial pressure was not significantly different in aged C57BL/6 mice compared with aged DBA/2J mice.

CONCLUSIONS

The reduced ocular blood flow in DBA/2J mice compared with C57BL/6 control mice suggests that ischemia or hypoxia should be considered as a possible contributing factor in the optic neuropathy in the DBA/2J mouse model of glaucoma.

Missing optomotor head-turning reflex in the DBA/2J mouse

PURPOSE

The optomotor reflex of DBA/2J (D2), DBA/2J-Gpnmb+ (D2-Gpnmb+), and C57BL/6J (B6) mouse strains was assayed, and the retinal ganglion cell (RGC) firing patterns, direction selectivity, vestibulomotor function and central vision was compared between the D2 and B6 mouse lines.

METHODS

Intraocular pressure (IOP) measurements, real-time PCR, and immunohistochemical analysis were used to assess the time course of glaucomatous changes in D2 retinas. Behavioral analyses of optomotor head-turning reflex, visible platform Morris water maze and Rotarod measurements were conducted to test vision and vestibulomotor function. Electroretinogram (ERG) measurements were used to assay outer retinal function. The multielectrode array (MEA) technique was used to characterize RGC spiking and direction selectivity in D2 and B6 retinas.

RESULTS

Progressive increase in IOP and loss of Brn3a signals in D2 animals were consistent with glaucoma progression starting after 6 months of age. D2 mice showed no response to visual stimulation that evoked robust optomotor responses in B6 mice at any age after eye opening. Spatial frequency threshold was also not measurable in the D2-Gpnmb+ strain control. ERG a- and b-waves, central vision, vestibulomotor function, the spiking properties of ON, OFF, ON-OFF, and direction-selective RGCs were normal in young D2 mice.

CONCLUSIONS

The D2 strain is characterized by a lack of optomotor reflex before IOP elevation and RGC degeneration are observed. This behavioral deficit is D2 strain-specific, but is independent of retinal function and glaucoma. Caution is advised when using the optomotor reflex to follow glaucoma progression in D2 mice.

Postnatal elongation of eye size in DBA/2J mice compared with C57BL/6J mice: in vivo analysis with whole-eye OCT

PURPOSE

To characterize postnatal changes in eye size in glaucomatous DBA/2J (D2) mice and in nonglaucomatous C57BL/6J mice (B6) in vivo by means of whole-eye optical coherence tomography (OCT).

METHODS

D2 (n = 32) and B6 (n = 36) mice were tested between 2 and 20 months of age in eight age bins. A custom time-domain OCT system with a center wavelength of 825 nm and an axial scan length of 7.1 mm produced axial A-scan interferograms at a rate of 20 A-lines/s with a resolution of 8 μm. Axial length (AL), corneal thickness (CT), anterior chamber depth (ACD), lens thickness (LT), vitreous chamber depth (VCD), and retinal thickness (RT) were measured in the optical axis and adjusted with corresponding refractive indices. Corneal curvature (CC) and IOP were also measured.

RESULTS

AL increased (P < 0.001) more in the D2 (21%) than in the B6 (9%) mice. There was an interaction effect (two-way ANOVA, P < 0.001) between age and strain for AL, CT, ACD, and VCD. In the D2 mice, the lens became dislocated posteriorly. Multiple regression analysis in the D2 mice revealed an independent effect of age and IOP (P ≤ 0.01) on axial length. CC steepened in the older D2 mice, whereas it flattened in the B6 mice.

CONCLUSIONS

In D2 mice, postnatal elongation of AL is larger than that in B6 mice and is associated with a greater increase in ACD and IOP, which seems to be a causal factor. The ease of use, short acquisition time, and noninvasiveness of whole-eye OCT make it suitable for routine use in longitudinal studies of mouse models.

Effect of general anesthetics on IOP in elevated IOP mouse model

Elevated intraocular pressure (IOP) is the best recognized risk factor for the pathogenesis of glaucoma and the extent of retinal ganglion cell (RGC) degeneration in glaucoma is closely correlated with the extent of IOP elevation. Therefore, accurately and reliably measuring IOP is critical in investigating the mechanism of pressure-induced RGC damage in glaucoma. However, IOP is measured under general anesthesia in most studies using mouse models and many anesthetics affect the IOP measurements in both human and animals. In the present study, we used a noninvasive approach to measure the IOP of mice with normal and elevated IOP. The approach used mice that were awake and mice that were under general anesthesia. Our results demonstrate that not only the behavioral training enables IOP measurement from conscious mice without using a restrainer, it also significantly improves the consistency and reliability of the IOP measurement. In addition, we provide a direct comparison between awake and anesthetized IOP measurements as a function of time after the induction of general anesthesia with several commonly used anesthetic agents. We found that all tested general anesthetics significantly altered the IOP measurements both in normal eyes and in those with elevated IOP. Therefore, we conclude that behavioral training of mice can provide an approach to measure awake IOP that does not require general anesthesia and thus produces reliable and consistent results.

C57BL/6J, DBA/2J, and DBA/2J.Gpnmb mice have different visual signal processing in the inner retina

PURPOSE

To characterize differences in retinal ganglion cell (RGC) function in mouse strains relevant to disease models. C57BL/6J (B6) and DBA/2J (D2) are the two most common mouse strains; D2 has two mutated genes, tyrosinase-related protein 1 (Tyrp1) and glycoprotein non-metastatic melanoma protein B (Gpnmb), causing iris disease and intraocular pressure (IOP) elevation after 6 months of age that results in RGC degeneration, and is the most widely used model of glaucoma. DBA/2J.Gpnmb(+) (D2.Gpnmb(+)) is the wild type for the Gpnmb mutation and does not develop IOP elevation and glaucoma.

METHODS

Young (2-4 months of age) B6, D2, and D2.Gpnmb(+) mice (n=6 for each group) were tested with pattern electroretinogram (PERG) in response to different contrasts and spatial frequencies. PERG amplitude and latency dependencies on stimulus parameters (transfer functions) were established for each mouse strain, together with corresponding thresholds for contrast and spatial resolution.

RESULTS

PERG analysis showed that B6, D2, and D2.Gpnmb(+) mice had comparable contrast threshold and spatial resolution. Suprathreshold spatial contrast processing, however, had different characteristics in the three strains. PERG amplitude and latency changes with increasing contrast were different between B6 and D2 as well as between D2 and D2.Gpnmb(+).

CONCLUSIONS

B6, D2, and D2.Gpnmb(+) mice have different characteristics of PERG spatial contrast processing consistent with different mechanisms of contrast gain control. This may imply differences in the activity of underlying PERG generators and synaptic circuitry in the inner retina.

Neurodegeneration in glaucoma: progression and calcium-dependent intracellular mechanisms

Glaucoma is an age-related optic neuropathy involving sensitivity to ocular pressure. The disease is now seen increasingly as one of the central nervous system, as powerful new approaches highlight an increasing number of similarities with other age-related neurodegenerations such as Alzheimer's and Parkinson's. While the etiologies of these diseases are diverse, they involve many important common elements including compartmentalized programs of degeneration targeting axons, dendrites and finally cell bodies. Most age-related degenerations display early functional deficits that precede actual loss of neuronal substrate. These are linked to several specific neurochemical cascades that can be linked back to dysregulation of Ca(2+)-dependent processes. We are now in the midst of identifying similar cascades in glaucoma. Here we review recent evidence on the pathological progression of neurodegeneration in glaucoma and some of the Ca(2+)-dependent mechanisms that could underlie these changes. These mechanisms present clear implications for efforts to develop interventions targeting neuronal loss directly and make glaucoma an attractive model for both interrogating and informing other neurodegenerative diseases.

The impact of intraocular pressure reduction on retinal ganglion cell function measured using pattern electroretinogram in eyes receiving latanoprost 0.005% versus placebo

PURPOSE

To evaluate the impact of intraocular (IOP) reduction on retinal ganglion cell (RGC) function measured using pattern electroretinogram optimized for glaucoma (PERGLA) in glaucoma suspect and glaucomatous eyes receiving latanoprost 0.005% versus placebo.

METHODS

This was a prospective, placebo-controlled, double masked, cross-over clinical trial. One randomly selected eye of each subject meeting eligibility criteria was enrolled. At each visit, subjects underwent five diurnal measurements between 8:00 am and 4:00 pm consisting of Goldmann IOP, and PERGLA measurements. A baseline examination was performed following a 4-week washout period, and repeat examination after randomly receiving latanoprost or placebo for 4-weeks. Subjects were then crossed over to receive the alternative therapy for 4 weeks following a second washout period, and underwent repeat examination. Linear mixed-effect models were used for the analysis.

RESULTS

Sixty-eight eyes (35 glaucoma, 33 glaucoma suspect) of 68 patients (mean age 67.4 ± 10.6 years) were enrolled. The mean IOP (mmHg) after latanoprost 0.005% therapy (14.9 ± 3.8) was significantly lower than baseline (18.8 ± 4.7, p<0.001) or placebo (18.0 ± 4.3), with a mean reduction of -20 ± 13%. Mean PERGLA amplitude (μV) and phase (π-radian) using latanoprost (0.49 ± 0.22 and 1.71 ± 0.22, respectively) were similar (p > 0.05) to baseline (0.49 ± 0.24 and 1.69 ± 0.19) and placebo (0.50 ± 0.24 and 1.72 ± 0.23). No significant (p > 0.05) diurnal variation in PERGLA amplitude was observed at baseline, or using latanoprost or placebo. Treatment with latanoprost, time of day, and IOP were not significantly (p > 0.05) associated with PERGLA amplitude or phase.

CONCLUSION

Twenty percent IOP reduction using latanoprost monotherapy is not associated with improvement in RGC function measured with PERGLA.