Optic Nerve Degeneration and Potential Neuroprotection: Implications for Glaucoma

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.

Neuroprotection of Retinal Ganglion Cells In Vivo Using the Activation of the Endogenous Cannabinoid Signaling System in Mammalian Eyes

Cannabinoid and glutamatergic signaling systems in the human retina coexist and greatly influence one another. Under glaucomatous conditions, excess levels of glutamate accrete in the retinal ganglion cell (RGC) layer. The present study tests the putative neuroprotective effect mediated by cannabinoids at the CB1 and CB2 receptors. In the first experiment, mice were given intravitreal injections of 160 nmol N-methyl-d-aspartic acid (NMDA) in one eye and saline in the paired eye. In the second experiment, both eyes were given NMDA, while one of the two was additionally given the cannabinoid agonist WIN 55,212-2. Ten days later, animals were perfused and the retinae were dissected as wholemounts and stained with Cresyl Violet. Quantitative analysis revealed that 70% of the neurons in the retinal ganglion cell (RGC) layer exposed to NMDA underwent cell death. The addition of the cannabinoid CB1/CB2 agonist doubled the number of neurons surviving the NMDA treatment. These data provide evidence that cannabinoids, either exogenous or endogenous, may be harnessed to provide protection from neurodegenerative diseases, including glaucoma, and from glutamate-induced, and potentially other forms of neurotoxicity, under chronic or acute conditions.

Mesoscopic cortical network reorganization during recovery of optic nerve injury in GCaMP6s mice

As the residual vision following a traumatic optic nerve injury can spontaneously recover over time, we explored the spontaneous plasticity of cortical networks during the early post-optic nerve crush (ONC) phase. Using in vivo wide-field calcium imaging on awake Thy1-GCaMP6s mice, we characterized resting state and evoked cortical activity before, during, and 31 days after ONC. The recovery of monocular visual acuity and depth perception was evaluated in parallel. Cortical responses to an LED flash decreased in the contralateral hemisphere in the primary visual cortex and in the secondary visual areas following the ONC, but was partially rescued between 3 and 5 days post-ONC, remaining stable thereafter. The connectivity between visual and non-visual regions was disorganized after the crush, as shown by a decorrelation, but correlated activity was restored 31 days after the injury. The number of surviving retinal ganglion cells dramatically dropped and remained low. At the behavioral level, the ONC resulted in visual acuity loss on the injured side and an increase in visual acuity with the non-injured eye. In conclusion, our results show a reorganization of connectivity between visual and associative cortical areas after an ONC, which is indicative of spontaneous cortical plasticity.

[Centrally acting cholinomimetics in the complex therapy of progressive glaucomatous optic neuropathy]

Many factors exist that are associated with higher risk of glaucoma progression. Arterial hypotension, low perfusion pressure, vasospastic syndrome, diabetes mellitus, myopia, etc. increase the need for neuroprotective therapy, which is aimed at stabilizing the pathological process and creating favorable conditions for maintaining visual functions. The aim of this study was to assess the therapeutic efficacy of Gliatilin as part of the complex treatment of progressive glaucomatous optic neuropathy.

MATERIAL AND METHODS

A total of 240 patients were randomly selected and divided into 2 groups, 120 patients each. Both groups were matched for age, somatic comorbidity, and the gravity of the glaucomatous process. Patient age averaged 71.3±1.6 years. Advanced glaucoma prevailed in both groups: 70.0 and 76.6% correspondingly. Neuroprotective therapy included drugs from different pharmacological classes so that different aspects of pathogenesis were addressed. Apart from that, patients from Group I first received intravenous Gliatilin (1000 mg/4ml, 12--15 doses) and then switched to oral (1 capsule b.i.d. for 4 months). All patients underwent standard ophthalmic examination and static perimetry.

RESULTS

No adverse effects were observed over the first two weeks of Gliatilin course, during which the patients stayed in the hospital. IOP level was normal and stable. Although neuroprotective therapy does not directly affect IOP, stability of the latter describes the dynamics of the glaucomatous process. When assessing changes in visual functions, particular attention was paid to the central visual field, foveolar and total light sensitivity, peripheral visual field, and MD and PSD indices. All mean values showed a tendency toward improvement, more pronounced in the Gliatilin group.

CONCLUSION

A complex therapy cannot be limited to a single drug only, and to make better decisions, one should consider not only ocular, but also general condition of the patient. Adjuvant Gliatilin in the complex therapy of progressive glaucoma is appropriate and efficient, especially in case of systemic atherosclerosis and cerebrovascular insufficiency. The frequency of stabilization therapy depends on the efficacy of the latest course and clinical manifestations of the glaucomatous process.

Chronic mild stress eliminates the neuroprotective effect of Copaxone after CNS injury

Copolymer (Cop)-1, also known as glatiramer acetate, is an active compound of Copaxone, a drug widely used by patients with multiple sclerosis (MS). Copaxone functions in MS through two mechanisms of action, namely immunomodulation and neuroprotection. Because the immune system is suppressed or altered in depressed individuals, and since depression is often associated with neurological conditions, we were interested in examining whether the neuroprotective effect of Copaxone persists under conditions of stress-induced depressive behavior. We exposed mice to unpredictable chronic mild stress for 4 weeks and then treated them with three doses of Copaxone at 3-day intervals, with the last dose given immediately before the mice underwent a crush injury to the optic nerve. Whereas nonstressed mice exhibited a strong neuroprotective response after Copaxone treatment, this effect was completely absent in mice that underwent chronic mild stress. Interestingly, when Copaxone was combined with Prozac, the neuroprotective effect of Copaxone was regained, suggesting that chronic mild stress interferes with the neuroprotective effect of Copaxone. These results may shed a light on mechanism of action of Copaxone and lead to new combined therapies for neurodegenerative and neuroinflammatory disorders.

Effect of ion pairing on in vitro transcorneal permeability of a Δ(9) -tetrahydrocannabinol prodrug: potential in glaucoma therapy

The aim of the present study was to evaluate and improve the in vitro transcorneal permeability characteristics of Δ(9) -tetrahydrocannabinol (THC) through prodrug derivatization and formulation approaches. In vitro corneal permeability of THC and its hemisuccinate (THC-HS) and hemiglutarate (THC-HG) ester prodrugs and WIN 55-212-2 (WIN), a synthetic cannabinoid, was determined using isolated rabbit cornea. The formulations studied included hydroxypropyl beta cyclodextrin (HPβCD) or randomly methylated beta cyclodextrin (RMβCD), as well as prodrug-ion-pair complexes with l-arginine or tromethamine. Corneal permeability of WIN was found to be two-fold higher than THC in the presence of HPβCD. THC-HS and THC-HG exhibited pH-dependent permeability. In the presence of HPβCD, at pH 5 (donor solution pH), both prodrugs exhibited six-fold higher permeability compared with THC. However, permeability of the prodrugs was about three-fold lower than that of THC at pH 7.4. RMβCD, at pH 7.4, led to a significant improvement in permeability. Formation of ion-pair complexes markedly improved the solubility and permeability of THC-HG (sevenfold and threefold greater permeability compared with THC and WIN, respectively) at pH 7.4. The in vitro results demonstrate that the use of an ion-pair complex of THC-HG could be an effective strategy for topical delivery of THC.

Nonpsychotropic cannabinoids, abnormal cannabidiol and canabigerol-dimethyl heptyl, act at novel cannabinoid receptors to reduce intraocular pressure

The objective of our study was to examine the pharmacology of the intraocular pressure (IOP)-lowering actions of the behaviorally inactive cannabinoids, abnormal cannabidiol (abn-CBD), and a cannabigerol analog, cannabigerol-dimethyl heptyl (CBG-DMH), in comparison to that of the nonselective cannabinoid 1 receptor (CB(1)R) and CB(2)R agonist, WIN55,212-2, in Brown Norway rats. The IOP was measured noninvasively using a hand-held tonometer in nonanesthetized animals. The IOP measurements were taken every 15 min for a period of 2 h after drug administration. All drugs were administered via intraperitoneal (i.p.) injections, and abn-CBD and CBG-DMH were also given topically. Both abn-CBD and CBG-DMH reduced IOP when administrated i.p. at doses of ≥2.5 mg/kg or topically at concentrations of 1%-2%. The IOP-lowering effects of abn-CBD and CBG-DMH were reduced by i.p. administration of O-1918 (2.5 mg/kg), a selective antagonist of the abn-CBD-sensitive cannabinoid-related receptor (CBx), but were unaffected by the CB(1)R antagonist, AM251 (2.5 mg/kg), or the CB(2)R antagonist, AM630 (2.5 mg/kg). In contrast, the IOP-lowering action of WIN55,212-2 was completely blocked by the CB(1)R-selective antagonist, AM251, and was unaffected by the CBx receptor antagonist, O-1918. However, similar to the nonpsychotropic cannabinoids, the ocular hypotensive actions of WIN55,212-2 were also insensitive to block by the CB(2)R antagonist, AM630. Consistent with this, the selective CB(2)R agonist, HU-308 (2 mg/kg) failed to reduce IOP in Brown Norway rats. Concurrent application of a dose of WIN55,212-2 that was subthreshold to reduce IOP (0.25 mg/kg), together with a topical dose of either abn-CBD (0.5%) or CBG-DMH (0.25%), respectively, potentiated the ocular hypotensive effect of either compound applied alone. This study demonstrates that the atypical cannabinoid, abn-CBD, and the cannabigerol analog, CBG-DMH, decrease IOP in the normotensive Brown Norway rat eye independent of CB(1)R or CB(2)R activation, via activation of CBx receptors. The enhanced decrease in IOP seen after coapplication of the CB(1)R agonist, WIN55,212-2, together with either abn-CBD or CBG-DMH, respectively, further suggests that the ocular pharmacodynamics of abn-CBD and CBG-DMH are mediated by receptor targets distinct from CB(1)R. These results indicate that both CBG-DMH and abn-CBD have the potential for further investigation as novel ocular hypotensive cannabinoids devoid of CB(1)R-mediated side-effects.

Repeated intraocular pressure measurement in awake Lewis rats does not bias retinal ganglion cell survival

PURPOSE

The TonoPen applanation tonometry is an established method for intraocular pressure (IOP) measurement. The IOP is one of the main variables affecting retinal ganglion cell (RGC) loss in experimental animal models in ophthalmology and the main risk factor for human glaucoma. In this study, we examined if IOP measurements with the TonoPen itself lead to retinal ganglion cell loss or any other possible retina damages, such as intraocular bleedings or ablation, in Lewis rats.

METHODS

Three groups of rats (n = 5 each) were formed. IOP monitoring, using a TonoPen XL, was performed on groups 1 and 3. Animals in groups 1 and 2 received funduscopies before and after one and two weeks of the study, in order to detect possible abnormalities. After two weeks, retinal flatmounts were stained to detect ganglion cells. RGCs were manually counted in eight predefined areas to compare mean RGC densities between groups 1 and 2 (IOP readings vs. no readings), using student t-test.

RESULTS

No significant difference in RGC density between animals that underwent IOP readings and controls could be observed (p = 0.8). As expected, no IOP alterations were monitored in groups 1 and 3 throughout the study. No retinal abnormalities, such as bleeding or retina ablation, were detectable.

CONCLUSION

We could detect no effects on retinal ganglion cell survival in Lewis rats or any other damages to the retina caused by IOP measurements using a TonoPen XL. This study proposes that repeated applanation tonometry does not affect RGC numbers, one of the main monitored variables in most glaucoma model studies. Therefore, the use of a TonoPen XL for repeated IOP monitoring in Lewis rats can be considered harmless.

Pattern electroretinography in a rat model of ocular hypertension: functional evidence for early detection of inner retinal damage

With the increasing use of the rat as an animal model for glaucoma and for the evaluation of neuroprotective treatments, there is a need for a sensitive test of retinal ganglion cell (RGC) function in this species. The aims of this study were to detect functional abnormalities of the inner retina in a rat model of high intraocular pressure (IOP) using the pattern electroretinogram (PERG), and to correlate them with morphometric analysis of RGC survival and the functional integrity of the inner retina. Unilateral ocular hypertension was induced in 17 Lewis rats through laser photocoagulation. Pattern ERGs were recorded prior to lasering and 3 weeks later, using a series of shifting patterns of decreasing spatial frequency projected directly onto the animals' fundus. IOP was measured at the same intervals, and the number of surviving RGCs estimated. Low amplitude PERG signals could be recorded in response to a narrow grating of 0.368 cycles per degree (cpd), and increased with stimulus size. Lasering caused mean (+/-s.d.) IOP to increase significantly from 18.3+/-4.5 (baseline) to 29.8+/-8.8 mmHg within 3 weeks (p<0.0001). At this time, PERG amplitudes were significantly reduced (p<0.05), declining an average of 45% compared to the normotensive, control eyes. No outer retinal damage was observed, but the mean number of RGCs decreased significantly (p<0.001), from 2 525.0+/-372.4 to 1 542.8+/-333.8 cells per mm2. This decrease in RGC number was significantly (p=0.03) correlated the decrease in PERG amplitude. The correlation between functional integrity of the inner retina and the rat PERG was further demonstrated by intravitreal tetrodotoxin injections, which temporarily abolished the PERG but did not affect outer retinal activity, reflected in the flash ERG. The evidence for early functional deficits, combined with tonometry and documentation of correlated ganglion cells loss, confirms the sensitivity of this diagnostic tool and the validity and importance of this animal model in glaucoma research.

Neuroprotective effects of erythropoietin on glutamate and nitric oxide toxicity in primary cultured retinal ganglion cells

Erythropoietin receptor (EpoR) is expressed in the central nervous system (CNS), however, no clear consensus has been obtained whether Epo acts as a prosurvival factor in neurons. Because retinal ganglion cell (RGC) death is a common cause of reduced visual function in several ocular diseases, we explored whether Epo might potentially be beneficial in protecting RGCs from glutamate and nitric oxide (NO)-induced cytotoxicity, using isolated RGCs by a two-step panning method. Brain-derived neurotrophic factor (BDNF) was used as a positive control. EpoR mRNA was expressed in isolated RGCs, and EpoR protein was expressed on the RGCs in the normal and ischemic retinas. Epo had less potential to improve the survival of primary RGCs in serum-free medium than BDNF. In these cells, BDNF, but not Epo, downregulated the expression of Bim, a proapoptotic Bcl-2 family member that plays a key role in cytokine-mediated cell survival, suggesting a possible mechanism for this difference. When RGCs were cultured with glutamate or an NO-generating reagent, the survival of RGCs was compromised, and Bcl-2 expression was decreased in these cells. Both Epo and BDNF significantly reduced RGC death induced by glutamate and NO. In agreement with this, these factors reversed the Bcl-2 expression. These findings suggest that Epo may be a potent neuroprotective therapeutic agent for the treatment of ocular diseases that are characterized by RGC death.

Evaluation of neuroprotective qualities of brimonidine during LASIK

PURPOSE

The effects of LASIK-induced increased intraocular pressure on the optic nerve and nerve fiber layer are poorly understood. This study evaluates the effect of LASIK on several optic nerve parameters, both structural (scanning laser polarimetry) and functional (automated perimetry). In addition, the potential neuroprotective effect of perioperative brimonidine is studied.

DESIGN

Randomized self-controlled, masked trial.

PARTICIPANTS

Fifty-one patients scheduled for routine, bilateral, myopic LASIK. Patients served as their own control and received brimonidine in one eye and placebo in their fellow eye.

METHODS

Patients were treated with the VISX Star S3 Excimer Laser. Patients were evaluated preoperatively and at 1 day, 1 month, and 3 months after LASIK. Topical brimonidine or control vehicle was administrated three times daily for 3 days before surgery. In addition, 1 drop of the respective study drug was instilled at the end of the procedure, and the patient continued the study drug three times daily for 3 weeks after surgery in the respective (randomized) eye.

MAIN OUTCOME MEASURES

Visual acuity, nerve fiber layer analysis, automated visual field, contrast sensitivity, color vision, and pupillary function.

RESULTS

Brimonidine did not change the outcome of any of the parameters analyzed in this study. There was not a statistically significant change in any of the visual field parameters (mean elevation or depression, pattern standard deviation, and corrected pattern standard deviation) measured in either the placebo or brimonidine group postoperatively. However, both the placebo and brimonidine group did show a statistically significant change in many of the direct Nerve Fiber Analyzer GDx (NFA GDx) measures. The average thickness, ellipse, and superior average were generally reduced in both groups. There was, however, no statistically significant change after LASIK in either group in indirect NFA GDx parameters such as symmetry, superior ratio, inferior ratio, or superior/nasal ratio. There was no significant change in optic nerve appearance, contrast sensitivity, or color vision between treatment groups before or after LASIK.

CONCLUSIONS

LASIK with or without brimonidine did not affect the structure or function of the parameters of the optic nerve studied. Direct NFA GDx measures were globally reduced after myopic LASIK; however, the ratio measures were generally unaffected. The reduction in direct, retinal nerve fiber layer measures, in the absence of change in ratio measures and other optic nerve parameters and absence of brimonidine effect, suggest an alteration in corneal birefringence after excimer laser ablation.

Cannabinoids in the treatment of glaucoma

The leading cause of irreversible blindness is glaucoma, a disease normally characterized by the development of ocular hypertension and consequent damage to the optic nerve at its point of retinal attachment. This results in a narrowing of the visual field, and eventually results in blindness. A number of drugs are available to lower intraocular pressure (IOP), but, occasionally, they are ineffective or have intolerable side-effects for some patients and can lose efficacy with chronic administration. The smoking of marijuana has decreased IOP in glaucoma patients. Cannabinoid drugs, therefore, are thought to have significant potential for pharmaceutical development. However, as the mechanism surrounding their effect on IOP initially was thought to involve the CNS, issues of psychoactivity hindered progress. The discovery of ocular cannabinoid receptors implied an explanation for the induction of hypotension by topical cannabinoid applications, and has stimulated a new phase of ophthalmic cannabinoid research. Featured within these investigations is the possibility that at least some cannabinoids may ameliorate optic neuronal damage through suppression of N-methyl-D-aspartate receptor hyperexcitability, stimulation of neural microcirculation, and the suppression of both apoptosis and damaging free radical reactions, among other mechanisms. Separation of therapeutic actions from side-effects now seems possible through a diverse array of novel chemical, pharmacological, and formulation strategies.

Prospects for relevant glaucoma models with retinal ganglion cell damage in the rodent eye

Retinal ganglion cell (RGC) death is the end result of practically all diseases of the optic nerve, including glaucomatous optic neuropathy. Understanding the factors determining susceptibility of the retina or the optic nerve to glaucomatous damage, and the means to prevent it, requires good animal models. Here we review the different, current models in rodents that have been used to study RGC damage, discuss their value, and their adequacy as models for human glaucoma.

Alpha-2 adrenergic receptor agonists are neuroprotective in experimental models of glaucoma

PURPOSE

The glaucomas are characterized by chronic progressive ganglion cell loss over many years. A drug with neuroprotective activity should increase the resistance of retinal ganglion cells (RGC) to chronic stress or injury and therefore enhance survival. Brimonidine is a highly selective and potent alpha-2 adrenergic receptor agonist, which lowers intra-ocular pressure (IOP) and is neuroprotective. Immunohistochemistry data have shown that the specific receptor targets, the alpha-2 receptors, are located in the inner retina.

METHODS

Brimonidine 0.1 mg/kg given intraperitoneally promoted RGC survival compared with vehicle using the optic nerve crush model even when administered up to 24 hours before injury. Using the chronic ocular hypertensive rat model, brimonidine 1 mg/kg/day (with osmotic pump) significantly prevented the loss of RGCs when compared with vehicle or timolol. This ability was due to the neuroprotective action of brimonidine, since it did not affect IOP. In addition, brimonidine 0.1 mg/day reached concentrations in the retina of Sprague-Dawley rats within 30 minutes of injection, which was sufficient to activate the alpha-2 receptor (> or = 2 nM) and maintained these concentrations for 6 hours.

CONCLUSIONS

Having demonstrated that: a. the specific receptor target of brimonidine is located in the retina, which is important for optic neuroprotection, b. the agent shows neuroprotective ability in animal models, c. pharmacological concentrations of the drug can be reached in the retina, clinical trial has been initiated to determine whether brimonidine is neuroprotective in patients with nonarteritic ischaemic optic neuropathy.