Phenytoin blocks retinal ganglion cell death after partial optic nerve crush

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.

Inducible rodent models of glaucoma

Glaucoma is one of the leading causes of vision impairment worldwide. In order to further understand the molecular pathobiology of this disease and to develop better therapies, clinically relevant animal models are necessary. In recent years, both the rat and mouse have become popular models in glaucoma research. Key reasons are: many important biological similarities shared among rodent eyes and the human eye; development of improved methods to induce glaucoma and to evaluate glaucomatous damage; availability of genetic tools in the mouse; as well as the relatively low cost of rodent studies. Commonly studied rat and mouse glaucoma models include intraocular pressure (IOP)-dependent and pressure-independent models. The pressure-dependent models address the most important risk factor of elevated IOP, whereas the pressure-independent models assess "normal tension" glaucoma and other "non-IOP" related factors associated with glaucomatous damage. The current article provides descriptions of these models, their characterizations, specific techniques to induce glaucoma, mechanisms of injury, advantages, and limitations.

The retinoprotective role of phenytoin

Phenytoin is a non-sedative barbiturate derivate and has been recently rediscovered as a neuroprotective and retinoprotective compound in patients affected by optic neuritis secondary to multiple sclerosis. However, currently there are still no neuroprotective compounds registered and available in the clinic. We reviewed the literature supporting the retinoprotective properties of phenytoin and analyzed the various approaches and definitions from the first research periods onwards. The retinoprotective role of phenytoin was already known in the 1970s, but only recently has this effect been rediscovered, confirming that it could indeed provide structural protection of the retinal cells.

Glioprotection of Retinal Astrocytes After Intravitreal Administration of Memantine in the Mouse Optic Nerve Crush Model

Background

In glaucoma, non-intraocular pressure (IOP)-related risk factors can result in increased levels of extracellular glutamate, which triggers a cascade of neurodegeneration characterized by the excessive activation of N-methyl-D-aspartate (NMDA). The purpose of our study was to evaluate the glioprotective effects of memantine as a prototypic uncompetitive NMDA blocker on retinal astrocytes in the optic nerve crush (ONC) mouse model for glaucoma.

Material/Methods

Optic nerve crush was performed on all of the right eyes (n=8), whereas left eyes served as contralateral healthy controls (n=8) in Balb/c/Sca mice. Four randomly assigned mice received 2-μl intravitreal injections of memantine (1 mg/ml) after ONC in the experimental eye. One week after the experiment, optic nerves were dissected and stained with methylene blue. Retinae were detached from the sclera. The tissue was immunostained. Whole-mount retinae were investigated by fluorescent microscopy. Astrocyte counts for each image were performed manually.

Results

Histological sections of crushed optic nerves showed consistently moderate tissue damage in experimental groups. The mean number of astrocytes per image in the ONC group was significantly lower than in the healthy control group (7.13±1.5 and 10.47±1.9, respectively). Loss of astrocytes in the memantine-treated group was significantly lower (8.83±2.2) than in the ONC group. Assessment of inter-observer reliability showed excellent agreement among observations in control, ONC, and memantine groups.

Conclusions

The ONC is an effective method for investigation of astrocytic changes in mouse retina. Intravitreally administered memantine shows a promising glioprotective effect on mouse retinal astrocytes by preserving astrocyte count after ONC.

Neuroprotection in glaucoma - Is there a future role?

In glaucoma, the major cause of global irreversible blindness, there is an urgent need for treatment modalities that directly target the RGCs. The discovery of an alternative therapeutic approach, independent of IOP reduction, is highly sought after, due to the indirect nature and limited effectiveness of IOP lowering therapy in preventing RGC loss. Several mechanisms have been implicated in initiating the apoptotic cascade in glaucomatous retinopathy and numerous drugs have been shown to be neuroprotective in animal models of glaucoma. These mechanisms and their potential treatment include excitotoxicity, protein misfolding, mitochondrial dysfunction, oxidative stress, inflammation and neurotrophin deprivation. All of these mechanisms ultimately lead to programmed cell death with loss of RGCs. In this article we summarize the mechanisms involved in glaucomatous disease, highlight the rationale for neuroprotection in glaucoma management and review current potential neuroprotective strategies targeting RGCs from the laboratory to the clinic.

Pharmacodynamics of memantine: an update

Memantine received marketing authorization from the European Agency for the Evaluation of Medicinal Products (EMEA) for the treatment of moderately severe to severe Alzheimer s disease (AD) in Europe on 17(th) May 2002 and shortly thereafter was also approved by the FDA for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with strong voltage-dependency and fast kinetics. Due to this mechanism of action (MOA), there is a wealth of other possible therapeutic indications for memantine and numerous preclinical data in animal models support this assumption. This review is intended to provide an update on preclinical studies on the pharmacodynamics of memantine, with an additional focus on animal models of diseases aside from the approved indication. For most studies prior to 1999, the reader is referred to a previous review [196].In general, since 1999, considerable additional preclinical evidence has accumulated supporting the use of memantine in AD (both symptomatic and neuroprotective). In addition, there has been further confirmation of the MOA of memantine as an uncompetitive NMDA receptor antagonist and essentially no data contradicting our understanding of the benign side effect profile of memantine.

Felbamate but not phenytoin or gabapentin reduces glutamate release by blocking presynaptic NMDA receptors in the entorhinal cortex

We have shown that a number of anticonvulsant drugs can reduce glutamate release at synapses in the rat entorhinal cortex (EC) in vitro. We have also shown that presynaptic NMDA receptors (NMDAr) tonically facilitate glutamate release at these synapses. In the present study we determined whether, phenytoin, gabapentin and felbamate may reduce glutamate release by blocking the presynaptic NMDAr. Whole cell patch clamp recordings of spontaneous excitatory postsynaptic currents (sEPSCs) were used as a monitor of presynaptic glutamate release. Postsynaptic NMDAr were blocked with internal dialysis with an NMDAr channel blocker. The antagonist, 2-AP5, reduced the frequency of sEPSCs by blocking the presynaptic facilitatory NMDAr, but did not occlude a reduction in sEPSC frequency by gabapentin or phenytoin. Felbamate also reduced sEPSC frequency, but this effect was occluded by prior application of 2-AP5. Thus, whilst all three drugs can reduce glutamate release, only the action of felbamate seems to be due to interaction with presynaptic NMDAr.

Rodent models for glaucoma retinopathy and optic neuropathy

Animal models are useful to elucidate the etiology and pathology of glaucoma and to develop novel and more effective therapies for the disease. Because of the substantial similarities between the rodent and primate eyes, and the advances of relevant study techniques, rat and mouse models of glaucoma have recently become popular as research tools. This review surveys research techniques used in the measurement of rodent intraocular pressure, and also the evaluation of pertinent morphologic, biochemical, and functional changes in the retina, optic nerve head, and optic nerve. This review further describes in detail the individual rodent models, some of which serve as surrogate models and do not entail ocular hypertension, whereas others involve transient or chronic increases of intraocular pressure. The technical considerations and theoretical concerns of these models, their advantages, and limitations, are also discussed.

Balance of purines may determine life or death of retinal ganglion cells as A3 adenosine receptors prevent loss following P2X7 receptor stimulation

The purines ATP and adenosine can act as a coordinated team of transmitters. As extracellular adenosine is frequently derived from the enzymatic dephosphorylation of released ATP, the distinct actions of the two purines can be synchronized. In retinal ganglion cells (RGCs), stimulation of the P2X7 receptor for ATP leads to increased intracellular Ca2+ and death. Here we define the contrasting effects of adenosine and identify protective actions mediated by the A3 receptor. Adenosine attenuated the rise in Ca2+ produced by the P2X7 agonist 3'-O-(4-benzoylbenzoyl)ATP (BzATP). Adenosine was also neuroprotective, increasing the survival of ganglion cells exposed to BzATP. The A3 adenosine receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronimide (Cl-IB-MECA) mimicked the inhibition of the Ca2+ rise, whereas the A3 antagonist 3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191) reduced the protective effects of adenosine. Both Cl-IB-MECA and a second A3 receptor agonist IB-MECA reduced the cell loss triggered by BzATP. The actions of BzATP were mimicked by ATPgammaS, but not by ATP. In summary, adenosine can stop the rise in Ca2+ and cell death resulting from stimulation of the P2X7 receptor on RGCs, with the A3 adenosine receptor contributing to this protection. Hydrolysis of ATP into adenosine and perhaps inosine shifts the balance of purinergic action from that of death to the preservation of life.

Can phenytoin prevent Alzheimer's disease?

Alzheimer disease (AD) is the most common neurodegenerative disease presenting with relentless memory loss. A number of factors contribute to comorbidity and mortality and seizures are among the most common ones. Seizures worsen existing demantia, and by causing falls lead to serious complications like fractures and intracranial hemorrhage. Hippocampus is one of the earliest affected structures in brain in AD. It undergoes atrophy followed by progressive neuron loss. Atrophy appears much before the clinical manifestations of AD develop. Hence, there is a potential for its prevention or reversal. Neuron loss in brains of AD have two important consequences: these include development of AD and seizures. Patients of AD have unusually high incidence of seizures and the first seizure episode in this population is considered to be an indication of treatment with anti-seizure drugs. Among many drugs that have been shown to prevent hippocampal atrophy is phenytoin. It is time tested and proven anticonvulsant that can be used to prevent the development of atrophy and consequent development of AD. Phenytoin can thus be exploited for its dual effects i.e. reversal of hippocampal atrophy and anti-seizure effects. Selection of patients, appropriate dose, duration, monitoring and drug safety are important issues to be addressed before the testing of this hypothesis can be considered. Ideal candidates will be those who are at risk of development of AD or those with milder symptoms.

Sodium channel blockade with phenytoin protects spinal cord axons, enhances axonal conduction, and improves functional motor recovery after contusion SCI

Accumulation of intracellular sodium through voltage-gated sodium channels (VGSCs) is an important event in the cascade leading to anatomic degeneration of spinal cord axons and poor functional outcome following traumatic spinal cord injury (SCI). In this study, we hypothesized that phenytoin, a sodium channel blocker, would result in protection of axons with concomitant improvement of functional recovery after SCI. Adult male Sprague-Dawley rats underwent T9 contusion SCI after being fed normal chow or chow containing phenytoin; serum levels of phenytoin were within therapeutic range at the time of injury. At various timepoints after injury, quantitative assessment of lesion volumes, axonal degeneration, axonal conduction, and functional locomotor recovery were performed. When compared to controls, phenytoin-treated animals demonstrated reductions in the degree of destruction of gray and white matter surrounding the lesion epicenter, sparing of axons within the dorsal corticospinal tract (dCST) and dorsal column (DC) system rostral to the lesion site, and within the dorsolateral funiculus (DLF) caudal to the lesion site, and enhanced axonal conduction across the lesion site. Improved performance in measures of skilled locomotor function was observed in phenytoin-treated animals. Based on these results, we conclude that phenytoin provides neuroprotection and improves functional outcome after experimental SCI, and that it merits further examination as a potential treatment strategy in human SCI.