Brimonidine's Neuroprotective Effects against Transient Ischaemia-Induced Retinal Ganglion Cell Death

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.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

Current and new pharmacotherapeutic approaches for glaucoma

INTRODUCTION

Glaucoma is the leading cause of irreversible blindness worldwide. Medical therapy is the main line of treatment of open-angle glaucoma (OAG) and ocular hypertension. Despite the expansion of the glaucoma lineup with the newly approved medication classes, many barriers and issues still exist with topical therapy. Novel non-topical drug delivery may address such issues, adding more options to glaucoma pharmacotherapy.

AREAS COVERED

This review outlines current topical therapies for glaucoma, in addition to new medications under trials. The issues with topical therapy, in general, are discussed, as well as the new non-topical drug delivery systems. The authors performed a comprehensive search for published studies on glaucoma medical therapy using the electronic database of PubMed and manual search for each medication and non-topical delivery options. The extra- and intraocular delivery methods and the new topical glaucoma medications under research are covered.

EXPERT OPINION

Medical management of glaucoma represents a challenge for both patients and clinicians. Noncompliance (e.g. difficulty with administering the eye drops), cost, side effects, and intraocular pressure fluctuation are the major problems with topical therapy. To overcome such barriers, research should continue in developing new medications and innovation of non-topical drug delivery systems.

Intravitreal stem cell paracrine properties as a potential neuroprotective therapy for retinal photoreceptor neurodegenerative diseases

Retinal degenerations are the leading causes of irreversible visual loss worldwide. Many pathologies included under this umbrella involve progressive degeneration and ultimate loss of the photoreceptor cells, with age-related macular degeneration and inherited and ischemic retinal diseases the most relevant. These diseases greatly impact patients’ daily lives, with accompanying marked social and economic consequences. However, the currently available treatments only delay the onset or slow progression of visual impairment, and there are no cures for these photoreceptor diseases. Therefore, new therapeutic strategies are being investigated, such as gene therapy, optogenetics, cell replacement, or cell-based neuroprotection. Specifically, stem cells can secrete neurotrophic, immunomodulatory, and anti-angiogenic factors that potentially protect and preserve retinal cells from neurodegeneration. Further, neuroprotection can be used in different types of retinal degenerative diseases and at different disease stages, unlike other potential therapies. This review summarizes stem cell-based paracrine neuroprotective strategies for photoreceptor degeneration, which are under study in clinical trials, and the latest preclinical studies. Effective retinal neuroprotection could be the next frontier in photoreceptor diseases, and the development of novel neuroprotective strategies will address the unmet therapeutic needs.

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.

Effects of brimonidine tartrate 0.1% ophthalmic solution on the pupil, refraction, and light reflex

This study aimed to assess changes in pupil size, uncorrected visual acuity, refraction, and the direct and consensual light reflexes after instillation of brimonidine 0.1% in healthy subjects. The investigation comprised 46 eyes of 23 healthy subjects with no eye diseases in whom brimonidine 0.1% was instilled in the right eye. Pupil size was measured quantitatively under photopic and scotopic conditions, uncorrected visual acuity, refraction, and direct and consensual light reflexes before and at 1, 6, and 24 h after instillation. We found No significant change was found in refraction or uncorrected visual acuity (P = 0.999 and P = 0.998, respectively). A significant reduction in pupil size was observed under scotopic conditions at 1 h and 6 h after instillation (P = 0.007 and P = 0.005, respectively). The rate of pupil contraction and constriction speed measured by light reflexes were significantly increased at 1 h and 6 h after instillation (P = 0.021 and P = 0.033, respectively). Brimonidine 0.1% induced a significant reduction in pupil size under scotopic conditions without a significant change in refraction or visual acuity due to suppression of the sympathetic nervous system.

A Masked, Randomized, Phase 3 Comparison of Triple Fixed-Combination Bimatoprost/Brimonidine/Timolol versus Fixed-Combination Brimonidine/Timolol for Lowering Intraocular Pressure

OBJECTIVE

To evaluate the efficacy and safety of triple fixed-combination bimatoprost 0.01%/brimonidine 0.15%/timolol 0.5% (TFC) versus dual fixed-combination brimonidine 0.2%/timolol 0.5% (DFC) in primary open-angle glaucoma and ocular hypertension.

METHODS

Patients with intraocular pressure (IOP) ≥23 and ≤34 mmHg were randomized to twice-daily TFC or DFC. The primary variable is the change in worse eye mean IOP from baseline at week 12 (modified intent-to-treat (mITT) population). Secondary endpoints are mean IOP and mean change from baseline at weeks 1, 2, 4, 8, and 12 (mITT population). TFC superiority was demonstrated if the primary variable favored TFC (p ≤ 0.05). Sensitivity analyses were conducted, and safety was assessed at all visits.

RESULTS

TFC (n = 93) provided greater IOP reductions from baseline than DFC (n = 97) at week 12 (treatment difference, 0.85 mmHg; p = 0.028) and all other visits. TFC was also superior to DFC in patients with high baseline IOP (i.e., IOP ≥ 25 mmHg; p ≤ 0.011). Conjunctival hyperemia, ocular irritation, and dry eye were reported more often with TFC (p ≤ 0.016); however, discontinuations for ocular adverse events were similar between treatments.

CONCLUSIONS

TFC demonstrated IOP-lowering benefits that outweigh the risk of predominantly mild ocular side effects, which may be particularly relevant in patients who require greater IOP lowering to prevent/delay disease progression. This trial is registered with ClinicalTrials.gov registry number: NCT01241240.

Pharmacotherapy of glaucoma

Glaucoma is a group of diseases involving the optic nerve and associated structures, which is characterized by progressive visual field loss and typical changes of the optic nerve head (ONH). The only known treatment of the disease is reduction of intraocular pressure (IOP), which has been shown to reduce glaucoma progression in a variety of large-scale clinical trials. Nowadays, a relatively wide array of topical antiglaucoma drugs is available, including prostaglandin analogues, carbonic anhydrase inhibitors, beta-receptor antagonists, adrenergic agonists, and parasympathomimetics. In clinical routine, this allows for individualized treatment taking risk factors, efficacy, and safety into account. A major challenge is related to adherence to therapy. Sustained release devices may help minimize this problem but are not yet available for clinical routine use. Another hope arises from non-IOP-related treatment concepts. In recent years, much knowledge has been gained regarding the molecular mechanisms that underlie the disease process in glaucoma. This also strengthens the hope that glaucoma therapy beyond IOP lowering will become available. Implementing this concept with clinical trials remains, however, a challenge.

Trophic factors in the pathogenesis and therapy for retinal degenerative diseases

Trophic factors are endogenously secreted proteins that act in an autocrine and/or paracrine fashion to affect vital cellular processes such as proliferation, differentiation, and regeneration, thereby maintaining overall cell homeostasis. In the eye, the major contributors of these molecules are the retinal pigment epithelial (RPE) and Müller cells. The primary paracrine targets of these secreted proteins include the photoreceptors and choriocapillaris. Retinal degenerative diseases such as age-related macular degeneration and retinitis pigmentosa are characterized by aberrant function and/or eventual death of RPE cells, photoreceptors, choriocapillaris, and other retinal cells. We discuss results of in vitro and in vivo animal studies in which candidate trophic factors, either singly or in combination, were used in an attempt to ameliorate photoreceptor and/or retinal degeneration. We also examine current trophic factor therapies as they relate to the treatment of retinal degenerative diseases in clinical studies.

Brimonidine in the treatment of glaucoma and ocular hypertension

Treatment in glaucoma aims to lower intraocular pressure (IOP) to reduce the risk of progression and vision loss. The alpha2-adrenergic receptor agonist brimonidine effectively lowers IOP and is useful as monotherapy, adjunctive therapy, and replacement therapy in open-angle glaucoma and ocular hypertension. A fixed combination of brimonidine and timolol, available in some countries, reduces IOP as effectively as concomitant therapy with brimonidine and timolol and offers the convenience of 2 drugs in a single eyedrop. Brimonidine is safe and well tolerated. Its most common side-effects are conjunctival hyperemia, allergic conjunctivitis, and ocular pruritus. The newest formulation of brimonidine, brimonidine-Purite 0.1%, has a higher pH to improve the ocular bioavailability of brimonidine. This formulation contains the lowest effective concentration of brimonidine and is preserved with Purite(R) to enhance ocular tolerability. Brimonidine-Purite 0.1% is as effective in reducing IOP as the original brimonidine 0.2% solution preserved with benzalkonium chloride. Recent results from preclinical and clinical studies suggest that brimonidine may protect retinal ganglion cells and their projections from damage and death independently of its effects on IOP. The potential for neuroprotection with brimonidine is an added benefit of its use in glaucoma and ocular hypertension.

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.

Localization of alpha 2 receptors in ocular tissues

Alpha 2 adrenergic agonists are used for controlling intraocular pressure (IOP) in the treatment of glaucoma. They have also been shown to be neuroprotective to retinal cells in a variety of injury models. Despite this significance, the localization of the three known alpha 2 adrenergic receptors has not been unequivocally established. The aim of this study was to determine the location of the three alpha 2 adrenergic receptors in ocular tissues using immunohistochemical techniques. New antibodies were generated and their specificity was determined using Western blotting and preadsorption. In the anterior segment of the eye alpha 2A immunoreactivity was located in the nonpigmented ciliary epithelium, corneal, and conjunctival epithelia. Alpha 2B staining was not apparent in these tissues. Alpha 2C immunostaining was present in the membrane of pigmented ciliary epithelium and corneal and conjunctival epithelial cells. In the rat retina, all three receptor subtypes were present but were differentially localized. Alpha 2A was present in the somata of ganglion cell layer and inner nuclear layer somas, alpha 2B was located in the dendrites and axons of most of the neurons as well as glia, while alpha 2C was present in the somata and inner segment of the photoreceptors. In human and monkey retinas, similar pattern of labeling for alpha 2A and 2B receptors were observed, while alpha 2B was additionally present in the membranes of many cell somata in addition to dendrites and axons. Alpha 2C labeling was much weaker but exhibited similar pattern to that observed in the rat. These data provide additional information on the location of the alpha 2 receptors in the anterior portion of the eye and present new information on their specific location in the retina. This offers insights into possible targets for adrenergic agonists in a therapeutic context.

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.

Antivasoconstrictor effect of the neuroprotective agent dexrazoxane in rat aorta

Dexrazoxane is used clinically to reduce the cardiotoxicity of anthracycline cancer chemotherapeutic agents, acting by an iron-chelating antioxidant mechanism. In a study designed to explore the possible mechanism of the recently described neuroprotective effect of the drug in cerebral ischemia, its influence on vascular reactivity was determined in rat aortic rings. Dexrazoxane was found to be devoid of direct contractile or relaxant activity and to have no influence on responses to acetylcholine or histamine (relaxation), or to angiotensin or serotonin (contraction). In contrast, it decreased contractions to norepinephrine, as evidenced by rightward displacement of the concentration-response curves. The effect was prevented by the removal of the endothelium and by the alpha(2)-adrenoceptor antagonist yohimbine; it was partially antagonized by the endothelium-derived depolarizing factor inhibitor clotrimazole, but was not affected by L-NAME or indomethacin, inhibitors of endothelial nitric oxide and prostacyclin production. The anti-contractile effect did not occur in rings stimulated with the alpha(1)-adrenoceptor agonist phenylephrine. It was concluded that dexrazoxane opposes norepinephrine vascular contraction by enhancing endothelial alpha(2)-adrenoceptor-mediated release of relaxing factor(s). The drug could thus offset the deleterious vasoconstriction elicited by the increased circulating catecholamines present during cerebral ischemia, and by this mechanism produce neuroprotection.

Neuroprotective effects of brimonidine treatment in a rodent model of ischemic optic neuropathy

Ischemic optic neuropathy (ION) is a common disorder caused by disruption of the arterial blood supply to the optic nerve. It can result in significant loss of visual acuity and/or visual field. An ischemic optic nerve injury was produced in rats by intravenous injection of Rose Bengal dye followed by argon green laser application to the retinal arteries overlying the optic nerve, causing a coagulopathy within the blood vessels and disruption of optic nerve and retinal perfusion. The effect of brimonidine tartrate eye drops on survival of retinal ganglion cell axons in this experimental paradigm was studied. One eye was treated and the contralateral eye served as a control. Four groups of animals were used for this study. Group 1 received 7 days of treatment with 0.15% brimonidine tartrate eye drops twice a day prior to the ischemic injury. Group 2 animals received 0.15% brimonidine tartrate eye drops twice a day for 14 days after photocoagulation injury. Animal groups 3 and 4 received eye drops of 0.9% NaCl twice a day either daily for 7 days before injury or daily for 14 days, respectively. All rats were sacrificed 5 months after the injury to ascertain long-term optic axon survival. Coagulopathy-induced optic nerve ischemia resulted in a 71% loss of optic axons. Treatment with brimonidine daily for the 7 days prior to the injury resulted in a greater survival of optic axons, with only a 56.1% loss compared to control. Brimonidine treatment every day for 14 days after the ischemic injury did not result in a significant rescue of optic axons compared to injury alone. In summary, the application of brimonidine eye drops for one week prior to an ischemic injury resulted in a statistically significant increase in survival of optic axons within the injured optic nerves. Brimonidine treatment of the eye after the ischemic injury did not result in axon rescue, and axon loss was similar to the injured optic nerves treated with saline only. These results suggest that brimonidine may have potential use for prevention of ION in at-risk patients.

Neuroprotective effect of the novel Na+/Ca2+ channel blocker NS-7 on rat retinal ganglion cells

PURPOSE

To investigate whether NS-7, 4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride, a novel Na(+)/Ca(2+) channel blocker, can protect the rat retina subjected to ischemia-reperfusion insult.

METHODS

To evaluate the protective effect of NS-7 against retinal damage, the drug was administered before and after ischemia-reperfusion. Damage to the retina was assessed by measuring the thickness of the inner plexiform layer (IPL) and the outer nuclear layer (ONL) of each eye. In a subsequent experiment, electroretinographic (ERG) evaluation was also used.

RESULTS

In histopathologic evaluation, ischemia-reperfusion injury caused a significant reduction of IPL thickness (measured as the IPL/ONL ratio). In the NS-7-treated group, retinal damage was partially prevented by a concentration of 0.25 mg/kg per day. In the ERG evaluation, ischemia-reperfusion injury caused a reduction of A- and B-wave amplitudes. NS-7 treatment significantly prevented the reduction of the B wave at a concentration of 0.1 or 0.3 mg/kg, while the reduction of the A wave was not significantly affected.

CONCLUSIONS

NS-7 has neuroprotective effects against retinal damage resulting from subjection to ischemia. In addition, NS-7 can be used as an agent for treating acute ischemic retinopathy, including diseases associated with very high intraocular pressure, such as acute angle-closure glaucoma.

Ocular distribution after topical instillation and potential neuroprotective effect after intravitreal injection of the calcium channel blocker iganipidine

PURPOSE

To investigate the effects of iganidipine, a new calcium antagonist on glutamate agonist-induced retinal damage.

METHODS

Iganidipine was injected with N-methyl-D-aspartate (NMDA) or kainic acid (KA) into the rat vitreous, and the retina was histologically examined. After co-injection with KA, the number of DiI-labeled retinal ganglion cells was also counted. Rabbits received unilateral instillation of 0.03% iganidipine twice daily for 14 days, and the iganidipine level in the posterior retina-choroid was determined by liquid chromatography/mass spectrometry.

RESULTS

Coadministration of iganidipine had no significant effect on NMDA-induced thinning of the inner plexiform layer but partly suppressed KA-induced thinning at final intravitreous concentrations of 10(-8) M or higher, which was confirmed by counting the ganglion cell number. The iganidipine level in the posterior retina-choroid was approximately 6.2 x 10(-7) M in the instilled eye, which was higher than on the contralateral side by 5.2 x 10(-7) M (p = 0.035).

CONCLUSIONS

Iganidipine suppressed KA-induced retinal damage in rats. This suppression was achieved with a lower concentration than that resulting from unilateral instillations of iganidipine in the ipsilateral posterior retina-choroid in rabbits.

Gold standard medical therapy for glaucoma: defining the criteria identifying measures for an evidence-based analysis

BACKGROUND

Over the past decade, several new medical therapies have become available for the treatment of primary open-angle glaucoma (POAG). A systematic evidence-based approach for identifying an optimal therapeutic agent is lacking.

OBJECTIVES

The aims of this review were to critically evaluate published treatment recommendations for POAG and, based on a systematic review of the literature, to develop criteria that would define a "gold standard" medical therapy that reflects new treatment advances and established therapeutic goals.

METHODS

A MEDLINE search spanning the years 1966 to 2002 and using the search terms gold standard, drug of choice, agent of choice, benchmark, ophthalmology, eye, and glaucoma was conducted and the results reviewed by a panel of 15 experts in the field of glaucoma. Published treatment recommendations for POAG were discussed. Criteria, anchored to medical evidence, for distinguishing a standard of medical therapy for POAG were defined.

RESULTS

The terms connoting a gold standard therapy were found in only 258 of approximately 368,000 ophthalmology-related citations and 53 of almost 23,000 glaucoma citations, validating the need to define therapeutic standards. The lack of recommendations for the use of new classes of ocular hypotensive agents was acknowledged. Criteria identified to evaluate intraocular pressure (IOP)-lowering agents as gold standards included the following: efficacy in reducing IOP consistently over a 24-hour period to a level that will preserve the visual field and protect the optic nerve without inducing tachyphylaxis and tolerance, paucity of local and systemic adverse effects, promotion of patient compliance, and applicability in diverse patient populations.

CONCLUSIONS

These criteria should be employed as measures for evidence-based analyses to evaluate available and future IOP-lowering medical therapies for POAG. The conceptual framework presented may be applicable to other therapeutic areas.

Possible involvement of a fibroblast growth factor 9 (FGF9)-FGF receptor-3-mediated pathway in adult pig retinal ganglion cell survival in vitro

The expression and potential roles of fibroblast growth factors (FGF) and their cognate FGF receptors (FGFR) in adult mammalian retinal ganglion cells (RGC) are poorly known. We show that FGFR-3 and FGFR-4 are especially pronounced on RGC and amacrine cell bodies in adult pig inner retinae both in vivo and in vitro. Western blotting revealed distinct profiles for each receptor. Expression of each FGFR and effects of the preferred ligand for FGFR-3, FGF9, upon RGC survival and neurite outgrowth were examined in primary retinal cell cultures: whereas there was no stimulation of neuritogenesis, RGC survival was promoted in a dose-dependent manner (ED(50) approximately 500 pg/ml, mean maximal increase of 60%) and could be completely blocked by addition of FGF9 neutralising antibody. Experiments with three additional FGF (FGF1, FGF2, and FGF4) showed no stimulation of RGC survival above control levels. Taken together, these data suggest that the ligand-receptor couple FGF9-FGFR-3 may function to promote survival of adult mammalian RGC, and their application might be beneficial in retinal degenerative diseases such as glaucoma.

Hypothesis for a common basis for neuroprotection in glaucoma and Alzheimer's disease: anti-apoptosis by alpha-2-adrenergic receptor activation

Recent studies have suggested glaucomatous loss of retinal ganglion cells and their axons in Alzheimer's disease. Amyloid beta peptides and phosphorylated tau protein have been implicated in the selective regional neuronal loss and protein accumulations characteristic of Alzheimer's disease. Similar protein accumulations are not present on glaucomatous retinal ganglion cells. Neurons die in both Alzheimer's disease and glaucoma by apoptosis, although the signaling pathways for neuronal degradation appear to differ in the two diseases. Alzheimer's disease features a loss of locus ceruleus noradrenergic neurons, which send axon terminals to the brain regions suffering neuronal apoptosis and results in reductions in noradrenaline in those regions. Activation of alpha-2 adrenergic receptors reduces neuronal apoptosis, in part through a protein kinase B (Akt)-dependent signaling pathway. Loss of noradrenaline innervation facilitates neuronal apoptosis in Alzheimer's disease models and may act similarly in glaucoma. Alpha-2 adrenergic receptor agonists offer the potential to slow the neuronal loss in both diseases by compensating for lost noradrenaline innervation.

The beta-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol

beta-adrenoceptor antagonists are used clinically to reduce elevated intraocular pressure in glaucoma which is characterised by a loss of retinal ganglion cells. Previous studies have shown that the beta(1)-selective adrenoceptor antagonist, betaxolol, is additionally able to protect retinal neurones in vitro and ganglion cells in vivo from the detrimental effects of either ischemia-reperfusion or from excitotoxicity, after topical application. The neuroprotective effect of betaxolol is thought not to be elicited through an interaction with beta-adrenoceptors, but by its ability to reduce influx of sodium and calcium through voltage-sensitive calcium and sodium channels. In the present study it is shown that the non-selective beta-adrenoceptor antagonists, metipranolol and timolol behave like betaxolol. When topically applied they all attenuate the detrimental effect of ischemia-reperfusion. Protection of the retina was determined by evaluating changes in the electroretinogram and by assessing the loss of mRNA for Thy-1, which is expressed in retinal ganglion cells. In addition, studies conducted on neurones in mixed retinal cultures demonstrated that metipranolol, betaxolol and timolol were all able to partially counteract anoxia-induced cell loss and viability reduction. The influence of timolol was, however, not significant. Within the confines of these investigations, an order of neuroprotective efficacy was delineated for the three beta-adrenoceptor antagonists: betaxolol>metipranolol>timolol. The ability of the beta-adrenoceptor antagonists to attenuate ligand-induced stimulation of calcium and sodium entry into neuronal preparations showed a similar order of effectiveness. In conclusion, the ability to confer neuroprotection to retinal neurones is a common feature of three ophthalmic beta-adrenoceptor antagonists (betaxolol, metipranolol and timolol). A comparison of the effectiveness of the individual compounds in protecting retinal cells in vivo was not possible in these studies. However, in vitro studies show that the capacity of the individual beta-adrenoceptor antagonists to act as neuroprotectants appears to relate to their capacity to attenuate neuronal calcium and sodium influx.

Rat retinal ganglion cell loss caused by kainate, NMDA and ischemia correlates with a reduction in mRNA and protein of Thy-1 and neurofilament light

Quantification of retinal ganglion cell (RGC) loss/survival following a defined insult to the retina is a prerequisite in order to allow a comparison to be made between the effectiveness of potential neuroprotective drugs. The purpose of the present study was to extend the characterisation of our previously published semiquantitative RT-PCR assay to assess RGC loss/survival. Comparisons were made between the total mRNA levels of the ganglion cell-specific markers Thy-1 and neurofilament light (NF-L) in the retina at specific times after an intravitreal injection of N-methyl-D-aspartate (NMDA) or kainate or after 45 min of ischemia/reperfusion and also between the levels of NF-L mRNA and protein at various times after NMDA injection. Changes in Thy-1 and NF-L immunoreactivities were also observed. NMDA, kainate and ischemia/reperfusion all caused a reduction in the retinal content of Thy-1 and NF-L mRNAs and immunoreactivities. An excellent correlation was observed between the levels of the two mRNAs after these treatments. After NMDA, loss of NF-L mRNA was shown to precede loss of NF-L protein but total loss of each marker was similar after 7 days. The results of the study demonstrate that injury and subsequent death of RGCs, which occurs after ischemia/reperfusion and after intraocular injection of NMDA or kainate, can be followed by measurement of total retinal levels of Thy-1 and NF-L mRNAs and NF-L protein. The assays provides accurate, practical and complementary methods for assessing the potential benefits of neuroprotective drugs on RGCs which have been injured by a variety of experimental modalities.

Brimonidine Purite and bimatoprost compared with timolol and latanoprost in patients with glaucoma and ocular hypertension

This 3-month multicenter, investigator-masked, parallel-group study compared brimonidine Purite and bimatoprost (brimP/bim) with timolol gel-forming solution and latanoprost (tim/latan) in 28 patients with open-angle glaucoma or ocular hypertension. IOP was measured at baseline and 2 hours after morning instillation at weeks 2, 4, and 12. The primary outcome measure was the mean IOP reduction from baseline. Secondary measures were the percentage of patients in each group who achieved specific low target pressures and the incidence of adverse events. Mean IOP at baseline was 24.8 mm Hg in each group. At follow-up visits, mean reductions from baseline ranged from 8.5 to 9.0 mm Hg with brimP/bim and from 7.5 to 7.7 mm Hg with tim/latan. More patients achieved low target pressures with brimP/bim. At week 12, 69.2% of brimP/bim patients and 27.3% of tim/latan patients had IOPs of 16 mm Hg or lower (P = .024). Both regimens were well tolerated, and adverse events were infrequent. The combination of brimonidine Purite and bimatoprost was well tolerated and at least as effective as Timoptic-XE and latanoprost in reducing IOP. More patients achieved low target IOPs with brimonidine Purite and bimatoprost than with Timoptic-XE and latanoprost. A larger study is needed to confirm these results.