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

Comparisons of α2-Adrenergic Agents, Medetomidine and Xylazine, with Pentobarbital for Anesthesia: Important Pitfalls in Diabetic and Nondiabetic Rats

Purpose: Anesthesia is necessary to conduct rodent electroretinograms (ERGs). We evaluated utility of the α2-agonist medetomidine versus xylazine for ERG studies in nondiabetic and diabetic rats. Pentobarbital was included as a comparator. Methods: Male Sprague-Dawley rats, with and without streptozotocin (STZ)-induced diabetes, were anesthetized with medetomidine (1 mg/kg), xylazine (10 mg/kg) (both with ketamine 75 mg/kg), or pentobarbital (70 mg/kg). The depth of anesthesia was assessed, and if adequate, scotopic ERGs were recorded. Blood glucose was monitored. Results: In nondiabetic rats, all three agents induced satisfactory anesthesia, but with differing durations: medetomidine > pentobarbital > xylazine. ERG responses were similar under medetomidine and xylazine, but relatively reduced under pentobarbital. Both α2-agonists (but not pentobarbital) elicited marked hyperglycemia (peak values 316.1 ± 42.6 and 300.3 ± 29.5 mg/dL, respectively), persisting for 12 h. In diabetic rats, elevated blood glucose concentrations were not affected by any of the agents, but the depth of anesthesia under medetomidine and xylazine was inadequate for ERG recording. Conclusions: In nondiabetic rats, medetomidine and xylazine elicited comparable effects on ERGs that differ from pentobarbital, but both perturbed glucose metabolism, potentially confounding experimental outcomes. In STZ-diabetic rats, neither α2-agent provided adequate anesthesia, while pentobarbital did so. Problems with α2-anesthetic agents, including medetomidine, must be recognized to ensure meaningful interpretation of experimental results.

Animal Models of LED-Induced Phototoxicity. Short- and Long-Term In Vivo and Ex Vivo Retinal Alterations

Phototoxicity animal models have been largely studied due to their degenerative communalities with human pathologies, e.g., age-related macular degeneration (AMD). Studies have documented not only the effects of white light exposure, but also other wavelengths using LEDs, such as blue or green light. Recently, a blue LED-induced phototoxicity (LIP) model has been developed that causes focal damage in the outer layers of the superior-temporal region of the retina in rodents. In vivo studies described a progressive reduction in retinal thickness that affected the most extensively the photoreceptor layer. Functionally, a transient reduction in a- and b-wave amplitude of the ERG response was observed. Ex vivo studies showed a progressive reduction of cones and an involvement of retinal pigment epithelium cells in the area of the lesion and, in parallel, an activation of microglial cells that perfectly circumscribe the damage in the outer retinal layer. The use of neuroprotective strategies such as intravitreal administration of trophic factors, e.g., basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) or pigment epithelium-derived factor (PEDF) and topical administration of the selective alpha-2 agonist (Brimonidine) have demonstrated to increase the survival of the cone population after LIP.

Oral Scutellarin Treatment Ameliorates Retinal Thinning and Visual Deficits in Experimental Glaucoma

Purpose: Intraocular pressure (IOP) is currently the only modifiable risk factor for glaucoma, yet glaucoma can continue to progress despite controlled IOP. Thus, development of glaucoma neurotherapeutics remains an unmet need. Scutellarin is a flavonoid that can exert neuroprotective effects in the eye and brain. Here, we investigated the neurobehavioral effects of scutellarin treatment in a chronic IOP elevation model. Methods: Ten adult C57BL/6J mice were unilaterally injected with an optically clear hydrogel into the anterior chamber to obstruct aqueous outflow and induce chronic IOP elevation. Eight other mice received unilateral intracameral injection of phosphate-buffered saline only. Another eight mice with hydrogel-induced unilateral chronic IOP elevation also received daily oral gavage of 300 mg/kg scutellarin. Tonometry, optical coherence tomography, and optokinetics were performed longitudinally for 4 weeks to monitor the IOP, retinal nerve fiber layer thickness, total retinal thickness, visual acuity, and contrast sensitivity of both eyes in all three groups. Results: Intracameral hydrogel injection resulted in unilateral chronic IOP elevation with no significant inter-eye IOP difference between scutellarin treatment and untreated groups. Upon scutellarin treatment, the hydrogel-injected eyes showed less retinal thinning and reduced visual behavioral deficits when compared to the untreated, hydrogel-injected eyes. No significant difference in retinal thickness or optokinetic measures was found in the contralateral, non-treated eyes over time or between all groups. Conclusion: Using the non-invasive measuring platform, oral scutellarin treatment appeared to preserve retinal structure and visual function upon chronic IOP elevation in mice. Scutellarin may be a novel neurotherapeutic agent for glaucoma treatment.

A Topical Formulation of Melatoninergic Compounds Exerts Strong Hypotensive and Neuroprotective Effects in a Rat Model of Hypertensive Glaucoma

Melatonin is of great importance for regulating several eye processes, including pressure homeostasis. Melatonin in combination with agomelatine has been recently reported to reduce intraocular pressure (IOP) with higher efficacy than each compound alone. Here, we used the methylcellulose (MCE) rat model of hypertensive glaucoma, an optic neuropathy characterized by the apoptotic death of retinal ganglion cells (RGCs), to evaluate the hypotensive and neuroprotective efficacy of an eye drop nanomicellar formulation containing melatonin/agomelatine. Eye tissue distribution of melatonin/agomelatine in healthy rats was evaluated by HPLC/MS/MS. In the MCE model, we assessed by tonometry the hypotensive efficacy of melatonin/agomelatine. Neuroprotection was revealed by electroretinography; by levels of inflammatory and apoptotic markers; and by RGC density. The effects of melatonin/agomelatine were compared with those of timolol (a beta blocker with prevalent hypotensive activity) or brimonidine (an alpha 2 adrenergic agonist with potential neuroprotective efficacy), two drugs commonly used to treat glaucoma. Both melatonin and agomelatine penetrate the posterior segment of the eye. In the MCE model, IOP elevation was drastically reduced by melatonin/agomelatine with higher efficacy than that of timolol or brimonidine. Concomitantly, gliosis-related inflammation and the Bax-associated apoptosis were partially prevented, thus leading to RGC survival and recovered retinal dysfunction. We suggest that topical melatoninergic compounds might be beneficial for ocular health.

Retinal Cell Protection in Ocular Excitotoxicity Diseases. Possible Alternatives Offered by Microparticulate Drug Delivery Systems and Future Prospects

Excitotoxicity seems to play a critical role in ocular neurodegeneration. Excess-glutamate-mediated retinal ganglion cells death is the principal cause of cell loss. Uncontrolled glutamate in the synapsis has significant implications in the pathogenesis of neurodegenerative disorders. The exploitation of various approaches of controlled release systems enhances the pharmacokinetic and pharmacodynamic activity of drugs. In particular, microparticles are secure, can maintain therapeutic drug concentrations in the eye for prolonged periods, and make intimate contact by improving drug bioavailability. According to the promising results reported, possible new investigations will focus intense attention on microparticulate formulations and can be expected to open the field to new alternatives for doctors, as currently required by patients.

Retinal biomarkers provide "insight" into cortical pharmacology and disease

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

Translational Pharmacology in Glaucoma Neuroprotection

Glaucoma is both the most common optic neuropathy worldwide and the most common cause of irreversible blindness in the world. The only proven treatment for glaucomatous optic neuropathy is lowering the intraocular pressure, achieved with a variety of pharmacological, laser, and surgical approaches. Over the past 2 decades there has been much basic and clinical research into achieving treatment of the underlying optic nerve damage with neuroprotective approaches. However, none has resulted in regulatory approval based on successful phase 3 studies. This chapter discusses the reasons for this "lost in translation" aspect of glaucoma neuroprotection, and outlines issues at the laboratory and clinical trial level that need to be addressed for successful development of neuroprotective therapies.

Brimonidine Enhances the Electrophysiological Response of Retinal Ganglion Cells through the Trk-MAPK/ERK and PI3K Pathways in Axotomized Eyes

PURPOSE

To investigate changes in retinal ganglion cell (RGC) activity by measuring the positive scotopic threshold response (pSTR) of the electroretinogram (ERG) in axotomized eyes after brimonidine injection.

METHODS

In 50 adult Sprague-Dawley rats, the left eye was axotomized and injected with phosphate buffered saline (PBS) or brimonidine and the contralateral right eye was left untreated. Scotopic ERGs were recorded simultaneously from both eyes on days 1, 2, 3, 7, and 10 after the intravitreal injection, and the amplitude of the a- and b-waves and the pSTR were measured. Surviving RGCs in the flat-mounted retinas were counted 10 days after axotomy. In addition to brimonidine, K252a (an inhibitor of tyrosine kinase phosphorylation of the Trk receptors), U0126 (a MAPK/ERK kinase inhibitor), and LY294002 (phosphoinositide 3-kinases [PI3Ks]) were also injected intravitreally into the left eye, and ERGs were recorded using the same protocol.

RESULTS

The pSTR amplitude increased significantly in the axotomized eyes with brimonidine, to 122.9 ± 5.0%, 161.8 ± 8.3%, and 133.6 ± 8.1% on days 1, 2, and 3 (P < 0.01), respectively, compared to the axotomized eyes treated with PBS (control). The increased pSTR amplitude returned to normal (103.6 ± 6.7%) on day 7, although there were a greater number of surviving RGCs in the treatment groups than in the controls. The intravitreal injection of K252a, U0126, or LY294002 significantly attenuated the increase in pSTR induced by intravitreal brimonidine (P < 0.01).

CONCLUSION

Intravitreal brimonidine enhanced the survival and electrophysiological activity of the RGCs in rats. The mechanism of this electrophysiological change may involve activation of the Trk-MAPK/ERK and Trk-PI3K signals.

Effects of timolol maleate, levobunolol and apraclonidine on intraocular pressure, pupil size, blood pressure and heart rate in beagles

ABSTRACT The aim of this study was to evaluate changes in intraocular pressure (IOP), pupil size (PS), blood pressure (BP), heart rate (HR), and ECG variables (Pms wave PmV, PR interval, QRS complex, RMV wave and QT intervals) over time during the instillation of 0.5% timolol, 0.5% levobunolol and 0.5% apraclonidine in clinically normal dogs. Ten adult beagles were used. Baseline values were measured at 8a.m., 2p.m. and 8p.m., for three consecutive days. A waiting period of 10 days between the administrations of each drug was established. For 15 consecutive days, the drug being tested was instilled in one eye of each dog twice a day (7a.m. and 7p.m.). The parameters were evaluated at the aforementioned times on days 3, 6, 9, 12 and 15. Data were statistically compared using the Bonferroni test and one-way repeated measures analysis of variance (P<0.05). The Pearson test was used to evaluate any correlation between QT interval, HR and BP. The tested drugs did not find a decrease in IOP. A significant decreased in PS was observed in almost all dogs following levobunolol administration, relative to the control eye. A significant decrease in HR was observed on day 3 following levobunolol treatment, while apraclonidine induced an increase on day 15. Blood pressure was reduced in all measurement time points following apraclonidine treatment. A negative correlation between QT interval and HR was only observed in dogs treated with timolol. In conclusion, levobunolol was the only drug that induced significant alterations in PS. Apraclonidine was the only drug that induced systemic hypotension. Timolol was the only drug to that induced a negative correlation between QT and HR.

Optogenetic approaches to retinal prosthesis

The concept of visual restoration via retinal prosthesis arguably started in 1992 with the discovery that some of the retinal cells were still intact in those with the retinitis pigmentosa disease. Two decades later, the first commercially available devices have the capability to allow users to identify basic shapes. Such devices are still very far from returning vision beyond the legal blindness. Thus, there is considerable continued development of electrode materials, and structures and electronic control mechanisms to increase both resolution and contrast. In parallel, the field of optogenetics--the genetic photosensitization of neural tissue holds particular promise for new approaches. Given that the eye is transparent, photosensitizing remaining neural layers of the eye and illuminating from the outside could prove to be less invasive, cheaper, and more effective than present approaches. As we move toward human trials in the coming years, this review explores the core technological and biological challenges related to the gene therapy and the high radiance optical stimulation requirement.

Brimonidine promotes axon growth after optic nerve injury through Erk phosphorylation

It is well known that axons of the adult mammalian central nervous system have a very limited ability to regenerate after injury. Therefore, the neurodegenerative process of glaucoma results in irreversible functional deficits, such as blindness. Brimonidine (BMD) is an alpha2-adrenergic receptor agonist that is used commonly to lower intraocular pressure in glaucoma. Although it has been suggested that BMD has neuroprotective effects, the underlying mechanism remains unknown. In this study, we explored the molecular mechanism underlying the neuroprotective effect of BMD in an optic nerve injury (ONI) model. BMD treatment promoted optic nerve regeneration by inducing Erk1/2 phosphorylation after ONI. In addition, an Erk1/2 antagonist suppressed BMD-mediated axonal regeneration. A gene expression analysis revealed that the expression of the neurotrophin receptor gene p75 was increased and that the expression of the tropomyosin receptor kinase B (TrkB) gene was decreased after ONI. BMD treatment abrogated the changes in the expression of these genes. These results indicate that BMD promotes optic nerve regeneration via the activation of Erk1/2.

Is elevated noradrenaline an aetiological factor in a number of diseases?

1 Here I put forth the hypothesis that noradrenaline (NA), which is a signalling molecule in the brain and sympathetic nervous system (SNS), is an aetiological factor in a number of diseases. 2 In a previous paper (Fitzgerald, Int. J. Cancer, 124, 2009, 257), I examined evidence that elevated NA is a factor in various types of cancer. Here I extend the argument to several other diseases, including diabetes mellitus, open-angle glaucoma, osteoarthritis and rheumatoid arthritis and asthma. 3 The principal hypothesis is that, largely as a result of genetics, elevated noradrenergic tone in the SNS predisposes a large number of individuals to a broad range of diseases. 4 For each of the above five diseases, I briefly examine the following four lines of evidence to assess the hypothesis: i) whether pharmacological studies in rodents that manipulate NA levels or receptors affect these diseases; ii) whether pharmacological manipulation of NA in humans affects these diseases; iii) whether bipolar disorder, excessive body weight, and hypertension, which may all three involve elevated NA, tend to be comorbid with these diseases and iv) whether psychological stressors tend to cause or exacerbate these conditions, since psychological stress is associated with increased release of NA. 5 The four lines of evidence tend to support the hypothesis.

α2-Adrenergic Receptors Are Present in Normal Human Conjunctiva

PURPOSE

Despite the passage of medications, including antiglaucoma drugs, through the ocular surface, and despite the increasing relevance of neurogenic inflammation in the ocular surface, the presence of some neuroreceptors in the conjunctiva has not been ascertained. This study describes the presence of alpha2-adrenergic receptors in normal human conjunctiva.

METHODS

Immunofluorescence microscopy, electrophoresis, and Western blot analyses were done in human conjunctival biopsies and rat control tissues. Antibodies against alpha2-adrenergic receptor subtypes alpha2A, alpha2B, and alpha2C were used.

RESULTS

Immunoreactivity for alpha2A- and alpha2B-, but not alpha2C-adrenergic receptors was evenly distributed in epithelial cells of human conjunctiva cryosections. Immunoreactive bands were detected for the three alpha2-adrenergic receptor subtypes: a major band of 48-50 kDa and fellow bands of 65-67 kDa.

CONCLUSIONS

Normal human conjunctival epithelial cells express alpha2A-, alpha2B-, and alpha2C-adrenergic receptors. Further studies to determine the functional implications of these receptors in ocular surface homeostasis are warranted.

A pilot study of topical treatment with an alpha2-agonist in patients with retinal dystrophies

PURPOSE

The aim of this study was to assess the neuroprotective effect of a topical alpha2-agonist in patients with retinal dystrophies.

METHODS

This study was a prospective, placebo-controlled, double-masked, randomized clinical trial. A total of 26 patients with retinal dystrophies were included. One (1) randomly selected eye was treated with brimonidine tartrate 0.2% twice-daily, while the fellow eye received artificial tears. Disease progression parameters tested at 6-8-month intervals throughout the study included Goldmann visual fields, contrast sensitivity, color vision, and fullfield electroretinography.

RESULTS

Seventeen (17) of the 26 recruited patients completed the study. Except for 1 patient with an 18-month follow-up, all patients were followed up for 24-36 months (mean, 29). At the conclusion of the study, there were no differences detected in visual acuity, color vision, and contrast sensitivity between the treated and control eyes. There was a trend, however, toward a lesser degree of visual field loss in the brimonidine-treated eyes. There was also a delay in the time required to reach a 25% visual field loss in the treated eyes. These differences were more pronounced in a subgroup of patients diagnosed as retinitis pigmentosa and with visual fields of 5 cm2 or more at baseline.

CONCLUSIONS

The findings of this pilot study suggest a trend for slower progression in the eyes of patients with retinal dystrophy when treated with brimonidine, according to one of the parameters that was studied (visual field loss). Further studies that include a larger number of patients and a longer follow-up period are needed to clarify and confirm the potential neuroprotective effect of alpha2-agonists in human retinal dystrophies.

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.

Alteration of retinal intrinsic survival signal and effect of α2–adrenergic receptor agonist in the retina of the chronic ocular hypertension rat

The purpose of this study is to examine the retinal expression of intrinsic cell survival molecules and to elucidate the effect of an α2-adrenergic receptor agonist in the chronic ocular hypertensive rat model. Chronic ocular hypertension was induced in both eyes of each rat by episcleral vein cauterization. Two five-microliter drops of the selective α2-adrenoceptor agonist brimonidine 0.2% (Alphagan; Allergan Inc., Irvine, CA, USA) were topically administered twice daily for up to eight weeks in one eye. The fellow eye received balanced salt solution as a control. Protein and mRNA expression were evaluated at 1, 4, and 8 weeks after injury. Retinal expression of BDNF, Akt, and GFAP was assessed using immunohistochemistry. Retinal levels of mRNA for BDNF, bcl-2, and bcl-xL were determined using semi-quantitative RT-PCR. Retinal ganglion cell (RGC) density was evaluated after retrograde labeling with 4-Di-10-ASP (DiA). A significant decrease in RGC density was observed in ocular hypertensive eyes. Cauterized eyes showed an increase in GFAP expression from one week after injury, and the expression of bcl-2, bcl-xL, and BDNF mRNA was also increased. Treatment of ocular hypertensive eyes with brimonidine resulted in a reduction in RGC loss, a decrease in the level of GFAP immunoreactivity, and an increment in BDNF mRNA and p-Akt expression. Brimonidine appears to protect RGCs from neurodegeneration through mechanisms involving α2-adrenergic receptor mediated survival signal activation and up-regulation of endogenous neurotrophic factor expression in the chronic ocular hypertensive rat retina.

Efeitos maleato de timolol 0.5% do cloridrato de dorzolamida 2%, e da associação de ambas na pressão intra-ocular

Avaliaram-se efeitos da dorzolamida do timolol e da combinação de ambos sobre pressão intra-ocular (PIO) de cães normais, além de alterações no olho contralateral, não-tratado. Foram utilizados 60 cães sadios, distribuídos em três grupos (G) de 20 animais. No primeiro grupo (GT), foi avaliada a ação do maleato de timolol 0,5% na PIO; no segundo (GD), a ação do cloridrato de dorzolamida 2%; e, no terceiro (GTD), o efeito da associação fixa timolol/dorzolamida. A PIO foi aferida utilizando-se tonômetro de aplanação (Tonopen®), uma hora antes e uma, duas, quatro, seis e oito horas após a instilação do colírio em análise no olho esquerdo. O efeito da associação timolol/dorzolamida foi mais intenso (27%) que os efeitos do timolol (21,9%) e da dorzolamida (22,4%) na redução da PIO. No olho contralateral, verificou-se redução de 7% no GT, 13,8% no GD e 13,6% no GTD, após quatro e duas horas da administração.

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.

Vitreous Concentration of Topically Applied Brimonidine-Purite 0.15%

PURPOSE

The aim of this study was to determine the vitreous brimonidine concentration of topically applied brimonidine-Purite 0.15%.

METHODS

Patients scheduled for a pars plana vitrectomy were invited to participate in this study after institutional review board (IRB) approval was obtained. Each patient was asked to apply brimonidine-Purite (0.15%) drops in the designated eye either every 12 h (b.i.d.; 9 patients) or every 8 h (t.i.d.; 10 patients) for the 2 weeks proceeding scheduled surgery. The importance of the last topical dose being 12 h (b.i.d. group) or 8 h (t.i.d. group) before the scheduled surgery, was emphasized. Four (4) patients served as controls and did not receive any drops. Vitreous (approximately 0.5-1.0 mL) was aspirated prior to opening the infusion line. Specimens were frozen at -68 degrees C until analyzed.

RESULTS

In the b.i.d. group, the mean concentration of brimonidine was 16.74 nM+/-10.33 standard deviation (range, 0.42-34.68 nM; median, 16.38); in the t.i.d. group, the mean concentration of brimonidine was 19.16 nM+/-15.40 standard deviation (range, 0.22-39.48 nM; median, 16.98). A significant difference was observed between the (no drug) control vitreous brimonidine levels and b.i.d. or t.i.d. vitreous levels (P<0.01, <0.01, respectively; n=4, 9, and 10, respectively); and in brimonidine levels between t.i.d. phakic patients and t.i.d. patients with posterior chamber lens (P=0.04; n=4 and 6, respectively).

CONCLUSIONS

Brimonidine-Purite 0.15%, topically applied b.i.d. or t.i.d. for 2 weeks prior to collection, acquired vitreous levels of brimonidine at or above the 2-nM concentration known to activate the neuroprotective alpha-2 receptor in animals.

Study of effects of antiglaucoma eye drops on N-methyl-D-aspartate-induced retinal damage

PURPOSE

To study the effects of antiglaucoma eye drops on N-methyl-D-aspartate (NMDA)-induced retinal damage.

METHODS

Several antiglaucoma eye drops, beta-blockers, alpha/beta-blockers, an alpha1-blocker, an alpha2-agonist, and a prostaglandin derivative, were topically administrated to NMDA-treated rat eyes daily for 2 weeks, and the retinal thickness, the number of retrograde-labeled retinal ganglion cells (RGCs), and the results of a cDNA microarray analysis were studied.

RESULTS

Intravitreal administration of NMDA caused a significant decrease in the thickness of the retinal layers and induced upregulation of glial fibrillary acidic protein (GFAP). Topical administration of beta-blockers (timolol, betaxolol, and carteolol) and a prostaglandin derivative (latanoprost) showed almost no significant effects on retinal thickness, the number of RGCs, or expression of GFAP. In contrast, the alpha/beta-blockers (nipradilol and levobunolol), the alpha1-blocker (bunazosin HCl), and the alpha2-agonist (brimonidine) showed preservation effects on retinal thickness and the number of RGCs, and marked suppression of NMDA-induced upregulation of GFAP. Among 1101 genes related to cellular regulatory mechanisms, the expression of two genes, both for insulin-like growth factors, (IGF-1) and ErbB3, was altered upon administration of the alpha/beta-blockers, the alpha1-blocker, and the alpha2-agonist.

CONCLUSION

Our present study suggests that modulations of the alpha-adrenergic receptor, alpha1-blocking and alpha2-stimulation, by antiglaucoma eye drops may cause beneficial effects on NMDA-induced retinal damage in the rat.

Neuroprotective effects of cardiotrophin-like cytokine on retinal ganglion cells

BACKGROUND

Premature neuronal cell death is a feature of numerous central nervous system and eye diseases, including glaucoma. Neurons (including retinal ganglion cells, RGCs) are protected by several neurotrophic factors, among those the IL-6 family of cytokines. Lately, a novel member of the IL-6 family of cytokines has been identified and cloned. This cytokine is known as novel neurotrophin-1/B-cell-stimulating factor-3 (NNT-1/BSF-3) or cardiotrophin-like cytokine (CLC). It shows neurotrophic as well as B-cell stimulatory effects.

METHODS

In this study, the neuroprotective properties of CLC on RGC loss in vivo were investigated.

RESULTS

CLC significantly protected RGCs from degeneration in both chosen models of retinal neuronal damage: optic nerve crush (P<0.01) and N-methyl-D-aspartate (NMDA) injection (P<0.001).

CONCLUSIONS

CLC shows neuroprotective effects on RGCs in vivo and might be a treatment option for chronic neurodegenerative eye diseases such as glaucoma. Clinical feasibility for the substance requires further investigation since the immunomodulatory and possible adverse effects have not yet been thoroughly characterized.

Efficacy and tolerability of 0.2% brimonidine tartrate for the treatment of acute non-arteritic anterior ischemic optic neuropathy (NAION): a 3-month, double-masked, randomised, placebo-controlled trial

INTRODUCTION

Neuroprotection may be an option in ischemic optic nerve disease. There have been promising reports about the neuroprotective ability of topical brimonidine in animal studies. Therefore, we tested whether 0.2% brimonidine tartrate could improve the outcome of patients with non-arteritic anterior ischemic optic neuropathy (NAION). The study was stopped after an interim analysis, having not proven its feasibility within practicable time frame.

METHODS

A 3-month, double-masked, placebo-controlled, randomised European multicenter trial conducted according to good clinical practice rules. Thirty-six patients (22 m, 14 f), older than 40 years with first eye involvement and typical signs of NAION were included in the study within the 1st week after visual loss (VA 0.05-1.0) and were randomized to treatment with either brimonidine 0.2% (Alphagan) or placebo. Visual acuity (VA, primary endpoint), visual field (VF, Humphrey 30-2 and Goldmann, quantified by a modified Esterman grid) and automated swinging flashlight test (SWIFT) were performed at baseline, 2 weeks, 4 weeks and 12 weeks after first visit. Primary analysis aimed at intention-to-treat group (ITT, n=29), secondary analysis to the per protocol population (PP, n=25). Tolerability and safety were tested in the safety group (n=36). A two-sample two-sided t-test was used for statistical analysis (alpha level at 0.05).

RESULTS

VA did not show statistically significant difference by treatment. There were non-significant trends for better visual field results in the brimonidine group. Adverse events consisting of local irritation were observed six times in the verum and three times in the placebo group. No serious adverse events occurred.

CONCLUSION

In contradiction to an open-labeled, retrospective study published by Fazzone et al., the results of this trial did not indicate any harmful effect of brimonidine in patients suffering from NAION. However, a statistically significant advantage for the patients receiving brimonidine tartrate could not be shown.

Human ganglion cells express the alpha-2 adrenergic receptor: relevance to neuroprotection

BACKGROUND/AIM

Alpha-2alpha adrenergic receptor (alpha(2)-AR) agonists are thought to be neuroprotective, preventing retinal ganglion cell death independent of pressure reduction. Previous studies have identified alpha(2)-ARs in rat retina. The authors aimed to demonstrate the presence and localisation of alpha(2)-ARs in human and rat retina and on the rat retinal ganglion cell line, RGC-5.

METHODS

Seven postmortem human and three postmortem rat eyes were paraformaldehyde fixed and frozen. RGC-5 cells were also paraformaldehyde fixed. The expression of alpha(2A)-ARs was determined by antibody immunofluorescence.

RESULTS

alpha(2A)-AR expression was identified in the human retina, on ganglion cells, and cells in the inner and outer nuclear layers (INL, ONL). Differential alpha(2A)-AR staining patterns in the INL and ONL suggest a further restriction to as yet unidentified neuronal subclasses. The RGC-5 cell line also expressed alpha(2A)-ARs in undifferentiated cells and an increased expression upon fully differentiated cells.

CONCLUSION

alpha(2)-AR agonists in addition to their pressure lowering effects in the eye, may act directly upon retinal neurons, including retinal ganglion cells. The presence of alpha(2)-ARs on the RGC-5 cell line allows future investigation of these possible direct effects using in vitro glaucoma model systems.

Advances in glaucoma therapeutics

Glaucoma is a major cause of irreversible blindness in the world. The prevalence of glaucomatous loss in vision will continue to grow as our populations age. Ocular hypertension is a major risk factor for the development of glaucoma and current glaucoma therapy is directed at lowering intraocular pressure. Several new ocular hypotensive agents have been introduced in the past several years providing a variety of treatment options. In addition, various classes of neuroprotective agents demonstrating activity in a wide variety of animal models have been proposed as potential new glaucoma therapeutics. Although these approaches will slow the progression of vision loss, they do not directly intervene in the disease process(es). Advances have been made attempting to understand the pathogenic pathways involved in glaucomatous damage to the eye and in methods to clinically measure glaucoma damage. An increased understanding of the pathophysiology of glaucoma will lead to the development of new therapeutic agents that intervene and perhaps even reverse glaucomatous damage to the eye. There also is a need to develop new methods to clinically measure glaucoma damage because, currently, considerable damage occurs before glaucoma is diagnosed and glaucoma remains underdiagnosed in the general population.

Adrenoceptor subtype-specific acceleration of the hypoxic depression of excitatory synaptic transmission in area CA1 of the rat hippocampus

The depression of excitatory synaptic transmission by hypoxia in area CA1 of the hippocampus is largely dependent upon the activation of adenosine A(1) receptors on presynaptic glutamatergic terminals. As well as adenosine, norepinephrine levels increase in the hypoxic/ischemic hippocampus. We sought to determine the influence of alpha- and beta-adrenoceptor (AR) activation on the hypoxic depression of synaptic transmission utilizing electrophysiological, pharmacological and adenosine sensor techniques. Norepinephrine depressed synaptic transmission and significantly accelerated the hypoxic depression of synaptic transmission. The alpha-AR agonist 6-fluoronorepinephrine mimicked both of these effects whilst the alpha(2)-AR antagonist yohimbine, but not the alpha(1)-AR antagonist urapidil, prevented the actions of 6-fluoronorepinephrine. In contrast, the beta-AR agonist isoproterenol enhanced synaptic transmission and only accelerated the hypoxic depression of transmission in hypoxia-conditioned slices in which the hypoxic release of adenosine is reduced. The effects of isoproterenol were blocked by the non-selective beta-AR antagonist propranolol and the selective beta(1)-AR antagonist betaxolol. Using an enzyme-based adenosine sensor we observed that the application of the beta-AR agonist resulted in increased extracellular adenosine during repeated hypoxia. Our results suggest that alpha(2)-AR activation facilitates the hypoxic depression of synaptic transmission probably via the known alpha(2)-AR-mediated inhibition of presynaptic calcium channels whereas beta(1)-AR activation does so via increased extracellular adenosine and greater activation of inhibitory adenosine A(1) receptors.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Randomised controlled trial comparing the effect of brimonidine and timolol on visual field loss after acute primary angle closure

AIM

To compare the effect of brimonidine and timolol in reducing visual field loss in patients with acute primary angle closure (APAC).

METHODS

In addition to standard acute medical treatment, patients presenting with APAC were randomised to either brimonidine 0.2% or timolol 0.5% upon diagnosis, then twice daily for 4 weeks. After laser peripheral iridotomy (LPI), subjects underwent three baseline perimetry tests during the first week, and then at weeks 4, 8, 12, and 16. Pointwise linear regression analysis was applied to the field series of each of these subjects starting with the third test (total of five tests per subject). Progression was defined as a significant regression slope (p<0.05) showing 1 dB per year or more of sensitivity loss at the same test location in the series. Patients were also compared for prevalence of abnormal fields at 16 weeks, which was defined as an abnormal glaucoma hemifield test result and/or corrected pattern standard deviation outside the 95% confidence limits.

RESULTS

59 subjects (31 in the brimonidine group; 28 in the timolol group) completed the study. There were 47 females (79.7%), the majority of subjects (94.9%) were Chinese and the mean age was 59.2 (SD 7.2) years. There were no significant differences between the two groups with respect to demographic features, presenting intraocular pressure (IOP), duration of symptoms, time from presentation to LPI, or mean IOP at each study visit. Over the 16 week study period, despite adequate statistical power, no difference was found between groups in terms of the number of patients with progressing locations, the mean number of progressing locations per subject, or the mean slope of the progressing locations. Nine (29%) subjects in the brimonidine group and 10 (35.7%) in the timolol group were found to have significant visual field defects at 16 weeks (p = 0.58). 15 out of these 19 subjects (78.9%) already had these visual field defects in the first week.

CONCLUSIONS

In the first 16 weeks after APAC, there was no difference in the prevalence of visual field defects or rate of visual field progression between brimonidine and timolol treated groups.

Ophthalmic drug discovery

Millions of people suffer from a wide variety of ocular diseases, many of which lead to irreversible blindness. The leading causes of irreversible blindness in the elderly--age-related macular degeneration and glaucoma--will continue to effect more individuals as the worldwide population continues to age. Although there are therapies for treating glaucoma, as well as ongoing clinical trials of treatments for age-related macular degeneration, there still is a great need for more efficacious treatments that halt or even reverse ocular diseases. The eye has special attributes that allow local drug delivery and non-invasive clinical assessment of disease, but it is also a highly complex and unique organ, which makes understanding disease pathogenesis and ocular drug discovery challenging. As we learn more about the cellular mechanisms involved in age-related macular degeneration and glaucoma, potentially, new drug targets will emerge. This review provides insight into some of the new approaches to therapy.

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.