Glutamate elevation in rabbit vitreous during transient ischemia-reperfusion

Traumatic Optic Neuropathy: Update on Management

Traumatic optic neuropathy is one of the causes of visual loss caused by blunt or penetrating head trauma and is classified as both direct and indirect. Clinical history and examination findings usually allow for the diagnosis of traumatic optic neuropathy. There is still controversy surrounding the management of traumatic optic neuropathy; some physicians advocate observation alone, while others recommend steroid therapy, surgery, or both. In this entry, we tried to highlight traumatic optic neuropathy's main pathophysiologic mechanisms with the most available updated treatment. Recent research suggests future therapies that may be helpful in traumatic optic neuropathy cases.

Outer retinal involvement in N-methyl-D-aspartate-induced inner retinal injury in rabbits assessed by optical coherence tomography

This study was aimed to investigate morphological alteration of the retina with N-methyl-D-aspartate (NMDA)-induced injury in rabbits by optical coherence tomography (OCT). The right and left eyes of a total of 12 rabbits received single-intravitreal injection of vehicle and NMDA, respectively. Four out of the 12 animals underwent OCT and quantification of plasma microRNA repeatedly (4, 48, and 168 hr after dosing), followed by ocular histopathology at the end of the study. Ocular histopathology was also conducted in the eyes collected 4 or 48 hr after dosing from 4 animals at each time period. OCT revealed hyper-reflective ganglion cell complex and thickened inner retina in NMDA-treated eyes 4 hr after dosing; the inner retina shifted to thinning at later time points. The eyes given NMDA also exhibited greater thickness of the outer retina, which contains photoreceptors, after treatment, and thickened and obscured ellipsoid zone 168 hr after dosing. The plasma levels of miR-182 and miR-183, which are known to be highly expressed in photoreceptors, were higher 4 hr after dosing than pre-dosing values. Histopathologically, NMDA-induced inner retinal damage was confirmed: single-cell necrosis was observed in the ganglion cell layer and the inner nuclear layer 4 hr after dosing, the incidence of which decreased thereafter. At 168 hr after dosing, reduced number of ganglion cells was noted. No change was histopathologically observed in the outer retina. In conclusion, our results suggest involvement of photoreceptors in NMDA-induced inner retinal injury. Additionally, OCT revealed acute inner retinal findings suggestive of temporary edema.

Ionic channel changes in glaucomatous retinal ganglion cells: multicompartment modeling

This research takes a step towards discovering underlying ionic channel changes in the glaucomatous ganglion cells. Glaucoma is characterized by a gradual death of retinal ganglion cells. In this paper, we propose a hypothesis that the ionic channel concentrations change during the progression of glaucoma. We use computer simulation of a multi-compartment morphologically correct model of a mouse retinal ganglion cell to verify our hypothesis. Using published experimental data, we alter the morphology of healthy ganglion cells to replicate glaucomatous cells. Our results suggest that in glaucomatous cell, the sodium channel concentration decreases in the soma by 30% and by 60% in the dendrites, calcium channel concentration decreases by 10% in all compartments, and leak channel concentration increases by 40% in the soma and by 100% in the dendrites.

Changes in transcript and protein levels of calbindin D28k, calretinin and parvalbumin, and numbers of neuronal populations expressing these proteins in an ischemia model of rat retina

Excessive calcium is thought to be a critical step in various neurodegenerative processes including ischemia. Calbindin D28k (CB), calretinin (CR), and parvalbumin (PV), members of the EF-hand calcium-binding protein family, are thought to play a neuroprotective role in various pathologic conditions by serving as a buffer against excessive calcium. The expression of CB, PV and CR in the ischemic rat retina induced by increasing intraocular pressure was investigated at the transcript and protein levels, by means of the quantitative real-time reverse transcription-polymerase chain reaction, western blot and immunohistochemistry. The transcript and protein levels of CB, which is strongly expressed in the horizontal cells in both normal and affected retinas, were not changed significantly and the number of CB-expressing horizontal cells remained unchanged throughout the experimental period 8 weeks after ischemia/reperfusion injury. At both the transcript and protein levels, however, CR, which is strongly expressed in several types of amacrine, ganglion, and displaced amacrine cells in both normal and affected retinas, was decreased. CR-expressing ganglion cell number was particularly decreased in ischemic retinas. Similar to the CR, PV transcript and protein levels, and PV-expressing AII amacrine cell number were decreased. Interestingly, in ischemic retinas PV was transiently expressed in putative cone bipolar cell types possibly those that connect with AII amacrine cells via gap junctions. These results suggest that these three calcium binding proteins may play different neuroprotective roles in ischemic insult by their ability to buffer calcium in the rat retina.

Agmatine as retinal protection from ischemia-reperfusion injury in guinea pigs

PURPOSE

To determine the neuroprotective effect of agmatine (Agm) on the retinas of guinea pigs subjected to a transient ischemia-reperfusion insult.

METHODS

Twenty-eight guinea pigs were randomly divided into four groups. Forty-five minutes before ischemic insult, the guinea pigs were intraperitoneally administered either Agm (50 mg/kg) (Agm 1) or saline (control 1 group) once, or twice separated by a 12-h interval (Agm 2; control 2). Transient ocular ischemia was achieved under general anesthesia by cannulating an anterior chamber maintainer connected to an infusion line of a semiflexible bottle. The saline reservoir pressure was increased by using a blood pressure tolls cuff to achieve an intraocular pressure (IOP) of 150 mmHg. This IOP was maintained for 90 min. Reperfusion was achieved by pulling off the anterior chamber maintainer. The animals in the Agm 1 and control 1 groups were killed at the end of the 4-h reperfusion period. The eyes were enucleated for histopathological (retinal thickness) and biochemical (thiobarbituric acid reactive substance, TBARS, and nitric oxide, NO) investigation. The animals in the Agm 2 and control 2 groups were killed at the end of a 24-h reperfusion period.

RESULTS

The mean retinal thickness of the animals in the Agm 1 (25.94 +/- 1.23 microm) and Agm 2 (24.49 +/- 0.88 microm) groups was lower than that of those in the control 1 (37.60 +/- 2.27 microm) and control 2 (36. 64 +/- 1.32 microm) groups (P < 0.05). The mean TBARS level of the animals in the Agm 1 (8.37 +/- 0.94 nmol/ml) and Agm 2 (8.01 +/- 0.97 nmol/ml) groups was lower than that of those in the control 1 (12.09 +/- 1.27 nmol/ml) and control 2 (12.09 +/- 1.27 and 11.72 +/- 1.63 nmol/ml) groups (P < 0.05). The mean NO level of the animals in the Agm 1 (100.77 +/- 6.20 nmol/ml) and Agm 2 (94.63 +/- 5.24 nmol/ml) was lower than that of those in the control 1 (131.77 +/- 4.61 nmol/ml) and control 2 (122.43 +/- 4.35 nmol/ml) groups (P < 0.05). There were positive correlations between the TBARS and NO levels and retinal thickness in the Agm and control groups.

CONCLUSION

Agmatine exerts a significant neuroprotective effect on guinea pig retinas after transient ischemia-reperfusion insult.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Changes in vitreous amino acid concentrations in a rabbit model of cataract surgery

PURPOSE

We previously reported marked elevation of glutamate, gamma-aminobutyric acid (GABA) and alanine in the vitreous after combined cataract surgery and vitrectomy for macular disorder. Photo-stimulation by the operating microscope and increased intraocular pressure (IOP) are possible causes of postoperative increases in amino acids. An animal model was used to verify this hypothesis.

METHODS

Forty-five coloured rabbits were studied; stress was applied to simulate cataract surgery. Simultaneous photic and pressure stress, photic stress alone, or pressure stress alone were exerted for 20 mins. The vitreous was extracted at the end of the stress period, or 10 mins or 30 mins later. Levels of six amino acids (asparate, glutamate, glycine, taurine, alanine and GABA) in the vitreous were analysed quantitatively.

RESULTS

Significant increases in vitreous glutamate concentrations in treated eyes were observed following simultaneous photic and pressure stress, as well as after pressure stress alone, whereas no difference was found after photic stress alone. The mean glutamate concentrations in treated eyes and fellow eyes, respectively, were 5.59 +/- 2.03 microM and 4.36 +/- 2.09 microM (p < or = 0.05) 10 mins after simultaneous photic and pressure stress, and 4.32 +/- 0.97 microM and 2.29 +/- 0.51 microM (p < 0.05) 10 mins after pressure stress alone. Taurine concentration was elevated by pressure stress alone at 0 min and at 10 mins post-stress. Alanine concentration was reduced by photic stress alone at 10 mins post-stress. In all protocols, no significant differences between treated and fellow eyes were observed at 30 mins after stress. Time-dependent changes were observed not only in the treated eyes, but also in untreated fellow eyes.

CONCLUSIONS

Elevations of vitreous glutamate in cataract surgery may be caused by pressure stress due to irrigation. Photic stress caused by the operating microscope has little influence. Sympathetic response may be involved in amino acid changes due to stress in cataract surgery.

Hypoxia-induced retinal ganglion cell death and the neuroprotective effects of beta-adrenergic antagonists

Hypoxia-induced retinal ganglion cell (RGC) death has been implicated in glaucomatous optic neuropathy. However, the precise mechanism of death signaling and how neuroprotective agents affect it are still unclear. The aim of this study is to characterize the mechanisms of hypoxia-induced apoptosis of cultured purified RGCs and to study the neuroprotective effects of beta-adrenergic antagonists. Rat RGCs were purified utilizing a modified two-step immuno-panning procedure. First, the extent of apoptosis in RGCs under hypoxia was quantified. Next, the effects of glutamate-channel antagonists (MK801 or DNQX), Bax inhibiting peptide (BIP), and beta-adrenergic antagonists (betaxolol, nipradilol, timolol or carteolol) on hypoxia-induced RGC death were investigated by the cell viability assay. Third, the effects of beta-adrenergic antagonists on hypoxia-induced increase of intracellular calcium concentrations ([Ca(2+)](i)) and the additional effect of NO scavenger to nipradilol were evaluated. Apoptotic RGC percentages under hypoxia were significantly increased compared to the control. The viability of RGCs under hypoxia was not affected by MK801 or DNQX, whereas it was increased in a dose-dependent manner with exposure to BIP, and to betaxolol, nipradilol, timolol, but not to carteolol. These effective beta-adrenergic antagonists showed no significant change in hypoxia-induced [Ca(2+)](i) levels. The NO scavenger alleviated neuroprotective effect by nipradilol. In conclusion, purified RGC damage induced by hypoxia involves Bax-dependent apoptotic pathway, but mostly independent of glutamate receptor-mediated excitotoxicity. Betaxolol, timolol and nipradilol showed a protective effect against hypoxia-induced RGC death, which was thought to be irrelevant either to calcium channel or beta-adrenoceptor blocking effects.

Alterations of amino acids and glutamate transport in the DBA/2J mouse retina; possible clues to degeneration

BACKGROUND

The DBA/2J mouse spontaneously develops ocular hypertension and time-dependent progressive retinal ganglion cell (RGC) loss. This study examines changes in amino acid levels in the vitreous, and changes in the expression of retinal glutamate transporters and receptors that occur during the progression of this pathology.

METHODS

Retinas were obtained from DBA/2J mice at ages 3, 6 and 11 months. C57BL/6 mice were used as age-matched controls. Vitreal amino acid content was measured with HPLC. Western blotting and immunohistochemistry were performed using specific antibodies against the glutamate transporters (GLAST, GLT-1v, EAAC-1) and glutamate receptors, particularly NMDA (NR1, NR2A, NR2B) and AMPA (GluR1, GluR2/3, GluR4) receptors.

RESULTS

HPLC showed retinal concentrations of glutamate, glutamine, glycine, alanine, lysine, serine, and arginine to be significantly higher in DBA/2J mice at 11 months of age compared to age-matched controls. Western Blots revealed a moderate decrease of GLAST and GLT-1v expression in DBA/2J mice at 6 and 11 months as compared to age-matched controls while there was no change in EAAC1. Immunohistochemically, no changes in expression of NMDA and AMPA receptors were seen.

CONCLUSION

Alterations of amino acid content and enhanced glutamate neurotransmission might be involved in the pathogenesis of retinal neurodegeneration in the DBA/ 2J mouse model of ocular hypertension. Moreover, these mice provide an animal model for studying excitotoxic retinal damage.

Glutamate level in optic nerve head is increased by artificial elevation of intraocular pressure in rabbits

Neurons can be damaged by the activation of glutamate receptors, but whether glutamate is related to the development of glaucomatous optic neuropathy is still controversial. The purpose of this study was to measure the acute changes in the glutamate levels in the optic nerve head (ONH) of rabbits induced by an artificial elevation of the intraocular pressure (IOP). A concentric microdialysis probe was inserted into the ONH of rabbits via the pars plana. The probe was perfused with Ringer's solution, and the levels of glutamate in 10-min dialysate samples were measured repeatedly using high-performance liquid chromatography. After the glutamate level was stabilized for at least 60 min, the IOP was adjusted to three levels; 120 mm Hg (n=11), 60 mm Hg (n=12), and 15 mm Hg (control group; n=11). The IOP was altered by changing the height of a bottle of Ringer's solution, which was connected to the anterior chamber by a 23-gauge needle. The IOP levels were maintained for 60 min, and the glutamate levels were determined every 10 min during the 60 min. The mean basal levels of glutamate in the 10-min dialysate were not significantly different among the three groups. The glutamate levels remained unchanged and stable in the controls, but elevation of the IOP significantly increased the level of the glutamate in the dialysate (IOP60, P=0.012; and IOP120, P=0.005: repeated measures ANOVA). Elevation of the IOP causes an increase in the glutamate levels in the ONH of rabbits. This suggests a possible interaction between glutamate metabolism and the IOP in the ONH.

Effects of poly(ADP-ribose) polymerase inhibitor on NMDA-induced retinal injury

PURPOSE

Excessive activation of poly(ADP-ribose) polymerase (PARP), a nuclear enzyme that is activated by DNA damage, leads to neuronal cell death through depletion of ATP. The purpose of this study was to determine whether inhibition of PARP has some neuroprotective effects on the N-methyl-D-aspartate (NMDA)-induced functional and morphological injury to the rabbit retina.

METHODS

Visually evoked potentials (VEPs) were recorded at different times after an intravitreal injection of NMDA (200, 660, and 2000 nmol) alone, or NMDA with 3-aminobenzamide (ABA, 200 nmol), a PARP inhibitor, or with MK-801 (200 nmol), an NMDA antagonist. The physiological changes were followed for 2 weeks, after which the eyes were enuculeated and prepared for histological examinations.

RESULTS

Intravitreal injections of NMDA reduced the amplitudes of rabbit VEPs and the number of cells in the retinal ganglion cell layer in a dose-dependent manner. No significant changes could be detected in the bright-flash electroretinograms (ERGs). Simultaneous injection of MK-801 (200 nmol) significantly diminished the changes induced by intravitreal NMDA. 3-Aminobenzamide (ABA) (200 nmol) also suppressed these changes, but its effects were less than those of MK-801.

CONCLUSIONS

NMDA-induced retinal damage can be detected by VEPs, and PARP inhibition has some neuroprotective effects on the NMDA-induced retinal damage.

Effectiveness of levobetaxolol and timolol at blunting retinal ischaemia is related to their calcium and sodium blocking activities: relevance to glaucoma

Glaucoma is a chronic optic neuropathy in which retinal ganglion cells die over a number of years. The initiation of the disease and its progression may involve an ischaemic-like insult to the ganglion cell axons caused by an alteration in the quality of blood flow. Thus, to effectively treat glaucoma it may be necessary to counteract the ischaemic-like insult to the region of the optic nerve head. Studies on the isolated optic nerve suggest that substances that reduce the influx of sodium would be particularly effective neuroprotectants. Significantly, of the presently used antiglaucoma substances, only beta-blockers can reduce sodium influx into cells. Moreover, they also reduce the influx of calcium and this would be expected to benefit the survival of insulted neurones. Betaxolol is the most effective antiglaucoma drug at reducing sodium/calcium influx. Our electroretinographic data indicated that topical application of levobetaxolol to rats attenuated the effects of ischaemia/reperfusion injury. Timolol was also effective but to a lesser extent. Based on these data we conclude that beta-blockers may be able to blunt ganglion cell death in glaucoma, and that levobetaxolol may be a more effective neuroprotectant than timolol because of its greater capacity to block sodium and calcium influx.

Possible involvement of endothelin-1 and nitric oxide in the pathogenesis of proliferative diabetic retinopathy

BACKGROUND

Overproduction of endothelin-1 (ET-1) and nitric oxide (NO) in the retina is demonstrated in experimental diabetic animals. To clarify the possible involvement of ET-1 and NO in the pathogenesis of diabetic retinopathy, the authors examined the vitreous levels of these principal endothelium-derived vasoactive substances in patients with proliferative diabetic retinopathy (PDR).

METHODS

Vitreous fluid was taken from patients with PDR (ET-1, n = 12; NO, n = 12) and from patients with macular holes as controls (ET-1, n = 10; NO, n = 10) at vitreous surgery. Endothelin-1 and NO metabolites were measured by radioimmunoassay and high-performance liquid chromatography based on the Griess method, respectively.

RESULTS

Endothelin-1 levels (mean +/- SE) were 21.5 +/- 1.7 pg/mL in the vitreous of patients with PDR and 16.7 +/- 0.7 pg/mL in the vitreous of patients with macular hole. There was a significant difference between patients with PDR and controls (P = 0.009, Mann-Whitney). Nitrate (NO3) was 49.8 +/- 5.0 micromol/L in patients with PDR and 24.2 +/- 2.8 micromol/L in patients with macula hole; it was also significantly elevated in patients with PDR (P = 0.004, Mann-Whitney), whereas nitrite (NO2) was not detected in this study.

CONCLUSION

These results indicate that ET-1 and NO may be related in the pathogenesis of PDR.

Pharmacodynamics of citicoline relevant to the treatment of glaucoma

Citicoline (exogenous CDP-choline) is a nontoxic and well-tolerated drug used in pharmacotherapy of brain insufficiency and some other neurological disorders, such as stroke, brain trauma, and Parkinson's disease. A few reports indicate that citicoline treatment may also be beneficial in glaucoma. Currently glaucoma is considered a neurodegenerative disease in which retinal ganglion cells (RGC) slowly die, likely in the apoptotic mechanism. Endogenous CDP-choline is a natural precursor of cellular synthesis of phospholipids, mainly phosphatydylcholine (PtdCho). Enhancement of PtdCho synthesis may counteract neuronal apoptosis and provide neuroprotection. Citicoline, when administered, undergoes a quick transformation to cytidine and choline, which are believed to enter brain cells separately and provide neuroprotection by enhancing PtdCho synthesis; similar effect may be expected to occur in glaucomatous RGC. Furthermore, citicoline stimulates some brain neurotransmitter systems, including the dopaminergic system, and dopamine is known as a major neurotransmitter in retina and postretinal visual pathways. In a double-blind, placebo-controlled study, treatment of glaucoma resulted in functional improvement in the visual system noted with electrophysiological methods. Development of citicoline as a treatment for glaucoma is indicated.

Vasospasm, its role in the pathogenesis of diseases with particular reference to the eye

Vasospasm can have many different causes and can occur in a variety of diseases, including infectious, autoimmune, and ophthalmic diseases, as well as in otherwise healthy subjects. We distinguish between the primary vasospastic syndrome and secondary vasospasm. The term "vasospastic syndrome" summarizes the symptoms of patients having such a diathesis as responding with spasm to stimuli like cold or emotional stress. Secondary vasospasm can occur in a number of autoimmune diseases, such as multiple sclerosis, lupus erythematosus, antiphospholipid syndrome, rheumatoid polyarthritis, giant cell arteritis, Behcet's disease, Buerger's disease and preeclampsia, and also in infectious diseases such as AIDS. Other potential causes for vasospasm are hemorrhages, homocysteinemia, head injury, acute intermittent porphyria, sickle cell disease, anorexia nervosa, Susac syndrome, mitochondriopathies, tumors, colitis ulcerosa, Crohn's disease, arteriosclerosis and drugs. Patients with primary vasospastic syndrome tend to suffer from cold hands, low blood pressure, and even migraine and silent myocardial ischemia. Valuable diagnostic tools for vasospastic diathesis are nailfold capillary microscopy and angiography, but probably the best indicator is an increased plasma level of endothelin-1. The eye is frequently involved in the vasospastic syndrome, and ocular manifestations of vasospasm include alteration of conjunctival vessels, corneal edema, retinal arterial and venous occlusions, choroidal ischemia, amaurosis fugax, AION, and glaucoma. Since the clinical impact of vascular dysregulation has only really been appreciated in the last few years, there has been little research in the according therapeutic field. The role of calcium channel blockers, magnesium, endothelin and glutamate antagonists, and gene therapy are discussed.