Betaxolol,a beta1-adrenoceptor antagonist,protects a transient ischemic injury of the retina

In the present study, we investigated the protective effects of the topical beta-adrenoceptor antagonist Betoptic((R)) (0.25% betaxolol) in the rat retina following the ischemic injury induced by a transient increase of intraocular pressure (IOP). Like other areas of the central nervous system, the retina is highly vulnerable to ischemic-induced injury. Ischemia was induced in the rat retina by raising the IOP above the systolic blood pressure for 60min. After an ischemia/reperfusion, the thickness of the retinal layers and the immunoreactivities of choline acetyltransferase (ChAT), gamma-amino butyric acid (GABA) and tyrosine hydroxylase (TH) were examined. After a reperfusion period of 7 days, the thickness of both the inner plexiform layer and inner nuclear layer was much decreased. After a reperfusion period of 14-28 days, the thickness of the outer nuclear layer decreased markedly. Moreover, the ChAT and TH immunoreactivity had almost completely disappeared in the retinas after 7 days, while GABA immunoreactivity remained for 28 days. These results suggest that the inner retinal layers are more susceptible to ischemic-induced injury than the outer retinal layer.Histological examination demonstrated protective effects of betaxolol on ischemic-induced retinal damage, which was more substantial in the inner retinal layer. When two drops of betaxolol, once before ischemic injury and twice daily for 28 days after ischemia, were continuously administered, the reductions in the retinal ChAT, GABA and TH immunoreactivities were significantly attenuated. The present study suggests that topically applied betaxolol is an efficient neuroprotective agent and prevents the retinal cell damage induced by ischemic injury in rats.     

Topically applied betaxolol attenuates NMDA-induced toxicity to ganglion cells and the effects of ischaemia to the retina.

The present results show that topically applied Betoptic(R)(0.5% betaxolol) to the rabbit or rat eye reaches the retina and can counteract the detrimental effects caused by ischaemia/reperfusion or N -methyl- d -aspartate (NMDA)-induced insults to the retina. Betaxolol is a beta(1)-adrenergic blocker but its neuroprotective action is generally thought to be due to its calcium channel blocking properties. Support for this view comes from studies on cultures of cortical neurones where it was found that betaxolol attenuated the NMDA-induced influx of(45)Ca(2+)while beta-adrenoreceptor agonists were ineffective.Topically applied Betoptic(R)to the rabbit eye was observed to reach the retina in maximal amounts within 60 min. Some of the substance was also found in the contralateral retina of the untreated eye suggesting that the agent reaches the retina by local systemic and retinal circulation. Concurrent treatment with Latanoprost(R)did not result in a greater amount of betaxolol reaching the retina.An ophthalmodynamometric procedure, which raises the intraocular pressure, was used to apply an ischaemic insult to the rabbit retina. After three days of reperfusion the b-wave of the electroretinogram was reduced by an average of 59% and the choline acetyltransferase immunoreactivity in the retina was almost obliterated. However, when experiments were carried out on animals which had been treated with one drop of Betoptic(R) twice daily for 4 weeks before ischaemia and also during the reperfusion phase, the reductions in both the b-wave of the electroretinogram and retinal choline acetyltransferase immunoreactivity due to ischaemia/reperfusion were greatly attenuated.Intravitreal injection of NMDA into the rat eye caused a decrease in the immunostaining for Thy-1 antigen which is associated with ganglion cells. The Thy-1 mRNA level was also reduced as was the mRNA for the common subunit of the NMDA receptor, the NR1 subunit. However, in animals subjected to a topical Betoptic(R)regime, before and after intravitreal injection of NMDA, the decreases in the mRNA levels of Thy-1 and NR1 were significantly attenuated.     

实验性急性高眼压对兔视网膜电图的影响

目的：检测家兔实验性急性高眼压不同压力状态下视网膜电图的变化。方法：采用视电生理检测仪测定家兔实验前,30mmHg(1mmHg=0.133kPa),60mmHg,90mmHg和120mmHg前房高压灌注45min及恢复正常眼压4h的视网膜电图（Flash Electroretinogram FERG)和振荡电位（Oscillatory Potentials,OPs)。结果：对照组和30mmHg组视电生理检测在实验前后无差异。60mmHg组在高压持续45min后,b波和OPs波振幅下降,4h后恢复正常。90mmHg和120mmHg组在高压45min后,FERG波形消失。4h后有不同程度恢复。结论：随着实验性高眼压压力的升高,家兔视网膜功能损伤加重,恢复能力减弱。     

Topical flunarizine reduces IOP and protects the retina against ischemia-excitotoxicity

PURPOSE: To determine whether topical application of flunarizine reduces intraocular pressure (IOP) and acts as a retinal neuroprotectant and to compare the effectiveness of flunarizine with betaxolol and nifedipine at reducing the influx of calcium and sodium. METHODS: Ischemia was delivered to the rabbit retina by raising the IOP. After 3 days, a flash electroretinogram (ERG) was recorded, and the retina processed for the localization of certain antigens. In the rat, N-methyl-D-aspartate (NMDA) was injected intravitreally, and 8 days later, the retinas were analyzed for the localization of Thy-1 or the relative amounts of mRNAs for antigens located to ganglion cells or photoreceptors. Rats and rabbits received topical flunarizine or vehicle before and after ischemia or NMDA. IOP was measured in rabbits after a single topical application of 2% flunarizine. Studies were conducted on isolated rat retinas, cortical cultures, and brain synaptosomes to compare the effectiveness of flunarizine with nifedipine and betaxolol at reducing the influx of calcium or sodium. RESULTS: Changes in rabbit retinal choline acetyltransferase and parvalbumin immunoreactivities and the b-wave of the ERG caused by ischemia-reperfusion were blunted by topical treatment with flunarizine. Similarly, NMDA induced reductions in Thy-1 immunoreactivity and mRNA for rat ganglion cell antigens (Thy-1 and neurofilament light form) were counteracted by topical application of flunarizine. Topical application of 2% flunarizine significantly lowered the IOP in rabbits over a period of 5 hours. Flunarizine was more effective than betaxolol and much stronger than nifedipine at attenuating veratridine-induced influx of sodium into synaptosomes. Nifedipine, flunarizine, and betaxolol all reduced the NMDA-induced influx of calcium into the isolated retina or cortical neurons, but betaxolol was the least effective. CONCLUSIONS: Topically applied flunarizine reduces IOP and attenuates injury to the whole of the retina, including the ganglion cells. The neuroprotective action of flunarizine is to reduce the influx of calcium and sodium into stressed neurons. The potent effect of flunarizine on sodium influx would be particularly protective to axons.     

溴莫尼定对视网膜缺血性损伤神经保护作用的实验研究

目的：探讨溴莫尼定(brimonidine)对视网膜缺血性损伤神经的保护作用。方法：新西兰大耳白兔32只，随机分为正常对照组、生理盐水治疗组、噻吗心安(timolol)治疗组、brimonidine治疗组，每组8只。后3组为损伤治疗组，通过生理盐水前房高压灌注的方法，制成视网膜缺血动物模型，在视网膜缺血前lh其结膜囊内分剐给予生理盐水、0．5％timolol眼液或0．2％brimonidine眼液局部治疗。在灌注后7d，观察图形视网膜电图(P-ERG)b波振幅变化，并进行组织形态学观察和视网膜神经节细胞(RGC)计数分析。结果：灌注后7d，3个损伤治疗组相对b波振幅恢复率为：7％、11％和64％，RGC标准丢失率为：43％、38％和12％，brimori-die治疗组视网膜组织形态结构接近正常对照组，而生理盐水治疗组和timolol治疗组视网膜内层组织结构损伤明显。结论：Brimonidine局部治疗对缺血诱导的视网膜结构和功能的损害有明显的神经保护作用。     

Ketamine suppresses ischemic injury in the rabbit retina.

Overstimulation of the N-methyl D-aspartate (NMDA) receptor has been implicated as a factor in the pathogenesis of ischemic injury in the central nervous system. The present study was undertaken to determine whether ketamine hydrochloride, a potent NMDA antagonist, could attenuate ischemic injury in the rabbit retina. Retinal ischemia was induced for 60 min in one eye of 18 albino rabbits by raising intraocular pressure above the systolic blood pressure. Three concentrations of ketamine, 0.5, 1.5, 5.0 mumol were dissolved in 20 microliters of saline solution and injected in the midvitreous in each eye of 14 rabbits 1 hr prior to ischemia. Four rabbits received saline solution as controls. The scotopic electroretinogram was monitored in each eye to assess the postischemic recovery of retinal function. A statistically significant reduction in the b-wave was detected in the eyes treated with saline (P less than 0.05), whereas the postischemic recovery of b-wave amplitude was enhanced by pretreatment with lower doses of ketamine. The highest dose depressed b-wave amplitude regardless of ischemia. Six rabbits underwent unilateral ocular ischemia under general anesthesia with ketamine. A small ameliorative effect was seen (P = 0.029). These results suggest that ketamine may alleviate ischemic injury in the rabbit retina, presumably by antagonizing the NMDA receptor-mediated toxicity. Thus, ketamine may have potential in the treatment of retinal vascular occlusive diseases. Moreover, a modified ischemic state may exist in experiments on ischemia conducted under general anesthesia with ketamine hydrochloride.     

Photoreceptors are preferentially affected in the rat retina following permanent occlusion of the carotid arteries

Carotid artery occlusion (two vessel occlusion; 2-VO) for 3 or 9 months causes a suppression of the electroretinogram. However, after 3 months the retinal morphology appears unaffected judging from the localisation of GABA, ChAT, alpha PKC, Thy-1 and GFAP immunoreactivities. Moreover, no difference in NMDA-R1, opsin or Thy-1 mRNA levels were detected. In contrast, after 9 months 2-VO photoreceptor degeneration occurred as indicated by thinning of the outer nuclear layer and reduced Ret-P1 immunoreactivity. All other immunoreactivities appeared normal. These findings were supported by analysis of retinal mRNA levels. We conclude that the major effect of prolonged 2-VO is photoreceptor degeneration.     

Topically applied betaxolol attenuates ischaemia-induced effects to the rat retina and stimulates BDNF mRNA

It has previously been reported that the beta(1)-adrenoceptor antagonist, betaxolol, can protect retinal neurones from ischaemia when applied topically. It has further been shown that betaxolol can reduce influx of both sodium or calcium into neurones through interaction at neurotoxin site 2 of the sodium channel and the L-type calcium channel, respectively. The present study sought to further investigate the neuroprotective mode of action of betaxolol in the rat retina. Rats were treated topically with L-betaxolol for 10, 5 and 1 min before ischaemia, induced by raising the intraocular pressure above systolic blood pressure for 45 min. This was followed by reperfusion of 3 or 5 days where L-betaxolol was applied topically twice daily. Ischaemia plus reperfusion caused both a loss of immunoreactivity for choline acetyl transferase (ChAT) and a marked reduction of the b-wave of the electroretinogram (ERG). Treatment, as described, with topical L-betaxolol, completely blunted the effects upon ChAT immunoreactivity and caused a significant reversal of the ERG changes. Furthermore, other rats treated topically with commercially available racemic betaxolol (Betoptic Solution, 0.5%) for 6 hr had raised levels of mRNA for brain derived neurotrophic factor (BDNF) but not for basic fibroblast growth factor (bFGF) in their retinas. The combined data provide further evidence that betaxolol can blunt the effects of ischaemia to the rat retina when applied topically just before the insult. Furthermore, the finding that retinal levels of BDNF mRNA are raised following topical betaxolol treatment shows that not only can this drug reach the retina but that it can also induce changes in expression of factors which are known, themselves, to provide neuroprotection to retinal neurones.     

家兔视网膜缺血-再灌注损伤后形态学的动态变化

目的　观察家兔视网膜缺血 -再灌注后视网膜结构的动态变化。方法　通过前房灌注加压至 16 .7k Pa,维持 1h,建立缺血模型 ,观察再灌注 2～ 14d内其结构变化及内层视网膜平均厚度的变化。结果　家兔视网膜在缺血 -再灌注后 2～ 14d内表现为神经细胞持续丢失 ,视网膜层次逐渐不清 ,萎缩、变薄。其中神经节细胞、神经纤维及视锥、视杆对缺血最敏感 ,外颗粒层次之 ,内颗粒层最能耐受缺血 -再灌注后的损伤。结论　视网膜缺血 -再灌注损伤是个持续性、进行性的损伤过程 ,与功能变化相一致     

Induction of a 55-kDa PKN cleavage product by ischemia/reperfusion model in the rat retina

PURPOSE: To investigate the physiological role of a protein kinase, PKN, and its relation to apoptosis in vivo. METHODS: An ischemia/reperfusion model of the rat retina was created by elevating the intraocular pressure. Retinal samples were obtained after ischemic insult (15-45 minutes) followed by reperfusion (1-7 days). The effect of ischemia on the fragmentation of PKN was examined by immunoblotting and immunocytochemical procedures using the antibody against PKN. N-methyl-D-aspartate (NMDA) or a caspase-3 inhibitor (DEVD-CHO) was administered intravitreally to investigate its effect on the induction of PKN fragmentation. The retinal cell loss in each sample was evaluated by toluidine blue staining. RESULTS: Ischemia induced a 55-kDa PKN cleavage fragment corresponding to the molecular size of the constitutively active fragment of PKN. The appearance of the cleavage fragment depended on the duration of reperfusion and correlated with the occurrence of retinal cell loss. Immunocytochemical analysis revealed that ischemia increased PKN immunoreactivity in the inner layers of the retina. DEVD-CHO significantly inhibited the appearance of the 55-kDa fragment and protected against retinal cell loss. The administration of NMDA also induced cleavage of PKN. CONCLUSIONS: PKN is specifically cleaved by caspase-3 or a related protease during apoptosis in vivo, and PKN cleavage is at least partially initiated by activation of the NMDA receptor.     

Anti-inflammatory effects of lutein in retinal ischemic/hypoxic injury: in vivo and in vitro studies

Purpose. Lutein protects retinal neurons by its anti-oxidative and anti-apoptotic properties in ischemia/reperfusion (I/R) injury while its anti-inflammatory effects remain unknown. As Müller cells play a critical role in retinal inflammation, the effect of lutein on Müller cells was investigated in a murine model of I/R injury and a culture model of hypoxic damage. Methods. Unilateral retinal I/R was induced by a blockade of internal carotid artery using the intraluminal method in mice. Ischemia was maintained for 2 hours followed by 22 hours of reperfusion, during which either lutein (0.2 mg/kg) or vehicle was administered. Flash electroretinogram (flash ERG) and glial fibrillary acidic protein (GFAP) activation were assessed. Lutein's effect on Müller cells was further evaluated in immortalized rat Müller cells (rMC-1) challenged with cobalt chloride-induced hypoxia. Levels of IL-1β, cyclooxygenase-2 (Cox-2), TNFα, and nuclear factor-NF-kappa-B (NF-κB) were examined by Western blot analysis. Results. Lutein treatment minimized deterioration of b-wave/a-wave ratio and oscillatory potentials as well as inhibited up-regulation of GFAP in retinal I/R injury. In cultured Müller cells, lutein treatment increased cell viability and reduced level of nuclear NF-κB, IL-1β, and Cox-2, but not TNFα after hypoxic injury. Conclusions. Reduced gliosis in I/R retina was observed with lutein treatment, which may contribute to preserved retinal function. Less production of pro-inflammatory factors from Müller cells suggested an anti-inflammatory role of lutein in retinal ischemic/hypoxic injury. Together with our previous studies, our results suggest that lutein protected the retina from ischemic/hypoxic damage by its anti-oxidative, anti-apoptotic, and anti-inflammatory properties.     

The effect of GABA and the GABA-uptake-blocker NO-711 on the b-wave of the ERG and the responses of horizontal cells to light

Background The effects of GABA in the retina have now become of special interest because the anti-epileptic drug vigabatrin, a GABA analogue, can cause visual field loss in humans. Vigabatrin inhibits the GABA-aminotransferase, which finally results in GABA accumulation in the extracellular space. The b-wave of the electroretinogram (ERG), which originates partly in on-bipolar cells, is influenced by both GABAergic horizontal cells (HCs) and GABAergic amacrine cells (ACs). Their influences, however, are difficult to separate. In an attempt to isolate the effect of GABAergic ACs, use has been made of the specific effect of the GABA-uptake-blocker NO-711, which blocks only the GABA transporter GAT1 of GABAergic ACs.Methods The ERG and the intracellular responses of HCs to light were recorded in the isolated rabbit retina, and the effects of GABA and NO-711, when added separately to the superfusate, were determined. Results GABA reduced significantly both the light responses of HCs and the b-wave. NO-711 enlarged the b-wave drastically, but did not affect the responses of HCs to light.Conclusions An increase in the extracellular GABA concentration decreases the b-wave; an impairment of the function of ACs increases the b-wave. These conditions are discussed in the context of the lack of consistent changes to the b-wave during therapy with vigabatrin.     

Granulocyte colony-stimulating factor facilitates recovery of retinal function following retinal ischemic injury

The purpose of the present study was to investigate whether systemically administered granulocyte colony-stimulating factor (G-CSF) can protect against acute ischemic reperfusion injury. Two groups of anesthetized adult male Lewis rats (n = 8 per group) were subjected to an acute (45 min) episode of retinal ischemic injury followed by subcutaneous administration of vehicle (5% dextrose) or G-CSF (0.1 mg/kg/day) once per day × 5 days. Prior to and one week following ischemic insult, retinal function was measured by scotopic electroretinography (ERG). Retinas were harvested and morphologically analyzed one week after ischemic insult. ERG a- and b-wave amplitudes were significantly reduced following ischemic reperfusion injury. G-CSF treatment attenuated ischemic-induced loss of retinal function. In control vehicle-treated rats, ischemic reperfusion injury elicited marked and selective thinning of inner retinal layers while only minimally affecting outer retinal layers. Therapeutically administered G-CSF minimized ischemic-mediated thinning of whole retina and inner retinal layers. G-CSF may be of therapeutic interest for the management of retinal ischemic disorders.     

Protective effect of trimetazidine in a model of ischemia-reperfusion in the rat retina

Trimetazidine is an anti-ischemic agent which is frequently prescribed as a prophylactic treatment of episodes of angina pectoris and as a symptomatic treatment of vertigo and tinnitus. It has also shown beneficial effects in models of visual dysfunction, but the mechanism(s) by which this occurs is as yet undefined. The present study was intended to evaluate the influence of trimetazidine on retinal damage induced by ischemia-reperfusion in a rat model. Retinal ischemia was induced by increasing intraocular pressure to 160 mm Hg for 60 min. Trimetazidine or buffer controls were administered 3 days before the ischemia or in the postischemic period. The degree of retinal damage was assessed after 15 and 30 days of reperfusion after the ischemic insult by histopathologic study according to Hughes' quantification of ischemic damage. Retinal ischemia led to significant reductions in thickness and cell number, mainly in the inner retinal layers. The results from the study demonstrate that treatment with intraperitoneally injected trimetazidine conferred significant protection against retinal ischemic damage. Better results were obtained in the pretreatment group after 15 days of reperfusion. Trimetazidine protects the rat retina from pressure-induced ischemic injury and might be considered a potential therapeutic modality for combating retinal ischemia.     

缺血后适应对视网膜缺血-再灌注损伤的保护作用

背景研究证明,缺血后适应（IPC）对多种组织器官的缺血缺氧损伤均有一定的抵抗作用,但其对视网膜缺血缺氧的作用仍受到关注。目的探讨IPC对大鼠视网膜缺血-再灌注损伤（RIRI）后视网膜结构和功能的保护作用。方法将36只健康雄性Wistar大鼠以随机数字表法分为正常对照组、伪手术组、缺血-再灌注组、IPC组。利用前房灌注生理盐水升高眼压至100mmHg（1mmHg=0．133kPa）维持60min的方法制备RIRI大鼠模型,实施IPC处理鼠亚分为再灌注后即刻、1min、10min组（即IPCⅠ组、IPCⅡ组、IPCⅢ组）,分别于实验后1d、7d行大鼠视网膜电图（ERG）检测,然后用过量麻醉法处死大鼠并制备视网膜切片,行苏木精-伊红染色,对各组大鼠视网膜厚度的变化和视网膜形态进行观察。采用SPSS13．0统计学软件的单因素方差分析对各组大鼠ERG各波振幅恢复率和视网膜厚度值的差异进行比较。结果实验后1d,与正常对照组大鼠比较,伪手术组大鼠视网膜结构接近正常,而缺血-再灌注组及IPCⅠ组、IPCⅡ组、IPCⅢ组大鼠视网膜均出现水肿,可见空泡变性,主要在内丛状层（IPL）及内核层（INL）。缺血-再灌注组及IPCⅠ组、IPCⅡ组、IPCⅢ组大鼠视网膜全层、INL、IPL及视网膜外层厚度值均明显高于正常对照组,差异均有统计学意义（均P〈0．05）。再灌注后7d,缺血-再灌注组大鼠视网膜全层厚度值明显低于正常对照组,差异均有统计学意义（均P〈0．05）,尤以INL、IPL显著。IPCⅠ组、IPCⅡ组、IPCⅢ组大鼠视网膜全层、INL、IPL及视网膜外层厚度值均明显高于缺血-再灌注组,差异均有统计学意义（均P〈0．05）。再灌注后7d,缺血-再灌注组、IPC各组大鼠ERG a波、b波和OPs振幅恢复率明显低于伪手术组和正常对照组大鼠,差异均有统计学意义（均P〈0．05）;而IPCⅠ组、IPCⅡ组、IPCⅢ组大鼠ERG a波、b波和OPs振幅恢复率明显高于缺血-再灌注组,差异均有统计学意义（均P〈0,05）。结论IPC对RIRI具有保护作用,在大鼠模型中,这种保护作用在再灌注后即刻至1min时最强。     

GABA agonists fail to protect the retina from ischemia-reperfusion injury

The purpose of this study was to test the hypothesis that ischemia/reperfusion injury in the rat retina may be ameliorated by reducing retinal metabolism with either hypothermia or inhibitory GABA agonists. The intraocular pressure of each right eye in rats was raised to 130 mmHg for 60 min with the left eye serving as normal control. The rats were divided into four groups in terms of drug and hypothermia treatment: (1) Untreated ischemia, (2) Hypothermia, (3) Baclofen/midazolam and (4) Baclofen/muscimol. Electroretinogram was recorded before ischemia and again after 10-day reperfusion. Histological analysis with H&E staining and cell counts was performed. Untreated ischemia/reperfusion resulted in severely reduced ERG responses. The ERG b-wave was reduced from 423 ± 144 μV to 130 ± 91 μV (mean ± SD, n = 5). With hypothermia the ERG b-wave was reduced from 499 ± 80 μV to 237 ± 111 μV (n = 4). With combinations of baclofen and midazolam the ERG b-wave was reduced from 432 ± 96 μV to 104 ± 67 μV (n = 7). In baclofen/muscimol treated eyes the ERG b-wave went from 426 ± 101 μV to 148 ± 118 μV (n = 6). The histological tissue damage was severe in untreated ischemia and the baclofen/midazolam and baclofen/muscimol groups, but less severe in the hypothermia group. The GABA agonists do not provide any protection in our ischemia/reperfusion model. Our results are consistent with earlier reports that hypothermia may be helpful in ischemic conditions in the retina.     

Riluzole improves functional recovery after ischemia in the rat retina

PURPOSE: Retinal ischemia leads to neuronal death. The effects of riluzole, a drug that protects against the deleterious effect of cerebral ischemia by acting on several types of ion channels and blocking glutamatergic neurotransmission, were investigated in a rat model of retinal ischemic injury. METHODS: Retinal ischemia was induced by increasing intraocular pressure above systolic blood pressure for 30 minutes. Electroretinograms were recorded before ischemia and at different periods of reperfusion. Riluzole was injected or topically applied to the eye before or after ischemia and twice daily during the reperfusion period. Retinas were harvested for histopathology (toluidine blue and silver-impregnation stainings, Tdt-dUTP terminal nick-end labeling [TUNEL] method) and immunohistochemistry for cytoskeletal glial fibrillary acid protein and c-jun NH2-terminal kinase (p-JNK). RESULTS: Ischemia for 30 minutes caused a reduction of a- and b-waves of the electroretinogram. Systemic and topical treatments with riluzole significantly enhanced the recovery of the reduced a- and b-waves after defined reperfusion times. Riluzole also prevented or attenuated ischemia-induced retinal cell death (necrosis and apoptosis) and reduced the activation of p-JNK, c-jun phosphorylation, and the increase of cytoskeletal proteins induced by ischemic injury. CONCLUSIONS: Riluzole acted in vivo as a potent neuroprotective agent against pressure-induced ischemia. Therefore, riluzole may be a major drug for use in protection against retinal injury.     

Efficacy and safety of memantine, an NMDA-type open-channel blocker, for reduction of retinal injury associated with experimental glaucoma in rat and monkey

Glutamatergic excitotoxicity has been implicated as a mechanism for injury in a variety of central nervous system pathologies, including glaucoma. Memantine, an NMDA-type glutamatergic open-channel blocker, has pharmacologic properties that make its efficacy greater under excitotoxic conditions, but lesser under normal conditions. Daily oral dosing for approximately 15 months with 4.0 mg/kg memantine in monkeys yielded plasma concentrations similar to those found in patients who received memantine treatment for Parkinson's disease. This same dose of memantine was not associated with any evidence of an effect on the normal function of the retina and central visual pathways, as indicated by measures of the electroretinogram (ERG) and visually-evoked cortical potential (VECP). Amplitude of the VECP response was reduced in eyes with experimentally induced glaucoma. When compared to vehicle-treated control animals, memantine-treated glaucoma eyes suffered significantly less reduction of VECP amplitude. Preliminary results in a rat model for experimental glaucoma also show that, when compared to control animals, systemic treatment with memantine (10 mg/kg/day) was associated with a significant reduction in glaucoma-induced loss of retinal ganglion cells.     

Glutamate elevation in rabbit vitreous during transient ischemia-reperfusion

PURPOSE: To investigate the response of the retina to an ischemic insult, we measured the levels of glutamate in the vitreous of rabbit eyes. METHODS: Ocular ischemia was induced in the vitreous of rabbit eyes by simultaneous ligature of the optic nerve, ciliary arteries, and extraocular muscles for 45 minutes. Contralateral eyes were subjected to a conjunctival peritomy to serve as sham-operated controls. Reperfusion was achieved by releasing the ligature. Eyes were enucleated at the end of the ischemic period or 15, 45, or 75 minutes after reperfusion. RESULTS: Analysis of the vitreous free amino acids showed a significant increase in glutamate levels in the operated eyes at the end of the ischemic period (P <.001) and after 15 minutes of reperfusion (P <.05) as compared with the contralateral, sham-operated eyes. Glutamine levels in the vitreous were unchanged throughout the study. CONCLUSIONS: These results show that glutamate, which is considered to be derived from synaptic release of the retinal neurons or accumulation due to a deterioration of glutamate uptake or a degradation system in the retina, was transiently elevated in the vitreous.     

Neuroprotective effect of citicoline against KA-induced neurotoxicity in the rat retina

We examined whether citicoline has neuroprotective effect on kainic acid (KA)-induced retinal damage. KA (6 nmol) was injected into the vitreous of rat eyes. Rats were injected intraperitoneally with citicoline (500 mgkg-1, i.p.) twice (09:00 and 21:00) daily for 1, 3 and 7 days after KA-injection. The neuroprotective effects of citicoline were estimated by measuring the thickness of the various retinal layers. In addition, immunohistochemistry was conducted to elucidate the expression of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH). Morphometric analysis of retinal damage in KA-injected eyes showed a significant cell loss in the inner nuclear layer (INL) and inner plexiform layer (IPL) of the retinas at the 1, 3 and 7 days after KA injection, but not in the outer nuclear layers (ONL). At 1 and 3 days after citicoline treatment, no significant changes were detected in the retinal thickness and immunoreactivities of ChAT and TH. The immunoreactivities of ChAT and TH had almost disappeared in the retina after 7 days of KA injection. However, prolonged citicoline treatment for 7 days significantly attenuated the reduction of retinal thickness and immunoreactivities of ChAT and TH. The present study suggests that treatment with citicoline has neuroprotective effect on the retinal damage due to KA-induced neurotoxicity.