Selective neuronal survival and upregulation of PCNA in the rat inner retina following transient ischemia

Longitudinal effects of ketamine on cell proliferation and death in the CNS of zebrafish

Zebrafish is known for its widespread neurogenesis and regenerative capacity, as well as several biological advantages, which turned it into a relevant animal model in several areas of research, namely in toxicological studies. Ketamine is a well-known anesthetic used both in human as well as veterinary medicine, due to its safety, short duration and unique mode of action. However, ketamine administration is associated with neurotoxic effects and neuronal death, which renders its use on pediatric medicine problematic. Thus, the evaluation of ketamine effects administration at early stages of neurogenesis is of pivotal importance. The 1-4 somites stage of zebrafish embryo development corresponds to the beginning of segmentation and formation of neural tube. In this species, as well as in other vertebrates, longitudinal studies are scarce, and the evaluation of ketamine long-term effects in adults is poorly understood. This study aimed to assess the effects of ketamine administration at the 1-4 somites stage, both in subanesthetic and anesthetic concentrations, in brain cellular proliferation, pluripotency and death mechanisms in place during early and adult neurogenesis. For that purpose, embryos at the 1-4 somites stage (10,5hours post fertilization - hpf) were distributed into study groups and exposed for 20minutes to ketamine concentrations at 0.2/0.8mg/mL. Animals were grown until defined check points, namely 50 hpf, 144 hpf and 7 months adults. The assessment of the expression and distribution patterns of proliferating cell nuclear antigen (PCNA), of sex-determining region Y-box 2 (Sox 2), apoptosis-inducing factor (AIF) and microtubule-associated protein 1 light chain 3 (LC3) was performed by Western-blot and immunohistochemistry. The results evidenced the main alterations in 144 hpf larvae, namely in autophagy and in cellular proliferation at the highest concentration of ketamine (0.8mg/mL). Nonetheless, in adults no significant alterations were seen, pointing to a return to a homeostatic stage. This study allowed clarifying some of the aspects pertaining the longitudinal effects of ketamine administration regarding the CNS capacity to proliferate and activate the appropriate cell death and repair mechanisms leading to homeostasis in zebrafish. Moreover, the results indicate that ketamine administration at 1-4 somites stage in the subanesthetic and anesthetic concentrations despite some transitory detrimental effects at 144 hpf, is long-term safe for CNS, which are newly and promising results in this research field.

AIBP protects retinal ganglion cells against neuroinflammation and mitochondrial dysfunction in glaucomatous neurodegeneration

Glaucoma is a leading cause of blindness worldwide in individuals 60 years of age and older. Despite its high prevalence, the factors contributing to glaucoma progression are currently not well characterized. Glia-driven neuroinflammation and mitochondrial dysfunction play critical roles in glaucomatous neurodegeneration. Here, we demonstrated that elevated intraocular pressure (IOP) significantly decreased apolipoprotein A-I binding protein (AIBP; gene name Apoa1bp) in retinal ganglion cells (RGCs), but resulted in upregulation of TLR4 and IL-1β expression in Müller glia endfeet. Apoa1bp^-/- mice had impaired visual function and Müller glia characterized by upregulated TLR4 activity, impaired mitochondrial network and function, increased oxidative stress and induced inflammatory responses. We also found that AIBP deficiency compromised mitochondrial network and function in RGCs and exacerbated RGC vulnerability to elevated IOP. Administration of recombinant AIBP prevented RGC death and inhibited inflammatory responses and cytokine production in Müller glia in vivo. These findings indicate that AIBP protects RGCs against glia-driven neuroinflammation and mitochondrial dysfunction in glaucomatous neurodegeneration and suggest that recombinant AIBP may be a potential therapeutic agent for glaucoma.

Ubiquinol promotes retinal ganglion cell survival and blocks the apoptotic pathway in ischemic retinal degeneration

Coenzyme Q10 (CoQ₁₀) protects retinal ganglion cells (RGCs) in experimental retinal ischemia and glaucoma by scavenging reactive oxygen species. We tested whether a diet supplemented with ubiquinol, the reduced form of CoQ₁₀, promotes RGC survival and blocks the apoptotic pathway in ischemic mouse retina induced by acute high intraocular pressure (IOP) elevation. Ubiquinol (1%) treatment significantly promoted RGC survival at 2 weeks after ischemia/reperfusion. The ubiquinol treatment significantly blocked activation of astroglial and microglial cells in the ischemic retina at 2 weeks. While the ubiquinol treatment significantly decreased active Bax protein expression in the ischemic retina, phosphorylation of Bad at serine 112 and Bcl-xL protein expression were preserved in the ubiquinol-treated ischemic retina at 12 h. Consistently, the ubiquinol treatment prevented apoptotic cell death by blocking caspase-3 cleavage. These results suggest that the ubiquinol enhances RGC survival by modulating the Bax/Bad/Bcl-xL-mediated apoptotic pathway in the ischemic retina. Ubiquinol has therapeutic potential for ameliorating elevated IOP-induced ischemic retinal degeneration.

Vulnerability study of myelinated and unmyelinated nerve fibers in acute ocular hypertension in rabbit

In the current study, it was aimed to evaluate the changes in myelinated and unmyelinated nerve fibers in retinal ischemia-reperfusion injuries caused by acute ocular hypertension and to determine the sequence of these changes. Adult healthy New Zealand white rabbits were randomized to the hemodynamic group [n=12; used to determine the optimal intraocular pressure (IOP) for the subsequent experiments] and the hypertension group (n=6; 70-mmHg hypertension induced in one eye). IOP was adjusted using a cannula and saline. Doppler ultrasound was used to measure the velocity of the optic artery under different intraocular pressures. Immunohistochemistry for myelin basic protein (MBP) was performed. Apoptosis of retinal cells was detected by terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) assay. Electron microscopy was used to investigate the changes in myelinated and unmyelinated nerve fibers. IOP of the hypertension eyes was maintained at 70.2±1.0 mmHg, while IOP of control eyes was 7–14 mmHg. Doppler ultrasound demonstrated an obvious decline of peak systolic velocity and an increase of resistance index of retinal bloodstream under a 70-mmHg IOP. MBP immunohistochemistry and electron microscopy demonstrated obvious injuries to the myelin fibers. TUNEL indicated a significantly higher apoptosis rate in the hypertension eyes compared with control eyes. The apoptosis rate of retinal ganglion cells and bipolar cells in unmyelinated regions was higher than in myelinated regions. In conclusion, an IOP of 70 mmHg led to incomplete retinal ischemia but was the threshold for retinal ischemia, leading to obvious injuries to the myelin fibers.

Inhibition of cyclophilin D by cyclosporin A promotes retinal ganglion cell survival by preventing mitochondrial alteration in ischemic injury

Cyclosporin A (CsA) inhibits the opening of the mitochondrial permeability transition pore (MPTP) by interacting with cyclophilin D (CypD) and ameliorates neuronal cell death in the central nervous system against ischemic injury. However, the molecular mechanisms underlying CypD/MPTP opening-mediated cell death in ischemic retinal injury induced by acute intraocular pressure (IOP) elevation remain unknown. We observed the first direct evidence that acute IOP elevation significantly upregulated CypD protein expression in ischemic retina at 12 h. However, CsA prevented the upregulation of CypD protein expression and promoted retinal ganglion cell (RGC) survival against ischemic injury. Moreover, CsA blocked apoptotic cell death by decreasing cleaved caspase-3 protein expression in ischemic retina. Of interest, although the expression level of Bcl-xL protein did not show a significant change in ischemic retina treated with vehicle or CsA at 12 h, ischemic damage induced the reduction of Bcl-xL immunoreactivity in RGCs. More importantly, CsA preserved Bcl-xL immunoreactivity in RGCs of ischemic retina. In parallel, acute IOP elevation significantly increased phosphorylated Bad (pBad) at Ser112 protein expression in ischemic retina at 12 h. However, CsA significantly preserved pBad protein expression in ischemic retina. Finally, acute IOP elevation significantly increased mitochondrial transcription factor A (Tfam) protein expression in ischemic retina at 12 h. However, CsA significantly preserved Tfam protein expression in ischemic retina. Studies on mitochondrial DNA (mtDNA) content in ischemic retina showed that there were no statistically significant differences in mtDNA content among control and ischemic groups treated with vehicle or CsA. Therefore, these results provide evidence that the activation of CypD-mediated MPTP opening is associated with the apoptotic pathway and the mitochondrial alteration in RGC death of ischemic retinal injury. On the basis of these observations, our findings suggest that CsA-mediated CypD inhibition may provide a promising therapeutic potential for protecting RGCs against ischemic injury-mediated mitochondrial dysfunction.

Protective effect of hesperetin and naringenin against apoptosis in ischemia/reperfusion-induced retinal injury in rats

Purpose. Hesperetin and naringenin are naturally common flavonoids reported to have antioxidative effects. This study was performed to investigate whether either hesperetin or naringenin has a protective effect against apoptosis on retinal ischemia/reperfusion (I/R) injury. Methods. Retinal I/R was induced by increasing the intraocular pressure to 150 mmHg for 60 minutes. Thirty-three male Wistar albino rats were randomised into 5 groups named control, I/R + sham, I/R + solvent (DMSO), I/R + hesperetin, and I/R + naringenin. Animals were given either hesperetin, naringenin, or the solvent intraperitoneally immediately following reperfusion. Thickness of retinal layers and retinal cell apoptosis were detected by histological analysis, tunel assay, and immunohistochemistry assay. Results. Hesperetin and naringenin attenuated the I/R-induced apoptosis of retinal cells in the inner and outer nuclear cells of the rat retina. Retinal layer thickness of the naringenin treatment group was significantly thicker than that of the hesperetin, sham, and solvent groups (P < 0.05). Conclusions. Hesperetin and naringenin can prevent harmful effects induced by I/R injury in the rat retina by inhibiting apoptosis of retinal cells, which suggests that those flavanones have a therapeutic potential for the protection of ocular ischemic diseases.

Measuring glutamate receptor activation-induced apoptotic cell death in ischemic rat retina using the TUNEL assay

Glutamate receptor activation-mediated excitotoxicity has been hypothesized to cause cell death in both acute and chronic neurodegenerative diseases including glaucoma. Although the precise mechanisms of ischemia-induced neuronal death are unknown, glutamate excitotoxicity-induced apoptotic cell death is considered to be an important component of postischemic damage in the retina. The blockade of apoptotic cell death induced by glutamate receptor activation provides strong evidence that glutamate excitotoxicity-induced apoptotic cell death may be a central mechanism of cell death in ischemic rat retina. We have shown that there is TUNEL-positive apoptotic cell death in the outer nuclear layer, inner nuclear layer, and ganglion cell layer of the ischemic rat retina at 12 h.

Alterations in photoreceptor-bipolar cell signaling following ischemia/reperfusion in the rat retina

Studies of retinal ischemia/reperfusion indicate a disparity between the anatomical and functional results; while a large number of rod bipolar cells remain postischemia, there is a significant reduction in the amplitude of the scotopic b-wave of the electroretinogram (ERG). We investigated the alterations in photoreceptor-bipolar cell signaling following ischemia/reperfusion and suggest a mechanism for the decrease in b-wave amplitude. A cation channel probe (agmatine, 1-amino-4-guanidobutane, AGB) was used to assess cellular ion channel activity in neurochemically identified cells secondary to endogenous glutamate release or pharmacological manipulations. By applying the "neurochemical truth point" principle (Sun et al. [2007a] J Comp Neurol, this issue), we have been able to confirm the loss of specific subpopulations of neurons. ERG was used to assess gross retinal function, with parameters of the ERG model providing insight into changes in the phototransduction cascade and sensitivity of postreceptoral glutamate receptors. Following ischemia/reperfusion, rod bipolar cells maintained 2-amino-4-phosphonobutyric acid-responsive metabotropic glutamate receptors and displayed no change in sensitivity to flashes of light as assessed by ERG. Therefore, the loss in b-wave amplitude is likely due to alterations in photoreceptoral glutamate release detected as a change in postsynaptic AGB permeation into rod bipolar cells. Bipolar cell to amacrine cell signaling was also altered. The robust AGB entry into cholinergic amacrine cells was virtually absent in retinas that had undergone ischemia/reperfusion but remained in the AII amacrine cells. Such results suggest a loss of glutamate receptors and/or a change in receptor subunit expression in subpopulations of inner retinal neurons. Although many cells retain their characteristic neurochemical labeling following ischemia/reperfusion, caution should be used when assuming cells participate in functional retinal circuits based solely on the persistence of neurochemical labeling.

Metabolic and functional profiling of the ischemic/reperfused rat retina

We quantitatively tracked the recovery in amino acid labeling and cation channel functionality within distinct retinal elements for up to 2 weeks after an ischemic insult. Pattern recognition analysis of multiple amino acid and agmatine (a cation channel probe; 1-amino-4-guanidobutane; AGB) immunocytochemical patterns was used to classify all neural elements within the retina. This classification was spatially complete and with single-cell resolution. By 48 hours of reperfusion the amino acid labeling pattern of virtually all cell populations had returned to near preischemic levels, with the exception of glutamine and alanine levels, which remained significantly higher in many cell populations. Classification resulted in a total of 18 statistically separable theme classes (including neurons, glia, and extraretinal classes), a reduction of 10 theme classes from the normal retina (Sun et al. [ 2007a, b] J Comp Neurol, this issue). In addition to the known selective losses of amacrine cell types within the inner nuclear layer, we now demonstrate a selective loss of theme classes representing cone bipolar cells within the bipolar cell population. While there was a recovery in the amino acid labeling pattern, there were persistent cation channel gating anomalies (as reflected by AGB labeling) within several theme classes, including the theme class representing all the remaining rod bipolar cells, suggesting aberrant neuronal function secondary to metabolic insult.

Aldose reductase deficiency prevents diabetes-induced blood-retinal barrier breakdown, apoptosis, and glial reactivation in the retina of db/db mice

In 15-month-old db/db mice, signs of diabetic retinopathy, including blood-retinal barrier breakdown, loss of pericytes, neuro-retinal apoptosis, glial reactivation, and proliferation of blood vessels, were evident. These changes in the diabetic retina were associated with increased expression of aldose reductase (AR). To further understand the role of AR in the pathogenesis of diabetic retinopathy, we generated db/db mice with an AR null mutation (AR-/- db/db). AR deficiency led to fewer retinal blood vessels with IgG leakage, suggesting that AR may contribute to blood-retinal barrier breakdown. AR deficiency also prevented diabetes-induced reduction of platelet/endothelial cell adhesion molecule-1 expression and increased expression of vascular endothelial growth factor, which may have contributed to blood-retinal barrier breakdown. In addition, long-term diabetes-induced neuro-retinal stress and apoptosis and proliferation of blood vessels were less prominent in AR-/- db/db mice. These findings indicate that AR is responsible for the early events in the pathogenesis of diabetic retinopathy, leading to a cascade of retinal lesions, including blood-retinal barrier breakdown, loss of pericytes, neuro-retinal apoptosis, glial reactivation, and neovascularization.

Contribution of Nitric Oxide-Producing Cells in Normal and Diabetic Rat Retina

PURPOSE

To examine the immunohistochemical localization of L-arginine and L-citrulline and determine where and how nitric oxide (NO) is produced in the normal and streptozotocin (STZ)-induced diabetic rat retinas.

METHODS

NO is produced when L-arginine is changed to L-citrulline by NO synthase (NOS). In normal and STZ-induced diabetic rats, using an immunohistochemical method, we examined the retinal distribution of L-arginine and L-citrulline after intracardiac perfusion. We studied the distribution of NOS after immersed fixation and analyzed the number of neuronal NOS (nNOS)-positive neurons.

RESULTS

We observed L-arginine localization in the internal limiting membrane (ILM), the ganglion cell layer (GCL), and the inner nuclear layer (INL). L-Arginine immunoreactivity in the diabetic rat retinas was found in the inner plexiform layer (IPL), as well as in the normal retina. L-Citrulline immunoreactivity in the normal and diabetic retinas was observed in the ILM, the GCL, the IPL, and the INL. nNOS staining in the normal and diabetic rat retinas was observed in the GCL, the IPL and the INL. The number of nNOS-positive amacrine cells was less in the diabetic rat retinas.

CONCLUSION

NO might be produced in the GCL and amacrine cells, which show immunoreactivity to L-arginine, L-citrulline, and nNOS. In the early stage of diabetic retinopathy in STZ rat retinas, diabetes disturbed the function of the nNOS-positive amacrine cells and reduced NO production via nNOS.

Correlations between neuronal nitric oxide synthase and muscarinic M3/M1 receptors in the rat retina

This study determined the different signal pathways involved in M1/M3 muscarinic acetylcholine receptor (mAChR) dependent stimulation of nitric oxide synthase (NOS) activity/cyclic GMP (cGMP) production and nNOS mRNA expression in rat retina. Exposure of the retina to different concentrations of carbachol caused an increase in NOS activity, cGMP production and phosphoinositol (PI) accumulation. The increase in NOS activity and cGMP content was blocked by L-NMMA and ODQ, respectively. Also, phospholipase C (PLC) and calcium/calmodulin (CaM) inhibition prevented the carbachol activation on NOS/cGMP pathways. Both, 4-DAMP and pirenzepine but not AF-DX 116 blocked the increase in NOS and cGMP induced by carbachol. Carbachol-stimulation of M1/M3 mAChR increased nNOS-mRNA levels associated with an increase of endogenous NO and cGMP production. The mechanism appears to occur secondarily to stimulation of PIs turnover via PLC. This triggers a cascade reaction involving CaM and soluble guanylate cyclase leading to NO and cGMP accumulation, that in turn, up regulates nNOS-mRNA gene expression. These results give novel insight into the mechanism involved in the regulation of nNOS-mRNA levels by mAChR activation of retina.

Altered membrane physiology in Müller glial cells after transient ischemia of the rat retina

Inwardly rectifying K+ (Kir) channels have been implicated in the mediation of retinal K+ homeostasis by Muller glial cells. To assess possible involvement of altered glial K+ channel expression in ischemia-reperfusion injury, transient retinal ischemia was induced in rat eyes. Acutely isolated Muller cells from postischemic retinae displayed a fast downregulation of their Kir currents, which began within 1 day and reached a maximum at 3 days of reperfusion, with a peak decrease to 20% as compared with control. This strong decrease of Kir currents was accompanied by an increase of the incidence of cells which displayed depolarization-evoked fast transient (A-type) K+ currents. While no cell from untreated control rats expressed A-type K+ currents, all cells investigated from 3- and 7-day postischemic retinae displayed such currents. An increased incidence of cells displaying fast transient Na+ currents was observed at 7 days after ischemia. These results suggest a role of altered glial Kir channel expression in postischemic neuronal degeneration via disturbance of retinal K+ siphoning.

Expression of clusterin in Müller cells of the rat retina after pressure-induced ischemia

We have investigated the expression and cellular localization of clusterin in the rat retina following ischemia induced by transiently increasing the intraocular pressure. In the normal retina, weak clusterin immunoreactivity was visible in Müller cell profiles located in the inner nuclear layer. Following ischemia and reperfusion, strong immunoreactivity appeared in Müller cell somata and processes up to 3 days postlesion. Quantitative evaluation by immunoblotting confirmed that clusterin expression continuously increased and showed a peak value at 3 days after ischemic injury (to 1300% of control levels), and then decreased again to 400% of controls at 4 weeks postlesion. Immunocytochemistry using antisera against clusterin or glutamine synthase combined with the TUNEL method or immunocytochemistry using antisera activated caspase 3 and electron microscopy revealed that some clusterin-labeled Müller cells underwent apoptotic cell death. Our findings demonstrate that some Müller cells die by apoptosis, and suggest that clusterin produced and released by Müller cell may play an important role in the pathogenesis of ischemic injury in the rat retina.

Neuronal susceptibility to damage: comparison of the retinas of young, old and old/caloric restricted rats before and after transient ischemia

Compared to young rats, old age increases susceptibility and caloric restriction decreases susceptibility for the loss of retinal ganglion cells and displaced amacrine cells following retinal ischemia/reperfusion. In retinas of old animals before ischemia, reactive gliosis, including activation of Muller cells, microglia and astrocytes, is increased compared to retinas from young and old/caloric restricted animals. Post-ischemia, the existing reactive gliosis in retinas of old animals is not neuroprotective and the reactive gliosis is even further increased in old animals compared to young or old/caloric restricted animals. In retinas from old/caloric restricted animals, inducible heat shock protein-70 and brain-derived neurotrophic factor increased more markedly after ischemia/reperfusion compared to retinas from young and old animals. Thus, compared to retinas in young animals, neurons of old animals may be more susceptible to cell death by secondary glial mechanisms after retinal ischemia/reperfusion. Caloric restriction in old animals is neuroprotective against damage in the retina following ischemia, perhaps by suppressing glial activity and by the neuroprotective effects of inducible heat shock protein-70 and brain-derived neurotrophic factor.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Insulin-like growth factor (IGF)-1 suppresses oligodendrocyte caspase-3 activation and increases glial proliferation after ischemia in near-term fetal sheep

Insulin-like growth factor (IGF-1) markedly increases myelination and glial numbers in white matter after ischemia in near-term fetal sheep; however, it is unclear whether this is due to reduced cell loss or increased secondary proliferation. Brain injury was induced in near-term fetal sheep by 30 minutes of bilateral carotid artery occlusion. Ninety minutes after the occlusion, fetuses were given, intracerebroventricularly, either a single dose of IGF-1 (either 3 or 30 micro g), or 3 micro g followed by 3 micro g over 24 hours (3 + 3 micro g). White matter was assessed 4 days after reperfusion. Three micrograms, but not 30 micro g of IGF-1 prevented loss of oligodendrocytes and myelin basic protein density (P < 0.001) compared to the vehicle-treated ischemia controls. No additional effect was observed in the 3 + 3 micro g group. IGF-1 treatment was associated with reduced caspase-3 activation and increased glial proliferation in a similar dose-dependent manner. Caspase-3 was only expressed in oligodendrocytes that showed apoptotic morphology. Proliferating cell nuclear antigen co-localized with both oligodendrocytes and astrocytes and microglia. Thus, increased oligodendrocyte numbers after IGF-1 treatment is partly due to suppression of apoptosis, and partly to increased proliferation. In contrast, the increase in reactive glia was related only to proliferation. Speculatively, reactive glia may partly mediate IGF-1 white matter protection.

Neuronal degradation in mouse retina after a transient ischemia and protective effect of hypothermia

Temporal profile of neuronal deaths in the mouse retina evoked by a transient retinal ischemia and the protective effect of hypothermia on such deaths were evaluated. A transient ischemic insult was induced in the mouse retina by elevating the intra-ocular pressure. The retina tissue responses after reperfusion were histopathologically detected by monitoring the retinal cell death in the ganglion cell layer and inner nuclear layer, using a sequential TUNEL-staining technique, and by measuring the inner retinal thickness. Elevation of intra-ocular pressure induced a time-related appearance of TUNEL-positive cells in the mouse inner retinas. Peak TUNEL staining occurred 12 h after reperfusion. Lowering mice body temperature to 35 degrees C, 33 degrees C and 29 degrees C during the ischemia period significantly inhibited DNA fragmentation of retinal neurons in a lowering temperature dependent manner. In this experiment, the inner retinal thickness was preserved in 29 degrees C group compared with that in 37 degrees C group. From these results, the 45-min transient ischemia and histopathological examination 12 h later provided a reproducible number of retinal neuronal deaths. Furthermore, hypothermic intervention showed a protective effect to salvage retinal neuronal cells from a transient ischemic insult.

Growth-associated protein 43 is up-regulated in the ganglion cells of the ischemic rat retina

We investigated the expression and cellular localization of growth-associated protein (GAP)-43 in the rat retina following ischemia induced by transiently increased intraocular pressure. In the normal retina, GAP-43 immunoreactivity was restricted to profiles in the inner plexiform layer. Following ischemia and reperfusion, immunoreactivity appeared in ganglion cells. The cell density of labeled ganglion cells peaked three days post-lesion and then decreased at seven days. Quantitative evaluation by immunoblotting confirmed that GAP-43 expression increased at three days (to 190% of control levels) and then slightly decreased at seven days. Our findings suggest that some ganglion cells have the potential to regenerate through the up-regulation of GAP-43 in the ischemic rat retina.

Sodium nitroprusside selectively induces apoptotic cell death in the outer retina of the rat

Sodium nitroprusside (SNP), an NO donor, was studied for its effects on apoptosis in rat retinal neurons. TUNEL-positive cells were observed in the outer nuclear layer (ONL), but not in the inner retina after SNP treatment. Inner retinal neurons died by necrosis. No photoreceptor cells were found in the ONL after seven days. Immunoblotting confirmed that neurnal NO synthase expression increased up to 5 days (approximately 170% of control levels), and then declined by 7 days, suggesting that NO induces apoptosis in the ONL, and that inner retinal neurons die by necrosis due to glutamate from damaged photoreceptors.

The regulatory expression of neuronal nitric oxide synthase in the ischemic rat retina

We investigated the expression and cellular localization of neuronal nitric oxide synthase (nNOS) in the rat retina, following ischemic injury induced by transient increase of intraocular pressure. In the normal retina, nNOS immunoreactivity was localized to certain populations of amacrine cells, displaced amacrine cells and a few bipolar cells. Following transient ischemia, retinal neurons expressing the immunoreactivity increased and peaked three days after reperfusion. Quantitative evaluation using immunoblotting confirmed that nNOS expression showed a peak value (500% of control levels) at 3 days, and then decreased again to 150% of controls by 4 weeks after reperfusion. Our findings suggest that this over-produced NO may act as a neurotoxic agent in the ischemic rat retina.

Up-regulated eNOS protects blood-retinal barrier in the L-arginine treated ischemic rat retina

Using immunoblot analysis and immunocytochemistry, we investigated expression and cellular localization of endothelial nitric oxide synthase (eNOS) and proliferating cell nuclear antigen (PCNA) in the l-arginine treated ischemic rat retina. In parallel, we tested whether the blood-retinal barrier was intact by immunocytochemistry using an antiserum against IgG. In the l-arginine-treated ischemic retina, the magnitude of the increased eNOS was higher, and PCNA was expressed in endothelial cells as well as in neurons in the inner retina during the whole experimental period. Finally, IgG leakage was not detectable in the l-arginine-treated ischemic retina. Our results clearly suggest that the increased NO production by eNOS may be essential for the survival of endothelial cells in the rat retina following transient ischemia.

Simultaneous management of blood flow and IOP in glaucoma

Factors other than intraocular pressure (IOP) elevation must be involved in initiation and progression of glaucoma. An additional element in disease causation may be ischemia in the retina and optic nerve head. Ischemic damage to neurons in the CNS is similar mechanistically and histopathologically to changes seen in glaucoma. Further, glaucoma patients with normal IOP show clear evidence for cerebral and ocular ischemia. Aging and atherosclerosis reduce the ability of the eye to autoregulate blood flow when ocular perfusion pressure changes: the dependence of blood flow on perfusion pressure links ischemia to IOP. Consequently, neuroprotective treatments for glaucoma should be designed to both reduce IOP and improve ocular nutrient delivery.

Nitric oxide is involved in sustained and delayed cell death of rat retina following transient ischemia

We have investigated the role of nitric oxide (NO) in the rat retina following ischemic injury induced by transient increase of intraocular pressure. The thickness of both the inner plexiform layer and inner nuclear layer decreased during early postischemic stages (up to 1 week). In late postischemic stages (2-4 weeks), the thickness of the outer nuclear layer (ONL) decreased markedly. Thus, mechanisms other than excitotoxic ones may contribute to postischemic retinal cell death. Treatment of rats with N(G)-nitro-L-arginine methyl ester, a nitric oxide synthase (NOS) inhibitor, significantly reduced ischemic damage. Our findings suggest that NO is involved in the mechanism of ischemic injury, and plays a key role in the delayed and sustained cell death in the ONL following transient retinal ischemia.