Role of nitric oxide during the initial phase of reperfusion after retinal ischemia in the rat

NCX 470 Exerts Retinal Cell Protection and Enhances Ophthalmic Artery Blood Flow After Ischemia/Reperfusion Injury of Optic Nerve Head and Retina

Purpose

The purpose of this study was to assess the retinal protective activity and ocular hemodynamics after NCX 470 (0.1%) compared to bimatoprost administered as the US Food and Drug Administration (FDA)-approved drug (Lumigan - 0.01% ophthalmic solution, LUM) and at an equimolar dose (0.072%, BIM) to that released by NCX 470.

Methods

Endothelin-1 (ET-1) induced ischemia/reperfusion injury model in rabbits was used. ET-1 was injected nearby the optic nerve head (ONH) twice/week for 6 weeks. Starting on week 3, the animals received vehicle (VEH), NCX 470, LUM, or BIM (30 µL/eye, twice daily, 6 days/week) until the end of ET-1 treatment. Intraocular pressure (IOP), ophthalmic artery resistive index (OA-RI), and electroretinogram (ERG) data were collected prior to dosing and at different time points postdosing. Reduced glutathione, 8-Hydroxy 2-deoxyguanosine, and Caspase-3 were determined in the retina of treated eyes. DNA fragmentation was determined by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining.

Results

ET-1 increased IOP (VEHIOP_Baseline = 20.5 ± 0.8 and VEHIOP_Week6 = 24.8 ± 0.3 mmHg) and OA-RI (VEHOA-RI_Baseline = 0.36 ± 0.02 and VEHOA-RI_Week6 = 0.55 ± 0.01) and reduced rod/cone responses over time. Oxidative stress, inflammation, and apoptotic markers increased in ET-1-treated eyes. NCX 470 prevented IOP (NCX 470IOP_Week6 = 18.1 ± 0.6 mmHg) and OA-RI changes (NCX 470OA-RI_Week6 = 0.33 ± 0.01) and restored ERG amplitude leaving unaltered the respective latency; these effects were only partially demonstrated by LUM or BIM. Additionally, NCX 470 reduced oxidative stress, inflammation, and apoptosis in the retinas of treated eyes. BIM and LUM were numerically less effective on these parameters.

Conclusions

NCX 470 repeated ocular dosing ameliorates ocular hemodynamics and retinal cell dysfunction caused by ischemia/reperfusion via nitric oxide- and bimatoprost-mediated mechanisms.

Translational Relevance

If confirmed in clinical setting our data may open new therapeutic opportunities to reduce visual field loss in glaucoma.

BPC 157 as a Therapy for Retinal Ischemia Induced by Retrobulbar Application of L-NAME in Rats

Providing NO-system importance, we suggest that one single application of the NOS-blocker L-NAME may induce retinal ischemia in rats, and that the stable pentadecapeptide BPC 157 may be the therapy, since it may interact with the NO-system and may counteract various adverse effects of L-NAME application. A rat retinal ischemia study was conducted throughout 4 weeks, including fundoscopy, behavior presentation, tonometry, and histology assessment. Retrobulbar L-NAME application (5 mg/kg; 0.5 mg/0.1 ml saline/each eye) in rats immediately produced moderate generalized irregularity in the diameter of blood vessels with moderate atrophy of the optic disc and faint presentation of the choroidal blood vessels, and these lesions rapidly progressed to the severe stage. The specific L-NAME-induced vascular failure points to normal intraocular pressure (except to very transitory increase upon drug retrobulbar administration). When BPC 157 (10 μg; 10 ng/kg, as retrobulbar application, 1 μg; 1 ng/0.1 ml saline/each eye) is given at either 20 min after L-NAME or, lately, at 48 h after L-NAME, the regular retrobulbar L-NAME injection findings disappear. Instead, fundoscopy demonstrated only discrete generalized vessel caliber irregularity with mild atrophy of the optic disc, and then, quite rapidly, normal eye background and choroidal blood vessels, which remain in all of the subsequent periods. Also, histology assessment at 1, 2, and 4 weeks shows that BPC 157 counteracted the damaged inner plexiform layer and inner nuclear layer, and revealed normal retinal thickness. The poor behavioral presentation was also rescued. Thus, while further studies will be done, BPC 157 counteracted L-NAME-induced rat retinal ischemia.

Functional and Structural Evaluation of Sildenafil in a Rat Model of Acute Retinal Ischemia/Reperfusion Injury

PURPOSE

Retinal ischemia is a common cause of visual impairment and blindness. Sildenafil, a PDE5 inhibitor which inhibits cGMP degradation and in turn prolongs the effect of nitric oxide, has been shown to be protective in a number of ischemia/reperfusion (I/R) injuries, as well as in neuronal damage. We hypothesized that treatment with sildenafil might be neuroprotective in a model of acute retinal I/R injury.

METHODS

Anterior chamber cannulation was performed to induce unilateral I/R injury in 38 Lewis rats. Animals received intraperitoneal injections of sildenafil (0.5 and 1 mg/kg once a day, for a period of 7 and 18 days, respectively), or saline. Electroretinography recordings, retinal ganglion cell (RGC) counts following retrograde labeling with fluorogold, histopathology, and immunohistochemistry (IHC) using antibodies against PDE5, NOS2, caspase-3, caspase-9, and Bcl-2 were conducted.

RESULTS

No significant differences in electroretinography, RGC counts, or retinal morphometry were observed between experimental eyes of sildenafil- and saline-treated animals. A tendency toward less necrosis in histopathology, and a slight trend toward lower PDE5, NOS2, and caspase-9 and higher Bcl-2 IHC scores were evident in experimental retinas of sildenafil-treated animals.

CONCLUSIONS

Electroretinography, RGC counts, and retinal morphometry failed to show any neuroprotective effect of sildenafil in acute retinal I/R injury in rats. A slight positive effect of sildenafil was qualitatively indicated by histopathology and IHC.

Endothelin-1 and nitric oxide levels in patients with glaucoma

AIMS

To investigate the levels of endothelin-1 (ET-1) and nitric oxide (NO) in the aqueous humor and plasma of human eyes with different types of glaucoma: primary open-angle glaucoma (POAG) and chronic closed-angle glaucoma (CCAG).

METHODS

Patients were classified into 3 groups: group I comprised 35 patients with POAG, group II comprised 25 patients with CCAG, and 30 patients with senile cataract (group III) were used as a control group. Aqueous humor and corresponding plasma were analyzed for ET-1 and NO concentrations by enzyme-linked immunosorbent assay. A Bonferroni correction for multiple comparisons was performed.

RESULTS

There was no significant difference in plasma levels of either ET-1 or NO metabolites between the groups studied. ET-1 and NO were significantly elevated in the aqueous humor of patients with CCAG and POAG compared to the corresponding value in patients with cataract (p < 0.001). ET-1 and NO concentrations in the aqueous humor were more marked in CCAG than in POAG. NO levels were correlated with ET-1 in the aqueous humor of patients with glaucoma (p < 0.001).

CONCLUSIONS

Increased concentrations of ET-1 and NO in aqueous humor may be useful with POAG and CCAG. In addition, ET-1 and NO may have useful metabolite levels in the aqueous humor of POAG and CCAG patients as a result of glaucoma damage and may not be a cause of it.

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.

Amelioration by topical bunazosin hydrochloride of the impairment in ocular blood flow caused by nitric oxide synthase inhibition in rabbits

We investigated whether topical instillation of an alpha(1)-adrenergic blocker would improve an insufficient blood supply in the optic nerve head (ONH) and visual function, in rabbits. The effect of systemic NOS inhibition on visual-evoked potentials (VEPs) and hemodynamics in ONH were determined. VEPs were recorded before and every 15 min during a 120-min observation period after an intravenous injection of 50 mg/kg N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor. Capillary blood flow in ONH was evaluated by the laser speckle method throughout the same period. Then, we investigated the effect of topical instillation of a recently developed alpha(1) adrenergic blocker, bunazosin hydrochloride (0.01%), 60 min prior to the intravenous L-NAME (50 mg/kg) on the changes by NOS inhibition. The VEP amplitudes were reduced by L-NAME (10, 20, and 50 mg/kg) in a dose-dependent manner, while the VEP implicit time was unchanged, and no significant changes were detected in the electroretinogram. The reductions in ONH capillary blood flow and VEP amplitudes caused by L-NAME (50 mg/kg) were significantly suppressed by an instillation of bunazosin hydrochloride. These results indicate that blocking alpha(1)-adrenergic receptors may ameliorate the impairments in blood flow and retinal function caused by NOS inhibition. The enhancement of basal vascular tone due to deprivation of continuous NO production may be diminished by this alpha(1)-adrenergic blocker.

Protective effects of timolol against the neuronal damage induced by glutamate and ischemia in the rat retina

The purpose of this study was to determine whether timolol, an ocular hypotensive drug, has retinal neuroprotective effects in experimental in vitro and in vivo models. For in vitro studies, we used retinal neuron cultures from rat embryos and purified retinal ganglion cells (RGCs) from newborn rats. In the former, neurotoxicity was induced using 1 mM glutamate and cell viability was assessed. In RGCs, neurotoxicity was induced using 25 microM glutamate for 3 days and cell viability was assessed. For the in vivo study, we used a rat model of retinal ischemic injury induced by elevating intraocular pressure (IOP) by raising the hydrostatic pressure. The retinal damage was evaluated by counting the number of cells in the ganglion cell layer (GCL) and by examining the a- and b-waves in the electroretinogram (ERG). For the intraocular distribution study, 0.5% [3H]timolol was topically applied to rat eyes, and these were enucleated after various intervals and divided into parts. Each part was combusted and the radioactivity measured. Timolol (0.1 and 1 microM) markedly reduced the glutamate-induced neuronal cells in retinal neuron cultures and in RGCs. After ischemic-reperfusion, both the number of cells in the GCL and a- and b-waves in the ERG decreased; however, topically applied 0.5% timolol reduced these effects. Topically applied 0.5% timolol was detected at a concentration of approximately 1 microg/g wet tissue in retina-choroid at 30 min after its application. In conclusion, timolol was effective against retinal neuron damage both in vitro and in vivo. Furthermore, topically applied timolol reached the retina-choroid. These findings suggest that timolol has a direct neuroprotective effect against retinal damage.

Loss of retinal ganglion cells following retinal ischemia: the role of inducible nitric oxide synthase

Following experimental, transient, retinal ischemia in the rat, there is loss of retinal neurons, which occurs over several weeks. Retinal ganglion cells (RGCs) are particularly susceptible and there is early, massive degeneration of these neurons after ischemia. We have determined the early mechanisms by which RGCs are killed following ischemia. Retinal ischemia/reperfusion was produced in rats by transient unilateral elevation of intraocular pressure above systolic blood pressure. Retinas were studied by immunohistochemistry for the presence of inducible nitric oxide synthase (NOS-2) at several time points post-ischemia and specific cell types were identified. Rats were also treated orally with L -N(6) -(1-iminoethyl)lysine 5-tetrazole amide (SC-51), a prodrug of an inhibitor of NOS-2 or with aminoguanidine (AG) for a period of 14 days. Retrograde labelling with Fluoro-Gold quantitated the loss of RGCs. NOS-2 was not present in the normal retina and was not present in the eyes that were contralateral to the ischemic eyes. Within 24hr after ischemia, polymorphonuclear leukocytes containing NOS-2 had entered the ganglion cell layer and surrounded RGCs. Within 5 days after ischemia, NOS-2 was present in many inner retina cells and in invading monocytes in the vitreous. Between 7 and 14 days post-ischemia, there were few hematogenous cells in the retina but NOS-2 was sparsely detectable in microglia and other cells of the inner retina. Two weeks after ischemia, rat eyes lost approximately 50% of the RGCs. Treatment with AG for 14 days following ischemia was partially neuroprotective; approximately 28% of the RGCs were lost. Treatment with SC-51 for 14 days following ischemia almost completely prevented the loss of RGCs. Thus, within 24hr following ischemia, polymorphonuclear leukocytes containing NOS-2 attack and kill neurons in the ganglion cell layer. For 2 weeks after ischemia, NOS-2 appears transiently in the retina in several different cell types at different times. Continuous pharmacological treatment with inhibitors of NOS-2 activity during the 2 weeks post-ischemia period provides significant neuroprotection against the loss of RGCs.

Neuroprotective effects of nipradilol on purified cultured retinal ganglion cells

PURPOSE

To investigate effects of nipradilol, a nonselective beta- andbgr;-and selective alpha1-receptor antagonist and a potential nitric oxide releaser, on retinal ganglion cells purified and cultured in a serum-free medium.

METHODS

Retinal ganglion cells were isolated from 2-day-old Sprague-Dawley rats by means of two-step panning. A series of nipradilol (10(-5), 10(-6), 10(-7), 10(-8), 10(-9), and 10(-10)-mol/L) or vehicle solutions were administered to the culture medium for 48 hours, and the survival rate of retinal ganglion cells was evaluated using a newly developed system that evaluates the survival rate in small and large retinal ganglion cells separately. The effects of timolol maleate or bunazosin (10(-5), 10(-6), 10(-7), and 10(-8) -mol/L) solutions on retinal ganglion cells survival were also evaluated. The survival rate was evaluated after 10(-5)-mol/L c-PTIO (2-[4-carboxyphenyl]-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide potassium salt), a nitric oxide scavenger, was administered to retinal ganglion cells with 10(-5)-mol/L nipradilol.

RESULTS

Nipradilol significantly increased the survival rate of both small and large retinal ganglion cells in a concentration-dependent manner compared with the controls. The maximum survival rate improvement of small and large retinal ganglion cells was 29.1% and 14.5%, respectively. Although timolol maleate and bunazosin did not affect the survival rate, 10(-5)-mol/L c-PTIO significantly inhibited the nipradilol-induced survival rate improvement by 69.9% in small retinal ganglion cells and by 91.6% in large retinal ganglion cells.

CONCLUSION

Nipradilol improves the survival rate of cultured postnatal rat retinal ganglion cells, and the nitric oxide generated from nipradilol may contribute to this effect.

Nitric oxide levels in the aqueous humor in cataract patients

PURPOSE

To determine the role of nitric oxide (NO) in the clinical manifestations of cataract in different age and etiological groups.

SETTING

Multicenter study, Taipei, Taiwan, Republic of China.

METHODS

Aqueous humor samples were obtained from 114 cataract patients in different etiological and age groups (eg, juvenile, traumatic, and age-related cataract). Using chemiluminescence assay, NO was detected through the light generated by ozone and NO interaction. The amount of NO production was calculated and compared among groups. The results were correlated with patient age.

RESULTS

The NO levels in aqueous humor were highest in the 12 patients with traumatic cataract and lowest in the 15 patients with juvenile cataract (47.59 micromol/L +/- 12.81 [SD] and 7.66 +/- 2.62 micromol/L, respectively; P <.001). Aqueous humor NO levels were correlated with age and were highest in patients older than 80 years (mean 38.78 +/- 6.29 micromol/L) and in those with mature cataract (mean 40.15 +/- 6.15 micromol/L) (P <.05).

CONCLUSIONS

The NO levels in the aqueous humor increased with age and traumatic events. The results suggest that NO could be a risk factor in cataract formation.

Elevated nitric oxide level in aqueous humor of patients with acute angle-closure glaucoma

Nitric oxide, a noxious and free-radical gas, plays a key role in vasodilatation, inflammation, immunity, and neurotoxicity. Studies have shown that a recently detected NO synthase inducible isoform, found in astrocytes in the optic nerve heads of glaucoma patients, could stimulate excessive production of NO. The purpose of our study was to elucidate the role of NO production in the pathogenesis of glaucoma. We measured the concentrations of NO in aqueous humor from 11 patients with acute angle-closure glaucoma (AACG) and 14 patients with cataract. All samples were collected from patients who were free of any other systemic disease. Utilizing a chemiluminescence assay, we found that in AACG patients the NO levels in aqueous humor samples (83.2 +/- 6.7 microM) were significantly higher than in cataract patients (27.1 +/- 3.6 microM; p < 0.001). We therefore concluded that excessive NO in AACG patients reflects the degree of ocular impairment demonstrated by concomitant retinal cell and optic nerve cell damage.

Effects of aging on the electroretinogram during ischemia-reperfusion in rats

The effects of aging on the electroretinogram (ERG) during ischemia-reperfusion were investigated in rats. Flash-elicited ERG (a-wave, b-wave, and oscillatory potentials (OPs)) was recorded in young (4 months old) and aged rats (over 18 months old) before, during, and after exposure to 30- or 120-min ischemia induced by increasing intraocular pressure to 80 mmHg. The choroidal blood flow, measured by means of laser Doppler flowmetry, decreased to 40 to 60% of the baseline value during ischemia. Young rats showed no significant difference in the amplitude of each ERG component during ischemia between 30- and 120-min ischemia groups; 78.0 +/- 4.9 vs. 76.1 +/- 3.6% for a-wave, 63.4 +/- 3.1 vs. 60.6 +/- 3.0% for b-wave, and 59.6 +/- 5.9 vs. 57.5 +/- 6.7% for SigmaOP. In aged rats, however, 120-min ischemia caused a greater decrease, to 56.7 +/- 3.1% of the baseline value, in the a-wave amplitude than 30-min ischemia did, to 70.8 +/- 3.2%. The reduction of each ERG component in both 30- and 120-min ischemia experiments was greater in aged rats than in young rats. The recovery time for the amplitude of each ERG component during reperfusion was longer in aged rats than in young rats. The latency of b-wave and the second component of OPs prolonged during ischemia, and recovery time for the latency was longer in aged rats than in young rats. These results suggest that the electrophysiological function of the retina is less tolerable against ischemia-reperfusion in aged rats than in young rats.

Effect of nitric oxide synthesis inhibition on post-occlusive choroidal blood flow in rats

Experiments were designed to study involvement of nitric oxide on vascular responses to ocular ischemia in the anesthetized rat. Anterior choroidal blood flow was measured using laser-Doppler flowmetry. In some experiments, cerebral cortical blood flow also was measured. Ischemia was produced by either occlusion of the cephalic blood supply or more locally via a ligature tightened around the eye stalk. Arterial blood pressure and choroidal blood flow was continuously measured before, during and after a 20 min ischemic challenge. Both methods of ischemia reduced choroidal blood flow (>90%) with no consistent ocular hyperemia seen upon reperfusion. No significant differences in response pattern between the two ischemia techniques were apparent. Treatment with the non-selective inhibitor of nitric oxide (L-NAME 2 mg/kg, i.v.) did not alter either basal choroidal blood flow or the pattern of reperfusion. A larger dose of L-NAME (50 mg/kg, i.v.) reduced both basal choroidal blood flow and the final reperfusion level (most likely due to continued depression of the basal ocular choroidal blood flow). Neither D-NAME nor the neuronal nitric oxide synthase inhibitor, 7-nitroindazole, altered basal anterior choroidal blood flow or the reperfusion pattern seen after reperfusion. The results confirm our previous observations that inhibition of endothelial nitric oxide lowers. basal choroidal blood flow in the rat eye. However, in contrast to the cerebral circulation where L-NAME greatly attenuates initial reperfusion to the cerebral cortex, inhibition of nitric oxide synthase does not appear to notably further influence anterior choroidal reperfusion.

Functional role of nitric oxide in regulation of ocular blood flow

Nitric oxide generated by three distinct enzyme systems appears to play a critical role in many diverse physiological processes. Using both conventional and immunohistochemical techniques, nitric oxide synthases have been identified throughout the body, including all regions of the eye. A large number of in vitro and in vivo preparations have been utilized showing nitric oxide to have an important role in regulation of regional ocular blood flow. Nitric oxide-mediated control of basal ocular blood flow is demonstrated by vasoconstriction seen in experiments where vascular endothelial cells are removed, or when nitric oxide synthase is inhibited. The endogenous source of nitric oxide in the eye appears to be both endothelial and neural. In addition, administration of drugs that can 'donate' nitric oxide produces vasodilation of the eye vasculature. Local vasodilation in response to illumination of the retina is controlled by generation and release of nitric oxide, whereas most other physiological adjustments of ocular blood flow (i.e., autoregulation and responses to altered blood gas levels) seem to be relatively independent of nitric oxide mechanisms. Nitric oxide is implicated in a variety of ocular pathophysiological states including uveitis, retinal ischemic disease, diabetes and glaucoma.