Brimonidine evokes heterogeneous vasomotor response of retinal arterioles: diminished nitric oxide-mediated vasodilation when size goes small

The potential of brimonidine for myopia treatment: Targeting MMP-2 to regulate choroidal thickness and control eye growth

Drug treatment studies are a focal point for identifying novel approaches to reduce myopia progression through basic science research. Here, we investigated the effects of various brimonidine administration routes and concentrations on form-deprivation myopia (FDM) progression, matrix metalloproteinase-2 (MMP-2), and collagen alpha1 chain of type I (COL1A1) expression in the retinal pigment epithelial (RPE)-choroid complex and sclera of guinea pigs. They demonstrate that brimonidine has the capacity to impede choroidal thinning induced by FDM, potentially through the induction of choroidal vasodilation. Additionally, we observed that brimonidine effectively counteracts FDM-induced downregulation of choroidal and scleral MMP-2 expression. Suppression of MMP-2 expression may reduce disruption of scleral and choroidal structural integrity which reduces declines in choroidal blood circulation and mitigates increases in ocular elongation. This research elucidates the effects of brimonidine on myopia progression, offering potential insights into therapeutic interventions for myopia.

Effect of brimonidine on vascular density and imagej-derived flow index of optic nerve head and macula in primary open angle glaucoma

PURPOSE

To study the effect of brimonidine on vascular density and flow index of optic nerve head (ONH) and macula in primary open angle glaucoma (POAG) using optical coherence tomography angiography (OCTA).

METHODS

Twenty-three brimonidine-naïve POAG patients were started on brimonidine. They underwent OCTA ONH and macula before commencing brimonidine and one month thereafter. Systemic arterial blood pressure (SABP) and intraocular pressure (IOP) were measured at each visit to calculate mean ocular perfusion pressure (MOPP). The OCT angiograms were analyzed using ImageJ software to calculate ONH and macular flow indices.

RESULTS

Thirty-seven eyes (23 patients) with a mean age of 56.7 ± 12.49 years were included of whom 60.8% were males. Brimonidine was associated with an increase in the superficial flow index (SFI) (P-value = 0.02) and optic nerve head flow index (ONHFI) (P-value = 0.01). Also, superficial vascular density (SVD) for whole image, superior-hemi and fovea increased (P-value = 0.03, 0.02, 0.03 respectively). ONH inferior-hemi vascular density decreased (P-value = 0.01) despite an increase in inferior quadrant retinal nerve fiber layer thickness (RNFLT) (P-value = 0.03). There was no statistically significant correlation between flow indices and MOPP at baseline and follow-up. A moderate negative correlation was found between SVD and DVD at the fovea and MOPP at baseline and follow-up (P-value = 0.03, 0.05) (P-value = 0.02, 0.01) respectively.

CONCLUSIONS

Brimonidine was associated with an increase in SFI, ONHFI and SVD indicating improved GCC and RNFL perfusion in POAG. Despite the increase in inferior quadrant RNFLT, the concomitant decrease in inferior-hemi ONHVD precluded a conclusion of hemodynamically-mediated improvement of RNFLT.

The effect of a brinzolamide/brimonidine fixed combination on optic nerve head blood flow in rabbits

PURPOSE

The purpose of this study was to investigate the effect of a 1% brinzolamide and 0.1% brimonidine fixed combination (BBFC) on ONH blood flow (BF) in rabbits.

METHODS

A crossover study was conducted on pigmented rabbits; a physiological saline solution, brinzolamide, or BBFC was administered for eight days. ONH BF, intraocular pressure (IOP) and systemic parameters were measured before the eighth day's first dose and at 6, 9, 12, and 14 hours after the dose. ONH BF was assessed using laser speckle flowgraphy, and mean blur rate (MBR) values were calculated. The percentage against baseline of each parameter was calculated, and intergroup comparisons were performed at each time point.

RESULTS

There were no significant differences in the percentage change in systemic parameters. At 6 hours after administration, the BBFC group showed a significantly higher percentage change in large vessel area-MBR (%MV) compared to the control group (98.6±16.8%MV vs. 81.3±7.9%MV, P = 0.03). On the other hand, the brinzolamide group did not show a significant difference. Both the brinzolamide and BBFC groups had significantly lower percentage change in IOP (%IOP) compared to the control group (90.6±5.0%IOP, 93.3±2.9%IOP, and 99.2±1.7%IOP, respectively, P < 0.01).

CONCLUSION

BBFC effectively reduces IOP and mitigates diurnal fluctuation-induced decreases in ONH BF.

Acute macular neuroretinopathy in a patient with birdshot chorioretinopathy after intravitreal triamcinolone suspension injection

PURPOSE

To report a case of acute macular neuroretinopathy (AMN) after intravitreal triamcinolone acetonide (TRIESENCE®) injection for cystoid macular edema secondary to birdshot chorioretinopathy.

METHOD

A case report.

PATIENT

A 62-year-old female.

RESULTS

The patient presented with acutely decreased vision and a ring scotoma around her central vision three days after intravitreal triamcinolone acetonide (TRIESENCE®) injection for cystoid macular edema in her right eye (OD) secondary to birdshot chorioretinopathy. She had undergone pars plana vitrectomy, cataract extraction, and secondary intraocular lens implantation OD three months prior to the recent injection. Best-corrected visual acuity (BCVA) was 20/1000 OD and 20/50 OS. Intraocular pressure was 21 mmHg OD and 12 mmHg OS. Fluorescein angiography demonstrated a hypofluorescent area in the perifoveal zone OD. Optical coherence tomography OD depicted hyperreflective areas in the outer nuclear layer, outer plexiform layer, and retinal pigment epithelium. We diagnosed her with AMN OD and started her on brimonidine three times a day OD. She came back a week later with resolved scotoma and her vision improved to 20/60 OD. Five weeks later, BCVA was 20/40 and Intraocular pressures (IOP) was 12 mmHg OD.

CONCLUSIONS AND IMPORTANCE

Intravitreal triamcinolone injection may be a cause of AMN with cystoid macular edema (CME) and borderline-high intraocular pressure. Brimonidine may be an effective treatment for these patients in the early course of the disease.

Comparative Assessment of Retinal Blood Flow Velocity Changes Following Brimonidine and Brinzolamide Administration Using Retinal Function Imaging

Purpose

Impaired ocular blood flow has been associated with the etiopathogenesis of glaucoma. Topical brimonidine lowers intraocular pressure, a major glaucoma risk factor. However, brimonidine's influence on retinal blood flow remains to be fully elucidated. Our aim was to compare the effect of topical brimonidine and brinzolamide administration on retinal blood flow velocity in second and third order vessels in healthy adults using the retinal function imager.

Methods

In 10 healthy probands between 23 and 32 years of age, one eye was randomly selected to receive 2 treatment rounds with 3 single doses of brimonidine 2 mg/mL and brinzolamide 10 mg/mL at 12-hour intervals each. The fellow eyes served as intra-individual controls. Immediately before the first drop and 2 hours after the last drop of each treatment round, all subjects were examined, including Goldmann tonometry, Pascal tonometry, assessment of retinal blood flow velocity using the retinal function imager, as well as blood pressure and pulse measurements.

Results

Intraocular pressure decreased significantly in treated eyes while remaining stable in control eyes, indicating reliable application of brimonidine and brinzolamide drops. In contrast, retinal blood flow velocities did not demonstrate any significant differences between groups after both treatment rounds.

Conclusions

Neither brimonidine nor brinzolamide appear to alter retinal blood flow velocity in a clinically relevant manner. The slight velocity changes detected in our study are likely physiologic fluctuations. Our findings do not support the rationale of a detrimental effect of topical brimonidine on ocular blood flow and hence brimonidine may be further administered for lowering intraocular pressure with the appropriate caution. However, our study is strongly limited by the small sample size and, thus, further research with larger cohorts of healthy volunteers and patients with glaucoma is needed to confirm the results.

Translational Relevance

The study provides information about the effect of the topically administered antiglaucoma medications brimonidine and brinzolamide on the ocular blood flow and its regulation. The findings indicate that beside the lowering of IOP there is no evidence for an additional effect on the development of glaucoma.

[Features of blood pressure in mice with a genetic defect of different subtypes of α1-adrenoreceptors]

Primary open-angle glaucoma is a major cause of irreversible blindness in all countries of the world. The pathogenesis of primary open-angle glaucoma has not been resolved. In addition to increased intraocular pressure, pathogenetic factors of this disease such as reduced systemic blood pressure, deterioration of ocular blood flow, as well as vascular dysregulation, reperfusion damage and oxidative stress of ocular tissues are now generally recognized. The role of α1-adrenoreceptors in regulation of, for example, ocular blood flow, blood flow in the retina, permeability of the vascular wall etc has already been described.

PURPOSE

To identify the effects of different subtypes of α1-adrenoreceptors on the level of systemic blood pressure in laboratory mice.

MATERIAL AND METHODS

Features of blood pressure of 36 laboratory mice older than 18 months having a genetic defect (lines ADRA-1A, ADRA-1D and ADRA-1D) of one of the subtypes of α1-adrenoreceptors (α1a, α1b, α1d) were studied. The control group included ten healthy laboratory mice of the same age. The blood pressure of mice was measured with modern high-precision and noninvasive method using the Coda^TM Standard system (Kent Scientific Corporation, U.S.A.).

RESULTS

The mice of the ADRA-1A line were revealed to have reduced systolic and diastolic blood pressure and maximum level of pulse pressure, as well as the greatest fluctuations of pulse pressure; for the ADRA-1B line - the highest levels of systolic and diastolic blood pressure; for the ADRA-1D line - the smallest systolic and diastolic blood pressure and the minimum values of pulse pressure.

CONCLUSION

The obtained results show the specific contribution of α1a, α1b and α1d subtypes of the adrenoreceptors in the maintenance of normal hemodynamic of an organism, which should be considered when studying pathogenesis of primary open-angle glaucoma.

The Effect of Brimonidine 0.1% on Disc Hemorrhage in Primary Open-Angle Glaucoma Patients

Background

This retrospective study evaluated the effect of adjunctive administration of brimonidine 0.1% on disc hemorrhage (DH) in patients with primary open-angle glaucoma or normal-tension glaucoma who were already treated with other anti-glaucoma drugs.

Methods

Patients with DH, before adjunctive therapy with brimonidine, were enrolled. Subjects were excluded if their treatment regimen was changed within 1 year after initiation of adjunctive therapy with brimonidine. We investigated the frequency of DH and intraocular pressure (IOP). Both parameters were compared before and after adjunctive administration of brimonidine.

Results

The frequency of DH before and after brimonidine administration was 0.67±0.68 and 0.31±0.72 times/year, respectively, with a significant decrease (P=0.01), and the mean IOP before and after brimonidine administration was 12.5±1.9 and 11.2±2.2 mmHg, respectively, (P=0.0006) with a significant reduction after adjunctive administration.

Conclusion

The results of this study supported the hypothesis that the frequency of DH is reduced by brimonidine alongside lowering of IOP.

OCT-Angiography: Mydriatic phenylephrine and tropicamide do not influence retinal microvasculature in macula and peripapillary region

Purpose

Optical coherence tomography angiography (OCT-A) enables visualization of retinal microcirculation. As a potential influence of mydriatic eye drops on retinal vessel density (VD) was proposed, the purpose of the present study was to investigate an influence of 5% phenylephrine and 0.5% tropicamide on macula and peripapillary VD.

Methods

30 eyes of 30 healthy persons were measured by en face OCT-A (Spectralis OCT II, Heidelberg Engineering, Heidelberg). Scans of the macula (12 sectors, region of interest, ROI: 6.10 mm²) and peripapillary region (4 sectors, ROI: 2.67 mm²) were performed before (-) and 30 minutes after application of phenylephrine 5% and tropicamide 0.5% (+) eye drops (scan size was 8.41 mm²). Macula microcirculation was quantified in 3 retinal layers (superficial vascular plexus (SVP), deep capillary plexus (DCP), intermediate capillary plexus (ICP)). Data analysis was performed with the Erlangen-Angio-Tool.

Results

(I) Mean VD was 33.03±2.3 (SVP), 23.53±2.9 (ICP) and 25.48±4.2 (DCP) before and 33.12±2.4 (SVP), 23.74±2.9 (ICP) and 25.82±4.0 (DCP) with mydriasis respectively. (II) Sectorial analysis: 30.63±2.9–34.45±2.9 (-) and 31.04±2.9–34.34±2.7 (+) in SVP; 22.61±2.9–24.93±3.2 (-) and 22.75±2.5–25.20±3.0 (+) in ICP; 24.56±4.7–26.45±3.4 (-) and 25.00±4.1–27.07±3.5 (+) in DCP. (III) Peripapillary region showed a mean VD of 31.82±3.8 before and 31.59±4.3 after mydriasis. Sectorial analysis of VD yielded a range of 31.04±4.1–32.65±3.8 (-) and 30.98±4.4–31.89±4.1 (+). (IV) Macula and peripapillary VD were not different before and after mydriasis (p>0.05).

Conclusion

Pharmacologic mydriasis did not influence retinal microcirculation in macula and peripapillary region enabling OCT-A scans with enhanced imaging process and scan quality.

[Selective α2 agonists in the treatment of glaucoma: neuroprotective properties and impact on ocular blood flow]

Glaucoma is the main cause of irreversible blindness in the world. The selective α2 adrenergic receptor agonist brimonidine holds an important place among the hypotensive eye drops. The second part of this review focuses on some important effects of brimonidine that characterizes it as a medication with direct neuroprotective multifactorial action, discusses its influence on ocular blood flow and highlights its capability to maintain normal autoregulation of ocular blood flow.

Hyperglycemia Enhances Constriction of Retinal Venules via Activation of the Reverse-Mode Sodium-Calcium Exchanger

Diabetes is associated with hyperglycemia and impairment of retinal microvascular function. However, the impact of hyperglycemia on retinal venular constriction remains unknown. We examined retinal venular responsiveness to endogenous vasoconstrictors and the contribution of the reverse-mode sodium-calcium exchanger (NCX) to these responses during hyperglycemia. Retinal venules were isolated from pigs with streptozocin-induced diabetes (2 weeks, in vivo hyperglycemia) and age-matched control pigs for vasoreactivity and molecular studies. For in vitro hyperglycemia, vessels from euglycemic pigs were exposed to high glucose (25 mmol/L) for 2 h, and 5 mmol/L glucose served as the control. Constrictions of venules from euglycemic pigs to endothelin-1 (ET-1), thromboxane analog U46619, and norepinephrine were mediated by ET_A, thromboxane, and α₂-adrenergic receptors, respectively, and were insensitive to reverse-mode NCX blockade (KB-R7943). In vivo hyperglycemia enhanced these vasoconstrictions without altering respective receptor mRNA expression. Similarly, in vitro hyperglycemia augmented venular constrictions. Enhanced vasoconstrictions during hyperglycemia were prevented by KB-R7943, while mRNA expression of venular NCX isoforms was unaltered. In vivo hyperglycemia increased vitreous levels of ET-1 but not thromboxane B₂ In conclusion, both in vitro and in vivo hyperglycemia enhance retinal venular responses to endogenous vasoconstrictors by activating reverse-mode NCX. Therapies targeting this vascular molecule may alleviate retinal complications during diabetes.

Constriction of Retinal Venules to Endothelin-1: Obligatory Roles of ETA Receptors, Extracellular Calcium Entry, and Rho Kinase

Purpose

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide implicated in retinal venous pathologies such as diabetic retinopathy and retinal vein occlusion. However, underlying mechanisms contributing to venular constriction remain unknown. Thus, we examined the roles of ET-1 receptors, extracellular calcium (Ca²⁺), L-type voltage-operated calcium channels (L-VOCCs), Rho kinase (ROCK), and protein kinase C (PKC) in ET-1-induced constriction of retinal venules.

Methods

Porcine retinal venules were isolated and pressurized for vasoreactivity study using videomicroscopic techniques. Protein and mRNA were analyzed using molecular tools.

Results

Retinal venules developed basal tone and constricted concentration-dependently to ET-1. The ET_A receptor (ET_AR) antagonist BQ123 abolished venular constriction to ET-1, but ET_B receptor (ET_BR) antagonist BQ788 had no effect on vasoconstriction. The ET_BR agonist sarafotoxin S6c did not elicit vasomotor activity. In the absence of extracellular Ca²⁺, venules lost basal tone and ET-1–induced constriction was nearly abolished. Although L-VOCC inhibitor nifedipine also reduced basal tone and blocked vasoconstriction to L-VOCC activator Bay K8644, constriction of venules to ET-1 remained. The ROCK inhibitor H-1152 but not PKC inhibitor Gö 6983 prevented ET-1-induced vasoconstriction. Protein and mRNA expressions of ET_ARs and ET_BRs, along with ROCK1 and ROCK2 isoforms, were detected in retinal venules.

Conclusions

Extracellular Ca²⁺ entry via L-VOCCs is essential for developing and maintaining basal tone of porcine retinal venules. ET-1 causes significant constriction of retinal venules by activating ET_ARs and extracellular Ca²⁺ entry independent of L-VOCCs. Activation of ROCK signaling, without involvement of PKC, appears to mediate venular constriction to ET-1 in the porcine retina.

Myopia-Inhibiting Concentrations of Muscarinic Receptor Antagonists Block Activation of Alpha2A-Adrenoceptors In Vitro

Purpose

Myopia is a refractive disorder that degrades vision. It can be treated with atropine, a muscarinic acetylcholine receptor (mAChR) antagonist, but the mechanism is unknown. Atropine may block α-adrenoceptors at concentrations ≥0.1 mM, and another potent myopia-inhibiting ligand, mamba toxin-3 (MT3), binds equally well to human mAChR M4 and α1A- and α2A-adrenoceptors. We hypothesized that mAChR antagonists could inhibit myopia via α2A-adrenoceptors, rather than mAChR M4.

Methods

Human mAChR M4 (M4), chicken mAChR M4 (cM4), or human α2A-adrenergic receptor (hADRA2A) clones were cotransfected with CRE/promoter-luciferase (CRE-Luc; agonist-induced luminescence) and Renilla luciferase (RLuc; normalizing control) into human cells. Inhibition of normalized agonist-induced luminescence by antagonists (ATR: atropine; MT3; HIM: himbacine; PRZ: pirenzepine; TRP: tropicamide; OXY: oxyphenonium; QNB: 3-quinuclidinyl benzilate; DIC: dicyclomine; MEP: mepenzolate) was measured using the Dual-Glo Luciferase Assay System.

Results

Relative inhibitory potencies of mAChR antagonists at mAChR M4/cM4, from most to least potent, were QNB > OXY ≥ ATR > MEP > HIM > DIC > PRZ > TRP. MT3 was 56× less potent at cM4 than at M4. Relative potencies of mAChR antagonists at hADRA2A, from most to least potent, were MT3 > HIM > ATR > OXY > PRZ > TRP > QNB > MEP; DIC did not antagonize.

Conclusions

Muscarinic antagonists block hADRA2A signaling at concentrations comparable to those used to inhibit chick myopia (≥0.1 mM) in vivo. Relative potencies at hADRA2A, but not M4/cM4, correlate with reported abilities to inhibit chick form-deprivation myopia. mAChR antagonists might inhibit myopia via α2-adrenoceptors, instead of through the mAChR M4/cM4 receptor subtype.

Phenylephrine Affects Peripapillary Retinal Vasculature-an Optic Coherence Tomography Angiography Study

Purpose: To evaluate and compare the effect of topical mydriatic eye drops (0.5% tropicamide alone, and a 0.5% tropicamide/0.5% phenylephrine mixture) on the retinal perfusion using optic coherence tomography angiography (OCTA).

Methods: The perfused vessel density in the macular and peripapillary areas of both eyes was measured in eight healthy subjects using OCTA and a split-spectrum amplitude decorrelation angiography algorithm (SSADA). Various parameters, including retinal vessel density, were obtained at baseline and again 30 min after the instillation of either the tropicamide or the tropicamide/phenylephrine mixture eye drops in the right eye.

Results: Following instillation of the tropicamide/phenylephrine mixture eye drops, there was a significant reduction of vessel density in the peripapillary area (from 89.63 ± 4.53% to 85.00 ± 4.14%, P = 0.034), but not in the macular areas. The mean effect size in the peripapillary area was −4.63%, and was not correlated with age, sex, or baseline vessel density. There was no reduction in vessel density in the macular or peripapillary areas after instillation of 0.5% tropicamide alone, and no reduction in vessel density in the contralateral (control) eyes.

Conclusion: Topical phenylephrine reduced the retinal vessel density within the peripapillary area, but not within the macular area. Therefore, studies using OCTA, especially those focusing on the peripapillary area, should take into account this effect of phenylephrine on vessel density.

Vascular and autonomic dysregulation in primary open-angle glaucoma

PURPOSE OF REVIEW

The purpose of this review is to discuss whether vascular dysfunction and autonomic dysfunction are related to primary open-angle glaucoma stratified by the intraocular pressure (IOP) level.

RECENT FINDINGS

Patients with primary open angle glaucoma (POAG) across the spectrum of IOP exhibit a variety of ocular and nonocular vascular abnormalities. Interestingly, common genetic variation in nitric oxide synthase 3 (NOS3) and the caveolin 1/caveolin 2 (CAV1/CAV2) genomic regions, which code for proteins involved in setting vascular tone, are associated with POAG. These markers seem to stratify with POAG subtypes stratified by sex or pattern of initial visual field loss and not by IOP level. Overall, it is clear that there is also cardiovascular autonomic dysfunction in high-tension glaucoma and normal-tension glaucoma but it is unclear if this dysfunction is more common in normal-tension glaucoma compared with high-tension glaucoma.

SUMMARY

Overall, POAG is likely a heterogeneous disease but stratifying cases by IOP level associated with initial optic nerve damage may be less useful than using other endophenotype approaches. Embracing the evidence suggesting systemic endothelial and autonomic dysfunction are operative in POAG will help us move beyond an IOP-centric view of the disease and facilitate 'tearing down the wall' that divides treating physicians and a better understanding of POAG pathogenesis.

Advance in the pathogenesis and treatment of normal-tension glaucoma

Normal-tension glaucoma (NTG) is a multifactorial disease where mechanical stresses and vascular alterations to the optic nerve head probably represent the key pathogenic moments. Although intraocular pressure (IOP) plays a crucial role in the retinal ganglion cell loss, the IOP reduction does not necessarily reduces the disease progression. Therefore, several IOP-independent factors such as glutamate toxicity, oxidative stress, autoimmunity, and vascular dysregulation have been considered in the pathogenesis of NTG. Numerous evidences documented an impairment of the ocular blood flow, involved both in the onset and progression of the disease. The IOP reduction remains the main strategy to reduce the damage progression in NTG. Recently, new treatment strategies have been proposed to improve the control of the disease. Neuroprotection is a rapidly expanding area of research, which represents a promising tool. In the present review, we summarize the recent scientific advancements in the pathogenesis and treatment of NTG.

Effect of brimonidine on retinal vascular autoregulation and short-term visual function in normal tension glaucoma

PURPOSE

To assess whether brimonidine 0.15% alters retinal vascular autoregulation and short-term visual function in normal tension glaucoma patients who demonstrate retinal vascular dysregulation.

DESIGN

Nonrandomized clinical trial.

METHODS

In this prospective study, 46 normal tension glaucoma patients not previously treated with brimonidine underwent retinal vascular autoregulation testing and visual function assessment using frequency doubling technology perimetry and equivalent noise motion sensitivity testing. We measured blood flow in a major temporal retinal artery with subjects seated and then while reclined for 30 minutes. Patients having a change in retinal blood flow with posture change outside the range previously found in healthy subjects were classified as having retinal vascular dysregulation. They were treated with brimonidine 0.15% for 8 weeks and designated for retesting.

RESULTS

Twenty-three patients demonstrated retinal vascular dysregulation at the initial visit. Younger age (P = .050) and diabetes (P = .055) were marginally significant risk factors for retinal vascular dysregulation. After the 8-week course with brimonidine, 14 of the 17 patients who completed the study showed a return of posture-induced retinal blood flow changes to levels consistent with normal retinal vascular autoregulation (P < .0001). We found no significant changes in frequency doubling technology perimetry or in motion detection parameters following treatment with brimondine (P > .09 for all tests performed).

CONCLUSIONS

Brimonidine significantly improved impaired retinal vascular autoregulation in normal tension glaucoma patients, but short-term alteration in visual function could not be demonstrated.

Effects of dorzolamide-timolol and brimonidine-timolol on retinal vascular autoregulation and ocular perfusion pressure in primary open angle glaucoma

PURPOSE

To assess whether dorzolamide 2%-timolol 0.5% (D/T) and/or brimonidine 0.2%-timolol 0.5% (B/T) alters retinal vascular autoregulation (RVA) and seated ocular perfusion pressure (sOPP) in primary open angle glaucoma (POAG) patients who demonstrate retinal vascular dysregulation (RVD) on timolol 0.5% alone.

METHODS

In this prospective, observer-masked, crossover study, 21 POAG patients with untreated intraocular pressure (IOP) >21 mmHg were treated for 6 weeks with timolol 0.5%. Subsequently, we measured inferior temporal retinal artery blood flow in the left eye with subjects seated and then while reclined for 30 min using the Canon Laser Blood Flowmeter. Subjects with a change in retinal blood flow in response to posture change outside of the range previously found in healthy subjects were designated as having RVD and randomized to either D/T or B/T for 6 weeks and re-tested. This was followed by treatment with the opposite medication.

RESULTS

Seven of the 21 subjects demonstrated RVD in response to posture change following timolol 0.5%. Multiple linear regression analysis indicated that lower sOPP was the main determinant of RVD (P=0.033). After treatment with D/T, all 7 converted from RVD to normal RVA status (P=0.001). Four of 6 subjects showed a similar return to normal RVA following B/T (P=0.066). Mid-morning sOPP was 41.1±5.5 mmHg post-timolol, 46.3±6.5 mmHg post-D/T, and 38.6±6.0 mmHg post-B/T (D/T vs. B/T, P=0.026).

CONCLUSIONS

D/T significantly improved RVA in POAG patients exhibiting RVD while on timolol 0.5% alone. D/T also increased sOPP compared to B/T. There was no significant difference (P=0.37) between D/T and B/T in improving RVA.

Soluble guanylate cyclase α1-deficient mice: a novel murine model for primary open angle glaucoma

Primary open angle glaucoma (POAG) is a leading cause of blindness worldwide. The molecular signaling involved in the pathogenesis of POAG remains unknown. Here, we report that mice lacking the α1 subunit of the nitric oxide receptor soluble guanylate cyclase represent a novel and translatable animal model of POAG, characterized by thinning of the retinal nerve fiber layer and loss of optic nerve axons in the context of an open iridocorneal angle. The optic neuropathy associated with soluble guanylate cyclase α1-deficiency was accompanied by modestly increased intraocular pressure and retinal vascular dysfunction. Moreover, data from a candidate gene association study suggests that a variant in the locus containing the genes encoding for the α1 and β1 subunits of soluble guanylate cyclase is associated with POAG in patients presenting with initial paracentral vision loss, a disease subtype thought to be associated with vascular dysregulation. These findings provide new insights into the pathogenesis and genetics of POAG and suggest new therapeutic strategies for POAG.

Innervation pattern of the preocular human central retinal artery

The central retinal artery (CRA) is the main vessel for inner retinal oxygen and nutrition supply. While the intraocular branches lack autonomic innervation, the innervation pattern of the extra-ocular part of this vessel along its course within the optic nerve is poorly investigated. This part however is essential for maintenance of retinal blood supply, in physiological and pathological conditions. Therefore, the aim of this study was the characterization of the autonomic innervation of the preocular CRA in humans with morphological methods. Meeting the Declaration of Helsinki, eyes of body or cornea donors were processed for single or double immunohistochemistry against tyrosine hydroxilase (TH), dopamine-β-hydroxylase (DBH), choline acetyl-transferase (ChAT), vesicular acetylcholine transporter (VAChT), neuronal nitric oxide synthase (nNOS), calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP), and cytochemistry for NADPH-diaphorase (NADPH-d). For documentation, light-, fluorescence-, and confocal laser-scanning microscopy were used. TH and DBH immunoreactive nerve fibres were detected in the CRA vessel wall, although a distinct perivascular plexus was missing. Further, nerve fibres immunoreactive for ChAT and VAChT were found, while CGRP, SP, and VIP were not detected. NADPH-d staining revealed scattered nerve fibres in the adventitia of the CRA and in close vicinity; however, nNOS-immunostaining could not confirm this finding. The CRA receives adrenergic and cholinergic innervations, indicating sympathetic and parasympathetic components, respectively. Remarkably, a peptidergic primary afferent innervation was missing. Since clinical results suggest an autoregulation of intraretinal vessels, further studies are needed to clarify the impact of CRA innervation for retinal perfusion.

Acute retinal ischemia inhibits endothelium-dependent nitric oxide-mediated dilation of retinal arterioles via enhanced superoxide production

PURPOSE

Because retinal vascular disease is associated with ischemia and increased oxidative stress, the vasodilator function of retinal arterioles was examined after retinal ischemia induced by elevated intraocular pressure (IOP). The role of superoxide anions in the development of vascular dysfunction was assessed.

METHODS

IOP was increased and maintained at 80 to 90 mm Hg for 30, 60, or 90 minutes by infusing saline into the anterior chamber of a porcine eye. The fellow eye with normal IOP (10-20 mm Hg) served as control. In some pigs, superoxide dismutase mimetic TEMPOL (1 mM) or vehicle (saline) was injected intravitreally before IOP elevation. After enucleation, retinal arterioles were isolated and pressurized without flow for functional analysis by recording diameter changes using videomicroscopic techniques. Dihydroethidium (DHE) was used to detect superoxide production in isolated retinal arterioles.

RESULTS

Isolated retinal arterioles developed stable basal tone and the vasodilations to endothelium-dependent nitric oxide (NO)-mediated agonists bradykinin and L-lactate were significantly reduced only by 90 minutes of ischemia. However, vasodilation to endothelium-independent NO donor sodium nitroprusside was unaffected after all time periods of ischemia. DHE staining showed that 90 minutes of ischemia significantly increased superoxide levels in retinal arterioles. Intravitreal injection of membrane-permeable radical scavenger but not vehicle before ischemia prevented elevation of vascular superoxide and preserved bradykinin-induced dilation.

CONCLUSIONS

Endothelium-dependent NO-mediated dilation of retinal arterioles is impaired by 90 minutes of ischemia induced by elevated IOP. The inhibitory effect appears to be mediated by the alteration of NO signaling via vascular superoxide.

Effect of brimonidine on retinal blood flow autoregulation in primary open-angle glaucoma

PURPOSE

To determine whether topically applied brimonidine affects the retinal hemodynamic autoregulatory response to posture change in patients with normal tension glaucoma.

METHODS

Six patients with normal tension glaucoma (primary open-angle glaucoma and maximum untreated intraocular pressure <22 mmHg) in each eye were studied. We retrospectively reviewed retinal hemodynamic data acquired when the patients were off and on treatment with brimonidine 0.15% (twice a day, both eyes) during the course of their care. At each testing session, vessel diameter and blood speed at the same site along the inferior temporal retinal artery of 1 eye were measured while sitting, while reclining for 30 min, and again while sitting using a retinal laser Doppler instrument. Blood flow was computed automatically. Brachial artery blood pressure and heart rate were also measured. The Wilcoxon signed rank test was used to assess the statistical significance of the differences in each measured parameter while subjects were on and off brimonidine.

RESULTS

Off brimonidine, the mean blood flow rate increased by 68.0%±34.3% (range: +17% to +108%) after 30 min in reclined posture compared to baseline-seated measures. On brimonidine, the mean blood flow rate increased by 8.9%±16.8% (range: -9.7 to +28.0%) after 30 min in reclined posture compared to baseline-seated measures. The difference in the posture-induced changes for blood flow rate while on brimonidine compared to off brimonidine was statistically significant (P=0.027).

CONCLUSIONS

Off brimonidine, the patients exhibited marked increases in retinal blood flow while reclining. On brimonidine, the hemodynamic changes were consistent with normal autoregulatory control of retinal blood flow.

Update on the role of alpha-agonists in glaucoma management

Glaucoma is the second most common cause of world blindness (following cataract) with estimated cases reaching 79.6 million by 2020. Although the etiology of glaucoma is multi-factorial, intraocular pressure (IOP) is the only modifiable factor in glaucoma management proven to alter the natural course of the disease. Among various classes of IOP-lowering medications currently available, alpha-adrenergic receptor agonists are used either as monotherapy, as second-line therapy, or in fixed combination with beta-blockers. Non-selective adrenergic agonists such as epinephrine and dipivefrin are infrequently used today for the treatment of glaucoma or ocular hypertension, and have been replaced by the alpha-2-selective agonists. The use of apraclonidine for IOP reduction in glaucoma or OHT is limited due to a high rate of follicular conjunctivitis. The alpha-2-selective agonist in use today is brimonidine. The brimonidine-purite formulations are preferred to brimonidine-benzalkonium chloride (BAC) formulations due better tolerability while maintaining similar efficacy. Brimonidine is also effective when used in combination with a beta-blocker. Using brimonidine-timolol fixed combination (BTFC) as first-line therapy has an added potential for neuroprotection. This would be a valuable strategy for glaucoma treatment, for patients who are intolerant of prostaglandin analogs, or for patients where prostaglandin analogues are contraindicated as first-line therapy, such as in patients with inflammatory glaucoma.

Functional and molecular characterization of the endothelin system in retinal arterioles

PURPOSE

Activation of the endothelin (ET) system has been implicated in the pathogenesis of retinal ischemic disease. Although ET-1, the predominant endogenous isoform of ET, has been shown to cause constriction of retinal vessels, the expression and functional significance of its synthesis and the involved specific ET receptors in retinal arterioles remain unknown. The authors examined the roles of ET(A) and ET(B) receptors and of endothelin-converting enzyme (ECE)-1 in ET-1-induced vasomotor responses of single retinal arterioles.

METHODS

To exclude systemic confounding effects, porcine retinal arterioles were isolated for vasoreactivity and molecular studies.

RESULTS

Isolated and pressurized retinal arterioles developed basal tone and constricted in a manner dependent on concentration to ET-1. ET-1 precursor big ET-1 elicited time-dependent vasoconstriction over 20 minutes, which was blocked by the ECE-1 inhibitor phosphoramidon. ET(A) receptor antagonist BQ123 inhibited most (approximately 90%) of vasoconstrictions to ET-1 and big ET-1. ET(B) receptor agonist sarafotoxin also elicited concentration-dependent constriction of retinal arterioles but with significantly less potency than ET-1. ET(B) receptor antagonist BQ788 abolished vasoconstriction to sarafotoxin but only slightly reduced responses to ET-1 and big ET-1. Protein and mRNA expressions of ET(A), ET(B), and ECE-1 were detected in retinal arterioles. Immunohistochemistry revealed ET(A) and ET(B) receptors predominantly in smooth muscle and ECE-1 predominantly in endothelium and smooth muscle.

CONCLUSIONS

ET-1 elicits constriction of retinal arterioles predominantly through the activation of smooth muscle ET(A) receptors. Endogenous production of ET-1 from vascular ECE-1 is sufficient to evoke ET(A) receptor-dependent constriction in retinal arterioles.

Neuroprotective effects of brimonidine treatment in a rodent model of ischemic optic neuropathy

Ischemic optic neuropathy (ION) is a common disorder caused by disruption of the arterial blood supply to the optic nerve. It can result in significant loss of visual acuity and/or visual field. An ischemic optic nerve injury was produced in rats by intravenous injection of Rose Bengal dye followed by argon green laser application to the retinal arteries overlying the optic nerve, causing a coagulopathy within the blood vessels and disruption of optic nerve and retinal perfusion. The effect of brimonidine tartrate eye drops on survival of retinal ganglion cell axons in this experimental paradigm was studied. One eye was treated and the contralateral eye served as a control. Four groups of animals were used for this study. Group 1 received 7 days of treatment with 0.15% brimonidine tartrate eye drops twice a day prior to the ischemic injury. Group 2 animals received 0.15% brimonidine tartrate eye drops twice a day for 14 days after photocoagulation injury. Animal groups 3 and 4 received eye drops of 0.9% NaCl twice a day either daily for 7 days before injury or daily for 14 days, respectively. All rats were sacrificed 5 months after the injury to ascertain long-term optic axon survival. Coagulopathy-induced optic nerve ischemia resulted in a 71% loss of optic axons. Treatment with brimonidine daily for the 7 days prior to the injury resulted in a greater survival of optic axons, with only a 56.1% loss compared to control. Brimonidine treatment every day for 14 days after the ischemic injury did not result in a significant rescue of optic axons compared to injury alone. In summary, the application of brimonidine eye drops for one week prior to an ischemic injury resulted in a statistically significant increase in survival of optic axons within the injured optic nerves. Brimonidine treatment of the eye after the ischemic injury did not result in axon rescue, and axon loss was similar to the injured optic nerves treated with saline only. These results suggest that brimonidine may have potential use for prevention of ION in at-risk patients.