Optometry and Vision Sciences - Research Publications

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    Reversibility of retinal ganglion cell dysfunction due to chronic IOP elevation.
    Zhao, D ; Wong, VHY ; He, Z ; Nguyen, CTO ; Jobling, AI ; Fletcher, E ; Chinnery, H ; Jusuf, P ; Lim, JKH ; Vingrys, AJ ; Bui, BV (Association for Research in Vision and Ophthalmology, 2018-07-01)
    Purpose : To determine the duration of chronic IOP elevation beyond which ganglion cell function can no longer recover using the mouse circumlimbal suture model. Methods : IOP elevation was induced in anaesthetized (isoflurane) adult male C57BL6/J mice by attaching a circumlimbal suture (nylon, 10/0) around the equator of one eye, with the contralateral eye serving as a control. The suture was left in place for 8, 12 and 16 weeks (n=27, 23 and 27), respectively, and animals underwent electroretinography and optical coherence tomography at these time points. In two other groups, the suture was removed after 8 and 12 weeks (n=26 and 28), and the capacity for recovery assessed 4 weeks later. IOP was measured weekly (Tonolab). Retinal ganglion cell (RGC) function (or integrity) was assessed with the positive scotopic threshold response (pSTR) and retinal nerve fibre layer (RNFL) thickness. Data (mean ± SEM) were compared using t-test (control vs. treatment) and one-way ANOVA (within groups). Results : IOP in sutured eyes was higher than control eyes (8wk: 17.1 ± 0.3 vs. 26.8 ± 0.6 mmHg, 12wk: 13.8 ± 0.3 vs. 19.5 ± 0.5 mmHg, 16wk: 17.1 ± 0.2 vs. 27.4 ± 0.6 mmHg; all P<0.001). After suture removal, IOP returned to levels comparable to control eyes (8+4wk: 16.9 ± 0.3 vs. 16.1 ± 0.3 mmHg; P=0.08, 12+4wk: 17.3 ± 0.2 vs. 17.1 ± 0.3 mmHg; P=0.5). With IOP elevation, RGC function declined to 75% ± 8% (8wk), 78% ± 7% (12wk) and 59% ± 4% (16wk, all P<0.001) of control eyes. RNFL thinning was also evident (8wk: 84% ± 4%, 12wk: 83% ± 5%; 16wk: 83% ± 3%; P<0.001) but no change in total retinal thickness was noted (P=0.33). Suture removal at week 8 facilitated full recovery of RGC function (97% ± 7%, P=0.9 vs. baseline) 4 weeks later. However, there was no recovery in RNFL thickness (87% ± 3%, P<0.001 vs. baseline). When the suture was removed at week 12, neither function (79% ± 9%, P<0.05) nor RNFL thickness recovered (89% ± 3%, P<0.01) 4 weeks later. Conclusions : RGC dysfunction can be recovered 4 weeks after an 8-week period of mild IOP elevation, but not after a 12-week period. Beyond 12 weeks, IOP reversal only served to prevent further functional decline. This identifies a critical chronic IOP duration that results in irreversible ganglion cell dysfunction. This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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    Evaluating retinal biomarkers in a mouse model of Parkinson's disease
    Nguyen, CTO ; Tran, K ; Lim, JKH ; Wong, VHY ; Shahandeh, A ; Vingrys, AJ ; Bui, BV ; Finkelstein, D (Association for Research in Vision and Ophthalmology, 2019-07-01)
    Purpose : The retina, an accessible outpouching of the central nervous system, may manifest cortical changes that occur with Parkinson’s disease (PD), lending itself as a potential biomarker. PD is characterised by reduced dopamine levels, a neurotransmitter found in amacrine cells. Human PD patients have also shown structural changes in the outer retina. This work aims to determine if retinal function and structure are altered in a murine model of PD and whether deficits can be ameliorated with L-DOPA treatment. Methods : A PD model was induced in adult C57BL6/J mice using MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 4x i.p. injections, 20mg/kg) and vehicle control and examined at day 21 and 45. Another MPTP group was administered L-DOPA (L-3,4-dihydroxyphenylalanine 0.2 mg/ml) or control in their drinking water and assessed at day 45 (n=12–15/group). In ketamine:xylazine anaesthetised (80:10mg/kg) mice full-field dark- and light-adapted electroretinography (ERG) was assessed to target dopamine-related responses. Optical coherence tomography (OCT) was used to quantify thickness of retinal layers. Retinal and cortical tissue were collected for immunohistochemical assessment of changes in tyrosine hydroxylase (TH)and imaged using confocal microscopy. Data (mean±SEM) were compared using unpaired ANOVA and t-tests as appropriate. Results : At day 21 no retinal changes were found. At day 45 dark and light adapted ERGs showed slower amacrine cell responses (oscillatory potential, p<0.05), a finding which reversed with L-DOPA treatment (p<0.05). Other components of the ERG were unchanged. TH staining showed a trend towards decreased retinal levels in MPTP mice but this did not reach significance (p=0.10). Reduced levels of TH were found in the ventral hippocampus of MPTP mice compared with control (p<0.05). OCT revealed thinning of the outer plexiform layer at day 45, and the L-DOPA group exhibited a thinning of the outer nuclear layer (p<0.05). Conclusions : This study shows for the first time that the MPTP model recapitulates key dopaminergic changes previously reported in humans. In particular, electroretinographic changes that correspond with dopaminergic retinal cells occur in the Parkinson’s model and reverse with therapeutic treatment. Structural thinning of the outer retinal layers also occur, which parallels some human findings. This work paves the way for retinal measures as preclinical screening tools in drug development.
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    How ganglion cell responses to IOP elevation are impacted by blood pressure and intracranial pressure
    Bui, BV ; van Koeverden, A ; He, Z ; Vingrys, AJ ; Nguyen, CTO ; Zhao, D (Association for Research in Vision and Ophthalmology, 2019-07-01)
    Purpose : The extent to which blood pressure or intracranial pressure modifies ganglion cell responses to acute intraocular pressure (IOP) elevation incompletely understood. Using the electroretinogram (ERG) we measure ganglion cell mediated responses in rat retina, whilst acutely modifying IOP, BP and ICP in a systematic manner. We quantify the relationship between ganglion cell function and ocular perfusion pressure (BP - IOP) at low, normal and high ICP. Methods : Six groups of adult Long-Evans rats (n=7-11 eyes/group, total animals = 25) were anaesthetised (60:5mg/kg ketamine:xylazine) and underwent acute pressure modification. A femoral artery and vein were cannulated for blood pressure measurement and manipulation (sodium nitroprusside to lower and angiotensin II to elevate pressure). ICP was set to -5, 5 or 25 mmHg via a dual cannula (30G infusion needle inside a 23G measurement needle) placed into the lateral ventricle (-1.5mm from bregma, ±2mm from midline) on the ipsilateral side to the cannulated eye (30G, vitreal chamber). At each ICP (-5, 5 or 25 mmHg) and BP setting (normal or high), IOP was raised from 10 to 90 mmHg in 10 mmHg steps (3 min each). At each IOP level ganglion cell function was assessed using the scotopic threshold response (-5 log cd.s/m2, 20 repeats). Data were compared using one- and two-way ANOVA. Results : Average blood pressure at baseline was similar for the normal blood pressure groups (ICP-5 93±3; ICP5 99±5; ICP25 105±3mmHg, p=0.8). There was significant BP elevation in all the high blood pressure groups (ICP-5 160±3; ICP5 157±3; ICP25 157±5mmHg p<0.001). Compared with normal blood pressure groups (32.0±2.0μV), animals with high blood pressure (24.5±1.8μV) had significantly smaller baseline STR amplitudes (p<0.01). There was also a significant ICP effect (p<0.01), with larger baseline amplitudes in the 25mmHg ICP group (34.8±1.6μV) compared with normal (26.4±2.5μV) and low ICP groups (23.9±2.5μV). The ocular perfusion pressure (BP-IOP) relationship fully could not account for difference in ganglion cell function between ICP levels. Conclusions : Ganglion cell function is dependent on ocular perfusion pressure, excessive low or high perfusion attenuates function. Higher intracranial pressure appears to protect against acute ocular perfusion stress.
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    Effect of intraocular pressure on vascular autoregulation of the mouse trilaminar network
    Lim, JKH ; Wu, RH ; Wong, VHY ; Vingrys, AJ ; Nguyen, CTO ; Bui, BV (ASSOC RESEARCH VISION OPHTHALMOLOGY INC, 2019-07-01)
    PURPOSE. The purpose of this study was to test the hypothesis that the superficial, intermediate, and deep retinal vascular plexus show different responses to intraocular pressure (IOP) elevation. METHODS. Anesthetized adult Long Evans rats (n = 14) were imaged using optical coherence tomography angiography (OCTA; Spectralis) at baseline (IOP 10 mm Hg) and in follow-up mode to examine the vasculature during IOP elevation (10 to 110 mm Hg, 10 mm Hg steps, each step 3 minutes). A 20° × 10° field was imaged. Vessel density within a 2D projection image was determined (%) for the superficial vascular complex (SVC), intermediate capillary plexus (ICP), and deep capillary plexus (DCP). Comparisons were made between layers using 2-way repeated measures ANOVA (layer versus IOP) following normalization to baseline (% relative to 10 mm Hg). RESULTS. The three vascular layers responded differently to IOP elevation. For IOPs between 40 and 60 mm Hg, DCP and ICP capillaries were significantly more resistant to IOP elevation than those in the SVC. When IOP was elevated above 70 mm Hg, all layers showed reduced vessel density. IOP induced change in SVC vessel density closely followed reductions in thickness of the inner retinal layers (nerve fiber, ganglion cell, and inner plexiform layer). This close relationship between reductions in tissue thickness and vessel density was less apparent for the ICP and DCP. CONCLUSIONS. These data show that the intermediate and deep vascular plexus in the rat retina have a greater capacity for autoregulation against mild IOP elevation but are more affected at high IOP.