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Carla J. Abbott, Tiffany E. Choe, Theresa A. Lusardi, Lin Wang, Claude F. Burgoyne, Brad Fortune; Substantial Thickening of the Rat Peripapillary Retinal Nerve Fiber Layer but No Alteration of Axonal Transport 1- and 2-weeks after an 8-hour Elevation of Intraocular Pressure (IOP). Invest. Ophthalmol. Vis. Sci. 2012;53(14):5083.
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To assess RNFL thickness (RNFLT) and axonal transport 1- and 2-weeks after an 8-hour acute IOP elevation in the rat.
Twenty adult male Brown-Norway rats were used. All procedures were performed under anesthesia (ketamine, xylazine, acepromazine 55:5:1 mg/kg IM or 2% isoflurane inhalation). IOP was manometrically elevated to 50mmHg for 8 hours in all right eyes; the left eyes served as untouched controls. RNFLT was measured by spectral domain optical coherence tomography (SDOCT, Spectralis, Heidelberg Engineering, GmbH) at baseline, then at 1-week (N=17) and 2-weeks (N=9) after the acute IOP event. Retrograde axonal transport was assessed in vivo by confocal scanning laser ophthalmoscopy (CSLO, Spectralis HRA) 24h after bilateral stereotactic injections into the superior colliculli (SC) of 2µl 1% cholera toxin b-subunit conjugated to AlexaFluor488 (CTB). Animals were sacrificed straight after CSLO for retinal microscopy. Anterograde transport was assessed by post mortem imaging of optic nerves (ON) and SC 24h after bilateral intravitreal injection of 2µl 1% CTB. CTB positive retinal ganglion cells (RGCs) were counted using Image J analysis of retrograde flatmounts and CSLO images of the central retina (flatmount 5.8mm², CSLO 4.1mm²). Fluorescence intensity was measured for the entire SC and the optic nerves in the anterograde assay. Rats with failed injections were excluded from transport analysis (n=4). Statistics included Wilcoxon paired t-tests (axonal transport) and 2-way repeated measures ANOVA (RNFLT).
RNFLT in the right eye increased by 22% (p<0.0001) from baseline (and fellow control) 1-week after the acute IOP event and was 10% thicker at 2-weeks (p<0.001). For anterograde transport at 1-week, there were no differences in fluorescence intensity of the ON (R 49.77±6.15a.u., L 51.51±4.60, p=0.79, n=3) or SC (R 80.40±6.57, L 76.99±10.66, p=0.50, n=3). Similarly, at 2-weeks there were no differences for ON (R 54.99±9.32%, L 56.88±9.44, p=0.44, n=5) or SC (R 95.19±16.18, L 86.56±19.50, p=0.19, n=5). For retrograde transport, there were no differences in density of CTB positive retinal ganglion cells (RGC) observed in vivo by CSLO at 1-week (R 1740±443mm-2, L 2415±260, p=0.25, n=4) or 2-weeks (R 1817±265, L 1582±915, p=0.75, n=4). The density of CTB positive RGCs in the flatmounts was equivalent at 1-week (R 2239±168mm-2, L 2314±348, p=0.875, n=4) and 2-weeks (R 2070±238, L 1950±325, p=0.625, n=3).
Acute IOP elevation to 50mmHg for 8 hours in the rat has no effect on either anterograde or retrograde axonal transport at either 1- or 2-weeks, but resulted in substantial RNFL thickening.
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