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S.J. McKinnon, H.A. Reitsamer, N.L. Ransom, M. Caldwell, J.M. Harrison, J.W. Kiel; Induction and Tonopen Measurement of Ocular Hypertension in C57BL/6 Mice . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3319.
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Purpose: To determine whether mouse intraocular pressure (IOP) could be measured non-invasively with a Tonopen, and whether ocular hypertension could be induced in wild-type C57BL/6 mice. Methods: To certify accuracy of Tonopen (Medtronic Xomed) measurements in mouse eyes, we performed calibration curves on anesthetized mice (n=5). IOPs between 10 and 45 mmHg were simultaneously measured by Tonopen (without the Ocufilm cover) and by pressure transducer after direct cannulation of the vitreous compartment. Ocular hypertension was induced in the right eye of adult wild-type C57BL/6 mice (n=7) by the Morrison method (Exp Eye Res 64:85), consisting of limbal injection of hypertonic saline (2.0 M) after placement of a modified plastic occlusion ring. IOP was measured in both eyes under topical anesthesia pre-operatively and on a weekly basis post-operatively using a Tonopen. After eight weeks, treated and control optic nerve were obtained and cross-sections stained with toluidine blue. Axon counts were obtained using the KS400 imaging system (Zeiss) and percent axon survival between treated and control eyes compared. Results: Mouse Tonopen IOP readings showed higher variability than that reported for rats; however, the measurements were reproducible and the correlation between Tonopen and cannulated transducer readings was excellent for single (PTonopen= 7.72 + 0.69*PCannula, R= 0.76) or group-averaged mice (PTonopen= 5.54 + 0.70*PCannula, R= 0.97; n= 5). Power analysis indicated that in order to detect a 2 mmHg IOP difference as significant (α= 0.05, power= 0.8), sample size (# of readings) must be 20. Of the seven treated eyes, six displayed elevations of IOP, for an overall conversion rate of 86%. After mild IOP exposures (mean= 156 mmHg-days, range= 132-204), the average (±SEM) axon count was 40,272.5 ± 3,663 for treated eyes and 50,078 ±1,121 for control eyes (n=6). Average % axon loss for the group was 20 ± 6% (range -1 to 46%). Mild damage (0-24% axon loss) was noted in 3 mice and moderate damage (25-49% axon loss) was noted in 2 mice. Conclusions: We are able to perform limbal injections of hypertonic saline, reliably and accurately measure IOP with a Tonopen, and produce optic nerve damage in mice, consistent with that seen in rat glaucoma models. By selecting specific transgenic mouse lines, one can determine if a gene mutation or knockout provides a protective or deleterious role in RGC apoptosis in the setting of chronic ocular hypertension. An inducible mouse glaucoma model is a powerful tool to isolate the relevant steps responsible for RGC death in glaucoma.
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