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Vicki Chrysostomou, Peter van Wijngaarden, Joseph Paul, Timothy Colgan, Ian A. Trounce, Jonathan G Crowston; Protection of the injured optic nerve with forced and voluntary exercise in aged mice: can there be too much of a good thing?. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2616. doi: https://doi.org/.
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We have previously shown that forced exercise by swimming effectively protects aged retinal ganglion cells (RGCs) of the mouse optic nerve against functional and structural loss after pressure-induced injury. Here, we test (i) if the protection we have previously seen with forced exercise can be replicated with a regimen of voluntary exercise, and (ii) the effect of various durations and time-courses of forced exercise that facilitate RGC functional protection.
At 12 months of age, C57BL/6J mice were assigned to forced (swimming) or voluntary (running wheels) exercise and control groups (n=20/group). Injury to RGCs was induced by unilateral elevation of intraocular pressure (50mmHg, 30min). RGC function was assessed before and after injury with the positive scotopic threshold response (pSTR) component of the electroretinogram.
Amplitudes of the pSTR in non-exercised control mice were reduced to 49.6±6.4% in response to injury. In contrast, pSTR amplitudes were significantly preserved at 79.3±4.0% in mice that underwent either swimming for 7 days post-injury (60min, 1/day) or in mice that had free access to a running wheel for 7 days post-injury (71.2±5.8%). Increasing the overall time-course of swimming to 60min, 1/day for 6 weeks pre- and 7 days post-injury did not confer any additional functional protection; mice that swam both before and after injury displayed pSTR amplitudes that were not significantly different from those seen in mice that only swam after injury (87.0±6.7% vs 79.3±4.0%). Doubling the daily duration of swimming (60min, 2/day) for 6 weeks pre- and 7 days post-injury abrogated the functional protection conferred by exercise. Amplitudes of the pSTR post-injury in mice that underwent this additional swimming were not significantly different from those seen in non-exercised animals (58.0±4.3% vs 52.3±14.8%). This regime of forced exercise was also associated with significant cardiac hypertrophy that was not observed in mice undergoing any other form or duration of exercise.
Forced and voluntary forms of exercise can provide functional protection to the optic nerve after an acute pressure-induced injury. There appears to be an optimal level of exercise above which there are little additional benefits. Further exceeding this level may result in the loss of any benefits or even have the potential to become harmful.
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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