Scientists brought forth a treatment that prevents the optic nerve injury, which occurs in glaucoma, a neurodegenerative disease, foremost cause of blindness.
Scientists at Washington University School of Medicine in St. Louis increased the resistance of optic nerve cells to damage by repeatedly exposing the mice to low levels of oxygen similar to those found at high altitudes.
The stress of the intermittent low-oxygen environment induces a protective response called tolerance that makes nerve cells - including those in the eye - less vulnerable to harm.
Stress is typically thought of as a negative phenomenon, but senior author Jeffrey M. Gidday, PhD, associate professor of neurological surgery and ophthalmology, and others have previously shown that the right kinds of stress, such as exercise and low-oxygen environments, can precondition cells and induce changes that make them more resistant to injury and disease.
Scientists previously thought tolerance in the central nervous system only lasted for a few days.
But last year Gidday developed a preconditioning protocol that extended the effects of tolerance from days to months.
By exposing mice to hypoxia, or low oxygen concentrations, several times over a two-week period, Gidday and colleagues triggered an extended period of tolerance. After preconditioning ended, the brain was protected from stroke damage for at least 8 weeks.
"Once we discovered tolerance could be extended, we wondered whether this protracted period of injury resistance could also protect against the slow, progressive loss of neurons that characterizes neurodegenerative diseases," Gidday says.
To find out, Gidday turned to an animal model of glaucoma, a condition linked to increases in the pressure of the fluid that fills the eye.
The only treatments for glaucoma are drugs that reduce this pressure; there are no therapies designed to protect the retina and optic nerves from harm.
For the new study, Yanli Zhu, MD, research instructor in neurosurgery, induced glaucoma in mice by tying off vessels that normally allow fluid to drain from the eye.
This causes pressure in the eye to increase. Zhu then assessed how many cell bodies and axons of retinal ganglion cells were intact after three or 10 weeks.
The investigators found that normal mice lost an average of 30 percent of their retinal ganglion cell bodies after 10 weeks of glaucoma.
But mice that received the preconditioning before glaucoma-inducing surgery lost only 3 percent of retinal ganglion cell bodies.
"We also showed that preconditioned mice lost significantly fewer retinal ganglion cell axons," Zhu says.
Neurologists are currently conducting clinical trials to see if stress-induced tolerance can reduce brain damage after acute injuries like stroke, subarachnoid hemorrhage or trauma.
Gidday hopes his new finding will promote studies of tolerance's potential usefulness in animal models of Parkinson's disease, Alzheimer's disease and other neurodegenerative conditions.
"Neurons in the central nervous system appear to be hard-wired for survival," Gidday says.
"This is one of the first steps in establishing a framework for how we can take advantage of that metaphorical wiring and use positive stress to help treat a variety of neurological diseases," Gidday added.
The study has been published online in Molecular Medicine.