From the article:
At the end of this “endurance exercise training” period, all mice were injected with 10 mg. of methamphetamine over a 24-hour period. […] In the sedentary group of mice, the small capillaries in the brain experienced increased oxidative stress, causing the blood brain barrier to become more permeable. Toxins and inflammatory cells previously prevented from crossing the blood brain barrier then had access to the brain. The exercise group showed no such changes.