Transient Pockets of Hypoxia in the Mammalian Brain
Evidence suggests that the mammalian brain is operating at the very edge of its capacity, supplied with just enough oxygen and nutrients to barely get by. That exercise produces measurable short-term gains in cognitive function, while blood flow is increased, is one point in favor of this view. Another is provided here, in which researchers note that it is entirely normal to observe transient areas of hypoxia in the brain at rest, and that the occurrence of these regions is diminished by the increased blood flow of exercise. It is an open question as to what to do with this finding: we can imagine future technologies that greatly increase the capacity of the blood to carry oxygen around the body, but equally it is also the case that mild hypoxia is actually beneficial. A little stress provokes better cell maintenance.
Using a bioluminescent oxygen indicator, researchers discovered a spontaneous, spatially defined occurrence of "hypoxic pockets" in the mouse brain. Their technique offers a way to learn more about brain oxygen tension (pO2), a measure of oxygen delivery and demand in brain tissue that changes dynamically but is not well understood. The findings could have implications for how rest and exercise affect pO2 in the human brain, including the role of these activities in conditions such as dementia.
The researchers used a genetically encoded bioluminescent oxygen indicator in mouse cortical astrocytes to track pO2 changes. Under resting conditions, pO2 changed often and included transient but sharply defined events of hypoxia that lasted several seconds to minutes and were spatially confined. Further research confirmed that the hypoxic pockets were caused by circulation changes in the brain's capillaries. During exercise, the area covered by hypoxic pockets in the mouse brain decreased by 52% compared to the brain during rest.
"Our study predicts that physical inactivity has direct effects on tissue pO2 by favoring capillary occlusions and increasing the number of hypoxic pockets. Conversely, simply increasing sensory input or locomotion rapidly suppress the occurrence of hypoxic pockets perhaps explaining the linkage between sedentary lifestyle and an increased risk of dementia." As pO2 decreases with age, the researchers also note that their technique might someday be used to determine if hypoxic pockets expand or last longer with age.