Astrocytes may Hold the Key to Exercise-Induced Cognitive Enhancement

By Emilie Reas, PLOS Neuroscience Community Editor

Forget expensive pills or exotic miracle supplements. Exercise may be the most effective – not to mention free and accessible – cognitive enhancer on the market. Research in humans has shown that physical activity can improve cognitive function and may help stave off dementia, yet the biological mechanisms underlying these benefits aren’t fully understood. Animal studies have made substantial progress on this front, demonstrating such positive responses to running as enhanced neurogenesis and elevated levels of neural growth factors. However, much of this research has been relatively narrowly focused, with particular attention devoted to neuronal changes and one notable brain region – the hippocampus. The hippocampus is selectively important for certain functions like learning and episodic memory, but exercise improves a range of cognitive processes, many of which depend on other, non-hippocampal brain regions. Therefore, researchers from Princeton University looked beyond the hippocampus and neurons to more thoroughly characterize the neural events that impart cognitive protection from physical activity. In their study recently published in PLOS ONE, Adam Brockett and colleagues report that running enhances performance on various cognitive tasks, improvements which may be mediated by changes in astrocytes, the lesser appreciated brain cells.

Running selectively boosts some cognitive functions

To manipulate levels of physical activity, rats were divided into a group of runners who were allowed free access to running wheels for 12 days, and another group of sedentary controls. Prior studies have shown that running improves performance on tasks requiring the hippocampus, like learning and memory. Here, the runners and non-runners were subjected to three tests to determine how exercise affects cognitive functions that are not dependent on the hippocampus. An object-in-place task, which tests how well rats remember the location of previously encountered objects, relies on the medial prefrontal cortex, hippocampus and perirhinal cortex. A novel object task, in which rats distinguish novel from familiar objects, selectively depends on the perirhinal cortex. Lastly, a set-shifting task, supported by the orbitofrontal and medial prefrontal cortices, measures attention and cognitive flexibility.

Compared to their non-runner companions, the runners performed better on the object-in-place test and on several measures of the set-shifting task. However, there were no differences between runners and non-runners in performance on the novel object recognition test. Of course, the cognitive benefits of running don’t end here, since many cognitive domains were not assessed in this test battery. But these findings highlight a striking selectivity of the brain-boosting powers of exercise. In particular, they suggest that running may enhance functions that specifically depend on the medial prefrontal and orbitofrontal cortices, along with the hippocampus, but it does not appear to modulate perirhinal-dependent functions.

Cognitive enhancement is linked to astrocytes

Although behavioral changes provide a window into the underlying neural events, they do not tell the complete mechanistic story. To directly examine how running affects the brain, the researchers assessed changes to both neuronal and non-neuronal brain cells. Running induced widespread neuronal changes, including higher levels of pre- and postsynaptic markers throughout the brain (including in the hippocampus and orbitofrontal, medial prefrontal and perirhinal cortices), and increased density and length of dendritic spines in the medial prefrontal cortex. While these effects suggest that exercise elicits generalized synaptic changes, they do not explain why particular cognitive functions are selectively boosted over others.

The researchers therefore looked for this crucial link to behavior in astrocytes. As Brockett explains, “We hypothesized that all cells likely change as a function of experience. We chose to focus on astrocytes because there is lots of evidence suggesting that astrocytes could be implicated in cognitive behavior. Loss of astrocytes correlate with impairment on a cognitive task and astrocytes connect the majority of neurons to blood vessels. They extend numerous processes that envelop nearby synapses, and gliotransmitters have been implicated directly in LTP-induction.”

Confirming their suspicions, in runners, astrocytes increased in size (Figure, A-B) and showed more contacts with blood vessels (Figure, C-D). But these changes only occurred in the hippocampus, medial prefrontal cortex and orbitofrontal cortex – critically, all regions that support the tasks showing running-related improvement. In contrast, running did not alter astrocytes in the perirhinal cortex, a region necessary for novel object recognition, which did not benefit from running. Thus, while running modified both neurons and astrocytes, the pattern of selective cognitive enhancements corresponded only with changes to astrocytes.

Implications for the active human

Although the varied and widespread cognitive benefits of exercise have long been appreciated, this study provides some of the first insight into the remarkable selectivity of these enhancements. Follow-up studies will help elucidate why, from both biological and evolutionary perspectives, running would demonstrate such selectivity. Might, for example, attention or task-switching abilities have been more important than object recognition for the efficiency of both animals and our persistence-hunting, distance-runner ancestors? Does running more heavily recruit certain brain regions over others, making them more susceptible to remodeling?

Given the cognitive and neurobiological differences between rats and humans, future research will be important to help extrapolate beyond rodents. Currently it’s unclear how different forms of exercise enjoyed by humans – for instance, swimming, yoga or strength training – uniquely influence distinct cognitive functions. According to Brockett,

“There is a lot of evidence that running has numerous beneficial effects on rodent and human cognitive functioning, but it is likely that aerobic exercise in general is responsible for these effects rather than running per se.”

Perhaps most notably, these findings add to the growing pool of studies underscoring the importance of astrocytes in neural processes that support cognition, and reveal a novel role for these cells in experience-dependent plasticity. As Brockett explains:

“Astrocytes are a unique cell type that haven’t been explored as much as neurons by the field of Neuroscience at large. Few studies have directly examined the role of astrocytes in complex behavior, and this was our first attempt at investigating this question.”

Any views expressed are those of the author, and do not necessarily reflect those of PLOS.

References

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Brockett AT, LaMarca EA, Gould E (2015) Physical Exercise Enhances Cognitive Flexibility as Well as Astrocytic and Synaptic Markers in the Medial Prefrontal Cortex. PLoS ONE 10(5): e0124859. doi:10.1371/journal.pone.0124859

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Emilie Reas received her PhD in Neuroscience from UC San Diego, where she used fMRI to study memory. As a postdoc at UCSD, she currently studies how the brain changes with aging and disease. In addition to her tweets for @PLOSNeuro she is @etreas.