NASA hopes to send the first round-trip, manned spaceflight to Mars by the 2030s. If the mission succeeds, astronauts could spend several years potentially being bombarded with cosmic rays—high-energy particles launched across space by supernovae and other galactic explosions. Now, a study in mice suggests that these particles could alter the shape of neurons, impairing astronauts’ memories and other cognitive abilities.
The concern about cosmic rays is a long-standing one, prompting NASA (and science fiction writers) to spend a lot of time discussing ways of protecting astronauts from them. (A buffer of water around the spacecraft’s hull is one popular idea.) But scientists don’t really know how much of a threat the radiation poses. It’s not feasible to study the effects of cosmic rays on real astronauts, such as those living in the International Space Station, because many variables, including the stress of living on a spaceship, can affect cognition, says Patric Stanton, a cell biologist at New York Medical College in Valhalla. It’s also impossible to control the level of radiation astronauts are exposed to, making it difficult to do rigorous experiments, he says.
To overcome those challenges, several NASA-funded research groups are testing cosmic radiation on mice. In the new study, published today in Science Advances, Charles Limoli, a molecular biologist at the University of California, Irvine, and colleagues took male mice to a particle accelerator at the NASA Space Radiation Laboratory in Upton, New York. There, they catapulted oxygen and titanium ions down a 100-meter transport tunnel and into the restrained rodents’ brains at roughly two-thirds the speed of light. The dose of high-energy particles resembled the radiation likely to pass through the unprotected hull of a spaceship over the course of a mission to Mars, Limoli says.
After letting the mice rest for 6 weeks, the team put them through a battery of cognitive tests, including a task that required the rodents to distinguish between familiar and novel objects, such as toys. The mice hammered by radiation were “severely compromised” on several measures compared with an unexposed control group, Limoli says. Control mice, for example, spent more time sniffing around a new item placed in their cages than investigating familiar objects—a sign that their ability to react to novelty was intact. Irradiated mice, in contrast, spent equal time exploring new and old items, suggesting their ability to learn and remember new information about their environment had been impaired.
Limoli’s team found alterations in the structure of the radiated rodents’ neurons as well. In the prefrontal cortex, a brain region associated with executive function, a range of high-level cognitive tasks such as reasoning, short-term memory, and problem-solving, neurons had 30% to 40% fewer branches, called dendrites, which receive electrical input from other cells.
The work “builds on a notable amount of literature showing that space radiation produces negative consequences on brain and behavior,” says Amelia Eisch, a neuroscientist at the University of Texas Southwestern Medicine Dallas. “The focus on a prefrontal cortical-linked task is particularly mission-relevant, since astronauts will need to have their executive functions in tip-top shape.”
Although the loss of dendrites appears drastic, many factors aside from radiation can quickly alter neuronal structure, Stanton notes. Simply depriving rodents of toys and other forms of environmental stimulation, for example, can cause dendrites and their spines to shrivel up, he says. “Luckily for you and I,” they appear to come back when environmental conditions change, he says, suggesting that brain-boosting drugs and cognitive training could counteract cosmic radiation’s ill effects.
Longer term studies are needed to establish whether the structural changes and behavioral deficits found in the mice last, Stanton says. For his own part, Limoli doesn’t think the findings are a “showstopper” for the Mars mission, but “there’s no reason to think that what we found in rodents wouldn’t hold in humans as well,” he says. “This is something NASA needs to consider.”
*Correction, 4 May, 12:57 p.m.: Amelia Eisch's discipline has been corrected.