By exposing rats to charged particle irradiation and monitoring changes in their brain cells, the researchers found evidence of damage as much as six months after the initial exposure. This has been dubbed “space brain.”
In particular, UCI’s Charles Limoli and colleagues found that exposure to highly energetic charged particles – much like those found in the cosmic rays that astronauts will be exposed to during extended spaceflights – causes significant long-term brain damage that results in cognitive impairments and dementia.
The research, published in Nature’s Scientific Reports, follows a similar study released last year showing shorter-term brain effects of galactic cosmic rays.
“This is not positive news for astronauts deployed on a two-to-three-year round trip to Mars,” said Professor Limoli.
“The space environment poses unique hazards to astronauts. Exposure to these particles can lead to a range of potential central nervous system complications that can occur during and persist long after actual space travel – such as various performance decrements, memory deficits, anxiety, depression and impaired decision-making. Many of these adverse consequences to cognition may continue and progress throughout life.”
As part of the study, the rodents were subjected to charged particle irradiation of fully ionized oxygen and titanium at the Nasa Space Radiation Laboratory at New York’s Brookhaven National Laboratory, before being sent to Limoli’s UCI lab.
Six months after exposure, the researchers found significant levels of brain inflammation and damage to neurons. Specifically, imaging revealed a reduction of dendrites and spines on these neurons. This showed the brain’s neural network was impaired and a drop in dendrites interferes with the transmission of signals among brain cells.
These deficiencies were similar to those used to signify poor performance on behavioural tasks designed to test learning and memory in humans.
In addition, the Limoli team discovered that the radiation affected “fear extinction” – a process in which the brain suppresses unpleasant and stressful memories and associations. An example given by the researchers is when someone who nearly drowns and has a fear of water learns to swim and enjoy water again.
“Deficits in fear extinction could make you prone to anxiety,” Limoli said, “which could become problematic over the course of a three-year trip to and from Mars.”
Most notably, he said, these six-month results mirror the six-week post-irradiation findings of a 2015 study he conducted that appeared in the May issue of Science Advances.
Similar types of more severe cognitive dysfunction are common in brain cancer patients who have received high-dose, photon-based radiation treatments. In other research, Limoli examined the impact of chemotherapy and cranial irradiation on cognition.
While dementia-like symptoms in astronauts would take months to appear, Professor Limoli said that the time required for a mission to Mars “is sufficient for such impairments to develop”. People working for extended periods on the International Space Station, however, do not face the same level of bombardment with galactic cosmic rays because they are still within the Earth’s protective magnetosphere.
Limoli’s work is part of Nasa’s Human Research Program.
“Recent studies from our laboratory have confirmed the adverse effects of cosmic radiation exposure on CNS function. Cognitive tasks used in the present study corroborate past findings and identify significant longer-term deficits in episodic, spatial, recognition memory and compound discrimination mapping [in] the frontal and temporal lobes containing the hippocampus, mPFC and perirhinal cortex,” the researchers wrote in their paper.
“These new data reveal additional detrimental effects related to fear extinction and anxiety, where charged particle irradiation compromises the ability to dissociate adverse events and outcomes. The inability to moderate reactions to certain unpleasant stimuli could elicit elevated stress, anxiety and otherwise disadvantageous responses in unexpected or emergency situations.”
The researchers continued that such conditions could be problematic for astronauts and their capability to efficiently operate over the course of a deep space mission. Furthermore, impairments in executive function point to further potential complications in carrying out complicated tasks or in decision-making in stressful situations.
“Unlike humankinds’ other great adventures, space is truly the final frontier. Our exploration of strange new worlds should not be hampered by the fear of cosmic radiation exposure, but rather, inspire robust efforts to advance our understanding of a tractable problem,” the study group concluded.
Partial solutions are being explored, Limoli said. Spacecraft could be designed to include areas of increased shielding, such as those used for rest and sleep. However, these highly energetic charged particles will traverse the ship nonetheless, he added, “and there is really no escaping them.”
Elsewhere, preventive treatments offer some hope. Development of more advanced engineering and biologic countermeasures designed to the protect host neuronal circuitry from cosmic radiation exposure are underway, and will be incorporated into deep space mission planning. In particular, Limoli’s group is working on pharmacological strategies involving compounds that scavenge free radicals and protect neurotransmission.