Previous studies suggested that radiation exposure could also cause cognitive impairment, including earlier onset of Alzheimer's-like dementia. Now Charles Limoli, a professor of radiation oncology at the University of California Irvine School of Medicine, and his team have demonstrated that even relatively low doses of cosmic rays will induce a specific series of neural abnormalities that could manifest themselves during a round-trip mission to Mars, which is predicted to last for two to three years.

“This is the first study, in my opinion, that really ties a lot of loose ends together and provides a mechanism for what’s going on to cause cognitive dysfunction,” says Limoli, whose team reports the results today in Science Advances.

To study the “mind numbing” effects of radiation, the researchers examined several groups of six-month-old mice—the approximate average age of astronauts in mouse years. The team blasted the mice with low or high doses of energetic charged particles similar to those found in galactic cosmic radiation. These particles displace electrons in living tissue that then trigger free radical reactions, which cause changes in the cells and tissues of the body. Although free radical reactions occur within milliseconds, the cellular abnormalities they cause take form over months or even years, so the researchers waited six weeks before testing the irradiated mice to allow the cellular mischief to unfold.

The results showed that irradiated mice were significantly impaired in their ability to explore new objects placed in their environment, a task that draws upon a healthy learning and memory system. “The animals that were exposed lost curiosity. They lost their tendency to explore novelty,” says Limoli.

Specifically, the team discovered radiation-induced structural changes in the medial prefrontal cortex, a brain region responsible for higher-order processes known to be engaged during memory tasks. Neurons in these impaired areas showed a reduction in the complexity and density of structures called dendrites, which act as antennae for incoming cellular messages and are essential for the efficient exchange of information throughout the brain. The research team also discovered alterations in PSD-95, a protein that is important for neurotransmission and is also associated with learning and memory.

The cellular changes in the dendrites were directly related to cognitive performance—the mice with the largest structural alterations had the poorest performance results. And although these deficiencies took time to manifest, they appear to be permanent.

Limoli notes that, while the work was done in mice, the damage seen in their study looks a lot like defects seen in human brains suffering from neurodegenerative conditions like dementia. “Because these types of changes have also been found in a range of neurodegenerative conditions and occur over the course of aging, it provides a logical backdrop for what radiation does to the brains of both rodents and humans,” says Limoli.

It's likely no one has seen these types of defects in today's astronauts because people working on the International Space Station are “protected by the Earth’s magnetosphere, which deflects anything that has a charge,” says Limoli. And while the astronauts that traveled to the moon were not protected by Earth’s magnetic embrace, their relatively short trips would have limited exposure levels to a fraction of those that would be experienced on a mission to Mars.

While the results of this experiment were striking, other experts emphasize that there is still a lack of sufficient data to make definitive conclusions about the effects of radiation of people. “A lot of the information we have has been extrapolated from studies of catastrophic events in World War II," says Nathan Schwadron, associate professor of space plasma physics at the University of New Hampshire. "We just don’t have a lot of knowledge about what happens to biological systems when exposed to high levels of radiation for prolonged periods. I think that there is a potential risk here, but we really just don’t understand it yet.”

So what is to be done? NASA is currently investigating more advanced shielding technologies that could better protect astronauts on long-term missions into deep space. Engineers could also alter the shielding capabilities within certain regions of the ship, such as where astronauts sleep, or fit people with specialized helmets for space walks, says Limoli.

Schwadron, whose research is primarily focused on the development advanced shielding, says the energy from galactic cosmic radiation is so high that it interacts with the shielding materials in potentially problematic ways. “What happens is that high-energy radiation hits the shield and then produces a bath of secondary particles. Neutrons are probably the primary example of this.” These high-energy particles can then interact with the body, inducing free radical reactions and subsequent tissue damage.

Moving forward, Limoli and his team plan to design experiments that more accurately simulate human exposure to galactic cosmic rays and investigate alternative underlying mechanisms and cell types that could contribute to the proliferation of cognitive deficits. He is also investigating pharmacological interventions that could protect brain tissue from this radiation.

“We have some promising compounds that will probably help quite a bit,” says Limoli. “This is not a deal breaker—it is something that we need to understand and be aware of so we are not caught off guard.”

Quelle: Smithsonian