Carbon Dating New Neurons in Old Brains

In 1963, the Partial Nuclear Test Ban Treaty was signed, banning nuclear tests underwater, in the atmosphere and in outer space. Although underground detonations were still permitted and several countries refused to sign the treaty, it nonetheless helped slash radioactive fallout. Prior to the test ban treaty, the fallout from nuclear tests showered radioactive atoms across the Earth. Among these radioactive atoms was carbon-14, which was absorbed by plants, which were eaten by humans, who then had the carbon-14 atoms integrated into their DNA.


Fortunately, it appears that this spike in atmospheric carbon-14 did not have a substantial negative effect on the health of most humans. (For certain groups of humans, and for certain ecosystems, there were devastating effects linked to radioactive fallout from nuclear tests.) In fact, Jonas Frisén, of the Karolinska Institute in Sweden, and his colleagues have actually found a use for this pulse of atmospheric carbon-14: determining the age of human cells. Using this technique, they recently discovered, as detailed in Cell, that new neurons are produced in the striatum of adult humans, an area of the brain that had previously been overlooked in the search for adult-born neurons: neurons produced by adult brains.

Old Brains, New Tricks

For many years, most scientists believed that the human heart and brain could not produce new cells in adulthood. But in the late 1990s, researchers began to find evidence of neurons produced past infancy. Several years later, Frisén and colleagues were able to show that the human heart produces new muscles cells throughout adulthood. They determined that as many as half of the cells in the heart are replaced during an average lifetime.


Scientists had missed these remarkable phenomena for so long because it is unethical to perform the sort of experiments that could demonstrate the genesis of new heart or brain cells in adults. For instance, one way to test such a thing in mice is to feed them radiolabeled food for a certain period of time. Any cells produced during this time will incorporate the radiolabeled material into their DNA, which is stable for the life of the cell. Later, when researchers find a radiolabeled cell, they will know that the cell was created while the mouse was being fed the labeled food. This is sometimes called "pulse" labeling.
Of course, researchers are not generally allowed to feed humans radioactive food. But, as Frisén and colleagues realized, humans born throughout much of the 20th century had already been exposed to a known pulse of radiolabeled food as a result of the spike in atmospheric carbon-14 starting in 1955, when nuclear testing began to surge, peaking in 1963, and then slowly declining in the ensuing decades.

Radiocarbon Dating and Bomb Pulse Dating

Most people are familiar with radiocarbon dating, a technique frequently used to determine the age of ancient organisms. This is similar to the method used by Frisén and colleagues in that it focuses on the amount of carbon-14 in a sample. Radiocarbon dating relies on the radioactive decay of carbon-14 over time. If you know the approximate ratio of carbon-14 to carbon-12 in the atmosphere when an ancient organism was alive and you can measure the current ratio in a sample of the organism, then, using the half-life of carbon-14, you can calculate its age.
However, with a half-life of nearly 6,000 years, carbon-14 does not decay fast enough to mark individual phases in the life of a human. Instead, Frisén and colleagues gathered data on the rapidly rising and falling concentration of carbon-14 in the atmosphere before and after the Partial Test Ban and compared it to the amount of carbon-14 found in the neuronal DNA of humans who had lived and died during that time.


They found that neurons — specifically interneurons — in the striatum appeared to be much younger than the individuals whose brains the samples had been taken from. For instance, they found that people born before 1955 had interneurons containing DNA with high concentrations of carbon-14, suggesting that the interneurons had been created after their birth, during the heyday of atomic bomb testing.

Huntington's Disease and Neurogenesis

The striatum is an area at the crossroads of many of the brain's essential functions. In fact, cell death in the striatum is a primary characteristic of Huntington's disease, which results in extreme loss of coordination and dementia. Knowing this, Frisén and colleagues decided to take a closer look at the neurons of deceased individuals with Huntington's disease. Intriguingly, they found that neurons throughout the brain, including the striatum, appeared to be about the same age as the individual, indicating that either adult neurogenesis is suppressed in Huntington's disease or that adult-born neurons are destroyed by the disease's progression.


Although the exact relationship between adult neurogenesis and Huntington's disease is not clear, this finding does suggest that it might be possible to encourage or protect adult-born neurons, so that they might replace striatal neurons ravaged by the disease. More generally, massive cell death is the hallmark of many brain diseases, and it is the hope of scientists like Frisén and colleagues that research on adult neurogenesis will enable us to devise therapeutic strategies harnessing the brain's own ability to replenish neurons.

Discussion Questions

Imagine you are an archaeologist 1,000 years in the future. Will the bomb pulse of carbon-14 make radiocarbon dating easier or harder?
Frisén and colleagues also examined the neurons of deceased cancer patients who had been treated with a radiolabeled medicine as part of their treatment. This data provided provisional support for adult neurogenesis in the striatum, but Frisén and colleagues felt that this alone was not sufficient to support their hypothesis. What are the weaknesses of such cancer evidence as compared to the bomb-pulse evidence?