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Patrick Guilfoile: The bomb on our brains

It has been an article of faith that the adult human brain is fated to get worse with age.

Conventional wisdom is that adults do not develop new nerve cells, and, as the old ones wear out, our mental function continues to go downhill.

However, the evidence about whether or not humans can develop new brain nerve cells has been limited. One study from the late 1990s relied on the use of a now-banned chemical to identify new nerve cells. This study showed that five patients, ranging in age from 57 to 72, produced new nerve cells in a region of the brain called the hippocampus (an area of the brain critical for memory formation). However, this research wasn’t able to determine the number or the rate at which these new nerve cells were produced, leaving important questions unanswered.

To gain a more detailed understanding of the production of new nerve cells in the brain, researchers at institutions in Europe and the United States studied the brains of 55 deceased individuals ranging in age from 19 to 92-years-old. These scientists exploited an unintended biological experiment. From 1945 to 1963, several nations exploded nuclear bombs above ground. The result of those detonations was elevated radioactivity in the atmosphere, and this radioactivity was incorporated into the genetic material of newly formed cells, including cells in the brain. This radioactivity acted like a date stamp, providing precise information about when new cells were “born,” based on the amount of radioactivity remaining in the atmosphere from the atomic bomb tests at the time of the cell’s “birth.” If scientists could measure the amount of radioactivity from the bomb in brain cells, they could tell whether new nerve cells were being formed and the rate at which they were formed. Technically, this was very difficult because the genetic material they had available for analysis was 50 to 100 times less than that normally required. Therefore, the scientists developed new procedures to get the analysis to work. They initially separated out the portions of the cell containing the genetic material (the DNA) from the rest of the cell, then separated out the DNA from the nerve cells, and converted it to carbon to measure the amount of radioactive carbon present.

From these experiments, the researchers determined that individuals developed new nerve cells in their hippocampus, to the tune of about 700 new cells per day, or nearly 2 percent of this region of the brain per year. This suggests an important role for new nerve cells in the human brain. It may be that the generation of new nerve cells maintains flexibility in dealing with new situations. It may also be that individuals less able to generate new nerve cells may be more susceptible to certain psychiatric conditions such as anxiety and depression. In any case, it is clear that the nuclear bomb tests in the mid-1900’s have opened a window to a new understanding of how our brains work.

More information is available in the article “Dynamics of Hippocampal Neurogenesis in Adult Humans”, Kirsty L. Spalding and others. Cell, 153, 1219—1227, June 6, 2013

PATRICK GUILFOILE has a Ph.D. in bacteriology and is the associate vice president at Bemidji State University.