Nuclear bombs mark newborn neurons
Radioactivity released into the air by Cold War nuclear tests has enabled Swedish scientists to probe how adult brains produce new nerve cells and whether they survive for any length of time.
The discovery in the 1990s that certain parts of the human brain retain the ability to produce new nerve cells into adulthood rocked the neuroscience world which had, prior to then, been firmly of the opinion that adult brains don't regenerate.
But although it became clear that new nerve cells could be produced, it wasn't known whether these cells survived for any appreciable length of time and therefore whether they had any important purpose or were just an anomaly.
Surprisingly, the Cold War has come to the rescue. The proliferation and testing of nuclear weapons in the 1950s and 60s increased the levels of radioactive carbon (carbon-14) in the atmosphere.
Taken in by growing plants and then channelled along the food chain, this radioactive blip has found its way into the brain cells of people alive both at the time and subsequently, date-stamping their DNA with an indelible marker.
Kirsty Spalding, from the Karolinska Institute in Sweden, has now used this radioactive label to carbon date nerve cells to discover - to within an accuracy of 1.5 years - how long these newborn neurones can live for, and how many of them there are.
Working on donated post-mortem brain samples from patients aged 19-92, the team isolated nerve cells from the hippocampus, one of three regions where new nerve cells were found to appear during adulthood.
The DNA was extracted from these cells and the amount of carbon-14 contained within them was quantified enabling them to be retrospectively birth-dated.
The team worked on the premise that, if a baby is born with a complement of brain cells that are to last it a lifetime, then the amount of carbon-14 in the DNA of the cells, which are not dividing, should fall over time.
But subjects the team studied, one of whom was 92 at the time of death, had nerve cell DNA levels of carbon-14 that were far higher than the levels of carbon-14 that would have been around when the person was born. This proves that they must have been adding new nerve cells to their hippocampi well into adulthood.
The results also show that, on average, 700 new nerve cells are added to the adult hippocampus each day which, over a year, amounts to just under 2% of the hippocampal nerve cells being replaced. Over a lifetime that means that about one third of the cells in this part of the brain turnover.
These newly-added cells aren't as long-lived as the neurones a person is born with, the team found. Models run by the Karolinska team show that they die off 7-10x faster than the starting cells.
As to the role of these replacement cells, the researchers speculate in their paper in Cell that they might be important in new memory formation, and specifically the separation of new from existing experiences. They also highlight the importance of exploring the part played by these nascent cells in psychiatric diseases, particularly depression which might affect their longevity...