New Nerves Sprout New View of Brain

Scientists have found that the brains of adult primates are not a dead end for nerve growth, as popularly thought, but in fact are able to grow new nerve cells. The results, reported in today's issue of the Proceedings of the National Academy of Sciences, hold out hope for finding new treatments for neurodegenerative diseases such as Parkinson's--a condition marked by a loss of functioning brain neurons.

In the 1960s, researchers showed that adult rats continually produce neurons in a brain region called the dentate gyrus, part of the hippocampus--an area that plays an important role in acquiring new memories. In a series of studies beginning in 1994, neuroscientist Elizabeth Gould of Rockefeller University in New York City and her co-workers extended this picture, showing that stressed rats release adrenal hormones that suppress new nerve cell production in the brain. But because these results had never been duplicated in primates, the additional nerve cell growth was thought to be a feature peculiar to the rodent brain.

Gould, now at Princeton University, and her colleagues have finally made the jump to primates. The researchers injected marmoset monkeys with a chemical marker called BrdU that becomes incorporated into the DNA of dividing cells, making newborn cells easy to spot. Three weeks later, they looked at brain tissue from the monkeys and found BrdU to be abundant in the dentate gyrus. To see if the newborn cells were indeed neurons, Gould and her colleagues used another marker called neuron-specific enolase. It turned out that 80% of the BrdU-labeled cells were also tagged with the enolase, confirming that the newborn cells were in fact neurons. Gould's team also noted a stress effect: Monkeys housed in close quarters with other monkeys ended up with one-third fewer BrdU-labeled cells.

The findings are "a great advance" that runs "against the prevailing wisdom," says Gerd Kempermann, a neuroscientist at the Salk Institute in La Jolla, California. "Everybody else hopes that you can make use of that [nerve cell production] to treat neurodegenerative diseases," such as Parkinson's disease, or even to encourage the aging brain to regenerate by stimulating the production of new nerve cells, he says. But Fred Gage, also at the Salk Institute, and others caution that it's too soon to jump from marmosets to people. Previous studies have shown that the ability of adult mice to grow new nerve cells in the hippocampus can be influenced by genetic background, Gage says, suggesting that this ability may vary widely across species.