For many years, neuroscientists thought that the brains of adult mammals don't grow additional neurons. Then studies showed that neurons pop up throughout life in some regions of the brain, including the hippocampus, an area important for building new memories. However, whether these new neurons actually do anything important has remained a mystery.

To address this question, behavioral neuroscientist Tracey Shors of Rutgers University in Piscataway, New Jersey, and colleagues treated rats with a drug called MAM that kills proliferating cells, including developing neurons. They then trained these rats, along with a group of normal rats, on a memory task known to require the hippocampus. In each trial, the rats heard a brief buzzing sound followed by a short delay and then a mild shock to the eyelid. After a few dozen repetitions, normal rats got wise and blinked when they heard the noise. However, the treated rats were much less likely to blink--even after hundreds of trials. Apparently, the rats didn't remember the experiences, the researchers report in the 15 March issue of Nature. The deficit did not appear to be due to the toxic side effects of MAM, the authors say, because the rats performed well on a memory task that doesn't involve the hippocampus.

The study answers a question that has lingered ever since the discovery that adult brains can grow new neurons, says neurobiologist Rob Malenka of Stanford University. Now it seems that the new neurons do have an important function, at least in the hippocampus. "It's an experiment that clearly needed to be done, and I admire them for actually doing it," Malenka says.

--GREG MILLER

Related site

Tracey Shors's Web site at Rutgers