Researchers have known for almost a decade that the adult brain produces new neurons. But a new study appearing yesterday (August 31) online in Nature Neuroscience gets a better look at what adult neurogenesis in two regions of the brain is actually for.

"I think the conclusions are really groundbreaking," Barbara Beltz, neuroscientist at Wellesley College who was not involved in the study, told The Scientist. Previous studies have examined the behavioral effects of neurogenesis, but they've only looked at one region of the brain at a time, whereas this study was able to compare what is happening in the two brain regions, Beltz added.

Adult neurogenesis occurs in two regions of the brain: The subventricular zone -- after which some neurons migrate to the olfactory bulb -- and the dentate gyrus of the hippocampus, which plays a role in spatial memory. The research team, led by Ryoichiro Kageyama at the School of Medicine at Kyoto University, Japan, used fluorescent and genetic markers to trace the birth of new neurons in the two brain areas.

They observed continual cell death in the olfactory bulb, suggesting that newly born neurons would be necessary to take their place. Conversely, neurons in the dentate gyrus did not die regularly, so neurogenesis wouldn't be necessary as a neuron replacement strategy.

The researchers then went on to examine the behavioral effects of these newly generated neurons by blocking their production with genetic ablation -- essentially, programming a gene into the mice that killed specific new neurons. When they knocked out neurogenesis in the olfactory bulb, they observed no deficits in smell memory, in contrast to results of some previous work. In the dentate gyrus, on the other hand, blocking neurogenesis resulted in problems with spatial memory, suggesting that while the new neurons aren't required for the area's functioning, they do affect an animal's behavior.

One of the strengths of the study, said Beltz, is that, unlike previous work that used ablation techniques that would affect other processes besides neurogenesis, ablating genetically ensured that the researchers targeted neurogenesis exclusively, excluding the possibility that they were observing behavioral effects from some other altered process.

The new results reveal even more mysteries about how these regions of the brain operate. "The surprise is that we get different outcomes in different [brain] tissues," said Beltz. The fact that the olfactory bulb needs a high level of cell turnover "brings up all sorts of new questions," she said. "If we think about olfactory odor memory, if we're turning over [neurons] all the time how do we maintain odor memories, if the majority of neurons die and are replaced?"

In addition, the two regions don't necessarily operate independently from each other; "there may be crosstalk between the two areas that needs to be pursued," Beltz added.