The adult brain requires a continuous stream of new neurons for the maintenance of regions controlling smell and spatial navigation, suggests a paper online in Nature Neuroscience.
Until a few years ago, it was thought that the mature brain gradually loses nerve cells but cannot make any new ones. This dogma has since been overthrown, but whether the few newly generated nerve cells play any important roles is not known.
Ryoichiro Kageyama and colleagues tackled this question by genetically modifying mice to synthesize a fluorescent protein in all adult-born nerve cells, and recording the numbers of fluorescent neurons over the course of one year. Over this time, nearly all neurons in a certain layer of the olfactory bulb -- crucial for smelling -- were replaced with new ones. In the hippocampus, which is essential for spatial learning and memory, about 15% new cells were added.
The authors then investigated whether stopping neurogenesis would interfere with the mice's ability to smell or learn. The team made another mouse line, producing a poisonous protein in all adult nerve cell precursors to kill them. With no new neurons available, the mice's olfactory bulbs shrank -- though their sense of smell surprisingly remained intact for at least four months, suggesting a lot of redundancy in the olfactory circuitry. In contrast, the mice's ability to remember how to navigate a particular kind of maze was lost.
Ryoichiro Kageyama (Kyoto University, Japan)
Abstract available online.
(C) Nature Neuroscience press release.
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