New brain cells are essential for learning

• 18:01 31 August 2008
• NewScientist.com news service
• Rachel Nowak

Far from being a completed masterpiece, some parts of the brain are works-in-progress, continuously churning out new cells. Now we may know why the brain goes to all that trouble. In mice at least, it appears that fresh brain cells are key to learning and memory.

By watching how genetically altered mice tried to learn and memorise the location of a hiding hole without the help of new brain cells, a team led by Ryoichiro Kageyama of Kyoto University in Japan has shown that new brain cells are essential for learning and memory.

"It was always unclear whether neurogenesis in the adult was essential for normal brain function, or whether it was an innocent bystander. This shows that it's essential," says Rodney Rietze of the Queensland Brain Institute in Brisbane, Australia.

Kageyama's team created a strain of mice engineered so that when they were given a drug, newly made brain cells in the hippocampus produced proteins that killed the cells. The hippocampus is essential for learning and memory.

The team then looked at how well the mice learned to find a hiding hole, a standard test of learning and memory. "Usually a mouse remembers the hole after one or two days' training, and will still remember it a week later. These mice took five or six days to remember, and then totally forget it one week later," says Kageyama.

Smelly test

In another test, the researchers cut the supply of new cells to the animals' olfactory bulb, the region of the brain that perceives odour. Although the olfactory bulb shrank, the ability of the mice to discriminate between very similar smells was not affected, nor was their ability to remember the smells four months later. That suggests new cells might not be as important for smell memory as for spatial memory.

But Kageyama says that alternative explanations are that the mice's abilities may worsen as their olfactory bulbs become more damaged over time, or that new cells in the olfactory bulbs are used for specific types of smell memories not tested in the experiment. For instance female mice remember the pheromonal scent of their mates, and will abort their foetuses if they smell a strange male.

The team also created mice in which new nerve cells glow green in order to test drugs that could be used to stimulate neurogenesis in people, in whom a reduced ability to produce new brain cells may account for age-related memory loss.

"Brain circuitry is not set in stone," says Pankaj Sah of the Queensland Brain Institute in Brisbane. "It's malleable and growing even in an adult. We've suspected it for a long while, but this is the first evidence that these new cells are doing something functional in learning and memory,"

Journal reference:

Nature Neuroscience (DOI: 10.1038/nn.2185)