PARTS of the brain are a work-in-progress, continuously churning out new cells – but why go to all that trouble? In mice at least, fresh brain cells are key to learning and memory.
“It was always unclear whether neurogenesis in the adult was essential for normal brain function. This shows that it is,” says of the Queensland Brain Institute in Brisbane, Australia.
Neurogenesis happens in all adult mammals, including humans, and of Kyoto University in Japan and colleagues wanted to know why. They genetically altered mice so that when they took a drug, genes for proteins that kill cells switched on only in new brain cells. These cells are mainly present in just two regions – the hippocampus, used for learning and memory, and the olfactory bulb, which processes smell signals.
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The team then gave the mice the drug and a month later watched as they found a hiding hole in a maze. The mice took five or six days of training before they were able to remember where the hole was and then they forgot it one week later, indicating that losing new brain cells impaired their ability to learn and remember (Nature Neuroscience, ). “Usually a mouse remembers the hole after one or two days’ training, and will still remember it a week later,” says Kageyama.
“Mice forgot where the hole was after a week, indicating that losing new brain cells impaired their ability to remember”
However, cutting the supply of new cells to the animals’ olfactory bulb did not affect the ability of the mice to discriminate between very similar smells, or to remember the smells four months later – even though the olfactory bulb did shrink. So new cells might not be as crucial in smell memory as in spatial memory.
But Kageyama says it is also possible that the mice’s abilities may degenerate 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 fetuses if they smell a strange male.
The team also created mice in which new nerve cells glow green. They plan to use them to test drugs that could be used to stimulate neurogenesis in people, where a reduced ability to produce new brain cells may account for age-related memory loss.
The Human Brain – With one hundred billion nerve cells, the complexity is mind-boggling. Learn more in our cutting edge special report.
