Newborn neurons join up in memory networks. The brain generates new neurons throughout life in the memory nexus known as the hippocampus. Some research hints that these young cells play key roles in memory formation. In the March issue of Nature Neuroscience, scientists at the University of Toronto and Universidade do Proto, Portugal, provide the first visual evidence that newborn neurons take their places in memory-forming circuits.
(For a full discussion of neurogenesis and memory, see “New Neurons in the Adult Brain: What’s the Point?” in the January/February issue of BrainWork, online at www.dana.org/books/press/brainwork/bw_0207_neurons.html.)
The investigators injected mice with bromodioxyuridine (BrdU), a green stain that fastens onto young neurons, and trained the animals in an underwater maze one, two, four, six, or eight weeks later. Then they stained the hippocampal tissue again, this time for a gene called Fos, a marker of neuronal activity.
“We reasoned that if the green cells expressed Fos more than other neurons did, it would indicate that the younger cells were being incorporated into memory networks,” says study author Paul Frankland of the Hospital for Sick Children, Toronto.
The prediction was correct. Not only were newborn neurons more active than their mature counterparts (cells that did not take up the green stain), but by the time the young cells were four to six weeks old they expressed Fos at nearly twice the rate of older cells—meaning that they were far more likely to contribute to memory formation.
In addition to supporting the belief that new neurons make a unique contribution to memory, the finding also established the peak time that these cells do their job.