Fred Gage of the Salk Institute for Biological Studies and his colleagues suspected that a lack of brain signals significantly impacted a new neuron's fate. Much like the new kid in school, a recently generated neuron must make friends--form synapses--within three weeks or it will not survive. To test this theory, the researchers created a virus capable of blocking the receptor for glutamate--a chemical involved in transferring information between brain cells. When injected into mice, the virus effectively cut off the glutamate receptor--N-methyl-D-aspartate (NMDA)--in new neurons, which were also marked with fluorescent dye to ensure tracking. In the absence of signals from surrounding cells, these did not last more than a few weeks. "The NMDA receptor modulates synapse formation and determines what pattern of input activity new neurons receive, which in turn determines survival," Gage explains.
The research published online yesterday by Nature proves that life in the hippocampus is as tough as high school. "The NMDA-receptor mediated event is a competition between mature cells vying for connectivity and young [ones] competing with both the mature cells and their peers to fit in," Gage says. "You are selecting for the cells that perform best in this environment." Previous studies have shown that new neurons thrive when mice are exposed to new stimulus and, combined with this finding, suggest that new learning may be dependent on the new kid on the block rearranging the neural neighborhood.