With the help of a whole arsenal of high-tech imaging technology and the electron tomography expertise of Mark. H. Ellisman, Ph.D., a professor at the National Center for Microscopy and Imaging Research at the University of California, San Diego, Toni then zoomed in at a nanometer scale and watched how the young and the old got acquainted.
He observed that between three and four weeks after injection of the virus newborn neurons sent out dendritic filopodia—tiny feelers that probe the environment. "When we analyzed them in three dimensions, the tip of the filopodia was preferentially associated with synapses already connected to other neurons," explains Toni.
However, as the new neurons matured, the tiny tips filled out and started to monopolize the synaptic connections. "That's what we believe is the crux of the study: the survival of new neurons may depend on the ability to compete out the older existing neurons," says Gage. Earlier studies had shown that if young neurons fail to receive signals from other brain cells they wither and die. By connecting to functional synapses, the newborn neurons ensure that they are not reaching out to deadbeats.
The Gage lab previously identified a subunit of the NMDA receptor, a protein complex that transduces signals sent by neighboring cells, as the newborn neurons' life-saving equipment. The NMDA receptor is activated by the neurotransmitter glutamate, a chemical released by neurons in order to transmit information to neighboring cells. Whenever the receptor picks up a glutamate signal it is stimulated and relays the signal. For young neurons that signal means survival.
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