At first, they gingerly sniff out pre-existing connections between brain cells but as the new neurons mature over time they get emboldened and muscle out the old guys. "Adding new neurons could be a very problematic process if newborn cells would make connections all over the place," explains Fred H. Gage, Ph.D., a professor in the Gene Expression Laboratory and the Vi and John Adler Chair for Research on Age-Related Neurodegenerative Diseases. "But if they are only replacing already existing connections there is less chance of error," he adds.
Neurons make contact via specialized structures called synapses. As a signal traveling along a nerve branch arrives at the pre-synaptic area, it releases a chemical signal. The signaling molecules travel across the synapse and induce a signal on the neighboring, receiving nerve fiber or dendrite. A typical neuron sports about 7,000 synapses through which it stays in touch with roughly 1,000 other cells. But just how young neurons make their presence known and hook up with already well-connected elders has been unclear.
"If you have hopes that one day neuronal stem cells can replace damaged neurons in neurodegenerative diseases such as Alzheimer's or Parkinson's disease you have to ensure that these cells make proper connections, form functional synapses and integrate into the rest of the brain," says postdoctoral fellow Nicolas Toni, Ph.D., who headed the current study.
To figure out how the newcomers do it, the Salk researchers injected a virus carrying the gene for green fluorescent protein