Listening to Gage, one gets the impression that the hippocampus is a dangerous place for a fledgling neuron trying to elbow its way into pre-existing networks. "It's rough in there!" he concedes. "The NMDA receptor-mediated event is a competition between mature cells vying for connectivity and young guys competing with both the mature cells and their peers to fit in. You are selecting for the cell that performs best in this environment."
The Gage lab previously showed that the rate at which new neurons emerge from stem cells depends on an animal's activity. "If you put animals in an enriched environment and give them access to running wheels, you increase survival of new brain cells," says Gage. "Now we show that stimulation may, in part, be mediated through the NMDA receptor."
Those studies had also shown that young and middle-aged "exercised" rats perform better on learning tasks such as maze swimming, indicating that new neurons are more than just a backup supply but actually enhance learning.
"Remarkably, new neurons are born in the hippocampus, a structure whose function is to acquire new information," says Gage. "That suggests that new cells are involved in how we learn."
This ongoing struggle for connections between young and mature neurons is apparently more than just a spectacle designed to keep Mother Nature amused: the fact that enhanced learning is correlated with adult neurogenesis suggests constant rearrangements within neural networks are absolutely necessary for learning to occur.
In fact, data emerging from studies in the Gage lab reinforces the commonly held belief that using one's brain cells is the best way to optimize brain funct