After decades of neglect by researchers more interested in know-it-all neurons, the brain cells classified as "glia" are getting some respect. They've been written off as support scaffolding for neurons or as caterers that provide nutrition. But now researchers have found that glia play an important role in setting up neural networks: They tell neurons to start talking to one another.
Neurons send and receive messages through connections called synapses, points of near-contact where neurons swap chemical signals. The first indication that glia boost synaptic communication came in 1997, when a team led by neurobiologist Ben Barres of Stanford University reported that neurons grown near glial cells called astrocytes were 10 times as responsive as neurons grown alone. They just didn't know why.
To find out, Barres's team, led by postdoc Erik Ullian, conducted dozens of experiments on neurons grown with glia or alone. The reason for the increased synaptic activity with glia became apparent when the researchers stained various proteins that neurons use to build synapses. Neurons exposed to glia had clustered these proteins into seven times as many synapses as did the isolated neurons. The team confirmed this count by tallying neurons under the microscope, where they saw that the synapses looked the same in both sets of neurons, they report in the 26 January issue of Science. The isolated neurons appear to contain all the building blocks necessary to build synapses, the researchers say, but they don't start construction until getting the go-ahead from glia.
No one expected glia to determine the number of synapses on a neuron, says Barres, but as neurobiologist Charles Stevens of the Salk Institute for Biological Studies in La Jolla, California, points out, the result fits well with the earlier report that glia increase synaptic activity. "Once you know it's true," Stevens says, "you believe it instantly." He suspects the research might one day be used in memory research. Researchers are convinced that the act of learning forges stronger synaptic connections, and it's possible that similar glial signals are involved.
The Barres lab