Just as some people become stubbornly attached to routine as they get older, so too do neurons settle into a strict regimen with time. New research shows that whereas neurons in young mice continuously make and break their connections with one another, neurons in middle-aged mice tend to solidify those contacts. The results support the contention that old brains aren't terribly pliable.
Neurons talk to each other at synapses: The axon of the speaking nerve cell sends its message across this tiny gap to the listening dendrite of another cell, which then passes the message along. Neurobiologists have long wondered whether the chain of communication is fixed like old-fashioned telephone lines or if the contacts are made and broken like the transient connections between cell phones and transmission towers. Last year, two studies that examined the receiving dendrites came to opposite conclusions: One revealed stable connections in older animals, and the other indicated dynamic ones (see Science, 4 April). Neuroscientist Jeff Lichtman of Washington University in St. Louis, Missouri, and colleagues decided to look instead at the transmitting axons.
The team watched synapses in mice engineered so that a few neurons glowed with a fluorescent yellow protein. The researchers used time-lapse photography to record changes at synapses in the salivary glands over several months. When they looked for neurons lengthening and retracting their extensions over the course of minutes, they found that about 70% of cells in month-old mice wiggled their axons, but that number dropped to 10% in mice older than 4 months. And less axon wiggling meant more stable synapses: About 70% of synapses stayed put in 1.5-month-old mice during a given month, whereas 80% were stable in older animals, the researchers report online 17 August in Nature Neuroscience.
Calling the study "quite important," neuroscientist Tobias Bonhoeffer of the Max Planck Institute of Neurobiology in Martinsried, Germany, says the work addresses a "fundamental question" in neurobiology: whether neurons actively make and break connections throughout an animal's lifetime. The data support other types of studies that suggests the brain settles down with age, Bonhoeffer says, although he adds he's surprised at just how stable the older synapses appear to be.