It's a general belief that the circuitry of young
brains has robust flexibility but eventually gets
"hard-wired" in adulthood. As Johns Hopkins researchers and
their colleagues report in the Nov. 8
issue of Neuron, however, adult neurons aren't quite
as rigidly glued in place as we suspect.
The investigators, led by David Linden, professor of
neuroscience in the School of Medicine,
took advantage of a new technique known as two-photon
microscopy that lets them literally see living
neurons going about their business in the intact brain. The
researchers injected fluorescent dye into
the brains of mice to light up a subset of neurons and then
viewed these neurons through a window
constructed in the skull of living, anesthetized mice.
They examined neurons that extend fibers (called
axons) to send signals to a brain region called
the cerebellum, which helps coordinate movements and
sensory information. Like a growing tree, these
axons have a primary trunk that runs upward and several
smaller branches that sprout out to the
sides.
But while the main trunk was firmly connected to other
target neurons in the cerebellum,
stationary as adult axons are generally thought to be, "the
side branches swayed like kite tails in the
wind," Linden said. Over the course of a few hours,
individual side branches would elongate, retract
and morph in a highly dynamic fashion. These side branches
also failed to make conventional
connections, or synapses, with adjacent neurons.
Furthermore, when a drug was given that produced
strong electrical currents in the axons, the motion of the
side branches stalled.
Why the brain would want such motile, nonconnected
branches is the next mystery to tackle.
Linden said he thinks they may present a second mechanism
for conveying information beyond
traditional synapses or assist in nerve regeneration,
quickly forming synapses should nearby nerves get
damaged. "The ability to make time-lapse movies of axons in
the living brain gives us a powerful tool to
explore axon regeneration that underlies neural recovery
following stroke or other brain trauma,"
Linden said.
The research was funded by the National Institute of
Mental Health; Ministry of Education,
Culture, Sports, Science and Technology of Japan; and Japan
Society for the Promotion of Science.
Authors on the paper are Masahiro Fukaya and Masahiko
Watanabe, all of Hokkaido University
School of Medicine; and Hiroshi Nishiyama and Linden, both
of Johns Hopkins.
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