To show that learning changes brain circuitry, scientists have relied on indirect evidence—before and after counts of the number of synapses in particular regions. To nab direct evidence of neural remodeling, Yukiko Goda (University of California, San Diego, CA) and colleagues used light to trigger a current through coupled neurons resting on a silicon chip. Then they observed the response of the actin cytoskeleton of the presynaptic and postsynaptic terminals, and were able to detect changes in individual synapses.
Repeated stimulation triggered action on both sides of the synapse. On the presynaptic side, actin networks changed shape to form projections, or puncta, some of which developed into functional presynaptic terminals with vesicles for recycling neurotransmitters. The postsynaptic side also sent out extensions that cozied up to the puncta that formed on the presynaptic side, thus completing the new synapses. This is the first direct demonstration of neural remodeling of brain synapses at the cellular level, says Goda. “We have a very conclusive demonstration of activity-induced remodeling,” she says. “We were able to capture it as it happened.” One of the key mysteries left to solve, she says, is how the electrical signal gets translated into morphological change. ▪