B ored dendritic spines look for new challenges, say David Richards, R. Anne McKinney (in work done at the University of Zurich, Switzerland), and colleagues.
Spines are small dendritic protrusions on which excitatory synapses connect to axons. Recent evidence suggested that spines are mobile even in adults. Richards et al. now suggest that this mobility allows for post-developmental synaptic rewiring.
The group found that mobile spines formed filopodium-like protrusions extending toward neighboring axons. The protrusions are a response to glutamate in situations when the spine is receiving little from its own axonal partner. Small amounts of glutamate caused the protrusions to extend toward the glutamate source. A large dose, however, repressed protrusion formation for up to 20 min. This may be due to the strong influx of calcium in excited synapses, which is known to inhibit cytoskeletal changes.
The prevalence of protrusions in inactive spines might allow them to seek out more active presynaptic partners. “If activity is very low, a spine gets restless,” says Richards. “But if it is close enough to another presynaptic terminal, some of [that terminal's] glutamate can diffuse and weakly activate the spine. Now it's found a [new] potential source of glutamate, so it heads in that direction.” This rewiring might explain how stroke sufferers are able to recover certain neurological functions.