Discovery of Silent Synapses May explain and Provide brain Plasticity in Adulthood

Amazing stuff! This could be a huge discovery!

"... Silent synapses are otherwise complete neuronal connections that lack a key signaling protein—AMPA receptors—that renders them inactive. They were thought to be unique to early development, as previous work found that the silent connections vanish by the time a mouse has reached adulthood. But researchers may have been looking in the wrong place. In young animals, silent synapses are formed from larger protrusions called dendritic spines. But in adults, they can be found on the ends of threadlike structures called filopodia, according to the new study. ..."

From the abstract:

"Newly generated excitatory synapses in the mammalian cortex lack sufficient AMPA-type glutamate receptors to mediate neurotransmission, resulting in functionally silent synapses that require activity-dependent plasticity to mature. Silent synapses are abundant in early development, during which they mediate circuit formation and refinement, but they are thought to be scarce in adulthood. However, adults retain a capacity for neural plasticity and flexible learning that suggests that the formation of new connections is still prevalent. Here we used super-resolution protein imaging to visualize synaptic proteins at 2,234 synapses from layer 5 pyramidal neurons in the primary visual cortex of adult mice. Unexpectedly, about 25% of these synapses lack AMPA receptors. These putative silent synapses were located at the tips of thin dendritic protrusions, known as filopodia, which were more abundant by an order of magnitude than previously believed (comprising about 30% of all dendritic protrusions). Physiological experiments revealed that filopodia do indeed lack AMPA-receptor-mediated transmission, but they exhibit NMDA-receptor-mediated synaptic transmission. We further showed that functionally silent synapses on filopodia can be unsilenced through Hebbian plasticity, recruiting new active connections into a neuron’s input matrix. These results challenge the model that functional connectivity is largely fixed in the adult cortex and demonstrate a new mechanism for flexible control of synaptic wiring that expands the learning capabilities of the mature brain."

Silent Synapses May Provide Plasticity in Adulthood | The Scientist Magazine® Inactive synapses—found on tiny neuronal projections called filopodia—are abundant in the adult brain, a mouse study suggests.

Filopodia are a structural substrate for silent synapses in adult neocortex (no public access, but article above contains link to PDF file)