Grownup mouse brains are filled with ‘silent synapses’

Learning large amounts of new information in infancy requires a large number of immature connections between readily available nerve cells for rapid memory formation. Known as silent synapses, these connections are inactive until called upon to help create memories, and are thought to exist primarily in the developing brain and disappear over time.But a new study shows that there are Many silent synapses in adult mouse brainthe researchers reported on Nov. 30 nature.

Neuroscientists have long wondered how the adult brain can have stable, long-term memories while maintaining Some flexibility, ability to create new memoriesa concept called plasticity (Serial Number: 7/27/12). These silent synapses may be part of the answer, says neuroscientist Jesper Sjöström of McGill University in Montreal, who was not involved in the study.

“Silent synapses are ready to connect,” he said, suggesting that by using those connections, rather than having to overwrite or destroy mature synapses already connected to memories, it might make it easier to store new memories in adulthood. “This means that there is much more room for plasticity in the mature brain than we previously thought.”

In a previous study, MIT neuroscientist Mark Harnett and colleagues Discovery of many long rod-like structures called filopodia in the brains of adult mice. This surprised Harnett because these protrusions are found primarily on nerve cells in the developing brain.

“Here, they’re present in adult animals, and we can see them clearly,” Harnett said. So he and his team decided to examine filopodia to see what role they play and whether they might be silent synapses.

The researchers used a technique to enlarge the brains of adult mice combined with high-resolution microscopy. Because nerve cell connections and the molecules called receptors that allow communication between connected cells are so small, these methods reveal synapses that past studies had missed.

The team looked for the classic sign of silent synapses: the presence of a receptor called NMDA and the absence of others, called AMPA receptors. Both types of receptors respond to the chemical messenger glutamate, but normally both need to be present for synapses to be active.

Of the more than 2,000 synapses the team studied, about 30 percent were filopodia, and almost all of them had features that suggested they might be silent synapses.

To test whether these connections are truly silent, the researchers turned to glutamate. The team found that artificially adding chemical messengers was not enough to activate synapses, suggesting that these connections are virtually silent.

Adding an electric current in addition to glutamate transformed these connections from immature to mature synapses. This is also what happens in the developing brain when a new memory is formed from a silent synapse.

It’s unclear whether silent synapses are also common in adult brains, but other scientists, including Harnett and Sjöström, think it’s likely. Researchers are now using the same technique on the human brain to find out.

Finding silent synapses in the adult brain could have implications for treating diseases like drug addiction.A study of rats given cocaine showed that Drug use creates more silent synapseswhich might come into play with quit symptom. If scientists can develop a way to control the number of silent synapses, they may be able to target conditions that exhibit abnormal levels of silent synapses.

What’s clear is that silent synapses may answer how the adult brain strikes a balance between keeping old memories and creating new ones, Harnett said. With this discovery, “all of a sudden, it’s a lot easier to figure out this trade-off.”