Introducing the brain: source of adult plasticity (Introduction)

by David Turell , Tuesday, March 14, 2023, 15:17 (407 days ago) @ David Turell

Study of filopodia:

https://www.the-scientist.com/news-opinion/silent-synapses-may-provide-plasticity-in-ad...

"The information we gather throughout the course of our lives—the quickest way to get to work, for instance, or the name of a friend’s new partner—is stored in synapses. In the adult brain, new synapses are thought to be formed from scratch as needed or through the modification of existing connections. Now, a study published November 30 in Nature unearths an abundance of ready-made ‘silent synapses’ which ripen upon neuronal stimulation.

"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.

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"In the new study, the researchers used ... brain slices of the visual cortex, then on other brain regions, in mice expressing fluorescently labeled synaptic proteins. They found that the tips of filopodia were indeed covered with AMPA-deficient synapses. Without AMPA, synapses cannot be activated, as the receptors clear magnesium ions that obstruct other receptors integral to synaptic transmission.

"They also realized that filopodia are far more widespread than they anticipated. They were found all over the brain and at levels ten times higher than previously described, making up 30 percent of the protrusions on a given dendritic branch. This suggests a similar proportion of synapses in the adult mouse brain are silent, waiting to be activated.

"To confirm that the synapses were in fact silent, the researchers released the neurotransmitter glutamate at the tips of filopodia to mimic activity in a neighboring neuron. Unlike the synapses on dendritic spines, which responded with a burst of electrical activity, the synapses on filopodia were unresponsive. When the team washed away the magnesium ions from the same filopodium—unblocking receptors typically activated by AMPA—the silent synapse discharged a babble of electricity.

“'It is a major advance to have this direct demonstration” that silent synapses are abundant in adult mice, says Yan Dong, a neuroscientist at the University of Pittsburgh who was not involved in the study.

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"The study may explain how the brain is able to learn new things without having to sacrifice existing connections, the researchers say. The ability of the brain to use different synapses “solves the plasticity versus flexibility dilemma,” says Harnett. If all the brain’s synapses are flexible, then you can’t preserve old information. But if they’re all stable, then it is difficult to learn new things, he says. Instead, the brain employs both: spiny synapses for stability and filopodia for flexibility."

Comment: if the adult brain does not produce new neurons (except in the hippocampus) so there had to be a way to keep plasticity in play.