Common brain cell plays key role in shaping neural circuit

Scientists have discovered that a common but mysterious class of brain cells play a crucial role in shaping neural circuit.
Neuroscientists from the Stanford University School of Medicine show that these cells, called astrocytes, actively refine nerve-cell circuits by selectively eliminating synapses - contact points through which nerve cells, or neurons, convey impulses to one another.
"This was an entirely unknown function of astrocytes," said Ben Barres, professor and chair of neurobiology and the study's senior author.
More than one-third of all the cells in the human brain are astrocytes. But until quite recently, their role in the brain has remained obscure.

The study was performed on brain tissue from mice, but it is likely to apply to people as well, Barres said.
The discovery adds to a growing body of evidence that substantial remodelling of brain circuits takes place in the adult brain and that astrocytes are master sculptors of its constantly evolving synaptic architecture.

The findings also raise the question of whether deficits and excesses in this astrocytic function could underlie, respectively, the loss of this remodelling capacity in old age or the wholesale destruction of synapses that erupts in neurodegenerative disorders, such as Alzheimer's and Parkinson's disease.
"Astrocytes are in the driver's seat when it comes to synapse formation, function and elimination," Barres said.

In previous studies, he and his colleagues have shown that astrocytes play a critical role in determining exactly where and when new synapses are generated.
The new study showed that astrocytes' synapse-gobbling behaviour persists into adulthood and is triggered by activity in the neurons.
This suggests astrocytes may be central to the constant fine-tuning and reconfiguring of brain circuits occurring throughout our lives in response to experiences such as learning, recollection, emotion and motion.
While a healthy brain's neurons remain intact for much a person's lifetime, the connections between them - the synapses - are constantly forming, strengthening, weakening or dying.

The study will be published in the journal Nature.