Children with autism have a surplus of synapses in the brain due to a slowdown in a normal “pruning” process during development, according to a study by neuroscientists.
The excessive synapses may have profound effects on how the brain functions because synapses are the points where neurons connect and communicate with each other.
A drug which restores normal synaptic pruning can improve autistic-like behaviours in mice, the researchers found. Autism usually appears in children and includes difficulty communicating and forming relationships and in using language.
Although the drug, rapamycin, has side effects “the fact that we can see changes in behaviour suggests that autism may still be treatable after a child is diagnosed, if we can find a better drug,” says the study’s senior investigator, David Sulzer, professor of neurobiology Columbia University Medical Center.
During normal brain development, a burst of synapse formation occurs in infancy, particularly in the cortex, a region involved in autistic behaviours.
Pruning eliminates about half of these cortical synapses by late adolescence. Synapses are known to be affected by many genes linked to autism, and some researchers have hypothesised that people with autism may have more synapses.
To test this hypothesis, the scientists examined brains from children with autism who had died from other causes. Thirteen brains came from children ages two to 9, and thirteen brains came from children ages 13 to 20. Twenty-two brains from children without autism were also examined for comparison.
“It’s the first time that anyone has looked for, and seen, a lack of pruning during development of children with autism,” says Dr Sulzer.
“Although lower numbers of synapses in some brain areas have been detected in brains from older patients and in mice with autistic-like behaviours.”
Clues to what caused the pruning defect were also found in the patients’ brains.
The autistic children’s brain cells were filled with old and damaged parts and were very deficient in a degradation pathway known as autophagy. Cells use autophagy (a term from the Greek for self-eating) to degrade their own components.
The researchers could restore normal autophagy and synaptic pruning, and reverse autistic-like behaviours in the mice, by administering rapamycin.
The study was published in the journal Neuron.
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