A study published last week in the journal Neuron shed new evidence on differences in brain development between individuals with and without autism. The researchers from Columbia University Medical Center compared brain tissue from subjects ages 2 to 20, who had died at an early age. About half of the subjects had been diagnosed with autism.
The researchers focused on an area of the brain’s temporal lobe involved in social behavior and communication. They found that the brains of children with autism had significantly more synapses than those of children without the diagnosis. This difference was more pronounced in the brains of older children and adolescents.
Synapses are spines found on neurons that that connect with other neurons, allowing them to communicate and pass information along a neural pathway. Infant brains contain a large amount of synapses, which are gradually pruned away as the brain develops. Connections that are necessary for optimal brain and physical functioning are strengthened, while extra synapses are pruned away, like weeds in a garden. This allows neural pathways that are useful to flourish, while others are eliminated. This study suggests that something is going wrong in the pruning process when a child has autism.
The researchers found that the brain tissue of typical 19 year-olds had 41% fewer synapses than toddlers, while that of adolescents with autism had only 16% fewer. Lisa Boulanger, molecular biologist from Princeton, says,
“More is not better when it comes to synapses, for sure, and pruning is absolutely essential. If it was overgrowth, you’d expect them to be different from the start, but because the synapse difference comes on so late, it’s probably pruning.”
Ralph Axel-Muller, neuroscientist at San Diego State University, has also seen evidence of over connectivity in autistic brains in brain scan studies. He says,
“Impairments that we see in autism seem to be partly due to different parts of the brain talking too much to each other. You need to lose connections in order to develop a fine-tuned system of brain networks, because if all parts of the brain talk to all parts of the brain, all you get is noise.”
An overabundance of synapses may also explain why individuals with autism are more likely to develop seizure disorders, since increased electrical activity may lead to seizures.
The Columbia team also studied mice with tuberous sclerosis complex, a rare genetic disorder that often co-occurs with autism. They found that mTOR, a protein that is necessary for neuronal pruning, was hyperactive. When they treated the mice with rapamycin, a drug that curbed the effects of mTOR, abnormal social behaviors were reduced.
While this shows what may be a promising new treatment for individuals with autism, researchers caution that it is too early to determine whether or not rapamycin will be an effective treatment for humans. Further studies will likely shed more light on the role of over connectivity, and help researchers to discover new, effective and safe treatments.