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Excess of synapses in the brain may contribute to development of autism

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Many children and adults diagnosed with autism spectrum disorder (ASD) experience a hyper-sensitivity to sensory input from the environment, such as light or sounds, and to social situations. A new study published Thursday in the journal Neuron may point to a neurological origin for such symptoms. Scientists from Columbia University have discovered that children diagnosed with autism have an excess number of neurons and synapses in the brain, possibly leading to many of the hallmark symptoms of autism, including over-sensitivity to sensory stimuli.

During a child's development, an abundance of neurons and synapses, or connections between neurons, in the brain are formed. This typically leads to more neurons than are functionally needed. As a child grows into adolescence, the brain engages in a process known as pruning which curbs the number of extra neurons and synapses present in the brain, specializing them into different functions, thereby increasing the overall efficiency of the brain. This process continues until a person reaches sexual maturity.

The researchers in the study analyzed 13 brains age two to nine diagnosed with autism, 13 brains age 13 to 20 diagnosed with autism and 20 undiagnosed brains, specifically looking at areas associated with communication and socialization. They found that the autistic brains contained drastically more synaptic density than the typical brains. Specifically, the densities of the typical brains declined by 41% between the first and second decades of life, due to pruning, whereas the densities of the ASD brains only reduced by 16%.

This indicates a stark deficit in the pruning mechanism of those diagnosed on the autism spectrum, rather than an overproduction of neurons. It may also point to why children are diagnosed with autism at approximately two to three years of age, as this is when the pruning process begins to take shape, rather than due to other factors such as vaccines.

Furthermore, the scientists identified a specific protein, mTOR, in mice which inhibits the pruning mechanism when active. They then tested a drug, rapamycin, to inhibit the protein and observed that they were able to restore the brain's pruning ability even after symptoms had already appeared. When the drug was not effective on mice with pruning deficits, behaviors did not improve. This confirms the link between lack of pruning and autism symptoms, although the scientists noted that human treatments with the drug are a long way off.

This study, in concert with those investigating genetic links to autism, are all mounting evidence confirming the neurological origins of autism. Identifying genes linked to pruning deficiencies may lead to earlier diagnosis, earlier intervention and more promising outcomes for those diagnosed on the autism spectrum.