Children with autism often show a heightened sensitivity to factors that don't usually bother others as much, such as loud talking, bright lights or a scratchy tag on a shirt.
Now Harvard and MIT neuroscientists have found what may be a critical link between autism and a key neurotransmitter that regulates inhibition and excitation in brain activity.
The study appears in a Dec. 17 paper in the journal Current Biology.
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The research team led by Caroline Robertson, a junior fellow of the Harvard Society of Fellows, used brain imaging coupled with a visual test known to trigger different reactions in the brains of people with autism and those without the disorder. The team showed that differences between the two groups were associated with a breakdown in the signaling pathway used by GABA, one of the brain's chief inhibitory neurotransmitters.
GABA plays a key role in inhibiting activity in the brain and past studies of animals with autism-like symptoms have found reduced GABA activity in the brain. Until now there has been no direct evidence for such a link in humans.
"This is the first time, in humans, that a neurotransmitter in the brain has been linked to autistic behavior - full stop," Robertson said in a Harvard University news release.
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"This theory that the GABA signaling pathway plays a role in autism has been shown in animal models, but until now we never had evidence for it actually causing autistic differences in humans."
The tests suggested that while GABA is present in the brains of autistic people, its ability to inhibit may be compromised. This malfunction could also be tied to seizures, another common symptom of autism.
"Autism is often described as a disorder in which all the sensory input comes flooding in at once, so the idea that an inhibitory neurotransmitter was important fit with the clinical observations," Robertson said. "In addition, people with autism often have seizures – there is a 20 to 25 percent co-morbidity between autism and epilepsy – and we think seizures are runaway excitation in the brain."
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While the finding is exciting and could help lead to a better understanding and perhaps better treatment for autism, experts caution it is a highly complicated condition and this is just one piece of a vast puzzle.
"I'm excited about this study, but there are many other molecules in the brain, and many of them may be associated with autism in some form," Robertson said.