By Robert Preidt, HealthDay Reporter

THURSDAY, Dec. 17, 2015 (HealthDay News) -- Scientists say they've discovered a specific chemical in the brain they believe is linked to autism.

The Harvard University researchers found that autistic behavior is associated with a breakdown in the signaling pathway used by a major inhibitory neurotransmitter called GABA.

"This is the first time, in humans, that a neurotransmitter in the brain has been linked to autistic behavior. 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," study leader Caroline Robertson said in a university news release.

The study was published Dec. 17 in the journal Current Biology.

In the study, Robertson's team used brain imaging plus a visual test known to trigger different reactions in the brains of people with autism and those without the disorder. They believe that similar tests could be used to screen young children for autism.

While GABA has long been suspected of being a factor in autism, there was no proof until now, the researchers said.

"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 comorbidity [when two conditions are present in a patient] between autism and epilepsy -- and we think seizures are runaway excitation in the brain," she added.

While the finding improves understanding of autism and could lead to new treatments that target the GABA pathway, it is only one piece of the autism puzzle, the researchers said.

"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. "We were looking at the GABA story, but we're not done screening the autistic brain for other possible pathways that may play a role. But this is one, and we feel good about this one."

Two autism experts were cautiously optimistic about the findings.

"The authors make extraordinary claims about the role of GABA in autism," said Alycia Halladay, chief science officer for the Autism Science Foundation. "However, if this theory holds true in other independent studies, it might lead to new ways to help some of the symptoms of autism spectrum disorders."

Dr. Matthew Lorber directs child and adolescent psychiatry at Lenox Hill Hospital in New York City. He noted that "the causes of autism have remained a mystery with many disproven theories since the late 1930s. There has been a lot of recent discussion about the role GABA plays, and this most recent study is showing less GABA activity in the brain of humans with autism."

He said the new research may be an added piece to the puzzle, but the overall origins of autism remain elusive.

"Although we do not have an answer to the actual cause of this disease, it is further evidence that the mystery of autism may have connections to the neurotransmitter GABA, and should motivate further research into this part of the scientific mystery," Lorber said.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more about autism.