In a new research, Harvard scientists for the first time have found link between a specific neurotransmitter in the brain and autistic behavior. The discovery is significant as it is likely to offer valuable new insights into understanding, diagnosing and even treating autism. The details of the study were published in a recent paper in Current Biology.
For this study, the research team led by Caroline Robertson, a Junior Fellow of the Harvard Society of Fellows used a visual test known to prompt different reactions in autistic and normal brains. The test results show that those differences are associated with a breakdown in the signaling pathway used by GABA, one of the brain’s chief inhibitory neurotransmitters.
Robertson remarked that it is the first instance when a neurotransmitter in the brain has been linked to autistic behavior in humans. In animal models it has been shown that the GABA signaling pathway plays a role in autism, but there was no evidence that it can cause autistic differences in humans.
Autism is a disorder characterized by a condition when all the sensory input comes flooding in at once. Hence, the researchers thought that an inhibitory neurotransmitter was important fit with the clinical observations. Also, autistic people suffer from seizures – there is 20 to 25 percent co-morbidity between autism and epilepsy – and it was suspected seizures are runaway excitation in the brain.
In order to find that evidence, Robertson and colleagues chose the binocular rivalry as it is an easily replicable test that produced consistently different results in those with and without autism. Typically, when seen from each eye the brain has two slightly different images an average is created out of the two for the single image we see every day. But, the binocular rivalry test forces each eye is take in very different images, with surprising results.
The end result is that one image is just suppressed entirely from visual awareness for a short period remarked Robertson. However, eventually the neurons that are forcing that inhibitory signal get tired, and it will switch until only the first image is seen. As that process repeats, the two images will rock back and forth. She explained that an average normal person might rock back and forth between the two images every three seconds; an autistic person might take twice as long. It takes them longer to switch between the two images.
As a part of the study, a brain imaging technique called magnetic resonance spectroscopy was applied to measure the levels of certain neurotransmitters in the brain. The researchers found that autistic people show normal levels of excitatory neurotransmitters, but their GABA was far lower than expected.
Robertson said that while it is not that there’s no GABA in the brain, but there’s some step along that pathway that’s broken. Fixing that pathway is the challenge. To explain further she added that there are two forms of GABA receptors, A and B, and the GABA A receptor can take multiple forms. This test may be used to look at the effectiveness of drugs to give a better idea about which of those receptors isn’t working properly, but it’s very complex.
Right now there is no way to diagnose autism in kids who cannot speak. However, before children talk, they see. So, this type of visual task can be used to screen children and see if there’s something imbalanced in their brain. Robertson cautioned that understanding the signaling pathway for GABA won’t be a cure-all for autism. There are many other molecules in the brain, and many of them can play a role in autism in some way. While the scope of this study is to look at the GABA story, but to screen the autistic brain, other possible pathways also need to be explored.