Vanderbilt scientists report that a disruption in serotonin transmission in the brain may be a contributing factor for autism spectrum disorder (ASD) and other behavioral conditions.
Serotonin is a brain chemical neurotransmitter that carries signals across the synapse, or gap between nerve cells. The supply of serotonin is regulated by the serotonin transporter (SERT).
Prior research discovered rare genetic variations can disrupt SERT function in children with ASD. In the new study, published in the Proceedings of the National Academy of Sciences, researchers report the creation of a mouse model that expressed and thereby confirmed the most common of these variations.
Researchers say a small biochemical change appears to cause SERT in the brain to go into “overdrive” and restrict the availability of serotonin at synapses.
“The SERT protein in the brain of our mice appears to exhibit the exaggerated function and lack of regulation we saw using cell models,” said Randy Blakely, Ph.D., director of the Vanderbilt Silvio O. Conte Center for Neuroscience Research.
“Remarkably, these mice show changes in social behavior and communication from early life that may parallel aspects of ASD,” noted first author Jeremy Veenstra-VanderWeele, M.D. The researchers conclude that a lack of serotonin during development may lead to long-standing changes in the way the brain is “wired.”
The association between autism and high levels of blood serotonin has been acknowledged for over 50 years. Researchers believe that as many as 30 percent of children with autism have elevated blood levels of serotonin, a finding described as “hyperserotonemia.”
Hyperserotonemia is the most consistently reported biochemical finding in autism, and is a highly inherited trait. Yet, the cause or significance of this “biomarker” has remained shrouded in mystery.
The current study unravels the puzzle by showing that a particular variant of a human SERT gene associated with autism, when inserted in mice, can produce hyperserotonemia in the mice.
The genetic variant makes the transporter more active, causing higher levels of serotonin to accumulate in platelets and therefore in the bloodstream. In the brain, overactive transporters should have the opposite effect — lowering serotonin levels at the synapse and producing behavioral changes relevant to autism. That’s exactly what the researchers observed.
Researchers admit that a mouse model cannot completely explain or reproduce the human condition. Neither does a single genetic variation cause autism. Experts believe the wide spectrum of autistic behaviors represents a complex web of interactions between many genes and environmental factors.
Nevertheless, animal models are critical to determine the source for developmental changes that are observed in ASD. In this case, researchers use mice to explore how altered brain serotonin levels during development may produce long-lasting changes in behavior and impact the risk for autism.
Source: Vanderbilt University