Just what is it about autism that produces the three hallmark behaviors of social impairment, language difficulties, and rigidity, or an "insistence on sameness'? Scientists at this year's Keystone meeting on the pathophysiology of autism in Santa Fe, NM, are looking for clues from a molecule we hear an awful lot about in discussions of non-autistic brain activity: Serotonin.

It turns out that a significant number of children with autism -- up to 30% -- have elevated levels of serotonin in their blood. The vast majority of that is stored in platelets, which carry a serotonin transporter, also known as SERT, which brings in serotonin from the gut. Not surprisingly, some doctors have found that selective serotonin reuptake inhibitors (SSRIs), the drugs-of-choice for depression and related symptoms, help treat some aspects of autism, such as anxiety and compulsive behaviors.

Randy Blakely from Vanderbilt University School of Medicine in Nashville did some digging in the gene banks from affected families, and found multiple gain-of-function alleles at the SERT locus on chromosome 17. When he and his team knocked out SERT in mice, they saw changes in cortical development, adding more evidence that brain disruptions in autism could stem, in part, from changes in SERT.

When the team looked more closely at the SERT variants and different behaviors in autism, they found that variants in this transporter were mostly linked to rigid and compulsive behaviors - that "insistence on sameness" domain. (Children with these behaviors typically repeat the same words, need rituals, and get attached to objects.) These findings suggest that serotonin plays a strong role in this aspect of the disorder, Blakely said. "This is a domain serotonin has more control over than other domains."