THURSDAY, June 25, 2004 (HealthDayNews) -- A glitch in brain circuitry may help explain why people with autism often display anti-social behavior.
So says a new report by Italian and French researchers that appears in the June 26 issue of Science.
For years, researchers have suspected that the brain's opioid system, involved in behaviors related to pain, pleasure and addiction, may somehow be involved in social interactions, or the lack of them, evident in those with autism.
Now, the European researchers report that mice genetically engineered to lack specific opioid receptors on the surface of brain cells responded differently when separated from their mothers. They also didn't respond, as normal mice did, to the opioid drug morphine, which normally reduces distress.
The social indifference displayed by people with autism and other "attachment" disorders may be related to the opioid signaling system, the researchers concluded.
"Brain opiates play a fundamental role in the reward circuitry -- that is to say they help the individual to understand what is good for them, in evolutionary terms," said study co-author Francesca D'Amato, a researcher at the CNR Institute of Neuroscience, Psychology and Psychopharmacology, in Rome.
Working with the mice, the researchers focused on the brain's opioid system, partially regulated by 'u-opioid' receptors lying on the surface of cells. They wanted to see if the u-opioid receptors also played a role in infant-mother attachment behavior.
To do so, they closely observed newborn mice genetically engineered to lack the receptors, watching to see how the mouse pups responded when separated from their mothers.
According to D'Amato's team, mice without the receptors made fewer distress calls to their mothers compared with normal mice.
Furthermore, when the researchers gave mice the opioid drug morphine, it reduced the distress of normal mice -- as expected -- but appeared to have no effect on mice lacking receptors.
People with autism have trouble interacting with others and appear aloof and socially indifferent, and D'Amato's team speculates that perhaps the opioid signaling system is to blame.
The researchers believe their study gives weight to the argument that opioid receptors are a "critical player" in attachment disorders, echoing the findings of previous research.
According to the Autism Society of America, autism affects nearly 1.5 million U.S. children and adults, with signs typically appearing during the first three years of life. The disorder especially affects normal brain development in areas of social interaction and communication skills. Individuals with autism typically have problems communicating, both verbally and nonverbally, and interacting with others in general.
"I think it's a very interesting study," said Andy Shih, director of research and programs at the National Alliance for Autism Research in Princeton, N.J. "As Dr. D'Amato correctly points out, the opioid receptor system has long been suspected to play a role in attachment behavior. Using knockout mice to demonstrate that is fairly convincing evidence."
Even so, he added, "the challenge of any animal study is how does it really correlate with human behavior." More research is needed, Shih emphasized.
For more on autism, visit the National Alliance for Autism Research.