Beyond unraveling the biology of that most basic of bonds, the work also may offer new leads to better understand autism, a disorder characterized by poor social attachment, scientists from Italy's National Research Center report.
At issue is the brain's opioid system, best known for its role in pain, pleasure and addiction. Opioid drugs like morphine act on that system to block physical pain.
But there have long been clues that the pathway plays a role in some basic emotional pain, too, because giving morphine to animals can decrease their social behaviors.
Thus, one theory behind autism's symptom of social indifference is that the brain might be incapable of forming strong social bonds without feedback from its opioid reward system -- like the pleasure a baby should feel from loving parental care.
To test that, Italian neuroscientist Francesca D'Amato and colleagues bred mice to lack a crucial opioid receptor in the brain, and compared them to normal baby mice.
First, they separated the newborns from their mothers for short periods. Normally that sparks nonstop shrieking from the babies. But the opioid-deficient pups hardly cried, the researchers report in Friday's edition of the journal Science.
And while normal mice babies always chose the nest built by their own mother over another mother's bed, only about a third of the opioid-deficient babies did.
Both types of mice could smell equally well, a key way the pups tell where they are and who's around. Nor were there differences in crying when the babies smelled threatening male mice nearby or were exposed to cold temperatures -- suggesting the opioid defect harmed only social bonding.
"It's a nifty little paper" that suggests an opioid role in attachment, said Dr. James T. Winslow, a neuroscientist at the National Institute of Mental Health.
But it's not proof, he cautioned. For example, the study doesn't show if the genetically altered pups cry less simply because of temperament, that they're more laid-back.
Previous studies of the opioid pathway and attachment yielded mixed results. Now, many scientists researching attachment disorders like autism focus more on other brain chemicals involved in social behavior, such as vasopressin and oxytocin.
The new study reintroduces the opioid concept, said Winslow -- adding that it would be fairly easy to test whether autism patients harbor abnormalities in opioid-receptor genes.
By Lauran Neergaard