Pleasure receptors best known for helping the body respond to morphine and opium may also hold the key to mother-child bonding, according to scientists.
Scientists have genetically engineered mice pups to lack these receptors, which are doorways into cells.
They found the mice pups were unable to properly bond to their mothers and did not show the natural distress when separated from them.
Francesca D'Amato, of the CNR Institute of Neuroscience, Psychobiology, and Psychopharmacology in Rome, and colleagues have used mice that had no mu-opioid receptor - a protein on cells vital to the brain's pain circuitry.
Normal baby mice will squeal when separated from their mothers and will squeal more if put into a strange environment.
But the mutant mice barely whimpered when this happened, the scientists report in the latest issue of the journal Science.
It was not a lack of fear - the mutated babies whimpered and squealed just as much as normal pups when they were put into a cold beaker or allowed to smell a strange male.
Mice pups are afraid of strange males because they often eat them.
Dr D'Amato and colleagues say their experiment supports a theory that pleasure and bonding are linked.
The opioid system may help babies bond with their mothers by helping them associate the smell of mum with pleasure.
The researchers say it could be that children with so-called attachment disorders, most notably seen in autism, lack properly functioning opioid systems.
They say if research shows this to be the case, the mutant mice would make good models for studying human autism and other attachment disorders.
--Reuters