Morphine acts on a part of the brain known as the opioid system, which is linked to pain, pleasure and addictive behaviors. The results of a mouse study published in today's issue of the journal Science suggest that the same brain circuitry plays a role in mother-infant bonding.

Previous animal research had implicated the opioid system in the forming of lasting attachments but exactly how it did this was unclear. In the new work, Francesca R. D'Amato of the CNR Institute of Neuroscience, Psychobiology and Psychopharmacology in Rome and her colleagues studied so-called knockout mice that lacked u-opioid receptors. The team observed newborn pups that had been separated from their mothers and found that mice lacking the receptors made fewer distress calls when they were abandoned than normal mice did. When the control animals were given morphine they calmed down, but the drug had no effect on the knockout mice. The researchers also gave the animals a choice between two cages, one they had been exposed to before and one that was unfamiliar. Whereas the normal mice all chose the familiar surroundings, two thirds of the knockout animals seemed content to bed down in a strange mother's nest.

The results suggest that u-opioid receptors are critical players in attachment disorders, the scientists note. In addition, they lend further support to the theory that malfunction of the opioid system could be to blame for the social indifference seen in autistic infants.