No fuzzy feeling. Mouse pups lacking a specific opioid receptor don't cry when separated from their mother.

Mice That Don't Miss Mom

Blind, deaf, and hungry, a newborn mouse can't care for itself. Take away its mother, and a pup will scream bloody murder to get help. But if the neuronal receptors that respond to morphine are also taken away, the pup just doesn't seem to care. The finding may shed some light on roots of emotional attachment disorders, as well as drug abuse.

The research, reported in the 25 June issue of Science, supports pharmacological evidence from a variety of animals that the opioid reward system helps wee ones bond with others. Opioids are best known as painkillers. They have other effects, too; morphine will turn the shrieks of lonely baby guinea pigs into whimpers.

Francesca D'Amato of the CNR Institute of Neuroscience, Psychobiology, and Psychopharmacology in Rome and colleagues tested infant attachment in mouse pups born to parents genetically designed to lack both copies of the μ-opioid receptor gene. First, the researchers removed mom from the living quarters and, 5 minutes later, subjected the pups to a new environment. Normal 8-day-old pups screamed incessantly when placed into a beaker with clean bedding; they screamed about half as much when the beaker contained old fluff that smelled like mom. The mutant mice, though, hardly screamed at all. The lack of screeching was not due to an inability to smell or react to adverse circumstances: When threatening males were near, the mutant pups squealed even more than the normal pups, and all pups freaked comparably when placed in a frigid beaker. In addition, the mutant pups weren't able to discriminate between moms--when given a choice between their own place or a strange mom's nest, all of the normal pups chose their own place. But only a third of the pups lacking the μ-opioid receptor went home.

Although the work is "compelling evidence" that opioids play a role in maternal reward, by no means are they the whole story, says neuroscientist Jaak Panksepp of Bowling Green State University in Ohio. The authors suggest extending this research to look at whether opioids contribute to attachment disorders such as autism.

Panksepp suggests that the finding may explain the prevalence of opioid drug abuse among youth: "The biggest societal bottom line of this research is ... the fact that our young people can feel the warmth of human love pharmacologically, if they are not getting enough of it from their social networks." That thought may leave you crying for mom.