Brain receptor shapes voles' family values.

The sexual and parental behavior of a species of wild rodents called mountain voles makes Murphy Brown look like a model mother.

Mountain, or "montane," voles live in isolated burrows and avoid other voles except to mate-which they do often and indiscriminately. Female montane voles usually abandon their pups soon after birth, and male montane voles never even see their offspring. Not that the pups themselves seem to mind: When a montane vole pup is plucked from its nest, it neither calls for its mother nor experiences a surge in stress-related hormones.

Two behavioral neuroscientists have now uncovered a clue that could explain this lack of family values. Thomas R. Insel and Lawrence E. Shapiro of the National Institute of Mental Health's facility in Poolesville, Md., have found that promiscuous montane voles have fewer receptors for the reproductive hormone oxytocin in key areas of their brains than do their monogamous, family-oriented cousins, the prairie voles.

The researchers say their finding - published in the July 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES - could have implications for human sexual and parental behavior. They say it could also shed light on the causes of autism, a brain disorder characterized by the diminished ability to interact and form relationships with other people.

Insel and Shapiro began studying voles after discovering the importance of oxytocin to the reproductive behavior of mice and rats. They turned to voles to confirm that their earlier findings were real, not artifacts caused by several generations of life in a laboratory.

The researchers maintained newborn voles of both species until the age of 3 months. They found that the voles' captive behavior mimicked that in the wild: The montane voles fought off other voles, while the prairie voles usually sat side by side with a mate.

When Insel and Shapiro stained brain slices taken from both groups of voles with radioactively labeled oxytocin, they found that prairie voles had three times the amount of oxytocin receptors in their prelimbic cortex - and seven times the amount of such receptors in their nucleus accumbens - as found in montane voles. The researchers also discovered similar differences between the brains of two other vole species - monogamous pine voles and polygamous meadow voles.

Moreover, they found that the concentration of oxytocin receptors in the brains of montane voles surged during the brief period following birth when the females cared for and nursed their young. In contrast, concentrations of receptors for two other brain chemicals important in social behavior remained constant among all of the species. And all of the vole species had roughly the same concentrations of the oxytocin hormone itself.

"This is evidence that oxytocin receptors may be very important for the social, 'affiliative' behaviors that make animals receptive to social attachments," Insel concludes. However, he adds, researchers. have not yet determined the exact functions of the brain regions he and Shapiro studied, although the areas are thought to play a role in reproduction.

Insel says a team of Swedish researchers recently reported that human forebrians bear a large concentration of oxytocin receptors. "The question now is whether the level of that receptor changes over time," he asserts. "Is it different before than after puberty? Does it vary in women at different times of the month?" He and Shapiro next plan to measure oxytocin receptor concentrations in the autopsied brians of autistic individuals to see if lower amounts might explain the social isolation of autism.

Cort Pedersen, a behavioral neuroscientist at the University of North Carolina in Chapel Hill, says the new study is "very convincing" that oxytocin receptors shape secual and parental behavior in rodents. He adds that the autism link "is an interesting and potentially clinically relevant idea."