Why are some people so sociable while others are loners or seemingly outright allergic to interactions with others?

A new study in mice by researchers at the Stanford University School of Medicine begins to provide an answer, pinpointing places and processes in the brain that promote socialization by providing pleasurable sensations when it occurs. The findings point to potential ways of helping people, such as those with autism or schizophrenia, who can be painfully averse to socializing.

The study, which was published Sept. 29 in the journal Science, details the role of a substance called oxytocin in fostering and maintaining sociability. The senior author is Robert Malenka, MD, PhD, professor and associate chair of psychiatry and behavioral science. The lead author is former postdoctoral scholar Lin Hung, PhD.

“Our study reveals news about the brain circuitry behind social reward, the positive experience you often get when you run into an old friend or meet somebody you like,” said Malenka, who has focused much of his research on an assembly of interacting nerve tracts in the brain collectively known as the reward circuitry.

“The reward circuitry is crucial to our survival because it rewards us for doing things that have, during our evolutionary history, tended to enhance our survival, our reproduction and the survival of our resulting offspring,” said Malenka, who holds the Nancy Friend Pritzker Professorship in Psychiatry and the Behavioral Sciences. “It tells us what’s good by making us feel good. When you’re hungry, food tastes great. When you’re thirsty, water is refreshing. Sex is great pretty much most of the time. Hanging out with your friends confers a survival advantage, too, by decreasing your chances of getting eaten by predators, increasing your chances of finding a mate and maybe helping you learn where food and water are.”

Because the reward system is so critical, it’s been carefully conserved over evolution and in many respects operates just the same way in mice as it does in humans, making mice good experimental models for studying it.

Far and away the most important component of the brain’s reward circuitry, Malenka said, is a nerve tract that runs from a structure deep in the brain called the ventral tegmental area to a midbrain structure called the nucleus accumbens. The ventral tegmental area houses a cluster of nerve cells, or neurons, whose projections to the nucleus accumbens secrete a substance called dopamine, altering neuronal activity in this region. Dopamine release in the nucleus accumbens can produce a wave of pleasure, telling the brain that the event going on is helpful for survival. Dopamine release in this region, and subsequent changes in activity there and in downstream neurons, also primes the brain to remember the events and the behaviors leading up to the chemical’s release.

This tract, so famous for reinforcing survival-enhancing behaviors such as eating, drinking and mating, has been infamously implicated in our vulnerability to drug addiction — a survival-threatening outcome resulting from drugs’ ability to inappropriately stimulate dopamine secretion in the tract. But understanding exactly how and under what natural conditions the firing of its dopamine-secreting nerves gets tripped off is a work in progress.

Earlier work has specifically implicated dopamine release in the nucleus accumbens in social behavior. “So, we knew reward circuitry plays a role in social interactions,” Malenka said. “What we still didn’t know — but now we do — was: How does this increased dopamine release during social interaction come about?”

It turns out that another chemical — oxytocin — is pulling the strings.

Oxytocin is sometimes called the “love hormone” because it’s thought to be involved in falling in love, mother-child bonding and sexual arousal in females, as well as lifetime pair-bonding of sexual mates am