Oxytocin is sometimes referred to as "the love hormone," but perhaps a better term might be "the social hormone." A new mouse study suggests that oxytocin may play a role in the warm, fuzzy feelings we get when we hang out with friends, and could also hold the key to treating the socialization issues that come with conditions like autism spectrum disorders or schizophrenia.

How the reward circuitry in the brain drives various human and animal behaviors is a hot topic in neuroscience. And oxytocin, with its ties to sex, bonding and anxiety, is a powerful component of this neurological network. Now, in a paper published in the journal Nature, a group of Stanford University researchers show that blocking oxytocin activity in a certain part of the mouse brain seems to dull the reward the animal gets from hanging out with littermates.

But before they started peering into brains, the scientists had to find a way to gauge the social life of mice.

"You can’t ask an animal, ‘did you have a good time with your friends?’” senior author Rob Malenka said in a phone interview. “You have to develop an assay – then it’s their behavior that tells you.”

To prime the mice for their experiment, Malenka and his colleagues set up what’s called a “conditioned place preference test.” Mice spent 24 hours in a cage with their littermates, then 24 hours in a cage all by themselves. On the third day, the mice were put in a little “house” made by connecting the previous two cages together, with a door that allowed the animals freedom to hang out in either room. The researchers watched to see which rooms the mice preferred to stay in.

To relate this to human experience: “If you go out with some friends to a bar, and you had good time, and the next day you go to this other bar by yourself where no one talks to you, chances are that on the third day you’re going to go back to the bar where you were hanging out with your friends,” Malenka explains.

Mice normally preferred the room where they had palled around with their littermates previously. When the scientists interrupted part of the reward pathway in the brain, they still moved around, but their preference for the social room vanished.

“We found that if we block the actions of oxytocin in this key area of the reward circuitry, called the nucleus accumbens, while they were in the same cage with their buddies, [and] ask them the next day, their behavior told us [socializing] wasn’t rewarding,” Malenka said.

But oxytocin isn’t the entire story. Malenka and his colleagues found that the hormone binds to other receptors that release serotonin, which both induces happy feelings and feeds back into the activity of the nucleus accumbens.

“Oxytocin is not acting by itself, but sort of dancing with serotonin,” Malenka said.

The next step for Malenka and his colleagues will be to try out similar experiments in mouse models of autism. It may be that autism disorders give rise to some sort of oxytocin-blocking effect in the nucleus accumbens. If so, there may be a way to treat social dysfunction by administering oxytocin, perhaps in combination with a drug like Prozac that boosts serotonin levels.

Researchers across the world are already trying out oxytocin treatments for schizophrenia and autism, but there’s not yet any solid verdict on the benefits or effectiveness of the treatment yet. When a group of Australian psychologists tried giving oxytocin to young autistic boys in a small trial published this past July, they saw no significant improvement in social interaction, emotion recognition, or general behavior. But another small study conducted by German researchers found oxytocin nasal sprays did produce a measurable effect on “social processing.”

The problem with administering oxytocin nasal sprays, according to Malenka, “is nobody really knows how it’s working or what the side effects are. If you’re giving something to a human being, it’s really prudent to understand how it’s working. This study provides evidence for one part of the brain that may be really important.”

SOURCE: Dolen et al. “Social reward requires coordinated activity of nucleus accumbens oxytocin and serotonin.” Nature published online ahead of print 11 September 2013.

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