Oxytocin, a brain chemical known for promoting social bonding and nurturing behavior, has been used in several studies to potentially treat disorders such as autism, but with inconsistent results. 

Yerkes National Primate Research Center Division Chief Larry Young and his research colleagues in Yerkes’ Division of Behavioral Neuroscience and Psychiatric Disorders as well as Emory’s Center for Translational Social Neuroscience found the dynamic response of neurons to oxytocin may vary depending on the past social experiences of the individual. 

The study was conducted in female prairie voles because they form lifelong bonds with their partners and focused on the nucleus accumbens because it plays an important role in the brain for pair bonding. Tissue from the nucleus accumbens was exposed to TGOT, a drug that mimics oxytocin signals. 

Robert Liu, PhD, professor of biology and director of Emory’s Neuroscience graduate program compared the electrical responses of neurons to oxytocin signals to an analog television, before and after the television is tuned to a station. “Before the animal forms a pair bond, oxytocin reduces the static noise: the neurons in the nucleus accumbens fire spontaneously less often,” said Liu. “But after an animal has been exposed to a partner, it increases the clarity of the signal from the station: the neurons gradually fire with greater strength – but only when electrically triggered.”

In an unexpected turn, researchers found that after bonding, oxytocin signals became coupled to endocannabinoids, molecules produced within the brain resembling the psychoactive substances found in cannabis. By blocking the endocannabinoids, the scientists could interfere with some aspects of the prairie voles pair interactions. 

Blocking endocannabinoid signals increased the likelihood the female vole would display a defensive upright posture, a sign of rejection, in the presence of their partner, but not toward a stranger. However, the pair-bonded animals still spent more time with their partner than a stranger. This reaction shows endocannabinoid signaling is modulating defensive interactions, rather than pair bonding. 

The study suggests the way oxytocin modulates brain circuits changes with prior experience, which may help explain inconsistent results from human studies involving oxytocin.