Oxytocin Can Suddenly Switch on Maternal Behavior

Injections of the "love" hormone, oxytocin, let non-mother rats suddenly hear the ultrasonic distress sounds of mothers’ pups, according to a study in mice published in Nature.

As oxytocin seems to awaken, and drive, other forms of social behavior too, this research could lead to therapy for social anxiety, post-traumatic stress disorder, post-partum depression, and other behavioral issues

"This is beautiful work indeed," Mizrahi Adi, Ph.D., told Bioscience Technology. Adi, a Hebrew University neurobiologist, was uninvolved in the study. "The paper's strength is that it shows how oxytocin is boosting the responses of the cortex in mothers to social stimuli--here, pup calls."

The study

Mouse pups emit ultrasonic distress calls when away from the nest. Mothers use these ultrasonic calls to seek them out, lift them by the scruff of the neck, and bring them home.

(Video Credit: Marlin et al.)

NYU neuroscientist Robert Froemke, Ph.D., and his team discovered that experienced mothers respond to distress cries via a well-orchestrated pattern of excitatory and inhibitory neural activity in the left auditory cortex. This response is lacking in the brains of virgin females. They generally can’t hear pup distress calls. But if oxytocin is delivered to the left auditory cortex at the same time as the calls, the brain activity and behavior of virgin females starts resembling that of mothers.

They suddenly seek out the pups, pick them up by the scruff of the neck, and bring them to their nests. “Amazingly,” wrote Emory University biologist Robert Liu in an accompanying “News and Views,” “naïve virgins that had been injected with oxytocin in the left auditory cortex began retrieving pups earlier than counterparts that received saline.”

The suddenly motherly non-mothers, furthermore, retained their new maternal instincts.

Froemke’s team also showed the left auditory cortex possesses a wealth of oxytocin receptors. This indicates the area may be primed to respond to socially relevant signals, and is very much like the asymmetry of human brain speech processing.

Groundbreaking

The study broke ground on a few fronts. Froemke told Bioscience Technology the field “previously knew that neural responses to pup calls were different in the maternal versus the virgin auditory cortex. However, our study is the first to provide a mechanism by which the virgin brain might be transformed into the maternal state, by the action of oxytocin paired with pup calls. Our study is also the first to determine which kinds of neurons directly respond to oxytocin. “

Liu wrote that while such “oxytocin-experience” interactions were suspected in the recognition of social odors through subcortical neurons, Froemke’s study offered the new insight that such interactions occur at the level of the sensory neocortex.

“There were three surprises” for his team, Froemke told Bioscience Technology. The first surprise was “how quickly oxytocin can act in the brain to both change synapses and neural responses, as well as enable maternal behavior--seconds physiologically, and hours behaviorally.”

The second surprise was “the remarkable temporal milliseconds-timescale precision of neural responses to pup calls in the maternal brain,” he said.

The third surprise was the fact that “oxytocin receptor expression and neural responses to pup calls are left-lateralized, meaning that the responses are stronger in the left auditory cortex than in the right hemisphere, perhaps due to the higher expression levels of oxytocin receptors in left female brains.”

All told, Froemke said: “We believe that our findings may provide a general mechanism by which oxytocin might enhance the salience of social information. But we don't plan on investigating other social behaviors in our lab. Any one behavior is very complex, and social behaviors in particular can be complicated and dynamic, which is in part what makes them so interesting. There are other labs studying other kinds of social behaviors, and that's in good hands. Instead we're interested in asking how the lateralization of receptor expression emerges, and what kinds of experiences or social stimuli, naturally lead to oxytocin release in new mothers.”

Adi noted to Bioscience Technology that “one interesting next step would be to find what role oxytocin plays in other brain circuits involved in motherhood and how downstream circuits to the cortex process social signals.”

Oxytocin is known as the “love” hormone because of the role it plays in mother/child and pair bonding.