Certain brain cells explain maternal calming effect

Scientists have identified neurons that become active when mouse pups interact with their mothers, seemingly reducing stress — and the same neurons may also be involved in the bond between human mothers and their young.

The zona incerta, located in the center of the mammalian brain, is thought to be involved in integrating sensory information. Studies in mice suggest that its role changes as mice develop. In infants, neurons in the zona incerta send information to other brain regions, such as the cortex, to promote neural growth elsewhere.

Its apparent shifting role led Yuexuan Li of the Yale School of Medicine and his colleagues to wonder whether this part of the brain might be involved in establishing a mother-infant bond.

To investigate, the researchers first assessed which neurons in the zona incerta become active when puppies, who are still dependent on their mothers’ milk, interact with their mothers. This involved surgically implanting fiber-optic probes in the brains of some of the puppies, allowing the researchers to detect the light emitted when neurons became active.

The researchers found that activated neurons express a hormone called somatostatin, which regulates several bodily functions by inhibiting the release of other hormones, including the stress hormone corticosterone.

Free social interactions between mothers and infants activated these neurons, but contact with toys did not.

The researchers were also interested in how other social interactions might affect this brain region: They found that contact with lactating females who were not the pups’ mothers, non-lactating females, siblings, or unrelated males also activated neurons, but not as much as contact with the pups’ mothers.

“Our results suggest that social interaction with the mother elicits the greatest response compared to the other social stimuli tested, with a difference of about 1.5-fold in mean response levels,” said Marcelo de Oliveira Dietrich, also from Yale University, a member of the research team.

In another part of the experiment, the team monitored the infants’ brain activity while they were socially isolated: during these periods, which lasted between 10 minutes and 12 hours, no neurons were active, but this changed when the infants were reunited with their mothers.

Reunion also reduced the puppies’ stress response, as measured by whether they made crying noises and released corticosterone.

Finally, the team wanted to see whether artificially activating neurons while the puppies were isolated could reduce stress in them: Activating the neurons using chemicals suppressed the puppies’ crying and blunted their release of corticosterone.

The researchers believe the zona incerta may be involved in early social relationships in mammals, as well as the development of other parts of the brain. “The distinct bond between infant and mother” is “a hallmark of mammals,” they write. The bond may be involved in the development of parts of the brain, with the zona incerta acting as “a nodal point that intertwines the elements that define mammalian biology.”

Robert Froemke of NYU Langone Health in New York says the study demonstrates that certain neurons “essentially serve to soothe the infant. But it’s still a little unclear how the infant senses its mother — which aspects of smell, touch or temperature are important,” Froemke says.

“Another open question is how much contact is needed to send a safety signal, and how long that contact lasts? What promotes healthy development, as opposed to neglect?” In humans, “visual and auditory input — the sight and sound of the caregiver — is also likely to be important, or perhaps more important, than olfactory cues,” he says.