A type of brain cell helps explain the calming influence of mothers

Scientists have identified neurons that are active when mouse pups interact with their mothers, appearing to reduce their distress. The same neurons may also be involved in human mother-child bonding.

The zona incerta, in the centre of mammal brains, is thought to be involved in integrating sensory information. Studies in mice suggest its role changes as they develop. In infants, neurons within the zona incerta send information to other brain regions, such as the cortex, to inform neural growth elsewhere.

With its role seeming to shift, Yuexuan Li at Yale School of Medicine and her colleagues wondered whether this brain part is implicated in establishing mother-infant relationships.

To investigate, they first assessed what neurons in the zona incerta become active when pups still reliant on milk interact with their mothers. This involved surgically implanting a fibre-optic probe into some pup brains. Through this, the researchers could detect light emitted when neurons fired up.

They found that the neurons that fired up express the hormone somatostatin, which regulates several bodily functions by hindering the release of other hormones, such as the stress hormone corticosterone.

Free social interactions between mothers and infants activated these neurons, whereas contact with a toy did not.

The researchers were also interested in how other social interactions might influence this brain region. They found that contact with a lactating female who wasn’t the pup’s mother, a non-lactating female, siblings or an unrelated male also activated the neurons, but to a lesser extent than the pups’ mothers.

“Our results suggest that social interactions with the mother induce the largest responses, around 1.5-fold difference on the averaged response level, compared to other tested social stimuli,” says team member Marcelo de Oliveira Dietrich, also at Yale.

In another part of the experiment, the team observed brain activity while the infants were socially isolated. During this time, which lasted from 10 minutes to 12 hours, the neurons weren’t activated, but this changed when they were reunited with their mothers.

The reunion also alleviated the pups’ stress responses, measured by whether they made crying-like sounds and released corticosterone.

Finally, the team wanted to see if artificially activating the neurons while the pups were isolated could alleviate their distress. Using chemicals to activate the neurons suppressed their crying and blunted corticosterone release.

The researchers think the zona incerta is probably involved in the development of early social relationships and other parts of the brain in mammals. “The distinct bond between the infant and its mother” is a “hallmark of mammals”, they write in their paper. Mother-child bonding may be involved in the development of parts of the brain, with the zona incerta acting as “a nexus that intertwines defining characteristics of mammalian biology”.

Robert Froemke at NYU Langone Health in New York says the study demonstrates how certain neurons “essentially act to calm the infant”. “It remains a bit unclear though exactly how the pup senses the mother (what aspects of smell, touch or perhaps temperature are important),” he says.

“Another open question is about how much contact is required for safety signals – how long do they last? What promotes healthy development, as opposed to what constitutes neglect?” In people, “it is likely that visual and acoustic inputs – the sights and sounds of caretakers – would also be important or perhaps more important than olfactory signals”, he says.