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• Posted September 20, 2023

A Baby Cries & Mom's Breast Milk Releases: New Study Could Explain Why

Many a new mom knows that when her newborn cries, her milk releases.

Now, animal research reveals a newly discovered brain circuit that may explain why that happens.

This new study showed that when a mouse pup starts crying, sound information travels to an area of its mother's brain called the posterior intralaminar nucleus of the thalamus (PIL).

This then sends signals to oxytocin-releasing brain cells (neurons) in another region called the hypothalamus, which is a control center for hormone activity.

While most of the time, these hypothalamus neurons are kept in check by proteins acting as gatekeepers to prevent wasted milk, after 30 seconds of continuous crying, signals from the PIL were found to build up and overpower these inhibitory proteins, triggering oxytocin release.

“Our findings uncover how a crying infant primes its mother's brain to ready her body for nursing,” said study co-lead author Habon Issa, a graduate student at NYU Langone Health, in New York City. “Without such preparation, there can be a delay of several minutes between suckling and milk flow, potentially leading to a frustrated baby and stressed parent.”

The oxytocin boost only occurs in mother mice and not in female mice who have never given birth. And the mothers' brain circuitry only responded to cries from her pup and not to computer-generated tones designed to mimic natural wails.

Issa said the study shows how sensory experiences like hearing directly activate oxytocin neurons in mothers; however, animal research doesn't always pan out in humans.

To study this, the scientists used a relatively new kind of molecular sensor called iTango. This measured actual oxytocin release from brain cells in real time.

Without that, previously researchers could only take indirect measurements using proxies because the hormone is small and degrades quickly.

In this study, the research team examined brain cell activity in dozens of female mice. Then they traced how sound information travels through different areas of the brain to trigger milk flow.

The team also looked at how the brain circuit affects parenting behavior.

Typically, the moms would quickly retrieve their pups when they strayed from the nest, no matter how many times they had to do this, Issa said.

But when the researchers chemically blocked the PIL from communicating with oxytocin neurons, the mice eventually tired and stopped fetching their pups.

They once again began fetching and caring for their offspring after the PIL system was turned back on.

“These results suggest that the crying-prompted brain circuit is not only important for nursing behavior, but also for maintaining a mother's attention over time and encouraging effective care of her young even when she is exhausted,” senior study author Robert Froemke, a professor of genetics in NYU Langone's department of neuroscience and physiology, said in a university news release.

Learning how oxytocin works may offer new ways to help human mothers who want to breastfeed but struggle to do so, Froemke said.

He cautioned that the researchers did not measure lactation itself, only the hormone release that prompts it.

The findings were published online Sept. 20 in the journal Nature. Funding for the study was provided by the U.S. National Institutes of Health, the Natural Sciences and Engineering Research Council of Canada and a Howard Hughes Medical Institute Faculty Scholarship.

More information

The U.S. Centers for Control and Prevention has more on breastfeeding.

SOURCE: NYU Grossman School of Medicine, news release, Sept. 20, 2023