A rodent study from the NYU Grossman School of Medicine shows that newborn cries stimulate mothers to release oxytocin, which helps breast milk secretion. This study highlights the important role that the release of this hormone plays in breastfeeding and ongoing maternal care, even when tired.
Led by researchers at NYU Grossman School of Medicine, the study explores centuries-old observations in humans and other mammals that when babies begin to forage for food, the sound of crying alone can prompt mothers to release breast milk. Decades of research have shown that this call for food, not the sucking itself, triggers a surge of oxytocin. However, the mechanism and purpose behind this cry-to-milk pipeline remain unclear.
According to research published today (September 20) in the journal Nature, when mouse pups start to cry, acoustic information is transmitted to an area in the mother's brain called the posterior thalamic nucleus (PIL). This sensory hub then sends signals to oxytocin-releasing brain cells (neurons) in another area called the hypothalamus, the control center for the hormone's activity.
Most of the time, these hypothalamic neurons are "locked" by proteins that act like gatekeepers, preventing false alarms and wasted milk. However, after 30 seconds of continuous crying, signals from PIL were found to build up and overwhelm these inhibitory proteins, triggering the release of oxytocin.
"Our findings reveal how a crying baby prepares the mother's brain to nurse," said Habon Issa, a graduate student at NYU Langone Health and co-first author of the study. "Without this preparation, there can be a delay of several minutes between sucking and milk flow, which can lead to frustration for the baby and stress for the parent."
The results also showed that the stimulating effect of oxytocin occurred only in female rats, but not in female rats that had never given birth. Furthermore, the mother rats' brain circuits responded only to the cries of their pups, not to computer-generated tones that mimicked natural cries.
Issa believes this study is the first to describe how sensory experiences such as hearing directly activate oxytocin neurons in the mother. She noted that the scientists used a relatively new molecular sensor called iTango to measure in real time how much oxytocin is actually being released by brain cells. Until now, she said, researchers could only make indirect measurements using surrogates because the hormone is small and degrades quickly.
For the study, the team examined brain cell activity in dozens of female mice. Then, in a form of "reverse engineering," they traced how sound information triggers milk flow through different areas of the brain.
Next, the team explored how this circuit affects parenting behavior. Typically, Issa said, when pups become lost or taken away from the nest, no matter how many times it happens, the mother quickly retrieves them. However, when the researchers chemically blocked PIL's communication with oxytocin neurons, the mice eventually became tired and stopped retrieving pups. Once the system was turned back on, the mothers overcame their fatigue and continued caring for their babies.
"These results suggest that cry-induced brain circuits are important not only for suckling behavior, but also for maintaining the mother's long-term attention and encouraging mothers to effectively care for their pups even when they are exhausted," said study senior author Robert Froemke, Ph.D., the Skirball Foundation Professor of Genetics in the Department of Neuroscience and Physiology at NYU Langone.
Froemke, who is also a professor in the Department of Otolaryngology-Head and Neck Surgery at NYU Langone, added that understanding how the oxytocin system works (and how it goes wrong) in our species may lead to new ways to help human mothers who want to breastfeed but have difficulty doing so.
Fromke, a member of NYU Langone's Neuroscience Institute, cautions that the researchers did not measure lactation itself, just the release of hormones that prompt lactation.