Like many newborn mammals, baby mice cry to get their mother’s attention. But the mother doesn’t instinctively recognize these calls; she must learn the sounds of her offspring—just as human parents must learn the cries of their infants. Now, a team of researchers has discovered that the hormone oxytocin, which has been tied to trust and maternal bonding, holds the key to how this learning occurs. Only after oxytocin tweaks the brain of a female mouse does she respond with a mother’s concern and attentiveness to crying pups.
“It’s an exciting study with implications that … could be helpful to certain disorders, such as autism,” says Larry Young, a neuroscientist at Emory University in Atlanta who was not involved in the work.
To understand the role oxytocin plays in a mother mouse’s brain, scientists at New York University School of Medicine first investigated how female mice in general respond to the distress calls of baby mice. Pups emit ultrasonic cries when they are separated from the nest, which sometimes happens when a mother carries her babies to a new location. (Moms change nest locations regularly to elude predators.) When a mother hears these cries, she runs to the lost pup, picks it up, and carries it back to her nest. Other scientists have shown that moms respond even to the distress cries of pups that aren’t their own, readily approaching loudspeakers that broadcast the calls. Most virgin female mice, though, couldn’t care less; they seem completely indifferent to the pups’ cries for help. And yet, some virgin females that have either been housed with a mother and her litter or have been injected with oxytocin will retrieve crying infants.
That discovery led the team’s leader, neuroscientist Robert Froemke, to suspect that oxytocin must help “transform the virgin brain into the maternal.” But how? The researchers knew from other studies that a maternal mouse’s memories of pups’ distress cries are stored in the brain’s auditory cortex, which controls the processing of sounds. The auditory cortex has two sides, a right and a left. In both sides, the team discovered oxytocin receptors and oxytocin producing neurons—something never known before. It also found that the left auditory cortex is especially rich in receptors for the hormone, suggesting that this part of the brain is specialized in recognizing social signals. “It’s very similar to the lateralized speech processing in human brains,” Froemke says. In most people, structures in the brain’s left hemisphere control speech.
The scientists confirmed the importance of the left auditory cortex in the retrieval behavior of maternal mice by blocking its activity with a drug: Treated moms largely ignored distressed babies.
And when the researchers injected oxytocin into the left auditory cortex of virgin female mice, the rodents began retrieving crying infants far sooner than did mice that were given only a saline shot, the team reports online today in Nature. In some mice given the oxytocin, the reaction was “almost instantaneous,” Froemke says. “They weren’t retrieving, and then—boom! They were.”
Based on other experiments the researchers carried out, they think that oxytocin helps the maternal mice learn and form a memory of the pups’ distress calls and attend to them. “It both makes sure her brain is paying attention to these social cues and transforms her behavior,” Young says.
Discovering this new role for oxytocin is groundbreaking, says Robert Liu, a neuroscientist also at Emory University who was not involved in the study. “We didn’t used to think about the role oxytocin may be playing inside the brain. But that’s where all the excitement [in this field] is now.” It’s likely, he and others think, that oxytocin is also involved in the brain’s processing of olfactory and visual signals. And, if that’s the case, the so-called love hormone may in time lead us to far better treatments for disorders such as autism and postpartum depression.