Infant mice are born blind, deaf and vulnerable. They spend their early lives snuggled under their mothers, barely moving. If mum is forced to move nests, the pups cling to her fur. And if they fall off, as often happens, they make distinctive high-pitched distress calls. When virgin females hear these calls, they ignore them. Experienced mothers, however, will always head back to retrieve their lost youngsters. Something changes their brain during their first forays into motherhood, turning these unimportant calls into irresistible cries.
That something is a hormone called oxytocin. Bianca Marlin from New York University have shown that when virgin females get a boost of oxytocin, they readily retrieve crying pups, just like experienced mothers. The hormone specifically affects a part of their brain that deals with sounds. It tunes the neurons there to make the cries of lost pups more “socially salient”. These sounds now grab attention, just like a name being spoken across the room in a loud party. Oxytocin makes them pop.
This is far from the first study to show that oxytocin plays an important part in the social life of mammals. The substance is released during labour and strengthens bonds between mothers and children. It also cements the connections between monogamous voles. For these reasons, oxytocin often gets plastered with trite and misleading names, like “hug hormone” or “moral molecule”. It is nothing of the sort. Although some psychological studies have found that it promotes trust, empathy, and co-operation, others have shown that it can foster envy, schadenfreude, favouritism, and distrust in different situations.
Rather than virtue incarnate, oxytocin is more of an all-purpose social molecule. It probably acts as a spotlight that draws our attention to social cues. We then react differently, depending on our temperament, or whether those social cues are positive or threatening.
That’s a more nuanced view of this much-hyped molecule, but it’s still a bit unsatisfying. As I wrote in 2012, “the problem with oxytocin research is that too many people have been focusing on cataloguing what it does… rather than how it works.” There’s been a lot of psychology and much less hard neuroscience. Thankfully, things are changing. Several teams of scientists are looking past the alliterative nicknames and actually working out how oxytocin affects neurons.
Marlin started by finding exactly where oxytocin exert its influence in the brain. The hormone works by attaching to a receptor protein, like a cable plugging into a socket. Marlin’s colleagues created an antibody that stick to the socket, revealing its presence. It turned out to be especially common in a mouse’s left auditory cortex—a region in the left side of their brain that processes sounds. If Marlin injected oxytocin into this area, or stimulated the local neurons to release the hormone themselves, she could turn negligent, inexperienced females into doting, pup-retrieving ones.
That result was surprising, says Robert Liu from Emory University, because no one had really linked this part of the brain to oxytocin before. And “it’s surprising that just putting oxytocin in that region could lead to the behaviour,” he adds. Then again, “it’s only in the last few years that people have really started looking at where and how oxytocin is acting on neurons.”
Marlin had the ‘where’ bit. To understand the ‘how’, she used electrodes to record the activity of individual neurons in the auditory cortex, to work out how they react to oxytocin. She found that the hormone strengthens excitatory signals that rouse neurons into a buzz of activity, while also suppressing inhibitory signals that would otherwise shush them into silence. This creates a window of time in which the auditory cortex becomes much more responsive to incoming information—in this case, the distress calls of pups.
Robert Froemke, who led the study, explains that the auditory cortex filters the sounds we hear, so that we can pay attention to the ones that matter. That’s why we can listen to a single person in a loud bar, or ignore the ticking of a clock at home. “In the naive virgin mice, the auditory cortex throws away the pup calls,” Froemke says. But when oxytocin hits, those once-ignored calls start to stand out.
“They seem to have shown that oxytocin transforms an otherwise irrelevant stimulus into one that grabs my immediate attention and requires my immediate action,” says Jennifer Bartz from McGill University. “It increases attention not only to social cues but also to their personal significance.”
The neurons of the auditory cortex also started responding to pup calls more consistently and synchronously. “If the neurons are firing disjointedly, they cancel each other out and create a lot of noise,” says Larry Young from Emory University. “This paper suggests that oxytocin causes these neurons to fire with perfect timing, amplifying their response to the pups’ calls.” Again, this makes the mother more attentive.
This only needs to happen once. Once the mums start paying attention to the distress calls, they can then retrieve their pups without any further pulses of oxytocin. They don’t continually need the hormone to be good parents. They just need it to initially point their brains at the right stimuli. “It may be learning or it may be unlocking an instinct,” says Froemke. “It’s not really clear what’s going on there.”
He now wants to know what triggers the initial burst of oxytocin that changes the brains of inexperienced females. Also, once oxytocin changes the brain, how does that lead to the actual act of retrieving a pup? Finally, does oxytocin affect other senses too? Does it make mothers more sensitive to the smell of babies, or to their touch? And what about humans and other animals? “In people, visual cues are important,” says Young. “Is this kind of thing happening in areas that process visual information?”
Young praises Marlin and Froemke’s work. “They have delved deep into the neural mechanism, more than anyone else has done,” he adds. “This kind of study, which gets into details and doesn’t attribute fluffy psychological traits to this molecule, is exactly what we need to move the field forward.”
Reference: Marlin, Mitre, D’Amour, Chao & Froemke. 2015. Oxytocin enables maternal behaviour by balancing cortical inhibition. Nature http://dx.doi.org/10.1038/nature14402
More on oxytocin:
- One Molecule for Love, Morality, and Prosperity?
- Oxytocin: Still Not a Moral Molecule
- Not a hug hormone – fish version of oxytocin acts as social spotlight
- No love for outsiders – oxytocin boosts favouritism towards our own ethnic or cultural group
- The dark side of oxytocin, much more than just a “love hormone”