Credit: Blake J. Lam / Princeton University
In mice, the oldest social memories can form at the age of three days and continue into adulthood, the scientists reported Jan.26 in the journal. Cell Reports. They showed that rat pups prefer their mates over unfamiliar mice as newborns and remember them after up to 100 days of separation – even though they prefer unfamiliar mice as adults.
First author Blake J. Lam from Princeton University: “I am really interested in studying the development of social memory, which is the memory that we have of other individuals including some facts and characteristics about them.” These early memories can form long-term social bonds that pave the way for social interactions later in life. To understand how and when social memories are first developed, Lahham turned to the memory of baby mice about their mothers.
With eyes and ears closed, three-day-old mice are pink, hairless, and translucent, just enough to see white milk in their stomachs. Although puppies at this age have poor motor skills, they have had no trouble identifying their caregivers, likely using scent. Because the animals have the potential to recognize their biological mother based on purely genetic factors, Lahham and colleagues placed the pups in their study under the heading of “foster care” with a different mouse mother, known as the caregiver, at birth.
When the puppy’s welder was placed between the caregiver and the unrelated mother of a mouse (a new mother), the animals ejaculated their small bodies, taking turns taking over the mother who raised her. They spent more time pointing their noses towards their caregivers, which is a sign of recognition and preference.
The authors found that these memories of their adoptive mothers continued into adulthood, with the mice retaining the ability to recognize their mothers even after they had been separated for more than 100 days. However, “there is this really cool behavioral shift once the animal is weaned,” says Lahham. “When the animal is no longer dependent on the mother caring for it, the animal prefers to investigate new mothers.” Meat suggested that adult mice might be more interested in investigating whether the unknown mouse was a threat or a potential mate.
To determine what their mothers’ memories looked like in a mouse brain, the researchers examined an area associated with social memory called CA2 in the hippocampus. They found that the pups of the mice showed more biological markers related to neural activity in a region of their brain after exposure to their caregiver mothers. Conversely, adults expressed more of these markers after exposure to new mothers, indicating that CA2 played a role in behavioral inversion.
They also found that temporarily turning off neuronal activity in the CA2 region in both pups and adults impaired their ability to distinguish between caregivers and new mothers. The next step for Lahham and co-workers is to explore how neighboring brain regions contribute to this phenomenon.
“Our life is made up of memories – but at the end of the day, all of our memories that mean a lot to us, that stimulate us, that fill us with joy or fear, are just nerve cells communicating with each other.” “This is great for me,” Lahham says.
This work was supported by a National Institute of Mental Health grant.
Cell Reports, Lahham et al .: “Newborn mice form lasting CA2-dependent memories of their mothers” https://www.cell.com/cell-reports/fulltext/S2211-1247(20)31657-0
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