Mouse pups recognize their caregiving mother and distinguish her from novel mothers as newborns and remember them after up to 100 days apart, according to a new study in the journal Cell Reports.
Laham et al. show that mice form memories of their mother within days after birth and that these memories persist into adulthood. Image credit: Laham et al., doi: 10.1016/j.celrep.2020.108668.
Some of the most enduring social connections begin when infants first recognize their caregivers, memories that form the basis of many family relationships.
It remains unknown whether these early social memories persist into adulthood in mice and, if so, which brain regions support them.
“I’m really interested in studying the development of social memory, which is the memory we have for other individuals including certain facts and features about them,” said Blake Laham, a Ph.D. candidate at the Princeton Neuroscience Institute.
“These early memories can form long-lasting social connections that set the stage for social interactions later in life.”
With closed eyes and ears, three-day-old mice are pink, hairless, and a tad translucent, just enough to see the white milk in their bellies.
Although pups at this age have poor motor skills, they had no problem recognizing their caregivers, likely using scent.
Because there is a possibility that animals may recognize their biological mother based purely on genetic factors, Laham and colleagues placed the pups in their study under foster care with a different mouse mother, known as the caregiving mother, at birth.
When the researchers put the pups between the caregiving mother and an unrelated mouse mother, the animals flailed their tiny bodies, rotating themselves to the mother that raised them.
They spent more time pointing their nose towards their caregivers, a sign of recognition and preference.
The scientists found that these memories for their foster mothers lasted into adulthood, with mice retaining the ability to recognize their mothers even after being separated for over 100 days.
“However, there’s this really fascinating behavioral transition once the animal is weaned,” Laham said.
“When the animal is no longer dependent on the caregiving mother, the animal prefers investigating novel mothers.”
“Adult mice might have more interest in investigating if an unknown mouse is a threat or a potential mate.”
To identify what these memories of their mothers look like in the mouse brain, the authors examined a region associated with social memory called CA2 in the hippocampus.
They found that mouse pups showed more biological markers related to neuronal activity in the brain region after exposure to their caregiving mothers.
On the contrary, adults expressed more of those markers after exposure to novel mothers, suggesting that CA2 played role in the behavioral flip.
The team also found that temporarily shutting down neuronal activity in the CA2 region in both mouse pups and adults impaired their ability to distinguish their caregivers from novel mothers.
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Blake J. Laham et al. 2021. Newborn mice form lasting CA2-dependent memories of their mothers. Cell Reports 34 (4): 108668; doi: 10.1016/j.celrep.2020.108668
