Study of Rodents Sheds Light on Nature-Nurture Debate

by Anna Kuchment

It’s no surprise that mothers exert a powerful influence over their children’s personalities. Nurturing parents are likely to rear offspring who grow up to be affectionate, attentive caretakers in their own right. Abusive mothers are likely to raise abusive children.

Until recently, scientists assumed that genes transferred such behavioral traits across generations. To their surprise, they’re now finding that children’s social circumstances, which can alter estrogen receptors in the brain, affect not only their own behavior but also that of their children and grandchildren.
 
Frances Champagne
Frances Champagne researches the effects of licking and grooming in rats.
Image credit: Charles E. Manley
“In a way, we’re reviving Lamarckian ideas,” says Frances Champagne, a behavioral neuroscientist and assistant professor in Columbia University’s Department of Psychology, referring to the 19th-century French naturalist whose theory on the inheritance of acquired characteristics has been widely discredited. “We’re addressing the issue of how social experiences become incorporated at a molecular level in such a way that they can affect not only one generation but multiple generations.”
 
Specifically, Champagne studies the effects of good and bad mothering in rodents. (Male rats do not contribute to the care of offspring.) In her research, she breeds rats and mice to be “high LG” (frequent lickers and groomers of their pups) or “low LG” (not very affectionate), then mixes the populations and watches what happens. She has found that, even when the offspring of non-nurturing mothers are placed with highly affectionate rats, they take on the characteristics of the adoptive parent and pass those nurturing traits on to their pups and grand-pups. This occurs because the pups’ early environment affects their epigenomes, the molecules that determine which genes are switched off and on. “High levels of nurturing seem to promote the activity of genes that make females maternal,” says Champagne.
 
Since the genome was decoded in 2003, many scientists have moved on to the study of the epigenome, which Champagne describes as “anything clustering around the DNA that controls whether it’s expressed or not.” Her group has focused specifically on estrogen, a hormone released late in pregnancy that activates genes promoting a host of maternal behaviors. The pups of good mothers grow up with active estrogen receptors that allow the hormone to bind to cells and turn on their “good parenting” genes. However, the female pups of less affectionate rats develop less active receptors that do not bind estrogen as effectively.
 
By and large, rats are very good mothers—a trait that first attracted Champagne to this work when she was a graduate student in the Neuroscience Program at McGill University in Montreal. The average rat gives birth to 16 to 20 babies, and an average female only has about 6 to 8 nipples. “You can imagine this is quite a juggling act,” Champagne says. “You watch them, and if one pup gets momentarily separated from the group, they retrieve them right away. They spend the majority of their time simply making sure the pups are getting enough warmth and milk. They’re very, very vigilant and, for the most part, all the pups survive, which really is quite an amazing feat when you think of how many mouths they have to feed.”
 
Now that her group has identified one of the pathways by which bad maternal instincts are transmitted, Champagne is hoping to find a drug therapy that will reverse those effects. “This is a big public health issue,” she says. Studies have shown that up to 70 percent of abusive parents were themselves abused, and 20 to 30 percent of abused infants will likely abuse their own kids.
 
On the other hand, children who bond with nurturing, affectionate caretakers develop elevated self-esteem, reduced anxiety levels and are likely to pass along the good parenting tendencies they were exposed to as infants. Not that a pill can substitute for a happy childhood, but it may keep troubled parents from passing their problems on to future generations.
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