Maternal care may leave brain legacy

Mother rats literally groom their daughters to be attentive or neglectful mothers themselves, concludes a team of neuroscientists at McGill University in Montreal.

Adult females who were frequently licked and groomed by their mothers behave similarly toward pups in their care. They also show heightened sensitivity to the hormone estrogen in brain regions devoted to maternal behavior, say Michael J. Meaney and his colleagues. This physiological effect of grooming suggests that a change in the female pup’s brain governs the animal’s own mothering styles, the team concludes in the Oct. 23 Proceedings of the National Academy of Sciences.

In contrast, Meaney’s group finds that adult females who were rarely licked and groomed by their mothers mirror that minimalist maternal style and exhibit relatively little estrogen sensitivity in mothering-related brain regions.

In earlier work, the scientists noted that maternal styles shape the young rats’ behavior regardless of genetics. For instance, if reared by conscientious adoptive mothers, female rats born to unresponsive mothers withstand stress and care for newborns just as effectively as do female rats born to dutiful mothers.

In its new study, Meaney’s group first found that virgin females who had been reared by attentive mothers more often licked and groomed pups placed in their presence than did virgin females who had been reared by tongues-off mothers.

Along with estrogen, the brain hormone oxytocin plays a key role in these maternal responses. Reports have linked oxytocin to sexual and social behavior in mammals (SN: 7/8/00, p. 23).

Six days after giving birth, females reared by attentive mothers displayed greater numbers of oxytocin receptors in several brain areas involved in maternal behavior than did females reared by unresponsive mothers. Other research indicates that surges in estrogen levels lead to increased numbers of oxytocin receptors in these parts of the brain.

There’s more evidence for the oxytocin-mothering link. When given a drug that blocks oxytocin receptors, new mothers licked and groomed their pups at consistently low rates, regardless of their own rearing histories, the scientists say. Yet only those females that were reared by attentive mothers showed a marked increase in oxytocin-receptor activity when treated with estrogen. Such findings show how nature and nurture intertwine during development, say the researchers.

“These experiments elegantly show that, in rats, mothers’ behavior powerfully influences their daughters’ genetic potential for maternal behavior,” comments Bruce S. McEwen of Rockefeller University in New York City.