Baby rats who get extra attention from their mothers cope better with stress later in life, according to a paper  published in tomorrow's issue of Science. Other neuroscientists say the findings emphasize the importance of an infant's environment and reveal how the brain chemistry that makes for a relaxed adult might develop.
In 1957, psychiatrist Seymour Levine showed that baby rats that had been taken away from their mother for 15 minutes each day grew up to be less nervous and make lower amounts of a stress hormone. He guessed that it was not the human handling, but the extra licking and grooming the pups received when returned to their anxious mothers, that destined the pups for greater tranquility.
In an attempt to test that theory and to explain the effects, neuroscientist Michael Meaney and his colleagues at McGill University in Montreal and Emory University in Atlanta began by watching many hours of videotapes of mother rats with their newborns. From the videos, they picked out two groups of rats--one of which naturally spent about twice as much time licking, grooming, and nursing their pups as the other. Later, they compared how the pups behaved when they had grown up and found that the extra nurturing had paid off. When put in a stressful situation, such as being crammed into a small plastic box, the pups who got more licking and grooming were more relaxed. These rats also produced about half as much glucocorticoid and adrenocorticotropic hormone--hormones that normally increase pulse and circulation rate in response to stressful situations.
The reason, the researchers discovered, was that these rats had developed more brain receptors for glucocorticoid, which provide a feedback mechanism that helps shut down excess hormone production. The study "takes what was mostly clinical lore and really shows a strong biological basis and experimental proof of these effects," says Myron Hofer, a developmental psychobiologist at the Columbia University College of Physicians and Surgeons in New York City. Next, Meaney and his team will try to learn which of the mothering behaviors really changes the baby rat's biochemistry and how it does so.
Human stress chemistry is similar to that of rats, and overproduction of glucocorticoid hormones can increase the risk of heart disease, diabetes, alcoholism, and other diseases, Meaney says. But no human studies have yet examined the impact of subtle differences in early childcare on stress hormones and susceptibility to disease.