Rat study links future stress, maternal care

BRIGHTON, England -- The way a mother cares for her baby can determine how stressed out the child will be as an adult because her nurturing can permanently change the way the infant's genes operate, new studies on rats suggest.

The studies, presented Sunday at a conference on the fetal and infant origins of adult disease, found that baby rats who were licked by their mothers a lot turned out to be less anxious and fearful as adults and produced lower levels of stress hormones than those who were groomed less.

The scientists found that the mothers' licking caused the baby's brain to crank up a gene involved in soothing the body in stressful situations.

Several human studies have found an association between a mother's nurturing and the future mental health of her children. The rat research, led by Michael Meaney, a professor of medicine at McGill University in Montreal, Canada, for the first time rigorously tested whether it really is the mother's behavior that makes the difference and showed what happens in the brain of the offspring to produce the adult characteristics.

However, it's unclear how the findings translate to humans, especially whether kissing and cuddling would be the equivalent behavior, experts said.

"That's where we start to become cautious," Meaney said. "In the rat, the key input is tactile, so it's very tempting to say tactile stimulation could do the same for humans, but we don't know that."

Rats are born with their eyes closed and are very sensitive to touch, whereas humans are born into a more complex environment with a greater range of sensory stimuli, he said.

Experts said that while the details of what maternal behaviors are important in humans might differ, they suspect the general principle and mechanism will prove true.

"This is a very important study. It's fabulous data -- really world-class," said Peter Gluckman, a professor of pediatric and perinatal biology at the University of Auckland in New Zealand, who was not involved in the research. "It shows us that the expression of genes in mammals can be permanently changed by how mothers and infants interact and how that can have long-term effects on behavior and psychiatric health."

The study involved more than 100 rats in various experiments.

"All the mothers nurture their pups, provide ample milk and the pups grow up perfectly well. But there is one behavior, called licking and grooming, that some mothers do much more than others -- four or five times as much," Meaney said.

Meaney set out to test whether baby rats who are licked more turn out differently from those who are licked and groomed less and if so, why.

"The pups who are licked more are less fearful, they produce less stress hormones when provoked and their heart rate doesn't go up as much, so they have a more modest stress response than the pups who are licked much less," he said.

The brain contains receptors for stress hormones such as cortisol. The more receptors there are, the more sensitive the brain is to cortisol and the easier it is for the brain to tell the adrenal glands when to stop cranking out the hormones. The receptors set the tone for how the body responds to stress.

Meaney found that the rats who were reared with much licking had more cortisol receptors in their brains than the others and he determined why and how. He examined the DNA of about 50 rats who were licked a lot and another 50 who were not.

"The receptors are made by genes. The gene was more active in adult rats who were reared by high-licking mothers than in those raised by low-licking mothers," he said.

To verify that it was the licking behavior that was causing the difference and not inherited genes, the scientists swapped the babies and mothers around, so that offspring of mothers that licked a lot were given to low-licking mothers and vice versa. The results were the same -- those who got licked a lot turned out to be less stressed out as adults, regardless of who their natural mother was, and the gene responsible for that cooler response was more active.

The scientists even took the mothers out of the picture altogether and stroked the baby rats with paint brushes.

"It does the same thing that maternal licking does," Meaney said.

The change in the production of the brain receptors was apparent by the second week of life and could be reversed by chemical manipulation in the rats, which suggests the changes would not be irreversible in humans, Meaney said.

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