May 2008

Parenting on Autopilot

Robert Bridges, a professor of biomedical sciences, is learning how neurobiology drives behavioral instincts in raising young

By Taylor McNeil

For most new parents, there seems to be an instinctive urge to hug their babies, coddle them and respond quickly when they cry out. But some people, of course, aren't the baby-cuddling type, and don't respond on cue as most others do. What makes the difference? It could be the experience they had as youngsters themselves, or it could be innate, nature overruling nurture. Up until now, we've never been able to truly know the difference.

“You work on the assumption that there are common biological mechanisms across mammals that regulate parental care,” says Professor Robert Bridges. Photo: Andy Cunningham

For Bob Bridges, this is the consuming question: what drives good parenting? An expert in the neurobiology of parental behavior, he's not so much interested in pathologies-what makes things go wrong. He wants to know what makes parenting go right. In his research, the nature-versus-nurture question is beginning to be answered.

Consider this: When females of many species have the functioning of a certain part of their brain compromised-it's the medial preoptic area in the hypothalamus-they suddenly do not show any of the typical signs of nurturing parental behavior. That part of the brain "is a necessary component" for successful parenting, says Bridges, professor of biomedical sciences and director of the reproductive biology division at the Cummings School of Veterinary Medicine. If the area is intact in a female that has not previously given birth and raised young, though, merely administering some of the hormone prolactin into this area of the brain will stimulate maternal behavior.

To understand the basic mechanics of the neurobiology of parenting, Bridges and his colleagues focus on the endocrine system, which regulates the hormones that act on the brain. They are researching how and where prolactin and other hormones act in the brain to stimulate the onset and continuation of parental behavior, and how the mother's reproductive experience changes her reliance on endocrine signals.

Bridges isn't working with humans, of course. He's using rats, which turn out to be a good analogue for humans, at least in this respect: the mothers are nurturing and will readily adopt foster children. How do the scientists measure maternal behavior in rats? Bridges lists four primary activities: retrieval of the young if they leave the nest; grouping the young together; licking them and crouching over them in a nursing position. If you're a female rat and do these things, Bridges considers that good mothering.

All this talk of mothers, though, begs the question: what about fathers? Finding the neurobiological roots of parental behavior in males turns out to be more difficult. That's mostly because species in which the males help raise the kids are the exception, not the rule. Sarah Blaffer Hrdy of the University of California at Davis, a contributor to Neurobiology of the Parental Brain (Academic Press, July 2008), which Bridges edited, notes that in some 90 percent of species, fathering means passing on genes-and nothing else. Males in a few species are indeed caregivers, such as marmosets and the occasional Homo sapiens, but they are few and far between.

From Brain to Behavior

Bridges' central working hypothesis is that the endocrine system induces and maintains maternal care behavior, and helps retain those abilities as well. Take the hormone prolactin. In collaborative studies with other researchers, mice were bred without brain receptors for prolactin, "and in those animals, you definitely see deficits in maternal tests," he says. He and his colleagues are searching for the exact mechanisms in the brain that translate the hormone into parental behaviors.

Take the behavior of foster mothers who have never given birth. Bridges removes baby rats, or pups, that have been fed by their birth mothers and presents them during the day to the foster mothers, and watches their response times. "After an average of five or six days of exposure, [the foster mothers] will start to show these same types of maternal behaviors," he says. "We're interested in what brings about this transition from being a slow responder to one similar to a female who gives birth and who is immediately responsive."

Prior reproductive experience and its impact on endocrine and neural functions in adult females have been given little attention by researchers, Bridges says. It's an area of study, he notes, where findings in the rat model could provide lessons for humans, and with the help of a recently awarded five-year, $1.7 million NIH grant, he and his team, which includes Elizabeth Byrnes, a research assistant professor in biomedical sciences, and David Grattan of the University of Otago, New Zealand, are delving into it now.

"We had shown in females that had given birth previously that circulating levels of hormones, including prolactin, seem to be reduced-they release less prolactin," he says. In fact, the same occurs in humans. Studies have shown that women who had given birth 10 years earlier experience the same effect. The question is what arises to compensate for the reduced amount of prolactin. Is it the result of a change or increase in the sensitivity of the brain to hormones, or is it due to learned behaviors reducing the need for endocrine involvement? Bridges and his colleagues are devising experiments to find out.

One aspect of the effects of parenting experience also relates to anxiety. Rats that have given birth and experience reduced hormone activity display less anxiousness, Bridges reports. He tests this using what's called an elevated plus maze. Shaped like a vertical plus sign, it has two different types of arms, one with walls, and one more open. "An animal that is defined as being anxious tends to stay in those areas with walls, and those that are less anxious tend to go out into the other arms and are more exploratory," Bridges says.

"We found that females who've given birth show reduced levels of anxiety later in life," he says. But not too much later. The researchers have also found that when they test similar rats when they become old, "they are acting more anxious. It's the grandmother-worry effect," Bridges says. "It depends on the age you test-you get different effects. We're trying to understand those mechanisms."

A Growing Field

Research into the neurobiology of parental behavior is a fairly new field, made possible in part by more sensitive hormone assays and the development of new techniques in molecular endocrinology and neuroscience. The research field itself is also growing. Bridges hosted the third Parental Brain Conference in Boston in June 2007, bringing together more than 150 researchers from around the world. This was the largest grouping of scientists attending this conference to date.

Those looking for easy parallels between human and other animals, though, need to be patient.

"Not everything going on in rats is going to go on in humans. It's naÔve to think that's the case," Bridges says. "But you work on the assumption that there are common biological mechanisms across mammals that regulate parental care. It's really up to the creativity of the scientists as to what questions you ask and how they may relate to what goes on in other species."

Taylor McNeil can be reached at taylor.mcneil@tufts.edu.

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