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Study: Brain development affects teens' behavior

Brain imaging and tests on hamsters show it's not hormones: Their brains just haven't finished growing yet.

Every parent dreads it.

Almost overnight a sweet, cheerful, obedient child mutates into a churlish monster prone to recklessness and unpredictable mood swings.

This is not The Exorcist. This is adolescence.

Parents and experts have always blamed the same hormones that catapult young bodies into adulthood for the sleeping until noon, the reckless driving, the drug use and the other woes of adolescence. But recent research shows that what's going on above teenagers' necks, not raging hormones, explains the changes.

Beginning around age 11, the brain undergoes major reorganization in an area associated with things like social behavior and impulse control. Neuroscientists figured this out only in the last few years, and the discovery has led them to see adolescence as a period when the developing brain is vulnerable to traumatic experiences, drug abuse and unhealthy influences.

"The adolescent brain is different. It's still growing," says Fulton Crews, a neuroscientist at the University of North Carolina in Chapel Hill.

Not long ago, neuroscientists thought that the brain stopped growing by the time a child entered nursery school. By then, it was thought, nearly all the brain's wiring had been connected and the only remaining task was to program that hardware.

But new brain imaging technologies have shattered that notion. Using techniques like MRI and positron emission tomography, or PET scanning, researchers have detected brain growth throughout childhood and well into adolescence.

Because their brains are not yet mature, adolescents do not handle social pressure, instinctual urges and other stresses the way adults do. That may explain in part why adolescents are so prone to unsavory or reckless behavior.

This year in the scientific journal Nature, researchers presented a series of time-lapse images depicting brain growth from age three to 15. The images showed a tangle of nerve cells sprouting in the part of the brain above the eyes, then a period of "pruning" after puberty, when about half of the new fibers are cut away to create an efficient network of circuits.

All this action happens in a part of the brain known as the prefrontal cortex, an area responsible for what neuroscientists call the "executive functions." Those functions are practically a laundry list of the qualities adolescents often lack _ goal-setting, priority-setting, planning, organization and impulse inhibition.

Adolescence is a time of risk-taking, says Lynn Ponton, a psychiatrist at the University of California-San Francisco and author of The Romance of Risk: Why Teenagers Do the Things They Do.

"A big part of adolescence is learning how to assess the risk in an activity," Ponton says. "Part of the reason teenagers aren't good at risk-taking is that the brain isn't fully developed."

Looked at that way, it is no big surprise that accidents are the leading cause of death among adolescents, or that teens are more likely to become crime victims than any other age group. It's no wonder that the vast majority of alcoholics and smokers get started during their teen years, or that a quarter of all people with HIV contract it before age 21.

It's no big secret that things like criminal records and sexually transmitted diseases can really mess up your life. But neuroscientists are learning that less serious stuff can also have lasting effects.

Scientists conduct most of their research on adolescent brain development using animals, because it would be unethical to experiment with human teens. Animals don't all go through a transitional period between childhood and adulthood, but most mammals do exhibit some kind of adolescence.

"They don't hang out at malls and spike their hair and stuff, but their social behavior and social structure changes dramatically," says Linda Spear of Binghamton University in New York.

Adolescent rats, for example, show more interest than adults do when strange objects are put into their cages. They start hanging out with their peers more, exploring their surroundings intensely and flitting from one activity to the next.

Craig Ferris, who studies golden hamsters at the University of Massachusetts Medical Center in Worcester, says that in the wild his study subjects enter adolescence when they are ejected from the nest at about 25 days of age. For about two weeks they wander the wheat fields of Syria, looking for a nest that will take them in or founding one of their own.

Ferris' experiments show that a golden hamster's experiences during this stage can determine how it will behave for the rest of its life. If an adolescent golden hamster is put in a cage with an aggressive adult for an hour each day, it will grow up to become a bully, picking on animals smaller than itself. But it will cower in fear around hamsters its own size.

Those golden hamsters raised in the presence of aggressive adults also grow up to have lower than normal levels of vasopressin, a chemical associated with aggression, in the brain's hypothalamus. And they sprout more receptors in the hypothalamus for serotonin, a chemical that blocks vasopressin.

Ferris and his colleagues aren't sure yet exactly what to make of the chemical changes they observe. But they are certain that at least for golden hamsters, the experience of being intimidated by an adult during adolescence has permanent effects.

"The take-home of all this stuff is that the brain is constantly interacting with the environment," Ferris says.

During adolescence, he and his colleagues hypothesize, the developing brain picks up cues from the environment and uses them to help determine "normal" behavior.

"If the environment provokes or encourages aberrant behaviors, those behaviors become the norm," says Jordan Grafman of the National Institute of Neurological Disorders and Stroke.

To neuroscientists, one of the most disturbing behaviors among today's adolescents is binge drinking. Studies have already shown that alcohol exposure in utero can have devastating effects on the developing brain, and many researchers fear the period of vulnerability could extend through childhood and into adolescence.

Researchers at the University of North Carolina recently decided to test the sensitivity of the adolescent brain to binge drinking by subjecting rats to an alcohol bender. Four times a day for four days, they gave both adolescent and adult rats 10 grams of alcohol per kilogram of body weight. After the rats had sobered up the researchers looked for brain damage and found more in adolescent rats compared to adults. Most importantly, the adolescents sustained damage in brain regions associated with addiction.

Researchers at Duke University found that adolescent brains respond more intensely to nicotine. The scientists injected rats with nicotine every day for more than two weeks, a dose comparable to what a typical smoker receives. In all of the rats the number of chemical receptors dedicated to nicotine increased _ a sign of addiction. But in adolescents, the number of nicotine receptors increased twice as much as adults.

"What we found is that the adolescent brain gets a lot more bang for the buck," says Theodore Slotkin, one of the scientists who performed the research.

A follow-up study published in the October issue of Brain Research showed that adolescent nicotine exposure caused permanent behavioral problems as well, especially for females. Even after two weeks with no nicotine, female rats were less interested in moving around and raising their young than counterparts who had never been exposed.

That may be because nicotine retards cell division in the hippocampus, a brain region that continues to grow into adulthood in females, but not males.

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